Method and apparatus for correcting a reflective lithography mask and reflective lithography mask

Abstract

This invention relates to a method for correcting a reflective lithography mask, wherein the mask includes a substrate and a reflective multilayer stack. The method includes: applying a first electromagnetic radiation to the mask to induce a first material change within the mask, which corrects a reflectivity of the mask. This invention further relates to an apparatus for correcting a reflective lithography mask and a reflective lithography mask itself.

Description

Multi - layer Reflectivity Modification The present invention relates to calibrating a reflective lithography mask. In particular, the present invention relates to a method for calibrating a reflective lithography mask, a device for calibrating a reflective lithography mask, and a reflective lithography mask. In the field of semiconductor technology, a variety of lithography processes are known for creating structures on a sample (such as a wafer). Lithography processes can include, for example, lithography, ultraviolet (UV) lithography, DUV lithography (i.e., lithography in the deep ultraviolet spectral region), EUV lithography (i.e., lithography in the extreme ultraviolet spectral region), X - ray lithography, and so on. Generally, a lithography process requires the use of a lithography mask (such as a photomask, an exposure mask, a reticule, etc.), which includes a pattern for imaging the desired structure on the sample. Imaging can be performed through a lithography exposure system that irradiates the lithography mask with exposure light such that the lithographic image of the mask is projected onto an ideal plane (e.g., the wafer plane). Lithography processes using reflective lithography masks have become increasingly important in the field of semiconductor technology. For example, EUV lithography may require the use of reflective lithography masks, where EUV lithography can produce increasingly smaller structures. The mechanism of a lithography process using a reflective lithography mask can include exposure light being partially absorbed by the pattern elements of the mask and reflected from the mask, where the reflected light is used to generate a corresponding lithographic image of the mask in the ideal plane. However, the mask properties of a reflective lithography mask may not always be ideal for a lithography process. If the mask is not ideal, the lithography results of the mask may also be不理想 or even incorrect (e.g., out of specification). For example, there may be mask errors (e.g., from mask manufacturing) in the mask, which can lead to optical errors in the lithographic image. For example, the mask properties of a reflective mask may deviate slightly such that the lithographic image may not be optimal compared to other lithography processes being performed to create structures on the sample. However, a lithography process using a reflective lithography mask (especially in an industrial environment) should have process results within known specifications. Therefore, there is a need to find improved methods for calibrating reflective lithography masks. Aspects described herein at least partially address the above - mentioned need. A first aspect relates to a method for calibrating a reflective lithography mask, including: applying a first electromagnetic radiation to the mask to induce a first material change within the mask, which modifies a reflectivity of the mask. For example, applying the first electromagnetic radiation may include initially shaping the first electromagnetic radiation, e.g., by setting one or more specific electromagnetic radiation parameters. For example, the specific electromagnetic radiation parameters may include the power, power density, energy, and / or flux of the first electromagnetic radiation. The specific electromagnetic radiation parameters may also include the pulse parameters of the first electromagnetic radiation (e.g., the pulse width, pulse repetition rate, and / or pulse density of the first electromagnetic radiation). In an example, the specific electromagnetic radiation parameters may include the wavelength and / or polarization of the first electromagnetic radiation. In an example, the specific electromagnetic radiation parameters may include the numerical aperture (of the optical system) associated with the first electromagnetic radiation. In addition, applying the first electromagnetic radiation may include guiding the first electromagnetic radiation to an ideal position within the photomask. For example, the guiding may be implemented by one or more optical elements that can affect the orientation and / or direction of the first electromagnetic radiation. The application of the first electromagnetic radiation may be performed such that an ideal spot size and / or spot shape of the first electromagnetic radiation appears at a specific depth within the photomask. For example, the application of the first electromagnetic radiation may include focusing the first electromagnetic radiation to an ideal spot size at a specific depth within the photomask. The spot form may be a two-dimensional (2D) geometric form in a plane perpendicular to the beam direction of the electromagnetic radiation (usually the focal plane). Generally, the 2D spot shape may have a circular shape or may be elliptical. In one example, the application of the first electromagnetic radiation may be performed such that an effective volume is formed within the photomask through the first electromagnetic radiation, where the conditions for inducing a change in the first material are met. For example, applying the first electromagnetic radiation may induce heat within the photomask (e.g., within the effective volume), where the heat induces a change in the first material. It should be noted that the heat may induce chemical and / or physical reactions that result in a change in the first material. In an example, the change in the first material may include the generation of a structure with a different material composition and / or density compared to the surrounding photomask material that has not undergone the change in the first material. For example, depending on the triggered reaction, the density may increase or decrease compared to the surrounding material. In an example, the change in the first material may include local melting of the material within the photomask. The change in the first material may include a permanent change. In an example, the change in the first material may persist for a long period of time. For example, the change in the first material may persist to the extent that the existence of the change in the first material can be verified over an extended time period (e.g., after 1 day, 1 week, 1 month, 1 year, 10 years, 20 years, etc.). The change in the first material within the photomask may affect the photomask such that the reflectivity of the photomask changes. For example, the different material composition and / or density of the change in the first material may have a critical value that affects the reflectivity of the photomask. Thus, by purposefully applying the first electromagnetic radiation to the photomask, the reflectivity of the photomask can be modified in a defined manner according to the disclosures described herein. For example, the method may include causing a change in a first material within the photomask to produce a desired (e.g., predetermined) reflectivity. It is noted that in the prior art, modifying the flatness of a reflective photomask in a defined manner is well known. For example, US2017010540A1 teaches locally depositing the energy of light pulses from a light beam into the substrate of an EUV photomask to produce a predetermined 3D profile of the EUV photomask. However, the method described herein is capable of adjusting the reflectivity of a reflective photomask to a defined state. In an example, the reflectivity of the photomask may include the reflectivity relative to the wavelength of the lithography process for which the reflective photomask is configured. For example, the photomask may be configured for a lithography process having a specific exposure wavelength λ E of a lithography process. Thus, during the lithography process, the photomask may be exposed to light having a specific exposure wavelength λ E to produce a lithographic image. Since the photomask is a reflective photomask, light having a specific exposure wavelength λ E will be reflected off the photomask. The application of the first electromagnetic radiation may cause a change in the first material such that the reflectivity of the photomask is modified for light having a specific exposure wavelength λ E Thus, the modified reflectivity may modify the lithographic image for a lithography process having a specific exposure wavelength. In this regard, this method can be applied to correct the lithographic image of the photomask. For example, by purposefully applying the first electromagnetic radiation to the photomask, the reflectivity of the photomask can be modified in a defined manner to correct (e.g., optimize) the lithographic image of the photomask. In an example, correcting a reflective photomask (as described herein) can also be regarded as correcting the lithographic image of the photomask. It is noted that the method described herein can implement a further degree of freedom to correct a reflective photomask (or its lithographic image) by adjusting the reflectivity of the photomask. For example, the prior art may be quite limited in accommodating other characteristics of a reflective photomask. For example, correcting a reflective photomask is well known, where the absorption of the reflective photomask is changed (e.g., through electron beam induced etching and / or deposition). However, the disclosure described herein addresses how to effectively adjust another key parameter of a reflective photomask - namely, the reflectivity of the reflective photomask. In an example, the first electromagnetic radiation may have a specific exposure wavelength λ that the photomask is configured for EDifferent wavelengths. That is, this can basically avoid the mask from reflecting the first electromagnetic radiation. In an example, the first electromagnetic radiation may include laser light. In an example, the first electromagnetic radiation may be applied to locally modify the reflectivity of the mask. For example, the method may include applying the first electromagnetic radiation to the mask such that the mask causes a local change in reflectivity (e.g., a spatially defined change in reflectivity). Thus, the reflectivity of the mask may be applicable, for example, only to a sub-region or sub-volume of the mask. Therefore, the methods described herein may not be limited to global changes in reflectivity. According to the methods described herein, various local reflectivities may be generated over the entire mask. In an example, the first material change may be regarded as a first type of material change. For example, the first electromagnetic radiation may be directed to multiple locations within the mask, where a first type of material change occurs at each location. At each location, a physical and / or chemical reaction may be based on the same mechanism (e.g., heat-induced changes in a specific material). However, at each location, the first electromagnetic radiation may be different (e.g., by changing one or more specific electromagnetic radiation parameters). This can enable different magnitudes of the first material change to be induced at each location. For example, depending on the specific electromagnetic radiation parameters selected, different degrees of density change may occur, where the induced material composition may be substantially the same. For example, the first type of material change may also be regarded as a first type of pixel that can be written into the mask. However, the methods described herein may not always be implemented such that the first material change can be regarded as a pixel spatially, such that the term pixel may not be applicable to all examples of the present invention. For example, the method may include applying the first electromagnetic radiation to a first location within the mask using a first set of specific electromagnetic radiation parameters. This may cause a first material change (or a first type of material change) having a first magnitude (e.g., a first local density increase). The method may further include applying the first electromagnetic radiation to a second location within the mask (different from the first set of specific electromagnetic radiation parameters) using a second set of specific