JP2007022903A - Method of finishing pre-polished glass substrate surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method by which the waviness generated in a glass substrate surface during pre-polishing are removed and the glass substrate is finished so as to have a highly flat surface. <P>SOLUTION: In the method of finishing a pre-polished glass substrate surface, the glass substrate is made of quartz glass containing a dopant and comprising SiO<SB>2</SB>as a main component. The finishing method comprises a step for measuring a concentration distribution of the dopant contained in the glass substrate and a step for measuring a surface shape of the glass substrate in the pre-polished state, wherein conditions for processing the glass substrate surface are set for each part of the glass substrate based on the results from the step for measuring the concentration distribution of the dopant contained in the glass substrate and the step for measuring the surface shape of the glass substrate. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method of finishing a surface of a pre-polished glass substrate, and more particularly to a glass substrate used for a reflective mask for EUV (Extreme Ultra Violet) lithography in a semiconductor manufacturing process. The present invention relates to a method for finishing a glass substrate surface that requires flatness of the glass.

Conventionally, in lithography technology, an exposure apparatus for manufacturing an integrated circuit by transferring a fine circuit pattern onto a wafer has been widely used. As integrated circuits become highly integrated, faster, and more functional, miniaturization of integrated circuits advances, and the exposure apparatus is required to image a high-resolution circuit pattern on the wafer surface with a deep focal depth. The wavelength of the exposure light source is being shortened. As an exposure light source, an ArF excimer laser (wavelength 193 nm) has started to be used further from the conventional g-line (wavelength 436 nm), i-line (wavelength 365 nm), and KrF excimer laser (wavelength 248 nm). Further, in order to cope with next-generation integrated circuits whose circuit line width is 100 nm or less, it is considered promising to use an F ₂ laser (wavelength 157 nm) as an exposure light source. It can only be covered.

  In such a technical trend, a lithography technique using EUV light as an exposure light source for the next generation is considered to be applicable over a plurality of generations of 45 nm and after, and has attracted attention. EUV light refers to light in the wavelength band of the soft X-ray region or the vacuum ultraviolet region, specifically, light having a wavelength of about 0.2 to 100 nm. At present, the use of 13.5 nm as a lithography light source is being studied. The exposure principle of this EUV lithography (hereinafter abbreviated as “EUVL”) is the same as that of conventional lithography in that the mask pattern is transferred using a projection optical system, but light is transmitted in the EUV light energy region. Since there is no material to be used, the refractive optical system cannot be used, and a reflective optical system is used (see Patent Document 1).

A mask used for EUVL basically includes (1) a glass substrate, (2) a reflective multilayer film formed on the glass substrate, and (3) an absorber layer formed on the reflective multilayer film. As the reflective multilayer film, one having a structure in which a plurality of materials having different refractive indexes with respect to the wavelength of exposure light are periodically stacked in the order of nm is used, and Mo and Si are known as representative materials. .
Further, Ta and Cr have been studied for the absorber layer. As the glass substrate, a material having a low coefficient of thermal expansion is required so that distortion does not occur even under EUV light irradiation, and the use of glass having a low coefficient of thermal expansion or crystallized glass having a low coefficient of thermal expansion has been studied. Hereinafter, in the present specification, a glass having a low thermal expansion coefficient and a crystallized glass having a low thermal expansion coefficient are collectively referred to as “low expansion glass” or “ultra-low expansion glass”.
The low expansion glass or ultra low expansion glass used as a mask for EUVL is quartz glass mainly composed of SiO ₂ , and TiO ₂ , SnO ₂ or ZrO ₂ is a dopant in order to lower the thermal expansion coefficient of the glass. Those added as are most widely used.

  The glass substrate is manufactured by processing and washing these glass and crystallized glass materials with high accuracy. When processing a glass substrate, it is usually pre-polished at a relatively high processing rate until the glass substrate surface has a predetermined flatness and surface roughness, and then using a method with higher processing accuracy or more processing. Finishing is performed so that the glass substrate surface has a desired flatness and surface roughness using processing conditions that increase accuracy.

Japanese translation of PCT publication No. 2003-505891

When processing a glass substrate for an EUVL mask, partial undulation may occur on the glass substrate surface. The present inventors have found that the occurrence of this undulation is caused by a partial difference in composition of the glass substrate, more specifically, by the concentration distribution of the dopant contained in the glass substrate. There is a possibility that the glass substrate surface may be swelled during both the preliminary polishing and the finishing process. However, during pre-polishing with a high processing rate, there is a possibility that a large undulation will occur on the surface of the glass substrate. When large waviness was generated during the preliminary polishing, it was difficult to remove the surface by finishing to obtain a desired flatness on the glass substrate surface. Further, the undulation generated during the preliminary polishing may grow into a larger undulation during the finishing process.
In order to solve the above-described problems, an object of the present invention is to provide a method for removing a waviness generated on the surface of a glass substrate during preliminary polishing and finishing the glass substrate to a surface having excellent flatness. And

To achieve the above object, the present invention is a method of finishing a pre-polished glass substrate surface,
The glass substrate contains a dopant and is made of quartz glass containing SiO ₂ as a main component, and a method of finishing the pre-polished glass substrate surface,
Measuring the concentration distribution of the dopant contained in the glass substrate;
A step of measuring a surface shape of the glass substrate after preliminary polishing, and a step of measuring a concentration distribution of a dopant contained in the glass substrate and a result obtained from a step of measuring the surface shape of the glass substrate. Then, the processing condition of the glass substrate surface is set for each part of the glass substrate, and a method of finishing the pre-polished glass substrate surface (hereinafter referred to as “the finishing method of the present invention”). I will provide a.

