This ionization takes the form of a luminescent cloud located
between the two electrodes, much like a neon tube. An electric current is
established between the two electrodes since part of the gas is now a
conductor. The positive ions are attracted to the negatively biased target
and the faster electrons are attracted to the anode. The positive ions
impinge on the target and, like the cue ball hitting a racked set of billiard
balls, dislodges atoms from the target. These atoms migrate toward the
substrate holder gradually coating the substrates.
As a result of the mechanical nature of sputtering, refractory substances
such as Ta, W, Mo and TiC can just as easily be sputtered as those with
lower melting points. Alloys are sputtered with a composition often
equal to that of the target material. NiCr films are readily deposited with
predictable stoichiometry, in contrast to evaporation methods where the
chromium first sublimes forming Cr-rich layers followed by Ni-rich
layers. To overcome this problem, dielectric materials may be sputtered
using an RF technique, discussed in 7.1.2.2.