KR20090049112A - Half-tone phase shift blankmask for haze reduction and it's manufacturing method - Google Patents

Abstract

The present invention relates to a method of manufacturing a binary blank mask and a photomask, and more particularly, to the manufacture of a binary blank mask for reducing Haze and a phase inversion blank for reducing Haze, and a binary mask for reducing Haze and a phase inversion mask for reducing Haze using the same. . In the binary blank mask for reducing Haze according to the present invention, a binary blank mask is produced in which less Haze is generated by controlling the properties of the light shielding film and the antireflection film through the composition of the light shielding film and the antireflection film, the film formation method, the sheet resistance, the residual stress and the surface treatment method. Is possible. In addition, in the Haze reduction phase inversion blank mask according to the present invention, the phase inversion blank mask is produced by controlling the characteristics of the phase inversion film through the composition of the phase inversion film, the film formation method, the sheet resistance, the residual stress and the surface treatment method. It is possible. As a result, when manufacturing the binary mask and the phase inversion mask by using the Haze reduction binary blank mask and Haze reduction phase inversion blank mask according to the present invention, it is possible to manufacture a binary mask and phase inversion mask of excellent quality, It has the effect of obtaining a binary mask and a phase inversion mask having stable characteristics.

Halftone Phase Inversion Blank Mask, Haze, Reactive Gas

Description

Half-tone phase shift blank mask and haze reduction and it's manufacturing method

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method for manufacturing a photomask and a mask blank. In particular, 248 nm KrF and 193 nm ArF lithography capable of achieving a critical dimension (CD) of 90 nm or less Pertains to photomasks and blank masks.

Photolithography for manufacturing a semiconductor integrated circuit is a method of forming a pattern using a UV mask or an electron beam, a blank mask in which a chromium-based light shielding film, an antireflection film, and a resist are sequentially stacked on a transparent substrate. Such photolithography technology has shortened the exposure light to 436 nm g-line, 365 nm i-line, 248 nm KrF, and 193 nm ArF as the pattern size is miniaturized.

In addition, cleaning liquids of various compositions are used to remove impurities on the surface of the mask blank and photomask during the manufacturing process. Such cleaning liquids include SC1 (NH ₄ OH / H ₂ O ₂ / H ₂ O) and SPM (H _2). SO ₄ / H ₂ O ₂ ), HPM (HCl / H ₂ O ₂ / H ₂ O), FPM (HF / H ₂ O ₂ ), DHF (Hf / H ₂ O), and the like, in particular of SC1 and SPM In many cases, a single mask blank and a photomask are used to manufacture a product. After the cleaning process using the cleaning solution, ammonia (NH ₄ OH), sulfuric acid (H ₂ SO ₄ ), and the like are adsorbed onto the mask surface. When the adsorbed material is exposed to a high-energy short wavelength source, photochemical reactions are known to produce haze defects called growth defects.

These Haze defects occur on the surface of the mask blank and photomask on the back side of the quartz substrate or on the surface of the functional film, the light shielding layer, the antireflecting layer or the phase shifting layer. Haze defects are very small at first, but they are also called growth defects because their number and size increase as the exposure dose increases. These defects not only shorten the life of the photomask, but also reduce productivity.

Adsorption of chemical ions on the mask surface is determined by the surface state of the thin film. If the surface of the thin film is electrically, chemically or physically unstable, more ions will be adsorbed, sharing electrons and bonding strength with each other, resulting in a stable state of low Gibbs Free energy. Therefore, there should be no chemical ions for Haze Free, which requires a stable thin film manufacturing process.

The present invention solves the above problems, and relates to a film formation technique and a surface treatment technique of a thin film in the method of manufacturing a photomask and mask blank. In particular, the film deposition technology that can minimize the concentration of surface adsorption ions through stable thin film deposition technology through the control of parameters such as thin film deposition technology, reactive gas type, reactive gas flow rate, sputtering target composition and thin film structure will be. Furthermore, the present invention relates to a method of manufacturing a mask blank and a photomask using a commercially available technique.

Accordingly, an object of the present invention is to provide a photomask and mask blank having high surface roughness and flatness, with a small amount of surface ion detection that affects growth defects and adhesion in high-end photomask and mask blanks. will be.