In the finishing method of the present invention, the correlation between the dopant concentration contained in the glass substrate and the processing rate of the glass substrate surface is obtained in advance,
It is preferable to set the processing conditions of the glass substrate surface for each part of the glass substrate using the measurement result of the dopant concentration distribution contained in the glass substrate and the correlation between the dopant concentration and the processing rate. .

In the finishing method of the present invention, the flatness of the glass substrate surface is determined from the measurement result of the surface shape of the glass substrate,
Based on the flatness of the glass substrate surface, it is preferable to set the processing conditions of the glass substrate surface for each part of the glass substrate.

  In the finishing method of the present invention, the glass substrate surface is processed using a processing method selected from the group consisting of ion beam etching, gas cluster ion beam etching, plasma etching, nano-ablation, and MRF (magnetorheological finishing). It is preferable.

In the finishing method of the present invention, the processing method is ion beam etching, gas cluster ion beam etching or plasma etching,
From the measurement result of the surface shape of the glass substrate, specify the width of the undulation present on the glass substrate surface,
It is preferable to perform processing using a beam having a beam diameter of FWHM (full width of half maximum) and less than the width of the waviness.
The FWHM value of the beam diameter is more preferably ½ or less of the waviness width.

In the finishing method of the present invention, the processing method is preferably gas cluster ion beam etching.
As a source gas for the gas cluster ion beam etching, it is preferable to use any mixed gas selected from the following group.
SF ₆ and O ₂ mixed gas, SF ₆ , Ar and O ₂ mixed gas, NF ₃ and O ₂ mixed gas, NF ₃ , Ar and O ₂ mixed gas, NF ₃ and N ₂ mixed gas, NF _3. Mixed gas of Ar, N ₂ It is more preferable to use any mixed gas selected from the following group as the source gas.
A mixed gas of SF ₆ and O _2, a mixed gas of SF ₆ , Ar and O _2, a mixed gas of NF ₃ and O _2, a mixed gas of NF ₃ , Ar and O ₂

In the finishing method of the present invention, the glass substrate is preferably made of low expansion glass having a thermal expansion coefficient of 0 ± 30 ppb / ° C. at 20 ° C.
In the finishing method of the present invention, the dopant is preferably TiO ₂ .

  In the finishing method of the present invention, the glass substrate preferably has a surface roughness (Rms) after preliminary polishing of 5 nm or less.

In the finishing method of the present invention, the dopant concentration distribution contained in the glass substrate and the surface shape of the glass substrate after preliminary polishing are measured, and the processing conditions of the glass substrate surface are determined based on the measurement results. Since it sets for every site | part, the waviness which arose on the glass substrate surface in the case of preliminary polishing can be removed effectively. In addition, since the processing conditions of the glass substrate surface are set for each part of the glass substrate based on the measurement result of the concentration distribution of the dopant contained in the glass substrate, new undulation occurs on the glass substrate surface during the finishing process. There is no risk that the waviness generated during the preliminary polishing will grow during the finishing process.
By using the finishing method of the present invention, the glass substrate can be processed into a surface excellent in flatness. Thereby, the glass substrate excellent in flatness which can respond also to the board | substrate for optical components etc. of the exposure apparatus for the next generation semiconductor manufacture whose line | wire width is 45 nm or less can be obtained.

  The finishing method of the present invention is a method for finishing a glass substrate surface after preliminary polishing. More specifically, it is a method of removing waviness generated on the surface of the glass substrate during preliminary polishing and finishing the glass substrate to a surface having excellent flatness.

  Pre-polishing is a procedure for processing a glass substrate surface to a certain degree of flatness and surface roughness at a relatively high processing rate before processing the glass substrate surface to a predetermined flatness and surface roughness. The pre-polished glass substrate is finished to have a predetermined flatness and surface roughness.

  The surface roughness (Rms) of the glass substrate after preliminary polishing is preferably 5 nm or less, and more preferably 1 nm or less. In this specification, when it says surface roughness, it means the surface roughness measured with the atomic force microscope about the area of 1-10 micrometers square. If the surface roughness of the glass substrate after preliminary polishing is more than 5 nm, it takes a considerable time to finish the surface of the glass substrate by the finish polishing method of the present invention, which causes an increase in cost.

  The processing method used for preliminary polishing is not particularly limited, and can be widely selected from known processing methods used for processing the glass surface. However, since a large area can be polished at a time by using a polishing pad having a high processing rate and a large surface area, a mechanical polishing method is usually used. The mechanical polishing method referred to here includes a chemical mechanical polishing method in which a polishing action by abrasive grains and a chemical polishing action by chemicals are used in addition to a polishing process only by a polishing action by abrasive grains. The mechanical polishing method may be either lapping or polishing, and the polishing tool and polishing agent to be used can be appropriately selected from known ones.

