The purpose of this paper is to apply characteristics of residual stress that
causes cantilever beams to bend for manufacturing a micro-structured gas flow
sensor. This study uses a silicon wafer deposited silicon nitride layers,
reassembled the gas flow sensor with four cantilever beams that perpendicular
to each other and manufactured piezoresistive structure on each
micro-cantilever by MEMS technologies, respectively. When the cantilever beams
are formed after etching the silicon wafer, it bends up a little due to the
released residual stress induced in the previous fabrication process. As air
flows through the sensor upstream and downstream beam deformation was made,
thus the airflow direction can be determined through comparing the resistance
variation between different cantilever beams. The flow rate can also be
measured by calculating the total resistance variations on the four
cantilevers.Comment: Submitted on behalf of EDA Publishing Association
  (http://irevues.inist.fr/handle/2042/16838

Chou, P. -Ch.

Fu, Lung-Ming

Hsueh, Tzu-Han

Lee, Chia-Yen

Ma, Rong-Hua

Tsai, Chien-Hsiung

Wang, Y. -H.

English

arXiv

Abstract- The purpose of this paper is to apply characteristics of residual stress that causes cantilever beams to bend for manufacturing a micro-structured gas flow sensor. This study uses a silicon wafer deposited silicon nitride layers, reassembled the gas flow sensor with four cantilever beams that perpendicular to each other and manufactured piezoresistive structure on each micro-cantilever by MEMS technologies, respectively. When the cantilever beams are formed after etching the silicon wafer, it bends up a little due to the released residual stress induced in the previous fabrication process. As air flows through the sensor upstream and downstream beam deformation was made, thus the airflow direction can be determined through comparing the resistance variation between different cantilever beams. The flow rate can also be measured by calculating the total resistance variations on the four cantilevers. I

Yu-hsiang Wang

Tzu-han Hsueh

Rong-hua Ma

Chia-yen Lee

Lung-ming Fu

Po-cheng Chou

Chien-hsiung Tsai

CiteSeerX

A Microcantilever-based Gas Flow Sensor for Flow Rate and Direction Detection

The purpose of this paper is to apply characteristics of residual stress that causes cantilever beams to bend for manufacturing a micro-structured gas flow sensor. This study uses a silicon wafer deposited silicon nitride layers, reassembled the gas flow sensor with four cantilever beams that perpendicular to each other and manufactured piezoresistive structure on each micro-cantilever by MEMS technologies, respectively. When the cantilever beams are formed after etching the silicon wafer, it bends up a little due to the released residual stress induced in the previous fabrication process. As air flows through the sensor upstream and downstream beam deformation was made, thus the airflow direction can be determined through comparing the resistance variation between different cantilever beams. The flow rate can also be measured by calculating the total resistance variations on the four cantilevers

Wang, Y.-H.

Chou, P.-Ch.

I-Revues

9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
A Microcantilever-based Gas Flow Sensor for Flow 
Rate and Direction Detection 
  
Yu-Hsiang Wang1, Tzu-Han Hsueh1, Rong-Hua Ma2, Chia-Yen Lee1*,  
Lung-Ming Fu3, Po-Cheng Chou4, Chien-Hsiung Tsai5 
1*Department of Mechanical and Automation Engineering, Da-Yeh University, Changhua, Taiwan. 
2Department of Mechanical Engineering, Chinese Military Academy, Kaohsiung, Taiwan. 
3Department of Materials Engineering, National Pingtung University of Science and Technology, Pingtung, Taiwan. 
4Department of Interior Design, Shu-Te University of Science and Technology, Kaohsiung, Taiwan 
5Department of Vehicle Engineering, National Pingtung University of Science and Technology, Pingtung, Taiwan. 
 
 
Abstract- The purpose of this paper is to apply characteristics 
of residual stress that causes cantilever beams to bend for 
manufacturing a micro-structured gas flow sensor. This study 
uses a silicon wafer deposited silicon nitride layers, reassembled 
the gas flow sensor with four cantilever beams that 
perpendicular to each other and manufactured piezoresistive 
structure on each micro-cantilever by MEMS technologies, 
respectively. When the cantilever beams are formed after 
etching the silicon wafer, it bends up a little due to the released 
residual stress induced in the previous fabrication process. As 
air flows through the sensor upstream and downstream beam 
deformation was made, thus the airflow direction can be 
determined through comparing the resistance variation between 
different cantilever beams. The flow rate can also be measured 
by calculating the total resistance variations on the four 
cantilevers. 
  
