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APS Technology
Plasma IAD - Advanced
Plasma Source (APS) - Precision optical coatings.
Introduction
Conventional evaporation
involves the e-beam evaporation of a material from a
crucible. The evaporant then condenses on substrates
thus building up a thin film. In order to promote densification
of the coating the depositing atoms need to receive
additional energy. Conventionally this additional energy
has been achieved by heating the substrates to ~200 C
and more recently by ion assisted deposition (IAD).
Plasma IAD with the Leybold APS (Advanced Plasma Source)
was introduced into the coating market in 1992.
APS technology produces
dense, environmentally stable films, and the ability
to deposit high performance coatings in the UV, visible
and near infra-red
onto glass and plastics. No discernible vacuum to air
shift, high temperature stability as well as low absorption
is experienced without the need for additional substrate
heating.
Technology
Review
Figure
1: Principle of the Plasma Ion Assisted Deposition
(APS)
Figure
2 : The inside of an APS chamber
Figure 1 illustrates the
principle of the plasma IAD operation. Apart from the
plasma source the coating chamber consists of a conventional
e-beam evaporation arrangement. The plasma source is
operated by applying a DC voltage between a large area
LaB6 cathode and cylindrical anode. The plasma consists
of argon ions and a reactive gas (typically oxygen)
which is introduced through a shower ring located on
top of the anode tube. The ions are magnetically accelerated
towards the substrate holder and transfer momentum to
the condensing film molecules thus increasing their
surface mobility and packing density (Figure 3)
Figure 3 - Illustration
of the densification of the coating by accelerated plasma
ions.
Material
Properties
Siltint routinely deposits
the following materials in its APS system;
· SiO2
Low index material, very low absorption in the UV, visible
and infra-red
· TiO2 High index material, very
low absorption in the visible and infra-red
· HfO2 High index material, particularly
suited to UV filters.
· ITO Transparent conducting oxide.
Research undertaken on
TiO2 layers has shown the high
obtainable refractive indices (n=2.37 @ 550nm) are very
insensitive to parameter variations and non-varying
with humidity and temperature cycling. The structure
of the APS deposited films, figure 4, is denser than
that of conventionally evaporated films.
Figure
4 - Microstructure of TiO2
deposited using conventional evaporation and plasma
IAD (APS)
Environmental stability
Our measurements have
shown that the films deposited using APS technology
are more tolerant to environmental instability.
Siltint is able to produce
edge filters that exhibit little to no shift with temperature
cycling, humidity, and other adverse environmental conditions.
Figure 5 illustrates the temperature behaviour of a
filter deposited using TiO2 and
SiO2 multilayers. Furthermore,
Dense Wavelength Division Multiplexers (DWDM) filters
have been deposited [2] in the 1550nm range that show
a temperature stability of 0.0023 nm / C
-> 0.0078 nm / C.
Figure
5: Temperature behaviour
of a filter deposited using APS technology
Temperature
Sensitive Substrates
The ability to produce
dense films on substrates without the need for heat
enables us to use the APS technology to deposit any
of our coatings (anti-reflection coatings -> Edge
filters) on plastic substrates such as polycarbonate.
Moreover, we are able to deposit the coatings on other
temperature sensitive substrates such as optical fibres
ends used in telecommunications.
Literature
Various papers have been
published by other groups demonstrating the performance
advantage gained by the use of Plasma IAD as listed
below:
1· "Advanced Broadband
AR Coatings in the Visible : A comparative study of
different deposition technologies", Bauer, H.H.,
et al, Proc SPIE - INT SOC OPT ENG, 1996 Vol 2776, p
138 - 143.
2· "Shift-free narrowband
filters for the UV-B region", Uhlig, H., et al,
Proc SPIE - INT SOC OPT ENG, 1996 Vol 2776, p.342 -
352.
3· "Plasma ion assisted
deposition : a powerful technology for the production
of optical coatings", Zoller, A., et al, Proc SPIE
1997, Vol 3133-23, p.196 - 204.
APS Technology on YOUR
Substrates
All Siltint coatings are
available deposited using APS technology. However, the
following coatings are, unless otherwise requested,
always deposited using APS technology to achieve the
highest degree of environmental and optical performance:
QUOTE
FOR THIS COATING ON MY SUBSTRATES
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