Purdue University Purdue e-Pubs Birck and NCN Publications Birck Nanotechnology Center 6-2014 All-Silicon Technology for High-Q Evanescent Mode Cavity Tunable Resonators and Filters Muhammad Shoaib Arif Birck Nanotechnology Center, Purdue University, [email protected] Dimitrios Peroulis Purdue University, Birck Nanotechnology Center, [email protected] Follow this and additional works at: http://docs.lib.purdue.edu/nanopub Part of the Nanoscience and Nanotechnology Commons Arif, Muhammad Shoaib and Peroulis, Dimitrios, "All-Silicon Technology for High-Q Evanescent Mode Cavity Tunable Resonators and Filters" (2014). Birck and NCN Publications. Paper 1638. http://dx.doi.org/10.1109/JMEMS.2013.2281119 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014 727 All-Silicon Technology for High-Q Evanescent Mode Cavity Tunable Resonators and Filters Muhammad Shoaib Arif, Student Member, IEEE, and Dimitrios Peroulis, Member, IEEE Abstract— This paper presents a new fabrication technology relatively large size, difficulty in planar integration, high power and the associated design parameters for realizing compact consumption, need of controlled temperature environment and widely-tunable cavity filters with a high unloaded quality for proper operation, and high cost [4]. The planar or 2-D factor ( Qu) maintained throughout the analog tuning range. This all-silicon technology has been successfully employed to tunable filter technology is another extensively explored demonstrate tunable resonators in mobile form factors in the approach. In these filters, transmission lines are loaded with C, X, Ku, and K bands with simultaneous high unloaded lumped tunable elements (varactors) for frequency tuning. quality factors (≥500) and tuning ratios (≥1:2). It is shown that Some of the notable tunable filter demonstrations are based on by employing high-precision micro-fabrication techniques and solid-state (e.g Schottky or p.i.n diode) [8]–[10], ferroelectric careful modeling, the measured RF and tuning performance of the fabricated device closely match the simulated results. The (e.g Barium-Strontium-Titanate (BST)) [11], [12], and micro- capability of monolithic (system-on-chip) integration, low-cost electromechanical systems (MEMS) [13]–[15] varactors. batch processing, and compatibility with CMOS processing is Planar filters have the advantages of small volume, high tuning some of the key advantages of this 3-D tunable filter fabrication ratio, easy on-chip integration and good power handling. technology over conventional approaches. This technology also However, their RF performance is suboptimal for SDR due has the potential to be extended to produce tunable resonators ≤ and filters in the millimeter wave region. [2013-0123] to their low-Q ( 200) [4], [16]. A third tunable filter family is the non-planar (3-D) Index Terms— Evanescent-mode cavity, MEMS, silicon, approach, which includes cavity-based technologies with inte- tunable resonator, electrostatic, tunable filter, high- Q. grated tuners. This approach is rapidly gaining popularity I. INTRODUCTION due to their significantly small size (compared to wavelength) and weight unlike half-wave cavity resonators, their ability to HE vision of the next-generation software defined radio achieve a higher Qu as compared to planar approach, wide T(SDR) is to have a single reconfigurable radio with tuning, large spurious-free region, high linearity and good multi-band and multi-standard compatibility, that can operate power handling [5], [17]–[26]. Of these 3-D technologies, effectively in dense electromagnetic radiation environments the capacitive-post loaded cavity filter technology has demon- by having the required frequency agility, necessary to com- strated state-of-the-art performance in the S and C-bands with municate using multiple waveforms in rapid succession. The a simultaneous high-Q (≥300) and tuning ratio (≥1:2) [5], SDR concept has been under development for several years [17], [18]. In the capacitive-post loaded filters, the resonant in both military and commercial sector [1], [2] but it is still frequency is highly sensitive to the parallel-plate capacitance far from complete. One main area requiring significant inno- between the post-top and cavity ceiling. By incorporating a vation is the tunable filters for their reconfigurable RF front- MEMS tuner directly above the capacitive post (as a moveable end (RFFE) [3]. The search for a tunable filter technology cavity ceiling), few micrometers of tuner displacement causes that completely fulfills the stringent SDR requirements like several gigahertz shift in the resonant