44 Experimental Methods
deposition, a negative bias is applied on the target material, from which the
surface atoms can be knocked loose. With a suciently low chamber pressure
the mean free path of atoms is long enough to allow for deposition on a nearby
substrate. The target to substrate distance is approximately 10-15 cm in our
chamber. While a suitable mean free path is desirable the pressure should still
be kept high enough for the plasma to be stable and a higher pressure means
a larger number of Ar ions bombarding the target. These requirements lead
to a pressure range of about 1-100 mTorr (or roughly 10��3 to 0.1 mBar). A
schematic representation of the above description can be seen in gure 3.1.
Figure 3.1: Schematic of the sputter deposition process. Ar gas, in red, is ionized
in a plasma to form Ar+, in pink. A negative bias is applied on the target material, in
grey, which causes the Ar+ ions to bombard the surface. Surface atoms from the target
are then knocked loose and deposit on the substrate, solid grey. Figure from 186.
The rate of material deposited depends to a large degree on energy of the incoming
Ar ions, the pressure in the chamber and the sputter yield of the target
material. The energy of Ar ions and chamber pressure are both easily controlled
and are important parameters in the optimization of a deposition process. The
sputter yield is a material parameter and can not be manipulated. This is particularly
important to realize for co-sputtering purposes where a certain lm
composition requires matching deposition rates from two or more targets. The
dierence in sputter yields can be seen in gure 3.2. In extreme cases it can
vary with up to a factor of seven for pure metals, which is the case for Si and
Ag.
There is a tendency for the yield to increase with increased lling of the dshells
in the transition metals. This is related to the energy transfer factor and
the fact that harder collisions increase the yield while the atoms with a more
open electronic structure has low yields. Fundamental studies on the sputter