electromagnetic radiation parameters. This may trigger a first material change (or a first type of material change) having a second magnitude, the second magnitude being different from the first magnitude (e.g., a second local density increase different from the first local density increase). Thus, at two locations within the mask, the same first type of material change may be caused, but to different extents. This can allow for a wide range of spatial adaptation of the mask reflectivity, and thus allow for a wide range of local correction of reflective lithography masks. In an example, two different locations that cause a first type of material change may also have the same order of magnitude of the first material change. In an example, a local density increase may be caused at a first location, while a local density decrease may be caused at a second location. In another example, local compression may be caused at a first location, while local expansion may be caused at a second location. In an example, a first type of material change (or a first material change) may be produced at three or more, four or more, ten or more, one hundred or more, and / or one thousand or more locations within a photomask. For example, a first type of material change may be produced for up to ten thousand locations, up to five thousand locations, up to one thousand locations, and / or up to one hundred locations within the photomask. In an example, a first type of material change can be induced in a continuous manner. For example, a first electromagnetic radiation can be guided through a region within the photomask in a scanning manner. For example, the first electromagnetic radiation can be guided continuously along one or more axes. During the continuous scanning, one or more specific electromagnetic parameters (as described herein) can be varied. Thus, a continuous change in the first type of material change can be induced according to the methods described herein. In an example, a first type of material change can be induced in a discrete manner. For the discrete manner, the first electromagnetic radiation can be guided to two or more locations of a region within the photomask, where the two or more locations can be spatially separated (e.g., they may not overlap). At each location, a first type of material change can be induced, where the first material change induced at each location is spatially separated from the first material change induced at another location. Thus, the first electromagnetic radiation can be stepped through the respective locations within the photomask to produce spatially isolated first type of material changes. In such an example, the first type of material change can also be regarded as a first type of pixel, for example. In an example, aspects of the methods described herein involving continuous and discrete manners can be combined for correcting a reflective lithography photomask. For example, a first region within the photomask can be corrected in a continuous manner, where a second region of the photomask can be corrected in a discrete manner. Subsequently, example values of specific electromagnetic radiation parameters that can be varied are described (e.g., depending on the desired magnitude of the first type of material change). In an example, a specific radiation parameter can include the wavelength of the first electromagnetic radiation. The wavelength of the first electromagnetic radiation can be between 150 nm and 1500 nm, between 500 nm and 1400 nm, between 800 nm and 1300 nm, and / or between 840 nm and 1100 nm. In an example, a specific radiation parameter may include the pulse width of the first electromagnetic radiation. The pulse width of the first electromagnetic radiation may be between 100 fs and 100 ns, between 500 fs and 50 ns, between 800 fs and 20 ns, and / or between 1 ns and 20 ns. The specific radiation parameter may include a pulse sequence including at least two pulses. The first pulse and at least one second pulse of the pulse sequence may be spaced apart by at least 10 ns in time. Typically, the pulses of the pulse sequence may be substantially equidistant in time. The pulses of the pulse sequence can be emitted at a specific frequency and / or in a non-periodic pattern. The same pulse sequence can be repeated at a certain frequency, for example, at a frequency of 100 kHz, which corresponds to a 10 µs period for a predetermined time. Alternatively, the first electromagnetic radiation may include a pulse sequence having a substantially continuous and / or periodic pulse sequence. In an example, a specific radiation parameter may include the spot size of the first electromagnetic radiation (inside the mask). The spot size of the first electromagnetic radiation may be between 20 µm and 200 µm, between 20 µm and 100 µm, and / or between 20 µm and 50 µm. Specifically, the spot size of the first electromagnetic radiation may have a circular form with a diameter between 20 µm and 200 µm, between 20 µm and 100 µm, and / or between 20 µm and 50 µm. In an example, a specific radiation parameter may include the power density of the first electromagnetic radiation. The power density of the first electromagnetic radiation may be between 10 mJ / cm 2 and 1000 mJ / cm 2 、10 mJ / cm 2 and 500 mJ / cm 2 、10 mJ / cm 2 and 200 mJ / cm 2 、10 mJ / cm 2 and 100 mJ / cm 2 . The power density of the first electromagnetic radiation may also be, for example, between 10 J / cm 2 and 1000 J / cm 2 、100 J / cm 2 and 1000 J / cm 2 、250 J / cm 2 to 1000 J / cm 2 、500 J / cm 2 to 1000 J / cm 2 between. It should be noted that adjusting the power density may be very beneficial for the methods described herein, as the magnitude of the first material change induced can, for example, be highly dependent on the power density of the first electromagnetic radiation. In an example, the method may include applying the first electromagnetic radiation to a first location within the photomask at a first power density. This may cause a first material change (or a first type of material change) having a first magnitude (e.g., a first local density increase). The method may further include applying the first electromagnetic radiation to a second location within the photomask at a second power density, which may be different from the first power density. This may trigger a first material change (or a first type of material change) having a second magnitude, which is different from the first magnitude (e.g., a second local density increase different from the first local density increase). In an example, a specific radiation parameter may include the pulse density of the first electromagnetic radiation. The pulse density may be a value of the number of pulses per area (e.g., in units of pulses / mm 2 )). For example, the pulse density may be from 1,000 to 10,000,000 pulses / mm 2 . For example, the pulse density may be from 1,000 to 10,000,000 pulses / mm 2 、from 1,000 to 5,000,000 pulses / mm 2 、from 1,000 to 1,000,000 pulses / mm 2 、from 1,000 to 500,000 pulses / mm 2 、from 1,000 to 100,000 pulses / mm 2 、and / or from 1,000 to 50,000 pulses / mm 2 . For example, the pulse density may be from 1,000 to 10,000,000 pulses / mm 2 、from 2,000 to 10,000,000 pulses / mm 2, 5,000 to 10,000,000 pulses / mm 2 , 10,000 to 10,000,000 pulses / mm 2 , and / or 100,000 to 10,000,000 pulses / mm 2 . In one example, a mask may include a substrate and a reflective multilayer stack. As described herein, a first electromagnetic radiation may be applied to such a mask to induce a first material change within the mask, which changes the reflectivity of the mask. For example, the substrate may be a base material of the reflective mask, which has various additional layers (e.g., a reflective multilayer stack) coupled thereto. In an example, the substrate may have basic mechanical and / or thermal functions for the mask (e.g., to implement secure clamping and / or fastening and / or thermal management of heat of the mask). In an example, the substrate may be a layer of the mask, which, through the design of the mask, may not substantially affect the optical image of the mask. For example, the substrate may not be designed to affect the reflectivity of the mask. However, the substrate may affect the operation of the reflective multilayer stack disposed on the front side of the mask, where the front side of the mask is where the absorption pattern is fabricated. For example, an uneven surface of the mask substrate on which the reflective multilayer stack is deposited may locally affect the reflectivity of the reflective multilayer stack. A coating having one or more layers may be applied to the back side of the mask substrate. The coating on the back side of the mask enables or facilitates clamping or fastening of the EUV mask during operation. For example, the reflective multilayer stack may be configured to act as a mirror for a specific exposure wavelength λ E of the mask (as described herein). Thus, the reflective multilayer stack may be, for example, the main contributor to the reflectivity of the reflective mask. The reflective multilayer stack may include one or more reflector layers, which may contribute to the reflectivity of the mask for the lithography process. For example, the reflective multilayer stack may be configured as a Bragg mirror (or distributed Bragg reflector). In one example, the first electromagnetic radiation may be applied such that the first material change occurs within the reflective multilayer stack. Accordingly, the methods described herein may include causing a first material change within a reflective multilayer stack of a photomask. Causing a first material change in a layer that makes a positive contribution to the reflectivity of the photomask can be highly beneficial for modifying the reflectivity of the photomask. Since the first material change is a material change of the reflective multilayer stack, the impact of the first material change on the reflectivity can be more significant. Thus, the reflectivity can be modified directly. Indirect modification can be avoided because the material being modified may not (by design) contribute to the reflectivity of the photomask. Such indirect modification may not always result in the desired correction of the lithographic image. According to the disclosure herein, a first type of material change can be caused at different locations within the reflective multilayer stack with different magnitudes. The basic physical process of non - linear absorption of ultrashort laser pulses at the focus of an EUV photomask is as follows: Absorption of laser energy by free electrons and energy exchange between electrons at a rate of υ ee ≈ 10 14 s -1 can raise the electron temperature T e to approximately 10,000 K. At the same time, the initial lattice temperature of the optical element T i remains at approximately the temperature of the substrate of the EUV photomask, which is typically room temperature. Subsequently, the electrons transfer energy to the lattice of the EUV photomask at a rate of υ ei ≈ 10 12 s -1 , which corresponds to a time scale of approximately 1 ps. This is the specific time for metal heating. Then, the absorbed energy is typically dissipated within the material according to the thermal diffusion coefficient of the material composition of the EUV photomask. Typically, the reflective multilayer stack includes multiple (usually 20 to 60) consecutive bilayers of molybdenum (Mo) and silicon (Si) layers, which have diffusion coefficients D Mo = 0.54 cm 2 / s and D Si = 0.88 cm 2 / s. Thus, for a diffusion length L of 7 nm or longer, diffusion (given by the equation t ~ L 2 / D description) typically occurs on a time scale longer than 1 ps. The present invention takes into account the relevance of different time scales of the basic physical process when determining the electromagnetic radiation applied to generate or cause (first) material changes within the EUV mask. In particular, the first and second pulses of a pulse train proven to be spaced 100 ns or less allow improved results to be obtained in modifying the material changes of the EUV mask. When the processed EUV mask is not yet in thermal equilibrium (e.g., through heat diffusion), keeping the time between pulses short enough allows the thermal energy of the second pulse to be introduced into the EUV mask shortly after the first pulse. Specifically, experiments conducted using a pulse train containing two pulses can reach a temperature of more than 50% at a depth of 200 nm within the EUV mask (especially within the reflective multilayer structure), compared to a control experiment with only one pulse (i.e., no pulse train), at substantially the same degree of damage (e.g., including compression and / or undesirable displacement of absorber pattern elements thereon). Therefore, the effect can reach deeper into the processing surface of the reflective multilayer structure of the EUV mask. Based on the important physical principles outlined above, better results can be expected by providing longer pulse trains. Specifically, a longer pulse train can include, for example, three, four, five, or more pulses in each pulse train. In one example, the first electromagnetic radiation can be applied such that the first material change occurs within the substrate. Thus, the methods described herein can include causing a first material change within the substrate to modify the reflectivity of the mask. For example, the first material change can be generated near the interface of the reflective multilayer stack (e.g., the reflective multilayer stack can border the substrate, forming a corresponding interface at the boundary). For example, the first material change can be generated at a first predetermined distance from the interface. The first predetermined distance can be considered close to the interface of the reflective multilayer stack. The first predetermined distance can be selected such that the first material change within the substrate can have an impact on the reflectivity of the mask (e.g., an impact on the reflectivity of the reflective multilayer stack). The value of the first predetermined distance can range between 0 μm and 50 μm, 0 μm and 30 μm, 0 μm and 30 μm, 0 μm and 10 μm, 0 μm and 5 μm, and / or 0 μm and 2 μm. As described above, the reflective multilayer stack can include 20 to 60 Mo and Si bilayers. The nominal thickness of the Mo layer is 2.9 nm, and the nominal thickness of the Si layer is 4.0 nm. The real parts of the refractive indices of Mo and Si are very similar. But their imaginary parts or extinction coefficients are very different: k Mo = 3.365, k Si = 0.005 (irradiation wavelength 800 nm). This translates to a penetration depth (pd) at which the irradiation intensity drops to pd Mo = 19 nm and pd Si = 12.7 µm of e -1 。This means that, at 800 nm, the Mo layer absorbs light much more efficiently than the Si layer, or in other words, almost all of the energy absorbed by the reflective multilayer stack is absorbed by the metallic Mo layer. The melting points (T m ) of: T m (Mo) = 2623 °C and T m (Si) = 1412 °C. As a result, the deeper the incident radiation penetrates the reflective multilayer stack of the EUV mask, the more sharply the light intensity of the light at 800 nm drops. The Mo layer of the first bilayer has absorbed approximately 40% of the incident energy, and the percentage drops to 25% (15%) for the second (third) bilayer. This means that the metallic elements of the reflective multilayer stack can absorb almost all of the incident light with wavelengths in the range of 400 nm to 1000 nm in several bilayers, regardless of whether the electromagnetic radiation is incident through the front side (with the absorption pattern element) or the back side (or the mask substrate side) of the reflective multilayer stack. Therefore, the first electromagnetic radiation incident on the reflective multilayer stack of the EUV mask can locally melt one or more Mo layers. The local melting of one or more Mo layers of the reflective multilayer stack can locally reduce or locally disrupt the reflectivity of the first or the first few bilayers adjacent to the front side or the back side of the reflective multilayer stack. This local first material change can locally modify (usually reduce) the reflectivity of the EUV mask. In another aspect, the first electromagnetic radiation having a pulse or a pulse sequence can locally cause or trigger a second material change by locally slightly compressing one or more bilayers of the reflective multilayer stack. The compressed portion of the reflective multilayer stack has a slightly shifted Bragg reflection condition. If the reflective multilayer stack is irradiated exactly at the Bragg wavelength, the reflectivity of the locally compressed portion decreases. However, if the wavelength distribution of the electromagnetic radiation is not optically aligned with the Bragg wavelength, the reflectivity of the locally compressed portion can increase or decrease compared to the untreated reflective multilayer stack. In one example, a mask may include a substrate and at least one additional layer, where applying first electromagnetic radiation causes a first material change to occur within the at least one additional layer. For example, the at least one additional layer may be designed to perform any function of the mask (e.g., optical, mechanical, and / or thermal functions, etc.). According to the methods described herein, the first material change may be applied to the at least one additional layer to modify the reflectivity of the mask. Thus, the first material change may not have to be applied to the substrate of the mask. Instead, the first material change may be applied to a reflective multilayer stack as described above. In one example, the at least one additional layer may include a layer of the reflective multilayer stack of the mask (as described herein). In an example, the reflectivity of the mask may be modified for a patterned area of the mask. For example, the patterned area may be an area of the mask where patterned elements (which may absorb the exposed light) are located. According to the disclosure described herein, the first electromagnetic radiation may be used to modify the reflectivity within the patterned area. Thus, in a lithographic image, the image of the patterned area may be corrected (e.g., other image areas may not be correctable). In an example, the reflective lithographic mask described herein may include a reflective lithographic mask for extreme ultraviolet (EUV) lithography. Such a mask may also be referred to as an EUV mask. In one example, the reflectivity may be modified such that at least one critical dimension (CD) of a lithographic image of the mask is adjusted. For example, the first type of material change induced may be located within the mask such that the CD of the lithographic image of the mask is corrected (e.g., improved). For example, by adjusting the reflectivity of the mask in a defined manner (as described herein), the intensity distribution of the lithographic image may be adjusted. The mechanism may be used to correspondingly adjust the CD of the lithographic image. For example, the CD may include the width of a patterned element of the mask in the lithographic image, which is defined by the corresponding intensity distribution. By adjusting the reflectivity of the mask, the CD of the mask (e.g., the corresponding intensity distribution) may be adjusted. The mechanism may also be used to accommodate the critical dimension uniformity (CDU) of the entire lithographic image. The modification of at least one CD may also be referred to as a CD correction process (CDC process). In this regard, the first electromagnetic radiation may be used to initiate the CDC process. In one example, the modification of at least one CD may also be at least partially based on the same principle known for transmissive photomasks. However, in a transmissive photomask, light transmission can be adapted to adjust for the CD. This principle for CD correction (CDC) for transmissive photomasks is disclosed in US9436080B2, US2020124959A1, US2012009511A1. In an example, US9436080B2 is incorporated herein by reference in its entirety. In an example, US2020124959A1 is incorporated herein by reference in its entirety. In an example, US2012009511A1 is incorporated herein by reference in its entirety. In one example, the first material change may further modify at least one other property of the photomask besides the reflectivity. The at least one other property of the photomask can be modified in a defined manner. For example, the method may include setting a defined reflectivity and a defined other property of the photomask via the first material change. As described herein, the first material change can be regarded as a first type of material change. To illustrate the corresponding example, a single first type of material change (e.g., a single spatially separated pixel) can be induced, which modifies the reflectivity of the photomask and at least one other property in a defined manner. In such an example, each first type of material change may thus contribute to both factors (the reflectivity of the photomask and at least one other property). For example, as described above, the first material change caused by applying the first electromagnetic radiation can be a local compression of at least one bilayer of the reflective multilayer stack. This type of material change may also cause a local modification of the EUV photomask reflectivity. In addition, the local compression of the reflective multilayer stack may also cause a displacement of the position of the absorption pattern element adjacent to the locally compressed portion. The first material change can be a reduction in the reflectivity of the first bilayer, preferably two bilayers, more preferably five bilayers, most preferably ten bilayers, either on the front side (pattern side) of the reflective multilayer stack of the EUV photomask or on the back side (substrate side) of the multilayer stack of the EUV photomask. To illustrate another example, in the first step, a first type of material change (e.g., a single spatially separated pixel) can be induced, which can essentially modify only the reflectivity. For the first step, a first set of specific electromagnetic radiation parameters can be used. The first step can include causing a first type of material change at a first location within the mask. In the second step, another first type of material change (e.g., an additional single spatially separated pixel) can be induced, which can essentially modify at least one other property of the mask. The first step can include causing a first type of material change at a second location within the mask. For the second step, a second set of specific electromagnetic radiation parameters can be used, which is different from the first set. For example, the power density of the first set of specific electromagnetic radiation parameters can be different from the power density of the second set of specific electromagnetic radiation parameters. For example, the pulse density and / or pulse sequence of the first set of specific electromagnetic radiation parameters can be different from the second set of specific electromagnetic radiation parameters. In an example, the first location can be different from the second location. However, it is also possible to consider that the second location is the same as the first location. In one example, the at least one other property can include at least one of the following: a compression of the mask; a position of a pattern element on the mask; a flatness of the mask. The local compression of the reflective multilayer stack of the EUV mask can include a local shift in the wavelength that satisfies the Bragg condition. Thus, as described above, the local wavelength shift can locally change the reflection at the compressed portion of the reflective multilayer stack. For example, the at least one other property can include a compression of the mask. The compression can include local compression, which can be introduced into the mask via the first electromagnetic radiation by changing the reflectivity of the mask. The compression can also be changed via the first electromagnetic radiation. In one example (as described herein), a single first type of material change can change the compression and thus can locally change the reflectivity of the reflective multilayer stack of the EUV mask. Thus, a single pixel can change the reflectivity and the degree of compression. It can be predetermined that the introduced change in reflectivity will result in a certain compression. This can be solved by adjusting the pixel such that (alternatively) the EUV mask undergoes an ideal expansion. In another example (also as described herein), in the first step, a first type of material change can be caused, which can mainly modify the reflectivity, and in the second step, a first type of material change can be caused, which can mainly modify the compression rate. For example, a first pixel can significantly change the reflectivity, where a second pixel can significantly change the compression rate. For example, the second pixel can introduce expansion into the mask. It can be predetermined that the introduced change in reflectivity will result in a certain compression on the mask. This can be solved by