CN117891123A

CN117891123A - Phase shift mask blank and photomask for EUV lithography

Info

Publication number: CN117891123A
Application number: CN202310202890.3A
Authority: CN
Inventors: 金用大; 李锺华; 梁澈圭
Original assignee: S&S Tech Co Ltd
Current assignee: S&S Tech Co Ltd
Priority date: 2022-10-13
Filing date: 2023-03-06
Publication date: 2024-04-16

Abstract

A photomask for EUV lithography is disclosed, comprising a reflective film, a cap film and a phase shift film sequentially formed on a substrate. The phase shift film includes a first layer containing niobium (Nb) and chromium (Cr) and a second layer containing tantalum (Ta) and silicon (Si). In the first layer, the content of niobium (Nb) is in the range of 20 to 50 at%, and the content of chromium (Cr) is in the range of 10 to 40 at%. The blank mask may implement excellent resolution and NILS, and implement low DtC.

Description

Phase shift mask blank and photomask for EUV lithography

Technical Field

The present disclosure relates to a photomask and a photomask, and more particularly, to a phase shifting photomask including a phase shifting film for shifting phase with respect to Extreme Ultraviolet (EUV) light to implement high resolution during wafer printing; and a photomask for EUV lithography prepared using the same.

Background

Photolithography for semiconductors has recently evolved from ArF, arFi Multiple (MP) lithography to Extreme Ultraviolet (EUV) lithography. A photomask blank for EUV lithography typically includes two thin films on a substrate, such as a reflective film for reflecting EUV light and an absorbing film for absorbing EUV light.

Recently, attempts have been made to develop a phase shift photomask capable of implementing a higher resolution than such a binary photomask having an absorbing film. The phase shift mask blanks have a higher normalized image log slope (normalized image log slope; NILS) than the binary mask blanks and thus reduce random defects due to shot noise effects during wafer printing. In addition, the phase shift mask blanks implement low clean (DtC), thereby increasing semiconductor throughput.

Fig. 1 is a view showing the basic structure of a phase shift photomask blank for EUV lithography. The phase shift mask blank for EUV lithography includes a substrate 102, a reflective film 104 formed on the substrate 102, a cap film 105 formed on the reflective film 104, a phase shift film 108 formed on the cap film 105, and a resist film 110 formed on the phase shift film 108.

In a phase shift photomask for EUV lithography, preferably, the phase shift film 108 is made of a material that is easy to prepare a photomask and has good performance during wafer printing. In this regard, ruthenium (Ru) has been studied as a material of the phase shift film 108, but the production stage has not been reached due to the following problems.

First, ruthenium (Ru) has a slow etch rate and thus it is difficult to implement a vertical pattern profile when the phase shift film 108 is etched.

Second, the cap film 105 under the phase shift film 108 is generally made of ruthenium (Ru), and it is difficult to ensure etching selectivity to the cap film 105 when the phase shift film 108 contains ruthenium (Ru) like the cap film 105. Therefore, when ruthenium (Ru) is used for both the cap film 105 and the phase shift film 108, an etch stop film is additionally required between the phase shift film 108 and the cap film 105, thereby causing problems of increasing complexity of film design, adding a process of forming the etch stop film, and cleaning and defect control of additional films. Furthermore, even for photomasks prepared using this photomask blank, cleaning and similar additional procedures are required. Such problems ultimately serve as a cause of reduced yield.

Third, ruthenium (Ru) has high surface reflectivity for Deep Ultraviolet (DUV) detection light having a wavelength of 193nm, and thus causes a problem of decreasing detection sensitivity during detection using DUV detection light. To solve this problem, oxygen (O) may be additionally used together with ruthenium (Ru). However, when oxygen (O) and ruthenium (Ru) are used together, a problem arises in that the refractive index (n) is increased. In addition, oxygen (O) oxidizes the cap film 105 made of ruthenium (Ru), thereby reducing the reflectivity of the stacked structure of the reflective film 104 and the cap film 105.

Fourth, ruthenium (Ru) is responsible for XeF during electron beam repair ₂ Has a slow repair speed, and thus causes a problem of bad patterns immediately after repair.

Disclosure of Invention

An aspect of the present disclosure is to provide a blank mask for Extreme Ultraviolet (EUV) that can solve the problems caused by materials conventionally used as a phase shift film, particularly ruthenium (Ru), while conforming to the characteristics required for the phase shift film.

