Microsyst Technol
DOI 10.1007/s00542-007-0489-8
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Qi Chen Ji Fang Hai-Feng Ji Kody Varahramyan
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Received: 29 April 2007 / Accepted: 25 November 2007
Ó Springer-Verlag 2007
Abstract Ultra-sensitive and selective moisture sensors
are needed in various industries for processing control or
environmental monitoring. As an outstanding sensor platform, surface-stress sensing microcantilevers have
potential application in moisture detection. To enlarge the
deflection of the microcantilever under surface stress
induced by specific reactions, a new SiO2 microcantilever
is developed which features a much lower Young’s modulus than conventional Si or SiNx microcantilevers. For
comparing SiO2 cantilever with Si cantilevers, a model of
the cantilever sensor is given by using both analysis and
simulation, resulting in good agreement with the experimental data. The results demonstrate the SiO2 cantilever
can achieve a much higher sensitivity than the Si cantilever. In order to fabricate this device, a new fabrication
process using isotropic combined with anisotropic dry
etching to release the SiO2 microcantilever beam by ICP
(Inductively Coupled Plasma) was developed and investigated. This new process not only obtains a high etch rate at
9.1 lm/min, but also provides good profile controllability,
and a flexibility of device design. Attributed to the high
sensitivity, a significant deflection amplitude of the surface
modified SiO2 microcantilever was observed upon exposure to 1% relative humidity. The SiO2 cantilevers are
promising for inexpensive and highly sensitive moisture
detection.
A1
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Q. Chen J. Fang (&) H.-F. Ji K. Varahramyan
Institute for Micromanufacturing, Louisiana Tech University,
911 Hergot St, Ruston, LA 71270, USA
e-mail: jfang@latech.edu
1 Introduction
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In recent years, moisture sensors have been widely used for
measurement and control in industrial or household environment. The sensing and control of moisture is very
important in various areas and processes of industries such
as chemical, petrochemical, food processing, agriculture,
textile industries, etc., as the presence of moisture is highly
undesirable and deteriorates the quality of the product.
Different types of moisture sensors have been reported,
such as capacitive humidity sensors (Nahar and Khanna
1998; Boisen et al. 2000; Kang and Wise 2000) resistive
humidity sensors (Barkauskas 1997; Chou et al. 1999;
Arshak and Twomey 2002), optical humidity sensors
(Somani et al. 2001), hygrometric humidity sensors and
gravimetric humidity sensors (Rittersma 2002). However,
the moisture sensor, which has the capability to detect low
relative humidity (RH) like 1% or lower than 1%, is rarely
reported. Environmental detection of RH often needs to be
performed at very low concentrations. Therefore, more
efforts should be paid to develop the sensitivity, linearity,
and stability of the moisture sensors.
Microcantilevers have proven to be an outstanding
sensor platform for extremely sensitive chemical, biological and environmental sensors (Chabot and Moreland 2003;
Thundat et al. 1997; Xu et al. 2002; Ji et al. 2001; Hansen
et al. 2001; Wu et al. 2001; Ji and Thundat 2002; Belaubre
et al. 2003; Fritz et al. 2000). In general, microcantilever
technology has several advantages besides high sensitivity.
The entire sensor could fit in an area less than a few millimeters. Because the cantilever bending signal is driven by
molecular recognition, the only power required is for the
detection and display, allowing use of light-weight battery
power or photovoltaic cells (Oden et al. 1996). Electronics
for operation and control are relatively simple and
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Micromachined SiO2 microcantilever for high sensitive
moisture sensor
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Author Proof
TECHNICAL PAPER
Q. Chen
e-mail: chenqi75@gmail.com
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to the adjacent silicon substrate (Chatzandroulis et al.
2002) and the low SiO2: Si etching selectivity in wet
etching (Tang et al. 2004). To completely release the
cantilever beam, the anisotropic dry etching must go
through the 500-lm-thick sacrificial silicon from the
backside of wafer. Common photoresist cannot be used as
an etching mask. The etching mask material must be
carefully selected. In our present work, we developed a dry
isotropic plasma etching with ICP for front-sided releasing
cantilever beam. Therefore, the entire fabrication process
of SiO2 cantilever was to divide the release process into
three dry etching steps, the 3 lm thick photoresist 1813 can
be an appropriate mask layer for releasing the cantilever
beam.
