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Design, fabrication and characterization of a miniaturized series-connected potentiometric oxygen sensor
Sensors and Actuators B 105 (2005) 312–321
Design, fabrication and characterization of a miniaturized series-connected potentiometric oxygen sensor R. Radhakrishnan a , A.V. Virkar a,∗ , S.C. Singhal b , G.C. Dunham b , O.A. Marina b a
Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112, USA b Pacific Northwest National Laboratory, Richland, WA 99352, USA Received 15 January 2004; received in revised form 27 May 2004; accepted 10 June 2004 Available online 24 July 2004
Abstract Miniaturization of potentiometric sensors facilitates connecting many sensors in series to amplify the output. Miniaturized seriesconnected potentiometric sensors were developed on a silicon (Si) wafer by microfabrication techniques. The sensors consist of a thin film yttria stabilized zirconia (YSZ) electrolyte and platinum (Pt) electrodes. The reference oxygen partial pressure is determined by a nickel–nickel oxide (Ni–NiO) mixture. The open circuit voltage (OCV) was tested in air at 300 ◦ C and was found to be lower than expected. The output of the net sensor increased almost linearly by connecting 10 sensors in series. Impedance spectroscopy was used to investigate the electrolyte and electrolyte–electrode interfaces using a two electrode configuration. © 2004 Published by Elsevier B.V. Keywords: Potentiometric oxygen sensor; Yttria stabilized zirconia; Nickel–nickel oxide; Platinum; Microfabrication; Impedance spectroscopy
1. Introduction Potentiometric electrochemical oxygen sensors are commonly used in a variety of applications including internal combustion engines, process control, industrial boilers, and metallurgical heat treatment furnaces [1–4]. These sensors are simple in their construction, operation and maintenance and allow quick and continuous measurements with high accuracy and reliability. A typical potentiometric oxygen sensor [5,6] consists of an oxygen ion conducting solid electrolyte, usually YSZ, and two electrodes, such as porous Pt, deposited on the two sides of the electrolyte. One of these is a reference electrode, exposed to a known oxygen partial pressure and the other is a working electrode, which is exposed to the medium with unknown oxygen partial pressure that needs to be measured. When the two electrodes are exposed to two different oxygen partial pressures and isolated from each other, the sensor develops an electromotive force (EMF). If the partial pressure of oxygen at the reference electrode is Pr and that at the working electrode is Pw , the EMF generated E is given by the Nernst equation: RT Pw E= ln (1) 4F Pr ∗ Corresponding author. Tel.: +1-801-581-3148; fax: +1-801-581-4816. E-mail address: [email protected] (A.V. Virkar).
0925-4005/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.snb.2004.06.014
where R is the gas constant, T the temperature in Kelvin and F the Faraday’s constant. These sensors usually operate at a temperature between 600 and 1000 ◦ C. The high operating temperature is due to the low oxygen ion conductivity of the electrolyte at temperatures lower than 600 ◦ C and also due to slow electrode kinetics [7]. With the advent of microfabrication technology, efforts have been made to make miniaturized sensors [8–10]. Miniaturized potentiometric oxygen sensors offer great advantages over their macroscopic counterparts such as lower operating temperature, lower power consumption, easy integration with the required electrical components, small size and reduced weight. But the use of two gaseous electrodes causes difficulties in the miniaturization due to the fact that it is difficult to separate the two gaseous electrodes. It also requires porous substrates, to allow the diffusion of the reference gas to the reference electrode. An alternate approach is to use a mixture of a transition metal and its oxide as the reference electrode, completely sealed from the working gas media. This offers great advantages in the microfabrication of the sensor. The possibilities of using various metal/metal oxide systems [6,11] have been explored and it was reported that the Ni–NiO system is a very suitable candidate. Tubular [12] and thin film [13] zirconia electrochemical oxygen sensors using Ni–NiO have been reported. A drawback of potentiometric sensors in general is that the output signal is low, when the ratio of the partial pressures
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at the two electrodes is low. At 600 K, the E changes by 12.93 mV for every e-fold (2.718-fold) change in Pw /Pr . Also the signal response is poorly resolved when the oxygen pressure on the working electrode varies very little. For example, in the combustion of gaseous fuels, knowledge of the oxygen concentration in the depleted air is required, and it is important to know if the residual oxygen concentration is say 10% rather than 11%. The difference in the measured value of E at 600 K is only 1.23 mV in this case. Often this signal is too small and may be undetected if the measuring instrument has poor sensitivity. A suitable approach for enhancing the signal consists of connecting a number of sensors in series. If there are a total of N sensors, connected in series, the total signal will be given by: NRT Pw E = NE = (2) ln 4F Pr Utilization of microfabrication techniques makes it feasible to connect many sensors in series that can be accommodated in a very small area. In this paper, design, fabrication by micromachining techniques, and testing of a new, series-connected miniaturized potentiometric oxygen sensor are discussed. This sensor is based on thin film YSZ electrolyte. The reference electrode is a porous Pt film in contact with a thin film of Ni–NiO. The working electrode is a porous platinum thin film. Since the microfabrication on silicon substrates is well studied, silicon was used as the substrate. Also the use of Si wafers as substrates offers the advantage of incorporating the related electronics on the same wafer. Since even intrinsic silicon wafers have some conductivity, a thin layer of silicon dioxide (SiO2 ) was initially thermally grown on the silicon wafer, which was later patterned and etched to isolate the sensors from each other. A second layer of SiO2 was sputter deposited and patterned at the end of the fabrication to isolate the Ni–NiO reference electrode from the ambient atmosphere.
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and 275 V, respectively. All depositions were conducted for 10 min. X-ray diffraction (XRD) using Phillips PW3040/60 X’Pert Pro MRD system was conducted on the films deposited on glass substrates at different oxygen partial pressures to determine the phases present in the films. Data analysis was accomplished using Jade 6 (Materials Data Inc., Livermore, CA) and reference data was obtained from the Powder Diffraction File Database (2001 Release, International Center for Diffraction Data, Newtown Square, PA). Rutherford back scattering (RBS) and nuclear reaction analysis (NRA) were carried out on the films deposited on the silicon wafer to analyze the composition and to measure the oxygen content. RBS experimental data were fitted with a theoretical model using the SIMNRA simulation code. Since the oxygen signal is superimposed on the Si signal in the RBS spectrum, quantification of oxygen is more difficult. Hence, 16 O(d, p)17 O nuclear reaction was used to measure the surface and the bulk oxygen content in the sample. 2.2. Porous Pt electrode Porous platinum films of 0.20 m thickness were sputtered at high chamber pressure using argon plasma in a dc sputtering chamber. This technique is simpler than other reported techniques [14] to produce porous platinum films and can produce chemically pure platinum films. Experiments were conducted to optimize the chamber pressure and it was found that deposition at 15 mTorr chamber pressure produces porous platinum films. Hence the electrode film was deposited at this pressure. The corresponding argon flow was 22.0 sccm. The current and voltage of the dc supply were maintained at 0.35 A and 430 V, respectively. The target to substrate distance was 22.5 cm. SEM micrographs were obtained at high magnification using a LEO 982 field emission scanning electron microscope (FESEM) to study the microstructure of the electrode. Elemental analysis of the Pt film deposited on a silicon wafer under the same condition was carried out using energy dispersive X-ray analysis (EDX).
