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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 24, NO. 4, AUGUST 2015
Optimization of Hot-Wire Airflow Sensors on an
Out-of-Plane Glass Bubble for 2-D Detection
Shuwei Liu, Shanshan Pan, Fei Xue, Lin Nay, Jianmin Miao, and Leslie K. Norford
Abstract— This paper presents design, analysis, fabrication,
and measurement of airflow sensors with three hot-wire resistors
on an out-of-plane glass bubble. The fabrication process is based
on etching cavities in silicon wafer, followed by anodic bonding of
a thin Pyrex glass wafer to the etched silicon wafer. The bonded
wafers are then heated inside a furnace at a temperature above
the softening point of the glass, and because of the expansion
of the trapped gas in the silicon cavities, the glass is blown
into three-dimensional (3-D) spherical glass bubbles. Resistors
patterned on the glass wafer above the cavities are elevated above
the base during the glass bubble blowing process. An optimization
analysis on the structure and geometry of the sensor, fabrication
process, and properties of multilayer thin-film resistors on glass
has been conducted in an attempt to improve the sensitivity.
[2014-0177]
Index Terms— Airflow sensor, fabrication, flow pattern, glass
bubble, sensor system, 3-D-MEMS.
I. I NTRODUCTION
F
LOW DETECTION is important in environmental
monitoring, since airflow characteristics in urban areas
affect pedestrian comfort, air quality, pollutant dispersion and
energy performance of buildings [1]. MEMS sensors can
reduce power consumption and have lower cost owing to the
nature of batch fabrication and small size. Micro scale airflow
sensors fabricated by MEMS technology are mainly based
on two principles, namely mechanical and thermal. Based on
a mechanical principle, microcantilevers can be employed as
sensing elements. The passage of airflow causes the cantilever
to deflect, leading to strain (stress) deformation detected by
the piezoresistive/piezoelectric mechanism [2], [3]. For sensors
based on the thermal principle, the measurement relies on the
detection of the convective heat transfer from an electrically
heated resistive sensing element to flowing fluid [4]. Without introducing any movable structure, sensors based on the
thermal principle are more commonly implemented because
of their fast response and robust and simple structure.
Manuscript received June 9, 2014; revised September 8, 2014; accepted
September 15, 2014. Date of publication October 13, 2014; date of current
version July 29, 2015. This work was supported by the Singapore National
Research Foundation through the Center for Environmental Sensing and Modeling, Singapore-Massachusetts Institute of Technology Alliance for Research
and Technology. Subject Editor P. M. Sarro.
S. Liu, S. Pan, F. Xue, L. Nay, and J. Miao are with the School of
Mechanical and Aerospace Engineering, Nanyang Technological University,
Singapore 639798 (e-mail: liushuwei8@gmail.com; pans0004@e.ntu.edu.sg;
xuefei@ntu.edu.sg; m070034@e.ntu.edu.sg; mjmmiao@ntu.edu.sg).
L. K. Norford is with the Massachusetts Institute of Technology, Cambridge,
MA 02139 USA (e-mail: lnorford@mit.edu).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2014.2360378
In order to detect airflow direction, the micro-resistor airflow sensors must have more than one sensing component to
establish a flow-dependent thermal gradient [5]. The majority
of micro-resistor airflow sensors for direction detection are
based on a calorimetric principle that employs more than four
sensing elements around a heating element to establish the
temperature gradient on a planar structure [6]. This needs
a very smooth and even surface of the sensor structure,
which requires a carefully designed packaging. Researchers
have reported that an out-of-plane airflow sensor structure has
manifested greater sensitivity than an in-plane sensor structure
by elevating the thermal element away from the bottom of the
speed boundary layer and therefore the thermal element is
exposed to greater fluid flow speed [7]. Therefore, our work
has focused on developing an airflow sensor for detection of
speed and direction based on an out-of-plane sensor structure.
