WO2008047886A1

WO2008047886A1 - Method of smoothing surface of substrate for euv mask blank, and euv mask blank obtained by the method

Info

Publication number: WO2008047886A1
Application number: PCT/JP2007/070391
Authority: WO
Inventors: Mika Yokoyama; Yoshiaki Ikuta; Katsuhiro Matsumoto
Original assignee: Asahi Glass Co., Ltd.
Priority date: 2006-10-13
Filing date: 2007-10-12
Publication date: 2008-04-24
Also published as: TW200830034A

Abstract

The present invention is to provide a method of smoothing a surface of a substrate for an EUV mask blank, having concave defects such as pits or scratches. The present invention relates to a method of smoothing a surface of a substrate for a reflective mask blank for EUV lithography, comprising applying a solution containing a polysilazane compound to a substrate surface having concave defects, and heating and curing the applied solution to form a silica coating (a coating comprising SiO2 as a main skeleton), thereby smoothing the substrate surface having concave defects.

Description

DESCRIPTION

METHOD OF SMOOTHING SURFACE OF SUBSTRATE FOR EUV MASK BLANK, AND EUV MASK BLANK OBTAINED BY THE METHOD

TECHNICAL FIELD

The present invention relates to a method of smoothing a surface of a substrate for a reflective mask blank for EUV (Extreme Ultra Violet) lithography (the mask blank is hereinafter referred to as "EUV mask blank") . More specifically, it relates to a method of smoothing a surface having concave defects of a substrate for an EUV mask blank.

The present invention further relates to a substrate for an EUV mask blank obtained by the smoothing method, and an EUV mask blank using the substrate.

BACKGROUND ART

A mask blank for EUV lithography (hereinafter referred to as "EUV mask blank) is produced by forming a reflective film and an absorption layer on an ultra- polished substrate in this order. The reflective film is most generally a multilayer reflective film comprising a Mo layer and a Si layer laminated alternately. If microasperity is present on a surface of a substrate used in the production of an EUV mask blank, it adversely affects the reflective film and the absorption layer formed on a substrate. For example, if microasperity is present on the substrate surface, the periodic structure of a multilayer reflective film formed on the substrate is perturbed, and if a pattern on a mask is transferred to a photosensitive organic film (the so-called photoresist film) on a Si wafer using an exposure device, a part of the desired pattern may be deficient, or superfluous pattern other than the desired pattern may be formed. A perturbation of periodic structure of a multilayer reflective film due to minute concavity and convexity present on a substrate is called a phase defect, and is serious problem. It is desirable that concavity and convexity having a predetermined size or bigger are not present on a substrate.

Non-Patent Documents 1 and 2 describe requirements relating to the defects of EUV masks and EUV mask blanks, and the requirements relating to those defects are very severe. Non-Patent Document 1 describes that when defects exceeding 50 nm are present on a substrate, it causes perturbation in the structure of the reflective coating, resulting in generating unexpected shape in a pattern which is projected on a resist on a Si wafer, and the presence of such defects is inadmissible. Non-Patent Document 1 further describes that surface roughness of a substrate is required to be less than 0.15 nm in terms of RMS (Root- Mean-Square) roughness in a pattern projected on a resist on a Si wafer in order to prevent roughness of line edges from being increased. Non-Patent Document 2 describes that it is inadmissible that defects exceeding 25 nm are present on a reticle coated with a reflective film, used in EUV lithography. Non-Patent Document 3 describes as to whether there is the possibility that defects having what degree of a size on a substrate are transferred. Non-Patent Document 3 describes that a phase defect has the possibility to change a line width of an image printed. It is further described therein that a phase defect having a surface bump having a height of 2 nm and FWHM (full width of half maximum) of 60 nm is a size which is a boundary condition as to whether the phase defect of this size is transferred and causes an inadmissible change of a line width, which is 20% of a 35 nm line width (140 nm line width on a mask) . Among microasperity present on a substrate surface, convex defects such as foreign matters (particles) or fibers can be removed by the conventional wet cleaning methods using hydrofluoric acid or aqueous ammonia, brush cleaning, precision polishing and the like. However, concave defects such as pits or scratches cannot be removed by those methods. Further, when the wet cleaning method using hydrofluoric acid or aqueous ammonia is applied, it is required to apply etching on a substrate surface slightly in order to remove convex defects from a substrate by a liftoff system, and as a result, new concave defects may be generated on a substrate surface. Even when brush cleaning is used in order to remove convex defects, new concave defects may be generated on a substrate surface.

Non-Patent Document 1: SEMI, P37-1102 (2002), "Specification for extreme ultraviolet lithography mask substrates"

Non-Patent Document 2: SEMI, P38-1102 (2002), "Specification for absorbing film stacks and multilayers on extreme ultraviolet lithography mask blanks"

Non-Patent Document 3: SPIE, vol. 4889, Alan Stivers, et. al., p.408-417 (2002), "Evaluation of the Capability of a Multibeam Confocal Inspection System for Inspection of EUVL Mask Blanks"

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method of smoothing a surface of a substrate having concave defects such as pits or scratches for an EUV mask blank in order to solve the above-described problems in the background art.

Another object of the present invention is to provide a substrate for an EUV mask blank obtained by the above method of smoothing a surface of a substrate.

Still another object of the present invention is to provide a substrate with a multilayer reflective film for an EUV mask blank and an EUV mask blank, using the substrate, for an EUV mask blank.

