Development of highly-sensitive and ultra-thin silicon stress sensor chips for wearable biomedical applications

• A fabrication method has been developed for thinning individual sensor chips using a reconfiguration method.
• Ultra-thin and highly-sensitive stress sensor chips with thickness of 35 μm have been achieved, and the sensitivity is 70 times better than metal strain gauge.
• The ultra-thin stress sensor chips have been used to measure pulse on wrist and orthodontic force of invisible aligners.

This paper reports the development of a highly-sensitive and ultra-thin silicon stress sensor chip (UTSC) and its applications for wearable sensors. Stress sensor chips are fabricated using CMOS technology, and after dicing the individual chips are reconfigured into a virtual wafer on a carrier wafer using temporary adhesive bonding. The reconfigured wafer is then thinned using mechanical grinding, polishing, and wet etching. After thinning, the sensor chips with thickness of 35 μm are laminated to a thin Kapton PI film, followed by de-bonding to separate the carrier wafer. Measurement results show that the UTSC is able to comply with curved surfaces, and the sensitivity is around 70 times that of metal strain gauge. The specifications of the UTSC are characterized in terms of linearity, repeatability, hysteresis, and zero drift. The UTSCs are demonstrated to measure human pulses on wrist and orthodontic forces of invisible aligners for dental treatment. The preliminary results show that the reconfigured method is applicable to thinning individual chips, and the UTSCs are flexible and sensitive enough for measurement of stress and strain on curved surfaces on human bodies.