Reflection type mask

Abstract

The present disclosure provides a reflection type mask having: a substrate; a multilayered film that is positioned on one surface of the substrate; and a pattern of an absorption layer that is positioned on the opposite surface of the multilayered film from the substrate, wherein the reflection type mask has a transfer pattern region that has the pattern of the absorption layer, and a light shielding region that is positioned at the periphery of the transfer pattern region and does not have the multilayered film or the absorption layer, and in which the substrate is exposed, the reflection type mask has a relief structure on the multilayered film-side surface of the substrate in the light shielding region, the depth D (nm) of recesses in the relief structure satisfies the following expression (1), and the pitch P (nm) of the recesses satisfies the following expression (2). (1): 37.5×n–7.5≤D≤37.5×n+7.5　(In expression (1), n=1, 3, or 5.) (2): Pmin≤P≤Pmax　(In expression (2), Pmin is represented by the following expression (3), and Pmax is represented by the following expression (4).) (3): Pmin=λmin / C　(4): Pmax=λmin / (NA / m) / C　(In expression (3) and expression (4), λmin is the minimum value of an applied wavelength, C is the pattern coefficient of the recesses of the relief structure, NA is the numerical aperture of a lens of a projection optical system of an exposure device that is used for the reflection type mask, and m represents the reciprocal of the magnification of said projection optical system. The minimum value of the applied wavelength is 130 nm. )

Description

Reflective mask

[0001] This disclosure relates to a reflective mask used in extreme ultraviolet (EUV) lithography.

[0002] In recent years, EUV lithography, an exposure technique using EUV light, has been seen as promising in the semiconductor industry due to the miniaturization of semiconductor devices. Reflective masks have been proposed as masks to be used in EUV lithography. A reflective mask is, for example, a mask having a substrate, a multilayer film arranged on one side of the substrate that reflects EUV light, and an absorbing layer arranged in a pattern on the side of the multilayer film opposite the substrate that absorbs EUV light. In EUV lithography, EUV light incident on a reflective mask is absorbed by the absorbing layer and reflected by the multilayer film, and the light image resulting from the reflection in the multilayer film is transferred onto the wafer through a reflective optical system.

[0003] When transferring patterns onto a wafer using a reflective mask, an exposure method called the step-and-repeat method is employed. The step-and-repeat method involves sequentially moving the wafer (steps) and repeatedly (repeats) exposing it. The exposure equipment used for the step-and-repeat method is called a stepper.

[0004] When exposure is performed using this step-and-repeat method, the transfer pattern regions where the absorption layer patterns are placed on the reflective mask are typically transferred as close together as possible to extract as many chips as possible from the wafer. Also, in the step-and-repeat method, a slightly wider area than the transfer pattern region is generally exposed. As a result, overlapping areas occur in adjacent exposure regions on the wafer. Hereafter, these overlapping areas may be referred to as multiple exposure regions. For example, in the case of a rectangular exposure region, the corners of one exposure region overlap with the other three exposure regions, resulting in four exposures.

[0005] In areas with multiple exposures, the exposure occurs multiple times, and as a result, most of the exposure light is absorbed by the absorption layer. Even if the exposure amount in a single exposure is not substantial enough to contribute to resolution, the total exposure amount can add up to a level that contributes to resolution. Consequently, unwanted patterns are formed, leading to the problem of not being able to obtain high-precision patterns.

[0006] Furthermore, in reflective masks, exposure light is incident from a direction perpendicular to the mask surface, typically at an angle of several degrees, usually around 6 degrees. Because the absorption layer has thickness, the oblique incidence of exposure light creates a shadow of the absorption layer pattern itself. This effect is called the shadow effect. The degree of the shadow effect varies depending on the orientation of the absorption layer pattern relative to the exposure light and affects the transfer dimensions to the wafer. This shadow effect problem has become particularly pronounced with the miniaturization of patterns in recent years.

[0007] To suppress such shadow effects, a thinner absorption layer is preferable. However, reducing the thickness of the absorption layer reduces the absorption of exposure light by the absorption layer, which exacerbates the pattern defects in the multiple exposure regions mentioned above.

[0008] Therefore, in reflective masks, it has been proposed to place a light-shielding region around the outer edge of the transfer pattern region to suppress reflected light from the outer edge region located on the outer edge of the transfer pattern region, which forms the multiple exposure region. The light-shielding region is also called a light-shielding frame or light-shielding band.

[0009] Furthermore, it is known that EUV light sources emit not only light in the EUV region but also light in the ultraviolet region called out-of-band (OoB) light. The resists used in EUV lithography are sensitive to vacuum ultraviolet light with wavelengths of around 100 nm to 300 nm, and are therefore sensitive to out-of-band light in this wavelength range.

[0010] For example, in the reflective mask having the above-described light-shielding region, since the substrate surface is exposed in the light-shielding region, out-of-band light is reflected on the substrate surface, transmitted through the substrate, and reflected by the conductive film disposed on the back surface of the substrate. As described above, since the multiple exposure region is exposed multiple times, the exposure amounts are added, so there is a concern that the resist may be exposed due to the reflection of out-of-band light.

[0011] Therefore, a reflective mask that reduces the reflection of out-of-band light has been studied. For example, in Patent Documents 1 to 3, a reflective mask in which a moss-eye structure body composed of a fine uneven pattern is formed in the light-shielding region has been proposed. Also, in Patent Documents 4 to 5, a reflective mask in which an uneven structure is formed in the light-shielding region has been proposed. Further, in Patent Document 6, a reflective mask in which a phase shift structure is formed in the light-shielding region has been proposed.

[0012] Japanese Patent No. 5930132, Japanese Patent No. 5953833, Japanese Patent No. 6319368, Japanese Patent No. 6728748, Japanese Patent No. 6852281, Japanese Unexamined Patent Application Publication No. 2018-44979

[0013] Generally, the moss-eye structure body is composed of a fine uneven pattern having conical protrusions with a size of several hundred nanometers. In the reflective mask having the moss-eye structure body in the above-described light-shielding region, when forming such a fine uneven pattern, it is difficult to control the shape of the conical protrusions, particularly the shape of the inclined surface which is the side surface of the conical protrusions.

[0014] Also, in the reflective mask having an uneven structure in the above-described light-shielding region, it is said that a reduction effect of the specular reflection component can be obtained by the diffraction phenomenon of the reflected light. However, if the pitch, depth, etc. of the uneven structure are not appropriate, phase oscillation is likely to occur, and it is difficult to control the reflectance in the wavelength range of out-of-band light. Also, if the pitch of the uneven structure is large, diffracted light from the uneven structure enters the pupil plane of the exposure apparatus, and the pattern is resolved on the condensing surface, and conversely, the reflectance increases.

[0015] Also, in a reflective mask having a phase shift structure in the above-described light shielding region, it is said that a low reflection effect can be obtained by imparting a phase difference to out-of-band light. However, there is the same problem as in the reflective mask having an uneven structure in the above-described light shielding region.

[0016] In view of the above circumstances, the present disclosure has been made, and the main object thereof is to provide a reflective mask capable of reducing reflection of out-of-band light in a light shielding region and realizing highly accurate pattern transfer.

[0017] One embodiment of the present disclosure is a reflective mask having a substrate, a multilayer film disposed on one surface of the substrate, and a pattern of an absorption layer disposed on a surface of the multilayer film opposite to the substrate, the reflective mask including a transfer pattern region having a pattern of the absorption layer, and a light shielding region disposed on an outer periphery of the transfer pattern region, not having the multilayer film and the absorption layer, and exposing the substrate, the light shielding region having an uneven structure on a surface of the substrate on the multilayer film side, a depth D (nm) of a concave portion of the uneven structure satisfying the following formula (1), and a pitch P (nm) of the concave portion satisfying the following formula (2). 37.5 × n - 7.5 ≤ D ≤ 37.5 × n + 7.5 (1) (In the above formula (1), n = 1, 3, 5.) P ｍｉｎ ≤ P ≤ P ｍａｘ (2) (In the above formula (2), P ｍｉｎ is represented by the following formula (3), and P ｍａｘ is represented by the following formula (4).) P ｍｉｎ = λ ｍｉｎ / C (3) P ｍａｘ = λ [[ID=2P]] ｍｉｎ / (NA / m) / C (4) (In the above formula (3) and the above formula (4), λ ｍｉｎ is the minimum value of the applicable wavelength, C is the pattern coefficient of the concave portion of the uneven structure, NA is the numerical aperture of the lens of the projection optical system of the exposure apparatus used for the reflective mask, and m represents the reciprocal of the magnification of the projection optical system. The minimum value of the applicable wavelength is 130 nm.)

[0018] The present disclosure has an effect of being able to reduce reflection of out-of-band light in a light shielding region and realizing highly accurate pattern transfer.

[0019] These are schematic plan views and cross-sectional views showing an example of a reflective mask in this disclosure. These are schematic cross-sectional views showing an example of a textured structure in a reflective mask in this disclosure. This is a schematic plan view showing another example of a textured structure in a reflective mask in this disclosure. This is a schematic plan view showing another example of a textured structure in a reflective mask in this disclosure. This is a schematic plan view showing another example of a textured structure in a reflective mask in this disclosure. This is a schematic cross-sectional view showing another example of a textured structure in a reflective mask in this disclosure. This is a process diagram illustrating a method for manufacturing a reflective mask in this disclosure. This is a process diagram illustrating a method for manufacturing a reflective mask in this disclosure. This is a graph showing the wavelength dependence of the refractive index of a low thermal expansion glass substrate. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 1. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 1. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 1. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 1. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 2. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 3. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 3. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 4. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 5. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 5. This is a graph showing the reflectance of the reflective mask in Example 6. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 7. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 7. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 7. This is a schematic cross-sectional view showing another example of the uneven structure in the reflective mask in this disclosure. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 8. This is a graph showing the simulation results of the reflectance of the reflective mask in Example 8.

[0020] Embodiments of this disclosure will be described below with reference to drawings and other illustrations. However, this disclosure can be implemented in many different ways and should not be interpreted as being limited to the embodiments described below. In addition, drawings may schematically represent the width, thickness, shape, etc. of each part compared to the actual form in order to make the explanation clearer, but these are merely examples and should not limit the interpretation of this disclosure. Furthermore, in this specification and each drawing, elements similar to those described above in previously shown drawings will be denoted by the same reference numerals, and detailed explanations may be omitted as appropriate. Moreover, terms such as "parallel," "orthogonal," and "identical," as well as values ​​such as length and angle, which specify the shape and geometric conditions and their degree, should not be interpreted as being bound by strict meaning, but should include a range that allows for the expectation of similar function.

[0021] In this specification, when describing a configuration in which one member is placed on top of another member, unless otherwise specified, the terms "on top" or "below" include both cases: when the other member is placed directly above or below the other member so as to be in contact with it, and when the other member is placed above or below the other member via yet another member. Similarly, when describing a configuration in this specification in which one member is placed on the surface of another member, unless otherwise specified, the terms "on the surface" or "on the surface" include both cases: when the other member is placed directly above or below the other member so as to be in contact with it, and when the other member is placed above or below the other member via yet another member.

[0022] The reflective mask described in this disclosure will be explained in detail below.

