Patterning of Hard Mask on (111)-Oriented BTO/LSMO Heterostructures on STO Substrates

Functional oxides exhibit properties that make them interesting for future electronic devices. Thin films of functional oxides can be routinely produced, but there lacks a standard technique for establishing nanoscale freestanding structures on insulating substrates. The nanostructures must be well defined and maintain bulk properties. In this work a recipe for establishing a two layer hard mask suitable for ion beam etching (IBE) has been developed. A chromium hard mask is patterned and deposited using electron beam lithography (EBL) and lift off. The pattern is transferred to an underlying carbon layer by oxygen reactive ion etching (RIE). Hard masks with hallbar patterns have been structured on 5 × 5 mm samples with thin films of La0.7Sr0.3MnO3 (LSMO) and BaTiO3 (BTO) on (111)-oriented SrTiO3 (STO) substrates. The hallbars have widths from 11μm and down to 130 nm, which are connected to 200× 200 μm contacts pads. A bilayer mask of PMMA and MMA with a total thickness of 240 nm is found to work successfully for lifting off 20 nm of chromium. Samples are prone to charging during electron beam lithography. Fine features are written using 100 pA beam current, while large features are defined at 10 nA. The hard mask patterned samples are prepared for IBE, which will transfer the hard mask pattern into the oxide thin film and substrate, realizing freestanding oxide structures. Followed by deposition of contacts and wirebonding, the devices can be used to investigate electrical transport properties.