To achieve the above objects, the present invention provides a method of smoothing a surface of a substrate for a reflective mask blank for EUV lithography, comprising applying a solution containing a polysilazane compound to a substrate surface having concave defects, and heating and curing the applied solution to form a silica coating (a coating comprising Siθ2 as a main skeleton) , thereby smoothing the substrate surface having concave defects (hereinafter referred to as ^Xλa smoothing method of a substrate for an EUV mask blank of the present invention") .

In the smoothing method of a substrate for an EUV mask blank of the present invention, it is preferred that the solution containing a polysilazane compound has a polysilazane compound concentration of from 0.05 to 2 wt%.

In the smoothing method of a substrate for an EUV mask blank of the present invention, it is preferred that the heating and curing are applied at a temperature from

150 to 500°C in an oxygen-containing atmosphere or a water vapor-containing atmosphere.

In the smoothing method of a substrate for an EUV mask blank of the present invention, it is preferred that the concave defects on the substrate surface have a depth of 30 nm or less.

In the smoothing method of a substrate for an EUV mask blank of the present invention, it is preferred that the concave defects after the heating and curing have a depth of 3 nm or less.

The present invention further provides a substrate for a reflective mask blank for EUV lithography, having a surface smoothed by the smoothing method of a substrate for an EUV mask blank of the present invention (hereinafter referred to as ^λλa substrate for an EUV mask blank of the present invention") .

The present invention further provides a substrate with multilayer reflective film for a reflective mask blank for EUV lithography, comprising the substrate for an EUV mask blank of the present invention.

The present invention further provides a reflective mask blank for EUV lithography, comprising the substrate for an EUV mask blank of the present invention. According to the smoothing method of a substrate for an EUV mask blank of the present invention, a surface of a substrate having concave defects for an EUV mask blank can be smoothed, thereby reducing the concave defects to a size which is not problematic in the production of an EUV mask blank. The degree to which the concave defects are reduced varies depending on the shape of the concave defects. However, one approximation is that according to the present invention, a surface of a substrate for an EUV mask blank, having concave defects having a depth of 30 nm or less which are often present on a film formation surface of a substrate for an EUV mask blank after performing surface polishing and cleaning can be smoothed, and the depth of concave defects can be reduced to 3 nm or less.

Convex defects present on a substrate surface can be removed by the conventional wet cleaning methods using hydrofluoric acid or aqueous ammonia, brush cleaning or precision polishing. However, when those methods are applied for the purpose of removing the convex defects, new concave defects may be generated on the substrate surface, but those concave defects can be reduced by the smoothing method of the substrate for an EUV mask blank of the present invention.

Further, in subjecting the substrate for an EUV mask blank to surface figuring such as polishing, concave defects may be generated as by-products. However, such concave defects can also be reduced by the smoothing method of the substrate for an EUV mask blank of the present invention.

Moreover, in the event that concave defects on a substrate surface are problem, when concave defects on the substrate are smoothed by any method, it is important that the smoothed surface by such a film material formation also has no concave defects. According to the smoothing method of a substrate for an EUV mask blank of the present invention, the smoothed surface becomes the condition that concave defects of a size which gives rise to the problem on the production of an EUV mask blank are not present thereon by using a silica coating (or a coating comprising Siθ2 as a main skeleton) obtained by heating and curing a polysilazane compound. It is thought that the silica coating formed by heating and curing a polysilazane compound is a dense and amorphous silica film.

Accordingly, according to the smoothing method of a substrate for an EUV mask blank of the present invention, a substrate for an EUV mask blank having excellent smoothness, in which concave defects having a size which gives rise to the problem on the production of an EUV mask blank are not present on the film formation surface, can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 (a) is an AFM image before forming a silica coating on the concave defect 10, and Fig. 1 (b) is an AFM image after forming a silica coating on the concave 10.

Fig. 2 (a) is an AFM image before forming a silica coating on the concave defect 11, and Fig. 2 (b) is an AFM image after forming a silica coating on the concave 11.

Fig. 3 (a) is an AFM image before forming a silica coating on the concave defect 12, and Fig. 3 (b) is an AFM image after forming a silica coating on the concave 12.

BEST MODE FOR CARRYING OUT THE INVENTION

The smoothing method of a substrate for an EUV mask blank of the present invention is described below.

The smoothing method of a substrate for an EUV mask blank of the present invention is used for the purpose of smoothing a surface of a substrate for an EUV mask blank, more specifically a substrate surface at the side that a multilayer reflective film and an absorption layer are formed in the production steps of an EUV mask blank (hereinafter referred to as "a film formation surface") . A side on which a film for electrostatic chuck for holding an EUV mask blank is formed may be smoothed by the method of the present invention. When the smoothing method of a substrate for an EUV mask blank of the present invention is applied to conduct a method of smoothing concave defects on a substrate, it comprises polishing a film formation surface of a substrate for an EUV mask blank previously provided, with polishing abrasive grains such as cerium oxide, zirconium oxide or colloidal silica, cleaning the film formation surface using an acidic solution such as hydrofluoric acid, hydrofluosilic acid or sulfuric acid, an alkali solution such as aqueous ammonia, or pure water, and drying the cleaned surface. When convex defects such as foreign matters or fibers are present on the film formation surface, the convex defects are removed by those procedures.

The smoothing method of a substrate for an EUV mask blank of the present invention is preferably used to a substrate surface on which concave defects are present on the film formation surface after surface polishing and cleaning. Extremely large concave defects are not present on the film formation surface after surface polishing and cleaning, and the depth of concave defects present on the film formation surface is at most 30 run.

