Masataka Umeno

Self-limiting oxidation of SiGe alloy on silicon-on-insulator wafers

Self-limiting oxidation of SiGe alloy on silicon-on-insulator wafers was investigated. For oxidation at 1000°C, oxidation stops completely after a few hours for the Si1−xGex (x=0.068–0.16) layers. For higher initial Ge concentrations of the SiGe layer, the oxidation saturated in a shorter oxidation time, whereas saturation was not observed for the oxidation at 900 and 1100°C. The authors propose a model for self-limiting oxidation, in which the oxidation saturation is governed by an interfacial Ge-rich layer that depends on the oxidation temperature and the initial Ge concentration.

Large-scale atomistic modeling of thermally grown SiO2 on Si(111) substrate

Large-scale SiO2/Si(111) models were constructed by introducing oxygen atoms in c-Si models in an atom-by-atom manner. Molecular dynamics calculation at a constant temperature was repeatedly carried out for the growing oxide model. By comparing the oxidation simulation of Si(111) substrate with that of Si(001) substrate performed previously, the influence of substrate orientation on the oxide structure was discussed. Owing to the significant feature of bonding arrangement within a Si bilayer in the Si(111) substrate, the inherent stress induced at the SiO2/Si interface by oxygen insertions is originally higher for the Si(111) oxidation than for the Si(001) oxidation, resulting in frequent changes in the bonding network. The resulting structure of bulk SiO2 on Si(111) has less strain and a lower density than that on Si(001), but involves a larger number of dangling bonds. The X-ray diffraction pattern calculated for the SiO2/Si(111) model exhibits a diffraction peak with a Laue-function-like profile on each of the crystal-truncation-rods from the 111 and 111 points, agreeing well with experimental results. These diffraction peaks indicate that the thermally grown SiO2 retains the residual order emanating from the {111} atomic planes in the original c-Si. Because of differences in the angles between the surface and the {111} atomic planes, the residual order within the SiO2 differs depending on the substrate orientation.

Effects of Thermal History on Residual Order of Thermally Grown Silicon Dioxide

By simulation of silicon oxidation and measurement of X-ray crystal-truncation-rod (CTR) scattering, the structures of silicon dioxide films grown at different temperatures and the structural changes due to thermal annealing have been investigated. Large-scale SiO2/Si(001) models were formed by introducing oxygen atoms, atom-by-atom, in crystalline Si from the surfaces. Molecular dynamics (MD) calculation at a constant temperature was repeatedly carried out for the growing oxide model. The intensity and position of the extra diffraction peak observed for the oxide, correlating with the residual order emanating from the parent Si crystal, depend on the growth temperature and change after thermal annealing. The peak intensity becomes smaller with increasing growth temperature. Thermal annealing monotonically decreases the peak intensity and shifts the position along the CTR, toward the lower angle side. There is a good agreement between the results of simulation and experiment. It is shown that (1) the oxide grown at a higher temperature has a lower degree of residual order, (2) thermal annealing decreases the residual order, ultimately leads to complete amorphization and never restores the ordering, and (3) the peak shift along the CTR corresponds to the volumetric expansion of the SiO2 in the surface-normal direction.

X-ray diffraction measurements of internal strain in Si nanowires fabricated using a self-limiting oxidation process

We demonstrate x-ray diffraction measurements of internal strain in Si nanowires that were fabricated using a self-limiting oxidation process. Rod-shaped scattering around the 111 Bragg point due to interference effects from the Si nanowires were observed, which are robust reflections for incoherent displacement of the wires. From the shifts of the scattering in reciprocal space, the strain was estimated to be 1.0–1.5×10−3 for the sample oxidized at 800°C for 300min.

Observation of Lattice Undulation of Commercial Bonded Silicon-On-Insulator Wafers by Synchrotron X-Ray Topography.

Synchrotron X-ray topographs of silicon-on-insulator (SOI) layers of two kinds of commercial bonded SOI wafers were obtained; one is fabricated by transferring an epitaxial layer over porous Si, and the other is processed using hydrogen-delamination-induced splitting. For the 2-µm-thick SOI layer of the former wafer, the quasi-periodical contrasts were observed in the topographs, which indicate that the lattice plane of the SOI layer undulated at spatial intervals of about 20 µm with a tilt angle of the order of ten arc seconds. It was also evident from the granular pattern in the topographs that the undulation existed in the SOI layer of 100 nm thickness for both of the wafers.

Synchrotron X-Ray Topography of Lattice Undulation of Bonded Silicon-On-Insulator Wafers

Lattice undulation of Silicon-on-Insulator (SOI) layers of bonded SOI wafers was observed by synchrotron X-ray topography. Patterns observed on topographs depended on the thickness of the SOI layer, the camera distance between a specimen and an X-ray film, and the diffraction geometry of the Laue and Bragg cases. The dependence was interpreted as the effects of the geometrical relation in reciprocal space among the Ewald sphere, the reciprocal lattice vector, and the surface normal direction. To confirm the origin of the pattern formation, the topographic images were simulated in the framework of the kinematical diffraction theory. Based on the simulation, it was found that a granular pattern observed in the 115 Bragg case was due to the divergence/convergence effect of X-rays diffracted from the undulated SOI layer.

Large-Area X-Ray Topographs of Lattice Undulation of Bonded Silicon-On-Insulator Wafers

X-ray topographs of bonded silicon-on-insulator (SOI) wafers were obtained using a synchrotron large X-ray beam. Wrinkled patterns in a micrometer scale were observed all over the wafers. Crumpled patterns were also observed, which were related to the lattice undulation at the average spatial interval of about 6 mm with the tilt angle of the order of ten arcsec. From the comparison with the topographs between the SOI layer and the substrate, it was also found that the warpage of the lattice plane of the SOI layer was different from that of the substrate.

White X-ray Topography of Lattice Undulation in Bonded Silicon-on-Insulator Wafers

The lattice undulation of a silicon-on-insulator (SOI) layer in bonded SOI wafers was observed by synchrotron white and monochromatic X-ray topographies. Pattern formation for white X-ray topography was discussed using the geometric relation among the Bragg streak and diffracted X-rays in reciprocal space. The diffraction images in white X-ray topographs were simulated using the angular distributions of the lattice inclination in SOI layers obtained by the analysis of monochromatic X-ray topographs. The results of this simulation were in very good agreement with observations including the dependences on camera distance and SOI layer thickness, indicating that the contrast is mainly formed by the divergence/convergence effect of the diffracted X-rays.

Thermal Stability and Electron Irradiation Damage of Ordered Structure in the Thermal Oxide Layer on Si

We investigated the thermal stability and the electron irradiation damage of the ordered structure in the thermal oxide layer on Si substrates. The diffraction peak from the ordered SiO 2 shifted to the lower angle side, and the intensity decreased after thermal annealing above 950°C, indicating the decrease in density and the disordering of the structure. The least-squares fitting analysis assuming the quasi-amorphous structure showed that the ordered SiO 2 was relatively stable at the SiO 2 /Si interface and the root-mean-square displacement of the atoms at the interface was 0.22 nm. The ordered structure was also degraded by electron irradiation at a dose of less than 6.6 C/cm 2 .