Method of decreasing wafer-in-process (WIP) errors in a lithographic process

Abstract

Disclosed is a method for determining a correction for an exposure on a substrate using an exposure apparatus, the method comprising: determining a correction for performing one or more first exposures on a substrate, the correction being at least partially for correcting an error introduced by at least one component of the exposure apparatus; dividing the correction into a first correction component and a second correction component; applying the first correction component and not applying the second correction component when exposing one or more first layers on the substrate prior to a maintenance action.

Description

METHOD OF DECREASING WAFER-IN-PROCESS (WIP) ERRORS IN A LITHOGRAPHICPROCESSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24217145.2 which was filed on 03 December 2024, 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

[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 andUS2010201963A1. 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, US20110043791 A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A andWO2013178422A1. 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] It is necessary to routinely perform maintenance actions on a lithographic apparatus so as to replace deteriorated components. It may be necessary to ramp-down (temporarily cease production) of one or more layers in the run-up to a maintenance action, as the fingerprints or impact of the replacement component compared to replaced component can cause large overlay errors when different layers of the same substrate have been exposed before and after the maintenance action.

[0007] It is desirable to mitigate the effect of these maintenance actions on productivity.SUMMARY OF THE INVENTION

[0008] The invention in accordance with a first aspect provides a method for determining a correction for an exposure on a substrate using an exposure apparatus, the method comprising: determining a correction for performing one or more first exposures on a substrate, the correction being at least partially for correcting an error introduced by at least one component of the exposure apparatus; dividing the correction into a first correction component and a second correction component; applying the first correction component and not applying the second correction component when exposing one or more first layers on the substrate prior to a maintenance action.

[0009] The invention in accordance with a second aspect provides a computer program comprising program instructions operable to perform the method of the first aspect or any optional example therein, when run on a suitable apparatus.

[0010] The invention in accordance with a third aspect provides a non-transient computer-readable storage medium carrying the computer program of the second aspect.

[0011] The invention in accordance with a fourth aspect provides an exposure apparatus configured to carry out the method of the first aspect or any optional example therein.

[0012] The invention in accordance with a fifth aspect provides a semiconductor product manufactured by the exposure apparatus according to the fourth aspect or any optional example therein.

[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:

[0015] Figure 1 depicts a lithographic apparatus;

[0016] Figure 2 illustrates schematically measurement and exposure processes in the apparatus of Figure 1;

[0017] Figure 3(a) conceptually illustrates the cancelling effect in overlay of wafer table fingerprints in each layer in the absence of a wafer table swap, and Figure 3(b) conceptually illustrates the performance impact in overlay of such a wafer table swap;

[0018] Figure 4 is a plot of cumulative lots of wafers produced against time, illustrating the concept of C-time; and

[0019] Figure 5 is a flowchart describing a method to reduce an overlay jump between consecutive exposed layers on a substrate in a scanner, in accordance with an embodiment of the present disclosure.DETAILED DESCRIPTION OF EMBODIMENTS

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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 thepatterning 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.

[0024] 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.

[0025] 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.

[0026] 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”.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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 desiredlocations, 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.

[0043] 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.

[0044] A lithographic apparatus or scanner requires regular maintenance actions, for example to replace hardware components which are subject to degradation over time. By way of a specific example, the wafer table of a scanner degrades and requires periodic replacement. Such hardware swaps and / or maintenance actions, can result in a performance impact.

[0045] Such a performance impact is conceptually illustrated in Figure 3. Overlay is an important parameter which describes the proper placement of a layer with respect to a previously exposed (lower) layer. Each wafer table imposes a wafer clamping impact or clamping fingerprint on a substrate clamped thereto, which should be corrected for. However, as a wafer table wears over time, the associated wafer clamping impact is affected (typically becoming greater) which will affect the positioning of exposed structures on the substrate. As such, the degradation of the wafer table (and other components) will lead to positional errors; these errors can be divided into lower frequency errors or correctable errors (CEs), which may be measured (using a suitable metrology tool) and corrected for within the scanner via a process correction loop (e.g., as known as the advanced process corrections or APC loop), and higher frequency errors or non-correctable errors (NCEs) which cannot be corrected via APC either because the effect of the errors cannot be measured using sufficiently fast metrology, captured by the models used to represent the metrology data and / or because the required corrections cannot be actuated within the scanner.

