WO2021144066A1

WO2021144066A1 - Substrate, patterning device and lithographic apparatuses

Info

Publication number: WO2021144066A1
Application number: PCT/EP2020/084787
Authority: WO
Inventors: Franciscus Godefridus Casper Bijnen; Tran Thanh Thuy VU; Krishanu SHOME
Original assignee: ASML Holding NV; ASML Netherlands BV
Current assignee: ASML Holding NV; ASML Netherlands BV
Priority date: 2020-01-16
Filing date: 2020-12-07
Publication date: 2021-07-22
Anticipated expiration: 2022-07-16
Also published as: TW202132899A

Abstract

A substrate, associated patterning device and a method. The substrate includes at least one periodic alignment mark and at least one neighboring structure. The alignment mark comprises at least a first part having a direction of periodicity in a first direction and at least a second part having a direction of periodicity in a second direction. Over the extent of a scan length of a scanning of a measurement spot over the alignment mark, the alignment mark is such that when a first region of the measurement spot comprises neighboring structure, a second region of the measurement spot comprises no parallel structure to the neighboring structure within the first region. The first region and second region comprise regions which correspond when one is rotated 180 degrees relative to the other within the measurement spot.

Description

SUBSTRATE. PATTERNING DEVICE AND LITHOGRAPHIC APPARATUSES

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of U.S. provisional patent application 62/961,950, which was filed on January 16, 2020 which is incorporated herein in its entirety by reference.

FIELD

[0002] The present description relates to methods and apparatus usable, for example, in the manufacture of devices by lithographic techniques, and to methods of manufacturing devices using lithographic techniques. The description relates to metrology devices, and more specifically metrology devices used for measuring position such as an alignment sensor and a lithography apparatus having such an alignment sensor.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of a die, one die, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. These target portions are commonly referred to as “fields”.

[0004] In the manufacture of complex devices, typically many lithographic patterning steps are performed, thereby forming functional features in successive layers on the substrate. A significant aspect of performance of the lithographic apparatus is therefore the ability to place the applied pattern correctly and accurately in relation to features laid down (by the same apparatus or a different lithographic apparatus) in previous layers. For this purpose, the substrate is provided with one or more sets of alignment marks. Each mark is a structure whose position can be measured at a later time using a position sensor or alignment sensor (both terms are used synonymously), typically an optical position sensor.

[0005] The lithographic apparatus includes one or more alignment sensors by which positions of marks on a substrate can be measured accurately. Different types of marks and different types of alignment sensors are known from different manufacturers and different products of the same manufacturer. A type of sensor used in a lithographic apparatus is based on a self-referencing interferometer as described in U.S. patent no. 6,961,116, which is incorporated herein its entirety by reference. Various enhancements and modifications of the position sensor have been developed, for example as disclosed in U.S. patent application publication no. 2015-261097, which is incorporated herein its entirety by reference.

SUMMARY

[0006] It would be desirable to improve on the alignment marks measured by an alignment sensor, in particular to improve one or more selected from: the size of the mark (e.g., decrease its size), the scan length over the mark, the speed of measurement of the mark, signal processing of the signal from the mark and/or the performance of the mark expressed in reproducibility and or accuracy.

[0007] In an aspect, there is provided a substrate comprising at least one periodic alignment mark and at least one neighboring structure, wherein the alignment mark comprises at least a first part having a direction of periodicity in a first direction and at least a second part having a direction of periodicity in a second direction, wherein, over the extent of a scan length of a scanning of a measurement spot over the alignment mark, the alignment mark is such that when a first region of the measurement spot comprises neighboring structure, a second region of the measurement spot comprises no parallel structure to the neighboring structure within the first region, and wherein the first region and second region comprise regions which correspond when one is rotated 180 degrees relative to the other within the measurement spot.

[0008] In an aspect, there is provided a substrate comprising at least one periodic alignment mark and at least one neighboring structure, wherein the alignment mark comprises at least a first region having a direction of periodicity in a first direction and at least a second region having a direction of periodicity in a second direction, and wherein the direction of periodicity of the alignment mark at each of its boundaries is non-parallel with a direction of at least one edge defined by the at least one neighboring structure adjacent the respective boundary.

[0009] In an aspect, there is provided a method for designing a periodic alignment mark with at least a first part having a direction of periodicity in a first direction and at least a second part having a direction of periodicity in a second direction, the method comprising: determining a layout of at least one neighboring structure and a reserved area therein for an alignment mark; and based on the orientation of the at least one neighboring structure adjacent the alignment mark, a scan length of a scan of a measurement spot of an alignment sensor to be used to measure the alignment mark and the size of the measurement spot, determining an alignment mark design which ensures that a direction of periodicity of the adjacent neighboring structure captured within a first region of the measurement spot is always non-parallel with the direction of periodicity of the alignment mark within a second region within the measurement spot, the first region and second region comprising regions which correspond when one is rotated 180 degrees relative to the other within the measurement spot.

