Abstract

Abstract. This paper investigates the design constraints and possibilities given when designing a micromechanical band pass filter for intermediate frequencies (e.g. 10 MHz). The class of filters are based on coupled clamped-clamped beams constituting an H-shaped structure. A primary beam can electrostatically be activated in one of its different harmonic modes, setting the filter center frequency. The motion is transferred to an accompanying beam of equal dimensions by a mechanical coupling beam. The placement or coupling points of the quarterwavelength coupling beam which connects the vertically resonating beams is critical with respect to the bandwidth of the filters. Of special concern has been to investigate realistic dimensions allowing the filters to be processed by an actual foundry process and to find out how the choice of materials and actual dimensions would affect the performance. IntroductionVarious MEMS (Micro Electro Mechanical System) technologies have shown their possibilities to bring forth components of quite different kinds, e.g. pressure sensors, gyroscopes or micro mirrors for projectors. A class of MEMS components can preferably be used in RF (Radio Frequency) systems, RF MEMS, enabling compact implementations and high performance [1]. Micromechanical parts can replace their electronic counterparts in effective ways. Most mature in the RF MEMS area are mechanical switches which have demonstrated very low insertion loss and high isolation [2]. Of special interest to our work is mechanically resonating (or vibrating) structures which can be used to implement oscillators, mixers or filters. By taking advantage of the mechanical properties high performance and miniaturization can be obtained by cost effective batch processing. Such MEMS resonators can replace off-chip crystals or can be used in implementing filter banks with unique selection properties [1].In this paper H-shaped filter structures based on coupled clamped-clamped (c-c) beams are studied. Figure 1 shows the 3-D model of our filter where one of the beams is electrostatically activated and output is taken from a second beam. The performance depends on various parameters related to the actual MEMS process, geometrical dimensions, material selection for the structural layer, offset voltage and placement of the coupling beam. A systematic way of choosing an appropriate set within the design constraints is vital. A commercial foundry process, [3], has been chosen to give a realistic parameter choice. In section 2 the design method is described, and results from a concrete design are shown and discussed in section 3, preceding the conclusion.

The smart integrated systems of tomorrow would demand a combination of micromechanical components and traditional electronics. On-chip solutions will be the ultimate goal. One way of making such systems is to implement the mechanical parts in an ordinary CMOS process. This procedure has been used to design an oscillator consisting of a resonating cantilever beam and a CMOS Pierce feedback amplifier. The resonating frequency is changed if the beam is bent by external forces. The paper describes central features of this procedure and highlights the design considerations for the CMOS-MEMS oscillator. The circuit is used as an example of a ''VLSI designer'' way of making future integrated micromechanical and microelectronic systems on-chip. The possibility for expansion to larger systems is reviewed.

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This paper demonstrates how micromechanical on-chip MEMS resonators can be used as higher-order mixer-filters in RF front-end WSN nodes. Vibrating FFSFRs (Free-free Square Frame Resonator) connected together can create 4th and 6th order mixer-filter responses. The output is further enhanced by an on-chip amplifier, thus reducing stray capacitances. These mixer-filters are fabricated utilizing a CMOS-MEMS approach where the movable MEMS structure is defined by the metal layers offered by the CMOS foundry and released using a few simple etch steps. The system is implemented in TSMC 0.35µm CMOS and was post-CMOS processed at NTHU in Taiwan. Detailed modeling, simulation and implementation of the system show the performance of these higher order MEMS resonator mixer-filters as a potential candidate to replace bulky offchip transceiver components.