SILICON DIOXIDE FILMS ON DIAMOND

Abstract: A method for producing oriented diamond films on a substrate. Diamond seed crystals are suspended in a slurry (1) and applied to the substrate surface (2). Heating (3) the substrate removes the slurry fluid and produces seed crystals whose (111) crystal planes are substantially parallel to the plane of the substrate. An oriented polycrystalline diamond film may be grown about the seed crystals by CVD (4).

Description: SILICON DIOXIDE FILMS ON DIAMOND Background of the Invention This invention relates to the fabrication of semiconductor material for use in electronic devices, and more particularly, to the fabrication of diamond semiconductor films. Diamond is a material with semiconductor properties that are superior to Silicon (Si), Germanium (Ge) or Gallium Arsenide (GaAs), which are now commonly used. In particular, diamond provides a higher band gap, a higher breakdown voltage and a greater saturation velocity which produces a substantial increase in its projected cutoff frequency and maximum operating voltage compared to devices fabricated from Si, Ge, or GaAs. Furthermore, diamond has the highest thermal conductivity of any solid at room temperature and excellent conductivity over a temperature range up to and beyond 500°k. Diamond therefore holds the potential Unfortunately, however, the advantages of diamond as a semiconductor have not been exploited because of the difficulty in forming electrical contacts on diamond surfaces which allow access to and contol of diamond semiconductor devices.. Although natural diamonds may be of device quality, they are of limited supply and size. Furthermore, most natural diamonds are insulators, and thus introduction, or doping, of electrically active impurities such as boron by ion implantation is required to render them useful as semiconductors. Doping via ion implantation has proven to be problematic in diamond. A review of this method may be found in Vavilov et al. "Electronic and Optical Processes in Diamond" copyright 1975, Nauka, Moscow. A more practical approach is to synthesize device quality diamonds on a desirable substrate by chemical vapor deposition (CVD) . In this technique, a gaseous mixture including a carbon supply, usually provided by methane and hydrogen, is pyrolized, or injected into a high frequency plasma, above a substrate surface. Carbon radical reactions produce diamond crystals on the substrate, while the hydrogen present is converted to atomic hydrogen which preferentially etches away graphite and thereby leaves a film which is predominately diamond. This method also allows for the possibility of doping by introducing electrically active impurities into the environment above the substrate which are then trapped in the diamond lattices so synthesized. The quality and growth rate of diamond films produced by CVD on substrates has been found in the past to be dependent on the nature of the substrate. In fact, it had been thought that only when the substrate is diamond itself may device quality films be grown. In this technique, known as homoepitaxy, the orientation of newly deposited diamond is coincident with that of the substrate. This method, however, imposes the disadvantages of cost, size and availability of natural diamonds. Furthermore, in the cases where films have been successfully formed not using diamond as the substrate, they have proven to be largely inhomogeneous, i.e, polycrystalline and exhibiting excessive crystal defects. The growth of diamond films about diamond seed crystals has been practiced in our laboratory. The advantage to this approach is that the seeds may be small, and may take advantage of more readily available natural or synthetic diamond grits. Furthermore, the rate of diamond growth by CVD is enhanced by employing seeds. Another method involves polishing silicon substrates with diamond powder and growing diamond films by CVD about the crystal residue which remains. In both techniques, the seed crystals are completely unoriented and the resulting films are polycrystalline and inhomogeneous. Summary of the Invention It is one aspect of the present invention to provide a method for orienting seed crystals on substrates by suspending the crystals in a slurry, applying the slurry to a substrate surface and then heating the surface to effect the preferential orientation of the crystals. These seed crystals may be diamond, boron nitride, or like material. It is another aspect of the present invention to provide a method of growing oriented diamond crystal films on substrates by applying seed crystals to a