International Journal of Scientific & Engineering Research Volume 2, Issue 10, Oct-2011 1
ISSN 2229-5518
Optimization of Capacitive MEMS Pressure
Sensor for RF Telemetry
Prince Nagpal, Manish Mehta, Kamaljeet Rangra, Ravinder Aggarwal
Abstract — This paper describes the capacitive pressure sensor design for biomedical applications like blood pressure measurement. The described pressure sensors provide high sensitivity even at low pressure range suitable for biomedical applications. Effects of varying different parameters on the pressure sensor performance have been studied. From the results, the pressure sensors with compatible parameters can be selected for specific requirements. These compact pressure sensors are made up of biocompatible materials and can be implanted easily inside body to be used for RF telemetry purpose.
Key Words— MEMS, Capacitive, Diaphragm, Biocompatible, Biomedical
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ENSORS are one of the most active applications of MEMS. With the achievements in miniaturization of MEMS devices, sensor’s applications have expanded
not only for industry purpose but also for biomedical applications. Pressure sensors are one of the various types of sensors, where pressure is to be measured. One of the types of pressure sensor is capacitive pressure sensor. The capacitive pressure sensing technique’s per- formance in respect of sensitivity is better than rest all other microsensing techniques viz. piezoresistive micro- sensing, piezoelectric microsensing and resonant micro- sensing. And so is applicable to wide range of applica- tions like automotive applications, industrial applica- tions, biomedical applications and general & consumer applications. The capacitive microsensing technique utilizes the diaphragm-deformation-induced capacit- ance change to convert the information of pressure into electrical signals. The capacitance is computed in a 3D model using C = ε * ∫ (1/h) dA [1]
As MEMS technology is promising to achieve com- pact sizes, here we have optimized the capacitive pres- sure sensor to be implanted inside the body for blood pressure measurement [2] like applications. MEMS technology offers the possibility to realize devices of hundreds of micrometers and diaphragms of thickness 1 micrometer only. Therefore these capacitive pressure sensors are required to respond very less changes in applied pressure. Some of the desired features to these capacitive pressure sensors include high sensitivity at
low pressures, biocompatibility and low profile. Using contamination insensitive [3] manufacturing process, smooth contacts [4] and additional touch points [1] sen- sitivity of capacitive pressure can be further improved.
In this paper, effect of varying different paramaters of the capacitive pressure sensors for biomedical applica- tions in pressure range from 0 to twice of atmospheric pressure has been studied.
In this paper we demonstate three different possible geometries for capacitive pressure sensor are circular, square & rectangular. These pressure sensors are de- signed by having a silicon substrate, a silica bridge and a biocompatible diaphragm [5]. Details of the pressure sensors design and effect of varying geometry, surface area of diaphragm [6], diaphragm thickness, diaphragm dimensions and diaphragm material are described.
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Prince Nagpal is currently pursuing masters degree program in electron- ics & communication engineering in JCD college of Engineering from KUK,India,E-mail: princenagpal@gmail.com,princenagpal@rediffmail.com
Asst. Professor Manish Mehta from JCD college of Engineering is current-
ly pursuing Ph.D. from Thapar University, Patiala, India,Email id: ma-
Dr. Kamaljeet Rangra is currently working as a Scientist at CEERI, Pila-
ni, India
Dr. Ravinder Aggarwal is currently Professor at Thapar University, Pa-
tiala, India
Fig.1 Subdomains of circular capacitive pressure sensor
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ISSN 2229-5518
The design of the circular capacitive pressure sensor analysed is shown in figure. On an silicon substrate of radius 283 µm and thickness 500 µm, a silica bridge has been mounted of thickness 300 µm on substrate perimeter of 30 µm width. A thin silicon diaphragm of 2 µm and 253.885 µm radius is than placed on mounted bridge to create a vaccum compartment beneath it.
The pressure sensor’s performance with three differ- ent geometries is studied as shown in Fig. 2 in accor- dance with the following Table 1
TABLE 1
DIMENSIONS OF THE PRESSURE SENSOR
circular, square and rectangular geometry respectively. Here we can see that for the same surface area of diaph- ragm and silicon material used, the circular geometry performs best among all three, regarding sensitivity in the pressure range from 0 to 200kPa.
