JP2019099415A - Silica glass sheet and method for reducing birefringence of silica glass sheet - Google Patents

Abstract

To provide a silica glass sheet used in optical lithography and having birefringence amount of a silica glass surface vertical to a transmission direction of a light simply controlled to 2.0 nm/cm or less, and a method for reducing birefringence of the silica glass sheet.SOLUTION: The silica glass sheet is a silica glass used in an optical lithography process with a vacuum ultraviolet light as a light source, content of fluorine is 1 wt.% to 5 wt.%, birefringence amount of a silica glass surface (utilized surface) vertical to a transmission direction of a light is simply controlled to 2.0 nm/cm or less when irradiated with an ultraviolet light with 193 nm as exposure wavelength of optical lithography, and birefringence when the silica glass surface in parallel to the transmission direction of the light is irradiated with a light vertically is larger than birefringence of the vertical silica glass surface.SELECTED DRAWING: None

Description

  The present invention relates to a method of reducing birefringence of a silica glass plate and a silica glass plate, for example, a method of reducing birefringence of a silica glass plate for a photomask and a silica glass plate that can be used for photolithography in the vacuum ultraviolet wavelength region.

  In recent years, in lithography technology, the demand for miniaturization of semiconductor devices has been increasing more and more, exposure has been shortened by shortening the exposure wavelength, or by immersion exposure technology in which pure water or the like is immersed between the lens and the wafer. A method of increasing the numerical aperture of the lens to be used is adopted.

  Assuming that the wavelength of exposure light is λ, the numerical aperture representing the lens performance of the exposure apparatus is NA, and the process constant is k1, the resolution R in photolithography can be expressed by the equation R = k1λ / NA. The resolution can be improved by reducing the process constant k1 while shortening the numerical aperture NA.

  Here, the exposure wavelength λ starts from the g-line (436 nm) of a mercury lamp, and so far i-line (365 nm), KrF excimer laser (248 nm) and ArF excimer laser (193 nm) are used to shorten the light source Has been advanced.

  The numerical aperture NA geometrically represents the size of the lens, and when the exposure light is narrowed by the lens and imaged on the wafer surface, NA = n · sin θ (n is a medium between the lens and the wafer The refractive index of, θ represents the opening angle of the light beam.

Here, when the medium between the lens and the wafer is air, the refractive index n = 1.0, but when this medium is changed to pure water, the refractive index of water to the ArF excimer laser wavelength is 1.44. Therefore, NA takes a value of 1.44 at the maximum.
In reality, the opening angle is not 0 degree, so NA can take a value of about 1.35. The kl factor is called a process constant determined by the optical system and resist performance, and the theoretical limit is 0.25.
Therefore, by using an ArF excimer laser with an exposure light wavelength of 193 nm and using an immersion exposure technique, it is possible to achieve a resolution of 43 nm with a numerical aperture of 1.35 and a kl factor of 0.3.

  And, as a member for photolithography using this ArF excimer laser, a silica glass plate is suitably used because it is excellent in low thermal expansion and light transmission.

As the performance required for the silica glass plate, in the case of using an ArF excimer laser, light resistance and the like in which the light transmittance is not deteriorated even when exposed to high energy light can be mentioned.
In addition, when immersion exposure is performed, the difference between the refractive index of pure water existing between the lens and the wafer and the refractive index of the resist is reduced, so that the opening angle of the light beam is increased, and the polarization effect becomes a problem.
For this reason, the silica glass plate is required to have low birefringence. When the silica glass plate has birefringence, the transmitted exposure light may cause polarization change, which may deteriorate the imaging performance.

As a method of reducing this birefringence, silica glass is maintained in a temperature range of 1000 ° C. to 1200 ° C., thermal strain is reduced, and it remains in the glass by cooling (annealing treatment) at a slow temperature lowering rate Distortion is removed and birefringence is reduced.
For example, Patent Document 1 shows that, in a silica glass having a fluorine content of 1000 wt ppm, the birefringence is set to 5 nm / cm or less at a wavelength of 633 nm by annealing.

