WO2025180728A1 - Method of monitoring an exposure process - Google Patents

Abstract

Disclosed is a method of monitoring an exposure process, comprising: obtaining a set of first monitoring data relating to a performance of a monitoring action of the exposure process using at least one first reference structure, the at least one first reference structure being in use for the monitoring action during exposures; obtaining at least one set of second monitoring data relating to a performance of the monitoring action of the exposure process using at least one second reference structure, the at least one second reference structure not being in use for the monitoring action during exposures; comparing the set of first monitoring data to the at least one set of second monitoring data; and determining, based on the comparing, whether to change from the at least first reference structure to the at least one second reference structure for the monitoring action during further exposures

Description

METHOD OF MONITORING AN EXPOSURE PROCESS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from EP application no. 24160056.8 which was filed on February 27, 2024 and which is incorporated herein in its entirety by reference.

FIELD OF INVENTION

[0002] The present invention relates to methods and apparatus for monitoring an exposure process, in particular, to a method of feedforward corrections and monitoring part health using redundant reference structures in a lithographic exposure process.

BACKGROUND

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

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

[0005] In other applications, metrology sensors are used for measuring exposed structures on a substrate (either in resist and/or after etch). A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications WO 2009/078708 and WO 2009/106279 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470 A, US20120123581A, US20130258310A, US20130271740A and

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

[0006] A known lithographic apparatus or “scanner” may comprise a measurement side and an exposure side. At the measurement side, the position of marks on the wafer may be measured relative to a common reference or fiducial. At the exposure side, the position and shape of the reticle is measured relative to the fiducial. In this manner, a common reference frame between the measurement side and exposure side is established. Thus reference frame may be used to align the measure side wafer measurements to the required exposure side exposure actuations.

[0007] The methods and/or locations for performing measurements on the fiducial respectively at the measure side and exposure side are different. Because of this difference, some errors (e.g., resultant from degradation of the fiducial) are only seen by the measure side and not by exposure side and vice versa. Examples of errors seen only by the measure side, are those due to degradation of the protective layer on the sensor or plate mark degradation. As the exposure side uses the measurement side measurements as reference, it cannot correct for these errors, which it is unable to measure, and as such these errors will be seen in the exposure on the wafer.

[0008] Changing from a degraded component to a new component or moving from degraded primary reference marks to redundant reference marks of the same component (e.g., of a fiducial) results in previously applied corrections (when the degraded component or the degraded primary reference marks were in use) being no longer valid for the new component or redundant reference marks. If the same corrections are used, the level of correction applied will be incorrect and causes a performance impact, also known as wafer-in-process (WIP) impact (see below for more details). Methods and apparatus for monitoring an exposure process are proposed herein to minimize or prevent such a WIP impact.

SUMMARY OF THE INVENTION

[0009] The invention in a first aspect provides a method of monitoring an exposure process, comprising: obtaining a set of first monitoring data relating to a performance of a monitoring action using at least one first reference structure, the at least one first reference structure being in use for the monitoring action during exposures; obtaining at least one set of second monitoring data relating to a performance of the monitoring action of the exposure process using at least one second reference structure, the at least one second reference structure not being in use for the monitoring action during exposures; comparing the set of first monitoring data to the at least one set of second monitoring data; and determining, based on the comparing, whether to change from the at least first reference structure to the at least one second reference structure for the monitoring action during further exposures.

[0010] In an example application, the invention monitors a measurement action within a lithographic device that periodically monitors secondary, redundant reference structures for the following two purposes: After the measurement is conducted, the lithographic device will compare the redundant structure against the primary structure. When the deviation between the two measurements becomes too large, then this can be used to trigger the structural switch from the primary structure to the secondary structure. The secondary structure’s state is always known due to the periodic measurements. Upon switching to the secondary structure, the difference between the old and new structures can be fedforward within the device. This way the WIP impact due to the switching will be minimized.

[0011] The invention in a second aspect provides an exposure apparatus operable to perform the method of the first aspect. The invention is a third aspect provides a semiconductor device manufactured using the exposure apparatus of the second aspect.

[0012] The invention in a third aspect provides a computer program comprising program instructions operable to perform the method of the first aspect, when run on a processing apparatus.

[0013] The above and other aspects of the invention will be understood from 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 shows a plan view of a substrate table which supports a wafer and two fiducial plates according to the prior art;

[0018] Figure 4 schematically illustrates a transmission image sensor according to the state of the art; [0019] Figure 5 schematically illustrates a single-field wavefront sensor according to the state of the art;

[0020] Figure 6 illustrates schematically a wafer-in-process (WIP) impact resulting from hardware swaps and/or maintenance actions during wafer processing;

[0021] Figure 7 is a flowchart describing a method of monitoring an exposure process; and

[0022] Figure 8 is a flowchart describing a method of determining whether to stop using redundant reference structures. DETAILED DESCRIPTION OF EMBODIMENTS

[0023] 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.

