The present invention relates to a diamond electron emission cathode and an electron emission source for use in electron beams and electron beam devices such as electron microscopes and electron beam exposure devices and in vacuum tubes such as traveling wave tubes and microwave tubes, and also relates to an electronic device using such cathode and source.
Because an electron bears a negative charge and has a very small mass, electron beams in which electrons are arranged to travel in one direction have the following special features. (1) The direction and the degree of convergence can be controlled by an electric or magnetic field. (2) Energy in a wide range can be obtained by acceleration and deceleration with an electric field. (3) Because the wavelength is short, the beam can be converged to a small diameter. Electron microscopes and electron beam exposure devices employing such special features have been widely used. As cathode materials for such devices, cheap W filaments or hexaborides, e.g., LaB6 which can produce an electron beam with a high brightness, are exemplified for thermal electron emission sources. Further, W with a sharpened tip that uses a tunnel phenomenon based on a quantum effect and ZrO/W using the Schottky effect based on electric filed application have been used as cathodes with a high brightness and a narrow energy width.
However, although W filaments are inexpensive, a problem associated therewith is that the service life thereof is extremely short (about 100 h). As a result, a replacement operation such as opening a vacuum container to the atmosphere or adjusting the optical axis of electron beam when the filament is broken has to be frequently performed. The service life of LaB6 is about 1000 h and longer than that of W filaments, but because it is used in devices in which beams with a comparatively high brightness are obtained, the replacement operation is most often performed by the device manufacturers and the cost thereof is high. A problem associated with ZrO/W that has a comparatively long service life of about one year and W with a sharpened tip that allows a higher brightness to be obtained is that the replacement cost is high.
Because electron microscopes are presently required to enable highly accurate observations of even smaller objects and because electron beam exposure devices have been advanced to a node size of less than 65 nm, the cathodes with even higher brightness and narrower energy width are needed.
Diamond is a material that meets such expectations. Diamond exists in a state with a negative electron affinity (NEA) or in a state with a positive electron affinity (PEA) less than that of metals with a small work function, as described in Non-Patent Document 1 and Non-Patent Document 2. By employing such an extremely unique physical property, it is possible to obtain electron emission with a high current density and to reduce the energy width, without requiring a high temperature in excess of 1000° C. as in W filaments, LaB6, or ZrO/W. Further, because the operation temperature is low, a long service life can be expected. In addition, because there is a microprocessing technology allowing a tip end diameter of 10 nm to be obtained, as described in Non-Patent Document 3, no problems are associated with increase in brightness. Since diamond has been found to have the aforementioned electron affinity, electron sources as described in Non-Patent Document 4 and Patent Document 1 have been proposed.
Non-Patent Document 1: F. J. Himpsel et al., Phys. Rev. B, Vol. 20, Number 2 (1979), 624-;
Non-Patent Document 4: W. B. Choi et al., J. Vac. Sci. Technol. B14 (1996) 2051-; and
However, the following problems are encountered when electron sources using diamond are employed in electronic microscopes or electron beam exposure devices that have become widely spread. Thus, structures in which a plurality of electron emission points, such as described in Non-Patent Document 3, are arranged side by side become plane electron sources and a small-diameter beam is difficult to obtain by converging. Mounting on the device is also a difficult task. No problems are associated with shape when Mo with a sharp tip is coated with diamond, as described in Non-Patent Document 4, but because of a polycrystalline structure there are differences among individual species and variations in electric characteristics. Because the structure suggested in Patent Document 1 is also a plane electron source, a converged beam is difficult to obtain. Such source is also difficult to mount on the device.
Accordingly, with the foregoing in view, it is an object of the present invention to provide an electron emission cathode and an electron emission source using diamond and having a high brightness and a small energy width that are suitable for electron ray and electron beam devices and vacuum tubes, in particular, electron microscopes and electron beam exposure devices, and to an electron microscope and an electron beam exposure device that use such electron emission cathode and electron emission source.
In order to resolve the above-described problems, the diamond electron emission cathode in accordance with the present invention adopts the following configuration.
(1) A diamond electron emission cathode in accordance with the present invention comprises single crystal diamond in at least part thereof, wherein the diamond electron emission cathode is in a columnar form having a sharpened acute portion in one place of an electron emitting portion and is constituted by at least two types of semiconductors that differ in electric properties, one of the types constituting the semiconductors is an n-type semiconductor comprising n-type impurities at 2×1015 cm−3 or higher, the other type is a p-type semiconductor comprising p-type impurities at 2×1015 cm−3 or higher, the p-type semiconductor and the n-type semiconductor are joined directly or indirectly via a layer of another type (for example, an intrinsic semiconductor layer), an electric potential that is negative with respect to the p-type semiconductor is applied with a pair of current introducing terminals to the n-type semiconductor so that electrons flow from the n-type semiconductor to the p-type semiconductor, a joining interface of the p-type semiconductor and n-type semiconductor is positioned close to the electron emitting portion, and the n-type semiconductor has a component in which electrons flow to the emitting portion.
(2) A diamond electron emission cathode of the present invention comprises single crystal diamond in at least part thereof, wherein the diamond electron emission cathode is in a columnar form having a sharpened acute portion in one place of an electron emitting portion and is constituted by at least two types of semiconductors that differ in electric properties, one of the types constituting the semiconductors is an n-type semiconductor comprising n-type impurities at 2×1015 cm−3 or higher, the other species is a p-type semiconductor comprising p-type impurities at 2×1015 cm−3 or higher, the p-type semiconductor and the n-type semiconductor are joined directly or indirectly via a layer of another type, an electric potential that is positive with respect to the p-type semiconductor is applied with a pair of current introducing terminals to the n-type semiconductor so that electrons flow from the p-type semiconductor to the n-type semiconductor, a joining interface of the p-type semiconductor and n-type semiconductor is positioned close to the electron emitting portion, and the n-type semiconductor has a component in which electrons flow to the emitting portion.
