JP2007265924A - Diamond electron source element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diamond electron source element having a sufficiently improved life characteristic because a surface of a diamond electron source terminated by hydrogen has a problem wherein termination hydrogen is separated by collision of hydrogen ions accelerated and having large energy and, as a result, electron emission capability rapidly declines. <P>SOLUTION: In this diamond electron source element, a surface of an electron emission part is formed with a diamond terminated by hydrogen atoms, and not less than 0.02% within the hydrogen atoms are deuterium atoms and/or tritium atoms. In addition, it is preferable that not less than 1.2% within carbon atoms constituting the foremost layer of the diamond are carbon atoms each having a mass number of 13 and/or carbon atoms each having a mass number of 14. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a diamond electron source element using diamond as an electron emission material.

When diamond is used as a semiconductor material, since the band gap is as large as about 5.5 eV, application to power devices and ultraviolet light emitting devices is expected. Further, because of this large band gap, the energy difference between the lower end of the conductor and the vacuum level is small, and further, as described in Non-Patent Document 1, negative electrons in which the vacuum level is at an energy position lower than the lower end of the conductor. Since a rare physical property called “Negative Electron Affinity” (NEA) has been clarified, application to electron-emitting devices is also expected.

The electron source currently used is a hot cathode electron source, LaB ₆ , which heats a metal having a heat function as high as 4.5 eV such as a tungsten filament but heat-resistant metal to a high temperature of around 2800 K to extract electrons into a vacuum. A hot cathode electron source for extracting electrons into a vacuum by using a material having a work function as small as 2.6 eV (lanthanum hexaboride) as high as about 1800 K, and a sharp tungsten needle like ZrO / W (zirconia tungsten) Zirconium oxide is coated to lower the work function to 2.7 eV, and a thermal field emission electron source that takes out electrons in a vacuum at a temperature of about 1800 K and a high electric field. A sharp tungsten needle is vacuumed at room temperature and a high electric field. There are field emission electron sources to be taken out. On the other hand, in diamond, if the negative electron affinity described above can be utilized well, an energy-saving electron source can be produced because there is no need for high temperature and high electric field. The electron source used has been proposed.

It has been found that a state in which diamond has a negative electron affinity, that is, a state in which electrons are likely to be emitted, occurs only when the state of the diamond surface is limited. The state is a state in which the diamond surface is covered with hydrogen atoms, that is, the terminal atoms on the diamond surface are hydrogen atoms. Conversely, if hydrogen is removed from the diamond surface, the merit of using diamond as an electron source is reduced. For this reason, the hydrogen partial pressure in the hermetic vessel is sealed in the range of 1 × 10 ⁻⁶ to 1 × 10 ⁻³ Torr as in the electron tube of Patent Document 3, or in the discharge or the like as in the discharge lamp of Patent Document 4. Application products equipped with means to maintain the hydrogen termination surface of the diamond electron emission cathode used have been proposed, such as having a hydrogen storage alloy to maintain the hydrogen partial pressure of the metal and suppressing the decrease in electron emission efficiency. ing.
FJ Himpsel et al., Phys. Rev. B, 20, 624-627p (1979) JP-A-4-67527 JP-A-5-152640 JP-A-10-116555 JP-A-2005-108564

However, when a diamond electron source cathode is used, it may be difficult to adjust the hydrogen partial pressure in the atmosphere as described above. For example, in an electron gun chamber maintained in an ultrahigh vacuum environment of about 10 ⁻⁵ to 10 ⁻⁷ Pa such as an electron microscope or an electron beam exposure machine, a diamond electron source is used instead of LaB ₆ or ZrO / W. This is the case. In this case, in order to extract electrons emitted from the diamond electron source as a beam, a voltage of several kV with respect to the diamond electron source or to accelerate the electron beam to a predetermined energy necessary for observation / exposure. An acceleration voltage of several tens of kV is applied. For this reason, hydrogen, which is dominant in the residual gas component in the ultra-high vacuum region, collides with the accelerated electrons and ionizes, and is accelerated toward the diamond electron source in the opposite direction to the electrons by the extraction voltage and acceleration voltage. Finally, it collides with the surface of the diamond electron source. The surface of the diamond electron source terminated with hydrogen is superior to the competitive electron source because the terminal hydrogen is desorbed by collision of hydrogen ions with high energy that are accelerated, and the electron emission capability declines rapidly. There was a problem that the life that can be said was not obtained.
Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to provide a diamond electron source element with sufficiently improved life characteristics.

