可撓曲軟性電子元件之設計製造與電性分析

Design, Fabrication and Characterization of Soft Electrical Devices on Flexible Substrate

孟杰新(Meena, Jagan Singh)

指導教授 ： 柯富祥

國立交通大學/工學院/奈米科技研究所/博士(2012年)

https://doi.org/10.6842/NCTU.2012.00045

摘要

可撓曲式電子元件技術是目前各科技強國積極發展之領域，經由研究其材料基本特性，有系統地設計與製備可撓式電子產品將影響日常生活型態。本篇研究內容將針對各項可撓曲式電子元件進行探討，其中包括：金屬-絕緣層-金屬 (MIM) 與金屬-絕緣層-半導體 (MIS) 電容以及有機薄膜電晶體 (OTFT) 等。
本篇論文第一部分，我們利用溶膠-凝膠 (sol-gel) 法旋轉塗佈高界電係數材料-HfO2於polyimide (PI) 基板，經由氧電漿 (O2 plasma) 處理此sol-gel薄膜製作膜厚為10 nm-HfO2之MIM電容。此方法在低溫 (250 oC) 製程之薄膜，利用原子力顯微鏡 (AFM) 與掃描式電子顯微鏡 (SEM) 實驗分析，得知此薄膜表面有良好的連續性和均勻性，在x射線光電子能譜 (XPS) 分析也指出此薄膜在經過O2 plasma處理後其化學特性與氧化程度有所改變。電性結果顯示此MIM電容具有低漏電流與高電容量之特性，且由於元件基板PI撓曲性高，經由100,000次以上與將近360o 折疊測試後，此元件仍可保持其電子特性。
第二部分實驗，我們嘗試製作p-type之poly-(3-hexylthiophene) (P3HT) 薄膜電晶體於PI基板上，並利用Si3N4-HfO2為此電子元件之絕緣層。此薄膜電晶體製程步驟如下：1. 使用簡單、便宜sol-gel技術製作膜厚為10 nm-HfO2之介電層於PI基板；2. 堆疊50 nm Si3N4於HfO2之上；3. 旋轉塗佈30 nm P3HT薄膜作為channel layer。此製作過程，由於Si4N4之作用有效地改善原先MIM電容漏電流之情況，在電性量測方面 (如：on-to-off ratio及saturation mobility等)，結果皆顯示其特性良好。此外，針對此電晶體之可撓性，我們也量測元件在不同曲度及折疊後之電性，詳細探討撓曲對元件所造成之影響。
第三部分內容，我們使用聚合物 (EO20-PO70-EO20; P123) 當作表面活性劑備製二氧化鈦 (nc-TiO2) 薄膜，以旋轉塗佈技術將P123-TiO2旋塗於PI基板製作成MIS電容結構。經由XRD、AFM與XPS等分析在270 °C退火5小時後觀察nc-TiO2表面形貌及化學組成等參數，實驗結果顯示薄膜經由P123的作用促使其表面結構平坦且均勻度良好，此外我們也針對半導體層 (pentacene) 表面進行探討，利用AFM與XRD實驗得知其表面為連續平面且均勻度高。在I-V特性量測分析方面，此MIS元件電性良好，在外加偏壓為 -5 V時，漏電流密度值為8.7×10-12 A cm-2，在1 MHz條件下，電容值為102.3 pF，且介電常數值為28.8。另外，針對此電子元件可撓性測試，我們量測元件在不同曲度的電性表現，發現經過30天置放後，此元件仍可保持其電子特性。
本論文最後一部分，我們將P123與聚苯乙烯 (polystyrene; PS) 兩種聚合物混合後當作製備有機薄膜層之材料 (P123-PS)，AFM分析得知此薄膜表面均勻度高，且對於大氣水分有良好抵抗能力，利用接觸角量測儀實驗顯示，此平面為疏水表面，且表面自由能約為7.12 mJ m-2。之後我們嘗試利用此P123-PS薄膜製作成MIM結構之電子元件於PI基板，經由I-V特性量測分析結果得知，其漏電流密度值低於1.07×10-11 A cm-2，電容值為88.2 nF cm-2，而介電常數值為2.7。此外，我們也利用此薄膜材料 (P123-PS) 當作絕緣層在PI基板上製作OTFT，實驗結果顯示其電性參數值saturation mobility: 0.16 cm2 V-1 s-1, on-to-off current ratio: 5×105，且經過拉伸與壓縮測試，此元件亦維持良好電子特性，表示其可撓性良好。

