Extreme ultraviolet lithography mask

Abstract

An extreme ultraviolet lithography mask (1), the mask (2) comprising: a multi-layer mirror (11), and a patterned absorber layer (12) over the multi-layer mirror (11), where the absorber layer (12) has a thickness (t) between 15 and 35 nm and a refractive index of at most 0.93.

Description

Technical Field

[0001] The present invention relates to the field of extreme ultraviolet lithography. More specifically, the present invention relates to apparatus, systems and methods for performing extreme ultraviolet lithography. Background Art

[0002] Due to the short wavelength of EUV radiation, EUV lithography apparatuses typically employ reflective optics, which means oblique illumination of the EUV exposure mask. Therefore, an EUV lithography mask, also called a reticle, comprises a reflector (usually a multilayer reflector) and a patterned absorption layer on the multilayer reflector. In the photolithography process, the mask is located between an EUV radiation source and a substrate (usually a semiconductor wafer) containing a photoresist, so that the EUV radiation from the source is reflected by the mask to form an image of the absorption layer pattern on the photoresist, thereby patterning the photoresist.

[0003] Mask 3D effects arise from the finite or non-zero thickness of the patterned absorption layer of the mask used in the lithography process. These mask 3D effects, caused by the oblique incident angle of the EUV radiation on the 3D mask, lead to shadowing effects and imperfect diffraction. Mask 3D effects lead to contrast degradation, feature-dependent best focus shifts, and telecentricity errors. These effects can lead to unexpected and often undesirable differences between the absorption layer pattern and the pattern imprinted in the photoresist. As a result, the critical dimensions of the features may not be accurately reproduced or may be more prone to random failures, resulting in reduced pattern accuracy and chip performance. As a result, mask 3D effects shorten the process window of operation of the lithography step and increase the defect rate in the chip manufacturing process. Due to the small process window and the random failures associated with imaging small mask features, EUV lithography cannot yet be pushed to low k1 imaging processes.

[0004] The EUV radiation sources used during the lithography process also contribute to mask 3D effects. Optimized sources for EUV lithography contain multiple poles to limit pattern placement errors by defocusing. Different poles of a multipole EUV radiation source are typically incident on the mask at different angles of incidence, and different poles typically produce EUV radiation with different polarization states. Using a multipole EUV source can result in diffraction pattern imperfections originating from these mask 3D effects, including monopole diffraction amplitude imbalance, inter-pole phase shift or image shift, and inter-pole spatial image amplitude imbalance.

[0005] As such, at critical mask dimensions at future technology nodes, the photolithography step is expected to be challenging due to the mask 3D effect. As a result, the defect rate of the photolithography step in the chip manufacturing process may be expected to increase, thereby reducing chip yield.

[0006]

[0006] Thus, there remains a need in the art for EUV lithography masks and methods of using such masks that address at least some of the issues discussed above. Summary of the invention

[0007] An object of the present invention is to provide an EUV lithography mask, an EUV lithography apparatus comprising the mask, and a method of patterning a photoresist using the mask to solve at least some of the above problems.

[0008] The above objectives are achieved by the method and device according to the present invention.

[0009] An advantage of embodiments of the present invention is that the contrast that can be achieved with an EUV lithography mask can be very high. An advantage of embodiments of the present invention is that 3D mask effects can be mitigated, such as contrast falloff, overfocus pattern or image shift, and best focus shift. Thus, embodiments of the present invention can reduce defect rates and increase throughput of (high numerical aperture) EUV lithography processes. Given a larger process window and reduced random failures, embodiments of the present invention can push the lithography steps during chip manufacturing to lower k1 imaging processes.

[0010] The advantage of the embodiment of the present invention is that, since the thickness of the absorption layer of the EUV lithography mask can be very small, the reflectivity of the mask can be very high, so that the EUV radiation dose required for EUV lithography is very low.

[0011] In a first aspect, the present invention relates to an extreme ultraviolet lithography mask, the mask comprising:

[0012] Multilayer mirrors, and

[0013] A patterned absorbing layer located above the multilayer mirror,

[0014] The thickness of the absorption layer is between 15 and 35 nm and the refractive index is at most 0.93.

[0015] In some embodiments, the refractive index is at most 0.92, such as at most 0.91, or such as at most 0.90, or such as at most 0.89. When the absorbing layer has a low refractive index, the contrast may be particularly high. For example, the refractive index may be between 0.85 and 0.93, such as between 0.87 and 0.92, or between 0.87 and 0.90. The refractive index may be a refractive index at a wavelength between 5 and 40 nm, for example at a wavelength of 13.5 nm, which is typically used for extreme ultraviolet lithography.

[0016] The absorption layer may be formed of a material having a refractive index of at most 0.93, such as Pt, PtMo, Pt2Mo, RuTa, Ru3Ta, Mo or Ru3Re. Other examples are Ru or Ru3W. However, the present invention is not limited to these materials.

