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A study of the sensing properties of thin film sensor to trimethylamine
Sensors and Actuators B 53 (1998) 8 – 12
A study of the sensing properties of thin film sensor to trimethylamine Guorui Dai * Department of Electronic Engineering, Jilin Uni6ersity, 79 Jiefang Road, Changchun, 130023, People’s Republic of China Received 12 September 1997; received in revised form 25 May 1998; accepted 2 June 1998
Abstract TiO2 –Fe2O3 sensing thin films have been deposited onto a ceramic substrates using TiCl4 and Fe(CO)5 as the precursors by means of plasma enhanced chemical vapour deposition (PECVD) technique. This type of thin film is used to fabricate the sensors. The gas sensing characteristics of the thin film sensors were measured for trimethylamine (TMA). In order to further improve the sensing properties, a small amounts of In is coated over the surface of TiO2 –Fe2O3 thin films. A high sensitivity and excellent selectivity of the thin film sensors to TMA was found. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Thin film sensor; Trimethylamine; Sensing characteristic
1. Introduction It is well-known that detection of fish freshness is the most important problem in the fish-processing industry. Some gaseous species such as trimethylamine (TMA), dimethylamine and ammonia are given off during the deterioration of fish after death [1]. One of the most widely used methods of testing freshness is chemically measuring the breakdown products of adenosine triphosphate-related compounds in the fish’s tissue [2], which requires a great deal of effort and much time. These facts suggest that the fish freshness can be monitored by a rapid, continuous and non-destructive way using a special gas sensor capable of detecting TMA [3]. Previous studies were directed towards investigating the effects of metal additives on the TMA-sensing properties of TiO2 sensors [4,5]. It was revealed that the sensitivity could be markedly improved by the addition of a small amounts of Ru or In to TiO2 specimen. At present, an interest of research has been shifted to thin-film sensors [6]. This approach has obvious advantages over conventionally fabricated gas sensors on ceramic substrates, e.g. small size, low costs due to automatic and batch production, low energy consumption and the possibility of integration with electronics. * Tel.: +86 431 8923189; fax: + 86 431 8923907; e-mail: [email protected]
Truly thin film gas sensors, deposited by conventional methods, have generally shown poor gas selectivity and poor stability [7]. The author thinks that the sensing property can be detected by a change in the electric conductance of the thin films and which is composed of the bulk conductance and surface conductance, then
&
d
G=
0
s(x)
W W(D− d) dx+ s0 L L
where L, W and D are the length, width and thickness of the sample, respectively. d is the thickness of the surface layer. s0 and s(x) are the bulk and surface conductivity, respectively. Therefore, the surface conductance depends on the concentration of the chemisorbed gas on the semiconductor and the bulk conductance depends on the structure of the double-layer thin films. In the present paper, the TiO2 –Fe2O3 thin films have been deposited by using PECVD technique and the sensing characteristics of the sensors were measured for TMA gas.
2. Experimental
2.1. Thin film preparation and structure analysis Thin film sensing materials, such as titanium dioxide (TiO2) and alpha hematite (a-Fe2O3), have been fabri-
0925-4005/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00286-X
G. Dai / Sensors and Actuators B 53 (1998) 8–12
9
Fig. 1. Experimental set-up for PECVD.