The substrate for an EUV mask blank is required to have high flatness and smoothness on the entire surface of the film formation surface. Specifically, the film formation surface of the substrate is required to have a smooth surface having RMS (root-mean-square) roughness of 0.15 nm or less and a flatness of 50 nm or less. Even though those required values are satisfied, concave defects referred to as pits and scratches may be present on the film formation surface.

When the size of the concave defects present on the film formation surface is very small, such concave defects may not adversely affect the EUV mask blank production. However, when concave defects having a certain size or bigger are present on the film formation surface, concave defects appear on the surface of the multilayer reflective film or the absorption layer formed on the film formation surface, and such concave defects may be considered as the defects of an EUV mask blank. Further, even when the concave defects do not appear on the multilayer reflective film surface or the absorption layer surface, phase defects may be formed by that the multilayer reflection structure is perturbed in those films.

The size of the concave defects which are considered as the defect of an EUV mask blank varies depending on a shape of a concave defect cannot be defined precisely. However, as one approximation, when concave defects having a depth exceeding 3 nm are present on the film formation surface of a substrate, the concave defects may appear on the surface of the multilayer reflective film or the absorption layer formed on the film formation surface, resulting in the defects of an EUV mask blank. Even when concave defects do not appear on the multilayer reflective film surface or the absorption layer surface, phase defects may be formed by that the multilayer reflection structure is perturbed in those films.

A substrate for an EUV mask blank requires excellent smoothness and flatness, and in addition to this, preferably low coefficient of thermal expansion (preferably

0±1.0xl0-⁸/°C, and more preferably 0+0.3xlO^"8/°C) . Specifically, examples of the substrate having low coefficient of thermal expansion include substrates made of glass having low coefficient of thermal expansion, such as Siθ2-TiO₂ system glass. However, the substrates are not limited to this, and a substrate made of a crystallized glass in which β-quartz solid solution has been precipitated can be used.

The substrate for an EUV mask blank preferably has excellent durability to cleaning solvents which are used in cleaning and the like of an EUV mask blank or a photomask after pattern formation.

Further, the substrate for an EUV mask blank preferably has high rigidity in order to prevent deformation of the substrate by film stress of the multilayer reflective film and the absorption layer formed on the substrate. In particular, the substrate having high Young's modulus of 65 GPa or more is preferable.

Size, thickness and the like of the substrate for an EUV mask blank are appropriately determined by its specifications and the like of a mask. Specific example of the substrate includes a substrate having an outer shape of about 6 inches (152.4 mm) square and a thickness of about 0.25 inch (6.3 mm) .

The smoothing method of a substrate for an EUV mask blank of the present invention comprises applying a solution containing a polysilazane compound to a film formation surface having concave defects of a substrate, and heating and curing the applied solution to form a silica coating (or a coating comprising SiO₂ as a main skeleton) , thereby smoothing the film formation surface having concave defects.

The present invention can use the following polysilazane compounds.

On example of the polysilazane compound that can be used includes an organic polysilazane compound having a structural unit A represented by the following general formula (A) and a structural unit B represented by the following general formula (B) . -Si- (NH) - (A)

R²

R³ -Si-O- (B)

R⁴

In the above organic polysilazane compound, the structural units (A) and (B) each are generally present in a form of A_n, and B_n, respectively, wherein m and n are a positive integer. The same applies to structural units (C) to (I) described hereinafter.

In the above general formulae (A) and (B) , R¹, R², R³ and R⁴ represent a group selected from the group consisting of an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, an aralkyl group, an alkylamino group, an alkylsilyl group and an alkoxy group, or hydrogen, with proviso that R¹ and R² are not simultaneously hydrogen, and R³ and R⁴ are not simultaneously hydrogen. The organic polysilazane compound may further have a structural unit C represented by the following general formula (C) .

R⁵ R⁸

-Si-R⁷-Si- (C)

R⁶ R⁹

In the above general formula (C) , R⁵, R⁶, R⁸ and R⁹ each represent a group selected from the group consisting of an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, an aralkyl group, an alkylamino group, an alkylsilyl group and an alkoxy group, or hydrogen, with proviso that R⁵, R⁶, R⁸ and R⁹ are not simultaneously hydrogen. R⁷ represents a divalent aromatic group.

The above organic polysilazane compound may further have at least one structural unit selected from the group consisting of a structural unit D represented by the following general formula (D) , a structural unit E represented by the following general formula (E) and a structural unit F represented by the following general formula (F) . -Si- (NH) - (D)

H

-Si- (NH) 1. 5 (E)

R¹⁴-S i- (NH) 0. 5^' (F)

In the above general formulae (D), (E) and (F), R¹⁰, R¹¹, R¹², R¹³ and R¹⁴ each represent a group selected from the group consisting of an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, an aralkyl group, an alkylarαino group, an alkylsilyl group and an alkoxy group.

The above organic polysilazane compound may further have at least one structural unit selected from the group consisting of a structural unit G represented by the following general formula (G) , a structural unit H represented by the following general formula (H) and a structural unit I represented by the following general formula (I) . R¹⁵

-Si- (NH) -R¹⁶- (NH) - (G)

H

-S i- (NH) -R¹⁹- (NH) - (H)

R

-Si- ((NH) -R²¹- (NH)) _J.₅- (I)

In the above general formulae (G), (H) and (I), R¹⁵, R¹⁷, R¹⁸ and R²⁰ each represent a group selected from the group consisting of an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, an aralkyl group, an alkylamino group, an alkylsilyl group and an alkoxy group, and R¹⁶, R¹⁹ and R²¹ each represent a divalent aromatic group. In the above general formulas (A) to (I), it is preferable that the alkyl group is a Cl to C3 alkyl group, the alkenyl group is a Cl to C2 alkenyl group, the cycloalkyl group is a C6 to C8 cycloalkyl group, the aryl group is a C6 to C8 aryl group, the aralkyl group is a Cl to C3 aralkyl group, the alkylamino group is a Cl to C3 alkylamino group, the alkylsilyl group is a Cl to C3 alkylsilyl group, and the alkoxy group is a Cl to C3 alkoxy group .

