Micro fingerprint sensor based on piezoresistive nanocomposite polymers

Since existing fingerprint sensors still couldn’t satisfy various requirements for daily information and communication infrastructure, other solutions with cheap price and acceptable reliability are necessary. Correspondingly, in this thesis, a compact pressure-type design using novel CNT-PDMS nanocomposite is proposed as one promising solution for low-cost fingerprint sensors. First, piezoresistive CNT-PDMS composite is successfully synthesized. The mechanical and electromechanical properties, including Young’s modulus, DC conductivity, and piezoresistive coefficients of this material are characterized. The impressive piezoresistive coefficient of CNT-PDMS composite is higher than that of traditional semiconductor materials by 3~4 orders of magnitude. Moreover, the curing property and the surface property of the composite are also studied. Then, conceptual design, basic fabrication methodology and process flow of the pressure sensor are presented. The piezoresistive phenomenon of the device is also simulated via FEA method with ANSYS. Consequently, the optimal geometry of the piezoresistor is obtained. In addition, the fabrication, packaging and testing of the pressure sensor is mainly discussed. Although currently there is still no compatible deposition technology to define conductive polymer on metal substrate in batch fabrication, a simultaneous deposition method with parylene mould is introduced. This should help improve the reliability and automation for the special deposition process. The resistance distribution for the fabricated CNT-PDMS piezoresistors is measured to be quite uniform (about 86.7% of the resistances range from 50kΩ to 200kΩ). And the fluctuation of the resistance of the sensor is found to be around 0.638%. Finally, layout design of the array type sensor, which is much simpler compared with the traditional ones and could be realized with micron level fabrication apparatus, is proposed, also based on the piezoresistivity of CNT-PDMS composite. The highest sensitivity of the fabricated piezoresistors is measured to be around - 0.575MPa-1. The negativity of the pressure sensitivity is qualitatively consistent with the simulation results