2. Experimental 2.3. YSZ electrolyte 2.1. Ni–NiO reference electrode In order to study and optimize the composition of Ni and NiO in the reference electrode, nearly 0.25 m thick Ni–NiO films were deposited on silicon and glass substrates by reactive dc sputtering from a Ni target using a mixture of oxygen and argon plasma. The oxygen content was varied between 2 and 4% in order to change the relative amounts of Ni and NiO in the film. During sputtering, the target to substrate separation was 15 cm. The chamber was pumped down to a base pressure of 10−6 Torr or less, and argon and oxygen gases were introduced into the chamber in order to keep the chamber pressure at around 2.5 mTorr during deposition. The current and voltage for the deposition were 0.5 A
Reactive sputter deposition of YSZ from a composite metallic target has been reported previously [15]. In this work, to study the YSZ films before use in the sensor, films of 1 m thickness were deposited on a (1 0 0) Si substrate as well as on glass slides by reactive dc sputtering using argon–oxygen atmosphere from a yttrium (Y) doped zirconium (Zr) target (16% Y). Several trial depositions were conducted to optimize the percentage of oxygen. This is important since too high an oxygen content will slow down or stop the deposition due to the oxidation of the target and too low an oxygen content will not completely oxidize the yttrium or zirconium, leading to a metal rich film. In order to control the oxygen content an optical monitoring system
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was used. The signal from the optical monitoring system was recorded. The monitor oscillates without any damping if the oxygen content is optimum. If the oxygen content is low, the oscillation dampens due to a metal rich film. If the oxygen content is high negligible deposition occurs and the oscillation becomes random. In order to ensure that the deposition at the substrate and the detector of the optical monitoring system were the same, the substrate and the detector were placed symmetrically with respect to the target. The target-substrate separation was maintained at 32.5 cm, as the optical monitor was at a fixed distance of 32.5 cm from the target. The chamber was pumped down to a base pressure below 10−6 Torr, and argon and oxygen gases were introduced into the chamber in order to maintain the chamber pressure at 2 mTorr during deposition. Since the chamber pressure affects the porosity in the film, it was kept as low as possible. The O2 percentage was adjusted to observe a steady oscillation on the monitor. The current and voltage during the deposition were maintained at 0.4 A and 325 V, respectively. The deposition was conducted for 5 h. XRD scans were obtained on the films deposited on glass slides to determine the YSZ phase. X-ray photoelectron spectroscopy (XPS) measurements using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe were conducted on the films deposited on a silicon wafer to determine the ratio of Zr, Y and O in the film and also to determine if the Zr and Y were fully oxidized. The microstructure and porosity of films deposited on silicon wafer were analyzed using the FESEM. 2.4. Sensor fabrication The sensors were fabricated on a silicon wafer (5 cm diameter). A set of six mask patterns was designed using AutoCAD and the patterns were transferred to an iron oxide mask. The masks were designed such that a single wafer can accommodate 100 individual sensors, in an array of 10 rows and 10 columns. The sensors in one row are connected in series. This creates 10 devices, each containing 10 individual sensors connected in series. These masks were then used for the photolithographic patterning of the wafer to define the various layers of the sensor. In general, photolithographic patterning of the wafer followed by sputter deposition of the materials and a lift-off process was used to fabricate the various layers of the sensor. To perform photolithography, positive photoresist Shipley 1818 was spin-coated on a silicon wafer at a spin speed of 2500 rpm and soft baked for 10 min at 100 ◦ C. The wafer was then aligned with respect to the required mask pattern in the mask aligner, exposed to UV light for 20 s and developed in Shipley MF-321 to remove the photoresist from the UV exposed regions. This opened a window for the deposition of the material. The required material was then sputter deposited. After sputter deposition, the wafer was washed in acetone to lift off the material that was deposited on top of the photoresist.
Fig. 1. Process flow for sensor fabrication.
The process flow adapted for fabrication described below is shown schematically in Fig. 1. The wafer (Fig. 1a) was initially oxidized to grow a 1.5 m thick oxide layer (Fig. 1b). The oxidized wafer was patterned using the first mask by photolithography, to define the sensor area and 1.2 m of oxide was etched away from these regions leaving a 0.3 m thick oxide layer at the bottom (Fig. 1c). This created a trench in the oxide film. The second mask was used to pattern for the reference electrode area and 2500 Å thick Ni–NiO layer was deposited by dc reactive sputtering in Ar–O2 atmosphere followed by the deposition of 850 Å porous platinum by dc sputtering in argon atmosphere. After deposition the wafer was washed in acetone to lift-off the unwanted Ni–NiO and Pt followed by washing in deionized (DI) water for 10 min (Fig. 1d). The wafer was then patterned using the third mask to define electrolyte regions and 1.2 m YSZ was deposited by dc reactive sputtering, followed by a lift-off process in acetone and a DI water rinse (Fig. 1e). In the next step the wafer was patterned with the fourth mask to define the gold connector layer, followed by
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taining the sensors is given in Fig. 2 and a cross-sectional SEM image of a single sensor (as fabricated, before testing) is shown in Fig. 3. 2.5. Sensor packaging
Fig. 2. Photograph of part of the wafer containing final devices.
dc reactive sputtering of gold, lift-off in acetone and DI water rinse (Fig. 1f). A thin Ti seed layer was used to get good adhesion of the gold film on the oxide surface. After the connecter regions were deposited, the wafer was patterned with a fifth mask for the working electrode region and 1500 Å porous platinum was deposited by dc sputtering followed by lift-off in acetone and DI water rinse (Fig. 1g). Finally the wafer was patterned using a sixth mask to deposit a layer of 1 m thick SiO2 by RF sputtering in the regions where the reference electrode was exposed to atmosphere, followed by lift-off in acetone and DI water rinse (Fig. 1h). At the end of each patterning and lift-off, the wafer was examined under an optical microscope to make sure that the layers were well patterned and that the lift-off process had not damaged the sensor layers. When the fabrication was completed the wafer contained 10 series connected sensors (10 rows) of 10 devices each. A photograph of a part of the wafer con-
The silicon wafer containing the sensors was diced in a wafer-dicing saw, to obtain one series-connected sensor per chip. The chip containing the sensor was then placed in a flat bonding pad having 10 legs, which were electrically isolated from each other, and held at the ends using two sets of screws and washers. Gold wires of 30 m diameter were used to wirebond (using a K&S wedge bonder) the sensor to the pad. One end of each wire was bonded to a single sensor. The other end of the wire was connected to one leg of the pad. Thus electrical connection was established between each sensor and a leg of the pad. Later, platinum wires were extended from the legs of the pad to connect to the measuring device. 2.6. EMF measurement and impedance analysis The packaged sensor was placed in a tube furnace with working electrode exposed to air and heated to 350 ◦ C. The open circuit voltage (OCV) was measured using a Keithley Electrometer with an input impedance of 5 × 1013 . The measurement was done as a function of the number of sensors at 300 ◦ C. The impedance analysis of a single sensor was carried out using a Solartron 1280 frequency response analyzer in combination with a Solartron 1286 potentiostat. Impedance spectra were obtained during heating upto 350 ◦ C and also during cooling.
Fig. 3. Cross-sectional image of a single sensor.
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Fig. 4. XRD spectra of sputter deposited Ni–NiO film using (a) 2% O2 –98% Ar, (b) 2.5% O2 –97.5% Ar, (c) 3% O2 –97% Ar, (d) 4% O2 –96% Ar plasma.
3. Results and discussion 3.1. Ni–NiO reference electrode The XRD spectra of films deposited using (a) 2% O2 –98% Ar, (b) 2.5% O2 –97.5% Ar, (c) 3% O2 –97% Ar and (d) 4% O2 –96% Ar plasma are given in Fig. 4. The film deposited at 2% O2 showed mostly Ni and almost no NiO. The film deposited at 4% O2 showed almost all NiO and no Ni in the XRD spectra. The O2 concentration was then varied between these limits to obtain nearly equal amounts of Ni and NiO. The ratio of Ni to NiO in the film deposited using 2.5% O2 /97.5% Ar sputtering gas was obtained from the areas of the Ni (1 1 1)/100% and NiO (2 0 0)/100% peaks and was found to be 60:40. The peak areas were determined by profile fitting the background-subtracted pattern over the 2θ range of 35–55◦ as shown in Fig. 5. The elemental composition in this film estimated from an RBS spectrum was 72% nickel and 27% oxygen and that of the substrate was 100% silicon.
Fig. 6. High resolution SEM image of sputter deposited Pt.
