EXPERIMENTAL DETAILS
Absorption spectrum
An Optical Solutions PS-2 fiber optics spectrometer and a Shimadzu model UV 3100 PC
spectrophotometer were used to gather data on vibrational absorptions. These data were
used to determine absorption loss as a function of wavelength. The data were collected by
placing the material in a 1–cm cuvette and scanning the wavelengths covering both the
UV-visible and Near-IR regions. In all cases a cuvette filled with carbon tetrachloride was
used as the reference sample. The raw absorption data at 1550 nm are expressed as loss in
dB/cm and are summarized in Table 2.
Refractive index, thermo-optic effect, and thermo-mechanical properties
Polymers films were spun on silicon wafers with film thickness ranging from 2 to 15 µm.
The refractive index of the materials was determined on these samples with a prism
coupler (Metricon model 2010) at two wavelengths, 633 and 1554 nm, measured at a
temperature of T =24 °C. The thermo-optic coefficient of the slab waveguide, dn/dT, was
determined from the temperature dependence of the refractive index of the thin film on the
silicon wafer substrate by spectral reflectance interferometry. A Filmetrics F-20 instrument
was used in conjunction with a temperature–controlled, enclosed sample stage, allowing
measurements in the temperature range from ~15 °C (limited by the dew point of the
ambient) and ~95 °C (limited by the Peltier element in the stage). Sample temperature was
measured on a dummy wafer with a thermocouple attached to its surface and symmetrically
mounted with respect to the sample under test on the sample stage. Temperature deviation
between the two positions was less than 1 °C over the temperature range of the
measurement. Spectral reflection intensity data was gathered over a wavelength range of
~700 to 950 nm using a single layer model with a dispersion described by a simple Cauchy
relation. From a linear least–squares regression to the data, i.e., refractive index vs.
temperature, the thermo-optic coefficient was obtained as the slope dn/dT. The data was
calibrated by comparison of the refractive index value at 633 nm to the value obtained with
the prism coupler at room temperature. Results for some siloxane–based materials are
given in Table 1.
Figure 4. Thickness change as a function of temperature for siloxane-based polymer
waveguide prepared by spin-coating on a silicon wafer. The least-squares regression line has
a slope of 3.36x10-3 °C-1 (r = 0.9992). From the slope, a value for the linear thermal expansion
coefficient of the device of CTE = 513 ppm/°C at 30°C is calculated.
Since the spectral reflection interferometer (Filmetrics F-20 instrument) provides not only refractive index
but at the same time also thickness data for the film under test, the linear expansion coefficient of the polymer
material in the particular device configuration, namely as a laterally highly constrained film, can also be
determined by this method. Figure 4 shows a plot of the thickness data vs. temperature; from the slope
of linear regression line, the thermal expansion coefficient for the waveguide device is calculated as
CTE = 5.13×10–4 °C–1 at 30 °C.
Silicone Materials for Optical Applications
ABSTRACT
Optical loss
Light propagating in a planar waveguide is scattered from inhomogeneities on its surface and inside of the
material. The change in intensity of this scattered light as a function of position along the waveguide in
propagation direction is due to the combined losses the light suffers in the waveguide and can be evaluated to
determine a value for the loss number of the device. Using a prism coupler (Metricon model 2010), light with
λ = 633 or 1554 nm was coupled into the waveguide and the scattered light measured with a sensor about 0.5
mm from the waveguide surface. A typical plot of the intensity as a function of position is shown in Figure 5.
The propagation loss determined for this waveguide is 0.58 dB/cm. The results obtained from this method
compare favorably to those obtained in Table 2.
Silicones are among the most suitable materials for optical telecommunication devices
due to their tolerance to high optical flux and their thermo-mechanical and
environmental stability; they also have excellent processability. This work focuses on
utilizing silicon-based branched resins and linear polymers for optical waveguides and
switches where both refractive index and thermo-optic coefficient need to be controlled
to the requirements of specific applications. Materials were synthesized with high
optical transmission bands between 1.3 and 1.6 µm by varying the amount of aliphatic
and aromatic C-H in the material. At the same time, the ratio of methyl to phenyl groups
also controls the refractive index in the range of nD = 1.4 to 1.6 precisely enough that
both core and cladding components (∆n < 0.5%) can be obtained. Films of 5 to 20 µm
thickness prepared on silicon substrates by spin-coating from solution were evaluated
by measuring refractive index, thermo-optic coefficient, optical loss, and film
uniformity both before and after exposure to high temperature and humidity. These
films can be patterned through a number of techniques to form the required features.
The resinous materials show very low birefringence and excellent resistance to heat
and moisture.
Figure 5. Plot of light intensity scattered from
a planar waveguide as a function of
propagation distance. Waveguide material:
Siloxane-based polymer. Wavelength λ = 1554
nm, first order propagation mode. The raw
data shows several broad peaks which are
due to dust particles on the waveguide
surface. The Straight line represents an
exponential fit to the data (with the peaks
excluded); the fit line is described by the
equation y = 205•exp(-0.134x) where y is the
intensity and x the sensor position. From
these data, a loss of 0.58 dB/cm is calculated.
INTRODUCTION
Silicone polymers have a long history of successful use in the electronics and aerospace
industries. Many of their properties such as very low ionic impurities, low moisture
absorption, and a wide range of use temperature, make them excellent materials choices
for applications in these markets. These properties, in conjunction with their excellent
optical clarity, make silicones highly suitable for meeting the material requirements of
the emerging photonics industry, particularly in the access and fiber-to-the-home
markets. Through easily achieved chemical modifications of the polymer repeat unit, the
optical, mechanical, and thermal properties of the polymers can be varied over wide
ranges and tuned to meet the requirements of specific applications. Silicones are also
useful as host matrices for guest molecules that have non-linear optical properties or
show an electro-optic effect. In the following text, examples are discussed for
representative systems that have been investigated to demonstrate the versatility of
silicone polymers.
