Sub-resolution grating in EUV imaging

Abstract

The present invention relates to a mask 100 for an extreme ultraviolet (EUV) lithography process in a high or hyper-numerical aperture range. The mask 100 comprises a mask pattern derived from a superposition of the two sub-layouts 120, 130. The first sub-layout 120 comprises a plurality of mask features intended for printing, wherein at least one feature is oriented along a first direction of the mask layout. The second sub-layout 130 includes one or more sub-resolution gratings 140 whose grating lines extend substantially in a second direction of the mask layout covering almost the entire width in the second direction. These sub-resolution gratings 140 are designed to be non-printable in a high or hyper-numerical aperture range. The invention aims to improve lithographic performance by optimizing a mask design under specific imaging conditions, such as optimal focus, highest contrast and depth-of-focus penetration pitch considerations.

Description

Technical Field

[0001] The present invention relates to the field of extreme ultraviolet imaging and lithography. More particularly, the present invention relates to masks, methods for designing masks, and methods for performing extreme ultraviolet lithography using these masks. Background Art

[0002] The field of semiconductor manufacturing has seen significant progress over the past few decades, with the miniaturization of electronic components being a key driver of technological advancement. One of the key steps in semiconductor manufacturing is the photolithography process, which involves transferring complex patterns onto silicon wafers to define the various features of integrated circuits. As the demand for smaller and more powerful electronic devices continues to grow, the semiconductor industry has been seeking ways to print smaller features with high contrast.

[0003] Projection lithography has been at the forefront of this effort, continuously evolving to utilize shorter wavelengths of light and larger numerical aperture (NA) optics to achieve finer resolution. The introduction of extreme ultraviolet (EUV) lithography with a wavelength of 13.5nm marked a major leap forward in this regard. However, the pursuit of higher resolution has led to the exploration of further increasing the NA. This presents a challenge because the depth of focus (DoF) is inversely proportional to the square of the NA. Therefore, in high NA (0.55) and ultra-NA (greater than 0.75) EUV lithography systems, the DoF becomes extremely limited.

[0004] In EUV lithography, the pattern to be printed on the wafer is encoded on a photomask. Due to the unique optical properties of EUV wavelengths, the structures on the mask must be multiples of the wavelength thickness, resulting in a three-dimensional mask topology. This complexity causes the optimal focus position to vary for different pattern pitches on the mask. Because all pitches must be printed simultaneously, some pitches will inevitably be out of focus, a situation that worsens with increasing NA.

[0005] The industry has explored various technical solutions to address the imaging issues caused by mask three-dimensionality (M3D) in EUV lithography. A common approach is to use sub-resolution assist features (SRAFs), thin lines placed parallel to the main features, to improve focus variation through pitch and increase the normalized image logarithmic slope (NILS). However, SRAF placement is complex and requires specialized tooling. In addition, SRAFs are sometimes unintentionally printed, resulting in increased defects and random failure rates.

[0006] Another approach to reducing best focus shift is to change the mask material. Low-refractive-index (low-n) absorbing materials with moderate absorption have been considered for their potential benefits, including improved reflectivity and diffraction efficiency, which can lead to sharper images. However, these materials also introduce new challenges, such as larger pitch best focus shifts, which can sometimes worsen the overall focus range compared to standard mask materials.

[0007] Despite these efforts, the industry still faces significant challenges in EUV mask technology. Differences in optimal focus at different pitches and the pursuit of higher NILS remain key issues that need to be addressed. As the semiconductor industry pushes the limits of EUV lithography, further advancements in mask technology are clearly necessary to overcome these challenges and enable the continued scaling of electronic devices. Summary of the Invention

[0008] It is an object of the present invention to improve the overall process window of multiple features when imaging using a numerical aperture of 0.55 or higher.

[0009] An advantage of the embodiments of the present invention is that EUV lithography can obtain an aligned best focus penetration pitch and a better image contrast penetration pitch.

[0010] An advantage of embodiments of the present invention is that they are compatible with any kind of mask. When using a mask-absorber stack, the reflectivity of the absorber stack can generally be chosen such that a sub-resolution pattern of a manufacturable size results in an appropriate background intensity for the absorber patterned with the sub-resolution features.

[0011] The above objectives are achieved by the method and device according to the present invention.

[0012] In a first aspect, the present invention relates to a mask for an EUV lithography process in the high or ultra-numerical aperture range, the mask comprising a mask pattern according to a mask layout, the mask layout being a superposition of a first mask sub-layout and a second mask sub-layout, the first mask sub-layout comprising a plurality of mask features to be printed, the plurality of mask features to be printed comprising at least one mask feature oriented along a first direction of the mask layout, the second mask sub-layout comprising one or more sub-resolution gratings having sub-resolution grating lines, the sub-resolution grating lines extending substantially in a second direction of the mask layout and over substantially their full width in the second direction of the mask layout, the one or more sub-resolution gratings being not printable for the high or ultra-numerical aperture range.

