Amorphous Silicon 2-TFT Pixel Circuits on Stainless Steel Foils

Abstract

AbstractWe 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 75m thick, with a peak-to-peak surface roughness of >1.2m. This rough, as-rolled, conductive surface needed a thick planarization and passivation (electrical isolation) layer. The surface was planarized with 1.6m of spin-on-glass, which reduced the roughness to ~0.3m peak-to-peak. A passivation layer of 0.6m of SiNx deposited by plasma-enhanced chemical vapor deposition was used to reduce leakage currents and capacitive coupling to the substrate. The 92m × 369m voltage-programmed pixel circuits employ a switching (Sw) TFT (W/L=50/5m), a driver (Dr) TFT (W/L=200/5m), 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 26A of current. The vertical stripe pixels were integrated into 48 × (4) × 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.