A review is presented of the recent advances in the study of oxygen precipitation and of the main properties of oxide precipitates in silicon. After a general overview of the system ‘‘oxygen in silicon,’’ the thermodynamics and the kinetics of the precipitate formation are treated in detail, with major emphasis on the phenomenology; subsequently, the most important techniques for the characterization of the precipitates are illustrated together with the most interesting and recent results. Finally, the possible influence of oxygen precipitation on technological applications is stressed, with particular attention to recent results regarding device yield. Actually, the essential novelty of this review rests on the attempt to give an extended picture of what has been recently clarified by means of highly sophisticated diagnostic methods and of the influence of precipitation on the properties of semiconductor devices.

Oxygen precipitation in silicon

A structural model for the oxygen‐related thermal donors produced at moderate temperatures (<500 °C) is presented, where electrical activity commences with clusters containing five or more oxygen atoms and arises from a silicon atom at the center of the cluster. The donor activity of a cluster is terminated upon the ejection of this central silicon atom in order to bring about stress relaxation. A large number of electrically active donor species are predicted, differing only in the number of oxygen atoms each species contains. This structural model satisfies the stringent symmetry requirements of electron paramagnetic resonance (EPR), explains the lack of hyperfine interaction with Si29 or O17 nuclei and the large multiplicity in the number of donor species observed by infrared spectroscopy and EPR. The proposed clusters are shown to be embryonic forms of the much larger rodlike defects examined in detail by electron microscopy. Chemical reaction theory is used to deduce the kinetic consequences of the s...

Oxygen‐related thermal donors in silicon: A new structural and kinetic model

It is well known that recombination lifetimes are significantly degraded in oxygen‐precipitated silicon. The possible sources for lifetime degradation are expected to be oxygen precipitates (OP’s), dislocation loops, stacking faults, and point defects associated with self‐interstitials generated during the oxygen precipitation process. From the results of an extensive experimental study using IR absorption, TEM, DLTS, and SPV (surface photovoltage), we have found that OP’s are mainly responsible for the observed lifetime degradation and that recombination at OP’s takes place through Si/OP interface states. In addition we have observed that the lifetime degradation is more severe in p‐Si than in n‐Si even for identical densities and sizes of OP’s. A model for recombination at OP’s is presented in terms of the surface recombination velocity at the Si/OP interface and their average density and size. To explain the lifetime difference between n‐Si and p‐Si we propose a band bending around OP’s caused by positive fixed charges in the OP’s.

Recombination properties of oxygen-precipitated silicon

The precipitation of oxygen in silicon has been studied in the temperature range 650 to 1050 degrees C using the techniques of chemical etching, IR absorption applied to the 9 mu m band (4.2K), and small-angle neutron scattering (SANS). The IR data and etch pit counts, relating to the number densities of precipitate particles, have been fitted to Ham's theoretical model for the diffusion limited growth of randomly distributed particles. Full allowance is made for the increase in size of the precipitates with increasing time and good agreement is found over the complete period starting from zero time. Values of the diffusion coefficient of oxygen so determined are in excellent agreement with other data obtained by different methods, including SANS measurements at 750 degrees C described in the present work. The combined data give D=0.11 exp(-2.51 eV/kT) cm2s-1. The SANS data also indicate that the precipitates are not spherical in shape after a short initial period, and they show that the initial concentration of particles nucleated decreases with annealing time. The solid solubility cs(T) is determined at each temperature and compared with previous measurements. The authors conclude that the best combination of results leads to cs(T)=2.6*1022 exp(-1.4 eV/kT) down to 850 degrees C. At lower temperatures cs decreases more slowly with decreasing temperature, indicating a possible change in the structure or the form of the precipitated SiO2 phase.

An infrared and neutron scattering analysis of the precipitation of oxygen in dislocation-free silicon

High resolution electron microscopy has been used to study the microstructural changes brought about by phoshorus gettering and intrinsic gettering of Ni in Si. Phosphorus gettering proceeds by the formation of SiP particles at the Si/phosphosilicate glass interface, which emit a large concentration of Si interstitials and result in the gettering of Ni to produce NiSi2 particles at the interface. Intrinsic gettering, under the conditions studied, also proceeds by the emission of Si interstitials due to oxygen precipitiation, with no dislocation generation, and results in the production of NiSi2 at the Si/SiO2 interface. It is suggested that the supersaturation of Si interstitials may be responsible for gettering in both cases.

Phosphorus gettering and intrinsic gettering of nickel in silicon

The early stages of oxygen segregation at dislocation and precipitation in the bulk have been investigated by high‐resolution electron microscopy in Czochralski grown silicon. Two kinds of precipitates are observed: a crystalline silica phase, coesite, and an amorphous phase. Both forms coexist after 650 °C heat treatment: the so‐called rodlike defects are in fact long 〈011〉 ribbons of coesite associated with interstitial dipoles. This crystalline form is favored by a high oxygen supersaturation and a low carbon content. Above 870 °C amorphous platelets of silica are formed on the {100} planes, whereas coesite is no longer observed but interstitial dislocation loops are always present. The strain produced by such precipitates is partially relaxed by Si interstitial emission, which explains the internal formation of dislocations. It is suggested that both forms are nucleated on two different species of nuclei. At 〈011〉 dislocation cores it is shown that the coesite phase is stabilized over a wide range of oxygen content or annealing temperature.

A. Bourret

Papers

Early stages of oxygen segregation and precipitation in silicon