A feature of the mask blank manufacturing method according to the present invention for achieving the above object is that a light shielding film, an antireflection film, a resist film is sequentially formed on a transparent substrate, and a phase inversion film, a light shielding film, an antireflection film, a resist film sequentially on the transparent substrate Characterized in that formed.

In particular, when the mask blank manufacturing method according to the present invention is used in the binary mask blank manufacturing process,

a1) forming a transparent substrate,

b1) forming a light shielding film on the transparent substrate,

c1) forming an anti-reflection film on the light shielding film,

d1) cleaning the anti-reflection film;

e1) preferably applying a resist on the anti-reflection film.

In addition, when the mask blank manufacturing method according to the present invention is used in the phase inversion mask blank manufacturing process,

a2) forming a transparent substrate,

b2) forming a phase inversion film on the transparent substrate,

c2) cleaning the phase shift film,

d2) forming a light shielding film on the phase inversion film,

e2) forming an anti-reflection film on the light shielding film,

f2) preferably applying a resist on the anti-reflection film to produce a phase inversion mask blank.

In addition, the binary photomask manufacturing method according to the present invention,

a3) unnecessary resist after forming a resist film pattern, an anti-reflection film pattern, or a light shielding film pattern on a binary mask blank manufactured by the above-described method (steps a1) to e1) through a process commonly used in manufacturing a binary photomask. It is desirable to produce a binary photomask by stripping the film pattern.

In addition, the method of manufacturing a phase inversion photomask according to the present invention,

a4) a resist film pattern, an anti-reflection film pattern, a light shielding film pattern, a phase inversion film pattern through a process generally used in manufacturing a phase inversion photomask for a phase inversion mask blank prepared by the above-described methods (a2) to f2)) It is preferable to produce a phase inversion photomask through the step of removing the unnecessary resist film pattern after forming the film.

Hereinafter, each step will be described in more detail.

In the a1) and a2) steps, the transparent substrate is made of soda lime, synthetic quartz or calcium fluoride (CaF ₂ ), and the ArF laser at I-line (365 nm), which is a lithography light source. And a substrate having a size of at least 5 inches having a transmittance of at least 85% or more at a (193 nm) wavelength. In addition, it has a birefringence of 4nm / 6.35mm or less.

The phase inversion film of b2) is formed by sputtering in a vacuum chamber in which an inert and active gas is introduced using a metal as a matrix, and cobalt (Co), tantalum (Ta), tungsten (W), and molybdenum (Mo) ), Chromium (Cr), vanadium (V), palladium (Pd), titanium (Ti), niobium (Nb), zinc (Zn), hafnium (Hf), germanium (Ge), aluminum (Al), platinum (Pt) ), Manganese (Mn), iron (Fe), ruthenium (Ru), antonium (Sb), nickel (Ni), cadmium (Cd), zirconium (Zr), tin (Sn), gallium (Ga), It is characterized in that the thin film containing at least one type of silicon (Si), wherein the inert gas is from the group consisting of argon (Ar), helium (He), neon (Ne), krypton (Kr) and xenon (Xe) At least one selected is used and the active gas is oxygen (O ₂ ), nitrogen (N ₂ ), carbon monoxide (CO), carbon dioxide (CO ₂ ), nitrous oxide (N ₂ O), nitrogen oxides (NO), nitrogen dioxide (NO). ₂ ), a group consisting of ammonia (NH ₃ ) and methane (CH ₄ ) Use at least one selected gas. In addition, a method such as DC sputtering, RF sputtering, or ion beam deposition may be used during the formation of the phase shift film, and power is characterized in that reactive sputtering is performed at 0.1 kW to 4 kW. In addition, during the sputtering, the pressure in the vacuum chamber is preferably performed at 10 ⁻¹ to 10 ⁻⁴ Pa.

In the steps b1) and e2), the light shielding film is a thin film including one or more selected from the group consisting of chromium, tungsten, tantalum, and titanium based on a metal, and selected from argon, helium, neon, xenon, and krypton. It is characterized by sputtering using at least one inert gas and at least one active gas selected from oxygen, nitrogen, carbon monoxide, carbon dioxide, nitrous oxide, nitrogen oxide, nitrogen dioxide, ammonia and methane.