[0010] Also disclosed is a lithographic apparatus being operable to perform a method as described herein. [0011] The above and other aspects of the invention will be understood from a consideration of the examples described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0013] Figure 1 depicts a lithographic apparatus;

[0014] Figure 2 illustrates schematically measurement and exposure processes in the apparatus of Figure 1;

[0015] Figure 3 is a schematic illustration of an alignment sensor adaptable according to an embodiment;

[0016] Figure 4 depicts a number of alignment marks according to embodiments of the invention; and

[0017] Figure 5 depicts a further example of an alignment mark according to an embodiment of the invention.

DETAIFED DESCRIPTION OF EMBODIMENTS

[0018] Before describing embodiments of the invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0019] Figure 1 schematically depicts a lithographic apparatus FA. The apparatus includes an illumination system (illuminator) IF configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a patterning device support or support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; two substrate tables (e.g., a wafer table) WTa and WTb each constructed to hold a substrate (e.g., a resist coated wafer) W and each connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W. A reference frame RF connects the various components, and serves as a reference for setting and measuring positions of the patterning device and substrate and of features on them.

[0020] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0021] The patterning device support MT holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support MT may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0022] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0023] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive patterning device). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” The term “patterning device” can also be interpreted as referring to a device storing in digital form pattern information for use in controlling such a programmable patterning device.

[0024] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0025] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0026] In operation, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and/or a beam expander. In other cases, the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0027] The illuminator IL may for example include an adjuster AD for adjusting the angular intensity distribution of the radiation beam, an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross section.

[0028] The radiation beam B is incident on the patterning device MA, which is held on the patterning device support MT, and is patterned by the patterning device. Having traversed the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WTa or WTb can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan.

[0029] Patterning device (e.g., mask) MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g., mask) MA, the patterning device alignment marks may be located between the dies. Small alignment marks may also be included within dies, in amongst the device features, in which case it is desirable that the markers be as small as possible and not require any different imaging or process conditions than adjacent features. The alignment system, which detects the alignment markers is described further below.

[0030] The depicted apparatus could be used in a variety of modes. In a scan mode, the patterning device support (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. Other types of lithographic apparatus and modes of operation are possible, as is well-known in the art. For example, a step mode is known. In so-called “maskless” lithography, a programmable patterning device is held stationary but with a changing pattern, and the substrate table WT is moved or scanned. [0031] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0032] Lithographic apparatus LA is of a so-called dual stage type which has two substrate tables WTa, WTb and two stations - an exposure station EXP and a measurement station ME A - between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps carried out. This enables a substantial increase in the throughput of the apparatus. The preparatory steps may include mapping the surface height contours of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. If the position sensor IF is not capable of measuring the position of the substrate table while it is at the measurement station as well as at the exposure station, a second position sensor may be provided to enable the positions of the substrate table to be tracked at both stations, relative to reference frame RF. Other arrangements are known and usable instead of the dual-stage arrangement shown. For example, other lithographic apparatuses are known in which a substrate table and a measurement table are provided. These are docked together when performing preparatory measurements, and then undocked while the substrate table undergoes exposure.

[0033] Figure 2 illustrates the steps to expose target portions (e.g. dies) on a substrate W in the dual stage apparatus of Figure 1. On the left hand side within a dotted box are steps performed at a measurement station MEA, while the right hand side shows steps performed at the exposure station EXP. From time to time, one of the substrate tables WTa, WTb will be at the exposure station, while the other is at the measurement station, as described above. For the purposes of this description, it is assumed that a substrate W has already been loaded into the exposure station. At step 200, a new substrate W’ is loaded to the apparatus by a mechanism not shown. These two substrates are processed in parallel in order to increase the throughput of the lithographic apparatus.

[0034] Referring initially to the newly-loaded substrate W’ , this may be a previously unprocessed substrate, prepared with a new photo resist for first time exposure in the apparatus. In general, however, the lithography process described will be merely one step in a series of exposure and processing steps, so that substrate W’ has been through this apparatus and/or other lithography apparatuses, several times already, and may have subsequent processes to undergo as well.

Particularly for the problem of improving overlay performance, the task is to ensure that new patterns are applied in exactly the correct position on a substrate that has already been subjected to one or more cycles of patterning and processing. These processing steps progressively introduce distortions in the substrate that must be measured and corrected for, to achieve satisfactory overlay performance. [0035] The previous and/or subsequent patterning step may be performed in other lithography apparatuses, as just mentioned, and may even be performed in different types of lithography apparatus. For example, some layers in the device manufacturing process which are very demanding in parameters such as resolution and overlay may be performed in a more advanced lithography tool than other layers that are less demanding. Therefore, some layers may be exposed in an immersion type lithography tool, while others are exposed in a ‘dry’ tool. Some layers may be exposed in a tool working at DUV wavelengths, while others are exposed using EUV wavelength radiation.

[0036] At 202, alignment measurements using the substrate marks PI etc. and image sensors (not shown) are used to measure and record alignment of the substrate relative to substrate table WTa/WTb. In addition, several alignment marks across the substrate W’ will be measured using alignment sensor AS. These measurements are used in one embodiment to establish a “wafer grid”, which maps very accurately the distribution of marks across the substrate, including any distortion relative to a nominal rectangular grid.

