JP5561164B2 - Optical member for photomask and method for manufacturing the same - Google Patents

Description

  The present invention relates to a photomask substrate used when manufacturing a flat panel display (hereinafter referred to as FPD) such as a liquid crystal panel, and a manufacturing method thereof.

  The FPD is manufactured through a process of forming FPD elements with high accuracy on the surface of a glass substrate. In this step, a photolithography technique is used. That is, a photomask having a mask pattern formed with high precision on the surface of a flat transparent substrate having excellent flatness is illuminated with exposure light, and an image of the mask pattern is displayed on a glass substrate on which a photoresist has been applied in advance. After the image is formed, a resist pattern is formed on the surface of the glass substrate by developing.

  By the way, in order to increase the screen size of FPD and increase the efficiency of production, the glass substrate for FPD has been increasing year by year, and along with this, the size of photomasks used for production has also been increasing. In the near future, the glass substrate will be an extremely large size of 2200 mm × 2500 mm, and accordingly, the photomask used to expose the mask pattern on the glass substrate has a diagonal length exceeding 1470 mm, for example, 1220 mm × 1400 mm and the thickness is 13 mm, which is extremely large. However, the increase in size is not limited to this, and a larger glass substrate and a photomask are required.

When the pattern formed on the photomask is exposed on the substrate, the exposure is performed with the photomask held horizontally. Quartz glass is known as a material used for such a photomask. Quartz glass has a linear thermal expansion coefficient of 5 × 10 ⁻⁷ / ° C., and is a material that is relatively less deformed by heat. However, when a volume change occurs due to the influence of ultraviolet rays or the like irradiated during exposure, it forms on the FPD substrate. Therefore, it is desirable to use a material that has very little thermal expansion. In addition, for example, ultraviolet rays having a wavelength of about 365 nm may be used at the time of exposure, and it is desired to have a high transmittance at such a short wavelength.

As a material having very little thermal expansion near room temperature, a material in which about 7.5% by weight of titania (TiO ₂ ) is added to quartz glass is known. The linear thermal expansion coefficient depends on the amount of titania added, and the linear thermal expansion coefficient is almost zero by setting it to 7.5% by weight. However, in the case of a composition near 7.5% by weight, the transmittance near a wavelength of 365 nm is less than 90%, which is not a sufficient transmittance. It has been proposed that such a material is used as a material for a reflective photomask for EUV, which takes advantage of the property of low expansion and requires high accuracy.

JP2007-182367

  An object of the present invention is to provide a photomask substrate having a practically sufficient transmittance even in the vicinity of a wavelength of 365 nm and less likely to thermally expand than quartz glass, and a method for manufacturing the same.

According to the first aspect of the present invention, an optical member in which TiO ₂ is added to synthetic quartz glass, the composition of the TiO ₂ is 3.0 to 6.5% by weight, and the transmittance at a wavelength of 365 nm. 0.05wt There Ri der 90% or more, the linear thermal expansion coefficient at 20 ° C. to 80 ° C. is 2.5 × ^{10 -7} / ℃ less, as impurities, Al and 0.1 wt · ppm or less, the Cu An optical member for a photomask is provided that includes ppm or less, Fe of 0.1 wt · ppm or less, Na of 0.05 wt · ppm or less, and K of 0.05 wt · ppm or less .

According to a second aspect of the present invention , there is provided a method for manufacturing a photomask optical member according to the first aspect, which is a method for manufacturing a photomask optical member, wherein TiO ₂ is mixed by mixing raw material gases. A synthesis step of synthesizing a quartz glass ingot containing, a molding step of forming the quartz glass ingot into a predetermined plate shape by pressurizing the quartz glass ingot while maintaining a predetermined temperature, and an oxidation after the molding step There is provided a method for producing an optical member for a photomask, comprising an oxidation treatment step of oxidizing titanium contained in the quartz glass by heating in an atmosphere.

