Microfabrication of capacitive pressure sensors using ferrofluid sacrificial layers

A novel micromachined capacitive pressure sensor that is fabricated using a liquid-phase sacrificial layer is reported. Ferrofluid serves as the sacrificial layer in the microfabrication process, enabling extremely simple, fast, and low-cost production of the sensor, while eliminating the need for photolithographic, bonding, and/or chemical processes. The entire sensor fabrication is performed at/near room temperature. The sensor is based on the 1.5×1.5-mm² stainless-steel chip with a 10-μm-deep capacitive cavity. A Parylene-C membrane with a Ti electrode is formed to seal the cavity by depositing it directly on top of the ferrofluid filled in the cavity. The ferrofluid is magnetically extracted from the cavity, suspending the membrane to form the sensing capacitor. A highly linear response with a sensitivity of 12.4 fF/KPa is obtained with the fabricated device. The temperature dependence of the sensor capacitance is experimentally characterized and reported as well.