Method of inline metrology in a lithographic process

Abstract

Disclosed is a method of determining exposure induced overlay errors comprising: exposing a reticle feature onto a sensor feature comprised on a sensor, wherein the reticle feature comprises at least one first reticle sub-feature comprising a first periodic pattern and at least one second reticle sub-feature comprising a second periodic pattern, and said sensor feature is complementary to said reticle feature. First and second signals resultant from said first reticle sub-feature being exposed onto said sensor feature said second reticle sub-feature being exposed onto said sensor feature are obtained. a first exposure induced overlay error in a first direction and a second exposure induced overlay error in a second direction are determined from said first signal and said second signal.

Description

METHOD OF INLINE METROLOGY IN A LITHOGRAPHIC PROCESSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24217159.3 which was filed on 03 December 2024, and which is incorporated herein in its entirety by reference.FIELD OF THE INVENTION

[0002] The present invention relates to methods and apparatus usable, for example, in the manufacture of devices by lithographic techniques, and to methods of manufacturing devices using lithographic techniques.BACKGROUND

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

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

[0005] In other applications, metrology sensors are used for measuring exposed structures on a substrate (either in resist and / or after etch). A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. In addition to measurement of feature shapes by reconstruction, diffraction basedoverlay 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 andWO2013178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.

[0006] 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 plate of a fiducial sensor arrangement. At the expose side, the position and shape of the reticle is measured relative to the fiducial plate. In this manner, a common reference frame between measure side and expose side is established. This reference frame may be used to align the measure side wafer measurements to the required expose side exposure actuations.

[0007] Presently, metrology to monitor a parameter of interest such as overlay is typically performed post-exposure. This may comprise exposing the pattern in-resist on a substrate and measuring the exposed in-resist pattern using, for example, a scatterometer.

[0008] It would be desirable to perform in-line metrology for a parameter of interest such as overlay.SUMMARY OF THE INVENTION

[0009] The invention in a first aspect provides a method of determining exposure induced overlay errors relating to an exposure apparatus, the method comprising: exposing at least a portion of a reticle feature, comprised on a reticle, onto a sensor feature comprised on a sensor, wherein the reticle feature comprises at least one first reticle sub-feature comprising a first periodic pattern and at least one second reticle sub-feature comprising a second periodic pattern, and said sensor feature is complementary to said reticle feature; obtaining a first signal resultant from said first reticle sub-feature being exposed onto said sensor feature and captured by said sensor; obtaining a second signal resultant from said second reticle sub-feature being exposed onto said sensor feature and captured by said sensor; and determining a first exposure induced overlay error in a first direction and a second exposure induced overlay error in a second direction from said first signal and said second signal.

[0010] The invention in a first aspect provides a reticle for exposing onto a sensor using an exposure apparatus, the reticle comprising at least one reticle feature, wherein the reticle feature comprises: at least one first reticle sub-feature comprising a first periodic pattern; and at least one second reticle subfeature comprising a second periodic pattern; wherein the reticle feature extends over substantially over a full extent of a reticle imaging area in at least a second direction of the exposure apparatus, wherein afirst direction of the exposure apparatus is the scan direction of the exposure apparatus and said second direction is the slit direction of the exposure apparatus.

[0011] The invention in a first aspect provides a sensor for an exposure apparatus comprising a sensor feature, wherein the sensor feature comprises: at least one first sensor sub-feature comprising a first periodic pattern; and at least one second sensor sub-feature comprising a second periodic pattern.

[0012] Also disclosed is a computer program and lithographic or exposure apparatus being operable to perform the method of the first aspect.

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

[0014] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:Figure 1 depicts a lithographic apparatus;Figure 2 illustrates schematically measurement and exposure processes in the apparatus of Figure 1; andFigure 3(a) schematically illustrates a reticle feature usable for methods disclosed herein and Figure 3(b) schematically illustrates a complementary sensor feature;Figure 4 shows example signal plots obtained when exposing respective sub-features of the reticle feature of Figure 3(a) onto the complementary sensor feature of Figure 3(b);Figure 5(a) schematically illustrates a reticle feature and specific metrology method using the same, according to a first example, and Figure 5(b) is a plot of an intensity signal corresponding to this first example;Figure 6 schematically illustrates a reticle feature and specific metrology method using the same, according to a second example, and Figure 6(b) is a plot of an intensity signal corresponding to this second example;Figure 7 schematically illustrates a reticle feature and specific metrology method using the same, according to a third example, and Figure 7(b) is a plot of an intensity signal corresponding to this third example;Figure 8 schematically illustrates a reticle feature and specific metrology method using the same, according to a fourth example, and Figure 8(b) is a plot of an intensity signal corresponding to this fourth example;Figure 9 schematically illustrates a reticle feature usable for methods disclosed herein and adapted to provide for parallel dose metrology; andFigures 10(a) - 10(d) each schematically illustrates a portion of a reticle feature usable for methods disclosed herein.DETAILED DESCRIPTION OF EMBODIMENTS

[0015] Before describing embodiments of the invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0016] Figure 1 schematically depicts a lithographic apparatus LA. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a patterning device support or support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; two substrate tables (e.g., a wafer table) WTa and WTb each constructed to hold a substrate (e.g., a resist coated wafer) W and each connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W. A reference frame RF connects the various components, and serves as a reference for setting and measuring positions of the patterning device and substrate and of features on them.

[0017] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0018] The patterning device support MT holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support MT may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0019] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0020] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive patterning device). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term“patterning device.” The term “patterning device” can also be interpreted as referring to a device storing in digital form pattern information for use in controlling such a programmable patterning device.

[0021] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0022] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to 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.

[0023] In operation, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and / or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0024] The illuminator IL may for example include an adjuster AD for adjusting the angular intensity distribution of the radiation beam, an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross section.

