Blankmask and Photomask for EUV lithography with Reflective Film

Abstract

A blank mask for EUV lithography comprises a reflective film having a multilayer structure formed on a substrate. The reflective film comprises a first pair comprising two layers with different refractive indices for EUV exposure light, and a second pair comprising two layers with different refractive indices for EUV exposure light. A structure in which the first pair is stacked multiple times is disposed in the lower region of the reflective film, and a structure in which the second pair is stacked multiple times is disposed in the upper region of the reflective film. The first pair is configured to exhibit a higher FWHM measurement value compared to the second pair, and the second pair is configured to exhibit a higher reflectance measurement value compared to the first pair. The number of stacking cycles of the second pair is 1 to 8. The reduction in reflectance of the reflective film is minimized, the FWHM is improved, and the interface between each layer is improved.

Description

Technology Field

[0001] The present invention relates to a blank mask and a photomask, and more specifically, to a blank mask for extreme ultraviolet lithography having a multilayer reflective film that reflects EUV exposure light of a wavelength of 13.5 nm, and a photomask manufactured using the same. Background Technology

[0003] FIG. 1 is a drawing illustrating an example of a blank mask for EUV lithography, and FIG. 2 is a drawing illustrating a photomask produced using the blank mask of FIG. 1.

[0004] A blank mask comprises a substrate (102), a reflective film (104) stacked on the substrate (102), an absorption film (106) formed on the reflective film (104), and a resist film (108) formed on the absorption film (106). The reflective film (104) is generally formed with a structure in which pairs of layers made of Mo and layers made of Si are stacked dozens of times, and functions to reflect incident exposure light. The absorption film (106) is typically formed of TaBN or TaBON material and serves to absorb incident exposure light. A phase inversion film that performs a phase inversion function may also be used instead of the absorption film (106). The resist film (108) is used to pattern the absorption film (106).

[0005] As the absorption film (106) is patterned into a predetermined shape as shown in FIG. 2, the blank mask is fabricated into a photomask, and EUV exposure light incident on the photomask is absorbed or reflected according to the pattern of the absorption film (106) and then irradiated onto a semiconductor wafer. A phase inversion film may be formed instead of or together with the absorption film (106), and other thin films such as an etch stop film or a hard mask film may be additionally formed. A capping film (not shown) containing Ru is generally formed on the upper surface of the reflection film (104) to protect the reflection film (104) during etching of the absorption film (106), etc.

[0006] In a blank mask for EUV, the reflective film (104) requires a very high reflectivity. The performance of the reflective film (104) is evaluated by reflectivity and FWHM (Full Width at Half Maximum), and for a high-quality reflective film (104), the reflectivity must be high and the FWHM must be wide. Here, FWHM refers to the FWHM in the reflectivity graph against wavelength. To improve reflectivity, the difference in refractive index between two layers forming a pair must be large, and the absorption coefficient (k) of the material must be low. To improve FWHM, the difference in refractive index between two layers forming a pair must be large. As a structure to satisfy these requirements, it is common for the reflective film (104) to be formed as a multilayer structure in which a pair of Mo / Si structures is stacked 30 to 50 times.

[0007] Si has the highest refractive index (n) among the group of materials capable of forming mass-producible thin films. Therefore, in order to further improve the FWHM compared to the reflective film (104) composed of Mo / Si pairs, a material with a lower refractive index than Mo must be introduced. However, among materials with a refractive index lower than Mo, there is no known material that has an absorption coefficient (k) lower than or similar to Mo. Although a method of forming the reflective film (104) with Ru / Si pairs has been studied, a decrease in reflectivity inevitably occurs because the absorption coefficient of Ru is about twice as high as that of Mo. Therefore, even if the FWHM of the reflective film (104) formed with Ru / Si pairs is greatly improved, its performance as a reflective film (104) is degraded due to the low reflectivity. In addition, in the stacked structure of Ru / Si pairs, the Ru-silicide reaction at the interface between the Ru layer and the Si layer is large, so an interlayer is formed at the interface, and the performance of the reflective film (104) is degraded compared to the structure of the Mo / Si pair. The problem to be solved

