TW202511698A - Interferometer configured to measure small structures - Google Patents

Abstract

An interferometer is configured to characterize a plurality of structures in a sample including High Aspect Ratio structures. The interferometer illuminates the sample with illumination optics having a numerical aperture that is smaller than the numerical aperture of the collection optics that collect the reflected light in order to increase the amount of light that penetrates the depth of the structures and improves the signal to noise ratio. During measurement, the focal position of the collection optics with respect to the sample is fixed, while an optical path difference between the sample light and the reference light is altered to scan the coherence fringes through the depth of the structures. Images of a region of the sample are collected at different optical path differences between the reference light and the sample light and are analyzed to characterize the structures in the sample.

Description

Interferometer configured to measure small structures

The subject matter described herein relates to interferometry and, more particularly, to methods and apparatus for measuring deep and narrow structures.

Interferometry is an established method for accurately measuring a variety of physical parameters including the surface shape or profile of solid objects. Multiple data frames are acquired, each with a different relative phase shift between the reference and test beams, and the data is processed by a computer to determine the relative path difference between reference and test with high accuracy.

The semiconductor and other similar industries are producing smaller and more complex structures that require non-contact evaluation during manufacturing. For example, Through Silicon Vias (TSV) are a type of structure produced to connect devices and chips in advanced packaging of integrated circuits. For example, TSVs can include both trenches and holes. Structures such as TSVs are sometimes referred to as High Aspect Ratio (HAR) because of their deep depth and narrow width. As complex structures such as TSVs continue to increase in depth and narrow in width, it becomes increasingly difficult or even impossible to measure the profile of these devices using conventional optical metrology methods. For example, with conventional interferometers, not enough light can penetrate deep into the structure and return to the detector to provide an adequate signal.

An interferometer is configured to characterize a plurality of structures in a sample including structures having a high aspect ratio. The interferometer illuminates the sample using an illumination optic having a numerical aperture that is smaller than the numerical aperture of a collection optic used to collect reflected light from the sample, thereby increasing the amount of light that penetrates deep into the structure to improve the signal-to-noise ratio. Additionally, the collection optic is focused at a fixed focal position that is constant relative to the sample, while the optical path difference between the sample light and the reference light is varied, or the periodicity of the spectrum of the light is modulated to scan coherence fringes through the depth of the structure during measurement. Images of a region of the sample are collected with different optical path differences between the reference light and the sample light and analyzed to characterize the structure in the sample.

In one embodiment, a method of characterizing a region of a sample including a plurality of structures using an interferometer includes: splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The method includes: illuminating the region of the sample with the sample light using an illumination optical element having a first numerical aperture; and focusing a collection optical element at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture. Scanning the sample through a depth of the sample by interference of reflected sample light from the sample with reflected reference light from the reference mirror to generate one or more coherent envelopes while not changing the focal position of the collection optics relative to the sample. The method further includes imaging, using the collection optics, reflected sample light from the sample that interferes with reflected reference light from the reference mirror at a plurality of depths of the one or more coherent envelopes to generate a plurality of images. The plurality of images of the region of the sample that includes the plurality of structures are analyzed to characterize the region of the sample.

In one embodiment, an interferometer configured for characterizing a region of a sample including a plurality of structures includes means for splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The interferometer further includes means for illuminating the region of the sample with the sample light using illumination optics having a first numerical aperture; and means for focusing collection optics at a focal position relative to the sample, the collection optics having a second numerical aperture greater than the first numerical aperture. The interferometer further includes: means for generating one or more coherent envelopes by interference of reflected sample light from the sample with reflected reference light from the reference mirror through depth scanning of the sample, while not changing the focal position of the collection optical element relative to the sample. The interferometer further includes: means for imaging the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror at a plurality of depths of the one or more coherent envelopes using the collection optical element to generate a plurality of images. The interferometer further includes: means for analyzing the plurality of images of the region of the sample including the plurality of structures to characterize the region of the sample.

In one embodiment, an interferometer configured for characterizing a region of a sample including a plurality of structures includes a beam splitter configured to split illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The interferometer further includes: an illumination optical element having a first numerical aperture, configured to illuminate the region of the sample with the sample light; and a collection optical element configured to focus at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture. An actuator is configured to generate one or more coherence envelopes by interference of reflected sample light from the sample with reflected reference light from the reference mirror through depth scanning of the sample, while not changing the focal position of the collection optical element relative to the sample. The interferometer further includes: a detector configured to image the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror at multiple depths of the one or more coherence envelopes using the collection optical element to generate multiple images. The interferometer further includes: at least one processor configured to analyze the multiple images of the region of the sample including the multiple structures to characterize the region of the sample.

In one embodiment, a method of characterizing a region of a sample including a plurality of structures using an interferometer includes: splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The method includes: illuminating the region of the sample with the sample light using an illumination optical element having a first numerical aperture; and focusing a collection optical element at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture. Changing an optical path difference between the sample light and the reference light while not changing the focal position of the collection optical element relative to the sample. The method further includes: using the collection optical element to image the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror by a plurality of optical path differences to generate a plurality of images. The plurality of images of the region of the sample including the plurality of structures are analyzed to characterize the region of the sample.

In one embodiment, a method of characterizing a region of a sample including a plurality of structures using an interferometer includes: splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror. The method includes: illuminating the region of the sample with the sample light using an illumination optical element having a first numerical aperture; and focusing a collection optical element at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture. The method includes: modulating the periodicity of the spectrum of the illumination light or interference light generated by interference of reflected sample light from the sample with reflected reference light from the reference mirror, while not changing the focal position of the collection optical element relative to the sample. The method further includes: using the collection optical element to image the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror with a plurality of changes in the periodic modulation of the spectrum of the light to generate a plurality of images. The plurality of images of the region of the sample including the plurality of structures are analyzed to characterize the region of the sample.

Interferometers such as low coherence interferometers (LCI) generate sample light reflected from a sample and reference light reflected from a reference surface. The reflected sample and reference light interfere, generating an interference pattern. When the optical path difference between the sample and reference light is less than the coherence length, the intensity of the wave interference increases. By scanning the focal position of the interferometer objective on the sample, the optical path difference between the sample and reference light is changed and can be recorded relative to the changed focal position. Using the recorded interference data about the focal position, the sample can be characterized, for example, the depth profile of the sample can be determined.

Interferometry has been used to accurately measure the profile of structures on a sample. However, the structures on a sample continue to shrink, creating challenges for conventional interferometry techniques. For example, LCI optical profilers can accurately measure the depth of vias such as trenches or holes that are over 10 µm wide. However, it is becoming increasingly difficult to measure vias with smaller widths using LCI techniques. For example, measuring the depth of a 5 µm wide via may present some difficulty, but for vias less than 5 µm wide, and especially 3 µm wide or less, depth measurements are not repeatable and may not be measurable, especially for deeper vias, e.g., 30 µm or greater depths.

As discussed herein, interferometers are configured to characterize small structures using illumination optics having a numerical aperture (NA) less than the NA of collection optics, such as high aspect ratio (HAR) through holes that may have a width of 5 µm or less and a depth of 30 µm or more. In some embodiments, for example, illumination light may be provided to an interferometer objective below an objective such that the NA of the illumination optics does not include the objective and the collection optics includes the objective. In some embodiments, illumination light may be provided through the objective, but the NA of the illumination light is reduced such that the illumination optics (including the objective) has a smaller NA than the NA of the objective used as the collection optics.

Additionally, the interferometer varies the optical path difference while maintaining the focal position at a fixed position relative to the sample, rather than scanning the focal position of the interferometer objective to vary the optical path difference between the reference light and the sample light. For example, the length of the reference optical path may be varied by moving the reference mirror or the beam splitter or a combination thereof, without changing the objective focal position. In another embodiment, the interferometer may modulate the periodicity of the spectrum used by the interferometer while maintaining the focal position at a fixed position relative to the sample, rather than scanning the focal position of the interferometer objective or reference mirror.

In some implementations, multiple reference optical paths may be used, with the length of each reference optical path configured based on a nominal position on the surface of the sample. Each of the multiple reference optical paths may be scanned simultaneously or sequentially to reduce the throughput time of the measurement.

FIG1 shows a schematic diagram of an interferometer 100 using an interference objective that is scanned perpendicularly relative to a sample to vary the optical path difference with a reference beam. The interferometer 100 may be a low coherence interferometer and includes a broadband light source 110 and a beam splitter 112 that directs light toward an interference objective 120. The interference objective 120 is illustrated as a Milhausen objective that includes an objective 122, a beam splitter 124, and a reference lens 126. The objective 122 directs light toward the sample 102, and the beam splitter 124 splits the light into a reflected reference beam 123 that is incident on and reflected from the reference lens 126, and a sample beam 125 that is incident on and reflected from the sample 102. The reflected reference beam 123 and the reflected sample beam 125r are recombined at the objective lens 122. The resulting interference beam is directed by the beam splitter 112 to the lens 142, which provides an image of the interference beam at the detector 140 coupled to the processing system 150. During measurement, the reflected reference beam 123 and the reflected sample beam 125r interfere when the difference in light is less than the coherence length.

The interferometer 120 is coupled to an actuator 128 controlled by a processing system 150 to adjust the vertical position of the interferometer 120. In operation, the interferometer 100 scans the interferometer 120 vertically relative to the sample as indicated by arrow 129 to change the optical path difference with the reference beam 123. In some embodiments, the sample 102 can be moved vertically (via a stage) to adjust the relative vertical position of the interferometer 120 and the sample 102. The resulting interference pattern is received by the detector 140 at different vertical heights and used by the processing system 150 to determine characteristics of the sample 102, such as the surface profile of the sample 102.

FIG. 2A illustrates an example of the operation of an interferometer objective 210 that scans the focal position on a sample to change the optical path difference. The interferometer objective 210 generates a sample beam 212 that is directed toward and reflected by the sample 202. The focal position 213 of the sample beam 212 is depicted as a dashed line. During measurement of the sample 202, the focal position 213 of the sample beam 212 is vertically scanned (moved) on the sample 202 as indicated by arrow 218 by moving the interferometer objective 210 toward and away from the sample (or moving the relative position of the sample 202).

2B shows a sample 202 with superimposed interference signals 220A, 220B, and 220C generated by scanning the focal position of the interferometer objective 210 on the sample 202 at different scan points. When the optical path difference between the reference beam and the sample beam is less than the coherence length, the intensity of the wave interference increases. By vertically scanning the focal position 213 on the sample 202 at multiple horizontal scan points and recording the resulting interference signal relative to the focal position, the depth profile of the sample 202 can be obtained.

3A and 3B illustrate a Mil-Laurence interferometer objective 310 that scans the focal position of a sample beam on a sample 302, as indicated by arrow 312. For example, sample 302 may include a complex structure and is illustrated as having a structure 304 that may be a hole or a groove. For example, FIG3A illustrates an interferometer objective 310 where the focal position of the sample beam is located at the top surface of sample 302, i.e., the top of structure 304. FIG3B illustrates the interferometer objective 310 with the focal position of the sample beam after scanning to the bottom of structure 304.

FIG4A shows a scanning electron microscope (SEM) cross-sectional profile 410 of a sample 402, a corresponding low coherence interferometer (LCI) cross-sectional profile 420 of the sample 402, and a corresponding correlation graph 430 showing LCI fringes. The LCI cross-sectional profile 420 and the corresponding correlation graph 430 can be generated, for example, by the interferometer 100 shown in FIG1 by vertically scanning the focal position of the sample beam on the sample 402. As shown, the sample 402 has a top surface 406 shown in dashed lines and a plurality of TSVs 404 having a bottom surface 408 shown in dashed lines. The TSVs 404 in FIG4A are 5 μm wide and have a depth of 10 μm.