According to an embodiment of the present disclosure, there is provided a blank mask for Extreme Ultraviolet (EUV) lithography including a substrate, a reflective film formed on the substrate, a cap film formed on the reflective film, and a phase shift film formed on the cap film, the phase shift film including: a first layer formed on the cap film and containing niobium (Nb) and a second layer formed on the first layer and containing one or more of tantalum (Ta) and silicon (Si).

The first layer may contain one or more of tantalum (Ta) and silicon (Si).

The first layer may further contain chromium (Cr).

The content of chromium (Cr) in the first layer may be in the range of 10 to 40 at%.

The first layer may further contain one or more of oxygen (O), nitrogen (N), and carbon (C).

The content of nitrogen (N) in the first layer may be in the range of 10 to 60 atomic%.

The first layer may have a thickness of 30 to 60 nm.

The first layer may have a thickness of 80% or more of the total thickness of the phase shift film.

The first layer may have a refractive index (n) of 0.925 to 0.935 and an extinction coefficient (k) of 0.015 to 0.025 with respect to exposure light having a wavelength of 13.5 nm.

The content of tantalum (Ta) in the second layer may be greater than or equal to 50 atomic%.

The second layer may contain one or more of oxygen (O), nitrogen (N), and carbon (C).

The second layer may further contain boron (B).

The content of boron (B) in the second layer may be in the range of 5 to 20 at%.

The second layer may have a thickness of 2 to 10 nm.

The second layer may have a surface reflectance of 40% or less for detection light having a wavelength of 193 nm.

The second layer may have a refractive index (n) of 0.940 to 0.960 and an extinction coefficient (k) of 0.025 to 0.035 with respect to exposure light having a wavelength of 13.5 nm.

The phase shift film may have a relative reflectivity of 6 to 15% with respect to the reflective film with respect to the exposure light having a wavelength of 13.5 nm.

According to another embodiment of the present disclosure, there is provided a photomask for Extreme Ultraviolet (EUV) lithography prepared using the foregoing photomask blank.

Drawings

The foregoing and/or various aspects will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram showing the basic structure of a conventional phase-shifting blank mask for Extreme Ultraviolet (EUV) lithography;

FIG. 2 is a diagram showing a phase-shift photomask blank for Extreme Ultraviolet (EUV) lithography according to the present disclosure; and is also provided with

Fig. 3 is a view showing a detailed configuration of the phase shift film of fig. 2.

Detailed Description

Hereinafter, the present disclosure will be described in more detail with reference to the accompanying drawings.

Fig. 2 is a view showing a phase shift blank mask for Extreme Ultraviolet (EUV) lithography according to the present disclosure, and fig. 3 is a view showing a detailed configuration of the phase shift film of fig. 2.

A phase shift mask blank for EUV lithography according to the present disclosure includes a substrate 202, a reflective film 204 formed on the substrate 202, a cap film 205 formed on the reflective film 204, a phase shift film 208 formed on the cap film 205, a resist film 210 formed on the phase shift film 208, and a conductive film 201 formed on a rear surface of the substrate 202.

The substrate 202 is configured as a low thermal expansion material (low thermal expansion material; LTEM) substrate having a low coefficient of thermal expansion in the range of 0±1.0x10-7/°c, preferably 0±0.3x10-7/°c to prevent deformation and stress of the pattern due to heat during exposure so as to be suitable for a glass substrate for a reflective photomask using EUV exposure light. As a material of the substrate 202, siO2-TiO 2-based glass, multicomponent glass ceramic, or the like can be used.

The reflective film 204 has a multilayer structure in which the refractive indices of the respective layers are different, and is used to reflect EUV exposure light. In particular, the reflective film 204 is formed by alternately stacking Mo layers and Si layers from 40 layers to 60 layers.

The cap film 205 is used to prevent an oxide film from being formed on the reflective film 204 so as to maintain the reflectivity of the reflective film 204 to EUV exposure light, and to prevent the reflective film 204 from being etched when the phase shift film 208 is patterned. In general, the cap film 205 is made of a ruthenium (Ru) -containing material. The cap film 205 is formed to have a thickness of 2 to 5 nm. When the thickness of the cap film 205 is less than or equal to 2nm, it is difficult to function as the cap film 205. When the thickness of the cap film 205 is 5nm or more, the reflectivity to EUV exposure light is reduced.

The phase shift film 208 shifts the phase of the exposure light and reflects the exposure light, thereby destructively interfering with the exposure light reflected from the reflective film 204. The phase shift film 208 includes a first layer 208a formed on the cap film 205 and a second layer 208b formed on the first layer 208a.

