Thin Glass processing with various Laser Sources ... For large scale processing in industry a laser...

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  • Thin Glass processing with various Laser Sources

    Adam R. Collinsa, David Milneb, Camilo Prietob, Gerard M. O'Connora a National University of Ireland, Galway, Ireland.

    b M-Solv Ltd, Oxford, United Kingdom.

    ABSTRACT

    Laser processing of thin glass has proven problematic due to the inefficient coupling of optical energy into glass and the difficulty achieving an economical processing speed while maintaining cut quality. Laser glass processing is pertinent to touch screen display, microfluidic, microoptic and photovoltaic applications. The results of the laser scribing of 110 μm thick alkali free glass with various laser sources are presented. The laser sources include a CO₂ laser, nanosecond UV laser and femtosecond IR laser. The contrasting absorption mechanisms are discussed. Cut quality and processing speed are characterised using SEM and optical microscopy techniques. Alternative laser techniques for thin glass processing are also considered.

    Keywords: Laser processing, glass processing, laser ablation, ultrashort laser, thin glass, dielectric, CO2 laser.

    1 INTRODUCTION

    Accurately structuring thin glass is of significant industrial interest due to the growing popularity of touch screen displays, microfluidic1, 2, microoptic3, 4 and photovoltaic5 applications. Glass has a good chemical resistance, high optical transparency and moderate flexibility for thicknesses below 200 µm. For this reason glass is suitable for many applications. The flexibility of thin glass offers an opportunity to substitute sheet-fed processing with a fully reel-to-reel process6. Here it will be possible to process continuous rolls of substrates decreasing processing time significantly. Lasers potentially offer a sustainable, reconfigurable and versatile solution for structuring thin glass. Ideally a laser scribe in glass would have a smooth cut face, no chipping or cracks along the cut edge and be completely reproducible. The overall processing speed must be >100mm/s for the process to be economical and disruptive to current glass cutting techniques.

    CO2 lasers are, at first glance, an ideal candidate for glass processing. A mature and economical technology which emit light at wavelengths strongly absorbed by glass substrates (10.6µm). The absorption coefficient for silica at this wavelength is estimated at 250cm-1 7. The photon energy of a CO2 laser (0.12eV) is in resonance with the excitation energy of the first vibrational level in a polyatomic molecule, typically between 0.01eV and 0.1eV7. Therefore absorption takes place through resonance absorption in SiO bonds and other impurities in the glass. CO2 lasers are generally available in long pulse or continuous wave output modes resulting in significant thermal diffusion and therefore large heat affected zones (HAZ). Common methods of CO2 laser glass processing are laser induced fracture and full body laser cutting. For large scale processing in industry a laser fracture technique first introduced by Kondratenko8 is commonly used. This technique is based on the fact that the tensile fracture stress of glass is lower than the compressive fracture stress by a factor of 10 due to flaws in glass being unable to amplify compressive stresses9. A glass substrate containing an edge crack glass is locally heated by a CO2 laser to a temperature just below the glass softening temperature causing compressive stresses in the substrate, insufficient to cause fracture. A coolant jet is subsequently applied to the heated region. Rapid cooling causes the stress to become tensile and induce “mode 1”9 fracture in the glass along the line heated by the laser. Mechanical force is usually required to ensure the crack has propagated through the entire substrate. This method is widely used in industry for processing glass of half a millimetre

    Laser-based Micro- and Nanoprocessing IX, edited by Udo Klotzbach, Proc. of SPIE Vol. 9351, 93511K · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2077217

    Proc. of SPIE Vol. 9351 93511K-1

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  • to several millimetres thickness. Processing speeds of 0.3m/s are reported for 1mm thick soda lime glass10. It is possible to cut curves using this method, however, the shape must be closed and begin at an edge. Separately, a full body laser cut can be achieved using a focused CO2 laser to ablate a trench through the entire substrate. Material removal in this case takes place boiling and vaporisation of the glass along with a coaxial air jet shearing molten glass through the cut10. Repetitively pulsed nanosecond lasers offer precise and low cost method for material processing. Glass processing with short pulse lasers is limited due to the negligible linear absorption of UV, VIS and NIR wavelengths (α

  • Figure 1: Ty

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  • was cooled by a cool air jet emitted from a compressed air vortex cooler (Meech). The cooler emitted an air jet with a temperature of approximately -5°C. Heat diffusion lengths were calculated using the expression7 ≈ 2 .

    Table 1: Laser settings used for glass scribing tests.

    2.2 Characterisation process

    Sample characterisation was carried out using SEM (FEI Phenom Desktop SEM), optical microscopy (Olympus STM-MJS2 measuring microscope) and white light interferometry techniques (Zygo Maxim-GP 200). The high aspect ratio of the scribes led to difficulties measuring the scribe depth. To characterise the scribe depth a cross sectioning technique was developed. The sample is first scribed with the laser at a specific setting. The sample is then turned over and scribed, at a low power, on the rear surface in a direction perpendicular to the first scribe. Mechanical force is used to fracture the sample along the rear side scribe. The sample is then mounted onto an angled stub with a carbon tab. This allows the cross section to be viewed directly on a SEM. Where required a sputter coater was used to deposit a thin gold coating (~40 nm) on the sample to reduce charging and improve image contrast.

    3 RESULTS

    3.1 CO2 Laser processing

    CO₂ laser full body laser cuts were made in glass. The high absorption coefficient results in the laser being heavily absorbed in a thin surface layer causing rapid heating. The long pulse duration allows time for significant thermal diffusion to occur. This results in a large HAZ, an edge burr (Figure 2) and in most cases catastrophic uncontrollable fracture of the glass. For a 40µs pulse we have a heat diffusion length of 11.9µm. Fracture usually occurs near the edge of the laser interaction zone and is caused by tensile stress induced by cooling due to thermal diffusion. The fracture can sometimes occur several seconds after the laser interaction. The substrate was completely cut and no mechanical force was required to separate the pieces.

    Laser Source CO2 Laser NS UV Laser FS IR Laser Wavelength (nm) 10600 355 1030 Pulse Duration 40 µs 12 ns 500 fs Average Power (W) 30 5.5 1.74 Repetition Rate (kHz) 10 30 10 Pulse Energy (µJ) 300 183 174 Spot Diameter 1/e² (µm) 39.4 (calculated) 16.4 59.7 Scan Speed (mm/s) 70 400 200 Overlap (SPA) 1.58 1.23 2.98 Fluence (J/cm²) 49.24 173 12.4 Intensity (TW/cm²) 1.23 X 10-6 0.014 24.8 Polarisation Linear Linear Linear

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