Dow Corning® WL-5150

Dow Corning Proprietary
Revised April, 2004
Dow Corning ® WL-5150
Photodefinable Spin-On
Silicone
Properties and Processing Procedures
Introduction
Dow Corning ® WL-5150 is a silicone
formulation which can be photopatterned
and cured using standard microelectronics
processing techniques. The silicone material
is prepared in mesitylene with a solution
viscosity of 450 cP. The standard method for
application of Dow Corning ® WL-5150 is
by spin coating although other methods of
application (such as spray coating and
extrusion coating) may also be applicable.
Application, patterning and cure of Dow
Corning ® WL-5150 is accomplished in a
simple six-step process that utilizes
commercially available equipment. Updated
information on the processing of the Dow
Corning ® WL-5150 materials is available
from your local Dow Corning
representative.
Material Properties
Dow Corning ® WL-5150 can be solvent cast
to form free standing monoliths or spincoated
onto substrates to form sub 50 µm
thick films. The mechanical properties of
Dow Corning ® WL-5150 thin film (20 µm)
and bulk samples (~ 2 mm thickness) are
listed in Table 1 for samples cured at 250°C
for 30 minutes.
The room temperature modulus of Dow
Corning ® WL-5150 has been measured by a
continuous stiffness (CSM) nanoindentation
technique on spin-coated thin films. Figure 1
shows a typical modulus vs. displacement
curve for a 20 µm thick film of Dow
Corning ® WL-5150 cured at 250°C for 30
minutes. The plot shows the change in the
modulus as the indenter tip penetrates into
the film surface up to a maximum
penetration depth of 2200 nm.
Modulus (GPa)
1
Mechanical Property Value
Nanoindentation Modulus (MPa) 301
Nanoindentation Hardness
(MPa)
9.5
Film Residual Stress on Si
Wafer (MPa)
2.6
Bulk Modulus (MPa at 25°C) 160
Bulk Tensile Strength (MPa) 6.0
Bulk Elongation (%) 37.6
CTE (ppm/°C) 236
Table 1. Mechanical properties of Dow
Corning ® WL-5150.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 500 1000 1500 2000 2500
Displacement Into Surface (nm)
Figure 1. Modulus vs. displacement curve
for Dow Corning ® WL-5150 (frequency =
75 Hz).
Dow Corning ® WL-5150 provides a low
modulus, low stress film after cure with a
film surface modulus of approximately 300
MPa. The increase in modulus with indenter
displacement into the film is due an
increasing substrate effect with penetration.
The dimensional change of Dow Corning ®
WL-5150 as a function of temperature is
shown in Figure 2. A linear expansion is
observed throughout the temperature region
of –100 to 300°C.
The thermal stability characteristics of Dow
Corning ® WL-5150 cured at 250°C for 30
minutes are listed in Table 2. Good thermal
stability and modulus retention have been
demonstrated to a temperature of 300°C.

Dow Corning Proprietary
Revised April, 2004
Dimension Change (µm/m)
1e+5
8e+4
6e+4
4e+4
2e+4
0
-100 -50 0 50 100 150 200 250 300
Figure 2. Dimension change vs. temperature
for Dow Corning ® WL-5150.
Condition Weight Loss (%)
Isothermal at
1.9
250°C/60 minutes
Isothermal at
3.5
300°C/60 minutes
Dynamic to 250°C
1.0
(10°C/minute)
Dynamic to 300°C
1.7
(10°C/minute)
Table 2. Thermal stability characteristics for
Dow Corning ® WL-5150.
The weight loss profile for Dow Corning ®
WL-5150 is shown in Figure 3 for a sample
cured at 250°C for 30 minutes.
Weight Percent Remaining
102
100
98
96
94
92
90
88
86
84
82
80
0
0 20 40 60 80 100 120
Time (minutes)
600
500
400
300
200
100
Figure 3. Thermal weight loss for a
250°C/30 minute cured Dow Corning ® WL-
Temperature (°C) (°C)
Percent Transmittance
Cured films of Dow Corning ® WL-5150
show excellent optical transparency down to
a wavelength of 220 nm. The UV-visible
spectrum for a 24 µm thick 250°C cured
film on a quartz substrate is shown in Figure
4.
