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https://www.sciencedirect.com/science/article/pii/S1877705814027301
DOI: 10.1016/j.proeng.2014.11.615
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©2014 by Elsevier. All rights reserved.
DIRETORIA DE TRATAMENTO DA INFORMAÇÃO Cidade Universitária Zeferino Vaz Barão Geraldo
CEP 13083-970 – Campinas SP Fone: (19) 3521-6493 http://www.repositorio.unicamp.br
Procedia Engineering 87 ( 2014 ) 188 – 191
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi: 10.1016/j.proeng.2014.11.615
ScienceDirect
EUROSENSORS 2014, the XXVIII edition of the conference series
Electrolyte-Insulator-Semiconductor structure for Pb
+detecting.
R. R. César
a,b*, A. D. Barros
a,b, R. O. Nascimento
c, O. L. Alves
c, I. Doi
a,b, J. A. Diniz
a,b,
J. W. Swart
a,baSchool of Electrical and Computer Engineering, University os Campinas, Campinas, 13083-852, Brazil bCenter of Semiconductor Components, University of Campinas, Campinas,
13083-870, Brazil
cLaboratory of Solid State Chemistry, University of Campinas, Campinas, 13081-970, Brazil
Abstract
Controlling the water quality has become an important issue nowadays, especially due to its contamination along the years which may cause significant damage to human health and in this context, lead deserves a great attention[1]. This work presents the results for lead detection in water using an Electrolyte-Insulator-Semiconductor (EIS) structure sensor[2]. As insulator was used TiO2 thin films pH measurements. This structure showed 41mV/pH sensitivity. In order to enhance the Pb+ detection an
additional cerium phosphate[3] layer was deposited over the TiO2 thin film as selective membrane for Pb+ measurements and the
device presented 40mV/100ppm sensitivity. © 2014 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of Eurosensors 2014.
Keywords: EIS; pH sensor; Chemical sensor; Lead; CeP
1. Introduction
With the shortage of drinking water and its contamination by industrial waste and sewage, controlling the water quality has become an important issue. Among the many contaminants, lead (Pb+)[1] deserves a great attention once
it can cause a lot of damage to human health from triggering kidney disorders, miscarriage, brain damage, increased blood pressure and some cancers[1] and therefore has to be monitored. In order to obtain a device for Pb+ detection
in water an electrolyte-insulator-solution sensor was built. The EIS sensor operates as a Metal-Oxide-Semiconductor capacitor but instead of having the metal contact, an electrolyte solution and a reference electrode are used to apply
* Corresponding author. Tel.: +55-19-3521-7282; fax: +55-19-3521-5226.
E-mail address: rodrigo22cesar@gmail.com/ angelicadenardi@gmail.com
© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
189 R.R. César et al. / Procedia Engineering 87 ( 2014 ) 188 – 191
voltage (Fig. 1.a)[2].
a b c
Fig.1. (a) shows the EIS structure schema of the test sample characterized; (b) shows a picture of the EIS structure with CeP deposited inside the SU8 well; (c) shows the SEM image of the CeP film.
For pH detection high frequency 1/C2 Normalized x Bias Voltage measurements is performed. Using these
measurements, the VFB values are extracted for each pH solution, making it possible to obtain the VFB x pH curve
and to calculate the sensitivity of the device for pH detection. The same method is used to detect different Pb+
concentrations in water solution. From the VFB x Lead concentration in particles per million curve is possible to
calculated the sensitivity of the device for Pb+ detection. The equation 1 describes the V
FB for EIS devices[3]:
SOL S I SS FB REF I W Q Q V E q C M F (1) Were EREF is the reference electrode potential , φ is the potential of the electrolyte/insulator interface, χsol is the
dipole potential of the solution surface, WS is work function of silicon, q is the electron charge, QI is the charge on
the surface of the oxide and QSS is the density of surface states.
2. Material and methods
2.1. Fabrication of EIS – Electrolyte Insulator Semiconductor for pH detection
The samples were manufactured using p-type Si wafers with 100 crystal orientations as substrates. First, the silicon wafer were cleaned by the RCA method[4], then 30 nm of metallic titanium was deposited by DC sputtering. The thin Ti film was oxidized at 960°C using 1000 sccm oxygen flow rate for 40 seconds in a rapid thermal process (RTP) oven[4]. After this step the TiO2 thin film was formed. Then the native oxide was removed from the bottom
of the wafer and 300 nm of aluminum was deposited to form the back contact. Then the samples were annealed for 10 minutes in a conventional oven using nitrogen gas. The last photolithographic step defines the wells with dimensions of 3mm x 3mm using SU8 (Fig. 1.b) where the solutions should be confined.
2.2. Fabrication of EIS – Electrolyte Insulator Semiconductor for Pb+ detection using cerium phosphate (CeP)
In order to enhance the Pb+ detection an additional (CeP)[3] layer was deposited inside the wells of the EIS
structure (Fig. 1.b). The solution of CeP, developed by Nascimento[3], is known to present affinity with Pb+
molecules and it was able to resolve between different Pb+ concentrations. In this work, we investigate how it
interacts in a field effect based device.
Figure 1.c shows a SEM image of the CeP film deposited inside the EIS well. The CeP film presents a fibrous structure which allows increasing the contact area between the test solution and the sensing layer, and therefore increasing the output response[5].
