CRISTOBALITE-CONTAINED SILICA GLASS, METHOD OF PRODUCING SAME 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 transmittance and a method of producing the same. Another object of the present invention is to provide a silica glass jig with a vapor-deposited film thereon. Therefore cristobalite-contained silica glass wherein alpha -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 alpha -cristobalite sphere or region is in the range of 0.1 mu m to 1000 mu m, and a content of the alpha -cristobalite is at least 10 wt.%. A method of producing the cristobalite-contained silica glass by heating a mixture of two kinds or more of crystalline silicon dioxide powder the melting points of which silicon dioxide are different from each other by 20 DEG C or more wherein the mixture contains the silicon dioxide having the highest melting point at a content in the range of 10 wt.% to 80 wt.% and is heated at temperatures in the range of the lowest melting point of the ingredients to a temperature lower than the highest melting point. 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: CRISTOBALITE-CONTAINED SILICA GLASS, METHOD OF PRODUCING SAME AND SILICA GLASS JIG MADE OF SAME Detailed Description of the Invention Industrial Field of Application The present invention relates to new cristobalite-contained silica glass, a method of producing the 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, a method of producing the α-cri- stobalite-contained silica glass 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 requi¬ red, 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 consti¬ tuent 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 ele¬ ment never comes out at any extent from the jig in a semiconductor device fabrication process CONRRMAπON -CQfir and thereby the jig does not contaminate a semiconductor device being fabricated, and se¬ condly 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 excel¬ lent in not only heat resistance, but purity has drawn attention. For example, a crystal-con¬ tained 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 amoφhous quartz. Another crystal-contained silica glass is proposed in Journal of the Ameri¬ can 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 sili¬ ca glass matrix, and thereby the mechanical strength is low and the thermal expansion coeffi¬ cient 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 expan¬ sion coefficients thereof, which are 1 x 10*/°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 patteming 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 contamina¬ ted 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. 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 puri¬ ty and high thermal expansion coefficient 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 shaφ-edged, three-dimensional region dispersed in the bulk of silica glass matrix and further the silica glass with a lower light trans¬ mission is obtained 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. The present inventors have also discovered a fact that the silica glass with all the characteristics above mentioned can be produced with ease by heating to melt a mixture of two or more kinds of cry¬ stalline silicon dioxide powder which have different melting points from each other by 20 °C or more at a temperature in a range. 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 investi¬ gated 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 a silica glass not only with a thermal expansion coefficient which is on the same order with that of a vapor-deposited thin film covering the surface of the silica glass jig, but also with high heat resistance is produced with ease by heating to melt a mixture of two or more kinds of crystalline silicon dioxide powder which have different melting points from each other by 20 β C or more at a temperature in a range, as is same as in the first aspect of the present invention. The present inventors have further 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 the silica glass jig, 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 par¬ ticles 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, accordingly, to provide crystobalite-contained silica glass having high purity, high heat resistance and large thermal expansion coefficient. It is another object of the present invention to provide a crystobalite-contained silica glass ha¬ ving high purity, high heat resistance, large thermal expansion coefficient and light transmittan- ce of 1 % or less at a wavelength in the region between 0,2 μm and 5 μm at a sample thick¬ ness of 4 mm. It is a further object of the present invention to provide a method of producing new crystobalite- contained silica glass. It is a still further object of the present invention to provide a method of producing crystobalite- contained silica glass with small-sized independent bubbles dispersed therein. It is another object of the present invention to provide a silica glass jig with high purity and high resistance to plasma etching. 