JP5131735B2 - Manufacturing method of surface emitter, manufacturing method of point emitter, and structure - Google Patents

Description

Field of Invention

  The present invention relates to a chemical vapor deposition process for growing a thin film of high density ReB6 nanowires on the surface of a high temperature material. The ReB6 nanowire thin film can be used as a high emission current density thermionic electron source in applications such as an electron microscope, an electron beam lithography system, an X-ray tube, a microwave tube, a free electron laser, and a particle accelerator. Two examples of the use of LaB6 as a planar electron source in a traveling wave tube and a point electron source in a TEM are shown in FIGS. 20 and 21, respectively.

Background of the Invention

Thermionic electron emission sources are widely used in various devices such as TV cathode ray tubes, electron microscopes, vacuum tubes, X-ray tubes, microwave tubes, nuclear accelerators and the like. An increase in the emission current density of the electron source directly leads to an increase in image brightness, an increase in spatial resolution, an increase in output, and a compact design of the entire device. ReB6, especially LaB6 and CeB6, has been found to exhibit the highest electron emission density of all cathode materials.
There are two current forms of ReB6 emitter material. One is a single crystal type and the other is a sintered polycrystalline block type. However, the size of LaB6 single crystals is limited to a few square millimeters in the emission region due to currently available growth techniques. Moreover, the discharge density of the sintered polycrystalline LaB6 is low. Thin films composed of high density ReB6 single crystal nanowires resembling “forests” formed by upright 1D nanosize ReB6 single crystals overcome these barriers.

  Accordingly, it is desirable to form a thin film of high density ReB6 nanowires on a substrate material that is stable at the ReB6 release operating temperature of about 1600 ° C. It is also expected that the material will not react with ReB6 nanowires on its surface. Recently, several studies on the synthesis of ReB6 nanowires on silicon substrates have been published (H. Zhang et al. 2005 & 2006). However, since silicon melts at 1400 ° C., it is not suitable for this purpose. In addition, although it was reported in 1978 that LaB6 whiskers grew on the graphite surface (S. Motojima et al. 1978), the whisker diameter was too large and the density on the graphite surface was too low. This is also not suitable for the purpose. Thus, the desired types of structures and their use as thermionic electron sources have not yet been reported to date.

  We have successfully developed a chemical vapor deposition process for depositing high density ReB6 nanowires on several high temperature substrate surfaces. This material appears to be very suitable for high current thermionic electron emission applications.

Cited references

[Citation 1 other than patent] Zhang, H., Zhang, Q., Tang, J., and Qin, LC, J. Am. Chem. Soc., 127, 2005, 2862.
[Citation 2 other than patent] Zhang, H., Tang, J., Zhang, Q., Zhao, G., Yang, G., Zhang, J., Zhou, O., and Qin, LC, Adv. Mater., 18, 2006, 87.
[Citation 3 other than patent] Motojima, S., Takahashi, Y., Sugiyama, K., J. Cryst. Growth, 44, 1978, 106.

  The object of the present invention is to produce high emission current density thermionic emitters by using rare earth hexaboride nanowires as electron emission units. The emitter can be either a point electron source or a planar electron source having a large emission area.

  There are two types of rare earth hexaboride nanowire thermionic emitters. The first type is a surface emitter, which can be formed by depositing a thin film rare earth hexaboride nanowire on the surface of a material that functions as a heater. The second type is a point emitter, which can be formed by attaching one rare earth hexaboride nanowire on the heater body.

Fabrication of the surface emitter of the present invention can be accomplished in two ways. The first method comprises the following steps:
(Step 1) A heater material made of one material selected from the group consisting of tungsten, molybdenum, and LaB6 is used for TEM, SEM, EBL system, X-ray tube, microwave tube, electron transistor, particle accelerator, and free electron laser. Process into cathode filament shape for any application.
Si layer (Step 2) Previous Symbol cathode filament surface, depositing a Pt layer in this order, to form a catalyst layer.
(Step 3) A rare earth hexaboride nanowire thin film is grown on the surface of the cathode filament through an alloy of Pt and Si formed by heating on the catalyst film by chemical vapor deposition.

