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Uniting the laser processing value chain

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The European Appolo project, which aims to establish connections within the industrial laser processing ecosystem, is drawing to a close. Gediminas Raciukaitis, head of the department for laser technologies at the Center for Physical Sciences and Technology in Lithuania, updates on the outcomes

The Appolo Hub is a research-oriented network where industrial partners can come to test, assess and implement novel laser micromachining solutions for their markets. The Hub is part of the European Appolo FP7 project, which began in September 2013 and is made up of 36 partners with laser application laboratories in Switzerland, Spain, Germany, Finland and Lithuania.

Its activities include technical, technological and economical assessment of new equipment supplied by project partners in 15 complex assessment value chains. Below are some of the outcomes.

Laser scribing in thin film solar cells

One of the main activities in Appolo was validating various approaches for laser scribing in thin film solar cells. Measurement of electrical properties of solar cells before and after laser scribing is the usual way to evaluate the impact of laser processing, but various effects influence the final electrical measurements, which can therefore be misinterpreted. New methods of inline electrical characterisation of thin film solar cells during laser scribing were developed by the Leibniz Institute of Surface Modification (IOM) and the Center for Physical Sciences and Technology (FTMC) in Lithuania.

Inline monitoring methods were designed by Lappeenranta University of Technology (LUT) in Finland and Amsys, and evaluated at FTMC for high speed and precise inline monitoring of laser scribing in CIGS solar cells. The technique used galvo and polygon scanners running at 50m/s.

The assessment of optimised high-throughput scribing processes for industrial patterning of CIGS solar cells and a suitably tailored laser system are also being investigated. Several types of validation experiments were made by Bern University of Applied Sciences (BUAS) using various lasers. High throughput scribing processes developed in Appolo were validated against the BUAS reference processes. Electrical performance of modules, made using the optimised high throughput P2 scribing process at 1,720mm/s, was comparable to the performance of the reference samples processed with a round focal spot at 90mm/s.

Several series of functional modules on a flexible substrate were produced and compared directly with reference modules.

Perovskite photovoltaics is a hot topic in the PV community. Perovskite-based devices not only allow potential low cost and simple manufacture of solar modules, but the technology can also be added to existing PV concepts like c-Si and CIGS to increase efficiency without substantially increasing production complexity. In order to allow fast implementation, a reliable and fast monolithic interconnection concept needs to be developed.

In a first step, studies of the performance of laser scribing with ultrashort laser sources were performed. To ensure that the laser scribes are made without damaging the material, a common protocol of best practice for handling, processing and storage of these perovskite samples was developed and implemented. A mini-module design, including an adapted fabrication process, was developed by IOM in collaboration with Swiss institute EMPA.

The current module efficiencies exceed 10 per cent and are almost comparable to their small reference cells, confirming the high quality of picosecond laser scribing.

The use of picosecond lasers for metallisation in flex 3D microelectronic devices was also validated, in this case with the full metallisation of a CIGS solar cell on steel flex substrate with fingers and busses deposited by laser-induced forward transfer (LIFT). The assessment showed the possibility of LIFT printing a silver grid on the commercial screen using picosecond lasers. The assessment demonstrated that lines can be made using fixed lens optics, a conventional galvoscanner, and a high-speed polygon scanner with a high aspect ratio.

The LIFT process was successfully validated by Universidad Politecnica de Madrid (UPM) for freeform front contact grid printing with picosecond lasers on silicon solar cells. The full metallisation of a front grid has been performed to check if it is suitable for printing on large areas.

High-performance laser texturing

The production of advanced packaging designs, as well as specialised moulding applications, for example, is an extremely fast-growing market. Printing and embossing rolls for packaging is now a standard technology, and engraving the rotating cylinder is a key element in modern industrial printing and moulding.

Laser texturing of steel was investigated at Bern University of Applied Sciences with average laser power exceeding 100W. The ablation process is scalable up to the several-hundred-watts regime, but is limited by plasma shielding and heat accumulation – reducing the overlap and enlarging the marking speed lowers heat accumulation – which leads to bumpy surfaces.

Dedicated nanostructures on moulds

Use of picosecond lasers in texturing moulds for automotive interior parts offers freedom of design. Picosecond lasers can texture surfaces with features in the micro and nanoscale, and such surfaces exhibit functional properties like superhydrophobicity, anti-glare, soft-touch and others. Laser texturing is also more environmentally friendly compared to traditional chemical etching, and it means new functional properties can be applied to surfaces.

Lightmotif and Centro Ricerche Fiat (CRF), a research arm of Fiat, have been co-operating with the goal of obtaining a method for producing polymer parts for car interiors. The parts have added functionality thanks to the micro- and nano-textured surface, achieved with a textured mould. The surfaces show a soft-touch effect that reduces the contact area of the skin with the polymer.

The visual appearance of the surfaces was improved step-by-step and finally resulted in a perfectly homogeneous surface texture. For automotive applications, further optimisation is needed. However, developments made within Appolo have resulted in

Laser-based selective metallisation of 3D-shaped polymers

Finally, a technology for writing electronic circuits directly on to polymers by modifying surface properties with a laser has been developed as part of the Appolo project. The technique reduces processing costs by at least three times compared with the current technology used in industry.

The Selective Surface Activation Induced by Laser (SSAIL) technique was developed at the Centre for Physical Sciences and Technology (FTMC) in Vilnius, Lithuania. Demonstrators were built by partners from CRF, BioAge and Elas. The new technique for selective surface plating can be applied to conventional plastics without any special additives, an advantage over the current technology, laser direct structuring, which uses additives mixed within the polymers. The additives increase the price of the raw plastic material by three to five times, meaning the SSAIL technology lowers processing costs by at least three-fold.

SSAIL is a three-step process: the first is surface modification by laser; second is chemical activation of modified areas; and the last step is metal deposition by electroless plating. The technology offers laser writing speeds of up to 4m/s, and therefore spatial plating pitch is kept narrow at 25µm.

Equipment validation

Laser companies Ekspla, Onefive, Lumentum and Sisma provided updated versions of their ultrashort pulse laser sources for testing at the Appolo Hub. Lithuanian firm Ekspla has developed a new concept of high average power ultrashort pulsed laser, a high repetition rate laser emitting picosecond pulses at 1,342nm. The company also demonstrated its Atlantic picosecond laser, which became the main working force in the Appolo Hub equipment pool. The laser offers 60W output power at 1,064nm and, with a repetition rate up to 1MHz, it is a good choice for industrial, high throughput material processing. It was demonstrated with thin film scribing, surface texturing and the LIFT process at different research laboratories within the Appolo Hub.

Lumentum and Onefive submitted systems for verification, Lumentum’s a 100W range 1ps laser for high efficiency surface texturing, while Onefive’s Genki model is a high pulse energy fibre laser.

Next Scan Technologies also supplied updated versions of its polygon line scanner with reduced focused beam spot and increase 
line length. The polygon scanner can operate at speeds of 100m/s.

Lithuanian system builder Elas upgraded its multifunctional laser processing system DuoMaster. The whole optical setup was reconstructed based on feedback from the assessment experiments. Lightmotif progressed with developing its laser texturing system, and the technology used for texturing moulds was transferred from laboratory-scale setup to a real industrial machine. Mondragon Assembly prepared a prototype for the LIFT process, meanwhile OSAI Automation and Sisma are working on integrating new processes into their machines.

Further results within the Appolo project will be presented during Laser World of Photonics in Munich in June. For more information, see: www.appolo-fp7.eu and www.appolohub.eu

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