Silicon mirrors resist laser heat to guide beams in high-value machining

The microelectromechanical systems (MEMS) mirrors are the result of a project involving the Fraunhofer Institute for Silicon Technology ISIT in Itzehoe and the Fraunhofer Institute for Material and Beam Technology IWS in Dresden.

“If this technology is accepted there is a lot of potential to use it,” Dresden-based group manager for laser cutting Patrick Herwig told The Engineer.

“It is really easy to integrate,” he added. “It is only a deflection mirror that can also be exchanged for a different frequency or whatever. It is a low-cost tool that allows high-quality processes.”

Potential customers could include laser process head suppliers, Herwig said.

Mass production should make the micromirrors much cheaper than comparable galvanometer scanners, he added.

Materials such as aluminium and high-strength steels are extensively used by the automotive and aircraft manufacturing industries.

Until now laser systems had to be adapted to individual materials, in many cases requiring special optics installed specifically for a particular process.

The micromirrors, which are etched from silicon, offer greater flexibility in processing. They are scanning mirrors that deflect the laser beam and guide it precisely over the workpiece.

It was previously only possible to use them with laser outputs of a few milliwatts, which was not enough for laser cutting and welding. Higher outputs would melt the mirrors.

Thanks to a protective coating developed for the project and a specially designed mounting, the new mirrors can withstand laser powers in the kilowatt range – enough to process aluminum or sheet steel.

The MEMS mirrors can be swivelled back and forth at high speeds, reaching frequencies of up to 100,000Hz. This allows the laser energy to be distributed more effectively than with conventional laser systems, whose mirrors swivel at about 1,000Hz.

The system can be used to weld aluminum and copper, using the heat input to control which metal heats up most, making up more of the melt.

The mirrors also make it easier to weld aluminum alloys. At present, aluminum weld seams are often porous because substances outgas from the alloys and form bubbles during the welding process.

With the micromirror, heat input can be controlled so that the melt remains liquid until the substances have completely outgassed.

The ability of the MEMS micromirrors to work with large laser outputs is not only due to their reflective coating, but also their size.

MEMS mirrors normally have a diameter of 1–2mm. The new mirrors have diameters of up to 2cm, enabling them to handle lasers with larger beam diameters and higher output.

For the developers, the challenge was to achieve high frequencies despite the size.

To solve the problem, the mirrors are operated in air-evacuated vacuum pods to minimise attenuation of the oscillating mirror.

“We hope to find a frequency where we can say it is the general best fit to all applications,” Herwig said. “We would have one fixed product usable for all applications.”