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Catering to the customer

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Matthew Dale learns about the latest demands being made of laser micromachining technology

A complex dimple texture applied to the curved flute surface of a drill. (Credit: Lightmotif)

Technology innovation is largely influenced by the demands of the customer. Such demands are leading to the emergence of increasingly complex products offering smaller footprint, lighter weight and higher functionality. Through speaking with multiple laser micromachining firms, I recently gained some insight into the ever-increasing list of demands being made of their technology when manufacturing complex products, and learned how the development of new laser micromachining systems featuring ultrafast lasers is enabling these demands to be addressed.

Customers want customisation

According to Dr Dimitris Karnakis, technical manager for R&D projects at Oxford Lasers, a UK-based provider of laser micromachining systems and services, one of the main manufacturing trends being seen right now is mass-customisation, where customers are looking to produce many different components using many different materials.

‘The complexity in design for manufacturing has increased, with more customisation now being required to meet market needs,’ he confirmed. ‘This customisation not only encompasses customers wanting different features on different parts, but also different features on the same part – you might now be required to cut, drill, mill and texture different areas of it.’

To make matters more complicated, Karnakis added that such customers are also wanting to produce parts using lots of different materials, meaning that in order to machine certain features – for example microholes – the laser beam is often required to go through multiple layers of different material, each layer requiring different laser parameters to process.

In addition, these customers also want to be able to produce these features with increasingly higher throughputs and increasingly smaller sizes, all the way down to the sub-10μm level, according to Karnakis – who remarked that this is actually approaching the optical resolution limit of what lasers are able to achieve.

Thankfully, in recent years ultrafast lasers have proven to be incredibly versatile tools capable of addressing most of these demands. Their ultrashort pulses can be used to deliver energy to the workpiece with such precision that they are able to ablate minimal amounts of material and produce microscopic surface features without causing damage to the surrounding material. In addition, their ability to couple energy into materials through non-linear optical absorption enables them to machine almost every type of material, making them an incredibly flexible manufacturing tool.

A pillar texture laser machined onto a curved surface. (Credit: Lightmotif)

The ultrafast sources used by Oxford Lasers, which are mostly used in micro-milling, micro-cutting, micro-drilling, and surface texturing applications, are diode-pumped solid-state lasers operating at wavelengths ranging from the ultraviolet to the infrared, and at average output powers up to 100W. As described in the Autumn issue of Laser Systems Europe, such high average powers are increasingly being considered for ultrafast laser systems, even though most of the materials processing applications they can be used for can already be achieved using 10-20W average power. This is because, by using innovative optics, the beams of higher average power ultrafast lasers can be split into multiple, lower-power beams, enabling larger areas of a workpiece to be processed quicker, or for multiple workpieces to be worked on simultaneously. This increases processing throughput dramatically, addressing one of the main customer demands highlighted by Karnakis.

New application: Micro-welding of dissimilar materials

A new micromachining application currently under development that will be taking full advantage of the capabilities of ultrafast lasers is the micro-welding of dissimilar materials, more specifically the direct bonding of transparent glass or crystalline substrates to metals.

The application is being developed within the Innovate-UK-funded project ‘Ultraweld’, which Oxford Lasers is currently coordinating with several academic and industrial partners: Heriot-Watt University; Centre for Process Innovation; Gooch & Housego; Coherent Scotland; Glass Technology Services; and Leonardo UK. The project is due to finish in June 2020.

‘So far we have successfully welded glass to glass and glass to metal using ultrafast lasers, soon we will be exploring the welding of glass to silicon and glass to ceramic,’ said Karnakis.

He continued by explaining that to perform the process, ultrashort laser pulses are guided through the transparent material towards the point where it meets the metal, where they then deliver a controlled heat input to join the two materials without the need of an interlayer between them. ‘Ultrafast lasers are key here as they precisely control heat input and keep the associated melting to a minimum, avoiding thermal stress related damage,’ he noted. ‘This protects the materials – particularly those which are brittle – so they don't break after the laser weld has taken place without compromising the bond strength.’

Micro-drilled 5μm holes in a tube with 1.5mm outer diameter (Credit: Oxford Lasers)

According to Karnakis, there is a clear demand for the micro-welding of dissimilar materials in aerospace, microelectronics, vacuum technology and other sectors that require precision bonding or hermetic sealing for device manufacturing.

