Silicon Oxy-nitride Coating by Plasma Enhanced Chemical Vapour Deposition for the Protection of Silver Mirrors

1
Silicon Oxy-nitride Coating by Plasma Enhanced
Chemical Vapour Deposition for the Protection of
Silver Mirrors

• C. Jariwala, P. Kikani, P.M. Raole, A. Chainani,
• S.K. Nema, and P.I. John
• Facilitation Centre for Industrial Plasma
  Technologies,
• Institute for Plasma Research, Gandhinagar,
• Gujarat, India.
• K. Mishra and K.R. Murali
• Space Applications Centre,
• Indian Space Research Organization, Ahmedabad,
  Gujarat, INDIA

2
Abstract

• Introduction to Silver coated mirror.
• Silicon oxynitride (SiON) coatings have been
  grown by Plasma Enhanced Chemical Vapor
  Deposition (PECVD) using safe organic precursor
  Hexamethyl Disilazane (HMDSN), as protective
  layer on silver coated mirrors.
• The stoichiometry and chemical composition of
  SiON protective layer has been analysied by X-ray
  Photoelectron Spectroscopy.
• The reflectance measurements performed on the
  SiON protected mirrors, which were subjected to
  salt spray test for corrosion resistance and to
  humidity test for environmental protection,
  demonstrate that the SiON coating provides good
  protection of the mirrors, without significant
  degradation in the infrared and visible
  reflectance.

3
Introduction to silver coated mirrors

• Evaporated silver coating on mirror substrate has
  the highest reflectivity from 400 nm through the
  infrared compared to other evaporated metal
  mirrors

• The highest reflectivity of silver coated mirror
  in the visible and infrared regions makes it the
  best choice as the optical components for
  following instruments Telescopes,
  Spectrometers, Cameras, Solar reflectors,
  Photocopiers etc.

4

• Evaporated silver coating on mirror substrate has
  the highest reflectivity from While freshly
  evaporated aluminum mirror has high reflection
  down to 200 nm as compare to silver coated mirror
  and it is used more commonly as mirrors for
  astronomical telescopes.

• On the other hand aluminum has a reflection notch
  at around 850 nm wavelengths due to inter-band
  transition and at which the reflectance, at best,
  is about 80. Furthermore, the average
  reflectance of aluminum in the visible portion
  around 92, as compare to 98 for silver, and
  hence losses for silver are less compare to
  aluminum.

Schematic view for light intensity losses in
Aluminum and Silver coated Mirror
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• Silver coated mirrors introduce less
  polarization in to an optical system than do
  other metallic reflectors, such as aluminum,
  rhodium etc.

G. Hass et al. Applied Optics, Vol. 14 (1975)
2639

• Reflectance of Al, Al coated l/2 thick Al2O3, Ag
  and Rh mirrors as a function of incidence angle
  at l540 nm

• Also, low Infrared emittance in the thermal
  infrared region compared to other metallic
  reflector results in silver coated mirrors most
  valuable in astronomical telescopes.

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Requirement of Protective layer for Silver Coated
mirrors

• Bare silver coating is quite reactive and its
  optical properties easily degraded by process of
  sulfide formation, oxidation etc on exposure to
  corrosive chemicals such as H2S, SO2, H2O2, etc,
  present in salt fog, acids and sulfur compounds
  in the atmosphere.
• It results in forming a thin layer of the
  corrosion product of silver, the most common
  corrosion product being silver sulfide(Ag2S).
• A recent study describes experiments on the
  degradation process of silver coated mirror in an
  operation environment showed that the sulfide
  formation and internal corrosion limit the useful
  silver coating life to 3-4 months.

D. Nahrstedt et al. Applied Optics, Vol. 35
(1996) 3680
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SiON dielectric thin film as a protective layer
for silver mirror

• It is well known that by depositing single- or
  multi-layer dielectric oxides and fluoride films
  on silver coated mirror surface serves the dual
  purpose of environmental protection and
  mechanical protection.
• The mathematical expression of reflectivity for
  such single dielectric thin film coated on
  metallic substrate is given by

Where, r1 (no-n1)/(non1) r2
(n1-n2)/(n1n2) d (2p
n1d1)/l

• Schematic view of dielectric thin film coating on
  metallic substrate

• The solution of mathematical expression of
  reflectivity leads to two extreme condition of
  reflectance

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• Case-I
• when, n1d1 l/2, 2l/2, 3l/2.
• ? d 180?, 360?, 540?, ? cos2d
  1
• This gives
• This corresponds to the reflectance of uncoated
  surface, a layer of thickness l/2 or multiples of
  l/2 is therefore optically absent film.
• Case-II
• when, n1d1 l/4, 3l/4, 5l/4.
• ? d 90?, 270?, 450?, ? cos2d -1
• This gives
• This corresponds to the anti-reflectance for
  noltn1ltn2 or nogtn1gtn2 and high reflectance for
  noltn1gtn2 conditions.