  According to the present invention, a photomask substrate having a practically sufficient transmittance such that the transmittance at a wavelength of 365 nm is 90% or more by adding 3.0 to 6.5 wt% titania to quartz glass. Material is realized.

Is a diagram illustrating the relationship between the transmittance and the linear thermal expansion coefficient and TiO ₂ concentration of the first embodiment. These are the flowcharts which show the manufacturing process of the photomask of Embodiment 2. FIG.

  Embodiments of the present invention will be described below.

[Embodiment 1]
An embodiment of the optical member of the present invention will be described. The optical member of this embodiment is an optical member obtained by adding 3.0 to 6.5% by weight of TiO ₂ to synthetic quartz glass (SiO ₂ ), and can take any shape. It is desirable that the optical member does not contain any substance other than TiO ₂ and SiO ₂ . For example, even when elements such as Al, Cu, Fe, Na, and K are included as impurities, for example, Al is 0.1 wt.ppm or less, Cu is 0.05 wt.ppm or less, and Fe is 0.1 wt. Preferably, ppm or less, Na is 0.05 wt · ppm or less, and K is 0.05 wt · ppm or less.

A method for creating the optical member of this embodiment will be described. First, prepare a synthetic furnace deposits intermediate comprising a mixture of SiO ₂ particles and Ti0 ₂ nanoparticle (soot body). The soot body is an aggregate of fine particles, and a method of making the aggregate transparent by heating the aggregate to a vitrification temperature or higher in an electric heating furnace or the like can be used.

To obtain a deposit intermediate of the present embodiment, in one synthetic furnace, to synthesize fine particles of SiO ₂ and Ti0 ₂ particles simultaneously, it creates a soot body of a mixture by mixing, clearing the soot body Can be synthesized. In this case, the one synthetic furnace, it is possible to use synthetic furnace having a second burner for synthesizing the first burner and the Ti0 ₂ particles to synthesize SiO ₂ particles. The first burner for synthesizing the SiO ₂ fine particles includes a source gas containing a silicon compound such as silicon tetrachloride (SiCl ₄ ), silicon tetrafluoride (SiF ₄ ), silane (SiH ₄ ), and a combustion-supporting gas ( Oxygen gas) and combustible gas (hydrogen gas) or the like and an inert gas are ejected, and a silicon compound is hydrolyzed in a flame to generate SiO ₂ glass fine particles. The second burner is composed of a raw material gas containing a titanium compound such as titanium tetrachloride (TiCl ₄ ), a combustion gas such as a combustion-supporting gas (oxygen gas) and a combustible gas (hydrogen gas), and an inert gas. And TiO ₂ glass fine particles are generated by hydrolyzing the titanium compound in the flame.

The SiO ₂ glass fine particles generated by the first burner and the TiO ₂ glass fine particles generated by the second burner are deposited on a deposition target provided obliquely above the two burners. By burning the first burner and the second burner simultaneously, a mixture of SiO ₂ glass particles and TiO ₂ glass particles is deposited on the target. The amount of TiO _{2 in} the composition can be changed by changing the amount ratio of SiO ₂ produced by the first burner and TiO ₂ produced by the second burner. For example, it can be changed by controlling the flow rate of the raw material gas introduced into the burner.

  Since the soot body of the mixture thus prepared is opaque, it becomes transparent by heating to 1300 ° C. or higher. A sample for measurement was prepared by cutting the transparent sample into a size of about 16 mm in diameter and 10 mm in thickness, polishing the surface of the sample, and then washing it. The transmittance was measured by using a Varian ultraviolet / visible / near infrared spectrophotometer Cary5 and measuring the transmittance at 365 nm (i-line).

