Quality control methods for imprint lithography

Abstract

A method for quality assessment in imprint lithography, based on analyzing the quality of an imprintable layer based on indirect detection of the viscosity of the imprintable layer. The method includes imprinting a dedicated test pattern including multiple pattern sections, including pattern indentations of different widths and / or depths. The viscosity of the imprintable layer affects how it flows to fill the lithographic imprint pattern, with higher viscosities resulting in uneven layer geometry (i.e., curvature or warping) (at the micro- or nano-scale) on each groove or ridge of the recessed pattern. This unevenness is detectable during optical inspection by thin film interference patterns caused by the passage of light through the uneven imprintable layer surface and back reflection from the underlying substrate, which is detectable in the captured optical image data. Thus, the viscosity of the imprintable layer is indirectly detectable by capturing optical image data of the test pattern. By using multiple different pattern sections of different widths and depths, an accurate quantitative measure of quality in terms of viscosity: can be identified.

Description

[Technical field]

[0001] The present invention relates to the field of imprint lithography, and in particular to a method for quality control assessment in that context. [Background technology]

[0002] Imprint lithography (IL) is a technique that uses an imprint cycle to form a patterned layer on a substrate. The patterned layer can function as or part of a functional substrate layer, or as a mask layer for patterning further layers on the substrate. An imprint cycle usually starts with the deposition of an imprint composition on a substrate surface to form an imprintable layer thereon. The imprintable layer, which is still formable at this point, is then brought into contact with the relief surface of a stamp, where the relief surface represents the pattern to be replicated in the imprintable layer. Since the applied imprintable layer is still formable, during this contact, the imprintable layer surface conforms (at least partially fits) to the shape of the relief surface due to the redistribution of material within the imprintable layer. Once conformity is achieved, the imprintable layer is solidified ("cured") while still in contact with the relief surface. Solidification can occur by stimulating or triggering a specific solidification process, for example, by removal of solvent and / or exposure to an external stimulus such as UV light. After solidification, the stamp (i.e., the relief surface) is removed from the solidified imprintable layer leaving a patterned layer, often referred to as the imprint layer, on the surface of the substrate, which has a pattern complementary to the pattern of the stamp relief surface.

[0003] One form of imprint lithography utilizes a flexible stamp having an elastomeric body and a relief surface that is brought into contact with an imprintable layer of a substrate. Imprinting of the imprintable layer by the stamp relief surface is due to capillary forces, rather than pressure, which cause a redistribution (flow) of material during the contacting step described above. Substrate Conformal Imprint Lithography™ ("SCIL") is an example of such a technique in which a silicone rubber stamp is used in combination with a sol-gel based imprint composition.

[0004] In SCIL, the stamp is locally bent to establish a first local contact with a sol-gel imprintable layer applied to a substrate, and this local contact spreads over the substrate area in a wave-propagating-like manner by successive local bending of the stamp during the contacting step. Once applied and in contact with the flexible stamp, the imprintable layer solidifies due to removal of solvent from the imprintable layer by the flexible stamp. The SCIL technique reduces or avoids changes in the relief surface structure during the contacting process. Moreover, the successive contacting routine does not allow the formation of air gaps / inclusions, resulting in high yields and improving productivity. Summary of the Invention [Problem to be solved by the invention]

[0005] In the context of large-scale imprint lithography manufacturing processes (e.g., semiconductor manufacturing), quality control means are needed to monitor the quality with which imprint cycles result in patterned layers. Preferably, such quality control can be used to check the quality of the various layers fabricated on the substrate. Quality control involves analysis of the layered structure at one or more stages through fabrication. Quality control analysis of the imprint lithography process involves analyzing the structural properties of the patterned imprintable layer.

[0006] Well-known methods for analyzing microstructures are microscopy, such as scanning electron microscopy (SEM) and atomic force microscopy (AFM). Other methods include white light interferometry. The appropriate method depends on the size of the object of interest. For nanostructures, SEM is more suitable, but is expensive and requires large machines with a large spatial footprint.

[0007] AFM provides detailed and accurate analysis of structures, but is time consuming. SEM can provide a detailed view of the surface of a structure, but its cost, complexity and large spatial footprint make it somewhat impractical for in-line quality control.

[0008] Improved approaches to quality control of imprint lithography processes are desirable. Such quality control analysis would preferably be able to be performed along the manufacturing process and would not require significant time and effort. The need develops from the realization that the ability of the imprintable layer to conform to the stamp relief surface is highly dependent on the quality of the imprint composition, as well as on the imprint conditions, such as the environmental humidity and temperature, in which the imprint cycle is performed. Such conforming properties may depend, for example, on the viscosity of the imprintable layer, but such properties cannot necessarily be inferred from the imprint composition. For example, sol-gel based imprint compositions and imprintable layers are typically composed of materials in a slowly changing equilibrium state, i.e., somewhat unstable and changing over time. Contact of the imprint composition may affect such equilibrium behavior. Thus, one important aspect is to monitor the quality of the imprint composition and / or the imprintable layer, as their ability to conform to the stamp relief surface affects the final resulting patterned layer. [Means for solving the problem]

[0009] The invention is defined by the claims.

[0010] The inventors have recognised that the structural or material quality of an imprintable layer can be assessed based on indirect detection of the viscosity or flow properties of the layer, which is an important parameter in assessing the material quality of the layer, and therefore the inventors have sought a quality control method that provides an indication of this viscosity or flow properties.

[0011] It is further appreciated by the inventors that the viscosity or flow properties of the layer can be assessed through analysis of the quality of the above-mentioned fitting process based on optically analysing the patterned layer (i.e. the imprintable layer after imprinting and solidification) to detect the morphology of the imprinted relief pattern.

[0012] In this regard, according to one aspect of the present disclosure, there is provided a quality control method for use in imprint lithography, the method comprising: providing a patterned stamp for patterning the imprintable layer, the stamp having a flexible stamp layer carrying a pattern in a relief shape defining a stamp pattern, the stamp pattern having a plurality of pattern sections, the pattern sections being spaced apart from one another, each pattern section having a pattern shape for forming at least one depression of a respective width different from the other pattern sections; applying a stamp to an imprintable layer deposited on a substrate, the imprintable layer comprising a curable imprint medium, whereby the imprintable layer is induced to conform to the stamp pattern; acquiring optical image data of an imprint pattern in the imprintable layer; and deriving a fit quality index indicative of a fit quality between the layer and the stamp based on the optical image data and based on knowledge of the different widths and / or depths of the recesses of the pattern sections.