The anti-reflection film in the steps c1) and f2) is a thin film containing at least one member selected from the group consisting of chromium, tungsten, tantalum, molybdenum silicide and titanium based on a metal, and includes argon, helium, neon, xenon, It is characterized by sputtering using at least one selected inert gas of krypton and at least one active gas selected from oxygen, nitrogen, carbon monoxide, carbon dioxide, nitrous oxide, nitrogen oxide, nitrogen dioxide, ammonia and methane.

When the mask blank and the photomask are manufactured through the thin film deposition technology according to the present invention, by minimizing the concentration of surface adsorption ions, the photomask can not only prolong the service life but also improve productivity, and manufacture high quality blank mask and the photomask. Is possible. In addition, it is possible to manufacture a mask blank capable of preventing growth defects.

The present invention will be described in detail by way of examples, but the examples are only used for the purpose of illustration and description of the present invention and are not used to limit the scope of the present invention as defined in the meaning or claims. Therefore, those skilled in the art will understand that various modifications and equivalent other embodiments are possible from the embodiments. Therefore, the true technical protection scope of the present invention will be defined by the technical details of the claims.

(Example 1)

Using a sputtering target made of a metal monoatomic material, a 6-inch transparent substrate having a surface area of 231.04 cm 2 was prepared, and a thin film was formed by using a DC magnetron sputtering device. The target used was a metallic chromium (Cr) target with a purity of 99.999%. The sputtering conditions used in the deposition were selected from at least one of Ar gas, 5-100 sccm, N ₂ gas 5-100 sccm, and CO ₂ gas 5-100 sccm. The power was 0.1-4 kW and the pressure was 10 ^-1. A thin film was formed by applying at -10 ^-4 Pa.

The thin film thus prepared was subjected to chemical treatment according to the cleaning process used in the blank mask and photomask manufacturing process. Ion Chromatography (IC) was used to analyze the adsorption ion concentration on the thin film surface after chemical treatment. Pretreatment of IC analysis was performed for 20 minutes at 120 ° C using JEIOTECH's Autoclave, and analysis was performed using DIONEX's DX-120s. In the case of IC, a mask was placed in an analytical container during analysis, and a certain amount of DI water was filled, followed by heating at 120 ° C. for 20 minutes to detect anions and cations dissolved in DI water through respective filters.

3 and 4 show the concentration of ions detected on the surface of the thin film according to the respective deposition conditions.

3 is an ion concentration value at the surface of the Cr thin film according to the content of the CO ₂ gas during thin film deposition. As the amount of gas of CO ₂ increases, the amount of ions detected on the Cr thin film surface decreases and then increases again from 50 sccm. This is due to the physical and chemical stability of the thin film. In general, the metal thin film has a different stability in the order of metal oxide film> metal film> metal nitride film, and has the lowest ion concentration value because it becomes the most stable thin film state at 50 sccm. 4 is a thin film surface ion concentration value according to the amount of N ₂ gas at the time of Cr thin film deposition. As described above, in the case of a metal thin film, the nitride film is the most unstable thin film because the atomic force in the thin film becomes unstable because smaller nitrogen atoms come and stick to the place where oxygen should be. Therefore, as the amount of nitrogen increases, the instability of the thin film becomes greater and the concentration of surface adsorption ions also increases.

5 is a haze test after accelerating to 30 kJ of energy using an ArF Laser. According to the Haze test according to the contents of CO ₂ and N ₂ gas, the smaller the total amount of ions detected in the Cr thin film, the lower the number of Haze defects. In general, Haze is generated by photochemical reaction between surface adsorption ions and exposure energy. When CO ₂ is 50 sccm and N ₂ is 10 sccm, the minimum amount of total ions adsorbed on the surface is the smallest. Because.

Therefore, the stability of the surface of the thin film can be controlled by controlling the flow rate of the gas used in the thin film deposition through Example 1, thereby developing a film forming technology of the Cr thin film reduced Haze.

(Example 2)

In Example 2, a thin film was formed in the same manner as in Example 1 for the metal target containing Mo and Si and the metal target containing Ta in Mo and Si. In addition, after the thin film was formed, a thin film was formed in the same manner as in Example 1. The gas used was 5 to 100 sccm for Ar gas and 5 to 100 sccm for N ₂ gas.

