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0910-A16-03-L09-03

Amorphous Silicon 2-TFT Pixel Circuits on Stainless Steel Foils Alex Z. Kattamis1, I-Chun Cheng1, Yongtaek Hong2,3, and Sigurd Wagner1 1 Princeton Institute for the Science and Technology of Materials & the Department of Electrical Eng, Princeton University, E-Quad Building, Olden Street, Princeton, NJ, 08544 2 Display Science & Technology Center, Eastman Kodak Company, 1999 Lake Road, Rochester, NY, 14626 3 New Address: School of EECS, Seoul National University, Kwanak P.O. Box 34, Seoul, 151-600, Korea, Republic of Abstract We have fabricated pixel circuits consisting of two bottom-gate staggered source-drain amorphous silicon thin-film transistors (a-Si:H TFTs) on flexible stainless steel foils. Stainless steel is attractive because it allows for high processing temperatures of >300°C and is a perfect barrier to oxygen and moisture. Our steel foils were 75µm thick, with a peak-to-peak surface roughness of >1.2µm. This rough, as-rolled, conductive surface needed a thick planarization and passivation (electrical isolation) layer. The surface was planarized with 1.6µm of spin-on-glass, which reduced the roughness to ~0.3µm peak-to-peak. A passivation layer of 0.6µm of SiNx deposited by plasmaenhanced chemical vapor deposition was used to reduce leakage currents and capacitive coupling to the substrate. The 92µm x 369µm voltage-programmed pixel circuits employ a switching (Sw) TFT (W/L=50/5µm), a driver (Dr) TFT (W/L=200/5µm), and a 2pF storage capacitor between the gate and source of the Dr TFT. With a supply voltage of VDD=20V and a drive bias of 20V the circuits deliver 26µA of current. The vertical stripe pixels were integrated into 48 x (4) x 48 arrays and passivated with SiNx. Anode metal of Al-1% Si was also deposited, preparing the displays for subsequent OLED fabrication. Pixel circuits with this performance can drive top-emitting organic light emitting diodes (OLEDs) and therefore can be used in backplanes for flexible, high-resolution, active-matrix OLED displays. Introduction Growing interest in flexible displays has stimulated research in thin-film transistor (TFT) backplanes on novel substrates such as plastics and stainless steel foils. Stainless steel foils have distinct advantages over plastic including high temperature process capability and chemical and moisture barrier properties [1]. It has been shown that hydrogenated amorphous silicon (a-Si:H) TFTs deposited at high temperatures (~300°C) tend to be more stable than their low temperature counterparts (~150°C) [2]. A chemical barrier is important for integrating with OLEDs, since OLEDs are sensitive to moisture.

One challenge when working with steel foil substrates is the surface roughness caused by rolling marks, often with peak-to-peak surface roughness of >1.2µm. Therefore the surface must be planarized using a spin-on material such as spin-on glass (SOG) [3, 4], benzocyclobutene [5], or possibly a spin-on polyimide. For the work described here we used a 1.6µm thick SOG. The metallic substrate would cause leakage currents