[0023] The reflective mask in this disclosure comprises a substrate, a multilayer film disposed on one side of the substrate, and a pattern of an absorption layer disposed on the side of the multilayer film opposite to the substrate, wherein the reflective mask has a transfer pattern region having the pattern of the absorption layer, and a light-shielding region disposed on the outer periphery of the transfer pattern region, which does not have the multilayer film or the absorption layer, and the substrate is exposed, and the light-shielding region has an uneven structure on the multilayer film side of the substrate, the depth D (nm) of the recesses of the uneven structure satisfies the following formula (1), and the pitch P (nm) of the recesses satisfies the following formula (2): 37.5 × n - 7.5 ≤ D ≤ 37.5 × n + 7.5 (1) (wherein n = 1, 3, 5) P ｍｉｎ ≤P ≤P ｍａｘ (2) (In the above formula (2), P ｍｉｎ P is expressed by the following equation (3), ｍａｘ P is expressed by the following formula (4). ｍｉｎ = λ ｍｉｎ / C (3) P ｍａｘ = λ ｍｉｎ / (NA / m) / C (4) (In equations (3) and (4) above, λ ｍｉｎ (where is the minimum applicable wavelength, C is the pattern coefficient of the recess in the above-mentioned uneven structure, NA is the numerical aperture of the lens in the projection optical system of the exposure apparatus used in the above-mentioned reflective mask, and m is the reciprocal of the magnification of the above-mentioned projection optical system. The minimum applicable wavelength is 130 nm.)

[0024] Figure 1(a) is a schematic plan view showing an example of a reflective mask in this disclosure, and Figure 1(b) is a cross-sectional view taken along line A-A in Figure 1(a). As illustrated in Figures 1(a) and 1(b), the reflective mask 1 includes a substrate 2, a multilayer film 3 disposed on one side of the substrate 2, a protective layer 4 formed on the side of the multilayer film 3 opposite to the substrate 2, a pattern of an absorption layer 5 disposed on the side of the protective layer 4 opposite to the multilayer film 3, and a conductive film 6 disposed on the side of the substrate 2 opposite to the multilayer film 3. The reflective mask 1 also includes a transfer pattern region 11 having the pattern of the absorption layer 5, and a light-shielding region 12 disposed on the outer periphery of the transfer pattern region 11, which does not have the multilayer film 3, protective layer 4, or absorption layer 5, and exposes the substrate 2. An uneven structure 7 is disposed on the side of the substrate 2 facing the multilayer film 3 in the light-shielding region 12. The uneven structure 7 utilizes the phase effect to reduce reflectivity to out-of-band light contained in the EUV light source. In other words, the uneven structure 7 provides a phase difference to the out-of-band light contained in the EUV light source.

[0025] The wavelength range of out-of-band light is approximately 130 nm to 400 nm. Furthermore, the wavelength range in which resists used in EUV lithography are sensitive is, for example, 150 nm to 250 nm and the surrounding wavelength range, and this varies depending on the composition of the resist. In addition, the low thermal expansion glass substrate used in reflective masks absorbs light with wavelengths of 285 nm or less, so for light with wavelengths of 285 nm or less, only reflection from the substrate surface needs to be considered. On the other hand, light with wavelengths of 285 nm or more is not absorbed by the low thermal expansion glass substrate, so reflection from the conductive film placed on the back surface of the substrate becomes large, making it difficult to control the reflection by the surface irregularities of the substrate alone.Therefore, in this disclosure, we consider reducing the reflection of out-of-band light in the wavelength range of 130 nm to 285 nm.

[0026] In this disclosure, a surface with unevenness is arranged on the substrate surface of the light-shielding region, and phase interference is utilized in which the reflected light from the upper surface of the protrusions of the unevenness structure and the reflected light from the lower surface of the recesses cancel each other out due to a phase difference.

[0027] As described above, the low thermal expansion glass substrate used in the reflective mask absorbs light with a wavelength of 285 nm or less, so for light with a wavelength of 285 nm or less, only reflection from the substrate surface needs to be considered. Therefore, the condition under which the reflected light from the upper surface of the convex parts of the uneven structure and the reflected light from the lower surface of the concave parts destructively cancel each other out is D = λ / 4 × n (where n is an odd number), where D is the depth of the concave part and λ is the wavelength. Also, as described above, in this disclosure, the wavelength range of out-of-band light is limited to the wavelength range of 130 nm to 285 nm. Furthermore, for the low thermal expansion glass substrate used in the reflective mask, the surface reflectivity of the low thermal expansion glass substrate itself increases sharply on the shorter wavelength side of the wavelength range of 130 nm to 285 nm, specifically below 150 nm. Therefore, it is particularly important to suppress the reflection of out-of-band light in the wavelength range below 150 nm. Accordingly, this disclosure focuses on the wavelength of 150 nm. When the wavelength λ is 150 nm, the condition under which the reflected light from the upper surface of the convex part of the uneven structure and the reflected light from the lower surface of the concave part destructively interfere with each other is D = λ / 4 × n = 150 / 4 × n = 37.5 × n (where n is an odd number). Furthermore, as described in the embodiments below, the inventors of this application performed simulations on the effect of the depth of the concave part on the reflectance and found that when D = 37.5 × n (n = 1, 3, 5), the reflectance is approximately 3% or less in the wavelength range from 130 nm to 285 nm. In addition, as described in the embodiments below, the inventors of this application performed simulations on the effect of the depth of the concave part on the reflectance and found that a slight deviation from the theoretical value calculated by the formula D = 37.5 × n (n = 1, 3, 5) is acceptable.Therefore, in this disclosure, the depth D (nm) of the concave part is assumed to satisfy the following formula (1). 37.5 × n - 7.5 ≤ D ≤ 37.5 × n + 7.5 (1) (In equation (1) above, n = 1, 3, 5.)

[0028] When the depth D of the recess satisfies equation (1) above, the reflected light from the bottom surface of the recess in the uneven structure and the reflected light from the top surface of the protrusion are in opposite phases, and the reflection of out-of-band light can be effectively reduced by the interference of these reflected lights.

[0029] Furthermore, in photolithography technology, the resolution R of the transfer pattern is expressed by the following Rayleigh formula: R = k1 × λ / NA (In the above formula, λ is the exposure wavelength, NA is the numerical aperture of the lens, and k1 is called the k1 factor and is a constant determined by the process.) Also, the resolution R is often expressed in terms of the half-pitch of the transfer pattern. In that case, the following formula holds: R = p / 2 = k1 × λ / NA (In the above formula, p is the pitch of the transfer pattern.) The pitch p of the transfer pattern is expressed by the following formula based on the above formula: p = k1 × 2 × λ ​​ / NA Also, when the transfer magnification is 1x, the pitch p of the transfer pattern is half the pitch P of the recesses of the uneven structure in the light-shielding region of the reflective mask, and is expressed by the following formula: P = 2p When the reduction magnification is 1 / m, it is expressed by the following formula. P = 2p / m = (λ / NA) / m = λ / (NA / m) Therefore, in this disclosure, the following equation (4) was derived as the maximum pitch of the recess P (nm) such that the first diffracted light does not enter the pupil of the exposure apparatus. P ｍａｘ = λ ｍｉｎ / (NA / m) / C (4) (In the above formula (4), λ ｍｉｎ λ represents the minimum applicable wavelength, C is the pattern coefficient of the recess, NA is the numerical aperture of the lens in the projection optical system of the exposure apparatus, and m is the reciprocal of the magnification of the projection optical system of the exposure apparatus.) In this disclosure, as described above, the wavelength range of out-of-band light is the wavelength range of 130 nm to 285 nm. Therefore, in the above equation (4), the minimum applicable wavelength λ ｍｉｎ The wavelength was set to 130 nm.

[0030] Furthermore, the fundamental principle in this disclosure, as described above, is to actively utilize the phase interference between the reflected light from the upper surface of the convex parts of the uneven structure and the reflected light from the lower surface of the concave parts. However, when the pitch of the concave parts becomes narrower, the wrapping and scattering near the edges of the concave parts increase, making it difficult for phase interference due to specular reflection to occur. This is particularly noticeable when the pitch of the concave parts is equal to or less than the wavelength of out-of-band light. Moreover, if the size of the concave parts is reduced, the reflectance increases on the shorter wavelength side within the wavelength range of 130 nm to 285 nm. Therefore, in this disclosure, for the pitch P (nm) of the concave parts, the following equation (3) was derived as the minimum pitch of the concave parts in which no primary diffracted light is generated from the mask. P ｍｉｎ = λ ｍｉｎ / C (3) (In the above formula (3), λ ｍｉｎ λ represents the minimum applicable wavelength, and C represents the pattern coefficient of the recess.) In this disclosure, as described above, the wavelength range of out-of-band light is limited to the wavelength range of 130 nm to 285 nm. Therefore, in the above equation (3), the minimum applicable wavelength λ ｍｉｎ The wavelength was set to 130 nm.

[0031] Then, the pitch P of the recess satisfies the following equation (2). ｍｉｎ ≤P ≤P ｍａｘ (2) (In the above formula (2), P ｍｉｎ This is expressed by the above equation (3), P ｍａｘ This is expressed by the above formula (4).

[0032] When the pitch P of the recess satisfies equation (2) above, the pitch P of the recess is relatively large, and while primary diffracted light is generated, the primary diffracted light does not enter the pupil of the exposure apparatus, and low reflection of out-of-band light can be achieved.

[0033] Furthermore, as mentioned above, the low thermal expansion glass substrate used in reflective masks absorbs light with wavelengths of 285 nm or less. Therefore, out-of-band light generated in the exposure apparatus is absorbed by the substrate and converted into heat. A rise in the temperature of the reflective mask causes thermal expansion of the reflective mask, which reduces the transfer accuracy, so the temperature of the substrate is strictly controlled. In contrast, when the pitch P of the recesses satisfies equation (2) above, the out-of-band light is emitted as first-order diffracted light. As a result, the amount of light energy absorbed by the substrate of the reflective mask and converted into heat can be slightly reduced. This allows for a slight reduction in the energy required to cool the substrate.

[0034] Furthermore, when a reflective mask is repeatedly washed and reused, the surface of the reflective mask may be dissolved by the cleaning solution. In the case of ultrasonic cleaning, the fine shape of the reflective mask may be physically destroyed. As a result, in the case of an uneven surface, the corners of the protrusions may become rounded, or the height of the protrusions (depth of the recesses) may change. Such changes in the shape of the uneven surface may change the reflective properties. In contrast, as mentioned above, when the pitch P of the recesses satisfies equation (2) above, the pitch P of the recesses is relatively large, and the uneven surface becomes a simple uneven surface with a relatively large period. Therefore, it is presumed that changes in the reflective properties will be less likely to occur due to changes in the shape of the uneven surface caused by repeated washing.

[0035] Furthermore, in the light-shielding region, the multilayer film and absorption layer are removed, exposing the substrate, which allows for a lower reflectivity of EUV light in the light-shielding region.

[0036] Therefore, when performing step-and-repeat exposure using the reflective mask described in this disclosure, the light-shielding effect of the light-shielding region of the reflective mask can be sufficiently obtained in the multiple exposure region of the wafer, making it possible to achieve high-precision pattern transfer.

[0037] The following describes the various components of the reflective mask in this disclosure.

[0038] 1. Uneven Structure The uneven structure in this disclosure is arranged on the substrate surface of the light-shielding region and utilizes a phase effect to reduce reflectivity to out-of-band light contained in the EUV light source. In other words, the uneven structure imparts a phase difference to the out-of-band light contained in the EUV light source.

