Reconfigurable 4 Pole Bandstop Filter based on RF-MEMS-loaded

Reconfigurable 4 Pole Bandstop Filter based on RF-MEMS-loaded Split Ring Resonators David Bouyge1, Aurélian Crunteanu2, Arnaud Pothier2, P. Olivier Martin2, Pierre Blondy2, Adolfo Velez1, Jordi Bonache1, J. Christophe Orlianges3, Ferran Martin1 1

CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain 2

XLIM -CNRS, University of Limoges, 123 Avenue Albert Thomas, 87060 Limoges, France

3

SPCTS-CNRS, University of Limoges, 123 Avenue Albert Thomas, 87060 Limoges, France

Abstract — A reconfigurable four-pole bandstop filter based on the combination of the split ring resonators (SRRs) and RFMEMS switches, operative at the X frequency band, is presented for the first time. The fabricated device consists on a microstrip line loaded with four pairs of SRRs, exhibiting four different resonance frequencies, and two capacitive ON/OFF switches placed between the inner and outer rings of each SRR. Through their electrostatic actuation, the resonance frequency of each SRR can be shifted and as a result filter bandwidth can be digitally controlled, or even suppressed. Good agreement between theory and experiment is achieved. Index Terms — Bandstop filters, cantilevers, RF-MEMS, split ring resonators, switching, tunable filters.

I. INTRODUCTION The synthesis of reconfigurable microwave and millimeter wave devices has been extensively studied in the last years for their high-potential integration in advanced communication systems for defense or space applications (antennas, multistandard communication systems, etc.). Moreover, intensive research efforts are supported by companies and academies to improve the size of these circuits. Within this context, the design of compact tunable bandstop filters based on split ring resonators (SRRs) or complementary SRRs (CSRRs) has been investigated widely these years. Indeed, SRRs are sub-wavelength (i.e. electrically small) resonators able to inhibit signal propagation in a narrow band in the vicinity of their resonance frequency. Recently, tunablility has been achieved by external electric and/or by temperature, in structures based on SRRs implemented by ferroelectric materials [1], [2]. However, tunable filters based on SRRs are generally realized by associating active semiconductor components. By placing varactor diodes between the inner and the outer rings, some of the authors proposed the synthesis of tunable microstrip bandstop filters [3], [4]. With this configuration, the diode capacitance dominates over the distributed capacitance between rings, and the resonance frequency of the structure can be electronically controlled. The resulting particle, called varactor-loaded split ring resonators (VLSRR),

has been subsequently used by other authors in microstrip configurations to realize compact tunable notch filters and resonators [5], [6]. Vélez et al. demonstrated that by loading a microstrip line with varactor-loaded complementary split ring resonators (VLCSRR), the structure exhibits a tunable rejection band that can be shifted to a tunable pass band with left handed wave propagation by adding a gap in the strip line [7]. This kind of particles can be very useful for the manufacturing of tunable devices in L and S frequency bands, but at higher frequencies, filter design will be limited by the large dimensions of varactor diodes. Furthermore, the performance of varactor-based tunable microwave systems is generally limited by losses, power consumption, and nonlinearity. In this context, successful integration of RF-MEMS switches is expected to be an enabling technology for many microwave applications, thanks to their low loss, near zero power dissipation, compactness and high linearity on wide bands. They are still prone to reliability troubles and moderate switching time but recent progress were done on these specific aspects [8]-[10]. The ability of this technology has been demonstrated over the past few years to provide an efficient solution to the tuning of microwave circuits [11], [12]. While many devices based on SRRs allowing to tune the central frequency of bandstop or bandpass filters have been realized, the originality of the 4-pole reconfigurable bandstop filter design presented in this paper lies in its digitallycontrolled bandwidth in X-Band. Moreover, the authors associate for the first time SRRs and cantilever-type RFMEMS in a microstrip configuration. Nevertheless, we would like to mention that Gil et al. [13] previously realized a tunable stopband filter, operating at Q-Band, by etching CSRRs in the central strip of a coplanar waveguide structure loaded with RF-MEMS variable capacitors integrated on top of them.

