Patent Application
20080160872
This application claims benefit of priority under 35 USC 119 based on Japanese Patent Application No. P2003-202518 filed Jul. 28, 2003, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a discharge electrode, a discharge lamp using the discharge electrode and a method for manufacturing the discharge electrode, and more particularly to a discharge electrode serving as a hot cathode, a discharge lamp using the discharge electrode and a method for manufacturing the discharge electrode.
2. Description of the Related Art
A hot cathode (discharge electrode), used for discharge lamps such as fluorescent lamps, emit electrons from its surface in an atmosphere of a discharge gas by being thermally heated under application of negative potential to its surface. The hot cathode widely utilizes a filament implemented by a thin refractory metal wire, formed into a coil configuration, which is heated by electric energy. Furthermore, thermionic emission is generally promoted as the work function of cathode material thereof is decreased, and thus a variety of metals or materials called emitter materials such as a barium (Ba)-based materials have been formed on the surface of the filament, by a coating method, an impregnation method, or the like, in order to reduce the work function of the filament material surface.
For example, in a fluorescent lamp, which is the most widely and generally used discharge lamp, the flow of electric current in the hot cathode involves the dissipation of energy, heating the whole system of the hot cathode, and the thermionic emission is initiated from the surface of the hot cathode. In earlier technology, the hot cathode was fabricated by coating tungsten filament with a barium-based emitter material. Earlier hot cathodes, or earlier discharge electrodes make it possible to emit electrons via a small drop of the cathode voltage, which supports the high luminous efficiency of earlier fluorescent lamps, whereas earlier fluorescent lamps are associated with the problem of short operation life. Moreover, to satisfy the requirements for high integration of devices and needs for miniaturization, the development of a high-performance hot cathode operating at an even lower temperature and with lower heat dissipation is required to meet the requirements thereof.
Recently, in Japanese Patent Application laid-open No. H10-698688 (hereinafter called “the first document”), a discharge lamp installing a specific hot cathode (discharge electrode) has been proposed, the specific hot cathode has a layer of particulate diamonds on the surface of the hot cathode material. Namely, particulate diamonds having an average particle diameter of 0.2 μm or less are coated on the surface of the hot cathode material in the first document.
Further, in Japanese Patent Application laid-open No. 2000-106130 (hereinafter called “the second document”), another discharge electrode for integrating into a low-pressure discharge lamp has been proposed. In the second document, fine diamond particles having a particle diameter of from 0.01 μm to 10 μm, preferably from 0.1 μm to 1 μm, are deposited on or impregnated into the surface of a tungsten coil. The diamond-deposited or -impregnated tungsten coil was integrated into the low-pressure discharge lamp as the discharge electrode. The objective of the second document was to suppress the deterioration of thermionic emission characteristics of the discharge electrode and to achieve long operation life of the low-pressure discharge lamp.
The techniques disclosed in the first and second documents, however, are not sufficient in efficient improvement because the applied power is mostly dissipated at the tungsten coil.
In view of these situations, it is an object of the present invention to provide a long life discharge electrode which allows adequate electrical conductivity from startup at room temperature and which enables efficient heating and thermionic emission, and to provide a discharge lamp using the discharge electrode, and further to provide a method for manufacturing the discharge electrode.
An aspect of the present invention inheres in a discharge electrode emitting electrons into a discharge gas, encompassing (a) an emitter encompassing a wide bandgap semiconductor having at 300 K a bandgap of 2.2 eV or wider, acceptor impurity atoms and donor impurity atoms being doped in the wide bandgap semiconductor, an activation energy of the donor impurity atoms being larger than the activation energy of the acceptor impurity atoms, and (b) current supply terminals configured to supply electric current to the emitter.
Another aspect of the present invention inheres in a discharge lamp encompassing (a) a discharge envelope in which a discharge gas is sealed, and (b) a discharge electrode disposed in the discharge envelope. Here, the discharge electrode embraces an emitter encompassing a wide bandgap semiconductor having at 300 K a bandgap of 2.2 eV or wider, acceptor impurity atoms and donor impurity atoms being doped in the wide bandgap semiconductor, an activation energy of the donor impurity atoms being larger than the activation energy of the acceptor impurity atoms; and current supply terminals configured to supply electric current to the emitter.
