JPWO2013084978A1 - Photomask substrate for EUV lithography made of titania-silica glass - Google Patents

Abstract

Photocatalyst for EUV lithography made of titania-silica glass, which has high flatness required for a photomask substrate for EUV lithography and can reduce the cost and processing time required for surface treatment such as plasma etching and ion beam etching. A mask substrate is provided.
In the photomask substrate shape of 152.4 × 152.4 × 6.35 mm, the refractive index homogeneity Δn within the central portion 142.4 × 142.4 mm of the substrate is 3 × 10 ⁻⁴ or less. The central portion of the 4 × 152.4 mm plane has a maximum or minimum refractive index within the range of 20 × 20 mm, and the central portion of the 152.4 × 152.4 mm plane is 142.4 × 142.4 mm. In the range, the difference between the highest position and the lowest position was set to 50 nm or less.

Description

The present invention relates to a photomask substrate for EUV lithography having high surface accuracy.

As is well known, high integration of semiconductor integrated circuits in recent years is remarkable. Along with this trend, the wavelength of the exposure light source in the lithography process at the time of manufacturing a semiconductor element has been shortened. At present, lithography using an ArF excimer laser (193 nm) has become mainstream. In the future, the life of lithography using ArF excimer laser such as immersion technology and double patterning will be extended to realize further high integration, and then to lithography using extreme ultraviolet (EUV) The transition is promising.

EUV lithography is expected to use a soft X-ray having a wavelength of 70 nm or less, particularly a wavelength near 13 nm as a light source. Since there is no highly transmissive substance at such a wavelength, a reflective optical system is employed in EUV lithography. At this time, reflection is performed by a reflective multilayer film such as silicon or molybdenum deposited on the substrate, but several tens of percent of the incident EUV light reaches the substrate without being reflected and is converted into heat. In EUV lithography in which the light source wavelength is extremely short as compared with conventional lithography techniques, even a slight thermal expansion due to heat reaching each member used in a lithography optical system such as a substrate adversely affects lithography accuracy. Therefore, the use of a low expansion material is essential for the photomask. As a low expansion material, silica glass doped with titania is known. Silica glass can be reduced in thermal expansion by adding a certain amount of titania.

In EUV lithography with a short light source wavelength, the unevenness of the photomask base material surface affects the reflection characteristics of the silicon and molybdenum multilayer reflective film formed on the substrate. There are roughly three types of indexes representing the unevenness of a photomask substrate for EUV lithography. High spatial frequency roughness HSFR (High-Spatial Frequency Roughness) of 1 μm pitch or less, Medium spatial frequency roughness MSFR (1 μm to 10 μm pitch) Mid-Spatial Frequency Roughness and Flatness. HSFR affects the power loss of the EUV light source, MSFR affects the scattering of the EUV light source, and the flatness affects the transfer position accuracy.

As a method for improving HFSR and MSFR, Patent Document 1 discloses that the striae direction is parallel to the reflecting surface and the surface roughness of each spatial frequency is improved by a method of penetrating from a TiO ₂ —SiO ₂ glass lump. Has been.

Patent Document 2 discloses that the deposition rotation speed of the VAD (Vapor phased Axial Deposition) method is optimized, the refractive index fluctuation width Δn caused by TiO ₂ segregation is reduced, and as a result, the MSFR is also improved.

However, these methods are limited to the improvement of striae caused by local TiO ₂ that affects HFSR and MSFR, and within the central exposure range 132.4 × 132.4 mm of a photomask for EUV lithography. There is no mention of roughness distribution or flatness.

JP 2008-201665 A JP 2010-275189 A JP 2006-240978 A JP 2010-13335 A

The present invention has been made to solve the above-mentioned problems of the prior art, has high flatness required for a photomask substrate for EUV lithography, and costs required for surface treatment such as plasma etching and ion beam etching. An object of the present invention is to provide a titania-silica glass photomask substrate for EUV lithography that can reduce processing time and the like.

As a result of intensive studies to solve the above problems, the present inventors have obtained a refractive index homogeneity and a refractive index distribution of titania-silica glass in order to obtain high flatness required for a photomask substrate for EUV lithography. The present inventors have found that it is necessary to consider the shape, and have made the present invention.

A photomask substrate for EUV lithography made of titania-silica glass according to the present invention has a photomask substrate shape of 152.4 × 152.4 × 6.35 mm, and a refractive index within the central portion 142.4 × 142.4 mm of the substrate. The homogeneity Δn [difference between maximum and minimum refractive index (Δn)] is 3 × 10 ⁻⁴ or less, and the refractive index is within a range of 20 × 20 mm in the center of 152.4 × 152.4 mm. The difference between the highest position and the lowest position (PV flatness within a range of 142.4 × 142.4 mm in the center in the 152.4 × 152.4 mm plane). ) Is 50 nm or less.

EUV lithography is expected to be applied to 32 nm and 22 nm node semiconductor fine processing techniques. It is very difficult to achieve high-accuracy surface flatness within an exposure range of 132.4 × 132.4 mm, which is required for a photomask substrate for EUV lithography, only by forming a prism from a conventional titania-silica glass ingot. Met.

