FEATURE
Issue: 

See-through lasing

Greg Blackman on the laser technology for cutting toughened display glass, as well as the various techniques for glass engraving

Cracked screens could be a thing of the past if smartphone manufacturers adopt sapphire to cover touchscreen displays. Apple, which already uses the material as a protective cover for the camera lenses and home button on its latest iPhone model, announced in November a supply deal with GT Advanced Technologies for growing the gemstone. GTAT will receive $578 million from Apple to build its production capacity of sapphire at its factory in Mesa, Arizona.

Sapphire is scratchproof and difficult to break, which makes it appealing as a cover glass for electronic devices like mobile phones. However, because it’s so hard, sapphire is difficult to process mechanically, requiring a diamond-tipped saw to cut. Non-contact laser dicing is, therefore, an attractive alternative for processing the material.

The same challenges apply to dicing toughened glass, called Gorilla Glass, which cover the displays of most of the smartphones made at the moment, and, again, laser processing is an excellent alternative to mechanical means. Frank Gäbler, director of marketing at laser manufacturer Coherent, commented: ‘There’s an interesting market for laser cutting the cover glass for flat panel displays. The cover glass is now very thin, ranging from 400 to 700µm thick, so traditional cutting methods don’t work any longer. Mobile phone manufacturers are investigating alternative methods to cut strengthened glass.’

At the end of 2012, Coherent bought Lumera Laser, which produces picosecond pulsed lasers. These lasers are part of machines from German company InnoLas Systems made specifically for cutting brittle materials like chemically strengthened display glass and sapphire. The machines use filament cutting developed by Filaser (Portland, Oregon). This is a type of ultrafast laser processing, whereby picosecond pulses are focused to a break line within the material to make the cuts. The process is fast – Filaser claims a throughput of more than 500mm/s – it has a low cut face roughness, meaning there is very little post-processing required, and the glass is cut without generating any microcracks, which means the bending strength is high (Filaser claims more than 150MPa).

‘In the past, CO2 lasers have also been used for cutting cover glass,’ remarked Gäbler. ‘CO2 lasers are relatively low cost, do the job, and are fast and reliable. The downside is that they can only cut straight lines. In the past, the straight cuts were made with a CO2 laser, while the corners were cut by another method, which wasn’t ideal. The new filament cutting technology is able to cut straight lines as well as curves.’

Gäbler says mobile phone manufacturers and display manufacturers currently use ultrafast laser technology for cutting display glass. Standard ultrafast laser processing ablates the material using multiple passes to produce a cut. Here, the laser is focused onto the surface to ablate the glass. However, an ablative process creates microcracks and is quite slow, Gäbler commented. Any microcracks need to be removed by a secondary polishing process.

Scientists at the Fraunhofer Institute for Laser Technology have been investigating the process to potentially fine tune the strategies for cutting glass with picosecond lasers. The researchers used a Trumpf picosecond laser, which, in the experiments, delivered a peak pulse energy of 40µJ at 10ps pulse duration. The work suggested potential mechanisms of why ultrafast lasers sometimes cause damage in the glass and it is hoped the results will be used to improve the process, both in terms of quality and speed.

Making a mark

When it comes to marking glass with a laser the process is different, as here the idea is to create a certain amount of fracturing within the glass to make the mark. The degree of fracturing, however, has to be controlled to avoid weakening the glass and to produce a legible mark.

CO2 lasers at 10.6µm are ideal for marking glass, because the material absorbs at this wavelength. David Earl, technical sales engineer at UK laser supplier Laser Lines explained that, by careful selection of laser parameters, glass fracturing can be controlled to achieve a high-quality mark: ‘It  depends on what effect you are looking for and the type of glass you are marking. If you’re looking to mark a high-definition image, then you’ve got to try and create a fairly small spot and a localised crack , which is not going to create thermal stressing.’

Laser Lines is a distributor for Synrad, which manufactures the 48 series, Firestar and Fenix Flyer CO2 laser markers, all ideal for marking glass. A 30W laser with an M2 value of around 1.2 would be ideal for this application.

Synrad has researched laser marking glass and has identified three methods to control the amount of fracturing caused by the laser radiation. Firstly, the mark can be made using multiple laser passes. This heats the material adjacent to the mark and forms a stress gradient to reduce the chances of secondary fracturing. The method is effective for marking soda lime and borosilicate glass.

A second method is by forming a series of ring fractures, created by the heating and cooling cycle using discrete laser spots. The method works well with common optical materials and tempered, chemically strengthened, or plain soda lime float glass. A different take on this method uses a larger spot size to produce a crazed surface fracture. It works best with high-quality glass.

‘It’s [glass marking] not a given science, as it depends on the type of glass and the quality of the glass,’ said Earl. The laser parameters for branding drinking glasses with a logo, for example, would be different to how a data matrix code would be engraved on a windowpane. ‘Because a windowpane is toughened, the glass tends to reflow rather than crack,’ Earl continued. ‘Whereas marking a soda glass or a soda-lime-type glass used for bulbs and tableware, you would get small fractures rather than a reflow.’

One way a mark can be created is by laying down single dots in a raster style. ‘It’s about getting the spacing and spot size of these dots right that give the image resolution,’ Earl noted. ‘But as you go up in spot size, you’ll get a different effect. A large spot size will give a frosting effect, whereas a small spot gives a very fine discrete cracked spot in the glass.’

Laser Lines has customers marking nucleation points onto the bottom of beer glasses or champagne glasses to encourage bubbles to form. Earl noted that the lens used in the system has to have a sufficient focal length to focus the beam onto the bottom of the inside of the glass. The laser produces a series of closely spaced dashes, which creates more surface texture. This increases the beer bubbling effect on the mark location.

Laser marking of window glass is different, says Earl. Here, vector lines are generally created – rather than discrete dots, which cause the glass to melt locally and reflow to create the mark. ‘Marking window glass is a big business,’ stated Earl, ‘which could be anything from a data matrix code to a customer logo or human readable text.’

There is alternative laser technology and methods for engraving glass, including excimer lasers emitting in the UV and even sub-nanosecond lasers, which, Gäbler says, can be used for marking very thin, high-quality glass. Coherent acquired Innolight last year for its sub-nanosecond laser technology, which has uses in glass engraving.

‘Sub-nanosecond lasers are capable of a diffractive marking, whereby the diffractive index in the glass is altered to make the mark without microcracks,’ explained Gäbler. Medical ampoules, for instance, can be marked in this way, as can glass bottles containing expensive products for anti-counterfeiting purposes.

‘These are applications which in the past have required expensive femtosecond lasers,’ said Gäbler. ‘However, now this can be achieved with a green sub-nanosecond laser, which is 15 per cent of the cost of a femtosecond laser. This new technology might open up new markets.’

While CO2 lasers remain a good tool for cutting and engraving glass, ultrafast laser technology is opening up new techniques. With mobile phone and display manufacturers potentially turning to sapphire as a cover glass material, there is a huge future market for ultrafast laser systems to cut this brittle material.

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