[0046] Without a wafer table swap, the wafer clamping impact changes sufficiently slowly such that there is essentially no significant change between exposures of different layers on a single wafer. Because overlay is a relative measure between two layers, NCEs resulting from this impact in each layer largely cancel themselves out. Referring to Figure 3(a), the lines for the first layer LI and second layer L2 represent a high frequency component of wafer grid impact of a degraded wafer table. Although this degradation results in a relatively large magnitude disturbance of the local placement of features in each exposure layer, these disturbances are typically sufficiently similar in each layer and cancel themselves out in overlay (i.e., the positional errors are the same in each layer meaning that misalignment between layers due to this effect is relatively small). Therefore, overlay NCEs due to this wafer table fingerprint in each layer will be small. By contrast, Figure 3(b) conceptually illustrates the situation should there be a wafer table swap between exposure of layer LI and L2. The new table results in an overall smaller wafer grid impact due to wafer table imperfection; however the impact of the oldtable is present in the layer LI exposure. Therefore, there is no longer a cancelling out of this impact, resulting in a significantly larger NCE overlay penalty. This may cause an APC non-correctable jump due to the difference in these fingerprints, which may be sufficiently large to affect yield (the fact that the errors are non-correctable means that these wafers cannot be recovered via rework).

[0047] Wafer table maintenance is only one example of a maintenance action (e.g., component replacement) which results in this mismatched fingerprint issue. Other maintenance actions which can result in mismatched fingerprints before and after the maintenance action include inter alia lens swaps of any of the lenses of the projection optics.

[0048] This overlay penalty (i.e. overlay jump) due to hardware maintenance during wafer processing (i.e., between exposing different layers on a wafer) is often referred to as a wafer-in-process (WIP) impact, as it affects wafers-in-progress (those with only some of the required layers exposed thereon) at the time of the maintenance action. One strategy to mitigate this WIP impact, is to “ramp-down” the exposure of a number of layers in the run-up to such a maintenance action, so as to reduce the number of wafers in progress at the time of the action.

[0049] Figure 4 illustrates the effect of such a ramp-down. Such a ramp-down typically comprises the ceasing of exposure of one or more layers in the weeks leading up to the maintenance action, e.g., ceasing exposure of each layer in turn from the bottom layer, at intervals of a few days to a couple or few weeks between the ramping down of each successive layer. It may be that not all layers are ramped down, and optimizing the number of layers which are to be ramped down is an aim of at least some of the methods disclosed herein. This ramping down of layers represents a loss of productivity with respect to continuing wafer production at the rate prior to beginning ramp-down.

[0050] In addition to a ramp-down impact, there will be an accompanying ramp-up impact (i.e., in comparison to full production rate) when production restarts following the maintenance action. For example, there is a need to restart production for each layer ramped down layer before production of layers higher in the stack can be started. In addition, the correction loop or APC loop needs to be restarted as there is no (or insufficient) metrology data for the post-maintenance system. Because of this, many of the first wafers in progress that are exposed post-maintenance action will be exposed out- of-spec and therefore will require reworking (stripped of the poorly exposed resist, re-covered and reexposed).

[0051] The combination of this ramp-down and ramp-up time leads to what is often referred to as C- time: the time impact of these ramp-down and ramp-up periods with respect to no ramp-up or rampdown. C-time is additional to A-time (the nominal downtime for the actual maintenance action) and B- time (a margin applied to the A-time).