That is, the titania-silica glass photomask substrate for EUV lithography of the present invention is refracted on a 142.4 × 142.4 mm surface, which is wider than an exposure range of 132.4 × 132.4 mm surface that reflects EUV light having a wavelength of 70 nm or less. The rate homogeneity Δn is 3 × 10 ⁻⁴ or less, and there is only one local maximum or minimum value of the refractive index in the center 20 × 20 mm of the substrate. Due to the homogeneity of this refractive index, the difference between the highest position and the lowest position in the 142.4 × 142.4 mm plane after high-precision polishing is 50 nm or less, and for EUV lithography with excellent transfer position accuracy. A photomask substrate can be easily obtained by a conventional polishing method. The reason for this seems to be that in the polishing by the double-side polishing machine, the member to be polished revolves with respect to the rotation center of the double-side polishing machine, and at the same time, is polished while rotating with respect to the center of the member.

In the present invention, in the photomask substrate for EUV lithography made of titania-silica glass, the in-plane refractive index distribution that reflects EUV light having a wavelength of 70 nm or less has central symmetry with respect to the refractive index pole. desirable. The central symmetry in the present invention refers to the extreme of the difference between the refractive index value of the extreme point and the refractive index value most different from the refractive index value of the extreme point in the in-plane refractive index distribution that reflects EUV light having a wavelength of 10 to 70 nm. It is defined that the ratio of the longest distance to the shortest distance from the pole to the constant refractive index curve is 2 or less with respect to the constant refractive index curve having a refractive index value different from the refractive index value by 1/10. That is, the refractive index value of the equal refractive index curve in the present invention can be obtained as follows.

When the extreme point of the in-plane refractive index distribution that reflects EUV light having a wavelength of 10 to 70 nm is a maximum point, (refractive index value at the maximum point) − ((refractive index value at the maximum point) − (minimum refraction in the same plane) Rate value)) / 10 (1)
When the extreme point of the in-plane refractive index distribution that reflects EUV light having a wavelength of 10 to 70 nm is a minimal point,
(Refractive index value at minimum point) + ((maximum refractive index value in the same plane) − (refractive index value at minimum point)) / 10 (2)

In the present invention, the titania-silica glass photomask substrate for EUV lithography preferably has an in-plane refractive index fluctuation of 1 × 10 ⁻⁴ / mm ² or less for reflecting EUV light having a wavelength of 10 to 70 nm. As described above, a photomask substrate for EUV lithography made of titania-silica glass having high surface accuracy can be obtained by considering the in-plane refractive index distribution that reflects EUV light having a wavelength of 10 to 70 nm. However, it is difficult to achieve high surface accuracy by the double-side polishing method when the refractive index changes rapidly on the reflecting surface. A steep refractive index variation region tends to accumulate distortion and the like, and the polishing rate is often different from other regions, which causes a decrease in surface accuracy. Accordingly, in the present invention, the in-plane refractive index variation for reflecting EUV light having a wavelength of 10 to 70 nm is more preferably 5 × 10 ⁻⁵ / mm ² or less.

The refractive index in the present invention can be measured by an oil-on-plate method using a Fizeau interferometer (ZYGO MARK IV) using a He—Ne laser having a wavelength of 632.8 nm as a light source. Specifically, oil having a refractive index equivalent to that of silica glass is filled between two parallel plates made of silica glass having a low refractive index distribution, and the refractive index distribution of the parallel plates is measured in advance. A titania-silica glass photomask substrate for EUV lithography whose both surfaces are polished is sandwiched between the two parallel flat plates, the oil is filled between the parallel flat plate and the member, and a titania-silica glass photomask substrate is mounted. Measure refractive index distribution including. The refractive index distribution of the titania-doped quartz glass member is measured by removing the refractive index distribution of only the parallel plate from the refractive index distribution including the titania-silica glass photomask substrate.

Moreover, the titania-silica glass photomask substrate for EUV lithography of the present invention has a maximum value and a minimum value of TiO ₂ concentration within the range of 142.4 × 142.4 mm in the central portion in the 152.4 × 152.4 mm plane. The difference in value is preferably 0.3% or less. By setting it as this range, high PV flatness can be achieved.
The TiO ₂ concentration in the present invention was measured using an electron probe microanalyzer (JEOL Ltd.).

It is preferable that the average linear thermal expansion coefficient in the room temperature level (10-30 degreeC) of the photomask substrate for EUV lithography made from a titania-silica glass of the present invention is in the range of -30 to +30 ppb / ° C. In this case, the room temperature level is a temperature range that is an operating temperature in EUV lithography. If the average linear thermal expansion coefficient is not within the above range, it may be unsuitable for use as a member for EUV lithography such as a photomask substrate for EUV lithography. The average linear thermal expansion coefficient can be measured with a precision thermal dilatometer manufactured by NETZSCH, and can be measured with a cylindrical sample having a diameter of 3.5 mm × 25 mm. A photomask substrate for EUV lithography formed from such titania-silica glass has a similar average linear thermal expansion coefficient.

The titania-silica glass photomask substrate for EUV lithography according to the present invention has a maximum value at an OH group concentration of 500 to 700 ppm within a range of 142.4 × 142.4 mm in the center of 152.4 × 152.4 mm. And the difference between the minimum values is preferably 10 ppm or less. By setting it as this range, high PV flatness can be achieved.
The OH group concentration in the present invention was measured using an infrared spectrophotometer (Nicolet). The quantification method is based on the method described in DM DODD and DB FRASER, Optical determination of OH in fused silica, Journal of Applied Physics, Vol. 37 (1966) p. 3911.