In the above general formulae (C), (G), (H) and (I), the divalent aromatic group is preferably an araklylene group or an arylene group, and particularly preferably a phenylene group, a tolylene group, a xylilene group, a benzilidene group, a phenethylidene group, an α- methylbenzilidene group, a cynnamylidene group or a naphthylene group.

Other preferred embodiment of R⁷, R¹⁶, R¹⁹ and R²¹ which are a divalent aromatic group includes a group represented by the following general formula.

R²² in the above general formula represents a halogen atom or a lower alkyl group, and preferably a Cl to C3 alkyl group. a is an integer of from 0 to 4, and Z is directly bonded, or represents a group represented by the following general formula.

R²³ in the above general formula represents a halogen atom or a lower alkyl group, and preferably a Cl to C3 alkyl group, b is an integer of from 0 to 4. Y is directly bonded, or represents a divalent bond. Preferably, Y is directly bonded, or is -0-, -S-, -CH₂- or -CH₂CH₂-.

R⁷, R¹⁶, R¹⁹ and R²¹ which are a divalent aromatic group each are particularly preferably a group selected from the group consisting of a phenylene group, a tolylene group, a xylilene group, a benzilidene group, a phenethylidene group, an α-methylbenzilidene group, a cynnamylidene group and a naphthylene group.

In the above organic polysilazane compound, when the number of Si-O bond is N_Si-o and the number of Si-N bond is Nsi-N/ N_Si-o/ (Nsi-N + N_Si-o) is preferably from 0.50 to 0.99, and particularly preferably from 0.80 to 0.95. When N_Si- o/ (Nsi-N + N_Si-o) is smaller than 0.50, there is the tendency that elastic modulus increases, and the compound becomes brittle. When it exceeds 0.99, crosslinking point in the organic polysilazane decreases, and hardness may become insufficient.

The organic polysilazane compound can be synthesized by conventional methods as described in, for example, JP-A- 2005-36089 and JP-A-2004-77874.

For example, the inside of a reaction vessel placed in a thermostatic chamber is replaced with dry nitrogen, and a mixture obtained by dissolving 47 g (0.222 mol) of phenyl trichlorosilane (PhSiCl₃), 56 g (0.222 mol) of diphenyl dichlorosilane (Ph₂SiCl₂), 3.8 g (0.033 mol) of methyl dichlorosilane (MeSiHCl₂) and 50 g (0.19 mol) of 1, 4-bis (dimethylchlorosilyl) benzene in 1,000 ml of xylene is introduced into the reaction vessel.

Temperature in the reaction vessel is set to -5°C, and a solution obtained by dissolving 13.0 g (0.7222 mol) of water in 1,000 ml of pyridine was poured in the reaction vessel at a rate of about 30 ml/min. At this time, reaction between a halosilane and water occurs simultaneously with the pouring, and the temperature in the vessel elevates to -2°C. After completion of the pouring of the mixed solution of water and pyridine, stirring is continued for 1 hour. Thereafter, ammonia is poured at a rate of 2 mol/min for 10 minutes for the purpose of completely reacting unreacted chlorosilane, followed by stirring. After completion of the reaction, dry nitrogen is blown into the vessel to remove unreacted ammonia, and the reaction mixture was filtered under pressure in a nitrogen atmosphere to obtain about 2,300 ml of a filtrate. When the filtrate is solvent substituted under reduced pressure, 100 g of an organic polysilazane compound al which is a colorless, transparent and viscous resin is obtained.

The organic polysilazane compound al obtained has a number average molecular weight of 2,200, and is an organic polysilazane compound having the structural unit A wherein R¹ and R² are a phenyl group, the structural unit B wherein R³ and R⁴ are a phenyl group, the structural unit C wherein R⁵, R⁶, R⁸ and R⁹ are CH₃, and R⁷ is a phenyl group, the structural unit D wherein R¹⁰ is CH₃, and the structural unit E wherein R¹¹ is a phenyl group.

When the organic polysilazane compound al is subjected to IR spectrum analysis, absorption based on N-H group at a wavelength of 3,350 cm^"1, absorption based on Si-H at 2,160 cm^"1, absorption based on Si-Ph group at 1,140 cm^"1, absorption based on Si-O at 1,060 to 1,100 cm^"1, absorption based on Si-H and Si-N-Si at 1,020 to 820 cm^"1, absorptions based on C-H at 3,140 cm^"1, 2,980 cm^"1 and 1,270 cm^"1, and absorptions based on C-H of a benzene ring at 810 cm^"1 and 780 cm^"1 are confirmed.

When ¹H-NMR spectrum of the organic polysilazane compound al is measured, absorptions of δ7.2 ppm (br, CeHs), 64.8 ppm (br, SiH), δl.4 ppm (br, NH) and δO .3 ppm (br, SiCHs) are confirmed.

The theoretical value of N_si-₀/ (N_si__N + N_si_₀) of the organic polysilazane compound al is 0.928.

