JP2006019200A - Field emission electron source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a field emission electron source having a maximum cathode current by raising the heat resistance temperature of the jointing part of a carbon nano material and a cathode member. <P>SOLUTION: The field emission electron source 10 has a thin wire member 2 consisting of a carbon nano material jointed at the tip of the cathode member 1 and electron is emitted from the tip of the thin wire member 2. A covering layer 3 of a metal having a melting point of 500°C or more is formed at the jointing part of the cathode member 1 and the thin wire member 2, thereby the jointing part is adhered by a metal membrane having heat resistance. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a field emission electron source that emits electrons from the tip of a cathode, and more specifically, an electric field in which a thin wire member made of a carbon nanomaterial is joined to a cathode tip portion in order to easily cause electron emission by field emission. It relates to a radiation electron source.
This electron emission electron source is used for an electron gun such as an electron microscope, an X-ray microscope, or a microfocus X-ray source.

Carbon nanomaterials typified by carbon nanotubes (CNT) (including carbon nanotubes, carbon nanofibers, carbon nanocoils, etc.) are excellent in chemical stability and structural integrity, and are influenced by contaminants. It has a number of excellent properties such as high mechanical strength and high electrical conductivity.
Therefore, applications are being studied in various fields such as electrodes of field emission displays (FEDs), cantilevers of scanning probe microscopes (SPMs), catalyst holding electrodes of fuel cells, and single electron transistors.

  In the field of the microscope, it has been proposed to use a carbon nanotube as a probe attached to the tip of a cantilever of a scanning probe microscope (SPM), and a probe having a small curvature radius at the tip is formed by joining CNT to the tip of the probe. Thus, a technique for capturing a high-resolution surface atomic image has been disclosed (see Patent Document 1).

  Further, the above document discloses a method for bonding CNT to the probe tip. That is, in the vacuum chamber, the CNT is brought into contact with the tip of the probe of the cantilever and the contact portion is irradiated with an electron beam to join the CNT. This bonding is performed in a vacuum by positioning under an electron microscope using an apparatus capable of processing at the nano level with a manipulator attached to the electron microscope, and irradiating an electron beam from the electron source of the electron microscope itself. It is formed by adhering hydrocarbons remaining on the joint portion.

On the other hand, the applicants have recently proposed that CNT is not used as a cantilever of a scanning probe microscope but used as an electron source such as an electron microscope (see Non-Patent Document 1). The electron microscope is equipped with an electron source for irradiating the sample with electrons, and the X-ray microscope is equipped with an electron source for exciting X-rays. Generally, a field emission electron source using a tungsten material as a cathode member is used as these electron sources. Applicants attach the CNT to the tip of the cathode member, apply a voltage with the field emission electron source in an ultra-high vacuum state of about 10 ⁻⁸ Pa, and emit electrons from the tip of the CNT, so that it is as narrow as possible Electrons are ejected from the region (the tip diameter is about several to several tens of nanometers) to improve the resolution of the electron microscope.
Japanese Patent No. 3441396 Proceedings of the 50th Applied Physics Related Conference Lecture No. 2, 28p-W-2, p. 1023

In a field emission electron source using carbon nanomaterials (thin wire members) such as CNT, the carbon nanomaterial is formed at the tip of a cathode member formed of a high melting point metal material such as tungsten (W) or molybdenum (Mo). They are joined so as to protrude, and form a thin wire member for electron emission.
As described above, this bonding is performed by attaching and adhering hydrocarbons remaining in a vacuum by electron beam irradiation. The heat-resistant temperature of this bonded portion is about 400 ° C. at most.

In the field emission electron source, electric field emission is generated by current flowing to the cathode member, the joint, and the carbon nanomaterial (thin wire member). At that time, the temperature of the cathode member, the joint portion, and the thin wire member rises due to self-heating due to energization.
In addition, the radiation current value can be stabilized by performing thermal field radiation that generates field radiation while actively heating the cathode member during field radiation.
In addition, when it is desired to remove residual gas such as H ₂ O adsorbed on the cathode member or the thin wire member, the adsorbed gas is desorbed by performing flushing for heating these members for several seconds to several hundred seconds. Can do.

As described above, the temperature of the junction rises due to self-heating due to the electric field radiation, and the temperature of the junction rises due to the thermal electric field radiation and flushing. However, when the temperature of the joined portion rises to 400 ° C. or higher, the bonded portion of the joined portion is melted and the joint is damaged.
Therefore, when performing field emission, thermal field emission, or flushing, it is necessary to limit the current flowing through the cathode so that the junction temperature does not rise above 400 ° C.

