Patent Application
20100101940
The present invention relates to a method for removing foreign matter from a glass substrate surface and, in particular, to a method for removing foreign matter from a glass substrate surface which is required to have a high flatness, such as glass substrates to be used for a reflective type mask for EUV (extreme ultraviolet) lithography in the semiconductor manufacturing process. More specifically, the invention relates to a method for removing foreign matter from a glass substrate surface to be processed by a method accompanied with beam irradiation or laser light irradiation on a glass substrate surface, such as ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion by means of laser light irradiation.
Also, the invention relates to a method comprising removing foreign matter from a glass substrate surface, and then processing the glass substrate surface by a method accompanied with beam irradiation or laser light irradiation on the glass substrate surface, such as ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion by means of laser light irradiation.
In the lithography technology, an exposure tool for manufacturing an integrated circuit by transferring a fine circuit pattern onto a wafer has hitherto been widely utilized. With the trend toward higher integration of an integrated circuit, higher speed and higher functionalization, the integrated circuit is becoming finer. For that reason, the exposure tool is required to achieve image formation of a circuit pattern on a wafer surface with high resolution and a long focal depth, and shortening of the wavelength of an exposure light source is being advanced. The exposure light source is further advancing from conventional g-rays (wavelength: 436 nm), i-rays (wavelength: 365 nm) and a KrF excimer laser (wavelength: 248 nm), and an ArF excimer laser (wavelength: 193 nm) is started to be used. Also, in order to cope with a next-generation integrated circuit whose circuit line width will become not more than 100 nm, the use of an F2 laser (wavelength: 157 nm) as an exposure light source is regarded as promising. However, it is considered that even this is able to cover only the generation with a line width of up to 70 nm.
Under such a technical trend, a lithography technology using EUV light as a next-generation exposure light source is considered to be applicable over plural generations after the line width of 45 nm and is attracting attention. The EUV light refers to light of a waveband of a soft X-ray region or vacuum ultraviolet region, and specifically means light having a wavelength of from about 0.2 to 100 nm. At present, the use of 13.5 nm as a lithography light source is investigated. The exposure principle of this EUV lithography (hereinafter abbreviated as “EUVL”) is identical with the conventional lithography in respect of transferring a mask pattern using a projection optical system. However, in an energy region of EUV light, since there is no material capable of transmitting the light therethrough, a refraction optical system cannot be used, but a reflective optical system is inevitably used (see Patent Document 1).
The mask to be used for EUVL is basically configured of (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 plural materials having a different refractive index against the wavelength of the exposure light are cyclically laminated at a nanometer scale is used, and Mo and Si are known as representative materials. Also, Ta and Cr has been investigated as the material for the absorber layer. As the glass substrate, in order that a strain may not be generated even upon irradiation with EUV light, a material having a low heat expansion coefficient is required, and the use of a glass having a low heat expansion coefficient or a crystallized glass having a low heat expansion coefficient has been investigated. In this specification, the glass having a low heat expansion coefficient and the crystallized glass having a low heat expansion coefficient are collectively called “low expansion glass” or “ultra-low expansion glass”.
As the low expansion glass or ultra-low expansion glass to be used as a mask for EUVL, a quartz glass mainly composed of SiO2 and to which TiO2, SnO2 or ZrO2 is added as a dopant for the purpose of reducing the heat expansion coefficient of glass is most widely used.
The glass substrate is manufactured by processing a raw material of such a glass or crystallized glass in high precision and washing it. In the case of processing the glass substrate, in general, the glass substrate surface is pre-polished at a relatively high processing rate until it comes to have predetermined flatness and surface roughness; foreign matter such as a polishing waste generated by pre-polishing is removed by washing; and the glass substrate surface is then finish-processed by a method with higher processing precision so as to have desired flatness and surface roughness. As the method with high processing precision to be used for finish-processing, a method accompanied with beam irradiation or laser light irradiation on a glass substrate surface, such as ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion by means of laser light irradiation, is preferably used.
However, there are the case where foreign matter which has not been completely removed by washing remains; and the case where foreign matter newly attaches onto the glass substrate surface after washing. When the glass substrate surface where such foreign matter exists is finish-processed by a method accompanied with beam irradiation or laser light irradiation on a glass substrate surface, such as ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion by means of laser light irradiation, there is a problem that the portion on the glass substrate surface where foreign matter exists is not processed, and the subsequent washing to remove foreign matter from the glass substrate surface results in generation of convex defects of the glass.
Patent Document 1: JP-T-2003-505891
In order to solve of the foregoing problems of the related art, an object of the invention is to provide a method for removing foreign matter from a glass substrate surface to be finish-processed by a method accompanied with beam irradiation or laser light irradiation on the glass substrate surface, such as ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion by means of laser light irradiation.
Another object of the invention is to provide a method in which after removing foreign matter from a glass substrate surface, the glass substrate surface is processed by a method accompanied with beam irradiation or laser light irradiation on a glass substrate surface, such as ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion by means of laser light irradiation.
