By Haibin Zhang, ESI.
Due to their excellent strength, chemical resistance, and optical transparency, glass materials are widely used in consumer electronics such as in flat panel TVs, laptops, and hand-held devices. Small to medium form factor glass panels (3 to 10 inch), in particular, are enjoying a very healthy annual growth of close to 20% thanks to the recent boom in high end smart phones, e-readers, and tablets.
An anatomy on most of these popular gadgets reveals several layers of glass panels: first is a strengthened piece of cover glass that has extremely high compressive stress to protect the devices from impacts, scratches, stains, and harmful chemicals. Underneath the cover is a thin layer of glass deposited with two-dimensional ITO patterns (in most cases) to support touch functions. Display glass such as LCD or OLED modules lies beneath these two layers and provides vivid images and video playbacks at high contrasts. Different functions of these glass substrates require different designs: on the cover glass curvilinear and internal features are needed to generate rounded corners, streamlined perimeters, speaker holes, and home buttons. Touch panel and display module generally only need straight cuts for panel singulation from a mother sheet.
Established glass machining methods include mostly mechanical scribing, manual breaking, and mechanical grinding and polishing. As the industry move forward, consumers demand better protection, function, design, and structures for these glass panels which lead to thinner and stronger substrates for reduced weight and volume. This has resulted in great challenges for mechanical processes in terms of cut quality, yield, and throughput. Laser glass machining tools provide non-contact machining with minimum impact and superior quality compared to the mechanical counterparts. As their power grows higher and price becomes cheaper lasers are increasingly used in glass machining applications.
Different lasers are used for machining glass depending on the functions and features of the panels. For straight line cutting in touch panel and display glass, CO2 lasers are used to trigger a controlled thermal cleaving (CTC) process thanks to their high linear absorption in glass at the 10.6um wavelength. A large tensile stress (up to 200 MPa) can be generated by following the CO2 ‘scribe’ beam with a quenching jet of water-air mist. This is enough to tear the glass surface open when a crack already exist – usually a surface crack initialized at the edge of the glass by a mechanical scribe or laser shot. The crack only penetrates partially into the substrate so the glass is still held together after this ‘scribe’ step. Usually a manual break follows to separate the glass which can be done by a breaker system or a human operator. At ESI we use a second laser beam (the ‘break beam’) to fully propagate the crack and singulate the panels in one step. This proprietary technology – laser full cut – reduces the process steps and increase cut facet quality.
Figure 1 shows a schematic process flow for the laser full cut method (a) and the high edge quality from a laser singulated 1.1-mm panel. Cutting speed ranges from 200 to 500 mm/s depending on the glass type and thickness.
When curvilinear and internal features are needed a laser direct ablation process is used. DPSS lasers in the nanosecond and picosecond pulse width can trigger efficient nonlinear absorption in glass due to their high peak intensities. Both IR (around 1um) and its second harmonic generation (~0.5 um in green) can be used to cut glass layer by layer but green wavelength gives better edge quality. At different pulse duration, a green picosecond laser (~2W, 200 kHz, 10ps) provided 10x higher edge quality (0.5 um surface roughness) but 10x slower material removal rate (5 um3/uJ) compared to a green nanosecond laser (~2W, 20 kHz, 10ns). Furthermore, for the chemically strengthened substrates such as the Corning Gorilla glass, only the picosecond laser was able to cut through without cracking the glass. Material removal rate of both ns and ps lasers scales with laser power and we have recently demonstrated effective cutting speed of ~3 mm/s through a 0.7-mm thick Gorilla glass by using a 50W green picosecond laser.
Figure 2 shows a 10-mm diameter hole and a 20x30mm rounded rectangle cut from a 0.7-mm thick Gorilla glass with high quality water-clear edge.
As the glass market evolve, glass substrates are getting even thinner and stronger than ever before. Laser machining tools with high quality cut and competitive throughput are expected to play more important roles in the consumer electronic device manufacturing processes.