The LCI fringes show an LCI cross-sectional profile 420 and show in correlation graph 430 that the intensity of the interference signal increases at the top surface 406 and the bottom surface 408 of the sample 402. The LCI fringes shown in correlation graph 430 are generated at two different locations, namely, a first location located outside the TSV 404 to measure the top surface 406 of the sample 402 and a second location located inside the TSV 404 to measure the bottom surface 408 of the TSV 404. By scanning the interferometer objective perpendicularly so that the focal position of the sample beam scans both the top surface 406 and the bottom surface 408 of the sample 402, the depth of the TSV can be determined based on the distance between the top fringe and the bottom fringe (corresponding to the top surface 406 and the bottom surface 408). Diffraction from the via may produce additional fringes, sometimes referred to as ghost fringes, which may further complicate the measurement. For relatively large TSVs, such as the 5 µm wide TSV shown in FIG. 4A, the ghost fringes are barely visible in the opening of the via or extending outside the bottom of the via as shown in the LCI cross-sectional profile 420.

However, as devices continue to shrink, measuring TSVs and other high aspect ratio structures using conventional LCI interferometry is becoming increasingly difficult or even impossible due to the narrowed width and large depth of the structures. For example, due to the narrow width of the TSV, conventional LCI interferometry, which scans the focal position of the sample beam across the depth of the TSV, cannot provide enough light to the bottom of the TSV to generate sufficient signal, and the presence of ghost fringes may further complicate the measurement.

4B and 4C show LCI stripes 450 and 460 generated from a sample having a high aspect ratio via. FIG. 4B shows LCI stripe 450 from a sample having a via having a width of 5 µm and a depth of 10 µm. As can be seen in FIG. 4B , stripes 452 from the top surface are clearly visible as are stripes 454 from the bottom of the via, and therefore, the depth of the via in the structure shown in FIG. 4B can be accurately measured. However, as the via narrows and/or deepens, the amplitude of the stripes from the bottom of the via decreases significantly. For example, FIG. 4C shows LCI stripe 460 from a sample having a via, such as a trench, having a width of 1.5 µm and a depth of 8 µm. As can be seen in FIG. 4C , the fringe 462 from the top surface is clearly visible, however, the amplitude of the fringe 464 from the bottom of the via is greatly reduced and may not be sufficient to accurately characterize the via. Specifically, it should be noted that the via depicted in FIG. 4C is relatively shallow, e.g., having an aspect ratio (width:depth) of about 1:5, and the amplitude of the fringe 464 at the bottom of the via is very weak. Measuring high aspect ratio (HAR) vias is desirable, which may have an aspect ratio of less than 1:20, e.g., 1:50. For known LCI measurements of vias, the typical aspect ratio limit for vias with diameters less than between 5 µm and 3 µm is about 1:12, and for narrower structures the limit is worse, e.g., 1:10 or less. Additionally, when the structure narrows to below, for example, 4 µm, additional diffraction from the via includes ghost fringes 466 that are strong at the via opening at the top surface location, which can further complicate the measurement.

In order to fully characterize a structure, such as a HAR structure, it has been found that the intensity of the fringing at the bottom of the via depends on a variety of factors, including the numerical aperture of the illumination light, whether the via is a hole or a trench, the diameter/width, depth, and polarization of the illumination light, the relative reflectivity of the sample relative to the reference lens, the spectral bandwidth of the illumination light (e.g., shorter wavelengths will travel farther in a narrow structure), and the position of the reference lens. Specifically, it has been found that maintaining the focal position of the objective lens at a single height while varying the optical path difference between the reference beam and the sample beam produces fringing with increasing fringing amplitude from the bottom of the structure. The optical path difference may be varied and low coherence fringes may be scanned through the depth of the object while maintaining the focal position of the objective lens (i.e., while holding the focal position of the objective lens fixed at one position), for example, by scanning a reference lens, scanning a beam splitter that generates reference light, or a combination thereof. In another embodiment, low coherence fringes may be generated and scanned through the depth of the object by modulating the periodicity of the spectrum of the light while maintaining the focal position of the objective lens at a single height. In some embodiments, the optical path difference may be varied and the periodicity of the spectrum of the light may be modulated to generate coherence fringes that are scanned through the depth of the object without moving the focal position of the objective lens. By scanning the coherent fringes through the depth of the object without moving the focal position of the objective, for example by scanning a reference lens or by modulating the periodicity of the spectrum of the light, diffraction that defocuses the objective can be avoided from affecting the imaging, for example to avoid changing the background intensity and/or affecting the visibility of the fringes.

Scanning low coherence fringes through the depth of an object by modulating the periodicity of the spectrum of a light source has a similar effect to varying the optical path difference while maintaining the objective lens focal position at the same height relative to the object. Modulation of the periodicity of the spectrum of light can be performed, for example, by modifying the broadband spectrum to be periodic and then varying the spectrum periodically in time while maintaining the focal position of the objective lens. For example, the periodicity of the spectrum of light can be modulated using a modulation interferometer that is combined with a main metrology interferometer in one of a number of ways, such as in a tandem interferometer configuration.

FIG5A illustrates an example of a modulation interferometer 500 that can be used to modulate the periodicity of a spectrum of light and can be combined with a metrology interferometer, such as in a tandem interferometer configuration. For example, the modulation interferometer 500 can receive light from a light source and modulate the spectrum of light provided as input light to the metrology interferometer, or the modulation interferometer 500 can receive light from a metrology interferometer and modulate the spectrum of light provided to a detector of the metrology interferometer. As shown, the modulation interferometer 500 can have a Michelson configuration, but other configurations can be used. Input light (e.g., from a light source or from a metrology interferometer) is received and provided to a beam splitter 504. The beam splitter 504 directs a portion of the light to a first return mirror 506 in the first arm and another portion of the light to a second return mirror 508 in the second arm. The light reflected from the first return mirror 506 and the second return mirror 508 is recombined at the beam splitter 504 and provided as output light (e.g., as input light to a metrology interferometer or to a detector of the metrology interferometer). As shown, the first return mirror 506 is coupled to an actuator 507, which is controlled by a processing system to change the position of the first return mirror 506, thereby changing the optical path difference between the reference arms of the modulation interferometer 500. If necessary, both the first return mirror 506 and the second return mirror 508 can be moved.

In another embodiment, instead of moving the first return mirror 506, an unbalanced fiber optic interferometer may be used to modulate the periodicity of the spectrum of light, for example, the fiber optic interferometer stretches one fiber relative to the other to change the balance of the interferometer, thereby changing the position of the internal reference plane and thereby modulating the periodicity of the spectrum of light.

Other techniques may be used to modulate the periodicity of the spectrum of light. For example, FIG. 5B illustrates a spectrum modulator 520 that may be used to modulate the periodicity of the spectrum of light and may be combined with a metrology interferometer. For example, the spectrum modulator 520 may receive light from a light source and modulate the spectrum of light provided as input light to a metrology interferometer, or the spectrum modulator 520 may receive light from a metrology interferometer and modulate the spectrum of light provided to a detector of the metrology interferometer. The spectrum modulator 520 receives input light and spectrally decomposes the spectrum of light using a dispersive optical element 522 such as a prism or a diffraction grating. The spectrum of the light is modulated using a spatial light modulator 524 to generate periodic changes in the spectrum of the light, and is recombined using a recombining optical element 526 (which may be a prism or a diffraction grating) to generate output light.

As discussed above, FIG5C illustrates low coherence fringes 550 used to generate a periodic spectrum. FIG5D illustrates an example of input light 562 from a source before modulation, and FIG5E illustrates an example of output light 564, showing a periodic spectrum after modulation. As shown in FIG5C, three sets of low coherence fringes 552, 554, and 556 are shown, each set being bounded by a coherent envelope. The outer stripes 554 and 556 are located at a distance D from the center stripe 552, and the distance D is varied to change the periodicity of the spectrum, for example, by changing the optical path difference of the reference arm in the modulation interferometer 500 or by modulating the spectrum using the spatial light modulator 524 in the spectrum modulator 520. As discussed above, by modulating the periodicity of the spectrum of light, or using other methods, the coherent envelope of the outer stripes 554 and/or 556 can be moved through the depth of an object, such as a through hole, while maintaining the focal position of the objective lens, which advantageously avoids mechanical scanning.

Additionally, interferometers can use designs that separate the illumination and imaging optics. For example, the optics design can use low numerical aperture (NA) optics to illuminate the sample while using high NA imaging optics. The imaging optics can be designed for best imaging in terms of resolution, such as high NA, distortion, chromatic aberrations, etc. The illumination optics, which can be a separate optic below the objective, can be designed for best wavefront quality, better collimation, such as lower NA. Defocusing of the collimating optics can additionally control the wavefront shape and shape the illumination beam. Wavelength bandwidth filters and polarization elements can also affect light propagation in structures such as HAR TSVs. The design of separate illumination and imaging optics allows the shape of the fringe modulation envelope to be controlled or enhanced, thereby allowing deeper vias to be characterized.

Figure 6 illustrates an example of beam collimation divergence in systems with different source sizes and lens focal lengths. Figure 6 shows three systems 610, 620, and 630 including a light source 602 that is adjusted by the size of the source aperture 614 (in systems 610 and 630) and the source aperture 624 (in system 620) and the numerical aperture NA (shown in terms of focal length) of optical elements 612 (having focal length fl1 in systems 610 and 620) and 632 (having focal length fl2 in system 630). As shown, the light source aperture 624 is larger than the light source aperture 614, thereby providing a larger light source 602, and the focal length fl2 of the optical element 632 is shorter than the focal length fl1 of the optical element 612, eg, the NA of the optical element 632 is larger than the NA of the optical element 612.

As shown in systems 610 and 620 using light source 602 and optical element 612, the size of the source aperture of source 602 affects the collimated beam divergence. For example, system 610 includes a relatively smaller source aperture 614 than a source aperture 624 of system 620, which results in the angular range (shown by angle α) of the collimated beam series (divergence) in system 610 being relatively smaller than the angular range (shown by β1) in system 620, e.g., β1>α. The smaller size of source aperture 614 in system 610 results in a smaller collimated beam divergence than that found in system 620 having a larger size source aperture 624. Thus, system 610 has better beam collimation than system 620, but provides less light for an optical element 612 of equivalent size.

Additionally, as shown for systems 610 and 630 using the same light source 602 and light source aperture 614, the NA (focal length) of the optical element affects the beam collimation divergence. For example, system 610 includes optical element 612 having a relatively longer focal length fl1 (smaller NA) than focal length fl2 (higher NA) of optical element 632 of system 630, which results in the angular range (shown by angle α) of the collimated beam series (divergence) in system 610 being relatively smaller than the angular range (shown by β2) in system 630, e.g., β2>α. The longer focal length fl1 (smaller NA) of optical element 612 in system 610 results in a smaller collimated beam divergence than that found in system 630 having a shorter focal length fl2 (larger NA). Thus, system 610 has better beam collimation than system 630, but provides less light for an equivalent sized light source 602 and light source aperture 614.

Therefore, separating the illumination optics design and the imaging optics design allows control of light propagation to structures such as vias without affecting the imaging optics. By reducing the NA of the illumination optics to, for example, nominally collimated light, more light can be provided to the bottom of the structure being measured, while using a high NA for the imaging optics allows better imaging of small lateral features on the object.