frequency. High-Q . low insertion loss (high-Q), high tuning ratio (T R), easy resonators/filters with an octave (1:2) tuning range or higher integration, high linearity, high power handling, small size and can therefore be realized in both microwave and millimeter low manufacturing cost is ongoing [3]–[5]. wave frequencies. Table I summarizes some simulated designs Several existing tunable filter technologies have been eval- of such capacitive-post loaded resonators with a maintained uated for SDR applications. The yttrium-iron-garnet (YIG) Q ≥ 500 throughout the frequency tuning. ≥ u crystal based tunable filters demonstrate a high-Q ( 1 000) It may be noted from Table I that fabrication tolerances with multiple octave tuning range [6], [7]. However, YIG become critical in realizing repeatable and reliable filters, filters are not well suited for portable devices due to their especially for millimeter wave applications. Existing imple- mentations are based on cavities machined in the metallic Manuscript received April 23, 2013; revised August 13, 2013; accepted September 1, 2013. Date of publication September 20, 2013; date of current or ceramic substrates using conventional tooling techniques, version May 29, 2014. This work was supported by DARPA under the and their tuners are attached using solder, physical pressure Purdue Microwave Reconfigurable Evanescent-Mode Cavity Filters Study. or conductive epoxy [5], [17], [18], [27]. A conventionally Subject Editor D. Elata. The authors are with the School of Electrical and Computer Engineering, machined cavity suffers from high fabrication uncertainties Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907 (≥±20 μm) which inhibits reliable scaling of capacitive- USA (e-mail: [email protected]; [email protected]). post loaded filter technology beyond X-band (a capacitive- Color versions of one or more of the figures in this paper are available ≤ μ online at http://ieeexplore.ieee.org. post radius of 100 m is needed with a precision of Digital Object Identifier 10.1109/JMEMS.2013.2281119 ≤±10 μm). Moreover, the uncertainty due to surface 1057-7157 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 728 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014 TABLE I TUNABLE CAPACITIVE-POST LOADED RESONATOR DESIGN EXAMPLES FOR MICROWAVE AND MILLIMETER WAV E roughness in the metallic or ceramic cavity substrates, and the uniformity issues inherent with the solder paste or conductive epoxy attachment like uneven application, shrinkage, and irregular grain size can cause substantial error in the desired post-ceiling gap (>2 μm) [5], [18], [27]. These uncertainties tend to create a significant disparity between the designed and fabricated performance. For instance, in the designs of Table I, a1-μm error in dmin due to epoxy attachment can lead to >15% reduction in the tuning ratio. Consequently, a fundamentally different fabrication tech- nology with tighter tolerances is needed for high-frequency applications. This requirement becomes the premise for all-silicon capacitive-post loaded tunable filter technology based on micro-fabrication. Several topologies for high-Q fixed-frequency (static) resonators and filters using micro- machined silicon cavities have already been demonstrated in the microwave and millimeter-wave regions [28]–[33]. Simi- larly, single-crystal-silicon has also been extensively explored as a choice material for MEMS actuators due to its superior mechanical properties [5], [34]–[37]. The all-silicon tunable filter technology aims to successfully exploit both the RF and mechanical advantages of silicon. Two noticeable performance outcomes demonstrated using this approach include 1) an X to Ku band capacitive-post loaded cavity tunable resonator with a tuning range of 9.5 to 17 GHz (1:1.87) and Qu of 650 to 950 using magnetostatic actuation [38], and 2) a C to K-band Fig. 1. (a) The assembly of three individually micro-fabricated constituents electrostatically tunable resonator demonstrating continuous to form an electrostatically-tuned all-silicon capacitive-post loaded cavity resonator. (b) Schematics and working principle. Tuning is achieved by tuning from 6.1 to 24.4 GHz (1:4) with a Qu varying from applying bias voltage to the electrode, causing a change in capacitive gap 300 to 1000 [39], which is state-of-the-art in the tunable filter between the cavity post and the Au/Si diaphragm due to electrostatic actuation. technology for simultaneous high-Q and wide tuning range. This paper details the groundwork, design methodology and the fabrication considerations involved in the implementation