Silicon as a Millimeter-Wave Monolithically Integrated Substrate - A New Look

Materials suitable for use as monolithic substrates are summarized. A study of the properties of silicon substrates as transmission line media shows that serious consideration should be given to them for use at mm-wave frequencies. It is concluded that for silicon resistivities of 2000 ohm-cm or greater, microstrip loss in silicon at mm-wave frequencies is only slightly higher than that in GaAs or alumina. The cross section width of a transmission line represents an appreciable part of a wavelength when microstrip is used as an impedance transformer at mm-wave frequencies. Therefore, substrate thickness (using the latest dispersion characteristics) is especially considered in circuit design. These effects on the design of 3-dB interdigitated and branch-line couplers are demonstrated. Fabrication of silicon IMPATT diodes operating up to 200 GHz has been ac­complished by novel techniques that maintain the silicon\u27s high resistivity. We report on diodes yielding 25 mW cw at 102 GHz, 16 mW cw at 132 GHz, and 1 mW at 195 GHz. The techniques described are ion implantation, laser annealing, unique secondary-ion mass spectrometry (SIMS) profile diagnostics, and novel wafer thinning. The utilization of these technologies paves the way for the pro­cessing of silicon monolithic mm-wave integrated circuits