All-Silicon Technology for High-Q Evanescent Mode Cavity Tunable

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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]

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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]Ð[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]Ð[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]Ð[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]Ð[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 technology 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 micromachined silicon cavities have already been demonstrated in the microwave and millimeter-wave regions [28]Ð[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]Ð[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 of the tunable resonator. Fig. 1(a) and (b) show the concept of capacitive-post loaded tunable cavity filters using all-silicon schematics and working principle of an electrostatically actu- approach. In the design section, simple analytical expressions ated all-silicon resonator respectively. The resonator is realized are presented to predict and optimize the Qu , tuning range by combining three individually microfabricated components: and biasing voltage. The fabrication section explains the steps 1) A gold-coated cavity with post etched in a bulk silicon needed to produce high quality tunable filters and to minimize substrate, 2) a gold-coated single-crystal-silicon diaphragm disparity between the simulated and measured performance. released by selective etching of a silicon-on-insulator (SOI) Some advantages of the all-silicon fabrication technology like substrate, 3) a micromachined silicon electrode post coated accuracy, high reliability, compatibility with the growing on- with a dielectric layer for electric isolation. The diaphragm chip RF technology and low-cost are briefly discussed to SOI is attached to the cavity through gold-gold thermo- emphasize its potential from commercial viewpoint. Finally, compression bonding using a gold bonding ring on the SOI notable performances of some tunable resonators fabricated substrate [38]. The thickness of this bonding ring defines the and demonstrated to date are summarized and compared with initial gap between the post-top and the moveable ceiling. some existing tunable microwave filter technologies. Tuning is achieved through electrostatic actuation of the diaphragm, when a bias voltage is applied to the Si electrode II. ALL-SILICON TECHNOLOGY OVERVIEW housed in the etched recess of the SOI handle. The depth of In the presented all-silicon approach, silicon is used as the the electrode post is kept lower than the SOI handle layer substrate material for fabricating all constituent components thickness, to create the initial biasing gap. This bias gap ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH-Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS 729

(SiNx ) layer. The silicon wet-etching mask is patterned in the nitride layer by selectively etching it in SF6 plasma using a photo-resist (PR) mask. The cavity and post of desired dimensions are formed using a well-characterized silicon wet- etching process. The nitride layer is stripped and the cavity is sputtered with a gold ﬁlm to provide uniform metal coverage to the sidewalls and the post.

B. Diaphragm Fabrication The Au/Si diaphragm fabrication begins with an SOI wafer of appropriate device layer thickness. First, a gold film is deposited on the SOI device layer. Then by using a PR mold, a Au bonding ring is created over the existing gold film using electroplating or lift-off process. The thickness of the bonding ring is defined by the initial post-ceiling gap. Backside lithography is carried out to pattern the PR mask, required for creating the diaphragm by etching away the SOI handle substrate using Deep Reactive Ion Etching (DRIE). The buried oxide of SOI acts as the etch-stop layer for DRIE. Finally, the oxide layer is removed using Buffered Oxide Etch (BOE) to release a smooth and flat gold-coated SCS diaphragm. Major diaphragm fabrication steps are shown in Fig. 2(b).

C. Bias Electrode Fabrication In order to get high precision in the electrode/ diaphragm gap for electrostatic actuation, the bias electrode is also microfabricated. The electrode fabrication process steps are similar to cavity wet-etching process and are shown in Fig. 2(c). A Si post of appropriate height and diameter is wet-etched in the Si substrate using a (SiNx ) mask. The nitride ﬁlm is removed and the bias electrode is uniformly coated with an insulation ﬁlm like PR to provide electrical isolation.

D. Assembly After their fabrication, the cavity and tuner parts are thor- oughly solvent-cleaned and UV-treated to remove any debris or organic contamination on the surface. They are subsequently aligned and bonded using Au-Au thermo-compression bonding carried out under high temperature (∼310 ¡C) and pressure (∼5 MPa). The assembly of these micro-machined parts to Fig. 2. Major fabrication steps for the constituent parts of the proposed all- form a single resonator is shown in Fig. 1(a). silicon resonator. (a) Si cavity with post, (b) Au coated single-crystal-silicon diaphragm and (c) Si bias electrode. III. A CAPACITIVE-POST LOADED EVANESCENT-MODE CAV I T Y RESONATOR DESIGN is determined from the desired filter tuning range through Craven [40] proposed that a cutoff guide (which can be electromechanical analysis of the diaphragm deflection. approximated as a lumped inductance), when appropriately Although the fabrication steps are customized based on loaded with capacitive obstacles at suitable intervals, acts as a specific designs as in [38] and [39], some important fabrication waveguide equivalent of a lumped element circuit filter. The steps which represent the core of this all-silicon approach are capacitive-post loaded evanescent-mode cavity resonators and given below. filters are designed on the same principle. The advantages include considerable size and weight reduction compared to a half-wave resonator with the same resonant frequency and a A. Cavity Fabrication large spurious-free region [28]. Fig. 3 shows the cross-section As shown in Fig. 2(a), the fabrication starts with a cavity of a typical capacitive-post loaded cylindrical cavity operating substrate Si wafer coated with a film of LPCVD silicon nitride in evanescent mode. The important design parameters are 730 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014

Fig. 3. Concept schematics of a capacitive post loaded cavity resonator showing important dimensions and reactive elements of the cavity.

