Trends in Industrial Laser Solutions

HANK HOGAN, CONTRIBUTING EDITOR

The demands on lasers grow as the need for precision in manufacturing and other applications increases. In general, lasers enable processes that cannot be performed economically in any other way — such as drilling holes, selectively removing material, or texturing a surface. Lasers also allow novel methods of imaging.

Such uses may require a variety of laser advancements, however, including shorter pulse width, higher repetition rate, varying wavelengths, greater power, better beam quality, and improved uptime. These needs must be met while simultaneously lowering initial and ongoing expenses. In response, vendors are improving current systems and researchers are developing new electro-optic lasers that offer much faster repetition rates.

The basic structure of a thin layer of material from a semiconductor device for transmission electron microscope analysis. The sample was cut and thinned by an ultrafast laser that is up to 10× faster than the previous ion milling approach. Courtesy of 3D-Micromac AG.

3D-Micromac AG of Chemnitz, Germany, which specializes in laser micromachining, is seeing the opportunities and challenges. “We are facing different requirements for the laser sources,” said Uwe Wagner, the company’s chief technology officer and acting managing director.

An integrated circuit sample thinned by laser micromachining at multiple positions as part of a microdiagnostics process. Courtesy of 3D-Micromac AG.

Ever smaller features are driving laser systems toward shorter wavelengths, if for no reason other than basic physics.

The company primarily targets photovoltaics, semiconductors, and MEMS systems manufacturing. For 3D-Micromac, increasing laser power is not their main concern, according to Wagner. “We are more looking into [improving] beam stability and the quality,” he said.

Micromachining

The company’s laser sample preparation tool for microstructure diagnostics and failure analysis is one example. Focused ion beam milling has traditionally been used to remove material and thereby prepare the sample to be analyzed by transmission electron microscopy. The milling process takes hours, however. Using a laser for most of the ablation speeds things up considerably, but the laser must not damage the surface of the sample. Hence, the solution is a low-power, or less than 10-W, ultrashort-pulse (USP) laser that is precisely controlled. Sample preparation is reduced to half an hour, perhaps a tenth of the time needed with milling.

This new use of laser micromachining represents opportunity, but challenges remain. Because the laser system has an industrial application, the system must be inexpensive to buy and operate, and it must be robust and easy to use. The laser must also be easy to integrate into a system that allows 3D-Micromac to supply a complete workflow — another way to cut costs.

The trend toward miniaturization of features affects parts both big and small, said Joris van Nunen, director of product marketing for the industrial picosecond lasers business unit of Santa Clara, Calif.-based Coherent Inc. Smaller parts are being made, and finer details are being machined on larger parts.

Cuts made by a picosecond UV laser in a stack representative of a typical OLED display show improvements in cut quality as the repetition rate is increased. Results of a study show that increasing repetition rate while maintaining average power improves throughput and quality, indicating a possible benefit to raising repetition rates even more. Courtesy of Cohent Inc.

Examples can be found as close as the nearest smartphone. A variety of lasers and processes are used to cut glass or sapphire display substrates and the active electronics that drive the display, including CW or near-CW CO2 and CO IR lasers, picosecond pulse micron lasers, and green or UV lasers of nanosecond or less pulse duration. The laser choice depends upon the material, application, and manufacturing requirements.

In general, smartphone screens are getting larger, and the usable part of the display should come as close to the edge as possible. As a result, manufacturing specifications are tightening, in some cases going to half the value they were before.

“Heat-affected zones are shrinking, so they basically go below 50 µm for a stack of [display] materials that are hundreds of microns thick,” van Nunen said. In the past, that figure may have been 100 µm or more.

For the lasers, this means pulse widths may be shorter, as this lowers the thermal impact. Another option is to go to a shorter wavelength, such as moving from the green to the UV, to increase optical absorption of the target. The best approach, though, is material dependent. Metals, for instance, benefit most from shorter femtosecond pulses, while polymers benefit from UV wavelengths at picosecond pulses. Thus, the ideal solution is tailored to the task.

A lab system top glass-cutting module, made more precise with the use of lasers. Courtesy of GFH GmbH.

Van Nunen said some applications will need more throughput in the future. One possible solution could be to split the beam into multiple spots, which could boost throughput by creating many work areas. Splitting the beam could also cut costs per part because only one high-quality laser would be needed instead of many. A multiple-spot approach could be combined with novel scanning techniques to push throughput even higher, thereby further reducing the cost per part, all things being equal.

Ever smaller features are driving laser systems toward shorter wavelengths, if for no reason other than basic physics, according Marco Mendes, director of laser applications engineering for the materials processing systems division of fiber laser maker IPG Photonics Corp. of Oxford, Mass. The size of the machining tool is set by the laser spot size, and shorter wavelengths lead to a smaller spot size. Mendes pointed to this as a reason that 1-µm wavelength lasers continue to take over applications historically performed by 10-µm wavelength lasers.