7A and 7B illustrate schematic diagrams of interferometers 700 and 700', respectively, which characterize structures on a sample by scanning coherent fringes through the depth of an object while maintaining the focal position of the collection optical element at a single height, and using illumination optical elements and collection optical elements with different effective NAs as discussed herein to make the illumination light more collimated than the collected sample light. Interferometer 700' is similar to interferometer 700, where similarly designated elements are the same and are sometimes collectively referred to herein as interferometer 700.

The interferometer 700 illustrated in FIG7A is configured to vary the optical path difference between the reference light and the sample light to scan a coherence fringe through the depth of the object while maintaining the focal position of the collection optics, and the interferometer 700′ illustrated in FIG7B is configured to modulate the periodicity of the spectrum of the light to scan a coherence fringe through the depth of the object while maintaining the focal position of the collection optics. In some embodiments, one or more aspects of the interferometer 700′ can be combined with the interferometer 700. The interferometer 700 can use a low NA illumination optic as the illumination optic and a high NA collection optic to produce illumination light that is more collimated than the collected sample light. In some implementations, the interferometer 700 may be an LCI interferometer, a coherent scanning interferometer, or the like.

As shown in FIG. 7A and FIG. 7B , interferometer 700 and interferometer 700′ respectively include optical heads 710 and 710′ (sometimes collectively referred to as optical heads 710), which include a light source 712 and a detector 720, and an interferometer objective 713, which includes a beam splitter 714, a reference lens 716, and an objective lens 718. In some embodiments, interferometer 700 can be an LCI interferometer, and light source 712 can have a spectral bandwidth and an angular bandwidth sufficient to generate low coherence fringes. In addition, light source 712 can be configurable based on the type of structure on the sample being measured. For example, the light source 712 can adjust at least one of the spectral bandwidth and the angular bandwidth, at least one of the cylindrical optical element and the spatial linear source, and polarization control, or any combination thereof based on the configuration of the measured structure. For example, the spectral and/or angular bandwidth and type of the optical element can be selected based on the shape, width, and depth of the through hole. For example, shorter wavelengths can be used to better propagate in narrow through holes than longer wavelengths, and using a wide spectral bandwidth can be used to better determine the coherence envelope, for example, as the spectral bandwidth increases, the coherence envelope is narrower. For example, as shown in Figure 6, the angular bandwidth can be used to control the divergence of a collimated beam. In addition, the use of cylindrical optical elements and/or linear light sources can be used to control the collimated divergence in the x and y directions, which can be beneficial for controlling trench measurements. Further, the polarization of light can be controlled in response to the shape, width, and depth of the through-hole. For example, circular polarization can be used for various through-holes, including holes and trenches with various orientations. In specific examples, if the trench has a specific orientation, linear polarization with the same orientation as the trench can be used to improve propagation in the trench. Additionally, the relative reflectivity of the sample relative to the reference mirror can be controlled, for example, by selecting mirrors of different reflectivities or by changing the splitting ratio of different arms of the interferometer. In some embodiments, the optical head 710 may include additional components, such as a beam splitter, a polarizer, a lens, etc. Additionally, as discussed herein, the relative position of the light source 712 in the optical head 710 can be varied. For example, as depicted, illumination light from the light source 712 is provided to the beam splitter 714 below the objective lens 718, but in some implementations, illumination light from the light source 712 can be provided above the objective lens 718 (as depicted by the objective lens 718 in dashed lines), for example, via a second beam splitter. The beam splitter 714 splits the illumination light and produces a reference light 715 that is directed to and reflected from a reference lens 716 and a sample light 717 that is directed to the sample 702.

In some embodiments, the optical head 710 may include additional components such as an aperture 719 to effectively reduce the NA of the illumination light. Although the interference lens 713 shown in Figures 7A and 7B has a Michaelson configuration, it should be understood that other types of interference lens configurations can be used in the optical head 710, including a Milhausen lens configuration and a Linnik lens configuration.

Illumination light from light source 712 is received by beamsplitter 714, which splits the illumination light and produces reference light 715 that is directed to and reflected from reference mirror 716 and sample light 717 that is incident on and reflected from sample 702. The reflected light from reference mirror 716 and sample 702 is recombined and interferes to produce interference light 722 that is received by detector 720, from which one or more characteristics of structure 704 on sample 702 can be measured, such as depth, width, or other profile parameters. Sample light 717 is focused by objective lens 718 in optical head 710, for example, at or near the top surface of sample 702.

The interferometer 700 maintains the focal position of the objective lens 718 relative to the sample 702, i.e., the focal position is fixed, while scanning the coherence fringe through the depth of the object during measurement of the sample 702. For example, as shown in FIG. 7A , the optical path difference between the reference light 715 and the sample light 717 is changed to scan the coherence fringe through the depth of the object. The optical path difference between the reference light 715 and the sample light 717 can be changed by increasing and/or decreasing the optical path length of the reference light 715 (as shown by arrow 721). For example, the optical path length of reference light 715 may be increased or decreased by moving reference mirror 716 or by moving beam splitter 714 (e.g., if interferometer objective 713 has a Michelaud-type objective configuration).

As shown in FIG7B , the periodicity of the spectrum of light can be modulated to scan coherent fringes through the depth of an object, for example using a modulation optical element 772 or 782, which can be similar to the modulation interferometer 500 shown in FIG5A or the spectrum modulator 520 shown in FIG5B . For example, the modulation optical element 772 can modulate the periodicity of the spectrum of light generated by the light source 712 provided to the beam splitter 714. In another example, the modulation optical element 782 can receive light from the beam splitter 714, for example via mirrors 784 and 786, and can modulate the periodicity of the spectrum of light provided to the detector 720, for example via mirror 788. In some implementations, a combination of the techniques depicted in Figures 7A and 7B may be used, including any combination of moving the reference mirror 716, moving the beam splitter 714, and modulating the periodicity of the spectrum of light.

Additionally, the interferometer 700 can use an illumination optics element having an effective NA for illumination light incident on the sample 702 (e.g., as illustrated by the shadow sample light 724) that is lower than the NA effective for the collected light from the sample 702 (e.g., as illustrated by the collected light 726). The illumination optics element for the illumination light can, for example, have a relatively low effective NA of less than 0.05 and can produce collimated or nearly collimated light, while the collection optics element for the collected light can have a relatively high effective NA of at least 0.2. For example, in some embodiments, the low NA illumination optics element can be one or more lenses (not shown) that provide illumination light to the interferometer objective 713 after the collection optics element in the optical path (i.e., between the objective 718 and the sample 702). Beam splitter 714 directs a portion of the illumination light to illuminate sample 702 and another portion of the illumination light to reference lens 716. In another embodiment, the effective NA of the illumination optics may be reduced by limiting the spatial extent of light source 712, e.g., using aperture 719 (e.g., as discussed in FIG. 6 ), and thus, for example, the low NA illumination optics may include objective lens 718 and aperture 719 to reduce the spatial extent of light source 712. There are different methods of reducing the NA of illumination optics, such as by using Kohler illumination, critical illumination, and other afocal or non-focused or critical illumination may be used to control the NA of the illumination optics.

Interferometer 700 is further configured to image a region of sample 702 including a plurality of structures 704 so that multiple structures 704 can be simultaneously characterized. For example, detector 720 can include an imaging tube lens that images a region of the sample received by detector 720 and can collect multiple images of the region as the optical path difference varies.

The detector 720 in the optical head 710 and other components of the interferometer 700, such as the light source 712, the stage 730, the chuck 732, the actuators for moving the reference mirror 716 and/or the beam splitter 714, the reference mirrors in the modulation optical elements 772 or 782, etc., can be coupled to a processing system 750, such as a workbench, a personal computer, a central processing unit, or other suitable computer system, or multiple systems. It should be understood that one processor, multiple separate processors, or multiple connected processors can be used, all of which are interchangeably referred to herein as the processing system 750. The processing system 750 can be included in the interferometer 700, connected to the interferometer, or otherwise associated with the interferometer. For example, the processing system 750 can control the positioning of the sample 702, such as by controlling the movement of the stage 730 on which the sample 702 is held. For example, the stage 730 may be capable of horizontal movement in either Cartesian (i.e., X and Y) coordinates or polar (i.e., R and θ) coordinates, or some combination of both. The stage 730 may also be capable of vertical movement along the Z coordinate, such as for focusing the objective 718 on or near the surface of the sample 702. In some embodiments, the stage 730 may be held stationary while the optical head 710 or a portion of the optical head 710 moves relative to the sample 702, or both the stage 730 and the optical head 710 (or a portion thereof) may move relative to the other. The processing system 750 can further control the operation of the chuck 732 on the stage 730 for holding or releasing the sample 702.

The processing system 750 can also collect and analyze data acquired from the optical head 710. As discussed herein, the processing system 750 can analyze the interferometer data to determine characteristics of the structures 704 on the sample 702, such as depth, width, or other profile parameters. The processing system 750 includes at least one processor 752 having a memory 754 and a user interface including, for example, a display 756 and an input device 758. A non-transitory computer usable storage medium 759 embodying computer readable program code can be used by the processing system 750 to enable the processing system 750 to control the optical interferometer 700 and perform functions including the analysis described herein. Data structures, software codes, etc. used to automatically implement one or more actions described in the present embodiment can be implemented by a person of ordinary skill in the art in accordance with the present disclosure and stored, for example, on a non-transitory computer-usable storage medium 759, which can be any device or medium that can store code and/or data for use by a computer system (such as at least one processor 752). The non-transitory computer-usable storage medium 759 can be, but is not limited to, a flash drive, magnetic and optical storage devices, such as a disk, a tape, an optical disk, and a DVD (digital versatile disk or digital video disk). Communication port 757 may also be used to receive instructions for programming processing system 750 to perform any one or more of the functions described herein, and may represent any type of communication connection, such as to the Internet or any other computer network. Communication port 757 may further export signals (e.g., having measurement or detection results and/or instructions) to another system (e.g., an external processing tool) in a feedforward or feedback process to adjust a process parameter associated with a sample manufacturing process step based on the measurement results. Additionally, the functionality described herein may be embodied in whole or in part in the circuitry of an application specific integrated circuit (ASIC) or a programmable logic device (PLD), and the functionality may be embodied in a computer-understandable descriptor language that may be used to create an ASIC or PLD that operates as described herein. Results from the data analysis may be stored, for example, in a memory 754 associated with the sample and/or provided to a user (e.g., via a display 756, an alarm, or other output device). Furthermore, results from the analysis may be fed back to the process equipment to adjust the appropriate patterning steps to compensate for any detected errors in the measurement or detection.

FIG8 shows a schematic diagram of an interferometer 800, which may be a low coherence interferometer and may be an implementation of the interferometer 700 shown in FIG7A. The interferometer 800 includes an interferometer objective lens that changes the reference optical path length to change the optical path difference between the reference light and the sample light, thereby scanning the coherence fringes through the depth of the object. Additionally, the interferometer 800 provides nominally collimated illumination light to provide a low NA for the illumination light incident on the sample and a high NA for the collected light from the sample 802. If desired, in addition to or instead of changing the optical path difference between the reference light and the sample light to scan the coherence fringes through the depth of the object, the interferometer 800 may include a modulating optical element as shown in Figures 7B and 5A and 5B.