I.  INTRODUCTION 
Flow measurement is a necessary task in such diverse fields 
as medical instrumentation, process control, environmental 
monitoring, and so forth. Many previous studies have 
demonstrated the successful applications of MEMS 
techniques to the fabrication of a variety of flow sensors 
capable of detecting both the flow rate and the flow direction 
[1-4]. Broadly speaking, MEMS-based gas flow sensors can 
be categorized as either thermal or non-thermal, depending 
upon their mode of operation. 
 Many thermally-actuated gas flow sensors apply some 
forms of resistors to evaluate local temperature changes [1-3]. 
In such sensors, the temperature difference between the 
sensing resistors varies as the gas flow passes through the 
sensor, and the comparison of the resistance variations among 
each resistors enables the gas flow direction to be determined. 
Adamec et al. [1] fabricated a multi-axis hotwire anemometer 
with four thermoresistor to evaluate flow direction with a 
power consumption of 25 mW. Kim et al. [2] presented a 
circular-type thermal flow sensor consists of a heater and 
circular-type temperature sensors to detect flow direction and 
velocity. However, the power consumption of this device 
exceeded 80 mW. Recently, non-thermal gas flow meters 
have been developed. These devices typically have the 
advantages of lower power consumption and an improved 
potential for integration with other sensors. Ozaki [4] 
fabricated an air flow sensor modeled on windreceptor hairs 
of insects which can detect low velocity of air flow, but the 
maximum measurable flow rate was just 2 ms-1. The 
practicality of the device for flow rate sensing applications 
was severely limited. Wang et al. [5-7] fabricated an air flow 
sensor with a free-standing microcantilever structure which 
has a high sensitivity (0.0284 Ω/ms-1) and a high velocity 
measurement limit (45 ms-1). In the current study, four 
cantilever beams are formed and the measured values of four 
piezoresistors on them are compared to determined the 
airflow direction. The flow rate can also be measured by 
calculating the total resistance variations on the four 
cantilevers. 
II.  DESIGN AND FABRICATION 
A.  Shape Design  
As shown in Fig. 1, the electrodes used to connect the 
piezoresistor to the external LCR meter are fabricated from 
gold since gold has a lower resistivity than platinum, and 
hence the resistance effect of the leads is reduced. The 
resistance R of the piezoresistor is given by 
                           
wt
l
A
l
R RR
ρρ
==                                   (1) 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1 Schematic illustration of gas flow sensor. 
 
where lR is the resistor length, ρ is the resistivity, A is the 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
cross-sectional area, w is the width and t is thickness. The 
derivative of (1) is 
                           
t
dt
w
dw
l
dld
R
dR
R
R −−+=
ρ
ρ              (2) 
where defines 
R
R
l l
dl
=ε , ldl ∆= , wdw ∆=  and tdt ∆= ,  
then it could be rearranged to )21( vR
dR
l
+≈
ε
                          
(3) 
As shown in Fig. 2, when air flows over the cantilever 
structure, the beams are deflected that causing a change in the 
cross-sectional area, and hence the resistance of the platinum 
resistor changes. The corresponding airflow velocity and 
direction can then be evaluated and determined simply by 
measuring the resistance change using an external LCR 
meter.  
B.  Shape Design 
Fig. 3 indicates the configuration and dimensions of the 
cantilever beams and platinum resistors considered in this 
study. In Fig. 3(a), the periphery of the black line indicates the 
cantilever beams. In Fig. 3(b) the black netted area 
corresponds to the platinum resistors. All units are µm. As 
shown in the figures, the gas flow sensor is assembled with 
four cantilever beams that perpendicular to each other and 
manufactured with piezoresistors on each micro-cantilever. 
C.  Fabrication  
Fig. 4 presents an overview of the fabrication process used 
for the current micromachined gas flow sensor. Initially, a 
low-stress nitride layer of thickness 1.0 µm was deposited on 
either side of a 500 µm thick double-side-polished silicon 
wafer. A thin layer of Cr (0.03 µm) was then deposited on the 
upper surface of the wafer to form an adhesion layer for the 
subsequent deposition of the platinum resistors. The platinum 
resistors (0.1µm thickness) were deposited using an electron 
-beam evaporation process. The same technique was then 
used to deposit a layer of Au (0.1µm thickness) on the 
resistors to serve as an electrode and to provide electrical 
leads. To form the micro-cantilever beams, the cantilever 
structure and a back-etching nitride mask were patterned in 
SF6 RIE plasma. In the etching process, the cantilever 
structure was released in a KOH etchant (40 wt %, 85 oC, 
supplied by J. T. Baker). Following the etching process, the 
total micro-cantilever beam thickness was 20 µm (nitride: 
1µm / silicon: 19 µm) and the cantilever beams exhibited an 
upward bending effect as a result of the stress relaxation 
caused by the thermal stress induced in the heating and  
 
 
 
 
 
 
 
 
 
Figure 2 Diagram of gas flow sensor during sensing operation. 
 
 
 
 
 
 
 
 
 
 
 
 
 
(a) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 (b) 
Figure 3 Configuration and dimensions of (a) cantilever beams and (b) 
platinum resistors.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4 Overview of fabrication process employed for proposed flow sensor. 
 
