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CERAC Coating Materials News
Volume 9,
Issue 1 March, 1999
In
this issue we examine a common problem found with optical coatings,
and discuss the various forms and consequences that this problem
exhibits as well as techniques for minimizing the effect.
-
- Water in Coatings
Thin film coatings grow with a density less than that
of the bulk form of the material and consequently possess a considerable
void volume. Density is dependent on the energetics of the layer
growth and the incorporation of gasses during deposition. In
review of earlier discussions, increased packing density is produced
by high substrate temperatures, ion bombardment of various forms,
or high impact energy of the primary species. Figure 1 is a SEM
microphoto of a thick CdTe film grown at 60° C substrate
temperature. When the temperature was raised to 200° C, the
film becomes featureless and apparently amorphous. Amorphous
growth, as opposed to crystalline or columnar microstructure,
is required to provide a high packing density and therefore lower
penetration of water vapor.
-
- (click on image to enlarge)
-
- Figure 1. SEM microphoto of
a CdTe layer grown at 60° C substrate temperature. Individual
well defined columns are 50Å diameter.
-
-
- High substrate temperature assists in removing surface contamination,
thereby conditioning the surface for nucleation, increases surface
mobility of the arriving adatoms, and can either decrease or
increase the water and other gas content of the atmosphere. Cases
of increased water content can result from wall and tooling outgassing
at high temperatures. Ion bombardment occurs when a plasma is
present in the vicinity of the substrate as in the many forms
of sputtering and plasma-assisted processes. High impact energies
are produced in sputtering or in ion-assisted deposition (IAD)
where the substrate is deliberately showered by high-energy argon
ions. The greater arrival energy of e-beam vs resistance-heated
evaporation generally results in denser films. These deposition
techniques are employed to densify deposited layers, however,
other means can be and are used towards this end.
-
- As a first step, effort must be made to remove as much water
from the deposition environment as possible. Chamber walls and
tooling hold surface water that is continually being released
to the deposition atmosphere. High rate exhaustion of this water
and other gasses through baking out under vacuum, cold trapping,
or pumping is required. Cryopumps, in contrast to diffusion pumps,
are better at removing water. Sometimes a cold trap is placed
in the coating volume to trap volatile water through condensation.
-
- The starting material and its form and preparation are important
in limiting the introduction of water. Absorbed and adsorbed
water and other gases are released in going from atmosphere to
vacuum. Some materials hold more water than others. Materials
that have been conditioned by pre-melting, fusing, or mixing
retain less water than materials that are simply sized into smaller
pieces of the parent crystallized material, for example. We are
all aware of the sometimes explosive showering and spitting that
occurs with dust and small particles in the crucible. All pure
fluoride compounds hold water, while few oxide compounds do.
IR materials such as ZnS, Ge, and other semiconductors generally
remain dry, as do metals. Many salts, used in far-IR coatings,
are hydrophilic and even hygroscopic and require special care,
for example cryopumping, to be successfully deposited with stable
film properties.
-
- When a film layer condenses with porosity instead of bulk
density, water vapor can diffuse in and occupy the internal voids
upon exposure of the layer to the atmosphere. The internal surfaces
are highly reactive and bonding forces are high. Several phenomena
can then transpire.
Spectral Shifting
- The effective refractive index of the layer increases with
increased diffusion of water vapor, causing a spectral shift
(longward). This change would be acceptable if it was permanent
and could be compensated for in anticipation, but some fraction
of the water bonds are weak, permitting water to escape upon
evacuation of the layer. This is a consistent problem for filters
and AR coatings that are intended for vacuum operation as in
space missions. Oxide compounds that grow with columnar microstructure
are notorious for this problem. Titania layers exhibit the effect
to a greater degree than silica layers because the latter tend
to grow as glassy films (amorphous). Hafnia films exhibit the
water-caused shift , and it is observed with nearly all oxide
compounds. The problem is especially visible with alumina films
combined with silica or titania layers. Blotchiness is reported
that requires the passage of days for absorption to complete
and achieve uniform appearance.
