Surface-mountable capacitive tactile sensors with flipped CMOS-diaphragm on a flexible and stretchable bus line

This paper describes a MEMS-CMOS integrated tactile sensor for surface mounting on a flexible and stretchable bus line. The sensor is featured by the following configurations; (1) A sensing diaphragm is formed on a CMOS substrate by backside etching, and (2) the CMOS substrate is flip-bonded to a special low temperature cofired ceramic (LTCC) substrate with Au through vias. The flipped CMOS substrate and the LTCC substrate were electrically and mechanically connected using Au-Au bonding, which also formed differential capacitive gaps. The structure and fabrication process are simple compared with the existing surface-mountable MEMS-CMOS integrated sensors. Bendable and stretchable bus lines were fabricated by etching metal, which is deposited on a polyimide substrate and the outer shape is then determined by laser cutting. The bus line with a silicone protection coat is capable of stretching up to 50%. The sensors covered with silicone worked even under 10% stretching of the bus line. The tactile sensors mounted on the surface of the flexible bus line were characterized in terms of force sensitivity. Also, smart functions such as threshold and adaptation operations and the configuration of sensor parameters were demonstrated.