[0037] At step 204, a map of substrate height (Z) against X-Y position is measured using the level sensor LS. Conventionally, the height map is used only to achieve accurate focusing of the exposed pattern. It may be used for other purposes in addition.

[0038] When substrate W’ was loaded, recipe data 206 were received, defining the exposures to be performed, and also one or more properties of the substrate and the patterns previously made and to be made upon it. To these recipe data are added the measurements of substrate position, wafer grid and height map that were made at 202, 204, so that a complete set of recipe and measurement data 208 can be passed to the exposure station EXP. The measurements of alignment data for example comprise X and Y positions of alignment targets formed in a fixed or nominally fixed relationship to the product patterns that are the product of the lithographic process. These alignment data, taken just before exposure, are used to generate an alignment model with parameters that fit the model to the data. These parameters and the alignment model will be used during the exposure operation to correct positions of patterns applied in the current lithographic step. The model in use interpolates positional deviations between the measured positions. A conventional alignment model might comprise four, five or six parameters, together defining translation, rotation and scaling of the ‘ideal’ grid, in different dimensions. Advanced models are known that use more parameters.

[0039] At 210, substrates W’ and W are swapped, so that the measured substrate W’ becomes the substrate W entering the exposure station EXP. In the example apparatus of Figure 1, this swapping is performed by exchanging the supports WTa and WTb within the apparatus, so that the substrates W, W’ remain accurately clamped and positioned on those supports, to preserve relative alignment between the substrate tables and substrates themselves. Accordingly, once the tables have been swapped, determining the relative position between projection system PS and substrate table WTb (formerly WTa) is all that is necessary to make use of the measurement information 202, 204 for the substrate W (formerly W’) in control of the exposure steps. At step 212, patterning device alignment is performed using the patterning device alignment marks Ml, M2. In steps 214, 216, 218, scanning motions and radiation pulses are applied at successive target locations across the substrate W, in order to complete the exposure of a number of patterns.

[0040] By using the alignment data and height map obtained at the measuring station in the performance of the exposure steps, these patterns are accurately aligned with respect to the desired locations, and, in particular, with respect to features previously laid down on the same substrate. The exposed substrate, now labeled W” is unloaded from the apparatus at step 220, to undergo etching or other processes, in accordance with the exposed pattern.

[0041] The skilled person will know that the above description is a simplified overview of a number of very detailed steps involved in one example of a real manufacturing situation. For example, rather than measuring alignment in a single pass, often there will be separate phases of coarse and fine measurement, using the same or different marks. The coarse and/or fine alignment measurement steps can be performed before or after the height measurement, or interleaved.

[0042] Measuring the position of the marks may also provide information on a deformation of the substrate on which the marks are provided, for example in the form of a wafer grid. Deformation of the substrate may occur by, for example, electrostatic clamping of the substrate to the substrate table and/or heating of the substrate when the substrate is exposed to radiation.

[0043] Figure 3 is a schematic block diagram of an embodiment of an alignment sensor AS.

Radiation source RSO provides a beam RB of radiation of one or more wavelengths, which is diverted by diverting optics onto a mark, such as mark AM located on substrate W, as an illumination spot SP. In this example the diverting optics comprises a spot mirror SM and an objective lens OL. The illumination spot SP, by which the mark AM is illuminated, may be slightly smaller in width (e.g., diameter) than the width of the mark itself.

[0044] Radiation diffracted by the mark AM is collimated (in this example via the objective lens OL) into an information-carrying beam IB. The term “diffracted” is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI, e.g. of the type disclosed in U.S. patent no. 6,961,116 mentioned above, interferes the beam IB with itself after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams in case more than one wavelength is created by the radiation source RSO. The photodetector may be a single element, or it may comprise a number of pixels, if desired. The photodetector may comprise a sensor array.

[0045] The diverting optics, which in this example comprises the spot mirror SM, may also serve to block zero order radiation reflected from the mark, so that the information-carrying beam IB comprises only higher order diffracted radiation from the mark AM (this is not essential to the measurement, but may improve a signal to noise ratio).

[0046] One or more intensity signals SI are supplied to a processing unit PU. By a combination of optical processing in the block SRI and computational processing in the unit PU, values for X- and/or Y-position on the substrate relative to a reference frame are output.

[0047] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques are used in conjunction with this to identify which period of, e.g., a sine wave is the one containing the marked position. The same process at coarser and or finer levels are repeated at different wavelengths for increased accuracy and/or for robust detection of the mark irrespective of the materials from which the mark is made, and materials on and or below which the mark is provided.

[0048] A mark, or alignment mark, may comprise a series of bars formed on or in a layer provided on the substrate or formed (directly) in the substrate. The bars may be regularly spaced and act as grating lines so that the mark can be regarded as a diffraction grating with a known spatial period (pitch). Depending on the orientation of these grating lines, a mark may be designed to allow measurement of a position along the X axis, or along the Y axis (which is oriented substantially perpendicular to the X axis). A mark comprising bars that are arranged at +45 degrees and/or -45 degrees with respect to both the X- and Y-axes allows for a combined X- and Y- measurement using techniques as described in United States patent application publication no. US 2009/195768, which is incorporated herein in its entirety by reference.

