An ultra-high vacuum apparatus

Abstract: A vacuum container is evacuated to be kept at an ultra-high vacuum. Valve means is provided for connecting and disconnecting the inside of the vacuum container with and from the outside thereof, and includes a vacuum-tight seal made of a high polymer having a water absorptivity of 0.1 % and less and an outgassing rate of 2.5x10⁻⁴ (Pa·ℓ/sec/cm² :at 200 °C) and less.

Description: BACKGROUND OF THE INVENTION This invention relates to an ultra-high vacuum apparatus, and more particularly to an ultra-high vacuum apparatus suited for use in a scanning electron microscope of field emission type and a vacuum-tight seal for use in such an apparatus. In a scanning electron microscope, it is general to use an electron gun of field emission type rather than that of thermionic emission type in order to obtain an electron beam with which a sample or specimen is irradiated. This is because an electron current density in the field emission is remarkably higher than that in the thermionic emission and the diameter of an electron beam source is extremely smaller in the field emission than in the thermionic emission. However, an ultra-high vacuum is required for obtaining a stable field emission current. Though a degree of vacuum of 10 -5 Pa and less is generally called an ultra-high vacuum, an ultra-high vacuum of 10 -7 Pa, and less, and typically 10 -7 Pa to 10 -8 Pa, is indispensable for obtaining a stable field emission current. According to a scanning electron microscope of field emission type, an electron gun chamber in which a field emission gun is arranged is evacuated by a vacuum pump such as a rotary pump through a specimen chamber in which a specimen is disposed and then is continued to be evacuated by a vacuum pump such as an ion pump, whereby the electron gun chamber is maintained at an ultra-high vacuum of the order of 10 -7 Pa to 10 -8 Pa, , while the specimen chamber is kept at 10 -6 Pa and more, and typically 10 -3 Pa to 10 -4 Pa. When the specimen is exchanged, the specimen chamber is made open to the atmosphere to replace the old specimen by a new one. In this case, it is desirable to isolate the electron gun chamber from the specimen chamber in vacuum continuum to prevent the electron gun chamber from being vacuum-broken. Therefore, a valve means is provided for connecting and disconnecting the electron gun chamber with and from the specimen chamber each time the specimen is exchanged. The valve means comprises a vacuum-tight seal, an opening for communicating the electron gun chamber and the specimen chamber with each other, and a manipulator for bringing the vacuum-tight seal into close contact with an end surface of the opening so as to close the same and for releasing the close contact between the vacuum-tight seal and the opening end surface so as to open the opening. Walls of the electron gun chamber and the specimen chamber are made of a metal such as stainless steel, including a member forming the opening. The vacuum-tight seal, however, is generally made of a material other than a metal to maintain the close contact between the vacuum-tight seal and the opening end surface even where the opening is closed and opened repeatedly. On the other hand, in advance of assembling an ultra-high vacuum apparatus, components thereof are individually baked, including the vacuum-tight seal. The apparatus is assembled with the baked components and then further baked in advance of formal specimen observations. The baking treatment is normally carried out at a high temperature of about 250 °C to 400 °C for outgassing, i. e. degassing the surface of the apparatus exposed to the vacuum atmosphere therein. Thus, the vacuum-tight seal should be made of a material resisting such a high temperature. In view of the foregoing, the vacuum-tight seal is generally made of polyimide or fluorocarbon polymer. Examples employing these materials for a vacuum-tight seal is disclosed in Japanese laid-open Nos. 153970/1991 and 234973/1991. However, the outgassing of polyimide or fluorocarbon polymer is larger than the outgassing of other structural materials of the ultra-high vacuum apparatus such as metallic materials like stainless steel. Comparing outgassing rate (Pa · 1/sec/cm 2 at 200 °C) under an identical condition, it is known that the outgassing rate of polyimide is approximately 1000 times as large as that of stainless steel. Therefore, employing polyimide for a vacuum-tight seal increases the baking time and the ultimate pressure. An ultra-high vacuum apparatus comprising the feature of the pre-characterising part of claim 1 is known from WO-A-91 08 412. The valve means of this prior art apparatus includes an elastomeric vacuum-tight seal. Further, a totally-aromatic-liquid-crystal-polyester is known under the trade name "EKONOL" from Sumitomo Chemicals Co. and the trade name "XYDAR TM " from Nippon Petrochemical Co.. This polyester has the following molecular structure formula: Such a substance is also employed in combination with a spherical