This invention relates generally to the fabrication of semiconductor devices, and more particularly, to a semiconductor device wafer base wherein a silicon carbide overlay layer is deposited on a suitable substrate.
An extensive technology of semiconductor devices has been developed based upon the properties of single crystal silicon and other similar materials which may be doped, heat treated and otherwise processed to produce adjacent layers and regions of varying electronic characteristics. The use of devices produced by silicon technology is generally limited to operation at ambient or, at most, low elevated temperatures, and in non-corrosive, inert environments. The temperature limitation is a consequence of the rapid diffusion of dopant and impurity species in the silicon, which in turn can substantially alter the character of the fabricated semiconductor device by diffusional degradation. The limitation to relatively inert environments results from the high chemical reactivity of silicon in many corrosive environments, which also can alter the character of the fabricated device. Silicon devices are also limited as to power level, frequency and radiation tolerance by the materials used therein.
For many applications, the temperature, environmental, and other use limitations on silicon devices may be overcome by the use of proper cooling and packaging techniques, but in other applications these limitations have prevented the use of silicon for integrated circuit technology. For example, in many space craft and aircraft applications, elevated temperatures may be encountered, and it is not always possible to insure that adequate cooling is provided. In high power applications, internal thermal transients in devices otherwise operating at ambient temperature can rapidly destroy the operability of the device unless extensive cooling is provided. Such cooling requires that the device be larger in size than might otherwise be necessary, in part defeating the purpose of the integrated circuit technology.
There has therefore been an on-going search over a period of twenty years to identify and develop a semiconductor technology based in other materials which allow the fabrication of devices for use at higher temperatures such as, for example, the range of at least about 400.degree. C.-600.degree. C., and in applications not amenable to the use of silicon. Because corrosive effects can be greatly accelerated at elevated temperatures, any such materials and devices must also exhibit excellent corrosion resistance over a range of pressures from vacuum to many atmospheres. Some generally desirable requirements of such materials have been identified, including large band gap, ability to be doped to produce regions of varying electronic characteristics, a high melting temperature, resistance to diffusion by undesired foreign atoms, thermal stability, chemical inertness, and the ability to form ohmic external contacts.
Silicon carbide was early identified as a candidate material meeting the indicated requirements. Silicon carbide has a high breakdown voltage, a relatively large band gap, and a thermal conductivity of more than three times that of silicon at ambient temperature. Silicon carbide is also resistant to the diffusion of impurity species. Silicon carbide may be processed by several techniques similar to those used in silicon device technology, and in many instances silicon carbide devices may be substituted at moderate and low temperatures for silicon devices. Silicon carbide semiconductor device technology therefore offers the opportunity for supplementing, and in some instances replacing, conventional silicon device technology.
For all of its potential, the promise of silicon carbide device technology has not been achieved simply because it has not been possible to produce unpolytyped silicon carbide single crystals of sufficient size to allow the fabrication of semiconductor devices. Small bulk single crystals of beta silicon carbide have been fabricated, but with dimensions no greater than about 2 millimeters. An alternative approach to the use of bulk single crystals, which is widely used in silicon device technology, is the preparation of device bases by epitaxially depositing an overlay of the desired semiconductor on a substrate which serves both to support the overlay layer and to provide a nucleation plane for epitaxial growth. Epitaxial growth of thin overlay layers of beta silicon carbide has been attempted on beta silicon carbide itself, previously carburized silicon, alpha silicon carbide and molybdenum with and without a liquid metal intermediate layer, and silicon.
All of these substrates and techniques have proved to be unsuitable, either because large single crystals of the substrates themselves cannot be prepared, because the resulting silicon carbide overlays could not be fabricated as sufficiently large single crystals, because a single stable crystal of beta silicon carbide polytype without any included alpha silicon carbide polytype could not be prepared, because of a large lattice parameter and thermal expansion mismatch between the beta silicon carbide and the substrate, or because of unsatisfactory surface morphology of the resulting beta silicon carbide overlay. In short, an intensive, worldwide effort to produce either bulk single crystals or epitaxial overlay single crystals of unpolytyped silicon carbide suitable for integrated circuit applications, with dimensions greater that a few millimeters, has failed completely. The potential for the use of silicon carbide has been verified by manufacturing devices in very small silicon carbide crystals, but this demonstration has not been extended to practical, large scale devices because of this limitation in fabricating sufficiently large single crystals of a single polytype or large area beta silicon carbide epitaxial layers suitable for device or integrated circuit processing.
Thus, there exists a need for some approach to utilizing silicon carbide semiconductor technology on a large scale in practical devices. The approach should allow the reproducible preparation of thin beta silicon carbide single crystals having lateral dimensions greater than several millimeters, so that large scale semiconductor arrays of devices can be processed and prepared on a single crystal. The approach should also be consistent with the utilization of the desirable characteristics of beta silicon carbide. That is, the fabrication technique should not adversely influence its thermal stability, corrosion resistance, and other desirable properties. The present invention fulfills this need, and further provides related advantages.
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