Fig. 5. Field comparison for a cylindrical and micromachined Si cavity: (a) Electric ﬁeld, mainly concentrated at the post, (b) Magnetic ﬁeld, circling around the post.

Fig. 4. Change in frequency and unloaded quality factor as a function of post gap. capacitive post radius (a), cavity radius (b), capacitive post height (h) and gap (d) between the post-top and cavity ceiling. The resonant frequency ( f0) can then be approximated by [32], [41] 1 f0 ≈ (1) 2π L0(C0 + C ff + C pp) Fig. 6. Frequency vs. gap plot for different shaped cavities of similar πa2 C = 0 (2) dimensions. pp d where L0 and C0 are the fixed equivalent cavity inductance and capacitance respectively. C pp and C ff are the parallel-plate cavity can be analytically predicted. The unique shape of the and fringing-field capacitances between the post and cavity micro-machined Si cavity and post, due to the anisotropic Si ceiling. When the post-height approaches the cavity-height wet-etching, makes it difficult to relate the Qu and resonant (highly loaded) such that C pp dominates other capacitances, frequency with cavity dimensions analytically. Therefore, the gap d becomes the main resonant frequency determining FEM based solvers like Ansys HFSS¨ [42] are initially used parameter, as shown in Fig. 4. even for preliminary design parameters. However, a com- Based on the fabrication technology, various cavity and parison of the simulated electric and magnetic field patterns post shapes have been explored such as cylindrical [5], [18], of a cylindrical to pyramidal cavity of similar dimensions square [17] and pyramidal [31], [32], [38], [39]. From the (a, b, h and d) show strong similarities (refer Fig. 5). As a design viewpoint, a cylindrical cavity and post is relatively result, the resonant frequency and Qu of these cavities are easy to model as it can be closely approximated as a small also found similar as shown in Fig. 6 and Fig. 7 respectively. segment of coaxial transmission line shorted at one end, and Hence, the analytical expressions obtained from cylindrical a series capacitor at the other [5]. Using this analogy the cavity analysis can provide a good starting point for our all- resonant frequency, tuning range and Qu for a cylindrical silicon cavity resonator designs. ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH-Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS 731

Fig. 7. Qu vs. frequency plot for different shaped cavities of similar dimensions.

A. Resonant Frequency and Tuning Ratio The LC-tank model from (2) can be expanded to relate the resonant frequency with cavity dimensions (a, b and h)as follows: 1 f0 ≈ (3) 2π L0(C parasitic + C pp) where

C parasitic = C0 + C ff (4) μ h L ≈ 0 ln(b/a) (5) 0 2π 2hπ0 C ≈ Fig. 8. Simulated resonant frequency and tuning dependance on cavity 0 ( / ) (6) ln b a dimensions for different frequency ranges. The Qu is ≥400 for all designs. C ff ≈ K1a0ln(h/d) (7) (a) post radius, (b) cavity radius, and (c) cavity height.

C parasitic is a capacitance that does not contribute to the sensitivity of resonant frequency f0 with change in gap d. other two dimensions because a directly impacts the dominant K1 is an empirically fitted constant which is sensitive to parallel-plate capacitance, whereas b and h affect the parasitic / / ≤ . a b ratio. For a b 0 3, this constant has been found to capacitance. For millimeter-wave applications, a capacitive be 2.78 [43]. The tuning ratio of the resonator can then be post of very small radius (10s of micrometers) with high computed by precision is needed. This constraint plays a major role in the choice of suitable fabrication technology for high frequency + π 2/ f C parasitic 0 a dmin T.R = max ≈ (8) applications. A lower parasitic capacitance compared to par- fmin 2 allel plate capacitance results in higher tuning range. C parasitic + 0πa /dmax 2) Minimum Gap (dmin): The parallel plate capacitance where dmin and dmax are the minimum and maximum gaps C pp, which is the dominant capacitance for tuning purposes, between post-top and the cavity ceiling respectively, achieved have an inverse relationship with d. Therefore, for the same through ceiling deflection. From (3) and (8), the f0 and T.R ceiling deflection, a higher tuning ratio can be achieved of the resonator depend on 1) cavity shape and dimensions, through lower dmin. 2) minimum gap dmin, and 3) ceiling deflection. 3) Ceiling Deflection: A higher actuation stroke corre- 1) Cavity Shape and Dimensions: The effects of a, b and sponds to better tuning ratio for the same dmin. The resonator h on the resonant frequency are highlighted in Fig. 8(a), frequency f0 tends to saturate as the post-ceiling gap increases (b) and (c) respectively. Reducing any of these dimensions, such that C pp approaches C parasitic. The ceiling deflections corresponds to lowering the overall capacitance and there- beyond this point has negligible impact on the tuning ratio. The fore causes the resonant frequency to increase. The resonant extent of ceiling deflection is determined by the the actuation frequency is more sensitive to the post radius a than the mechanism and the stiffness of the ceiling. The stiffness k 732 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014