[0009] The present invention was devised to solve the above problems, and the objective of the present invention is to derive the most efficient structure of a reflective film from three perspectives: minimization of reflectance reduction, improvement of FWHM, and improvement of the interface. means of solving the problem

[0011] To achieve the above objective, the present invention provides a blank mask for EUV lithography having a multilayer reflective film formed on a substrate. The reflective film comprises a first pair comprising two layers with different refractive indices for EUV exposure light, and a second pair comprising two layers with different refractive indices for EUV exposure light. A structure in which the first pair is stacked multiple times is disposed in a lower region of the reflective film, and a structure in which the second pair is stacked multiple times is disposed in an upper region of the reflective film. The first pair exhibits a higher FWHM measurement value compared to the second pair, and the second pair exhibits a higher reflectance measurement value compared to the first pair. The number of stackings of the second pair is 1 to 8 times.

[0012] It is more preferable that the number of stacking times of the second pair is 3 to 6 times.

[0013] The above second pair may have one or more structures among Mo / Si, Mo / Y, Mo / Zr, Nb / Si, Nb / Y, and Zr / Si.

[0014] The first pair above may have one or more structures among Ru / Si, Ru / Y, and Ru / Zr.

[0015] At least a portion of each layer constituting the above-mentioned reflective film contains one or more additional materials among C, O, N, and B.

[0016] The above additional material is included in a region adjacent to the interface of each layer.

[0017] A capping layer may be formed on the uppermost layer of the above-mentioned reflective film.

[0018] According to another aspect of the present invention, a photomask produced using a blank mask having the above configuration is provided. Effects of the invention

[0020] According to the present invention, a blank mask having a reflective film configuration in which the reduction in reflectivity is minimized, the FWHM is improved, and the interface between each layer is improved is provided. Brief explanation of the drawing

[0022] FIG. 1 is a schematic diagram illustrating the structure of a conventional general EUV lithography blank mask. FIG. 2 is a drawing illustrating a photomask produced using the blank mask of FIG. 1. Figure 3 is a graph showing the measured values ​​of FWHM and reflectance when the number of uppermost Mo / Si pairs increases by one, based on a state in which a total of 40 pairs are formed with 17 Ru / Si pairs and 23 Mo / Si pairs. Figure 4 is a graph showing the product of the FWHM measurement and the reflectance (R) measurement in the graph of Figure 3. Figure 5 is a graph showing the measured values ​​of FWHM and reflectance as Mo / Si pairs are added one by one to the top, based on a state where 24 Ru / Si pairs are formed at the bottom. Figure 6 is a graph showing the product of the FWHM measurement and the reflectance (R) measurement in the graph of Figure 5. Figure 7 is a graph showing the measured values ​​of FWHM and reflectance as the topmost pair among the Ru / Si pairs is sequentially replaced with Mo / Si pairs one by one, based on the state where 24 Ru / Si pairs are formed. FIG. 8 is a graph showing the product of the FWHM measurement and the reflectance (R) measurement in the graph of FIG. 7. Specific details for implementing the invention

[0023] The present invention is described in more detail below. Since the structure of the blank mask in the present invention is identical to the structure of the conventional blank mask described above, its illustration and description are omitted and are used by reference as an explanation of the present invention. The laminated structure of the reflective film of the present invention is described below.

[0024] Under ideal conditions, the FWHM of a reflective film with a structure in which 40 Mo / Si pairs are stacked is 0.67. Since higher reflectivity is desirable, conventional methods have utilized Mo with a low absorption coefficient (k) through Mo / Si pairs. However, as previously mentioned in the description of the prior art, to improve FWHM, it is necessary to adopt other materials even if it results in some reduction in reflectivity. For the improvement in FWHM to be meaningful even at the expense of reduced reflectivity, the improvement must be at least 20%. Furthermore, even if reflectivity is sacrificed to improve FWHM, a reflectivity of 65% or higher is generally required, and for next-generation reflective films with high NA, a reflectivity of 60% or higher is required.