He added that designing manufactured products with smaller features makes control of beam quality more important because it affects spot size. Highly precise removal of material via ablation also requires accurate control of how much is removed with each pulse, meaning pulse duration and power must be tightly controlled. This need for precision extends beyond the laser itself.

Beam quality

“Positional and dimensional accuracy of the laser-machined parts will depend on the workstation motion platform as well as beam delivery,” Mendes said.

At the same time, these systems must reduce operational costs and the overall cost of ownership. Combining high wall-plug efficiency lasers with innovative and simple designs could accomplish this, Mendes said. Greater efficiency means a need for less electricity. A simpler design lowers costs because the system uses fewer components. It may also improve uptime by eliminating potential problems, such as misaligned bulk optics.

Mendes predicted continued improvements in all aspects of fiber laser performance. These improvements will allow lasers to find homes in new applications — such as micromachining and micro- welding — displacing incumbent technologies and enabling new uses.

Bill Holtkamp — sales manager for micro business for disk and fiber laser maker TRUMPF North America in Santa Clara, Calif. — echoed that better beam quality, along with innovations, could lead to novel applications. Improved beam quality makes it easier to split the beam for efficiency and cost sharing.

Beamsplitting can be used to increase the proficiency of material removal. For instance, cutting or machining over a large area with a minimal heat-affected zone can be performed with a thin laser line. Optically this presents a challenge, which can be solved by using diffractive optical elements (DOEs) to split the beam into 100-plus spots. These spots can be used individually or overlapped to form a line, Holtkamp said.

“DOEs can also produce other spatial beam profiles that can be useful for increasing process efficiency for cutting, welding, and surface texturing,” he said.

Pulses and bursts

Other novel applications — such as an ultrafast laser with the ability to change pulse duration while keeping spatial beam quality and pointing static — could arise from these innovations. This capability would allow process engineers to tailor pulses and scanner movement. TRUMPF will demonstrate this technology early in 2019, Holtkamp said.

Beyond pulse duration, it is important to control the temporal sequence and shape of pulses to meet demands for precision, said Herman Chui, senior director of product marketing for Santa Clara, Calif.-based Spectra-Physics. The solid-state laser maker is a business unit of MKS Instruments Inc.

As costs go down, lasers are finding their way into new applications, such as surface texturing and micropatterning.

Adjusting the number of pulses in a burst along with their energy and shape can improve material removal rates and process control. The ability to trigger pulses on demand in a way that is synchronized with motion is also helpful. “You might have to accelerate around a curve,” Chui said. “Being able to control the timing is really critical, so that you’re not bunching pulses when motion slows down or speeds up.”

New areas of application

As costs go down, lasers in other industries are finding their way into new applications, such as surface texturing and micropatterning. These treatments may, for example, direct liquids in biofluidic or biomimetic applications by making a liquid move in one direction rather than another. Reduced costs allow applications in completely new areas, such as collecting fog in new ways to make water. Unlike current fog-collecting techniques that require large sheets of canvas and large installations, laser patterning may work with much smaller areas while still obtaining useful amounts of water.

Finally, innovations point to the emergence of new laser technologies. Using common electronics, physicists at the National Institute of Standards and Technology (NIST) built a laser that pulses at 100× the repetition rate of common ultrafast lasers. Such lasers are based on mode-locked technology. In contrast, the NIST team used an electro-optic technique to carve a continuous beam directly into pulses, according to David Carlson, a researcher and the lead author of a September 2018 Science article that described the work.

The group developed a method to filter out electronic noise, combining it with silicon nitride nanophotonic devices. This allowed them to use an inexpensive fiber optic laser as a source, yet still produce pulses comparable in noise level, bandwidth, and duration to a standard ultrafast laser, said Carlson.

A microwave cavity, which reduces electronic noise, is key to a new, higher-repetition-rate laser technology, as demonstrated by researchers. Courtesy of NIST.

The new laser technology enables much higher repetition rates. As with other laser techniques, this leads to lower peak intensities — the result of spreading the total average power over more pulses. Such improvements may not matter, however, for applications such as biological imaging, where the need is to keep intensity low to avoid tissue damage. There, the new technology offers other benefits.

“You can potentially acquire data significantly faster, and that’s a nice advantage of the significantly higher repetition rate,” Carlson said, adding that the NIST research team is investigating just such a use.

These innovations bode well for lasers in precision applications. Increasing performance in terms of throughput, along with plunging costs, means that manufacturers are considering using lasers in places where lasers would have been too expensive before. Manufacturing toolmakers are also creating new systems to displace other approaches, such as using a laser instead of focused ion beam milling.

3D-Micromac’s Wagner said, “There are new markets and completely new applications that are opening up.”