Method for aligning, overlaying, configuring marks, manufacturing pattern forming devices and patterning marks

Abstract

A resonant amplitude grating mark has a periodic structure configured to scatter radiation (602) of wavelength λ incident (600) on the alignment mark. The scattering is primarily due to coupling of the incident radiation into a waveguide mode in the periodic structure. The effective refractive index (n) of each portion of the periodic structure is l , n2) and length (d l , d2) are configured to provide an optical path length (n1d1+n2d2) of the unit cell in the periodic direction, the optical path length (n1d1+n2d2) being equal to an integer multiple (mλ) of the wavelength present in the spectrum of the radiation. The effective refractive index (n l , n2) and length (d l , d2) is also configured to provide an optical path length (n2d2) of the second portion in the periodic direction, wherein the optical path length (n2d2) is 0.30 to 0.49 times the wavelength presented in the spectrum of the radiation.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 19186248.1 filed on July 15, 2019 and European application 19186925.4 filed on July 18, 2019, which are incorporated herein by reference in their entirety. Technical Field

[0003] The present disclosure relates to a mark, an overlay target, and an associated method of aligning and determining overlay error that may be used in the fabrication of devices, such as by photolithography. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may project a pattern (also often referred to as a "design layout" or "design"), for example, at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] In order to project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of the radiation determines the minimum size of a feature that can be formed on a substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm, and 13.5nm. Compared to a lithographic apparatus using, for example, radiation with a wavelength of 193nm, a lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4-20nm (e.g., 6.7nm or 13.5nm) can be used to form smaller features on a substrate.

[0006] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD=k1×λ / NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce on a substrate a pattern similar in shape and size to that planned by the circuit designer to achieve specific electrical functions and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These fine-tuning steps include, for example, but are not limited to, optimization of NA, customized illumination schemes, use of phase-shifted patterning devices, various optimizations of the design layout (such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout), or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] Accurate placement of patterns on substrates is a major challenge in reducing the size of circuit components and other products that can be produced by lithography. In particular, the challenge of accurately measuring features that have been laid down on the substrate is a critical step in being able to align successive layers of features accurately enough when overlaying to produce working devices with high yields. Typically, in today's submicron semiconductor devices, the so-called overlay is achieved within tens of nanometers and can be as low as a few nanometers in the most critical layers.

[0008] During lithography, it is often desirable to make measurements of the structures created, for example for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, which are often used to measure critical dimensions (CD), and specialized tools for measuring the accuracy of the overlay, alignment of two layers in a device. More recently, various forms of scatterometers have been developed for use in the field of lithography. These devices direct a beam of electromagnetic radiation onto a target and measure one or more properties of the scattered electromagnetic radiation (e.g., intensity at a single reflection angle as a function of wavelength; intensity at one or more wavelengths as a function of reflection angle; or polarization as a function of reflection angle) to obtain a diffraction "spectrum" from which properties of interest of the target can be determined.

[0009] Conventional binary phase grating marks rely on interference between diffracted light from the top of the line and the bottom of the space. The diffraction height of the light depends on the grating depth. The interference between the + and – diffraction orders is detected by an alignment sensor to determine the alignment position. For an ideal mark without any asymmetry, the alignment position deviation (APD) should theoretically be 0. However, due to imperfections in processes such as etching, chemical mechanical polishing (CMP), deposition, etc., the mark can be deformed in many different ways. Typical mark deformations can be simplified and classified into three main types of asymmetry, including floor tilt (FT), side wall angle (SWA) and top tilt (TT). The mark depth may also be different from the nominal value due to process reasons. When one or more of these asymmetries appear in the printed mark, this will result in an alignment error, i.e. APD. Due to the interferometric nature of the phase grating, the impact of asymmetry on APD also depends largely on the mark depth.

[0010] Overlay targets used to measure overlay error are also subject to the same deformation problem. This can cause the measured overlay error to be inaccurate.

[0011] International Patent Publication No. WO2019081091A1 (the entire contents of which are incorporated herein by reference) describes these problems and discloses a technical solution including a resonant amplitude mark. A mark is formed on a planar substrate, the mark including a periodic structure configured to scatter radiation incident on a surface plane of the alignment mark, the surface plane being parallel to the plane of the substrate, the scattering being primarily due to the excitation of resonant modes in the periodic structure parallel to the surface plane. The problem with this approach is that it is limited to design rules based on resonance. Summary of the invention

[0012] It is desirable to have marks (typically alignment marks and overlay targets) that are less sensitive to mark asymmetry due to process-induced deformation within wide design rule tolerances. It is also desirable to provide methods of alignment, overlay measurement, configuring marks used in said alignment and overlay measurement, manufacturing a patterning device including said less sensitive marks, and providing said less sensitive marks to a planar substrate. It is also desirable to have a metrology system configured to perform the method of alignment and overlay measurement, and a tool including said metrology system, such as a lithography apparatus or a stand-alone metrology tool.

[0013] According to a first aspect of the present invention, there is provided an alignment method, the method comprising the following steps: providing an alignment mark formed on or in a planar substrate, the mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part having a first length and a first effective refractive index along the periodic direction, the second part having a second length and a second effective refractive index along the periodic direction, the second effective refractive index of the second part within its optical path range being lower than the first effective refractive index; irradiating the alignment mark with radiation having a predetermined wavelength, wherein the effective refractive index and length of each part of the alignment mark are configured to provide: a) an optical path length of the unit cell in the periodic direction that is equal to an integer multiple of the predetermined wavelength, and b) an optical path length or optical path length of the second part in the periodic direction that is 0.35 to 0.45 times the predetermined wavelength, so that scattering occurs mainly due to excitation of a waveguide mode in the periodic structure; detecting radiation generated by the irradiation and scattered by the alignment mark; and determining the position of the alignment mark using the detected radiation.

