JPH0227643A - Stabilizing heat electric field radiation electron gun - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for stabilizing a thermal field emission electron gun used in an electron microscope or the like.

[Conventional technology and issues]

A so-called Zr/W thermal field emission electron gun, which has a coating layer made of zirconium and oxygen on the tip of a tungsten single crystal needle tip with an axial orientation of <100>, has a narrower energy width and lower brightness than a hot cathode. It has the characteristics of high energy consumption and long life. Because of this characteristic, Zr/W thermal field emission electron guns are used in various electron beam application equipment such as electron microscopes.

The stability of the emission current of a Zr/W thermal field emission electron gun depends on the shape of the tip of its needle tip.

(100) at the tip of the needle tip as shown in Figure 3 (A).
When a flat part of the surface (hereinafter referred to as a facet) is formed, the radiation current is extremely stable, but this facet shape collapses and becomes a simple curved shape as shown in Figure 3 (B). In this case, periodic fluctuations in the radiation current as shown in FIG. 4 occur. The optimum operating temperature for a Zr/W thermal field emission electron gun is said to be 1800℃ [Reference: L.W. Swanson and Day Tag: Applications of Surface Science (LJ, S.
wans.

n and D, Tuggle: Applicat
ions of 5 surface 5 science), Volume 8 (1981), pp. 185-196]. However, in order to maintain the facet shape at this temperature, it is necessary to apply a fairly high electric field, and the total emitted current at this time reaches 100 to several 100 μA, and the energy width increases. In addition, when operating the electron gun in a region where the total emitted current is as low as several tens of μA,
It is difficult to apply such a high electric field, and the facet shape of the tip of the needle tip cannot be maintained.
Difficult to operate stably.

The present invention aims to solve this problem and provide a method for stabilizing a Zr/W thermal field emission electron gun.

[Means for Solving the Problem] In order to achieve the above object, the present inventors have conducted various studies on the relationship between the operating conditions and stability of a thermal field emission electron gun, and have found that the following processing can be carried out. The inventors have discovered that a stable radiation current can be obtained for a long time even in operation in a not so high electric field, that is, the total radiation current is on the order of several tens of microamperes, leading to the present invention.

That is, the present invention provides a thermal field emission electron gun equipped with a tungsten single crystal needle tip whose axial direction is <100>, and a coating layer made of zirconium and oxygen provided at the tip of the needle tip. This is a method for stabilizing a thermal field emission electron gun, characterized in that the steps are performed in the following order:

1st step - Temperature of needle tip to 1750 or more than 1900
An electric field of 0.15 V/Å or more and less than 0.3 V/Å is applied.

2nd step--The temperature of the needle tip is set to 1600 or above 17
less than 0.00, and 0.05V/Å or more and 0.15V/Å
Apply an electric field of less than

The present invention will be explained in detail below. First, the cathode tip, which became curved in the first step, is regenerated into a facet shape. The electric field at this time is 0.15V/Å or more
．． It is less than 3V/Å. However, the electric field is defined by the following formula.

Here, F: electric field (V/in), Vex: extraction voltage (V), r: radius of curvature of tip tip (human), d: Distance between tip tip and extraction electrode It is (enter).

This is because if the electric field is less than 0.15 V/A, the facets cannot be reproduced, and if the electric field is more than 0.3 V/A, there is a risk of damaging the tip of the needle tip. Note that the temperature of the needle tip at this time must be 1,750 or more and less than 1,900. If it is less than 1750, it will take too much time to regenerate the facets, and if it is more than 1900, the cathode is likely to be damaged.

After the facets at the tip of the thermal field emission cathode are regenerated in the first step, the electric field and the temperature of the cathode are lowered in the second step. If only the electric field is lowered, the regenerated facet shape will not be maintained and will change to a curved shape again, so it is important to lower the temperature of the cathode at the same time. At this time, the temperature is in the range of 1600 to 1750, and the electric field is 0.05V/Å to 0.15V.
It is only necessary to lower the radiation current in the range below 0.0000000000000000000000000000000 though the irradiation current is less than 100%, respectively. At a temperature of 1,750 or more or an electric field of less than 0.05 V/Å, the effect of maintaining the facet shape is poor, and at a temperature of less than 1,600 or an electric field of 0.15 V/Å or more, the noise and energy width of the emitted current increases. It is from.

