Extensive research has been done on carbon nanotube field-effect transistors (CNTFETs), which may revolutionize electronics. Single-walled carbon nano-tubes (SWNTs) act as the channel between the source and the drain of the transistor device. CNTFETs can have very high on/off current ratios, but the presence of metallic CNTs, in addition to semiconducting CNTs, reduces the on/off ratio significantly. Therefore, the CNT thin film has a major influence on the transistor behavior and must be carefully developed.

Standard microprocessing techniques were used to Fabricate CNTFET Devices. The CNT films created in the first three processing runs were processed to create back-gated CNTFETs with silicon as the gate, gold electrodes as the source and drain, and CNTs as the channel.
The purpose of this research was to determine the most useful parameters for the controlled deposition of CNTs, optimized for the fabrication of CNTFETs. The main goal was to create CNTFETs with reproducible, predictable properties. Solution deposition serves as an alternative to the chemical vapor deposition (CVD) growth of nanotubes. It is faster and simpler, and can be conducted at room temperature, unlike CVD, which requires temperatures up to 900 °C. Ultimately, this process will help to increase nanoelectronic device manufacturability, enabling large-scale fabrication of CNT-based electronic devices. Deposition of CNTs from solution may also permit additional approaches to functionalizing CNTFETs to make sensors.

A total of three processing runs, which varied a number of process parameters, were completed for this study. The resulting CNT films were characterized to explore the reproducibility and uniformity of CNT films. The CNT solution types were pristine SWNTs solubilized with surfactant, COOH functionalized SWNTs, and aminopyrene non-covalently functionalized SWNTs with a dispersant. For each run, the devices were processed using the three solutions in parallel.

Various sample cleaning methods were evaluated because the resulting CNT films and substrates were noticeably covered with residue after solution deposition. One method was rinsing (while spinning) or soaking the sample with IPA, and perhaps water, for varying lengths of time, then drying it off with nitrogen.

In other experiments, the spin-processor speeds were varied to determine how the tube density changed with spin speed, if at all. Dynamic spins at speeds of 500, 1,000, 3,000, and 8,000 RPM were used to make CNT films. Static spins were compared to dynamic spins at different speeds using pristine CNT solutions and COOH-functionalized CNT solutions to see if preferential alignment of tubes was attainable. The number of drops of solution deposited during dynamic spins was varied to see if this impacted tube density. Another attempt at controlling tube density involved the comparison of a sample created from one static spin; a sample created from three consecutive static spins, allowing the sample to dry between spins; and a sample created from three consecutive static spins, placing the sample on a hot plate at 65 °C for 30 seconds between spins.

The CNT films created in the first three processing runs were processed to create back-gated CNTFETs with silicon as the gate, gold electrodes as the source and drain, and CNTs as the channel. Following fabrication, samples were characterized using the scanning electron microscope (SEM) to observe CNT distribution in the gaps between device electrodes.

Each CNTFET device on each sample was measured for transistor behavior to evaluate the solution deposition conditions for all the samples. The qualitative observations on the electrical properties of the devices include any post-processing treatments on certain samples and their subsequent electrical properties. The most successful samples were evaluated as having a high percentage of semiconducting devices and high on/off ratios.

The solution-based process for depositing CNTs onto substrates for processing into FETs, while amenable to using tubes of different functionalizations (e.g., COOH, aminopyrene, and pristine tubes), requires optimization for each type of tube. To this point, the aminopyrene non-covalently functionalized CNTs produced the most reproducible, predictable CNTFET devices. These samples had the highest yield in semiconducting devices. However, different types of tube functionalization may be useful for facilitating further functionalization of the CNTFETs in order to produce CNTFET-based chemical sensors. The deposition of CNTs from solution will provide additional flexibility for chemically modifying the tubes for sensing various analytes.

This work was done by Natalie Salaets and Matthew Ervin of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp  under the Manufacturing & Prototyping category. ARL-0082

This Brief includes a Technical Support Package (TSP).
Solution Deposition Methods for Carbon Nanotube Field-Effect Transistors

(reference ARL-0082) is currently available for download from the TSP library.

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