Therefore, the cathode current cannot be increased sufficiently to increase the field emission, and cannot be sufficiently stabilized by the thermal electric field emission at a high temperature. It was not possible to perform this, and it was difficult to desorb the gas in a short time.

  Therefore, an object of the present invention is to provide a field emission electron source that can be used at a higher temperature by improving the heat resistance of the field emission electron source.

The field emission electron source of the present invention, which has been made to solve the above-mentioned problems, is a field emission device formed such that a fine wire member made of a carbon nanomaterial is joined to the tip of a cathode member, and electrons are emitted from the tip of the fine wire member. In the electron source, a metal coating layer having a melting point of 500 ° C. or higher is formed at the junction between the cathode member and the thin wire member.
According to this invention, the thin wire member made of the carbon nanomaterial is joined to the tip of the cathode member, and further, a metal coating layer having a melting point of 500 ° C. or higher is formed at the joint. Then, the adhesive force of the metal coating layer is utilized to increase the adhesion between the cathode member and the fine wire member, and to increase the heat-resistant temperature to the melting point temperature of the coating layer.

In the field emission electron source, as the metal coating layer having a temperature of 500 ° C. or higher, aluminum (Al) having a melting point of 660 ° C. and silver (Ag) having a melting point of 961 ° C. can be used. The metal coating layer may be made of a metal having a melting point of 1000 ° C. or higher to further increase the heat resistant temperature. Here, as the metal having a melting point of 1000 ° C. or higher, for example, gold (Au) having a melting point of 1100 ° C., melting point of 1500 ° C. or higher, that is, titanium (Ti) at 1700 ° C., platinum (Pt) at 1800 ° C., 1900 Chromium (Cr) and vanadium (V) can be used.
Furthermore, iridium (Ir, melting point 2400 ° C.), tantalum (Ta, melting point 3000 ° C.), molybdenum (Mo, melting point 2600 ° C.), tungsten (W, melting point 3400 ° C.), etc., which are so-called refractory metals having a melting point of 2000 ° C. or higher. You may make it improve heat resistance more.
In addition, although the single substance is generally used for the metal which forms a coating layer, an alloy may be sufficient.

In the electron source, the metal coating layer has a laminated structure of a first layer made of a metal having good wettability to the cathode member and a second layer made of a metal having a melting point higher than that of the metal of the first layer. You may make it form.
According to this, by forming a metal having good wettability with respect to the cathode member as the first layer, the bondability with the cathode member is improved, and further, the metal having a melting point higher than that of the first layer is changed to the second layer. As a result, the heat resistance is increased. This simultaneously improves the bondability and heat resistance between the cathode member and the fine wire member.
Here, for the first layer, an appropriate material is selected in relation to the metal used for the second layer. For example, when the second layer is tungsten (W) or molybdenum (Mo), platinum ( Pt) is preferably used.

  In the electron source, the same material as that of the cathode member may be formed as a coating layer, or a metal having substantially the same coefficient of thermal expansion may be formed as the coating layer. According to this, by using a material having the same or almost the same coefficient of thermal expansion, it is possible to suppress the occurrence of thermal stress in the joint portion even if the temperature changes. To prevent the carbon nanomaterial (thin wire member) from peeling off.

According to the present invention, the heat resistance of the field emission electron source can be increased by forming a metal coating layer having a melting point of 500 ° C. or more at the junction, and a larger current than before can be applied to the junction. And strong field radiation can be generated. Moreover, sufficient effects can be exhibited by performing current stabilization by thermal electric field radiation and removal of adsorbed gas by flushing at a higher temperature than ever.
Further, if a metal having a melting point of 1000 ° C. or higher is used for the coating layer, the heat resistant temperature can be further increased. If a refractory metal having a melting point of 2000 ° C. or higher is used, the heat resistant temperature can be further increased.

Further, the metal coating layer is formed by forming a laminated structure of a first layer made of a metal having good wettability to the cathode member and a second layer made of a metal having a higher melting point than the metal of the first layer, While improving heat resistance, the adhesive force of a cathode member and a coating layer can be improved, and the bondability of a cathode member and a thin wire member can be improved.
Further, by forming the same metal as the cathode member as the coating layer or forming a metal having substantially the same thermal expansion coefficient as the coating layer, it is possible to prevent the thin wire member from being peeled off from the cathode member due to the peeling of the coating layer.