[0052] Figure 4 is a plot of the cumulative number of lots #lots against time showing production rate for a production period (solid line) which includes a ramp-down period, maintenance action and ramp- up period. The A+B time is the period of the actual maintenance action including margin, during which productivity is nil (machine down-time). The dotted line represents a nominal production rate had therebeen no ramp-down or ramp-up time, and production continued at a constant rate till the maintenance action and again immediately on recommencing production after the action. C-time is the time difference between the two plots after completion of ramp-up and production has reached an approximately steady rate. It may be appreciated that C-time is typically much greater than A+B-time, more so than illustrated in this plot.

[0053] A proposed strategy for resolving this WIP impact issue is to use a combination of an “early swap” and “WIP-less recovery” so as to reduce ramp-down and therefore C-time (ideally to zero). An early swap strategy comprises monitoring and predicting the buildup of potential WIP impact (i.e. it is desirable to perform this swap “just-in-time”, e.g., such that the maintenance action is performed at or near the latest time for which the predicted non-correctable portion of the WIP impact is minimal (e.g., that any degradation impact that contributes to WIP impact can be corrected via machine or scanner actuation).

[0054] WIP-less recovery comprises using adapted scanner calibrations in order to recover back to the fingerprint before the swap instead of resetting the fingerprint to “zero”. This may comprise measuring the “pre -post” fingerprint difference, i.e., the difference in product drift (e.g., difference in parameter of interest or overlay fingerprints) between using the old component and the new component (e.g., immediately before and immediately after the maintenance action). As such, this may comprise measuring a first fingerprint (e.g., immediately or shortly) before the maintenance action and then measuring a second (equivalent) fingerprint (e.g., immediately or shortly) after the maintenance action. Each of these fingerprints may, for example, describe variation or spatial distribution of a parameter of interest such as overlay across the wafer (or a portion thereof). The difference of these fingerprints (the pre-post fingerprint difference) may be used as a delta in control loops to provide for WIPless recovery after a component replacement. Over time, this delta can be gradually tuned down to zero as the replacement component degrades. Provided the maintenance action is performed sufficiently early, the pre -post fingerprint difference comprises no, or at least an acceptably small, non-correctable portion and therefore can be at least substantially minimized via scanner actuation (one or more control loops).

[0055] ‘ ‘Just-in-time” swapping of parts heavily relies on part availability and operational flexibility of the user. Supply chain or fab operational issues may lead to unscheduled delays in performance of the maintenance action and therefore further deterioration of the hardware part which is to be replaced. This may lead to a WIP impact that is beyond recovery for the aforementioned “WIP less recovery”, e.g., for which the “pre-post” fingerprint difference comprises a significant non-correctable portion such that it cannot be minimized sufficiently via scanner actuation. As a result, costly WIP mitigation may be required (e.g., ramp-down) or else there may be a significant yield loss for the affected lots. Additionally, part-to-part deviation due to manufacturing tolerances may lead to additional WIP impact contribution even if the replaced part has otherwise been swapped “in time”.

[0056] As such, a method is proposed in accordance with the first aspect, in which only a percentage of the errors are corrected before replacing a component. Subsequently, after replacing the component, the remaining percentage of the errors existing after replacing the component, are corrected.

[0057] The machine may be a lithography apparatus or scanner and said machine performance may relate to imaging performance of the lithography apparatus when imaging a pattern on a substrate, e.g., in terms of a parameter of interest indicative of machine / imaging performance such as overlay. In particular, the performance impact metric may describe imaging performance for the parameter of interest when a first action (e.g., imaging of a first layer) is performed on the substrate using the old machine component and a second action (e.g., imaging of a second layer) is performed on the (same) substrate using the replacement machine component.

[0058] A non-correctable performance impact metric may comprise a difference of a first non- correctable spatial distribution or first non-correctable fingerprint (e.g., non-correctable portion of the first fingerprint) imposed by the old component and a second non-correctable spatial distribution or second non-correctable fingerprint (e.g., non-correctable portion of the second fingerprint) imposed by the replacement component.

