JPS5818211Y2 - electron gun - Google Patents

Description

[Detailed description of the invention] The present invention relates to an electron gun used in a scanning electron microscope, etc.
In particular, it relates to a thermionic emission type electron gun.

Generally, in an electron gun such as a scanning electron microscope or a transmission electron microscope, a cathode made of a thermionic emission material such as tungsten or boride is heated to a high temperature (for example, about 2000') to emit thermionic electrons. Thermionic electrons are emitted from the cathode by the anode, which is kept at a high positive potential with respect to the cathode, and the Wehnelt electrode, which is placed around the cathode and is kept at a negative potential with respect to the cathode. Thermionic emission type electron guns that control the flow of electrons are widely used.However, when such electron guns are used for a long time, the inner wall surface of the Wehnelt electrode (especially the inner wall surface near the electron passage opening) A film or whisker of cathode material is formed on the surface of the cathode, making thermionic emission unstable.

This is due to the following reasons.

That is, when the cathode is heated to a high temperature for a long period of time, the space formed by the Wehnelt electrode is filled with vaporized gas molecules of the cathode material, and these vaporized gas molecules are ionized by thermionic electrons from the cathode and become positive ions.

Therefore, the cations are attracted to the Wehnelt electrode, which is kept at a negative potential with respect to the cathode, and are deposited on the inner wall surface.

A film or whisker of cathode material is formed.

At this time, in order to increase the brightness, the tip of the cathode is placed close to the Wehnelt electrode, and as a result of the formation of the whiskers, a microdischarge or its precursor phenomenon occurs between the cathode and the Wehnelt electrode, and thermionic emission occurs. becomes unstable.

However, in the past, there was no effective means for suppressing the growth of whiskers etc. formed on Wehnelt electrodes.
For this reason, the electron gun had to be disassembled and the film and whiskers removed after each relatively short period of time.

Therefore, handling is very troublesome.

``The present invention aims to solve such inconveniences, and will be explained in detail below based on the drawings.

The drawing is a configuration diagram showing an embodiment of the present invention, and 1 is a cathode formed of, for example, lanthanum hexaborium (LaB6), to which a high negative voltage is applied from a high voltage power source 2, The cathode is also held between conductive support members 4a and 4b via graphite members 3a and 3b.

The support member is attached to an insulator fixed to the upper end of the electron gun chamber (not shown), and a thermal power source 5 is connected between the support members.
is connected.

Reference numeral 6 denotes a Wehnelt electrode placed close to the cathode 1, and the Wehnelt electrode is maintained at a negative potential with respect to the cathode 1 due to the voltage drop across the bias resistor 7.

8 is the Uninel)! An anode at ground potential placed opposite the pole.

Reference numeral 9 denotes an annular auxiliary electrode disposed around the outer periphery of the cathode 1, and this auxiliary electrode is fixed to the inner wall of the Wehnelt electrode 6 via an insulator 10.
1, it is maintained at a negative potential of several hundred volts or more with respect to the Wehnelt electrode.

Further, the auxiliary electrode 9 is placed at a predetermined distance from the cathode 1 (at least between the cathode and the Wehnelt electrode) in order to prevent discharge from easily occurring between the auxiliary electrode 9 and the cathode 1 even if some whiskers or the like are formed on the electrode 9. (a distance greater than or equal to ).

In the above-described configuration, when current is applied from the heating power source 5 between the support members 4a and 4b, the graphite members 3a and 3b
generates heat, the cathode 1 is heated to a desired temperature, and thermionic electrons are emitted from the cathode.

The emitted thermoelectrons are accelerated by the electric field between the anode 8 and taken out as an electron beam.

On the other hand, evaporation occurs due to the high temperature heating of the cathode 1, and the evaporated gas molecules are ionized by thermionic electrons and become positive ions.

Since the auxiliary electrode 9 is kept at a more negative potential than the Wehnelt electrode 6, the cations are attracted not only to the Wehnelt electrode but also to the auxiliary electrode 9 and adhere to the surface of the electrode 9.

By doing so, the attachment points of the evaporated gas molecules are dispersed, so that the rate at which a cathode material coating or whiskers are formed on the inner wall surface of the opening 12 portion of the Wehnelt electrode 6 is significantly slowed down.

Therefore, there is no need to frequently disassemble and clean the electron gun as in the past, and handling becomes easier, which has great practical effects.

[Brief explanation of the drawing]

The drawings illustrate an embodiment of the present invention. In the figure, 1 is a cathode, 2 is a high-voltage power source, 3a and 3b are graphite, 4a and 4b are conductive support members, 5 is a heating power source, and 6 is a Wehnelt electrode. 7 is a bias resistor, 8 is an anode, and 9 is an auxiliary electrode. 10 is an insulator, and 11 is a power source.

Claims (1)

[Scope of utility model registration request] a cathode; a means for heating the cathode; a Wehnelt electrode placed close to the cathode; an anode placed opposite the Wehnelt electrode; and a means for thermally heating the cathode. What is claimed is: 1. An electron gun comprising: a power supply for increasing the electron beam speed; and an annular auxiliary electrode provided inside the Wehnelt electrode and kept at a negative potential with respect to the Wehnelt electrode.