The field emission electron source of the present invention will be described below with reference to the drawings.
FIG. 2 is a diagram showing a configuration of a cathode member portion of a field emission electron source according to an embodiment of the present invention, and FIG. 1 is a diagram showing an overall configuration of the field emission electron source.

The field emission electron source 10 includes a cathode member 1 having a sharp tip, a thin wire member 2 made of a carbon nanomaterial fixed to the cathode member 1 at a joint 7 near the tip of the cathode member 1, and a joint. 7, a coating layer 3 made of a metal film covering the tip portion of the cathode member 1, a filament 4 that supports the cathode member 1 and is electrically connected to the cathode member 1, and an insulating base that supports the filament 4 It is comprised with the stand 5. FIG.

The cathode member 1 uses a tungsten needle having a diameter of about 0.3 mm, and the tip thereof is sharpened to a radius of curvature of about 100 nm by electrolytic polishing.
Note that the tip of the cathode member 1 does not necessarily need to be sharpened, but when the thin wire member 2 is attached under observation with an electron microscope, the attachment work to the cathode member is simpler if the tip is sharpened.

The carbon nanomaterial that is the thin wire member 2 is produced by a known arc discharge method or CVD method. Here, carbon nanotubes having a diameter of about 5 to 50 nm and a length of about 200 nm are used as the carbon nanomaterial. In place of the hollow carbon nanotubes, carbon nanofibers in which graphene is packed to the center or coil-shaped carbon nanocoils may be used.

In addition, the thin wire member 2 may actually use one carbon nanomaterial, or may be one in which several are bundled to be substantially one. For example, when strength is required as in the case of use as an electron source for an X-ray microscope, a structure in which several are bundled into one is advantageous.
For the coating layer 3, a refractory metal having a melting point of 2000 ° C. or more, specifically, tungsten (W) is used. However, not limited to tungsten (W), iridium (Ir), tantalum (Ta), or molybdenum (Mo) may be used. Further, even when gold (Au) or platinum (Pt), which is a metal having a melting point of 1000 ° C. or higher, is used, heat resistance can be improved as compared with the conventional case. Similarly, even when a metal having a melting point of 500 ° C. or higher is used, it is possible to form a field emission electron source having excellent heat resistance as compared with conventional bonding using hydrocarbons.
In addition, the joining method of the cathode member 1 and the thin wire member 2, and the formation method of the coating layer 3 are mentioned later.

  For the filament 4, tungsten (W) similar to the cathode member 1 is used. Both ends of the linear filament 4 function as struts, and the filament struts 4 a are fixed to an insulating base 5. The filament support 4a is used as a voltage application terminal. In addition, if the filament support | pillar 4a is used as a single cathode and a voltage is applied between the opposing anodes which are not shown in figure, it can be used as a field emission electron source which does not utilize a thermoelectron.

The base 5 is made of a disk-shaped insulating material. The filament support 4 a is fixed so as to penetrate the base 5. When the base 5 fixes the filament support 4a, the filament 4, the cathode member 1, and the thin wire member 2 are supported by the insulating base 5 in a conductive state.
Further, the normal direction of the disk surface of the base 5 and the axial direction of the cathode member 1 are set to be as parallel as possible. Thereby, if the axial direction of the cathode member is determined with reference to the surface direction of the base 5, the adjustment becomes easy.

  In the present embodiment, the insulating base 5 is used as a support for the filament support 4a. This is because baking is performed using the filament 4 as described above. In the case where baking is not necessary, instead of the insulating base 5, a conductive cathode may be used to form a single cathode, and the cathode member 1 may be directly attached to the base.

(Bonding of cathode member and carbon nanomaterial (thin wire member))
Next, the process for attaching the carbon nanomaterial (thin wire member 2) to the cathode tip in the field emission electron source of the present invention will be described. FIG. 3 is a diagram for explaining a joining process of joining the thin wire member 2 to the tip portion of the cathode member 1.