  Waviness refers to those having a period of 5 to 30 mm among the periodic irregularities present on the glass substrate surface. The finishing method of the present invention is a method for effectively removing waviness generated on the surface of a glass substrate during preliminary polishing.

  The finishing method of the present invention is suitable for finishing a glass substrate for a reflective mask for EUVL that can cope with high integration and high definition of an integrated circuit. The glass substrate used in this application is a glass substrate having a small coefficient of thermal expansion and small variations, and is preferably made of low expansion glass having a coefficient of thermal expansion at 20 ° C. of 0 ± 30 ppb / ° C., More preferably, it is made of ultra-low expansion glass having a thermal expansion coefficient at 0 ° C. of 0 ± 10 ppb / ° C.

As such low expansion glass and ultra-low expansion glass, quartz glass containing SiO ₂ as a main component and having a dopant added to lower the thermal expansion coefficient of the glass is most widely used. Note that dopant added to lower the thermal expansion coefficient of the glass is typically a TiO _2. Specific examples of the low expansion glass and ultra low expansion glass to which TiO ₂ is added as a dopant include ULE (registered trademark) code 7972 (manufactured by Corning).

In the finishing method of the present invention, the glass constituting the glass substrate is quartz glass containing SiO ₂ as a main component, and a dopant is added. A typical example is quartz glass to which TiO ₂ is added in order to lower the thermal expansion coefficient of the glass. However, not limited thereto, the glass constituting the glass substrate, a quartz glass as a main component SiO _2, may be one dopant is added for any other purpose. Hereinafter, in the present specification, quartz glass containing SiO ₂ as a main component and added with some dopant is collectively referred to as “doped quartz glass”.
As doped quartz glass to which a dopant is added for the purpose other than lowering the thermal expansion coefficient, for example, La ₂ O ₃ , Al ₂ O ₃ , ZrO ₂ or N is added to increase the absolute refractive index of the glass. In order to increase the laser resistance of the doped quartz glass and glass, doped quartz glass to which F is added can be used.

The dopant content of the doped quartz glass varies depending on the type of dopant and the purpose of including the dopant. In order to lower the thermal expansion coefficient of glass, in the case of doped quartz glass to which TiO ₂ is added, it is preferable to contain 1 to 12% by mass of TiO ₂ in terms of mass% with respect to SiO ₂ . If the content of TiO ₂ is less than 1% by mass, the thermal expansion coefficient of the glass may not be sufficiently lowered. When the content of TiO ₂ exceeds 12 wt%, the thermal expansion coefficient becomes large in the negative side, less than -30ppb / ℃. The content of TiO ₂ is more preferably 5 to 9% by mass.

Note that the finishing method of the present invention can be applied to glass substrates other than doped quartz glass as long as the waviness generated on the glass substrate surface during preliminary polishing can be removed by the procedure described below. is there. Therefore, it is considered that the finishing method of the present invention can also be applied to low expansion crystallized glass containing TiO ₂ or ZrO ₂ as crystal nuclei.
The shape, size, thickness, and the like of the glass substrate are not particularly limited, but in the case of a substrate for a reflective mask for EUVL, the shape is a plate-like body having a rectangular or square planar shape.

The finishing method of the present invention has the following two measurement steps.
-Step of measuring the concentration distribution of dopant contained in the glass substrate (measurement step 1)
-Step of measuring the surface shape of the glass substrate after preliminary polishing (Measurement step 2)

In the measurement step 1, the concentration distribution of the dopant contained in the glass substrate is measured. Here, the concentration distribution of the dopant contained in the glass substrate is not the concentration distribution of the dopant in the thickness direction of the glass substrate, but the two-dimensional shape when the flat glass substrate is viewed as a two-dimensional shape having no thickness. It means the dopant concentration distribution in each part of the dimension shape, that is, the concentration distribution in the plane of the flat glass substrate. It is assumed that the dopant concentration distribution in a plane parallel to the glass substrate at an arbitrary thickness of the glass substrate is equivalent to the measured surface concentration distribution of the glass substrate.
Therefore, the measurement result obtained from the measurement step 1 is a dopant concentration distribution map (hereinafter referred to as “dopant concentration distribution map”) indicating the concentration of the dopant in each part of the two-dimensional shape.

The method used for measuring the concentration distribution of the dopant can be appropriately selected according to the type of dopant. For example, the concentration distribution of the dopant contained in the glass substrate can be measured by fluorescent X-ray analysis of the glass substrate surface. In addition, when the dopant is TiO ₂ , there is a correlation between the TiO ₂ concentration and the refractive index of the glass substrate. Therefore, from the refractive index distribution measurement by transmission wavefront measurement with a laser interference flatness measuring device, The concentration distribution of contained TiO ₂ can also be obtained nondestructively.

  It is considered that the concentration distribution of the dopant contained in the glass substrate is substantially the same before and after the preliminary polishing. Therefore, the measurement process 1 may be performed before preliminary polishing or may be performed after preliminary polishing. However, when a laser interference flatness measuring apparatus is used, the surface shape of the glass substrate after preliminary polishing can be measured with the same apparatus, so that it is preferable to perform after the preliminary polishing.