[0017] In some embodiments, the thickness of the absorption layer is between 15 and 30 nm, for example between 20 and 25 nm. When the thickness of the absorption layer is within these ranges, the achievable contrast ratio may be particularly high.

[0018] In a second aspect, the present invention relates to an extreme ultraviolet lithography apparatus. The apparatus comprises an extreme ultraviolet lithography mask according to an embodiment of the first aspect of the invention. The apparatus comprises a multipolar extreme ultraviolet radiation source. The apparatus further comprises a stage for holding a substrate comprising a photoresist to be patterned. The apparatus is configured to expose the poles of the multipolar extreme ultraviolet radiation source so that the extreme ultraviolet radiation from each pole is reflected by the mask to form an image of an absorption layer pattern on the photoresist (if present), thereby patterning the photoresist. The apparatus is adapted to compensate for image shifts between images formed by different poles, the image shifts being caused by different angles of incidence and / or polarities of the extreme ultraviolet radiation from different poles on the mask.

[0019] In various embodiments, the multipole extreme ultraviolet radiation source is adapted to control the irradiance intensity to within 0.8 and 1.0 times I0, where I0 is given by:

[0020]

[0021] The irradiation intensity may be the irradiation intensity incident on the mask. absorber is the thickness of the absorption layer of the EUV lithography mask according to an embodiment of the first aspect of the present invention; t absorber,conventional is the thickness of a conventional absorption layer, for example, a conventional absorption layer having a thickness of 60 nm and formed of tantalum; conventional absorber thickness The conventional absorbing layer has a thickness t absorber The irradiation intensity or exposure dose used in the case of ; k is the extinction coefficient of the absorption layer of the EUV lithography mask according to the embodiment of the first aspect of the present invention at the wavelength λ; λ is the wavelength of the EUV radiation. conventional absorber thickness I0 may depend on the photolithography system used, so I0 may also depend on the photolithography system used. However, on the mask, I conventional absorber thickness Typically between 20 and 100 mJ / cm 2 In the range of, for example, 30 mJ / cm 2 .

[0022] The multipolar extreme ultraviolet radiation source can be any type of extreme ultraviolet ray source having at least two poles, such as a dipole, quadrupole or octopole extreme ultraviolet radiation source. In other words, the multipolar extreme ultraviolet radiation source can be decomposed into a plurality of poles or each group of at least one pole, such as a plurality of monopoles. Different poles of the extreme ultraviolet radiation source typically produce extreme ultraviolet radiation having different polarization states. In addition, the different poles of the extreme ultraviolet radiation source are typically spatially offset from each other, so the angles of incidence of radiation from different poles on the mask may be different. This may result in the image shift between the images formed by different poles.

[0023] Any technique for compensating for the image shift may be applied.

[0024] In various embodiments, the device adapted to compensate for the image shift may include optimizing an extreme ultraviolet radiation source to compensate for or minimize the image shift. Franke, Joern-Holger et al., International Conference on Extreme Ultraviolet Lithography (International Extreme Ultraviolet Lithography Conference 2019) Volume 11147, SIPE, 2019, "Improving exposure latitudes and aligning best focus through pitch by curingM3D phase effects with controlled aberrations (correcting the M3D phase effect by using controlled aberrations, improving exposure latitudes and aligning the best focus through spacing)" describes this method of compensating for image shift. The optimization of the source may include providing an adapted pupil shape to the source, and may provide aberrations to the radiation from the source. The aberrations may be introduced by an extreme ultraviolet radiation source that is asymmetric in design. However, due to the angle-dependent nature of the mask 3D effect (which is significant for thin low-refractive-index absorption layers), source asymmetry may result in an extreme-dependent mask 3D effect, which may be difficult to compensate for by controlled aberrations.

[0025] In a preferred embodiment, the device adapted to compensate for the image shift preferably includes the device being adapted to perform polarized exposure lithography. For example, the polarized exposure lithography is described in Franke, Joern-Holger, Timothy A. Brunner and Eric Hendrickx, "Dual monopole exposure strategy to improve extreme ultraviolet imaging", Journal of Micro / Nanopatterning, Materials, and Metrology 21.3 (2022): 030501-030501. In these embodiments, the device may include a controller adapted to continuously expose different groups of at least one pole of a multipole extreme ultraviolet radiation source, and the compensating for the image shift includes optimizing the alignment of the radiation relative to the photoresist between consecutive exposures. When the multipole EUV radiation source comprises more than two poles, the set may comprise two or more poles of the multipole EUV radiation source, wherein the controller may be adapted to expose two or more poles in each set simultaneously.