In particular, the titania-silica glass used in the present invention forms a photomask substrate having a surface roughness (Rms) after polishing of 0.30 nm or less, preferably 0.20 nm or less, more preferably 0.15 nm or less. can do. The surface roughness (Rms) can be measured with an atomic force microscope. For example, when the photomask substrate is a 152.4 mm × 152.4 mm square substrate, the central portion of the substrate surface is 142.4 mm × 142. The surface roughness (Rms) in a 4 mm square region is preferably in the above range.

Further, the EUV lithography photomask substrate includes, for example, a region of the photomask substrate actually used at the time of exposure of a 152.4 mm × 152.4 mm square EUV lithography photomask (the central portion of the photomask substrate surface is 142.4 mm). A high accuracy is also required for the flatness of the area of (× 142.4 mm square) and the flatness of each 1 mm ² area within the above 142.4 mm × 142.4 mm square area. According to the present invention, a photomask substrate for EUV lithography that satisfies the required high accuracy can be formed.

From the titania-silica glass used in the present invention, the difference (PV flatness) between the highest position and the lowest position in the region of the central part 142.4 mm × 142.4 mm square after polishing is 50 nm or less, A photomask substrate for EUV lithography having a thickness of preferably 40 nm or less, more preferably 30 nm or less can be formed. In addition, these PV flatnesses are the highest position in each region of 1 mm ^{2 in} the region of 142.4 mm × 142.4 mm square or the region of 142.4 mm × 142.4 mm square. The difference from the lowest position can be evaluated by measuring with a laser interferometer. If the PV flatness is not within the above range, the surface shape required for the photomask substrate for EUV lithography may not be satisfied.

In addition, the difference (PV flatness) between the highest position and the lowest position in the above-mentioned substrate surface central portion 142.4 mm × 142.4 mm square region and the substrate surface central portion 142.4 mm × 142.4 mm square region The difference (PV flatness) between the highest position and the lowest position for each 1 mm ² region is strongly correlated with in-plane refractive index fluctuations that reflect EUV light having a wavelength of 70 nm or less. Therefore, it is possible to apply a homogenization treatment to remove striae by applying a zone melting method so that shear stress acts in a direction perpendicular to the growth axis of the titania-silica glass ingot and the glass body. desirable. The titania-silica glass ingot after the removal treatment is subjected to a zone melting method so that a shear stress acts in a direction perpendicular to the growth axis of the glass body to remove striae to remove OH group concentration and titania concentration. It is preferable to perform a homogenization treatment for flattening.

Further, after the homogenization treatment, it is preferable to further change the direction of the homogenization treatment axis to the titania-silica glass body and perform a second homogenization treatment by a zone melting method. After the second homogenization treatment, it is preferable to heat deform and mold so that gravity is applied in the second homogenization treatment axial direction.

In the homogenization treatment, glass support rods having a linear expansion coefficient of 0.0 × 10 ⁻⁷ / ° C. or more and 6.0 × 10 ⁻⁷ / ° C. or less at 0 to 900 ° C. at both ends of the titania-silica glass body. It is preferable to hold by a pair of rotatable holding means and to perform a homogenization process.

Further, in the homogenization treatment, both ends of the titania-silica glass body are held by a pair of rotatable holding means, and a part of the titania-silica glass body is heated with a burner, and the pair of rotatable By moving the burner while giving a large rotational difference to the holding means, a shear stress is applied in a direction perpendicular to the growth axis of the titania-silica glass body, and the striae is removed to remove the OH group concentration and titania concentration. It is preferable to perform a homogenization process for achieving homogenization. In the homogenization step, it is preferable that the method of giving a large rotation difference to the pair of rotatable holding means is to reversely rotate the pair of rotatable holding means. It can be obtained by mirror-polishing titania-silica glass with a double-side polishing machine.

The titania-silica glass ingot is an oxyhydrogen formed at the tip of the burner by supplying a combustible gas containing hydrogen gas and a combustion-supporting gas containing oxygen gas to a burner provided in a silica glass production furnace and burning it. In the flame, silicon source material gas and titanium source material gas are supplied, and silicon source material gas and titanium source material gas are hydrolyzed to produce silicon oxide, titanium oxide and their composite fine particles. An ingot is produced by adhering and growing on a target disposed in front, the obtained ingot is hot-molded into a predetermined shape, the molded ingot is annealed, and further slowly cooled The titania-silica glass photomask substrate for EUV lithography of the present invention is The fluctuations in the supply flow rates of the combustible gas, the combustion-supporting gas, the silicon source material gas, and the titanium source material gas are controlled within ± 1% / hr and introduced as a gas that circulates in the silica glass manufacturing furnace. The temperature of each of the air, the exhaust from the silica glass manufacturing furnace, and the ambient air around the silica glass manufacturing furnace is controlled within ± 2.5 ° C., the target is rotated at a rotation speed of 5 rpm or more, and the fine particles are It can be obtained by depositing on a target.

Both a vertical type and a horizontal type can be used as a titania-silica glass ingot production furnace, but the rotational speed of a target such as a seed material is 5 rpm or more, preferably 15 rpm or more, more preferably 30 rpm or more. This occurs in structurally and compositionally nonuniform regions such as striae and strain in the titania-silica glass, depending largely on the temperature nonuniformity of the portion where the titania-silica glass grows on the rotating target. Because. Therefore, by increasing the number of revolutions of the target and making the temperature of the portion where the titania-silica glass grows uniform, the occurrence of structurally and compositionally nonuniform regions of titania-silica can be suppressed. In addition, although the upper limit of the rotation speed of a target is selected suitably, it is 200 rpm or less normally.