Another second method comprises the following steps:
(Step 1) The chemical vapor deposition is performed on the surface of the heater material made of one material selected from the group consisting of tungsten, molybdenum, and LaB6, which is previously deposited with the Si layer and the Pt layer in this order and coated with the formed catalyst. A rare earth hexaboride nanowire thin film is grown through an alloy of Pt and Si formed by heating on the catalyst film by the above method .
(Process 2) Cathode filament for any of TEM, SEM, EBL system, X-ray tube, microwave tube, electronic transistor, particle accelerator, free electron laser, heater material coated with rare earth hexaboride nanowire thin film Process into shape.
The structure of the present invention is a structure used as a surface emitter for any of TEM, SEM, EBL system, X-ray tube, microwave tube, electron transistor, particle accelerator, and free electron laser. , Tungsten, molybdenum, LaB6, a heater material made of one material , a catalyst layer formed by depositing a Si layer and a Pt layer in this order on the surface of the heater material , and the heater material A rare earth hexaboride nanowire thin film grown on a Pt and Si alloy by heating on the catalyst film by a chemical vapor deposition method on the surface.

The manufacture of the point emitter of the present invention includes the following steps:
(Step 1) A Si layer and a Pt layer are deposited in this order in advance , and a chemical vapor deposition method is performed on a substrate made of one material selected from the group consisting of tungsten, molybdenum, and LaB6 coated with the formed catalyst. A rare earth hexaboride nanowire thin film is grown through an alloy of Pt and Si formed by heating on the catalyst film .
(Step 2) One rare earth hexaboride nanowire of the rare earth hexaboride nanowire thin film is attached to the tip of a needle-like heat source made of one material selected from the group consisting of tungsten, molybdenum, and LaB6.
(Step 3) A refractory metal layer is deposited on the contact portion of the rare earth hexaboride nanowire / needle heat source to enhance the attachment of the rare earth hexaboride nanowire / needle heat source.
The structure of the present invention is a structure manufactured by the method for manufacturing a point emitter described above and used as a point emitter for any of the TEM, SEM, and EBL systems, and includes tungsten, molybdenum, LaB6. A needle-like heat source made of one material selected from the group consisting of: one rare-earth hexaboride nanowire attached to the tip of the needle-like heat source, and a rare-earth hexaboride nanowire / needle-like heat source contact portion And a deposited refractory metal layer.

[Ellipsis]
ReB6: rare earth hexaboride LaB6: lanthanum hexaboride CeB6: cerium hexaboride GdB6: gadolinium hexaboride TEM: transmission electron microscope SEM: scanning electron microscope EBL: electron beam lithography

The structure of the ReB6 nanowire thin film is similar to “forest”, and the trees constituting the forest are similar to nanowires. The effective surface area is over 10,000 times the area of the area on which the thin film is deposited. Since the effective electron emission region after heating to the emission temperature includes not only the upper surface of the nanowire but also its side surface, the effective emission region exceeds 10,000 times that of a conventional ReB6 emitter having only one plane as the emission surface. Theoretically, it is expected that ReB6 nanowire thin film emitters produce emission densities exceeding 10,000 times. Such a mechanism is basically a 3D emission type.
In order to simplify the manufacturing process, the ReB6 nanowire thin film can be deposited directly on the heater body, resulting in higher mechanical stability and higher thermal efficiency.

  There are two types of rare earth hexaboride nanowire thermionic emitters. The first type is a surface emitter, which can be formed by depositing a thin film rare earth hexaboride nanowire on the surface of a material that functions as a heater. The second type is a point emitter, which can be formed by attaching one rare earth hexaboride nanowire on the heater body.

Fabrication of the surface emitter can be achieved in two ways. The first method comprises the following steps:
(Step 1) With formation of a desired cathode filament shape according to a specific application.
Examples of the filament material described herein include graphite, rare earth hexaboride such as LaB6 and CeB6, metal carbides such as ZrC, HfC, NbC, TiC, TaC and VC, and refractory metals such as W, Mo and Ta.
(Step 2) Deposition of the catalyst film on the filament surface.
Examples of the catalyst described here include Pt, Si, C, Au, Fe, or a combination of these elements. For example, an alloy formed of Pt and Si.
(Step 3) Growth of rare earth hexaboride nanowire thin film on filament surface by chemical vapor deposition.
The chemical vapor deposition method described here introduces a mixture of BCl3 gas, H2 gas, and an inert gas such as Ar and N2, vaporization of a rare earth chloride salt disposed in the center of the reaction zone of the tubular furnace, and It is described as a reaction between the above reactants represented by the formula: BCl 3 + ReCl 3 = ReB 6 + HCl and deposition of ReB 6 nanowires on a substrate placed downstream of the gas flow flowing to the reaction zone. The substrate is a filament coated with the above-described catalyst film.

