A Capacitance Level Sensor Design and Sensor Signal Enhancement
Proceedings of the 2011 6th IEEE International Conference on Nano/Micro Engineered and Molecular Systems February 20-23, 2011, Kaohsiung, Taiwan
A Capacitance Level Sensor Design and Sensor Signal Enhancement Yu-Ting Li, Chih-Ming Chao, Kerwin Wang Department of Mechatronics Engineering, National Changhua University of Education, Taiwan E-mail: [email protected]
length and the side area of electrodes, one can improve fringing field effect and sensitivity. The design improvement process of fringing field sensor relies on a good understanding of the physics principles, numerical simulation tools or analytical methods [9, 10]. In general, design variables include the geometry and the material properties of electrodes [11]. In order to enhance sensor sensitivity, this paper also investigates the influence of hydrophilicity of the electrode surface on capacitive measurement. An atmospheric-pressure nitrogen-plasma surface treatment method has been introduced for electrode surface modification. This method is widely adopted by many industries for surface cleaning and modification [12-14].
Abstract—This paper introduces a novel fringe field capacitive sensor. To enhance the sensor signal, various sensor electrode design strategies and a surface modification method are introduced, discussed and compared. The sensors are characterized by both simulations and experiments. The experiment results demonstrate the sensor capability of water level measuring. In addition, fringing field sensor of the field strength across the target by the dielectric strength can be used to measure the dielectric constant or component of a solution. The experiment results show that nitrogen plasma surface modification can improve the sensitivity from 69pF/mm to 97pF/mm. Keywords: Fringing electric field, atmospheric-pressre plasma, level sensor.
I.
capacitance
sensor,
In following sections, the electrode geometry design, simulation analysis, fabrication, measurement results, and a related application to level sensing are presented.
INTRODUCTION
Capacitive sensors are capable of measuring various physical parameters. They are widely used in industrial, biomedical, and instrumental electronic devices, such as proximity sensors, pressure sensors, accelerometers, displacement sensors, water level sensors, torque sensors, etc[1-5]. In general, capacitive sensors consist of two parallel electrodes and dielectric material, when the electrode overlapping area, distance, or dielectric constant is affected by temperature, humidity or stress, one can take advantage on the changing of capacitance to measure these physic parameters. The widely used applications include hygrometer, manometer accelerometers, and etc... Capacitive sensor is different from other types of sensors, such as resistive, electromagnetic, or stress sensors, it provides non-contact and low energy consumption measurement[1]. Most of the capacitive sensors have fringing filed effect, based on the consideration of linearity and sensitivity; people utilize less fringing field effect to detect external physical parameters. The first fringing effect capacitive sensor consists of two electrodes placed in the middle insulator[6]. This simple electrode design provides highly sensitive fringing field measurement capability.
II. DESIGN AND SIMULATION The optimal design of a fringe filed capacitive sensor depends on the meshed electrode pore size and electrode thickness being considered. For the determination of the pore size and density, a numerical simulation include all these variables, has to be studied. A two dimensional COMSOL simulation results suggest that increasing the effective edge length of these electrodes can promote fringing field effect. For a given electrode area, increasing the fractal dimension can actually extend the effective electrode edge length of electrodes to enhance the fringing field effects. (a)
(b)
Aluminum Oxide
70μm
When the device operates at high frequency, fringing field capacitance have non-negligible influence to circuit, in terms of signal process, succession of other scholars developed employed micro-fabrication process to manufacture fringing capacitive sensors[7, 8]. If the overlapping area between two electrodes is reduced, it will reduce parallel capacitance; conversely, increasing the edge
978-1-61284-777-1/11/$26.00 ©2011 IEEE
40μm
Silicon
(c)
(d)
(e)
(f)
Figure. 1. Schematic diagram of various electrode designs
888
pore densities are 2209 pores/mm2 and 1019 pores/mm2, for sensor Type I and sensor Type II, respectively. The fill factor of the top electrode for sensor Type I is 15% smaller than sensor Type II, however, the effective electrode edge length of Type I is twice longer than Type II, therefore, sensor Type I has greater fringing effect.
