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. Collins a , David Milne b , Camilo Prieto b , Gerard M. O'Connor a 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 COlaser, 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, CO 2 laser. 1 INTRODUCTION Accurately structuring thin glass is of significant industrial interest due to the growing popularity of touch screen displays, microfluidic 1, 2 , microoptic 3, 4 and photovoltaic 5 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 process 6 . 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. CO 2 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 CO 2 laser (0.12eV) is in resonance with the excitation energy of the first vibrational level in a polyatomic molecule, typically between 0.01eV and 0.1eV 7 . Therefore absorption takes place through resonance absorption in SiO bonds and other impurities in the glass. CO 2 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 CO 2 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 Kondratenko 8 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 stresses 9 . A glass substrate containing an edge crack glass is locally heated by a CO 2 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 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/19/2015 Terms of Use: http://spiedl.org/terms

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

Page 1: Thin Glass processing with various Laser Sources...For large scale processing in industry a laser fracture technique first introduced by Kondratenko 8 is commonly used. This technique

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 touchscreen displays, microfluidic1, 2, microoptic3, 4 and photovoltaic5 applications. Glass has a good chemical resistance, highoptical 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 laserscribe 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 atthis 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 aregenerally available in long pulse or continuous wave output modes resulting in significant thermal diffusion andtherefore large heat affected zones (HAZ). Common methods of CO2 laser glass processing are laser induced fractureand 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 thanthe 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 haspropagated 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

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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<<1cm-1) in glass due to the large bandgap7. Absorption takes place through bulk defects, surface states and quasi-free seed electrons. UV lasers are more suitable for short pulse laser glass processing due to the higher photon energy of a UV photon allowing stronger absorption in material defects. Defects are also generated by repeated laser irradiation. Laser induced defects are referred to as incubation centres. These can include colour centres, vacancies, broken bonds and molecular fragments7. Other less dominant absorption pathways include multiphoton ionisation and avalanche ionisation. During multiphoton ionisation two or more photons are absorbed simultaneously and the sum of their energies is sufficient to promote an electron from a bonding to a non-bonding state. The free electrons generated are highly absorbing of further photons through inverse bremsstrahlung. Excited free electrons can ionise additional electrons in a positive feedback process known as avalanche ionisation. Scribing of high aspect ratio features is problematic for nanosecond scale pulses due to attenuation of the incident laser by ablated material confined within the trench. The key features of ultrashort lasers are the ability to reach the high intensities required for non-linear absorption in glass at moderate pulse energies and highly localised energy deposition11-13. Ultrashort lasers can manufacture a range of structures in dielectrics, such as trenches, bevels, local surface or bulk changes in refractive index 14 and high aspect ratio drilled holes15. The initial interaction is mediated by strong multiphoton ionisation and avalanche ionisation. The dynamic relationship between the ionisation processes has been modelled by several authors13, 16-18. The optical properties of an ionised dielectric surface will change dynamically over the course of the laser pulse with the effects peaking approximately 100–500 fs after the commencement of the laser material interaction 16, 19. The surface plasma will heavily attenuate the incident beam through linear absorption and a fluence dependent increase in surface reflectivity19. Breakdown of the material occurs when the density of free electrons reaches a critical value, typically taken as the density where the plasma becomes reflecting of IR wavelengths, approximately 1021 cm-³ 13. Excited electrons will equilibrate with the lattice within a few picoseconds20. Rapid heating of the substrate leads to melting, vaporisation and material ejection. Sub-micron ablation precision is possible with femtosecond pulses21 due to the absence of thermal effects and deterministic damage threshold opening up micro and nano scale processing opportunities. This laser, combined with a CNC scanning system, allows complex features to be quickly and precisely machined on a dielectric surface or bulk substrate. Techniques for improving feature quality and processing speeds are of interest especially for industrial applications.

Other novel and hybrid laser processes have shown promise for thin glass processing. Using an ultrashort pulse laser and a small focused spot size elongated light filaments can be formed in glass. By scribing a perforated path the sample will self-cleave along the defined path22. A filament is formed when the laser intensity is high enough that the nonlinear refractive index becomes significant. Considering a Gaussian shaped laser spot, we have a higher refractive index in the centre and a lower refractive index towards the edges. This spatially dependant refractive index acts as a 'lens' cancelling diffraction and focusing the laser into a filament23. Sugioka et al.24 showed an improvement in feature quality and processing speed when using a dual laser multi wavelength process. These effects are not considered in this work. Lasers are versatile tools for material processing. This study examines the quality and speed of laser glass processing for common laser sources. Each laser has a unique absorption mechanism dependent on the laser parameters. This results in contrasting edge quality and processing speed.

2 EXPERIMENTAL METHOD

2.1 Experiment Setup

The lasers used in the experiments were a Coherent diamond Gem-60 CO2 laser (10.6 μm), a Spectra Physics high peak power oscillator (HIPPO) laser (1064nm) and an Amplitude Systemes s-Pulse laser (1030nm). The HIPPO laser emits a 1064nm wavelength beam with a pulse duration of 15ns. A third harmonic generation head was attached to convert this to a 355nm output with a 12ns pulse duration. For all setups the laser was incident on the sample from above (Figure 1). Samples were mounted on a CNC Z stage to position the sample in the laser focus. The laser was scanned across the sample using a galvo scanner system except for the CO2 laser setup where the stage was moved relative to the laser. Borosilicate glass substrates from 2 suppliers were used for the tests. The samples had a thickness of 110µm.