Generation of structurally and compositionally nonuniform areas of titania-silica glass stabilizes each of silicon source gas, titanium source gas, flammable gas and combustion-supporting gas used in the production of titania-silica glass. It can be suppressed by supplying. Therefore, in the production method of the present invention, the fluctuations in the supply flow rates of the silicon source material gas, the titanium source material gas, the combustible gas, and the combustion supporting gas are within ± 1% / hr, preferably ± 0.5. % / Hr, more preferably within ± 0.25% / hr.

A known organosilicon compound can be used as the silicon source gas. Specifically, chlorine-based silane compounds such as silicon tetrachloride, dimethyldichlorosilane, and methyltrichlorosilane, tetramethoxysilane, tetraethoxysilane, and methyltrichlorosilane are used. Alkoxysilanes such as methoxysilane can be used.

Also, a known compound can be used as the titanium source material gas. Specifically, titanium halides such as titanium tetrachloride and titanium tetrabromide, tetraethoxy titanium, tetraisopropoxy titanium, tetra-n-propoxy Titanium alkoxides such as titanium, tetra-n-butoxy titanium, tetra-sec-butoxy titanium, tetra-t-butoxy titanium, and the like can be used.

On the other hand, as the combustible gas, a gas containing hydrogen is used, and further, a gas using a gas such as carbon monoxide, methane, propane or the like is used as necessary. On the other hand, a gas containing oxygen gas is used as the combustion-supporting gas.

In order to obtain the titania-silica glass photomask substrate for EUV lithography of the present invention, the growth axis of the titania-silica glass ingot produced by the direct method shown in Patent Document 4 is used as a homogenization treatment axis. By applying the zone melting method shown in Patent Document 3 so that the shear stress works in the direction perpendicular to the surface, the striae can be completely removed in one direction, and at the same time, the titania concentration and OH group concentration can be homogenized. I can do it.

Further, the direction of the homogenization treatment axis is changed with respect to the titania-silica glass subjected to the homogenization treatment using the growth axis of the titania-silica glass ingot produced by the direct method shown in Patent Document 4 as the homogenization treatment axis. By performing the homogenization treatment by the second zone melting method, a titania-silica glass body having no striae in three directions and having a more uniform titania concentration distribution and OH group concentration distribution can be obtained.

In the homogenization treatment, glass support rods having a linear thermal expansion coefficient of 0.0 × 10 ⁻⁷ / ° C. or more and 6.0 × 10 ⁻⁷ / ° C. or less at 0 to 900 ° C. at both ends of the titania-silica glass body. It is preferable to hold it with a pair of rotatable holding means via a gap and perform a homogenization treatment.

Further, in the homogenization treatment, both ends of the titania-silica glass body are held by a pair of rotatable holding means, and the pair of rotations can be performed while part of the silica / titania glass body is ignited by a burner. By moving the burner while giving a large rotational difference to the holding means, a shear stress is applied in a direction perpendicular to the growth axis of the silica-titania glass body, and the striae is removed to homogenize the titania concentration. It is preferable to perform a homogenization treatment. In the homogenization step, it is preferable that the method of giving a large rotation difference to the pair of rotatable holding means is to reversely rotate the pair of rotatable holding means. The titania-silica glass tends to increase or decrease the OH group concentration by heating. For this reason, if the heating time in the homogenization step is long, the effect of improving the OH group concentration distribution is insufficient, and in some cases, it may worsen. In view of this, in order to make the OH group concentration more uniform, it is preferable to shorten the heating time for one treatment, specifically, to increase the moving speed of the burner. Further, by increasing the rotation difference to increase the stirring efficiency, a sufficient homogenizing effect can be achieved even if the processing time is short.

The pair of rotatable holding means are preferably left and right chucks provided on a lathe. Moreover, the said homogenization process can also be repeated in multiple times as needed.

Further, after maintaining and annealing in the atmosphere at 700 to 1,300 ° C. for 1 to 200 hours, the titania-silica glass is subjected to homogenization by gradually cooling to 500 ° C. at a rate of 1 to 20 ° C./hr. It is preferable to place the body in a forming crucible rotating in a furnace having a temperature gradient of 1.5 ° C./cm or more at 1,700 ° C., and perform hot prismatic molding.

After processing the outer peripheral surface from a prismatic titania-silica glass body into a 152.4 × 152.4 mm prism body, slicing to a 6.7 mm thickness, polishing on both sides with cerium oxide for 6 hours, It is preferable to change to colloidal silica and further polish for 1 hour to process into a photomask substrate shape of 152.4 × 152.4 × 6.35 mm.

According to the present invention, titania-silica has high flatness required for a photomask substrate for EUV lithography, and can reduce the cost and processing time required for surface treatment such as plasma etching and ion beam etching. There is a remarkable effect that a glass photomask substrate for EUV lithography can be provided.

It is a flow which shows the homogenization process outline | summary of this invention. It is a schematic explanatory drawing which shows in principle the step 106 in the homogenization process of the method of this invention. It is a schematic explanatory drawing which shows in principle the step 108 in the homogenization process of the method of this invention. It is a schematic explanatory drawing which shows in principle the step 110 in the homogenization process of the method of this invention. The structure of the burner used by the Example and the comparative example is shown, (a) is the schematic which shows a titania-silica glass ingot manufacturing apparatus, (b) is a cross-sectional view of the oxyhydrogen flame burner used for this.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the accompanying drawings. However, the illustrated examples are illustrative only, and various modifications can be made without departing from the technical idea of the present invention. .

