Ceramics-type vacuum vessel and a method of manufacturing thereof

Abstract: A vacuum vessel is provided in which the main portion of a wall including an annular wall portion (1) and a plate-wall portion (2) is formed of ceramics such as silicon nitride. Because the generation of gas such as hydrogen is reduced from the wall of the ceramics vessel, extremely high vacuum can be generated in the interior of the vacuum vessel. Also, because the wall of the vacuum vessel has a high permeability with respect to a magnetic field and an electric field, the vacuum vessel can he applied to a particle accelerator as a vessel that can control in high precision charged particles therein with an electromagnetic field.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a vacuum vessel suitable for obtaining ultrahigh vacuum or extremely high vacuum needed in semiconductor manufacturing apparatus and particle accelerators, and a method of manufacturing thereof. Description of the Related Art In manufacturing a semiconductor device having its integration density drastically increased, even a minute defect at the time of a thin film formation process will result in a definite damage in the performance of the device. Therefore, the need arises of an extremely high vacuum which is higher than an ultrahigh vacuum of pressure in a thin film deposition apparatus for the purpose of preventing contamination in the grating lattice due to foreign elements as well as introduction of a minute dust which may cause defect. Realization of an extremely high vacuum is indispensable not only in the semiconductor field but also in the field of particle accelerators used in nuclear fusion reactors for the purpose of maintaining a long lifetime of accelerated particles. A research for achieving extremely high vacuum is under study in various fields. In order to produce ultrahigh vacuum or extremely high vacuum, an evacuating system that can achieve a lower pressure and that has a large exhaust capacity is required. Suppressing the generation of gas from the inner wall of a vacuum vessel and prevention of leakage from the joint portion of the vacuum vessel are particularly important factors for attaining ultrahigh vacuum or extremely high vacuum. The wall of a conventional vacuum vessel was formed of stainless steel or aluminium alloy. A vacuum vessel formed mainly of such materials exhibited a great amount of gas generation from the surface and also from the inside of the wall during evacuation. The main component of the generated gas is water in a relatively low vacuum level where baking is not carried out, and is hydrogen when baking is carried out and water removed. Although the amount of gas generation can be reduced by raising the baking temperature, the baking temperature of a metal vessel is limited to approximately 300°C. It was therefore considered impossible to completely suppress gas generation by baking. Various methods for suppressing gas generation other than by baking are considered, such as using stainless steel of low hydrogen occlusion manufactured by dissolving a metal material of low impurities under vacuum, processing the inner wall of aluminium alloy by discharging in a mixed gas of argon and oxygen to form an oxide film, or a combination of these methods and applying a mirror-finishing process to the inner wall formed of stainless steel or aluminium alloy. The gas generation can be reduced considerably by combining these methods and baking. It has been reported that an extremely high vacuum on the order of 1.3·10⁻¹¹ Pa (10⁻¹³Torr) was obtained with a vacuum vessel made of stainless steel or aluminium alloy. However, generation of hydrogen gas was exhibited from the wall of such vessels, so that the eventually obtained vacuum pressure was limited by the hydrogen gas. The development of a vacuum vessel with extremely low gas generation is desired. In the field of a particle accelerator, an electric field or a magnetic field is applied in the vacuum vessel for controlling the motion of the charged particles. In the present status where the coil for generating an electromagnetic field is provided outside of the vacuum vessel, a vacuum vessel formed of either stainless steel or aluminium alloy which is reliable of shielding magnetic field and magnetic field has the problem of disabling the control of the accelerated particles at high precision. It was impossible to form a vacuum vessel accommodating a coil to solve this problem because of limitations associated with materials and shapes of the vessel. An approach can be considered to use a vacuum vessel made of glass that has a low hydrogen occlusion and that easily passes electric field and magnetic field for the precise control of accelerated particles. However, reliability with respect to weight exerted on the wall of the vessel during evacuation is low because the strength of glass is low and easily broken. Furthermore, glass begins to soften at the time of baking, or may crack on account