Sheng-Hsiang Tseng

A CMOS Micromachined Capacitive Tactile Sensor With High-Frequency Output

This paper describes the design and characterization of a CMOS-micromachined tactile sensing device that can be utilized for fingerprint recognition. The complete post micromachining steps are performed at die level without resorting to a wafer-level process, providing a low-cost solution for production. The micromechanical structure has an area of 200 mum by 200 mum and an initial sensing capacitance of 153 fF. An oscillator circuit is used to convert the pressure induced capacitance change to a shift in output frequency. The circuit has a measured initial frequency at 49.5 MHz under no applied force. The total frequency shift is 14 MHz with a corresponding mechanical displacement of 0.56 mum and a capacitance change of 63 fF, averaging a capacitive sensitivity of 222 kHz/fF. The measured spring constant is 923N/m, producing a force sensitivity of 27.1 kHz/muN

Implementation of a monolithic capacitive accelerometer in a wafer-level 0.18 µm CMOS MEMS process

This paper describes the design, fabrication and characterization of a complementary metal-oxide-semiconductor (CMOS) micro-electro-mechanical-system (MEMS) accelerometer implemented in a 0.18 µm multi-project wafer (MPW) CMOS MEMS process. In addition to the standard CMOS process, an additional aluminum layer and a thick photoresist masking layer are employed to achieve etching and microstructural release. The structural thickness of the accelerometer is up to 9 µm and the minimum structural spacing is 2.3 µm. The out-of-plane deflection resulted from the vertical stress gradient over the whole device is controlled to be under 0.2 µm. The chip area containing the micromechanical structure and switched-capacitor sensing circuit is 1.18 × 0.9 mm2, and the total power consumption is only 0.7 mW. Within the sensing range of ±6 G, the measured nonlinearity is 1.07% and the cross-axis sensitivities with respect to the in-plane and out-of-plane are 0.5% and 5.8%, respectively. The average sensitivity of five tested accelerometers is 191.4 mV G−1with a standard deviation of 2.5 mV G−1. The measured output noise floor is 354 µG Hz−1/2, corresponding to a 100 Hz 1 G sinusoidal acceleration. The measured output offset voltage is about 100 mV at 27 °C, and the zero-G temperature coefficient of the accelerometer output is 0.94 mV °C−1 below 85 °C.

A Highly Sensitive CMOS-MEMS Capacitive Tactile Sensor

This paper describes the design and characterization of a capacitive tactile sensor fabricated in a conventional CMOS process. To achieve a high capacitive sensitivity, an oscillator circuit is adopted to convert the pressure induced capacitive change to an output frequency shift. The complete post micromachining steps are performed on a CMOS die without resorting to a wafer process. The pressure-sensing membrane has a total size of 200 µ m × 200 µ m with an initial sensing capacitance of 153 fF. Experimental results show an initial frequency output at 48.96 MHz under no applied load. The total frequency shift is 13.5 MHz with a corresponding membrane displacement of 0.56 µ m and a capacitance change of 63 fF, averaging 0.21 MHz/fF. The measured force sensitivity is 26.1 kHz/µ N.

A CMOS MEMS thermal sensor with high frequency output

This work presents the design and characterization of a CMOS-integrated thermal sensor that features a novel oscillator-based sensing interface to achieve a high thermoelectric sensitivity. The thirty pairs of thermocouples are made of n+/p+ polysilicon of a standard 0.18-mum CMOS to produce a large thermoelectric voltage. The produced thermoelectric voltage is used to control the bias current of a high-frequency oscillator circuit and causes a shift in the output frequency. Experimental result indicates the 544-MHz oscillator has a temperature sensitivity of 46.3 kHz/degC.

CMOS micromachined structures using transistors in the subthreshold region for thermal sensing

In this paper, we describe the design and characterization of a CMOS micromachined suspended structure with a PMOS transistor operated in the subthreshold region for thermal sensing. The sensor has an area of 70 µm by 70 µm and the sensing transistor is placed inside a suspended plate of 50 µm by 50 µm. The device is fabricated by successive dry etching steps on a CMOS die. For device characterization, each sensor has an embedded polysilicon heater capable of providing a heating efficiency of 0.84 °C mW−1. The measured MOS output resistance changes from 108.3 MΩ to 90.4 MΩ for a temperature rise of 5.5 °C, which is equivalent to a measured temperature coefficient of resistance −3% °C−1, much better than the value of transistors operated in saturation. The measured input-referred noise voltage from the pre-amp is 7 µV Hz−1/2, equivalent to an input-referred noise temperature of 14.9 mK Hz−1/2.

Study of CMOS micromachined self-oscillating loop utilizing a phase-locked loop-driving circuit

This work describes the design and characterization of integrated CMOS (complementary metal oxide semiconductor) oscillators comprising a capacitively transduced micromechanical resonator and a phase-locked loop (PLL) driving circuit. Three oscillator schemes are studied and compared, including direct feedback, direct feedback containing a PLL and hybrid direct feedback plus a PLL. PLL is known for its capability in automatic tuning and tracking of a reference signal. Inclusion of a PLL is beneficial for sustaining oscillations at resonant frequencies within its capture range. The micromechanical resonator has a measured resonant frequency of 117.3 kHz. The CMOS PLL circuit has a closed-loop bandwidth of 1.8 kHz with a capture range between 111 kHz and 118.4 kHz. The start-up times for oscillation are shortened in the two schemes utilizing a PLL, since it provides an initial driving signal at its free-running frequency. The lock-in time is also reduced by increasing the proportion of PLL drive in the hybrid scheme. The measured noises for the three oscillator schemes are similar with a value of −75 dB below the resonant peak at a 10 Hz offset.

Ultra-low power boost DC-DC converter with integrated MEMS resonator

Complementary metal-oxide-semiconductor micro-electromechanical system (CMOS MEMS) resonators provide considerable advantages in size, cost, and power consumption over their crystal-based counterparts. However, the need for external high bias-voltage to drive the MEMS structure has limited the application of CMOS MEMS in portable electronic applications. This paper proposes an ultra-low power boost DC-DC converter with integrated MEMS resonator. The proposed DC-DC converter and MEMS resonator are integrated into a single chip, and the high bias-voltage required for the MEMS resonator is self-provided. The substantial reduction in the size of the resulting device as well as its low power requirements make it ideal for portable applications. A working prototype of the device was fabricated using a UMC 0.18-μm 60-V bipolar-CMOS-DMOS with a MEMS post-process. Measurement results show that the power dissipation of the boost DC-DC converter is less than 15 μW under various input voltages. Comparisons of high bias-voltage from an external supply with an internal DC-DC converter demonstrate the performance of the design. A preserved reset signal at a different clock frequency also verifies the resistance of the proposed design to process variation.

A CMOS capacitive micromechanical oscillator driven by a phase-locked loop

This work describes the design and characterization of integrated CMOS (complementary metal oxide semiconductor) oscillators comprising of a capacitively transduced micromechanical resonator and a phase-locked loop (PLL) driving circuit. Inclusion of a PLL is beneficial for sustaining oscillations at resonant frequencies within the PLL capture range. The micromechanical resonator has a measured resonant frequency of 117.3 kHz. The CMOS PLL circuit has a closed-loop bandwidth of 1.8 kHz with a capture range between 111 kHz to 118.4 kHz. The start-up times for oscillation are shortened in the scheme utilizing a PLL as it provides an initial driving signal at its free-running frequency. The measured noises for the presented two oscillator schemes are -75 dB below the resonant peak at a 10-Hz offset.