Alignment method and associated alignment and lithographic apparatus

Abstract

A method and associated apparatus for performing position measurements on alignment marks comprising a first periodic structure having a direction of periodicity along at least a first direction is disclosed. The method comprises acquiring signal data relating to the position measurements and fitting the signal data to determine a position value. The fitting step uses either a modulation fit or a background envelope period fit.

Description

Technical Field

[0001] [Cross - reference to Related Applications] This application claims the priority of European Application No. 20176845.4 filed on May 27, 2020, the entire content of which is incorporated herein by reference.

[0002] [Technical Field] The present invention relates to a method and apparatus that can be used in the manufacture of devices by, for example, lithography technology, and a method for manufacturing a device using lithography technology. The invention relates to a measurement device, more specifically a measurement device such as an alignment sensor used for measuring a position, and a lithography apparatus having such an alignment sensor.

Background Art

[0003] A lithography apparatus is an apparatus for applying a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device, also referred to as a mask or reticle, may be used to generate the circuit patterns to be formed on each layer of the IC. This pattern can be transferred onto a target portion (e.g., a portion of a die, one die or a plurality of dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is typically by imaging onto a layer of radiation - sensitive material (resist) provided on the substrate. Generally, a single substrate includes a network of adjacent target portions that are successively patterned. These target portions are generally referred to as "fields".

[0004] In the manufacture of complex devices, typically many lithography patterning steps are performed, and functional features are formed in successive layers on a substrate. Accordingly, an important aspect of the performance of a lithographic apparatus is the ability to accurately place the applied pattern with respect to features arranged in a previous layer (by the same apparatus or a different lithographic apparatus). For this purpose, one or more sets of alignment marks are provided on the substrate. Each mark is a structure whose position can later be measured using a position sensor or alignment sensor (both terms are used synonymously), typically an optical position sensor.

[0005] A lithographic apparatus includes one or more alignment sensors that can accurately measure the position of marks on a substrate. Different types of marks and different types of alignment sensors are known from different manufacturers and different products of the same manufacturer. The type of sensor that is widely used in current lithographic apparatuses is based on a self-referencing interferometer as described in US6961116 (den Boef et al). Various improvements and modifications of position sensors are being pursued, as disclosed for example in US2015261097A1. The content of all these publications is incorporated herein by reference.

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0006] In IC manufacturing, where both reticle space and measurement time are expensive, it is becoming increasingly desirable to reduce the size of alignment marks and the time taken to measure the alignment marks. Reducing the size of the alignment marks results in an increase in the influence of neighboring structures on the alignment position, as it reduces the scan length across the mark. The position measured from such small marks is of poor reproducibility and accuracy due to one or both of the short scan length and the influence of neighboring structures, especially when measured at the desired speed and direction for detecting two position directions in a single scan. It is desirable to improve the reproducibility and accuracy in the measurement of such marks.

Means for Solving the Problem

[0007] The invention in the first aspect provides a method for performing position measurement on an alignment mark having a first periodic structure with a periodic direction along at least a first direction. The method includes acquiring signal data related to the position measurement and fitting the signal data to determine a position value. The fitting step uses either a modulation fit or a background envelope period fit.

[0008] The invention in the second aspect provides a method for performing position measurement on a bidirectional alignment mark having a first periodic structure provided together with a second periodic structure having a periodic direction in a second direction different from the first direction. The mark is configured to be scanned obliquely with respect to the first direction and the second direction. The method includes, at least, acquiring signal data related to the position measurement, the signal data including a first direction component related to a first effective pitch detected during the oblique scan and a second direction component of the signal data related to a second effective pitch detected during the oblique scan, and applying correction for signal disturbance due to vibration at a frequency difference corresponding to a difference between a first frequency in the signal data related to the first effective pitch and a second frequency in the signal data related to the second effective pitch.

[0009] The invention in the third aspect provides a method for performing position measurement on an alignment mark having a first periodic structure with a periodic direction along at least a first direction. The method includes acquiring signal data related to the position measurement and generating an oblique projection operator that acts on the signal data to block signal data caused by crosstalk of a proximity structure in the vicinity of the alignment mark.

[0010] A computer program, a measuring device, and a lithography device capable of executing the method according to any of the above aspects are also disclosed.

[0011] The above and other aspects of the invention will be understood from the consideration of the examples described below.

Brief Description of the Drawings

[0012] Hereinafter, embodiments of the invention will be described by way of example only with reference to the following accompanying drawings.

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Modes for Carrying Out the Invention

[0013] Before describing the embodiments of the invention in detail, an example of the environment in which the embodiments of the invention may be implemented is presented for reference.

[0014] 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 or DUV radiation), a patterning device support or support structure (e.g., a mask table) MT connected to a first positioner PM configured to support a patterning device (e.g., a mask) MA and to accurately position the patterning device in accordance with certain parameters, two substrate tables (e.g., wafer tables) WTa and WTb each connected to a second positioner PW configured to hold a substrate (e.g., a wafer coated with resist) W and 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 formed in the radiation beam B by the 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 the positions of the patterning device and the substrate, and the features thereon.

[0015] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, and controlling the radiation.

[0016] The patterning device support MT holds the patterning device in a manner that depends on conditions such as the orientation of the patterning device, the design of the lithographic apparatus, and whether the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques for holding the patterning device. The patterning device support MT may be, for example, a fixed or movable frame or table as required. The patterning device support may ensure that the patterning device is in a desired position, for example, with respect to the projection system.

[0017] As used herein, the term "patterning device" should be broadly construed to mean any device that can be used to form a pattern in a cross-section of a radiation beam in order to, for example, generate a pattern in a target portion of a substrate. Note that the pattern formed in the radiation beam need not exactly correspond to the desired pattern in the target portion of the substrate, for example, when the pattern includes phase-shifting features or so-called assist features. Generally, the pattern formed in the radiation beam corresponds to a particular functional layer in a device such as an integrated circuit being generated in the target portion.

[0018] As shown here, the apparatus may be of the transmissive type (e.g., using a transmissive patterning device). Alternatively, the apparatus may be of the reflective type (e.g., using a programmable mirror array of the type described above, or using a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, programmable LCD panels. The use of the term "reticle" or "mask" here may be interpreted as synonymous with the more general term "patterning device". The term "patterning device" may also be interpreted to mean a device that stores pattern information in digital form for use in controlling such programmable patterning devices.

[0019] As used herein, the term "projection system" should be construed broadly to encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof, suitable for other factors such as the exposure radiation in use, the use of an immersion liquid, or the use of a vacuum. The use of the term "projection lens" herein may be construed as synonymous with the more general term "projection system".

[0020] The lithographic apparatus may be of a type in which at least a portion of a substrate is covered by a liquid such as water having a relatively high refractive index, to fill the space between the projection system and the substrate. The immersion liquid may also be applied to other spaces in the lithographic apparatus, such as between the mask and the projection system. Immersion techniques are well known for increasing the numerical aperture of the projection system.

[0021] During operation, the illuminator IL receives a radiation beam from the radiation source SO. For example, if the source is an excimer laser, the source and the lithographic apparatus may be separate. In such cases, the source is not construed as forming part of the lithographic apparatus, and the radiation beam is passed from the source SO to the illuminator IL via a beam delivery system BD including, for example, suitable directing mirrors and / or a beam expander. In other cases, for example if the source is a mercury lamp, the source may be integrated with the lithographic apparatus. The source SO and the illuminator IL may be represented as a radiation system, optionally together with the beam delivery system BD.

[0022] The illuminator IL may include, for example, an adjuster AD, an integrator IN, and a condenser CO for adjusting the angular intensity distribution of the radiation beam. The illuminator may be used to condition the radiation beam to have a desired uniformity and intensity distribution in cross-section.

