- Email: [email protected]
Semiconductor sensors for fluorine detection — optimization for low and high concentrations
Sensors and Actuators B 65 Ž2000. 270–272 www.elsevier.nlrlocatersensorb
Semiconductor sensors for fluorine detection — optimization for low and high concentrations L. Bartholomaus ¨ a
a,)
, A.A. Vasiliev b, W. Moritz
a
Humboldt-UniÕersity Berlin, Inst. of Phys. Chemistry, Bunsenstr. 1, 10117 Berlin, Germany b RRC ‘‘KurchatoÕ Institute’’, 123182 Moscow, Russian Federation Accepted 4 August 1999
Abstract For the detection of fluorine, two different preparation methods for semiconductor gas sensors were developed, the first for concentrations between 10 and 1000 ppm Žtype I. and another for concentrations between 0.01 and 10 ppm Žtype II.. The sensitivity of type I sensors is about 116 mVrlgŽpŽF2 ... It is possible to detect gas concentrations down to 0.1 ppm using this sensor. The main disadvantage is that the sensor response kinetics depends strongly on concentration. The sensors response is fast for measurement of high concentrations Žbetween 10 and 1000 ppm. but the response time is not acceptable for the detection of small concentrations. Type II sensors show a sensitivity of 28 mVrlgŽpŽF2 .. for gas concentrations between 0.006 and 10 ppm. This sensor is very fast in the detection of small concentrations of gas. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Fluorine; LaF3 ; ISFET
1. Introduction The determination of fluorine is very important for environmental monitoring. The highest tolerable level is, for example, 0.1 ppm in Germany. For the detection of fluorine, electrochemical cells are produced by several companies w1x. They all have a liquid electrolyte as the main disadvantage. Recently we suggested to detect fluorine using a MIS structure with the ionic conductor LaF3 as the sensitive layer w2,3x. A problem of this structure was the bad reproducibility of the sensitivity Ž60–200 mVrlgŽpŽF2 .... This was solved with a special pretreatment of the sensor in 1000 ppm of fluorine. After this pretreatment, a stable sensitivity was found between 0.1 and 1000 ppm w4x. The remaining disadvantage of this sensor was the dependence of the response kinetics of the system on the gas concentration. The sensor response kinetics was getting rather slow with smaller concentrations of fluorine. Therefore, our main aim was to investigate this response
)
Corresponding author. Tel.: q49-30-2093-5550; fax: q49-30-20935553; e-mail: [email protected]
Fig. 1. Scheme of a sensor based on n-SirSiO2 rSi 3 N4 rLaF3 rplatinum.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 3 3 7 - 8
L. Bartholomaus ¨ et al.r Sensors and Actuators B 65 (2000) 270–272
271
Table 1 Type I
Type II
–Vacuum deposition of 250 nm LaF3 –Platinum deposition 10 nm –2 min synthetic air at temperatures higher than 3008C –24 h in 1000 ppm fluorine in synthetic air
–Vacuum deposition of 250 nm LaF3 –Platinum deposition 30 nm –24 h in 10 ppm fluorine in synthetic air at a flow rate of 10 mlrmin
kinetics and to develop another preparation of sensors for measurement of low concentrations.
For the sensor based on LaF3rPt, different sensitivities were found depending on the history of the sensor. Therefore, two methods of preparation to get stable sensitivity were developed ŽTable 1.. Each of these leads to advantages, one for high and the other for low concentrations of gas.
The sensors were investigated between 0.1 and 1000 ppm of fluorine in synthetic air for type I and between 0.006 and 10 ppm for type II. The sensitivity was 115.8 mVrlgŽpŽF2 .. " 2.3 mV for type I and 28.0 mVrlgŽpŽF2 .. " 0.5 mV for type II ŽFig. 2.. Furthermore, the potential of both types differs by 400–500 mV at 10 ppm. For the characterization of the dynamics of the response, the time t50 was used. This is the time for 50% of the potential shift after a change in concentration. The influence of the concentration on the response kinetics is given for both preparations ŽFig. 3.. It is important to note that the dynamic response depends only on the concentration after the change of gas mixture but not on the initial concentration. In the double logarithmic scale, we found a slope of y0.51 for the type I sensor. As a consequence, this sensor is fast enough for measurements of high concentrations Žbetween 10 and 1000 ppm. but the response time is not acceptable for the detection of small concentrations between 0.1 and 10 ppm. For type II, the response kinetics does have a concentration dependence with a slope of y0.52 but is much faster than the type I sensors. The dependence of the response kinetics on the thickness of platinum was investigated too. After type I preparation, the sensors response time was influenced by the thickness of platinum Žthickness range between 5 and 100 nm.. The linear fitting of the t50 time in dependence on thickness in double logarithmic scale leads to function with a slope of 0.48. For diffusion-limited response kinetics, a function with a slope of 2 was expected. Therefore, simple diffusion is not a rate-determining step.