Such diamond electron emission cathode contains almost no dissimilar materials. Therefore, cathode fracture during heating and cooling caused by the difference in thermal expansion coefficients is prevented. The term “almost” used herein indicates the case where the cathode shape of the diamond electron emission cathode does not depend on any material other than diamond. Accordingly, this excludes the case where the cathode shape, except the below-described heating section, mainly depends on a material other than diamond, as in the shape in which diamond is coated on Mo having a sharp tip, as described in Non-Patent Document 4. Further, because at least part of the diamond electron emission cathode is composed of single crystal diamond, the doping concentration of n-type impurities that are necessary to use diamond as a cathode material can be controlled in the process of vapor-phase growing the diamond, this control being difficult with the polycrystalline materials.
The diamond electron emission cathode in accordance with the present invention contains n-type impurities at 2×1015 cm−3 or higher. This value indicates the value obtained in the vicinity of the electron emitting portion. When n-type impurities are contained at such concentration, electrons are supplied into the conduction band of the diamond. Therefore, the work function is substantially small and electron emission is possible at a high current density.
Further, the diamond electron emission cathode in accordance with the present invention preferably has a structure having an electrically conductive layer or semiconductor layer containing n-type impurities at 2×1015 cm−3 or higher in at least a portion on the diamond containing p-type impurities at 2×1015 cm−3 or higher. In the diamond, the level of p-type impurities is shallow and a low resistance can be obtained. Therefore, in this case, a lower resistance value of the entire cathode can be obtained.
By making the entire diamond electron emission cathode in a columnar form with one sharpened acute section having an electron emitting portion, the drop of voltage at the cathode is decreased, whereby the electron emission efficiency can be increased. As a result, an electron emission cathode and an electron emission source using diamond and having a narrow energy width and a high brightness can be realized.
(3) In the diamond electron emission cathode in accordance with the present invention, one type constituting the diamond semiconductors is preferably an intrinsic semiconductor constituted by diamond with a carrier concentration of 1×109 cm−3 or less at 300 K. By forming an intrinsic semiconductor layer, electrons can be emitted more effectively.
(4) It is preferred that the diamond electron emission cathode in accordance with the present invention have a heating section formed therein. By forming a heating section in the diamond electron emission cathode in accordance with the present invention, the removal of moisture or the like that has adhered to the surface of the electron emitting portion necessary for stabilizing the emission current can be performed in an easy manner by heating via the heating section.
(5) In the diamond electron emission cathode in accordance with the present invention, the electron emitting portion is preferably formed by the n-type semiconductor. With the n-type semiconductor, the amount of electrons in the electron flow can be increased and the electron emission characteristic is improved.
(6) In the diamond electron emission cathode in accordance with the present invention, the electron emitting portion is preferably formed by the p-type semiconductor. The p-type semiconductor has a low work function and the electrons flow with good efficiency. Therefore, a good electron emission characteristic can be obtained.
(7) In the diamond electron emission cathode in accordance with the present invention, the electron emitting portion is preferably formed by the intrinsic semiconductor.
(8) In the diamond electron emission cathode in accordance with the present invention, it is preferred that the p-type semiconductor comprise a bulk crystal synthesized by vapor phase growth, and the n-type semiconductor and/or the above-mentioned intrinsic semiconductor comprise a thin-film crystal synthesized by vapor phase growth. By synthesizing a diamond semiconductor by vapor phase growth, the introduction of impurities is reduced and a high-quality bulk crystal is obtained, whereby the electron emission efficiency is increased.
(9) In the diamond electron emission cathode in accordance with the present invention, the length in the short side direction of the diamond electron emission cathode is preferably 0.05 mm or more to 2 mm or less and the aspect ratio thereof is preferably 1 or more. Such a shape allows the diamond electron emission cathode to be easily mounted on electron beam devices such as electron microscopes and electron beam exposure devices.
The “short side direction” as referred to herein indicates a total width of the bottom section on the side opposite to the electron emitting portion of the diamond electron emission cathode. When the diamond electron emission cathode is a rectangular parallelepiped, this direction indicates a short side of the width of the bottom section. The “aspect ratio” as referred to herein is a ratio of the length in the longitudinal direction to that in the short side direction, where the length from the tip end of the electron emitting portion to the bottom section on the opposite side is taken as the longitudinal direction.
(10) In the diamond electron emission cathode in accordance with the present invention, at least one plane of the planes which form the electron emitting portion as a tip apex in the sharpened acute section is preferably formed by a (111) crystal plane [including off-planes within ±7° from the (111) just plane].
The stable growth plane in the vapor phase growth is a (100) plane or (111) plane, but the n-type impurity intake efficiency of the (111) plane in the vapor phase growth process is 10 or more times that of the (100) plane. It means that the (111) crystal plane of diamond enables high-concentration doping with n-type impurities, a metallic conduction can be easily obtained, and electron emission at a high current density can be obtained. Therefore, in the case where the electron emitting portion has a (111) crystal plane, a high-brightness electron emission cathode can be easily obtained.
