Directly induced ablation of metal thin films by ultrashort laser pulses

Abstract

ABSTRACT Molybdenum films on a glass substrate are ablated from the glass side by picosecond laser pulses at fluences below 1 J/cm², without damage. Thin films of chromium, titanium and platinum with thicknesses between 200 nm and 1 µm were examined to investigate the underlying ablation mechanisms. For molybdenum an influence of the intermediate buffer layer was observed. Ablation from the gla ss side clearly has higher abla tion efficiency and a better structural quality in contrast to metal side patterning. A m odel will be presented, in whic h the ablation characteristics are connected with the mechanical ductility of the metal. Keywords: Thin film, laser ablation, laser scribing, laser structuring, ultrafast laser, picosecond laser, 1. INTRODUCTION The patterning of thin metal films is applied in many fields from circuit board fabrication, semiconductor chip fabrication to component trimming, mask repair or marking. If the size of the substrate is small enough and the pattern to be written remains the same for a large number of samples, photolithography as a parallel process (all the features at the same time) is the technology of choice. However, if the pattern to be written changes frequently, only sequential technologies offer the necessary flexibility. In this case E-beam- or laser writing have their field of application for trimming, marking or flexible manufacturing. But even when the pattern remains unchanged for many samples, photolithography and e beam writing are not suited for large substrates as they are used in thin film photovoltaics (size ~ 1 m²). Here laser scribing is used, especially for the patterning of the monolithical serial interconnects. Molybdenum films with thicknesses below 1 µm on a glass substrat e serve as the p- contact of CIS thin film solar cells. These films have to be separated to achieve a galvanic isolation of adjacent cells. The patterning of the molybdenum films with pulsed laser beams has been investigated by several groups [1-7]. Currently in production lines, the molybdenum is patterned with pulse durations in the nanoseconds range. We have shown that it is advantageous to ablate the molybdenum with picoseconds laser pulses applied from the glass side. By this lift off process high quality grooves are written, without pronounced thermal effects and free from thermal damages[8]. Two observations seemed to be worth further investigations: The ablated spots have very clean and regular edges and complete disks of the ablated metal were found (see Figure 2 ). To get more insight in the underlying mechanisms we compare in this paper the ablation behaviour of chromium, titanium and platinum with that of molybdenum.