J. Micromech. Microeng. 7 (1997) 145–147. Printed in the UK PII: S0960-1317(97)83256-1
The mechanical properties of the
rubber elastic polymer
polydimethylsiloxane for sensor
applications
JCL
¨
otters†, W Olthuis,PHVeltink and P Bergveld
MESA Research Institute, University of Twente, PO Box 217, 7500 AE Enschede,
The Netherlands
Presented on 21 October 1996, accepted for publication on 9 April 1997
Abstract. Polydimethylsiloxane (PDMS) is a commercially available physically and
chemically stable silicone rubber. It has a unique flexibility with a shear elastic
modulus
G
≈ 250 kPa due to one of the lowest glass transition temperatures of
any polymer (
T
g
≈−125
◦
C). Further properties of PDMS are a low change in the
shear elastic modulus versus temperature (1.1 kPa
◦
C
−1
), virtually no change in
G
versus frequency and a high compressibility. Because of its clean room
processability, its low curing temperature, its high flexibility, the possibility to
change its functional groups and the very low drift of its properties with time and
temperature, PDMS is very well suited for micromachined mechanical and chemical
sensors, such as accelerometers (as the spring material) and ISFETs (as the ion
selective membrane). It can also be used as an adhesive in wafer bonding, as a
cover material in tactile sensors and as the mechanical decoupling zone in sensor
packagings.
1. Introduction
Polydimethylsiloxane (PDMS) is a commercially available
clean room compatible type of silicone rubber with a wide
range of applications. It is currently used, for instance, as
the mechanical interconnection layer between two silicon
wafers [1], as ion selective membranes on ISFETs [2] and
as the spring material in accelerometers [3]. Other possible
applications are its use as the top elastomer on a tactile
sensor such as that described in [4] without influencing
the sensitivity of the device and as flexible encapsulation
material in order to mechanically and chemically decouple
sensors from their environment [5]. Furthermore, it could
be used in sensors with integrated electronics due to its low
curing temperature.
Some physical and chemical attributes of PDMS are,
compared to other polymers [6], a low glass transition
temperature (T
g
≈−125
◦
C [6]), a unique flexibility
(the shear modulus G may vary between 100 kPa and
3 MPa [6]), very low loss tangent (tanδ 0.001), small
temperature variations of the physical constants (except
for the thermal expansivity, α ≈ 20 × 10
−5
K
−1
[7]),
high dielectric strength (∼14 V µm
−1
[7]), high gas
permeability, high compressibility, usability over a wide
† Tel: +31-53-4892755. Fax: +31-53-4892287. E-mail address:
j.c.lotters@el.utwente.nl
temperature range (at least from −100
◦
Cupto+100
◦
C
[8]), low chemical reactivity (except at extremes of pH)
and an essentially non-toxic nature.
This paper describes, the processing of PDMS, the layer
thickness versus spin rate, the variation of its shear modulus
G with frequency and temperature and its adhesive strength
to polished tungsten (after curing); experimental results are
also discussed.
2. Preparation of the PDMS structures
The materials used were polydimethylsiloxane PS851 from
ABCR [8] ((methacryloxypropyl)methylsiloxane), photo-
initiator DMAP (2, 2-dimethoxy 2-phenylacetophenone)
and TMSM from Aldrich (trimethoxysilylpropylmethacry-
late).
One wt% photo-initiator DMAP (powder) was sprin-
kled into 1 wt% xylene and the solution was shaken in
a Sarstedt CM-9 machine at 1400 rpm for about 1 h (the
temperature was kept at 60
◦
C). The mixture was not used
instantly; it had to be kept overnight. The silicon wafer on
which the PDMS was spun was cleaned and wet oxidized.
A mixture of 89.5% toluene, 10% TMSM and 0.5% demi-
water was heated to 60
◦
C. The wafers were kept in this
mixture for one minute so that the methacryl groups present
at the wafer surface became attached to the methacryl
0960-1317/97/030145+03$19.50
c
1997 IOP Publishing Ltd 145
JCL
¨
otters
et al
Figure 1. Thickness of polysiloxane PS851 at different spin
rates and spin times.
groups of the PDMS. The wafer was rinsed with demi-
water to remove surplus TMSM and then was spun dry.
The PDMS was spun upon the wafers with a spin rate
varying between 1000 and 5000 rpm and with two spin
times, 20 and 60 s. After spinning, the PDMS layer was
covered with Mylar foil of 23 µm thickness to avoid the
presence of oxygen near the PDMS which would disable
the cross linking process to occur and to prevent the PDMS
sticking to the mask. Thereafter, the PDMS sample was
exposed to UV light for 40 s via a mask and the PDMS
which was exposed to the UV light was cross linked. The
Mylar foil was then removed and the PDMS was developed
in xylene for 30 s, rinsed with isopropanol and spun dry.
3. Measurement results and discussion
3.1. Thickness versus spin rate
The thickness of the PDMS structures was measured with a
DEKTAK II surface profiler with needle radius 250 µm and
stylus force 0.1 mN. The measurement results are shown in
figure 1. Thicknesses greater than 40 µm can be obtained
by applying several layers of PDMS on top of each other.