[0023] The radiation beam B is incident on a patterning device MA held on a patterning device support MT and is patterned by the patterning device. The radiation beam B passing through the patterning device (e.g., a mask) MA passes through a projection system PS that focuses the beam onto a target portion C of a substrate W. By using a second positioner PW and a position sensor IF (e.g., an interferometer device, a linear encoder, a 2D encoder, or a capacitive sensor), the substrate table WTa or WTb can be accurately driven, for example, to place different target portions C along the path of the radiation beam B. Similarly, a first positioner PM and other position sensors (not shown in FIG. 1) can be used to accurately position the patterning device (e.g., a mask) MA with respect to the path of the radiation beam B, for example, after mechanical retrieval from a mask library or during a scan.

[0024] The patterning device (e.g., a mask) MA and the substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. The illustrated substrate alignment marks occupy dedicated target portions, but they may be placed in the spaces between the target portions (these are known as scribe line alignment marks). Similarly, in a situation where a plurality of dies are provided on the patterning device (e.g., a mask) MA, the mask alignment marks may be placed between the dies. Small alignment marks may be included within the die together with the device features. In this case, it is desirable that the markers are as small as possible and do not require different imaging or processing conditions from the adjacent features. An alignment system for detecting the alignment markers is described further below.

[0025] The apparatus shown can be used in various modes. In the scan mode, while a pattern formed in the radiation beam is projected onto the target portion C, the patterning device support (e.g., mask table) MT and the substrate table WT are scanned simultaneously (i.e., 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 magnification and image inversion characteristics of the projection system PS. In the scan mode, the maximum size of the exposure field limits the width (in the non-scan direction) of the target portion in a single dynamic exposure, and the length of the scan operation determines the height (in the scan direction) of the target portion. As is well known, other types of lithographic apparatus and operating modes are also possible. For example, the step mode is known. In so-called "maskless" lithography, the pattern is changed while the programmable patterning device is held stationary and the substrate table WT is driven or scanned.

[0026] Combinations and / or variations of the usage modes described above, or entirely different usage modes, may be utilized.

[0027] The lithography apparatus LA may be of the so-called dual stage type, having two substrate tables WTa, WTb, and two stations (exposure station EXP and measurement station MEA) where the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, the other substrate is mounted on the other substrate table at the measurement station and various preparation steps are carried out. Thereby, the throughput of the apparatus is increased significantly. The preparation steps may include mapping the surface height profile 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 cannot measure the positions of the substrate tables at the measurement station and the exposure station, a second position sensor may be provided to enable tracking of the positions of the substrate tables at both stations with respect to the reference frame RF. Other arrangements are known and available instead of the dual stage arrangement shown. For example, other lithography apparatuses are known that provide a substrate table and a measurement table. These are docked together when performing a preparation measurement and the docking is released while the substrate table is being exposed.

[0028] Figure 2 illustrates the steps for exposing a target portion (e.g., a die) on a substrate W in the dual stage apparatus of Figure 1. The steps executed at the measurement station MEA are within the left dotted box, and the steps executed at the exposure station EXP are shown on the right. As described above, during operation, one of the substrate tables WTa, WTb is at the exposure station and the other is at the measurement station. For the purposes of this description, it is assumed that the substrate W is already mounted in the exposure station. In step 200, a new substrate W' is loaded into the apparatus by a mechanism (not shown). These two substrates are processed in parallel and in sequence to increase the throughput of the lithography apparatus.

[0029] First, regarding the newly loaded substrate W', it may be an untreated substrate that is prepared together with a new photoresist for the first exposure in the apparatus. However, generally, the described lithography process is merely one step in a series of exposure and processing steps, and the substrate W' may already have passed through this apparatus and / or other lithography apparatuses multiple times, or may pass through subsequent processes. In particular, the task for improving overlay performance is to ensure that a new pattern is applied at an exact position on a substrate that has already gone through one or more cycles of pattern formation and processing. These processing steps progressively introduce into the substrate the distortions that must be measured and corrected in order to achieve sufficient overlay performance.

[0030] The previous and / or subsequent pattern formation steps may, as described above, be performed in other lithography apparatuses, or may be performed in different types of lithography apparatuses. For example, some layers in a device manufacturing process with very high requirements for parameters such as resolution and overlay may be processed in more advanced lithography tools than other layers with relatively low requirements. Thus, some layers may be exposed in immersion type lithography tools, and other layers may be exposed in "dry" tools. Some layers may be exposed in tools operating at DUV wavelengths, and other layers may be exposed using EUV wavelength radiation.

[0031] In 202, alignment measurements using substrate marks P1, etc. and an image sensor (not shown) are used to measure and record the alignment of the substrate with respect to the substrate tables WTa / WTb. In addition, several alignment marks across the substrate W' are measured using the alignment sensor AS. These measurements are used in one embodiment to establish a "wafer grid" that very accurately maps the distribution of marks across the substrate, including the distortion with respect to a reference rectangular grid.

[0032] In step 204, a map of the wafer height (Z) with respect to the XY position is measured using the level sensor LS as well. Generally, the height map is used only to achieve accurate focusing of the exposure pattern. It may be used for other purposes as well.

[0033] When the substrate W’ is mounted, recipe data 206 that defines not only the exposure to be performed but also the characteristics of the wafer, the patterns formed previously, and the patterns to be formed hereafter is received. The measurement results of the wafer position, wafer grid, and height map obtained in 202 and 204 are added to these recipe data, and a complete set 208 of recipe and measurement data can be passed to the exposure station EXP. The measurement results of the alignment data include, for example, the X and Y positions of the alignment targets formed in a fixed or nominally fixed relationship with respect to the product pattern that is the product of the lithography process. These alignment data acquired immediately before exposure are used to generate an alignment model having parameters for fitting the model to the data. These parameters and the alignment model are used during the exposure operation to correct the position of the pattern applied in the current lithography step. The model in use interpolates the position deviation between the measured positions. Conventional alignment models have four, five, or six parameters that define the translation, rotation, and scaling of an “ideal” grid in different dimensions. More advanced models that use more parameters are also known.

[0034] At 210, the wafers W’ and W are exchanged, and the measured substrate W’ becomes the substrate W and enters the exposure station EXP. In the apparatus example of FIG. 1, this exchange is carried out by exchanging the supports WTa and WTb within the apparatus. Since the substrates W and W’ remain held in the correct positions on their respective supports, the relative alignment between the substrate table and the substrate is maintained. Therefore, once the table is exchanged, only by determining the relative position between the projection system PS and the substrate table WTb (previously WTa), the measurement information 202, 204 can be used for the substrate W (previously W’) in the control of the exposure step. At step 212, reticle alignment is performed using the mask alignment marks M1, M2. At steps 214, 216, 218, in order to complete the exposure of a number of patterns, the scan operation and radiation pulses are applied at successive target positions across the substrate W.

[0035] By using the alignment data and height map obtained at the measurement station during the execution of the exposure step, these patterns are accurately aligned, in particular, with respect to the desired positions relative to features previously placed on the same substrate. The exposed substrate labeled W” is removed from the apparatus at step 220 and is subjected to etching or other processing according to the exposure pattern.

[0036] Those skilled in the art will understand that the above description is a simplified overview of a number of very detailed steps that can be implemented in an example of an actual manufacturing site. For example, instead of measuring alignment in a single pass, there are often other phases of coarse and fine measurements using the same or different marks. Coarse and / or fine alignment measurement steps can be performed before, after, or during height measurement.

[0037] The measurement of the position of the marks may provide information regarding the deformation of the substrate on which the marks are provided, for example, in the form of a wafer grid. The deformation of the substrate can occur, for example, due to electrostatic clamping of the substrate to the substrate table and / or heating of the substrate when the substrate is exposed to radiation.