[0025] The radiation beam B is incident on the patterning device MA, which is held on the patterning device support MT, and is patterned by the patterning device. Having traversed the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table Wta or WTb can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan.

[0026] 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 illustratedoccupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g., mask) MA, the mask alignment marks may be located between the dies. Small alignment marks may also be included within dies, in amongst the device features, in which case it is desirable that the markers be as small as possible and not require any different imaging or process conditions than adjacent features. The alignment system, which detects the alignment markers is described further below.

[0027] The depicted apparatus could be used in a scan mode, where the patterning device support (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. In so-called “maskless” lithography, a programmable patterning device is held stationary but with a changing pattern, and the substrate table WT is moved or scanned.

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

[0029] Lithographic apparatus LA is of a so-called dual stage type which has two substrate tables WTa, WTb and two stations - an exposure station EXP and a measurement station MEA - between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps carried out. This enables a substantial increase in the throughput of the apparatus. The preparatory steps may include mapping the surface height contours of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. If the position sensor IF is not capable of measuring the position of the substrate table while it is at the measurement station as well as at the exposure station, a second position sensor may be provided to enable the positions of the substrate table to be tracked at both stations, relative to reference frame RF. Each substrate table WTa, WTb may comprise a fiducial sensor arrangement FS for performing metrology to coordinate reticle alignment at the exposure station with wafer alignment at the measurement station. Other arrangements are known and usable instead of the dual-stage arrangement shown. For example, other lithographic apparatuses are known in which a substrate table and a measurement table are provided. These are docked together when performing preparatory measurements, and then undocked while the substrate table undergoes exposure.

[0030] Figure 2 illustrates the steps to expose target portions (e.g. dies) on a substrate W in the dual stage apparatus of Figure 1. On the left hand side within a dotted box are steps performed at ameasurement station MEA, while the right hand side shows steps performed at the exposure station EXP. From time to time, one of the substrate tables Wta, WTb will be at the exposure station, while the other is at the measurement station, as described above. For the purposes of this description, it is assumed that a substrate W has already been loaded into the exposure station. At step 200, a new substrate W’ is loaded to the apparatus by a mechanism not shown. These two substrates are processed in parallel in order to increase the throughput of the lithographic apparatus.

[0031] Referring initially to the newly-loaded substrate W’, this may be a previously unprocessed substrate, prepared with a new photo resist for first time exposure in the apparatus. In general, however, the lithography process described will be merely one step in a series of exposure and processing steps, so that substrate W’ has been through this apparatus and / or other lithography apparatuses, several times already, and may have subsequent processes to undergo as well. Particularly for the problem of improving overlay performance, the task is to ensure that new patterns are applied in exactly the correct position on a substrate that has already been subjected to one or more cycles of patterning and processing. These processing steps progressively introduce distortions in the substrate that must be measured and corrected for, to achieve satisfactory overlay performance.

[0032] The previous and / or subsequent patterning step may be performed in other lithography apparatuses, as just mentioned, and may even be performed in different types of lithography apparatus. For example, some layers in the device manufacturing process which are very demanding in parameters such as resolution and overlay may be performed in a more advanced lithography tool than other layers that are less demanding. Therefore some layers may be exposed in an immersion type lithography tool, while others are exposed in a ‘dry’ tool. Some layers may be exposed in a tool working at DUV wavelengths, while others are exposed using EUV wavelength radiation.

[0033] At 202, alignment measurements using the substrate marks Pl etc. and image sensors (not shown) are used to measure and record alignment of the substrate relative to substrate table Wta / WTb. In addition, several alignment marks across the substrate W’ will be measured using alignment sensor AS. These measurements are used in one embodiment to establish a “wafer grid”, which maps very accurately the distribution of marks across the substrate, including any distortion relative to a nominal rectangular grid.

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

[0035] When substrate W’ was loaded, recipe data 206 were received, defining the exposures to be performed, and also properties of the wafer and the patterns previously made and to be made upon it. To these recipe data are added the measurements of wafer position, wafer grid and height map that were made at 202, 204, so that a complete set of recipe and measurement data 208 can be passed to the exposure station EXP. The measurements of alignment data for example comprise X and Y positions of alignment targets formed in a fixed or nominally fixed relationship to the product patterns that arethe product of the lithographic process. These alignment data, taken just before exposure, are used to generate an alignment model with parameters that fit the model to the data. These parameters and the alignment model will be used during the exposure operation to correct positions of patterns applied in the current lithographic step. The model in use interpolates positional deviations between the measured positions. A conventional alignment model might comprise four, five or six parameters, together defining translation, rotation and scaling of the ‘ideal’ grid, in different dimensions. Advanced models are known that use more parameters.

[0036] At 210, wafers W’ and W are swapped, so that the measured substrate W’ becomes the substrate W entering the exposure station EXP. In the example apparatus of Figure 1, this swapping is performed by exchanging the supports Wta and WTb within the apparatus, so that the substrates W, W’ remain accurately clamped and positioned on those supports, to preserve relative alignment between the substrate tables and substrates themselves. Accordingly, once the tables have been swapped, determining the relative position between projection system PS and substrate table WTb (formerly Wta) is all that is necessary to make use of the measurement information 202, 204 for the substrate W (formerly W’) in control of the exposure steps. At step 212, reticle alignment is performed using the mask alignment marks Ml, M2. In steps 214, 216, 218, scanning motions and radiation pulses are applied at successive target locations across the substrate W, in order to complete the exposure of a number of patterns.

[0037] By using the alignment data and height map obtained at the measuring station in the performance of the exposure steps, these patterns are accurately aligned with respect to the desired locations, and, in particular, with respect to features previously laid down on the same substrate. The exposed substrate, now labeled W” is unloaded from the apparatus at step 220, to undergo etching or other processes, in accordance with the exposed pattern.

