Jena, Germany – Long, narrow structures in the micron scale in thin metal foils of <100μm thickness are used for shadow masks, electrical contact bars, medical devices, and other purposes. The production of such structures requires processes that induce minimal mechanical and thermal stress to the structures while providing economic feasibility.
Laser micro-cutting is a non-contact, cost-effective process if heat accumulation from the laser and the mechanical force from a process assist gas can be controlled. For economic efficiency, it is necessary to perform the process free of burr or debris so that post-processing costs can be saved.
Femtosecond laser processing
The process requirements mentioned above call for laser pulse widths in the femtosecond range. Femtosecond pulses enable new machining qualities that result from a non-thermal ablation effect. On a femtosecond time scale, material ablation works similarly to vaporization by breaking of chemical bonds, but without a melting step. Peripheral areas of the ablation zone are hardly affected because the thermal penetration depth, which depends on the pulse duration, is below 10nm for femtosecond pulses. This generates excellently processed surfaces in the ablation zone and, in fact, obtains the best possible burr-free cut edges.
Regarding heat accumulation, ablation threshold fluence has to be considered. Due to this material-specific value, not all of the laser pulse energy is converted into ablation. In the peripheral areas of the beam focus and in deeper material layers, the fluence does not reach the threshold, which results in residual heat input. Since the threshold fluence typically scales with the inverse square root of the pulse width, the heat input per femtosecond pulse is much lower than for longer pulses or continuous-wave lasers. By means of an adapted beam deflection strategy, it is possible—even for high pulse-repetition rates of several hundred kilohertz—to keep the local heat accumulation at levels sufficiently low to facilitate processing of thermally sensitive materials.
Furthermore, non-thermal ablation in the femtosecond range requires only low pressures of a process assist gas, so there is no risk that the gas flow deforms delicate structures.
Experimental cutting results
Flat copper and titanium sheets measuring 20 to 40μm thick were used to demonstrate cutting of bars 20 to 100μm wide and 1 to 2cm long (FIGURE). Tests were performed on a lab laser machining setup comprising a 10W femtosecond laser (Jenoptik's JenLas femto 10) and a galvanometer scanner. Since the selected components are industry-proven, the results can be transferred directly to industrial applications. Straight cuts were made in single- and multi-pass modes to find optimum ablation conditions. The cut quality was evaluated by the sharpness of the cut edge and thermal deformation or distortion of the bars.
|Copper foil with a 1cm-long bar of 20 × 20μm2 cross-section was cut at 13mm/s without any thermal distortion.|
Femtosecond laser requirements
Beam quality is decisive for excellent focusability and high cutting precision. A thin-disk laser like the JenLas femto 10 has a beam quality very close to the theoretical limit. Thin disks allow perfect cooling and thermal control, which leads to high beam quality. And the thin-disk laser is insensitive to back-reflection from the workpiece.
Cutting speed may be scalable with the pulse repetition rate and pulse energy before cut quality is impacted. The 10W laser with tunable pulse repetition rate that was used in the application delivers, for instance, 50μJ pulse energy at 200kHz, which ensures sufficient energy margin for parallel cutting with a beamsplitter.
This article was written by Susanna Friedel and Nikolas von Freyhold of Jenoptik, Jena, Germany; www.jenoptik.com/lm.