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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 (OlympusSTM-MJS2 measuring microscope) and white light interferometry techniques (Zygo Maxim-GP 200). The high aspectratio 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 overand scribed, at a low power, on the rear surface in a direction perpendicular to the first scribe. Mechanical force is usedto 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 thermaldiffusion 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 cansometimes 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 1030Pulse Duration 40 µs 12 ns 500 fs Average Power (W) 30 5.5 1.74Repetition Rate (kHz) 10 30 10Pulse Energy (µJ) 300 183 174Spot Diameter 1/e² (µm) 39.4 (calculated) 16.4 59.7 Scan Speed (mm/s) 70 400 200Overlap (SPA) 1.58 1.23 2.98Fluence (J/cm²) 49.24 173 12.4Intensity (TW/cm²) 1.23 X 10-6 0.014 24.8 Polarisation Linear Linear Linear

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Page 5: Thin Glass processing with various Laser Sources...For large scale processing in industry a laser fracture technique first introduced by Kondratenko 8 is commonly used. This technique

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Page 6: Thin Glass processing with various Laser Sources...For large scale processing in industry a laser fracture technique first introduced by Kondratenko 8 is commonly used. This technique

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The stochastic nature of nanosecond laser ablation can be seen from the optical microscope image presented in Figure 3 (a). After 20 laser passes parts of the sample are ablated nearly entirely through the substrate while an adjacent parts of the sample are visibly unaffected by the laser. Figure 4 (b) shows a cross section of a substrate scribed with 10 laser passes. The substrate appears to be scribed from both the front and the rear surface. The laser initially passed through the front surface and was absorbed at the rear surface where some impurity was present. After repeated passesincubation centres were formed in the front surface and allowing coupling of the laser energy. In excess of 50 passes are required for a consistent cut through the substrate. The edge quality of the cut glass is seen in Figure 3 (c), (d). The edge shows significant chipping and high roughness but is free from micro cracks and is reproducible. No spontaneous stray fracture is occurring. For a 40ns laser the heat diffusion length is 0.21µm.

Figure 3: Microscope and SEM images of thin glass substrates processed by nanosecond UV laser. Image (a) shows a microscope image of a glass sample after irradiation with 20 passes. Image (b) shows an SEM image of a cross section in aregion affected by 10 laser passes showing a partial scribe at the front and rear surface of the substrate. Images (c) and (d)

show an SEM image of the edge quality of a nanosecond UV laser full body cut from the front and rear surfaces respectively.The sample is tilted by 45° relative to the detector.

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3.2 Nanosecond UV Laser processing

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spontaneous fracture. Edge quality is poor (Figure 3 (c), (d)) with considerable chipping occurring. At low number ofpasses only parts of the sample which contain defects or contaminations will absorb the laser. Defects in the glass arerandomly distributed and therefore the initial ablation is highly stochastic in nature. At high numbers of passes we begin to see incubation effects such as colour centres forming and enabling coupling of laser energy into the substrate where itwas previously not occurring. Due to the unpredictable nature of the initiation of the ablation it is not suitable for scribing blind trenches, holes or other shallow features in a glass substrate. For example it is not possible to predictwhether ablation will occur at the front or the rear surface as is the case for image Figure 3 (b). Initially the laser passes through the defect free front surface of the glass. Once it reaches the rear surface defect states and impurities enablecoupling of the laser energy into the substrate and we have ablation. After repeated laser passes we begin to see incubation effects on the front surface and which then cause absorption of the laser pulses leading to ablation at the frontsurface.

Femtosecond laser processing of glass is highly accurate and deterministic. There is no HAZ due to the absence of thermal diffusion during the interaction of the laser with the substrate. There is no cracking occurring at the cut edge and chipping is minimal (Figure 4 (c), (d)). Due to the deterministic nature of the ablation femtosecond lasers are suitable for scribing blind trenches, holes or other shallow features in a glass substrate. High density scribing of features is possible due to the negligible HAZ. The processing speed (3.33mm/s) is well below what would be required for ultrashort lasers to be considered a market disruptor for glass cutting. Increasing the applied fluence has little beneficialeffects on the processing speed. Figure 5 shows that as fluence is increased no corresponding increase in ablation depth occurs. For scribes made with 40 and 50 passes we in fact see a decrease in ablation depth. Statistical scatter in theresults also increases with fluence. There is some evidence of beam distortion discussed by Klimentov et al.27 visible in the trench shape for high fluence scribes. Increasing the fluence may cause significant ionisation of the ambient air abovethe interaction zone. This will result in distortion and attenuation of the incident beam which may be causing a decreasein ablation depth.

5 CONCLUSION

We have shown the significant differences in cut quality and speed depending on the laser source used. We haveattributed these differences to the laser parameters and contrasting absorption mechanisms taking place in the material.For laser processing of glass the choice of laser is dependent on process requirements and budget constraints. Full body laser cutting of thin glass is an order of magnitude too slow to be economical in industry despite the other advantages oflaser processing over mechanical cutting. Glass cleaving processes, such as the filamentation process introduced by Hossieni et al.22, show promise for enabling high speed and high quality cuts in thin glass.

ACKNOWLEDGMENTSThis work was conducted under the framework of the Irish Government's Programme for Research in Third

Level Institutions Cycle 5, National Development Plan 2007–2013 with the assistance of the European Regional Development Fund. Laser-Connect is a Marie Curie Industry-Academia Partnership and Pathways (IAPP) Project funded under EU FP7-2009-people-IAPP 251542.

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[2] K. Conlisk, S. Favre, T. Lasser et al., “Application of reconfigurable pinhole mask with excimer laser tofabricate microfluidic components,” Microfluidics and Nanofluidics, 10(6), 1247-1256 (2011).

[3] Y. Cheng, K. Sugioka, K. Midorikawa et al., “Three-dimensional micro-optical components embedded in photosensitive glass by a femtosecond laser,” Optics letters, 28(13), 1144-1146 (2003).

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