标题: 新颖的溶胶凝胶沈积法与电浆氧化生成技术应用于可挠式透明薄膜电晶体之研究
Novel Techniques of Sol-Gel Deposition and Plasma Oxidation Growth Treatment for Transparent Flexible Thin-Film Transistors
作者: 朱铭清
柯富祥
Chu, Min-Ching
Ko, Fu-Hsiang
材料科学与工程学系奈米科技硕博士班
关键字: 薄膜电晶体;液晶显示器;透明金属氧化物半导体;软性电子;溶胶-凝胶沈积技术;电浆氧化生成技术;氧化锌薄膜电晶体;氧化锆高介电闸极介电层;Thin-film transistor;liquid-crystal displays;transparent metal-oxide semiconductors;flexible electronics;sol–gel deposition technique;plasma oxidation growth treatment technique;zinc oxide (ZnO) TFT;zirconium dioxide (ZrO2) gate dielectric
公开日期: 2016
摘要: 为了达到低成本、重量轻、面积大、功耗低和节能减碳的目标,新的显示技术已经被薄膜电晶体液晶显示器(Thin-Film Transistor Liquid-Crystal Displays, TFT-LCDs)所主宰。许多下一世代的显示器技术,如:非晶矽(Amorphous-Silicon,α-Si)、低温多晶矽(Low Temperature Poly-Silicon,LTPS)、以及透明金属氧化物半导体(Transparent Metal-Oxide Semiconductors,TMOSs)等相继被开发。同时,以各种物理与化学技术沈积具有透光性与可挠性的关键半导与闸极介电层体薄膜,亦广泛地被研究。而在这些已被报导过的沈积技术中,以溶液生成薄膜的沈积方法备受各方瞩目,因其具有制程简单、化学成本低、产量高等优势,大幅地提升了其于未来兼具高性能与低成本的软性电子产品中的地位。然而,以溶胶-凝胶技术沈积而成的薄膜亦面临着待解决的挑战:提升超薄TMOSs的电荷载子密度,以及改善闸极介电层于多次弯曲测试下的可靠度特性。为了顺利推动下一世代透明可挠式的显示器,本论文致力于研发新颖的‘溶胶-凝胶技术’与‘电浆氧化生成技术’,并进一步将两项技术结合,确实改善超薄氧化锌薄膜的电荷载子密度,以及高介电闸极氧化层的可靠度。

首先,本研究着重于超薄(3.7 nm)、透明并且高品质的氧化锌(Zinc Oxide,ZnO)薄膜,应用于TFT内作为载子传输的半导体层。此ZnO薄膜主要是以醋酸锌溶胶-凝胶溶液所制成。在后续各种退火温度条件下,300至700°C成功地制成了适当电性的TFT。然而,分别经过800与900°C的退火处理后,其电性特性明显受到氧空缺的流失而退化。此n型ZnO TFT的电性特性如下所述:介于0.47至1.78 cm2/V-s的电荷迁移率,5.7 × 105 至1.6 × 106的开关电流比,以及9.7至17.3 V的临限电压范围。接着对电荷迁移率与开关电流比作长达100天的可靠度测试,证实了此溶胶-凝胶技术制成的超薄ZnO TFT之有效性。接着我们进一步将‘电浆氧化生成技术’结合于溶胶-凝胶技术制成的超薄ZnO TFT中,藉由电浆处理控制ZnO薄膜的电荷载子密度。电浆氧化生成效应解释了临限电压的偏移以及载子迁移率的变异。而电浆与ZnO薄膜表面于分子等级的互相影响,以及薄膜的行为表征,皆藉由X光光电子能谱术O 1s图谱分析研究和证实。这个制程在缺陷和掺杂浓度的低阶变动是十分敏感的。而氧电浆处理成功地致使了ZnO TFT开启电压的偏移,并降低了两个数量及以上的漏电流。

另一方面,我们亦致力以‘溶胶-凝胶技术’开发制程简便与低成本的超薄氧化锆(Zirconium Dioxide,ZrO2)高介电闸极介电层,旋转涂布于可挠式塑胶软板上,制成金属-介电层-金属(Metal-insulator-Metal,MIM)的电容元件。此ZrO2薄膜于适当的氧电浆处理后以250°C退火,此ZrO2电容于5 V偏压下展现了极低的漏电流密度(9.0 × 10-9 A/cm2),而最大的电容密度于1 MHz时为13.3 fF/μm2 。此介电层性能的提升归功于相对低温且功率为30 W的氧电浆处理技术,成功地使溶胶-凝胶法制作的ZrO2薄膜完全地氧化。