[0059] The performance impact metric may be determined from first metrology data relating to the machine / scanner comprising the old component (e.g., shortly or immediately) prior to the maintenance action and second metrology data relating to the machine / scanner comprising the replacement component (e.g., shortly or immediately) after the maintenance action.

[0060] The first metrology data and second metrology data may comprise overlay data (e.g., overlay fingerprint data). Alternatively, or in addition, the first metrology data and second metrology data may comprise inline metrology data; such inline metrology data may comprise for example, inter alia: wafer table qualification data, levelling data, lens model drift data.

[0061] A problem which may arise in the exposure of a substrate by an exposure apparatus, is that components of the exposure apparatus may degrade with time. In particular, as components age, or wear, they may degrade and begin to cause NCEs to accumulate, which may reduce the overlay and affect the yield of an end product (e.g. an integrated circuit component, a memory chip, etc.). As a consequence of such degradations, a maintenance action may be required, as discussed above. The present disclosure recognizes that subsequent to performing a maintenance action, there may be a jump in the overlay. In this context an overlay jump (e.g., a jump in non-correctable overlay error) may comprise a sudden and / or step change in (e.g., non-correctable) overlay subsequent to the maintenance action.

[0062] The current method of reducing such overlay jumps is to include a ramp-down period, then perform the maintenance action and have a ramp-up period, as described above. The result of this being the introduction of the so-called C-time, as described above and depicted in relation to Figure 4. The present disclosure is generally related to wafers in progress, i.e., those having one or more layers exposed pre-action and one or more layers exposed post-action.

[0063] Consequently, a method is sought to reduce or eliminate C-time while reducing (i.e. correcting for) the overlay jump resulting from the performance of a maintenance action. The proposed method may be implemented within a “WIPless” procedure, e.g., where production is not ramped down (or fewer wafers and / or layers are ramped down compared to non WIPless procedures) prior to the maintenance action.

[0064] To address this issue, for substrates for which one or more layers (i.e., one or more first layers) are to be exposed prior to a maintenance action and one or more layers (i.e., one or more second layers) are to be exposed after the maintenance action (e.g., wafers-in-progress), it is proposed to divide a determined correction into a first correction component and a second correction component and applying the first correction component and not the second correction component when exposing the one or more first layers (i.e., the layers exposed prior to the maintenance action). The second correction component can instead be applied to correct the exposure of the one or more second layers (i.e., the layers exposed after the maintenance action).

[0065] In this manner, a portion of the correction which is determined to correct (at least partially) for a deteriorated component is actually applied when this deteriorated component has been replaced or refurbished.

[0066] Advantageously, by applying only a portion of the correction when exposing layers prior to the maintenance action and the remainder of the correction after the maintenance action, the sudden, typically large, overlay jump resulting from the performance of the maintenance action is reduced. This makes a WIPless strategy more feasible to implement, reducing yield loss and / or rework which may be inherent in such a strategy. By using the specific example of a wafer table swap, the method may improve matching by reducing the overlay actuation error of the flatness difference (which manifests as an overlay jump) between new and old wafer tables.

[0067] Note that dividing a correction into first and second correction components may comprise dividing the magnitude of the correction into the first and second correction components.

[0068] Further advantages may include improving imaging / fading (i.e. image contrast) performance as a result of dividing the correction into a first correction component and a second correction component, as this reduces the magnitude of the correction that is to be applied subsequent to the maintenance.

[0069] In context herein, the one or more first layers and the one or more second layers define only a relative order of these layers with respect to the maintenance action, rather than an actual order of exposure on the wafer. One or more first layers may comprise any number of layers exposed before the action and one or more second layers may comprise any number of layers exposed after the action. It may or may not be the case that any of the “one or more first layers” is the actual first layer on the wafer. Similarly, it may or may not be the case that any of the “one or more second layers” is the actual second layer on the wafer.

