JP2006324119A - Electron gun - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electron gun equipped with an electron source in which there is no deviation of optical axes between an electrostatic lens and an electromagnetic lens, variation of lens effect of the electrostatic lens is small even if acceleration voltage and extraction voltage fluctuate widely, and the electromagnetic lens can always focus a high-brightness electron beam and output it. <P>SOLUTION: In order to put the electromagnetic lens 3 near the electron source 1 and decrease the focal distance of the electromagnetic lens 3, pole distance between a bottom magnetic pole and a top magnetic pole is reduced, and the magnetic poles and electrodes are formed separately each other so that distance between the electrodes can be larger than that between the magnetic poles. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to enhancement of the performance of electron guns mounted on scanning electron microscopes, transmission electron mirrors, and their application devices.

  In order to irradiate a specimen with a large-current electron beam in an electron microscope and its application device, it is necessary to use even electrons with a large angle emitted from the electron source. If the aberration of the electron gun lens for emitting the electron beam is large, it becomes impossible to form a small probe with a large current on the sample surface and form an irradiation electron beam with good parallelism.

  Usually, a field emission electron source using field emission electrons or a Schottky electron source using Schottky emission electrons is used as an electron source from which a high-brightness electron beam can be obtained. As an electron gun lens equipped with these electron sources, an electrostatic lens composed of an extraction electrode and an anode electrode is generally used. An acceleration voltage V0 is applied to the electron source, and an extraction voltage V1 is applied between the extraction electrode facing the electron source. By adjusting the extraction voltage and controlling the electric field strength on the surface of the electron source, the electron It is possible to control the intensity of the electron beam that causes field emission or Schottky emission from the source. The electron beam that has passed through the extraction electrode is emitted from the anode electrode, which is normally set to the ground potential, with energy corresponding to the acceleration voltage V0. This extraction electrode and anode electrode form an electrostatic lens to give the electron beam a converging action. The applied voltage of these two electrodes is determined by the acceleration voltage and the extraction voltage, so the focusing lens action must be adjusted. I can't.

  In order to control the focusing effect of the electron beam with this electron gun lens, it can be realized by adding one more electrode. Generally, however, the electrostatic lens has a large aberration, and an electron source is used to take a large current. If the emission angle on the side is made large, the luminance of the electron beam is greatly deteriorated due to the aberration of the lens. Alternatively, in order to adjust the lens action, the lens action of the irradiated electron beam can be adjusted by arranging a condenser lens after the electron gun lens. However, in order for an electron beam to enter the condenser lens, the electron beam must be emitted from the electrostatic lens under nearly parallel conditions, resulting in a strong lens action on the electrostatic lens. The aberration becomes large.

  These problems are solved by superimposing a magnetic lens on the electron gun lens.

be able to. For example, (Patent Document 1) includes a magnetic lens 3 including an extraction electrode 4 as a part of a magnetic pole as shown in FIG. 6, and an electrostatic lens 2 formed by a terminal end of the magnetic lens 3 and an anode electrode 5. A configuration for accelerating to a desired acceleration voltage is disclosed. Further, for example, (Patent Document 2) discloses a configuration in which the extraction electrode 4 and the anode electrode 5 are disposed in the magnetic lens 3 as shown in FIG.

JP 59-42748 JP 10-188868

  In order to prevent the sample from being damaged or charged, an accelerating voltage of several kV or less is often used in electron microscope observation. (Patent Document 1) does not describe an acceleration voltage lower than the extraction voltage. Further, since the magnetic lens and the electrostatic lens are separated, the optical axis shift between the electrostatic lens and the magnetic lens becomes a problem. (Patent Document 2) describes an acceleration voltage lower than the extraction voltage, but the interval between the electrostatic lens 2 formed from the extraction electrode 4 and the anode electrode 5 is smaller than the magnetic pole interval. In such a configuration, the lens action of the electrostatic lens 2 greatly changes with respect to changes in the extraction voltage and the acceleration voltage, and the lens action of the electrostatic lens 2 is inevitably used under a large condition. There was a problem that the aberration of the entire electron gun was increased.