As shown in Fig.1 (a-f), six electrode number arrangements, 1, 2, 4, 8, 16, and 32, respectively are simulated. In the two dimensional simulation, the total length of the top electrode is held at 40μm, the dielectric thickness is 0.3μm, the total length of the bottom electrode is 70μm. To increase the number of electrodes in a finite design space, it can effectively enhance the fringing field capacitance. The simulation result is shown in Fig. 2.
Master pore
Capacitance (nF)
6.0 5.7
80 or 150μm pores
5.4 5.1
10μm mesh holes
4.8 4.5 0
5
10
15
20
25
30
35
numbers of electrodes Figure. 2. Different kinds of electrode designs and their capacitances. Aluminum electrode
Based on the outcomes from simulation results, to verify this design approach, two different kinds of pore sizes and densities for capacitive sensor are designed and fabricated, which provide two effective electrode edge lengths to enhance the fringing capacitance. The capacitance sensor parameters are listed in Table 1. The master pore width of sensor Type I and sensor Type II are 80μm and 150μm, respectively.
Oxide
P-type single crystal silicon
TABLE 1 SPECIFICATIONS OF THE CAPACITANCE LEVEL SENSOR Type I
Type II
Wafer thickness
540μm
Silicon lattice direction
(100)
Resistance
25.2~42.5(ohm-cm)
Doping element
Boron
Aluminum thickness
0.3μm
Oxide thickness Master pore size
0.3μm 80μm
Internal hole width Master pore density Internal small pore density Area size Edge length
Type II: 150μm master pores with internal small 10μm mesh holessensor designs. Figure. 3. A schematic diagram of two different capacitive Type I: 80μm master pores with internal small 10μm mesh holes
150μm 10μm
8 pores/mm2
III.
2pores/mm2 2
2209 pores/mm 2
1019 pores/mm
A. Fabrication This paper utilized surface microfabrication process to manufacture meshed capacitive sensor, as shown in Fig. 4. Capacitor plates spillover the arc-shaped fringing field line from high potential states to low potential states. Due to electric field lines intend to focus at the edge of a conductive electrode, therefore, multi-mesh electrodes can increase the fringing field effect of within a given design area.
7214996μm /mm 8511005μm2/mm2 88360μm/mm2
FABRICATION AND EXPERIMENT
2
2
40760μm/mm2
Both of them have internal hole with small aperture, 10μm in width. The master pore densities of the top aluminum electrodes are 8 pores/mm2 and 2 pores/mm2, internal small
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B. Experiment Our experiment results in Fig. 6 demonstrated that the top electrode with 80µm pore yields more than 60% capacitance enhancement than the electrode with 150µm pore which was very close to the theoretic estimation and finite element analysis. Fringing capacitance contributed from the pore edge
Figure. 4. Schematic diagram of a multi- pore capacitive sensor
Capacitance(nF)
The process started with a cleaned and heavily boron doped 4 inch p-type single crystal silicon wafer. The silicon substrate was functional as a ground electrode. A 0.3μm thick thermal growth oxide was served as a dielectric layer for capacitive sensing. A 0.3μm thick aluminum layer was also evaporated on the top of the silicon dioxide wafers to make the top electrode. A 2μm photoresist (PR) layer was spin-coated and exposure mesh pattern on the photoresist to define the sacrifice layer, and then utilized aluminum etchant solution to form the fringe sensing electrode. The process consisted of only one masking step. The fabrication steps with corresponding substrate cross-sections in the device process sequence are illustrated in Fig. 5. After post process, the wafer will be section to chip and etch contact windows to connecting electrode.
