Protective coatings for interference filters made of porous silicon

Article abstract of DOI:10.1002/pssa.200306527

We have tested different kinds of protective coatings for interference filters made of porous silicon (PS). While thin SiO₂ films deposited by various kinds of chemical vapor deposition (CVD) provided reasonable protection against moisture, best results were obtained for Plexiglas coatings. They left the filter characteristics essentially unchanged and also delivered negligible ageing effects in a period of 180 days.

Full text of DOI:10.1002/pssa.200306527

phys. stat. sol. (a) 197, No. 2, 370–373 (2003) / DOI 10.1002/pssa.200306527
Protective coatings for interference filters made
of porous silicon
R. Corban, H. Bousack, and H. G. Bohn*
Institut für Schichten und Grenzflächen, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
Received 11 March 2002, accepted 30 September 2002
Published online 26 May 2003
PACS 42.79.Ci, 42.79.Wc, 78.66.Db, 81.15.Gh
We have tested different kinds of protective coatings for interference filters made of porous silicon (PS).
While thin SiO2 films deposited by various kinds of chemical vapor deposition (CVD) provided reason-
able protection against moisture, best results were obtained for Plexiglas coatings. They left the filter
characteristics essentially unchanged and also delivered negligible ageing effects in a period of 180 days.
1 Introduction It is well known that multilayers can be made from porous silicon [1]. This allows the
simple and cost effective fabrication of superlattices serving as optical interference filters in a wide spec-
tral range. The remaining absorption of the Si limits these filters in the visible part of the spectrum (VIS)
to reflectors which can be made e.g. as small band or laterally graded mirrors. In the infra-red (IR) re-
gime where absorption of PS becomes negligible also transmission filters can be produced. Nevertheless
they have to be made from free standing PS-films (or extremely thin Si-wafers) as the free charge carri-
ers in the underlying highly doped substrate would prevent the transmission of light. The Research Cen-
ter Jülich (FZJ) has set up a project (named PorSiLux) which aims at the development of commercially
available optical filters from PS. In this context we have routinely produced transmission filters with area
as large as 3 cm2 and minimum thickness as low as 30 µm.
For practical applications one has to consider that, due to its huge surface area, PS is vulnerable in
particular to oxidation, humidity or other kinds of environmental influences which might change the
filter characteristics. So an effective protection of the filters will be required.
The effect of oxidation on PS interference filters has already been reported in the literature [2]. After
preparation of the superlattice a thin layer of native oxide readily forms at ambient air, typically 10 nm
thick. This layer slowly grows thereby leading to an ageing of the filters. We have extended the investi-
gation to almost 180 days. Fig. 1 shows the shift of the central wavelength of three different Bragg mir-
rors with time. There is a decrease of about 3.6% per year which seems to be negligible for many appli-
cations. For higher stability protective measures have to be taken. As suggested in Ref. [2] the thickness
of the SiO2 layer can be increased by raising the oxidation temperature and/or duration. Eventually this
completely converts PS to porous SiO2. Obviously this leads to stable filters. However, due to the con-
siderably smaller index of refraction of SiO2 compared to that of Si, they shift to smaller wavelengths.
This then poses a design problem on the filters as the material parameters of the resulting multilayer
system like layer thickness, porosity and the related index of refraction are not known initially. As a
consequence a protection is required which essentially conserves the characteristics of the filters.
* Corresponding author: e-mail: h.bohn.@fz-juelich.de
© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0031-8965/03/19705-0370 $ 17.50+.50/0

phys. stat. sol. (a) 197, No. 2 (2003)
371
1.0
0.8
0.6
0.4
0.2
0.0
660
as prepared
H2O(liq) after 14 days
640
620
600
H2O(liq) after 5 days
0
30
60
90 120 150 180
Time / d
200
400
600
800
1000
Wavelength / nm
Fig. 2 Influence of liquid water on the reflectance
Fig. 1 Influence of ambient air on three different Bragg
spectrum of an unprotected Bragg filter.
filters. The filters are characterized by their maximum
wavelengths λ
.
max
2 Experimental details The PS samples were formed on highly doped (10 mΩ cm) p+-type substrates
leading to mesoporous Si. Further details can be found in Ref. [2]. The optical spectra were recorded
using a Perkin–Elmer Lambda900 spectrometer. In order to test environmental influences the samples
were exposed to ambient air for long times (up to 180 days) or simply stored in de-ionized water. The
latter served as an accelerated test for the influence of moisture.
Capping layers of SiO2, i.e. layers which cover the top surfaces of the filter but do not penetrate into
the pores were deposited in two different ways of CVD [3]: plasma enhanced chemical vapor deposition
(PECVD) and low pressure chemical vapor deposition (LPCVD). In the latter case Tetraethylorthosili-
cate (TEOS) is used to form a silicon dioxide layer on the surface of the filter under low pressure and
high temperature conditions. The deposition rate is a function of the partial pressure of TEOS and the
deposition temperature. As a third technique spin-coating of Polymethylmethaacrylat (PMMA = Plexi-
glas) was employed. For the CVD techniques the layer thickness is controlled by the deposition rate and
time, for the spin-coating by the spinning rotation speed.
3 Experimental results Figure 2 shows the influence of water on the reflectance spectrum of an un-
protected Bragg mirror. After 5 days immersion into water the spectrum is severely disturbed and after
14 days it is almost completely corrupted. Obviously for applications in wet environment optical filters
made of PS have to be protected.
3.1 PECVD coating As an example the effect of a 100 nm thick layer of SiO2 deposited by PECVD on
a Bragg reflector is shown in Fig. 3. Relative to the as prepared state the additional layer has only a mi-
nor effect on the central wavelength of the mirror. As a beneficial side effect the layer forms an antire-
flection coating [4] which considerably reduces the side lobes. This effect could be optimized by adopt-
ing the layer thickness. After storage of the filter for 14 days in water the spectrum remains essentially
unchanged except for a slight shift to smaller wavelengths. This shift is continuous in time. Apparently
the filter protection is not perfect. However, for most practical application in humid environment this
type of coating seems to be sufficient, as first tests of filters in a relative humidity of 99% show. It
should be noted that the PECVD technique is equally well applicable to free standing filters where both
sides have to be coated. This is shown in Fig. 4 where the transmission spectrum of a free standing Bragg
mirror designed at a wavelength of 1700 nm, i.e. in the near-IR, is plotted. Deposition of 150 nm
PECVD-SiO2 on each side does not influence the spectrum.
3.2 LPCVD coating The long term behavior of two different Bragg mirrors coated with LPCVD-SiO2
is shown in Fig. 5 where the central wavelength of the reflection peak is plotted vs. time. Filter 1 (upper