incorporating the pixel into the mask, which essentially changes the compression of the mask to an ideal state. In addition, it can be predetermined that the introduced local compression will locally cause a decrease in the reflectivity of the reflective multilayer stack of the EUV mask. For example, at least one other characteristic may include the position of the pattern element on the mask. The position of the pattern element can be changed by modifying the reflectivity of the mask through the first electromagnetic radiation. Therefore, the position of the pattern element can also be modified by the first electromagnetic radiation. Specifically, by locally changing the reflectivity of the reflective multilayer stack, it has been possible to change the position of the pattern element using the first electromagnetic radiation. In an example (as described herein), a single change in the first type of material can modify the reflectivity and position of the pattern element. For example, a single pixel can modify the reflectivity and position of the pattern element. It can be predetermined that the introduced change in reflectivity, particularly the change in reflectivity of the reflective multilayer stack, will result in a specific change in the pattern element. This can be addressed by adjusting the pixel such that (as an alternative) the desired position of the pattern element appears. In another example (also as described herein), in a first step, a change in the first type of material can be induced, which mainly changes the reflectivity, and in a second step, a change in the first type of material can be induced, which can mainly modify the position of the pattern element. For example, a first pixel can significantly modify the reflectivity, where a second pixel can significantly modify the position of the pattern element. For example, the second pixel can introduce an expansion of the mask (to the reflective multilayer stack), which can move the pattern element to the desired position. It can be predetermined that the introduced change in reflectivity will result in a specific change in the position of the pattern element. This can be compensated for by incorporating the pixel into the reflective multilayer stack of the mask, which significantly affects the position of the pattern element. For example, at least one other characteristic may include the flatness of the mask. The flatness may include the deviation from the ideal flat surface plane of the mask. The flatness of the mask may have been changed by the first electromagnetic radiation by modifying the reflectivity of the mask or by locally modifying the reflectivity of the reflective multilayer stack deposited on the EUV mask substrate. Therefore, the flatness can also be modified by the first electromagnetic radiation. In an example (as described herein), a single change in the first type of material can modify the reflectivity and flatness. For example, a single pixel can modify the reflectivity and flatness. It can be predetermined that the introduced change in reflectivity will result in a specific change in the flatness. This can be addressed by adjusting the pixel such that (as an alternative) the desired flatness appears. In another example (also as described herein), in a first step, a first type of material change can be induced, which can mainly change the reflectivity, and in a second step, a first type of material change can be induced, which can mainly change the flatness. For example, a first pixel can significantly modify the reflectivity, while a second pixel can significantly modify the flatness. For example, the second pixel may introduce an expansion or compression to the mask, i.e., a mask substrate, a reflective multilayer stack, or both that can adapt to the flatness of the mask. The induced change in reflectivity can be predetermined to result in a specific change in flatness. This can be compensated by incorporating the pixel into both the substrate, the reflective multilayer stack, or the EUV mask that substantially affects the flatness. In an example, the curvature of the mask as part of the flatness may not be modified. That is, the curvature of the mask can be addressed through corresponding clamping. The modifications to the degree of compression, the position of the pattern elements, and / or the flatness described herein can also be referred to as a registration correction process (REGC process). In this regard, a first electromagnetic radiation can be used to initiate the REGC process. Such a registration correction process (REGC process) can also be at least partially based on the corresponding technical principles described in the following documents US9436080B2, US2020124959A1, US2012009511A1 (as mentioned, they are incorporated herein by reference in their entirety). In an example, the method may further include: applying a second electromagnetic radiation to the mask to induce a second material change within the mask, such that at least one characteristic of the mask is modified. Applying the second electromagnetic radiation to the mask can include applying the second electromagnetic radiation to the reflective multilayer stack of the EUV mask to cause a second material change within the reflective multilayer stack. All aspects described herein for the first electromagnetic radiation can also be correspondingly applied to the second electromagnetic radiation. All aspects described herein for the first material change within the mask can also be correspondingly applied to the second material change within the mask. Applying the second electromagnetic radiation can be regarded as a further process step for correcting the reflective lithography mask. For example, the second electromagnetic radiation can be applied to the mask in a continuous and / or discrete manner (as described herein for the first electromagnetic radiation) to further correct the mask. In one example, the second material change can also be regarded as a specific type of material change. For example, the second electromagnetic radiation can be directed to multiple positions within the mask, where a specific type of material change occurs at each position. At each position, the physical and / or chemical reaction may be based on the same mechanism (e.g., heat-induced change of a specific material). However, at each position, the second electromagnetic radiation may be different (e.g., by changing one or more specific electromagnetic radiation parameters of the second electromagnetic radiation). This can enable different magnitudes of the second material change to be induced at each position. For example, depending on the selected specific electromagnetic radiation parameters, different degrees of density change may occur, where the induced material composition can be substantially the same. The specific type of material change can be the same as the first type of material change. For example, the physical and / or chemical reaction caused by the second electromagnetic radiation can be substantially the same as the reaction caused by the first electromagnetic radiation. This may be the case when the first electromagnetic radiation and the second electromagnetic radiation are applied to the same material of the mask, such as both the mask substrate, the reflective multilayer stack disposed on the front side of the mask substrate, or an EUV mask. In another example, the specific type of material change can be different from the first type of material change. For example, the physical and / or chemical reaction caused by the second electromagnetic radiation can be substantially different from the reaction caused by the first electromagnetic radiation. This may be the case when the first electromagnetic radiation and the second electromagnetic radiation are applied to different materials of the mask, such as the mask substrate or the reflective multilayer structure. In an example, the specific electromagnetic radiation parameters of the second electromagnetic radiation can be different from the specific electromagnetic radiation parameters of the first electromagnetic radiation. For example, the power density of the second electromagnetic radiation can be different from the power density of the first electromagnetic radiation. For example, the pulse density of the second electromagnetic radiation can be different from the pulse density of the first electromagnetic radiation. In one example, the second electromagnetic radiation can be applied such that the second material change occurs within the substrate. For example, the first electromagnetic radiation can be applied such that the first material change occurs within the reflective multilayer stack, where the second electromagnetic radiation can be applied such that the second material change occurs within the substrate. For example, the first electromagnetic radiation can be applied such that the first material change occurs within the substrate, where the second electromagnetic radiation can be applied such that the second material change occurs within the substrate. In one example, the second electromagnetic radiation can be applied such that the second material change occurs within the reflective multilayer stack. For example, the first electromagnetic radiation can be applied such that the first material change occurs within the reflective multilayer stack, where the second electromagnetic radiation can be applied such that the second material change occurs within the reflective multilayer stack. For example, a first electromagnetic radiation may be applied such that a first material change occurs within a substrate, where a second electromagnetic radiation may be applied such that a second material change occurs within a reflective multilayer stack. In one example, at least one characteristic may be modified to reduce a deviation of the mask introduced by applying the first electromagnetic radiation. For example, a second electromagnetic radiation may be used to cause a second material change within the mask, which may compensate (e.g., minimize or eliminate) the deviation. In one example, at least one characteristic that may be modified by the second electromagnetic radiation may include at least one of the following: an error in the mask that cannot be corrected during lithography; a position of a pattern element on the mask; a compression of the mask; a flatness of the mask; a reflectivity of the mask; at least one critical dimension of a lithographic image of the mask. In an example, at least one characteristic may be modified to reduce a deviation introduced by applying the first electromagnetic radiation. For example, at least one characteristic (which may be modified by a second electromagnetic radiation) may include an error of the mask that cannot be corrected during lithography. For example, some errors may be corrected by an exposure system (e.g., through a scanner) during lithography. In this regard, an error of the mask that may be incorrect during lithography may be referred to as an uncorrectable error (NCE) in the lithography field. The application of the second electromagnetic radiation may be used to trigger a second material change that may change the amount of NCE. For example, the second material change may be used to reduce the NCE introduced by the first electromagnetic radiation. For example, at least one characteristic (which may be changed by a second electromagnetic radiation) may include a degree of compression of the mask. Compression of the mask may be introduced by applying the first electromagnetic radiation. In an example, the first electromagnetic radiation may not have addressed the compression. Thus, the second electromagnetic radiation may be used to trigger a second material change, which may reduce the undesirable compression of the mask. As described herein, modification of the compression may include a REGC process. For example, at least one characteristic (which may be modified by a second electromagnetic radiation) may include a position of a pattern element on the mask. The position of the pattern element may be changed by applying the first electromagnetic radiation. In an example, the position of the pattern element may not have been addressed by the first electromagnetic radiation. Thus, the second electromagnetic radiation may be used to trigger a second material change, which may move the pattern element to an ideal position. As described herein, modification of the position of the pattern element may include a REGC process. It is noted that the method may further include changing the positions of a plurality of pattern elements. Adjustment of the positions of the pattern elements (as described herein) may be