Analysis of internal modified-layer formation mechanism into silicon single crystal with nanosecond laser

Abstract

When a permeable nanosecond pulsed laser is focused into the interior of a silicon wafer and is scanned in the horizontal direction, a belt-shaped high dislocation density layer that consists of a partially polycrystalline region that is formed at an arbitrary depth in the wafer. By applying tensile stress perpendicularly to this belt-shaped modified-layer, the silicon wafer can be separated easily into individual chips without creating any damage to the wafer surface compared to the conventional blade dicing method, because the internal cracks spread from the modified layer up and down to the surfaces. This technology is called "stealth dicing" (SD), and attracts attention as a novel dicing method in semiconductor industries. The purpose of this study is to clarify the formation mechanism of modified layer. A coupling problem composed of focused laser propagation in a silicon single crystal is examined, considering laser absorption, temperature rise, and heat conduction, with particular attention to an experimental result that the absorption coefficient varies with temperature. Simple thermal stress analysis was also conducted based on those results. As a result, the formation mechanism of the modified layer could be explained clearly. It was seen that the temperature dependence of absorption coefficient is the most important factor of the modified layer formation. The present analysis can be applied to find the optimum laser irradiation condition for SD method.