Picosecond Laser 4-Beam Interference Ablation of Metal Films for Microstructuring

By: B. Voisiat, M. Gedvilas, G. Raciukaitis

Laboratory for Applied Research, Center for Physical Science and Technology, Lithuania

Properties of the surface highly depend on its relief. The best example is a lotus leaf with the “self-cleaning”, hydrofobic feature. Micro- and nano-structuring with specific patterns and at localized places is useful to control wettability, lubrication and other properties of the surface. Artificial structures borrowed from the nature are applied in automotive industry as well as in photonics and bio-medical research. A new growing application area is frequency-selective surfaces which are working as a filter for electro-magnetic waves in THz, infrared or visible range depending on the period and the feature size of the fabricated surface. They can be produced by laser- structuring of metal films.

Laser direct writing is a flexible method but the performance on practically large areas is too low when fine structures should be produced. Laser patterning using interference of several beams is capable of producing the sub-wavelength features not limited by a beam spot size and is an effective method of forming two-dimensional and three-dimensional structures.

In this work the interfering laser beams were applied to pattern thin metallic films on the glass substrate. Periodic structures were fabricated on thin metal films (chromium, aluminum, gold, copper and silver) deposited on the glass substrate using 4-beam laser interference patterning. 2D-gratings with a period of 5 µm and consisting of circular holes were fabricated when no phase difference was introduced between symmetrically arranged beams. The area where holes were ablated using only one laser pulse depended on the metal and its thickness. It was as large as 200×200 μm in copper and 450×450 μm in aluminum. The final shape of the structure ablated in metal films was found to be dependent on the phase difference between beam pairs as well as on the irradiation intensity versus the ablation threshold. The material was removed from a substrate in areas where the local intensity (laser fluence) exceeded the threshold value. Consequently by varying irradiation intensity and phase difference between the beams different kinds of periodic patterns can be formed on thin metal layers. Varying the phase difference between beam pairs and laser intensity relative the ablation threshold the “chess-board” or “segmented net” consisting of rectangular metal inlands with small gaps between them were fabricated by a single pulse expose of the Cr film. More examples of periodical structures modeled and produced by 4-beam interference ablation technique are presented in this work and show flexibility of the method in periodical structure formation.

Further work for improving the formation of periodic structures using beam interference ablation should be done. Lateral stability of produced structures required higher phase stability and irradiation uniformity over the interference area. Shaped laser beams with flat-top lateral energy distribution should be applied to ensure homogeneity of the structure.