In the measurement step 2, the surface shape of the glass substrate after preliminary polishing is measured. The surface shape of the glass substrate is measured as the flatness of the glass substrate surface. Here, the flatness of the glass substrate surface means the flatness, that is, the height difference in each part of the glass substrate surface.
Therefore, the measurement result obtained from the measurement step 2 is a flatness map (hereinafter referred to as “flatness map”) indicating the height difference in each part of the glass substrate surface.

  The flatness of the glass substrate surface can be measured using, for example, a laser interference type flatness measuring machine. In the examples shown later, the flatness of the surface of the glass substrate was measured using a G310S Fizeau laser interference flatness measuring machine (Fujinon). However, the means for measuring the flatness of the surface of the glass substrate is not limited to this, and the height difference of the surface of the glass substrate is measured using a laser displacement meter, ultrasonic displacement meter or contact displacement meter, and the measurement is performed. You may obtain | require flatness using a result.

The finishing method of the present invention sets the processing conditions on the surface of the glass substrate for each part of the glass substrate based on the results obtained from the measurement step 1 and the measurement step 2. Hereinafter, in this specification, setting the processing conditions on the surface of the glass substrate for each portion of the glass substrate is simply referred to as “setting the processing conditions of the glass substrate”.
In the finishing method of the present invention, the processing conditions for the glass substrate are set based on both the result obtained from the measurement step 1 and the result obtained from the measurement result 2. However, in order to facilitate understanding, the setting of the processing conditions based on the result obtained from the measurement step 1 and the setting of the processing conditions based on the result obtained from the measurement step 2 will be described separately below. .

  When processing conditions are set based on the results obtained from the measurement step 1, the correlation between the dopant concentration contained in the glass substrate and the processing rate on the glass substrate surface (hereinafter referred to as “correlation between the dopant concentration and the processing rate”). And the processing conditions are set using the result obtained from the measurement step 1 and the correlation between the dopant concentration and the processing rate.

The present inventors have found that when processing a glass substrate made of doped quartz glass, there is some correlation between the dopant concentration and the processing rate.
For example, in the case of doped quartz glass containing TiO ₂ as a dopant, when the doped quartz glass is processed under constant processing conditions, it is between the dopant concentration X (wt%) and the processing rate Y (μm / min). There is a correlation represented by the following formula (1).
Y = a · X + b (1)
In formula (1), a and b represent variables.

FIG. 1 is a graph showing a correlation between a dopant concentration and a processing rate for a doped quartz glass containing TiO ₂ as a dopant. In the case of using a gas cluster ion beam etching as a processing method, a machine using cerium oxide is shown. The correlation when using polishing is shown. The procedure for creating FIG. 1 is shown below.
Prepared test samples (20 × 20 × 1 mmt) made of doped quartz glass containing 0%, 3.1%, 5.1%, 6.9%, 8.7% of TiO _{2 in} mass% with respect to SiO ₂ did. These test samples having different TiO ₂ concentrations are processed under the same conditions to obtain the processing rate, and the relationship between the TiO ₂ concentration and the processing rate is plotted in FIG. In FIG. 1, the processing rate is shown as a normalized processing rate, where the processing rate when the TiO ₂ concentration is 0 mass% is 1.

Gas cluster ion beam etching and mechanical polishing were performed under the following conditions.
Gas cluster ion beam etching Source gas: SF ₆ 1.25%, O ₂ 24%, Ar 74.75%
Acceleration voltage: 30 kV
Ionization current: 50 μA
Beam diameter (FWHM value): 10 mm or less Dose amount: 6.2 × 10 ¹⁵ ions / cm ²
Mechanical polishing Abrasive: Cerium oxide (Speedfam CO85 (Showa Denko H-3))
Polishing pad: Cerium impregnated polyurethane pad (Roder Nitta MHC14B)
Polishing apparatus: double-side polishing machine The processing rate shown in FIG. 1 was obtained from the change in weight of the test sample before and after processing using a weight method.

When gas cluster ion beam etching is performed under the above processing conditions, the above equation (1) is obtained from the following equation (1-1) from FIG. On the other hand, when mechanical polishing with cerium oxide is performed, the above formula (1) is obtained as formula (1-2).
Y = 0.0522X + 1.0449 (1-1)
Y = 0.0306X + 1.0188 (1-2)

About the processing method and processing conditions used for the finish processing of the glass substrate surface, if a graph similar to FIG. 1 is prepared, the dopant concentration distribution map obtained from the measurement step 1 and the formula (1) are used to The processing amount of the substrate surface can be set for each part of the glass substrate. However, since Formula (1) shows the relationship between the dopant concentration contained in the glass substrate under a constant processing condition and the processing rate of the glass substrate surface, the processing condition that can be used for setting is only the processing time. It is.
The concept used for the dopant concentration distribution, in a two-dimensional glass substrate having no thickness, where the coordinates of the glass substrate are (x, y), the dopant concentration distribution map obtained from measurement step 1 is C (x, y) (mass%). The processing amount of the glass substrate is expressed as W (x, y) (μm), and the processing time is expressed as T (x, y) (min). Note that W (x, y) indicates a processing amount (planned processing amount) at a position of the coordinates (x, y) of the glass substrate, and is a constant. For example, when processing the part of the coordinate (x, y) of the glass substrate by 5 μm, W (x, y) = 5 μm.
The relationship between W (x, y) and T (x, y) is expressed by the following equation (2).
T (x, y) = W (x, y) / (a * C (x, y) + b) (2)
Therefore, when setting the processing conditions of a glass substrate based on the result obtained from the measurement process 1, the processing conditions of a glass substrate, specifically, processing time should just be set according to Formula (2).