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 80μm pore
150μm pore
Figure. 6. A Comparison of the capacitance of the different mesh capacitive sensors
An atmospheric-pressure nitrogen-plasma surface treatment were introduced to improve the electrode wettability and the water contact angle are reduced from 77.1 o to 12.1o, as show in Fig. 7. The sensor surface wettability would increase the immersive area, and it can contribute to the enhancement of fringe filed capacitance at nanoscale.
(a) thermal oxidation
(b) thermal evaporation (aluminum)
(c) photoresist spin-on coating coating
(a) Contact angle = 77.1o θ
o (b) Contact angle = 12.1
θ
(d) exposure and development Figure. 7. Photograph of cross section of aluminum hydrophilic and hydrophobic (a) before N2 plasma treatment (b) after N2 plasma treatment
(e) wet etching (aluminum) W 20µm
Silicon
10µm
Oxide
0.3µm 10μm
Al
After N2 plasma treatment of the electrode surface would be more hydrophilic, it employed 150µm pore electrodes for plasma treatment, and utilized deionize water to test capacitance under different liquid level. The sensor with N2 plasma treatment demonstrates 38%~ 59% enhancements than the sensor without. It also improves the sensor sensitivity from 69pF/mm to 97pF/mm. The fringe field level sensor is capable of measuring the water level, as shown in Fig. 8.
0.3µm
Photoresist
Figure 5. Fabrication processes of the fringing field capacitive sensor (W: the pore size, 80μm or 150μm, can be customized according to the sensor designs.)
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REFERENCES [1] L. Baxter, Capacitive Sensors: Design and Applications. New York: Wiley-IEEE Press 1997. [2] Z. Zhao-Hua, Z. Yan-Hong, L. Li-Tian, and R. Tian-Ling, "A novel MEMS pressure sensor with MOSFET on chip," in IEEE Sensor, pp. 1564-1567, 2008. [3] W. Hernandez, "Improving the Real-Time Response of an ADXL202 Accelerometer Placed in a Car Under Performance Tests by using Adaptive Filtering," in ICSPC. IEEE International Conference on Signal Processing and Communications, Dubai, pp. 1247-1250, 2007. [4] F. N. Toth, G. C. M. Meijer, and M. van der Lee, "A planar capacitive precision gauge for liquid-level and leakage detection," IEEE Transactions on Instrumentation and Measurement, vol. 46, pp. 644-646, 1997. [5] A. M. Madni, J. B. Vuong, D. C. H. Yang, and H. Bin, "A Differential Capacitive Torque Sensor With Optimal Kinematic Linearity," IEEE Journal Sensors, vol. 7, pp. 800-807, 2007. [6] B. E. Noltingk, "A novel proximity gauge," Journal of Physics E: Scientific Instruments, vol. 2, pp. 356-360, 1969. [7] R. C. Luo and Z. Chen, "Modeling and implementation of an innovative micro proximity sensor using micromachining technology," in IEEE/RSJ Initernational Conference on Intelligent Robots and Systems, Yokohama, pp. 1709-1716, 1993. [8] R. C. Zhenhai Chen ; Luo, "Design and implementation of capacitive proximity sensor using microelectromechanical systems technology," IEEE Transactions on Industrial Electronics, vol. 45, pp. 886 - 894 1998. [9] X. B. Li, V. V. Inclan, G. I. Rowe, and A. V. Mamishev, "Parametric modeling of concentric fringing electric field sensors," in Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 617620, 2005. [10] M. C. Hegg and A. V. Mamishev, "Influence of variable plate separation on Fringing Electric Fields in Parallel-Plate Capacitors," in IEEE International Symposium on Electrical Insulation, Indianapolis, USA, pp. 384-387, 2004. [11] X. B. Li, S. D. Larson, A. S. Zyuzin, and A. V. Mamishev, "Design principles for multichannel fringing electric field sensors," IEEE Sensors Journal, vol. 6, pp. 434-440, 2006. [12] H. M. L. Tan, T. Akagi, and T. Ichiki, "Localized Plasma Treatment of Poly(dimethylsiloxane) Surfaces and Its Application to Controlled Cell Cultivation," Journal of Photopolymer Science and Technology, vol. 19, pp. 245-250, 2006. [13] L. Jun, L. Xue-bin, T. Tao, L. Shu-hui, L. Tong, W. Yan-li, Z. Bei-ping, L. Qiang, Z. Xin-ping, and Z. Yan, "Experimental Research on Biological Nitrogen Removal in a Modified SBR Treating Municipal Wastewater," in Bioinformatics and Biomedical Engineering, 2008. ICBBE, The 2nd International Conference on, pp. 1090-1093, 2008. [14] A. L. Thangawng, R. S. Ruoff, M. A. Swartz, and M. R. Glucksberg, "An ultra-thin PDMS membrane as a bio/micro–nano interface: fabrication and characterization," Biomed. Microdevices vol. 9, pp. 587595, 2007.
