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THE IMPACT OF COMPUTERS ON THE DESIGN AND MANUFACTURE OF
OPTICAL MULTILAYER COATINGS DURING THE PAST 50 YEARS
J.A. (George) Dobrowolski
National Research Council of Canada, Ottawa, Ontario, Canada (retired)
Presented at the 50th SVC TechCon in Louisville, KY, in the Optical
Coatings session on May 2, 2007
ABSTRACT
Fifty years ago, with very few exceptions, optical multilayer coating
designs were composed of layers of quarter wave optical thicknesses. To
obtain a given performance, thin film designers usually varied the number of layers and the refractive indices in a multilayer. Design methods
based on vectors, and on various charts and diagrams, were often too
tedious to apply when problems needed to be specified at many wavelengths and were at the same time complex enough to require a larger
number of layers for their solution. Also, the monitoring of the thicknesses of non-quarter wave layers was difficult then. Philip Baumeister’s
1958 paper in which he described the use of a computer optimization
method to refine the performance of an approximate starting design to
obtain a short wavelength cut-off filter was a major breakthrough in our
field. However, the solution asked for the use of non-quarter wave layers.
This work was followed by further developments in numerical design
as well as in more sophisticated monitoring procedures, also based on
the use of computers. Since that time, progress in numerical methods,
deposition techniques and in-situ measurement has made possible the
reliable automatic manufacture of optical thin film systems with very
complex spectral transmittance or reflectance characteristics consisting
of many tens, or even hundreds of layers. In this talk an attempt will be
made to review these developments.
in nature. The powerful Herpin equivalent index concept was first
described in 1947 [8, 9].
Nevertheless, with the above tools most of the generic filter types
that we know today had been designed and produced. However, the
systems were two-material quarter- or multiple quarter wave solutions,
except for antireflection coatings in which more than two materials
were used.
In 1957 optical thin film systems were mostly deposited by evaporation, a process that in those days was not very well controlled. This
necessitated the use of in-situ measurements of the layer thicknesses.
Quartz crystal monitors at the time were more primitive than they
are today. They did not provide a direct measurement and so required
calibration and, like all indirect methods, they were not very reliable in
the presence of fluctuations in the evaporant plume. The direct optical
monitoring of layers of quarter wave thickness was simpler because the
end-point of the deposition of each layer occurred at a maximum or
minimum of T or R (Figure 1a). Thus this method did not require any
absolute measurements. Even though at the termination point the measurement was least sensitive to thickness errors, Macleod and Pelletier
showed later that a thickness error compensation mechanism occurs
that leads to good results [10]. This effect it is still of great importance
to-day. Certain narrow-band high rejection telecom filters consist of in
excess of a hundred layers and they can only be successfully produced
if the thicknesses of certain groups of layers in such systems are controlled by the quarter wave monitoring method.
DESIGN AND MANUFACTURE OF OPTICAL MULTILAYERS
IN 1957
Before 1957 most optical multilayer systems consisted of quarter waveor multiple quarter wave layers. The most important exceptions were
two-layer V-type antireflection coatings and metal-dielectric Fabry-Perot
interference band pass filters. There were two impediments to the use
of non-quarter wave systems: the design and the monitoring of the thicknesses of such layers.
The two-by-two matrix representation of a layer for the calculation of the properties of thin film systems was well-known by then so,
theoretically, calculations on non-quarter wave systems could have been
performed [1,2]. However, computers were not widely available then
and performing calculations on anything but the simplest systems was
tedious with hand-cranked or electrical calculators. The tools available
for the design of optical thin film systems at the time included Smith
and Kard charts, admittance and circle diagrams, and vector methods
[3,4]. These methods were very useful for systems consisting of a few
layers, and they provided a lot of insight into the way such systems
worked, but they were sometimes very approximate. Exact explicit
expressions for the reflectances of one, two and three-layer systems
were published [5-7]. For systems consisting of more layers, various
series expansions for systems of equal optical thickness which yielded
the refractive indices of the individual layers have been described,
as have expressions for the reflectances of periodic systems [3,4].
