JPS5931173B2 - Field emission type electron generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a field emission type electron generator, and particularly to a field emission type electron generator suitable for a scanning electron microscope.

As is well known, field emission type electron generators concentrate a high electric field on the sharp tip (tip) of a cathode, and use that electric field to emit electrons from the tip. Since it is possible to obtain an electron beam with a small spot size, it has come to be used in scanning electron microscopes in place of conventional thermionic emission type electron generators.

In a scanning electron microscope, the emitted electron stream must be highly stabilized.

In a field emission type electron generator, the field emission electron flow, that is, the emission, largely depends on the degree of vacuum near the chip, so when the field emission type electron generator is used in a scanning electron microscope.

The entire electron generating device is lo-9~l□t.

Efforts are being made to create an ultra-high vacuum on the order of Torr.

In general, emissions follow a pattern in which emissions gradually decrease from the start of operation, stabilize at a certain point, and then gradually increase after the stabilization time has elapsed. Despite great efforts to create an ultra-high vacuum for the entire device, the stabilization time for emissions is extremely short.

As is well known, in a field emission type electron generator, there is an extraction electrode called an extraction electrode that is necessary to concentrate a high electric field on the tip of the cathode so that the electrons are emitted from the tip. An anode having a minute aperture is provided.

Therefore, some of the field emission electrons emitted from the chip will pass through the micro-aperture, while the remaining electrons will directly hit the anode, thereby releasing the gas adsorbed or attached to it from the anode. .

In fact, it is said that this emitted gas is the cause of shortening the stabilization time of the above-mentioned emissions, and the basis for this is that the emitted gas is ionized by the impact of electrons from the chip, resulting in positive ions. This is required because the electric field near the chip becomes stronger because it is attracted to the chip that is at a negative potential, which increases emissions.

For this reason, conventional attempts have been made to degas the anode by electron bombardment or the like before use, or to make the anode itself from a material that releases gas by the bombardment of electrons emitted from the chip. It is being

However, it is possible that gas molecules emitted from the anode are attached to or adsorbed on the chip, and as a result, the electric field is concentrated in that area, which is one of the causes of unstable emissions. .

To eliminate this cause, it is possible to place a cold trap near the cathode to trap the gas generated from the anode and directed toward the chip.

According to this, although the gas trap effect itself is extremely strong, there is a major problem in that the scanning electron microscope mirror image is distorted or distorted due to the influence of vibrations caused by the boiling of the refrigerant. In order to prevent the gas molecules emitted from the anode from adhering to or adsorbing on the chip, the anode is degassed or the anode body is made of a material that releases gas by electron bombardment. However, it was found that this was still insufficient.

It is also possible to use a field emission electron generator, evacuating the entire device with a pump with high pumping capacity, or using it in combination with a means of covering the inner surface of the device wall with a gas adsorbing material, but these methods will reduce the overall energy consumption of the device. Although it is useful for improving the average degree of vacuum, it is still difficult to effectively prevent gas molecules emitted from the anode from adhering to or adsorbing on the chip, although the overall size of the device increases. It turned out to be insufficient.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a field emission type electron generator that is effective in stabilizing emissions over a long period of time in the various examples described above without causing the vibration problems described above. There is a particular thing.

The present invention focuses on the concept that gas molecules emitted from the anode are attached to or adsorbed on the chip, and as a result, the electric field is concentrated in that area, which may cause the emission to become unstable. In addition to being equipped with a means to degas the anode in the same way as the anode, it is also equipped with a gas adsorbent that does not use a refrigerant to adsorb the gas emitted from the anode and existing near the chip. have

According to the present invention, by degassing the anode, not only almost no gas is released from the anode, but even so, most of the gas that is emitted from the anode by electron bombardment and becomes present near the cathode is It is adsorbed by a gas adsorbent.

As a result, a good degree of vacuum is maintained near the cathode tip, and almost no gas from the anode adheres to the tip.

Therefore, in the various examples described above, the emissions are kept stable for a long time.

According to the results of experiments, as will be described later, it was found that the stabilization time of emissions was approximately twice as long as in the various examples described above.

As the gas adsorbent, active substances such as T, tZr+TbtBa, etc. can be used, and since these have the function of adsorbing gas without using a refrigerant, the vibration problem mentioned above can be avoided. It doesn't happen at all.

According to the invention, for activating the gas adsorbent,
A heating element embedded in the gas adsorbent is provided and is arranged to heat the gas adsorbent to activate it and to degas it to the anode.

Therefore, the configuration is simple and Q-economical.

FIG. 1 shows an embodiment of a field emission type electron generator according to an embodiment of the present invention.

In this figure, the upper end of a device wall 2 having an exhaust port 1 is formed as a seven flange 3, to which a seven flange 5 is removably attached with screws 6 via a vacuum seal 4.