Other examples of the organic polysilazane compound that can be used include organic polysilazane compounds disclosed in JP-A-2005-36089 and JP-A-2004-77874.

An inorganic polysilazane compound having a structural unit J represented by the following general formula (J) can also be used as the polysilazane compound.

H

-S i- (NH) - (J)

H

In the above inorganic polysilazane compound, the structural unit J is generally present in a form of Ji wherein 1 is a positive integer. 1 is generally from 10 to 10,000, and typically from 10 to 200. In the above structural unit J, its terminal group is not particularly limited, but is generally a silyl group, a methyl group, an amino group, a methoxy group, an alkoxy group or a trimethylsilyl group. Further, to bond to other component such as an organic polysilazane compound, the structural unit J may have a carboxyl group, an amino group, a hydroxyl group, a carbonyl group or the like as the terminal group .

Specific example of the inorganic polysilazane compound having the structural unit J includes perhydropolysilazane. Production method of the perhydropolysilazane is described in, for example, JP-A-60- 145903 and D. Seyferth, et al . , Communication of Am. Cer. Sor., c-13, January (1982).

For example, a gas blowing pipe, a mechanical stirrer and a Dewar condenser are fitted to a four-necked flask having an inner volume of 1 liter, and the inside of the reactor is substituted with deoxidated dry nitrogen. 490 ml of dry pyridine is introduced into the four-necked flask, and the flask is ice-cooled.

51.9 g of dichlorosilane is added to the flask to form a white solid adduct (SiH₂Cl₂-2C₂HsN) . The reaction mixture is ice-cooled, and 51.0 g of ammonia purified by passing through a sodium hydroxide pipe (absorption pipe) and activated carbon is blown in the flask while stirring the reaction mixture. The reaction mixture is then heated to 100⁰C.

After completion of the reaction, the reaction mixture is centrifuged, and washed using dry pyridine. The reaction mixture is filtered in a dry nitrogen atmosphere to obtain 850 ml of a filtrate. A solvent is removed from 5 ml of the filtrate to obtain 0.102 g of a resinous, solid perhydropolysilazane.

The perhydropolysilazane obtained has a number average molecular weight of 1,100 when measured with GPC (calculated as polystyrene) .

When IR spectrum analysis is conducted, absorptions based on N-H at wavelengths of 3,390 cm^"1 and 1,180 cm^"1, absorption based on Si-H at 2,170 cm^"1, and absorption based on Si-N-Si at 1,040 to 800 cm^"1 are confirmed.

The organic polysilazane compound having the above- described structural units A to I and the inorganic polysilazane compound having the structural unit J can be used in combination. In this case, the blending proportion of those compounds can optionally be selected according to the purpose, but the blending proportion of the inorganic polysilazane compound is preferably 90 parts by mass or less, and more preferably 50 parts by mass or less, per 100 parts by mass of the sum of the organic polysilazane compound and the inorganic polysilazane compound. When the blending proportion is changed, hardness of a film obtained by applying a solution containing the polysilazane compound and firing the applied solution changes. Therefore, it is preferred to appropriately select the blending proportion of those compounds according to need.

The polysilazane compound used in the present invention preferably has a number average molecular weight of from 500 to 2,500 approximately.

The polysilazane compound is hydrolyzed by reacting with a substance having a hydroxyl group. For this reason, water or an alcoholic solvent cannot be used as a solvent of the polysilazane compound. Further, a ketone type solvent and an ester type solvent dissolve water, and are not preferred. The solvent of the polysilazane compound is preferably high boiling aromatic solvents and mineral terpenes from the points of solubility, stability and coating properties, and may be ether type solvents such as dibutyl ether.

In the present invention, the solution for applying to a film formation surface of a substrate for an EUV mask blank is preferably that concentration of the polysilazane compound is from 0.05 to 2 wt%. When the concentration of the polysilazane compound is less than 0.05 wt%, film thickness of a silica coating (or a coating comprising SiO₂ as a main skeleton) formed by applying a solution containing the polysilazane compound, followed by heating and curing is too thin, and as a result, the effect of smoothing a film formation surface having concave defects becomes insufficient. When the concentration of the polysilazane compound exceeds 2 wt%, uniformity in thickness of a silica coating (or a coating comprising SiO₂ as a main skeleton) formed by applying a solution containing the polysilazane compound, followed by heating and curing deteriorates, resulting in deterioration of properties of a substrate for an EUV mask blank, such as flatness and surface roughness of a substrate. Further, coating properties of the solution containing the polysilazane compound deteriorate, although depending on a coating method used.

The solution for applying to a film formation surface of a substrate for an EUV mask blank may contain other components according to need, in addition to the polysilazane compound and the solvent. Examples of the other components that can be contained in the solvent include catalysts and polymerization initiators, as a promoter of the conversion reaction of from the polysilazane compound to silica. Specific examples of such catalysts include Pd compounds as an inorganic catalyst. Further, examples of organic catalysts include amine type catalysts. When those catalysts are applied, it is thought that curing at further low temperature, for example, curing at room temperature, is possible. Another example includes UV initiators which are added in proceeding the conversion reaction of from the polysilazane compound to silica by irradiation with ultraviolet rays (UV) . When UV irradiation is applied, it is thought that curing at room temperature is possible.

The solution containing the polysilazane compound can be selected from the commercially available products. Examples of such commercially available products include ALCEDAR COAT, the products of Clariant (Japan) K. K.

