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July

Polygon scanners boost lasing speeds tenfold

July/August 2014 Volume Volume 7 Issue Issue 4

Ronald D Schaeffer - PhotoMachining, Rick Slagle - Techsolmarketing.com

By Ronald D. Schaeffer, Ph.D.rschaeffer@photomachining.com, PhotoMachining Inc. and Rick Slagle rick@TechSolMarketing.com Technical Solutions Marketing, LLC

The standard way of delivering a low-divergence, well-collimated, Gaussian laser beam to a target is via galvanometers. The beam enters an aperture and encounters two nearly orthogonal mirrors mounted on servos that work together to direct the beam through a refractive, flat-field lens and onto the target. The laser and optical system determine field size, which is limited by the focal length of the lens, the scan angle and clipping of the beam inside the objective housing. Typically, the usable field size is about 0.6X the focal length.

A polygon scanner (left) allows laser scanning speeds of up to 100 m/sec., whereas fixed-beam and galvo-based lasers typically operate at scan rates of 10 m/sec. Image courtesy Next Scan Technology.

Despite the widespread use of galvos, the fastest ones have a positioning speed of 1,500 discrete positions per second. Many lasers can operate at hundreds of kilohertz and at repetition rates of up to several megahertz, meaning even the fastest galvos are too slow for certain lasing applications.

Polygon scanners were developed to handle some of these lasing applications. They can provide scanning speeds of up to 100 m/sec. By comparison, fixed-beam and galvo-based laser approaches typically offer scan rates of 10 m/sec.

How scanners work

Polygon scanners, first used commercially in laser printers in the mid-1970s, incorporate rotating polygon (multifaceted) mirrors that spin at a high, constant speed. A laser beam hits the mirrors and is directed along a line. The scanner writes one line of a bitmap image at a time—a process called raster scanning—while the target moves beneath the beam. (To watch a video of polygon scanning, visit www.nextscantechnology.com/applications.)

Galvo-based scan heads focus the laser beam with lenses, whereas polygon scanners rely exclusively on reflective optics. The beam deflects off one face of the rotating polygon onto the primary mirror, which, in turn, deflects the beam to a secondary mirror that delivers it to the target. Depending on the timing of the laser pulse in relation to the polygon mirror position, the beam can hit the primary mirror anywhere on its face, which determines where along the scan line the laser exposure occurs on the substrate.

With a galvanometer-based lasing system, two nearly orthogonal mirrors mounted on servos direct the laser beam through a refractive, flat-field lens and onto a stationary target. A polygon scanner incorporates rotating polygon mirrors that spin at a high, constant speed. Reflective optics direct the beam to a moving target. Images courtesy Next Scan Technology.

A polygon scanner’s primary and secondary mirrors are nonspherical in design, providing diffraction-limited performance. The design yields a tiny spot size (down to 5µm), maintains beam roundness and is fully telecentric, meaning it preserves a perpendicular beam across the entire scan surface.

Much like the mirrors of a large Cassegrain telescope, a polygon scanner’s mirrors are economically scalable compared to refractive optics. This makes it possible to have a 300mm field of view in the scan direction, which is good for large substrates such as a 12" semiconductor wafer, while still maintaining a small spot size and good beam quality.

Under normal operating conditions, “jitter”—undesirable fluctuations in the time between laser pulses—may occur. These fluctuations are insignificant in applications like cutting and scribing. But jitter can be problematic when drilling, surface patterning or performing other operations requiring precise spot placement. The effects of jitter can be offset by a master controller that reads an encoder to determine the polygon facet location and then synchronizes the laser firing on a pulse-by-pulse basis.

Applications

Polygon scanners are unsuitable for most laser machining jobs. The pulse rates required for most lasing tasks are too low for polygon scanning.

Polygon scanning is intended for a few, specialized applications requiring high throughput, accuracy and repeatability over a relatively large area—from about 50mm square to more than 1 meter square.

Polygon scanner applications include:

2.5D surface shaping. There is a great deal of interest in using lasers to modify the surface of a material in order to change its inherent characteristics. An example is the manufacture of hydrophobic automotive headlights and windshields. The process causes the surfaces of these vehicle components to repel water. Surface shaping also is used in the production of high-precision tooling employed by the security-printing industry.

Thin-film patterning. An ideal application for a megahertz laser/polygon scanner system is patterning the transparent conducting oxide (TCO) layer of smartphone and tablet glass and, increasingly, touch-screen computer monitors and televisions.

The laser doesn’t machine away the TCO layer. It attacks the TCO/glass interface, creating a “micro-explosion” that lifts off the thin film. Pulse energies of only a few microjoules are sufficient.

Hole drilling. Thanks to advanced controls for regulating speed and pulse timing, polygon scanning can be used for percussion hole drilling. If the laser is set at a low pulse-repetition rate, such as 200 kHz, the scan speed separates the fired pulses. Multiple passes turn the process into a percussion-drilling operation. With its highly repeatable spot placement, thousands of holes per second can be drilled. An additional benefit is that a range of laser sources can be used, from continuous-wavelength and nanosecond-pulse fiber lasers to ultrafast-pulse lasers.

A polygon scanner was used for percussion laser drilling of this high-density hole pattern. Images courtesy Bern University of Applied Science.

New applications for polygon scanners are expected to come online in the next few years. A European consortium was recently formed to promote the technology. Called the UltrafastRazipol Project (www.razipol.eu), the group’s initial goal is to develop a 500w, 20- to 40-MHz laser for the high-speed manufacturing of lab-on-a-chip microfluidic assemblies on large (8"-dia.) substrates. Polygon scanners will be vital in delivering the required high-pulse-rate laser beams. µ

— R. Schaeffer & Rick Slagle

Ronald D. Schaeffer, Ph.D., is CEO of PhotoMachining Inc., a high-precision laser job shop and systems integrator in Pelham, N.H. Rick Slagle is the managing partner of Technical Solutions Marketing LLC, Maynard, Mass. Polygon scanner manufacturer Next Scan Technology NV, The Netherlands, has contracted Slagle’s firm to conduct market research and develop business in North America. Telephone: (508) 331-8347. E-mail:  rick@TechSolMarketing.com.