Laser-Ablation-Induced Concentric Ring Structures

Formation of concentric ring structures on substrate surface by 3rd harmonic Nd:YAG laser ablation is investigated. Experiments show that most of the laser energy on the substrate surface is distributed at the center and the outmost ring. Optical microscope and scanning electron microscope (SEM) observations reveal that concentric ring structures are only formed on the substrate surface at the positions between the focus plane and the lens. Both the outmost ring diameter and the ring number increase with the defocus distance, however, they are not dependent on laser fluence. This phenomenon could be attributed to the lens pupil truncation and its aberration. Laser energy concentrated at the outmost ring can be used to punch via microholes with the diameter determined by the defocus distance. This technique provides a high-speed, high-efficiency manufacturing method for via microhole array fabrication with low-energy laser ablation.