In order to achieve the foregoing objects, the invention is to provide a method for removing foreign matter from a glass substrate surface, which comprises subjecting the glass substrate surface to gas cluster ion beam etching at an accelerating voltage of from 5 to 15 keV.
Also, the invention is to provide a method for removing foreign matter from a glass substrate surface, which comprises subjecting the glass substrate surface to gas cluster ion beam etching with, as a gas source, at least one gas selected from the group consisting of O2, Ar, B, CO2, N2, N2O and a boron hydride.
In this invention, each of the methods described in the preceding two paragraphs is hereinafter referred to as “foreign matter removal method of the invention”.
In the foreign matter removal method of the invention, it is preferable that the gas cluster ion beam etching is performed under a condition that the etching amount is not more than 20 nm.
In the foreign matter removal method of the invention, it is preferable that the glass substrate is made of a low expansion glass having a heat expansion coefficient at 20° C. or at from 50 to 80° C. of 0±30 ppb/° C.
In the foreign matter removal method of the invention, it is preferable that the glass substrate surface before performing the gas cluster ion beam etching has a surface roughness (Rms) of not more than 5 nm.
In the foreign matter removal method of the invention, it is preferable that the gas cluster ion beam etching is performed under a condition that a cluster size is 2,000 or more.
In the foreign matter removal method of the invention, it is preferable that the gas cluster ion beam etching is performed while keeping an angle formed by a normal line of the glass substrate and a gas cluster ion beam to be made incident to the glass substrate surface at from 3 to 60 degrees.
Here, it is preferable that the gas cluster ion beam etching is performed while keeping the glass substrate surface in a state facing downward relative to the horizontal direction by from 3 to 60 degrees.
Also, the invention is to provide a method for processing a glass substrate surface, which comprises the steps of:
removing foreign matter on the glass substrate surface by the foreign matter removal method of the invention; and
processing the glass substrate surface by a processing method selected from the group consisting of ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion (this method will be hereinafter referred to as “processing method (1) of the invention”).
In the processing method (1) of the invention, it is preferable that the processing method is gas cluster ion beam etching.
Here, it is preferable that the gas cluster ion beam etching in the processing step is performed at an accelerating voltage exceeding 15 keV, using, as a source gas, a mixed gas selected from the group consisting of: a mixed gas of SF6 and O2; a mixed gas of SF6, Ar and O2; a mixed gas of NF3 and O2; a mixed gas of NF3, Ar and O2; a mixed gas of NF3 and N2; and a mixed gas of NF3, Ar and N2.
It is preferable that the source gas is any one mixed gas selected from the group consisting of: a mixed gas of SF6 and O2; a mixed gas of SF6, Ar and O2; a mixed gas of NF3 and O2; and a mixed gas of NF3, Ar and O2.
Also, the invention is to provide a method for processing a glass substrate surface, which comprises the steps of:
measuring a flatness of the glass substrate surface;
removing foreign matter on the glass substrate surface by the foreign matter removal method of the invention; and
processing the glass substrate surface by a processing method selected from the group consisting of ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion,
wherein, in the step of processing the glass substrate surface, a processing condition of the glass substrate surface is set up for each site of the glass substrate on the basis of a result obtained from the step of measuring a flatness (this method will be hereinafter referred to as “processing method (2) of the invention”).
In the processing method (2) of the invention, it is preferable that the processing method is ion beam etching, gas cluster ion beam etching or plasma etching; a width of waviness existing on the glass substrate surface is specified on the basis of a result obtained from the step of measuring a flatness of the glass substrate surface; and the glass substrate surface is processed with a beam having a beam diameter of not more than the width of the waviness in terms of FWHM (full width of half maximum) value.
Here, it is preferable that the FWHM value of the beam diameter is not more than ½ of the width of the waviness.
In the processing method (2) of the invention, it is preferable that the processing method is gas cluster beam etching; and the gas cluster ion beam etching in the processing step is performed at an accelerating voltage exceeding 15 keV, using, as a source gas, a mixed gas selected from the group consisting of: any one mixed gas of SF6 and O2; a mixed gas of SF6, Ar and O2; a mixed gas of NF3 and O2; a mixed gas of NF3, Ar and O2; a mixed gas of NF3 and N2; and a mixed gas of NF3, Ar and N2.
It is more preferable that the source gas is any one mixed gas selected from the group consisting of: a mixed gas of SF6 and O2; a mixed gas of SF6, Ar and O2; a mixed gas of NF3 and O2; and a mixed gas of NF3, Ar and O2.
In the processing methods (1) and (2) of the invention, it is preferable that subsequent to the step of processing the glass substrate surface, a second processing step is performed for improving a surface roughness of the glass substrate surface.
It is preferable that as the second processing step, gas cluster ion beam etching is performed at an accelerating voltage of 3 keV or more and less than 30 keV, using, as a source gas, an O2 gas singly or a mixed gas of O2 and at least one gas selected from the group consisting of Ar, CO and CO2.