The first layer 208a is made of a material containing niobium (Nb). In this case, the content of niobium (Nb) in the first layer 208a may be 20 to 50 at%. When niobium (Nb) is less than 20 at%, the etching rate is significantly reduced, and it is difficult to implement a vertical pattern profile. Furthermore, the refractive index (n) and extinction coefficient (k) are relatively high, and thus there are limitations not only in the final improvement of NILS and DtC, but also problems of lowering the reflectivity and increasing the thickness. When niobium (Nb) is 50 at% or more, chemical resistance to chemicals for cleaning, such as sulfuric acid, is reduced.

The first layer 208a is formed of a material based on chlorine (Cl) ₂ ) And in particular in the absence of oxygen (O) ₂ ) Is etched under the conditions of (2). Thus, when the first layer 208a is etched, particularly over-etched, damage to ruthenium (Ru) contained in the cap film 205 under the first layer 208a is reduced, thereby minimizing problems such as reflectance reduction. Thus, no additional thin film, such as an etch stop film, is required.

In addition, the first layer 208a has a composition that is relatively free of oxygen (O ₂ ) Is based on chlorine (Cl) ₂ ) To improve vertical pattern profile. Furthermore, the first layer 208a is relative to XeF during electron beam repair ₂ Easy repair and excellent electron beam repair selectivity with respect to the capping film 205 containing ruthenium (Ru).

The first layer 208a may be made of a material containing one or more of tantalum (Ta), silicon (Si), and chromium (Cr). Preferably, the first layer 208a is made of a material containing niobium (Nb) and chromium (Cr), and in this case, the content of chromium (Cr) in the first layer 208a is 10 to 40 at%. When the content of chromium (Cr) is less than or equal to 10 at%, chemical resistance to chemicals used for cleaning, such as sulfuric acid, is reduced. When the content of chromium (Cr) is 40 at% or more, the etching rate is significantly reduced and thus it is difficult to implement the pattern profile.

The first layer 208a may further contain one or more of oxygen (O), nitrogen (N), and carbon (C). Preferably, the first layer 208a further comprises nitrogen (N). In this case, the content of nitrogen (N) in the first layer 208a may be 10 to 60 atomic%. When the content of nitrogen (N) is less than or equal to 10 at%, the refractive index (N) is relatively high and thus it is difficult to improve NILS. When the content of nitrogen (N) is 60 at% or more, the reaction for forming a thin film does not effectively occur, thereby reducing the sputtering efficiency.

The first layer 208a has a thickness of 30 to 60nm, preferably 45 to 60 nm.

The first layer 208a has a thickness of 50% or more, preferably 80% or more of the total thickness of the phase shift film 208. Accordingly, the second layer 208b has a thickness of 50% or less, preferably 20% or less, of the total thickness of the phase shift film 208.

The first layer 208a has a refractive index (n) of 0.925 to 0.935 and an extinction coefficient (k) of 0.015 to 0.025 with respect to exposure light having a wavelength of 13.5 nm. When the refractive index (n) is greater than or equal to 0.935, it is difficult to significantly improve NILS and DtS. Therefore, the lower the refractive index (n), the better. However, it is difficult to make the refractive index (n) lower than or equal to 0.925 due to the material properties. When the extinction coefficient (k) is less than or equal to 0.015, the relative reflectance becomes higher, thereby causing a problem of forming a double image pattern. Therefore, the higher the extinction coefficient (k), the better. However, it is difficult to make the extinction coefficient (k) higher than or equal to 0.025 due to the material properties.

To improve the pattern profile, the first layer 208a may be formed as a continuous film or a multi-layer film, wherein the composition is continuously modified. For example, in order to increase the etching rate of the first layer 208a in the depth direction, the content of nitrogen (N) may be increased in the depth direction of the first layer 208a, or the content of nitrogen (N) or niobium (Nb) may be increased in a portion of the first layer 208a adjacent to the cap film 205. Thus, the footing and the like are reduced while patterning, thereby making it possible to form a vertical pattern profile.

To form the first layer 208a, a single sputtering method or a co-sputtering method may be used. When a single sputtering method is used, the sputtering target may have a composition of Nb: cr=30 to 70 atom% to 70 to 30 atom%, preferably Nb: cr=40 to 60 atom% to 60 to 40 atom%. Using the co-sputtering method, sputtering can be performed using NbCr, cr, and Nb as targets, and the composition of the thin film can be determined by controlling the power of each sputtering program.