[0039] (1) Depth of the recess D In this disclosure, the depth D (nm) of the recess satisfies the following formula (1): 37.5 × n - 7.5 ≤ D ≤ 37.5 × n + 7.5 (1) In the above formula (1), n ​​= 1, 3, or 5. Among these, n = 1 or 3 is preferred, and n = 1 is more preferred. That is, it is more preferable that the depth D (nm) of the recess satisfies the following formula (1-1): 37.5 - 7.5 ≤ D ≤ 37.5 + 7.5 (1-1) The depth of the recess in the uneven structure is the distance shown by D in Figure 2.

[0040] (2) Pitch of the recess P In this disclosure, the pitch P (nm) of the recess satisfies the following formula (2). ｍｉｎ ≤P ≤P ｍａｘ (2) (In the above formula (2), P ｍｉｎ P is expressed by the following equation (3), ｍａｘ P is expressed by the following formula (4). ｍｉｎ = λ ｍｉｎ / C (3) P ｍａｘ = λ ｍｉｎ / (NA / m) / C (4) (In equations (3) and (4) above, λ ｍｉｎ (where is the minimum applicable wavelength, C is the pattern coefficient of the recess, NA is the numerical aperture of the lens in the projection optical system of the exposure device, and m is the reciprocal of the magnification of the projection optical system of the exposure device. The minimum applicable wavelength is 130 nm.)

[0041] Currently, the numerical aperture (NA) of the projection optical system lenses in commercially available EUV lithography systems is 0.33, and the magnification in the x-direction and y-direction of the projection optical system are 1 / 4x. Furthermore, EUV lithography systems with an NA of up to 0.55 have been developed, and the magnification in the x-direction and y-direction of the projection optical system are 1 / 4x and 1 / 8x, respectively. When the magnification in the x-direction and y-direction of the projection optical system are different, and the pattern shape of the recess is a line-and-space pattern, the relationship between the longitudinal direction of the line-and-space pattern and the incident direction of the exposure light should be considered, and the reciprocal of the magnification of the projection optical system, m, should be substituted into equation (4) above.

[0042] The pattern coefficient C of the recess is appropriately determined according to the pattern shape and arrangement of the recess and the numerical aperture NA of the lens of the projection optical system of the exposure apparatus. When the pattern shape of the recess is a line and space pattern, the pattern coefficient C is 1.

[0043] Furthermore, if the pattern shape of the recess is a hole pattern and the arrangement of the recess is a hexagonal lattice arrangement as shown in Figure 5(a), when the NA is 0.33, the magnification in the x direction is 1 / 4, and the magnification in the y direction is 1 / 4, the reflective mask is transferred as is by 1 / 4, so diffracted light is generated with a value of √3 / 2 × P. Therefore, the pattern coefficient C is √3 / 2.

[0044] Furthermore, if the pattern shape of the recess is a hole pattern and the arrangement of the recess is a hexagonal lattice arrangement as shown in Figure 5(a), when NA is 0.55, the magnification in the x direction is 1 / 4 times, and the magnification in the y direction is 1 / 8 times, the reflective mask is deformed and transferred as shown in Figure 5(b). Also, if we calculate the base angle θ of an isosceles triangle with height a = √3 / 4 × p and base length b = p using the "right triangle solution method", we get θ = tan - 1(a / b / 2) = 40.89°. Similarly, if we estimate the length of the normal NL from the hypotenuse and base angle of the isosceles triangle in a small right triangle between the isosceles triangle and the normal NL, we get the following equation: Length of the hypotenuse of the isosceles triangle = Length of the normal / cos(45 - 40.89) = 1.0026 × Length of the normal In other words, the length of the hypotenuse of the isosceles triangle is only 0.26% longer than the length of the normal NL. This is negligible. In other words, the diffracted light can be considered to originate at 1P and to originate at an oblique angle of √7 / 4 × P where the same phase information is aligned in a straight line. Therefore, the pattern coefficient C is set to √7 / 4.

[0045] Thus, the pattern coefficient C is appropriately determined according to the pattern shape and arrangement of the recesses and the numerical aperture NA of the lens of the projection optical system of the exposure apparatus.

[0046] If the pattern shape of the recess is a line and space pattern, P is calculated by the above formula (3). ｍｉｎ and P calculated by the above formula (4) ｍａｘ This is shown in Table 1 below.

[0047]

[0048] When the pattern shape of the recess is a hole pattern and the arrangement of the recess is a hexagonal lattice arrangement, P is calculated by formula (3) above. ｍｉｎ and P calculated by the above formula (4) ｍａｘ This is shown in Table 2 below.

[0049]

[0050] The pitch of a recess in an uneven structure refers to the distance from the center of one recess to the center of an adjacent recess, and is the distance indicated by P in Figure 2.

[0051] (3) Shape of the recess The pattern shape of the recess is not particularly limited as long as it can provide a phase difference to out-of-band light, and any shape can be used. Furthermore, the pattern shape of the recess is preferably a regular pattern. Specifically, line and space patterns and hole patterns can be used. Figure 3(a) is an example in which the recess 7a and protrusion 7b are line and space patterns. Figures 3(b) to 3(d) are examples in which the recess 7a is a hole pattern.

[0052] In line and space patterns, line shapes include straight lines, wavy lines, zigzag lines, and so on.

[0053] In a hole pattern, the plan view shape of the hole is not particularly limited and may include, for example, a rectangle, triangle, hexagon, circle, or ellipse. Furthermore, if the plan view shape of the hole is polygonal, the corners of the polygon may be rounded. The plan view shape of the hole refers to the plan view shape of the upper opening of the recess.

[0054] Furthermore, in the hole pattern, the arrangement of holes is not particularly limited, and examples include square grid arrangements, rectangular grid arrangements, staggered arrangements, and hexagonal grid arrangements. In the hole pattern, Figure 4(a) shows an example of a square grid arrangement, Figure 4(b) shows an example of a staggered arrangement, and Figure 4(c) shows an example of a hexagonal grid arrangement.

[0055] The cross-sectional shape of the recess is not particularly limited and can be rectangular, tapered, or tapered. In the cross-sectional shape of the recess, Figure 2 is an example of a rectangle, Figure 6(a) is an example of a tapered shape, and Figure 6(b) is an example of a tapered shape. In the uneven structure of an actual reflective mask, as shown in Figure 6(c), rounding may occur at the points where the top surface and the inclined surface intersect, and where the inclined surface and the bottom surface intersect, and although not shown, the top surface, inclined surface, and bottom surface may each have roughness. The boundaries where the top surface and the inclined surface intersect, and the boundaries where the inclined surface and the bottom surface intersect may be defined as the intersection points of straight lines obtained by approximating each surface with a straight line in a cross-sectional view. The area to be approximated may be defined by setting a threshold to limit the range, for example, in the vertical direction.

[0056] In a plan view, the ratio of the area of ​​the surface parallel to the surface of the substrate opposite to the multilayer film within the unit region of the uneven structure to the area of ​​the unit region of the uneven structure is appropriately set according to the cross-sectional shape of the recess.

[0057] In this specification, "unit region of uneven structure" refers to a region corresponding to a repeating unit of the uneven structure.

[0058] Furthermore, as shown in Figure 1(a), for example, the "side of the substrate opposite to the multilayer film" is usually parallel to the surface of the substrate on the multilayer film side in the transfer region. Here, the "side of the substrate on the multilayer film side in the transfer region" will be referred to as the "surface of the substrate where the multilayer film is located." Therefore, the "surface parallel to the side of the substrate opposite to the multilayer film" can be rephrased as the "surface parallel to the surface of the substrate where the multilayer film is located." Conversely, the "surface parallel to the surface of the substrate where the multilayer film is located" can be rephrased as the "surface parallel to the side of the substrate opposite to the multilayer film."

[0059] Furthermore, "the surface parallel to the surface of the substrate opposite to the multilayer film in a unit region of the uneven structure," that is, "the surface parallel to the surface on which the multilayer film of the substrate is located in a unit region of the uneven structure," includes the upper surface of the convex portion (the surface parallel to the surface on which the multilayer film of the substrate is located in the convex portion) and the lower surface of the concave portion (the surface parallel to the surface on which the multilayer film of the substrate is located in the concave portion). "In a plan view, the ratio of the area of ​​the surface parallel to the surface of the substrate opposite to the multilayer film in a unit region of the uneven structure to the area of ​​the unit region of the uneven structure" can be rephrased as "in a plan view, the ratio of the sum of the area of ​​the upper surface of the convex portion parallel to the surface of the substrate opposite to the multilayer film in a unit region of the uneven structure and the area of ​​the lower surface of the concave portion parallel to the surface of the substrate opposite to the multilayer film in the unit region of the uneven structure to the area of ​​the unit region of the uneven structure." Furthermore, the phrase "in a plan view, the ratio of the area of ​​the surface parallel to the surface of the substrate opposite to the multilayer film in the unit region of the uneven structure to the area of ​​the unit region of the uneven structure" can be rephrased as "in a plan view, the ratio of the sum of the area of ​​the upper surface of the convex portion parallel to the surface on which the multilayer film of the substrate is located and the area of ​​the lower surface of the concave portion parallel to the surface on which the multilayer film of the substrate is located to the area of ​​the unit region of the uneven structure."

[0060] In this case, the two surfaces of the unit region of the uneven structure—the top surface of the protrusion and the bottom surface of the recess—are also parallel to each other.

[0061] When the cross-sectional shape of the recess is a forward taper shape, in a plan view, the ratio of the area of ​​the surface parallel to the surface of the substrate opposite to the multilayer film in the unit region of the uneven structure to the area of ​​the unit region of the uneven structure is preferably 40% or more and less than 100%, more preferably 60% or more and less than 100%, and even more preferably 80% or more and less than 100%. In particular, when the cross-sectional shape of the recess is a forward taper shape, and in a plan view, the ratio of the area of ​​the recess at the intermediate depth to the area of ​​the unit region of the uneven structure to the area of ​​the substrate opposite to the multilayer film in the unit region of the uneven structure is preferably 60% or more and less than 100%, and more preferably 80% or more and less than 100%. Furthermore, when the cross-sectional shape of the recess is a forward taper shape, and in a plan view, the ratio of the area at the intermediate depth of the recess to the area of ​​the unit region of the uneven structure is 0.4 or more and 0.6 or less, the ratio of the area of ​​the surface parallel to the surface opposite to the multilayer film of the substrate in the unit region of the uneven structure is preferably 40% or more and less than 100%, more preferably 60% or more and less than 100%, and even more preferably 80% or more and less than 100%. Note that when the cross-sectional shape of the recess is rectangular, the ratio of the area of ​​the surface parallel to the surface opposite to the multilayer film of the substrate in the unit region of the uneven structure to the area of ​​the unit region of the uneven structure is 100%.

[0062] Furthermore, when the cross-sectional shape of the recess is an inverse tapered shape, in a plan view, the ratio of the area of ​​the surface parallel to the surface opposite to the multilayer film of the substrate in the unit region of the uneven structure to the area of ​​the unit region of the uneven structure is preferably more than 100% and 160% or less, more preferably more than 100% and 140% or less, and even more preferably more than 100% and 110% or less. In particular, when the cross-sectional shape of the recess is an inverse tapered shape, and in a plan view, the ratio of the area of ​​the recess at the intermediate depth to the area of ​​the unit region of the uneven structure to the area of ​​the unit region of the uneven structure is 0.5 or more and 0.7 or less, the ratio of the area of ​​the surface parallel to the surface opposite to the multilayer film of the substrate in the unit region of the uneven structure is preferably more than 100% and 125% or more, and more preferably more than 100% and 110% or less.