[0013] The sub-resolution grating pitch (SRG pitch) of the one or more sub-resolution gratings (SRGs) may typically be selected to exceed the resolution limit of the EUV lithography process. Thus, the sub-resolution gratings do not interfere with the intended mask pattern by remaining unprinted.

[0014] The size of the sub-resolution grating lines can be selected based on optimal focus. This allows for optimized focus across different pitches, improving the overall quality of the lithography process. The size of the sub-resolution grating lines can also be selected based on optimal contrast. This offers the advantage of enhanced image contrast.

[0015] The sub-resolution grating pitch of the one or more sub-resolution gratings can be selected based on the focal depth penetration pitch of the pattern to be printed. The sub-resolution grating pitch of the one or more sub-resolution gratings can also be selected based on the highest contrast penetration pitch of the pattern to be printed. This allows high contrast to be achieved at various pitches of the pattern to be printed.

[0016] The combination of the sub-resolution grating pitch and the sub-resolution grating line size of the one or more sub-resolution gratings may thus be selected based on the optimal depth of focus penetration pitch and / or the highest contrast penetration pitch of the pattern to be printed.

[0017] The second mask sub-layout may not include sub-resolution gratings at gap locations between the plurality of mask features in the first sub-layout.

[0018] In some embodiments, the first direction may be the same as the second direction.

[0019] In some embodiments, the first direction may be different from the second direction.

[0020] The grating lines can pass through essentially all mask features.

[0021] In various embodiments, the first direction may be perpendicular to the second direction.

[0022] In various embodiments, the mask can be used in an EUV lithography tool using anamorphic optics in which the demagnification in the X direction is less than the demagnification in the Y direction. The first direction along which the plurality of mask features extend can correspond to the Y direction, and the second direction can correspond to the X direction.

[0023] An advantage of embodiments of the present invention is that they are compatible with various shapes of illumination sources used in EUV scanners or lithography.

[0024] An advantage of the embodiments of the present invention is that the use of a sub-resolution grating stabilizes the 0th-order phase penetration pitch of the pattern to be printed.

[0025] An advantage of embodiments of the present invention is that, by using a sub-resolution grating, image contrast, such as NILS, a measure of image contrast, is substantially uniformly high across all pitch regions of the pattern to be printed. Such NILS cannot be achieved without using a sub-resolution grating. Although a slight reduction in NILS is observed at fine pitches, overall, the NILS is substantially more uniform and high across all pitch regions of the pattern to be printed.

[0026] An advantage of embodiments of the present invention is that they are immediately adaptable to high numerical aperture lithography at currently manufacturable sizes.

[0027] An advantage of embodiments of the present invention is that they can be applied to fabricate 2D metal patterns, such as logic, as well as 1D metal designs including L / S and line ends or 1.5D metal designs including landing pads.

[0028] An advantage of embodiments of the present invention is that they can be applied to imaging of 2D geometries like contact holes or logic via layers.

[0029] An advantage of embodiments of the present invention is that they may enhance depth of focus by reducing the complexity of the best focus problem that typically dominates source selection.An advantage of embodiments of the present invention is that they help to increase depth of focus, which is a key factor when using, for example, super-numerical aperture lithography.

[0030] An advantage of embodiments of the present invention is that these embodiments can be combined with other techniques such as fading correction, allowing further optimization of the imaging and the mask stack used simultaneously.

[0031] An advantage of embodiments of the present invention is that the mask type can be used with a highly reflective absorber, so that the best focus shift of isolated features is mitigated by the SRG, and so that the inter-electrode shift is mitigated by fading correction. In this way, all features can be optimized for aligned best focus, high NILS, and, for example, high total reflectivity and low intensity in dark areas.

[0032] An advantage of embodiments of the present invention is that the use of a sub-resolution grating results in an overall brighter mask.

[0033] In a second aspect, the present invention relates to a computer-implemented method for designing a mask layout for an EUV lithography process in a high or ultra-numerical aperture range, the method comprising defining a mask layout of a mask pattern as a superposition of a first mask sub-layout and a second mask sub-layout, the first mask sub-layout comprising a plurality of mask features to be printed, the plurality of mask features to be printed comprising at least one mask feature oriented along a first direction of the mask layout, the second mask sub-layout comprising one or more sub-resolution gratings having sub-resolution grating lines, the sub-resolution grating lines extending substantially in a second direction of the mask layout and extending substantially over the entire width in the second direction, the one or more sub-resolution gratings being not printable for the high or ultra-numerical aperture range.

