JP2006032195A - Electron emission source - Google Patents

Abstract

The present invention provides an electron emission source suitable for an electron beam exposure apparatus, an Auger spectroscopic apparatus, etc., and having a small total radiation current amount even in a high angular current density operation.
A cathode comprising a tungsten or molybdenum single crystal needle provided with a supply source for diffusing one or more metal elements selected from the group consisting of 2A group, 3A group, and 4A group, and the single crystal An electron emission source comprising a suppressor electrode having a hole through which the needle protrudes, wherein an end of the single crystal needle has a truncated cone shape having a full cone angle of 25 ° to 95 °, An electron emission source characterized in that the position of the boundary portion between the conical surface of the end portion and the side surface of the single crystal needle is not projected from the hole of the suppressor electrode.
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Description

The present invention relates to an electron emission source used for electron beam application equipment such as a scanning electron microscope, Auger electron spectroscopy, an electron beam exposure machine, and a wafer inspection apparatus, and more particularly to an electron emission source suitable for an electron beam exposure machine.

In recent years, in order to obtain an electron beam with a longer life and higher brightness than a hot cathode, an electron emission source using a cathode in which a tungsten-crystal needle-like electrode is provided with a coating layer of zirconium and oxygen has been used. (Hereinafter referred to as a ZrO / W electron emission source) (see Non-Patent Document 1).
D. Tugg1e, J. et al. Vac. Sci. Techno 1.16, p1699 (1979).

A ZrO / W electron emission source is provided with a coating layer made of zirconium and oxygen (hereinafter referred to as a ZrO coating layer) on a tungsten single crystal needle-shaped cathode having an axial orientation of <100>, and the ZrO coating layer The work function of the <100> plane of the tungsten single crystal is reduced from 4.5 eV to about 2.8 eV, and only the minute crystal plane corresponding to the <100> plane formed at the tip of the cathode is an electron. Since it becomes an emission region, it has a feature that an electron beam with higher brightness than that of a conventional hot cathode can be obtained and has a long life. Further, it is more stable than a cold field emission electron source, operates at a low vacuum, and is easy to use (see Non-Patent Document 2).
M.M. J. et al. Fransen, "On the E1 Electron-Optica 1 Properties of the ZrO / W Schottky Electron Emitter", ADVANCES IN IMAGEING AND ELECTRON PHYSICS, VOL. III, p91-166.1999 by Academic Press.

As shown in FIG. 1, the ZrO / W electron radiation source is a <100> orientation of tungsten that emits an electron beam to a predetermined position of a tungsten filament 3 provided on a conductive terminal 4 fixed to an insulator 5. The needle-like cathode 1 is fixed by welding or the like. A part of the cathode 1 is provided with a supply source 2 of zirconium and oxygen. Although not shown, the surface of the cathode 1 is covered with a ZrO coating layer.

Since the cathode 1 is energized and heated by the filament 3 and is generally used at a temperature of about 1800 K, the ZrO coating layer on the surface of the cathode 1 is consumed by evaporation. However, since zirconium and oxygen diffuse from the supply source 2 and are continuously supplied to the surface of the cathode electrode 1, the ZrO coating layer is maintained as a result.

The tip of the cathode 1 of the ZrO / W electron radiation source is used by being disposed between the suppressor electrode 6 and the extraction electrode 7 (see FIG. 2). A negative high voltage is applied to the cathode 1 with respect to the extraction electrode 7, and a negative voltage of about several hundred volts is applied to the suppressor electrode 6 with respect to the cathode 1, thereby suppressing excess current from the filament 3. .

The ZrO / W electron radiation source is 0.1 to 0.2 mA / sr in a length measurement SEM or wafer inspection apparatus used at a low acceleration voltage because the probe current is stable and the spread of the energy width is suppressed. It is operated at an angular current density of.

On the other hand, an electron beam exposure apparatus, an Auger spectroscopic apparatus, and the like are operated at a high angular current density of about 0.4 mA / sr because throughput is important. In such applications that place importance on throughput, higher angular current density operation is desired, and operation at an angular current density as high as 1.0 mA / sr may be required.

