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CERAC Coating Materials News
Volume 8, Issue 4
December, 1998
- Improving Coatings: Techniques,
Processes, and Testing
Introduction
The durability of thin films for optical as well as
mechanical applications is always a prime concern to the user
and the coater. We have approached this subject several times
and from different perspectives in CMN. We have discussed how
materials, deposition techniques and processes influence the
mechanical properties that produce wear resistance such as hardness,
adhesion, and strength, and the commonly used methods for evaluating
these properties. Similarly, the starting material composition
and deposition process parameters determine the optical properties
of index, absorption, wavelength stability, etc. In this issue,
we discuss work done with ternary compound mixtures toward improving
zirconia film layers. Finally, we include a brief discussion
on roll coating to mass produce optical coatings for many applications.
- Example:
Study of an Optical Coating with Electrical and Mechanical Requirements
-
- Ternary Composites Produce Stable Oxide
Coatings
In these issues of CMN, we have often extolled the
optical, mechanical, and stability advantages that mixed materials
provide over pure compounds [1]. Such materials include fluoride
and oxide compound mixtures that are evaporated or sputtered
from solid solutions of the composite starting materials. The
general properties achieved in the deposited film layer are controlled
crystallinity and better homogeneity, which are responsible for
uniform and greater index value (higher packing density), lower
internal stress, greater surface smoothness, greater stability
to environmental stresses, etc. The presence of the second component
(“dopant”) results in modifying the film microstructure,
specifically discouraging micro-crystal growth or columnar structure.
-
- Zirconia is a material that has received a lot of attention
because its films exhibit hardness, high temperature durability,
and high refractive index. However, in its pure starting state,
it is difficult to evaporate or sputter and produce films with
consistent index, high packing density, and low stress. A crucial
requirement for modern optical film technology is to have wavelength-stable
coatings, i.e., coatings that do not shift as a function of environmental
humidity (due to void filling when exposed to moisture). As is
the case with many oxide and fluoride compounds, very high substrate
temperatures (>200°C) are required to build high packing
density, but then the film stress is increased. When a solid
solution is made with one of the many possible glass-forming
oxides (see ref. 2 for examples), improved mechanical and optical
properties, including laser damage resistance, are achieved.
Two-compound mixtures produce partial stabilization of the crystalline
nature, often containing two phases, and therefore not truly
amorphous films. Previous researchers reported that ZrO2 could be stabilized
to a cubic phase by adding MgO, and that by adding Al2O3 to ZrO2 a tetragonal
phase is established [3]. Improved mechanical properties are
generally observed. Little work has been done on ternary composites
until a recent publication [4] that reports a thorough study
of the MgO-Al2O3-ZrO2 solid solution material (CERAC M-1126).
-
- Sahoo and Shapiro [4] observed that when both Al2O3 and MgO are present, the two phases above
disappear and an amorphous film results. Their study demonstrates
that the ZrO2-composite films exhibit
nearly invariant wavelength shifts with humidity exposure, very
low stress, high optical transparency between at least 2000 nm
and 300 nm (substrate cut-off), and good mechanical properties.
They caution that the material must be slowly and thoroughly
melted to prevent it from growing out of the crucible during
e-beam heating.
-
- Sahoo and Shapiro studied the optical, compositional, and
scattering properties as functions of oxygen pressure, deposition rate, and substrate
temperature. The general dependencies found were as follows.
Even at ambient, the films showed sufficiently high packing density
that no air-to-vacuum shift was observed. Index homogeneity was
good at both ambient and the maximum temperature of 237°C,
but in between these temperatures, there is apparently a phase
change that reduces both index and absorption. The oxygen pressure
was <5 E-05 mbar (base) and the rate was 4 Å/s.
-
- With a substrate temperature of 162°C and 5 E-05 mbar
oxygen pressure, the highest indices (~1.85) were obtained at
deposition rates between 8 and 10 Å/s. Extinction coefficient
was low at rates <10 Å/s.