Fig. 9. Electrostatic actuation schematics of Au/Si diaphragm when voltage is applied to the bias electrode. for a circular diaphragm of radius R and thickness t under uniform loading can be calculated by [9]: 3 16π Et k = k + k = + 4πσt (9) 3R2(1 − γ 2) where E is the Young’s modulus, γ is the Poisson’s ratio and σ is the residual stress in the diaphragm. The k component of stiffness depends upon material properties and dimensions, whereas k depends mainly upon the fabrication technology and is generally the dominant stiffness contributing mechanism due to difficulty in fabricating low-stress diaphragms. In case of a Au/Si diaphragm, which is extensively used in this work, the Au thickness tAu and residual stress σAu are the dominant stiffness contributors due to the stress-free nature of crystalline silicon [5]. Therefore the k part of the stiffness in (9) is generally several times higher than k and we can approximate the diaphragm stiffness to be k ≈ k = 4πσAutAu (10) Electrostatic actuation is the most widely used MEMS actuation scheme due to its near-zero power consumption, design- simplicity and high-speed [9]. Fig. 9 shows the schematic of an electrostatically actuated Au/Si bi-morph diaphragm. When a voltage V is applied to the bias electrode, the electrostatic force pulls the diaphragm towards the electrode by a displacement x. The diaphragm stiffness k creates a reactionary restoring force proportional to this displacement. At equilibrium, the electrostatic and restoring forces balance each other i.e. A V 2 kx = bias 0 2 (11) 2(g0 − x) Fig. 10. Bias voltage vs. deflection for different parameters of Au/Si diaphragm when g0 is 45 μm. (a) Residual stress in Au film, (b) Au film where Abias is the bias electrode area and g0 is the initial gap thickness, (c) diaphragm radius, and (d) SOI device layer thickness. between the bias electrode and the un-deflected diaphragm. Using this parallel-plate actuation of diaphragm, a continuous B. Quality Factor deflection can be achieved for only one-third of the actuation The unloaded quality factor of a cylindrical evanescent- gap (g0/3), followed by the “pull-in” instability. Therefore, for analog resonator designs, the initial gap between the mode cavity can be analytically approximated using the fol- lowing equation [44]: diaphragm and bias electrode should be, at least, three times the required diaphragm deflection to achieve the desired tuning R ( 1 + 1 ) 1 ≈ s a b + 2 range. The bias voltage needed to achieve the required deflec- b (12) Qu 2πμf0 ln( ) h tion therefore depends mainly upon k, A and g0.From(9) a and (11), the Au/Si diaphragm parameters σAu , tAu, R and tSi where Rs is the sheet resistance of the cavity sidewalls. should be carefully considered in the design process to deter- From (12), the Qu of the resonator at a particular frequency mine the bias voltage for the required diaphragm deflection as is mainly dependant on: 1) cavity dimensions and 2) sheet showin in Fig. 10. resistance. ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH-Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS 733

Fig. 11. Qu variation with the change in cavity radius b, for two different Fig. 12. Simulated Qu variation with cavity height h at 15 and 22 GHz. post heights at 5 GHz.