[0038] The skilled person will know that the above description is a simplified overview of a number of very detailed steps involved in one example of a real manufacturing situation. For example rather than measuring alignment in a single pass, often there will be separate phases of coarse and fine measurement, using the same or different marks. The coarse and / or fine alignment measurement steps can be performed before or after the height measurement, or interleaved.

[0039] In the 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 sensor arrangement. For example, the fiducial sensor arrangement may comprise a fiducial sensor on a wafer stage or chuck (more generally a substrate support) and a plurality of reference structures (e.g., a set of marks), comprising respectively one or more reference structures or marks on the substrate support (more specifically on a fiducial plate on the substrate support) and one or more reference structures or marks on the reticle and / or reticle stage.

[0040] On the measurement side, an alignment sensor and / or leveling sensor illuminates the one or more substrate support marks on the fiducial surface during alignment and leveling. As a result, thealigned 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 mark is measured on the fiducial plate, enabling the shape, and therefore any drift in this shape over time, of the fiducial plate 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.

[0041] On the exposure side, the reticle alignment step of step 212 aligns the reticle with respect to the fiducial sensor arrangement. This may be achieved by using the fiducial sensor on the fiducial sensor arrangement 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 expose side, e.g., lens corrections based on lens aberration measurements (wavefront measurements) which may also be performed by the fiducial sensor.

[0042] In performing these measurement steps 202, 204, 212, each with respect to the fiducial sensor arrangement, the position of the reticle with respect to the wafer can be determined.

[0043] 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 plate surface shape measured on the measurement side.

[0044] 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 in 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 plate is subject to temporal drift. As such, the measurement side metrology is performed routinely on the fiducial plate to characterize / model this drift. In this manner, any changes to the fiducial plate over time will be taken into account at the exposure side. Monitoring of this drift works under the assumption that the measured fiducial plate will be seen the same on the measurement side and exposure side, such that the same temporal fiducial position drift is captured.

[0045] These measurements have direct impact on overlay; any variation, e.g., such as may be induced by system noise, translates into overlay variance. The fiducial sensors are used to determine the relative alignment position of the reticle with respect to the wafer and to determine / calibrate imaging performance.

[0046] Presently, it is known to use the aforementioned fiducial sensor arrangements to measure contributions to a parameter of interest such as overlay resultant from exposure contributors (i.e., as opposed to contributions from the wafer table, wafer itself, resist etc.). For example, one such test may determine the impact of deflection of the pellicle during scanning on overlay. A pellicle is a very thinmembrane which acts as a physical barrier to prevent contamination of the reticle, and is typically used on some lithography scanners. Such a test requires the exposure of different layers on a wafer (e.g., for different pellicle deflections), developing the resist and measuring the overlay on a dedicated tool (e.g., a scatterometer). Alternatively, reference wafers can be used, the reference wafer comprising fixed marks already formed thereon. A layer of resist is then exposed on top of these fixed marks. Again, the resist needs to be developed prior to the overlay metrology.

[0047] This present procedure takes a lot of time, in particular due to the need to expose, develop and then measure patterns in-resist. Furthermore, a procedure using physical wafers means that the overlay measurements will also be dependent on factors other than those resultant from the exposure itself; e.g., in the pellicle example described, there may be overlay contributions from wafer clamping, properties of the resist, properties of the wafer etc., in addition to the pellicle deflection.

[0048] It is therefore desirable to be able to measure contributors to a parameter of interest such as overlay, as part of an inline calibration without the need for in-resist exposures.

[0049] A proposed method may use a feature on a reticle comprising at least a pair of sub-features, each said sub-feature comprising a differently oriented (e.g., oblique) periodic pattern. An oblique periodic pattern describes a periodic pattern wherein the individual periodic elements are aligned with neither the scan direction (Y -direction) nor the slit direction (X-direction). Each sub-feature may extend along the scan direction. The scan direction is the direction that the exposure slit of the scanner (exposure apparatus or lithographic apparatus) moves along during scanning and the slit direction is the longer dimension of the exposure slit (i.e., perpendicular to the scan direction).

[0050] The first sub feature and second sub-feature of each pair of sub-features may comprise mutually perpendicularly oriented oblique angles. For example, a first sub-feature of each pair of sub-features may comprise an oblique pattern at 45 degrees to the Y-direction and a second sub-feature of each pair of sub-features may comprise an oblique pattern at 135 degrees to the Y-direction.

[0051] The feature may comprise multiple pairs of sub-features, e.g., alternating between the first subfeature and second sub-feature in the slit direction. Adjacent sub-features may be spaced apart on the reticle, e.g., with blocking sub-features / regions therebetween to avoid or reduce crosstalk. Alternatively, the regions between adjacent sub-features may be used for monitoring other parameters of interest such as dose, e.g., they may be at least partially transparent / transmissive. By measuring the total dose simultaneously during exposure, it may be possible to correct e.g., for pulse-to-pulse variations of the source and / or dose inhomogeneity.

[0052] The proposed reticle feature may extend substantially over the whole of the imaging area of the reticle. However, in examples where the entire reticle is not scanned, this is not necessary.

[0053] In addition to the aforementioned oblique pattern, additional patterns per sub-feature may be included. For example, each sub-feature may comprise portions having different pattern orientation. Alternatively, or in addition, each sub-feature may include sub-segmentation (e.g., each individual periodic element of the pattern may be segmented in some manner) for at least some of the individualperiodic elements. Where sub-segmentation is provided, each sub-feature may comprise portions having different sub-segmentation. Alternatively, or in addition, each sub-feature may comprise portions having different periodicity. In this manner, different aberration features / components (e.g., Zemike components) may be addressed. Providing different periodicities also enables the Nonius principle to be employed to improve accuracy or capture.

[0054] Alternatively, or in addition, locally along the scan direction, the reticle sub-feature may also be varied in terms of unit cell, e.g., to provide an intensity modulation during scanning by subsegmentation. In doing this, the reticle sub-feature may comprise the same base grating pitch and orientation to match the sensor sub-feature, but having subsegmentation that is varied along the scan direction. This modulates intensity, and therefore address different Zemikes etc.