But these methods were still too cumbersome to use on problems in
which transmittances T or reflectances R were specified at very many
different wavelengths and when the systems consisted of many layers.
Furthermore, they often asked for refractive indices that did not exist
Figure 1. Control of layer thicknesses, a) quarter wave monitoring, b) most sensitive
wavelength method [34].
continued on page 36
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IMPACT OF COMPUTERS
continued from page 33
PROGRESS IN OPTICAL THIN FILM DESIGN
The use of only quarter wave layers in the design of non-absorbing optical multilayer coatings came at a great cost. The performance of a multilayer depends on its construction parameters,
nm, ns, di, ni, i=1, 2, … L
and on the overall thickness of the multilayer. Here nm, ns are the
refractive indices of the incident medium and substrate, and di, ni are
the metric thickness and refractive index of the i-th layer of an L-layer
system. When one chooses to deposit only quarter wave layers, one thus
neglects half of the available construction parameters. This is serious,
because the quality of a solution depends on the overall thickness of the
layer system and on the number of construction parameters.
With regard to the status of refractive indices as variables in thin film
design, Alexander Tikhonravov has shown in 1987 with his maximum
principle that, for problems with normal incidence of light and for solutions having the same optical thickness, the performance is not improved
if more than the two extreme refractive indices of the available coating
materials are used [11,12]. However, in the two-material solutions, the
total number of layers will usually be larger and some of the individual
layer thicknesses may be smaller. This follows intuitively from the Herpin
equivalent index concept [8, 9]. The maximum principle does no longer
hold if the problem is specified for non-normal incidence of light polarized in more than one direction.
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in a systematic manner. But the real breakthrough came in 1958 when
Philip Baumeister showed how, by using computer refinement methods,
the secondary reflectance ripples of a quarter wave stack could be
dramatically reduced to produce a good cut-off filter without the need
to increase the number of layers (Figure 2) [13]. Baumeister used the
damped least squares method in his optimization.
Refinement
When computers became more widely available, they were first used
for the analysis of the spectral and angular properties of more complex
layer systems. Some people started to perform trial-and-error calculations in which the thicknesses of some layers of the system were varied
Figue 2. Baumeister’s cut-off filter produced by the refinement of layer thicknesses.
Calculated performances (a), (b) and the construction parameters (Table 1) before and
after refinement [13].
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Table 1. Construction Parameters
Number of
layers
Refractive
index
substrate
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
medium
1.52
1.38
2.30
1.38
2.30
1.38
2.30
1.38
2.30
1.38
2.30
1.38
2.30
1.38
2.30
1.38
2.30
1.38
1.00
Optical thickness
A. Start
B. Refined
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
0.1550
0.1500
0.1500
0.2792
0.1630
0.1477
0.1506
0.1509
0.1440
0.1434
0.1495
0.1500
0.1427
0.1417
0.1514
0.1550
0.1406
0.1379
0.1907
0.0000
Since that time, many more optimization methods have been used for
optical thin film design. These can be classified into methods in which
zero, first and second order derivatives are evaluated during the refinement process. Most refinement methods require the minimization of a
single-valued function that represents at any moment of the design process the performance of the system. Many people call this function the
merit function M, others prefer the term error function. Quite frequently
it is defined in the following way:
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overall thickness is sufficiently large for the problem on hand [16].
In the fifties and early sixties work was started in different laboratories on more powerful thin film design methods that did not require
starting designs. Such computer programs are commonly called synthesis methods. In Russia, two groups published papers on such design
methods [17,18]. Although it was easy to think of problems for which
these methods would fail, they nevertheless inspired researchers at the
NRCC to produce methods that would be more successful.
The Comprehensive Search method requires no starting design.