The total number of atoms per unit area of the Ni–NiO film was estimated to be 28.2 × 1017 atoms/cm2 . The average oxygen concentration in the sample estimated from a nuclear reaction spectrum is 7.58 × 1017 atoms/cm2 . This value is in agreement with that obtained from RBS measurement (27% of 28.20 × 1017 atoms/cm2 is 7.60 × 1017 atoms/cm2 ). This film, deposited using 2.5% O2 –97.5% Ar plasma was used as the solid reference electrode film in oxygen sensors. 3.2. Platinum electrode Platinum films deposited under different chamber pressures showed different morphologies when examined under high magnification SEM. The film deposited at 1–5 mTorr chamber pressure was fully dense. As the chamber pressure was increased above 10 mTorr, the film appeared to be porous. Increasing the chamber pressure also decreased the deposition rate significantly. The platinum electrode used in the sensor was deposited at 15 mTorr and showed pores of 10–20 nm size with grains of nearly 100 nm (Fig. 6). On heating the film to 350 ◦ C the morphology changed, and the pores became larger. EDX spectra of the sample showed peaks corresponding to silicon substrate and platinum only. 3.3. YSZ electrolyte
Fig. 5. Curve fitting to estimate the Ni and NiO content of the film (b) in Fig. 4.
The rate of deposition of the YSZ film was low, mainly due to the large separation between the target and the substrate. The films appeared translucent. As shown in Fig. 7, the XRD pattern obtained from the film was consistent with reference data for cubic YSZ (PDF 30-1468). The enhanced intensity of the (2 0 0) peak indicates preferred orientation in the 1 0 0 direction. The XPS analysis of the YSZ film gave a Y to Zr atomic ratio of 0.19, consistent with that in Y0.16 Zr0.84 O2 . High energy resolution scans showed that the binding energies are consistent with that of fully oxidized Zr and Y. SEM imaging
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Fig. 7. XRD spectrum of the sputter deposited YSZ.
of the film deposited on a Si wafer was done to study the morphology and porosity of the film. Fig. 8 shows an SEM image (top view) of YSZ sputter deposited on silicon. The top view of the as-deposited film did not show any porosity and appeared to be very dense even at very high magnification. The cross-sectional image (Fig. 3) showed columnar growth of the grains. This also explains the enhanced intensity of (2 0 0) peak in the XRD pattern (Fig. 7), indicating a preferred grain orientation. 3.4. EMF measurement The measured EMF of a single sensor and that of several sensors connected in series while heating the sensor to 350 ◦ C from room temperature were assessed. For the single sensor, a stable EMF of 117 mV was measured at 275 ◦ C which remained steady upto 300 ◦ C. This tempera-
Fig. 8. High resolution SEM image of sputter deposited YSZ.
ture is about 300◦ lower than that required by typical oxygen sensors using a thick zirconia electrolyte. This significant drop in the operating temperature can be attributed to the thin film electrolyte used in this microfabricated sensor. But the measured EMF for a single sensor was lower than the calculated Nernst voltage of 900 mV under the experimental conditions. This is most likely due to poor sealing of the Ni–NiO reference electrode, which may have resulted in air leakage to the reference electrode. The microstructure of YSZ after heating at 350 ◦ C showed open grain boundaries (Fig. 9). Also since the cross-section of the YSZ film shows columnar growth, the grain boundaries may extend all the way through the thickness of the film, which could act as a leakage path for air from the working to the reference electrode side. By changing the sputtering conditions, it may
Fig. 9. SEM image of sputter deposited YSZ after heating at 350 ◦ C for 1 h.
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Fig. 10. Sensor voltage as a function of the number of sensors in series at 300 ◦ C in air. Fig. 11. Complex impedance spectrum of a single sensor obtained at 300 ◦ C in air.
be possible to change the electrolyte microstructure, which should help the sealing of the reference electrode. Efforts are underway in our lab to develop dense 1 m thick YSZ that does not show the columnar growth by RF sputtering using YSZ target. The EMF of sensors connected in series increased almost linearly with the number of sensors, Fig. 10, in agreement with Eq. (2). In the case of 10 sensors connected together, the EMF was 778 mV at 300 ◦ C. This concept of increasing the sensor signal by connecting many sensors in series is demonstrated for the first time. Upon heating the sensor to 350 ◦ C the voltage decreased slightly. The decrease in voltage continued while maintaining the sensor at 350 ◦ C. Despite this observed lower EMF and decay, it is demonstrated that microfabricated sensors in principle can offer significant advantages over the conventional sensors, in that the operating temperature can be reduced below 300 ◦ C and that the sensor signal can be increased by connecting many sensors in series.
Fig. 12. Impedance spectra of a single oxygen sensor at 350–280 ◦ C in air.
3.5. Impedance spectroscopy Impedance spectroscopy was used to investigate the electrolyte and electrolyte–electrode interfaces. Complex
Fig. 13. Arrhenius plot for the high-frequency and low-frequency resistance estimated from the impedance spectra shown in Fig. 12.
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impedance spectra were obtained in the temperature range 250–350 ◦ C, while heating or cooling the sensor. As illustrated in Figs. 11 and 12, spectra given as Nyquist plots contained at least two semi-circles: one was in the high-frequency region, a high-frequency arc (HFA), and the second was in the low frequency region, a low-frequency arc (LFA). Both the arcs were depressed. In Fig. 11, the low frequency arc is more depressed compared to the high frequency arc and is asymmetric. The asymmetric nature of the LFA may be due to the fact that this arc represents both the electrodes since no reference electrode was used in obtaining the spectrum. Both arcs were found to be temperature dependent. The LFA dominated the spectra obtained at all temperatures. Fig. 12, obtained while cooling the sensor shows that the HFA increased in size as the temperature was lowered. Resistance Rh at various temperatures was estimated from the HFAs in this figure. These estimated Rh values for various temperatures are given as an Arrhenius plot in Fig. 13. The apparent activation energy corresponding to this resistance was calculated from the slope of this plot and was equal to 0.85 eV. This value is close to that of the YSZ electrolyte. Thus, it is possible that the HFA reflects the YSZ resistance. From the plot of Fig. 12, resistance at 310 ◦ C was estimated to be 35 k corresponding to a resistivity of 2.2 M cm. Resistivity values of 1–1.5 M cm are reported [16] for doped zirconia at this temperature. It can also be noted that this arc goes through the origin. The grain and grain boundary arcs are not resolvable. This could be due to the columnar growth of YSZ film, in which the grain boundary is parallel to the measurement direction.
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Fig. 14. Variation of the complex impedance spectra of a single oxygen sensor with time at 350 ◦ C in air. (1) after stabilizing the temperature at 350 ◦ C, (2) 10 min after 1, (3) 25 min after 1, (4) 45 min after 1.
The size of the LFA also increases as the temperature decreases (Fig. 12). The resistance of LFA estimated by fitting the data at various temperatures is also plotted in Fig. 13. The activation energy corresponding to this curve is 1.5 eV, which is a reasonable value for platinum electrodes on YSZ at this temperature range [17]. Hence, this low frequency curve can be associated with the electrode process. It was also noticed that the size of the LFA increased while the temperature was maintained at 350 ◦ C (Fig. 14). This indicates either electrode polarization due to charging of the electrode–electrolyte interface or de-lamination of the electrode from the electrolyte on heating due to poor adhesion between the two. Poor interface contact was confirmed by
Fig. 15. Cross-section of the YSZ–Pt interface of the sensor after heating at 350 ◦ C.
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examining the sensor cross-section in SEM after heating the sensor at 350 ◦ C (Fig. 15). Comparing the spectra obtained while heating up the sensor from room temperature (Fig. 11) and that obtained while cooling from 350 ◦ C (Fig. 12), it can be noticed that the high frequency arc became smaller at the same temperature while cooling and the low frequency arc became larger. This lowering in size of the high frequency arc may be due to the change in zirconia microstructure as can be noticed from the SEM of YSZ before and after heating (Figs. 8 and 9) with associated increased conductivity. The increased size of the low frequency curve may be due to the degradation of the electrodes.
