JP2019048728A - Silica glass member - Google Patents

Abstract

To provide a silica glass member in which the distortion is further suppressed and the birefringence is reduced by containing a larger amount of fluorine, and to provide a method for producing the same.SOLUTION: The silica glass member according to the present invention is a silica glass used in a photolithographic process using vacuum ultraviolet light as a light source and is characterized in having a fluorine content of 1.5 wt% or more and 5 wt% or less, and a birefringence of 2 nm/cm or less at the photolithography exposure wavelength of 193 nm.SELECTED DRAWING: None

Description

  The present invention relates to a silica glass member, for example, a silica glass member for a photomask 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, and the exposure wavelength λ is 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 substrate for photolithography using this ArF excimer laser, a silica glass substrate is suitably used because it is excellent in low thermal expansion and light transmittance.

Examples of the performance required for the silica glass substrate include, 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.
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 effect of polarization becomes a problem.
Therefore, the silica glass substrate is required to have low birefringence. If the silica glass substrate 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 the birefringence is 5 nm / cm or less at a wavelength of 633 nm in silica glass having a fluorine content of 1000 wt ppm by annealing. Moreover, it is known that patent document 2 sets the birefringence to 2 nm / cm or less at a wavelength of 200 nm or less in silica glass having a fluorine content in the range of 100 ppm to 1000 ppm by annealing.

  By the way, the annealing treatment for removing the strain of the silica glass is usually performed in the atmosphere, however, in the annealing treatment performed in the atmosphere, the Si-F bond in the silica glass is thermally broken and the fluorine content is An altered layer is generated. The presence of the altered layer makes it difficult to reliably remove the strain in the silica glass, even when annealing is performed. That is, when the non-uniform distribution occurs in the fluorine content, it causes the non-uniformity of the density distribution in the silica glass to cause distortion.

  In order to solve this problem, Patent Document 3 discloses a technique of heat-treating silica glass containing fluorine in an atmosphere containing a fluorine compound to make the birefringence at a wavelength of 633 nm 10 nm / cm or less. ing.

Japanese Patent Application Laid-Open No. 2002-60227 JP 2000-264671 A JP 2003-192363 A

By the way, as described above, when the non-uniform distribution of the fluorine content occurs, it causes the non-uniformity of the density distribution in the silica glass to generate a strain.
If this phenomenon is applied to the conventional annealing process, thermal strain can be removed while holding at a high temperature above the annealing point, but if it is annealed it will cause breakage and recombination of Si-F bond also during annealing. As a result, new distortion occurs. Further, as the temperature is higher, the breaking of the Si-F bond is promoted, so that the generation of strain tends to occur.

  Thus, the fluorine-containing silica glass which has been subjected to the annealing treatment to reduce birefringence has a fluorine content of 100 to 1000 ppm and an annealing treatment temperature of 1000 to 1200 ° C. There is a problem that a degenerated layer having a different fluorine content is likely to be formed, which in turn makes it difficult to reduce birefringence.

  In addition, heat treatment in an atmosphere containing a fluorine compound requires a device for removing harmful gases not only in the glass step but also in the annealing step, which tends to increase the size of the device and increase the manufacturing cost.

The present invention has been made to solve the above-described technical problems, and it is an object of the present invention to provide a silica glass member in which the distortion is further suppressed and the birefringence is reduced by containing a larger amount of fluorine. .
In addition, the present invention was made to solve the above technical problems, and by carrying out the annealing step at a lower temperature and in the atmosphere, it is possible to produce silica without using a device for excluding harmful gases and the like. It aims at providing a manufacturing method of a glass member.

  The silica glass member 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 1.5 wt% or more of fluorine The birefringence is 2 nm / cm or less at 193 nm which is an exposure wavelength of photolithography.