[0025] The reflective film (104) of the present invention has a structure in which a first pair of layers having different refractive indices is stacked multiple times and disposed in a lower region of the reflective film, and a second pair of layers having different refractive indices is stacked multiple times and disposed in an upper region of the reflective film. Here, having different refractive indices means, for example, cases such as Mo / Si pairs or Ru / Si pairs.

[0026] At this time, the first pair at the bottom is configured to show a higher FWHM measurement value compared to the second pair at the top, and the second pair is configured to show a higher reflectance measurement value compared to the first pair. In the above example, the reflectance of the Mo / Si pair is higher than that of the Ru / Si pair, and the FWHM of the Ru / Si pair is lower than that of the Mo / Si pair. Therefore, the Ru / Si pair constitutes the first pair at the bottom, and the Mo / Si pair constitutes the second pair at the top. In short, based on a structure in which dozens of pairs of Mo / Si structures are stacked as in the conventional method, the reflective film (104) of the present invention has a structure in which the lower region is replaced with Ru / Si pairs.

[0027] When silicide at the interface is not considered, the structure determined to be the optimal point through simulation is a structure of 24 pairs of Ru / Si and 40 pairs of Mo / Si. However, regarding the superposition interference effect that increases reflectivity, there is an appropriate depth from the outermost surface of the reflective film (104) where the interference effect occurs effectively. Therefore, if the material with a low absorption coefficient (k) is used only up to that depth, the optimal point can be selected from both the perspectives of reflectivity and FWHM.

[0028] When the reflective film (104) has a stacked structure of Ru / Si pairs as the basic configuration (first pair), the reflectivity improves and the FWHM decreases each time a pair of Mo / Si structures is added to the top. This trend is maintained until the Mo / Si structures reach 1 to 8 pairs, after which the degree of improvement in reflectivity gradually decreases. Therefore, it is desirable that the second pairs above the first pairs be formed in a maximum of 8 pairs. More preferably, the second pairs are formed in 3 to 6 pairs.

[0029] In the present invention, the Mo / Si pair is formed with a Mo layer having a thickness of 3 nm and a Si layer having a thickness of 4 nm, and the Ru / Si pair is formed with a Ru layer having a thickness of 3.3 nm and a Si layer having a thickness of 3.8 nm. The difference in the thickness of Si in the Mo / Si pair and the Ru / Si pair, as well as the difference in the thickness of the Mo layer and the Ru layer in each pair, is due to the difference in the refractive indices of Mo and Ru. The optimal thickness is selected based on the formula nd=λ / 4, which is the result of the thickness being controlled by the difference in refractive index. That is, the optimal thickness of each material layer is set according to the refractive index of the constituent material. Generally, setting the thickness as described above is known to be the optimal thickness for the composition of a reflective film.

[0030] The actual thickness of each formed layer does not exactly match the thickness set above due to tolerances in the layer formation process, and generally, a tolerance of approximately 10% relative to the designed thickness occurs. In addition, depending on the product or manufacturer, the design specifications themselves may differ from the thickness above. However, even if the thickness of each layer is set differently from the thickness mentioned above, the same applies to the fact that an optimal point can be obtained by configuring the second pair at the top with 8 pairs or fewer, preferably 3 to 6 pairs.

[0032] Below, we will examine this with reference to Figures 3 to 8.

[0033] In the case of a Mo / Si 40-pair structure, the ideal reflectance is 73.9%, while the measured value in an actual manufactured product is 65%. Through this, it can be seen that there is a difference of about 0.88 times between the ideal reflectance and the reflectance of the actual product. Considering this, it can be seen that the reflective film of the present invention, in which Mo / Si pairs and Ru / Si pairs are employed together, requires a reflectance of 68.2% or higher when measured in an ideal state.

