WO2021151565A1

WO2021151565A1 - Metrology method and associated metrology and lithographic apparatuses

Info

Publication number: WO2021151565A1
Application number: PCT/EP2020/084943
Authority: WO
Inventors: Tijmen Pieter COLLIGNON; Georgios TSIROGIANNIS
Original assignee: Asml Netherlands B.V.
Priority date: 2020-01-28
Filing date: 2020-12-07
Publication date: 2021-08-05
Also published as: TW202132906A

Abstract

Disclosed is a method of determining a freeform shape impact parameter which describes the impact of a freeform shape of a substrate on a clamped substrate coordinate grid corresponding to said substrate subsequent to clamping onto a substrate support. The method comprises obtaining first measurement data relating to the substrate when in a first clamped state on said substrate support and second measurement data relating to the substrate when in a second clamped state on said substrate support. The freeform shape impact parameter is determined from the first measurement data and the second measurement data.

Description

METROLOGY METHOD AND ASSOCIATED METROLOGY AND LITHOGRAPHIC

APPARATUSES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 20154167.9 which was filed on January 28, 2020 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention 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 invention relates more particularly to metrology sensors and lithography apparatuses having such a metrology 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 critical 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, typically an optical position sensor. 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.

[0005] In other applications, metrology sensors are used for measuring exposed structures on a substrate (either in resist and/or after etch). A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications WO 2009/078708 and WO 2009/106279 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and

WO2013178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.

[0006] In the known lithographic apparatus, each substrate or wafer to be exposed is loaded on a substrate support (or wafer table) on which the substrate is supported during the exposure of a patterned beam of radiation. To clamp the substrate on the substrate support a clamping device is provided, which is arranged to provide a clamping force. In a known lithographic apparatus a vacuum clamping device is used as a clamping device. Such a vacuum clamping device provides a vacuum force with which the substrate is clamped on the supporting surface of the substrate support. In the case that a substrate is straight, the substrate will be clamped on the support surface without any substantial internal stresses in the substrate. Other clamping devices (e.g., suitable for use inside vacuum environments such as is required for EUV scanners), an electrostatic clamping force is applied to clamp the wafer to the support. [0007] However, substrates may not be flat in their freeform (i.e. unclamped) state, but for instance may be warped in a number of shapes, such as a corrugated shape, a cylindrical shape, a dome shape, a saddle form or another shape. This may be caused by the production method used to make the substrate, or by pre- or post-exposure processes to which the substrates are subjected during the manufacture. Examples of these contributors are: etching, thin film deposition, chemical-mechanical planarization and thermal anneal. These processes can introduce stress or stress changes in the thin films on top of the silicon wafers, resulting in significant wafer grid distortions.

[0008] When a warped substrate, for instance a dome-shaped substrate, is clamped on a substrate support for instance by means of a vacuum or electrostatic clamp, the substrate may first contact with the substrate support at the outer circumference of the substrate and thereafter over the rest of the surface of the substrate. Due to the clamping force the substrate is forced into a substantially straight form, while the actual clamping starts at the outer circumference of the substrate. As a result stresses may be induced in the substrate when it is clamped on the supporting surface. This results in in-plane distortion (IPD), which if not adequately corrected for in an alignment procedure may have a negative influence on the quality of the integrated circuits through degradation of parameters such as critical dimension and overlay. [0009] Known methods for correcting for IPD require knowledge of the freeform, unclamped wafer shape. At present this is typically achieved by performing an off-line measurement, whereby the substrate is completely removed from the clamp of the lithographic apparatus and transported to a separate wafer measurement device which directly measures the freeform shape of the wafer. Once the freeform wafer shape has been determined directly in this off-line manner, a physical or computational model (e.g. finite element model) which simulates the material properties of the wafer is then used to predict the IPD for the given wafer shape will which arise when the wafer is clamped back in the lithographic apparatus for the next part of the fabrication process. Once the IPD has been determined the lithographic apparatus can then be configured to apply compensation, thereby preventing significant degradation in the quality of the printed circuit.

