Method of performing maintenance action on lithographic apparatus

Abstract

A machine maintenance method performed after a maintenance action in which an old machine component has been replaced by a replacement machine component is disclosed. The method includes determining a performance impact metric that describes an impact on machine performance caused by replacing the old machine component with the replacement machine component; and degrading the replacement component such that at least an uncorrectable portion of the performance impact metric is reduced and / or minimized, the uncorrectable portion being a portion that cannot be corrected by a correction mechanism of the machine and / or a process performed using the machine.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 23153863.8 filed on January 30, 2023, which is incorporated herein by reference. Technical Field

[0003] The present invention relates to methods and apparatus that can be used to manufacture devices, for example, by photolithographic techniques, and to methods of manufacturing devices using photolithographic techniques. Background Art

[0004] A photolithographic apparatus is a machine that applies a desired pattern to a substrate, typically to a target portion of the substrate. A photolithographic apparatus can be used, for example, to manufacture integrated circuits (ICs). In this case, a pattern forming device (alternatively referred to as a mask or reticle) can be used to generate a circuit pattern to be formed on a single layer of the IC. This pattern can be transferred to a target portion (e.g., comprising a portion of a die, a die, or multiple dies) on a substrate (e.g., a silicon wafer). The pattern is typically transferred by imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will contain a network of continuously patterned adjacent target portions. These target portions are typically referred to as "fields."

[0005] In the manufacture of complex devices, many lithographic patterning steps are typically performed to form functional features in successive layers on a substrate. Therefore, a key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place the applied pattern relative to features arranged in a previous layer (by the same apparatus or by a different lithographic apparatus). 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 comprises one or more alignment sensors by means of which the position of the mark on the substrate can be accurately measured. Different types of marks and different types of alignment sensors are known from different manufacturers and from different products of the same manufacturer.

[0006] In other applications, metrology sensors are used to measure exposed structures on substrates (in resist and / or after etching). A fast and non-invasive form of specialized inspection tool is a scatterometer, in which a radiation beam is directed onto a target on the substrate surface and the properties of the scattered or reflected beam are measured. Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. In addition to measuring feature shapes by reconstruction, such devices can be used to measure diffraction-based overlay, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark field imaging of diffraction orders enables overlay measurement of smaller targets. Examples of dark field imaging metrology can be found in international patent applications WO2009 / 078708 and WO2009 / 106279, which are incorporated herein by reference in their entirety. Further developments of this technology are described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A, and WO2013178422A1. These targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Composite grating targets can be used to measure multiple gratings in a single image. The contents of all of these applications are incorporated herein by reference.

[0007] Periodically, it is necessary to perform maintenance operations on a lithographic apparatus to replace damaged components. When different layers of the same substrate have been exposed before and after the maintenance operation, the fingerprint or impact of the replacement component may result in large overlay errors compared to the replaced component, necessitating a mid-run ramp-down (temporarily stopping production) of one or more layers to perform the maintenance operation.

[0008] It is desirable to mitigate the impact of these maintenance actions on productivity. Summary of the Invention

[0009] In a first aspect, the present invention provides a method for machine maintenance, which includes, after performing a maintenance action in which an old machine part has been replaced by a replacement machine part, performing the following steps: determining a performance impact metric, which describes the impact on machine performance caused by replacing the old machine part with the replacement machine part; and degrading the replacement part so that at least an uncorrectable part of the performance impact metric is reduced and / or minimized, the uncorrectable part being a part that cannot be corrected by the correction mechanism of the machine and / or the process performed using the machine.

[0010] These and other aspects of the present invention can be understood through a consideration of the embodiments described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0012] Figure 1 depicts the lithographic apparatus;

[0013] Figure 2 Schematically shows Figure 1 measurement and exposure processes in the device;

[0014] Figure 3 (a) conceptually illustrates the effect of eliminating the overlay of wafer stage fingerprints in each layer without wafer stage swapping, and Figure 3 (b) conceptually illustrates the performance impact of such stage exchange in overlay;

[0015] Figure 4 is a graph of cumulative batches of wafers produced versus time, illustrating the concept of C-time; and

[0016] Figure 5 is a flow chart describing a method for reducing wafer-in-progress (WIP) impact of maintenance actions by de-staging replacement machine components according to an embodiment; and

[0017] Figure 6 is a diagram for use based on Figure 5 Flowchart of a method for predicting the impact of uncorrectable WIP using online metric data in a method. DETAILED DESCRIPTION

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

[0019] Figure 1A lithographic apparatus LA is schematically shown. The apparatus comprises 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., mask table) MT configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to accurately position the patterning device according to certain parameters; two substrate tables (e.g., wafer stages) WTa and WTb, each configured 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 according to 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 the patterning device MA onto a target portion C (e.g., comprising 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 the positions of the patterning device and substrate, and features thereon.