‘For example, one of our partners, CPI, is an OLED manufacturer interested in micro-welding for hermetic sealing in order to reduce the cost of device encapsulation by removing or minimising the need for high-value materials and manufacturing steps,’ he said. ‘We have also seen some interest in this from manufacturers of glass micro-electromechanical systems (MEMs).’

Within Ultraweld, Oxford Lasers was tasked with building a dedicated laser system prototype that will demonstrate micro-welding in an industrial environment for many different material combinations. Having now developed the system, the firm is currently demonstrating it on both high-value aerospace and flexible electronics applications. Once the project is completed, a commercial laser system capable of micro-welding will be available from the company for those who want to explore this application.

I've worked on a lot of development projects in my life, and this is the one project that stands out where from very early on we have experienced a lot of commercial interest,’ commented Karnakis. ‘Many people are ready to use the new micro-welding system now that it has become available. We believe this is going to have considerable uptake in industry and provide several tangible benefits to our customers.’

Surface texturing: Large future market potential

Another laser micromachining application currently gaining increasing interest within industry is surface texturing, which involves the very precise ablation of materials to create surface structures – such as tiny indents or pillars – on the micro scale. These structures can be used to change the physical properties of surfaces, such as their coefficient of friction, or the way they interact with water or light.

‘This is a micromachining application with large future market potential, and is the one that I am personally most excited about,’ remarked Max Groenendijk, CEO at Lightmotif, a Dutch manufacturer of ultrafast laser micromachining systems.

According to Groenendijk, multiple companies are currently in ongoing development projects in which they are learning about what laser texturing is able to offer them, enabling them to decide which textures will best suit their applications. ‘In the next few years however, I believe we will see these textures introduced onto real parts being sold commercially,’ he said. ‘They will even be producible on complex parts, as a lot of the development we’ve done over the past 10 years has been towards getting these textures onto curved surfaces. If these textures are used on injection moulds, for example, they could then be applied in mass production.’

A texture machined into a hard ceramic material. (Credit: Lightmotif)

Surface texturing has previously been achievable using nanosecond lasers, for which texturing injection moulds for parts such as vehicle dashboards has already been an established application in the automotive industry for a number of years. Groenendijk noted that automotive is the largest market for nanosecond laser texturing, and that while ultrafast lasers could be used to produce such textures with even higher precision and at smaller scales, ‘the difficulty here is that due to the size of the parts involved you would need a really large machine, and although improvements are welcome, they are not allowed to cost much more in this market.’

Lightmotif is therefore first focussing on the texturing of smaller parts with its machines, such as devices for the medical industry, where the functional surface properties offered by texturing are becoming increasingly sought after, according to Groenendijk – such as for optimising the surfaces of implants and electrodes. Sectors such as this, where texturing hasn’t previously been used and can add a lot of value, are where most of Lightmotif’s current customers are.

Adaptive micro-milling

In addition to surface texturing, another application currently looking very promising for Lightmotif is micro-milling, particularly for applications creating very accurately shaped tools – for example stamp tools or coining tools for industrial metalworking, which are often made of very hard materials such as cemented tungsten carbide.

‘These tools often need to have very intricate patterns or shapes, and here the laser has proven to be a very good solution,’ said Groenendijk. ‘Up until now these stamping tools have had to be produced using electrical discharge machining (EDM), which was the only process that was previously able to make them with the required smoothness and accuracy. This was a multi-stage, very lengthy – over six hours – and very expensive process.’

A stamp tool being machined by laser micro-milling. (Credit: Lightmotif)

While many laser companies have attempted to address micro-milling for tool production with their technology, Groenendijk noted that a lot of them have failed, due to them not being able to achieve the required high level of accuracy. Lightmotif was able to overcome this issue, however, by developing a process called ‘adaptive micro-milling’.

‘Although machining with ultrashort pulse lasers is an accurate and reproducible process, variations in the process conditions can lead to depth errors of a few per cent of the machined depth,’ said Groenendijk. ‘For structures that are hundreds of micrometres deep these errors may become significant.’

In adaptive micro-milling, integrated sensor technology is used to periodically measure the depth of laser processing and automatically correct for any deviations that occur, enabling depth accuracy to be improved significantly.

