Low loss suspended membrane on low resistivity silicon and its applications to millimetre-wave passive circuits

This paper presents the fabrication process of a thin suspended membrane on normal low resistivity silicon (LRSi), which is then utilized to design millimeter-wave (mm-wave) transmission line and passive circuits with low loss. First, two redistribution metal layers are formed on a plain LRSi wafer with benzocyclobutene in-between. Then, the wafer is thinned down and etched from backside with a specific pattern. Based on this process, a volume is introduced underneath the front side structure and the lossy LRSi substrate is replaced by air, which is the ideal substrate with no loss. Using this method, the substrate loss that contributes the majority part of total loss in mm-wave circuits on LRSi is eliminated. As the operating frequency goes into mm-wave region, the circuit size is scaled down so that the suspended portion is so small that it will not increase the mechanical instability of the structure. Therefore, the proposed suspended membrane on LRSi technology provides an excellent platform to construct mm-wave components. A U-shaped slotline resonator formed on the first redistribution layer is proposed and its quality factors are studied in detail. A second-order bandpass filter is designed and implemented by cascading two such U-shape slotline resonators. The measured frequency responses exhibit good filtering performance, with fractional bandwidth of 13.9% at 126.8 GHz, midband insertion loss of 3.9 dB and passband return loss larger than 14.2 dB. It demonstrates that the proposed suspended membrane on LRSi technology is an excellent candidate for designing low loss mm-wave components.