glassy carbon as member for scroll type compressors or vacuum pumps in US-A-5 124 397. If this prior art sealing member is used in an ultra-high vacuum apparatus, however, a degradation performance is turned out due to the compressive stress occurring each time the vacuum is offset. SUMMARY OF THE INVENTION It is the object of the present invention to provide an ultra-high vacuum apparatus which is capable of shortening a baking time for outgassing treatment and/or lowering an ultimate pressure, wherein no degradation of the ultra-high vacuum occurs due to the repeated change between a high and a low vacuum condition in the apparatus, and a vacuum tight seal therefor. This object is met by an apparatus according to claim 1 and a seal according to claim 6. Preferred embodiments are disclosed in the depending claims. The vacuum-tight seal according to the invention is suitable for a ultra-high vacuum apparatus and has at least a coating portion which has a water absorptivity of 0.1 % and less and/or an outgassing of 2.5 x 10 -4 Pa·ℓ/sec/cm 2 :at 200°C and less. The foregoing and other objects, features and advantages of the present invention will become apparent from the description of preferred embodiments of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a schematic pictorial view, partly in section, of an ultra-high vacuum apparatus showing an embodiment according to the present invention. FIG.2. is a graph showing a relationship between thermal deformation temperatures and water absorptivities for various vacuum-tight seal materials. FIG.3 is a graph showing a relationship between water absorptivities and outgassing rates for various vacuum-tight seal materials. FIG.4 is a graph showing relationship between temperatures and outgassing rates for various vacuum-tight seal materials. FIG.5 is a graph comparing ultimate vacuums for vacuum-tight seal materials having different water absorptivities. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, a vacuum container 1 is constituted by a first vacuum container 2 to which a vacuum pump such as an ion pump 4 is connected and a second vacuum container 3 to which a vacuum pump 5 such as a rotary pump is connected, the first and second vacuum containers 2 and 3 being communicated with each other through an opening 14 of a wall partitioning the vacuum container 1 into the first and second vacuum containers 2 and 3. Valve means is provided which comprises a rod 6 being in threaded-engagement with the upper wall of the first vacuum container 2 so as to extend from the outside of the first vacuum container 2 to the inside thereof, a handle 7 secured at one end of the rod 6, a valve body 9 secured at the other end of the rod 6, a bellows 8 fixed between the valve body 9 and the upper wall of the first vacuum container 2 in a vacuum-tight manner, and a vacuum-tight seal 10 mounted on the lower end surface of the valve body 9 with a stopper screwed into the lower end of the valve body 9 at the central portion thereof. The bellows 8 serves to convert rotation of the handle 7 and hence the rod 6 into rectilinear motion of the valve body 9 in an axial direction of the rod 6 while preventing the valve body 9 from rotating, whereby the vacuum-tight seal 10 is brought into close contact with an end surface 13 of the opening 14 which functions as a valve seat and released from the close contact with the valve seat 13 so as to close and open the opening 14. The vacuum-tight seal 10 is of a ring shape and is provided with a substrate having a circular section and a coat formed thereon. The substrate may be made of a metal or high polymer resisting a high temperature of about 250 °C and more, while the coat is made of a high polymer as described below in detail. The whole vacuum-tight seal 10 may be made of a high polymer as described below in detail without being divided into the substrate 11 and the coat 12 as shown in Fig. 1. When the opening is opened, the insides of the first and second vacuum containers 2 and 3 are evacuated by the vacuum pump 5. When the insides of both the vacuum containers reach a predetermined degree of vacuum, the first vacuum container 2 is evacuated by the vacuum pump 4 in addition to the vacuum pump 5 so that the first vacuum container 2 is kept at an ultra-high vacuum and the second vacuum container 3 is kept at a degree of vacuum lower than the first vacuum container 2. The first vacuum container 2 may be an electron gun chamber in which an electron gun of field emission type is arranged, while the second vacuum container 3 may be a specimen chamber in which a specimen to be irradiated with a field-emitted electron beam is disposed for scanning electron microscopic observations. In this case, the inside of the first vacuum container 2 is maintained at a degree of vacuum of the order of 10 -7 Pa to 10 -8 Pa , while the second vacuum container 3 is maintained at a degree of vacuum of 10 -6 Pa and more, and typically 10 -3 Pa to 10 -4 Pa. If the second vacuum container 3 is made open to the atmosphere after