600 500 400
Start Frozen
Active
SDR
Target
300
100
Wavelength (nm)
T (%) a) b)
80
60
40
20
0
40
30
20
10
3
Layer number
QWOT
2
1
0
Advanced thin-film software techniques
improve design-to-fabrication workflow
TATIANA AMOTCHKINA, MICHAEL TRUBETSKOV,
ALEXANDER TIKHONRAVOV, and JENNIFER KRUSCHWITZ
Advanced thin-film software can assist
designers in finding a solution for al-
most any design problem, meeting challenging specified spectral characteristics
with the highest accuracy. There is no
assurance, however, that design solu-
tions arrived at in this way can be fabricated using available thin-film-coating
monitoring and deposition equipment.
Keeping practical issues in mind, developers of advanced software have fo-
cused their efforts on developing a variety of algorithms aimed at the automatic
synthesis of feasible design solutions
and on the implementation of design
tools addressing special classes of de-
sign problems. Here, we discuss tech-
niques developed at OptiLayer Software
that lead to coating designs suitable for
specific monitoring techniques and deposition processes while retaining the
coating’s excellent spectral properties.
Design of blocking
and edge filters
Currently, experienced engineers invent
their own empirical approaches to avoid
complicated design solutions obtained
by basic optimization techniques and
to acquire designs of a desired struc-
ture. In many cases, to design broad-
band blocking filters (short-pass and
long-pass filters) or fil-
ters blocking sidebands
of Fabry-Perot bandpass-
es, nearly quarter-wave
stacks are preferable be-
cause these designs are
more straightforward to manufacture.
For many of these types of designs,
the absence of thin layers is an advantage for fabrication systems that cannot
deposit layers thinner than 15 nm with
good substrate uniformity. Many layers within these designs can be optically
checked without changing the monitor-
ing wavelength and still have advantageous cut-off conditions. We have de-
veloped a sensitivity-directed refinement
(SDR) algorithm that allows obtaining
near-quarter-wave solutions in 1 to 2 s.
As a starting design, a quarter-wave
stack or a combination of such stacks
should be specified with initially frozen
layer thicknesses. At each iteration, the
SDR algorithm activates several layers
using merit-function-sensitivity analy-
sis and performs the refinement with
respect to active thicknesses only. This
algorithm concept was devised by Ulf
Brauneck, an optical coating design-
er at Schott Schweiz-Schott Suisse SA.
However, manual application of this
algorithm is time consuming and re-
quires considerable designer experience,
especially when the number of layers is
high and not just one, but two or three
layers are active. We have developed an
automated version of this approach that
allows obtaining several nearly quarter-wave design solutions using differ-
ent algorithm settings: number of ac-
tive layers, possibility of using needle
variations, and so on.
1
Here, for illustration, we design a
blocking filter with target transmit-
tance less than 1% in the spectral ranges from 265 to 332 nm and 445 to
535 nm and more than 99% in the
range from 370 to 390 nm [see Fig.
New coating-design algorithms
automatically produce designs suitable
for real-life monitoring techniques and
deposition processes, while retaining the
coating’s excellent spectral properties.
