JP2007265685A - Probe for electron source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a probe for an electron source capable of emitting an electron beam even if a voltage to be applied is a low voltage, and capable of emitting an electron beam having uniform energy, high directivity, and high focusing ability. <P>SOLUTION: This probe for an electron source has a sharpened tip 30, and is constituted so that a tungsten atom 31 exists on the vacuum side of the tip 30, and an oxygen atom 32 exists on the inside of the tip 30 of a probe adjacently to the tungsten atom 31, and the probe for the electron source can be formed by applying oxygen adsorption and heat treatment and applying a positive voltage to the tip 30 comprising single crystal tungsten. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electron source probe that can be used in an electron microscope, an electron spectrometer, an electron diffractometer, and the like and can emit an electron beam with uniform energy and high directivity and focusability.

  An electron microscope, an electron spectroscopic device, an electron diffraction device, and the like are provided with an electron source that emits an electron beam, and the electron beam is emitted. As a method for generating an electron beam, a method of extracting thermoelectrons at a high temperature, a method of emitting electrons by applying a high voltage to the tip of a sharpened needle, a method of exciting electrons with light, and the like are used.

Electron beams are required to have uniform energy and high directivity and focusing. In this respect, a method of emitting electrons by applying a high voltage to the tip of a sharpened needle is excellent. In the conventional field emission electron source, since a high voltage is applied to a sharpened tungsten probe and electrons in the tungsten are taken out in a vacuum by a tunnel effect, it is necessary to apply a high voltage. In addition, according to this method, since the electron beam is emitted according to the tunnel probability, the emitted electron beam has a certain energy distribution.
The present inventor has produced a probe for an electron source that can make the opening angle of an electron beam about 4 ° using a probe in which a tungsten wire is sharpened by electrolytic polishing with respect to a field emission electron source. It was announced in Patent Document 1.

Kyushu University Graduate School of Science and Technology Vol. 23, No. 1, pages 9-14

  However, as an electron source, it is desirable to lower the voltage applied to the probe, and it is expected that an electron beam with uniform energy and high directivity and focusing will be emitted.

  The present invention has been made to solve the above problems, and can emit an electron beam even when a voltage to be applied is low, and has a uniform directivity and a high focusing property. It is an object of the present invention to provide an electron source probe capable of emitting.

  In order to solve the above problems, the probe for an electron source according to the present invention has a sharpened tip, and tungsten atoms exist on the vacuum side of the tip, and the probe is adjacent to the tungsten atoms. The structure is characterized in that oxygen atoms are present inside the tip.

  A structure in which tungsten atoms are present on the vacuum side of the tip and oxygen atoms are present on the inner side of the probe tip adjacent to the tungsten atoms, a dipole is formed at the tip, and this local The work function is extremely lowered in a region where the value is close to zero. Therefore, the electron beam is emitted even at a low voltage in the direction in which the dipole is formed, and the electron beam is emitted at an extremely narrow angle of 2 °.

  Such a probe for an electron source can be formed by applying oxygen adsorption and heat treatment to the tip portion made of single crystal tungsten and applying a positive voltage.

Oxygen adsorption is preferably performed by introducing oxygen at 10 L or more and 10,000 L or less. If it is less than 10 L, tungsten oxide is not sufficiently formed, which is not preferable, and if it exceeds 10000 L, the tungsten oxide layer becomes too thick. Here, L is Langmuir and is an amount determined by the product of the degree of vacuum and the introduction time. 1 L corresponds to introducing gas at a pressure of 1 × 10 ⁻⁶ Torr (1.33 × 10 ⁻⁴ Pa) for 1 second.

  The heat treatment is preferably performed at a temperature of 200 ° C. or higher and 800 ° C. or lower. If the temperature is lower than 200 ° C., tungsten oxide is not sufficiently formed, which is not preferable. If the temperature exceeds 800 ° C., the radius of curvature at the tip of the probe is not preferable.

  The positive voltage is preferably applied when the electric field generated at the tip of the probe is 1 V / nm or more and 100 V / nm or less. If it is less than 1 V / nm, it will not be possible to cause migration of atoms, and if it exceeds 100 V / nm, field evaporation will occur, which is not preferred.

  According to the present invention, an electron source probe capable of emitting an electron beam even when the applied voltage is low and capable of emitting an electron beam with uniform energy and high directivity and convergence. Can be realized.