the opening 14 is closed, the observed specimen can be exchanged for a new one. After the completion of the specimen exchange, the second vacuum container 3 is closed and then evacuated. When the inside of the second vacuum container 3 become to maintained at the original degree of vacuum, the opening 14 is opened. A specimen exchanging device is omitted in Fig. 1 because it is well known. A scanning electron microscope of field emission type is well known and thus it is also neglected in Fig. 1. The inventors have found through their experimental study the fact that, if the whole vacuum-tight seal or the coat thereof is made of a high polymer having a high polymer having a water absorptivity of 0.1 % and less and/or an outgassing 2.5x10 -4 (Pa · ℓ/sec/cm 2 : at 200 °C) and less, an ultimate pressure can be lowered and a time to achieve the ultimate pressure can be shortened. In addition, it has also been confirmed that, taking thermal resistivity, workability and durability into consideration, totally-aromatic-liquid-crystal-polyester is especially effective for the foregoing high polymer. Totally-aromatic-liquid-crystal-polyester is on the market as the trade name "EKONOL®" from Sumitomo Chemicals Co. and the trade name "XYDAR™" from Nippon Petrochemical Co., and has the following molecular structure formula: Turning again to Fig.1, the ultimate pressure P in the vacuum container 2 is expressed by the following equation, where S is the total pumping speed of the vacuum pumps 4 and 5, Q1 is an outgassing rate from the surface of the vacuum-tight seal 10 exposed to the vacuum, and Q2 is the total outgassing rate from the surfaces exposed to the vacuum other than the surface of the vacuum-tight seal 10, respectively: P = ( Q1 + Q2 ) / S Therein, the outgassing rates Q2 is negligible when the vacuum chambers 11, 12, etc. are sufficiently outgassed by baking treatment. Consequently, the ultimate pressure P is proportional to the outgassing rate Q1 of the vacuum-tight seal 10. FIG.2 is a graph showing a relationship between thermal deformation temperatures and water absorptivities of various vacuum-tight seal materials, i. e. sealants, wherein both the letters A and B indicate the EKONOL and the XYDAR, respectively, the letter C fluorocarbon resin containing filler, the letter D fluorocarbon resin without filler, and the letter E polyimide. It is understood that the fluorine-contained resin is of low water absorptivity and is effective for a sealant unless the thermal deformation temperature is considered. Considering thermal deformation temperatures (the test method thereof is of ASTM D648-82), the fluorine-contained resin may be excluded from the promising sealant, and the totally-aromatic-liquid-crystal-polyester may be suitable for the ultra-high vacuum sealant. Further, the totally-aromatic-liquid-crystal-polyester has a low water absorptivity (less than 0.1%) as compared with the polyimide E (0.7%), and is confirmed to be effective for the ultra-high vacuum sealant. The water absorptivity is measured after 24 hours of immersing the sealant into distilled water held at 23 °C as prescribed by ASTM D570. It is desirable that the water absorptivity measured thus is kept substantially unchanged after measured. Totally-aromatic-liquid-crystal-polymer is advantageous in that the absorptivity thereof is substantially stable after measured. FIG.3 is a graph showing a relationship between water absorptivities and outgassing rates of sealants, wherein the letters E1 and A1 indicate the outgassing rates of polyimide (water absorptivity is 0.7%) and totally-aromatic-liquid-crystal-polyester (water absorptivity is less than 0.1%) used for sealants respectively in an initial, or first, baking treatments for outgassing at a temperature of 200°C, and the letters E2 and A2 indicate the outgassing rates of polyamide and totally-aromatic-liquid-crystal-polyester respectively at a temperature of 200°C in the successive, or second, baking treatments after the initial baking followed by breaking the vacuum to the atmospheric pressure. It is apparent from the figure that the reductions in outgassing rates in the second baking treatments against in the first depend on water absorptivities and that the magnitude of reduction in outgassing rate in the second baking treatment for totally-aromatic-liquid-crystal-polyester (approximately 2.5x10 -4 Pa·ℓ/cm 2 :at 200°C), which has a low water absorptivity, is larger than the amount of reduction for polyimide which has a high water absorptivity. This means that performing baking treatments with the components of an apparatus apiece in advance and rebaking treatment with the apparatus after assembling thereof substantially reduces the baking treatment time at the time of operating the apparatus; and that applying the sealant in accordance with the present invention to an ultra-high vacuum apparatus such as field emission type electron microscopes substantially improves operating efficiency thereof. The inside of the column of a field emission type