[0055] This reticle feature may be used with a sensor arrangement on the substrate stage comprising a complementary (e.g., matching) sensor feature. For example, the sensor arrangement may comprise the fiducial sensor arrangement described above. However, a dedicated sensor arrangement may be used instead. Note that the sensor arrangement being “on the substrate stage” also includes the possibility of the sensor arrangement being mounted on a carrier, such as a wafer, being supported by the substrate stage.

[0056] The sensor arrangement may comprise a 1-dimensional array, e.g., a detector (e.g., photodiode) for each sub-feature, to obtain an intensity signal per sub-feature. However, a 2D array may also be used, wherein outputs of two or more (e.g., relatively smaller) photodiodes (per sub-feature) are grouped together to sample an intensity signal per sub-feature. An advantage of this is that a small photodiode is faster than a larger photodiode. Therefore, a cluster of multiple small photodiodes replacing a single larger photodiode will be faster. However, each individual diode will need respective readout electronics.

[0057] The alignment sensor of the scanner may be used in a calibration step to calibrate the position of the sensor feature (e.g., a fiducial feature), in a similar manner as presently performed in stage alignment.

[0058] Alternatively, the fiducial feature could also be used to calibrate the alignment sensor position (Stage Alignment).

[0059] Figure 3(a) illustrates a reticle feature which may be comprised on a reticle, and Figure 3(b) a complementary sensor feature which may be located on a sensor.

[0060] The reticle feature 300 comprises at least one pair of reticle sub-features, and in the illustration shown, multiple repetitions of pairs of reticle sub-features 305, each pair of reticle sub-features comprising a respective first reticle sub-feature 310 and second reticle sub-feature 315. Each reticle sub-feature 310, 315 extends along the Y -direction (scan direction) with the multiple reticle sub-features 310, 315 repeating along the X-direction (slit direction).

[0061] Each first reticle sub-feature comprises a first periodic pattern shown in closer detail at 320. This detail 320 shows that each individual periodic element 325 of the first pattern is oriented at a firstoblique angle a with respect to Y. Similarly, each second reticle sub-feature comprises a second periodic pattern shown in closer detail at 330. This detail 330 shows that each individual periodic element 335 of the second pattern is oriented at a second oblique angle (3 with respect to Y. The first oblique angle a and second oblique angle may be mutually perpendicular, e.g., a=45 degrees and (3=135 degrees.

[0062] Adjacent reticle sub-features 310, 315 may be separated by reticle blocking regions 340.

[0063] The sensor may comprise at least one photodiode under each said sensor sub-feature, configured to generate an intensity signal per sensor sub-feature.

[0064] The sensor feature 345 may be complementary to the reticle feature (e.g., comprise a matching pattern), and as such comprise corresponding pairs of sensor sub-features 350, each pair of sensor subfeatures 350 comprising a respective first sensor sub-feature 355 and second sensor sub-feature 360, the sensor sub-features 355, 360 being separated by sensor blocking regions 365. The first and second sensor sub-features 355, 360 may comprise a corresponding pattern respectively to the first and second reticle sub-features 310, 315.

[0065] The proposed methods comprise imaging (exposing) the reticle feature onto the sensor comprising the sensor feature. The reticle feature is measured against the sensor feature, the latter acting as a reference, to determine local overlay errors in both X and Y introduced by the (dynamic) exposure. In this manner, an intensity signal (against time) is obtained for each reticle sub-feature and corresponding sensor sub-feature.

[0066] Each signal, in the absence of exposure induced overlay error, will comprise a periodic wave, e.g., a sine wave or similar, of substantially constant phase. It can be appreciated that, even in the absence of overlay, the signals may not be perfect sine waves. In the presence of higher diffracted orders, the signal will remain be periodic, but higher frequency content may be present, causing each signal to deviate from a sine wave. The methods described below will assume a single spatial frequency (sine wave signal) for simplicity, although the skilled person will realise this does not need to be the case to apply the methods disclosed herein (the higher frequency content can be dealt with in a similar manner as in alignment). Any change of phase (phase shift) in the signal is indicative of a local overlay error. Comparison of the signals from the first sub-feature(s) and second sub-feature(s) enables the different overlays (i.e., X overlay and Y overlay) to be distinguished. More specifically, a shift of both of these signals in the same direction is indicative of a local overlay error in a first direction (e.g., Y for the examples shown) and a shift of both of these signals in opposite directions is indicative of a local overlay error in a second direction (e.g., X in the examples shown).

[0067] Local phase (and therefore any phase shifts) may be extracted from each signal using any suitable local phase fit method, such as presently used in extracting position in alignment. Examples of localized phase fits used in the context of alignment can be found, for example, WO2022156978 Al (e.g., quadrature detection) and US12032305B2 (description on background periodic envelope fit and extraction of local APD). Both of these documents are incorporated herein by reference. Thesedocuments are only examples which describe methods of phase fitting and any suitable phase fit method can be used.

[0068] Figure 4 illustrates a method for determining an X-direction overlay error and Y-direction overlay error using the method described herein. Two plots are shown, the top plot 400 is for a first signal comprising intensity In against time t and / or scan position Y for a first reticle sub-feature as imaged on the corresponding first sensor sub-feature of the sensor feature, and the bottom plot 405 is the equivalent plot of a second signal for the second reticle sub feature imaged on the second sensor sub-feature.

