PROC 24th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2004). VOL I,NI<. SERBIA AND MONTENEGRO, 16-19 MAY, 2004

MEMS Tunable Capacitors and Switches for RF Applications Th.G.S.M. Rijks, J.T.M. van Beek, P.G. Steeneken, M.J.E. Ulenaers, P. van Eerd, J.M.J. Den Toonder, A.R. van Dijken, J. De Coster, R. Puers, J.W. Weekamp, J.M. Scheer. A. Jourdain, and H.A.C. Tilmans - RF MEMS capacitive switches and tunable capacitors have been realized in an industrialized thin-film process developed for manufacturing high-quality inductors and capacitors. Combining integrated passives with high-perfoimance tuning and switching elements on the same die offers a potential for building a new generation of RF front-ends for hand-held mobile communication. Capacitive switches with an insenion loss of 0.4 dB and an isolation of 17 dB at I GHz have been demonstrated. Dual-gap relay type tunable capacitors have been fabricated that show a contimiour and reversible tuning ratio of 12 together with a quality factor larger than 150 at frequencies higher than 0.5 GHz. These are the highest tuning ratio and quality factor reported to date. A 0-level packaging concept that is compatible with the fabrication technology has been adopted.

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I. INTRODUCTION Fig. 1: Dual-band RF transmit module as a system-in-.-package: the multilayer laminate substrate (10x8 mm’) with interconnect and embedded passives carries active IC’s, passive IC’s (indicated in black squares), and SMD components on top [3].

In RF transceiver front-ends for wireless mobile communication accurate and high-quality passive circuits are of prime importance, especially in the transmit section, where high power levels are handled. The efficiency of the transmit circuitry, consisting of a power amplifier, impedance matching networks, harmonics filters, and switches, is to a large extent determined by the performance of the passives (including the switches) that constitute most of these circuits. Besides a high efficiency, resulting in a low power consumption, small size and low cost are important drivers in the market of RF modules for mobile communication terminals. A vely promising route of integrating high-quality passives in a cost-effective manner is using passive integration in a system-in-a-package approach [1,2]. In this approach the passives are integrated on a chip using a dedicated technology, which is then combined with the active IC’s in a modular fashion. In this way, technologies

and processes can easily be mixed and matched leading to an optimum of performance, cost, and size of the RF module. This approach is illustrated in Fig. 1, showing an example of a RF transmit module [3]. The multilayer laminate substrate provides interconnect and embedded inductors, and acts as a mechanical carrier for the active IC’s, passive IC’s, and surface mount device (SMD) components. The passive dies, indicated in the black squares, are manufactured in the Philips PASSIT” process and mounted by flip-chip assembly [4]. A next step to bring down module size and cost would be implementing a reconfigurable front-end in which adaptive components like tunableiswitchable capacitors and switches enable parts of (mainly passive) circuits to be used for more than one frequency band. In addition, these adaptive networks can be used to actively optimize the efficiency of the front-end as a function of the transmit power and the antenna impedance. RF MEMS variable capacitors and switches are very promising as tuning or switching elements due to their superior characteristics in terms of insertion loss, power consumption, and linearity a s compared to their semiconductor counterparts. They offer the opportunity to realize adaptive RF circuits such as, for example, adaptive impedance matching circuits, tunable antennas, and VCO

Th.G.S.M. Rijks, J.T.M. van Bsek, P.G. Stccnekcn, M.J.E. Ulcnaers, P. van Eerd, J.M.J. den Toonder, and A.R. van Dijkcn are with Philips Research, Prof. Holstlaan 4, 5656 AA Eindhoven, the Netherlands, E-mail: [email protected] J. De Coster and R. h e r s are with the Depanment of Electrical Enginccnng (ESAT-MICAS), Catholic University Leuven, Belgium J.W. Weekamp and J.M. Schcer arc with the Philips Centre of Industrial Technology, Eindhovcn, The Nctherlands A. Jourdain and H A C . Tiimans arc with Interuniversity Microelectronics Centcr (IMEC), Leuven, Belgium 0-7803-8166-1/04/$17.000 2004 IEEE

