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Review of modern techniques to generate
antireflective properties on thermoplastic polymers
Ulrike Schulz
Modern optical applications need solutions for providing polymer surfaces with antireflective properties.
The problems involved in coating comprise thermal limitations, incompatible mechanical properties of
coating and substrate materials, and interaction between polymers and plasma. As an alternative for
coating, antireflective properties on polymers can also be obtained by hot embossing or by ion etching of
surface structures. My objective is to provide the criteria for choosing suitable deposition or structuring
methods based on an understanding of plasma-, radiation-, and ion-induced surface phenomena; material
compatibility; mechanical and environmental performance; and cost issues. The potential to produce
antireflective interference coatings is documented for plasma-enhanced physical- and chemical-vapor-
deposition methods, including modern hybrid techniques, as well as for solgel wet-chemical processes.
The review about state-of-the-art coatings focuses on the thermoplastic acrylic, polycarbonate, and
cycloolefin polymers. © 2006 Optical Society of America
OCIS codes: 310.1210, 310.6860, 160.5470, 310.1860.
1. Introduction
Injection-molded or hot-embossed polymer optics will
replace glass optics whenever improved properties or
lower cost can be achieved with plastic parts. The
availability of antireflection (AR) coatings or alterna-
tive ways to reduce the reflection of polymer surfaces
plays an important role in this development. AR
properties are required to increase the share of trans-
mitted light, to improve the contrast of displays, and
to avoid the formation of ghost images in imaging and
illuminating systems. AR coatings have been well
established in glass optics for many years, while the
coating of polymers is a fast-growing field at present.
Applications include plastic eyeglass lenses as well as
optical lenses for sensors inside cars, photographic
equipment of mobile phones, and a growing number
of shields and covers to protect screens and displays.
The formation of AR coatings on plastic substrates
is connected with many problems. First of all, the
mechanical and thermal properties of polymers differ
from those of typical inorganic thin-film materials.
Second, a variety of chemical compositions have to be
taken into account on polymer surfaces. Some prop-
erties of the most important rigid thermoplastics
used for optics are shown in Table 1.
1,2
The different
chemical compositions can lead to various reactions
when they come in contact with plasma or with chem-
icals. The most significant threat to the long-term
stability of coated plastics is posed by environmental
factors such as UV irradiation and varying humidity
conditions, which result in slow changes of bulk or
interface properties of polymers after coating. In ad-
dition, the permissible temperature for coating pro-
cesses is limited. This is a difficulty for classical
evaporation processes just as for wet-chemical coat-
ings, which require high temperatures for hardening.
As a consequence, coating technologies that are well
established for glass cannot be used for polymers, and
different polymers normally require specialized pro-
cess parameters.
Only a small number of review articles about coat-
ing on plastics deal with the general task.
3–6
The
most extensive experience about abrasion-resistant
AR coatings has been gathered in the field of eyeglass
coating.
7–10
Several articles and books specialize on
plasma treatments of polymers.
11–15
The main focus
of this paper is on the discussion of procedures and
techniques for obtaining AR properties on rigid ther-
moplastics for optical applications. This concerns
dielectric interference coatings and AR surface struc-
tures. The most important thermoplastics for opti-
cal applications will each be discussed separately,
The author is with the Fraunhofer Institat für Angewandte
Optik und Feinmechanik, Albert-Einstein Strasse 7, 07745 Jena,
Germany. His e-mail address is schulzul@iof.fhg.de.
Received 8 March 2005; revised 18 October 2005; accepted 1
November 2005.
0003-6935/06/071608-11$15.00/0
© 2006 Optical Society of America
1608 APPLIED OPTICS 兾 Vol. 45, No. 7 兾 1 March 2006
including developments in science and technology
during the past ten years.
2. Principles for Reducing the Reflection of
Polymer Surfaces
A. Interference Layers
The basic theory behind generating AR coatings by
using the interference effect has been well known for
many years, and approved multilayer designs are
available.