As shown, the interferometer 800 includes a light source 810 and one or more lenses 811 to produce collimated or nearly collimated light received by an interference objective 820. The interference objective 820 is shown in FIG. 8 as a Michaelisoscope, which includes an objective lens 822, a beam splitter 812, and a reference lens 826, but other types of objectives may also be used. The collimated light is received by the interference objective 820 below the objective lens 822 (i.e., between the objective lens 822 and the sample 802). The beam splitter 812 splits the collimated light into a reference light 823 incident on and reflected from the reference lens 826 and a sample light 825 incident on and reflected from the sample 802. Reference mirror 826 is coupled to an actuator 828, which is controlled by at least one processor 850 to adjust the position of reference mirror 826. In operation, interferometer 800 moves reference mirror 826 as indicated by arrow 827 to change the optical path difference between reference light 823 and sample light 825. In some embodiments, an assembly 830 including, for example, reference mirror 826, beam splitter 812, and light source 810 having one or more lenses 811 can be coupled to actuator 831 and moved as indicated by arrow 832 to change the optical path difference between reference light 823 and sample light 825. Objective lens 822 serves as a collection optical element and focuses on or near the top surface of sample 802, as shown by collected light 829. The focal position of the collection optics (i.e., objective 822) does not change, i.e., the focal position is fixed relative to the sample 802, while the optical path difference is changed by moving the reference mirror 826. With collimated or nearly collimated illumination light provided below the objective 822, the interferometer 800 can use low NA illumination optics (e.g., lens(es) 811) for the (e.g., nominally collimated) sample light 825 while using high NA collection optics (e.g., objective 822) for the collected light 829.

Reference light 823 and sample light 825 reflected from sample 802 are recombined at beam splitter 812. The resulting interference light is directed by beam splitter 812, objective lens 822, and lens 842 (e.g., an imaging tube lens) to detector 840, which is coupled to at least one processor 850, which may be part of processing system 750. The resulting interference pattern is received by detector 840 at different optical path differences and used by at least one processor 850 to determine characteristics of sample 802, such as the surface profile of sample 802.

Interferometer 800 uses a broadband light source 810 with a scanning reference mirror 826 to achieve depth resolution determined by the location of the coherence envelope of a depth scan through the object. The coherence envelope is the propagation distance over which a coherent wave (e.g., an electromagnetic wave) maintains a specified degree of coherence. When the optical path difference between reference light 823 and sample light 825 differs by less than the coherence length, the waves interfere strongly. As discussed in Figures 5A and 5B and Figure 7B, in some embodiments, in addition to or in lieu of a scanning reference mirror 826, the periodicity of the spectrum of, for example, the light provided by the broadband light source 810 or the interfering light provided by the beam splitter 812 can be modulated to pass the coherent envelope of the depth scan of the object. At least one processor 850 receives the reflected optical data from each single scan point, which is interpreted as an interference pattern along the scan and recorded as a depth profile. If polarization states are introduced in the light beam, the interferometer 800 can simultaneously collect data over a large field of view, including a region of the sample 802 of several structures to be measured, for example by imaging on a detector array (e.g., a CCD array or a polarization pixelated camera). Each pixel in the image corresponds to a position in the region of the sample 802, and each image can correspond to a different optical path difference, which corresponds to depth.

FIG9 shows an embodiment of an interferometer objective that can be used as the interferometer objective 820 in the interferometer 800 shown in FIG8. FIG9 shows an interferometer objective 900 (which can be a Michaelson-type interferometer objective) that includes an objective lens 902, a beam splitter 904, and a reference lens 906 that moves as indicated by the arrows to change the optical path difference. Collimated or nearly collimated illumination light is provided by a light source (not shown) through one or more lenses 908 and received by the interferometer objective 900 below the objective lens 902 (i.e., between the objective lens 902 and the sample 901). Beam splitter 904 splits the illumination light into reference light 905 incident on and reflected from reference mirror 906 and sample light 907 incident on and reflected from sample 901. As shown by collection light 909, a collection optical element such as objective lens 902 focuses on or near the top surface of sample 901 and maintains the focal position while changing the optical path difference by moving reference mirror 906. In some embodiments, reference mirror 906, beam splitter 904, and the source of illumination light can be coupled to an actuator and moved vertically as indicated by the dashed arrow to change the optical path difference between reference light 905 and sample light 907. As depicted, because sample light 907 has low NA illumination optics (lens 908 ) (eg, nominally collimated), more light is provided to the bottom of the structure of sample 901 than light provided via objective 902 .

FIG10 shows a schematic diagram of an interferometer 1000, which may be a low coherence interferometer and may be an implementation of the interferometer 700 shown in FIG7A. The interferometer 1000 includes an interferometer objective lens that changes the reference optical path length to change the optical path difference between the reference light and the sample light, thereby scanning the coherence fringes through the depth of the object. Additionally, the interferometer 1000 uses low NA illumination optics to provide illumination light incident on the sample and uses high NA imaging optics to provide collected light from the sample 1002. If desired, in addition to or instead of changing the optical path difference between the reference light and the sample light to scan the coherence fringes through the depth of the object, the interferometer 1000 may include a modulation optical element as shown in Figures 7B and 5A and 5B.

As shown, the interferometer 1000 includes a light source 1010 (which may be a broadband light source) and one or more lenses 1011 and apertures 1012 to reduce the aperture size of the light source 1010. The interferometer 1000 may use Kohler illumination, for example. The illumination light is directed by a beam splitter 1014 to an interferometer objective 1020. The interferometer objective 1020 is shown in FIG. 10 as a Milhausen objective, which includes an objective lens 1022, a beam splitter 1024, and a reference lens 1026, but other types of objectives may also be used. As shown by the cone of sample light 1025, the illumination light is received by the objective lens 1022 and focused on or near the top surface of the sample 1002. Because of the reduced aperture size of the light source 1010, the illumination light incident on the sample 1002 is provided by the objective 1022, which has an effective low NA. However, the objective 1022 collects reflected light (e.g., an image) from the sample 1002 at a higher NA, as shown by the collected light 1029. Thus, as shown, the interferometer 1000 uses low NA optics for the sample light 1025, but uses high NA optics for the collected light 1029.

The beam splitter 1024 splits the illumination light into a reference light 1023 incident on and reflected from a reference mirror 1026 and a sample light 1025 incident on and reflected from the sample 1002. The length of the reference optical path in the interference objective 1020 can be changed by moving the reference mirror 1026 and/or the beam splitter 1024. For example, the reference mirror 1026 can be coupled to an actuator 1028 controlled by at least one processor 1050 to adjust the position of the reference mirror 1026. Additionally or alternatively, the beam splitter 1024 can be coupled to an actuator 1028 controlled by at least one processor 1050 to adjust the position of the beam splitter 1024. In operation, the interferometer 1000 maintains (i.e., keeps fixed relative to the sample 1002) the focal position of the objective lens 1022 while varying the optical path difference by, for example, moving the reference lens 1026 and/or the beam splitter 1024. The reference lens 1026 (and/or the beam splitter 1024) may be scanned as indicated by the arrows to vary the optical path difference between the reference light 1023 and the sample light 1025.

Reference light 1023 and sample light 1025 reflected from sample 1002 are recombined at beam splitter 1024. The resulting interference light is directed by objective 1022, beam splitter 1014, and lens 1042 to detector 1040, which is coupled to at least one processor 1050. The resulting interference pattern is received by detector 1040 at different optical path differences and used by at least one processor 1050 to determine characteristics of sample 1002, such as a surface profile of sample 1002. For example, at least one processor 1050 can be part of processing system 750 shown in Figures 7A and 7B.

Interferometer 1000 uses a light source 1010 with a reference mirror 1026 (which may be a scanning reference mirror) and/or a beam splitter 1024 to achieve depth resolution determined by the location of the coherence envelope of a depth scan through the object. The coherence envelope is the propagation distance over which a coherent wave (e.g., an electromagnetic wave) maintains a specified degree of coherence. When the optical path difference between reference light 1023 and sample light 1025 differs by less than the coherence length, the wave interference is strong. As discussed in Figures 5A and 5B and Figure 7B, in some embodiments, in addition to or in place of the scanning reference mirror 1026, the periodicity of the spectrum of the light provided by the broadband light source 1010 or the interfering light provided by the beam splitter 1024 can be varied to scan the coherent envelope through the depth of the object. At least one processor 1050 receives the reflected optical data from each single scan point, which is interpreted as an interference pattern and recorded as a depth profile. If orthogonal polarization states are introduced in the light beams, the interferometer 1000 can simultaneously collect data over a large field of view, including a region of the sample 1002 of several structures to be measured, for example by imaging on a detector array such as a CCD array or a polarization pixelated camera. Each pixel in the image corresponds to a position in the region of the sample 1002, and each image can correspond to a different optical path difference, which corresponds to depth.

11A to 11C illustrate various embodiments of interferometer objectives that may be used as the interferometer objective 1020 in the interferometer 1000 shown in FIG. 10 .

11A shows an interference objective 1100 (which may be a Michelaud-type interference objective) including an objective lens 1102, a beam splitter 1104, and a reference lens 1106, wherein the reference lens 1106 and/or the beam splitter 1104 are moved to change the optical path difference as indicated by the arrows. Illumination light is provided by a light source (not shown) having a reduced aperture size and is received by the interference objective 1100 and focused by the objective lens 1102 as shown by sample light 1107. The beam splitter 1104 splits the illumination light into reference light 1105 incident on and reflected from the reference lens 1106 and sample light 1107 incident on and reflected from the sample 1101. The objective lens 1102 is focused on or near the top surface of the sample 1101 as shown by the collected light 1109 to image the sample 1101 using relatively high NA optical elements and maintain the focal position while changing the optical path difference by moving the reference lens 1106, the beam splitter 1104, or both the reference lens 1106 and the beam splitter 1104.

FIG11B shows an interference objective 1120 (which may be a Michaelson-type interference objective) including an objective lens 1122, a beam splitter 1124, and a reference lens 1126 that moves as indicated by the associated arrows to change the optical path difference. Similar to the interference objective 1100 discussed in connection with FIG11A, illumination light is provided by a light source (not shown) having a reduced aperture size and is received by the interference objective 1120 and focused by the objective lens 1122 as indicated by sample light 1127. The beam splitter 1124 splits the illumination light into reference light 1125 incident on and reflected from the reference lens 1126 and sample light 1127 incident on and reflected from the sample 1121. The objective lens 1122 is focused on or near the top surface of the sample 1121 as shown by the collected light 1129 to image the sample 1121 using relatively high NA optical elements and maintain the focal position while changing the optical path difference by moving the reference mirror 1126, or by moving the beamsplitter 1124 and the reference mirror 1126 vertically together as shown by the dotted arrow.

11C shows a Linnik-type interferometer objective 1150, which includes matching objective lenses 1152 and 1153, a beam splitter 1154, and a reference lens 1156 that moves as indicated by the arrows to change the optical path difference. Illumination light is provided by a light source (not shown) with a reduced aperture size and is received by beam splitter 1154, which splits the illumination light into reference light 1155, which is focused on reference lens 1156 using objective lens 1153, and sample light 1157, which is focused on sample 1151 by objective lens 1152 and reflected from the sample. Objective lens 1153 and reference lens 1156 can be moved together to change the optical path difference, while in some embodiments, only reference lens 1156 is moved. In some embodiments, as shown by the dashed arrows, the beam splitter 1154, the objective lens 1153, and the reference lens 1156 are all moved vertically. The objective lens 1152 is focused on or near the top surface of the sample 1151 as shown by the collected light 1159 to image the sample 1151 using relatively high NA optical elements and maintain the focus position while changing the optical path difference by moving the reference lens 1156 and the objective lens 1153.

In some implementations, multiple reference paths may be used. For example, if a single reference path is used to measure a structure having multiple levels (e.g., a top surface and a bottom surface), the scan used to change the reference optical path to produce a fringe pattern corresponding to each level may be long, which will reduce throughput. By using multiple reference paths, with each reference path configured for a different level, the scan length can be greatly reduced.