Still another aspect of the present invention inheres in a method for manufacturing a discharge electrode encompassing (a) depositing a wide bandgap semiconductor layer on a substrate to form a composite structure, the wide bandgap semiconductor layer having at 300 K a bandgap of 2.2 eV or wider; (b) doping simultaneously acceptor impurity atoms and donor impurity atoms in the wide bandgap semiconductor layer, an activation energy of the donor impurity atoms being larger than the activation energy of the acceptor impurity atoms; and (c) electrically connecting current supply terminals to the wide bandgap semiconductor layer, the current supply terminals being configured to supply electric current to the wide bandgap semiconductor layer.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the present invention in practice.
Various embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that the same or similar reference numerals are applied to the same or similar parts and elements throughout the drawings, and the description of the same or similar parts and elements will be omitted or simplified.
Generally and as it is conventional in the representation of electron devices, it will be appreciated that the various drawings are not drawn to scale from one figure to another nor inside a given figure, and in particular that the layer thicknesses are arbitrarily drawn for facilitating the reading of the drawings.
In the following description specific details are set forth, such as specific materials, process and equipment in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known manufacturing materials, process and equipment are not set forth in detail in order not to unnecessary obscure the present invention. The technical principles of this invention can be altered in various manners within the scope of claims.
Prepositions, such as “on”, “under” and “beneath” are defined with respect to a planar surface of the supporting member, regardless of the orientation in which the supporting member is actually held. A layer is on another layer even if there are intervening layers.
A discharge lamp pertaining to a first embodiment of the present invention, as indicated in
Of the pair of discharge electrodes, the discharge electrode of the left side in
Similarly, the other of the pair of discharge electrodes, i.e., the right-hand discharge electrode of
The conductive films (contact films) 23a, 24a; 23b, 24b can use nickel (Ni) film, tungsten (W) film, titanium (Ti) film, chromium (Cr) film, tantalum (Ta) film, molybdenum (Mo) film, gold (Au) film, and the like. Or, an alloy film, a compound film, a multi-layer film (composite film) and the like, composed of a combination of a plurality of metals thereof, can be employed. For example, a multi-layer film such as titanium-platinum-gold Au) film, titanium-nickel-gold (Ti/Ni/Au) film, or titanium-nickel-platinum-gold, (Ti/Ni/Pt/Au) film, or the like can be selected.
Moreover, in an application field in which the contact resistance between the stem leads 21a, 22a and the wide bandgap semiconductor layer 1a or between the stem leads 21b, 22b and the wide bandgap semiconductor layer 1b is allowed to be high, as it is appropriate, the conductive films (contact films) 23a, 24a; 23b, 24b, and/or the amorphous layers (amorphous contact regions) directly beneath the conductive films (contact films) may be omitted.
The wide bandgap semiconductor layers 1a, 1b are doped with both acceptor impurity atoms having a relatively small activation energy and donor impurity atoms having a relatively large activation energy. Furthermore, the impurities are doped in such a way that the concentration NA of the acceptor impurity is smaller than the concentration ND of the donor impurity. Here, a “wide bandgap semiconductor” stands for a semiconductor material having a wider bandgap Eg than silicon (Si) having a bandgap Eg of about 1.1 eV at 300 K, germanium arsenide (GaAs) having a bandgap Eg of about 1.4 eV at 300 K) and the like, which have been studied earlier and have achieved progressive commercialization in the semiconductor industry. For example, typical wide bandgap semiconductors include, at 300 K, zinc telluride (ZnTe) having a bandgap Eg of about 2.2 eV, cadmium sulfide (CdS) having a bandgap Eg of about 2.4 eV, zinc selenide (ZnSe) having a bandgap Eg of about 2.7 eV, gallium nitride (GaN) having a bandgap Eg of about 3.4 eV, zinc sulfide (ZnS) having a bandgap Eg of about 3.7 eV, diamond having a bandgap Eg of about 5.5 eV and aluminium nitride (AlN) having a bandgap Eg of about 5.9 eV. In addition, silicon carbide (SiC) is also an example of a wide bandgap semiconductor. At 300 K, bandgap Eg of about 2.23 eV for 3C—SiC, about 2.93 eV for 6H—SiC, and about 3.26 eV for 4H—SiC have been reported, and a variety of SiC polytypes are usable for the wide bandgap semiconductor layers 1a, 1b. Various mixed crystals implemented by the combinations of two or three species, or a ternary compound or a quaternary compound of the aforementioned wide bandgap semiconductors are permissible for the wide bandgap semiconductor layers 1a, 1b. Of these wide bandgap semiconductors and the mixed crystals thereof, in particular, the wide bandgap semiconductor having a bandgap Eg at 300 K of 3.4 eV or more, and the mixed crystals thereof, are preferable for the thermionic emission sources (emitters), because the negative electron affinity of the wide bandgap semiconductor becomes significant as the bandgap Eg increases.