[0034] In various embodiments, the method may include repeating the definition of the mask layout for multiple sub-resolution grating pitches and / or sizes of sub-resolution grating lines of one or more sub-resolution gratings, thereby obtaining different mask layouts depending on the size of the sub-resolution grating lines; simulating lithographic imaging of the mask pattern for each of the different mask layouts depending on the sub-resolution pitches and the size of the sub-resolution grating lines, thereby obtaining imaging quality parameters; and selecting the mask layout with the best imaging quality parameters. The imaging quality parameters may be, for example, the depth of focus penetration pitch and / or the highest contrast penetration pitch of the pattern to be printed. This embodiment provides the advantage of enabling selection of the best mask layout penetration simulation, which may improve lithographic results.

[0035] The method may further comprise simulating lithographic imaging based on multi-pass unipolar exposure or implanted aberrations.This embodiment provides the advantage of taking various lithographic conditions into account, which may lead to a more robust and reliable mask design.

[0036] In a third aspect, the present invention relates to a system for determining lithographic processing conditions for an EUV lithography process in the high or ultra-numerical aperture range, the system comprising input means for obtaining characteristics of an illumination source and a lithographic pattern to be created, and processing means programmed to define a mask layout of a mask pattern as a superposition of a first mask sub-layout and a second mask sub-layout, the first mask sub-layout comprising a plurality of mask features to be printed, the plurality of mask features to be printed comprising at least one mask feature oriented in a first direction of the mask layout, the second mask sub-layout comprising one or more sub-resolution gratings having sub-resolution grating lines extending substantially in a second direction of the mask layout and extending over substantially the entire width of the mask layout in the second direction, the sub-resolution gratings also being not printable for the high or ultra-numerical aperture range. Furthermore, the system typically comprises an output system for outputting the designed or selected mask layout.

[0037] The processing device may be further programmed to repeat the definition of the mask layout for a plurality of sub-resolution grating pitches and / or sizes of the sub-resolution grating lines, thereby obtaining different mask layouts according to the sub-resolution grating pitches and according to the sizes of the sub-resolution grating lines; simulate lithographic imaging of the mask pattern for each of the different mask layouts according to the sub-resolution grating pitches and the sizes of the sub-resolution grating lines, thereby obtaining imaging quality parameters; and select the mask layout having the best imaging quality parameters. The imaging quality parameters may be, for example, a depth of focus penetration pitch and / or a maximum contrast penetration pitch of the pattern to be printed.

[0038] The system may be implemented as a computer program product which, when executed on a processing device, performs one of the methods according to the second aspect of the invention.

[0039] In a further aspect, the present invention relates to a data carrier storing a computer program product which, when executed on a processing device, performs one of the methods according to the second aspect of the invention. Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims as appropriate, and not just as explicitly set out in the claims.

[0040] While devices in the art are constantly improving, changing, and evolving, the present inventive concepts are believed to represent a substantially new and inventive advancement involving a departure from prior practice, thereby providing a more efficient, stable, and reliable device of this nature.

[0041] The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, which illustrate the principles of the invention by way of example. This description is given for illustrative purposes only and does not limit the scope of the invention. The reference figures cited below refer to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 FIG. 4 is a schematic diagram of a mask for an EUV lithography process according to an embodiment of the present invention.

[0043] Figure 2 is a flow chart of a computer-implemented method for designing a mask layout for an EUV lithography process according to an exemplary embodiment of the present invention.

[0044] Figure 3 A system for designing a mask layout for an EUV lithography process according to an embodiment of the present invention.

[0045] Figures 4 to 8An example of using a sub-resolution grating in a mask layout to obtain an improved EUV lithography mask and the advantages that accrue therefrom is shown.

[0046] The same reference numbers in different drawings refer to the same or similar elements. DETAILED DESCRIPTION

[0047] The present invention will be described with respect to specific embodiments and with reference to certain drawings, but the invention is not limited thereto but only by the claims. The drawings described are merely illustrative and non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. Dimensions and relative dimensions do not correspond to actual reductions in practice of the invention.

[0048] The terms first, second, third, etc. in the specification and claims are used to distinguish between similar elements and not necessarily to describe a sequence in time, space, level, or in any other manner. It is understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein are capable of operation in a sequence different from that described or illustrated herein.

[0049] Additionally, the terms top, over, and the like in the specification and claims are used for descriptive purposes and not necessarily for describing relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein are capable of operation in orientations other than those described or illustrated herein.