FUTURE WORK AND SUMMARY
Novel silicone materials have been synthesized that lend themselves to new, demanding applications of
photonic devices. In summary, silicone materials should be ideal candidates for use in devices such as arrayed
waveguide gratings (AWGs), variable optical attenuators (VOAs), modulators, UV laser diodes, and also for
LED encapsulation. Future work will focus on reducing loss via material synthesis as well as demonstrating
direct patterning of waveguides using UV functional groups on silicon; for these latter applications, sidewall
smoothness and good feature resolution will be key properties. Also, work will continue towards device
design, and correlation of device performance to materials properties.
SILICONE POLYMERS
LIMITED WARRANTY INFORMATION – PLEASE READ CAREFULLY
The information contained herein is offered in good faith and is believed to be accurate. However,
because conditions and methods of use of our products are beyond our control, this information
should not be used in substitution for customer’s tests to ensure that Dow Corning’s products are
safe, effective, and fully satisfactory for the intended end use. Suggestions of use shall not be taken
as inducements to infringe any patent.
Dow Corning’s sole warranty is that the product will meet the Dow Corning sales specifications in
effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund
of purchase price or replacement of any product shown to be other than as warranted.
DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED
WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY.
DOW CORNING DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL
DAMAGES.
A. Norrisa*, J. DeGroot, Jr.a,
F. Nishidaa, U. Pernisza,
N. Kushibikib, T. Ogawab
Dow Corning is a registered trademark of Dow Corning Corporation.
a
Dow Corning Corporation,
P.O. Box 994 Midland, MI 48686
We help you invent the future is a trademark of Dow Corning Corporation.
©2003 Dow Corning Corporation. All rights reserved.
Printed in USA
FPH35958
Dow Corning Toray Silicone,
Chiba, Japan
b
Form No.75-1007-01
One key advantage of silicone polymers is that many of the required properties (optical
and thermo-mechanical) are adjustable through controlled synthesis, and these polymers
can provide cost-effective material and processing options, particularly where low
temperature processing is required. Silicone polymers allow high flexibility for optical
integration by providing a wide range of refractive indices. Typical linear polymers have
a very low glass transition temperature (Tg < –70 °C), a high coefficient of thermal
expansion (CTE), and a large negative dn/dT; therefore, they are excellent candidate
materials for thermo-optical switching devices. The resinous materials show very low
birefringence and excellent resistance to heat and moisture. Both linear and resinous
silicones are expected to possess the requisite reliability in photonic device applications.
Chemical nature of silicon materials
Attributes for optical applications
Silicones can be considered a “molecular hybrid” between glass and organic linear polymers.
As shown in Figure 1, if there are no R groups, only oxygen, the structure is inorganic glass
(called a Q-type Si). If one oxygen is substituted with an R group (i.e. methyl, ethyl, phenyl,
etc.) a resin or silsequioxane (T-type Si) material is formed. These silsequioxanes are more
flexible than the Q-type materials. Finally, if two oxygen atoms are replaced by organic
groups a very flexible linear polymer (D-type Si) is obtained. The last structure shown
(M-type Si) has three oxygen atoms replaced by R groups, resulting in an end cap structure.
Because the backbone chain flexibility is increasing as R groups are added, the modulus
of the materials and their coefficients of thermal expansion (CTE) also change. This has
direct implications for the thermo-optic coefficient, dn/dT, which will be discussed in a
subsequent section.
Silicones are excellent candidates for short-haul telecom and data communication
applications because of their high optical transmission in all relevant wavelengths regions.
They have low absorption in the UV-visible/NIR range from 300 to 1300 nm which includes
the data communications band at 850 nm, and also from 1300 to 1600 nm, although in this
range the actual behavior depends on the particular chemical composition of the polymer;
see Table 1. Similarly, chemical substituents on the Si atom change the refractive index of
the material between 1.4 and 1.6 in the same wavelength range. This provides for control of
the refractive index difference between core and cladding.
Figure 1. Effect of R group substitution on Si in siloxane-based polymeric materials.
0 (Q)
1 (T)
2 (D)
3 (M)
O
R
R
R
O Si O
O Si O
O Si O
O Si R
O
O
R
R
R1SiO3/2
R2SiO2/2
SiO4/2
Silica
Glass
Resins
Silsesquioxanes
The thermo-optic coefficient, dn/dT, is affected by both, the ratio of organic groups to
oxygen linkages, i.e., T vs D, as defined in Figure 1, and the cross-link density which is
controlled by the cure chemistry. This property is correlated with the coefficient of thermal
expansion, CTE, of the material in a device application. Another important feature of
siloxanes is their low birefringence, usually below 2×10–4, which is due to their low glass
transition temperature and high compliance (low modulus).
The stability of siloxane–based materials to high optical flux has been demonstrated in an
endurance experiment using a 1310 nm laser with 200 mW optical power output coupled
into a single mode fiber (6 µm core diameter) where the radiation propagated through a
siloxane gel across a 5 µm gap.
R3SiO1/2
Linear polymers
Figure 3. Map of refractive index (vertical axis) vs. thermo-optic coefficient dn/dT for a range
of di-, tri-, and tetrafunctional siloxanes and resins. Also shown are the values (across broken
axes) of crystalline silicon and of fused quartz (SiO2)
One advantage of siloxane chemistry is the wide variety of cure systems available including
condensation or an addition types of curing. The most common cure system for materials used
in electronics and photonics applications is the addition cure of SiH to Si-Vinyl. This reaction
is typically catalyzed by Pt and can be accelerated with heat. This type of cure system can be
formulated as a one–part or two–part product and it is a neutral cure system that releases no
cure by-products. A typical example of this type is shown in Figure 2. Another cure system
that is advantageous for photonic applications is an ultraviolet (UV) light cure. Silicone
materials, both linear polymers and resins, can be synthesized with various UV–curable
functional groups attached; these polymers cure upon exposure to light of appropriate
wavelength. A key advantage of this approach is the ability to pattern waveguides directly
using lithographic techniques and equipment.