4. Conclusions Miniaturized, series-connected potentiometric thin film oxygen sensors were developed by microfabrication technology and tested. The sensor OCV was tested in air at 300 ◦ C and the sensor was found to be functional at this temperature. This is a significant improvement in lowering the operating temperature compared to traditionally fabricated sensors. Amplification of the sensor output by connecting many sensors in series is also demonstrated. By connecting 10 sensors in series the output voltage is increased almost linearly. The potential developed by a single sensor was much lower than expected. This is likely due to poor sealing of the Ni–NiO reference electrode resulting in an air leakage to the reference electrode through the grain boundary channels. Impedance analysis of the sensor was also performed using a two electrode configuration to elucidate the electrode, electrolyte and interface electrochemical properties and kinetics. Efforts are underway to improve the design of the sensor, to obtain better electrolyte microstructure and also to improve the electrode–electrolyte interface.
Acknowledgements This work was supported in part by University of Utah Technology Innovation Grant (TIG). Part of the research described in this paper was performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the US Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory in Richland, Washington. Authors would like to acknowledge D.C. Stewart, Pacific Northwest National Laboratory for help with sputtering, M.H. Engelhard, Pacific Northwest National Laboratory for XPS, D.E. McCready, Pacific Northwest National Laboratory for XRD, T. Thevuthasan, Pacific Northwest National Laboratory and S. Shuttandan, Pacific Northwest National Laboratory for RBS and NRA and J.S. Young, Pacific Northwest National Laboratory for SEM. R. Radhakrishnan would like to acknowledge the discussions he had with Dr.
M.S. Miller, University of Utah, Dr. Yi Jiang, University of Utah, Dr. G.Y. Lin, University of Utah, Dr. T.J. Amstrong, University of Utah, Dr. L. Saraf, Pacific Northwest National Laboratory and Dr. L.R. Pederson, Pacific Northwest National Laboratory. References [1] D. Eddy, Physical principles of zirconia oxygen exhaust gas sensors, IEEE Trans. Vehicle Technol. VT-23 (4) (1974) 125–128. [2] W. Fleming, Zirconia oxygen sensor an equivalent circuit model, SAE Paper 800020, Presented at the 1980 SAE Congress, Detroit, Michigan, February 28, 1980. [3] D.E. Williams, P. McGeehin, Solid state gas sensors and monitors, Electrochemistry 9 (1984) 246–290. [4] G. Vitter, P. Foster, M. Lahlou, F.J. Gutierrez Monreal, Use of an oxygen mini-gauge for monitoring domestic and medium sized boilers, Solid State Ionics 9–10 (1983) 1273–1276. [5] M. Benammar, Techniques for measurement of oxygen and air-to-fuel ratio using zirconia sensors—a review, Meas. Sci. Technol. 5 (1994) 757–767. [6] W.C. Maskell, B.C.H. Steele, Solid state potentiometric oxygen gas sensors, J. Appl. Electrochem. 16 (1986) 475–489. [7] K.P. Jagannathan, S.K. Tikku, H.S. Ray, A. Ghosh, E.C. Subbarao, in: E.C. Subbarao (Ed.), Solid State Electrolytes and their Applications, Plenum Press, New York, 1980, pp. 201–259. [8] G.W. McLaughlin, K. Braden, B. Franc, G.T.A. Kovacs, Microfabricated solid-state dissolved oxygen sensor, Sens. Actuators B 83 (2002) 138–148. [9] M. Dilhan, D. Esteve, A.M. Gue, O. Mauvais, L. Mercier, Electrochemical oxygen microsensors, Sens. Actuators B 2627 (1995) 401–403. [10] J.F. Currie, A. Essalik, J.-C. Marusic, Micromachined thin film solid state electrochemical CO2 , NO2 and SO2 gas sensors, Sens. Actuators B 59 (1999) 235–241. [11] F.J. Gutierrez Monreal, G. Vitter, Measurement of low oxygen pressures with a solid state electrolyte miniaturized sensor, J. Phys. E: Sci. Instrum. 16 (1983) 361. [12] A.G. Mortimer, G.P. Reed, Development of a robust electrochemical oxygen sensor, Sens. Actuators B 24–25 (1995) 328–335. [13] J.W. Bae, J.Y. Park, S.W. Hwang, G.Y. Yeom, K.D. Kim, Y.A. Cho, J.S. Jeon, D. Choi, Characterization of yttria-stabilized thin films prepared by radio frequency magnetron sputtering for a combustion control oxygen sensor, J. Electrochem. Soc. 147 (2000) 2380– 2384. [14] L. Maya, G.M. Brown, T. Thundat, Porous platinum electrodes derived from the reduction of sputtered platinum dioxide films, J. Appl. Electrochem. 29 (1999) 883–888. [15] T. Tsai, S.A. Barnett, Bias sputter deposition of dense yttria stabilized zirconia films on porous substrates, J. Electrochem. Soc. 142 (9) (1995) 3084–3087. [16] J.R. McDonald, Impedance Spectroscopy, Wiley, New York, 1987. [17] S.P. Yoon, S.W. Nam, S.-G. Kim, S.-A. Hong, S.-H. Hyun, Characteristics of cathodic polarization at Pt/YSZ interface without the effect of electrode microstructure, J. Power Sources 115 (2003) 27–34.
Biographies Rajesh Radhakrishnan obtained his Master of Science (M.Sc.) in Physics from Indian Institute of Technology, Madras, in 1995 and Master of Technology (M.Tech) degree in Materials Science and Engineering from Indian Institute of Technology, Bombay, in 1997. He is currently pursuing a Ph.D. in Materials Science and Engineering at the University of
R. Radhakrishnan et al. / Sensors and Actuators B 105 (2005) 312–321 Utah, Salt Lake City. His research interests include materials and electrochemical aspects of fuel cells and oxygen sensors, microfabrication and semiconductor devices. Professor Anil V. Virkar is the Chair of Department of Materials Science and Engineering, University of Utah, Salt Lake City. He obtained a Bachelor’s degree in Metallurgical Engineering from Indian Institute of Technology in 1967, an M.S. degree in Engineering Mechanics from Louisiana State University, and Ph.D. degree in Materials Science from Northwestern University in 1973. His present research areas include solid state electrochemistry, and electrochemical devices. He has authored over 160 scientific publications, and received 30 patents. Dr. Subhash C. Singhal is a Battelle fellow and Director, Fuel Cells, at Pacific Northwest National Laboratory. He obtained a B.S. in Physics, Chemistry, and Mathematics from Agra University in 1963, a B.E. in Metallurgy from Indian Institute of Science in 1965 and a Ph.D. in Materials Science and Engineering from the University of Pennsylvania in 1969. Dr. Singhal joined the Energy Science and Technology Directorate at PNNL in April 2000 after having worked at Siemens Westinghouse Power Corporation (formerly Westinghouse Electric Corporation) for over 29 years where he led an internationally recognized group in solid oxide fuel cell (SOFC) technology. He has authored over 75 scientific publications, edited 13 books, received 13 patents, and given over 190 plenary, keynote and other invited presentations worldwide. At PNNL, Dr. Singhal is responsible for providing senior technical, managerial,
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and commercialization leadership to the lab’s fuel cell program. His research interests are in advanced materials for high temperature energy conversion systems, and solid state electrochemistry and electrochemical devices, especially solid oxide fuel cells. Glen Dunham received a Bachelor of Arts in Chemistry and a Bachelor of Science in Physics from Pacific Lutheran University in 1980, and a Master of Science in Materials Science from Washington State University in 1983. Prior to joining PNNL, his research was primarily in III–V photovoltaics, including fabricating one of the most efficient solar cells of its time. At PNNL, his unique interest has been developing microfabrication capabilities directed toward microfluidics, chemical and biological sensing, and terahertz optics. Recently, he has begun applying microfabrication solutions to national security problems in biometrics. He holds two patents with several pending, and has published in the fields of chemical sensing, piezoelectrics, polymer thin films, dielectric materials, and microfabrication. Dr. Olga A Marina is a senior research scientist at the Pacific Northwest National Laboratory. She received her Ph.D. in chemistry from Boreskov Institute of Catalysis and held post-doctoral positions at Risø National Laboratory, Denmark and at University of Patras, Greece. Her research interests include development of advanced materials for solid oxide fuel cells, sensors and electrolyzers, electrocatalysis and fuel processing. She co-authored over 30 research papers.