本论文中结合了制程简易与低成本的‘溶胶-凝胶技术’与‘电浆氧化生成技术’,成功地开发了高电荷载子密度的超薄氧化锌薄膜,以及高可靠度的氧化锆高介电闸极氧化层。此电浆氧化生成技术使溶胶-凝胶生成的ZnO与ZrO2薄膜成为于下一世代软性电子中重点材料的候选人之一,以满足生医感测、发光二极体、以及可挠性面板日益增长的需求。
For the goal of low-cost, lightweight, large area, low power, energy efficiency and carbon reduction, the novel display technologies have been mostly dominated by thin-film transistor liquid-crystal displays (TFT-LCDs). Therefore, many next-generation displays such as amorphous-silicon (α-Si), low temperature poly-silicon (LTPS), and transparent metal-oxide semiconductors (TMOSs) TFT-LCDs have been widely developed. Meanwhile, various physical and chemical techniques are reported to deposit the critical semiconductor and gate dielectric layer with transparency and flexibility. Among these reported techniques, sol–gel derived thin-film deposition methods have been attracted lots of attention because of the simplicity, low chemical cost, and high throughput that enable the future fabrication of high-performance and low-cost electronics devices. However, sol–gel derived TFTs are facing numerous challenges, such as charge carrier density of ultra-thin TMOSs and reliability issue of gate dielectric under repeated bending test. To smoothly promote the next-generation displays with transparency and flexibility, this study has devoted to the development of novel techniques of sol–gel deposition and plasma oxidation growth treatment.

A low-cost, utra-thin (3.7 nm), transparency and high-quality zinc oxide (ZnO) film was successfully demonstrated as the carrier transporting and semiconducting layer for TFT devices. The ZnO ultra-thin film was spin-coated from zinc acetate sol-gel solution. Among various processing temperatures, the electrical property of the fabricated TFT verified the devices could be successfully achieved from suitable annealing temperature of 300-700°C. However, the higher treatment temperature of 800-900°C deteriorated the transistor property due to the loss of oxygen vacancy. The electrical properties of these ZnO-based n-type TFTs were obtained as follows: the mobility (μsat) ranged from 0.47 to 1.78 cm2/V-s, the on/off current ratio ranged from 5.7 × 105 to 1.6 × 106, and the threshold voltage ranged from 9.7 to 17.3 V. The long-term (100 days) characterization for the evaluation of the ultra-thin ZnO TFT reliability on the mobility and on/off current ratio strongly suggested the effectiveness of solution-processed ultra-thin film transistors. Also, a change in the charge carrier density of ZnO films for control the functioning of TFTs has been studied by oxygen (O2) plasma techniques. This effect was interpreted in terms of a threshold voltage shift and the variation in carrier mobility. The plasma-surface interaction on the molecular level and the behavioral characterization of ZnO films were investigated by X-ray photospectroscopy of the O 1s region. This process was highly sensitive at low-level variations in defect and doping density. O2 plasma treatment leads to a shift of turn-on voltage and a reduction of the off current by more than two orders of magnitude in ZnO TFTs.

In addition, a new flexible metal–insulator–metal (MIM) capacitor using 9.5-nm-thick ZrO2 film on a plastic polyimide substrate based on a simple and low-cost sol–gel precursor spin-coating process has been demonstrated. The as-deposited ZrO2 film under suitable treatment of O2 plasma and then subsequent annealing at 250°C exhibits superior low leakage current density of 9.0 × 10-9 A/cm2 at applied voltage of 5 V and maximum capacitance density of 13.3 fF/μm2 at 1 MHz. The as-deposited sol–gel film was completely oxidized when we employed O2 plasma at relatively low temperature and power (30 W), hence enhancing the electrical performance of the capacitor.

This proposed efficient combination of sol–gel solution method and O2 plasma oxidation growth treatment to fabricate transparent ZnO and ZrO2 ultra-thin film was relatively simple and cost-effective technique, and could be used as a new candidate of material for next-generation electronic devices to meet the growing demand of small feature bioelectronic sensor, light emitting diode and flexible panel.
URI: http://etd.lib.nctu.edu.tw/cdrfb3/record/nctu/#GT079652803
http://hdl.handle.net/11536/138373
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