[0034] Figure 3 is a graph showing the measured values ​​of FWHM and reflectance when the number of Mo / Si pairs at the top increases by one, based on a state where a total of 40 pairs are formed, consisting of 17 pairs of Ru / Si and 23 pairs of Mo / Si. That is, in Figure 3, 0 on the horizontal axis represents the case where there are 0 pairs of Mo / Si, 17 pairs of Ru / Si, and 23 pairs of Mo / Si from the top of the reflective film; 1 on the horizontal axis represents the case where there is 1 pair of Mo / Si, 17 pairs of Ru / Si, and 22 pairs of Mo / Si; and 2 on the horizontal axis represents the case where there are 2 pairs of Mo / Si, 17 pairs of Ru / Si, and 21 pairs of Mo / Si. In Figure 3, the left side of the vertical axis is indicated by blue dots on the graph as the measured value of FWHM, and the right side of the vertical axis is indicated by black dots as the measured value of reflectance. In addition, in Figure 3, the lower limit of the value at which the improvement in FWHM is considered meaningful compared to a conventional reflective film composed only of Mo / Si pairs is indicated by a blue dotted line. As the number of Mo / Si pairs on the upper part of the reflective film increases, the reflectivity increases and the FWHM decreases, and from the point where it exceeds 8, it deviates from the lower limit of the FWHM.

[0035] FIG. 4 is a graph showing the product of the FWHM measurement and the reflectance (R) measurement in the graph of FIG. 3. It can be seen that the value of FWHM * R is maintained above a certain level up to about 10 second pairs on the upper part of the reflective film (104), but decreases thereafter. However, considering the lower limit of FWHM as in FIG. 3, it is preferable that the number be within 8.

[0036] Figure 5 is a graph showing the measured values ​​of FWHM and reflectance as Mo / Si pairs are added one by one to the top, based on a state where 24 pairs of Ru / Si are formed at the bottom. That is, Figure 5 shows the measured values ​​as the number of second pairs of Mo / Si structures is sequentially increased on top of a state where 24 pairs of first pairs of Ru / Si structures are stacked at the bottom. It can be seen that reflectance improves as the number of second pairs increases. In addition, the FWHM is highest when composed only of first pairs, and the FWHM gradually decreases as Mo / Si is added. It can be seen that a high FWHM can be secured when the second pairs are stacked 8 times or less. From the perspective of reflectance, a reflectance of at least 68.2% as described above can be secured when the number of stacks of second pairs is 3 or more, and from the perspective of FWHM, a high FWHM can be secured when the number of stacks of second pairs is 6 or less.

[0037] FIG. 6 is a graph showing the product of the FWHM measurement and the reflectance (R) measurement in the graph of FIG. 5. It can be seen that the value of FWHM * R is maintained above a certain level up to about 8 times the number of second pairs on the upper part of the reflective film (104), but thereafter it decreases rapidly.

[0038] Figure 7 is a graph showing the measured values ​​of FWHM and reflectance as the topmost pair among the Ru / Si pairs is sequentially replaced with Mo / Si pairs one by one, based on a state where 24 Ru / Si pairs are formed. Specifically, the reflective film is composed of a total of 24 pairs, with the number of first pairs decreasing by one from the topmost pair and the number of second pairs increasing by one from zero pairs, and the FWHM and reflectance for these are measured. From the perspective of FWHM, a more desirable FWHM can be obtained when the second pair is stacked six times or less.

[0039] FIG. 8 is a graph showing the product of the FWHM measurement and the reflectance (R) measurement from the graph of FIG. 7. It can be seen that the value of FWHM * R is maintained above a certain level up to about 8 times the number of second pairs on the upper part of the reflective film (104), but thereafter it decreases rapidly.