When setting a processing condition based on the result obtained from the measurement step 2, the flatness of the glass substrate surface obtained from the measurement step 2 is obtained, and the processing condition is set based on the flatness.
As described above, the measurement result is obtained from the measurement step 2 as a flatness map. Similarly to [0039], when the coordinates of a two-dimensional glass substrate are (x, y), the flatness map is represented as S (x, y) (μm). The processing time is expressed as T (x, y) (min). When the processing rate is Y (μm / min), these relationships are expressed by the following formula (3).
T (x, y) = S (x, y) / Y (3)
Therefore, when the machining conditions are set based on the result obtained from the measurement step 2, the machining conditions, specifically, the machining time is set according to the equation (3).

The finishing method of the present invention combines a setting of processing conditions based on the result obtained from the measurement step 1 and a setting of processing conditions based on the result obtained from the measurement step 2 in the form of a glass substrate. Set the machining conditions.
Similarly to the above, the coordinates of the two-dimensional glass substrate are (x, y), the dopant concentration distribution map obtained from the measurement step 1 is C (x, y) (mass%), and the flatness obtained from the measurement step 2 is the same. When the degree map is S (x, y) (μm) and the machining time is T (x, y) (min), these relationships are expressed by the following formula (4).
T (x, y) = S (x, y) / (a × C (x, y) + b) (4)
Therefore, when the machining conditions are set based on the results obtained from the measurement process 1 and the measurement process 2, the machining conditions, specifically the machining time, are set according to the equation (4).

  The processing method used for the finishing method of the present invention is not particularly limited as long as it is a processing method that can set the processing conditions of the glass substrate based on the results obtained from the measurement step 1 and the measurement step 2. That is, there is no particular limitation as long as it is a method that can sufficiently reduce the range processed once and can easily set the processing conditions based on the results of the measurement step 1 and the measurement step 2. Therefore, mechanical polishing using a polishing pad having a small diameter, for example, a polishing pad having a diameter of about 2 cm may be used. However, since it is easy to set the processing conditions based on the results of the measurement step 1 and the measurement step 2, for example, according to the equation (4), the ion beam etching, gas cluster ion It is preferable to use a processing method selected from the group consisting of beam etching, plasma etching, nanoablation, and MRF (magnetorheological finishing).

  Among the processing methods described above, methods involving beam irradiation on the glass substrate surface, specifically, ion beam etching, gas cluster ion beam etching, and plasma etching are based on the results obtained from measurement step 2 above. Substrate processing conditions can be further set. Hereinafter, this setting procedure will be specifically described.

  When this setting procedure is performed, the width obtained by the measurement step 2 is used to specify the width of the undulation existing on the glass substrate surface. When it says the width | variety of a wave | undulation, it means the length of a recessed part or a convex part in the uneven | corrugated shape which exists periodically on the glass substrate surface. Therefore, the width of the swell is usually ½ of the swell period. When a plurality of undulations having different periods are present on the glass substrate surface, the width of the undulation having the smallest period is defined as the width of the undulation existing on the glass substrate surface.

  As described above, the measurement result obtained from the measurement step 2 is a flatness map indicating the height difference at each part of the glass substrate surface. Therefore, the width of the waviness existing on the glass substrate surface can be easily specified from the flatness map.

  Dry etching is performed using a beam having a beam diameter equal to or smaller than the width of the waviness, based on the waviness width specified in the above procedure. Here, the beam diameter is based on a FWHM (full width of half maximum) value. Hereinafter, in this specification, the term “beam diameter” means the FWHM value of the beam diameter. In the finishing method of the present invention, it is more preferable to use a beam having a beam diameter of ½ or less of the waviness width. If a beam having a beam diameter equal to or smaller than the width of the waviness is used, it becomes possible to concentrate the beam on the waviness existing on the surface of the glass substrate, and the waviness can be effectively removed.

  When using the above-described method involving irradiation of the glass substrate surface with a beam, that is, ion beam etching, gas cluster ion beam etching or plasma etching, the beam needs to be scanned on the surface of the glass substrate. This is because, in order to set the processing conditions of the glass substrate, it is necessary to make the range in which the beam is irradiated at one time as small as possible. In particular, when a beam having a beam diameter equal to or less than the width of the waviness is used, it is necessary to scan the beam on the surface of the glass substrate. As a method of scanning the beam, raster scan and spiral scan are known, but any of these may be used.

  In the finish polishing method of the present invention, when the processing time (in this case, the beam irradiation time) is set according to the above equation (4), the irradiation time of T (x, y) at the coordinates (x, y). To be obtained. That is, the set irradiation time T (x, y) is obtained by determining the relative motion speed of the glass substrate and the beam in consideration of the beam intensity profile, the scan pitch, and the dose. Similarly, in the case of using mechanical polishing, the relative processing speed of the glass substrate and the polishing pad is determined in consideration of the polishing amount distribution per unit time of the small-diameter polishing pad, so that the set processing time T ( x, y) is obtained.

