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 family of double donors with only slightly differing binding energies can be generated in silicon containing oxygen. In the 30 years since they were discovered the microscopic structure of these defects has not been unravelled in spite of being investigated with all the tools of solid state physics.

Thermal double donors in silicon

The objective of this article is to review our understanding of the properties of oxygen impurities in Czochralski silicon that is used to manufacture integrated circuits (ICs). These atoms, present at a concentration of ~1018 cm-3, occupy bond-centred sites (Oi) in as-grown Si and the jump rate between adjacent sites defines `normal' diffusion for the temperature range 1325 - 330 °C. Anneals at high temperatures lead to the formation of amorphous SiO2 precipitates that act as traps for fast diffusing metallic contaminants, such as Fe and Cu, that may be inadvertently introduced at levels as low as 1011 cm-3. Without this `gettering', there may be severe degradation of fabricated ICs. To accommodate the local volume increase during oxygen precipitation, there is parallel generation of self-interstitials that diffuse away and form lattice defects. High temperature (T > 700 °C) anneals are now well understood. Details of lower temperature processes are still a matter of debate: measurements of oxygen diffusion into or out of the Si surface and Oi atom aggregation have implied enhanced diffusion that has variously been attributed to interactions of Oi atoms with lattice vacancies, self-interstitials, metallic elements, carbon, hydrogen impurities etc. There is strong evidence for oxygen-hydrogen interactions at T < 500 °C and the formation of fast diffusing O2 dimers. These observations have led to significant advances in understanding the growth and structures of small oxygen clusters, identified with the so-called thermal donor and shallow thermal donor defects. There is a need to improve this understanding because the temperatures of device processing will continue to decrease as the size of future device features decreases below the lower end of the sub-micron range, currently close to 0.18 µm.

https://iopscience.iop.org/article/10.1088/0953-8984/12/25/201/pdf

Oxygen diffusion and precipitation in Czochralski silicon

As‐grown Czochralski silicon samples with different oxygen concentrations have been heated at temperatures in the range 350–500 °C. Oxygen loss during anneals at low temperatures (T≤400 °C) is shown to follow second‐order kinetics and measurements led to values of oxygen diffusivity that were larger than normal by a factor of ∼3, assuming the capture radius for dimer formation was 5 A. Variations in the rate of [Oi] loss during more extended anneals could be explained if oxygen diffusion was initially enhanced but tended to its normal value as the anneals progressed. Much greater initial enhancements were derived from similar measurements for samples which had been hydrogenated by a heat treatment in H2 gas at 1300 °C for 30 min followed by a rapid quench to room temperature, and the enhancements were consistent with values derived from measurements of the relaxation of stress‐induced dichroism. At higher temperatures (T≥450 °C) the measured rates of [Oi] loss were less than the expected rate of Oi‐Oi interaction and tended to vary with increasingly high powers of [Oi]. Modeling of the clustering process demonstrated that the reductions could be explained if the oxygen dimers were present in a quasiequilibrium concentration throughout the anneals. The establishment of this equilibrium appears to require that oxygen dimers diffuse much more rapidly than isolated Oi atoms. The kinetics of oxygen loss over the whole range of temperatures can then be explained if dimer clustering leads mainly to increases in concentrations of agglomorates containing large numbers (≥8) of oxygen atoms. It is therefore possible to account for thermal donor (TD) formation based on the formation of different sizes of oxygen clusters, although the possibility that self‐interstitials are involved in TD formation is not excluded.

http://users.uoa.gr/~hlontos/pdf/magazines_articles/A43-J.Appl.Phys.%2077,1427%20(1995).pdf

Oxygen loss during thermal donor formation in Czochralski silicon: New insights into oxygen diffusion mechanisms

We have studied the process of buried oxide formation as a function of implantation and annealing conditions. Concentrating on substoichiometric implants (<1×1018 O/cm2), we varied the implantation energies from 100 keV to 1 MeV. Some apparent precipitation of SiO2 similar to that observed in Czochralski‐grown silicon occurs on implantation. This means that formation of the buried oxide layer and perfection of the overlying crystalline Si layer depend more strongly on the substrate temperature during the implant than on the annealing temperature.

Mechanisms of buried oxide formation by ion implantation

Prolonged annealing of Czochralski silicon at 485°C leads to the formation of a number of defects which is accompanied by a reduction in the concentration of oxygen interstitials in the matrix. High‐resolution electron microscopy has been used to investigate the structure of these defects which are basically of three types: (1) ribbonlike, (2) looplike defects which result in local lattice strains and hence are also visible in conventional transmission electron microscopy, and (3) dark regions visible in high‐resolution micrographs with no lattice strains. Based on image characteristics and a comparison of the reduction in interstitial oxygen, from infrared spectroscopy, with the sizes of the ribbons, estimated from high‐resolution micrographs, it is proposed that the ribbonlike defects are, in fact, the coesite phase which forms at prolonged anneals of the same material at the higher temperatures of 630–650°C. Using a simple model for the diffusion of oxygen to ribbons, the diffusivity of oxygen in silic...

Precipitation of oxygen at 485 °C: Direct evidence for accelerated diffusion of oxygen in silicon?

SUMMARY

Prolonged annealing of Cz-silicon at 758 K results in the formation of ribbon-like oxygen precipitates on {311} planes interpreted as the coesite phase. From this an enhancement of the oxygen diffusion coefficient by more than three orders of magnitude is inferred. Excess selfinterstitials are accommodated in extrinsic dislocation loops on {111} planes (‘loopites’) and probably also in ‘blob’-like defects visible in high resolution micrographs with little detectable lattice strain. Evidence is presented that coesite ribbons can nucleate on {311} steps on loopites and vice versa.

W. Bergholz

Papers

Precipitation of oxygen at 485 °C: Direct evidence for accelerated diffusion of oxygen in silicon?

Enhanced oxygen diffusion and precipitation in silicon