10mm
Figure. 8. A Comparison of the level sensor capacitance on DI water level measurement (before and after plasma treatment)
Moreover, compared with two different kinds of pore structures capacitive sensor at different ratios of deionize water and IPA mixture. In this experiment, the 80µm pore electrode has better sensitivity than 150 µm pore electrode for liquid composition sensing, as shown in Fig. 9. IPA proportion 0% 50% 100%
IPA proportion 0% 50% 100%
Capacitance (nF)
1.4 1.2 1 0.8 0.6
0.4 0.2 0
Electrodes with 80μm master pore
Electrodes with 150μm master Pore
Figure. 9. Compare the different proportions of DI water and IPA solution
IV.
CONCLUSION
This paper presents a novel fringe field capacitive sensor with surface process technology using high-density mesh electrode structure to enhance fringing field effect. The atmospheric-pressure nitrogen-plasma surface treatment can increase the original capacitance of 38%~59%, and sensor sensitivity from 69pF/mm to 97pF/mm. The experiment results demonstrate the sensor capable of water level measuring. ACKNOWLEDGMENT This work was supported by the National Science Council, NSC-98-2221-E-018-012-MY2 and Ministry of Education Advisory Office Grant, Taiwan, R.O.C.
891
A Capacitance Level Sensor Design and Sensor Signal Enhancement Yu-Ting Li, Chih-Ming Chao, Kerwin Wang Department of Mechatronics Engineering, National Changhua University of Education, Taiwan E-mail: [email protected]
length and the side area of electrodes, one can improve fringing field effect and sensitivity. The design improvement process of fringing field sensor relies on a good understanding of the physics principles, numerical simulation tools or analytical methods [9, 10]. In general, design variables include the geometry and the material properties of electrodes [11]. In order to enhance sensor sensitivity, this paper also investigates the influence of hydrophilicity of the electrode surface on capacitive measurement. An atmospheric-pressure nitrogen-plasma surface treatment method has been introduced for electrode surface modification. This method is widely adopted by many industries for surface cleaning and modification [12-14].
Abstract—This paper introduces a novel fringe field capacitive sensor. To enhance the sensor signal, various sensor electrode design strategies and a surface modification method are introduced, discussed and compared. The sensors are characterized by both simulations and experiments. The experiment results demonstrate the sensor capability of water level measuring. In addition, fringing field sensor of the field strength across the target by the dielectric strength can be used to measure the dielectric constant or component of a solution. The experiment results show that nitrogen plasma surface modification can improve the sensitivity from 69pF/mm to 97pF/mm. Keywords: Fringing electric field, atmospheric-pressre plasma, level sensor.
I.
capacitance
sensor,
In following sections, the electrode geometry design, simulation analysis, fabrication, measurement results, and a related application to level sensing are presented.