[0010] Such known methods are however disadvantageous since they require the wafer to be removed (unclamped) from the substrate holder of the lithographic apparatus and transported to a different measurement apparatus each time the freeform wafer shape and hence IPD is to be determined/updated. Each time the wafer is unclamped from the substrate holder there is a risk of damaging the wafer. Further, this increases the production time since the wafer must be fully re- aligned each time it is brought back to the substrate holder of the lithographic apparatus from the off-line measuring apparatus to determine the wafer freeform shape.

SUMMARY

[0011] The invention in a first aspect provides a method for determining a freeform shape impact parameter which describes the impact of a freeform shape of a substrate on a clamped substrate coordinate grid corresponding to said substrate subsequent to clamping onto a substrate support; the method comprising: obtaining first measurement data relating to the substrate when in a first clamped state on said substrate support; obtaining second measurement data relating to the substrate when in a second clamped state on said substrate support; and determining the freeform shape impact parameter from said first measurement data and said second measurement data.

[0012] Also disclosed is a computer program, metrology sensor, alignment sensor and a lithographic apparatus being operable to perform the method of the first aspect.

[0013] The above and other aspects of the invention will be understood from a consideration of the examples described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 depicts a lithographic apparatus;

Figure 2 illustrates schematically measurement and exposure processes in the apparatus of

Figure 1; Figure 3 is a schematic illustration of an alignment sensor adaptable according to an embodiment of the invention; and

Figure 4 is a flow diagram showing a method for determining a freeform shape impact parameter in accordance with embodiments described herein.

DETAILED DESCRIPTION

[0015] 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.

[0016] Figure 1 schematically depicts a lithographic apparatus LA. The apparatus includes an illumination system (illuminator) IL 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.

[0017] 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.

[0018] 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.

[0019] 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. [0020] 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. [0021] 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”.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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 IP (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.

[0026] Patterning device (e.g., mask) MA and substrate W may be aligned using mask 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 mask 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.

[0027] 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.

[0028] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0029] 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 MEA - 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] At step 204, a map of wafer height (Z) against X-Y position is measured also using the level sensor FS. Conventionally, the height map is used only to achieve accurate focusing of the exposed pattern. It may be used for other purposes in addition.

[0035] When substrate W’ was loaded, recipe data 206 were received, defining the exposures to be performed, and also properties of the wafer and the patterns previously made and to be made upon it. To these recipe data are added the measurements of wafer 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.

[0036] At 210, wafers 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, reticle alignment is performed using the mask 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.

[0037] 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.

[0038] 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.

[0039] In the manufacture of complex devices, typically many lithographic patterning steps are performed, thereby forming functional features in successive layers on the substrate. A critical 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 in previous layers (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks. Each mark is a structure whose position can be measured at a later time using a position sensor, typically an optical position sensor. The position sensor may be referred to as “alignment sensor” and marks may be referred to as “alignment marks”.

[0040] A lithographic apparatus may include one or more (e.g. a plurality of) alignment sensors by which positions of alignment marks provided on a substrate can be measured accurately. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of the position sensor have been developed, for example as disclosed in US2015261097A1. The contents of all of these publications are incorporated herein by reference.

[0041] 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 well-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 US2009/195768A, which is incorporated by reference.

[0042] 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.

[0043] 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 or vacuum clamping of the substrate to the substrate table and/or heating of the substrate when the substrate is exposed to radiation.

[0044] Figure 3 is a schematic block diagram of an embodiment of a known 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 diameter than the width of the mark itself. [0045] 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 US6961116 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.

[0046] 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 improves signal to noise ratios).

[0047] 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 Y-position on the substrate relative to a reference frame are output.

[0048] 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 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.

[0049] It is known that the substrates or wafers on which structures are exposed are not in themselves completely flat. The freeform wafer shape defined by this unflatness has an impact on alignment and therefore printing placement (and as such, overlay and edge placement error). To mitigate for this it is desirable to measure the freeform wafer shape and determine its impact on the clamping grid. More specifically, variations in high order free-form wafer shape cause high order variations in the clamping grid. To capture this high order wafer-to-wafer variation, many alignment marks would have to be measured during wafer align, which has a negative impact on speed and throughput. Other solutions include measuring the freeform wafer shape directly using a stand-alone shape measuring tool. However, this is slow, cannot provide inline measurements and does not provide any data on the impact of the shape on the clamping grid, only the shape itself.