Design, fabrication and characterization of a miniaturized series-connected potentiometric oxygen sensor R. Radhakrishnan a , A.V. Virkar a,∗ , S.C. Singhal b , G.C. Dunham b , O.A. Marina b a
Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112, USA b Pacific Northwest National Laboratory, Richland, WA 99352, USA Received 15 January 2004; received in revised form 27 May 2004; accepted 10 June 2004 Available online 24 July 2004
Abstract Miniaturization of potentiometric sensors facilitates connecting many sensors in series to amplify the output. Miniaturized seriesconnected potentiometric sensors were developed on a silicon (Si) wafer by microfabrication techniques. The sensors consist of a thin film yttria stabilized zirconia (YSZ) electrolyte and platinum (Pt) electrodes. The reference oxygen partial pressure is determined by a nickel–nickel oxide (Ni–NiO) mixture. The open circuit voltage (OCV) was tested in air at 300 ◦ C and was found to be lower than expected. The output of the net sensor increased almost linearly by connecting 10 sensors in series. Impedance spectroscopy was used to investigate the electrolyte and electrolyte–electrode interfaces using a two electrode configuration. © 2004 Published by Elsevier B.V. Keywords: Potentiometric oxygen sensor; Yttria stabilized zirconia; Nickel–nickel oxide; Platinum; Microfabrication; Impedance spectroscopy
1. Introduction Potentiometric electrochemical oxygen sensors are commonly used in a variety of applications including internal combustion engines, process control, industrial boilers, and metallurgical heat treatment furnaces [1–4]. These sensors are simple in their construction, operation and maintenance and allow quick and continuous measurements with high accuracy and reliability. A typical potentiometric oxygen sensor [5,6] consists of an oxygen ion conducting solid electrolyte, usually YSZ, and two electrodes, such as porous Pt, deposited on the two sides of the electrolyte. One of these is a reference electrode, exposed to a known oxygen partial pressure and the other is a working electrode, which is exposed to the medium with unknown oxygen partial pressure that needs to be measured. When the two electrodes are exposed to two different oxygen partial pressures and isolated from each other, the sensor develops an electromotive force (EMF). If the partial pressure of oxygen at the reference electrode is Pr and that at the working electrode is Pw , the EMF generated E is given by the Nernst equation: RT Pw E= ln (1) 4F Pr ∗ Corresponding author. Tel.: +1-801-581-3148; fax: +1-801-581-4816. E-mail address: [email protected] (A.V. Virkar).
0925-4005/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.snb.2004.06.014
where R is the gas constant, T the temperature in Kelvin and F the Faraday’s constant. These sensors usually operate at a temperature between 600 and 1000 ◦ C. The high operating temperature is due to the low oxygen ion conductivity of the electrolyte at temperatures lower than 600 ◦ C and also due to slow electrode kinetics [7]. With the advent of microfabrication technology, efforts have been made to make miniaturized sensors [8–10]. Miniaturized potentiometric oxygen sensors offer great advantages over their macroscopic counterparts such as lower operating temperature, lower power consumption, easy integration with the required electrical components, small size and reduced weight. But the use of two gaseous electrodes causes difficulties in the miniaturization due to the fact that it is difficult to separate the two gaseous electrodes. It also requires porous substrates, to allow the diffusion of the reference gas to the reference electrode. An alternate approach is to use a mixture of a transition metal and its oxide as the reference electrode, completely sealed from the working gas media. This offers great advantages in the microfabrication of the sensor. The possibilities of using various metal/metal oxide systems [6,11] have been explored and it was reported that the Ni–NiO system is a very suitable candidate. Tubular [12] and thin film [13] zirconia electrochemical oxygen sensors using Ni–NiO have been reported. A drawback of potentiometric sensors in general is that the output signal is low, when the ratio of the partial pressures
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at the two electrodes is low. At 600 K, the E changes by 12.93 mV for every e-fold (2.718-fold) change in Pw /Pr . Also the signal response is poorly resolved when the oxygen pressure on the working electrode varies very little. For example, in the combustion of gaseous fuels, knowledge of the oxygen concentration in the depleted air is required, and it is important to know if the residual oxygen concentration is say 10% rather than 11%. The difference in the measured value of E at 600 K is only 1.23 mV in this case. Often this signal is too small and may be undetected if the measuring instrument has poor sensitivity. A suitable approach for enhancing the signal consists of connecting a number of sensors in series. If there are a total of N sensors, connected in series, the total signal will be given by: NRT Pw E = NE = (2) ln 4F Pr Utilization of microfabrication techniques makes it feasible to connect many sensors in series that can be accommodated in a very small area. In this paper, design, fabrication by micromachining techniques, and testing of a new, series-connected miniaturized potentiometric oxygen sensor are discussed. This sensor is based on thin film YSZ electrolyte. The reference electrode is a porous Pt film in contact with a thin film of Ni–NiO. The working electrode is a porous platinum thin film. Since the microfabrication on silicon substrates is well studied, silicon was used as the substrate. Also the use of Si wafers as substrates offers the advantage of incorporating the related electronics on the same wafer. Since even intrinsic silicon wafers have some conductivity, a thin layer of silicon dioxide (SiO2 ) was initially thermally grown on the silicon wafer, which was later patterned and etched to isolate the sensors from each other. A second layer of SiO2 was sputter deposited and patterned at the end of the fabrication to isolate the Ni–NiO reference electrode from the ambient atmosphere.
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and 275 V, respectively. All depositions were conducted for 10 min. X-ray diffraction (XRD) using Phillips PW3040/60 X’Pert Pro MRD system was conducted on the films deposited on glass substrates at different oxygen partial pressures to determine the phases present in the films. Data analysis was accomplished using Jade 6 (Materials Data Inc., Livermore, CA) and reference data was obtained from the Powder Diffraction File Database (2001 Release, International Center for Diffraction Data, Newtown Square, PA). Rutherford back scattering (RBS) and nuclear reaction analysis (NRA) were carried out on the films deposited on the silicon wafer to analyze the composition and to measure the oxygen content. RBS experimental data were fitted with a theoretical model using the SIMNRA simulation code. Since the oxygen signal is superimposed on the Si signal in the RBS spectrum, quantification of oxygen is more difficult. Hence, 16 O(d, p)17 O nuclear reaction was used to measure the surface and the bulk oxygen content in the sample. 2.2. Porous Pt electrode Porous platinum films of 0.20 m thickness were sputtered at high chamber pressure using argon plasma in a dc sputtering chamber. This technique is simpler than other reported techniques [14] to produce porous platinum films and can produce chemically pure platinum films. Experiments were conducted to optimize the chamber pressure and it was found that deposition at 15 mTorr chamber pressure produces porous platinum films. Hence the electrode film was deposited at this pressure. The corresponding argon flow was 22.0 sccm. The current and voltage of the dc supply were maintained at 0.35 A and 430 V, respectively. The target to substrate distance was 22.5 cm. SEM micrographs were obtained at high magnification using a LEO 982 field emission scanning electron microscope (FESEM) to study the microstructure of the electrode. Elemental analysis of the Pt film deposited on a silicon wafer under the same condition was carried out using energy dispersive X-ray analysis (EDX).