Fig. 3 Stress Distribution for Circular Geometry
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C irc ular
S qu are
R ec ta ng ular
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Fig. 4 Stress Distribution for Square Geometry
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142
0 50 100 150 200
Pre ssure ( kPa )
Fig. 2 Performance Graph of Capacitive Pressure Sensors
The Simulation is carried out by COMSOL Multi- physics 3.5 software, where capacitance is computed from the knowledge of deformation, induced by ap- plied pressure, using expression C = (ε * 4)/(50µm + W2). Fig. 2 shows the comparative change in capacit- ance as applied pressure is varied for three geometries circular, square and rectangular of same surface area. Fig.3, Fig.4 and Fig.5 shows the Stress distribution for
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Fig. 5 Stress Distribution for Rectangular Geometry
International Journal of Scientific & Engineering Research Volume 2, Issue 10, Oct-2011 3
ISSN 2229-5518
1 4 4. 6
1 4 4. 4
1 4 4. 2
1 4 4. 0
1 4 3. 8
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0 5 1 0 15 20
P ress ure (k Pa )
1 µm
2 µm
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5 µm
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S ili co n
T ita n iu m
S ta i n le s s S te e l
0 50 1 00 1 5 0 20 0
P re s su re (k P a )
Fig. 6 Performance graph of circular geometry for diaphragm thickness variation
Fig. 9 Performance graph of circular geometry for diaphragm material variation
14 4 .2
1 µ m
2 µ m
3 µ m
4 µ m
5 µ m
1 49 .0
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S ilic o n
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S ta in l e s s S t e e l
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P re s s u re (k P a )
1 44 .0
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0 5 0 1 00 15 0 2 00
P re s su re (k P a )
Fig. 7 Performance graph of square geometry for diaphragm thickness variation
Fig. 10 Performance graph of square geometry for diaphragm material variation
1 4 3. 70
1 4 3. 65
1 µ m
2 µ m
3 µ m
4 µ m
5 µ m
14 5 .4
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S ilic o n
T ita n iu m
S ta in le s s S te e l
1 4 3. 60
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1 4 3. 55
14 4 .4
14 4 .2
1 4 3. 50
1 4 3. 45
14 4 .0
14 3 .8
14 3 .6
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0 5 1 0 1 5 2 0
P re s s u re (k P a )
14 3 .4
14 3 .2
0 50 1 0 0 15 0 2 0 0
P re s s u re (k P a )
Fig. 8 Performance graph of rectangular geometry for diaph- ragm thickness variation
Fig. 11 Performance graph of rectangular geometry for diaph- ragm material variation
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International Journal of Scientific & Engineering Research Volume 2, Issue 10, Oct-2011 4
ISSN 2229-5518
3 10
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5 0 0 µm
6 0 0 µm
7 0 0 µm
0 50 1 0 0 15 0 2 0 0
P re s s u re (k P a )
Fig. 6, Fig. 7 and Fig. 8 shows the results of varying diaph- ragm’s thickness from 1 µm to 5 µm for all three geometries considered.
Fig. 9, Fig. 10 and Fig. 11 shows the results of capacitive pressure sensor performance for four different biocompatible diaphragm materials for all three geometries considered.
Fig. 12 and Fig. 13 show the results of capacitive pressure
sensor performance for different surface areas of circular and square geometry.
Fig. 14 shows the results of capacitive pressure sensor per- formance for different dimensions of rectangular geometry.
Fig. 12 Performance graph of circular geometry for diaphragm surface area variation
I specially thanks to CEERI, Pilani and Thapar Universi- ty, Patiala for providing their researches. Researches in [1], [2], [5] contributed great information for this paper.
3 30
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4 5 0 µ m
5 5 0 µ m
6 5 0 µ m
0 50 1 0 0 15 0 2 0 0
P r e s s u r e ( k P a )
Three different geometries of capacitive pressure sen- sor have been demonstrated. The capacitive pressure sensors are optimized regarding geometry, diaph- ragm material, diaphragm thickness, diaphragm sur- face area and diaphragm dimension ratio. The opti- mized sensors are suitable for biomedical applica- tions in the low pressure range.
[1] Haojie Lv, Qiang Guo, and Guoqing Hu, “A Touch Mode
Fig. 13 Performance graph of square geometry for diaphragm surface area variation
8 1 0 * 2 5 0 (µ m )
Capacitive Pressure Sensor With Long Linear Range and High Sensitivity”, In Proceedings of the 3rd IEEE Int. Conf. on Nano/Micro Engineered and Molecular Systems, pp.796-
800, January. 6-9, 2008.
[2] Darrin J. Young, “An RF-Powered Wireless Multi-Channel
Implantable Bio-Sensing Microsystem,” In 32nd Annual In-
1 54
1 52
1 50
1 48
1 46
1 44
1 42
6 7 5 * 3 1 0 (µ m )
5 0 0 * 4 0 5 (µ m )
0 50 1 0 0 15 0 2 0 0
P re s s u re (k P a )
ternational Conference of the IEEE EMBS Buenos Aires, pp.
6413-6416, Aug.31-Sep.4, 2010.
[3] C.C. Wang, B. P. Gogoi, D. J. Monk and C. H. Mastrangelo, “Contamination Insensitive Differential Capacitive Pressure Sensor”, IEEE, pp. 551-555, 2000.
[4] Jeahyeong Han and Mark A. Shannon, “Smooth Contact
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[5] Jeahyeong Han, Junghoon Yeom, Junghyun Lee, and Mark A. Shannon, “Smooth Contact Mode Capacitive Pressure Sensor with Polyimide Diaphragm”, Conference IEEE SEN- SORS, pp. 1468-1471, 2007.
Fig. 14 Performance graph of rectangular geometry for diaph-
ragm dimension ratio variation
[6] Norhayati Soin and Burhanuddin Yeop Majlis, “An Analytic Study on Diaphragm Behavior for Micromachined Capaci- tive Pressure Sensor”, in Proc. ICSE2002, pp. 502–505,
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