[0049] The alignment sensor scans each mark optically with a spot of radiation to obtain a periodically varying signal, such as a sine wave. The phase of this signal is analyzed to determine the position of the mark and, hence, of the substrate relative to the alignment sensor, which, in turn, is fixated relative to a reference frame of a lithographic apparatus. So-called coarse and fine marks may be provided, related to different (coarse and fine) mark dimensions, so that the alignment sensor can distinguish between different cycles of the periodic signal, as well as the exact position (phase) within a cycle. Marks of different pitches may also be used for this purpose.

[0050] In performing alignment by measuring the position of alignment marks on the substrate using an alignment sensor, it would be desirable to reduce the area (footprint) of the alignment marks, so that many of them could be accommodated all over the substrate; including in-die, between product structures, where substrate space is “expensive”. It is therefore desirable, in a scanning-type alignment sensor (e.g., which scans an underfilled spot over the mark to generate a signal for the SRI), to reduce the length of the required scan length over the mark to maintain sufficient accuracy and/or reproducibility (repro). In addition, it remains desirable to perform alignment detection on the same mark in X and Y directions (e.g., both directions parallel to the substrate plane) to further reduce the area used (and optionally to decrease alignment time and increase throughput when both directions are measured simultaneously).

[0051] To achieve this, it is proposed that the alignment mark is of a comparable size with respect to the spot size of the measurement spot of the alignment sensor. As such, it is proposed that the largest extent (e.g., the largest of length in the X and Y directions) of the mark be no more than 50%, no more than 40%, no more than 30%, no more than 20%, no more than 10%, or no more than 5% larger than the largest extent of the spot (e.g., width or diameter). The mark may be square (or more generally rectangle) with a side length being only fractionally greater or even the same size as the largest extent of the spot. It may be that the mark is smaller than the spot size. In each case, the result is that when scanned, the spot is overfilled on the mark; i.e., a neighboring structure to the alignment mark is also included within the spot during at least part of the measurement scan. [0052] A problem with an overfilled measurement is that the neighboring structure impacts on the measurement accuracy. When mark exclusion zones are just larger than the size of the spot (in both directions), scanning over one or more neighboring structures cannot be avoided which affects the measured position of the mark. This is not desirable and should be reduced or minimized. One approach is to reduce the scan length. However short scan length of a few detection cycles results in bad repro. Typically, the repro is improved when more cycles are scanned, but then the impact from one or more neighboring structures on the aligned position increases.

[0053] It is therefore proposed to provide a two dimensional alignment mark wherein, for the full extent of an alignment scan length (e.g., a proposed or predetermined scan length used for an alignment measurement) of a measurement spot over the alignment mark, a direction of periodicity of any neighboring structure (e.g., product structure which may comprise a periodic pattern) captured within the measurement spot is always non-parallel with the alignment mark at a corresponding 180 degree rotated interfering region within the measurement spot. In this way, for all interfering pairs of regions within the measurement spot for which one region of a pair comprises at least one neighboring structure, the other region of the interfering pair will comprise no parallel structure. [0054] The concept is based on the fundamental operation of the self-referencing interferometer SRI which forms the heart of many alignment sensors. When partially scanning over a mark with a neighboring structure present, the operation of the sensor may be envisaged as follows. An image is made and interfered with a 180 degrees rotated copy of that image. When these two images are overlapped and have different (non-parallel) orientation lines (local gratings) they give no or very weak signal. However, when the two images have parallel lines (at same pitch) they give a strong signal. In all the examples described below, it can be observed that, for short scan lengths, the at least one neighboring structure is always non-parallel with the part of the mark with which it is interfering with after rotation of the image at 180 degrees; i.e., the corresponding 180 degree rotated position within the measurement spot.

[0055] In an embodiment, this may be achieved by providing two dimensional alignment marks wherein the direction of periodicity at each edge is non-parallel with the direction of the at least one neighboring structure adjacent that edge. The proposed alignment mark may therefore be optimized with respect to the orientation of at least one neighboring structure (e.g., a product structure which may comprise a periodic pattern).

[0056] Figure 4 illustrates a number of proposals for a small alignment mark with X and Y detectability designed to minimize crosstalk from one or more neighboring structures. In each case, a direction of periodicity of a neighboring structure is non-parallel-with the 180 degrees in-spot-rotated mark structure. For example, each edge may be non-parallel with the neighboring structure adjacent that edge.

[0057] Figure 4(a) shows a target arrangement suitable for embedding within at least one neighboring structure comprising Y-oriented neighboring structure NSy (shown in Figure 4(a) at left and right sides) and X-oriented neighboring structure NSx (shown in Figure 4(a) at top and bottom sides). The target comprises a square area having been divided into four triangular regions defined by the two diagonal axes. Also shown is the measuring spot MS at the extreme left of its scan length. The alignment mark is designed such that any neighboring structure within the measurement spot (here neighboring structure NSy) within a first region IR of a pair of corresponding interfering regions of the measurement spot MS, is always non-parallel with the alignment mark orientation within the second region IR’ of the pair of corresponding interfering regions. The corresponding pair of regions IR, IR’ are corresponding in the sense that they will overlap when one is 180 degrees in- spot-rotated relative to the other.

