JPH0316735B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thermionic electron gun used in scanning electron microscopes, electron beam exposure devices, and the like.

[Prior Art] In scanning electron microscopes and the like, a thermionic electron gun having a so-called hairpin cathode or a cathode made of lanthanum hexaboride (LaB ₆ ) or the like is used.

FIG. 7 shows a conventional thermionic electron gun having a hairpin type cathode. In the figure, 1 is a filament formed by bending a tungsten wire into a hairpin shape. A voltage of, for example, -20 Kv is applied to the filament 1 from a high voltage power source (not shown), and a heating current is supplied from a heating power source (not shown). 2 is a Wehnelt electrode, and a negative voltage -Φ with respect to the filament 1 is applied to the Wehnelt electrode 2. 3 is an anode kept at ground potential, and a lens field is formed between the cathode 1 and the anode 3. 2a and 3a are circular holes provided in the Wehnelt electrode 2 and anode 3, respectively.

[Problems to be Solved by the Invention] In such a conventional thermionic electron gun, when the potential -Φ of the Wehnelt electrode 2 with respect to the filament 1 is changed, the brightness β of the electron gun changes as shown by the curve A in FIG.
Changes as shown in . However, in Figure 8,
The horizontal axis shows voltage Φ, and the vertical axis shows brightness β. As is clear from this diagram, Φ is approximately 80v
When the brightness β is at a maximum, the brightness β is at a maximum. Now, in FIG. 9, which shows an enlarged view of the A-A arrow view in FIG. , 0, the trajectory of the electron beam EB in the plane of 0 is as shown in FIGS. 10a and 10b, respectively, when Φ is about 80V. In both figures, the same components as in FIG. 7 are given the same numbers.

In Figures 10a and b, the direction of θ=π/2 (direction perpendicular to the bending of the tungsten wire)
The width of the electron beam flux at the crossover position U is shown as W(π/2), and the width at the position U in the direction of θ=0 is shown as W(0). This figure 10 a, b
As is clearer, since W(π/2) and W(0) are relatively large and W(0)>W(π/2), the electron beam probe has an enlarged electron beam crossover image. The cross-sectional shape becomes a relatively large ellipse as shown in FIG. 11, and such an electron beam cannot be used. Therefore, in the past, an electron beam probe with a relatively small cross section was obtained and used by setting Φ to about 26 V at the expense of brightness to create a state with less astigmatism.

An object of the present invention is to solve such conventional drawbacks and provide a thermionic electron gun that can obtain an electron beam probe with a small cross-sectional shape and rotational symmetry even when the brightness is at approximately maximum. .

[Means for Solving the Problems] In order to achieve such an object, the present invention includes a cathode for generating thermionic electrons, a heating means for the cathode, and a Wehnelt electrode having a hole in the center, In a thermionic electron gun in which the radius of curvature of the cathode tip surface differs depending on the angle θ formed with a reference plane including the axis, the radius of curvature with the angle θ as a variable is expressed as r(θ), and the Wehnelt in the direction of the angle θ is expressed as r(θ). The width of the electrode hole is R(θ)
The hole of the Wehnelt electrode is formed out of rotational symmetry so as to satisfy the following relationship: r(θ)·R(θ)=constant for any θ.

[Operation of the Invention] Next, the basic idea of the present invention will be explained based on FIGS. 12 to 13.

Now, if the radius of curvature r of the tip surface of the tungsten filament 1 is expressed using the angle θ as a variable, then
As shown by the dotted line P in FIG. 12, it is approximately elliptical. That is, the polarity radius r(θ) has a minimum value defined by the wire diameter of the wire itself at θ = π/2, and a maximum value in the direction of θ = 0 (the direction of bending of the wire). . Therefore, the width of the hole of the Wehnelt electrode in the angle θ direction is expressed as R(θ),
The hole of the Wehnelt electrode is made into an ellipse as shown by the solid line Q in FIG. 12 so as to satisfy the following relationship: r(θ)·R(θ)=constant for any given θ. In this way, when the voltage Φ that maximizes the brightness is applied, the EB trajectories of the electron beam in the planes θ=π/2 and θ=0 will be as shown in Figure 13a and b, respectively. . In FIG. 13, the same components as in FIG. 7 are given the same numbers. As is clear from Figure 13a and b, θ=
In the direction of π/2, since the width R (π/2) of the hole 4a of the Wehnelt electrode 4 is wider than before, the extraction voltage leaks out from the anode 3 side, and the voltage leaks along the surface of the filament 1. Electron beams emitted in a direction away from the optical axis are bent toward the optical axis earlier than in the past. In addition, in the direction of θ=0, the width R(0) of the hole 4a of the Wehnelt electrode 4
Since the electron beam is narrower than before, the electron beam emitted along the surface of the filament 1 in a direction away from the optical axis is also bent toward the optical axis at an earlier stage than before. Therefore, the crossover positions of the electron beam EB coincide in the directions of θ=π/2 and θ=0, and the widths W(π/2) and W(0) are also relatively small and equal to each other.

In addition, in FIGS. 13a and 13b, dotted lines indicate the shape of the hole in the conventional Wehnelt electrode.

[Example] Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 1 is for showing an example of the case where the present invention is implemented in a thermionic gun having a tungsten filament. In FIG. 1, the same components as in FIG. 9 are given the same numbers. There is.

