Self-aligned, Capillarity-assisted Lithography for Electronics (SCALE), a New Strategy for Printed Electronics

Abstract

With the impending development of the Internet of Things (IOT) and wearable technology, the consumer electronics market is subject to enhanced demand for flexible circuits. Printing of electronic inks is regarded as a promising route to realize low-cost, high-throughput manufacturing of flexible electronic devices for a variety of novel applications. Roll-to-roll printing, in particular, can significantly improve the throughput and further reduce production costs. Although several printing techniques, such as inkjet printing, aerosol jet printing and gravure printing, are compatible with roll-to-roll processing, there are several key technical challenges when making flexible circuits with excellent electrical performance and high yield by roll-to-roll printing. First, materials registration is a significant challenge when building multi-layer devices by printing on a moving web. Misalignment of different material layers may degrade device performance or cause electrical shorts. Patterning small features less than 10 μm is another technical challenge for printed electronics. These two challenges limit the industrial application of roll-to-roll printing. To address these two challenges, a novel method termed SCALE (Self-aligned, Capillarity-Assisted Lithography for Electronics) has been developed to fabricate multiple components of integrated circuits. SCALE utilizes micro-imprinting to create a complex network of circular ink receivers and small capillary channels on the top surface of a plastic substrate. When inks are printed into the receivers by a drop-on-demand inkjet printhead, they spontaneously flow under capillary forces into all the capillary channels connected to the receivers. Film deposition occurs upon drying of the inks. Different films can be layered on top of one another by delivering each ink sequentially into receivers with overlapping ink receivers or capillary channels. Since receivers have diameters on the order of 100 μm, the precision required to deliver ink is substantially relaxed. Consequently, this process is more suitable for printing on a moving web and more compatible with high-throughput, roll-to-roll processing. In this thesis, fabrication of fully-printed resistors and low-pass RC filters via this self-aligned strategy is presented. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was used as the resistive material, and silver was used for the electrodes. Using SCALE, fully inkjet-printed, self-aligned resistors were achieved with resistance values ranging over five orders of magnitude while keeping the overall dimensions of the devices constant. SCALE was then employed to build low-pass RC (resistor-capacitor) filters with cutoff frequency from 0.4 - 27 kHz and excellent operational stability. Self-aligned, fully printed diodes on plastic substrates were also demonstrated using SCALE in this thesis. A new pattern design for devices in a vertically stacked structure is reported, which incorporated flow control structures to realize better control of ink flows and to improve device yield. Printed diodes exhibited outstanding rectification ratios (>1E4) and excellent stability against repeated bending. Overall, the work in this thesis expands the potential of self-aligned inkjet printing for producing fully printed electronic circuits.