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Klaus Kleine and Michael LaHa of Coherent Inc review a new type of cutting laser, femtosecond and the ultra-short pulse (USP) industrial laser, which can cut feature sizes of tens of microns, with virtually no heat affected zone.

There is an increasing need to produce high precision, miniaturised components across several industries such as medical devices, automotive manufacturing and microelectronics. In many cases, traditional cutting methods cannot deliver the required combination of feature resolution and cut quality for these applications.

Even lasers, which have historically offered the highest level of cutting precision, sometimes produce an unacceptably large heat affected zone (HAZ), that is, a region where melting or microcracking in surrounding material degrades part quality and/or performance.

This article explores a new type of cutting laser, the ultra-short pulse (USP) industrial laser, which can cut feature sizes of tens of microns, with virtually no HAZ, and presents some specific examples of the use of this technology in medical device manufacturing.

Figure 1: Laser micromachining; in drilling, cutting and texturing applications, the use of lasers with shorter pulse widths avoids some of the limitations associated with longer pulse widths, including thermal effects and recast debris and surface micro-cracking.

USP Laser Advantages

Most precision micromachining lasers have pulse durations of 40 to 60 nanoseconds (10^-9 seconds).  But, when cut quality is of primary importance, such as for producing very smooth HAZ-free edges, or when processing thin or delicate substrates, these lasers are not always optimum.

In these cases, even shorter pulse durations, in the picosecond (10^-12 seconds) realm, can offer superior results.  This is because with this very short pulse duration, this is an athermal process which doesn’t cause unwanted heat to spread into surrounding material and cause a HAZ. And, the fact that the ejected material consists of very small particles (eg: as atoms) means that picosecond laser pulses do not produce recast, therefore leaving clean, smooth surfaces.

USP lasers are typically characterized by much lower pulse energies than nanosecond lasers, but with very high pulse repetition rates – usually in the 1 to 50 MHz range. So, each pulse removes a minute amount of material with minimal thermal damage, enabling unmatched depth control. Yet, the high pulse repetition rate delivers sufficient overall material removal speeds for many tasks.

Figure 2: A nitnol stent, cut with the Coherent Monaco femtosecond laser, shown at two different magnifications.  Notice the precision, edge quality and clean smooth surfaces in each case.

Femtosecond Lasers

Recently, interest has grown in using lasers with even shorter pulsewidths, specifically in the femtosecond (10^-15 seconds) domain. In medical device manufacturing in particular, three factors have driven this trend. The first is the growing need for increased miniaturization, superior edge quality and surface smoothness.

The extremely short pulse duration of femtosecond lasers further increases the advantages of athermal processing described previously. This is particularly valuable when processing thin films and delicate materials where no HAZ can be tolerated.

A second reason is the increasing use of mixed and layered materials, eg: bioabsorbable plastics on metal, or polyimide on glass. Femtosecond lasers produce very high peak pulse powers, which, in turn, drive non-linear (multiphoton) absorption in the material. Unlike traditional (linear) absorption, this is less dependent upon wavelength; the femtosecond laser can process virtually any material, even if it is transparent, such as glass. This allows coated and laminated substrates to be processed in a single step, enabling streamlined and lower cost fabrication in many cases.

Finally, femtosecond lasers are becoming increasingly attractive to industrial users because of recent improvements in their performance, lifetime, reliability and cost of ownership characteristics. Originally, femtosecond lasers were used exclusively for scientific applications. But, in the last few years, femtosecond laser manufacturers like Coherent have implemented a new laser material, ytterbium-doped fibre, which is scalable to much higher power. And because the laser material is in the form of a fibre, this new generation of industrial femtosecond lasers has simpler internal design and construction, leading to lower costs and significantly increased reliability.

For example, the Monaco series from Coherent provides up to 60W of processing power in a compact (667 mm x 360 mm x 181 mm) sealed package, which, given its lower capital cost and increased reliability, makes femtosecond laser processing economically competitive for a host of enabling applications in a variety of industries.  Moreover, these capabilities these lasers bring are available at several levels of integration. Options include standalone lasers, laser sub-systems (light “engines”) with scanning/focusing optics, complete machines with integrated part handling, and even complete solutions with custom software pre-optimized for a specified set of results for particular applications.

Figure 3: Three examples of stainless steel surface texturing produced with the Coherent Monaco femtosecond laser.

Cutting Precision Medical Devices

Many medical devices are formed from tubular blanks; common tasks are to make cylindrical cuts as well as produce intricate patterns for cardiovascular and peripheral stents. Testing has shown that femtosecond laser cutting delivers superior feature consistency and residual strength.

For this application, the laser is typically integrated in a workstation in which the blank is mounted to a moving stage with 4-axes of motion, (three translation and one rotation). The use of a femtosecond laser allows for kerf cutting of tube stock or flat stock material with micrometre-scale precision and tolerances. Processing is sometimes accompanied by high-pressure co-axial assist gas to help remove vaporised ablation debris when cutting thicker-walled materials.

For surface texturing of contoured materials, such as catheter balloons, or surface ablation of flat stock materials such as stainless steel, another approach is used. Here, a 2D scanner workstation is usually the optimum solution, employing a high-speed 2-axis galvanometer scanner to cover a 20cm radius. The use of a femtosecond laser enables highly precise results with sub-micrometre depth control.

Yet another approach has been optimized to perform tasks such as precision hole drilling in irrigated ablation tip catheters with controlled wall taper, precision placement of slots and grooves, or to create complex shapes in tubes or flat stock material. In this case, the workstation contains a 5-axis trepanning scan head with co-axial assist gas along with a 5-axis motion control system. Again the femtosecond laser provides sub-micrometre dimensional precision and clean surfaces with typically no need for post-processing.

Conclusion

Many industries face a challenge to produce ever smaller and more precise components, while simultaneously reducing cost. Ultra-short pulse laser micromachining supports this trend in several ways, since it naturally delivers small features without damaging, heating, cracking or otherwise affecting the bulk material, while its minimisation of debris and recast material almost completely eliminates the cost of post-processing cleaning.

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