Sub-micromachining using femtosecond pulse laser interference phenomenon

Abstract

This research work sets out to achieve submicromachining capabilities on the femtosecond pulse laser which surpasses the conventional laser micromachining method of using the direct laser beam to machine. The aim is to achieve laser ablated features of 200nm and below with this novel research concept. With ultrafast pulse lasers such as the femtosecond pulse laser, subspot size micromachining is possible because of their precise threshold value. However, the sub-spot size ablation mechanism has reached its lowest limit when it hit the 1/1 Oth of the laser focal spot size range. Up to now, the smallest laser micromachined feature reported is 200nm in diameter, or approximately 8% of the original laser spot size of 2.5um. The aim of this research work is to break through this limit and obtain submicron features of below 200nm with the femtosecond pulse laser. To achieve this, a novel concept of using the phenomenon of femtosecond pulse laser interference to machine is proposed. The laser beam from the femtosecond pulse laser is first interfered to obtain a circular interference beam. The central bright fringe from the generated interference pattern which has a sharper beam profile is then used to machine. With this method, the effective ablation spot size is reduced, subsequently reducing the size of the feature size it ablates. One of the attractive characteristics of ultrafast pulse lasers is its precise threshold value. It is this feature which enables the ablation of sub-spot size features on these lasers. And by applying this same concept, only a fraction (a part of the central bright fringe) of the interfered beam within which the fluence is greater than the threshold of the material, can be made to machine.