Cold ablation liberates thin films from HAZ

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Lasers that fire for as little as a ten billionth of a second provide advantages with many materials, such as no heat affected zones or melting with cold ablation, as revealed at a recent AILU workshop. Rob Coppinger reports

These fast lasers, lasers that fire, or pulse, as quickly as ten or millionths of a billionth of a second, were a focus of the Association of Laser Users’ (AILU) annual 'Ultra precision laser manufacturing systems, technologies and applications workshop'.  

Held on 13 September at the University of Cambridge’s Centre for Industrial Photonics, users and suppliers of laser systems heard about fast lasers micro machining thin films, metals, polymers and glass for the medical, microelectronics and other sectors. Toughened glass and OLED thin films both have large markets in mobile computing and phones and were the subject of presentations at the AILU meeting.  

Fast lasers and OLEDs have been investigated as part of a 36-month €15.5m European Union project that recently ended. Called Fast2light, its final report is being written at the moment according to Dr Alan Ferguson,s sales director for Oxford Lasers, a laser micromachining company and Fast2light participant. 

In the Fast2light work a laser was used to pattern thin film, ablating away 100nm layers. It does this using pico and femtosecond pulse widths, firing for billionths and millionths of a billionth of a second respectively. It is the rapid application of these short pulses that causes cold ablation. 

Cold ablation is where little or no heat is absorbed by the area around the target, avoiding HAZ creation. The target that is heated can change from a solid to a gas or vapour avoiding molten states that result in problematic debris. With cold ablation instead of melting, an explosive expansion of the target area sees it liberated from surrounding material and then that target vaporises, avoiding the melt. Speaking at the AILU workshop Ferguson explained that tailoring of the ablated area was needed however and that the ‘substrate would determine how much tailoring is needed.’

Cutting holes in toughened glass with these pico and femtosecond fast lasers was part of the presentation from Bob Hainsey, research and development senior director for microtechnology manufacturing laser processes specialist ESI. Hainsey explained that increases of up to 50 per cent in mobile phone glass manufacturing throughput could be achieved with fast lasers. But he declined to specify the type of laser the firm uses beyond confirming they were picosecond lasers. Toughened glass, such as the popular mobile phone component Gorilla glass, receive surface treatment that stops crack propagation within the top 50µm. ESI’s laser system has demonstrated it can cut a 10mm hole without a heat affected zone in a 0.7mm thick piece of glass the same size as those used for mobile phones. Hainsey explained that the key was to use ‘shorter pulse widths’ in combination with ‘higher power’.

The importance of fast lasers to the mass production of future mobile phones was underlined by Nokia Research Center’s materials science principal researcher Chris Bower. He gave a keynote speech at the AILU workshop about how laser processing was needed for the surface treatment of future handsets’ materials and components.

As well as AILU and its Micro:Nano special interest group, the event was organised with the help of the University of Cambridge’s Institute for Manufacturing and the UK government’s Knowledge Transfer Network (KTN) for nanotechnology and its KTN for electronics, sensors and photonics.