Ultrashort pulse (USP) lasers for precision micromachining can minimize or avoid undesirable thermal effects that limit feature resolution, edge quality and, ultimately, product functionality. This is because longer pulse (nanosecond) and continuous-wave (CW) lasers have a photothermal interaction with materials, creating a heat affected zone (HAZ) which limits the minimum size of features that can be cleanly machined. It can also cause functional and structural problems, e.g., damage to underlying semiconductor circuitry, or microcracking that weakens glass touchscreens. And, in external marking of consumer products, the HAZ can appear as charring or discoloration compromising the value of the final device. In addition, photothermal scribing, drilling and cutting often produce an edge burr consisting of recast debris, which may require post-process removal.
In contrast, lasers with shorter pulse widths can significantly reduce these unwanted thermal effects and minimize the HAZ, because these effects are dominated by the relationship between laser pulse duration and the thermal diffusion times of the target material. Moving to the picosecond (ps) regime, the short pulse duration often means that the vaporized material carries away any unwanted heat before it can spread and cause a HAZ, a condition sometimes called thermal confinement. And, the fact that the ejected material consists of very small particles – as small as atoms – means that picosecond laser pulses do not produce recast debris, often eliminating the need for post-processing such as edge grinding, chemical cleaning, etc. In the automotive industry, these qualities enable contoured holes to be drilled in fuel nozzles to support higher fuel efficiency, i.e., greener vehicles.
In several industries there is an overarching trend for increased miniaturization together with increased functionality. This fuels a growing interest in using even shorter duration pulses, in the femtosecond (fs) range, where the advantages of shorter pulses are even more pronounced. In materials with high thermal conductivity, particularly many metals and diamond, the thermal diffusion time can be picoseconds, or less. Thus moving from the ps domain to the fs operating regime delivers a measurable reduction in HAZ.
Femtosecond lasers can also process a wider range of materials than any other laser. That's because the high peak power of fs lasers drives non-linear absorption. In this extreme domain, materials absorb the laser light even if they are nominally transparent at the wavelength of the laser! The absorbed photons strip off electrons, instantly breaking the bonds holding the material together, in contrast to the usual photothermal process powered by longer pulse lasers.
Industrial Femtosecond Lasers
Traditional fs lasers were targeted at scientific researchers and had features, cost, and performance characteristics appropriate for this market. But now, some manufacturers have developed a new generation of fs lasers for industrial users based on an exciting new material – ytterbium doped fiber. This material supports much higher powers than previous femtosecond lasers, and is typically packaged in a master oscillator/power amplifier (MOPA) internal configuration. The fiber gain medium supports simpler internal design and construction, leading to lower costs and significantly increased reliability. Moreover, ytterbium fiber does not yet represent a mature technology, so the performance of these new lasers continues to advance at a steady pace, particularly in average power.
These lasers are typically characterized by much lower pulse energies than nanosecond lasers, but with very high pulse repetition rates – usually in the 1-50 MHz range. The high pulse rate delivers overall material removal rates that provide market enabling throughput for many target materials. An example of these new lasers is the Coherent Monaco series which currently provides up to 60 watts of processing power in a compact (670 mm × 360 mm × 180 mm) sealed package.
These new fs lasers also support burst mode operation which has proved very effective in materials processing with ps lasers. While average power determines throughput, it is the pulse energy that determines whether the laser can cut or not, relative to material strength and thickness. Simply stated, higher pulse energy enables processing thicker materials. Monaco not only features extremely high pulse energy at 80 μJ/pulse, it also provides burst mode. Here the laser emits a burst of pulses closely spaced in time that acts as a “super-pulse” with an effective pulse energy of > 320 μJ.
Advances in Picosecond Lasers
While the developments in fs lasers have been more dramatic, there have also been important recent developments in ps lasers for industrial applications, in two different areas. There is increasing interest in using ps lasers for lower power applications such as drilling small holes and marking in the automotive, semiconductor and medical device industries. This has created a demand for ps lasers at modest power levels, where lower cost of ownership is just as important as other parameters, such as pulse-to-pulse stability. An example of this new class of ps lasers is the Rapid NX series from Coherent, which deliver several watts of near infrared output in a very compact package.
At the other end of the power spectrum, there is a host of applications where high average power is needed to support high throughput and/or where significant material ablation must be performed. Examples of these new higher power ps lasers are the HyperRapid NX series from Coherent, which can deliver over 100 watts in the near infrared. The high power and beam quality of these lasers supports efficient harmonic generation to support a growing number of ps applications that require visible (532 nm) or ultraviolet (355 nm) wavelengths.