Polymer
+
Crosslinker
R H H R
-Si - C - C - Si R H H R
Catalyst
Thermo-optical properties
The change in refractive index with temperature is described by the thermo-optic (T-O)
coefficient, dn/dT. Polymers have a higher T-O coefficient than glass (see Figure 3), so
applications requiring this attribute are particularly suited to polymeric materials. However,
it has been observed that the bulk dn/dT may not always represent the thin film dn/dT value
because the latter depends on film thickness. When the film is constrained in the x-y plane on
a substrate, the expansion in the z-direction will be greater than the linear bulk value (which
is 1/3 of the volumetric CTE). This phenomenon results in a dn/dT that is smaller than in an
unconstrained material. Table 1 compares the dn/dT values of bulk materials and thin films;
also listed are the linear CTE values of the corresponding bulk materials obtained by thermal
mechanical analysis (TMA) measurements between room temperature and 150 °C.
Table 1. CTE vs. dn/dT for materials with different substituents on the
siloxane polymer backbone.
Linear CTE,
ppm/°C
bulk
dn/dT, °C-1
thin film
Dimethylsiloxane
325
-5x10-4
-3.6x10-4
Dimethyl-methylphenylsiloxane
265
-4.4x10-4
-3.3x10-4
Methyl-phenylsiloxane
250
-3.8x10-4
-3.0x10-4
Phenyl-T resin
(below & above Tg)
179
221
In progress
-2.1x10-4
-2.6x10-4
Polymer waveguides
Silicone polymers have many attributes that make them materials of choice for polymer
waveguides. Spin coating can be done using available equipment and techniques. They can
be patterned using either reactive ion etch (RIE) or direct photopattern (if UV curable). They
can also be synthesized to have a wide range of refractive indices and can be made low loss.
Because molecular composition can be used to maintain tight control over refractive index,
the required ∆n between core and cladding can be easily obtained for waveguiding
applications. The loss values shown in Table 2 were obtained on various neat polymers from
the NIR absorption spectrum measured in a cuvette with 1 cm path length. In all cases the
loss values observed at 1310 nm were less than those observed at 1550 nm.
Figure 2. Typical siloxane polymer cure system leaving no by-products. The chains protruding
from the silicon atoms signify siloxane oligomers.
R
H - Si
R
APPLICATIONS AND KEY PROPERTIES
Sample
Cure chemistry
R H
-Si - C = CH2
R
cycling, and high temperature storage exposure showed little or no degradation of optical
characteristics (refractive index measured at 633 and 1554 nm); after 2000 h, less than
0.2% / 0.2% change in refractive index was detected.
Reliability of silicones
Silicones can be synthesized to very high purity, with very low ionic content. If needed,
silicones can be made with very low content of volatiles. Silicones are typically very
permeable to gas and liquid vapors; however, they are also very hydrophobic. These two
attributes result in silicones having very low moisture pick-up in an environmental exposure
under 85 °C / 85% R.H. (also referred to as 85/85) test conditions. Silicone elastomers
(formulated with fused silica) have been shown to pick up less than 0.2 weight percent of
water when exposed to 85/85 conditions or by complete submersion in water. This is
significantly less than found with typical organic polymers. Because of this property, device
packages that use silicone encapsulant have been shown to pass JEDEC level 1 handling.
Reliability tests of different silicone elastomers and resins under 85/85 conditions, thermal
Table 2. Propagation loss in optical waveguide materials calculated from vibrational IR
absorption data for various siloxane-based polymers and resins with different chemical
substituents.
Siloxane Type
Loss at 1550 nm [dB/cm]
Dimethyl
0.67
Dimethyl/Methylphenyl copolymer
0.66
Methylphenyl
0.62
Fluorosilicone - 1
0.54
Fluorosilicone - 2
0.35
Phenyl resin - 1
0.49
Phenyl resin - 2
0.39
Chemical nature of silicon materials
Attributes for optical applications
Silicones can be considered a “molecular hybrid” between glass and organic linear polymers.
As shown in Figure 1, if there are no R groups, only oxygen, the structure is inorganic glass
(called a Q-type Si). If one oxygen is substituted with an R group (i.e. methyl, ethyl, phenyl,
etc.) a resin or silsequioxane (T-type Si) material is formed. These silsequioxanes are more
flexible than the Q-type materials. Finally, if two oxygen atoms are replaced by organic
groups a very flexible linear polymer (D-type Si) is obtained. The last structure shown
(M-type Si) has three oxygen atoms replaced by R groups, resulting in an end cap structure.
Because the backbone chain flexibility is increasing as R groups are added, the modulus
of the materials and their coefficients of thermal expansion (CTE) also change. This has
direct implications for the thermo-optic coefficient, dn/dT, which will be discussed in a
subsequent section.
Silicones are excellent candidates for short-haul telecom and data communication
applications because of their high optical transmission in all relevant wavelengths regions.
They have low absorption in the UV-visible/NIR range from 300 to 1300 nm which includes
the data communications band at 850 nm, and also from 1300 to 1600 nm, although in this
range the actual behavior depends on the particular chemical composition of the polymer;
see Table 1. Similarly, chemical substituents on the Si atom change the refractive index of
the material between 1.4 and 1.6 in the same wavelength range. This provides for control of
the refractive index difference between core and cladding.
Figure 1. Effect of R group substitution on Si in siloxane-based polymeric materials.
0 (Q)
1 (T)
2 (D)
3 (M)
O
R
R
R
O Si O
O Si O
O Si O
O Si R
O
O
R
R
R1SiO3/2
R2SiO2/2
SiO4/2
Silica
Glass
Resins
Silsesquioxanes
The thermo-optic coefficient, dn/dT, is affected by both, the ratio of organic groups to
oxygen linkages, i.e., T vs D, as defined in Figure 1, and the cross-link density which is
controlled by the cure chemistry. This property is correlated with the coefficient of thermal
expansion, CTE, of the material in a device application. Another important feature of
siloxanes is their low birefringence, usually below 2×10–4, which is due to their low glass
transition temperature and high compliance (low modulus).