The relationship between the concentration of TiO ₂ and the transmittance at a wavelength of 365 nm is shown in FIG. FIG. 1 shows the relationship between the concentration of TiO ₂ added to the synthetic quartz glass and the linear thermal expansion coefficient, and the relationship between the concentration of TiO ₂ and the transmittance at a wavelength of 365 nm. The composition of each sample was investigated using a fluorescent X-ray analyzer, and the TiO ₂ concentration of the composition was plotted on the horizontal axis in FIG. There are five types of TiO ₂ concentrations from 0.5 to 9.5% by weight, and the transmittance of synthetic quartz glass not containing TiO ₂ is also described as a reference example. The transmittance of the synthetic quartz glass not added with TiO ₂ was 92.9%. The transmittance decreased with increasing TiO ₂ concentration, and decreased to 89.0% at 7.5% by weight. The transmittance value shown in FIG. 1 is a value including the reflectance of a sample having a thickness of 10 mm. As a substrate used for a transmissive photomask that uses i-line (wavelength 365 nm) as exposure light, a transmittance of 90% or more is ensured in order to expose a high-definition and high-contrast pattern. desirable. The relationship between TiO ₂ concentration and the transmittance shown in FIG. 1, the TiO ₂ concentration that ensures transmittance of 90% or more, preferably in the range of 6.5 wt% or less.

In addition, since the size of the mask is increased in the photomask used for FPD pattern exposure, the influence of the positional deviation of the exposed pattern due to thermal expansion of the photomask cannot be ignored. For this reason, the material with a small thermal expansion coefficient in the temperature environment to be used is preferable. The linear thermal expansion coefficient was measured by defining the length change rate ΔL / L ₀ (referred to as the linear expansion coefficient) from the sample length L ₀ at room temperature and the temperature change ΔL. This linear expansion coefficient (ΔL / L ₀ ) temperature curve was measured by the laser interferometry, and the linear thermal expansion coefficient α was determined by the following formula (1).
α = (1 / L _o ) · (dL / dT) ... (1)
When TiO ₂ is 3.0% by weight, the linear thermal expansion coefficient is 2.5 × 10 ⁻⁷ / ° C., which is half the value of quartz glass without adding TiO ₂ , so when used in the same temperature environment In addition, it can be expected that the alignment accuracy is improved twice. It is also possible to double the size (length) while maintaining the same alignment accuracy.

Thus, it can be seen that the TiO ₂ concentration at which the transmittance is 90% or more and the linear thermal expansion coefficient is 2.5 × 10 ⁻⁷ / ° C. or less is 3.0 to 6.5 wt%. Even within this range, if TiO ₂ is 3.0 to 5.0% by weight, the transmittance is higher, and if TiO ₂ is 5.0 to 6.5% by weight, the linear thermal expansion coefficient is even lower. . Conventionally, quartz glass to which TiO ₂ was added was considered to be unable to be used as an optical member used for transmission at a wavelength of 365 nm or less because the transmittance cannot be secured, but as described above, 3.0 to 6 It has been found through experiments that thermal expansion can be suppressed to ½ or less of conventional quartz glass while ensuring a practically sufficient transmittance in a TiO ₂ concentration range of 0.5 wt%.

In addition, it is known that in the quartz glass to which TiO ₂ is added, the larger the content of Ti ^{3+ in} the constituent titanium element, the more the absorption, and even in the composition where the TiO ₂ concentration is around 6.5% by weight, It can be expected that the internal absorption is reduced and the transmittance is improved by reducing the content of Ti ³⁺ . However, it is known that the wavelength of the absorption edge tends to shift to the longer wavelength side as the TiO ₂ concentration increases. For example, when used at a wavelength of 300 nm to 400 nm, the TiO ₂ concentration or Ti ³⁺ There is a risk that the transmittance is rapidly lowered due to variation in the content of. For this reason as well, in order to provide an optical member for a photomask having a stable and sufficient transmittance at a wavelength of 365 nm, the TiO ₂ concentration is preferably set to 6.5% by weight or less.