3. Electrical characterization 3.1. EIS measurement for pH detection
For this measurements, the EIS sensor was placed in a probe station, was used a pseudo gold reference electrode in contact with the electrolyte and the backside contact of the EIS stayed in contact with the probe station holder. The contact area of the EIS sensor with the solution is determined by the dimensions of the wells, which were 9mm2. In order to measure the EIS as pH sensor, we used reference calibration solutions for commercial pHmeters.
The measurements have been performed in dark room at ambient temperature.
Figure 2.a shows the 1/C2 Normalized x Bias Voltage curve for pH 4, pH 7 and pH 10. It is possible to note
(Fig.2.a) that the measurements made with pH 4 solution presents a lower VFB value then the pH 10 solution, as
described in the literature[2].
Using the measurements showed in Fig. 2.a it was possible to extract the VFB values for each pH solution, and to
obtain the VFB x pH curve (Fig.2.b) in order to calculate the sensitivity of 41mV/pH.
a b
Fig. 2. (a) shows the 1/C2 normalized x Voltage curve used to obtained V
FB for the tested pH solutions; (b) shows the VFB versus pH curve used to estimate the sensitivity of the EIS sensor to detect pH.
3.2. EIS measurement for Pb+ detection
In order to measure the EIS for Pb+, we tested different Pb+ concentrations in water solution. Figure 3.a shows the
1/C2 Normalized x Bias Voltage curve for 750ppm, 500ppm and 250ppm. According to the literature[2], the
measurements made with 750ppm solution presents a higher VFB value then the 250ppm solution (Fig.3.a), due to
the higher concentration of Pb+ ions in the 750ppm solution.
From Fig. 3.a we extracted the VFB values for each concentration and we obtained the VFB x Lead concentration
curve (Fig.3.b) to calculate the sensitivity of the device. For the studied sample we determined the sensitivity of 40mV/100ppm for the device and it’s correlation coefficient (R2) 0,99.
This is a significant result when compared to actual literature, because the tests were done using Pb+ diluted in
water. Other papers also have good results, but the detection is not made directly in water, like I. Ion et al[6] who developed an electrode using polyvinyl chloride and obtained a sensitivity of 28,5mV/dec with a R2 of 0,99 and M.
B. Ali et al[7] developed an ISFET using β-cyclodextrin polysiloxane as selective membrane for detecting lead and obtained a sensitivity of 40mV/dec.
191 R.R. César et al. / Procedia Engineering 87 ( 2014 ) 188 – 191
a b
Fig. 3. (a) shows the 1/C2 normalized x Voltage curve used to obtained V
FB for the tested Pb+ solutions; (b) shows the VFB versus lead concentration curve used to estimate the sensitivity of the EIS sensor to detect Pb+.
Conclusion
The paper showed the results in obtaining EIS structures for pH measurements using TiO2 as dielectric thin film.
The 1/C2 Normalized x Bias Voltage was made for different pH solutions I order to calculate the V FB. The
sensitivity extracted from the curves of the VFB values for each pH solution was 41mV/pH.
The same EIS structure was used to sense Pb+ in different water solutions but instead of only having the TiO 2
thin film in the interface between the electrolyte and the oxide, an additional CeP film was deposited in order to provide a better sensitivity. The 1/C2 Normalized x Bias Voltage was made for 750ppm, 500ppm and 250ppm
solutions. From this curve the VFB values were extracted for each Pb+ concentration. Using these VFB values was
possible to determine the sensitivity of 40mV/100ppm for the device and it’s correlation coefficient equal to 0,99. This is a good result if compared to literature due to the fact that the measure has been made in water solution. Acknowledgements
The authors would like to thank CCS staff for samples processing. The work is supported by FAPESP, CAPES, CNPq and Namitec and PodiTrodi.
References
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Applied Sciences Campus Jülich Department of Applied Sciences and Technology, November, 2005.
[3] R. Nascimento, Mineral paper (e-paper) as a substrate for the production of functional organo-inorganic Nanocomposites, PhD Thesis, School of Chemistry, University of Campinas, 2013;
[4] Barros A. D., Developement of TiOx and ZnO thin films for ISFET and SAW devices, PhD Thesis, School of Electrical and Compuer Engineering, University of Campinas, 2013;
[5] J. Y. Oh, H.-J. Jang, W.-J. Cho, and M. S. Islam, “Highly sensitive electrolyte-insulator-semiconductor pH sensors enabled by silicon nanowires with Al2O3/SiO2 sensing membrane,” Sensors Actuators B Chem., vol. 171–172, pp. 238–243, Aug. 2012.
[6] I. Ion, A. Culetu, J. Costa, C. Luca, and A. C. Ion, “Polyvinyl chloride-based membranes of 3,7,11-tris (2-pyridylmethyl)-3,7,11,17-tetraazabicyclo [11.3.1] heptadeca-1(17),13,15-triene as a Pb(II)-selective sensor,” Desalination, vol. 259, no. 1–3, pp. 38–43, Sep. 2010. [7] M. B. Ali, R. Kalfat, H. Sfihi, J. . Chovelon, H. B. Ouada, and N. Jaffrezic-Renault, “Sensitive cyclodextrin–polysiloxane gel membrane on
