Surface acceleration during dry laser cleaning of silicon

We report on measurements of the surface acceleration for the application of dry laser cleaning. For that purpose, industrial silicon samples were irradiated by a frequency-doubled Q-switched Nd:YAC laser. The surface displacement was measured by a heterodyne interferometer and recorded by a digital storage oscilloscope. Several hundreds of shots were averaged to give smooth displacement curves which could be derived numerically. The experiments show that the highest accelerations, which are thought to be responsible for the cleaning, occur on the time scale of the laser pulse. Simple theoretical models are in good agreement with the experimental data. The maximal displacement depends only on the deposited energy, while the maximal acceleration shows also a strong dependence from the temporal pulse shape. This knowledge allows one to optimize the pulse shape for the cleaning process.