JPH044690B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in heated cathode field emission electron guns.

Field emission electron guns used as sources of high-intensity electron beams are broadly classified into cold cathode type and heated cathode type. As shown in the figure. In FIG. 1, two stems 2 passing through an insulator 1 are shown.
Both ends of a hairpin type tungsten filament 3 are welded to the lower end of each, and an emitter 4 is further welded to the tip of the filament. An extraction electrode 5 for generating a strong electric field at the tip of the emitter 4 is provided below the emitter 4, and an anode 6 having a ground potential is arranged below the extraction electrode 5. Emitsuta 4
Between the extraction electrode 5 and the anode 6, voltages Ve and Va are applied to the emitter 4 by power supplies 7 and 8, respectively, and an output current of a heating power supply 9 is supplied between both ends of the filament. It is configured as follows. With this configuration, when electric field is emitted while the filament is energized and the emitter 4 is heated to 1,500 to 1,800 degrees, gas adsorption to the tip of the emitter can be suppressed to a minimum, and the degree of vacuum in the electron gun chamber can be reduced to a cold cathode type electric field. If it is comparable to the emission electron gun, it has the advantage that a stable field emission current can be obtained over a longer period of time. However, electrons A are generated by field emission from the emitter tip as the filament is heated.
In addition to (its emission current value is Ia), thermionic electrons B (its emission current value is Ib) are emitted from the side of the emitter and the filament 3, and their proportion to the total current value emitted from the cathode is several dozen. It can even reach %. Most of these thermoelectrons cannot pass through the extraction electrode 5 or the anode 6, and do not contribute to the electron beam extracted from the electron gun. Therefore, the thermionic electrons B only contribute to emitting adsorbed gas from the extraction electrode by irradiating the surface of the extraction electrode 5 facing the emitter, making the operation of the heated field emission electron gun unstable. It was the cause.

The purpose of the present invention is to solve these problems and provide a heated field emission electron gun that suppresses the adverse effects of thermionic electrons. an emitter; a heating power source that supplies the filament with a current that heats the emitter to a temperature of 1500°C or more; an anode provided below the emitter and kept at ground potential; a power source that applies a negative high potential to the emitter; In a device equipped with an extraction electrode provided between emitters and to which a positive voltage Ve is applied to the emitter, a thermionic control electrode is provided to which a voltage Vc satisfying the condition 0<Vc<Ve is applied to the emitter, The lower end surface of the thermionic control electrode is arranged above the lower end of the emitter, and the diameter of the opening of the thermionic control electrode is smaller than the length of the thermionic emission region in the filament. That is.

FIG. 2 shows a device according to an embodiment of the present invention.
Components with the same reference numerals as in FIG. 1 represent the same components. In the device shown in FIG. 2, compared to the device shown in FIG. 1, a thermionic control electrode 10 is newly provided above the lower end of the emitter 4, and a voltage Vc is applied between it and the emitter by a power source 11. . this voltage
Since Vc is lower than the voltage applied to the extraction electrode (e.g. Ve=5KV, Vc=0.5-3KV),
It has almost no effect on the strong electric field for field emission formed at the tip of the emitter. Therefore, the emission path of electron A due to field emission is also almost the same as in the case of FIG. The diameter d of the opening of the thermionic control electrode 10 is smaller than the maximum length D of the region where the filament 3 generates thermionic electrons. Therefore, most of the thermionic electrons generated from the filament 3 and the thermionic electrons generated from the side surface of the emitter 4 are deflected toward the surface of the thermionic control electrode 10 and absorbed, and the heat passing through the opening of the control electrode 10 is absorbed. The amount of electrons is negligible. Thermionic control electrode 10
Thermionic electrons that irradiate the electrode surface also release the gas adsorbed on the electrode surface, but since the accelerating voltage Vc for thermionic electrons that irradiate the electrode is lower than the extraction voltage Ve of the extraction electrode 5, the gas release effect is weak. Furthermore, since the region from which the gas is emitted is above the emitter tip where field emission occurs, the rate at which the emitted gas is adsorbed again to the emitter tip is extremely low.

As described above, the device shown in FIG. 2 can reduce the adverse effect of heating electrons on the stabilization of the field emission electron gun, but the device shown in FIG. The degree of improvement can be determined by expressing the extent to which the energy of electron impact on each electrode is reduced using the following equation. That is, in FIG. 2, the energy of the electron impact received by each electrode is Ve×Ia+Vc×Ib. On the other hand, in the device shown in Figure 1, the energy of the electron impact received by the extraction electrode is Ve・(Ia+Ib), so in the device shown in FIG. 2, the energy of the electron impact received by the extraction electrode is only (Ve−Vc)・Ib. will decrease.

As described above, according to the heated cathode field emission electron gun of the present invention, a stable high-brightness electron beam can be obtained even when the pressure in the electron gun chamber is relatively high, so it can be used in scanning electron microscopes that require high resolution. Great effects can be obtained when used as an electron beam source.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a conventional heated field emission electron gun, and FIG. 2 is a schematic diagram showing an apparatus according to an embodiment of the present invention. 1... Insulator, 2... Stem, 3... Filament, 4... Emitter, 5... Extraction electrode, 6
... Anode, 7, 8 ... Power supply, 9 ... Heating power supply, 1
0...Power source for thermionic control.

Claims (1)

[Claims] 1. An emitter attached to the tip of a hairpin-shaped filament, a heating power source that supplies the filament with a current that heats the emitter to a temperature of 1500°C or more, an anode provided below the emitter and kept at ground potential, and a In an apparatus equipped with a power source that provides a negative high potential and an extraction electrode that is provided between the anode and the emitter and applies a positive voltage Ve to the emitter, 0<Vc< with respect to the emitter.
A thermionic control electrode to which a voltage Vc that satisfies the condition of Ve is provided, the lower end surface of the thermionic control electrode is disposed above the lower end of the emitter, and the diameter of the opening of the thermionic control electrode is set in the filament. A heated cathode field emission electron gun characterized in that the length is smaller than the length of the thermionic emission region.