Organic thin film transistors (TFTs) have been of great interest lately because of their potential applications in flexible systems, enabling devices such as electronic skins or implantable medical devices. With the ability to bend these new systems comes the question of how bending affects device perform. Consequently, thicker oxide layers are desirable because they are less likely to be stretched thin when flexed, preventing tunneling processes and high leakage currents. High-k dielectrics, such as the cubic pyrochlore Bi1.5Zn1Nb1.5O7 (BZN), have the potential to improve the reliability of this technology because they allow for a thicker film without de- creasing capacitive coupling.
In this work, we investigated how the operating characteristics, like capacitance, change when devices are flexed. When the BZN is bent, strain is introduced into the crystal structure which can affect the dielectric constant. To explore this, we fabricated MIM capacitors and measured capacitance at different degrees of curvature to extract the dielectric constant. The capacitors were fabricated with a reactive sputtered BZN. Frequently, BZN is annealed at temperatures of 500-700°C; however, many flexible substrates, such as the Kapton polyimide films used here, are not compatible with such high-temperatures. Without annealing, the BZN was amorphous with a dielectric constant of around 30 as compared to values up to 200 found in crystalline BZN.
We found that when the devices were bent to the radii of curvature, the capacitance dropped to 85-95% of the original capacitance when flat. As there was no apparent change in thickness or area of the devices, we’ve attributed this to a change in dielectric constant caused by strain in the crystal structure altering the alignment of electric dipoles in the material. When the devices were again laid flat, the capacitance returned to 95-99% of the original value. The information found in the MIM capacitor could be used to infer how device bending would affect behavior of a BZN-based OTFT for flexible applications.