Download Multispectral Optical Coatings Are Tough, Versatile for IR

Tech Feature
Multispectral Optical Coatings
Are Tough, Versatile for IR Applications
Hybrid diamondlike carbon (h-DLC) coatings for multispectral use
combine the hardness of protective DLC coatings with the multispectral
functionality of high-end IR coatings.
BY MICHAEL DEGEL AND ELVIRA GITTLER
JENOPTIK OPTICAL SYSTEMS GMBH
Optical coatings are
used in numerous industrial applications for optical
components. Besides the optical properties,
the mechanical properties of these coatings
play a significant role in the functionality
of the optical component. Thin layers made
of amorphous hydrogenous carbon (a-C:H)
have outstanding mechanical characteristics, including high hardness, and therefore have a high environmental resistance.
However, their optical performance is limited because of the single-layer design of
these thin-layer coatings.
Amorphous hydrogenous carbon is gen-
erally known as diamondlike carbon
(DLC). This term was originally established to point out the very high hardness
of these types of coatings and to distinguish this technology from other types,
such as graphite coatings.
Other characteristics of a-C:H coatings include high abrasion resistance and
transmission in the infrared wavelengths;
the coatings also are chemically inert for
corrosive chemicals, organic solvents, and
concentrated acids and bases. Because
of these properties, other industries have
found applications for a-C:H coatings;
Figure 1. Standard single-layer diamondlike carbon (DLC) coating compared with optimized antireflection
coating (ARC). DLC coatings are vital to optical systems because of their chemical and mechanical
characteristics and IR transmittance as coating material for filter and antireflection layers.
Images courtesy of Jenoptik.
e.g., the high hardness, abrasion resistance
and low coefficient of friction make a-C:H
layers an ideal surface treatment for automobile transmission parts and hand tools.
Because of their chemical resistance and
biocompatibility, a-C:H layers are compatible with artificial joints in human medical
applications.
DLC coatings play a significant role
in optical systems because of their mechanical and chemical properties and their
infrared transmittance as coating material for antireflection and filter layers.1,2,3
However, the optical performance of a
DLC coating is similar to that of a singlelayer coating, which limits the overall performance of an optical system. Figure 1
shows a standard DLC setup.
A look at other commonly used materials for conventional single-layer infrared coatings also shows a large disparity
between the mechanical properties and
the spectral performance of these materials. Dielectric materials such as fluorides
(PbF2, ThF4, BaF2) and chalcogenides
(Sb2S3, ZnSe, PbS), which have outstanding optical properties, offer low hardness
and environmental resistance. Oxide materials have significantly better mechanical properties; however, because of their
high infrared absorption, they can be used
only in very thin layers. Semiconductor
materials such as germanium and silicon, commonly used in infrared coating
systems, offer only moderate mechanical
properties.
The process for applying single-layer
coatings is very stable, but their spectral
performance is limited as a result of simple physics. For example, for a single-layer
antireflection coating (ARC), minimal re-
Reprinted from the March 2013 issue of PHOTONICS SPECTRA © Laurin Publishing
flection can be optimized only at a specific
wavelength, significantly narrowing the
working bandwidth of the optical system
(Figure 2). This is a key advantage of mul-
tilayer coating designs, which enable a
much broader working bandwidth.
Hybrid DLC coatings combine the durability of a DLC coating with the spectral
Figure 2. Layer setup of single-layer DLC coating. Minimal reflection can be optimized only at a specific
wavelength, thus narrowing the working bandwidth of the optical system.
performance of dielectric materials in a
multilayer design, resulting in an abrasionresistant, environmentally stable optical
coating that performs over a broader working waveband. This is achieved by adding
a layered dielectric system under an a-C:H
layer, aptly described as an h-DLC coating.2,3,4
Multilayer systems
for combined benefits
The development goal for the h-DLCARCs was to improve the spectral performance over the mid-wavelength IR (3 to
5 µm) or long-wavelength IR (8 to 12 µm)
wave bands, for the substrate materials
germanium and silicon, over the performance provided by a standard DLC singlelayer application.
A multilayer system, with up to 50 layers on each substrate surface, using the
described approach is shown in Figure 3.
The complexity of this design, and
the material mix, led to new approaches
in technology development. Standard parameters were not adequate for adhesion
between the a-C:H and dielectric layer, as
shown in Figure 4. Using the established
Figure 4. Hybrid-DLC without adapted coating
parameters after rubber and wiper test. Standard
Figure 3. Layer system of hybrid-DLC coating. This multilayer system has up to 50 layers
parameters were not sufficient for adhesion
on each substrate surface.
between the a-C:H and dielectric layer.
Amorphous hydrogenous carbon is generally
known as diamondlike carbon (DLC).
Tech Feature
Multispectral Coatings
Figure 5. Impacts/influences while combining a multilayer dielectric layer system
with a single-layer DLC coating.
