Cristobalite-contained silica glass, and silica glass jig made of same

Abstract: An object of the present invention is to provide silica glass having high purity, high heat resistance, large coefficient of thermal expansion, and low light transmittanc. Another object of the present invention is to provide a silica glass jig with a vapor-deposited film thereon. Therfore Cristobalite-contained silica glass wherein α-cristobalite in the shape of a small sphere or a small, round-edged or sharp-edged, three-dimensional region is dispersed in the silica glass matrix, the diameter of each α-cristobalite sphere or region is in the range of 0.1 µm to 1000 µm, a content of the α-cristobalite is at least 10 wt %, and small-sized independent bubbles which are additionally dispersed in the silica glass matrix. The silica glass jig is made of the cristobalite-contained silica glass and the surface of the jig is covered with a vapor-deposited thin film which has a property of being resistant to plasma etching, and which is made of a material of almost the same coefficient of thermal expansion as that of the silica glass. The silica glass jig has no chance to generate particles therefrom and thereby to contaminate a silicon wafer.

Description: Industrial Field of Application The present invention relates to new cristobalite-contained silica glass and a silica glass jig made of the silica glass the surface of which is covered with a vapor-deposited thin film. More particularly, it relates to cristobalite-contained silica glass having α-cristobalite in the shape of a small sphere or a small, round-edged or sharp-edged, three-dimensional region dispersed in the silica glass matrix and a silica glass jig made of the silica glass best used in a plasma etching process of semiconductor device fabrication. Prior Art Silica glass has been heretofore used in fields such as of a jig for use in semiconductor device fabrication and the like wherein high purity, heat-resistance and chemical resistance are required, since silica glass is generally good in heat-resistance, chemical resistance and further good in processability and cleanablity. With the recent advance in a packing density of constituent elements in a chip of a very-large-scale integrated circuit, conditions under which a jig is used has been getting severer, so that it has become earnestly required that firstly any element never comes out at any extent from the jig in a semiconductor device fabrication process and thereby the jig does not contaminate a semiconductor device being fabricated, and secondly the jig is not subjected to thermal deformation, even at a treating temperature of higher than 1000 °C. In such a technical situation as this, crystal-contained silica glass which is excellent in not only heat resistance, but purity has drawn attention. For example, a crystal-contained silica glass is proposed in a published document of Unexamined Patent Application No. Hei 3-45029, which silica glass is produced by sintering a mixture of crystalline quartz and amorphous quartz. Another crystal-contained silica glass is proposed in Journal of the American Ceramic Society, Vol. 42, No. 6, pp 263-270, June 1, 1959, in which a crystal phase is created in the silica glass matrix by doping aluminum and the like as a crystal forming agent. However, since the crystal-contained silica glass described in Unexamined Patent Application No. Hei 3-45029 is produced by sintering, gaps are generated among small crystals in the silica glass matrix, and thereby the mechanical strength is low and the thermal expansion coefficient is smaller than that of silica glass. When a jig made of such crystal-contained silica glass is used in a chemical vapor deposition process of a semiconductor device fabrication and is cooled in a cooling process, a thin polysilicon or silicon nitride film deposited on the surface of the jig shrinks to a great extent and becomes to have cracks therein, since the thermal expansion coefficients thereof, which are 1 x 10 -6 /°C or lager, are by far larger than that of the silica glass. Dust generated by peeling-off due to the cracks of part of the film causes the following problems. The dust is floating in the atmosphere of the chemical vapor deposition process to contaminate a semiconductor device being fabricated. The cracks further advances into the bulk of the jig to finally break down the jig. Because the crystal-contained silica glass proposed in the Journal of the American Ceramic Society above mentioned uses a crystal forming agent to form a crystal phase in the silica glass matrix, it has a fault that the crystal forming agent out-diffuses impurities to thereby contaminate a semiconductor device being fabricated, when the jig is used in a temperature higher than 1000 °C. What's more, with the same