‘We started developing this technology in 2015 as part of an EU project conducted with Fraunhofer Institute for Production Technology, Lumentum, multiple sensor system manufacturers and two end-users,’ said Groenendijk. ‘In the past year however, we have brought that to a level where it can be integrated in a production machine.’

Lightmotif’s OP2 5-axis micromachining system applying a micro-texture to a curved mould insert. (Credit: Lightmotif)

Such a machine will be brought to market next year by KLM Microlaser, a new company established by both Lightmotif and Kern Microtechnik, a manufacturer of high-end machine tools and one of the companies with which Lightmotif developed adaptive micro-milling. Groenendijk believes that those currently using EDM to make accurately shaped tools will switch to laser processing in the coming years, and expects this to be a considerably sized market.

The two firms have also seen interest from the market in combining ultrafast laser processing with conventional milling using machine tools, which according to Groenendijk is much faster than milling with lasers.

‘The removal rates of conventional milling are so high that you could never do it with a laser,’ he remarked. ‘People are talking about bringing multi-kW ultrashort pulse lasers to the market to speed up milling applications, however to be honest I don’t see this being successful. It’s already difficult to achieve such a process with a 10-20W laser.

‘However, lasers can be used to create features with very intricate sizes and curved shapes, which is very difficult to do with machine tools.

Kern Microtechnik and the newly established KLM Microlaser are therefore looking to combine their expertise to develop a complete solution that combines the speed of conventional milling with the precision of ultrafast laser processing. ‘We already see interest in the market for such combinations, for example in the automotive industry,’ said Groenendijk.

Overall, for conventional milling – rather than high-precision micro-milling – Groenendijk believes that machine tools won’t see much competition from laser technology. He explained that lasers will however offer new possibilities when very hard materials have to be milled: ‘Here the [machine] tools tend to be really expensive, you’ll spend hundreds of euros for just one mill, and then you have to discard the tool after machining just one part. Here is where lasers will provide a big advantage compared to conventional milling.’

Ultra-thin glass cutting for flexible displays

This year saw the introduction the foldable smartphone to the consumer market – devices with tablet-size screens that can be folded down into the standard footprint of a mobile phone.

In order to be able to fold, rather than being made of glass – as is the case with standard smartphones – the screens of these new foldable smartphones have to instead be made using a flexible polymer material. Compared to glass however, this material is much more susceptible to scratching – to the point where it can be marred with just a fingernail – meaning the polymer screens of foldable smartphones will be much less durable in the hands of the general public than their inflexible glass counterparts.

This may not be an issue for long, however, as according to The Electronic Times1, Samsung – whose new ‘Galaxy Fold’ devices use a polymer screen – has decided to use ‘ultra-thin glass’ for the screen of its next foldable smartphone, which will likely be released during the first half of next year.

According to Rene Liebers, product manager for display and smart glass technologies for laser micromachining firm 3D-Micromac, ultra-thin glass is a flexible material between 25µm to 100µm thick, which in addition to enabling the foldable and rollable functionality of future display technology, could also open up a range of new opportunities in the production of sensors and OLED lighting devices.

Flexible ultra-thin glass could enable rollable and foldable functionality in future display technologies (Credit: 3D-Micromac)

He explained that during the manufacture of products featuring ultra-thin glass, multiple resizing processes must take place: ‘This includes sheet cutting from rolled raw glass, cutting to standardised sheets/formats directly after a drawing procedure and – product depending – a final shaping, often called “dicing” or “shaped outline cutting”. This final resizing poses an additional challenge due to several coatings and microstructures that are applied to the glass surface before cutting.’

Through several research projects, 3D-Micromac was able to optimise ultrashort pulse laser technology for such ultra-thin glass cutting applications. ‘The use of ultrashort pulsed lasers ensures an efficient and stable separation performance,’ remarked Liebers. ‘It is a suitable method that can achieve high productivity, a high level of automation for high production volumes, and stable edge quality throughout 24/7 production.’

As a result 3D-Micromac now offers its microSHAPE system for the cutting of ultra-thin glass, which is able to process ultra-thin glass substrates at thicknesses down to 30µm.

[1] http://english.etnews.com/20191016200001

Case study: High pulse energy femtosecond laser with tunable GHz and MHz burst for efficient material processing

Ultrashort laser pulses have been well known as an attractive tool for material processing for more than 20 years now. Femtosecond pulses in particular provide a confined laser-material interaction, minimal heat-affected zone and melt-free treatment, leading to advanced processing precision. Leading laser manufacturers such as Light Conversion are continuously working on improvements to further enhance the efficiency of laser micro-machining in terms of cost of ownership, flexibility and tunability.