[0013] An embodiment of the present invention is based on analyzing the quality of an imprintable layer based on an indirect detection of its viscosity. The viscosity of the imprintable layer affects how it flows to fill the lithographic stamp pattern upon stamping, with higher viscosities causing the geometry of the imprintable layer to be non-flat (i.e., curved or warped) (at the micro- or nano-scale) at each depression / groove or each ridge of the imprint pattern. This non-flatness is detectable upon optical inspection, for example by spectroscopic (e.g., color) analysis, by optical effects caused by incident light on or through a non-flat reflective surface. For example, the non-flatness is detectable upon optical inspection by thin film interference patterns caused by the passage of light through a non-flat thin imprintable layer surface and back reflection from the underlying substrate, which is detectable in the captured optical image data. By providing an imprint pattern with multiple spatially separated regions, each defining an imprint depression of a different width, a more accurate quantitative measure of viscosity-related quality can be identified. In particular, the "fill" quality of the layer within the grooves / ridges is expected to be different for grooves / ridges of different sizes, and the fill quality versus ridge size function will be different for different layer viscosities.

[0014] The stamp can be a stamp dedicated to quality control or it can be a stamp used in the main manufacturing process, for example in the latter case the stamp pattern described above can be provided in one spatial area of ​​the functional stamp, i.e. the test area of ​​the main stamp, similar to an alignment test block in a mass printing process.

[0015] Each of the aforementioned pattern sections occupies a separate and respective spatial region of the stamp surface, which may also be referred to as a pattern area.

[0016] In some examples, the imprintable layer may be a curable imprintable layer. In this case, the method may further comprise the step of curing the imprintable layer for a predetermined time after applying the stamp, thereby forming a hardened imprint pattern on a surface of the imprintable layer, and the optical image data is acquired after curing. Other curing methods are, for example, hot embossing, which does not require a curing step.

[0017] With regard to the geometry or shape of the recesses mentioned above, the concept of the present invention does not depend on any particular geometry or shape. The important condition is to provide different widths and / or depths in different spatial sections of the stamp pattern, so that the different filling qualities at each different recess depth or width can be compared. It is preferred that the geometry or shape of the overall outline of at least one recess is the same in all pattern sections, and only the width or depth differs between the various pattern sections. This allows easier comparison of the sections.

[0018] In one particular set of embodiments, it is proposed to provide a respective line grating relief pattern in each pattern section to form a respective grating of imprinted lines when that section is applied to the imprintable layer.Thus, in this example, the at least one recess comprises at least one elongated linear recess, i.e. linear groove.

[0019] In particular, according to at least one set of embodiments, each pattern section may include a relief feature defining a respective grating of parallel lines for forming line depressions in the imprintable layer, the grating having a grating pitch and a grating duty cycle that together define a uniform grating line width of each of the line depressions, and a grating depth that defines a depth of each of the line depressions. Grating duty cycle is a term in the art and refers to the ratio of the grating ridge width to the grating pitch (wherein the pitch is also known as the grating period). If a grating is understood as an alternating pattern of line depressions (grooves) and line ridges, and the grating period or pitch is the distance covered by one depression and one ridge, then the grating duty cycle is essentially the fraction of the periodic distance covered by the width of the ridge (as opposed to the line width of the depression).

[0020] Advantages of a line grating per pattern section include providing a simple, uniform geometry for each pattern section (thus making the resulting imprint patterns easily comparable between sections) and simple fabrication.

[0021] The grating of each of the plurality of pattern sections may have a different respective line pitch compared to the other pattern sections, and preferably each of the plurality of pattern sections has at least a different line pitch resulting in a different grating recess line width compared to the other pattern sections, and optionally further has a different recess line depth.

[0022] In one advantageous set of embodiments, a plurality of sets of gratings are provided, each set providing a grating imprinted at the same common depth, the depth varying between the sets. For example, the plurality of pattern sections may include at least two or more gratings of a first set and two or more gratings of a second set, the first set of gratings including lines at a first depth and the second set of gratings including lines at a second depth, each of the first and second sets of gratings including respective gratings of a plurality of different line pitches and / or grating duty cycles. In this manner, a plurality of grating pitches at a plurality of different depths are achieved.

[0023] In terms of the scale of the recess width, the width of at least one recess in each pattern section can be between 0.5 and 200 micrometers.

[0024] More specifically, with respect to the set of embodiments in which each pattern section includes a respective line grating, the recess line width of each grating can be between 0.5 and 200 micrometers.

[0025] With regard to determining the fit quality indicator, generally this may be done based on comparing one or more optical characteristics of the light reflection pattern from each of the pattern sections.

[0026] In some embodiments, determining the fit quality indicator may include detecting from the optical image data a spatial color distribution or pattern exhibited by each pattern section, and determining the fit quality indicator based on the color pattern of each pattern section and based on knowledge of the width and / or depth of at least one recess in each pattern section.

[0027] As briefly mentioned above, the viscosity of the imprintable layer affects how it flows to fill the lithographic imprint pattern, with higher viscosity resulting in non-uniform geometry (i.e., curvature or warping) of the imprintable layer (at the micro- or nano-scale) above each groove or ridge. This non-uniformity is detectable upon optical inspection due to optical effects caused by light incident on a non-planar reflective surface, e.g., thin film interference patterns of light passing through the non-planar layer. One manifestation of this is a detectable color change. Thus, the viscosity of the imprintable layer is indirectly detectable by capturing the color pattern of reflected light from each pattern section. More generally, the method can include spectroscopic analysis of the optical image data based on detecting the spectral or light frequency patterns exhibited by each pattern section.

[0028] In some embodiments, the method may further include detecting a color variation pattern across the width of at least one recess from the color pattern of each pattern section.

[0029] The method may further include determining, for each pattern section, a color non-uniformity metric representative of a degree of color non-uniformity across the width of at least one recess based on the detected color variation across the width of the recess.

[0030] In some embodiments, the method may further include determining a fit quality indicator based on comparing the color uniformity metric of each pattern section against a predefined criteria, such as a threshold range.

[0031] In some embodiments, determining the fit quality indicator may include identifying a maximum depression width among different depression widths of a plurality of pattern sections for which a corresponding color uniformity metric meets the predefined criteria.

