Making a clean break
Laser makers are receiving orders and increased interest in ultrashort pulse laser technology from consumer electronics manufacturers for cutting brittle materials like glass and sapphire. Cutting glass for mobile phone screens with ultrafast lasers has been recognised has having ‘big potential’, according to Jochen Deile, a product line manager at Coherent, but adoption has been slow up until this point. Now, however, Deile commented that Coherent has received its first significant orders for production equipment to cut glass for the next generation of consumer electronic products, adding that the technology is currently being implemented in production.
Other ultrafast laser suppliers report a similar trend of greater adoption of laser technology in the consumer electronics sector. French ultrashort pulse laser maker Amplitude Systèmes is working with mobile phone manufacturers, according to the company’s vice president of sales, Vincent Rouffiange, and over the past three years has been developing processes for machining transparent materials such as glass or sapphire with femtosecond laser pulses. Dr Herman Chui, senior director of product marketing at Spectra-Physics Lasers, now owned by MKS Instruments, commented that some ultrafast laser processes are already in use in consumer electronics production, but went on to say that adoption is not broad-based yet. ‘There’s a lot more potential for using ultrafast lasers in consumer electronics,’ he said.
The potential for ultrashort pulse lasers – those operating at picosecond or femtosecond pulse durations – to replace mechanical tools for cutting glass in consumer electronics is certainly there. At such short pulse durations, the energy put into the part with each pulse of laser light is huge; ultrafast lasers are capable of a process known as cold ablation, whereby material is ablated without putting any heat into the surrounding part, all because of the properties of the laser pulses.
Machining glass with an ultrashort pulse laser is slightly different to machining metal, and there are different ways of creating a cut in glass with a laser. One of these is filamentation, which Deile described as a ‘mature process’ and one that is being ‘adopted right now’ by consumer electronics manufacturers. Here the ultrashort pulses don’t ablate the glass, but modify it to create a filament inside the glass that serves as a weak point to make the break.
‘There are various laser cutting processes employed, but in terms of cutting screen glass, we think laser filamentation is the best process to do that,’ Deile said. Coherent-Rofin recommends using a picosecond laser, and offers its SmartCleave lasers for glass filamentation.
Mechanical methods, using a diamond-tipped saw to scribe and then snap the glass, followed by grinding and polishing to get the surface finish, are the predominant means of cutting screen glass for consumer electronics. However, these are limited to cutting in straight lines and they also generate glass particles, which is particularly damaging for clean-room environments in electronics production.
Sheets of glass can be cut with laser filamentation, a technique that is being adopted by consumer electronics manufacturers. Credit: Coherent
‘The main reason for using a laser is that you can eliminate process steps and therefore save money,’ explained Chui, referring to the laser’s ability to produce a very good finish, meaning that post-processing steps like grinding and polishing are not required to some extent.
Other advantages of using a laser over mechanical machining are that the laser gives greater flexibility, noted Deile. A laser isn’t limited to cutting in straight lines and can cut more complex geometries, a big plus for cutting the curved screens found on the latest models from a number of phone providers. ‘A laser process can bring a lot of benefits to cutting shaped glass,’ he said.
Laser filamentation is also flexible in terms of the material mix, Deile added. The laser can cut many different types of glass at different thicknesses, as well as stacks of glass with several layers of either the same or different material. It doesn’t generate dust or particles, because it’s a process that modifies the glass and then separates the glass along a filament. It also gives much higher yield compared to a mechanical process, as well as being more efficient – the laser consumes less power than mechanical processing tools.
‘A laser system can produce many more parts than one CNC machine, and one laser system can replace many tens of CNC machines,’ Deile stated. Mechanical processes also require water for cooling and separation, which brings complications – you have to recycle the water, clean and dry the part, all of which is not found using laser processing.
Choice of pulse duration
Picosecond (10-12 seconds) pulses are the best choice for filamentation, in terms of the interaction between the laser pulse and the glass, according to Deile. Longer laser-material interaction times are required in order to sustain the filament. Coherent-Rofin uses a burst mode when running its picosecond laser for filamentation, producing a burst of several pulses with a few tens of nanoseconds between them.
‘The type of laser used depends on the process quality needed,’ explained Chui. There’s also the laser wavelength to consider; a shorter wavelength can focus to a smaller spot and cut finer features. However, there’s a conversion loss moving to shorter wavelengths, so on a cost per watt basis the shorter wavelength is significantly more expensive, according to Chui. In general, shorter wavelengths are better, but economically it might not make sense. ‘A UV nanosecond laser will give a really tight cut,’ he said. ‘If the feature size is less important, but minimising heat affected damage is crucial, then you might go to infrared picosecond or infrared femtosecond.’