The object of the present invention can be achieved by the following method.
First, a magnetic lens is arranged in the vicinity of the electron source, and the magnetic pole interval between the lower magnetic pole and the upper magnetic pole is shortened in order to shorten the magnetic lens. Next, in order to reduce the change in the action of the electrostatic lens with respect to the change in the voltage applied to the extraction electrode and the anode electrode, it is necessary to increase the distance between the entrance of the extraction electrode and the end portion of the anode electrode. is there. Here, if the magnetic pole and the electrode are the same electromagnetic pole, the magnetic pole interval is widened and the magnetic lens cannot be made short-focused. Therefore, a structure in which the magnetic pole and the electrode are separated from each other may be configured such that the electrode interval is larger than the magnetic pole interval. That is, it is only necessary to provide means for focusing the electron beam emitted by applying the extraction voltage with the magnetic lens in a section where the electron beam is accelerated or decelerated to a desired energy between the extraction electrode and the anode electrode.

According to the present invention, there is no deviation of the optical axis between the electrostatic lens and the magnetic lens, the change in the lens action of the electrostatic lens is small even when the acceleration voltage and the extraction voltage fluctuate greatly, and the magnetic lens always provides high brightness. An electron gun equipped with an electron source capable of focusing and emitting an electron beam can be realized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a configuration for explaining the operation of this embodiment. In this embodiment, a field emission electron source 1 is mounted as an electron source. The field emission electron source 1 is supported by a voltage introduction terminal 17 fixed by a flange 16 and is structured to be suspended in the lens barrel from above. The flange 16 can be moved in the horizontal direction by the alignment mechanism 15, and the horizontal position of the field emission electron source 1 and the extraction electrode 4 can be adjusted. An acceleration voltage V0 is applied to the field emission electron source 1. The extraction electrode 4 is supported by a cylindrical support portion 9 made of an insulator. By applying an extraction voltage V1 between the field emission electron source 1 and the extraction electrode 4, an electron beam 101 by field emission is emitted. The emission current from the field emission electron source 1 is usually adjusted in the range of 1-100 μA. The electron beam 101 accelerated by the extraction electrode 4 reaches the acceleration voltage after passing through the anode electrode 5. Here, the electron beam 101 receives a lens action due to a leakage magnetic field between the upper magnetic pole 6 and the lower magnetic pole 7 of the magnetic lens 3. Further, the electron beam 101 is subjected to the focusing action of the electrostatic lens 2 formed by the extraction electrode 4 and the anode electrode 5.

  FIG. 4 shows details of the structure of the electrostatic lens. If the acceleration voltage V0 is changed, the strength of the electrostatic lens changes greatly. When the extraction voltage V1 is smaller than the acceleration voltage V0, the electrostatic lens is in an acceleration condition and mainly receives the lens action on the extraction electrode 4 side. On the other hand, when the lead-out voltage V1 is larger than the acceleration voltage V0, the deceleration condition is established, and a strong lens action is received on the anode electrode 5 side. Even if the acceleration voltage V0 is constant, the electrostatic lens action changes if the extraction voltage V1 changes. Before field emission from the field emission electron source 1, it is necessary to clean the surface of the electron source by a surface heating process called flushing. This flushing process increases the radius of curvature of the tip of the field emission electron source 1. As a result, the extraction voltage V1 for obtaining a constant emission current for each flushing process gradually increases. Normally, when the electron source is new, the extraction voltage V1 immediately after the flushing is around 3 kV. However, as the use is repeated, the extraction voltage V1 immediately after the flushing increases to 6 kV or more.

  Immediately after the flushing process, the surface of the electron source is clean and the work function of the surface is small, but the work function of the surface gradually increases due to surface adsorbed molecules, and the field emission current decreases. Therefore, in order to increase the emission current that has decreased during use, it is necessary to adjust the extraction voltage V1 to be increased by, for example, 1 kV or more immediately after flushing. As a function of the electrostatic lens used in this embodiment, the electron beam 101 is drawn from the field emission electron source 1 and a desired acceleration voltage V0 is applied. Also, the influence of the change of the lens action on the change of the extraction voltage V1 and the acceleration voltage V0 is small.