[0032] With regard to deriving a quality indicator, finally the quality assessment seeks to determine whether the layer viscosity is such that the quality of fit with the stamp is at a minimum level required for the particular lithographic operation to be performed. A higher fit quality allows finer spatial resolution of the imprinted pattern. The detected color pattern is characteristic of the physical engagement between the stamp and the imprintable layer depending on the degree of geometric uniformity of the generated recesses. The detected color pattern is related to the refraction of light through the imprintable layer at the base of the recesses of the pattern section. In particular, the color across the width of a given recess is expected to vary according to a specific variation function or pattern that differs depending on the degree of uniformity of the recesses of the pattern section. Thus, the quality of the fit of a given pattern section can be indicated by the color pattern of that section and an overall fit quality indicator of the imprintable layer can be derived from an evaluation of the quality of the layer across the entire set of pattern sections. The quality of a given pattern section can be determined depending on the degree of uniformity of the color along the width of the recesses of that section. For example, the color pattern of each pattern section can be used to derive a pattern section quality metric, each pattern section quality metric can be compared to a threshold level or range, and an overall fit quality indicator can be determined depending on which of the pattern sections have an associated pattern section quality metric that meets the defined threshold. For example, as described above, a maximum depression width of different depression widths of a plurality of pattern sections for which a corresponding color uniformity metric meets the predetermined criteria can be identified.

[0033] In this context, it should be noted that color can be defined quantitatively using a variety of color metrics, including RGB color values ​​(or other standard color codes), or simply based on corresponding light frequencies or wavelengths or spectral composition.

[0034] In some embodiments, the method may further include detecting a degree of color uniformity from the color pattern of each pattern section for one or more of the peaks of the imprinted pattern section and the non-imprinted regions surrounding the pattern.

[0035] In some embodiments, for each pattern section, the degree of detected color uniformity of the peaks of the imprinted pattern section or the non-imprinted areas surrounding the pattern can be used as said predefined criterion.

[0036] In some embodiments, determining the fit quality indicator may include determining, for each pattern section, from the pattern of color variation across the width of at least one recess in the pattern section, the degree of height non-uniformity across the base of at least one recess, based on the assumption that the color variation is indicative of an optical thin film interference pattern of incident light through the imprintable layer.

[0037] In some embodiments, the method may further include comparing the derived degree of height non-uniformity to at least one threshold and deriving a fit quality indicator based thereon.

[0038] In some embodiments, the aforementioned optical image data may be obtained using an optical microscope, which may have a magnification of at least 1000 times.

[0039] In some embodiments, the imprintable layer may comprise a resist layer, for example a mask layer.

[0040] In some embodiments, the stamp can be a substrate conformal imprint lithography (SCIL) stamp.

[0041] A further aspect of the invention provides an imprint lithography stamp having a patterned surface for patterning an imprintable layer, the stamp comprising a flexible stamp layer carrying a pattern of relief features defining a stamp pattern, the stamp pattern comprising a plurality of pattern sections, the pattern sections being spaced apart from one another, each pattern section comprising a pattern feature for forming one or more depressions of different respective widths compared to the other pattern sections.

[0042] In some embodiments, each pattern section may comprise a relief feature defining a respective grid of parallel lines for forming line recesses in the imprintable layer, the grid having a grid pitch defining a width of each of the line recesses and a grid depth defining a depth of each of the line recesses.

[0043] In some embodiments, the grating of each of the multiple pattern sections may have a different respective line pitch and / or depth compared to the other pattern sections.

[0044] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. [Brief description of the drawings]

[0045] For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying schematic drawings, which are not to scale. [Figure 1] 1A-1C are schematic diagrams illustrating examples of patterned stamps for use in forming imprint patterns for quality control analysis. [Diagram 2] 13A-13C are schematic diagrams illustrating further examples of patterned stamps for use in forming imprint patterns for quality control analysis. [Diagram 3] 1A-1C show steps for forming an imprint pattern as part of an exemplary quality control method. [Figure 4] FIG. 1 shows optical analysis of imprint patterns formed for quality control evaluation. The figure shows an image of a first sample A and an image of a second sample B. The numbers 2 through 256 all indicate test pattern width values ​​for a single depth value. [Diagram 5] 1 illustrates an example quality method disclosed herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] The present invention will now be described with reference to the drawings.

[0047] The detailed description and specific examples set forth exemplary embodiments of the devices, systems and methods are intended for purposes of illustration only and are not intended to limit the scope of the claims. These and other features, aspects and advantages of the devices, systems and methods herein will become better understood from the following description, appended claims and accompanying drawings, in which the same reference numbers are used throughout the figures to denote the same or similar parts.

[0048] The present disclosure provides a method for quality assessment in imprint lithography based on analyzing the quality of an imprintable layer based on indirect detection of its viscosity. The method includes imprinting a dedicated test pattern including multiple pattern sections, including pattern recesses of different widths and / or depths. The viscosity of the imprintable layer affects how it flows to fill the lithographic imprint pattern, with higher viscosity resulting in a non-flat layer shape (i.e., curvature or warping) (at the micro- or nano-scale) on each groove or ridge of the recess pattern. This non-flatness is detectable during optical inspection by a thin film interference pattern caused by the passage of light through the non-flat imprintable layer surface and back reflection from the underlying substrate, which is detectable in the captured optical image data. Thus, the viscosity of the imprintable layer is indirectly detectable by capturing optical image data of the test pattern. By using multiple, various pattern sections of different widths and / or depths, an accurate quantitative measure of viscosity-related quality can be identified.

[0049] 1, 3(a) and 3(b) show schematic examples of imprint lithography stamps 12 for use in imprint lithography quality control methods. The stamp may in some examples be a substrate conformal imprint lithography (SCIL) stamp as used in substrate conformal imprint lithography (SCIL) as previously described in SCIL, where a somewhat viscous imprintable layer is brought into contact with a relief surface of a flexible (usually rubber, e.g., PDMS) stamp. Upon contact, the relief features of the stamp, which typically have sub-micrometer dimensions, are filled with the somewhat viscous material of the imprint layer. This process may be referred to as a conformation process of the imprintable layer to the relief surface. After a good conformance is achieved, the imprintable layer is solidified while in contact with the stamp. Once solidification is complete, the stamp is removed or released, leaving a patterned layer having a relief pattern complementary to the pattern of the relief stamp. However, other imprint methods and systems may be used. More detailed descriptions of apparatus, systems and imprint compositions for carrying out exemplary imprint methods, and methods of using the same, are described in EP 3126909A and WO2020 / 0099265, each of which is incorporated by reference.

[0050] Referring again to Figures 1, 3(a) and 3(b), the stamp includes a flexible layer 14 formed of an elastomeric material, such as rubber. In some examples, the flexible layer 14 includes a flexible thin glass or metal sheet 14 bonded to an elastomeric material layer 17. The layer carries on an exposed surface a pattern of relief features 22 that define a stamp pattern. The stamp pattern includes a plurality of pattern sections 16a-j, the pattern sections being spaced apart from one another. Each section of Figure 1 along the horizontal direction has a plurality of relief features 22 spaced apart from one another by gaps 24, as shown in Figures 3(a) and 3(b). Although ten pattern sections are shown in Figure 1, this is by way of example only and more or fewer pattern sections may alternatively be provided. Each pattern section 16 includes a relief feature 22 for forming at least one recess 44 in the imprintable layer, the recess 44 having a respective width 25 and / or depth 27 that differs compared to those of the other pattern sections.

