JPS5948318B2 - Shinkuhaikisouchi - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a vacuum evacuation system for a charged particle beam device such as an electron microscope, a scanning electron microscope, or an ion microanalyzer. The present invention relates to a temperature control device (hereinafter referred to as B, G, pump).

In particle beam devices such as microscopes or ion microanalyzers using field emission electron guns, the degree of vacuum in the sample chamber in which the sample is placed must often be about 10-6 Torr.

If a molecular pump is used as an evacuation means during sample exchange, it is possible to continuously evacuation from the atmosphere to a vacuum level of 10-6 Torr or less, but there are problems with noise, cost, and vibration, so for the above purposes, it is not possible to use a molecular pump. is a rotary pump (hereinafter referred to as R, P
, is written as .

) and an oil diffusion pump (hereinafter referred to as D and P) is used.

In other words, from the atmosphere to about 1O-2 Torr, R, P,
The vacuum is evacuated at P, and then the vacuum is evacuated to a lower level at P.

However, if D or P is used, there is a risk that oil will diffuse into the vacuum, so it is not preferable especially in equipment using a field emission electron gun that requires a clean vacuum.

Also, there are disadvantages such as the need to run water for cooling.

If a sorption pump is used in place of D and P, a clean vacuum can be created, but the need for liquid nitrogen poses problems in terms of maintenance and operability of the device.

1O-2 Torr from the atmosphere as an exhaust means during sample exchange
After exhausting with R and P, pump B and G to about 1O-
By adopting an evacuation system that evacuates to 5 Torr and then uses an ion pump to evacuate to a lower vacuum, it is possible to create a clean vacuum without using water or liquid nitrogen.

B, G, pumps are based on the principle that when an alloy of getter materials Zr and AI is heated, surrounding gas molecules are adsorbed and internally diffused, and the temperature required for this is approximately 200 to 400°C. be.

Also, the degree of vacuum reached by B, G, and pump alone is approximately 10=
It is about Torr.

Therefore, if R and P are evacuated to 1O-2 Torr, B, G, and the getter material of the pump are 200~4
If heated to 00℃, the exhaust will be pumped out to about 1O-5 Torr, so if you use an ion pump together, it will be evacuated to about 1O-6 Torr.
The degree of vacuum below R is relatively short (depending on the volume, but when evacuating the sample chamber of a small scanning electron microscope, the time required to evacuate from atmospheric pressure to 1O-6 Torr is 5 to 6 minutes. .

) can be made. Therefore, in principle B,
G.Although it is good to always heat the getter material of the pump to 200-400℃, B.G.The life of the pump is closely related to its heating time, and the longer the heating time, the shorter the life. It has this drawback.

In addition, the B, G, and pumps differ depending on their exhaust capacity, but the power required for heating is approximately 300 W for those with a capacity suitable for the above-mentioned small scanning electron microscope, so from the point of view of power saving, constant heating is required. It is uneconomical to do so.

For the reasons stated above, it can be said that the most efficient way to use the pumps is to operate the B, G, and pumps only when evacuation is performed from atmospheric pressure, that is, when exchanging samples.

Here, B, G, the getter material of the pump is structurally
The thermal time constant is very large, for example, the final temperature reached is 8
Even if heated with a heater to a temperature of 00℃, the getter material Zr and AI alloy will rise from room temperature to 200-400℃.
It takes about 2 minutes for it to heat up.

Furthermore, for the above-mentioned reasons, even if heating is stopped after the temperature reaches 200 to 400°C, the temperature of the getter material alloy continues to rise further, and there is a risk of overheating.

If the getter material has a large thermal time constant, it will take time to cool down.

For example, once heated to 300°C, the temperature drops to 280°C in about 1 to 2 minutes.

Therefore, once the temperature reaches 300° C., for example, sufficient exhaust capacity can be achieved just by preheating without any control to keep the temperature constant.

Therefore, for example, the required heating time for B, G, and the getter material of the pump to reach 300°C is measured in advance, and the heating time required for each time the temperature of the getter material of B, G, and pump reaches 300°C is measured, and the heating time is determined in advance at a certain level each time the sample is replaced. All you have to do is heat it for a while.

However, even with this method, problems still remain.

This is because the temperatures of the getter materials of B, G, and pumps are not always at room temperature when exchanging samples.

If samples are exchanged frequently, B, G, and pumps may not be completely cooled down, so there is a risk that they may overheat if heated for a certain period of time.

The present invention solves the above-mentioned problems and provides a method for safely and efficiently using B, G, and pumps during sample exchange.

In order to achieve the above object, the present invention detects the temperatures of the getter materials of B, G, and the pump immediately before heating.