Below, this invention is demonstrated based on the embodiment.
FIG. 1 shows the configuration of an apparatus for observing the structure of a probe according to an embodiment of the present invention.
In FIG. 1, a probe 1 made of tungsten single crystal sharpened by electrolytic polishing is spot-welded to a tantalum wire 2 and attached to a probe holder 3. A lead-in electrode 4 is provided in front of the tip of the probe 1, and a microchannel plate 5 is disposed in front of the lead-in electrode 4. A fluorescent screen 6 is arranged to face the microchannel plate 5, and a field ion microscope image of the probe 1 and a field emission electron beam shape are observed by the CCD camera 7.

  The probe 1, the tantalum wire 2, the probe holder 3, the lead-in electrode 4, the microchannel plate 5, and the fluorescent screen 6 are accommodated in an ultrahigh vacuum chamber 8, and the ultrahigh vacuum chamber 8 includes a helium cylinder 9 and An oxygen cylinder 10 is attached.

  A probe constant voltage power supply 12 is connected to the probe 1, and when a negative voltage is applied to the probe 1, the field emission electron beam 11 is extracted from the probe 1. A microchannel plate power supply 13 is connected to the microchannel plate 5, and a fluorescent screen power supply 14 is connected to the fluorescent screen 6. A voltage of +1000 V to +1300 V is applied to the microchannel plate 5, and A voltage of +2000 V to +4000 V is applied to observe the shape of the field emission electron beam 11.

  FIG. 2 shows the structure of an electrolytic fusing device that sharpens the probe. A ring-shaped platinum electrode 21 is used as the cathode, and the probe 1 made of tungsten single crystal is used as the anode. A 2N sodium hydroxide aqueous solution is used as the electrolytic solution, the platinum electrode 21 is installed so that the liquid surface is lifted by surface tension, and the probe 1 is installed at the center thereof. When the electrolysis proceeds and the probe 1 is cut, the electrolysis is terminated. Confirm that the tip of the created probe is sharp with an optical microscope. An optical microscope image of the tip of the probe 1 sharpened by this electrolytic fusing device is shown in FIG.

The probe 1 sharpened in this way is introduced into an ultra-high vacuum chamber having a pressure of 2 × 10 ⁻¹¹ Torr (2.67 × 10 ⁻⁹ Pa), and the temperature is 400 ° C. for 1 to 12 hours. Then, the sample is kept at 400 ° C., oxygen is introduced into the vacuum chamber at 1 × 10 ⁻⁷ Torr (1.33 × 10 ⁻⁵ Pa), and oxygen adsorption is performed for 1 hour. By field ion microscope observation, a voltage condition in which a special structure, which will be described later, appears, and oxygen adsorption is repeated until the special structure appears. When a special structure appears by field ion microscope observation, it is cooled to liquid nitrogen temperature. With respect to the probe 1 subjected to this treatment, helium of about 1 × 10 ⁻⁵ Torr (1.33 × 10 ⁻³ Pa) is introduced into the ultrahigh vacuum chamber in the apparatus shown in FIG. A positive voltage is applied to 1 and observation with a field ion microscope is performed.

  FIG. 4 shows an image obtained by observation with a field ion microscope. 4 (A) and 4 (B) are typical field ion microscope images of the probe before oxygen adsorption, (A) is as it is after electrolytic fusing, and (B) is heated to 1000 ° C. After that. Both show three-fold rotational symmetry, and it can be seen that the tip of the probe is the (111) plane. In the image of (B), the radius of curvature of the tip of the probe is larger than that of (A) due to heating.

  The image in FIG. 4A can be observed stably under a field condition where a voltage of +1.91 kV is applied, and field evaporation did not occur under this condition. FIGS. 4C1 to 4C4 show how the field ion microscope image after the probe is subjected to oxygen adsorption and heat treatment changes with time, and the formation of oxides. Thus, the image is different from that of clean single crystal tungsten. The surface of the probe on which the oxide was formed was unstable even under electric field conditions where the applied voltage was as low as +1.8 kV, and changed from (C1) to (C4) in 4 minutes. As described above, it was confirmed by observation with a field ion microscope that the structure of the surface of the probe on which the oxide was formed was changed with a relatively low electric field.

  It was found that when the oxidation heat treatment and field ion microscope observation were repeated several times, a structural change accompanied by a significant increase in the intensity of the field ion microscopic image, which was different from a normal structural change, occurred. This structural change will be described with reference to FIG.

  FIG. 5 is a field ion microscope image measured every second. The bright spots indicated by the arrows in the figure are relatively weak in FIGS. 5A to 5E, but suddenly become brighter in FIG. 5F. In FIGS. 5 (h) and 5 (i), in order to adjust the brightness, the voltage of the microchannel plate is lowered to lower the brightness. As a result, the plurality of bright spots visible in FIGS. 5A to 5E are much weaker than the bright spots suddenly brightened, and these bright spots are visible in FIG. 5I. Only the bright spots that suddenly become brighter are visible.