[0014] According to a second aspect of the present invention, an alignment method is provided, the method comprising the following steps: providing an alignment mark formed on or in a planar substrate, the mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part has a first length and a first effective refractive index along the periodic direction, the second part has a second length and a second effective refractive index along the periodic direction, the second effective refractive index of the second part within its optical path range is lower than the first effective refractive index; irradiating the alignment mark with broadband radiation including a predetermined wavelength; An alignment mark, wherein the effective refractive index and length of each portion of the alignment mark are configured to provide: a) an optical path length of the unit cell in the periodic direction that is equal to an integer multiple of a predetermined wavelength, and b) an optical path length of the second portion in the periodic direction that is 0.35 to 0.45 times the predetermined wavelength, so that scattering of the predetermined wavelength occurs primarily due to excitation of a waveguide mode in the periodic structure; filtering radiation generated by the illumination and scattered by the alignment mark to select wavelengths in the periodic structure that correspond to excitation of the waveguide mode parallel to the surface plane; detecting the filtered radiation; and determining the position of the alignment mark using the detected radiation.

[0015] According to a third aspect of the present invention, there is provided an overlay measurement method, the method comprising: providing an overlay mark formed on or in a planar substrate, the overlay mark comprising a lower mark superimposed on an upper mark, the upper mark having the same pitch as the lower mark, the lower mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part has a first length and a first effective refractive index along the periodic direction, the second part has a second length and a second effective refractive index along the periodic direction, and the second part is in its optical path The second effective refractive index in the range is lower than the first effective refractive index; using radiation with a predetermined wavelength to irradiate the overlapping mark, wherein the effective refractive index and length of each part of the lower mark are configured to provide: a) an optical path length of the unit cell in the periodic direction that is equal to an integer multiple of the predetermined wavelength, and b) an optical path length of the second part in the periodic direction that is in the range of 0.35 times to 0.45 times the predetermined wavelength, so that scattering mainly occurs due to excitation of waveguide modes in the periodic structure; detecting radiation generated by the irradiation and scattered by the overlapping mark; and using the detected radiation to determine the overlap at the position of the overlapping mark.

[0016] According to a fourth aspect of the present invention, there is provided a method for configuring a mark on or in a planar substrate, the planar substrate having a periodic structure, the periodic structure being configured to scatter radiation incident on the mark when subjected to alignment or overlay measurement performed on the planar substrate, the method comprising: obtaining the wavelength of the radiation incident on the mark; and configuring the periodic structure based on the excitation of a waveguide mode of the periodic structure parallel to a surface plane of the planar substrate when irradiated by incident radiation at the wavelength.

[0017] According to a fifth aspect of the present invention, there is provided a method for manufacturing a pattern forming device, wherein the pattern forming device is used to provide a mark comprising a periodic structure to a planar substrate, the method comprising: obtaining a wavelength of radiation for illuminating the planar substrate when subjected to alignment or overlap measurement; configuring the periodic structure so that when the periodic structure is provided to the planar substrate: a) scatters incident radiation when subjected to alignment or overlap measurement; and b) couples incident radiation at the wavelength to a waveguide mode of the periodic structure parallel to a surface plane of the substrate; and providing the configured mark to the pattern forming device during a manufacturing process of the pattern forming device.

[0018] According to a sixth aspect of the present invention, there is provided a method for providing a mark to a planar substrate, wherein the mark has a periodic structure, and the periodic structure is configured to scatter radiation incident on the mark when subjected to alignment or overlap measurement, the method comprising: obtaining a pattern forming device manufactured according to the method of the fifth aspect of the present invention; obtaining a planar substrate; and using the pattern forming device and the planar substrate to provide a mark to the planar substrate during a lithography process.

[0019] According to a seventh aspect of the present invention, there is provided a measurement system, wherein the measurement system is configured to execute the method according to any one of the first to third aspects of the present invention.

[0020] According to an eighth aspect of the present invention, there is provided a lithographic apparatus or a metrology tool, the lithographic apparatus or the metrology tool comprising a metrology system according to the seventh aspect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0022] - Figure 1 depicts a schematic overview of a lithographic apparatus;

[0023] - Figure 2 Diffraction from a conventional phase grating is depicted;

[0024] - Figure 3 Diffraction from an amplitude grating is depicted;

[0025] - Figure 4 The grating in a resonant state is depicted;

[0026] - Figure 5 The resonance amplitude mark (RAM) is depicted;

[0027] - Figure 6 depicts a Guided Mode Marker (GMM) according to an embodiment of the present invention;

[0028] - Figure 7 The field distribution for a single trench at each of four different critical dimensions is depicted;

[0029] - Figure 8 Describes the Figure 7 Wafer quality (WQ) and alignment position deviation (APD) as a function of alignment mark depth in the presence of 1 nm floor tilt asymmetry for each of the four different critical dimensions shown in FIG.

[0030] - Fig. 9 is a flowchart of a method of alignment according to an embodiment of the present invention; and

[0031] - Fig.10 is a flowchart of a method for overlay error measurement according to an embodiment of the present invention. DETAILED DESCRIPTION

[0032] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a support structure (e.g., a mask stage) T configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to specific parameters, a substrate stage (e.g., a wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W 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 to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0033] In operation, the illuminator IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0034] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as applicable with respect to the exposure radiation used and / or other factors such as the use of immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0035] The lithographic apparatus may be of a type in which at least a portion of the substrate may 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, which is also referred to as immersion lithography. More information on immersion technology is given in U.S. Pat. No. 6,952,253 and PCT Publication No. WO99-49504, which are incorporated herein by reference.

[0036] The lithographic apparatus LA may also be of a type having two (dual stage) or more substrate tables WT and, for example, two or more support structures T (not shown). In such a "multi-stage" machine, the additional tables / structures may be used in parallel, or preparatory steps may be performed on one or more tables while one or more other tables are being used to expose a design layout of the patterning device MA onto a substrate W.

[0037] In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA) which is held on a support structure (e.g. a mask table T) and is patterned by the patterning device MA. After traversing the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position sensor IF (e.g. an interferometer arrangement, a linear encoder, a 2D encoder or a capacitive sensor), the substrate table WT can be accurately moved, e.g. in order to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and possibly another position sensor (in Figure 1 The mask MA may be aligned with the substrate W using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the illustrated substrate alignment marks occupy dedicated target portions, they may be located in spaces between target portions (these are called scribe lane alignment marks).

