Thermal field emission electron gun

There is disclosed a long-lived thermal field emission electron gun for use in a scanning electron microscope. The gun has a tungsten tip. The surface of this tip is coated with zirconium, zirconium oxide, titanium or titanium oxide. A wire member is mounted above the front end of the tungsten tip to prevent the coating of zirconium or other material from slipping off.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a scanning electron microscope and, more 
particularly, to a thermal field emission electron gun comprising an 
emitter tip that is heated and applied with an electric field to eject 
electrons from the tip. 
2. Description of the Prior Art 
A thermal field emission electron gun comprising an emitter tip that is 
heated and applied with an electric field to eject electrons from the tip 
is used as an electron gun in a scanning electron microscope or other 
similar instrument. In the 1970s, an electron gun using a Schottky type 
emitter was developed as this kind of electron gun. This type of electron 
gun is described in detail in J. Vac. Sci. Technol., 16, p. 1704 (1979). 
The emitter of the Schottky type electron gun is shown in FIG. 1, where to 
pins 2 and 3 are attached to a ceramic insulator disk 1. A tungsten wire 4 
bent into a hairpin is stretched between the two pins 2 and 3. A 
cylindrical tungsten tip 5 of a single crystal is welded to the protruding 
front end of the tungsten wire 4. Usually, this tungsten tip 5 consists of 
a (100) single crystal and has a diameter of about 125 .mu.m. 
This tungsten tip 5 is obtained by welding the single-crystal tungsten to 
the tungsten wire 4 in the form of a hairpin and then electrolytically 
polishing the front end of the single-crystal tungsten into a needle-like 
form. This needle-like tip makes an angle of approximately 10 to 
30.degree.. 
After the tungsten tip 5 is machined as described above, it is coated with 
zirconium hydride (ZrH.sub.2), 6, and sintered. The zirconium hydride 
coating 6 is oxidized by the heating and becomes a mass of zirconium (Zr) 
or zirconium oxide (ZrO.sub.2), which is essential to forming a monolayer 
of zirconium at the tip of the single-crystal tungsten. The (100) plane of 
the thermal field emission electron gun with such a coating has a 
decreased work function and so the gun exhibits high brightness and long 
life. 
The Schottky type emitter is fabricated by the processing described above. 
The principle of operation of the electron gun using this emitter is 
described by referring to FIG. 2. A heating power supply 7 is mounted 
between the two pins 2 and 3, which in turn are affixed to the insulator 
disk 1. This power supply 7 electrically energizes the tungsten wire 4 via 
the pins 2 and 3. 
A suppressor electrode 8 is mounted to the insulator disk 1 so as to cover 
the disk 1 except for the front end of the tungsten tip 5. A suppressor 
power supply 9 applies a negative potential to the suppressor electrode 8 
with respect to the tungsten tip 5. An extraction electrode 10 is mounted 
close to the front end of the tungsten tip 5. An extraction voltage is 
applied to the extraction electrode 10 from an extraction voltage source 
11. An accelerating electrode (not shown) is positioned in front of the 
extraction electrode 10 such that an accelerating voltages is applied 
between the tungsten tip 5 and the accelerating electrode. 
In the structure described above, a heating current is supplied to the 
tungsten wire 4 from the heating power supply 7 to heat the tungsten wire 
4 up to approximately 1800 K. The extraction voltage, normally about 1 to 
6 kV, is applied between the tungsten tip 5 and the extraction electrode 
10 from the extraction voltage source 11. 
As a result, electrons are extracted from the front end of the tungsten tip 
5, accelerated to 3 kV, for example, by the accelerating electrode (not 
shown), and sharply focused onto a specimen by condenser lenses and an 
objective lens. The suppressor electrode 8 is put at a negative potential 
with respect to the tungsten tip 5 by the application of voltage from the 
suppressor power supply 9. Consequently, the thermal emission from the 
tungsten tip 5 is suppressed except from the front end. 
