MEMS ultrasonic sensors for anemometry applications on MARS

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Thacker, Mayank Bhupendra
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The design, development, and fabrication of a 67.5 kHz capacitive micromachined ultrasonic transducer (CMUT) suited for Martian anemometry is presented in this thesis. The CMUT is a clamped membrane structure that vibrates on the application of an electric field, generating ultrasound. The Martian environment is 95 % carbon dioxide (CO2) with atmospheric pressures around ~ 600 Pa. This produces a high mismatch at the device-air interface leading to higher ultrasonic signal attenuation. To have lower signal attenuation in such conditions, the device's operating frequency is limited to 100 kHz. The CMUTs capable of generating frequencies less than 100 kHz either needed a larger Silicon area or higher operating voltages. This is a problem for the portability and the battery operation of the devices, respectively. The devices presented in this thesis are designed and fabricated using low-cost commercially available surface micromachining technique, PolyMUMPs. PolyMUMPs process from MEMSCAP inc. is a three-layer polysilicon surface micromachining process. The CMUT devices proposed in this thesis make use of two out of three polysilicon layers. COMSOL Multiphysics simulations were used to analyze the device's critical design parameters like the operating frequency, breakdown voltage, and frequency response. These simulations helped investigate the operability of devices under the Martian environment. Simulation results show that the designed single cell 170-μm radius membrane had a resonant frequency of 68 kHz. The device exhibits a static displacement of ~90 nm under 16 V DC bias. Using the developed single cell model, an 8×10 array CMUT anemometer was fabricated and evaluated that resonated at 67.5 kHz frequency in the lab environment. This proposed CMUT anemometer can operate for a supply <38 V. The acoustic performance of the device was evaluated by using a commercial air-coupled capacitive microphone, CAP1, and with another similar CMUT chip in the lab environment. Successful transmit-receive of ultrasound from the developed 2D array to CAP1 and another CMUT chip for separation in the range of 1-16 cm were performed from the pitch-catch experiments. The parameters like the speed of sound, near/far field transition point, and attenuation were found for each case. The experimental results show that the parameters can be accurately measured this developed technology with a ±2% accuracy. Anemometry using the developed CMUT chips was performed using the pitch-catch experimental setup having a 10 cm separation between the two chips. Air pressure was directed towards the chips to imitate wind-like conditions. The pressurized air was directed to the transmitter first and later on the receiver to have the anemometry measurements. Accurate measurements were made with the help of the CMUT system. The pressurized air was later directed at different angles, and it was found that the chips using the proposed CMUT devices were responsive to the angular winds. This property enables the CMUT chips to determine the wind direction when connected as a sonic anemometer. A comparison of the CMUT-based system with the commercial anemometer was made and same results were obtained.
MEMS, Ultrasound, CMUT, MARS, Anemometry, Pitch - catch