(11) In the diamond electron emission cathode in accordance with the present invention, a surface of the diamond constituting the electron emitting portion is preferably terminated with hydrogen atoms.
By terminating a surface of the diamond constituting the electron emitting portion with hydrogen atoms, the electron affinity is reduced and the efficiency of electron emission characteristic can be increased. Terminating 50% or more of the dangling bonds of the surface carbon of diamond with hydrogen atoms is especially effective.
(12) In the diamond electron emission cathode in accordance with the present invention, a specific resistance of the n-type semiconductor at 300 K is preferably 300 Ωcm or less.
With the diamond electron emission cathode of such a resistivity, electrons are efficiently supplied to the portion containing n-type impurities. As a result, high-density electron emission is possible and a high-brightness electron emission cathode is obtained. It is especially preferred that a specific resistance in the vicinity of the electron emitting portion be within the above-descried range.
(13) In the diamond electron emission cathode in accordance with the present invention, a tip end diameter or a tip end curvature radius of the sharpened acute section is preferably 30 μm or less.
By making the tip end portion, which forms the electron emitting portion, in such a small size, an electron emission cathode with a higher brightness can be obtained.
(14) In the diamond electron emission cathode in accordance with the present invention, it is preferred that the electron emitting portion have a protruding structure, a tip end diameter of the protrusion be 5 μm or less, and an aspect ratio thereof be 2 or more.
When only the electron emitting portion of the entire diamond single crystal serving as an electron emission cathode has such a sharpened acute shape, it is possible to realize a high-brightness diamond thermal field emission cathode or diamond field-emission cathode that can be easily mounted on an electron microscope or electron beam exposure device.
(15) In the diamond electron emission cathode in accordance with the present invention, a temperature during electron emission is preferably 400 K or more to 1200 K or less. With the temperature during electron emission below 400 K, a sufficient emission current cannot be obtained and if the temperature exceeds 1200 K, a long serving life cannot be obtained. It is even more preferred that the temperature during electron emission be 400 K or more or 900 K or less.
(16) In the diamond electron emission cathode in accordance with the present invention, an electron beam with an energy width of 0.6 eV or less is preferably emitted from the electron emitting portion. A diamond electron emission cathode with a good electron beam can be provided.
(17) In the diamond electron emission cathode in accordance with the present invention, the heating section preferably has a metal layer. Because the presence of a metal layer decreases electric resistance, the source voltage used for heating can be decreased and the diamond electron emission cathode is suitable for mounting on electron beam devices such as electron microscopes and electron beam exposure devices.
(18) In the diamond electron emission cathode in accordance with the present invention, it is preferred that the surface of the diamond electron emission cathode be covered with a metal layer and the shortest distance from the electron emitting portion to an end section of the metal layer be 500 μm or less. If the distance of 500 μm is exceeded, the electron emission characteristic is degraded. Furthermore, the distance from the electron emitting portion is preferably 100 μm or less.
(19) A diamond electron emission source in accordance with the present invention is a structure for mounting the diamond electron emission cathode in accordance with the present invention on an electron microscope or electron beam exposure device, this structure comprising the diamond electron emission cathode in accordance with the present invention, an insulating ceramic, and a pair of terminals for supplying an electric current to the diamond electron emission cathode, wherein a resistance value between the terminals is preferably 10Ω or more to 3 kΩ or less. In this case, the diamond electron emission cathode in accordance with the present invention can be mounted without special modifications on the power source system of electron beam devices where the conventional cathode material has been used.
(20) A diamond electron emission source in accordance with the present invention is a structure comprising the diamond electron emission cathode in accordance with the present invention, an insulating ceramic, and a pair of terminals for supplying an electric current to the diamond electron emission cathode, wherein a resistance value between the terminals is preferably 10Ω or more to 700Ω or less.
(21) A diamond electron emission source in accordance with the present invention is a structure for mounting the diamond electron emission cathode in accordance with the present invention on an electron microscope or electron beam exposure device, this structure comprising the diamond electron emission cathode in accordance with the present invention, and a pair of supporting terminals that clamp an insulating ceramic, fix it to the insulating ceramic and supply an electric current to the diamond electron emission cathode, wherein the supporting terminals are brought into direct contact with the diamond electron emission cathode.
In such diamond electron emission source, the diamond electron emission cathode, which is a novel cathode material, can be easily installed in the electron beam devices that have been using w filaments, LaB6, or sharpened W, ZrO/W, which are the conventional cathode materials. Moreover, due to the structure in which the supporting terminals directly clamp the diamond electron emission cathode, the optical axes can be easily aligned during fabrication of the diamond electron emission source and the possibility of displacement or separation during use is very low.
(22) The pair of terminals or the pair of supporting terminals in the diamond electron emission source in accordance with the present invention preferably have a melting point of 1700 K or less. Because electron emission from diamond is possible at a temperature lower than that of W filaments, LaB6, or ZrO/W, a metal with a low melting point can be used and an electron emission source can be constructed by using a low-cost metal material.
(23) An electron microscope in accordance with the present invention has installed therein the diamond electron emission cathode or diamond electron emission source in accordance with the present invention. Because the diamond electron emission cathode or diamond electron emission source in accordance with the present invention makes it possible to obtain an electron beam with a high current density, high brightness, and low energy width, observations can be performed at a magnification ratio higher than that in the electron microscopes using the conventional cathode materials.