2. Experimental 2.3. YSZ electrolyte 2.1. Ni–NiO reference electrode In order to study and optimize the composition of Ni and NiO in the reference electrode, nearly 0.25 m thick Ni–NiO films were deposited on silicon and glass substrates by reactive dc sputtering from a Ni target using a mixture of oxygen and argon plasma. The oxygen content was varied between 2 and 4% in order to change the relative amounts of Ni and NiO in the film. During sputtering, the target to substrate separation was 15 cm. The chamber was pumped down to a base pressure of 10−6 Torr or less, and argon and oxygen gases were introduced into the chamber in order to keep the chamber pressure at around 2.5 mTorr during deposition. The current and voltage for the deposition were 0.5 A
Reactive sputter deposition of YSZ from a composite metallic target has been reported previously [15]. In this work, to study the YSZ films before use in the sensor, films of 1 m thickness were deposited on a (1 0 0) Si substrate as well as on glass slides by reactive dc sputtering using argon–oxygen atmosphere from a yttrium (Y) doped zirconium (Zr) target (16% Y). Several trial depositions were conducted to optimize the percentage of oxygen. This is important since too high an oxygen content will slow down or stop the deposition due to the oxidation of the target and too low an oxygen content will not completely oxidize the yttrium or zirconium, leading to a metal rich film. In order to control the oxygen content an optical monitoring system
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was used. The signal from the optical monitoring system was recorded. The monitor oscillates without any damping if the oxygen content is optimum. If the oxygen content is low, the oscillation dampens due to a metal rich film. If the oxygen content is high negligible deposition occurs and the oscillation becomes random. In order to ensure that the deposition at the substrate and the detector of the optical monitoring system were the same, the substrate and the detector were placed symmetrically with respect to the target. The target-substrate separation was maintained at 32.5 cm, as the optical monitor was at a fixed distance of 32.5 cm from the target. The chamber was pumped down to a base pressure below 10−6 Torr, and argon and oxygen gases were introduced into the chamber in order to maintain the chamber pressure at 2 mTorr during deposition. Since the chamber pressure affects the porosity in the film, it was kept as low as possible. The O2 percentage was adjusted to observe a steady oscillation on the monitor. The current and voltage during the deposition were maintained at 0.4 A and 325 V, respectively. The deposition was conducted for 5 h. XRD scans were obtained on the films deposited on glass slides to determine the YSZ phase. X-ray photoelectron spectroscopy (XPS) measurements using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe were conducted on the films deposited on a silicon wafer to determine the ratio of Zr, Y and O in the film and also to determine if the Zr and Y were fully oxidized. The microstructure and porosity of films deposited on silicon wafer were analyzed using the FESEM. 2.4. Sensor fabrication The sensors were fabricated on a silicon wafer (5 cm diameter). A set of six mask patterns was designed using AutoCAD and the patterns were transferred to an iron oxide mask. The masks were designed such that a single wafer can accommodate 100 individual sensors, in an array of 10 rows and 10 columns. The sensors in one row are connected in series. This creates 10 devices, each containing 10 individual sensors connected in series. These masks were then used for the photolithographic patterning of the wafer to define the various layers of the sensor. In general, photolithographic patterning of the wafer followed by sputter deposition of the materials and a lift-off process was used to fabricate the various layers of the sensor. To perform photolithography, positive photoresist Shipley 1818 was spin-coated on a silicon wafer at a spin speed of 2500 rpm and soft baked for 10 min at 100 ◦ C. The wafer was then aligned with respect to the required mask pattern in the mask aligner, exposed to UV light for 20 s and developed in Shipley MF-321 to remove the photoresist from the UV exposed regions. This opened a window for the deposition of the material. The required material was then sputter deposited. After sputter deposition, the wafer was washed in acetone to lift off the material that was deposited on top of the photoresist.
Fig. 1. Process flow for sensor fabrication.
The process flow adapted for fabrication described below is shown schematically in Fig. 1. The wafer (Fig. 1a) was initially oxidized to grow a 1.5 m thick oxide layer (Fig. 1b). The oxidized wafer was patterned using the first mask by photolithography, to define the sensor area and 1.2 m of oxide was etched away from these regions leaving a 0.3 m thick oxide layer at the bottom (Fig. 1c). This created a trench in the oxide film. The second mask was used to pattern for the reference electrode area and 2500 Å thick Ni–NiO layer was deposited by dc reactive sputtering in Ar–O2 atmosphere followed by the deposition of 850 Å porous platinum by dc sputtering in argon atmosphere. After deposition the wafer was washed in acetone to lift-off the unwanted Ni–NiO and Pt followed by washing in deionized (DI) water for 10 min (Fig. 1d). The wafer was then patterned using the third mask to define electrolyte regions and 1.2 m YSZ was deposited by dc reactive sputtering, followed by a lift-off process in acetone and a DI water rinse (Fig. 1e). In the next step the wafer was patterned with the fourth mask to define the gold connector layer, followed by
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taining the sensors is given in Fig. 2 and a cross-sectional SEM image of a single sensor (as fabricated, before testing) is shown in Fig. 3. 2.5. Sensor packaging
Fig. 2. Photograph of part of the wafer containing final devices.
dc reactive sputtering of gold, lift-off in acetone and DI water rinse (Fig. 1f). A thin Ti seed layer was used to get good adhesion of the gold film on the oxide surface. After the connecter regions were deposited, the wafer was patterned with a fifth mask for the working electrode region and 1500 Å porous platinum was deposited by dc sputtering followed by lift-off in acetone and DI water rinse (Fig. 1g). Finally the wafer was patterned using a sixth mask to deposit a layer of 1 m thick SiO2 by RF sputtering in the regions where the reference electrode was exposed to atmosphere, followed by lift-off in acetone and DI water rinse (Fig. 1h). At the end of each patterning and lift-off, the wafer was examined under an optical microscope to make sure that the layers were well patterned and that the lift-off process had not damaged the sensor layers. When the fabrication was completed the wafer contained 10 series connected sensors (10 rows) of 10 devices each. A photograph of a part of the wafer con-
The silicon wafer containing the sensors was diced in a wafer-dicing saw, to obtain one series-connected sensor per chip. The chip containing the sensor was then placed in a flat bonding pad having 10 legs, which were electrically isolated from each other, and held at the ends using two sets of screws and washers. Gold wires of 30 m diameter were used to wirebond (using a K&S wedge bonder) the sensor to the pad. One end of each wire was bonded to a single sensor. The other end of the wire was connected to one leg of the pad. Thus electrical connection was established between each sensor and a leg of the pad. Later, platinum wires were extended from the legs of the pad to connect to the measuring device. 2.6. EMF measurement and impedance analysis The packaged sensor was placed in a tube furnace with working electrode exposed to air and heated to 350 ◦ C. The open circuit voltage (OCV) was measured using a Keithley Electrometer with an input impedance of 5 × 1013 . The measurement was done as a function of the number of sensors at 300 ◦ C. The impedance analysis of a single sensor was carried out using a Solartron 1280 frequency response analyzer in combination with a Solartron 1286 potentiostat. Impedance spectra were obtained during heating upto 350 ◦ C and also during cooling.
Fig. 3. Cross-sectional image of a single sensor.
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Fig. 4. XRD spectra of sputter deposited Ni–NiO film using (a) 2% O2 –98% Ar, (b) 2.5% O2 –97.5% Ar, (c) 3% O2 –97% Ar, (d) 4% O2 –96% Ar plasma.
3. Results and discussion 3.1. Ni–NiO reference electrode The XRD spectra of films deposited using (a) 2% O2 –98% Ar, (b) 2.5% O2 –97.5% Ar, (c) 3% O2 –97% Ar and (d) 4% O2 –96% Ar plasma are given in Fig. 4. The film deposited at 2% O2 showed mostly Ni and almost no NiO. The film deposited at 4% O2 showed almost all NiO and no Ni in the XRD spectra. The O2 concentration was then varied between these limits to obtain nearly equal amounts of Ni and NiO. The ratio of Ni to NiO in the film deposited using 2.5% O2 /97.5% Ar sputtering gas was obtained from the areas of the Ni (1 1 1)/100% and NiO (2 0 0)/100% peaks and was found to be 60:40. The peak areas were determined by profile fitting the background-subtracted pattern over the 2θ range of 35–55◦ as shown in Fig. 5. The elemental composition in this film estimated from an RBS spectrum was 72% nickel and 27% oxygen and that of the substrate was 100% silicon.
Fig. 6. High resolution SEM image of sputter deposited Pt.