The device is characterized in that the heating time is calculated based on the detected temperature assuming a thermal time constant, and the getter materials of B, G, and the pump are heated with a heater.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Since it is difficult to directly convert temperature to time, first convert temperature to voltage.

Then convert this voltage to time.

Examples of devices that convert temperature into voltage include thermistors and thermocouples.

Although there are many methods for converting voltage into time, one method that can be implemented relatively easily is shown in FIG.

In the figure, 1 is a power source, 2 is a resistor, and 3 is a capacitor, which constitute a charging circuit.

4 is a reference power supply, 5 is a comparator, and 6 is a flip-flop.

That is, the output of the charging circuit and the reference power source are compared by the comparator 5 and output, and the output is taken out by a flip-flop.

In the above circuit, the voltage of power supply 1 is E, the resistor 2 is R, the capacitor 3 is C, and the initial voltage of capacitor 3 is E.

Then, the voltage ec across the capacitor 3 is ec=E-
(E-Eo)exp(-■), and when switch 7 is closed at time 1=0,
ec changes as shown in waveforms 8 and 9 shown in FIG.

In the figure, waveform 8 is when Eo is zero, and waveform 9 is when Eo is zero.
This shows the case where is of a certain size.

From the results shown in the figure, it will be understood that the higher the initial voltage Eo of the capacitor 3, the shorter the time it takes to reach the voltage level Es of the reference power source 4.

When the voltage ec of the capacitor 3 reaches the reference voltage Es, the comparator 5 jumps and inverts, so in the case of waveform 8, the output of the comparator 5 becomes, for example, waveform 10.

However, the timing of time t==0 cannot be known from waveform 10.

A flip-flop 6 is used for this purpose.

That is, at the same time as the switch 7 of the charging circuit is closed, a trigger is given to the flip-flop to make its output, in this case, a low level.

If the output of the comparator 5 is inverted, that is, if waveform 8 is generated, waveform 11 is obtained by setting the flip-flop 6 to "High" k-bell.

Since the time ts of the waveform 11 obtained in this way depends on the initial voltage Eo of the capacitor 3, the temperature of the getter material of B, G and the pump is converted to the voltage Eo as described above, and the flip-flop is The object of the present invention can be achieved by heating the getter materials of B, G, and the pump with a heater only during the "High" period.

FIG. 3 shows a specific embodiment of the present invention.

In the same figure, 13 is B, G is the pump heater, 12 is B
, G.

Pump getter material, 14 is a temperature detector such as a thermocouple,
15 is an amplifier, and 17 is a power source for the heater.

In this configuration, when the timing switch 19 is closed for a short time, the relay 18 is driven and the contacts 7 and 1 are closed.
6 opens briefly.

B, G, the temperature of the getter material 12 of the pump is detected by the detector 14
For example, Eo is converted into a voltage at the output of the amplifier 15
Therefore, when the contact 7 closes, the voltage of Eo is shifted to the capacitor 3.

On the other hand, when the contact 16 closes, the flip-flop 6 reverses its polarity to turn on the power supply 17 of the heater 13.
B, G, the getter material 12 of the pump begins to heat up.

Then, as time passes and the voltage ec of the capacitor 3 reaches the voltage value of the reference power supply 4, the comparator 5 is inverted, so the flip-flop 6 is reset and the heating of the getter material 12 of B, G and the pump is stopped. .

Of course, if the voltage of the capacitor is higher than the voltage of the reference power supply 4 (for example, when the temperature of B, G, and pump is 300°C), the flip-flop 6 will be reset immediately even if the timing switch 19 is pressed. B, G, and pump 12 will not be heated.

As described in detail above, according to the present invention, B, G, and B, regardless of the temperature immediately before heating the pump.

The pump can always be operated within the optimum temperature range.

Therefore, the risk of overheating the getter material is eliminated, resulting in a longer life of the getter pump and economical power consumption.

[Brief explanation of the drawing]

Figure 1 is a circuit block diagram for converting voltage to time.
FIG. 2 is a waveform diagram of FIG. 1, and FIG. 3 is a diagram showing one embodiment of the present invention.

Claims (1)

[Claims] 1. A getter temperature detection means for detecting the temperature of the getter material of the non-evaporative getter pump, and the output of the detection means,
In the vacuum evacuation device that controls the heating temperature of the getter, the getter material is optimal as a getter pump based on the signal E0 of the temperature detection means at the time of starting the pump, the reference signal Es, and the thermal time constant CR of the getter material. A vacuum evacuation device characterized in that it is configured to determine a heating time necessary to function in a temperature range and to control heating of the getter for the determined heating time.