[0050] It should be noted that the term "comprising," also used in the claims, should not be interpreted as being limited to the means listed thereafter; it does not exclude other elements or steps. Thus, the term should be interpreted as specifying the presence of the stated features, integers, steps, or components as mentioned, but does not exclude the presence or addition of one or more other features, integers, steps, or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and means B" should not be interpreted as being limited to devices consisting only of components A and B. This means that for the present invention, the only relevant components of the device are A and B. Thus, the term "comprising" covers cases where only the stated features are present as well as cases where these features and one or more other features are present. The word "comprising" according to the present invention therefore also includes an embodiment in which the other components are absent. In this application, when the word "comprising" is used to describe an embodiment, it will be understood that alternative versions of the same embodiment, in which the term "comprising" is replaced by "consisting of," are also included within the scope of the present invention.

[0051] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in various embodiments" in various places throughout this specification do not necessarily all refer to the same embodiment, but may refer to different embodiments. Furthermore, in one or more embodiments, the particular features, structures, or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure.

[0052] Similarly, it should be appreciated that in the description of exemplary embodiments of the present invention, various features of the present invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding understanding of one or more of the various inventive aspects. However, this method of disclosure should not be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. On the contrary, as reflected in the appended claims, inventive aspects lie in fewer features than all of the features of a single preceding disclosed embodiment. Accordingly, the claims appended to the specific embodiments are hereby expressly incorporated into this specific embodiment, with each claim itself representing a separate embodiment of the present invention.

[0053] Furthermore, although some embodiments described herein include some features included in other embodiments but do not include other features included in other embodiments, as will be understood by those skilled in the art, combinations of features from different embodiments are intended to fall within the scope of the present invention and form different embodiments. For example, in the appended claims, any of the claimed embodiments may be used in any combination.

[0054] In addition, some of the embodiments are described herein as methods or combinations of elements of methods that can be implemented by a processor of a computer system or by other devices that implement the functions. Thus, a processor with the necessary instructions for executing such methods or elements of a method forms a device for executing the method or elements of a method. In addition, the elements described herein of the device embodiments are examples of devices for implementing the functions performed by the elements that achieve the purposes of the present invention.

[0055] In the description provided herein, numerous specific details are set forth. However, it should be understood that embodiments of the present invention may be practiced without these specific details. In other instances, well-known methods, structures, and techniques are not shown in detail to avoid obscuring an understanding of this specification.

[0056] The following items are provided solely to assist in understanding the present invention.

[0057] When referring to "masks for EUV lithography processes," we are referring to photomasks specifically designed for use in extreme ultraviolet (EUV) lithography systems operating at high or ultra-high numerical apertures (NA). When referring to high numerical apertures (NA), we are referring to NAs greater than 0.5. When referring to ultra-high numerical apertures, we are referring to NAs greater than 0.55.

[0058] When referring to the "pitch of one or more sub-resolution gratings" or sub-resolution grating pitch, this refers to the distance between the centers of adjacent lines or features within the sub-resolution grating pattern. The sub-resolution grating pitch is selected to exceed the resolution capabilities of the EUV lithography process, meaning that the individual lines or features of the grating will not be noticeably printed on the substrate. Examples of specific embodiments include sub-resolution grating pitches that are larger than the minimum feature size that can be resolved by the lithography system, which may vary depending on the specific capabilities of the EUV lithography equipment used.

[0059] When referring to "inter-pole shifting," we are referring to a technique that uses pattern shifting produced by illumination from different illumination poles to push the resolution limit.

[0060] As used herein, unless otherwise specified, the term "sub-resolution grating line size" refers to the width or critical dimension of the individual body lines or features that make up the sub-resolution grating pattern on the mask.

[0061] The present invention will now be described in detail by describing several embodiments of the present invention. Obviously, other embodiments of the present invention can be configured according to the knowledge of those skilled in the art without departing from the technical teachings of the present invention, and the present invention is only limited by the terms of the appended claims.

[0062] In a first aspect, the present invention relates to a mask for EUV lithography processes in the high or ultra-high numerical aperture range. While masks for EUV lithography are typically designed for a specific numerical aperture (NA), the masks described herein can be made into hybrid masks, i.e., for use with different tools having different NAs. In this case, the mask frame can, for example, include different alignment marks for tools with different NAs.

[0063] The mask according to an embodiment of the present invention can be any type of EUV lithography mask. It can be a bright field mask or a dark field mask. The mask can be made of any suitable material. In some embodiments, the mask can be made of a low-n material, but the embodiment is not limited thereto.

[0064] The mask to be formed based on the determined mask design can be fabricated using any conventional EUV mask fabrication technology. The mask can be made using electron beam writing technology. In some embodiments, the mask features and one or more sub-resolution gratings defined below can be implemented in multiple processing steps or simultaneously, such as by electron beam writing and etching.