[0069] In the absence of overlay error in X and Y, each of first signal 410 and second signal 415 is a sine wave of constant phase. In a first time window 420, there is an overlay shift in Y (scan direction). In this period the measured signal 425 from the first reticle sub-feature and the measured signal 430 from the second reticle sub-feature each undergo a phase shift in the same direction (the dotted line represents the original zero overlay signal 410, 415), which is indicative of an Y overlay shift. In a real example, it may be appreciated that the phase shift will typically not manifest as a sudden jump, but rather a more gradual change of phase. In a second time window 435 there is an overlay shift in X (slit direction). In this period the measured signal 440 from the first reticle sub-feature and the measured signal 445 from the second reticle sub-feature each undergo a phase shift in opposite directions, which is indicative of an X overlay shift.

[0070] A number of specific implementations of this basic concept will now be described in combination with Figures 5 to 8. In each case, a reticle feature 500 such as has been described is shown as being exposed via an exposure slit 510. Each of these Figures only shows a single signal (e.g., the first signal or second signal). It can be appreciated that both of the first signals and second signals will be similar other than the direction of any change in phase in the presence of x and / or y overlay as has been described.

[0071] Figure 5(a) and 5(b) represents an example where the entire reticle is captured. On Figure 5(a) this is indicated by the arrow 520 which describes the extent of the reticle exposed onto the sensor, and the depiction 530 of the full reticle area 540 and the imaged reticle area (dotted line) 550. This may be achieved, for example, by maintaining the sensor stationary under the projection optics (objective lens) and scanning the reticle.

[0072] A plot 560 of one of the resultant signals 570 is shown in Figure 5(b). This will be a high frequency signal 570, which may have a frequency on the order of 103Hz to 106Hz depending on the feature pitch and scan speed.

[0073] Such an example may represent an ideal case, where apparatus specifications make it practical, as the entire reticle area is scanned. However, this approach requires a laser source having a sufficient pulse frequency, e.g., which is at least on the order of the expected signal frequency. If this is not the case, the signal will be difficult to analyze (requiring multiplication with the temporal emission profile). Therefore, this approach is best suited to continuous wave sources or those which are a sufficientlyclose approximation thereof. For many present scanner sources, the repetition frequency is much lower and therefore some of the other solutions described below may be preferred.

[0074] Figures 6(a) and 6(b) shows an example where only a smaller region of the reticle is scanned. In Figure 6(a), this region is depicted by the arrow 620 which describes the extent of the reticle exposed onto the sensor, and the imaged reticle area (dotted line) 650 with respect to the full reticle area 640 on reticle depiction 630. This may be achieved, for example, by scanning the sensor with (i.e., in the same direction as) the reticle under the projection optics, but with a respective speed differential. The movement of sensor (i.e., wafer stage) and reticle is synchronized to obtain a signal from only a portion 620, 650 of the reticle.

[0075] A plot 660 of the resultant signal 670 is shown in Figure 6(b). The frequency of the signal 670 is orders of magnitude lower than the signal 570 (e.g., on the order of 10-lOOOHz), depending on feature pitch and the relative speed between reticle and sensor.

[0076] Figures 7(a) and 7(b) show an example where a portion of the reticle is sampled discretely to build an intensity signal 770. In Figure 7(a), the imaged portion of the reticle is again indicated by the arrow 720 and the dotted region 750 with respect to the full reticle area 740 in the representation 730. In this example, the sensor and reticle move together at the same velocity during each sample, such that each sample comprises (in the absence of an overlay contributor) a single intensity value. Between each sample, one or both of the reticle and sensor will moved to impose a relative positional shift between reticle and sensor, such that each sample comprises a measurement of a different point on the reticle (as indicated by the dots next to arrow 720). The portion of the reticle measured by this method may be similar to the method described in relation to Figures 6(a) and 6(b) (but measured at discrete points rather than continuously).

[0077] A plot 760 of the resultant signal 770 is shown in Figure 7(b). The signal may be built from the discretely measured intensity values sampled at the different positions with respect to the exposed image on the sensor.

[0078] Figures 8(a) and 8(b) show an example where only a single snapshot is measured from a single position 820. As such, referring to the representation 830 in Figure 8(a), only a very small region 850 of the full reticle area 840 is captured in this example.

[0079] In this method, the sensor and reticle may be moved together in synchronization with the same velocity beneath the projection optics. A plot 860 of the resultant signal 870 is shown in Figure 8(b). The signal comprises a single measured intensity value. The relative position of the reticle with respect to the sensor is maintained such that the measured intensity value In is measured only at one point on the reticle corresponding approximately to maximum sensitivity of intensity with position Y. In the absence of overlay, this measured intensity value remains constant. Any change in intensity therefore can be translated into an overlay value. The change in intensity will be the same for the two sub-features when the overlay error is in a first direction (e.g., Y), and equal and opposite when the overlay error is in a first direction (e.g., X).

[0080] Figure 9 is an example reticle feature 900 similar to the reticle features described herein (i.e., comprising first and second sub-features 910, 920 with mutually orthogonal oblique periodic patterns. However, the areas between the sub-features may comprise reticle transmissive regions (e.g., transparent regions) 930 through which intensity and therefore dose may be measured. This may be used to measure intensity variation of the source over time (e.g., pulse-to-pulse variation). As such, the sensor feature may also comprise sensor transmissive regions which are complementary to said reticle transmissive regions.

[0081] Figure 10 illustrates a number of different possible arrangements for the first reticel sub-feature and second reticle sub-feature. These examples may be useful for addressing certain aberrations, and / or tailoring the frequency of the detected first and second signals. In each case, the sensor sub-features will correspond.

[0082] In Figure 10(a), the first periodic pattern of first sub-feature 1010 and second periodic pattern of second sub-feature 1020 comprise different features.

[0083] In Figure 10(b), the first periodic pattern of first sub-feature 1030 and second periodic pattern of second sub-feature 1040 do not have ‘opposite orientations’ (i.e., angles are not mutually perpendicular). The two sub-features are still oriented in different directions, so as to respond differently to X and Y perturbations. From the two different signals (first signal and second signal) it is possible to uniquely solve for the two unknowns (overlay in each of X and Y).

