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Tunable passband T-filter with electrostatically-driven polysilicon micromechanical resonators
Sensors and Actuators A 117 (2005) 115–120
Letter to the Editor
Tunable passband T-filter with electrostatically-driven polysilicon micromechanical resonators Dimitri Galayko a,∗ , Andreas Kaiser a , Bernard Legrand a , Lionel Buchaillot a , Dominique Collard b , Chantal Combi c b
a IEMN-ISEN departement, 41, boulevard Vauban, 59046 Lille, France CIRMM/IIS-CNRS, Institute of Industrial Sciences, The University of Tokyo, Japan c STMicroelectronics, Via Tolomeo, 1, 20010 Cornaredo, Milano, Italy
Received 14 January 2004; received in revised form 9 June 2004; accepted 10 June 2004 Available online 22 July 2004
Abstract A 2.4 MHz tunable bandpass T-filter built from three micromechanical silicon-micromachined electrostatically-driven resonators is demonstrated for the first time. Non-linear properties of electrostatical transducers are used to obtain a voltage-control of the transmission zero and pole distribution; a novel charge biasing technique is employed for electrostatical transducer biasing. Transmission zero frequency tuning is demonstrated. © 2004 Elsevier B.V. All rights reserved. Keywords: T-filter; Passband; Electrostatically-driven resonators; Transmission zero; Tunable
1. Introduction Integrated silicon-micromachined electrostatically-driven tunable micromechanical filters are potential candidates to replace bulky SAW filters in IF stages of radioreceivers. Up to now only simple coupled-resonator filters have been presented, with “all pole” transmission characteristics [1,2]. The presented structures were equivalent to RLC electrical ladder networks including series RLC sections (i.e., elementary resonators) in horizontal branches of the ladder. However, transmission zeros placed at adjacent channel frequencies can considerably alleviate requirements toward overall IF filter selectivity. In the electrical domain, this can be achieved by incorporating series RLC networks (elementary resonators) in vertical (parallel) ladder branches. In the elementary case of three-resonators filter this yields a so-called T-topology (Fig. 2); widely used with BAW and quartz resonators [3], this architecture has never been realized with electrostatically-driven micromechanical resonators. This letter presents a tunable T-filter built from three micromechanical electrostatically-driven resonators realized in a thick-film epitaxial polysilicon micromachining tech∗ Corresponding author. Tel.: +33 320 30 40 46; fax: +33 320 30 40 51. E-mail address: [email protected] (D. Galayko).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.06.002
nology of STMicroelectronics characterized by a 15 m thickness structural layer [4]. Unlike its counterparts based on quartz or BAW resonators, this filter requires external high-impedance DC biasing for the electrostatical transducers. In our structure, this is achieved using a novel constant-charge biasing technique. The measurement results firstly demonstrate a filter transmission characteristic with transmission zero, whose frequency is controlled by a bias voltage.
2. Elementary resonator operation The filter uses three resonators with identical geometry (Fig. 1a). Resonators include a vibrating element implemented with a clamped–clamped beam vibrating in lateral mode (parallel to substrate), and a pair of input–output electrostatical transducers formed by the walls of the signal electrodes and the resonator. DC biasing is necessary to linearize the quadratic transducing function and to increase the electromechanical coupling factor of the transducers. The AC input voltage is applied to the input transducer, creating a variable input force on the vibrating element. The resonator response is transformed to current by the output transducer. In the electrical domain such a resonator is seen as a series RLC network with three parasitic capacitors (Fig. 1b). The motional parameters of the resonator depend both on
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Fig. 1. Elementary resonator: (a) mechanical architecture, (b) equivalent electrical network.
resonator geometry and the bias voltage, making it possible to tune the resonance frequency. The important numerical parameters of resonator used in the implemented filter are given in Fig. 1. A detailed study of the device can be found in [4–6].
