(Invited) All-Carbon Integrated Circuits for Flexible/Stretchable Electronics
Authors: Ohno Yutaka 
Published: February 2020

Flexible and stretchable electronics are attracting much attention because of the variety of potential applications from flexible e-papers though wearable healthcare devices. Among various kinds of electronic materials, carbon nanotube thin films have advantages in flexibility, stretchability, and performance because of the excellent electronic and mechanical properties. Low cost manufacturing is also possible with printing techniques due to the good processability of carbon nanotube films. In the presentation, I will talk about our recent works on flexible and stretchable devices based on carbon nanotube thin films for realizing wearable healthcare electronics, including high-mobility all-carbon thin-film transistors and integrated circuits fabricated on a plastic or PDMS film. The simple fabrication processes based on micro-patterning technique of CNT films and high-throughput printing techniques will also be presented.

First, we have developed a method to realize high-mobility CNT thin films, based on a gas-phase filtration and transfer process. CNTs were grown by a floating-catalyst CVD technique. The CNT network was collected by filtering through membrane filters at room temperature. The CNT network was transferred from a membrane filter to the substrate with electrodes of TFTs. This technique enables to form CNT films with high mobility and controllable threshold voltage on a plastic film. A TFT fabricated on a Si substrate with the transfer technique exhibited a high mobility of 634 cm2/Vs with on/off ratio of 6x106. Various integrated circuits such as basic logic gates, ring oscillators, and flip-flops were demonstrated on a transparent plastic film. The operation speed of the present IC was evaluated with the ring oscillator to be 12 μs/gate. Since the delay time is dominated by the parasitic capacitances of overlap regions of the source-gate and drain-gate electrodes, there is a room to improve the operation speed. By modifying the device structure to eliminate the overlap capacitance, the gate delay was improved to be 2.3 μs/gate.

CNT films can also be used as electrodes and interconnections, which are flexible, transparent, stable, and free of rare metals. By using CNT transparent conductive films, we realized all-carbon TFTs and ICs, in which the electrodes and interconnections consist of CNT film and the insulators consist of PMMA. All carbon ICs were operated at relatively low voltage of 5 V even for the thick polymer gate insulator. The all-carbon devices exhibit unique moldability, which is important property of plastic materials to produce plastic products from toys through electronics devices and medical devices. We demonstrated three-dimensional dome-shape devices formed by the thermo-pressure forming technique, in which devices were stretched bi-axially by up to 18%.

We also fabricated stretchable and transparent all-carbon transistors on PDMS substrates. One-dimensional tensile strain test showed a small degradation in drain current as 8 % under the tensile strain of 20%. We also confirmed that the device worked even for 40% tensile strain, and the drain current returned to the initial value when the strain was released.

Printing process is also attractive to fabricate devices on a plastic film at low cost. We have introduced high-throughput flexographic printing technique, which is a kind of high-speed typographic printing technique with a flexible relief plate made of photopolymer, in the fabrication process of CNT TFT. Bottom-gate-type CNT TFTs were fabricated by fully lithography-free and nonvacuum process on a PEN film.