In this article, we present a novel guard-armed SiO2
cantilever sensor for high sensitive moisture detection. To
compare the Si and SiO2 cantilevers, the deflections of
both microcantilever beams induced from uniform surface-stress are analyzed. The simulation results proved
that the SiO2 cantilever beam has a higher sensitivity
compared to the Si beam. A comparison experiment
between Si and SiO2 microcantilevers was done by
exposing both to the same concentration of aminoethanethiol solution. The experimental results also fit perfectly
with the simulation results. The moisture sensor based on
the surface modified SiO2 cantilever beam showed a good
response to 1% RH.
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inexpensive. Microcantilever arrays could be used to
record numerous properties, gain specificity and provide
replication (Quist and Badia 2003). Therefore, developing
a high sensitivity moisture sensor based on microcantilevers will have promising applications in the chemical
petroleum and transportation industries.
Microcantilevers can undergo bending due to molecular
adsorption or absorption, by confining the adsorption and
absorption to one side of the cantilever where the sensing
material is coated. Adsorption or interaction of the analyte
will change the surface characteristics of the microcantilever or the film volume on the cantilever, and result in
bending of the microcantilever. This concept has already
been used to demonstrate the feasibility of chemical
detection of a number of vapor phase analytes as well as
highly sensitive detection of chemical and biological species in solutions. Most of these microcantilever sensors
were made of silicon. Due to the relatively large Young’s
modulus of silicon material, the bending response of the
silicon microcantilever is too weak to be measured when
surface-stress change is rather small. The silicon dioxide
(SiO2) microcantilever can provide a much higher
mechanical sensitivity compared to its silicon or silicon–
nitride counterparts, as thermally grown SiO2 film features
a much lower Young’s modulus than single-crystalline
silicon or silicon nitride (Tang et al. 2004; Bao 2000). It
can be clearly seen that (http://www.memsnet.org/material)
lists Young’s modulus values of thermal SiO2, single
crystalline Si and LPCVD (low pressure chemical vapor
deposition) Si3N4 as 57–79, 170 and 290–380 GPa,
respectively. The high mechanical sensitivity leads to a
large surface-stress-induced bending of the cantilever.
Many methods have been developed in the field of microcantilever fabrication in order to meet a wide range of
specifications. These specifications include beam size,
thickness, connectivity, beam protection, materials, alignment, and cost. Optimizing fabrication process usually
needs careful evaluation of advantages and trade-offs. A
key step of the fabrication process for cantilever sensors is
release of the suspended microcantilever beam at the final
stage of the fabrication process. Wet etching always gets
involved in the fabrication of microcantilever structures for
the releasing the cantilever beam from the substrate.
However, surface tension-induced stiction during the
release and drying step is a big challenge for the wet
etching process. Although some preventive measures are
taken to avoid this problem (Chatzandroulis et al. 2002),
especially in the fabrication of SiO2 microcantilevers due
to the low etching selectivity of SiO2: Si using either KOH
or EDP etchants, the processing conditions are hard to
control (Tang et al. 2004). Recently, we used a plasma
anisotropic dry etching process to release the SiO2 microcantilever beams, avoiding stiction of the released structure
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Microsyst Technol
2 Device design and simulation
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A schematic diagram of the SiO2 microcantilever structure
designed is shown in Fig. 1. The device consists of two
adjacent SiO2 cantilever beams, connecting wings on both
sides, and three guard arms. The dimensions of the
designed SiO2 microcantilever beams are 250 lm in
length, 100 lm in width, and 1 lm in thickness. The
connecting wings were connected with the adjoining cantilever bodies after the beams were released, so that all the
cantilevers were held on the substrate for further processing. The guard arm was designed to protect the
microcantilever beam from damage by collision during
separation and handling procedures.
