High performance TFT with MICC poly-Si on flexible metal foil

In this study, we investigated the bias-induced changes in the performance of the poly-Si thin-film transistors (TFTs) on flexible metal foil by metal-induced crystallization using a cap layer (MICC). Active-matrix liquid crystal display (AMLCD) with polycrystalline Si thin-film transistors (poly-Si TFTs) on glass have been widely used. However, active-matrix LCDs on glass have disadvantage of fragile and heavy. Therefore, new substrates such as plastic and metal foils are strongly required to make a rugged and lightweight flat panel display. In this work we selected metal foil as a substrate for poly-Si TFT. [1,2] In case of plastic substrate, the shrinkage and elongation by gas permeation through plastic is a main issue. Therefore, a gas barrier is necessary to project the device from the penetration of moisture to the plastic, which is not needed for metal foil. Beside, the thermal stress induced when the inorganic layers are deposited on, can be significantly reduced when steel foil is used. On the other hand, in case of steel foil, the process temperature can be as high as 9001) because the melting point of steel foil is ∼1400°C. The flexible metal foil used in this experiment was 50μm-thick foils of 304 stainless steel (Fe/Cr/Ni 72/18/10 wt.%) which was polished by Chemical Mechanical Polishing (CMP). However, the surface roughness is high for the SS metal foil. The RMS roughness of SS surface changed from 1200Å to 30Å after CMP of the SS foil. Figure 1 shows the AFM images of the metal foils before and after CMP. The p-channel MICC poly-Si TFTs were fabricated with a self-aligned coplanar structure on metal foil. The detail fabrication procedures are as followings; The a-Si was crystallized by MICC [3,4]. Then, SiN_x cap layer was etched by a BOB solution (NH ₄F+HF+H₂O). The poly-Si was defined for active islands, and then a 150 nm-thick SiO₂ and a 150 nm-thick gate metal were deposited. After patterning both gate metal and SiO₂ on a channel, the source and drain regions was doped at 280°C for ohmic contact in an ion shower system, and then the samples were annealed at 450°C for 2 hrs for dopant activation. Finally, contact holes were opened through 400nm-thick of interlayer and then source/drain metal was fined. The performances of the TFTs were measured with the source electrode grounded.In this study, the threshold voltage was defined by the gate voltage giving the drain current of W/L × 10nA at V_ds=-01.V. The threshold voltage shift is defined by the difference of the threshold voltage before and after bias stress. Figure 3 shows the field-effect mobility, threshold voltage and gate voltage swing of MICC poly-Si TFTs on metal foil plotted as a function of gate bias stress time at y_G= +20V. There is no change in the drain current, field-effect mobility and threshold voltage. This indicates that there is no charge trapping at the gate insulator of the SiO₂ and at the Si/SiO₂ interface by gate bias stress. We studied the bias-induced changes in the performance of the MICC poly-Si TFTs on flexible metal foil. The p-channel MICC poly-Si TFT on flexible metal foil exhibited a field-effect mobility of 75.1cm²/Vs, threshold voltage of -3.9V, gate voltage swing of 0.9V/deo., and minimum off current of 10^-12A/μm at V _ds=-0.1V. The MICC poly-Si TFTs on metal foil has very stable performance against gate bias stress as well as hot-carrier bias stress. The experimental results on bias stress effects on the MICC poly-Si TFT on metal foil indicate that the MICC poly-Si on metal foil can be applied to stable TFTs for driving AMOLED.