Chapter 5 Conclusion
5.1 Summary
In this thesis, the LTPS high-k TFT devices have been successfully fabricated by the gate-last process. Thanks to the high-k gate dielectric, all devices have the excellent sub-threshold swing which approximately in 135~150 mV/dec. and low threshold voltage about 1-V. Under HCS and PBTI stressing, we found that this gate dielectric, HfO2, prepared by e-gun, is not good enough and easy to trap charges.
Compared with control devices, all the F-implanted samples under HCS and PBTI show a better electrical characteristic, in terms of less threshold voltage shift under HCS and BPTI stressing, low gate leakage current in HCS and the low gate dielectric capacitance. Based on these observations, we can conclude that the fluorines diffuse into gate dielectric and passivate the defect in it. Second, have better sub-threshold swing than control both in HCS and PBTI, suggesting that strong Si-F bonds can passivate the deep states at the interface. Third, less degradation of transconductance after both HCS and PBTI implies that tail states in grain boundary have been effectively passivated by strong Si-F bond. Finally, the off state current after PBTI under higher temperature of F-implanted TFTs is found better than control one, which means that drain side junction defects have been passivated due to fluorine implanted. From the above result, F-implant into TFT can significantly improve the performance of TFTs.
For the N-Implanted TFTs, the performance does not show the same improvement as those in F-implanted devices. However, it still exhibits better characteristic in the sub-threshold swing compared with control and F-implanted
deices after PBTI. Our result show that fluorine does not passivate the donor like state after PBTI, but it does not see in the nitrogen after the same PBTI stress.
Hence, the LTPS high-k TFTs with F- and N-implantation have been investigated under HCS and PBTI stressing in this thesis. The F-implanted LTPS High-k TFTs have the better performance and reliability than N-implanted ones. Hence, the fluorine implantation seems to be a promising approach to obtain a high performance LTPS high-k TFTS.
Since high-k gate dielectric, HfO2, deposited by e-gun, is not good enough in this study, how to improve the integrity of HfO2 in the low temperature process is worthy for future study.
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Biography
I was born in September 1, 1983 in Tainan city of R. O. China. When I studied in my high school, Tainan Fist Senior High School, my teacher who teaches physics bring me into the interesting world. From then on, I was really love to find out the physical mechanism behind the phenomena. In 2005, I completed degree of Bachelor of Science in Applied Physics at National Chia-Yi University, R. O. China. I recommended myself into the Institute of Electrophysics at National Chiao-Tung University, R. O. China at the same year. Here, I was major in semiconductor device and process in T.S Chao's group. And my research was focus on how to improve the performance and reliability of TFTs. In this two year, I have been published the related results in several Letter and Conference. Now, I finished my research and received degree of master in physics at NCTU on June, 2007.