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단결정 실리콘 박막을 이용한 유연한 유기 발광 소자 제작 및 특성 평가

단결정 실리콘 박막을 이용한 유연한 유기 발광 소자 제작 및 특성 평가
Other Titles
Fabrication and Characterization of Flexible Organic Light Emitting Diode Devices Using single crystal silicon membrane
Alternative Author(s)
Kim, Sung-Jun
Issue Date
LCD를 대체할 차세대 디스플레이 소자로서 주목받고 있는 OLED는 유기물 (단분자 또는 고분자) 박막에 주입된 전자-정공에 의한 발광현상으로 1963년에 Pope등에 의해 anthracene의 단결정으로부터 처음 발견되었으며, 그 후 1987년에 Kodak사의 Tang등이 발광층과 전하 수송층으로 각각 Alq3와 TPD라는 이중층의 유기물 박막을 형성하여 효율과 안정성이 개선된 녹색의 발광 현상을 확인한 이후로 단분자를 이용한 OLED를 개발하려는 노력이 본격적으로 시작되었다. 또한 1990년에 영국 Cambridge 대학에서 PPV라는 π-공액성(conjugated) 고분자 박막으로부터 EL 특성을 관찰하여 고분자를 이용한 OLED를 개발하려는 연구가 동시에 진행되고 있다. OLED는 자체 발광형이기 때문에, LCD에 비하여 시야각, contrast가 우수하며 Back-light가 불필요하여 경량 박형이 가능하고 소비전력 측면에서도 유리하다. 또한, 응답속도가 빨라 동영상 구현에 적합하며 전부 고체이기 때문에 외부충격에 강하고 제조 cost측면에서도 저렴하다. 향후, 잠재 수요 면에서도 초고속 정보화 사회 및 멀티미디어 시대로 접어든 최근의 시대 상황에 발맞추어 매년 성장률이 30%이상의 급속한 신장세를 나타낼 것으로 예상된다. 이러한 특성 때문에 OLED는 휘거나 접을 수 있는 디스플레이, 바로 Flexible Display를 구현할 수 있는 가장 강력한 디스플레이 방법이라고 생각한다. OLED가 Flexible Display로 나아가기 위해서는 기존의 ITO/Glass 기판과는 다른 유연하고 투명한 새로운 유형의 기판을 필요로 한다. 본 논문에서는 Glass를 대체할 기판으로써 나노 스케일의 실리콘 박막을 제안하였다. 이는 향후 Glass 기판을 대체하는 효과뿐만 아니라 실리콘 기판에 도펀트를 도핑하여 일함수를 ITO와 유사하게 맞추어, ITO 또한 대체할 수 있는 효과도 기대되고 있다. 이를 위하여, 본 연구실이 보유하고 있는 Bonding 방법을 사용한 SOI Wafer 제조 방법을 응용하여 새로운 SON 웨이퍼를 제작하였고 특별한 형태의 장치(JIG)의 문제점을 보완한 KOH Bath를 개발하였다. 이렇게 하여 만든 나노 스케일의 실리콘 박막위에 여러 유기물(α-NPB, Alq3, LiF)층과 Al을 차례로 적층하여 나노 스케일의 실리콘 박막 위에 형성시킨 OLED의 특성을 평가하였다.; Organic light emitting diodes (OLED) using the phenomenon of electroluminescence (EL) of organic materials [1,4] are getting more attractive device for near-future flat panel display applications as they have advantages such as potentially low manufacturing costs, small thickness, light weight, low power consumption and superior viewing angle. In addition to those advantages, flexible OLED (FOLED), namely OLED with mechanical flexibility, is strongly desired for the application of rollup, advertising, automotive, head-mounting display, which can be affixed to any curved surfaces. [5-7] These FOLEDs have been fabricated on unbreakable thin plastic substrate. However, if they can be fabricated not on plastic substrate but on single crystal silicon [8] that is enough thin to have mechanical flexibility and transparency for visible light, it may provide us other advantages to fabricate smaller and higher-performance devices through the most-modern semiconductor device fabrication technology, and to enable bottom-emission mode on it. The materials used in OLEDs rapidly degrade when exposed to moisture and oxygen, so they should be encapsulated with another barrier material. [9] Permeability of single crystal silicon is extremely low compared to that of plastic, therefore, using single crystal silicon membrane as a substrate provides great advantage to realize long-lived OLED display devices. In this letter, we report a thinning technology to produce flexible and transparent single crystal silicon membrane and demonstrate successful fabrication of an OLED on the membrane. We have employed SOI wafer to obtain a thin and transparent single crystal silicon in the first stage. The buried oxide layer of a SOI wafer acts as a etching-stop layer since it have an etch selectivity as high as 200:1 between the porous silicon and the oxide layer in 30% KOH at 80℃ degree-C, which conformed through both paper and experimental result [10] After conducting patternability with photolithography and Dry etching process, SOI wafer was etched from the backside with 30% KOH. During wet etching process, the nitride layer deposited on both sides of the wafer acted as a mask against etchant solution and the substrate silicon in the window region on backside was completely removed until the buried oxide layer of the wafer was exposed. Though the buried oxide layer of a SOI wafer clearly is worked as a etching-stop layer by different etch rate between silicon and oxide, oxide layer is also etched by KOH which means that etch stop layer consist of only oxide layer not enough to do. While a window region on backside is etched by KOH, the etching solution occurs breakage of oxide layer and damages a thin and uniform top silicon that we have gained finally from SOI wafer. Also silicon membrane of a window region could be broken by tensile stress occurred during etching process so that the yield of a silicon film using SOI wafer is a low level. Therefore we devise SON Wafer to be inserted nitride layer and employed material to produce a transparent single crystal silicon. To get various patternability, positive photo resistance (PR) is deposed the thickness of 1.7um on cutting substrate with spincoater. Then soft baking, photolithography, development, and hard baking consecutively progress. Lithography process enable fabrication of silicon membrane with various square size. PR coated on cutting wafer is employed as a mask of dry etching for removing passivation layer. We act over etching around 0.3um as etching selectivity between coated PR and passivation layer consist of oxide and nitride is 1:1. The passivation layer by dry etching was completely removed and the silicon of window area etched by etching solution until nitride layer was exposed. The remaining oxide layer and nitride layer on the both side were stripped with hydrofluoric-acid (HF) if necessary. After fabricating silicon membrane of various thickness, measurements of silicon transmittance were performed by optical equipment. Also, we empolyed Atomic force microscope(AFM) and Transmission electron microscope(TEM) to investigate RMS of silicon and the structure of SON wafer. It can be said that we have established the basis of nano-thinning technology to produce highly flexible and transparent single crystal silicon membranes to realize OLED display devices with a new concept. In conclusion, we have developed a specially designed SON wafer and it made us possible to produce a flexible and transparent single crystal silicon membrane with various size. An OLED was successfully fabricated on the membrane, which demonstrates feasibility of flexible OLED display devices on silicon substrate.
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