Research Of InGaZnO Thin Film Transistor Based On Plasma Assistant Process

Recently,the back-panel driver technique basing on InGaZnO(IGZO)thin film transistor(TFT)have been attracted many attentions from research institutions and companies due to its merits of high mobility,low process temperature,excellent large-scale uniformity,and compatibility of existed a-Si product lines.Since the widely application of IGZO TFTs in the area of AMOLED and the boosted requirements of display quality,people have proposed higher demands on its process and performance.On the one hand,the temperature of IGZO process should be further decreased to satisfy the requirement of flexibility display;On the other hand,as the high resolution and high fresh rate need higher performance of IGZO TFT,the mobility,subthreshold swing and stability should be further improved.Hence,in this manuscript we focus on the plasma process related IGZO TFTs,and study on the improving performance,reducing the process temperature and enhancing the stability of IGZO TFT.The main results of study including:(1)We proposed a capacitively coupled plasma(CCP)assistant sputtering method to fabricate high performance IGZO TFT at only 100?post-annealing temperature with mobility of 26.03 cm²Vs,threshold voltage of 2 V and subthreshold swing of 0.33V/decade.To further modulate the performance of CCP assistant IGZO TFT,we employed the buffer layer between IGZO and gate insulator.Such buffer layer is fabricated by inductively coupled plasma-plasma enhanced chemical vapor deposition(ICP-PECVD),which could adjust the components of buffer layer via varying the ratio of precursor.The buffer layer could induce the hydrogen doping during the process of post CCP assistant sputtering IGZO.Furthermore,the post-annealing process could also induce the hydrogen atoms diffusion from buffer layer into the IGZO thin film.Since the hydrogen could act as electronic donors to provide extra electrons into the conduct band in IGZO,the mobility of IGZO TFT have been increased.Besides,we also using CCP assistant sputtering to fabricate the C axis orientation IGZO without thermal heating.The proposed methods in this chapter have the potential to apply in the flexible display.(2)Since the IGZO TFTs have suffered from the stability under the air,we focus on the origin of this issue-O₂ and H₂O absorption at back channel of IGZO TFT to carry out the research.We employ ICP-PECVD method,with precursors HMDSO and O₂ to fabricate organosilicon thin film as the IGZO TFT back-channel passivation layer(PVL).By the hydrogen doping effect during the PVL deposition,the IGZO TFT electrical properties have been improved.Compared with not using PVL IGZO TFT,mobility of the device up to 11.99 cm²/Vs from the 17.78 cm²/Vs,the subthreshold swing decreased from 0.63 V/decade to 0.41 V/decade,and the I_on/I_off increased from10⁶ to 10⁷.In addition,we also used FTIR,D-SIMS and other characterization methods to analyze the internal mechanism of performance improvement in detail.On the other hand,owing to the excellent barrier property of the PVL to water and oxygen,the threshold voltage drift phenomenon of IGZO TFT under positive and negative bias voltage is significantly improved after the introduction of the PVL,and it shows a good recovery property after bias voltage test.This method using organosilicon as PVL has the advantages of low process temperature,high visible light transmittance and good stability,which opens a new way to realize high performance and stable IGZO TFT.(3)For IGZO TFT in the potential application of advanced electronic devices,further improving the key electrical indicators,such as subthreshold swing should be considerate.We employed the plasma oxidation of the SiN_x gate dielectric which could dramatically reduce the subthreshold swing of IGZO TFT to 0.097 V/decade.Basing on the XPS analysis,an oxygen rich layer is formed on the SiN_x surface,which could effectively reduce the interface trap between SiN_x.and IGZO.In addition,we also perform the negative bias illumination stress(NBIS)for the plasma oxidation SiN_xIGZO TFT.After 7200 s NBIS test,owing to the reduce of oxygen vacancy between IGZO and SiN_x interface,the shift of threshold voltage is greatly reduced from-4.75 V to-0.37 V.The proposed method provides a potential method to achieve low subthreshold swing IGZO TFT.(4)During our study on IGZO TFT,we found that the extracted subthreshold swing has a large discrete,even in the same material system.Through further statistical analysis of the work related to different gate dielectric materials and their subthreshold swing,we find that the sampling interval between subthreshold swing and gate voltage presents a certain variation rule.In order to explain the reasons for this phenomenon,we use the different gate voltage sampling interval on the measurement of the IGZO TFT,and the forward/center difference method by Taylor expansion have been analyzed.Furthermore,to get a precision result on extracting subthreshold swing with proper sampling interval,we proposed an empirical formula to estimate the proper sampling interval of gate voltage,which provides the reference guidance for the similar related work in the future.