[0024] Figure 1 schematically depicts an exposure apparatus such as 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 supports (e.g., a wafer stage) 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.

[0025] 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.

[0026] The patterning device support MT holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support MT may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0027] 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.

[0028] 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.

[0029] 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”.

[0030] 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 fdl 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.

[0031] 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.

[0032] 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.

[0033] 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 support 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.

[0034] 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 alignment marks be as small as possible and not require any different imaging or process conditions than adjacent features. The alignment system, which detects the alignment marks is described further below.

[0035] The depicted apparatus could be used in a variety of modes. In a scan mode, the patterning device support (e.g., mask stage) MT and the substrate support 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 support 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 support WT is moved or scanned.

[0036] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0037] Lithographic apparatus LA is of a so-called dual stage type which has two substrate supports Wta, WTb and two stations - an exposure station EXP and a measurement station MEA - between which the substrate supports can be exchanged. While one substrate on one substrate support is being exposed at the exposure station, another substrate can be loaded onto the other substrate support 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 marks on the substrate using an alignment sensor AS. If the position sensor IF is not capable of measuring the position of the substrate support 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 support to be tracked at both stations, relative to reference frame RF. Other arrangements are known and usable instead of the dualstage arrangement shown. For example, other lithographic apparatuses are known in which a substrate support and a measurement support are provided. These are docked together when performing preparatory measurements, and then undocked while the substrate support undergoes exposure.

[0038] 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 supports 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.

[0039] 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.

[0040] 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.

[0041] 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 support 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.

[0042] 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.

[0043] 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.

[0044] 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 supports and substrates themselves. Accordingly, once the tables have been swapped, determining the relative position between projection system PS and substrate support 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.

[0045] By using the alignment data and height map obtained at the measuring station in the performance of the exposure steps, these patterns are accurately aligned with respect to the desired locations, and, in particular, with respect to features previously laid down on the same substrate. The exposed substrate, now labeled W” is unloaded from the apparatus at step 220, to undergo etching or other processes, in accordance with the exposed pattern.

[0046] 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.

[0047] In the flow of Figure 2, on the measurement side, the wafer alignment data measured at step 202 and the wafer height/leveling data measured at step 204 are both measured with respect to a reference arrangement or fiducial. For example, the fiducial may comprise one or more fiducial sensors and a plurality of reference structures (e.g., a set of marks) on a wafer stage or chuck (more generally a substrate support) and a mask stage (more generally a patterning device support).

[0048] Figure 3 is a plan view of a substrate table which supports a wafer and two fiducial plates according to the prior art. In this example arrangement, each fiducial plate FP1, FP2 comprises two reference structures RSI, RS2 and one fiducial sensor FS1, FS2. The reference structures RSI, RS2 may be configured for wafer stage measurement at the measurement side while the fiducial sensors FS1, FS2 may be configured for reticle alignment at the exposure side. The fiducial plates FP1, FP2 may be mounted to the top surface of the substrate table WT (e.g., which may be an example of the substrate tables WTa or WTb of the apparatus depicted in Figure 1). For example, the fiducial plates FP1, FP2 may be mounted at diagonally opposite positions outside the area covered by the wafer W. Each fiducial plate FP 1 , FP2 may be made of a highly stable material with a very low coefficient of thermal expansion, e.g., Invar, and has a flat reflective upper surface which carries the reference structures RSI, RS2 (e.g., gratings) used in alignment processes. Fiducial sensors FS1, FS2 are used to determine directly the vertical and horizontal position of the aerial image of the projection lens. Further information about fiducials can be gleaned in the U.S. patent US7019815B2, which is hereby incorporated by reference in its entirety.

[0049] On the measurement side, an alignment sensor and/or leveling sensor illuminates the one or more reference structures or marks on the fiducial surface during alignment and leveling (typically both will be performed; however the concepts disclosed herein are individually applicable to measurement- to-exposure matching in one or both dimensions parallel to the substrate plane and/or the direction perpendicular to the substrate plane). As a result, the aligned wafer position in the wafer plane is described with respect to the fiducial by the wafer alignment data (measurements of alignment structures or marks on the wafer) and/or the wafer position in the direction perpendicular to the wafer plane is described with respect to the fiducial by the leveling data. Typically more than one reference structure is measured on the fiducial, enabling the shape, and therefore any drift in this shape over time, of the fiducial surface to be characterized or modeled. This may be done using alignment modeling techniques, e.g., to measure and model an aligned position deviation (APD) with respect to nominal or expected positions of each mark assuming a perfectly flat surface.