[0051] In the illustrated example, each pattern section 16 includes a relief feature 22 for forming at least one depression 44 of a respective combination of width 25 and depth 27 that differs compared to those of the other pattern sections.

[0052] In the illustrated example, each pattern section 16 comprises a relief feature 22 defining a respective grating of parallel lines along a vertical direction in the plane of the drawing of FIG. 1 to form linear depressions 44 in the imprintable layer. Each grating has a grating pitch p (also known as grating period) defined along a horizontal direction in the plane of the drawing of FIG. 1. Each grating has a duty cycle. Duty cycle refers to the ratio of grating ridge width to grating pitch p. If a grating is understood as an alternating pattern of line depressions (grooves) and line ridges, and the grating period or pitch is the distance covered by one depression and one ridge, then the grating duty cycle is essentially the fraction of the periodic distance covered by the width of the ridge (as opposed to the line width of the depressions). The pitch and duty cycle together define the uniform grating line width of each of the line depressions, and the grating depth defines the depth of each of the line depressions.

[0053] In some preferred examples, each section has a single width 25, depth 27 and pitch for the features 22 in its pattern. Preferably, there are at least two adjacent features 22 per section separated from one another in the relief feature surface of the stamp by gaps 24, and more preferably, there are at least three such features 22 per section separated from one another by gaps 24.

[0054] In this example, the plurality of pattern sections 16 includes a first set of gratings 18a and a second set of gratings 18b, where the first set of gratings includes lines of a first depth and the second set of gratings includes lines of a second depth different from the first depth, and each of the first set and the second set of gratings includes gratings of a plurality of different line pitches and line widths, respectively. For example, each grating can have a recess line width between 0.5 μm and 200 μm. In other words, each pattern section is configured to form recesses on lines having a width between 0.5 μm and 200 μm. Regarding the respective depths of the first set of gratings 18a and the second set of gratings 18b, these can be, for example, a depth between 75 nm and 300 nm. In some examples, the depths can be selected as any two of 75 nm, 150 nm, and 300 nm.

[0055] In the example of FIG. 3, two sets of gratings 18a, 18b are shown, however in further examples more sets of gratings may be provided, for example three sets of gratings or more than three sets of gratings.

[0056] Regarding the number of line depressions 24 in each grating, this can in fact be any number greater than one, but the reliability of the optical analysis (described below) can be improved if each grating provides a greater number of line depressions.

[0057] The stamp is used to perform quality control assessments in the context of an imprint lithography process: More particularly, the stamp is for assessing the quality of an imprintable layer that is lithographically imprinted as part of the imprint lithography process. Thus, in some examples, the stamp can be used to assess the quality of the imprint composition from which the imprintable layer is derived.

[0058] One aspect of the present disclosure is a patterned stamp for use in a quality control method. Another aspect of the invention is a quality control method using a patterned stamp, or the use of the stamp for a quality control method. The basic steps of the method are now described.

[0059] The method also includes using, and optionally providing, a patterned stamp 12 carrying a pattern of relief features defining a stamp test pattern as defined herein. The stamp test pattern can be, for example, as described above with reference to Figure 1. More generally, the stamp test pattern comprises a plurality of pattern sections 16, the pattern sections being spaced apart from one another, each pattern section comprising a pattern feature 22 for forming at least one recess 24 of a respective width and, optionally, a different depth compared to the depths of the other pattern sections.

[0060] The method comprises applying a stamp to an imprintable layer deposited on a substrate, the imprintable layer comprising an imprintable medium, whereby the imprintable layer is induced to conform to the stamp pattern.

[0061] If the imprintable medium is a curable imprintable medium, the method further comprises curing the imprintable layer for a period of time to form a hardened imprint pattern in the surface of the imprintable layer. However, a curable imprintable medium is not required. Another alternative is a hot embossing process which does not require a curing step.

[0062] The method further comprises obtaining optical image data of the imprint pattern formed in the imprintable layer.

[0063] The method further includes deriving a fit quality indicator indicative of a quality of the fit between the layer and the stamp based on the optical image data and based on knowledge of the different recess widths and / or depths of the aligned pattern sections.

[0064] 2 shows a further example of a patterned stamp 12 according to one or more embodiments. The stamp can be a substrate conformal imprint lithography (SCIL) stamp in some examples.

[0065] In this example, the stamp pattern includes three pattern blocks 20a, 20b, and 20c. Each pattern block includes a plurality of sets 18 of stamp pattern sections 16. Each pattern section 16 includes a stamp relief shape that defines one or more depressions of a particular width. In this example, each pattern section 16 includes a relief shape that defines a respective line grating of a particular pitch and groove line width. One exemplary set 18a of pattern sections 16 (in the form of a line grating) is shown in FIG. 2, item 18a. Within each set 18 of gratings, each grating 16 has a different pitch and line width w measured along the double-arrowed line indicated by l. The pitch varies from a minimum pitch to a maximum pitch, thereby forming a series of line depressions of different widths when the stamp is applied. In this example, the pitch of the gratings 16 varies from a minimum pitch of 0.5 μm to a maximum pitch of 200 μm. In some examples, the number of periods (i.e., the number of lines) in each grating also varies as the pitch varies, such that a grating with a higher pitch has a smaller period. The minimum number of lines can be two or three.

[0066] Each block 20a, 20b, 20c can correspond to a different imprint depth, so that all of the sets 18 of pattern sections 16 within one block 20 are of the same imprint depth as defined in Figure 3(a). In the illustrated example, the first block has a depth of 75 nm, the second block 20b has a depth of 150 nm, and the third block 20c has a depth of 300 nm. These are merely examples and do not limit the scope of the inventive concept in any way.

[0067] In some examples, the sets 18 of line gratings may also vary between each other in their grating duty cycle. For example, FIG. 2 shows a design in which within each block, there are multiple sets 18 of gratings 16, each set of gratings containing gratings of the same range of pitches and line widths, but with varying grating duty cycles (meaning the percentage of each grating period that the ridge portion of the grating extends spatially, as opposed to the recesses). In the example of FIG. 2, each block 20 contains two identical groups 21 of three sets 18 of grating sections 16, with the sets 18 in each group being identical to each other, except for their grating duty cycles. For example, the three sets 18 of gratings within each group 21 may have respective grating duty cycles of 0.33, 0.5, and 0.66 (this is just one example). Providing gratings with different duty cycles allows for testing flow or viscosity at the beginning and end of each imprint pattern.