The second layer 208b may be made of a material containing one or more of tantalum (Ta) and silicon (Si). In addition, the second layer 208b may further contain one or more of oxygen (O), nitrogen (N), and carbon (C). Preferably, the second layer 208b is made of TaON.

The content of tantalum (Ta) in the second layer 208b is 25 atomic% or more, preferably 50 atomic%. When the content of tantalum (Ta) is low, it is difficult to ensure etching selectivity to the first layer 208a.

The second layer 208B may further contain boron (B). By adjusting the content of boron (B), it is possible to control the refractive index (n) and extinction coefficient (k) of the second layer 208B. The content of boron (B) in the second layer 208B may be 5 to 20 atomic%. The second layer 208B has reduced chemical resistance at a boron (B) content of 20 at% or more, and has increased film stress and reduced etching rate at a boron (B) content of 5 at% or less.

The second layer 208b is etched by a fluorine (F) -based gas. Thus, the second layer 208b has an etch selectivity to the underlying first layer 208a. In this case, the second layer 208b may be generally formed without oxygen (O ₂ ) But optionally contains oxygen (O) ₂ )。

The second layer 208b has a thickness of 2 to 10nm, preferably a thickness of not more than 5 nm.

The second layer 208b may have a surface reflectance of less than or equal to 40%, preferably 35%, for detection light having a wavelength of 193 nm. Therefore, during detection using Deep Ultraviolet (DUV) detection light, it is impossible to improve the contrast with the reflective film 204 and the cover film 205.

The second layer 208b has a refractive index (n) of 0.940 to 0.960 and an extinction coefficient (k) of 0.025 to 0.035 with respect to exposure light having a wavelength of 13.5 nm. In order to have a refractive index (n) of 0.960 or more, the content of oxygen (O) needs to be increased in the second layer 208b, thereby reducing reproducibility and program stability of the thin film. In addition, in order to have a refractive index (n) of 0.940 or less, the content of oxygen (O) needs to be reduced in the second layer 208b, thereby reducing the etching selectivity to the first layer 208a. In order to have an extinction coefficient (k) of 0.025 or less or 0.035 or more, the content of oxygen (O) needs to be excessively increased or decreased, thereby causing the problems as described above.

The phase shift film 208 has a relative reflectance of 6 to 15% with respect to the reflective film 204 with respect to exposure light having a wavelength of 13.5 nm. Here, the relative reflectivity refers to the ratio of the reflectivity of the phase shift film 208 to the reflectivity of the structure in which the reflection film 204 and the cap film 205 are stacked. Further, the phase shift film 208 has a phase shift amount of 180 ° to 220 °, preferably 185 ° to 220 °.

With this previous configuration, the total phase shift amount and reflectivity of the phase shift film 208 are changed depending on the first layer 208a. The second layer 208b differs from the first layer 208a in etch properties, and thus the second layer 208b acts as a hard mask for etching the first layer 208a when the phase shift film 208 is patterned. Further, the second layer 208b has a low reflectance, and thus it is possible to easily perform detection using DUV detection light after the pattern is finally formed.

The resist film 210 is configured as a chemically amplified resist (chemically amplified resist; CAR). The resist film 210 has a thickness of 40 to 100nm, preferably 40 to 80 nm.

The conductive film 201 is formed on the rear portion of the substrate 202. The conductive film 201 has a low sheet resistance value to improve close contact between the electronic chuck and a photomask for EUV lithography and to prevent generation of particles due to friction with the electronic chuck. The conductive film 201 has a sheet resistance of 100deg.OMEGA/≡or less, preferably 50Ω/≡or less, more preferably 20Ω/≡or less. The conductive film 201 may be provided in the form of a single film, a continuous film, or a multilayer film. The conductive film 201 may contain mainly chromium (Cr) or tantalum (Ta), for example.

The procedure for preparing a photomask using a blank mask having the aforementioned configuration is as follows.

First, the resist film 210 is patterned, and then the second layer 208a is etched by fluorine-based gas via the resist film pattern. Next, the resist film pattern is removed, and the pattern of the second layer 208a is used as an etching mask to etch the first layer 208a with a chlorine-based gas. In this case, the second layer 208b is part of the phase shift film 208, but acts as a hard mask film (hard mask) to the first layer 208a during the etching procedure.

Examples

In the following examples, examples of preparing phase shift blanks and photomasks according to the present disclosure will be described.