As shown in FIG. 3A, the tip portion of the cathode 1 is electrolytically polished so as to have a radius of curvature of several tens to several hundreds of nm.
Subsequently, as shown in FIG. 3B, the carbon nanomaterial which is the thin wire member 2 is fixed to the sharpened cathode member 1 by the method disclosed in Patent Document 1 introduced as the prior art. That is, positioning under the electron microscope using a device capable of processing at the nano level with a manipulator attached to the electron microscope, and irradiating the electron beam from the electron source of the electron microscope itself, the carbonization remaining in the vacuum The fine wire member 2 is joined at the joint 7 by attaching and bonding hydrogen.
At this time, by fixing the tip of the thin wire member 2 so as to protrude from the tip of the cathode member 1, the lines of electric force are concentrated on the tip of the thin wire member 2, and electric field radiation is emitted from the tip of the thin wire member 2. Can occur.
Subsequently, as shown in FIG. 3C, a coating layer 3 made of a tungsten metal film is formed on the tip surface of the cathode member 1 including the joint portion 7. The coating is performed by, for example, a well-known thin film forming method such as an electron beam evaporation method or a laser ablation method.

In the joining portion 7 formed in this way, the cathode member 1 and the thin wire member 2 are bonded together by the adhesion force of the tungsten coating layer 3, so that the joining portion 7 is simply joined rather than the case of joining with the adhesion force of hydrocarbons. The thin wire member 2 can be strongly adhered.
Furthermore, since the junction 7 formed in this way is covered with the coating layer 3 made of tungsten (W) having high heat resistance, the field emission electron source 10 is heated by self-heating, thermal field emission, and flushing. Even if it does, the junction part 7 will not be damaged until it becomes more than melting | fusing point of tungsten (W), and it can be set as the structure excellent in heat resistance.
Accordingly, the limit temperature due to self-heating has been about 400 ° C. in the past joint, but by forming the tungsten coating layer 3, the limit temperature can be increased to 1500 ° C. or higher, and as a result, the electric field is increased. The maximum cathode current during emission can be increased. Moreover, the temperature at the time of thermal electric field radiation can be set to 1500 ° C. or more, and the current value can be stabilized. Further, by performing flushing at 1500 ° C. or higher, the adsorbed gas can be effectively removed in a short time.

  Furthermore, since tungsten is used for the cathode member 1 and tungsten is also used for the coating layer 3, the cathode member 1 and the coating layer 3 have the same coefficient of thermal expansion. Therefore, even when a temperature change occurs, peeling of the coating layer 3 due to thermal stress is unlikely to occur, the thin wire member 2 can be stably fixed, and the life as an electron source can be extended.

FIG. 4 is a diagram showing a configuration of a cathode member portion of a field emission electron source according to another embodiment of the present invention. In FIG. 4, the same components as those in FIG. The overall configuration of the present embodiment is the same as FIG.
In this field emission electron source 10, a buffer layer 3 a is formed as a first layer at the tip portion of the cathode member 1 including the joint portion 7, and a metal having a melting point higher than that of the buffer layer 3 a on the buffer layer 3 a. A coating layer 3 made of a film is formed as a second layer.
The buffer layer 3a is made of platinum (Pt) having excellent wettability with respect to the tungsten (W) cathode member 1, and the coating layer 3 is made of tungsten (W).

  By forming such two metal coating layers, the bondability between the carbon nanomaterial and the tungsten (W) coating layer, which is a refractory metal, can be enhanced.

  The present invention can be used in a field emission electron source in which a thin wire member made of a carbon nanomaterial is bonded to a cathode member.

BRIEF DESCRIPTION OF THE DRAWINGS The whole block diagram of the field emission electron source which is one Embodiment of this invention. The figure which shows the structure of the cathode member front-end | tip part of the field emission electron source which is one Embodiment of this invention. The figure explaining the manufacturing process of the cathode front-end | tip part in the field emission electron source of FIG. The figure which shows the structure of the cathode front-end | tip part of the field emission electron source which is other embodiment of this invention.

Explanation of symbols

1: Cathode member (tungsten cathode member)
2: Fine wire member (carbon nanomaterial)
3: Metal coating layer 4: Filament 4a: Filament support 5: Base 7: Junction part 10: Field emission electron source

Claims (4)

In a field emission electron source formed by joining a thin wire member made of a carbon nanomaterial to the tip of a cathode member and emitting electrons from the tip of the fine wire member, the melting point is 500 at the joint between the cathode member and the fine wire member. A field emission electron source characterized by forming a metal coating layer having a temperature of not lower than ° C. The field emission electron source, wherein the metal coating layer is a refractory metal having a melting point of 1500 ° C. or higher. The metal coating layer has a laminated structure of a first layer made of a metal having good wettability to the cathode member and a second layer made of a metal having a higher melting point than the metal of the first layer. The field emission electron source according to claim 1. 2. The field emission electron source according to claim 1, wherein the metal covering layer is made of the same metal as the cathode member or a metal having substantially the same coefficient of thermal expansion.

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