1) Cavity Dimensions: Two general rules can be employed 1) Dimensional Accuracy: As can be seen from Fig. 8, for to maximize the quality factor of the resonator on the basis millimeter wave applications the capacitive post dimensions of cavity dimensions, provided it does not affect the desired are in the order of 10s of micrometers, and a small deviation tuning range: 1) increasing b or h results in higher quality in post size can cause a major shift in resonant frequency factor because it allows more volume for energy to be stored and tuning performance. The resolution of existing machining in a resonator, as shown in Fig. 11 and Fig. 12 respectively, as techniques is inadequate for these tight tolerances. Silicon long as b and h are much smaller than the resonant wavelength, wet micro-machining, when carried out in a well charac- 2) when the post radius a is approximately 0.28× the cavity terized, temperature and concentration controlled etch-bath, radius b, which can be mathematically validated from (12). can provide the precise dimension control necessary for this application. High-precision characterization tools like laser 2) Sheet Resistance: Qu is inversely proportional to the resistance offered by the cavity walls to the RF current which confocal microscope or profilometer are required to accurately is approximated by: monitor to dimensions. In our work, laser confocal microscope LEXTTM from Olympus¨ is used for the etch characterization ρ 1 which has the capability to measure cavity dimensions with Rs = ≈ (13) t σδ sub-micrometer precision [45]. The etch-depth (Fig. 13(a)) and 1 δ = √ (14) post-area (Fig. 13(b)) is measured at regular intervals and πμf0σ plotted against etch time (Fig. 13(c)), to extract the lateral and vertical etch rates. These etch rates can then be used for where ρ and σ are the cavity metal resistivity and conductivity fabricating cavities with post of desired dimensions with high respectively. The metal thickness t can be replaced by the precision. skin depth δ, provided t δ. Therefore highly conductive metals like Cu, Au and Ag are desirable for high-Q resonator 2) Sidewall Roughness: In transmission lines, RF losses fabrication. increase with surface roughness and are especially pronounced at higher frequencies when the roughness is comparable to skin-depth [46], [47]. Similarly, since the RF current concen- IV. FABRICATION TECHNOLOGY trates to the “skin” of the cavity sidewalls and post, the surface The important considerations in determining the suitabil- roughness increases the sheet resistance of the cavity and ity of all-silicon technology for realizing capacitive-post degrades the Qu. To estimate the impact of roughness on Qu loaded tunable microwave filters include precision, reliability, at different frequencies in a capacitive-post loaded resonator, compatibility and cost. we have used HFSS simulations and found a Qu degradation trend with increased roughness, as expected. The impact of roughness on Qu is higher at high frequencies as shown in A. Precision Fig. 14. Since the purpose of these simulations is to measure A successful fabrication technology should be accurate the trend not the actual values, the roughness is approximated and consistent. In order to realize high frequency capacitive- by uniformly spaced ridges in the cavity base, sidewalls, and post loaded tunable filters, precision is mainly needed in post as shown in the inset of Fig. 14. When the cavities are 1) dimensions, 2) surface profile (roughness), and 3) integra- fabricated using conventional machining, the tooling process tion of constituent parts. leaves its tracks and irregular marks on the cavity sidewalls and 734 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014

Fig. 13. Cavity with post wet-etch characterization using laser confocal microscope. (a) Step height measurement with side wall proﬁle, (b) superim- posed images of post-area as it reduces with etching time, showing symmetric etching around the post. (c) Plot of etch depth and post area vs. etch time.

Fig. 15. Surface, sidewalls and capacitive-post proﬁles from a conventionally machined cylindrical cavity in bulk copper (Top) and anisotropic wet-etched Si cavity (Bottom). Both cavities are 6-mm wide with a post-top diameter of 200-μm.

silicon, cavity/ posts sidewalls with mirror-like surface ﬁnish and sub-micrometer roughness can be obtained. Fig. 15 shows sidewall roughness comparison of a conventionally machined cavity with post in bulk copper to a micro-machined Si cavity of similar dimension. 3) Cavity-Ceiling Attachment: High conductivity, uniformity and accuracy are three essential requirements in the cavity-ceiling integration. A highly conductive attachment will offer less resistance to the RF current whereas a uniform Fig. 14. Simulated effect of surface roughness on the resonator Qu at different frequencies. (Inset) Roughness is approximated in HFSS by attachment will result in symmetric current distribution. Both introducing uniformly spaced ridges of roughness height and width. of these factors impact the resonator Qu. In the ceramic cavities, the cavity-tuner attachment is done using conductive silver epoxy [5], [18] with an approximate conductivity of 6 post, which are extremely difﬁcult to polish in small cavities 2 × 10 S/m [48]. The Qu of a resonator with Cu or Au as (few millimeters) with a fragile rough post in their center. the cavity metal suffers a substantial degradation (more than Depending upon the equipment precision, this roughness can 30% simulated for Au vs. epoxy) in Qu due to this bonding be in the order of 2Ð20 μm. Using the wet chemical etching of metal conductivity variation (Fig. 16). Another disadvantage ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH-Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS 735

Fig. 18. Fabricated parts of 15.5 GHz static resonator (a) cavity with post, (b) static ceiling with feed lines and electroplated gold bonding ring.

Fig. 16. Impact of bonding material conductivity on the resonator Qu.

Fig. 19. Simulated and measured performance of the all-silicon static ﬁlter designed for 15.5 GHz. The black dashed curve shows the post measurement simulation to match the measured resonant frequency for Qu and d0 comparison.

