BE1032505B1 - Reflectometer system and method for determining a reflection - Google Patents

Abstract

A reflectometer system (1, 10) for analyzing a sample (2) comprising a measuring radiation system comprising a measuring radiation source (3) for generating a measuring beam (30); a reflectance sensor (4) for detecting an intensity of the measuring beam (30) after reflection by the sample (2), if present; an alignment radiation source (5) for generating an alignment beam (50); optical elements (61, 62) configured for directing said measuring beam (30) and/or said alignment beam (5) toward the sample (2), if present; and a position sensor (7) for measuring a position of the alignment beam (50) on said position sensor (7) after reflection from the sample (2), the reflectometer system (1, 10) being adapted to determine an alignment of the system (1, 10) relative to the sample (2) based on the measured position of the alignment beam (50), a beam width of the alignment beam (50) being smaller than a beam width of the measurement beam (30), the reflectometer system (1, 10) being configured such that, when an alignment of the reflectometer system (1, 10) is optimized, at least a portion of a travel path of the measurement beam (30) is substantially parallel to at least a portion of a travel path of the alignment beam (50).

Description

1 BE2024/5189

Reflectometer system and method for determining a reflection

Technical field of the invention

The present invention relates to a reflectometer system and a method for determining the reflectance of a sample.

Background of the invention

Reflectometry is a widely used technique for analyzing the properties of a sample by measuring the intensity of light reflected from it. This method is crucial in several industries, including semiconductor manufacturing, materials science, and optics, where accurate measurements of reflectance can provide valuable information about the composition, structure, and quality of a material.

In reflectometry, accurate alignment of the sample to the measurement beam is essential for obtaining reliable and reproducible results. Incorrect alignment can lead to significant errors in reflectance measurements, as the angle of incidence and the position at which the light beam strikes the sample surface can significantly affect the overall, or spectral, intensity of the reflected light.

One of the challenges in the field is the difficulty of achieving and maintaining accurate alignment, especially when a series of samples must be measured and when there is a variety of sample sizes, shapes and types.

The alignment process can be time-consuming and complex, especially if high accuracy is required.

2 BE2024/5189

Despite advances in reflectometry technology, these challenges emphasize the need for further improvements in the field. There is a continuing demand for more efficient, accurate, and user-friendly systems that can provide reliable reflectance measurements while simultaneously minimizing the risk of errors due to sample misalignment. Therefore, further advances in this field are still needed to address these challenges.

Summary of the invention

It is an object of embodiments of the present invention to provide a good system and method for reflectometry that can provide accurate alignment of a sample for reflectance measurements by using a narrow alignment beam in combination with a position sensor.

This objective is achieved by a reflectometer system and a method for determining the reflectance of a sample.

It is an advantage of embodiments of the present invention that a precise and accurate determination of the alignment, and, in turn, a precise and accurate alignment, of a sample relative to a reflectometer system can be achieved.

The aforementioned precise and accurate determination of the alignment can be achieved by using a narrow alignment beam, which allows for accurate determination of the position of the sample relative to the measurement beam. Because the diameter of the

3 BE2024/5189, where the alignment beam can be significantly smaller than that of the measurement beam, a highly accurate determination of the alignment beam's location on the position sensor can be achieved. Embodiments of the present invention can thus facilitate the accurate determination of whether the reflectometer system is properly aligned, e.g., whether the system is aligned within predetermined specifications. Moreover, this can, in turn, result in a more accurate alignment process, which can be beneficial for accurate reflectance measurements.

In a first aspect, the present invention relates to a reflectometer system for analyzing a sample. The reflectometer system comprises a measurement radiation system including a measurement radiation source for generating a measurement beam.

The reflectometer system comprises a reflectance sensor for detecting the intensity of the measurement beam after reflection from the sample, if present, an alignment radiation source for generating an alignment beam, a position sensor for measuring a position of the alignment beam on said position sensor after reflection from the sample, and optical elements configured for directing said measurement beam and/or said alignment beam toward the sample, if present, or for directing said alignment beam toward the position sensor.

The reflectometer system is adapted to determine an alignment of the system with respect to the sample based on the measured position of the alignment beam. A beam width of the

4 BE2024/5189 alignment beam is smaller than the beamwidth of the measurement beam. These beamwidths can be determined on the sample and/or the position sensor.

The said reflectometer system for analyzing the sample may serve to determine the reflectance of the sample, or to determine properties, such as composition or surface properties, of the sample based on the determined reflectance of the sample, or properties of a coating present on the surface of the sample.

The reflectometer system is configured such that, when the reflectometer system is aligned, at least a portion of the measurement beam's path is substantially parallel to at least a portion of the alignment beam's path. When the reflectometer system is aligned, the measurement beam's path and the alignment beam's path are preferably at least substantially parallel to the sample. For example, both beams may be substantially parallel when incident on and after reflection from the sample, or the alignment beam incident on the sample may be substantially parallel to the measurement beam after reflection from the sample. This configuration facilitates accurate and straightforward alignment, or alignment determination, of the reflectometer system because it ensures that both beams impact the sample in a similar manner.

In embodiments, the alignment beam path of travel may substantially overlap with the measurement beam path of travel by at least a

> BE2024/5189 part of their trajectories towards the sample, when said alignment is optimized. This overlapping of the beam trajectories can further improve the accuracy of the alignment, or of the alignment determination, by ensuring that the beams follow a common path over a portion of their trajectory towards the sample. These embodiments can also ensure that both beams interact with the sample in a similar manner, further improving the reliability of the determined alignment and of the alignment process.

In embodiments, the measurement beam and the alignment beam may be aimed at an angle of up to 16°, preferably up to 12°, and typically close to 8° relative to normal incidence on the sample, if present, when said alignment is optimized. In preferred embodiments, the measurement beam and the alignment beam may be aimed at a substantially perpendicular incidence on the sample, if present, when said alignment is optimized. This configuration may further facilitate determining the alignment of the reflectometer system, as it can ensure that both beams impact the sample in a similar manner. Moreover, in this configuration, the alignment, or the exact position of the sample along a direction of travel of the beams, may not be very critical to the measured reflectance.