[0063] Furthermore, if the cross-sectional shape of the recess is an inverse taper shape, the ratio of the area of ​​the surface parallel to the surface opposite to the multilayer film of the substrate in the unit region of the uneven structure to the area of ​​the unit region of the uneven structure is greater than 100%. That is, if the cross-sectional shape of the recess is an inverse taper shape, in a plan view, the ratio of the sum of the area of ​​the upper surface of the convex portion parallel to the surface opposite to the multilayer film of the substrate in the unit region of the uneven structure and the area of ​​the bottom surface of the recess parallel to the surface opposite to the multilayer film of the substrate in the unit region of the uneven structure to the area of ​​the unit region of the uneven structure is greater than 100%. This is because, as shown in Figure 6(b), if the cross-sectional shape of the recess is an inverse taper shape, in a plan view, a part of the upper surface of the convex portion and a part of the bottom surface of the recess overlap each other. In a plan view, the region where a part of the upper surface of the convex portion and a part of the bottom surface of the recess overlap each other corresponds to the region of the inclined surface.

[0064] When the cross-sectional shape of the recess is a forward taper or reverse taper, the side surface of the recess 7a becomes an inclined surface, as shown in Figures 6(a) and 6(b). When the cross-sectional shape of the recess is a forward taper or reverse taper, if the ratio of the area of ​​the surface parallel to the surface opposite to the multilayer film of the substrate in the unit region of the above-mentioned uneven structure is within the above range, the ratio of parallel surfaces becomes relatively large and the ratio of inclined surfaces becomes small in the recess, so phase interference of reflected light is more likely to occur. Therefore, the reflectance of out-of-band light in the wavelength range of 130 nm to 285 nm can be effectively reduced.

[0065] Furthermore, when the cross-sectional shape of the recess is rectangular, in a plan view, the ratio of the area of ​​the recess to the area of ​​the unit region of the uneven structure is preferably greater than 0.3 and less than 0.78, more preferably between 0.325 and 0.775, and even more preferably between 0.42 and 0.575. When the above area ratio is within the above range, the ratio of the bottom surface in the recess becomes relatively large, making it easier for phase interference of reflected light to occur. Therefore, the reflectance of out-of-band light in the wavelength range of 130 nm to 285 nm can be effectively reduced.

[0066] In the case of a line-and-space pattern, the width of the recesses and the width of the protrusions preferably satisfy the above-mentioned area relationship, for example, being 120 nm or more and 220 nm or less, and also 140 nm or more and 180 nm or less.

[0067] In the case of a hole pattern, the width of the recess preferably satisfies the above-mentioned area relationship, for example, it is 150 nm or more and 300 nm or less, and may be 180 nm or more and 240 nm or less. In the case of a hole pattern, the width of the recess refers to the length of one side if the plan view shape of the recess is a square or triangle, the length of the shorter diagonal if the plan view shape of the recess is a hexagon, the length of the longer side if the plan view shape of the recess is a rectangle, the diameter if the plan view shape of the recess is a circle, and the major axis if the plan view shape of the recess is an ellipse.

[0068] As shown in Figure 6(c), the corners 7c of the bottom surface of the recess 7a may be rounded. That is, in cross-sectional view, the corners 7c of the bottom surface of the recess 7a may be rounded. When the depth D (nm) of the recess satisfies 37.5 - 7.5 ≤ D ≤ 37.5 + 7.5, it is preferable that the radius of curvature of the corners of the recess is greater than 0 nm and less than or equal to 35 nm. As described in the embodiments below, when the corners of the bottom surface of the recess are rounded, it can be considered a variation of the case where the cross-sectional shape of the recess is a forward tapered shape. If the radius of curvature of the corners of the recess is within the above range, there will be a relatively large area in the recess with an inclination angle close to the parallel plane of the bottom surface, so that phase interference of reflected light is more likely to occur. Therefore, the reflectance of out-of-band light in the wavelength range of 130 nm to 285 nm can be effectively reduced.

[0069] The method for forming the uneven structure will be described later.

[0070] 2. Light-shielding region In this disclosure, the light-shielding region is a region where the substrate is exposed and does not have a multilayer film or an absorption layer. In a reflective mask, if a protective layer or buffer layer is arranged between the multilayer film and the absorption layer, the light-shielding region will be a region that does not have a protective layer or buffer layer. A textured structure is arranged on the substrate surface of this light-shielding region.

[0071] The reflectance of the light-shielding region is not particularly limited as long as the exposure amount in the multiple exposure region of the wafer is set to an amount that does not contribute to resolution when exposure is performed using a step-and-repeat method with the reflective mask of this disclosure. Specifically, it is preferable that the reflectance of out-of-band light with wavelengths of 130 nm to 285 nm is 3.0% or less. Note that the above reflectance is the reflectance for each wavelength.

[0072] The light-shielding region may be located on the outer periphery of the transfer pattern region. The light-shielding region may be located on a part of the outer periphery of the transfer pattern region, or it may be located on the entire outer periphery of the transfer pattern region. In particular, in order to effectively suppress the occurrence of defective patterns in the multiple exposure region, it is preferable that the light-shielding region be located on the entire outer periphery of the transfer pattern region.

[0073] The shape of the light-shielding area can be any shape that allows it to be positioned around the outer edge of the transfer pattern area, but it is usually frame-shaped.

[0074] The dimensions of the light-shielding area are not particularly limited as long as they prevent the occurrence of defective patterns in the multiple exposure area, and can be appropriately adjusted based on the dimensions of the reflective mask, the dimensions of the exposure area when performing step-and-repeat exposure using the reflective mask described herein, etc.

[0075] 3. Multilayer film The multilayer film in this disclosure is arranged on one side of the substrate and reflects EUV light in EUV lithography using a reflective mask in this disclosure.

[0076] As materials for the multilayer film, those commonly used for multilayer films in reflective masks can be used. Among these, materials with extremely high reflectivity to EUV light are preferred, as they can enhance contrast when using a reflective mask. For example, a Mo / Si periodic multilayer film is typically used as a multilayer film that reflects EUV light. In addition, as multilayer films that can obtain high reflectivity in a specific wavelength range, for example, a Ru / Si periodic multilayer film, a Mo compound / Si compound periodic multilayer film, a Si / Nb periodic multilayer film, a Si / Mo / Ru periodic multilayer film, a Si / Mo / Ru periodic multilayer film, a Si / Ru / Mo / Ru periodic multilayer film can also be used.

[0077] The thickness of each layer constituting the multilayer film and the number of layers stacked vary depending on the materials used and are adjusted as appropriate. For example, as a Mo / Si periodic multilayer film, a multilayer film can be used in which 40 to 60 layers each of Mo and Si films, each with a thickness of a few nanometers, are stacked.

[0078] The thickness of the multilayer film is, for example, between 280 nm and 420 nm. Examples of methods for depositing the multilayer film include ion beam sputtering and magnetron sputtering.

[0079] 4. Absorption Layer The absorption layer in this disclosure is arranged in a pattern on the surface of the multilayer film opposite to the substrate and absorbs EUV light in EUV lithography using a reflective mask in this disclosure.

[0080] The material for the absorption layer is not particularly limited as long as it can absorb EUV light. For example, materials mainly composed of Ta, TaN, or Ta, or materials mainly composed of Cr and containing at least one component selected from N, O, or C can be used. Furthermore, TaSi, TaSiN, TaGe, TaGen, WN, TiN, etc., can also be used.

[0081] Methods for forming the absorption layer include, for example, magnetron sputtering, ion beam sputtering, CVD, and vapor deposition. Typically, photolithography and electron beam lithography are used to form the absorption layer in a patterned manner. Specifically, an absorption layer is formed on a substrate with a multilayer film, a resist layer is formed on this absorption layer, the resist layer is patterned, the absorption layer is etched using the resist pattern as a mask, and the remaining resist pattern is removed to form the absorption layer in a patterned manner. General methods can be used for photolithography and electron beam lithography.

[0082] 5. Protective Layer In this disclosure, a protective layer may be placed between the multilayer film and the absorption layer. The protective layer is provided to prevent oxidation of the multilayer film and to protect the reflective mask during cleaning. When the outermost surface of the multilayer film is a Si film or Mo film, the presence of a protective layer can suppress oxidation of the Si film or Mo film. If the Si film or Mo film is oxidized, the reflectivity of the multilayer film may decrease. When the buffer layer described later is placed on the side of the multilayer film opposite to the substrate, the protective layer and the buffer layer are usually placed in that order on the side of the multilayer film opposite to the substrate.

[0083] The material for the protective layer is not particularly limited as long as it exhibits the above-mentioned functions, and examples include Si and Ru.

[0084] Furthermore, the thickness of the protective layer is, for example, between 2 nm and 15 nm. Methods for depositing the protective layer include sputtering.

[0085] 6. Buffer Layer In this disclosure, a buffer layer may be placed between the multilayer film and the absorption layer. The buffer layer is provided to suppress damage to the underlying multilayer film. The presence of the buffer layer makes it possible to suppress damage to the underlying multilayer film when the absorption layer is pattern-etched by methods such as dry etching.

[0086] The buffer layer material should have high etching resistance, and typically, a material with different etching properties from the absorption layer, i.e., a material with a high etching selectivity ratio with the absorption layer, is used. The etching selectivity ratio of the buffer layer and the absorption layer is preferably 5 or higher, more preferably 10 or higher, and even more preferably 20 or higher. Furthermore, the buffer layer material is preferably low-stress and has excellent smoothness. In particular, the root mean square roughness Rq of the buffer layer is preferably 0.3 nm or less. The method for measuring the root mean square roughness Rq will be described later. From this viewpoint, the buffer layer material is preferably microcrystalline or amorphous. Examples of such buffer layer materials include SiO ２ Al ２ O ３ Examples include Cr and CrN.

[0087] Furthermore, the thickness of the buffer layer is, for example, between 2 nm and 25 nm.

[0088] Examples of methods for depositing the buffer layer include magnetron sputtering and ion beam sputtering. When using Cr, it is preferable to deposit Cr on the multilayer film using RF magnetron sputtering with a Cr target in an Ar gas atmosphere.

[0089] If the buffer layer is located on the side opposite to the substrate of the multilayer film, the exposed buffer layer may be peeled off after patterning the absorption layer. General buffer layer peeling methods can be used, such as dry etching.

[0090] 7. Low-reflection layer In this disclosure, a low-reflection layer may be arranged on the side opposite to the multilayer film of the absorption layer. The low-reflection layer is provided to increase the detection sensitivity during mask pattern inspection.

[0091] The material for the low-reflection layer can be any material that has low reflectivity to inspection light, such as tantalum oxide (TaO), oxynitride (TaNO), or tantalum boron oxide (TaBO). The thickness of the low-reflection layer is, for example, between 5 nm and 30 nm.