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 se¬ miconductor 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 shaφ-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. The first aspect of the present invention is directed to the cristobalite-contained silica glass wherein small-sized independent bubbles are additionally contained and uniformly dispersed in the silica glass matrix, the diameter of each bubble being 500 μm or smaller. The first aspect of the present invention is directed to a method of producing the cristobalite- contained silica glass 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-con¬ tained 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 shaφ-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 ma¬ trix and can be adjusted to a value equal to or larger than 1 x lOΥC which is a thermal expan¬ sion 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 impro¬ vement 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-cristobalitβ is gradually grown. The re-heating ti¬ me changes in dependence on a content of α-cristobalite. when a content of the silicon dioxi- depowder 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 incoφoration of independent small-sized bubbles in the silica glass matrix, a degree of opaqueness can be increased. A diameter of each bubble dispersed in the matrix is preferably 500 μm or smaller and a concen¬ tration 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 re¬ duced 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 bub¬ bles 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 hea¬ ting 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 at¬ mosphere, 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 tem¬ perature 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 at¬ mosphere 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 can be 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 coo¬ ling 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 dif¬ ferent 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 di¬ oxide can be still different from each other depending on the mineral veins from which the sili¬ con dioxides are produced. In a heating process of the method mentioned above, the silicon dioxide powder having the lo¬ west 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 for¬ ming 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 regi¬ on 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 di¬ oxide 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 according to the present invention, a carbon-made mold, a ceramic-made mold and a transparent quartz tube inserted in the molds just mentioned are named as heat resi¬ stant 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 at¬ mosphere 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,O 3 , SiC, Si 3 N<, Si, Carbon, or the like is named. The thermal expansion coefficients of these metals or ceramics are in the range of 1 x 10 " VC to 1 x 10 " °C. It is requi¬ red, 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 puφose, it is further necessary that sizes of the α-cristobalite regions and a content of the α-cristobalite are property adju¬ sted. 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 hea¬ ted 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 ob¬ tained by a following equation (1). α ( β C- 1 ) = (ΔlJLo ) x ( dUdt ) (1), 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 lO^ 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 Example 1 Two kinds of crystalline silicon dioxide powder of lota CGU - Fine having a grain size range of 100 μm to 250 μm and lota 6 Fine having a grain size range of 100 μm to 250 μm both produ¬ ced by Unimin Coφoration 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 nitro¬ gen 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 β C in 30 min, from the 1200 β C to 1630 β 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. Ob¬ servation by an electron microscope was schematically drawn in Fig. 2 to show the feature the¬ reof. 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. Example 2 The cristobalite-contained silica glass produced in 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 regi¬ ons and the thermal expansion coefficient, light transmission, and bending strength of the cri¬ stobalite-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. Example 3 Cristobalite-contained silica glass was produced in the same way as in Example 1 with an exception that a mixture of crystalline silicon dioxide powder was prepared from lota CGU Fine and lota 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. Example 4 The cristobalite-contained silica glass produced in 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 iden¬ tified as α-cristobalite by the x ray diffraction analyzer. Example 5 Cristobalite-contained silica glass was produced in the same way as in 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 cristoba¬ lite-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 lota CGU Fine produced by Unimin Coφoration was charged in a silica glass tube inserted in a carbon made mold and the powder was heated in a nitrogen atmosphere from room tempera¬ ture to 1000 β C in 90 min, from the 1000 β C to 1200 β C in 30 min, from the 1200 β C to 1630 °C in 90 min, and from the 1630 β C to 1720 β 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 expan¬ sion coefficient, light transmission, and bending strength of the opaque silica glass thus produ¬ ced 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 lota 6 Fine produced by Unimin Coφoration was charged in a silica glass tube inserted in a carbon made mold and the powder was heated in a nitrogen atmosphere from room tempera¬ ture to 1000 °C in 90 min, from the 1000 °C to 1200 β 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 