  For example, the lead-out electrode of the electrostatic lens has a mortar shape in which the lead-out electrode inlet 21 has a diameter D1 and the lead-out electrode outlet has a diameter D2 (D2> D1). The terminal portion has a mortar shape with a diameter D4 (D3> D4). The diameter of the extraction electrode entrance 21 is about 1 mm, and the potential in the vicinity of the optical axis of the electrode entrance 21 is substantially equal to the extraction voltage. An electrode hole having a diameter of about 1 mm or a fixed diaphragm 23 is disposed at the end portion 22 of the anode electrode, and the potential near the optical axis of the electrode end portion 22 is substantially equal to the anode voltage. For a given extraction voltage V1 and acceleration voltage V0, the greater the distance L between the extraction electrode entrance portion 21 and the anode electrode termination portion 22, the more the electric field strength between the electrodes decreases, thereby focusing the electrostatic lens. The lens action is reduced. Here, if the mortar-shaped apex angles θ1 and θ2 are processed into a shape of 108 °, there is a condition that the spherical aberration becomes small. However, depending on whether the apex angle is mainly used under a deceleration condition or an acceleration condition, Optimal conditions will give different results. Therefore, the section in which the electron beam is accelerated or decelerated to a desired energy, here, the interval between the extraction electrode entrance portion 21 and the ground electrode termination portion 22 is set to be, for example, 20 mm or more.

FIG. 5 shows details of the structure of the magnetic lens 3. The characteristics of the magnetic lens are substantially determined by the distance SM between the upper magnetic pole 6 and the lower magnetic pole 7, and the sum SD = SM + DM1 + DM2 of the inner diameter DM1 of the upper magnetic pole and the inner diameter DM2 of the lower magnetic pole. The shorter the focal length, the smaller the chromatic aberration and spherical aberration. When the excitation of the magnetic lens is increased, the focal length of the magnetic lens can be reduced to about 1/4 of SD. Further, under the same focal length condition, the spherical aberration becomes smaller when operated under the strong excitation condition. In this embodiment, since the magnetic lens 3 is arranged outside the lens barrel, DM1 and DM2 become large. Therefore, in order to shorten the focal length of the magnetic lens 3, the distance SM between the upper magnetic pole 6 and the lower magnetic pole 7 is set to be, for example, 10 mm or less, and the lens main surface position of the magnetic lens is further brought closer to the field emission electron source 1 side. Therefore, the lower end portion of the upper magnetic pole is set to be almost the same height as the electron source.

  As described above, the lens action of the electrostatic lens is small even when the extraction voltage and the acceleration voltage change, and in order to be able to control the focusing action mainly by the magnetic field lens, The distance L between the portion 21 and the anode electrode terminal portion 22 may be set to a length of 20 mm or more, and the distance SM between the upper magnetic pole 6 and the lower magnetic pole 7 of the magnetic lens may be set to 10 mm or less. In general, the upper end 24 of the lower magnetic pole 7 of the magnetic lens 3 may be arranged closer to the electron source 1 than the anode terminal 22 of the electrostatic lens 2.

The vicinity of the electron source 1 is separated from the electrostatic lens 2 by the support portion 9 and the extraction electrode 4, and is differentially evacuated almost independently by a vacuum pump 11 such as an ion pump. By providing a plurality of exhaust holes 12 outside the anode electrode 5 of the electrostatic lens 2 within a range that does not affect the electrostatic lens action, the inside of the electrostatic lens is evacuated using a vacuum pump 13. At the time of baking, the entire lens barrel is baked at 200 ° C. or more, and the extraction electrode portion is heated to about 400 ° C. with the internal heater 14 for a long time, so that the vicinity of the electron source 1 is minus 8 to the power during the operation of the electron microscope. It is kept at a vacuum level of Pascal. The coating material for the coil portion 15 of the magnetic lens 3 is made of polyimide or polyamideimide, and can be baked out without being removed at a temperature of 200 ° C. or higher.
With the above configuration, the lens action of the electrostatic lens is small even when the acceleration voltage and the extraction voltage fluctuate greatly, and it becomes possible to control the focusing of the electron beam with a magnetic lens arranged outside the lens barrel. An electron gun capable of always emitting a high-intensity electron beam can be realized.

