Method of lithography and associated apparatus

Abstract

Disclosed is a method for exposing structures onto a plurality of substrates using at least one lithographic apparatus. The method comprises: measuring each substrate of the plurality of substrate in accordance with a set of measurement timings for measurement actions comprised in measuring each substrate; and performing a plurality of exposures on each of said substrates; wherein variation 5 in said set of measurement timings is reduced and / or minimized per substrate of said plurality of substrates.

Description

METHOD OF LITHOGRAPHY AND ASSOCIATED APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23217936.6 which was filed on December 19, 2023 and which is incorporated herein in its entirety by reference.FIELD OF THE INVENTION

[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.BACKGROUND ART

[0003] A lithographic apparatus or exposure 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] The lithographic apparatus or exposure apparatus may be of a type wherein an optical projection system is used to project a pattern imparted on a radiation beam by a patterning device, for instance a mask, onto a target portion of a substrate, as will be explained later. An optical projection lithographic apparatus may furthermore 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 lithographicapparatus, 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. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0006] In device manufacturing methods using lithographic apparatus, an important factor is the yield, i.e. the percentage of correctly manufactured devices, is the accuracy within which layers are printed in relation to layers that have previously been formed. This is known as overlay and the overlay error budget will often be lOnm or less. To achieve such accuracy, the substrate must be aligned to the reticle pattern to be transferred with great accuracy.

[0007] It is desirable to improve lithography techniques so as to improve performance in terms of a parameter of interest such as overlay.SUMMARY OF THE INVENTION

[0008] The invention in a first aspect provides a method for exposing structures onto a plurality of substrates using at least one lithographic apparatus; the method comprising: measuring each substrate of the plurality of substrate in accordance with a set of measurement timings for measurement actions comprised in measuring each substrate; and performing a plurality of exposures on each of said substrates; wherein variation in said set of measurement timings is reduced and / or minimized per substrate of said plurality of substrates.

[0009] The above and other aspects of the invention will be understood from a consideration of the examples described below.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0011] Figure 1 depicts a lithographic apparatus; and

[0012] Figure 2 illustrates schematically measurement and exposure processes in the apparatus of Figure 1.DETAILED DESCRIPTION OF EMBODIMENTS

[0013] 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.

[0014] Figure 1 schematically depicts an exposure apparatus or 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 certainparameters; 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.

[0015] 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.

[0016] 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 exposure 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.

[0017] 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.

[0018] 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.

[0019] 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”.

[0020] The exposure 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 exposure 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.

[0021] In operation, the illuminator IL receives a radiation beam from a radiation source SO. The source and the exposure 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 exposure 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 exposure 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.

[0022] The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as G-outcr and o-inner, respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross section.

[0023] 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.

[0024] The exposure apparatus may comprise an aberration sensor for the verification of an aberration fingerprint of the projection system PS. In an embodiment such an aberration fingerprint, i.e. aberrations per field point of the projection system PS, may be determined using a such wavefront aberration sensor. A wavefront aberration sensor of a known type, for instance such as described in US2002 / 0001088, incorporated herein by reference, may be used. Such a wavefront aberration sensor may be based on the principle of shearing interferometry and comprises a source module and a sensor module. The source module may comprise a patterned layer of chromium that is placed in the object plane (i.e. where during production the pattern of the patterning means is) of the projection system PS and has additional opticsprovided above the chromium layer. The combination provides a wavefront of radiation to the entire pupil of the projection system PS. The sensor module may comprise a patterned layer of chromium that is placed in the image plane of the projection system (i.e. where during production the substrate W is) and a camera that is placed some distance behind said layer of chromium. The patterned layer of chromium on the sensor module diffracts radiation into several diffraction orders that interfere with each other giving rise to an interferogram. The interferogram is measured by the camera. The aberrations in the projection lens can be determined by software based upon the measured interferogram. The wavefront aberration sensor may be configured to transfer information with respect to the aberration fingerprint towards the control unit.

[0025] Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, 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.

[0026] 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 exposure 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.

[0027] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0028] Exposure 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 thesubstrate 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 exposure 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.

[0029] The exposure apparatus control unit LACU which controls all the movements and measurements of various actuators and sensors to receive substrates W and reticles MA and to implement the patterning operations. Exposure apparatus control unit LACU also includes signal processing and data processing capacity to implement desired calculations relevant to the operation of the apparatus, e.g., based on inter alia level sensor LS data, alignment sensor AS data and feedback metrology data (e.g., one or more of inter alia overlay, focus, dose, critical dimension data). In practice, control unit LACU may be realized as a system of many sub-units, each handling the real-time data acquisition, processing and control of a subsystem or component within the apparatus.

[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 exposure 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 layersthat 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 Pl 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 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.