of thermal stress caused by nonuniformity of the baking temperature, and is therefore not practical for usage. US-A-4 712 074 discloses a vacuum chamber for containing particle beams comprising a ceramic pipe wherein the basic mechanical structure for the vacuum chamber is fabricated from alumina of 94 to 99 % purity (column 3, lines 35-36). In contrast, present claim 1 excludes the use of aluminium oxide. EP-A-0 415 398 relates to a ceramic electric-discharge lamp including a vacuum vessel for maintaining vacuum in the interior of the lamp wherein the main portion for maintaining a vacuum consists essentially of ceramics. However, the vacuum vessel does not comprise a plurality of members and a connection layer provided between said faces of said plurality of members. The publication Salmang H. and Scholze H. "Keramik, Teil 1 : Allgemeine Grundlagen und wichtige Eingenschaften", New York 1982, provides background information about ceramic including a definition of ceramics. EP-A-0 334 000 describes a microwave plasma chemical deposition process for the production of a film containing mainly silicon and/or other group IV elements. A bell jar for use in a plasma treating apparatus is described, the bell jar may consist of ceramics including Al₂O₃. FR-A-2 595 876 discloses a tube for laser generation, the tube consists of material on the basis of aluminium nitride. SUMMARY OF THE INVENTION An object of the present invention is to provide a vacuum vessel reduced in generation of gas such as hydrogen which causes a rise in vacuum pressure. Another object of the present invention is to provide a vacuum vessel for a particle accelerator having sufficient mechanical strength and that can control acceleration of charged particles at high precision. A further object of the present invention is to provide a method of manufacturing a vacuum vessel applicable to manufacturing apparatus of a semiconductor device and particle accelerators. According to the present invention, a vacuum vessel is provided for maintaining vacuum space in the interior, having the features of claim 1. In the present invention, the main portion for holding vacuum space includes a wall forming a vacuum vessel. The vessel according to the present invention can have the joint portion for connecting, for example, an evacuating system or the portion for providing accessories such as a vacuum gauge or a window, formed of a material other than ceramics, such as metal of stainless steel and aluminium alloy. According to the present invention, ceramics includes oxide based ceramics such as, mullite, and partially stabilized zirconia, and non-oxide ceramics such as silicon nitride (Si₃N₄) and silicon carbide (SiC). Although aluminium oxide can be considered as the ceramics, aluminium oxide has relatively low strength and toughness in ordinary temperature, and a relatively high coefficient of thermal expansion of approximately 7 x 10⁻⁶/K. Therefore, aluminium oxide is not so suitable for manufacturing a large vacuum vessel for a particle accelerator that carries out baking at the time of usage. From the standpoint of strength and coefficient of thermal expansion in ordinary and high temperature, silicon nitride is most preferable for the formation of a vacuum vessel in the present invention. The main portion for holding vacuum space such as the wall, the inner wall in particular, of the vacuum vessel consists of ceramics having a strength significantly greater than that of glass at ordinary and high temperature, and that has an amount of gas generation such as hydrogen significantly lower than that of a metal such as stainless steel and aluminium alloy during production of vacuum. The main portion formed of ceramics can be baked at a temperature higher than that of a conventional one. Because ceramics has a high permeability of electric field and magnetic field, accelerated particles can be controlled at high precision when a ceramics-type vacuum vessel is used as a particle accelerator. An arbitrary electric field and/or magnetic field can be applied within the vessel. The vacuum vessel of the present invention is applicable for producing an ultrahigh vacuum (10⁻⁸-10⁻⁶Pa) or an extremely high vacuum (at most 10⁻⁸Pa). According to another aspect of the present invention, a method of manufacturing a vacuum vessel using the step of claim 8 is provided. According to this method, a plurality of members consisting essentially of ceramics and having bonding surfaces of a flatness of not more than 1µm are prepared. Ceramics powder having an average particle diameter of not more than 1µm is sandwiched between the faces of the plurality of members to be subjected to a heating process for connecting the plurality of members. The faces between each of the plurality of members are strongly adhered to each other by the heating process. The flatness of not more than 1µm used here means that the degree of undulation and unevenness of the finished surface is within 1µm in the entire bonding