16,17
In principle, there is no difference be-
tween the optical performance required for optical
parts consisting of a transparent polymer material
and that of any inorganic glass. The simplest case for
obtaining AR properties on a substrate material with
refractive index n
s
at a defined wavelength
0
is to
deposit a thin film with a lower refractive index n and
a thickness that is one quarter of that wavelength
[quarter-wave (QW) layer]. In theory, a QW layer
of magnesium fluoride with n ⫽ 1.38 may reduce
the reflectance of a polycarbonate (PC) surface 共n
s
⫽ 1.56兲 to ⬃1% at 500 nm. However, fluoride coat-
ings cannot be recommended for polymers because of
their high tensile growth stress and the unsatisfac-
tory mechanical properties of fluoride thin films de-
posited at a low substrate temperature.
18
The most
frequently used broadband AR coatings for the visible
spectral range nowadays are adapted from a quarter–
half–quarter design and consist of four to six lay-
ers.
19–20
Experience in producing such “classical”
broadband AR coatings on polymers exists mainly in
the field of eyeglass coating in which the AR layers
and hydrophobic top coatings are arranged on top of
hard coatings of several micrometers thickness.
21,7,8,10
Usually, the hard coatings are lacquers based on si-
loxane. Thus the hard-coated surface, rather than the
more sensitive polymer itself, forms the substrate for
vacuum deposition of the AR coating.
The recommendation in applying special optimized
AR designs to bare thermoplastics rather than well-
applied standard designs developed for glass results
from the practical problems with plastic coating. For
common thermoplastics, the following requirements
should be taken into account for generating AR de-
signs:
● The layer materials and thicknesses have to be
adjusted with consideration of the mechanical and
thermal film stresses and the heat accumulation dur-
ing the deposition process.
● The layer materials have to be selected to
achieve radiation protection of interfaces or of the
bulk polymer, depending on the polymer substrate
type.
● The process parameters for the materials used
have to be suitable to avoid damage to the polymer
bulk and interfaces during thin-film deposition. As a
consequence, the refractive index available may be
restricted practically.
● The change in the optical properties of interfa-
cial zones due to plasma treatments sometimes has to
be taken into account for the design calculation.
● The designs should be suitable for integrating
additional functions such as improved scratch resis-
tance, barrier function, antistatic function, or hydro-
phobic properties.
Modern software design tools are available to gen-
erate special broadband AR designs, taking into ac-
count the requirements of an individual polymer and
its field of application.
22,23
The design type known
under the trade name AR-hard was developed espe-
Table 1. Properties of Transparent Thermoplastic Materials
a
Material Brand Name
n
D
(587.6 nm)
Abbc
Value V
D
Light
Transmittance
(%)3mm
Tensile
Modulus
(MPa)
Density
(g兾cm3)
Water
Absorption (%)
24 h, 23 °C
Deflection
Temp. (°C)
ASTM D648
Poly(methyl
methacrylate)
Plexiglas 7N 1.491 58 92 3200 1.19 0.3 95
Polycarbonate Makrolon LQ2647 1.585 30 91 2400 1.2 0.12 124
Apec HT9351 1.566 88 2300 1.15 0.2 173
Polycycloolefin Topas 5013 1.533 58 92 3100 1.02 ⬍0.01 123
Zeonex E48R 1.53 56 92 2500 1.01 ⬍0.01 122
Zeonex 480R 1.525 56 92 2200 1.01 ⬍0.01 123
Zeonor 1020R 1.53 92 2100 1.01 ⬍0.01 101
Arton FX4727 1.523 52 92 3000 1.08 0.05 110
Apel 5014DP 1.543 56 90 3200 1.04 0.09
b
125
Polysulfone Udel P-1700 1.634 23 84 2480 1.24 0.3 174
Polyethersulfon Ultrason E2010 1.65 80 2700 1.37 2.1
c
208
Polyamide Trogamid CX7323 1.516 45 89 1400 1.02 0.3 122
d
a
From manufacturers’ published data sheets and Refs. 1 and 2.
b
ASTM D570 (one week).
c
Saturation at 23 °C.
d
ISO75-2 (0.45 MPa load).
1 March 2006 兾 Vol. 45, No. 7 兾 APPLIED OPTICS 1609
cially for plastic optics to use a higher total thickness
for AR coatings with high scratch resistance.
