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are usually used as the precursors and can be diluted into a carrier gas such as hydrogen (H2) and argon.
Researchers have proposed several models of reactors with horizontal and vertical flow of the gases and different designs of the suscep- tors, including non-rotating and rotating sub- strates and multi-wafer with planetary rota- tion with hot-wall or cold-wall configurations. The hot-wall reactor looks superior because of its accrued efficiency, uniform heating due to reduced temperature gradients, and more effective transformation of precursors into lighter products.
The typical growth temperature and growth
rate are 1,500˚C to 1,650˚C and 5–15 μmh–1.
The CVD growth process for SiC usually consists of (a) in situ etching and (b) the main epitaxial growth step. In situ etching is performed with pure H2, HCl∕H2, hydrocarbon/H2, or SiH4 ∕H2 at very high temperature, typically the same temperature as that of the main growth. The purpose of in situ etching is to remove the below-surface damage and to obtain regular step structures. Immediately after etching, main growth of n-type or p-type SiC (or their multilayers) is performed.
We have seen that an important aspect of the quality of a crystal is related to the presence of lattice defects that adversely impact the operation of devices manufactured from that crystal. This is due to the introduction of new elec- tron levels into the bandgap which, in turn, interfere with carriers in both the valence and conduction bands. To obtain high-quality materials, it is imperative to use a substrate with the lowest defect density and an epitaxial process capable of reducing the concentration of defects in the substrates. Figure 3 shows a schematic of a hot-wall CVD reactor.
FIGURE 3: SCHEMATIC OF CVD REACTOR
A common method for high-temperature heating is to use a radio-frequency induction coil, which contains a slab of graphite that is heated by the induced alternating current generated by an alternating magnetic field. The substrate is laid on the heated piece called the susceptor. Many steps, each one of them well-controlled by optimized reactor geometry, are involved: mass transport of precursors and reactants, absorption/desorption on the surface with conse- quent diffusion to growth sites, and, finally, mass transport for removing by-products. Instead of using separate precur- sors whereby one species provides silicon and the other sup- plies carbon, single-source precursors such as tetramethyl silane, or Si(CH3))4, have been successful.
In silicon semiconductors, doping can be achieved through high-temperature diffusion steps. In the case of SiC, how- ever, only ion implantation followed by annealing is possible. Because the high energy associated with ion implantation causes damage in the crystal (point defects), another doping method is to add a third precursor during epitaxial growth (in situ doping) to get a large range of concentrations for both n-type and p-type (1,014–1,019 cm–3). This is rather rare for other wide-bandgap semiconductors, such as GaN, for which p-type doping is difficult. An important factor in SiC is the deposition across the reactor walls, which depletes the gas from precursors. This effect is important to accurately esti- mate growth rates.
Gianfranco Di Marco
is a marketing communication manager at STMicroelectronics.
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REFERENCES
1V. D. Heydemann, N. Schulze, D. L. Barrett, and G. Pensl. “Growth of 6H and 4H silicon carbide single crystals by the modified Lely process utilizing a dual-seed crystal method.”Institut für Angewandte Physik, Universität Erlangen-Nürnberg, Erlangen, Germany (1996).
2F. La Via, M. Camarda, and A. La Magna. “Mechanisms of growth and defect properties of epitaxial SiC,” Applied Physics Reviews 1, 031301 (2014).
3Ranbir Singh. “Reliability and performance limitations in SiC power devices.” Microelectronics Reliability 46 (2006) 713–730.
4bit.ly/3hrOYCz
Technology Analysis The Fabrication Processes and Substrate Features of Silicon Carbide