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

[0021] The patterning device support MT holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions (e.g., whether 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 can be, for example, a frame or a table that can be fixed or moved as needed. The patterning device support can ensure that the patterning device is in a desired position, for example, relative to the projection system.

[0022] The term "patterning device" as 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 so as to produce 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. Typically, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device (e.g., an integrated circuit) being created in the target portion.

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

[0024] The term "projection system" as used herein should be broadly interpreted to include any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation used, or to 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 synonymous with the more general term "projection system."

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

[0026] In operation, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In this case, the source is not considered to form part of the lithographic apparatus, and the radiation beam is transferred from the source SO to the illuminator IL by means of a beam delivery system BD comprising, 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. If desired, the source SO and illuminator IL, together with the beam delivery system BD, may be referred to as a radiation system.

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

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

[0029] The lithographic apparatus may include an aberration sensor for verifying the aberration fingerprint of the projection system PS. In one embodiment, such a wavefront aberration sensor can be used to determine the aberration fingerprint, i.e., the aberration of each field point of the projection system PS. A known wavefront aberration sensor, such as that described in US 2002 / 0001088, can be used. Such a wavefront aberration sensor can be based on the principle of shearing interferometry and include a source module and a sensor module. The source module can include a patterned layer of chromium placed in the object plane of the projection system PS (i.e., where the pattern of the patterning device is located during manufacturing) and having additional optics provided above the chromium layer. This combination provides a radiation wavefront to the entire pupil of the projection system PS. The sensor module can include a patterned chromium layer placed in the image plane of the projection system (i.e., where the substrate W is located during production) and a camera positioned a distance behind the chromium layer. The patterned chromium layer on the sensor module diffracts the radiation into multiple diffraction orders that interfere with each other, thereby generating an interference pattern. The interference pattern is measured by the camera. Aberrations in the projection lens can be determined by software based on the measured interference pattern. The wavefront aberration sensor may be configured to transmit information about the aberration fingerprint to the control unit.

[0030] Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device (e.g., mask) MA and substrate W. Although the substrate alignment marks shown occupy dedicated target portions, they can be located in the spaces between target portions (these are called scribe alignment marks). Similarly, where more than one die is provided on the patterning device (e.g., mask) MA, the mask alignment marks can be located between the dies. In device features, small alignment marks can also be included within the die, in which case it is desirable that the marks be as small as possible and not require any imaging or processing conditions that are different from those of adjacent features. An alignment system for detecting alignment marks is described further below.

[0031] The described apparatus can be used in various modes. In scan mode, the pattern forming device support (e.g. mask table) MT and the substrate table WT are scanned synchronously while the 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 pattern forming device support (e.g. mask table) MT can be determined by the (reduction) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width of the target portion in a single dynamic exposure (in the non-scanning direction), while the length of the scanning movement determines the height of the target portion (in the scanning direction). As is known in the art, other types of lithographic apparatus and operating modes are also possible. For example, stepping modes are known. In so-called "maskless" lithography, the programmable pattern forming device remains stationary but has a changing pattern, and the substrate table WT is moved or scanned.

[0032] Combinations and / or variations on the above-described modes of use or entirely different modes of use may also be employed.

