Silicon Based Vertical Micro-Coaxial Transition for High Frequency Packaging Technologies

A through wafer vertical micro-coaxial transition flushed in a silicon substrate has been designed, fabricated, and tested. The transition has been designed using radio frequency (RF) coaxial theory and consists of a 100μm inner diameter and a 300 μm outer diameter, which corresponded to a 1:3 inner/outer diameter ratio. The transition’s through silicon structure has been achieved using standard photolithography techniques and Bosch’s process for deep reactive ion etching (DRIE). The coaxial vias of the transition have been successfully metalized with a diluted silver paste using a novel filling method. To measure the behavior of the transition at high frequencies, coplanar waveguide (CPW) lines matched at 50 ohms have been integrated on the front and backside of the device. Measurement results show that the transition demonstrate good results with a reflection coefficient better than -10 dB at high frequencies from 15 GHz-to-60 GHz. Results also indicate that the transition has good signal transmission with less than -1.8 dB insertion loss up to 65 GHz. By eliminating the need for rigorous bonding techniques, the transition is a low-cost and durable design that can produce high input/output ratios ideal for commercial products.