Fabrication of submicron microwave bipolar transistors by direct write electron beam lithography

To study the effects of reducing the lateral dimensions of bipolar transistors to values below 1 ¿m, a production process based on optical lithography, is adapted to fabricate submicron sized interdigitated microwave transistors. The emitter, base contact, and metallization patterns are defined by direct write electron beam lithography. For alignment of the patterns, special registration marks on (111) Si-wafers, compatible with further silicon processing have been developed. With a Philips EBPG/03 Beamwriter the resulting overlay accuracy is between 50 and 100 nm. The emitter and base contact resist patterns are transferred to a SiO2 layer by reactive ion etching. The exposure dose latitude for the definition of the Ti/Pt/Au metallization pattern is severely limited by the proximity effect and a high contrast electron beam resist is needed. To tansfer by sputter etching, the resist pattern into a high quality submicron metallization pattern, Ti is used as an etch mask.