[0033] The lithographic apparatus LA is of a so-called dual-stage type, having two substrate tables WTa, WTb and two stages (an exposure stage EXP and a measurement stage MEA) between which the substrate table can be exchanged. While one substrate on one substrate table is being exposed on the exposure stage, another substrate can be loaded onto the other substrate table on the measurement stage and various preparatory steps are performed. This significantly increases the throughput of the apparatus. The preparatory steps may include mapping the surface height profile of the substrate using a level sensor LS and measuring the positions of alignment marks on the substrate using an alignment sensor AS. If the position sensor IF cannot measure the position of the substrate table when the substrate table is at the measurement stage and the exposure stage, a second position sensor may be provided to enable the position of the substrate table to be tracked relative to the reference frame RF at the two stages. Other arrangements are known and can be used instead of the dual-stage arrangement shown. For example, other lithographic apparatuses in which a substrate table and a measurement stage are provided are known. These are docked together when performing preparatory measurements and then undocking when the substrate table undergoes exposure.

[0034] Figure 2 Shown in Figure 1 200 . The steps of exposing a target portion (e.g. a die) on a substrate W in a dual-stage apparatus of FIG. 201 . On the left hand side within the dotted box are the steps performed at the measurement stage MEA, while the right hand side shows the steps performed at the exposure stage EXP. Depending on the time, one of the substrate tables WTa, WTb will be at the exposure stage and the other at the measurement stage, as described above. For the purpose of description, it is assumed that the substrate W has already been loaded into the exposure stage. In step 200, a new substrate W' is loaded into the apparatus by a mechanism not shown. The two substrates are processed in parallel to increase the output of the lithography apparatus.

[0035] Reference is first made to a newly loaded substrate W', which may be a previously unprocessed substrate, being prepared in the apparatus with a new photoresist for the first exposure. However, typically, the described lithographic process will be just one step in a series of exposure and processing steps, such that substrate W' has already passed through the apparatus and / or other lithographic apparatus multiple times, and may also have subsequent processes. Specifically for the problem of improving overlay performance, the task is to ensure that a new pattern is applied precisely at the correct location on a substrate that has already undergone one or more patterning and processing cycles. These processing steps gradually introduce distortions into the substrate that must be measured and corrected to achieve satisfactory overlay performance.

[0036] As previously mentioned, previous and / or subsequent patterning steps can be performed in other lithographic equipment, and can even be performed in different types of lithographic equipment. For example, some layers in the device fabrication process that have very high requirements for parameters such as resolution and overlay can be performed in more advanced lithographic tools than other layers with lower requirements. Thus, some layers can be exposed in immersion lithography tools, while other layers are exposed in "dry" tools. Some layers can be exposed in tools operating at DUV wavelengths, while other layers are exposed using EUV wavelength radiation.

[0037] At 202, alignment measurements using substrate marks P1 and an image sensor (not shown) are used to measure and record the alignment of the substrate relative to substrate table WTa / WTb. Furthermore, alignment sensor AS is used to measure a plurality of alignment marks on substrate W'. In one embodiment, these measurements are used to create a "wafer grid" that very accurately maps the distribution of the marks on the substrate, including any deformations relative to the nominal rectangular grid.

[0038] In step 204, a level sensor LS is also used to measure a map of the wafer height (Z) versus X-Y position. Typically, the height map is used only to achieve precise focus of the exposure pattern. However, it can be used for other purposes.

[0039] When substrate W' is loaded, recipe data 206 is received, which defines the exposure to be performed, as well as the properties of the wafer and the pattern previously produced and created thereon. The measurements of the wafer position, wafer grid, and height map taken at 202 and 204 are added to these configuration data so that a complete set of configuration and measurement data 208 can be transferred to the exposure stage EXP. The measurements of alignment data include, for example, the X and Y positions of alignment targets, which are formed in a fixed or nominally fixed relationship relative to the product pattern that is the product of the lithography process. These alignment data, acquired just before exposure, are used to generate an alignment model with parameters that adapt the model to the data. These parameters and the alignment model are used during the exposure operation to correctly position the pattern applied in the current lithography step. The model used interpolates the positional deviations between the measured positions. Conventional alignment models can include four, five, or six parameters that together define the translation, rotation, and scaling of the "ideal" grid in different dimensions. Advanced models using more parameters are known.

[0040] At 210, wafers W' and W are exchanged so that the substrate W' to be measured becomes the substrate W entering the exposure stage EXP. Figure 1 In the example apparatus of , the exchange is performed by exchanging supports WTa and WTb within the apparatus so that substrates W, W' remain precisely clamped and positioned on those supports to maintain the relative alignment between the substrate table and the substrate itself. Therefore, once the stages have been exchanged, determining the relative position between the projection system PS and substrate table WTb (formerly WTa) is necessary to utilize measurement information 202, 204 of substrate W (formerly W') in the control of the exposure steps. In step 212, reticle alignment is performed using mask alignment marks M1, M2. In steps 214, 216, 218, scanning motions and radiation pulses are applied at successive target locations on substrate W in order to complete the exposure of a plurality of patterns.