Figure 6. Carbon bond structure. Carbon’s chemical composition is where the softer three-bond
and harder four-bond characteristics show up.
Figure 7. Standard and hybrid-DLC coated substrates after windshield-wiper test according to TS 1888.
standards TS 1888 (windshield-wiper test)
and MIL-C-48497A as testing directives,
the coating passed the tape and boil tests
but failed the abrasion and wiper tests, two
of the main benefits of a DLC-coated optical component. Therefore, new parameters
had to be established.
The combination of a hard (DLC) layer
and a soft (dielectric) layer system on a
thermally expanding substrate material
led to some technical challenges. At issue
were the grid tension between the substrate
and the coating (stress resulting from the
different thermal coefficients of expansion
between the substrate and the coating),
the “eggshell effect” of the layer system,
the tension between the dielectric and the
a-C:H layer, and the adhesion of the a-C:H
layer to the dielectric layer (Figure 5).
The eggshell effect can occur with a
material stack where the outer layer is
significantly harder than the inner layers.
If force is applied from the outside, the
harder outer layer, or “shell,” cracks, while
the inner layers remain intact.
To address these problems, adjustments
first were made to the hardness of the DLC
and the dielectric layers to eliminate the
eggshell effect and tension difficulties.
The biggest focus was to adjust the hardness of the DLC relative to the rest of the
coating system, increasing the DLC layer’s elasticity while retaining its mechanical properties.
It was found that the hardness and elasticity (E-module) correlate in a defined
manner. For DLC layers, a ratio between
0.1 and 0.16 generated the best results. The
chemical composition of carbon (Figure 6)
is where the softer three-bond and harder
four-bond characteristics are shown. By
adjusting the arrangement of these two
bonding types, the desired ratio of hardness to elasticity was achieved, and the
eggshell effect was eliminated.
The challenge of adhering the different
layers to each other and to the substrate
was solved using specific buffer layers that
support the adhesion between the different
elements with their diversity in surface
properties.
After overcoming the described material behaviors regarding adhesion, tension
and hardness/elasticity, the coating performance was then proved in application
usage, and the process was shown to be repeatable in a manufacturing environment.
Figure 7 shows the results achieved after
The eggshell effect can occur with a material stack where
the outer layer is significantly harder than the inner layers.
Multispectral Coatings
Tech Feature
Hybrid-DLC coated optics.
Figure 8. Comparison of the measured reflection of DLC and hybrid-DLC in the 7.5- to 11.5-µm
spectral range.
Meet the authors
Michael Degel is product manager for infrared
optics and systems at Jenoptik Optical Systems
in Jena, Germany; email: michael.degel@jen
optik.com. Elvira Gittler is business manager of
IR coatings at Jenoptik; email: elvira.gittler@
jenoptik.com.
Figure 9. Hybrid DLC has been developed for dual-band (MWIR [mid-wavelength] and LWIR [long-wavelength
infrared]) applications, including IR image-guided systems.
adapting the coating parameters on silicon
substrates.
Because the idea for h-DLC has its
roots in the improvement of optical properties of standard DLC coatings, the following measurement was taken from two
coated substrates to show the optical effect of this new technology.
Although a standard single-layer DLC
has an average reflection of 2.6 percent
for the long-wavelength IR spectral
range, it could be lowered with the multilayer system to a value of 0.95 percent
(Figure 8), which would lead to a higher
degree of efficiency and fewer reflec-
used to provide protection with only a
narrow AR window can now be used as a
multifunctional tool including protection,
a broadband AR window or a dual-band
filter, which leads to higher performance
and potentially fewer optical components
in the lens system, saving optical design
space and weight.
tions (“ghosts”) inside an optical system.
Promising results from the h-DLC development and different customer requests
have led to the idea of applying a highly
durable filter function to the optical surface. This provides even more possibilities to optical and system designers by
having a preoptimized optical surface that
filters the spectral range related to the performance of the system; for example, for
recently developed dual-band applications
such as infrared image-guided systems
(Figure 9).
This means that an optical element that
References
1. O.M. Kutsay et al (2001). Diamond-like carbon films in multilayered interference coatings for IR optical elements. Diamond and
Related Materials, Vol. 10, pp. 1846-1849.
2. Y. Pan et al (September 2009). Design and
fabrication of ultra broadband infrared
antireflection hard coatings on ZnSe in the
range from 2 to 16 μm. Infr Phys Tech, pp.
193-195.
3. J. Robertson (May 2002). Diamond-like
amorphous carbon. Mat Sci Engr: R: Reports, pp. 129-281.
4. X. Yao et al (June 9, 2006). Infrared durable
protective/antireflection coatings with high
performance on Ge and Si substrates. Proc
SPIE, Vol. 6149.
5. Ingmar Brenosch, Jena (2012). Research on
the optical and mechanical performance
of hybrid amorphous hydrogenous carbon
layers on germanium optics in the infrared
spectral range. Student thesis.