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In-process monitoring of laser ablation on thin steel membranes by multispectral shape-from-shading

: Jacobs, Paul-Philipp; Nelsen, Bryan; Taudt, Christopher; Rudek, Florian; Hartmann, Peter


Soskind, Yakov G. (Ed.) ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Photonic Instrumentation Engineering VI : 2-7 February 2019, San Francisco, California, United States
Bellingham, WA: SPIE, 2019 (Proceedings of SPIE 10925)
Paper 109250E, 7 S.
Conference "Photonic Instrumentation Engineering" <6, 2019, San Francisco/Calif.>
Fraunhofer IWS ()

Laser ablation of thin membranes for industrial applications such as Micro-Electrical-Meachanical Systems has high demands regarding the process stability, alignment, surface shape and surface roughness. In the production of laser structured membranes with thicknesses in the single-digit μm-range ablation monitoring is therefore desired at every stage of the manufacturing process. This works presents a shape-from-shading approach where the surface of microstructured circular steel membranes is illuminated by two different light sources in order to generate sufficient surface reflection data from which a three-dimensional depth profile is reconstructed. By rotational scanning of the sample under examination, data is gathered from different angles and wavelengths at the same time. The advantage of this novel approach is the gain in acquisition speed as the spectrally encoded angle information can be acquired within one camera frame. Data processing is performed on the R, G, and B channels of the recorded image in parallel. In experiments, steel membranes with thickness of 2:1 μm and ablated structures with approximately 1:5 μm depth were examined during the structuring process. In order to compare the results of the in-line metrology approach, the surface topography of the laser-ablated samples were characterized on a confocal laser microscope. A discussion of the implications of the results regarding the usability of the metrology approach in industrial use cases concludes the work.