Wüest M. 51 Wykes M. 82 Yamaguchi M. 17 Ybarra G. 129 Yubero F ...

JUNE 26 MONDAY MORNING
ETCHC-MoM-OR.5 OES TIME-RESOLVED CHARACTERIZATION OF THE
DEPOSITION OF MULTILAYERED COATINGS. J. Romero, A. Lousa. Departamento de
Física Aplicada y Optica, Universidad de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Catalunya,
Spain.
Multilayered structures with nanometric period and manocomposites materials are probably the most
promising alternatives to improve the properties of conventional coatings used for mechanical applications.
Multilayered structures are generally formed by alternatively piling two materials in a periodic
sequence ABABAB….., with a period Λ. Most of the multilayered structures referred in the literature
present mechanical properties that surpass those of their individual materials: increase of
hardness, elastic limit and toughness, and reduction of internal stresses.
We have developed a process for the deposition of multilayered structures by r.f. magnetron sputtering
in a continuous process. A single cathode with a metallic target (Cr) is used, and the alternate
deposition of two materials is achieved by periodically alternating the working gas composition. In
this way, the deposition of metal/nitride, metal/carbide and nitride/carbide multilayers can be
achieved just switching the gas composition between pure Ar and an Ar+N 2 , pure Ar and an
Ar+CH 4 , and Ar+N 2 mixture and Ar+CH 4 respectively.
Cr/CrN multilayered coatings with bilayer periods (Λ) between 120 and 2 nm were deposited by r.f.
magnetron sputtering (13.56 MHz) on Si wafers. The coatings were deposited by reactive sputtering
from a 3-in. diameter pure Cr target (99.99% purity) with a r.f. input power of 100 W and a targetsubstrate
distance of 5 cm. Two independent mass-flow meters controlled each gas flux (Ar and N 2 )
during deposition. Chromium thin films were deposited at 1.0 Pa pure Ar pressure, while chromium
nitride films were produced in Ar–N2 reactive mixtures where the total working pressure was 1.2 Pa
and the nitrogen partial pressure was 0.2 Pa. The Cr/CrN multilayers were deposited alternating both
experimental conditions. Cr and CrN growth rates were 0.8 and 0.6 μm/h, respectively; slow enough
to be able to produce multilayers in the nanometric range
Two nitrogen-related optical signals from plasma de excitation (N 0 2 = 337.1 nm and N + 2 = 391.4 nm,
were found to be the most easily monitored to appreciate changes in N 2 chamber presence. These
two well-defined nitrogen optical emission lines were measured in a time-resolved way during multilayer
deposition in order to monitor nitrogen in chamber residence time responses to the cyclic
switches in its flow.
Exponential time responses were found for cyclic switches, with relaxation times of 1.1 s for the N 2
switch off and 0.9 s for the switch on. These times are less than 4% of the deposition time corresponding
to a bilayer for bilayer periods down to 6-9 nm. Hence, for the multilayers with bilayer
thicknesses higher than 9 nm a well defined multilayer structure is expected, while for values lower
than 6 nm an increasing influence of interlayers and materials mixing is more probable to occur.
The multilayer structure was confirmed by TEM and LA-XRD measurements for bilayer periods
higher than 6 nm, what confirms the in-situ predictions of our OES measurements. The highest hardness
of the coatings set, 28 GPa, was obtained for the coating with the theoretic thinnest bilayer period
(Λ=2.1 nm). This maximum hardness was more than 100% higher than the expected value from
the rule-of-mixtures applied to CrN and Cr coatings
In summary, time-resolved OES plasma measurements are a useful tool to monitoring in-situ the
formation of well defined multilayer structures in a continuous process, with multilayer periods
down to a few nanometres.
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