JPS6216501B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electron gun that can suppress beam current drift caused by thermal expansion of a Wehnelt electrode.

In electron microscopes and the like, a thermionic emission type electron gun as shown in FIG. 1 is widely used. That is, the filament 2 is supported by the filament stand 1, and the filament stand 1 is pressed by screws 3a, 3.
A Wehnelt electrode 6 is fixed to the protrusion 4a in the base 4 by b and has an electron beam passage hole 5.
is fixed to the base 4 so as to surround the filament 2 by means of screws 7 formed at its ends. The base body 4 is fixed to an insulator 8 installed in an electron gun chamber (not shown). In such an electron gun, radiant heat from the filament 2 is generated by heating the filament 7 with electricity for electron emission.
and heat conduction, the Wehnelt electrode 6
Then, the base body 4 is heated and expands excessively over a long period of time. Therefore, the distance between the filament 2 and the electron beam passage hole 5 of the Wehnelt electrode 6 changes, causing a drift of the beam current over a long period of time (more than one hour) as shown by the solid line in FIG. Note that the change in beam current shown by the solid line a in FIG. 2
This is due to a change in the distance between the electron beam passage hole 5 of the Wehnelt electrode and the electron beam passage hole 5 of the Wehnelt electrode.

The present invention aims to prevent such long-term changes in beam current, and will be explained in detail below based on the embodiment shown in FIG. In FIG. 3, the same numbers as in FIG. 1 indicate the same components.

That is, in the present invention, the filament stand 1 holding the filament 2 is directly attached to the base 4 as in the conventional case.
It is characterized in that it is fixed to a protrusion 4a inside the base body 4 via a cylindrical support member 9 with set screws 3a and 3b, as shown in FIG. 3, without being fixed inside. 10a and 10b are push screws for fixing the filament stand 1 to the tip of the support member 9.

In such a configuration, if a support member 9 whose thermal expansion α ₁ is larger than the thermal expansion α ₂ of the member constituting the Wehnelt electrode 6 is used, the thermal expansion of the Wehnelt electrode 6 can be corrected by the thermal expansion of the support member 9. can. This point will be briefly explained below.

First, the Wehnelt electrode 6 in such a configuration
The base point for thermal expansion on the side and the filament 2 side is the fixed part of the support member 9 to the base body 4, that is, the base point on the base body 4
This serves as an engagement surface C between the protrusion 4a and the support member 9.

Therefore, the electron beam passing hole 5 of the Wehnelt electrode 6
The parts are the Wehnelt electrode 6 and the upper end of the base 4,
In other words, from the engagement surface C to the tip f of the Wehnelt electrode 6
move upward by the amount of thermal expansion (elongation) corresponding to the length l ₂ . Further, the tip of the filament 2 similarly moves upward by the amount of thermal expansion of the filament 2, the protruding portion of the filament supporting electrode rod, and the support member 9. Therefore, if the amount of movement due to thermal expansion of the Wehnelt electrode and the upper end of the base is made equal to the elongation due to thermal expansion of the filament, the filament supporting electrode rod, and the support member, the distance between the electron beam passage hole of the Wehnelt electrode and the tip of the filament is Since it can always be kept constant, the drift of the beam current can be kept low.

Here, the electron beam passage hole 5 of the Wehnelt electrode 6
Considering the movement of the parts, since the base body 4 and the Wehnelt electrode 6 are usually formed of the same metal material, the upper end of the base body can be substantially regarded as a part of the Wehnelt electrode. On the other hand, when moving the filament tip side, first the filament 2
Since the beam reaches a certain high temperature in a short period of time, its influence subsides in a short period of time, as shown by the solid line a in Figure 2, and can be ignored against long-term beam current fluctuations. . Furthermore, since the length of the protruding portion of the filament-supported electrode rod is very short, the thermal expansion of the protruding portion can be ignored.
Therefore, it can be considered that the movement of the filament tip is substantially determined by the thermal expansion of the support member 9 alone. At this time, since the support member 9 is placed inside the Wehnelt electrode 6, the length of the support member is shorter than the Wehnelt electrode, and the temperatures of the support member and the Wehnelt electrode are maintained at approximately the same level. As a result, the coefficient of thermal expansion of the support member is selected to be larger than the coefficient of thermal expansion of the Wehnelt electrode, and the engagement surface e between the support member and the filament base 1 is selected from the engagement surface c.
The thermal expansion of the Wehnelt electrode corresponding to the length _l2 may be compensated for by the thermal expansion of the support member corresponding to the length _l1 .

In the present invention described in detail above, when supporting the filament inside the Wehnelt electrode, a support member having a coefficient of thermal expansion larger than that of the Wehnelt electrode is interposed, so that the thermal expansion of the Wehnelt electrode is controlled by the support member. This can be corrected by thermal expansion, and the distance between the electron beam passage hole of the Wehnelt electrode and the filament tip can always be kept constant. Therefore, as shown by the broken line in FIG. 2, the drift of the beam current can be suppressed to a low level, and the beam current can be kept stable for a long time.

FIG. 4 shows another embodiment of the present invention, in which the Wehnelt electrode 6 is fixed to the base 4 and the support member 9 is fixed to the base 4 on substantially the same plane. It shows the case.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing a thermionic emission type electron gun, FIG. 2 is a view showing beam current drift, FIG. 3 is a cross-sectional view showing an embodiment of the present invention, and FIG. 4 is a cross-sectional view showing an embodiment of the present invention. FIG. 1: Filament stand, 2: Filament, 3
a, 3b, 10a and 10b: push screw, 4: base, 5: electron beam passage hole, 6: Wehnelt electrode,
8: Insulator, 9: Support member.

Claims (1)

[Claims] 1 A cylindrical base 4, a filament 2 that emits electrons, a cylindrical support member 9 that substantially holds the filament at one end and is attached to the inside of the base at the other end, and surrounds the filament. A Wehnelt electrode 6 is attached to the tip of the base body so that the Wehnelt electrode and the support member elongate in the same direction with respect to the fixed point between the base body and the support member due to thermal expansion caused by heating the filament. None, and by making the coefficient of thermal expansion of the supporting member larger than the coefficient of thermal expansion of the member constituting the Wehnelt electrode, the elongation of the Wehnelt electrode tip from the base point and the elongation of the filament tip are made approximately equal. An electron gun characterized by the following.