The entire fabrication process was completed using a Fujifilm Dimatix DMP-283113 printer for deposition. Individual printed material layers were thermally cured in an oven.

Device Structure

The general device structure is shown in Fig. 1, with 1a showing the device layout and 1b showing the cross section of the transistor. A top gate structure was used in order to protect the SWCNT thin film and provide repeatable printability14. Without a protective top layer, the SWCNT thin film can dissolve on contact with environmental contaminants and liquids.

Figure 1 (a) A composite optical microscope image of the final completed device used for high frequency testing. G is the gate electrode, S is the source electrode, while the D’s are the common drain electrode. The structure is designed to allow RF testing with 500 micron pitch GSG probes. (b) A cross section of the top gate transistor design. Source, drain, and gate were all printed using nanoparticle silver solution. Full size image

The substrate used was 500HN Kapton. This substrate provides a very high temperature tolerance ranging from −269 °C to 400 °C15, allowing for a wide range of compatible inks, while retaining high flexibility. Additionally, these properties make it an ideal substrate for use in aerospace and military applications.

First, SWCNT thin film structures were formed on the substrate using the techniques outlined in subsequent sections. Once the thin film was formed, the source and drain were printed on top, with one terminal using silver nanoparticle ink from Novacentrix Inc (JS-B40G) and the other using non-aqueous silver nanoparticle ink from UTDots Inc (UTDAg40IJ). The opposite polarities of inks cause the generation of a narrow, non-diminishing gap with a finite length. This process is discussed later in this paper. Silver was printed on top of the SWCNT thin film in order to maximize the contact surface area between the SWCNT network and the silver transmission lines. The source and drain were then cured at 200 degrees Celsius. Once this step was completed, a dielectric material (AZ5214 photoresist from MicroChemicals) was printed over the source, drain, and gap16. Finally, a gate was printed on top of the structure using the JS-B40G silver nanoparticle ink from Novacentrix.

The capacitance per unit area for the dielectric material was experimentally determined. Printing usually forms layers of very consistent heights, due to its high degree of control over the quantity of ink deposited in each layer. To find the value for C diel , a simple 100 μm × 100 μm silver patch was printed on top of a 1-micron thick layer of AZ5214. The capacitance across this structure was then measured allowing us to calculate C diel to be ~9.3 nF/m2. The relatively low capacitance is achieved due to the relatively thick gate dielectric and lower dielectric constant of AZ5214.

SWCNT Thin Film Printing

In the device outlined above the semiconductor consists of a network of single walled carbon nanotubes. Recent efforts for printed transistors have focused on the usage of semiconducting CNTs due to their exceptionally high mobilities17, 18. While other printable semiconductors have been used, none have shown the potential for the multiple GHz performance that CNTs have. While other bioFET devices have shown outstanding properties in terms of subthreshold swing and low gate voltage19, these devices have so far been unable to break into the GHz switching realm, whereas CNT devices have.

Typically, clean room processed SWCNT based transistors use some form of aligned SWCNTs20. This provides optimal performance while using the fewest number of CNTs. However, in recent years, the cost of CNTs has dropped to a point that using the minimal number is not necessarily a major concern from a cost standpoint21. While aligned thin films do provide significantly better performance22, 23, unaligned thin films are much easier to produce. Thus, efforts to ink jet print CNTs have largely focused on unaligned CNT networks14, 18, 24.

Past efforts to print these CNTs have largely been done using aerosol printer. This process has met with solid results18, 25, 26, but is not as flexible in terms of applications as ink deposition. Previously, our group formulated a mixture of CHP and SWCNTs which allowed the SWCNTs to be printed using a deposition printer17. CHP is ideal from a printing aspect for this application, as it has both a relatively low boiling point of 154 °C that is stable at room temperature, and a viscosity well suited to use in the Dimatix printing system24. However, while the CHP provides good print performance, subsequent layers must deal with the extreme hydrophobicity of the CNTs themselves. This means that the first layer of solution printed must be the only layer, as the CHP will be pushed away from the print site by the CNTs. To avoid this, a proprietary non-aqueous solution was developed with properties similar to the CHP. CNTs were dispersed via sonication in this proprietary solution at a 20% concentration by weight, leading to a solution like the one shown in Fig. 2a. This allowed for CNT printing without dealing with the aqueous/non-aqueous interactions of the CHP and CNTs.