Unlike conventional ones, the hole provided in the Wehnelt electrode 2 has a shape close to an ellipse, and the major axis R (π/2)
is 4.5 mm, and the minor axis R(0) is 1.5 mm. Further, the direction of the major axis is perpendicular to the bending direction of the filament 1.

With this configuration, the brightness β of the thermionic electron gun changes as a function of the voltage Φ, as shown by curve A in Figure 2, but if Φ is set to about 22 V, which is the value at which the brightness β becomes maximum, Even with this setting, as is clear from the optical diagrams shown in FIGS.
In order to alleviate the astigmatism of the lens field formed between the anode 3 and the anode 3, the cross-sectional shape of the electron beam probe is rotationally symmetrical, with a diameter of about 1/3 of the conventional one, as shown in Figure 3. It can be done.

Next, an embodiment in which the present invention is applied to a thermionic electron gun using an emitter made of lanthanum hexaboride will be described with reference to FIG.

If you continue to use lanthanum hexaboride emitters,
Due to the so-called faceting phenomenon during emitter evaporation, the tip of the emitter, which was initially formed into a conical shape, becomes square pyramidal as shown in FIG. Therefore, the radius of curvature r at the tip of the emitter
(θ) is θ as shown by the dotted line P in FIG.
The maximum value r in the direction of = ±nπ/2 (n is an integer)
max, θ=π/4±nπ/2 (n is an integer)
has a minimum value r min at .

Therefore, in this embodiment, the holes in the Wehnelt electrode are shown by solid lines Q in FIG. In this way, the above-mentioned equation (1) is substantially satisfied, so that the same effect as the embodiment shown in FIG. 1 can be achieved.

In this embodiment, substantially the same effect can be achieved even if the hole of the Wehnelt electrode is made square as shown in FIG. 6 for convenience of processing.

[Effects of the Invention] As is clear from the above description, in the thermionic electron gun based on the present invention, the radius of curvature at the tip surface of the cathode differs depending on the angle θ formed with the reference plane including the optical axis. , when the radius of curvature with the angle θ as a variable is expressed as r(θ), and the width of the hole of the Wehnelt electrode in the direction of the angle θ is expressed as R(θ), approximately r(θ) for any θ. )・R(θ) = Since the hole of the Wehnelt electrode is formed out of rotational symmetry so as to satisfy a certain relationship, astigmatism of the lens field between the cathode and anode occurs when the brightness is maximized. Therefore, the crossover of the electron beam from the electron emission source can be reduced to about 1/3 compared to the conventional one, and a rotationally symmetrical structure can be achieved. Therefore, since the thermionic electron gun according to the present invention can provide a high-luminance, narrow electron beam probe, image quality can be improved when used in a scanning electron microscope or the like.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the relationship between brightness and Wehnelt electrode voltage in the thermionic gun shown in FIG. 1, and FIG. 3 is a diagram showing the present invention. Figure 4 to show the cross-sectional shape of an electron beam probe obtained by a thermionic gun based on
Figure 5 is a diagram for showing another embodiment of the present invention.
The figure is a diagram showing a modification by using an emitter using lanthanum hexaboride as the cathode material, Figure 6 is a diagram showing another embodiment of the present invention, and Figure 7 is a diagram showing a conventional thermionic gun. FIG. 8 is a diagram showing the relationship between brightness and Wehnelt electrode voltage in a conventional thermionic electron gun, and FIG. 9 is a diagram showing A- in FIG. 8.
Figures 10a and 10b, which are enlarged views of the A arrow view, are θ=π/2 and θ=0, respectively, in the conventional thermionic electron gun.
Diagram for showing the trajectory of the electron beam in the direction, 1st
Figure 1 is a diagram showing the cross-sectional shape of an electron beam probe obtained by a conventional thermionic electron gun, and Figure 12 is a diagram showing the curvature r (θ) of the cathode tip surface and the width R ( FIGS. 13a and 13b are diagrams showing the trajectory of the electron beam in the θ=π/2 and θ=0 directions, respectively, to show the effect of the present invention. 1: filament, 2, 4: Wehnelt electrode, 2a, 4a, 5: hole of Wehnelt electrode,
3: Anode part, C: Optical axis, X: Reference plane.

Claims (1)

[Claims] 1. A cathode for generating thermionic electrons, a heating means for the cathode, and a Wehnelt electrode having a hole in the center, and a tip of the cathode according to an angle θ formed with a reference plane including an optical axis. In a thermionic electron gun whose surfaces have different radii of curvature, the radius of curvature with angle θ as a variable is expressed as r(θ),
When the width of the hole of the Wehnelt electrode in the direction of the angle θ is represented by R(θ), the Wehnelt electrode is adjusted so that approximately r(θ)・R(θ)=constant relationship is satisfied for any θ. A thermionic electron gun characterized in that the hole is formed out of rotational symmetry. 2. The cathode is formed by bending a wire into a hairpin shape, and the hole of the Wehnelt electrode has its short diameter in the direction of the bending where the radius of curvature is maximum on the tip surface of the hairpin cathode. death,
The thermionic electron gun according to claim 1, wherein the thermionic electron gun has a substantially elliptical shape having a major axis in a direction perpendicular to the above direction.