[0041] By using the alignment data and height map obtained at the measurement stage during the exposure step, these patterns are accurately aligned relative to desired locations, particularly relative to features previously placed on the same substrate. At step 220, the exposed substrate, now labeled "W," is unloaded from the apparatus for etching or other processing based on the exposed patterns.

[0042] Those skilled in the art will appreciate that the above description is a simplified overview of the many, more detailed steps involved in one example of a real-world manufacturing situation. For example, rather than measuring alignment in a single pass, there will typically be separate stages for coarse and fine measurements, using the same or different markers. The coarse and / or fine alignment measurement steps can be performed before or after the height measurement, or interleaved.

[0043] Lithographic equipment or scanners require regular maintenance, such as replacing hardware components that degrade over time. As a specific example, a scanner's wafer stage degrades and requires regular replacement. This hardware replacement and / or maintenance can impact performance.

[0044] This performance impact is Figure 3 FIG1 is a schematic diagram of a wafer stage and a process diagram of a wafer stage. The wafer stage is shown conceptually in FIG1 . Overlay is an important parameter that describes the proper placement of a layer relative to the previously exposed (lower) layer. Each wafer stage imposes a wafer clamping effect or clamping fingerprint on the substrate to which it is clamped, which should be corrected. However, as the wafer stage wears over time, the associated wafer clamping effect is affected (usually becomes larger), which will affect the positioning of the exposed structures on the substrate. In this way, degradation of the wafer stage (and other components) will lead to position errors; these errors can be divided into lower frequency errors or correctable errors (CE), which can be measured (using appropriate metrology tools) and corrected within the scanner via a process correction loop (e.g., known as an Advanced Process Correction or APC loop), and higher frequency errors or uncorrectable errors (NCE), which cannot be corrected via APC because the effect of the error cannot be measured using sufficiently fast metrology captured by the model used to represent the metrology data and / or because the required correction cannot be actuated within the scanner.

[0045] In the absence of wafer stage exchange, the wafer clamping effect changes slowly enough that there is essentially no significant variation between exposures of different layers on a single wafer. Because overlay is a relative measure between two layers, the NCEs produced by this effect in each layer largely cancel each other out. Figure 3 (a) The lines for the first layer L1 and the second layer L2 represent the high-frequency components of the wafer grid effect of a degraded wafer stage. While this degradation results in relatively large amplitude perturbations in the local placement of features in each exposed layer, these perturbations are generally similar enough in each layer to cancel themselves out in the overlay (i.e., the positional errors in each layer are the same, meaning that the misalignment between layers due to this effect is relatively small). Therefore, the overlay NCE due to this wafer stage fingerprint in each layer will be small. In contrast, Figure 3 (b) Conceptually illustrates the situation should a wafer stage be swapped between exposures of layers L1 and L2. Due to wafer stage imperfections, the new stage results in an overall smaller wafer grid effect; however, the effect of the old stage is present in the layer L1 exposure. Consequently, this effect is no longer offset, resulting in significantly larger NCE overlay losses. Due to the difference in these fingerprints, this can lead to uncorrectable jumps in APC that can be large enough to impact yield (the fact that the errors are uncorrectable means these wafers cannot be recovered through reprocessing).

[0046] Wafer stage maintenance is only one example of a maintenance action (e.g., component replacement) that can lead to the described mismatched fingerprint problem. Other maintenance actions that can lead to mismatched fingerprints before and after the maintenance action include, among others, lens swapping of any lens of the projection optics.

[0047] This overlay loss due to hardware maintenance during wafer processing (i.e., between exposing different layers on a wafer) is often referred to as the wafer-in-process (WIP) effect, because it affects wafers in progress (those on which only some desired layers are being exposed) at the time of the maintenance action. One strategy to mitigate this WIP effect is to "down-expose" multiple layers at the initiation of such a maintenance action, so as to reduce the number of wafers in process at the time of the action.

