以濺鍍技術製備ITO透明導電薄膜成長在不同基材其應力行為及基材效應對其光學性質之研究

Abstract

ITO(Indium Tin Oxide) 是具有高導電性且於可見光範圍內具備高透明特性之材料，普遍廣泛被應用於光電產業及研究方面上。近年來隨可撓性光電元件之發展，為求輕量化及薄型化，以高分子材料作為元件之基板則有受到愈來愈多討論與重視之趨勢，這是由於它們具有質輕、價廉、耐衝擊、可製備成各種尺寸且具有可撓性等優勢。然而，在於塑膠基板上鍍覆透明導電膜(ITO)，所造成之問題在於導電膜內造成明顯應力之形成，主要問題源自於高分子基材之高熱膨脹係數；由於塑膠基板與ITO之間的熱膨脹係數差異將會造成薄膜產生應力方面的問題，過量應力之存在往往造成ITO薄膜導電度不佳、透明度下降、表面破裂或其它缺陷，進而使得元件可靠度不佳甚而整個失效。因此，對於ITO膜沉積於不同基材上其應力行為之了解對其元件應用及其可靠度之關係逐漸形成高度重視之議題。 本論文主要探討以直流式濺鍍系統法(DC Sputter Stystem)，於不同基材包含矽(Si)、聚对苯二甲酸乙二酯 (PET) 及聚醯亞胺 (PI) 薄膜上沉積ITO膜，利用真空彎柄儀 (Bending Beam System)在於溫度循環測試下探討其應力行為。實驗結果顯示，室溫下所沈積ITO膜在於不同基材形成之殘留應力皆呈現張應力，然而應力值大小決定於基材之楊氏係數，兩者之間有強烈之關係。沉積於PET高分子基材時ITO膜具有最小應力之形成，然而在於Si基材上具有最大之應力值其範圍為80~330MPa。不同之應力形成主要在於高分子基材具備較低楊氏係數因而在於沉積過程中基材產生塑性變形進而減低ITO膜內應力之產生。進一步結果顯示ITO膜在於高分子基材上所造成表面破裂之臨界應力值分別於PET上為19.2MPa及在於PI上為75MPa。 本研究同時進一步討論製程溫度之影響。提高沉積溫度至200℃，分別在於Si及PI兩種不同基材。有別於室溫下所沉積之ITO膜，結果顯示提高製程溫度將明顯降低其應力值，主要因素可歸咎於在於高溫製程下將提高離子轟擊效應及增加導電膜內結晶程度之改善。 其次本論文研究ITO膜在於不同基材上在於可見光範圍內其穿透度及帶狀能隙距離的大小(band gap)，探討基材效應所造成之應力對其光學性質之影響。利用UV-VIS光譜儀量測結果顯示ITO膜在於三種不同基材(glass, PET及PI)在於可見光範圍內其穿透度皆為接近90%。而在於band gap量測上發現當ITO在於高分子基材上時其能隙距離有窄化之趨勢。於兩種不同之高分子基材比較上，ITO/PET具有較大之能隙主要具有較低之張應力，意謂當應力值被提升時，帶狀能隙距離的大小亦隨之降低。

Indium tin oxide (ITO) is a common transparent conductive layer in optoelectronic devices because of its high electrical conductivity and transparent in visible wavelength. Recently, with the development of flexible optoelectronic devices, polymer material is widely employed as the substrate in these devices due to its potential advantages of high flexibility, low cost, and light weight. However, once ITO film is formed onto polymer substrate, there is a significant stress induced in the ITO film because of polymer’s high coefficient thermal of expansion (CTE). A large film stress may result in poor durability, low conductivity, or low transparency. Thus, understanding the ITO film stress behavior onto various substrates is highly needed for its applications and reliability in flexible devices. In this study, the DC sputtered ITO film stress as a function of deposition temperature was investigated under multiple thermal cycling by using a bending beam technique. On various substrates such as Si, polyethylene terephthalate (PET) and ODPA-BAPP polyimide (PI), the room temperature formed ITO films exhibit a tensile stress. It was found that the film stress level strongly depends on the Young’s modulus of substrate. ITO stress achieves the lowest level as the film was deposited on PET. ITO film on Si has highest stress in the range of 80 – 330 MPa. The difference in stress level of ITO on various substrates is due to the fact that polymer substrate with lower Young’s modulus can plastically deform under ITO film stress, resulting in stress reduction of the ITO film. The critical stresses for cracking of ITO on polymer substrates were obtained as well, 19.2 MPa for ITO/PET and 75 MPa for ITO/PI, respectively. Different from the ITO films deposited at room temperature, ITO films deposited at 200 ºC on Si and PI are under less tensile stresses. The compressive tendency of film stress with increase in the deposition temperature can be attributed to a stronger ion peening effect and an improvement of film crystallinity. Beside the study of film stress behavior, transmittance and optical band gap of ITO film on glass, PET and PI substrates were investigated in order to understand any substrate effect on these importance characteristics of ITO film. An UV-visible spectrophotometer system was used to examine the transmittance of ITO film. The results show that ITO films on glass, PET and PI have similar transmittance nearly 90 % in the visible light region. However, the film optical band gap tends to narrow when coating on polymer substrates. Moreover, ITO on PET has larger optical band gap than ITO on PI presumably due to the less tensile stress of film on PET substrate.