W

(a)

(b) h w

C

Hi

G G0

Fig. 1. Layout of the all-integrated 4-pole reconfigurable bandstop filter (a) and overscale view of one RF-MEMS-loaded SRR with a part of 2 2 the microstrip line (b). The total size of the device is 6.4 × 14 mm . The dimensions of the cantilever-type MEMS are h × w = 200 × 150 µm . Width and distance between rings are C = 300 µm and G = 30 µm. The gap between SRRs and the microstrip line is G0 = 50 µm. The side length of the SRRs in the longitudinal direction is W = 1940 µm.

II. DESIGN OF THE RECONFIGURABLE 4-POLE BANDSTOP FILTER The topology of the 4-pole reconfigurable bandstop filter is depicted in Fig. 1(a). It consists on a 50 Ω microstrip transmission line loaded with eight SRRs integrating MEMS switches. SRRs are symmetrically placed along the transmission line, so the filter contains four cells of two identical SRRs which exhibit the same resonance frequency. The difference between SRRs called A, B, C and D is the side length Hi of the external ring (Fig. 2(b)): HA = 1430 µm, HB = 1475 µm, HC = 1530 µm and HD = 1580 µm. This configuration provides a bandstop behavior with four poles corresponding to the resonance frequencies fA, fB, fC and fD of the SRRs of cells A, B, C and D, respectively. Two capacitive cantilever-type MEMS switches are integrated between the inner and outer rings of each resonator. The external ring is the anchor of the cantilevers and the internal ring, under them and covered by a thin dielectric layer (with dielectric constant r = 9.8), acts as a common DC actuation electrode. We observe that the integration of these switching elements does not increase the dimensions of the structure. On the contrary, the two capacitances between the cantilever and the inner ring increase the coupling between rings, what results in a decrease of the resonance frequency of the SRRs, and hence in an improvement of the electrical size of the particle. The 2 area of each MEMS-loaded resonator is about λg / 32. A. Dynamic reconfigurability principle Our approach consists on integrating MEMS switches in each SRR to allow or not the signal transmission in the line at the resonance frequencies of the corresponding SRR. As shown in Fig. 1, the filter design integrates resistive lines and polarization pads used for the electrostatic actuation of the RF-MEMS switches. The common DC ground signal is supplied to all external rings through the transmission line and

resistive lines while each internal ring act as a DC independent electrode.

(a)

(b)

Fig. 2. Simulated insertion (solid lines) and return (dotted lines) losses when all MEMS are at up-state (blue) and all MEMS are at down state (red) (a). Simulated responses of the device for different combinations of switches actuated (b).

Owing the actuation of switches of one or several cells, the bandstop filter presents zero, one, two, three or four poles. As a result the bandwidth of the stopband can be digitally tuned. Taking into account that both SRRs of one cell must always