After confirming the stability of the very bright bright spots generated in this way, the shape of the field emission electron beam was observed. The result is shown in FIG.
FIG. 6 shows the observation of the shape of the field emission electron beam by applying a negative voltage to the probe. FIGS. 6A1 and 6B1 show the two probes before the oxidation heat treatment. The shape of the field emission electron beam. Since the radius of curvature of the probe is small, an electron beam is obtained at a relatively low voltage of −200 V to −300 V, and the emission direction of the electron beam is approximately in the direction of the probe axis <111>.

  The shape of the field emission electron beam after each of the probes is subjected to oxidation heat treatment to produce the bright bright spot shown in FIG. 5 (i) is shown in FIGS. 6 (A2), (A3), (B2), (B3 ). An electron beam is obtained at a very low voltage of −3 V to −4 V compared to that before the oxidation heat treatment. Further, the opening angle of the electron beam is as thin as 2 ° or less, and the emission direction coincides with the <111> direction. The shape of the electron beam was not a circle but a hexagon.

  As shown in FIG. 7, the electron beam is emitted by such a low voltage because an oxide is formed at the tip 30 of the probe by the adsorbed oxygen, and a structural change is caused by the electric field in the field ion microscope. This is probably because tungsten atoms 31 indicated by white circles exist on the side, and oxygen atoms 32 indicated by black circles exist on the inner side of the tip of the probe. Due to this structure, a dipole is formed at the probe tip 30, the work function is extremely lowered in this local region, and becomes a value close to zero, and the voltage of the electron beam is low in the direction in which the dipole is formed. But it will be released. As a result, the electron beam is emitted at an extremely narrow angle with an opening angle of 2 °.

  As described above, since the probe of the present invention emits electrons based on an extremely low work function, the applied voltage for electron emission can be small, and the energy dispersion of the emitted electron beam is very small. It is possible to emit a coherent electron beam with a uniform energy state. Further, since the electron beam emission site is a narrow atomic level region at the tip of the probe, the convergence is extremely high. Therefore, it has excellent properties as an electron beam source in terms of emission angle and opening angle. If the opening angle is large, it becomes necessary to narrow the emitted electron beam once, and aberrations occur when the electron beam is narrowed. However, in the probe of the present invention, the opening angle is small at the time of emission. Such a problem does not occur.

  The present invention is used in an electron microscope, an electron spectroscopic device, an electron diffraction device, and the like, and can emit an electron beam even when a voltage to be applied is low, and has uniform energy and high directivity and convergence. It can be used as a probe for an electron source capable of emitting an electron beam.

It is a figure which shows the structure of the apparatus for observing the structure of the probe which concerns on embodiment of this invention. It is a figure which shows the structure of the electrolytic fusing apparatus which sharpens a probe. It is a figure which shows the optical microscope image of the front-end | tip of the probe sharpened with the electrolytic fusing apparatus. It is a figure which shows the image by a field ion microscope observation. It is a figure which shows the electric field Oion microscope image measured every 1 second. It is a figure which shows the result of having observed the shape of the field emission electron beam by applying a negative voltage to a probe. It is a figure which shows the structure of a probe tip.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Probe 2 Tantalum wire 3 Probe holder 4 Lead-in electrode 5 Microchannel plate 6 Fluorescent screen 7 CCD camera 8 Ultra-high vacuum chamber 9 Helium cylinder 10 Oxygen cylinder 11 Field emission electron beam 12 Constant voltage power supply for probe 13 Microchannel plate Power supply 14 Phosphor screen power supply 21 Platinum electrode 30 Probe tip 31 Tungsten atom 32 Oxygen atom

Claims (5)

  Electrons characterized in that the tip has a sharp shape, tungsten atoms are present on the vacuum side of the tip, and oxygen atoms are present on the inner side of the probe tip adjacent to the tungsten atoms. Source probe.   An electron source probe formed by subjecting a tip portion made of single crystal tungsten to oxygen adsorption and heat treatment and applying a positive voltage.   The probe for an electron source according to claim 2, wherein the oxygen adsorption is performed by introducing oxygen at 10L or more and 10,000L or less.   4. The electron source probe according to claim 2, wherein the heat treatment is performed at a temperature of 200 ° C. or higher and 800 ° C. or lower.   5. The electron source probe according to claim 2, wherein the positive voltage is applied when an electric field generated at a tip of the probe is 1 V / nm or more and 100 V / nm or less.

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