handbook of modern sensors

544 18 Sensor Materials and Technologies
may be used to deposit thin and relatively thin (often referred to as “thick”) layers
of films on a substrate or semiconductor wafer. Among them, the frequently used are
the spin-casting, vacuum deposition, sputtering, electroplating, and screenprinting.
18.2.2 Spin-Casting
The spin-casting process involves the use of a thin-film material dissolved in a volatile
liquid solvent. The solution is pored on the sample and the sample is rotated at a high
speed. The centrifugal forces spread the material, and after the solvent evaporates,
a thin layer of film remains on the sample. This techniques is often used for the
deposition of organic materials, especially for fabricating humidity and chemical
sensors. The thickness depends on the solubility of the deposited material and the
spin film and typically is in the range from 0.1 to 50 µm. Because the process relies
on the flow of the solution, it may not yield a uniform film or can form island (filmfree
areas) when the sample has a nonflat surface. In addition, the material may have
tendency to shrink. Nevertheless, in many cases, it is a useful and often the only
acceptable method of deposition
18.2.3 Vacuum Deposition
A metal can be converted into gaseous form and then deposited on the surface of the
sample. The evaporation system consists of a vacuum chamber (Fig. 18.5) where a
diffuse pump evacuates air down to 10 −6 –10 −7 torr of pressure. A deposited material
is placed into a ceramic crucible which is heated by a tungsten filament above the
metal melting point. An alternative method of heating is the use of an electron beam.
Fig. 18.5. Deposition of a thin metal film in a vacuum chamber.