Fig. 2. Sensitivity to fluorine for type I and type II sensors.
Fig. 3. Dependence of t50 time from concentration for type I and type II sensors.
2. Experimentals The semiconductor substrates used were n-SirSiO 2r Si 3 N4 . Using a vacuum evaporating technique, these samples were coated with LaF3 . Platinum gate contacts were produced by sputtering in argon. The scheme of the sensor is given in Fig. 1. A computer-controlled measuring and gas mixing system was able to characterize the sensors in different concentrations of fluorine in synthetic air by High-Frequency Capacitance–Voltage method. The sensors were prepared in two different ways. The preparation steps are given in Table 1. The mechanism was studied using impedance spectroscopy. Single crystals of LaF3 were coated with platinum on one side. The backside contact was an electrolyte Ž1 M NaCl and 0.1 M NaF.. A Zahner Imd 5 impedance spectrometer was used.
3. Results and discussion
272
L. Bartholomaus ¨ et al.r Sensors and Actuators B 65 (2000) 270–272
4. Conclusions Two different methods to produce sensors were developed. It depends on the concentration range, witch preparation should be used. So it is possible to combine both sensor types to have a system suitable for measuring fluorine concentrations between 0.006 and 1000 ppm. The mechanism of type II sensors can be described by an electrochemical reaction with an exchange of two electrons: F2 q 2ey 2Fy
m
Fig. 4. Impedance spectroscopy on a single crystal of LaF3 with platinum electrode after 24 h of preconditioning in 1000 ppm of fluorine.
With type II sensors, a fast sensor with a sensitivity of 28 mVrlgŽpŽF2 .. for low concentrations was obtained. But after contact with gas concentrations of more than 100 ppm, the sensitivity becomes higher and not reproducible. In addition, the sensor becomes slower. In contrast to this, the type I sensor is stable in this range of concentrations. To study the mechanism of the sensors, impedance spectroscopy was used. Fig. 4 is an example for spectra on a single crystal of LaF3 at different concentrations. The platinum gate on the crystal was exposed for 24 h to 1000 ppm fluorine as for the type I thin film sensor. The sensitivity between 0.1 and 1000 ppm was the same as for thin film type I sensors. To describe the spectra, an equivalent circuit with an exchange resistance in parallel to an interface capacitance was used. A linear dependence of the resistance from concentration in double logarithmic scale with a slope of y0.498 from 0.1 to 100 ppm is analogue to an electrochemical reaction.
This leads to a sensitivity of 28 mVrlgŽpŽF2 .. according to Nernst law. For type I sensors, the mechanism is more complex and cannot be described by a two-electron electrochemical reaction.
Acknowledgements This work was done with the financial support of DFG.
References w1x Tooru Ichichi, Shoichi, Masaaki Ishizuka ŽRiken Keiki., Potentiostatic Electrolytic Acid Gas Sensor, JP 07, 55,768, March 3, 1995. w2x W. Moritz, S. Krause, A.A. Vasiliev, D.Yu. Godovski, V.V. Malyshev, Monitoring of HF and F2 using a field effect sensor, The Fifth International Meeting on Chemical Sensors, Rome, July 11–14, 1994 w3x W. Moritz, S. Krause, A.A. Vasiliev, D.Yu. Godovski, V.V. Malyshev, Sens. Actuators, B 24–25 Ž1995. 194. w4x W. Moritz, S. Krause, L. Bartholomaus, ¨ T. Gabusjan, A.A. Vasiliev, D.Yu. Godowski, V.V. Malyshev, Silicon Based Sensor for Fluorine Gas, Orlando American Chemical Society Meeting, Orlando, USA, August 25–30, 1996.