[0070] The error introduced by at least one component of the exposure apparatus may arise at least partially from a deterioration of the component of the exposure apparatus. The deterioration of the components may be a caused by aging and / or wear of the component of the exposure apparatus.

[0071] The maintenance action of the present disclosure may comprise replacing the component of the exposure apparatus. For example, it may be determined (for example via metrology) that a wafer table requires replacing, perhaps because of degradation of at least a part of the water table. In another example, it may be determined that a lens of the projection optics of the exposure apparatus requires replacement.

[0072] Alternatively, the maintenance action of the present disclosure may comprise reconditioning the component of the exposure apparatus. Reconditioning includes a process of correcting for errors resulting from a component of the exposure apparatus by performing a process to the component to remove or reduce the errors of the component. For example, for a wafer table, flatness reconditioning may be performed to wear or flatten the burls of the table in a fashion similar to that of the old table.

[0073] The correction in accordance with the first aspect may be divided into the first correction component and the second correction component based on a predefined criterion. Examples of such criteria may include, for example, may comprise minimizing overlay actuation errors and / or minimizing fading error. For the first example, it can be appreciated that correction actuation may be subject to limitations which cause overlay actuation error (e.g. by reaching the range limit of actuation of a component such as an actuator lens element). The first and the second correction components may be determined as those which minimize the sum of overlay actuation errors, (e.g., dividing the correction into first and second correction components such that the actuator does not reach the range limit). For the second criterion example, it can be appreciated that corrections actuation introduces fading and thus affects the imaging quality. Therefore, the correction may be divided so that the fading error caused by correction actuation which affects imaging of the one or more first layers and the one or more second layers is minimized by balancing between the first and the second correction components. These two criteria may both be used and therefore may be balanced in a suitable manner.

[0074] The first correction component and the second correction component may each comprise approximately 50% of the correction. Alternatively, the first correction component may be less than at least one of 50%, 40%, 30%, 20%, 10% or 5% of the full correction. Further alternatively, the first correction component may be more than at least one of 50%, 60%, 70%, 80%, 90%, or 95% of the full correction. The skilled reader would understand that the fraction assigned to the first correction component may be any number such that there is a division of the correction between a first and second correction component. Furthermore, the skilled reader would understand that the exact fraction chosen may be specific to a particular application. For example, if it is determined that another error is to be accounted for during the second exposure (e.g. a wafer table thermal matching correction is to be applied to a new wafer table component) is large, then it may be determined that a greater portion may be assigned to first correction component than the second correction component.

[0075] The respective fractions of the correction assigned to the first correction component and / or to the second correction component may be user-definable. To implement this, for example, the lithographic apparatus (i.e. the exposure apparatus) and / or any standalone computation station for determining corrections / controlling one or more lithographic apparatuses, may comprise a user interface for enabling the user to define the magnitude of one or both of these correction components.

[0076] The component may comprise a substrate table (i.e. wafer table) for the substrate. Alternatively, the component may comprise, for example, a lens of the projection optics of the exposure apparatus; a final lens element of a projection lens; or a sensor fiducial plate configured to align a reticle to a stage. The skilled reader would understand that any component of the exposure apparatus is envisioned to be within the scope of the present disclosure, provided degradation (i.e. wear or aging) of such component can lead to NCEs requiring a maintenance action to be performed, which may result in an overlay jump and where replacement of said component would require a maintenance process that includes ramping down and then ramping up, thereby leading to C-time, as described above.

[0077] The correction may be determined by measuring at least one previously exposed substrate to obtain measurement data; determining said error from said measurement data; and determining the correction to at least partially correct for said error. The measurement may comprise measuring by the exposure apparatus and / or one or more associated tools.

[0078] The correction of the error may be selected to reduce the error to a predefined acceptability threshold. For example, the predefined acceptability threshold may be an acceptable yield loss, or otherwise an acceptable reduction in performance in an end product. The skilled reader would understand that other predefined acceptability thresholds may be chosen.