24,25
The
basic idea of AR-hard is to treat symmetrical layer
periods as equivalent layers in order to replace layers
with practically unobtainable low refractive indices
共1.1 ⬍ n ⬍ 1.5兲. The replacements can be performed
by using layer sequences with a total phase thickness
of three or more QWs. A basic example is an arrange-
ment of symmetrical three-layer periods, each having
three times the QW optical thickness 共3QW兲. Each
period consists of a very thin high-index layer H
sandwiched between two thick low-index layers L.
Figure 1 shows this principle schematically. The
equivalent layers build up a so-called step-down de-
sign, matching the refractive index of the substrate to
that of the air.
26
A remarkable improvement in the performance of
broadband AR coatings has been accomplished over
the past fifteen years. The designs have changed from
the classic quarter–half–quarter design to the so-
called multicycle (MC) designs. Cycles
27
or clusters
28
denote a semiperiodic refractive-index profile previ-
ously observed in optimal solutions of broadband AR
coatings for particular sets of refractive indices and
wavelength ratios. For example, the bandwidth of
AR-hard designs can be broadened if layer sequences
with greater total phase thickness are used in the
step-down arrangement instead of the three-layer pe-
riods [schematic shown in Fig. 1(b)].
29
Figure 2 shows
the residual reflectance available with AR-hard layer
stacks for different bandwidths.
B. Inhomogeneous Layers and Surface Structures
Over the past decade, different attempts have been
made to produce broadband AR properties that apply
single layers in which the refractive index varies
gradually from that of the bulk material to unity. The
principle is to mix the available low-index material
(e.g., glass or silica) with air on a subwavelength
scale. That calls for a compromise between optical
and mechanical properties. Layers with decreasing
refractive index from the substrate site to air can be
achieved by porous coatings or by stochastic and pe-
riodic surface structures as schematically shown in
Fig. 3.
30
To implement AR properties in a certain
spectral range by surface structures, one must ensure
that the spacing of a suitable array is smaller than
the wavelength concerned but that the depth is at
least a significant fraction of that wavelength.
31
Optical properties of such surface structures can be
Fig. 1. Schematic illustration of the scratch-resistant AR coating called AR-hard, which is suitable for low-index polymer substrates.
Arrangements are of (a) symmetrical periods with 3 QW optical thickness each and (b) nonsymmetrical layer stacks with 4 QW optical
thickness. Both arrangements build up step-down designs of equivalent layers: H, high-index material; L, low-index material.
Fig. 2. Performance of AR-hard coatings for different spectral
bandwidths, applying basic layer arrangements as shown in Fig.
1(a) (curve 1) and Fig. 1(b) (curve 2). Both examples are calcu-
lated for a Zeonex substrate (n ⫽ 1.53) without a back surface.
1610 APPLIED OPTICS 兾 Vol. 45, No. 7 兾 1 March 2006
calculated approximately by application of the effec-
tive medium theory (EMT). To describe continuous
profiles, I separated the gradient layer is separated
into homogeneous thin films and regarded as an in-
terference layer.
32
3. Manufacturing Methods
A. Vacuum Coating
Evaporation of oxide materials is still the most com-
monly used method for manufacturing optical coat-
ings. Naturally, coating on plastics has to be conducted
at temperatures below the heat distortion temperature
of the plastic. For most thermoplastics this means a
temperature limit of ⬃120 °C (Table 1). Only porous
coatings would be obtained from thin-film materials
evaporated at the low substrate temperature required.
During the past decade, important innovations have
been established for coating plastics. Present-day coat-
ing technologies are based on the application of ion
assistance in evaporation processes and the possibility
of using cold plasma in chemical-vapor-deposition
(CVD) processes.
33–35,9
As an example of an ion-
assisted process, the plasma-ion-assisted deposition
(PIAD) technique carried out with an APS 904 coating
plant (Leybold Optics) may be mentioned here.
9
Dur-
ing the PIAD process, the growing film is bombarded
with argon ions emitted from the advanced plasma
source (APS). The ion energy is in a range between
⬃60 and 180 eV, but for coating plastics only volt-
ages below 120 eV can be recommended. Plasma-
assisted evaporation processes for the deposition of
AR layers on plastic optics are applied worldwide in
industry.