[0068] By providing each elongated block with two identical groups 21 of grating sets 18, one at each end of the block, this makes it possible to test the imprint quality at different times during imprint.

[0069] Purely by way of example, in the illustrated example, each set 18 of gratings 16 may have a length l between 15 mm and 20 mm. For example, in one prototype built by the inventors, the length was 17.1 mm (by way of non-limiting example only).

[0070] As further guidance, in the illustrated example, each set 18 of gratings 16 can have a width w between 1 mm and 5 mm. For example, in one prototype built by the inventors, the width was 3 mm (by non-limiting example only).

[0071] The horizontal spacing between each adjacent grating section 16 in a given set 18 of grating sections may be, for example, 75 μm to 125 μm, such as 100 μm. The horizontal spacing between adjacent pattern blocks 20a, 20b, 20c may be, for example, 1 mm to 5 mm, such as 3 mm.

[0072] It should be noted that according to at least one set of embodiments, the master stamp mold can be provided, for example, in silicon or another suitable material, from which multiple versions of the same flexible stamp can be formed with the same stamp pattern. The flexible stamp itself can be formed from an elastomeric material, while the master can be formed from a more rigid material, for example, etched into silicon.

[0073] For the sake of schematic illustration, Figures 1 and 2 show an exemplary stamp 12 in which the stamp pattern covers a majority of the surface area of ​​the stamp compliant layer 14. In other words, the stamp is a dedicated quality control stamp. For example, the stamp can be applied to dedicated test pieces at intervals throughout the manufacturing process to periodically test the quality of the imprintable layer used in the lithographic imprinting part of the manufacturing process. However, in other examples, the test stamp pattern may be provided to cover a minority portion of the surface area of ​​the primary stamp used in the lithographic imprinting step of the primary manufacturing process. For example, this can form a test area of ​​the imprint pattern, so that the quality of the imprintable layer can be evaluated by optical analysis of any piece manufactured throughout the manufacturing process. This is more advantageous to facilitate in-line (manufacturing line) quality control. Changing the stamp during a print run can be very time consuming.

[0074] A method of imprinting the test stamp 12 and optically analysing the resulting imprint pattern is then further described with reference to Figures 3a-d and 4. Further details of printing processes in which the present technology can be used are disclosed in EP3126909A and WO2020 / 0099265.

[0075] A schematic diagram of an exemplary stamp 12 is shown in Figure 3(a). The stamp includes a flexible stamp layer 14 carrying a pattern of relief features 22 that define the stamp pattern. In the schematic diagram of Figure 3, only one pattern portion is visible, the relief features 22 for forming depressions 44 of a common width in the imprintable layer, thereby forming a patterned layer. However, as discussed above, the stamp may include multiple pattern sections, each of which forms depressions of different widths.

[0076] The substrate can be, for example, a semiconductor substrate such as silicon, but can be any other substrate. The substrate can be flat or it can be curved to some degree.

[0077] The imprintable layer may be formed from a deformable material, where deformable means at least deformable while being imprinted with a flexible, pliable stamp, such as an elastomeric rubber stamp. Such a deformable layer may be referred to as an imprint resist layer. An example of a deformable layer is a sol-gel based layer, i.e. a layer comprising a sol-gel based material, or a sol-gel + nanoparticle based material. Other deformable layers include materials based solely on nanoparticles. Other examples include UV curable materials, such as epoxy cured or acrylate cured materials. Organic materials (with crosslinking by UV light or another initiator), or hybrid inorganic-organic materials may also be used. As a further example, the imprintable layer may be formed from a glassy material, such as silicon monoxide (SI-O). Suitable examples of such materials are disclosed in WO 2009 / 141774 A1, but it will be appreciated that any suitable resist material may be used. Further examples of suitable resist materials are disclosed, for example, in US 2004 / 0261981 A1, WO 2005 / 101466 A2, US 2005 / 0230882 A1, US 2004 / 0264019, as well as in the non-patent publication Advanced Materials, 1998, Vol. 10(8), page 571.

[0078] The imprintable layer may be applied to the substrate in a deposition process such as, for example, spin coating, spraying, dip coating, blade coating, or any other deposition method. Preferably, such a method is capable of depositing an imprintable layer having a substantially uniform thickness on the substrate.

[0079] The stamp 12 is applied to an imprintable layer 32 disposed on a substrate 34, the imprintable layer comprising a curable imprintable medium, whereby the imprintable layer is induced to conform to the stamp pattern. This step is shown diagrammatically in Figure 3(b). By way of example, the imprintable layer 32 can be a sol-gel based resist layer, e.g. a mask layer, for use as part of a semiconductor manufacturing process. For example, the imprintable layer 32 can be a nanoglass resist layer.

[0080] Application of the stamp causes material of the imprintable layer 32 to redistribute by flowing from the areas between the features 22 and the substrate into the gaps 24 in the stamp relief pattern, as indicated by the solid curved arrows in Figure 3(b).

[0081] After the stamp 12 is applied, it is held in place on the imprintable layer 32 for an appropriate and selected imprinting period to allow the imprintable layer material to perfectly conform to the stamp relief pattern, after which the layer is solidified in a curing process. After curing for the predetermined period of time, the stamp 12 is removed, leaving behind a hardened imprint pattern 42 in the patterned or imprinted layer 36.

[0082] Depending on the quality of the imprintable layer 32, the resulting imprint pattern in the imprinted layer 36 will vary in quality. A higher quality imprintable layer will result in a higher quality (better or more complete) fit of the layer 32 to the stamp pattern. FIG. 3(c) shows an example of a cured imprint pattern on the imprint layer 36 of a high quality imprintable layer 32. FIG. 3(d) shows a further example of a lower quality cured imprint pattern in the imprint layer 36 resulting from a lower quality imprintable layer 32. Clearly, in the case of FIG. 3(d), the depressions 44 and protrusions 46 do not have a flat top shape as based on the stamp relief shape. In contrast, the depressions 44 and protrusions 46 have an uneven shape. This may be due to an incomplete flow of the imprintable layer material at the selected imprint conditions. Typically, the imprint composition and the imprintable layer deposited therefrom are specified to provide a high quality imprint layer 36 when used at specific imprint conditions, including one or more of imprint duration, humidity, pressure and temperature. The dependence of the shape of the patterned layer 36 on the quality of the imprint composition and / or the imprintable layer 32 and the conditions for imprinting such as humidity, temperature or pressure allows for quality control monitoring of the imprint composition and the imprintable layer through monitoring of appropriate shape parameters. For example, the imprint conditions such as humidity, pressure and temperature can be monitored separately and / or kept constant.

