Ultrashort pulse laser scribing of thin flexible glass

Abstract

Laser scribing of thin glass has proven problematic due to inefficient optical absorption and difficulty achieving economical processing speeds while maintaining edge quality. Laser processing of glass is pertinent to touch screen, display, microfluidic, microoptic and photovoltaic applications. At thicknesses <100μm glass benefits from added flexible functionality. In addition to high optical transparency, electrical insulation and good chemical resistance, thin glass is a preferable material choice for many applications. Thin flexible glass offers an opportunity to substitute sheet-fed with reel-to-reel processing, reducing processing time and material handling issues. Unique absorption and thermalisation mechanisms associated with ultrashort pulse ablation have opened new opportunities for laser material processing, especially for optically transparent materials such as glass. A robust and reconfigurable thin flexible glass cutting technique, compatible with reel-to-reel manufacturing, has yet to be established. Initially this work benchmarks laser ablative processing of glass. Laser sources including a CO₂ laser, short pulse UV laser and an ultrashort pulse IR laser, are used. The contrasting absorption and material removal mechanisms produce diverse processing results. It was concluded that ultrashort pulse lasers are the most suitable for full body ablative processing of thin glass, due to precise non-linear absorption mechanisms and minimal thermal effects. Cross sections of glass which were scribed with a P polarised laser (relative to the trench wall) showed damage regions extending away from the trench walls, and correlated damage on the rear surface. This is indicative of damage caused by light transmission through the walls of the trench. The damage was reduced by rotating the polarisation to S polarised, due to the increased reflectance from the trench walls. It was found that S polarised light also required less passes to ablate through the glass substrate. A processing window capturing the peak of the polarisation effect was identified. An optical model was developed to predict the effect of polarisation on the intensity distribution reaching the rear surface of the glass. The model showed that S polarised light confined a greater amount of light in the trench. Consequently we see an increased fluence incident on the central region of the trench. Even with precise control of parameters, laser processing of thin glass speed is an order of magnitude below the required level. An alternative laser scribing method, which utilises surface stress raisers to enable controlled mechanical fracture of glass, was developed. An ultrashort laser source is used to precisely pattern elliptical recesses on the sample surface. The apex of an ellipse concentrates tensile stresses in a brittle material. Depending elliptical dimensions the stress concentration factor can be several tens in magnitude. A beam delivery system was designed to produce a focused elliptical spot. When scanned, the system generates a plurality of separated aligned elliptical recesses across the glass surface. The orientation of the ellipses defines a preferred scribing path. Tensile stress can be applied orthogonally to the path to cause mode I fracture. The quality of the right angular cuts in thin flexible glass, processed with this method, are of higher quality and strength than are possible with a full body laser cut. Curved scribed are possible with this technique by rotating the cylindrical lens along an arc while the laser is scanned in a curved path. The stress field around a stress raiser was analysed using the FEM. A non-contact method for fracturing scribed brittle substrates was developed. The process uses compressed-air jets, controlled by high-speed valves, to produce mechanical resonance and induce a bending stress in the glass substrate. If the stress is sufficient the substrate will fracture along the scribed line. The resonant frequency of the beam was studied analytically by modelling the substrate as a beam with both ends fixed. FEM analysis on the beam was also performed to compare with the analytical results. The optical setup for the mechanically inspired scribing process is simple, low cost and compatible with reel-to-reel manufacturing platforms. Consequently the stress raiser process, together with the resonant fracture technique, offers an alternative to other processes which employ high numerical aperture optics for thin glass scribing.