Functional flexible thin film electronics

The field of flexible electronics has advanced rapidly in recent years. This dissertation reports progress on a few components for flexible electronics with a special focus on process development, system integration, flexibility testing, and improvements in flexibility. In this dissertation, a rigid-substrate-compatible, lamination- and transfer-free process is developed to fabricate ZnO thin film transistors (TFTs) on very thin (~5 microm) solution-cast polyimide substrates. The flexible ZnO TFTs have very similar electrical characteristics to ZnO TFTs fabricated on rigid glass substrates. Typical TFT mobility is > 12 cm2/V·s for a gate electric field of 2 MV/cm. Inverters and 51-stage ring oscillators have been demonstrated using ZnO TFTs on thin flexible substrates. After a simple mechanical release, most TFTs show unchanged electrical characteristics on the freestanding flexible substrates. The performance of the flexible TFTs also remains the same when bent to radii between 7 mm concavely and 5 mm convexly.;Vanadium oxide (VOX) films have been studied as a temperature sensing material. Motivated by its potential of high-rate and uniform deposition, we investigated using RF diode sputtering to deposit VOX films as an alternative to the currently most commonly used ion-beam deposition. We found that process control for bolometer-grade VOX is very difficult. Small changes in oxygen-to-argon inlet ratio result in dramatic changes in the resistivity of the deposited VOX films, and a positive feedback mechanism drives depositions performed without active control to become either metallic or high resistivity films. We found that the local oxygen partial pressure near the reactively sputtered target varies with the oxidation of the target and is thus a strong candidate for an active control mechanism for reproducible reactive RF diode sputtering of VOX thin films.;As efforts have taken place towards integrating more functions into flexible electronic systems, flexible organic light emitting diodes (OLEDs) have been demonstrated on the same thin flexible substrate as for the aforementioned flexible ZnO TFTs. The OLEDs use sputtered ITO anodes, evaporated organic layers, and evaporated Al top cathodes. Green light emission is confirmed for both on-rigid-carrier and freestanding flexible OLEDs. A direct patterning technique based on substrate surface energy contrast for parylene films as an encapsulation layer for electronic devices, including OLEDs, is also reported.;The mechanical flexibility of flexible electronics is investigated in the context of testing strategies and the experimental results of different types of measurements. Besides the commonly reported static bending test, the strategies and apparatuses for repeated bending tests by push-toflex and roller-flex are developed with the capability of in-situ electrical measurement while bending or flexing. Our flexible ZnO TFTs survive more than 50,000 repeated bending cycles with a bending radius of 3.5 mm with very little change in electrical characteristics. With a smaller bending radius of 1.6 mm, the modified TFTs with Ti gate survived more than 10,000 bending cycles. For a better understanding of device failure mechanisms, numerical simulations for strain and stress distributions in flexible structures under various stretching and bending conditions are carried out using COMSOL Multiphysics. Means to improve device flexibility have also been proposed based on the experimental and simulation results.;The work reported in this dissertation represents developments in multiple aspects for flexible electronics and helps to pave the pathway to ubiquitous flexible electronics. This dissertation also provides a foundation for further study and development of the fabrication process and flexibility improvements for flexible electronic devices and systems.