Fabrication process changes for performance improvement of an RF MEMS resonator : conformable contact lithography, Moiré alignment, and chlorine dry etching

Abstract

This thesis presents fabrication process improvements for a RF MEMS resonator for the purpose of improving the quality factor (Q) and extending the frequency range. The process changes include the use of conformable contact lithography (CCL) and chlorine-based dry etching for improved fine-feature patterning and Moiré -based alignment techniques to allow for a non-self-aligned process. The resulting control over feature size and structure are expected to improve Q and enable higher frequency resonators. A CCL process utilizing moir6 alignment marks is described. An automated Moiré -based alignment system using Labview software is presented which demonstrates sub-100 nm alignment accuracy for a single alignment mark. A full-wafer alignment experiment is described that demonstrates average pattern placement errors of ... for the x- and y-directions respectively. The experimental limitations are analyzed and suggested improvements to the system are detailed. Chlorine dry etching experiments are conducted in order to produce a straight sidewall etch through the "stack" of resonator materials (chrome, aluminum nitride, and molybdenum). A combination of Cl₂/0₂, Cl₂/Ar, and CF₄0₂ plasmas at low pressure (2 mTorr), high microwave/source power (500W), and a moderate DC bias (-150V) demonstrates a straight sidewall angle (>80⁰ measured from horizontal) with no undercut for all layers of the stack. RF resonators fabricated with these process modifications are presented. An average overlay error of 55 nm (110 nm min-max) is recorded for 11 devices located closest to the line between the alignment marks in an aligned release of the resonators. The design modifications enabled by the new process are described and the prospect for higher-frequency devices and higher-Q device performance is discussed.