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.
Microphone membranes tthat have theeir frequency response uniiquely tailoreed to specificc applications are typicallyy produced in an unautomated and expensive manuufacturing prrocess. A combination off holographic stroboscopyy, numerical simulation and laser structuriing is appliedd to shift the resonant frequencies of thee membrane too their desired values given an unknown tension across the microphone membrane due to manuufacturing tollerances. Thee uncharacterized microphone membrane is driven thrrough physical contact withh a piezo. Thhe piezo is swept through a range of frequencies and the full surface profile oscillations are recorded using stroboscopic digitaal holographyy techniques. These resonant displacemennt maps will bee used, in combination with finite differeence eigenvaluue simulationss and perturbattion theory, to determine the preloadedd tension profile across the membrane. Given a desired responsee function of the membrane,, a new membrane mass-density profile can be tailoreed to match the current meembrane to thee requirements of the micropphone. A 20 WW ns Q-switchhed laser steerred by galvanometer mirrorrs will be usedd to restructuree the mass density of the membrane to meeet the design requirements of the micropphone.
Laser-line scanners have become ubiquitous in many forms of automation and measurement systems. Despite this fact, these systems are still susceptible to speckle or interference on rough scattering surfaces. Many scanning systems must be calibrated to the material being analyzed to obtain their full potential. In general, post-processing algorithms are used in most modern line-scanning devices in order to smooth out speckle and enhance the resolution through sub-pixel interpolation. However, these post-processing techniques come at a cost of increased CPU time and a subsequent decrease in bandwidth and resolution. in this paper, a low-cost, high-resolution solution to generating speckle-free sharply focused laser lines is presented. The key to this technique relies on only removing the spatial coherence in one dimension using a 1-D cylindrical lens array as a beam homogenizer. This beam homogenizer is then wrapped around and rotated about a central axis in order to remove the temporal component on the laser's coherence. Since the plane-wave-like behavior is maintained along one dimension, this beam can still be sharply focused to a line. however, the spatial coherence and temporal coherence are reduced to the point that speckle is minimally visible.