The effects of off-axis substrate orientation on MOSFET characteristics

The effects of off-axis substrate orientation on MOSFET performance and reliability are examined. As the <100> wafer is tilted off-axis around the <011> axis, two principal effects are observed. First, for current flow normal to the axis of rotation, inversion-layer mobility is lower than for current flow in the parallel direction. This mobility difference is due to anisotropy in the inversion-layer effective mass as well as increased surface roughness in the normal direction. Second, because surface roughness enhances nonuniform oxidation, thin spots in the gate oxide are generated which increase the susceptibility of the gate oxide to defect-related failure. The performance anisotropy may possibly be alleviated by rotating substrates in the <001> direction, such that the normal and parallel directions possess equivalent inversion-layer effective mass and degrees of surface roughness. The oxide quality may possibly be improved by using a two-step oxidation procedure, where an intermediate high-temperature anneal is used to smooth the oxide.<>