[0038] Figure 3 is a schematic block diagram of an embodiment of a known alignment sensor AS. The radiation source RSO provides a beam RB of radiation of one or more wavelengths that is deflected by a deflection optical element as an illumination spot SP onto a mark such as mark AM located on substrate W. In this example, the deflection optical element comprises a spot mirror SM and an objective lens OL. The illumination spot SP that illuminates mark AM may have a diameter slightly smaller than the width of the mark itself.

[0039] The radiation diffracted by mark AM is collimated (in this example, via objective lens OL) into an information-carrying beam IB. The term "diffracted" is intended to include zero-order diffraction (which may be represented as reflection) from the mark. For example, a self-referencing interferometer SRI of the type disclosed in the aforementioned US6961116 interferes with beam IB, after which the beam is received by a photodetector PD. Additional optical elements (not shown) may be included to provide a separate beam when multiple wavelengths are generated by radiation source RSO. The photodetector may be a single element or, if desired, may comprise a number of pixels. The photodetector may comprise a sensor array.

[0040] In this example, the deflection optical element comprising spot mirror SM may serve to block the zero-order radiation reflected from the mark so that information-carrying beam IB contains only higher-order diffracted radiation from mark AM (this is not essential for the measurement but improves the signal-to-noise ratio).

[0041] The intensity signal SI is supplied to a processing unit PU. A combination of optical processing in block SRI and arithmetic processing in unit PU outputs values of the X and Y positions on the substrate relative to a reference frame.

[0042] A single measurement of the type illustrated only fixes the position of the mark within a specific range corresponding to one pitch of the mark. A coarser measurement technique is used in combination to identify in which period of the sine wave the mark position is included. The same process at a coarse and / or fine level is repeated at different wavelengths for improved accuracy and / or robust detection of the mark, regardless of the material on which the mark is made and the material provided above and / or below the mark.

[0043] The mark or alignment mark may comprise a series of bars provided on a substrate, or (directly) formed in a substrate, or formed on or in a layer. The bars may be arranged at regular intervals to function as grating lines so that the mark can be regarded as a diffraction grating having a well-known spatial period (pitch). The mark may be designed to enable measurement of a position along the X-axis or the Y-axis (substantially orthogonal to the X-axis) depending on the direction of these grating lines. A mark comprising bars arranged at +45 degrees and / or -45 degrees with respect to both the X and Y axes enables combined X and Y measurements using the technique described in US2009 / 195768A, which is incorporated herein by reference.

[0044] The alignment sensor optically scans each mark with a spot of radiation to acquire a periodically varying signal such as a sine wave. The phase of this signal is analyzed to determine the position of the mark and thus the position of the substrate relative to the alignment sensor, which is fixed with respect to the reference frame of the lithographic apparatus. So-called coarse and fine marks regarding different (coarse and fine) mark dimensions may be provided so that the alignment sensor can identify different cycles of the periodic signal and the exact position (phase) within the cycle. Marks of different pitches may be used for this purpose.

[0045] When performing alignment by measuring the position of alignment marks on a substrate using an alignment sensor, it is desirable to reduce the area (footprint) of the alignment marks, including within "expensive" die across the wafer (e.g., between product structures), so that many of them can be adapted across the entire wafer. Thus, for a scan type alignment sensor (e.g., one that scans underfilled spots on the mark to generate a signal for SRI), it is desirable to reduce the required scan length on the mark in order to maintain sufficient accuracy and reproducibility (repro). In addition to alignment marks with a fixed period that provide a one-dimensional position (X or Y) for each scan, there are also diagonal marks that enable simultaneous detection in the X and Y directions (e.g., both directions parallel to the substrate surface) to reduce alignment time and increase throughput. Such diagonal marks may comprise two segments with mark structures at an angle of + / - 45 degrees.

[0046] Currently, for underfilled spots on the mark, an alignment fit (e.g., a least squares fit) based on the fast Fourier transform (FFT) is used to determine the alignment position from the alignment signal. This is equivalent to performing a least squares fit of sine and cosine signals. The alignment position of the mark is calculated by determining the phase from the fitted sine and cosine signals and combining this with the average stage position during the scan.

[0047] For smaller marks, the entire sensor beam does not impinge on the alignment mark, so the alignment signal is not perfectly periodic. The resulting signal is a periodic signal with a varying amplitude (or envelope) and a varying DC component (or background signal).

[0048] Regarding overlay performance, there are several issues when using a periodic fit on such alignment marks. Such issues include reproducibility (repro) by the envelope of the alignment signal, poor accuracy due to crosstalk effects from structures adjacent to the alignment mark, and poor reproducibility due to crosstalk between pitches with different angles and / or pitches for binary periodic oblique marks (in the case of multiple pitch marks). The reproducibility of binary pitch alignment marks is significantly worse than that of single pitch gratings, mainly due to the mixing (crosstalk) between the two effective pitches that enables the calculation of X and Y positions for binary pitch oblique marks. This is particularly significant for short scan lengths (e.g., 20 μm or less, 15 μm or less, or 12 μm or less) on the alignment mark.

[0049] In the first embodiment, it is proposed that, in order to effectively cope with the alignment signal envelope while fitting the signal to the alignment position, an algorithm based on modulation fit is used instead of an algorithm based on FFT. The following two approach examples based on modulation fit are described: Hilbert fit (HF). This algorithm is based on the frequency domain and uses band - pass filtering, Hilbert transform, and complex demodulation to determine the alignment position deviation (APD); Sine modulation fit (SMF). This algorithm is based on the time domain and uses real cosine demodulation and low - pass filtering to determine the APD.

[0050] Figure 4 is a flowchart describing the HF - based algorithm, which is the complex demodulation of the Hilbert transform of the band - filtered alignment signal. In step 400, the zero - phase band - pass filter "h bandpass " is used for the measured signal "I(x)" to separate the diffraction order signals. In step 410, the filtered signal is complex Hilbert - transformed, and in step 420, complex demodulation is performed. As a specific example, steps 400, 410, and 420 may be performed as a single step according to the following equations.

Number

[0051] In step 430, the local apd signal "apd" as (x) is calculated using a complex angle, for example, by the following equation.

Number

[0052] In step 440, the overall APD "APD" hann is calculated by integrating over the Hamming window "w" as (x) (the Hamming window center is optimized by performing multiple repetitions).

Number

[0053] Figure 5 is a flowchart describing an SMF-based algorithm with real cosine demodulation and real cosine demodulation of the alignment signal "I(x)" with a low-pass filter for removing harmonics. In step 500, real demodulation is performed using sine and cosine signals, and in step 510, the zero-phase low-pass filter "h" lowpass is applied to block the sidebands. For example, these steps may be expressed as follows.

Number

[0054] In step 520, the local APD signal "apd" as (x) is calculated as follows using "atan2".

Number

[0055] In step 530, the overall APD "APD" is calculated by integrating over the Hamming window "w hann (x)" according to the following formula (the Hamming window center is optimized by performing multiple repetitions). as Of course, in each of these examples, the described approach can be used for the Y-direction signal "I(y)" to perform alignment in the Y direction.

Equation

[0056] In other embodiments, a background envelope period fit (BEPF) may be performed that includes a least squares fit of a plurality of modulated sine, cosine, and DC signals for all orders within the alignment signal. This concept is based on extending the periodic signal model with the enveloped periodic signal and the background signal. This enables the algorithm to handle small mark signals without using filtering and modulation techniques. In addition, the extended model allows the oblique projection technique to be used for the fit coefficients that can actively cancel crosstalk interference for the alignment signal for a particular proximity structure and / or crosstalk between the pitches of the bidirectional marks. In the former case, the projection operator can be calibrated using offline mark and sensor model simulations to simulate marks with a particular crosstalk structure in proximity, or by actual measurement of such marks. Thus, this approach can simultaneously address the problem of poor reproducibility due to crosstalk between the envelope of the alignment signal and the binary pitch, and the problem of poor accuracy due to crosstalk effects from structures in proximity to the alignment marks.