recent advance in a packing density of constituent elements in a chip and the trend for finer patterning in a chip of a very-large-scale integrated circuit, precision processing on a finer scale of the order of submicrons has become applied to a silicon wafer and the traditional etching treatment of the silicon wafer has to also change to the plasma etching treatment. In the plasma etching treatment, a silica glass jig has a risk for being etched itself in a special gas atmosphere and as a result, the silicon wafer has a risk to be contaminated by generated particles from the jig. In light of the above fault, a silica glass jig the surface of which is covered with a vapor deposited thin film made of metal or ceramic thereon has been proposed, but since the thermal expansion coefficient of silica glass is usually smaller than those of the metal or ceramic, the thin film contracts to a great extent and thereby has a problem that a crack occurs in the thin film when it is used in a high temperature treatment of a silicon wafer. This problem has blocked the practical application of a thin film on the surface of a jig. In FR-A-2 291 953 a refractory material is specified which contains amorphous silica as well as 45 - 60 wt.% of a crystalline phase of silica. The refractory material is made by mixing powders of amorphous and crystalline powders, whereby the partides dimensions are between 75 µm and 150 µm and sintering the mixture. JP-A 1 126 238 as interpreted by a PAJ-abstract describes a silica glass member containing cristobalite. Cristobalite crystals having sizes between 5 µm and 1000 µm are obtained by growing strain seeds which are formed in the glass matrix. From the paper "A new material - quartz light -diffusing glass", published in "Glass and Ceramics", vol. 48, no. 7/8, 1991, pages 285 - 287, a light scattering quartz glass is known. The light scattering is achieved by producing a uniform distribution of bubbles in the glass mass. The bubbles have a size of 10 µm to 100 µm and a concentration between 200.000 - 700.000 cm -3 . From DD-A-295 619 as interpreted by the Derwent abstract, Accession no. 92-115129 a ceramic moulding is known consisting of quartz sand, cristobalite and alkali-alkaline earth-alumino borosilicate glass. JP-A 62 202 826 as interpreted by a PAJ-abstract describes a method for producing high-quality glass at low cost. by mixing SiO 2 -powder with a phase transition promotor, heat treating to achieve a growing of cristobalite crystals and melting the crystalline phase in vacuum to obtain a high quality transparent or functional glass. Problem the Invention Seeks to Solve In consideration of the current state of the technology, the present inventors have investigated with extreme efforts about development of a silica glass having high heat resistance, high purity, high thermal expansion coefficient and a low light transmittance and as a result, reached a discovery that silica glass having the characteristics above mentioned can be produced by incorporating a-crystobalite in the shape of a small sphere or a small, round-edged or sharp-edged, three-dimensional region dispersed in the bulk of silica glass matrix and further by dispersing small-sized independent bubbles at a content in a range in the matrix as compared with that of the silica glass with no independent bubbles. A first aspect of the present invention has been completed on the basis of such knowledge as mentioned above. In consideration of the current state of the technology, the present inventors have also investigated with extreme efforts about development of a silica glass jig made of a silica glass best used in a plasma etching process of semiconductor device fabrication. The present inventors have discovered facts that when a thin film made of metal or ceramic which is excellent in resistance to plasma etching is deposited on the surface of a jig of the above described silica glass, the thin film has no chance for cracks to occur therein. Thereby, the silica glass jig with the thin film thereon can prevent not only generation of particles but also particle-contamination of a silicon wafer. Thus, a long lasting jig for use in a plasma etching treatment can be obtained. A second aspect of the present invention has been completed on the basis of the discoveries mentioned above. It is an object of the present invention to provide a crystobalite-contained silica glass having high purity, high heat resistance, large thermal expansion coefficient and light transmittance of 1 % or less at a wavelength in the region between 0,2 µm and 5 µm at a sample thickness of 4 mm. It is a further object of the present invention to provide a silica glass jig having less of a chance to contaminate a silicon wafer by particles originated from a thin film deposited thereon in a semiconductor