Micromachined printheads for the direct evaporative patterning of organic materials

Abstract

Organic optoelectronic devices are appealing for low-performance applications on very low cost and flexible substrates, due to their low-temperature processing. However, it still remains a challenge to develop suitable fabrication techniques to pattern organic thin films on low-cost, large-area substrates. The two techniques used commercially are inkjet printing of polymers, which limits the morphology and performance of devices, and shadow-masking of vacuum sublimation for small molecule materials, which is not scalable to large-area substrates. In this thesis, we investigate the use of MicroElectroMechanical Systems (MEMS) to provide new ways of patterning organic materials deposited by an evaporative process. We present the design, fabrication, modeling and characterization of two generations of micromachined printheads developed to expand the possibilities of printing of organic optoelectronics. The design and fabrication of a compact electrostatic actuator enabling the first generation of printhead is first presented. It is then used to actuate a microshutter, and modulate the flux of evaporated organic materials in a vacuum chamber. We prove the feasibility of evaporative printing of small molecular organic materials at resolutions of the order of 800 dpi with high-throughput on large areas.

(cont.) We demonstrate that MicroElectroMechanical Systems can be used to pattern organic thin films in a way that combines the advantages of ink-jet printing and thermal evaporation. We also present the design and fabrication of a microevaporator for molecular organics, and show its suitability for the ambient printing of devices on low-cost substrates, without the limitations of ink-jet printing due to the drying of solvent on the substrate. We demonstrate the feasibility of using an array of pores in a membrane to capture molecular organic materials delivered by a solvent and an integrated microheater to release them by evaporation onto a substrate. This second generation of printhead enables evaporative printing of organic materials at ambient pressure. This thesis also provides a study of the failure of thin film platinum heaters used in the second generation printheads. We study the effect of current level, temperature, presence of a membrane, anneal conditions, and adhesion layer thickness on the failure of the heaters.