Ferrofluid sacrificial microfabrication of capacitive pressure sensors

A novel production approach to the fabrication of capacitive micropressure sensors is reported. A magnetic fluid known as ferrofluid is used as the liquid-phase sacrificial layer in the microfabrication process, enabling extremely simple, fast, and low-cost production of the sensors while eliminating the need for photolithographic, bonding, and/or chemical processes. The entire sensor fabrication is performed at/near room temperature. The sensors are designed to be constructed on the 1.5 × 1.5-mm² stainless-steel chip, being micromachined to have capacitive cavities with 10-30~μm depths. A Parylene-C membrane with a titanium 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 after the formation of the membrane, suspending it to establish the sensing capacitor. A highly linear response 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.