Fabrication of ultra-thin silicon stress sensor chips with high flexibility and high sensitivity

We propose a method for fabrication of ultra-thin stress chips (UTSC) by dicing before thinning method on chip-scale level, discuss attributes of UTSC, and report their applications in bio-medical areas, including pulse monitoring and orthodontic force measurement. Using an edge protection method and a wafer transfer technology, silicon stress sensor chips with size of 2 mm× 2 mm fabricated using CMOS technology have been reconfigured as a virtual wafer and thinned from 350 μm to 35 μm. The thinned individual chips have been laminated with a flexible polyimide substrate and wire-bonded to the electric interconnects embedded in the polyimide substrate. Measurement results show that the sensitivity of the UTSC is about 70 times larger than that of metal strain gauge. The flexibility and the high sensitivity of the UTSC enable the measurement of human wrist pulse and orthodontic force in 1:1 teeth model. These preliminary results demonstrate a possible solution for sensor integration on flexible substrates using dicing before thinning.