Photoplastic NEMS with an Encapsulated Polysilicon Piezoresistor
Nitin S. Kale, Sudip Nag, R. Pinto, V. Ramgopal Rao
Centre for Nanoelectronics, IIT Bombay, Mumbai, India
Department of Electrical Engineering, IIT Bombay, Mumbai, India.
Abstract- We demonstrate a photoplastic NEMS device that
has an encapsulated polysilicon piezoresistor. The crucial
temperature limitation of depositing a polysilicon film on a
polymer was overcome by employing a novel Hotwire CVD
process. In this paper, we report the fabrication and
characterization of a novel polymeric cantilever with an
embedded piezoresistor, which exploits the low Youngs modulus
of a polymer and high gauge factor of polysilicon. Such devices
are widely used for sensing biochemicals.
I.
Gas
Manifold
MFC
Exhaust
line
Filament
Assembly
LLTC
Shaft
INTRODUCTION
Microcantilever based biosensors have attracted
considerable interest to monitor a specific substance in
applications such as clinical analysis, environmental control
and industrial processes [1]. Adsorption of molecules onto the
surface of the cantilever generates a small magnitude (5mN/m
to 0.5 N/m) surface stress, resulting in a bending of the
cantilever to the tune of a few tens of nanometers [2 - 4]. The
detection mechanisms could be optical (measurement of the
deflection), or measurement of a change in resonant frequency
of the cantilever due to mass change [5], or even piezoresistive
(change in resistance due to bending) [6, 7]. Optical readout
scheme to measure the deflection of affinity cantilevers suffers
from two major problems: (i) realignment of the laser system
when performing measurement in liquids that have different
refractive index; (ii) measurement is not possible in opaque
liquids [3]. On the other hand, in a piezoresistive detection
scheme, laser alignment is not required and issues of changing
refractive index and processing in opaque liquids simply
disappear. As the signal from the piezoresistive detection
scheme is an electrical signal, one can build appropriate
electronics around it to amplify and process the signal.
However, the piezoresistive scheme is less sensitive and
noisier than the optical detection scheme [3].
In this paper, for the first time, we present the
fabrication and characterization of a novel polymeric
cantilever, with an embedded polysilicon piezoresistor. These
cantilevers can be used for sensing of biochemicals. The
polymer used to fabricate the structural layer and the
encapsulation layer was SU-8, which is a negative tone
chemically amplified near UV photoresist. It is highly resistant
to chemicals and hence can be used as a component material
[8]. Also, a method for modification of SU-8 surface has
already been invented so as to make it amenable for
biomolecule immobilization [9]. Fabricating a structural layer
using this material has several advantages: (i) it is a low cost,
low thermal budget process in which only spin coating, baking,
Reactor
Slit
Valve
Gate Valve
View Port
Pirani Gauge
Fig. 1. HWCVD cluster tool [10]
patterning and development is required. Thus one does not
need to fabricate a structural layer through a CVD process; (ii)
SU-8 has a low Young’s modulus (5 GPa) and thus has a
higher propensity for bending, compared to, for example a
nitride (150 to 350 GPa), for a given surface stress.
II. PHOTOPLASTIC PIEZORESISTIVE DEVICE FABRICATION
A. HWCVD Method
The critical unresolved technology issue for the integration
of a high gauge factor polysilicon material in a SU-8 matrix
was the thermal budget limitation of polymers. Both the Lowpressure & Plasma-enhanced CVD processes, normally used
for polysilicon deposition, had to be ruled out for polymeric
cantilevers because of the temperature limitations. We resolved
this crucial issue by employing polysilicon, deposited by the
low temperature Hotwire CVD (HWCVD) method, as the
piezoresistive material. The figure of the cluster tool for
depositing polysilicon (and other films) at a low substrate
temperature via the HWCVD process is shown in Fig. 1. The
tool can also be employed to modify the surface of SU-8 by
attaching amino groups to it via the dissociated ammonia
treatment process; and make SU-8 amenable for antibody
immobilization.