fabrication process. Means to Solve the Problem The first aspect of the present invention is directed to cristobalite-contained silica glass wherein α-cristobalite in the shape of a small sphere or a small, round-edged or sharp-edged, three-dimensional region is dispersed at a content of at least 10 wt % in the silica glass matrix and the diameter of each sphere or region is in the range of 0.1 µm to 1000 µm, wherein small-sized independent bubbles are additionally contained and uniformly dispersed in the silica glass matrix. The diameter of each bubble is preferably 500 µm or smaller. The second aspect of the present invention is directed to a silica glass jig the surface of which is covered with a metal or ceramic thin film wherein the silica glass of the jig is cristobalite-contained silica glass of the present invention. The cristobalite-contained silica glass of the present invention has a-crystobalite in the shape of a small sphere or a small, round-edged or sharp-edged, three-dimensional region dispersed in the silica glass matrix, a diameter of each small sphere or region is in the range of 0.1 µm to 1000 µm and a content of the cristobalite is in the range of 10 wt % to 99 wt %. The thermal expansion coefficient of silica glass is increased by containing the cistobalite phase in the matrix and can be adjusted to a value equal to or larger than 1 x 10 -6 /°C which is a thermal expansion coefficient of a vapor-deposited film formed in a chemical vapor deposition process. When the diameter of each α-cristobalite sphere or region is less than 0.1 µm, not only improvement of heat resistance of, but also increase in thermal expansion coefficient of the α-cristo-balite-contained silica glass are not observed and when the diameter is larger than 1000 µm, the mechanical strength of the α-cristobalite-contained silica glass is reduced to an extent at which it cannot be used in a practical use. When a content of the α-cristobalite-contained silica glass is less than 10 wt %, increase in the thermal expansion coefficient is negligible small and when a content is larger than 99 wt %, cracks are generated in a body of the α-cristobalite-contained silica glass. In the case that the α-cristobalite-contained silica glass according to the present invention is produced by heating to melt a mixture of two or more kinds crystalline silicon dioxide powder the melting points of which are different from each other by 20 °C or more, with a content of α-cristobalite more than 80 wt %, cracks usually occur in a body of the α-cristobalite-contained silica glass, but even with a content of α-cristobalite of 99 wt %, it is possible that cracks do not occur in a body of the α-cristobalite-contained silica glass, if it is re-heated at a temperature of 1000 °C or higher and thereby a crystal of α-cristobalite is gradually grown. The re-heating time changes in dependence on a content of α-cristobalite. when a content of the silicon dioxidepowder having the highest melting point in the mixture is larger, the re-heating time can be decreased and vice versa. Any atmosphere can be chosen in the heat treatment. The cristobalite-contained silica glass becomes opaque due to the presence of crystobalite phase in the matrix, for cracks are generated between a crystal of cristobalite and the silica glass matrix and the cracks scatter incident rays. With additional incorporation of independent small-sized bubbles in the silica glass matrix, a degree of opaqueness is increased. A diameter of each bubble dispersed in the matrix is preferably 500 µm or smaller and a concentration is preferably in the range of 100 bubbles/cm 3 to 100.000 bubbles/cm 3 . The cristobalite-contained silica glass which, in addition to the above characteristics, has a density of 2.0 g/cm 3 or more and a three-point bending strength of 150 kg/cm 2 or more can be effectively used as a material of a jig for use in semiconductor device fabrication. The light transmission can be reduced to a smaller value by re-heating to grow a crystal of α-cristobalite to an greater degree and it can be adjusted to a value of 1 % or less. It is possible to manufacture a jig such as an excellent heat-ray scattering member from the crystal-contained silica glass with such a low light transmission as this. According to the present invention, a content of independent bubbles in the matrix can be lowered to a value of the order of one tenth in comparison with that of a traditional opaque quartz glass, while the degree of opaqueness is still better than that of the traditional opaque quartz glass. The cristobalite-contained silica glass is produced in the following steps of first charging a raw material mixture of two or more kinds of crystalline silicon dioxide powder the melting points of which are different from each other by 