Effect of passivation layer on back channel etching InGaZnO thin film transistors

Abstract

Amorphous indium gallium zinc oxide (IGZO) Thin Film Transistor (TFT) are widely applied in active-matrix display industry because of their excellent stability,low off-current，high field-effect mobility and good process compatibility.Among IGZO TFT device structures,Back channel etching (BCE) is favorable due to low production cost,short channel length and small SD-to-gate capacitance.in this work,BCE IGZO TFTs have been prepared with the passivation layer of silicon dioxide (SiO2),polyimide (PI) or SiO2-PI stacked structure to study their difference in back channel hydrogen impurities and its diffusion behavior.In comparison with the conventional SiO<sub>2</sub> passivation BCE TFT,the performance of PI passivation TFT have been improved greatly,the saturation field effect mobility increases from 4.7 to 22.4 cm<sup>2</sup>/Vs,subthreshold swing decreases from 1.6 to 0.28 V/decade,the an on-off current ratio rises dramatically from 1.1×10<sup>7</sup> to 1.5×10<sup>10</sup>.After the SiO<sub>2</sub> passivation layer is substituted with PI,the I<sub>off</sub> decreases from 10<sup>-11</sup>A to 10<sup>-14</sup>A,which indicates the lower amount of shallow-level donor states of hydrogen impurities that might be explained by the following three mechanisms:First,during the film formation process of PI,the direct incorporation of hydrogen-related radicals from SiH<sub>4</sub> precursor into the back channel is avoided;Second,the hydrogen content in the PI film is lower and harder to diffuse into the back channel;Third,the hydrogen impurities of back channel that is introduced by the H<sub>2</sub>O<sub>2</sub>-based etchant during the SD etching process could diffuse more easily toward the PI layer.The TFTs with PI passivation layer also shows the less electrical degradation after 380 °C annealing treatment and better output performance,which affirms less defects and higher quality of the back channel.The bias stabilities of PI devices were improved comprehensively,especially negative bias illumination stability with the threshold voltage shift from-4.8 V to-0.7 V,which might be attributed to the disappearance of hydrogen interstitial sites and hydrogen vacancies that is charged positively at the back channel.PI passivation layer is effective to avoid back channel hydrogen impurities of BCE TFT and is promising for broad application in the display industry.