Enhanced piezoelectric tactile sensing behaviors of high-density and low-damage CF₄-plasma-treated IGZO thin-film transistors coated by P(VDF-TrFE) copolymers

Piezoelectric pressure sensing behaviors of high-density and low-damage CF₄-plasma-treated indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) coated by poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) films have been investigated. The CF₄ plasma treatment was performed on the IGZO channel by using an inductively coupled plasma (ICP) system with a quartz filter. Prior to the fabrication of devices, X-ray diffractometer (XRD) and atomic force microscopy (AFM) were applied to identify the enhancement in the crystallinity of P(VDF-TrFE) films on the plasma-fluorinated IGZO films. With the analyses of X-ray photoelectron spectroscopy (XPS), it is proved that the fluorine radicals reacted with the metal-oxygen bonds will increase the oxygen vacancies in IGZO films, contributing to an enhancement in field effect mobility (μ_FE) and drain current (I_DS) of IGZO TFTs. Furthermore, the fluorine atoms diffused from the IGZO channel into the bottom SiO₂ layer were confirmed by secondary ion-mass spectroscopy (SIMS), reducing the interfacial and oxide charges of the devices for a negative shift in threshold voltage (V_t). Under a 0.5-kg applied force press/release cyclic test, a 4.8-fold increase in drain current response of 1-min CF₄-plasma-treated piezoelectric pressure sensors was optimized, because of the enhanced electrical behaviors of IGZO TFTs and an increase in aligned dipole moments of P(VDF-TrFE) copolymers. The promising results make the piezoelectric pressure sensors with high-density and low-damage CF₄-plasma-treated IGZO TFTs coated by P(VDF-TrFE) copolymer suitable for future high-performance tactile sensing applications.