Multi-Wafer VPE Growth of Highly Uniform SiC Epitaxial Layers
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achieve satisfactory uniformity and reduce run-to-run variability, it was necessary to modify many of the original reactor components [6]. The improvements in uniformity and variability resulting from those modifications were sufficient to allow modest production goals for device quality SiC epitaxial layers to be exceeded in the previous year. Significantly higher volumes of epitaxial layers with more stringent specifications for dopant concentration and layer thickness values and uniformities will be required to meet current and future production goals. Indeed, production specifications are likely to require that all areas of all epitaxial layers be within ±10% of target values for both dopant concentration and thickness. Several more reactor modifications will be summarized which have allowed those production specifications to be realized with high yield. EXPERIMENT
The VPE reactor used to grow the SiC epitaxial layers discussed within has been described in detail elsewhere [3,6,7]. A cross sectional view of the reactor is shown in figure 1. The reactor consists of an inductively heated, mechanically rotated susceptor with seven wafer holders (satellites). The satellites are themselves rotated on the susceptor by means of gas foil levitation [5]. The satellite rotation is desirable to reduce upstream/downstream variation in growth. The susceptor rotation averages inhomogeniety in the RF induction heating and geometry of the non-rotating reactor components. However, susceptor rotation does not directly 161
Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
affect inhomogenieties that result from anisotropic conductivity in the graphite susceptor, dimensional nonuniformity of the machined and coated susceptor, and misalignment, i.e. tilt or eccentricity, of the susceptor on its support. All of those sources of inhomogeniety can be minimized but are difficult to eliminate. A graphite foil-on-foam ceiling [8] separates the growth region from the waterD cooled reactor chamber. The ceiling is passively heated by radiation from the -1600 0 C susceptor creating a pseudo hot-wall effect. The hot ceiling reduces supersaturation and condensation of silicon vapor in the reactor. In Figure 1. Stylized cross section of VPE a previous communication [6], it was reported reactor. A) Gas Injector, B) Ceiling, C) that the graphite foil-on-foam ceilings could Gas Collector, D) Susceptor, E) Wafer only be used for tens of runs. However, Holder, F)Rotation Axis recently, ceiling lifetime has been extended to nearly 100 runs, which has increased epitaxial layer production efficiency. Finally, a perforated gas collector terminates the outer diameter of the susceptor, the growth region, and the ceiling. The gas collector, made from graphite foam or molybdenum, ensures laminar flow of the gases in the growth chamber. Typical conditions for SiC epitaxial layer growth in this reactor have been described in detail elsewhere [6]. All of the growths reported in this study were performed at reduced pressure, susceptor tempe
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