[0050] It is therefore proposed to use information from loading and re-loading the same wafer on a substrate support (wafer table) using different clamping strategies (e.g., a fast clamping strategy where the vacuum flow or electrostatic force is applied immediately (e.g., normal clamping without a relaxation period) and a slow clamping strategy where the wafer is first allowed to mechanically relax on the wafer table for a period without any clamping force applied) to estimate free-form wafer shape variation impact on clamping grid (clamped wafer grid). In particular, it is proposed that the different loading strategies comprise a first clamped state or fast clamped state (e.g., with vacuum flow or electrostatic force depending on the clamp type applied immediately on loading) and a second clamped state or slow clamped state, where the wafer is state where the wafer is loaded on the table with no clamping force applied (e.g., no vacuum flow or electrostatic force such that it is free supported and unclamped), and is allowed to mechanically relax onto the wafer table for a period of time before being clamped for measurement.

[0051] In an embodiment, the estimate of the freeform wafer shape variation impact on the clamping grid is determined from a difference of the grid measured for the first clamping strategy and the grid loaded for the second clamping strategy (for the same wafer and wafer table), this difference being referred to herein as the freeform shape impact parameter. A similar measurement strategy has been used for wafer table qualification (with a low order model), e.g., to determine whether a wafer table is of sufficient quality and/or assess the impact of the wafer table on the clamped wafer shape.

[0052] In an embodiment, the freeform shape impact parameter may be measured using an alignment sensor on alignment marks; i.e., to measure the wafer respectively at the first loaded state and second loaded state, prior to calculation of the difference. As such, it may be measured from alignment marks on the substrate (although alignment sensors can measure other structures which is within the scope of this disclosure). As such, the proposed method may be used as part of, or to enhance or complement, alignment of a wafer prior to an exposure within a scanner. Where this is the case, the alignment sensor may be part of the scanner (e.g,, alignment sensor AS in Figure 1), whether part of a specialist measure stage (e.g., of a two stage system as illustrated) or a single stage on which alignment and exposure is performed. Alternatively (or in addition), alignment may be performed on a stand-alone alignment station, outside of the scanner. Alternatively or in addition, the freeform shape impact parameter may be measured using another metrology tool such as a scatterometer based metrology tool more commonly used for measurement of overlay and or focus in process control subsequent to exposure. Where this is the case, the measurements may be made on any suitable metrology target or on product structures. [0053] Figure 4 is a flow diagram describing a proposed method of measuring the freeform shape impact parameter. At step 400, a wafer W having a degree of unflatness (exaggerated here) is taken by e-pins E, which receive and lower the wafer onto the wafer table WT. At step 405, the wafer W is clamped normally (e.g., with a high vacuum flow or high electrostatic force F). At step 410, first measurements are performed of the clamped wafer W (e.g., using an alignment sensor AS). The wafer W is unloaded and reloaded onto e-pins E at step 415 and lowered onto the wafer table WT without clamping force applied (step 420); e.g., no vacuum flow or no electrostatic force and allowed to mechanically relax onto wafer table WT for a period of time. At step 425 the wafer is measured again (e.g., after the relaxing period). Finally, at step 430, the freeform shape impact parameter is determined as the difference between the measurements of steps 410 and 425. [0054] The freeform shape impact parameter provides valuable information on the impact of freeform wafer shape variation on the wafer’s clamped state. This can improve monitoring of wafer-to wafer variable content of the clamped wafer grid for a particular wafer table. .

[0055] In an embodiment, the methods described herein may be used to optimize wafer alignment strategies; e.g., on a per-lot basis. Such a method may comprise initially grouping wafers of a lot according to the likelihood that they will have a similar freeform shape. In practical term, this largely comprises grouping the wafers according to the ingot from which it was cut, and preferably its position within the ingot. Other relevant factors may comprise the wafers processing context (processing history) and therefore, for example, the specific deposition chamber or other tool it has been subjected to. [0056] In an embodiment, a method may comprise using the ingot information provided by the wafer scribe ID which allows for optimized wafer alignment strategies. Such an approach may comprise the following:

• Organize each lot to comprise only wafers from the same ingot and from similar position (e.g., information provided by the wafer scribe ID) and optionally the same processing context, to ensure a high likelihood of a similar freeform wafer shape for all wafers within each respective lot.

