Periodic lines and holes produced in thin Au films by pulsed laser irradiation

Abstract

A technique for the fabrication of periodic lines and holes in thin gold films (5–50nm thick) on a glass substrate by irradiation with two and four intersecting beams of a nanosecond pulsed laser is demonstrated. The two- or four-beam interference creates a one-dimensional or two-dimensional periodically alternating intensity distribution, respectively. It is shown that the threshold intensity necessary for the fabrication of periodic structures is not a monotonous function of increasing film thickness and also depends on the period of the structure. In films thicker than 17nm a unique redistribution of the film material was observed at the high intensity lines or spots. At these “hot” locations we observed the formation of tall and narrow ridges and rims caused by the hydrodynamic flow of the molten metallic film in the optically induced temperature gradient, with subsequent crystallization in the “cold” regions. A model was developed which allows us to calculate the temperature distribution around irradiated regions of the film as a function of time and position for various film thicknesses, periodicities, and beam intensities, taking into account the film reflectivity and the beam intensity profile. From the temperature distribution we calculate the expected film morphology subsequent to laser irradiation and find good agreement with the measured distances between the ridges and the rim diameters.