[0092] 8. Substrates The substrates used in this disclosure can be those generally used for reflective masks, and for example, glass substrates are preferably used. Glass substrates are particularly suitable as substrates for reflective masks because they provide good smoothness and flatness. Examples of glass substrate materials include quartz glass and amorphous glass with a low coefficient of thermal expansion (e.g., SiO2). ２ -TiO ２ Examples include crystallized glass with precipitated β-quartz solid solutions (such as glass systems). Metal substrates such as silicon and Fe-Ni Invar alloys can also be used.

[0093] To obtain high reflectivity and transfer accuracy for reflective masks, the root mean square roughness Rq of the substrate is preferably 0.2 nm or less. The root mean square roughness Rq is measured using an atomic force microscope in accordance with JIS B0601:2013.

[0094] Furthermore, in order to obtain high reflectivity and transfer accuracy of the reflective mask, the flatness of the substrate is preferably 100 nm or less. Flatness is a value that indicates the surface warp (amount of deformation) as shown by TIR (Total Indicator Reading). This value is the absolute value of the height difference between the highest point on the substrate surface above the focal plane and the lowest point below the focal plane, when the focal plane is defined by the least squares method based on the substrate surface. The flatness is the flatness in a 142 mm square area. Flatness is measured using an oblique incidence interferometer. For example, the "UltraFlat" manufactured by Tropel can be used as an oblique incidence interferometer.

[0095] 9. Conductive Film In this disclosure, the conductive film may be arranged on the side of the substrate opposite to the multilayer film. The conductive film is provided to attract the reflective mask in this disclosure to the electrostatic chuck of the exposure apparatus. Having such a conductive film makes it possible to easily and firmly fix the reflective mask to the exposure apparatus during exposure, thereby improving pattern transfer accuracy and manufacturing efficiency.

[0096] The material for the conductive film is not particularly limited as long as it is generally used for conductive films in reflective masks. For example, metals or metal compounds such as Cr and CrN that exhibit conductivity can be used.

[0097] Furthermore, the thickness of the conductive film is, for example, between 30 nm and 150 nm.

[0098] Methods for depositing conductive films include sputtering. Furthermore, when forming conductive films in a patterned manner, methods such as sputtering via a mask, photolithography, and electron beam lithography can be used.

[0099] 10. Applications The reflective masks in this disclosure are preferably used as reflective masks for lithography using EUV as the exposure light.

[0100] 11. Method for Manufacturing a Reflective Mask The method for manufacturing a reflective mask according to this disclosure includes, for example, a preparation step of preparing a mask blank having a substrate, a multilayer film, and an absorption layer in that order; an absorption layer patterning step of patterning the absorption layer; a light-shielding region formation step of forming a light-shielding region on the outer periphery of a transfer pattern region having the pattern of the absorption layer, in which the multilayer film and the absorption layer are removed and the substrate is exposed; and a textured structure formation step of forming a textured structure on the substrate surface of the light-shielding region.

[0101] Figures 7(a) to 7(c) and 8(a) to 8(d) are process diagrams showing an example of a method for manufacturing a reflective mask according to this disclosure. First, as shown in Figure 7(a), a mask blank 20 is prepared by sequentially laminating a multilayer film 3, a protective layer 4, and an absorption layer 5 on a substrate 2 (mask blank preparation step). Next, as shown in Figure 7(b), the absorption layer 5 is patterned to form a transfer pattern region 11 having the pattern of the absorption layer 5 (absorption layer patterning step). This yields a reflective mask intermediate 30 having a substrate 2, a multilayer film 3 formed on the substrate 2, a protective layer 4 formed on the multilayer film 3, and a pattern of the absorption layer 5 formed on the protective layer 4, and having a transfer pattern region 11 having the pattern of the absorption layer 5. Next, as shown in Figure 7(c), the absorption layer 5, protective layer 4, and multilayer film 3 on the outer periphery of the transfer pattern region 11 are removed to form a light-shielding region 12 where the substrate 2 is exposed (light-shielding region formation step). Next, as shown in Figure 8(a), a resist layer 31 is formed to cover the pattern of the absorption layer 5. Next, although not shown, an electron beam is irradiated onto the resist layer 31 in the light-shielding region 12 to draw a pattern, and then it is developed to form a pattern of the resist layer 31 in the light-shielding region 12, as shown in Figure 8(b). Next, as shown in Figure 8(c), the substrate 2 is etched using the pattern of the resist layer 31 as a mask to form a bumpy structure 7. Then, as shown in Figures 8(c) to 8(d), the pattern of the resist layer 31 is removed. In this way, a reflective mask 1 having a bumpy structure 7 in the light-shielding region 12 is obtained.

[0102] Each step in the method for manufacturing a reflective mask as described in this disclosure will be explained.

[0103] (1) Mask blank preparation process: In the mask blank preparation process, for example, a commercially available mask blank may be used, or a mask blank may be manufactured.

[0104] (2) Absorption layer patterning process The method for patterning the absorption layer has been described in the section on absorption layers above, so the explanation will be omitted here.

[0105] (3) Light-shielding region formation process The method for forming the light-shielding region is not particularly limited as long as it can partially remove the absorption layer and multilayer film, etc., and expose the substrate. Examples include photolithography and electron beam lithography. Specifically, a resist layer is formed to cover the pattern of the absorption layer, the resist layer is patterned, the absorption layer and multilayer film, etc. are etched using the resist pattern as a mask to expose the substrate, and the remaining resist pattern is removed. General methods can be used for photolithography and electron beam lithography.

[0106] (4) The process for forming an uneven structure is the same as the process for forming a phase shift structure in the method for manufacturing a reflective mask described in Japanese Patent Application Publication No. 2018-44979.

[0107] This disclosure is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that is substantially identical to the technical idea described in the claims of this disclosure and achieves similar effects is included within the technical scope of this disclosure.

[0108] [Example 1] The effect of the depth of the recesses in the uneven structure on the reflectivity was determined by simulation.

[0109] The simulation was performed using the actual reflective mask structure and the optical system of an EUV lithography apparatus as models. The reflective mask structure consisted of a 6-inch square, 0.25-inch thick synthetic quartz substrate with a low coefficient of thermal expansion, on which a multilayer film consisting of 40 pairs of Si and Mo films (4.2 nm / 2.8 nm thick) was formed. A protective layer made of Ru film (2.5 nm thick) was formed on the multilayer film, and an absorption layer made of TaN film (55 nm thick) and a low-reflection layer made of TaO film (15 nm thick) were formed in a line-and-space pattern on the protective layer. Furthermore, there was a light-shielding region where the low-reflection layer, absorption layer, protective layer, and multilayer film were removed to expose the substrate. The substrate surface of the light-shielding region had an uneven surface structure including convex and concave parts of a 1:1 line-and-space pattern or an uneven surface structure including concave parts of a hole pattern. For line-and-space patterns, the recessed structure was modeled under the following conditions: the cross-sectional shape of the recesses was rectangular, the pitch of the recesses was 360 nm, the width of the recesses was 180 nm, and the depth of the recesses was 37.5 × n (n = 1 to 8). For hole patterns, the recessed structure was modeled under the following conditions: the cross-sectional shape of the recesses was rectangular, the planar shape of the recesses was rectangular, the arrangement of the recesses was a hexagonal grid, the pitch of the recesses was 360 nm, the width of the recesses was 200 nm, 237 nm, or 268 nm, and the depth of the recesses was 37.5 × n (n = 1 to 8). The illumination conditions were a point source, an incident angle of 6°, and a wavelength range of 130 nm to 300 nm. For the refractive index of the synthetic quartz substrate, the wavelength dependence of the refractive index of the synthetic quartz substrate shown in Figure 9 was considered. Furthermore, for line-and-space patterns, simulations were performed for two cases: one where the longitudinal direction of the uneven line-and-space pattern is perpendicular to the direction of incidence of the exposure light, and another where the longitudinal direction of the uneven line-and-space pattern is parallel to the direction of incidence of the exposure light.

[0110] The simulation results for the case where the uneven structure is a line-and-space pattern are shown in Figures 10(a) to 10(d) and 11(a) to 11(d). In the figures, "horizontal" refers to the case where the longitudinal direction of the line-and-space pattern of the uneven structure is perpendicular to the direction of incidence of the exposure light, and "vertical" refers to the case where the longitudinal direction of the line-and-space pattern of the uneven structure is parallel to the direction of incidence of the exposure light. The simulation results for the case where the recesses are a hole pattern are shown in Figures 12(a) to 12(d) and 13(a) to 13(d). In the figures, Reference is a reflective mask that does not have an uneven structure in the light-shielding region. In Figures 12 and 13, r represents the area ratio of the hole pattern. The area ratio of the hole pattern is the ratio of the area of ​​the recesses within a unit region to the area of ​​the unit region of the uneven structure. When the width of the recess is 200 nm, the area ratio is 0.36; when the width of the recess is 237 nm, the area ratio is 0.5; and when the width of the recess is 268 nm, the area ratio is 0.64.

[0111] The simulation results showed that, for both line-and-space patterns and hole patterns, when the depth of the recess was (150 / 4) × n = 37.5 × n (n = 1, 3, 5), the reflectance was approximately 3% or less in the wavelength range from 130 nm to 285 nm.

[0112] Furthermore, as shown in Figure 10(a), when the depth of the recess was 37.5 nm (n=1), the reflectance decreased relative to the Reference in the entire wavelength range from 130 nm to 300 nm. In particular, the reflectance was 1% or less in the wavelength range from 130 nm to 200 nm. On the other hand, as shown in Figure 10(b), when the depth of the recess was 75 nm (n=2), the reflectance increased below 150 nm, decreased slightly at 225 nm, and then decreased to a minimum at 300 nm. This clearly shows that a phase effect is obtained. Also, when the recess was a hole pattern, as shown in Figure 12(a), in the range of area ratio (ratio of the area of ​​the recess within a unit region to the area of ​​the unit region of the uneven structure) from 0.5 (width of the recess: 237 nm) to 0.64 (width of the recess: 268 nm), the reflectance was 1% or less in the wavelength range from 130 nm to 200 nm. Furthermore, when the above-mentioned area ratio (the ratio of the area of ​​the recesses within a unit region to the area of ​​the unit region of the uneven structure) was 0.36 (width of the recess: 200 nm), the reflectance was 2.5% or less in the wavelength range of 130 nm to 200 nm.

[0113] [Example 2] The effect of the depth of the recesses in the uneven structure on the reflectivity was determined by simulation.

[0114] The simulation was performed using a model of an actual reflective mask structure and the optical system of an EUV lithography apparatus. The reflective mask structure was modeled as follows: a 6-inch square, 0.25-inch thick synthetic quartz substrate with a low coefficient of thermal expansion had a multilayer film consisting of 40 pairs of Si and Mo films (4.2 nm / 2.8 nm thick), a protective layer made of Ru film (2.5 nm thick) formed on the multilayer film, an absorption layer made of TaN film (55 nm thick) and a low-reflection layer made of TaO film (15 nm thick) formed on the protective layer in a line-and-space pattern, and a light-shielding region where the low-reflection layer, absorption layer, protective layer, and multilayer film were removed to expose the substrate, and the substrate surface of the light-shielding region had a bumpy structure including 1:1 line-and-space patterned protrusions and recesses. For the bumpy structure, the recesses were modeled under the conditions that the cross-sectional shape of the recesses was rectangular, the pitch of the recesses was 400 nm, the width of the recesses was 200 nm, and the depth of the recesses was λ / 4 (λ = 130 to 170). Furthermore, the illumination conditions were set to a point light source, an incident angle of 5.3552°, and a wavelength range of 130 nm to 300 nm. The NA of the projection optical system of the exposure apparatus was set to 0.55 (magnification 1 / 4 in the x direction and 1 / 8 in the y direction). For the refractive index of the synthetic quartz substrate, the wavelength dependence of the refractive index of the synthetic quartz substrate shown in Figure 9 was considered. In addition, simulations were performed for the case where the longitudinal direction of the line-and-space pattern of the uneven structure and the incident direction of the exposure light are perpendicular.