[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 exposure process. These alignment data, taken just before exposure, are used to generate an alignment model with parameters that fit the alignment 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 an exposure apparatus, it is known to control machine settings, e.g. lens settings and (reticle and substrate) stage settings, during exposures onto a substrate to optimize the projected image in terms of one or more placement parameters of interest, e.g., parameters related to placement of structures on the substrate. Such placement parameters of interest may comprise parameters on which device functionality and yield are dependent. In particular, the machine settings typically controlled include inter alia stage settings, lens control settings (e.g., to correct for lens aberration) and dose control settings. Stage settings may describe a planned or set stage movement such as described by various stage setpoints determined by the control hardware and software (e.g., as controlled by lithographic apparatus control unit or exposure apparatus control unit LACU). These may relate to stage positioning, speed, and / or acceleration in each of the substrate plane (e.g., for placement / overlay control) and perpendicular to the substrate plane (e.g., for focus control).

[0040] In this manner, parameters of interest such as one or more of overlay, focus, and imaging (e.g., critical dimension (CD)) may be optimized. Such an optimization may comprise some trade-off between some of these parameters, but which ensure that each of these parameters of interest remain within- specification (e.g., within a tolerance indicative of a yielding or functional device). As such, “optimize” in the context of this disclosure may simply mean improve and / or ensure that the parameter is within- specification. Alternatively, or in addition, “optimize” may mean optimizing towards a target projection (e.g., an optimized or ideal projection) onto the substrate (e.g., where there is a single ideal setting rather than a specification range. Similarly “minimize” in the context of this disclosure may simply mean reduce, e.g., minimizing variation may be understood to reduce variation and / or to reduce said variation as much as is practically possible.

[0041] Of particular relevance to this disclosure are placement parameters of interest relating to structure placement, e.g., overlay.

[0042] In addition to these optimizations, exposure sequencing is performed for control of hardware actions, e.g., where such hardware actions form part of an exposure sequence describing the sequencing and timing of exposures on one or more substrates. As such, exposure sequencing may describe the sequence and timing of each hardware action performed by the exposure apparatus during exposure ofa substrate or plurality of substrates (e.g., a lot and / or substrates performed on the same or similar exposure apparatus).

[0043] Exposure sequencing during actual exposure (e.g., on the exposure side or exposure station of a dual-stage lithographic apparatus such as illustrated in Figure 1) is typically determined and / or optimized to maximize the number of exposed dies over time, and therefore throughput, and then maintained stable from one lot to the next lot for subsequent projections on the same substrate, so as to optimize overlay. As such, the sequencing is typically determined for the substrate exposure to be as fast as possible. The respective times and / or durations of the hardware actions on the critical path determine the time that a sequence will take. It may be that an exposure sequencing aims to minimize or remove any delays and / or speed reductions (e.g., unless explicitly enforced such as to reduce defectivity on immersion systems).

[0044] However, it can be appreciated that measurement sequencing, i.e., timings on the measure side (measurement station) of a dual-stage lithographic apparatus are typically not maintained static, but rather are allowed to have some intrinsic variability. While typically every substrate is optimized for speed on each of the measure side and expose side, there is typically more variety on the measure side, resulting in more measure side time variations. For example, non-default measurements, such as process dependency gain / offset correction or global level contour measurements may be performed unpredictably or non-routinely. Individual measurements can fail unpredictably, resulting in a redo or fallback. Substrates may be delivered to the measure side with a delay which can result in a different start time of the measure sequence. Machine hardware and / or software can be upgraded or changed, resulting in a different measure sequence or faster stages with different timings. Another variable timing may be the clamping time of a substrate on a vacuum clamp. The build-up of the vacuum may have an associated variability.

[0045] The inventors have determined that this measurement timing variability has an associated quality impact (e.g., overlay impact) on the exposed patterns.

[0046] For example, variation in the time to clamp the substrate has an impact on temperature and / or stress in the substrate. In addition, exposures performed on lithographic apparatuses or scanners, such as illustrated in Figure 1, suffer from so-called measure-to-expose (M2E) crosstalk. Movements of the measure side (measurement station) chuck create pressure waves which influence hardware elements such as the lens at the exposure side (exposure station), which ultimately impacts overlay. This overlay impact manifests mainly in an increase in overlay variation and is particularly pronounced when the measure side chuck has to accelerate and decelerate frequently, for example during Fine Wafer Alignment (FIWA) measurements. Therefore, for example, the moment that an alignment sequence and / or leveling sequence begins has an impact on overlay.

[0047] Another issue which may be addressed by the methods disclosed herein is the crosstalk in the reverse direction, i.e., expose-to-measure (E2M) crosstalk, where exposure side actions and movement have a negative impact on the measurements on the measure side. This will also affect overlay.

