Chengchen Gao

A monolithic integration multifunctional MEMS sensor based on cavity SOI wafer

This paper reports a monolithic integration multi-functional MEMS sensor for acceleration and pressure measurement based on cavity SOI wafer. The piezoresistive sensor uses a beam-block-membrane structure which is not only sensitive to acceleration, but also can be used as a gauge pressure transducer to detect low pressure. A miniature absolute pressure sensor is embedded to the accelerometers mass block without increasing in chip area. This gapless sensor is designed for dust environment. The fabrication process allows thin membrane design to achieve high sensitivity in small chip size. The measure range of the accelerometer can be adjusted by controlling etching depth in manufacturing process after layout design and CMOS process. A 100g measurement range accelerometer which can be used as a 5kPa gauge pressure transducer and an 180μm×180μm 500kPa absolute pressure sensor are integrated in a 1.7mm×1.7mm die for tire pressure monitoring system and fabricated in 6 inch production line. The actual device shows the sensitivity of 0.164mV/g/V to acceleration and 0.0524mV/kPa/V to absolute pressure in 3.3V supply voltage.

Design and fabrication of a mems capacitive accelerometer based on double-device-layers SOI wafer

This paper presents a capacitive MEMS accelerometer with highly symmetric sandwich structure (Glass-Si-Glass). In order to get highly symmetric beam-mass structure (silicon middle-layer), a double-device-layer SOI (D-SOI) wafer, which has identical buried oxides (BOX) and device layers on both sides of a thick handle layer was adopted in fabrication. The fabrication process produced proof mass with though wafer thickness (1mm) to increase the sensitivity of the accelerometer. Two layers of single crystal silicon beams with highly uniform dimension suspended the proof mass from both sides symmetrically. The highly symmetric beam-mass structure reduced the cross axis sensitivity and rotational influences of the microaccelerometer dramatically. Two glass cap wafers with capacitance electrodes were anodic bonded with middle-layer wafer to form the capacitances. Initial capacitances designed to be 80pF were measured in the range of 75.03~86.94pF. The amplitude of capacitances variation up to 55pF/±1g was measured.

A SOI sandwich differential capacitance accelerometer with low-stress package

Thermal stress and device bending have significant effect on the performance of MEMS sensors. In this paper, a MEMS sandwich differential capacitance accelerometer with low-stress package is presented. The accelerometer is based on a thin silicon middle layer which has been bonded with two glass electrode plates. A metal hermetic package case is used for reliability and noise immunity. The purpose is to simplify manufacturing process and reduce thermal influence. The methods of reducing thermal stress and deflection were evaluated. The thickness of glass electrode plates was optimized for low-stress. A silicon multi-point supporting frame which could reduce thermal stress between glass electrode plate and metal case was designed and simulated. The stress model in this study provides useful information for sandwich structures. Test results in actual device show it has the sensitivity of 0.1124V/g and 0.435% nonlinearity error in test range of 0~50g, 0.02%/°C zero temperature drift without temperature compensation.

High temperature pressure sensor using a thermostable electrode

High temperature pressure sensors have been widely applied in modern industry. However, the performance of the pressure sensor is largely dependent on the thermal stability of ohmic contact electrode. This paper presents a TiSi2/Ti/TiN/Pt/Au multilayer electrode. Linear transmission line method (TLM) and voltmeter-ammeter method have been used to measure the electronic properties of the electrode at high temperature. The test results show that the multilayer electrode can be used in fabricating high temperature pressure sensor. Moreover, a high temperature piezoresistive pressure sensor is designed using this multilayer electrode. ANSYS software and finite element method (FEM) have been used to analyze the stress distribution, sensitivity and nonlinearity. To verify this design, the pressure sensor is fabricated based on silicon on insulator (SOI) wafer. The pressure sensor is measured across the range of 30-150kPa and the temperature range is 25-500°C. The test results show that the ohmic contact electrode and the pressure sensor are able to work at high temperature.