B. Device Fabrication
To fabricate the structure, a series of deposit, pattern and
etch steps are employed. Fig. 2 (a to m) illustrate the main
steps of the 5 mask fabrication process. First, a 2 inch diameter
<100> oriented, silicon wafer was cleaned with piranha (1:3
H2O2: H2SO4), followed by the RCA cleaning procedure.
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Silicon dioxide, which was later employed as a sacrificial
layer, was then sputtered on the wafer. Next, a 1.8 µm SU-8
2002 layer was spun and patterned on the substrate. This step
yielded the bottom layer, i.e. structural layer of the cantilever
(Fig. 2(b &i)). Polysilicon was deposited on the SU-8/substrate
using the HWCVD process; the substrate heater temperature
was maintained at 170 °C [10]. Patterns on the polysilicon film
were then defined and the film was etched by dissolving the
unprotected polysilicon in HNA (hydrofluoric acid + nitric acid
+ deionized water). The resultant pattern yielded the
piezoresistive polysilicon layer shown in Fig. 2(d & j).
thereby further lowering the manufacturing cost. Images of the
SU-8/polysilicon/SU-8 piezoresistive cantilevers captured
using an optical microscope are shown in Fig. 3.
Fig. 3. Optical images of a die with SU-8/polysilicon/SU-8 cantilever (a)
single cantilever (b) cantilevers in half-bridge configuration (c) gold pads
and track lines (d) entire die
III. PHOTOPLASTIC PIEZORESISTIVE DEVICE
CHARACTERIZATION
Fig. 2. Process sequence to fabricate SU-8/poly/SU-8 cantilevers
Next, a 100 nm film of chrome-gold was deposited on the
polysilicon/SU-8/substrate. Thus, the appropriately patterned
chrome-gold layer formed a contact pad to make an electrical
contact with the polysilicon layer (Fig. 2(e & k)). Another
layer of SU-8 (thickness was 0.9 µm) that encapsulated the
polysilicon was then defined which formed the top layer of the
cantilever. Encapsulation of the polysilicon piezoresistor is
essential so that the wet biological processes do not short the
piezoresistors. The top SU-8 layer is also the immobilization
layer of the device. This SU-8 layer can be subjected to
hotwire CVD induced dissociated ammonia treatment [9] so as
to make it amenable for antibody immobilization. The base of
the cantilever die was fabricated using SU-8 2100 which is a
highly viscous and therefore ‘thick’ negative photoresist (Fig.
2(g & m)). The die was released from the substrate by etching
the sacrificial (sputtered) oxide in hydrofluoric acid (Fig. 2(h)).
When the acid dissolved the oxide completely, the dies simply
lifted off from the substrate. An important advantage of this
method is that it does not ‘consume’ the substrate as in surface
or bulk micromachining. Hence the substrate is reusable,
Mechanical characterization of the cantilever was
performed to measure its stiffness. Electromechanical
characterization of the cantilever was performed to quantify the
change in resistance of one of the levers for a given deflection
at its tip.
