JPS6016700B2 - Hot cathode electron source device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electron microscope and its similar devices.
The present invention relates to a hot cathode electron source device.

La & hot cathode electron sources have higher brightness (5-1M sound) and longer life than tungsten/hairpin hot cathode electron sources, so they can be used in a high vacuum (10-6 on) atmosphere.
Despite the disadvantage that the equipment is expensive, it has been used in some cases.

Conventional LaB hot cathodes are generally used at a fixed temperature of 1800° to 190°K, and have a lifespan of 100 to several tens of degrees, which is longer than tungsten hairpin hot cathodes, but as a hot cathode used in a high vacuum atmosphere, Their short lifespan hindered their widespread use. The purpose of the present invention is to extend the life of the B6 hot cathode without causing any practical problems by varying the heating temperature of the LaB6 hot cathode in accordance with the diameter of the electron beam irradiated onto the sample. be.

FIG. 1 shows a schematic explanatory diagram of a scanning electron microscope.

The electron beam 2 irradiated from the hot cathode 1 passes through electron lenses 3 and 4.
The spot size of the electron beam 2 is reduced to a required size and irradiated onto the sample 5. Further, the deflection coils 6 and 7 cause the reduced electron beam spot to scan the sample 5 in a plane. The electron beam spot irradiated onto the sample 5 generates information about the shape of the sample, such as secondary electrons. Detector 10
, the signal detected and amplified by the intensifier 11 is sent to the cathode ray tube 12.
A grid is introduced, the electron beam 13 of the cathode ray tube 12 is controlled, and the brightness of the phosphor on the cathode ray tube surface is changed. The deflection coil 8 of the cathode ray tube 12 is driven by a deflection power source 9 in synchronization with the deflection coils 6 and 7. The sample image displayed on the cathode ray tube is a ratio L/
It will be expanded. In scanning electron microscopes and similar devices, the ability to observe high-magnification images, that is, the resolution, is one of the important performances, and the resolution is limited by the size of the spot of the electron beam irradiated onto the sample.

If the spot size is small, the image quality will not be bad even if the scanning limit on the sample is made small, but if the spot size is large, the image will become blurry and unclear unless the scanning limit is made large. , so there is no point in increasing the magnification.
That is, in order to improve the resolution, it is desirable to make the spot size as small as possible, but on the other hand, the amount of electron beam irradiated to the sample decreases in proportion to the spot size. Now, as shown in Figure 2, the current irradiated to the sample is
If the brightness of the electron source is B (A/.str), sample 5
Assuming that the half angle as looking into the aperture 15 does not change, the diameter d of the electron source.

and the electron beam spot diameter dS on the sample, the ratio M=d. The following relationship exists between /ds and the current ls applied to the sample.
IS = B (OS/M) 2, 4 = m2 / must-have = KBd......{1} However,
K=20 steals/4M=d.

/dS=8S/8. Solid angle = bamboo (11 cos8); bamboo 8
2'1} formula shows that as the spot diameter of the electron beam on the sample becomes smaller, the current irradiated to the sample decreases.

For this reason, as the spot diameter is made smaller, the amount of signals such as secondary electrons decreases, the S/N ratio with the noise generated in the detector 10 and intensifier 11 becomes worse, and the spot diameter becomes smaller by d. However, the result is poor image quality and poor resolution. For this reason, in a tungsten hairpin type hot cathode, the spot diameter of the electron beam is generally set to 50 to 100 A, and the sample current is set to about 2 to 5 x 10-12 A. Since the sample current is proportional to the brightness of the electron beam source, using a B6 hot cathode, which has several times the brightness compared to a tungsten hairpin type, increases the signal amount and reduces the spot diameter of the electron beam irradiating the sample. It is also possible to do this, making it possible to create a high-resolution library. However, when using the Yama B6 hot cathode,
It is necessary that the degree of vacuum near this hot cathode is better than 10-6 Tom. In order to maintain this high vacuum and increase the operating rate of the equipment, it is important to extend the life of the hot cathode as much as possible and minimize the frequency of exposure to the atmosphere for replacement of the cathode. is necessary. Also LaB
Hot cathodes are expensive, which is also one of the reasons why a longer life is desired. The life of the hot B6 hot cathode is determined by the evaporation rate of the hot B6.

An example of the relationship between evaporation rate and hot cathode temperature is shown in Figure 3.
The evaporation rate at 00 k is shown as a line V. Line B shows the relationship between brightness and cathode temperature. When observing specimen images at high magnification in scanning electron microscopes and similar devices, it is useful to use a high-brightness electron source.

However, at magnifications of several thousand times or less, which are normally used for observation, conventional tungsten. Sufficient sample irradiation current was obtained even with a pin-shaped hot cathode. The deflection L of the cathode ray tube in Fig. 1 is set to the 100 side, and from 50,00 to 1,00
The resolution of the cathode ray tube was set to 1/1500, and the spot diameter ds (
That is, ds=1/1500) and high magnification (50,00
Table 1 shows the sample current ratio when the spot diameter is at each magnification, based on the sample current when the spot diameter is 50 A at the time of 0x magnification.

Table 1 As shown in Table 1, if the sample current at 50,000 times is 1, then at 5,00 times the sample current is obtained 6.7 times.