INTRODUCTION
Capacitive sensors are capable of measuring various physical parameters. They are widely used in industrial, biomedical, and instrumental electronic devices, such as proximity sensors, pressure sensors, accelerometers, displacement sensors, water level sensors, torque sensors, etc[1-5]. In general, capacitive sensors consist of two parallel electrodes and dielectric material, when the electrode overlapping area, distance, or dielectric constant is affected by temperature, humidity or stress, one can take advantage on the changing of capacitance to measure these physic parameters. The widely used applications include hygrometer, manometer accelerometers, and etc... Capacitive sensor is different from other types of sensors, such as resistive, electromagnetic, or stress sensors, it provides non-contact and low energy consumption measurement[1]. Most of the capacitive sensors have fringing filed effect, based on the consideration of linearity and sensitivity; people utilize less fringing field effect to detect external physical parameters. The first fringing effect capacitive sensor consists of two electrodes placed in the middle insulator[6]. This simple electrode design provides highly sensitive fringing field measurement capability.
II. DESIGN AND SIMULATION The optimal design of a fringe filed capacitive sensor depends on the meshed electrode pore size and electrode thickness being considered. For the determination of the pore size and density, a numerical simulation include all these variables, has to be studied. A two dimensional COMSOL simulation results suggest that increasing the effective edge length of these electrodes can promote fringing field effect. For a given electrode area, increasing the fractal dimension can actually extend the effective electrode edge length of electrodes to enhance the fringing field effects. (a)
(b)
Aluminum Oxide
70μm
When the device operates at high frequency, fringing field capacitance have non-negligible influence to circuit, in terms of signal process, succession of other scholars developed employed micro-fabrication process to manufacture fringing capacitive sensors[7, 8]. If the overlapping area between two electrodes is reduced, it will reduce parallel capacitance; conversely, increasing the edge
978-1-61284-777-1/11/$26.00 ©2011 IEEE
40μm
Silicon
(c)
(d)
(e)
(f)
Figure. 1. Schematic diagram of various electrode designs
888
pore densities are 2209 pores/mm2 and 1019 pores/mm2, for sensor Type I and sensor Type II, respectively. The fill factor of the top electrode for sensor Type I is 15% smaller than sensor Type II, however, the effective electrode edge length of Type I is twice longer than Type II, therefore, sensor Type I has greater fringing effect.
As shown in Fig.1 (a-f), six electrode number arrangements, 1, 2, 4, 8, 16, and 32, respectively are simulated. In the two dimensional simulation, the total length of the top electrode is held at 40μm, the dielectric thickness is 0.3μm, the total length of the bottom electrode is 70μm. To increase the number of electrodes in a finite design space, it can effectively enhance the fringing field capacitance. The simulation result is shown in Fig. 2.
Master pore
Capacitance (nF)
6.0 5.7
80 or 150μm pores
5.4 5.1
10μm mesh holes
4.8 4.5 0
5
10
15
20
25
30
35
numbers of electrodes Figure. 2. Different kinds of electrode designs and their capacitances. Aluminum electrode
Based on the outcomes from simulation results, to verify this design approach, two different kinds of pore sizes and densities for capacitive sensor are designed and fabricated, which provide two effective electrode edge lengths to enhance the fringing capacitance. The capacitance sensor parameters are listed in Table 1. The master pore width of sensor Type I and sensor Type II are 80μm and 150μm, respectively.
Oxide
P-type single crystal silicon
TABLE 1 SPECIFICATIONS OF THE CAPACITANCE LEVEL SENSOR Type I
Type II
Wafer thickness
540μm
Silicon lattice direction
(100)
Resistance
25.2~42.5(ohm-cm)
Doping element
Boron
Aluminum thickness
0.3μm
Oxide thickness Master pore size
0.3μm 80μm
Internal hole width Master pore density Internal small pore density Area size Edge length
Type II: 150μm master pores with internal small 10μm mesh holessensor designs. Figure. 3. A schematic diagram of two different capacitive Type I: 80μm master pores with internal small 10μm mesh holes
150μm 10μm
8 pores/mm2
III.