[0115] The simulation results are shown in Figures 14(a) and 14(b). In the figures, Reference is a reflective mask that does not have an uneven structure in the light-shielding region.

[0116] Figure 14(a) shows the wavelength dependence of reflectance. The depth of the recess was varied in 2.5 nm increments from 42.5 nm to 32.5 nm, centered around 37.5 nm. As the depth of the recess decreased, the minimum value of the reflectance shifted to shorter wavelengths. Furthermore, in the wavelength range from 130 nm to 285 nm, when the depth of the recess was in the range of 37.5 nm to 37.5 ± 5 nm, the reflectance was 2.5% or less. In particular, in the wavelength range from 130 nm to 230 nm, the reflectance was 2% or less. From Figure 14(b), it was found that the recess depth at which the reflectance is approximately 3% or less in the wavelength range from 130 nm to 285 nm is 37.5 ± 7.5 nm.

[0117] [Example 3] The effect of the shape of the recesses in the uneven structure on the reflectivity was determined by simulation.

[0118] The simulation was performed using the actual reflective mask structure and the optical system of an EUV lithography apparatus as models. The reflective mask structure consisted of a 6-inch square, 0.25-inch thick synthetic quartz substrate with a low coefficient of thermal expansion, on which a multilayer film consisting of 40 pairs of Si and Mo films (4.2 nm / 2.8 nm thick) was formed. A protective layer made of Ru film (2.5 nm thick) was formed on the multilayer film, and an absorption layer made of TaN film (55 nm thick) and a low-reflection layer made of TaO film (15 nm thick) were formed in a line-and-space pattern on the protective layer. Furthermore, there was a light-shielding region where the low-reflection layer, absorption layer, protective layer, and multilayer film were removed to expose the substrate, and the substrate surface of the light-shielding region had an uneven structure including convex and concave parts of a 1:1 line-and-space pattern. The uneven structure was modeled under the following conditions: the cross-sectional shape of the recesses was rectangular, forward tapered, or reverse tapered; the pitch of the recesses was 400 nm; the width of the upper opening of the recesses was between 0 nm and 400 nm; the width of the bottom surface of the recesses was between 0 nm and 400 nm; and the depth of the recesses was 37.5 nm (n=1). The illumination conditions were a point light source, an incident angle of 5.3552°, and a wavelength range of 130 nm to 300 nm. The NA of the projection optical system of the exposure apparatus was set to 0.55 (magnification 1 / 4 in the x direction, 1 / 8 in the y direction). For the refractive index of the synthetic quartz substrate, the wavelength dependence of the refractive index of the synthetic quartz substrate shown in Figure 9 was considered. In addition, simulations were performed for the case where the longitudinal direction of the line-and-space pattern of the uneven structure and the incident direction of the exposure light are perpendicular.

[0119] The simulation results are shown in Figures 15(a) to 15(e). In Figure 15, Area ratio is the ratio of the area at the intermediate depth of a recess to the area of ​​a unit region of the uneven structure in a plan view. In the case of a line-and-space pattern, Area ratio can also be said to be the ratio of the width at the intermediate depth of a recess to the pitch of a recess. Slope area ratio is the ratio of the area of ​​the inclined surface of a recess to the area of ​​a unit region of the uneven structure in a plan view. In the case of a line-and-space pattern, Slope area ratio can also be said to be the ratio of the width of the inclined surface of a recess to the pitch of a recess in a cross-sectional view. Slope area ratio is a positive value when the cross-sectional shape of the recess is a forward taper shape, and a negative value when the cross-sectional shape of the recess is an inverse taper shape.

[0120] From the simulation results, it was found that when the cross-sectional shape of the recess is a forward taper shape, the reflectance in the wavelength range of 130 nm to 285 nm is approximately 3% or less when the Area ratio is between 0.3 and 0.7, and the Slope area ratio is 0.4 or less. In other words, when the ratio of the area at the intermediate depth of the recess to the area of ​​the unit region of the unevenness is between 0.3 and 0.7, and the ratio of the area of ​​the plane of the unevenness structure (the surface parallel to the substrate) to the area of ​​the unit region of the unevenness structure (the sum of the area of ​​the upper surface of the convex part parallel to the substrate and the area of ​​the lower surface of the recess parallel to the substrate) is between 0.6 and less than 1, the reflectance in the wavelength range of 130 nm to 285 nm is approximately 3% or less. Furthermore, it was found that the reflectance in the wavelength range of 130 nm to 285 nm is approximately 3% or less when the Area ratio is between 0.4 and 0.6, and when the Slope area ratio is 0.6 or less. In other words, when the ratio of the area at the intermediate depth of the recess to the area of ​​the unit region of the uneven structure is between 0.4 and 0.6, and the ratio of the area of ​​the plane of the uneven structure (the surface parallel to the substrate) to the area of ​​the unit region of the uneven structure (the sum of the area of ​​the upper surface of the convex part parallel to the substrate and the area of ​​the lower surface of the recess parallel to the substrate) is between 0.4 and 1, the reflectance in the wavelength range of 130 nm to 285 nm is approximately 3% or less.

[0121] On the other hand, when the cross-sectional shape of the recess is an inverse tapered shape, it was found that the reflectance in the wavelength range of 130 nm to 285 nm is approximately 3% or less when the Area ratio is 0.5 to 0.7 and the Slope area ratio is -0.25 or greater. In other words, when the ratio of the area at the intermediate depth of the recess to the area of ​​the unit region of the uneven structure is 0.5 to 0.7, and the ratio of the area of ​​the plane of the uneven structure (the surface parallel to the substrate) to the area of ​​the unit region of the uneven structure (the sum of the area of ​​the upper surface of the convex part parallel to the substrate and the area of ​​the bottom surface of the recess parallel to the substrate) is greater than 1 and 1.25 or less, the reflectance in the wavelength range of 130 nm to 285 nm is approximately 3% or less.

[0122] Furthermore, regarding the line-and-space pattern, Figure 16(a) shows the simulation results of the reflectance at multiple wavelengths with respect to the Area Ratio when the cross-sectional shape of the recess is rectangular. As mentioned above, the Area Ratio on the horizontal axis is the ratio of the area at the intermediate depth of the recess to the area of ​​the unit region of the uneven structure. When the cross-sectional shape of the recess is rectangular, the area of ​​the recess is constant regardless of the depth, so the Area Ratio can also be said to be the ratio of the area of ​​the recess to the area of ​​the unit region of the uneven structure. In addition, from the wavelength range of 130 nm to 300 nm, 130 nm and 150 nm were selected as short wavelengths, and 270 nm and 285 nm were selected as long wavelengths, and a graph of the average reflectance at these four wavelengths is shown in Figure 16(b). Furthermore, Figure 16(b) shows a graph of the absolute value of the rate of change of reflectance with respect to the area ratio (the absolute value of the slope of the graph of the average reflectance) for the average reflectance at the four wavelengths mentioned above. Note that in Figure 16(a), the simulation results for reflectance at 285 nm and 270 nm were almost the same, so the simulation results for reflectance at 270 nm are not included.

[0123] From the simulation results shown in Figure 16(a), it was found that when the cross-sectional shape of the recess is rectangular, the reflectance in the wavelength range from 130 nm to 285 nm is approximately 3% or less when the ratio of the area of ​​the recess to the area of ​​the unit region of the uneven structure is greater than 0.3 and less than 0.78. Furthermore, from the average reflectance results at four wavelengths shown in Figure 16(b), it was found that the overall reflectance in the wavelength range from 130 nm to 285 nm is low when the ratio of the area of ​​the recess to the area of ​​the unit region of the uneven structure is between 0.5 and 0.55, and that a ratio greater than half is preferable. Furthermore, from the results of the rate of change of reflectance with respect to the area ratio shown in Figure 16(b), it was found that when the ratio of the area of ​​the recesses to the area of ​​the unit region of the uneven structure is greater than 0.5, the change in reflectance with respect to the change in the area of ​​the recesses is smaller compared to when the ratio of the area of ​​the recesses to the area of ​​the unit region of the uneven structure is less than 0.5, and the change in reflectance is even smaller when the ratio of the area of ​​the recesses to the area of ​​the unit region of the uneven structure is greater than 0.5 and less than 0.7.

[0124] [Example 4] The simulation was performed in the same manner as in Example 3, except that the structure of the reflective mask was changed from a bumpy structure including convex and concave parts of a line and space pattern to a bumpy structure including concave parts of a hole pattern. The bumpy structure was modeled under the following conditions: the cross-sectional shape of the concave parts is rectangular, the planar shape of the concave parts is rectangular, the arrangement of the concave parts is a hexagonal grid arrangement, the pitch of the concave parts is 360 nm, the width of the concave parts is 200 nm, 237 nm, or 268 nm, and the depth of the concave parts is 37.5 × n (n = 1 to 8).

[0125] Figure 17(a) shows the simulation results of the reflectance at multiple wavelengths with respect to the Area Ratio when the cross-sectional shape of the recesses is rectangular for a hexagonal lattice arrangement of holes. The Area Ratio on the horizontal axis is, as described above, the ratio of the area at the intermediate depth of the recess to the area of ​​the unit region of the uneven structure. When the cross-sectional shape of the recess is rectangular, the area of ​​the recess is constant regardless of the depth, so the Area Ratio can also be said to be the ratio of the area of ​​the recess to the area of ​​the unit region of the uneven structure.

[0126] In the case of the hexagonal lattice arrangement of holes, only data with an area ratio between 0.36 and 0.64 is shown. However, as shown in Figure 17(b), the simulation results revealed that the reflectance shows a trend that is almost the same as that of the line-and-space pattern described above.

[0127] [Example 5] The effect of the pitch of the recesses in the uneven structure on the reflectivity was determined by simulation.

[0128] The simulation was performed using the actual reflective mask structure and the optical system of an EUV lithography apparatus as models. The reflective mask structure consisted of a 6-inch square, 0.25-inch thick synthetic quartz substrate with a low coefficient of thermal expansion, on which a multilayer film consisting of 40 pairs of Si and Mo films (4.2 nm / 2.8 nm thick) was formed. A protective layer made of Ru film (2.5 nm thick) was formed on the multilayer film, and an absorption layer made of TaN film (55 nm thick) and a low-reflection layer made of TaO film (15 nm thick) were formed in a line-and-space pattern on the protective layer. Furthermore, there was a light-shielding region where the low-reflection layer, absorption layer, protective layer, and multilayer film were removed to expose the substrate, and the substrate surface of the light-shielding region had an uneven structure including convex and concave parts of a 1:1 line-and-space pattern. The uneven structure was modeled under the following conditions: the cross-sectional shape of the recesses was rectangular, the pitch of the recesses was between 330 nm and 3000 nm, the width of the recesses was half the length of the recess pitch, and the depth of the recesses was 37.5 nm (n=1). The illumination conditions were a point light source, an incident angle of 6°, and a wavelength range of 130 nm to 300 nm. The NA of the projection optical system of the exposure apparatus was set to 0.55 (magnification 1 / 4x in the x direction, 1 / 8x in the y direction) or 0.33 (magnification 1 / 4x). For the refractive index of the synthetic quartz substrate, the wavelength dependence of the refractive index of the synthetic quartz substrate shown in Figure 9 was considered. Simulations were also performed for the case where the longitudinal direction of the line and space pattern of the uneven structure is perpendicular to the incident direction of the exposure light, and for the case where the longitudinal direction of the line and space pattern of the uneven structure is parallel to the incident direction of the exposure light.

