Transfer of Epitaxial Strained Multiferroic Thin Films on Millimeter Scale Using Rigid Substrate Epoxy Method

Multiferroic oxide thin films have diverse applications in the fields of integrated photonics, acoustic sensors, and memory devices. These devices are typically fabricated on substrates such as fused silica, lithium niobate, and silicon. However, these substrates can pose significant challenges for epitaxial growth due to their lattice structures. Pulsed Laser Deposition (PLD), a well-known technique for epitaxial thin film growth, is heavily limited by substrate selection due to its lattice parameter matching requirements, usually leading to growths being performed on SrTiO₃ (STO) or LaAlO₃ (LAO) substrates. However, a novel approach to enabling other substrates has attracted attention in recent years: film transfer. In this method, the insertion of a water-soluble layer of Sr₃Al₂O₆ (SAO) allows for epitaxial film growth while also enabling liftoff through dissolution of this sacrificial layer. The film of interest can be supported during release and transfer by attaching flexible polymer layers such as polypropylene carbonate (PPC) or poly(dimethylsiloxane) (PDMS) before dissolving the sacrificial layer. After adhering the film to the new substrate, the polymer layers can be removed. However, challenges associated with epitaxial strain have not yet been solved. Namely, when a highly strained film is released from the growth substrate, the substrate clamping effect is lost, allowing the film to wrinkle or curl to relax the strain. These deformations of the surface lead to cracks in the final transferred film, limiting the area of continuous film that can be successfully transferred. In this work, we report a new process where the rigid target substrate is adhered to the film, in this case the multiferroic Bi₃Fe₂Mn₂O_x (BFMO) material, using epoxy prior to releasing the film from the growth substrate. This creates a “sandwich” with the BFMO film between the original substrate and new substrate. The sacrificial layer is then dissolved, leaving the film attached to the new substrate. Since the film is not allowed to flex at any time during the process, the strain cannot be relaxed to cause surface deformation, resulting in large crack-free areas of transferred film—on the scale of several millimeters rather than several hundred microns as achieved by the polymer transfer method with strained films. Finally, we will demonstrate that the transfer process preserves crystal quality and therefore the multiferroic properties of the film.