Controlling depth and distance of the hole formations at the bottom of laser-scribed trenches in silicon using fs-pulses

Ultrafast-laser dicing of thin silicon wafers: strategies to improve front- and backside breaking strength

“…This has the following consequences. The remaining half of the wafer can be cut with less pulse energy, because the cutting depth scales with the pulse energy [17]. In addition, the ratio between spot radius w 0 and wafer thickness d changes, and thus the ratio between the absorbed fluences on the plain surface at the first scan F ap and on the sidewall at the final scan F as decreases.…”

“…The projected area on the sidewalls becomes elliptic and the absorbed fluence on the sidewall F as decreases. The length of the cut sidewall w s , which corresponds to the large axis of the projected ellipse, can be estimated in terms of the beam radius w 0 and the cutting depth d using the Pythagorean theorem [17]. Besides, the steep sidewalls guide and trap reflected light inside the trench and thus increase absorption continuously up to A s = 1.…”

“…Besides, the steep sidewalls guide and trap reflected light inside the trench and thus increase absorption continuously up to A s = 1. Ablation stops when the absorbed fluence on the sidewall F as reaches the ablation threshold fluence F th [5,17,23,24]. The ablation threshold fluence F th is defined the minimum energy density to initiate material removal.…”

“…It should be large enough to avoid thermal heating and directed material ejections in the trench [17], but small enough to ablate a closed line when the beam is translated across the sample. Recent studies on surface ablation demonstrated that the highest ablation quality can be achieved if the ratio between the pulse distance and the focus radius is set to d p w 0 = v scan (f rep w 0 ) ≈ 1 [27,28].…”

“…This could be related to the following phenomena. Self-organizing periodic holes arise during scribing of Si [16,17], which cause periodic kerfs at the lower sidewalls and backside after cutting the chips. The kerfs act as stress concentrators, which reduces the breaking strength of the chip backside [6,8,18].…”