In embodiments, the alignment radiation source may be positioned adjacent to the reflectance sensor such that the

6 BE2024/5189, the alignment beam is directed in a direction nearly opposite to the direction of the measurement beam incident on the reflectance sensor after reflection from the sample, if present. This arrangement can prevent the alignment beam from interfering with the measurement beam by preventing it from being detected by the reflectance sensor, thus improving the reliability of the reflectance measurements.

In embodiments, the alignment beam source may be located adjacent to the measurement beam source, or incorporated into the measurement beam source, such that the alignment and measurement beams can follow similar paths. In these embodiments, the alignment beam source and the measurement beam source may have similar interactions with any optical elements and the sample. This arrangement may allow for equivalent beam paths for the alignment and measurement beams, while allowing the position sensor to measure the position of the alignment beam on the position sensor, e.g., by filtering or removing the measurement beam signal based on different radiation properties for the alignment beam than for the measurement beam (e.g., by having a higher intensity for the alignment beam than for the measurement beam, and/or a different spectral content, and/or by pulsing alternately, etc.).

In embodiments, the measurement beam and the alignment beam can be directed with non-perpendicular incidence on the sample, if present, when said alignment is optimized. In embodiments, the measurement beam and the alignment beam can be directed

7 BE2024/5189 is oriented at an angle greater than zero, preferably from 1° to 20°, more preferably from 3° to 10°, relative to the perpendicular incidence on the sample, if present, when said alignment is optimized. In these embodiments, the beam path of the measurement beam and the alignment beam can take a V-shape, and the beam path of the beams to the sample typically does not overlap with the beam path of the same beams after reflection from the sample. An advantage of these embodiments is that the number of beam splitters can be limited, so that measurement times can be reduced. The inventors have discovered that in such a reflectometer system, the reproducibility and accuracy of measurements can be particularly good. Preferably, the beam paths are parallel, or they largely overlap over at least a portion of their paths. For example, the alignment beam's path to the sample may be substantially parallel or substantially overlap with the measurement beam's path after reflection from the sample, or, in other words, the alignment and measurement beams may travel in opposite directions along substantially the same path along at least part of their respective travel paths. In these embodiments, a beam splitter can be used to direct the alignment beam toward the position sensor.

In embodiments, the diameter of the alignment beam on the sample, if present, can be less than half the diameter of the measurement beam on the sample. This smaller diameter of the alignment beam can lead to a more accurate determination

8 BE2024/5189 of the sample alignment. In embodiments, the diameter or width of the alignment beam on the sample, if present, can be less than 20%, preferably less than 10%, of the diameter or width of the measurement beam on the sample. This further reduction of the alignment beam diameter can provide even greater accuracy in determining the alignment.

In embodiments, when said alignment is optimized, the alignment beam and/or the measurement beam, at least at the sample, may be substantially collimated. In embodiments, when said alignment is optimized, the alignment beam and/or the measurement beam, at least at the sample, may have a beam divergence with a magnitude, i.e. absolute value, of at most 16°, preferably at most 12°, more preferably at most 8°. In embodiments, when said alignment is optimized, the alignment beam and/or the measurement beam, at least at the sample, may have a beam convergence with a magnitude, i.e. absolute value, of at most 16°, preferably at most 12°, more preferably at most 8’.

The alignment source can be any type of light source, such as a light-emitting diode or a laser. In preferred embodiments, the alignment radiation source can be a laser. Using a laser can provide a narrow-width alignment beam, which can be advantageous for achieving precise alignment.

In embodiments, the measurement radiation source may be a diffuse light source. A diffuse light source may be a

3 BE2024/5189 provide uniform illumination of the sample, which can be important for accurate reflectance measurements.

For example, when a diffuse light source is used, the intensity of the measurement beam can be substantially uniform across the beam width. This type of light source is often used in reflectometer systems to provide uniform illumination over a large area of the sample. Examples of diffuse light sources include optic lamps, fluorescent lamps, and light-emitting diodes with diffusing optics (including both lenses and mirror systems, which can be continuous or patterned). In embodiments, the measurement beam source may comprise an Ulbricht sphere. A

An Ulbricht sphere can help create a highly diffuse and uniform light source, further improving the quality of reflectance measurements. An Ulbricht sphere, also called an integrating sphere, is often used in conjunction with a light source to produce diffuse illumination for reflectance measurements in a reflectometer system.

For precise alignment, using a laser as the alignment radiation source is advantageous, while using a diffuse light source, such as an Ulbricht sphere, for the measurement radiation source is advantageous for generating uniform illumination over the sample surface, which is desirable for precise reflectance measurements.

In embodiments, the position sensor may comprise at least two pixels. In embodiments, the position sensor may comprise a two-dimensional photodetector, such as a photodetector comprising a two-dimensional array of pixels. The use of a

10 BE2024/5189 A two-dimensional photodetector can enable more detailed spatial information to be captured regarding the position of the alignment beam, which can be beneficial for fine-tuning the alignment. In embodiments, the photodetector can be selected from a charged-coupled device and a complementary metal-oxide-semiconductor sensor. Integrating a two-dimensional photodetector, such as a charged-coupled device or a complementary metal-oxide-semiconductor sensor, as a position sensor is advantageous because it allows for the detection of the position of the alignment beam on the position sensor, typically with high accuracy and resolution. In embodiments, the photodetector can be selected from position-sensitive devices (PSDs) or optical position sensors (OPSs), making it possible to measure the position of a light spot in one or two dimensions on a sensor surface. Such devices may require only a limited number of connector pins (e.g., only 5), while still being able to determine the (one- or two-dimensional) position of a light spot.

In embodiments, the diameter of the alignment beam on the position sensor can be less than half the diameter of the measurement beam on the position sensor. This smaller diameter of the alignment beam can lead to a more accurate determination of the sample alignment. In embodiments, the diameter or width of the alignment beam on the position sensor can be less than 20%, preferably less than 10%, of the diameter or width.

11 BE2024/5189 of the measurement beam on the position sensor. This further reduction of the alignment beam diameter can provide even greater accuracy in determining alignment.