It is preferable that as the second processing step, mechanical polishing using a polishing slurry is performed at a surface pressure of from 1 to 60 gf/cm2.
Also, the invention is to provide a glass substrate obtained by the processing method of the invention, wherein the substrate surface has a flatness of not more than 50 nm, and is free from a convex glass defect having a height exceeding 1.5 nm.
According to the invention, in the case where a glass substrate surface is finish-processed by a method accompanied with beam irradiation or laser light irradiation on the glass substrate surface, such as ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion by means of laser light irradiation, it is possible to prevent the generation of a convex defect of the glass on the glass substrate surface after processing and to process the glass substrate surface into a surface having excellent flatness and surface roughness.
The reference numerals used in the drawings denote the following, respectively.
1: Substrate
10: Processing surface
The foreign matter removal method of the invention is concerned with a method for removing foreign matter from a glass substrate surface to be finish-processed (this glass substrate surface will be also referred to as “processing surface”), namely the processing surface before finish-processing, by a method accompanied with beam irradiation or laser light irradiation on a glass substrate surface, such as ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion by means of laser light irradiation.
Such a processing surface is washed before performing the finish-processing. On that occasion, there is the case where foreign matter which has not been completely removed by washing remains; or the case where foreign matter newly attaches onto the glass substrate surface after washing. The foreign matter removal method of the invention is aimed to remove such foreign matter.
The foreign matter which is a target of the foreign matter removal method of the invention refers to a material attaching onto the processing surface by a van der Waals force but not a material fixing onto the processing surface by a chemical bond, and its size is usually from about 1 to 2 μm or smaller.
The glass substrate which is a target of the foreign matter removal method of the invention is a glass substrate for a reflective type mask for EUVL capable of mainly coping with higher integration and higher definition of an integrated circuit. The glass substrate to be used for this application is a glass substrate having a small heat expansion coefficient and a small scattering thereof. The glass substrate is preferably made of a low expansion glass having a heat expansion coefficient at 20° C. or at from 50 to 80° C. of 0±30 ppb/° C., and more preferably made of an ultra-low expansion glass having a heat expansion coefficient at 20° C. or at from 50 to 80° C. of 0±10 ppb/° C.
The glass substrate is not particularly limited with respect to the shape, size and thickness and the like. In the case of a substrate for a reflective type mask for EUVL, its shape is a rectangular plate-shaped body in view of a rectangular planar shape.
It is preferable that in the glass substrate which is a target of the foreign matter removal method of the invention, the processing surface is preliminarily processed so as to have predetermined flatness and surface roughness.
Though ion beam etching, gas cluster ion beam etching, plasma etching and nano-abrasion to be used for preliminary processing of the processing surface are able to process the glass substrate surface into a surface with excellent flatness and surface roughness, these processing methods are inferior to the conventional mechanical polishing in view of a processing rate, especially a processing rate in the case of processing a glass substrate surface with a large area. On the other hand, the foreign matter removal method of the invention, which is described below in detail, is a method for removing foreign matter existing on the processing surface without substantially processing the processing surface. For that reason, it is preferable that prior to performing the foreign matter removal method of the invention, the processing surface is preliminarily processed so as to have predetermined flatness and surface roughness by a processing method having a relatively high processing rate.
The processing method to be used for the preliminary processing is not particularly limited but can be widely chosen among known processing methods to be used for processing a glass surface. However, by using a polishing pad having a large processing rate and a large surface area, it is possible to perform polishing processing with a large area at once, and therefore, a mechanical polishing method is usually used. The mechanical polishing method as referred to herein includes, in addition to polishing processing only by means of a polishing function with an abrasive grain, a method of using a polishing slurry which utilizes a polishing function with an abrasive grain and a chemical polishing function with a chemical in combination. The mechanical polishing method may be any of lapping and polishing, and a polishing tool and an abrasive to be used can be appropriately chosen from known ones. When the mechanical polishing method is used, for the purpose of making the processing rate large, it is preferable in the case of lapping that the lapping is performed at a surface pressure of from 30 to 70 gf/cm2, and more preferably at a surface pressure of from 40 to 60 gf/cm2; whereas it is preferable in the case of polishing that the polishing is performed at a surface pressure of from 60 to 140 gf/cm2, and more preferably at a surface pressure of from 80 to 120 gf/cm2. The lapping is preferably performed so as to give a lapping amount of from 100 to 300 μm, and the polishing is preferably performed so as to give a polishing amount of from 1 to 60 μm.
In the case of performing preliminary processing, the processing surface after the preliminary processing preferably has a surface roughness (Rms) of not more than 5 nm, and more preferably not more than 1 nm. The surface roughness as referred to in this specification means a surface roughness measured by an atomic force microscope with respect to an area of from 1 to 10 μm square. When the surface roughness of the glass substrate after the preliminary processing exceeds 5 nm, it takes a considerably long period of time for finish-processing the processing surface so as to have predetermined flatness and surface roughness after performing the foreign matter removal method of the invention, and therefore, such becomes a factor in the increase of costs.