[0057]

[0058] ​FIG. 6 is a flowchart depicting such a method. At step 600, the BEPF model matrix is computed using an appropriate model that enables fitting of a cosine and sine profile that depends on scan length multiples, pitch, order, where the amplitude is described by a set of parameters.

[0059] The first model may take the following form. [Number]

[0060] The second model may take the following form. [Number] The models describe the background signal, envelope, and periodic alignment signal by another set of parameters. More specifically, for each model: · The first line is a background signal model with a DC and background term (in the second model including the semi - order undulations μ, ν, and model coefficients α, β b , γ b ); · The second line includes an envelope signal for the cosine part of the alignment signal (model coefficients δ o , ζ o , η eo , θ eo for the first model, and model coefficients δ o , ε ob , ζ ob for the second model); · The third line includes an envelope signal for the sine part of the alignment signal (model coefficients κ o , λ o , μ eo , ν eo for the first model, and model coefficients η o , θ ob , ι ob for the second model). The second of these models has orthogonal background and envelope functions, and an overall condition number of the model matrix that is improved compared to the first model.

[0061] In step 610, laser noise normalization LNN may be performed on the difference and sum signals "I diff " "I sum ". This step may comprise the following sub-steps: · By using the background model of BEPF, calculate the background signals of "I diff " and "I sum "; · Normalize "I diff " and "I sum " together with the background signal; · Calculate the g factor (g) as the ratio of the RMS-normalized "I sum " to the RMS-normalized "I diff "; · Calculate the normalized laser noise signal "(I diff + g * I sum ) / (1 + g)"; · Divide the original alignment signal "I diff " by the laser noise signal (e.g., the normalized laser noise obtained from the ratio of the normalized difference and sum background signals from the BEPF model).

[0062] In step 620, the core fit of BEPF is performed. The core fit algorithm may use the pre-calculated inverse matrix "M as = M inv x I) to calculate the fit coefficient (e.g., the fit signal "c inv ". This speeds up the calculation compared to the regular least squares fit algorithm.

[0063] In one embodiment, for the second model specifically described, an envelope model "E" may be used to fit the stage position measurement (ΔSPM) signal. This may include fitting a position difference signal "dX", which is determined as the difference between the wafer stage position and the wafer stage set point signal (e.g., the expected position) (step 645). This stage position measurement "ΔSPM" signal describes the actual stage position relative to the expected position (e.g., the set point subtracted from the measured "SPM") such that the fitted "ΔSPM" signal can be combined with the result of the core fit to generate an alignment signal. One reason for this is to enable the operation of the fit coefficient rotation algorithm (see step 630 below). The core fit also calculates the fit residual energy. After this step, the raw alignment signal data becomes unnecessary. The fitting model for "ΔSPM" may be substantially similar to the already described envelope or background model, for example, as follows.

Number

[0064] Subsequently, in step 630, for example, a fit coefficient rotation algorithm such as an oblique projection matrix may be applied for crosstalk correction. The rotation step corrects for the fact that the expected position of the mark (i.e., the position where the scan is performed) does not match the true position of the mark (i.e., the position being sought). The purpose of rotating the fit coefficients is to move the alignment scan closer to the true mark position by moving from the expected mark position to the measured mark position. This only moves the center of the scan closer to the actual mark position and, in principle, does not change the alignment position as a correction. More details about this optional step are provided below.

[0065] In step 640, the APD signal is calculated from the phase-sensitive relationship of the cosine and sine amplitude components so as to be performed for a regular Fourier fit. For example, according to the following steps, the shape of the envelope expressed as the sum of sine and cosine over a set of scan length parts; the amplitude described by a set of parameters offset by the DC value is considered. · Calculate the envelope signal for the number of uses; for the described second model, this may comprise:

Number

Number

[0066] In step 650, a repeating Hann window may be applied with the modulation fit example to calculate the total APD signal "APD" as ". Since the APD signal for small marks behaves asymmetrically, there is an APD effect if the center of the Hann window is not close to the alignment position. The repetition of the Hann window placement may be performed until the resulting APD comes within a threshold (e.g., 1 picometer) from the alignment position. If convergence does not occur within a threshold number of repetitions (e.g., 5 times), an error is considered and the process stops.

[0067] Alternatively, instead of the Hann window repetition, step 630 may comprise shifting the alignment signal model by rotating the fit coefficients. In this implementation, a single rotation is sufficient. The purpose of this step is to shift the position of the alignment signal to the center of the scan. Compared to the repeating Hann window, this implementation is expected to give a more accurate alignment position, have a lower computational footprint, and enable the KPI to be calculated more easily and accurately. In such an embodiment, a simpler fixed Hann window may be applied. The process has the following four steps: ·Calculate the APD evaluation of the unrotated fit coefficient; ·Apply a shift to the background signal model; ·Apply a shift to the envelope; ·Apply a shift to the order (carrier). Since all fit coefficients are composed of cosine and sine functions, the shift can be implemented as a rotation of a cosine and sine pair. The described model has the desired extrapolation behavior by design, and since a Hann window may be used for APD calculations that become zero at the end of the scan, the influence of extrapolated APDs is expected to be negligible.

[0068] In step 660, an expected position (setpoint) may be added to the total APD to determine the alignment position "pos".

[0069] Optionally, in step 670, the BEPF fit enables, for example, the determination of in - mark KPIs such as the maximum correntropy criterion (MCC subspace) by the following formula: [Number] The MCC may consist almost entirely of sensor noise. In addition to the total MCC, signal distortion may be included in the MCC subspace. Other KPIs may be wafer quality (WQ) conditions. The raw WQ may be calculated as follows: rawWQ = sqrt(mean(E cos· 2 +E sin· 2 )); Definition based on the rms envelope rawWQ = max(sqrt(E cos· 2 +E sin· 2 )); Other definition based on the maximum order signal envelope

[0070] The core idea regarding the MCC's KPIs is to divide the BEPF model space into subspaces that cover the expected alignment signals and various different distortion types. In MTDF, a model parameter base transformation matrix including the MCC subspace division can be defined. Only the KPIs that can be verified need to be calculated online; other KPIs can be calculated in an offline tool from the recorded fit coefficients. Examples of MCC subspace components may include, for example: Background signal: Slope + low frequency + sub-harmonic oscillation; Slope + symmetric + asymmetric; Envelope per order: Slope + symmetric + asymmetric; Phase per order: Symmetric + asymmetric; Reproducibility: ARC component; Crosstalk: ACC component; Order comparison: Mismatch between orders (envelope, phase); Periodic signal: Deviation from pure periodicity; Small mark: Deviation from the expected signal subspace. Next, for the MCC subspace, nanometer KPIs can also be calculated from the APD signal by calculating the (weighted) standard deviation from the symmetric and asymmetric parts of the APD signal.

[0071] The BEPF fit can be used in combination with two-dimensional marks (e.g., bidirectional). Such two-dimensional marks may comprise co-located or superimposed x-direction and y-direction gratings (e.g., in two different layers or in a single layer) that are read using a diagonal or tilted scan (e.g., not parallel to either x or y), and / or may comprise multiple diagonal gratings co-located or superimposed at different angles that are scanned along the X or Y axis.

[0072] Figure 7 is a flowchart depicting such a two-dimensional embodiment. Steps 700, 710, 720, and 745 are substantially similar to the corresponding steps 600, 610, 620, and 645.

[0073] Core fit 720 may be performed on intensity I along with an additional SPM fit (as described above in the one-dimensional target embodiment) to generate a fit signal c.

