Surface-passivated high-resistivity silicon as a true microwave substrate

This paper addresses the properties of a surface-passivated (enhanced) high-resistivity silicon (HRS) substrate for use in monolithic microwave technology. The detrimental effects of conductive surface channels and their variations across the wafer related to the local oxide and silicon/silicon-dioxide interface quality are eliminated through the formation of a thin amorphous layer at the wafer surface. Without passivation, it is found that the surface channels greatly degrade the quality of passive components in HRS by masking the excellent properties of the bulk HRS substrate and by causing a spread in parameters and peak values across the wafer. Moreover, it is seen that the surface passivation leads to excellent agreement of the characteristics of fabricated components and circuits with those predicted by electromagnetic (EM) simulation based on the bulk HRS properties. This is experimentally verified for lumped (inductors and transformers) and distributed (coplanar waveguide, Marchand balun) passive microwave components, as well as for a traveling-wave amplifier, through which also the integration of transistors on HRS and the overall parameter control at circuit level are demonstrated. The results in this paper indicate the economically important possibility to transfer microwave circuit designs based on EM simulations directly to the HRS fabrication process, thus avoiding costly redesigns.