[0079] Figure 5 is a flowchart describing an example method 500 for implementing the concepts disclosed herein. In a metrology step 510, one or more previous wafers are measured to obtain metrology data 515. The wafers measured at metrology step 510 may comprise wafers of one or more previous lots (e.g., one or more of the immediately preceding lots) on which one or more layers have been exposed (e.g., at least the one or more first layers within the context of this disclosure). The metrology data 515 may comprise overlay data for example, and may be measured by a scatterometer. At step 520, a correction 525 is determined for an error characterized within the metrology data 515. The correction 525 may also correct for an error characterized by inline (e.g., alignment) metrology performed immediately prior to the exposure step 545 (below). Such an error may be a result, at least in part, of a degraded component. At step 530, the correction 525 is divided into a first correction component 535 and a second correction component 540. At step 545, an exposure of one or more first layers is performed on one or more substrates (e.g., a lot) for which at least one subsequent layer (one or more second layers) will be exposed subsequently to a maintenance action. The exposure 545 is performed as corrected by the first correction component 535, but not the second correction component. At step 550, a maintenance action is performed, e.g., to replace or refurbish the deteriorated component. At step 555, one or more second layers are exposed on the wafer / lot previously exposed at step 545.This exposure step 555 uses the second correction component 540. As is conventional, this step may optionally also use a post-action correction 560 determined from measurements of previous lots exposed subsequent to the maintenance action (if any) and as such the correction may be a combination of second correction component 540 and post-action correction 560.

[0080] Optionally, there may be a step 565 of determining the ratio 570 of the first correction component 535 and second correction component 540. The dividing step 530 to divide correction 525 can then be performed in accordance with this ratio 570. A suitable interface may be provided (e.g., as part of the lithographic apparatus / scanner, or an offline processing station) for selecting the desired ratio 570.

[0081] In accordance with the first aspect and any examples of the first aspect, the method may further comprise exposing said one or more first layers using at least said first correction component. Furthermore, the method may even further comprise exposing said one or more second layers using at least said second correction component.

[0082] In a further optional example, not depicted in relation to Figure 5, a second aspect of the present disclosure may comprise a computer program comprising program instructions operable to perform the method of the first aspect of the present disclosure, or any of its optional examples, when run on a suitable apparatus.

[0083] In a further optional example, not depicted in relation to Figure 5, a third aspect of the present disclosure may comprise a non-transient computer-readable storage medium carrying the computer program of the second aspect.

[0084] In a further optional example, not depicted in relation to Figure 5, a fourth aspect of the present disclosure may comprise an exposure apparatus configured to carry out the method of the first aspect of the present disclosure, or any of its optional examples.

[0085] In a further optional example, not depicted in relation to Figure 5, a fifth aspect of the present disclosure may comprise a semiconductor product manufactured by the exposure apparatus according to the fourth aspect of the present disclosure, or any of its optional examples.

[0086] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0087] 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.

[0088] 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.

