Morphological Instability of High Ge Percent SiGe Films Grown by Ultra-High Vacuum Chemical Vapor Deposition

The Ge content in SiGe bipolar transistors (HBTs) has been steadily increasing for the past few decades in order to meet higher frequency targets. Problems exist with higher Ge content films due to the strain from the pseudomorphic growth on Si substrates. In addition to relaxation by misfit dislocation at high Ge contents, SiGe films have been observed to exhibit morphological instability or surfaces waves. This phenomenon, known for some time, produces surface corrugations several nm in height that can have detrimental effects on SiGe devices. While historic solutions to the problem involved lowering the growth temperature, hot-wall ultra-high vacuum chemical vapor deposition presents a unique challenge due to the inability to quickly change temperatures during growth. Therefore the process parameters must be optimized for a constant growth temperature for all layers in the device structure. In this report, the process parameters including film growth rate and temperature will be described in their effect on reducing and eliminating the surface waves.