As described above, since the fluorine content is 1.5 wt% or more and 5 wt% or less, the annealing process can be performed at less than 1000 ° C. by further reducing the viscosity, and as a result, the Si—F bond is disconnected and recut. Binding 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, a degenerated layer in which the fluorine content has changed is hard to occur, and the birefringence can be made 2 nm / cm or less at 193 nm which is the exposure wavelength of photolithography.
Here, when the content of fluorine is less than 1.5 wt%, the viscosity of the glass becomes high, and it becomes necessary to carry out annealing at 1000 ° C. or higher. It is likely to produce altered alteration layers, which is undesirable. If the content of fluorine exceeds 5 wt%, slight breakage of Si-F bond occurs even at a low temperature of 1000 ° C. or less, and the fluorine content is The width of the maximum value of the central part of the glass and the minimum value of the peripheral part is more than 0.5 wt%, which is not preferable because 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, it is desirable that the range between the maximum value and the minimum value of the fluorine content is 0.5% or less and the OH group concentration is 10 ppm or less.
If the width (difference) between the maximum value and the minimum value of the fluorine content exceeds 0.5% in the glass, a homogeneous fluorine content is not maintained, so that an altered layer with a change in the fluorine content is generated. In many cases, it is difficult to reduce birefringence.
Since fluorine in glass is easily exchanged with OH groups, there is a trade-off relationship that the higher the concentration of OH groups, the lower the fluorine content. Therefore, when the OH group concentration is more than 10 ppm, it may be difficult to make the fluorine concentration 1.5 wt% or more.

Further, the method for producing silica glass according to the present invention made to solve the above technical problems comprises the steps of producing a porous silica matrix, and the content of fluorine in the matrix is 1.5 wt% or more and 5 wt%. %, The step of doping with fluorine so as to be less than 10%, the step of heating to make it transparent, and the above-mentioned base material is once cooled and processed into shape after the step of fluorine doping and making it transparent. And heating the silica glass to have a birefringence of 2 nm / cm or less.
As described above, since the annealing can be performed at less than 1000 ° C. in the atmosphere, the silica glass member can be easily manufactured without using a harmful gas removing device or the like.

Here, it is preferable that fluorine doping is performed by heating the base material in a fluorine compound gas-containing atmosphere.
Moreover, as for the said silica glass, the width | variety of the maximum value of a fluorine content and minimum value is 0.5% or less, and it is desirable that OH group concentration is 10 ppm or less.

According to the present invention, it is possible to obtain a silica glass member having a fluorine content of 1.5 wt% or more and 5 wt% or less and a birefringence of 2 nm / cm or less at 193 nm which is an exposure wavelength of photolithography.
Further, according to the present invention, after the porous silica body (soot) is fluorine-doped and transparentized, annealing can be performed in the air at a temperature range of less than 1000 ° C., and the fluorine content is 1 A silica glass member having 5 wt% or more and 5 wt% or less and having a birefringence of 2 nm / cm or less at an exposure wavelength of 193 nm for photolithography can be manufactured.
Such a silica glass member can be suitably used, for example, as a photomask substrate used in a double patterning exposure process using an ArF excimer laser as a light source.

The silica glass member of the present invention is characterized in that the fluorine content is 1.5 wt% or more and 5 wt% or less, and the maximum birefringence is 2 nm / cm 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 Patent Document 1, since the fluorine content is 100 to 1000 ppm, the annealing temperature is set to 1000 ° C. to 1200 ° C. It is considered that this corresponds to the temperature range above the strain point or the annealing point.

In the present invention, by setting the fluorine content to 1.5 wt% to 5 wt%, the viscosity can be further reduced to enable annealing at less than 1000 ° C., and as a result, breakage of Si—F bond It avoids the occurrence of recombination.
That is, by setting the fluorine content to 1.5 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 difficult to form an altered layer resulting from a change in the fluorine content.
Preferably, the content of fluorine is 1.5 wt% or more and 5 wt% or less.

And, it is important to note that, when annealing is performed in a state where viscosity is uniform, for example, hardening starts from a relatively high viscosity portion during annealing, and stress in the glass is not released or deteriorated. It is possible to suppress the generation of new strain by the layer, and the reduction of birefringence can be easily achieved.
Moreover, in order to suppress the formation of the altered layer in which the fluorine content has changed, it is not necessary to perform the treatment in the atmosphere containing the fluorine compound, and it is not necessary to use a device for removing harmful gas or the like.

Furthermore, it is desirable that the range of the maximum value and the minimum value of the fluorine content in the silica glass be 0.5 wt% or less.
The maximum value and the minimum value of the fluorine content are the values after the annealing treatment, but if the distribution of the fluorine content is not uniform in the glass before annealing, uniform annealing treatment becomes difficult. Not desirable.