The foreign matter removal method of the invention is characterized in that by performing gas cluster ion beam (hereinafter referred to as “GCIB”) etching under a specified condition that gives a low etching amount with respect to the processing surface (this condition will be hereinafter referred to as “low etching condition”), foreign matter is removed from the processing surface while making the processing amount of the processing surface extremely low.
The GCIB etching as referred to herein is a method in which a reactive substance (source gas) which is in a gaseous state at normal temperature and atmospheric pressure is jetted in a pressurized state into a vacuum apparatus via an expansion type nozzle to form a gas cluster, which is then ionized upon irradiation with an electron, and the resulting GCIB is irradiated on a target to achieve etching. The gas cluster is usually constituted of a massive atomic group or molecular group composed of several thousand atoms or molecules. In the foreign matter removal method of the invention, by performing the GCIB etching on the processing surface, when the gas cluster comes into collision with the processing surface, a multiple collision effect is generated due to a mutual action with the solid, whereby the foreign matter is removed from the processing surface. Then, since GCIB etching is performed on the processing surface under a low etching condition, the processing surface is not substantially processed.
In the foreign matter removal method of the invention, by specifying a site on the processing surface where foreign matter exists, the GCIB etching may be selectively performed in this site. However, it is difficult to specify a site where a fine foreign matter having a size of from about 1 to 2 μm or smaller and to selectively perform the GCIB etching in this site. Therefore, in general, the GCIB etching is performed on the whole of the processing surface under a low etching condition. In that case, it is necessary that GCIB is scanned on the processing surface. As a method of scanning GCIB, luster scanning and spiral scanning are known, and any of these methods may be used.
In a first embodiment of the foreign matter removal method of the invention, in order to perform GCIB etching on the processing surface under a low etching condition, an accelerating voltage for applying to accelerating electrodes is controlled at from 5 to 15 keV. In that case, the source gas which is used in performing the GCIB etching for the purpose of finish-processing the glass substrate surface may be a conventionally used source gas. Specific examples of such a conventional source gas include SF6, NF3, CHF3, CF4, C2F6, C3F8, C4F6, SiF4 and COF2. These gases may be used singly or in admixture.
By controlling the accelerating voltage to be applied to accelerating electrodes at from 5 to 15 keV, even in the case where a conventionally used source gas is used in performing the GCIB etching for the purpose of finish-processing the processing surface, the etching amount on the processing surface is sufficiently low, and foreign matter existing on the processing surface can be removed without substantially processing the processing surface. In the case where the accelerating voltage is less than 5 keV, when the gas cluster comes into collision with the processing surface, the kinetic energy is small so that foreign matter existing on the processing surface cannot be removed depending upon the size of the foreign matter. Specifically, foreign matter having a size of from about 1 to 2 μm cannot be removed. In the case where the accelerating voltage exceeds 15 keV, when the gas cluster comes into collision with the processing surface, the kinetic energy is large. Therefore, an etching action on the processing surface becomes more remarkable than an action to remove a fine foreign matter existing on the processing surface; and likewise the case where the processing surface on which foreign matter exists is finish-processed by GCIB etching, only a portion of the processing surface where foreign matter exists remains without being etched, resulting in a problem that a convex defect is generated on the processing surface from which foreign matter has been removed by washing or the like.
The accelerating voltage is more preferably from 5 to 10 keV.
In a second embodiment of the foreign matter removal method of the invention, in order to perform GCIB etching on the processing surface under a low etching condition, the GCIB etching is performed using, as a gas source, at least one gas selected from O2, Ar, B, CO2, N2O and a boron hydride (for example, BH3 and B4H10). When such a gas species comes into collision with the processing surface, it hardly causes a chemical reaction, and an action to etch the processing surface is extremely weak. In the case where the GCIB etching is performed using such a gas species as the source gas, foreign matter existing on the processing surface can be removed without substantially processing the processing surface.
In the second embodiment of the foreign matter removal method of the invention, the accelerating voltage for applying a gas species having an extremely weak etching action to accelerating electrodes is not particularly limited. However, what the accelerating voltage is 15 keV or more is preferable from the standpoint of removing foreign matter existing on the processing surface without substantially processing the processing surface. The accelerating voltage is more preferably 20 keV or more, and further preferably 30 keV or more. In the case where the accelerating voltage is less than 15 keV, when the gas cluster comes into collision with the processing surface, the kinetic energy is small, and therefore, there is a possibility that foreign matter existing on the processing surface cannot be removed depending upon the size of the foreign matter. However, even when the accelerating voltage is less than 15 keV, by adjusting the cluster size, dose amount, irradiation time and the like, foreign matter existing on the processing surface can be removed.