On the LTEM substrate, 40 pairs of Mo/Si layers were stacked to form a reflective film, and then a cap film was formed of ruthenium (Ru) to have a thickness of 2.5 nm. In the structure in which the reflective film and the cap film were stacked, the reflectance measured by an EUV reflectometer with respect to EUV light having a wavelength of 13.53nm was exhibited as 64.57%, the half (peak height) width (full width at half maximum; FWHM) was exhibited as 0.57nm, and the center wavelength (center wavelength; CWL) was exhibited as 13.52nm. Therefore, there is no problem in using this structure as a reflective film and a cover film.

Before forming the double-layer phase shift film, the first layer and the second layer of the phase shift film are formed separately and then their refractive index (n) and extinction coefficient (k) are measured with respect to exposure light having a wavelength of 13.53 nm.

TABLE 1

Sputtering conditions for phase shift films and n & k measured under the conditions

Regarding some of the examples of the layer of the phase shift film formed under the conditions shown in table 1, the etching properties thereof were evaluated using an ICP dry etcher. The evaluation results are shown in table 2 below.

TABLE 2

Etching properties of materials for phase shifting films

Based on the measured etching properties of n, k and phase shift films, a first layer and a second layer of phase shift film are sequentially formed on the cap film. In this case, the film growth conditions were based on example 3 and example 12, and the first layer and the second layer were grown as films having a thickness of 50nm and a thickness of 4nm, respectively. The result of measuring the reflectance of the phase shift film grown as described above with respect to DUV detection light having a wavelength of 193nm exhibited 33%, and it was confirmed that the contrast was higher compared to the reflectance of 63% that the stacked structure of the reflective film and the cap film had during detection using DUV detection light. Then, a resist film was finally coated on the phase shift film to a thickness of 100nm, thereby completely preparing a phase shift mask blank.

Using the phase shift photomask prepared as described above, a photomask was prepared by the following procedure. First, a resist film pattern is formed by an electron beam writing and developing process. Next, the second layer of the phase shift film is etched by fluorine-based gas via the resist film pattern. Subsequently, the first layer of the phase shift film is etched by a chlorine-based gas to completely prepare a phase shift photomask. Then, the result of measuring the cross-sectional inclination angle of the phase shift film pattern was revealed to be 86 °, and it was confirmed that an excellent pattern profile could be implemented.

With respect to the photomask prepared as described above, wafer simulation was performed as follows. Simulation was performed using a staggered contact hole pattern of 17 nm. Thus, the relative reflectance of the phase shift film exhibited 10 °, and the phase shift amount exhibited 199 °. Furthermore, NILS exhibits 1.95 and DtC exhibits 114.7mJ.

Comparative example

In this comparative example, a procedure of preparing a photomask having a phase shift film containing ruthenium (Ru) and a procedure of preparing a photomask using the prepared photomask will be described.

First, a reflective film and a cover film are generally formed as those of the foregoing embodiments. Next, an etching stopper film is formed to have a double-layer structure including a TaBN layer and a TaBO layer, and a phase shift film containing only ruthenium (Ru) is formed on the etching stopper film. Then, a hard mask film containing TaBO is formed on the phase shift film, and a resist film is finally formed on the hard mask film, thereby finally completing a photomask blank.

In the foregoing procedure, the phase shift film and the cap film contain the same material, and thus the etch stop film is formed to have a double layer structure to minimize damage to the cap film when the etch stop film is etched. In other words, since it is highly likely that the Ta-based material is oxidized and the material for etching the Ru-containing phase shift film contains a chlorine-based gas and oxygen, the TaBN layer is first formed and then the TaBO layer is additionally formed to control the oxide film on the surface of the TaBN.

Next, a phase shift film containing ruthenium (Ru) is formed on the etch stop film, and a hard mask film containing TaBO is additionally formed to improve resolution and CD linearity.

This phase shift blank mask structure according to the comparative example additionally includes an etch stop film and a hard mask film and is more complex than those of the previous embodiments.

The method of preparing a photomask via a phase shift photomask is as follows.

First, as in the previous embodiment, a resist film pattern is formed by an electron beam writing and developing process. Next, the resist film pattern is used as an etching mask to form a hard mask film pattern of TaBO. In this case, a fluorine (F) -based gas is used as the etching gas. Then, the resist film pattern is removed, and then the hard mask film pattern is used as an etching mask to be formed of a resist film based on chlorine (Cl) ₂ ) Is a gas and oxygen (O) ₂ ) The gas etches the phase shift film. Next, the fluorine (F) -based gas is again used to etch the upper layer of TaBO in the etch stop film. In this case, the hard mask film is removed. Next, in the absence of oxygen (O ₂ ) In the case of reuse of chlorine-based (Cl) ₂ ) To etch the lower layer of the TaBN in the etch stop film, thereby finally completing the phase shift photomask.