When the width between the maximum value and the minimum value of the fluorine concentration exceeds 0.5%, it is not possible to obtain a silica glass member excellent in refractive index homogeneity. On the other hand, if the fluorine content in the silica glass member is made uniform, the homogeneity of the refractive index can be expected, but since it is difficult to make it completely homogeneous in industry, the width of the maximum value and the minimum value of the fluorine concentration is 10 ppm The above is preferable.
Therefore, it is desirable that the range between the maximum value and the minimum value of the fluorine concentration be 10 ppm or more and 0.5% or less.

  The maximum value of the fluorine concentration refers to the maximum value of the fluorine concentrations measured at each point in the use area. The minimum value of the fluorine concentration means the minimum value of the fluorine concentrations measured at each point in the use area. Further, the range between the maximum value and the minimum value of the fluorine concentration means the difference between the maximum value and the minimum value of the fluorine concentration.

The OH group concentration is preferably 10 ppm or less. If the OH group concentration is more than 10 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 10 ppm, it may be difficult to make the fluorine concentration 1.5 wt% or more, which is not preferable.

The said silica glass member is manufactured by the manufacturing method demonstrated below.
For example, it can be manufactured by a so-called VAD method in which a silica glass forming raw material is subjected to flame hydrolysis to form a porous silica matrix (soot) and then a transparentizing treatment is performed (for example, JP-A 2001-342027) See official gazette).
That is, after the porous silica base material (soot) is formed, it is treated in a mixed gas atmosphere in which an inert gas such as helium and a SiF ₄ gas are mixed to dope fluorine, and then a fluorine-containing atmosphere (mixed A silica glass member is manufactured by clarifying (clarifying treatment) under a gas atmosphere) and further performing a specific annealing treatment.

More than 5 vol% to 35 vol% of the fluorine concentration (concentration ratio of SiF ₄ gas) in the mixed gas in fluorine doping, which is made after forming the porous silica matrix (soot), is preferably 10 vol% to 35 vol% Preferably, 25 vol% to 35 vol% is particularly preferred.
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 1.5 wt% or more and 5 wt% or less by adjusting the concentration ratio of SiF ₄ gas in the mixed gas and the firing temperature.
At this time, the OH group concentration is 10 ppm or less. When the OH group concentration is more than 10 ppm, it may be difficult to make the fluorine concentration 1.5 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 10 ppm or less.

The specific heat treatment in the present invention is a step of annealing treatment at less than 1000 ° C. after the clarification treatment.
The 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 1.5 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. Perform between ° C.

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 member 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 member obtained in this manner has a fluorine content of 1.5 wt% or more and 5 wt% or less, and a maximum birefringence of 2 nm / cm or less.

In addition to the soot method described above, the porous silica matrix may be formed by a sol-gel method.
Also, according to the method of forming the porous silica matrix, the raw material gas for producing silica and the fluorine compound gas are introduced into the flame with the combustion supporting gas and the flammable gas and reacted to deposit silica glass fine particles on the 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.

  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 performed the fluorine dope process hold | maintaining at 1200 degreeC for 2 hours.

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 was once cooled to room temperature and sliced to a thin plate having a thickness of 6.4 mm, and then heated to 990 ° C. over 5 hours, then held for 30 minutes and then cooled to 400 ° C. for 60 hours.

After that, it was naturally cooled and the thin plate was recovered. It was 3.2 wt% when the fluorine content in the obtained glass was evaluated. Further, the range between the maximum value and the minimum value of the fluorine content was 0.3%, and the OH group concentration was 1 ppm.
After mirror polishing the obtained silica glass member, birefringence at a wavelength of 193 nm was measured using an optical measurement device (HINDS Exicor DUV). As a result, the birefringence was 1.2 nm / cm. The results are shown in Table 1

Example 2
A silica glass member was obtained in the same manner as in Example 1 except that the fluorine doping treatment was performed in Example 1 in which the ratio of SiF ₄ to He mixed gas was 20 vol%: 80 vol%.
Thereafter, the same test and evaluation as in Example 1 were performed. The fluorine content was 2.2 wt%. Further, the range between the maximum value and the minimum value of the fluorine content was 0.2%, and the OH group concentration was 2 ppm. Furthermore, the birefringence at a wavelength of 193 nm was measured to be 1.9 nm / cm. The results are shown in Table 1.