[0026] The different groups of at least one pole typically include different poles of the poles of the multipolar extreme ultraviolet radiation source. Each of the different groups of at least one pole can include an equal number of poles, or in other words, the poles of the source can be evenly distributed among the multiple groups. For example, the two poles of a dipole extreme ultraviolet radiation source can be divided into two groups of poles, each group including one of the two poles. As another example, the four poles of a quadrupole extreme ultraviolet radiation source can be divided into two groups of poles, such that each group includes two of the four poles. Alternatively, the four poles can be divided into four groups of poles, such that each group of poles includes one of the four poles.

[0027] Any optical component of the device may be moved to optimize the alignment. Preferably, the controller may be adapted to move the EUV lithography mask to optimize the alignment. The controller may be adapted to move a stage for holding a substrate including a photoresist to be patterned to optimize the alignment. The controller may be adapted to move the EUV source and / or any optical component, such as a mirror, to direct radiation from the source to the mask and from the mask to the photoresist to be patterned, thereby optimizing the alignment. The movement may be performed between consecutive exposures, so that a good overlap between the images produced in the consecutive exposures may be obtained.

[0028] In a third aspect, the invention relates to a method for patterning a photoresist. The method comprises performing photolithography by exposing individual poles of an EUV radiation source so that the EUV radiation from each pole is reflected by an EUV lithography mask according to any embodiment of the first aspect of the invention to form an image of an absorption layer pattern on the photoresist, thereby patterning the photoresist. The method comprises compensating for image shifts between images formed by different poles, the image shifts being caused by different angles of incidence and / or polarities of the EUV radiation from different poles on the mask. The image shifts may be caused by a mask 3D effect.

[0029] Any technique for compensating for the image shift may be applied. The technique may include determining the image shift between different poles of the source, or obtaining a predetermined image shift. The technique may include modifying the characteristics of the device to compensate for or reduce the image shift. Determining the image shift may be performed in any manner known to those skilled in the art, such as by theoretical modeling or experimental testing. In various embodiments, the controller of the device may be adapted to determine the shift, or a device external to the device (e.g., when designing the device) may be used to determine the shift.

[0030] Determining how the image shift can best be compensated can be performed by modifying the features of the device differently for lithography performed on different dies on the wafer and determining which die has the best (e.g., smallest) image shift. Then, in a method according to an embodiment of the invention, compensation for the image shift can be performed in the same manner as compensation is performed on the die with the best image shift.

[0031] In various embodiments, the feature that can be modified can be the extreme ultraviolet radiation source, for example, the pupil or aberration introduced into the radiation from the source can be optimized, optimized to compensate for, for example, minimize the image shift. In these embodiments, the optimization can be performed when designing the device so that the device can include an optimized extreme ultraviolet radiation source.

[0032] In various embodiments, modifying the characteristics of the device may include adjusting or optimizing the alignment of the device, such as in polarized exposure lithography, to compensate for the image shift. In various embodiments, modifying the characteristics of the device may include configuring a controller based on the determined image shift to optimize the alignment of the device, or to perform polarized exposure lithography to compensate for the image shift. Alternatively, the controller may be preconfigured, such as during installation of the device, to optimize the alignment of the device, or to perform polarized exposure lithography to compensate for the image shift.

[0033] The mask of the present invention is particularly suitable when different poles of a multipole EUV source are not exposed all at once, but are exposed successively in groups of at least one pole, i.e. when used in a polarization exposure technique. Preferably, said performing lithography comprises performing polarization exposure lithography, comprising successively exposing different groups of at least one pole of a multipole EUV radiation source, and said compensating said image shift comprises optimizing the alignment of the radiation relative to the photoresist between the consecutive exposures. Preferably, optimizing said alignment comprises moving the position of the mask between the consecutive exposures of different poles.

[0034] In various embodiments, the extreme ultraviolet radiation is incident on the mask at an intensity of 0.8 to 1.0 times I0, where I0 is given by:

[0035]

[0036] 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 and with features of other dependent claims as appropriate and not just as explicitly set out in the claims.

[0037] While devices in the art are constantly improving, changing, and evolving, it is believed that the present inventive concepts represent a substantially new and original advancement involving a departure from prior practice, thereby providing a more efficient, stable, and reliable device of this nature.

[0038] The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description in conjunction with the accompanying drawings, which illustrate the principles of the present invention by way of example. This description is given for illustrative purposes only and does not limit the scope of the present invention. The reference figures cited below refer to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a schematic diagram of a vertical cross section of a mask according to an embodiment of the first aspect of the present invention.

[0040] Figure 2A is a schematic diagram of a lithography apparatus according to an embodiment of the second aspect of the present invention.

[0041] Figure 2B is a front view of a multipole extreme ultraviolet source that can be used in a lithography apparatus.

[0042] Figure 3A and Figure 3B is a plot of an image of a patterned absorbing layer on a photoresist, plotted as radiation intensity as a function of position.

[0043] Figure 4A and Figure 4Bis a plot of contrast as a function of absorber layer thickness for absorber layers formed of Ta and a material having a refractive index of 0.90, respectively.