An application of the solution ^'containing the polysilazane compound can be performed by conventional methods, spin coating, dipping, roller coating, bar coating, spray coating, brush coating and the like. Of those, spin coating or dipping is preferable, and spin coating is particularly preferable, from the point that the solution can be applied uniformly to the film formation surface. When spin coating is selected, rotation speed is preferably 2,000 rpm or higher rpm in order to obtain uniform film formation.

After the application of the solution containing the polysilazane compound, the substrate is heated and cured in an oxygen-containing atmosphere or a water vapor-containing atmosphere. As a result, the polysilazane compound reacts with oxygen or water vapor in the atmosphere, and converts to silica, thereby a silica coating (or a coating comprising SiO₂ as a main skeleton) is formed on the film formation surface of the substrate. The oxygen-containing atmosphere used herein means that oxygen is present in the atmosphere, and oxygen concentration in the atmosphere is not particularly limited. The water vapor-containing atmosphere used herein means that water vapor is present in the atmosphere, and water vapor concentration in the atmosphere is not particularly limited. Therefore, the heating and curing may be performed in air.

The heating and curing temperature varies depending on the kind of the polysilazane compound used and the concentration of the polysilazane compound in the solution, but the heating and curing is preferably performed at a temperature of from 150 to 500°C. When the heating and curing temperature is lower than 150°C, the conversion from the polysilazane compound to silica does not proceed sufficiently, and as a result, a silica coating (or a coating comprising SiO₂ as a main skeleton) having a sufficient thickness to smooth concave defects cannot be formed. When the heating and curing temperature exceeds

500°C, baking (heating and curing) temperature is higher than the strain point of the substrate, although depending on a substrate material, and as a result, the substrate may deform.

The heating and curing temperature is more preferably from 150 to 400°C.

When a promoter of the conversion reaction, such as a catalyst or a polymerization initiator, is used as described before, it is thought that curing at significantly low temperature, for example, curing at room temperature, is possible. Further, when UV irradiation is applied, it is thought that curing at room temperature is possible .

In the present invention, the film formation surface having concave defects is smoothed by forming a silica coating (or a coating comprising SiO₂ as a main skeleton) , specifically a coating which is considered as a dense and amorphous silica coating, on the film formation surface having concave defects of a substrate for an EUV mask blank. The film formation surface after the formation of a coating means a surface of a silica coating (or a coating comprising Siθ₂ as a main skeleton) . In the case that the film formation surface having concave defects is smoothed, the film formation surface, that is, a coating surface, is not necessary to be a condition that concave defects are not present at all, and it is sufficient if only the concave defects present on the coating surface are smoothed to a size such that there is no problem as the substrate for an EUV mask blank.

The size of concave defects which are defects of an EUV mask blank varies depending on the shapes of concave defects, and cannot be completely defined. As one approximation, when concave defects having a depth exceeding 3 nm are present on the film formation surface of a substrate, the concave defects appear on the surface of a multilayer reflective film or an absorption layer, which is formed on the film formation surface, and may become defects of an EUV mask blank. Even when concave defects do not appear on the surface of the multilayer reflective film or the absorption layer, phase defects may be caused by that the structure is perturbed in those films. For this reason, the depth of concave defects present on the coating surface is preferably 3 nm or less, and more preferably 1 nm or less.

The coating surface may have poor surface roughness as compared with the film formation surface of the substrate for an EUV mask blank, although depending on the components, the film thickness, the film formation conditions and the like of the coating formed. Stated differently, the surface roughness of the film formation surface may deteriorate by the formation of the coating. In this case, the surface roughness of the film formation surface (coating surface) can be improved by conventional methods that improve the surface roughness but do not create new concave defects. A specific example of such methods preferably includes mechanical polishing at low pressure, called touch polishing. Also, the surface roughness can be improved by applying wet etching. or dry etching by controlling the etching rate and/or the etching time.

When an EUV mask blank is produced using the substrate for an EUV mask blank of the present invention, the multilayer reflective film and the absorption layer may be formed in this order on the coating surface formed by the above procedures using conventional film formation methods, specifically sputtering methods such as a magnetron sputtering method or an ion beam sputtering method.

The multilayer reflective film is not particularly limited so far as it has the desired properties as the multilayer reflective film of an EUV mask blank. The properties particularly required in the multilayer reflective film are that it is the film having high EUV light reflectivity. Specifically, when the surface of a multilayer reflective film is irradiated with light of EUV light wavelength region, the maximum value of the light reflectivity in the vicinity of a wavelength of 13.5 nm is preferably 60% or more, and more preferably 65% or more.

Examples of the multilayer reflective films satisfying the above properties include a Si/Mo multilayer reflective film comprising a Si film and a Mo film laminated alternately, a Be/Mo multilayer reflective film comprising a Be film and a Mo film laminated alternately, a Si compound/Mo compound multilayer reflective film comprising a Si compound layer and a Mo compound layer laminated alternately, a Si/Mo/Ru multilayer reflective film comprising a Si film, a Mo film and a Ru film laminated in this order, and a Si/Ru/Mo/Ru multilayer reflective film comprising a Si film, a Ru film, a Mo film and a Ru film laminated in this order.

The procedure for film-forming the multilayer reflective film may be a procedure generally carried out in film-forming a multilayer reflective film using a sputtering method. For example, when a Si/Mo multilayer reflective film is formed using an ion beam sputtering method, it is preferred that a Si film is formed so as to have a thickness of 4.5 nm at an ion accelerating voltage of from 300 to 1,500 V and a film-formation rate of from 0.03 to 0.30 nm/sec, using a Si target and using Ar gas

(gas pressure: 1.3xlO^~2 Pa to 2.7xlO^~2 Pa) as a sputtering gas, and a Mo film is then formed so as to have a thickness of 2.3 nm at an ion accelerating voltage of from 300 to 1,500 V and a film-formation rate of from 0.03 to 0.30 nm/sec, using a Mo target and using Ar gas (gas pressure: 1.3xlO^~2 Pa to 2.7xlO^~2 Pa) as a sputtering gas. Considering the foregoing procedures asone cycle, a Si film and a Mo film are laminated with 40 to 50 cycles.- Thus, a Si/Mo multilayer reflective film is formed.

