Effect of debris on the silicon wafering for solar cells

The wafers used by the photovoltaic industry are mostly produced by multi-wire slurry sawing. One of the key factors determining the wafer quality (presence of saw marks and chips, increased roughness, wafer thickness variations and wafer strength) is the abrasive slurry. For cost reduction, the slurry is regularly exchanged and the debris it contains is removed in a recycling operation. To optimise the slurry usage, it is of utmost importance to understand the effects of the silicon debris concentration in the slurry. This was studied by sawing several bricks one after the other with the same slurry. It was found that when the amount of debris is too high (more than 4% of the slurry volume), saw marks appear on the wafers and they become more fragile. Finally, a first qualitative model explaining the apparition of the saw marks and the reduction of wafer strength is proposed.

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► Four half-batches of silicon were sawn in a row without changing the slurry.
► At a low level, the silicon debris has no effect on the wafer properties.
► Over 4% of the slurry volume, the debris creates saw marks.
► The saw marks appear first at the wire exit side of the wafers.
► Wafers with saw marks are much weaker than wafers without.