[0048] Due to these measurement timing variations, not all substrates in the lot are subject to the same timing, resulting in different effects on different substrates. This leads to overlay penalties, e.g., due to different heating effects or different crosstalk effects per substrate. This results in a substrate-to- substrate error or overlay variation. Such substrate-to-substrate variation cannot be addressed by standard process control strategies (e.g., advanced process control APC) which can only correct average errors over a number of substrates (e.g., over one or more lots).

[0049] To address this issue, it is proposed to provide common measurement sequencing describing a set measurement timings (e.g., measure side timing) per substrate (e.g., for all substrates of one or more lots). As such, the proposed measurement sequencing may describe a common set of measurement timing setpoints and / or durations for all substrates of one or more lots. As such, it is proposed to reduce and / or minimize variation in set of measurement timings (describing the timing of a set of measurement actions performed on e.g., the measurement station) over all substrates of one or lots of substrates; thereby, minimizing any variation of measurement timing wafer-to-wafer.

[0050] Such an approach may further comprise providing common expsure sequencing describing a set exposure timings (e.g., exposure side timing) per substrate (e.g., for all substrates of one or more lots).

[0051] The proposed approach is particularly suitable for dual-stage lithographic apparatuses which are subject to M2E crosstalk and or E2M crosstalk. However, the methods disclosed herein are not limited to only dual-stage lithographic apparatuses, and may be implemented on single-stage apparatus for which at least the issue of variable clamping duration is still applicable. As such, on single stage machines, variable timings per wafer can also lead to overlay impact.

[0052] It is proposed that such a method may be implemented by using a synchronization bus or “sync” bus of the lithographic apparatus for every action. For example, a fixed timing (e.g., including fixed timing setpoints and / or durations) for each measurement action of the set of measurement actions can be set via the sync bus. It can be appreciated that presently, no timing maximum or set measurement action duration is ever set; instead the time for each measurement action is simply minimized on a per- substrate basis. For example, all entities attached to the sync bus upfront negotiate on the scan duration. Each entity provides its fastest possible time.

[0053] The sync bus is an electronic synchronization arrangement, comprising an electrical bus to which all synchronized entities are connected. It sends a pulse when the synchronized action should start, which can be picked up simultaneously by all entities. By upfront aligning on (initial setting of) the total duration of each synchronized action, it can be ensured that the action is synchronized to begin at the same time and have the same duration.

[0054] As such, the measurement timing per wafer may actually result in a timing delay to one or more measurement actions. In other words, one or more of said measurement actions may be performed with a deliberately longer duration than a nominal duration, e.g., where a nominal duration may be associated with and / or determined for optimized throughput for that wafer.

[0055] The method may comprise adapting at least one aspect of the measurement timing for a particular substrate during a measurement of the substrate so as to reduce and / or minimize variation in the measurement timing of this substrate with respect to the measurement timing of preceding and / or subsequent substrates of a lot, and / or substrates performed on the same or similar exposure apparatus.

[0056] For most of the substrates only a small deviation of timing is anticipated. Therefore the proposed method will have very little impact on throughput. To minimize throughput further, the proposed methods may be applied to only certain layers, e.g., the most critical layers having an associated small process window.

[0057] It is known that a sequence delay (e.g., a “hiccup”) during exposures may result in a horizontal distortion (e.g., this is particularly the case for immersion systems). Having such a delay in only a single layer and not in the next layer results in an overlay impact. To prevent track / reticle hiccups it is also proposed, in certain embodiments, to only begin measuring each substrate when it is known (e.g., to the lithographic apparatus or other controller) that the immediately subsequent substrate can be properly loaded in time (in accordance with the measurement timings). In other words, measurement of substrate n will not begin until it is known that substrate n+1 can be loaded on schedule (where n is an indexing integer for substrates of a lot). Because the wafer handler loads a few substrates at a time before loading each substrate onto the measure stage, the lithographic apparatus can be made aware of whether the next substrate can be loaded in time.

[0058] In this way, the measurement sequence (measurement timing) and exposure sequence (exposure timing) can be synchronized such that variation in this synchronicity is minimized over the substrate exposure. As such, while each exposure action of an exposure sequence is being performed for each wafer, the same measurement action will be being performed on the next wafer on the measurement stage. This means that the combined exposure sequencing and measurement sequencing is maintained constant for all substrates. In this way, the crosstalk effect (M2E or E2M crosstalk) should be the same for all substrates.

[0059] To ensure that all substrates are run in synchronization and according to the set measure side and exposure side timings, dummy substrates may be loaded onto at least one of the chucks if and when it is deemed necessary. For example, if substrate x+1 is held up too long in the substrate handler, and substrate x is already on the measure side (to make sure substrate x-1 was exposed in time), then measure side actions may be performed on a dummy substrate. In this case, substrate x will be subject to the same crosstalk effects as all other substrates.