  FIG. 2 shows a configuration for explaining the operation of this embodiment. In this embodiment, a field emission electron source 1 is mounted as an electron source. The field emission electron source 1 is supported by a voltage introduction terminal 17 fixed by a flange 16 and has a structure suspended from the upper part in a lens barrel. The distance between the field emission electron source 1 and the extraction electrode 4 is set between 2 mm and 5 mm, for example. Although it is desirable that the upper magnetic pole of the magnetic lens 3 be close to the electron source, if a magnetic material such as Kovar is disposed near the electron source, for example, in a support portion, the magnetic field is disturbed, and the magnetic field leaks to the electron source. It is desirable to reduce this. For this purpose, a structure is adopted in which the inner diameter of the upper magnetic pole 6 is reduced to reduce leakage of the magnetic field on the electron source side. The inner diameter DM1 of the upper magnetic pole 6 of the magnetic lens 3 is set to 15 mm or less, for example. The distance between the upper magnetic pole 6 and the lower magnetic pole 7 is set to 10 mm or less, and the inner diameter DM2 of the lower magnetic pole 7 is increased to 20 mm or more, so that the operation is performed under the strong excitation condition with a short focal length of about 10 mm. Conditions are obtained such that the chromatic aberration coefficient and the spherical aberration coefficient are 10 mm or less. In order to reduce the inner diameter of the upper magnetic pole 6, the upper magnetic pole 6 is disposed in the lens barrel. The upper magnetic pole 6 is spaced by several millimeters from the outer magnetic path 19 outside the lens barrel and generates a magnetic resistance, but does not change the amount of magnetic flux.

  The upper magnetic pole 6 is supported by a spacer 10 via an insulator 9, and the extraction electrode 4 is fixed inside the upper magnetic pole 6. The extraction electrode 4 includes a non-magnetic conductive member in part or in whole so as not to act as a part of the upper magnetic pole 6. With such a configuration, the functions of the extraction electrode 4 and the upper magnetic pole 6 can be separated, and the operations of the electrostatic lens and the magnetic lens can be optimized independently. That is, the lens action of the electrostatic lens that is generally determined by the distance between the entrance portion 21 of the extraction electrode 4 and the end portion 22 of the anode electrode 5 can be adjusted independently of the optimization of the magnetic pole shape described above. In addition, by setting the diameter of the opening of the extraction electrode in a range from 1 mm to 2 mm, for example, the emission angle of the electron beam emitted from the field emission electron source 1 is limited in advance and the opening is made small. Since the space on the electron source side from the electrostatic lens can be evacuated almost independently, the vicinity of the electron source can be maintained in an ultra-high vacuum. An extraction voltage V <b> 1 is applied to the extraction electrode 4 and the upper magnetic pole 6 through a voltage introduction terminal 20. The lower magnetic pole 7 of the magnetic lens 3 is fixed to the outer magnetic path 19, and the outer magnetic path 19 has the same ground potential as that of the lens barrel.

  The outer magnetic path 19 is divided into upper and lower parts, and the coil 8 can be exchanged. Vacuum seals are provided above and below the spacer 10 between the outer magnetic paths 19 to separate the coil 8 in the atmosphere from the vacuum. With respect to the spacer 10, the central axes of the upper magnetic pole 6 and the lower magnetic pole 7 are designed with a certain tolerance, for example, mechanical accuracy of 10 microns or less. The electrostatic lens 2 includes an extraction electrode 4 and an anode electrode 5 that include a part or all of a nonmagnetic conductive member. As the distance between the upper surface of the extraction electrode 4 and the bottom surface of the anode electrode 5 becomes longer, the electrostatic lens action becomes smaller. Therefore, the terminal portion 22 of the anode electrode 5 is arranged closer to the sample side than the upper end portion 24 of the lower magnetic pole of the magnetic lens 3. The upper end 24 of the anode electrode 5 is fixed at substantially the same height as the upper end of the lower magnetic pole 7 and is in electrical contact therewith.

