The role of etching anisotropy in the fabrication of freestanding oxide microstructures on SrTiO3(100), SrTiO3(110), and SrTiO3(111) substrates

The release process for the fabrication of freestanding oxide microstructures relies on appropriate, controllable, and repeatable wet etching procedures. SrTiO (STO) is among the most employed substrates for oxide thin films growth and can be decomposed in HF:water solution. Such a process is strongly anisotropic and is affected by local defects and substrate cut-planes. We analyze the etching behavior of SrTiO substrates having (100), (110), and (111) cut-planes during immersion in a 5% HF:water solution. The etching process over the three substrates is compared in terms of pitting, anisotropy, macroscopic etch rate, and underetching effects around HF-resistant (La,Sr)MnO thin film micropatterns. The release of targeted structures, such as the reported (La,Sr)MnO freestanding microbridges, depends on the substrate crystallographic symmetry and on the in-plane orientation of the structures themselves along the planar directions. By comparing the etching evolution at two different length scales, we distinguish two regimes for the propagation of the etching front: an intrinsic one, owing to a specific lattice direction, and a macroscopic one, resulting from the mixing of different etching fronts. We report the morphologies of the etched SrTiO surfaces and the geometries of the underetched regions as well as of the microbridge clamping zones. The reported analysis will enable the design of complex MEMS devices by allowing to model the evolution of the etching process required for the release of arbitrary structures made of oxide thin films deposited on top of STO.