[0044] Figure 5A and Figure 5B is a plot of NILS as a function of the thickness of the absorbing layer for dipole and monopole radiation, respectively, for absorbing layers formed of materials with a wide range of refractive indices n.

[0045] Figure 6 is a plot of absolute absorption reflectance as a function of absorption layer thickness for absorption layers formed from materials having a wide range of extinction coefficients.

[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-restrictive. In the drawings, for illustrative purposes, the size of some of the elements may be exaggerated and not drawn to scale. The dimensions and relative dimensions do not correspond to actual reductions in the practice of the invention.

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

[0049] It should be noted that the term "comprising" 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. Therefore, the term "comprising" covers the case where only the stated features are present as well as the case where these features and one or more other features are present. The word "comprising" according to the present invention therefore also includes an embodiment as being absent of other components. Thus, the scope of the expression "device comprising means A and 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 in the device are A and B.

[0050] References throughout this specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" 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, as would be apparent to one of ordinary skill in the art from this disclosure, the particular features, structures, or characteristics may be combined in any suitable manner.

[0051] Similarly, it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the 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 those expressly recited in each claim. On the contrary, as reflected in the appended claims, the inventive aspects lie in fewer features than all of the features of a single preceding disclosed embodiment. Thus, the claims appended to the specific embodiments are hereby expressly incorporated into the specific embodiments, with each claim itself representing a separate embodiment of the invention.

[0052] In addition, 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, the combination of features of different embodiments is 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 can be used in any combination.

[0053] 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 functions. Thus, a processor with necessary instructions for executing such methods or elements of methods forms a device for executing the method or elements of the method. In addition, the elements described herein of the device embodiments are examples of devices for implementing the functions performed by the elements that implement the purpose of the present invention.

[0054] 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 the understanding of this specification.

[0055] The invention will now be described by a detailed description of several embodiments of the invention. Obviously, other embodiments of the invention can be configured according to the knowledge of those skilled in the art without departing from the technical teaching of the invention, and the invention is only limited by the terms of the appended claims.

[0056] In the embodiments of the present invention, when optimization or optimization is mentioned, it means "improvement relative to the conventional or existing situation". This improvement may lead to the best or just improvement, so further improvement is still possible, such as by adjusting other conditions.

[0057] In a first aspect, the present invention relates to an extreme ultraviolet lithography mask. Such a mask is generally used for extreme ultraviolet lithography. The mask comprises a multilayer reflector and a patterned absorption layer on the multilayer reflector. According to an embodiment of the present invention, the thickness (t) of the absorption layer is between 15 and 35 nm, for example between 15 and 30 nm, and the refractive index is at most 0.93. Such an absorption layer can be formed of, for example, Pt, PtMo, Pt2Mo, RuTa, Ru3Ta, Mo or Ru3Re.

[0058] In another aspect, the present invention relates to an extreme ultraviolet lithography system comprising such a mask as described in the first aspect. Such an extreme ultraviolet lithography system may include a multipole extreme ultraviolet radiation source and a stage for holding a substrate including a photoresist to be patterned. The device may be configured to expose the poles of the multipole extreme ultraviolet radiation source so that the extreme ultraviolet radiation from each pole is reflected by the mask to form an image of the absorption layer pattern on the photoresist (if present), thereby patterning the photoresist. The device is also adapted to compensate for image shifts (δx) between images formed by different poles, the image shifts (δx) being caused by different angles of incidence and / or polarities of the extreme ultraviolet radiation from different poles on the mask.

[0059] The multipole EUV radiation source may be adapted to control the radiation intensity to within 0.8 and 1.0 times I0, where I0 is given by:

[0060]

[0061] In some embodiments, there may be a controller for controlling the multipole EUV radiation source accordingly.

[0062] In yet another aspect, the present invention relates to a method for performing extreme ultraviolet lithography, the method comprising using a mask as described in the first aspect. Other features of this method may include steps corresponding to the features of the corresponding extreme ultraviolet lithography system described in the second aspect.

[0063] By way of illustration, embodiments of the present invention are not limited thereto, and a number of standard and optional features or method steps will now be described using a number of examples.

[0064] Example: Extreme Ultraviolet Lithography Masks

[0065] refer to Figure 1 , which is a schematic diagram of a vertical cross section of an EUV lithography mask 1 according to an embodiment of the first aspect of the present invention. The EUV lithography mask 1 comprises a multilayer reflector 11 for reflecting EUV radiation incident on a top surface 110 of the multilayer reflector 11. The multilayer reflector 11 comprises multiple layers of alternating materials having different refractive indices and effectively functions as a Bragg reflector.

[0066] The EUV lithography mask 1 further comprises a patterned absorption layer 12 located on or above the multilayer mirror 11, on or above the top surface 110. The patterned absorption layer 12 has a thickness t between 15 and 35 nm in a direction orthogonal to the top surface 110 of the multilayer mirror 11. The thickness can be determined, for example, by scanning electron microscopy (such as cross-sectional scanning electron microscopy) or atomic force microscopy.

