Substrate, patterning device and metrology apparatus

Abstract

A method for determining a focus parameter value for exposing at least one structure on a substrate is disclosed. The method comprises obtaining measurement data relating to a measurement of the at least one structure, wherein the at least one structure comprises a single periodic structure per measurement location; and decomposing the measurement data into component data comprising one or more components of the measurement data. At least one of the components is processed to extract processed component data having a reduced dependency on non-focus related effects; and determining a value of the focus parameter from the processed component data. Associated apparatus and patterning device are also disclosed.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to European application 20177328.0, filed on 29 May 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to methods and apparatus for use in device fabrication, such as by photolithography, and to methods for fabricating devices using photolithography. The invention also relates to metrology apparatus, and more specifically, to performing focused measurements using metrology devices (such as alignment sensors) typically used for measuring position, and to photolithography apparatus having such alignment sensors. Background Technology

[0004] A photolithography apparatus is a machine that applies a desired pattern onto a substrate (typically a target portion of the substrate). For example, photolithography apparatus can be used in the fabrication of integrated circuits (ICs). In this case, a patterning apparatus, alternatively called a mask or photomask, can be used to generate a circuit pattern to be formed on a single layer of the IC. This pattern can then be transferred onto a target portion (e.g., a portion of a die, one or more dies) on a substrate (e.g., a silicon wafer). Typically, the transfer of the pattern is performed by imaging the pattern onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will contain a grid of adjacent target portions that are patterned sequentially. These target portions are often referred to as “fields.”

[0005] In the fabrication of complex devices, numerous photolithographic patterning steps are typically performed to form functional features in successive layers on the substrate. Therefore, a crucial aspect of the performance of a photolithography apparatus is its ability to correctly and accurately align the applied pattern relative to features placed in previous layers (by the same or different photolithography apparatuses). For this purpose, the substrate is provided with one or more sets of alignment marks. Each mark is structured such that its position can be measured later using a position sensor or alignment sensor (both terms are used synonymously), typically an optical position sensor.

[0006] The lithography apparatus includes one or more alignment sensors that can accurately measure the position of marks on the substrate. Different types of marks and different types of alignment sensors are known to come from different manufacturers and different products from the same manufacturer. One type of sensor widely used in current lithography apparatuses is based on a self-reference interferometer as described in US6961116 (den Boef et al.). Various improvements and modifications to the position sensor have been developed, such as those disclosed in US2015261097A1. All of these disclosures are incorporated herein by reference.

[0007] It is known to use such alignment sensors to measure focus settings for exposing specific types of focus-sensitive alignment marks or focus marks. For example, during focus testing used in tool identification, setup, and recovery procedures, many of these marks may be exposed and read. Such procedures can take several hours, during which the tool is offline. Reducing the time required for these procedures would be beneficial. Summary of the Invention

[0008] The present invention provides, in a first aspect, a method for determining a focus parameter value for at least one structure on an exposure substrate, comprising: obtaining measurement data relating to a measurement of the at least one structure, wherein the at least one structure comprises a single periodic structure at each measurement location; decomposing the measurement data into component data comprising one or more components of the measurement data; processing at least one of the components to extract processed component data having a reduced dependence on non-focus-related effects; and determining the focus parameter value based on the processed component data.

[0009] In a second aspect, the present invention provides a method for determining focus parameter values ​​for at least one structure on an exposure substrate, comprising: obtaining measurement data relating to a measurement of the at least one structure, wherein the at least one structure comprises a single periodic structure at each measurement location; obtaining a trained model that has been trained to infer focus from the measurement data; and determining the focus parameter values ​​based on the trained model and the measurement data.

[0010] In a third aspect, the present invention provides a pattern forming apparatus comprising a plurality of periodic features for forming a periodic structure on a substrate, each of the periodic features comprising alternating first segments and second segments, wherein a suitable subset of the first segments comprises focus-sensitive segments.

[0011] In a fourth aspect, the present invention provides a processing apparatus comprising: a processor; and a program memory including a computer program, the computer program including program instructions operable to perform the method according to the first or second aspect.

[0012] A photolithography apparatus is also disclosed, comprising: an alignment sensor; a pattern forming apparatus support for supporting a pattern forming apparatus; a substrate support for supporting a substrate; and a processing apparatus according to the fourth aspect.

[0013] The above and other aspects of the invention will be understood in light of the considerations given in the examples described below. Attached Figure Description

[0014] Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which:

[0015] Figure 1 The photolithography equipment is described;

[0016] Figure 2 Schematic map shows Figure 1 Measurement and exposure processes in the equipment;

[0017] Figure 3 This is a schematic diagram of a first alignment sensor applicable according to an embodiment;

[0018] Figure 4 An example of a focus curve is shown, including a graph of the alignment offset ao relative to the defocus amount dZ, with the measurement mark included.

[0019] Figure 5 A possible structure including a focus marker that is sensitive to focus is shown;

[0020] Figure 6 Including (a) for markers without overlap error and (b) for markers with overlap error, graphs of the original alignment sensor signal and its first third harmonic component versus scan length;

[0021] Figure 7 The figure illustrates a focus mark that can be read using an alignment sensor according to an embodiment of the present invention; and

[0022] Figure 8 It is a graph of the raw alignment signal (amplitude relative to position) from a single focus mark for three different defocus values. Detailed Implementation

[0023] Before describing the embodiments of the present invention in detail, it is helpful to provide example environments in which the embodiments of the present invention can be implemented.

[0024] Figure 1 A lithography apparatus LA is schematically depicted. The lithography apparatus includes: an irradiation system (irradiator) IL configured to modulate a radiation beam B (e.g., UV radiation or DUV radiation); a patterning apparatus support or support structure (e.g., a mask stage) MT configured to support a patterning apparatus (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning apparatus according to specific parameters; two substrate stages (e.g., wafer stages) WTa and WTb configured to hold a substrate (e.g., a wafer coated with resist) W, respectively, and connected to a second positioner PW configured to accurately position the substrate according to specific parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the radiation beam B by the patterning apparatus MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0025] The irradiation system may include various types of optical components for guiding, shaping, or controlling radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof.

