Robert J. Falster

On the Properties of the Intrinsic Point Defects in Silicon: A Perspective from Crystal Growth and Wafer Processing

Taking into account a wide variety of recent results from studies of silicon crystal growth and high temperature wafer heat treatments, a consistent picture of intrinsic point defect behavior is produced. The relevant point defect parameters: diffusivities, equilibrium concentrations and the details of the interaction of vacancies with oxygen are deduced. This set of parameters successfully explains a very wide array of experimental observations covering the temperature range 900–1410 °C. These experimental observations, which are reviewed here, include the properties of grown-in microdefects and vacancy-controlled oxygen precipitation effects in rapidly cooled wafers. The analysis of point defect behavior from observations of high temperature phenomena such as these has the great advantage of relative simplicity and transparency.

The engineering of intrinsic point defects in silicon wafers and crystals

Using new techniques which manage intrinsic point defect concentrations during the growth of silicon crystals and subsequent processing of silicon wafers, it is possible to achieve two highly desirable results. First, using techniques which maintain crystals sufficiently close to equilibrium during growth such that it is possible to grow large diameter CZ silicon crystals which are completely free of agglomerates of vacancies (voids) or self interstitials (dislocation loops). Secondly, through the control of quenched-in vacancy concentration profiles in thin silicon wafers during rapid thermal annealing processes, it is possible to obtain ideal oxygen precipitation behavior in silicon wafers. Such vacancy concentration engineered wafers are effectively programmed to produce robust defect distributions suitable for internal gettering (IG) applications. The performance of such wafers is independent of all parameters previously important to engineering of IG structures: oxygen concentration, crystal growth method and even the application in which the wafer is used. These two new types of material represent important simplifications to the successful defect engineering of silicon wafers in advanced IC applications. It is shown that, taken together, results from both crystal growth and wafer processing studies place a large number of constraints on the parameter set used to describe the properties of the vacancies and self interstitials in silicon. A model for the oxygen precipitation enhancement in the MDZ process is given.

SPATIAL VARIATIONS IN OXYGEN PRECIPITATION IN SILICON AFTER HIGH TEMPERATURE RAPID THERMAL ANNEALING

Spatial variations of oxygen precipitation have been studied in silicon wafers submitted to rapid thermal annealing (RTA) in nitrogen ambients at a temperature above 1150 °C prior to a two-step precipitation treatment. The samples submitted to high temperature preannealing show a consistent enhancement of oxygen precipitation, which is dependent on the RTA time and temperature. Oxygen precipitate density measurements show that oxygen precipitation is not homogeneous inside the wafer, but peaks near the surfaces, with a minimum in the bulk. The high treatment in N2 results in the formation of defects which act as precursors for the subsequent nucleation and growth of oxygen precipitates. There is strong evidence that these precursors are generated by the thermal nitridation of the silicon surface and subsequent vacancy injection into the bulk of the wafer.

Parameterisation of injection-dependent lifetime measurements in semiconductors in terms of Shockley-Read-Hall statistics: An application to oxide precipitates in silicon

Injection-dependent minority carrier lifetime measurements are a valuable characterisation method for semiconductor materials, particularly those for photovoltaic applications. For a sample containing defects which obey Shockley-Read-Hall statistics, it is possible to use such measurements to determine (i) the location of energy levels within the band-gap and (ii) the ratios of the capture coefficients for electrons and holes. In this paper, we discuss a convenient methodology for determining these parameters from lifetime data. Minority carrier lifetime is expressed as a linear function of the ratio of the total electron concentration to the total hole concentration for p-type (or vice versa for n-type) material. When this is plotted on linear scales, a single-level Shockley-Read-Hall centre manifests itself as a straight line. The gradient and intercepts of such a plot can be used to determine recombination parameters. The formulation is particularly instructive when multiple states are recombination-ac...

Oxygen-dislocation interactions in silicon at temperatures below 700 °C: Dislocation locking and oxygen diffusion

The locking of dislocations by oxygen atoms in Czochralski–silicon at temperatures between 350 and 700 °C has been studied. Both experimental and theoretical investigations were carried out for different oxygen concentrations, different annealing times (from 10 to 3×107 s), and different point defect concentrations. It was found that the unlocking stress of dislocations at low temperatures follows similar trends to those previously observed at higher temperatures and is determined by annealing temperature, time, and oxygen concentration. However, in the present temperature range, experimental results indicate an enhanced transport of oxygen to dislocations. Numerical simulations solving the diffusion equation for oxygen transport to the dislocations show that the effective diffusivity of oxygen at lower temperatures diverges from “normal” diffusivity of oxygen. We have shown that oxygen transport can be as much as three orders of magnitude higher than that which would be assumed by extrapolation of the “n...

