As seen in last week's issues of the journal Nature, work at the
University of Illinois at Urbana-Champaign (UIUC) has brought engineers closer
production quality flexible ICs. Utilizing the popular carbon nanotube
(CNT), UIUC's circuit is not only flexible, but much faster than typical
organic circuitry. Organics have previously been too slow for use in devices
like high speed displays.
The UIUC circuits are made using a combination of transfer printing and
lithography techniques. First, the CNTs are deposited over the surface of a
flexible substrate. The nanotubes are not forcefully aligned in any way, but
due to their dispersion, form a conductive mat, similar to the University of
microelectrode. Next, gold electrodes and various circuit components are
applied over the substrate and CNT coating. Finally, to prevent the CNTs from
bridging connections and short circuiting the network, soft lithography is used
to cut channels between the electrodes, severing any nanotube connections that
may bridge them.
One hurdle that organic circuitry has faced is a much lower operating speed
than standard silicon devices. In order to be useful in high-speed circuitry,
transistors must be capable of switching on and off thousands or millions of
times per second. While the UIUC circuits haven't breached the megahertz
barrier, their current speed of several kilohertz is more than enough for
simple devices and RFID sensors and on par with that of current LCD circuitry.
There are no “high tech” processes involved in UIUC's circuits. Transfer
printing and simple lithography techniques have been around for decades. While
creating uniform nanotubes is difficult outside of a laboratory, it is
certainly not an uncommon procedure at this time. The simplicity of the
fabrication process favors mass production, should a few small developments be
Jet printed nanotubes made headlines back in 2006 when Rensselaer Polytechnic
Institute and University of Oulu scientists used a standard ink jet printer to
print a popular picture of Albert Einstein using ink made of dissolved CNTs.
Since then, jet printing has become widely used in CNT research. Further
refinement to printing processes could benefit the UIUC circuits by virtue of
printing a more precise pattern of nanotubes. This could eliminate the need for
the lithography step of fabrication entirely.
The nanotube printing process is not alone in areas that could see improvement.
High precision methods of nanotube production are needed to insure that the
nanotubes used are of high purity and similar mechanical dimensions. In
conjunction with a refined printing process, this could benefit the UIUC
circuits both in ease of production and in quality management.
Mass produced flexible displays and wearable computers are not far off. Technological
advances like those at the University of Illinois are moving these futuristic
devices further from books and movies and closer to our homes and pockets.