A thermal driving and piezoresistive sensing MEMS cantilever resonator has been proposed and developed to construct trace gas detection sensors. The problem of integrating vibration structure, transducers and electric elements is the main concern in the design and fabrication of the resonator. In this paper, the parameters and the configuration of the resonator are discussed, the fabrication process and the test results are presented. Finite Element Analysis (FEA) has been carried out to optimize the configuration of the resonator to obtain high sensitivity and efficiency with a uniform temperature distribution that is propitious to the function of the gas sensing material. The fabrication process is based on direct bonding silicon-on-insulator (SOI) wafer and inductive coupled plasma (ICP) etching technology, which conciliate the semiconductor processes and the micromaching processes, and provide precise control of the resonator parameters. The experimental test results of the fabricated resonator agreed well with the calculation and simulation results and demonstrated that the proposed resonator was qualified to construct trace gas detection sensors

Wei, Gao

Ying, Dong

Zheng, You

Caltech Authors - Main

Direct Bonding SOI Wafer Based Cantilever Resonator for Trace Gas Sensor Application

Proceedings of the 2009 4th IEEE International
Conference on Nano/Micro Engineered and Molecular Systems
January 5-8, 2009, Shenzhen, China
Direct Bonding SOl Wafer Based MEMS Cantilever Resonator for
Trace Gas Sensor Applicaiton
Dong Ying, Gao Wei, You Zheng
Department ofPrecision Instruments and Mechanology, Tsinghua University, China
II. RESONATOR CONFIGURATION
A. Cantilever Dimension
The operation principle of the resonant gas sensor is based
on the following model [4]:
B. Transducers Arrangement
Thermal driving and piezoresistive sensing mode is adopted
by the resonator. The heating resistors and the piezoresistors
are integrated with the cantilever to construct a MEMS
resonator. After the resonator is fabricated and packaged into a
chip, the gas sensing material will be deposited onto the
cantilever to construct a gas sensor. The position of heating
resistors, piezoresistors and gas sensing layer on the surface of
the cantilever affects the performance of the resonator and the
sensor indeed. Trade-offs must be made in the consideration of
the transducers arrangement.
where m and f are the mass and resonant frequency of the
cantilever respectively, 11m is the additional mass loading on
the cantilever that is caused by the specific gas molecules being
trapped in the sensing layer attached on the surface of the
cantilever, and 3f is the changes offdue to the mass loading.
Therefore, through monitoring the frequency change 3j, mass
loading 11m can be measured, and accordingly information
about the gas concentration can be obtained.
It can be seen from the model that higher resonant
frequency and less mass of the cantilever resonator provide
better sensitivity for the sensor. Therefore the design principle
of the cantilever dimension is that under the precondition of
providing enough space to arrange the vibration driving and
sensing elements on the cantilever surface, to get higher
frequency and less mass. Taking the fabrication factors into
consideration, the dimension of the cantilever with rectangle
cross section is 1000f.!m long, 400f.!m wide and 10f.!m thick.
The density of silicon is 2.23x 10-15kg/f.!m3, then the mass of the
cantilever is about 9f.!g. Calculation using classical formula [4]
shows that the fundamental frequency of the cantilever is about
14kHz. With a frequency resolution of 1Hz which can be easily
obtained by an ordinary counter, a mass resolution of 1~2ng
can be obtained according to Eq. (1). Therefore trace gas
detection can be expected with this cantilever dimension.
1 ~m
----
2 m
8f
f
Abstract - A thermal driving and piezoresistive sensing MEMS
cantilever resonator has been proposed and developed to
construct trace gas detection sensors. The problem of integrating
vibration structure, transducers and electric elements is the main
concern in the design and fabrication of the resonator. In this
paper, the parameters and the configuration of the resonator are
discussed, the fabrication process and the test results are
presented. Finite Element Analysis (FEA) has been carried out to
optimize the configuration of the resonator to obtain high
sensitivity and efficiency with a uniform temperature distribution
