MEMS tunable capacitors have been fabricated in a thin-film technology for passive integration. Using a dual-gap relay- type design, continuous and reversible ...
RF MEMS TUNABLE CAPACITORS WITH LARGE TUNING RATIO Th.G.S.M Rijkr', J.T.M. van Beek', P.G. Steenekenl, M J E . Ulenaers', J. De Costes andR. Puers2
'Philips Research Eindhoven, The Netherlands 2 Catholic University Leuven, Belgium
,mechanical suspension ,
ABSTRACT
MEMS tunable capacitors have been fabricated in a thin-film technology for passive integration. Using a dual-gap relaytype design, continuous and reversible capacitance tuning with a tuning ratio up to 17 has been demonstrated, while requiring an actuation voltage of only 20 V. A quality factor of 150 to 500 has been measured in the frequency range of 1 to 6 GHz, making these devices very suitable as building blocks in many RF applications. These are the highest tuning ratio and quality factor reported to date for parallel-plate tunable capacitors.
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Figure I : Schematic view of a dual-gap relay-type tunable capacitor.
1. INTRODUCTION Tunable capacitors are important building blocks in many radio-frequency (W) applications such as voltage-controlled oscillators, tunable filters and antennas, and adaptive impedance matching circuits [I-31. Most of these applications require a high quality factor and a wide tuning range. Nowadays semiconductor bipolar varactors are generally applied, using the voltage-dependent capacitance of a reversed pn junction. Although this type of varactors can provide considerable capacitance tuning, they usually suffer from a large series resistance that increases with increasing capacitance tuning. Therefore it is fundamentally impossible to realize a large tuning ratio combined with an appreciable quality (Q) factor [4]. MOS varactors are more promising for RF applications, especially when processed in a silicon-oninsulator technology. A tuning ratio of 6.8, defined as the ratio of the maximum and the minimum capacitance, and a Q factor of IS0 at I GHz have been reported [5]. Considerably higher quality factors can be achieved with tunable capacitors in MEMS (Micro-electromechanical systems) technology, making them suitable for application beyond 1 GHz. A MEMS tunable capacitor in its simplest appearance is an air capacitor with a fixed and a movable electrode. The capacitance is tuned by varying the air gap andor the electrode overlap area by electrostatic actuation. Electrostatic actuation is preferred for its inherent speed and its low power consumption, typically pw's, although this form of actuation requires high voltages, i.e., typically 10-50 V. A wide variety of MEMS tunable capacitors, manufactured by hulk andor surface micro-machining, has been reported in literature [2, 4,6-121. For continuous tuning typical tuning ratios range from 1.35 to 3, depending on the
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design and the choice of materials. The highest tuning ratios have been reported by Tsang et al. [I21 (tuning ratio = 5.3), Xiao et al. [8] (tuning ratio = 6), and Bonvick et al. [ I l l (tuning ratio = 8.4). Tsang et al. report on relay-type tunable capacitors in a thin-film technology based on poly-silicon. The latter two have realized tunable capacitors in a SOI-like approach using hulk micro-machining. All these siliconbased devices generally suffer from a low Q factor and most of them require a large actuation voltage (up to 75 V) andor a large actuation area. High Q factors can he achieved in metal-based MEMS tunable capacitors. In this paper we report on MEMS tunable capacitors fabricated in a thin-film technology for passive integration. It will he shown that a very large tuning ratio can he achieved fogether with a high Q factor, while requiring only a moderate actuation voltage. Due to the high quality factor and low power consumption these MEMS capacitors are very suitable as tuning elements in RF front-ends for hand-held mobile communication devices.