required to achieve an ideal coverage during a lithography process. For example, the overlay may include an on-product overlay (OPO) known in the art. In an example, the first and / or second electromagnetic radiation may be used to adjust the OPO to an ideal value (e.g., such that the OPO is within a predetermined specification). For example, at least one characteristic (which can be modified by a second electromagnetic radiation) can include the flatness of a photomask. The flatness of the photomask may have been changed by applying a first electromagnetic radiation. In an example, the flatness of the photomask may not have been resolved by the first electromagnetic radiation. Thus, the second electromagnetic radiation can be used to cause a second material change, which can bring the flatness to an ideal state. As described herein, the modification of the flatness can include the REGC process. In an example, the second electromagnetic radiation can modify the flatness without including bending (since bending can be resolved by corresponding clamping). For example, at least one characteristic (which can be modified by a second electromagnetic radiation) can include the reflectivity of a photomask. For example, the first electromagnetic radiation can cause a first material change, which imparts a first reflectivity change to the photomask. Subsequently, the second electromagnetic radiation can cause a second material change, which imparts a second reflectivity change to the photomask, and then can result in an overall modification of the reflectivity of the photomask. As described herein, two or more characteristics of a photomask can be changed by a second electromagnetic radiation. For example, the second electromagnetic radiation can be used to modify errors of the photomask and the position of pattern elements that cannot be corrected during lithography. For example, the second electromagnetic radiation can be used to modify the position of pattern elements and the flatness of the photomask. It is noted that the second electromagnetic radiation can cause an REGC process, which can modify the registration and / or flatness of the photomask. In an example, after measuring at least one characteristic of the photomask by applying the first electromagnetic radiation, the photomask is measured. Based on the measurement results, at least one characteristic can be changed by applying the second electromagnetic radiation. In an example, the first electromagnetic radiation and the second electromagnetic radiation can be applied to the photomask simultaneously. In an example, the second electromagnetic radiation can be applied after applying the first electromagnetic radiation to the photomask. In an example, the first electromagnetic radiation can be applied from one side of the photomask that includes a pattern element for lithography, and / or the second electromagnetic radiation can be applied from a back side of the photomask. The side of the photomask that includes the pattern element can be regarded as the front side of the photomask. The back side of the photomask is the opposite side of the front side. For example, the first step may include applying a first electromagnetic radiation to the front side of the photomask to change the reflectivity of the photomask. The second step may include applying a second electromagnetic radiation to the back side of the photomask to modify the reflectivity of the photomask. For example, such a process can be used to adapt the CD of the photomask to the front-side process and the back-side process. In an example, the front-side process may cause a first material change in the reflective multilayer stack, where the back-side process may also cause a second material change in the reflective multilayer stack. The first and second material changes may be spatially separated (e.g., located at different depths in the multilayer stack). Additionally, other combinations can be envisioned where the front-side process and the back-side process may cause respective material changes in the multilayer stack and the substrate or only in the substrate. As described herein, the second electromagnetic radiation applied from the back side can also be used to adjust other characteristics of the photomask. In one example, the second electromagnetic radiation can be applied from a side of the photomask that includes the pattern elements for lithography; and / or the first electromagnetic radiation can be applied from the back side of the photomask. Thus, the back-side process can be utilized to apply the first electromagnetic radiation, where the second electromagnetic radiation can be applied through the front-side process. In one example, the method further includes: applying a third electromagnetic radiation to the photomask to induce a third material change in the photomask such that at least one characteristic of the photomask is modified. All aspects described herein for the first and / or second electromagnetic radiation can also be correspondingly applied to the third electromagnetic radiation. All aspects described herein for the first and / or second material changes in the photomask can also be correspondingly applied to the third material change in the photomask. Applying the third electromagnetic radiation can be regarded as a further process step for correcting the reflective photomask. As described herein, the third electromagnetic radiation can be applied to the photomask in a continuous and / or discrete manner (as described herein for the first electromagnetic radiation) to further correct the photomask. For example, the third electromagnetic radiation can be used to correct the deviations caused by the first and / or second electromagnetic radiation. For example, the first electromagnetic radiation can be applied such that a first material change occurs in the reflective multilayer stack, where the second electromagnetic radiation can be applied such that a second material change occurs in the substrate, where the third electromagnetic radiation can be applied such that a third material change occurs in the reflective multilayer stack. For example, the first electromagnetic radiation can be applied such that a first material change occurs in the reflective multilayer stack, where the second electromagnetic radiation can be applied such that a second material change occurs in the substrate, where the third electromagnetic radiation can be applied such that a third material change occurs in the reflective substrate. For example, the first electromagnetic radiation can be applied such that a first material change occurs in the reflective multilayer stack, where the second electromagnetic radiation can be applied such that a second material change occurs in the reflective multilayer stack, where the third electromagnetic radiation can be applied such that a third material change occurs in the substrate. For example, a first electromagnetic radiation may be applied such that a first material change occurs within a substrate, where a second electromagnetic radiation may be applied such that a second material change occurs within the substrate, and where a third electromagnetic radiation may be applied such that a third material change occurs within the substrate. In one example, applying a specific type of electromagnetic radiation (or a combination of specific types of electromagnetic radiation) as described herein may be considered a writing process, and generally, this application may be implemented as a process block (e.g., in an industrial environment). A second aspect relates to an apparatus for correcting a reflective lithography mask, comprising: means for applying a first electromagnetic radiation to the mask to induce a first material change within the mask, the first material change correcting a reflectivity of the mask. The means may further include a control unit, and the control unit may include a computer program for performing the method of the first aspect. Thus, the control unit may control the means to perform the method of the first aspect. For example, the control unit may be configured to adapt the means for applying the first electromagnetic radiation such that it causes a first material change within a multilayer stack and / or within a substrate. A third aspect relates to a reflective lithography mask, wherein the mask is corrected by the method described in the first aspect. Specifically, the third aspect relates to a reflective lithography mask, wherein the reflective multilayer stack of the reflective lithography mask is corrected by the method of the first aspect. A fourth aspect relates to a computer program for performing the method of the first aspect. Note that the method steps (or computer program steps) described herein may include all aspects described herein, even if not explicitly described as method steps but rather with reference to a device (or apparatus or system). Furthermore, the apparatus (or system or device) outlined herein may include components for implementing all aspects outlined herein, even if these aspects may be described throughout in method steps (or computer program steps). Regardless of whether the method steps, computer programs, and / or components described herein can be implemented using hardware, software, firmware, and / or combinations thereof. If implemented using software / firmware, the functions can be stored on a computer-readable medium or transmitted as one or more instructions or codes on a computer-readable medium. The computer-readable medium includes both computer storage media and communication media, including any medium that facilitates the transfer of a computer program in different places. The storage media can be any available media accessible by a general-purpose or special-purpose computer. By way of example and not limitation, such computer-readable storage media can include RAM, ROM, EEPROM, FPGA, CD / DVD or other optical disc storage, magnetic disk storage or other magnetic storage components, or can be any other medium that is used to carry or store the required code in the form of instructions or data structures and is accessible by a general-purpose or special-purpose computer, or a general-purpose or special-purpose processor. The control unit described herein can also be implemented using hardware, software, firmware, and / or combinations thereof, for example, through one or more general-purpose or special-purpose computers, and / or general-purpose or special-purpose processors. Figure 1 shows an example reflective lithography mask M for EUV lithography. Subsequently, the example reflective lithography mask is referred to as EUV mask M. However, the teachings described herein can be applied to any reflective lithography mask (and can be不限于特定类型的反射微影光罩). The EUV mask M of Figure 1 can include a substrate S. The reflective multilayer stack ML can include alternating reflector layers (schematically shown in Figure 1). The reflective multilayer stack ML can be configured to be reflective for light used in EUV lithography. For example, the reflective multilayer stack ML can be reflective for a specific exposure wavelength of 13.5 nm. The reflective multilayer stack can be configured as a Bragg mirror. In an example, the reflective multilayer stack ML can have a boundary with the substrate S. This boundary can also be regarded as the interface I between the reflective multilayer stack ML and the substrate S. The EUV mask M can further include a cover layer C. The cover layer C can be adjacent to the reflective multilayer stack ML. For example, the cover layer C can serve as a protection for the reflective multilayer stack ML. It should be noted that there is an unclear expression "不限于特定类型的反射微影光罩" in the translation of , which may need to be further clarified according to the accurate meaning.The EUV mask M may further include one or more pattern elements P. The pattern element P may be configured to absorb a specific exposure wavelength used during EUV lithography. Thus, the one or more pattern elements can be used to generate a lithographic image with a desired pattern. In the example of FIG. 1, the pattern element P may include three pattern element layers P1, P2, and P3. The first pattern element layer P1 may include an anti-reflective coating. The second pattern element layer P2 may include a material that substantially contributes to absorbing the specific exposure wavelength. The second pattern element layer P2 may be regarded as an absorption layer. The third pattern element layer P3 may be a stabilizing layer (e.g., for mechanical, optical, and / or thermal purposes). It should be noted that the present disclosure is not limited to the schematic