  Moreover, it is known that patent document 2 sets the birefringence amount to 2 nm / cm or less at a wavelength of 200 nm or less in silica glass having a fluorine content of 100 ppm to 1000 ppm by annealing treatment. Furthermore, in the embodiment of this patent document 2, in order to remove the thermal strain remaining in the transparent quartz glass ingot, the temperature is raised from room temperature to 1100 ° C. for 3 hours in an air furnace and then held for 10 hours. It is described to cool to 950 ° C. at a temperature drop rate of 0 ° C./hr, to turn off the power and to cool in the air. And, it is described that the birefringence of the obtained synthetic quartz glass block is approximately 1 nm / cm, and the strain is removed by annealing.

Japanese Patent Application Laid-Open No. 2002-60227 JP 2000-264671 A

By the way, since silica glass has an irregular amorphous structure, it is difficult to remove strain in silica glass having an amount of birefringence of 2.0 nm / cm or less even by annealing, as shown in, for example, Patent Document 2 Generally, by using an electric furnace or the like having a uniform temperature distribution, the entire silica glass member is heated, and low birefringence is achieved by performing gradual slow cooling while maintaining a uniform temperature distribution. Was achieved.
However, in this long-time annealing process, Si—F bonds in silica glass are thermally broken during the process, and a denatured layer having a changed fluorine content is generated in the outer peripheral portion. In this case, the degenerated layer has different viscosity because it has a different fluorine concentration, which causes distortion to generate birefringence and exerts an adverse effect, so long-time annealing is not preferable.

Under these circumstances, the present inventor has intensively studied a silica glass plate having a birefringence of 2.0 nm / cm or less and a method of reducing birefringence.
In this research, based on the recognition that the inventors of the present invention have importance in the case of silica glass used in lithography, birefringence when vacuum ultraviolet light is incident on the surface of silica glass which is a working surface is important. We examined the method of annealing treatment.

That is, as described above, since the conventional annealing treatment uniformly heats and gradually cools the entire silica glass member, in the light transmission direction as a result when the silica glass member is considered as a three-dimensional object On the other hand, a glass having low birefringence in all directions of three directions, that is, the parallel direction (Z direction) and the vertical direction (X and Y directions) was produced.
Therefore, in order to make the amount of birefringence in the vertical direction on the plane of incidence of vacuum ultraviolet light, which is an important surface in silica glass used in lithography, to be 2.0 nm / cm or less, the present inventor It tried to control the variation of the structure.
Specifically, the amount of birefringence when irradiating the working surface (incident surface) with vacuum ultraviolet light is made different from the amount of birefringence when irradiating a surface perpendicular to the incident surface (when irradiating the incident surface) It is found that the birefringence of the surface perpendicular to the light transmission direction can be made very low by making the birefringence amount of the light smaller than the birefringence amount when irradiated to the surface perpendicular to the present invention. The

  The present invention has been made to solve the above technical problems, and in a silica glass plate used in sography, a surface where the amount of birefringence when light is incident from the working surface intersects the working surface perpendicularly. It is an object of the present invention to provide a silica glass plate having a birefringence amount smaller than the birefringence amount when light is incident from the above and a wavelength of 2.0 nm / cm or less and a method for reducing birefringence of the silica glass plate.

  The silica glass plate according to the present invention, which was made to solve the above technical problems, is a silica glass used in a photolithographic process using vacuum ultraviolet light as a light source, and contains 1 wt% to 5 wt% of fluorine In the following, when ultraviolet light of 193 nm, which is the exposure wavelength of photolithography, is irradiated, the birefringence of the silica glass surface (working surface) perpendicular to the light transmission direction is 2.0 nm / cm or less, and The amount of birefringence when light is irradiated perpendicularly to the silica glass surface parallel to the light transmission direction is characterized by being larger than the birefringence amount of the perpendicular silica glass surface. In the present application, 1 wt% of fluorine means 1.0 wt% and 5 wt% of fluorine means 5.0 wt%.

As described above, in the present invention, the silica glass used in the optical lithography transmits light in order to make the birefringence amount from the incident surface which is the working surface of the vacuum ultraviolet light be 2.0 nm / cm or less. It controls the variation of the amorphous structure of the silica glass surface perpendicular to the direction.
On the other hand, since the dispersion of the amorphous structure of the silica glass surface parallel to the light transmission direction is not precisely controlled, the birefringence when irradiating the parallel silica glass surface is the birefringence of the perpendicular silica glass surface. However, the effect is small in the use of the photolithographic process.