  Among the methods involving beam irradiation on the glass substrate surface described above, it is preferable to use gas cluster ion beam etching because the surface roughness is small and the surface can be processed with excellent smoothness.

  Gas cluster ion beam etching is to form a gas cluster by ejecting a gaseous reactive substance (source gas) at normal temperature and normal pressure in a pressurized state through an expansion nozzle into a vacuum device, This is a method of etching an object by irradiating a gas cluster ion beam ionized by electron irradiation. A gas cluster is usually composed of a massive atomic group or molecular group consisting of thousands of atoms or molecules. In the finishing method of the present invention, when gas cluster ion beam etching is used, when a gas cluster collides with the glass substrate surface, a multi-body collision effect occurs due to the interaction with the solid and the glass substrate surface is processed.

When gas cluster ion beam etching is used, source gases include SF ₆ , Ar, O ₂ , N ₂ , NF ₃ , N ₂ O, CHF ₃ , CF ₄ , C ₂ F ₆ , C ₃ F ₈ , and C _4. Gases such as F ₆ , SiF ₄ , and COF ₂ can be used alone or in combination. Among these, SF ₆ and NF ₃ are excellent as source gases in terms of chemical reaction that occurs when they collide with the surface of the glass substrate. Therefore, a mixed gas containing SF ₆ or NF ₃ , specifically, SF ₆ and O ₂ mixed gas, SF ₆ , Ar and O ₂ mixed gas, NF ₃ and O ₂ mixed gas, or NF ₃ , Ar and O ₂ mixed gas, NF ₃ and N ₂ mixed gas, NF ₃ , A mixed gas of Ar and N ₂ is preferred. In these mixed gases, a suitable mixing ratio of each component varies depending on conditions such as irradiation conditions, but is preferably as follows.
SF ₆ : O ₂ = 0.1 to 5%: 95 to 99.9% (mixed gas of SF ₆ and O ₂ )
SF ₆ : Ar: O ₂ = 0.1 to 5%: 9.9 to 49.9%: 50 to 90% (mixed gas of SF ₆ , Ar and O ₂ )
NF ₃ : O ₂ = 0.1 to 5%: 95 to 99.9% (mixed gas of NF ₃ and O ₂ )
NF ₃ : Ar: O ₂ = 0.1 to 5%: 9.9 to 49.9%: 50 to 90% (mixed gas of NF ₃ , Ar and O ₂ )
NF ₃ : N ₂ = 0.1 to 5%: 95 to 99.9% (mixed gas of NF ₃ and N ₂ )
NF ₃ : Ar: N ₂ = 0.1 to 5%: 9.9 to 49.9%: 50 to 90% (mixed gas of NF ₃ , Ar and N ₂ )
Among these mixed gases, a mixed gas of SF ₆ and O _2, a mixed gas of SF ₆ , Ar and O _2, a mixed gas of NF ₃ and O ₂ , or a mixed gas of NF ₃ , Ar and O ₂ is preferable.

  The cluster size, ionization current applied to the ionization electrode of the gas cluster ion beam etching apparatus to ionize the cluster, acceleration voltage applied to the acceleration electrode of the gas cluster ion beam etching apparatus, and dose amount of the gas cluster ion beam Irradiation conditions can be appropriately selected according to the type of source gas and the surface properties of the glass substrate after preliminary polishing. For example, in order to remove waviness from the surface of the glass substrate and improve the flatness without excessively deteriorating the surface roughness of the glass substrate, the acceleration voltage applied to the acceleration electrode is 15 to 30 kV. Is preferred.

  Since the finishing method of the present invention sets the processing conditions of the glass substrate based on the results obtained from the measurement step 1 and the measurement step 2, the swell generated on the glass substrate surface during preliminary polishing can be effectively removed. The glass substrate can be processed into a surface with excellent flatness. If the finishing method of this invention is used, the flatness of the surface of a glass substrate can be improved to 50 nm or less.

When the finishing method of the present invention is carried out, the surface roughness of the glass substrate may be somewhat deteriorated depending on the surface properties of the glass to be processed and the beam irradiation conditions. Depending on the specifications of the glass substrate, the finishing method of the present invention may achieve a desired flatness, but may not be processed with a desired surface roughness. For this reason, in order to improve the surface roughness of a glass substrate, after the implementation of the finishing method of the present invention, further finishing processing may be performed.
For the finishing process performed for such a purpose, it is preferable to use gas cluster ion beam etching. Accordingly, when gas cluster ion beam etching is used in the finishing method of the present invention, further finishing processing is performed as second gas cluster ion beam etching with different irradiation conditions such as source gas, ionization current, and acceleration voltage. Specifically, the second cluster ion beam etching is performed more slowly using a lower ionization current or a lower acceleration voltage than the gas cluster ion beam etching for the purpose of removing the undulation generated during the preliminary polishing. It is preferable to carry out under conditions. Specifically, for example, the acceleration voltage is preferably 3 kV or more and less than 30 kV, and more preferably 3 to 20 kV. As the source gas, it is difficult to cause a chemical reaction when it collides with the surface of the glass substrate. Therefore, gases such as O ₂ , Ar, CO, and CO ₂ can be used alone or in combination. Of these, O ₂ and Ar are preferred.