36,37
Magnetron sputter deposition is another method of
physically depositing inorganic materials. Because of
its comparatively high heat and radiation emissions,
the sputtering of dielectrics is critical for coating ther-
moplastics. Nevertheless, some new concepts use the
advantage of high precision for the production of AR
layers on eyeglasses. Closed-field magnetron sputter-
ing has been described as a flexible low-temperature
process suitable for optical coatings on plastics.
38
Sputter deposition processes of AR coatings are also
applied, following a plasma-enhanced CVD (PECVD)
process step that deposits a scratch-resistant lay-
er.
39,40
Such arrangements may include dc magne-
tron sputter deposition of silica and titania as well as
reactive magnetron sputtering of silica and silicon
nitride from a single silicon target. Sputtering is also
a basic technology in web coaters for flexible sub-
strates, in which case the undesired heating of the
polymers can be prevented by cooling the web over a
drum.
41,42
CVD is based on the decomposition and chemical
reaction of gaseous compounds in vacuum near the
substrate surface. One of the reaction products is a
solid, which precipitates onto the surface forming a
thin film. PECVD techniques used to deposit coatings
on plastics use a microwave (MW) or radio frequency
(rf) plasma for activating the reacting gases.
43,44
Because they avoid high temperatures and plasma
emissions, pulsed processes will become useful for
coating plastics. An innovative plasma deposition
process for thermoplastics is provided by the German
Schott Hicotec Company.
45,46
In the plasma-impulsed
CVD (PICVD) process, the gaseous precursor is de-
composed by a pulsed MW plasma (Fig. 7.4) in Ref.
47. A short ignition of the plasma is repeated many
times until the required layer structure has built up.
The process is already being used in production for
coating light reflectors and eyeglasses. For complex
optical multilayers, PECVD processes do not deliver
the same thickness accuracy and homogeneity as
physical-vapor-deposition (PVD) processes. More-
over, the materials that are suitable as precursors are
limited and have to be handled carefully.
B. Wet-Chemical Coating (Solgel Coating)
Wet-chemical coatings can be deposited by dip coat-
ing or spin coating on rigid flat or slightly curved
substrates. Injection-molded parts of complex forms
are not suitable for this deposition method. The solgel
process classically involves the use of inorganic salts
or metal alkoxides as precursors. Typically, an ele-
vated temperature up to 400 °C is needed for hydro-
lysis and polycondensation to accomplish networking
and chemical decomposition until the oxide is for-
med.
48
It is essential for coating polymers to avoid
high temperatures for curing or using radiation-
induced cross-linking reactions for hardening. Typi-
cal materials used to improve the abrasion resistance
of thermoplastics are organically modified silanes.
Silicate-based inorganic–organic hybrid polymers
(ORMOCERs) have attracted considerable attention
owing to their optical and mechanical properties.
49
The synthesis entails a chemical modification of or-
ganic components to covalently attach them to the
inorganic network.
The development of AR coatings using wet-chemical
processes comprises mainly single-layer and two-layer
systems. Procedures for generating porous layers are
baking and burning out the organic particles, which
Fig. 3. Schematic illustration of surfaces with decreasing refrac-
tive index from the substrate site to air: (a) porous coating, (b)
stochastic surface structure, (c) periodic surface structure.
1 March 2006 兾 Vol. 45, No. 7 兾 APPLIED OPTICS 1611
are in a suspension within a solgel coating,
50
or
the unmixing of a binary polymer blend during spin
coating.
51
These processes are described mainly for
temperature-stable substrates. The preparation of
multilayers by solgel is complex, because the deposit-
ing and hardening steps have to be repeated many
times. The basic research in this field is focused on the
coating material rather than on the kind of polymer
substrate or the coating design.
52
For example, the
following AR coatings are results of basic research and
are described in the specified references:
● A multilayer for poly(methyl methacrylate)
(PMMA) substrates that consists of a hard coating,
three-layer AR system and a water-repellent top
coating.
53
● A four-layer coating for plastics from polymer-
ized nonorganic titanium-containing and silicon-con-
taining solutions. Each layer is cured by thermal
treatment before the next layer is applied.