12 illustrates a structure 1200 having a top surface 1202 and a bottom surface 1204, with corresponding correlation maps 1210 and 1230 generated by interferometry, as discussed herein. The correlation maps 1210, 1230 include top stripes 1212, 1232 corresponding to the top surface 1202 of the structure 1200 and bottom stripes 1214, 1234 corresponding to the bottom surface 1204 of the structure 1200.

With a single reference optical path, the scan used to change the reference optical path will have a length at least as great as the entire depth of structure 1200. For example, single reference mirror 1220 may scan distance 1222 throughout the depth of structure 1200 to produce correlation graph 1210, where top stripe 1212 corresponds to a first location at top surface 1202, and bottom stripe 1214 corresponds to a second location at bottom surface 1204, but is drawn along the same line. The depth of structure 1200 may be determined based on the distance between top stripe 1212 and bottom stripe 1214 corresponding to top surface 1202. If a single reference optical path is used, it may be desirable to adjust the range (ie, distance 1222) scanned by reference mirror 1220, for example, to correspond to the depth of structure 1200.

However, by using multiple reference paths, separate scans can be used at the levels of interest, and it is not necessary to scan the entire depth of structure 1200. For example, as shown, first reference mirror 1240 scans distance 1242 (which is only a portion of full distance 1222), and second reference mirror 1244 separately scans distance 1246 (which is only a portion of full distance 1222), resulting in correlation graph 1230 in which top stripe 1232 corresponds to a first location at top surface 1202 (resulting from the scan by first reference mirror 1240), and bottom stripe 1234 corresponds to a second location at bottom surface 1204 (resulting from the scan by second reference mirror 1244). It will be appreciated that because the interferometer images the entire area, the scans of the first reference mirror 1240 and the second reference mirror 1244 can be performed without relative lateral movement between the optical elements and the sample, and can be performed simultaneously (or sequentially) as discussed in Figures 13A-13C. The depth of the structure 1200 can be determined based on the distance between the top stripe 1232 and the bottom stripe 1234, which can be calibrated based on the known difference between the lengths of the reference optical paths. Multiple reference optical paths can be scanned simultaneously or sequentially. Because each reference optical path scans a much shorter distance than when using a single reference optical path, the acquisition time can be significantly reduced. Additionally (or alternatively), for example, as discussed in connection with FIGS. 5A-5C , and 7B , the periodicity of the spectrum of light can be modulated to further reduce (or eliminate) the distance that reference mirrors 1240 and 1244 are physically scanned.

13A, 13B, and 13C illustrate different implementations of generating multiple reference optical paths that may be used with a McArthur or Linnik type interferometer objective as shown in FIGS. 9, 11B, and 11C discussed herein in conjunction with the interferometers 700, 800, 100 discussed herein.

As shown in FIG13A , an interference objective 1300 includes multiple reference optical paths with multiple reference lenses. The interference objective 1300 includes an objective lens (not shown), a first beam splitter 1304, a second beam splitter 1306, and two reference lenses 1308 and 1310 that move separately as indicated by arrows to change the optical path difference in each reference optical path. Illumination light 1303, shown as collimated, is provided by a light source (not shown) and received by the interference objective 1300. The first beam splitter 1304 splits the illumination light 1303 into reference light 1305 and sample light 1307, which is incident on and reflected from a sample 1301. Second beam splitter 1306 directs a first portion of reference light 1305 to first reference mirror 1308 and directs a second portion of reference light 1305 to second reference mirror 1310. The lengths of the reference optical paths associated with first reference mirror 1308 and second reference mirror 1310 may correspond to nominal positions at different levels in the proximity structure, such as top surface 1202 and bottom surface 1204 as shown in FIG12. Reference mirrors 1308 and 1310 may be independently movable to vary the optical path difference of each reference optical path. In some implementations, second beam splitter 1306 can be a dichroic beam splitter that directs different spectral ranges of reference light 1305 to first reference mirror 1308 and second reference mirror 1310. A detector, such as detector 720, 840, or 1040 in FIG. 7A, FIG. 7B, FIG. 8, or FIG. 10, can include another dichroic beam splitter and separate sensors to simultaneously detect fringes generated in response to each reference optical path. Alternatively, first reference mirror 1308 and second reference mirror 1310 can be scanned sequentially so that fringes generated by one reference optical path can be collected before fringes generated by another reference optical path. In some implementations, a flip mirror can be used to prevent collection of reference light from second reference mirror 1310 when scanning first reference mirror 1308, and to prevent collection of reference light from first reference mirror 1308 when scanning second reference mirror 1310.

As shown in FIG13B , the interference objective 1320 includes a plurality of reference optical paths having a plurality of reference surfaces. The interference objective 1320 includes an objective lens (not shown), a first beam splitter 1324, a first reference lens 1326, which is movable and acts as a beam splitter, which directs a portion of incident light to the first beam splitter 1324 and directs another portion of the light to a second reference lens 1330 that is movable separately from the first reference lens 1326. Illumination light 1323, shown as collimated, is provided by a light source (not shown) and received by the interference objective 1320. The first beam splitter 1324 splits the illumination light 1323 into reference light 1325 and sample light 1327, which is incident on and reflected from the sample 1321. A first reference mirror 1326, which may be a partially transparent film, reflects a first portion of the reference light 1305 back to the first beam splitter 1324, and transmits a second portion of the reference light 1305 to the second reference mirror 1330, and reflects and returns through the first reference mirror 1326. The first reference mirror 1326 may be movable as indicated by the associated arrows, and the second reference mirror 1330 may be movable as indicated by the associated arrows. The lengths of the reference optical paths associated with the first reference mirror 1326 and the second reference mirror 1330 may correspond to nominal positions at different levels in the structure, such as the top surface 1202 and the bottom surface 1204 shown in FIG. Reference mirrors 1326 and 1330 can be independently moved to change the optical path difference of each reference optical path. In some embodiments, second reference mirror 1330 can be moved with first reference mirror 1326 while scanning first reference mirror 1326, and then can be moved separately to scan second reference mirror 1330. In some embodiments, first reference mirror 1326 can be a dichroic beam splitter that reflects one spectral range of reference light 1325 back to first beam splitter 1324 and transmits a different spectral range of reference light 1325 to second reference mirror 1330. As discussed in Figure 13A, a detector, such as detector 720, 840, or 1040 in Figures 7A, 7B, 8, or 10, may include a dichroic beam splitter and separate sensors to simultaneously detect fringes generated in response to each reference optical path. Alternatively, the first reference mirror 1326 and the second reference mirror 1330 may be scanned sequentially so that fringes generated by one reference optical path are collected before fringes generated by another reference optical path.

As shown in Figure 13C, the interference objective 1350 includes multiple reference optical paths with multiple reference surfaces produced by a single optical element. The interference objective 1350 includes an objective lens (not shown) and a glass plate 1356 having a partially reflective front surface used as a first reference mirror 1358 and a reflective rear surface used as a second reference mirror 1360. Illumination light 1353, shown as collimated, is provided by a light source (not shown) and received by the interference objective 1350. A first beam splitter 1354 splits the illumination light 1353 into reference light 1355 and sample light 1357, which is incident on and reflected from the sample 1351. The front surface of the glass plate 1356 serves as a first reference mirror 1358 and reflects a first portion of the reference light 1305 back to the first beam splitter 1354, and transmits a second portion of the reference light 1305 to the rear surface of the glass plate 1356, which serves as a second reference mirror 1360 and reflects the reference light 1305. The glass plate 1356 can be moved as indicated by the associated arrows to move the first reference mirror 1358 and the second reference mirror 1360. The length of the reference optical path associated with the first reference mirror 1358 and the second reference mirror 1360 (i.e., the distance between the front surface and the rear surface of the glass plate 1356) can correspond to the nominal positions of different levels in the structure, such as the top surface 1202 and the bottom surface 1204 shown in FIG. In some embodiments, glass plate 1356 can be a dichroic beam splitter that reflects one spectral range of reference light 1355 back to first beam splitter 1354 and transmits a different spectral range of reference light 1355 to second reference mirror 1360. As discussed in FIG13A, a detector, such as detector 720, 840, or 1040 of FIG7A, FIG7B, FIG8, or FIG10, can include a dichroic beam splitter and separate sensors to simultaneously detect fringes generated in response to each reference optical path. Alternatively, the first reference mirror 1358 and the second reference mirror 1360 may be scanned sequentially so that fringes produced by one reference optical path are collected before fringes produced by the other reference optical path.

Figures 14A, 14B, and 14C show examples of correlation graphs for vias of different sizes as seen with a single pixel scan. In the correlation graphs shown in Figures 14A to 14C, the top stripes are ghost stripes and can be used with the bottom stripes to generate an approximate via depth, but the stripes generated from the top surface of the sample (i.e., the outside of the via) will be used for accurate depth measurement, and the bottom stripes and ghost stripes (i.e., the top stripes shown in Figures 14A, 14B, and 14C) shown in Figures 14A, 14B, and 14C will be ignored.

FIG. 14A shows correlation graphs for a 5 μm wide by 50 μm deep through hole. Correlation graph 1402 is, for example, from a sample scan obtained by an interferometer with a known scanning interferometer objective, i.e., the focal position of the scanning objective, while correlation graph 1404 is from a sample scan obtained by an interferometer with a scanning reference lens as discussed herein. As can be seen, both correlation graphs 1402 and 1404 have visible fringes 1402t and 1404t, respectively, corresponding to ghost fringes generated at the top surface of the sample. However, the correlation graph 1402 produced with the conventional scanning interferometer has a significant background intensity variation as indicated by the tilt to the right, and has a low contrast of the bottom stripe 1402b corresponding to the bottom of the through hole, which may be the result of diffraction and multiple reflections within the structure. The background intensity is the sum of the intensities of the reference, object, and any other beams at the detected position. On the other hand, the correlation graph 1404 produced with the scanning reference lens has a constant background intensity value and has a good contrast of the bottom stripe 1404b corresponding to the bottom of the through hole.

FIG14B shows correlation graphs for a 3 µm wide by 30 µm deep via. For example, correlation graph 1412 is from a sample scan obtained with an interferometer having a known scanning interferometer objective, i.e., the focal position of the scanning objective, while correlation graph 1414 is from a sample scan obtained with an interferometer having a scanning reference lens as discussed herein. As can be seen, both correlation graphs 1412 and 1414 have visible fringes 1412t and 1414t, respectively, corresponding to ghost fringes generated at the top surface of the sample. However, the correlation graph 1412 produced by the conventional scanning interferometer has a significant background intensity variation as indicated by the tilt to the right, and the bottom stripe corresponding to the bottom of the via is not visible, which may be the result of diffraction and multiple reflections within the structure. On the other hand, the correlation graph 1414 produced by the scanning reference lens has a constant background intensity value and has good contrast of the bottom stripe 1414b corresponding to the bottom of the via.

FIG. 14C shows a correlation graph for a 3 µm wide by 50 µm deep through-hole. The correlation graph 1424 is, for example, from a sample scan obtained by an interferometer with a scanning reference lens as discussed herein. The corresponding correlation graph obtained by an interferometer with a conventional scanning interferometer objective is not shown because no useful fringe corresponding to the bottom of the through-hole is produced. However, as can be seen, the correlation graph 1424 produced using the scanning reference lens shows a visible fringe 1424t corresponding to a ghost fringe produced at the top surface of the sample, with a constant background intensity value, and with good contrast of a bottom fringe 1424b corresponding to the bottom of the through-hole.