In the thermal field emission electron gun of the construction shown in 
FIG. 2, the sintered zirconium 6' (Zr of ZrO.sub.2) coated on the tungsten 
tip 5 wears down with the lapse of time. This phenomenon is especially 
conspicuous at the interface between the tungsten tip 5 and the zirconium 
mass 6'. This interface is shown in the cross section of FIG. 3. That is, 
a gap 12 is created between the tungsten tip 5 and the zirconium mass 6'. 
If this gap 12 forms and grows to about 10 .mu.m, the zirconium mass 6' 
slips off because the tungsten tip 5 is cylindrical in shape. This stops 
the generation of the electron beam from the tungsten tip 5. This slip 
occurs earlier than the wear of the zirconium itself and thus determines 
the life of this kind of thermal field emission electron gun. 
SUMMARY OF THE INVENTION 
In view of the foregoing circumstances, the present invention has been 
made. It is an object of the invention to provide a thermal field emission 
electron gun in which a mass of zirconium is prevented from slipping off, 
thus prolonging the lifetime. 
This object is achieved by a thermal field emission electron gun comprising 
a tungsten tip, a coating formed on the surface of the tip above the front 
end of the tip, and a slip preventive member mounted above the front end 
of the tungsten tip. This slip preventive member prevents the coating from 
slipping off. 
In one feature of the invention, one or more grooves are formed above the 
front end of the tungsten tip described above to prevent the coating from 
slipping off. 
Other objects and features of the invention will appear in the course of 
the description thereof, which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 4, there is shown main portions of a thermal field 
emission electron gun in accordance with the present invention. It is to 
be noted that like components are denoted by like reference numerals in 
various figures and that those components which have been already 
described in connection with FIGS. 1 and 2 will not be described in detail 
below. 
A tungsten wire 13 is spot-welded to a central portion of the tungsten tip 
5. A slurry of zirconium hydride (ZrH.sub.2), or zirconium hydride 
dispersed in a solvent, is applied to this wire 13. The slurry is sintered 
by heating at about 1800 K. As a result, a mass of zirconium 6' is 
obtained. 
The electron gun constructed as described above is operated. Normal heating 
at about 1800 K wears down the zirconium mass 6' in a normal manner. In 
this embodiment, however, if a gap of about 30 .mu.m is formed at the 
interface between the zirconium mass 6' and the tungsten tip 5, the wire 
13 prevents the zirconium mass 6' from slipping off, otherwise the 
generation of the electron beam would be stopped. Thus, a long-lived 
electron gun is provided. This zirconium hydride coating is wider in 
surface area than in the case of FIG. 1. Therefore, it is easier to apply 
the slurry of zirconium hydride. At the same time, the zirconium hydride 
is less likely to slip off prior to the sintering. 
Referring next to FIG. 5, there is shown an electron gun that is similar to 
the electron gun shown in FIG. 4 except that a mesh member 14 made of fine 
wires of tungsten is used instead of the wire 13. This mesh member 14 is 
wound around the tungsten tip 5 and spot-welded to it. This mesh member 14 
yields the same advantages as the wire shown in FIG. 4. 
Referring next to FIG. 6, there is shown an electron gun that is similar to 
the electron gun shown in FIG. 4 except that a groove 15 about 30 .mu.m 
deep is formed in the tungsten tip 5 by electropolishing or other method 
instead of using a wire or a mesh member. Zirconium hydride is applied to 
this groove. The mass of zirconium 6' does not slip off until the inside 
diameter of a gap formed in the groove 15 reaches the diameter of the 
needle-like tip 5. 
Referring now to FIG. 7, there is shown a further electron gun in 
accordance with the present invention. In this embodiment, a plurality of 
grooves 16 prevent the mass of zirconium from slipping off in the same way 
as the single groove in the embodiment illustrated in FIG. 6. 
While some preferred embodiments of the present invention have been 
described, it is to be understood that the invention is not limited to 
them. Rather, various changes and modifications are possible. In the 
embodiments described above, zirconium is coated on the tungsten tip and 
sintered. Instead, zirconium oxide, titanium or titanium oxide may be 
applied with equal utility to the tungsten tip. 
Having thus described our invention with the detail and particularity 
required by the Patent Laws, what is desired protected by Letters Patent 
is set forth in the following claims.