In forming a multilayer reflective film, it is preferred that film formation is conducted while rotating a substrate using a turntable for a substrate in order to obtain uniform film formation.

To prevent the surface of the multilayer reflective film from being oxidized, the top layer of the multilayer reflective film is preferably a layer comprising a material which is difficult to be oxidized. A layer comprising a material which is difficult to be oxidized functions as a cap layer of the multilayer reflective film. Example of the layer comprising a material which is difficult to be oxidized includes Si layer. When the multilayer reflective film is Si/Mo film, the top layer can be functioned as a cap layer by forming the top layer with a Si layer. In this case, the cap layer has a film thickness of preferably ll.O±l.O nm.

In the present description, a substrate having a multilayer reflective film formed on the coating surface by the above procedures is called ^ΛNa substrate with multilayer reflective film for an EUV mask blank". When an EUV mask blank is produced by using the substrate with the multilayer reflective film, the absorption layer is formed on the multilayer reflective film formed by the above procedures, or a cap layer in the case where the top layer of the multilayer reflective 'film is the cap layer, using conventional film formation methods, specifically sputtering methods such as a magnetron sputtering method or an ion beam sputtering method.

Examples of the material constituting the absorption layer formed on the multilayer reflective film include materials having high absorption coefficient to EUV light, specifically Cr, Ta and their nitrides. Of those, TaN is preferable for the reasons that it is apt to be amorphous and its surface is smooth. The absorption layer has a thickness of preferably from 50 to 100 nm. The film formation method of the absorption layer is not particularly limited so far as it is a sputtering method, and may be any of a magnetron sputtering method or an ion beam sputtering method.

When TaN layer is formed as the absorption layer using an ion beam sputtering method, it is preferred to form the film so as to have a thickness of from 50 to 100 nm at a voltage of from 300 to 1,500V and the film formation rate of from 0.01 to 0.1 nm/sec, using a Ta target and using N₂ gas (gas pressure: 1.3xlO^~2 Pa to 2.7xlO^~2 Pa) as a sputtering gas.

In film-forming the absorption layer using a sputtering method, it is preferred to conduct the film formation while rotating a substrate using a turntable for a substrate in order to obtain uniform film formation.

A buffer layer may be formed between the multilayer reflective film and the absorption layer. Examples of the materials constituting the buffer layer include Cr, Al, Ru, Ta and their nitrides; and further include SiO₂, Si3N₄ and Al₂O₃. The buffer layer has a thickness of preferably from 10 to 60 nm.

EXAMPLES

The present invention will be illustrated in greater detail with reference to the following Examples .

In the Examples, a solution containing a polysilazane compound was applied to a surface of a substrate for an EUV mask blank by spin coating, and was heated and cured in air to form a silica coating on the substrate surface. The respective conditions in this procedure are as follows. Substrate for EUV Mask Blank

SiO₂-TiO₂ system glass substrate: a product of the Asahi Glass Co., Ltd., part number 6025 A substrate which is polished to have a surface roughness of 0.15 run or less and flatness of 100 run or less was used. A kapton tape was adhered to a part of the substrate surface.

Coefficient of thermal expansion: 0.2xl0^"7/°C Young's modulus: 67 GPa

Strain point Ts: 1,100⁰C

Size: outer shape 6 inches (152.4 mm square), thickness 6.3 mm Solution Containing Polysilazane Compound

Name: ALCEDAR COAT NN310-20 (a product of the Clariant (Japan) K. K.)

Number average molecular weight: 900

Solvent: xylene

Polysilazane compound content: 20 wt%

Density: 0.92 (catalog value at a polysilazane compound content of 20 wt%)

Viscosity: 1.10 cp (catalog value at a polysilazane compound content of 20 wt%)

The above solution was diluted with xylene to adjust such that the polysilazane compound content is 10 wt%, 5 wt% and 2 wt%. Spin Coating

Filter: 0.5 μm

Spin coating conditions: 2,000 rpm, 10 sec. 2 cc of the polysilazane compound-containing solution was spin coated on a substrate surface under the above conditions . Heating and Curing

The substrates after spin coating were placed in a clean oven, and heated and cured at 200°C for 1 hour in air.

After heating and curing, a kapton tape was peeled from the substrate surface, and a height difference between the tape-peeled area (original surface) and the silica coating formed area thereon was measured with a non-contact three-dimensional surface shape measurement equipment (Zygo NewView) . As a result, it was confirmed that film thickness of the silica coating is 150 nm (polysilazane compound content: 10 wt%), 50 nm (polysilazane compound content: 5 wt%) and 20 nm (polysilazane compound content: 2 wt%) . This result shows that film thickness of the silica coating formed on the substrate surface can be controlled by changing the polysilazane compound content in the solution.

Six substrates for EUV mask blanks having the same composition as above were provided, and concave defects on the respective substrate surfaces were inspected by a defect inspection tool. Vickers markings were given around concave defects detected by the defect inspection tool, and the shapes of the concave defects were measured by an atomic force microscope (AEM) . The results obtained are shown in Table 1 below.

