Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With movement of substrate or vapor or gas supply means...
Reexamination Certificate
1999-10-08
2004-11-30
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With movement of substrate or vapor or gas supply means...
C117S105000, C117S900000, C117S902000, C117S935000, C117S951000
Reexamination Certificate
active
06824611
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the high temperature growth of large single crystals, and in particular relates to methods and apparatus for the growth of high-quality single crystals of silicon carbide.
BACKGROUND
Silicon carbide is a perennial candidate for use as a semiconductor material. Silicon carbide has a wide bandgap, a low dielectric constant, and is stable at temperatures far higher than temperatures at which other semiconductor materials, such as silicon, become unstable. These and other characteristics give silicon carbide excellent semiconducting properties. Electronic devices made from silicon carbide can be expected to perform, inter alia, at higher temperatures, faster speeds and at higher radiation densities, than devices made from other commonly used semiconductor materials such as silicon.
Those familiar with solid-state physics and the behavior of semiconductors know that a semiconductor material must have certain characteristics to be useful as a material from which electrical devices may be manufactured. In many applications, a single crystal is required, with low levels of defects in the crystal lattice, along with low levels of unwanted chemical and physical impurities. If the impurities cannot be controlled, the material is generally unsatisfactory for use in electrical devices. Even in a pure material, a defective lattice structure can prevent the material from being useful.
Silicon carbide possesses other desirable physical characteristics in addition to its electrical properties. It is very hard, possessing a hardness of 8.5-9.25 Mohs depending on the polytype [i.e., atomic arrangement] and crystallographic direction. In comparison, diamond possesses a hardness of 10 Mohs. Silicon carbide is brilliant, possessing a refractive index of 2.5-2.71 depending on the polytype. In comparison, diamond's refractive index is approximately 2.4. Furthermore, silicon carbide is a tough and extremely stable material that can be heated to more than 2000° C. in air without suffering damage. These physical characteristics make silicon carbide an ideal substitute for naturally occurring gemstones. The use of silicon carbide as gemstones is described in U.S Pat. Nos. 5,723,391 and 5,762,896 to Hunter et al.
Accordingly, and because the physical characteristics and potential uses for silicon carbide have been recognized for some time, a number of researchers have suggested a number of techniques for forming crystals of silicon carbide. These techniques generally fall into two broad categories, although it will be understood that some techniques are not necessarily so easily classified. The first technique is known as chemical vapor deposition (CVD) in which reactants and gases are introduced into a system within which they form silicon carbide crystals upon an appropriate substrate.
The other main technique for growing silicon carbide crystals is generally referred to as the sublimation technique. As the designation “sublimation” implies, sublimation techniques generally use some kind of solid silicon carbide starting material, which is heated until the solid silicon carbide sublimes. The vaporized silicon carbide starting material is then encouraged to condense on a substrate, such as a seed crystal, with the condensation intended to produce the desired crystal polytype.
One of the first sublimation techniques of any practical usefulness for producing better crystals was developed in the 1950s by J. A. Lely, and is described in U.S. Pat. No. 2,854,364. From a general standpoint, Lely's technique lines the interior of a carbon vessel with a silicon carbide source material. By heating the vessel to a temperature at which silicon carbide sublimes, and then allowing it to condense, re-crystallized silicon carbide is encouraged to deposit along the lining of the vessel.
The Lely sublimation technique was modified and improved upon by several researchers. Hergenrother, U.S. Pat. No. 3,228,756 (“Hergenrother '756”) discusses another sublimation growth technique, which utilizes a seed crystal of silicon carbide upon which other silicon carbide condenses to grow a crystal. Hergenrother '756 suggests that in order to promote proper growth, the seed crystal must be heated to an appropriate temperature, generally over 2000° C. in such a manner that the time period during which the seed crystal is at temperatures between 1800° C. and 2000° C. is minimized.
Ozarow, U.S. Pat. No. 3,236,780 (“Ozarow '780”) discusses another unseeded sublimation technique which utilizes a lining of silicon carbide within a carbon vessel. Ozarow '780 attempts to establish a radial temperature gradient between the silicon carbide lined inner portion of the vessel and the outer portion of the vessel.
Knippenberg, U.S. Pat. No. 3,615,930 (“Knippenberg '930”) and U.S. Pat. No. 3,962,406 (“Knippenberg '406”) discuss alternative methods for growing silicon carbide in a desired fashion. The Knippenberg '930 patent discusses a method of growing p-n junctions in silicon carbide as a crystal grows by sublimation. According to the discussion in this patent, silicon carbide is heated in an enclosed space in the presence of an inert gas containing a donor type dopant atom. The dopant material is then evacuated from the vessel and the vessel is reheated in the presence of an acceptor dopant. This technique is intended to result in adjacent crystal portions having opposite conductivity types thereby forming a p-n junction.
The Knippenberg '406 patent discusses a three-step process for forming silicon carbide in which a silicon dioxide core is packed entirely within a surrounding mass of either granular silicon carbide or materials that will form silicon carbide. The packed mass of silicon carbide and silicon dioxide is then heated. The system is heated to a temperature at which a silicon carbide shell forms around the silicon dioxide core, and then further heated to vaporize the silicon dioxide from within the silicon carbide shell. Finally, the system is heated even further to encourage additional silicon carbide to continue to grow within the silicon carbide shell.
Vodakov, U.S. Pat. No. 4,147,572 discusses a geometry oriented sublimation technique in which solid silicon carbide source material and seed crystals are arranged in a parallel close proximity relationship to another.
Addamiano, U.S. Pat. No. 4,556,436 (“Addamiano '436”) discusses a Lely-type furnace system for forming thin films of beta silicon carbide on alpha silicon carbide which is characterized by a rapid cooling from sublimation temperatures of between 2300° C. and 2700° C. to another temperature of less than 1800° C. Addamiano '436 notes that large single crystals of cubic (beta) silicon carbide are simply not available and that growth of silicon carbide or other materials such as silicon or diamond is rather difficult.
Hsu, U.S. Pat. No. 4,664,944, discusses a fluidized bed technique for forming silicon carbide crystals which resembles a chemical vapor deposition technique in its use of non-silicon carbide reactants, but which includes silicon carbide particles in the fluidized bed, thus somewhat resembling the sublimation technique.
German (Federal Republic) Patent No. 3,230,727 to Siemens Corporation discusses a silicon carbide sublimation technique in which the emphasis of the discussion is the minimization of the thermal gradient between a silicon carbide seed crystal and silicon carbide source material. This patent suggests limiting the thermal gradient to no more than 20° C. per centimeter of distance between source and seed in the reaction vessel. This patent also suggests that the overall vapor pressure in the sublimation system be kept in the range of between 1 and 5 millibar and preferably around 1.5 to 2.5 millibar.
Davis, U.S. Pat. No. Re. 34,861 (“Davis '861”) discuss a method of forming large device quality single crystals of silicon carbide. This patent presents a sublimation process enhanced by maintaining a constant polytype compo
Kordina Olle Claes Erik
Paisley Michael James
Cree Inc.
Kunemund Robert
Summa & Allan P.A.
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