DEPOSITION AND ETCHING OF REFRACTORY OHMIC CONTACTS TO GAAS FOR SELF-ALIGNED TECHNOLOGY

Refractory ohmic contacts to n-type GaAs have been developed for heter ojunction bipolar transistors (HBTS) processed with self-aligned techn ology and including high-temperature anneals. We report the results on W and MoGe deposition, annealing and the reactive ion etching (RIE) o perations necessary for HBTS' self-aligned processing. These refractor y metallizations were fabricated on n-type epitaxial GaAs layers with a carrier concentration of 10(18) cm-3 . Ge was evaporated initially b y an electron beam technique, while Mo and W films were deposited sepa rately by sputtering in RF plasma. The MoGe ohmic contact formation is due to both an n+ layer created at the GaAs contact interface by Ge o verdoping and molybdenum germanide stable phase formation. Low specifi c contact resistivities were obtained with experiments combining As-do ped Ge and As overpressure. It was then possible to reduce the resisti vity to 2 x 10(-7) OMEGA cm2. Reactive ion etching of W, Mo and Ge is also described. Etch rates are compared using either photoresists or P t masking materials. A Pt mask was found to be much more suitable than photoresist because it allows higher selectivities and etch rates. Su rface contamination on the GaAs surface is also discussed and could be minimized for optimal device performance.