The stability of siloxane–based materials to high optical flux has been demonstrated in an
endurance experiment using a 1310 nm laser with 200 mW optical power output coupled
into a single mode fiber (6 µm core diameter) where the radiation propagated through a
siloxane gel across a 5 µm gap.
R3SiO1/2
Linear polymers
Figure 3. Map of refractive index (vertical axis) vs. thermo-optic coefficient dn/dT for a range
of di-, tri-, and tetrafunctional siloxanes and resins. Also shown are the values (across broken
axes) of crystalline silicon and of fused quartz (SiO2)
One advantage of siloxane chemistry is the wide variety of cure systems available including
condensation or an addition types of curing. The most common cure system for materials used
in electronics and photonics applications is the addition cure of SiH to Si-Vinyl. This reaction
is typically catalyzed by Pt and can be accelerated with heat. This type of cure system can be
formulated as a one–part or two–part product and it is a neutral cure system that releases no
cure by-products. A typical example of this type is shown in Figure 2. Another cure system
that is advantageous for photonic applications is an ultraviolet (UV) light cure. Silicone
materials, both linear polymers and resins, can be synthesized with various UV–curable
functional groups attached; these polymers cure upon exposure to light of appropriate
wavelength. A key advantage of this approach is the ability to pattern waveguides directly
using lithographic techniques and equipment.
Polymer
+
Crosslinker
R H H R
-Si - C - C - Si R H H R
Catalyst
Thermo-optical properties
The change in refractive index with temperature is described by the thermo-optic (T-O)
coefficient, dn/dT. Polymers have a higher T-O coefficient than glass (see Figure 3), so
applications requiring this attribute are particularly suited to polymeric materials. However,
it has been observed that the bulk dn/dT may not always represent the thin film dn/dT value
because the latter depends on film thickness. When the film is constrained in the x-y plane on
a substrate, the expansion in the z-direction will be greater than the linear bulk value (which
is 1/3 of the volumetric CTE). This phenomenon results in a dn/dT that is smaller than in an
unconstrained material. Table 1 compares the dn/dT values of bulk materials and thin films;
also listed are the linear CTE values of the corresponding bulk materials obtained by thermal
mechanical analysis (TMA) measurements between room temperature and 150 °C.
Table 1. CTE vs. dn/dT for materials with different substituents on the
siloxane polymer backbone.
Linear CTE,
ppm/°C
bulk
dn/dT, °C-1
thin film
Dimethylsiloxane
325
-5x10-4
-3.6x10-4
Dimethyl-methylphenylsiloxane
265
-4.4x10-4
-3.3x10-4
Methyl-phenylsiloxane
250
-3.8x10-4
-3.0x10-4
Phenyl-T resin
(below & above Tg)
179
221
In progress
-2.1x10-4
-2.6x10-4
Polymer waveguides
Silicone polymers have many attributes that make them materials of choice for polymer
waveguides. Spin coating can be done using available equipment and techniques. They can
be patterned using either reactive ion etch (RIE) or direct photopattern (if UV curable). They
can also be synthesized to have a wide range of refractive indices and can be made low loss.
Because molecular composition can be used to maintain tight control over refractive index,
the required ∆n between core and cladding can be easily obtained for waveguiding
applications. The loss values shown in Table 2 were obtained on various neat polymers from
the NIR absorption spectrum measured in a cuvette with 1 cm path length. In all cases the
loss values observed at 1310 nm were less than those observed at 1550 nm.
Figure 2. Typical siloxane polymer cure system leaving no by-products. The chains protruding
from the silicon atoms signify siloxane oligomers.
R
H - Si
R
APPLICATIONS AND KEY PROPERTIES
Sample
Cure chemistry
R H
-Si - C = CH2
R
cycling, and high temperature storage exposure showed little or no degradation of optical
characteristics (refractive index measured at 633 and 1554 nm); after 2000 h, less than
0.2% / 0.2% change in refractive index was detected.
Reliability of silicones
Silicones can be synthesized to very high purity, with very low ionic content. If needed,
silicones can be made with very low content of volatiles. Silicones are typically very
permeable to gas and liquid vapors; however, they are also very hydrophobic. These two
attributes result in silicones having very low moisture pick-up in an environmental exposure
under 85 °C / 85% R.H. (also referred to as 85/85) test conditions. Silicone elastomers
(formulated with fused silica) have been shown to pick up less than 0.2 weight percent of
water when exposed to 85/85 conditions or by complete submersion in water. This is
significantly less than found with typical organic polymers. Because of this property, device
packages that use silicone encapsulant have been shown to pass JEDEC level 1 handling.
Reliability tests of different silicone elastomers and resins under 85/85 conditions, thermal
Table 2. Propagation loss in optical waveguide materials calculated from vibrational IR
absorption data for various siloxane-based polymers and resins with different chemical
substituents.
Siloxane Type
Loss at 1550 nm [dB/cm]
Dimethyl
0.67
Dimethyl/Methylphenyl copolymer
0.66
Methylphenyl
0.62
Fluorosilicone - 1
0.54
Fluorosilicone - 2
0.35
Phenyl resin - 1
0.49
Phenyl resin - 2
0.39
Chemical nature of silicon materials
Attributes for optical applications
Silicones can be considered a “molecular hybrid” between glass and organic linear polymers.
As shown in Figure 1, if there are no R groups, only oxygen, the structure is inorganic glass
(called a Q-type Si). If one oxygen is substituted with an R group (i.e. methyl, ethyl, phenyl,
etc.) a resin or silsequioxane (T-type Si) material is formed. These silsequioxanes are more
flexible than the Q-type materials. Finally, if two oxygen atoms are replaced by organic
groups a very flexible linear polymer (D-type Si) is obtained. The last structure shown
(M-type Si) has three oxygen atoms replaced by R groups, resulting in an end cap structure.