7 thermo-compression bonding (σAu ≈ 4.1 × 10 S/m). Au-Au thermo-compression bonding has been successfully employed in several micromachined static resonators and ﬁlters [28], [29], [31], [32], and [49]. The role of a bonding ring in this all- silicon approach is very important in accurately establishing the initial-gap (d0). The thickness of the bonding ring can be Fig. 17. (a) Fabrication uncertainty in d0 and C pp due to cavity and ceiling accurately controlled using well-characterized metal deposi- surface roughness and ﬂatness. All imperfections assumed on cavity side for tion methods commonly used in microfabrication, like e-beam simplicity. (a) Imperfect cavity-ceiling attachment due to surface roughness evaporation, sputtering and electroplating. only, (b) post roughness and (c) non-parallel mating surfaces.

B. Reliability of epoxy based cavity-ceiling attachment with rough cavity The reliability of a contact based MEMS (DC and capacitive surfaces, is the high-uncertainty in determining the post-ceiling switches) has been challenged by problems like stiction, gap (d0) and parallel-plate capacitance (C pp). This is explained fatigue and creep and dielectric charging [9]. Special tech- in Fig. 17. In an all-silicon approach, we eliminate the issue niques like use of novel contact material, modified actuation of roughness of mating surfaces by exploiting the smoothness waveform and proprietary fabrication processes have been and flatness of the Si wafers which are received polished and developed to mitigate these failure mechanisms and currently leveled to sub-micrometer accuracy at the foundries. With Au several switch technologies have reliably demonstrated over as the cavity metal, several precise and conductive attachment a billion switching cycles [50]Ð[53]. Contact-less MEMS are methods are available including the highly conductive Au-Au preferred in the tunable filter applications where analog tuning 736 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014

Fig. 22. The measured tuning response (S11) of a weakly coupled, electro- Fig. 20. The measured tuning response (S21) of a weakly coupled, magnetostatically tuned all-silicon resonator designed for 12−24 GHz applications. statically tuned all-silicon resonator designed for 8−18 GHz applications [38]. The inset shows the schematics of the measured device. The inset shows the schematics of the measured device.

Fig. 21. Fabricated parts and assembly of electrostatic all-silicon tunable resonator. range is desired. In contact-less MEMS, creep and hysteresis are the two main failure mechanisms that can reduce the device repeatability and reliability [5]. The piezoelectric actuation Fig. 23. Tuning enhancement from 12−24 GHz (1:2) to 6.1−24.4 GHz (1:4) of a copper clad lead-zirconate-titanate (PZT) diaphragm as for the same resonator design by using Parylene-N dielectric spacer ﬁlm [39]. used in [18] suffer from substantial hysteresis. Whereas electrostatic actuation of gold coated single-crystal-silicon (SCS) substrate [38]. In addition, coaxial pin feed for ﬁlters has diaphragm used in this work has been demonstrated to be also been successfully investigated and demonstrated in this almost creep-free with no hysteresis in the analog tuning all-silicon approach [39], to show the compatibility of this range [5]. technology with the modular RF designs.

C. Ease of Integration D. Cost The monolithic integration of the RF components minimizes All-silicon filters can be reliably and economically produced fabrication complexity and interconnect losses [28], [29], using standard microfabrication equipment and processes. [32], [49], and [54]. Silicon has become the most highly Unlike conventional filter fabrication methods which required used RF substrate with the popularity of CMOS RFICs, individual machining and assembling of each filter, it is a therefore the compatibility of filter fabrication with CMOS wafer-level batch fabrication approach which is fast, consistent processes is highly desirable. The all-silicon filter technology and cost-effective. has this unique edge over the previous cavity based tunable filter approaches that it allows system-on-chip integration with V. P ERFORMANCE ANALYSIS CMOS and SOI CMOS RFICs. Two variations of planar We summarize the results of three different all-silicon res- integration have been demonstrated where the RF feed can onator designs, to demonstrate the versatility of this approach either be on the SOI diaphragm (Fig. 1) or on the cavity along with its performance. ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH-Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS 737

Fig. 24. All-Si tunable resonators performance comparison with other tunable resonator/ﬁlter technologies [5], [14], [17], [18], [22], [23], [38], [39], [56]Ð[58]. The comparison is based on the highest Qu reported at their highest f0 and the tuning ratio.