[0065] The mask includes a mask pattern, which is usually a pattern formed on a substrate by a mask material. Thus, the mask pattern is made according to a mask layout.

[0066] According to an embodiment of the present invention, the mask layout is a superposition of a first mask sub-layout and a second mask sub-layout.

[0067] In other words, a mask layout is the result of superimposing or combining multiple individual patterns or sub-layouts. A first mask sub-layout includes multiple mask features to be printed. The multiple mask features to be printed include at least one mask feature oriented along a first direction of the mask layout. This refers to the portion of the mask pattern that includes the primary features intended to be transferred to a wafer or substrate during the photolithography process. These features represent the actual pattern or device structure desired in the final manufactured product. Examples of specific embodiments include, but are not limited to, lines, spaces, contact holes, and other geometric shapes corresponding to components of an integrated circuit or other device.

[0068] The second mask sub-layout comprises one or more sub-resolution gratings having sub-resolution grating lines. The one or more sub-resolution gratings may have different sub-resolution grating spacings or the same sub-resolution grating spacings. If different sub-resolution grating spacings are used for different sub-resolution gratings, optimization may be performed for each of them. The sub-resolution grating lines of the one or more sub-resolution gratings extend substantially along a second direction of the mask layout and extend over substantially the entire width in the second direction of the mask layout. The one or more sub-resolution gratings are not printable for the high or super numerical aperture range for which the mask is used. Therefore, the sub-resolution grating lines only include grating lines whose dimensions are below the resolution limit of the lithography system so that they will not be printed as clear features on the wafer.

[0069] The final mask layout used to produce the corresponding mask can be represented in a binary format in the final representation, i.e. the combination of mask features and sub-resolution grating lines is represented only as 0 and 1. In this case, 0 represents a dark area that absorbs light (=absorptive area), while 1 represents a bright area that reflects light (=reflective area).

[0070] Figure 1 A mask layout 110 is shown, which diagrammatically illustrates the corresponding pattern on the substrate on which the mask 100 is formed. Figure 1In , the mask layout 110 is a superposition of a first mask sub-layout 120 comprising mask features intended to be printed and a second mask layout 130 comprising one or more sub-resolution gratings 140 that are not intended to appear as printed features in the final substrate.

[0071] In some embodiments, the first direction can be the same as the second direction. An example of such an embodiment is combining a horizontal metal design with a pitch of 20 nm with a sub-resolution grating having sub-resolution grating lines with a sub-resolution grating pitch of 10 nm. This embodiment has the advantage of resolving the 2-bar critical dimension asymmetric penetration focusing that occurs in the horizontal metal design (lines along the X direction) by using a manufacturable sub-resolution grating extending in the X direction, even though a sub-resolution grating extending in the Y direction would not otherwise be manufacturable.

[0072] In some embodiments, the first direction may be different from the second direction. This covers, for example, the case where the pattern design to be printed is rotated relative to the sub-resolution grating direction.

[0073] In some embodiments, the first direction and the second direction can be perpendicular to each other. In such embodiments, another aspect of EUV lithography can be exploited. In EUV lithography, increasing the NA means that the angle at which the radiation hits the mask increases, resulting in a loss of reflectivity, which leads to printing problems. This problem has been solved in the field by using anamorphic optics. The reflective optics of the EUV system used do not uniformly reduce the pattern to be printed, but instead reduce the pattern to be printed differently in one direction than in a direction perpendicular to that direction. A typical reduction used in EUV lithography systems is that the reduction in the X direction is less than the reduction in the Y direction. An example of such a system includes a reduction factor of 4x in the X direction and 8x in the Y direction.

[0074] Because the mask size is relatively large in the Y direction, it allows for a relaxed and designer-friendly layout. Utilizing this anamorphic optical design system, in some embodiments, sub-resolution grating lines can be easily inserted in the Y direction (i.e., lines extending along the X direction) at their pitch without risking mask rule check (MRC) violations. It does not cause unwanted printability issues for sub-resolution features because it uses patterns that exceed resolution limits. Thus, in some embodiments, a first direction along which multiple mask features extend corresponds to the Y direction, while a second direction corresponds to the X direction.

[0075] In some embodiments according to the present invention, the design of the mask is further adjusted by selecting the sub-resolution grating pitch and the size of the sub-resolution grating lines based on factors such as best focus, highest contrast and depth of focus penetration spacing of the pattern to be printed. These selections allow optimization of the lithography process because they enhance image quality and uniformity for various spacings. According to some specific embodiments, the size of the grating lines of one or more sub-resolution gratings is selected based on optimal imaging performance. This may vary depending on the mask tone and mask stack (absorber thickness, refractive index of the absorbing material). The sub-resolution grating pitch of one or more sub-resolution gratings is selected to exceed the resolution limit, which ensures that there is no risk of unwanted sub-feature printability on the wafer. In addition, it also takes into account the feasibility of mask manufacturing. If the maximum sigma-y point of the illumination pupil can be reduced to a smaller value, the sub-resolution grating pitch can be further relaxed or fine-tuned.