-
- Oxygen pressure has a strong influence on refractive index
and extinction coefficient. The best values for 162°C substrate
and 8 Å/s conditions were obtained at pressures of 5 E-05
to 10 E-05 mbar (3.7-7.5 E-05 Torr). Similarly surface roughness
was low. The interesting observation was the relationship between
zirconium content in the films and oxygen pressure. Zirconium
content increased as pressure increased above ~6 E-04 mbar, but
simultaneously index decreased, while the other components remained
somewhat constant with pressure. So a good starting range for
the parameters seems to be: substrate temperature 125 - 180°C,
pressure 5 E-05 mbar, rate 8 Å/s. In summarizing Sahoo
and Shapiro’s work, we related the preferred parameters,
but advise the user to experiment with the material and to establish
optimum parameters and procedures in his own evaporation system
(good advice for starting with any unfamiliar evaporation materials).
-
- Sputter Deposition Roll Coating
Large area, high volume thin film coatings are produced by roll-to-roll
web coating for many optical and electrical applications. The
most common substrate is PET film, which is then laminated to
glass. Large glass sheets can also be continuously sputter coated.
Typical products are architectural and automotive windows, coatings
(AR and EMI shielding) for CRT and flat panel displays, transparent
conductors for touch panels, reflectors for lighting, flexible
circuit boards, etc. We summarize some items of interest from
a recent paper that traces the development of roll coater sputtering
and reviews some of the developments [7].
-
- The mainstay sputter technique for many years has been DC
planar magnetron sputtering. High deposition rates are achieved
for metals, but the process is not suitable for dielectrics because
the oxidative reaction required to produce oxide compound films
causes target oxidation and system arcing and consequently instability
and inefficiency. The development of alternate sputtering from
two cathodes operating in the AC mode at mid-kHz frequencies
overcame the oxidation and arcing problems.
-
- In the case of oxide film production, the problem of composition
control was severe because the process is basically an unstable
one where a stable operating point exists on a steep curve transitioning
between metal-rich and oxidized target regions. To achieve efficient,
high rate sputtering, it was necessary to invent methods for
sensitive monitoring of the conditions in the plasma environment
and rapid-response process control. Two methods are now used:
monitor the intensity of specific plasma emission lines of the
metallic species, or monitor the partial pressure of oxygen in
the coating region. The first method is implemented by using
the signal intensity from the metal line emission to control
the oxygen flow. Since oxygen consumption is determined by the
oxidation rate of the metal atoms in the plasma and on the target
surface and the evacuation speed of the pumps, the control is
complicated. The second method measures the partial pressure
of oxygen and uses this signal to control target power. A complication
present is that oxygen consumption is not uniform throughout
the chamber because reaction, pumping, and flow rates vary locally.
Consequently, multiple sensors must be used.
-
- Roll coating technology for mass producing functional and
decorative coatings has advanced rapidly in a short period, and
there is no end to improvement possibilities in sight.
-
- References
1. CMN V2 No. 2, April-June 1996;
CMN V6 No. 1, Jan.-Mar 1996; V1, No. 3, Jul.-Sept. 1991.
2. CMN V8 No. 3 (1998).
3. C. M. Gilmore, C. Quinn, S. B. Quadri, C. R. Gosset, and E.
F. Skelton, J. Vac. Sci. Techol., A5, 2085 (1987).
4. N. K. Sahoo and A. P. Shapiro, Appl. Opt. V37, 8043 (1998).
5. W-F Wu and B-S. Chiou, Applied Surface Science 115, 96 (1997).
6. CERAC stock nos.: ITO targets: SS-242, SS-245; Si targets:
SS-352.
7. J. B. Fenn, W. C. Kittler, D. Lievens, R. Ludwig, G. Phillips,
and A. Taylor, SVC 41st Tech. Conf. Proc., 463, April 1998.
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