[0050] On the exposure side, the reticle alignment step (e.g., step 212 of Figure 2) aligns the reticle with respect to the fiducial. This may be achieved by using the fiducial sensor on the fiducial to measure illumination (e.g., from the exposure source) via one or more reference structures or marks on the reticle and/or reticle stage. In addition to reticle stage positioning, other corrections may be performed at the exposure side, e.g., lens corrections based on lens aberration measurements (wavefront measurements) which may also be performed by the fiducial sensor.

[0051] Till now, several different types of sensors have been used as fiducial sensors. These may include transmission image sensors (TIS), single-field wavefront sensors (SWS), and multi-field wavefront sensors (MWS). Fiducials may be configured such that each fiducial sensor is located underneath one respective reference structure or mark (e.g., grating) and configured to measure the amount of light passing through the reference structure or mark. Referring back to Figures 1 and 2, in the reticle alignment step 212, the light source SO emits exposure radiation beam B which passes through the beam delivery system BD, the illuminator IL, the reticle MA, the projection system PS (and optionally immersion liquid) before being incident on the fiducial plate (not shown) which is mounted on the substrate support (or wafer stage or chuck) WTa, WTb. The substrate support WTa, WTb moves and tilts until the fiducial sensors in the fiducial plate detect a maximum amount of light. How to move and tilt is based on the retrieved measurement data. Once the detected light is maximized at the fiducial sensors, the reticle alignment is completed. TIS, SWS and MWS are described below.

[0052] A TIS is a sensor that is used to measure the position at substrate level of a projected aerial image of a mark pattern at the mask (reticle) level. The projected image at substrate level may be a line pattern with a line width comparable to the wavelength of the exposure radiation. The TIS measures these mask patterns using a transmission pattern with a photocell underneath it. The sensor data may be used to measure the position of the mask with respect to the substrate support in six degrees of freedom (three in translation and three in rotation).

[0053] Figure 4 schematically illustrates a transmission image sensor (TIS) according to the state of the art. The projection beam PB is incident on a first object for example a first grating GO in the mask MA. The first grating GO comprises a plurality of openings arranged for creating an image from the projection beam PB. The openings in the first grating GO each emit a radiation beam originating from the projection beam PB. The radiation beams emitted by the plurality of openings in first grating GO, pass through a lens for example, the projection lens system PS. The optical properties of such projection lens system are such that an image GO’ of first grating GO, is formed at a given plane below the projection lens system PS. The TIS is positioned below the projection lens system PS. The TIS comprises a slot pattern G1 and a photo sensor PH device. The slot pattern G1 is an opening over the photo sensor PH device which has the shape of a slit or a square. Advantageously, applying a pattern on the opening over the photo sensor PH device increases the number of edges which may increase the signal level and thus the signal/noise ratio of the photo sensor PH.

[0054] The TIS may be arranged on a substrate support, such as the substrate table WT depicted in Figure 3 or the substrate tables WTa or WTb of the apparatus depicted in Figure 1. The TIS allows accurate positioning of the wafer relative to the position of the projection lens system PS and the mask MA in three orthogonal directions X, Y, and Z. By scanning along these three directions the intensity of the image GO’ can be mapped as a function of the XYZ position of the transmission image detector, for example in an image map (a 3D map), which comprises the coordinates of sampling locations and the intensity sampled at each location. From the 3D map, a processor connected to the TIS can derive the position of the image by using for example a parabolic fit of the top position using a least squares fitting method. Further information about TIS provided on a lithography scanner apparatus can be gleaned in the U.S. patent US8482718B2, which is hereby incorporated by reference in its entirety.

[0055] A SWS is an interferometric wavefront measurement system that may perform static measurements at wafer level on the wavefront of the exposure radiation beam (e.g., the radiation beam B shown in Figure 1). The wavefront measurement data may be used to determine the aberrations of a lens or lens assembly (e.g., the projection system PS of the apparatus depicted in Figure 1) up to a high order. In the meantime, the wavefront measurement data also allows the position of the reticle stage (e.g., the patterning device support MT of the apparatus depicted in Figure 1) with respect to the SWS (and therefore the fiducial) to be determined. A SWS may be implemented as an integrated measurement system used for system initialization and calibration. Alternatively, it may be used for monitoring and recalibration “on-demand”.

[0056] Figure 5 is schematically illustrates a SWS according to the state of the art. This SWS 20 has a shearing grating structure 21 as a radiation-receiving element, supported by a transmissive plate 22, which may be made of glass or quartz. A quantum conversion layer (or fluorescent layer) 23 is positioned immediately above a camera chip 25 (a radiation-detecting element), which is in turn mounted on a substrate 28. The substrate 28 is connected to the transmissive plate 22 via spacers 26 and bonding wires 27 that connect the radiation-detecting element to external instrumentation. An air gap is located between the quantum conversion layer 23 and the transmissive plate 22. Further information on SWS provided on a lithography scanner apparatus can be gleaned from U.S. patents US7333216B2, US6650399B2, US8947637B2, and US7282701B2, which are hereby incorporated by reference in their entirety.