An insulator 7 is provided at the center of the flange 5 for vacuum resistance.

The internal space 8 of the device wall 2 is formed as a vacuum chamber, and a lead rod 9a is inserted from the outside of the vacuum chamber 8 into the inside through the insulator 7.
, 9b and 10a, 10b extend in a vacuum-proof manner.

A cathode support rod 12a is fixed to an insulator 11 at the tips of the lead rods 9a and 9b.

12b is supported by screws or the like, and the cathode support rod 12a,
A cathode 13 is supported at the tip of 12b by point contact.

The cathode 13 consists of a hairpin-shaped cathode base 13a and a tip 13b in point contact therewith.

An anode 14 having a minute electron passage hole is placed opposite the cathode 130, and is attached to the device wall 2.

Heating body lead rods 15a, 1 are attached to the lead rods 10a, 10b.
5b is supported by a screw or the like, and the lead rod 15a.

A heating element (heater) 16 is connected to 15b.

The heating body 16 is formed in the shape of a coil spring around the axis (electron beam axis) connecting the chip 13b and the electron passage hole of the anode 14, and is embedded in the gas adsorbent 18 via an insulator 17. There is.

That is, the heating body 16 and the gas adsorption body 18 are integrally constructed.

The gas adsorbent 18 is composed of active substances such as T1+ Z r pTh, Ba, etc.

In the above, the inside of the vacuum chamber 8 is supplied by an ion pump (not shown) through the exhaust port 1! 1) 10-9~10” Tor
It is evacuated to a high vacuum of about r.

In this condition, the lead rod 10 at 10 bv
When a current is passed through the heating element 16, the heating element generates heat,
The gas adsorbent 18 is heated to, for example, about 1000°C.

This activates the gas adsorbent 1-8.

At the same time, the anode 14 is also heated by the heating element 16 to approximately the same temperature as the gas adsorbent 18, thereby degassing the anode 14.

After the gas adsorbent 18 has been sufficiently activated and the anode 14 has been sufficiently degassed, a negative voltage is applied to the tip 13b of the cathode 13 through the lead rods 9a and 9b with respect to the anode 14 (earth potential). 10 to the chip 13b.
When a high electric field of about 7 V/cIrL is concentrated, electrons are emitted from the chip 13b and pass through the electron passage hole of the anode 14. .

The electrons passing through the electron passage hole of the anode 14 are part of the electrons emitted from the chip 13b, and most of them impact the anode 14.

As a result, gases still attached to or adsorbed on the anode 14 even after the degassing treatment is performed are released.

This gas is emitted in various directions, but especially in the tip 13.
The gas heading toward the b side tends to adhere to the chip 13b.

However, since the gas adsorbent 18 is placed near the chip, most of the gas is adsorbed by the gas adsorbent 18.

Therefore, the degree of vacuum near the step 13b is
8 is improved compared to the case without the gas, and almost no gas is attached or adsorbed to the chip 13b (the field emission electron flow, that is, the emission is stabilized over a long period of time).

According to the experimental results, when a Mo plate was used as the anode 14 and an At-Zr alloy was used as the gas adsorbent 18, when the emission current was 20 μ, the emission stabilization time was about 6 to 10 hours, and the above-mentioned various It was about twice as high as the example.

As mentioned above, the gas adsorbent has T1+ZrtTh, Ba
These active substances have the function of adsorbing gas without using a refrigerant.

Therefore, the problem of vibration that occurs when using a refrigerant does not occur at all.

Activation of the gas adsorbent 18 and degassing of the anode 14 may be performed by separate means, but in FIG. 1 both are performed by the heating member 16 alone.

Therefore, the structure is simpler than the former, and it is also extremely advantageous economically.

In FIG. 1, the heating element 16 is shaped like a coiled spring, but a plate-shaped element may be used instead.

As is clear from the above description, according to the present invention, the above object is completely achieved.

In addition, since the activation of the gas adsorbent and the degassing treatment of the anode are performed by a single means, the structure is simple and economical.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of a field emission type electron generator according to the present invention (one embodiment).
Anode, 18... Gas adsorbent, 16...
heating body.

Claims (1)

[Claims] 1. Constructed to emit electrons from the tip by an electric field between the tip of the cathode and the anode in a vacuum, and extract the electrons through the electron passage hole of the anode, and degassing the anode. In the field emission type electron generator configured as above, a gas adsorbent that does not use a refrigerant for adsorbing gas emitted from the anode and existing near the tip, and a gas adsorbent for activating the gas adsorbent. a heating element embedded in the gas adsorbent, the heating element being arranged to heat the gas adsorbent to activate it and to degas (heat) the anode. A field emission type electron generator.