The BCl3 gas described here is used at a flow rate of 0.1 to 1 L / min (equivalent to air).
The H2 gas described here is used at a flow rate of 0.1 to 1 L / min (equivalent to air).
The Ar gas described here is used at a flow rate of 0.1 to 1 L / min (equivalent to air).
The N2 gas described here is used at a flow rate of 0.1 to 1 L / min (equivalent to air).
As the ReCl3 salt described here, LaCl3, CeCl3, GdCl3, YCl3, or a mixture thereof can be used. More preferably, it is an anhydride of the above-mentioned salt.
ReCl3 described herein can be replaced with ReBr3, more specifically LaBr3, CeBr3, GdBr3, YBr3, or a combination thereof, and more preferably an anhydride of the aforementioned salt.

Another second method involves the following steps.
(Step 1) Growth of rare earth hexaboride nanowire thin film on desired heater material surface previously coated with catalyst. The heater material and catalyst type are the same as described in the first method. This method is the same chemical vapor deposition method as described in the first method.
(Step 2) Adjustment of a heater material coated with a rare earth hexaboride thin film to a desired cathode shape.
The structures described above are used as planar electron sources for TEM, SEM, EBL systems, X-ray tubes, microwave tubes, electron transistors, particle accelerators, and free electron lasers.

The manufacture of point emitters is described by the following process.
(Step 1) Formation of a rare earth hexaboride nanowire thin film on a suitable substrate by chemical vapor deposition. The deposition process is the same as described above.
(Step 2) Attachment of one rare-earth hexaboride nanowire to the tip of a sharp needle-like heat source.
Examples of the heat source material described here include graphite, rare earth hexaboride such as LaB6 and CeB6, metal carbide such as ZrC, HfC, NbC, TiC, TaC and VC, and refractory metals such as W, MO and Ta.
The method described herein for forming needles from these materials may be electrochemical etching using a suitable etching solution.

The attachment of the nanowire to the needle described herein can be performed using a micromanipulator platform monitored with an optical microscope, as illustrated in FIG. After the nanowire and the tungsten (W) needle come into contact with each other, an electric current can be passed through the nanowire to cut it from the growth substrate.
(Step 3) Strengthening of the attachment portion by depositing a refractory metal layer on the nanowire / needle tip contact portion.
Examples of the metal described here include W, Ta, and Mo. Deposition methods include, for example, focused ion beam deposition or electron beam lithography deposition.
The structure described above is used as a point electron source for TEM, SEM and EBL systems.

[Synthesis]
3 g of LaCl3 anhydrous beads are placed inside a quartz boat placed in the center area of the quartz furnace tube. A substrate of Si, graphite, W or Mo was placed downstream of the gas flow to the quartz boat. All substrates except Si were first coated with a Si layer and then with a Pt layer. The Si substrate was coated only with a Pt layer. The furnace was evacuated to 0.001 torr and pure hydrogen was introduced into the furnace tube. Subsequently, the gas pressure was maintained at 0.1 atm with a hydrogen atmosphere. The furnace was heated to 1200 ° C. before introducing BCl 3 gas through the induction quartz tube into the vicinity of the LaB 6 salt. After maintaining the reaction for 5 minutes, the supply of BCl3 was stopped and cooled to room temperature. The configuration of the furnace tube is illustrated in FIG. The experimental parameters are summarized in Table 1.