EUV mask M of FIG. 1. For example, the EUV mask may not necessarily include a pattern element having three pattern element layers. The EUV mask may also have a pattern element consisting of only one pattern element layer (e.g., only one absorption layer) or may include only two pattern element layers. The EUV mask M may further include a backside layer B. The backside layer B may be adjacent to the substrate S. The backside layer B may be conductive to achieve improved clamping. During EUV lithography, the EUV mask M (as shown in FIG. 1) can be irradiated with exposure light W from the front side of the mask that includes the pattern element P. As described above, the exposure light may include a specific exposure wavelength (e.g., 13.5 nm). The exposure light may be partially absorbed by the pattern element P and may partially enter the reflective multilayer stack ML. Since the reflective multilayer stack can be used as a mirror, for regions that do not include pattern elements, the exposure light is reflected from the EUV mask. In other words, if the exposure light W is not absorbed by the pattern element P, the reflective multilayer stack ML ensures that the light is reflected away from the mask. Therefore, the reflectivity of the reflective multilayer stack ML may have a significant impact on the lithographic image of the EUV mask M. Thus, the reflective multilayer stack can act as a major contributor to the reflectivity of the EUV mask. Errors in the reflectivity of the EUV mask M may have an adverse effect on the quality of the lithographic image of the EUV mask. In some cases, it may therefore be necessary to adjust the EUV mask (e.g., its reflectivity). FIG. 2 shows a first example of a corrected mask according to the method described herein. In this example, a corrected EUV mask M is shown. As described, the method may include applying a first electromagnetic radiation to the mask to cause a first material change within the mask that changes the reflectivity of the mask. In the example of FIG. 2, a first material change C1 is caused in the reflective multilayer stack ML. In this example, the first material change C1 may include discrete pixels 1, 2, 3, 4, 5 that have been written into the reflective multilayer stack. Thus, the method is implemented in a discrete manner (as described herein). Each pixel 1, 2, 3, 4, 5 can be a spatially separated material change compared to the surrounding (substantially unmodified) material of the reflective multilayer stack ML. Each pixel may have an impact on the local reflectivity of the EUV mask M. For example, each pixel can locally change the reflectivity of the reflective multilayer stack ML. A first electromagnetic radiation has been applied such that for each pixel, a physical and / or chemical reaction is induced that can change the local density. For example, each pixel may have a higher local density than the surrounding material. Each increase in local density can locally adapt the reflectivity of the EUV mask M. Each pixel may contribute differently to the local reflectivity change (as described herein). For example, the first pixel 1 has been irradiated with a first electromagnetic radiation of a first set (as described herein) having specific radiation parameters. The second pixel 2 has been irradiated with a first electromagnetic radiation of a second set (as described herein) having specific radiation parameters. The first set may be different from the second set such that the local reflectivity change of the first pixel 1 may be different from the local reflectivity change of the second pixel 2. For example, the power density used to generate the first pixel 1 may be different from the power density used to generate pixel 2. In FIG. 2 and the following FIGS. 3, 6, and 7, the pixels or more generally, the induced first material change, may preferably be generated near the front side (pattern side) and / or the back side (mask substrate side). As described above, the metal layers of the bilayer of the reflective multilayer stack typically have a high optical absorption, which in turn hinders the generation of material changes only in the middle part of the reflective multilayer stack. FIG. 3 shows a second example of a corrected mask according to the method described herein. In this example, the corrected EUV mask M is shown. In the example of FIG. 2, a first material change C1 will also be induced in the reflective multilayer stack ML. However, in this example, the first material change C1 exists in a continuous manner. That is, in FIG. 3, the method is implemented in a continuous manner (as described herein). In a continuous method, the pixels generated via the first electromagnetic radiation may overlap such that spatially separated material changes may not always exist. For example, a continuous change in the material within a specific region of the reflective multilayer stack ML may be generated. As described herein, the material changes induced within the reflective multilayer stack can be used to correct at least one CD of the lithographic image of the EUV mask M. For example, the first material change can locally change the reflective multilayer stack, which can locally change the reflectivity of the EUV mask, which can adapt the CD of the optical image generated using the EUV mask. For example, the induced first material change C1 can adapt the intensity distribution of the EUV mask M, thereby correcting the image of the pattern element P. Applying a first electromagnetic radiation within a reflective multilayer stack ML may thus include (or be referred to as) a CDC process. FIG. 4 shows a third example of a corrected reticle according to the method described herein. In this example, a corrected EUV reticle M is shown. In the example of FIG. 2, a first material change C1 is induced in the substrate S. In this regard, discrete pixels are shown (however, the method can also be applied in a continuous manner). In this example, the first material change C1 is induced near the interface I (the boundary between the reflective multilayer stack ML and the substrate S). As described herein, the first material change can thus be considered to be close to the reflective multilayer stack ML (or the interface I near the reflective multilayer stack ML). As shown in the figure, the first material change C1 may be located within a first predetermined distance d1 from the interface I. The first predetermined distance d1 can be selected such that the first material change C1 in the substrate can affect the reflectivity of the EUV reticle M (as described herein). For example, the pixels induced in the substrate S with the first material change can cause the reflective multilayer stack ML and thus cause a local modification of the EUV reticle M. As described above, the bilayer of the reflective multilayer stack ML closely determines the front surface and determines the reflectivity of the reflective multilayer stack ML. Therefore, the effect of the material change C1 of the reticle substrate S is partially shielded by the lower bilayer of the reflective multilayer stack ML. The value of the first predetermined distance may range between 0 μm and 50 μm, 0 μm and 30 μm, 0 μm and 30 μm, 0 μm and 10 μm, 0 μm and 5 μm, and / or 0 μm and 2 μm (as described herein). In another example, the first predetermined distance may be less than 50% of the substrate height, less than 30% of the substrate height, less than 20% of the substrate height, less than 10% of the height of the substrate S, and / or less than 5% of the height of the substrate S. The height of the substrate S may include the substrate height defined by the extension of the substrate along the normal vector of the reflective lithography reticle (e.g., the EUV reticle M). The height of the substrate S is also denoted as height h in FIG. 1. S . Returning to the example of FIG. 4, the first predetermined distance d1 can be varied for each pixel. For example, the distance of the first pixel to the interface I may be different from the distance of the second pixel to the interface I. This can provide greater freedom for correcting a reflective lithography reticle (e.g., an EUV reticle). The first predetermined distance d1 may also be referred to as the writing distance. Thus, for each pixel (or each type of first material change), a specific writing distance to the interface can be selected. This can implement a further degree of freedom in affecting the reticle correction. For example, the resulting reflectivity change can be further changed by adjusting the writing distance of the pixels. In the example, the pixels induced by the first material change C1 near the interface I may have different functions. Different functions can be triggered by specific electromagnetic radiation parameters of different sets of the first electromagnetic radiation. For example, the first pixel El may mainly contribute to the ideal reflectivity change of the EUV mask M. The second pixel E2 may mainly contribute to the ideal registration change of the EUV mask (e.g., the change in the position of the pattern element P). For example, the first pixel E1 can be induced by first electromagnetic radiation with a certain power density, such that a local density increase of the mask material can be caused. The local density increase can (locally) adjust the reflectivity of the EUV mask M. This can be used to adapt the CD of the lithography image to an ideal state. However, the local density increase may cause isotropic compression within the EUV mask M. This may lead to undesirable registration changes (e.g., undesirable changes in the position of the pattern element P) and / or undesirable flatness changes. The second pixel E2 (in this regard) is caused by a specific pulse density of the first electromagnetic radiation, such that a local expansion of the mask material can be caused. The local expansion (locally) corrects the undesirable registration changes and / or undesirable flatness changes caused by the first pixel E1 (which is used for CD change). It should be noted that the method may include generating multiple pixels within the EUV mask M and may not be limited to two pixels. For example, a first number of pixels can be used to generate ideal reflectivity changes, where a second number of pixels can be used to generate ideal registration changes and / or flatness changes of the EUV mask (as described herein). For example, a second number of pixels can be generated to compensate for the deviation caused by the first number of pixels. Thus, simultaneous modification of the reflectivity of the reflective multilayer stack ML and modification of the EUV mask compression can be implemented. According to the disclosure described herein, the pixel characteristics can vary according to the power density or pulse density of the first electromagnetic radiation, as well as the writing distance of the pixel to the interface I. For example, if the power density is higher than a predetermined power threshold, a reflectivity change can be caused (e.g., via local compression). For example, if the pulse density is selected within a predetermined pulse density range, a registration change and / or flatness change can be caused (e.g., via local expansion). Adjusting the writing distance of the pixel can further change its induced effect on the EUV mask M. For example, the writing distance can further adapt to the caused reflectivity change, caused registration change, and / or flatness change. The method described herein can thus implement a wide degree of freedom to correct the EUV mask M. FIG. 5 shows a fourth example of a mask corrected according to the method described herein. In this example, the corrected EUV mask M is shown. In this example, the first electromagnetic radiation is applied to the reflective multilayer stack ML to cause a first material change C1 in the reflective multilayer stack ML. For example, the pixel that causes the first material change C1 can be as described with respect to FIG. 2 (or FIG. 3). Implement writing pixels into the reflective multilayer stack ML to correct the reflectivity of the EUV mask M. However, the first material change C1 may have introduced an undesirable deviation of the mask. Therefore, the method may include applying a second electromagnetic radiation to the substrate to initiate a second material change C2, and the second material change C2 may compensate for the deviation caused by the first material change C1. For example, the pixels of the second material change C2 may compensate for the non-correctable error (NCE) caused by the first material change C1. For example, the pixels of the second material change C2 may compensate for the registration change