STRAIN RELIEF MECHANISM FOR DAMAGE GROWTH DURING HIGH-DOSE, O+ IMPLANTATION OF SI

Ion-induced damage accumulation and growth during separation by implantation of oxygen (SIMOX) processing were studied. Silicon wafers were implanted with 450 keV oxygen ions at an elevated temperature with doses of 0.8 X 10(18) and 1.1 X 10(18) cm-2. At the lower dose, the silicon overlayer was found to be highly strained but free of dislocations, while a distinct band of dislocations was observed in the top Si layer at the higher dose. The occurrence of this band is shown to correlate with strain relief in the overlayer. Rutherford backscattering spectrometry, cross-section transmission electron microscopy, and x-ray diffraction were used to characterize this damage so that its role in releasing the accumulated strain during ion implantation could be better understood. Additional insight was gained into the nature of the damage formed at the different doses by studying the thermal stability at 900-degrees-C. Markedly different thermal behaviors were observed and are correlated to changes in the strain state of each sample. These results strongly suggest that dislocation formation in the Si overlayer during the SIMOX process is in response to strain accumulation in the lattice and that dislocation-free layers can be formed by appropriate intervention prior to the yield point. This mechanism for dislocation formation is thought to be generally operative under extreme irradiation conditions and, therefore, will be important to other ion-beam synthesis processes such as buried silicide formation.