[0038] Embodiments of the present invention provide a novel mark fabricated as a binary grating. A general architecture for a mark design that is insensitive to asymmetry is provided. A general architecture for a mark design that is insensitive to depth variations is also provided. The novel mark design requires only a single wavelength to mitigate the effects of process-induced mark asymmetry. In addition, the alignment signal strength (wafer quality WQ) can be tuned simply by adjusting the pitch of the mark. Since the mark is insensitive to process-induced mark asymmetry, the mark simplifies the manufacture of reference wafers, thereby providing a "golden" reference wafer for wafer-to-wafer error correction.

[0039] Embodiments provide a novel binary mark design that is insensitive to most types of asymmetries (FT, SWA) within wide design rule tolerances. The binary mark design is insensitive to depth variations and therefore insensitive to process fluctuations that cause any asymmetry (FT, SWA, TT).

[0040] Before considering the novel marking, a conventional phase grating ( Figure 2 ), and the principle of the amplitude grating will be described in combination with the markings disclosed in WO2019081091A1 ( Figure 3) and the principle of resonance generation ( Figure 4 ).

[0041] Figure 2 Diffraction of a conventional phase grating is depicted. Radiation 200 of wavelength λ is irradiated onto a periodic structure 210 formed on a planar substrate 212, which in this example is a grating shown in cross-section. The gaps between the ridges 210 form grooves extending down to the substrate 212 with a depth d. This (d) is the mark depth. Interference between scattered radiation 204, 206 reflected at the top and bottom of the grating 210, respectively, helps to produce scattered radiation 202 for an optimal thickness d. Thus, diffraction occurs via modulation of the phase of the reflected wave. The grating introduces a periodic modulation of the wavefront.

[0042] Figure 3 Diffraction of an amplitude grating is depicted. Radiation 300 of wavelength λ illuminates the grating 310. The radiation 302 diffracted by the grating with periodic holes in the reflective film depends only on the period Λ. The reflective film is equivalent to a set of point sources 304. In contrast to a phase grating, diffraction occurs via modulation of the amplitude of the reflected wave rather than modulation of the phase. Like a phase grating, an amplitude grating introduces a periodic modulation of the wavefront.

[0043] Figure 4 A grating in a resonant state is depicted. Radiation 400 of wavelength λ illuminates the grating 510. The incident radiation 400 is resonantly excited as counter-propagating waves 408 in the plane of the grating. The grating itself introduces the required momentum.

[0044] Figure 5 A resonant amplitude mark (RAM) according to the disclosure of WO2019081091A1 is depicted. The mark is formed on a planar substrate 512. The mark has a periodic structure configured to scatter radiation 502 of wavelength λ incident 500 onto a surface plane 506 of the alignment mark. The surface plane 506 is parallel to the plane of the substrate. The scattering is mainly due to the excitation of a resonant mode 508 in the periodic structure that is parallel to the surface plane.

[0045] A periodic structure has a repeating unit cell that is located along a periodic direction (in Figure 5 504) is divided into adjacent first and second parts 510 and 504.

[0046] The first portion 510 has a first length (L l ) and the first effective refractive index (n s The second portion 504 has a second length (L2) along the periodic direction and a second effective refractive index (n d), wherein the second effective refractive index (n d ) is lower than the first effective refractive index.

[0047] The effective refractive index of each part (n s , n d ) and lengths (L1, L2) are configured to provide an optical path length (n) of the unit cell in the periodic direction s L1+n d L2), which is equal to an integer multiple of the wavelengths present or present in the spectrum of the radiation (mλ).

[0048] The wavelength of the incident radiation may be predetermined so that it matches the resonance design rule. Alternatively, broadband radiation may be incident on the marker and then the alignment sensor frequency filter may be tuned to select the resonant mode wavelength.

[0049] The effective refractive index of each part (n s , n d ) and the lengths (L1, L2) are also configured to provide an optical path length (n d L2), which is equal to half of an integer multiple of the wavelength present in the radiation spectrum (kλ / 2). These are the conditions that match the radiation wavelength to the grating material boundary conditions to support resonance.

[0050] In this example, the optical path length of the second portion in the periodic direction (n d L2) is equal to half the wavelength present in the spectrum of the radiation (λ / 2), so there is only one antinode of the resonant mode in the second portion 504, i.e. k = 1. When k>1, there are an odd number of antinodes, but the even number of antinodes cancel each other, leaving only one antinode that contributes to scattering, but with reduced efficiency.

[0051] A mark is formed on a planar substrate such as a wafer W, such as Figure 1 As depicted in P1 and P2.

[0052] The radiation diffracted by the mark does not contain information about the mark profile, but only about the mark position on the wafer. The mark may be referred to as a resonant amplitude mark (RAM). The term is chosen to emphasize the superiority of such RAM over conventional alignment marks based on phase gratings (e.g., reference 1). Figure 2 The terms "marker" and "grating" may be used interchangeably. The grating may be a one-dimensional (1-D) grating, such as that described in reference Figure 5 The invention is described by way of example, but is not limited to 1-D gratings. The invention can be applied to 2-D gratings having an effective refractive index and length in two periodic directions configured to support resonance.

[0053] In this marker design, the radiation from the alignment sensor excites two counter-propagating waves in the grating plane. These two waves form a so-called "standing wave", i.e. a resonant mode in the grating plane. Figure 4 As described, the two counter-propagating waves do not propagate through the mark depth but stay on the mark surface and are therefore not affected by the mark depth.

[0054] This resonant mode effectively leaks light into the grating order, like any other type of mark, and can be captured in much the same way as a conventional mark, so no new sensor design is required. In fact, the sensor will not be able to tell whether the light comes from the RAM or a conventional mark, with the advantage that, because the resonant mode is located in the grating plane, the light coming from the RAM does not contain information about the mark profile, but only about its position. In fact, in the case where the light diffracted in the grating order (the radiation properties of which do not depend on the mark depth) comes from a point source positioned in a periodic manner on the grating surface, the mark appears as an amplitude mark. This is similar to the case of periodic slits opened in a reflective opaque film, as described in reference . Figure 3 as described.

