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tEchnology scan hEads Does this


scan?


greg Blackman on laser scanner technology, including three-axis scan heads and developments for ultrafast lasers


‘M


ost laser applications began with a fixed laser beam and a positioning stage that moved the part underneath the beam.


This is way too slow to be economically viable for most applications,’ says Georg Hofner, chief executive officer at Scanlab, which manufactures laser scanning devices. Those applications where fast means economically viable range from marking to solar power, and scanning can make the difference,. The widespread adoption of solar power is


going to rely on engineers increasing the sunlight conversion efficiency and making the photovoltaic cell manufacturing process much cheaper. Hofner explains how a scanning system manages to be fast enough to deliver economic improvements to the likes of photovoltaics. ‘The scanning system is only moving small mirrors, which positions the beam large distances relative to the movement of the mirror. Scanning heads are therefore dynamically much more advanced than what you can do with a linear stage.’


18 ElEctro optics l DECEMBER 2011/JANUARY 2012


One example of this speed is the recently completed Solasys project. Coordinated by the Fraunhofer Institute for Laser Technology in Aachen, Germany, Solasys demonstrated 10,000 holes per second laser drilling in metal wrap- through (MWT) and emitter wrap-through (EWT) techniques. This technology is designed to lower the proportion of a crystalline silicon solar cell covered by conducting wires and thereby improve efficiency.


Other scanning laser applications are eye


surgery and diagnostic retinal scanning. These have to be very precise with excellent reliability and accuracy. Industrial laser processing is another sector. The scanners themselves can range in price and complexity, from simple scan heads for marking costing $500, to three-axis, high-end systems that might reach $50,000. Two-axis systems have an x and y axis perpendicular to one another. As the x and y axes are typically closed-loop and servoed, they can be programmed to cover any spot in a scanning area. ‘The type of laser processing will depend on the size of the aperture,’ explains Red Aylward, president of Cambridge Technology. Larger apertures with a large mirror will give a small, focused spot, but the beam will be manipulated at a slower speed. It depends on the application as to the size of the aperture used. ‘These mirrors move extraordinarily fast,’


Aylward states. ‘Scanning systems can drill 3,000 holes in a circuit board in one second with a small spot size and very fine positioning. The beam has to accelerate to a position, stop, drill the hole, accelerate to the next spot and drill the hole, 3,000 times a second.’ Cambridge Technology provides 28 models of galvanometers, including open-loop, closed- loop and resonant varieties, plus scan heads, controllers, and scanning subsystems. They cover clear apertures from 1 to 100mm, laser powers from milliwatts to several kilowatts, and wavelengths from deep UV to the far infrared. Most galvanometers are closed-looped systems, which have a positioning feedback device built into them. ‘A lot of the performance of the galvo is dependent upon the precision of the feedback device, in terms of its accuracy, its thermal stability, its noise floor, and repeatability,’ says Aylward.


adding a z-axis Adding a third axis makes the system more complex, but allows 3D geometries to be processed. In two axis systems, an f-theta lens is used to alter the focal point dynamically as the beam sweeps over the workpiece. David Freihofer, director, optical scanning components at Cambridge Technology, explains: ‘As the galvo


www.electrooptics.com


Image courtesy of Cambridge Technology


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