Very Low Temperature (Cyclic) Deposition / Etch of In-Situ Raised Sources and Drains

Abstract

We have developed an innovative 500°C process for the selective deposition of SiGe:B Raised Sources and Drains (RSDs). We have first of all studied on blanket Si wafers the in-situ boron doping of SiGe with Si2H6, GeH4 and B2H6. A growth rate increase by a factor higher than 4 together with a Ge concentration decrease from 45% down to 28% occurred as the diborane mass-flow increased (at 500°C, 20 Torr). Very high substitutional boron concentrations were achieved (~ 5x1020 cm-3) in layers that were single crystalline and flat. Adding large amounts of HCl to the gaseous mixture did not yield the selectivity aimed for on SiO2-covered Si wafers, however. To that end, we have thus benchmarked various 500°C Cyclic Deposition / Etch (CDE) processes. 12 cycles CDE processes were characterized by HCl etch rates of poly-SiGe:B that were either too low to be of any practical use or yielded 3 dimensional SiGe:B layers on Si(001). Straightforward Deposition / Etch (DE) processes, with the HCl selective etch of poly-SiGe:B carried out at 740 Torr (i.e. atmospheric pressure), enabled us by contrast to achieve selectivity on SiO2 while retaining single crystalline if slightly rough SiGe:B layers. Those DE processes were tested on patterned Silicon-On-Insulator substrates with gate stacks. Longer HCl etch times than the ones identified on blanket wafers were key in getting rid of poly-SiGe:B on top of dielectrics covered surfaces; rather smooth, facetted SiGe:B RSDs were obtained in the end.