substrate surface in a slurry, heating the substrate surface to effect the preferential orientation of the seed crystals with respect to the substrate and growing a crystal film about the seed crystals. It is yet another aspect of the present invention to provide a method of growing diamond films on a substrate surface by suspending the crystals in a slurry, applying the crystals to a grated surface to control the orientation of the seeds with respect to rotation about an axis perpendicular to the surface, heating the surface to affect the preferential orientation of crystals and then growing a crystal film about the seed crystals Yet another object of the present invention is to provide a crystal film on a substrate which is a mosaic of single crystal diamonds whose (111) planes are substantially parallel. Brief Description of the Drawing A preferred embodiment of the present invention is described in the accompanying drawings, in which: Fig. 1 is a flow diagram showing four preferred steps of the oriented diamond crystal process of the present invention; Fig. 2 is a perspective view of a diamond crystal seeded onto a flat substrate in practice of process steps 1-3 of Fig. 1, showing a preferred crystal orientation on the substrate; Fig. 3 is a graphic representation of an X-ray diffraction intensity pattern obtained from diamond crystals oriented on a quartz substrate according to the present invention, where X-ray intensity is plotted on the Y-axis against the Bragg angle 2Θ; Fig. 4 is a graphic representation of the distribution of the orientations of the crystals' (111) plane relative to the substrate plane, before (dotted line) and after (solid line) step 3 of Fig. 1, where diffraction intensity is plotted on the Y-axis in arbitrary units and degree of tilt of crystals' (111) plane relative to the substrate plane is plotted along the X-axis; Fig. 5a is a schematic view of seed crystals on a substrate surface before step 4 of the present invention; Fig. 5b is a schematic view showing growth of the seed crystals during the early stage of step 4 of the present invention; Fig. 5c is a schematic view showing formation of a crystal film after step 4 of the present invention; Fig. 6 is a perspective view of a flat surface, seeded with diamond crystals in practice of the present invention wherein the orientation pt the (ill) planes is depicted; Fig. 7 is a perspective view of a grated surface, seeded with diamond crystals in practice of the present invention wherein certain planes and directions are depicted; and Fig. 8 is a side cross-sectional view of a vertical semiconductor device having a textured polycrystalline film grown in practice of the present invention on a conductive substrate. Description of the Preferred Embodiment Referring to Figure 1, there is provided a flow diagram of the preferred method for oriented diamond crystal growth, including suspending seed crystals in a slurry, applying the slurry to the surface of a prepared substrate, heating the substrate and drawing off the slurry fluid, growing a film of diamond crystals on the treated substrate, by CVD. As a result of practicing the steps of the above invention, a diamond crystal film can be grown on a substrate. In particular, steps 1-3 of this procedure effect a desired orientation of seed crystals on the substrate surface. These seed crystals may be, for example, diamond or boron nitride or any other crystal whose crystal structure and behavior is similar to diamond. In Step 4, a single crystal film is grown around these oriented seeds preferably by CVD. 5 In preparation for practice of the first step of the invention, synthetic or natural diamond grit is prepared for use, preferably by immersion in a bath of boiling sulfuric acid and ammonium persulfate at approximately 300°C for 10 minutes. Thereafter, after 0 the wash solution is decanted from the grit, the grit is next rinsed with deionized water, concentrated hydrofloric acid and again with deionized water. The grit is thus made ready for use in the first step of the present invention. Grit sizes from less than 2μm up 5 to lOOμm have proven to yield satisfactory results in laboratory tests of the present invention. It has been found that the prepared diamond seed crystals tend to adhere to one another, forming clumps which may inhibit their orientation on the 0 prepared substrate. The first step of the present method is directed to separating or declumping, the seed crystals. In particular, the washed seeds are suspended in a slurry, preferably by mixture of 0.l grams of grit per 10 ml of a 40,000:1 solution of water