Foldable smartphones such as the Samsung Galaxy Fold and Huawei Mate X began hitting the market in 2019. The devices offer the screen size of a small tablet, while being able to fold down to the form-factor of a standard
smartphone.
This functionality, however, was achieved at the expense of screen durability, as the standard smartphone cover glass had to be replaced with a polymer that, while flexible, proved more susceptible to damage such as
scratching.
Last year saw the release of Samsung’s Galaxy Z Fold2, which the firm claims has a foldable screen made from flexible ultra-thin glass.
While this is factually correct, the screen still requires a polymer cover – pre-installed on the device – to protect the glass from damage. This means the user is still required to interact with a polymer surface that scratches easily.
According to René Liebers, product manager for display and glass systems at laser micromachining integrator 3D-Micromac, the demand is therefore rising for durable ultra-thin cover glass of around 30μm in thickness (smartphone cover glass is typically 450 to 550μm thick) that can function independently and substitute this glass-polymer sandwich. While this cover glass is available, due to its high cost it is not yet suited for mass production or widespread use.
‘The thermal, optical, chemical and mechanical properties of ultra-thin glass make it a much more suitable cover material for flexible displays than polymers or glass-polymer stacks,’ said Liebers. ‘It also grants enjoyable haptics, which is, of course, relevant for touch-screen devices.’ In addition to folding smartphone technology, flexible ultra-thin glass could be used to form displays that blend seamlessly with vehicle interiors, following the curves of a car dashboard, for example.
However, despite being a suitable cover material for flexible displays, ultra-thin glass is still very fragile and has limited flexibility, which makes it challenging to manufacture and handle.
Available manufacturing methods
There are a number of techniques manufacturers currently use to process standard smartphone glass, the high rigidity of which makes it easy to handle and cut. Classical scoring and breaking processes using diamond tips or wheels work well, as do typical grinding and polishing post-processing steps used to provide the necessary finish.
For fragile ultra-thin glass, however, such mechanical techniques do not suffice. Not only do these methods require the application of force and produce particles – both of which are detrimental to ultra-thin glass – but they are also limited in their ability to create curved edges. This means they could only be used to produce traditional square or rectangular displays, rather than futuristic flowing displays for vehicle interiors. In addition, edge polishing or grinding is not feasible for ultra-thin glass, due to its low mechanical stability.
Ultra-thin glass with thickness below 30µm cut using a USP laser. (Image: 3D-Micromac)
Other processing techniques glass manufacturers use include those using laser technology, which of course removes the possibility of mechanical tool wear. Crack-based thermal laser separation using CO2 lasers is a well-established technique. However, similar to the mechanical methods it is also limited in its curvature processing, according to Liebers. Another alternative is ablative laser cutting using UV lasers, which, while being able to provide good curvature and edge quality, has a very low throughput, in addition to producing particles. The fragility of ultra-thin glass, in combination with the limited freeform capability of the above processing methods, will therefore prevent them from being used to manufacture futuristic displays.
‘Fully-integrated, nearly invisible displays made from ultra-thin glass are the future,’ said Liebers. ‘The display unit itself can stay rectangular, but the touch unit and cover has to be more flexible in its shape, so freeform
cutouts become increasingly important for the cover material.’
A new era for freeform glass processing
Thankfully, in recent years ultrashort-pulse (USP) lasers and their associative beam delivery technologies have emerged and matured.
These technologies are defined, as their name suggests, by their extremely short pulse duration in the range of picoseconds (10-12s) all the way down to hundreds of femtoseconds (10-15s). At such short durations, the energy delivered to the workpiece with each pulse is huge. Despite this, a very small heat-affected zone is achieved due to almost no heat being generated in the surrounding material. This capability makes USP lasers the perfect solution for processing ultra-thin glass, via a process known as filamentation. Here, the ultrashort pulses do not ablate the glass, but rather modify it to create a filament inside the glass that serves as a weak point to make a clean break.
