Examination of Pitting Formation in Rapid Thermal Annealing of

Pitt ing at the periphery of refractory gates was observed fol lowing rapid thermal annealing (RTA) of capped self-al igned GaAs MESFETs. The pitting was attributed to enhanced outdiffusion of Ga and As at the interface between the gate edge and the cap layer. RTA with an arsenic overpressure, using arsine as a source, prohibited pitting and surface degradation in both capped and uncapped wafers. Higher mobil it ies were obtained for the ion implanted wafers annealed caplessly with arsenic overpressure, rather than for the wafers capped with SiO2 or PSG. Conversely, higher activation was measured for the capped wafers, rather than for uncapped ones. Both effects can be related to preferential Ga outdif-fusion from the substrate into the capping layer re.suiting in the formation of Ga vacancies. The increased presence of Ga vacancies reduces mobility, but enhances activation, since they serve as substitutional sites for Si dopants. Processing of self-aligned GaAs MESFET and HFET (heterostructure FET) devices involves high temperature anneals for the activation of the ion-implanted species (1). RTA (rapid thermal annealing) is a preferred method of dopant activation compared to the conventional furnace anneal process due to the reduction of short channel e