關鍵字

可撓性基板 ； 氧電漿 ； 薄膜電晶體 ； 溶膠-凝膠

並列摘要

Flexible technology is rapidly exploding worldwide and it will revolutionize all aspects of our everyday life, since it will lead to fundamental breakthroughs in the way materials, devices and systems are understood, designed and manufactured. In this thesis, the advances and prospects of nanotechnology in the fields of novel active and functional thin films and nanocomposite-materials to be used for the production of flexible electronic devices, such as flexible metal-insulator-metal (MIM), metal-insulator-semiconductor (MIS) capacitors and organic thin-film transistor (OTFT), will be described.
In first section, we focused on the oxygen (O2) plasma affected sol-gel spin-coating process to fabricate a new MIM capacitor comprising a 10 nm-thick high-k thin dielectric HfO2 film on a flexible polyimide (PI) substrate. We observed the as-deposited sol–gel film was completely oxidized when employing O2 plasma in together annealing at a relatively low temperature (ca. 250 oC), thereby enhancing the electrical performance. An O2 plasma mechanism was proposed to explain the surface oxidation of the HfO2 sol-gel film. The surface morphology of this HfO2 film was investigated using atomic force microscopy (AFM) and scanning electron microscopy (SEM), which confirmed that continuous and crack-free film growth had occurred. We employed X-ray photoelectron spectroscopy (XPS) at both high and low resolution to examine the chemical composition of the film subjected to various treatment conditions. The shift of the XPS peaks towards higher binding energy revealed that O2 plasma treatment was the most effective process for the complete oxidation of hafnium atoms at low temperature. To investigate the insulator properties of the HfO2 film, we deployed it in sandwiched like cross sectional i.e., MIM structured capacitor, which exhibited a low leakage current density and a maximum capacitance density. The real-life flexibility study of the insulator properties indicated the excellent bendability of our MIM capacitor and the flexible PI substrate could be bent up to 100,000 times and folded to near 360o without any deterioration in its electrical performance.
In second section, we demonstrated a new and fully flexible Si3N4-HfO2 stacked poly-(3-hexylthiophene) p-type OTFT on PI substrate. The success of the TFT manufacturing adopts a very simple and cost-effective sol-gel spin-coating technique to obtain 10-nm high-k HfO2 as dielectric layer over PI; 50-nm Si3N4 as the most efficient passivation layer on top of HfO2 film; and bendable 30-nm P3HT channel film by spin-coating method. The origin of unsatisfactory leakage current in MIM and TFT structures could be effectively suppressed by means of Si3N4 film as the efficient passivation layer. The bottom-gate TFT demonstrated the on-to-off ratio 2×104 for drain current and good saturation mobility (0.041 cm2 V-1s-1). The proposed devices were examined in convex and concave types of various radii of curvature (Rc) in order to explore the manufacturing feasibility and electrical reliability of the flexible TFT for practical applications. Additionally, various folding times and environmental stability on aforementioned devices with respective to electrical performances were also evaluated.
In third section, we have synthesized triblock copolymer surfactant, EO20-PO70-EO20 (i.e. P123)-based nanocrystalline (nc)-TiO2 thin film on organic flexible PI sheet for their application in organic MIS device. The nc-TiO2 film over PI was successfully deposited for the first time by a systematic solution proceeds dip-coating method and by the assistance of triblock copolymer surfactant. The effect of annealing temperature (270 °C, 5 h) on the texture, morphology and time-induced hydrophilicity was studied by XRD, AFM, and XPS, respectively, to examine the chemical composition of the film and the contact angle. The semiconductor-dielectric interface of pentacene and nc-TiO2 films was characterized by current-voltage and capacitance-voltage measurements. This interface measurement in cross-link MIS structured device yielded a low leakage current density of 8.7×10-12 A cm-2 at -5 V, maximum capacitance of 102.3 pF at 1 MHz and estimated dielectric constant value of 28.8.
In last section, an organic–organic blend thin–film has been synthesized by solution deposition of triblock copolymer (Pluronic P123, EO20-PO70-EO20) and polystyrene (PS), hereafter named P123-PS for blend film. AFM result revealed that the optimized blend P123–PS film was uniform, crack-free, and highly resistant to moisture absorption. Time-induced contact angle measurements for P123–PS surface was also evaluated by using contact angle meter, which showed excellent hydrophobic surface with surface free energy to about 7.12 mJ m-2. The dielectric properties of P123–PS were characterized in cross linked MIM structured device over PI substrate showed a low leakage current density of 1.07×10-11 A cm-2, large capacitance of 88.2 nF cm-2 and dielectric constant of 2.7. In addition, we demonstrate OTFT device on flexible PI substrate by using P123–PS as insulator layer and pentacene as channel layer. The OTFT showed good saturation mobility (0.16 cm2 V-1 s-1) and on-to-off current ratio of 5×105. The OTFT should functions under bending condition; the flexibility tests for two types of bending modes (tensile and compressive) were also performed successfully.

並列關鍵字

Flexible Substrate ； Oxygen Plasma ； Thin-Film Transistor ； Sol-Gel Technique

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