The electrical properties and moisture
absorptivity for Dow Corning ® WL-5150
are listed in Table 3.
100
90
80
70
60
50
40
30
20
10
0
100 200 300 400 500 600 700 800 900
Wavelength (nm)
Figure 4. UV-visible spectrum for a cured
film of Dow Corning ® WL-5150.
Electrical Property Value
Dielectric Constant (1 MHz) 3.2
Dissipation Factor (1 MHz) 0.007
Leakage Current (A/cm 2 ) 1 x 10 -11
Breakdown Voltage (V/cm) 3.9 x 10 5
PCT Moisture Absorption (%) 0.24
Table 3. Electrical properties and moisture
absorptivity for Dow Corning ® WL-5150
measured on monolithic specimens.
Material Shipping and Storage
Dow Corning ® WL-5150 is shipped cold
with blue ice at a temperature of
approximately 5°C. If the solution arrives at
its destination warm please contact your
local Dow Corning representative. Upon
receipt the solution should be immediately
stored at a temperature of –15°C. Prior to
use the solution should be allowed to
equilibrate to room temperature.
5150 sample analyzed under helium. During use Dow Corning ® WL-5150 should
not be allowed to remain at room
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Dow Corning Proprietary
Revised April, 2004
temperature for more than 3-4 days to
maintain good performance.
Material Waste Handling
In disposing of Dow Corning ® WL-5150 or
rinse and developing solvents containing
Dow Corning ® WL-5150 it is important that
these do not come into contact with basic
materials (including ammonia, alkyl amines,
ammonium hydroxides, alkyl ammonium
hydroxides, metal hydroxides) as well as
oxidizing metals and oxidizing agents.
These in combination with water can result
in the evolution of hydrogen gas from the
composition. Consult the MSDS provided
for additional information on materials
waste handling.
Processing
A step-by-step summary of processing for
Dow Corning ® WL-5150 is presented
schematically in Figure 5 and in Table 4.
1
Spin Coat
2
Soft Bake
3
UV Exposure
4
Post Exposure Bake
5
Solvent Develop
6
Hard Bake (Cure)
Figure 5. Application, patterning and cure
process steps for Dow Corning ® WL-5150.
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The process consists of six general process
steps to arrive at a cured patterned film. The
target thickness is approximately 22 µm.
Process Step Standard Conditions
1. Spin Coat
a) Spread
b) Spin
c) Edge Bead
500 rpm/10 sec.
500-2500 rpm/30 sec.
1500 rpm/30 sec.
2. Soft-bake Hot Plate
110°C/120 seconds
3. UV Exposure i-line or Broadband
1000 mJ/cm 2
4. Post-exposure
Hot Plate
Bake
150°C/120 sec.
5. Development Stoddard Solvent
Automated
Development Process
(see Step 5 comments)
6. Hard Bake Oven (Air, N2 or
vacuum)
150°C/120 min. or
180°C/60 min. or
250°C/30 min.
Table 4. Processing conditions for Dow
Corning ® WL-5150.
Step 1. Coating Process
Surfaces for coating Dow Corning ® WL-
5150 should be free of surface impurities
prior to coating. It is recommended that the
spin-coaters used to deposit the solutions be
equipped with a backside rinse capability
and bowl exhaust. The coater should also be
equipped with two dispense heads, one for
dispensing Dow Corning ® WL-5150 and a
second for dispensing a solvent for edge
bead removal.
Depending on the size of the substrate
approximately 8-10 mL of Dow Corning ®
WL-5150 is applied to a wafer in either
static or dynamic dispense modes. The
wafer speed is then set to 500 rpm for 10
seconds to allow the solution to spread
across the wafer. The speed is then increased
to achieve a uniform film coating across the
wafer. A speed of 1500 rpm is
recommended to achieve a film thickness of

Dow Corning Proprietary
Revised April, 2004
approximately 22 µm after full processing
(completion of steps 1-6). Expansion of the
spin speed range between 500 and 3000 rpm
allows for final fully processed film
thicknesses ranging from 40 to 15 µm
respectively. During the spin step it is
recommended that a backside rinse be
implemented to eliminate polymer string
(cotton candy) formation at the wafer edge.