  With the above configuration, there is no deviation of the optical axis between the electrostatic lens and the magnetic lens, and even if the acceleration voltage and the extraction voltage fluctuate greatly, the change in the lens action of the electrostatic lens is small. An electron gun equipped with a field emission electron source that can focus and emit a bright electron beam can be realized.

  FIG. 3 shows a configuration for explaining the operation of this embodiment. A Schottky electron source 31 is mounted as an electron source. The center axis of the Schottky electron source 31, suppressor electrode 32, and extraction electrode 4 is supported by a voltage introduction terminal 17 fixed to the flange 16 while maintaining a mechanical accuracy of a certain tolerance, for example, 20 microns or less. It is structured to be suspended in the lens barrel from the top. The Schottky electron source 31 is heated to a temperature of about 1800 K, and the suppressor electrode 32 is disposed at a position retracted several hundred μm from the tip of the Schottky electron source 31, and minus several hundred V with respect to the Schottky electron source 31. And has a function of returning the thermoelectrons emitted simultaneously with the Schottky electrons to the electron source side. In the field emission electron source, since the distance between the electron source and the extraction electrode 4 is several mm or more, adjusting the electron source and the extraction electrode 4 independently of each other is more advantageous for optical axis adjustment. In the electron gun equipped with the source 31, since the Schottky electron source 31, the suppressor 32, and the extraction electrode 4 are arranged at a distance of 1 mm or less, the positions of each other are fixed in advance with a mechanical accuracy of several tens of μm or less. It is said.

  The voltage introduction terminal 17 is fixed to the flange 16, and the flange 16 can be moved in the horizontal direction by the alignment mechanism 15, and the optical axis with the magnetic lens 3 fixed to the lens barrel side can be adjusted. In order to reduce the inner diameter of the upper magnetic pole 6, the upper magnetic pole 6 is disposed in the lens barrel. The upper magnetic pole 6 and the outer magnetic path outside the lens barrel have a gap of several millimeters and generate a magnetic resistance, but do not change the amount of magnetic flux. The potential of the upper magnetic pole 6 in the lens body is set to either the same potential as that of the lens body or the same potential as that of the extraction electrode 4, but the alignment electrode 15 is used for the upper magnetic pole fixed to the lens barrel. Is movable in the horizontal direction, and depending on the position of the extraction electrode, there may be a condition that the upper magnetic pole 6 and the extraction electrode 4 are close to each other. Therefore, it is desirable that the upper magnetic pole has the same potential as the extraction electrode. In order to set the upper magnetic pole 6 at the same potential as that of the extraction electrode 4, an extraction voltage is set as a structure in which a potential is independently applied to the upper magnetic pole 6 by a voltage introduction terminal, or the extraction electrode 4 is formed on the upper magnetic pole 6 as a spring 33 The potential of the extraction electrode 4 can be set to the upper magnetic pole 6 by making the contact through this material.

  The lower magnetic pole 7 of the magnetic lens 3 is fixed to the outer magnetic path 19, and the outer magnetic path 19 has the same ground potential as that of the lens barrel. The outer magnetic path 19 is divided into upper and lower parts, and the coil 8 can be exchanged. Vacuum seals are provided above and below the spacer 10 between the outer magnetic paths 19 to separate the coil 8 in the atmosphere from the vacuum. With respect to the spacer 10, the central axes of the upper magnetic pole 6 and the lower magnetic pole 7 are designed with a certain tolerance, for example, mechanical accuracy of 10 microns or less. The electrostatic lens 2 includes an extraction electrode 4 and an anode electrode 5. The longer the distance between the entrance portion 21 of the extraction electrode 4 and the end portion 22 of the anode electrode 5, the smaller the electrostatic lens action, so that the end portion 22 of the anode electrode 5 is the upper end portion 24 of the lower magnetic pole of the magnetic lens 3. The upper end portion 24 of the anode electrode 5 is fixed at substantially the same height as the upper end portion of the lower magnetic pole 7 and is in electrical contact. While the anode electrode 5 is fixed to the lens barrel, the extraction electrode 4 can be moved in the horizontal direction by the alignment mechanism 15. By increasing the distance between the anode electrode 5 and the extraction electrode 4, the electrostatic lens action is remarkably reduced, so that the degree of resolution degradation due to the axial shift of the electrostatic lens is negligible.