[0076] As an illustration, the embodiments of the present invention are not limited thereto, and the allowable sub-resolution grating pitch can be expressed as

[0077]

[0078] Where λ is the wavelength of the EUV lithography process, and σ Ymax is the maximum sigma-y point of the illumination pupil. For example, if the maximum sigma-y point of the illumination pupil σ Ymax is 0.8, then the minimum spacing can be 10 nm, and if the maximum sigma-y point σ of the illumination pupil Ymax is 1, the minimum pitch can be 9nm. If the current minimum width achievable on a mask using e-beam writing technology is 24nm to 32nm, this results in 3-4nm (image level magnification is 8x). For 0.55NA, the resolution limit in the Y direction is 12.3nm pitch at the wafer level, so 98.2nm pitch at the mask level. For 0.75NA, the resolution limit in the Y direction is 9nm pitch at the wafer level and 72nm pitch at the mask level. Therefore, the printable features on the mask are much smaller than the minimum period that can be imaged by high NA or ultra-NA tools, especially if the grating is oriented as a horizontal line (8x magnification).

[0079] According to some embodiments, a sub-resolution grating having grating lines extending substantially across the full width of the mask is implemented in some areas of the mask, but no sub-resolution grating is provided in areas where there are gaps between mask features to be printed, thereby simplifying the design where a grating is not required.

[0080] In a second aspect, the present invention relates to a computer-implemented method for designing a mask layout for an EUV lithography process in a high or ultra-numerical aperture range. The method may produce a mask for EUV lithography processing as described in the first aspect. According to an embodiment of the present invention, the method comprises defining a mask layout of a mask pattern as a superposition of a first mask sub-layout and a second mask sub-layout. The first mask sub-layout comprises a plurality of mask features to be printed, the plurality of mask features to be printed comprising at least one mask feature oriented along a first direction of the mask layout. The second mask sub-layout comprises one or more sub-resolution gratings having sub-resolution grating lines, the sub-resolution grating lines extending substantially in a second direction of the mask layout and extending over substantially the entire width of the mask layout in the second direction, the one or more sub-resolution gratings being not printable for the high or ultra-numerical aperture range.

[0081] In an exemplary embodiment, Figure 2 As shown, a method is disclosed, wherein the method 200 includes the following steps.

[0082] In a first step, the method 200 comprises repeatedly defining a mask layout 210 having the mask layout features as described above but for a plurality of sub-resolution grating pitches and / or sizes of sub-resolution grating lines of one or more sub-resolution gratings, thereby obtaining different mask layouts depending on the sub-resolution grating pitches or the sizes of the sub-resolution grating lines.

[0083] In a second step, method 200 includes simulating lithographic imaging 220 of mask patterns for different mask layouts based on the size of the sub-resolution grating lines. Image quality parameters can be derived from these simulations. These can be, for example, the optimal depth-of-focus penetration pitch and / or the highest contrast penetration pitch for the pattern to be printed, but other image quality parameters may also be included.

[0084] In a third step, the method 200 includes selecting 230 a mask layout having optimal imaging quality parameters.

[0085] It should be noted that these steps may be performed at least partially in parallel, and based on the first evaluation, certain mask layouts may no longer be considered, while other mask layouts may be defined and simulated with priority.

[0086] The method can be implemented as an optimization method, thereby obtaining an improved mask layout.

[0087] According to an embodiment of the present invention, this involves defining 210 a plurality of mask layouts having different sub-resolution grating pitches and / or sizes of sub-resolution grating lines, simulating lithographic imaging 220 of each layout, and selecting 230 the layout with the best depth of focus and / or highest contrast penetration pitch. In addition to these optimizations, the method can also take into account different lithographic conditions, such as multi-pass monopole exposure or injection aberrations. Further optimization of the mask design based on other known optimization processes can also be implemented simultaneously or in addition.

