Magazine Articles
The articles below are from the Winter 2008 Issue of the AILU Magazine
Ultrafast laser patterning of OLEDs for solid-state lighting
Traditionally, laser micromachining has relied on Q-switched Diode-Pumped Solid State (DPSS), excimer and CO2 lasers of typically nanosecond (ns) pulse duration or longer. These lasers offer relatively high average power and can achieve high volume removal rates harnessing a combined etching effect from laser and laser-induced plasma processes. As a result, micro-drilling, scribing and fine cutting applications have benefited on an industrial scale. However, the driving mechanism even at short wavelengths is strongly thermal in nature. This arises largely as a result of secondary plasma heating of the target and substrate thermal conduction, and it limits fine control of the ablation process. Undesirable side effects near the irradiated regions such as micro-cracking or edge chipping, burr formation and particle debris that often accompany the micromachining process confirm this claim. As a result, successful laser machining with ns lasers is still viewed by many as a ‘black art’.
In contrast to the nanosecond case, the fluences (incident pulse energy per unit area) typically employed for ultrafast laser micro-machining, are kept close to the ablation threshold to avoid any laser plasma-assisted etching. Ultrafast-laser micromachining is deterministic, highly reproducible and inherently precise. By utilising the very high laser repetition rates available from modern ultrafast lasers (up to few MHz), throughput is maximised. A very promising laser candidate for OLED patterning uses picosecond laser pulses at very low incident fluences near the single-pulse ablation threshold (few tens of mJ/cm2).
Where conventional nanosecond lasers seem to have failed, ultrafast lasers enable highly reproducible thin-film patterning of OLEDs on flexible substrates. Among the perceived benefits is excellent depth resolution and debris-free machining which could facilitate a new disruptive micro-manufacturing technology.
Dimitris Karnakis
Oxford Lasers Ltd, Didcot, UK
IMAGE: Picosecond laser scribed ITO on a flexible substrate with 532 nm 9 ps pulses. (top) 4 μm holes 100 nm deep; (bottom) 5 times slower scan speed.
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Laser-assisted cold spray process
Thick metal coatings are currently deposited via two well established process routes, Laser or arc based cladding, and thermal spray. A new coating technique known as Laser-assisted Cold Spray (LCS), which aims to expand on the capabilities of the two process routes currently available, is under development at the University of Cambridge in the UK. LCS is a development of the Cold Spray process (CS) in which coatings are built up from powder particles which are entrained within a gas stream and accelerated through a de Laval nozzle, impacting the substrate at supersonic speeds that exceed a material dependent critical velocity.
In LCS, a laser heats the deposition site so that deposition occurs at above two thirds of the melting point of the material. At these temperatures the deposition site is considerably softened: this reduces the critical velocity and allows bonding to occur on impact at velocities around half those found in cold spray (< 500 ms-1) even when depositing materials such as titanium, which are difficult to process using cold spray.
LCS has been proven to be a viable method for the processing of metal coatings. Trials with a range of materials have shown that it is possible to deposit dense, low impurity content coatings with build rates in excess of those normally found in laser cladding when similar power levels are employed (25 g min-1 for 1 kW). The relatively low temperature process route has been found to reproduce many of the advantages of CS while the reduced operating costs should make this technology suitable for a wider variety of applications.
Andrew Cockburn, Matthew Bray and Bill O'Neill
Cambridge University
IMAGE: Screen shot showing the deposition of a titanium track
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