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, mechanical sumension

aluminum

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.

tunable C (d,) Figure 4: Schematic view of a dual-gap relay-type tunable capacitor. The actuation capacitors with a large air gap are separated from the F S capacitor with a small air gap. Bumps at the edges of the structllre prevent pull-in of the actuation capacitors

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passivation Fig. 2 : Cross-sectional view ofthe PASSTMprocess tank circuits where low losses and high linearity are key issues [2,5]. In this paper we present electrostatically actuated MEMS tunable and switchable capacitors as part of a passive integration strategy. It will be shown that large tuning and switching ratios can he achieved together with a high quality factor. Combining these with high quality inductors and fixed capacitors on the same die offers a potential for building a new generation of RF front-ends for hand-held mobile communication.

11. DESIGN AND FABRICATION As a technological and functional platform for the development of RF MEMS an industrialized low-cost process for passive integration is used: the Philips' PASSIT" process [1,4]. This thin-film technology on highohmic silicon combines three metal layers and two dielectric layers in only five mask steps to form highquality integrated inductors and capacitors. A process cross-section is shown in Fig. 2. For the realization of RF MEMS, the standard process is slightly modified and extended with surface micro-machining as a hack-end module. The resulting process is a very simple and costeffective approach to inanufacture RF MEMS capacitive switches and tunable capacitors together with high-quality inductors and fixed capacitors on the same die [6]. Additionally, the process is fully compatible with the

Fig. 3: Ci-oss-sectionalview of a capacitive switch in PASSTh' technology, using two of the available metal layers. The native aluminum oxide (black) covers a11 metal surfaces that are exposed to air.

standard semiconductor infrastructure. A cross-section of a RF MEMS capacitive switch in P A S S T Mis shown in Fig. 3, using two of the available metal layers. 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 aluminum (IN). The top electrode, which is used as the structural layer, consists of 5 pm of an aluminum alloy (INT). This alloy has been selected for its high hardness and low creep [7]. Both the silicon nitride and silicon oxide layer act as sacrificial layers to create an air gap of 1.4 pin between the top and bottom electrode. The native aluminum oxide, covering the metal layers, is used as a dielectric and avoids shorting of the RF electrodes. This thin dielectric layer facilitates a high capacitance density when the top and bottom electrode are in contact. In fact. a capacitance density up to 400 pF/mrn2 has been measured in MEMS capacitors with aluminum oxide as a dielectric layer. The aluminum oxide provides electrical isolation between the electrodes without limiting the capacitance density as this is found to be governed by the surface roughness on each of the contacting surfaces. Electrostatic actuation is preferred for its inherent speed and its low energy consumption, typically I nJ per switching cycle. A drawback is the high actuation voltage. typically in the range 5-SO V. Another drawback in conventional parallel plate MEMS capacitors is the maximum tuning ratio of 1.5 due to the so-called pull-in

Fig. 5 : Cross-sectional view of a dual-gap relay-type tunable capacitor in P A N T Mtechnology. All three metal layers are used to realize two different air paps. The native aluminum oxide un the metal surface is indicated in black.