However, in the ZrO / W electron emission source, (1) an angular current density of about 1.0 mA / sr is the upper limit at the time of high angular current density operation, and (2) at this time, it is applied between the cathode and the extraction electrode. The extraction voltage is as high as 4 kV or more, the electric field strength at the tip of the chip is remarkably high at 0.4 to 1.0 × 10 ⁹ V / m, and the failure frequency due to arc discharge increases (see Non-Patent Document 3).
D. W. Tugg1e, J. et al. Vac. Sci. Techno1. B3 (1), p220 (1985).

In order to solve this drawback, an electron emission source comprising a tungsten cathode 1, a suppressor electrode 6, and an extraction electrode 7 is characterized in that the top of the cathode 1 has a truncated cone shape having a surface having a diameter of 5 μm or more. A radiation source has been proposed (see Patent Document 1).
Japanese Patent Application No. 2003-37920.

The ZrO / W electron emission source (1) has an angular current density of about 1.0 mA / sr at the maximum at the time of high angular current density operation, and (2) the extraction voltage applied to the cathode and the extraction electrode at this time. Is as high as 4 kV or more, the electric field strength at the tip of the chip is remarkably increased to 0.4 to 1.0 × 10 ⁹ V / m, and the failure frequency due to arc discharge increases.

An electron radiation source using a truncated-cone-shaped tungsten cathode that solves these problems operates at a high angular current density of 1 mA / sr or higher at a low extraction voltage, and the total radiation current is as low as several tens of μA, resulting in high reliability. Has characteristics.

However, when the extraction voltage is increased in order to operate at a higher angular current density, there is a problem that the surplus current increases suddenly from a certain extraction voltage and the total radiation current increases to about several hundred μA.

This increase in surplus current induces degassing from the constituent device members, causes a reduction in the degree of equipment vacuum, and impairs reliability.

As a result of various studies in view of the above circumstances, the present inventor has found that the cause of the increase in surplus current is due to electron emission from the boundary between the side surface of the truncated cone-shaped cathode and the conical surface, and the electrode The present inventors have obtained the knowledge that the above-mentioned problems can be solved by adjusting the arrangement of the present invention, and have reached the present invention.

That is, the present invention provides a cathode comprising a tungsten or molybdenum single crystal needle provided with a supply source for diffusing one or more metal elements selected from the group consisting of 2A group, 3A group and 4A group, An electron emission source comprising a suppressor electrode having a hole for projecting a single crystal needle, wherein the end of the single crystal needle has a truncated cone shape having a full cone angle of 25 ° to 95 °, and the single crystal An electron emission source characterized in that the position of the boundary between the conical surface at the end of the needle and the side surface of the single crystal needle is not protruded from the hole of the suppressor electrode, preferably the diameter of the upper surface of the truncated cone Is from 5 μm to 200 μm in the electron emission source.

Further, the present invention is an electron emission source characterized in that the metal element is zirconium or titanium, and preferably, the <100> orientation of the single crystal constituting the cathode and the direction of the normal to the upper surface Is an electron emission source characterized in that the angle difference is within 2 °.

The electron emission source of the present invention does not project the position of the boundary between the conical surface of the cathode and the side surface of the rod from the hole of the suppressor electrode, thereby maintaining the operation at a high angular current density, and the total radiation current is a comparative example. It is one order of magnitude smaller and can be expected to have high reliability. In addition, the electron emission source of the present invention operates at an angular current density almost equal to that of a conventionally known electron emission source, and the total emission current at that time is smaller than that of a conventionally known electron emission source, and high reliability is expected. Obviously, it is possible to obtain a radiation current with a high operating angular current density.

As a specific embodiment of the present invention, a conical portion 8 is provided by mechanical polishing or electrolytic polishing at the end of a rod-like cathode 1 of tungsten or molybdenum single crystal <100> orientation, and a diamond abrasive is applied at the apex thereof. Polishing with a coated polishing film or applying a focused gallium ion beam device to cut the apex and provide a flat portion 9.

In the present invention, when xenon difluoride gas is introduced when processing with a focused gallium ion beam, the processing time can be shortened. Moreover, the full angle of the cone part 8 is 25 degrees or more and 95 degrees or less, and the diameter of the flat part 9 (or upper surface part) is 5-200 micrometers. Furthermore, it is preferable that the normal line of the flat electron emission surface formed at the end of the cathode 1 is within an angle difference of 2 ° or less with the <100> orientation.

