Transfer printing methods for fabricating thin-film electronics on nonconventional substrates [electronic resource]
Keywords: Mechanical Engineering.

Thin-film electronics refer to devices composed from thin-films of active semiconductors, dielectrics and metallic connects that are deposited over a supporting substrate, frequently silicon or glass. Thin-film electronics are widely used for applications, ranging from solar cells, batteries, transistors, light emitting diodes to sensors. Fabrication of thin-film electronics on nonconventional substrates (e.g., papers, polymers, fabrics and metal sheets) presents exciting opportunities for realizing the next-generation of electronics, such as flexible displays, transparent touchscreen panels, wearable solar cells and bio-integrated electronics. However, fabrication of thin-film electronics on nonconventional substrates faces a significant challenge due to the mismatch between the device fabrication conditions and the tolerable processing conditions for the nonconventional substrates in terms of maximum temperature and chemical compatibility. To overcome this challenge, two transfer printing methods are developed, which involve the following main steps: (1) fabricating the thin-film electronics on a conventional silicon wafer using standard fabrication methods; (2) peeling off the entire thin-film electronics from the silicon wafer; and (3) finally attaching the thin-film electronics to nonconventional substrates. This general approach eliminates the need for further fabrication steps on the nonconventional substrates; therefore overcoming the above processing condition mismatch challenge. The first transfer printing method relies on the differences in adhesion to transfer thin-film electronics from weakly adhesive donor silicon wafer to more strongly adhesive receiver substrates. To demonstrate this, the silicon nanowire based thin-film resistors were used. Electrical characterization of the transferred thin-film electronics shows that ohmic contacts are maintained between silicon nanowires and metal electrodes, and are successfully demonstrated for applications of flexible piezoresistive sensors and temperature sensors with reliable performance. Nevertheless, about 30~50% of the devices are damaged during the peel-off process due to the high mechanical stresses. To resolve this problem, the second transfer printing method is developed, named water-assisted transfer printing method or peel-and-stick process, which relies on the phenomenon of water penetrating into the interface between Ni film and SiO2 layer. As model systems for the peel-and-stick process, we use hydrogenated amorphous silicon thin-film solar cells in addition to silicon nanowire based thin-film resistors, diodes and transistors as model systems. The transfer yield is improved to 100%, and the transferred thin-film electronics maintain their original geometries and device performances with high fidelity. We hypothesize that the peel-and-stick process is closely related to the water-assisted subcritical debonding phenomenon. So, we further experimentally investigate the critical adhesion energy of the Ni-SiO2 interface where the phenomenon of water penetrating occurs, and confirm that the critical interfacial adhesion energies are significantly reduced by 70~80% in the presence of water, compared to those in ambient environment. We believe that the peel-and-stick process has great potential for the manufacturing of high-performance thin-film electronics on diverse flexible/transparent nonconventional substrates.