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effect [XI. The tuning ratio is defined as C(V)/C(O), the ratio between the capacitance C a t actuation voltage V and the capacitance at an actuation voltage of 0 V. Pull-in occurs if the displacement of the suspended electrode exceeds 113 of the air gap at 0 V. If.the actuation voltage exceeds the pull-in voltage the suspended top electrode collapses on the bottom electrode and the air gap reduces to zero. This pull-in effect can be avoided in a dual-gap relaytype tunable capacitor, schematically shown in Fig. 4 [9]. In the cross-section it is shown that the actuation capacitors are separated from the RF capacitor. There is a small air gap d, between the RF electrodes and a large air gap d2 between the actuation electrodes. If d2 > 3 4 continuous tuning of the complete air gap d, can be achieved. The tuning ratio is determined by the capacitance density in the up state (at 0 V) and the capacitance density in the down state. As the suspended top electrode and its mechanical suspension are not expected to act a s an ideally rigid plate, special bumps are designed to avoid pull-in at the edges of the structure. Dual-gap relay-type tunable capacitors can he realized in the PASSITMprocess by utilizing all three metal layers. A cross-section of a dual-gap relay-type tunable capacitor, according to the design of Fig. 4, is shown in Fig. 5. Now, the top electrode and structural layer consists of INT locally combined with INS (0.5 pm of aluminum). Removal of the silicon nitride between INS and IN creates the sinall air gap of d, = 0.4 p i n By removing both sacrificial layers between INT and IN again creates the large air gap of d2= I .4 pm.

111. MEASUREMENT RESULTS AND DISCUSSION A . Cupocitive switches Fig. 6 shows a scanning electron microscopy (SEM) photograph of an RF MEMS capacitive switch, of the type shown in Fig. 3. The mechanical suspension of the switch

Fig 6: SEM photograph of a capacitive switch of the ’ypc shown in Fig. 3..

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Fig. 7: Measured capacitance versus actuation voltage of a 325x325 pm2 capacitive switch similar to the switch shown in Fig. 6 . the arrows indicate the ramping direction of the actuation voltage. has been designed to minimize the deformation due to the difference in thermal expansion coefficient between the aluminum and the silicon substrate [IO]. Rapid etching of the sacrificial layer is facilitated by the access holes in the top electrode. The switch has been designed as a shunt switch as part of a 50 0 coplanar wave guide (CPW). A DC voltage between top and bottom electrode, superimposed on the RF signal, is used to change the height of the air gap, therewith changing the effective capacitance. As explained in the previous section, this design is not attractive for application as a tunable capacitor, due to the pull-in phenomenon. If the acttiation voltage exceeds the pull-in voltage, the MEMS capacitor can be used as a capacitive switch (or switched capacitor). Experimental results of the capacitance versus the actuation voltage of a MEMS capacitor, with a siniilar design as in Fig. 6 , are shown in Fig. 7. The capacitance has been derived from a four-point impedance nieasurement at 1 MHz using a HP4275A LCR meter. The substrate capacitance at 1 MHz has been de-embedded by using the results of a measurement of the scatteriny parameters at 1 GHz [5]. The capacitance of the CPW has been de-embedded by using the scattering parameters at 1 GHz of a CPW of the same length, but without a MEMS switch. The electrode area of the switch is 325x325 pti?. The pull-in voltage is 10.6 V and is determined by the electrode area, the initial air gap, and the spring constant O S the suspension springs. The release voltage is around I \I. The arrows indicate the sweep direction of the actuation voltage. The different curves for sweeping the actuation voltage upward and downward are well-known for parallelplate capacitors and are caused by the fact that the electrostatic force is inversely quadratic in the gap height whereas the spring force scales linearly with the deflection. The capacitance measured at 0 V (up state) is 0.71 pF, the down-state the capacitance is 16.7 pF resulting i n a capacitance density of 164 pF1mm’. The switching ratio, defined as the quotient o f t h e capacitance in the down

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switch are shown, as measured with a HP8753D vector network analyzer. When the capacitor is in the up state (at 0 V), the switch is electrically closed. An insertion loss of 0.4 and 0.95 dB at 1 and 2 GHz, respectively, has been measured, corresponding to an up-state capacitance of thcMEMS switch of 1.2 pF. In the down state (at 7 V), the switch is electrically open showing an isolation of 17 and 23 dB at 1 and 2 GHz, respectively. This corresponds to a down-state capacitance of 40.5 pF and a down-state capacitance density of 288 pF/mm2. The switching ratio is 33.8. Capacitive RF switches reported in literature usually have been designed for frequencies exceeding 5 to IO GHz [ll-131. They generally show a very low insertion loss: mainly governed by the CPW losses, but a poor isolation below 5 GHz. However, in terms of the down-state capacitance density and switching ratio they are similar to the devices presented here, i.e., 230 pF/mm2 and 94, using an initial air gap of 3.6 pm [12], and 260 pF/mm’ and 38 for an initial air gap of 1.5 pm [13].