In order to solve the above problems, the diamond electron source element according to the present invention uses diamond whose surface of the electron emission portion is terminated with hydrogen atoms, and 0.02% or more of the hydrogen atoms are deuterium. Atom (D) and \ or tritium atom (T).
In this diamond electron source element, deuterium atoms (D) and / or tritium atoms (T) among the hydrogen atoms covering the diamond surface of the electron emission part are present in the deuterium atoms (D) on the earth. More than 0.02% is included. There are three types of hydrogen atoms on the earth, namely light hydrogen atoms (H), deuterium atoms (D), and tritium atoms (T). A negative electron affinity can be obtained regardless of which of these terminal atoms. On the other hand, the mass number differs from that of light hydrogen atom (H) by 2 times for deuterium atom (D) and 3 times by tritium atom (T), and is bonded to carbon atom (C) on the outermost layer of diamond. The C—D bond and the C—T bond have a higher binding energy than the C—H bond, and are not easily broken. Therefore, light hydrogen gas (H ₂ ), which is dominant as a residual gas in an ultrahigh vacuum environment of 10 ⁻⁵ Pa or less, is ionized by electron impact and becomes light hydrogen ions (H ⁺ ), colliding with the hydrogen-terminated diamond surface. When the proportion of deuterium atoms (D) and tritium atoms (T) among the terminal hydrogen atoms on the diamond surface is 0.02% or more, which is at least larger than the existence ratio on the earth, ⁺ ) As a result of strong impact and maintaining the hydrogen-terminated surface for a relatively long time, the life characteristics of the diamond electron source element are improved. Here, in the present invention, “hydrogen” refers to all isotopes having mass numbers 1, 2, and 3, “light hydrogen (H)” is mass number 1, and “deuterium (D)” is mass number 2. , “Tritium (T)” refers to a mass number of 3.

In the diamond electron source element according to the present invention, 1.2% or more of the carbon atoms (C) constituting the outermost layer of diamond in the electron emission portion have a mass number of 13 ( ^13C ) and / or a mass number of 14. It is a carbon atom ( ¹⁴ C).
In this diamond electron source element, among the carbon atoms (C) on the diamond outermost surface of the electron emission portion, carbon atoms having a mass number of 13 ( ¹³ C) and / or carbon atoms having a mass number of 14 ( ¹⁴ C) are It is contained 1.2% or more which is larger than the abundance ratio of carbon atoms ( ^13C ) having a mass number of 13 above. On the earth three carbon atoms (C), i.e., carbon atoms ⁽¹² C) of the mass number 12, carbon atoms ⁽¹³ C) of the mass number 13, there are carbon atoms ⁽¹⁴ C) of the mass number of 14 However, since these chemical properties are the same, even if these proportions are different, the chemical properties of diamond are the same. On the other hand, since the mass numbers are different as in the hydrogen described above, the ¹³ C—H bond and the ¹⁴ C—H bond are less likely to be cleaved than the ¹² C—H bond because the bond energy is higher. Similarly ¹² C-D toward the ¹³ C-D bond and ¹⁴ C-D bonds than bonds, ¹² C-T than the binding ¹³ C-T bond and ¹⁴ C-T bond bonding energy towards the large cutting It is hard to be done. Accordingly, the proportion of carbon atoms ( ¹³ C) having a mass number of 13 and carbon atoms having a mass number of 14 ( ¹⁴ C) in the carbon atoms (C) of the outermost layer of diamond is at least a carbon atom having a mass number of 13 on the earth ( ¹³ When the content is 1.2% or more, which is greater than the proportion of C), the impact to light hydrogen ions (H ⁺ ) can be further enhanced, and the life of the diamond electron source element is further improved.

  ADVANTAGE OF THE INVENTION According to this invention, the electron source element which can obtain the lifetime characteristic improved in the diamond electron source element used in electron beam apparatuses, such as an electron microscope and an electron beam exposure machine, is provided.

  Hereinafter, preferred embodiments of a diamond electron source element according to the present invention will be described in detail with reference to the accompanying drawings, but the present invention is not limited to the following embodiments. In the description of the drawings, the same reference numerals are given to the same elements, and duplicate descriptions are omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.