Note that the electric field in the second step is also defined by the above equation.

In this way, the thermal field emission electron gun can be stabilized, and thereafter a stable radiation current can be obtained for a long time.

〔Example〕

Examples of the present invention will be described below. FIG. 1 is a circuit diagram of an embodiment. A tungsten single crystal with an <ioo> orientation having a diameter of 0.125 mm was used for the needle tip 1, and the radius of curvature of the tip was 0.4 μm. The needle tip is fixed to the heating filament 2 by welding.

The needle tip protrudes 0.25 mm from the suppressor 3. The heating filament and suppressor are fixed via an insulating base 4. The distance between the tip of the needle tip and the center of the surface of the extraction electrode 5 is 0.35 m5.

A ground electrode 6 is provided at a distance of 48- from the extraction electrode, and a Faraday cup 7 is provided at the tip of the ground electrode 6.

The needle tip 1 is heated by supplying electricity to the heating filament 2 from the heating power source 8. -300V is applied to the suppressor 3 by the suppressor power supply 9. In this embodiment, all voltages are expressed based on the potential of the needle tip 1.

Further, an extraction voltage (Vex) is applied to the extraction electrode 5 by an extraction power source 10 . The total radiation current (It) is measured by the ammeter 11, and the probe current (Ip) is measured by the ammeter 12. The temperature of the needle tip 1 is measured using a radiation thermometer (not shown) with an emissivity of 0.44.

The experimental results using the above device are shown in the graph of Figure 2.The temperature of the needle tip was 1800K until 80 hours after the start of operation.
The extraction voltage is 1.7kV, that is, the electric field (F) is 0.11
It was operated at V/Å. At this time, the total radiation current is 35 to 40
Periodic fluctuations were repeated in the range of μA and the probe current in the range of 2 to 6 nA. (Part (a) in the figure).

Next, for 80 to 100 hours, as the conditions for the first step, the temperature was 1800, and an electric field of 3 OkV, that is, 0.20 V/Å, was applied.

After 90 hours, the total emission current was 115 μA and the probe current was stable at 23 nA (part (b) in the figure).
.

Therefore, after 100 hours, the temperature was lowered to 1700K, and at the same time the extraction voltage was increased to 1.7kV, that is, the electric field was reduced to 0.11V.
/Å (second step, part (C) in the figure)
In this state, the radiation current became extremely stable, and after that, it was confirmed that stable operation was maintained for 1000 hours.

In this embodiment, the suppressor voltage was kept constant at -300 Ω, but the electric field applied to the thermal field emission electron gun can also be adjusted by changing this.

〔Effect of the invention〕

According to the method of the present invention, the function of the thermal field emission electron gun, which had been unstable up to that point, can be restored and the gun can be operated stably for a long period of time.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram for implementing the method of the invention. FIG. 2 is a graph showing an embodiment of the present invention. Figure 3 (
A) and (B) are perspective views of the tip of the needle-like tip, and FIG. 4 is a graph showing changes in radiation current over time. Code 1 - Needle tip 2 - Heating filament 3 - Suppressor 4 - Insulating base 5 - Extraction electrode 6 - Grounding electrode 7 - Faraday cup 8 Heating power supply 9 - Suppressor power supply l〇 - Extraction power supply 11-・Ammeter 12・-・Ammeter

Claims (1)

[Claims] A thermal field emission electron gun equipped with a tungsten single crystal needle tip whose axial direction is <100> oriented and a coating layer made of zirconium and oxygen provided at the tip thereof is provided by the following first method. 1. A method for stabilizing a thermal field emission electron gun, characterized in that a step and a second step are performed in this order. 1st step... Temperature of the needle tip is 1750K or higher19
00K and an electric field of 0.15 V/Å or more and less than 0.3 V/Å is applied. 2nd step...The temperature of the needle tip is 1600K or higher17
The temperature is less than 50K, and an electric field of 0.05 V/Å or more and less than 0.15 V/Å is applied.