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Fig. 8: Insertion loss (dark) and isolation (light) of a 375x375 pin’ capacitive switch versus frequency. and the up state, is 23.5. It is demonstrated here that the native aluminum oxide can be a stable dielectric for aluininuin based MEMS capacitors. It has been found that the native aluminum oxide breaks down at a DC voltage of 10-12 V. Down-state capacitance densities up to 400 pF/mm2 have been measured in this type of devices. This can be recalculated to a remaining air gap of 22 nm between the electrodes in the down state. With atomicforce microscopy an average roughness of 8-10 nm has been ~neasuredon each of the contacting surfaces, which can account for an effective air gap of 16-20 nm. Therefore, the capacitance density in the down state (and therewith the switching ratio) is determined by the surface roughness of the electrodes and is not limited by the dielectric properties of the aluminum oxide. In addition, if the suspended top electrode is not completely flat, larger effective air gaps might occur. In the device of Fig. 7, the down-state capacitance density of 164 pF/mm2 corresponds IO an effective air gap of 54 nm. In Fig. 8 the insertion loss (dark line) and isolation (light line) of a 375x375 pm2 MEMS capacitive shunt

B. firnoble capacitors

Fig. 9 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 as a result of thermal stressing. A rigid design of the central part of the suspended electrode, a compact arrangement of the RF electrodes and the separate actuation electrodes, and antipull-in bumps assure an efficient transfer o f the electrostatic force from the actuation capacitors to the RF capacitor. Fig. 10 shows in a close-up the two different gaps of0.4 and 1.4 pm in the RF and actuation region. The tunable capacitor ha5 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 CPW.

Fig. 10: SEM picture showing the W O different gaps in the dualgap hinable capacitor.

Fig. 9: Top view of a dual-gap relay-type tunable capacitor of the type shown in Figs. 3 and 4

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Fig. 1 2 Height profile in the up state, measured an optical profiler, along the central axis of the suspended eiccrrodr of capacitors A, B, and C. The dashed line represents Ihe hcipht profile in case of a perfectly flat electrode. The structure in the center of the beam is related to the etching holes and tlie overlap

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(W Fig. I I : Capacitance and tuning ratio versus acruation voltage of nominally (a) 0.09 (capacitor A), (b) 0.16 (capacitor B), and (c) 0.47 pF (capacitorC) tunable capacitors. V

Four-point impedance versus actuation voltage ineasnreinents have been carried out at 1 MHz using a HP4275A LCR meter. One-port ground-signal-ground RF scattering parameter measurements between 1 MHz and 6 GHz have been performed using a HP8753D network analyzer. For accurate determination of the capacitance and qiiality (Q) factor of the tunable capacitor, the substrate capacitance and the capacitance of the CPW have been deembedded. Measurements of the capacitance versus the actuation voltage C(V) for three different sizes of tunable capacitors are shown in Fig. 11. The right-hand axis shows the tuning ratio. In the following the capacitors will be referred to as devices A, B, and C. The designed capacitance values in