3. T-filter architecture An T-filter is composed of three elementary resonators (Fig. 2). The series resonance of the resonators 1 and 2 and the parallel resonance of the resonator 3 generate the filter passband poles, and parallel resonance of the resonators 1 and 2 and the series resonance of the resonator 3 generate the filter transmission zeros [3]. For each resonator the ratio between the series and parallel resonance frequencies (fS and fP ) depends on the ratio between the motional and parallel capacitances (CX and CC ): fP = fS CX /CC + 1 (1)
voltage Vbias , transducer’s area S, transducer’s gap d0 ) and the mechanical stiffness of the vibrating element k0 : fS = f0 1 −
2 ξ (S/d 3 ) Vbias 0 0 k0
(2)
Details on T-filter design issues can be found in [3]. Although this book principally addresses electrical filter design, the proposed techniques can be re-used with mechanical filters by employing electromechanical analogy (approach demonstrated in [1,5]). By contrast with piezoelectric electromechanical transducers, electrostatical transducers need a DC voltage biasing for linear-mode operation. At the same time, the biasing should preserve the high-impedance state of the signal node A.
Series resonance frequency is defined by the vibrating element resonance frequency f0 , the transducer parameters (bias
Fig. 2. Architecture of an elementary T-filter.
Fig. 3. Architecture of the realized filter. CA is a parasitic parallel-toground capacitor.
D. Galayko et al. / Sensors and Actuators A 117 (2005) 115–120
4. T-filter realization In the realized filter (Fig. 3) the resonating beams are biased by DC voltages Vr1 –Vr3 . The external electrodes of the input–output transducers are zero biased thanks to the resistors RL and RS providing a DC path to ground. High-impedance voltage biasing of node A could be achieved using a high-value choke inductor or resistor. It is not practical here: (i) high-value elements are undesirable in integrated circuits, (ii) external connections to this node dramatically increase its parasitic capacitance. Instead we propose to use the technique of constant-charge biasing of electrostatic transducers [2,5]. It employs a micromechanical switch to connect node A with a DC voltage source for a time enough to charge all node capacitances (transducer and parasitic). This operation allows to define an initial potential of node A. When the switch is off, the node charge remains
117
constant but redistributes among the resonators when they move. This redistribution modifies the potential of node A and so the force generated by each transducer thus creating a mechanical coupling between the resonators [5,6]. The CX /CC ratio of elementary resonators is relatively high, resulting in parallel resonance frequency of resonators approximately 100 kHz above the series resonance frequency (cf. Eq. (2)). In fact, the intrinsic CC capacitance is due to the electrostatic coupling between the resonator electrodes, and intrinsically weak (5–7 fF, cf. Fig. 1). However, in the realized filter prototype the resonator 3 has a relatively high external capacitance due to the 0.36 pF parasitic parallel-to-ground capacitance of node A (CA in Fig. 3) which yields a pole-to-zero offset of ∼1.5 kHz for this resonator. Therefore, the filter will have only one useful transmission zero close to the passband generated by the series resonance of the resonator 3; two others (generated
Fig. 4. Photo of the realized filter with enlarged view of an elementary resonator and switch.
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Stoppers protect the switch actuators from the short-circuit; in difference with the gap reduction mechanism, the motion of the contact electrode is limited by the node A plane, not by the stoppers. The direct proximity of the switch with the node A allows to minimize the parasitic capacitances on the latter. The actuation voltage equals 40 V [2].
5. Filter fabrication Fig. 5. Section of a device in thick-film epitaxial polysilicon technology.
by the parallel resonance of the resonators 1 and 2) appear far from the passband and cannot be observed. To approach the parallel resonance frequency of the resonators 1 and 2 close to the center of the passband, external coupling capacitances should be used; this will add two additional useful transmission zeros and increase the filter selectivity. The photograph in Fig. 4 presents the overall view of filter, and enlarged views of an elementary resonator and switch. The complex geometry of the transducer electrodes realizes a post-fabrication reduction of the large lateral transducer gaps obtained at fabrication. For this the resonator electrodes are released, joined to a soft released spring and attracted close to the beam by an electrostatic actuation [4]. The electrode motion is limited by stoppers. The gap is reduced from 3.0 m down to 0.5 m. The mechanical switch is realized on the same chip as the resonators, its operation is similar to that of the gap reduction mechanism. The contact electrode is released, joined to a soft spring and pushed with a pair of electrostatic transducers so get in contact with a plane of the node to be biased (node A).
A vertical section of a device fabricated in thick-film epitaxial polysilicon technology is presented in Fig. 5. A device is built with two polysilicon layers, one is the 15 m thickness structural layer, the second is 0.45 m thickness buried layer used for anchoring and interconnect (cf. also Fig. 4). The devices are machined in the structural layer by anisotropic dry etching. The mobile parts are released using time-controlled releasing technique.