• For the first wafer in each lot, determine the freeform shape impact parameter (e.g., by performing the method described by Figure 4) using a dense wafer alignment sampling scheme for each of the measurement steps.

• Identify areas of the wafer where large wafer load grid issues may be expected and adapt or optimize alignment strategy accordingly for the remaining wafers in the lot.

[0057] The last step may comprise identifying areas or regions where the impact of the freeform wafer shape is greatest and scheduling increased alignment metrology effort for such regions for the remaining wafers in the lot, so as to better characterize the resultant grid distortions. As such, an alignment sampling scheme may be optimized, e.g., per lot (or per wafer) based on the freeform shape impact parameter. Identification of regions with high wafer shape impact can be achieved by any suitable statistical method, for example; specific examples may include a component analysis such as principal component analysis. For example, freeform shape impact parameter (which may comprise a vector field over the wafer) might be decomposed and the gradient found.

[0058] The number of wafers measured in this manner may differ from one per lot. Optionally, for example, only one wafer per ingot, or half an ingot is measured densely to obtain the freeform shape impact parameter over the whole wafer. In such an embodiment, other wafers (e.g., one per lot) within the ingot or half ingot wafer group may be measured less densely; e.g., with targeted measurements in areas with the expected greatest degree of unflatness (e.g., as determined from the densely measured wafer) to obtain a less dense or targeted freeform shape impact parameter. The alignment strategy can then be adapted per lot as before. Other embodiments may have a freeform shape impact parameter measured for more than one wafer per lot; e.g., dense for the first wafer and less dense/targeted for one or more (or even all) wafers in a lot.

[0059] In addition to optimizing alignment strategy, the methods described herein and therefore the freeform shape impact parameter may be used in determining feedforward corrections to the exposure process e.g., to correct for in plane distortion IPD of the wafer or otherwise. Alternatively or in addition, the freeform shape impact parameter may be used in optimizing post-exposure metrology effort (e.g., overlay metrology), for example, to concentrate measurements in regions identified as having higher freeform wafer shape impact on the clamped grid and therefore having an increased risk of overlay error. Additionally whole lots or wafers may be scheduled for increased or decreased metrology depending on the value of the freeform shape impact parameter determined for that lot/wafer.

[0060] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0061] 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 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] Other aspects of the invention are set out in the following numbered clauses.

1. A method of determining a freeform shape impact parameter which describes the impact of a freeform shape of a substrate on a clamped substrate coordinate grid corresponding to said substrate subsequent to clamping onto a substrate support; the method comprising: obtaining first measurement data relating to the substrate when in a first clamped state on said substrate support; obtaining second measurement data relating to the substrate when in a second clamped state on said substrate support; and determining the freeform shape impact parameter from said first measurement data and said second measurement data.

2. A method as claimed in clause 1 , wherein said determining the freeform shape impact parameter comprises determining a difference of said first measurement data and said second measurement data.

3. A method as claimed in clause 1 or 2, wherein said first clamped state comprises a clamped state for which a clamping force is applied normally to said substrate to clamp it to the substrate support.

4. A method as claimed in clause 3, wherein the clamping force comprises a vacuum force imposed by a vacuum generating flow.

5. A method as claimed in clause 3, wherein the clamping force comprises an electrostatic force.

6. A method as claimed in any preceding clause, wherein said second clamped state comprises an initial free supported state for which no clamping force is actively applied to said substrate for a relaxation period of time between loading and clamping for measurement, said substrate being freely supported by the substrate support during the relaxation period.

7. A method as claimed in any preceding clause, wherein the first measurement data and second measurement data each comprise alignment data having been measured by an alignment sensor.

8. A method as claimed in any of clauses 1 to 6, wherein the first measurement data and second measurement data each comprise post-exposure metrology data having been measured by an post exposure metrology tool.

9. A method as claimed in any preceding clause, further comprising optimizing an alignment strategy based on said freeform shape impact parameter.