cated onto the silicon wafer or ceramic tube substrates by using PECVD technology. The schematic diagram of the PECVD reaction system is shown in Fig. 1. Titanium tetrachloride (TiCl4) and pentacarbonyl iron [Fe(CO)5] reactants are used as the precursors. The chamber was pumped down to approximately 0.26 Pa and then the desired amounts of oxygen and precursor vapours were introduced into the chamber. The substrate temperature was maintained at 200°C during the thin film deposition. The radio frequency is 13.56 MHz with 300 W power. According to S.P. Mukherjee’s report [8] oxygen may undergo the following reactions with electrons by the resonance capture process: O2 +e − O2 O2− O − +O at 4.53 eV net. The TiO2 –Fe2O3 thin films were deposited by the plasma excited decomposition reaction as follows: TiCl4 +2O TiO2 +2Cl2
tal gold electrodes on the outer wall of a ceramic tube. Electrical contacts were made with 0.05 mm gold wires attached to the gold electrodes (the structure of the film is shown in Fig. 2). The thin film sensors were set up in a 0.18 m3 glass test chamber, kept under a continuous flow of fresh air for 10 min before measurement. The operation temperature was maintained at 170°C, when the heating voltage was 4.0 V. During the electrical measurements, approx. 4.0 V was supplied to either of the coils for heating the sensors and the circuit voltage (V= 10 V) was applied across the sensor and load resistance (RL = 2 KV) connected in series. The signal voltage across the load resistance, which changed with sort and concentration of gas was measured. The gas sensitivities to TMA, H2, C2H5OH, gasoline gas, CO and CH4 were measured. A given amount of each gas was injected into the chamber and mixed by a fan for 30 s. The sensitivity to gases, S, is defined as S= Vg/Va, where Va and Vg are the voltage drop across the load resistance in air and testing gases/air mixture, respectively.
4Fe(CO)5 +26O2Fe2O3 +20CO2 In general, the deposition rate of the thin films is between 10 and 15 nm min − 1 under typical deposition conditions. In-depth profiles of the TiO2 – Fe2O3 films were measured by means of Auger electron spectroscopy (AES) combined with sputtering etching by argon ions. AES measurements were carried out with a PHI model 550 electron spectrometer. According to Yuji Takao’s report, surface modification of the thin film sensors has been carried out by impregnating the thin films with an aqueous solution of In(NO3)3, followed by drying and heating at 400°C in air for 12 h.
3. Results and discussion
3.1. AES analysis of TiO2 –Fe2O3 thin films Fig. 3 shows the surface elemental composition of
2.2. Gas-sensiti6e characteristic measurements TiO2 –Fe2O3 films were deposited between interdigi-
Fig. 2. Scheme of film structure.
10
G. Dai / Sensors and Actuators B 53 (1998) 8–12
Fig. 4. Relationship between gas sensitivity and operating temperature at various TMA concentrations.
Fig. 3. AES spectra of TiO2 –Fe2O3 thin films: (a) Auger electron spectrum of the films; and (b) In-depth profiles of elemental composition in the films
the operating temperature for various TMA concentrations. As shown in Fig. 4, for 100, 500 and 1000 ppm of TMA, the optimum of the operating temperature is 230°C. The gas sensitivity of the sensors increases with an increase in the operating temperature. The operating temperature range of 140 230°C of the thin film sensors is much lower than that of the sintered Ru/TiO2 sensor [9]. This property is useful for us to decrease the heating power in practice. Fig. 5 shows the relationship between the gas sensitivity and TMA concentrations. Furthermore, we have found that the gas sensitivity of the sensor increases with increasing TMA concentration. A low detecting
the TiO2 –Fe2O3 thin films, obtained by means of AES analyses, indicating that the double layer films consist of oxygen, titanium and iron in Fig. 3 (a). An in-depth profile of TiO2 – Fe2O3 thin films can be seen as shown in Fig. 3 (b). An abrupt interface appears between the Fe2O3 thin films and the silicon substrate. In the thin films, TiO2 – Fe2O3, interaction with each other to form a large transition range during the film deposition process is observed. The transition range is considered to be an important factor in controlling charge transfer of the thin film sensors.
3.2. Sensing characteristics of the thin film sensors In general, the sensitivity of the sensors is affected by the operating temperature. The higher temperature enhances surface reaction of the thin films and gives higher sensitivity in a temperature range. Fig. 4 shows the relationship between the gas sensitivity and
Fig. 5. Gas sensitivity as a function of TMA concentration at 170°C.
G. Dai / Sensors and Actuators B 53 (1998) 8–12
11
Fig. 8. Response and recovery characteristics of the thin film sensor to 300 ppm TMA.
