High-Sensitivity Encoder-Like Micro Area-Changed Capacitive Transducer for a Nano-g Micro Accelerometer

Abstract

Encoder-like micro area-changed capacitive transducers are advantageous in terms of their better linearity and larger dynamic range compared to gap-changed capacitive transducers. Such transducers have been widely applied in rectilinear and rotational position sensors, lab-on-a-chip applications and bio-sensors. However, a complete model accounting for both the parasitic capacitance and fringe effect in area-changed capacitive transducers has not yet been developed. This paper presents a complete model for this type of transducer applied to a high-resolution micro accelerometer that was verified by both simulations and experiments. A novel optimization method involving the insertion of photosensitive polyimide was used to reduce the parasitic capacitance, and the capacitor spacing was decreased to overcome the fringe effect. The sensitivity of the optimized transducer was approximately 46 pF/mm, which was nearly 40 times higher than that of our previous transducer. The displacement detection resolution was measured as 50 pm/√Hz at 0.1 Hz using a precise capacitance detection circuit. Then, the transducer was applied to a sandwich in-plane micro accelerometer, and the measured level of the accelerometer was approximately 30 ng/√Hz at 1Hz. The earthquake that occurred in Taiwan was also detected during a continuous gravity measurement.

Highly Sensitive Signal Processing Devices for Capacitive Transducers of Micromechanical Accelerometers

“…It uses high-frequency harmonic oscillators, the frequencies of which are limited only by the parameters of the CMOS-technology used and power dissipation requirements. In accordance with Equations (11) and (13), high frequency of the oscillators increases sensitivity and expands dynamic range of MEMS-accelerometers with the considered SPD. According to Figure 9, functionality of this frequency-based SPD is saved even for RMS noise of input signals (N RMS = 0.5 V) higher than amplitude of these signals (0.1 V).…”

“…In the voltage-based or charge-based SPD, these voltage fluctuations are significantly amplified by the high-gain amplifier. This requires an increase of the proof mass to minimize the noise-floor and expand the dynamic range R D [7,8,13]. In contrast to this, there is no amplifier in the open-loop frequency-based SPD ( Figure 2).…”

“…An important advantage of the considered SPD project is a possibility of increasing the frequency of the oscillators to maximum values determined by the CMOS-technology used and the permissible level of phase noise. According to Equations (11) and (13), higher frequency of the SPD oscillators provides an increase in sensitivity and dynamic range of the accelerometer.…”

“…There are many kinds of transducers in present micromechanical accelerometers, such as capacitive, piezo-resistive, electromagnetic, optical, thermal convective, acoustic waves resonant, field emission, tunnel transducers and so on [6]. The micromechanical accelerometers with capacitive transducers are widely used due to relatively simple transducer structure, high-sensitivity, reliability, thermal stability, excellent linearity, resolution, bandwidth, good compatibility with integrated circuits technology, and consequently low cost [7][8][9][10][11][12][13][14].…”

“…Moreover, the switching-capacitor charge-based technique is conventionally used in closed-loop SPD to effectively suppress flicker noise [19]. Therefore, voltage control closed-loop SPD are frequently used today especially for high-sensitivity and high accuracy MEMS accelerometers [8][9][10][11][12][13]. However, there are the following disadvantages of voltage control closed-loop signal processing devices:…”