In addition, since the dispersion of the amorphous structure of the silica glass surface perpendicular to the light transmission direction is controlled, the dispersion of the crystal structure of the vertical plane and the parallel plane with respect to the conventional vacuum ultraviolet light incident plane is controlled. Compared to the case, it can be easily achieved by a time-suppressed annealing process.
As a result, as a silica glass plate used for the photolithographic process which uses vacuum ultraviolet light as a light source, a silica glass plate comparable to the conventional silica glass plate can be obtained.

In addition, since the fluorine content is 1 wt% or more and 5 wt% or less, annealing can be performed at less than 1000 ° C. by further reducing the viscosity, and as a result, breakage and recombination of Si-F bonds can be avoided. .
Since this Si-F bond is less likely to be broken, the distribution of fluorine content in silica glass during annealing does not change, and homogeneous fluorine content can be maintained, so the viscosity in the glass remains uniform. It can be annealed. That is, it is difficult to form a denatured layer due to a change in the fluorine content, and when the 193 nm light which is the exposure wavelength of photolithography is irradiated, the birefringence of the silica glass surface perpendicular to the light transmission direction is 2 nm / cm. It can be

Here, when the content of fluorine is less than 1 wt%, the viscosity of the glass becomes high, and it becomes necessary to carry out annealing at 1000 ° C. or higher, so that breakage of Si-F bond occurs and the content of fluorine changes. Unfavorable results in the formation of altered layers.
On the other hand, when the content of fluorine exceeds 5 wt%, it is not preferable because cleavage of Si-F bond slightly occurs even at a low temperature of 1000 ° C. or less and a so-called altered layer is easily formed. Preferably, the content of fluorine is 1.5 wt% or more and 5 wt% or less.

Here, the OH group concentration is desirably 50 ppm or less.
Since fluorine in glass is exchanged with OH groups, there is a trade-off relationship that the higher the OH group concentration, the lower the fluorine content. Therefore, when the OH group concentration is more than 50 ppm, it may be difficult to make the fluorine concentration 1 wt% or more, which is not preferable. More preferably, the OH group concentration is 10 ppm or less.

The method for reducing birefringence of a silica glass plate according to the present invention, which was made to solve the above technical problems, relates to a method for reducing light in silica glass having a fluorine content of 1 wt% to 5 wt% and a thickness of 10 mm or less. The surface of silica glass (that is, the working surface) perpendicular to the direction of transmission of the light is specified, and heating is performed with a heater from a distance of 1/3 or less of the thickness of the silica glass with respect to the surface of the normal silica glass. .
As described above, the annealing process can be easily performed because the annealing process is performed by specifying the silica glass surface perpendicular to the light transmission direction and heating the perpendicular silica glass surface with a heater.

  According to the present invention, in the silica glass plate used in the sography, the birefringence amount of the silica glass surface (working surface) perpendicular to the light transmission direction is 2.0 nm / cm or less, and the light transmission direction It is possible to obtain a silica glass plate in which the amount of birefringence when irradiated with light perpendicular to the surface of silica glass parallel to the above is larger than the amount of birefringence of the surface of the normal silica glass. Further, by implementing the method for reducing the amount of birefringence of a silica glass plate according to the present invention, a silica glass plate having a birefringence amount of 2.0 nm / cm or less perpendicular to the light transmission direction can be easily obtained. be able to.

The silica glass plate of the present invention is a silica glass used in a photolithography process using vacuum ultraviolet light as a light source, and has a fluorine content of 1 wt% or more and 5 wt% or less and 193 nm as an exposure wavelength of photolithography. When ultraviolet light is irradiated, the birefringence amount of the silica glass surface (working surface) perpendicular to the light transmission direction is 2.0 nm / cm or less and with respect to the silica glass surface parallel to the light transmission direction It is characterized in that the amount of birefringence when vertically irradiated with light is larger than the amount of birefringence of the perpendicular silica glass surface.
In particular, according to the present invention, silica glass used in photolithography has an amount of birefringence of 2.0 nm / cm or less in a plane perpendicular to the incident direction of vacuum ultraviolet light, while birefringence in the plane parallel to the incident direction is It is characterized in that it is larger than the birefringence in the vertical plane, and the amount of birefringence is not set to 2.0 nm / cm or less.
The amount of birefringence in the parallel plane may be 2.5 nm / cm or more, but preferably, the amount of birefringence in the parallel plane is 2.5 nm / cm or less.

