Millimeter-Wave “Reflectionless” Filters and Diplexers Based on Silicon Micromachining

Reflectionless filters are two-port networks that ideally present matched terminations at both filter ports to all frequencies, within both the filter’s passband and stopband. Standard lossless filter structures reflect out-of-band power back to the input, which can profoundly impact the performance of heterodyne instruments and contribute to generation of spurious sidebands, unwanted gain compression, reduced dynamic range or instability. Reflectionless filters, conversely, direct out-of-band power to one of the filter’s two “internal” resistive terminations. If either of these internal resistors is converted into a port, then the resulting circuit forms a matched frequency diplexer This paper reports a prototype lowpass “quasi-reflectionless” filter and diplexer architecture that exploits integration of passive elements on a silicon-on-insulator platform. The design employs suspended high-impedance transmission lines for the filter inductive elements as well as integrated metal-insulator-metal (MIM) capacitors and thin-film titanium resistors on a 15 mm thick high-resistivity silicon substrate. As proof-of-concept implementations, a filter designed for operation in the 0—220 GHz band with 3 dB roll-off at 150 GHz and a diplexer architecture designed for the 0—220 GHz band with target cross-over frequency at 170 GHz are described. The filter and diplexer are implemented using a silicon-on-insulator (SOI) micromachining process that has been used broadly as a platform for integration. Fabrication utilizes frontside lithography, sputter deposition and electroplating processes to form thin-film titanium resistors (nominal impedance of 50 W, 2 mm10 mm, 50 nm thick), metal-insulator-metal capacitors (radii of 4.6 mm and 3.8 mm with SiO2 insulator thickness of 200 nm) and meandered high-impedance microstrip transmission lines (width of 3 mm, lengths of 180 mm and 90 mm) on a 15 mm thick, high-resistivity (>20 kW-cm) silicon membrane. After the topside features of the filter and diplexer have been formed, the SOI wafer is bonded topside-down to a temporary carrier to allow backside processing. Scattering parameter measurements of the filter and diplexer both show the desired lowpass characteristic from lowband input port to output, but with a softer roll-off than anticipated from electromagnetic finite-element analysis and circuit simulations of the structures. Modeling suggests this soft roll-off characteristic is associated with parasitic inductance and resistance contributed by the metallized ground via. These parasitic elements associated with the via-to-ground are not observed in the electromagnetic simulation of the filter or diplexer and the observation that they occur consistently in the model for each circuit node connected to ground suggest they are associated with the electrical interface of the circuits to the tests fixtures used to characterize them. Moreover, it is also hypothesized that these parasitic elements are associated with discontinuities between the filter/diplexer metallized ground plane (on the silicon backside) and the metallized measurement housing platform to which the circuits are affixed for measurement. Disruptions to ground currents flowing beneath the silicon-supported microstrip circuit and the suspended transmission lines (where the measurement housing itself serves as the ground) will contribute to parasitics associated with the ground connection, consistent with the observed measurements.