【作者】 徐华；

【导师】 巩雄；

【作者基本信息】 华南理工大学 ， 材料物理与化学， 2014， 博士

【副题名】金属/半导体界面和柔性薄膜晶体管

【摘要】 近年来，氧化物薄膜晶体管(TFTs)由其在有源矩阵发光二极管(AMOLED)显示中的潜在应用而受到了广泛的关注。传统的非晶硅TFTs由于迁移率较低、阈值电压漂移较严重，无法满足电流型驱动方式的OLED显示屏；而多晶硅TFTs虽然具有较高的迁移率和较好的稳定性，但是，其晶界的存在使得均匀性较差，从而影响了其在AMOLED显示中的应用，特别是在大尺寸显示屏中的应用。而氧化物TFTs，不但具有较高的迁移率，而且可见光透明、均匀性好、稳定性好，特别是可以低温制备，有望实现低成本的柔性显示。因此，开展氧化物薄膜晶体管的研究就有重大的现实意义。本论文首先对氧化物薄膜晶体管中有源层的制备条件进行探索，获得了各工艺参数对器件性能的影响。根据氧化物半导体的特点，重点研究了薄膜制备中的氧含量和溅射气压的影响；结合钝化层的沉积和后退火处理，研究了后退火温度和退火气氛对器件性能的影响。此外，针对源漏电极的沉积条件对器件性能的影响，详细研究了金属钼(Mo)电极和氧化铟锌(IZO)有源层的界面特性。发现随着溅射功率的增加，器件的阈值电压负向移动，迁移率降低。借助X射线光电子能谱，分析了Mo/IZO界面的扩散现象以及高功率对IZO薄膜的影响。为制备高性能的氧化物薄膜晶体管提供了指导方向。其次，背沟道刻蚀型薄膜晶体管由于具有相对简单的工艺，制备成本相对较低，在实际应用中具有巨大的吸引力。但是氧化物半导体和传统的硅基材料不同，其很容易被大部分刻蚀液刻蚀，较难实现该结构的氧化物薄膜晶体管。针对氧化物半导体耐酸碱等刻蚀液的能力较差，基于前面章节的研究结果，本论文通过采用多层钼-铝-钼(Mo/Al/Mo)的源漏电极结构，以双氧水(H2O2)和低浓度的氢氧化钾(KOH，0.5wt%)分别作为Mo和Al的刻蚀液，制备了高性能的IZO-TFT。其饱和迁移率为11.3cm2V-1s-1，较小的亚阈值摆幅(0.24V/decade)，电流开关比大于108，在+10V和-10V的栅偏压下器件的阈值电压漂移量仅为0.4V和-0.2V。该Mo/Al/Mo电极结构不仅和有源层的接触良好，而且具备低电阻率和耐高温特性。H2O2对Mo和IZO具有较高的刻蚀选择比，而且对IZO的损伤较小；0.5wt%的KOH能较好地刻蚀Al而几乎不刻蚀Mo。本文还对H2O2刻蚀Mo薄膜的机理进行了分析，实现了以湿法背沟道刻蚀方案制备高性能氧化物薄膜晶体管。另外，本论文针对目前低温制备高质量的绝缘薄膜存在的问题，提出了低温阳极氧化法制备氧化铝(Al2O3)薄膜在柔性衬底上应用的可能，研究了其制备的关键工艺。对柔性聚萘二甲酸乙二醇酯(PEN)衬底上制备的Al2O3薄膜特性进行了详细的表征，其具有较好的台阶覆盖性和较低的表面粗糙度，特别是在较厚的栅极上制备绝缘层时有明显的优势；该Al2O3薄膜还表现出了优异的电特性和机械性能。总之，以阳极氧化法制备的Al2O3薄膜表现出和原子层沉积(ALD)方式制备的Al2O3薄膜相媲美的性能。另外，以阳极氧化法制备的栅绝缘薄膜被成功地应用到柔性薄膜晶体管的制备当中，所制备的薄膜晶体管表现出了良好的性能。本文还尝试了在柔性透明的PEN衬底上以阳极氧化的方式制备氧化铝(Al2O3)栅绝缘层薄膜阵列，基于前面对氧化物薄膜晶体管等方面的研究，成功制备了5inch的AMOLED彩色柔性显示屏。该显示屏不仅能正常显示静态图片，而且可以播放动态视频，在一定的曲率半径下，不会影响到显示效果；展现出了阳极氧化法制备的绝缘薄膜在低温和柔性电子领域的巨大应用潜力。最后，透明电极作为现代功能薄膜的重要组成部分，其图案化工艺更是决定了其应用的领域。众所周知，传统的氧化铟锡(ITO)薄膜随厚度增加而呈结晶特性，而结晶的ITO薄膜只能被强酸刻蚀，严重限制了其应用领域。为制备低阻值、低粗糙度、易刻蚀的透明电极，我们首先对ITO薄膜制备中各工艺参数的影响展开研究，获得了制备高性能透明导电ITO薄膜的关键参数。本论文首次采用多层的复合结构，以非晶透明的IZO薄膜作为插入层，有效地抑制了氧化ITO薄膜随着厚度增加呈结晶生长的特性。制备的多层复合薄膜不仅具有较低的电阻率(~2×10-4Ωcm)、高可见光透过率(~85%)、低表面粗糙度(~0.52nm)，而且可以被草酸等弱酸刻蚀。该方法解决了低阻透明导电薄膜不易刻蚀的难题，拓宽其应用领域。更多还原