[0058] In an embodiment, including many of the embodiments disclosed herein, this may be achieved by having the direction of periodicity in the regions perpendicular to that of the neighboring structure of the corresponding peripheral edge of the region (i.e., the edge shared by both region and full target region and therefore adjacent neighboring structure). However, the skilled person will recognize that other arrangements are possible to achieve the same effect. Figure 4(b) shows a complementary design with all the orientations reversed (X for Y and Y for X).

[0059] Figures 4(c) and 4(d) show complementary designs for embedding in at least one neighboring structure all oriented in a single direction (X-direction NSx in Figure 4(c) and Y-direction NSy in Figure 4(d)). In each case, the mark comprises a square-in-square arrangement with the inner square region having the same orientation as the neighboring structure, and the outer region being perpendicular to the inner region and neighboring structure. Note that the inner region need not necessarily be square, only be a region oriented with the neighboring structure and surrounded by a (respectively) perpendicularly oriented region.

[0060] Figures 4(e) and 4(f) show complementary arrangements which are alternative to the arrangements depicted in Figures 4(a) and 4(b) respectively. In each case they comprise a combination of the targets depicted in Figure 4(a) and 4(b) arranged square-in-square, with Figure 4(e) having the target of Figure 4(a) in its central region and the target of Figure 4(b) in its outer region. The target of Figure 4(f) is the reverse of this.

[0061] Figure 4(g) shows an arrangement where two adjacent sides of the target are surrounded by X-direction neighboring structure NSx and the other two adjacent sides of the target are surrounded by Y-direction neighboring structure NSy. Here the target comprises an outer obliquely oriented region (e.g., oriented at 45 degrees) and an inner obliquely oriented region, oriented perpendicular to the outer region (e.g., oriented at 135 degrees). Here the inner region is rhomboid in shape, although this is exemplary. The measurement spot MS is shown on this example, to illustrate an example relative size with respect to the target.

[0062] When scanning close to the center of the mark for any of the marks illustrated in Figure 4, the impact of the surrounding structure on measured position APD is minimized. This allows sufficient scan length (e.g., with some scanning over the neighboring structure) for acceptable repro. Note that the embodiments of Figure 4 are purely exemplary and there are many other arrangements which may be envisaged within the scope of the disclosure and dependent on the arrangement of neighboring structures.

[0063] Figure 5 illustrates another embodiment which may be used to help ensure that a neighboring structure is non-parallel-with 180 degrees in-spot-rotated alignment mark structure. This embodiment may be an alternative to, or used in combination with, other embodiments described herein. In this embodiment, a smaller alignment mark with respect to the exclusion zone (region reserved for alignment without product structure) is proposed. Figure 5 shows a smaller mark within the exclusion zone (x direction dimension EZc labelled), leaving a significant gap between the mark and neighboring structure NSx. Also shown is the measurement spot MS comprising a pair of corresponding interfering regions (e.g., corresponding when one is 180 degrees in-spot-rotated relative to the other). It can be seen that when the first region IR comprises neighboring structure NSx the corresponding region IR’ comprises no structure and therefore there will be no interference signal. It should be noted however, the benefits of a small mark will come at a cost of signal strength and repro.

[0064] An effect of one or more of the proposals described herein is up to an order of magnitude suppression of the impact from a surrounding structure on the measured alignment position when using a self-referencing interferometer (SRI) based tool. The self-referencing interferometer of an alignment sensor creates overlap of rotationally opposite areas of the mark around the scan position. By having the mark periodicity non-parallel with 180 degrees in-spot-rotated adjacent neighboring structure, the at least one neighboring structure does not interfere and therefore does not contribute to the alignment signal (or at least the contribution is much lower). This effect can be used to suppress the impact of surrounding structure in the same layer, from a lower layer or from a higher layer. [0065] It is proposed that the methods may be used in combination with one or more of an improved fit method and a smaller spot.

[0066] A method for designing an alignment mark is also proposed, the method comprising: determining a layout of at least one neighboring structure (e.g., product structure) and a reserved area therein for an alignment mark; and based on the orientation of neighboring structure adjacent the alignment mark, a scan length of a measurement spot of an alignment sensor used to measure the alignment mark and the size of the measurement spot, determining an alignment mark design which ensures that a direction of periodicity of adjacent neighboring structure captured within the measurement spot is always non-parallel with a direction of periodicity of alignment mark within a corresponding 180 degree rotated interfering position within the measurement spot. This can be achieved by noting the orientation of the adjacent neighboring structure which will be captured over a proposed scan and choosing the alignment mark orientation appropriately for the alignment mark within the corresponding interfering region of the spot.

[0067] It is proposed that the concepts described may enable X and/or Y detection using underfilled marks from a single scan on a small (e.g., 50x50 mhi or smaller or 2500 mhi² or smaller) mark. Such a method may require only a 4 to 10 mhi scan length for sufficient reproducibility.