[0026] The patterning apparatus MT holds the patterning apparatus in a manner dependent on the orientation of the patterning apparatus, the design of the lithography equipment, and other conditions such as whether the patterning apparatus is kept in a vacuum environment. The patterning apparatus support MT can employ mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning apparatus. The patterning apparatus support MT can be a frame or a table; for example, it can be fixed or movable as needed. The patterning apparatus support ensures that the patterning apparatus (e.g., relative to the projection system) is positioned as desired.

[0027] As used herein, the term "patterning apparatus" should be broadly interpreted to refer to any apparatus that can be used to impart a pattern to the cross-section of a radiation beam in order to generate a pattern in a target portion of the substrate. It should be noted that, for example, if the pattern includes phase-shifting features or so-called auxiliary features, the pattern imparted to the radiation beam may not correspond precisely to the desired pattern in the target portion of the substrate. Typically, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device (such as an integrated circuit) generated in the target portion.

[0028] As depicted herein, the device may be of the transmissive type (e.g., using a transmissive pattern forming apparatus). Alternatively, the device may be of the reflective type (e.g., employing a programmable mirror array of the type mentioned above, or employing a reflective mask). Examples of pattern forming apparatuses include masks, programmable mirror arrays, and programmable LCD panels. Any term “mask” or “pattern” used herein may be considered synonymous with the more general term “pattern forming apparatus.” The term “pattern forming apparatus” may also be interpreted as a means of storing pattern information in digital form for use in controlling such a programmable pattern forming apparatus.

[0029] As used herein, the term "projection system" should be interpreted broadly to include any type of projection system, including refractive, reflective, reflective-refractive, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation used or other factors such as the use of immersion liquids or vacuum. Any term "projection lens" used herein may be considered synonymous with the more general term "projection system."

[0030] The photolithography apparatus can also be of the type in which at least a portion of the substrate can be covered by a liquid (e.g., water) having a relatively high refractive index to fill the space between the projection system and the substrate. An immersion liquid can also be applied to other spaces within the photolithography apparatus, such as the space between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of a projection system.

[0031] In operation, the irradiator IL receives a radiation beam from a radiation source SO. For example, when the source is an excimer laser, the source and the lithography apparatus can be separate entities. In such cases, the source is not considered part of the lithography apparatus, and the radiation beam is transmitted from the source SO to the irradiator IL by means of a beam delivery system BD comprising, for example, suitable directional mirrors and / or beam expanders. In other cases, for example, when the source is a mercury lamp, the source can be an integral part of the lithography apparatus. The source SO, the irradiator IL, and the beam delivery system BD, if necessary, can be collectively referred to as the radiation system.

[0032] The irradiator IL may, for example, include an adjuster AD, an integrator IN, and a concentrator CO for adjusting the angular intensity distribution of the radiation beam. The irradiator can be used to adjust the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0033] The radiation beam B is incident on the patterning apparatus MA held on the patterning apparatus support MT and patterned by the patterning apparatus. Having traversed the patterning apparatus (e.g., a mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto the target portion C of the substrate W. The substrate stage WTa or WTb can be accurately moved, for example, to position different target portions C within the path of the radiation beam B, by means of a second positioner PW and a position sensor IF (e.g., an interferometric device, linear encoder, 2D encoder, or capacitive sensor). Similarly, for example, after mechanical retrieval from a mask library or during scanning, the first positioner PM and another position sensor (… Figure 1 (Not explicitly shown) for accurately positioning the pattern forming apparatus (e.g., mask) MA relative to the path of the radiation beam B.

[0034] The patterning apparatus (e.g., a mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks occupy dedicated target portions, they can be located in the space between multiple target portions (these multiple target portions are referred to as scribing alignment marks). Similarly, in cases where more than one die is disposed on the patterning apparatus (e.g., a mask) MA, mask alignment marks can be located between the dies. Smaller alignment marks can also be included within the dies, between device features, in which case it is desirable that the marks be as small as possible and do not require any imaging or process conditions different from adjacent features. An alignment system for detecting these alignment marks is further described below.

[0035] The described apparatus can be used in various modes. In scanning mode, the pattern forming apparatus support (e.g., mask stage) MT and substrate stage WT are scanned synchronously while the pattern to be applied to the radiation beam is projected onto the target portion C (i.e., single dynamic exposure). The speed and direction of the substrate stage WT relative to the pattern forming apparatus support (e.g., mask stage) MT can be determined by the magnification (reduction) and image inversion characteristics of the projection system PS. In scanning mode, the maximum size of the exposure field limits the width of the target portion (along the non-scanning direction) in a single dynamic exposure, while the length of the scanning motion determines the height of the target portion (along the scanning direction) C. Other types of lithography apparatus and operating modes are possible, as is known in the art. For example, stepping mode is known. In so-called “maskless” lithography, the programmable pattern forming apparatus is kept stationary but has a changing pattern, and the substrate stage WT is moved or scanned.

[0036] Alternatively, the above usage patterns or combinations and / or variations of completely different usage patterns may be adopted.

[0037] The lithography apparatus LA belongs to the so-called dual-platform type, which has two substrate stages WTa and WTb, and two stations—an exposure station EXP and a measurement station MEA—where the substrate stages can be exchanged between the exposure and measurement stations. While a substrate on one stage is exposed at the exposure station, another substrate can be loaded onto the other substrate stage at the measurement station, and various preparatory steps can be performed. This enables a significant increase in the apparatus's throughput. The preparatory steps may include mapping or drawing the surface height profile of the substrate using a level sensor LS and measuring the position of alignment marks on the substrate using an alignment sensor AS. If the position sensor IF cannot measure the position of the substrate stage while it is simultaneously at the measurement and exposure stations, a second position sensor can be provided to enable tracking of the substrate stage's position relative to the reference frame RF at both stations. Other arrangements are known and available instead of the illustrated dual-platform arrangement. For example, other lithography apparatuses in which substrate stages and measurement stages are arranged are known. These substrate stages and measurement stages are mated together during preparatory measurements and then disengaged during exposure of the substrate stage.

[0038] Figure 2 The diagram illustrates the use of in Figure 1 The steps for exposing a target portion (e.g., a die) on a substrate W in a dual-platform lithography apparatus are described. The left-hand side within the dashed box shows the steps performed at the measurement station MEA, while the right-hand side shows the steps performed at the exposure station EXP. Typically, one of the substrate stages WTa and WTb will be located at the exposure station, while the other substrate stage will be located at the measurement station, as described above. For the purposes of this specification, it is assumed that the substrate W has already been loaded into the exposure station. At step 200, a new substrate W' is loaded into the apparatus via a mechanism not shown. Both substrates are processed in parallel to increase the throughput of the lithography apparatus.

