Investigation Of Thin Film Transistors Based Ultrasonic Fingerprint Identification Technology

In this dissertation,a large area,low cost,and high security ultrasonic fingerprint identification sensor is developed based on thin film transistor(TFT)arrays.The main research and results are summarized as follows:(1)PVDF piezoelectric films:The blending of PVDF-Tr FE with reduced graphene oxide(r GO),the screen printing,and the in-situ polarization,the preparation of large area and performance enhanced piezoelectric film was realized.The addition of r GO into PVDF-Tr FE enhanced the dipole interaction,which significantly improved the d₃₃coefficient from 27 p C/N to 34 p C/N,the the open circuit voltage from 4.7 V to 8.0 V,and the short circuit current form 0.37?A to 0.6?A,respectively.Besides,the principle to generate and receive ultrasonic waves by PVDF-Tr FE was studied,which serves as the fundamentals of ultrasonic fingerprint identification sensor arrays.(2)TFT array sensors:The low temperature polysilicon(LTPS)process was selected to realize the large area and low cost circuit design.Then,the PVDF-Tr FE and relevant circuits were tested to verify and investigate the influential factors.The 38decoder and multiple channel circuits were selected to solve the extended response time caused by large TFT area,which may facilitate the ultrasonic sensor applications.(3)High frequency and high voltage piezoelectric drivers:The Class D LCLC convertor was designed to realize the high voltage of PVDF-Tr FE piezoelectric film driver.A higher output value of 376 V was generated by the multilevel resonant circuits.With this driving voltage,the ultrasonic echo signal was 970 m V,much larger than the360 m V signal generated by the Class D LC resonant converter.Thus,the convertor can drive the PVDF-Tr FE film with higher output voltage and higher echo signal,which may promote the ultrasonic fingerprint recognition technology.(4)Ultrasonic fingerprint module integration:The integration of the PVDF-Tr FE film,the TFT sensor circuit,and the high voltage and high frequency driver into modules was investigated.The modules were controlled by the software and operated in time sequence flow to generate/receive ultrasound waves and complete the fingerprint image data acquisition.Different module structures of different Tx/Rx laminations were evaluated and the homologous Tx/Rx with far-field lamination structure was recommended for OLED under-screen fingerprint applications.