6. Experimental results The filter was terminated by a 50 resistor at the input and a 1 k resistor at the output port, the output voltage was amplified by a 20 dB gain voltage amplifier. The filter was tested in a vacuum chamber under 13.33 Pa air pressure. At the used bias voltages, the individual resonator impedance is about 30–40 k , thus correct filter termination would require source and load impedances of hundreds kiloohms at filter ports. In our case this was impossible to realize because of the large parasitic capacitances of the measurement setup. For this reason the low measured filter
Fig. 6. Filter transmission characteristics: Vn = 18.10 V, Vr2 = 20.5 V.
D. Galayko et al. / Sensors and Actuators A 117 (2005) 115–120
transmission −(21 + 20) dB does not represent the insertion loss of a correctly terminated filter. Thus, the main damping mechanism is related to the internal losses of resonators defined by their quality factor (5000 measured at 0.1 Torr pressure). The control of the transmission characteristic shape is achieved in the following way. Voltages Vr1 , Vr2 allow to correct the resonance frequency mismatch of the resonators 1 and 2 due to the fabrication tolerances. Thanks to the voltage Vn , we can control the coupling factor between these resonators, thus the passband pole distribution which defines the filter bandwidth [2]. This voltage was fixed to 18.10 V so to obtain a maximally-flat passband when the resonators have a quality factor of 5000. Detailed characterization of the bandwidth control of a two-resonator charge-biased passband filter (without transmission zero) has been presented in [7], a 8.9 maximum-to-minimum bandwidth control ratio has been achieved. Vr3 controls directly the series resonance frequency of resonator 3, thus the filter zero frequency. The plots in Fig. 6 present three measured filter transmission characteristics for different bias voltages of resonator 3 (Vr3 ), demonstrating the possibility to control the transmission zero frequency. We should note a very high sensitivity of the filter characteristic with respect to the bias voltages (10 mV precision was required in the experiment). This can be explained by the very high overall quality factor of the filter and thus by the very high required relative precision of the pole and zero implementation.
7. Conclusions This work shows that complex filtering architectures having large tuning potential are possible with electrostatically-driven micromechanical resonators. Thank to the constant-charge biasing of electrostatical transducers, filter networks constructed with piezoelectrically-driven resonators are realizable with electrostatically-driven resonators. We demonstrated a prototype of a 2.4 MHz electrostatically-driven T-filter with reconfigurable transmission characteristic. For the first time, zero frequency tuning was demonstrated for a microelectromechanical passband filter.
Acknowledgements This study was funded by the European Union under contract number IST-1999-10945.
References [1] F.D. Bannon, J.R. Clark, C.T.-C. Nguyen, High-Q HF microelectromechanical filters, IEEE J. Solid-State Circ. 35 (10) (2000) 512–526.
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[2] D. Galayko, et al., Electrostatical coupling-spring for micro-mechanical filtering applications, in: Proceedings of the International Symposium on Circuits and Systems, Bangkok, Thailand, May 25–28, 2003, pp. III-530–III-533. [3] A.I. Zverev, Handbook of Filter Synthesis, Wiley, New York, 1967. [4] D. Galayko, et al., Design, realisation and test of micro-mechanical resonators in thick-film silicon technology with postprocess electrode-to-resonator gap reduction, J. Micromech. Microeng. 13 (2003) 134–140. [5] D. Galayko, Filtres microélectromécaniques micro-usinés en polysilicium couche épaisse et leur application au filtrage en fréquence intermédiaire, Ph.D. Dissertation, University Lille-I, Lille, France, 2002. [6] T.-C. Clark, Nguyen, Micromechanical signal processors, Ph.D. Dissertation, University of California at Berkeley, USA, 1994 [7] D. Galayko, A. Kaiser, B. Legrand, L. Buchaillot, D. Collard, C. Combi, Microelectromechanical coupled-resonator IF filters with variable bandwidth in thick-film epitaxial polysilicon technology. in: R. Plana, A. Müller, D. Daskalu (Eds.), MEMS technologies for millimeter wave devices and circuits, Editura Academiei Romˆane, Bucharest, pp. 96–111. ISBN: 973-27-1087-X.