In theory it finds, from a large number of combinations of layers with
different refractive indices and thicknesses, that system which has a
performance that is close to the global optimum solution to the problem
[19]. This assumes that there is a solution that consists of a number
of layers small enough so that the computation speed of the computer
used is sufficient to find it in a reasonable time. Figure 3 shows the
performance of an antireflection coating for the He-Ne laser wavelength
(λ=0.6328 µm) that reduced the substrate reflectance for unpolarized
light to less than 1% for all angles of incidence up to 60° [20].
For problems that required many more layers for a solution, the
Gradual Evolution method was proposed [19]. It was an extension of
the comprehensive search method and it permitted solutions to much
more complicated problems to be found. As already mentioned above,
to find a solution to a complicated problem, the designer must have not
only enough layers L at his disposal but also a sufficiently large overall
optical thickness of the layer system. Because the solution is gradually
evolved by adding a number of layers to the design in several stages, the
solution found will no longer be close to the global optimum solution,
but it can still consist of a reasonable number of layers. The example in
Figure 4 is a linear filter in which the transmittance changes approximately from 1.0 to 0.0 within the 0.4–0.7 µm spectral region.
where QiT, Qi are the desired target and current values of the i-th
quantity of interest and δQi is the acceptable tolerance for that particular quantity. Qi can represent any one of a multitude of properties of a
multilayer and the merit function can be defined in terms of a combination of different types of Qi because the use of δQi in the denominator
normalizes all the diverse quantities [14]. However, most frequently Qi
represents T or R defined at different wavelengths, angles of incidence
and planes of polarization.
An investigation to find which optimization method is the best for
refinement of optical thin films not surprisingly did not reveal any one
preferred routine – the best method depends not only on the particular
problem but also on the stage in the optimization process [15]. Most thin
film design programs provide a choice of several optimization methods.
Synthesis
One problem with optimization methods is that it helps to have a starting
design with a performance that is a good first estimate of the required
performance. For very complex problems choosing a good starting design
is not an obvious task. Some people found good solutions to problems by
refining several systems with an appropriate number of layers but with
randomly selected thicknesses. Baumeister, for example, suggested that
a suitable starting point for refinement would be a multilayer system
composed of the type (0.1H 0.1L)N, where H, L correspond to quarter
wave optical thicknesses of the highest and lowest refractive indices
the available coating materials, and where N is large enough so that the
Figure 3. A wide-angle antireflection coating for the He-Ne laser designed by the comprehensive search method [20].
continued on page 38
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IMPACT OF COMPUTERS
continued from page 37
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materials. The refractive indices of these individual layers are then,
one at a time, replaced by the refractive index of that of two or more
pre-selected coating materials which yields the lowest value of the merit
function. This process is very fast because during these calculations the
optical thicknesses of the individual layers are not varied. The above
procedure is repeated for all the layers of the system until the performance stabilizes. At that point adjacent layers with the same refractive
index are merged and the system is refined to further improve its performance. The method can be extended to include absorbing layers [24].
In Figure 6 is shown an infrared black absorber filter that was designed
by the Flip-Flop synthesis method.
Figure 4. A filter with a linearly varying transmittance curve designed by the gradual
evolution method [19].
The Minus Filter method decomposes a required spectral transmission curve into a number of minus filters that are placed in series, each
having a suitable attenuation and half width [21]. The minus filters
consist of quarter wave layers of several refractive indices centered on
appropriate wavelengths. Alfred Thelen has shown how to design such
minus filters [22]. In many instances the selected minus filter can be
deposited on top of each other on one surface of a substrate and then
refined to further improve the performance of the composite layer
system. Figure 5 shows a notch filter in which the transmission and
rejection regions have the same width on a wavelength scale [21].
Figure 5:. A notch filter designed by the minus filter method [21].
Bill Southwell described the Flip-Flop method that makes use of
a generic starting design of a suitable overall thickness that has been
subdivided into a large number of very thin layers of equal optical thicknesses [23]. These layers are usually composed of one or two different
38
Figure 6. A black infrared absorber coating designed by the flip-flop method [24].