A <100> -oriented rod made of tungsten or molybdenum single crystal functions as a cathode, and its surface is covered with a metal element selected from the groups 2A, 3A, and 4A and oxygen. Taking the Zr-O system as an example, zirconium hydride is pulverized and mixed with an organic solvent to form a paste, which is applied to a part of the cathode, in an oxygen atmosphere of about 1 × 10 ¹⁶ Torr. The cathode is heated to thermally decompose ZrH ₂ and further oxidized to form a supply source of zirconium and oxygen, and the surface of the cathode is coated with zirconium and oxygen.

This cathode is arranged between the extraction electrode 7 and the suppressor electrode 6, a negative high voltage of several kilovolts is applied to the extraction electrode with respect to the cathode 1. And the cathode 1 is heated to 1500 to 1900K to emit electrons. According to the present invention, the position of the boundary portion 19 between the conical surface of the cathode and the side surface of the single crystal needle is the suppressor electrode 6. Do not protrude from the surface 20 of the hole.

If the boundary portion 19 protrudes from the hole surface 20 of the suppressor electrode 6, if the extraction voltage is increased, the surplus current suddenly increases from a certain extraction voltage, and the total radiation current reaches about several hundred μA. It will increase. This is considered to be due to the fact that, when a high extraction voltage is applied, an electric field that can radiate electrons is generated in the boundary portion 19 and is radiated as a surplus current.

In the present invention, since the position of the boundary portion 19 does not protrude from the hole surface 20 of the suppressor electrode 6, the shielding effect of the electric field by the suppressor works effectively on the boundary portion 19, and it is possible to suppress excess current. It becomes.

The electron emission source of the present invention operates at a high angular current density of 1 mA / sr or more, and prevents the boundary between the conical surface of the cathode and the side surface of the single crystal needle from protruding from the hole surface of the suppressor electrode. Thus, the shielding effect of the electric field works effectively on the boundary portion, and the surplus current is extremely low, so that the reliability is high.

(Examples 1 and 2, Comparative Example) A filament made of tungsten was fixed to a conductive terminal brazed to an insulator by spot welding. A conical portion with a full angle of 90 ° is formed at the end of a single crystal tungsten chip of <100> orientation using a diamond paste and a polishing disk, and further polished with a polishing film coated with a diamond abrasive at the apex of the conical portion. A flat part having a diameter of 20 μm was formed. This rod was attached to the filament by spot welding. This rod functions as a cathode. In Example 1, a single crystal tungsten chip having a diameter of 0.4 mm and in Example 2 and Comparative Example having a diameter of 0.3 mm were used.

Zirconium hydride was pulverized and mixed with isoamyl acetate to form a paste, which was applied to a part of the cathode. After the isoamyl acetate evaporated, it was introduced into the apparatus shown in FIG.

The tip of the cathode is disposed between the suppressor electrode and the extraction electrode. The distance between the suppressor electrode and the extraction electrode was 0.8 mm, the hole diameter of the extraction electrode was 0.8 mm, and the hole diameter of the suppressor electrode was 0.8 mm. The distance between the cathode tip and the suppressor electrode was 0.15 mm in Example 1 and Comparative Example, and 0.10 mm in Example 2. With this electrode configuration, the boundary between the conical surface of the cathode and the side surface of the single crystal needle is located 0.03 mm deep in both the first and second embodiments with respect to the hole surface of the suppressor electrode. The boundary portion is located at a position protruding 0.02 mm from the hole surface of the suppressor electrode.

The filament is connected to a filament heating power source and further connected to a high voltage power source, and a negative high voltage, that is, an extraction voltage Vex is applied to the extraction electrode. The suppressor electrode is connected to a bias power source, and a negative voltage and a bias voltage Vb are further applied to the cathode and the filament. The total emission current It from the electron source is measured by an ammeter placed between the high voltage power source and ground. The electron beam emitted from the tip of the cathode passes through the hole of the extraction electrode and reaches the fluorescent screen. There is an aperture (small hole) in the center of the fluorescent plate, and the probe current Ip passing through and reaching the cup-shaped electrode is measured by a microammeter. If the solid angle calculated from the distance between the aperture and the tip of the cathode and the inner diameter of the aperture is ω, the angular current density is Ip / ω.