FIG. 15 illustrates, by way of example, a collection 1500 of multiple images (shown as images 1500A, 1500B, and 1500C) generated by an interferometer as discussed herein, such as interferometer 700. Each image in collection 1500 is a data frame that includes a full field of view of a sample having a plurality of structures 1502, such as vias, as collected during a scan. For example, image 1500A is generated when the reference mirror is in a position where interference fringes are not visible at this given scan time/frame. The full field of view may be, for example, 0.45 mm x 0.45 mm and includes an area of the sample including a plurality of structures 1502, such as vias. Multiple images of an area of the sample are generated in set 1500, each image having a different optical path difference produced by scanning a reference mirror, a beam splitter, or changing the spectrum provided by the light source, or any combination thereof. It should be understood that although Figure 15 shows three images in set 1500, a set of images from a scan may include a significantly larger number of images.

FIG16 is a flow chart illustrating a method for characterizing a region of a sample including a plurality of structures using interferometry, which method may be performed by any of the interferometers 700, 800, and 1000 illustrated in FIGS. 7A, 7B, 8, and 10, respectively, using any of the interferometric objectives discussed with respect to FIGS. 9 and 11A-11C and/or the modulation of the periodic spectrum discussed with respect to FIGS. 5A-5E, for example. In some embodiments, the plurality of structures may be at least one of trenches, holes, and high aspect ratio (HAR) structures. For example, the HAR structures may have a width of 5 μm or less and a depth of 30 μm or more.

As shown in block 1602, the method includes splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by the reference mirror, for example, as shown by the beam splitter 714 in Figures 7A and 7B. For example, the components for splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by the reference mirror may include a beam splitter positioned below the objective lens (i.e., the beam splitter is positioned between the objective lens and the sample), such as the beam splitters 812, 904, 1024, 1104, and 1124 in Figures 8, 9, 10, 11A, and 11B, or a beam splitter positioned above the objective lens (i.e., the objective lens is positioned between the beam splitter and the sample), such as the beam splitter 1154 in Figure 11C.

As indicated at block 1604, the method includes illuminating a region of the sample with sample light using an illumination optical element having a first numerical aperture, such as illustrated by sample light 724 in FIGS. 7A and 7B. Components for illuminating an area of a sample with sample light using an illumination optical element having an aperture of a first numerical value may include, for example, an optical element positioned to provide illumination light below a collection lens, such as (multiple) lens/beamsplitter/sample light 811/812/825 and 908/904/907 shown in Figures 8 and 9, respectively, or may include, for example, an optical element positioned to provide illumination light before a collection lens, such as aperture/objective lens/sample light 1012/1022/1023 shown in Figure 10 and objective lens/sample light 1102/1107, 1152/1157 shown in Figures 11A, 11B, and 11C.

At block 1606, the method includes focusing a collection optical element at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture, for example, as depicted by collected light 726 in Figures 7A and 7B. In some embodiments, the collection optical element can be one of a Michael bioscope, a Millau objective, or a Linnik objective. Components for focusing the collecting optical element at a focal position relative to the sample, the collecting optical element having a second numerical aperture greater than the first numerical aperture may include, for example, objective lenses/collected light 822/829, 902/909, 1022/1029, 1102/1109, 1122/1129, and 1152/1159 shown in Figures 8, 9, 10, 11A, 11B, and 11C, respectively.

At block 1608, the method includes generating one or more coherence envelopes by depth scanning the sample by interference of reflected sample light from the sample with reflected reference light from a reference mirror while not changing the focal position of the collection optical element relative to the sample, for example, as shown by arrow 721 in Figures 7A and 7B. One or more coherent envelopes generated by interference of reflected sample light from the sample and reflected reference light from the reference mirror for depth scanning through the sample, while not changing the focal position of the collection optical element relative to the sample, may include, for example, components for changing the optical path difference between the sample light and the reference light, such as one or more moving reference mirrors 716, 826, 906, 1026 shown in FIGS. 8, 9, 10, 11A, 11B, and 11C. , 1106, 1126, and 1156, and/or the moving beam splitters 1024 and 1104 shown in Figures 10 and 11A, and/or the light source 810 or 1010 in Figures 8 and 10 for changing the spectrum of the illumination light, or a periodic component for modulating the spectrum of the illumination light or the interference light generated by interference between the reflected sample light from the sample and the reflected reference light, such as the modulation interferometer 500 or the unbalanced fiber optic interferometer discussed with reference to Figures 5A to 5E.

As shown in block 1610, the method includes imaging reflected sample light from a sample that interferes with reflected reference light from a reference mirror at multiple depths of one or more coherent envelopes using a collecting optical element to produce multiple images, such as illustrated by the interference light 722 and detector 720 shown in FIGS. 7A and 7B and 15 . Components for imaging reflected sample light from a sample that interferes with reflected reference light from a reference mirror at multiple depths of one or more coherent envelopes using collecting optical elements to produce multiple images may include, for example, objective lens 822, lens 842, and detector 840 as shown in Figure 8, or objective lens 1022, lens 1042, and detector 1040 in Figure 10.

As indicated at block 1612, the method includes analyzing a plurality of images of a region of a sample including a plurality of structures to characterize the region of the sample, for example, as discussed with reference to the processing system 750 shown in Figures 7A and 7B. The means for analyzing a plurality of images of a region of a sample including a plurality of structures to characterize the region of the sample may include, for example, at least one processor 752, 850, and 1050 of Figures 7A, 7B, 8, and 10 that may be configured with computer readable code as discussed herein.

In some embodiments, the method may generate one or more coherent envelopes by causing interference between reflected sample light from the sample and reflected reference light from a reference mirror through depth scanning of the sample by, for example, changing the optical path difference between the sample light and the reference light using one or more moving reference mirrors 716, 826, 906, 1026, 1106, 1126, and 1156 shown in FIGS. 8 , 9 , 10 , 11A , 11B, and 11C and/or the moving beam splitters 1024 and 1104 shown in FIGS. 10 and 11A and/or the light source 810 or 1010 that changes the spectrum of the illumination light in FIGS. 8 and 10 .

In some implementations, the method can further include providing illumination light to the interferometer in an optical path between the collection optics and the sample, e.g., as discussed with reference to FIGS. 8 and 9. For example, components for providing illumination light to the interferometer in an optical path between the collection optics and the sample can include, for example, light source 810, lens 811, and beam splitter 812 shown in FIG. 8, or lens/beam splitter 908/904 shown in FIG. 9. In one example implementation, the optical path difference between the sample light and the reference light can be changed by moving the illumination optics and the reference lens while not changing the focal position of the collection optics relative to the sample, e.g., as discussed with reference to assembly 830 in FIG. 8. The means for moving the illumination optics and the reference lens may include, for example, actuator 831 as shown in Fig. 8. In one embodiment, the illumination optics may include collimating optics and a beamsplitter that splits the illumination light into sample light and reference light, for example, as illustrated by lenses/beamsplitters 811/812 and 908/904 shown in Figs. 8 and 9, respectively.

In some implementations, the method can change the optical path difference between the sample light and the reference light by moving a reference mirror, moving a beamsplitter that splits the illumination light into sample light and reference light, and modulating at least one of the spectrum of the illumination light, while not changing the focal position of the collection optical element relative to the sample, for example, as discussed with reference to Figures 7A, 7B, 8, and 10.

In some embodiments, the method may further include splitting the reference light to generate a second reference light to be incident on and reflected by a second reference mirror; changing a second optical path difference between the sample light and the second reference light while not changing a focal position of the collecting optical element relative to the sample, wherein the second optical path difference is different from the optical path difference; imaging the reflected sample light from the sample that interferes with the reflected second reference light from the second reference mirror by a plurality of second optical path differences using the collecting optical element to generate a second set of multiple images; and analyzing the second set of multiple images of a region of the sample including a plurality of structures to characterize the region of the sample, for example, as discussed with reference to FIG. 12 and FIGS. 13A to 13C. For example, components for splitting the reference light to generate the second reference light to be incident on and reflected by the second reference mirror may include, for example, the second beam splitter 1306, the first reference mirror 1326, and the glass plate 1356 shown in FIG. 13A, FIG. 13B, and FIG. 13C, respectively. Components for changing the second optical path difference between the sample light and the second reference light while not changing the focal position of the collection optical element relative to the sample, wherein the second optical path difference is different from the optical path difference, may include, for example, the second reference mirrors 1310, 1330, 1360 shown in FIG. 13A, FIG. 13B, and FIG. 13C, respectively. The means for imaging the reflected sample light from the sample that interferes with the reflected second reference light from the second reference lens by a plurality of second optical path differences using a collection optical element to generate a second set of multiple images may include, for example, the objective lens 822, lens 842, and detector 840 in FIG8, or the objective lens 1022, lens 1042, and detector 1040 in FIG10. The means for analyzing the second set of multiple images of a region of the sample including a plurality of structures to characterize the region of the sample may include, for example, at least one processor 752, 850, and 1050 in FIG7A, FIG7B, FIG8, and FIG10 that may be configured with computer readable code as discussed herein.

For example, reflected sample light from a sample and reflected reference light from a reference mirror that interfere with each other by an optical path difference in a plurality of images generate top interference fringes corresponding to a top surface of the sample, for example, as shown in top fringes 1232 in correlation graph 1230 in FIG. 12 , and reflected sample light from a sample and reflected second reference light from a second reference mirror that interfere with each other by a second optical path difference in a second set of multiple images generate bottom interference fringes corresponding to bottoms of a plurality of structures, for example, as shown in bottom fringes 1234 in correlation graph 1230 in FIG. 12 .

For example, the illumination light may have a spectral bandwidth, and the split reference light may direct a first spectral range to be incident on a reference mirror and a second spectral range to be incident on a second reference mirror, for example, as discussed with reference to FIG12 and FIG13A to FIG13C. Additionally, imaging the interfering sample light and the reference light may use a first sensor, and imaging the interfering sample light and the second reference light may use a second sensor, for example, as discussed with reference to FIG12 and FIG13A to FIG13C.

In some embodiments, the method may further include reducing the numerical aperture of the light source that produces the illumination light, and providing the illumination light to an optical path of an interferometer having a beam splitter that directs the illumination light to an objective lens; wherein the illumination optical element having a first numerical aperture includes the numerical aperture of the light source and the objective lens, and wherein the collection optical element having a second numerical aperture greater than the first numerical aperture includes the objective lens, for example, as discussed with reference to Figures 10, 11A, 11B, and 11C. The means for reducing the numerical aperture of the light source that produces the illumination light may include, for example, aperture/objective lens 1012/1022 shown in Figure 10, or the aperture may be adjusted at the lens, or may be further adjusted in an intermediate image of the source or anywhere along the beam. Components for providing illumination light into the optical path of an interferometer having a beam splitter that directs the illumination light to the objective lens may include, for example, beam splitters 1014, 1154 shown in Figures 10 and 11C.

In some implementations, the illumination light may have at least one of a spectral bandwidth and an angular bandwidth to produce low coherence fringes in multiple images, for example, as discussed with reference to light source 712 in Figures 7A and 7B.

In some implementations, an illumination source that generates illumination light can be configured to vary one or more of the following in the illumination light: at least one of spectral bandwidth and angular bandwidth; at least one of a cylindrical optical element and a spatially linear light source; and polarization control, for example, as discussed with reference to light source 712 in Figures 7A and 7B.

In some embodiments, the one or more coherence envelopes may be scanned over a range at least as large as the depth of the plurality of structures, e.g., as discussed with reference to FIG. 12. For example, in some embodiments, the range may be adjustable, e.g., as discussed with reference to FIG. 12. In some embodiments, the method may further include: detecting top interference fringes corresponding to a top surface of the sample in the plurality of images; detecting bottom interference fringes corresponding to bottoms of the plurality of structures in the plurality of images; and determining the depths of the plurality of structures based on a distance between the top interference fringes and the bottom interference fringes, e.g., as discussed with reference to FIG. 12 and FIG. 15. Components for detecting top interference fringes corresponding to the top surface of the sample in multiple images; components for detecting bottom interference fringes corresponding to the bottom of multiple structures in multiple images; and components for determining the depth of multiple structures based on the distance between the top interference fringes and the bottom interference fringes may include, for example, at least one processor 752, 850, and 1050 in Figures 7A, 7B, 8, and 10 that can be configured using computer-readable code as discussed herein.