Effect of rapid thermal annealing on recombination centres in boron-doped Czochralski-grown silicon

Rapid thermal annealing in a belt furnace results in a dramatic change of the recombination properties of boron-doped Czochralski silicon: (1) the lifetime degraded by applying a prolonged illumination at room temperature was significantly improved, (2) after subsequent dark recovery, the lifetime has a remarkably high value, and (3) the permanent recovery, by annealing at 185 °C under illumination, is enormously accelerated, and the finally achieved stable lifetime acquires a record value of 1.5 ms, as compared to 110 μs after permanent recovery of not-annealed reference samples.

Gettering thresholds for transition metals by oxygen‐related defects in silicon

This letter reports a qualitative study of the gettering of technologically important transition metal contaminants by a wide variety of distributions of oxygen precipitates and related defects in silicon. Various metals were diffused into specially prepared silicon wafers containing densities of oxygen precipitates ranging between 105 and 2×1010 cm−3. The precipitates were of a variety of sizes both with and without punched‐out dislocation networks and associated stacking faults. Following previous work and using the Haze Test to monitor gettering activity, a threshold in precipitate density has been determined for the complete gettering of Cu and Ni (about 1×105 and 3×106 cm−3, respectively). No influence of precipitate size (above an as yet to be determined minimum) or of the presence of punched‐out dislocations or stacking faults could be determined for these two metals, the fastest diffusing of the 3d group. Comments on the gettering of Fe are made.

Quantification of Defect Dynamics in Unsteady-State and Steady-State Czochralski Growth of Monocrystalline Silicon

Most common microdefects in Czochralski silicon, voids and dislocation loops, are formed by agglomeration of point defects, vacancies, and self-interstitials, respectively. Dynamics of formation and growth of the microdefects along with the entire crystal pulling process is simulated. The Frenkel reaction, the transport and nucleation of the point defects, and the growth of the microdefects are considered to occur simultaneously. The nucleation is modeled using the classical nucleation theory. The microdefects are approximated as spherical clusters, which grow by a diffusion-limited kinetics. The microdefect distribution at any given location is captured on the basis of the formation and path histories of the clusters. The microdefect type and size distributions in crystals grown under various steady states as well as unsteady states are predicted. The developed one-dimensional model captures the salient features of defect dynamics and reveals significant differences between the steady-state defect dynamics and the unsteady-state defect dynamics. The model predictions agree very well with the experimental observations. Various predictions of the model are presented, and results are discussed.

The effect of oxide precipitates on minority carrier lifetime in p-type silicon

Supersaturated levels of interstitial oxygen in Czochralski silicon can lead to the formation of oxide precipitates. Although beneficial from an internal gettering perspective, oxygen-related extended defects give rise to recombination which reduces minority carrier lifetime. The highest efficiency silicon solar cells are made from n-type substrates in which oxide precipitates can have a detrimental impact on cell efficiency. In order to quantify and to understand the mechanism of recombination in such materials, we correlate injection level-dependent minority carrier lifetime data measured with silicon nitride surface passivation with interstitial oxygen loss and precipitate concentration measurements in samples processed under substantially different conditions. We account for surface recombination, doping level, and precipitate morphology to present a generalised parameterisation of lifetime. The lifetime data are analysed in terms of recombination activity which is dependent on precipitate density or on the surface area of different morphologies of precipitates. Correlation of the lifetime data with interstitial oxygen loss data shows that the recombination activity is likely to be dependent on the precipitate surface area. We generalise our findings to estimate the impact of oxide precipitates with a given surface area on lifetime in both n-type and p-type silicon.

On the locking of dislocations by oxygen in silicon

Abstract Locking of dislocations by oxygen atoms in Czochralski silicon has been investigated both experimentally and theoretically. Experiments were performed at annealing temperatures between 700 and 850°C for different annealing times and different oxygen concentrations. These showed five distinct regimes for the unlocking stress as a function of annealing time. First the unlocking stress increases almost linearly with time and then saturates. The saturation stress, the time needed to reach saturation and the duration of saturation depend on the annealing conditions and oxygen content. Following the saturation a rapid increase and a second saturation of the unlocking stress with increasing annealing time were observed. Finally after long anneals the locking effect is much reduced. From the temperature dependence of the first saturation stress the interaction energy between an oxygen atom and a dislocation has been deduced and it is shown that the change in entropy when an oxygen atom is trapped at a dislocation is significant. The transport of oxygen to dislocations has also been investigated by solving the diffusion equation numerically. For these calculations both trapping and emission of oxygen atoms from the dislocation core have been considered.