2pores/mm2 2
2209 pores/mm 2
1019 pores/mm
A. Fabrication This paper utilized surface microfabrication process to manufacture meshed capacitive sensor, as shown in Fig. 4. Capacitor plates spillover the arc-shaped fringing field line from high potential states to low potential states. Due to electric field lines intend to focus at the edge of a conductive electrode, therefore, multi-mesh electrodes can increase the fringing field effect of within a given design area.
7214996μm /mm 8511005μm2/mm2 88360μm/mm2
FABRICATION AND EXPERIMENT
2
2
40760μm/mm2
Both of them have internal hole with small aperture, 10μm in width. The master pore densities of the top aluminum electrodes are 8 pores/mm2 and 2 pores/mm2, internal small
889
B. Experiment Our experiment results in Fig. 6 demonstrated that the top electrode with 80µm pore yields more than 60% capacitance enhancement than the electrode with 150µm pore which was very close to the theoretic estimation and finite element analysis. Fringing capacitance contributed from the pore edge
Figure. 4. Schematic diagram of a multi- pore capacitive sensor
Capacitance(nF)
The process started with a cleaned and heavily boron doped 4 inch p-type single crystal silicon wafer. The silicon substrate was functional as a ground electrode. A 0.3μm thick thermal growth oxide was served as a dielectric layer for capacitive sensing. A 0.3μm thick aluminum layer was also evaporated on the top of the silicon dioxide wafers to make the top electrode. A 2μm photoresist (PR) layer was spin-coated and exposure mesh pattern on the photoresist to define the sacrifice layer, and then utilized aluminum etchant solution to form the fringe sensing electrode. The process consisted of only one masking step. The fabrication steps with corresponding substrate cross-sections in the device process sequence are illustrated in Fig. 5. After post process, the wafer will be section to chip and etch contact windows to connecting electrode.
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 80μm pore
150μm pore
Figure. 6. A Comparison of the capacitance of the different mesh capacitive sensors
An atmospheric-pressure nitrogen-plasma surface treatment were introduced to improve the electrode wettability and the water contact angle are reduced from 77.1 o to 12.1o, as show in Fig. 7. The sensor surface wettability would increase the immersive area, and it can contribute to the enhancement of fringe filed capacitance at nanoscale.
(a) thermal oxidation
(b) thermal evaporation (aluminum)
(c) photoresist spin-on coating coating
(a) Contact angle = 77.1o θ
o (b) Contact angle = 12.1
θ
(d) exposure and development Figure. 7. Photograph of cross section of aluminum hydrophilic and hydrophobic (a) before N2 plasma treatment (b) after N2 plasma treatment
(e) wet etching (aluminum) W 20µm
Silicon
10µm
Oxide
0.3µm 10μm
Al
After N2 plasma treatment of the electrode surface would be more hydrophilic, it employed 150µm pore electrodes for plasma treatment, and utilized deionize water to test capacitance under different liquid level. The sensor with N2 plasma treatment demonstrates 38%~ 59% enhancements than the sensor without. It also improves the sensor sensitivity from 69pF/mm to 97pF/mm. The fringe field level sensor is capable of measuring the water level, as shown in Fig. 8.
0.3µm
Photoresist
Figure 5. Fabrication processes of the fringing field capacitive sensor (W: the pore size, 80μm or 150μm, can be customized according to the sensor designs.)