[0039] First, consider the newly loaded substrate W'. This substrate can be a previously untreated substrate, prepared with a new photoresist for the first exposure in the apparatus. However, typically, the described lithography process will only be one step in a series of exposure and processing steps, such that the substrate W' has passed through this apparatus and / or other lithography apparatuses several times, and may also undergo subsequent processes. In particular, to address the issue of improving overlap performance, the task will be to ensure that the new pattern is correctly applied to the correct position on the substrate that has already undergone one or more cycles of patterning and processing. These processing steps gradually introduce deformations into the substrate, which must be measured and corrected to achieve satisfactory overlap performance.

[0040] Pre- and / or subsequent patterning steps (as just mentioned) can be performed in other lithography equipment, and even in different types of lithography equipment. For example, in device fabrication, some layers with very high requirements for parameters such as resolution and overlap can be processed in more advanced lithography tools compared to other layers with less stringent requirements. Therefore, some layers can be exposed in immersion lithography tools, while others are exposed in "dry" tools. Some layers can be exposed in tools operating at DUV wavelengths, while others are exposed using EUV wavelength radiation.

[0041] At point 202, alignment measurements using the substrate marker P1 and an image sensor (not shown) are used to measure and record the alignment of the substrate relative to the substrate stage WTa / WTb. Additionally, an alignment sensor AS is used to measure several alignment marks across the entire substrate W'. In one embodiment, these measurements are used to establish a "wafer grid" that maps the distribution of the marks across the entire substrate very accurately, including any distortions relative to the nominal rectangular grid.

[0042] At step 204, the horizontal sensor LS is also used to measure the wafer height (Z) map relative to the XY position. Typically, height mapping is used only to achieve accurate focusing of the exposed pattern. However, height mapping can be used for other purposes.

[0043] When substrate W' is loaded, configuration data 206 is received, which defines the exposure to be performed and also defines the properties of the wafer, the previously fabricated pattern, and the pattern to be fabricated on the wafer. Measurements of wafer position, wafer grid, and height mapping performed at 202 and 204 are added to this configuration data, allowing a complete set of configurations and measurement data 208 to be passed to the exposure station EXP. The alignment data measurements include, for example, the X and Y positions of the alignment target formed in a fixed or nominally fixed relationship with the product pattern (the product of the lithography process). This alignment data, acquired just before exposure, is used to generate an alignment model with parameters that fit 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 used interpolates positional deviations between the measured positions. A typical alignment model may include four, five, or six parameters that together define the translation, rotation, and scaling of the "ideal" grid in different dimensions. A high-level model using more parameters is known.

[0044] At 210, wafers W' and W are swapped so that the substrate W' being measured becomes the substrate W entering the exposure station EX.Figure 1 In the example apparatus, this interchange is performed by swapping the supports WTa and WTb within the apparatus, ensuring that the substrates W and W' remain accurately clamped and positioned on those supports to maintain relative alignment between the substrate stage and the substrate itself. Therefore, once the stage has been interchanged, determining the relative position between the projection system PS and the substrate stage WTb (formerly WTa) is necessary for controlling the exposure steps using measurement information from the substrate W (formerly W'), steps 202 and 204. At step 212, mask alignment is performed using the mask alignment marks M1 and M2. In steps 214, 216, and 218, scanning motion and radiation pulses are applied to consecutive target locations across the entire substrate W to complete the exposure of multiple patterns.

[0045] By using alignment data and height mapping obtained at the measurement station during the exposure step, these patterns are precisely aligned relative to the desired location, and more specifically, relative to features previously placed on the same substrate. At step 220, the exposed substrate, now labeled "W", unloaded from the device, undergoes etching or other processes according to the exposed pattern.

[0046] Those skilled in the art will recognize that the above description is a simplified overview of many very detailed steps involved in an example of a real manufacturing scenario. For instance, instead of measuring alignment in a single stroke or pass-through, there are often separate stages of coarse and fine measurements using the same or different markings. The coarse alignment measurement steps and / or fine alignment measurement steps can be performed before or after the height measurement, or interleaved with the height measurement.

[0047] Measuring the position of the markings can also provide information related to deformation of the substrate, which may be disposed on the substrate, for example, in the form of a wafer grid. Deformation of the substrate may occur, for example, by electrostatically clamping the substrate to the substrate stage and / or by heating the substrate when it is subjected to radiation.

[0048] Figure 3 This is a schematic block diagram of a known embodiment of an alignment sensor AS. A radiation source RSO provides a beam RB having one or more wavelengths, which is directed by a steering optics device as an illumination spot SP onto a marker (such as a marker AM located on a substrate W). In this example, the steering optics includes a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP (on which the marker AM is illuminated) may be slightly smaller than the width of the marker itself.

[0049] Radiation diffracted by the marker AM (in this example, through the objective lens OL) is collimated into the information-carrying beam IB. The term "diffraction" is intended to include zero-order diffraction from the marker (zero-order diffraction may be referred to as reflection). A self-referenced interferometer SRI (e.g., of the type disclosed in US6961116 mentioned above) causes the beam IB to interfere with itself, after which the beam is received by a photodetector PD. In cases where the radiation source RSO generates more than one wavelength, additional optics (not shown) may be included to provide discrete beams. The photodetector may be a single element, or it may include multiple pixels (if desired). The photodetector may include a sensor array.

[0050] The steering optics (which in this example include the speckle mirror SM) can also be used to block the zero-order radiation reflected from the marker, so that the information-carrying beam IB includes only the higher-order diffraction radiation from the marker AM (this is not necessary for measurement, but improves the signal-to-noise ratio).

[0051] The intensity signal SI is supplied to the processing unit PU. Through a combination of optical processing in the frame SRI and computational processing in the unit PU, the values ​​of the X and Y positions on the substrate relative to the reference frame are output.

[0052] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques can be used in conjunction with the single measurement to identify which period of the sine wave contains the marked position. To increase accuracy and / or robustness in detecting the mark, regardless of the material on which the mark is made or on or under which materials it is placed, the same process can be repeated at different wavelengths at coarser and / or finer levels.

