Developments of thick-metal inductors and applications to reactive lumped-element low-pass filter circuits

Abstract

Strong demands for smaller, cheaper, and multifunction wireless systems have put very stringent requirements on passive devices, such as inductors and capacitors. This is especially true considering the size and weight of most radio frequency (RF) transceivers are mainly due to passives. RF micro-electro-mechanical-systems (MEMS) passives are addressing this issue by offering lower power consumption and losses, higher linearity and quality (Q)-factors, potential for integration and miniaturization, and batch fabrication. These advantages position RF MEMS passives as good candidates to replace conventional passives. Further, they also open an opportunity for using the passives as building blocks for lumped element-based RF circuits (e.g. Flters, couplers, etc.) which could replace the more-bulky distributed-element circuits.

This thesis presents the design, simulation, fabrication using the deep X-ray lithography process, and testing of thick-metal RF inductors and their applications to lumped-element low-pass Filter (LPF) circuits. The 70-um tall single-turn loop inductors are structurally compatible to a pre-existing RF MEMS capacitor concept and allow the two device types to be fabricated together. This compatibility issue is crucial if they would be used to construct more complex RF circuits.

At a 50-Ohm inductive reactance point, test results show Q-factors of 17- 55, self-resonant frequencies (SRF) exceeding 11 GHz, and nominal inductances of 0.4- 3 nH for 1-loop inductors and Q-factors of 11- 42, SRFs of 4- 22 GHz, and inductances of 0.8- 5.5 nH for 2-loop inductors. Further, test results reveal that high conductivity metals improve the Q-factors, and that low dielectric-constant substrates increase the SRFs.

In terms of LPFs, measurements show that they demonstrate the expected third-order Chebyshev response. Two nickel Filters on a quartz glass substrate show a 0.6-dB ripple with 3-dB frequencies (f-3dB) of 6.1 GHz and 11.9 GHz respectively. On an alumina substrate, they exhibit a 1.4-dB ripple with f-3dB of 5.4 GHz and 10.6 GHz respectively. The filters are 203- 285 um tall and feature 6- 6.5 um wide capacitance air gaps. These dimensions are different than the original designs and the filter performances were shown to be somewhat sensitive to these discrepancies. Compared to a distributed approach, the lumped-element implementations led to an area reduction of up to 95%.