Ultrathin gate oxide CMOS on (111) surface-oriented Si substrate

The properties of ultrathin gate oxides in the direct-tunneling regime and the characteristics of the related CMOS transistors on a (111) surface-oriented Si substrate were investigated and compared with those on a (100) substrate for the first time. It was confirmed that low field mobility of n-MOSFETs on the (111) substrate is smaller than that on the (100) substrate and that of p-MOSFETs on (111) is larger than that on (100) until the direct-tunneling gate oxide regime. It has been found that most of the electrical properties of MOSFETs, with the notable exception of mobility, become almost identical for (100) and (111) substrates when the oxide thickness is reduced to less than 2.0 nm. Some of the properties are quite different between the two substrates for the thicker oxide case. It has been found that the reliability of hot carrier injection and time-dependent dielectric breakdown (TDDB) of the oxides and MOSFETs on the (111) substrate is slightly better than that on the (100) substrate. In addition, the characteristics and reliability of oxides and MOSFETs on a wafer tilted 4° from (100) axis were investigated. It was found that there are few differences in the mobility between (100) and (100) 4° off substrates for both n- and p-MOSFET cases. The reliability of oxides or MOSFETs on the wafer was identical to that on normal (100) substrate. These results suggest that ultrathin gate oxide MOSFETs on Si surfaces with various orientations are likely to have practical applications. This is good news for possible future new structures of MOSFETs such as vertical or three-dimensional (3-D) MOSFETs