We first presented a spherical sensor structure with four hotwire resistors [4], followed by work in which we obtained
high sensitivity for airflow speed and direction detection from
a cylindrical sensor structure with three manually-assembled
MEMS-based hot-wire resistors arranged 120 degrees apart
on the structure [8]. The flow direction measurement is
based on the relative output difference of the three sensing
elements in response to temperature variation induced by
airflow. To improve the assembly effectiveness, we propose
to fabricate three self-assembled micro hot-wire resistors on a
borosilicate glass bubble that provides good thermal isolation.
This paper is organized in five sections. Section II discusses
the design and analysis of sensors with micro hot-wire resistors
on an out-of-plane glass bubble structure, with focus on the
sensor design in Section II-A, the geometry of the sensor
base affecting the flow pattern in Section II-B, the design
of geometry of the metal-line resistors on the glass bubble
in Section II-C, and the non-uniform thickness of the
glass bubble and associated metal-line fracture analysis
in Section II-D. In Section III, fabrication results and discussion are presented. Section IV presents the airflow measurement results, followed by a conclusion in Section V.
II. D ESIGN AND A NALYSIS
A. Sensor Design
A glass bubble was previously introduced for the creation
of spherical cells for resonators [9] and micro-reactors [10].
We propose to employ this technique for the fabrication of
resistors on an out-of plane structure illustrated in Fig. 1.
Cavities with depth of 500 μm and diameter of 2 mm by
DRIE (deep reactive ion etching) are created in a 1 mm
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LIU et al.: OPTIMIZATION OF HOT-WIRE AIRFLOW SENSORS
941
Fig. 2. Model of airflow speed in front of the glass bubble on the square
base in CFD modeling at incident angles of (a) 0° and (b) 30°.
Fig. 1. (a) Micro hot-wire resistors on square base before glass blowing;
(b) micro hot-wire resistors on square base after glass blowing.
thick silicon wafer, followed by anodic bonding of a glass
wafer with thickness of 300 μm onto the etched silicon wafer.
Three serpentine-shaped resistors 120° apart are patterned
on the glass wafer above the cavities (Fig. 1(a)), with the
two ends connected to metal pads. After the dicing of the
bonded wafers, individual chips are heated inside a furnace
at a temperature above the softening point of 820 °C of the
Pyrex glass [11]. The resistors are then elevated while the
glass is blown into a bubble, as a result of the expansion of
the trapped gas in the silicon cavities (Fig. 1(b)). To avoid the
distortion of flow pattern, wire bonding bumps on the metal
pads should be away from the glass bubble. Therefore, the
side length of the square base is designed as 8 mm and the
metal pads are placed close to the edges of the base. Resistors
are designed with short and thin wire to reduce thermal mass,
and the dimension of the three resistors is designed the same
way for obtaining electrical resistance of 70 ohm. During
measurement, the heat transfer between the resistor and the
air relies on the speed of air flowing over the resistor. In
the following subsection, analysis will be carried out on three
geometric structure factors influencing the speed experienced
by the hot-wire resistor, namely the sensor base, the glass
bubble height and the position of the resistor on the glass
bubble.
B. Geometry of Sensor Base
At the measurement and calibration stage, sensors are
placed in a wind tunnel where the airflow direction is fixed
and speed can be varied. The sensor’s response to airflow
direction is characterized through measuring the outputs of
Fig. 3.
Airflow speed in front of the glass bubble at incident angles
of 0° and 30°.
resistors at different angles of incidence between the airflow
and the sensor by rotating the sensor in the wind tunnel. The
ideal measuring condition requires the flow pattern to remain
constant regardless of the incident angle to make sure that the
outputs reflect the change of airflow direction. However, the
axisymmetric glass bubble is resting on a non-axisymmetric
square base produced in the straight-line wafer dicing process
which could alter the flow pattern. To examine the influence
of the non-axisymmetric base, the airflow patterns around the
structure are modeled at incident angles of 0° and 30°, as
illustrated in Fig.2 (a) and (b), respectively. The incident angle
of 30° is formed by rotating the sensor clockwise by 30°. The
discrepancy between the airflow speed in front of the bubble
at incident angle of 0° and 30° is found in Fig. 3 for a glass
bubble of 300 μm in height and an input airflow speed U0
of 10m/s. This is caused by the immersion of the glass bubble
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 24, NO. 4, AUGUST 2015
Fig. 4. Cross-sectional view of flow pattern around high-dome and low-dome
glass bubbles.
