Patterning functional oxides: Some failures and solutions in fabricating a Hall bar

Functional oxides exhibit diverse useful properties, opening attractive prospects for electronic, magnetic, and optoelectronic devices. However, the chemical instability of these oxides with some microfabrication processes, especially when dealing with thin films (<40 nm) or micrometer-scale lateral dimensions, can pose a considerable challenge. Here, we describe the fabrication process of a Hall bar device used as a test structure for thin functional oxide films. This process employs photolithography and dry etching, utilizing standard clean-room materials and methods. The device layout and fabrication process are described in detail, including conclusions and lessons gained from several failed attempts during process development. A key lesson is the advantage of employing an ultrathin protective layer to protect the oxide film surface. In addition, we encountered two additional fabrication challenges: photoresist hardening during dry etching and edge accumulation of photoresist during spin-coating. Effective solutions to these issues are described. We hope that our lessons and conclusions, not limited to functional oxides, will help to shorten the process development for other materials and devices.