Edge bead removal is conducted by spinning
the wafer at speeds not faster than the spinon
speed and applying mesitylene or a
Stoddard type of solvent (such as Air
Products Negative Resist Developer ® ) to the
topside of the wafer to remove the bead
formed at the wafer edge. The solvent can
be applied using a dispenser or manually by
syringe.
Step 2. Soft-bake
After coating, the film produced from Dow
Corning ® WL-5150 is subjected to a low
temperature bake to remove residual solvent.
The standard process is to heat the wafer on
a hotplate to a temperature of 110°C for a
period of 2 minutes. After the soft-bake the
film will remain tacky; however, the
tackiness can be reduced by further heating
the film on a hotplate at a temperature of
110°C for up to 5 minutes.
It may be desirable to perform an oven
rather than hotplate soft-bake. Any oven
process will need to be optimized to the
equipment used. If such an approach is used
it is important that the coatings remain in a
horizontal rather than vertical orientation to
avoid a downward flow of the material on
the substrate.
Step 3. UV Exposure
UV-light exposure of films prepared from
Dow Corning ® WL-5150 causes the
activation of a photo-sensitive compound in
the formulation that results in selective
cross-linking (setting) of the UV-light
exposed regions during the post-exposure
bake (Step 4). As the photoactivation and
subsequent cross-linking takes place in the
light exposed regions, dissolution and
4
removal of the non-cross-linked regions in
Step 5 results in a negative image of the
original photomask. Films prepared from
Dow Corning ® WL-5150 can be exposed to
patterned UV-light using a mask aligner or a
stepper. Due to the surface tackiness of the
films after soft-bake direct contact of the
films with the UV exposure equipment
components must be avoided. For UV-light
exposure using a mask aligner a typical
proximity gap of 20-40 µm is used. It is
recommended that the UV-light exposure be
conducted using either i-line or broadband
radiation. The sensitivity of the films to g-
and h-line radiation is low and exposure at
these wavelengths is not recommended. The
standard dose of broadband UV-light from a
medium pressure Hg lamp for films with a
thickness of 15 to 40 µm is 1000 mJ/cm 2 .
Doses in the range of 600 to 1000 mJ/cm 2
can be used but may result in slightly
reduced film retention. Exposure to doses
lower than 600 mJ/cm 2 will result in low or
no film retention.
Step 4. Post-Exposure Bake
Following Step 3 it is necessary to subject
the film to a post UV-light exposure bake.
Omitting this step will result in the complete
removal of both the irradiated and nonirradiated
portions of the film during the
development step. It is during this step that
the UV-light irradiated portions of the film
undergo a cross-linking process rendering
those areas insoluble in the developing
solvent. The standard condition for the postexposure
bake is placement of the substrate
onto a hotplate at 150ºC for a period of at
least 90 seconds and not exceeding 180
seconds. The standard hot-plate exposure is
150ºC for 120 seconds. Exposure of the film
to a temperature of 150ºC for periods longer
than 180 seconds can result in a significant
increase of film residue (scumming) in the
non-exposed regions. Exposure at 150ºC for
periods less than 90 seconds can result in
insufficient cross-linking and a significant
reduction in the final film thickness after
development.

Dow Corning Proprietary
Revised April, 2004
Step 5. Development
The solvent used to develop the negative
image in the film is a Stoddard type of
solvent such as Negative Resist Developer ®
(NRD) that is available from Air Products.
This solvent can be used as both developer
and rinse solvent (Step 1). Alternatively,
mesitylene has also shown to be a good
development solvent for Dow Corning ®
WL-5150. After the post UV-light exposure
bake (Step 4), the negative image pattern
can be developed by the following process
using an automated coater with two dispense
heads, one for NRD and one for IPA:
1. 15-second static dispense of NRD to coat
the wafer.
2. 10 second dynamic dispense (200 rpm)
of NRD to rinse wafer.
3. 60 second NRD puddle.
4. 10 second NRD dynamic rinse at 500
rpm.