in the boundary layer of the base, making the sensor very
sensitive to the different boundary layer properties caused by
the angle of incidence. In order to minimize the change of flow
pattern with the angle of incidence, one approach is to create
a high glass bubble and place the resistors away from the
bottom of the speed boundary layer. Another way is to change
the square base to an axisymmetric circular base by adding a
circular frame around the square base in the packaging. In the
following simulation, symmetrical boundaries are established
by building a 1mm thick circular base with diameter of 13mm
for the glass bubble.
Theoretically, the airflow sensor would be more sensitive if
the glass bubble could stick out of the speed boundary layer
because the flow speed, U , above the boundary layer is higher
and thus can create more distinct voltage change per unit of
input flow speed change. The simulation will test two models,
a low-dome (300 μm) glass bubble and a high-dome (1 mm)
glass bubble. The simulation is performed to study the flow
pattern around glass bubbles at different horizontal cut planes
where the resistor can be placed. Isothermal flow simulations
employing the k − ω turbulence model are performed [12].
The scaled residuals are less than 10−5 at convergence. Fig. 4
shows a cross-sectional view of flow patterns around the highdome and low-dome glass bubbles. Cut plane 1 corresponds to
a plane height of 100 μm in the low-dome glass bubble. Cut
planes 2 and 3 correspond to plane heights of 100 μm and
500 μm, respectively, in the high-dome glass bubble. Speed
variations with respect to the angle between the point on a cut
plane and the airflow direction are plotted in Fig. 5.
The variation of normalized speed (U /U0 ) at the 100 μm
cut plane of the low-dome glass bubble for U0 of 2.5m/s and
10m/s can be seen in Fig 5(a). Note that the magnitude of
speed around the glass bubble increases with input speed, and
the normalized speed increases with the input speed indicating
that the sensitivity of sensor will increase with the input
speed. Fig.5 (b) compares speeds at different cut planes of
the high-dome and the low dome glass bubbles. For the highdome glass bubble, the difference between the maximum and
minimum speed at the cut plane of 500 μm is 6.70 m/s, higher
than the lower cut plane of 100 μm and the cut plane of
the low-dome glass bubble. It can be concluded from the
simulation that higher glass bubbles with elevated resistor
Fig. 5. CFD modeling of the speed variation with respect to the angle
between the point on cut plane and input airflow direction: (a) the normalized
speed at 100 μm plane of the low-dome glass bubble for input speed U0
of 2.5m/s and 10m/s; (b) the speed at 500 μm plane and 100 μm plane on
the high dome and 100 μm plane on the low dome when U0 is 10m/s.
positions could potentially improve the sensing accuracy and
sensitivity. Hence attempts will be made to fabricate higher
glass bubbles with elevated micro-resistor positions.
C. Geometry of Resistor on Glass Bubble
The design guideline proposed in the previous section
specifies the height of the glass bubble and the height of the
resistors on the glass bubble sidewall. However, resistor metal
lines are patterned on the wafer before the blowing of glass
bubbles. As a result, the relationship between the locations of
the resistors on the 3D bubble and the locations of the resistors
on the 2D wafer plane needs to be established.
As illustrated in Fig. 6, point P with initial radius r on the
planar surface of the cavity shifts to P with height of h on
the glass bubble with height of h 0 . Assuming the formed glass
bubble is a symmetric structure, the wall of the glass bubble is
LIU et al.: OPTIMIZATION OF HOT-WIRE AIRFLOW SENSORS
Fig. 6.
943
Geometry of glass bubble.
Fig. 8.
