By Ark-Chew Wong, John R. Clark, & Clark T.-C. Nguyen
Micro-electro-mechanical (MEMS) bandpass filters with frequencies in the low VHF range (e.g., 35MHz) have recently been demonstrated with bandwidths less than 0.2 percent and associated insertion losses less than 2-DB - performance that rivals, and in some cases surpasses, off-chip crystal and surface acoustic wave (SAW) filters used in present-day wireless transceivers. Given their compatibility with IC fabrication technologies, MEMS filters can potentially serve as direct-on-chip replacements for their bulky, off-chip SAW and crystal counterparts, allowing substantial size and power reduction in wireless transceivers, perhaps even paving the way for single-chip implementations. In order for this to become a commercial reality, however, the operating frequency of MEMS filters must be extended.
Figure 1: Scanning electron micrograph of a 68MHz spring-coupled mircro-electro-mechanical filter with key components and
dimensions identified
In this work, tunable, two-pole micro-electro-mechanical (MEMS) filters for telecommunications applications are demonstrated with center frequencies in the range 50-68 MHz and bandwidths less than 2 percent, all with insertion losses less than 9dB. The 68MHz filter represents the highest frequency yet attained by a MEMS two-pole filter. The performance was attained by a combination of dimensional scaling and the following key design features: (1) electrodes allowing in situ localized annealing of devices to enhance the resonator quality factor (Q-factor) and thus minimize filter loss, and (2) additional electrodes flanking the input and output, allowing voltage controlled tuning of each individual resonator frequency to correct for process variations in this scaled technology.
As shown, this is a two-pole filter consisting of two polycrystalline silicon (poly-Silicon) MEMS resonators with identical geometries (and hence, identical resonance frequencies), coupled by a flexural-mode beam attached to each resonator at low-velocity locations (Figure 1). Three electrodes underlie each resonator: one for input and output coupling, and two symmetrically placed for frequency tuning via voltage-controllable electrical stiffness (i.e., prestress). In addition, electrodes to the anchors of each resonator are also included to allow localized annealing of the filter structure. The annealing (heating or bakeout) serves as a key anticontamination measure used before (and sometimes during) filter operation under nonideal test vacuum environments.
To operate the filter, the structure is biased to a DC voltage, and an AC input signal is applied to the input electrode. When the excitation frequency falls within the filter passband, the whole structure is forced into vibration, at which point an output voltage is generated across the output resistor. In effect, electrical signals at the input are converted to mechanical signals, processed in the mechanical domain, then reconverted to electrical signals at the output, ready for further processing by subsequent electrical stages.
In this work, ANSYS University Research (ANSYS Multiphysics) was used to understand the annealing of the structures. The device solid model was meshed using coupled-field (SOLID98) elements. These elements support structural, thermal, electric, and magnetic degrees of freedom (DOF). For this simulation, only structural, thermal, and electric DOF were used to model the behavior of the resonators at various applied annealing voltages. When a voltage is applied across the device - since the poly-Silicon has a finite resistance - a current flows, which produces Joule heating and subsequent thermal expansion of the device.Temperatures up to 400°C are generated (Figure 2).
Figure 2: Thermal tomography over a 50 MHz filter as predicted by finite element simulation using ANSYS.
Figure 3: Measured frequency characteristics for a 50MHz micro-electro-mechanical resonator (i) before and (ii) after in situ localized anneal-activation.
ANSYS Academic products can also be used to simulate the electrostatic-mechanical operation of the device and to compute the passband characteristics. Both structural and electrostatic physics domains are set up by the user, and the software automates the coupling between them.