[Table 1]

[Feature]
After the experiment, all samples were removed for SEM inspection. The substrates were all covered with a thick layer of blue-violet material, which was later found to be LaB6 single crystal nanowires. The diameter of the nanowire is from a few nanometers to less than 1 micron. The length of the nanowire is several microns to several tens of microns. Representative SEM images of LaB6 nanowires grown on Si, graphite, W, and Mo substrates are presented in FIGS. By controlling the deposition conditions, it is possible to form a layer of LaB6 crystal film on the substrate prior to the growth of LaB6 nanowires. By this method, LaB6 nanowires can be epitaxially grown on the LaB6 crystal film. SEM images of these structures are shown in FIGS. All nanowires are aligned vertically with the underlying LaB6 grains.

  LaB6 nanowires were scraped off the substrate and suspended in ethanol. A drop of suspension was dropped on a TEM copper grid coated with lacy carbon and subjected to TEM inspection. A representative TEM image is shown in FIG. All nanowires were found to have flat sides and top surfaces. These surfaces all terminate at the (100) plane of the LaB6 crystal. It has been found that hemispherical catalyst particles remain at the tips of some nanowires, and the results are shown in FIG. The EDX spectra shown in FIGS. 14b and c measured at points B and C marked in FIG. 14a also show that the catalyst particles are composed of Si and Pt elements.

[Manufacture of emitter structure]
It is proposed to configure a LaB6 nanowire electron source as illustrated in FIG. LaB6 nanowire thin films are deposited on tungsten (W) filaments. Heating is performed by passing a heating current directly through the tungsten filament. In order to accelerate the electrons emitted from the LaB6 nanowire, the anode is placed slightly away from the cathode.

  In order to construct a point electron source such as an electron gun used in an electron microscope, a 0.1 mm tungsten wire was electrochemically etched in a NaOH solution (1 Mol / l), and the tip diameter was about 200 nm. A needle was formed. Next, under the control of the 3D microstage, the tip of the tungsten needle was brought close to one LaB6 nanowire grown on the substrate. Next, the selected nanowire was attached, and after applying a DC voltage of 30 V between the substrate and the tungsten needle, the tungsten needle was picked up. Subsequently, a tungsten thin film was deposited on the junction between the nanowire and the tungsten needle using FIB. The tip of the tungsten needle was imaged using SEM both before and after FIB welding. The optical microscope assisted 3D microstage system is designed to pick up a single LaB6 nanowire using an electrochemically sharpened tungsten needle.

  FIG. 16 is a schematic diagram of the system. As shown in FIG. 17, the Si substrate on which LaB6 nanowires are grown is placed in the center, while the tungsten needle is gradually brought closer to one LaB6 in the bundle using a microstage. After the tungsten needle is brought close to the selected nanowire, a direct current voltage of about 30 V is applied between the needle and the Si substrate to generate an electrostatic force for attachment. After cutting the LaB6 nanowire from the substrate by mechanical force, the needle is transferred to the FIB chamber for adhesion layer deposition. FIG. 18 shows two SEM images of the tip of the tungsten needle immediately before and immediately after FIB “welding”. A tungsten metal layer is deposited in the circled area in the left image where the LaB6 nanowire is only loosely attached to the top of the tungsten needle by van der Waals interaction, and the resulting structure is shown in the right image. It is shown. This is expected to strengthen the bond and greatly improve thermal and electrical conductivity due to the presence of the metal layer. FIG. 19 shows LaB6 nanowires welded onto a pointed tungsten needle by a similar process.

Explanatory drawing about a chemical vapor deposition mechanism. Low magnification SEM image of LaB6 nanowires grown on a silicon substrate. High magnification SEM image of LaB6 nanowires grown on a silicon substrate. SEM image of LaB6 nanowires grown on a silicon substrate. Side view. Low magnification SEM image of LaB6 nanowires grown on a graphite substrate. High magnification SEM image of LaB6 nanowires grown on a graphite substrate. Low magnification SEM image of LaB6 nanowires grown on a tungsten substrate. High magnification SEM image of LaB6 nanowires grown on a tungsten substrate. Low magnification SEM image of LaB6 nanowires grown on a molybdenum substrate. High magnification SEM image of LaB6 nanowires grown on a molybdenum substrate. Low magnification SEM image of LaB6 nanowires grown on a LaB6 substrate. High magnification SEM image of LaB6 nanowires grown on LaB6 substrate. (A) High resolution TEM image and diffraction pattern of LaB6 nanowire oriented in the <001> direction of its crystal structure. (B) A diffraction pattern obtained from one nanowire exhibiting <001> orientation. (C) High-resolution image showing the tip corner of the nanowire. (A) TEM image of LaB6 nanowires grown with the aid of a Pt / Si catalyst. (B) and (c) are EDX spectra measured at points B and C in (a), indicating that the catalyst particles are Pt / Si alloys. Explanatory drawing of the direct heating type ReB6 nanowire thin film thermionic electron emitter which used tungsten as a heating filament. Explanatory drawing which shows the process of attaching one nanowire to the front-end | tip of a tungsten chip. Photo of electrochemically etched tungsten needle approaching LaB6 nanowires controlled by microstage. SEM image of one LaB6 nanowire emitter welded to a flat heat source by FIB process. One LaB6 nanowire welded onto a pointed heat source. Example 1: Planar electron source: Traveling wave tube equipped with a LaB6 cathode for generating high output high frequency for satellite communication. Example 2: Point electron source: JOEL2100 transmission electron microscope equipped with a LaB6 electron gun.