2. DESIGN AND FABRICATION It is well known that the continuous tuning ratio of a conventional parallel-plate MEMS capacitor with electrostatic actuation is fundamentally limited to 1.5, due to the so-called pull-in effect. If the actuation voltage exceeds the pull-in voltage, the air gap reduces to less than 2/3 of the original air gap and the suspended top electrode collapses on the bottom electrode. This pull-in effect can be avoided in a dual-gap relay-type tunable capacitor with an air gap dl between the RF electrodes and d2 between the actuation electrodes. Fig. 1 shows a schematic view of the dual-gap design. If d2 > 3dl continuous tuning of the complete air gap d , can he achieved [ 6 ] . In this case the tuning ratio is determined by the initial air gap d , and the capacitance density that can he achieved upon closing the gap. As the suspended top electrode and its mechanical suspension is not
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passivation Figure 2: Cross-sectional view of the PASSfMprocess. expected to act as an ideally rigid plate, special humps are designed to avoid pull-in at the edges of the structure. As a technological and functional platform for the development of RF MEMS an industrialized low-cost process for passive integration has been used the Philips PASSTMprocess. A process cross-section is shown in Fig. 2. This thin-film technology on high-ohmic silicon combines three metal layers and two dielectric layers in five mask steps to form integrated inductors and capacitors [13, 141. For the realization of RF MEMS, the standard process has heen slightly modified and extended with surface micromachining as a hack-end module. This process results in a very simple and cost effective approach to manufacture RF MEMS switches and variable capacitors together with highquality inductors and fixed capacitors on the same die. In addition the process is fully compatible with the standard semiconductor infrastructure. A schematic cross-section of (part of) a dual-gap relaytype tunable capacitor in PASSTMis shown in Fig. 3. The substrate is high-ohmic silicon (p > 5 kncm) in order to suppress RF losses in the substrate. The bottom electrode consists of 0.5 pm of aluminum (IN). The top electrode, which is used as the structural layer, consists of INT ( 5 Fm of an aluminum alloy) locally combined with INS (0.5 pm of aluminum). The dielectric layers act as sacrificial layers in order to create an air gap of d, = 0.425 Km between INS and IN and d2 = 1.425 pm between WT and IN. The native aluminum oxide, covering the metal layers, is used as a dielectric and avoids shorting of the RF electrodes. This few
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Figure 3: Cross-sectional view of (part o j a dual-gap relay-type tunable capacitor. The native aluminum oxide covers all metal surfaces that are exposed to air.
nanometer thick dielectric layer facilitates a high capacitance density when the top and bottom electrode are in contact. Capacitance densities up to 400 pF/mm2 have been measured in MEMS capacitors with aluminum oxide as a dielectric layer, mainly governed by a roughness of X-10 nm on each of the contacting surfaces [IS]. Fig. 4 shows an example of a dual-gap relay-type tunable capacitor. The suspended electrode is attached to the substrate at four anchor points, through a mechanical suspension that is designed to limit deformation due to the difference in thermal expansion coefficient between aluminum and the silicon substrate. A rigid design of the central part of the suspended electrode, a compact arrangement of the RF electrodes and the separate actuation electrodes, and the anti-pull-in humps assure an efficient transfer of the electrostatic force from the actuation capacitors to the RF capacitor. Holes in the suspended electrode facilitate rapid etching of the sacrificial layers. The tunable capacitor has been designed as a shunt capacitor: the suspended electrode is connected to ground. The actuation voltage is not limited by the breakdown voltage of the thin aluminum oxide layer as there is no DC voltage over the RF capacitor. The capacitor is contacted through a 50 Q conlanar wave guide (CPW).
Figure 4: Top view of a dual-gap relay-type tunable capacitor.
3.
MEASUREMENT RESULTS AND DISCUSSION
Four-probe impedance versus actuation voltage measurements have been carried out at 1 MHz using a HP4275A LCR meter and a Keithley 617 DC voltage source. One-port ground-signal-ground RF scattering parameter measurements between 1 MHz and 6 GHz have heen performed using a HE'8753D Network analyzer. For accurate determination o f the capacitance and Q factor of the tunable capacitor, the capacitance of the CPW has been de-embedded using a CPW of the same length without a MEMS capacitor. Measurements of the capacitance versus the actuation voltage C(v) of three different tunable capacitors are shown in Fig. 5. The right-hand axis shows the tuning ratio C(v)/C(O). The nominal capacitance values are 0.16 pF, 0.22
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Figure 6: Qualify factor versus fiequency of a 0.17 pF tunable capacitor, derived fiom scattering parameter measurements. The solid line is a guide to the eye.