54
● An alcohol-based inhomogeneous AR layer pro-
duced by soaking in acidic or alkaline etchant to dis-
solve oxide colloids partly from the layer.
55
● A single composite layer containing organic flu-
oropolymers.
56
● A broadband AR and scratch-resistant coating
made from tantalum and silicon oxide–based layers.
The baking step temperature does not exceed 150 °C
during 30 min so that various substrates can be
coated.
57
● An interference two-layer AR coating compris-
ing alkoxide or acrylate compounds of Ti, Al, or Zn as
the high-index material and hydrolysable organic sil-
icon compounds as the low-index material.
58
● A two-layer AR coating containing polymeriz-
able nanoparticles modified with alkoxy-silanes and
a photoinitiator.
59
● An antistatic two-layer AR consisting of a solgel
alkoxide polymeric material and a colloidal indium
tin oxide material.
60
C. Antireflective Surface Structures
First, periodic subwavelength surface structures with
AR properties were observed in nature on the eyes of
the night-flying moth.
6
Artificial AR surface structures
on polymers may have the potential to be inexpensive
if they can be replicated by embossing. Initially, tech-
nical solutions were generated by using the interfer-
ence pattern at the intersection of two coherent beams
of light from a laser.
62
Today, master structures for
surface areas up to ⬃0.5 m
2
can be generated in a
holographic optical process, and efforts are in prog-
ress to increase this area. The outstanding etching
behavior of PMMA concerning low-pressure plasma
is used to generate a stochastic AR structure directly
on the polymer surface.
63
All structures described can
be transferred to polymer surfaces by hot or cold em-
bossing by using nickel replicas. Figure 4 shows
atomic force microscopy (AFM) pictures of AR surface
structures generated on PMMA by (a) hot embossing
of a periodic moth-eye structure and (b) ion etching.
Stochastic structures suitable for AR can also be gen-
erated directly on the tool for hot embossing by anodic
oxidation of aluminum directly on the tool
64
or by
depositing very rough PVD coatings.
65
The basic dis-
advantage of AR structures is their mechanical weak-
ness. The surfaces have to be handled very carefully,
and it is practically impossible to clean them. On the
other hand, excellent optical properties and cost is-
sues will further promote their industrial application
onto protected or build-in optical parts.
4. General Problems for Coating Polymers
A. Interaction between Polymers and Plasma and Effects
on Coating Adhesion
Interactions between polymers and plasma; normally
cannot be avoided during modern vacuum-coating
processes. Please note that emissions from plasma
can influence the adhesion of coatings on polymers in
different ways. It has been well described in the lit-
erature that adhesion is simultaneously related to
the complementary effects of roughness, surface en-
ergy, surface stabilization reactions, and the possible
formation of covalent bonds at the coating–polymer
interface. An adhesion improvement by plasma treat-
ments has been variously attributed to improved wet-
tability, to surface cross-linking, or to interfacial
diffusion in a depth of typically 5–50 nm.
66–68
On the
other hand, when an adhesive bond breaks at a low
Fig. 4. AFM images (1 m ⫻ 1 m) of AR surface structures on PMMA: (a) hot-embossed periodic structure and (b) stochastic structure
produced by direct ion etching.
1612 APPLIED OPTICS 兾 Vol. 45, No. 7 兾 1 March 2006
applied stress, the fracture may have occurred ex-
actly at the interface, in a thin layer close to the
interface, or as a cohesive fracture in the bulk phase.
If poor coating adhesion is observed because of a co-
hesive fracture in the bulk, the main problem may be
the weak boundary polymer layer formed during an
unwanted plasma exposure or an applied plasma
treatment. In this case efforts should be directed to-
ward strengthening this weak boundary layer rather
than to increasing the interfacial attraction.
Electromagnetic radiation at wavelengths below
200 nm has enough energy to break any polymer
bond, and for wavelengths below ⬃120 nm the photon
energy is sufficient to ionize most organic mole-
cules.