[0089] 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.Other aspects of the invention are set out in the following clauses:1. A method for determining a correction for an exposure on a substrate using an exposure apparatus, the method comprising: determining a correction for performing one or more first exposures on a substrate, the correction being at least partially for correcting an error introduced by at least one component of the exposure apparatus; dividing the correction into a first correction component and a second correction component; applying the first correction component and not applying the second correction component when exposing one or more first layers on the substrate prior to a maintenance action.2. The method of clause 1 , further comprising: applying the second correction component when exposing one or more second layers on the substrate subsequent to the maintenance action.3. The method of clauses 1 or 2, wherein the error introduced by at least one component of the exposure apparatus is from a deterioration and / or aging of the component of the exposure apparatus.4. The method of any preceding clause, wherein the maintenance action comprises replacing the component with a new component.5. The method of any preceding clause, wherein the maintenance action comprises reconditioning the component of the exposure apparatus.6. The method of any preceding clause, wherein the correction is divided into the first correction component and the second correction component based on at least one predefined criterion.7. The method of clause 6, wherein the at least one predefined criterion comprises one or more of a minimisation of overlay actuation error and / or a minimisation of fading error.8. The method of any preceding clause, wherein the first correction component comprises between 30% and 70% of the correction.9. The method of any preceding clause, wherein the first correction component comprises between 30% and 60% of the correction.10. The method of any preceding clause, wherein the first correction component comprises between 40% and 60% of the correction.11. The method of any preceding clause, wherein the first correction component comprises less than 50% of the correction.12. The method of any of clauses 1 to 10, wherein the first correction component comprises more than 50% of the correction.13. The method of any of clauses 8 to 12, wherein the second correction component comprises a remaining part of the correction not comprised within the first correction component.14. The method of any preceding clause, wherein the component comprises a substrate table for the substrate.15. The method of any of clauses 1 to 13, wherein the component comprises a lens of the projection optics of the exposure apparatus.16. The method of any preceding clause, wherein the correction is determined by: measuring at least one previously exposed substrate to obtain measurement data; determining said error from said measurement data; and determining the correction to at least partially correct for said error.17. The method of any preceding clause, wherein the correction of the error is selected to reduce the error to a predefined acceptability threshold.18. The method of any preceding clause, further comprising exposing said one or more first layers using at least said first correction component.19. The method of any preceding clause, further comprising exposing said one or more second layers using at least said second correction component.20. A computer program comprising program instructions operable to perform the method of any of clauses 1 to 19, when run on a suitable apparatus.21. A non-transient computer-readable storage medium carrying the computer program of clause 20.22. An exposure apparatus configured to carry out the method of any of clauses 1 to 19.23. The exposure apparatus of clause 22, further comprising: a user interface for receiving a user input for determining a ratio of said first correction component and said second correction component.24. A semiconductor product manufactured by the exposure apparatus according to clauses 22 or 23.

[0090] 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 determining a correction for an exposure on a substrate using an exposure apparatus, the method comprising: determining a correction for performing one or more first exposures on a substrate, the correction being at least partially for correcting an error introduced by at least one component of the exposure apparatus; dividing the correction into a first correction component and a second correction component; applying the first correction component and not applying the second correction component when exposing one or more first layers on the substrate prior to a maintenance action.

2. The method of claim 1, further comprising: applying the second correction component when exposing one or more second layers on the substrate subsequent to the maintenance action.

3. The method of claims 1 or 2, wherein the error introduced by at least one component of the exposure apparatus is from a deterioration and / or aging of the component of the exposure apparatus.

4. The method of any preceding claim, wherein the maintenance action comprises replacing the component with a new component.

5. The method of any preceding claim, wherein the maintenance action comprises reconditioning the component of the exposure apparatus.

6. The method of any preceding claim, wherein the correction is divided into the first correction component and the second correction component based on at least one predefined criterion.

7. The method of claim 6, wherein the at least one predefined criterion comprises one or more of a minimisation of overlay actuation error and / or a minimisation of fading error.

8. The method of any preceding claim, wherein the component comprises a substrate table for the substrate.

9. The method of any of claims 1 to 7, wherein the component comprises a lens of the projection optics of the exposure apparatus.

10. The method of any preceding claim, wherein the correction is determined by:measuring at least one previously exposed substrate to obtain measurement data; determining said error from said measurement data; and determining the correction to at least partially correct for said error.

11. The method of any preceding claim, further comprising exposing said one or more first layers using at least said first correction component.

12. The method of any preceding claim, further comprising exposing said one or more second layers using at least said second correction component.

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

14. An exposure apparatus configured to carry out the method of any of claims 1 to 12.

15. A semiconductor product manufactured by the exposure apparatus according to claim 14.

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