20 °C or more, the silicon dioxide powder having the highest melting point of which is mixed at a content of 10 wt % to 80 wt %, and a particle size of each powder is in the range of 10 µm to 1000 µm in a heat resistant mold, and second heating the mold and the charged mixture at a temperature between the lowest melting point and a temperature less than the highest melting point of the ingredients of the mixture. In the process, it is preferable to take such a heating and cooling conditions that in an inert atmosphere, firstly the mixture is heated at a first heating speed of 10 °C/min to 50 °C/min from room temperature to 1000 °C, secondly at a second heating speed of 10 °C/min or lower from the 1000 °C up to a first temperature in the range of the lowest melting point to a second temperature of 10 °C higher than the lowest melting point, thirdly slowly from the first temperature up to a third temperature lower than the highest melting temperature, and fourthly the mixture is constantly kept at the third temperature. In the same process wherein nitrogen gas flows at a flow rate of 5 l/min to 20 l/min in the atmosphere thereof, cristobalite-contained silica glass having independent bubbles dispersed in the silica glass matrix which bubbles has a diameter of 500 µm or smaller and a concentration of 100 bubbles/cm 3 to 100,000 bubbles/cm 3 is produced. In the above mentioned method of production, a-quartz is used as crystalline silicon dioxide and the structural form of α-quartz transits to β-quartz, β-tridymite and to β-cristobalite when heating to melt it and if the β-cristobalite is heated at a temperature of the melting point or higher, the crystal changes into a liquid to be in a vitrified state. Silica glass is obtained by cooling down the β-cristobalite in the vitrified state to room temperature to reach a supercooled condition. The melting point of β-cristobalite is 1723 °C according to a technical document. However, the present inventors have discovered facts that melting points of crystalline silicon dioxides produced from mines are different from each other bt 20 °C or more. A fact of the different melting points can be probably explained by such a reasoning that if the natural silicon dioxides in an investigation all are β-cristobalite, the structural form of crystal of each silicon dioxide can be still different from each other depending on the mineral veins from which the silicon dioxides are produced. In a heating process of the method mentioned above, the silicon dioxide powder having the lowest melting point is first molten to form a silica glass matrix and on the other hand the silicon dioxide powder having the highest melting point remains unmelted to form cristobalite regions. Thus produced crystal-contained silica glass contains neither impurities such as a crystal forming agent or gaps formed by sintering and thereby is high both in purity and in mechanical strength, since it is produced by neither sintering nor doping a crystal forming agent. The structural form of α-cristobalite is one form of SiO 2 crystal. A fact that the size of a crystal region is smaller than the size of each particle of the crystalline silicon dioxide powder having the highest melting point seemingly comes from a cause that particles of the crystalline silicon dioxide powder are further pulverized to smaller sizes in a vitrification process. It can be seen from the above explanation that a kind of crystalline silicon dioxide chemically synthesized having a different melting point by 20 °C or more from those of the other kinds of crystalline silicon dioxide in the mixture. In a method for producinfg the silica glass according to the invention a carbon-made mold, a ceramic-made mold and a transparent quartz tube inserted in the molds just mentioned are named as heat resistant molds for use in charging a mixture of crystalline silicon dioxide powder. In the case of using a transparent quartz tube, high purity of the raw material charged can be maintained and at the same time silica glass produced does not have a chance for the bulk form to collapse. In these points, a transparent quartz tube as a mold is excellent. An electric-melting method is adopted as a heating means for melting, since it has a little chance for OH groups to mixed in silica glass, and thereby the silica glass being highly heat-resistant is produced. As an atmosphere in the method, an inert gas atmosphere such as a nitrogen atmosphere or a vacuum atmosphere is generally used. When a mold is made of carbon, an inert gas atmosphere is specially required. but when a mold is made of ceramic, a gas in the atmosphere is not so much specialized. The body of a jig according to the present invention is made of crystal-contained silica glass produced in the method above mentioned. The surface of the