  With the above configuration, there is no deviation of the optical axis between the electrostatic lens and the magnetic lens, and even if the acceleration voltage and the extraction voltage fluctuate greatly, the change in the lens action of the electrostatic lens is small. An electron gun equipped with a Schottky electron source capable of focusing and emitting a bright electron beam can be realized.

In addition, by using the electron gun according to the present invention in an electron beam application apparatus such as an inspection apparatus,
Even if the acceleration voltage and the extraction voltage fluctuate greatly, the change in the lens action of the electrostatic lens is small, and a magnetic lens can always focus and emit a high-brightness electron beam, which contributes to improvement in resolution.

  Although the anode potential is described as the ground potential in the first to third embodiments, the present invention can be realized without departing from the object of the present invention even under a condition where the anode voltage is not the ground potential.

The figure which shows the structure of the mirror electron microscope which becomes the 1st Example of this invention. The figure explaining the structure of the 2nd Example of this invention. The figure explaining the structure of the 3rd Example of this invention. The figure explaining the structure of an electrostatic lens. The figure explaining the structure of a magnetic lens. The figure explaining a prior art example. The figure explaining a prior art example.

Explanation of symbols

  1: field emission electron source, 2: electrostatic lens, 3: magnetic lens, 4: extraction electrode, 5: anode electrode, 6: upper magnetic pole, 7: lower magnetic pole, 8: coil, 9: support, 10: spacer 11: vacuum pump, 12: exhaust hole, 13: vacuum pump, 14: internal heater, 15: alignment mechanism, 16: flange, 17: voltage introduction terminal, 18: bellows, 19: external magnetic circuit, 20: voltage introduction Terminal: 21: Entrance of lead electrode, 22: Terminal end of anode electrode, 23: Fixed aperture, 24: Upper end of lower magnetic pole, 31: Schottky electron source, 32: Suppressor, 33: Spring, 101: Electron beam, 102: optical axis.

Claims (5)

An electrostatic lens including an electron source, an extraction electrode that extracts an electron beam from the electron source, and an anode electrode that sets the electron beam to a desired acceleration voltage, and a magnetic lens that focuses the electron beam in the vicinity of the electron source In an electron gun with
The extraction gun and the anode electrode include a nonmagnetic conductive member, and the magnetic pole of the magnetic lens is disposed closer to the electron source than the terminal end of the anode electrode. An electron source,
An electrostatic lens including an extraction electrode that extracts an electron beam from the electron source and an anode electrode that sets the electron beam to a desired acceleration voltage;
In an electron gun including a magnetic lens for focusing the electron beam in the vicinity of the electron source, the electron beam is focused in an acceleration or deceleration period until the electron beam is set to a desired acceleration voltage by the anode electrode. An electron gun characterized by that. An electron source,
An electrostatic lens including an extraction electrode that extracts an electron beam from the electron source and an anode electrode that sets the electron beam to a desired acceleration voltage;
In an electron gun provided with a magnetic lens for focusing the electron beam in the vicinity of the electron source,
An electron gun comprising the magnetic lens between the extraction electrode and the anode electrode. The electron gun according to claim 3,
The magnetic lens includes a first magnetic pole and a second magnetic pole,
The distance between said 1st and 2nd magnetic pole is smaller than the distance between the entrance part of said extraction electrode, and the terminal part of an anode electrode, The electron gun characterized by the above-mentioned. The electron gun according to any one of claims 1 to 3,
The electron gun is a field emission electron source or a Schottky electron source.

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