caused by the first material change C1. For example, the pixels of the second material change C2 may compensate for the flatness change caused by the first material change C1. For example, the pixels of the second material change may compensate for the flatness change caused by the first material change C1, where the bending change of the EUV mask may not be compensated. For example, the pixels of the second material change C2 may compensate for the registration change and flatness change caused by the first material change C1. For example, the pixels of the second material change C2 may be generated at a second predetermined distance d2 from the interface I. In this example, since the second material change C2 may not be used to affect the reflectivity, the pixels may not have to be positioned near the interface I. Therefore, the second material change C2 may be initiated deeper within the substrate. However, it is also conceivable to cause the second material change C2 in this example near the interface I (at a first predetermined distance d1 from the interface I). As described herein, the application of the second electromagnetic radiation may include (or be referred to as) a REGC post-process to compensate for the deviation caused by the first change C1. For example, the first material change C1 may introduce a certain amount of quantifiable error into the lithographic image of the EUV mask M. For example, the quantifiable error may be measured. The quantifiable error may include, for example, NCE, overlay error, or any other type of known and quantifiable lithographic error. It has been found that through the REGC post-process described herein, the quantifiable error caused by the first material change C1 can be reduced by at least 90%. Through certain settings of the REGC post-processing, the quantifiable error caused by the first material change C1 can even be reduced by at least 99% (even 100% in some examples). FIG. 6 shows a fifth example of a corrected mask according to the method described herein. In this example, the corrected EUV mask M is shown. In this example, the first electromagnetic radiation causes a first material change C1 in the reflective multilayer stack ML, where the second electromagnetic radiation also causes a second material change C2 in the reflective multilayer stack. For example, the pixels that cause the first material change C1 can be as described with respect to FIG. 2 (or FIG. 3). For example, the pixels that cause the second material change C1 can also be as described with respect to FIG. 2 (or FIG. 3). One or more pixels of the first material change C1 can be arranged in an approximate plane that is approximately parallel to the plane of one or more pixels of the second material change C2. In the example of FIG. 6, the pixels of the first material change C1 can be generated via a front-side process (as described herein). In this regard, the front side can be the side of the EUV mask M that includes the pattern element P. For example, the radiation source of the first electromagnetic radiation can be located on the front side of the EUV mask M for the front-side process. In the example of FIG. 6, the pixels of the second material change C2 can be generated via a back-side process (as described herein). In this regard, the back side can be the side of the EUV mask M that is opposite to the side including the pattern element P. For example, the radiation source of the second electromagnetic radiation can be located on the back side of the EUV mask M for the back-side process. For example, the front-side process and the back-side process can be performed simultaneously. In such an example, the first radiation source at the front side can generate the first electromagnetic radiation, where the second radiation source located at the back side can generate the second electromagnetic radiation. For example, the front-side process and the back-side process can be performed sequentially. In such an example, it is conceivable that the EUV mask M is positioned accordingly (e.g., rotated to the right) to perform the front-side process or the back-side process, respectively. In the example of FIG. 6, the first material change C1 can be used to modify the reflectivity of the EUV mask M. For example, the first material change C1 can modify the CD of the optical image of the EUV mask M. For example, applying the first electromagnetic radiation can be a CDC process (as described herein). In the example of FIG. 6, the second material change C2 can be used to further modify the reflectivity of the EUV mask M. For example, the second material change C2 can modify the CD of the optical image of the EUV mask M. For example, applying the second electromagnetic radiation can also be a CDC process (as described herein). Such a method (causing the first and second material changes in the reflective multilayer stack ML) can effectively reduce the non-correctable error (NCE). For example, compared to using only the first material change, the NCE may be less when using the front-side and back-side methods. In another example, the second material change C2 can compensate for the NCE introduced by the first material change C1 (and vice versa). It should be noted that the example of FIG. 6 may include a front-side CDC process (causing a first material change C1) and a back-side CDC process (causing a second material change C2). FIG. 7 shows a sixth example of a reticle being corrected according to the method described herein. In this example, the corrected EUV reticle M is shown. In this example, a first electromagnetic radiation is applied to the EUV reticle M to cause a first material change C1 within the reflective multilayer stack ML. A second electromagnetic radiation is applied to the EUV reticle M to cause a second material change C2 within the reflective multilayer stack ML. A third electromagnetic radiation is applied to trigger a third material change C3. In this example, the first material change C1 and the second material change C2 may include the material changes as described with respect to FIG. 6. The process described in FIG. 6 can correct the CD of the lithographic image of the reticle. However, this may cause a deviation of the EUV reticle M (e.g., that did not exist before). For example, the modification of the reflectivity of the EUV reticle (as described herein) may cause undesirable characteristics of the reticle. For example, the process of FIG. 6 may cause undesirable registration changes and / or undesirable flatness changes of the EUV reticle M. Such undesirable characteristics can be compensated for by causing a third material change C3 within the substrate S. For example, the pixels of the third material change C3 can compensate for the uncorrectable errors (NCE) caused by the first material change C1 and / or the second material change C2. For example, the pixels of the third material change C3 can compensate for the registration changes caused by the first material change C1 and / or the second material change C2. For example, the pixels of the third material change C3 can compensate for the flatness changes caused by the first material change C1 and / or the second material change C2. For example, the pixels of the third material change can compensate for the flatness changes caused by the first material change C1 and / or the second material change C2, where the bending changes of the EUV reticle may not be compensated. For example, the pixels of the third material change C3 can compensate for the registration changes and flatness changes caused by the first material change C1 and / or the second material change C2. As described herein, the application of the third electromagnetic radiation may include (or be referred to as) a REGC post-process to compensate for the deviation caused by the first and second material changes. For example, a first material change C1 and a second material change C2 may introduce a certain amount of quantifiable error into the lithography image of the EUV mask M. For example, the quantifiable error is measured. The quantifiable error may include, for example, NCE, overlay error, or any other type of known and quantifiable lithography error. It has been found that through the REGC post-process described herein, the quantifiable error caused by the first material change C1 can be reduced by at least 90%. Through certain settings of the REGC post-processing, the quantifiable error caused by the first material change C1 can even be reduced by at least 99% (and even 100% in some examples). Generally speaking, the methods described herein can implement the correction of the EUV mask M such that the lithography image of the EUV mask M is within the predetermined specifications. Further concepts of the disclosure described herein are given below. As described, the CDC process can be followed by the REGC process to compensate for parts of the scanner that cannot be corrected (e.g., those of NCE). In an example, the CDC process can be combined with the REGC process to correct the registration and CD problems of the EUV mask M. For example, the first pixel type can mainly correct the registration problem, where the second pixel type can mainly correct the CD problem of the mask (as described herein). In one example, a combination of front-side and back-side CDC processes can be performed to minimize the remaining scanner-uncorrectable terms (e.g., those of NCE). The concepts described herein can improve the versatility of the process and can improve the desired goals of CD, registration, and / or flatness of the EUV mask M. For example, the REGC process in the substrate can be used to compensate for the NCE generated by the CDC process (which, for example, includes material changes in the reflective multilayer stack ML). For example, the REGC process can be used to compensate for the registration changes caused by the CDC process (which, for example, includes material changes in the reflective multilayer stack ML). For example, the REGC process can be used to compensate for the flatness changes caused by the CDC process (which, for example, includes material changes in the reflective multilayer stack ML). For example, the REGC process can be used to compensate for the flatness changes caused by the CDC process (which, for example, includes material changes in the reflective multilayer stack ML). However, the REGC process may not be able to compensate for the bending changes of the EUV mask M. For example, the REGC process can be used to simultaneously compensate for the registration and flatness changes caused by the CDC process (which, for example, includes material changes in the reflective multilayer stack ML). For example, the REGC process can be used in combination with the CDC process (wherein the two processes can cause, for example, a change in the material in the substrate) to simultaneously compensate for high-frequency line CD problems, REG problems, and / or flatness problems. For example, a combination of front-side and back-side CDC processes can be used to reduce the NCE generated by each CDC process. For example, the REGC process can be used in combination with the front-side CDC process and the back-side CDC process, wherein the REGC process can correct CD problems, REG problems, and / or flatness problems (possibly introduced by the front-side CDC process and the back-side CDC process). 1: Pixel 2: Pixel 3: Pixel 4: Pixel 5: Pixel B: Back-side layer C: Cover layer C1: First material change C2: Second material change C3: Third material change d1: First predetermined distance d2: Second predetermined distance E1: First pixel E2: Second pixel h S : Height I: Interface M: Mask ML: Reflective multilayer stack P: Pattern element P1: Pattern element layer P2: Pattern element layer P3: Pattern element layer S: Substrate W: Exposure lamp Figure 1 shows an example reflective lithography mask for EUV lithography. Figure 2 shows a first example of a calibration mask according to the present disclosure. Figure 3 shows a second example of a calibration mask according to the present disclosure. Figure 4 shows a third example of a calibration mask according to the present disclosure. Figure 5 shows a fourth example of a calibration mask according to the present disclosure. Figure 6 shows a fifth example of a calibration mask according to the present disclosure. Figure 7 shows a sixth example of a calibration mask according to the present disclosure. 1: Pixel 2: Pixel 3: Pixel 4: Pixel 5: Pixel B: Back-side layer C1: First material change I: Interface M: Mask ML: Reflective multilayer stack P: Pattern element P1: Pattern element layer P2: Pattern element layer P3: Pattern element layer S: Substrate W: Exposure lamp