The total number of atoms per unit area of the Ni–NiO film was estimated to be 28.2 × 1017 atoms/cm2 . The average oxygen concentration in the sample estimated from a nuclear reaction spectrum is 7.58 × 1017 atoms/cm2 . This value is in agreement with that obtained from RBS measurement (27% of 28.20 × 1017 atoms/cm2 is 7.60 × 1017 atoms/cm2 ). This film, deposited using 2.5% O2 –97.5% Ar plasma was used as the solid reference electrode film in oxygen sensors. 3.2. Platinum electrode Platinum films deposited under different chamber pressures showed different morphologies when examined under high magnification SEM. The film deposited at 1–5 mTorr chamber pressure was fully dense. As the chamber pressure was increased above 10 mTorr, the film appeared to be porous. Increasing the chamber pressure also decreased the deposition rate significantly. The platinum electrode used in the sensor was deposited at 15 mTorr and showed pores of 10–20 nm size with grains of nearly 100 nm (Fig. 6). On heating the film to 350 ◦ C the morphology changed, and the pores became larger. EDX spectra of the sample showed peaks corresponding to silicon substrate and platinum only. 3.3. YSZ electrolyte
Fig. 5. Curve fitting to estimate the Ni and NiO content of the film (b) in Fig. 4.
The rate of deposition of the YSZ film was low, mainly due to the large separation between the target and the substrate. The films appeared translucent. As shown in Fig. 7, the XRD pattern obtained from the film was consistent with reference data for cubic YSZ (PDF 30-1468). The enhanced intensity of the (2 0 0) peak indicates preferred orientation in the 1 0 0 direction. The XPS analysis of the YSZ film gave a Y to Zr atomic ratio of 0.19, consistent with that in Y0.16 Zr0.84 O2 . High energy resolution scans showed that the binding energies are consistent with that of fully oxidized Zr and Y. SEM imaging
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Fig. 7. XRD spectrum of the sputter deposited YSZ.
of the film deposited on a Si wafer was done to study the morphology and porosity of the film. Fig. 8 shows an SEM image (top view) of YSZ sputter deposited on silicon. The top view of the as-deposited film did not show any porosity and appeared to be very dense even at very high magnification. The cross-sectional image (Fig. 3) showed columnar growth of the grains. This also explains the enhanced intensity of (2 0 0) peak in the XRD pattern (Fig. 7), indicating a preferred grain orientation. 3.4. EMF measurement The measured EMF of a single sensor and that of several sensors connected in series while heating the sensor to 350 ◦ C from room temperature were assessed. For the single sensor, a stable EMF of 117 mV was measured at 275 ◦ C which remained steady upto 300 ◦ C. This tempera-
Fig. 8. High resolution SEM image of sputter deposited YSZ.
ture is about 300◦ lower than that required by typical oxygen sensors using a thick zirconia electrolyte. This significant drop in the operating temperature can be attributed to the thin film electrolyte used in this microfabricated sensor. But the measured EMF for a single sensor was lower than the calculated Nernst voltage of 900 mV under the experimental conditions. This is most likely due to poor sealing of the Ni–NiO reference electrode, which may have resulted in air leakage to the reference electrode. The microstructure of YSZ after heating at 350 ◦ C showed open grain boundaries (Fig. 9). Also since the cross-section of the YSZ film shows columnar growth, the grain boundaries may extend all the way through the thickness of the film, which could act as a leakage path for air from the working to the reference electrode side. By changing the sputtering conditions, it may
Fig. 9. SEM image of sputter deposited YSZ after heating at 350 ◦ C for 1 h.
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Fig. 10. Sensor voltage as a function of the number of sensors in series at 300 ◦ C in air. Fig. 11. Complex impedance spectrum of a single sensor obtained at 300 ◦ C in air.
be possible to change the electrolyte microstructure, which should help the sealing of the reference electrode. Efforts are underway in our lab to develop dense 1 m thick YSZ that does not show the columnar growth by RF sputtering using YSZ target. The EMF of sensors connected in series increased almost linearly with the number of sensors, Fig. 10, in agreement with Eq. (2). In the case of 10 sensors connected together, the EMF was 778 mV at 300 ◦ C. This concept of increasing the sensor signal by connecting many sensors in series is demonstrated for the first time. Upon heating the sensor to 350 ◦ C the voltage decreased slightly. The decrease in voltage continued while maintaining the sensor at 350 ◦ C. Despite this observed lower EMF and decay, it is demonstrated that microfabricated sensors in principle can offer significant advantages over the conventional sensors, in that the operating temperature can be reduced below 300 ◦ C and that the sensor signal can be increased by connecting many sensors in series.
Fig. 12. Impedance spectra of a single oxygen sensor at 350–280 ◦ C in air.
3.5. Impedance spectroscopy Impedance spectroscopy was used to investigate the electrolyte and electrolyte–electrode interfaces. Complex
Fig. 13. Arrhenius plot for the high-frequency and low-frequency resistance estimated from the impedance spectra shown in Fig. 12.
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impedance spectra were obtained in the temperature range 250–350 ◦ C, while heating or cooling the sensor. As illustrated in Figs. 11 and 12, spectra given as Nyquist plots contained at least two semi-circles: one was in the high-frequency region, a high-frequency arc (HFA), and the second was in the low frequency region, a low-frequency arc (LFA). Both the arcs were depressed. In Fig. 11, the low frequency arc is more depressed compared to the high frequency arc and is asymmetric. The asymmetric nature of the LFA may be due to the fact that this arc represents both the electrodes since no reference electrode was used in obtaining the spectrum. Both arcs were found to be temperature dependent. The LFA dominated the spectra obtained at all temperatures. Fig. 12, obtained while cooling the sensor shows that the HFA increased in size as the temperature was lowered. Resistance Rh at various temperatures was estimated from the HFAs in this figure. These estimated Rh values for various temperatures are given as an Arrhenius plot in Fig. 13. The apparent activation energy corresponding to this resistance was calculated from the slope of this plot and was equal to 0.85 eV. This value is close to that of the YSZ electrolyte. Thus, it is possible that the HFA reflects the YSZ resistance. From the plot of Fig. 12, resistance at 310 ◦ C was estimated to be 35 k corresponding to a resistivity of 2.2 M cm. Resistivity values of 1–1.5 M cm are reported [16] for doped zirconia at this temperature. It can also be noted that this arc goes through the origin. The grain and grain boundary arcs are not resolvable. This could be due to the columnar growth of YSZ film, in which the grain boundary is parallel to the measurement direction.
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Fig. 14. Variation of the complex impedance spectra of a single oxygen sensor with time at 350 ◦ C in air. (1) after stabilizing the temperature at 350 ◦ C, (2) 10 min after 1, (3) 25 min after 1, (4) 45 min after 1.
The size of the LFA also increases as the temperature decreases (Fig. 12). The resistance of LFA estimated by fitting the data at various temperatures is also plotted in Fig. 13. The activation energy corresponding to this curve is 1.5 eV, which is a reasonable value for platinum electrodes on YSZ at this temperature range [17]. Hence, this low frequency curve can be associated with the electrode process. It was also noticed that the size of the LFA increased while the temperature was maintained at 350 ◦ C (Fig. 14). This indicates either electrode polarization due to charging of the electrode–electrolyte interface or de-lamination of the electrode from the electrolyte on heating due to poor adhesion between the two. Poor interface contact was confirmed by
Fig. 15. Cross-section of the YSZ–Pt interface of the sensor after heating at 350 ◦ C.
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examining the sensor cross-section in SEM after heating the sensor at 350 ◦ C (Fig. 15). Comparing the spectra obtained while heating up the sensor from room temperature (Fig. 11) and that obtained while cooling from 350 ◦ C (Fig. 12), it can be noticed that the high frequency arc became smaller at the same temperature while cooling and the low frequency arc became larger. This lowering in size of the high frequency arc may be due to the change in zirconia microstructure as can be noticed from the SEM of YSZ before and after heating (Figs. 8 and 9) with associated increased conductivity. The increased size of the low frequency curve may be due to the degradation of the electrodes.