With an illustrative example for diamond, doping can be selected such that the concentration NA of acceptor impurity atoms is smaller than the concentration ND of donor impurity atoms—in a concentration of boron (B) as the acceptor impurity ranging from about 1015 cm−3 to about 1019 cm−3 to the corresponding concentration of phosphorus (P) as the donor impurity ranging from about 1016 cm−1 to about 1021 cm−3.
The insulating substrates 7a, 7b, which are adapted for the supporting members in the discharge electrodes relating to the first embodiment, can be made of quartz glass or ceramic such as alumina (Al2O3).
The fluorescent layer 10 applied to a portion of the inner wall of the discharge envelope 9 emits visible rays, after receiving the radiation of ultraviolet rays, which are generated by discharge in the discharge envelope 9. In addition to the discharge gas 11, the inside of the discharge envelope 9 includes a necessary, given amount of mercury (mercury particle) for establishing the mercury discharge. As discharge gas 11 for aiding lighting, an inert gas such as argon (Ar), neon (Ne), xenon (Xe), or the like can be used; the pressure of the inside of the discharge envelope 9 is set, for example, at from about 5.3 kPa to about 13 kPa. In addition, a percentage of hydrogen gas (H2) is preferably mixed into an inert gas.
As discussed above, in the discharge electrode of the discharge lamp pertaining to the first embodiment, the emitters configured to emit electrons by resistive heating is implemented by the wide bandgap semiconductor layers 1a, 1b, in which both acceptor impurity atoms having a relatively small activation energy and donor impurity atoms having a relatively large activation energy are doped.
As shown in
Current flow of the holes 3 heats restively the wide bandgap semiconductor layers 1a, 1b themselves to about 700K to about 800K;
Namely, the increase of temperature by resistive heating changes the inactive donor impurity atoms 4i to the activated donor impurity atoms 4a. In this activated energy state at elevated temperature, electrons being bound to the donor impurity atoms (activated state) 4a are supplied to the conduction band so as to establish n-type conduction. In other words, in the wide bandgap semiconductor layers 1a, 1b, which are heated to a temperature ranging from about 700K to about 800K, sufficient number of electrons 6, required for thermionic emission, are generated as majority carriers.
In this manner, according to the first embodiment, because a simple configuration implemented by each of the wide bandgap semiconductor layers 1a, 1b alone enables a sequence of steps of, from the p-type conduction at the starting point of the heating, the resistive heating by the p-type conduction, the change of conduction types associated with the increase of the temperature, the resistive heating by the n-type conduction and the following thermionic emission by the n-type conduction, the electric power dissipation is minimize in the discharge electrodes. Therefore, the high efficiency and low-temperature hot cathode (thermionic cathode) can be achieved with a simple structure. That is, according to the discharge electrodes pertaining to the first embodiment, the simultaneous donor/acceptor doping effect in the wide bandgap semiconductor layers 1a, 1b causes electric current to efficiently flow through the wide bandgap semiconductor layers 1a, 1b from the start of the resistive heating, which efficiently establishes the elevated temperature state, thereby facilitating the electron conduction in the n-type conduction regime suited to the thermionic emission.
In addition, in
Furthermore, the wide bandgap semiconductor layers 1a, 1b do not necessarily cover all of the top surfaces of the insulating substrates 7a, 7b uniformly, and may also selectively be formed on portions of the top surfaces of the insulating substrates 7a, 7b so as to delineate specific wiring patterns, such as a straight stripe shape, a zigzag shape, or a meandering filament.
