Fast Dry Fabrication Process with Ultra-Thin Atomic Layer Deposited Mask for Released MEMS-Devices with High Electromechanical Coupling

A fast, dry micro fabrication process combining atomic layer deposition, electron beam lithography and cryogenic deep reactive ion etching (DRIE) is presented. The process utilizes atomic layer deposited ultra-thin (t_ax < 5 nm) aluminum oxide (Al₂O₃) films as hard mask for the DRIE that is used for both vertical cavity and dry release etching. The use of ultra-thin films can be explained by the extremely high selectivity (1:70 000) between Al₂O₃ and silicon. The process rules and limitations are carefully analyzed and in order to test the process, multiresonant tuning fork resonators are fabricated.