Development of a proximity sensor with vertically monolithic integrated inductive and capacitive sensing units

This study designs and implements a proximity sensor consisting of inductive and capacitive sensing units. These two sensing units are vertically monolithic integrated on a single chip using the micro-fabrication processes. In addition, low-temperature fabricated nanoporous anodic aluminum oxide (np-AAO) is employed as the dielectric layer to enhance the performance of capacitive sensing. The characteristics of the presented vertically monolithic integrated inductive and capacitive proximity sensor are as follows: (1) enlarged sensing distance of conductive objectives: capacitive sensing unit for short distance detection and inductive sensing unit for long distance detection, (2) non-conductive object can be detected by the capacitive sensing unit, (3) fringe effect capacitive sensing is enhanced by the spiral coil electrode and (4) np-AAO has good dielectric properties (the dielectric constant is 11.9 in this study) for capacitive sensing. In application, various materials (including metal, plastic and a human finger) have been successfully detected by the presented sensor. Preliminary results demonstrate that the typical fabricated proximity sensor has a sensing range of 0.5–5 mm for the metal rod. In comparison, the inductive and capacitive sensing units have the sensing ranges of 1.5–5 and 0.5–3 mm, respectively. Moreover, the non-conductive plastic rod can be detected by the capacitive sensing unit.