[0057] In contrast to a SWS which measures at a single field point, a MWS allows for a parallel measurement of the wavefront of the exposure radiation beam at multiple field points throughout the projection (or exposure) slit and therefore increases throughput by reducing the time taken for measurements. The output of a wavefront measurement may comprise an array of Zemike coefficients for each measured field point. The MWS may contribute to the overlay, focus, imaging, and throughput performance of the scanner. Further information about MWS provided on a lithography scanner apparatus can be gleaned from the international patent application publication W02022/100930A, which is hereby incorporated by reference in its entirety.

[0058] In performing these measurement steps 202, 204, 212, each with respect to the fiducial, the position of the reticle with respect to the wafer can be determined.

[0059] The exposure side metrology uses the measurement side metrology as a baseline and assumes that the measurements are correct. For example, the exposure side will correct exposure side measurements and/or models (e.g., for reticle shape and/or lens aberrations) based on the fiducial surface shape measured on the measurement side.

[0060] An initial calibration step, sometimes referred to as measurement-to-exposure matching, is typically performed during set-up of the lithographic apparatus. Simplistically, this matching is done using “stage alignment” measurements on the measurement side, and “reticle alignment” measurements on the exposure side. This matching calibrates any difference between the exposure side fiducial metrology to the measurement side fiducial metrology due to the different methods used at each side. During production, the fiducial is subject to temporal drift. By way of example and referring back to Figure 3, the reference structures (e.g., RSI or RS2) of the fiducial (e.g., FP1 or FP2) are used by the alignment sensor at the measurement side to do the horizontal wafer stage alignment. Such reference structures are prone to degradation after being exposed to DUV light. Degradation of these reference structures (e.g., RSI or RS2) causes the measurement side of the scanner to provide an incorrect reference state for the (e.g., lens) calibration at the exposure side. This is because the scanner assumes that the measured fiducial will be seen the same on the measurement side and exposure side, such that the same fiducial temporal drift is captured.

[0061] However, because the measurement methods and measured structures on each side differ this is not always the case, and differences in the measured drift will be detected on each side in practice. In particular, because only the measurement side measures (reference structures or marks (e.g., RSI or RS2) on) the surface of the fiducial, rather than using cameras/detectors (e.g., TIS, SWS, or MWS) within the fiducial, anything which degrades this surface and/or the marks thereon will be detected only on the measurement side. Such degradation includes degradation of the marks themselves and/or of the fiducial coating.

[0062] Once the measurement side metrology is modeled, the modeled data will comprise a main fiducial description component describing the fiducial surface shape and an additional nuisance component resulting from the fiducial degradation. On the exposure side, this additional nuisance component will not be measured, only the main component. The exposure side control will therefore incorrectly assume that this additional nuisance component has resulted from exposure side error (e.g., reticle, lens and/or reticle stage error) and determine erroneous corrections for this (e.g., via reticle, lens and/or reticle stage control/actuation). These erroneous “corrections” will be directly responsible for imaging errors in the final product.

[0063] An exposure 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 fiducial (e.g., the reference structures or marks thereon) of a scanner degrades and requires periodic replacement. Such hardware swaps and/or maintenance actions, can result in a performance impact. Replacing a hardware component (e.g., a fiducial) can cause a long scanner downtime and may not be the most cost-effective way to address the issue (particularly for those expensive components such as fiducials). Thus, many hardware components in the scanner (e.g., fiducials) are provided with redundant reference structures or marks. When primary (or currently used) reference structures or marks are found to have degraded, redundant reference structures or marks will be used in placement of the degraded ones for subsequent wafer processing. Since changing from the primary reference structures or marks to the redundant reference structures or marks can be achieved solely by software, a hardware swap is no longer necessary (before the redundant reference structures or marks reach their end of life). The use of redundant reference structures or marks reduces the scanner downtime and increases the lifespan of the fiducial (and thus reduces the cost).

[0064] However, the issue with such an approach is that it is unable to accurately determine when the primary reference structures or marks start to degrade and when the redundant marks should be used. Changing from the primary marks to the redundant marks too late results in more wafers in process being negatively impacted, whereas swapping them too early effectively shortens the lifespan of the fiducial as the imaging errors in the final product resulting from the degraded fiducial may still be acceptable (i.e. within specification). [0065] Overlay is an important parameter which describes the proper placement of a layer with respect to a previously exposed (lower) layer. The degradation of the primary reference structures or marks of a fiducial (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. The APC loop is indiscriminate of where the errors come from. It is an aggregated correction of all necessary corrections, from all scanner modules and process related components. This means that if one component is swapped, the contribution of that component to APC is unknown. So, the correct APC level cannot be easily determined. Also, APC is slow. It is averaged over many wafers and captures the average effect over a longer period of time. If the loop is reset, it will take, in some cases, multiple days to get back to the optimal level of correction. [0066] When the primary reference structures or marks are degraded, the resultant disturbances are captured and corrected by the CEs in APC. At some point, APC alone is unable to catch these errors, because the temporally and spatially high-frequent NCEs also become large. At this moment, a switch to redundant reference structures or marks is performed. In this case, even though the NCEs are minimal, the CEs used for the primary reference structures or marks are no longer valid. If the same CEs are used, the level of correction applied will be incorrect. In order to mitigate this, APC is reset, and dummy wafers are run in order to capture the correct APC amount. The running of these dummy wafers, and the need to do an APC reset is known as WIP impact. Such a performance impact is conceptually illustrated in Figures 6(a) and 6(b).