surface. For example, a bonding surface having a flatness of not more than 1 µm means that the bonding surface exists between two parallel planes not more than 1 µm apart from each other, according to Japanese Industrial Standard B 0021 (1984). In manufacturing a ceramics-type vacuum vessel, a method of bonding ceramics portions of a simple configuration formed by sintering is effective because ceramice can not be easily formed in a compact of a complex configuration with a high cost for work after sintering. A method of using glass having a coefficient of thermal expansion approximating that of the ceramics matrix to be bonded is known as one method of bonding ceramics portions to each other. However, the bonding strength according to this method is low and is at most, 100MPa. Therefore, the bonded portion is easily separated on account of the thermal stress due to a slight difference in coefficients of thermal expansion with the matrix at the time of bonding or baking. The method according to the present invention includes the steps of forming a plurality of ceramics components implementing the wall of a vacuum vessel by a normal sintering method, interposing ceramics powder formed of ultrafine particles having an average particle diameter of not more than 1µm, preferably not more than 0.5µm between the surfaces of each of the plurality of ceramics components, and applying a heating process to bond them to each other. According to this method, the interlayer between the surfaces of bonding components can he reduced significantly in thickness because ultrafine particles are used. Because generation of a bonding layer having a different coefficient of thermal expansion can be suppressed significantly, and because a strong bond can be obtained by reaction between the ceramics portions and the particles, the bonded portion will not be separated even if baking is repeatedly carried out during the usage of the vessel. Ceramics powder having an average particle diameter greater than 1µm is reduced in reactivity, so that a sufficient bonding strength can not be achieved. Furthermore, if a particle having a particle diameter greater than 1µm is used, a gap will remain in the bonded portion, leading to a possibility of leakage. Ceramics powder for forming an interlayer of the bonding portion may be of a single substance or a mixed powder of a plurality of substances as long as it has a high reactivity and wettability with respect to the ceramics forming the vessel component and can form a bonding layer of high strength by reaction. For example, for a vessel component formed of Si₃N₄, powder constituted of only Aℓ₂O₃ or a mixed powder is preferably used such as Y₂O₃-Aℓ₂O₃-SiO₂ or Si₃N₄-Y₂O₃-Aℓ₂O₃-SiO₂ which is a component similar to a grain boundary to be formed by sintering. When the vessel component is formed of non-oxide ceramics, various sintering aids are added into the ceramics material for manufacturing the vessel components. In this case, it is necessary to select the substance of the ceramic powder taking into consideration the components of the sintering aids. In bonding using ultrafine particles of ceramics powder according to the present invention, a gap may remain in the bonding portion to cause leakage if the smoothness of the surface of the bonding component is low because the gap will not be easily filled by fusion such as in the case of glass. In order to achieve bonding with no leakage, it is necessary to set the average particle diameter of the ultrafine particles to not more than 1µm as described above, as well as applying a finishing process to the surface of the component to be bonded in high precision to have a flatness of not more than 1µm, preferably not more than 0.5µm. A typical working method for finishing the surface in high precision is an abrasion work and the like using a lapping machine of high precision. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of a ceramics-type vacuum vessel according to an embodiment of the present invention. Fig. 2 is a side view of a ceramics-type vacuum vessel according to another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Referring to Fig. 1, by sintering Si₃N₄ powder using Y₂O₃-Aℓ₂O₃ as a sintering aid, an annular wall portion 1 of a right circular cylinder configuration of 200mm in outer diameter x 180mm in inner diameter x 600mm in length and having both ends open, and a disk-type plate-wall portion 2 of 200mm in diameter x 5mm in thickness having two holes of 40mm in diameter therein were formed. The bonding surface of one end (annular side face having a width of 20mm) of the annular wall portion 1 constituted of Si₃N₄ sintered body and the bonding surface of the plate-wall portion 2 (the outer peripheral portion of the surface having a width of 20mm) were processed to have a flatness of not more than 0.5µm by a lapping process of diamond abrasive grains. Then, Aℓ₂O₃ ultrafine particle powder having an average particle diameter of 0.07µm was interposed