Semiconductor sensors for fluorine detection — optimization for low and high concentrations L. Bartholomaus ¨ a
a,)
, A.A. Vasiliev b, W. Moritz
a
Humboldt-UniÕersity Berlin, Inst. of Phys. Chemistry, Bunsenstr. 1, 10117 Berlin, Germany b RRC ‘‘KurchatoÕ Institute’’, 123182 Moscow, Russian Federation Accepted 4 August 1999
Abstract For the detection of fluorine, two different preparation methods for semiconductor gas sensors were developed, the first for concentrations between 10 and 1000 ppm Žtype I. and another for concentrations between 0.01 and 10 ppm Žtype II.. The sensitivity of type I sensors is about 116 mVrlgŽpŽF2 ... It is possible to detect gas concentrations down to 0.1 ppm using this sensor. The main disadvantage is that the sensor response kinetics depends strongly on concentration. The sensors response is fast for measurement of high concentrations Žbetween 10 and 1000 ppm. but the response time is not acceptable for the detection of small concentrations. Type II sensors show a sensitivity of 28 mVrlgŽpŽF2 .. for gas concentrations between 0.006 and 10 ppm. This sensor is very fast in the detection of small concentrations of gas. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Fluorine; LaF3 ; ISFET
1. Introduction The determination of fluorine is very important for environmental monitoring. The highest tolerable level is, for example, 0.1 ppm in Germany. For the detection of fluorine, electrochemical cells are produced by several companies w1x. They all have a liquid electrolyte as the main disadvantage. Recently we suggested to detect fluorine using a MIS structure with the ionic conductor LaF3 as the sensitive layer w2,3x. A problem of this structure was the bad reproducibility of the sensitivity Ž60–200 mVrlgŽpŽF2 .... This was solved with a special pretreatment of the sensor in 1000 ppm of fluorine. After this pretreatment, a stable sensitivity was found between 0.1 and 1000 ppm w4x. The remaining disadvantage of this sensor was the dependence of the response kinetics of the system on the gas concentration. The sensor response kinetics was getting rather slow with smaller concentrations of fluorine. Therefore, our main aim was to investigate this response
)
Corresponding author. Tel.: q49-30-2093-5550; fax: q49-30-20935553; e-mail: [email protected]
Fig. 1. Scheme of a sensor based on n-SirSiO2 rSi 3 N4 rLaF3 rplatinum.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 3 3 7 - 8
L. Bartholomaus ¨ et al.r Sensors and Actuators B 65 (2000) 270–272
271
Table 1 Type I
Type II
–Vacuum deposition of 250 nm LaF3 –Platinum deposition 10 nm –2 min synthetic air at temperatures higher than 3008C –24 h in 1000 ppm fluorine in synthetic air
–Vacuum deposition of 250 nm LaF3 –Platinum deposition 30 nm –24 h in 10 ppm fluorine in synthetic air at a flow rate of 10 mlrmin
kinetics and to develop another preparation of sensors for measurement of low concentrations.
For the sensor based on LaF3rPt, different sensitivities were found depending on the history of the sensor. Therefore, two methods of preparation to get stable sensitivity were developed ŽTable 1.. Each of these leads to advantages, one for high and the other for low concentrations of gas.
The sensors were investigated between 0.1 and 1000 ppm of fluorine in synthetic air for type I and between 0.006 and 10 ppm for type II. The sensitivity was 115.8 mVrlgŽpŽF2 .. " 2.3 mV for type I and 28.0 mVrlgŽpŽF2 .. " 0.5 mV for type II ŽFig. 2.. Furthermore, the potential of both types differs by 400–500 mV at 10 ppm. For the characterization of the dynamics of the response, the time t50 was used. This is the time for 50% of the potential shift after a change in concentration. The influence of the concentration on the response kinetics is given for both preparations ŽFig. 3.. It is important to note that the dynamic response depends only on the concentration after the change of gas mixture but not on the initial concentration. In the double logarithmic scale, we found a slope of y0.51 for the type I sensor. As a consequence, this sensor is fast enough for measurements of high concentrations Žbetween 10 and 1000 ppm. but the response time is not acceptable for the detection of small concentrations between 0.1 and 10 ppm. For type II, the response kinetics does have a concentration dependence with a slope of y0.52 but is much faster than the type I sensors. The dependence of the response kinetics on the thickness of platinum was investigated too. After type I preparation, the sensors response time was influenced by the thickness of platinum Žthickness range between 5 and 100 nm.. The linear fitting of the t50 time in dependence on thickness in double logarithmic scale leads to function with a slope of 0.48. For diffusion-limited response kinetics, a function with a slope of 2 was expected. Therefore, simple diffusion is not a rate-determining step.
