Nanotechnology-Enabled Sensors

Fig. 4.17 A diagram of a simple diode sputtering system.
4.4 Physical Vapor Deposition (PVD) 159
The sputtering gas can be inert, such as argon. In this case, the deposited
thin film’s composition is the same as the target (perhaps with different
crystal structure). Reactive sputtering takes place when the deposited film
is formed by chemical reaction between the target material and a gas,
which is introduced into the vacuum chamber. Oxide and nitride films can
also be fabricated in this way. However, the deposition rate can be very
different. The deposition rate depends on parameters such as the softness
of the target, the target to substrate distance, the power applied and the energy
applied to the target. The applied energy can be in a range of several
Watts to a few thousands of Watts depending on the area of the target and
the deposition rate required. The deposition rate can be as small as several
nanometers to several micrometers per hour.
In practice, a variety of techniques are used to modify the plasma properties.
For instance, the ion density can be tailored to optimize the sputtering
conditions with a radio frequency (RF - 13.56 MHz) power source and
the application of modulating voltage at the target. Furthermore, the target
material itself plays a significant role when choosing the sputtering system.
For instance, a diode sputterer cannot be used for the deposition of insulating
materials due to charge builds up on insulating targets. This problem
can be solved by using an RF source, for which the sign of the anodecathode
bias alternates at a high rate. RF sputtering is used to deposit