[Embodiment 2]
In this embodiment, a method obtained by improving the photomask optical member manufacturing method described in the first embodiment will be described. As described above, when quartz glass having a small expansion coefficient by containing TiO ₂ is used as an optical member for a photomask, it is preferable that internal absorption by TiO ₂ is small. When a material with high internal absorption is used as an optical member for a photomask, the exposure light irradiated onto the wafer decreases due to the occurrence of a photomask temperature increase due to the absorbed exposure light or a decrease in the transmittance of the photomask. In order to prevent this, measures such as increasing the power of the exposure light on the light source side are required. Therefore, a material with as little increase in internal absorption as possible is desired as compared with quartz glass not containing TiO ₂ . As described in the first embodiment, the present inventor suppresses thermal expansion while ensuring sufficient transmittance by setting the content of TiO ₂ within the range of 3.0 to 6.5% by weight. I was able to. The inventor conducted further research and found a method capable of producing an optical member for a photomask having high transmittance.

Quartz glass containing TiO ₂ is known to absorb more as the content of Ti ^{3+ in} the constituent titanium elements increases, and absorption is ^achieved by changing Ti ³⁺ to Ti ⁴⁺ by oxidation. Can be reduced. Such oxidation can be performed by annealing at a temperature of about 1000 ° C. in an oxidizing atmosphere such as air. Further, absorption by Ti ³⁺ can be obtained with high accuracy by measuring the transmittance in the vicinity of a wavelength of 420 nm.

  The low-expansion optical member created in Embodiment 1 is likely to exert a low-expansion effect when used for a large photomask having a diagonal dimension exceeding 1470 mm. This is because a larger photomask has a larger expansion / contraction length (expansion amount) due to thermal expansion. As a photomask used for projecting a pattern for FPD, for example, a large-sized photomask having a size of 1220 mm × 1400 mm, a thickness of 13 mm, and a weight exceeding several tens of kg has been put into practical use. When used as a substrate, the effect of low expansion is easily exhibited.

Usually, a photomask substrate having such a size is manufactured by the following process. First, quartz glass containing TiO ₂ is prepared by synthesis. For example, a mixture of SiO ₂ fine particles and TiO ₂ fine particles is prepared in a synthesis furnace, and the resulting mixture is heated to a temperature equal to or higher than the vitrification temperature in an electric heating furnace to obtain an ingot of a photomask substrate. The ingot obtained in this synthesis step is obtained while being ejected from a burner that ejects a raw material onto a deposition target. In order to make this a flat photomask substrate, the ingot is cut so that the upper surface is flat, accommodated in a carbon mold, and pressurized while heating in an inert gas atmosphere. The flat quartz glass is formed by deforming. The quartz glass thus formed is ground into a predetermined shape after cooling and the surface is polished to obtain a quartz glass substrate for a photomask. In order to use as a photomask, a light shielding film made of Cr is further formed on one surface used as a mask, and the light masking film is partially removed to form a pattern to be projected, thereby completing the photomask. To do.

A method for manufacturing the photomask optical member of Embodiment 2 will be described with reference to FIG. First, quartz glass containing a predetermined concentration of TiO ₂ is synthesized (S1: synthesis step). From the result of Embodiment 1, TiO ₂ is preferably contained in the silica glass at 3.0 to 6.5% by weight. In this synthesis step, either a soot method or a direct method can be used. For example, silicon compound source gas, titanium compound source gas, combustion-supporting gas, and gas containing combustion gas are ejected from a multi-tube burner, reaction is performed in a flame, and glass particles are placed on a rotating target. Synthetic methods that deposit and melt can be used. SiCl ₄ , SiF ₄ , SiH _4, etc. are used as the raw material gas for silicon oxide, TiCl ₄ is used as the raw material gas for the titanium compound, oxygen is used as the combustion-supporting gas, and hydrogen is used as the combustion gas. it can. The TiO ₂ concentration can be adjusted by adjusting the mixing ratio of the silicon oxide source gas (SiCl ₄ , SiF ₄ , SiH _4, etc.) and the titanium compound source gas (TiCl _4, etc.). In addition, it is also possible to employ the synthesis method disclosed in JP-A-10-279319 and JP-A-11-292551. In the case of using the soot method, a quartz glass ingot is obtained by further making it transparent (S2: transparency process), and an amount of quartz glass necessary for producing one photomask substrate from this ingot is obtained. cut.