53

the up state are 0.09 (device A), 0.16 (device B), and 0.47 pF (device C). Capacitance tuning has been demonstrated resulting in maximum tuning ratios in the range of 8.1 to 17.1 at a maximum actuation voltage between 18 and 20 V. It is shown in Fig. 11 that the up-state capacitance is considerably lower than the designed value. The height profile of the suspended electrode has been measured along its central axis using a Wyko optical profiler. The results, shown in Fig. 12, reveal an upward displacement and bending of the electrode. From these data the actual air gap d , (see Fig. 4) can he calculated. For devices B and C the air gap d, in the up state is around 1.4 and 1.2 pin, respectively. This is at least three times larger than the designed air gap and acconnts for the measured capacitance values. Due to the bending, the air gap d2 of the actuation capacitors depends on the position along the beam. Fig. 12 evidences that the design of the mechanical suspension of the top electrode insufficiently compensates for the lhermal mismatch between the aluminum beam and the silicon substrate. Therefore, deformation of the top eleclrode can occur during processing at elevated temperatures. Device A originates from a different wafer, and shows a considerably smaller deformation, resulting in d , = 0.7 pm. The behavior as a function of the voltage of devices A, B, and C is very different from the expected characteristic of the tuning ratio, as shown in Fig. 13. The displacement x of the suspended electrode as a fiinctiou of the actuation voltage V has been calculated by solving Eq. 1, which describes the equilibriiiin between the electrostatic force and the restoring spring force. &,AV' kx= (1) 2(d, -x)'

In this equation

GO is the permittivity in free space and A is the actuation area. The tuning ratio is calculated from _ C ( V )%AI (2) c(0) ( d , - .r(V)) '

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z

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0.2

0.4 0.6 Normalized Voltage

0.8

0

1

1

2

3 4 Frequency (GHz)

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Fig. 14: Quality factor versus frequency of device A, at 0 V (light curve) and I8 V (dark curve).

Fig. 13: Calculated hlning ratio versus the normalized actuation voltage, based on d,ld, = 3.5 and an up-state and downstate capacitance densities of 20.8 and 400 pFlmm', respectively.

in which A I is the area of the RF capacitor. In Fig. 13 the actuation voltage has been normalized by a factor d((Skd:)/(27~+A)), corresponding to the pull-in voltage. The devices have been designed with d2/dl= 3.5 so pull-in does not occur. The maximum tuning ratio is around 19, based on the expected up-state and down-state capacitance densities of 20.8 and 400 pF/mm'. The differences between the calculated and measured characteristics will be explained in t e r m of the deviations in air gaps and the deformation of the suspended electrode. Two voltage regimes can be distinguished in the C(Q cttrves of Fig. 11. Below roughly 6 V (device A) and 1 I V (devices B and C) the capacitance increases slowly with increasing voltage. In the up state the average air gap d2 of the achtation capacitors is smaller than 3dl due to the drfonnation of the suspended electrode. Nevertheless, the bending of the beam causes the anti-pull-in humps to touch-down first and prevent pull-in a t this stage. After the bumps have landed on the bottom the beam flattens when increasing the voltage, thereby decreasing the air gaps. The ratio of d, and d' in this state determines whether pull-in occurs. The average ratios dJdI of the devices A, B, and C are 3.7 (l.Ipin/O.3pm), 2.2 (1.3ptd0.6pm), and 2.4 (I .2 pmi0.5 pm), respectively. Therefore, devices B and C potentially show a pull-in effect. The capacitance increases progressively as the air gap becomes smaller. The second voltage regime starts when the actuation voltage exceeds 6 V (device A) or 11 V (devices B and C). This transition voltage, indicated in fig. I I by the dashed line, is higher for devices B and C than for device A due to their larger air gaps (see Fig. 12). In this regime the centre part of the suspended electrode touches down. When further increasing the voltage the effective air gap still decreases resulting in an almost linear increase of the capacitance. In this regime the slope of the C(V) curve is believed to he mainly determined by the flatness of the suspended electrode. From the C(Q cnrves of Fig. 11 the effective air gap at the transition voltage is aroitnd 0.1, 0.1, and 0.2 pm for devices A, 8, and C, respectively. The large

