180GHz microstrip ring resonator bandpass filter on micromachined silicon substrate

A 180 GHz bandpass filter utilizing dual mode stepped impedance ring resonator is reported. A monolithic integrated microstrip design is proposed and fabricated on a silicon substrate. To excite appropriate resonant mode and to alleviate difficulties in micro-fabrication process, the second-order resonance of the circular ring resonator is adopted in the filter design. The whole design includes a conductor-backed coplanar waveguide (CBCPW) to guide signals into and out of the filter. Coplanar waveguide (CPW)-microstrip transition structure between CBCPW and central ring is considered for grounding to avoid having via metal layers bridging the top and bottom electrodes. This design also eliminates the need for wire bonding. Between the microstrip and ring resonator is a capacitive coupling gap for its simple design and easy to modify for different frequency operation. Aluminum is used for the top and bottom electrodes and the thickness is controlled at 600 nm. A (110) p-type 150 μm double side polished silicon wafer is used as the substrate. The thickness of the wafer is etched down to 100 μm from backside by inductively coupled plasma (ICP) dry etching to insure optimal waveguide transmission at the operation frequency. Since the backside surface roughness of substrate is shown to be much less than 600 nm, the thin film quality of subsequent conductor layer is guaranteed. The result from the CST simulation shows that a 3.5 dB insertion loss at 181 GHz and 3 dB passband from 176 to 186 GHz can be obtained.