Eiichi Asayama

Effect of Heavy Boron Doping on Oxygen Precipitation in Czochralski Silicon Substrates of Epitaxial Wafers

The effect of heavy boron doping on oxygen precipitation in Czochralski silicon substrates of epitaxial wafers has been studied with transmission electron microscopy observations and a preferential etching method. Prolonged isothermal annealing between 700 and 1000°C for up to 700 h was performed on p/p+ (5–20 mΩ cm) and p/p− (10 Ω cm) wafers. It was found that, with an increase in boron concentration, the precipitate density increased, and the precipitates could nucleate at a higher temperature. The growth process of platelet precipitates was also investigated and compared with the process in polished p− wafers. It was confirmed that precipitate growth rate in p/p+ wafers was higher than that in p− wafers, and precipitate nucleation in p/p− wafers was delayed compared with p/p+ wafers. The precipitate growth in p/p+ wafers was determined to be reaction‐limited, which differed from the diffusion‐limited growth in p− wafers.

OXYGEN DIFFUSION IN HEAVILY ANTIMONY-, ARSENIC-, AND BORON-DOPED CZOCHRALSKI SILICON WAFERS

The effect of dopant-type, antimony (Sb), arsenic (As), and boron (B), on the outdiffusion of oxygen in heavily doped Czochralski (Cz) silicon wafers has been investigated using secondary ion mass spectroscopy. The results indicate that, although oxygen diffusion in Cz silicon is retarded in heavily B- and As-doped wafers during low temperature annealing (800 °C), it is not influenced by heavy Sb doping. This indicates that charge effects and atom size effects have negligible influence on the diffusion of oxygen. The B and As diffusion retardation effect is attributed to the existence of dopant-oxygen complexes. The oxygen solubility was largest in the most heavily B-doped samples annealed at low temperature.

Effect of Heavy Boron Doping on Oxide Precipitate Growth in Czochralski Silicon

Oxide precipitate growth in boron‐doped Czochralski silicon wafers with resistivities ranging from 6 to 40 mΩ cm has been studied following prolonged annealing from 800 to 1000°C. Transmission electron microscopy revealed that (i) the growth rate of oxide platelet precipitates is proportional to the square root of time in 40 mΩ cm samples and (ii) the precipitate morphology changes from plate to polyhedral and strain around the precipitate decreases during annealing at 900°C in 6, 9, and 18 mΩ cm samples. These results indicate that changes in precipitate morphology occur because the oxygen precipitation and boron atom size effects are enhanced by increasing boron concentration.

Behavior of Defects in Heavily Boron Doped Czochralski Silicon

Defect behaviors in heavily boron doped silicon, with boron concentration ranging from 7.1×1017 to 3.1×1019 atoms/cm3, were studied using transmission electron microscopy. The oxygen precipitation and the change of the precipitate morphology from plate to polyhedral were observed to be enhanced with increasing boron concentration. However, in wafers with the highest boron concentration, precipitate density decreased, and aggregations of small polyhedral precipitates on the {110} planes were observed. It was also found that as the boron concentration was increased, the precipitate size reduced and the type of secondary induced defects changed from punching-out dislocations to stacking faults due to the reduction in a crystal lattice strain.

Nitrogen effect on grown-in defects in Czochralski silicon crystals

The formation behavior of grown-in voids during crystal growth was investigated for nitrogen-doped Czochralski silicon crystals by means of a new quantitative defect evaluation method using a bright-field infrared-laser interferometer. Crystal quenching techniques were employed to study void formation and it was found that in crystals grown without nitrogen doping, the total amount of vacancies composing the voids did not change during the crystal growth halt. In nitrogen-doped crystals, however, the total amount of vacancies composing the voids increased during the crystal growth halt. These results indicate that nitrogen makes excess vacancies remain after void formation in crystals grown without a growth halt.

Oxide Precipitate‐Induced Dislocation Generation in Heavily Boron‐Doped Czochralski Silicon

Dislocation generation associated with oxide precipitates in heavily boron-doped Czochralski silicon wafers with resistivities of 9, 18, and 40 mΩ cm has been studied following prolonged isothermal annealing from 800 to 1000°C. Transmission electron microscopy observations revealed (i) the critical precipitate size for punched-out dislocations to form in 9 and 18 mΩ cm wafers was smaller than 40 nm in 9 and 18 mΩ cm wafers, while larger than 55 nm in 40 mΩ cm samples; (ii) the precipitate density was higher than 10 12 cm -3 in 9 and 18 mΩ cm wafers, and below 10 11 cm -3 in 40 mΩ cm wafers annealed at 800 and 900°C, respectively. The strain around a precipitate was estimated and it was concluded that the higher supersaturation of silicon interstitials in the 9 and 18 mΩ cm wafers was due to the higher precipitate density, which in turn was likely to be the main cause of the reduction in critical precipitate size.