[0053] The markings or alignment marks may comprise a series of strips formed on or within a layer disposed on the substrate, or (directly) formed in the substrate. These strips are regularly spaced and serve as grating lines, such that the markings can be considered as diffraction gratings with a well-known spatial period (pitch). Depending on the orientation of these grating lines, the markings can be designed to allow measurements along the x-axis or along the y-axis (with the y-axis oriented approximately perpendicular to the x-axis). Markings comprising strips arranged at +45 degrees and / or -45 degrees relative to both the x-axis and y-axis allow for combined x- and y-measurements using techniques as described in US2009 / 195768A (US2009 / 195768A is incorporated herein by reference).

[0054] The alignment sensor optically scans each mark with a radiation spot to obtain 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 in turn fixed relative to the reference frame of the photolithography apparatus. So-called coarse and fine marks involving different (coarse and fine) mark sizes can be provided, allowing the alignment sensor to distinguish different periods of the periodic signal and, more precisely, the exact position (phase) within a period. Marks with different pitches can also be used for this purpose.

[0055] The published patent application US 2009 / 0135389A1 describes a known method for measuring the focus of a photolithography projection apparatus using, for example, an alignment sensor, which is incorporated herein by reference. First, a wafer is exposed with a complete wafer-covered (verification) field at a predetermined optimal focus offset FO. The purpose of the focus offset FO is to expose the wafer at the most focus-sensitive portion of the so-called focus profile, as will be explained in more detail below. A mask used to expose multiple verification fields on a test substrate may include multiple focus markers (e.g., in an area of ​​250 markers), which, when exposed, produce corresponding markers on the substrate that can be read using an alignment sensor to infer the focus settings during exposure.

[0056] Therefore, the markings (on the mask) can include shredded strips with a range of linewidths and pitches. Due to the specific dimensions of the structure, the linewidth of the shredded strips will be sensitive to focus, according to the well-known Bossung principle. As a result, when such markings are exposed without focus, the exposed markings on the wafer will have an alignment offset. This offset can be used to determine the focus error (or defocusing amount) using an alignment sensor or other suitable measurement device.

[0057] To take advantage of this, a predetermined focus offset can be used to expose the verification field. The specific focus offset is determined by observing the focus profile that has already been exposed on the lithography equipment. Figure 4 A curve is shown, which is a graph of the alignment offset ao of the measured mark relative to the defocus amount dZ. Figure 4The useful focus offset FO is shown on the graph. Around the focus offset FO, a focus range (indicated by a rectangle with a reference 400) is defined, for which there is a unique relationship between the alignment offset and the defocus amount dZ. The focus offset FO ensures that all exposed focus marks will be within the usable focus range 400 throughout the exposure. For example, this means that the predetermined focus offset FO cannot be too close to the dZ = 0 axis, because the alignment offset as a function of dZ is zero, which is unusable for this method. In the embodiment, the focus offset FO is chosen to be in the middle of the working range 400 where the focus curve is approximately linear, because this is the region of maximum focus sensitivity to alignment.

[0058] After exposing the verification field, the test substrate is developed, and the alignment offset of each verification mark in the verification marks is measured using, for example, a standard alignment sensor present in the system. Next, the measured alignment offset of each verification mark in the verification marks is converted into defocus data using a so-called transposed focus curve. This transposed focus curve can be determined by transposing a portion of the focus curve of the lithography apparatus corresponding to the working range indicated by rectangle 400. The transposed focus curve is essentially the focus sensitivity of the apparatus.

[0059] Figure 5 The illustration shows an example of suitable focus markings (as seen on a mask), which is currently used in the method described above. The markings on the mask include alignment-type markings, where a portion (e.g., half) of each line structure in the line structure is "shredded" (subdivided into sub-gratings or sub-periodic features) rather than continuous. Thus, marking 500 includes periodic features or gratings, where each individual line or grating feature includes a solid portion 510 and a shredded portion 520.

[0060] Based on the Poisson curves of those structures, the imaging quality of the shredded portion 520 depends on contrast. Therefore, the effective alignment position seen by the alignment sensor is any position between the center of the continuous line structure and the center of the complete line structure. Thus, the alignment offset plotted as a function of defocus behaves similarly to the Poisson curve. The position of the peak on the resulting curve indicates optimal focus, while the height of the peak indicates the degree of "fading" caused by dynamic errors. As an algorithm, such a method may include:

[0061] 1. Expose the periodic focus marker at a given defocus point;

[0062] 2. Read this mark using an alignment sensor (AS);

[0063] 3. Fit the original periodic AS signal to a sine wave with the same period; the phase shift of this sine wave... Depends on the out-of-focus value (f) during exposure.

[0064] 4. Read the phase shift from the measurement results. This phase shift... This can be referred to as alignment offset.

[0065] 5. The focus curve AO(f) is well approximated by the parabola near the optimal focus (BF).

[0066] Therefore, BF is measured by fitting the top of the focusing curve.

[0067] One drawback of the current method is the phase shift. (That is, alignment offset AO) also depends on other asymmetry contributions (such as overlap error OV), and not only on focus-induced asymmetry;

[0068] To remove overlapping contributions, measurements at each measurement location can be taken of two marks that are mirror-symmetric relative to each other. This is as follows: Figure 5 As shown, there exists a right marker 500R and a left marker 500L. For these two markers, the overlap error is the same, while the focus dependence is opposite (due to the L / R symmetry). Mathematically:

[0069] The phase shift of the right-marked 500R is as well as

[0070] The phase shift of the left marker 500L is

[0071] Therefore, by obtaining the difference between the left and right alignment offsets (phase shifts), overlap errors can be eliminated, and a signal that depends solely on the focus can be obtained.

[0072] This method has several drawbacks. The large total marker size (comprising two separate markers (L and R)) doubles the required readout time and mask / substrate area. The total time spent on focus calibration and verification is significantly impacted by this. Furthermore, current forms of signal processing for alignment marker measurements require the two markers to cancel out any potential overlap, assuming the overlap of the two sub-markers will be equal (which may not be the case). The measurement signal includes noise and other artifacts. This method now decomposes the signal into Fourier components and calculates the phase difference. The periodicity assumption of the signal is not robust to aliasing problems (for Fourier decomposition, the entire period of the signal, or integer multiples of its periods, is not taken); this can lead to errors in calculating alignment offset, resulting in defocusing or out-of-focus phenomena.