In some embodiments, the method can be performed by depth scanning of the sample to obtain one or more coherent envelopes generated by interference between reflected sample light from the sample and reflected reference light from a reference mirror by modulating the periodicity of the spectrum of illumination light or interference light generated by interference between reflected sample light from the sample and reflected reference light, for example using a modulation interferometer 500 or an unbalanced fiber optic interferometer as discussed with reference to FIGS. 5A to 5E .

FIG17 is a flow chart illustrating a method for characterizing a region of a sample including a plurality of structures using interferometry, which method may be performed, for example, by any of the interferometers 700, 800, and 1000 illustrated in FIG7A, FIG8, and FIG10, respectively, using the interferometric objectives discussed with respect to FIG9 and FIG11A-11C. In some embodiments, the plurality of structures may be at least one of trenches, holes, and high aspect ratio (HAR) structures. For example, the HAR structures may have a width of 5 μm or less and a depth of 30 μm or more.

As shown in block 1702, the method includes splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by the reference mirror, for example, as shown by the beam splitter 714 in FIG. 7A. For example, the components for splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by the reference mirror may include a beam splitter positioned below the objective lens (i.e., the beam splitter is positioned between the objective lens and the sample), such as the beam splitters 812, 904, 1024, 1104, and 1124 in Figures 8, 9, 10, 11A, and 11B, or a beam splitter positioned above the objective lens (i.e., the objective lens is positioned between the beam splitter and the sample), such as the beam splitter 1154 in Figure 11C.

As depicted at block 1704, the method includes illuminating a region of the sample with sample light using an illumination optical element having a first numerical aperture, for example, as depicted as sample light 724 in FIG. 7A. Components for illuminating an area of a sample with sample light using an illumination optical element having an aperture of a first numerical value may include, for example, an optical element positioned to provide illumination light below a collection lens, such as (multiple) lens/beamsplitter/sample light 811/812/825 and 908/904/907 shown in Figures 8 and 9, respectively, or may include, for example, an optical element positioned to provide illumination light before a collection lens, such as aperture/objective lens/sample light 1012/1022/1023 shown in Figure 10 and objective lens/sample light 1102/1107, 1152/1157 shown in Figures 11A, 11B, and 11C.

At block 1706, the method includes focusing a collection optical element at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture, for example, as depicted by collected light 726 in FIG 7A. In some embodiments, the collection optical element can be one of a Michaelisoscope, a Milbescope, or a Linnik objective. Components for focusing the collecting optical element at a focal position relative to the sample, the collecting optical element having a second numerical aperture greater than the first numerical aperture may include, for example, objective lenses/collected light 822/829, 902/909, 1022/1029, 1102/1109, 1122/1129, and 1152/1159 shown in Figures 8, 9, 10, 11A, 11B, and 11C, respectively.

At block 1708, the method includes changing the optical path difference between the sample light and the reference light while not changing the focal position of the collection optical element relative to the sample, for example, as shown by arrow 721 in Figure 7A. Means for changing the optical path difference between the sample light and the reference light while not changing the focal position of the collection optical element relative to the sample may include, for example, the moving reference mirrors 716, 826, 906, 1026, 1106, 1126, and 1156 shown in Figures 8, 9, 10, 11A, 11B, and 11C, and/or the moving beam splitters 1024 and 1104 shown in Figures 10 and 11A, and/or the light source 810 or 1010 in Figures 8 and 10 that changes the spectrum of the illumination light.

As shown in block 1710, the method includes imaging the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror by a plurality of optical path differences using a collection optical element to generate a plurality of images, for example, as shown in FIG. 7A and the interference light 722 and the detector 720 shown in FIG. 15. The components for imaging the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror by a plurality of optical path differences using a collection optical element to generate a plurality of images may include, for example, the objective lens 822, the lens 842, and the detector 840 in FIG. 8, or the objective lens 1022, the lens 1042, and the detector 1040 in FIG.

As indicated at block 1712, the method includes analyzing a plurality of images of a region of a sample including a plurality of structures to characterize the region of the sample, for example, as discussed with reference to the processing system 750 shown in FIG7A. Means for analyzing a plurality of images of a region of a sample including a plurality of structures to characterize the region of the sample may include, for example, at least one processor 752, 850, and 1050 of FIG7A, FIG8, and FIG10 that may be configured with computer readable code as discussed herein.

FIG. 18 is a flow chart illustrating a method for characterizing a region of a sample including a plurality of structures using interferometry, which method may be performed, for example, by the interferometer 700' shown in FIG. 7B using modulation of the periodic spectrum discussed with respect to FIG. 5A-5E. In some embodiments, the plurality of structures may be at least one of trenches, holes, and high aspect ratio (HAR) structures. For example, the HAR structures may have a width of 5 μm or less and a depth of 30 μm or more.

As shown in block 1802, the method includes splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by the reference mirror, for example, as shown by the beam splitter 714 in FIG. 7B. For example, the components for splitting the illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by the reference mirror may include a beam splitter positioned below the objective lens (i.e., the beam splitter is positioned between the objective lens and the sample), such as the beam splitters 812, 904, 1024, 1104, and 1124 in Figures 8, 9, 10, 11A, and 11B, or a beam splitter positioned above the objective lens (i.e., the objective lens is positioned between the beam splitter and the sample), such as the beam splitter 1154 in Figure 11C.

As depicted at block 1804, the method includes illuminating a region of the sample with sample light using an illumination optical element having a first numerical aperture, for example, as depicted as sample light 724 in FIG. 7B. Components for illuminating an area of a sample with sample light using an illumination optical element having an aperture of a first numerical value may include, for example, an optical element positioned to provide illumination light below a collection lens, such as (multiple) lens/beamsplitter/sample light 811/812/825 and 908/904/907 shown in Figures 8 and 9, respectively, or may include, for example, an optical element positioned to provide illumination light before a collection lens, such as aperture/objective lens/sample light 1012/1022/1023 shown in Figure 10 and objective lens/sample light 1102/1107, 1152/1157 shown in Figures 11A, 11B, and 11C.

At block 1806, the method includes focusing a collection optical element at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture, for example, as depicted by collected light 726 in Figure 7B. In some embodiments, the collection optical element can be one of a Michaelisoscope, a Milbescope, or a Linnik objective. Components for focusing the collecting optical element at a focal position relative to the sample, the collecting optical element having a second numerical aperture greater than the first numerical aperture may include, for example, objective lenses/collected light 822/829, 902/909, 1022/1029, 1102/1109, 1122/1129, and 1152/1159 shown in Figures 8, 9, 10, 11A, 11B, and 11C, respectively.

At block 1808, the method includes modulating the periodicity of the spectrum of illumination light or interference light generated by interference of reflected sample light from the sample with reflected reference light from the reference mirror without changing the focal position of the collection optical element relative to the sample, for example, as shown by arrow 721 in FIG. 7B. Means for modulating the periodicity of the spectrum of illumination light or interference light generated by interference of reflected sample light from the sample with reflected reference light from the reference mirror without changing the focal position of the collection optical element relative to the sample may include, for example, the modulation interferometer 500 or the unbalanced fiber interferometer discussed with reference to FIGS. 5A to 5E.

As shown in block 1810, the method includes: using a collecting optical element to image the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror with multiple changes in the periodic modulation of the light spectrum to produce multiple images, for example, as shown by the interference light 722 and the detector 720 shown in Figures 7B and 15. Components for imaging the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror by multiple changes in the periodic modulation of the light spectrum using collecting optical elements to produce multiple images may include, for example, the objective lens 822, lens 842, and detector 840 in Figure 8, or the objective lens 1022, lens 1042, and detector 1040 in Figure 10.

As indicated at block 1812, the method includes analyzing a plurality of images of a region of a sample including a plurality of structures to characterize the region of the sample, for example, as discussed with reference to the processing system 750 shown in FIG7B. The means for analyzing a plurality of images of a region of a sample including a plurality of structures to characterize the region of the sample may include, for example, at least one processor 752 of FIG7B that may be configured with computer readable code as discussed herein.

Those of ordinary skill in the art will appreciate that the foregoing embodiments of the present disclosure provide many alternatives and modifications, which are also considered to be within the scope of the present disclosure. For example, the entire disclosure is described in the absence of light polarization, but if desired, orthogonal polarized light in an interfering beam can be used and a polarization pixelated camera can be used to analyze the image. The above description is intended to be illustrative and non-limiting. For example, the above examples (or one or more aspects thereof) can be used in combination with each other. Other embodiments can be used as reviewed by those of ordinary skill in the art. In addition, various features can be grouped together, and less than all features of the specific disclosed embodiments can be used. Therefore, the following aspects are hereby incorporated into the above description as examples or implementations, wherein each aspect is independently a separate implementation, and it is expected that such implementations can be combined with each other in various combinations or arrangements. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.

100: interferometer 102: sample 110: broadband light source 112: beam splitter 120: interferometer lens 122: objective lens 123: reflected reference beam 124: beam splitter 125: sample beam 125r: reflected sample beam 126: reference lens 128: actuator 129: arrow 140: detector 142: lens 150: processing system 202: sample 210: interferometer lens 212: sample beam 213: focal position 218: arrow 220A, 220B, 220C: interference signal 302: sample 304: structure 310: Milhaud interferometer; interferometer 312: arrow 402: sample 404: through-silicon via 406: top surface 408: bottom surface 410: scanning electron microscope cross-sectional profile 420: low coherence interferometer cross-sectional profile 430: low coherence interferometer stripe correlation diagram 450: low coherence interferometer stripe 452: top stripe 454: bottom stripe 460: low coherence interferometer stripe 462: top stripe 464: bottom stripe 466: ghost stripe 500: modulation interferometer 504: beam splitter 506: return mirror 507: actuator 508: return mirror 520: spectrum modulator 522: dispersion optical element 524: spatial light modulator 526: recombinant optical element 550: low coherence stripe 552: low coherence stripe; middle stripe 554: low coherence stripe; outer stripe 556: low coherence stripe; outer stripe 562: input light 564: output light 602: light source 612,632: optical element 610,620,630: system 614: light source aperture 624: light source aperture 700,700': interferometer 702: sample 704: structure 710,710': optical head 712: light source 713: interference objective 714: spectrometer 715: reference light 716: reference lens 717: sample light 718: objective lens 719: aperture 720: detector 721: arrow 722: interference light 724: shadow sample light; sample light 726: collected light 730: stand 732: chuck 750: processing system 752: processor 754: memory 756: display 757: communication port 758: input device 759: non-temporary computer-usable storage medium 772,782: modulated optical element 784,786,788: mirror 800: interferometer 802: sample 810: light source 811: lens 812: beam splitter 820: interferometer lens 822: objective lens 823: reference light 825: sample light 826: reference lens 827: arrow 828: actuator 829: collected light 830: assembly 831: actuator 832: arrow 840: detector 842: lens 850: processor 900: interferometer lens 901: sample 902: objective lens 904: spectrometer 905: reference light 906: reference lens 907: sample light 908: lens 909: collected light 1000: interferometer 1002: sample 1010: light source 1011: lens 1012: aperture 1014: spectrometer 1020: interferometer 1022: objective lens 1023: reference light 1024: spectrometer 1025: sample light 1026: reference lens 1028: actuator 1029: collected light 1040: detector 1042: lens 1050: Processor 1100: Interference objective 1101: Sample 1102: Objective 1104: Spectrometer 1105: Reference light 1106: Reference lens 1107: Sample light 1109: Collected light 1120: Interference objective 1121: Sample 1122: Objective 1124: Spectrometer 1125: Reference light 1126: Reference lens 1127: Sample light 1129: Collected light 1150: Interference objective 1151: Sample 1152,1153: Matching objective 1154: Spectrometer 1155: Reference light 1156: reference mirror 1157: sample light 1159: collected light 1200: structure 1202: top surface 1204: bottom surface 1210: correlation map 1212: top stripe 1214: bottom stripe 1220: reference mirror 1222: distance 1230: correlation map 1232: top stripe 1234: bottom stripe 1240: first reference mirror 1242: distance 1244: second reference mirror 1246: distance 1300: interferometer 1301: sample 1303: illumination light 1304: beam splitter 1305: reference light 1306: second beam splitter 1307: sample light 1308: reference mirror; first reference mirror 1310: reference mirror; second reference mirror 1320: interference objective 1321: sample 1323: illumination light 1324: first beam splitter 1325: reference light 1326: first reference mirror 1327: sample light 1330: second reference mirror 1350: interference objective 1351: sample 1353: illumination light 1354: first beam splitter 1355: reference light 1356: glass plate 1357: sample light 1358: First reference mirror 1360: Second reference mirror 1402: Correlation graph 1402b: Bottom stripe 1404: Correlation graph 1404b: Bottom stripe 1402t, 1404t: Visible stripe 1412: Correlation graph 1414: Correlation graph 1412t, 1414t: Visible stripe 1414b: Bottom stripe 1424: Correlation graph 1424t: Visible stripe 1424b: Bottom stripe 1500: Image collection 1500A, 1500B, 1500C: Image 1502: Structure 1602: Process block 1604: Process block 1606: Process block 1608: Process block 1610: Process block 1612: Process block 1702: Process block 1704: Process block 1706: Process block 1708: Process block 1710: Process block 1712: Process block 1802: Process block 1804: Process block 1806: Process block 1808: Process block 1810: Process block 1812: Process block fl1: focal length fl2: focal length α: angle β1: angle β2: angle

