The gettering behavior of polysilicon back seal (PBS) and internal gettering (IG) with isothermal annealing (600–1000° C) was systematically investigated for Fe contamination by deep level transient spectroscopy (DLTS). There was a clear dependence of the PBS gettering efficiency on the PBS deposition temperature and on annealing temperatures used in the gettering processes. The use of lower deposition temperatures and lower gettering temperatures resulted in a higher gettering efficiency. IG efficiency has a clear dependence on size and density of the oxygen precipitate. In the case of a bulk micro defect (BMD) density of 105 cm-2, it was necessary for the platelet oxygen precipitate size to be larger than 200 nm, while a polyhedral oxygen precipitate size of 100 nm was sufficient in obtaining IG effects for an Fe contamination level of 1012 atoms/cm3. The gettering efficiency has a clear correlation with the volume of the oxygen precipitates per unit volume of the silicon wafers. These results suggest that Fe atoms are gettered within the oxygen precipitates and not in the area surrounding them.
The evaluation of B gettering for Fe impurities in p/p+ Si epitaxial wafers was carried out, after intentional Fe contamination, by measuring the Fe concentration in the epitaxial layer using deep level transient spectroscopy (DLTS). As the surface [Fe] before diffusion was increased, [Fe] in epitaxial layer also increased. As B concentration in the p+ substrate was raised, B gettering efficiency became higher. On comparison of the experimental results with the segregation gettering model, it was concluded that B gettering for Fe does not occur at a high temperature such as 800° C or 1100° C. B gettering for Fe can be inferred to occur below 600° C during the cooling process.
The formation and dissolution of Ni and Cu silicides were investigated to determine effective intrinsic gettering (IG) for low temperature processes. Ni formed silicides easily at low contamination levels and these silicides formed the nuclei for OSF during subsequent annealing at temperatures above 1000°C. Ni silicides were dissolved and gettered during low temperature deposition of a poly-back seal (PBS) at 620°C, whereas Cu silicides, once formed, easily induced secondary defects on further annealing even at low temperatures and could not be dissolved or gettered by PBS. The sizes and densities of oxygen precipitates necessary to intrinsically getter Ni and Cu contamination levels of 1012atoms/cm2 were also investigated with respect to generation lifetime. Cu contamination at this level did not degrade generation lifetime or gate oxide integrity (GOI) yield. For Ni contaminated samples, a strong dependence of generation lifetime on both oxygen precipitate density and size was observed. Effective IG for Ni during a low temperature process was demonstrated using a 2-step low temperature process simulation.
The nature and radial distribution of crystallographic micro-defects in CZ-grown silicon crystal wafers which exhibit OSF ring were investigated. With the difference in copper decoration XRT image and in temperature dependence of oxgen precipitation, four coaxial ring regions were clearly identified. In the OSF ring region, oxgen precipitation takes place even at relatively high temperatures around 1, 100°C. Gold diffusion experiments were also carried out, for which the ‘kick-out mechanism’ was considered to reveal the density distribution of sinks for interstitial silicon. A semi-quantitative schematic model describing the density distribution of the precipitation nuclei as a function of critical size of the nuclei for each of the four ring regions is derived. The nuclei that are stable at high temperature act as sinks for interstitial silicon atoms, generating stacking faults.
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