Hereinafter, based on an Example, this invention is demonstrated more concretely. However, the present invention is not limited to this.
(Reference example)
As a workpiece, an ingot of TiO ₂ doped quartz glass (TiO ₂ 7% by mass dope) manufactured by a known method is prepared, and the length is 153.0 mm × width 153.0 mm × thickness using an inner peripheral slicer. A 6.75 mm plate was cut into a plate material sample made of TiO ₂ -doped quartz glass. Then, using a # 120 diamond grindstone with a commercially available NC chamfering machine, chamfering was performed so that the outer diameter was 152 mm and the chamfering width was 0.2 to 0.4 mm, to obtain a glass substrate sample. .

The TiO ₂ concentration distribution of the glass substrate sample was measured by fluorescent X-ray analysis. FIG. 2 is a graph showing the TiO ₂ concentration distribution in the diagonal direction of the glass substrate sample. The horizontal axis of the graph indicates the distance from the center of the substrate.
The expected processing amount distribution when the glass substrate sample having the TiO ₂ concentration distribution shown in FIG. 2 was mechanically polished as preliminary polishing was calculated using the equation (1-2). In the calculation, the processing amount with a TiO ₂ concentration of 7% by mass was set to 5 μm (reference processing amount).
Y = 0.0306X + 1.0188 (1-2)
X: dopant concentration (wt%), Y: polishing rate (μm / min)
FIG. 3 is a graph showing the expected machining amount distribution obtained by the above procedure. FIG. 4 is a graph showing an expected cross-sectional profile in the diagonal direction of the glass substrate sample after preliminary polishing, which is a result of subtracting the expected processing amount shown in FIG. 3 from the reference processing amount of 5 μm. 3 and 4, the horizontal axis of the graph indicates the distance from the substrate center.

The glass substrate sample described above was pre-polished using mechanical polishing with cerium oxide.
The polishing material, polishing pad, and polishing apparatus are the same as those used when creating FIG. 1, and the processing rate and processing time are 5 μm for the processing amount of the doped quartz glass having a TiO ₂ concentration of 7 mass%. Condition.
After preliminary polishing, the surface of the glass substrate sample was measured using an interference flatness meter (G310S Fizeau type laser interference flatness measuring machine (Fujinon)). The surface shape of the glass substrate sample created from the measurement results is shown in FIG. In the surface shape of the glass substrate after mechanical polishing, the long-period shape and the sagging of the outer periphery depend on the characteristics of the polishing pad used, the processing conditions for mechanical polishing, and the like. For this reason, the cross-sectional profile in the diagonal direction of the glass substrate sample after approximating the third-order aberration function and removing the tilt, power, astigmatism, coma aberration, and spherical aberration is shown in FIG. In FIG. 6, the horizontal axis of the graph indicates the distance from the substrate center.
As is clear from the comparison between FIG. 4 and FIG. 6, the position and height of the unevenness (waviness) in the expected cross-sectional profile after preliminary polishing and the cross-sectional profile after actual mechanical polishing are very good. It was consistent.

From the state shown in FIG. 4, an expected cross-sectional profile in the diagonal direction of the glass substrate sample after finishing using gas cluster ion beam etching was created.
Specifically, the expected processing amount distribution when the glass substrate sample having the TiO ₂ concentration distribution shown in FIG. 2 is finished by the gas cluster ion beam etching method was calculated using the equation (1-1). In the calculation, the processing amount with a TiO ₂ concentration of 7% by mass was set to 0.8 μm (reference processing amount).
Y = 0.0522X + 1.0449 (1-1)
X: dopant concentration (wt%), Y: processing rate (μm / min)
Subsequently, an expected cross-sectional profile of the glass substrate after finishing was created by subtracting the expected processing amount obtained by the above procedure from the expected cross-sectional profile shown in FIG. FIG. 7 is a graph showing an expected cross-sectional profile (diagonal direction) of the glass substrate after finishing. In FIG. 7, the horizontal axis of the graph indicates the distance from the substrate center.

On the other hand, the glass substrate sample preliminarily polished by the above procedure was further processed using gas cluster ion beam etching. The conditions for gas cluster ion beam etching are the same as the conditions used when creating FIG. 1, and the processing rate and processing time are 0.8 μm for the polished amount of doped quartz glass with a TiO ₂ concentration of 7 mass%. The conditions were as follows.
The surface of the glass substrate sample after finishing was measured using an interference flatness meter. FIG. 8 shows a diagonal cross-sectional profile of the glass substrate sample after approximating the third-order aberration function to the obtained measurement results and removing the tilt, power, astigmatism, coma aberration, and spherical aberration. In FIG. 8, the horizontal axis of the graph indicates the distance from the center of the substrate.
As is clear from the comparison between FIG. 7 and FIG. 8, the position and height where the projections and depressions exist are in good agreement with the expected cross-sectional profile after finishing and the cross-sectional profile after actual finishing. It was.