In some embodiments, the photodetector can have a photosensitive area larger than the beamwidth of the alignment beam at the photodetector. These embodiments can ensure that the entire alignment beam is captured by the photodetector, which is important for accurate position measurement. Furthermore, this can enable the detection of larger deviations or misalignments of the sample.

In embodiments, the photodetector's pixel width can be smaller than the alignment beam's beamwidth at the photodetector. This pixel size configuration can enable the photodetector to resolve the small spot size of the alignment beam, which may be preferable for accurate alignment detection.

In embodiments, the position sensor is adapted to distinguish the alignment beam from the measurement beam.

In embodiments, a wavelength of the measurement beam may differ from a wavelength of the alignment beam. This difference in wavelengths can help distinguish between the two beams, reducing the chance of crosstalk and improving measurement accuracy. In embodiments, the reflectance sensor is adapted to separate or filter out the wavelength of the alignment beam. For example, the reflectance sensor can

12 BE2024/5189 include a spectral filter for filtering out the wavelength of the alignment beam.

For example, the reflectance sensor can have a higher sensitivity to the wavelength of the measurement beam than to the wavelength of the alignment beam.

In embodiments, the wavelength of the measurement beam can be separated by the position sensor.

Preferably, the measurement beam can be refracted by a grating, and the refracted measurement beam can be projected onto a one- or two-dimensional CCD, so that the measurement beam can be spectrally separated from the alignment beam, thereby allowing the measurement beam to be detected separately from the alignment beam.

For example, the position sensor may be adapted to sense light at the wavelength of the alignment beam and not sense light at the wavelength of the measurement beam, such as by using a bandpass filter adapted to pass the alignment beam and block the measurement beam.

Additionally, a laser or light emitting diode can be chosen that has a narrow wavelength bandwidth that can be easily separated from the wavelength content of the measurement radiation source or that has a wavelength content that lies outside the detection wavelength range of the reflectance sensor.

In embodiments, pulses from the measurement beam and/or pulses from the alignment beam on the one hand and/or pulses on the reflection sensor and/or pulses on the position sensor on the other hand can be alternated.

In embodiments, the measurement radiation source may be configured to

13 BE2024/5189 can generate pulses of the measurement beam and/or the alignment radiation source can be configured to generate pulses of the alignment beam, and, simultaneously or synchronized with the source, the reflectance sensor and/or the position sensor can be configured to detect the measurement and/or alignment beam in a pulsed manner. Preferably, the measurement beam is continuous (i.e., not pulsed) to prevent changes in the spectral content of the measurement beam that may occur as a result of said pulsing. The reflectance sensor can be adapted to measure the measurement beam in a pulsed manner (e.g., continuously), whereby the alignment beam is pulsed between the pulses or periods during which the reflectance sensor measures the measurement beam. In embodiments, the alignment radiation source can be adapted to provide a pulsed alignment beam, and the reflectance sensor can be adapted to separate or filter out the alignment beam. Alternating the pulses can temporarily separate the measurement and alignment beams, allowing the position sensor and/or reflectance sensor to distinguish between the measurement and alignment beams. This can reduce potential interference and improve the overall accuracy of the system.

In embodiments, the reflectance sensor is adapted to detect the intensity of the measurement beam after it has been reflected by the sample.

The signal from the reflectance sensor can be used to quantify the amount of reflected light, which is related to the reflectivity of the sample.

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Examples of reflectance sensors include photodiodes, photomultiplier tubes, and spectrophotometers.

In embodiments, the reflectometer system may further comprise a beam splitter for separating a portion of the measurement beam before reflection from the sample, if present, and a reference sensor for detecting the intensity of the separated measurement beam. Typically, a beam splitter is an absorption-limited optical element that transmits a fraction of the incident radiation and reflects a fraction of the incident radiation.

This fraction can typically be any value between 5% and 95%. If absorption is low, the sum of the transmission and reflection fractions will approach 100%. The reference sensor can be, for example, a photodiode, a photomultiplier tube, or a spectrophotometer. This function can provide a reference signal for the intensity of the measurement beam, which can then be used to calibrate the system and ensure accurate reflection measurements.

In embodiments, the reflectometer system includes a controller for receiving the determined position from the position sensor. The controller may be adapted to compare the determined position with a predetermined area or range, and when the determined position falls within the predetermined area or range, the alignment can be considered optimized.

In embodiments, the reflectometer system may include a sample holder for holding the sample, the sample holder being adapted to move, e.g., rotate, the sample for

15 BE2024/5189 optimizes the aforementioned alignment based on the measured position of the alignment beam on the position sensor. This capability can allow mechanical adjustment of the sample orientation to achieve optimal alignment, which can be crucial for accurate reflection measurements.

In embodiments, the reflectometer system may further comprise control means for changing the configuration of the optical elements to improve alignment of the reflectometer system based on the measured position of the alignment beam on the position sensor. This feature may enable dynamic adjustment of the optical path to correct misalignments, thereby increasing the system's flexibility and accuracy. Optical components whose configuration can be adjusted to improve alignment typically include all optical components on which, when alignment is optimized, both the alignment beam is reflected along its path from the alignment radiation source to the position sensor, and the measurement beam is reflected along its path from the measurement radiation source to the reflectance sensor.

In embodiments, the reflectometer system may further comprise a controller for automated control of the steering means based on the measured position of the alignment beam on the position sensor.

Automating the alignment process can improve the efficiency and repeatability of the system

16 BE2024/5189 increase, reducing the need for manual intervention.

In embodiments, at least three selected from, preferably each of, the measurement radiation system, the alignment radiation source, the reflectance sensor, and the position sensor may be located in the same housing, with the reflectometer system configured to analyze the sample located outside said housing. In embodiments, the reflectometer system may be configured to analyze the sample located at a distance from the housing greater than the beam width of the measurement beam, e.g., at least 5 times the beam width, or even more than 10 times the beam width. These embodiments may facilitate flexibility and compatibility between the reflectometer system and the various types of samples that can be analyzed.