[0055] With proper design, this makes the mark less sensitive to the presence of layers stacked beneath it, and the mark can also be used as an overlapping target (see Figure 17 of WO2019081091A) or used in a mark stack (see Figure 16 of W02019081091A).

[0056] For the RAM configuration, it is desirable to efficiently couple to the resonant mode and to efficiently leak the mode into the sensor plane.

[0057] These are provided by using the following design rules:

[0058] The optical path of a grating unit cell is equal to an integer multiple of the wavelength:

[0059] n S L1+n d L2=mλ (1)

[0060] And the optical path of the low refractive index material (space) is equal to half the wavelength:

[0061] n d L2=kλ / 2, (2)

[0062] Where L1+L2=Λ(pitch) and k is an integer, preferably k=1. According to these two simple design rules, different pitches can be used for specific colors according to the sensor specifications (numerical aperture, NA). Therefore, for a fixed wavelength λ, an increase in the mark pitch Λ will result in a larger duty cycle.

[0063] In embodiments of the present invention, diffraction similarly occurs primarily at the grating top interface, and very little light is diffracted from the grating bottom interface. Figure 6 Guided mode markers (GMMs) are described as one-dimensional photonic crystals used to define nominal design rules.

[0064] refer to Figure 6 , the k vector along the x direction in the air is

[0065]

[0066] The k vector along the x direction in silicon is

[0067]

[0068] The period of the 1D photonic crystal is

[0069] d=d1+d2.

[0070] The dispersion relation of a 1D photonic crystal is

[0071]

[0072] Using k x =0 and To solve for the eigenmode

[0073] We get the nominal design rule:

[0074]

[0075]

[0076] Next, we change the nomenclature to match that of equations (1) and (2).

[0077] In a part with a lower effective refractive index (e.g., air):

[0078] Change d2 to L2, and n2 to n d .

[0079] In the portion with a higher effective refractive index (e.g., Si):

[0080] Change d1 to L1, and n1 to n s .

[0081] Thus, equating λ with λ (italics), and m with m (italics),

[0082] Will as well as

[0083] becomes

[0084] as well as

[0085] therefore,

[0086]

[0087] This is the same design rule as equation (2).

[0088] also,

[0089]

[0090] Will Add to both sides:

[0091]

[0092]

[0093] n s L l +n d L2=mλ;m=1,2,3…

[0094] This is the same design rule as equation (1). Therefore, the waveguide theory framework obtains the same nominal design rules (equations (3) and (4)) as the resonant mode method disclosed in WO2019081091A1 (equations (1) and (2)). However, using waveguide theory shows that the design rules of this notation are no longer limited to the nominal design rules based on resonance.

[0095] Figure 6 A guided mode mark (GMM) according to an embodiment of the present invention is depicted. The mark is formed on a planar substrate 612. The mark has a periodic structure configured to scatter radiation 602 of wavelength λ incident 600 on the alignment mark. The scattering is primarily due to coupling of the incident radiation into waveguide modes in the periodic structure.

[0096] The periodic structure has a repeating unit cell that is arranged along a periodic direction (in Figure 6 The cross section of FIG. 6 is in the x direction; from left to right) and is divided into adjacent first and second parts 610 and 604.

[0097] The first portion 610 has a first length (d l ) and the first effective refractive index (n l). The second portion 604 has a second length (d2) and a second effective refractive index (n2) along the periodic direction, wherein the second effective refractive index (n2) of the second portion within the optical path thereof is lower than the first effective refractive index.

[0098] The effective refractive index of each part (n l , n2) and length (d l , d2) are configured to provide an optical path length (n1d1+n1d1) of the unit cell in the periodic direction that is equal to an integer multiple of the wavelength (mλ) present in the radiation spectrum.

[0099] The wavelength of the incident radiation may be predetermined so that it matches the waveguide theoretical design rules. Alternatively, broadband radiation may be incident on the tag and the alignment sensor frequency filter may then be tuned to select the wavelength associated with the desired waveguide mode.

[0100] The effective refractive index of each part (n l , n2) and length (d l , d2) is also configured to provide an optical path length (n2d2) of the second portion in the periodic direction, which is preferably equal to 0.30 to 0.49 times (more preferably 0.35 to 0.45 times) the wavelength present in the spectrum of the radiation. These are conditions that match the wavelength of the radiation with the boundary conditions of the grating material to support the generation of the waveguide mode.

[0101] A mark is formed on a planar substrate such as a wafer W, such as Figure 1 As depicted in P1 and P2.

[0102] The radiation diffracted by the mark does not contain information about the mark profile, but only about the mark position on the wafer. The mark may be referred to as a guided mode mark (GMM). This terminology is chosen to emphasize the superiority of such a GMM over conventional alignment marks based on phase gratings (e.g., reference Figure 2 described) and reference Figure 5 The different operating principles of the RAM described. "Mark" and "grating" are used interchangeably. The grating can be a one-dimensional (1-D) grating, such as reference Figure 6 The invention is described by way of example, but is not limited to 1-D gratings. The invention can be applied to 2-D gratings having an effective refractive index and length in two periodic directions configured to support resonance.

[0103] With proper design, this makes the mark less sensitive to the presence of layers stacked beneath it, and the mark can also be used as an overlapping target (see Figure 17 of WO2019081091A) or used in a mark stack (see Figure 16 of W02019081091A).

[0104] Once the nominal design rules are met, light will be coupled primarily into the waveguide mode, which is primarily confined and propagates within the silicon ridge and evanescently coupled to the silicon ridge adjacent to the ridge through the trench (see Figure 7 =Field distribution with CD=0.5*λ in , where CD is the width of the air trench). Therefore, FT will have a minimal effect on the APD. SWA will only slightly affect the guided mode in case of unrealistically large mark depths. TT is located at the grating top interface, which will affect the coupling of light into the waveguide mode, mainly manifesting as a general shift. Since in all three types of asymmetry, diffraction mainly occurs at the grating top interface, the APD caused by asymmetry is less dependent on the mark depth than traditional mark types.

