Amorphous-silicon thin-film transistors made at 280°C on clear-plastic substrates by interfacial stress engineering

Ke Long I-Chun Cheng Alexis Kattamis Helena Gleskova Sigurd Wagner James C. Sturm Abstract — A process temperature of ~300°C produces amorphous-silicon (a-Si) thin-film transistors (TFTs) with the best performance and long-term stability. Clear organic polymers (plastics) are the most versatile substrate materials for flexible displays. However, clear plastics with a glass-transition temperature (Tg) in excess of 300°C can have coefficients of thermal expansion (CTE) much larger than that of the silicon nitride (SiNx) and a-Si in TFTs deposited by plasma-enhanced chemical vapor deposition (PECVD). The difference in the CTE that may lead to cracking of the device films can limit the process temperature to well below that of the Tg of the plastic. A model of the mechanical interaction of the TFT stack and the plastic substrate, which provides design guidelines for avoid cracking during TFT fabrication, is presented. The fracture point is determined by a critical interfacial stress. The model was used to successfully fabricate a-Si TFTs on novel clear-plastic substrates with a maximum process temperature of up to 280°C. The TFTs made at high temperatures have higher mobility, lower leakage current, and higher stability than TFTs made on conventional low-Tg clear-plastic substrates.