Research
Master's Thesis on the Development of a Piezoelectric MEMS Pressure Sensor for Quench Detection in Cryogenic Superconductors
Quenches are an unwanted phenomenon that occurs when superconducting magnets (or coils) lose their superconducting efficiency, and return to their normal state. This is either caused by a local temperature increase, exceeding a maximum current density, or an increase in local mechanical strain. In high energy applications In high energy applications quenches, and specifically the rapid switch from a zero-resistance state to a normal state, can cause substantial damage.
Initial analysis suggests that quench pressures could be as high as 1-10 atmospheres (100 kPa – 1MPa). Commercial acoustic MEMS microphones would saturate or possibly be destroyed at by these pressure pulses, and at cryogenic temperatures. The purpose of this thesis is to design, demonstrate and lay out the fabrication method for a MEMS pressure sensor that could operate at 4K and can sense transient pressure pulses on the order of 1 to 10 atm.
The sensor features a piezoelectric stack consisting of aluminum nitride as the piezoelectric, sandwiched between a top and bottom molybdenum electrode layer (Mo/AlN/Mo). These are deposited on a silicon-on-insulator (SOI) wafer with a thermal oxide layer grown on top. The insulator/buried oxide is etched out to create an airgap that the piezoelectric layer will deflect into (mimicking a clamped plate). The sensor functions by reading the amount of charge induced for a given deflection in the piezo layer. For a given transient stress source, such as a pressure spike caused by a quench, the AlN piezoelectric layer will deflect and generate a charge. Depending on the capacitance measured, directly related to the induced charge, between top and bottom electrodes one can measure the intensity of the pressure spike.
I've created the additive manufacturing method/ process for fabricating the sensor. This includes creating masks using LayoutEditor, writing process parameters for plasma depositions, developing charts of different photoresists, and acids for lithography/patterning and etching. I completed COMSOL stress analysis for given deflections and combined electromagnetic analysis to generate charts of induced charge verses pressure in order to determine the sensitivity of the device. Though I wasn't able to finish the fabrication of the device during the time of my masters, I illustrated the packaging, dicing, and use of test structures I also designed in order to validate the quality of each layer.
Co-Author/Contributor,
"Cryogenic Testing of Microphones and Preamplifiers for MEMS Quench Detection in Superconducting Magnets " 2021
Continuation of quench detection in superconducting systems research. Designed and developed preamplifier circuits to be used with commercial MEMS in cryogenic testing.
Previous testing showed that MEMS with built in amplifier ASIC failed at cold temperatures. Purpose of this research was to test each part separately. Preamplifier was designed using commercial op-amps. Results showed we were able to operate in temperatures as low as 13K.
Attended the Acoustical Society of America's 181st meeting as a contributor to the work.
Contributor,
"Characterization of MEMS Acoustic Sensors and Amplifiers in Cryogenic Fluids for Quench Detections Applications in HTS CICC," 2021
Contributor of research article published in IEE Transactions on Applied Superconductivity. Collaborated directly with professors from Tufts University as well as MIT and Tanner Research. Created cryogenic testing rig and testing method for Micro-Electromechanical Sensor (MEMS) pressure sensors.
Fabrication of Piezoelectric Aluminum Nitride Thin Films, 2019
Research conducted at the Tufts University Micro/Nanofabrication Facility with professor Robert White.
Over the course of this research position, I developed a processing method for the fabrication of aluminum nitride thin films. A three layer piezoelectric-electrode stack, consisting of a layer of aluminum nitride between two layers of molybdenum, was sputtered onto silicon wafers. I designed surface acoustic wave (SAW) relay lines, as well as test structures for resistive testing, and patterned them onto the top molybdenum layer through the use of a photoresist and wet etching. Two different wet etches were tested. Resistive tests as well as etch rate were measured in order to optimize the aluminum nitride fabrication process. See results of research below.
