Inline process analysis and control using optical measurement for the ultra-short pulse laser ablation of CAD designed microstructures

Generating microstructures using ultra-short laser pulses reducing the thermal influence to the work piece is a rapidly growing technology field. The physical connections for material laser interactions with short laser pulses are often too complex for industrial processes. Scientific results for a single pulse ablation can hardly be transferred to industrial applications for volume ablation. Finding a suitable parameter setting is a challenge because of the numerous process influences, such as pulse duration, repetition frequency or scanning parameters. An automated inline process control supports the analysis of material reactions to different laser parameter settings. A database that includes these reactions can be used to identify essential process variables. These variables can be used to generate a laser process sequence for the ablation of a structure based on a CAD generated model. In addition this database assists to analyze possible errors in the manufacturing process before the processing starts. These errors may be differences between a CAD generated model and the laser fabricated work piece and may result in inaccurate edge structures or incorrect ablation depth. Recurring measurements as taking pictures of the treated surface or analyzing the ablation depth and roughness offer a high potential for an automated process. The Laser Center of the University of Applied Sciences Muenster (LFM) presents a system design that involves a picosecond laser (8 ps, 800 kHz repetition rate) with scanning optics for the generation of microstructures, as well as microscopes (field view 2 to 10 mm) and chromatic sensors (z-resolution 0.2 to 1 µm) for the surface inspection of the generated microstructures. This machine design is used to generate a database including changeable laser settings and its material reactions such as ablation depth and roughness. Using algorithm for database evaluation it is possible to identify process influences which lead e.g. to a defined roughness and ablation depth. Combining this database with a CAD designed microstructure it is possible to evolve a process sequence for the laser ablation process. It can be shown that the information of the database helps to predict the deviation of the processed structure to the designed one. Using this feature the design of microstructures can be changed to reduce these deviations before processing, which decrease waste of material and developing time of the structure.