[0084] Figure 10(c) illustrates an extreme example of Figure 10(b) where the second periodic pattern of second sub-feature 1060 is not oblique (aligned with the scan direction). However, because the first periodic pattern of first sub-feature 1050 is obliquely oriented, it remains possible to uniquely overlay in each of X and Y. The second sub-feature 1060 enables direct measurement of overlay in Y only; however, with this determined, the first signal enables both overlays to be distinguished.

[0085] In Figure 10(d), the first periodic pattern of first sub-feature 1070 and second periodic pattern of second sub-feature 1080 are both non-oblique (e.g., design rules or another reason may imposes a limit that only X and Y oriented periodic patterns can be used). From the first sub-feature 1070 it is possible to determine overlay in Y, as the reticle is scanned along the scan direction. Any of the approaches described above may be used. For the second sub-feature 1080, an approach similar to that described in relation to Figure 8 may be used as the reticle is not scanned in X. As such, this may comprise positioning the sensor along the X direction (perpendicular to the scanning direction) at a sensitive part of the response curve and monitoring for changes in intensity.

[0086] The concepts disclosed herein enable the inline monitoring of exposure induced overlay errors without requiring in-resist exposures. This enables, for example, the impact of different exposure conditions to be assessed, thereby enabling dynamic behavior to be studied.

[0087] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0088] 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.

[0089] 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.