In many cases, adsorption-induced bending of cantilever beams can be predicted by a model shown in Fig. 2a,
in which the beam tip was under a uniformly distributed
loading, r. The unit of r is N/m, and r [ 0 is tensile. A
concentrated moment M applied at the cantilever beam’s
free end can represent this surface-stress effect. Therefore,
the radius of microcantilever curvature R can be derived
as
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1 6Drð1 mÞ
¼
R
Et2
ð1Þ
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Microsyst Technol
this as a uniformly distributed axial stress sw along the
beam’s neutral axis and a uniformly distributed bending
moment m along the beam. In terms of this, they derived
the governing equation as
ymax ¼
ð2Þ
OO
ð4Þ
This second model can solve the problems, which the
first model cannot explain. A beam module (L = 250 lm,
w = 100 lm, t = 1 lm) was built in CoventorWare
(Coventer Inc., Cary, NC, USA) to compute the
mechanic deformation induced by uniform surface stress
through the finite element method (FEM). When applying
boundary conditions to the meshed model, we divided the
load force applied on the beam surface into a uniform
concentrated force in each element, which is equivalent to
the condition described in the aforementioned second
model. If the cantilever is made of silicon dioxide, the
Young’s modulus E = 73.0 GPa, the Poisson’s ratio
m = 0.17, then the maximum deflection of the silicon
dioxide beam appeared at the end of the beam with the
value 16.49 nm. On the other hand, if the cantilever beam
is made of silicon, the Young’s modulus E = 169.0 GPa,
the Poisson’s ratio m = 0.3, then the maximum deflection of
the cantilever beam is 6.96 nm. The simulation results of
beam deflection are shown in Fig. 3. The SiO2 beam can
give a 2.37 times larger bending amplitude under the same
conditions compared with the silicon beam. A higher
sensitivity could be achieved through using the silicon
dioxide instead of silicon as the cantilever beam.
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3 Device fabrication
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3.1 Investigation of ICP isotropic plasma etching
for beam release
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In addition to conventional optical lithography, SiO2 wet
etching and ICP plasma dry etching, a new cantilever beam
release processing was developed for the device fabrication. This process includes two steps. The cantilever beam
was first patterned by anisotropic dry etching, followed by
the isotropic plasma dry etching to release the cantilever
beam completely. In the first step, anisotropic etching was
applied to open a window and to ensure the fluorine
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Although Stoney’s formula serves as a cornerstone for
curvature based analysis and a technique for the
measurement of surface stress, it does not agree well
with the experiment data for structures under large
deflection or ‘‘thick’’ film/coating on the substrate (Klein
2000; Freund et al. 1999). In addition, the above model
regards the surface stress as a moment applied at the
beam’s free edges does not take into account the influence
of the surface stress on structure stiffness. Thus, Zhang
et al. (2004) provided a new model which is demonstrated
in Fig. 2b. This model assumes an area stress s uniformly
distribute on the top surface of the beam. They modeled
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L2 3L2 ð1 mÞ
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Here, E and m represent the cantilever beam’s Young’s
modulus and the Poisson’s ratio, respectively. t stands for
the thickness of the cantilever beam. Dr represent differential surface stress, and Dr = r+-r-, r+ and r- are
surface stresses on the top and bottom surfaces of the
cantilever beam, respectively.
In fact, the Eq. (1) is the famous Stoney’s formula. In
terms of the radius of curvature R, the deflection at the end
of the cantilever beam can be expressed as
and boundary conditions as
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yð xÞjx¼0 ¼ 0
>
>
>
dy
>
>
<
dx x¼0 ¼ 0
d 2 y
dx2 x¼L ¼ 0
>
>
>
>
3
>
: E I ddxy3 þm ¼ 0
ð3Þ
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Fig. 1 Schematic diagram of the novel SiO2 microcantilever design
d4 y
d2 y
dy
swðL
xÞ
þ sw ¼ 0
dx4
dx2
dx
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Fig. 2 Scheme diagram for theoretical analysis
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Fig. 3 The deflection contour of cantilever beam. a SiO2. b Si
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radicals could react efficiently with the silicon underneath
the SiO2 beam during the isotropic plasma etching. The
silicon isotropic etching procedure is modified from the
standard Bosch process. In this modified process, the passivation step was dismissed, and SF6 was the only process
gas. Because no passivation layer was formed along the
sidewall of the trench, the ions reacted freely with the
silicon atoms under the mask horizontally. The isotropic
etching was realized largely by ion-enhanced chemical
etching.