[0074] In this embodiment, the core fit may use a model modified for the envelope as compared to the first BEPF model or the second BEPF model disclosed above. This is because two orders are sequentially required in the two-dimensional fit to determine two alignment positions in X and Y. In such cases, the common basic period for the two orders is often too large, and only one or two basic periods fit within the scan length. The BEPF models of the forms described above thus tend not to result in an envelope model above DC. In the modified model, the concept of the basic period is discarded, and the maximum envelope order is based on the signal period rather than the common basic period. Thus, the envelope coefficient “sin or cos(2πox?L baseperiod )” may be replaced by a coefficient based on the signal period, the maximum envelope order is allowed to be higher than the common basic period, and the envelope can be extended up to half of the actual signal period rather than half of the basic period.

[0075] One result of the above modified model is that the condition of this two-dimensional BEPF model deteriorates. To address this, normalization may be applied. In one embodiment, the normalization may comprise a secondary normalization for biasing the fitting towards a solution with a smooth envelope function. More specifically, in one embodiment, the normalization may take the following form:

Equation

[0076] Normalization is performed on both the degrees of these envelope functions and the second derivative with respect to the background signal. Since the envelope and background signal models include the sum of cosine and sine functions, an analytical expression for integration is obtained. This approach is important for the two-dimensional BEPF model, but can also provide improved stability for the one-dimensional BEPF model described above. Thus, this normalization may be applied to the fitting of the described embodiments of the one-dimensional BEPF model.

[0077] In step 725, a correction step may be performed that includes performing one or more of active reproducibility correction (ARC), order leakage correction (OLC), and weak semi-degree correction (WOC). Each of ARC, OLC, and WOC is described below. Note that this step may be performed after the rotation step 730, or may be split and performed before and after the rotation step 730. For example, one or more of these corrections may be performed before step 730, and one or more of these corrections (e.g., those not performed previously) may be performed after this step. However, it is simpler to perform ARC on the unrotated fit coefficients before step 730 (even though it can be converted to a post-rotation correction in theory).

[0078] Steps 730 and 770 correspond to steps 630 and 660 (of course, the rotation in the fit coordinate step 730 is a two-dimensional rotation). In step 740, in the step corresponding to step 640, the APD signal is determined in the X and Y directions. Thus, there are two channels for each direction.

[0079] Step 745 includes a coordinate transformation step of transforming from the APD values determined in the oblique coordinate system to the X / Y (Cartesian) coordinate system. In step 750, a (e.g., fixed) Hann window may be applied to each channel. In step 760, the expected position (setpoint) in each direction may be added to the respective total APD for the channel in order to determine the alignment position “pos”.

[0080] Ideally, for performing BEPF fitting, the scan should provide an even signal period for all diffraction orders (one - dimensional and two - dimensional). The half - order beat frequency may be included only in the background signal and removed only by the envelope signal.

[0081] FIG. 8 shows an example of a mark having such two - dimensional or bidirectional detectability of X and Y superimposed. FIG. 8(a) shows a bidirectional mark 800 comprising an X - direction grating 810 co - located (e.g., overlapped in a single layer or two layers) with a Y - direction grating 820. FIG. 8(b) shows how such a mark may be scanned. The radiation spots of the alignment radiation during scanning in the direction of the arrow are shown at the first position 830 and the second position 830’. The physical pitches of the X - direction grating 810 and the Y - direction grating 820 are the same in this embodiment, but may be different. Regardless of the physical pitch, the scan angle α (with respect to x) is oblique so as to enable separation of the X and Y signals, each having a different effective detection pitch in the scan direction. Here, the angle α causes 12 lines of the X - direction grating 810 to be scanned while 8 lines of the Y - direction grating 820 are scanned (e.g., α = 33.7 degrees), and thus the detection pitch ratio X:Y in the scan direction is 2:3. Of course, such a mark may be oblique with respect to X and Y and scanned along one of X and Y.

[0082] Figure 9 shows a second example of a two-dimensional or two-way mark having such detectability of X and Y. The mark comprises sub-targets arranged to form four triangular sections or a square or rectangular target. The sub-targets comprise two X-targets 900 and two Y-targets 910. In one embodiment, it is proposed that the mark be read at a small scan angle in order to increase the frequency difference Δf, i.e., reproducibility, between the two alignment signal components. In this case, ARC can be used to enhance performance, but it is not necessarily required. ARC is described below.

[0083] Further embodiments, which may be implemented in combination with other two-dimensional embodiments disclosed herein or may be implemented independently, comprise active reproducibility correction (ARC). Due to the use of two pitches for two-dimensional marks that differ in angle and / or pitch (e.g., as illustrated in FIG. 8 or FIG. 9), frequency oscillations close to the frequency difference Δf between the two alignment signal components resulting from the two pitches adversely affect reproducibility. This is particularly significant at the slow scan speeds used to measure small marks. A strong signal amplitude difference (sub-segmentation / polarization effect) also has a large impact.

[0084] Figure 10 illustrates the problems addressed by ARC. Two main orders at the main alignment signal frequencies (the target pitch frequencies at X and Y respectively) f 1 and f 2 are shown. Since there are two frequencies in the alignment signal, the sensitivity to vibrations at the frequency difference "Δf = f 2 - f 1 " of the measured position (repro) is increased. This also shows that by fitting two additional orders or "ARC orders", the perturbations at Δf can be detected and corrected, as illustrated in FIG. 10. Thus, each of these main orders has two additional frequency components or "ARC orders" f 1 ±Δf and f2 It has ±Δf. This means that f 1 is modulated on f 2 and vice versa, disturbing both alignment signal frequencies. That is, by modulating each main order on the ARC order, the order f 1 is modulated on the frequency component f 2 -Δf, and the order f 2 is modulated on the frequency component f 1 +Δf. For example, using a least squares fit approach, it is not possible to distinguish the effects of the true alignment signal and the Δf oscillation. However, in addition to the errors in f 1 and f 2 , Δf also results in frequency components at f 1 -Δf (= 2f 1 - f 2 ) and f 2 +Δf (= 2f 2 - f 1 ). Since the alignment marks themselves do not generate these frequency components, they can be used to detect and correct the effects of Δf on reproducibility. Three ways to achieve this are described:

[0085] The first approach involves adding the alignment signal disturbance due to the oscillation at the frequency difference Δf to the alignment signal model and fitting them together to the alignment signal. This generates an extended alignment signal model that includes the effects of the complete Δf oscillation as a model function.

[0086] The second approach involves generating a slant projection operator that acts on the raw alignment signal samples (e.g., the input difference, the sum intensity signal "I sum ", the difference and sum of complementary diffraction orders each with "I diff ") and blocking the alignment signal disturbance due to the oscillation at the frequency difference Δf through the alignment signal model function. This may be performed, for example, at step 630 of FIG. 6, step 725 of FIG. 7, or equivalent steps (not shown) of the method of FIGS. 4 or 5.

[0087] The second approach may comprise performing the following steps: · Calculating the alignment signal influence of the Δf vibration; · Blocking the Δf influence and generating a projection matrix that passes only the pure alignment order; · Applying the projection matrix to the alignment signal before alignment fitting.

[0088] The projection matrix may be calculated / optimized in the following (at least) three ways: · Based on dynamic transfer function simulation, the transfer function can be calculated from the alignment signal. From this, the disturbance to the alignment signal caused by the frequency difference Δf can be determined, and a projection matrix that blocks this specific disturbance can be designed; · Based on static scan test data from the scanner, the projection matrix may be optimized to achieve the best predicted reproducibility; · By using a raw reproducibility scan (i.e., a reproducibility scan that includes the raw alignment signal), the projection matrix may be optimized to achieve the best measured reproducibility.

[0089] This projection operator approach has the advantage that it can be used before any fitting algorithm (e.g., periodic fit, HF, SMF, BEPF), whereas the first approach can only be used only when fitting an alignment signal model (periodic fit, BEPF).