[0073] The original alignment signal contains more information that could be used to track other parameters / changes between different markers and the field, but this information is not currently being utilized. The method disclosed below utilizes this additional information.

[0074] Therefore, many focus measurement methods have been proposed that can measure focus from a single marker while still mitigating the effects of overlap errors. Compared to currently used information, this method utilizes additional information available from the measured periodic raw signal from the alignment sensor (AS).

[0075] In the first embodiment, the amplitudes of these harmonic components are used when determining the focus, instead of or excluding the first harmonic. Phase shift (and higher-order harmonics). The inventors have determined that these amplitudes do not suffer from overlap errors, which is the opposite of what happens to the phase of harmonic components. This embodiment also discloses a new marking design that better utilizes this method.

[0076] Figure 6 The diagram illustrates this point. Figure 6 The alignment sensor signal AS is shown in (a) the case without overlap error and (b) the case with overlap error. sig Relative to scan length AS sl (a) is a graph. In each graph, the original signal AS is shown. raw and its preceding third harmonic component AS 1st AS 2nd AS 3rd In the current method, only the phase of the first harmonic is used. As an alignment offset (with phase starting from the left mark) (Combination). When overlap error exists ( Figure 6 (b)), phase Influenced by a significant amount (shifted), making However, the amplitudes of the harmonics (e.g., A1 of the first harmonic and A2 of the second harmonic, respectively) do not suffer from overlap error (i.e., they do not change as the original signal is shifted backward or forward).

[0077] Therefore, it is suggested that by deriving the focus from the amplitude rather than the phase shift, a focus value that is insensitive to overlap can be inferred from a single marker. In this way, an approximate 50% gain in readout time can be achieved for all focus tests.

[0078] It is understandable that these amplitudes may be affected by amplitude drift and variations in the original AS signal. For example, the amplitude of the original AS signal may be affected by variations in the intensity of the AS light source or by variations in the gain of the electronics used to generate the original signal. Since readouts in focus tests can last for several hours, it is conceivable that the overall AS output may not be stable enough to reliably retrieve the focus value using a single amplitude signal.

[0079] However, refer to Figure 6 It can be seen that the focus-dependent signal given by the ratio r(f) = A2(f) / A1(f) between the first harmonic amplitude A1(f) and the second harmonic amplitude A2(f) is an overlap-independent focus signal, which is also independent of the amplitude drift of the original signal from the alignment sensor. Thus, if the amplitude of the original signal changes by a factor C (e.g., due to drift in the AS light source), both amplitudes A1 and A2 change by the same factor (becoming CA1 and CA2), and the ratio r = CA2 / CA1 = A2 / A1 remains constant. Therefore, the proposed index according to the embodiment is a suitable combination of the amplitudes of at least two harmonics of the alignment signal; for example, the ratio r(f) between the amplitudes of the two harmonic components of the original AS signal. Any other combination that produces an overlap-independent focus signal can also be used, which can also eliminate the dependence on the amplitude of the original signal.

[0080] Figure 7 This is a schematic diagram of a proposed marking for utilizing the alternative focus signal r(f) according to an embodiment. Such a marking maximizes the focus response of the amplitude ratio signal r(f). In other words, such a marking provides a good focus curve (e.g., a curve with sufficient capture range and amplitude) for the amplitude ratio signal r(f). Current focus markings are designed to provide a good focus curve. That is, a good focus curve in terms of phase shift (or alignment offset), but not necessarily a good focus curve in terms of amplitude ratio r(f).

[0081] The markings 700 on the photomask include a periodic structure, wherein one of every two bars is focus-sensitive. Thus, the markings can comprise a repeating pattern of solid bars 710 separated by space 705 and focus-sensitive bars or diced bars 720; for example, with a 50% duty cycle. The principle of this design is to have a marking whose harmonic content (i.e., its amplitude spectrum) can be altered by focusing. When the diced bar 720 is exposed at a dose greater than to be cleared in unfocused Oof state, the effect in the resist 725 is closer to the effect of space (fully exposed resist), making it read as blank or space. Therefore, the effect on the wafer is similar to that of a marking with a period P and a 25% duty cycle (e.g., a repetition of three blanks 730 and one solid bar 735). On the other hand, when the diced bar 720 is exposed at optimal focus BF, its effect in the resist is more similar to the effect of the bars (and therefore will be read as such). Therefore, under optimal focus, the mark 740 on the wafer will behave more closely to a mark with two bars 735 and two blanks 730 in period P, that is, more closely to a mark with period P / 2. This behavior will change the harmonic spectrum of the original AS signal. The first harmonic (periodic P) in the out-of-focus case will be higher than the first harmonic (periodic P) in the optimal focus case, while the second harmonic of the signal in the optimal focus case will increase (P / 2 periodicity).

[0082] The specific layout may be related to Figure 7 The markings shown are different. Figure 7 The markings shown are merely examples. Any type of alignment mark can be used that is designed to increase the response of at least two harmonics (or other combinations) of the measured signal. Such markings on a mask can include (but are not limited to) alternating reflective and absorptive segments (e.g., blanks and bars, respectively), wherein one or more bars (and / or one blank) comprises a focus-sensitive sub-segmented bar (chopped bar). The chopped bar modulates the resist height after PEB in a focus-sensitive manner, while the full bar and blank modulate the resist height almost equally by focusing. In this way, the spectral content of the exposure mark is modulated by focusing. For example, the mark can have bars of P / 3 length, which are designed to modulate the third harmonic. Alternatively, the mark can have two sets of features of different sizes, such as alternating length P / 3 features and length P / 2 features, to modulate the second and third harmonics. Alternating sizes can be applied to both focus-sensitive (chopped) bars and solid bars. It should be understood that the optimal signal may not necessarily be the ratio of second harmonic amplitude to first harmonic amplitude, but sometimes it may be other ratios or combinations of harmonic amplitudes.

[0083] After exposure-before baking (PEB), the diced area is almost completely exposed. This means that during PEB, the resist in this area will be almost completely deprotected, thus reducing the resist height. The opposite behavior is observed when the resist is exposed with optimal focus. Here, the portion of the resist below the diced area remains protected and will hardly shrink. This loss of thickness allows the resist to be read out by the alignment sensor after PEB because the height variation in the resist acts as a diffraction grating (so-called "latent readout" because the resist does not need to be developed to provide an alignment signal). Note that the AS signal includes harmonic components of the marking itself.