Dry etching with ICP has been extensively studied for
the fabrication of high aspect ratio micromechanical
structures. Among the available process schemes and recipes, the Bosch process (Laermer and Schilp 1996;
Laermer et al. 1999) has made high aspect ratio silicon
structures with vertical sidewalls possible (Quevy et al.
2002). The standard Bosch process is an anisotropic etching procedure and works as follows. It starts with a shallow
etching step under a low pressure SF6 plasma generated
with an RF source, fluorine radicals generated in SF6
plasma then react with silicon to form volatile SiFx, and
continues with a passivation step enhanced via an ICP
source, where C4F8 gas molecules are dissociated to form a
polymeric layer over the exposed silicon surface. During
the subsequent etch step, this layer is preferentially
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withdrawn on the horizontal surface rather than on the
sidewalls because of ionic physical etching. Both steps
repeat until complete etching of the silicon layer is
accomplished. Experiments and optimization settings for
anisotropic plasma etching with ICP system have been
reported (Aachboun and Ranson 1999; Aachboun et al.
2000).
To take advantage of high etching rate with ICP, we
investigated isotropic plasma etching for the release of
suspended structure from a silicon substrate. A high-density ICP system (Alcatel 601E) was used to conduct deep
dry plasma etching investigation. We have applied different isotropic etching process recipes to 16 processed wafers
for 10 min, respectively, Table 1 shows the processing
parameters and the sample profiles. Pure SF6 is the only
process gas in the silicon isotropic etching. The etch rate,
undercut rate, and isotropic ratio have been taken into
consideration to obtain an optimal set of plasma etching
parameters. In fact, for the isotropic etch, we hope to
achieve higher etch rates as well as better isotropic ratios at
the same time. Therefore, recipe 13 in Table 1 (46 mTorr
pressure, 1,800 W source power, substrate power 20 W,
300 sccm SF6 flow rate, and 20°C chuck temperature) was
adopted as the best one. For this recipe, the etching rate
was about 9.1 lm/min, uniformity was 92% and isotropic
ratio was 66%. Using this process, the cantilever beam was
successfully released from the substrate.
3.2 Fabrication process
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The starting material was a commercially available (Silicon
Inc.) double side polished 4-in. (100-mm), \100[ silicon
wafer with 500 lm thick, and a 1 lm thick SiO2 layer on
both surfaces. To achieve the structure of microcantilever
described previously, four masks were designed and
applied. Among these, mask 2 was specifically designed for
cantilever beam release by isotropic plasma dry etching
combined with an anisotropic etching process in Fig. 4.
Here, Fig. 4b depicts an enlarged part of the mask pattern in
Fig. 1a and the related dimension of masks 1 and 2. The
photoresist pattern of mask 2 served as an etching mask in
the ICP etching process to release the microcantilever beam
from bulk silicon. In order to protect the edges of the cantilever beam, the photoresist pattern covering on the
cantilever beam was about 5 lm longer along the three
edges than the SiO2 beam. This also compensated for any
slight alignment errors. To ensure that the cantilever beam
released completely and the foot of the beam remained at
the expected position after release, the photoresist pattern at
the foot of beam was designed at 60 lm longer than in mask
1. Figure 5 illustrates the main steps of the fabrication
process. The fabrication process is described as follows:
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3.2.2 Wafer backside etching 1: pattern cantilever base
with guard arm
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A 3 lm thick 1813 photoresist layer was spun on the
backside of the wafer and then patterned with a
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Shipley 1813 positive tone photoresist was spun on one
surface of a silicon wafer. A microcantilever beam pattern was transferred with mask 1 to the photoresist layer
on the front side of the wafer by a standard photolithography process and then the SiO2 cantilever shapes
were defined by wet etching with Buffered Oxidation
Etchant (BOE HF:NH4F = 1:6 in volume). In the mean
time, the entire SiO2 layer on the backside was etched
off. The photoresist was then cleaned by acetone and DI
water (Fig. 5a).