890
REFERENCES [1] L. Baxter, Capacitive Sensors: Design and Applications. New York: Wiley-IEEE Press 1997. [2] Z. Zhao-Hua, Z. Yan-Hong, L. Li-Tian, and R. Tian-Ling, "A novel MEMS pressure sensor with MOSFET on chip," in IEEE Sensor, pp. 1564-1567, 2008. [3] W. Hernandez, "Improving the Real-Time Response of an ADXL202 Accelerometer Placed in a Car Under Performance Tests by using Adaptive Filtering," in ICSPC. IEEE International Conference on Signal Processing and Communications, Dubai, pp. 1247-1250, 2007. [4] F. N. Toth, G. C. M. Meijer, and M. van der Lee, "A planar capacitive precision gauge for liquid-level and leakage detection," IEEE Transactions on Instrumentation and Measurement, vol. 46, pp. 644-646, 1997. [5] A. M. Madni, J. B. Vuong, D. C. H. Yang, and H. Bin, "A Differential Capacitive Torque Sensor With Optimal Kinematic Linearity," IEEE Journal Sensors, vol. 7, pp. 800-807, 2007. [6] B. E. Noltingk, "A novel proximity gauge," Journal of Physics E: Scientific Instruments, vol. 2, pp. 356-360, 1969. [7] R. C. Luo and Z. Chen, "Modeling and implementation of an innovative micro proximity sensor using micromachining technology," in IEEE/RSJ Initernational Conference on Intelligent Robots and Systems, Yokohama, pp. 1709-1716, 1993. [8] R. C. Zhenhai Chen ; Luo, "Design and implementation of capacitive proximity sensor using microelectromechanical systems technology," IEEE Transactions on Industrial Electronics, vol. 45, pp. 886 - 894 1998. [9] X. B. Li, V. V. Inclan, G. I. Rowe, and A. V. Mamishev, "Parametric modeling of concentric fringing electric field sensors," in Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 617620, 2005. [10] M. C. Hegg and A. V. Mamishev, "Influence of variable plate separation on Fringing Electric Fields in Parallel-Plate Capacitors," in IEEE International Symposium on Electrical Insulation, Indianapolis, USA, pp. 384-387, 2004. [11] X. B. Li, S. D. Larson, A. S. Zyuzin, and A. V. Mamishev, "Design principles for multichannel fringing electric field sensors," IEEE Sensors Journal, vol. 6, pp. 434-440, 2006. [12] H. M. L. Tan, T. Akagi, and T. Ichiki, "Localized Plasma Treatment of Poly(dimethylsiloxane) Surfaces and Its Application to Controlled Cell Cultivation," Journal of Photopolymer Science and Technology, vol. 19, pp. 245-250, 2006. [13] L. Jun, L. Xue-bin, T. Tao, L. Shu-hui, L. Tong, W. Yan-li, Z. Bei-ping, L. Qiang, Z. Xin-ping, and Z. Yan, "Experimental Research on Biological Nitrogen Removal in a Modified SBR Treating Municipal Wastewater," in Bioinformatics and Biomedical Engineering, 2008. ICBBE, The 2nd International Conference on, pp. 1090-1093, 2008. [14] A. L. Thangawng, R. S. Ruoff, M. A. Swartz, and M. R. Glucksberg, "An ultra-thin PDMS membrane as a bio/micro–nano interface: fabrication and characterization," Biomed. Microdevices vol. 9, pp. 587595, 2007.
10mm
Figure. 8. A Comparison of the level sensor capacitance on DI water level measurement (before and after plasma treatment)
Moreover, compared with two different kinds of pore structures capacitive sensor at different ratios of deionize water and IPA mixture. In this experiment, the 80µm pore electrode has better sensitivity than 150 µm pore electrode for liquid composition sensing, as shown in Fig. 9. IPA proportion 0% 50% 100%
IPA proportion 0% 50% 100%
Capacitance (nF)
1.4 1.2 1 0.8 0.6
0.4 0.2 0
Electrodes with 80μm master pore
Electrodes with 150μm master Pore
Figure. 9. Compare the different proportions of DI water and IPA solution
IV.
CONCLUSION
This paper presents a novel fringe field capacitive sensor with surface process technology using high-density mesh electrode structure to enhance fringing field effect. The atmospheric-pressure nitrogen-plasma surface treatment can increase the original capacitance of 38%~59%, and sensor sensitivity from 69pF/mm to 97pF/mm. The experiment results demonstrate the sensor capable of water level measuring. ACKNOWLEDGMENT This work was supported by the National Science Council, NSC-98-2221-E-018-012-MY2 and Ministry of Education Advisory Office Grant, Taiwan, R.O.C.
891