-
- An example of the shift between atmospheric humidity (~50%)
and dry nitrogen purging illustrates the phenomenon [1]. A thirty-layer
stack of titania and silica quarterwaves deposited by e-beam
on a 250° C substrate was examined. The startling observation
was that an 8 nm shift of the spectral edge of the coating was
observed to occur within 1-1.5 min, and was completely reversible.
The porosity of the 30-layer stack evidently was quite high.
Water Bands
- Another phenomenon that occurs is the appearance of spectral
absorption water bands, specifically in the 2.8 to 3.2 µm
and 6.0 to 7.4 µm regions of the IR. These absorptions
can be troublesome for certain applications and must be minimized.
The depths of the absorption bands can be used as a measure of
the success of the deposition parameters in compacting the layer.
Since IR coatings nearly always require a fluoride compound for
the low index component, and fluorides notoriously retain water,
efforts must be made to reduce the water content of the layers
[2]. These efforts take the forms of deposition by e-beam at
high substrate temperatures, IAD, preconditioning the material
to drive as much water off as possible, using mixed materials,
and water pumping. Generally, slow heating of the starting material
has a significant effect in driving the water out. Fluoride sputtering
techniques are being developed, but are not as straightforward
or hazard-free as sputtering oxide compounds. Therefore, starting
material preparation and pre-deposition conditioning are the
processes steps that should routinely become part of the deposition
parameter set.
-
- If a material is hydrophilic, small particle sizes with their
greater surface area must be avoided. It is preferable to start
with larger chunks. Under the shutter and in vacuum, the material
is slowly heated, and, if e-beam is used, the low power beam
must be swept rapidly. When outgassing stops or a fused surface
is formed, the material is ready for evaporation. Some materials
behave very well in terms of low outgassing and absence of particulate
showering (spitting). CERAC IRX® and IRB™ are especially
notable for these properties. These materials are solid solutions
of mixed fluoride compounds. Silicon monoxide also behaves well
because of the way it is formed as a starting material and its
condensation as an amorphous microstructure. Materials that melt
produce layers that absorb less water than materials that sublime.
Stress Effects
- Another effect of water absorption / desorption is changes
in the nature and magnitude of layer stress. Fluoride films often
develop tensile stress when exposed to atmospheric moisture.
This effect is reversible to the degree that the volatile (weakly
bound) component is exchanged between vacuum and air. In some
cases, the sign and magnitude of the stress can move in directions
that improve the integrity of the coating upon exposure to air.
The coating is said to have “seasoned”.
-
- In the case of multi-layer coatings, changes in total stress
level can result in crazing, cracking, and loss of adhesion to
the substrate. The change occur over hours or days depending
on the diffusion rate. It has been observed that a coating that
appears sound upon venting can spontaneously break up after some
time of residence in normal atmosphere. The MIL-spec humidity
soak is designed to detect this tendency. The preventative measure
is to insure that the materials of the multi-layer grow with
amorphous form to prevent moisture penetration, and form strong
mutual bonds. This is not always accomplished with high substrate
temperature because high temperatures promote large grain growth
with accompanying larger interstitial spaces. Slow growth under
high vacuum conditions can help some materials, but obviously
not oxides that require reactive combination with a substantial
background pressure of oxygen. IAD is useful under these deposition
conditions, and has been observed to reduce stress mainly because
columnar growth microstructure is discouraged in the presence
of high impact energies.
-
Laser Damage Threshold
- There is evidence that the absorbed water lowers the LDT
of oxide coatings for near-IR laser applications. Water has an
absorption bands near 830 nm, 905 nm, 955 nm, 1.34 µm,
and 1.37 µm. Heating by high-energy irradiation can cause
debonding and mechanical effects that result in coating destruction.
Again, densifying methods are required to increase the LDT.
References
- 1. Samuel F. Pellicori and Herbert L. Hettich,
Appl. Opt. V27, No. 15, 3061 (1988).
2. S. F. Pellicori and E. Colton, Thin Solid Films, 209, 109
(1992).
3. Fink, Yoel, Winn, Fan, Chen, Michel, Joannopolous, Thomas,
Science, 282, 1679 (1998).
4. Angus Macleod, Macleod Medium, V7, No. 1 (1999).
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