新穎的溶膠凝膠沈積法與電漿氧化生成技術應用於可撓式透明薄膜電晶體之研究

Abstract

為了達到低成本、重量輕、面積大、功耗低和節能減碳的目標，新的顯示技術已經被薄膜電晶體液晶顯示器（Thin-Film Transistor Liquid-Crystal Displays， TFT-LCDs）所主宰。許多下一世代的顯示器技術，如：非晶矽（Amorphous-Silicon，α-Si）、低溫多晶矽（Low Temperature Poly-Silicon，LTPS）、以及透明金屬氧化物半導體（Transparent Metal-Oxide Semiconductors，TMOSs）等相繼被開發。同時，以各種物理與化學技術沈積具有透光性與可撓性的關鍵半導與閘極介電層體薄膜，亦廣泛地被研究。而在這些已被報導過的沈積技術中，以溶液生成薄膜的沈積方法備受各方矚目，因其具有製程簡單、化學成本低、產量高等優勢，大幅地提升了其於未來兼具高性能與低成本的軟性電子產品中的地位。然而，以溶膠－凝膠技術沈積而成的薄膜亦面臨著待解決的挑戰：提升超薄TMOSs的電荷載子密度，以及改善閘極介電層於多次彎曲測試下的可靠度特性。為了順利推動下一世代透明可撓式的顯示器，本論文致力於研發新穎的『溶膠－凝膠技術』與『電漿氧化生成技術』，並進一步將兩項技術結合，確實改善超薄氧化鋅薄膜的電荷載子密度，以及高介電閘極氧化層的可靠度。

首先，本研究著重於超薄（3.7 nm）、透明並且高品質的氧化鋅（Zinc Oxide，ZnO）薄膜，應用於TFT內作為載子傳輸的半導體層。此ZnO薄膜主要是以醋酸鋅溶膠－凝膠溶液所製成。在後續各種退火溫度條件下，300至700°C成功地製成了適當電性的TFT。然而，分別經過800與900°C的退火處理後，其電性特性明顯受到氧空缺的流失而退化。此n型ZnO TFT的電性特性如下所述：介於0.47至1.78 cm2/V-s的電荷遷移率，5.7 × 105 至1.6 × 106的開關電流比，以及9.7至17.3 V的臨限電壓範圍。接著對電荷遷移率與開關電流比作長達100天的可靠度測試，證實了此溶膠－凝膠技術製成的超薄ZnO TFT之有效性。接著我們進一步將『電漿氧化生成技術』結合於溶膠－凝膠技術製成的超薄ZnO TFT中，藉由電漿處理控制ZnO薄膜的電荷載子密度。電漿氧化生成效應解釋了臨限電壓的偏移以及載子遷移率的變異。而電漿與ZnO薄膜表面於分子等級的互相影響，以及薄膜的行為表徵，皆藉由X光光電子能譜術O 1s圖譜分析研究和證實。這個製程在缺陷和摻雜濃度的低階變動是十分敏感的。而氧電漿處理成功地致使了ZnO TFT開啟電壓的偏移，並降低了兩個數量及以上的漏電流。

另一方面，我們亦致力以『溶膠－凝膠技術』開發製程簡便與低成本的超薄氧化鋯（Zirconium Dioxide，ZrO2）高介電閘極介電層，旋轉塗布於可撓式塑膠軟板上，製成金屬-介電層-金屬（Metal-insulator-Metal，MIM）的電容元件。此ZrO2薄膜於適當的氧電漿處理後以250°C退火，此ZrO2電容於5 V偏壓下展現了極低的漏電流密度（9.0 × 10-9 A/cm2），而最大的電容密度於1 MHz時為13.3 fF/μm2 。此介電層性能的提升歸功於相對低溫且功率為30 W的氧電漿處理技術，成功地使溶膠－凝膠法製作的ZrO2薄膜完全地氧化。

本論文中結合了製程簡易與低成本的『溶膠－凝膠技術』與『電漿氧化生成技術』，成功地開發了高電荷載子密度的超薄氧化鋅薄膜，以及高可靠度的氧化鋯高介電閘極氧化層。此電漿氧化生成技術使溶膠－凝膠生成的ZnO與ZrO2薄膜成為於下一世代軟性電子中重點材料的候選人之一，以滿足生醫感測、發光二極體、以及可撓性面板日益增長的需求。

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.