Next, an example of a homogenization treatment for obtaining a photomask substrate for EUV lithography made of titania-silica glass according to the present invention will be described. In the method of the present invention, when strict homogeneity is required, it is preferable to perform homogenization treatment a plurality of times by changing the homogenization treatment axis. FIG. 1 shows that homogenization processing for removing striae by applying a zone melting method so that shear stress acts in a direction perpendicular to the growth axis of the glass body is performed on two homogenization processing axes, respectively. It is a flowchart which shows a preferable example in a case.

As shown in FIG. 1, a titania-silica glass body is produced by a direct method (step 100), and a zone melting method is applied so that shear stress acts in a direction perpendicular to the growth axis of the glass body. A homogenization process is performed (step 102: first homogenization process). Thereafter, the homogenized glass body is formed into a spherical glass body (step 104: first molding process), and the spherical glass body is changed so as to change the axis (step 106: holding process). The glass body is stretched while being heated (step 108: stretching process) and formed into a glass body having a cylindrical shape suitable for homogenization (step 110: second molding process), and then again by the zone melting method. By performing the homogenization process (step 112: second homogenization process) and by molding the glass body (step 114: third molding process), there is no striae in three directions, A very homogeneous titania-silica glass is obtained. In the illustrated example, the titania-silica glass body is produced by the direct method in Step 100. However, the titania-silica glass body can be produced by a known method other than the direct method.

FIG. 2 is a schematic explanatory view showing step 106 in principle. As shown in FIG. 2, the spherical glass body 25 is replaced by separating the molded spherical glass body 25 from the glass support bar 30 and attaching the glass support bar 30 again so that the axis changes. In FIG. 2, reference numeral 42a is a homogenization process axis in the first homogenization process, and reference numeral 42b is a homogenization process axis in the second homogenization process. Reference numeral 31a is a growth axis. The holding method is not particularly limited, but as shown in FIG. 2, the spherical glass body 25 separated from the glass support rod 30 is rotated by approximately 90 degrees, and the first homogenization processing shaft 42a and the second homogenization processing are performed. The shaft 42b is preferably installed so as to be substantially orthogonal.

FIG. 3 is a schematic explanatory view showing step 108 in principle. As shown in FIG. 3, the glass body 21 is stretched by increasing the distance between the left and right chucks 32 a and 32 b while heating the held spherical glass body 25 with a burner 34.

FIG. 4 is a schematic explanatory view showing step 110 in principle. As shown in FIG. 4, by moving the burner 24 while twisting the stretched glass body 21 while giving a difference in the number of rotations of the left and right chucks 32a and 32b, the entire glass body is formed into a columnar shape. The rod-shaped glass body 23 which is molded and has a substantially circular cross section is obtained.

By applying homogenization to the molded rod-shaped glass body 23, a homogeneous titania-silica glass free from striae in three directions from which striae has been mechanically removed is produced. By molding the homogenized glass body (step 114), a homogeneous titania-silica glass molded into a desired shape such as a cylindrical shape is obtained. In step 114, when the glass body is molded in the molding furnace, it is preferable that the glass body is installed so that gravity is applied in the direction of the second homogenization processing shaft 42b and is heated and deformed by its own weight.

The titania-silica glass obtained by the above-described method and subjected to homogenization treatment with multiple axes is completely free of striae in three directions, and also has extremely improved homogeneity. Therefore, the reflective optical system for EUV lithography It satisfies the high linear expansion coefficient homogeneity required as a photomask substrate material.

The titania-silica glass used in the present invention is hot-formed at 1,500 to 1,800 ° C. for 1 to 10 hours in order to obtain a photomask substrate shape. In hot forming, a furnace having a temperature gradient of 1.5 ° C./cm or more and 10.0 ° C./cm or less is used in the furnace at 1,700 ° C. The temperature gradient of the temperature distribution in the furnace in the present invention is defined as the average temperature gradient obtained from the temperature difference from the highest temperature range when the maximum temperature range in the furnace is 1,700 ° C. to the position 500 mm above the furnace. To do. Further, it is desirable to perform hot forming without load. Furthermore, when titania-silica glass is square at the center of the bottom of the crucible or when the bottom of the crucible is square, it is installed at the intersection of the diagonal lines or at the center of gravity.

The hot-formed titania-silica glass is annealed and further slowly cooled. These annealing treatment and slow cooling treatment have the effect of reducing the strain in the titania-silica glass generated by hot forming. Known annealing conditions can be used, and the temperature may be maintained at 700 to 1,300 ° C. in the atmosphere for 1 to 200 hours. Also, the slow cooling treatment conditions can be known conditions. For example, cooling from the annealing temperature to a temperature of 500 ° C. may be performed at a rate of 1 to 20 ° C./hr.

After processing titania-silica glass that has been annealed and annealed to a predetermined size by grinding or slicing as appropriate, silicon oxide, aluminum oxide, molybdenum oxide, silicon carbide, diamond, cerium oxide, colloidal silica, etc. It is possible to form a photomask substrate for EUV lithography by polishing with a double-side polishing machine using this polishing agent.

EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated concretely, this invention is not limited to the following Example.

