NH3 and NO + NH3 Annealing of 4H-SiC Trench MOSFETs: Device Performance and Reliability

As the gate oxide (GOX)-SiC interface of SiC MOSFETs is crucial for device performance, it requires special attention. To improve interface quality, device performance, and reliability, commonly, a postoxidation anneal (POA) is applied. Different gas compositions can be used to passivate the SiC interface and bulk oxide. We study the effects of ammonia (NH3) and a new combined NO + NH3 POA on 4H-SiC trench MOSFET test structures and compare them to the common nitric oxide (NO) POA. Our studies are not only limited to device performance represented by channel mobility but also contain reliability studies in the form of bias temperature instability (BTI), sweep hysteresis, Fowler-Nordheim (FN) tunneling and GOX breakdown. We find that both, NH3 and NO + NH3 result in higher mobility than NO annealed test structures. On the other hand, ammonia containing POAs deteriorate device reliability, e.g., leading to larger BTI and higher tunneling currents. As both can be detrimental in certain applications, they must be optimized. We show that NO + NH3 anneals may offer an acceptable compromise between the characteristics of NO and NH3 with regard to tunneling and device performance. However, it shows more positive BTI (PBTI) drift than NH3 and NO. We will show that a full characterization of the GOX is essential to evaluate, optimize, and benchmark new annealing strategies.