Biographies Dimitri Galayko was born in Odessa, Ukraine, in 1976. He received the engineering degree from the State Polytechnique University of Odessa (Ukraine) in 1998, Master degree from the National Institute of Applied Sciences of Lion (France) in 1999 and PhD degree from the Science and Technology University of Lille (France) in 2002. Since then, he has worked as postdoctoral researcher at Institut d’Electronique Microelectronique et Nanotechnologie (Lille, France). His research has been focused on design of microelectromechanical filters for RF applications and architecture-level design of electronic systems for radiocommunications using MEMS devices. Andreas Kaiser was born in Freiburg, Germany, in 1958. He received the engineering diploma from ISEN (Catholic University of Lille), France, in 1984, and the PhD degree from the University of Lille in 1990. In 1990 he joined the Centre National de la Recherche Scientique (CNRS) where he is heading the analog IC design group at the Institut d’Electronique, de Microelectronique de Nanotechnologies (IEMN). He is also an Associate Professor with the electronics department of ISEN. He served as Programme Chairman of the European Solid State Circuits Conference in 1995. His research interests are analog, RF and mixed signal IC design, CAD and MEMS. Bernard Legrand was born in 1973. He received the B.S. in 1991, the electrical engineer degree and the M.S. in electronics in 1996, and the Ph.D. degree in electronics from the Université des Sciences et Technologies de Lille in 2000. From 1996 to 2000 he was working on semiconductor nanostructures such as InAs quantum dots and silicon nanowires and was involved in their fabrication and characterization using scanning probe microscopes (AFM and STM). In 2000, he joined the silicon microsystem group of IEMN (Institut d’Electronique, de Microelectronique et de Nanotechnologie) in Lille. His research was first focussed on electrical characterization of MEMS resonators. He is currently working on mechanical sensors and actuators in liquid environment for BioMEMS applications at the cellular level. Lionel Buchaillot received the Diplˆome d’Etudes Approfondies (DEA) degree in 1991 in Material Sciences and the PhD degree in Mechanical Engineering in 1995 from the Université de Franche-Comté, Besançon, France. Between 1995 and 1997, he has been with the Laboratory for Integrated MicroMechatronic Systems (LIMMS-CNRS-IIS) as a JSPS fellow working on thin film shape memory alloys actuators for MEMS. In 1997, he worked as a R&D engineer for the SFIM Company (now
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SAGEM) and AVIAC Technologies Company. In 1998, he joined the CNRS in the ISEN Department of IEMN. Since 2001, he is the Head of the “Silicon-based MEMS” research group at IEMN. His research focuses RF MEMS; MEMS reliability; and MEMS for nanotechnology. Dominique Collard was born in Cambrai, France, in 1958. He received the engineering degree from Institut Supérieur d Electronique du Nord (ISEN), Lille, France, in 1980, and the Ph.D. degree from the University of Lille, Lille, France, in 1984. From 1985 to 1986, he was with Toshiba ULSI Research Center, Kawasaki, Japan, as Visiting Scientist. He entered the Centre National de la Recherche Scientifique (CNRS) as Senior Researcher in 1988, and settled a research group on process and device simulation at ISEN and Institut d Electronique et de Micro-dlectronique du Nord (IEMN), Lille. His own research activity focused on silicon oxidation simulation and related stress effects. From 1995 to 1997, he was Director of the Laboratory for Integrated Micromechatronic Systems (LIMMS), Tokyo, Japan, a joint CNRS laboratory with the Institute of Industrial Science of the University of Tokyo. Within LIMMS, he worked on silicon-based electrostatic actuator for device alignment system and self
assembling 3D technology. In 1997, he joined IEMN as CNRS Research Director and settled a silicon microsystem group, working on microactuators and their integration in silicon microtechnology. Since November 2000, he has also been Director of the Center of International Research on MicroMechatronics CIRMM/CNRS, a joint center between IIS, The University of Tokyo, and the CNRS to promote joint research projects between French and Japanese laboratories. Chantal Combi received her degree in Subnuclear Physics from University of Milan in 1998. Meanwhile she worked at CERN (Geneve-CH) on the electrical and noise characterization of the silicon vertex-detectors, in 1998 he joined ST Microelectronics R&D Lab., Italy. She started to work on test structures to characterize the structural and sacrificial layer for MEMS. In the last 4 years she contributed to develop the design of RF MEMS for telecommunication system (switch, varcap, resonator and filters, high Q inductors and RF package). She has been involved in the MELODICT project on the integration of RF MEMS passives in the mobile phone and nowadays in other European project on RF MEMS. She has made some patents on RF MEMS devices.
