JPH0612973A - Method for operating thermoelectric field radiating cathode - Google Patents

Abstract

PURPOSE:To emit a stable electron beam by heating a thermoelectric field emitting cathode in a specific condition to volatilize a volatile, object from a hot cathode constitutional member, and then increasing drawout voltage to emit the electron beam. CONSTITUTION:A thermoelectric field emitting cathode, comprising a tungsten monocrystal needle electrode of <100> axial azimuth having a coating layer consisting of Zr and oxygen and a suppressor electrode formed of conductive material with 5X10<-10>m<2>/s or less diffusion speed of hydrogen gas at 600 deg.K, is heated while holding drawout voltage to 1kv or less and a vacuum degree to 1X10<-8>Torr, to volatilize a volatile object from a thermoelectric field emitting cathode constitutional member. Then, the drawout voltage is increased to emit an electron beam.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal field emission cathode used in electron microscopes, length measuring machines, electron beam exposure machines, electron beam testers and the like.

[0002]

2. Description of the Related Art A thermoelectron emitter made of LaB ₆ is used as a stable high-brightness electron source, but it has not reached the radiation conditions required for an ultrahigh-speed electron beam exposure apparatus with higher brightness. Not in. Therefore, in recent years, the axis direction has become <10.
The so-called Zr in which a coating layer made of zirconium and oxygen is provided on a needle electrode of a tungsten single crystal having a 0> orientation.
The O / W thermal field emission cathode has higher brightness than the conventional hot cathode.
It is used because it has a long life and is stable and easy to use as compared with a cold field emission cathode.

[0003]

The above-mentioned thermal field emission cathode is set in a predetermined circuit and evacuated to a vacuum, and then a predetermined electric field is applied while raising the temperature of the thermal field emission cathode to emit an electron beam. However, for stable operation, it is important to secure a limited temperature range and a good vacuum degree. In general, it is necessary to operate in a vacuum having a temperature range of 1400 to 1900 ° K and a vacuum degree of better than 10 ^-8 Torr.

Even when operated under such a condition, there are problems that an electron beam is not easily generated at some time and that a discharge phenomenon occurs at some time to damage the thermal field emission cathode.

The present invention has been made in view of these problems, and it is an object of the present invention to provide a method for operating a thermal field emission cathode which is less likely to cause a discharge phenomenon and can operate efficiently. In particular, the inventors have found that the material of the suppressor electrode, the extraction voltage at the time of startup and the degree of vacuum are important factors, and as a result of intensive investigations, the present invention has been completed.

[0006]

That is, a feature of the present invention is to provide a needle-shaped electrode of a tungsten single crystal with a <100> orientation in the axial direction, which has a coating layer made of zirconium and oxygen.
Hydrogen gas diffusion rate at 00 ° K is 5 × 10 ^-10 m ² / s
A thermoelectric field emission cathode composed of a suppressor electrode formed of a smaller conductive material is heated while keeping the extraction voltage at 1 kV or less and the vacuum degree at 1 × 10 ^-8 Torr or less. It is a method of operating a thermal field emission cathode, characterized in that, after volatilization of volatile matter, an extraction voltage is further raised to emit an electron beam.

[0007]

[Function] Factors for stably extracting the electron beam from the needle electrode of the thermal field emission cathode are the material of the suppressor electrode, the temperature of the needle electrode, the surface condition, the electric field strength of this portion, the vacuum degree of the atmosphere, and the like. To be At the so-called start-up stage which leads to the operating conditions of the thermal field emission cathode, the oxides and the like adhering to the surface of the needle electrode and the suppressor electrode are volatilized, and the surface of each electrode is cleaned. Also,
Gas is also generated from inside the suppressor electrode.

Generation of gas from the inside of the suppressor electrode causes a part of the emitted electrons to collide with the suppressor electrode due to a rise in the temperature of the needle electrode and the application of an electric field, or a part of the suppressor electrode due to thermal radiation from the cathode. The fact that the gas component stored in the suppressor electrode is released by heating the is causing a discharge phenomenon.

Of the stored gases, hydrogen gas has a high diffusion rate, which is the largest cause of the discharge phenomenon. The inventors have continued to study the generation of this hydrogen gas and have found that the above-mentioned discharge phenomenon can be avoided.