(24) An electron beam exposure device in accordance with the present invention has installed therein the diamond electron emission cathode or diamond electron emission source in accordance with the present invention. Because the diamond electron emission cathode or diamond electron emission source in accordance with the present invention makes it possible to obtain an electron beam with a high current density, high brightness, and low energy width, the device of the present invention makes it possible to draw a fine pattern at a higher throughput than in the electron beam exposure devices using the conventional cathode material.
With the present invention, it is possible to realize an electron emission cathode and electron emission source that use diamond and have a high brightness and a small energy width and suitable for electron beam and electron beam devices and vacuum tubes, in particular electron microscopes and electron beam exposure devices. Furthermore, by using the electron emission cathode and electron emission source, it is possible to realize an electron microscope enabling high-magnification observations and an electron beam exposure device in which fine patterns can be drawn at a high throughput.
The preferred modes for carrying out the diamond electron emission cathode, electron emission source, electron microscope, and electron beam exposure device in accordance with the present invention will be described hereinbelow in greater detail with reference to the appended drawings. In the explanation of the drawings, identical elements will be assigned with identical reference symbols and the redundant explanation will be omitted. The dimensional ratio in the drawings does not necessarily match that in the description.
The formation of the diamond electrically conductive layer 12 is preferably performed by epitaxial growth based on a vapor phase synthesis method in order to reduce spread or individual difference in electric conduction characteristic between the diamonds that produce a large effect on electron emission characteristic. The growth is preferably performed by a plasma CVD method that uses microwaves and can control impurity concentration with high accuracy. In the diamond electrically conductive layer 12, n-type impurities are contained at 2×1015 cm−3 or higher, electrons are supplied to the conduction band of diamond, and electron emission can be conducted at a high current density. As a result, an electron emission cathode with a high brightness is obtained. In order to obtain an electron emission cathode with even higher brightness, the concentration of n-type impurities is preferably 2×1019 cm−3 or higher. When the impurities are contained at such a high concentration, the distance between the donors in the diamond crystal becomes extremely small, and the electric conduction mechanism starts making a transition from the semiconductor conduction to metal conduction in the diamond electrically conductive layer 12.
Because the room-temperature resistance starts to decrease abruptly, the voltage drop at the diamond electron emission cathode itself during electron emission decreases. Therefore, electron emission with a higher current density becomes possible and a high-brightness electron emission cathode is obtained. At this time, for example, P or S is used as the n-type doping element. For example, H2 or CH4 is used as a starting material gas employed for vapor phase deposition, and PH3 or H2S is used as a doping gas. The synthesis can be performed under the following conditions. Thus, in the case of P doping, the numerical ratio of C atoms and H atoms in the vapor phase is C/H=0.005-10% and the numerical ratio of P atoms and C atoms is P/C=10−4-100%. In the case of S doping, C/H=0.005-10% and the numerical ratio of S atoms and C atoms is S/C=10−2-100%. The temperature conditions are 600-1300° C. for both P and S doping. The diamond 10 is in a columnar form with a length in the short side direction of 0.05 mm or more to 2 mm or less and an aspect ratio of 1 or more, so that the electron emission cathode can be installed in an electron gun chamber produced as an electron extraction structure of an electron microscope or electron beam exposure device.
With such a structure, it is important that a voltage be applied so as to move electrons from the n-type layer toward the p-type layer and that the electrons move in this direction.
In
In
In both case, a chip voltage in the forward direction and a sharpened tip end are important. No such phenomenon occurs with the flat tip end.
a) and (b) show one sharpened acute section having an electron emitting portion and illustrate a diamond electron emission cathode composed of two types of diamond semiconductors with different electric properties. One of the two types constituting the diamond semiconductors is a p-type layer comprising p-type impurities at 2×1015 cm−3 or higher and the other type of semiconductor is an n-type semiconductor comprising n-type impurities at 2×1015 cm−3 or higher. A potential positive with respect to the p-type layer is applied to the n-type layer so as to create a direction of electron flow from the p-type layer to the n-type layer. Furthermore, the p-type layer has a component in which electrons move toward the electron emitting portion. In the configuration shown in
Even when an intrinsic semiconductor layer (also referred to hereinbelow as “i-type layer”) that is composed of an intrinsic semiconductor is present in the center, the explanation can be the same as related to
a)-(d) show one sharpened acute section having an electron emitting portion and illustrate a diamond electron emission cathode composed of diamond semiconductors of two types with different electric properties. One of the types constituting the diamond semiconductors is a p-type layer comprising p-type impurities at 2×1015 cm−3 or higher and the other type of semiconductor is an n-type semiconductor comprising n-type impurities at 2×1015 cm−3 or higher. The configuration also has an inner i-type layer. The electron emitting portion in the configuration shown in
The configuration shown in
In the case where a potential is applied in the reverse direction in the structure shown in
The diamond electron emission cathode shown in
The diamond electron emission cathode preferably uses a diamond comprising p-type impurities at 2×1015 cm−3 or higher. The above-described effect can be obtained at even larger scale by selecting a diamond comprising n-type impurities at 2×1015 cm−3 or higher as the first semiconductor 15, selecting a diamond comprising p-type impurities at 2×1015 cm−3 or higher as the second semiconductor 16, and applying a bias voltage with a pair of current introducing terminals. Furthermore, because the level of p-type impurities in diamond is shallow and a low resistance can be obtained, in this case the resistance value of the entire cathode becomes even lower. As a result, a voltage drop at the cathode decreases, whereby the electron emission efficiency increases. A B-doped single crystal diamond obtained by high-temperature high-pressure synthesis can be advantageously used.
