| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Wednesday Sessions |
| Session TF-WeM |
| Session: | Plasma-based Film Growth, Etching, & Processing |
| Presenter: | Sungmin Kim, Seoul National University, Korea, Republic of Korea |
| Authors: | S. Kim, Seoul National University, Korea, Republic of Korea J.M. Kim, Seoul National University, Korea, Republic of Korea H.J. Kim, Seoul National University, Korea, Republic of Korea |
| Correspondent: | Click to Email |
Amorphous oxide semiconductor (AOS) such as InGaZnOx (IGZO) and ZnSnOx (ZTO) attracts an attention as a new channel material for thin film transistors (TFT). It has outstanding properties such as high field effect mobility (>10cm2/V-s) as well as transparency and uniformity. Therefore, display industry has named it as a prime candidate of channel material for large-sized and flexible display devices. However, considering that substrates in use for flexible display devices are polymer materials like PEN or PI, AOS has a critical drawback that it requires annealing process over 350℃. This is a quite low temperature compared to conventional silicon-based process(over 800℃ ), but the process temperature must be under 200℃ to prevent deformation of polymer substrates.
In this study, the fabricated ZTO TFTs have a ZTO bottom gate and ITO top contacts, and all of them were made by RF magnetron sputtering system. And we replaced post annealing step with heating up the sputter stage up to 200℃ during sputtering and forming 10% oxygen atmosphere. Electrical properties such as threshold voltage (Vth), mobility and subthreshold swing (SS) of ZTO TFTs were compared to convince that our new process could replace post annealing over 350℃ . Analyses for morphology and chemical compositions are also conducted.
Saturation mobility of ZTO TFTs with 350℃ annealing process and with our novel process were measured to be 17.4 and 11.7cm^2/V-s, respectively. Post annealing at 200℃ cannot activate ZTO TFTs at all, but by heating up the sputter stage, heating of 200℃ was sufficient to activate TFTs. There was a little reduction of mobility, but 11.7cm^2/V-s is still good enough giving that mobility of 10cm^2/V-s is enough for large sized display (>70 inches). And SS values of 350℃ annealed sample and one sputtered at 200℃ was 0.31V/decade and 0.50 V/decade, respectively. Also, chemical composition and surface roughness was analyzed by XPS and AFM. These results also show that our new low temperature process is suitable for manufacturing AOS TFTs.
The main reason of this phenomenon can be served by combination of plasma and thermal energy in sputter chamber. Generally, it is obvious that as-deposited AOS channel needs oxygen curing to connect broken bonds between metal and oxygen atoms. While Ar/O2 plasma in sputter chamber redeem relatively low thermal energy(200℃ ), curing reaction immediately occurs on as-deposited ZTO. Consequently, we made ZTO TFTs below 200℃ process, rather than post annealing over 350℃ . Performances of them were still applicable for next generation display. But it shows a little bit poor performance, so there must be further study to advance properties