Roughness of TFT Gate Metallization and its Impact on Leakage, Threshold Voltage Shift and Mobility
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Roughness of TFT Gate Metallization and its Impact on Leakage, Threshold Voltage Shift and Mobility A. Nathan, B. Park, A. Sazonov, and R.V.R. Murthy* Electrical and Computer Engineering, University of Waterloo Waterloo, Ontario, Canada N2L 3G1 [email protected] *currently at: PerkinElmer Optoelectronics, Vaudreuil, Québec, Canada J7V 8P7 ABSTRACT A comparison of the performance of aluminium (Al)-gated thin film transistors (TFTs) is presented in which we varied its sputter deposition conditions, such as deposition temperature, process pressure, and power. Gate films deposited at 30˚C/5mTorr/300W yield TFT characteristics with low leakage current (~ 10 fA at low VDS), an ON/OFF ratio better than 108, and a mobility of 1.1 cm2/Vs. In contrast, films deposited at 150˚C/10mTorr/400W, yield a significant degradation in leakage current (~ 1 pA) and mobility (0.77 cm2/Vs). The degradation stems from the high surface roughness of the a-SiNx:H gate insulator, and hence the TFT channel, caused by hillock formation on the Al gate. In addition, the high roughness leads to a correspondingly large shift in threshold voltage. After one-hour bias stress of +25 V applied to the gate, the shift in threshold voltage is ∆VT ~ 5 V, as compared to the small shift of ∆VT ~ 2.3 V associated with the smoother gate. Also included in our comparison is a TFT whose Al gate is now capped with 20 nm of molybdenum (Mo) to minimize propagation of the gate surface roughness to the active channel. Its cross sectional topography shows the interface smoothness to be as good or better, to yield improved leakage and stability characteristics. INTRODUCTION Although the presently employed high refractory metals such as molybdenum (Mo) and chromium (Cr) are stable materials, their highly resistivity results in RC gate delays that impose constraints on the array size. Higher conductivity metals such as copper (Cu) and aluminum (Al) are highly desirable, but their use is generally constrained by process considerations. Copper typically suffers from poor adhesion to glass and reacts with silicon and other PECVD films. These problems are typically overcome, although at the cost of increased process complexity, by depositing the Cu on indium tin oxide (ITO) coated glass for increased adhesion [1] or through self-passivation of the copper surface with Cr2O3 to avoid reactions during the PECVD process [2]. With Al, although its resistivity is higher than that of Cu, its adhesion to glass substrates is far better, but it can suffer from hillock generation induced by subsequent process steps that are relatively high in temperature. In TFT fabrication, these process steps include plasma-enhanced chemical vapor deposition (PECVD) of amorphous silicon nitride (a-SiN:H) dielectric and aSi:H active layers. This thermal processing, as we will observe in the TFT samples considered here, undermine surface smoothness to increase the leakage current, increase the shift in threshold voltage, and to reduce the field effect mobility. Although the film roughness can be
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