Laser assisted particle removal from silicon wafers

Abstract

We have studied the removal of submicrometer particles from silicon wafers by the steam laser cleaning (SLC) and dry laser cleaning (DLC) processes. These processes are currently being investigated as new promising cleaning technologies for complementing traditional methods in industrial applications. For SLC a thin liquid layer (e.g. a water-alcohol mixture) is condensed onto the substrate, and is subsequently evaporated by irradiating the surface with a short laser pulse. The DLC process, on the other hand, relies only on the laser pulse, without application of a vapor jet. Using well-characterized monodisperse polystyrene and silica particles as well as irregularly shaped alumina particles with diameters down to 60 nm we have systematically investigated the efficiency of the two processes. The influence of laser pulse duration from the nanosecond to the femtosecond range was studied. For the DLC we were able to measure the acceleration of the silicon surface due to thermal expansion for DLC. Our results demonstrate that for the gentle cleaning of silicon wafers the SLC is a very efficient method and for particles smaller in diameter than 400 nm it is superior to DLC. This is due to lower cleaning thresholds in laser fluence for SLC compared to DLC for the removal of small particles. DLC may cause serious surface damage by field enhancement under the contaminants, an effect that has only rarely been taken into account in laser cleaning studies so far and is also discussed here.