Tech Briefs Magazine - September 2021 - PIT-10

The Hidden Effects of Optical Coating Stress
O
ptical coatings are critical for
enhancing the transmission, reflection,
or polarization properties of optical
systems (Figure 1). However, any
time a coating is deposited on an optic it
imparts some level of stress onto the surface.
This stress can deform the optical
surface and lead to wavefront errors in
reflected or transmitted light. In many
applications this effect is negligible, and
a compensation coating layer can be
added to the back surface of the optic to
cancel out some of these effects, but
coating-caused surface deformation can
be an important issue in precise laser
optics systems.
Many modern optical coating techniques,
such as magnetron sputtering
and ion beam sputtering, utilize highenergy
material deposition. While these
technologies can form coatings with
unique advantages, they can result in a
significant amount of stress in both the
coating and substrate. Truly understanding
the differences between coating
types and how coating stress can impact
your application is essential for ensuring
that high-precision laser applications
achieve desired performance.
Why Care About CoatingInduced
Stress?
The stress imparted onto optical surfaces
from coating can lead to issues
with surface flatness, impacting the
transmitted or reflected wavefront error
of coated optics (Figure 2). Depending
on the coating materials used, this stress
can either be compressive (causing the
surface to bow upwards from their
edges) or tensile (causing the surfaces to
bend downwards from their edges).
Compressive stresses are preferred relative
to tensile ones because tensile film
stress can lead to the film itself cracking,
but both types can lead to issues if the
surface is significantly deformed.[1]
These effects are more significant
when the optics are thin and/or particularly
wide. Thicker optics with smaller
diameters are more resistant to deformation.
Compensation coatings on the
back side of optics have been shown to
reduce up to 90% of surface deformation,
but in some circumstances different
coatings are needed on each surface
of an optic.[1] In this case, the coatings
would be imparting different levels of
stress and would not compensate for
each other as well.
In highly sensitive applications, the
remaining deformation after applying a
compensation coating could still be
enough to noticeably degrade system
performance. When the surface figure of
optics is highly controlled (such as when
a λ/20 surface is specified), coatinginduced
surface deformation is more
noticeable. In lower precision components
(such as those with a λ/4 surface
figure specification), deformation from
coating stress is likely negligible.
Figure 1. A wide variety of optical coatings are used to achieve desired spectral and polarization performance
in optical systems, but the act of depositing thin films onto an optical surface imparts some
level of stress and could deform the surface.
SF2
tc
ts
(PV)
D
Substrate
Coating
Figure 2. This exaggerated depiction of the effects of coating induced stress shows how the optical surface
can bend either upwards or downwards. Relationships for modeling this behavior are given in
Equations 1, 2, and 3.
10
Cov
ToC
Modeling Coating-Induced Stress
and Deformation
The amount of surface deformation
resulting from coating-induced stress is
dependent on the surface figure and
dimensions of the optic, substrate material
properties, and coating type.[2]
These relationships can be used to create
tools that estimate the change in surface
figure before and after coating an
optic, as well as whether that will have a
noticeable impact on system performance.
The resulting radius of curvature
(R) of a flat optic after coating can be
approximated using:
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Photonics & Imaging Technology, September 2021

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Tech Briefs Magazine - September 2021

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