• In present study we have used the condition of an
  optically absent film using optical thickness of
  n1d1 l/2 for l 550 nm, which gives a physical
  thickness of approximately d1 1610 Å for the
  SiON (n1.7) thin film.

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SiON thin film deposition by PECVD

• PECVD is widely used process for thin film
  synthesis, in these processes, the gas is
  activated by electronic impact, whereas for
  conventional thermal chemical vapour deposition
  (CVD), the gas decomposition is due to heating.

Schematic View of a PECVD system
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Characterization of SiON thin film

• X-ray Photoelectron Spectroscopic analysis of
  SiON thin film on Silver mirror

• O 1s core level _at_ 532 eV, in the range of oxide
  species
• Si 2p core level _at_ 102.8 eV, which is typically
  belongs to Si-oxynitride
• (for Si elemental Si 2p core level _at_ 99 eV)
• N 1s core level _at_ 398.1 eV and is similar to the
  oxynitride film
• deposited using SiH4 and N2O/NH3

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• Salt spray and humidity test of SiON protected
  silver mirror

• The ASTM B-117 salt spray test for corrosion
  resistance was performed using water (5 NaCl) in
  a closed chamber on a set of SiON protected
  silver mirrors for 72 hours, shows good
  protection of silver mirror without significant
  degradation of reflectance
• The humidity test has been conducted for
  environmental durability of a set of SiON
  protected silver mirrors using water kept at 60º
  C in closed chamber. The reflectance of such
  tested mirrors was measured for specific time
  interval exposures.

• The measured reflectance curve for SiON protected
  silver coated mirror has reflectance above 85
  even after 11 days exposed to humid environment
  of water.
• The formation of oxidized/corrosive product on
  protected silver mirror was found to be clearly
  absent.

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Comparison of SiON protected mirror with
unprotected, Al2O3 protected and
Ta2O5/SiO2/Ta2O5/Al2O3 multi-layer protected
silver mirror after exposed to humid environment
of water kept at 60º C and 40 º C
D. Song et al. Applied Optics, Vol. 24 (1985) 1164
13
Photographs of SiON , Al2O3 and
Ta2O5/SiO2/Ta2O5/Al2O3 multi-layer protected and
unprotected silver mirrors after exposure of 1
year in normal lab environment.
Al2O3 protected silver mirror after exposure of
1 year at Kit Peak National Observatory, USA
Unprotected silver mirror after exposure of 1
year in lab. environment
Multi-layer protected silver mirror after
exposure of 1 year at Kit Peak National
Observatory, USA
SiON protected silver mirror after exposure of 1
year in lab. environment
D. Song et al. Applied Optics, Vol. 24
(1985) 1164
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Conclusions

• Silicon oxynitride thin film grown on silver
  mirror as protective layer by PECVD process
• XPS studies confirmed the chemical composition of
  grown SiON thin film
• Corrosion and humidity test along with
  reflectance measurement shows usefulness of SiON
  coating as protective layer on Silver mirror
  without significant degradation

15
Acknowledgement

• The authors would like to thank M/s. Light Guide
  Optics for providing us nickel based
  electroplated silver mirrors

Thank you
16
D. K. Burge et al. Applied Optics, Vol. 12 (1973)
42

• 2Ag H2S ? Ag2S H2

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• In recent years, significant attention has been
  devoted to SiON thin film due to possibility for
  changing the film properties between Silicon
  oxide (SiO2) and Silicon nitride (Si3N4) as
  composition changes.

Ehn hc/l

• SiON is very promising material for both optical
  and electrical applications such as optical
  wave-guide, optical devices like band pass
  filters, Anti-reflection coating, decorative
  coatings and due to better resistance against
  dopent penetration (e.g. boron penetration from
  p poly-Si gate in to substrate) and higher
  dielectric strength make SiON an alternative gate
  dielectric against SiO2.

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• The thickness of the film has been confirmed by
  comparing with a calibrated colour chart for
  identical depositions on single crystal polished
  Si test substrates kept adjacent to the actual
  mirror.
• Since SiON thick films on Si give different
  colour hues depending on the thickness, the
  corresponding thickness can be estimated using
  standard colour thickness chart

S.M. Sze, Physics of Semoconductor Devices, New
York, John Wiley, 2nd Edition, 1981.