Hereinafter, the characteristics of the blank mask according to the comparative example were measured as follows. First, the reflectance of the phase shift film was measured with respect to the detection light having a wavelength of 193 nm. Thus, the reflectivity exhibited 45.2%, which showed a decrease in contrast compared to the previous embodiment. This means that it is not appropriate to form the phase shift film only from ruthenium (Ru), and oxygen (O) is required in addition to ruthenium (Ru).

Next, the profile of the phase shift film pattern formed by the foregoing procedure was measured. Thus, the cross-sectional inclination of the pattern exhibited 70 °, and the validation profile was inferior compared to the previous embodiment.

According to the present disclosure, it is possible to implement the characteristics, i.e. high resolution and NILS required for phase shift blanks for EUV during wafer printing, and implement low DtC.

Further, according to the present disclosure, the phase shift film is made of a material that does not contain ruthenium (Ru), and thus does not cause problems that occur when ruthenium (Ru) is contained. Therefore, the procedure for preparing the photomask is relatively simple, and the structure of the etch stop film under the phase shift film is not required to improve the yield when preparing not only the blank mask but also the photomask.

While the details of the present disclosure have been described above via several embodiments of the present disclosure with reference to the accompanying drawings, the embodiments are for illustrative and descriptive purposes only and are not to be construed as limiting the scope of the present disclosure defined in the appended claims. It will be appreciated by those of ordinary skill in the art that various changes and other equivalent embodiments can be made from these embodiments. Accordingly, the scope of the present disclosure should be defined by the technical subject matter of the appended claims.

Claims

1. A photomask for Extreme Ultraviolet (EUV) lithography includes a substrate, a reflective film formed on the substrate, a cap film formed on the reflective film, and a phase shift film formed on the cap film,

the phase shift film includes:

a first layer formed on the cap film and containing niobium (Nb), an

A second layer formed on the first layer and containing one or more of tantalum (Ta) and silicon (Si).

2. The photomask blank of claim 1, wherein the content of niobium in the first layer is in the range of 20 to 50 atomic percent.

3. The photomask of claim 2, wherein the first layer comprises one or more of tantalum (Ta) and silicon (Si).

4. The photomask blank of claim 2, wherein the first layer further comprises chromium (Cr).

5. The photomask blank according to claim 4, wherein the content of chromium (Cr) in the first layer is in the range of 10 to 40 atomic%.

6. The photomask of claim 5, wherein the first layer further comprises one or more of oxygen (O), nitrogen (N), and carbon (C).

7. The photomask blank according to claim 6, wherein the content of nitrogen (N) in the first layer is in the range of 10 to 60 atomic%.

8. The photomask blank of claim 7, wherein the first layer has a thickness of 30 to 60 nm.

9. The photomask of claim 8, wherein the first layer has a thickness of 80% or more of the total thickness of the phase shift film.

10. The photomask blank of claim 9, wherein the first layer has a refractive index (n) of 0.925 to 0.935 and an extinction coefficient (k) of 0.015 to 0.025 with respect to exposure light having a wavelength of 13.5 nm.

11. The photomask blank according to any one of claims 1 to 10, wherein the content of tantalum (Ta) in the second layer is greater than or equal to 50 atomic%.

12. The photomask of claim 11, wherein the second layer contains one or more of oxygen (O), nitrogen (N), and carbon (C).

13. The photomask of claim 11, wherein the second layer further comprises boron (B).

14. The photomask blank according to claim 13, wherein the content of boron (B) in the second layer is in the range of 5 to 20 atomic%.

15. The photomask blank of claim 11, wherein the second layer has a thickness of 2 to 10 nm.

16. The photomask of claim 11, wherein the second layer has a surface reflectance of 40% or less for detection light having a wavelength of 193 nm.

17. The photomask blank of claim 11, wherein the second layer has a refractive index (n) of 0.940 to 0.960 and an extinction coefficient (k) of 0.025 to 0.035 with respect to exposure light having a wavelength of 13.5 nm.

18. The photomask blank according to claim 1, wherein the phase shift film has a relative reflectance of 6 to 15% with respect to the reflective film with respect to exposure light having a wavelength of 13.5 nm.

19. A photomask prepared using the photomask for Extreme Ultraviolet (EUV) lithography of claim 1.