jig is covered with a thin film of 2 nm or more thick made of metal or ceramic vapor-deposited by means of CVD method or the like which film has an excellent resistance to plasma etching. As metal or ceramic of which the thin film is made, Al 2 O 3 , SiC, Si 3 N 4 , Si, Carbon, or the like is named. The thermal expansion coefficients of these metals or ceramics are in the range of 1 x 10 -6 /°C to 1 x 10 -5 /°C. It is required, therefore, that a thermal expansion coefficient of the crystal-contained silica glass as a material of a jig is selected so as to be in the range. For the purpose, it is further necessary that sizes of the α-cristobalite regions and a content of the α-cristobalite are properly adjusted. Such an adjustment are properly effected by gradually reheating the crystal-contained silica glass as produced in the above mentioned method at a temperature of 1000 °C or higher to grow crystal regions both in size and in content to a desired extent. A silica glass jig of the present invention has a little risk to suffer from etching even in a plasma etching process applied to semiconductor device fabrication, wherein a degree of etching is only at one-tenth of that of a traditional jig or less and besides, no particle is generated from a vapor-deposited thin film in a CVD process. Embodiments of the Invention The present invention will be in concrete terms described in reference to examples below, but it should be understood that the present invention is not limited to the specific embodiments thereof except as defined in the appended claims. Physical properties described in the examples are measured in ways as described here. Thermal expansion coefficient of cristobalite-contained silica glass: Micro constant load dilatometer made by Rigaku Denki K. K. was used. A specimen was heated from room temperature to 700 °C at a heating speed of 2 °C/min and a distance between two points is measured in the course of heating up. Coefficients of thermal expansion were obtained by a following equation (1). α ( °C -1 ) = (ΔL/Lo ) x ( dL/dt ) where Lo is the length of a sample at a reference temperature ( 25 °C ), ΔL is an increment of the length at a temperature of measurement, dt is an infinitesimal difference in temperature ( 20°C ) and dL is an increment of the length in the difference in temperature dt. Coefficients of thermal expansion were computed by means of a central difference method with dt = 20°C. The results are shown in Fig. 1. The average coefficient of thermal expansion was obtained from Fig. 1, which was 1.5 x 10 -6 /°C. A strip of silica glass is used as a reference sample. A sample has a cross-section of 3 mm square and a length of 15 mm. Size of a cristobalite region: A scanning electron microscope is used to determine the size of each cristobalite region. Light transmittance: A ratio of the energy of light transmitted through a sample to the energy of light radiated on a surface of the sample is measured where the sample was prepared in the shape of a small plate having a thickness of 2 mm by slicing a block with a diamond saw blade and the ratio was measured by a infrared spectrophotometer IR-700 made by Nihon Bunkou Kougyou K. K. Structural form of a crystal: An x ray diffraction analyzer was used to determine the structural form of a crystal. Bending strength: A three-point bending test machine was used to determine the bending strength of a sample at room temperature. Examples Reference Example 1 Two kinds of crystalline silicon dioxide powder of Iota CGU - Fine having a grain size range of 100 µm to 250 µm and Iota 6 Fine having a grain size range of 100 µm to 250 µm both produced by Unimin Corporation were mixed at a ratio of 80:20 in a ball mill made of silica glass. The powder mixture was charged in a silica glass tube inserted in a carbon made mold, then the air lying in the gaps between the particles of the powder mixture was removed by vacuum suction. After completion of air removal, the space in the silica glass tube was filled with nitrogen gas. In the nitrogen atmosphere the powder mixture was heated from room temperature to 1000 °C in 90 min, from the 1000 °C to 1200 °C in 30 min, from the 1200 °C to 1630 °C in 90 min, and from the 1630 °C to 1720 °C in 180 min and then kept at a constant temperature of the 1720 °C for 1 h. The powder mixture thus treated became cristobalite-contained silica glass. In the cristobalite-contained silica glass, α-cristobalite in the shape of a small sphere of about 40 µm in diameter was observed at a content of 30 wt % in the silica glass matrix. Observation by an electron microscope was schematically drawn in Fig. 2 to show the feature thereof. The sizes of crystalline regions, thermal expansion coefficient, light transmission, and bending strength were measured on samples of the