Deep-patterning of complex oxides by focused ion beam with PMMA-assisted hybrid protective layer

Studying novel properties of complex oxides in nanoscale has become a popular research interest. Nanofabrication of complex oxides without damaging its intrinsic structure, however, is still challenging. In this work, we investigated the commonly used focused ion beam (FIB) technique for deep-patterning SrTiO 3 (STO) using Cr as a surface protective layer and found that it was insufficient in protecting STO against the damage caused by the FIB beam tail effect. We further developed a new method for effectively deep-patterning STO using FIB. Our approach adopted a hybrid surface layer of Cr and polymethyl methacrylate (PMMA) to protect the STO surface during the FIB milling process against the damage caused by the beam tail. This PMMA-assisted hybrid protective layer can effectively prevent the damage resulting from the energetic ion beam, as verified by high-resolution transmission electron microscopy characterization. It was found that PMMA is not spun off during the FIB process but forms bubbles and likely absorbs the energy from the ion beam during this process. At the same time, a thin Cr layer of this hybrid served as a charge-releasing path and kept the patterning precise. This mechanism is very different from simply using Cr as a scarifying surface layer for ion bombardment.