[0088] In a third aspect, a corresponding system for performing the method as described in the second aspect is disclosed. The system for determining the lithographic processing conditions includes an input device for obtaining the characteristics of the illumination source and the lithographic pattern, and a processing device for defining the mask layout to be used. It also includes an output device for outputting the defined or selected mask layout. In some embodiments, the processing device can be programmed to define mask layouts with different sub-resolution grating spacings and sub-resolution grating line sizes, simulate the lithographic imaging of each layout, and select the layout that provides the best depth of focus penetration spacing and / or the highest contrast penetration spacing, thereby providing a further systematic method for mask design. Such a system can be implemented as a computer program product and a data carrier storing such a program, respectively. These aspects can be specifically designed to implement the execution of the method described in the second aspect of the present invention, thereby facilitating the design and optimization of the mask layout of the EUV lithography process. Figure 3 An exemplary system is shown in , illustrating a system 300 having an input device 310 , a processing device 320 , and an output device 330 .

[0089] By way of illustration, the embodiments are not limited thereto, and the present invention will be further described through some examples and considerations, showing standard and / or optional features of the embodiments of the present invention and corresponding advantages.

[0090] In the first example, Figure 4 The effect of adding sub-resolution gratings of various widths to the mask features on the mask is shown in . The results are shown from the perspective of best focus and from the perspective of NILS. Figure 4 Shown (from left to right - top row) are the best focus offset penetration pitch of the mask features, the 3 sigma of the best focus variation, and the 1-0 order phase offset for a low-n mask. The sub-resolution grating pitch used is SRGpitch = 9 nm at the wafer level. In the left image of the top row, the width of the sub-resolution grating lines is varied from 0 nm (no sub-resolution grating) to 4 nm at the wafer level, with the arrow indicating the direction of increasing SRG line width.

[0091] Figure 4Also shown (from left to right - bottom row) are the NILS penetration spacing of the mask features, the NILS as a function of spacing and defocus when no sub-resolution grating is used, and the NILS as a function of spacing and defocus when a sub-resolution grating with an SRG pitch of 9 nm and a sub-resolution grating line width of 3 nm is used. In the left panel of the bottom row, the arrow indicates the direction of increasing SRG width (no SRG line, SRG width = 1 nm to SRG width 4 nm). In the middle and right panels of the bottom row, the arrows indicate the direction of increasing NILS.

[0092] From these results, it can be seen that applying a sub-resolution grating in the mask results in a favorable lithographic process when using such a mask.

[0093] In a second example, the impact of adjusting the use of sub-resolution gratings in a mask layout according to imaging conditions for EUV lithography on commonly applied mask rule checking is shown. In the example shown, the layout includes horizontal sub-resolution grating lines perpendicular to vertical main features. Figure 5 On the left side it is shown that using sub-resolution assist features separated from the vertical pattern will result in strict mask rule checking, while Figure 5 On the right side it is shown that a sub-resolution grating line cutting through all mask features avoids the mask rule checking problem.This case is illustrated for using a sub-resolution grating as a dark field mask. Figure 5 Representations of sub-resolution gratings for dark-field and bright-field masks are shown. In the case of the bright-field mask, the tones of the sub-resolution grating are inverted to improve imaging performance.

[0094] Apart from Figure 6 In addition to the transmittance plotted at the bottom, Figure 6 A representation of the mask features is also shown in the upper portion. Figure 6 , mask features 122 and sub-resolution grating 140 are indicated in the upper portion showing the mask drawn by layer. Furthermore, in the lower portion showing the mask drawn by transmission, reflective portions 152 (caused by the multiple layers) and absorbing portions 154 (caused by the absorber) are shown.

[0095] In yet another example, optimization of the size and spacing of sub-resolution grating lines is shown. In this example, different sub-resolution sizes at the wafer level are evaluated as a function of different sub-resolution grating spacings. Figure 7Figure 2 shows the case where no sub-resolution grating is used, and the case where sub-resolution gratings with sub-resolution grating line widths of 1 nm, 2 nm, 3 nm, and 4 nm at the wafer level are used. This case is shown for sub-resolution grating pitches of 7 nm, 8 nm, and 9 nm at the wafer level. In the accompanying figure, the arrows indicate the direction of increasing NILS. As can be seen from the figure, the best performance is achieved for both image contrast and best focus penetration spacing for a sub-resolution grating pitch of 9 nm at the wafer level and a sub-resolution grating line size of 3 nm at the wafer level.

[0096] In the fourth example, a 1D metal design layout ( Figure 8 The resulting image ( Figure 8 The effect of Figure 8 The left part shows the result of the mask design without sub-resolution grating, while Figure 8 The right panel shows the results of a mask design with a sub-resolution grating. As can be seen, the sub-resolution grating is not visible in the image, but significantly improves the pattern image. It can also be seen that the background intensity is reduced, thereby reducing the propensity for defects in EUV lithography.