[0105] However, surprisingly, using the waveguide theory framework, we also observed a smaller APD in the simulation for the case of CD = 0.4*λ. We can see from the field distribution that the light can still couple to the waveguide mode, but not as well as for the case of CD = 0.5*λ. This shows that the design rules disclosed in WO2019081091A1 and represented by equations (1) to (4) do not need to be met exactly. In this case, a very small part of the light will propagate in the groove, but with a small effective mode index. Due to the small effective mode index, the geometric asymmetry will introduce less phase asymmetry. The small effective mode index effect leads to a small APD.

[0106] Another effect of small APD that also contributes to FT is the small CD effect. In this mark type, the duty cycle (spatial:baseline) is less than 50%. Therefore, the same degree of mark asymmetry will result in a smaller shift in the center of gravity of the mark. This effect applies to both s-polarization and p-polarization.

[0107] Figure 7 The field distribution for a single trench for each of four different critical dimensions (CD) is depicted, where CD is the width of the air trench. A "-" represents a negative value of the electric field. A "+" represents a positive value of the electric field. On both sides and below the trench, the black pattern represents alternating negative and positive electric field values ​​in the silicon.

[0108] Figure 8 Describes the Figure 7 Wafer quality (WQ) and alignment position deviation (APD) as a function of alignment mark depth d (from 0.2λ to 2λ) in the presence of 1 nm floor tilt asymmetry for each of the four different critical dimensions shown in FIG. WQ is the alignment signal strength.

[0109] Can be obtained from Figure 8 The results in the following table are obtained.

[0110]

[0111] The table shows that within a wide range of CD the mark is insensitive to the mark depth, since (for a mark depth range from 0.2λ to 2λ) at a typical FT value of 1 nm this results in smaller simulated APD variations for both cd=0.5λ and cd=0.4λ.

[0112] Fig. 9 is a flow chart of an alignment method according to an embodiment of the present invention. The alignment method has the following steps.

[0113] 902 (MRK): An alignment mark formed on a planar substrate is provided.

[0114] 904 (ILL): Illuminating the alignment mark with radiation of a predetermined wavelength.

[0115] 906 (DET): Detects radiation caused by illumination and scattered by alignment marks.

[0116] 908 (APD): Use the detected radiation to determine the position of the alignment mark (APD).

[0117] The alignment mark has a periodic structure configured to scatter radiation incident on the alignment mark. The scattering is primarily due to coupling of the incident radiation into waveguide modes in the periodic structure.

[0118] Fig.10 is a flow chart of a method for measuring overlay error according to an embodiment of the present invention. The method for determining overlay error has the following steps.

[0119] 1002 (TGT): An overlay target formed on a planar substrate is provided. The overlay target has a lower mark overlapping an upper mark, the upper mark having the same pitch as the lower mark. The lower mark has a periodic structure configured to scatter radiation incident on a surface plane of the lower mark. The surface plane is parallel to the substrate plane. The scattering is primarily due to coupling of the incident radiation to a waveguide mode. The upper mark has a periodic structure configured to scatter radiation of a predetermined wavelength, but the scattering is not primarily due to coupling of the incident radiation to a waveguide mode in the periodic structure of the upper mark.

[0120] 1004 (ILL): Illuminating an overlapping target with radiation of a predetermined wavelength.

[0121] 1006 (DET): Detects radiation caused by illumination and scattered by overlapping targets.

[0122] 1008 (OV): Use the detected radiation to determine the overlay error OV between the upper mark and the lower mark.

[0123] refer to Fig. 9 and Fig.10 , the periodic structure has a repeated unit cell, and the repeated unit cell is divided into adjacent first and second parts along a periodic direction. The first part has a first length and a first effective refractive index along the periodic direction. The second part has a second length and a second effective refractive index along the periodic direction, wherein the second effective refractive index of the second part within its optical path range is lower than the first effective refractive index. The effective refractive index and length of each part are configured to provide an optical path length of the unit cell in the periodic direction, and the optical path length is equal to an integer multiple of the wavelength presented in the radiation spectrum. The effective refractive index and length of each part are configured to provide an optical path length of the second part in the periodic direction, and the optical path length is preferably equal to 0.30 times to 0.49 times (more preferably 0.35 times to 0.45 times) the wavelength presented in the spectrum.

[0124] Compared with the disclosure of WO2019081091A1, embodiments of the present invention have several advantages within a wider design rule tolerance.

[0125] The alignment and overlay measurement methods are less complex; for alignment purposes only a single wavelength is required because for the GMM, within wide tolerances, WQ and APD do not vary as a function of mark depth.

[0126] The alignment and overlay measurement methods are more accurate; in the case of process-induced asymmetry, especially in the case of FT, the APD obtained for GMM is very small; for a typical value of FT = 1 nm, the APD is less than 0.5 Angstrom within a relatively wide range.

[0127] Within wider tolerances, the alignment and overlay measurement methods are faster, especially in the case of a tunable light source that can provide only one color at a time.

[0128] Markers and targets can be used as golden reference wafers for wafer-to-wafer error correction because they are insensitive to asymmetry within wider limits.

[0129] There is an improvement in the copper dual damascene structure; the presence of the layer below the GMM mark has limited impact on the signal, thus allowing a more stable APD or OV readout within wider margins.

[0130] Similar to what is described with reference to FIG. 17 of WO2019081091A1, the GMM may be used as a bottom grating for an overlay target to reduce the effects of the layer below the mark that affects the overlay readout signal within a wider margin.

[0131] Furthermore, embodiments of the present invention are compatible with smaller marks within wider limits.

[0132] The embodiment provides a sub-angstrom APD with FT = 1 nm.

[0133] The embodiment provides a sub-nanometer APD for a practical mark depth range of [0.1-1λ] with SWA=1 degree.

