Abstract
Ultra-low pressure measurement is necessary in many areas, such as high-vacuum environment monitoring, process control and biomedical applications. This paper presents a novel approach for ultra-low pressure measurement where capacitive micromachined ultrasonic transducers (CMUTs) are used as the sensing elements. The working principle is based on the resonant frequency shift of the membrane under the applied pressure. The membranes of the biased CMUTs can produce a larger resonant frequency shift than the diaphragms with no DC bias in the state-of-the-art resonant pressure sensors, which contributes to pressure sensitivity improvement. The theoretical analysis and finite element method (FEM) simulation were employed to study the relationship between the resonant frequency and the pressure. The results demonstrated excellent capability of the CMUTs for ultra-low pressure measurement. It is shown that the resonant frequency of the CMUT varies linearly with the applied pressure. A sensitivity of more than 6.33 ppm/Pa (68 kHz/kPa) was obtained within a pressure range of 0 to 100 Pa when the CMUTs were biased at a DC voltage of 90% of the collapse voltage. It was also demonstrated that the pressure sensitivity can be adjusted by the DC bias voltage. In addition, the effects of air damping and ambient temperature on the resonant frequency were also studied. The effect of air damping is negligible for the pressures below 1000 Pa. To eliminate the temperature effect on the resonant frequency, a temperature compensating method was proposed.
Highlights
Ultra-low pressure measurement is of great importance in areas of high-vacuum environment monitoring, process control and biomedical applications, which is a challenging problem as the measured pressure is extremely small.[1,2,3] With recent advancement in MEMS technology, numerous investigations focused on silicon-based micromachined pressure sensors due to their small size, high sensitivity and low-energy consumption
This paper presents a novel approach for ultra-low pressure measurement where capacitive micromachined ultrasonic transducers (CMUTs) are used as the sensing elements
A sensitivity of more than 6.33 ppm/Pa (68 kHz/kPa) was obtained within a pressure range of 0 to 100 Pa when the CMUTs were biased at a DC voltage of 90% of the collapse voltage
Summary
Compared with the clamped diaphragm or diaphragm-beam resonant elements in the state-of-the-art resonant pressure sensors,[6] CMUT membranes (thin plates in most cases) under DC bias voltages are able to achieve a larger resonant frequency shift for the same applied pressure. This is due to the fact that the pressure can generate larger changes in the deflection and stresses of the CMUT membrane under bias voltage than the diaphragm or diaphragm-beam elements without DC bias voltage. The effects of air damping and temperature on the sensing performance were studied
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