[0090] The third approach uses the physical relationship between the fitted ARC order and the disturbance on the corrected order to correct the fitted ARC order (e.g., f 1 -Δf) to the corrected order (e.g., f 2) is mapped directly onto it. In such an embodiment, the fitted ARC order is rotated and partially inverted to determine the correction. Scaling of the correction is not required: the magnitude of the perturbation on the principal order is, in theory, equal to the magnitude of the fitted ARC order. The rotation angle depends only on the phases of the other principal orders. More specifically, the perturbation ΔI for each order is as follows:

Number

Number

Number

[0091] For best performance, the perturbation signal for the first approach or the projection matrix for the second approach may use the following alignment signal knowledge (e.g., as part of calibration): · kx + ky for all orders: These are known from the mark design. These are the two-dimensional spatial frequencies of the alignment signal components. The two-dimensional alignment signal is given by the sum over "i" of the following:

Number

[0092] The two-dimensional BEPF fit in combination with two-dimensional marks was found to be affected by the scan offset. The fit is particularly sensitive to the horizontal scan offset. The following two phenomena were identified as the root causes: · Order leakage: The two-dimensional BEPF fit needs to fit at least two orders. The order envelope in the combination with the envelope model in the BEPF fit results in crosstalk between the orders indicating the influence of the scan offset. The aforementioned option of normalization may also have an impact on the order leakage. · Weak semi-orders: Depending on the tool actually used for measurement, weak semi-orders can be generated. This is particularly prominent in alignment tools based on self-referencing interferometers. These weak semi-orders do not physically exist on the pupil plane of the optical element, but are generated by self-referencing signal formation. The weak semi-orders depend on the beam size: there are no weak semi-orders for an infinite alignment beam, but they become non-zero for a finite beam size. The smaller the beam size, the stronger these weak semi-orders tend to be. The semi-order fringes appearing on the periodic marks are examples of such weak semi-orders. When the weak semi-orders overlap by occurring at the same or a similar frequency as the actual order of the BEPF fit, the influence of the scan offset occurs. The weak semi-orders are typically too weak to be fitted and corrected like the ARC. Since the weak semi-orders depend on the strong orders, they may be artificially synthesized from the strong orders: the weak semi-orders are the result of the combination of two actual orders, and the amplitude of the weak semi-orders is proportional to the square root of the product of the amplitudes of the actual orders. Therefore, the phase signal of the weak semi-orders is equal to the average phase signal of the two actual orders.

[0093] A set of correction methods for correcting this scan offset effect due to order leakage and weak sub-orders is described. These methods may be referred to as order leakage correction (OLC) and weak sub-order correction (WOC), and may be applied, for example, at step 730 of FIG. 7. OLC may comprise compensating for leakage from one or more orders to other affected orders, for example, by canceling the leakage by mixing the affected order into the order being affected. This mixing may cancel or compensate for the effect of the leakage on the alignment position by using an appropriate mixing gain value. The mixing gain may be calibrated by measuring the effect of the horizontal and vertical scan offsets on the two-dimensional mark and directly optimizing the mixing gain such that the APD effect becomes zero (i.e., minimizes the APD effect) and / or such that the scan offset effect is minimized.

[0094] WOC works similarly, but the mixing includes a synthetically mixed weak sub-order corresponding to the generated weak sub-order. When the alignment signal has balanced diffraction orders, the electric field (image) of the alignment mark may be described as follows:

Number

[0095] The alignment signal may be described as follows:

Number

Number

[0096] Amplitude An + and A n - is known in the same way as beam factor G nn Thus, the strong orders are fitted so that their intensities can be calculated. The weak semi-orders are calculated (synthesized), and calibration is performed to correct the strong orders for the influence of the weak semi-orders. This method may comprise mixing the strong orders with the synthesized weak semi-orders and optimizing the mixing gain in the same way as OLC so that the APD influence becomes zero and / or the scan offset influence is minimized. Thus, OLC and WOC may be performed as a single correction.

[0097] For completeness, if the alignment signal has orders that are not balanced, the alignment signal may be described as follows: [Number] The alignment signal is described as follows: [Number] And the modulation beam factor G for the weak semi-orders nm is described as follows: [Number]

[0098] The OLC and WOC methods are suitable for suppressing the influence of scan offsets when the scan offset is less than 100 nm. For larger scan offsets, these corrections may be insufficient. Furthermore, when calibrated on unsubsegmented marks or marks with different segmentations, it has been observed that OLC / WOC cannot correct subsegmented marks. Also, the finiteness of the alignment mark grid and additional effects due to corners in the mark design contribute to the scan offset influence. To address this, a more empirical method that overcomes the limitations of the OLC / WOC method is described. In such a method, the correction applied to the fit coefficients may be described by the following horizontal scan offset correction model: [Number] where g k is a scaling function that determines how the correction scales with respect to the wafer quality wq of the order being fitted (a signal amplitude metric equal to the RMS signal amplitude or other signal amplitude metrics may be used), x lat and x long are the horizontal and vertical scan offset coordinates, and d knm comprises correction fit coefficients for each scaling function and terms of a polynomial for the scan offset. In practice, only a few terms of the polynomial for the scan offset coordinates x lat and x long are required, and since the scan alignment position is affected only by the asymmetric part of the horizontal scan offset influence on the rotated alignment signal, the polynomial terms can be restricted to only those that are asymmetric (i.e., where m + n is odd). By performing scans with multiple scan offsets on multiple marks that capture relevant segmentation aspects including WQ / signal amplitude imbalance and other mark characteristics that contribute to the horizontal scan offset influence, d knm may be calibrated. For example, amplitudes A n , A mFor a weak semi-degree (e.g., proportional) based on the square root of a product, an appropriate set of scaling functions g k may be determined. For all scan offsets and marks, a correction fit coefficient d knm may be optimized to minimize the variation of the rotated fit coefficient.

[0099] BEPF (in both one-dimensional and two-dimensional variations) has been observed to be affected by periodic jumps when the color-to-color shift is large (e.g., when the image shifts from the color-to-color of the measured radiation used). There is a risk that the alignment signal is rotated to the wrong signal period, generating an inaccurate alignment position. To address this, the following two improvements, which may be used alone or in combination, are proposed:

[0100] The first improvement may include position correction based on (inline) color-to-color offset for the marks and colors used (and, optionally, an offset specific to the mark type: a small mark-specific offset added to the greater difference in color-to-color and polarization-to-polarization present in the sensor). In this context, color-to-color includes any variation between measurement channels and thus may include variation between wavelengths in addition to polarization. Thus, each measurement channel may relate to a different wavelength, polarization, or combination thereof. The correction may be performed on the APD that is calculated prior to rotation of the fit coefficients. Thus, this approach includes calibrating the color-to-color offset and correcting this offset. Large color-to-color offset values are the main cause of periodic jumps and can thus be avoided. The second improvement may include unwrapping of the APD signal (local phase signal) to avoid periodic jumps in the APD signal. Typically, large phase shifts occur near the start or end of the scan, so the center of the APD signal is the most robust. For this reason, it is proposed that the unwrapping be done in two steps, starting at each timing passing through the center of the scan to the left and right. Phase unwrapping and unwrapping functions are well known and those skilled in the art can easily adapt standard approaches to the manner in which the unwrapping is done from the center.

[0101] In the first embodiment, it may be performed in combination with BEPF (or an embodiment of another alignment signal model). A further embodiment is a crosstalk mitigation method represented as Active Crosstalk Correction (ACC). The product structure adjacent to the mark may have an impact on the alignment signal (APD) that varies due to process and stack variations.

[0102] ACC may be based on the oblique projection onto the fit coefficients (e.g., of an explicitly described model or any other suitable model) by the BEPF fit algorithm described above. The proposed ACC method may include the following steps: ·Simulate (or measure) the alignment signal influence; ·Generate a projection matrix for blocking the BEPF alignment signal influence; ·Apply the projection matrix to the BEPF fit coefficients.