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effective air gap of device C points at a large deformation of the centre part of the electrode. This actually prevents pull-in. Device B, however, exhibits a pull-in effect. The resulting hysteresis in the C O curve of Fig. 1 I(b) is very small. Using d2/dl= 2.2 it can he calculated that the pull-in voltage and the release voltage differ only 5 % for this ratio of air gaps [ 141. At the maximum applied voltage, capacitance densities of 212, 152, and 84 pF/mm2 have been measured for devices A, B, and C. This is considerably smaller than expected on the hasis of a surface roughness of 10 ntn. These capacitance densities correspond to remaining effective air gaps o f 42, 58, and 105 nm. The incomplete closure of the air gap is ascribed to the deformation of the suspended electrode. When attempting to improve the contact of the RF electrodes by applying even higher voltages, pull-in occurs in the actuation capacitors due to downward bending of the top electrode. This results in breakdown of the native aluminum oxide and shorting of the actuation capacitors. More compact designs with a more efficient transfer the actuation force to the RF capacitor or better control over the deformation of the suspended electrode could result in a higher down-state capacitance density. Very high maximum tuning ratios of 12, 17, and 8 have been measured for the devices under investigation. These values are boosted by the low capacitance density in the up-state as a result of the deformation of the suspended electrode. Nevertheless, only very little or no hysteresis has been observed resulting in reversible tuning of the capacitance. In addition, the high capacitance density that can be achieved when using the native aluminum oxide as a dielectric strongly contributes to the high tuning ratios. Fig. 14 shows the Q factor as a function of the frequency deduced from a one-port scattering parameter measurement on device A. Again, the reactive part of the connecting CPW has been de-embedded. Q factors larger than 150 have been obtained for frequencies above 0.5 GHz. 'Increasing the achiation voltage from 0 to 18 V results in a decrease in Q factor from 600 to 250 at 2 GHz, due to the increasing capacitance.

in a silicon-on-insulator technology [17]. Again, there is a trade-off between tuning ratio and Q factor. A tuning ratio of 6.8 has been reported together with a maximum Q(l GHz) of 50 at the minimum capacitance of 1.8 pF. On the other hand, a tuning ratio of 2.6 together with a maximum Q(1 GHz) of 1 I O at the minimum capacitance of 1.8 pF have been reported . A wide variety of MEMS tunable capacitors, manufactured by bulk and/or surface micro-machining, has heen reported in literature [9, 16, 18-25]. For continuous tuning typical hining ratios range from 1.35 to 3, depending on the design and the choice of materials. The highest tuning ratios have been reported by Tsang e/ a/. [24] (hining ratio = 5.3), Xiao et al. [20] (tuning ratio = 6), and Borwick e/ al. [23] (tuning ratio = 8.4). Tsang et al. report on relay-type tunable capacitors in a thin-film technology based on poly-silicon. The latter hvo have realized tunable capacitors in a SOI-like approach using bulk micromachining. All these silicon-based devices generally suffer from a low Q factor and most of them require a largr actuation voltage (up to 75 V) a n d o r a large actuation area. We have demonstrated here that in metal-based MEMS tunable capacitors a tuning ratio of 12 or more can be achieved together with a high Q Factor, while requiring only a moderate actuation voltage.

IV. PACKAGING

Fig. 15: Illustration of the IRS technique for hermetic MEMS packaging. (a) A cap with a solderring is aligned and prebonded to the device wafer. Through an indent in the soldcrring the cavity is evacuated and subsequently filled with any gas. (b) Through reflowing the solder the cavity is sealed hermetically.