  Next, the cut-out quartz glass is formed into a flat plate shape by heating and pressing (S3: forming step). In the molding process, a rectangular carbon-shaped mold is prepared, quartz glass is accommodated in the space in the mold, heated to near 1600 ° C. in a nitrogen gas atmosphere, and given pressure is maintained while maintaining this temperature. Mold into a flat plate shape and cool to room temperature. Since the surface of the quartz glass after molding may have a portion that reacts with the deposits at a high temperature or bubbles, etc., each surface is ground to a size to be used as a photomask after the molding process (S4: grinding process). . In the grinding process, the thickness of the quartz glass is preferably 20 mm or less.

Following the grinding step, the transmittance of the flat quartz glass is measured (S5: transmittance inspection step). In order to accurately measure the transmittance, the surface of the portion to be measured needs to be a polished surface. For example, only the vicinity of the corner of a flat plate may be polished and the transmittance of this portion measured. Alternatively, a test piece cut out from the same quartz glass lump after molding may be created, and the measurement of the transmittance of this test piece may be used instead. The transmittance can be measured using Cary 5 of Varian. The wavelength to be measured is preferably around 365 nm or 420 nm, which is the wavelength of exposure light when the photomask is used in an exposure apparatus. Since the absorption by Ti ³⁺ is remarkable in the vicinity of the wavelength of 420 nm, it is possible to expect an accurate measurement reflecting the influence of absorption by Ti ³⁺ .

Based on the transmittance value thus measured, the conditions (temperature, oxidizing gas pressure, annealing time, etc.) for the next annealing process are selected. In particular, from the viewpoint of manufacturing process management and productivity, it is preferable that sufficient oxidation can be performed in as short an annealing time as possible. Regarding the conditions of the transmittance and the annealing step, it is possible to obtain the annealing conditions necessary for the oxidation by conducting a preliminary experiment and select the conditions from the measured transmittance. In the preliminary experiment, for example, a plurality of samples with the TiO ₂ concentration as a variable is prepared (for example, a plurality of samples selected from the range of 3.0 to 6.5% by weight in quartz glass), and annealing conditions (temperature, It is preferable to conduct an annealing experiment using time and oxidizing gas pressure as variables and measure the transmittance, Ti ³⁺ concentration, etc. before and after annealing. The Ti ³⁺ concentration can be measured by ESR (Electron Spin Resonance).

  In the above method, the procedure for determining the conditions of the annealing process after performing the transmittance inspection process is set, but when the prescribed annealing conditions are set and the measured transmittance is within a predetermined range, May be processed under prescribed annealing conditions. Further, when the characteristics of the quartz glass ingot obtained in the synthesis step are stable, the next step annealing can be performed under the prescribed annealing conditions without performing the transmittance inspection.

Next, oxidation treatment is performed to oxidize Ti ³⁺ and reduce internal absorption (S6: annealing step). A flat quartz glass is accommodated in a heat-resistant furnace and heated while introducing an oxidizing gas (for example, air). In the annealing step, it is preferable that the whole is efficiently oxidized in order to sufficiently oxidize Ti ³⁺ inside the flat quartz glass. In Embodiment 2, since the grinding process for processing into a flat plate having the same thickness as that used as the photomask is performed before the annealing process, the oxidation can be efficiently performed in a short time. In order to efficiently oxidize the two surfaces of the flat quartz glass through which the exposure light passes, it is preferable to heat the two surfaces so that both surfaces are in sufficient contact with the oxidizing gas. For this reason, it is preferable that the support member supporting the flat quartz glass in the annealing step is not in contact as much as possible. For example, it may be arranged so that the surface of the flat plate is in the vertical direction, and the peripheral end surface may be contacted and supported by the support member. In the annealing step, it is preferable to set the temperature to 1200 ° C. or lower in order to prevent deformation of the quartz glass. After cooling to room temperature after the annealing step, the transmittance may be measured again to confirm the effect of oxidation.