electron microscope is classified into sections by vacuum levels for the purpose of use and a plurality of ultra-high-vacuum valves for pre-pumping are provided to attain each level of vacuum pressures. Therefore, introduction of the sealant in accordance with the present invention to the ultra-high vacuum valves improves working efficiency in baking outgassing treatments. FIG.4 is a graph showing a relationship between temperatures and outgassing rates for various kinds of vacuum sealants, wherein the letters E1, F1, A1 indicate outgassing rates of polyimide, stainless steal, totally-aromatic-liquid-crystal-polyester, respectively, used for sealants in the initial baking treatments, and the letters E2 and A2 indicate outgassing rates of polyimide and totally-aromatic-liquid-crystal-polyester respectively used for sealants in the successive, or second, baking treatments after the initial baking treatments followed by breaking a vacuum condition to the atmospheric pressure. According to the figure, the outgassing rates of totally-aromatic-liquid-crystal-polyester in the second baking treatment show substantially the same values as the outgassing rates of stainless steel. FIG.5 is a graph showing a comparison of ultimate vacuums obtained by substituting the outgassing rate for polyimide (B) or totally-aromatic-liquid-crystal-polyester (A) into the equation (1) described above, wherein both the ultimate pressure and the time to achieve ultra-high vacuum for totally-aromatic-liquid-crystal-polyester, having a low water absorptivity, are smaller than those for polyimide, and showing that employing totally-aromatic-liquid-crystal-polyester as sealants may shorten the baking treatment time for outgassing. According to the embodiments of the present invention, as described above, the following advantageous effects are obtained: (1) Baking treatment time can be shortened since the outgassing, or outgassing rate, of the sealant in accordance with the present invention is small. (2) The time to achieve an ultra-high-vacuum in operating an apparatus can substantially be reduce when baking treatments are performed with components thereof apiece in advance and again after assembling thereof. (3) An availability factor of an ultra-high-vacuum apparatus can substantially be improved since the baking treatment time to recover an ultra-high-vacuum condition after breaking a vacuum to the atmosphere pressure for changing specimens or the like is shortened by approximately 24 hours against the conventional treatment time. (4) Adhering totally-aromatic-liquid-crystal-polyester to the surfaces of substances, having high outgassing rates and thermal resistances, widens the field of applications thereof and makes it possible to construct seals in any shapes.

Claims: An ultra-high vacuum apparatus comprising a vacuum container (2), means (4, 5) for evacuating the container (2) so as to maintain the inside thereof at an ultra-high vacuum, and valve means (6-14) for connecting and disconnecting the inside of the container (2) with and from the outside, the valve means (6-14) including a vacuum-tight seal (10-12), characterised in that at least a coating portion (12) of said vacuum-tight seal (10-12) is made only of a totally-aromatic-liquid-crystal-polyester having a water absorptivity of 0.1 % or less and/or an outgassing rate of 2.5 x 10 -4 Pa.ℓ/s·cm -2 at 200°C or less. The apparatus according to claim 1, wherein said vacuum-tight seal (10-12) comprises a substrate (11) substantially made only of said totally-aromatic-liquid-crystal-polyester. The apparatus according to claim 1 or 2, wherein said totally-aromatic-liquid-crystal-polyester has the following structure formula: The apparatus according to any of claims 1 to 3, wherein said vacuum-tight seal (10-12) has been degassed by a baking treatment. The apparatus of claim 4, wherein said totally-aromatic-liquid-crystal-polyester has a thermal deformation temperature higher than the temperature of said baking treatment. A vacuum-tight seal for use in ultra-high vacuum, characterised by comprising a coating portion (12) made only of a totally-aromatic-liquid-crystal-polyester having a water absorptivity of 0.1 % or less and/or an outgassing rate of 2.5 x 10 -4 Pa·ℓ/s·cm -2 at 200°C or less. The seal according to claim 6, further comprising a substrate (11) substantially made only of said totally-aromatic-liquid-crystal polyester. The seal according to claim 6 or 7, wherein said totally-aromatic-liquid-crystal-polyester has the following structure formula: Use of the apparatus according to any of claims 1 to 5 or of the seal according to any of claims 6 to 8 in an ultra-high vacuum on the order of 10 -7 Pa and less.

Inventors: HIROKI MINORU, ISHIDA YASUHIKO, KIMURA HIDEITI, KOBARI TOSHIAKI, NAKAIZUMI YASUSHI, OKURA AKIMITSU, SATOU OSAMU, HIROKI, MINORU, ISHIDA, YASUHIKO, KIMURA, HIDEITI, KOBARI, TOSHIAKI, NAKAIZUMI, YASUSHI, OKURA, AKIMITSU, SATOU, OSAMU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: F16K25
IPC codes: F16K51, C09K3, F16J15