MEMS switches and tunable capacitors require 0-level encapsulation to protect their fragile moving parts during wafer separation, assembly, and final use. As also moisture is considered to be detrimental for the device performance, hermetic sealing of the device cavity is preferred. A method for fabrication of hermitically sealed cavities has been described in [26]. Using the so-called ‘Indent Reflow Sealing’ (IRS) technique, illustrated in Fig. 15, a glass or silicon cap is placed on top of the MEMS device using a reflow soldering process. A metal ring is layed out around the MEMS device. A similar ring is processed on the cap with a solder layer on top (Fig. I5(a)). Next, the cap is

Although the MEMS devices have been processed on high-ohmic silicon substrates, substrate losses cannot be neglected. Capacitive coupling of the RF signal into the lossy substrate results in an increased equivalent series resistance, especially at low frequencies when the impedance of the MEMS device is large compared to the parasitic path through the substrate [3, IS]. This limits the Q factor below typically 2 GHz. When the capacitance of the tunable capacitor increases, the substrate losses become less dominant. That is why the drop in Q factor with increasing capacitance (between 0 and 18 V) is much lower than the factor of 12 expected on the basis of the tuning ratio. This is especially evident in the low frequency range. However, the substrate parasitics litnit the frequency range of application of these devices to roughly 0.5 GHz or higher. For comparison, nowadays semiconductor bipolar varactors are generally applied as tunable capacitors, 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 Q factor [16]. MOS varactors are more promising for RF applications, especially when processed

Fig. 16: Example of a MEMS capacitive switch with 0-level encapsulation using a 100 pm AF45 glass cap.

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ESSCIRC 2003, Estoril, Pormeal, - Seat. 16-18 2003, 26,2003. [6]J.T.M. van Beek er a/., MRS Synip. Proc. Vu1 783, paper B3.1.1., MRS Fall Meeting, Dec. 1-5, Boston, 2003 [7] J.M.1. den Toondcr er il., to be publishcd, MRS Spr-ing A4eefing April 12-16 2004, San Francisco. [8] G.M. Rebeiz, RF MEMS: Theory Design, and Technoloxy, New Jersey: John Wiley & Sons, 2003. [9] J. Zou, C.Liu, J. Schutt-Aine, J.H. Chen, and S.M. Kang, IEDMZOOO Tech. Dig.,403,2000. [IOIH. Nieminnen, T. Ryhanen, V. Ermolov, S . Silanto, US potent 110.6557413B2. [II]H.A.C. Tilmans, H. Ziad, H. lansen, 0. Di Monaco, A. Jourdain, W. De Raedt, X. Rottenberg, E. De Backer, A . Decaussemaker, and K. Baert, Proc. IEDM 2001, 921-924, Washington DC, Dec. 3-5,2001. [I21 C.L. Goldsmith, Z. Yao, S. Eshelman, and D. Denniston; IEEE Microwwe and Guided Wave Lerr. 8, 269-271, 1998. [I31 J.B. Muldavin and G.M. Rebeiz, IEEE Trans. On Microii~ave Theory and Techn. 48, 1045-1052, 2000. (I41 See e.g. H.A.C. Tilmans, EUROSENSORS XVI, 16"' European Conference on Solid-Srare Trmsducers, Sept. 1518 2002, Prague, Czech Republic, 1-34, 2002. [I5]A.B.M. Jansman, I.T.M. van Beek, M.H.W.M. van Delden, A.L.A.M. Kemmeren, A. den Dekker, and F.P. Widdershaven, Digest ESSDERC 2003, Estoril Portugal. Sept. 16-18, 2003. 1161J.J. Yao, J Micron?ech.Microeng. IO, R9, 2000. [17]K. Shen, F. P. S. Hui, W.M.Y. Wong, Z. Chen, J. Lau, P.C.H. Chan, and P.K. KO, IEEE Trans. Elec. De". 48, 289, 2001. [ 181 A. Dec and K. Suyama, IEEE Trans. Microwave Theoiy uud Techn. 46,2587, 1998. [I91 H. Nieminen, V. Ermolov, K. Nybergh, S. Silanto, and T. Ryhsnen, J. Micrornech. Microeng. 12, 177, 2002. [201Z. Xiao, W. Peng, R.F. Wolffenbuttel, and K.R. Farmer. Dipex1