Letter to the Editor
Tunable passband T-filter with electrostatically-driven polysilicon micromechanical resonators Dimitri Galayko a,∗ , Andreas Kaiser a , Bernard Legrand a , Lionel Buchaillot a , Dominique Collard b , Chantal Combi c b
a IEMN-ISEN departement, 41, boulevard Vauban, 59046 Lille, France CIRMM/IIS-CNRS, Institute of Industrial Sciences, The University of Tokyo, Japan c STMicroelectronics, Via Tolomeo, 1, 20010 Cornaredo, Milano, Italy
Received 14 January 2004; received in revised form 9 June 2004; accepted 10 June 2004 Available online 22 July 2004
Abstract A 2.4 MHz tunable bandpass T-filter built from three micromechanical silicon-micromachined electrostatically-driven resonators is demonstrated for the first time. Non-linear properties of electrostatical transducers are used to obtain a voltage-control of the transmission zero and pole distribution; a novel charge biasing technique is employed for electrostatical transducer biasing. Transmission zero frequency tuning is demonstrated. © 2004 Elsevier B.V. All rights reserved. Keywords: T-filter; Passband; Electrostatically-driven resonators; Transmission zero; Tunable
1. Introduction Integrated silicon-micromachined electrostatically-driven tunable micromechanical filters are potential candidates to replace bulky SAW filters in IF stages of radioreceivers. Up to now only simple coupled-resonator filters have been presented, with “all pole” transmission characteristics [1,2]. The presented structures were equivalent to RLC electrical ladder networks including series RLC sections (i.e., elementary resonators) in horizontal branches of the ladder. However, transmission zeros placed at adjacent channel frequencies can considerably alleviate requirements toward overall IF filter selectivity. In the electrical domain, this can be achieved by incorporating series RLC networks (elementary resonators) in vertical (parallel) ladder branches. In the elementary case of three-resonators filter this yields a so-called T-topology (Fig. 2); widely used with BAW and quartz resonators [3], this architecture has never been realized with electrostatically-driven micromechanical resonators. This letter presents a tunable T-filter built from three micromechanical electrostatically-driven resonators realized in a thick-film epitaxial polysilicon micromachining tech∗ Corresponding author. Tel.: +33 320 30 40 46; fax: +33 320 30 40 51. E-mail address: [email protected] (D. Galayko).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.06.002
nology of STMicroelectronics characterized by a 15 m thickness structural layer [4]. Unlike its counterparts based on quartz or BAW resonators, this filter requires external high-impedance DC biasing for the electrostatical transducers. In our structure, this is achieved using a novel constant-charge biasing technique. The measurement results firstly demonstrate a filter transmission characteristic with transmission zero, whose frequency is controlled by a bias voltage.
2. Elementary resonator operation The filter uses three resonators with identical geometry (Fig. 1a). Resonators include a vibrating element implemented with a clamped–clamped beam vibrating in lateral mode (parallel to substrate), and a pair of input–output electrostatical transducers formed by the walls of the signal electrodes and the resonator. DC biasing is necessary to linearize the quadratic transducing function and to increase the electromechanical coupling factor of the transducers. The AC input voltage is applied to the input transducer, creating a variable input force on the vibrating element. The resonator response is transformed to current by the output transducer. In the electrical domain such a resonator is seen as a series RLC network with three parasitic capacitors (Fig. 1b). The motional parameters of the resonator depend both on
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Fig. 1. Elementary resonator: (a) mechanical architecture, (b) equivalent electrical network.
resonator geometry and the bias voltage, making it possible to tune the resonance frequency. The important numerical parameters of resonator used in the implemented filter are given in Fig. 1. A detailed study of the device can be found in [4–6].
3. T-filter architecture An T-filter is composed of three elementary resonators (Fig. 2). The series resonance of the resonators 1 and 2 and the parallel resonance of the resonator 3 generate the filter passband poles, and parallel resonance of the resonators 1 and 2 and the series resonance of the resonator 3 generate the filter transmission zeros [3]. For each resonator the ratio between the series and parallel resonance frequencies (fS and fP ) depends on the ratio between the motional and parallel capacitances (CX and CC ): fP = fS CX /CC + 1 (1)
voltage Vbias , transducer’s area S, transducer’s gap d0 ) and the mechanical stiffness of the vibrating element k0 : fS = f0 1 −
2 ξ (S/d 3 ) Vbias 0 0 k0
(2)
Details on T-filter design issues can be found in [3]. Although this book principally addresses electrical filter design, the proposed techniques can be re-used with mechanical filters by employing electromechanical analogy (approach demonstrated in [1,5]). By contrast with piezoelectric electromechanical transducers, electrostatical transducers need a DC voltage biasing for linear-mode operation. At the same time, the biasing should preserve the high-impedance state of the signal node A.