According to the foregoing first embodiment and second embodiment of the foreign matter removal method of the invention, since GCIB etching is performed on the processing surface under a low etching condition, foreign matter existing on the processing surface can be removed without substantially processing the processing surface. The low etching condition as referred to herein is preferably a condition under which the etching amount is not more than 20 nm, and more preferably a condition under which the etching amount is not more than 10 nm.
In the first embodiment and second embodiment of the foreign matter removal method of the invention, the irradiation condition, for example, a cluster size, an ionizing current to be applied to ionization electrodes of a GCIB etching apparatus for ionizing the cluster and a dose amount of GCIB can be appropriately chosen in accordance with the kind of a source gas, the accelerating voltage to be applied to accelerating electrodes and the like. It is preferable that the GCIB etching is performed under a condition that a cluster size is 2,000 or more. When the cluster size is 2,000 or more, since a relatively large gas cluster comes into collision with the processing surface, it is expected that an effect for removing foreign matter existing on the processing surface is enhanced due to the multiple collision effect. The cluster size is more preferably 3,000 or more, and especially preferably 5,000 or more.
In the first embodiment and second embodiment of the foreign matter removal method of the invention, it is preferable that GCIB is irradiated on the processing surface from an oblique direction thereto. When GCIB is irradiated on the processing surface from an oblique direction thereto, it is expected that an effect for removing foreign matter existing on the processing surface is enhanced due to the multiple collision effect.
The angle θ is kept more preferably at from 10 to 60 degrees, and further preferably at from 30 to 60 degrees.
In the case where GCIB is irradiated on the processing surface from an oblique direction thereto, as illustrated in
The processing method (1) of the invention includes a step of removing foreign matter on a glass substrate surface by the foregoing foreign matter removal method of the invention (this step will be hereinafter referred to as “foreign matter removal step”); and a step of processing the glass substrate surface by a processing method selected from the group consisting of ion beam etching, GCIB etching, plasma etching and nano-abrasion (this step will be hereinafter referred to as “processing step”).
In the case where the processing surface is pre-polished so as to have predetermined flatness and surface roughness, after removing foreign matter existing on the processing surface by the foregoing foreign matter removal method of the invention, the processing surface is finish-processed by a processing method selected from the group consisting of ion beam etching, GCIB etching, plasma etching and nano-abrasion.
In order to prevent the attachment of a new foreign matter onto the processing surface after the foreign matter removal step, it is preferable that the foreign matter removal step and the processing step are performed in the same chamber or performed in chambers placed side by side in such a manner that the substrate can be conveyed without being discharged from the apparatus. In the case of using GCIB etching in the processing step, it is preferable to use the same GCIB etching apparatus in the foreign matter removal step and the processing step.
Of the foregoing processing methods, it is preferable to use GCIB etching because the surface can be processed so as to have a small surface roughness and excellent smoothness.
In the case of using the GCIB etching, a gas such as SF6, Ar, O2, N2, NF3, N2O, CHF3, CF4, C2F6, C3F8, C4F6, SiF4 and COF2 can be used singly or in admixture as a source gas. Of these, SF6, NF3, CHF3, CF4, C2F6, C3F8, C4F6, SiF4 and COF2 are excellent as the source gas from the standpoint of a chemical reaction which occurs when the gas cluster comes into collision with the processing surface. Above all, mixed gases containing SF6 or NF3, specifically a mixed gas of SF6 and O2, a mixed gas of SF6, Ar and O2, a mixed gas of NF3 and O2, a mixed gas of NF3, Ar and O2, a mixed gas of NF3 and N2 and a mixed gas of NF3, Ar and N2 are preferable for reasons that the etching rate is high and that the processing tact is enhanced. In such a mixed gas, though a favorable mixing ratio of the respective components varies with a condition such as an irradiation condition, the following are preferable.
SF6/O2=0.1 to 5%/95 to 99.9% (a mixed gas of SF6 and O2)
SF6/Ar/O2=0.1 to 5%/9.9 to 49.9%/50 to 90% (a mixed gas of SF6, Ar and O2)
NF3/O2=0.1 to 5%/95 to 99.9% (a mixed gas of NF3 and O2)
NF3/Ar/O2=0.1 to 5%/9.9 to 49.9%/50 to 90% (a mixed gas of NF3, Ar and O2)
NF3/N2=0.1 to 5%/95 to 99.9% (a mixed gas of NF3 and N2)
NF3/Ar/N2=0.1 to 5%/9.9 to 49.9%/50 to 90% (a mixed gas of NF3, Ar and N2)
Of these mixed gases, a mixed gas of SF6 and O2, a mixed gas of SF6, Ar and O2, a mixed gas of NF3 and O2 and a mixed gas of NF3, Ar and O2 are preferable.