The diamond electron emission cathode shown in
A sharpened acute section having one electron emitting portion will be explained below.
It is preferred that in the surfaces of the diamond 10 constituting the diamond electron emission cathode shown in
The specific resistance of the portion of the diamond electron emission cathode containing n-type impurities at 300 K is preferably 300 Ωcm or less. In this case, electrons can be effectively supplied to the portion containing n-type impurities, high-density electron emission is possible, and a high brightness electron emission cathode can be obtained.
The tip end diameter or tip end curvature radius of the electron emitting portion 11 is preferably 30 μm or less. By making the tip end of the electron emitting portion of such a small size, an electron emission cathode with a higher brightness can be obtained. Furthermore, a smaller focusing point can be maintained when the electron emitting tip is used for a tip of thermal electron emission. If the tip end diameter or pointed top curvature radius is more than 30 μm, the focus point is very difficult to decrease in size and special design should be paid to the optical system of the electron beam device. In order to obtain a sharp tip end, it is preferred that a diamond electrically conductive layer 12 be formed by vapor phase synthesis and then machining be conducted by polishing or ion etching. Furthermore, if the tip end diameter is 5 μm or less, an electric field of 104 V/cm or more can be easily obtained at the tip end of the electron emitting portion 11. Therefore, this tip end diameter is a threshold at which the cathode can be advantageously used as a thermal filed emission cathode. Even more preferred that the tip end diameter be 1 μm or less. In this case, an electric field of 107 V/cm or more can be easily obtained at the tip end of the electron emitting portion 11 and, therefore, the cathode can be advantageously used as a field-emission cathode.
Furthermore, the electron emitting portion 11 may have a protruding structure 13, as shown in
Emitting an electron beam with an energy width of 0.6 eV or less under an applied voltage of 0.5 kV or more to 100 kV or less also may be a specific feature of the diamond electron emission cathode in accordance with the present invention. Such diamond electron emission cathode as a replacement for cathodes from the conventional materials can provide a good electron beam.
Further, the diamond electron emission source shown in
The electron microscope in accordance with the present invention has installed therein the diamond electron emission cathode or diamond electron emission source in accordance with the present invention and enables observations at a higher magnification ratio than the electron microscopes using the conventional cathode materials. When the diamond electron emission cathode in accordance with the present invention is so shaped that it can be used as a thermal electron emission cathode and installed in an electron microscope, observations of fine configurations can be performed at a magnification ratio higher than that attained when LaB6 is used. When the diamond electron emission cathode is so shaped that it can be used as a thermal field emission cathode and installed in an electron microscope, observations of fine configurations can be performed at a magnification ratio higher than that attained when ZrO/W is used.
Alternatively when the diamond electron emission cathode is so shaped that it can be used as a field-emitter and installed in an electron microscope, observations of fine configurations can be performed at a magnification ratio higher than that attained when sharpened W is used.
The electron beam exposure device in accordance with the present invention has installed therein the diamond electron emission cathode or diamond electron emission source in accordance with the present invention and enables the drawing of fine patterns with a throughput higher than that of electron beam exposure devices using the conventional cathode materials. When the diamond electron emission cathode in accordance with the present invention is so shaped that it can be used as a thermal electron emission cathode and installed in an electron beam exposure device, fine patterns can be drawn with a throughput higher than that attained when LaB6 is used. When the diamond electron emission cathode is so shaped that it can be used as a thermal field emission cathode and installed in an electron beam exposure device, fine patterns can be drawn with a throughput higher than that attained when ZrO/W is used.
The diamond electron emission cathode, electron emission source, electron microscope, and electron beam exposure device in accordance with the present invention will be described below in greater detail based on embodiments thereof.
Single crystal diamond substrates obtained by high-pressure synthesis and single crystal substrates obtained by vapor phase synthesis, all having the shape of an elongated rectangular parallelepiped, were subjected to sharpening in order to prepare samples having a sharp tip end, as shown in
The tip end diameter could be roughly adjusted during polishing, and representative samples with a tip end diameter of about 1 μm and about 10 μm were prepared. The tip end can be also sharpened to a submicron level by diamond re-growth or vertical etching using plasma or ions.
When an n-type layer was formed on the (111) plane, the methane concentration (CH4/H2), phosphorus concentration (PH3/CH4), pressure and substrate temperature were set to 0.03-0.05%, 0.002-20%, 100 Torr and 870° C., respectively. Under such conditions, the low-resistance n-type layer could be formed only on the (111) plane and practically no film was attached to other planes, or films with a somewhat higher resistance were formed thereon. Such films with a high resistance had a bonding strength lower than the epitaxial diamond film on the (111) plane and could be removed by treating in hydrogen plasma comprising oxygen at −0.1% or less. It goes without saying, that the n-type film and high-resistance film were formed only on the surface of the substrate facing the plasma and could not be formed on the rear surface of the substrate.
When an n-type layer was formed on the (100) plane, the methane concentration, phosphorus concentration and pressure were set to 0.5%, 1-10% and 10-50 Torr, respectively. Although the layer formation rate varied within this pressure range, similar films could be formed. Under such conditions, a high-resistance film could be formed only on the (111) plane, but this high-resistance film on the (111) plane could be etched selectively by hydrogen plasma containing oxygen, as mentioned above.
