A four-stack Bruce model BDF-4 furnace bank with cantilever loading is used for low pressure chemical vapour deposition on both 50 mm and 100 mm wafers. Computer control of temperature and gas flow is identical to that for the ox/diff furnaces.
At present dedicated tubes and processes are in place for silicon nitride, polysilicon and low-temperature oxide deposition. Silicon nitride deposition is carried out using SiH2Cl2/NH3 chemistry at 820°C. Polysilicon is deposited from silane at temperatures between 550 and 625°C (below 600°C amorphous silicon layers are formed). The polysilicon can be doped in-situ to solid solubility using either phosphine or diborane. Oxide is deposited at 405°C from silane/oxygen. Doping is accomplished by adding trimethyl borate (TMB) or phosphine. Typically BPSG with 5 to 6 mass percent boron and phosphorus is used as a poly-metal dielectric. This BPSG can be flowed at 900°C to give good surface planarization.
The silicon nitride and undoped polysilicon processes give better than +/- 10% uniformity on 100 mm diameter wafers. Uniformity of doped polysilicon and doped glass deposition is dependent on doping concentration.
There is interest in adding a TEOS tube to provide a conformal oxide deposition capability.
It is worth noting that the Bruce furnaces used for oxidation/diffusion and LPCVD operations were formerly used in industrial production of the CMOS3 technology at Northern Telecom Electronics in Ottawa.