[0083] Shape interrogation of the patterned layer can be used to monitor the quality during repeated imprint cycles by checking whether selected shape parameters remain within specified tolerances or change to assume values ​​outside such tolerances. Parameters and methods for monitoring these are described herein below.

[0084] For the higher quality imprintable layer 32 used to obtain the imprint layer 36 of FIG. 3(c), its viscosity is lower than that of the imprintable layer 32 that resulted in the imprint layer 36 of FIG. 3(d), resulting in an improved quality of fit between the layer and the stamp. This is manifested in the imprint pattern 42 of the imprint layer 36 including depressions 44 that have a more uniform height at the base of the depressions, i.e. are flatter, i.e. less convex, at this base. It further results in a more uniform height at the top of the peaks 46 of the imprint pattern, i.e. a flatter, less concave base. Without wishing to be bound by theory, the viscosity change to higher viscosities in sol-gel based imprint compositions and imprintable layers may be due to, for example, more extensive gel formation, which generally involves an increase in the chain length of the polymer and crosslink density in the composition and / or layer.

[0085] 3(d), there is greater non-uniformity in the height of the layer 36 at the bases of the imprint pattern recesses 44 and at the tops of the imprint pattern peaks 46. The higher viscosity impairs proper filling of the l-line spaces 24 of the stamp pattern under the desired or selected imprint conditions, resulting in a poor quality match between the stamp and the layer.

[0086] The quality of the fit of the imprintable layer 32 to the stamp pattern can be optically assessed by capturing optical image data of the resulting imprint pattern 42 in the imprinted layer 36. The image data can be obtained, for example, using an optical microscope or other optical detection system capable of imaging at least a portion of the test pattern and providing the image data to a user via a suitable image display interface or image processing system; by way of example, an optical microscope can be used at a magnification of about 1000x.

[0087] In imprintable layer 32 that has a poor quality fit with stamp 12, non-uniformities in the layer surface height at the bases of the depressions 44 and the tops of the peaks 46 result in changes in the optical properties of light reflected from the pattern at these locations. In particular, light incident on the stamp pattern may pass through the thin imprint layer 36 material, reflect off of the substrate 34, and then be received by an optical detection system. This results in image data that includes a thin film interference color pattern that varies in color across the width of the depressions 44 due to the height variations at the bases of the depressions and / or the height variations at the tops of the peaks 46.

[0088] In particular, the spectral characteristics of light reflected from a textured surface can be expected to differ from the spectral characteristics of light reflected from a more uniform surface. This allows the degree of surface roughness at the valleys and peaks to be detected based on the capture of optical image data and spectral (e.g. spectroscopic) analysis of the image data. From this, the quality of the fit between the layer associated with the imprintable layer 32 and the stamp can be inferred, and thus the quality of the imprintable layer 32. This can be extrapolated to the quality of the imprint composition.

[0089] As an example, determining the fit quality may include detecting from the acquired optical image data a spatial color pattern or distribution exhibited by each pattern section 16 of the stamp pattern (see above discussion regarding various pattern sections), and the method may further include determining an indicator of the fit quality based on the color pattern of each pattern section 16 and based on knowledge of the width and / or depth of at least one recess 24 in each pattern section.

[0090] More specifically, the method can include detecting from the color pattern of each pattern section 16 a color variation pattern across the width of at least one recess 44 of the pattern section. For each pattern section, a color uniformity metric can be determined, which represents a degree of color uniformity across the width of the at least one recess based on said detected color variation across the width of the recess. A fit quality indicator can be determined based on comparing the color uniformity metric of each pattern section against a predefined standard, such as a threshold range. For example, a perfectly flat recess base would indicate an optimal fit, which would be associated with a perfectly uniform color pattern (high color uniformity metric). A color uniformity threshold level can be set that is associated with an acceptable (within a tolerance range) degree of surface non-uniformity, which corresponds to an acceptable fit quality. In some examples, the color uniformity threshold level can be set based on the color uniformity of the non-imprinted areas surrounding the pattern 42 (which are flat). In other words, the color variation of the grooves of the grid pattern can be compared to the color variation of the flat areas outside the grid pattern.

[0091] The variation in color across the width of a given depression 44 (e.g., along the horizontal direction in the drawing plane of Figs. 3(c) and 3(d) and Figs. 4A and 4B) indicates a non-uniform surface of the layer 36 in this depression 44, which causes an interference pattern resulting in color variations in the reflected light due to non-uniformity in the height of the layer 36 across the width. This non-uniformity in layer height is caused by excess material due to restricted flow of the layer material into the gap 24 of the imprint pattern of the stamp. For example, referring to Fig. 3(b), the region of the depression 44 will be the first to experience problems with flow behavior because the space between the stamp and the wafer is the smallest, which impedes the flow of material more than, for example, the flow of material in the gap 24 where there is more space. Thus, the resulting layer surface across region 44 will be convex, and for example, material in the center of the depression will have the most difficulty flowing out to the edges. While problems with region 44 are encountered, region 46 is still flat and therefore may exhibit a uniform interference color. Therefore, preferably, the quality monitoring uses at least the areas of recesses in the test pattern, such as the depressions 44. If the flow becomes more restricted, for example due to an increase in viscosity, the areas 46 will at some point show problematic flow, in which case they will become convex as they cannot deliver enough material, and therefore the stamp will deform in these areas as well.

[0092] In some examples, determining the fit quality indicator can include identifying a maximum recess width among the different recess widths of the plurality of pattern sections whose corresponding color uniformity metric meets said predetermined criteria, in other words, the widest recess width that still results in an acceptably uniform layer thickness is identified.

[0093] In some examples, determining the fit quality indicator may include determining, for each pattern section, from the pattern of color variation across the width of at least one recess in the pattern section, the degree of height non-uniformity across the base of at least one recess, based on an assumption that the color variation is indicative of a thin film interference pattern of incident light through the imprintable layer.

[0094] Instead of, or in addition to, analyzing the color uniformity of the valleys, the color uniformity of the peaks of the pattern can be analyzed.

[0095] More specifically, the method may include detecting the color of the non-imprinted areas surrounding the imprint pattern 42 from the color pattern of each pattern section 16 .

[0096] In some examples, the color non-uniformity of the non-imprinted regions surrounding the pattern 42 can be used as a reference against which the color non-uniformity of the recesses 44 and / or peaks 46 is compared. The non-imprinted regions preferably do not have features 22, and thus during imprinting, this region is largely unaffected by the imprinting step. Thus, due to the initial deposition process (e.g., spin coating, blade coating, or other) of the imprintable layer 32, the non-imprinted regions are expected to be flat (i.e., of uniform surface height). Thus, the color uniformity of this region can be compared to the color uniformity of the base of the recesses and / or the color uniformity of the top of the peaks to determine the degree of surface height non-uniformity.