that is propitious to the function of the gas sensing material. The
fabrication process is based on direct bonding silicon-on-
insulator (SOl) wafer and inductive coupled plasma (ICP)
etching technology, which conciliate the semiconductor processes
and the micromaching processes, and provide precise control of
the resonator parameters. The experimental test results of the
fabricated resonator agreed well with the calculation and
simulation results and demonstrated that the proposed resonator
was qualified to construct trace gas detection sensors.
Keywords -cantilever, gas sensor, resonator, SOl wafer
I. INTRODUCTION
Micromachined cantilevers have been proposed for a
variety of sensor applications [1,2]. With a suitable driving
mechanism cantilever can operate in resonant vibrating mode
where causes of the resonant frequency shift can be detected
with a frequency tracking mechanism. Several combinations of
vibration driving and frequency tracking mechanisms can be
applied to the micromachined cantilevers [3, 4]. For portable
sensor application, it is necessary to integrate the driving and
tracking transducers into the cantilever to form a MEMS
resonator. Micromachined monocrystalline silicon cantilevers
have inherently high quality factor (Q), providing the resonator
with high frequency resolution. It has been demonstrated that
micromachined resonant cantilever sensors can achieve
nanogram-level to picogram-Iever mass sensitivity, which
attracts intensive interests for bio-chemical applications [5].
However, fabricating cantilever structure, transducers and
electric paths and pads from a single crystal wafer involves
complicated processes. Efforts have to be made to compromise
the feasibility of the processes and the performances of the
device [6]. We have developed a MEMS cantilever resonator
for on-site trace gas detection application. Thermal driving and
piezoresistive sensing combination was used. A fabrication
process based on direct bonding SOl wafer was proposed to
solve conflicts in different processes and to improve control
precision of the device fabrication. In this paper the design,
fabrication and test results of the MEMS cantilever resonator
are presented.
This project isfunded by the National Nature Science Foundation of
China (NSFC) (Project code: 50605040).
*Contact author: Dong Ying
Dept. Precision Instruments and Mechanology, Tsinghua University
Beijing 100084, China
Phone: 8610-62776000-803
Email: dongy@tsinghua.edu.cn
978-1-4244-4630-8/09/$25.00 ©2009 IEEE 134
It is well known that stress increases to the most at the root
of deflected cantilever, therefore to obtain the maximal signal
to noise ratio the piezoresistors should be positioned in the area
close to the root. Another common knowledge is that the
highest efficiency can be obtained if the cantilever is driven at
its free end. Also it has been understood that the resonant
frequency has the most sensitivity to the mass loading
occurring at the tip of the cantilever [7]. Meanwhile the
temperature elevation and distribution on the cantilever due to
heating has impact on the performance of the gas sensing
material. Generally to say, proper temperature with a uniform
distribution is propitious to the function of the gas sensing
material.
FEA simulation has been carried out in order to optimize
the transducers arrangement. The option we made is, two
identic slits divide the cantilever into three areas and from the
tip to the root of the cantilever there are gas sensing layer,
heating resistors and piezoresistors ordinally in each of the
three areas. The slits obstruct the heat conduction and produce
uniform temperature elevation in the gas sensing area.
c. Heating Resistors
The thermal driving element is composed of four shunt-
wound resistor strips stretching along the width of the
cantilever, as sketched in Fig.1 where dashed lines indicate
electrical connection. Using shunt-wound resistors instead of a
single resistor can avoid local heating due to the nonuniform
doping of the resistor strip. Each of the strips is 100J.! long and
25J.!m wide and has 15J.!m space in between. Given the sheet
resistance of the doping process 80'"100nJO, the overall
resistance of the thermal driving element is about 80~ lOOn.
Through electrical-thermal-mechanical couple simulation by
ANSYS, near 1J.!m deflection at the tip of the cantilever can be
produced under the Ie standard voltage 5V.
.......•....•...•....•