TABLE 1

Concave defect (unit: nm )

Width (Min) Width (Max) Depth Shape

Concave defect 1 145 156 22.1 Pit

Concave defect 2 108 242 14.5 Pit

Concave defect 3 100 203 12.5 Pit

Concave defect 4 98 294 8.4 Pit

Concave defect 5 83 268 20.2 Pit

Concave defect 6 102 229 6.4 Pit

A solution containing the polysilazane compound (polysilazane compound content: 2 wt%) was applied to the surfaces of six substrates provided above by spin coating in the same procedures as above, and heated and cured to form a silica coating on the surfaces of the substrates. After heating and curing, there was an attempt to measure shapes of the concave defects on the surfaces of the silica coating using an AFM. However, all of the concave defects present on the substrate surfaces were covered completely with the silica coating, and their positions could not be confirmed. As is apparent from Table 1, it was confirmed by the AFM analysis that the formation of the silica coating on the substrate surfaces through spin coating of the solution containing the polysilazane compound (polysilazane compound content: 2 wt%) followed by heating and curing smoothes concave defects having various shapes and reduces their depths to give a smooth surface having an RMS (Root-Mean-Square) roughness, which shows the degree of being indistinguishable from the surface roughness of the substrate, of 0.15 nm or lower. Similar to the above, six substrates for EUV mask blanks were provided, and the shapes of the concave defects detected by a defect inspection tool were measured by an atomic force microscope

(AEM) . The results obtained are shown in Table 2 below. The solution containing the polysilazane compound

(polysilazane compound content: 0.2 wt%) was applied by spin coating to the surfaces of six substrates provided above in the same procedures as above, and heated and cured to form a silica coating on the surfaces of the substrates . After heating and curing, shapes of the concave defects on the surfaces of the silica coating were measured by an AFM. The results obtained are shown in Table 2 below. TABLE 2

Concave defect (unit: nm)

Before formation of silica coating After formation of silica coating

Width (Min) Width (max) Depth Shape Depth

Concave defect 7 211 443 6.0 Scratch 5.2

Concave defect 8 130 463 4.7 Scratch 3.2

Concave defect 9 91 228 17.2 Scratch 8.4

Concave defect 10 89 190 16.1 Pit 7.2

Concave defect 11 113 153 15.6 Pit 8.9

Concave defect 12 99 165 12.0 Pit 10.0 As is apparent from Table 2, according to the present invention, it was confirmed that concave defects having various shapes can be smoothed and their depths can be reduced. In Table 2, the depth of the concave defects after formation of the silica coating exceeds 3 nm. However, taking the width and shape of the concave defects into consideration, it can be considered that these concave defects are not problematic in the production of EUV mask blanks, because these concave defects can be smoothed and their depths can be reduced to an undetectable level of the AFM by forming again a silica coating on the substrate surface through spin coating of the solution containing the polysilazane compound (polysilazane compound content: 0.2 wt%) followed by heating and curing in the same procedures as above.

AFM images of the concave defects 10 to 12 before and after formation of the silica coating are shown in Figs. 1 to 3. Fig. 1 (a) is an AFM image of the concave defect 10 before the formation of the silica coating, and Fig. 1 (b) is an AFM image of the concave defect 10 after the formation of the silica coating. Fig. 2 (a) is an AFM image of the concave defect 11 before the formation of the silica coating, and Fig. 2(b) is an AFM image of the concave defect 11 after the formation of the silica coating. Fig. 3 (a) is an AFM image of the concave defect 12 before the formation of the silica coating, and Fig. 3 (b) is an AFM image of the concave defect 12 after the formation of the silica coating. As is apparent from the drawings, according to the present invention, concave defects having various shapes can be smoothed and their depths can be reduced, without causing new concave defects and without causing remarkable increase of surface roughness.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

This application is based on Japanese Patent Application No. 2006-280173 filed October 13, 2006, and the contents thereof are herein incorporated by reference.

Claims

1. A method of smoothing a surface of a substrate for a reflective mask blank for EUV lithography, comprising applying a solution containing a polysilazane compound to a substrate surface having concave defects, and heating and curing the applied solution to form a silica coating (a coating comprising SiO₂ as a main skeleton) , thereby smoothing the substrate surface having concave defects.

2. The method of smoothing a surface of a substrate for a reflective mask blank for EUV lithography as claimed in claim 1, wherein the solution containing a polysilazane compound has a polysilazane compound concentration of from 0.05 to 2 wt%.

3. The method of smoothing a surface of a substrate for a reflective mask blank for EUV lithography as claimed in claim 1 or 2, wherein the heating and curing are conducted at a temperature from 150 to 500°C in an oxygen- containing atmosphere or a water vapor-containing atmosphere.

4. The method of smoothing a surface of a substrate for a reflective mask blank for EUV lithography as claimed in any one of claims 1 to 3, wherein the concave defects on the substrate surface have a depth of 30 nm or less.

5. The method of smoothing a surface of a substrate for a reflective mask blank for EUV lithography as claimed in any one of claims 1 to 4, wherein the concave defects after the heating and curing have a depth of 3 nm or less.

6. A substrate for a reflective mask blank for EUV lithography, having a surface smoothed by the method as claimed in any one of claims 1 to 5.

7. A substrate with a multilayer reflective film for EUV lithography, comprising the substrate for a reflective mask blank for EUV lithography as claimed in claim 6.

8. A reflective mask blank for EUV lithography, comprising the substrate for a reflective mask blank for EUV lithography as claimed in claim 6.