[0068] The present invention can also be characterized by the following clauses:

1. A substrate comprising at least one periodic alignment mark and at least one neighboring structure, wherein the alignment mark comprises at least a first part having a direction of periodicity in a first direction and a second part having a direction of periodicity in a second direction, wherein, over the extent of a scan length of a scanning of a measurement spot over the alignment mark, the alignment mark is such that when a first region of the measurement spot comprises neighboring structure, a second region of the measurement spot comprises no parallel structure to the neighboring structure within the first region, and wherein the first region and second region comprise regions which correspond when one is rotated 180 degrees relative to the other within the measurement spot.

2. The substrate as stated in clause 1, wherein the second region comprises a portion of the alignment mark, the portion of the alignment mark comprising a periodicity which is non-parallel to the neighboring structure within the first region.

3. The substrate as stated in clause 2, wherein the neighboring structure has a periodic character, and the portion of the alignment mark comprises a periodicity which is non-parallel to the periodicity of the neighboring structure within the first region.

4. The substrate as stated in any of clauses 1-3, wherein the direction of periodicity of the alignment mark at each of its one or more boundaries is non-parallel with a direction of at least one edge defined by the neighboring structure adjacent the respective boundary.

5. The substrate as stated in any of clauses 1-4, wherein the first direction is perpendicular to the second direction.

6. The substrate as stated in clause 5, wherein: adjacent to each of the boundaries of the alignment mark, the direction of periodicity of the neighboring structure is substantially in the first direction; the first part comprises an inner region having a direction of periodicity in the first direction; and the second part surrounds the first part and has a direction of periodicity in the second direction.

7. The substrate as stated in clause 5, wherein: adjacent to each of a first opposing pair of boundaries of the alignment mark, the direction of periodicity of the neighboring structure is in the first direction; adjacent to each of a second opposing pair of boundaries of the alignment mark, the direction of periodicity of the neighboring structure is in the second direction; and the alignment mark comprises: a first pair of portions, each portion adjacent a respective one of the boundaries of the second opposing pair of boundaries and having a direction of periodicity in the first direction, and a second pair of portions, each portion adjacent a respective one of the boundaries of first opposing pair of boundaries and having a direction of periodicity in the second direction.

8. The substrate as stated in clause 7, wherein the alignment mark is rectangular and each of the portions comprises a triangle defined by one of the boundaries and diagonals of the rectangle.

9. The substrate as stated in clause 8, wherein each of the portions is further divided into sub portions, such that: each of the first pair of portions has a direction of periodicity in the first direction for an outer sub-portion and a direction of periodicity in the second direction for an inner sub-portion, and each of the second pair of portions has a direction of periodicity in the second direction for an outer sub-portion and a direction of periodicity in the first direction for an inner sub-portion.

10. The substrate as stated in clause 4, wherein: the neighboring structure has a periodic character; adjacent to each of a first adjacent pair of boundaries of the alignment mark, a direction of periodicity of the neighboring structure is in a third direction; adjacent to each of a second adjacent pair of the boundaries of the alignment mark, a direction of periodicity of the neighboring structure is in a fourth direction, perpendicular to the third direction; the first part comprises an inner region having a direction of periodicity in the first direction; and the second part surrounds the first region and has a direction of periodicity in the second direction.

11. The substrate as stated in clause 10, wherein the third direction and fourth direction are respectively parallel with a first coordinate axis and a second coordinate axis of a coordinate system of the substrate.

12. The substrate as stated in any of clauses 1-11, wherein the first direction and second direction are respectively parallel with a first coordinate axis and a second coordinate axis of a coordinate system of the substrate.

13. The substrate as stated in any of clauses 1-12, wherein the size of the alignment mark is 50 pm x 50 pm or smaller or 2500 pm² or smaller.

14. The substrate as stated in any of clauses 1-13, wherein the alignment mark is relatively small with respect to an area reserved for it, the area comprising no neighboring structure, such that there is a gap between the alignment mark and neighboring structure; and when the first region of the measurement spot comprises the neighboring structure, the second region of the measurement spot comprises the gap.

15. The substrate as stated in clause 14, wherein a dimension of the alignment mark is less than 70% of the equivalent dimension of the area reserved for it.

16. A substrate comprising at least one periodic alignment mark and at least one neighboring structure, wherein the alignment mark comprises at least a first part having a direction of periodicity in a first direction and at least a second part having a direction of periodicity in a second direction, and wherein the direction of periodicity of the alignment mark at each of its boundaries is non parallel with a direction of at least one edge defined by the neighboring structure adjacent the respective boundary.

17. The substrate as stated in clause 16, wherein the first direction is perpendicular to the second direction.

18. The substrate as stated in clause 16 or clause 17, wherein the neighboring structure has a periodic character, and the direction of periodicity of the alignment mark at each of the boundaries is perpendicular to the direction of periodicity of the neighboring structure adjacent that boundary.