The discharge electrode pertaining to the first embodiment does not need to attach an extra filament for resistive heating, and therefore the structure is simple; a simple manufacturing process as described below enables mass production, thus being capable of reducing manufacturing costs. With reference to
(a) First, a parallel plate slab or a substrate is prepared for a supporting member 7. The supporting member 7 may be an insulating substrate, more specifically, an alumina (Al2O3) substrate. And, a wide bandgap semiconductor layer 1 is epitaxially grown on the top surface of the supporting member 7 by a chemical vapor deposition (CVD) technique as shown in
(b) Next, a titanium-gold (Ti/Au) composite layer or the like is delineated by the lift-off process to form an ion implantation mask. Ar ions (Ar+) are selectively implanted on the top surface of the wide bandgap semiconductor layer 1 using the ion implantation mask at an acceleration energy EACC=40 keV and a dose amount φ=1016 cm−2. During the ion implantation, the temperatures of the insulating substrate 7 and the wide bandgap semiconductor layer 1 are kept at room temperature (25° C.). Then, after removal of the ion implantation mask, the resultant material is heat treated at 400° C. to produce an amorphous layer (amorphous contact region). Although a case of Ar+ ion implantation has been described, the ion shall not be limited to Ar+ alone, and a variety of ions are acceptable for the formation of the amorphous layer. For example, element ions of inert gases such as krypton (Kr+), xenon (Xe+) and the like, and carbide-forming element ions such as Ti+, Ta+, W+, Si+, N+, B+, and the like can be used. Of these, if N+ and B+ are implanted to the lattice positions of diamond, these may serve, respectively, as a donor and an acceptor. Rather, it can be considered that implanted N+ and B+ form the carbides (compounds) NC1-x and BC1-x at the top surface of diamond in such a high dose implantation condition of φ ranging from 1015 cm−2 to 1016 cm−2.
(c) Then, a mask is aligned on the exact position directly above the amorphous layer (amorphous contact region) so as to establish the lift-off process. Namely, after a successive vacuum evaporation method or a successive sputtering method for continuously depositing a Ti film, a Pt film and an Au film so as to implement Ti/Pt/Au multi-layer film, the Ti/Pt/Au multi-layer film is delineated by the lift-off process to provide respective patterns of the conductive films (contact films) 23a, 24a; 23b, 24b, 24c . . . , as shown in
(d) Next, an oxide film (SiO2 film) with a thickness of 500 nm to 1 μm is deposited on the whole top surface of the wide bandgap semiconductor layer 1 by CVD. Furthermore, a photoresist film is applied to the upper part of the oxide film and is delineated by photolithography. Subsequently, the oxide film is selectively etched using the delineated photoresist film as an etching mask. After patterning the oxide film, the photoresist film is removed. Using the delineated oxide film as an etching mask, the wide bandgap semiconductor layer 1 is selectively etched by reactive ion etching (RIE) using oxygen gas (O2 gas), at spaces between the conductive films (contact films) 24c and 23a, between the conductive films (contact films) 24a and 23b, and so on until the insulating substrate 7 is exposed. The space between the conductive films (contact films) 24c and 23a, the space between the conductive films (contact films) 24a and 23b, and so on become the dicing lines Dj−1, Dj, Dj+1 . . . . As a result, along the dicing lines Dj−i, Dj, Dj+1, . . . , the dicing grooves are formed. When the composite structure (1, 7) is cut along the dicing grooves with diamond blade or the like so as to divide into a plurality of chips, and a plurality of “composite electrode-bodies” each having a desired chip size are cut out.