[0067] The patterned absorption layer 12, i.e., the material forming the patterned absorption layer 12, has a refractive index of at most 0.93. The refractive index can be determined using an extreme ultraviolet reflectometer. For example, the extreme ultraviolet reflectometer technique described in Ciesielski, Richard et al. "Determination of optical constants of thin films in the EUV", Applied Optics 61.8 (2022): 2060-2078, can be used to determine the refractive index of a material. For example, the model described therein (which corrects any experimental parameters) can be used to derive the refractive index of a material from an extreme ultraviolet reflection experiment. The experimental details may correspond to those described therein (where the materials mentioned are replaced by the materials whose refractive index is to be determined), but this is not necessary because the model corrects any experimental parameters that may vary between different experiments. For example, the refractive index can be determined at a wavelength between 5 and 40 nm, for example at a wavelength of 13.5 nm, which is commonly used for extreme ultraviolet lithography.

[0068] As used herein, the refractive index used in the context of the present invention refers to the real part of the refractive index corresponding to the refraction (thus, in the case of a complex-valued refractive index, the imaginary part corresponding to the extinction coefficient is ignored).

[0069] Example: Extreme Ultraviolet Lithography Apparatus

[0070] 2 is a schematic diagram of an EUV lithography apparatus 5 according to an embodiment of the second aspect of the present invention. The EUV lithography apparatus 5 includes an EUV lithography mask 1 according to an embodiment of the first aspect of the present invention. The EUV lithography apparatus 5 can implement a method according to an embodiment of the third aspect of the present invention.

[0071] In the example shown, extreme ultraviolet radiation 20 from a multipole extreme ultraviolet radiation source 2 is directed to an extreme ultraviolet lithography mask 1 via an optical element 31. The extreme ultraviolet radiation 20 incident on the mask 1 is reflected by the mask 1. As known to those skilled in the art, the reflected extreme ultraviolet radiation 200 includes radiation reflected and / or diffracted by the mask 1 in different diffraction orders, including, for example, the zeroth diffraction order (i.e., undiffracted radiation) and radiation diffracted in the first diffraction order.

[0072] The reflected radiation 200 reflected by the mask 1 is collected by the projection optical device 32 and directed onto the photoresist 61 of the substrate 6 located on the stage 7 of the EUV lithography apparatus 5, thereby projecting the image of the pattern of the patterned absorption layer 12 of the mask 1 onto the photoresist 61. The photoresist 61 is sensitive to the EUV radiation 200, so that the pattern of the patterned absorption layer 12 of the mask 1 can be patterned onto the photoresist 61. The patterning may include translating the mask 1 and the substrate 6 simultaneously to continuously transfer different parts of the pattern of the mask 1 into the substrate 6. For example, the translation may be implemented using a controller 8.

[0073] Although in this example, the optical assembly 31 and the projection optical device 32 are Figure 2A In the figure, they are simplified as a single concave mirror, but the present invention is not limited to this.

[0074] Also refer to Figure 2B , which is a front view of a multipolar EUV radiation source 2, which in this example is a dipole EUV radiation source 2. EUV radiation source 2 emits EUV radiation, which may be radiation with a wavelength between 5 and 40 nm, preferably radiation with a wavelength of 13.5 nm, which is typically used for EUV lithography. Multipolar EUV radiation source 2 comprises or may be decomposed into a plurality of poles 21 and 22, or a single pole 21 and 22, each generating EUV radiation with a different polarization state. Furthermore, as can be observed, poles 21 and 22 are spatially offset from each other. Due to the different polarization states of the radiation emitted by different poles 21 and 22, and due to their spatial offset relative to each other, the different angles of incidence of radiation from different poles 21, 22 on mask 1, combined with the non-zero thickness of the patterned absorption layer 12 of mask 1 and the different paths of radiation 21, 22 from different poles through the multilayer reflector 11 of mask 1, a mask 3D effect may occur and thus lead to defects in the diffraction pattern. This may lead in particular to monopole diffraction amplitude imbalance, inter-pole phase shift or image shift and inter-pole spatial image amplitude imbalance.

[0075] Also refer to Figure 3A , where the aerial image of the absorption layer 12 projected onto the photoresist 61 is plotted as the radiation intensity as a function of the position on the photoresist 61. The inter-pole phase offset typically results in an offset δx between the (spatial) image 301 generated by the radiation from the first pole 21 onto the photoresist 61 and the (spatial) image 302 generated by the radiation from the second pole 22 onto the photoresist 61.

[0076] Unlike the present invention, when multipole (e.g., dipole) illumination is used for illumination without compensating for this image shift δx, the actual (aerial) image 300 formed on the photoresist 61 is a combination of images 301 and 302 that are shifted relative to each other by δx. Therefore, the actual image 300 thus formed is widened compared to the individual images 301 and 302 generated by each pole 21 and 22 alone, resulting in a reduction in contrast.