4. Conclusions Miniaturized, series-connected potentiometric thin film oxygen sensors were developed by microfabrication technology and tested. The sensor OCV was tested in air at 300 ◦ C and the sensor was found to be functional at this temperature. This is a significant improvement in lowering the operating temperature compared to traditionally fabricated sensors. Amplification of the sensor output by connecting many sensors in series is also demonstrated. By connecting 10 sensors in series the output voltage is increased almost linearly. The potential developed by a single sensor was much lower than expected. This is likely due to poor sealing of the Ni–NiO reference electrode resulting in an air leakage to the reference electrode through the grain boundary channels. Impedance analysis of the sensor was also performed using a two electrode configuration to elucidate the electrode, electrolyte and interface electrochemical properties and kinetics. Efforts are underway to improve the design of the sensor, to obtain better electrolyte microstructure and also to improve the electrode–electrolyte interface.
Acknowledgements This work was supported in part by University of Utah Technology Innovation Grant (TIG). Part of the research described in this paper was performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the US Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory in Richland, Washington. Authors would like to acknowledge D.C. Stewart, Pacific Northwest National Laboratory for help with sputtering, M.H. Engelhard, Pacific Northwest National Laboratory for XPS, D.E. McCready, Pacific Northwest National Laboratory for XRD, T. Thevuthasan, Pacific Northwest National Laboratory and S. Shuttandan, Pacific Northwest National Laboratory for RBS and NRA and J.S. Young, Pacific Northwest National Laboratory for SEM. R. Radhakrishnan would like to acknowledge the discussions he had with Dr.
M.S. Miller, University of Utah, Dr. Yi Jiang, University of Utah, Dr. G.Y. Lin, University of Utah, Dr. T.J. Amstrong, University of Utah, Dr. L. Saraf, Pacific Northwest National Laboratory and Dr. L.R. Pederson, Pacific Northwest National Laboratory. References [1] D. Eddy, Physical principles of zirconia oxygen exhaust gas sensors, IEEE Trans. Vehicle Technol. VT-23 (4) (1974) 125–128. [2] W. Fleming, Zirconia oxygen sensor an equivalent circuit model, SAE Paper 800020, Presented at the 1980 SAE Congress, Detroit, Michigan, February 28, 1980. [3] D.E. Williams, P. McGeehin, Solid state gas sensors and monitors, Electrochemistry 9 (1984) 246–290. [4] G. Vitter, P. Foster, M. Lahlou, F.J. Gutierrez Monreal, Use of an oxygen mini-gauge for monitoring domestic and medium sized boilers, Solid State Ionics 9–10 (1983) 1273–1276. [5] M. Benammar, Techniques for measurement of oxygen and air-to-fuel ratio using zirconia sensors—a review, Meas. Sci. Technol. 5 (1994) 757–767. [6] W.C. Maskell, B.C.H. Steele, Solid state potentiometric oxygen gas sensors, J. Appl. Electrochem. 16 (1986) 475–489. [7] K.P. Jagannathan, S.K. Tikku, H.S. Ray, A. Ghosh, E.C. Subbarao, in: E.C. Subbarao (Ed.), Solid State Electrolytes and their Applications, Plenum Press, New York, 1980, pp. 201–259. [8] G.W. McLaughlin, K. Braden, B. Franc, G.T.A. Kovacs, Microfabricated solid-state dissolved oxygen sensor, Sens. Actuators B 83 (2002) 138–148. [9] M. Dilhan, D. Esteve, A.M. Gue, O. Mauvais, L. Mercier, Electrochemical oxygen microsensors, Sens. Actuators B 2627 (1995) 401–403. [10] J.F. Currie, A. Essalik, J.-C. Marusic, Micromachined thin film solid state electrochemical CO2 , NO2 and SO2 gas sensors, Sens. Actuators B 59 (1999) 235–241. [11] F.J. Gutierrez Monreal, G. Vitter, Measurement of low oxygen pressures with a solid state electrolyte miniaturized sensor, J. Phys. E: Sci. Instrum. 16 (1983) 361. [12] A.G. Mortimer, G.P. Reed, Development of a robust electrochemical oxygen sensor, Sens. Actuators B 24–25 (1995) 328–335. [13] J.W. Bae, J.Y. Park, S.W. Hwang, G.Y. Yeom, K.D. Kim, Y.A. Cho, J.S. Jeon, D. Choi, Characterization of yttria-stabilized thin films prepared by radio frequency magnetron sputtering for a combustion control oxygen sensor, J. Electrochem. Soc. 147 (2000) 2380– 2384. [14] L. Maya, G.M. Brown, T. Thundat, Porous platinum electrodes derived from the reduction of sputtered platinum dioxide films, J. Appl. Electrochem. 29 (1999) 883–888. [15] T. Tsai, S.A. Barnett, Bias sputter deposition of dense yttria stabilized zirconia films on porous substrates, J. Electrochem. Soc. 142 (9) (1995) 3084–3087. [16] J.R. McDonald, Impedance Spectroscopy, Wiley, New York, 1987. [17] S.P. Yoon, S.W. Nam, S.-G. Kim, S.-A. Hong, S.-H. Hyun, Characteristics of cathodic polarization at Pt/YSZ interface without the effect of electrode microstructure, J. Power Sources 115 (2003) 27–34.
Biographies Rajesh Radhakrishnan obtained his Master of Science (M.Sc.) in Physics from Indian Institute of Technology, Madras, in 1995 and Master of Technology (M.Tech) degree in Materials Science and Engineering from Indian Institute of Technology, Bombay, in 1997. He is currently pursuing a Ph.D. in Materials Science and Engineering at the University of
R. Radhakrishnan et al. / Sensors and Actuators B 105 (2005) 312–321 Utah, Salt Lake City. His research interests include materials and electrochemical aspects of fuel cells and oxygen sensors, microfabrication and semiconductor devices. Professor Anil V. Virkar is the Chair of Department of Materials Science and Engineering, University of Utah, Salt Lake City. He obtained a Bachelor’s degree in Metallurgical Engineering from Indian Institute of Technology in 1967, an M.S. degree in Engineering Mechanics from Louisiana State University, and Ph.D. degree in Materials Science from Northwestern University in 1973. His present research areas include solid state electrochemistry, and electrochemical devices. He has authored over 160 scientific publications, and received 30 patents. Dr. Subhash C. Singhal is a Battelle fellow and Director, Fuel Cells, at Pacific Northwest National Laboratory. He obtained a B.S. in Physics, Chemistry, and Mathematics from Agra University in 1963, a B.E. in Metallurgy from Indian Institute of Science in 1965 and a Ph.D. in Materials Science and Engineering from the University of Pennsylvania in 1969. Dr. Singhal joined the Energy Science and Technology Directorate at PNNL in April 2000 after having worked at Siemens Westinghouse Power Corporation (formerly Westinghouse Electric Corporation) for over 29 years where he led an internationally recognized group in solid oxide fuel cell (SOFC) technology. He has authored over 75 scientific publications, edited 13 books, received 13 patents, and given over 190 plenary, keynote and other invited presentations worldwide. At PNNL, Dr. Singhal is responsible for providing senior technical, managerial,
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and commercialization leadership to the lab’s fuel cell program. His research interests are in advanced materials for high temperature energy conversion systems, and solid state electrochemistry and electrochemical devices, especially solid oxide fuel cells. Glen Dunham received a Bachelor of Arts in Chemistry and a Bachelor of Science in Physics from Pacific Lutheran University in 1980, and a Master of Science in Materials Science from Washington State University in 1983. Prior to joining PNNL, his research was primarily in III–V photovoltaics, including fabricating one of the most efficient solar cells of its time. At PNNL, his unique interest has been developing microfabrication capabilities directed toward microfluidics, chemical and biological sensing, and terahertz optics. Recently, he has begun applying microfabrication solutions to national security problems in biometrics. He holds two patents with several pending, and has published in the fields of chemical sensing, piezoelectrics, polymer thin films, dielectric materials, and microfabrication. Dr. Olga A Marina is a senior research scientist at the Pacific Northwest National Laboratory. She received her Ph.D. in chemistry from Boreskov Institute of Catalysis and held post-doctoral positions at Risø National Laboratory, Denmark and at University of Patras, Greece. Her research interests include development of advanced materials for solid oxide fuel cells, sensors and electrolyzers, electrocatalysis and fuel processing. She co-authored over 30 research papers.