5. 10 second NRD dynamic rinse at 1000
rpm.
6. 10 second IPA dynamic rinse at 1000
rpm.
7. 30 second spin dry at 3000 rpm.
During development a portion of material in
the exposed regions is removed in addition
to the non-irradiated regions. This results in
the post-developed film having a final
thickness less than the original thickness
achieved after the soft-bake step. The
percentage of the final film thickness
relative to the thickness after the soft-bake
step (Step 2) is referred to as the film
retention. The film retention for fully
processed (Steps 1-6) films from Dow
Corning ® WL-5150 is in the range of 80 to
90 percent with the higher values obtained
for the thicker films.
Dow Corning ® WL-5150 is an organic
solvent developable film. Water or aqueous
alkali types of developers cannot be used.
Step 6. Hard Bake
The objective of the hard bake is to
complete the film cure process resulting in
5
CSM Modulus (MPa)
maximization of the cure level and
mechanical properties. Curing of Dow
Corning® WL-5150 films is a result of an
addition cure mechanism that does not
involve the evolution of cure by-products.
This feature results in negligible shrinkage
(~ 2%) and low stress after cure. The
standard cure process for Dow Corning®
WL-5150 films is 250ºC for 30 minutes in
an oven. The cure atmosphere can be air,
nitrogen or vacuum. No heating or cooling
ramps are necessary to bring the films to the
cure temperature. Films can also be cured at
a temperature of 180ºC for 60 minutes. The
final modulus as a function of cure condition
is shown in Figure 6 for 250ºC/30 minute
and 180ºC/60 minute cures and well as a
third condition of 180ºC/60 minutes
followed by 3 excursions to 260ºC for 1
minute each.
400
300
200
100
0
250°C/
30 min
180°C/
60 min
180°C/1 hr +
3 x 260°C/1 min.
Figure 6. CSM modulus against cure
condition for Dow Corning ® WL-5150
films.
Lithographic Process Performance
The lithographic performance for fully
processed 20 µm thick films of Dow
Corning ® WL-5150 is summarized in Table
5. The results were generated for films
processed using a mask aligner and standard
process conditions for steps 1 through 6.
Key performance features include good
resolution down to 15 µm sized pads, a
sidewall slope of 60º and minimal shrinkage
during cure.

Dow Corning Proprietary
Revised April, 2004
Process Property Value
Film Thickness Range 15 to 40 µm
Minimal Feature Size 15 µm
Aspect ratio (ht/w)

Dow Corning Proprietary
Revised April, 2004
films cured at 180°C for 1 hour can be
removed by immersion for approximately 40
minutes, and films cured at 250°C for 30
minutes can be removed by immersion for
approximately 90 minutes. Following film
removal, the substrate should be washed
with IPA and water.
Dow Corning ® WL-1975 cannot be used as
a developing solvent for Step 5 as it will
remove the entire film.
Rework of Dow Corning ® WL-5150 films
can results in the evolution of trace amounts
of hydrogen gas requiring good ventilation
for processing. Please consult the MSDS
provided with Dow Corning ® WL-1975 for
additional details.
Plasma and Reactive Ion Etch Processing
Films derived from Dow Corning ® WL-
5150 contain silicone and as a result are
sensitive to O2 and air plasmas. Excessive
exposure to these plasmas may cause the
films to crack. O2 and air plasmas at low
exposure dose may be used to increase the
surface energy of the films to improve the
adhesion and wettability to other electronics
materials.
Following processing and cure,
films derived from Dow Corning ® WL-5150
may leave a small amount of polymer
residue on the surface of the developed
regions of the substrate. This residue is not
visible to the eye and is typically less than
2000 Å thick. To remove this unwanted
material a reactive ion etching process may
be used. As Dow Corning ® WL-5150
contains silicone it is not desirable to use a
pure O2 etch. An etching gas combination of
SF6/O2 is recommended with an optimum
SF6 level of 50 percent. After SF6/O2 etch, a
water wash is recommended to avoid
contamination of the substrate. Additionally,
CF4/O2 gas systems have been demonstrated
as an effective etch system. Please ask your
local Dow Corning representative for the
latest detailed results in etching processes.
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