Sound Pressure Level Sensor: Design and Fabrication

MEMS acoustic pressure sensor plays an important role in the measurement of sound pressure level in military equipment, satellite launch vehicle and communication industry. MEMS acoustic sensor is based on the principle of piezoelectricity. After comparison between different piezoelectric material, Aluminum nitride is the most suitable material for acoustic sensor because of its CMOS compatibility. In the present approach the design and simulation of ALN based MEMS acoustic sensor has been done using MEMS CAD tool COVENTORWARE. The thickness of Si-diaphragm consists with AlN layer sandwiched between two Al electrodes has been optimized for higher SPL range 120-180 dB. The sensitivity of the sensor was simulated and found to be 175µV/Pa. microtunnel was also designed for pressure compensation and protection of diaphragm in MEMS acoustic sensor. The microtunnel and cavity formed during diaphragm design gives the lower cut-off frequency of the device .which was found to be 0.01Hz.the device was designed for the frequency range 0.01 Hz to 10 kHz. To get the higher cut-off frequency of the device, the resonance frequency of the structure was calculated analytically and found to be 41.9 kHz. For linear operation of the device the higher cut-off frequency should for below the resonance frequency.In order to achieve the exact dimension of the microtunnel and Si–diaphragm, the deep reactive ion technique (DRIE) instead of TMAH was used for Si bulk-micromachining. In the fabrication process 6inch diameter Si (p-type <100>) wafer of 600µm-thick was used. The fabricated structure of microtunnel and diaphragm were characterized using microscope having high magnification, Dektak 6M surface profiler and SEM (scanning electron microscope). A deposition process of piezoelectric layer AlN was developed using DC magnetron sputtering technique. After developing the layer it was deposited on processed wafer for fabrication of MEMS acoustic sensor. The device was fabricated up to the structure, thermal SiO2-Al-PECVD SiO2-AlN-PECVD SiO2-Al on a 25µm thick Si diaphragm fabricated using DRIE technique. The characterizations of each layer used in fabrication of the device were formed using Dektak 6M surface profiler. The fabricated device will be used to measure the sound pressure level during satellite launch vehicle.