With an ever-increasing demand for efficient and power-saving devices, wide bandgap (WBG) semiconductors have emerged as potential candidates for next-generation power devices. In particular, silicon carbide (SiC) has attracted significant research attention as it is the only compound semiconductor capable of facilitating the growth of native silicon oxide (SiO2). Since the metal-oxide-semiconductor (MOS) structure exists in most power devices, the gate oxide’s reliability and quality have been considered a critical design issue. Additionally, optimizing the quality of the oxide gate SiO2/SiC interface requires improvements to the conventional gate oxide processes.
At present, post-oxidation annealing (POA) is one of the most commonly used methods to optimize the SiO2/SiC interface. In most cases, POA uses nitrogen oxide (NO) and nitrous oxide (N2O) as annealing gases, known as the nitridation process. The nitridation process has proved effective for improving the interface between 4H-SiC and SiO2 due to the formation of strong Si≡N bonds and nitrogen-assisted removal of carbon-oxide compounds from the interface. Besides the nitridation process, many other methods to improve channel mobility, such as nitrogen implantation, have been proposed.
Nevertheless, NO annealing is the most commonly used in industries owing to its effectiveness in decreasing the interface state density and improving gate oxide quality. However, there are contradictory reports that NO annealing could produce hole traps in SiO2 to facilitate gate oxide degradation. In addition, the time-dependent dielectric breakdown (TDDB) characteristics are susceptible to oxidation temperature, time and ambient. Although this is well known, there are limited studies on the impact of various oxidation conditions on the TDDB of the gate oxide.
Herein, Professor Bing-Yue Tsui, Miss Yi-Ting Huang, Professor Tian-Li Wu and Professor Chao-Hsin Chien from National Yang Ming Chiao Tung University investigated the effects of different oxidation processes on the TDDB of gate oxide integrity on 4H-SiC. The gate oxide under investigation was grown by dry and wet oxidation processes followed by NO POA. Different parameters, including the gate oxide thickness, electron tunneling barrier height, interface state density, TDDB and breakdown field, as well as the current-voltage and capacitance-voltage characteristics, were measured and discussed in detail. Finally, the TDDB performance of the oxide gate was evaluated. The work is currently published in the research journal, Microelectronics Reliability.
The authors findings revealed that more nitrogen atoms were absorbed into wet oxide than dry oxide under similar NO POA conditions. Thus, the interface state density was passivated effectively with more nitrogen incorporation. At the same time, the electron tunneling barrier at the SiO2/SiC interface increases close to the ideal barrier height, while the positive charge in the oxide increases.. Whereas wet oxidation resulted in an increased breakdown field, NO annealing exhibited a less significant effect on the breakdown field, though the differences were practically limited. Regarding the TDDB reliability, a decrease in the 10-year-projected intrinsic breakdown field with an increase in the NO annealing time was reported for dry and wet annealing conditions.
In a nutshell, the present study reported the fabrication of MOS capacitors on 4H-SiC with different NO annealing processes and conditions and a subsequent investigation of the TDDB performance of the oxide gate. From the findings, NO annealing proved to be more effective in reducing the density of the interface and hole traps induced by the carboxyl defects. However, excessive nitrogen could induce hole traps leading to significant deterioration of the TDDB reliability. As a result, the authors recommended carefully adjusting the NO annealing process to achieve improved interface quality and TDDB reliability properties simultaneously. In a statement to Advances in Engineering, Professor Bing-Yue Tsui, first author explained their findings contribute to producing high-performance WBG semiconductor materials for energy-efficient power devices.
Tsui, B., Huang, Y., Wu, T., & Chien, C. (2021). Time-dependent dielectric breakdown of gate oxide on 4H-SiC with different oxidation processes. Microelectronics Reliability, 123, 114186.