Sensitivity Enhencement in Monolithical CMOS-MEMS Tactile Force and Proximity Sensors Using 3D Toroidal Coils

This study presents a novel design of inductive tactile force and capacitive proximity sensors by integrating toroidal-coil (formed by numerous solenoid-coils) and planar-coil vertically, along with interdigitated electrodes (IDE) horizontally (Fig. 1 ). Based on the TSMC <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.18\mu \mathrm{m}$</tex> 1P6M CMOS platform, the proposed coils are available to be leveraged vertically by multiple metal layers and VIAs, which helps the miniaturization of two coil types footprint. Using the toroidal-coil in the sensor integration, magnetic flux leakage is reduced at the edges of the planar-coil, improving magnetic field density and thereby increasing sensitivity of the tactile force sensor. Furthermore, by mitigating flux leakage, the toroidal-coil minimizes electromagnetic cross-talk between the force and proximity sensors, ensuring more reliable dual-sensor operation. Measurements demonstrate that the proposed tactile force sensor achieves a sensitivity of 0.98 nH/N for sensing range 0 to 5.5 N, compared to 0.49 nH/N in the reference design. Additionally, the proposed proximity sensor exhibits only a 1.5% sensitivity drop, in stark contrast to a 9% drop observed in the reference design.