10. A method as claimed in clause 9, wherein said optimizing an alignment strategy comprises: determining one or more higher impact regions where the impact of the freeform shape of on the clamped substrate coordinate grid is relatively greater; and scheduling more and/or a greater density of measurements for said one or more higher impact regions

11. A method as claimed in any preceding clause, wherein a plurality of substrates have been arranged into groups for which the freeform shape of each substrate is expected to be similar; and determining said freeform shape impact parameter for only a subset of each group.

12. A method as claimed in clause 11, comprising assuming the same freeform shape impact parameter for other substrates in each respective group

13. A method as claimed in clause 11 or 12, wherein said subset comprises only one substrate per group.

14. A method as claimed in any of clauses 11 to 13, wherein each group corresponds to an exposure lot.

15. A method as claimed in any of clauses 11 to 14, wherein each group comprises substrates having been cut from the same ingot. 16. A method as claimed in clause 15, wherein each group comprises substrates having been cut from the same portion of said ingot.

17. A method as claimed in clause 15 or 16, wherein each group comprises substrates having been subject to the same processing history. 18. A method as claimed in any preceding clause, comprising determining a correction for a subsequent exposure process based on said freeform shape impact parameter.

19. A method as claimed in any preceding clause, further comprising optimizing a post-exposure metrology strategy based on said freeform shape impact parameter.

20. A method as claimed in any preceding clause, further comprising performing said measurements to obtain said first measurement data and said second measurement data.

21. A computer program comprising program instructions operable to perform the method of any of clauses 1 to 19, when run on a suitable apparatus.

22. A non-transient computer program carrier comprising the computer program of clause 21.

23. A processing system comprising a processor and a storage device comprising the computer program of clause 21.

24. A metrology tool operable to perform the method of any of clauses 1 to 20.

25. A metrology tool as claimed in clause 24, comprising an alignment sensor.

26. A lithographic apparatus comprising: a patterning device support for supporting a patterning device; a substrate support for supporting a substrate; and the alignment sensor of clause 25.

Claims

2. A method as claimed in claim 1, wherein said determining the freeform shape impact parameter comprises determining a difference of said first measurement data and said second measurement data.

3. A method as claimed in claim 1, wherein said first clamped state comprises a clamped state for which a clamping force is applied normally to said substrate to clamp it to the substrate support.

4. A method as claimed in claim 3, wherein the clamping force comprises a vacuum force imposed by a vacuum generating flow.

5. A method as claimed in any preceding claim, wherein said second clamped state comprises an initial free supported state for which no clamping force is actively applied to said substrate for a relaxation period of time between loading and clamping for measurement, said substrate being freely supported by the substrate support during the relaxation period.

6. A method as claimed in any preceding claim, wherein the first measurement data and second measurement data each comprise alignment data having been measured by an alignment sensor.

7. A method as claimed in any of claims 1 to 6, wherein the first measurement data and second measurement data each comprise post-exposure metrology data having been measured by an post exposure metrology tool.

8. A method as claimed in any preceding claim, further comprising optimizing an alignment strategy based on said freeform shape impact parameter.

9. A method as claimed in claim 9, wherein said optimizing an alignment strategy comprises: determining one or more higher impact regions where the impact of the freeform shape of on the clamped substrate coordinate grid is relatively greater; and scheduling more and/or a greater density of measurements for said one or more higher impact regions.

10. A method as claimed in any preceding claim, wherein a plurality of substrates have been arranged into groups for which the freeform shape of each substrate is expected to be similar; and determining said freeform shape impact parameter for only a subset of each group.

11. A method as claimed in any preceding claim, further comprising optimizing a post-exposure metrology strategy based on said freeform shape impact parameter.

12. A computer program comprising program instructions operable to perform the method of any of claims 1 to 11, when run on a suitable apparatus.

13. A non- transient computer program carrier comprising the computer program of claim 12.

14. A processing system comprising a processor and a storage device comprising the computer program of claim 12.

15. A metrology tool operable to perform the method of any of claims 1 to 11.

16. A metrology tool as claimed in claim 15, comprising an alignment sensor.

17. A lithographic apparatus comprising: a patterning device support for supporting a patterning device; a substrate support for supporting a substrate; and the alignment sensor of claim 16.