[0097] Note that the different depths 27 in the stamp pattern shown in Figure 2 allow the same stamp to be used to test imprintable layers of different thicknesses. Determining the fit quality of an individual layer may require analysis of the color pattern of a pattern section at only a single depth.

[0098] In some instances, color patterns or distributions in both the peaks 46 and valleys 44 of each pattern section can be detected, which can provide additional information. Color uniformity in both the peaks and valleys can be determined. The initial effects of impeded material flow are present initially in the lower portion 44 of the pattern, and later in both the lower portion 44 and the higher portion 46. By examining the non-uniformity in the higher and lower portions of the pattern, the fit quality can be more accurately assessed.

[0099] It should be noted that the surface height non-uniformity tends to be concave at the bottom of the depressions 44 and convex at the top of the peaks 46. During imprinting, material flows from the protruding areas to the raised areas in the stamp. If the flow is impeded due to the material properties of the layer, the material in the protruding stamp cannot be fully extruded and the rubber stamp deforms and curves under capillary forces. At the edges of the pattern, the material needs to flow only a short distance and therefore most easily reaches the full height and deepest pattern depth. From the center of the protrusions / depressions, the material needs to flow the longest distance and therefore is most impeded. If the viscosity is too high, not all the material can flow or be transferred between the stamp and the substrate, which results in a convex / concave face surface relief pattern.

[0100] Although the above description refers to capturing the color pattern and colors of the dips and peaks, more generally the method can include capturing the spectral pattern and spectral characteristics of the portions of the pattern that correspond to the dips and peaks.

[0101] The method can include determining a quantitative fit quality indicator, which can be calculated based on determining the degree of color non-uniformity across the width of each of the recesses 44 or peaks 46 of the imprint pattern 42. The degree of color non-uniformity across the recess width for each grating pitch can then be compared to one or more predefined thresholds for that pitch to derive an indicator of fit quality between the layer and the stamp. For example, a look-up table can record a set of different thresholds corresponding to different quantitative values ​​of the fit quality indicator.

[0102] A reference color pattern can be measured and stored at the beginning of a production lot using a set of imprint conditions and a test imprint cycle. During subsequent imprint cycles, on further substrates, one or more color patterns of the test pattern can be measured and compared to the reference color pattern. Then, if the deviation exceeds a threshold, it can be determined that the quality is too low to continue with the same resist (optionally after verifying that the imprint conditions have not changed relative to the conditions during the definition of the reference color pattern). At that point, an indication of resist change is given. Alternatively, the imprint conditions can be changed so that the effects of changed resist properties can be compensated to a desired extent.

[0103] It should be noted that the above method, including stamp application, curing and optical data acquisition, should be carried out under controlled environmental conditions, e.g., controlled humidity, pressure and / or temperature. If the material of the imprintable layer is photosensitive, the light intensity conditions should also be controlled.

[0104] By way of illustration, FIG. 4 shows two exemplary sets A, B of optical image data of a series of test patterns 2-256 for two sample imprintable layers 32 imprinted with a test stamp 12 according to an embodiment of the present disclosure. The numbers 2-256 indicate all test pattern periods as a single feature depth value. Image data set B corresponds to a first sample after 4 hours of curing and left at a temperature of 20° C. Image data set A corresponds to a second sample after 24 hours of curing (aging), again left at a temperature of 20° C. The two image data sets are labelled (at the top) according to the period (in microns) of each grating.

[0105] In the first sample image data B, there is good color uniformity (and therefore good layer quality) across the line width for grating periods up to 32 microns (32 and below show uniform color across each line width). For the second sample at 24 hours, the material viscosity is high (due to aging) and color uniformity across the line width is only good up to periods of 8 microns. Note, for example, in image dataset A at test sections 16, 32, 64, 128 and 256, the depressions and peaks have similar dark colors, but the edges of the depressions / peaks show lighter color variations, indicative of a situation where the depressions and peaks have convex and concave surface profiles, similar to FIG. 3(d). In the corresponding test patterns 16 and 32 in image dataset B, the colors of the depressions and peaks are different and the color difference within the depressions or within the peaks is smaller than expected for a situation similar to FIG. 3(c).

[0106] This experiment demonstrates the principle that the layer viscosity, and therefore the quality of fit between the layer and the stamp, is detectable in the color pattern non-uniformity when the imprinted layer is illuminated. Finding the maximum recess width at which the color uniformity is high provides an indication of the layer's quality of fit.

[0107] The image datasets are reproduced in black and white for the purposes of this application, but in fact contain full color information, and their representation here is limited in terms of black and white reproduction and is for illustrative purposes only.

[0108] Each image data set represents a stamp containing multiple pattern sections, each containing a line grating with a different respective pitch, the line grating pitch varying from 2 μm to 256 μm in each sample.

[0109] FIG. 5 shows an exemplary method 500 as disclosed herein. In step 510, a first substrate coated with an imprintable layer is imprinted with a stamp having a test pattern as disclosed herein. For example, a substrate and stamp as described with reference to FIGS. 3(a)-3(d) are used in a SCIL process as described herein above. The imprinting can be performed using systems and apparatus as described herein for SCIL. During the imprinting step (FIGS. 3(a) and 3(b)), the imprinting conditions such as environmental temperature, pressure, and preferably humidity can be kept constant and determined as required using methods known in the art. The data can be stored in the memory of a computer or other data processor. The result of the imprinting is an imprinted layer as represented by layer 36 on substrate 34 in FIG. 3(c).

[0110] In step 520, one or more test pattern areas in the imprinted layer 36 are analyzed. This includes optical interrogation as described herein to measure color variation across the depressions and / or peaks of the test pattern. For this purpose, a camera and / or a spectrometer may be used to record and / or analyze an image or a portion thereof. Any image may be stored in the memory of a computer or other data processor. Standard color measurement methods and principles may be used to determine the color of a particular portion of the test pattern. The data may be stored in the memory of a computer or other data processor. The color measurement may be performed across the depressions and / or peaks of the test pattern or a plurality thereof. In some examples, the analysis includes the generation of a quality indicator representative of the color variation across a portion of the depressions and / or peaks. This indicator may be used and optionally stored as a reference quality indicator if the color variation is acceptable or deemed representative of the imprint pattern according to an accepted specification. Such reference images, reference color variations, and reference indicators may thus be used as a threshold for a minimum quality of the imprintable layer.

