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Abstract
Weak van der Waals bonding in molecular organic semiconductors allows depositing them without lattice matching on a variety of substrates, (e.g. glass, steel foil, and plastic), for low-cost, large-area device applications. As the device performance improves, lowering fabrication costs becomes important. Organic Vapor Phase Deposition (OVPD) and Organic Vapor Jet Printing (OVJP) may accomplish this, while also presenting scientifically interesting mechanisms of thin-film growth.
In OVPD, a hot inert carrier gas picks up molecular organic vapor and transports it into a hot-wall chamber, where the vapor selectively physisorbs onto a cooled substrate. The film deposition rate, uniformity, composition and morphology are controlled through the source and substrate temperatures, carrier gas flow rate, the source cell and the deposition chamber pressures. The composition and morphology of the deposited films bear directly on the electrical and optical device performance. Theory, simulation, and experiments are used to understand the mechanisms governing OVPD and demonstrate the method's capabilities.
Applications like full-color displays or transistor circuits require lateral patterning of the active organic thin films. Because organic semiconductors are typically incompatible with conventional patterning methods (e.g. photolithography), alternative techniques are employed. In-situ patterning using shadow-masks is studied. For OVPD, this involves Monte-Carlo modeling of molecular transport in confined geometries, where the apparatus dimensions are on the order of the molecular mean free path. Optimum operating conditions (e.g. pressure, mask-substrate separation) and mask aperture geometry are suggested and verified by patterning experiments.
Using the knowledge thus gained, OVJP is developed. Here, the light carrier gas mixes with the heavier organic molecules and is rapidly ejected through a collimating nozzle onto a proximally located cooled substrate. OVJP proceeds entirely in the gas phase, eliminating the shortcomings associated with liquid-based ink jet printing, enabling high-resolution, rapid and direct printing of molecular organic semiconductors. A theory of the flow is developed and verified by direct simulation Monte-Carlo models and printing experiments, showing how pressure gradients, nozzle geometry, distance to the substrate, and choice of carrier gas control the pattern resolution. A high performance pentacene TFT is printed at ultra-high local deposition rate.