Because the backbone chain flexibility is increasing as R groups are added, the modulus
of the materials and their coefficients of thermal expansion (CTE) also change. This has
direct implications for the thermo-optic coefficient, dn/dT, which will be discussed in a
subsequent section.
Silicones are excellent candidates for short-haul telecom and data communication
applications because of their high optical transmission in all relevant wavelengths regions.
They have low absorption in the UV-visible/NIR range from 300 to 1300 nm which includes
the data communications band at 850 nm, and also from 1300 to 1600 nm, although in this
range the actual behavior depends on the particular chemical composition of the polymer;
see Table 1. Similarly, chemical substituents on the Si atom change the refractive index of
the material between 1.4 and 1.6 in the same wavelength range. This provides for control of
the refractive index difference between core and cladding.
Figure 1. Effect of R group substitution on Si in siloxane-based polymeric materials.
0 (Q)
1 (T)
2 (D)
3 (M)
O
R
R
R
O Si O
O Si O
O Si O
O Si R
O
O
R
R
R1SiO3/2
R2SiO2/2
SiO4/2
Silica
Glass
Resins
Silsesquioxanes
The thermo-optic coefficient, dn/dT, is affected by both, the ratio of organic groups to
oxygen linkages, i.e., T vs D, as defined in Figure 1, and the cross-link density which is
controlled by the cure chemistry. This property is correlated with the coefficient of thermal
expansion, CTE, of the material in a device application. Another important feature of
siloxanes is their low birefringence, usually below 2×10–4, which is due to their low glass
transition temperature and high compliance (low modulus).
The stability of siloxane–based materials to high optical flux has been demonstrated in an
endurance experiment using a 1310 nm laser with 200 mW optical power output coupled
into a single mode fiber (6 µm core diameter) where the radiation propagated through a
siloxane gel across a 5 µm gap.
R3SiO1/2
Linear polymers
Figure 3. Map of refractive index (vertical axis) vs. thermo-optic coefficient dn/dT for a range
of di-, tri-, and tetrafunctional siloxanes and resins. Also shown are the values (across broken
axes) of crystalline silicon and of fused quartz (SiO2)
One advantage of siloxane chemistry is the wide variety of cure systems available including
condensation or an addition types of curing. The most common cure system for materials used
in electronics and photonics applications is the addition cure of SiH to Si-Vinyl. This reaction
is typically catalyzed by Pt and can be accelerated with heat. This type of cure system can be
formulated as a one–part or two–part product and it is a neutral cure system that releases no
cure by-products. A typical example of this type is shown in Figure 2. Another cure system
that is advantageous for photonic applications is an ultraviolet (UV) light cure. Silicone
materials, both linear polymers and resins, can be synthesized with various UV–curable
functional groups attached; these polymers cure upon exposure to light of appropriate
wavelength. A key advantage of this approach is the ability to pattern waveguides directly
using lithographic techniques and equipment.
Polymer
+
Crosslinker
R H H R
-Si - C - C - Si R H H R
Catalyst
Thermo-optical properties
The change in refractive index with temperature is described by the thermo-optic (T-O)
coefficient, dn/dT. Polymers have a higher T-O coefficient than glass (see Figure 3), so
applications requiring this attribute are particularly suited to polymeric materials. However,
it has been observed that the bulk dn/dT may not always represent the thin film dn/dT value
because the latter depends on film thickness. When the film is constrained in the x-y plane on
a substrate, the expansion in the z-direction will be greater than the linear bulk value (which
is 1/3 of the volumetric CTE). This phenomenon results in a dn/dT that is smaller than in an
unconstrained material. Table 1 compares the dn/dT values of bulk materials and thin films;
also listed are the linear CTE values of the corresponding bulk materials obtained by thermal
mechanical analysis (TMA) measurements between room temperature and 150 °C.
Table 1. CTE vs. dn/dT for materials with different substituents on the
siloxane polymer backbone.
Linear CTE,
ppm/°C
bulk
dn/dT, °C-1
thin film
Dimethylsiloxane
325
-5x10-4
-3.6x10-4
Dimethyl-methylphenylsiloxane
265
-4.4x10-4
-3.3x10-4
Methyl-phenylsiloxane
250
-3.8x10-4
-3.0x10-4
Phenyl-T resin
(below & above Tg)
179
221
In progress
-2.1x10-4
-2.6x10-4
Polymer waveguides
Silicone polymers have many attributes that make them materials of choice for polymer
waveguides. Spin coating can be done using available equipment and techniques. They can
be patterned using either reactive ion etch (RIE) or direct photopattern (if UV curable). They
can also be synthesized to have a wide range of refractive indices and can be made low loss.
Because molecular composition can be used to maintain tight control over refractive index,
the required ∆n between core and cladding can be easily obtained for waveguiding
applications. The loss values shown in Table 2 were obtained on various neat polymers from
the NIR absorption spectrum measured in a cuvette with 1 cm path length. In all cases the
loss values observed at 1310 nm were less than those observed at 1550 nm.
Figure 2. Typical siloxane polymer cure system leaving no by-products. The chains protruding
from the silicon atoms signify siloxane oligomers.
R
H - Si
R
APPLICATIONS AND KEY PROPERTIES
Sample
Cure chemistry
R H
-Si - C = CH2
R
cycling, and high temperature storage exposure showed little or no degradation of optical
characteristics (refractive index measured at 633 and 1554 nm); after 2000 h, less than
0.2% / 0.2% change in refractive index was detected.
