Correlation of the microstructure and processing conditions of ultra-thin oxygen-implanted silicon-on-insulator materials

Abstract

The effect of implantation dose and annealing conditions on the microstructure of ultra-thin SIMOX materials formed by 65 keV ion implantation were investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Rutherford backscattering spectrometry (RBS), and optical microscopy. The implantation dose has a strong effect on the microstructure in both the as-implanted and annealed samples. The dominant defects observed in the as-implanted samples were multiply faulted defects (MFDs) near the upper interface and {113} defects beneatht he buried oxide (BOX) layer. The BOX layer started to form continuously at the dose of 7.0x10¹⁷/cm² after implantation. The most noticeable microstructural feature observed in the as-implanted samples was the mixed structure of silicon and oxygen precipitates which formed around the oxygen projected range. The structure, observed in the samples with dose in the range of 3.5 to 5.0x10¹⁷/cm², was found to be the precursor for the formation of silicon islands inthe samples after annealing. For the annealed samples, the dose range of 3.5 to 5.0x10¹⁷/cm² was established as the optimum for the BOX layer to form continuously without silicon islands. At doses above 2.5x10¹⁷/cm², the BOX layer formed continuously with silicon islands. The dose dependence of the defect densities in the top Si layers of the annealed samples was investigated. The dose of 3.5x10¹⁷/cm² was found to contain the lowest density of defects in the top Si layer. Above and below this dose, the defect density increased. The effect of intermediate-temperature annealing on microstructural evolution was investigated. The MFDs and the {113} defects were completely eliminated at 1100°C and 1200°C, respectively. It was found also that the redistribution process for oxygen and silicon interstitials during annealing was initiated at 1100°C, which also recovered the crystallinity of the top Si layer and developed the formation of the BOX layer. Above 900°C, oxygen precipitates in the top Si layer grew in size while they decreased in number with increasing temperature, an indication of Ostwald ripening. The effect of final annealing temperature and surface capping on the microstructure were also investigated. Annealing at 1300°C for 6 hours restored completely the crystal quality of the top Si layer and produced a continuous and uniform BOX layer. While the surface capping during annealing preserved the thickness of the top Si layer, it adversely affected the BOX layer formation especially at much lower doses. It led also to a slightly higher density of defects in the top Si layer by stabilizing defects which otherwise would have been eliminated during the high-temperature annealing. Additionally, the uncapped samples showed slightly lower density of Si islands in the BOX layer. Oxygen from the annealing ambient diffused in the uncapped samples through the thin top Si layer, which helped the BOX layer grow laterally and lowered the Si island density. The correlations between processing conditions and the microstructure of as-implanted and annealed material were established.