between the bonding surfaces of the annular wall portion 1 and the plate-wall portion 2 to be subjected to a heating process for one hour at 1750°C in nitrogen atmosphere for preliminary bonding. Then, an HIP (Hot Isostatic Pressing) process was applied for one hour at 1700°C in a nitrogen gas of 1000 atmospheres to completely bond the plate-wall portion 2 to one end of the annular wall portion 1. The obtained bonding strength was not less than 700MPa according to another model test carried out which is a value approximating that of the matrix. This value is drastically higher than the value of 50MPa in the case where sealing glass is used. Next, a stainless steel flange 3 having an inner diameter of 180mm was bonded to the other end of the annular wall portion 1 having an annular side face, and stainless steel flanges 4 and 5 respectively having an inner diameter of 40mm were bonded around the two holes of the plate-wall portion 2 in communication respectively to obtain a ceramics-type vacuum vessel. Each of flanges 3-5 wad formed of clean stainless steel obtained by being dissolved under vacuum. The flanges have a structure such that the exposed area in the interior of the vacuum vessel is as small as possible at the bonding portion. Furthermore, oxidation or the surface of the flange was carried out to reduce generation or hydrogen. The bonding of flanges 3, 4, and 5 with the annular wall portion 1 and the plate-wall portion 2 was carried out by interposing a layer including Ni allowing plastic deformation for reducing thermal stress between the surfaces, followed by brazing using silver-copper brazing alloy containing titanium. A titanium sublimation pump with two stages of molecular pumps as auxiliary pumps of an evacuating system was connected to the flange 3 of the obtained vacuum vessel. The vessel of the titanium sublimation pump was formed of clean stainless steel obtained by being dissolved under vacuum, and the inner wall thereof was mirror-finished by an electrolytic process, followed by an oxidation process. An extractor type vacuum gauge and a quadrupole mass spectrometer were connected to flanges 4 and 5, respectively, to complete a vacuum system. The entire vacuum system was baked for ten hours at 300°C. After cooling, the titanium sublimation pump was actuated and the pressure and the composition of the remaining gas were measured. Also, a leakage test was carried out with a He leak detector. For the purpose of comparison, a vacuum system having a structure similar to the above-described vacuum system was provided using a vacuum vessel having a structure similar to that of the above-described vacuum vessel and formed of clean stainless steel obtained by being dissolved under vacuum instead of the Si₃N₄ sintered body Then, similar tests were carried out. The results are shown in the following Table 1. It can be appreciated from the above Table 1 that the vacuum vessel having the wall formed of Si₃N₄ sintered body according to the present embodiment has the generation of hydrogen greatly reduced to obtain a lower achieved pressure in comparison with a conventional vacuum vessel having the wall formed of clean stainless steel. To examine the reliability of the vacuum vessel of the present embodiment, particularly the reliability of the bonded portion, the baking process was repeated for ten times, However, no leakage was observed, and only a tendency of a slight decrease in achieved pressure was seen. The reason why hydrogen accounts for the greatest amount in the remaining gas even in the vacuum vessel having the wall formed of Si₃N₄ sintered body may be due to the existence of the stainless steel portion remaining in the inner wall even though the area is small. Also, the reason why a definite proportional relationship between the measured values of the achieved pressure and the quadrupole mass spectrometer is not observed may be due to the linearity destroyed because of approximating the measurement limit of the vacuum gauge. Embodiment 2 Referring to Fig. 2, an annular wall portion 1 and a plate-wall portion 2 similar to those of the first embodiment were provided. Similarly, cylindrical portions 6 and 7 of a right circular column of Si₃N₄ sintered body having 45mm in outer diameter x 40mm in inner diameter x 100mm in length and having both ends open were formed. As in the first embodiment, the annular wall portion 1 and the plate-wall portion 2 were bonded. Simultaneously, the surroundings of each of the two holes in the plate-wall portion 2 and the other end side surfaces of cylinders 6 and 7 were finished to have a flatness of 0.3µm respectively. They were bonded together as in the first embodiment using Aℓ₂O₃ ultrafine particle having an average diameter of 0.07µm. A stainless steel flange 3 identical to that in the first embodiment was bonded to the other end of the annular wall portion 1, and stainless flanges 4 and 5 identical to those in the first embodiment were bonded to the one end side surfaces