A Study on the 4H-SiC MOSFETs Channel Mobility Degradation Mechanism

• Main Authors: Wang, Kuo-Cheng, 王國丞
• Other Authors: Tsui, Bing-Yue
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/01803534002397652091

碩士 === 國立交通大學 === 電子工程學系 電子研究所 === 102 === Silicon carbide (SiC) is suitable for fabricating high power semiconductor devices because of its wide bandgap and high thermal conductivity. Unfortunately, low channel mobility occurs on the 4H-SiC MOSFETs due to the high SiO2/SiC interface state density (Dit) which leads to coulomb scattering. Therefore, if we want to enhance channel mobility, we must reduce Dit at first. This thesis is divided into two parts. The first part is using low thermal budget method to reduce the Dit of MOS capacitors. The second part is the study on the 4H-SiC MOSFETs channel mobility degradation mechanism. We tried some low thermal budget methods to reduce Dit, such as depositing a PE-oxide and N2O oxidation at 1100 ℃ as gate dielectric. But these methods didn’t reduce Dit as effective as our group used ammonia plasma treatment in 2012. Before executing this method in MOSFETs fabrication processes. We should examine the thermal stability of the hydrogen passivation effect after additional high temperature processes. Although ammonia plasma treatment can reduce the Dit, but the passivated interface can’t sustain processes with temperatures higher than 500 C. Therefore, NH3 plasma treatment after device fabrication should be evaluated. That is NH3 plasma treatment should be post-gate-pattering. We fabricated the MOS capacitor by NH3 plasma treatment post-gate-pattering. Fortunately, the post-gate-patterning NH3 plasma treatment does reduce Dit for the enough time of ammonia plasma treatment. Hence we could use this method in the MOSFETs fabrication process. For the process of fabrication MOSFETs, we coated different kinds of photo resistors to form graphite capping layer. They are FH-6400 and S-1813, respectively. Samples using FH-6400 show better surface roughness than S-1813. This is because the graphite capping layer formed by coating FH-6400 is thicker than S-1813 which leads to better protective effect. Therefore, sample has higher channel mobility with the smoother surface roughness. However, our samples have very low channel mobility and only slightly improvement channel mobility even if samples were exposed ammonia treatment. Despite of we excluding RSD series resistance method. Therefore, to investigate the reason of leading mobility degradation is studied in this thesis. To reduce surface roughness, we fabricated the MOSFETs with slightly chemical etching SiC surface before growing gate dielectric. However the surface morphology becomes worse after gate oxidation process. The higher surface roughness of samples leads to worse channel mobility. This result might be the lattice defect caused by p-well implantation. The defect region is etched with higher rapid which leads to different orientation. Therefore the oxidation rate is difference which leads to highly surface roughness. Ammonia plasma treatment can passivate the interface defects so that the threshold voltage is reduced and the subthreshold swing is improved. To further investigate the degradation of the channel mobility, high temperature measurement was executed. The result shows that the channel mobility will enhance with higher temperature. This is because the mobility is dominated by the coulomb scattering mobility, which has positive temperature dependence. Mobility of samples after etching process don’t increase with higher temperature obviously. This is because the surface roughness scattering which dominates the mobility mechanism. Body effect was also measured by giving various body voltages. The result shows the channel mobility will decrease with the more negative VBs bias. As VBS becomes more negative, a stronger vertical electric field would exert on the electrons in the inverted channel. Hence, electrons will suffer stronger surface scattering and coulomb scattering which lead to mobility degradation. This is the first time for our group to fabricate 4H-SiC MOSFETs. Although the electrical properties of our samples are needed to be improved in the future, but from the study of mobility degradation mechanism, we realized the reasons of our MOSFETs channel mobility degradation. We expect better performance of our 4H-SiC MOSFETs by solving these problems in the future.