Amplitude Systèmes offers a femtosecond (10-15 seconds) process for cutting glass. It launched its Glass module at Photonics West 2017 in January, the result of three years’ work on machining transparent materials such as glass or sapphire with femtosecond laser pulses. The method Amplitude Systèmes employs is different to the standard ultrafast laser processing technique used on metals, and it also differs to filamentation. ‘Metals are much easier to understand in terms of ultrafast processing; with materials like glass and sapphire, the technique is based on non-linear absorption,’ explained Amplitude Systèmes’ Rouffiange.
Amplitude Systèmes’ research into the effects of ultrafast pulses on transparent materials was driven by industry requirements, largely from the consumer electronics sector, according to Rouffiange, for cutting strengthened and unstrengthened glass, as well as sapphire.
The company’s Glass module produces a narrow and elongated beam so that the pulse energy is absorbed in a very thin channel, typically 2µm wide, throughout the thickness of the material. The process doesn’t ablate material, explained Rouffiange; instead the high peak power of the femtosecond pulses produces micro-cracks inside the glass or sapphire and, by applying a small amount of stress, the sheet is split in a given direction.
Amplitude Systemes has developed its Glass module for cutting glass or sapphire with femtosecond pulses
The Glass module shapes the beam geometrically to deposit the same energy across the thickness of the material, as well as temporally by applying a sequence of pulses. In this way, the technique applies local stress inside the material to make the cut, which Rouffiange noted is low roughness and free from chips.
The femtosecond pulses produce a different effect on glass and sapphire than picosecond pulses, according to Rouffiange. Instead of producing local modification, or voids, the technique produces regular oriented and controlled micro-cracks. This gives a high-quality cut.
Glass and sapphire are transparent at 1,030nm wavelength. However, the high intensity of femtosecond pulses causes a multiphoton absorption at the focal point. The peak power of the pulse is therefore quite important, Rouffiange explained, which is why femtosecond pulses are used.
Amplitude Systèmes typically receives requests to cut glass a few hundreds of microns thick for consumer electronics, but has shown that its technique is effective at cutting glass 2mm thick. There needs to be sufficient energy deposition across the thickness of the material, and therefore the energy of the laser and the length of the elongated beam have to be adapted depending on the thickness of the material.
Amplitude Systèmes’ setup can be arranged so that either the beam is fixed and the sample moves, or, if the sample is large and cannot be manipulated, then the Glass module and laser can be moved on a gantry.
‘Our feeling is that ultrafast processing of glass could be implemented really soon in consumer electronics, because the process improves yield,’ commented Rouffiange, referring to Amplitude Systèmes’ technique. ‘In the next two quarters some mobile phone manufacturers could potentially implement these techniques in their production.’
Yield is important, but so is the speed of cutting, Rouffiange noted. Amplitude Systèmes can achieve 3m/s cutting speed with its femtosecond laser.
Work to be done
While ultrafast lasers are beginning to be adopted in consumer electronics production, it’s still early days for the technology. ‘There’s a lot of process development that has to happen to adopt laser technology,’ commented Chui. ‘It’s one thing to demonstrate that you can do the process and another to implement it in high volume manufacturing. There’s a lot of work on that front.
‘On the laser front, it’s getting the throughput up and the costs down,’ he continued. ‘For most of these applications, that’s the key thing. It’s already economical in terms of cost per throughput for some processes, and in other cases there needs to be more progress made.’
Deile from Coherent noted that ultrashort pulsed lasers are not the bottleneck in terms of throughput. ‘The bottleneck is really on the machine side; we cannot move axes, cannot manipulate the workpiece fast enough to take advantage of what lasers can do, in most cases,’ he said.
Femtosecond infrared lasers can now reach 100W, which is relatively high power – MKS Instruments demonstrated its Spectra-Physics Spirit 1030-100 at Laser World of Photonics in Munich in June, which can deliver femtosecond pulses at 100W of infrared power. ‘There’s research working at much higher powers, but the problem with going to powers higher than 100W is that, with the shorter pulses, you need to go to a higher repetition rate to produce the result,’ explained Chui. ‘You can’t put too much energy into a very short pulse at a slow repetition rate without causing damage, which is exactly what you’re trying to avoid.
‘The challenge is that the motion systems to handle the fast repetition rates have reached their limits, galvo scanners in particular,’ he added. ‘People are looking at other options, such as polygon scanners, to get faster scanning speeds.’
There are other machining steps in manufacturing consumer electronics where ultrashort pulse lasers can play a role, such as cutting or drilling small holes or features. These features typically need straight side walls, according to Deile, so a process called bottom-up ablation is used, whereby the angle of the cut edge can be controlled to create tapers.
Top-down ablation using ultrashort pulsed lasers is also possible and used for instance to produce a chamfer, to remove any sharp edges, so that the screen is less susceptible to fracturing upon impact. ‘This is a process that the industry is also looking to adopt to replace mechanical polishing and grinding,’ Deile remarked.
‘If you look at how mature these processes are and the state of adoption into production, then filamentation is a mature process and is being adopted right now,’ Deile added. ‘Bottom-up ablation is also being adopted at the moment, while top-down ablation for chamfering is still currently under development.’