6 is a concentration of ions detected on the surface of the MoSi thin film according to the content of N ₂ . When the nitrogen content is 10 sccm, the total ion concentration is 14.21 ppbv. As the nitrogen content is increased, the total ion concentration increases, but starts to saturate around 70 sccm. 7 is a concentration of ions detected on the surface of the MoTaSi thin film according to the content of N ₂ . When the nitrogen content is 10 ppbv, the total ion concentration is 5.21 ppbv, and as the nitrogen content is increased, the total ion concentration increases, and the nitrogen content starts to saturate around 70 sccm. Comparing FIG. 6 with FIG. 7, it can be seen that the concentration of surface ions also varies according to the composition of the thin film. In general, Mo and Si form an unstable bond. When a thin film is formed using a target composed of Mo and Si, the thin film also becomes an unstable thin film. However, as Ta is added here, the unstable bond between Mo and Si can be made into a stable bond, and when a thin film is formed using a target composed of these three metals, a more stable thin film can be formed. Therefore, the concentration of ions detected on the surface of the thin film was significantly reduced when the thin film was formed by using the target consisting of Mo, Si and Ta, compared to the thin film formed using the target consisting of Mo and Si. Haze experiment was performed on these thin films as in Example 1.

8 shows the number of Haze defects after ArF Laser acceleration in MoSi and MoTaSi thin films. As the N ₂ content was increased, the number of Haze defects increased in both films. In the case of MoTaSi, Haze defects did not occur when the N ₂ content was 0 to 20 sccm. In addition, Haze defects were smaller in MoTaSi than in MoSi.

Therefore, by controlling the composition of the sputtering target, that is, the composition of the thin film, it is possible to implement the same optical properties and to develop a thin film with reduced Haze.

(Example 3)

In the same manner as in Example 2, a thin film was formed and subjected to chemical treatment, followed by surface treatment to reduce Haze. After surface treatment, it was accelerated using an ArF laser and subjected to Haze inspection. At this time, the surface treatment was using a Rapid Thermal Process (RTP) and the process pressure was 7.6 x 10 ^-2 torr. The heat treatment temperature was performed at 500 degreeC for 10 minutes.

Fig. 9 shows the number of Haze defects after ArF laser acceleration of surface treated MoSi and MoTaSi thin films. The number of Haze defects was drastically reduced compared to the thin film without surface treatment. In particular, HaTe defects did not occur in the case of MoTaSi until the nitrogen content was 50 sccm. On the other hand, in case of MoSi, the number of Haze defects was reduced, but Haze occurred in all sections.

Therefore, it was possible to develop a thin film technology that can reduce the Haze generation through the surface treatment. In addition, the surface treatment may include not only RTP but also all techniques capable of changing the stable state of the surface. This may include hot-plates, furnaces, vacuum bakeries, etc., and may also include surface treatment technology using plasma.

(Example 4)

As in Examples 1 and 2, the sheet resistance and Haze generation degree according to the reactive gas flow rate of the thin film formed through the reactive DC sputtering method were examined.

Total amount of reactive gas Sheet resistance Cr [Ω / □] MoSi [MΩ / □] MoTaSi [MΩ / □] 10 sccm 13.52 1.20 1.41 20 sccm 13.98 1.46 1.43 30 sccm 14.05 1.51 1.47 40 sccm 14.57 1.79 1.53 50 sccm 20.59 1.84 1.95 60 sccm 30.49 2.02 2.51 70 sccm 40.18 2.19 2.56 80 sccm 50.67 2.27 2.57 90 sccm 51.29 2.31 2.62 100 sccm 53.96 2.37 2.68

The sheet resistance of the thin film was measured using AMT CMT-SR3000 series. The sheet resistance increased with increasing nitrogen content in Cr, MoSi, and MoTaSi thin films. In case of Cr, the sheet resistance of the thin film increases rapidly as the content of the reactive gas exceeds 40 sccm. In addition, Cr has a smaller sheet resistance than MoSi and MoTaSi by more than 5 orders, which is large because the bond between atoms in MoSi and MoTaSi thin films cannot be formed unstable. MoSi shows a sharp increase in sheet resistance at 30 sccm and a saturation at 90 sccm. MoTaSi, on the other hand, exhibits a sharp increase in sheet resistance at 50 sccm and saturation at 70 sccm. This is because MoTaSi forms a thin film structure having a more stable bond than MoSi.