Therefore, if you want to observe 5,000 folds, 50,00
It can be observed with the brightness reduced to 1/6/7 compared to the 0x condition. In a 5,00M section, the brightness of the thermionic hairpin filament is 3 to 5 x A/.

Since it is known from the conventional equipment that str is sufficient, the cathode temperature of 5,00 pm is 17000 pm from Figure
It turns out that k is good. The evaporation rate in this case is “200
If you compare it with 0ok, it will be 1/100, and if you compare it with ]90k, it will be 1/30, which means that the lifespan will be 100 times longer than 3M moss. Furthermore, when observing at 50,000 times magnification, in order to obtain the same sample current as when observing at 5,00 pm, it is necessary to /of.
A brightness of sV is required, and from Figure 3 it is 1950-2000.
It can be seen that a cathode temperature of 0 k is required. When using conventional thermionic hair pin filament, 3 x 1 A/
Earth. Since the str was at the limit, in observation with 50,000 yen moss, the conventional brightness of 4 to 100 g was obtained, making it possible to observe a bright image with a good S/N ratio. Also, when dividing by 10,00, the sample current is 1/1.7 of 50,000 times, which is 2 to 3 x 10-6/1.731 to 1.7 x 1 pA.
/Su. str is sufficient, the cathode temperature is 1850 to 19000K, which can be lowered by about 10pK, and the life span is extended about 5 times. In Table 1, the relationship between the spot flea ds and the sample current is proportional to the square of ds, so if the sample current of 50A is 1, then at 130A it is (130/5
0)2=6.7 times. Conventional B6 hot cathodes have always been used at a temperature that provides the necessary brightness at maximum magnification.

It is used more than 90% of the time.

When the magnification is 10,00 or less, the cathode temperature is set to the appropriate temperature (
If you have a mechanism that can set the temperature to the minimum temperature necessary to obtain the required beam diameter and sample current at that time, it is possible to extend the lifespan of 10 to several 1 needle sounds without any deterioration in performance. becomes.

4 and 6 are schematic diagrams of embodiments of the present invention. Figure 4 shows the step switching method, and the amount of DC power is 5.
The output of r. , r2 .

As a result, the input voltage of the DC-DC converter 20, and therefore the current value supplied to the hot cathode 1, is increased or decreased to control its brightness. Therefore, by switching the changeover switch 17 to an appropriate position, the heating temperature of the B6 hot cathode can be set to the minimum necessary value in accordance with changes in lens magnification or required irradiation beam diameter. Figure 5 is Figure 4
A variable resistor VR is used instead of the series resistor for voltage division to enable continuous switching, and its operation is exactly the same as the case of FIG. In these cases, the index of the selector switch 17 or the variable resistance slider 17A can be expressed by the lens magnification or the required spot diameter, and if the switch or the variable resistor is linked with the current selector switch of the electronic lens. , to help improve operability).

In addition, it usually has a slow evaporation rate, has a long life, and can provide the same brightness as a tungsten hairpin hot cathode1.
The cathode temperature is set at 600oK to 1,800oK, and the cathode temperature is raised only when the high-intensity electron source is required, and automatically after a certain period of time (for example, after the use of the electron beam device is stopped). , it is more effective to extend the life of the hot cathode by allowing the temperature to return to the normal set temperature.

FIG. 5 is an example. In FIG. 6, the contacts SI and S2 of the relay 23 are normally in contact with the black contact side, and the base potential of the transistor 19 is set to a value corresponding to a temperature of, for example, 1600 to 1800 ok by the voltage dividing resistors R1 and R2. ing. When the set switch 22 is opened during high-magnification observation, the relay 23 is energized and the contacts S1 and S2 are switched to the white contact side, and the hot cathode 1 is heated by a current whose magnitude corresponds to the contact position of the changeover switch 17. , and at the same time, the relay 23 is self-held by the contact S2. When the power supply 15 is turned off after the observation, the self-holding of the relay 23 is released and the hot cathode 1 returns to its initial state, so that the hot cathode 1 returns to its normal setting state. The above explanation is about scanning exposure microscopes, but transmission electron microscopes are also heated in the same way. like B6
The features of the electron source can be utilized.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are schematic diagrams explaining the principle of the present invention, FIG. 3 is a chart showing the relationship between cathode temperature, brightness, and evaporation rate, and FIGS. 4 to 6 each illustrate the implementation of the present invention. FIG. 3 is an example circuit diagram.

1... Hot cathode, 15... DC power supply, 16... Jena diode, 17... Changeover switch, 20... DC-
DC converter, 23...relay. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6

Claims (1)

[Claims] 1. Equipped with a LaB_6 hot cathode that is heated in a high vacuum, a power supply device that supplies current to the hot cathode, means for accelerating the generated electron beam, and means for focusing the electron beam. In the hot cathode electron source device, a means is provided to link the heating temperature of the hot cathode with switching of the electron beam focusing means, and the heating temperature of the LaB_6 hot cathode is adjusted so that the smaller the beam diameter is, the higher the temperature is. In addition, a hot cathode electron source device characterized in that the temperature is controlled so that the larger the beam diameter, the lower the temperature. 2 The heating temperature of the hot cathode is usually set to a relatively low temperature,
2. The hot cathode electron source device according to claim 1, characterized in that when reducing the beam diameter, the temperature is switched to high temperature, and after a predetermined time has elapsed, it is automatically returned to the normal setting state.