Explanation of symbols

1. BC ₁₃ gas + H ₂ gas 2. 2. furnace tube 3. Tubular furnace 4. Rare earth chloride source Growth substrate

Claims (5)

A method for manufacturing a surface emitter comprising the following steps 1 to 3:
(Step 1) A heater material made of one material selected from the group consisting of tungsten, molybdenum, and LaB6 is used for TEM, SEM, EBL system, X-ray tube, microwave tube, electron transistor, particle accelerator, and free electron laser. Process into cathode filament shape for any application.
Si layer (Step 2) Previous Symbol cathode filament surface, depositing a Pt layer in this order, to form a catalyst layer.
(Step 3) A rare earth hexaboride nanowire thin film is grown on the surface of the cathode filament through an alloy of Pt and Si formed by heating on the catalyst film by chemical vapor deposition. A method of manufacturing a surface emitter comprising the following steps 1 and 2:
(Step 1) An Si layer and a Pt layer are deposited in this order in advance, and a chemical vapor phase is formed on the surface of the heater material made of one material selected from the group consisting of tungsten, molybdenum, and LaB6 coated with the formed catalyst film. A rare earth hexaboride nanowire thin film is grown through an alloy of Pt and Si formed by heating on the catalyst film by a growth method .
(Process 2) Cathode filament for any of TEM, SEM, EBL system, X-ray tube, microwave tube, electronic transistor, particle accelerator, free electron laser, heater material coated with rare earth hexaboride nanowire thin film Process into shape. A structure used as a surface emitter for any of TEM, SEM, EBL system, X-ray tube, microwave tube, electron transistor, particle accelerator, free electron laser,
A heater material made of one material selected from the group consisting of tungsten, molybdenum, and LaB6;
A catalyst layer formed by depositing a Si layer and a Pt layer in this order on the surface of the heater material;
A rare earth hexaboride nanowire thin film grown on a surface of the heater material through an alloy of Pt and Si formed by heating on the catalyst film by a chemical vapor deposition method . A method for producing a point emitter comprising the following steps 1 to 3:
(Step 1) A Si layer and a Pt layer are deposited in this order in advance , and a chemical vapor deposition method is performed on a substrate made of one material selected from the group consisting of tungsten, molybdenum, and LaB6 coated with the formed catalyst. A rare earth hexaboride nanowire thin film is grown through an alloy of Pt and Si formed by heating on the catalyst film .
(Step 2) One rare earth hexaboride nanowire of the rare earth hexaboride nanowire thin film is attached to the tip of a needle-like heat source made of one material selected from the group consisting of tungsten, molybdenum, and LaB6.
(Step 3) A refractory metal layer is deposited on the contact portion of the rare earth hexaboride nanowire / needle heat source to enhance the attachment of the rare earth hexaboride nanowire / needle heat source. A point emitter manufactured by the method of manufacturing a point emitter according to claim 4, wherein the structure is used as a point emitter for any of TEM, SEM and EBL systems,
A needle-shaped heat source made of one material selected from the group consisting of tungsten, molybdenum, and LaB6;
One rare earth hexaboride nanowire attached to the tip of the needle-shaped heat source;
And a refractory metal layer deposited on the contact portion of the rare earth hexaboride nanowire / needle-like heat source.