Claims

1. A mask (100) for EUV lithography processes in the high or ultra-numerical aperture range, the mask (100) comprising a mask pattern according to a mask layout (110), the mask layout being a superposition of: a first mask sub-layout (120) comprising a plurality of mask features to be printed, the plurality of mask features to be printed comprising at least one mask feature oriented along a first direction of the mask layout, a second mask sub-layout (130) comprising one or more sub-resolution gratings (140) having sub-resolution grating lines extending substantially in a second direction of the mask layout (110) and over substantially the entire width of the mask layout (110) in the second direction, the one or more sub-resolution gratings (140) being not printable for the high or ultra-numerical aperture range, The mask layout (110) is the result of superimposing the sub-layouts (120, 130). The size of the sub-resolution grating lines is selected based on the best focus and / or highest contrast for the respective pitches of the mask features to be printed.

2. The mask (100) according to claim 1, characterized in that The combination of sub-resolution grating pitch and size of the sub-resolution grating lines of the one or more sub-resolution gratings (140) is selected based on an optimal depth of focus penetration pitch and / or a highest contrast penetration pitch.

3. The mask (100) according to any one of the preceding claims, characterized in that The second mask sub-layout (130) does not include sub-resolution gratings at interstitial locations between the plurality of mask features in the first sub-layout (120).

4. The mask (100) according to any one of claims 1 to 2, characterized in that The first direction is different from the second direction, and wherein the sub-resolution grating lines traverse substantially all mask features, or The first direction is perpendicular to the second direction.

5. The mask (100) according to the preceding claim, characterized in that The mask is one of a dark field mask or a bright field mask.

6. Use of the mask (100) according to any of the preceding claims in EUV lithography in the high or ultra-numerical aperture range and in an anamorphic optical system, in which the demagnification in the X direction is smaller than the demagnification in the Y direction, wherein a first direction along which the plurality of mask features extend corresponds to the Y direction and the second direction corresponds to the X direction.

7. A computer-implemented method (200) for designing a mask layout for an EUV lithography process in a high or ultra-numerical aperture range, the method comprising: A mask layout (110) of mask patterns is repeatedly defined as a superposition of: a first mask sub-layout (120) comprising a plurality of mask features to be printed, the plurality of mask features to be printed comprising at least one mask feature oriented along a first direction of the mask layout, and a second mask sub-layout (130) comprising one or more sub-resolution gratings (140) having sub-resolution grating lines extending substantially in a second direction of the mask layout and extending over substantially the entire width of the mask layout (110) in the second direction, the one or more sub-resolution gratings (140) being not printable for the high or ultra-numerical aperture range, The mask layout (110) is the result of superimposing the sub-layouts (120, 130). wherein said repeated definition is performed for a plurality of sub-resolution pitches and / or sizes of sub-resolution grating lines of said one or more sub-resolution gratings, thereby obtaining different mask layouts, simulating lithographic imaging (220) of the mask pattern for each of the different mask layouts based on the sub-resolution pitch and size of the sub-resolution grating lines to obtain image quality parameters, and The mask layout (230) is selected based on best focus and / or highest contrast for each pitch of the mask features to be printed.

8. The computer-implemented method (200) of claim 7, wherein: The method (200) also includes simulating lithographic imaging based on multiple passes of unipolar exposure or implant aberrations.

9. A system (300) for determining lithographic processing conditions for an EUV lithographic process in a high or ultra-high numerical aperture range, the system comprising: - input means (310) for obtaining the characteristics of the illumination source and the lithographic pattern to be created, - a processing device (320) programmed to: A mask layout (110) of mask patterns is repeatedly defined as a superposition of: a first mask sub-layout (120) comprising a plurality of mask features to be printed, the plurality of mask features to be printed comprising at least one mask feature oriented along a first direction of the mask layout, and a second mask sub-layout (130), the second mask sub-layout comprising one or more sub-resolution gratings (140) having sub-resolution grating lines extending substantially in a second direction of the mask layout (110) and extending over substantially the entire width of the mask layout (110) in the second direction, the sub-resolution gratings (140) further being not printable for the high or ultra-numerical aperture range, The mask layout (110) is the result of superimposing the sub-layouts (120, 130). wherein the repetitive definition is performed for a plurality of sub-resolution grating pitches and sizes of the sub-resolution grating lines, thereby obtaining different mask layouts (110) depending on the sub-resolution grating pitches and depending on the sizes of the sub-resolution grating lines, simulating lithographic imaging of the mask pattern of each of the different mask layouts (110) according to the sub-resolution pitch and the size of the sub-resolution grating lines to obtain image quality parameters, and selecting a mask layout (110) based on best focus and / or highest contrast for each pitch of the mask features to be printed, as well as - an output device (330) for outputting the selected mask layout (110).

10. A data carrier storing a computer program product which, when executed on a processing device, performs the method according to any one of claims 7 to 8.

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