A 6 to 24 GHz Continuously Tunable, Microfabricated, High-Q Cavity Resonator With Electrostatic MEMS Actuation Muhammad Shoaib Arif and Dimitrios Peroulis Birck Nanotechnology Center, School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA. Abstract—An all-silicon capacitive-post loaded cavity tunable resonator, continuously tunable from C to K frequency bands is presented for the first time. All parts of the resonator are fabricated using silicon microfabrication techniques. The presented device is tuned electrostatically from 6.1 to 24.4 GHz (4:1 tuning range) with a measured unloaded quality factor (Q u ) from 300-1,000. The resonator includes an evanescent capacitive- post section with a thin film of low-loss dielectric (Parylene-N) as a spacer between the optically smooth, gold coated silicon post-top and single-crystal-silicon diaphragm to create sub- micrometer initial gaps. Consequently, the resonator achieves a capacitance density of 47 pF/mm 2 at its lowest frequency. This is 5.6× higher than the state-of-the-art tunable filter designs. It is considered that precision and tight tolerances associated with the microfabrication techniques hold the key in achieving high- Q, widely tunable filters. To the best of the authors’ knowledge, this is the widest tuning high-Q resonator presented today. Index Terms—Evanescent-mode cavity, tunable filter, res- onator, silicon, microfabrication, MEMS, electrostatic actuation. I. I NTRODUCTION Widely tunable filters with low insertion loss (high-Q u ) are essential components in realizing next generation software defined radios (SDR) and reconfigurable RF-front ends [1]. Evanescent-mode (loaded) cavity based tunable resonators and filters are receiving widespread interest in microwave community due to their small size (compared to their half- wave counterparts), high quality factors, and their potential for wide tuning range using integrated micro-electromechanical systems (MEMS) [2]. Several tunable filter technologies have successfully been demonstrated based on this approach or its variations [1]–[5]. For a capacitive-post loaded cavity, the tuning range is proportional to the ratio of parallel-plate capacitances be- tween the post and moveable ceiling at its minimum and maximum actuation gaps [2]. Due to the 1/x relationship between capacitance and gap, higher capacitance density at the minimum actuation gap yields higher tuning range for same or comparable actuation strokes. For this reason, smallest possible initial gaps have been targeted in the high tuning ratio (≥ 2:1) designs like [1] and [4]. In the work of both Leo et al. [1] and Moon et al. [4], the minimum gap is restricted by the surface roughness (∼2 µm) of the post tip, limiting their maximum capacitance density values to 4.43 pF/mm 2 and 8.4 pF/mm 2 respectively as these cavities were fabricated in a ceramic polymer composite substrate using conventional machining techniques. In [5], the authors of this work have shown that by using silicon as the fabrication Fig. 1. (a) Conceptual design of widely tunable, coaxial-pin-fed all-silicon capacitive-post loaded cavity resonator, tuned using electrostatic actuation. (b) Constituent parts of the assembled resonator. (c) Close-up image of the post tip showing Parylene-N spacer film between the post top and moveable diaphragm, a key element in achieving wide tuning range. material and employing well characterized microfabrication techniques, this gap can be controlled with higher precision than the conventional machining. Using these microfabrication techniques, this work demonstrates the first 4:1 tunable cavity resonator, with a maximum capacitance density of 47 pF/mm 2 (at 0.5 µm minimum gap) which is 5.6× higher than the state- of-the-art. II. DESIGN AND MODELING The proposed resonator’s design schematic is presented in Fig. 1(a). A capacitive-post loaded cavity resonator, excited by a feed pin, is tuned by electrostatic actuation of a gold-coated silicon-on-insulator (SOI) diaphragm. The resonator assembly consists of four distinct parts as shown in Fig. 1(b): 1) A metallic fixture that houses an SSMA connector for pin feed. 2) A gold-coated silicon wet-etched cavity with capacitive post and a via at its base for pin feed. 3) An SOI substrate with a gold-coated silicon diaphragm in the center. A patch of Parylene-N film deposited on diaphragm gold as spacer film. 4) A bias electrode with a post made from silicon wet-etching. The post fits in the recess of the SOI handle layer and its height determines the initial gap between the diaphragm and electrode. The electrode is electrically isolated from the SOI U.S. Government work not protected by U.S. copyright