Strain Balanced Self-Supporting Single-Crystalline LiNbO₃ Thin Films for Flexible Electronics

Functional single-crystalline films with mechanical flexibility have attracted intensive interest due to excellent material quality and the wide applications in flexible electronics. However, the free-standing single-crystalline films with the thickness in sub-micrometer range usually deform due to insufficient mechanical strength or internal stress. This study introduces a strain balanced model (SBM) of a sandwich structure and an ion slicing-based strain compensation bonding method for fabricating ultrathin but self-supporting single-crystalline thin films. Based on the SBM and the strain compensation bonding method, a centimeter-scale strain balanced LiNbO₃ (LN) thin film (SB-LNTF) consisting of two pieces of 550 nm single-crystalline LN film and an intermediate layer of benzocyclobutene is successfully fabricated. In additional to flat, bendable, transparent, and lightweight, the fabricated ultrathin (<10 µm) SB-LNTF also exhibits excellent self-supporting property. Standard piezoresponse force microscopy amplitude butterfly curve and a 180° phase switching associated with ferroelectric behavior of LN film are observed, which confirm its high crystal quality of the ion sliced LiNbO₃ thin film. A flexible acoustic resonator demonstrated on SB-LNTF shows strong resonances. In principle, the strain compensation bonding method is also applicable to epitaxial lift-off films.