When the annealing step is completed, a polishing step is performed using an abrasive such as colloidal silica (S7), and a quartz glass substrate for a photomask is completed. In the conventional quartz glass manufacturing process, an annealing process for removing strain may be performed between the synthesis process and the grinding process, but the ingot shape has a thickness of several hundred mm or more. Even when oxidized by heating in the inside, in order to sufficiently oxidize even Ti ³⁺ inside, further oxidation for a longer time is required. If the oxidation is insufficient, sufficient transmittance may not be obtained due to absorption by Ti ³⁺ . In the second embodiment, since the oxidation annealing process is performed after the molding process, the inside can be efficiently oxidized in a relatively short time. Therefore, the optical member for the photomask having a light transmittance is relatively short. Can be manufactured in time. Further, it is more preferable to perform an annealing process for oxidation treatment after a grinding process for removing unnecessary portions on the surface after the molding process.

  INDUSTRIAL APPLICABILITY The present invention is useful for a transmissive photomask optical member that transmits ultraviolet light having a wavelength of 300 nm or more, and particularly for a large photomask optical member having a diagonal dimension exceeding 1470 mm.

Claims (8)

An optical member obtained by adding TiO ₂ to synthetic quartz glass,
The composition of the TiO ₂ is 3.0 to 6.5% by weight,
Ri der transmittance at a wavelength of 365nm is 90% or more,
The linear thermal expansion coefficient at 20 ° C. to 80 ° C. is 2.5 × 10 ⁻⁷ / ° C. or less,
Impurities include Al of 0.1 wt.ppm or less, Cu of 0.05 wt.ppm or less, Fe of 0.1 wt.ppm or less, Na of 0.05 wt.ppm or less, and K of 0.05 wt.ppm or less. Optical member for photomask.   It is a manufacturing method of the optical member for photomasks according to claim 1,
  By mixing the source gas, TiO ₂ A synthesis step of synthesizing a quartz glass ingot containing
  A molding step of forming the quartz glass ingot into a predetermined shape in a flat plate shape by pressurizing the ingot while maintaining a predetermined temperature;
  The manufacturing method of the optical member for photomasks including the oxidation process process of oxidizing the titanium contained in the said quartz glass by heating in an oxidizing atmosphere after the said formation process.   The quartz glass ingot is made of TiO. ₂ The manufacturing method of the optical member for photomasks of Claim 2 containing 3.0 to 6.5 weight%.   4. The method for producing an optical member for a photomask according to claim 2, wherein the oxidation treatment step is annealing a flat quartz glass having a thickness of 20 mm or less.   Further comprising a transmittance inspection step between the synthesis step and the oxidation treatment step, and based on the transmittance measured by the transmittance inspection step, the temperature, time and gas pressure of the oxidation atmosphere of the oxidation treatment step The method for producing a photomask optical member according to any one of claims 2 to 4, wherein any one of the above is determined.   The optical member for a photomask according to any one of claims 2 to 5, further comprising a grinding step of removing a surface of the flat quartz glass after the forming step, and performing the oxidation treatment step after the grinding step. Manufacturing method.   The manufacturing method of the optical member for photomasks as described in any one of Claims 2-6 which further includes the grinding | polishing process of grind | polishing the surface which light permeate | transmits when using as a photomask after the said oxidation process process.   The method for producing an optical member for photomask according to any one of claims 2 to 7, wherein the photomask optical member has a rectangular shape, and the length of the diagonal line is 1470 mm or more.

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