A boron-doped layer was formed as the p-type film by using B2H6 gas. The p-type film could be formed at a methane concentration of 0.1-5% and a diboran concentration (B2H6/CH6) of 10 ppm-10000 ppm. The film could be formed at a pressure and substrate temperature of 20-160 Torr and 750-1000° C., respectively, and the films were formed at representative pressure and temperature of 100 Torr and close to 870° C. Obviously, the films could be formed on the front surface of the substrates and could not be formed on the rear surface.
Methods for fabricating chips of various structures are shown in
A sample in which an undoped layer (i-type layer) was formed on the n-type layer was also prepared as a separate sample. In this case, no i-type layer was formed on the portion that was to be brought into contact with the electrodes. The effect was demonstrated by forming this layer on the tip end. The i-type layer as referred to herein indicates a film formed on the insulating substrate and having a carrier concentration at a level of 109 cm−3 that cannot be measured.
The results relating to electron emission characteristics of the samples are shown in Tables 1-6 below. In all the cases, poor characteristics were obtained in the samples without the chip voltage and the samples without protrusions. Separately from the tests illustrated by the tables, the materials with difference tip end diameter and height were studied, and the characteristics tended to improve when the tip end diameter was 30 μm or less and the protrusion height exceeded 50 μm.
As described hereinabove, because plane orientation in the sample is an extremely important factor, substrates in which plane orientation cannot be controlled in the chip sample, for example polycrystalline substrates, are unsuitable from the standpoint of the present invention. Not only single crystal substrates, but also high-orientation substrates or hetero-substrates can be also used, provided that the plane orientation can be controlled.
All the tests were conducted on chips with hydrogen-terminated surface, and the emission current characteristic of the chips with hydrogen-terminated surface was improved by almost 30-50% with respect to that of the chips with oxygen-terminated surface. Furthermore, the length in the short side direction of the chip was set mainly to 0.05-1 mm; properties could not be evaluated at a length more than 2 mm and less than 0.05 mm. The resistance between the terminals was set to 10Ω-3 kΩ; no electron emission was possible when the resistance was outside this range. The resistance between the terminals could be controlled by the thickness of the n-type layer or p-type layer or the doping concentration, and even when the layer thickness or concentration were constant, the resistance still could be controlled by a metal coating.
The metal coating usually was formed individually on both the p-type layer and the n-type layer, or only on one layer. The coating was effective when the film resistance of the layers was high. For example, the metal coating had the structure shown in
As another method for metal coating, a method of coating so that the p-type layer and n-type layer become conductive at the same time was also found to be effective. If the layers are made conductive with the coating metal, no potential is applied to the p-type layer or n-type layer. Accordingly, such approach was thought to be ineffective. However, in practice, since the coating metal was thin, a resistance component was existent, an electric current flowed to the metal portion under the applied voltage and an electric potential could be generated at the n-type layer side and p-type layer side. In this case, the n-type layer or p-type layer operated effectively in the case of a low concentration, the resistance between the terminals could be reduced to 700Ω or less, and a characteristic similar to that shown in Table 5 could be maintained. At a higher resistance, the characteristic degraded. On the other hand, when the resistance between the terminals was 10Ω or less, the desired electric potential in the pn junction could not be ensured and the emission characteristic also greatly degraded.
Single crystal diamond substrates obtained by high-pressure synthesis and single crystal substrate obtained by vapor phase synthesis, all having the shape of an elongated rectangular parallelepiped, were subjected to sharpening in order to prepare samples having a sharp tip end, as shown in
The tip end diameter could be roughly adjusted during polishing, and representative samples with a tip end diameter of about 1 μm and about 10 μm were prepared. The tip end can be also sharpened to a submicron level by diamond re-growth or vertical etching using plasma or ions.
The conditions of forming a n-type layer on the (111) plane, forming a n-type layer on the (100) surface, and forming a p-type film were identical to those of Example 1.
All the substrates were boron-containing substrates with p-type conductivity. An n-type film was formed on the (111) plane under the conditions relating to the (111) plane on the sharpened samples of the shape shown in
The film formed on the remaining planes was removed by the above-described plasma treatment. As a result, a single chip with a p-type tip end portion and an n-type film formed half way to the tip end portion was produced. Further, the produced samples in which the p-type and n-type films were stacked as shown in
An n-type film was formed under the n-type film formation conditions on the (111) plane on the sharpened samples of the shape shown in
As a result, an emitter having an n-type layer and a p-type layer tip end was produced in all the samples and the same structure as described above in which an electric current was caused to flow in the pn forward direction was obtained. With such a structure, the substrate could be heated under certain chip voltage and current conditions.
The results relating to electron emission characteristics of the samples are shown in Tables 7-11 below. In all the cases, poor characteristics were obtained in the samples without the chip voltage and the samples without protrusions. The characteristics tended to improve when the tip end diameter was 30 μm or less and the protrusion height exceeded 50 μm, in the same manner as in Example 1.
As described hereinabove, because plane orientation in the sample is an extremely important factor, substrates in which plane orientation cannot be controlled in the chip sample, for example polycrystalline substrates, are unsuitable from the standpoint of the present invention. Not only single crystal substrates, but also high-orientation substrates or hetero-substrates can be also used, provided that the plane orientation can be controlled.
Finally, an undoped i-type layer was formed on the p-type layer to obtain an npi structure with a pn-junction. In sample (c) and sample (d) with a chip structure in which the tip end was an i-type layer, the emission current was increased by 20-50%. The structure in which the i-type layer was present on the tip end of the outermost surface was effective. Using the single crystal substrate obtained by vapor phase growth increased the emission current value by about 30% with respect to that attained by using the single crystal diamond substrate obtained by high-pressure synthesis.
