Periodic silicon surfaces: I. Kinetics of atomic steps. II. Strain induced by oxidation

Evolution of surface profiles and step contours toward equilibrium have been studied on one- and two-dimensional gratings fabricated on Si(001) surface by atomic force microscopy (AFM) and low-energy electron microscopy (LEEM). By annealing at 900-1200{dollar}spcirc{dollar}C, quasi-sinusoidal profiles with extensive (001) flats at the extrema are developed. On 1-D gratings, straight steps parallel to the grating lines plus background steps due to the wafer miscut are observed. On 2-D gratings, concentric closed step loops are formed. The dimensions of the flats decreases as the temperature increases. Above the surface roughening temperature of 1200{dollar}spcirc{dollar}C, the flats are not formed. This temperature dependence of the dimension is mainly due to the increase in the surface entropy. From the analysis of the dimensions and the shapes of the step contours on a 2-D grating, it is deduced that the attachment-detachment process at the step limits the surface diffusion. At 1000-1150{dollar}spcirc{dollar}C where sublimation of surface atoms by a step flow mechanism becomes significant, the flats at the minima of a 2-D grating become larger than those at the maxima, while on a 1-D grating the dimensions of the extrema are approximately the same. The closed step loops formed on 2-D grating act as barriers to the flow of background steps, leading to the asymmetric surface profile. With an appropriate choice of starting structures, 10 x 10 {dollar}mu{dollar}m step-free regions have been created at the minima of a 2-D grating.; In addition to the studies above, the strain induced by the thermal oxidation of Si nanostructures was measured by high-resolution synchrotron X-ray diffraction. The state of strain induced by a 5-15 nm thick oxide grown on 150-200nm diameter cylinders was observed. The results are compared with an analysis using the semiconductor process program T-SUPREM. The observed strain in the Si cylinders is the sum of at least two contributions: the stress built up during the oxide growth and that due to the thermal expansion difference of Si and SiO{dollar}sb2{dollar}. Increase in the oxide thickness changes the sign of the strain in the Si cylinder from tensile to compressive due to the increase in the oxide stress component.