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pull-in effect and evidences fully reversible tuning of the .- capacitance. 0.8 It is shown in Fig. 5 that the capacitance at 0 V is E considerably lower than the designed value. Optical Q 0.6 interferometry reveals a slight upward bending of the 0 suspended electrode resulting in an air gap of the RF a 0.4 capacitor that is approximately twice the designed air gap. Two voltage regimes can be distinguished in the C(V) 0.2 curves: below roughly 11-12 V the capacitance shows a progressive increase as a function of the voltage until the top 04 CO electrode first touches the bottom electrode of the RF capacitor. When further increasing the voltage the effective 0 5 10 15 20 air gap decreases even more, resulting in a further increase of v (V) the capacitance. In this regime the slope of the C(v) curve is determined by the morphology of the contacting surfaces, the 1.8 flatness of the suspended electrode and the details of the 1.6 design. Tuning ratios of 8, 12, and 17 have been measured I.4 for the different sizes of capacitors. At the maximum applied voltage a capacitance density 1.2 of 84 to 152 pF/mm2 has been measured, decreasing with E P 1 increasing electrode area. It is considerably smaller than 0 0.8 expected on the basis of a surface roughness of IO nm, 0.6 measured by atomic force microscopy. Capacitance densities of 84 and 152 pF/mm2 correspond to remaining effective air 0.4 gaps of 105 and 58 nm, respectively. The incomplete closure 0.2 of the air gap could be explained by a slight deformation of 04 $0 the suspended electrode, which has a larger impact on the 0 5 10 15 20 capacitance when the eleceode area is larger. That is why a larger tuning ratio is measured for small capacitors. When v (VI attempting to improve the contact of the RF electrodes by Figure 5: Capacitance and tuning ratio versus actuation applying even higher voltages, pull-in occurs in the actuation voltage of nominally 0.16pF (a), 0.22pF @), and 0.47pF (c) capacitors due to yielding of the top electrode, resulting in tunable capacitors. breakdown of the native aluminum oxide and shorting of the actuation capacitors. pF and 0.47 pF, respectively. The actuation area of the 0.47 Fig. 6 shows the quality factor as a function of pF capacitor is approximately 10 % larger than that of the frequency deduced from a one-port scattering parameter 0.16 and 0.22 pF capacitor. Continuous tuning has been measurement. Large Q factors have been obtained for demonstrated resulting in a maximum tuning ratio of 17.1 at frequencies larger than 1 GHz. Although the MEMS devices an actuation voltage of 20 V. The absence of hysteresis when have been processed on high-ohmic silicon substrates, ramping up and down the voltage reflects the absence of the
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substrate losses cannot be neglected. Capacitive coupling of the RF signal into the lossy substrate limits the Q factor, especially at low frequencies, when the impedance of the MEMS device is small as compared to the parasitic path ’ through the substrate [15, 161. At present, this effect limits the frequency range of application to roughly 1 GHz or higher.
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4. CONCLUSIONS
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’! Metal-based MEMS tunable capacitors have been fabricated in a thin-film process on high-ohmic silicon. Continuous and I reversible tuning with a tuning ratio up to 17 has been demonstrated using a maximum actuation voltage of 20 V. A quality factor between 150 and 500 has been obtained in the frequency range of 1 to 6 GHz. These are the highest tuning ’ ratio and quality factor reported to date for MEMS tunable ’ capacitors. In view of the high quality factor, these MEMS , tunable capacitors are very suitable as building blocks in many RF applications. ~
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ACKNOWLEDGMENT This work has been carried out as p a t of the IST project ‘MEMSZTUNE’.
REFERENCES I
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