An Ultra-thin CMOS Sensor for In-plane Stress Sensing

Ultra-thin chips combine flexibility and mechanical stability of silicon (20 µm thick) and maturity of CMOS technology. However, circuit design on bendable dies brings up new challenges that are not typically present in rigid electronics. Due to bending, variable stress emerges in silicon, changing the mobility of carriers, which may lead to inaccuracy or failure of the circuit. In order to obtain a robust, precise system, bending must be considered in the design phase prior to fabrication. This work focuses on development of a precision flexible CMOS in-plane stress sensor to measure the complex stress-displacement relationship of a Fin Ray robotic gripper without introducing nonlinearity. Within this framework, a stress-aware mixed-signal design flow is developed for the first time, demonstrating the feasibility of high performance, complex circuitry on ultra-thin chips, in turn flexible foils.
Key points of this work:

In-plane stress effect is maximized and irrelevant tensor components are filtered out by sensor element circuit configuration. Analysis of linearity and temperature dependence of sensor elements is performed.
A stress insensitive temperature measurement method is developed. Localized temperature inquiry is enabled by a small footprint circuit, which is crucial considering the large lateral thermal resistance of ultra-thin chips.
The readout circuit is desensitized to stress by using stress insensitive components in the feedback loop and additional layout considerations.
The biasing circuitry is developed for minimum stress sensitivity, which enables standalone sensor operation without external components.
The digital design flow is modified with minimum interference on the conventional steps avoiding extended compilation and simulation times.