The same housing can refer to a single physical enclosure or chassis containing at least three components of the reflectometer system. The fact that these components are housed in the same housing indicates that at least three components—the measurement radiation system, the alignment radiation source, the reflectance sensor, and the position sensor—are all integrated into the same structural unit, enhancing the stability and portability of the reflectometer system.

In a second aspect, the present invention relates to an arrangement comprising the reflectometer system of embodiments of the

17 BE2024/5189, first aspect, and a vacuum system, e.g., a vacuum sputtering system, comprising a vacuum chamber containing an optical window. The optical window may be provided in an interface flange. The reflectometer system is located outside the vacuum chamber and is configured such that the measurement beam and the alignment beam pass through the optical window for analyzing a sample, if present, in the vacuum chamber.

In a third aspect, the present invention relates to a method for determining the reflectance of a sample in a reflectometer system, comprising directing an alignment beam toward the sample to reflect the alignment beam, measuring a position of the reflected alignment beam on a position sensor, determining an alignment of the reflectometer system based on the measured position, directing a measurement beam, having a beam width larger than that of the alignment beam, toward the sample to reflect the measurement beam, and measuring an intensity of the reflected measurement beam to determine said reflectance. The reflectometer system is configured such that, when an alignment of the reflectometer system is optimized, at least a portion of a propagation path of the measurement beam is substantially parallel to at least a portion of a propagation path of the alignment beam.

Determining the alignment of the reflectometer system can be followed by optimizing the alignment of the

18 BE2024/5189 reflectometer system, based on the aforementioned determined alignment. This alignment optimization can be performed before measuring the intensity of the reflected measurement beam for determining the aforementioned reflectance. Alternatively, the alignment beam and the measurement beam can be simultaneously directed at the sample, and the determination, and optionally the optimization, of the alignment can be performed during the aforementioned measurement of the intensity of the reflected measurement beam for determining the aforementioned reflectance. Alternatively, no alignment optimization is performed. For example, the determined position can be compared with a predetermined area or range, and if the determined position falls within the aforementioned predetermined area or range, the measured reflectance can be assumed to be accurate, while if the determined alignment falls outside the aforementioned predetermined area or range, the measured reflectance can be assumed to be inaccurate.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims and with features of other dependent claims where appropriate and not solely as explicitly set out in the claims.

The above and other features, properties and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying

19 BE2024/5189 drawings, which illustrate the principles of the invention by way of example. This description is provided for illustrative purposes only and does not limit the scope of the invention. The reference numerals below refer to the attached drawings.

Short description of the drawings

FIG. 1 is a schematic representation of a reflectometer system according to embodiments of the present invention.

FIG. 2 is a schematic representation of the reflectometer system with optimized sample alignment according to embodiments of the present invention.

FIG. 3 is a schematic representation of a reflectometer system with integrated components within a housing according to embodiments of the present invention.

FIG. 4 is a schematic representation of a position sensor used in the reflectometer system according to embodiments of the present invention.

FIG. 5 is a schematic representation of a reflectometer system according to embodiments of the present invention, wherein the beams are incident on a sample at non-perpendicular incidence.

In the various figures, the same reference marks refer to the same or analogous elements.

Description of illustrative embodiments

The present invention will be described with reference to certain embodiments and with reference to certain drawings, but the invention

20 BE2024/5189 is not limited to this, but only by the conclusions.

The drawings described are schematic only and not restrictive. In the drawings, the dimensions of some elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and relative sizes do not represent actual reductions in practice of the invention.

Furthermore, the terms "first," "second," "third," and the like are used in the description and the claims to distinguish between similar elements and not necessarily to describe a series, whether in time, space, arrangement, or otherwise. It should be understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein may operate in other sequences than described or illustrated herein.

Furthermore, the terms "top," "bottom," "above," "below," and the like are used in the description and claims for descriptive purposes and not necessarily to describe relative positions. It should be noted that the terms used in this way are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein may operate in orientations other than those described or illustrated herein.

It should be noted that the term ‘comprising’ used in the claims should not be construed as limited to the means listed below; it does not exclude other elements or steps.

21 BE2024/5189. It should therefore be interpreted as specifying the presence of the named features, integers, steps, or components referred to, but does not exclude the presence or addition of one or more other features, integers, steps, or components, or groups thereof. The term "comprising" is therefore understood to mean both the situation in which only the named features are present and the situation in which these features and one or more other features are present. The word "comprising" according to the invention therefore also implies, as one specific embodiment, that no further components are present. The scope of the expression "a device comprising means A and B" should therefore not be interpreted as limited to devices consisting only of components A and B.

It means that with respect to the present invention the only relevant components of the device are A and B.

Likewise, it should be noted that the term "coupled" should not be interpreted as limited to direct connections only. The terms "coupled" and "connected" can be used together with their derivatives. It should be noted that these terms are not intended as synonyms for each other. The scope of the expression "an establishment A coupled to an establishment

B' may therefore not be limited to installations or systems where an output of installation A is directly connected to an input of installation

B. It means that there is a route between an output of A and an input of B, which may involve other facilities or resources. 'Linked'

22 BE2024/5189 may mean that two or more elements are in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but still cooperate or interact with each other.

Reference in this specification to “a particular embodiment” or “an embodiment” means that a particular property, structure, or feature described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the occurrences of the phrases “in a particular embodiment” or “in an embodiment” at various points in this specification do not necessarily all refer to the same embodiment, but they may. Furthermore, the specific properties, structures, or features may be combined in any suitable manner, as will be apparent to those skilled in the art to which this disclosure applies, in one or more embodiments.

Likewise, it is to be understood that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped into a single embodiment, figure or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects.

However, this method of disclosure should not be construed to reflect an intention that the claimed invention requires more features than are expressly set forth in each claim.

On the contrary, as the following conclusions reflect, the inventive aspects lie in less than all

23 BE2024/5189 features a single previously disclosed embodiment. The claims following the detailed description are therefore expressly incorporated into this detailed description, with each claim standing alone as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features contained in other embodiments, it is intended that combinations of features from different embodiments fall within the scope of the invention and constitute different embodiments as will be understood by one skilled in the art.