3.2. Shear modulus and loss tangent versus frequency
and temperature
A thick cylindrical polysiloxane structure of height 1.2 mm
and radius 8 mm was fabricated on a silicon substrate to
measure the shear modulus G. The shear elastic modulus
G can be divided into a real part G
0
and an imaginary part
G
00
[8]: G = G
0
+ iG
00
. The loss tangent tan δ is equal to
G
00
/G
0
.
The variation in G due to changes in frequency and
temperature can be measured with the Bohlin rheometer
system. The Bohlin can apply frequencies in the range
between 0.005 and 30 Hz and temperatures between 0
◦
and
70
◦
C.
The measurement system consists of two parallel cir-
cular discs with the polysiloxane structure in between. The
lower disc applies a certain torque with a certain frequency
to the structure, a torsional force measuring device is con-
nected to the upper disc and this measures the resulting
movement of the polysiloxane due to the applied force.
Figure 2. Shear modulus
G
0
(bottom) and loss tangent
(top) versus shear rate.
Figure 3. Shear modulus
G
0
versus temperature, at
frequencies between 0.005 and 30 Hz.
The shear modulus G
0
and the loss tangent tanδ
versus applied frequency are shown in figure 2; G
0
versus
temperature is shown in figure 3.
Figure 2 shows that the shear modulus is independent of
the applied frequency, which is typical for a rubberelastic
material. The loss tangent is so small that its value was
determined by the accuracy of the equipment rather than
by its actual value, so it can be concluded that these are the
absolute maximum values. Figure 3 shows that the shear
modulus increases with temperature, which is typical for
rubberelastic materials at the ‘rubbery plateau’ [8]. The
variation of the loss tangent with temperature was not
measured because of the inaccuracy of the equipment used.
3.3. Adhesion of PDMS to an oxidized silicon wafer
A primer like TMSM should be used as the coupling agent
between an organic polymer like PDMS and a mineral
substrate such as oxidized silicon. The primer is able
to chemically react with the silicon oxide surface and it
contains at least one other functional group that can react
with the PDMS during curing (the methacrylate group in
146
Mechanical properties of PDMS
the case of TMSM). In this way the primer acts as a linker
to bind the silicon oxide surface to the PDMS covalently
[5]. The adhesion between the silicon oxide and the PDMS
is very strong—it was not possible to separate the PDMS
from the wafer by manual peel tests.
3.4. Adhesion of cured PDMS to polished tungsten
The surface forces for elastomers which are responsible for
adhesion between a cured elastomer and a rigid smooth
surface can arise from (1) van der Waals forces, (2)
electrostatic forces and (3) hydrogen bonds. It is not clear
which forces provide the major source of bonding, but most
observations are in favour of the van der Waals forces.
The more flexible the polymer and the less rough the rigid
solid surface on which the cured polymer is put, the better
the adhesion: when the average surface roughness is less
than 0.33 µm and G ≤ 250 kPa the relative adhesion
is higher than 50% [9]. To test the adhesive strength,
polished tungsten cubes of 3 × 3 × 3mm
3
with a mass of
520 mg and an average surface roughness of 0.3 µm were
placed upon several PDMS structures with areas varying
from 4.7 × 10
−8
m
2
up to 4.7 × 10
−7
m
2
and accelerations
up to 160 m s
−2
were applied to the constructions with a
Gearing and Watson GWV20 shaker unit. Thus, adhesive
strengths up to 180 kPa were observed.
4. Conclusions
Polydimethylsiloxane is the most flexible polymer with a
shear modulus G ≈ 250 kPa at room temperature. The
shear modulus is independent of the applied frequency
but linearly dependent on the temperature with a slope of
1.1 kPa
◦
C
−1
. The loss tangent tan δ, which has a value
tan δ 0.001 according to the literature, could not be
determined due to the inaccuracy of the equipment used.
The results show that PDMS is a rubberelastic material.
For rubberelastic materials, Young’s modulus E ≈ 3G,so
here E ≈ 750 kPa.
When a primer is used, a very good adhesion is obtained
between oxidized silicon and PDMS. Furthermore, due to
the low surface energy and high flexibility of PDMS a good
adhesion is obtained between cured PDMS and polished
surfaces with an average roughness of less than 0.33 µm.
Adhesive strengths up to 180 kPa were observed in this
case.
PDMS is commercially available with a selection of
functional groups which allows various curing schemes to
enable patterning, bonding and chemical selectivity. Apart
from known applications as mechanical interconnection
layers between two silicon wafers, ion selective membranes
on ISFETs and spring material in accelerometers, new
applications for PDMS could be a flexible top elastomer
in tactile sensors (without influencing the sensitivity of the
device) and a flexible encapsulation material in order to
mechanically and chemically decouple sensors from their
environment.
Acknowledgment
The authors would like to thank Mr J G Bomer and Mr
A J Verloop for their assistance in device preparation and
the Dutch Technology Foundation (STW) for its financial
support.
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