[0067] Referring to Figure 6(a), the lines for the first layer LI and second layer L2 represent a high frequency component of wafer alignment impact of degraded primary reference structures or marks of a fiducial. 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 in each layer will be small.

[0068] By contrast, Figure 6(b) conceptually illustrates the situation should there be a switch or move from primary reference structures or marks to redundant reference structures or marks between exposure of layer LI and L2. The switch or move to redundant reference structures or marks results in an overall smaller wafer alignment impact due to fiducial imperfection; however the impact of the degraded primary reference structures or marks 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).

[0069] One strategy to mitigate the 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 (wafers with only some of the required layers exposed) at the time of the action. 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. This ramping down of layers represents a loss of productivity with respect to continuing wafer production at the rate prior to beginning ramp-down.

[0070] 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 ramped down layer before production of layers higher in the stack can be started.

[0071] The implementation of ramping-down and ramping-up the exposure of a number of layers may allow the APC correction to better converge to the needed correction.

[0072] Another strategy to mitigate the WIP impact is the so called WIPless approach where a first snapshot of the scanner is taken prior to the maintenance action and upon completion of the maintenance action, a second snapshot of the scanner is taken. Then, the difference between the first and second snapshots is determined and subsequently fed-forward to the scanner. More specifically, the first snapshot of the scanner contains the information about a correction amount which has been applied by the APC prior to the maintenance action to correct a shift amount (e.g., an overlay error) of the exposure result from a target exposure result due to the fiducial degradation. After the maintenance action and with a new fiducial installed, the impact of the fiducial degradation on the exposure result is no longer present but the previously applied APC corrections are still in place. As explained above in connection with Figures 6(a) and 6(b), this results in an increase of the shift amount of the exposure result from the target result. The difference between the pre-maintenance shift amount and the post-maintenance shift amount is used to determine an offset correction amount which will then be fed-forward to the scanner to compensate for the performance change before and after the maintenance action. As such, the scanner accepts the old APC as if no maintenance action was performed. Further information about such a WIPless approach can be gleaned from the U.S. patent US11385549B2, which is hereby incorporated by reference in its entirety.

[0073] To address the problem with the prior art, a method of monitoring an exposure process is proposed. The proposed method addresses the issue by actively monitoring the state of the redundant reference structures (e.g., of a fiducial) when the primary structures (e.g., of the same fiducial) are in use for a monitoring action beam of the exposure process during lithographic exposures. The scanner may periodically or intermittently measure the redundant reference structures. After the primary reference structures and the redundant reference structures are both measured, the scanner may compare the two measurements to determine a measurement deviation which may reflect a state of degradation (e.g., of the fiducial). When such a measurement deviation becomes too large, the scanner may be triggered to initialize an action (e.g., a software action) to change from the primary reference structures to the redundant (clean) reference structures. The state of the redundant reference structures is always known due to the periodic or intermittent measurements. Upon switching to the redundant reference structures or marks, the deviation between the two measurements may be fed-forward to the scanner. As such, the WIP impact due to the swap of the reference structures may be minimized or avoided.

[0074] While the description below is largely described in terms of substrate alignment and/or reticle alignment and reference structures therefor; e.g., at least one first reference structure and (redundant or backup) at least one second reference structure, the concepts disclosed are not necessarily limited to only such arrangements. The concepts may relate to any monitoring action performed as part of an exposure process and/or using an exposure apparatus, which uses at least one (first) reference structure and, for example, where the at least one (first) reference structure is subject to degradation and/or drift over time, and where at least one redundant or backup (second) reference structure is provided. The at least one redundant or backup (second) reference structure may be provided to take the place of the at least one (first) reference structure (e.g., to be measured or otherwise referenced instead of the at least one (first) reference structure); for example when the at least one (first) reference structure is deemed to have degraded and/or drifted too much.

[0075] Figure 7 is a flowchart describing a method of monitoring an exposure process in accordance with an embodiment. In this embodiment, the method 700 may comprise the following four main steps 710-740 and may be carried out in an exposure apparatus or scanner.