[0090] 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.Aspects of the invention are set out in the following clauses:1. A method of determining exposure induced overlay errors relating to an exposure apparatus, the method comprising: exposing at least a portion of a reticle feature, comprised on a reticle, onto a sensor feature comprised on a sensor, wherein the reticle feature comprises at least one first reticle sub-feature comprising a first periodic pattern and at least one second reticle sub-feature comprising a second periodic pattern, and said sensor feature is complementary to said reticle feature; obtaining a first signal resultant from said first reticle sub-feature being exposed onto said sensor feature and captured by said sensor; obtaining a second signal resultant from said second reticle sub-feature being exposed onto said sensor feature and captured by said sensor; and determining a first exposure induced overlay error in a first direction and a second exposure induced overlay error in a second direction from said first signal and said second signal.2. A method as set out in clause 1, wherein said sensor is comprised on a substrate support of the exposure apparatus.3. A method as set out in clause 2, wherein said sensor comprises a fiducial sensor.4. A method as set out in clause 1, 2 or 3, wherein said sensor is comprised on a substrate being supported by a substrate support of the exposure apparatus.5. A method as set out in any preceding clause, wherein said sensor feature comprises at least a first sensor sub-feature comprising the first periodic pattern and a second sensor sub-feature comprising the second periodic pattern.6. A method as set out in any preceding clause, wherein at least one of said first periodic pattern and said second periodic pattern comprises an oblique periodic pattern.7. A method as set out in any preceding clause, wherein said first periodic pattern comprises a first oblique periodic pattern and said second periodic pattern comprises a second oblique periodic pattern.8. A method as set out in clause 7, wherein the first oblique periodic pattern comprises a first oblique angle with respect to the first direction, the first oblique angle being neither parallel nor perpendicular with said first direction.9. A method as set out in clause 8, wherein said first oblique angle is 45 degrees with respect to said first direction.10. A method as set out in clause 8 or 9, wherein the second oblique periodic pattern comprises a second oblique angle with respect to said first direction, the second oblique angle being different to said first oblique angle.11. A method as set out in clause 10, wherein said first oblique angle and said second oblique angle differ by 90 degrees.12. A method as set out in any preceding clause, wherein said first periodic pattern is aligned with said first direction and said second periodic pattern is aligned with said second direction; and wherein said sensor is positioned with respect to the reticle at a position corresponding substantially to a maximum sensitivity of the second signal, the second exposure induced overlay error being determined from a change in magnitude of said second signal.13. A method as set out in any preceding clause, wherein at least each said first reticle sub-feature is spaced apart from each adjacent second reticle sub-feature on said reticle.14. A method as set out in clause 13, comprising blocking regions between adjacent reticle sub-features.15. A method as set out in any preceding clause, wherein each said reticle sub-feature extends along the first direction of the exposure apparatus.16. A method as set out in any preceding clause, wherein said reticle feature comprises a plurality of first reticle sub-features and a plurality of second reticle sub-features, arranged such that the first reticle sub-features alternate with said second reticle sub-features.17. A method as set out in clause 16, wherein said first reticle sub-features alternate with said second reticle sub-features in said second direction.18. A method as set out in any preceding clause, wherein said first direction comprises a scan direction of the exposure apparatus and said second direction comprises a slit direction of the exposure apparatus.19. A method as set out in any preceding clause, wherein said reticle feature comprises one or more reticle transmissive regions, and said sensor feature comprises corresponding sensor transmissive regions.20. A method as set out in clause 19, comprising determining intensity and / or dose of exposure radiation used to perform the exposing step via radiation exposed onto the sensor via the reticle transmissive regions and sensor transmissive regions.21. A method as set out in any preceding clause, comprising differentiating between said first exposure induced overlay error and said second exposure induced overlay error by determining whether a first change of the first signal and a second change of the second signal are in the same direction or in opposite directions.22. A method as set out in clause 21, wherein said first change and comprises a change in phase and / or change in magnitude of said first signal and said second change comprises a change in phase and / or change in magnitude of said second signal.23. A method as set out in any preceding clause, wherein said reticle feature extends substantially over a full imaging area of the reticle.24. A method as set out in any preceding clause, wherein said exposing step comprises scanning a full imaging area of the reticle.25. A method as set out in any preceding clause, wherein said exposing step comprises maintaining the sensor substantially stationary and scanning the reticle.26. A method as set out in any of clauses 1 to 23, wherein said exposing step comprises exposing only a portion of an imaging area of the reticle.27. A method as set out in clause 26, wherein said exposing step comprises synchronizing motion of the sensor and reticle, such that there is a velocity differential between the sensor and reticle.28. A method as set out in clause 27, wherein the velocity differential is at least an order of magnitude smaller than the velocity of the reticle with respect to a fixed reference of the exposure apparatus.29. A method as set out in clause 26 wherein said exposing step comprises: synchronizing motion of the sensor and reticle, such that there is the sensor and reticle move at the same velocity; and sampling one or more discrete samples of said first signal and second signal.30. A method as set out in clause 29, comprising sampling a plurality of said discrete samples of said first signal and second signal, each at a different relative displacement between said reticle and sensor; and constructing said first signal and second signal from said plurality of said discrete samples of said first signal and second signal.31. A method as set out in any preceding clause, comprising determining said first exposure induced overlay error and second exposure induced overlay error from a change in phase of said first signal and second signal.32. A method as set out in clause 29, wherein said sampling is performed at only one position on the reticle with respect to the sensor; and determining said first exposure induced overlay error and second exposure induced overlay error from a change in magnitude of said first signal and second signal.33. A method as set out in clause 32, wherein said one position corresponds substantially to a maximum sensitivity of each said first signal and second signal with reticle position in said first direction.34. A method as set out in any preceding clause, wherein each of said first reticle sub-feature and second reticle sub-portion each comprise portions having a varied pattern orientation.35. A method as set out in any preceding clause, wherein each of said first reticle sub-feature and second reticle sub-portion each comprise reticle sub-segmentation.36. A method as set out in clause 35 wherein each of said first reticle sub-feature and second reticle sub-portion each comprise portions having different reticle sub-segmentation.37. A method as set out in any preceding clause, wherein each of said first reticle sub-feature and second reticle sub-portion each comprise portions having different periodicity.38. A reticle for exposing onto a sensor using an exposure apparatus, the reticle comprising at least one reticle feature, wherein the reticle feature comprises: at least one first reticle sub-feature comprising a first periodic pattern; and at least one second reticle sub-feature comprising a second periodic pattern; wherein the reticle feature extends over substantially over a full extent of a reticle imaging area in at least a second direction of the exposure apparatus, wherein a first direction of the exposure apparatus is the scan direction of the exposure apparatus and said second direction is a slit direction of the exposure apparatus.39. A reticle as set out in clause 38, wherein at least one of said first periodic pattern and said second periodic pattern comprises an oblique periodic pattern.40. A reticle as set out in clause 38 or 39, wherein said first periodic pattern comprises a first oblique periodic pattern and said second periodic pattern comprises a second oblique periodic pattern.41. A reticle as set out in clause 40, wherein the first oblique periodic pattern comprises a first oblique angle with respect to the first direction, the first oblique angle being neither parallel nor perpendicular with said first direction.42. A reticle as set out in clause 41, wherein said first oblique angle is 45 degrees with respect to said first direction.43. A reticle as set out in clause 41 or 42, wherein the second oblique periodic pattern comprises a second oblique angle with respect to said first direction, the second oblique angle being different to said first oblique angle.44. A reticle as set out in clause 38, wherein said first oblique angle and said second oblique angle differ by 90 degrees.45. A reticle as set out in any clause 38, wherein said first periodic pattern is aligned with said first direction and said second periodic pattern is aligned with said second direction.46. A reticle as set out in any of clauses 38 to 45, wherein at least each said first reticle sub-feature is spaced apart from each adjacent second reticle sub-feature on said reticle.47. A reticle as set out in clause 46, comprising blocking regions between adjacent reticle sub-features.48. A reticle as set out in any of clauses 38 to 47, wherein each said reticle sub-feature extends along the first direction of the exposure apparatus.49. A reticle as set out in any of clauses 38 to 48, wherein said reticle feature comprises a plurality of first reticle sub-features and a plurality of second reticle sub-features, arranged such that the first reticle sub-features alternate with said second reticle sub-features.50. A reticle as set out in clause 49, wherein said first reticle sub-features alternate with said second reticle sub-features in said second direction.51. A reticle as set out in any of clauses 38 to 50, wherein said first direction comprises a scan direction of the exposure apparatus and said second direction comprises a slit direction of the exposure apparatus.52. A reticle as set out in any of clauses 38 to 51, wherein said reticle feature comprises one or more reticle transmissive regions, and said sensor feature comprises corresponding sensor transmissive regions.53. A reticle as set out in any of clauses 38 to 52, wherein said reticle feature extends substantially over a full imaging area of the reticle.54. A reticle as set out in any of clauses 38 to 53, wherein each of said first reticle sub-feature and second reticle sub-portion each comprise portions having a varied pattern orientation.55. A reticle as set out in any of clauses 38 to 54, wherein each of said first reticle sub-feature and second reticle sub-portion each comprise reticle sub-segmentation.56. A reticle as set out in clause 55 wherein each of said first reticle sub-feature and second reticle subportion each comprise portions having different reticle sub-segmentation.57. A reticle as set out in any of clauses 38 to 56, wherein each of said first reticle sub-feature and second reticle sub-portion each comprise portions having different periodicity.58. A sensor for an exposure apparatus comprising a sensor feature, wherein the sensor feature comprises: at least one first sensor sub-feature comprising a first periodic pattern; and at least one second sensor sub-feature comprising a second periodic pattern.59. A sensor as set out in clause 58, wherein at least one of said first periodic pattern and said second periodic pattern comprises an oblique periodic pattern.60. A sensor as set out in clause 58 or 59, wherein said first periodic pattern comprises a first oblique periodic pattern and said second periodic pattern comprises a second oblique periodic pattern.61. A sensor as set out in clause 60, wherein the first oblique periodic pattern comprises a first oblique angle with respect to a scan direction of the exposure apparatus, the first oblique angle being neither parallel nor perpendicular with said first direction.62. A sensor as set out in clause 61, wherein said first oblique angle is 45 degrees with respect to said scan direction.63. A sensor as set out in clause 61 or 62, wherein the second oblique periodic pattern comprises a second oblique angle with respect to said first direction, the second oblique angle being different to said first oblique angle.64. A sensor as set out in clause 63, wherein said first oblique angle and said second oblique angle differ by 90 degrees.65. A sensor as set out in any clause 58, wherein said first periodic pattern is aligned with said scan direction of the exposure apparatus and said second periodic pattern is aligned perpendicular to said scan direction.66. A sensor as set out in any of clauses 58 to 65, wherein at least each said first sensor sub-feature is spaced apart from each adjacent second sensor sub-feature on said sensor.67. A sensor as set out in clause 66, comprising blocking regions between adjacent sensor sub-features.68. A sensor as set out in any of clauses 58 to 67, wherein each said sensor sub-feature extends along the first direction of the exposure apparatus.69. A sensor as set out in any of clauses 58 to 68, wherein said sensor feature comprises a plurality of first sensor sub-features and a plurality of second sensor sub-features, arranged such that the first sensor sub-features alternate with said second sensor sub-features.70. A sensor as set out in clause 69, wherein said first sensor sub-features alternate with said second sensor sub-features in said second direction.71. A sensor as set out in any of clauses 58 to 70, wherein said first direction comprises a scan direction of the exposure apparatus and said second direction comprises a slit direction of the exposure apparatus.72. A sensor as set out in any of clauses 58 to 71, wherein said sensor feature comprises one or more transmissive regions.73. A sensor as set out in any of clauses 58 to 72, comprising a fiducial sensor.74. A sensor as set out in any of clauses 58 to 73, comprising a single photodiode for each said subs- feature.75. A sensor as set out in any of clauses 58 to 73, comprising a group of photodiodes for each said subs- feature.76. A method as set out in any of clauses 1 to 37, wherein said sensor comprises the sensor of any of clauses 58 to 75.77. A computer program comprising program instructions operable to perform the method of any of clauses 1 to 37, when run on a suitable apparatus.78. A non-transient computer program carrier comprising the computer program of clause 77.79. A processing system comprising a processor and a storage device comprising the computer program of clause 77.80. An exposure apparatus being configured to perform the method of any of clauses 1 to 37.81. An exposure apparatus as set out in clause 80, further comprising: a reticle support for supporting a reticle; projection optics for projecting a pattern onto the reticle; a substrate support for supporting a substrate; and a sensor as set out in any of clauses 58 to 75.82. An exposure apparatus as set out in clause 81, wherein the reticle comprises the reticle of any of clauses 38 to 57.83. A semiconductor device manufactured using the exposure apparatus of clause 80, 81 or 82.