of cylinders 6 and 7, respectively, by interposing Ni therebetween, respectively, and by using silver-copper brazing alloy containing titanium as in the first embodiment to form a vacuum vessel. Furthermore, as in the first embodiment, a titanium sublimation pump, an extractor type vacuum gauge, and a quadrupole mass spectrometer were connected to flanges 3, 4 and 5, respectively to complete a vacuum system. Cylinders 6 and 7 and flanges 4 and 5 were cooled at 300°C for protecting the vacuum gauge and the mass spectrometer, while the vacuum vessel was baked for ten hours at 600°C. Then, the entire vacuum system was cooled after the baking process, and tests similar to those of the first embodiment were carried out. The results are shown in Table 2. It can be appreciated from the second embodiment that the achieved pressure and the relative intensity of the mass spectrometer are reduced significantly as a result of baking at a high temperature in comparison with the first embodiment. According to the present invention, a high vacuum vessel can be provided that has sufficient mechanical strength in ordinary and high temperature, that has the amount of gas generation greatly reduced such as hydrogen causing a rise in the vacuum achieved pressure, and that has high reliability with respect to a repetitive baking process for preventing gas generation. The vacuum vessel has an achieved pressure lower than that of a vacuum vessel formed of stainless steel or aluminium alloy, and can attain extremely high vacuum using an evacuating system of high performance to be applicable to fields such as of semiconductor manufacturing apparatus. The vacuum vessel has a high permeability of electric field and an magnetic field in addition to a low achieved pressure. Therefore, the vacuum vessel is also applicable as a vacuum vessel that can control accurately charged particles by an externally provided coil in the field of particle accelerators.

Claims: Vacuum vessel made out of members of ceramic material being assembled by a connection material characterized in that the plurality of members is formed essentially of ceramic material other than aluminium oxide, said members having bonding surfaces of not more than 1µm in surface flatness, whereby the undulation and the unevenness of each of the said bonding surfaces is such that the bonding surface exists between two parallel planes not more than 1 µm apart from each other and that the connection material is formed of a layer of sintered ceramics powder provided between said bonding surfaces of said plurality of members, said sintered ceramic powder having an average particle diameter of not more than 1µm. Vacuum vessel according to claim 1, wherein said plurality of members consist essentially of silicon nitride. Vacuum vessel according to claim 1, said vacuum vessel maintaining vacuum that is not more than 10⁻⁶Pa in pressure. Vacuum vessel according to claim 1, said vacuum vessel applying at least an electric field or a magnetic field to the vacuum space. Vacuum vessel according to claim 1, said vacuum vessel forming a portion of a particle accelerator. Vacuum vessel according to claim 1, forming a portion of a semiconductor device manufacturing apparatus. Vacuum vessel according to claim 1, wherein the main portion for maintaining vacuum has a structure of a plurality of members essentially formed of ceramics bonded together. Method of manufacturing a vacuum vessel characterized by the steps of preparing a member including a plurality of members formed essentially of ceramics and having bonding surfaces of not more than 1µm in surface flatness, whereby the undulation and the unevenness of each of the said bonding surfaces is such that the bonding surface exists between two parallel planes not more than 1 µm apart from each other and interposing ceramics powder having an average particle diameter of not more than 1µm between said bonding surfaces of said plurality of members, and carrying out a heating process to bond said plurality of members. Method according to claim 8, wherein said plurality of members is formed essentially of silicon nitride. Method according to claim 8, wherein said ceramics powder is essentially formed of at least a material selected from the group consisting of Al₂O₃, Y₂O₃, SiO₂, and Si₃N₄. Method according to claim 8, wherein the surface flatness of said bonding surfaces is not more than 0.5µm, and the average particle diameter of said ceramics powder is not more than 0.5µm.

Inventors: ABE TOTSUYA, MIYAKE MASAYA, MURAKAMI YOSHIO, TAKEUCHI HISAO, YAMAKAWA AKIRA, ABE, TOTSUYA, MIYAKE, MASAYA, MURAKAMI, YOSHIO, TAKEUCHI, HISAO, YAMAKAWA, AKIRA, Abe, Totsuya, c/o Naka Res. Lab. of JAPAN ATOMIC, MIYAKE, MASAYA, C/O ITAMI WORKS OF SUMITOMO, Murakami, Yoshio, c/o Naka Res.Lab.of JAPAN ATOMIC, Takeuchi, Hisao, c/o Itami Works of Sumitomo, YAMAKAWA, AKIRA, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1992
Main classification: H05H7
IPC codes: F16J12, C04B37, H05H7