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3.2.1 Pattern the SiO2 cantilever beam
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Fig. 4 Pattern of the PR (photoresist) as a mask for cantilever beam
release
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Isotropic ratio at edge
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Isotropic ratio at center
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m)
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Etch rate at center (lm/m)
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5.6
Etch uniformity
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Etch depth y at edge (lm)
Undercut x at center (lm)
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Etch depth y at center (lm)
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Chuck temperature (°C)
Substrate power (W)
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SF6 Flow rate (sccm)
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Source power (W)
Pressure (mTorr)
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1
Recipe ID
Table 1 Process parameters and etching profiles
1,800
1,800
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Microsyst Technol
Fig. 5 Process flow for SiO2 microcantilever fabrication. a Pattern
SiO2 microcantilever beam by BOE etching. b Pattern the guide arm
by ICP etching. c Pattern the connecting wing by ICP etching.
d Microcantilever beam released by ICP isotropic etching
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photolithography process to form the base of the cantilever
sensor. The photoresist pattern served as a mask for ICP
plasma etching to create the guard arm in the next fabrication step. The wafer was etched by about 70 lm by the
ICP anisotropic etching process (Fig. 5b). The etching
depth of about 70 lm determined the thickness of the
connecting arm. This etched area will be completely
opened through the wafer after releasing the cantilever
beam.
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3.2.3 Wafer backside etching 2: pattern cantilever
connecting wing and guard arm
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A 3 lm layer of photoresist 1813 was spun on the backside
of the wafer again and then plasma anisotropic etching
applied to etch about 260 lm. This etching step is also
applied to etch the connecting wings, guard arms, and open
window from the backside of the substrate (Fig. 5c).
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3.2.4 Cantilever beam release
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A new process, which involves two steps, was developed to
release the SiO2 cantilever beam. The cantilever beam was
first patterned by anisotropic dry etching and followed by
the isotropic plasma dry etching to completely release the
cantilever beam. In the first step, anisotropic etching was
applied to open a window and to ensure that the fluorine
radicals can react efficiently with the silicon underneath the
SiO2 beam during the isotropic plasma etching. Then, the
isotropic plasma etching was applied to release the SiO2
beam from the bulk silicon substrate.
A 3 lm thick film of photoresist 1813 was spun on the
front side of the wafer and then the front side of the wafer
was patterned with mask 2 (Fig. 4). The photoresist pattern
served as an etching mask for ICP etching process to release
the microcantilever beam from the bulk silicon. The cantilever beam was released by two plasma dry etching steps:
90-lm thick anisotropic etching and then isotropic etching
until all microcantilever beams were released (Fig. 5d). The
processing time for releasing the cantilever was 20 min.
The optimal recipe for isotropic etching in this step was
chosen by the following investigation and discussions.
Figure 6 shows the scanning electron microscopy
(SEM) pictures of one cantilever. The configuration of the
several devices is shown in Fig. 7. The connecting wing
between two adjacent microcantilever bodies should be
about 50–70 lm thick, so that the thickness is not only
sufficient to connect them together, but also easily broken
just by applying a little force. The cantilevers can easily be
coated with a sensing polymer or films for different
chemical or biological sensing applications.
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Fig. 6 SEM image of rectangular cantilever beam
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4 Measurement
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The above simulation result indicates that a SiO2 cantilever
has the potential to give a larger deflection amplitude than
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Another advantage of this process is that it is easy for us
to handle the problem associated with the photoresist mask,
which occurred in our previous experiment. During an
anisotropic dry etching go through an entire wafer with
ICP, the photoresist mask was damaged or etched off
because the ions bombard the surface of the photoresist.