[0076] At step 710, the exposure apparatus may be operable to perform a monitoring action of the exposure process using at least one first reference structure to obtain a set of first monitoring data, the at least one first reference structure being in use for the monitoring action during lithographic exposures. The monitoring action may comprise monitoring a radiation beam or a state of a component of the exposure apparatus that directly affects the performance of lithographic exposures.

[0077] The at least one first reference structure may comprise at least one reflective grating and/or at least one transmissive grating. Referring back to Figure 3, the at least one first reference structure may comprise reference structures RSI, RS2; e.g., reflective gratings configured for wafer stage alignment at a measurement side of the exposure apparatus. Alternatively, the at least one first reference structure may comprise the gratings of the fiducial sensors (e.g., grating G1 of the TIS shown in Figure 4, grating 21 of the SWS shown in Figure 5); e.g., transmissive gratings configured for transmitting at least a portion of the incident radiation beam.

[0078] In an embodiment, the at least one first reference structure may be comprised in a fiducial mounted on a substrate support (e.g., as shown in Figure 3). In other embodiments, the at least one first reference structure may be comprised in a different component of the exposure apparatus, such as for example a patterning device (e.g., the mask MA shown in Figure 1).

[0079] Step 710 may comprise measuring the at least one first reference structure during a measurement action of the exposure process to obtain the set of first monitoring data. For example, the measuring may be performed at the measurement side of the exposure apparatus using a sensor such as an alignment sensor or level sensor. The set of first monitoring data may provide a calibration reference for measurements performed during an exposure action of the exposure process. The set of first monitoring data may comprise metrology data in relation to a position of the at least one first reference structure.

[0080] The measurements performed during the exposure action of the exposure process may comprise, for example, measurements of a position of a patterning device with respect to the fiducial (see reticle alignment described above). Alternatively or additionally, the measurements performed during the exposure process may comprise measurements of one or more optical parameters of a radiation beam used to perform the exposure action. The one or more optical parameters of the radiation beam may comprise, for example, a wavefront of the radiation beam.

[0081] The measurements performed at the exposure side of the exposure apparatus may be made using one or more sensors comprised in the fiducial. For example, the fiducial may comprise at least one TIS configured for measuring and aligning a position of a patterning device with respect to the fiducial.

[0082] Alternatively or additionally, the fiducial may comprise at least one SWS or MWS configured for measuring a wavefront of the radiation beam in the exposure apparatus. The obtained wavefront data may be used to determine aberrations of a lens or a lens assembly (e.g., the projection system PS shown in Figure 2) of the exposure apparatus, based on which corrections may be further determined and subsequently applied to the lens or lens assembly during subsequent lithographic exposures. Since the wavefront measurements at the exposure side are made by the fiducial sensors rather than the at least one first reference structure, the degradation of the at least one first reference structure detected at the measurement side may not be detected equally (or at all) at the exposure side. Consequently, using the set of first monitoring data (e.g., fiducial metrology data) obtained from the measurement side to provide a calibration reference for the measurements performed at the exposure side may lead to imaging errors in the final product. This issue may be addressable by replacing the degraded (first) reference structures with clean redundant (second) reference structures (see method steps below).

[0083] At step 720, the exposure apparatus may be operable to perform the monitoring action of the exposure process using at least one second reference structure to obtain at least one set of second monitoring data, the at least one second reference structure not being in use for the monitoring action during lithographic exposures. It may not be necessary to perform step 720 every time step 710 is performed. Step 720 may be performed in an intermittent or periodical manner (e.g., in the case where the at least one second reference structure is used as redundant structures). [0084] The at least one second reference structure may be substantially identical to the at least one first reference structure in terms of design parameters (e.g., dimensions, materials). The at least one second reference structure may be configured to provide redundancy to the at least one first reference structure. The at least one second reference structure may be comprised in the same component (e.g., fiducial) of the exposure apparatus as the at least one first reference structure.

[0085] In an embodiment, the at least one first reference structure and the at least one second reference structure may both be comprised in a fiducial mounted on a substrate support. In this embodiment, step 720 may comprise measuring the at least one second reference structure either during the measurement action of the exposure process or periodically (e.g., measuring the second reference structures once per day) to obtain at least one set of second monitoring data. For example, the measuring may be performed at the measurement side of the exposure apparatus using a sensor such as an alignment sensor or level sensor. The at least one set of second monitoring data may comprise metrology data in relation to a position of the at least one second reference structure. The at least one second reference structure may be measured repetitively, e.g., in a periodical or intermittent manner. Thus, each individual measurement of the at least one second reference structure may result in one set of the at least one set of second monitoring data.

[0086] In the embodiment where the at least one first reference structure and the at least one second reference structure are both comprised in a fiducial mounted on a substrate support, step 720 may comprise periodically measuring the at least one second reference structure by an alignment sensor or level sensor at the measurement side of the exposure apparatus to obtain a plurality of sets of second monitoring data. Each set of second monitoring data may comprise metrology data in relation to a position of the at least one second reference structure.