Comparing with Example 1, in the protruding chip with an np-junction, the characteristics can be said to be better with a p-type tip end than an n-type tip end. Furthermore, using a boron-doped bulk substrate obtained by vapor phase synthesis improved the characteristics over those obtained by using a boron-doped substrate obtained by high-pressure synthesis.
All the tests were conducted on chips with hydrogen-terminated surface, and the emission current characteristic of the chips with hydrogen-terminated surface was improved by almost 30-50% with respect to that of the chips with oxygen-terminated surface. Furthermore, the length in the short side direction of the chip was set mainly to 0.5-1 mm; the properties could not be evaluated at a length more than 2 mm and less than 0.05 mm. The resistance between the terminals was set to 10Ω-3 kΩ; no electron emission possible when the resistance was outside this range. The resistance between the terminals could be controlled by the thickness of the n-type layer or p-type layer or the doping concentration, and when the layer thickness or concentration were constant, the resistance still could be controlled by a metal coating.
30
<1
<1
As shown in
With this method chips having with a tip end in the form of an n-type layer and a pin stack were fabricated.
Similarly to Example 1, a sample in which a boron-doped p-type layer was formed prior to the formation of the i-type layer was also prepared. Although similar structures of a pin-stack type were obtained, the advantage of the latter structure was that the junction surface could use a CVD film in which the impurities of the p-type layer were accurately controlled. Another feature identical to that of Example 1 was that the produced samples in which the p-type and n-type films were stacked as shown in figures (a), (b) had a structure in which the electrodes were brought into contact individually with the n-type and p-type films in the base portion of the chip and the chip current was induced in the pn forward direction. The electric field was applied across the junction surface. Usually, no electric current flows in the i-type layer, but if an electric field is applied in a state where this layer is sandwiched between a p-type layer and n-type layer, the current flows through the i-type layer. The electrodes that were in contact with the p and n-type layers were connected to the metal that was clamping the chip. With such a structure, the substrate can be heated under certain chip voltage and current conditions.
A sample in which an undoped layer (i-type layer) was formed on the n-type layer was also prepared as a separate sample. In this case, no i-type layer was formed on the portion that was to be brought into contact with the electrode. The effect was demonstrated by forming this layer on the tip end. The i-type layer as referred to herein indicates a film formed on the insulating substrate and having a carrier concentration at a level of 109 cm−3 that cannot be measured.
The results relating to electron emission characteristics of the samples are shown in Tables 12-14 below. In all the cases, poor characteristics were obtained in the samples without the chip voltage and the samples without protrusions. Similarly to Example 1, the characteristics tended to improve when the tip end diameter was 30 μm or less and the protrusion height exceeded 50 μm. As described hereinabove, because plane orientation in the sample is an extremely important factor, substrates in which plane orientation cannot be controlled in the chip sample, for example polycrystalline substrates, are unsuitable from the standpoint of the present invention. Not only single crystal substrates, but also high-orientation substrates or hetero-substrates can be also used, provided that the plane orientation can be controlled.
As shown in Table 15, using a single crystal substrate obtained by vapor phase growth increased the emission current value with respect to that attained by using a single crystal diamond substrate obtained by high-pressure synthesis, and it was also effective to perform vapor phase synthesis on a single crystal obtained by high-pressure synthesis.
All the tests were conducted on chips with hydrogen-terminated surface, and the emission current characteristic of the chips with hydrogen-terminated surface was improved by almost 30-50% with respect to that of the chips with oxygen-terminated surface. Furthermore, the length in the short side direction of the chip was set mainly to 0.5-1 mm; properties could not be evaluated at a length more than 2 mm and less than 0.05 mm. The resistance between the terminals was set to 10Ω-3 kΩ; no electron emission was possible when the resistance was outside this range. The resistance between the terminals could be controlled by the thickness of the n-type layer or p-type layer and the doping concentration, and even when the layer thickness or concentration were constant, the resistance still could be controlled by a metal coating.
<1
<1
<1
<1
As shown in
Similarly to Example 2, a sample in which a boron-doped p-type layer was formed prior to the formation of the i-type layer was also prepared. Although similar structures of a pin-stack type were obtained, the advantage of the latter structure was that the junction surface used a CVD film in which the impurities of the p-type layer were accurately controlled. Another feature identical to that of Example 2 was that the produced samples in which the p-type and n-type films were stacked as shown in figures (a), (b) had a structure in which the electrodes were brought into contact individually with the n-type and p-type films in the base portion of the chip and the chip current was induced in the pn forward direction. The electric field was applied across the junction surface. Usually, no electric current flows in the i-type layer, but if an electric field is applied in a state where this layer is sandwiched between a p-type layer and n-type layer, the current flows through the i-type layer. Further, the electrodes that were in contact with the p and n-type layers were connected to the metal that was clamping the chip. With such a structure, the substrate can be heated under certain chip voltage and current conditions.
A sample in which an undoped layer (i-type layer) was formed on the n-type layer was also prepared as a separate sample. In this case, no i-type layer was formed on the portion that was to be brought into contact with the electrode. The effect was demonstrated by forming this layer on the tip end. The i-type layer as referred to herein indicates a film formed on the insulating substrate and having a carrier concentration at a level of 109 cm−3 that cannot be measured.
The results relating to electron emission characteristics of the samples are shown in Tables 15-17 below. In all the cases, poor characteristics were obtained in the samples without the chip voltage and the samples without protrusions.