69
But only radiation that is selectively abor-
bed by a chosen polymer can initiate chemical effects
on that polymer. The penetration depth of the radi-
ation depends on the absorption coefficient and varies
between a few nanometers and some millimeters. Im-
portant for coating processes are vacuum UV (VUV)
radiation at wavelengths below 180 nm. Several in-
vestigations show that cross-linking reactions during
low-pressure plasma treatments are initiated mainly
by VUV radiation.
70–73
Modern PVD processes often use ion sources for
substrate activation and the densification of growing
films. Besides causing radiation effects, the ions typ-
ically initiate etching processes that are associated
with changes in surface topography
74
and refractive
index.
75,44
In addition, interlayers such as nonstoichiometric
oxides of chromium and silicon are discussed to im-
prove coating adhesion on polymers.
76–78
The forma-
tion of covalent bonds between atoms of the polymer
structure and a metallic atom is the most widely
accepted mechanism for high adhesion forces.
79–80
B. Mechanical and Thermal Stresses
Residual stresses in thin films are due to the mechan-
ical growth stress of the thin-film material and the
thermal mismatch between the film and the sub-
strate when the system is heated during film deposi-
tion and cooled from its fabrication temperature
to room temperature.
81–83
As is known from many
studies, the mechanical growth stress of thin oxide
layers depends mainly on the parameters of the
vacuum-deposition process and can be adjusted wit-
hin a certain range.
84–86
Even though many investigations into stress in
coatings have been made, less is known about the
special case of inorganic coatings on polymer sub-
strates. A different total-stress behavior compared
with coatings on inorganic glass has to be considered
for oxide coatings deposited on polymer substrates,
because of the difference in the thermal expansions of
substrate and film, which is the origin of high-level
thermal stress. The thermal expansion coefficient of a
polymer is typically 1 order of magnitude higher than
that for inorganic layers. During PVD and PECVD
processes, the substrate temperature is determined
as the combination of heat received from the evapo-
ration sources and the plasma sources used in the
process.
87,88
For example, the thermal load on a sub-
strate during PIAD results from both the power of the
electron-beam (e-beam) gun and the thermal emis-
sion of the ion source. The e-beam power, which
depends on the evaporation temperature, melting be-
havior, and thermal conductivity of the materials, is
much higher for most high-index materials than for
the low-index SiO
2
. An increase in the maximum
temperature on the polymer surface occurs with in-
creasing film thickness at constant ion energy as
shown for an e-beam-evaporated TiO
2
layer in Fig. 5.
Therefore differences between the stress levels at the
coating–polymer interface and on the outer surface
have to be expected. It has to be considered in a rough
evaluation that a temperature change during the
deposition process may generate a stress component
of ⬃5–20 MPa兾K. The stress is tensile during heat-
ing and becomes compressive by cooling.
88
For the
complex stress behavior of coatings on polymer sub-
strates to be understood, the temperature has to be
controlled carefully during the deposition process.
More precise calculations are possible by using com-
plex thermal-stress models that take into account the
elasticity theory.
89–91
Mechanical coating stress can make it difficult to
produce coated plastics that do not show cracking or
ablation during their later life. Figure 6 shows a typ-
ical stress cracking observed on PMMA coated with
1 m SiO
2
. Although the coatings themselves have
compressive growth stress, the stress cracking caused
by thermal tensile stress can occur during the deposi-
tion of thick layers on polymer substrates. Figure 6(a)
shows typical buckling of a coating in the form of
“worm tracks” caused by a high compressive growth
stress in combination with insufficient coating adhe-
sion. Note that this kind of damage typically occurs if
Fig. 5. Temperature development measured in situ and simulta-
neously on a polymer substrate, a glass substrate, and a metallic
substrate holder during IAD of a single-layer of TiO
2
.
1 March 2006 兾 Vol. 45, No. 7 兾 APPLIED OPTICS 1613
the coating adhesion is low. In Fig. 6(b) the coating has
cracked during the deposition process because of high
temperature (tensile-stress cracking). The mechanical
growth stress of coatings can often be limited by re-
stricting the film thickness and by optimizing the dep-
osition parameters, such as the level of ion energy in
the case of IAD,
85
or by using water vapor as a reactive
gas.