A. 15.5 GHz Static Resonator magnetostatically actuated all-silicon resonator. The resonator A proof-of-concept resonator is fabricated to check the fea- tuned from 9.5 to 17 GHz with the Qu of 650Ð950 when the ± sibility of our all-silicon approach for making high frequency coil current was changed between 70 mA. capacitive-post loaded cavity filters with high accuracy and − desired performance. The resonator is designed for a fixed C. 12 24 GHz Resonator With Electrostatic Actuation resonant frequency of 15.5 GHz with a Qu of 780, when the The electrostatic actuation of a Au/Si diaphragm is post-ceiling gap is fixed to 10-μm. It is excited with a planar employed for a 12Ð24 GHz all-silicon resonator. A diaphragm short-ended CPW feed on the cavity ceiling, that enters the deflection from 2Ð15 μm from the post is required to achieve cavity via feed-through slots etched on cavity top sidewalls. the desired tuning range, with a simulated Qu of 600Ð1350. The fabricated cavity and ceiling parts are shown in Fig. 18. The cavity is excited using a coaxial pin inserted through The measured device demonstrated a Qu of 700 at 15.71 GHz a via etched in Si cavity bottom. The fabricated parts and which corresponds to 90% of the simulated Qu with ≤2% resonator assembly is shown in Fig. 21. From the measured deviation from targeted frequency, as shown in Fig. 19. The reflection (S11) parameters presented in Fig. 22, the resonator extracted d0 from simulations is 10.6 μm which corresponds continuously tuned from 12.3 to 23.7 GHz when the bias to ≤1 μm fabrication uncertainty in dmin. voltage is ramped to 370 V. A further increase in voltage resulted in a discrete frequency jump to 25 GHz due to electrostatic pull-in effect. An unloaded quality factor of − B. 8 18 GHz Resonator With Magnetostatic Actuation 510Ð1020 is extracted through data post-processing using In [38], the all-silicon tunable filter technology is demon- reflection method [55]. strated for the first time using magnetostatic MEMS actuation. 1) Tuning Range Enhancement: The tuning range of this The cavity and diaphragm are fabricated and assembled using electrostatic is enhanced by introducing a thin (0.5 μm) low- the procedure described earlier in section II. However, to loss Parylene-N (r = 2.65, tan δ = 0.0006 at 1 MHz) implement magnetostatic actuation of the diaphragm, magnetic spacer film between the post and the diaphragm as demon- field interactions between a PCB mounted current carrying strated in [39]. This Parylene-N film accurately allows sub- coil and a small permanent magnet attached to the back of micrometer gaps between the cavity and the post, not easily the diaphragm are exploited. Using bi-directional magnetosta- possible with an air-gap. Moreover, the high relative permit- tic actuation, the diaphragm is designed to actuate between tivity of Parylene-N further enhances the capacitive density at 5Ð40 μm from the post to achieve 8 to 18 GHz tuning with small deflections. With this modified approach, the fabricated a Qu of 950Ð1500. The resonator is fed by an open-ended resonator demonstrated a tuning range from 6.1 to 24.4 GHz microstrip line patterned at the back of the cavity. The field (see Fig. 23) with a Qu of 300Ð1000. To the best of authors’ is coupled through a rectangular slot etched inside the cavity knowledge, this is the highest reported tuning ratio (1:4) to gold. A coplanar waveguide (CPW) to microstrip transition is date in the MEMS based tunable filter technology. incorporated to perform on-wafer probing. Fig. 20 shows the A comparison has been made between the existing tunable measured transmission (S21) parameters of a weakly coupled, filter technologies with the all-silicon approach, based on the 738 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014 reported performance in terms of highest operating frequency, [7] Teledyne Microwave Solutions. [Online]. Available: unloaded quality factor and tuning ratio in Fig. 24. It can be http://www.teledynemicrowave.com [8] Y.-H. Shu, J. Navarro, and K. Chang, “Electronically switch- seen that the performance of all-silicon resonators is superior able and tunable coplanar waveguide-slotline band-pass filters,” to all other approaches demonstrated to date. IEEE Trans. Microw. Theory Tech., vol. 39, no. 3, pp. 548Ð554, Mar. 1991. [9] G. Rebeiz, RF MEMS, Theory, Design and Technology.NewYork,NY, VI. CONCLUSION USA: Wiley, 2003. [10] K. Entesari and G. M. Rebeiz, “RF MEMS, BST, and GaAs varactor The capacitive-post loaded 3-D filter technology has great system-level response in complex modulation systems: Research Arti- potential for realizing compact, high-Q (≥500) and widely cles,” Int. J. RF Microw. Comput.