Electronic Material Properties - und Geowissenschaften ...

The spectrum shows that apart from C, hydrogen has been implanted. In the plasma,
different hydrogen-containing ionic species are present, such as CH4 + , CH3 + , and
CH2 + . The H profile starts at a certain level at the surface, and drops almost an order
of magnitude in depth. The carbon profiles of samples, treated with different process
times, are compared in Fig. 2. They were extracted from the C-12 signal. A strict
quantitative comparison is not possible, as the secondary ion yield might slightly
differ, however, one can state that the amount of incorporated carbon increases with
process time. Also, the shape of the depth profiles change. At the lowest process
time, the profile drops monotonely with depth.
A process time of 1 h leads to a typical PIII implantation profile: a slight drop below
the surface, followed by an increase, and a slope down to the background level.
When the process time is doubled, the profile changes further. A plateau seems to be
formed, indicating a possible saturation effect. The situation with tantalum is similar.
Fig. 3 shows the depth profile of five masses, apart from the above mentioned H, C,
and O, it is Ta-181, and Ta-181 C-12. Again, a typical PIII implantation profile is
observed.
Fig. 3: SIMS element
profile of Ta, treated
by pulse biasing for 2
h
The carbon profiles are compared in Fig. 4. An increase in process time leads to an
increase in the amount of implanted C. It can be assumed that the carbide film is
shallower in comparison to Ti. The thickness of the carbide zone is dependent on
mass and density of the sample element, because the projected range of the carbon
ions is a direct function of substrate mass and density. For a full acceleration voltage
of 20 kV and CH4 + ions, the energy of the C atom is 15 keV, while it is 1.25 for each
H atom, as the molecule breaks up upon impact and the single atoms take a part of
the kinetic energy of the molecule, according to their mass. Following SRIM
calculations, the projected range of 15 keV carbon ions in Ti is 32.5 nm, while it is
only 14.6 nm in Ta. Other species, such as CH2 + show accordingly a slightly higher
energy. However, a part of the ions have a lower kinetic energy, as they do not
experience the full acceleration voltage or they collide with molecules from the gas
atmosphere. The sum of all energies, as well as sputtering effects, lead to the
observed depth profiles.
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