handbook of modern sensors

18.2 Surface Processing 545
On a command from the control device, the shutter opens and allows the metal
atoms emanated from the molten metal to deposit on the sample. Parts of the sample
which remain free of the film are protected by the mask. The film thickness is determined
by the evaporation time and the vapor pressure of the metal. Hence, materials
with a low melting point are easy to deposit (e.g., aluminum). In general, vacuumdeposited
films have large residual stress and thus this technique is used mainly for
depositing only thin layers.
Because the molten material is virtually a point source of atoms, it may cause both
nonuniform distribution of the deposited film and the so-called shadowing effect
where the edges of the masked pattern appear blurry. Two methods may help to
alleviate this problem. One is the use of multiple sources where more than one crucible
(often three or four) is used. Another method is the rotation of the target.
When using vacuum deposition, one must pay attention to the introduction of
spurious materials into the chamber. For instance, even a miniscule amount of oil
leaking from the diffuse pump will result in the burning of organic materials and
codeposition on the sample of such undesirable compounds as carbohydrates.
18.2.4 Sputtering
As in the vacuum-deposition method, sputtering is performed in a vacuum chamber
(Fig. 18.6); however, after evacuation of air, an inert gas, such as argon or helium,
is introduced into the chamber at about 2 × 10 −6 to 5 × 10 −6 torr. An external highvoltage
dc or ac power supply is attached to the cathode (target), which is fabricated
of the material which has to be deposited on the sample. The sample is attached to the
anode at some distance from the cathode.Ahigh voltage ignites the plasma of the inert
Fig. 18.6. Sputtering process in a vacuum chamber.