[0129] The simulation results are shown in Figures 18 and 19. In the figures, the case where the longitudinal direction of the line-and-space pattern of the uneven structure is perpendicular to the direction of incidence of the exposure light is indicated as "horizontal," and the case where the longitudinal direction of the line-and-space pattern of the uneven structure is parallel to the direction of incidence of the exposure light is indicated as "vertical."

[0130] The simulation results showed that the wavelength at which reflectivity is high and the pitch of the recesses change depending on the NA of the projection optical system of the exposure apparatus. The wavelength at which reflectivity is high was consistent with the relationship between wavelength λ and recess pitch P calculated from the value of equation (4). Furthermore, the high reflectivity is due to diffracted light being incident on the pupil of the exposure apparatus. In addition, the maximum reflectivity increases as the pattern is resolved on the wafer, becoming higher than the reflectivity when no uneven structure is formed. Also, when NA = 0.55, it was found that the wavelength at which diffracted light interferes and the pitch of the recesses change depending on the difference in magnification in the x-direction and the y-direction. In Figures 18(a) and 18(b), λ = NA × pitch = 0.33 × (2000 / 4) = 165 (nm) and λ = NA × pitch = 0.33 × (3000 / 4) = 247.5 (nm). In Figure 19(a), λ = NA × pitch = 0.55 × (2000 / 8) = 137.5 (nm) and λ = NA × pitch = 0.55 × (3000 / 8) = 206.25 (nm). In Figure 19(b), λ = NA × pitch = 0.55 × (1000 / 4) = 137.5 (nm) and λ = NA × pitch = 0.55 × (2000 / 4) = 275 (nm).

[0131] [Example 6] A reflective mask was actually fabricated. An optically polished synthetic quartz substrate with a size of 6 inches square and a thickness of 0.25 inches and a low coefficient of thermal expansion was used. On one side of the synthetic quartz substrate, a 4.2 nm Si film was deposited using a Si target by ion beam sputtering, followed by a 2.8 nm Mo film deposited using a Mo target. This was stacked for 40 periods to form a multilayer film of Mo and Si. Then, a 2.5 nm Ru film was deposited on the outermost Mo film of the multilayer film to form a protective layer. Next, on the Ru film, a 55 nm thick TaN film was formed as an absorption layer using a Ta target by DC magnetron sputtering in a mixed gas atmosphere of Ar and nitrogen. Then, on the TaN film, a 15 nm thick TaO film was formed as a low-reflection layer using a Ta target by DC magnetron sputtering in a mixed gas atmosphere of Ar and oxygen. Furthermore, a conductive film was formed on the other side of the synthetic quartz substrate by DC magnetron sputtering using a Cr target under a mixed gas atmosphere of Ar and nitrogen, with a CrN film thickness of 30 nm. This resulted in obtaining a mask blank.

[0132] Next, an electron beam resist (FEP171, manufactured by Fujifilm Electronic Materials Corporation) was applied to the TaO film constituting the low-reflection layer described above, and a resist pattern for forming a transfer pattern was formed using an electron beam lithography apparatus. Then, the TaO film exposed from the openings of the resist pattern was CF ４ Dry etching with gas, and then the TaN film is removed with Cl ２ The Ru film constituting the protective layer was exposed by dry etching with gas. Afterward, the resist pattern was removed.

[0133] Next, an i-line resist (THMR-iP3500, manufactured by Tokyo Ohka Kogyo Co., Ltd.) was applied to the entire surface of the TaO film constituting the low-reflection layer, and a resist pattern was formed to remove a portion of the reflective layer as a light-shielding region by drawing using a laser writing device (ALTA-3000, manufactured by Applied Materials, Inc.). Next, the TaO film exposed from the opening of the resist pattern was treated with CF ４ Dry etching with gas, then the TaN film is removed with Cl ２Dry etching with gas, followed by etching of the Ru film and multilayer film using CF ４ The surface of the synthetic quartz substrate was exposed by dry etching with gas, forming a light-shielding region with a width of 5 mm.

[0134] Next, after removing the resist used to form the light-shielding region, an electron beam resist (FEP171, manufactured by Fujifilm Electronic Materials Corporation) was applied to a thickness of 150 nm, and an electron beam lithography system was used to form a resist pattern in the light-shielding region that would create an uneven structure including recesses of the hole pattern. Subsequently, CF ４ and CHF ３ Dry etching using a mixed gas was used to etch the substrate surface in the light-shielding region, forming a textured structure on the substrate that included recesses for the hole pattern. For the textured structure, the cross-sectional shape of the recesses was rectangular, the planar shape of the recesses was square, the side length of the square was 240 nm, the arrangement of the recesses was a hexagonal lattice, and the pitch of the recesses was 400 nm. Measurements using an atomic force microscope (AFM) showed that the depth of the recesses was approximately 37 nm.

[0135] The reflectivity of the obtained reflective masks was measured using synchrotron radiation. The results are shown in Figure 20. In the figure, Reference is a reflective mask that does not have a textured structure in the light-shielding region.

[0136] [Example 7] The effect of changes in the shape of the recesses in the uneven structure on the reflectivity was determined by simulation.

[0137] When reflective masks are repeatedly washed and reused, the surface of the reflective mask may dissolve due to the cleaning solution. In the case of ultrasonic cleaning, the fine structure of the reflective mask may be physically destroyed. Therefore, if the cross-sectional shape of the recess is rectangular, the corners of the protrusions may become rounded, or the height of the protrusions (depth of the recesses) may change. Such changes in the shape of the uneven structure can alter the reflective properties, potentially resulting in a reflectance that exceeds the design value.

[0138] In the simulation, the height of the convex portion (depth of the concave portion) was assumed to remain relatively constant, and the modeling was performed under the condition that the cross-sectional shape of the concave portion changes from rectangular to a forward tapered shape as the corners of the convex portion are rounded off. In this case, the height of the convex portion (depth of the concave portion) and the width of the bottom surface of the concave portion were kept constant, and the conditions were set such that the Area Ratio (ratio of the area at the intermediate depth of the concave portion to the area of ​​the unit region of the convex-convex structure in a plan view) increases, and the Slope Area Ratio (ratio of the area of ​​the inclined surface of the concave portion to the area of ​​the unit region of the convex-convex structure in a plan view) also increases. The specific conditions are shown below.

[0139] The simulation was performed using the actual reflective mask structure and the optical system of an EUV lithography apparatus as models. The reflective mask structure consisted of a 6-inch square, 0.25-inch thick synthetic quartz substrate with a low coefficient of thermal expansion, on which a multilayer film consisting of 40 pairs of Si and Mo films (4.2 nm / 2.8 nm thick) was formed. A protective layer made of Ru film (2.5 nm thick) was formed on the multilayer film, and an absorption layer made of TaN film (55 nm thick) and a low-reflection layer made of TaO film (15 nm thick) were formed in a line-and-space pattern on the protective layer. Furthermore, there was a light-shielding region where the low-reflection layer, absorption layer, protective layer, and multilayer film were removed to expose the substrate, and the substrate surface of the light-shielding region had an uneven structure including convex and concave parts of a 1:1 line-and-space pattern. The uneven structure was modeled under the following conditions: the cross-sectional shape of the recesses was rectangular or a forward tapered shape, the pitch of the recesses was 400 nm, the width of the upper opening of the recesses was between 0 nm and 400 nm, the width of the bottom surface of the recesses was 200 nm, 160 nm, 120 nm, and 240 nm, and the depth of the recesses was 37.5 nm (n=1). The illumination conditions were a point light source, an incident angle of 5.3552°, and a wavelength range of 130 nm to 300 nm. The NA of the projection optical system of the exposure apparatus was set to 0.55 (magnification 1 / 4 in the x direction, 1 / 8 in the y direction). For the refractive index of the synthetic quartz substrate, the wavelength dependence of the refractive index of the synthetic quartz substrate shown in Figure 9 was considered. In addition, simulations were performed for the case where the longitudinal direction of the line-and-space pattern of the uneven structure and the incident direction of the exposure light are perpendicular.

[0140] The simulation results are shown in Figures 21 to 24. Figure 21 shows the simulation results when the width of the bottom surface of the recess is 200 nm, Figure 22 shows the simulation results when the width of the bottom surface of the recess is 160 nm, Figure 23 shows the simulation results when the width of the bottom surface of the recess is 120 nm, and Figure 24 shows the simulation results when the width of the bottom surface of the recess is 240 nm.

[0141] Figure 21 shows that when the cross-sectional shape of the recess is a rectangle with an area ratio of 0.5, the cross-sectional shape of the recess changes from a rectangle to a tapered shape, and the angle of the inclined surface of the recess increases, that is, as the slope area ratio increases, the reflectance on the short wavelength side decreases once, and then the overall reflectance increases, with the reflectance becoming 3% or less in the wavelength range from 130 nm to 285 nm.

[0142] Furthermore, Figure 22 shows that when the cross-sectional shape of the recess is a rectangle with an area ratio of 0.4, the cross-sectional shape of the recess changes from a rectangle to a forward tapered shape, and the angle of the inclined surface of the recess increases. In other words, as the slope area ratio increases, the reflectance on the short wavelength side decreases once, and then the overall reflectance increases, resulting in a reflectance of 2.7% or less in the wavelength range from 130 nm to 285 nm.

[0143] Furthermore, Figure 23 shows that when the cross-sectional shape of the recess is rectangular with an area ratio of 0.3, the reflectance is about 3% at short wavelengths. However, as the cross-sectional shape of the recess changes from rectangular to a tapered shape, and the angle of the inclined surface of the recess increases, that is, as the slope area ratio increases, the reflectance decreases, especially at short wavelengths. Subsequently, the overall reflectance increases, and the reflectance becomes 2.6% or less in the wavelength range from 130 nm to 285 nm.

[0144] Furthermore, Figure 24 shows that when the cross-sectional shape of the recess is rectangular with an area ratio of 0.6, the cross-sectional shape of the recess changes from rectangular to a tapered shape, and the angle of the inclined surface of the recess increases, that is, as the slope area ratio increases, the overall reflectivity increases in the wavelength range from 130 nm to 285 nm. Subsequently, when the upper opening of the recess becomes 400 nm and the uneven structure has no upper surface of the convex portion parallel to the substrate, the reflectivity exceeds 3%.