As is apparent from the above results, undulation caused by the dopant concentration distribution of the glass substrate occurs on the surface of the glass substrate after preliminary polishing, and this undulation cannot be completely removed by finishing. However, the position and height of this swell agree very well with the expected value determined from the dopant concentration distribution and the correlation between the dopant concentration and the processing rate.
In the finishing method of the present invention, since the processing conditions of the glass substrate are set based on the results obtained from the measurement step 1 and the measurement step 2, the undulation generated during the preliminary polishing is completely removed, It is considered that the surface can be finished to a surface with excellent flatness.

FIG. 1 is a graph showing the correlation between dopant concentration and processing rate for doped quartz glass containing TiO ₂ as a dopant. FIG. 2 is a graph showing the TiO ₂ concentration distribution (diagonal direction) of the glass substrate sample of the example. FIG. 3 is a graph showing the expected amount of processing during preliminary polishing for the glass substrate sample of the example. FIG. 4 is a graph showing an expected cross-sectional profile (diagonal direction) of the glass substrate sample after preliminary polishing for the glass substrate sample of the example. FIG. 5 shows the surface shape of the glass substrate sample obtained from the result of measuring the glass substrate surface after preliminary polishing using an interference flatness meter for the glass substrate sample of the example. FIG. 6 is a graph showing a cross-sectional profile (diagonal direction) of the glass substrate sample after preliminary polishing for the glass substrate sample of the example. FIG. 7 is a graph showing an expected cross-sectional profile (diagonal direction) of the glass substrate sample after finishing for the glass substrate sample of the example. FIG. 8 is a graph showing a cross-sectional profile (diagonal direction) of the glass substrate sample after finishing for the glass substrate sample of the example.

Claims (12)

A method for finishing a pre-polished glass substrate surface,
The glass substrate contains a dopant and is made of quartz glass containing SiO ₂ as a main component, and a method of finishing the pre-polished glass substrate surface,
Measuring the concentration distribution of the dopant contained in the glass substrate;
A step of measuring a surface shape of the glass substrate after preliminary polishing, and a step of measuring a concentration distribution of a dopant contained in the glass substrate and a result obtained from a step of measuring the surface shape of the glass substrate. Then, the processing conditions of the glass substrate surface are set for each part of the glass substrate, and a method of finishing the pre-polished glass substrate surface is provided. Obtain a correlation between the dopant concentration contained in the glass substrate and the processing rate of the glass substrate surface in advance,
Using the measurement result of the dopant concentration distribution contained in the glass substrate and the correlation between the dopant concentration and the processing rate, the processing conditions of the glass substrate surface are set for each part of the glass substrate, A method for finishing a pre-polished glass substrate surface according to claim 1. Obtain the flatness of the glass substrate surface from the measurement results of the surface shape of the glass substrate,
3. The pre-polished glass substrate surface according to claim 1, wherein a processing condition of the glass substrate surface is set for each portion of the glass substrate based on flatness of the glass substrate surface. How to process.   The processing of the glass substrate surface is carried out using a processing method selected from the group consisting of ion beam etching, gas cluster ion beam etching, plasma etching, nano-ablation, and MRF (magnetic theoretical finishing). A method for finishing a pre-polished glass substrate surface according to any one of the above. The processing method is ion beam etching, gas cluster ion beam etching or plasma etching,
From the measurement result of the surface shape of the glass substrate, specify the width of the undulation present on the glass substrate surface,
5. The method of finishing a pre-polished glass substrate surface according to claim 4, wherein processing is performed using a beam having a beam diameter of FWHM (full width of half maximum) and less than the width of the waviness.   6. The method for finishing a pre-polished glass substrate surface according to claim 5, wherein a FWHM value of the beam diameter is ½ or less of the width of the waviness.   The method for finishing a pre-polished glass substrate surface according to claim 4 or 5, wherein the processing method is gas cluster ion beam etching. The method for finishing a pre-polished glass substrate surface according to any one of claims 5 to 7, wherein any one of the mixed gases selected from the following group is used as a source gas for the gas cluster ion beam etching.
SF ₆ and O ₂ mixed gas, SF ₆ , Ar and O ₂ mixed gas, NF ₃ and O ₂ mixed gas, NF ₃ , Ar and O ₂ mixed gas, NF ₃ and N ₂ mixed gas, NF ₃ , mixed gas of Ar and N ₂ The glass substrate polishing method according to claim 8, wherein any one of the mixed gases selected from the following group is used as the source gas.
A mixed gas of SF ₆ and O _2, a mixed gas of SF ₆ , Ar and O _2, a mixed gas of NF ₃ and O _2, a mixed gas of NF ₃ , Ar and O ₂   The method for finishing a pre-polished glass substrate surface according to any one of claims 1 to 9, wherein the glass substrate is made of low expansion glass having a thermal expansion coefficient at 20 ° C of 0 ± 30 ppb / ° C. The method for finishing a pre-polished glass substrate surface according to claim 1, wherein the dopant is TiO ₂ .   The method for finishing a pre-polished glass substrate surface according to any one of claims 1 to 11, wherein the glass substrate has a surface roughness (Rms) after pre-polishing of 5 nm or less.