[0084] Through specific examples, for each defocus value and Figure 7 The example notation can be illustrated as the second harmonic increases and the first harmonic decreases as the image moves from out of focus to optimal focus. Therefore, the ratio r(f) between the second and first harmonics produces the focus curve that is largest at optimal focus. This curve can be used to determine optimal focus in the resist by sampling it at several out-of-focus locations and then finding the out-of-focus value at which the curve reaches its maximum value (e.g., by fitting the sampled data with a parabola).

[0085] A second embodiment will now be described, which processes the raw alignment signal collected from only one marker at each measurement location while decoupling overlap from focus. This embodiment also demonstrates good performance of the markers using this design, avoiding any marker design variations (except, optionally, the use of a second mirror marker at each measurement location).

[0086] This embodiment includes unsupervised learning methods. Such methods may include applying linear or nonlinear dimensionality reduction methods to measurement data associated with different measurement locations and focus offsets, in order to decouple common sensor noise, overlap, and other artifacts from the underlying signal that has a desired relationship with the focus offset. Suitable dimensionality reduction methods may include, for example, principal component analysis (PCA), isometric mapping, or unified manifold approximation and projection (UMAP).

[0087] The method may include obtaining measurement data comprising raw alignment signals from different measurement locations, each raw alignment signal corresponding to one of a plurality of similar single marker structures already formed at said different locations with varying focus offsets. This measurement data may then be stacked in the following matrix form:

[0088]

[0089] Where m represents the total number of measurement positions with different focus offsets on the wafer, and n is the total length of the discretized alignment signal. The rows of the Data matrix are the alignment measurement results for each marked location on the wafer, and the columns are the discretized x-values ​​of each alignment signal.

[0090] Dimensionality reduction methods are applied to this data matrix to decouple common sensor noise, overlap, and other artifacts from the underlying signal that has the desired relationship with focus offset. The data matrix can then be decomposed into two matrices A and B:

[0091] Data (m×n) =A (m×n′) B (n′×n) +mean(Data), where n′≤n (2)

[0092] Matrix A is a representation of the Data matrix in a space with an orthogonal basis, possibly having a dimension n' lower than n. The method then involves examining matrix A to identify which column has a desired relationship with the focus offset, e.g., a parabolic relationship. This column represents purified alignment or focus signal data, extracted from the noisy and potentially contaminated original alignment signal. It has been observed that mapping to only the first component is sufficient to achieve a very accurate representation of the defocus amount, without significant contributions from other factors. A focus curve can then be constructed by plotting the selected column A against the set focus offset. The maximum value of the selected column can be found from the curve, and the optimal focus (BF) can be defined as its corresponding focus offset.

[0093] Figure 8 The raw alignment readout signals from a single structural marker are shown for different defocus levels. It can be observed that different defocus levels encode different shapes into the alignment signal. As already described, current methods use phase shifting to achieve a scalar alignment offset, which is then used to create a focus curve to estimate optimal focus (or unknown focus in a focus uniformity verification test (FUV)). However, this approach is heavily influenced by the actual unknown overlap of the markers. As discussed in the previous section, we use our unsupervised learning technique to map the shape of the alignment signal while remaining insensitive to phase / x-shift.

[0094] It has been shown that even when the data is noisy and the markers have varying overlap offsets, using linear PCA to map the original alignment signal (to one-dimensional space) is sufficient to construct an accurate focus curve and infer optimal focus. Nonlinear methods (e.g., isometric mapping and UMAP) can be shown to construct sharper or more defined curves and provide more discriminative optimal focus determination.

[0095] In another embodiment, Independent Component Analysis (ICA) is proposed for multiple measurements with different focus offsets, based on the assumption that overlap and measurement noise are independent of focus. Using this assumption, the original alignment signal can be decomposed into a (weighted) focus signal and other components. To find the focus components, constraints can be added to the optimization of the ICA method to find signals with certain desired characteristics; for example, range and sign changes corresponding to the applied set focus value. The signal can then be processed as in the previous example to obtain the optimal focus value.

[0096] An implementation example utilizing supervised learning methods will now be described. Such methods can be based on focusing tests on multiple fields with different induced focusing shifts (such a test layout can be similar to that described above). Figure 8 (and the focus test layout described in the FUV layout). Once exposed and measured, the original alignment signal AL i and the corresponding (known) set focus value f i This data can then be used to train a suitable model to infer focus from the raw alignment data. Such a model can include, for example, a kernel regression model or a simple neural network (e.g., with only one hidden layer). The input data can include per-sample (e.g., each individual structural label at the measurement location): a row vector containing quantized / discrete values ​​over the length (per x) of the alignment measurement, with corresponding reference labels (setting the focus offset value for each sample). In an embodiment, the total dataset can be divided into training, validation, and test sets to avoid overfitting.

[0097] After training, the trained (e.g., nonlinear) model can be stored and used to infer focus from the original alignment signal (from a single focus marker; e.g., a verification field in an FUV (focus uniformity verification) test).

[0098] In summary, the concepts disclosed herein describe several methods for readout using aligned sensors, making measurements insensitive to overlap and eliminating the need for two mirror-symmetric markers (left and right). This reduces the readout time for all tests utilizing the focusing test method by a factor of two, i.e., to half. This (by reducing setup and qualification time) reduces system assembly cycle time, reduces maintenance time whenever the machine requires such testing (e.g., recovery, recalibration, or user-specific testing), resulting in increased availability and reduced substrate surface area of ​​the focusing markers on the mask.

[0099] A pattern forming apparatus is also disclosed, comprising a plurality of periodic features for forming periodic structures on a substrate, each periodic feature comprising alternating first segments and second segments, wherein a suitable subset of the first segments comprises focus-sensitive first segments. The focus-sensitive segments comprise sub-segmented segments. The first segments may comprise absorbing segments and the second segments may comprise reflecting segments; or the first segments may comprise reflecting segments and the second segments may comprise absorbing segments. The first segments may comprise alternating insensitive first segments and the focus-sensitive first segments, the insensitive first segments not having significant focus sensitivity. The focus-sensitive segments approximate corresponding blanks on the substrate when substantially not exposed with focus, and approximate corresponding stripes on the substrate when exposed with optimal focus.