cristobalite-contained silica glass and the results are shown in Table 1. Besides, the measurement of x ray diffraction is shown in Chart 1 and from the chart, the structural form of the crystal regions was identified as α-cristobalite, as shown in Chart 1. Reference Example 2 The cristobalite-contained silica glass produced in Reference Example 1 was further heated at 1400 °C for 60 h. An α-cristobalite region had grown to that of about 100 µm in diameter. Observation by the electron microscope is schematically shown in Fig. 3. The sizes of the cristobalite regions and the thermal expansion coefficient, light transmission, and bending strength of the cristobalite-contained silica glass were measured and the results are shown in Table 1. The structural form of a crystal zone in the silica glass matrix was identified as α-cristobalite by the x ray diffraction analyzer. Reference Example 3 Cristobalite-contained silica glass was produced in the same way as in Reference Example 1 with an exception that a mixture of crystalline silicon dioxide powder was prepared from Iota CGU Fine and Iota 6 Fine at a ratio of 50:50. The sizes of cristobalite regions and thermal expansion coefficient, light transmission, and bending strength of the cristobalite-contained silica glass thus produced were measured and the results are shown in Table 1. The structural form of a crystal region in the silica glass matrix was identified as α-cristobalite by the x ray diffraction analyzer. Reference Example 4 The cristobalite-contained silica glass produced in Reference Example 3 was further heated at 1400 °C for 20 h. An α-cristobalite region had grown to that of about 100 µm in diameter. The sizes of the cristobalite regions and the thermal expansion coefficient, light transmission, and bending strength of the cristobalite-contained silica glass thus produced were measured and the results are shown in Table 1. The structural form of a crystal region in the silica glass matrix was identified as α-cristobalite by the x ray diffraction analyzer. Example 1 Cristobalite-contained silica glass was produced in the same way as in Reference Example 1 with an exception that the powder mixture was molten in an atmosphere in which nitrogen gas flew at a rate of 10 l/min. In the matrix of the produced cristobalite-contained silica glass, independent bubbles of diameters of 160 µm or smaller were observed at a concentration of 68.000 bubbles/cm 3 and the density was measured at 2.115 g/cm 3 . The sizes of cristobalite regions and the thermal expansion coefficient, light transmission, and bending strength of the cristobalite-contained silica glass thus produced were measured and the results are shown in Table 1. The structural form of a crystal region in the silica glass matrix was identified as α-cristobalite by the x ray diffraction analyzer. Comparative example 1 Iota CGU Fine produced by Unimin Corporation was charged in a silica glass tube inserted in a carbon made mold and the powder was heated in a nitrogen atmosphere from room temperature to 1000 °C in 90 min, from the 1000 °C to 1200 °C in 30 min, from the 1200 °C to 1630 °C in 90 min, and from the 1630 °C to 1720 °C in 180 min and kept at a constant temperature of the 1720 °C for 1 h. The powder thus treated became opaque silica glass. The thermal expansion coefficient, light transmission, and bending strength of the opaque silica glass thus produced were measured and the results are shown in Table 1. Non-existence of a crystalline region in the silica glass matrix was confirmed as a result of an x ray diffraction analysis. A rocking curve obtained is shown in Chart 2. Comparative example 2 Iota 6 Fine produced by Unimin Corporation was charged in a silica glass tube inserted in a carbon made mold and the powder was heated in a nitrogen atmosphere from room temperature to 1000 °C in 90 min, from the 1000 °C to 1200 °C in 30 min, from the 1200 °C to 1630 °C in 90 min, and from the 1630 °C to 1720 °C in 180 min and kept at a constant temperature of the 1720 °C for 1 h. The powder thus treated became opaque silica glass. The thermal expansion coefficient, light transmission, and bending strength of the opaque silica glass thus produced were measured and the results are shown in Table 1. Non-existence of a crystalline region in the silica glass matrix was confirmed as a result of an x ray diffraction analysis. Comparative example 3 The thermal expansion coefficient, light transmission, and bending strength of silica glass produced by an oxygen-hydrogen melting method were measured and the results are shown in Table 1. Non-existence of a crystalline region in the silica glass matrix was confirmed as a result of an x ray diffraction analysis. Tests on thin films on silica glass made jigs Thin films were coated on disks made of the crystal-contained