.......•....•....•....•
Figure 1. Sketch of the thermal driving element
D. Piezoresistors
The piezoresistors to construct the Wheatstone bridge are
usually 1 ~ 5 kn for pressure or stress measurement
applications. The resistor strips will be too long to be
positioned within the cantilever if the same doping process as
the heating resistors is used to fabricate the piezoresistors.
Therefore the resistor strips must be made into zigzag shape
just like those in pressure sensors. For the convenience of the
connection and the compactness of the arrangement, three
parallel resistor strips are connected in serial. Each of the strips
is 150J.!m long and 15J.!m wide and has 15J.!m space in between,
as sketched in Fig.2. Because the connection path are vertical
to the strips, reverse piezoresistive effect will be arosed. To
eliminate the reverse effect, the resistance of the connection
paths must be much smaller than the strips, which means they
must be heavy doped. With this strategy the overall resistance
of a piezoresistor is approximately three times the resistance of
the resistor strip which is about 800~ 1000n if the same
doping process as the heating resistors is used, and therefore
2.4~3kn can be obtained.
....-------IV'.~..,.'.......•.•.... •.•:,.. ..•.~ '..... ...• Connection pathConnectionpail--------~
Figure 2. Sketch of the piezoresistor
Four piezoresistors are connected to construct a Wheatstone
bridge in the way shown in Fig.3 where the dashed lines
indicate electrical connection and PI"'P5 indicate electrodes.
P5 is the grounding, P2 is the input port, PI and P4 are output
ports, and an adjustable resistor will be connected between P2
and P3 to eliminate the unbalance of the bridge due to the non-
uniform resistance of the piezoresistors, which is unavoidable
in doping process.
Pl ':
:~ :::::::::::::::·~ · ·il I Ir!
I :
...,..:
r·ir-----"-Ill
: I
,... '-------~~I
P4 _ 1
P5 ; :
Figure 3. The construction of wheatstone bridge from the piezoersistors
E. Simulation Results
The configuration of the cantilever resonator is shown in
Fig.4. Space is reserved to deposit gas sensing material later.
FEA simulation has been carried out to evaluate this
configuration. The fundamental frequency obtained is about
14.2kHz. Fig.5 shows the FEA simulation result of the
temperature distribution on the cantilever with 100mW driving
power. It can be seen that uniform temperature elevation is
achieved in the gas sensing area. The stress produced at the
root of the cantilever is about 3MPa which can provide sizable
piezoresistive signal.
135
Figure 4. Skech of the cantilever resonator configuration
o 37.2'i4 ?i .487 111. 731 148.975
16.622 55.6615 93.109 130.353 1157.5517
Figure 5. FEA simulation of temperature distribution on the cantilever with
IOOmW driving power
III. FABRICATION
sal wafer was used as the start material for the fabrication
of the resonator. The advantages of sal to MEMS are
summarized in reference [8]. In this project, the sal wafer is
developed through directly bonding technique: an oxidated
silicon wafer is bonded onto a non-oxidated silicon wafer and
then grinded and polished to form a crystal-oxide-crystal
sandwich structure, as show in Fig.6 (a). The thin crystal is as
the front of the wafer and usually called device silicon, while
the thick crystal is as the back of the wafer and called the
substrate silicon.
The resistors, conduct paths and electrodes were fabricated
on the device silicon. Boron implantation technology was used
to produce resistors. A light doping process of lOOQ/D sheet
resistance and a heavy doping process of lOQ/D sheet
resistance were used. Aluminium thin film was deposited by
electron beam evaporation technology to make the conduct
paths and electrodes. To obtain ideal doping distribution and
real Ohm contact, anneal was performed in the processes.
The micromachining of the cantilever structure was
accomplished by inductive coupled plasma (ICP) etching
technology. The sal wafer was etched from the front to form
the cantilever dimension and subsequently from the back to
provide the hanging height for the cantilever. The oxide
between the device silicon and the substrate silicon acting as
stop layer of silicon etching was etched in the end to release the
cantilever structure. Actually the cantilever was developed
through ICP etching successively the nitride film, the oxide
film and the device silicon from the front of the wafer, and
afterwards the substrate silicon and the oxide layer from the