[Figure 1] shows a schematic diagram of an interferometer with a learned interferometer. [Figure 2A] shows the operation of scanning the focal position of the learned interferometer. [Figure 2B] shows a sample with superimposed interference signals generated by scanning the focal position. [Figure 3A] and [Figure 3B] show the Mirau interferometer at different focal positions relative to the sample. [Figure 4A] shows the scanning electron microscope (SEM) cross-sectional profile of the sample, the corresponding low coherence interferometer (LCI) cross-sectional profile, and the corresponding correlation diagram showing the LCI stripes. [FIG. 4B] and [FIG. 4C] show LCI fringes generated from a sample having high aspect ratio vias with a via width of 5 µm and 1 µm, respectively. [FIG. 5A] shows a schematic diagram of a modulation interferometer that modulates the spectrum of light provided to a metrology interferometer. [FIG. 5B] shows a schematic diagram of a modulation interferometer that modulates the spectrum of light received from a metrology interferometer. [FIG. 5C] shows fringes generated by modulating the spectrum of light. [FIG. 5D] shows input light before the spectrum of light is modulated by the modulation interferometer. [FIG. 5E] shows output light showing a periodic spectrum after the spectrum of light is modulated by the modulation interferometer. [Figure 6] shows an example of beam collimation divergence in a system with different source sizes and lens focal lengths. [Figure 7A] shows a schematic diagram of an interferometer with low NA illumination optics and high NA collection optics that characterizes structures on a sample by varying the optical path difference between reference and sample light while maintaining the focal position of the collection optics. [Figure 7B] shows a schematic diagram of an interferometer with low NA illumination optics and high NA collection optics that characterizes structures on a sample by modulating the periodicity of the spectrum of light while maintaining the focal position of the collection optics. [Figure 8] shows a schematic diagram of an interferometer providing illumination light to an interference objective below the objective. [FIG. 9] illustrates an embodiment of an interferometer objective that can be used with the interferometer shown in FIG. 8. [FIG. 10] illustrates a schematic diagram of an interferometer that provides illumination light to the interferometer objective through an objective lens. [FIG. 11A] to [FIG. 11C] illustrate various embodiments of an interferometer objective that can be used with the interferometer shown in FIG. 10. [FIG. 12] illustrates a comparison of scanning a single reference optical path and scanning multiple reference optical paths. [FIG. 13A], [FIG. 13B], and [FIG. 13C] illustrate different embodiments of generating multiple reference optical paths that can be used with the interferometers shown in FIG. 7A, FIG. 7B, FIG. 8, and FIG. 10. [FIG. 14A], [FIG. 14B], and [FIG. 14C] illustrate examples of correlation plots of vias of different sizes as seen in a single pixel scan of an interferometer as discussed herein. [FIG. 15] illustrates a full field of view of a sample in multiple images produced by an interferometer as discussed herein. [FIG. 16] to [FIG. 18] illustrate flow charts of a method for characterizing a region of a sample comprising multiple structures using an interferometer as discussed herein.

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Claims (17)

A method for characterizing a region of a sample containing a plurality of structures using an interferometer, the method comprising: splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror; illuminating the region of the sample with the sample light using an illumination optical element having a first numerical aperture; focusing a collection optical element at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture; changing the optical path difference between the sample light and the reference light while not changing the focal position of the collection optical element relative to the sample; Using the collecting optical element to image the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror by a plurality of optical path differences to generate a plurality of images; and Analyzing the plurality of images of the region of the sample containing the plurality of structures to characterize the region of the sample. The method of claim 1, further comprising providing the illumination light to the interferometer in an optical path between the collection optical element and the sample. A method as in claim 1, wherein changing the optical path difference between the sample light and the reference light while not changing the focal position of the collecting optical element relative to the sample includes moving the reference mirror and moving at least one of a beam splitter that splits the illumination light into the sample light and the reference light. The method of claim 1 further comprises: splitting the reference light to generate a second reference light to be incident on and reflected by a second reference mirror; changing the second optical path difference between the sample light and the second reference light while not changing the focal position of the collecting optical element relative to the sample; using the collecting optical element to image the reflected sample light from the sample and the reflected second reference light from the second reference mirror that interfere with a plurality of second optical path differences to generate a second set of multiple images; and analyzing the second set of multiple images of the region of the sample containing the plurality of structures to characterize the region of the sample. A method as claimed in claim 4, wherein the second optical path difference is different from the optical path difference, and wherein the reflected sample light from the sample and the reflected reference light from the reference mirror that interfere with each other due to the optical path difference in the multiple images generate top interference fringes corresponding to a top surface of the sample, and wherein the reflected sample light from the sample and the reflected second reference light from the second reference mirror that interfere with each other due to the second optical path difference in the second set of multiple images generate bottom interference fringes corresponding to the bottoms of the multiple structures. The method of claim 1, further comprising: reducing the numerical aperture of the light source generating the illumination light; providing the illumination light to the optical path of the interferometer having a beam splitter that directs the illumination light to the objective lens; and wherein the illumination optical element having the first numerical aperture includes the numerical aperture of the light source and the objective lens; wherein the collection optical element having the second numerical aperture greater than the first numerical aperture includes the objective lens. The method of claim 1, wherein the plurality of structures comprises at least one of a trench, a hole, and a high aspect ratio (HAR) structure, wherein the HAR structure has a width of 5 μm or less and a depth of 30 μm or greater. The method of claim 1 further comprises: Detecting top interference fringes corresponding to the top surface of the sample in the plurality of images; Detecting bottom interference fringes corresponding to the bottom of the plurality of structures in the plurality of images; and Determining the depth of the plurality of structures based on the distance between the top interference fringes and the bottom interference fringes. An interferometer configured for characterizing a region of a sample comprising a plurality of structures, the interferometer comprising: a means for splitting illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror; a means for illuminating the region of the sample with the sample light using an illumination optical element having a first numerical aperture; a means for focusing a collection optical element at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture; a means for changing the optical path difference between the sample light and the reference light while not changing the focal position of the collection optical element relative to the sample; A component for imaging the sample light reflected from the sample that interferes with the reference light reflected from the reference mirror by a plurality of optical path differences using the collecting optical element to generate a plurality of images; and A component for analyzing the plurality of images of the region of the sample containing the plurality of structures to characterize the region of the sample. An interferometer as claimed in claim 9, wherein the component for splitting the illumination light into sample light and reference light is positioned between the collecting optical element and the sample. An interferometer as claimed in claim 9, wherein the component for changing the optical path difference between the sample light and the reference light while not changing the focal position of the collecting optical element relative to the sample performs at least one of moving the reference mirror and moving a beam splitter that splits the illumination light into the sample light and the reference light. The interferometer of claim 9 further comprises: a component for splitting the reference light to generate a second reference light to be incident on a second reference mirror and reflected by the second reference mirror; a component for changing the second optical path difference between the sample light and the second reference light without changing the focal position of the collecting optical element relative to the sample; a component for generating a second set of multiple images by imaging the reflected sample light from the sample and the reflected second reference light from the second reference mirror that interfere with a plurality of second optical path differences using the collecting optical element; and a component for analyzing the second set of multiple images of the region of the sample containing the plurality of structures to characterize the region of the sample. An interferometer as claimed in claim 12, wherein the second optical path difference is different from the optical path difference, and wherein the reflected sample light from the sample and the reflected reference light from the reference mirror that interfere with the optical path difference in the multiple images produce top interference fringes corresponding to a top surface of the sample, and wherein the reflected sample light from the sample and the reflected second reference light from the second reference mirror that interfere with the second optical path difference in the second set of multiple images produce bottom interference fringes corresponding to the bottoms of the multiple structures. The interferometer of claim 9 further comprises: a component for reducing the numerical aperture of the light source generating the illumination light; a component for providing the illumination light to the optical path of the interferometer that guides the illumination light to the objective lens; and wherein the illumination optical element having the first numerical aperture comprises the numerical aperture of the light source and the objective lens; wherein the collection optical element having the second numerical aperture greater than the first numerical aperture comprises the objective lens. The interferometer of claim 9, wherein the plurality of structures comprises at least one of a trench, a hole, and a high aspect ratio (HAR) structure, wherein the HAR structure has a width of 5 μm or less and a depth of 30 μm or greater. The interferometer of claim 9 further comprises: a component for detecting top interference fringes corresponding to the top surface of the sample in the plurality of images; a component for detecting bottom interference fringes corresponding to the bottom of the plurality of structures in the plurality of images; and a component for determining the depth of the plurality of structures based on the distance between the top interference fringes and the bottom interference fringes. An interferometer configured to characterize a region of a sample comprising a plurality of structures, the interferometer comprising: a beam splitter configured to split illumination light into sample light to be incident on and reflected by the sample and reference light to be incident on and reflected by a reference mirror; an illumination optical element having a first numerical aperture configured to illuminate the region of the sample with the sample light; a collection optical element configured to focus at a focal position relative to the sample, the collection optical element having a second numerical aperture greater than the first numerical aperture; an actuator configured to change the optical path difference between the sample light and the reference light while not changing the focal position of the collection optical element relative to the sample; a detector configured to utilize the collecting optical element to image the reflected sample light from the sample that interferes with the reflected reference light from the reference mirror by a plurality of optical path differences to generate a plurality of images; and at least one processor configured to analyze the plurality of images of the region of the sample containing the plurality of structures to characterize the region of the sample.