expan¬ sion coefficient, light transmission, and bending strength of the opaque silica glass thus produ¬ ced 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 pro¬ duced 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 re¬ sult of an x ray diffraction analysis. Tests on thin films on silica glass made Has Thin films were coated on disks made of the crystal-contained silica glasses produced in Ex¬ amples 1 to 5 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 and comparative examples are respectively numbered as 6 to 10 and 4 to 6. i ) A polysiiicon film of 50 μm thick was vapor-deposited from SiH 4 as a raw material at 600 β 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 sili¬ con 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 cry¬ stal-contained silica glass respectively produced in 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 Examples 1 and 2 ( which are respectively expres¬ sed with Examples 6 and 7 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 sur¬ faces 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. Table 1 Table 1 ( Continued ) Table 2 Com Ex 4 Cam Ex 5 Cam Ex 6 i Poly Si fil l peeled peeled peeled ii Si3N4 film peeled peeled peeled Table 2 { Continued ) iii SiC film glass deformed glass deformed!glass deformed no vapor Depo no vapor Depo no vapor Depo applicable applicable applicable iv BN film glass deformed glass deformediglass deformed no vapor Depo no vapor Depo no vegpor Depo applicable applicable applicable Table 3

Claims: Claims 1. 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. 2. Cristobalite-contained silica glass claimed in claim 1 wherein small-sized independent bubbles are additionally dispersed in the silica glass matrix. 3. Cristobalite-contained silica glass claimed in claim 2 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 . 4. Cristobalite-contained silica glass claimed in claim 3 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. 5. Cristobalite-contained silica glass claimed in any of claims 1 to 4 wherein the silica glass has a coefficient of thermal expansion of 1 x 10* C or more. 6. Cristobalite-contained silica glass claimed in any of claims 2 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. 7. A method of producing cristobalite-contained silica glass wherein two kinds or more of crystalline silicon dioxide powder the melting points of which are different by 20 °C or more from each other are used to prepare a mixture thereof, the silicon dioxide having the highest melting point is contained in the mixture at a content of 10 wt % to 80 wt %, and the mixture is heated at a temperature in the range of the lowest melting point of the ingredients to a temperature lower than the highest melting point thereof. 8. A method of producing cristobalite-contained silica glass claimed in claim 7 wherein the mixture is heated at a first heating speed in the range of 10 °C/min to 50 °C/min from room temperature up to 1000 °C and further at a second heating speed of 10 °C/min or less from the 1000 °C up to a temperature between the lowest melting temperature of the ingredients and 10 °C higher the lowest melting point, and still further at a third slow heating speed up to a temperature lower than the highest melting point of the ingredients. 9. A method of producing cristobalite-contained silica glass claimed in claims 7 or 8 wherein a particle diameter of the crystalline silicon dioxide powder is in the range of 10 μm to 1000 μm. 10. A method of producing cristobalite-contained silica glass claimed in claims 7 or 8 wherein the mixture is heated in an atmosphere of nitrogen flowing at a flow rate in the range of 5 l/min to 20 l/min so that small-sized independent bubbles dispersed are obtained in the silica glass matrix. 11. 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, and the surface of the jig is covered with a vapor-deposited thin film made of metal or ceramic. 12. A silica glass jig claimed in claim 11 wherein small-sized independent bubbles are additionally dispersed in the silica glass matrix. 13. A silica glass jig claimed in claim 12 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 . 14. A silica glass jig claimed in claim 13 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. 15. A silica glass jig claimed in any of claims 11 to 14 wherein the silica glass has a thermal expansion coefficient of 1 x lO^ C or more. 16. A silica glass jig claimed in any of claims 12 to 14 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. 17. A silica glass jig claimed in any of claims 11 to 14 and 16 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 % and a thermal expansion coefficient of the silica glass is 1 x IO^ C or more. 18. A silica glass jig claimed in claim 15 wherein the silica glass has the 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 %. 19. A silica glass jig claimed in any of claims 11 to 18 wherein the surface of the silica glass jig is covered with a thin film having a thermal expansion coefficient between 1 x lO^ C to 1 x lO ' V'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
Applicants: ENDO MAMORU, HERAEUS QUARZGLAS, INAKI KYOICHI, SEGAWA TOHRU, SHINETSU QUARTZ PROD, YOSHIDA NOBUMASA, ENDO, MAMORU, HERAEUS QUARZGLAS GMBH, INAKI, KYOICHI, SEGAWA, TOHRU, SHIN-ETSU QUARTZ PRODUCTS CO., LTD., YOSHIDA, NOBUMASA
Application year: 1996
Main classification: C03C14
IPC codes: C03B20, C03B32, C03C3, C03C14