Micromachined integrated pressure-thermal sensors on flexible substrates

This paper presents the design, modeling and simulation of micromachined, integrated pressure-thermal sensors on flexible polyimide substrates. Finite element simulations were performed with polycrystalline silicon as the piezoresistor material on a suspended Si3N4 layer. These piezoresistors are connected to each other in a half-bridge Wheatstone configuration using flexible aluminum interconnects. Several different designs of integrated thermal-pressure sensors as well as pressure-only sensors were simulated to compute the sensor figures of merit such as the percentage change in piezoresistance in response to normal pressure, piezoresistor Wheatstone-bridge output voltage for varying skin curvature, bolometric response to broadband infrared radiation, thermal time constant and thermal conductance of the micromachined structures hosting the sensors to the substrate. For a perpendicular uniform pressure application of 50 kPa, a maximum Wheatstone-bridge output of 7.59 mV was computed for 1 V bias, corresponding to a piezoresistance change of 1.52%. When the skin is bent to a curvature of 2.2 mm, a maximum Wheatstone-bridge output voltage of 70 mV was calculated for the case when the sensors are aligned along the axis of bending. Thermal and optical calculations performed on the integrated thermal-pressure sensors showed a thermal time constant as low as 12.8 mu s for a 1.9 mu m thick silicon nitride membrane layer, with a responsivity of 270 V W-1 to a broad-band infrared radiation. This would be appropriate for applications requiring fast response but not high sensitivity. Integrated sensors on a thinner silicon nitride membrane layer of 0.5 mu m, on the other hand, exhibited responsivity as high as 2000 V W-1, with a response time of 626 mu s.