Internal Modification of Ultra Thin Silicon Wafer by Permeable Pulse Laser

In this study, "stealth dicing" (SD) was applied to ultra thin wafers 50 μm in thickness. A coupling problem composed of focused laser propagation in single crystal silicon, along with laser absorption, temperature rise and heat conduction was analysed by considering the temperature dependence of the absorption coefficient. When the depth of the focal plane is too shallow, the laser is also absorbed at the surface as the thermal shock wave reaches the surface. As a result, not only is an internal modified layer generated but ablation occurs at the surface as well. When the laser is focused at the surface, strong ablation occurs. Ablation at the surface is unfavorable because of the debris pollution and thermal effect on the device domain. It was concluded that there is a suitable depth for the focal plane so that the thermal shock wave propagates inside the wafer only. The optimum irradiating conditions such as pulse energy, pulse width, spot radius, and depth of focal plane can be estimated theoretically also for ultra thin wafer.