[0134] Embodiments are insensitive to mark depth, (eg, with an APD variation of ˜0.08 nm for FT and ˜0.4 nm for SWA), thus facilitating higher etch / polish / deposition tolerances of lithography processing tools.

[0135] Further embodiments provide methods for configuring marks used in alignment and / or overlay measurements, such as methods applied to the process of designing marks. The design / configuration of the marks is intended to obtain the excitation of the desired waveguide mode when irradiated with radiation having a specific wavelength. In a first step, the marks are configured by using a computer program and a computer system suitable for providing the design of the marks so that the marks follow the design rules associated with the excitation of the desired waveguide mode. In a second step, a patterning device is manufactured, the patterning device comprising at least one mark compatible with the design rules. In a third step, the marks are provided to a (planar) substrate by exposing the patterning device obtained by the manufacture to a planar substrate using any suitable lithography process. Typically, the patterning device is then loaded into a lithography apparatus, the planar substrate being provided with a photosensitive layer (resist coating) and subsequently loaded into the lithography apparatus. After exposing the patterning device to the coated planar substrate and developing the resist, the marks are provided to the substrate. Subsequently, an etching process can be applied to etch the mark pattern into the planar substrate material.

[0136] Additionally, the measurement system can be configured to perform Fig. 9 and Fig.10 The method of alignment and / or overlay measurement described in . The metrology system may be an alignment or overlay measurement system within a lithography apparatus or a metrology tool (such as an integrated or stand-alone scatterometer tool). The metrology system is configured to provide radiation of a wavelength suitable for exciting a waveguide mode associated with a mark provided to a planar substrate.

[0137] Other embodiments are disclosed in the following numbered list:

[0138] 1. A method of performing alignment or overlay measurement on a mark, the mark comprising a periodic structure, the mark being formed on or in a planar substrate, the method comprising:

[0139] irradiating the marking with radiation having a predetermined wavelength;

[0140] detecting radiation generated by the irradiation and scattered by the marker; and

[0141] Using the detected radiation to determine a position or overlap value,

[0142] Characterized in that the predetermined wavelength and the periodic structure are configured to provide scattering of radiation by excitation of a waveguide mode in the periodic structure parallel to a surface plane of the planar substrate.

[0143] 2. The method according to aspect 1, wherein the periodic structure has a repeated unit cell, and the repeated unit cell is divided into adjacent first and second parts along a periodic direction, wherein:

[0144] The first portion has a first length along the periodic direction and a first effective refractive index;

[0145] The second portion has a second length and a second effective refractive index along the periodic direction, and the second effective refractive index of the second portion within the optical path of the second portion is lower than the first effective refractive index;

[0146] The optical path length of the unit cell in the periodic direction is equal to or close to an integer multiple of the predetermined wavelength; and

[0147] An optical path length of the second portion in the periodic direction is in a range of 0.35 to 0.45 times the predetermined wavelength.

[0148] 3. A method according to aspects 1 or 2, wherein detecting comprises: a) filtering radiation scattered by the marker to select only scattered radiation having a wavelength close to or equal to a predetermined wavelength; and b) detecting the filtered radiation.

[0149] 4. A method according to any preceding aspect, wherein the mark is a lower mark of an overlay mark, the overlay mark consisting of the lower mark superimposed on the upper mark, the upper mark having the same pitch as the lower mark.

[0150] 5. An alignment method, the method comprising the following steps:

[0151] An alignment mark formed on or in a planar substrate is provided, the mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part has a first length and a first effective refractive index along the periodic direction, the second part has a second length and a second effective refractive index along the periodic direction, the second effective refractive index of the second part within the optical path range thereof is lower than the first effective refractive index, and

[0152] illuminating the alignment mark with radiation having a predetermined wavelength, wherein the effective refractive index and length of portions of the alignment mark are configured to provide: a) an optical path length of the unit cell in the periodic direction equal to an integer multiple of the predetermined wavelength, and b) an optical path length of the second portion in the periodic direction between 0.35 and 0.45 times the predetermined wavelength, such that scattering occurs primarily due to excitation of a waveguide mode in the periodic structure;

[0153] detecting radiation generated by the illumination and scattered by the alignment mark; and

[0154] The detected radiation is used to determine the position of the alignment mark.

[0155] 6. An alignment method, the method comprising the following steps:

[0156] Providing an alignment mark formed on or in a planar substrate, the mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part has a first length and a first effective refractive index along the periodic direction, the second part has a second length and a second effective refractive index along the periodic direction, and the second effective refractive index of the second part within its optical path range is lower than the first effective refractive index;

[0157] illuminating the alignment mark with broadband radiation including a predetermined wavelength, wherein the effective refractive index and length of portions of the alignment mark are configured to provide: a) an optical path length of the unit cell in the periodic direction equal to an integer multiple of the predetermined wavelength, and b) an optical path length of the second portion in the periodic direction of 0.35 to 0.45 times the predetermined wavelength, such that scattering of the predetermined wavelength occurs primarily due to excitation of a waveguide mode in the periodic structure;

[0158] filtering radiation generated by the illumination and scattered by the alignment marks to select wavelengths in the periodic structure corresponding to excitation of a waveguide mode parallel to the plane of the surface;

[0159] Detecting filtered radiation; and

[0160] The detected radiation is used to determine the position of the alignment mark.

[0161] 7. An overlap measurement method, the method comprising:

[0162] Providing an overlapping mark formed on or in a planar substrate, the overlapping mark comprising a lower mark overlapping an upper mark, the upper mark having the same pitch as the lower mark, the lower mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part has a first length and a first effective refractive index along the periodic direction, the second part has a second length and a second effective refractive index along the periodic direction, and the second effective refractive index of the second part within its optical path range is lower than the first effective refractive index;

[0163] irradiating the overlapping marks with radiation having a predetermined wavelength, wherein the effective refractive index and length of a portion of the lower mark are configured to provide: a) an optical path length of the unit cell in the periodic direction that is equal to an integer multiple of the predetermined wavelength, and b) an optical path length of the second portion in the periodic direction that is in the range of 0.35 to 0.45 times the predetermined wavelength, such that scattering occurs primarily due to excitation of a waveguide mode in the periodic structure;

[0164] detecting radiation generated by the irradiation and scattered by the overlying markers; and

[0165] The detected radiation is used to determine the overlap at the locations of the overlap marks.