[0048] Figure 4 The effect of such ramping is shown. This ramping typically involves stopping exposure of one or more layers for several weeks until maintenance action is required, for example, stopping exposure of each layer in sequence, starting from the bottom layer, with intervals of several days to several weeks between ramping down each successive layer. Not all layers may be ramped down, and optimizing the number of layers to be ramped down is an objective of at least some of the methods disclosed herein. Such ramping down of layers represents a loss in productivity relative to continuing wafer production at the rate before ramping down was initiated.

[0049] In addition to the ramp-down effect, there will be a concomitant ramp-up effect (i.e., compared to full production rate) when production is restarted after a maintenance action. For example, production of each layer that was ramped down will need to be restarted before production of the layers higher in the stack can begin. Furthermore, the calibration loop or APC loop will need to be restarted because there is no (or insufficient) metrology data for the post-maintenance system. As a result, many of the first wafers exposed after ongoing maintenance will be poorly exposed, requiring rework (stripping of the poorly exposed resist, re-overlaying, and re-exposure).

[0050] This combination of ramp-down and ramp-up times results in what is commonly referred to as C-time: the effect of these ramp-down and ramp-up periods relative to the time without ramp-up or ramp-down. C-time is in addition to A-time (the nominal downtime for the actual maintenance action) and B-time (the margin applied to A-time).

[0051] Figure 4is a graph of the cumulative number of batches versus time, showing the production rate (solid line) for the production period, including the ramp-down period, maintenance action, and ramp-up period. The A+B time is the period of actual maintenance action, including margin, during which the production rate is zero (machine downtime). The dashed line represents the nominal production rate in the absence of ramp-down or ramp-up time, with production continuing at a constant rate until the maintenance action and then immediately restarting after the action when production is resumed. The C-time is the time difference between the two curves after the ramp-up is complete and production has reached an approximately steady-state rate. As will be appreciated, the C-time is typically much greater than the A+B-time, and is larger than shown in the diagram.

[0052] The proposed strategy for addressing the WIP impact problem is to use a combination of "early swap" and "no WIP recovery" to reduce ramp-down and thus reduce C time (ideally to zero). The early swap strategy includes monitoring and predicting the establishment of potential WIP impact so that swaps are "on time," e.g., so that maintenance actions are performed at or near the nearest time when the predicted uncorrectable portion of the WIP impact is minimal (e.g., any degrading effects that contribute to the WIP impact can be corrected via machine or scanner actuation).

[0053] WIP-free recovery involves using adapted scanner calibration to restore to the fingerprint prior to the swap, rather than resetting the fingerprint to "zero." This can include measuring a "before-after" fingerprint difference, i.e., using the difference in product drift (e.g., the difference in the parameter of interest or the overprint fingerprint) between the old and new parts (e.g., immediately before and after the maintenance action). Thus, this can include measuring a first fingerprint before (e.g., immediately or shortly after) the maintenance action, followed by measuring a second (equivalent) fingerprint after (e.g., immediately or shortly after) the maintenance action. Each of these fingerprints can, for example, describe the variation or spatial distribution of a parameter of interest, such as the overprint across a wafer (or a portion thereof). The difference between these fingerprints (the before-after fingerprint difference) can be used as an increment in a control loop to provide WIP-free recovery after a part swap. Over time, this increment can be gradually adjusted down to zero as the replaced part degrades. If the maintenance action is performed early enough, the before-after fingerprint difference does not include, or at least includes, an acceptably small uncorrectable portion and can therefore be at least substantially minimized via scanner actuation (one or more control loops).

[0054] The "just in time" exchange of parts is largely dependent on the part availability and operational flexibility of the user. Supply chain or manufacturing operation issues may lead to unscheduled delays in the performance of maintenance actions and, therefore, further degradation of the hardware part to be replaced. This may result in a WIP impact that goes beyond the recovery of the "WIP less recovery" described above, for example, for which the "before and after" fingerprint difference includes a significant uncorrectable portion such that it cannot be adequately minimized via scanner actuation. As a result, expensive WIP mitigation (e.g., ramp-down) may be required, or there may be significant yield losses for the affected batches. Furthermore, part-to-part deviations due to manufacturing tolerances may result in additional WIP impact contributions even if the replaced part has otherwise been exchanged "just in time."