Further describing the features of the present invention, the variation of the crystal structure of the silica glass surface perpendicular to the light transmission direction is controlled to make the birefringence amount of the perpendicular silica glass surface 2.0 nm / cm or less, The birefringence amount of the parallel silica glass surface is formed to be larger than the birefringence amount of the perpendicular silica glass surface without controlling the dispersion of the amorphous structure of the silica glass surface parallel to the transmission direction.
As described above, even when the birefringence amount of the silica glass surface parallel to the light transmission direction is larger than the birefringence amount of the perpendicular silica glass surface, the influence thereof in the use of the photolithography process is small, and Similar to silica glass members, they are characterized in that they can be used without deterioration.

In addition, since the variation in the amorphous structure of the silica glass surface parallel to the light transmission direction is not controlled, it can be carried out easily and with a shorter time as compared with the conventional annealing process.
Specifically, by heating and annealing the upper and lower surfaces of the silica glass plate, the birefringence of the silica glass surface perpendicular to the light transmission direction can be 2.0 nm / cm or less.

Moreover, this silica glass has a fluorine content of 1 wt% or more and 5 wt% or less.
As described above, the annealing treatment of the silica glass is performed to remove the strain and the like due to the thermal stress in the silica glass. As the method, it is made by heating for a fixed time above the annealing point of silica glass and annealing to the distortion point or less.

Here, the strain point is a temperature at which the viscosity of the silica glass is 10 ^14.5 dPas, at which temperature viscous flow virtually does not occur, and below this temperature strain in the glass can not be removed. Further, the annealing point is a temperature at which the viscosity is 10 ¹³ dPas, which is a temperature suitable for removing the internal strain generated by the glass processing.

It is known that the viscosity of silica glass changes depending on the fluorine content, and the viscosity decreases when the fluorine content is high, and conversely, the viscosity is relatively high at a portion where the fluorine content is low.
In the present invention, by setting the fluorine content to 1 wt% to 5 wt%, the viscosity can be further reduced to enable annealing at less than 1000 ° C., and as a result, the Si—F bond is broken and recombined. To avoid the occurrence of
That is, by setting the fluorine content to 1 wt% to 5 wt%, the viscosity can be reduced, and annealing can be performed at a lower temperature. Moreover, since the annealing treatment is performed at a lower temperature, breaking of the Si-F bond is unlikely to occur, and during the annealing treatment, the distribution of the fluorine content in the silica glass does not change, and the homogeneous fluorine content can be maintained. As a result, it is hard to produce the degenerating layer which arises by changing fluorine content. Preferably, the content of fluorine is 1.5 wt% or more and 5 wt% or less.
The fact that the fluorine content is preferably 1 wt% or more and 5 wt% or less is described in Japanese Patent Application No. 2017-172645 previously filed by the present applicant.

The OH group concentration is preferably 50 ppm or less.
If the OH group concentration is more than 50 ppm, the viscosity is influenced by the distribution of not only fluorine but also OH groups, making uniform annealing difficult.
Further, since fluorine in glass is exchanged with OH groups, there is a trade-off relationship that the fluorine content decreases as the OH group concentration increases. Therefore, when the OH group concentration is more than 50 ppm, it may be difficult to make the fluorine concentration 1 wt% or more, which is not preferable.
In addition, it is known that the OH group concentration of the glass which does not contain fluorine is generally about 1000-1300 ppm.

Next, the said silica glass board is manufactured by the manufacturing method demonstrated below.
In the present invention, silica glass having a fluorine concentration of 1.0 wt% or more and 5.0 wt% or less can be produced by a liquid phase method called a so-called vapor phase method called sol-gel method or a so-called VAD method.
In the method of forming a porous silica base material, a raw material gas for producing silica and a fluorine compound gas are introduced into a flame from a combustion supporting gas and a flammable gas and reacted to deposit silica glass fine particles on a target. Although there is a so-called direct method of vitrifying, it is not preferable because the distribution of fluorine content generated when depositing silica glass particles on the target tends to be uneven.
However, even if the direct method is used, the method for producing silica glass according to the present invention is applied, as long as the nonuniformity of the content distribution of fluorine generated when depositing the silica glass fine particles on the target is alleviated. be able to.

