A study on photolithographic patterning for organic electronics

摘要 本研究的目的在於使用光微影技術(photolithography)於有機發光材料上產生高解析度圖樣(pattern)，以應用於有機發光二極體顯示器之製程。現有圖樣有機發光材料的技術，如使用噴墨列印技術與蔭罩（shadow mask）等，皆無法達到大面積、快速、高解度圖樣的理想。若現今工業應用以臻成熟之光微影技術可應用於有機發光材料，則對有機發光二極體顯示器之商業化將有極大助益。本實驗分成兩個部份。第一部份將討論如何改善現有光微影技術，增進其圖樣之最小尺寸。第二部份將研究如何應用光微影技術來圖樣有機發光材料。 第一部份我們利用化學增幅型光阻(chemically amplified photo-resist)經由後顯影(post-development)的紫外光照射，來縮減其光阻本身的特徵尺寸(feature size)，達到近60%的縮減程度。我們發現光阻尺寸縮減之機制如下。 化學增幅型光阻經由強紫外光照射及烘烤後，其保護官能基經由去保護(deprotecting)過程，分解成氣體逸散出高分子外，因而產生自由體積(free volume)。經由強紫外光照射後的高分子產生交聯(crosslink)，而擠壓其自由體積，導致高分子本身的體積縮小而使光阻的特徵尺寸縮減。使用此方法可以有效加強248奈米光微影技術的解析度極限，使其能圖樣50奈米以下特徵尺寸。 第二部份我們使用MEH-PPV共軛高分子(conjugated polymers)來作為有機發光二極體之發光層，並以光微影進行圖樣。在進行光微影步驟前，我們使用原子層沉積技術(atomic layer deposition)沉積厚度約1.5奈米的氧化鋁層在共軛高分子上。此氧化鋁層構成一層完全的保護層，有效避免高分子與光微影製程中使用之溶劑接觸。 研究結果發現，受氧化鋁層保護的共軛高分子，在經由光微影製備成發光二極體後，在電壓10V下非但電流效率與發光強度沒有降低，反而增加為控制組之3倍與10倍。此現象之成因應為：氧化鋁層除擔任保護層功能外，也伴演著緩衝層(buffer layer)的功用。緩衝層在有機發光二極體中能夠增加電子注入，導致亮度與電流效率的大幅提升。 本研究突破了現有微影技術的解析極限，達到50奈米以下特徵尺寸，並且實現了使用光微影技術圖樣發光材料的目標。Abstract The objective of this study is to realize high-resolution patterning of organic electronic materials by photolithography, with a goal of facilitating the development of commercially viable manufacturing processes for producing organic light-emitting diode (OLED) displays. Current patterning technologies such as inkjet printing and shadow-mask deposition are inadequate for commercializing organic electronics due to their limitations on resolution, throughput, and displays size. Photolithography, well-developed for industrial patterning, is well positioned to overcome the challenge of patterning for organic electronics. We developed a photolithographic method for patterning organic electroluminescent materials for OLED displays, and demonstrated its practicality. In the first part of our study, we determined the mechanism of a ~60% reduction in feature size, discovered with chemically amplified (CA) photo-resists, upon post-development UV irradiation. When a CA photo-resist is treated with post-development UV irradiation, it is both deprotected and cross-linked. Deprotection of the protecting groups turns them into gases to escape the photoresist, increasing the free volume of the photoresist. Crosslinking of the photoresist contracts the increased free volume, inducing a macroscopic reduction in the feature size. This technique enhances the resolution limitation for 248-nm photolithography from 90-nm feature sizes to sub-50-nm feature sizes. In the second part of our study, we fabricated OLED displays with photolithography, using MEH-PPV as the electroluminescent materials. Before the photolithographic process, a 15-cycle ALD Al2O3 layer was overcoated onto the MEH-PPV layer to isolate the MEH-PPV surface from direct contact with the solvents used in the photolithographic process. We observed that OLED devices, whose MEH-PPV layer was passivated with an Al2O3 film and was photolithographically patterned thereafter, showed 3-fold increase in efficiency and 10-fold increase in emissive intensity, compared to the control. This enhancement of performance may result from the Al2O3 film’s dual functions: protecting and buffering. An Al2O3 film is known to function as a buffer layer, which reduces the barrier height for the injection of electrons in OLEDs, leading to dramatic increases in current efficiency and emissive intensity. Through our study, we have demonstrated the extendability of the 248-nm photolithography to feature sizes smaller than 50 nm. We have also demonstrated photolithographic patterning for OLEDs with a passivating ALD Al2O3 layer, achieving enhanced device performance.Contents Acknowledgement………………………………….………………….…I Abstract (English)………………………………….…………………….II Abstract (Chinese)………………………………………………………IV Contents……………………………………………………………….…V List of Tables……………………………………………………...