Efficiency of cavity gettering in single and in multicrystalline silicon wafers

Multicrystalline silicon wafers are submitted to He ion implantations at various energy levels and fluences in the ranges 40–120 keV and 1016–1017 cm−2, respectively. Cavities are formed after annealing at 900°C for 1 h to 4 h in argon flow and induce a segregation gettering. For comparison the wafers experience a Al–Si gettering after the deposition of a backside 1 μm thick Al wafer and annealing at 900°C for 4 h in argon. Some wafers are voluntarily contaminated by nickel atoms and diffusion lengths of minority carriers are determined by SPV technique and measured before and after gettering. Float zone single crystalline wafers were also investigated for comparison. It is found that gettering by cavities works well provided the fluence is 1017 cm−2, although the capture of impurities by the cavities is limited by the saturation of their internal surface. Al–Si alloying appears more efficient. This is most likely due to the high solubility of metal atoms in the liquid Al–Si alloy formed during the annealing. The two gettering techniques can protect the wafers during processing steps.