The irradiation condition, for example, a cluster size, an ionizing current to be applied to ionization electrodes of a GCIB etching apparatus for ionizing the cluster and a dose amount of GCIB can be appropriately chosen in accordance with the kind of the source gas, the surface properties of the processing surface, the purpose of finish-processing and the like. For example, in the case where finish-processing is performed for the purpose of improving the flatness of the processing surface after the preliminary processing, it is preferable that the accelerating voltage to be applied to accelerating electrodes exceeds 15 keV; and for the purpose of improving the flatness of the processing surface without excessively deteriorating the surface roughness, it is preferable that the accelerating voltage exceeds 15 keV and is not more than 30 keV.
Also, in the processing step, in the case of using GCIB etching, it is necessary that GCIB is scanned on the processing surface. As a method of scanning GCIB, luster scanning and spiral scanning are known, and any of these methods may be used.
The processing method (2) of the invention includes a step of measuring a flatness of a glass substrate surface (this step will be hereinafter referred to as “flatness measuring step”); a step of removing foreign matter on the processing surface by the foregoing foreign matter removal method of the invention (this step will be hereinafter referred to as “foreign matter removal step”); and a step of processing the processing surface by a processing method selected from the group consisting of ion beam etching, GCIB etching, plasma etching and nano-abrasion (this step will be hereinafter referred to as “processing step”), wherein in the processing step, a processing condition of the processing surface is set up for each site of the processing surface on the basis of a result obtained from the flatness measuring step.
In the case of performing preliminary processing and finish-processing by ion beam etching, GCIB etching, plasma etching or nano-abrasion for the purpose of processing the processing surface of a glass substrate, for example, the processing surface of a glass substrate for a mask for EUVL, there may be the case where partial waviness exists on the processing surface after the preliminary processing. The waviness as referred to herein means irregularities having a cycle of from 5 to 30 mm among cyclic irregularities existing on the processing surface.
It is difficult to remove such waviness by means of the finish-processing so as to have a desired flatness on the processing surface. Also, there may be the case where the waviness generated in the preliminary processing grows into larger waviness in the course of finish-processing.
The processing method (2) of the invention is a method in which such waviness generated on the processing surface after the preliminary processing is removed, and the processing surface is finish-processed into a surface with excellent flatness.
In the processing method (2) of the invention, for the purpose of setting up a processing condition of the processing surface for each site of the processing surface on the basis of a result obtained from the flatness measuring step, it would be better that the flatness measuring step is performed prior to the processing step. The flatness measuring step may be performed after the foreign matter removal step. However, in order to prevent the attachment of a new foreign matter onto the processing surface after the foreign matter removal step, it is preferable that the flatness measuring step is performed prior to the foreign matter removal step.
Also, in order to prevent the attachment of a new foreign matter onto the processing surface after the foreign matter removal step, it is preferable that the foreign matter removal step and the processing step are performed in the same chamber or performed in chambers placed side by side in such a manner that the substrate can be conveyed without being discharged from the apparatus. In the case of using GCIB etching in the processing step, it is preferable to use the same GCIB etching apparatus in the foreign matter removal step and the processing step.
In the flatness measuring step, the flatness in each site of the processing surface, namely a difference of altitude is measured. Accordingly, the result obtained from the flatness measuring step becomes a flatness map showing a difference of altitude in each site of the processing surface (this will be hereinafter referred to as “flatness map”).
The flatness in each site of the processing surface can be, for example, measured by a laser interference type flatness measuring device. However, it should not be construed that the invention is limited thereto. The flatness map may be prepared using a measurement result obtained by measuring a difference of altitude in each site of the processing surface using a laser displacement gauge, an ultrasonic displacement gauge or a contact type displacement gauge.
In the processing method (2) of the invention, after performing the flatness measuring step and the foreign matter removal step, the processing condition of the processing surface is set up for each site of the processing surface on the basis of a result obtained from the flatness measuring step.
As described previously, the result obtained from the flatness measuring step becomes a flatness map. In the case where the coordinates of the processing surface as a two-dimensional planar shape are defined as (x, y), the flatness map is expressed by S(x, y) (μm). The processing time is expressed by T(x, y) (min). In the case where the processing rate is defined as Y (μm/min), the relationship of these is expressed by the following equation.
T(x, y)=S(x, y)/Y
Accordingly, in the case where the processing condition of the processing surface is set up for each site of the processing surface on the basis of a result obtained from the flatness measuring step, the processing condition, specifically the processing time is set up for each site of the processing surface according to the foregoing equation.
In the processing step, in the case of using a method accompanied with beam irradiation onto the processing surface, specifically in the case of using ion beam etching, GCIB etching or plasma etching, the processing condition of the processing surface can be set up for each site of the processing surface on the basis of a result obtained from the flatness measuring step. A setup procedure for this is hereunder specifically described.
In the case of performing this setup procedure, the width of the waviness existing on the processing surface is specified using a result obtained from the flatness measuring step. The width of the waviness as referred to herein means a length of a concave portion or a convex portion in the convex-concave shape existing cyclically on the processing surface. Accordingly, the width of the waviness is usually ½ of a cycle of the width of the waviness. In the case where a plural number of waviness having different cycles exit, the width of the waviness having the smallest cycle is taken as the width of the waviness existing on the processing surface.