[Example 1]
The burner described in Patent Document 4 shown in FIG. 5 was used. 5, in FIG. 5 (a), 1 is a silicon tetrachloride supply pipe, 2 is a titanium tetrachloride supply pipe, 3 is a flow meter, 4, 5 and 6 are hydrogen gas supply pipes, 7, 8, 9 and 10 are oxygen gas supply pipes, 11 is an oxyhydrogen flame burner, 12 is an oxyhydrogen flame, 13 is titania-doped silica fine particles, 14 is a support, and 15 is an ingot. FIG. 5B is a cross-sectional view of the burner 11, and the burner 11 has an outer shell tube 22 outside the quintuple tube 16 composed of the nozzles 17 to 21. The center nozzle (first nozzle) 17 is supplied with silicon tetrachloride and titanium tetrachloride from the supply pipes 1 and 2 and supplied with oxygen gas from the oxygen supply pipe 10. . Note that an inert gas such as an argon gas can be supplied if necessary. Further, oxygen gas is supplied to the second nozzle 18 and the fourth nozzle 20 from the oxygen gas supply pipes 7 and 8, and hydrogen gas is supplied to the third nozzle 19 and the fifth nozzle 21 from the hydrogen gas supply pipes 4 and 5. Is done. Further, hydrogen gas is supplied from the hydrogen gas supply pipe 6 to the outer shell tube 22, and oxygen gas is supplied from the oxygen gas supply pipe 9 to the nozzle 23.

The gas shown in Table 1 is supplied to each nozzle of the main burner, and SiO ₂ and TiO ₂ produced by the hydrolysis reaction of silicon tetrachloride and titanium tetrachloride in the oxyhydrogen flame are sent to the tip of the quartz glass burner. A titania-silica glass ingot was manufactured by adhering to a target material that retreated at 10 mm / hr while rotating at 50 rpm. In addition, a sub-burner that applied an oxyhydrogen flame to the side of the ingot was used simultaneously with the main burner. At this time, the flow rate variation of various gases was ± 0.2% / hr. Moreover, the temperature fluctuation | variation of the air supplied to a titania-silica glass manufacturing furnace, the gas exhausted, and the external temperature of a manufacturing furnace was +/- 1 degreeC.

Silica having a linear expansion coefficient of 0 ° C. to 900 ° C. at both ends in the growth axis direction of the titania-silica glass body at both ends in the growth axis direction of the obtained titania-silica glass ingot is 5 × 10 ⁻⁷ / ° C. A glass support bar was welded, and both ends of the support bar were fixed with a lathe chuck.

While the left and right chucks of the lathe were rotating synchronously at 50 rpm, the left end of the titania-silica glass body was ignited with an oxyhydrogen burner to form a melting zone. After confirming that the melting zone was formed, the rotation of the chuck on the right lathe was rotated at 150 rpm opposite to the rotation direction of the left chuck, and a strong shear stress was applied to stir the melting zone. At the same time, the melting zone was moved by moving the burner to the right at a speed of 20 mm / min to homogenize the entire titania-silica glass body (homogenization process in the first direction). The homogenization process was again performed in the same direction by the same operation, and the homogenization process was performed twice in total.

After the homogenization treatment, the rotation directions of both chucks were aligned and rotated synchronously at 50 rpm, and the burner was returned to the left end of the rod-like titania-silica glass body and ignited to melt. After confirming that the titania-silica glass body was melted, the silica lathe titania glass body was crushed by pressing and narrowing the chuck of the right lathe and molded into a spherical shape having a diameter of about 190 mm (first molding step).

Both ends of the spherical glass body (diameter: about 190 mm) that had undergone the molding process were cut off from the support rod, placed on a table with one cut surface down, and the support rod was welded again to both sides of the ball. Since the axis connecting both ends of the separated ball is the axis of the first homogenization treatment, the axis connecting the newly welded support rods is orthogonal to the axis of the first homogenization (holding process) ). The whole glass body was strongly heated with a burner flame while rotating the entire ball in synchronism with both support rods at 20 rpm, and the whole glass body was melted. After confirming that the entire glass body was melted, both chucks of the lathe were separated to stretch the glass body (stretching step).

For the irregularly stretched titania-silica glass body, the rotation speed of the chuck on the right side of the lathe is increased to 40 rpm, the rotation speed between the chucks is given a differential, and the titania-silica glass body is slowly twisted. The burner was moved to the right side at a speed of 20 mm / min while forming a cylindrical shape with the above and the gap between both chucks was increased to increase the diameter of the glass body, thereby forming the entire glass body into a cylindrical shape with a diameter of about φ70 mm ( Second molding step). In this case, the rotation directions of both chucks are the same.

The titania-silica glass body was subjected to a homogenization treatment by the same operation as the first homogenization treatment (second homogenization treatment step). The axis in the homogenization process in this case is orthogonal to the axis in the first homogenization process. The titania-silica glass body after the second homogenization treatment was formed into a spherical shape by the same operation as in the first forming step.