First, a manufacturing example of the diamond electron source element of the present invention will be described in the order of manufacturing steps.
-Step I-
A sharpened diamond single crystal 1 as shown in FIG. 1 is prepared. The diamond single crystal may be produced by high-temperature high-pressure synthesis or gas-phase synthesis, or may be natural. In addition, the crystal quality can be any of the a type with almost no impurities mixed, the Ia type or Ib type mixed with nitrogen, the IIb type mixed with boron, or the Ib type mixed with boron. A maximum size of 2 mm × 2 mm × 5 mm can be suitably used. The sharpening process may be formed using any means such as mechanical polishing, electric discharge machining, reactive ion etching (RIE), and focused ion beam (FIB). The tip 2 of the sharpened portion is formed by a plurality of inclined surfaces. In the diamond electron source element of the present invention, it is desirable that at least one of the surfaces constituting the sharpened portion is a {111} plane. For example, as shown in FIG. 1, four {111} planes 3 may be formed. On the {111} plane, phosphorus, which is an n-type impurity, can be efficiently doped by vapor phase growth, so that an n-type diamond semiconductor capable of obtaining excellent electron emission characteristics can be formed efficiently.

-Step II-
As shown in FIG. 2, the phosphorus-doped diamond thin film 4 is formed by epitaxial growth on the four {111} planes that form the sharpened tip 5. A microwave plasma CVD method can be suitably used as the growth method. As the source gas, hydrogen (H ₂ ) gas, methane (CH ₄ ) gas, and phosphine (PH ₃ ) gas can be preferably used. In order to produce the diamond electron source element of the present invention, methane (CH ₄ ) 1.2% or more of the carbon atoms (C) in the gas has a mass number of 1
3 ( ¹³ C) and / or methane (CH ₄ ) gas having a mass of 14 carbon atoms ( ¹⁴ C) is used. In this way, the grown phosphorus-doped diamond thin film 4 has 1.2% or more of carbon atoms (C) constituting the diamond having a mass number of 13 carbon atoms ( ¹³ C) and / or a mass number of 14 carbon atoms ( As a result of ¹⁴ C), 1.2% or more of the carbon atoms (C) constituting the outermost layer of the phosphorus-doped diamond thin film 4 are carbon atoms having a mass number of 13 ( ¹³ C) and / or a mass number of 14 Of carbon atoms ( ¹⁴ C). The phosphorus (P) concentration in the phosphorus-doped diamond thin film 4 is 5 × 10 ¹⁵ cm ⁻³ or more suitable for the diamond electron source element of the present invention, but more preferably 5 × 10 ¹⁹ cm ⁻³ or more. Since the resistance value of the thin film 4 becomes lower, it is preferable. The film thickness of the phosphorus-doped diamond thin film 4 is preferably 1 μm or more in consideration of the following sharpened portion shape correction step, and if it is 10 μm, it sufficiently functions as the electron source element of the present invention. Instead of the phosphorus-doped diamond thin film 4, a boron-doped diamond thin film may be grown.

-Step III-
As shown in FIG. 2, the shape of the sharpened portion is corrected, and a spherical sharpened portion having a curvature radius of about 1 μm is formed as the electron emitting portion 5. Depending on the use of the diamond electron source element, there is no problem even if the radius of curvature is about 10 μm. The machining method can be any machining means such as machining, electric discharge machining, RIE, and FIB as in the sharpening process described above.

-Step IV-
The electron emission portion 5 is exposed to a plasma generated with hydrogen gas containing 0.02% or more of deuterium (D ₂ ), and 0.02% or more of the electron emission portion is deuterium atoms (D) and / or tritium. Terminate with an atom (T). By exposing the electron emission part 5 to plasma for about 1 hour, the surface is terminated with hydrogen atoms, and 0.02% or more of the hydrogen atoms are deuterium atoms (D) and / or tritium atoms (T). Become. As a method for activating hydrogen gas, a method using a hot filament in addition to plasma can be suitably used. Also, hydrogen termination is possible even if heat treatment is performed in a high-temperature hydrogen atmosphere.

-Process V-
As for the phosphorus-doped diamond thin film 6 on the two {111} faces facing each other as shown in FIG. 3, electrons are supplied to the phosphorus-doped diamond thin film 6 so that only the electron emission portion 5 at the very tip is exposed. The metal 7 to be deposited is deposited. Since the diamond electron source element supplies electrons to the phosphorus-doped diamond thin film 6 by the metal 7 and is heated by energization or heating due to electron emission, it is preferable to use a deposited metal having a melting point of 1,000 ° C. or higher. In particular, the metal forming carbide (carbide), for example, titanium (Ti), tungsten (W), molybdenum (Mo), etc. has a Schottky characteristic that is ohmic due to contact with the phosphorus-doped diamond thin film 6 when the carbide is formed. Since it becomes possible to efficiently supply electrons to the phosphorus-doped diamond in order to approach, it can be suitably used.