-Aided Eng., vol. 18, no. 1, pp. 86Ð98, tunable (≥1:2) filters suitable for microwave and millimeter Jan. 2008. [11] G. Subramanyam, A. Zaman, N. Mohsina, F. Van Keuls, F. Miranda, and wave SDR applications. However, this performance is strongly R. Romanofsky, “A narrow-band ferroelectric tunable bandpass filter dependant on the fabrication technology due to tight design for K-band applications,” in Proc. Asia-Pacific Microw. Conf., 2000, tolerances. Conventional fabrication methods used for real- pp. 938Ð941. [12] J. Nath, D. Ghosh, J.-P. Maria, A. Kingon, W. Fathelbab, P. Franzon, and izing capacitive-post loaded filters have suboptimal precision M. Steer, “An electronically tunable microstrip bandpass filter using thin- which impedes reliable scaling of these filters beyond X-band. film Barium-Strontium-Titanate (BST) varactors,” IEEE Trans. Microw. The presented all-silicon technology utilizes microfabrica- Theory Tech., vol. 53, no. 9, pp. 2707Ð2712, Sep. 2005. [13] J. Zou, C. Liu, J. Schutt-Aine, J. Chen, and S.-M. Kang, “Development tion tools and techniques to achieve the desired accuracy. of a wide tuning range MEMS tunable capacitor for wireless commu- This paper focused on design, optimization and fabrication nication systems,” in IEDM Tech. Dig., Dec. 2000, pp. 403Ð406. techniques for the all-silicon capacitive-post loaded cavity [14] K. Entesari and G. Rebeiz, “A 12Ð18-GHz three-pole RF MEMS tunable filter,” IEEE Trans. Microw. Theory Tech., vol. 53, no. 8, pp. 2566Ð2571, resonators to simultaneously achieve high-Q and tuning ratio. Aug. 2005. Three different RF feed variations are presented to show the [15] C. Nordquist, C. Goldsmith, C. Dyck, G. Kraus, P. Finnegan, F. Austin, diversity in the all-silicon approach. Some notable technology and C. Sullivan, “X-band RF MEMS tuned combline filter,” Electron. Lett., vol. 41, no. 2, pp. 76Ð77, Jan. 2005. demonstrations include 1) a 9.5Ð17 GHz (1:1.87) magnetosta- [16] D. Pozar, Microwave Engineering. New York, NY, USA: Wiley, 2004. tically tunable all-silicon resonator with a Qu of 650 to 950, [17] H. Joshi, H. Sigmarsson, D. Peroulis, and W. Chappell, “Highly and 2) an electrostatically tunable 12.3Ð23.7 GHz (1:1.93) res- loaded evanescent cavities for widely tunable high-Q filters,” in Proc. IEEE/MTT-S Int. Microw. Symp., Jun. 2007, pp. 2133Ð2136. onator with a Qu of 510 to 1020. The tuning ratio of the elec- [18] S. Moon, H. Sigmarsson, H. Joshi, and W. Chappell, “Substrate inte- trostatically actuated resonator has been doubled to 6.1 to 24.4 grated evanescent-mode cavity filter with a 3.5 to 1 tuning ratio,” IEEE GHz (1:4) by introducing a low-loss dielectric film between Microw. Wireless Compon. Lett., vol. 20, no. 8, pp. 450Ð452, Aug. 2010. [19] X. Liu, L. Katehi, W. Chappell, and D. Peroulis, “A 3.4Ð6.2 GHz the post and diaphragm to achieve a sub-micrometer capacitive continuously tunable electrostatic MEMS resonator with quality factor gap. This two octave (C to K-band) tunable resonator with of 460Ð530,” in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2009, a high-Q of 300 to 1000 is state-of-the-art in the present pp. 1149Ð1152. [20] X. Liu, L. Katehi, W. Chappell, and D. Peroulis, “Power handling capa- tunable filter technology. With this supreme RF performance bility of high-Q evanescent-mode RF MEMS resonators with flexible and advantages like high reliability, compatibility with the on- diaphragm,” in Proc. APMC, Dec. 2009, pp. 194Ð197. chip RF technology and a low cost batch production, it is [21] W. Yan and R. Mansour, “Micromachined millimeter-wave ridge waveguide filter with embedded MEMS tuning elements,” in IEEE MTT- believed that the all-silicon filters will have a significant contri- S Int. Microw. Symp. Dig., 2006, pp. 1290Ð1293. bution in the realization of next-generation reconfigurable RF [22] W. D. Yan and R. Mansour, “Tunable dielectric resonator bandpass filter front-ends. with embedded MEMS tuning elements,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 1, pp. 154Ð160, Jan. 2007. [23] S. J. Park, I. Reines, C. Patel, and G. Rebeiz, “High-Q RF-MEMS ACKNOWLEDGMENT 4Ð6 GHz tunable evanescent-mode cavity filter,” IEEE Trans. Microw. Theory Tech., vol. 58, no. 2, pp. 381Ð389, Feb. 2010. The authors would like to thank N. Ragunathan, [24] R. Stefanini, M. Chatras, A. Pothier, J. C. Orlianges, and P. Blondy, Dr. H. Sigmarsson, and Dr. X. Liu for their technical help. 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National University of Science and Technology, Asia Pacific Microw. Conf., Dec. 2009, pp. 575Ð578. Islamabad, Pakistan, in 1999, and the master’s [38] M. Arif, W. Irshad, X. Liu, W. Chappell, and D. Peroulis, “A high- degree in electrical and computer engineering from Purdue University, West Lafayette, IN, USA, in Q magnetostatically-tunable all-silicon evanescent cavity resonator,” in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2011, pp. 1Ð4. 