[0111] In step 530, a further substrate having a further imprintable layer is imprinted using the conditions and imprint composition defined for the reference imprint layer. During the further imprint cycle, one or more further test patterns are analyzed as described for step 520. A separate test substrate can be used, but alternatively or additionally the test pattern can be part of the actual production stamp. The latter allows for a more efficient use of time during production and also allows for the test results to be compared with the actual production geometric patterns that are also present on such substrates. The further test pattern images, analysis and quality indicators can be stored in memory and / or compared with the corresponding ones of the reference test pattern. The results can again be stored in memory. If the comparison reveals that the results exceed a quality threshold (e.g. a quality indicator higher than a minimum quality indicator), the further imprinting of the imprintable layer can continue. Conversely, if the comparison reveals that the further test pattern results fall below the quality threshold, a warning can be generated, stored and / or output to a user or operator. Alternatively, the imprint cycle can be interrupted until the imprint composition is changed or the imprint conditions are changed.

[0112] Any of the analysis results may be output to a user using a suitable output interface, such as a display interface connectable to a computer or other data processor, which may be part of an imprinting apparatus, such as a SCIL imprinting apparatus or system.

[0113] Any one of the methods disclosed herein can be implemented in a computer or data processor executable code. Such code can be part of a computer or data processor suitable for or configured to control an imprint system and / or an optical imaging system for an imprint system. The code can be stored in a non-transitory medium such as an optical memory (CD, DVD, Blu-ray, etc.) known in the art, an electrical memory (EPROM, EEPROM, Flash, SRAM, DRAM, etc.) known in the art, or a magnetic medium such as a magnetic hard disk drive known in the art. Other storage methods can also be used.

[0114] The computer or other data processor may be designed according to principles known in the art having processing circuitry capable of communicating with a memory via a communication bus and protocol as known in the art. The processing circuitry is configured to execute code to enable a user to input controls and store the results of the analysis as described herein, as well as generate and output such results to a user via a suitable output interface, such as, for example, one or more displays as known in the art.

[0115] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

[0116] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0117] When the term "adapted for" appears in the claims or the specification, it is meant to be synonymous with the term "configured to."

[0118] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. 1. A quality control method for use in imprint lithography, comprising: using a patterned stamp to pattern the imprintable layer, said stamp comprising a flexible stamp layer carrying a pattern of relief features defining a stamp pattern, said stamp pattern comprising a plurality of pattern sections, said pattern sections spaced apart from one another, each pattern section including a pattern feature for forming at least one recess of a respective width different compared to that of the other pattern sections; applying the stamp to an imprintable layer disposed on a substrate, the imprintable layer comprising a hardenable imprintable medium, whereby the imprintable layer is induced to conform to the stamp pattern; acquiring optical image data of an imprint pattern formed in the imprintable layer; - deriving a fit quality indicator indicative of a quality of fit between a layer and a stamp based on the optical image data and based on knowledge of widths and / or depths of different recesses of the pattern section; Optionally, providing said match quality indicator to a user; The method according to claim 1,

2. 2. The method of claim 1 , wherein each pattern section has a relief feature defining a respective grid of parallel lines for forming linear depressions in the imprintable layer, the grid having a grating pitch and a grating duty cycle that together define a uniform grating linewidth of each of the linear depressions, and a grating depth that defines a depth of each of the linear depressions.

3. 3. The method of claim 2, wherein the grating of each of the plurality of pattern sections has a different respective line pitch compared to other pattern sections, the different respective line pitch resulting in a different grating line width compared to other pattern sections.

4. 4. The method of claim 3, wherein the plurality of pattern sections includes at least a first set of two or more gratings and a second set of two or more gratings, the first set of gratings including lines of a first depth and the second set of gratings including lines of a second depth, and each of the first and second sets of gratings including gratings of a plurality of different line pitches and / or grating duty cycles, respectively.

5. 5. The method of claim 2, wherein the grating line width of each grating is between 0.5 μm and 200 μm.

6. 4. The method of claim 3, further comprising: detecting a spatial color distribution or pattern exhibited by each pattern section from the optical image data; determining an indicator of fit quality based on the color pattern of each pattern section and based on knowledge of the width and / or depth of at least one of the recesses of each pattern section; 6. The method according to claim 1 , comprising:

7. 7. The method of claim 6, further comprising detecting a pattern of color variation from the color pattern of each pattern section across the width of at least one of the recesses.

8. 8. The method of claim 7, further comprising determining for each pattern section a color uniformity metric indicative of a degree of color uniformity across the width of at least one of the recesses based on the detected color variation across the width of the recesses.

9. The method of claim 8 , further comprising determining the fit quality indicator based on comparing the color uniformity metric of each pattern section against a predetermined criteria, such as a threshold range.

10. 10. The method of claim 9, wherein determining the fit quality indicator comprises identifying a maximum depression width among different depression widths of the plurality of pattern sections for which a corresponding color uniformity metric meets the predetermined criteria.

11. 11. The method of claim 8, further comprising detecting from the color pattern for each pattern section a degree of color uniformity of one or more of the peaks of the imprinted pattern section and non-imprinted areas surrounding the pattern.

12. 12. A method according to claim 11, directly or indirectly dependent on claim 9, wherein for each pattern section, the detected degree of colour uniformity of the peaks of the imprinted pattern section or of the non-imprinted areas surrounding the pattern is used as the predetermined criterion.

13. 12. A method according to any one of claims 7 to 11, wherein determining the fit quality indicator comprises, for each pattern section, determining a degree of height non-uniformity across a base of at least one of the recesses from the pattern of colour variation across a width of at least one of the recesses in the pattern section, based on the assumption that the colour variation is indicative of a thin film interference pattern of incident light through the imprintable layer.

14. The method of claim 1 , wherein the stamp is a Substrate Conformal Imprint Lithography (SCIL) stamp.

15. 1. An imprint lithography stamp having a patterned surface for patterning an imprintable layer, the stamp having a flexible stamp layer carrying a pattern of relief features defining a stamp pattern, the stamp pattern including a plurality of pattern sections, the pattern sections being spaced apart from one another, each pattern section including a pattern feature for forming one or more depressions of a respective width and / or depth that differs compared to that of the other pattern sections.

16. each pattern section has a relief feature defining a respective grid of parallel lines for forming linear depressions in said imprintable layer, said grid having a grating pitch and a grating duty cycle which together define a uniform grating linewidth of each of said linear depressions, and a grating depth which defines a depth of each of said linear depressions; The stamp of claim 15.

17. A computer program product running on a computer or data processor connected to an imprint system having an optical imaging device, the computer program product causing the imprint system to carry out a method according to any one of claims 1 to 14.

18. A non-transitory medium comprising the computer program of claim 17.

19. 20. A computer or data processor comprising a memory having a non-transitory medium according to claim 18.

20. 20. An imprint system comprising a non-transitory medium according to claim 18 and / or a computer or data processor according to claim 19.

Images