Measurements of the ns-laserpulse induced expansion of (111) silicon below and above the melting threshold on the nanosecond time scale

The surface displacement of 111-silicon wafers of 0.675 mm and 3.05 mm thickness, respectively, during intense ns laser irradiation is determined on the nm-vertical and ns-time scale using an optimized Michelson interferometer. The surface dynamics is observed below as well as above the melting threshold. We show that the obtained surface expansion below the melting threshold is in good agreement with theoretical heat transfer calculations. Additionally, thickness-dependent bending in the samples is illustrated and the acoustic reflections from the backside of the sample are observed. Maximum thermal displacement at the first expansion plateau is around 6 nm before melting occurs. We show that qualitative agreement between experimental results and simulation above the melting threshold can be established for the first time by taking the phase shift change in the reflection for molten silicon into account

Nanosecond laser pulse induced vertical movement of thin gold films on silicon determined by a modified Michelson interferometer

The vertical movement of a 40 nm thin Au film on a silicon substrate during intense nanosecond (ns) laser irradiation is determined on the nm vertical and ns time scales using an optimized Michelson interferometer. The balanced setup with two detectors uses the inverse interference signal and accounts for transient reflectivity changes during irradiation. We show that a change in phase shift upon reflection must be taken into account to gain quantitative results. Three distinct fluence regimes can be distinguished, characterized by transient reflectivity behavior, dewetting processes and film detachment. Maximum displacement velocities are determined to be 0.6 m/s and 1.9 m/s below and above the melting threshold of the metal, respectively. Flight velocities of detaching liquid films are found to be between 30 and 70 m/s for many nanoseconds

Acoustic laser cleaning of silicon surfaces

We investigate the detachment of small particles from silicon surfaces by means of acoustic waves generated by laser-induced plasma formation at the back side of the sample. It is demonstrated that sufficiently high acoustic intensities can be reached to detach particles in the submicron regime. In order to study this acoustic laser cleaning in more detail, we have developed an interference technique which allows one to determine the elongation and acceleration of the surface with high temporal resolution, the basis for an analysis of the nanomechanical detachment process, which takes place on a temporal scale of nanoseconds. We find that the velocity of the detaching particles is significantly higher than the maximum velocity of the substrate surface. This indicates that not only inertial forces, but also elastic deformations of the particles, resulting from the acoustic pulse, play an important role for the cleaning process