May 3, 2017

Laser and glass—a good team

					Laser and glass

Whether it’s in processing ultra-thin glass components, cutting, welding and forming glass or manufacturing high-resolution polysilicon displays, lasers are the instrument of choice.

Glass barely thicker than a human hair is making inroads into electronics. There, these flexible glass layers, no more than 30 μm thick, serve as covers for fingerprint sensors, camera chips, lenses in smartphones and tablets, and sometimes as the basis for microelectronic sensor systems in cars or industrial plants.

Traditional mechanical glass processing is unable to cope with these wafer-thin glass elements. When they encounter sawblades or diamond-studded rollers they form microcracks or simply break. Even the flame pressure of a glass burner is often too much for this micrometer-fine material. The instrument of choice is therefore lasers. Because they work by applying precisely controllable energy, are massless and use no tools, straight or curved cuts are just as feasible as clean bores. Research at the Hanover Laser Center shows that edge chipping can be completely prevented by using varying brief laser pulses. Ever more exact numerical simulations enable the researchers to identify the optimal process parameters.

Key technology for high-resolution displays

The manufacture of high-resolution smartphone displays and OLED screens not only calls for the precise laser processing of ultra-thin glass. In order to construct displays that are as light and slim as possible, manufacturers construct the micrometer-thin functional layers on a polymer base instead of on glass. However, the process does still call for glass, which serves as the stabilizing substrate on which to construct the displays, layer by layer. Short-wave excimer lasers then carefully separate the delicate functional sandwich from its glass substrate. Coherent has optimized this “cold” laser lift-off process to such an extent that a few thousand ultra-brief laser pulses are sufficient to separate a surface with five dozen 5" displays from its glass substrate.

But there is another reason why lasers are indispensable to display manufacturers. The base on which these new, high-resolution screens are constructed is high purity polysilicon. This is created using a laser process in which a glass substrate coated with amorphous silicon is exposed to high-power UV lasers. Nanosecond pulses of 308-nanometer light waves act on the material, causing it to melt and crystallize. This produces the finest polycrystalline silicon layers that are 2,000 times thinner than a human hair. Coherent is constructing so-called Vyper/LineBeam systems for this purpose, each of which, each month, can process an area equivalent to three soccer pitches. Without this technology, Coherent says there would be no TFT displays based on polysilicon—and therefore neither ultra-high-definition (ultra HD) displays nor any market prospects for brilliant, energy-efficient OLED technology.

Lasers increase quality and cost potential in glass processing

For drilling, cutting, shaping and welding flat glass and glass tubes, lasers also ensure an increase in productivity and quality. Previously, it was necessary to use specific process gases in order to heat different types of glass to the required processing temperature; for instance, quartz glass called for costly energy-intensive hydrogen. With lasers, the temperatures are easier and usually faster to achieve. The processes can also be controlled more precisely by varying the focus and duration of the laser pulses. Used together with contact-free temperature sensors, lasers pave the way to closed control circuits in glass processing. They therefore provide the basis for process automation, which extends through to defined cooling. The latter has a decisive influence on the inherent stresses in the processed glass. Laser-treated glass can usually withstand mechanical loads two or three times higher than mechanically processed glass.

Alongside the high and readily reproducible quality, automated laser processes have yet more advantages: Since lasers have no chemical influence on the glass and immediately create unbroken edges, several of the usual cleaning and post-processing steps can often be dispensed with. All in all, rising productivity and the elimination of process gases ensure that laser systems used in glass processing will pay for themselves very quickly.