JPS6113114B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an exhaust system using turbo molecules in an electron microscope. In order to reduce contamination of samples in electron microscopes and the like, it is necessary to keep the inside of the microscope body clean (oil-free). For this purpose, instead of the conventionally commonly used oil diffusion pump,
Turbo molecular pump (hereinafter simply referred to as molecular pump)
It is possible to use However, since such molecular pumps in principle require a high-speed rotation mechanism, there is a risk that the resolution of the electron microscope will decrease due to the vibrations generated by the rotation mechanism, and sufficient countermeasures against vibrations are required. Become. For this purpose, it is conceivable to firmly fix the molecular pump to the floor. This method has the advantage that the molecular pump can be installed directly under the mirror, but it requires foundation work and is very difficult to handle, as foundation work must be done every time the mirror is installed. This has the disadvantage of being troublesome and increasing costs. In order to prevent this drawback, it is possible to install the molecular pump at a position quite far from the mirror, but this inevitably requires the use of a long exhaust pipe, which increases the exhaust resistance and increases the exhaust time. It has some drawbacks. However, as a result of various experiments to solve this problem, the inventor of the present invention decided to suspend the molecular pump from the main exhaust pipe fixed to the pedestal via a bellows, as shown in the accompanying drawing. We found that when supported, the vibrations of the molecular pump do not affect the resolution. Therefore, the features of the present invention include a mirror body, an exhaust pipe connected to the mirror body, a pedestal placed on the floor via a vibration isolator to hold the mirror body and the exhaust pipe, and the A molecular pump is provided for evacuating the inside of the mirror body through an exhaust pipe and a bellows, and the molecular pump is substantially suspended from the pedestal by the bellows and is made non-contact with the floor side. A weight corresponding to the back pressure that the molecular pump receives by creating a vacuum inside the mirror body is attached to the molecular pump. Hereinafter, the present invention will be explained in detail based on the drawings. The attached drawing is a schematic diagram showing an embodiment of the present invention, and numeral 1 indicates a mirror body of a transmission electron microscope, and the mirror body is attached to a pedestal 2. The frame 2 has four columns 4a, 4b, 4c, 4d equipped with a vibration isolator 3.
(4c and 4d not shown) on the floor 5,
It is placed. 6 is placed behind the mirror body 1, and sub-exhaust pipes 7a, 7b, 7 are provided in each chamber of the mirror body.
The main exhaust pipe communicates with the main exhaust pipe 6 through c.
is attached to the pedestal 2, and the lower end 6a of the main exhaust pipe 6 protrudes from the lower surface of the pedestal 2.
8 is a molecular pump, which has an inlet port 9;
The lower end 6a of the main exhaust pipe 6 is connected to the lower end 6a of the main exhaust pipe 6 via a bellows 10 made of stainless steel, for example. In this case, the lower end of the molecular pump 8 is floating above the floor 5. In other words, the molecular pump is suspended and supported by the main exhaust pipe 6 via the bellows 10. Further, the discharge port 11 of the molecular pump 8 is connected to an oil rotary pump via a pipe (not shown). Reference numeral 12 denotes a weight attached to the molecular pump 8, and this weight is used to cancel the upward suction force of the molecular pump 8 due to back pressure when the interior of the mirror body 1 is evacuated. In other words, when the molecular pump 8 is operated, the bellows 10 is used in a substantially free length state. 13a and 13b are wires for preventing the bellows 10 from expanding due to the weight 12 when the inside of the mirror body 1 leaks and the back pressure to the molecular pump 8 is released.
The flange 14 of the bellows when bent into a letter shape.
fixed to part a, and the other wire 13b passes through one wire 13a to the flange 14 of the bellows.
It is fixed to part b. Both wires are molecular pumps 8
When the back pressure exerted by the molecular pump and the load of the weight 12 (actually including the load of the molecular pump 8) are balanced, they are separated from each other as shown in the figure, and when the back pressure exerted by the molecular pump disappears, both are separated. are configured to engage. If the molecular pump 8 is suspended from the main exhaust pipe fixed to the pedestal 2 in this manner via the bellows 10, an oil diffusion pump that does not require the rotating mechanism commonly used at present can be used. The same resolution was obtained. This is because the vibration of the molecular pump 8 is attenuated by the bellows 10, so the vibration of the molecular pump causes the microscope body (this body consists of the mirror body 1, pedestal 2, supports 4a, 4b, and exhaust pipes 6, 7).
This can be understood from the fact that the amplitude value of the microscope body due to the vibration transmitted through the floor is larger than the amplitude value of the microscope body (including all of a, 7b, 7c, etc.). This point will be briefly explained below. The specifications of the apparatus used in the experiment are as follows. a Transmission electron microscope Acceleration voltage 100KV Total weight 750Kg b Vibration isolator Spring constant 500Kg/cm Quantity 4 piecesc Axial flow molecular pump Speed 43000rpm Vibration 715Hz Vibration width 0.04μ (actual measurement) Pump weight 6Kg d Bellows Material Stainless Steel Mountain Number 22 Spring constant 9.5Kg/cm Effective diameter 9.4cm First, the amplitude a1 transmitted from the floor to the microscope body is expressed by the following formula. a1=a|1/1-(f/fn1) ² |...(1) Here, a is the swing width of the floor, f is the frequency of the floor, and fn1 is the natural frequency of the vibration isolator. The frequency f of the floor
According to actual measurements, the amplitude a is f=13Hz, a=2.5×
10 ^-4 cm, and the natural frequency fn1 of the vibration isolator is fn1
= 1/2π√g·N·K ₀ /W. Note that g is the gravitational acceleration, N is the number of vibration isolators used, K ₀ is the spring constant of the vibration isolators, and W is the weight of the microscope body. Therefore, the natural frequency fn1 of the vibration isolator is determined from the above specifications. becomes. Therefore, the value of the amplitude a1 of the microscope body relative to the floor is (1)
From the formula, a1=a|1/1-(f/fn1) ² |=2.5× ^10-4 |1/1-(13/8.1) ² |=1.5× ^10-4 cm. On the other hand, the amplitude _a2 of the microscope body when the molecular pump is attached is expressed by the following equation. a ₂ = a ₀ | 1/1-(f ₁ /fn ₂ ) ² | ...(2) Here, a ₀ is the deflection due to vibration of the molecular pump,
f ₁ is the natural frequency of the molecular pump, and fn1 is the natural frequency of the bellows. The deflection a ₀ due to vibration of the molecular pump is a ₀ = a ₃
It is expressed as ×k/K ₀ . Note that a ₃ is the amplitude of the molecular pump, K ₀ is the spring constant of the vibration isolator described above, and k is the spring constant of the bellows. Therefore, the deflection a ₀ is determined from the above specifications as a ₀ =4×10 ⁻⁶ ×9.5/500=7.6×10 ⁻⁸ cm.
Also, the natural frequency fn1 of the bellows is

