Biomedical Engineering Reference
In-Depth Information
Optoelectronic devices require a transparent conductive layer with high electrical con-
ductivity and low visible light absorption. This layer can be prepared from a wide variety
of materials, including doped semiconducting oxides of tin, indium, zinc, or cadmium and
titanium nitride ceramics, as well as metals such as silver and gold (59). Among them, the
most widely adopted transparent conducting oxides (TCO) are that of tin (SnO
2
:F), zinc
(ZnO
2
:F), and indium (In
2
O
3
:Sn, ITO).
For flexible device applications, TCO layers must be deposited at low temperatures
because plastic substrates are thermally sensitive. Fluorine-doped tin dioxide (SnO
2
:F) is
most common due to its low manufacturing cost; however, it requires high deposition
temperatures and is very difficult to etch; hence, it is now widely used for glass sub-
strates. Fluorine-doped zinc oxide (ZnO
2
:F) can be deposited at lower temperatures and
has the greatest light transparency. Despite these qualities, it suffers from rather low con-
ductivity, precluding its use from sensitive electronics like optical sensors and flat-panel
displays. Although indium is a costly raw material and its tin-doped oxide (ITO) is some-
what more difficult to etch than zinc oxide, it allows very low-deposition temperatures
(
200°C) and high conductivity, making it ideal for deposition on thin flexible plastic
substrates (60).
The properties of a TCO depend not only on its chemical composition but also on the
method used for its preparation. Numerous methods are available for TCO deposition.
Some of the most common methods used for flexible substrate coating include chemi-
cal vapor deposition, magnetron sputtering, and pulsed-laser deposition (61-63).
Chemical vapor deposition is suitable for large-scale applications and can provide very
high deposition rates; however, many chemicals tend to be toxic or unstable.
Magnetron sputtering provides high deposition rates at low temperatures and can pro-
vide uniform coatings over large areas. Pulsed laser deposition is a relatively new
process and well suited for research laboratories. Some industrial implementations are
able to pattern the electrode directly and eliminate the etching process, but this method
is costly and limited to small areas.
17.2.2.2 Fabrication of Indium Tin Oxide Patterned Polyethylene Terephthalate Film
In this work, the flexible substrate is made of a 175-
m-thick PET film. This material is com-
monly used for flat panel displays, touch screens, and flexible electronic circuits because it is
inexpensive, mechanically durable, and provides high light transmission (typically
86%).
The ITO electrode pattern is designed and then transferred to an image, which is then used
to develop a photomask. A conductive ITO layer is deposited onto the substrate via pulsed
laser deposition (Sheldahl Inc.). The ITO-coated film has surface resistance of 35
/sq. The
2 mm and
is separated from the neighboring pixels by 1 mm. An independent ITO-coated wire with a
width of 300
flexible ITO electrode is imprinted as a 4
4 pixel array pattern. Each pixel is 2
m is connected to each pixel. These wires lead to two sets of terminals along
the sides. Each terminal is 1
3 mm, separated by 1 mm from each other. The overall area of
a patterned ITO electrode is 15 by 23 mm.
17.2.3
Device Fabrication
17.2.3.1 Principles of Electrophoretic Sedimentation Fabrication
Purple membrane patches, like many other cell membrane materials, maintain a small poten-
tial across the membrane. This small potential arises from differences in charge densities of
each side. The C- and N-terminals of the polypeptide chain are located on opposite sides of
the membrane. The cytoplasmic side of the membrane is negative with respect to the extra-
cellular side at pH values between 5 and 7; the polarity reverses when the pH falls below