As described hereinabove, because plane orientation in the sample is an extremely important factor, substrates in which plane orientation cannot be controlled in the chip sample, for example polycrystalline substrates, are unsuitable from the standpoint of the present invention. Not only single crystal substrates, but also high-orientation substrates or hetero-substrates can be also used, provided that the plane orientation can be controlled.
Further, as shown in Table 16, using a single crystal substrate obtained by vapor phase growth increased the emission current value with respect to that attained by using a single crystal diamond substrate obtained by high-pressure synthesis, and it was also effective to perform vapor phase synthesis on a single crystal obtained by high-pressure synthesis. The pin-type chips with a sandwiched i-type substrate were found to have better characteristics than the pn-type chips.
All the tests were conducted on chips with hydrogen-terminated surface, and the emission current characteristic of the chips with hydrogen-terminated surface was improved by almost 30-50% with respect to that of the chips with oxygen-terminated surface. Furthermore, the length in the short side direction of the chip was set mainly to 0.5-1 mm; properties could not be evaluated at a length more than 2 mm and less than 0.05 mm. The resistance between the terminals was set to 10Ω-3 kΩ; no electron emission was possible when the resistance was outside this range. The resistance between the terminals could be controlled by the thickness of the n-type layer or p-type layer and the doping concentration, and even when the layer thickness or concentration were constant, the resistance still could be controlled by a metal coating.
As shown in
Samples in which an i-type layer was formed by vapor phase synthesis prior to forming the n-type layer and p-type layer in a similar way to Example 2 were also formed. Although similar structures of a pin-stack type were obtained, the characteristics improved when the junction surface used a CVD film. Another feature identical to that of Example 2 was that the produced samples in which the p-type and n-type films were stacked had a structure in which the electrodes were brought into contact individually with the n-type and p-type films in the base portion of the chip and the chip current was induced in the pn forward direction. The electric field was applied across the junction surface. Usually, no electric current flows in the i-type layer, but if an electric field is applied in a state where this layer is sandwiched between a p-type layer and n-type layer, the current flows through the i-type layer. Further, the electrodes that were in contact with the p and n-type layers were connected to the metal that was clamping the chip. With such a structure, the substrate can be heated under certain chip voltage and current conditions.
The i-type layer as referred to herein indicates a film formed on the insulating substrate and having a carrier concentration at a level of 109 cm−3 that cannot be measured.
The results relating to electron emission characteristics of the samples are shown in Tables 18-20 below. In all the cases, poor characteristics were obtained in the samples without the chip voltage.
As described hereinabove, because plane orientation in the sample is an extremely important factor, substrates in which plane orientation cannot be controlled in the chip samples, for example polycrystalline substrates, are unsuitable from the standpoint of the present invention. Not only single crystal substrates, but also high-orientation substrates or hetero-substrates can be also used, provided that the plane orientation can be controlled.
Further, as shown in Table 19, using a single crystal substrate obtained by vapor phase growth increased the emission current value with respect to that attained by using a single crystal diamond substrate obtained by high-pressure synthesis, and it was also effective to perform vapor phase synthesis on a single crystal obtained by high-pressure synthesis. The pin-type chips with a sandwiched i-type substrate were found to have better characteristics than the pn-type chips.
All the tests were conducted on chips with hydrogen-terminated surface, and the emission current characteristic of the chips with hydrogen-terminated surface was improved by almost 30-50% with respect to that of the chips with oxygen-terminated surface. Furthermore, the length in the short side direction of the chip was set mainly to 0.5-1 mm; properties could not be evaluated at a length more than 2 mm and less than 0.05 mm. The resistance between the terminals was set to 10Ω-3 kΩ; no electron emission was possible when the resistance was outside this range. The resistance between the terminals could be controlled by the thickness of the n-type layer or p-type layer or the doping concentration, and when the layer thickness or concentration was constant, the resistance still could be controlled by a metal coating.
Samples were prepared in the same manner as in Example 1 and Example 3. This time, electron emission was studied by applying a negative potential to the p-type side and a positive potential to the n-type side, i.e., in the direction opposite that in Example 1, etc. With the chip voltage opposite that of Examples 1, 3, practically no current flowed, and without the electric current, the electron emission characteristic was not good, similarly to that in the case where there was no chip voltage. However, when a certain fixed potential was exceeded, the current started flowing and the electron emission characteristic improved accordingly. The results are shown in Tables 21-25.
A good electron emission characteristic was obtained even in the case of reverse-direction potential. However, the characteristic was not better that in the case of a forward-direction potential.
All the tests were conducted on chips with hydrogen-terminated surface, and the emission current characteristic of the chips with hydrogen-terminated surface was improved by almost 30-50% with respect to that of the chips with oxygen-terminated surface. Furthermore, the length in the short side direction of the chip was set mainly to 0.5-1 mm; properties could not be evaluated at a length more than 2 mm and less than 0.05 mm. The resistance between the terminals was set to 10Ω-3 kΩ; no electron emission was possible when the resistance was outside this range. The resistance between the terminals could be controlled by the thickness of the n-type layer or p-type layer or the doping concentration, and even when the layer thickness or concentration were constant, the resistance still could be controlled by a metal coating.
Number | Date | Country | Kind |
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2005-178163 | Jun 2005 | JP | national |
2005-257452 | Sep 2005 | JP | national |
2005-257791 | Sep 2005 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2006/312261 | 6/19/2006 | WO | 00 | 4/16/2007 |