Series resonance frequency is defined by the vibrating element resonance frequency f0 , the transducer parameters (bias
Fig. 2. Architecture of an elementary T-filter.
Fig. 3. Architecture of the realized filter. CA is a parasitic parallel-toground capacitor.
D. Galayko et al. / Sensors and Actuators A 117 (2005) 115–120
4. T-filter realization In the realized filter (Fig. 3) the resonating beams are biased by DC voltages Vr1 –Vr3 . The external electrodes of the input–output transducers are zero biased thanks to the resistors RL and RS providing a DC path to ground. High-impedance voltage biasing of node A could be achieved using a high-value choke inductor or resistor. It is not practical here: (i) high-value elements are undesirable in integrated circuits, (ii) external connections to this node dramatically increase its parasitic capacitance. Instead we propose to use the technique of constant-charge biasing of electrostatic transducers [2,5]. It employs a micromechanical switch to connect node A with a DC voltage source for a time enough to charge all node capacitances (transducer and parasitic). This operation allows to define an initial potential of node A. When the switch is off, the node charge remains
117
constant but redistributes among the resonators when they move. This redistribution modifies the potential of node A and so the force generated by each transducer thus creating a mechanical coupling between the resonators [5,6]. The CX /CC ratio of elementary resonators is relatively high, resulting in parallel resonance frequency of resonators approximately 100 kHz above the series resonance frequency (cf. Eq. (2)). In fact, the intrinsic CC capacitance is due to the electrostatic coupling between the resonator electrodes, and intrinsically weak (5–7 fF, cf. Fig. 1). However, in the realized filter prototype the resonator 3 has a relatively high external capacitance due to the 0.36 pF parasitic parallel-to-ground capacitance of node A (CA in Fig. 3) which yields a pole-to-zero offset of ∼1.5 kHz for this resonator. Therefore, the filter will have only one useful transmission zero close to the passband generated by the series resonance of the resonator 3; two others (generated
Fig. 4. Photo of the realized filter with enlarged view of an elementary resonator and switch.
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D. Galayko et al. / Sensors and Actuators A 117 (2005) 115–120
Stoppers protect the switch actuators from the short-circuit; in difference with the gap reduction mechanism, the motion of the contact electrode is limited by the node A plane, not by the stoppers. The direct proximity of the switch with the node A allows to minimize the parasitic capacitances on the latter. The actuation voltage equals 40 V [2].
5. Filter fabrication Fig. 5. Section of a device in thick-film epitaxial polysilicon technology.
by the parallel resonance of the resonators 1 and 2) appear far from the passband and cannot be observed. To approach the parallel resonance frequency of the resonators 1 and 2 close to the center of the passband, external coupling capacitances should be used; this will add two additional useful transmission zeros and increase the filter selectivity. The photograph in Fig. 4 presents the overall view of filter, and enlarged views of an elementary resonator and switch. The complex geometry of the transducer electrodes realizes a post-fabrication reduction of the large lateral transducer gaps obtained at fabrication. For this the resonator electrodes are released, joined to a soft released spring and attracted close to the beam by an electrostatic actuation [4]. The electrode motion is limited by stoppers. The gap is reduced from 3.0 m down to 0.5 m. The mechanical switch is realized on the same chip as the resonators, its operation is similar to that of the gap reduction mechanism. The contact electrode is released, joined to a soft spring and pushed with a pair of electrostatic transducers so get in contact with a plane of the node to be biased (node A).
A vertical section of a device fabricated in thick-film epitaxial polysilicon technology is presented in Fig. 5. A device is built with two polysilicon layers, one is the 15 m thickness structural layer, the second is 0.45 m thickness buried layer used for anchoring and interconnect (cf. also Fig. 4). The devices are machined in the structural layer by anisotropic dry etching. The mobile parts are released using time-controlled releasing technique.