19. The substrate as stated in any of clauses 16-18, wherein: adjacent to each of the boundaries of the alignment mark, a direction of periodicity of the neighboring structure is substantially in the first direction; the first part comprises an inner region having a direction of periodicity in the first direction; and the second part surrounds the first part and has a direction of periodicity in the second direction.

20. The substrate as stated in any of clauses 16-18, wherein adjacent to each of a first opposing pair of the boundaries of the alignment mark, the direction of periodicity of the neighboring structure is in the first direction; adjacent to each of a second opposing pair of the boundaries of the alignment mark, the direction of periodicity of the neighboring structure is in the second direction; and the alignment mark comprises: a first pair of portions, each portion adjacent a respective one of the boundaries of the second opposing pair of boundaries and having a direction of periodicity in the first direction, and a second pair of portions, each portion adjacent a respective one of the boundaries of first opposing pair of boundaries and having a direction of periodicity in the second direction.

21. The substrate as stated in clause 20, wherein the alignment mark is rectangular and each of the portions comprises a triangle defined by one of the boundaries and diagonals of the rectangle.

22. The substrate as stated in clause 21, wherein each of the portions is further divided into sub portions, such that: each of the first pair of portions has a direction of periodicity in the first direction for an outer sub-portion and a direction of periodicity in the second direction for an inner sub-portion, and each of the second pair of portions has a direction of periodicity in the second direction for an outer sub-portion and a direction of periodicity in the first direction for an inner sub-portion.

23. The substrate as stated in clause 16 or clause 17, wherein: the neighboring structure has a periodic character; adjacent to each of a first adjacent pair of boundaries of the alignment mark, a direction of periodicity of the neighboring structure is in a third direction; adjacent to each of a second adjacent pair of the boundaries of the alignment mark, a direction of periodicity of the neighboring structure is in a fourth direction, perpendicular to the third direction; the first part comprises an inner region having a direction of periodicity in the first direction; and the second part surrounds the first region and has a direction of periodicity in the second direction.

24. The substrate as stated in clause 23, wherein the third direction and fourth direction are respectively parallel with a first coordinate axis and a second coordinate axis of a coordinate system of the substrate.

25. The substrate as stated in any of clauses 16-24, wherein the first direction and second direction are respectively parallel with a first coordinate axis and a second coordinate axis of a coordinate system of the substrate.

26. The substrate as stated in any of clauses 16-25, wherein the alignment mark is relatively small with respect to an area reserved for it, the area comprising no neighboring structure, such that there is a gap between the alignment mark and neighboring structure.

27. The substrate as stated in clause 26, wherein a dimension of the alignment mark is less than 70% of the equivalent dimension of the area reserved for it.

28. The substrate as stated in any of clauses 16-27, wherein the size of the alignment mark is 50 pm x 50 pm or smaller or 2500 pm² or smaller.

29. A patterning device comprising a plurality of patterns for forming alignment marks on a substrate, the patterning device being configured to pattern a substrate to obtain the substrate as stated in any of clauses 1-28.

30. A method for designing a periodic alignment mark with at least a first part having a direction of periodicity in a first direction and a second part having a direction of periodicity in a second direction, the method comprising: determining a layout of at least one neighboring structure and a reserved area therein for an alignment mark; and based on the orientation of the at least one neighboring structure adjacent the alignment mark, a scan length of a scan of a measurement spot of an alignment sensor to be used to measure the alignment mark and the size of the measurement spot, determining an alignment mark design which ensures that a direction of periodicity of the adjacent neighboring structure captured within a first region of the measurement spot is always non-parallel with a direction of periodicity of the alignment mark within a second region within the measurement spot, the first region and second region comprising regions which correspond when one is rotated 180 degrees relative to the other within the measurement spot.

31. The method as stated in clause 30, comprising noting the orientation of the adjacent neighboring structure which will be captured during the scan and choosing the alignment mark orientation to be non-parallel to the adjacent neighboring structure for the alignment mark within the corresponding 180 degree rotated region.

32. The method as stated in clause 30 or clause 31, further comprising exposing the periodic alignment mark on a substrate, the substrate also comprising the neighboring structure at least when all exposure steps are complete.

33. The method as stated in clause 32, wherein the alignment mark comprises any of the alignment marks comprised on the substrate of any of clauses 1-28.

34. A lithographic apparatus operable to perform the method as stated in clause 32 or clause 33.

35. A lithographic apparatus as stated in clause 34, comprising: a patterning device support for supporting a patterning device as stated in clause 29; and a substrate support for supporting a substrate.

[0069] Further, as will be appreciated, embodiments of the mark described herein are formed on a substrate. Accordingly, in an embodiment, a patterning device will have one or more patterns to form an embodiment of the mark. For example, a mask can have one or more patterns formed thereon that when projected onto a resist-coated substrate form pattern on the substrate for formation of the mark. [0070] While specific embodiments have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0071] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that embodiments of the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0072] The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 1-100 nm), as well as particle beams, such as ion beams or electron beams.

[0073] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. Reflective components are likely to be used in an apparatus operating in the UV and/or EUV ranges.

[0074] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, an embodiment of the invention may take the form of a computer program containing one or more sequences of machine -readable instructions to implement all or part of a method as described herein, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program therein.