  In the VAD method, a hydrolysis reaction of a gas using silicon tetrachloride as a raw material is caused to form a porous silica base material called soot. Thereafter, a fluorine addition treatment and a densification treatment are performed in a fluorine gas atmosphere represented by silicon tetrafluoride gas.

The fluorine gas atmosphere in the fluorine doping, which is performed after forming the porous silica matrix (soot), is formed of a mixed gas of fluorine gas and He gas, and the fluorine concentration in the mixed gas (concentration ratio of SiF ₄ gas) is More than 5 vol% to 35 vol% is preferable, 10 vol% to 35 vol% is more preferable, and 25 vol% to 35 vol% is particularly preferable.
Moreover, 1000 degreeC-1300 degreeC are preferable, and, as for introduction | transduction temperature of mixed gas, 1100 degreeC-1200 degreeC is more preferable. When the introduction temperature is less than 1000 ° C., diffusion of fluorine into the glass structure may be slow and not sufficiently doped. On the other hand, if the temperature is higher than 1300 ° C., sintering of soot may start, and diffusion of fluorine into the glass structure may be inhibited.

The fluorine concentration in the obtained silica glass member can be made 1 wt% or more and 5 wt% or less by adjusting the concentration ratio of the SiF ₄ gas in the mixed gas and the firing temperature.
At this time, the OH group concentration is 50 ppm or less. When the OH group concentration is over 50 ppm, it may be difficult to make the fluorine concentration 1 wt% or more.
This is due to the trade-off relationship that fluorine is easily exchanged with OH groups, and the concentration of fluorine decreases as the concentration of OH groups increases. Therefore, the concentration ratio of SiF ₄ gas in the mixed gas and the firing temperature are adjusted to make the OH group concentration 50 ppm or less.

In the sol-gel method, hydrolysis reaction is caused using tetraethoxysilane represented by silicon alkoxide, tetramethoxysilane or the like as a raw material to gelate and dry to form a porous silica.
For the addition of fluorine, it is also possible to use a fluoride for the catalyst used to accelerate the hydrolysis reaction, or it can be achieved by impregnating the gel with a fluorine compound such as fluoroalcohol. Further, densification is performed by firing in a vacuum or helium atmosphere. For example, when hydrofluoric acid is used as the fluoride used as a catalyst, the fluorine concentration in the glass is 1 wt% or more and 5 wt% by setting 0.05 mol to 0.3 mol of hydrofluoric acid with respect to 1 mol of tetraethoxysilane. It can be less than%. Moreover, when using fluoro alcohol, a 2,2,2- trifluoro ethanol etc. can be used and it is glass by impregnating the solution diluted with ethanol to 60 wt%-80 wt% for 5 to 40 hours at 60 degreeC. The concentration of fluorine in the medium can be 1 wt% or more and 5 wt% or less.

The silica glass thus obtained is processed into a plate shape if necessary, and after clarifying a plane perpendicular to the light transmission direction, annealing is performed in an electric furnace or the like.
In the electric furnace, flat heaters for heating are disposed at the top and bottom. That is, a plane heater directly heats the surface (upper and lower surfaces) perpendicular to the light transmission direction. A carbon heater can be used as this flat heater. The heating surface of the heater has a sufficient width with respect to the surface to be heated of silica glass, and the entire surface to be heated is uniformly determined by heating.

This annealing treatment needs to be performed at a temperature of less than 1000 ° C. and a viscosity of 10 ^14.5 dPa · s or less. The annealing temperature is determined by the temperature characteristics of silica glass. For example, since the strain point of silica glass having a fluorine concentration of 1 wt% or more and 5 wt% or less is less than 1000 ° C., the annealing temperature is usually less than 1000 ° C., preferably 800 ° C. or less, more preferably 600 ° C. to 400 ° C. Do between.

By heating the cooled silica glass with a heater or the like and holding it at, for example, a temperature of 400 ° C. for about 50 hours, the silica glass eliminates local distortion while maintaining the low density state. It becomes possible. At temperatures below 400 ° C., the effect of annealing can not be expected.
The strain point is a temperature at which the viscosity becomes 10 ^14.5 dPa · s, a temperature at which viscous flow of the silica glass can not practically occur, and corresponds to the lower limit temperature in the slow cooling zone.
Therefore, the viscosity at 1000 ° C. of the silica glass plate of the present invention is preferably 10 ^14.5 dPa · s or less, more preferably 10 ^13.0 dPa · s or less.