Reliability of silicones
Silicones can be synthesized to very high purity, with very low ionic content. If needed,
silicones can be made with very low content of volatiles. Silicones are typically very
permeable to gas and liquid vapors; however, they are also very hydrophobic. These two
attributes result in silicones having very low moisture pick-up in an environmental exposure
under 85 °C / 85% R.H. (also referred to as 85/85) test conditions. Silicone elastomers
(formulated with fused silica) have been shown to pick up less than 0.2 weight percent of
water when exposed to 85/85 conditions or by complete submersion in water. This is
significantly less than found with typical organic polymers. Because of this property, device
packages that use silicone encapsulant have been shown to pass JEDEC level 1 handling.
Reliability tests of different silicone elastomers and resins under 85/85 conditions, thermal
Table 2. Propagation loss in optical waveguide materials calculated from vibrational IR
absorption data for various siloxane-based polymers and resins with different chemical
substituents.
Siloxane Type
Loss at 1550 nm [dB/cm]
Dimethyl
0.67
Dimethyl/Methylphenyl copolymer
0.66
Methylphenyl
0.62
Fluorosilicone - 1
0.54
Fluorosilicone - 2
0.35
Phenyl resin - 1
0.49
Phenyl resin - 2
0.39
EXPERIMENTAL DETAILS
Absorption spectrum
An Optical Solutions PS-2 fiber optics spectrometer and a Shimadzu model UV 3100 PC
spectrophotometer were used to gather data on vibrational absorptions. These data were
used to determine absorption loss as a function of wavelength. The data were collected by
placing the material in a 1–cm cuvette and scanning the wavelengths covering both the
UV-visible and Near-IR regions. In all cases a cuvette filled with carbon tetrachloride was
used as the reference sample. The raw absorption data at 1550 nm are expressed as loss in
dB/cm and are summarized in Table 2.
Refractive index, thermo-optic effect, and thermo-mechanical properties
Polymers films were spun on silicon wafers with film thickness ranging from 2 to 15 µm.
The refractive index of the materials was determined on these samples with a prism
coupler (Metricon model 2010) at two wavelengths, 633 and 1554 nm, measured at a
temperature of T =24 °C. The thermo-optic coefficient of the slab waveguide, dn/dT, was
determined from the temperature dependence of the refractive index of the thin film on the
silicon wafer substrate by spectral reflectance interferometry. A Filmetrics F-20 instrument
was used in conjunction with a temperature–controlled, enclosed sample stage, allowing
measurements in the temperature range from ~15 °C (limited by the dew point of the
ambient) and ~95 °C (limited by the Peltier element in the stage). Sample temperature was
measured on a dummy wafer with a thermocouple attached to its surface and symmetrically
mounted with respect to the sample under test on the sample stage. Temperature deviation
between the two positions was less than 1 °C over the temperature range of the
measurement. Spectral reflection intensity data was gathered over a wavelength range of
~700 to 950 nm using a single layer model with a dispersion described by a simple Cauchy
relation. From a linear least–squares regression to the data, i.e., refractive index vs.
temperature, the thermo-optic coefficient was obtained as the slope dn/dT. The data was
calibrated by comparison of the refractive index value at 633 nm to the value obtained with
the prism coupler at room temperature. Results for some siloxane–based materials are
given in Table 1.
Figure 4. Thickness change as a function of temperature for siloxane-based polymer
waveguide prepared by spin-coating on a silicon wafer. The least-squares regression line has
a slope of 3.36x10-3 °C-1 (r = 0.9992). From the slope, a value for the linear thermal expansion
coefficient of the device of CTE = 513 ppm/°C at 30°C is calculated.
Since the spectral reflection interferometer (Filmetrics F-20 instrument) provides not only refractive index
but at the same time also thickness data for the film under test, the linear expansion coefficient of the polymer
material in the particular device configuration, namely as a laterally highly constrained film, can also be
determined by this method. Figure 4 shows a plot of the thickness data vs. temperature; from the slope
of linear regression line, the thermal expansion coefficient for the waveguide device is calculated as
CTE = 5.13×10–4 °C–1 at 30 °C.
Silicone Materials for Optical Applications
ABSTRACT
Optical loss
Light propagating in a planar waveguide is scattered from inhomogeneities on its surface and inside of the
material. The change in intensity of this scattered light as a function of position along the waveguide in
propagation direction is due to the combined losses the light suffers in the waveguide and can be evaluated to
determine a value for the loss number of the device. Using a prism coupler (Metricon model 2010), light with
λ = 633 or 1554 nm was coupled into the waveguide and the scattered light measured with a sensor about 0.5
mm from the waveguide surface. A typical plot of the intensity as a function of position is shown in Figure 5.
The propagation loss determined for this waveguide is 0.58 dB/cm. The results obtained from this method
compare favorably to those obtained in Table 2.
Silicones are among the most suitable materials for optical telecommunication devices
due to their tolerance to high optical flux and their thermo-mechanical and
environmental stability; they also have excellent processability. This work focuses on
utilizing silicon-based branched resins and linear polymers for optical waveguides and
switches where both refractive index and thermo-optic coefficient need to be controlled
to the requirements of specific applications. Materials were synthesized with high
optical transmission bands between 1.3 and 1.6 µm by varying the amount of aliphatic
and aromatic C-H in the material. At the same time, the ratio of methyl to phenyl groups
also controls the refractive index in the range of nD = 1.4 to 1.6 precisely enough that
both core and cladding components (∆n < 0.5%) can be obtained. Films of 5 to 20 µm
thickness prepared on silicon substrates by spin-coating from solution were evaluated
by measuring refractive index, thermo-optic coefficient, optical loss, and film
uniformity both before and after exposure to high temperature and humidity. These
films can be patterned through a number of techniques to form the required features.
The resinous materials show very low birefringence and excellent resistance to heat
and moisture.
Figure 5. Plot of light intensity scattered from
a planar waveguide as a function of
propagation distance. Waveguide material:
Siloxane-based polymer. Wavelength λ = 1554
nm, first order propagation mode. The raw
data shows several broad peaks which are
due to dust particles on the waveguide
surface. The Straight line represents an
exponential fit to the data (with the peaks
excluded); the fit line is described by the
equation y = 205•exp(-0.134x) where y is the
intensity and x the sensor position. From
these data, a loss of 0.58 dB/cm is calculated.