For example, in the following claims, any of the claimed embodiments may be used in any combination.

Additionally, some embodiments are described herein as a method or combination of method elements that can be implemented by a processor of a computer system or by other means to perform the function. Thus, a processor having the necessary instructions for performing such a method or a portion of a method constitutes a means for performing the method or a portion of a method. Furthermore, a component of an apparatus embodiment described herein is an example of a means for performing the function performed by the component for the purpose of practicing the invention.

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The description herein sets forth various specific details. However, it will be understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this description.

The following terms are provided solely to aid in understanding the invention.

As used herein, and unless otherwise indicated, the term “beam width” refers to the diameter or size of a beam of light or other electromagnetic radiation at a particular location along the path of the beam’s travel, e.g., as measured at a sample surface or at a position sensor. The width may be the full width at half-maximum or the beam width at half power, or alternatively the 1/e? width, which are parameters known in the art for determining beam width. For an asymmetric (e.g., non-circular) beam profile, the width may be the smallest width at a particular location along the path of the beam’s travel. Beam width is an important parameter in determining the resolution and precision of the reflectometer system. The term “smaller beam width” indicates a narrower beam, typically used for alignment purposes, while “larger beam width” indicates a wider beam, typically used for measurement purposes.

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As used herein, and unless otherwise indicated, the term “beam divergence” refers to the angle through which a beam of light or other electromagnetic radiation spreads out as it travels from its source or after passing through an optical system. As used herein, and unless otherwise indicated, the term “beam convergence” refers to the angle through which a beam of light or other electromagnetic radiation narrows or converges as it travels toward a focal point. The angle of beam convergence or divergence can be calculated, for example, by the magnitude, or absolute value, of 2xarctan((wl-w2)/{(2x1)), where w1 and w2 are the beam widths at two different locations along the beam's path of travel, and l is the distance between said two different locations along the beam's path of travel.

As used herein, and unless otherwise specified, a collimated beam refers to a beam of light or other electromagnetic radiation that has parallel rays and therefore does not diverge or converge over a distance. For example, stating that the alignment beam is at least "substantially collimated" at the sample may indicate that the alignment beam maintains a substantially consistent beam width as it travels toward and impinges on the sample.

The invention will now be described in detail by describing various embodiments of the invention. It is understood that other embodiments of the invention may be configured according to the knowledge of those skilled in the art.

26 BE2024/5189 in the field, without departing from the technical teaching of the invention, the invention being limited only by the terms of the appended claims.

In a first aspect, the present invention relates to a reflectometer system for analyzing a sample. The reflectometer system comprises a measurement radiation system including a measurement radiation source for generating a measurement beam.

The reflectometer system comprises a reflectance sensor for detecting the intensity of the measurement beam after reflection from the sample, if present, an alignment radiation source for generating an alignment beam, a position sensor for measuring a position of the alignment beam on said position sensor after reflection from the sample, and optical elements configured for directing said measurement beam and/or said alignment beam toward the sample, if present, or for directing said alignment beam toward the position sensor.

The reflectometer system is adapted to determine the alignment of the system relative to the sample based on the measured position of the alignment beam. A beam width of the alignment beam is smaller than a beam width of the measurement beam. These beam widths can be determined on the sample and/or on the position sensor. The reflectometer system is configured such that, when the alignment of the reflectometer system is optimized, at least a portion of a path of travel of the measurement beam is substantially parallel.

27 BE2024/5189 to at least part of a progress route of the alignment bundle.

In a second aspect, the present invention relates to an arrangement comprising the reflectometer system of embodiments of the first aspect and a vacuum system comprising a vacuum chamber containing an optical window. The reflectometer system is located outside the vacuum chamber and is configured such that the measurement beam and the alignment beam pass through the optical window to analyze a sample, if present, in the vacuum chamber.

In a third aspect, the present invention relates to a method for determining the reflectance of a sample in a reflectometer system, comprising directing an alignment beam toward the sample to reflect the alignment beam, measuring a position of the reflected alignment beam on a position sensor, determining an alignment of the reflectometer system based on the measured position, directing a measurement beam, having a beam width greater than that of the alignment beam, toward the sample to reflect the measurement beam, and measuring an intensity of the reflected measurement beam to determine said reflectance. The reflectometer system is configured such that, when an alignment of the reflectometer system is optimized, at least a portion of a propagation path of the measurement beam is substantially parallel to at least a portion of a propagation path of the alignment beam.

28 BE2024/5189

Reference is made to FIG. 1, which is a schematic representation of a reflectometer system (1) in accordance with embodiments of the present invention for analyzing the reflectance of a sample (2) with high precision. In FIG. 1, the sample (2) is not properly aligned with the reflectometer system (1).

The system (1) is designed to optimize the alignment of the sample (2) to ensure accurate measurements. The reflectometer system (1), as shown in FIG. 1, comprises a measurement radiation system with a measurement radiation source (3) generating a measurement beam (30). The measurement beam (30) propagates, towards the sample (2), through optical elements that may include two beam splitters (61, 62). The reflectometer system (1) may comprise further optical elements, such as mirrors or lenses, to direct the measurement beam (30) towards the sample (2).

The measurement beam (30), after reflecting from the sample (20), is reflected by the first beam splitter (61) to a reflectance sensor (4) when the alignment is optimized. The reflectance sensor (4) is provided to detect the intensity of the measurement beam (30) after it is reflected from the sample (2). As can be observed, in FIG. 1, the measurement beam (30) does not impinge on the reflectance sensor (4) because the sample (2) is not aligned with the reflectometer system (1).

In the illustrated example, the second beam splitter (62) separates a portion of the measurement beam (30) before reflecting it through the sample (2) to a reference sensor (33), as shown in

29 BE2024/5189

FIG. 1. Alternatively, the system (1) may use the first beam splitter (61), or may include yet another beam splitter, to separate a portion of the measurement beam (30) before it reflects off the sample (2) to a reference sensor (33). The reference sensor (33) detects the intensity of this separated measurement beam (32), and provides a reference signal for calibrating the intensity of the measurement beam (30) detected by the reflectance sensor (4). In the illustrated example, the controller (8) may receive signals from the reference sensor (33) and from the reflectance sensor (4) and may determine a reflectance of the sample (2) based on these signals.