6. Experimental results The filter was terminated by a 50 resistor at the input and a 1 k resistor at the output port, the output voltage was amplified by a 20 dB gain voltage amplifier. The filter was tested in a vacuum chamber under 13.33 Pa air pressure. At the used bias voltages, the individual resonator impedance is about 30–40 k , thus correct filter termination would require source and load impedances of hundreds kiloohms at filter ports. In our case this was impossible to realize because of the large parasitic capacitances of the measurement setup. For this reason the low measured filter
Fig. 6. Filter transmission characteristics: Vn = 18.10 V, Vr2 = 20.5 V.
D. Galayko et al. / Sensors and Actuators A 117 (2005) 115–120
transmission −(21 + 20) dB does not represent the insertion loss of a correctly terminated filter. Thus, the main damping mechanism is related to the internal losses of resonators defined by their quality factor (5000 measured at 0.1 Torr pressure). The control of the transmission characteristic shape is achieved in the following way. Voltages Vr1 , Vr2 allow to correct the resonance frequency mismatch of the resonators 1 and 2 due to the fabrication tolerances. Thanks to the voltage Vn , we can control the coupling factor between these resonators, thus the passband pole distribution which defines the filter bandwidth [2]. This voltage was fixed to 18.10 V so to obtain a maximally-flat passband when the resonators have a quality factor of 5000. Detailed characterization of the bandwidth control of a two-resonator charge-biased passband filter (without transmission zero) has been presented in [7], a 8.9 maximum-to-minimum bandwidth control ratio has been achieved. Vr3 controls directly the series resonance frequency of resonator 3, thus the filter zero frequency. The plots in Fig. 6 present three measured filter transmission characteristics for different bias voltages of resonator 3 (Vr3 ), demonstrating the possibility to control the transmission zero frequency. We should note a very high sensitivity of the filter characteristic with respect to the bias voltages (10 mV precision was required in the experiment). This can be explained by the very high overall quality factor of the filter and thus by the very high required relative precision of the pole and zero implementation.
7. Conclusions This work shows that complex filtering architectures having large tuning potential are possible with electrostatically-driven micromechanical resonators. Thank to the constant-charge biasing of electrostatical transducers, filter networks constructed with piezoelectrically-driven resonators are realizable with electrostatically-driven resonators. We demonstrated a prototype of a 2.4 MHz electrostatically-driven T-filter with reconfigurable transmission characteristic. For the first time, zero frequency tuning was demonstrated for a microelectromechanical passband filter.
Acknowledgements This study was funded by the European Union under contract number IST-1999-10945.
References [1] F.D. Bannon, J.R. Clark, C.T.-C. Nguyen, High-Q HF microelectromechanical filters, IEEE J. Solid-State Circ. 35 (10) (2000) 512–526.
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[2] D. Galayko, et al., Electrostatical coupling-spring for micro-mechanical filtering applications, in: Proceedings of the International Symposium on Circuits and Systems, Bangkok, Thailand, May 25–28, 2003, pp. III-530–III-533. [3] A.I. Zverev, Handbook of Filter Synthesis, Wiley, New York, 1967. [4] D. Galayko, et al., Design, realisation and test of micro-mechanical resonators in thick-film silicon technology with postprocess electrode-to-resonator gap reduction, J. Micromech. Microeng. 13 (2003) 134–140. [5] D. Galayko, Filtres microélectromécaniques micro-usinés en polysilicium couche épaisse et leur application au filtrage en fréquence intermédiaire, Ph.D. Dissertation, University Lille-I, Lille, France, 2002. [6] T.-C. Clark, Nguyen, Micromechanical signal processors, Ph.D. Dissertation, University of California at Berkeley, USA, 1994 [7] D. Galayko, A. Kaiser, B. Legrand, L. Buchaillot, D. Collard, C. Combi, Microelectromechanical coupled-resonator IF filters with variable bandwidth in thick-film epitaxial polysilicon technology. in: R. Plana, A. Müller, D. Daskalu (Eds.), MEMS technologies for millimeter wave devices and circuits, Editura Academiei Romˆane, Bucharest, pp. 96–111. ISBN: 973-27-1087-X.