Several people have described inverse Fourier transform methods
for the design of optical thin film coatings. In their original implementations these methods could be only applied to problems in which the
desired spectral reflectance or transmittance curve could be described
by an expression that could be integrated [25-28]. The solutions
were inhomogeneous layers that could be readily approximated by
homogeneous layer systems. However, the need for an expression that
could be integrated was rather limiting. At the NRCC a Numerical
Inverse Fourier Transform method was developed that accepted the
desired spectral curve in tabular form [29]. This was the first time that
it was demonstrated that, for normal incidence of light, filters could
be designed with transmission or reflection curves having any desired
spectral profile, provided that the spectral region over which they were
defined was reasonable. In Figure 7 is shown a filter designed by this
method with a spectral reflectance curve that corresponds to the silhouette of the Parliament Buildings in Ottawa.
When refining an optical multilayer system, the process can come to
a stop before a satisfactory solution has been obtained. This is because
either the number of layers is not enough, or the overall thickness of the
layer system is not sufficient. Originally Alexander Tikhonravov’s Needle
method was designed to overcome this impasse [30]. In this method
a very thin layer is inserted in the most effective position within a
multilayer, in order to increase the number of construction parameters,
so that the performance of the system could continue to be optimized
through refinement. However, the needle method is much more than
that. It is a true synthesis method that does not need a starting design.
It is probably currently the most commonly used synthesis method in
commercial thin film design software. Figure 8 shows a universal AR
coating designed by the needle method which is effective on any glass
substrate with a refractive index ns satisfying 1.48<ns<1.75 [31,32].
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tion of a step in the refractive index of a layer will result in the largest
improvement in the performance.
In addition to the above thin film synthesis programs for the design
of optical multilayer coatings it is also possible to use various global
minimum seeking programs that have been developed for the solution of
general engineering problems [15]. Routines of this type that have been
used in the past for thin film design include the Generalized Simulated
Annealing-, the Monte Carlo Simulated Annealing-, the Revised NelderMead Simplex- and the Genetic Algorithm methods. However, experience
has shown, that even simple refinement methods, when used with a
number of appropriate starting designs with random layer thicknesses,
will likely yield satisfactory results more quickly.
PROGRESS IN DEPOSITION TECHNOLOGIES
Figure 7. A filter with a spectral reflectance corresponding to the silhouette of the Parliament
Buildings, Ottawa designed by the numerical inverse Fourier transform method [29].
It should be mentioned, that even though such powerful design
methods are now available, new thin film synthesis methods continue to
be proposed. For example, only at the last Annual Meeting of the Society
of Vacuum Coaters, a team from the Ecolé Polytechnique in Montreal
proposed the Refractive Index Step method, which the authors say,
was inspired by the needle method (Figure 9) [33]. In it, an algorithm
repeatedly finds the optimum position in the system where the introduc-
To take full advantage of the enhanced power of the new design tools,
improvements in two areas were crucial. One was the ability to deposit
layers of different materials in a reproducible manner with properties
that do not change in time. The other was the precise control of the
thicknesses of the layers during the deposition, a subject that will be
discussed in the next section.
The first of these two topics is not the main subject of this paper,
but it is so important for the further development of the subject, that
it has to be mentioned. In 1957 optical thin films were most commonly
deposited from resistance-heated sources or by e-gun evaporation.
Resistance heated sources had rather steady angular evaporation
characteristics, which means that it was relatively easy to obtain uniform
films by this process. But they were mostly useful for the deposition of
fluorides, sulphides and other semi-conducting materials that evaporated
or sublimed at relatively low temperatures. If the substrates could be
heated during the deposition to sufficiently high temperatures, dense
films were obtained whose properties did not change on exposure to the
continued on page 40
Figure 8. Universal antireflection coatings for substrates with an index ns satisfying 1.48<ns<1.75, designed by the needle method [31].
