Hiroshi Takeno

Dynamic Behavior of Intrinsic Point Defects in Fz and Cz Silicon Crystals

Detaching the crystals from the melt revealed the real state and dynamic change of intrinsic point defects. From this observation it is confirmed that the predominant point defects near the melting point are vacancies. Clusters of interstitial-type dislocation loops (CDL) are eliminated by taking an extremely low growth rate under 0.2 mm/min and nitrogen doping. The anomalous oxygen precipitate (AOP) of the crystals grown in a nitrogen ambient is enhanced. AOP and ring-oxidation induced stacking fault (R-OSF) coexist at the periphery in nitrogen doped crystals. Almost all crystals have defect boundaries between periphery and center region which may be attributed to a stress field in the crystals during growth.

Enhanced nucleation of oxide precipitates during Czochralski silicon crystal growth with nitrogen doping

Thermal stability of oxide precipitate nuclei has been investigated for Czochralski silicon crystals with nitrogen doping. The experimental result indicates that generation of the grown-in oxide precipitate nuclei stable over 800 °C is enhanced by nitrogen doping. On the other hand, even though we confirmed this existence, doped nitrogen shows no influence on further oxide precipitate nucleation during the isothermal annealing at 600 °C after an epitaxial silicon growth process. Thus, it is found that the nitrogen doping only enhances the oxide precipitate nucleation at higher temperature during crystal cooling. The enhanced precipitate nucleation during the cooling is considered to be through excess vacancies which are suppressed to agglomerate by nitrogen.

Diffusivity of oxygen in Czochralski silicon at 400–750 °C

Diffusivity of oxygen in Czochralski silicon crystal in the temperature range of 400–750 °C has been determined from macroscopic oxygen precipitation behavior. The oxygen diffusivities at several nucleation temperatures from 400 to 750 °C were deduced from precipitated oxygen concentrations after a series of precipitate growth heat treatments, 800 °C/4 h and 1000 °C/16 h, using an extended nucleation theory. The measured oxygen diffusivity at 450–650 °C is 2–4×10−14 cm2/s, independent of the temperature, and considerably larger than the generally accepted normal diffusivity of Di=0.13 exp(−2.53 eV/kT). Moreover, the diffusivity at 450 °C is found to be roughly proportional to the interstitial oxygen concentration. It is suggested that this dependence of oxygen diffusivity on interstitial oxygen concentration can be explained by a model involving fast diffusing oxygen molecules.

Temperature-dependent retardation effect of dopants on oxygen diffusion in heavily doped Czochralski silicon

The influence of boron (B), arsenic (As), and antimony (Sb) on oxygen diffusivity at 500–800 °C was investigated in heavily doped Czochralski silicon wafers with resistivities below 0.02 Ω cm. The oxygen diffusivity was determined from the outdiffusion profile measured by secondary ion mass spectrometry after prolonged heat treatments. It was found that the heavily doped As and Sb reduce the oxygen diffusivity more at lower temperature. The increases in the activation energy for diffusion were found to be about 0.64–0.68 and 1.40 eV for As and Sb doping, respectively. Heavy B doping, however, exhibited anomalous temperature dependence showing a reduction rate peak around 600–700 °C, supposedly due to enhanced formation of immobile oxygen aggregates.

Evaluation of Microdefects in As-Grown Silicon Crystals

Microdefects, revealed as ‘flow patterns’ by preferential etching using Seccos etchant, in as-grown silicon crystals have been investigated by means of a transmission electron microscopy and a preferential etching. In as-grown CZ crystals, grown at the pulling speeds of 0.4 or 1.4 mm/min, dislocation loops and clusters were observed with TEM. The dislocation loops in both crystals are interstitial type. From a thermal behavior of flow patterns by heat treatments, we confirmed that the defects revealed as flow patterns in CZ crystals do not have a similar nature of that in D-defect region of FZ crystals.