[Example 3]
A silica glass member was obtained in the same manner as in Example 1 except that the fluorine doping treatment was performed in Example 1 in which the ratio of SiF ₄ to He mixed gas was 40 vol%: 60 vol%.
Thereafter, the same test and evaluation as in Example 1 were performed. The fluorine content was 4.8 wt%. Further, the range between the maximum value and the minimum value of the fluorine content was 0.5%, and the OH group concentration was 1 ppm. The birefringence at a wavelength of 193 nm was measured to be 0.6 nm / cm. The results are shown in Table 1.

Example 4
A silica glass member was obtained in the same manner as in Example 1 except that the fluorine doping treatment was performed in Example 1 in which the ratio of SiF ₄ to He mixed gas was 15 vol%: 85 vol%.
Thereafter, the same test and evaluation as in Example 1 were performed. The fluorine content was 1.5 wt%. Further, the range between the maximum value and the minimum value of the fluorine content was 0.2%, and the OH group concentration was 3 ppm. Furthermore, the birefringence at a wavelength of 193 nm was measured to be 2.0 nm / cm. The results are shown in Table 1.

Comparative Example 1
A silica glass was obtained in the same manner as in Example 1 except that a fluorine doping process was performed in Example 1 in which the ratio of SiF ₄ to He mixed gas was 10 vol%: 90 vol%. Thereafter, the same test and evaluation as in Example 1 were performed. As a result, the fluorine content was 0.8 wt%. Further, the range between the maximum value and the minimum value of the fluorine content was 0.1%, and the OH group concentration was 6 ppm. Furthermore, the birefringence at a wavelength of 193 nm was measured to be 2.2 nm / cm. The results are shown in Table 1.

Comparative Example 2
A silica glass was obtained in the same manner as in Comparative Example 1 except that a fluorine doping process was performed in Comparative Example 1 in which the ratio of SiF ₄ to He mixed gas was 50 vol%: 50 vol%. Thereafter, the same test and evaluation as in Comparative Example 1 were performed. As a result, the fluorine content was 6.0 wt%. Further, the range between the maximum value and the minimum value of the fluorine content was 0.6%, and the OH group concentration was 1 ppm. Furthermore, the birefringence at a wavelength of 193 nm was measured to be 3.1 nm / cm. The results are shown in Table 1.

Comparative Example 3
A silica glass member was obtained in the same manner as in Comparative Example 1 except that a fluorine doping process was performed in which the ratio of SiF ₄ to He mixed gas was 12 vol%: 88 vol% in Comparative Example 1. Thereafter, the same test and evaluation as in Example 1 were performed. The fluorine content was 1.2 wt%. Further, the range between the maximum value and the minimum value of the fluorine content was 0.1%, and the OH group concentration was 4 ppm. Further, the birefringence at a wavelength of 193 nm was measured to be 2.1 nm / cm. The results are shown in Table 1.

  The silica glass member of the present invention can be suitably used for photolithography using vacuum ultraviolet light such as ArF excimer laser (193 nm) or F2 laser (157 nm) as a light source. In particular, it is excellent as a photomask substrate used in a double patterning exposure process using an ArF excimer laser (193 nm) as a light source.

Claims (5)

A silica glass used in a photolithographic process using vacuum ultraviolet light as a light source, wherein the content of fluorine is 1.5 wt% or more and 5 wt% or less,
A silica glass member having a birefringence of 2 nm / cm or less at 193 nm which is an exposure wavelength of photolithography.   The silica glass member according to claim 1, wherein the width between the maximum value and the minimum value of the fluorine content is 0.5 wt% or less, and the OH group concentration is 10 ppm or less. Producing a porous silica matrix;
Doping the base material with fluorine so that the fluorine content is 1.5 wt% or more and 5 wt% or less;
A process of heating and clarifying,
After the fluorine doping and the clarifying step, the temperature of the base material is once lowered and processed into a shape, and then heated in the air at less than 1000 ° C .;
Including
A method for producing a silica glass member, comprising obtaining a silica glass having a birefringence of 2 nm / cm or less.   The method for producing a silica glass member according to claim 3, wherein the fluorine doping is performed by heating the base material in a fluorine compound gas-containing atmosphere.   The silica according to claim 3 or 4, wherein the width of the maximum value and the minimum value of the fluorine content is 0.5 wt% or less and the OH group concentration is 10 ppm or less. Method of manufacturing a glass member