The growing field of printed electronics combines
liquid functional materials with state-of-the-art printing
equipment to create semiconductor components and
electronic circuits. The resulting devices are functionally
similar to their traditional silicon-based counterparts.
However, they're also less expensive and have a number
of unique features that open the door to a wide range of
new electronic applications, from tiny "smart labels" to
full-body-sized medical imaging equipment.
Printing techniques
Printed semiconductor
technology delivers a sharp increase in productivity by building
on a variety of established and familiar printing techniques.
But while traditional graphic arts must only look
good to the naked eye, electronics require precise electrical,
mechanical, and optical properties. No matter which printing
technology is selected, printed-electronics companies
demand state-of-the-art equipment and procedures.
Virtually any printing technique can be adapted for
semiconductor manufacturing, but certain processes
better suit particular materials or applications. Each
technology has its pros and cons. In practice, multiple
printing processes may be combined in series to produce
a single device.
Ink jet
One of the most popular technologies in
printed electronics is ink-jet printing. A typical ink-jet
printer has several print heads (one for each color or ink
type), each with dozens of tiny nozzles that spray ink onto
the substrate. Because it is a fully digital technology, it
does not require any tooling. An electronic design can be
directly converted into a printing file. It lends itself well to
rapid prototyping and customized batch production, but it
also can be used in a high-volume environment.
Ink-jet printing has many advantages, including fairly
high resolution (80- to 100-μm lines), flexibility, relatively
low cost, and compatibility with almost any type of substrate.
Printed electronics is driving further equipment
development, as the newest ink-jet heads may be capable
of 20-μm feature sizes, which would greatly expand
the use of ink-jet technology in electronics.
Screen printing
Another common technique in
printed electronics is screen printing. A screen consists
of a finely woven porous fabric or metal mesh stretched
over a frame. A stencil on top of the screen blocks off the
areas where ink should not pass. The screen is placed on
top of the substrate, and ink is applied. A rubber blade
pushes the ink through the open areas of the screen, and
the screen is then lifted away.
Screen printing can be used with a variety of substrates.
It's also possible to deposit thick films in a single
pass. On the other hand, it cannot be used to
deposit extremely thin layers. It was once considered a
very low-resolution technique, but state-of-the-art
screens can achieve features as small as 40 μm, with
sharper edges than ink-jet.
Nanoimprint lithography
A relatively new
technology that could be used in printed electronics is
nanoimprint lithography. Based on traditional photolithography
techniques used in the graphic arts, nanoimprint
begins with a three-dimensional stamp.
A layer of liquid resist material is either spin-coated or
dropped onto the substrate. The stamp is pressed onto
the resist (Fig. 1a), and the material is hardened (Fig. 1b)
with either heat or UV exposure. When the stamp is
removed (Fig. 1c), the hardened resist maintains the
shape of the stamp. The residual layer of resist may then
be etched away (Fig. 1d).
The patterned resist can then be used as a mask to
pattern subsequent layers and then be dissolved. Alternately,
the resist material, properly formulated, can
itself be a functional layer in the finished device. This
promises to be an excellent high-resolution technique. Resolution is limited only by the
stamp-making process and could
be as small as 20 nm - several
orders of magnitude below the resolution
of ink-jet or screen print. The
challenge may be formulating resist
materials that also have the desired
electrical and optical properties.
New materials
Because
graphics printing is done on a wide
variety of surfaces, today's commercial
printing technology can print on
nearly any material. The technology
is ready to handle anything from
thick glass to rough paper or plastic
to thin plastic film. Even curved surfaces
are possible.
This ability provides a number of
advantages for electronics. Instead
of being bound by thick, rigid silicon
substrates, electronic components
and circuits can be ultrathin, lightweight,
bendable, and transparent.
Substrate size is limited primarily by
the printing technology, and commercial
roll-to-roll printing equipment
can print on surfaces 2 m
wide and kilometers long.
The adoption of printing techniques
requires liquid functional
materials - conductors, semiconductors,
insulators, and so forth.
Although printed electronics is
often discussed as if it were synonymous
with organic electronics, in
practice, both organic and inorganic
materials may be used.
Printable inorganic materials
include metallic nanoparticle materials
such as silver and semiconducting
materials like quantum
dots in solution. The organic materials
are based on the Nobel Prizewinning
discovery in the 1970s
that conjugated polymers have
semiconducting properties. These
materials can be tailored to have
application-specific electrical and
optical characteristics.