[0166] 8. A method of configuring a mark on or in a planar substrate, the planar substrate having a periodic structure, the periodic structure being configured to scatter radiation incident on the mark when subjected to alignment or overlay measurements performed on the planar substrate, the method comprising:

[0167] obtaining the wavelength of radiation incident on the mark; and

[0168] The periodic structure is configured based on the occurrence of a waveguide mode that couples incident radiation into the periodic structure, parallel to the surface plane of the planar substrate.

[0169] 9. The method according to aspect 8, wherein:

[0170] The periodic structure has a repeated unit cell, and the repeated unit cell is divided into adjacent first and second parts along a periodic direction, wherein the first part has a first length and a first effective refractive index along the periodic direction, and the second part has a second length and a second effective refractive index along the periodic direction, and the second effective refractive index of the second part within the optical path range thereof is lower than the first effective refractive index; and

[0171] The configuration of the periodic structure is based on the following arrangements: a) the optical path length of the unit cell in the periodic direction is equal to an integer multiple of the wavelength, and b) the optical path length of the second portion in the periodic direction is in the range of 0.30 to 0.45 times the wavelength.

[0172] 10. A method for manufacturing a patterning device for providing a marking comprising a periodic structure to a planar substrate, the method comprising:

[0173] obtaining a wavelength of radiation used to illuminate a planar substrate when subjected to alignment or overlay measurement;

[0174] The periodic structure is configured such that when provided to a planar substrate:

[0175] a) scatter incident radiation when subjected to alignment or overlay measurements; and

[0176] b) coupling incident radiation at said wavelength into a waveguide mode of the periodic structure parallel to the surface plane of the substrate; and

[0177] During the manufacturing process of the patterning device, the patterning device is provided with configured markings.

[0178] 11. A method of providing a planar substrate with a mark having a periodic structure, the periodic structure being configured to scatter radiation incident on the mark when subjected to alignment or overlay measurements, the method comprising:

[0179] Obtaining a pattern forming device manufactured according to the method of aspect 10;

[0180] obtaining a planar substrate; and

[0181] A patterning device and a planar substrate are used in a lithographic process to provide markings to the planar substrate.

[0182] 12. A measurement system, the measurement system being configured to perform the method according to any one of aspects 1 to 7.

[0183] 13. A lithographic apparatus or a metrology tool comprising the metrology system according to clause 12.

[0184] 14. A computer program comprising instructions for performing the method according to any one of aspects 1 to 11.

[0185] 15. A computer system configured to execute the computer program according to aspect 14.

[0186] 16. A mark formed on a planar substrate, the mark comprising a periodic structure configured to scatter radiation incident on the alignment mark, the scattering being primarily due to coupling the incident radiation into a waveguide mode in the periodic structure, wherein the periodic structure has a repeating unit cell, the repeating unit cell being divided into adjacent first and second portions along a periodic direction,

[0187] - the first portion has a first length along the periodic direction and a first effective refractive index,

[0188] - the second portion has a second length along the periodic direction and a second effective refractive index, the second effective refractive index of the second portion within the optical path of the second portion being lower than the first effective refractive index,

[0189] wherein the effective refractive index and length of each section are configured to provide:

[0190] 1) the optical path length of the unit cell in the periodic direction is equal to an integer multiple of the wavelength present in the spectrum of the radiation, and

[0191] 2) The optical path length of the second portion in the periodic direction is 0.30 to 0.49 times the wavelength present in the spectrum of the radiation.

[0192] 17. A marking according to aspect 16, wherein the effective refractive index and length of each portion are configured to provide: an optical path length of the second portion in the periodic direction of 0.35 to 0.45 times the wavelength present in the spectrum of the radiation.

[0193] 18. A substrate comprising the marking according to aspect 16 or aspect 17.

[0194] 19. An overlapping target, comprising a lower mark according to aspect 16 or aspect 17, the lower mark overlapping with an upper mark, the upper mark having the same pitch as the lower mark, and the lower mark comprising a periodic structure, wherein the periodic structure is configured to scatter radiation, and the scattering is not mainly caused by the incident radiation coupling into the waveguide mode in its periodic structure.

[0195] 20. A substrate comprising the overlay target according to aspect 19.

[0196] 21. An alignment method, the alignment method comprising the following steps:

[0197] - According to the mark of aspect 16 or aspect 17, providing an alignment mark formed on a planar substrate;

[0198] - irradiating the alignment mark with radiation;

[0199] - detecting radiation generated by the illumination and scattered by the alignment marks; and

[0200] - Using the detected radiation to determine the position of the alignment marks.

[0201] 22. A method for determining an overlay error, the method comprising the steps of:

[0202] - providing an overlay target formed on a planar substrate, the overlay target comprising a lower mark overlying an upper mark, the upper mark having the same pitch as the lower mark,

[0203] in:

[0204] - the lower marking comprises a marking according to aspect 16 or aspect 17; and

[0205] - the upper marking comprises a periodic structure configured to scatter radiation, but the scattering is not primarily due to coupling of the incident radiation into a waveguide mode in its periodic structure;

[0206] -Use radiation to illuminate overlapping targets;

[0207] - detecting radiation caused by illumination and scattered by overlapping targets; and

[0208] - Using the detected radiation to determine the overlay error between the upper and lower marks.

[0209] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as substrate processing in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc. The skilled person will understand that in the context of such alternative applications, any use of the term "wafer" or "field" / "die" herein may be considered synonymous with the more general term "substrate" or "target portion", respectively. Before or after exposure, the substrates referred to herein may be processed in, for example, a track or coating and developing system (a tool that typically applies a resist layer to a substrate and develops the exposed resist), a metrology tool, and / or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, a substrate may be processed more than once, for example to create a multi-layer IC, so the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0210] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, such as imprint lithography, and is not limited to lithography where the context permits. In imprint lithography, the topography in the patterning device defines the pattern created on the substrate. The topography of the patterning device may be pressed into a layer of resist provided to the substrate, and the resist on the substrate is then cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist is cured, the patterning device is removed from the resist, leaving a pattern in the resist.

