Fabrication of silicon microwave transistors with one micron emitter stripe widths

Double-diffused epitaxial npn silicon transistors having base layer thicknesses of <0.15 micron and emitter stripe widths of 1 micron have been fabricated. These devices, designed for linear amplification in the low microwave range, exhibit fTup to 7 GHz and up to 4 dB of maximum available power gain at 4 GHz. Several major fabrication problems were encountered during the development of these devices. Emitter-collector shorts can result from dislocations and precipitations introduced into the silicon during base and emitter diffusions. This problem is minimized by using very shallow diffused layers: the present structures employ 0.35µ boron-diffused base layers and 0.2µ phosphorus-diffused emitters and only a small and usually negligible number of defects are obtained. Inadequate resolution of the photoresist can result from non-uniform exposure caused by constructive interference of the incident light with that reflected at the silicon surface. This difficulty is alleviated either by incorporating a thin metallic film between the SiO2and the photoresist or by using a modified photoresist with increased optical density. The formation of electrical base-emitter shorts by the overlay contact is minimized by employing an extremely thin layer of phosphorus glass for the emitter diffusion source, thus avoiding excessive interaction with protective SiO2.