Fig. 7. Height h of point P on glass bubble as a function of the initial
radius r with varied h 0 .
P
very thin and the thickness is uniform, the arc length l from
to the bottom and the arc length l0 from the top to the bottom
form the following relationship:
l
r0 − r
=
(1)
l0
r0
where r0 is the radius of circular cavity. In order to achieve
high structural symmetry and avoid surface tension induced
instability during glass blowing, h 0 ≤ r0 is desired [13], and
therefore an elliptical cross section is employed for estimation
of h. Because h 0 is comparable to r0 , l0 can be approximated
by the general equation [14]:
π h 20 + r02
(2)
l0 =
2
2
In Fig. 6, the arc length l is obtained from:
ϕ0 2 2
l=
+ h 0 cos ϕ dϕ
r0 sin ϕ
(3)
0
Note that ϕ0 =arctan(r0/h 0 tanθ ), and θ is the central angle
in the coordinate for the point P (r0 cos ϕ0 , h 0 sin ϕ0 ) on
the arc. By combining (1)-(3), ϕ0 can be derived. After
substituting ϕ0 into (4), h is developed as a function of h 0 ,
r0 , and r .
h = h 0 sin ϕ0
(4)
Fig. 7 plots the height h of point P as a function of
the initial radius r , for h 0 of 300 μm, 500 μm, 800 μm,
Geometry of glass bubble wall with non-uniform thickness.
Fig. 9. Relations between the thickness of glass bubble wall and initial
radius r with varied bubble height h 0 .
and 1000 μm and fixed r0 of 1000 μm. For example, if a
resistor with the length of 240 μm is to be placed at the
height approximately 400 μm to 700 μm on the glass bubble
of 1000 μm height, the metal line should be positioned from
r1 of 530 μm to r2 of 770 μm on the planer surface.
D. Non-Uniform Thickness of Glass Bubble
In practical case, under the influence of gravity, the viscous
nature of glass at softening temperature causes the glass to
drain to the side of structure [10], [15]. This results in a nonuniform thickness that gives rise to surface stress gradients that
stretch the metal lines at different levels and lead to possible
fracture and breakage of the metal lines. Meanwhile, P will
shift to a lower position than that predicted by Fig. 7, in which
the thickness of the bubble wall is assumed as uniform. This
will be proved in the fabrication section.
The non-uniform thickness of the glass bubble wall is
illustrated in Fig. 8. The relation is predicted by (5) in
reference [16], which is reported to provide a good match
with experimental value of thickness at the side of the glass
bubble [17]:
d = d0
r04 + r 2 h 20
r02 (r02 + h 20 )
2
(5)
where d0 is the initial thickness of glass wafer. Based on (5),
Fig. 9 plots the theoretical thickness of the glass bubble wall
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 24, NO. 4, AUGUST 2015
Fig. 10. Strain ε as a function of initial radius r with varied bubble height h 0 .
against the initial radius r with varied height of bubble h 0 .
As h 0 increases, the thickness of the bubble wall at the top
becomes much thinner than that at the base of glass bubble,
leading to potentially high stress gradient at the position close
to the top.
Assume the flow of softening glass is biaxial elongational
on the glass bubble surface, the true strain ε is ln(d/d0) [18].
By combining (5) with the true strain formula, ε is correlated
with the geometric parameters in (6):
ε = ln
r04 + r 2 h 20
2
r02 (r02 + h 20 )
(6)
Fig.10 plots strain ε with respect to initial radius r based
on (6) with varied h 0 . As a smaller r corresponds to a higher
position on the glass bubble, the strain level increases with the
height of position on the glass bubble surface and indicates that
metal lines at those positions suffer from larger stress and are
more likely to break.
The threshold strain level ε0 causing fracture of metal lines
is unique for each type of multi-layer thin films at certain
temperature. As the metal lines are attached to the glass
surface, the correlation between the fracture of metal lines and
the strain of viscous glass needs to be empirically analyzed
to estimate values of r and h 0 that ensure the intact form of
metal lines on the side of glass bubble. The initial radius r as
a function of h 0 based on ε0 is then derived as:
eε0 r04 + eε0 r02 h 20 − r04
(7)
r=
h 20
This method will be illustrated in the fabrication section.
