【作者】 刘畅；

【导师】 赵毅；

【作者基本信息】 吉林大学 ， 微电子学与固体电子学， 2021， 博士

【摘要】 近年来,基于InGaZnO（IGZO）薄膜晶体管（Thin Film transistor,TFT）的背板驱动技术,由于其具有迁移率高、大面积均匀性好、可见光透过率优异、兼容现有a-Si TFT制程以及可应用于柔性显示等优点而被众多科研机构以及公司所关注。随着IGZO TFT在有源矩阵式显示等领域的广泛应用以及对显示品质要求的不断提高,人们对IGZO TFT的制程以及性能提出了更为严苛的要求。一方面,为了满足柔性显示的驱动电路的需求,匹配柔性衬底的耐受温度以及降低研发成本,IGZO TFT的制备温度需要进一步的降低;另一方面,随着显示需求的逐步升级,高分辨率、高刷新率的显示应用对IGZO TFT的迁移率、亚阈值摆幅以及稳定性等指标提出了更高的要求。因此,本论文的主要工作以基于等离子体辅助制程的IGZO TFT为主线,利用等离子体辅助相关方法,从降低IGZO薄膜沉积温度、提高IGZO TFT器件性能、增强IGZO TFT器件的稳定性等几个角度出发,探索研究实现低制备温度、高性能、高稳定性的IGZO TFT的方法,主要研究成果包括以下内容:（1）针对IGZO TFT面向低玻璃化温度廉价柔性衬底上的应用,扩展IGZO TFT应用于柔性显示时衬底的选择范围,我们提出利用电容耦合等离子体辅助方法在100℃的后退火温度下制备了IGZO TFT。相较于未引入等离子体辅助方法所制备的IGZO TFT,在同一退火温度下引入等离子体辅助方法制备的IGZO TFT各项电学性能指标有了较大幅度的提高。我们利用这一方法,在40 W辅助等离子体功率、100℃的后退火温度条件下,获得了迁移率高达26.03 cm²/V·s,阈值电压为2.00 V,亚阈值摆幅为0.33 V/decade的高性能IGZO TFT。为了进一步调控利用电容耦合等离子体辅助方法制备的IGZO TFT的电学特性,我们引入了低温制备的组分可调缓冲层对IGZO TFT中的栅介质层进行修饰,这种组分可调的栅介质缓冲层是利用电感耦合-等离子体增强化学气相沉积系统在70℃的条件下实现沉积,通过控制沉积过程中的O₂流量,可以对栅介质缓冲层的成分进行调节。在后续的等离子体辅助制备IGZO沟道层的过程中,这种栅介质缓冲层中的H原子可以在辅助等离子体轰击的作用下掺杂至正在沉积过程中的IGZO薄膜内,从而改善了IGZO TFT的电学性能。此外,在IGZO TFT制备的后退火过程中,在浓度梯度的作用下,栅介质缓冲层中残留H原子也可以通过扩散作用掺杂至IGZO薄膜中。我们所提出的这一方法所研制的IGZO TFT具有工艺温度低、电学性能优异的特点,为未来高性能柔性显示提供了一种全新的思路。另一方面,我们还利用电容耦合等离子体辅助方法同时结合高Zn组分的IGZO,在无需后退火的条件下,成功制备了具有C轴结晶取向的IGZO TFT,然而受限于这种方法所制备薄膜的Zn含量较高,迁移率仅有6.05 cm²/V·s,其性能有待进一步提高。（2）目前,由于未钝化的IGZO TFT普遍存在的空气中偏压稳定性的问题,我们针对IGZO TFT产生阈值电压漂移问题的根源—背沟道水氧吸附进行了研究。我们提出利用电感耦合—等离子体增强化学气相沉积法,以HMDSO为前驱体在大约80℃的温度下制备了有机硅薄膜作为IGZO TFT的背沟道钝化层。得益于这种背沟道钝化层对水氧的优良阻隔特性,在引入了背沟道钝化层后IGZO TFT的正负偏压下阈值电压漂移现象得到了明显的改善,并且在偏压测试后展现了良好的恢复特性。针对在负偏压稳定性的测试过程中观察到的I_ds电流异常现象,我们从陷阱捕获/发射的角度对这一现象进行了详细阐述。此外,我们利用在沉积背沟道钝化层过程中由前驱体HMDSO的氧化分解引起的H掺杂效应,实现了IGZO TFT电学特性的提高。相较于未使用有机硅钝化层的IGZO TFT,器件的迁移率从11.99 cm²/V·s提高至17.78 cm²/V·s,亚阈值摆幅从0.63 V/decade降低至0.41 V/decade,而开关比则从10⁶提高至10⁷。我们利用傅立叶红外光谱,动态二次离子质谱等多种表征手段详细分析了性能提高的内在机理。这种利用有机硅作为钝化层的方法具有工艺温度低、可见光透过率高兼容全透明TFT、稳定性好等优点,为实现稳定的高性能IGZO TFT提供了一种新的方法。（3）为了满足对IGZO TFT的日益提高的性能需求,进一步降低亚阈值摆幅等关键电学指标,我们提出利用电容耦合等离子体氧化SiN_x栅介质的方法大幅降低了IGZO TFT的亚阈值摆幅。在引入了等离子氧化SiN_x栅介质后,我们获得了亚阈值摆幅仅有0.097 V/decade的低亚阈值摆幅IGZO TFT。通过对SiN_x表面的X射线光电子能谱（XPS）测试结果进行分析我们发现,引入等离子氧化SiN_x栅介质过程后,会在SiN_x表面形成一层富氧层,通过角分辨XPS对SiN_x/IGZO界面附近进行测试,证明了这一预植入的富氧层可以有效地抑制SiN_x/IGZO界面处的氧空位的形成,从而降低了SiN_x栅介质与IGZO沟道层界面处的缺陷态密度。除此之外,我们还对引入等离子体氧化栅介质前后的IGZO TFT在光照条件下的负偏压稳定性进行了测试。测试结果表明,得益于SiN_x/IGZO界面处的氧空位的减少,引入等离子体氧化SiN_x栅介质层的IGZO TFT在7200 s负偏压光照测试后阈值电压漂移量从-4.75 V大幅降低至-0.37 V。而我们利用电导法对界面处的缺陷态密度进行了测量估算,进一步证明了等离子体氧化SiN_x栅介质过程的引入可以使得界面缺陷态密度降低,从而令IGZO TFT的电学性能以及稳定性有了大幅度的提高。（4）在我们对IGZO TFT的研究过程中,发现国际上不同课题组所制备的结构相近的IGZO TFT所获得的亚阈值摆幅不尽相同,离散程度很高,通过进一步对不同栅介质材料以及亚阈值摆幅的相关工作进行统计分析,我们发现亚阈值摆幅与栅电压的采样间隔呈现一定的变化规律。为了探究出现这一现象的原因,我们利用了不同的栅电压采样间隔对制备的IGZO TFT进行了测试,并通过泰勒展开对前向差分以及中心差分法提取的亚阈值摆幅进行了分析,解释了出现上述离散现象的原因。此外,我们提出了一个兼顾测试效率以及测试准确性的栅电压采样间隔的经验公式,对同行的相关工作提供了一定参考作用。更多还原

【Abstract】 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.更多还原