Biographies Dimitri Galayko was born in Odessa, Ukraine, in 1976. He received the engineering degree from the State Polytechnique University of Odessa (Ukraine) in 1998, Master degree from the National Institute of Applied Sciences of Lion (France) in 1999 and PhD degree from the Science and Technology University of Lille (France) in 2002. Since then, he has worked as postdoctoral researcher at Institut d’Electronique Microelectronique et Nanotechnologie (Lille, France). His research has been focused on design of microelectromechanical filters for RF applications and architecture-level design of electronic systems for radiocommunications using MEMS devices. Andreas Kaiser was born in Freiburg, Germany, in 1958. He received the engineering diploma from ISEN (Catholic University of Lille), France, in 1984, and the PhD degree from the University of Lille in 1990. In 1990 he joined the Centre National de la Recherche Scientique (CNRS) where he is heading the analog IC design group at the Institut d’Electronique, de Microelectronique de Nanotechnologies (IEMN). He is also an Associate Professor with the electronics department of ISEN. He served as Programme Chairman of the European Solid State Circuits Conference in 1995. His research interests are analog, RF and mixed signal IC design, CAD and MEMS. Bernard Legrand was born in 1973. He received the B.S. in 1991, the electrical engineer degree and the M.S. in electronics in 1996, and the Ph.D. degree in electronics from the Université des Sciences et Technologies de Lille in 2000. From 1996 to 2000 he was working on semiconductor nanostructures such as InAs quantum dots and silicon nanowires and was involved in their fabrication and characterization using scanning probe microscopes (AFM and STM). In 2000, he joined the silicon microsystem group of IEMN (Institut d’Electronique, de Microelectronique et de Nanotechnologie) in Lille. His research was first focussed on electrical characterization of MEMS resonators. He is currently working on mechanical sensors and actuators in liquid environment for BioMEMS applications at the cellular level. Lionel Buchaillot received the Diplˆome d’Etudes Approfondies (DEA) degree in 1991 in Material Sciences and the PhD degree in Mechanical Engineering in 1995 from the Université de Franche-Comté, Besançon, France. Between 1995 and 1997, he has been with the Laboratory for Integrated MicroMechatronic Systems (LIMMS-CNRS-IIS) as a JSPS fellow working on thin film shape memory alloys actuators for MEMS. In 1997, he worked as a R&D engineer for the SFIM Company (now
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SAGEM) and AVIAC Technologies Company. In 1998, he joined the CNRS in the ISEN Department of IEMN. Since 2001, he is the Head of the “Silicon-based MEMS” research group at IEMN. His research focuses RF MEMS; MEMS reliability; and MEMS for nanotechnology. Dominique Collard was born in Cambrai, France, in 1958. He received the engineering degree from Institut Supérieur d Electronique du Nord (ISEN), Lille, France, in 1980, and the Ph.D. degree from the University of Lille, Lille, France, in 1984. From 1985 to 1986, he was with Toshiba ULSI Research Center, Kawasaki, Japan, as Visiting Scientist. He entered the Centre National de la Recherche Scientifique (CNRS) as Senior Researcher in 1988, and settled a research group on process and device simulation at ISEN and Institut d Electronique et de Micro-dlectronique du Nord (IEMN), Lille. His own research activity focused on silicon oxidation simulation and related stress effects. From 1995 to 1997, he was Director of the Laboratory for Integrated Micromechatronic Systems (LIMMS), Tokyo, Japan, a joint CNRS laboratory with the Institute of Industrial Science of the University of Tokyo. Within LIMMS, he worked on silicon-based electrostatic actuator for device alignment system and self
assembling 3D technology. In 1997, he joined IEMN as CNRS Research Director and settled a silicon microsystem group, working on microactuators and their integration in silicon microtechnology. Since November 2000, he has also been Director of the Center of International Research on MicroMechatronics CIRMM/CNRS, a joint center between IIS, The University of Tokyo, and the CNRS to promote joint research projects between French and Japanese laboratories. Chantal Combi received her degree in Subnuclear Physics from University of Milan in 1998. Meanwhile she worked at CERN (Geneve-CH) on the electrical and noise characterization of the silicon vertex-detectors, in 1998 he joined ST Microelectronics R&D Lab., Italy. She started to work on test structures to characterize the structural and sacrificial layer for MEMS. In the last 4 years she contributed to develop the design of RF MEMS for telecommunication system (switch, varcap, resonator and filters, high Q inductors and RF package). She has been involved in the MELODICT project on the integration of RF MEMS passives in the mobile phone and nowadays in other European project on RF MEMS. She has made some patents on RF MEMS devices.