[0087] When the at least one second reference structure are used as redundant reference structures, the at least one set of second monitoring data are typically not used for the monitoring action during lithographic exposures. For example, the at least one set of second monitoring data are typically not used to provide a calibration reference for the measurements performed during an exposure action of the exposure process and therefore may not be involved in the exposure process.

[0088] At 730, the exposure apparatus may be operable to compare the set of first monitoring data to the at least one set of second monitoring data. For example, this comparing may comprise determining a comparison value, e.g., a difference value or any other suitable comparison metric for comparing the two datasets).

[0089] In the case where the at least one second reference structure are periodically measured, the exposure apparatus may compare the set of first monitoring data with every set of second monitoring data after the data has been obtained.

[0090] In an embodiment, step 730 may comprise determining a first position of the at least one first reference structure and a second position of the at least one second reference structure; and determining a difference value between the first position and the second position. [0091] At step 740, the exposure apparatus may be operable to determine, based on the comparing of step 730, whether to change from the at least one first reference structure to the at least one second reference structure for the monitoring action during further lithographic exposures.

[0092] In an embodiment, step 740 may comprise comparing a comparison (e.g., difference) value determined at step 730 to a threshold value. If the determined comparison value is equal to or greater than the threshold value, the exposure apparatus may be operable to change from the at least one first reference structure to the at least one second reference structure. A larger comparison (e.g., difference) value is indicative that the at least one first reference structure have degraded and/or drifted with respect to the at least one second reference structure, and therefore should be replaced.

[0093] After having changed from the at least one first reference structure to the at least one second reference structure for the monitoring action during further lithographic exposures, the at least one set of second monitoring data (e.g., the most recent set of second fiduciary metrology data) may then be used to provide a calibration reference for measurements performed during the exposure action of the exposure process. The exposure apparatus may be operable to determine a correction for the exposure process based on the determined comparison value.

[0094] In some embodiments, method 700 may further comprise a step of determining whether to stop using the at least one second reference structure for the monitoring action during further lithographic exposures.

[0095] In an embodiment, such determination may be based on a predefined lifespan of the at least one second reference structure.

[0096] In an alternative embodiment, whether to stop using the at least one second reference structures may be determined by a different method, e.g., method 800 shown in Figure 8. Method 800 may comprise the following four steps 810-840:

[0097] Step 810: performing the monitoring action of the exposure process using the at least first reference structure to obtain at least one set of third monitoring data, the at least one first reference structure not being in use for the monitoring action during lithographic exposures.

[0098] Step 820: performing the monitoring action of the exposure process using the at least one second reference structure to obtain the at least one set of second monitoring data, the at least one second reference structure being in use for the monitoring action during lithographic exposures.

[0099] Step 830: determining a comparison (e.g., difference) value by comparing the at least one set of third monitoring data to the at least one set of second monitoring data.

[0100] Step 840: determining, based on the determined comparison value, whether to stop using the at least one second reference structures for the monitoring action during further lithographic exposures. For example, the decision to stop using the at least one second reference structure may occur when the determined comparison value is small and/or below a second threshold value (i.e., when the (originally redundant) second structures are as degraded as the (primary or original) first reference structures), [0101] When the at least one second reference structure have reached or exceeded the predefined lifespan, the exposure apparatus may indicate a need of scheduling and/or performing a maintenance action on the at least one first reference structures and the at least one second reference structures and/or to replace both reference structures. In some cases, this may mean a maintenance action and/or a replacement of a component or module of the exposure apparatus which comprises both sets of degraded reference structures.

[0102] It will be appreciated that method 700 may be equally applicable to situations where the at least one first reference structure are not used at the measurement side but at the exposure side instead. For example, the at least one first reference structure may be the transmissive gratings of fiducial sensors (e.g., grating G1 of the TIS shown in Figure 4, grating 21 of the SWS shown in Figure 5). Since fiducial sensors are only used for measurements performed at the exposure side, the degradation of their transmissive gratings is not detectable by the alignment sensor at the measurement side. As such, the set of first monitoring data provides a correct calibration reference. However, when such degraded fiducial sensors are used to measure e.g., one or more parameters of the radiation beam, the degradation is detectable in the measurements at the exposure side. This again results in differences between the measurement side and the exposure side and thus imaging errors in the final product.

[0103] The concepts herein are equally applicable to a single stage exposure apparatus or scanner used in combination with a stand-alone alignment tool or alignment station, where the alignment station is the measurement side and the scanner is the exposure side. As such, an exposure apparatus arrangement may comprise a dual stage exposure apparatus or single stage exposure apparatus with stand-alone alignment station.

