Aluminum oxide mask fabrication by focused ion beam implantation combined with wet etching

A novel aluminum oxide (Al₂O₃) hard mask fabrication process with nanoscale resolution is introduced. The Al₂O₃ mask can be used for various purposes, but in this work it was utilized for silicon patterning using cryogenic deep reactive ion etching (DRIE). The patterning of Al₂O₃ is a two-step process utilizing focused ion beam (FIB) irradiation combined with wet chemical etching. Gallium (Ga⁺) FIB maskless patterning confers wet etch selectivity between the irradiated region and the non-irradiated one on the Al₂O₃ layer, and mask patterns can easily be revealed by wet etching. This method is a modification of Ga⁺ FIB mask patterning for the silicon etch stop, which eliminates the detrimental lattice damage and doping of the silicon substrate in critical devices. The shallow surface gallium FIB irradiated Al₂O₃ mask protects the underlying silicon from Ga⁺ ions. The performance of the masking capacity was tested by drawing pairs consisting of a line and an empty space with varying width. The best result was seven such pairs for 1 μm. The smallest half pitch was 59 nm. This method is capable of arbitrary pattern generation. The fabrication of a freestanding single-ended tuning fork resonator utilizing the introduced masking method is demonstrated.