in soap, such 25 as common microelectronics cleaning soap. Alternate slurry solutions include silicone base diffusion pump oil in trichloroethylene. The slurry mixture is then subjected to an ultrasonic vibratory mixer to suspend the grit. 30. The substrate, which may be, for example, a standard Si wafer, is preferably prepared by cleaning in an oxygen plasma asher. Other substrates having metal or quartz surfaces also may be used. The substrate is then wetted across its surface by application of the slurry (step 2) in any conventional manner, such as by use of a dropper or the like. Next, the wafer is heated (step 3) until the solvent is removed and the surface is visually dry. Such heating has been achieved by use of an ordinary laboratory hot plate, where the substrate is placed on the hot plate surface and is raised to approximately 200°C for approximately two minutes. The diamond growth CVD process (step 4) involves heating the substrate to 900°C and passing a gaseous mixture of, for example, 99% hydrogen and 1% methane, through a 2.75 GHz discharge above the substrate surface to create a plasma. Since pure diamond is an insulator, it may be necessary to introduce or dope electrically active impurities to render the diamond useful as a semiconductor. To create boron-doped crystal films for use in P-type semiconductor devices, B 2 H g may be added to the gas during CVD at chosen concentrations in the parts per million range. To create phosphorus-doped crystal films for use in N-type semiconductor devices, PH 3 may be added to the gas during CVD at chosen concentrations in the parts per million range. As a result, diamond films may be produced with a chosen level of doping. Another CVD method of growing diamond is to flow a mixture of hydrogen and a hydrocarbon gas, such as methane, past a hot tungsten filament located in close proximity to the substrate surface. This method of diamond CVD growth was described originally by S. Matsumoto et al. in the Japan Journal of Applied Physics, vol. 21, page L183, (1982), with recent elaborations by Hirose and Terasawa, in Japan. J. Appl. Phys. vol, 25, p. L519 (1986). Referring now to the perspective view of Fig. 2, a diamond crystal seeded onto a flat substrate in practice of process steps 1-3, and prior to step 4, is conceptually presented. Here, the desired orientaton of the (111) plane of the seed crystal is shown with respect to the substrate surface. In particular, it has been found that seeded crystals after the heating of step 3 and prior to the CVD of step 4, are oriented such that the (111) plane of the crystal (using the Miller indices referencing method) will lie parallel to the substrate plane. Turning to Figure 3, a graphic representation of an X-ray diffraction pattern obtained from diamond crystals applied to a quartz substrate by the present method, and substantially as oriented in Fig. 2, is shown, where X-ray intensity is plotted in arbitrary units on the Y-axis and diffraction angle 2Θ is plotted on the X-axis, where 2Θ represents the Bragg angle. It will be understood by those skilled in the art that diffraction from the (111) planes of the crystals is almost exclusively experienced, as represented by the extreme spike at 43.9°. This graph therefore clearly shows that the method of steps 1-3 indeed brings about the desired orientation shown in Fig. 2. Turning next to a graphic representation of Fig. 4, the distribution of crystals with (111) crystal planes oriented relative to the substrate plane is presented before and after the heating of step 3. In this graph, X-ray diffraction intensity is plotted along the Y-axis in arbitrary units, while the degree of tilt of the crystals' (111) plane relative to the substrate plane is plotted along the X-axis. The mildly curvilinear dotted line represents diffraction intensity after the step 2 application of the seed crystals to a substrate but before the heating of step 3, and shows some amount of (111) plane orientation. The sharply parabolic curve (in solid line) represents vastly improved diffraction intensity after the annealing of step 3, suggestive of vastly improved crystal orientation. It will thus be appreciated that the heating process of step 3 substantially improves the conformity of the seed crystal orientation on the substrate surface. The film growth process of the invention (step 4) is shown in detail in Figs. 5a-c. Fig. 5a is a schematic view of seed crystals on a substrate surface in practice of the invention, after step 3 but before step 4. These crystals, as seeded, obtain the preferred orientation shown in Fig. 2. In