  The silica glass plate obtained in this manner has a fluorine content of 1 wt% or more and 5 wt% or less, and the birefringence amount in a plane perpendicular to the light transmission direction is 2 nm / cm or less.

In this annealing process, it is desirable that the upper and lower heaters (heating surfaces) of the electric furnace be in contact with or close to a plane perpendicular to the light transmission direction.
The distance between the silica glass plate and the heating surface of the heater is preferably t / 3 or less, where t is the thickness of the silica glass plate.
When the distance between the heating surface and the working surface of the silica glass plate exceeds t / 3, the silica glass plate is three-dimensionally heated to uniform heat, which is not preferable.

In this way, the surface in a distance in contact with or close to the heater is heated and gradually cooled to reduce the amount of birefringence, whereby a low birefringence of 2.0 nm / cm or less can be achieved.
On the other hand, the other surface (surface perpendicular to the working surface) has a high birefringence of more than 2.0 nm / cm without sufficient heating and no reduction in the amount of birefringence. For example, the amount of birefringence is 2.5 nm / cm or more, and this is a level at which sufficient imaging performance can not be exhibited for use in the lithography process if it is a working surface.

  EXAMPLES Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited by the examples shown below.

Example 1
SiCl ₄ as a glass forming material was hydrolyzed in an oxyhydrogen flame, and the formed silica fine particles were deposited on a target made of quartz glass to obtain a porous silica matrix (soot) having a diameter of 250 mm and a length of 450 mm. .
Then, the porous put silica preform material (soot) in the furnace, in He gas atmosphere flow rate 20L / min, the temperature was elevated up to 1200 ° C. at a heating rate of 400 ° C. / h, the atmospheric gas SiF ₄ 30 vol% It switched to the mixed gas of + He 70 vol% (flow volume 15 L / min), and it hold | maintained at 1200 degreeC for 2 hours, and performed the fluorine dope process.
After completion of the fluorine doping treatment, the atmosphere is kept as it is, the temperature is raised to 1450 ° C. at a temperature rising rate of 300 ° C./h, and held for 1 hour at 1450 ° C. A silica glass ingot was obtained.
The ingot is once cooled to room temperature, square-shaped and sliced to form a thin plate of 6.4 mm thick, and then inserted into an electric furnace having movable upper and lower surface heaters so that the distance between the heater and the silica glass plate is 2 mm. Thereafter, the temperature was raised to 1100 ° C. over 3 hours, and after holding for 30 minutes, the temperature was lowered to 400 ° C. over 20 hours. After that, it was naturally cooled and the thin plate was recovered.

It was 3.0 wt% when the fluorine content in the obtained glass was evaluated.
After mirror polishing the obtained silica glass plate, birefringence at a wavelength of 193 nm was measured using an optical measurement device (HINDS Exicor DUV).
As a result, the birefringence amount of the silica glass surface perpendicular to the light transmission direction at the time of lithography exposure was 1.5 nm / cm. On the other hand, the birefringence of the silica glass surface parallel to the light transmission direction was 2.7 nm / cm. Moreover, it was 4 ppm when OH group of the mirror-polished glass was measured using the infrared spectrophotometer.

Example 2
After mixing and stirring 1 mole of tetraethoxysilane, 5 moles of water, 3 moles of ethanol, and 0.15 moles of hydrofluoric acid, hydrolysis is allowed to proceed in a room at 15 ° C., and then the solution is gelled and then dried in a dryer at 70 ° C. The gel strength was improved.
Furthermore, the obtained gel was dried at 80 ° C. for 7 days to obtain a porous silica body having a diameter of 100 mm and a length of 100 mm. Then, the silica glass member was obtained by baking at 1300 degreeC in helium atmosphere.
The obtained glass is sliced into a thin plate of 6.4 mm thick and inserted into an electric furnace having movable upper and lower surface heaters, and the distance between the heater and the silica glass plate is set to 1 mm. The temperature was raised over time, and after holding for 30 minutes, the temperature was lowered to 400 ° C. over 25 hours. After that, it was naturally cooled and the thin plate was recovered.