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IMPACT OF COMPUTERS
continued from page 39
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by computer analysis so that the derivative of the transmittance with
respect to thickness of the partial system including this layer had the
highest possible value (Figure 1b) [34]. At this wavelength a given error
in the transmittance corresponded to the smallest thickness error.
Figure 9. Design of a cut-off filter by the refractive index step method using a quarter-wave
stack as a starting design [33].
atmosphere. However, the above materials were relatively soft and not
suitable for front surface use. Oxide films were much harder, but they
required e-gun evaporation. As a rule, when deposited by this process,
they resulted in porous layers that adsorbed water and changed their
properties in air. Also, it was a challenge with this process to maintain
good thickness uniformity over large areas.
In the last 50 years huge strides were made in this area. First, ion
assisted e-gun evaporation resulted in denser coatings. Various magnetron-sputtering processes were developed that in addition to producing
even denser coatings, made achieving good uniformity across large areas
quite simple. Further, these processes are so steady and reproducible
that for many applications, after suitable calibration, simple timing
could be used to control the thicknesses of the layers. The speed of magnetron deposition is now approaching that of e-gun evaporation. Finally,
ion-beam sputtering shows much promise for the future – coatings produced by this process have excellent properties, but the reproducibility
of the deposition rate, the layer uniformity and system maintenance still
need to be improved. Without the developments in deposition technology
it would have been impossible to achieve the present state of the art in
the manufacture of optical coatings.
PROGRESS IN THICKNESS CONTROL DURING THE LAYER
DEPOSITION
After the spectacular demonstrations that thin film design methods can
be used to find good solutions to quite complex filtering problems, much
research was done to improve methods for the control of non-quarter
wave layer systems.
At the NRCC a quarter wave layer monitoring technique was developed to control non-quarter wave layer thicknesses (Figure 10) [34].
This required the substrates to be stationary and the sources to be
rotating. A monitoring glass (a) was placed next to the real substrate
(b) that was in front of an adjustable rotating shutter (c) that could
intercept a different fraction of the incident vapour for each layer. The
resistance-heated sources (d) were mounted on a rotating commutator
ring. This method worked very well for soft coating materials. But it was
too difficult to adapt this method to e-gun evaporation of hard oxide
layers.
The “most sensitive wavelength” monitoring method was then developed in which the monitoring wavelength for each layer was chosen
40
Figure 10: Apparatus built at the NRCC for the use of the quarter wave monitoring method
for the control of the thicknesses of non-quarter wave layers [34].
Many other groups proposed increasingly more sophisticated monitoring methods. The most significant contributions to this field probably
came from Emil Pelletier’s group in Marseille. They published a number
of papers that described various developments in wide-band optical
monitoring in which the measured spectra of the gradually evolving
system were compared to the calculated spectra of the system at the
end of each layer. The deposition of the layer was terminated when the
best fit between the two curves occurred. Figure 11 shows the calculated
spectral transmittances for wide-band monitoring purposes of a two
transmission peak band pass filter at different stages of its manufacture
[35,36].
At this point it was not difficult to predict the future trend. Figure
12 shows the process envisioned at the NRCC in 1976 [20]. The starting
point was a filter design that was optimized to be relatively insensitive to
random thickness errors. It was assumed that the all-dielectric systems
would be produced by a stable deposition process, that the optical constants of the coating materials were repeatable from run to run and that
they were reliably determined by spectrophotometry or ellipsometry, and
that the main unknowns would be the thicknesses of the layers. After the
completion of each layer, the spectral transmittances would be measured
and from these measurements the thicknesses of the layers deposited
thus far determined. This step would be followed by the refinement of the
thicknesses of the remaining layers of the system to compensate for the
errors in the thicknesses of the layers produced thus far. This proposed
deposition with real-time re-optimization process was first implemented
at the NRCC in the early 1990-ties, after well-behaved and reproducible
RF and AC magnetron sputtering sources became available. The process
was completely automatic and, once initiated, it could proceed without
supervision until the layer system was complete [37-39]. Figure 13 shows
the target, design and measured spectral reflectances of a two-material
60-layer coating that has a spectral reflectance that corresponds to the
silhouette of the Taj Mahal and that was designed by the needle synthesis
method by Pierre Verly [38].