[0091] 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

CLAIMS1. A method of determining exposure induced overlay errors relating to an exposure apparatus, the method comprising: exposing at least a portion of a reticle feature, comprised on a reticle, onto a sensor feature comprised on a sensor, wherein the reticle feature comprises at least one first reticle sub-feature comprising a first periodic pattern and at least one second reticle sub-feature comprising a second periodic pattern, and said sensor feature is complementary to said reticle feature; obtaining a first signal resultant from said first reticle sub-feature being exposed onto said sensor feature and captured by said sensor; obtaining a second signal resultant from said second reticle sub-feature being exposed onto said sensor feature and captured by said sensor; and determining a first exposure induced overlay error in a first direction and a second exposure induced overlay error in a second direction from said first signal and said second signal.

2. A method as claimed in claim 1, wherein said sensor is comprised on a substrate support of the exposure apparatus.

3. A method as claimed in claim 1 or 2, wherein said sensor is comprised on a substrate being supported by a substrate support of the exposure apparatus.

4. A method as claimed in any preceding claim, wherein said sensor feature comprises at least a first sensor sub-feature comprising the first periodic pattern and a second sensor sub-feature comprising the second periodic pattern.

5. A method as claimed in any preceding claim, wherein at least one of said first periodic pattern and said second periodic pattern comprises an oblique periodic pattern.

6. A method as claimed in any preceding claim, wherein said first periodic pattern comprises a first oblique periodic pattern and said second periodic pattern comprises a second oblique periodic pattern.

7. A method as claimed in any preceding claim, wherein said first periodic pattern is aligned with said first direction and said second periodic pattern is aligned with said second direction; and wherein said sensor is positioned with respect to the reticle at a position corresponding substantially to a maximum sensitivity of the second signal, the second exposure induced overlay error being determined from a change in magnitude of said second signal.

8. A method as claimed in any preceding claim, wherein said reticle feature comprises a plurality of first reticle sub-features and a plurality of second reticle sub-features, arranged such that the first reticle sub-features alternate with said second reticle sub-features.

9. A method as claimed in any preceding claim, comprising differentiating between said first exposure induced overlay error and said second exposure induced overlay error by determining whether a first change of the first signal and a second change of the second signal are in the same direction or in opposite directions.

10. A reticle for exposing onto a sensor using an exposure apparatus, the reticle comprising at least one reticle feature, wherein the reticle feature comprises: at least one first reticle sub-feature comprising a first periodic pattern; and at least one second reticle sub-feature comprising a second periodic pattern; wherein the reticle feature extends over substantially over a full extent of a reticle imaging area in at least a second direction of the exposure apparatus, wherein a first direction of the exposure apparatus is the scan direction of the exposure apparatus and said second direction is a slit direction of the exposure apparatus.

11. A reticle as claimed in claim 10, wherein at least one of said first periodic pattern and said second periodic pattern comprises an oblique periodic pattern.

12. A computer program comprising program instructions operable to perform the method of any of claims 1 to 9, when run on a suitable apparatus.

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

14. An exposure apparatus being configured to perform the method of any of claims 1 to 9.

15. A semiconductor device manufactured using the exposure apparatus of claim 14.

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