Formation of near-surface melt films on glass ceramics due to ultrashort laser pulses

摘要

Glass ceramics are highly specialized composite materials, which have a partly polycrystalline and a partly glassy state. Due to their special properties such as good mechanical strength, low thermal expansion, and excellent thermal shock resistance, they are especially well known for their use in consumer goods industry. But also in the high-tech sector, like optics or microsystems technology, the applications for glass ceramics are constantly growing. Simultaneously, the continuing miniaturization of microelectronic components requires precise, high-resolution processing methods. While mechanical processing is limited due to the brittle-hard properties of the material, ultrashort pulse lasers can serve as an ideal tool for this purpose. The pulse durations in the pico- and femtosecond range are known to enable a highly precise and gentle processing with very small thermal load for the workpiece. Therefore, the process is often referred to as cold ablation. It has been known for some time, however, that if pulse energies and repetition rates are sufficiently high, heat accumulation effects can occur. In this article, we report on surface modifications on glass ceramics arising during femtosecond ablation. Using Low Temperature Co-fired Ceramics (LTCC) as an example, we show that even at low repetition rates of 100 kHz and moderate average laser power below 3 W several micrometer thick molten patterns can emerge. Another peculiarity of the observed phenomenon lies in the increase of the vitreous layer with decreasing pulse duration. The dependence of the effect on the material structure is investigated by means of X-ray diffraction (XRD) measurements.