[0077] In the present invention, the image offset δx is compensated. There are several methods known in the art to compensate for this image offset δx between the images 301 and 302 generated by different poles 21 and 22.

[0078] As a first example, polarization exposure techniques may be used to compensate for these image shifts δx.

[0079] In short, in the polarization exposure technique, different poles 21, 22 or different groups 21, 22 of at least one pole of the multipole extreme ultraviolet radiation source 2 are exposed successively. The controller 8 can control the extreme ultraviolet radiation source 2 to perform the consecutive exposures. Between the consecutive exposures of different poles 21, 22, or between the consecutive exposures of different groups of at least one pole 21, 22, the alignment of the radiation 200 relative to the photoresist 61 can be optimized.

[0080] Specifically, in the first step, a first group of at least one pole (in the example shown, the first pole 21) can be exposed first (while the other poles 22 of the source 2 are not exposed) to project a first image 301 of a pattern or a specific portion of the pattern of the absorption layer 12 onto the photoresist 61.

[0081] Then, in a second step, the exposure may be terminated and the components of the apparatus 5 may be moved or realigned. The movement may be performed by the controller 8 and may include movement of the mask 1, the substrate 6 or the EUV source 2, for example.

[0082] The shifting or realignment is performed so that subsequently in a third step, when the second pole 22 is exposed (while the first pole 21 is not exposed), the alignment of the radiation 200 relative to the photoresist 61 is optimized so that the second image 302 of the pattern of the absorption layer 12 or of the specific portion of the pattern on the photoresist 61 overlaps better with the first image 301 that would have been on the photoresist 61 without the shifting and realignment. In other words, the shifting or realignment is performed so that when the second pole 22 is exposed, the offset between the first image 301 and the second image 302 is smaller than the image offset δx that would have been in the absence of the shifting or realignment. Preferably, the shifting or realignment is performed so that the images generated by different poles (e.g. the first and second images) coincide, such as Figure 3B As shown (wherein the first image 301 is represented by dots and the second image 302 is represented by stripes).

[0083] Although in the example shown, a dipole EUV source 2 is used, in which the first pole 21 and the second pole 22 are exposed separately and successively, the present invention is not limited thereto. In fact, the EUV source 2 can be any type of multipole EUV source 2, in which, in a first step, one group of at least one pole 21, 22 of the multipole EUV source 2 can be exposed, and after the second step of the movement or realignment, in a third step, another group of at least one pole 21, 22 of the multipole EUV source 2 can be exposed. The EUV source 2 can include more than two groups of at least one pole 21, 22, which can be exposed successively, wherein the movement or realignment can be performed between each two consecutive exposures.

[0084] As a second example, the device 5 being adapted to compensate for the image shift δx may include adapting the extreme ultraviolet radiation source 2. For example, the source 2 may have an adapted pupil shape, and the radiation 20 from the source 2 may have an aberration. The aberration may be introduced by the extreme ultraviolet radiation source being asymmetric in design. In this second example, by adapting the pupil shape of the source 2 and introducing the aberration accordingly, the pole-specific phase error causing the image shift δx may be partially mitigated, while all poles 21, 22 of the multipolar extreme ultraviolet radiation source 2 may be exposed simultaneously, which may simplify lithography. For example, adapting or optimizing the source 2 may include determining an image shift δx between different poles 21, 22 of the source 2, and modifying the source 2 to reduce the determined image shift δx. The adapting or optimizing the source 2 is typically performed when designing the device 5. In this example, the extreme ultraviolet radiation source 2 may be adapted so that the shift δx between the images of the different poles 21, 22 is reduced compared to the image shift δx that would otherwise exist when using a symmetric extreme ultraviolet radiation source with a symmetric pupil and no aberrations.

[0085] Example: Simulating Contrast Loss for Different Absorption Layers

[0086] Mask absorption layer material and thickness optimization can be performed to restore the imperfect diffraction originating from the mask 3D effect. In fact, the extent to which the contrast is affected by compensating the interpolar phase shift or image shift depends on the characteristics of the absorption layer of the EUV lithography mask used. The mask according to an embodiment of the first aspect of the invention is optimized so that a particularly good contrast can be achieved when applying a lithography technique that compensates for the image shift.

[0087] When using multipole EUV radiation sources for lithography, three mechanisms can cause defects in the diffraction pattern:

[0088] Monopole diffraction amplitude imbalance (also called a1 / a0 imbalance)

[0089] Inter-pole phase shift (also called P2P shift) or the image shift between images formed by different poles

[0090] The inter-polar spatial image amplitude imbalance (which is negligible and is further neglected here).

