E-Beam IAD APS PARMS IBS
Spectral
Performance Stable Stable Stable Very Stable
Coating Stress Low-Medium Medium-High Medium-High High
Repeatability Med-High High High Very High
Layer Density Med-High High High Very High
Layer
Smoothness Med-High High High Very High
Process Time Fast Intermediate Slow-Intermediate Slow
UV Capability High Medium-High Medium Low-Medium
Substrate
Geometry Very Versatile Versatile Limited Limited
Relative Price € €€ €€ €€€
Table 11.5: The key parameters of common coating technologies show
that the ideal coating technology for a given situation is 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)
www.edmundoptics.eu/LO 37
onto optical surfaces and forms uniform, low-stress layers of specifi c
designed thicknesses. IAD e-beam coatings feature low losses in the
ultraviolet (UV) spectrum and high laser-induced damage thresholds
(LIDTs) in the near infrared (NIR) spectrum. This technique also off ers
more fl exibility for coating design than other methods, as it can use the
widest range of useable materials. IAD e-beam evaporative deposition
machines also produce coatings at a lower cost than other methods and
accommodate larger coating chamber sizes. This coating technology is
ideal for situations when fl exibility and cost are the top priorities over
high performance. Depending on the exact ion source used, this technique
can result in coatings with lower densities, limited smoothness
and refl ectivity, and less repeatable properties. This can make precise
layer thickness control more challenging than when using ion beam or
magnetron sputtering. For this reason, IAD e-beam evaporative deposition
cannot create extremely low- or high-refl ectivity coatings.
Ion Beam Sputtering
Ion beam sputtering (IBS) is a highly-repeatable coating technology that
creates coatings of very high optical quality and stability. During IBS,
a high-energy beam of ions bombards a target of the desired coating
material, causing target atoms to “sputter” off the target (Figure 11.15).
The target atoms experience a signifi cant kinetic energy (~10’s to 100’s
eV), which causes them to form a dense, hard, and smooth fi lm on the
surface of optical components,7 One of the main advantages of IBS is
that it allows for precise monitoring and control of parameters including
layer growth rate, oxidation level, and energy input, resulting in highly
repeatable coatings. High-speed substrate rotation also contributes to
exceptional layer thickness accuracy. This allows IBS to create some of
the most demanding optical coatings, including ultra-low loss mirrors
with refl ectivity values above 99,9%, chirped mirrors for ultrafast laser
applications, and fi lters with very sharp spectral transitions. The performance
of IBS coatings is also less aff ected by environmental factors,
such as temperature and humidity, than that of other coating technologies.
However, there are several drawbacks to IBS coatings, including
higher stress and loss in the UV spectrum. Slower growth rates and
smaller chamber sizes also lead to a signifi cantly higher relative cost
than other coating methods.
Advanced Plasma Sputtering
Advanced plasma sputtering (APS) is a modifi ed version of IAD e-beam
evaporative deposition that benefi ts from advanced automated processing
capabilities. APS utilizes a hot cathode DC glow discharge plasma
instead of an ion beam to deposit coating material. The plasma fi lls the
entire coating chamber, releasing target ions and depositing them on
optical surfaces. APS results in smooth, dense, and hard coatings that
off er more stable optical properties than IAD e-beam while keeping IAD
e-beam’s high level of versatility. APS can also deposit coatings in volume
at a similar price structure to IAD e-beam evaporative deposition,
making it preferable when large volumes of coatings with slightly more
demanding performance requirements. However, APS experiences
higher stress, has more loss in the UV spectrum, and requires iterative
process development which leads to a slightly higher cost compared to
IAD e-beam evaporative deposition. In many aspects, APS, along with
magnetron sputtering, can be considered an intermediate solution for
many parameters between IAD e-beam evaporative deposition and IBS.
Plasma Assisted Reactive Magnetron Sputtering
Plasma assisted reactive magnetron sputtering (PARMS) is another
plasma generation-based coating technology. A glow discharge plasma
is generated like in APS, but a magnetic fi eld “confi nes” it near the target
instead of fi lling the entire coating chamber. The plasma accelerates positive
ions onto the target, ejecting target atoms which deposit onto optical
surfaces. PARMS operates at a relatively low chamber pressure at high effi
ciency because of the confi nement of the plasma. This low pressure reduces
setup time and allows for more economical coating of high-volume
optics. Thin-fi lm coatings formed by PARMS are hard and dense due to
reactive gasses added to improve the stoichiometry of the coatings. PARMS
is highly repeatable, but not as highly repeatable as IBS. However,
PARMS has a higher throughput, making it an appealing middle ground
between the high price and performance of IBS with more economical
coating technologies such as IAD e-beam evaporative deposition.
ION Gun
Substrates
Target
Figure 11.15: IBS is a highly-controllable process which utilizes a highenergy
ion gun to sputter material off a target onto rotating substrates,
resulting in very accurate and repeatable optical coatings
References
1. Willey, Ronald R. Field Guide to Optical Thin Films. SPIE Optical
Engineering Press, 2006.
2. Greivenkamp, John E. Field Guide to Geometrical Optics. SPIE Optical
Engineering Press, 2004.
3. Paschotta, Rüdiger. Encyclopedia of Laser Physics and Technology,
RP Photonics, October 2017, www.rp-photonics.com/encyclopedia.html.
4. “Field, Ella S., et al. “Repair of a Mirror Coating on a Large Optic for
High Laser-Damage Applications Using Ion Milling and over-Coating
Methods.” Laser-Induced Damage in Optical Materials: 2014, July 2016,
doi:10,1117/12,2067920.
5. Collett, Edward. Field Guide to Polarization. SPIE Optical Engineering Press, 2012.
6. Vandendriessche, Stefaan. “No One-Size-Fits-All Approach to Optical
Coatings.” Photonics Spectra, Photonics Media, December 2016.
7. “IBS Mirror Coatings for Highly Demanding Applications.” Photonics
News, Laser Components Group, August 2016, www.lasercomponents.
com/uk/news/ibs-mirror-coatings-for-highly-demanding-applications/.
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