For applications where the laser is working constantly, 24/7, the processing rate is the most critical characteristic. A recent study on improving material removal rates has attracted high scientific and industrial interest1. It was shown that high ablation efficiency and quality can be reached by using a burst regime. Having tunable bursts allows the process to be optimised even further. A special burst regime (called BiBurst) allows burst-in-burst operation. It was developed by Light Conversion and is patent pending. The femtosecond lasers PHAROS and CARBIDE can generate bursts with different numbers of pulses within a configurable intensity envelope. If sub-pulses within the burst have equal amplitudes and their durations are the same, regardless of the interval between them, then this produces an effective GHz and MHz laser repetition rate. Many experiments demonstrate that given a specific set of process parameter settings, the micro-machining throughput can be an order of magnitude higher when using burst modes as compared to even the best results of the conventional kHz case. Excess thermal damage is also evident in some cases, which can also be useful in special applications such as glass-to-glass welding.

 

Figure 1: GHz burst pulse train envelope in different configurations. From left to right: declining, quasi-flat, inclining.

Configurable burst pulse train envelope (from declining to quasi-flat to inclining – see Figure 1) is useful for finding the best recipes for polishing, brittle material drilling, cutting, deep engraving, selective ablation, transparent materials volume modification, welding, hidden marking and surface functional structuring.

The study of copper ablation (see Figure 2) by laser pulses at 64.5MHz intra-burst pulse repetition rate revealed that, in the best case, the ablation efficiency could be improved by at least 20 per cent compared to the single-pulse regime for similar beam-size-optimised regimes2. All other numbers of pulses per burst demonstrated lower ablation efficiency. To our best knowledge, with the beam-size-optimised and three pulses per burst processing, we have achieved the highest ever published laser milling ablation efficiency of copper by ultrashort pulses − 4.8μm3/μJ. Another advantage of burst mode compared to the single-pulse regime ablation was lower surface roughness of the bottom of the ablated cavities. The lowest surface roughness achieved by the single-pulse regime was several times higher than that measured for burst mode. In the range of pulse duration between 210fs and 10ps, the ablation efficiency increased by 32 per cent for longer pulses, and micromachining quality improved. In conclusion, the usage of bursts of pulses for laser micro-processing of copper is advantageous only when three pulses per burst are used – the ablation efficiency and quality are increased compared to the single-pulse regime.

Figure 2: Photo of 3D milled sample. The maximum ablation efficiency of 4.8µm3/µJ and ablation rate of 176µm3/µs (10.5mm3/min) was obtained, with the average optical power of 36W, three pulses per burst. The quality of the processed surface at the optimal processing parameters for the highest ablation efficiency was evaluated by measuring the surface roughness Ra, and it was 0.4µm. Inset: SEM image.

Burst modes were also shown to be more efficient for micromachining through-holes. The MHz case is the best setting overall, the GHz case is not as efficient as the MHz regime, though still generally improved (in terms of ablated volume per unit of energy) compared with a typical 100kHz femtosecond laser. We attribute these differences to changing material properties over time during multiple pulse impingement, as theoretically investigated by multiple parties. The decrease in fabrication efficiency for the GHz case is believed to be caused by unwanted beam-plasma interaction effects (reignition and scattering). Overall, this new type of laser looks like a promising tool for material micromachining and could possibly open new frontiers for faster micromachining while maintaining a similar micromachining quality. There are already several PHAROS laser systems in the UK utilising this innovative BiBurst feature, all having been installed and fully supported by Photonic Solutions Ltd.

[1] S. Butkus, D.Paipulas, M.Barkauskas, K.Neimontas and V.Sirutkaitis “Comparison of GHz, MHz and kHz Femtosecond Burst Mode Micromachining of Invar Foils” Proceedings of LPM2017 - the 18th International Symposium on Laser Precision Microfabrication

[2] A.Žemaitis, P.Gečys, M.Barkauskas, G.Račiukaitis and M.Gedvilas, "Highly-efficient laser ablation of copper by bursts of ultrashort tuneable (fs-ps) pulses", Scientific Reports, volume 9, Article number: 12280 (2019).

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