2009. [39] M. S. Arif and D. Peroulis, “A 6 to 24 GHz continuously tunable, micro- He is currently pursuing the Ph.D. degree in elec- fabricated, high-Q cavity resonator with electrostatic MEMS actuation,” trical and computer engineering at Purdue Univer- in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2012, pp. 1Ð3. sity. His current research interests include novel RF Q [40] G. Craven, C. K. Mok, and R. F. Skedd, “Integrated microwave systems MEMS devices and micro-machined high- tunable employing evanescent mode waveguide components,” in Proc. 1st Eur. filters for reconfigurable RF front-ends. Microw. Conf., Sep. 1969, pp. 285Ð289. [41] X. Gong, W. Chappell, and L. P. B. Katehi, “Reduced size capacitive defect EBG resonators,” in IEEE MTT-S Int. Microw. Symp. Dig.,vol.2. Jun. 2002, pp. 1091Ð1094. [42] ANSYS Inc. [Online]. Available: http://ansys.com Dimitrios Peroulis (S’99–M’04) received the Ph.D. [43] X. Liu, “High-Q RF-MEMS tunable resonators and filters for recon- degree in electrical engineering from the University figurable radio frequency front-ends,” Ph.D. dissertation, Dept. Electr. of Michigan, Ann Arbor, MI, USA, in 2003. He Eng., Purdue Univ., West Lafayette, IN, USA, Jul. 2010. has been with Purdue University, West Lafayette, [44] H. Joshi, “Multiband RF bandpass filter design,” Ph.D. dissertation, IN, USA, since August 2003, where he is currently School Electr. Comput. Eng., Purdue Univ., West Lafayette, IN, USA, leading a group of graduate students on a variety May 2010. of research projects in the areas of RF MEMS, [45] Olympus Corporation. [Online]. Available: http://www.olympus- sensing, and power harvesting applications, as well global.com as RF identification (RFID) sensors for the health [46] G. Brist, S. Hall, and T. Liang, “Non-classical conductor losses due to monitoring of sensitive equipment. He has been a copper foil roughness and treatment,” in Proc. Conf. APEX IPC Printed Principle Investigator (PI) or a co-PI in numerous Circuits Expo. Designers Summit, Anaheim, CA, USA, Feb. 2005, projects funded by government agencies and industry in these areas. He pp. 1Ð11. is currently a Key Contributor in two Defense Advanced Research Project [47] A. Deutsch, C. Surovic, R. Krabbenhoft, G. Kopcsay, and B. Chamber- Agency (DARPA) projects at Purdue, which focus on very high quality lin, “Prediction of losses caused by roughness of metallization in printed- (Q ≥ 1000) RF tunable filters in mobile form factors (DARPA Analog circuit boards,” IEEE Trans. Adv. Packag., vol. 30, no. 2, pp. 279Ð287, Spectral Processing Program, Phases I, II, and III) and on developing May 2007. comprehensive characterization methods and models for understanding the [48] Creative Materials Inc. [Online]. Available: http://www. viscoelasticity/creep phenomena in high-power RF MEMS devices (DARPA creativematerials.com M/NEMS ST Fundamentals Program, Phases I and II). Furthermore, he leads [49] L. Harle and L. Katehi, “A vertically integrated micromachined filter,” the experimental program on the Center for the Prediction of Reliability, IEEE Trans. Microw. Theory Tech., vol. 50, no. 9, pp. 2063Ð2068, Integrity, and Survivability of Microsystems (PRISM) funded by the National Sep. 2002. Nuclear Security Administration. In addition, he heads the development of [50] R. Johnson. Omron Boosts MEMS Medical Sensor Production the MEMS technology in a U.S. Navy project funded under the Technology [Online]. Available: http://www.eetimes.com/news/latest/showArticle. Insertion Program for Savings program focused on harsh-environment wireless jhtml?articleID=221601199 microsensors for the health monitoring of aircraft engines. He has authored [51] WiSpry [Online]. Available: http://www.wispry.com or coauthored more than 110 refereed journal and conference publications in [52] MEMtronics [Online]. Available: http://www.memtronics.com the areas of microwave integrated circuits and antennas. [53] Radant MEMS [Online]. Available: http://www.radantmems.com Dr. Peroulis was the recipient of the 2008 National Science Foundation [54] C. Tavernier, R. Henderson, and J. Papapolymerou, “A reduced-size CAREER Award. His students have been the recipients of numerous Student silicon micromachined high-Q resonator at 5.7 GHz,” IEEE Trans. Paper Awards and other student research-based scholarships. He has been Microw. Theory Tech., vol. 50, no. 10, pp. 2305Ð2314, Oct. 2002. the recipient of Eight Teaching Awards including the 2010 HKN C. Holmes [55] D. Kajfez, Q Factor Measurements Using MATLAB. Norwood, MA, MacDonald Outstanding Teaching Award and the 2010 Charles B. Murphy USA: Artech House, 2011. Award, which is Purdue University’s highest undergraduate teaching honor.