[0091] Optimizing the absorber layer material in terms of optical properties (refractive index and extinction coefficient) can suppress the mask 3D effect. In addition, optimizing the absorber layer material can enable the development of EUV lithography masks that can be used at reduced EUV radiation doses necessary to complete the lithography steps compared to doses used in the prior art, thereby increasing the throughput of EUV scanners. However, in order to obtain these advantages of optimized absorber layer materials, the absorber layer thickness must also be carefully optimized.

[0092] The inventors have studied important imaging metrics to select the best characteristics of the absorber layer. Specifically, the characteristics of the absorber layer according to embodiments of the present invention are optimized so that the mask 3D effect has little impact on the lithography step and furthermore the exposure dose necessary to complete the lithography step is also small.

[0093] Exposures with a dipole EUV source were simulated, specifically for generic leaf-shaped dipole illumination (pupil fill factor = 20%), and for P28 vertical line and space, half-pitch mask CD under high NA (NA = 0.55) imaging. Figure 4A 2 is a plot of the contrast achieved with a mask having an absorption layer formed of Ta (ie, tantalum) having a refractive index of 0.95 at a wavelength of 13.5 nm (solid line). Figure 4B is a plot (solid line) of the contrast achieved with a mask having an absorbing layer formed of an absorbing material with a refractive index of 0.90. Figure 4A and Figure 4B Of the two, the dashed line represents the contrast loss due to the image shift caused by the inter-pole phase shift, while the dot-dashed line represents the contrast loss due to the monopole diffraction amplitude imbalance.

[0094] If available Figure 4A As observed in , for an absorption layer formed of Ta, both image shift and monopole diffraction amplitude imbalance mechanisms lead to a reduction in contrast, and it was found that the optimal absorption layer thickness (at which the contrast is high) is in the range of 60 to 70 nm.

[0095] However, if Figure 4B As observed in Figure 2, in the case of low-n absorber layers, the driver of defects in the diffraction pattern can be mainly attributed to image shifts when the absorber thickness is greater than about 15 nm. From these simulations, it can be seen that when using a dipole radiation source for lithography, without compensating for the image shift or interpolar phase shift, lowering the refractive index of the absorber layer compared to that of Ta only results in good contrast when the thickness of the patterned absorber layer is above about 35 to 50 nm.

[0096] However, thinner (less than 35 nm) low-n mask absorber layers have the potential to significantly reduce the exposure dose of the lithography step and are therefore preferred. Although it seems that only the contrast of thicker low-n mask absorber layers is large when using a dipole radiation source, it can be seen from the simulations that the high contrast is largely due to the image shift between the different poles. In fact, for absorber layer thicknesses greater than about 15 nm, the contribution of the monopole diffraction amplitude imbalance is quite limited. Therefore, it can be seen from these simulations that when applying lithography techniques that compensate for the image shift between images formed by different poles, and when using a mask according to an embodiment of the present invention, which has an absorber layer with a thickness of 15 to 35 nm and a low refractive index, the contrast achievable in lithography can be very good.

[0097] Example: Polarization exposure using a mask with a low n-absorbing layer

[0098] Simulations were performed to evaluate the effect of the polarization exposure technique described above, in particular to compensate for image shifts between images formed by different poles, on the contrast that can be achieved in lithography using masks having low n-absorbing layers.

[0099] Simulations were performed for both dipole and monopole radiation (pupil fill factor = 10%) for P20 vertical lines and spaces, half-pitch mask CD at high NA imaging (NA = 0.55). The monopole radiation can be the result of a split-polarization exposure technique using a dipole source, where both monopoles of the dipole source are exposed one at a time.

[0100] refer to Figure 5A and Figure 5B , which are plots of the simulated NILS (Normalized Image Log Slope, which is a measure of contrast, with higher NILS indicating a sharper image) as a function of the absorber thickness, over a range of refractive indices, Figure 5A The case is dipole radiation and Figure 5B The case of is monopolar radiation. A mask including an absorbing layer formed of a material with a refractive index ranging from 0.88 to 0.95 was simulated.

[0101] From the above Figure 4B It can be clearly seen that the imperfect diffraction of the low-n absorber with a thickness in the range of 15 to 35 nm is mainly caused by a large image shift, which can be compensated or even eliminated by the polarization exposure technique. Therefore, in the case of a low-n absorber, the absorber thickness can be further reduced, thereby solving the above technical problems and achieving significant imaging benefits.

[0102] like Figure 5A As shown, when using P20V line and space dipole radiation, even for low-n materials, ie materials with a low refractive index n, it is preferred to use an absorption layer thickness of 35 to 40 nm (where NILS is maximum) to prevent contrast degradation due to mask 3D effects.

[0103] In contrast, Figure 5B As shown, in the case of unipolar exposure, such as that used in polarization exposure technology, for low n absorber layer materials, the thickness can be reduced to between 15 and 35 nm. Figure 5B As observed in , the simulations unexpectedly predict good NILS for absorber layers with thicknesses between 15 and 35 nm when the refractive index of the absorber layer is at most 0.93, especially compared to the NILS for thicker low-n absorber layers.

