Ultrafast laser micromachining of through-drill trenches in silicon wafers

Silicon wafer processing is crucial in the semiconductor industry, especially in applications requiring precision through-trench fabrication. This study investigates the impact of various laser fluence, scan counts, and focal point dynamics on the trench width for both the entrance (top) and exit (bottom) surfaces using ultrafast laser machining on silicon wafers. Experiments revealed that while the trench width at the entrance remains nearly constant, the width at the exit increases with higher laser fluence and scan counts, stabilizing once a complete through-drill state is reached. Additionally, dynamic focal point strategy (step processing) experiments at laser fluences of 20 and 30 J/cm², and total scan counts of 300 and 600 were conducted. It was found that when the scan count was below the through-drill threshold, increasing the number of steps led to a wider trench bottom. However, when the scan count exceeded the threshold, the trench width at the bottom reached its maximum at two steps and then decreased with additional steps. Furthermore, the effect of polarization direction on machining quality was assessed, showing improved trench uniformity when the polarization was perpendicular to the trench direction. These findings suggest that a 2-step processing approach, with each step meeting the through-drill scan threshold and maintaining perpendicular polarization to the trench, optimizes micro-machining quality and minimizes taper.