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Figure 11. Monitoring information for the control of the deposition of a two-peak interference filter by the wide-band monitoring method [35,36].
Of course, significant contributions to such processes have also been
made in many other laboratories [see, for example, references 40-42].
In particular, a combined team from the University of Rochester and
the Moscow State University, headed respectively by Doug Smith and
Alexander Tikhonravov, achieved promising results when they attempted
to do the same with the much more complicated e-gun evaporation
process [43].
It is obvious that even better results would be obtained if more
accurate measurements could be made during the deposition process. A
number of laboratories have pioneered the use of in-situ ellipsometric
measurements which, in addition to the estimation of the thicknesses of
the layers, permit the real-time determination of their optical constants
[see, for example, references 42,44-47]. This is particularly important
when the multilayer systems contain thin metal films – it is often crucial
to determine accurately the extinction coefficients of the metal layers.
Figure 12. Proposed process for the deposition of optical multilayer coatings with real-time
re-optimization [20].
continued on page 42
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continued from page 41
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produce a coating with very complex T and R curves. In the best solution the experimentally produced curves on the average differed from
the target values by only 0.98% (Figure 15).
Figure 13. A filter with a spectral reflectance that corresponds to the silhouette of the Taj
Mahal produced on an automatic RF magnetron sputtering system with real-time re-optimization [39].
STATE - OF-THE -ART OF DESIGN AND MANUFACTURE OF
OPTICAL MULTILAYER COATINGS IN 2007
Several thin film design methods exist today that will provide the layer
systems required to obtain almost any normal incidence spectral transmittance or reflectance for normal incidence of light, providing that
the curve is specified over a reasonable range of wavelengths. However,
computer methods are presently used for much more than just finding
numerical solutions to a given design problem. They are now routinely
used to perform various calculations such as the determination of the
optical constants of coating materials and any inhomogeneities in
deposited layers, the simplification of solutions through the removal of
insignificant layers, calculations for the de-sensitization of layer systems
to thickness and optical constant errors, random thickness and optical
constants error analysis, the statistical modelling of monitoring strategies, the control of the deposition process, as well as the post-deposition
evaluation of the layer systems. When all these processes are encompassed, it is indeed proper to talk about the “computer manufacturing” of
optical multilayer coatings.
It should be stated at this point that a number of very good commercial thin film design software packages exist that provide such
computational power [48,49,50]. Today it does not make any sense for a
casual designer to write his or her own software.
The design of thin film filters for non-normal angles of incidence,
especially when the filter is specified for both planes of polarization, can
still be a challenge. Nevertheless, a number of innovative solutions for
reflectors, polarizers, beam splitters and antireflection coatings (Figure
14), operating over a wide range of wavelengths and angles of incidence
have been found in the past ten years.
In the past, from time to time, competitions have been held to
test the state-of-the-art of thin film design [51-56]. More recently,
Manufacturing Problems are being held at the OSA Topical Meetings on
Optical Interference Coatings (OIC) that test the-state-of-the-art of manufacturing. At the 2001 OIC meeting in Banff, participants were asked to
design and produce a filter with spectral transmittance and reflectance
curves that corresponded to the silhouette and its reflection in a lake of
a mountain range in the Canadian Rockies [57]. This problem required
non-absorbing coating materials and tested the ability of a laboratory to
42
Figure 14: Theoretical and measured performance of two wide-angle antireflection
coatings [65,66].