[0075] The breadth and scope of the present invention should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

2. The substrate as claimed in claim 1, wherein the second region comprises a portion of the alignment mark, the portion of the alignment mark comprising a periodicity which is non-parallel to the neighboring structure within the first region.

3. The substrate as claimed in claim 2, wherein the neighboring structure has a periodic character, and the portion of the alignment mark comprises a periodicity which is non-parallel to the periodicity of the neighboring structure within the first region.

4. The substrate as claimed in any of claims 1-3, wherein the direction of periodicity of the alignment mark at each of its one or more boundaries is non-parallel with a direction of at least one edge defined by the neighboring structure adjacent the respective boundary.

5. The substrate as claimed in any of claims 1-4, wherein the first direction is perpendicular to the second direction.

6. The substrate as claimed in claim 5, wherein: adjacent to each of the boundaries of the alignment mark, the direction of periodicity of the neighboring structure is substantially in the first direction; the first part comprises an inner region having a direction of periodicity in the first direction; and the second part surrounds the first part and has a direction of periodicity in the second direction.

7. The substrate as claimed in claim 5, wherein: adjacent to each of a first opposing pair of boundaries of the alignment mark, the direction of periodicity of the neighboring structure is in the first direction; adjacent to each of a second opposing pair of boundaries of the alignment mark, the direction of periodicity of the neighboring structure is in the second direction; and the alignment mark comprises: a first pair of portions, each portion adjacent a respective one of the boundaries of the second opposing pair of boundaries and having a direction of periodicity in the first direction, and a second pair of portions, each portion adjacent a respective one of the boundaries of first opposing pair of boundaries and having a direction of periodicity in the second direction.

8. A substrate comprising at least one periodic alignment mark and at least one neighboring structure, wherein the alignment mark comprises at least a first part having a direction of periodicity in a first direction and at least a second part having a direction of periodicity in a second direction, and wherein the direction of periodicity of the alignment mark at each of its boundaries is non-parallel with a direction of at least one edge defined by the neighboring structure adjacent the respective boundary.

9. The substrate as claimed in claim 8, wherein the first direction is perpendicular to the second direction.

10. The substrate as claimed in claim 8 or claim 9, wherein the neighboring structure has a periodic character, and the direction of periodicity of the alignment mark at each of the boundaries is perpendicular to the direction of periodicity of the neighboring structure adjacent that boundary.

11. The substrate as claimed in any of claims 8-10, wherein: adjacent to each of the boundaries of the alignment mark, a direction of periodicity of the neighboring structure is substantially in the first direction; the first part comprises an inner region having a direction of periodicity in the first direction; and the second part surrounds the first part and has a direction of periodicity in the second direction.

12. The substrate as claimed in any of claims 8-10, wherein adjacent to each of a first opposing pair of the boundaries of the alignment mark, the direction of periodicity of the neighboring structure is in the first direction; adjacent to each of a second opposing pair of the boundaries of the alignment mark, the direction of periodicity of the neighboring structure is in the second direction; and the alignment mark comprises: a first pair of portions, each portion adjacent a respective one of the boundaries of the second opposing pair of boundaries and having a direction of periodicity in the first direction, and a second pair of portions, each portion adjacent a respective one of the boundaries of first opposing pair of boundaries and having a direction of periodicity in the second direction.

13. The substrate as claimed in claim 8 or claim 9, wherein: the neighboring structure has a periodic character; adjacent to each of a first adjacent pair of boundaries of the alignment mark, a direction of periodicity of the neighboring structure is in a third direction; adjacent to each of a second adjacent pair of the boundaries of the alignment mark, a direction of periodicity of the neighboring structure is in a fourth direction, perpendicular to the third direction; the first part comprises an inner region having a direction of periodicity in the first direction; and the second part surrounds the first region and has a direction of periodicity in the second direction.

14. A patterning device comprising a plurality of patterns for forming alignment marks on a substrate, the patterning device being configured to pattern a substrate to obtain the substrate as claimed in any of claims 1-13.

15. A method for designing a periodic alignment mark with at least a first part having a direction of periodicity in a first direction and a second part having a direction of periodicity in a second direction, the method comprising: determining a layout of at least one neighboring structure and a reserved area therein for an alignment mark; and based on the orientation of the at least one neighboring structure adjacent the alignment mark, a scan length of a scan of a measurement spot of an alignment sensor to be used to measure the alignment mark and the size of the measurement spot, determining an alignment mark design which ensures that a direction of periodicity of the adjacent neighboring structure captured within a first region of the measurement spot is always non-parallel with a direction of periodicity of the alignment mark within a second region within the measurement spot, the first region and second region comprising regions which correspond when one is rotated 180 degrees relative to the other within the measurement spot.

16. The method as claimed in claim 15, further comprising exposing the periodic alignment mark on a substrate, the substrate also comprising the neighboring structure at least when all exposure steps are complete.

17. The method as claimed in claim 16, wherein the alignment mark comprises any of the alignment marks comprised on the substrate of any of claims 1-13.