In addition, the relationship between sheet resistance and Haze generation tendency also has a tendency similar to that of Examples 1 and 2. That is, it can be seen that there is a difference in the Haze generation tendency depending on the flow rate in the reactive gas.

(Example 5)

Cr thin films, MoSi thin films, and MoTaSi thin films prepared through Example 1, Example 2, and Example 4 were prepared. In order to control the transmittance of the thin film, the thin film was formed while controlling Ar gas, N ₂ gas, and CO ₂ gas in the range of 0 to 100 sccm. A blank mask was prepared by coating a resist film on a Cr thin film, a MoSi thin film, and a MoTaSi thin film manufactured under the above conditions by a general blank mask method. At this time, the resist used in the phase inversion blank mask was chemically amplified resist (CAR).

The phase inversion mask was manufactured by the process of patterning, etching, strip, etc. which are a conventional photomask manufacturing method with respect to the prepared phase inversion blank mask.

Photomask evaluation was performed about the produced phase inversion mask.

10 was tested for CD performance using Immersion Lithography using the phase inversion mask. The MoTaSiN phase inversion mask had a smaller CD error than the MoSiN phase inversion mask. In addition, the phase inversion mask having a transmittance of 1 to 2% showed a very good CD error of 1 nm level. In the two phase inversion masks, the CD error tended to increase as the transmittance increased.

Therefore, through Example 5, it is possible to manufacture a phase inversion mask for imaging lithography.

(Example 6)

Through Example 1, Example 2, and Example 4, a thin film was formed on the Cr thin film, the MoSi thin film, and the MoTaSi thin film, and the amount of Ar was fixed at 100 sccm. After thin film formation, the residual stress in the thin film was measured according to the amount of reactive gas.

Thin film material N ₂ [sccm] CO ₂ [sccm] Residual stress [MPa] Haze Counts [EA / ㎠] CrCO 0 100 50.12 2.12 CrCON 10 90 51.34 2.45 CrCON 20 80 51.29 2.95 CrCON 30 70 53.04 3.68 CrCON 40 60 54.78 4.18 CrCON 50 50 59.49 9.19 CrCON 60 40 61.78 11.55 CrCON 70 30 68.95 16.73 CrCON 80 20 73.97 21.04 CrCON 90 10 79.45 26.18 CrN 100 0 86.49 33.18

As a result of measuring the residual stress of the Cr film, the higher the content of nitrogen, the larger the residual stress. On the other hand, as the CO ₂ content increases, the residual stress tends to decrease. In the case of metal compounds, metal oxides form the most stable bonds. As the amount of CO ₂ increases, the amount of oxygen atoms that can bond with metal atoms increases, so that the concentration of metal oxides in the thin film becomes more stable. Because you will have. However, as the content of nitrogen increases, the bond becomes unstable because the atoms of nitrogen react in place of the atoms of oxygen in the bond between metal and oxygen. This is because the size of the nitrogen atom is relatively small and the electrical characteristics are different from each other. Therefore, as the residual stress of the thin film increases, the thin film structure has a very unstable structure. As shown in the results of Examples 1 to 3, more ions are adsorbed to generate a large number of Haze.

N ₂ [sccm] Residual stress [MPa] Haze Counts [EA / ㎠] MoSiN MoTaSiN MoSiN MoTaSiN 0 64.15 21.04 4.01 0 10 65.12 21.67 5.24 0 20 66.19 22.06 6.31 0 30 65.12 22.79 7.07 1.01 40 70.12 23.05 12.12 2.50 50 75.23 25.11 20.65 5.23 60 85.12 25.98 20.54 5.98 70 96.19 28.85 26.32 12.31 80 99.19 29.97 27.75 12.98 90 104.74 30.74 29.88 13.51 100 107.12 31.21 31.18 14.24

The residual stress and Haze test results of MoSiN and MoTaSiN thin films showed similar trends with those of Cr thin films. In the case of MoSiN thin films, the residual stress increased rapidly when the nitrogen content was above 30 sccm. This is due to the nitrogen content of Mo-N, Si-N and Mo-Si-N bonds to form a stable bond is gradually collided with each other as the concentration of these bonds increases, the residual stress of the thin film increases. This tendency can be observed in MoTaSiN, and MoTaSiN is considered to have a more stable thin film structure than MoSiN by adding Ta.