III. FABRICATION
A. Fabrication Results
As discussed in Section II, higher glass bubbles with
elevated resistor positions could potentially improve the sensing accuracy and sensitivity. The main challenge of fabrication
is to form symmetrical glass bubble with desired height while
retaining intact and stable metal trace on the bubble surface
after stretching in the glass blowing step.
Fig. 11. Fabrication process. (a) Prepare 1mm thick <100> P-type silicon
wafer. (b) Create cavity by photolithography and DRIE process. (c) Distribute
TiH2 power in cavities. (d) Anodic bonding of Pyrex wafer and silicon wafer.
(e) Define patterns of metal line on photoresist via lift-off process. (f) Dice
bonded wafer. (g) Annealing at high temperature.
To form high-dome glass bubbles, TiH foaming agent is
first utilized in the glass blowing process, during which TiH
releases H2 in thermal decomposition at high temperature [10].
Two types of stacked thin-films have been used in the process
to fabricate metal-line resistors. One type is Ti (20 nm)/
Pt (200 nm)/Au (200 nm), noting that the structural material
of the resistor, Pt, has a high TCR (temperature coefficient
of resistance) of 3900 ppm/°C. The other type is Ti (20 nm)/
Cu (200 nm)/Au (200 nm) in which ductile Cu is selected
as the structural material. Ti layer acts as an adhesion layer
between the glass substrate and the other metal layers in the
deposition process and the Au layer as the top layer prevents
the oxidation and corrosion.
In Fig. 11 (a), four-inch silicon wafers with 1 mm thickness are cleaned in piranha solution (H2 SO4 + H2 O2 ) for
15 minutes to remove the organic residues and prepared for
fabrication. The DRIE process is then carried out to etch the
silicon to obtain cylindrical cavities with depth of 500 μm
and diameter of 2 mm (Fig. 11(b)) [19]. TiH2 powder of
2–5 microgram is distributed into each cavity for forming a
glass bubble with height around 1mm before anodic bonding
with a 300 μm thick glass wafer at a temperature of 400 °C
and atmospheric pressure of 1000 mbar (Fig. 11 (c) and (d)).
LIU et al.: OPTIMIZATION OF HOT-WIRE AIRFLOW SENSORS
945
After the wafers are bonded, the metal lines of the resistors
are lithographically defined on five micrometer thick AZ9260
photoresist layers above the cavities. Stacked thin-films are
deposited on the wafers. After the deposition of the metal
layers, the lift-off process is used to pattern the resistors
on the glass wafers (Fig. 11 (e)). A protective coating layer
of 10 μm thick photoresist is spin-coated on the surface to
protect the resistors before performing dicing. After dicing,
chips are then heated inside a furnace. A slow temperature
ramp rate or long annealing time induces the distortion of
geometrical shape of bubbles. Hence the ramp rate for the
annealing is set as 60 °C/min and temperature of 900 °C above
the softening point of the glass is maintained for 30 s. After
the annealing, rapid cooling of the bubble (approximately less
than 30 sec) is achieved by turning off the heating current
and opening the horizontal split tube furnace immediately to
freeze the expected shape of bubbles. Once the temperature
of the glass reaches below its softening temperature, the
differential pressure across the glass bubble is negligible to
deform it. As a result of the expansion of generated H2 gas,
the glass is blown into glass bubbles with the resistors on the
sidewall (Fig. 11 (g)).
B. Discussion
With a foaming agent, glass bubbles with height around
1 mm can be easily formed. However, asymmetry of structures
has been found on glass bubbles with height more than 1.5 mm
and this phenomenon is in accordance with the prediction
in [13] that if the glass bubble structure is blown beyond,
h 0 > r0 , the drastically decreased surface tension forces
aggravate the effect of any perturbation on the geometric
shape, leading to instability within the blown glass structure.