That is, the diffusion rate at 600 degrees K is 5 × 10 5.
Discharge occurs by using a conductive material smaller than ^-10 m ² / s for the suppressor electrode and heating it while keeping the extraction voltage below 1 kV and the pressure below 1 × 10 ^-8 Torr before starting. It is possible to volatilize volatiles from the hot cathode constituent member without the need. After that, a stable electron beam can be extracted by increasing the extraction voltage.

The electric field strength acting on the electrode surface is formed by the needle-shaped electrode and the suppressor electrode, and it is considered that if this electric field is too strong, a discharge phenomenon will occur through volatile matter and generated gas. On the other hand, if the electric field strength acting on the electrode surface is too low, the electron beam cannot be easily extracted, and it only takes time for mischief.

Therefore, as described above, in order to practically start up the hot cathode and perform stable operation, the electric field strength on the electrode surface is such that volatile matter and evolved gas are quickly desorbed from the needle electrode and suppressor electrode surfaces. It is important that they are tuned apart.

[0013]

EXAMPLES AND COMPARATIVE EXAMPLES Examples of the present invention will be specifically described below with reference to the accompanying drawings.

FIG. 1 is a sectional view showing the structure of the thermal field emission cathode of the present invention. The needle-shaped electrode 1 made of a tungsten single crystal having an axis orientation of <100> is provided with a coating layer made of zirconium and oxygen on the suppressor electrode 2 so that a predetermined dimensional accuracy is maintained. ing.
The needle electrode 1 is a tungsten wire 3 that heats the needle electrode 3.
The tungsten wire is also welded and fixed to the metal support 5 fixed to the insulator 4. Here, the suppressor electrode is formed of a conductive material having a diffusion rate of hydrogen gas at 600 ° K of less than 5 × 10 ^-10 m ² / s.

Using a suppressor electrode made of the material shown in Table 1, a thermal field emission cathode was prototyped and assembled into the circuit of FIG. 3 to examine the electron emission characteristics. The startup of the thermal field emission cathode at this time was performed according to a typical practical startup schedule shown in FIG. That is, after reducing the pressure to 1 × 10 ^-8 Torr, the temperature was raised to 1500 ° C in 10 minutes while heating, and 1800 ° C after 10 minutes.
The temperature was raised to. After 20 minutes, the extraction voltage was raised to 0.5 kV, and after 5 minutes, to 1 kV. After this, pull out voltage 3kV
And radiated an electron beam. As a result, as shown in Table 1, in Experiment Nos. 1 to 14, there was no discharge phenomenon, and it was possible to start up smoothly, showing a total radiation current of 100 μA to 200 μA and showing good operation. Nos. 15 to No. 18 were not able to operate well due to discharge.

[0016]

[Table 1]

[0017]

According to the operating method of the present invention, a trouble such as a discharge phenomenon does not occur when the thermoelectric field emission cathode is operated under practical startup conditions, and a stable startup and good operating condition are obtained. Therefore, it is particularly effective for applications such as an ultra-high speed electron beam exposure apparatus.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a thermal field emission cathode of the present invention.

FIG. 2 is a typical start-up schedule of the thermal field emission cathode of the present invention.

FIG. 3 is an explanatory diagram of a circuit for evaluating electron emission characteristics of a thermal field emission cathode.

[Explanation of symbols]

 1 Tungsten single crystal needle electrode 2 Suppressor electrode 3 Tungsten wire 4 Insulator 5 Metal support 6 Screw 7 Extraction voltage 12 Bias voltage 13 Total radiation current 14 Heating current

Claims (1)

[Claims] 1. A needle electrode of a tungsten single crystal having a <100> orientation with a coating layer of zirconium and oxygen, and a diffusion rate of hydrogen gas at 600 ° K of 5 × 10 5.
^-10 m ² / s A thermoelectric field emission cathode consisting of a suppressor electrode made of a conductive material smaller than ¹⁰ m ² / s, with an extraction voltage of 1
The method is characterized in that heating is performed at a vacuum degree of 1 × 10 ^-8 Torr or less to kV or less to volatilize volatile substances from the thermoelectric field emission cathode constituent member, and then an extraction voltage is further increased to emit an electron beam. Method of operating a thermal field emission cathode.