Stiffness characterization of the cantilever was
performed by the technique developed by Serre et al [11]. The
technique is based on the measurement of the deflection of the
tip of the sample cantilever; due to a load applied on it, by an
AFM cantilever. Molecular Imaging AFM was used to
measure the stiffness (i.e. its spring constant) of the SU8/polysilicon/SU-8 cantilever. The AFM cantilever chosen was
a silicon nitride structure whose nominal stiffness was 0.12
N/m (i.e KAFM=0.12 N/m). The AFM cantilever was made to
approach the tip of the SU-8/polysilicon/SU-8 cantilever. To
obtain the approach curve i.e. the ‘AFM Force vs Distance
spectroscopy’ the deflection vs distance sweep was run. The
plot for the approach curve i.e. ‘Force vs distance
spectroscopy’ as the AFM cantilever approaches the SU8/poly/SU-8 cantilever is shown in Fig. 4(a). This information
used in conjunction with the ‘calibrate’ software tool of the
AFM is used to extract the deflection of the AFM tip in
nanometers per Volt. The calibration information is also
depicted in the text box as y = - 0.026 + 5.85. Next, the
measurement involving actual bending of the probe cantilever
i.e. ‘AFM Calibrated Force vs Distance Spectroscopy’ was
carried out. The deflection of the AFM cantilever in
nanometers (nm) plotted as a function of the distance traveled
by the scanner tube (in nm) is shown in Fig. 4(b). Using the
stiffness data of the AFM cantilever (KAFM=0.12 N/m) we
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calculate the spring constant or stiffness of the SU-8/poly/SU-8
cantilever to be 0.25 N/m.
nulled node. This clearly established a piezoresistive behaviour
by the encapsulated polysilicon layer.
y = -0.026x + 5.85
Fig. 4(a). AFM Force vs distance spectroscopy
Fig. 5. Change in voltage across the deflected cantilever vs deflection
at the tip of the cantilever.
Therefore, we have successfully characterized the fabricated
polymer cantilever that has an encapsulated polysilicon
piezoresistor. Its stiffness was determined. For its
electromechanical characterization, one of the two cantilevers
of the die, that formed a balanced (and nulled) half bridge, was
deflected and a change in voltage across it was measured,
thereby establishing piezoresistive behaviour.
y = 0.673x + 292.87
Fig. 4(b). AFM Calibrated Force vs Distance Spectroscopy
An important performance parameter of a
piezoresistive cantilever is its deflection sensitivity. It is a
measure of the relative change in resistance as a function of
cantilever deflection. The measurement is performed by
deflecting the sample cantilever by a micromanipulator. The
relative change in resistance is measured using a highly
sensitive
in
house
developed
electromechanical
characterization system. Three micromanipulators of a Karl
Suss PM8 probe station were used to probe the three gold pads,
while the 4th micromanipulator of the probe station was used to
deflect the cantilever. The plot of the measured change in
voltage as the cantilever is deflected is shown in Fig. 5. We
observed that the voltage measured at the measurement node
changed as the cantilever was deflected. This occurred because
the deflected cantilever got strained; due to which its resistance
changed, thus causing a ‘voltage measure’ at the previously
Thus we demonstrate, for the first time, a photoplastic
NEMS device that has an encapsulated polysilicon
piezoresistor. The device is designed and fabricated so that it
can fit into the nose of an AFM; therefore it can be used in
conjunction with an AFM’s liquid cell for biochemical sensing
applications. Antibodies, such as anti-myoglobin, can be
immobilized on one surface of the cantilever inside the AFM’s
liquid cell. The scheme can be used to detect myoglobin, a
cardiac marker, which is released when a person has a heart
attack.
5.
CONCLUSION
In this paper we have reported, for the first time, the
fabrication of a polymeric microcantilever with an
encapsulated polysilicon piezoresistor, using a 5 mask process.
The process to fabricate the die was developed in such a way
that it can be lifted off the silicon substrate. Hence the silicon
substrate is reusable. The polysilicon piezoresistor is fully
encapsulated by SU-8, so that the wet biological processes do
not short the piezoresistors. The devices were characterized for
their mechanical and electro-mechanical behaviour. The spring
constant of the fabricated SU-8/polysilicon/SU-8 cantilevers
was measured to be 0.25 N/m. The piezoresistive properties of
these cantilevers with an encapsulated HWCVD polysilicon
piezoresistor, were demonstrated and characterized.
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ACKNOWLEDGMENT
Nitin S. Kale gratefully acknowledges the financial support
provided by the National Program on Smart Materials and
Structures and the Nanotechnology project; both funded by the
Government of India.
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