Method for sorting silicon wafers according to their bulk lifetime

Abstract

The invention relates to a Czochralski-type method for sorting wafers obtained by cutting a single-crystal silicon ingot, the method being implemented when the wafers are in an as-cut state or in a shaped-surface state. The method comprises the following steps: a) measuring the majority free charge carrier concentration in at least one area of each wafer; b) calculating the thermal donor concentration in said area of each wafer, on the basis of the majority free charge carrier concentration; c) calculating the charge carrier lifetime limited by the thermal donors in said area of each wafer, onthe basis of the thermal donor concentration; d) determining a bulk lifetime value for the charge carriers in each wafer on the basis of the lifetime limited by the thermal donors; e) comparing the bulk lifetime value or a normalised bulk lifetime value with a threshold value; and f) discarding the wafer when the bulk lifetime value or the normalised bulk lifetime value is lower than the threshold value.

Description

technical field [0001] The present invention relates to a method for sorting silicon wafers before they are cut from an ingot and before they are used in the manufacture of semiconductor devices such as solar cells. Background technique [0002] About 10% to 20% of monocrystalline silicon wafers obtained by the Czochralski method are incompatible with the manufacture of high-efficiency solar cells. Indeed, these wafers contain defects that limit the lifetime of the photogenerated charge carriers and lead to a reduction in efficiency, which can be as high as 4% (absolute). [0003] Life-limiting defects include thermal donors (DT). Thermal donors are aggregates generated by interstitial oxygen contained in silicon (ie, oxygen atoms occupy interstitial positions in the crystal lattice). They are formed during ingot drawing when the temperature of the silicon is between 350°C and 500°C. [0004] The wafer which has the highest donor concentration and thus can limit its effic...

AI Technical Summary

Problems solved by technology

[0010] Additionally, this method of sorting wafers does not allow for optimal optimization of the fabrication of homojunction cells, as it requires that the first step of the fabrication method be completed in each case One step: Diffusion of the emitter (e.g. 10 seconds at 800°C)

This step represents unnecessary overhead when wafers are eventually discarded

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