silica glasses produced in Reference Examples 1 to 4 and Example 1 and on disks made of traditional silica glass in Comparative examples 1 to 3 in the following conditions. Observations of the coating states of the thin films by the electron microscope are described in Table 2, where additional examples are respectively numbered as Reference examples 5-8, Example 2 and comparative examples 4-6. i ) A polysilicon film of 50 µm thick was vapor-deposited from SiH 4 as a raw material at 600 °C. ii ) An Si 3 N 4 film of 50 µm thick was vapor-deposited from a mixture of mono-silane and ammonia as raw materials at about 300 °C. iii ) An SiC film of 50 µm thick was vapor-deposited from a mixture of methane and silicon tetrachloride as raw materials at about 1200 °C. iv ) A BN film of 50 µm thick was vapor-deposited from a mixture of BF 3 and ammonia as raw materials at about 1600 °C. Five trial tests were additionally conducted by actually using silica glass jigs made of the crystal-contained silica glass respectively produced in Reference Examples 1 and 2, and of the silica glass produced in Comparative example 3 with no thin film thereon, and silica glass jigs made of the crystal-contained silica glass produced in Reference Examples 1 and 2 ( which are respectively expressed with Reference Examples 5 and 6 in Table 3 ) with vapor-deposited Si 3 N 4 films of 50 µm thick thereon as holders for the lower electrode plate in a plasma etching apparatus for one month. The surfaces of the five holders and the coating conditions of the two thin films after the one month use were observed and the results are shown in Table 3. Ref. Example 1 Ref. Example 2 Com. Ex 3 Ref. Example 5 Ref. Example 6 Rough Surface some some much none none Etching little little much none none Coating State - - - good no peeling good no peeling

Claims: Cristobalite-contained silica glass wherein α-cristobalite regions are dispersed at a content of at least 10 wt % in the silica glass matrix and the diameter of each region is in the range of 0.1 µm to 1000 µm, and wherein small-sized independent bubbles are additionally dispersed in the silica glass matrix. Cristobalite-contained silica glass claimed in claim 1 wherein the diameter of each bubble is 500 µm or smaller and a concentration of the bubbles is in the range of 100 bubbles/cm 3 to 100,000 bubbles/cm 3 . Cristobalite-contained silica glass claimed in claim 2 wherein the silica glass has a density of 2.0 g/cm 3 or more and a bending strength of 150 kg/cm 2 or more. Cristobalite-contained silica glass claimed in any of claims 1 to 3 wherein the silica glass has a coefficient of thermal expansion of 1 x 10 -6 /°C or more. Cristobalite-contained silica glass claimed in any of claims 1 to 4 wherein the silica glass has a light transmittance of 1 % or less at a wavelength in the region between 0,2 µm and 5 µm of a 4 mm thickness sample. A silica glass jig wherein the jig is made of cristobalite-contained silica glass, α-cristobalite regions are dispersed at a content of at least 10 wt % in the silica glass matrix and the diameter of each region is in the range of 0.1 µm to 1000 µm, wherein small-sized independent bubbles are additionally dispersed in the silica glass matrix, and the surface of the jig is covered with a vapor-deposited thin film made of metal or ceramic. A silica glass jig claimed in claim 6 wherein the diameter of each bubble is 500 µm or smaller and a concentration of the bubbles is in the range of 100 bubbles/cm 3 to 100.000 bubbles/cm 3 . A silica glass jig claimed in claim 6 wherein the silica glass has a density of 2.0 g/cm 3 or more and a bending strength of 150 kg/cm 2 or more. A silica glass jig claimed in any of claims 6 to 8 wherein the silica glass has a thermal expansion coefficient of 1 x 10 -6 /°C or more. A silica glass jig claimed in claim 9 wherein the silica glass has α-cristobalite regions the diameter of which each is at least 1 µm, and which are dispersed at a content in the range of 10 wt % to 99 wt %. A silica glass jig claimed in any of claims 6 to 10 wherein the surface of the silica glass jig is covered with a thin film having a thermal expansion coefficient between 1 x 10 -6 /°C to 1 x 10 -5 /°C and the thin film has a property of high resistance to plasma etching.

Inventors: ENDO MAMORU, INAKI KYOICHI, SEGAWA TOHRU, YOSHIDA NOBUMASA, ENDO, MAMORU, INAKI, KYOICHI, SEGAWA, TOHRU, YOSHIDA, NOBUMASA, Endo, Mamoru c/o Shin-Etsu Quartz Prod. Co. Ltd., Inaki, Kyoichi c/o Shin-Etsu Quartz Prod. Co. Ltd., Segawa, Tohru c/o Shin-Etsu Quartz Prod. Co. Ltd., Yoshida, Nobumasa Shin-Etsu Quartz Prod. Co. Ltd.
Applicants: HERAEUS QUARZGLAS, SHINETSU QUARTZ PROD, HERAEUS QUARZGLAS GMBH & CO. KG, SHIN-ETSU QUARTZ PRODUCTS CO., LTD.
Application year: 1996
Main classification: C03C14
IPC codes: C03C14, C03C3