372
R. Corban et al.: Protective coatings for interference filters made of porous silicon
1.0
1.0
0.8
0.6
0.4
0.2
0.0
SiO2 - 100 nm
SiO2 - 150 nm
0.8
H2O - 14 days
as prepared
as prepared
0.6
0.4
0.2
0.0
200
400
600
800
1000
1200
1600
2000
2400
Wavelength / nm
Wavelength / nm
Fig. 3 Influence of 100 nm SiO2-coating (PECVD) on a
Bragg filter and the protection against water.
Fig. 4 Transmission spectra of a free standing IR
Bragg filter: As prepared and after PECVD coating
with 150 nm SiO2 on both surfaces.
curve) was stored in wet air (relative humidity 99%) at room temperature whereas filter 2 (lower curve)
was stored at ambient air. The capping layer introduces a 5% reduction of the wavelength, which in the
case of the wet environment recovers to a large extend within a few days. Apart from these initial effects
both filters are then perfectly stable during the time of the investigation (170 days). A layer thickness as
low as 50 nm is sufficient for the protection of the filters. Unfortunately this deposition technique is not
applicable to thin free standing filters because of mechanical stresses introduced both by the elevated
temperature and the low pressure.
3.3 PMMA spin-coating In order to test the effect of PMMA coatings a 300 nm thick PMMA layer
(corresponding to a speed of 3000 rpm) was deposited onto a Bragg mirror. Figure 6 shows the reflec-
tance spectrum of this sample in the as prepared state, immediately after deposition of the PMMA, and
after storage in water for 14 days. One can see that PMMA coating has a negligible influence on the
position of the spectrum and provides an effective protection against water.
1.0
0.8
0.6
0.4
0.2
0.0
700
650
600
550
500
PMMA 300 nm
as prepared
H2O - 14 days
as prepared
SiO2-Coating
as prepared
SiO2-Coating
200
400
600
800
1000
0
30
60
90 120 150 180
Time / days
Wavelength / nm
Fig. 5 Long-time investigation of the shift of the
maximum wavelength for two SiO2-coated (LPCVD)
Bragg filters. Filter 1 (upper curve) had a layer thickness
of 50 nm and was stored in wet air of 99% relative hu-
midity. Filter 2 (lower curve) had a layer thickness 100
nm and was stored in ambient air at room temperature.
Fig. 6 Influence of a 300 nm PMMA-coating on a
Bragg filter and its protection against water.

phys. stat. sol. (a) 197, No. 2 (2003)
373
4 Conclusions Aiming at the commercialization of optical filters made of porous Silicon both the long
term stability of such filters as well as several types of protective layers for applications in wet environ-
ments have been explored.
The long term stability in ambient air turned out to be sufficient without any further measures except
when high precision filters are required. The ageing due to surface oxidation was found to be as low as
3.6% per year.
In wet environments thin capping layers of SiO2 or Plexiglas provide good protection. The filters are
only slightly (30 nm for LPCVD coating) or even negligibly affected within the accuracy of the meas-
urements (for PECVD and PMMA coatings). The long term stability after LPCVD coating is excellent.
With respect to protection against moisture best results were obtained for PMMA coating.
For free standing filters only PECVD was found to be applicable. The application of PMMA spin-
coating technique has not yet been tested.
As a beneficial side effect suitable choice of the layer thickness leads to an antireflection coating
thereby improving the quality of the filters.
Acknowledgements The authors gratefully acknowledge the members of the PorSiLux-team M. Banzet,
E. Brauweiler-Reuters, K. Rumpold, G. Schleinkäfer, and J. Vogel for their expert technical assistance as well as
many helpful discussions. This work is supported under grant number 01SF9963 by the German BMBF.
References
[1] M. Thönissen et al., Solid State Phenom. 54, 65 (1997).
[2] M. Krüger et al., Opt. Commun. 146, 309, (1998).
[3] K. K. Schuegraf, Handbook of Thin-Film Deposition Processes and Techniques (Noyes Publications, Park
Ridge, NJ, U.S.A., 1988).
[4] A. Thelen, Design of Optical Interference Coatings (McGraw-Hill, NY, U.S.A., 1988).