Modeling and Characterization of a Pull-in Free MEMS Microphone

Abstract

In this study, we examine the feasibility of designing a MEMS microphone employing a levitation based electrode configuration. This electrode scheme enables capacitive MEMS sensors that could work for large bias voltages without pullin failure. Our experiments and simulations indicate that it is possible to create robust sensors properly working at high DC voltages, which is not feasible for most of the conventional parallel plate electrode-based micro-scale devices. In addition, the use of larger bias voltages will improve signal-to-noise ratios in MEMS sensors because it increases the signal relative to the noise in read-out circuits. This study presents the design, fabrication, and testing of a capacitive microphone, which is made of approximately 2 µm thick highly-doped polysilicon as a diaphragm. It has approximately 1 mm 2 surface area and incorporates interdigitated sensing electrodes on three of its sides. Right underneath these moving electrodes, there are fixed fingers having held at the same voltage potential as the moving electrodes and separated from them with a 2 µm thick air gap. The electronic output is obtained using a charge amplifier. Measured results obtained on three different microphone chips using bias voltages up to 200 volts indicate that pull-in failure is completely avoided. The sensitivity of this initial design was measured to be 16.1 mV/Pa at 200 V bias voltage, and the bandwidth was from 100 Hz to 4.9 kHz.

Large-Stroke Capacitive MEMS Accelerometer Without Pull-In

“…Our expectation for the sensitivity was to increase with the voltage on the side electrodes. In a different publication of our research group [18], where the repulsive electrode configuration is used to make a MEMS microphone, the sensitivity increases with the voltage on the side electrode. In that study, where the same electrical circuit (Fig.…”

“…This nature of the electrostatic force eliminates the pull-in possibility between the moving electrode and the underlying fixed electrode [4], [15], [18]. Aside from eliminating pull-in, another advantage of the repulsive approach is that it extends the travel range of the microstructure more than initial fabrication gap and therefore provides a transduction scheme to make a new class of MEMS sensors and actuators.…”

Large-Signal Equivalent-Circuit Model of Asymmetric Electrostatic Transducers

“…Standard MEMS microphones [12], as well as CMUTs [13], are based on a membrane that bends upon application of an electric field, so the equation of a parallel-plate capacitor imposes certain limitations. A more recent development, the pull-in-free microphone from Ozdogan et al [25], is based electrostatic levitation and holds no resemblance to the springcapacitor system. This transducer was therefore modelled with a displacement-varying capacitance following a polynomial expression.…”

Dynamic Behavior of T-beam Resonator With Repulsive Actuation

“…Repulsive electrode configuration is a multi-electrode capacitive scheme that addresses the issues associated with conventional two-electrode designs [7,8]. Since their introduction by Lee et al [7], this scheme has been explored to realize different MEMS sensors and actuators such as Micro-Mirrors [9,10], microphones [11], accelerometers [12], MEMS switches [13] and resonators [8]. In this study, we investigate the feasibility of using repulsive electrode configuration to build a MEMS gas sensors.…”

“…This force is upward, meaning that it pushes the microstructure away from the bottom electrode [8,22]. This nature of the electrostatic force eliminates the pull-in possibility between the moving and bottom electrodes [11,23]. Because the beam goes further away from the bottom electrode when a DC voltage is applied on the side electrode, the effective gap between the moving and bottom electrodes increases [8].…”