Low temperature epitaxial silicon growth using electron cyclotron resonance plasma deposition
The development of a process for the low temperature (<600° C) growth of epitaxial silicon is an important technological issue. Conventional growth processes involve temperatures in excess of 1000° C. At these temperatures autodoping and impurity redistribution limit the feature size achievable in VLSI fabrication. As the typical feature sizes move into the submicron region, new processes for epitaxial silicon deposition will be needed. Another application for a low temperature growth process is the fabrication of solar cells on inexpensive metallurgical grade silicon wafers. Impurity diffusion from the wafer during conventional epitaxial silicon growth limits the quality of the solar cells if expensive high purity wafers are not used. We have used electron cyclotron resonance (ECR) plasma deposition to grow high quality epitaxial silicon films on silicon wafers. This growth technique relies on the deposition of silicon from a highly energetic hydrogen and silane plasma. The presence of the hydrogen in the plasma provides reactive etching of the silicon surface during growth. This reduces the oxygen and carbon contamination in the film as well as increasing the number of available growth sites on the surface by displacing the adsorbed hydrogen. By optimizing the growth pressure, substrate temperature, microwave power, substrate bias and silane to hydrogen ratio we have developed a process which provides enhanced growth rates and good uniformity at temperatures (425-575° C) significantly below those used in conventional processes. The structural and electrical properties of the films have been characterized using SEM, TEM, Raman spectroscopy, UV reflectance, spreading resistance profiles, Hall mobility measurements, and both four-point probe and van der Pauw resistivity measurements.