back of the wafer. The thickness of the cantilever was precisely
guaranteed by the thickness of the device silicon.
The resistors, contacts, paths, pads and cantilever structure
were patterned through lithography. The whole fabrication
process included six lithographic steps defining the light
doping areas, heavy doping areas, Ohm contacting areas, shape
of electric paths and pads, front etching areas and back etching
areas respectively. Therefore six lithographic masks (Ml~M6)
had to be developed. The fabrication processes of the resonator
are schematicly presented in Fig.6.
(a) The direct bonding sal wafer
(b) Lightly doping Boron (Ml)
(c) Heavily doping Boron (M2)
(d) Depositing and developing oxide film (M3)
(e) Depositing and developing AI film (M4)
136
(f) Depositing nitride film
Figure 8. The packaged chip of the MEMS cantilever resonator
(h) ICP etching from the back (silicon, oxide) (M6)
(g) ICP etching from the front (nitride, oxide, silicon) (M5)
IV. TEST RESULTS
The amplitude-frequency characteristics of the cantilever
resonator at different levels of excitation have been
investigated. Fig.9 shows the amplitude-frequency
characteristic of the resonator in ambient air under 3V driving
voltage. The resonant frequency measured is approximate
14.92 kHz and the quality factor (Q) is approximate 200.
According to the tested parameters of the resonator, nanogram-
level mass sensitivity can be expected, which provides the
feasibility for trace gas detection. The cantilever resonator also
shows advantages of high efficiency and stability in the test
experiments.
••
Figure 7. The SEM picture of the cantilever resonator
Figure 6. The schematilc fabrication processes of the resonator
Figure 9. Tested amplitude-frequency characteristic of the resonator
14.92kHz
0.1' , , , , ,", , , , I
14.4 14.5 14.6 14.7 14.8 14.9 15 15.1 15.2 15.3 15.4
0.2
V. CONCLUSIONS
The significant advantage of thermal driving and
piezoresistive sensing mode over other operation modes of
MEMS cantilever resonator is comparative testing signal can
be produced with much lower driving voltage (below 5V in this
case), which permits practical application and provides
opportunity of IC integration. The designed configuration for
the MEMS cantilever resonator optimizes the sensitivity and
the efficiency of the resonator and can create uniform
•
•
Silicon Nitride
Silicon Oxide
Heavy-doped Boron
D
D
Aluminum
Light-doped Boron
Monocrystalline Silicon
Fig.7 is the SEM picture of the resonator fabricated from
the SOl wafer. The developed wafer was diced, sifted and
package into chips. Fig.8 is the picture of a resonator chip.
137
temperature distribution for the function of the gas sensing
material. SOl wafer provides a solution to the integrated
fabrication of microstructure, transducers and electronics, and
the precise control of the resonator parameters. The test results
of the fabricated resonator agreed well with the theoretical
calculation and FEA prediction. The resonator showed
relatively high quality factor (approximate 200 in the air) in the
experiment. According to the results, the proposed resonator is
qualified to construct high sensitivity gas sensor.
With proper gas sensing material and close-loop vibration
control circuit, the MEMS cantilever resonator will be made
into specific trace gas sensors.
ACKNOWLEDGMENT
This project is supported by the National Nature Science
Foundation of China (NSFC, No.50405060). The fabrication of
the resonator was accomplished by the Institute of
Semiconductors, Chinese Science Academy (CSA). We are
grateful to Professor Ji An and Dr. Ning Jin for their advices
and contributions.
REFERENCES
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[2] C. Ziegler, "cantilever-based biosensors," Anal. Bioanal. Chern., vol.
379,pp.946-959,2004.
[3] G. Stemme, "Resonant silicon sensors," J. Micromech. Microeng., vol. 1,
pp. 113-125, 1991
[4] O. Brand and H. Baltes, "Micromachined resonant sensors- an
overview," Sensors update, vol. 4, pp. 3-51, 1999
[5] H. P. Lang and C. Gerber, "Mechanical Sensors for the Nanoworld,"
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[7] Zhou Qin, "Study on MEMS Chemical Gas Sensor Based on Resonating
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138


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Direct Bonding SOI Wafer Based Cantilever Resonator for Trace Gas Sensor Application

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