Design and fabrication of air-bridge CPW using porous silicon and MEMS technology

This paper proposes a three-dimensional structure for an RF passive device using a thick oxidized porous silicon layer (OPSL). The OPS air-bridge was fabricated using an anodic reaction, reactive ion etching (RIE), tetramethyl ammonium hydroxide (TMAH) etching, and a multistep oxidation process. The problem of the high dielectric loss of a waveguide on silicon can be solved using a thick OPS air-bridge. The three dimensional structures were fabricated using a 20 wt.% TMAH solution at 80 degrees C for 3 h, and the thickness of the micromachined OPS air-bridge was 10 mu m. A coplanar waveguide (CPW) structure was also fabricated on the OPSL air-bridge for RF application. The fabricated structure was 2 mm in length, and the width of the signal line and the gap between the ground and the signal lines were 100 mu m and 20 mu m, respectively. This process is well compatible with conventional complementary metal oxide semiconductor (CMOS) fabrication process without post-processing, and does not require an additional mask for silicon etching.