Fig. 2. Sensitivity to fluorine for type I and type II sensors.
Fig. 3. Dependence of t50 time from concentration for type I and type II sensors.
2. Experimentals The semiconductor substrates used were n-SirSiO 2r Si 3 N4 . Using a vacuum evaporating technique, these samples were coated with LaF3 . Platinum gate contacts were produced by sputtering in argon. The scheme of the sensor is given in Fig. 1. A computer-controlled measuring and gas mixing system was able to characterize the sensors in different concentrations of fluorine in synthetic air by High-Frequency Capacitance–Voltage method. The sensors were prepared in two different ways. The preparation steps are given in Table 1. The mechanism was studied using impedance spectroscopy. Single crystals of LaF3 were coated with platinum on one side. The backside contact was an electrolyte Ž1 M NaCl and 0.1 M NaF.. A Zahner Imd 5 impedance spectrometer was used.
3. Results and discussion
272
L. Bartholomaus ¨ et al.r Sensors and Actuators B 65 (2000) 270–272
4. Conclusions Two different methods to produce sensors were developed. It depends on the concentration range, witch preparation should be used. So it is possible to combine both sensor types to have a system suitable for measuring fluorine concentrations between 0.006 and 1000 ppm. The mechanism of type II sensors can be described by an electrochemical reaction with an exchange of two electrons: F2 q 2ey 2Fy
m
Fig. 4. Impedance spectroscopy on a single crystal of LaF3 with platinum electrode after 24 h of preconditioning in 1000 ppm of fluorine.
With type II sensors, a fast sensor with a sensitivity of 28 mVrlgŽpŽF2 .. for low concentrations was obtained. But after contact with gas concentrations of more than 100 ppm, the sensitivity becomes higher and not reproducible. In addition, the sensor becomes slower. In contrast to this, the type I sensor is stable in this range of concentrations. To study the mechanism of the sensors, impedance spectroscopy was used. Fig. 4 is an example for spectra on a single crystal of LaF3 at different concentrations. The platinum gate on the crystal was exposed for 24 h to 1000 ppm fluorine as for the type I thin film sensor. The sensitivity between 0.1 and 1000 ppm was the same as for thin film type I sensors. To describe the spectra, an equivalent circuit with an exchange resistance in parallel to an interface capacitance was used. A linear dependence of the resistance from concentration in double logarithmic scale with a slope of y0.498 from 0.1 to 100 ppm is analogue to an electrochemical reaction.
This leads to a sensitivity of 28 mVrlgŽpŽF2 .. according to Nernst law. For type I sensors, the mechanism is more complex and cannot be described by a two-electron electrochemical reaction.
Acknowledgements This work was done with the financial support of DFG.
References w1x Tooru Ichichi, Shoichi, Masaaki Ishizuka ŽRiken Keiki., Potentiostatic Electrolytic Acid Gas Sensor, JP 07, 55,768, March 3, 1995. w2x W. Moritz, S. Krause, A.A. Vasiliev, D.Yu. Godovski, V.V. Malyshev, Monitoring of HF and F2 using a field effect sensor, The Fifth International Meeting on Chemical Sensors, Rome, July 11–14, 1994 w3x W. Moritz, S. Krause, A.A. Vasiliev, D.Yu. Godovski, V.V. Malyshev, Sens. Actuators, B 24–25 Ž1995. 194. w4x W. Moritz, S. Krause, L. Bartholomaus, ¨ T. Gabusjan, A.A. Vasiliev, D.Yu. Godowski, V.V. Malyshev, Silicon Based Sensor for Fluorine Gas, Orlando American Chemical Society Meeting, Orlando, USA, August 25–30, 1996.