The obtained titania-silica glass spheres were placed at the intersection of 155 mm × 155 mm prismatic bottom and diagonal lines in an electric furnace with a temperature gradient at 1,700 ° C. of 2.5 ° C./cm at 6 ° C. at 1,700 ° C. Hot molding was performed by heating for a period of time. Thereafter, it was kept at 1,150 ° C. for 150 hours in the air, annealed, and then gradually cooled to 500 ° C. at a rate of 5 ° C./hr. The annealed molded body is ground to a 152.4 mm × 152.4 mm prism shape, and the prismatic molded body is sliced to a thickness of 6.7 mm to be ground to a photomask substrate. As a result of measuring the refractive index distribution in the 4 mm square, the difference between the maximum refractive index value and the minimum refractive index within the range of 142.4 mm × 142.4 mm was 2.0 × 10 ⁻⁴ . The refractive index distribution shape was a shape having a minimum point within a central portion of 20 × 20 mm in a 152.4 mm × 152.4 mm square surface. There were no other extreme points and no inflection curve was observed.

Using a suede type polishing cloth and a cerium oxide abrasive on a titania-silica glass EUV lithography substrate with a refractive index of 6.7 mm, using a 12B type double-side polishing machine (Fujikoshi Kikai Kogyo Co., Ltd.) After polishing for 6 hours, the abrasive was changed to colloidal silica and polished for 0.5 hours.
The difference (PV flatness of the exposure utilization area) between the highest position and the lowest position in the produced substrate surface center part 142.4 mm × 142.4 mm square area was measured using a laser interferometer. The results are shown in Table 2. The PV flatness was 25 nm, which was very small.
Table 2 shows the maximum value and the minimum value as a result of measuring the average linear thermal expansion coefficient at 10 points in the range of 10 to 30 ° C. on the diagonal line in the 152.4 mm × 152.4 mm square surface of the produced substrate.

An electron probe microanalyzer is applied from the central part of a titania-silica glass substrate having a flatness of 152.4 mm × 152.4 mm square and a thickness of 6.7 mm to the central part of the substrate surface of 142.4 mm × 142.4 mm square part. The TiO ₂ concentration (manufactured by JEOL Ltd.) was measured. Table 2 shows the difference between the maximum value and the minimum value of the TiO ₂ concentration. The density difference was very low at 0.2%.

TiO ₂ concentration measured 152.4 mm × 152.4 mm square, 6.7 mm thick titania-silica glass substrate center to substrate surface center 142.4 mm × 142.4 mm square The OH group concentration was measured with a photometer (Nicolet). Table 2 shows the maximum and minimum values of the OH group concentration and the difference between the maximum and minimum values. The concentration difference was very low at 3 ppm.

The obtained titania-silica glass photomask substrate for EUV lithography has only one minimum point of refractive index in the center of the surface that reflects EUV light with a wavelength of 70 nm or less, and the refractive index distribution is at the minimum point. On the other hand, it had central symmetry and the refractive index variation was small and good. The PV flatness in the region of 142.4 mm × 142.4 mm square at the center of the polished photomask substrate surface was small, and the most suitable photomask substrate for EUV lithography was obtained.

[Example 2]
Using the burner shown in FIG. 5, the gas shown in Table 1 was supplied to each nozzle of the main burner, and SiO ₂ produced by the hydrolysis reaction of silicon tetrachloride and titanium tetrachloride in an oxyhydrogen flame and An ingot of titania-doped quartz glass was manufactured by adhering TiO ₂ to a target material set at the tip of a quartz glass burner and rotating at 50 rpm and retreating at 10 mm / hr. At this time, the flow rate variation of various gases was ± 0.2% / hr. Moreover, the temperature fluctuation of the air supplied to the titania-doped quartz glass manufacturing furnace, the exhausted gas, and the outside temperature of the manufacturing furnace was ± 1 ° C.

The obtained titania-silica glass ingot was subjected to homogenization treatment and molding treatment in the same manner as described in Example 1, polished in the same manner as described in Example 1, and 152.4 mm × 152.4 mm square, A photomask substrate for EUV lithography made of titania-silica glass having a thickness of 6.7 mm was obtained.

Table 2 shows the maximum value and the minimum value as a result of measuring the average linear thermal expansion coefficient at 10 points in the range of 10 to 30 ° C. on the diagonal line in the 152.4 mm × 152.4 mm square surface of the produced substrate.

The difference between the highest position and the lowest position (PV flatness of the exposure utilization area) in the area of the produced substrate surface center part 142.4 mm × 142.4 mm square was measured by the method described in Example 1. The PV flatness was 45 nm, which was very small.

The flatness was measured from the central part of the photomask substrate for titania-silica glass EUV lithography having a thickness of 152.4 mm × 152.4 mm square and a thickness of 6.7 mm to the central part of the substrate surface of 142.4 mm × 142.4 mm. The measurement was performed by the method described in Example 1. Table 2 shows the difference between the maximum value and the minimum value of the TiO ₂ concentration. The difference in density was very low at 0.3%.

TiO ₂ concentration measured 152.4 mm × 152.4 mm square, 6.7 mm thick titania-silica glass substrate center to substrate surface center 142.4 mm × 142.4 mm square The OH group concentration was measured with a photometer (Nicolet). Table 2 shows the maximum and minimum values of the OH group concentration and the difference between the maximum and minimum values. The concentration difference was as low as 5 ppm.

The substrate obtained in Example 2 was suitable as a photomask substrate for EUV lithography.

[Comparative Example 1]
A titania-silica glass ingot was obtained in the same manner as described in Example 1.

The obtained ingot made of titania-silica glass was subjected to a molding process without being homogenized, and polished in the same manner as described in Example 1 to obtain a 152.4 mm × 152.4 mm square and a thickness of 6.7 mm. A titania-silica glass substrate was obtained.