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
10 15 20 25 30 35
Airflow velocity (ms-1)
To
ta
l r
es
ist
an
ce
 v
ar
ia
tio
n 
(Ω
)
Airflow angle = 135°
Airflow angle = 180°
 
 
 
 
 
 
 
 
 
 
 
Figure 5 SEM image of gas flow sensor.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6 The relative position between the initial air flow direction and the 
sensor. 
 
cooling steps performed before etching. This phenomenon 
causes an initial beam deformation position which assists the 
flow rate sensing function. To increase the reliability of the 
micro-cantilever, a small thickness of silicon was retained to 
act as an understructure. Fig. 5 presents a scanning electron 
microscope (SEM) image of a typical freestanding structure 
fabricated in this study. 
III.  RESULTS AND DISCUSSION 
A systematic investigation was conducted into the 
performance of the fabricated micro gas flow sensors. The 
characterization of the sensors was carried out with a rotary 
table (LC-PR60, TanLian E-O Co., Ltd., Taiwan) in a wind 
tunnel, and the relative position between the air flow direction 
and the sensor is shown in Fig. 6. The variation in the sensor 
resistance as the airflow passed over the cantilever was 
measured using an LCR meter (WK4230, Wayne Kerr 
Electronics Ltd., Taiwan). For reference purposes, a Pitot 
tube flow meter was also used to measure the velocity of the 
airflow. 
A.  Flow Rate  
For the of measurement of the flow rate in the wind tunnel,  
the relationship between the sum of the absolute value of 
resistance variation in all the four piezoresistors on the beams 
and flow rate is shown in Fig. 7. It shows that as the airflow 
direction was at 180 o or 135 o direction, the sum of the 
absolute values of the resistance variation was equal and it 
just depended on the flow rate. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 7 The relationship between the airflow velocity and the total 
resistance variation when airflow direction was 135 o, and 180 o. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8 The relationship between the airflow angle and the resistance 
variation when airflow velocity was (a)15 ms-1 [○], (b)20 ms-1 [∆], (c)25 
ms-1 [□] and (d) 30 ms-1 [◊ ].  
 
B.  Flow Direction  
For the measurement of the flow direction, a strong 
dependence of the measurement was found on the 
geometrical setup of the sensor axes. It is necessary to assure 
the cantilevers precise location with the angle 90o. As the 
airflow pass through the gas flow sensor from 180 o, the 
largest resistance variation was caused by the downwind 
cantilever deformation, and the least resistance variation was 
caused by upwind cantilever deformation. The resistance 
variation of cantilever beams that perpendicular to the airflow 
direction are almost equal to zero. The measured values at 
different flow rate, 30, 25, 20 and 15 m/s, are shown in Fig. 8. 
IV.  CONCLUSIONS 
The current study develops a MEMS-based gas flow sensor 
featuring a free-standing micro-cantilever structure. In the 
sensing operation, the air flow velocity is detected by 
measuring the change in resistance of the piezoelectric 
resistors deposited on the cantilever beams as the beam 
deforms under the effect of the passing airflow and the 
airflow direction can be obtained by comparing the resistance 
variation difference between the upstream and downstream 
cantilever beams to evaluate the airflow angle. 
ACKNOWLEDGMENT 
-3
-2
-1
0
1
2
3
-180 -135 -90 -45 0 45 90 135 180
Airflow angle (deg)
R
es
ist
an
ce
 v
ar
ia
tio
n 
(Ω
)
30 m/s
25 m/s
20 m/s
15 m/s
0o180o
90o
270o
45o135o
225o 315o
9-11 April 2008 
©EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
The authors would like to thank the financial support 
provided by the National Science Council in Taiwan (NSC 
96-2221-E-212-037 and NSC 95-2218-E-006-022).  
REFERENCES 
[1]  R. Adamec, D. V. Thiel, P. Tanner; "MEMS wind direction 
detection: from design to operation", Proc. Institute of Electrical and 
Electronics Engineers Sensors Conference, Toronto, pp. 340-343, 
2003. 
[2]  S. Kim, T. Nam, S. Park, “Measurement of flow direction and 
velocity using a micromachined flow sensor”, Sensors and Actuators 
A. 114, 312–318, 2004. 
[3]  K. A. A. Makinwa, J. H. Huijsing, “Constant power operation of a 
two-dimensional flow sensor using thermal sigma–delta modulation 
techniques”, in: Proc. IEEE-IMTC, Budapest, pp. 1577–1580, 2002.  
[4]  Y. Ozaki, T. Ohyama, T. Yasuda and I. Shimoyama, “An air flow 
sensor modeled on wind receptor hairs of insects”. MEMS 2000 , pp. 
531–536, 2000.  
[5] Y. H. Wang, C. Y. Lee, C. M. Chiang, “A MEMS-based Air Flow 
Sensor with a Free-standing Microcantilever Structure”, Sensors, 7, 
pp. 2389-2401, 2007. 
[6]  R. H. Ma, M. C. Ho, C. Y. Lee, Y. H. Wang, L. M. Fu, 
“Micromachined Silicon Cantilever Paddle for High-Flow-Rate 
Sensing”, Sensors and Materials, Vol. 18, No. 8, pp. 405-417, 2006. 
[7]  R. H. Ma, C. Y. Lee, Y. H. Wang, H. J. Chen, 
“Microcantilever-based weather station for temperature, humidity 
and flow rate measurement”, Microsyst. Technol, in press, 2008. 
 


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A Microcantilever-based Gas Flow Sensor for Flow Rate and Direction
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A Microcantilever-based Gas Flow Sensor for Flow Rate and Direction Detection

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