86
5. Experience Gained with Selected Polymer Materials
A. Poly(methyl methacrylate)
PMMA is still the main thermoplastic material used
for large lenses such as condensers, Fresnel lenses,
and projection TV lenses. Characteristics of PMMA
are low birefringence, outstanding hardness, high
light-deterioration resistance, and high Abbe num-
ber. For vacuum coating, however, this material is
problematic. When exposed to VUV radiation, PMMA
shows a degradation on its outer surface that de-
pends strongly on the wavelength of irradiation. Pho-
tons with energies of ⬃8.5 eV cause the methyl ester
group to split off, whereas radiation below this wave-
length can break the main polymer chain.
92,93
Most
types of plasma exposure cause weak boundary lay-
ers, of decomposition products on the surface that
dramatically lessen the adhesion of subsequently
evaporated layers. Many different plasma and ion
treatments have been investigated for PMMA.
94–99
Some strong plasma-treatment conditions are capa-
ble of removing the ester groups nearly completely
and of creating a polyolefinlike surface composition.
These treatments have been described to improve the
coating adhesion on PMMA.
100–103,45
The adhesion
layers investigated include organic compounds, sili-
con oxides, as well as thin-metal layers.
104–107
Despite the problems, AR coatings on PMMA are
available on the market. Suitable technologies are
the PICVD process of SCHOTT HiCotec,
45
the pro-
cess of NAGASE Company,
53
and a plasma-IAD pro-
cess of Fraunhofer Institut für Angewandt Optik und
Feinmechanik (IOF).
108
The latter suggests the dep-
osition of a VUV protective layer without any pres-
ence of plasma as the first step in vacuum deposition.
After that, a coating of type AR-hard can be deposited
by using plasma-assisted e-beam evaporation.
Widely used on PMMA are several types of sub-
wavelength AR structures, as indicated in Subsection
3.C. The stochastic AR nanostructure shown in Fig.
4(b) can be generated directly on PMMA by applying
ion bombardment in vacuum.
63
Figure 7 shows the
transmission of a PMMA sample after such a treat-
ment. The low residual reflection remains colorless at
normal as well as at oblique angles of light incidence.
B. Bisphenol-A Polycarbonate
Polycarbonate of bisphenol A (PC) (Makrolon, Lexan)
is especially important for optical disks and for
automobile applications because of its high-impact
strength. The characteristics of PC are a high refrac-
tive index and a low Abbe number. The disadvantages
of the material are high birefringence, low mechanical
hardness, and sensitivity to UV radiation. PC is much
softer than PMMA and needs a hardening coating sev-
eral micrometers thick to pass the rubber test accord-
ing to ISO9211. Various efforts are underway to
develop scratch-resistant transparent layers. On auto-
mobile parts and eyeglasses, one scratch-resistant sol-
gel coating is based on highly filled nanocomposite
materials.
109
Protective coatings on PC based on silica
and siloxanes [hexamethyldisiloxane (HMDSO) or tet-
raethoxysilane (TEOS)] can be obtained by applying
Fig. 6. Cracking behavior of coatings on polymers: (a) buckling of a coating as a consequence of compressive growth stress and insufficient
coating adhesion and (b) tensile-stress cracking caused by different thermal expansions of the substrate and coating.
1614 APPLIED OPTICS 兾 Vol. 45, No. 7 兾 1 March 2006
different PECVD processes
110,111
and IAD of silica.
112
Different plasma treatments are successfully applied
to increase the coating adhesion on polycarbon-
ate.
113,114
Optical coatings produced by PECVD that
consist of SiN
x
and SiO
2
are described
115,116
along
with classical oxide layer stacks.
25
The degradation of polycarbonate caused by global
UV radiation is well investigated, because polycar-
bonates are prone to yellowing.
117–119
Most commer-
cially used PC types are protected by stabilizers in
the bulk material. It has to be pointed out that the
additives do not protect the interfaces from UV-
induced degradation. The adhesion of coatings can
suffer during a later irradiation of coated parts, un-
less the coating blocks the radiation at wavelengths
below ⬃380 nm. For outdoor applications, coatings on
PC have to be provided with UV-protective layers to
prevent gradual destruction of the interfaces.