[0100] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in ways other than those described.

[0101] As used herein, the terms “radiation” and “beam” include all types of electromagnetic radiation, including: ultraviolet (UV) radiation (e.g., having wavelengths of 365 nm, 355 nm, 248 nm, 193 nm, 157 or 126 nm or more) and extreme ultraviolet (EUV) radiation (e.g., having wavelengths in the range of 1 nm to 100 nm), as well as particle beams, such as ion beams or electron beams.

[0102] Where context permits, the term "lens" can refer to any one or a combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components. Reflective components can be used in equipment operating in the UV and / or EUV range.

[0103] The breadth and scope of this invention should not be limited by any of the exemplary embodiments described above, but should be defined only by the appended claims and their equivalents.

[0104] Other aspects of the invention are set forth in the following numbered aspects:

[0105] 1. A method for determining focusing parameter values ​​for at least one structure on an exposed substrate, comprising:

[0106] Obtain measurement data relating to the measurement of the at least one structure, wherein the at least one structure comprises a single periodic structure at each measurement location;

[0107] The measurement data is decomposed into component data, which includes one or more components of the measurement data;

[0108] Process at least one of the components to extract processed component data with reduced dependence on non-focused correlation effects; and

[0109] The focusing parameter value is determined based on the processed component data.

[0110] 2. According to aspect 1, wherein:

[0111] The decomposition step includes decomposing the measurement data to obtain at least two harmonic components of the measurement data; and

[0112] The processing steps include determining a combination of the amplitudes of each of the at least two harmonic components.

[0113] 3. According to aspect 2, wherein the combination includes the ratio of the amplitudes of each of the at least two harmonic components.

[0114] 4. According to aspect 2 or 3, wherein the harmonic components include a first harmonic component and a second harmonic component.

[0115] 5. The aspect according to any one of aspects 2 to 4, wherein the at least one structure comprises at least one periodic structure, the at least one periodic structure increasing the response of the combination of the amplitudes of the at least two harmonic components to the focusing parameter.

[0116] 6. The aspect according to aspect 5, including:

[0117] The at least one periodic structure is exposed using a mask comprising at least one corresponding periodic feature having alternating first and second segments, wherein an appropriate subset of the first segments comprises focus-sensitive first segments.

[0118] 7. According to aspect 6, wherein the focus-sensitive segment includes a sub-segment segment.

[0119] 8. According to aspect 6 or 7, wherein the first segment includes an absorbing segment and the second segment includes a reflecting segment; or the first segment includes a reflecting segment and the second segment includes an absorbing segment.

[0120] 9. The aspect according to any one of aspects 6 to 8, wherein the first segment comprises alternating insensitive first segments and focus-sensitive first segments, the insensitive first segments not having significant focus sensitivity.

[0121] 10. The aspect according to any one of aspects 6 to 9, wherein the focus-sensitive segment approximates a corresponding blank on the substrate when exposed substantially without focus, and approximates a corresponding stripe on the substrate when exposed with optimal focus.

[0122] 11. The aspect according to any one of aspects 6 to 10, comprising:

[0123] Multiple structures are exposed with different focusing parameter values;

[0124] The structure is measured to obtain the measurement data;

[0125] The measurement data is used to determine the focusing curve; and

[0126] The focusing curve is used to deduce the optimal value of the focusing parameter in the resist.

[0127] 12. According to aspect 1, wherein:

[0128] The measurement data is related to different measurement locations and the structure formed by setting values ​​for variations in the focusing parameters;

[0129] The decomposition and / or processing steps include applying linear or nonlinear dimensionality reduction methods to decouple potential signals that have a desired relationship with the focusing parameters from the non-focused correlation effects.

[0130] 13. According to aspect 12, wherein the dimensionality reduction method includes at least one of the following: principal component analysis, isometric mapping or unified manifold approximation and projection.

[0131] 14. The method according to aspect 12 or 13, wherein the method comprises:

[0132] The measurement data are stacked into an m x n data matrix, where m is the total number of measurement locations and n is the length of each discrete measurement signal from each measurement location;

[0133] The data matrix is ​​decomposed into at least two decomposition matrices, wherein the first decomposition matrix of the at least two decomposition matrices includes the representation of the data matrix in a space having an orthogonal basis; and

[0134] Determine the columns of the first decomposition matrix that display the expected relationship with the focusing parameters.

[0135] 15. The aspect according to aspect 14, comprising: constructing a focus curve by plotting the determined columns of the first decomposition matrix relative to set focus parameter values; and

[0136] The optimal focusing parameter values ​​are determined based on the focusing curve.

[0137] 16. According to aspect 12, wherein the dimensionality reduction method includes independent component analysis.

[0138] 17. The aspect according to aspect 16, comprising: performing the independent component analysis on measurement data including multiple measurements with different focusing parameter values, based on the assumption that the non-focused correlation effect is independent of the focusing parameter.

[0139] 18. According to aspect 16 or 17, wherein the decomposition step includes decomposing the measurement data into focusing parameter components and other components.

[0140] 19. According to aspect 17, wherein the decomposition step includes, in order to find the focusing parameter components, applying constraints to independent component analysis optimization such that the optimization seeks to identify signals having expected characteristics indicative of the focusing parameters.

[0141] 20. According to aspect 19, wherein the expected characteristics include variations in range and sign corresponding to the applied set focus parameter value.

[0142] 21. A method for determining focusing parameter values ​​for exposing at least one structure on a substrate, comprising:

[0143] Obtain measurement data relating to the measurement of the at least one structure, wherein the at least one structure comprises a single periodic structure at each measurement location;

[0144] Obtain a trained model that has been trained to infer focus from the measurement data; and

[0145] The focusing parameter value is determined based on the trained model and the measurement data.

[0146] 22. The method according to aspect 21, wherein the trained model comprises a kernel regression model or a neural network.

[0147] 23. The aspect according to aspect 21 or 22 includes an initial step of training the trained model with respect to training measurement data; the training measurement data includes measurement data relating to a plurality of individual periodic structures formed at different measurement locations and with set values ​​for variations in the focusing parameters, the training measurement data being labeled with the set values.

[0148] 24. According to aspect 23, wherein the training measurement data comprises a row vector for each measurement position, the row vector containing discrete values ​​over the length of a measurement signal corresponding to the measurement position, the discrete values ​​being labeled with the corresponding set values.

