ESE PhD Thesis Defense: “Multiferroic Micro Electromechanical Systems for Magnetic Sensing and Wireless Power Transfer in Biomedical Applications”

Multiferroic micro-electromechanical systems (MEMS) enable small, room temperature, low power magnetic sensing and wireless power transfer (WPT) in biomedical applications. Current biomagnetic sensing relies on sensitive magnetometers like superconducting quantum interference devices (SQUIDs), but their reliance on cryogenic temperatures is undesirable.

This thesis presents the theory, design, microfabrication, and characterization of multiferroic MEMS magnetic sensors and WPT devices. Iron cobalt/silver (Fe₅₀Co₅₀/Ag) magnetostrictive material is coupled to piezoelectric aluminum nitride (AlN) to form a multiferroic sensor. Low frequency biomagnetic signals are upconverted around the length-extensional beam’s 7-16 MHz mechanical resonance to provide Q enhancement to the sensitivity. The up conversion exploits a nonlinear phenomenon of magnetostrictive materials with applied mechanical strain. For two devices studied, modulated sensitivities of 58.4 mA/T and 37.7 mA/T were observed along with resolutions of 5.03 nT/√Hz and 2.72 nT/√Hz over a bandwidth larger than the biomagnetic frequency spectrum (0.1Hz to 1kHz). The sensors’ sensitivity was limited by Duffing nonlinearity and the relatively low piezoelectric coefficients of AlN.

To improve sensitivity, magnetoelectric sensors were fabricated using (Fe_0.5Co_0.5)_0.92Hf_0.08 coupled to 28% aluminum scandium nitride (Al_0.72Sc_0.28N). Increasing sensitivity improved the resolution from 5.03 nT/√Hz to 2.16 nT/√Hz. To delay the onset of thermal Duffing nonlinearity, various anchoring tether lengths were explored in Fe_0.5Co_0.5/Ag – AlN magnetoelectric sensors to provide better heat conduction away from the structure. Also, silicon dioxide (SiO₂) was added to compensate the temperature coefficient of frequency (TCF). Larger achievable strain was verified before the onset of Duffing nonlinearity, providing increased modulation of the Fe_0.5Co_0.5/Ag and a resolution of 1.11 nT/√Hz, an 86% improvement when compared to a long tether device with the same layer stack (8.02 nT/√Hz) and a 78% improvement over the initial (Fe₅₀Co₅₀/Ag) – AlN long tether devices with no SiO₂ thermal compensation.

WPT measurements were taken using (Fe₅₀Co₅₀/Ag) – AlN magnetoelectric devices. By sending a magnetic field at the device resonance frequency, optimal WPT can be achieved. Devices were packaged with a magnetic bias circuit and the output power was measured. For a device at 7.44MHz, an output power of 126.8 nW and a power density of 1196.2 uW/mm³ is projected when measuring with both electrodes.