At the 2004 OIC meeting in Tucson the manufacturing problem
tested the ability of the participants to produce a 70:30 beam splitter
for unpolarized light effective over a wavelength range of 0.45<λ<0.65
µm and operating at an angle of incidence of 60° (Figure 16) [58]. A
number of very different solutions were submitted. In the best of these
the average measured deviation from the target values in the wavelength range specified was only 0.79%. Another lesson learned during
that exercise was that to make further progress in the manufacture
of oblique incidence coatings, the accuracy of measurements at such
angles has to be improved. It is very appropriate that measurement
problems are being again held in order to focus on this topic [59-62].
Figure 15. Manufacturing problem at the 2001 OIC meeting in Banff: a filter with spectral
transmittance and reflectance curves that corresponded to the silhouette and its reflection
in a lake of a mountain range in the Canadian Rockies [57].
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Figure 16: Manufacturing problem at the 2004 OIC meeting in Tucson: a 70/30 polarizing
beam splitter for 60º incidence [58].
At the 2007 OIC meeting, also to be held in Tucson, the manufacturing problem will test the ability to deposit accurate thin metal layers.
The problem is to produce a coating that looks orange and blue in light
reflected from the two surfaces, but appears grey in transmitted light.
Lastly, a forecast for the future. Let us suppose that a solution to a
problem has been found that, for a certain set of materials and overall
thickness, is a global minimum. Let us also assume that up to a certain
point in the deposition process all the theoretical thicknesses of the
layers have been deposited accurately, except that the thickness of the
last layer has been exceeded. By re-optimization the thicknesses of
the remaining layers of the system some of the damage caused by this
error can be reduced. However, since the original solution was a global
minimum, the performance of the original system cannot be completely
recovered with the same number of layers. It follows that the only way to
achieve the global minimum solution is not to make any errors.
At the NRCC an ion beam sputtering system was used to make a
filter with a spectral reflectance curve that followed the silhouette of
the Palace of the Shoguns, in Kyoto (Figure 17) [63,64]. The departure
of the theoretically calculated transmittance curve from the target value
was 0.45%. This would also be the best possible performance of an experimentally produced system, providing that no errors were made during its
manufacture. When the system was deposited without the real-time reoptimization of the remaining layers of the system, the average departure
of the measured curve from the theoretical value was 3.27%. With re-optimization, the departure was reduced to 1.28%. When the thicknesses that
were overshot were etched down with the ion beam, the departure from
the target value was only 0.77%. To reduce this residual discrepancy still
further, a more accurate measurement system would have to be installed.
This procedure will become practical once instrument manufacturers
develop ion beam guns with more repeatable performance and less need
for maintenance.
CONCLUSIONS
This article attempted to show that advances in computer technologies,
when combined with the progress in deposition methods and thin film
monitoring techniques, have resulted in the ability to manufacture complex optical multilayer systems with an accuracy that could hardly have
been imagined 50 years ago. This, in turn, has lead to a widespread use of
optical coatings in a range of consumer-oriented products and sophisticated scientific and technological devices.
Figure 17: Manufacturing with ion beam etching: a filter with a spectral transmittance
corresponding to the silhouette of the Palace of the Shoguns in Kyoto (see text) [63,64].
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J.A. (George) Dobrowolski is a Researcher
Emeritus at the National Research Council of
Canada. Main interests: optical filters in general,
and the development of methods for the design and
construction of optical multilayer systems, in particular. He is also interested in new technological and
consumer-oriented applications of optical coatings.
Author of eight handbook chapters, and author or
co-author of about 140 written publications and 33
US patents. He has also given numerous courses at
the University of Rochester; the SPIE, the SVC and
at the OSA. He is a member of the SPIE and SVC and
a fellow of the OSA. George received the 1987 Joseph
J.A. (George) Dobrowolski
Fraunhofer Award and the 1996 David Richardson
Medal of the OSA, and the 1997 Medal of Achievement
in Industrial and Applied Physics of the Canadian Association of Physicists. He is the
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44
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