(e) Next, “a composite electrode-body (7a, 1a)” is selected from the plurality of “composite electrode-bodies”. Furthermore, stem leads 21a, 22a, portions close to the centers of which are fixed to a glass ball (bead) 62a, are provided. Then, the stem lead 21a is contacted at an angular portion of bent portions thereof with the bottom surface of the insulating substrate 7a opposing to the conductive film (contact film) 23a and pinches “the composite electrode-body (7a, 1a)” from both the sides by elastic force. Similarly, a stem lead 22a is contacted at an angular portion of bent portions thereof with the bottom surface of the insulating substrate 7a opposing to the conductive film (contact film) 24a and pinches the composite electrode-body (7a, 1a) from both the sides by elastic force. While the illustration is omitted in
(f) Next, as illustrated in
(g) Then, the vacuum pump 81 is operated to evacuate air within the glass tube 9 for achieving a specific ultimate pressure, by opening the vacuum exhaust passage via the exhaust magnet valve 84 and the transfer valve 83, with the glass tube 9, equipped with the pair of discharge electrodes, being connected to the pumping head 86. Thereafter, a small amount of mercury is sealed in the glass tube 9 together with a specified discharge gas 11 such as argon from the gas supply source 82 through the transfer valve 83 and the intake magnet valve 85. Further, subsequently, the proximity of the narrow portion 66B and the glass ball 62b is heated with a gas burner or the like to melt the glass tube 9 and the glass ball 62b and weld both, thus forming the other sealed portion 67B of the discharge lamp. Subsequently, removal of the unnecessary portions outside the sealed portions of the glass tube provides the discharge lamp shown in
According to the method for manufacturing a discharge lamp pertaining to the first embodiment of the present invention, because of no need for attaching an extra filament for resistive heating, dicing the wide bandgap semiconductor layer 1 collectively formed on a large insulating substrate 7 along the dicing lines Dj−i, Dj, Dj+1, . . . and pinching by elastic force both the ends thereof with the stem leads 21a, 22a or the stem leads 21b, 22b alone enables the manufacturing of a discharge electrode, thereby permitting mass production and reduction of manufacturing costs.
In addition, the method for manufacturing the discharge lamp described above is an example, and other different manufacturing methods are of course possible, including modifications thereof. For example, in the above-described embodiment, although the wide bandgap semiconductor layer 1 is blanketly grown on the large insulating substrate 7 and a plurality of resulting bodies are divided along dicing lines Dj−1, Dj, Dj+1, . . . , a plurality of chips, or chip-likely divided insulating substrates 7a, 7b . . . are provided firstly, and wide bandgap semiconductor layers 1a, 1b, . . . may individually be formed on the chip-likely divided insulating substrates 7a, 7b . . . .
As shown in
Although illustration is omitted, on a surface of the wide bandgap semiconductor rod 12 directly below the conductive films (contact films) 31a, 31b, amorphous layers (amorphous contact regions) are formed, respectively. As such, the conductive films 31a, 31b each make a low-contact-resistance ohmic contact to the outer peripheries close to both ends of the wide bandgap semiconductor rod 12. Materials for the conductive films 31a, 31b can be selected from the group including Ni, W, Ti, Cr, Ta, Mo, Au, and the like. Further, the combinations of materials listed in the group can be employed as the conductive films 31a, 31b. For example, multi-layer film s such as Ti/Pt/Au and Ti/Ni Au as well as Ti/Ni/Pt/Au and the like, which were discussed in the discharge lamp relating to the first embodiment can be employed as the conductive films 31a, 31b in the second embodiment. However, in a specific application field that permits relatively high contact resistance of an electrode may omit, as necessary, the conductive films 31a, 31b, and/or the amorphous layers (amorphous contact regions).
Then, the lead wire 13a electrically connected to the left end of the wide bandgap semiconductor rod 12 is supported by a suspension wire 14a; the lead wire 13b electrically connected to the right-hand end of the wide bandgap semiconductor rod 12 is supported by a suspension wire 14b. Further, the suspension wires 14a, 14b each are welded to stem pins 15a, 15b fixed to a stem 16, which fixes the wide bandgap semiconductor rod 12 to the stem 16 to implement a discharge electrode. Here, the lead wire 13a, the suspension wire 14a and the stem pin 15a serve as one of the pair of current supply terminals for supplying electric current to the emitter made of the wide bandgap semiconductor rod 12; the lead wire 13b, the suspension wire 14b and the stem pin 15b function as another of the pair of current supply terminals for supplying electric current to the emitter made of the wide bandgap semiconductor rod 12.