present the same resonance frequency, we obtain a 4-bit (called A, B, C and D) reconfigurable filter. B. Filter responses The reconfigurable filter operates at the X-Band. It has been simulated by using the Agilent Momentum electromagnetic simulator. The ON/OFF RF-MEMS switches have been designed to provide a ratio between up-state and down-state capacitances of 10, which leads to a shift of the resonance frequencies of the resonators from X-band to L-Band. The simulated S-parameters of the device are displayed in Fig. 2. When none of the switches are actuated, at up-state, the rejection is higher than 20 dB in a 0.7 GHz range. The structure provides four poles at fD = 9.73 GHz, fC = 9.92 GHz, fB = 10.15 GHz and fA = 10.36 GHz (Fig. 2(b)). When all the switches are actuated, at down-state, insertion losses are less than 1 dB and return losses higher than 20 dB in a range from 3 GHz to 16 GHz. The curves on Fig.2 (b) present other filter responses corresponding to different bit-combinations. According to the number of switches actuated, we can tailor the bandwidth of the rejection band. It is shown that besides allowing the digitally tuning of the bandwidth of the filter, this kind of structure authorizes also the digitally tune of the central frequency of a two pole bandstop filter. III. FABRICATION AND MEASUREMENTS The device was fabricated in a clean room environment using low-cost, batch and standard micro fabrication process. First, the actuation electrodes are realized by the thermal evaporation of a Cr / Au thin layer on a 250 μm-thick Sapphire substrate (εr = 9.8). They are covered by a 0.4 µmthick Al2O3 dielectric layer deposited by PECVD. It follows the lift-off of a 50 nm-thick doped Carbon layer, deposited by reactive laser ablation, to realize the 20 KΩ resistive lines. The suspended parts of the structure are defined by the pattern of a 0.5 µm-thick sacrificial PMGI resist. The metallization is done using a Cr / Au seed layer, gold-electroplated up to 1.5 μm. Next a 90Å Cr stress layer is deposited and patterned on the foldable areas, in order to provide an appropriate stress gradient in the foldable areas. Finally, the device is released and dried in a critical point drying system. The fabricated microwave device has been mounted in a metallic box using SMA connectors to make RF measurement, as shown in Fig. 3. To avoid damaging the device during the characterization, the polarization pads are connected to external ports by wire bonding. Filter performance was characterized by means of an Agilent 8710 network analyzer in the 1-20 GHz frequency range (the used calibration does not take into account losses induced by the SMA connectors). The simulated and measured insertion and return losses of the filter with all MEMS in the up-state (non-actuated) are presented in Fig. 4(a). As expected, the filter exhibits a four

pole rejection band around 10 GHz and the rejection is higher than 20 dB on a 0.72 GHz frequency range. There is good agreement between simulation and experiment, except that out of the stopband measured insertion losses are higher and return losses are lower than those predicted by the simulation. This is due to the connection between the transmission line of the filter and the two SMA connectors. Future measurements using a commercial microstrip to coplanar transition will be realized to improve this aspect.

Fig. 3. Photograph of the fabricated reconfigurable filter mounted in a metallic housing with two SMA connectors for microwave performance measurement and with external electrical wires for RFMEMS actuation through DC voltage.

Other filter measured responses corresponding to different combinations of switches simultaneously actuated with 60 Volts (reported in Table I) are depicted in Fig. 4 (b). The number of poles of the stopband corresponds to the number of non-actuated switches. The digital reconfigurability principle is then validated. TABLE I SUMMARY OF RF-MEMS SWITCHES COMBINATIONS “0”: MEMS AT UP-STATE “1”: MEMS AT DOWNSTATE Measure 1 2 3 4

A 0 0 0 1

Bit value B C 0 0 0 0 0 1 0 1

D 0 1 1 1

IV. CONCLUSIONS An all-integrated reconfigurable four-pole bandstop filter based on the combination of split ring resonators and capacitive RF-MEMS switches has been presented for the first time. MEMS integration improves the compactness of the filter and allows to digitally control the bandwidth of the

rejection band in the X-Band. A new device will be fabricated and measured using microstrip to coplanar transitions. Other responses of the filter for different combinations of switches actuated will be presented to the conference with higher performances expected in terms of out of band insertion and return losses.

(a)

(b)

Fig. 4. Simulated (blue) and measured (red) insertion (solid lines) and return (dotted lines) losses when all switches are at up-state (a). Simulated and measured responses of the device for different combinations of switches actuated (b).

ACKNOWLEDGEMENT This work has been supported by Spain-MEC (project contract TEC2007-68013-C02-02 METAINNOVA) and by Spain-MCI (project CONSOLIDER-INGENIO 2010 CSD2008-00066). Ferran Martin is in debt with the ICREA Foundation for giving him an ICREA Academia award. The research activities of David Bouyge are financed by the Universitat Autonoma de Barcelona. Finally, special thanks go to D. Passerieux for help in measuring the devices.

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