Practical Computer Simulation Technique to Predict Oxygen Precipitation Behavior in Czochralski Silicon Wafers for Various Thermal Processes

A practical computer simulation technique has been developed to predict oxygen precipitation behavior in Czochralski silicon wafers during various thermal processes. In this simulation, an empirical factor is introduced in the initial and boundary conditions of the Fokker-Planck equation of the oxygen precipitation in order to make up an incomplete assumption of a homogeneous nucleation process proposed by Schrems et al. 1 The empirical factor is constructed as a function of heat-treatment temperature and interstitial oxygen concentration so as to describe characteristic phenomena of the precipitation nucleation processes in the 450 to 800°C range. Futhermore, an experimentally measured t ermal history during a crystal growth process, which strongly influence the oxygen precipitation behavior in the subsequent thermal process, has been taken into consideration. The calculated results agree fairly well with the experimental results for a variety of thermal processes. This semi-empirical simulation technique thus provides an advantageous tool for industrial optimization of the oxygen precipitation characteristics.

Deep-Level Luminescence in Czochralski-Grown Silicon Crystals after Long-Term Annealing at 450°C

Isothermal annealing at 450°C for more than 100 h was performed for Czochralski-grown silicon crystals. We detected a photoluminescence (PL) band at around 0.86 eV at room temperature. Temperature dependence of PL spectra indicates that the 0.86 eV band has an origin distinct from that of the 0.885 eV line observed at liquid helium temperature. Spectral shape analysis using the curve-fitting technique indicates that at least two different electronic levels are formed by oxygen precipitation during annealing. The peak energy position and half-width of the 0.86 eV band were different from those of a similar 0.86 eV band resulting from two-step annealing at 450°C and subsequently at 650°C. The difference in the spectral shape is ascribed to the structural change of oxygen aggregates. We believe that PL at room temperature is sensitive to the morphology of these aggregates.

Tem Observation Of Grown-In Defects In CZ-Si Crystals And Their Secco Etching Properties

Grown-in defects detected by IR laser scattering tomography (LSTDs) in Czochralski-grown Si crystals were identified by transmission electron microscopy (TEM) with a special defect positioning technique. The basic structure of the LSTD was revealed to be a composite of two or three incomplete octahedral voids with the 100–300nm total size. The TEM images of the defect showed existence of 2∼4nm-thick walls surrounding the voids. These thin-walls are considered to be made of oxide, SiO x . These LSTDs are indeed dominant grown-in defect species in most of the commercial CZ-Si walers. The LSTD after 1200°C oxidation was also observed by TEM. The resulting image shows that the defect changed from void to filled oxide precipitate by the high temperature heat treatment. On the other hand, in very slowly pulled crystals with ∼0.4mm/min rate, interstitial type dislocation loops were observed as major defect species. Non-agitated Secco etching of these grown-in defects delineates “flow patterns” (FPs) or pits without the flow patterns. The FP forming property is shown to disappear by oxidation at temperature above 1150°C, while the defect itself remains stable. This implies that the grown-in defects lose their chemical properties to form FPs by the high-temperature oxidation. It is further revealed that the grown-in defects, which once lost the FP forming property by the high-temperature oxidation, can form FPs again by an intentional Cu contamination. Thus a possible FP formation factor is Cu decoration at the grown-in defect site. Defect formation model of the as-grown twin-type LSTD is also proposed.

Effect of point defects on oxygen aggregation in Si at 450 °C

The oxygen aggregation process at 450 °C has been investigated by comparing the distributions of the interstitial oxygen, the thermal donor (TD), and the defect responsible for the photoluminescence (PL) line at 0.767 eV in a rapidly cooled Czochralski‐grown Si crystal in which point defects are frozen‐in nonuniformly in the growth direction. The formations of the TD and the 0.767‐eV PL defect are delayed in the vacancy‐rich region, which is not explained by the oxygen variation. The present result leads us to suggest that the oxygen aggregation is retarded by excess vacancies, which is consistent with the reported models for the oxygen aggregation at 450 °C.

Observation of microdefects in As-grown Czochralski silicon crystals by synchrotron radiation topography

Microdefects in as-grown Czochralski silicon crystals have been investigated by means of X-ray topography using 60 keV radiation from a synchrotron radiation source. In crystals grown at the pulling rate of 0.4 mm/min, very fine defects were observed and their density was about 1×103/cm3, which was in agreement with the result obtained by the flow pattern technique using Secco preferential etching. The strain field of the defects was analyzed from the topographic image height of defects using the kinematical diffraction theory. The values of the defect strength that gave a measure of the size of the defects were from 2 to 40×10-20 m3.