Turnkey Tools Enable Real Applications
Developing a micromachining application that processes the maximum amount of material in the minimum time requires a careful interplay between part or beam motion and laser power modulation/pulsing characteristics. Increasingly, users look to laser or laser system suppliers to lead this effort, often by supplying integrated tools optimized for the specific task type. These can include scanning optics, part loading and positioning mechanics, control software, and, increasingly, the “process recipe” of software parameters required to perform the desired task within the target process window. While this same trend is seen across many industrial laser applications, it is even more important with USP lasers where there is a much smaller body of knowledge in the industry.
The new Exact family of tools from Coherent epitomize this approach. Based on many years of Rofin (now Coherent) expertise in turnkey systems and applications support, these make maximum use of modular technology to yield a series of specialized tools (e.g., ExactCut, ExactWeld, etc.) on a common, cost-effective platform. In terms of optional hardware, the processing head often includes a vision system, and/or autofocus hardware, and/or some other type of process monitoring technology. When equipped with a ps or fs laser source, these tools are usually supplied with a heavy granite base, since most applications for these USP lasers emphasize micron-level precision. All the motion mechanics and optics are similarly optimized for extreme precision.
USP Laser Tools in Action
SmartCleave® Glass Cutting: One of the most successful applications of USP lasers is in filament cutting of glass, increasingly used as both structural and functional elements in smartphones and other consumer products. Mechanical methods cannot easily produce the small cutouts and tight curves these applications often need, and they cannot cut chemically strengthened glass used in touchscreens.
In the SmartCleave process, the high peak intensity created by a focused USP laser produces self-focusing of the beam due to the nonlinear Kerr optical effect. This self-focusing further increases power density, until, at a certain threshold, a low-density plasma is created in the material. This plasma lowers the material refractive index in the center of the beam path and causes the beam to defocus. If the beam focusing optics are properly configured, this focusing/defocusing effect can be balanced to repeat periodically and form a stable filament which extends over several millimeters in depth through an optically transparent material. In order to achieve a continuous cut, these laser-generated filaments are produced close to each other by a relative movement of the work piece with respect to the laser beam. The typical filament diameter is in the range of 0.5 μm to 1 μm, enabling very high precision cutting.
In the case of strengthened glass, SmartCleave with a ps laser is sufficient to separate the glass in a single step with high edge quality that requires no postprocessing. Moreover, speed is as high as 2000 mm/s depending on glass thickness. For conventional glass, a localized thermal shock is applied by closely following the USP laser beam with a jet of cold air, or sometimes a focused infrared (CO2) laser. Again, the edges exhibit no microcracking and need no grinding, and there is no residual edge stress. This is critical, because even when force is applied to the center of a glass panel, any break usually initiates at the edge.
The new generation of fs lasers can drive the same process. However, the non-linear absorption of fs pulses means that the same laser can cut disparate transparent materials, all with high edge quality. So laminates (e.g. glass coated with polymide) that are increasingly used in microelectronics and medical devices can be cut in a single pass. The example here shows an edge view of 20 μm of polyimide on 0.5 mm glass, cut with a femtosecond laser with 40 watts of average power and a pulse width ~350 fs. The surface roughness was < 350 nm, as measured with an AFM.
Miniature Metal Gears: USP lasers are ideal for cutting small shapes from thin metal substrates. A standout example involves the small gears used in wristwatches and other precision mechanical systems. The ExactCut is ideal for these tasks with near infrared ps lasers such as the HyperRapid NX being the preferred choice. This combination delivers the requisite edge quality without any post processing requirement, and with no risk of any thermal distortion or discoloration of the gears. A similar setup with an ultraviolet version of the same laser is used to cut the tiny electrodes used in certain neurosurgical implantables.
Surface Texturing of Metals: Lasers have long been used to texture metal surfaces for a variety of reasons. This includes texturing (titanium) dental and other implant surfaces to match the dimensions of osteocyte cells and thereby support optimum bone fusion. It has also included the use of carbon dioxide and excimer lasers to texture the cylinder liners of diesel engines to lower friction and thus extend life and increase fuel economy.
Most early applications used the laser to produce a random pattern of surface features appropriate to the application. But the high peak power and high repetition rates of the new ps and fs lasers enables these to be used to deterministically produce surface features of specific dimensions and in a controlled pattern. A major area of interest for this type of texturing is to manipulate the tribological properties of moving parts in diesel and gasoline engines where a dimpled pattern (like a golf ball) retains lubrication, minimizes friction and wear, and helps manufacturers in their relentless quest for increased fuel efficiency.
To summarize, USP laser materials processing is a rapidly growing area that is extremely dynamic in terms of the core laser technology. In addition, ongoing advances in laser packaging and integrated tools make these cutting edge products cost effective and practical for the many applications that can benefit from their unique capabilities.
This article was written by Thomas Schreiner, Ernst Treffers, and Michael LaHa, Coherent, (Santa Clara, CA). For more information, contact Mr. Schreiner at