Represented by [Formula]. Note that N ₁ is the number of bellows used, and W ₁ is the force (back pressure) that the bellows receives when the inside of the casing is exhausted. This force W ₁ is the effective cross-sectional area of the bellows (π/4・D ² cm ² ) multiplied by the atmospheric pressure (approximately 1 Kg/cm ² ), and from the above specifications, W ₁ = π/4×9.4 ² ×1 = 69Kg. Therefore, the natural frequency fn1 of the bellows is becomes. Therefore, the amplitude _a2 of the microscope body transmitted from the molecular pump is _a2 = _a0 |1/1-( _f1 / _fn2 ) ² |=7.6× ^10-8 |1/1-(715/1. 8) ² | =4.82×10 ^-13 cm. As is clear from this, the amplitude of the microscope is 1.5 × 10 ^-4 cm due to the vibration of the floor.
The vibration of the molecular pump is less than 4.82×
10 ^-13 cm, it can be seen that the resolution is determined by the vibration from the floor and the vibration of the molecular pump has no effect. As detailed above, in the present invention, the molecular pump is suspended from the pedestal side using a bellows to make the molecular pump non-contact with the floor side, and the molecular pump is allowed to exhaust the inside of the mirror body. Since a weight commensurate with the back pressure applied to the molecular pump is attached, the bellows is not compressed by the back pressure applied to the molecular pump and can be used in a substantially free length state. As a result, the vibrations of the molecular pump can be attenuated by the bellows, so there is no need to fix the molecular pump on the floor with foundation work or install it away from the mirror body, which is required in the past. This makes it easy to evacuate the inside of the mirror body, and it is highly practical. In the above description, the case where the present invention is applied to a transmission electron microscope has been described, but the present invention can be similarly applied to a scanning electron microscope or the like.

[Brief explanation of the drawing]

The accompanying drawings are schematic diagrams showing an embodiment of the present invention. 1: Mirror body, 2: Frame, 3: Vibration isolator, 4a and 4b: Support column, 5: Floor, 6: Main exhaust pipe, 7a, 7b
and 7c: subexhaust pipe, 8: molecular pump, 9: suction port, 10: bellows, 11: discharge port, 12: weight, 13a and 13b: wire, 14a and 14
b: flange.

Claims (1)

[Claims] 1. A mirror body, an exhaust pipe connected to the mirror body, a pedestal mounted on the floor via a vibration isolator to hold these mirror bodies and the exhaust pipe, and and a molecular pump for evacuating the interior of the mirror body, the molecular pump being substantially suspended from the pedestal by the bellows so as not to contact the floor side, and evacuating the interior of the mirror body. An exhaust system for an electron microscope or the like, characterized in that a weight commensurate with the back pressure that the molecular pump receives is attached to the molecular pump.