[0060] Occasionally, a substrate may be exposed with an unplanned or undesired variation in the measurement and / or exposure sequencing (e.g., at least one different timing to the set timings and therefore to the other substrates). Unpredicted variations may occur due to, for example, the use of fall back scenarios. For example, stage alignment normally has a fixed timing. However, if a measurement fails, for example due to a water droplet on the measurement plate, the measurement may be retried at a different position. Such variations cannot be predicted in advance. As such, it is proposed that affectedsubstrates may be marked and / or tracked when such a timing variation is identified or encountered. In this way, such substrates may be distinguished going forward, and appropriate action taken (e.g., metrology performed to assess the effect and if necessary rework).

[0061] Such a method enables wafer specific (wafer-to-wafer) errors (e.g., those which result from measure side timing variation) to be reduced. Presently, process control may utilize process control loops (e.g., advanced process control APC loop) to correct for this type of issue. However APC can only remove the average error, and not wafer specific errors. A theoretical alternative approach may comprise attempting to measure all overlay related effects in the lithographic apparatus and trying to compensate for them by via feedforward correction on the measured time difference or a layer specific calibration. Such an approach would be very complex and difficult with a very high metrology overhead.

[0062] The proposed methods may, for example, be performed using the exposure apparatus control unit LACU shown and described in relation to Figure 1.

[0063] Also disclosed is an integrated circuit manufactured by an exposure apparatus as disclosed herein and / or according to an exposure process as disclosed herein.

[0064] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] The breadth and scope of the present invention should not be limited by any of the abovedescribed exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A method for exposing structures onto a plurality of substrates using at least one lithographic apparatus; the method comprising: measuring each substrate of the plurality of substrate in accordance with a set of measurement timings for measurement actions comprised in measuring each substrate; and performing a plurality of exposures on each of said substrates; wherein variation in said set of measurement timings is reduced and / or minimized per substrate of said plurality of substrates.

2. A method as claimed in claim 1, wherein said set of measurement timings is maintained substantially constant for each of said substrates.

3. A method as claimed in claim 1 or 2, comprising adapting at least one aspect of the set of measurement timings for a substrate being measured during said measurement step so as to reduce and / or minimize variation in the set of measurement timings of the substrate being measured with respect to a respective set of measurement timings of a preceding substrate and / or subsequent substrate of said plurality of substrates.

4. A method as claimed in any preceding claim, wherein said at least one lithographic apparatus comprises a single lithographic apparatus or a plurality of similar lithographic apparatuses.

5. A method as claimed in any preceding claim, wherein said plurality of substrates are all comprised in at least one lot.

6. A method as claimed in any preceding claim, wherein said exposure step comprises performing a plurality of exposures on each of said substrates in accordance with a set of exposure timings for exposure actions comprised in exposing each substrate; and wherein said set of exposure timings is maintained constant for exposing each of said substrates.

7. A method as claimed in claim 6, wherein said set of measurement timings and set of exposure timings is synchronized such that per-substrate variation in this synchronicity is minimized during exposure of each substrate.

8. A method as claimed in claim 7, comprising only beginning said measurement step for each substrate when it is known that the next substrate to be measured can be properly loaded in time to maintain said synchronicity and / or minimize per-substrate variation in said synchronicity.

9. A method as claimed in claim 7 or 8, comprising occasionally loading a dummy substrate onto a substrate support of the at least one lithographic apparatus to maintain said synchronicity and / or minimize per-substrate variation in said synchronicity.

10. A method as claimed in any preceding claim, wherein said at least one lithographic apparatus comprises at least one dual-stage lithographic apparatus, such that said measuring step is performed on a measurement station of said dual-stage lithographic apparatus and said exposure step is performed on an exposure station of said dual-stage lithographic apparatus.

11. A method as claimed in any preceding claim, wherein said a fixed timing for each measurement timing of the set of measurement timings is imposed via a synchronization bus of the at least one lithographic apparatus.

12. A method as claimed in any preceding claim, comprising marking and / or tracking any substrate subject to a variation in said set of measurement timings.

13. A method as claimed in any preceding claim, wherein said set of measurement timings relate at least to at least one time and / or duration relating to movement of a stage of the lithographic apparatus and / or to at least one time and / or duration relating to a clamping of each said substrate to a substrate support of the lithographic apparatus.

14. A computer program comprising program instructions operable to perform the method of any of claims 1 to 13, when run on a suitable apparatus.

15. A non- transient computer program carrier comprising the computer program of clause 14.

16. A processing arrangement comprising: a non-transient computer program carrier comprising a computer program as claimed in claim 14; and a processor operable to run the computer program comprised on said non-transient computer program carrier.

17. An exposure apparatus comprising the processing arrangement of claim 16.

18. An exposure apparatus being operable to perform the method of any of claims 1 to 13.

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