Table 2 shows the maximum value and the minimum value as a result of measuring the average linear thermal expansion coefficient at 10 points in the range of 10 to 30 ° C. on the diagonal line in the 152.4 mm × 152.4 mm square surface of the produced substrate.

The difference between the highest position and the lowest position (PV flatness of the exposure utilization area) in the area of the produced substrate surface center part 142.4 mm × 142.4 mm square was measured by the method described in Example 1. The PV flatness was 55 nm, which was slightly large.

Example 1 is described by multiplying the center part of the substrate made of titania-silica glass having a flatness of 152.4 mm × 152.4 mm square and a thickness of 6.7 mm from the center part of the substrate surface 142.4 mm × 142.4 mm square part. It measured by the method of. Table 2 shows the difference between the maximum value and the minimum value of the TiO ₂ concentration. The density difference was slightly high at 0.5%.

TiO ₂ concentration measured 152.4 mm × 152.4 mm square, 6.7 mm thick titania-silica glass substrate center to substrate surface center 142.4 mm × 142.4 mm square The OH group concentration was measured with a photometer (Nicolet). Table 2 shows the maximum and minimum values of the OH group concentration and the difference between the maximum and minimum values. The concentration difference was high at 15 ppm.

The substrate obtained in Comparative Example 1 was not suitable as a photomask substrate for EUV lithography.

[Comparative Example 2]
A titania-silica glass ingot was obtained in the same manner as described in Example 2.

The obtained ingot made of titania-silica glass was subjected to a molding process without being homogenized, and polished in the same manner as described in Example 1 to obtain a 152.4 mm × 152.4 mm square and a thickness of 6.7 mm. The photomask substrate for EUV lithography made of titania-silica glass was obtained.

Table 2 shows the maximum value and the minimum value as a result of measuring the average linear thermal expansion coefficient at 10 points in the range of 10 to 30 ° C. on the diagonal line in the 152.4 mm × 152.4 mm square surface of the produced substrate.

The difference between the highest position and the lowest position (PV flatness of the exposure utilization area) in the area of the produced substrate surface center part 142.4 mm × 142.4 mm square was measured by the method described in Example 1. The PV flatness was high at 65 nm.

Example 1 is described by multiplying the center part of the substrate made of titania-silica glass having a flatness of 152.4 mm × 152.4 mm square and a thickness of 6.7 mm from the center part of the substrate surface 142.4 mm × 142.4 mm square part. It measured by the method of. Table 2 shows the difference between the maximum value and the minimum value of the TiO ₂ concentration. The density difference was high at 0.7%.

TiO ₂ concentration measured 152.4 mm × 152.4 mm square, 6.7 mm thick titania-silica glass substrate center to substrate surface center 142.4 mm × 142.4 mm square The OH group concentration was measured with a photometer (Nicolet). Table 2 shows the maximum and minimum values of the OH group concentration and the difference between the maximum and minimum values. The concentration difference was slightly high at 20 ppm.

The substrate obtained in Comparative Example 2 was not suitable as a photomask substrate for EUV lithography.

As shown in Table 2, the PV flatness can be reduced to 50 nm or less by setting the refractive index distribution to 3 × 10 ⁻⁴ or less. Moreover, a high PV flatness can be achieved by setting the TiO ₂ concentration difference to 0.3% or less and the OH group concentration difference to 10 ppm or less.

1: Silicon tetrachloride supply pipe, 2: Titanium tetrachloride supply pipe, 3: Flow meter, 4, 5, 6: Hydrogen gas supply pipe, 7, 8, 9, 10: Oxygen gas supply pipe, 11: Oxyhydrogen flame Burner, 12: oxyhydrogen flame, 13: titania-doped silica fine particles, 14: support, 15: ingot, 16: 5-fold tube, 17, 18, 19, 20, 21: nozzle, 22: outer shell tube, 23: Nozzle, 25: spherical glass body, 21: stretched glass body, 23: molded rod-shaped glass body, 30: glass support rod, 31a: growth axis, 32a, 32b: chuck, 34: burner, 42a: first Homogenization processing axis, 42b: second homogenization processing axis.

Claims (4)

In the photomask substrate shape of 152.4 × 152.4 × 6.35 mm, the refractive index homogeneity Δn within the central portion 142.4 × 142.4 mm of the substrate is 3 × 10 ⁻⁴ or less,
It has only one point of the maximum or minimum value of the refractive index in the range of 20 × 20 mm in the central part in the 152.4 × 152.4 mm plane,
EUV lithography made of titania-silica glass characterized in that the difference between the highest position and the lowest position is 50 nm or less within the range of 142.4 × 142.4 mm in the central portion in the 152.4 × 152.4 mm plane. Photomask substrate. The difference between the maximum value and the minimum value of the TiO ₂ concentration is 0.3% or less within a range within a center portion of 142.4 × 142.4 mm in a 152.4 × 152.4 mm plane. Item 2. A photomask substrate for EUV lithography made of titania-silica glass according to Item 1.   3. The titania-silica glass EUV lithography photomask substrate according to claim 1, wherein an average linear thermal expansion coefficient is −30 to +30 ppb / ° C. in a range of 10 to 30 ° C. 4.   The OH group concentration is 500 to 700 ppm within the range of the central part 142.4 × 142.4 mm in the 152.4 × 152.4 mm plane, and the difference between the maximum value and the minimum value of the OH group concentration is 10 ppm or less. The photomask substrate for EUV lithography made of titania-silica glass according to any one of claims 1 to 3.

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