[0055] Therefore, a method is proposed in which a replacement or new part is intentionally degraded as part of a maintenance action, for example using an accelerated degradation action. This degradation can be such that at least the uncorrectable portion of the front-to-back fingerprint difference (the WIP effect of the maintenance action) is minimized and / or reduced, for example, such that the front-to-back fingerprint difference is substantially fully correctable, for example by scanner actuation.

[0056] In one embodiment, the proposed method may include performing, after performing a maintenance action in which an old machine part has been replaced by a replacement machine part: determining a performance impact metric describing the impact on machine performance caused by replacing the old machine part with the replacement machine part; and downgrading the replacement part so that at least an uncorrectable portion of the performance impact metric is reduced and / or minimized, the uncorrectable portion being the portion that cannot be corrected by a correction mechanism of the machine and / or a process performed using the machine.

[0057] Such correction mechanisms may involve the actuation of the machine, the software or model of the machine (e.g., there may be limitations in what can be corrected in the machine due to some balance of compromises that may not be optimized for the problem at hand, or "under-modeling" of the data for various reasons), and / or the control loop of the machine. For example, with respect to APC uncorrectable errors, these are not necessarily limited only by the actuation of the scanner, but also by the correction (software) interface and the sampling and modeling of the scanner data. It will therefore be appreciated that the APCNCE and the scanner NCE are not necessarily identical.

[0058] The machine may be a lithographic apparatus or a scanner, and the machine performance may relate to imaging performance of the lithographic apparatus when imaging a pattern on a substrate, for example, in terms of a parameter of interest representing machine / imaging performance, such as overlay error. In particular, when a first action (e.g., imaging of a first layer) is performed on a substrate using an old machine component and a second action (e.g., imaging of a second layer) is performed on the (same) substrate using a replacement machine component, the performance impact metric may describe the imaging performance of the parameter of interest.

[0059] The method may include determining an uncorrectable performance impact metric comprising an uncorrectable portion of the performance impact metric, and downgrading a replacement component to reduce and / or minimize the uncorrectable performance impact metric. Alternatively or additionally, the method may include downgrading a replacement component to reduce and / or minimize a (full) performance impact metric, i.e., comprising the correctable portion of the performance impact metric.

[0060] The uncorrectable performance impact metric can include a difference between a first uncorrectable spatial distribution or a first uncorrectable fingerprint imposed by the old component (e.g., an uncorrectable portion of the first fingerprint) and a second uncorrectable spatial distribution or a second uncorrectable fingerprint imposed by the replacement component (e.g., an uncorrectable portion of the second fingerprint). Thus, the degradation step can change the second uncorrectable fingerprint so that it more closely matches the first uncorrectable fingerprint.

[0061] The performance impact metric can be determined from first metric data associated with the machine / scanner including the old part before (e.g., shortly or immediately) the maintenance action and second metric data associated with the machine / scanner including the replaced part after (e.g., shortly or immediately) the maintenance action.

[0062] The first metrology data and the second metrology data may include overlay data (e.g., overlay fingerprint data). Alternatively or additionally, the first metrology data and the second metrology data may include inline metrology data; such inline metrology data may include, for example, wafer stage qualification data, leveling data, lens aberration drift data, among others. Figure 6 An example method for converting the online metrology data into a WIP impact fingerprint (a difference between before and after fingerprints) is described.

[0063] It will be appreciated that the second measurement data need not be actually measured, but rather (or in addition) may be predicted based on the first measurement data and a modelling step (a suitable model which can predict the second (later) measurement data from the first (anterior) measurement data).

[0064] The degradation step may be performed in situ, ie on a machine (eg scanner) comprising the replacement component, and the replacement component is installed. Alternatively, the degradation step may be performed iteratively, wherein a (eg uncorrectable) performance impact metric is determined for each iteration.

[0065] Intentionally downgrading a new part to more closely match the old one may seem counterintuitive, as it intuitively seems that such a downgraded new part would have a significantly shortened lifespan. However, while there may be some reduction in part lifespan and machine availability, this impact will be less than the C-time impact without such degradation, especially when part replacement is delayed for any reason; this C-time impact can manifest as weeks or months of productivity impact. Furthermore, part lifespan has different aspects; for example, one aspect affects run-to-run performance (e.g., wafer load grid), while another affects the WIP impact during part exchange. The downgrading process can be implemented to affect the latter aspect, but not necessarily the former.