Figure 2 (a) A vial of SWCNT ink with a 20% CNT concentration by weight. (b) SEM image of a CNT film post-annealing. While there is residual from the solvent, conductivity is not impacted. Full size image

Once the solution is deposited on the substrate, the proprietary solution is annealed away via thermal annealing. Figure 2b shows an SEM image of a multilayer CNT thin film after the annealing step. The final resistance of the CNT thin film was 200 kΩ, which was in line with past papers using aerosol printing18.

Chemical Gapping Process

One of the primary challenges in fabricating a transistor via any printing methodology is the achievable minimum channel length. Traditionally, inkjet printed electronics achieves this gap between the source and drain by merely not printing a space in a transmission line. In theory this yields a gap where the width is limited by the resolution of the printer. Given that a printer like the Dimatix has a resolution of 10–30 µm depending on drop volume13, this provides a small gap, but by no means a short channel compared to photolithographic methods in a CMOS foundry4, 5.

In practice, this gap often has to be much larger. Not only does print position have fairly significant error bars, but this issue is further exacerbated by affinity of inks employed. Thus, if the source and drain are printed using the same ink, and the chemical force is greater than the surface energy of the substrate, the ink will pull across the source and drain, causing closure of the gap and creating a short circuit.

The concept of using surface effects to control gap size is not inherently new11, 12, 27. However, past efforts have focused on the usage of some sort of self-aligning monolayer. This potentially places major restrictions on both materials and substrates that could be used. Additionally, the technique has not been ported to a roll to roll technique, limiting its potential for mass production. In order to combat these disadvantages, the technique outlined below focuses instead on creating the chemical effect with multiple wet layers. This allows for the selection of any two inks with opposite chemical properties. For the specific device produced here, conductive silver inks with opposing chemistries were selected due to their easy commercial availability. Both inks form silver thin films when cured. However, one is a hydrophobic, non-aqueous hydrocarbon silver nano-particle solution while the other is a hydrophilic, aqueous silver nanoparticle solution. What this means is that the two inks cannot mix, similar to the way water and oil will separate when poured in the same glass.

By printing the two inks in a single layer side by side, the chemical force would produce the gap as shown in Fig. 3a. In the printing process, the two ink sections were printed right next to each other with no gap from the printing process. However, the chemical forces between the two inks introduce a micron to sub-micron order gap.

Figure 3 (a) Illustration of the chemical forces between the inks. A small gap can be accomplished with any ink pairing with opposing chemistries, not just silver nanoparticle solutions. (b) Two parallel lines forming a chemical gap. The lines are electrically isolated. (c) A zoomed in image of an average chemical gap. Results like this were observed across multiple devices. (d) SEM photograph taken at 16880× magnification of the smallest observed electrically isolated gap. Full size image

One major advantage of this method over previously listed short channel methods is its flexibility and tunability. In our experiments, Kapton was used with two off-the-shelf inks. However, by using custom formulated silver inks, various solvent combinations could be used to introduce gaps with a specific length. Gap size would theoretically be controlled by the net polarity difference between the chosen solvents. This also allows the technique to be adapted to other substrates that have surface coatings that might affect the polarity of the substrate, and thus the interactions of the inks with it. As far as substrate limitations go, the primary issue would relate to surface roughness. At a certain surface roughness the ability of the inks to move apart might become impeded. However, this effect was not observed on Kapton, Glass or PET, which covers three of the most common substrates used in printed electronics.

Additionally, unlike most other short channel methods that have been developed for these types of devices, this technique is fully roll to roll compatible, and relies on the usage of inkjet deposition as opposed to gravure28 or offset methodologies29. Inkjet technology does not require the use of any kind of stencil or mask to produce devices, allowing designs to be changed at essentially no cost. This is in essence one of the core promises of inkjet deposition technology, which this technique does not interfere with in any way.

Additionally, this method has the added benefit of being highly reliable. Rather than relying on a mechanical process, this methodology relies on a chemical process which should be consistent as long as the formulation of the inks does not change. Thus, it is possible to print this gap much smaller without having conduction between the two sides of the gap while still maintaining print reliability.