[0104] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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

1. A method of monitoring an exposure process, comprising: obtaining a set of first monitoring data relating to a performance of a monitoring action of the exposure process using at least one first reference structure, the at least one first reference structure being in use for the monitoring action during exposures; obtaining at least one set of second monitoring data relating to a performance of the monitoring action of the exposure process using at least one second reference structure, the at least one second reference structure not being in use for the monitoring action during exposures; comparing the set of first monitoring data to the at least one set of second monitoring data; and determining, based on the comparing, whether to change from the at least first reference structure to the at least one second reference structure for the monitoring action during further exposures.

2. A method as claimed in claim 1, wherein the step of comparing the set of first monitoring data to the at least one set of second monitoring data comprises determining a first comparison value between the set of first monitoring data and the at least one set of second monitoring data.

3. A method as claimed in claim 2, wherein the step of determining whether to change from the at least one first reference structure to the at least one second reference structure comprises: comparing the determined first comparison value to a threshold value; and if the determined first comparison value is equal to or greater than the threshold value, changing from the at least one first reference structure to the at least one second reference structure.

4. A method as claimed in claim 2 or 3, wherein after having changed from the at least one first reference structure to the at least one second reference structure for the monitoring action during further exposures, the method further comprises determining a correction for the exposure process based on the determined first comparison value.

5. A method as claimed in any preceding claim, wherein the step of obtaining at least one set of second monitoring data, the step of comparing and the step of determining are performed periodically or intermittently, each set of the at least one set of second monitoring data resulting from a respective monitoring action.

6. A method as claimed in any preceding claim, wherein each of the at least one first reference structure and the at least one second reference structure comprises at least one reflective grating and/or at least one transmissive grating.

7. A method as claimed in any preceding claim, wherein the at least one first reference structure and the at least one second reference structure are both comprised on a fiducial mountable on a substrate support of an exposure apparatus.

8. A method as claimed in claim 7, wherein the step of obtaining a set of first monitoring data comprises measuring the at least one first reference structure during a measurement action of the exposure process to obtain the set of first monitoring data, thereby providing a calibration reference for measurements performed during an exposure action of the exposure process.

9. A method as claimed in claim 8, wherein the step of obtaining at least one set of second monitoring data comprises measuring the at least one second reference structure during the measurement action of the exposure process to obtain said at least one set of second monitoring data.

10. A method as claimed in claim 8 or 9, wherein after having changed from the at least one first reference structure to the at least one second reference structure for the monitoring action during further exposures, the method further comprises using the at least one set of second monitoring data to provide the calibration reference for measurements performed during the exposure action of the exposure process.

11. A method as claimed in claims 8 to 10, wherein the set of first monitoring data comprises metrology data in relation to a position of the at least one first reference structure and the at least one set of second monitoring data comprises metrology data in relation to a position of the at least one second reference structure.

12. A method as claimed in any of claims 8 to 11, wherein the measurements performed during the exposure action of the exposure process comprise measurements of a position of a patterning device with respect to the fiducial.

13. A method as claimed in any of claims 8 to 12, wherein the measurements performed during the exposure action of the exposure process comprise measurements of one or more optical parameters of a radiation beam used to perform the exposure action.

14. A method as claimed in claim 13, wherein the one or more optical parameters of the radiation beam comprises a wavefront of the radiation beam.

15. A method as claimed in any of claims 8 to 14, wherein the measurements performed on the exposure side of the exposure apparatus are made by one or more following sensors of the fiducial: at least a transmission imaging sensor (TIS); at least a single-field wavefront sensor (SWS); and at least a multi-field wavefront sensor (MWS),

16. A method as claimed in any preceding claim, wherein after having changed from the at least one first reference structure to the at least one second reference structure for the monitoring action during further exposures, the method further comprises determining whether to stop using the at least one second reference structure for the monitoring action during further exposures based on a predefined lifespan of the at least one second reference structure.

17. A method as claimed in any of claims 1 to 15, wherein after having changed from the at least one first reference structure to the at least one second reference structure for the monitoring action during further exposures, the method further comprises: performing the monitoring action of the exposure process using the at least one first reference structure to obtain at least one set of third monitoring data, the at least one first reference structure not being in use for the monitoring action during exposures; performing the monitoring action of the exposure process using the at least one second reference structure to obtain the at least one set of second monitoring data, the at least one second reference structure being in use for the monitoring action during exposures; determining a second comparison value by comparing the at least one set of third monitoring data to the at least one set of second monitoring data; and determining, based on the determined second comparison value, whether to stop using the at least one second reference structure for the monitoring action during further exposures.

18. A method as claimed in claim 16 or 17, further comprising scheduling and/or performing a maintenance action on the at least one first reference structure and the at least one second reference structure and/or to replace both based on said determining step.

19. An exposure apparatus operable to perform the method of any preceding claim.

20. A semiconductor device manufactured using the exposure apparatus of claim 19.