‘In general, it is very hard to find a comparable technology to filamentation, especially in terms of heat input and thermal side effects,’ remarked Liebers. ‘The process has numerous advantages, including the ability to create freeform shapes and cutouts. A high-end edge quality is achieved at cutting speeds of up to 1,500mm/s, without the introduction of tension, particles or the need for additional post-processing.’
Scanning electron microscope image of ultra-thin cover glass cut using a USP laser. (Image: 3D-Micromac)
The mechanics of ultra-thin glass processing using USP lasers are detailed in a white paper recently authored by Liebers. Meanwhile, he shared with Laser Systems Europe how established the technology is for display manufacturing, as well as the developments that could potentially drive its success further in these applications.
‘Currently USP lasers are already well established at display manufacturers, both in 24/7 production as well as R&D,’ said Liebers. ‘Yet there is still room for more uptake. The main issues are the cost of the technology and
the necessary process knowledge required, when using it to develop new products.’
There is exceptionally high potential for USP laser processing in industry, with numerous laser sources and beam delivery optics already available on the market.
However, as Liebers made clear, the investment cost still needs to come down for the technology to experience broader adoption. In addition, there is still a lack of well-educated engineers that can use the technology effectively in new applications. ‘The necessary knowhow is very specific,’ said Liebers. ‘Integrators, such as 3D-Micromac, currently have to put large amounts of effort into process development for increasingly complex tasks during the customer evaluation period for this technology.’
When 3D-Micromac provides a USP laser system to display manufacturers (the firm has supplied some of the biggest names in display manufacturing), it sells its expertise in addition to the equipment.
‘We have the knowhow to process ultra-thin glass, as well as the tools required,’ Liebers confirmed. ‘The customer doesn’t just buy a laser machine, but a complete solution, including the ideal settings for processing their materials. In addition to performing the integration, we give the customer basic training that tells them how to use the machine and how to optimise it for further projects or products.’
The laser cutting system microSHAPE, by 3D-Micromac, is perfectly suited to cutting ultra-thin glass using a USP laser source. (Image: 3D-Micromac)
This training can only go so far, however, and never equates to the many years required for 3D-Micromac to teach its employees the required knowhow for USP laser processing. This makes matters challenging for the customer, when looking to reconfigure their systems for future applications. For display manufacturers in particular, this is almost certainly inevitable, as the lifecycle of their products is typically one to two years.
‘As initially they are buying a specialised tool with specific process knowhow, if they then need to optimise the tool for a different application in the future, they need to bring the knowledge in-house by educating their workforce,’ said Liebers. ‘In addition to teaching them about the laser, they’ll need to teach them the fundamentals of laser-material interaction and the many variations of dedicated optics that can be used to influence the process. Companies will therefore have to invest a lot of money and time into growing their employees from a starting point up to being a specialist.'
Unfortunately, it is a combination of this high investment in both equipment and education that deters customers from adopting USP lasers as part of their production process.
‘It is often an issue that a laser machine is not going to a glass manufacturer because they don’t have the money to invest in an engineer to run it,’ confirmed Liebers. ‘For them it’s necessary that the machine is very intuitive and easy to use.’
A potential solution could be embedding the specific processing knowhow in the machine software, in order to make it easier to use by employees who aren’t as knowledgeable in USP laser processing. This is far easier said than done.
‘They would want to be able to select options for ultra-thin glass or thick glass from a database, and then have the machine automatically configure to be able to process those materials,’ said Liebers. ‘This could be a good solution to establish USP laser processing in industry to a further degree – in addition to the initial investment cost coming down – but, it is very challenging to put this knowhow into software form. This has been done for other laser processes, but further work is required before this can be done for USP laser processing.’
It is certainly clear that USP lasers will be the go-to processing tool for ultra-thin glass once the material reaches the required cost level for use in the mass production of consumer displays. However, the barriers of high investment cost and required process knowhow must be overcome to encourage further adoption of USP laser processing, while ensuring its longevity.