[0104] The reduction in thickness results in a more linearly reflective absorber layer. Thus, for an absorber layer formed of a low-n material and having a thickness of 15 to 25 nm, the exposure dose is expected to be reduced by 62.5%-37.5%, respectively, while the P20V line and space contrast will also be improved by about 30%, compared to a prior art low-n mask layer with a thickness of 40 nm when using dipole illumination.

[0105] In the case of thin absorber layers, the absorber reflectivity can be much higher than that of conventional low-n masks, i.e., up to 35%. Figure 6 As shown, by adjusting the extinction coefficient k of the absorbing layer material, for an exemplary absorbing layer with a refractive index of 0.90 and a thickness of 22 nm, the reflectivity of the absorbing layer can be adjusted from 30% to 8%.

[0106] In general, it can be seen from the simulation that when the polarization exposure technique is combined with the mask according to the embodiment of the present invention, a particularly large imaging benefit can be obtained.

[0107] However, combining masks according to embodiments of the present invention with other techniques for reducing inter-pole phase shifts is also expected to produce good contrast. However, due to the angle-dependent nature of the mask 3D effect (which is important for thin, low-n masks), which is best compensated by polarized exposure techniques, contrast is expected to be best optimized when using polarized exposure methods.

[0108] It is understood that although preferred embodiments, specific structures and configurations, and materials have been discussed herein for the apparatus according to the present invention, various changes or modifications in form and detail may be made without departing from the scope of the present invention. Steps may be added or deleted from the method within the scope of the present invention.

Claims

1. An extreme ultraviolet lithography mask (1), the mask (2) comprising: a multilayer reflector (11), and a patterned absorption layer (12) located above the multilayer reflector (11), The absorbing layer (12) has a thickness (t) between 15 and 35 nm and a refractive index of at most 0.

93.

2. The EUV lithography mask (1) according to claim 1, characterized in that: The thickness of the absorption layer (12) is between 15 and 30 nm.

3. The EUV lithography mask (1) according to any one of the preceding claims, characterized in that The absorption layer (12) is formed of Pt, PtMo, Pt2Mo, RuTa, Ru3Ta, Mo or Ru3Re.

4. The EUV lithography mask (1) according to any one of the preceding claims, characterized in that The refractive index is between 0.85 and 0.

93.

5. An extreme ultraviolet lithography apparatus (5), comprising: The EUV lithography mask (1) according to any one of claims 1 to 4, a multipolar extreme ultraviolet radiation source (2), and a stage (7) for holding a substrate (6) including a photoresist (61) to be patterned, wherein the apparatus (5) is configured to expose the respective poles (21, 22) of the multipolar EUV radiation source (2) so that EUV radiation (20) from each pole (21, 22) is reflected by the mask (1) to form an image (300, 301, 302) of the absorption layer pattern thereon when the photoresist (61) is present, thereby patterning the photoresist (61), and The device (5) is adapted to compensate for an image shift (δx) between images (301, 302) formed by different poles (21, 22), the image shift (δx) being caused by different angles of incidence and / or polarities of extreme ultraviolet radiation (20) from different poles (21, 22) on the mask (1).

6. The extreme ultraviolet lithography apparatus (5) according to claim 5, characterized in that: The multi-pole extreme ultraviolet radiation source (2) is adapted to control the irradiation intensity within the range of 0.8 to 1.0 times I0, where I0 is given by the following formula:

7. A method for patterning a photoresist (61), the method comprising: The photolithography is performed by exposing the respective poles (21, 22) of the extreme ultraviolet radiation source (2) so that the extreme ultraviolet radiation (20) from each pole (21, 22) is The EUV lithography mask (1) according to any one of claims 1 to 4 is reflected to form an image (300, 301, 302) of the absorption layer pattern on the photoresist (61), thereby patterning the photoresist (61), The method comprises compensating for an image shift (δx) between images (301, 302) formed by different poles (21, 22), the image shift (δx) being caused by different angles of incidence and / or polarities of extreme ultraviolet radiation (20) from different poles (21, 22) on the mask (1).

8. The method according to claim 7, characterized in that Performing lithography includes performing polarized exposure lithography, including successively exposing different sets of at least one pole (21, 22) of the multipole extreme ultraviolet radiation source (2), The compensating for the image shift (δx) comprises optimizing the alignment of the radiation (200) relative to the photoresist (61) between successive exposures.

9. The method according to claim 8, characterized in that Optimizing the alignment comprises shifting the position of the mask (1) between successive exposures of different poles (21, 22).

10. The method according to any one of claims 7 to 9, characterized in that The extreme ultraviolet radiation (20) is incident on the mask (1) with an intensity of 0.8 to 1.0 times I0, where I0 is given by:

11. The method according to any one of claims 7 to 10, characterized in that The image shift (δx) is caused by the mask 3D effect.

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