The system (1) also includes an alignment radiation source (5) which produces an alignment beam (50) with a smaller beam width than the measurement beam (30). The alignment radiation source (5) can be any type of radiation source (5), such as a laser diode or LED. In the illustrated example, the alignment beam (50) is reflected by the first beam splitter (61) to direct the alignment beam (50) towards the sample (2). The first beam splitter (61) is arranged such that the alignment beam (50) incident on the sample (2) and the measurement beam (30) incident on the sample (2) are substantially parallel. In the illustrated example, the alignment beam (50), after being reflected by the sample (2), continues through the first beam splitter (61) and is reflected by the second beam splitter (62) towards a

30 BE2024/5189 position sensor (7), which measures the position of the alignment beam (50) on the position sensor (7) after said reflection on the second beam splitter (62).

Since the alignment beam (50) incident on the sample (2) and the measurement beam (30) incident on the sample (2) are approximately parallel, the beams (30, 50) are also approximately parallel after reflection from the sample (2).

The reflectometer system (1) uses the information about the location of the alignment beam (50) on the position sensor (7) to determine the alignment of the sample (2). The position sensor (7) may comprise any type of position-sensitive device. The position sensor (7) may comprise, for example, a 1D or 2D array with at least two pixels, any type of camera, or any type of position-sensitive device. The position sensor (7) may comprise a two-dimensional photodetector (7), such as a charged-coupled device or a complementary metal-oxide-semiconductor sensor, preferably with a pixel width smaller than the beamwidth of the alignment beam (50). Furthermore, the position sensor (7) may comprise, for example, a position-sensitive device (PSD) or optical position sensor (OPS), allowing the position of a light spot on a sensor surface to be measured in one or two dimensions.

This allows the photodetector (7) to determine the small spot size of the alignment beam (50) for accurate alignment detection.

The smaller diameter of the alignment beam (50) allows for a more accurate determination of the sample alignment (2).

31 BE2024/5189

The controller (8) can receive information about a location of the alignment beam (50) on the position sensor (7). The alignment beam (50) can be distinguished from the measurement beam (30) in several ways. In the illustrated example, the alignment beam (50) and the measurement beam (30) are parallel but spatially separated, so that the position sensor (7) can detect the alignment beam (50) separately from the measurement beam (30). Alternatively, the alignment beam (50) can have a different spectral content, or wavelength, than the measurement beam (30). The position sensor (7) can be adapted to distinguish the wavelength of the alignment beam (50) from the wavelength of the measurement beam (30), or the system (1) can include a spectral filter to allow the alignment beam (50) to pass toward the position sensor (7) but block the progression of the measurement beam (30) to the position sensor (7). Alternatively, pulses from the measurement beam (30) may be alternated with pulses from the alignment beam (50), so that the position sensor (7) may be able to distinguish between the beams (30, 50) due to the different timing of the beams (30, 50) incident on the position sensor (7).

Based on the determined location of the alignment beam (50) on the position sensor (7), e.g., by comparing the determined location to a predetermined location or by determining whether the determined location falls within a predetermined area or range, the alignment of the reflectometer system (1) can be optimized. In the illustrated example

32 BE2024/5189 the sample (2) can be moved, e.g. rotated. The sample (2) can be placed, for example, in a sample holder (not shown) which can be triggered by the control (8) to move, e.g. to rotate the sample (2).

In FIG. 2, the reflectometer system (1) is shown after said rotation of the sample (2) so that alignment is optimized, for example, so that the alignment beam (50) falls within a predefined area on the position sensor (7). In the illustrated example, when alignment is optimized, the measurement beam (30) and the alignment beam (50) are incident perpendicularly on the sample (2).

In particular when the position sensor (7) is located further away from the sample (2) than the reflection sensor (4) for measuring the reflected source signal, a high accuracy of said alignment can be achieved. The length of the propagation path of the alignment beam (50) from the sample (2) to the position sensor (7) is preferably greater than the length of the propagation path of the measurement beam (30) from the sample (2) to the reflection sensor (4) (when the alignment of the system (1) is optimized).

In the illustrated example, the alignment beam source (5) is positioned next to the reflection sensor (4) to prevent interference between the alignment beam (50) and the measurement beam (30). When the alignment is optimized, the alignment beam (50) actually travels in a nearly opposite direction to the

33 BE2024/5189 measurement beam (30), and it is unlikely that the alignment beam (50) will impinge on the reflectance sensor (4). In addition, the beams (30, 50) are parallel to each other at least at the sample (2), which can facilitate proper alignment of the reflectometer system (1) based on the detection of the alignment beam (50) by the position sensor. More specifically, in the illustrated example, when the alignment is optimized, as in FIG. 2, the alignment beam (50), from the alignment radiation source (5) via the first beam splitter (61) to the sample (2), and the measurement beam (30), from the sample (2) via the first beam splitter (61) to the reflectance sensor (4), propagate along substantially parallel paths.

In the illustrated example, the method for determining the reflectance of a sample (2) comprises directing the alignment beam (50) onto the sample (2), measuring the position of the reflected alignment beam (50), determining the alignment of the system (1) based on this measurement, and directing the measurement beam (30) onto the sample (2) to measure the intensity of the reflected measurement beam (30). The directing of the alignment beam (50) and the measurement beam (30) onto the sample (2) may overlap in time. Although the alignment of the system (1) can be optimized based on the determined alignment, this is not essential. Instead, the determined alignment can be used, for example, to calibrate the measured reflectance. Alternatively, the measured reflectance can be made acceptably or sufficiently accurate

34 BE2024/5189 deemed when the determined position of the alignment beam (50) on the position sensor (7) falls within a predetermined range.