Design of a novel low cross-axis sensitivity micro-gravity sandwich capacitance accelerometer

This paper presents a novel design of the highly symmetric beam-mass structure for out-of-plane differential capacitance accelerometer. In this design, the thick proof mass is mirror symmetrically suspended by eight L-shape beams form both top and bottom sides. By this approach, the sensing mode of the accelerometer is successfully decoupled from other variation modes without tradeoff between sensing capacitance and structure flexibility. Accelerometer adopting this structure achieves a differential capacitance sensitvity of 54.38pF/g. The capacitance instability is 0.95µg (30min) in normal pressure, and the close-loop sensitivity is1.01v/g.

Anemometer for detection of very low speed air flow with three-dimensional directionality

This paper shows measurement of very low speed air flow with a new sensor. This micro-machined sensor contains two monolithically integrated orthogonal cells making it prospective to realize three-dimensional (3D) detection of the direction and velocity of air flow at one point. Each cell (vent) has a five-wire configuration, formed by five resistive beams of platinum (with pitch of 354 µm), sensitive to two direction. It was measured from 16.7 mm/s to 167 mm/s, showing good repeatability and demonstrating its directionality. Velocity distribution was simulated, and results corresponded to the theoretical lower limit of velocity of 50 mm/s due to natural convection, the induced angle dependent offset voltage of which can be taken into account to make a more accurate computation of lower velocity.

Design of an ultra-low pressure sensor based on the growth of graphene on silicon dioxide surface

In this paper, we present the theoretical analysis and calculation results for characterization of a graphene diaphragm applied for capacitive ultra-low pressure sensor structure, where a few layers of graphene diaphragm with radius of 8 µm and total thickness of 2 nm, the graphene diaphragm is suspended over a circular cavity. In this paper, the graphene film was fabricated by an atmospheric-pressure chemical vapor deposition approach to directly form large-area uniform graphene film on silicon dioxide layer. Calculation results show that this capacitive pressure sensor is able to provide 164aF/Pa pressure sensitivity in an ultralow pressure range from 1Pa to 5Pa.

Anemometer with three-dimensional directionality for detection of very low speed air flow and acoustic particle velocity detecting capability

We develop a micro-fabricated anemometer with three-dimensional directionality based on thermal Thomas flowmeter principle, which is capable of measuring very low airflow speed. This sensor has three layers of resistive platinum beams. Experiments on speed lower than 200 mm/s are conducted, showing acceptable repeatability and obvious directionality. The speed of airflow is tested with the volumetric flow rate. To further explore its potential, we expose it to acoustic plane waves of frequency range from 200 Hz to 1.38 kHz in a standing wave tube, showing consistent planar directivity of figure of eight. Velocity sensitivity at 1 kHz and 120 dBC measures 4.4 mV-s/m, with relative sensitivity of 27.0 dB.

Design of a graphene capacitive pressure sensor for ultra-low pressure detection

This paper reports the design and theoretical calculation of a capacitive ultra-low pressure sensor, which is based on circular suspend graphene diaphragm array. The atom scale thickness and high mechanical strength of suspend graphene diaphragm both contribute to ultra-low pressure measure range. As the size of single suspend graphene diaphragm is limited by fabrication process, a parallel connected sensor array is designed to achieve detectable capacitance change and redundancy. In calculation, a sensor array with 40,000 5μm radius suspend graphene diaphragm cells in 4mm×4mm size can provide 288fF/Pa sensitivity to pressure load.

Design and simulation of corrugated diaphragm applied to the MEMS fiber optic pressure sensor

In this paper, we propose a corrugated diaphragm applied to the fiber optic pressure sensor which needs a wide sensing range and a large deflection. The corrugated diaphragm can effectively improve the sensitivity of the sensor has been proved by ANSYS simulation. The stress concentration is weakened after the silicon is chamfered into corrugated diaphragm by isotropic etching process. The sensitivity of the corrugated diaphragm can reach to 0.44μ m/MPa which is two times larger than that of the same size planar diaphragm. The maximum sensing stress is up to 14MPa.