Therefore, in order to consistently fabricate symmetric and
stable bubble structures for cavities with radius r0 of 1mm,
the optimal height of bubble should be controlled to be less
than 1mm.
Fig. 12(a) shows a glass bubble with height of 1480 μm and
Ti/Pt/Au resistor metal lines spanning a height from 317 μm
to 643 μm, which is lower than the predicted position from
460 μm to 880 μm when the thickness of the bubble wall
is assumed to be uniform. Discontinuity and deformation
are observed from the metal line trace on the glass bubble.
When the height of bubble is reduced to 634 μm, fractures
are eliminated but deformation of metal trace is still present
in Fig. 12(b).
By applying the methodology of analyzing the fracture of
metal thin-film and glass strain level in Section II-D, the
threshold strain level ε0 causing fracture is extrapolated in
Figure 13. In this work, the corresponding ε0 is −1.2 for
the Ti/Pt/Cu multi-layer thin film. Based on (7), Fig. 14 plots
fracture/breakage of Ti/Pt/Au metal line fabricated in this work
by considering structural parameters r and h 0.
The poor adhesion of Ti/Pt/Au metal lines to the glass
surface could be explained by the thermally induced residual
stress due to the different coefficient of temperature expansion
(CTE) between the multi-layer thin film and the glass base.
Table I lists the material properties of the metal [20], and
Fig. 12. SEM images of Ti/Pt/Au multi-layer thin film on the glass bubble:
(a) bubble with height of 1.48mm; (b) bubble with height of 634 μm.
Fig. 13. Extrapolated threshold strain level ε0 based on measured fracture
points of Ti-Pt-Au on bubbles with varied heights.
shows that Pt has a larger CTE than glass. Since the Ti layer is
very thin and diffuses into the Pt layer in the annealing process,
Pt plays a dominant role in the thermal expansion match.
As sensors are employed to detect airflow, poor adhesion
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 24, NO. 4, AUGUST 2015
Fig. 16.
Fig. 14.
Estimation of fracture/breakage of Ti/Pt/Au metal line.
TABLE I
TABLE OF M ATERIAL P ROPERTIES
Principle of detection of airflow speed and direction.
Cu, resulting in an exponential decrease of CTE of Cu with
increasing temperature near its melting point [21]. Therefore,
although Pt potentially provides high sensitivity, Cu is chosen
as the structural material for the hot-wire airflow sensors with
out-of-plane bubble structure.
IV. M EASUREMENT AND R ESULTS
Fig. 15. SEM images of Ti/Cu/Au multi-layer thin film on the glass bubble.
between the thin-films and the glass base could lead to
detachment or peel-off of the thin-films. It should be noted that
although the CTE of Cu is much higher than glass, a metal
line of Ti/Cu/Au thin film in Fig. 15 shows good adhesion
to the glass base. This is because the annealing is conducted
at 900 degree that is very close to the melting temperature of
A sensor with Ti/Cu/Au resistors with height of 200 μm on
a glass bubble with an approximate height of 500 μm is tested.
The measured resistances of the three resistors are 66.9 ,
62.0 and 51.3 wit average TCR of 1033 ppm/°C between
25 °C and 100 °C. The principle of detection of airflow speed
and direction is illustrated in Fig.16. A detailed description of
the principle can be found in [8]. Each resistor is connected
to a constant temperature anemometry (CTA) circuit channel
based on a Wheatstone bridge. Constant temperature means
that the circuit actively keeps the resistance of each resistor
constant by a closed-loop feedback. The voltage outputs,
Vo1, Vo2 and Vo3 are processed by an algorithm, and then
the corresponding airflow speed and direction are derived.
To generate the algorithm, sensors need to be calibrated by
conducting measurement in wind tunnel and voltage outputs
are measured at different speeds and directions.
During the calibration, the sensor chip is placed on the top
of a pillar, which is mounted on a rotary station for flow
direction characterization. The sensor is positioned in a wind
tunnel such that the airflow is parallel to the sensor surface.
