As-grown defects in 6-inch-diameter Czochralski-silicon crystals grown under different crystal growth rate conditions (0.4, 0.7, 1.1 mm/min) were studied by means of preferential etching and IR light-scattering tomography (LST). Grown-in defect images were classified into four types as follows: (a) flow patterns (wedge-shaped etch pits), (b) IR-defect images observed by LST, (c) ringlike distributed small pits, and (d) large pits. It was found by secondary ion mass spectrometry that IR defects are oxygen precipitates. Large pit defects were identified by transmission electron microscopy as large dislocation loops with a length of about 30 µm. At growth rates from 0.7 mm/min to 1.1 mm/min, flow pattern defects and IR defects coexist inside a ringlike distributed oxidation-induced stacking fault (ring-OSF) region. However, at growth rates less than 0.7 mm/min, large pit defects were observed in the region outside the ring. Characteristic ringlike distributed small pit defects were observed on the outer periphery of the ring region. Flow pattern defects were annihilated during annealing at 1100°C, while IR defects were stable at 1250°C.
It is widely recognized that heavy metal impurities such as Fe, Ni, and Cu degrade silicon device performance and yield. Gettering techniques of internal gettering (IG) by oxide precipitates, 1-10 boron gettering, [5][6][7][9][10][11] and various external gettering (EG) [5][6][7][8]12,13 are used for the removal of metal contaminants from surface active regions to ensure high device performance. Further advances in device processing will probably require double-side polishing of wafers in order to achieve higher flatness. For such wafers, implementation of EG at the wafer back surface is obviously difficult to realize. Furthermore, it is known that boron gettering cannot be expected for Ni contamination. 7 Therefore, IG is crucial for Ni contamination in the next generation of device processes. However, IG behavior for Ni contamination is not fully understood. Shabani et al. 8 and Kageyama et al. 9 emphasized that Ni was very difficult element to be gettered by IG. On the other hand, Falster and Bergholz 3 found that Ni was easily gettered by IG when the oxide precipitates were not visible by the etching method or X-ray topography. To control IG for Ni contamination, it is important to investigate the IG effect as a function of precipitate density and size.The main process for future device fabrication is a low temperature process using high-energy ion implantation whose maximum temperatures are below 1050ЊC. In some cases, the precipitate size becomes less than the detection limit of the preferential etching method. This brings about the difficulty in the prediction of the IG effect in the target wafers.In this paper, we have studied the IG behavior for Ni contamination. On the basis of the experimental and calculated results, the critical size of the oxide precipitate for the IG effect was determined. We have also developed the method of IG prediction for Ni contamination in actual device processes by comparing the calculated precipitate size with determined critical size. Experimental and ResultsThe samples used were lightly boron doped (4.5-6.5 ⍀ cm) p-type, 6 in., Czochralski silicon (100) wafers. Oxygen concentration of the as-received wafers was (16.25 Ϯ 0.25) ϫ 10 17 atom/cm 3 (old ASTM). These wafers were initially contaminated with Ni of 5 ϫ 10 11 atom/cm 2 by a spin-coating method. 14 Ni concentration was determined with total reflection X-ray fluorescence. This contamination level should be far higher than that in actual device processes. The contaminated wafers were subjected to isothermal annealing at 800, 900, 950, and 1000ЊC for up to 16 h in a N 2 ambient. This isothermal annealing plays the roles of Ni drive-in annealing and oxide precipitate generation with certain density and size at the same time. After isothermal annealing, the wafers were pulled from the horizontal furnace at a rate of 10 cm/min. In the pulling process, the wafers were cooled from annealing temperature to room temperature at an average rate of Յ40ЊC/min. This cooling rate supplies enough time for Ni atoms to diffuse from t...
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 degradation of gate oxide integrity (GOI) by metal impurities on Si wafers was studied. Ni and Cu tended to precipitate at the Si surface after high-temperature annealing. When these precipitates existed before gate oxidation, they penetrated into the gate oxide film and degraded GOI. Fe tended to remain in the oxide film after oxidation and degraded GOI. The degradation was observed for annealed samples when the surface metal concentration exceeded 1.0×1012 atoms/cm2 of Ni or 5.0×1012 atoms/cm2 of Cu. It was also observed with contamination of 1.0×1013 atoms/cm2 of Fe without annealing.
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