Shinsuke Sadamitsu

Dependence of the Grown-in Defect Distribution on Growth Rates in Czochralski Silicon

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.

Degradation of Gate Oxide Integrity by Metal Impurities

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.

Analysis of Local Lattice Strains Around Plate-Like Oxygen Precipitates in Czochralski-Silicon Wafers by Convergent-Beam Electron Diffraction

Convergent-beam electron diffraction (CBED) is used to study lattice strain around plate-like oxygen precipitates in Czochralski (CZ)-grown silicon. Local lattice strain determined from higher-order Laue zone (HOLZ) patterns shows that compressive and tensile stress fields exist near the precipitates. The spatial variation of local lattice strain and lattice rotation is visualized in a defocused large angle CBED disc, or a convergent-beam imaging (CBIM) disc.

Axial Microscopic Distribution of Grown-in Defects in Czochralski-Grown Silicon Crystals

The axial microscopic distribution of grown-in defects in Czochralski silicon was studied by means of IR light scattering tomography (LST) and preferential etching. IR scattering defects (defects observed with LST) were found to degrade the gate oxide integrity yield, and the axial density distribution of IR scattering defects and flow patterns (wedge-shaped etch patterns) fluctuated with oxygen concentration fluctuations along the growth axis. However, the defect density did not depend directly on oxygen concentration. It is considered that the formation of IR scattering defects is related to the solid-liquid intertace temperature fluctuations.

Transmission Electron Microscopy Observation of Defects Induced by Fe Contamination on Si(100) Surface

The behavior of Si(100) surface defects induced by Fe contamination was studied with transmission electron microscopy. After annealing at 1150°C for 1 hour and a subsequent heat treatment at 850°C for 2 hours, Fe-containing precipitates were observed in Si substrate in close vicinity to the interface of Si and SiO2 formed during the annealing. One of these precipitates is identified as FeSi-type silicide. In addition, the inclusions which were confirmed to be Fe3O4 or γ-Fe2SiO4 were observed in the surface thermal SiO2 layer. These results demonstrate that Fe atoms diffuse into the Si substrate during annealing at 1150°C and precipitate at a Si/SiO2 interface, while Fe atoms left on the surface form inclusions in the surface SiO2 layer. Under an additional thermal oxidation at 1000°C, oxidation-induced stacking faults were formed. They were not decorated at all in contrast with those induced by Cu or Ni contamination.

Gettering Characteristics of Heavy Metal Impurities in Silicon Wafers with Polysilicon Back Seal and Internal Gettering

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.

A Model for the Formation of Oxidation-Induced Stacking Faults in Czochralski Silicon

The mechanism of oxidation-induced stacking fault (OSF) formation in Czochralski silicon (CZ-Si) crystals was investigated by transmission electron microscopy observations of the initial stages of OSF growth. OSFs were observed to be always generated at one of the edges of platelet oxygen precipitates. We observed previously that these platelet oxygen precititates had an expansive strain field in the direction parallel to the precipitate plate and a compressive strain field normal to the plate. Silicon self-interstitials having compressive strain are probably attracted to the expansive strain field of the precipitates, and condense to form stacking faults. A new model for OSF generation is presented taking into consideration the strain field around self-interstitials and oxygen precipitates.

TEM Observation of Defects Induced by Cu Contamination on Si(100) Surface

The behavior of a Si(100) surface defect induced by intentional Cu contamination was studied with transmission electron microscopy. After annealing at 1150°C for 1 hour in N2 atmosphere, colony precipitates lying along (110) planes were observed only on the surface of the wafer. These precipitates were estimated to be Cu6Si-type silicides by the selected area diffraction pattern. During additional annealing in an oxidation atmosphere, stacking faults were formed from each of colony precipitates. This indicated that colony precipitates were the nucleus of oxidation-induced stacking faults.

Internal gettering for Ni contamination in Czochralski silicon wafers

Internal gettering (IG) behavior for Ni contamination in Czochralski silicon wafers was studied. The wafers were initially contaminated with Ni, and then isothermally annealed between 800 and 1000°C for up to 16 h. The density of Ni‐silicides at the polished surfaces and the density of oxide precipitates at the cleaved surfaces were obtained by the preferential etching method with Wright etchant. It was confirmed that (i) with an increase in annealing time, Ni‐silicide density decreased and became less than the detection limit, and (ii) oxide precipitates were not detected in some of the wafers, in which Ni‐silicides were not detected. The density of oxide precipitates with their size less than the detection limit was obtained after additional annealing at 1000°C for 16 h, and the size of precipitates was obtained by calculations with assuming diffusion‐limited growth. The critical size of oxide precipitates for the IG effect was defined as the size above which the Ni‐silicides were not detected. It was concluded that (i) the critical size decreased with an increase in precipitate density and (ii) the critical size became less than the detection limit of approximately 200 nm by the etching method, when the precipitate density was higher than about .

Generation of Oxidation-Induced Stacking Faults in Czochralski-Grown Silicon Crystals Exhibiting a Ring-like Distributed Stacking Fault Region

The radial density distribution of oxidation-induced stacking faults (OSFs) and bulk micro defects (BMDs) in Czochralski-grown silicon crystals having a ring-like distributed OSF (ring-OSF) region was characterized after low-temperature pre-annealing followed by high-temperature oxidation, using optical microscopy, IR light scattering tomography and Fourier transform IR spectroscopy. Stacking faults were observed to have grown during oxidation in various radial regions of the crystal other than the ring-OSF region, depending on the radial distributions of BMDs determined by the pre-annealing conditions. The ability of these BMDs grown during pre-annealing to nucleate OSFs with subsequent oxidation is determined by the strain surrounding them. However, to realize OSF nucleation, the strain must lie within specific limits controlled by the size of the BMD.