The position at which hot-wire resistor 1 is facing the airflow
direction is marked as 0°. Flow direction characteristics are
analyzed as the supporting pillar is rotated in the wind tunnel.
Fig. 17 compares the voltage output from channel 1 under
two conditions: the sensor with square base (left) and the
sensor with circular base (right), for which an acrylic circular
frame with thickness of approximately 1 mm is added around
the square base to make the base axisymmetric. An initial
voltage of 1900 mV is applied to channel 1. When the sensor
voltages are measured for a wind-tunnel airflow speed of
10 m/s, unexpected valleys appear in the voltage profile of
sensor with square base. Errors are introduced due to the
change of flow pattern when change incident angle in the
course of rotation (discussed and illustrated in Section II-B).
The distorted output voltage with unexpected valleys in the
LIU et al.: OPTIMIZATION OF HOT-WIRE AIRFLOW SENSORS
947
by the result tested at an airflow speed of 2.5m/s in Fig. 17.
This is because in the experiments, the surface features on the
base, such as soldering bumps and connecting wires, as well
as the resistors are submerged in the airflow boundary layer.
The flow is inevitably disturbed by those surface features,
which generate noise in the output signal. As the speed
around the glass bubble decreases, noise arising from the
disturbance becomes comparable to the output signal, leading
to an inconsistency of output characteristic at low airflow
speed and resulting in difficulties in conducting calibration
and generating algorithm for low airflow speed and direction
detection at this stage. In the next step to further optimize
the sensor on an out-of-plane bubble structure, the effort will
be placed on reducing the base area and elevating resistors
to a higher position where the noise generated by the surface
feature on the base is minimized.
V. C ONCLUSION
Fig. 17. Comparison of measuredvoltage output from channel 1 at speeds
of 2.5 m/s and 10m/s when square base and circular base was used,
respectively.
A hot-wire airflow sensor with a novel out-of-plane
glass bubble structure is introduced in this paper. As highlighted in the simulation and verified in the measurement,
non-axisymmetric square bases produced in the wafer
dicing process introduce errors in the measurement and those
errors can be corrected by converting the square base into
an axisymmetric circular shape. Based on the analysis of the
dimension of glass bubbles and positions of hot-wire resistors
on glass bubbles for the purpose of increasing sensitivity,
the optimization of fabrication process highlights fabricating
elevated thin-film metal wires on glass bubbles with a high
dome under the condition of mechanical and thermal stress at
high temperature. Measurement in initial steps demonstrates
the capability of micro hot-wire resistors on a glass bubble in
detecting airflow and presents preliminary measurement results
that provide inspiration for further design and fabrication of
self-assembled micro hot-wire resistors on an out-of-plane
glass structure for detecting airflow speed and direction.
Fig. 18. Measured maximum voltage change V0 as a result of direction
change when two wind speeds 5m/s and 10m/s are applied.
R EFERENCES
curve has been corrected and a good match is observed
between the voltage change and the simulated speed variation
for a glass bubble on a circular base with a smooth surface.
This indicates that micro hot-wire resistors on the glass bubble
can capture the speed difference around the glass bubble and
therefore the three channels will output direction-dependent
voltages on which an algorithm for generating airflow direction
relies. As indicated in Fig. 17, the maximum voltage difference
V0 as a result of direction difference has been compared
with that obtained from another sensor with resistors at height
of 330 μm in Fig. 18. It has been proven that resistors
at elevated position provide higher voltage output at varied
airflow speed, thus increasing sensitivity.
