The effect of the spot diameter in nanosecond excimer laser percussion drilling of silicon wafer was investigated. The aim was the efficient production of Through-Silicon-Vias (TSV), in recently demonstrated Silicon-on-Diamond (SoD) material (see the figure). Results show a tenfold increase of the ablation efficiency when decreasing the spot diameter from 220 mm to 9 mm at constant fluence in the range 7.5 J/cm2 to 13.2 J/cm2. Such effect is absent when using 60 ps deep-UV laser pulses. An original model has been developed to explain the findings in terms of plume shielding effect on the laser pulse. The presented results indicate that the spot size should be taken into great account when performing laser percussion drilling in silicon with ns pulses. This finding may also have impact in efficient laser drilling of “Emitter wrap-through” (EWT) and “Metallisation wrap through” (MWT) back-contact solar cell.