JPS628393Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a baking device for a sublimation pump used for evacuation of an electron microscope, etc., which requires a high vacuum and a highly clean vacuum.

In such a sublimation pump, a titanium filament is heated to high temperature and sublimated to form a titanium film (getter surface) on the inner wall of the pump, and gas molecules in the pump container are adsorbed to the getter surface through a chemical reaction, and the pump is activated. It is something that performs an action. Therefore, before operating the pump, it is necessary to perform so-called baking, in which the inner wall of the pump is baked to release the gas molecules adsorbed on the getter surface, and the oil rotary pump is used to exhaust the gas molecules. This baking is usually done for a certain period of time (for example, about 1 hour) determined by the handler based on economic considerations.

However, since the amount of gas molecules adsorbed on the getter surface varies each time, when the amount of gas molecules is small, the vacuum in the exhaust pipe connecting the sublimation pump and the oil rotary pump improves, and the flow changes from the viscous flow region to the intermediate flow region. The oil vapor from the oil rotary pump, which has a high vapor pressure, flows back into the sublimation pump and adheres to the inner wall of the pump. When this oil vapor adheres, it sublimates the titanium filament and adheres (evaporates) to the inner wall of the pump.
Since the titanium film is peeled off, there is an inconvenience that the pumping speed is reduced.

The purpose of the present invention is to solve such inconveniences, and the present invention has a configuration including a sublimation pump, an oil rotary pump that roughly pumps the inside of the sublimation pump via an exhaust pipe, A valve for shutting off between the sublimation pump and the oil rotary pump, a vacuum gauge for generating a signal according to the degree of vacuum in the exhaust pipe, a means for baking the sublimation pump, and the sublimation pump. and means for monitoring the signal from the vacuum gauge during baking of the pump and closing the valve when the degree of vacuum in the exhaust pipe reaches the degree of vacuum in the intermediate flow region or higher. This is characterized by preventing oil vapor from flowing back into the sublimation pump.
The present invention will be explained in detail below based on an embodiment shown in the drawings.

The attached drawings are schematic diagrams showing an embodiment of the present invention, and 1 is a sublimation pump using a titanium filament 2. The pump 1 is connected to an evacuated chamber 4 of an electron microscope or the like via a main exhaust pipe 3, and is connected to an oil rotary pump 6 via a sub-exhaust pipe 5. The main exhaust pipe 3 and the sub-exhaust pipe 5 are provided with valves 7 and 8, respectively. 9 is a baking power supply for supplying a baking current to the titanium filament 2; 10 is a sublimation power supply for supplying a sublimation current to the titanium filament 2;
connected to.

Reference numeral 12 denotes a vacuum gauge that generates a signal corresponding to the degree of vacuum in the sub-exhaust pipe 5, and the output signal is sent to a comparison circuit 13 and compared with a reference signal from a reference power supply 14.
The comparator circuit 13 sends an output signal to the driver of the valve 8 to close the valve 8 when the two signals match. Note that several titanium filaments 2 are actually installed.

To bake the sublimation pump 1, first, the valve 7 is closed, the valve 8 is opened, and the inside of the sublimation pump is evacuated by the oil rotary pump 6. In this state, the changeover switch 11
is connected to a baking power source 9 as shown in the drawing, and the titanium filament 2 is heated by supplying a baking current. This filament 2
As the inner wall of the sublimation pump is heated by radiation, degassing takes place. However, as baking progresses, the auxiliary exhaust pipe 5
When the vacuum inside moves from the viscous flow region to the intermediate region, the value of the output signal from the vacuum gauge 12 becomes larger than the value of the reference signal from the reference power source 14.
3 sends an output signal to the driver of valve 8 to close valve 8. This prevents oil vapor from the oil rotary pump 6 from entering the sublimation pump 1. Furthermore, when the vacuum in the sub-exhaust pipe 5 deteriorates again due to degassing due to heating of the titanium filament 2 and enters the viscous flow region, the output signal from the comparator circuit 13 is stopped, so the valve 8 is opened again and the sub-exhaust pipe 5 is discharged. The gas molecules inside the pump are evacuated. This operation is repeated thereafter to complete baking.

By the way, the following relationship is derived by Knudsen.

That is, if the inner diameter of the tube is D, the length is L, and the pressure inside the tube is P, then the exhaust conductance of this tube is C.
(l/s) is given by C=12.1(D ³ /L)J (1).

However, the factor J in the above formula is the following formula: J=1+271DP+4790(DP) ² /1+31
6DP, and the second as a function of DP (Torr cm)
The values are shown in the figure.

Now, the flow in a pipe can be classified into molecular flow, intermediate flow, and viscous flow.The regions where each of molecular flow, intermediate flow, and viscous flow are dominant depend on the value of DP as shown in Figure 2. Become.

Based on the results derived by Knudsen above, if we choose, for example, DP = 2 × 10 ^-1 (Torr cm) as the representative value of the intermediate flow region, and apply this to the sub-exhaust pipe in the example, we can obtain this value. The pressure P ₀ at which the sub-exhaust pipe becomes an intermediate flow region is expressed as P ₀ =2×10 ⁻¹ /D ₀ . Here, D ₀ is the inner diameter of the sub-exhaust pipe. Now, when a sub-exhaust pipe with an inner diameter D ₀ of 8 mm is used, this pressure P ₀ becomes P ₀ =2.5×10 ^-1 (Torr). Therefore, the degree of vacuum in the sub-exhaust pipe is 2.5×
The reference value of the reference power source may be set so that valve 8 is closed when the value reaches 10 ^-1 Torr.

As described above, in this invention, when the vacuum in the sub-exhaust pipe reaches a degree of vacuum at which oil vapor from the oil rotary pump flows backward (intermediate vacuum degree), the valve automatically closes and the sublimation pump and oil rotary pump Since the gap is blocked between the two, it is possible to prevent the titanium film from being peeled off due to oil vapor adhering to the inner wall of the sublimation pump, and it is possible to prevent a decrease in the pumping speed.

In the above description, a case has been described in which the inner wall of the sublimation pump is heated by passing a baking current through the titanium filament, but instead of this, a heating coil may be wound around the wall of the sublimation pump.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing the relationship between each region of molecular flow, intermediate flow, and viscous flow and the value of DP. 1: Sublimation pump, 2: Titanium filament, 3: Main exhaust pipe, 4: Exhaust chamber,
5: Sub-exhaust pipe, 6: Oil rotary pump, 7, 8: Valve, 9: Baking power supply, 10: Sublimation power supply,
11: Selector switch, 12: Vacuum gauge, 13: Comparison circuit, 14: Reference power supply.

Claims (1)

[Scope of utility model registration request] a sublimation pump, an oil rotary pump that roughly pumps the inside of the sublimation pump through an exhaust pipe, a valve for isolating between the sublimation pump and the oil rotary pump, and a valve inside the exhaust pipe. a vacuum gauge that generates a signal according to the degree of vacuum; a means for baking the sublimation pump; and a means for monitoring the signal from the vacuum gauge while baking the sublimation pump to determine whether the degree of vacuum in the exhaust pipe is intermediate and means for closing the valve when a vacuum level in the flow region is reached.