[0103] The specific correction matrix for active crosstalk correction representing a set of parameters of the BEPF model may take the following form (for example, for the second model described above):

Number

[0104] Only the correction for the enveloped sine and cosine fit coefficients is important. This is the most common linear correction form for the fit coefficients. In practice, only one or a few relevant fit coefficients should be selected for correction, and the matrix M becomes very sparse. This approach requires knowledge around the mark. Various calibration methods are possible, and this approach may be based on pure simulation. The actual optimization may also be an optimization for the overlay data.

[0105] Alternatively, the ACC may be applied using a method similar to the ARC embodiment described above; that is, implemented as a projection operator for the raw alignment signal to block specific crosstalk effects on the alignment signal before alignment fitting. In this way, the ACC becomes independent of the fit algorithm used and is thus applicable to other modulation fit algorithms disclosed herein. Further, the ARC and ACC can be combined into a single projection matrix that mitigates both the Δf reproducibility disturbance and the crosstalk disturbance from the raw alignment signal.

[0106] It is proposed that the described concept may enable detection from a single scan (e.g., XY) on a small (e.g., 50x50 μm) mark using an underfilled mark. Such a method requires only a scan length of only 12 μm for sufficient reproducibility.

[0107] Although specific embodiments of the invention have been described above, it is understood that the invention may be practiced in ways different from those described.

[0108] Although specific reference may have been made in the context of optical lithography to the use of embodiments of the invention, it is understood that the invention may be used in other applications such as imprint lithography and is not limited to optical lithography where the context allows. In imprint lithography, the topography in the patterning device defines the pattern that is created on the substrate. The topography of the patterning device may be pressed into a resist layer supplied to the substrate, and the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is removed from the resist in which the pattern remains after the resist has been cured.

[0109] As used herein, the terms “radiation” and “beam” encompass all types of electromagnetic radiation including ultraviolet (UV) radiation (e.g., having a wavelength of about 365, 355, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 1 - 100 nm), particle beams such as ion beams or electron beams.

[0110] Where the context allows, the term “lens” may represent each or a combination of various types of optical components including refractive, reflective, magnetic, electro - magnetic and electrostatic optical components. In devices operating in the UV and / or EUV range, reflective components are likely to be used.

[0111] The width and scope of the present invention should not be limited by any of the foregoing exemplary embodiments, but should be determined only according to the following claims and their equivalents.

[0112] The invention may be further described using the following items. 1. A method of performing position measurement on an alignment mark having a first periodic structure with a periodic direction along at least a first direction, acquiring signal data regarding the position measurement; fitting the signal data to determine a position value; comprising: The fitting step is a method using either a modulation fit or a background envelope periodic fit. 2. The method according to item 1, wherein the fitting step uses a Hilbert modulation fit. 3. The method according to item 2, wherein the Hilbert modulation fit comprises complex demodulation of the Hilbert transform of the band-filtered signal data. 4. The method according to item 1, wherein the fitting step uses a sine modulation fit. 5. The method according to item 4, wherein the sine modulation fit comprises real sine demodulation and real cosine demodulation of the low-pass filtered signal data. 6. The fit comprises the background envelope periodic fit, The background envelope periodic fit comprises extending a periodic signal model that describes the signal data together with an enveloped periodic signal and a background signal. The method according to item 1. 7. The method according to item 6, wherein the background envelope periodic fit comprises fitting a plurality of modulated sine, cosine, and DC signals for all orders in the signal data. 8. The method according to item 7, wherein the fit of the plurality of modulated sine, cosine, and DC signals comprises a least squares fit. 9. The method according to any one of items 6 to 8, comprising dividing the periodic signal model space into subspaces covering expected signal data values and various different distortion types. 10. The method according to item 9, comprising determining a model parameter basis transformation matrix including the subspaces. 11. The alignment mark comprises a bidirectional alignment mark including the first periodic structure provided together with a second periodic structure having a periodic direction in a second direction different from the first direction, The method further comprises scanning the alignment mark obliquely with respect to the first direction and the second direction such that a first direction component of the signal data is distinguished from a second direction component of the signal data by having different effective pitches in the scan direction. The method according to any one of items 6 to 10. 12. The alignment mark comprises a bidirectional alignment mark including four triangular sections or sub - marks provided to form a square or rectangular mark, The sub - marks include two of the first periodic structures and two second periodic structures having a periodic direction in a second direction different from the first direction. The method further comprises scanning the alignment mark obliquely with respect to the first direction and the second direction to increase a frequency difference between the first direction component and the second direction component. The method according to any one of items 6 to 10. 13. The method according to item 11 or 12, comprising applying a correction for signal disturbance due to vibration at a frequency difference corresponding to a difference between a first frequency in the signal data with respect to a first effective pitch of the different effective pitches and a second frequency in the signal data with respect to a second effective pitch of the different effective pitches. 14. The method according to item 13, wherein the correction for signal disturbance is applied by adding the signal disturbance at the frequency difference to the periodic signal model and fitting them together to the signal data. 15. The method according to item 13, wherein the method comprises characterizing signal frequency components corresponding to the signal disturbance and mapping the characterized signal frequency components onto a degree to be corrected, such that the correction for the signal disturbance is applied. 16. The method according to item 15, wherein the mapping comprises rotating the characterized signal frequency components and partially inverting the rotated signal frequency components. 17. Compensating for leakage from one or more diffraction orders to a main diffraction order for the signal data, wherein the method comprises mixing the one or more diffraction orders into the main diffraction order with a mixing gain, wherein the mixing gain is optimized to minimize the influence of the leakage on the position value. The method according to any one of items 6 to 16. 18. Further comprising synthesizing weak semi-orders corresponding to generated weak semi-orders that affect the signal data, wherein the mixing comprises additionally mixing the synthesized weak semi-orders into the main diffraction order such that the optimized mixing gain also minimizes the influence of the weak semi-orders on the position value. The method according to item 17. 19. The method according to any one of items 6 to 18, comprising generating a fit coefficient correction algorithm that acts on the fit coefficients of the periodic signal model to block the influence on the signal data from crosstalk of a proximity structure adjacent to the alignment mark. 20. The method according to item 19, wherein the fit coefficient correction algorithm comprises a oblique projection operator. 21. The method according to item 19 or 20, wherein the fit coefficient correction algorithm acts only on the fit coefficients corresponding to the enveloped periodic signal. 22. The method according to item 19, 20 or 21, comprising a first step of selecting only a subset of the fit coefficients based on knowledge of the proximity structure. 23. Determining horizontal scan offset correction from an amplitude metric of the fitted signal data scaled by a scaling function, a correction fit coefficient for each scaling function, and a horizontal scan offset correction model based on terms of a polynomial for horizontal and vertical scan offsets. The method includes optimizing the correction fit coefficient to minimize variations in the rotated fit coefficients obtained from the fit coefficient correction algorithm for different scan offsets and / or alignment marks. The method according to any one of items 19 to 22. 24. Normalizing the background envelope period fit according to any one of items 6 to 23 to bias the fitting towards a solution with a smooth envelope function. 25. Calibrating a wavelength-dependent contribution in the signal data and correcting the position value for the wavelength-dependent contribution according to any one of items 6 to 24. 26. Performing unwrapping of the signal data towards each end of the scan from the center of the scan across the alignment mark according to any one of items 6 to 25. 27. Applying the correction includes using additional frequency components in the signal data caused by the signal disturbance in the frequency difference to detect and determine the correction for the signal disturbance according to the method described in item 13. 28. Generating an oblique projection operator that blocks the signal disturbance due to vibrations in the frequency difference while acting on the signal data and passing through a model function that describes the signal data at the first and second frequencies according to the method described in item 26 or 27. 29. The method according to item 28, wherein the oblique projection operator can further block the influence of crosstalk signals in the signal data from crosstalk of a proximity structure adjacent to the alignment mark. 30. The method according to any one of items 1 to 28, comprising generating a skew projection operator that acts on the signal data and blocks the signal data resulting from crosstalk of a proximity structure in proximity to the alignment mark. 31. A method of performing position measurement for a bidirectional alignment mark comprising a first periodic structure provided with a second periodic structure having a periodic direction in a second direction different from a first direction, wherein the mark is configured to be scanned obliquely with respect to the first direction and the second direction, acquiring signal data regarding the position measurement, comprising at least a first direction component regarding a first effective pitch detected during the oblique scan, and a second direction component of the signal data regarding a second effective pitch detected during the oblique scan; applying correction for signal disturbance due to vibration at a frequency difference corresponding to a difference between a first frequency in the signal data regarding the first effective pitch and a second frequency in the signal data regarding the second effective pitch; and comprising the method. 32. The method according to item 31, wherein applying the correction comprises using an additional frequency component in the signal data caused by the signal disturbance at the frequency difference to detect and determine the correction for the signal disturbance. 33. The method according to item 31 or 32, comprising generating a skew projection operator that acts on the signal data and blocks the signal disturbance due to vibration at the frequency difference while passing through a model function that describes the signal data at the first frequency and the second frequency. 34. The method according to item 33, wherein the skew projection operator can further block the influence of a crosstalk signal in the signal data from crosstalk of a proximity structure in proximity to the alignment mark. 35. A method of performing position measurement for an alignment mark comprising a first periodic structure having a periodic direction along at least a first direction, Obtaining signal data related to the position measurement; Acting on the signal data to generate a skew projection operator that blocks the signal data caused by crosstalk of a proximity structure close to the alignment mark; A method comprising the steps of: 36. A step of scanning a measurement beam across at least a part of the alignment mark; A step of detecting scattered radiation scattered by the alignment mark to obtain the signal data; The method according to any one of items 1 to 35, comprising the steps of: 37. The method according to any one of items 1 to 36, wherein the scan length in the scanning step is less than 15 μm. 38. A computer program comprising computer-readable instructions executable to perform the method according to any one of items 1 to 37 39. A processor and an associated storage medium, wherein the storage medium comprises the computer program according to item 38 such that the processor can execute the method according to any one of items 1 to 37. 40. A measuring device comprising the processor and the associated storage medium according to item 39 so as to be able to execute the method according to item 36 or 37. 41. A lithography apparatus comprising the measuring device according to item 40. 42. A patterning device support for supporting a patterning device, and A substrate support for supporting a substrate, Comprising: The measuring device can determine an alignment position for one or both of the patterning device support and the substrate support. The lithography apparatus according to item 41.

