High Speed Imaging of Laser Hole Micro Drill in Aluminum Thin Films

Micro hole drilling with semi ultrafast lasers may create phenomena which are different than regular boiling and heat transfer mechanisms. In this paper high speed shock waves generated during nanosecond laser drilling of 1000 nm aluminum films are imaged. Extremely fast heat transfer from the laser to the metal film causes very fast homogeneous boiling of the metal film and extremely quick evaporation of the material introduces high-speed shockwaves. This phenomenon is different than regular heterogenous boiling since there is not enough time for the boiling bubbles to be generated. One way to study the heat transfer during laser drilling, is to study the shockwave generated during this process. Even though the shockwave is occurring in the air and not in the drilled material, the actual data obtained from the shockwave may later be employed in measurement of mass transfer during laser drilling. Shockwave images are analyzed and the position of the shockwaves are measured. Using least square method for a power law shape, a power law function is fitted on the experimental data. The velocity of the shockwave is also calculated by differentiating the power law function for shock position. Since the data for radius of the shockwave is not available for time equal to zero, a scale analysis is employed to estimate the initial velocity of the shockwave. The speed of the shockwave is hence estimated to be as high as 6042 meters per second.