Low-Resistivity Amorphous Silicon for Contacts Using Low-Temperature Rapid Thermal Annealing

ABSTRACTFabrication of SiGe heterojunction bipolar transistors (HBTs) requires a low thermal budget to avoid relaxation of the strained SiGe base layer. Ion implantation is one of the most widely used techniques to achieve contacts. However, due to thermal budget constraints, low temperature rapid thermal annealing (RTA) cycles to activate these implants are insufficient to anneal out all of the implant damage. Polysilicon contacts provide an alternative to ion implantation, but are typically annealed at high temperatures (>950°C) to achieve low sheet resistivity. In this study, amorphous silicon and polycrystalline silicon films were implanted with boron, arsenic, or phosphorus and RTA'd at temperatures from 800°C to 950°C and compared to single crystal silicon with identical implants and RTA cycles. The films were characterized using four-point probe, Hall measurements, TEM (transmission electron microscopy), and SIMS (secondary-ion mass-spectrometry). TEM analysis shows that the amorphous deposition produces larger grains upon RTA due to more rapid grain growth than the polycrystalline deposition. The sheet resistance for the amorphous deposited films is much lower than that of the polycrystalline deposition for all implant conditions. Activations of the implants indicate that the arsenic and phosphorus segregate to the grain boundaries, while the boron does not. The segregation is more significant for the polycrystalline films than for the amorphous films and can be explained by the grain boundary area. For contacts to the SiGe HBT, which requires a low thermal budget, an amorphous deposited silicon film is advantageous over a polycrystalline film at low annealing temperatures because it has lower sheet resistance, less segregation to the grain boundaries, and produces larger grains.