Fig. 5a, the preferred orientation of the seed crystals is indicated by the horizontal hash marks drawn across the cross-section of the crystals. Fig. 5b is a schematic representation of the early stage of the CVD process of step 4, where it will be seen that the seeded crystals become enlarged as new diamond material is deposited on the seed surface. The new diamond material is also favorably oriented in coincidence with the orientation of the seed crystals, as indicated by the horizontal hash marks of Fig. 5b, coincident with the hash marks of 5a. As shown in Figure 5c, the resulting grown crystals merge and a crystalline film is formed upon completion of the CVD process. This textured film is characterized by a multiplicity of crystals A, B, C grown from individual seeds a, b, c, all of whose (ill) planes are similarly oriented. At the point where adjacent crystals A, B, C merge, there is the possibility that crystal defects may occur due to the differing rotational orientation of the seeds about their <111> axis. These defects are shown schematically in Figure 5c as the substantially vertical lines intermediate the grown seed crystals. Turning now to Fig. 6, a flat substrate surface, seeded with diamond crystals, is shown in perspective view. These crystals are indicated to have dissimilar orientations with respect to rotation about an axis perpendicular to the (111) plane. It will be appreciated that these crystals may have irregular shapes. However, regular tetrahedral crystal shapes are drawn in Figs. 6 and 7 for ease of indicating the orientations of the (111) planes. Growth of diamond films by CVD from such crystals may lead to crystal defects at the point at which film growth from adjacent crystals meet. This leads to a textured crystal film of many crystals (i.e., polycrystalline film) grown from adjacent seeds whose (111) planes are all similarly oriented. Nevertheless, and quite surprisingly, these defects are not so substantial as to be disabling. An alternative embodiment of the present invention is to apply the method of steps 1-4 to a surface which has been previously patterned in the form of a grating, as disclosed in the perspective view of Figure 7. Employing a grated substrate surface orients the seed crystals with respect to rotation relative to each other and thus reduces or eliminates the crystal defects arising from orientation mismatch between adjacent seed crystals. In this manner a polycrystalline film with fewer defect boundries may be formed. The present invention may be favorably employed in the growth of vertical semiconductor devices. These devices are characterized by vertical current flow through the device. Turning now to Fig. 8, there is shown a side cross-sectional view of a vertical semiconductor device having a textured polycrystalline film grown on an ungrated conductive substrate (such as of nickel or carbon) , created by the present method of crystal film growth. In Fig. 8, the now familiar grain boundaries of the textured film will be seen, where it will be appreciated that a grating ^ pattern has been etched into the surface of the prepared polycrystalline film, and where the device is provided with an emitter, base, and collector. As will be further appreciated, the vertical axes of the crystals of the textured film are within a few degrees of the substrate normal, where rotational orientations about the normal axis have not been controlled. For purposes of this device, the film has been doped with boron sufficient to render it a suitable semiconductor. Ohmic contacts may be created by conventional means.. Finally, metal is evaporated on all horizontal grating surfaces without metallizing the vertical walls of the grating, thus creating a Schottky base and contacts on the tops of the grating. This vertical device is only one of several devices which may be created in practice of the present invention. The preferred process is as follows: 1. growing an oriented diamond-like polycrystalline film on a conducting substrate of nickel, carbon, or the like; 2. creating ohmic contact surfaces; 3. creating a grating in the grown film with ion-beam assisted etching; and 4. evaporating metal (such as aluminium) on the horizontal surfaces of the grating to form an emitter and base. The technique of ion beam etching of step 3. above has been particularly set forth in the Journal of Vacuum Science and Technology, vol. 313, p. 416 (1985). Finally, it should be appreciated that in randomly oriented crystal films, as provided by prior art methods, substantial doping variations occur between crystals in the film. This occurs because the doping concentration is dependent upon the orientation of the growing crystal. By providing a film in which the crystal planes are consistently textured, however, the doping concentration in the present invention is both uniform and predictable. Several modifications and variations of the present invention are possible when considered in the light of the above teachings. It is therefore understood that the scope of the present invention is not to be limited to the details disclosed herein, may be practiced otherwise than is as specifically described, and is intended only to be limited by the claims appended hereto:

Claims: Claims 1. A method of orienting crystals on a substrate surface, comprising the steps of: suspending seed crystals in a slurry; applying said slurry to said substrate surface; and heating the seeded substrate to remove the slurry fluid and to orient the seed crystals on said substrate such that a common plane of the crystals is substantially parallel to the plane of said substrate. 2. The method of claim 1, wherein said seed crystals are diamond and wherein said common plane is the (111) plane.. 3. The method of claim 1, wherein said seed crystals are BN. 4. The method of claim 2, further comprising the step of cleaning said seed crystals before the step of suspending said seed crystals in a slurry. 5. The method of claim 2, wherein said seed crystals are 100 μm or smaller. 6. The method of claim 2, wherein said substrate is non-diamond. 7. The method of claim 2, wherein said substrate is a metal. 8. The method of claim 2, wherein said substrate is silicon. 9. The method of claim 2, wherein said substrate is quartz. 10. The method of claim 1, wherein said slurry fluid is a mixture of water and soap. 11. The method of claim 1, wherein said slurry fluid is a mixture of hydrocarbon oil and trichloroethylene. 12. A method of producing an oriented diamond crystal film on a substrate, comprising the steps of: suspending seed crystals in a slurry; applying said slurry to said substrate surface; heating the seeded substrate to remove the slurry fluid and to orient the seed crystals on said substrate such that a common plane of the crystals is substantially parallel to the plane of the substrate; and growing a film about said seed crystals. 13. The method of claim 12, wherein said seed crystals are diamond and said common plane is the (111) plane. 14. The method of claim 12, wherein said seed crystals are BN. 15. The method of claim 13, further comprising the step of cleaning said crystals before applying to said substrate surface in said slurry. 16. The method of claim 13, wherein said seed crystals are 100 μm. 17. The method of claim 13, wherein said substrate is a non-diamond surface. 18. The method of claim 13, wherein said substrate is a metal. 19. The method of claim 13, wherein said substrate is a silicon wafer. 20. The method of claim 13, wherein said substrate is quartz. 21. The method of claim 12, wherein said slurry fluid is a mixture of water and soap. 22. The method of claim 12, wherein said slurry fluid is a mixture of hydrocarbon oil and trichloroethylene. 23. The method of claim 12, wherein said surface is grated to allow preferential orientation of said seed crystals with respect to rotation about an axis perpendicular to said (111) plane. 24. The method of claim 12, wherein said substrate is conductive and further comprising the steps of: creating ohmic contact surfaces on said film; creating a grating in the film with ion-beam assisted etching, and evaporating metal on the horizontal surfaces of the grating. 25. The method of claim 24, wherein said substrate is nickel and said evaporated metal is aluminum. 26. The method of claim 24, wherein said substrate is carbon and said evaporated metal is aluminum. 27. A diamond film comprising: diamond crystals whose (111) planes are parallel, said film being supported on a substrate. 28. The film of claim 27, wherein said crystals are aligned to form a mosaic. 29. The film of claim 27, wherein said crystals are aligned to form a textured film. 30. The device of claim 27, wherein the substrate is non-diamond. 31. The device of claim 27, wherein the substrate is a metal. 32. The device of claim 27, wherein the substrate is silicon.

Inventors: EFREMOW NIKOLAY N, GEIS MICHAEL W, SMITH HENRY I, EFREMOW, NIKOLAY, N., GEIS, MICHAEL, W., SMITH, HENRY, I.
Applicants: MASSACHUSETTS INST TECHNOLOGY, MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Application year: 1989
Main classification: B01D9
IPC codes: B01D9, C23C16, C30B7, C30B25, C30B28, C30B29, H01L21