Aspect Ratio Dependent Scalloping Attenuation in Drie and an Application to Low-Loss Fiber-Optical Switches

The scalloping in the Bosch’s process[ 1] attenuates dependently on the aspect ratio: The scalloping factor, undercut length l
u
divided by etched depth in each Bosch’s cycle l
d
, attenuates drastically from the depth where the aspect ratio exceeds 1:1. The phenomenon is not found in the case of large openings at the same depth. Theoretical reason can be attributed to the transportation limit of radicals. This Aspect-Ratio-Dependent-Scalloping-Attenuation (ARDSA) is useful for the applications in which scallop-free surface is crucial, such as fiber-optical switches[ 2]. A simple two-step DRIE, proposed as a contour-lithography method[ 3], can make use of the ARDSA: By firstly etching the scalloping-critical surface using narrow trench openings and dummy structures and secondly etch the dummy, microstructure having scallop-free walls are obtainable. The authors studied scalloping attenuation dependency through trenches having the openings from sub-micron to hundreds of microns. After which a fiber-optical reflector having scallop-free vertical walls was fabricated. Scallop-free surfaces made by the contour lithography method showed +2.4dB better reflectivity.