Lathe-based Printing for Versatile Fabrication of Thin-film Electronics

Abstract

It is undeniable that the digital technology revolution has brought our world closer together in many ways. Hardware, specifically electronics hardware, has been a major driver of the interconnected age as the physical bodies that are needed to support the digital world. This accelerated technical progress doesn’t seem to be slowing down, and there is persistent demand for more electronics to support increasingly sophisticated applications. Continued progress in electronics is opening doors to an expansive landscape of new applications that are challenging to produce with traditional fabrication technologies and are better accommodated by additive manufacturing approaches for printed electronics. Printed devices can offer better physical flexibility and design versatility with less complex and energy-intensive fabrications processes. With printing of thin-film electronics, technologies can be integrated into everyday objects more easily and parallel progress can be made on other ways novel electronics can be deployed and utilized. The work put forth in this dissertation presents contributions to the technical capabilities of printed electronics by way of advancing aerosol jet printing (AJP) with the use of a versatile lathe-based method. AJP is a printing technique that is capable of patterning solution-processed inks into films as thin as 100 nm using direct write (DW) computer-aided design (CAD) to deposit a wide variety of materials. AJP has been shown as a very capable method for printed electronics with demonstrations spanning the breath of electronic devices; however, there are serious obstacles to scaling up AJP and carrying its demonstrated successes to a commercially ready level. These challenges arise from difficulties in rapidly printing onto curved surfaces, controlling printed film thickness, and achieving reliable morphologies for thin-film printed electronics. By incorporating a specially designed lathe-like mechanism that changes the motion of the AJP system, we uncover a way to overcome these processing issues with AJP and open opportunities to develop better printed electronic devices. Specifically, we will present the design and manifestation of a lathe motion system that enables AJP in cylindrical-coordinates at any print speed up to 3500 mm/s. This versatile, lathe-based AJP fabrication approach is demonstrated for a variety of use cases with specific benefit to each, beginning with the printing of a conformal graphene sensor directly onto the surface of a catheter balloon, which was only made possible by inflating the balloon on the lathe and printing around its circumference. Then, we found a way to overcome film morphology issues with AJP by using high-speed rotation to improve control over film formation with centripetal force, which reduces ink spreading at all deposition rates. This improved film control gave the ability to tune film thickness for any type of material without compromising film quality, resulting in more consistent films for devices. To this end, we coupled improved film consistency with high print speeds to create a fabrication flow for outputting many devices in an array within a very short print time. Demonstrations of the versatility of lathe-based AJP include a fully-printed MoS2 memristor crossbar array that was validated in electrical behavior through multiple testing protocols. With better control over the film thickness, and consistent yield of functioning devices, we determined the influence of MoS2 thickness in the device and proved that a decrease in thickness lowered the required reset voltage significantly, thus reducing the power consumption of these devices for use in memory storage. Another demonstration involved assessment of the difficulties with developing printed dielectric materials and pointed to issues with film morphology as being a leading cause of poor dielectric functionality. We used lathe-based AJP to overcome morphology issues for a printed zirconia (ZrO2) dielectric with good uniformity and dense film packing; however, we also discovered substantial problems with trapped moisture in the films causing erratic electrical behavior. The extent of these moisture-related problems within our printed zirconia were fully characterized, with validation of high-k capacitance within the dielectric, and a temperature-free vacuum-based approach was used for clearing moisture out of the film, which then did not reabsorb into the film even in humid conditions because of zirconia’s moisture resilience. Using the novel lathe-based AJP platform, significant scientific challenges for thin-film printing were overcome and some long-standing scientific questions were answered, particularly related to the ionic behavior of printed dielectric films. What’s more, challenges were overcome that have previously affected this DW non-contact printing technique, including achieving truly conformal printing onto curved substrates, slow print speeds causing low yield and throughput, and polymer binders and film morphology affecting the development of printable dielectrics. However, there is more work to be done in expanding the use of this technology and employing it to bring more printable, and potentially sustainable, thin-film materials to printed electronics. Looking forward, the framework presented herein can be utilized to improve printed electronics for applications like medical devices, memory storge, and high-k gate oxides in thin-film transistors. These applications will all benefit from using lathe-based AJP in both low- and high-speed configurations to enable curved conformal printing onto any number of substrates with an arsenal of diverse materials and inks.