[0041] Meanwhile, in a configuration including these Ru / Si pairs and Mo / Si pairs, yttrium (Y) and zirconium (Zr), which are metals with an absorption coefficient lower than 0.006, can be considered as materials to replace silicon (Si) constituting the Si layer within each pair. When Si is replaced with Y or Zr, there is no silicide reaction at the interface between metals, so the intermixing layer is converted from a silicide form to an alloy form, and since it is formed thinner in the alloy form than in the silicide form, the formation of the intermixing layer is significantly improved as a result.

[0042] In addition, niobium (Nb), which has an absorption coefficient (k) and refractive index (n) similar to Mo, can be considered as a material to replace Mo in Mo / Si structural pairs. (In this invention, the values ​​of n and k refer to values ​​for EUV exposure light with a wavelength of 13.5 nm.) Although it is difficult to view Nb as bringing about effects such as improved properties compared to Mo, it can be adopted at least from the same perspective as employing Mo. However, in the case of an Nb / Zr pair configured such that an Nb layer and a Zr layer form a pair, it is difficult to expect an improvement in reflectance because the difference in refractive index between Nb and Zr is small.

[0043] Considering all these factors, as constituent materials of the reflective film for implementing the present invention, the lower first pair may have one or more structures among Ru / Si, Ru / Y, and Ru / Zr, and the upper second pair may have one or more structures among Mo / Si, Mo / Y, Mo / Zr, Nb / Si, Nb / Y, and Zr / Si.

[0044] At least a portion of each layer constituting the reflective film may contain one or more additional materials selected from C, O, N, and B. It is preferable that the additional material be included in a portion adjacent to the interface of each layer.

[0045] A capping layer may be formed on the uppermost layer of the above-mentioned reflective film. By being formed of Ru, the capping layer functions to protect the reflective film from etching material during the etching process in which a thin film, such as an absorption film or a phase inversion film on the upper part of the reflective film, is patterned.

[0047] Although the present invention has been specifically described above through embodiments with reference to the drawings, the embodiments are used merely for the purpose of illustrating and explaining the invention and are not intended to limit the meaning or the scope of the invention as described in the claims. Therefore, those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible from the embodiments, and the true technical scope of protection of the invention should be determined by the technical details of the claims.

Claims

Claim 1 delete Claim 2 A blank mask for EUV lithography having a multilayer reflective film formed on a substrate, wherein the reflective film comprises a first pair comprising two layers with different refractive indices for EUV exposure light and a second pair comprising two layers with different refractive indices for EUV exposure light, wherein a structure in which the first pair is stacked multiple times is disposed in a lower region of the reflective film and a structure in which the second pair is stacked multiple times is disposed in an upper region of the reflective film, wherein the first pair shows a higher FWHM measurement value in a reflectance graph with respect to wavelength compared to the second pair, and the second pair shows a higher reflectance measurement value compared to the first pair, and the stacking count of the second pair is 3 to 6 times. Claim 3 A blank mask for EUV lithography according to claim 2, wherein the second pair has one or more structures among Mo / Si, Mo / Y, Mo / Zr, Nb / Si, Nb / Y, and Zr / Si. Claim 4 A blank mask for EUV lithography according to claim 3, wherein the first pair has one or more structures selected from Ru / Si, Ru / Y, and Ru / Zr. Claim 5 A blank mask for EUV lithography according to any one of claims 2 to 4, characterized in that at least a portion of each layer constituting the reflective film includes one or more additional materials selected from C, O, N, and B. Claim 6 A blank mask for EUV lithography according to claim 5, characterized in that the additional material is included in a region adjacent to the interface of each layer. Claim 7 A blank mask for EUV lithography according to claim 2, further comprising a capping layer formed on the uppermost layer of the reflective film. Claim 8 A photomask produced using a blank mask according to any one of claims 2 through 4 and 7.

Images