[0149] 25. According to aspect 23 or 24, wherein the training measurement data is divided into a training set, a validation set, and a test set to avoid overfitting during the training.

[0150] 26. According to any of the preceding aspects, wherein the measurement data includes alignment data measured using an alignment sensor.

[0151] 27. A pattern forming apparatus comprising a plurality of periodic features for forming a periodic structure on a substrate, each of the periodic features comprising alternating first segments and second segments, wherein a suitable subset of the first segments comprises a focus-sensitive first segment.

[0152] 28. The pattern forming apparatus according to aspect 27, wherein the first segment comprises alternating insensitive first segments and focus-sensitive first segments, the insensitive first segments not having significant focus sensitivity.

[0153] 29. A computer program comprising program instructions operable to perform the method according to any one of aspects 1 to 26 when run on a suitable device.

[0154] 30. A non-transitory computer program carrier, the carrier comprising the computer program according to aspect 29.

[0155] 31. A processing apparatus, comprising:

[0156] processor; and

[0157] A program memory comprising a computer program, the computer program comprising program instructions operable to perform the method according to any one of aspects 1 to 5 or 12 to 25.

[0158] 32. A photolithography apparatus, comprising:

[0159] Align with the sensor;

[0160] A pattern forming apparatus support member, the pattern forming apparatus support member being used to support the pattern forming apparatus;

[0161] Substrate support member, the substrate support member being used to support the substrate; and

[0162] The processing apparatus according to aspect 31.

[0163] 33. The lithography apparatus according to aspect 32; also operable to perform the method according to any one of aspects 6 to 11.

[0164] 34. The lithography apparatus according to aspect 32 or 33 is also operable to use the alignment sensor to measure the at least one structure to obtain the measurement data.

[0165] 35. The lithography apparatus according to any one of aspects 32 to 34, wherein the pattern forming apparatus support includes the pattern forming apparatus according to aspect 27 or 28 supported thereon.

[0166] 36. A substrate comprising a plurality of periodic structures, said plurality of periodic structures having been exposed on said substrate using a patterning apparatus according to aspect 27 or 28.

Claims

1. A method for determining focusing parameter values ​​for at least one structure on an exposed substrate, comprising: Obtain measurement data relating to the measurement of the at least one structure, wherein the at least one structure comprises a single periodic structure at each measurement location; The measurement data is decomposed into component data, which includes one or more components of the measurement data; Process at least one of the components to extract processed component data with reduced dependence on non-focused correlation effects; and The focusing parameter value is determined based on the processed component data.

2. The method according to claim 1, wherein: The decomposition step includes decomposing the measurement data to obtain at least two harmonic components of the measurement data; and The processing steps include determining a combination of the amplitudes of each of the at least two harmonic components.

3. The method according to claim 1, wherein: The decomposition step includes decomposing the measurement data to obtain at least two harmonic components of the measurement data; and The processing steps include determining a combination of phases for each of the at least two harmonic components.

4. The method according to claim 3, wherein, The combination includes the phase difference of each of the at least two harmonic components.

5. The method according to any one of claims 3 or 4, wherein, The at least one structure includes at least one periodic structure that increases the response of the combination of the phases of the at least two harmonic components to the focusing parameters.

6. The method according to claim 2, wherein, The combination includes the ratio of the amplitude of each of the at least two harmonic components.

7. The method according to any one of claims 2 to 4 and 6, wherein, The harmonic components include the first harmonic component and the second harmonic component.

8. The method according to claim 2, wherein, The at least one structure includes at least one periodic structure that increases the response of the combination of the amplitudes of the at least two harmonic components to the focusing parameter.

9. The method of claim 8, comprising: The at least one periodic structure is exposed using a mask comprising at least one corresponding periodic feature having alternating first and second segments, and Wherein, an appropriate subset of the first segment includes the first segment that is sensitive to focus.

10. The method of claim 9, comprising: Multiple structures are exposed with different focusing parameter values; The structure is measured to obtain the measurement data; The measurement data is used to determine the focusing curve; as well as The focusing curve is used to deduce the optimal value of the focusing parameter in the resist.

11. The method according to claim 1, wherein: The measurement data is related to different measurement locations and the structure formed by setting values ​​for variations in the focusing parameters; The decomposition and / or processing steps include applying linear or nonlinear dimensionality reduction methods to decouple potential signals that have a desired relationship with the focusing parameters from the non-focused correlation effects.

12. The method according to claim 11, wherein, The dimensionality reduction method includes at least one of the following: principal component analysis, isometric mapping, or unified manifold approximation and projection.

13. The method according to claim 11 or 12, wherein, The method includes: The measurement data are stacked into an m x n data matrix, where m is the total number of measurement locations and n is the length of each discrete measurement signal from each measurement location; The data matrix is ​​decomposed into at least two decomposition matrices, wherein the first decomposition matrix of the at least two decomposition matrices includes the representation of the data matrix in a space having an orthogonal basis; and Determine the columns of the first decomposition matrix that display the expected relationship with the focusing parameters.

14. A method for determining focusing parameter values ​​for exposing at least one structure on a substrate, comprising: Obtain measurement data relating to the measurement of the at least one structure, wherein the at least one structure comprises a single periodic structure at each measurement location; Obtain a trained model that has been trained to infer focus from the measurement data; and The focusing parameter value is determined based on the trained model and the measurement data.

15. The method according to claim 14, wherein, The trained model includes a kernel regression model or a neural network.

16. The method of claim 14 or 15, comprising an initial step of training the trained model with respect to training measurement data; the training measurement data comprising measurement data relating to a plurality of individual periodic structures formed at different measurement locations and with set values ​​for variations in the focusing parameters, the training measurement data being labeled with the set values.

17. The method according to claim 14 or 15, wherein, The measurement data includes alignment data measured using an alignment sensor.

18. A computer program comprising program instructions operable to perform the method according to any one of claims 1 to 17 when run on a suitable device.

19. A processing apparatus, comprising: processor; and A program memory, the program memory including a computer program, the computer program including program instructions operable to perform the method according to any one of claims 1 to 17.

20. A photolithography apparatus, comprising: Align with the sensor; A pattern forming apparatus support member, the pattern forming apparatus support member being used to support the pattern forming apparatus; A substrate support member for supporting a substrate; and The processing apparatus according to claim 19.

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