JPS5918540A - Field emission cathode - Google Patents

Abstract

PURPOSE:To enable the mass production, with the same specifications, of a field emission cathode consisting of a pin-type chip and a filament that holds it and simplifying the structure of the cathode by selecting one from groups Ti, Zr, and Hf as the filament material of the cathode. CONSTITUTION:At least one metal selected from groups Ti, Zr, and Hf or an alloy containing these is used as the filament 6 of a field emission cathode consisting of a W single crystal pin-type chip 2 of azimuth (100) and the filament 6 that can hold and heat the chip. This cathode is mounted on a vacuum device and the W chip is purified by the high temperature flushing. Then the metals Ti, Zr, and Hf are diffused on the tip of the W chip from the filament section by the aging treatment. As a result, the cathode with good beam directivity and stable emission current can be mass-produced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electron source useful in electron beam application equipment such as electron microscopes.

A monoatomic layer of T is formed on the surface of a tungsten (W) needle-like tip.
When metals such as i, z, r, and Hf are attached and heat treated in a vacuum containing a trace amount of oxygen, the work function of the (1001 plane) decreases, and electron emission occurs preferentially from this crystal plane. Electron emitting cathodes have good electron beam directionality and stable emission current, so they are beginning to be used in various electron beam application devices.A trace amount of oxygen is naturally extracted from residual gas in vacuum during the heat treatment process. Therefore, in order to realize the above cathode, only T', z ”
Efforts have been made to find out how to properly supply metals such as eHf to the W needle tip. A typical method conventionally used is shown in Fig. 1. In (a), a lump 3 of a hydride such as Zr is attached to the root of a W needle-shaped tip, and the filament 1 is heated with electricity to remove this compound. Evaporate it W
Method for attaching Zr metal atoms to the surface of a needle-like tip, (b
) is a method of wrapping a thin wire 4 of T;, Zr, Hf, etc. around a part of the filament 1 and evaporating it; (C)
Another method is to provide an evaporation source 5 of 'I'i, Zr, Hf, etc. around the field emission cathode. .

These cathodes have a more complex structure than field emission cathodes that do not contain Zr, Hf, etc., and this poses a problem in producing large quantities of cathodes of the same standard. Ta. Especially Ti, zr.

In this type of field emission cathode with a metal such as Hf attached, the surface of the needle-like tip must be constantly covered with a thin metal layer of about a monoatomic layer during operation, and the operating conditions must be strictly regulated. However, the complexity of the structure of conventional cathodes has been an obstacle.

The object of the present invention is to provide an electron-emitting cathode with a simplified structure, which has been developed to mass-produce electron-emitting cathodes coated with 'pi, Zr, )(f) according to the same standard. It is in.

The filament of a field emission cathode using a W chip is usually made of high-melting point metals such as W, MO, Ta, etc., because it is necessary to clean the W chip by heating it in a vacuum to a high temperature of 2000 C or more before operation of the cathode. is used. During the cleaning process, impurities evaporate and W atoms on the surface diffuse, resulting in a clean and smooth chip. Even in a field emission cathode of the type in which Ti, zr, y(f is attached to the tip of the W tip,
This is also the reason why W has conventionally been exclusively used as a filament material. However, according to detailed experiments by the present inventors, Ti, zr.

1500 for a type of field emission cathode with Hf deposited.
The W needle-shaped tip can be cleaned even at heating temperatures below t:'.
Moreover, it has been found that it is possible to deposit the metal on the chip surface with good reproducibility to the extent of a monoatomic layer. This is interpreted to be because the presence of metals such as TI, Zr, and Hf on the W surface lowers the surface energy [7], and surface diffusion can occur even at low temperatures. Therefore, in this case, the reason why a high melting point material such as W must be used as the filament material disappears.

The present invention has been made with this point in mind, and is characterized by using any one of Ti, Zr, and I-If as the filament material. Here, since the filament and the metal element supply source can be shared, the structure of the electron-emitting cathode can be greatly simplified. The melting points are T i (166s
c), Z r (ls52c).

Hf (222oc). Furthermore, the filament material is not limited to the above-mentioned single metal, and its melting point is 150
If it is 0tl:' or more, an alloy is also possible. The metal elements used as alloys must have a higher melting point than Ti, ZrJ (f), and a lower vapor pressure than Ti, Zr, and Hf. Examples of such metal elements include 11
(u, le, Qs, Ir, Nb.

MO, Ta, and W are preferable, and MO, Ta, and W are more preferable from the viewpoint of ease of manufacturing the alloy, workability, economic efficiency, and the like. When a filament made of an alloy of these metals and Tj, Zr, or Hf is heated, Ti, Zr, or Hf is preferentially evaporated, and this is supplied to the tip of the W emitter.

A specific example of such an alloy is w-Ti.

Examples include W-Zr and W-Hf. Also Ti, zr.

Alloys of Hf, such as Ti-Zr and Ti-Hf.

Zr-)(f, etc. are also used.

Examples will be explained below.

Example 1 Figure 2 is a structural diagram of an electron emitting cathode, l is <100>
Orientation (2) W single crystal chip, 2 is Ti, Zr.

Or a filament made of Hf. This cathode was installed in a vacuum device, and the W chip was first cleaned by high-temperature flushing, and then Ti and Zr were removed from the filament by aging treatment.

Ht was diffused onto the tip of the gold metal W tip. Table 1 shows a comparison of appropriate cleaning temperatures and aging when using each metal filament. In addition,
The oxygen partial pressure in vacuum during the experiment is also shown. As is clear from this table, the filament material contains Ti and 7. r, H
It is clear that by using metal wires such as f, it is possible to stably operate an electron emitting cathode whose W chip surface is covered with these metals. When we tried electron-emitting cathodes using filaments of Ti, Zr, and Hf as electron sources for electron microscopes, we found that all of them operated stably and fully utilized the features of cathodes coated with these metals. Ta.

Example 2 An electron emitting cathode having a structure similar to that shown in FIG. 2 was prepared using W-25 at% Ti as the filament material.

It was produced using a W-5at%Zr+W10at%Hf alloy. Table 2 shows the cleaning temperature, aging temperature, and oxygen partial pressure in vacuum. When an alloy is used for the filament, the aging temperature is about 10% higher than when a single metal is used, but all of them were able to operate stably as an electron source.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the structure of an electron-emitting cathode produced by a conventional method, and FIG. 2 is a diagram showing the structure of an electron-emitting cathode manufactured in an example of the present invention. DESCRIPTION OF SYMBOLS 1...W filament, 2...(100)W needle-like tip 3...zr hydride, 4...Ti, zr, H
Thin line such as r, 5...'pi, Zr, or) if
Evaporation source, 6 kudzu 1 figure

Claims (1)

[Claims] 1. A field emission cathode consisting of a tungsten needle tip and a filament that can hold and heat the tip,
Ti as the filament material. A field emission cathode characterized by using at least one metal selected from the group consisting of zr and )(f) or an alloy containing the same.