Tunable Bandpass Filter using RF MEMS Variable Capacitors Md. Fokhrul Islam1, M. A. Mohd. Ali2 and B. Y. Majlis2

1,2Department of Electrical, Electronic and System Engineering Faculty of Engineering and Built Environment

Universiti Kebangsaan Malaysia 1engr_tutul96@yahoo.com

Introduction

The growing number of multi-standard and multi-application telecommunication systems has led to the development of new tunable filter topologies. Tunable filters are integral components in a variety of radar and modern multi-band communication systems [1]. The conventional tunable filters typically utilize YIG resonators, active resonators or varactors as the tuning element. However, these varactor based tuning filters have low Q values due to high series resistance of diodes. The development in microelectromechanical system (MEMS) technology allows new and innovative design of the tunable bandpass filters [2]-[3]. In this paper, the concept in [4] is extended to design radio frequency (RF) MEMS tunable filters for wireless application.

Basic Filter Topology In order to design the tunable BPF, a parallel coupled BPF has been designed on microstrip platform using coupled half-wave resonators as shown in figure 1. Microstrip technology has been used for simplicity and ease of fabrication [5]-[6]. Although miniaturized end coupled bandpass filters are widely used for tunable applications, owing to the loose coupling of their resonators they account for high insertion losses and poor performance. This bandpass filter is designed by following the design procedure based on the even- and odd-mode impedances of the coupled lines [7], and is further optimized using IE3D [8].

Tuning Principle The concept of tunability is incorporated into the basic filter design by a set of low loss RF MEMS variable capacitors. The cross section of a metal membrane variable capacitor is shown in figure 2. The RF MEMS capacitor consists of a lower electrode which is fabricated as a filter circuit and a thin aluminum membrane suspended over the electrode. The membrane is connected directly to grounds on either side of the electrode. The air gap between the two conductors determines the MEMS capacitor off-capacitance. With no applied actuation potential, the residual tensile stress of the membrane keeps it suspended above the RF path as shown in figure 2a. When using this variable capacitor as a filter circuit, a dc-bias voltage pV is applied to the membrane conductor, while an ac excitation signal

tVv iii ωcos= is applied to the underlying electrode. The difference voltage ( pi Vv − ) is effectively applied across the underlying electrode to suspended membrane, generating a force between the stationary electrode and the moveable beam given by [9].

( )xCVv

xEF pid ∂

∂−=∂∂= 2

21

978-1-4244-2642-3/08/$25.00 ©2008 IEEE

tVxCtV

xCVVV

xCF i

iiip

ipd ωω 2cos

4cos

42

222

∂∂+

∂∂−

���

����

�+

∂∂= (1)

The first term in equation (1) represents an off-resonance dc force that statically bends the beam. The second term constitutes a force at the frequency of the input signal, amplified by the dc-bias voltage pV which creating a variable capacitance between the electrode and suspended beam. The third term in (1) represents a term capable of driving the beam into vibration, if pV is very large compared with iV , this term is greatly suppressed. Figure 2b demonstrates the actuated state of the beam which determines the MEMS on-capacitance. By virtue of changing the position of the membrane with an applied DC voltage, the capacitance of this RF MEMS device can be changed over a significant capacitance range.

Electromechanical Simulation The commercial software ANSYS, that uses finite element method is used as the electromechanical simulation tool for designing the MEMS device. It provides interaction between the electrostatic field analysis and structural analysis. Figure 3 shows electro-mechanical coupling field analysis of the beam to an applied voltage of 8 Volts. As can be seen, a voltage of 7.5 Volts is strong enough to attract the beam to a maximum displacemnt of 0.5 μm which is 1/3 of the original airgap.

Electromagnetic Simulation The filter was simulated in the layout window of IE3D software, including the silicon nitride (Si3N4) layer. The insertion loss and return loss parameters of the tunable filter are shown in the figure 4. Two states of the MEMS bridge were simulated: In the UP state, when the bridges are at a height of 1.5 μm the filter has a center frequency of 9.8 GHz with a 3dB bandwidth of about 45%. In the DOWN state, when the bridges are at the minimum height of 1.0 μm the center frequency shifts to a value of about 8.76 GHz with a 3dB bandwidth of about 69%. The 3dB bandwidth of the filter tunes towards the left side of the response from 45% to 69% for corresponding variations in the height of the bridge will be controlled by the actuation voltage. This filter is widely used today in radar, satellite and terrestrial communications, and electronic countermeasure applications, both militarily and commercially.

Fabrication The filters are fabricated on a 2×3 cm2 silicon substrate, with 600 μm thickness and dielectric constant �r = 11.9. The fabricated filter occupies a chip area of 12 × 3 mm2 shown in figure 5. Surface micromachining techniques were utilized to fabricate the filters. During the development of the process many changes were made to optimize it and some results did not fulfill the expectations, however every change demonstrates the viability of the fabrication.

Conclusion This paper described an original tunable bandpass filter topology. This tunable filter is obtained by associating a microstrip bandpass filter and MEMS variable capacitors. Such an association allows tuning simultaneously and independently both central frequency and

bandwidth. A design technique has been introduced which allows these devices to be eventually used to make high performance tunable filters using only capacitor tuning. Further works will present a set of measurements to fully characterize the filter, including: insertion loss, isolation, among other physical properties. Acknowledgements: The authors would like to thank the Ministry of Science, Technology and Innovation (MOSTI) of Malaysia for supporting this work under the eScienceFund 03-01-02-SF0254.

References [1] E. Foum et al., “Bandwidth and central frequency control on tunable bandpass filter by

using MEMS cantilevers” IEEE MTT-S Digest, 2003, pp. 523-526, 2003. [2] Y. Liu, A. Borgioli, A.S. Nagra, Robert A. York, “Distributed MEMS transmission

lines for tunable filter applications”, Int. J. RF Microwave Comput. Aided Eng., Special Issue 11, pp. 254-260, 2001.

[3] A. Tamijani, L. Dussopt, G.M. Rebeiz, “Miniature and tunable filters using MEMS capacitors”, IEEE Trans. Microwave Theory Tech. vol. 51, pp. 1878-1885, 2003.

[4] Md. Fokhrul Islam, M. A. Mohd. Ali and B. Y. Majlis, “Parallel coupled microstrip bandpass filter for x-band applications”, DIUJST Vol. 2, Iss. 2, pp. 27-31 July 2007.

[5] WU H.-W., WENG M.-H., SU Y.-K., YANG R.-Y. and HUNG C.-Y, “Spurious suppression of a parallel coupled microstrip bandpass filter with simple ring EBG cells on the middle layer”, IEICE TRANS. ELECTRON., vol.E89-C, np.4, pp. 568-570, April 2006.

[6] T. C. Edwards and M.B. Steer, Foundation of Interconnect and Microstrip Design, John Wiley & Sons Ltd., 2000.

[7] D. M. Pozar, Microwave engineering, John Wiley & Sons Inc., 2005. [8] Zeland Software, IE3D Full Wave Electromagnetic Simulator V12.14, 2007. [9] M. Lobur, T. Sviridova and K. Baybakov, “RF MEMS: filter model”, in Proc.

TCSET’2004, Lviv-Slavsko, Ukraine, pp. 92-93, 24-28 February 2004.

Figure1. Top View of the Basic BPF Structure

Figure 2. Cross-Section of an RF MEMS Capacitor in the (a) Unactuated and (b) Actuated Positions

Figure 3. Pull-In Voltage vs Displacement Characteristics

Figure 4. Transmission Characteristics of the Tunable Filter

Figure 5. Photograph of Fabricated Tunable Bandpass Filter