[0145] When considering a change where the area of ​​the upper opening of the recess increases, it was found that if the area ratio of the recess to the convex portion in a plan view is less than 1, the reflectance first decreases as the area of ​​the upper opening of the recess increases, and then begins to increase when the area of ​​the upper opening of the recess increases further. When the area ratio of the recess to the convex portion in a plan view is greater than 1, the area of ​​the upper surface of the convex portion parallel to the substrate is small. As the upper opening of the recess increases and the uneven structure approaches a shape where there is no upper surface of the convex portion parallel to the substrate, the reflectance increases in all wavelength ranges from 130 nm to 300 nm, and the reflectance exceeds 3%. From these results, it was found that when considering repeated cleaning of a reflective mask, if the cross-sectional shape of the recess is rectangular, the characteristic change in reflectance is less likely to occur when the corners of the convex portion are rounded and the cross-sectional shape of the recess changes from rectangular to a tapered shape.

[0146] [Example 8] The effect of changes in the rounded shape of the bottom surface of the recesses in the uneven structure on the reflectivity was determined by simulation.

[0147] The simulation was performed using the actual reflective mask structure and the optical system of an EUV lithography apparatus as models. The reflective mask structure consisted of a 6-inch square, 0.25-inch thick synthetic quartz substrate with a low coefficient of thermal expansion, on which a multilayer film consisting of 40 pairs of Si and Mo films (4.2 nm / 2.8 nm thick) was formed. A protective layer made of Ru film (2.5 nm thick) was formed on the multilayer film, and an absorption layer made of TaN film (55 nm thick) and a low-reflection layer made of TaO film (15 nm thick) were formed in a line-and-space pattern on the protective layer. Furthermore, there was a light-shielding region where the low-reflection layer, absorption layer, protective layer, and multilayer film were removed to expose the substrate, and the substrate surface of the light-shielding region had an uneven structure including convex and concave parts of a 1:1 line-and-space pattern. Regarding the uneven structure, the model was created under two conditions: the upper cross-sectional shape of the recess is rectangular, the recess pitch is 400 nm, the recess width is 200 nm, and the recess depth is 37.5 nm (n=1); and the recess pitch is 200 nm, the recess width is 100 nm, and the recess depth is 37.5 nm (n=1). In this case, as shown in Figure 25, the model was created under conditions where the radius of curvature R1 of the rounded corner 7c of the bottom surface of the recess 7a was 0 nm, 5 nm, 15 nm, 25 nm, and 35 nm. The illumination conditions were a point light source, an incident angle of 5.3552°, and a wavelength range of 130 nm to 300 nm. The NA of the projection optical system of the exposure apparatus was set to 0.55 (magnification 1 / 4 in the x direction, 1 / 8 in the y direction). For the refractive index of the synthetic quartz substrate, the wavelength dependence of the refractive index of the synthetic quartz substrate shown in Figure 9 was considered. The simulation results are shown in Figures 26(a), 26(b), and 27.

[0148] Figure 26(a) shows the wavelength dependence of reflectance when the pitch of the recesses is 400 nm and the width of the recesses is 200 nm. There is almost no difference between the case where the radius of curvature R1 is 0 nm and the case where the radius of curvature R1 is 5 nm, so the case where the radius of curvature R1 is 0 nm is omitted. It was found that as the radius of curvature R1 increases, the reflectance on the shorter wavelength side decreases from 150 nm, and the reflectance on the longer wavelength side increases from 150 nm. Figure 26(b) shows the reflectance at wavelengths of 130 nm, 210 nm, and 285 nm when the radius of curvature is changed from 0 nm to 35 nm. As mentioned above, there is almost no difference between the case where the radius of curvature is 0 nm and the case where the radius of curvature is 5 nm, but it was found that when the radius of curvature changes from 5 nm to 35 nm, the reflectance changes by about 0.3% depending on the wavelength. From this, it was found that the wavelength dependence of reflectance can be adjusted by the rounded shape of the corners of the bottom surface of the recesses. This is thought to correspond to the depth of a part of the recess in the uneven structure being shallower. This can be considered a modified example in which a part of the recess is curved, relating to the effect of the depth of the recess in the uneven structure on reflectivity, as shown in Example 2 above. On the other hand, this can also be considered a modified example in which a part of the forward tapered shape of the recess is curved, relating to the effect of the shape of the recess in the uneven structure on reflectivity, as shown in Example 3 above.

[0149] Figure 27 shows the wavelength dependence of reflectance when the pitch of the recesses is 200 nm and the width of the recesses is 100 nm. As the area of ​​the plane of the recesses within a unit region decreases relative to the area of ​​the unit region of the uneven structure, the overall reflectance increases. However, it was found that the trend of the reflectance decreasing on the shorter wavelength side from 150 nm and increasing on the longer wavelength side from 150 nm remains the same.

[0150] This disclosure provides the following invention: [1] A reflective mask having a substrate, a multilayer film disposed on one side of the substrate, and a pattern of an absorption layer disposed on the side of the multilayer film opposite to the substrate, the reflective mask having a transfer pattern region having the pattern of the absorption layer, and a light-shielding region disposed on the outer periphery of the transfer pattern region, which does not have the multilayer film or the absorption layer, and where the substrate is exposed, the light-shielding region having an uneven structure on the multilayer film side of the substrate, the depth D (nm) of the recesses of the uneven structure satisfying the following formula (1), and the pitch P (nm) of the recesses satisfying the following formula (2): 37.5 × n - 7.5 ≤ D ≤ 37.5 × n + 7.5 (1) (wherein of formula (1), n ​​= 1, 3, 5.) P ｍｉｎ ≤P ≤P ｍａｘ (2) (In the above formula (2), P ｍｉｎ P is expressed by the following equation (3), ｍａｘ P is expressed by the following formula (4). ｍｉｎ = λ ｍｉｎ / C (3) P ｍａｘ = λ ｍｉｎ / (NA / m) / C (4) (In equations (3) and (4) above, λ ｍｉｎ∫ is the minimum applicable wavelength, C is the pattern coefficient of the recess in the above-mentioned uneven structure, NA is the numerical aperture of the lens of the projection optical system of the exposure apparatus used in the above-mentioned reflective mask, and m is the reciprocal of the magnification of the projection optical system. The minimum applicable wavelength is 130 nm.) [2] The reflective mask according to [1], wherein the cross-sectional shape of the recess is a forward taper shape, in a plan view, the ratio of the area at the intermediate depth position of the recess within the unit region to the area of ​​the unit region of the uneven structure is 0.3 or more and 0.7 or less, and in a plan view, the ratio of the sum of the area of ​​the upper surface of the protrusion parallel to the surface of the substrate opposite to the multilayer film in the unit region of the uneven structure to the area of ​​the bottom surface of the recess parallel to the surface of the substrate opposite to the multilayer film in the unit region of the uneven structure is 60% or more and less than 100%. [3] The reflective mask according to [1], wherein the cross-sectional shape of the recess is an inverse tapered shape, and in a plan view, the ratio of the area at the intermediate depth of the recess within the unit region to the area of ​​the unit region of the uneven structure is 0.5 or more and 0.7 or less, and in a plan view, the ratio of the sum of the area of ​​the upper surface of the protrusion parallel to the surface of the substrate opposite to the multilayer film within the unit region of the uneven structure to the area of ​​the bottom surface of the recess parallel to the surface of the substrate opposite to the multilayer film within the unit region of the uneven structure is greater than 100% and 125% or less. [4] The reflective mask according to [1], wherein the cross-sectional shape of the recess is rectangular, and in a plan view, the ratio of the area of ​​the recess within the unit region to the area of ​​the unit region of the uneven structure is greater than 0.3 and less than 0.78. [5] The reflective mask according to any one of [1] to [4], wherein the depth D (nm) of the recess of the above uneven structure satisfies the following formula (1-1): 37.5 - 7.5 ≤ D ≤ 37.5 + 7.5 (1-1) In a cross-sectional view, the corners of the bottom surface of the recess are rounded, and the radius of curvature of the corners of the recess is greater than 0 nm and less than or equal to 35 nm. [6] The reflective mask according to any one of [1] to [5], wherein the substrate has a conductive film on the surface opposite to the multilayer film.

[0151] 1...Reflective mask 2...Substrate 3...Multilayer film 4...Protective layer 5...Absorbing layer 6...Conductive film 7...Rubber structure 7a...Recess 7b...Protrusion 11...Transfer pattern area 12...Light-shielding area

Claims

1. A reflective mask having a substrate, a multilayer film disposed on one surface of the substrate, and a pattern of an absorption layer disposed on the surface of the multilayer film opposite to the substrate, the reflective mask having a transfer pattern region having the pattern of the absorption layer and a light-shielding region disposed on the outer periphery of the transfer pattern region, having no multilayer film and absorption layer, and exposing the substrate, the surface of the substrate on the multilayer film side of the light-shielding region having an uneven structure, the depth D (nm) of the concave portion of the uneven structure satisfying the following formula (1), and the pitch P (nm) of the concave portion satisfying the following formula (2). A reflective mask. 37.5×n−7.5≦D≦37.5×n+7.5 (1) (In the above formula (1), n = 1, 3, 5.) P ｍｉｎ ≦P≦P ｍａｘ (2) (In the above formula (2), P ｍｉｎ is represented by the following formula (3), and P ｍａｘ is represented by the following formula (4).) P ｍｉｎ = λ ｍｉｎ / C (3) P ｍａｘ = λ ｍｉｎ / (NA / m) / C (4) (In the above formula (3) and the above formula (4), λ ｍｉｎ is the minimum value of the applicable wavelength, C is the pattern coefficient of the concave portion of the uneven structure, NA is the numerical aperture of the lens of the projection optical system of the exposure apparatus used for the reflective mask, and m represents the reciprocal of the magnification of the projection optical system. The minimum value of the applicable wavelength is 130 nm.) 2. The reflective mask according to claim 1, wherein the cross-sectional shape of the recess is a forward tapered shape, in a plan view, the ratio of the area of ​​the intermediate depth position of the recess within the unit region to the area of ​​the unit region of the uneven structure is 0.3 or more and 0.7 or less, and in a plan view, the ratio of the sum of the area of ​​the upper surface of the protrusion parallel to the surface of the substrate opposite to the multilayer film within the unit region of the uneven structure to the area of ​​the bottom surface of the recess parallel to the surface of the substrate opposite to the multilayer film within the unit region of the uneven structure is 60% or more and less than 100%.

3. The reflective mask according to claim 1, wherein the cross-sectional shape of the recess is an inverse tapered shape, in a plan view, the ratio of the area at the intermediate depth of the recess within the unit region to the area of ​​the unit region of the uneven structure is 0.5 or more and 0.7 or less, and in a plan view, the ratio of the sum of the area of ​​the upper surface of the protrusion parallel to the surface of the substrate opposite to the multilayer film within the unit region of the uneven structure to the area of ​​the bottom surface of the recess parallel to the surface of the substrate opposite to the multilayer film within the unit region of the uneven structure is more than 100% and 125% or less.

4. The reflective mask according to claim 1, wherein the cross-sectional shape of the recess is rectangular, and in a plan view, the ratio of the area of ​​the recess within the unit region to the area of ​​the unit region of the uneven structure is greater than 0.3 and less than 0.

78.

5. The reflective mask according to claim 1, wherein the depth D (nm) of the recess of the uneven structure satisfies the following formula (1-1): 37.5 - 7.5 ≤ D ≤ 37.5 + 7.5 (1-1) In a cross-sectional view, the corner of the bottom surface of the recess is rounded, and the radius of curvature of the corner of the recess is greater than 0 nm and less than or equal to 35 nm.

6. A reflective mask according to any one of claims 1 to 5, wherein the substrate has a conductive film on the side opposite to the multilayer film.

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