…….IX List of Figures…………………………………………………………... X Chapter 1………………………………………………………………….1 Introduction ……………………………………………………………...1 1.1 Background and motivation …………………………………...…1 Chapter 2………………………………………………………………….7 2.1 Background on photolithography…………………………………….7 2.2 Motivations…………………………………………………………...9 2.3 Experiments…………………………………………………………14 2.3.1 Materials……………………………………………………..16 2.3.2 Instruments…………………………………………………..17 2.3.3 Preparation of PVPh solution………………………………..17 2.3.4 Shrinkage of chemically amplified photo-resists, poly-(vinyl-phenol), and I-line resist in Z-direction after UV irradiation……………………………………………………17 2.3.5 Characterizations…………………………………………….18 2.3.5.1 Solubility of exposed chemically amplified photo-resists, poly-(vinyl-phenol), and I-line resist...18 2.3.5.2 FTIR of three commercial positive DUV-Resists…..18 2.3.5.3 NMR of exposed PVPh…………………….……….18 2.4 Results……………………………………………………………....20 2.4.1 Enhancement of resolution for positive 248-nm resists by post-development UV irradiation …………………………..20 2.4.2 Shrinkage of chemically amplified photo-resists, poly-(vinyl-phenol), and I-line resist in Z-direction after UV irradiation……………………………………………………24 2.4.3 Chemical changes of chemically amplified photo-resists after strong UV irradiation (3000mJ/cm2)………………………..26 2.5 Discussions ………………………………………………………....29 2.5.1 Enhancement of resolution for positive 248-nm resists by post-development UV irradiation…………………………...29 2.5.2 Factors affecting shrinkage of polymers after UV irradiation…………………………………………………....31 2.5.3 Chemical changes of chemically amplified photo-resists after strong UV irradiation (3000mJ/cm2)………………………..32 2.5.4 A model for shrinkage of chemically amplified photo-resists after post-development UV irradiation…………………...…34 25.5 The capability for the technique utilizing UV irradiation…….38 2.6 Conclusions……………………………….………………………...40 Chapter 3 ………………………………………………………………..41 3.1 Motivation and method……………………………………………...41 3.2 Experiment ………………………………………………………….49 3.2.1 Materials and instruments……………………………………50 3.2.2 Formation of ALD Al2O3 thin layers………………………...52 3.2.3 Formation of a polymer LED with 1.5-nm natural-oxidation Al2O3 ultra-thin layer between an active layer and cathode...54 3.2.4 Property of the ALD Al2O3 on MEH-PPV: A barrier screening…………………………………………………….54 3.2.5 H2O and TMA effects on MEH-PPV films..55 3.2.6 Fabrication of polymer LEDs with 15-cycle ALD Al2O3 ultra-thin layer between an active layer and cathode…….….56 3.2.7 Photopatterning by photolithography ……………………….58 3.2.8 Characterization of devices with and without ultra-thin ALD Al2O3 layers………………………………………….….…..58 3.3 Results…………………………………………………………..…..59 3.3.1 Barrier screening……………………………………….……59 3.3.2 H2O effects on MEH-PPV films…………………………….60 3.3.3 TMA effects on MEH-PPV films……………………………63 3.3.4 Polymer LEDs with ALD Al2O3 ultra-thin layers……………66 3.3.5 The role of 1.5-nm natural-oxidation Al2O3 thin layer in the interface of MEH-PPV and the cathode…………………….70 3.3.6 Polymer LEDs with ALD Al2O3 thin layer after photolithographic process…………………………………...72 3.4 Discussions …………………………………………………………75 3.4.1 The role of Al2O3 ultra-thin layer in the interface of a PLED with MEH-PPV as the electroluminescent materials……….76 3.4.2 H2O effects on MEH-PPV films…………………………….78 3.4.3 TMA effects on MEH-PPV films……………………………79 3.4.4 Polymer LEDs with ALD Al2O3 ultra-thin layers……………81 3.4.5 Polymer LEDs with ALD Al2O3 thin layer after photolithographic process…………………………………...83 3.5 Conclusions…………………………………………………………84 Chapter 4 ………………………………………………………………..86 Conclusions and future works 4.1 Conclusions…………………………………………………………86 4.2 Future works………………………………………………………...87 4.21. Nucleation of ALD Al2O3 on MEH-PPV surface………………...87 4.2.2 An improved method to reduce the damage caused by TMA…….87 4.2.3 Multi-color patterns……………………………………………….9