'Thin films & coatings' Roadmap - Nano Mahidol

2.3.4.5 Description of main applications
Thin Films Transistors (TFT)
It is a field effect transistor made by depositing thin films for the metallic contacts,
semiconductor active layer, and dielectric layer. For instance, thin films are used as a
layer of electrically insulating material (usually SiO2) called the gate channel. Above
the channel there’s the Gate that provides the electric field to switch on/off the
transistor. The trend has been to make transistors faster by making them smaller
(e.g. shortening the gate length thus shortening the path to be followed by charged
particles) and reducing the thickness of the gate channel (to strengthen the gate field
and maintains the maximum current even with less thickness). The problem to solve
is that making the gate channel thinner allows current to leak, thus wasting power,
heating the transistor and reducing its performance.
One approach to tackling this is by introducing appropriate dielectrics (called
interlayer dielectric / intermetal dielectrics. The performance criteria of such materials
are low dielectric constant, thermal stability and processability. Low k dielectric
materials (dielectric constant less than that of SiO2 (k < 3.9)) are increasingly
becoming important in IC technology, especially for fabricating dynamic random
access memories (DRAM). Some of the potential candidate materials to be used are
polyimides, polyindan, poly(arylether)s, poly(silsesquioxane)s and
poly(benzoxazole)s.
Over the last years, research has focused on deformation of Si crystals (strained Si)
for improved performance. With regard to thin films, the strain is produced depositing
layers of strained material (e.g. silicon-germanium) over the wafer surface.
There’s the possibility to exploit thin-film technology in large surface area. Thin-film
technology is CMOS compatible and therefore enables integration with Si integrated
circuits. Some applications have already been developed: thermal actuators,
resonators and air gap Thin Film Transistors (TFTs; based on polymorpohous Si or
nanocrystalline ZnO). As described below, other TFT applications are under
development.
Large area displays
Thin film silicon is well established for many large-area electronic devices such as
LCD or OLED displays. Besides the improvements described below, the possibility
to apply Si thin films over a polymer flexible substrate (instead of glass) could
ultimately lead to flexible devices broadening the range of potential (high addedvalue)
applications.
Liquid Crystal Displays (LCD): They consist of a liquid crystal solution between two
sheets of polarizing material. Each crystal either blocks or let light pass through
depending on its alignment. These displays do require a backlight to work. A thin film
transistor is used to stimulate a single crystal cell (pixel). LCDs already have
significant market shares (many well-established applications such as calculators,
lap-tops, mobile phones, etc.). Light Emitting diodes are considered as an alternative
light source for LCD.
32 Roadmap report on
Thin films and coatings