Semiconductor device manufacturing: process – Introduction of conductivity modifying dopant into... – Ion implantation of dopant into semiconductor region
Reexamination Certificate
2000-03-30
2002-06-18
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Introduction of conductivity modifying dopant into...
Ion implantation of dopant into semiconductor region
Reexamination Certificate
active
06406983
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a process for the thermal annealing of at least one implantation-doped silicon carbide semiconductor in a gas stream.
silicon carbide (SiC), preferably in monocrystalline form, is a semiconductor material with outstanding physical properties which make that semiconductor material of interest particularly for optoelectronics, high temperature electronics and power electronics. While silicon carbide light-emitting diodes are already commercially available, there are not yet any commercial silicon carbide-based power semiconductor components. That is primarily due to the elaborate and expensive production of suitable silicon carbide substrates (wafers) and the more difficult process technology in comparison with silicon.
One of the problems is presented by the doping of monocrystalline silicon carbides. Due to the high temperatures required, which are in excess of 1800° C, it is practically impossible to dope silicon carbide by diffusion, unlike the case with silicon. Monocrystalline silicon carbide is therefore doped either by adding dopants during growth, in particular during sublimation growth (PVD) or chemical vapor deposition (CVD), or by implanting dopant ions (ion implantation).
The implantation of dopant ions in monocrystalline silicon carbide substrates or in a previously grown silicon carbide epitaxial layer allows targeted lateral variation of the dopant concentration, thereby making it possible to produce semiconductor components with a surface patterned in a planar manner. That constitutes a basic precondition for the fabrication of most semiconductor components. However, a problem with doping by implantation is the crystal defects (lattice defects, crystal imperfections) which are created in the silicon carbide crystal of the epitaxial layer by the dopant atoms implanted with high kinetic energy and which impair the electronic properties of the implanted semiconductor region and therefore of the whole component. Moreover, the dopant atoms or atomic residues are not incorporated optimally in the silicon carbide crystal lattice after implantation, and therefore only some of them are electrically activated.
Processes have therefore been developed for annealing the crystal defects created by the implantation by using heat treatment and, at the same time, for obtaining a high activation coefficient of the dopant atoms (so-called thermal annealing).
On one hand, an article in “IEEE Electronic Device Letters”, Vol. 13, 1992, pages 639 to 641 discloses a process for the thermal annealing of a 6H-silicon carbide semiconductor region, which is n-doped by implantation of nitrogen ions at high implantation temperatures of between 5000° C., and 1000° C., in a 6H-silicon carbide epitaxial layer that is p-doped with aluminum. In that process, the 6H-silicon carbide semiconductor is treated at a constant annealing temperature of between 1100° C., and 1500° C., in an argon atmosphere. In order to prevent the surface from being destroyed by uncontrolled evaporation with the formation of craters and cavities, the 6H-silicon carbide semiconductor is introduced into a crucible made of silicon carbide. During the heat treatment, the surface of the 6H-silicon carbide semiconductor is in equilibrium with the silicon carbide atmosphere within the crucible.
On the other hand, an article in “Applied Surface Science”, Vol. 99, 1996, pages 27 to 33 describes the influence of the gas composition during the cooling operation of a chemical vapor deposition process (LPCVD=Low Pressure Chemical vapor Deposition) on silicon carbide semiconductors. The cooling operation starts at a maximum temperature of 1450° C., which is thus comparable to the temperatures during thermal annealing after ion implantation. Therefore, the results that are obtained can also be transferred to the thermal annealing processes after ion implantation. In the investigations cited, it was ascertained that at temperatures of above 1000° C., in vacuum or under a protective gas, the silicon carbide atomic layers near the surface are depleted of silicon, and a thin graphite layer can form on the surface of the silicon carbide semiconductor. If, on the other hand, the same process is carried out under a pure hydrogen atmosphere, then the result is a virtually stoichiometric surface.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a process for the thermal annealing of implantation-doped silicon carbide semiconductors, which is improved in comparison with the heretofore-known processes of this general type and in which the formation or the clustering of undesirable crystallographically oriented steps is reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for the thermal annealing of at least one implantation-doped silicon carbide semiconductor in a gas stream, which comprises holding the at least one silicon carbide semiconductor with a carrier within a container; conducting the gas stream within the container causing the gas stream to contact regions of the carrier and the container; forming the carrier and the container of a material selected from the group consisting of at least one metal and at least one metal compound, at least in the regions; and supplying practically no carbon to the at least one silicon carbide semiconductor through the gas stream.
The annealing process is thus to be configured in such a way that practically no carbon is supplied to the at least one silicon carbide semiconductor through the gas stream. In this connection, “practically no carbon”is to be understood as a smaller proportion of carbon than that which corresponds to the equilibrium partial pressure of carbon or carbon-containing components (e.g. SiC
2
) over the silicon carbide semiconductor at the respective process temperature.
In this case, the invention is based on the insight that the crystallographically oriented steps which are always present in misoriented silicon carbide surfaces of layers applied epitaxially, for example, or of monocrystalline substrates and which ideally have a height of just one to two monolayers, cluster in an undesirable manner up to a step height of approximately 50 nm (step bunching) due to thermally activated surface redistribution. This takes place if, during the thermal annealing operation, the silicon carbide semiconductor is in equilibrium with a silicon carbide atmosphere or the gas stream supplied contains proportions of carbon which are at least comparable to this equilibrium state. Many small crystallographically oriented steps conglomerate in this case to form a few high crystallographically oriented steps. The small crystallographically oriented steps having a height of approximately two monolayers are an unavoidable consequence of the misorientation of the base silicon carbide crystals which is necessary for epitaxial layer growth. It has been found that the step growth described can be considerably restricted by reducing the proportion of foreign carbon in the gas stream, that is to say the proportion of carbon which is supplied externally to the silicon carbide semiconductor.
Consequently, when the gas stream is provided according to the invention, the step heights which result after thermal annealing are significantly smaller than in the prior art, in particular at least a factor of three smaller.
In accordance with another mode of the invention, at least the surface of the doped region of the silicon carbide semiconductor is exposed to a gas stream which preferably contains at least one inert gas and/or nitrogen and/or hydrogen. The gas stream composition can be changed during annealing, for example from an inert gas composition to a hydrogen-containing composition or even into practically pure hydrogen. Argon or helium with proportions by volume of up to approximately 100% are advantageously used as the inert gases.
In accordance with a further mode of the invention, a preferred variant of the process control resides i
Hölzlein Karlheinz
Rupp Roland
Wiedenhofer Arno
Bowers Charles
Greenberg Laurence A.
Infineon - Technologies AG
Lerner Herbert L.
Smoot Stephen W.
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