Laser Spot Micro-Welding of Thin-Film Coated Silicon Wafers

Lasers are used to generate micro-welds on Si wafers for electronic and photonic materials applications. The temperatures within the weld pool generally approach or exceed the boiling point for the materials used, and the high heating and cooling rates can cause metal expulsion and evaporation of critical alloying elements.  Furthermore, sufficient mixing and alloying must take place during welding to meet device performance requirements.  To avoid thermally-induced damage to the Si substrate and to minimize processing time to meet throughput requirements, short pulses on the order of hundreds of nanoseconds to a few microseconds are used.  These short pulse durations and large power densities generate large vapor recoil pressures across the molten pool surface, which can cause surface deformation, resulting in undesirable surface morphologies and poor weld quality.  Here, mathematical modeling is used to evaluate the transition between laser/material interaction modes as a function of pulse duration and power density.  Process maps are generated to predict the parameter space in which optimal welding can be conducted while avoiding significant metal expulsion and microstructural degradation of the Si substrate.