INTRODUCTION
Silicone polymers have a long history of successful use in the electronics and aerospace
industries. Many of their properties such as very low ionic impurities, low moisture
absorption, and a wide range of use temperature, make them excellent materials choices
for applications in these markets. These properties, in conjunction with their excellent
optical clarity, make silicones highly suitable for meeting the material requirements of
the emerging photonics industry, particularly in the access and fiber-to-the-home
markets. Through easily achieved chemical modifications of the polymer repeat unit, the
optical, mechanical, and thermal properties of the polymers can be varied over wide
ranges and tuned to meet the requirements of specific applications. Silicones are also
useful as host matrices for guest molecules that have non-linear optical properties or
show an electro-optic effect. In the following text, examples are discussed for
representative systems that have been investigated to demonstrate the versatility of
silicone polymers.
FUTURE WORK AND SUMMARY
Novel silicone materials have been synthesized that lend themselves to new, demanding applications of
photonic devices. In summary, silicone materials should be ideal candidates for use in devices such as arrayed
waveguide gratings (AWGs), variable optical attenuators (VOAs), modulators, UV laser diodes, and also for
LED encapsulation. Future work will focus on reducing loss via material synthesis as well as demonstrating
direct patterning of waveguides using UV functional groups on silicon; for these latter applications, sidewall
smoothness and good feature resolution will be key properties. Also, work will continue towards device
design, and correlation of device performance to materials properties.
SILICONE POLYMERS
LIMITED WARRANTY INFORMATION – PLEASE READ CAREFULLY
The information contained herein is offered in good faith and is believed to be accurate. However,
because conditions and methods of use of our products are beyond our control, this information
should not be used in substitution for customer’s tests to ensure that Dow Corning’s products are
safe, effective, and fully satisfactory for the intended end use. Suggestions of use shall not be taken
as inducements to infringe any patent.
Dow Corning’s sole warranty is that the product will meet the Dow Corning sales specifications in
effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund
of purchase price or replacement of any product shown to be other than as warranted.
DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED
WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY.
DOW CORNING DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL
DAMAGES.
A. Norrisa*, J. DeGroot, Jr.a,
F. Nishidaa, U. Pernisza,
N. Kushibikib, T. Ogawab
Dow Corning is a registered trademark of Dow Corning Corporation.
a
Dow Corning Corporation,
P.O. Box 994 Midland, MI 48686
We help you invent the future is a trademark of Dow Corning Corporation.
©2003 Dow Corning Corporation. All rights reserved.
Printed in USA
FPH35958
Dow Corning Toray Silicone,
Chiba, Japan
b
Form No.75-1007-01
One key advantage of silicone polymers is that many of the required properties (optical
and thermo-mechanical) are adjustable through controlled synthesis, and these polymers
can provide cost-effective material and processing options, particularly where low
temperature processing is required. Silicone polymers allow high flexibility for optical
integration by providing a wide range of refractive indices. Typical linear polymers have
a very low glass transition temperature (Tg < –70 °C), a high coefficient of thermal
expansion (CTE), and a large negative dn/dT; therefore, they are excellent candidate
materials for thermo-optical switching devices. The resinous materials show very low
birefringence and excellent resistance to heat and moisture. Both linear and resinous
silicones are expected to possess the requisite reliability in photonic device applications.
EXPERIMENTAL DETAILS
Absorption spectrum
An Optical Solutions PS-2 fiber optics spectrometer and a Shimadzu model UV 3100 PC
spectrophotometer were used to gather data on vibrational absorptions. These data were
used to determine absorption loss as a function of wavelength. The data were collected by
placing the material in a 1–cm cuvette and scanning the wavelengths covering both the
UV-visible and Near-IR regions. In all cases a cuvette filled with carbon tetrachloride was
used as the reference sample. The raw absorption data at 1550 nm are expressed as loss in
dB/cm and are summarized in Table 2.
Refractive index, thermo-optic effect, and thermo-mechanical properties
Polymers films were spun on silicon wafers with film thickness ranging from 2 to 15 µm.
The refractive index of the materials was determined on these samples with a prism
coupler (Metricon model 2010) at two wavelengths, 633 and 1554 nm, measured at a
temperature of T =24 °C. The thermo-optic coefficient of the slab waveguide, dn/dT, was
determined from the temperature dependence of the refractive index of the thin film on the
silicon wafer substrate by spectral reflectance interferometry. A Filmetrics F-20 instrument
was used in conjunction with a temperature–controlled, enclosed sample stage, allowing
measurements in the temperature range from ~15 °C (limited by the dew point of the
ambient) and ~95 °C (limited by the Peltier element in the stage). Sample temperature was
measured on a dummy wafer with a thermocouple attached to its surface and symmetrically
mounted with respect to the sample under test on the sample stage. Temperature deviation
between the two positions was less than 1 °C over the temperature range of the
measurement. Spectral reflection intensity data was gathered over a wavelength range of
~700 to 950 nm using a single layer model with a dispersion described by a simple Cauchy
relation. From a linear least–squares regression to the data, i.e., refractive index vs.
temperature, the thermo-optic coefficient was obtained as the slope dn/dT. The data was
calibrated by comparison of the refractive index value at 633 nm to the value obtained with
the prism coupler at room temperature. Results for some siloxane–based materials are
given in Table 1.
Figure 4. Thickness change as a function of temperature for siloxane-based polymer
waveguide prepared by spin-coating on a silicon wafer. The least-squares regression line has
a slope of 3.36x10-3 °C-1 (r = 0.9992). From the slope, a value for the linear thermal expansion
coefficient of the device of CTE = 513 ppm/°C at 30°C is calculated.