However, in the experiments, we observe a discrepancy
between the corrected voltage output and the simulated speed
when the airflow speed is lower than 10m/s. This discrepancy
increases with the decrease of the airflow speed, illustrated
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Shuwei Liu received the B.S. degree in electronics engineering from the University of Electronic
Science and Technology, Chengdu, China, in 2005;
the M.S. degree in electronics engineering from
Shanghai Jiao Tong University, Shanghai, China, in
2008; and the Ph.D. degree from the School of
Mechanical and Aerospace Engineering, Nanyang
Technological University (NTU), Singapore. She is
currently a member of the research staff with NTU
the Singapore-MIT and Alliance for Research and
Technology Centre, Singapore. Her research interests include microelectromechanical systems, microfabrication, sensors and
systems for environmental monitoring, and energy harvesting.
Shanshan Pan received the B.Eng. degree in
environmental engineering from Nanyang Technological University, Singapore, in 2010, where she
is currently with the School of Mechanical and
Aerospace Engineering, and is currently pursuing the
Ph.D. degree with the Singapore-MIT Alliance
Program, with a focus on urban airflow. Her current
interests include airflow sensor applications.
Fei Xue received the B.S. degree in automation from
the Nanjing University of Posts and Telecommunications, Nanjing, China, in 2008, and the M.S. degree
in engineering and technology from Northumbria University, Newcastle, U.K., in 2011. From
2011 to 2012, he was a Research Assistant with
the Shenzhen Institutes of Advanced Technology,
Chinese Academy of Sciences, Shenzhen, China.
He is currently a Research Engineer with the School
of Mechanical and Aerospace Engineering, Nanyang
Technological University (NTU), and the Intelligent
Systems Centre, Singapore. The Intelligent Systems Centre is an applied
research center jointly set up by the Singapore Technologies Engineering
and NTU.
His research interests include wireless sensor network, embedded operating
system, microelectromechanical systems signal conditioning, computer vision,
and data management.
Lin Nay received the M.Sc. degrees in microelectromechanical systems (MEMS) from École
Supérieure d’Ingénieurs en Électrotechnique et Électronique, Paris, France, and Nanyang Technological University (NTU), Singapore, in 2006, and the
Ph.D. degree from NTU in 2014.
His research concentrated on analysis and design
of MEMS, thermal management of electronics and
MEMS components, and CNT-based MEMS and
nanoelectromechanical systems. He is currently a
Research Associate with the Mechanobiology Institute, National University of Singapore, Singapore. His current research interests include nanofabrication for biological applications and TSV interconnect
technologies.
Jianmin Miao received the Dipl.-Ing. and Dr.Ing.
(Hons.) degrees in microelectromechanical systems
(MEMS) from the Darmstadt University of Technology, Darmstadt, Germany. After several years
spent in the industry for sensor/MEMS development,
he joined the Nanyang Technological University,
Singapore, in 1998, to establish the Micromachines
Centre as the Founding Director, where he is currently a tenured faculty member with the School
of Mechanical and Aerospace. He has collaborated
with the MIT faculty at the Singapore-MIT Alliance
for Research and Technology, Singapore, since 2008. He has authored or
co-authored over 350 papers in journals and conferences, several books
and book chapters, and holds 15 patents. He is an Editorial Board Member of the Journal of Micromechanics and Microengineering, and was the
Chair and Co-Chair of the MEMS/nanotechnology international conferences,
and a Technical Committee Member of international conferences, including
the IEEE International Conference on Micro Electro Mechanical Systems
and the International Conference on Solid-State Sensors, Actuators, and
Microsystems. He was invited by several international MEMS/nanotechnology
conferences as a plenary speaker, a keynote lecturer, and an invited talker.
Leslie K. Norford received the Ph.D. degree
in mechanical and aerospace engineering from
Princeton University, Princeton, NJ, USA, in 1984.
He is currently a Professor of Building Technology
with the Department of Architecture, Massachusetts
Institute of Technology, Cambridge, MA, USA, and
the Lead Investigator of the Center for Environmental Studies, an interdisciplinary research group
in the Singapore-MIT Alliance for Research and
Technology, Singapore. His current research interests include monitoring performance of mechanical
and electrical equipment in buildings, optimization techniques as applied
to design and operation of buildings and their mechanical systems, and
measurements and simulations of the interaction of buildings with urban
environments and electricity grids.