Claims

1. A method of performing position measurement on an alignment mark comprising a first periodic structure having a periodic direction along at least a first direction, the method comprising: acquiring signal data relating to the position measurement; fitting the signal data to determine a position value; wherein the fitting step uses a background envelope period fit; the background envelope period fit comprises extending a periodic signal model that describes the signal data together with an enveloped periodic signal and a background signal; the alignment mark comprises a bidirectional alignment mark comprising the first periodic structure provided with a second periodic structure having a periodic direction in a second direction different from the first direction; the method further comprises scanning the alignment mark obliquely with respect to the first direction and the second direction such that a first direction component of the signal data is distinguished from a second direction component of the signal data by having different effective pitches in a scan direction.

2. The alignment mark comprises a bidirectional alignment mark comprising four triangular sections or sub-marks provided to form a square or rectangular mark, the sub-marks comprising two of the first periodic structures and two second periodic structures having a periodic direction in a second direction different from the first direction; the method further comprises scanning the alignment mark obliquely with respect to the first direction and the second direction to increase a frequency difference between a first direction component and a second direction component, The method according to claim 1.

3. The method according to claim 1 or 2, comprising applying a correction for signal disturbance due to vibration in a frequency difference corresponding to a difference between a first frequency in the signal data relating to a first effective pitch of the different effective pitches and a second frequency in the signal data relating to a second effective pitch of the different effective pitches.

4. The method according to claim 3, wherein the correction for signal disturbance is applied by adding the signal disturbance in the frequency difference to the periodic signal model and fitting them together to the signal data.

5. The method according to claim 4, wherein a signal frequency component corresponding to the signal disturbance is characterized, and the characterized signal frequency component is mapped onto a correction order to be corrected, whereby the correction for the signal disturbance is applied.

6. comprising compensating for leakage from one or more diffraction orders to a main diffraction order for the signal data, the method comprising mixing the one or more diffraction orders into the main diffraction order with a mixing gain, the mixing gain being optimized to minimize the influence of the leakage on the position value, The method according to any one of claims 1 to 5.

7. further comprising synthesizing a weak semi-order corresponding to a generated weak semi-order affecting the signal data, the mixing comprising additionally mixing the synthesized weak semi-order into the main diffraction order such that the optimized mixing gain also minimizes the influence of the weak semi-order on the position value, The method according to claim 6.

8. The method according to any one of claims 1 to 7, comprising generating a fit coefficient correction algorithm acting on the fit coefficients of the periodic signal model to block the influence on the signal data from crosstalk of a proximity structure in proximity to the alignment mark.

9. determining a horizontal scan offset correction from a horizontal scan offset correction model based on terms of a polynomial for an amplitude metric of signal data fitted based on a background envelope period fit scaled by a scaling function, a correction fit coefficient for each scaling function, and horizontal and vertical scan offsets, the method comprising optimizing the correction fit coefficients to minimize variations in rotated fit coefficients obtained from the fit coefficient correction algorithm for different scan offsets and / or alignment marks, The method according to claim 8.

10. The method according to any one of claims 1 to 9, comprising calibrating a wavelength-dependent contribution in the signal data and correcting the position value for the wavelength-dependent contribution.

11. Applying the correction comprises using additional frequency components in the signal data resulting from the signal disturbance at the frequency difference to detect and determine the correction for the signal disturbance, the method of claim 3.

12. The method according to claim 10 or 11, comprising generating a projection operator that acts on the signal data and blocks signal disturbances due to vibrations at the frequency difference through a model function that describes the signal data at a first frequency and a second frequency.

13. The method according to claim 12, wherein the projection operator can further block the influence of crosstalk signals in the signal data from crosstalk of a proximity structure proximate to the alignment mark.

14. A method of performing a position measurement for a bidirectional alignment mark comprising a first periodic structure provided with a second periodic structure having a periodicity direction in a second direction different from a first direction, wherein the mark is configured to be scanned obliquely with respect to the first direction and the second direction, obtaining signal data for the position measurement comprising at least a first direction component of a first effective pitch detected during the oblique scan and a second direction component of the signal data of a second effective pitch detected during the oblique scan; applying a correction for signal disturbances due to vibrations at a frequency difference corresponding to a difference between a first frequency in the signal data for the first effective pitch and a second frequency in the signal data for the second effective pitch; comprising a method.

15. Applying the correction comprises using additional frequency components in the signal data resulting from the signal disturbance at the frequency difference to detect and determine the correction for the signal disturbance, the method of claim 14.

16. The method according to claim 14 or 15, comprising generating a projection operator that acts on the signal data and blocks the signal disturbance due to vibrations at the frequency difference through a model function that describes the signal data at the first frequency and the second frequency.

17. The method according to claim 16, wherein the skew projection operator can further block the influence of crosstalk signals in the signal data from crosstalk of a proximity structure close to the alignment mark.

18. A computer program comprising computer-readable instructions executable to perform the method according to any one of claims 1 to 17.

19. A processor and an associated storage medium, wherein the storage medium comprises the computer program according to claim 18 such that the processor can execute the method according to any one of claims 1 to 17.

20. A measurement device comprising the processor and the associated storage medium according to claim 19 such that the method according to claim 17 can be executed.

21. A lithography apparatus comprising the measurement device according to claim 20.

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