As described previously, the measurement result obtained from the flatness measuring step is a flatness map showing a difference of altitude in each site of the processing surface. Accordingly, it is possible to easily specify the width of the waviness existing on the processing surface from the flatness map.
Ion beam etching, GCIB etching or plasma etching is performed with a beam having a beam diameter of not more than the width of the waviness on the basis of the width of the waviness as specified in the foregoing procedure. The beam diameter as referred to herein is based on FWHM (full width of half maximum) value. In this specification, when the beam diameter is referred to, it means the FWHM value of the beam diameter. In the processing step, it is more preferable to use a beam having a beam diameter of not more than ½ of the width of the waviness. By using a beam having a beam diameter of not more than the width of the waviness, it is possible to irradiate a beam while concentrating on the waviness existing on the processing surface and to effectively remove the waviness.
In the processing step, when a method accompanied with the beam irradiation on the processing surface is used, namely in the case of using ion beam etching, GCIB etching or plasma etching, it is necessary that a beam is scanned on the processing surface. This is because in order to set up a processing condition of the processing surface for each site of the processing surface, it is required to make the range to be irradiated with a beam at one time small as far as possible. In particular, in the case of using a beam having a beam diameter of not more than the width of the waviness, it is necessary to scan the processing surface with the beam. As a method of scanning with a beam, luster scanning and spiral scanning are known, and any of these methods may be used.
Of the foregoing processing methods, it is preferable to use GCIB etching because the surface can be processed so as to have a small surface roughness and excellent smoothness.
In the case of using the GCIB etching, the source gas and irradiation condition are the same as those described regarding the processing method (1) of the invention.
When the processing step according to the processing methods (1) or (2) of the invention is performed, there may be the case where the surface roughness of the processing surface is somewhat deteriorated depending upon the properties of the processing surface or the irradiation condition of a beam. Also, there may be the case where even when a desired flatness can be achieved in the foregoing processing step, the surface cannot be processed so as to have a desired surface roughness depending upon specifications of the glass substrate. For that reason, it is preferable that subsequent to the foregoing processing step (hereinafter referred to as “first processing step”), a second processing step for the purpose of improving the surface roughness of the processing surface is performed.
In the second processing step, GCIB etching can be used. In that case, the GCIB etching is performed by changing an irradiation condition such as a source gas, an ionizing current and an accelerating voltage from those used in the GCIB etching to be used in the foreign matter removal method and the GCIB etching to be used in the first processing step. Specifically, the GCIB etching is performed under an irradiation condition such that the etching amount is lower than that in the GCIB etching to be used in the first processing step. In comparison with the GCIB etching to be used in the first processing step, the GCIB etching is performed under a more gentle condition using a lower ionizing current or a lower accelerating voltage. More specifically, the accelerating voltage is preferably 3 keV or more and less than 30 keV, and more preferably from 3 to 20 keV. Also, it is preferable to use, as a source gas, an O2 gas singly or a mixed gas of O2 and at least one gas selected from the group consisting of Ar, CO and CO2 from the standpoint that when the source gas comes into collision with the processing surface, it hardly causes a chemical reaction. Above all, it is preferable to use a mixed gas of O2 and Ar.
Also, in the second processing step, mechanical polishing using a polishing slurry, which is called touch polishing, can be performed at a low surface pressure of from 1 to 60 gf/cm2. In the touch polishing, a glass substrate is set interposed between polishing plates each provided with a polishing pad made of a non-woven fabric, a woven fabric or the like, and the polishing plates are relatively rotated against the glass substrate while feeding a slurry adjusted so as to have predetermined properties, thereby polishing processing the processing surface at a surface pressure of from 1 to 60 gf/cm2.
As the polishing pad, for example, BELLATRIX K7512, manufactured by Kanebo, Ltd. is useful. As the polishing slurry, it is preferable to use a colloidal silica-containing polishing slurry; and it is more preferable to use a polishing slurry containing colloidal silica having an average primary particle size of not more than 50 nm and water and adjusted so as to have a pH in the range of from 0.5 to 4. The surface pressure of polishing is from 1 to 60 gf/cm2. When the surface pressure exceeds 60 gf/cm2, it is impossible to process the processing surface to a desired surface roughness due to the generation of a scratch scar on the substrate surface or the like. Also, there is a possibility that a rotation load of the polishing plates becomes large. When the surface pressure is less than 1 gf/cm2, it takes a long period of time for the processing, and hence, such is not practically useful. Also, when the surface pressure is less than 30 gf/cm2, it takes a long period of time for the processing. Therefore, it is preferable that after processing at a surface pressure of from 30 to 60 gf/cm2 to some extent, the surface is finish-processed at a surface pressure of from 1 to 30 gf/cm2.
