Ultra-Thin Flexible CMOS Stress Sensor
February 24, 2015, ISSCC, San Francisco—Yigit Mahsereci from the University of Stuttgart presented work on a hybrid system-in-foil that takes advantage of both thin-film electronics and silicon integration. The combination provides the industry with new capabilities that neither technology can provide alone. The main trick is to thin the silicon to 10-20 um where the silicon becomes as flexible as any substrate material.
The first application for the technology is a pressure sensor array for a bionic gripper. The 3-prong gripper is a 3-D printed, complex structure that is pneumatically actuated to adapt to many shapes. The addition of stress sensing to the gripper provides information about the object’s shape and position, while providing handling information on positive locking and safe handling. The sensor also measures temperature.
The complex gripper is a deformable triangular structure so the stress measurements have to be indirect. Further exacerbating the control issues are the non-linear and non-monotonic nature of the stress. As a result, the design emphasizes the linear and localized in-plane stress components with efforts to maximize the stress effect of the sensors while minimizing the stress impact on the signal processing circuits, with temperature compensation. The non-uniform stress on the chip have to be considered in the chip layout.
The basic stress sensor uses MOS sensing elements to generate currents linearly proportional to the in-plane shear and normal stress difference components. The sensing elements subtract the currents from two orthogonal NMOS ((100)-(010)) and PMOS ((110)-(110)) transistors using current mirrors. The difference currents are then digitized following a track and hold which offers low input impedance and high linearity and is stress insensitive.
The stress sensors change capacitance by 1.6 fF from a nominal value of 4.47 pF. The stress on the sensors influences the operating region, transconductance, and output resistance. These changes have to be accounted for in the integrator and track-and-hold as well as in the final layout. The 10-bit A/D converter must also work with the applied stress, but fortunately the R-2R resistor ratios are constant. The stress-aware floorplan keeps the sensing elements away from the edges and the area for the DAC very small to minimize changes in the resistor ratios.
The sensors are calibrated and operated in a configurable integration time. The physical stack affects the stress value due to differences in thickness and stiffness. The stress-aware controller changes timing based on stress direction and device orientation. The synthesis of the design emphasized large setup and hold margin and allowed for clock uncertainty through additional buffers.
The total system uses large-area thin-film components and ultra-thin chips to achieve flexibility. The integration of active sensors and signal processing that is stress aware enables very good localization and object definition, based on variations in readings from the multiple sensors in the system.