aligned and pre-bonded to the device wafer. An indent in the solder ring makes it possible to evacuate the cavity and, if desired, fill it with any gas, e.g., diy nitrogen, prior to reflowing the solder. After reflow, the cavity that contains the MEMS device i s sealed hernietically (Fig. 15(b)). Fig. 16 shows an example of a packaged capacitive switch using an AF45 glass cap with a thickness of 100 pm. The switch is connected to the outside through RF underpasses. The solder pads around the device are meant for I-level assembly using solder balls. A more detailed description of the packaging and the performance of the packaged components will be published elsewhere [27].

CONCLUSIONS We have demonstrated that RF MEMS capacitive switches and tunable capacitors can he manufactured in a thin-film process that integrates high-quality inductors and capacitors on high-ohmic silicon (PAWTM).Combining integrated passives with high-performance tuning and switching elements offers a potential for realizing miniaturized reconfigurable RF frontends. A high switching ratio and a high capacitance density in the down state yields switches with a typical insertion loss of 0.4 d B and an isolation of I 7 d B at 1 GHz. Dunlgap relay-type tunable capacitors have been fabricated that show continuous and reversible tuning with a tuning ratio up to 12 together with a quality factor as high as 150 for frequencies higher than 0.5 GHz. These are the highest tuning ratio and quality factor reported to date for MEMS tunable capacitors. A 0-level packaging concept that is compatible with the fabrication technology has been adopted.

Proc. Solid Slate Sensor, Acruuto,: and Mkmzyx/ev~s

Workhop, Hilton Head Island, South Carolina, June 2-6. 2002,346,2002. [21] 1. De Coster; R. Puers, H.A.C. Tilmans, J.T.M. van Beek: and Th.G.S.M. Rijks, TRANSDUCERS, 12" hi. CO,$ on Solid-Stare Sensors. Acruarors and Microsvsrems June 9- I?. 2003, Vol. 2, 1784, 2003. [22] D. Peroulis and L P.B. Katehi, IEEE M7TS 2003 Digesr. 1793.2003. [23] R.L. Bonvick 11% P.A. Stupar, J.F. DeNatale. R. Anderson. and R. Erlandson. IEEE Trans. Microwove Theow m d Teclvi. 51,315, 2003. [24]T.K. Tsang, M.N. El-Gama], W.S. Best, and H.J. De Los Santos. Microwave Journal. A i iIP 2003.22.2003. . . [25JA.J. Gallant and D. Wood, J Micromech. Microeng. 13. S17S-S182,2003. [26] H.A.C. Tilmans, M.D.J. van de Peer, and e. Beyne, Joumol of Micr.omeclra,rical Systems 9, 206-2 17.2000. (271 A. Jourdain el al.. to be published.

ACKNOWLEDGMENT This work has been carried out as part of the IST Project MEMSZTUNE under number IST-2000-2823 1,

REFERENCES [ I ] N.Pulsford, RF Design Magazine Nov. 2002,40-48.2002. [2] H.A.C. Tilmans, W. De Raedt, and E. Bcyne, Journal of Micromechanics and Microengineering 13, S139-Sl63,

2003.

[3] Courtesy of Philips Semiconductors [4] 1.T.M van Beek, M.W.H.M. van Delden, A.B.M lansman, A. Boogaard, and A. Kemmeren, Proc. IMAPS 2001, Baltimore USA, Oct. 9-11 2001,467-470,2001. [5]Th.G.S.M. Rijks, J.T.M. van Beek, M.J.E. Ulenaers, I. De Coster, R. Puers, A. den Dekkker, and L. van Teeffelen,

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MEMS tunable capacitors and switches for RF ...

The native aluminum oxide un the metal surface is indicated in black. 50 ... This pull-in effect can be avoided in a dual-gap relay- type tunable capacitor ...

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