The average primary particle size of colloidal silica is preferably less than 20 nm, more preferably less than 15 nm, and especially preferably less than 10 nm. When the average primary particle size of colloidal silica exceeds 50 nm, it is difficult to process the processing surface so as to have a desired surface roughness. Also, from the viewpoint of painstakingly managing the particle size, it is desirable that the colloidal silica does not contain a secondary particle which is formed through coagulation of the primary particle as far as possible. In the case where the colloidal silica contains a secondary particle, its average particle size is preferably not more than 70 nm. The particle size of colloidal silica as referred to herein is a particle size obtained by measuring an image with a magnification of from 15 to 105×103 times by SEM (scanning electron microscope).
The content of colloidal silica in the polishing slurry is preferably from 10 to 30% by mass. When the content of colloidal silica in the polishing slurry is less than 10% by mass, there is a possibility that the polishing efficiency may become worse, whereby economic polishing is not attained. On the other hand, when the content of colloidal silica exceeds 30% by mass, since the use amount of colloidal silica increases, there is a possibility of causing a trouble from the viewpoints of costs and washability. The content of colloidal silica in the polishing slurry is more preferably from 18 to 25% by mass, and especially preferably from 18 to 22% by mass.
When the pH of the polishing slurry is made to fall within the foregoing acidic range, namely the pH is made to fall within the range of from 0.5 to 4, it is possible to subject the processing surface to chemical and mechanical polishing processing, thereby achieving efficient polishing processing of the processing surface with good smoothness. That is, the convex portions of the processing surface are softened by an acid of the polishing slurry, and therefore, the convex portions can be easily removed by mechanical polishing. According to this, not only the processing efficiency is enhanced, but a glass waste which has been removed off by the polishing processing is softened, and therefore, the generation of a new damage due to the glass waste or the like is prevented. When the pH of the polishing slurry is less than 0.5, there is a possibility that corrosion is generated in a polishing machine to be used for touch polishing. From the viewpoint of handling properties of the polishing slurry, the pH is preferably 1 or more. In order to obtain a sufficient chemical polishing processing effect, the pH is preferably not more than 4, and especially preferably in the range of from 1.8 to 2.5.
The pH adjustment of the polishing slurry can be achieved by adding an inorganic acid or an organic acid singly or a combination thereof. Examples of the inorganic acid which can be used include nitric acid, sulfuric acid, hydrochloric acid, perchloric acid and phosphoric acid. Of these, nitric acid is preferable in view of easiness of handling. Also, examples of the organic acid include oxalic acid and citric acid.
As water to be used for the polishing slurry, pure water or ultrapure water from which foreign matter has been removed is preferably used. That is, pure water or ultrapure water which substantially has not more than one fine particle having a maximum size, as measured by a light scattering mode using laser light or the like, of 0.1 μm or more per mL is preferable. When more than one foreign matter per mL is incorporated regardless of the material quality or shape, there is a possibility that a surface defect such as a scratch and a pit is formed on the processing surface. The foreign matter in pure water or ultrapure water can be removed by, for example, filtration or ultrafiltration with a membrane filter, but it should not be construed that the removal method of foreign matter is limited thereto.
In the glass substrate as processed by the processing methods (1) or (2) of the invention, the processing surface has excellent flatness and surface roughness; the flatness of the processing surface after processing is not more than 50 nm; and a convex defect of the glass having a height exceeding 1.5 nm does not exist on the processing surface. The flatness of the processing surface after processing is more preferably not more than 30 nm, and further preferably not more than 20 nm.
The glass substrate as processed by the processing method of the invention is favorable as an optical device to be used in an optical system of an exposure tool for semiconductor manufacture, especially an optical device to be used in an optical system of a next-generation exposure tool for semiconductor manufacture with a line width of not more than 45 nm because the processing surface has excellent flatness and surface roughness. Specific examples of such an optical device include lenses, diffraction gratings, optical membrane bodies and composite bodies thereof, for example, lenses, multi-lenses, lens arrays, lenticular lenses, fly-eye lenses, aspheric lenses, mirrors, diffraction gratings, binary optics devices, photomasks and composite bodies thereof.
Also, the glass substrate as processed by the processing method of the invention is favorable as a photomask and a mask blanks for manufacturing this photomask, especially a reflective type mask for EUVL and a mask blanks for manufacturing this mask.
The light source of the exposure tool is not particularly limited and may be a conventional laser capable of emitting g-rays (wavelength: 436 nm) or i-rays (wavelength: 365 nm). However, light sources of a shorter wavelength, specifically light sources having a wavelength of not more than 250 nm are preferable. Specific examples of such a light source include a Kr F excimer laser (wavelength: 248 nm), an ArF excimer laser (wavelength: 193 nm), an F2 laser (wavelength: 157 nm) and EUV (wavelength: 13.5 nm).
While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
This application is based on Japanese Patent Application No. 2007-172274 filed Jun. 29, 2007, and the contents thereof are herein incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-172274 | Jun 2007 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP08/57553 | Apr 2008 | US |
| Child | 12648481 | US |