The fabrication process we developed calls for dividing the
release process into three dry etching steps. Meanwhile, the
release time is greatly reduced at the higher isotropic
etching rate (9.1 lm/min). Therefore, the 3 lm thick
photoresist 1813 can be an appropriate mask layer for
releasing the cantilever beam.
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Fig. 7 SEM image of SiO2 microcantilevers
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Fig. 9 The bending response of the SiO2 and Si microcantilevers
upon exposure to a 10-5 M solution of aminoethanethiol in ethanol
Fig. 10 The response of cantilever beam coated with a layer of ethyl
acetate to relative humidity of 1%
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a silicon cantilever as chemical or biological sensors. To
compare their deflections, a SiO2 cantilever and a silicon
cantilever of the same size were deposited with a 20 nm
thick layer of gold on one side for further measurement,
because thiol compounds, such as aminoethanethiol solution, form a monolayer on the gold surface that would
deflect the microcantilever on whose surface by a gold
layer has been deposited (Fritz 2000). The measurement
experiment setup is shown in Fig. 8. The experiments were
performed in a flow-through liquid cell that held the microcantilever which was coated with gold as the sensing
material. The deflection of the cantilever beam was
detected with an atomic force microscopy (AFM) apparatus, where a laser beam is focusing at the free end of the
cantilever beam. The corresponding displacement of the
reflecting laser beam was detected by a position-sensitive
photodetector, while the deflection was caused by the
monolayer formed on the gold surface.
Figure 9 shows that the deflection of the SiO2 microcantilever upon exposure to 10-5 M of aminoethanethiol
solution was approximately 220 nm, while that for the
silicon microcantilever was approximately 90 nm. Therefore, the bending value of SiO2 microcantilever was 2.44
times larger than that of silicon microcantilever under the
same experimental surface-stress situation. This experimental result matched perfect with our above simulation
result, in which the bending ratio was 2.37 times.
As for the moisture sensor, the experiments were performed in a flow-through gas cell that held the
microcantilever which was coated by a layer of ethyl
acetate as the sensing material. Before testing, the N2 was
flowed overnight to dry the system chamber to RH \\
1.0%. When the baseline was reached, a tiny amount of
water (DRH = 1.0) was injected into the chamber. A
humidity meter (HM34, The Vaisala Corporation) was
online to monitor the RH. As the moisture returned to the
initial environment, the microcantilever released the
bending and bounced back to its original position (Fig. 10).
Figure 10 shows that the new moisture sensor responds
immediately about 75 nm deflection when the humidity
increases to 1% and releases bending spontaneously with a
decrease in the humidity. The excellent real time
monitoring property gives the device a promising application in environments requiring high sensitivity.
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A high sensitivity guard-armed SiO2 cantilever-based
sensor platform was successfully designed, modeling-analyzed, micro-fabricated, and tested experimentally. A frontsided microfabrication process has been developed, which
was valuable for fabricating SiO2 cantilever beams and
releasing other suspending parts from the silicon substrate.
The new cantilever’s design and fabrication process solve
some major issues, which occurred in the conventional
design and fabrication, such as cantilever release, etching
mask endurance, and beam protection. Based on the
developed ICP isotropic release process, a special mask
was designed. The cantilever beam was completely
released with a 3 lm thickness photoresist layer and
20 min etching time by ICP plasma dry etching (46 mTorr
pressure, 1,800 W source power, substrate power 20 W,
300 sccm SF6 flow rate, and 20°C chuck temperature).
Both simulation and experimental results proved that the
SiO2 microcantilever can achieve higher sensitivity than
the equivalent Silicon microcantilever due to its lower
Young’s modulus. With specific modification on the surface of the sensing cantilever, the sensors have been shown
experimentally to detect 1% RH. It can be seen that using
the SiO2 microcantilever as a platform with appropriate
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Fig. 8 Measurement setup based on optical methodology
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sensing material is able to detect the RH much lower than
1%.
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