  As mentioned above, although the preparation example of the diamond electron source element of the present invention has been shown in the order of the production process, the shape is not limited to the above-mentioned shape, Any shape of the diamond electron source element using diamond as the electron emission portion, such as a shape using the convex and concave portions of the surface of the polycrystalline diamond as an electron emission point, is within the scope of the present invention.

According to the prior art, the carbon atom (C) in the methane (CH ₄ ) gas used when the phosphorus-doped diamond thin film produced in Step II is synthesized by vapor phase growth has a mass number of 12C on the earth.
Of carbon atoms ( ¹² C), mass 13 carbon atoms ( ¹³ C) and mass 14 carbon atoms ( ¹⁴ C) are present in the same proportion, and carbon atoms ( ¹² C) of mass ¹² C are 98.89. %, Carbon atoms having a mass number of 13 ( ¹³ C) are 1.1%, and carbon atoms having a mass number of 14 ( ¹⁴ C) are negligible. Therefore, the carbon atom (C) constituting the outermost layer of the phosphorus-doped diamond thin film synthesized by the prior art is constituted by the isotope ratio.

  Further, according to the prior art, the hydrogen atoms in the hydrogen gas used in Step IV are the same as the existing proportions of light hydrogen atoms (H), deuterium atoms (D) and tritium atoms (T) on the earth. The light hydrogen atom (H) is 99.983%, the deuterium atom (D) is 0.016%, and the tritium atom (T) is very small. Therefore, if the electron emission portion is terminated by the conventional technique, the hydrogen atoms on the diamond surface of the electron emission portion are configured with the isotope ratio.

  The diamond electron source element fabricated by the conventional technology was measured for the time dependence of the electron emission current value by applying an acceleration voltage while heating the diamond electron source element by energization heating in an ultra-high vacuum environment. Not found out. In addition, the time dependence of the ultrahigh vacuum light hydrogen partial pressure, which was measured simultaneously in this experiment, also revealed that hydrogen desorption from the surface of the diamond electron source element was the cause of poor life characteristics.

The inventors considered the cause of hydrogen desorption, that is, the cause of the breakage of the bond between the carbon atom on the diamond surface and the terminal hydrogen atom, i: due to heat by current heating or electron flow, ii: due to electron emission, iii: Assuming that it was due to ion bombardment, we conducted extensive research to identify the cause. As a result, it was found that iii was most effective for the life characteristics. Furthermore, the ions are mostly light hydrogen ions (H ⁺ ) generated by collision of light hydrogen (H ₂ ) gas, which is dominant as a residual gas in an ultra-high vacuum environment, with accelerated emitted electrons, It was found that the light hydrogen ions (H ⁺ ) are accelerated in the direction opposite to the electrons by the acceleration voltage, collide with the surface of the diamond electron source element, and desorb the terminal hydrogen atoms.

As a result of intensive studies by the inventors for this measure, in order to withstand the impact of light hydrogen ions (H ⁺ ) and to improve the life characteristics, the terminal hydrogen atoms have the same chemical properties as light hydrogen (H). However, it has been found that it may be terminated with a deuterium atom (D) or tritium atom (T) having a large mass number and different physical properties. An effect is obtained when the coverage of deuterium atoms (D) and tritium atoms (T) in the electron emission part is 0.02% or more, and more particularly, when these coverages are 1% or more, it is particularly prominent. It was found that the characteristics were improved. This has the effect of increasing the binding energy, and even if light hydrogen ions (H ⁺ ) collide with deuterium atoms (D) or tritium atoms (T), the energy is returned to light hydrogen ions (H ⁺ ). Deuterium atoms (D) and tritium atoms (T) are related to the difficulty in receiving energy. Then, further Noting that may be strongly bonded to the diamond outermost layer of carbon and hydrogen atoms, and the diamond outermost layer of carbon atoms, the carbon atoms of a mass number 12 ⁽¹² C) mass number 13 rather than When carbon atoms ( ¹³ C) and carbon atoms having a mass number of 14 ( ¹⁴ C) are used, terminations with deuterium (D) and tritium (T) are found to be more effective.