It was 2.5 wt% when the fluorine content in the obtained glass was evaluated.
After mirror polishing the obtained silica glass plate, birefringence at a wavelength of 193 nm was measured using an optical measurement device (HINDS Exicor DUV).
As a result, the birefringence amount of the silica glass surface perpendicular to the light transmission direction at the time of lithography exposure was 1.8 nm / cm. On the other hand, the birefringence of the silica glass surface parallel to the light transmission direction was 3.0 nm / cm. Moreover, it was 15 ppm when the glass OH group mirror-polished was measured using the infrared spectrophotometer.

Comparative Example 1
In the same manner as in Example 1, a silica glass ingot having a diameter of 150 mm and a length of 200 mm was obtained. The ingot was temporarily cooled to room temperature, square-shaped, sliced, and inserted into a table-top electric furnace having a 6.4 mm thick iron-chromium wire heater. After setting the distance between the heater and the silica glass plate to 50 mm, the temperature was raised to 1100 ° C. over 3 hours, and after holding for 30 minutes, the temperature was lowered to 400 ° C. over 20 hours. After that, it was naturally cooled and the thin plate was recovered.
It was 3.0 wt% when the fluorine content in the obtained glass was evaluated.
After mirror polishing the obtained silica glass plate, birefringence at a wavelength of 193 nm was measured using an optical measurement device (HINDS Exicor DUV).
As a result, the birefringence of the silica glass surface perpendicular to the light transmission direction during lithography exposure was 3.2 nm / cm. On the other hand, the birefringence of the silica glass surface parallel to the light transmission direction was 3.6 nm / cm. Moreover, it was 5 ppm when OH group of the mirror-polished glass was measured using an infrared spectrophotometer.

Comparative Example 2
A glass was produced in the same manner as in the process of Example 1 to obtain a silica glass body. Insert into an electric furnace with a movable upper and lower surface heater, and set the distance between the heater and the silica glass plate to 10 mm. Then raise the temperature to 1100 ° C over 3 hours, hold for 30 minutes and then take 230 hours to 400 ° C. The temperature dropped. After that, it was naturally cooled and the thin plate was recovered.

It was 2.9 wt% when the fluorine content in the obtained glass was evaluated.
After mirror polishing the obtained silica glass plate, birefringence at a wavelength of 193 nm was measured using an optical measurement device (HINDS Exicor DUV).
As a result, the birefringence amount of the silica glass surface perpendicular to the light transmission direction at the time of lithography exposure was 2.4 nm / cm. On the other hand, the birefringence of the silica glass surface parallel to the light transmission direction was 3.7 nm / cm. Moreover, it was 6 ppm when OH group of the mirror-polished glass was measured using an infrared spectrophotometer.

Thus, in the example of the present invention, a silica glass plate having a birefringence amount of 1.5 nm / cm and 1.8 nm / cm perpendicular to the light transmission direction can be obtained in a short time. . At this time, the birefringence amount of the parallel silica glass surface was 2.7 nm / cm and 3.0 nm / cm.
There was no particular problem even if this silica gas member was used during lithographic exposure.

  The silica glass plate of the present invention can be suitably used for photolithography using vacuum ultraviolet light as a light source. In particular, it is excellent as a photomask substrate used in a multi-patterning exposure process using an ArF excimer laser (193 nm) as a light source.

Claims (3)

Silica glass used in photolithography processes using vacuum ultraviolet light as a light source,
At a fluorine content of 1 wt% to 5 wt%,
When irradiated with ultraviolet light of 193 nm, which is the exposure wavelength of photolithography,
When the birefringence of the silica glass surface (working surface) perpendicular to the light transmission direction is 2.0 nm / cm or less and light is irradiated perpendicularly to the silica glass surface parallel to the light transmission direction A silica glass plate characterized in that the birefringence amount of is larger than the birefringence amount of the perpendicular silica glass surface.   An OH group concentration is 50 ppm or less, The silica glass board of Claim 1 characterized by the above-mentioned. In a silica glass sheet having a fluorine content of 1 wt% or more and 5 wt% or less and a thickness of 10 mm or less,
Identify the silica glass surface perpendicular to the light transmission direction,
A method for reducing birefringence of a silica glass plate, comprising heating with a heater from a distance of 1/3 or less of the thickness of the silica glass with respect to the vertical silica glass surface.