【Abstract】 Recently, oxide thin-film transistors （TFTs） have been acctracting greatanntention due to the application in active matrix organic light emitting diode（AMOLED）displays. Because of the low mobility and serious threshold voltage (Vth)shift, the traditional amorphous silicion (a-Si) is insufficient to drive the OLED whichdriven by current. To be the poly-silicion (p-Si), although it has high field-effectmobility, good electrical stability, suffering from the low uniformity caused by theexistence of grain boundaries, which are restricted to application in AMOLED,especially for the lager-size displays. On the other hand, oxide semiconductor exhibitreasonably high mobility, transparent in visible light, good uniformity and excellentstability, especially of the low-temperature deposition, which shows the potential torealize the low-cost flexible displays. The oxide TFTs combine the advantages of a-Siand p-Si TFTs, and might be an alternative approach to realize reliable and highresolution AMOLEDs.Firstly, we explore the performance of the TFTs influenced by the preparation ofthe active layer. Consider to the characteristics of the oxide semiconductor, oxygen inthe sputtering atmosphere and sputtering pressure were mainly studied. Combinationof passivation layer and annealing ambient, the influence of the annealing temperatureand ambient were studied also. Then, we found that as the increase of the sputteringpower of molybdenum (Mo), the Vthshift negative and the mobility decrease. Bymeans of X-ray photoelectron spectroscopy (XPS), we analyzed the diffusionphenomenon in the interface of Mo/IZO. These parts provide the guidance forfabrication high performance TFTs.Secondly, the TFT with back-channel-etch structure has great attention due to therelatively simple process and low-cost. But, differ to the conventional silicon-basedmaterials; the oxide semiconductors are very weak in most of the wet etchants. Basedon the results of the previous section in this paper, a stacked structure of Mo/Al/Mowas adopted as the source/drain electrodes and patterned by a wet-etch-method. TheMo and Al were etched by H2O2and KOH, respectively. Good etching profile withfew residues on the channel was obtained. The TFT exhibited a field effect mobility ofas high as11.3cm2V-1s-1, a sub-threshold swing of only0.24V/decade, and anon-to-off current ratio of larger than108. Furthermore, the threshold voltage shiftedonly0.4V and-0.2V for the device under positive (+10V) and negative (-10V) gate bias stress for2.5h, respectively. These results indicate that the method of wet etchingS/D electrodes is feasible for IZO-TFT and has the potential to reduce themanufacturing cost of oxide semiconductor-based TFT backplanes.In addition, aiming at the problems existing in the preparation of high qualitydielectric at low-temperature, we try to form the alumina (Al2O3) on flexible substrateby anode-oxidization. The Al2O3on polyethylene naphthalate (PEN) substrate wasfabricated at room temperature, and the key process factors were inverstigated indetail. The anolized Al2O3shown better side face coverage characteristics than theSiO2deposited by plasma enhanced chemical vapor deposition (PECVD), especiallyfor the thick gate electrode. Additionally, the anolized Al2O3exhibited excellentelectrical and mechanical characteristics. In all, it shown the comparable performanceto the Al2O3thin film formed by ALD. Meanwhile, we fabricated the TFTs with theanolidze Al2O3as gate insulator, which demonstrates the promising application of theTFTs in flexible AMOLED displays. Based on the previous research, we succefullyfabricated a5inch color flexible AMOLED display on PEN substrate, which exhibitsgood image quality in both flat and bent states. It is demostrated that the anodizedAl2O3with tremendous potential in low-temperature and flexible electronics.Finally, transparent conductor oxides (TCO) play an important role in AMOLEDdisplays. As we all known, as the increase of the thickness, the ITO thin-film tends tobe crystalline. Unfortunately, the crystallized ITO film will encounter a patterningproblem, because it can only be etched by strong acids, such as aqua regia, which aredestructive to the underlying films. In order to prepare the low-roughness andeasily-etched transparent electrodes, a layer of ultra-thin IZO was inserted betweenITO films. It was found that this as-deposited TCO film with IZO insertion layers wasamorphous and easily-etched even in weak acids. Furthermore, the surface roughnessof this multilayer TCO film was only0.52nm, much lower than that of the ITOmonolayer film with the same thickness. After annealing at250oC in air, a low sheetresistance of～9.6Ω/and a reasonably transmittance of～85%in visible range wereobtained. This method expanded the application of the TCOs significantly.更多还原