Subsequently, the inside of the apparatus was placed in an ultrahigh vacuum of 3 × 10 ⁻¹⁰ Torr (4 × 10 ⁻⁸ Pa), and the filament was energized to heat the cathode to 1800 K, and ZrH ₂ was pyrolyzed to form metallic zirconium. Further, oxygen gas was introduced to make the inside of the apparatus 3 × 10 ⁻⁶ Torr (4 × 10 ⁻⁴ Pa) to oxidize metal zirconium to form a supply source of zirconium and oxygen.

Again, the inside of the apparatus was placed in an ultrahigh vacuum of 3 × 10 ⁻¹⁰ Torr (4 × 10 ⁻⁸ Pa), the bias voltage Vb = 300 V was applied to the suppressor while maintaining the cathode at 1800 K, and then the extraction voltage Vex was set to 2. When a high voltage of 5 kV was applied and held for several hours and the radiation current was stabilized, the total radiation current It and the probe current Ip at each extraction voltage Vex were measured, and the angular current density was calculated.

FIG. 4 shows the measurement results of the extraction voltage-angular current density and the total radiation current. In the comparative example in which the boundary between the conical surface of the cathode and the side surface of the single crystal needle protrudes from the surface of the hole of the suppressor electrode, the total radiation current caused by the surplus current when the extraction voltage Vex is 1.2 kV or more. It is increasing rapidly. On the other hand, in Example 1 and Example 2 in which the boundary between the conical surface of the cathode and the side surface of the single crystal needle is not projected with respect to the surface of the hole of the suppressor electrode, the extraction voltage Vex is fully radiated up to 2.5 kV. It is clear that the current shows a low value of several tens of μA, and the surplus current is suppressed.

The electron emission source of the present invention does not project the position of the boundary between the conical surface of the cathode and the side surface of the rod from the hole of the suppressor electrode, thereby maintaining the operation at a high angular current density, and the total radiation current is a comparative example. It is one order of magnitude smaller and can be expected to have high reliability, and is useful as an electron source for electron exposure, including many electron sources. In addition, the electron emission source of the present invention operates at an angular current density almost equal to that of a conventionally known electron emission source, and the total emission current at that time is smaller than that of a conventionally known electron emission source, and high reliability is expected. Obviously you can. In addition, there is a feature that a radiation current having a higher operating angular current density can be obtained as compared with a conventional ZrO / W electron radiation source, and an electron source for electron exposure is useful as many electron sources at the beginning.

The structural diagram of a ZrO / W electron emission source. The block diagram of the evaluation apparatus of an electron emission characteristic. The enlarged view of a cathode. Measurement results of extraction voltage-angular current density and total radiation current (Examples 1 and 2 and Comparative Example).

Explanation of symbols

1: Cathode 2: Supply source 3: Filament 4: Conductive terminal 5: Insulator 6: Suppressor electrode 7: Extraction electrode 8: Conical part (of the truncated cone part) 9: Flat part (upper surface part of the truncated cone part)
10: Fluorescent plate 11: Aperture 12: Cup-shaped electrode 13: Micro current meter 14 for probe current measurement 14: Bias power source 15: High voltage power source 16: Filament heating power source 17: Ammeter for total radiation current measurement 18: Radiation electron beam 19: Cathode 20 between the conical surface and the side surface of the electrode: hole surface of the suppressor electrode

Claims (4)

A single crystal needle of tungsten or molybdenum is provided with a cathode provided with a supply source for diffusing one or more metal elements selected from the group consisting of 2A group, 3A group and 4A group, and the single crystal needle is protruded An electron emission source comprising a suppressor electrode having a hole, wherein the end of the single crystal needle has a truncated cone shape having a full cone angle of 25 ° to 95 °, and a cone at the end of the single crystal needle An electron emission source, wherein a position of a boundary portion between the surface and the side surface of the single crystal needle is not protruded from the hole of the suppressor electrode. 2. The electron emission source according to claim 1, wherein the diameter of the upper surface of the truncated cone is not less than 5 μm and not more than 200 μm. 3. The electron emission source according to claim 1, wherein the metal element is zirconium or titanium. 4. The electron according to claim 1, wherein the <100> orientation of the single crystal constituting the cathode and the direction of the normal of the upper surface are an angle difference within 2 °. Radiation source.

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