Since the spectral reflection interferometer (Filmetrics F-20 instrument) provides not only refractive index
but at the same time also thickness data for the film under test, the linear expansion coefficient of the polymer
material in the particular device configuration, namely as a laterally highly constrained film, can also be
determined by this method. Figure 4 shows a plot of the thickness data vs. temperature; from the slope
of linear regression line, the thermal expansion coefficient for the waveguide device is calculated as
CTE = 5.13×10–4 °C–1 at 30 °C.
Silicone Materials for Optical Applications
ABSTRACT
Optical loss
Light propagating in a planar waveguide is scattered from inhomogeneities on its surface and inside of the
material. The change in intensity of this scattered light as a function of position along the waveguide in
propagation direction is due to the combined losses the light suffers in the waveguide and can be evaluated to
determine a value for the loss number of the device. Using a prism coupler (Metricon model 2010), light with
λ = 633 or 1554 nm was coupled into the waveguide and the scattered light measured with a sensor about 0.5
mm from the waveguide surface. A typical plot of the intensity as a function of position is shown in Figure 5.
The propagation loss determined for this waveguide is 0.58 dB/cm. The results obtained from this method
compare favorably to those obtained in Table 2.
Silicones are among the most suitable materials for optical telecommunication devices
due to their tolerance to high optical flux and their thermo-mechanical and
environmental stability; they also have excellent processability. This work focuses on
utilizing silicon-based branched resins and linear polymers for optical waveguides and
switches where both refractive index and thermo-optic coefficient need to be controlled
to the requirements of specific applications. Materials were synthesized with high
optical transmission bands between 1.3 and 1.6 µm by varying the amount of aliphatic
and aromatic C-H in the material. At the same time, the ratio of methyl to phenyl groups
also controls the refractive index in the range of nD = 1.4 to 1.6 precisely enough that
both core and cladding components (∆n < 0.5%) can be obtained. Films of 5 to 20 µm
thickness prepared on silicon substrates by spin-coating from solution were evaluated
by measuring refractive index, thermo-optic coefficient, optical loss, and film
uniformity both before and after exposure to high temperature and humidity. These
films can be patterned through a number of techniques to form the required features.
The resinous materials show very low birefringence and excellent resistance to heat
and moisture.
Figure 5. Plot of light intensity scattered from
a planar waveguide as a function of
propagation distance. Waveguide material:
Siloxane-based polymer. Wavelength λ = 1554
nm, first order propagation mode. The raw
data shows several broad peaks which are
due to dust particles on the waveguide
surface. The Straight line represents an
exponential fit to the data (with the peaks
excluded); the fit line is described by the
equation y = 205•exp(-0.134x) where y is the
intensity and x the sensor position. From
these data, a loss of 0.58 dB/cm is calculated.
INTRODUCTION
Silicone polymers have a long history of successful use in the electronics and aerospace
industries. Many of their properties such as very low ionic impurities, low moisture
absorption, and a wide range of use temperature, make them excellent materials choices
for applications in these markets. These properties, in conjunction with their excellent
optical clarity, make silicones highly suitable for meeting the material requirements of
the emerging photonics industry, particularly in the access and fiber-to-the-home
markets. Through easily achieved chemical modifications of the polymer repeat unit, the
optical, mechanical, and thermal properties of the polymers can be varied over wide
ranges and tuned to meet the requirements of specific applications. Silicones are also
useful as host matrices for guest molecules that have non-linear optical properties or
show an electro-optic effect. In the following text, examples are discussed for
representative systems that have been investigated to demonstrate the versatility of
silicone polymers.
FUTURE WORK AND SUMMARY
Novel silicone materials have been synthesized that lend themselves to new, demanding applications of
photonic devices. In summary, silicone materials should be ideal candidates for use in devices such as arrayed
waveguide gratings (AWGs), variable optical attenuators (VOAs), modulators, UV laser diodes, and also for
LED encapsulation. Future work will focus on reducing loss via material synthesis as well as demonstrating
direct patterning of waveguides using UV functional groups on silicon; for these latter applications, sidewall
smoothness and good feature resolution will be key properties. Also, work will continue towards device
design, and correlation of device performance to materials properties.
SILICONE POLYMERS
LIMITED WARRANTY INFORMATION – PLEASE READ CAREFULLY
The information contained herein is offered in good faith and is believed to be accurate. However,
because conditions and methods of use of our products are beyond our control, this information
should not be used in substitution for customer’s tests to ensure that Dow Corning’s products are
safe, effective, and fully satisfactory for the intended end use. Suggestions of use shall not be taken
as inducements to infringe any patent.
Dow Corning’s sole warranty is that the product will meet the Dow Corning sales specifications in
effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund
of purchase price or replacement of any product shown to be other than as warranted.
DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED
WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY.
DOW CORNING DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL
DAMAGES.
A. Norrisa*, J. DeGroot, Jr.a,
F. Nishidaa, U. Pernisza,
N. Kushibikib, T. Ogawab
Dow Corning is a registered trademark of Dow Corning Corporation.
a
Dow Corning Corporation,
P.O. Box 994 Midland, MI 48686
We help you invent the future is a trademark of Dow Corning Corporation.
©2003 Dow Corning Corporation. All rights reserved.
Printed in USA
FPH35958
Dow Corning Toray Silicone,
Chiba, Japan
b
Form No.75-1007-01
One key advantage of silicone polymers is that many of the required properties (optical
and thermo-mechanical) are adjustable through controlled synthesis, and these polymers
can provide cost-effective material and processing options, particularly where low
temperature processing is required. Silicone polymers allow high flexibility for optical
integration by providing a wide range of refractive indices. Typical linear polymers have
a very low glass transition temperature (Tg < –70 °C), a high coefficient of thermal
expansion (CTE), and a large negative dn/dT; therefore, they are excellent candidate
materials for thermo-optical switching devices. The resinous materials show very low
birefringence and excellent resistance to heat and moisture. Both linear and resinous
silicones are expected to possess the requisite reliability in photonic device applications.