As in the case of the discharge electrode of the discharge lamp pertaining to the first embodiment, both acceptor impurity atoms having a comparatively small activation energy and donor impurity atoms having a comparatively large activation energy are doped to the wide bandgap semiconductor rod 12 in such a way that the concentration NA of the acceptor impurity atoms is smaller than the concentration ND of the donor impurity atoms.
In the second embodiment, the discharge lamp shown in
In the discharge electrode of the discharge lamp pertaining to the second embodiment, the wide bandgap semiconductor rod 12 itself serves as a resistive heating material, and therefore the lead wires 13a, 13b can be wound around both ends only, and do not need to be wound around the whole surface of the wide bandgap semiconductor rod 12.
As indicated in
The discharge electrode encompasses cap-shaped conductive films (electrode layers) 19a, 19b selectively formed at the outer peripheries of both edges of the wide bandgap semiconductor layer (emitter) 17, an electrode pin 20a welded at the conductive film (electrode layer) 19a, and an electrode pin 20b welded at the conductive film (electrode layer) 19b. While the illustration is omitted, amorphous layers (amorphous contact regions) are formed in proximate regions of the outer peripheral surfaces at both edges of the wide bandgap semiconductor layer 17 directly beneath the inner wall of each of cap-shaped conductive films 19a, 19b. Hence, the conductive films 19a, 19b each form a low-contact-resistance ohmic contact to the outer peripheries of both end vicinities of the wide bandgap semiconductor layer 17. The conductive films 19a, 19b can utilize any one of Ni, W, Ti, Cr, Ta, Mo, Au, and the like and any combination of these metals. The combination of these metals can include multi-layer film s such as Ti/Pt/Au and Ti/Ni/Pt/Au as well as Ti/Ni/Pt/Au and the like, which were discussed in the discharge lamps relating to the first and the second embodiments.
The electrode pin 20a connected to the left end of the cylindrical (or prism-shaped) composite electrode-body (17, 18) through the conductive films 19a, 19b is supported by a suspension wire 14a; the electrode pin 20b connected to the right-hand end of the composite electrode-body (17, 18) is supported by a suspension wire 14b. Further, the suspension wires 14a, 14b each are welded to stem pins 15a, 15b fixed to a stem 16, which fixes the composite electrode-body (17, 18) to the stem 16. The combination of these elements (17, 18, 19a, 19b, 20a, 20b, 14a, 14b, 15a, 15b, 16) implements the discharge electrode of the third embodiment.
Here, the conductive film (electrode layer) 19a, the electrode pin 20a, the suspension wire 14a and the stem pin 15a function as one of the pair of current supply terminals for supplying electric current to the emitter made of the wide bandgap semiconductor layer 17; the conductive film (electrode layer) 19b, the electrode pin 20b, the suspension wire 14b and the stem pin 15b function as another of the pair of current supply terminals for supplying electric current to the emitter made of the wide bandgap semiconductor layer 17.
As in the case of discharge electrodes of discharge lamps concerning the first and the second embodiments, both acceptor impurity atoms having a comparatively small activation energy and donor impurity atoms having a comparatively large activation energy are doped to the wide bandgap semiconductor layer 17 so that the concentration NA of the acceptor impurity atoms is smaller than the concentration ND of the donor impurity atoms.
As shown in
The discharge electrode of the third embodiment can readily be fabricated by means of a CVD process, or the like that involves depositing the wide bandgap semiconductor layer 17 on the insulating core member 18 and then dividing the resulting material, as appropriate, into the required length. As a matter of course, a plurality of insulating core member 18 may first be provided, each of the insulating core members 18 having the required length for use, and subsequently the wide bandgap semiconductor layer 17 can be coated on the respective insulating core members 18 by a CVD process, or the like as well.
Various modifications will become possible for those skilled in the art after receiving the teaching of the present disclosure without departing from the scope thereof.
The first to the third embodiments described thus far have primarily discussed hot cathodes. However, electron emissions from the discharge electrodes shall not be limited to pure thermionic emissions, but may involve effects caused by electric fields.
Thus, the present invention of course includes various embodiments and modifications and the like which are not detailed above. Therefore, the scope of the present invention will be defined in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2003-202518 | Jul 2003 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10899153 | Jul 2004 | US |
| Child | 12010934 | US |