Claims

1. A method for correcting a reflective photomask (M), wherein the photomask (M) includes a substrate (S) and a reflective multilayer stack (ML), the method comprising: A first electromagnetic radiation is applied to the photomask (M) to induce a first material change (C1) within the photomask, which modifies a reflectivity of the photomask (M); a second electromagnetic radiation is applied to the photomask to induce a second material change (C2) within the photomask (M), such that at least one characteristic of the photomask is modified.

2. The method of claim 1, wherein the application of the first electromagnetic radiation causes the first material change (C1) to occur within the reflective multilayer stack (ML).

3. The method of claim 1, wherein the application of the first electromagnetic radiation causes the first material change (C1) to occur within the substrate (S).

4. The method of any one of claims 1 to 3, wherein the reflectivity is modified such that at least one key dimension of a lithography image of the photomask (M) is adjusted.

5. The method of any one of claims 1 to 3, wherein the first material change (C1) further modifies at least one other property of the photomask (M) besides the reflectivity.

6. The method of claim 5, wherein the at least one other characteristic includes at least one of the following: a compression of the photomask (M); a position of a pattern element on the photomask (M); a flatness of the photomask (M).

7. The method of any one of claims 1 to 3, wherein the application of the second electromagnetic radiation causes the second material change (C2) to occur within the substrate (S).

8. The method of any one of claims 1 to 3, wherein the application of the second electromagnetic radiation causes the second material change (C2) to occur within the reflective multilayer stack (ML).

9. The method of any one of claims 1 to 3, wherein the at least one characteristic is modified to reduce a deviation of the photomask (M) introduced by the application of the first electromagnetic radiation.

10. The method of any one of claims 1 to 3, wherein the at least one characteristic comprises at least one of the following: an error of the photomask (M) that cannot be corrected during lithography; a position of a pattern element on the photomask (M); a compression of the photomask (M); a flatness of the photomask (M); the reflectivity of the photomask (M); at least one critical dimension of a lithographic image of the photomask (M).

11. The method of any one of claims 1 to 3, wherein the first electromagnetic radiation is applied from one side of the photomask (M) including a pattern element (P) for lithography, and / or wherein the second electromagnetic radiation is applied from a rear side of the photomask (M).

12. The method of any one of claims 1 to 3 further comprises: applying a third electromagnetic radiation to the photomask to induce a third material change (C3) within the photomask (M) such that at least one characteristic of the photomask is modified.

13. An apparatus for calibrating a reflective photomask (M), comprising: means for applying a first electromagnetic radiation to the photomask to induce a first material change (C1) within the photomask, the first material change (C1) modifying a reflectivity of the photomask (M); and for applying a second electromagnetic radiation to the photomask to induce a second material change (C2) within the photomask (M) such that at least one characteristic of the photomask is modified.

14. A reflective photomask (M), wherein the photomask is corrected by any one of claims 1 to 12.

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