1 is a cross-sectional view of a binary blank mask made by the present invention,

2 is a cross-sectional view of a phase inversion blank mask manufactured by the present invention,

3 is a result of measuring the surface ion concentration of Cr according to the CO ₂ Gas Ratio,

4 is a result of measuring the surface ion concentration of Cr thin film according to the N ₂ Gas Ratio,

5 is a result of measuring the number of Haze defects of the Cr thin film after the ArF Laser acceleration,

6 is a result of measuring the surface ion concentration of the MoSi thin film according to the N ₂ Gas Ratio,

7 is a result of measuring the surface ion concentration of MoTaSi thin film according to N ₂ Gas Ratio,

8 is a result of measuring the number of Haze defects of the MoSi & MoTaSi thin film after ArF Laser acceleration,

9 is a result of measuring the number of Haze defects of the MoSi & MoTaSi thin film according to the surface treatment,

10 is a test result of the CD performance for the MoSiN & MoTaSiN phase inversion mask.

<Description of Symbols for Major Parts of Drawings>

10: transparent substrate 20: phase inversion film

30: light shielding film 40: antireflection film

60: resist film

100: binary mask blank produced by the present invention

200: phase inversion mask blank produced by the present invention

Claims (11)

In a halftone phase inversion blank mask in which a phase inversion film, a light shielding film, an antireflection film, and a resist film are sequentially formed on a transparent substrate, Half-tone phase inversion blank mask, characterized in that the flow rate of the reactive gas is 75vol% or less of the total gas flow rate when the phase shift film is formed through a sputtering method to minimize the occurrence of Haze defects. In a halftone phase inversion blank mask in which a phase inversion film, a light shielding film, an antireflection film, and a resist film are sequentially formed on a transparent substrate, Halftone phase inverted blank mask, characterized in that the phase inversion film essentially contains Mo, Ta, Si at the same time to minimize the occurrence of Haze defects The method according to claim 1 or 2, At least one inert gas selected from the group consisting of argon (Ar), helium (He), neon (Ne), krypton (Kr) and xenon (Xe), oxygen (O2) and nitrogen ( At least one reactive gas selected from the group consisting of N2), carbon monoxide (CO), carbon dioxide (CO2), nitrous oxide (N2O), nitrogen oxides (NO), nitrogen dioxide (NO2), ammonia (NH3) and methane (CH4) Halftone phase inversion blank mask, characterized in that used The method according to claim 1 or 2, Halftone phase inverted blank mask, characterized in that the phase inversion film is formed by a method selected from the group consisting of DC sputtering, RF sputtering, ion beam deposition and atomic layer deposition. The method according to claim 1 or 2, The phase inversion film has a composition of 1 to 80 at% of metal, 0 to 80 at% of nitrogen, 0 to 80 at% of oxygen, 0 to 50 at% of carbon, and 0 to 30 at% of hydrogen. Halftone phase inversion blank mask In the halftone phase inversion blank mask composed of at least one film selected from the group consisting of a phase inversion film, a light shielding film, an antireflection film, and a resist film on a transparent substrate, Halftone type phase inverted blank mask, characterized in that the sheet resistance of the phase inversion film and / or the light shielding film and / or the anti-reflection film is 10 to 10 ⁷ Ω / □ to reduce the generation of haze and to minimize the effects of electron charging during electron beam exposure. The method of claim 6, The half-tone phase inversion blank mask, wherein the total flow rate of the reactive gas is 70 vol% or less of the total gas flow rate when the phase inversion film and / or the light shielding film and / or the anti-reflection film are formed. In the halftone phase inversion blank mask composed of at least one film selected from the group consisting of a phase inversion film, a light shielding film, an antireflection film, and a resist film on a transparent substrate, Halftone type phase inverted blank mask, characterized in that the residual stress of the phase inversion film and / or the light shielding film and / or the antireflection film is 1 GPa or less for reducing Haze occurrence and reducing defects. The method of claim 8, Halftone phase inverted blank mask characterized in that a heat treatment is performed in forming the phase inversion film and / or the light shielding film and / or the antireflection film The method of claim 1, Halftone phase inversion blank mask, characterized in that the transmittance in the exposure wavelength of the phase shift film is 0.001 to 10%. A method for manufacturing a halftone phase shift blank mask for manufacturing the halftone phase shift blank mask of claim 1

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