Analysis of modified layer formation into silicon wafer by permeable nanosecond laser

Abstract

When a permeable nanosecond pulse laser which is focused into the inside of a silicon wafer is scanned in the horizontal direction, a belt-shaped high dislocation density layer including partially polycrystalline region is formed at an arbitrary depth in the wafer. Applying tensile stress perpendicularly to this belt-shaped modified-layer, silicon wafer can be separated easily into individual chip without creating any damage to the wafer surface comparing with the conventional blade dicing method, because the cracks that spread from the modified layer up and down progress to the surface. This technology is called “stealth dicing” (SD), and attracts attentions as a novel dicing technology in semiconductor industries. The purpose of this study is to clarify the formation mechanism of modified layer. We paid attention to an experimental result that the absorption coefficient varies with temperature. We analyzed a coupling problem composed of focused laser propagation in a silicon single crystal, laser absorption, temperature rise, and heat conduction. Simple thermal stress analysis was also conducted based on those results. As a result, formation mechanism of the modified layer could be explained clearly. 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, and it is a future subject to confirm it experimentally. It was supported by the present analysis that the problem of thermal effect on the active region can be solved by the SD method. SD method for wafer dicing is original firstly and it is valuable that formation mechanism of the modified layer in SD method was clarified theoretically.When a permeable nanosecond pulse laser which is focused into the inside of a silicon wafer is scanned in the horizontal direction, a belt-shaped high dislocation density layer including partially polycrystalline region is formed at an arbitrary depth in the wafer. Applying tensile stress perpendicularly to this belt-shaped modified-layer, silicon wafer can be separated easily into individual chip without creating any damage to the wafer surface comparing with the conventional blade dicing method, because the cracks that spread from the modified layer up and down progress to the surface. This technology is called “stealth dicing” (SD), and attracts attentions as a novel dicing technology in semiconductor industries. The purpose of this study is to clarify the formation mechanism of modified layer. We paid attention to an experimental result that the absorption coefficient varies with temperature. We analyzed a coupling problem composed of focused laser propagation in a silicon single crystal, laser absorp...