Abstract
High performance LTPS TFTs with mobility above 100cm2/V-sec and on/off current ratio beyond 108 have been fabricated by both excimer laser crystallization (ELC) and metal-induced lateral crystallization (MILC) methods. Comparing the two different processed LTPS TFTs, the MILC TFTs exhibit higher leakage current, which may be due to Ni contamination and more intra-granular defects, but ELC TFTs suffer from poor uniformity due to the narrow process window. The NH3 plasma passivation improves not only device performance but also device-performance uniformity. The passivation efficiency of MILC TFTs is higher than that of ELC TFTs, which is owing to the micro grain structures resulting from the two different crystallization methods. Temperature effects of different processed LTPS TFTs have been investigated especially for LTPS TFTs operating on glass substrates, which possess low thermal-conductivity. Ni contamination and grain boundary effect are observed in symmetric MILC TFTs, especially in short channel devices. With raising temperature, the performance of LTPS TFTs is degraded, which means increased leakage current and decreased on current, leading to small on/off current ratio. Some methods should be created to reduce the temperature rise when devices operate on glass substrate. At 75°C, carriers have higher thermal energy, and higher current brings about more impact ionization. Therefore, device at 75°C degrading more than at 25°C. At 125°C, thermal scattering becomes dominant, and mobility in is reduced. Beside, lattice vibration due to high temperature also reduce the probability of impact ionization. As a result, hot-carrier effect is suppressed. For devices before plasma treatment, hot-carrier degradation is due to states created by impact ionization. In bias-temperature instability, devices under hot-carrier effect degrade most at 75°C because of the competition of two factors: high current from high thermal energy, and low mobility from thermal scattering and lattice vibration. And larger degradation is observed in ELC devices because of its surface roughness. Self-heating effect and positive gate-bias degradation both degrade the devices by channel state creation and electron trapping in oxide. The two degradation are both suppressed at high temperature due to the reduction of mobility, only devices under self-heating effect degrades more because of the high current. ELC devices also degrade more due to its surface roughness. Off-state degradation is due to channel state creation and causes ION, IOFF degradation. And the degradation is more serious at higher temperature because of lower mobility. And ELC TFTs’ surface roughness causes more degradation. Devices under negative gate-bias degrade by channel state creation and hole trapping in oxide. And as temperature rises, more states are created, thus more degradation is observed. And ELC TFTs’ surface roughness causes more degradation. For devices after plasma treatment, the effect of H+ on device degradation: Under hot-electron bias, Si-H weak bonding is broken, leading to dangling bonds. The number of broken Si-H bonding increases with temperature, and causing more degradation. Under self-heating effect and positive gate-bias stress, H+ in oxide is driven into grain boundaries, and H+ transportation is enhanced by high temperature. Further passivation reduces trap density. Under negative gate-bias, Si-H weak bonding breaking involves in device degradation, and there is more broken bonding at high temperature. Hot-carrier degradation is the most serious degradation mechanism. Therefore, device operation in such bias condition should be avoided.