Tech Briefs Magazine - September 2021 - PIT-12

Es is the substrate elasticity modulus
(Pa), Ec is the coating elasticity modulus
(Pa), ts is the substrate thickness (µm), tc
is the coating thickness (µm), νs is the
substrate Poisson's ratio (unitless), and
νc is the coating Poisson's ratio (unitless).
This shows that the coating material
making up different coating layers
impacts the level of stress and deformation.
Equation 1 assumes that the stressinduced
deformation is spherical, the
mechanical properties of the coating
materials are averaged from bulk values,
and that the coating is applied to only
one surface.
This radius of curvature resulting
from the coating-induced stress can
then be used to determine the surface
flatness after coating (SF2) in units of
either waves or µm.[2]
   





SF1 is the surface flatness before coating
(waves or µm) and D is the substrate
diameter (µm). R is assumed to be significantly
greater than D. The percent
change between the starting flatness
and flatness after coating can then be
given by:
    




The smaller SF1 is, the higher the %
change in flatness will be, and the more
significantly it will make a difference in
the end system. Exactly what % change
in flatness is acceptable varies widely
from application to application, but a
general rule of thumb is that a % change
of 20% or greater is considered a failure.[2]
Tools used to model this behavior
typically draw from historical data to
approximate the material properties of
different coating types. Because these
simulations rely on so many assumptions,
they are great starting places for
understanding how surface figure will
be affected by coating, but their exact
values may vary slightly from real-world
results.
Figure 3. Ion beam sputtering (IBS) can produce optical coatings with some of the best spectral performance, but
the technique's high-energy material deposition results in more coating stress than other techniques.
E-Beam IAD
Coating Stress
Low-Medium
Spectral Performance Stable
Repeatability
Layer Density
Layer Smoothness
Process Time
UV Capability
Relative Price
Med-High
Med-High
Med-High
Fast
High
Substrate Geometry Very Versatile
$
APS
Medium -High
Stable
High
High
High
Intermediate
Medium-High
Versatile
$$
PARMS
Medium -High
Stable
High
High
High
Medium
Limited
$$
High
Very Stable
Very High
Very High
Very High
Slow-Intermediate Slow
Low-Medium
Limited
$$$
Table 1. Coating stress and other key parameters differ between common coating technologies, making the
ideal coating technology for a given situation highly application-dependent (E-Beam IAD: ion-assisted electron-beam
evaporative deposition, IBS: ion beam sputtering, APS: advanced plasma deposition, and PARMS:
plasma assisted reactive magnetron sputtering)
12
Cov
ToC
IBS
How Coating Techniques Differ
Different coating technologies impart
different amounts of stress during
coating deposition. Techniques
involving high-energy material
deposition, such as magnetron
sputtering and ion beam sputtering,
tend to result in the most
stress, and therefore deformation.
Table 1 compares the key characteristics
of common optical coating
technologies, with coating
stress highlighted in the first row.
Ion beam sputtering can
achieve the highest spectral performance
out of the four techniques
shown in Table 1, so it is
often used for high-reflectivity
laser mirrors. However, this technique
also imparts the most coating
stress, so the stress-induced
deformation models shown are
the most applicable to ion beam
sputtered optics (Figure 3). These
components also tend to have the
tightest surface figure specifications,
further adding to the importance
of understanding deformation
from coating-induced stress
for these components. For optics
coated using lower coating stress
techniques like electron-beam
evaporative deposition, the effects
of coating stress are likely negligible.
For
high-precision applications
requiring high-energy material
deposition coatings, understanding
how coating stress can impact
surface figure and system performance
is critical. The equations
shown above and online calculators
based on these relationships
are a good starting place for
understanding if these effects will
cause a noticeable impact in your
Photonics & Imaging Technology, September 2021
http://www.abpi.net/ntbpdfclicks/l.php?202109TBNAV
Tech Briefs Magazine - September 2021

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