Semrock Master Catalog 2018

TECHNICAL NOTE
Sputtered Thin-film Coatings
Optical thin-film coatings can be deposited by a variety of methods. Traditionally the most popular methods for depositing multilayer coatings –
required for higher-performance mirrors and filters – include thermal and electron-beam (e-beam) evaporation and ion-assisted e-beam evaporation
(IAD). These have been in use for many decades. Films evaporated without ion-assist have several significant shortcomings that largely stem from
the porosity of the resulting films. They are often referred to as “soft” coatings, because they are not very durable, they absorb water vapor which
results in wavelength shifting, they also shift with temperature changes, and they can exhibit noticeable scattering. With additional energy from an
ion gun directed at the substrate during the physical vapor deposition process, IAD coatings are sometimes referred to as “semihard” since they
are appreciably more dense, resulting in significantly better durability and lower moisture absorption, temperature shifting, and scattering. With all
evaporated film processes, variations in the vapor “plume” during the deposition process make it challenging to control the rate and uniformity with
high precision, thus making it difficult to manufacture large volumes of complex filters with a high number of precise-thickness layers.
Fluorophores
Single-band
Sets
Deposition Process
Resulting Thin Films
Electron-beam /
Thermal Evaporation
Physical Vapor Deposition
Ion-assisted Electron-beam
Evaporation (IAD)
Energetic Physical Vapor
Deposition
Sputtering
Energetic Physical Vapor
Deposition
Variable deposition rates Variable deposition rates Extremely stable deposition rates
Variable spatial uniformity Variable spatial uniformity Controllable spatial uniformity
Soft coatings Semi-hard coatings Hard, dense coatings
Low durability Moderate to high durability Very high durability
Hygroscopic (absorb moisture) Minimally hygroscopic Impervious to humidity
Appreciable temperature shifting Low temperature shifting Very low temperature shifting
Some scattering Low scattering Very low scattering
Some absorption Low absorption Very low absorption
Low film stress Film stress Reproducible film stress
Multiband
Sets
Cubes
Laser
Sets
In contrast, Semrock manufacturers all of its optical filters with a deposition process called sputtering. This state-of-the-art technology was originally
developed for coating precise ferrite thin films for magnetic disk drive heads, and then gained a reputation in the optics arena for fabrication of
extremely low-loss mirrors for ring-laser gyroscope applications. In the late-1990’s it was adapted to manufacture the highest-performance optical
filters for wavelength-division multiplexing in the booming fiber-optic telecommunications industry. Sputtering produces hard refractory oxide thin
films – as hard as the glass substrates on which they are coated. This stable process is renowned for its ability to reproducibly deposit many hundreds
of low-loss, reliable thin-film layers with high optical-thickness precision.
One way to clearly see the difference among soft evaporated films, the more robust films
produced with IAD, and the very dense, low-scattering films resulting from the sputtering process is to
study the film surface morphology closely. Atomic force microscopy reveals surface characteristics
indicative of the packing density of the films. The graph below shows results from a study that
compared the three main deposition methods as well as two other less-common modified processes
[1]. Films were coated on substrates with a starting root-mean-square (RMS) surface roughness below
0.5 Å. Only sputtering produces highly multi-layered films with sufficient packing density to result in
surface roughness comparable to that of the starting substrate.
RMS Surface Roughness (Å)
12
10
8
6
4
2
e-beam
evaporation
ion-assisted
e-beam
evaporation
(IAD)
plasma
enhanced
IAD
substrate
roughness
ion
plating
ion-assisted
ion-beam
sputtering
0
[1] “Optical Morphology: Just How Smooth Is That Surface?,”
C. Langhorn and A. Howe, Photonics Spectra (Laurin Publishing),
June 1998.
Assist
Ion Beam
Source
Target
Substrate
Deposition
Ion Beam
Source
A perceived limitation of the sputtering process has always been throughput – the excellent
performance came at the expense of slow deposition rates and limited coating areas. For the
established applications of disk drive heads and telecom filters with dimensions of only one to several
mm at most this limitation was not too severe. However, it was considered a show-stopper for costeffective
production of larger filters Door
in higher volumes. Semrock broke
through this limitation by turning
sputtering into a true high-volume
manufacturing platform for large
(dimensions of inches) very high layer count optical filters.
And we did this without compromising the optical performance for which
sputtering was renowned, resulting from dense, low-scattering thin film layers
of extreme optical-thickness precision. Semrock made ground-breaking
developments in process technology to boost rates and uniformity, and we
are continually improving the process even today. And our highly advanced
deposition-control technology based on the proprietary hardware, algorithms, and
software of Semrock’s “optical monitoring” system enables repeatable deposition
of many hundreds of thin film layers of even arbitrary thickness for complex filters
with superb spectral features.
Pump
Port
Ion-assisted Ion-beam Sputtering
NLO
Filters
Individual
Filters
Dichroic
Beamsplitters
Tunable
Filters
43
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