Antireflection coating of microstructures by anodizing and influence on the dimensional accuracy of structures
Effective micro- and macro-optical sensory constructions require an anti-reflective coating to prevent internal stray light. Otherwise the scattered light leads to measurement noise, false signals or sensor saturation affecting the functionality. Optical apertures in the range of d = 300 µm and less, which can be prepared as pinhole or slit by UP-machining, have high demands in the single-digit micrometre range on roundness and dimensional accuracy to avoid aberrations. The structural quality should not be influenced adversely during the coating process. One possible, and with less R = 4 % reflectivity in the visible range, competitive coating is black Al2O3 anodization. Due to its ceramic fabric and the bonding to the substrate it is particularly resistant. The layer can be produced directly on ultra-precisely manufactured aluminium (ACP 5080) structures that is suitable for anodizing. The coating process is a sequence of etching in 10 % NaOH, the anodizing step in 17 % H2SO4 and a colouration process. Dimensional losses occur in particular in the first two steps. During the etching process there is only a removal of the surface. The surface modification during the anodizing results in a removal with simultaneous layer growth. The superposition of the additive and subtractive contributions defines the total loss. In particular tiny cracks on the edge of bodies in the ceramic material may occur due to the resulting structural weakening. Regarding a compensation of those losses, predictions about the erosion behaviour have to be enabled. Depending on the anodic current, the etching and anodization time, the resulting removal rates can be influenced. However, a reduction in the material removal results in an increased reflectivity. An application-related compromise of both parameters should be aimed. The influence of parameters on these conflicting requirements was examined in order to derive design-related technological recommendations.