C. Other Polymers
The group of transparent thermoplastics suitable for
optical parts comprises certain types of acrylics, poly-
carbonates, polyamides, polysulfones, polystyrene,
and recently cycloolefin polymers (COPs, types of
Zeonex, Zeonor, Apel, Arton, and Topas).
120,121
For
most of them, published experience made with regard
to their properties for coating did not exist until now.
Some results validated for IAD of optical coatings can
be presented here from our own experience. For some
COPs (Zeonex E480, Zeonex 48R, Topas 5013) coated
with scratch-resistant AR layers, high adhesion and
outstanding environmental properties have been ob-
served.
122
COPs show only negligible water absorp-
tion and higher thermal stability compared with
PMMA. It is estimated that these materials have the
potential to replace PMMA particularly for precision
optical applications in which thick dielectric coatings
are required. Similarly, recent transparent poly-
amides (Trogamid CX, Grillamid) can be vacuum
coated without adhesion problems. In addition, trans-
parent materials with thermal stabilities up to
180 °C have been investigated concerning their prop-
erties for vacuum coating, with promising results for
the slightly yellow colored polyethersulfone Ultra-
son.
123
The application range of polyamides, as well
as of the polyethersulfone, may be restricted by its
high water absorption.
6. Summary and Outlook
At present, the development of plastic optics is sub-
ject to enormous pressure from applications. Under
the conditions of globalization, the time needed to
develop a new marketable product becomes inces-
santly shorter. The introduction of easy-to-form poly-
mer materials becomes important for cost savings in
the production of consumer electronics and automo-
bile parts as well as of complex-shaped, high-end pre-
cision optical parts.
Up to now, vacuum-coating processes were the
most appropriate techniques for producing interfer-
ence coatings that need a thickness precision in the
range of 1 or 2 nm. The use of different low-pressure
plasmas to assist the processes has become quite
common. Plasma treatments are applied to modify
the layer materials in a desired way. At the same
time, various interactions with the polymer surfaces
have to be taken into account. To minimize the risk
and to reduce development costs for the optical parts,
one should focus applications that need complex coat-
ings on those polymers that provide the highest pro-
cess stability. It becomes apparent with optical lenses
made from new materials such as Zeonex, rather
than from PMMA.
At present, there is a demand for techniques that
create AR properties on rigid plastic parts in mass
production. Particularly for applications in which
glass is to be replaced by plastics to reduce product
costs, expensive coating processes are counterproduc-
tive. Reactive magnetron sputtering is identified
as a possible way to increase the yield of a vacuum-
coating process. Sputtering provides an area source
that is ideal for scaling processes to any production
volume. Also, the hot embossing of “moth eyes” has
proved to be highly cost effective, especially for mass
production, and may be a suitable alternative to coat-
ing procedures. However, the application range of AR
structures is limited because the polymer surfaces
become soft and soil sensitive. The evaluation of new
materials described in patents has demonstrated the
potential of solgel techniques for polymer coating.
There is an ever-increasing number of new materials
that are more compatible with plastic substrates than
inorganic materials are. But up to now, manufacturing
methods to produce high-precision interference layers
by using solgel processes are still uncommon. In con-
clusion, it is estimated that the requirements of optical
technologies will stimulate the industry toward fur-
ther advancement in process techniques for making
AR polymer surfaces.
Fig. 7. PMMA with a stochastic AR structure, showing spectral
transmission at normal and tilted incidence of light.
1 March 2006 兾 Vol. 45, No. 7 兾 APPLIED OPTICS 1615
I thank my friends and colleagues for many helpful
discussions in the field of interference filters and poly-
mer coating and for the comments that improved the
quality of this paper: George Dobrowolski, Angus Ma-
cleod, Ludvig Martinu, Hans Pulker, Uwe Schallen-
berg, Robert Schaffer, Ric Shimshock, Ron Willey,
Norbert Kaiser, and all my other colleagues from the
optical coatings department at Fraunhofer IOF in
Jena. The research was supported by the Bundesmin-
isterium für Wissenschaft und Forschung BMBF (con-
tract 03N3118) and by the Thüringer Ministerium für
Wissenschaft, Forschung und Kunst TMWFK (B409-
04005).
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