FIG. 3 introduces a variation on the reflectometer system (1) where the measurement radiation source (3), the alignment radiation source (5), the reflectance sensor (4), and the position sensor (7) are all housed within the same housing (9). In the illustrated example, the controller (8) is modified to control the position of the first beam splitter (61). The orientation of the first optical element (61) can be adjusted by a combination of rotating and translating the beam splitter (61). The system (1) is aligned when the alignment beam (50) strikes a predetermined location on the position sensor (7), ensuring that the measurement beam (30) is incident on the reflectance sensor (4). The reflectometer system (1) is configured such that, when the position of the alignment beam (50) is incident on a predetermined location of the position sensor (7), the reflectometer system (1) is aligned so that the measurement beam (30) is incident on the reflectance sensor (4). In the illustrated example, when the alignment is optimized (as shown in FIG. 3), the beams (30, 50) are approximately parallel to the sample (2). In the illustrated example, when the alignment is optimized (as shown in FIG. 3), the beams (30, 50) may impinge on the sample (2) at an angle that deviates from normal incidence. This may be a result of the

35 BE2024/5189 optimising the alignment, not by optimising the alignment of the sample (2), but of several optical components, in particular, as in the illustrated example, the first beam splitter (61). Since both the alignment beam (50) is reflected along its path from the alignment radiation source (5) towards the position sensor (7), and the measurement beam (30) is reflected along its path from the measurement radiation source (3) towards the reflection sensor (4), realigning the first beam splitter (61) ensures that both the path of the alignment beam (50) towards the position sensor (7) and the path of the measurement beam (30) towards the reflection sensor (4) are realigned. In this case, the optimization of the alignment of the alignment beam (50) can be combined with the optimization of the alignment of the measurement beam (30), or, in other words, the direction of travel of both beams (30, 50) can be adjusted simultaneously so that when the position of the alignment beam (50) on the position sensor (7) is optimized, the alignment of the measurement beam (30) relative to the reflection sensor (4) is also optimized.

In the illustrated example, the sample (2) is located outside the enclosure (9). The beams (30, 50) can leave the enclosure (9) through an opening in the direction of the sample (2) and after reflection by said sample (2), the beams (30, 50) can re-enter the enclosure (9) through said same opening (90). Therefore, in the

36 BE2024/5189 The illustrated example may be difficult to align the sample (2) by, e.g., rotating the sample (2). However, this configuration may have the advantage that the housing (9) and the components of the system (1) located within it can be easily installed, while still allowing the alignment of the system (1) to be optimized independently of the specific orientation of the housing (9) relative to the sample (2).

FIG. 4 illustrates a typical position sensor (7) comprising a two-dimensional array (70) of pixels (71), e.g., in the illustrated example, 12x12 pixels (71). The narrow spot (51) for the alignment beam provides high accuracy for determining the alignment of the reflectometer system (1), unlike the large spot (31) for the measurement beam. The different spot sizes result from the different widths of the beams. Determining a location of a center of the wide spot (31) on the position sensor (7) can result in a large error for the determined location. Furthermore, in practice, a dimension of the larger spot (31) for the measurement beam may be larger than the photosensitive area (70) of the position sensor (7).

In the illustrated example, the alignment of the sample or the system is considered to be optimized when the narrow spot (51) for the alignment beam is incident on the center of the position sensor (7). Therefore, in the illustrated example, the alignment is considered optimized.

37 BE2024/5189

Instead, when alignment is not optimized, the position sensor (7) can detect the alignment beam spot (51) at a location away from the center of the position sensor (7). For example, as in the example of FIG. 1, the sample (2) may be slightly tilted or misaligned. The system alignment can be optimized in that case, with alignment being considered optimized when the narrow alignment beam spot (51) is incident on the center of the position sensor (7). Alternatively, the reflectance sensor signal can be accepted within certain limits of deviation from the narrow spot (51) on the position sensor (7), the reflectance sensor signal can be corrected depending on the reading signal from the position sensor (7), or the reflectance sensor signal can be ignored if it is assumed that alignment is not optimized.

Reference is made to FIG. 1, which is a schematic representation of another reflectometer system (10) in accordance with embodiments of the present invention for analyzing the reflectance of a sample (2) with high precision.

The reflectometer system (10) comprises a measurement beam source (3) for generating a measurement beam (30) and an alignment beam source (5) for generating an alignment beam (50). Here, the diameter of the alignment beam (50) is smaller than the diameter of the measurement beam (30), e.g. at least at the sample. In the illustrated example, when

38 BE2024/5189 The reflectometer system (10) is aligned, with the measurement beam (30) and the alignment beam (50) impinging non-perpendicularly on the sample (2). As a result, the beam paths of the beams (30, 50) assume a V-shape. In the illustrated example, the measurement beam (30) and the alignment beam (50) follow nearly parallel beam paths (but in opposite directions) over at least a portion—in the illustrated example, over most—of their travel paths. Consequently, the effect of the sample orientation (2) on both beams (30, 50) is similar, which can facilitate alignment. However, parallel beam paths are not essential, and instead, the beam paths can be non-parallel, which can reduce crosstalk.

In the illustrated example, the reflectometer system (10) comprises a position sensor (7). The position sensor (7) measures a position of the alignment beam (50) on the position sensor (7) after reflection from the sample (2), which can be used to determine whether the reflectometer system (10) is correctly aligned. Furthermore, the reflectometer system (10) comprises a sensor (33) for detecting the intensity of the split measurement beam (32). The reflectometer system (10) further comprises a reflection sensor (4) for detecting an intensity of the measurement beam (30) after reflection from the sample (2).

It is an advantage of the illustrated reflectometer system (10), in which the beams (30, 50) are incident at non-perpendicular incidence, that the number

39 BE2024/5189 beam splitters (62) can be limited. In the illustrated example, a single beam splitter (62) is used to divert the alignment beam (50) to the position sensor (7), and to divert a portion of the measurement beam (30) to a sensor (33) for detecting the intensity of the split measurement beam (32). Instead, in the other examples above, an additional beam splitter was used to divert the measurement beam (30) after reflection by the sample (2) from the path of the measurement beam (30) before said reflection, and to direct the alignment beam (50) towards the sample (2) at normal incidence. The fewer beam splitters (62) used, the higher the signal at the sensors (4, 7) can be, and the shorter the measurement times can be required to achieve accurate and reproducible measurements.