[0066] Figure 5 is a flow chart describing a method according to an embodiment. In step 500, measurements are performed (e.g., online) before and after a maintenance action (part replacement). Alternatively or in addition, measurements can be performed before the maintenance action and post-maintenance data predicted based on this measurement data. In step 510, the resulting first fingerprint L1 and second fingerprint L2 are determined, and the difference between the before and after fingerprints is thus determined. The steps may include initially determining the uncorrectable parts of these fingerprints and the uncorrectable before and after fingerprint differences or WIP effects (although the method may use complete fingerprints). These steps may additionally (optionally) take into account any drift control mechanism used, which may reduce the WIP effect.

[0067] In step 520, a component degradation step may be performed to (quickly) degrade the replacement component. The component degradation step may be performed with the replacement component in situ, i.e. mounted on the scanner. The details of such a component degradation step will depend on the component being replaced. For example, for a wafer stage, component degradation may comprise a version of "flatness trimming", e.g. wearing down or flattening the stage's burls in a manner similar to the old stage. For a replacement lens element, a burn-in procedure may be used. This may comprise a series of pseudo-exposures at high doses, e.g. selecting illumination settings and field sizes that are representative of those that may be used in wafer production on the scanner and that are known to affect the performance parameter in question. In the latter case, a specific reticle with a pre-programmed attenuation pattern may be used to prevent the introduction of "parasitic" fading.

[0068] The method may proceed directly to step 540, where a no-WIP recovery process is performed as already described, e.g., the control loop is not fully reset, but rather a new before-after fingerprint difference between the first fingerprint L1 and a new second fingerprint L2′ (e.g., an uncorrectable portion thereof) is used, where the new second fingerprint L2′ is the fingerprint of the now-degraded replacement component that was degraded as in step 520. In this way, step 540 may calibrate the final before-after fingerprint difference back to the before-after fingerprint difference prior to replacement.

[0069] Optionally, based on the determination of step 530 as to whether the (uncorrectable) before-after fingerprint difference is sufficiently minimized, the method may iteratively repeat steps 500 to 520. In this way, the steps may be performed in an iterative manner to converge towards a desired end result.

[0070] An advantage of the described approach is that generic components can be degraded within the scanner. This contrasts with known approaches, where replacement components can be prefabricated to more closely resemble the worn component being replaced. This can have significant advantages. For example, accelerated degradation will more closely resemble the actual degradation mechanism of the old component and may be better suited for approximating degradation fingerprints and / or emulating actual degradation patterns, as ex-situ manufacturing often has quite significant limitations / tolerances. The proposed approach is more flexible and can accommodate delayed (or premature) maintenance actions due to the use of generic components as a starting point.

[0071] It will be appreciated that the new part need not be significantly damaged, but only sufficiently damaged to bring the fingerprint delta between the old part and the replacement part within the correctable range of the calibration model and the corresponding actuator. This will also reduce the amount of machine time required. Thus, this approach can take into account (e.g., at step 530) the potential for correction without WIP recovery.

[0072] Figure 6 is a flow chart illustrating a method for predicting uncorrectable WIP effects based on inline metrology data 600 or scanner metrology / monitoring data (e.g., wafer stage qualification data, leveling data, lens aberration drift data, etc.). For example, Figure 5 This method is used in step 510 of the method. The inline metrology data 600 can be transformed 610 into the effects of overlay (or other parameters of interest) using an appropriate model (e.g., a physical transformation model). An example of a physical transformation model is a "Z2XY" model that determines the distortion-induced overlay based on leveling data and aberration sensitivity (which can be calculated using lithography simulation).