[0211] The terms "radiation" and "beam" as used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 nm-20 nm), as well as particle beams, such as ion beams or electron beams.

[0212] The term "lens," where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0213] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in a manner other than that described. For example, the present invention may take the form of a computer program comprising one or more machine-readable instruction sequences describing the above methods, or a data storage medium (e.g., a semiconductor memory, a magnetic disk, or an optical disk) in which such a computer program is stored.

[0214] The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that the invention described may be modified without departing from the scope of the following claims. In addition, it should be understood that the structural features or method steps shown or described in any embodiment herein may also be used in other embodiments.

Claims

1. An alignment method, the method comprising the following steps: Providing an alignment mark formed on or in a planar substrate, the mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part has a first length and a first effective refractive index along the periodic direction, the second part has a second length and a second effective refractive index along the periodic direction, and the second effective refractive index of the second part within its optical path range is lower than the first effective refractive index; illuminating the alignment mark with radiation having a predetermined wavelength, wherein the effective refractive index and length of portions of the alignment mark are configured to provide: a) an optical path length of the unit cell in the periodic direction that is equal to an integer multiple of the predetermined wavelength, and b) an optical path length of the second portion in the periodic direction that is 0.35 to 0.45 times the predetermined wavelength, such that scattering occurs primarily due to excitation of a waveguide mode in the periodic structure; detecting radiation generated by the illumination and scattered by the alignment mark; as well as The detected radiation is used to determine the position of the alignment mark.

2. An alignment method, the method comprising the following steps: Providing an alignment mark formed on or in a planar substrate, the mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part has a first length and a first effective refractive index along the periodic direction, the second part has a second length and a second effective refractive index along the periodic direction, and the second effective refractive index of the second part within its optical path range is lower than the first effective refractive index; illuminating the alignment mark with broadband radiation including a predetermined wavelength, wherein the effective refractive index and length of each portion of the alignment mark are configured to provide: a) an optical path length of the unit cell in the periodic direction equal to an integer multiple of the predetermined wavelength, and b) an optical path length of the second portion in the periodic direction of 0.35 to 0.45 times the predetermined wavelength, such that scattering of the predetermined wavelength occurs primarily due to excitation of a waveguide mode in the periodic structure; filtering radiation generated by the illumination and scattered by the alignment mark to select a wavelength corresponding to excitation of a waveguide mode in the periodic structure parallel to the surface plane; Detection of filtered radiation; as well as The detected radiation is used to determine the position of the alignment mark.

3. An overlap measurement method, the method comprising: Providing an overlapping mark formed on or in a planar substrate, the overlapping mark comprising a lower mark overlapping an upper mark, the upper mark having the same pitch as the lower mark, the lower mark comprising a periodic structure, wherein the periodic structure has a repeated unit cell, the repeated unit cell is divided into adjacent first and second parts along a periodic direction, the first part has a first length and a first effective refractive index along the periodic direction, the second part has a second length and a second effective refractive index along the periodic direction, and the second effective refractive index of the second part within its optical path range is lower than the first effective refractive index; illuminating the overlay mark with radiation having a predetermined wavelength, wherein the effective refractive index and length of each portion of the lower mark are configured to provide: a) an optical path length of the unit cell in the periodic direction that is equal to an integer multiple of the predetermined wavelength, and b) an optical path length of the second portion in the periodic direction that is in the range of 0.35 to 0.45 times the predetermined wavelength, such that scattering occurs primarily due to excitation of a waveguide mode in the periodic structure; detecting radiation generated by the irradiation and scattered by the overlying markers; as well as The detected radiation is used to determine overlap at the location of the overlap marks.

4. A method of configuring a mark on or in a planar substrate, the mark having a periodic structure configured to scatter radiation incident on the mark when subjected to alignment or overlay measurements performed on the planar substrate, the method comprising: obtaining a wavelength of radiation incident on the mark; as well as The periodic structure is configured based on the excitation of a waveguide mode of the periodic structure parallel to the surface plane of the planar substrate when illuminated by incident radiation at the wavelength.

5. The method according to claim 4, wherein: The periodic structure has a repeated unit cell, and the repeated unit cell is divided into adjacent first and second parts along a periodic direction, wherein the first part has a first length and a first effective refractive index along the periodic direction, and the second part has a second length and a second effective refractive index along the periodic direction, and the second effective refractive index of the second part within the optical path range thereof is lower than the first effective refractive index; and The configuration of the periodic structure is based on the following arrangements: a) the optical path length of the unit cell in the periodic direction is equal to an integer multiple of the wavelength, and b) the optical path length of the second portion in the periodic direction is in the range of 0.30 to 0.45 times the wavelength.

6. A method for manufacturing a patterning device for providing a planar substrate with a mark comprising a periodic structure, the method comprising: obtaining a wavelength of radiation used to illuminate the planar substrate when subjected to alignment or overlay measurement; The periodic structure is configured such that when provided to the planar substrate: a) scatters incident radiation when subjected to said alignment or said overlay measurement; and b) coupling incident radiation at said wavelength into a waveguide mode of the periodic structure parallel to a surface plane of said substrate; as well as During a manufacturing process of the patterning device, the patterning device is provided with configured markings.

7. A method for providing a mark to a planar substrate, the mark having a periodic structure configured to scatter radiation incident on the mark when subjected to alignment or overlay measurement, the method comprising: Obtaining a pattern forming device manufactured by the method according to claim 6; obtaining the planar substrate; as well as The patterning device and the planar substrate are used in a lithographic process to provide the planar substrate with the markings.

8. A measurement system, the measurement system being configured to perform the method according to any one of claims 1 to 3.

9. A lithographic apparatus or a metrology tool comprising the metrology system according to claim 8.

10. A computer program product comprising instructions for executing the method according to any one of claims 1 to 7.

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