Such an effect was difficult to find in the vacuum region of less than 10 ⁻⁵ Pa, but it was not until the discovery of an electron beam generation test from the diamond electron source element in the ultrahigh vacuum region of 10 ⁻⁵ Pa or more. It is a thing. In particular, the higher the operating temperature of the diamond electron source element, or the higher the emission current value, the lower the threshold of desorption energy of terminal hydrogen atoms due to collision of ion light hydrogen (H ⁺ ). Become. Specifically, since the effect appears greatly when the operating temperature of the diamond electron source is 250 ° C. or higher, or the emission current value is 1 μA or higher, an experiment was performed at a temperature lower than 250 ° C. and an emission current value lower than 1 μA. If it was hard to headline.

The diamond electron emission cathode, electron emission source, electron microscope, and electron beam exposure machine of the present invention will be described more specifically based on examples.
The diamond electron source elements (1) to (15) according to the present invention shown in FIG. The diamond single crystal is a high-temperature high-pressure synthetic type IIa type, and the size is 0.6 × 0.6 × 2.5 mm.

A phosphorus-doped diamond thin film 6 was formed by epitaxial growth on the four {111} faces forming the sharpened tip 5. The film thickness was 2 μm, the phosphorus concentration in the thin film was 1 × 10 ²⁰ cm ⁻³ , the radius of curvature of the electron emission portion 5 was 1 μm, and the metal 7 for supplying electrons was Mo. As a comparative example, a diamond electron source element (16) was produced in the same manner as the diamond electron source elements (1) to (15) except that the steps II and IV were changed to the prior art. Table 1 shows the lifetimes of the diamond electron source elements (1) to (16). The term “lifetime” as used herein refers to the time during which the electron emission current value is half of the initial value. The measurement environment is the same for all (1) to (16). The “termination type” in Table 1 indicates the type of isotope ratio of the termination hydrogen atom of the electron emission portion 5 of each diamond electron source element. In this embodiment, 4 of A to D shown in Table 2 A type was made. The “outermost layer type” indicates the isotope ratio type of the diamond outermost layer carbon atom in the electron emission portion 5 of each diamond electron source element, and four types a to d shown in Table 3 were produced.

As a result, when 0.02% or more of the terminal hydrogen atoms in the electron emission portion are deuterium atoms (D) and \ or tritium atoms (T), the lifetime characteristics are excellent as compared with the prior art. I understood it. It was also found that the life characteristics were better as the proportion of deuterium atoms (D) and \ or tritium atoms (T) was larger. Furthermore, in the case where 1.2% or more of carbon atoms (C) constituting the outermost diamond layer of the electron emission portion are carbon atoms having a mass number of 13 ( ^13C ) and carbon atoms having a mass number of 14 ( ^14C ) As a result, it was found that the life characteristics are superior to those of the prior art. And it turned out that a lifetime characteristic is excellent, so that the carbon atom ( ^13C ) of mass number ¹³ and the carbon atom ( ^14C ) of mass number 14 are large.
As can be seen from the above embodiments, the diamond electron source element according to the present invention has excellent lifetime characteristics, and can be suitably used for electron beams and electron beam equipment such as electron microscopes and electron beam exposure machines. Moreover, it can be used for vacuum tubes such as traveling wave tubes and microwave tubes.

It is a perspective view which shows the sharpened diamond single crystal. It is a perspective view which shows the diamond electron source element based on this invention. A diamond electron source element in which a metal for supplying electrons is deposited.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Diamond single crystal 2 Tip of sharp tip 3 Four {111} faces 4,6 Diamond thin film 5 Tip of sharp tip 7 Metal

Claims (3)

It is a diamond electron source element, the surface of the electron emission part is formed by diamond terminated with hydrogen atoms, and 0.02% or more of the hydrogen atoms are deuterium atoms and / or tritium atoms. A diamond electron source element. A diamond electron source element, wherein 1.2% or more of carbon atoms constituting the outermost layer of diamond in an electron emission portion are carbon atoms having a mass number of 13 and / or a mass number of 14 Electron source element. A diamond electron source element, wherein the surface of the electron emission portion is formed of diamond terminated with hydrogen atoms, and 0.02% or more of the hydrogen atoms are deuterium atoms and / or tritium atoms. A diamond electron source element, wherein 1.2% or more of carbon atoms constituting the outermost layer of the diamond are carbon atoms having a mass number of 13 and / or a mass number of 14.

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