[0073] In the next step, the scanner's excitation model is used to calculate 620 the uncorrectable error portion of the WIP impact or overlay impact (the difference between the before and after fingerprints). This step may include incorporating the uncorrectable impact into a decomposition of the node overlay performance to predict the total overlay impact under realistic process control performance assumptions. Node overlay performance can be evaluated using conventional production data from multiple batches collected across different layers before and after maintenance actions. This data can be used to assess variability across the product and its improvement due to maintenance actions (e.g., new scanner hardware). Because conventional production data is used, it does not include any WIP impact. This data can then be combined with the expected WIP impact determined based on the scanner data (e.g., Z2XY data). This can be achieved through a (e.g., root mean square) determination of overlay impact, or by combining the variability fingerprint across the product with the WIP impact fingerprint and then using die-in-spec statistical methods to determine the potential yield impact. Through such methods, for example, through gradient-based calculations (Z2XY), the leveling data can be converted into predicted overlay data and, therefore, into WIP impact.

[0074] It will be appreciated that the correctable error portion of the WIP effect may also be determined by performing steps 600 and 610, followed by a variation of step 620, wherein the correctable error portion of the overlay effect is calculated (instead of the uncorrectable portion). Additionally, future trends may be predicted by extrapolating the WIP effect into the future.

[0075] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0076] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, it should be understood that the present invention can be used in other applications, such as imprint lithography, and is not limited to optical lithography, where the context permits. In imprint lithography, the topography in a patterning device defines the pattern produced on a substrate. The topography of the patterning device can be pressed into a resist layer provided to the substrate, whereupon the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. The patterning device is removed from the resist, leaving a pattern therein after the resist cures.

[0077] As used herein, the terms "radiation" and "beam" 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 ultraviolet (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.

[0078] The term "lens", where the context permits, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components. Reflective components may be used in devices operating in the UV and / or EUV range.

[0079] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

1. A machine maintenance method comprising, after performing a maintenance action in which an old machine component has been replaced by a replacement machine component, performing the following steps: determining a performance impact metric describing an impact on machine performance caused by replacing the old machine component with the replacement machine component; and The replacement component is downgraded such that at least an uncorrectable portion of the performance impact metric is reduced and / or minimized, the uncorrectable portion being the portion that is not correctable by a correction mechanism of the machine and / or a process performed using the machine.

2. A method according to claim 1, wherein the machine comprises an exposure apparatus, and the effect on the performance of the machine can comprise an effect on the imaging performance of the exposure apparatus when imaging a pattern on a substrate. The method of claim 2 , wherein the performance impact metric is described in terms of a parameter of interest indicative of the imaging performance. The method of claim 3 , wherein the parameter of interest is overlay accuracy.

5. The method of claim 3 or 4, wherein the performance impact metric describes machine performance for the parameter of interest when a first action is performed on the substrate using the old machine component and a second action is performed on the substrate using the replacement machine component.

6. A method according to any one of the preceding claims, wherein the old machine part and the replacement machine part each comprise a substrate table.

7. The method of claim 6, wherein the degrading step comprises wearing and / or flattening burls of the replacement machine component in a manner similar to the old machine component.

8. The method according to any one of claims 1 to 5, wherein the old machine part and the replacement machine part each comprise a lens of a projection optical system of the machine.

9. The method of claim 8, wherein the degrading step comprises performing a plurality of dummy exposures.

10. The method of claim 9, wherein the plurality of dummy exposures are performed at a higher dose level than a dose level used during normal exposures.

11. A method according to any preceding claim, wherein the performance impact metric comprises a difference between a first spatial distribution associated with the old component and a second spatial distribution associated with the replacement component.

12. A method according to any one of the preceding claims, comprising determining an uncorrectable performance impact metric from the performance impact metric, the uncorrectable performance impact metric comprising an uncorrectable portion of the performance impact metric; and wherein the degradation step comprises degradation of the replacement component to reduce and / or minimize the uncorrectable performance impact metric.

13. The method according to any of the preceding claims, comprising determining the performance impact metric from first measurement data and second measurement data, the first measurement data relating to the machine including the old component before the maintenance action, and the second measurement data relating to the machine including the replacement component after the maintenance action.

14. A method according to any preceding claim, wherein the step of degrading is performed on the machine comprising and having the replacement component installed.

15. The method according to any one of the preceding claims, further comprising: determining the performance impact metric or an uncorrectable portion of the performance impact metric after performing the degrading step; as well as The performance impact metric, or an uncorrectable portion of the performance impact metric, is used within a control loop to control the machine using the replacement component.

16. The method of any one of the preceding claims, further comprising performing the maintenance action to replace the old component with the replacement component.

Images