In the present embodiment, it may be preferable that the transmission level of the beam splitter (62) is greater than its reflection level.

It should be noted that while preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope of this invention. Steps may be added to or deleted from methods described within the scope of this invention.

Claims (19)

40 BE2024/5189 Conclusions 1.- Reflectometer system (1, 10) for analyzing a sample (2), comprising: a measuring radiation system comprising a measuring radiation source (3) for generating a measuring beam (30), a reflection sensor (4) for detecting the intensity of the measuring beam (30) after reflection by the sample (2), if present, an alignment radiation source (5) for generating an alignment beam (50), and a position sensor (7) for measuring a position of the alignment beam (50) on said position sensor (7) after reflection by the sample (2), and wherein the reflectometer system (1, 10) is adapted to determine an alignment of the system (1, 10) with respect to the sample (2) based on the measured position of the alignment beam (50), wherein a beam width of the alignment beam (50) is smaller than a beam width of the measuring beam (30), the reflectometer system (1, 10) being so is configured such that, when an alignment of the reflectometer system (1, 10) is optimized, at least a portion of a travel path of the measurement beam (30) is substantially parallel to at least a portion of a travel path of the alignment beam (50). A BE2024/5189 2. Reflectometer system (1, 10) according to claim 1, wherein, when the alignment is optimized, the path of travel of the alignment beam (50) substantially overlaps with the path of travel of the measurement beam (30) over at least part of their paths of travel to the sample (2). 3. Reflectometer system (1) according to claim 1 or 2, wherein, when the alignment is optimized, the measurement beam (30) and the alignment beam (50) are directed at an angle of maximum 16°, preferably maximum 12°, such as typically 8° with respect to the normal incidence on the sample (2), if present. 4. Reflectometer system (1, 10) according to any of the preceding claims, wherein the alignment radiation source (5) is positioned adjacent to the reflection sensor (4) such that the alignment beam (50) is directed in a substantially opposite direction relative to a direction of the measurement beam (30) incident on the reflection sensor (4) after reflection by the sample (2), if present. 5. A reflectometer system (1, 10) according to any preceding claim, wherein the diameter of the alignment beam (50) on the sample (2), if present, is less than half the diameter of the measurement beam (30) on the sample (2). 42 BE2024/5189 6. Reflectometer system (1, 10) according to any of the preceding claims, wherein the position sensor (7) comprises a two-dimensional photodetector. 7. Reflectometer system (1, 10) according to any of the preceding claims, wherein a wavelength of the measuring beam (30) differs from a wavelength of the alignment beam (50). 38. Reflectometer system (1, 10) according to claim 7, wherein the reflectance sensor (4) is adapted to separate or filter out the wavelength of the alignment beam (50). 9. Reflectometer system (1, 10) according to any of the preceding claims, wherein pulses of the measuring beam (30) and/or pulses of the alignment beam (50) on the one hand and/or pulses on the reflection sensor (4) and/or pulses on the position sensor (7) on the other hand are alternated. Reflectometer system (1, 10) according to any of the preceding claims, wherein the alignment radiation source (5) is adapted to provide a pulsed alignment beam (50) and the reflectance sensor (4) is adapted to separate or filter out the alignment beam (50). 11. Reflectometer system (1, 10) according to any of the preceding claims, further comprising a beam splitter (62) for separating a portion 43 BE2024/5189 of the measurement beam (30) before reflection by the sample (2), if present, and a sensor (33) for detecting the intensity of the split measurement beam (32). 12. A reflectometer system (1, 10) according to any preceding claim, comprising a sample holder for holding the sample (2), the sample holder being adapted to move the sample (2) to optimize said alignment based on the measured position of the alignment beam (50). 13. A reflectometer system (1, 10) according to any preceding claim, further comprising optical elements (61, 62) configured for directing said measurement beam (30) and/or said alignment beam (5) towards the sample (2), if present, and/or for directing said alignment beam (5) towards the position sensor (7). 14. A reflectometer system (1, 10) as claimed in claim 13, further comprising control means for changing the configuration of the optical elements (61, 62) to improve alignment of the reflectometer system (1, 10) based on the measured position of the alignment beam (50) on the position sensor (7). 15. Reflectometer system (1, 10) according to claim 14, further comprising a control (8) for automated control of the steering means on 44 BE2024/5189 based on the measured position of the alignment beam (50) on the position sensor (7). 16. Reflectometer system (1, 10) according to any of the preceding claims, wherein at least three selected from the measuring radiation system (1, 10), the alignment radiation source (5), the reflection sensor (4) and the position sensor (7) are located in a same housing (9), the reflectometer system (1, 10) being configured to analyze the sample (2) located outside said housing (9). 17. Reflectometer system (1, 10) according to claim 16, wherein the reflectometer system (1, 10) is configured to analyze the sample (2) located at a distance from said housing (9) greater than the beam width of the measurement beam (30), e.g. at least 5 times the beam width, or even more than 10 times the beam width. An arrangement comprising the reflectometer system (1, 10) according to any of the preceding claims, and a vacuum system comprising a vacuum chamber comprising an optical window, the reflectometer system (1, 10) being located outside the vacuum chamber and being arranged such that the measurement beam and the alignment beam pass through said optical window for analyzing a sample (2), when present, in the vacuum chamber. 45 BE2024/5189 19. Method for determining the reflectance of a sample (2) in a reflectometer system (1, 10), comprising: directing an alignment beam (50) toward the sample (2) to reflect the alignment beam (50), measuring a position of the reflected alignment beam (50) on a position sensor (7), determining an alignment of the reflectometer system (1, 10) based on the measured position, directing a measurement beam (30), having a beam width larger than that of the alignment beam (50), toward the sample (2) to reflect the measurement beam (30), and measuring an intensity of the reflected measurement beam (30) to determine said reflectance, the reflectometer system (1, 10) being configured such that, when an alignment of the reflectometer system (1, 10) is optimized, at least a portion of a travel path of the measurement beam (30) is substantially parallel is on at least part of a progression route of the alignment bundle (50).