JPS581505B2 - Denkai Housiya Denshijiyuu - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the structure of a field emission electron gun, and particularly to a cathode protection device.

Field emission electrons are obtained by applying a strong electric field of about 107 [v/CrrL] to a tungsten needle whose tip has a small radius of curvature.

The emitted electron flow from a clean cathode after degassing the cathode exhibits a change in external diameter as shown in FIG.

This change in the radiated electron flow is due to gas adsorption on the cathode surface. By gas adsorption to the clean cathode, the work function of the cathode surface increases and the radiated electron flow decreases.

At the beginning, the radiation electron flow decreases quickly (this region is called the initial reduction region). When the gas adsorption of almost monoatomic atoms ends, the decrease in the electron flow stops, and a nearly constant radiation electron flow is obtained (
This region is called a stable region and can last up to 8 hours at a vacuum degree of 10"-10 Torr).

After that, the emitted electron flow gradually increases, and the pulse-like current changes gradually increase, eventually causing a discharge in vacuum at the cathode, damaging the cathode, and making it impossible to obtain field emitted electrons again.

(This region is called an indeterminate region, and if the strong electric field applied to the cathode is cut off and the cathode is degassed before the vacuum discharge occurs, the process shown in FIG. 1 can be traced again.

The present invention provides a field emission electron gun that detects the time when the cathode should be degassed using a simple method and prevents damage to the cathode.

Since the radiation electron flow increases in the unstable region, methods such as cutting off the strong electric field applied to the cathode when it reaches 150% of the value in the stable region have been considered, but the change in the radiation electron flow is very large. Since it is slow and requires a storage device, it is a large-scale process.

Since pulse-like current changes gradually increase in an unstable region, the present invention is a method of detecting only pulse-like current changes and detecting the time of cathode degassing from this detected amount.

FIG. 2 shows an embodiment of the invention.

The cathode 1 and the first anode 2 are housed in a high vacuum container 3.

By turning on the switch 6, the first anode voltage 5 is applied and electrons are emitted from the tip of the cathode 1.

Pulse current changes in the emitted electron flow are detected by a pulse current detector 4.
is detected, amplified by an amplifier 7, and when a predetermined value is reached, an alarm 8 is turned on.

This alarm allows the operator to turn off the switch 6, thereby preventing damage to the cathode 1.

FIG. 3 shows another embodiment of the present invention in which instead of turning on the alarm 8 of the previous embodiment, a relay coil 9 is turned on and the switch 6 is turned off in conjunction with this.

FIG. 4 shows another embodiment in which the output of the pulse current detector 4 is amplified by the amplifier 7, converted to DC by the AC/DC converter 10, and integrated by the integrator 11 until the output of the integrator 11 reaches a predetermined value. When this occurs, the relay coil 9 is turned on and the switch 6 is turned off.

The detection timing can also be adjusted by changing the time constant of the integrator 110.

By controlling using the integral value in this manner, malfunctions caused by external noise etc. can be reduced.

Similarly, it is also possible to count the output of the pulse current detector 4 and control based on the counted value.

FIG. 5 shows another embodiment.

In the above description, a structure without the second anode was described for the sake of simplicity, but in this embodiment, the second anode 14 is provided.

This example shows an example in which a field emission electron gun is used as an electron gun for a scanning electron microscope.

The alternating current component (noise) contained in the field emission electron flow is 4 to 5% even in the stable region, and in a scanning electron microscope, noise appears in the scanned image and deteriorates the image.

For this reason, when used in a scanning electron microscope, a correction circuit is used to erase the scanned image.

When an extraction voltage of several kilovolts is applied between the cathode and anode 2, electrons are emitted from the cathode 1.

These electrons are further accelerated to an arbitrary voltage by applying an accelerating voltage 12 to the second anode 14.

The first anode 2 and the second anode 14 form a pair of electrostatic lenses, and the electron beam is focused by this electrostatic lens.

With commonly used voltage distribution, the beams passing through the electrostatic lens are almost parallel.

The correction signal detector is formed by a first aperture 15, a second aperture 16, and a secondary electron detector 17.

The radiation angle from the cathode is approximately 10 mr due to the hole of the first diaphragm 15.
and is further limited to approximately 6 mr by the second aperture 16.

The electrons that have passed through the second aperture 16 are focused by an electromagnetic lens 20 and scanned over a sample 21 by a deflection coil 19 as scanning electrons.

Secondary electrons generated by electrons passing through the first aperture 15 and colliding with the second aperture 16 are detected by the detector 17 and become a correction signal.

On the other hand, secondary electrons and the like generated from the sample 21 are detected by the detector 18 and become information signals.

The information signal and the correction signal are divided by the correction device 22,
The output becomes a brightness modulation signal of a cathode ray tube 24 having a deflection coil 23 synchronized with the deflection coil 19 to form a scanning image.

With this configuration, the content of AC components is reduced to 1
/3-1/5.

The pulse-like current change included in this correction signal detector 17 is amplified by the amplifier 7, and when it reaches a predetermined value, the relay coil 9 is turned on, and the relay 13 is switched to increase the output voltage 5.
is separated from the first anode 2.

By providing a pulse current detector as described above, unstable regions of field emission electron flow can be detected easily and accurately, and the industrial contribution is enormous.

[Brief explanation of the drawing]

Figure 1 shows the change in field emission electron flow over time, Figures 2 and 3,
FIGS. 4 and 5 show an embodiment of the present invention. 1 is a cathode, 2 is a first anode, 3 is a high vacuum container, 4 is a pulse current detector, 5 is a power source for the first anode, 6 is a switch, 7 is an amplifier, and 8 is an alarm.

Claims (1)

[Scope of Claims] 1. In a field emission electron gun having at least a field emission cathode and an anode that applies a strong positive electric field to the cathode, the total amount emitted from the cathode is used as information for controlling the strong electric field applied to the cathode. A detection means is provided for detecting a pulse-like fluctuation component of the electron flow after the electron flow or a part of the electron flow has decreased, and the strong electric field applied to the cathode is blocked or reduced by the output of the detection means. A field emission electron gun characterized by comprising: 2. Claim 1, characterized in that the device is provided with means for integrating the output of the detector and interrupting or reducing the strong electric field applied to the cathode when the integrated value reaches a predetermined value. Field emission electron gun described. 3. A patent characterized in that a means is provided for counting the output of the detection means and for blocking or reducing the strong electric field applied to the cathode when the total counted value or the counted value per time reaches a predetermined value. A field emission electron gun according to claim 1.