26 September 2012
Titanium dioxide was the compound used to manufacture the first solid-state memristor device in 2008, and this technology is advancing rapidly, with commercial devices expected within the next few years. Memristor based memory is known as non-volatile storage, as the memory elements maintain their state even in the absence of power, which has obvious advantages for efficiency, cost and environmental concerns.
While memristors have clear uses for standard solid-state storage, when combined with flexible substrates they can also be applied to other thin-film technology, such as solar cells, keypads and displays.
Traditional fabrication processes, such as lithography, require complicated equipment, are time consuming, and manifest limitations in throughput and substrate materials. Printed electronics can overcome these shortcomings with decreased process times and improved area of fabrication, especially on flexible (plastic) substrates. Unfortunately, the resolution and performance of the fabricated devices are reduced compared to traditional techniques, but recent trends in materials sciences will help to improve this, and printing methods are becoming commercially viable.
Previous inkjet printing technologies (piezo and thermal) have had certain limitations, such as thermal problems, the inability to generate patterns smaller than the nozzle diameter, ejection frequency and clogging. To address these issues, the Korean team turned their attention to electrohydrodynamic printing.
Electrohydrodynamic jet printing, or EHD printing, is a relatively new kind of non-contact printing technology, in which liquid is pulled from a nozzle when a high electric field is applied. At a certain limit of electric field, the hemispherical meniscus in the nozzle deforms into a liquid cone (known as a cone-jet) by overcoming the surface tension forces. For deposition purposes, a single jet with a very small diameter can be ejected from the apex of the cone and then printed onto the substrate. This printing or jetting technology is widely used in the fabrication of printed electronics devices (patterning and thin-film deposition), bio-applications (drug delivery, DNA sampling), mass spectrometry and micro/nanofibre applications (electrospinning).
Kyung-Hyun Choi, lead author of the team’s Letter, explained in detail: “Electrohydrodynamic jet printing uses electric field energy to pull the liquid from the nozzle instead of thermal or acoustic energy” he said. “Due to this pull,” Choi continued, “a large ratio between nozzle diameter and jet diameter (micro/nano size) can be achieved, which helps in achieving the patterns with smaller dimensions compared to nozzle size”. Choi also added that the pull process means that “EHD printing is widely used to eject high viscosity inks containing functional materials, which is a limitation in traditional inkjet printing”. Finally, another major benefit of EHD jetting technology is that it can be used in continuous jetting, drop-on-demand and atomisation of particles (electrospray) with the same deposition setup and only modifying the operating conditions.
The team’s methods are not limited to flexible memory devices. Using EHD printing, Choi and the group are involved in the development of printed microsensors, organic LEDs and TFTs, CIS solar cells and organic photovoltaic devices. Specifically, Choi explained that “we are in the process of integrating a memristor as a transistor with an OLED device, and this can find its application in large area display panels as a next-generation technology”.
Given the relatively recent development of the technology, circuit jet printing is a field with huge potential for development and application. For example, for the large flexible printed electronics industry, there is a need to adopt a fabrication technique that has mechanical flexibility and also potential for monolithic integration of different electronic devices in a cost effective fashion. Choi explained that “EHD can play a vital role in the development of printed electronics on a large scale by integrating it with roll to roll printing systems. Also it can be easily hybridised with other conventional thin-film fabrication techniques and can lower the overall cost of fabrication.”
The Letter presenting the results on which this article is based can be found on the IET Digital Library.
Browse or search all papers in the latest or past issues of Electronics Letters on the IET Digital Library.