Fig. 6. Gas sensitivity as a function of operating temperature to different gases at the same concentration of 300 ppm.
concentration of TMA is 30 ppm and the gas sensitivity is approx. 3 at 170°C. This type of thin film sensors was also tested for their sensitivity to other gases. Fig. 6 shows their sensitivity to TMA. NH3, C2H5OH, gasoline gas, CH4, CO and H2 at the same concentration of 300 ppm as a function of operating temperature. These experimental results indicate that the TiO2 – Fe2O3 thin film sensors are very sensitive to TMA at 140 – 230°C, but not sensitive to CH4, CO and H2. The discrimination between TMA gas and NH3, C2H5OH, gasoline gas was expressed by a separation coefficient, C, calculated as follows
C= (VTMA − Va)/(Vx − Va) where VTMA, Va and Vx are voltage drop across the load resistance in TMA, air and test gases. Fig. 7 shows the relationship between the separation coefficient and the operating temperature at 300 ppm of the test gases. The separation coefficient is over 4.0 at 140°C. Therefore, the sensors of TiO2 –Fe2O3 thin films exhibits both high sensitivity and excellent selectivity to TMA gas. Fig. 8 shows the typical gas-response characteristic of the sensors of TiO2 –Fe2O3 thin films. After an introduction of 300 ppm of TMA gas, the response appears immediately. The 90% response time is within 5 s. The 90% recovery time is 20 s.
4. Conclusions Based upon the present analysis, we have shown that the sensor of TiO2 –Fe2O3 thin films prepared by PECVD technique provides the best sensitivity, selectivity and response to TMA gas. Thus it is concluded that the method based on double layer thin films modified by the addition of In may be useful for TMA detection. Studies of the sensing mechanism of the surface modification and TMA gas detection are in progress.
Acknowledgements
Fig. 7. Relationship between separation coefficient of TMA with respect to C2H5OH, NH3, gasoline and operating temperature.
The author wishes to thank Professor Kaiji Zhen for helpful discussion and associate Professor Guan Yuguo, Nan Jin, Han Zheng and Xu Xiulai who took some part in the work.
12
G. Dai / Sensors and Actuators B 53 (1998) 8–12 by a semiconductive Ru/TiO2 sensor, J. Electrochem. Soc. 5 (10) (1988) 2539 – 2540.
References [1] M. Egashira, Y. Shimizu, Y. Takao, Enhancement of trimethylamine sensitivity of semiconductor gas sensors by ruthenium, Chem. Lett. 195 (3) (1988) 389–392. [2] T. Saito, K. Arai, M. Matsuyoshi, A new method for estimating the freshness of fish, Bull. Jpn. Soc. Sci. Fish 24 (1959) 749 – 750. [3] Y. Takao, Y. Iwanaga, Y. Shimizu, M. Egashira, Trimethylamine-sensing mechanism of TiO2-based sensors 1, Sensors and Actuators B 10 (1993) 229–234. [4] Y. Takao, K. Fukuda, Y. Shimizu, M. Egashira, Trimethylaminesensing mechanism of TiO2-based sensors 2, Sensors and Actuators B 10 (1993) 235 –239. [5] M. Egashira, Y. Shimizu, Y. Takao, Trimethylamine sensor based on semiconductive metal oxides for detection of fish freshness, Sensors and Actuators B 1 (1990) 108–112. [6] V. Demarne, A. Grisel, An integrated low-power thin-film CO gas sensor on silicon, Sensors and Actuators 13 (1988) 301–313. [7] J.C. Anderson, Thin film transducers and sensors, J. Vac. Sci. Technol. A 4 (3) (1986) 610–616. [8] S.P. Mukherjee, P.E. Evans, The deposition of thin films by the decomposition of tetra-ethoxy silane in a radio frequency glow discharge, Thin Solid Films 14 (1972) 105–118. [9] Y. Shimizu, Y. Takao, M. Egashira, Detection of freshness of fish
.
Biography Guorui Dai, was born in February 1940 in Liaoning province. In July 1963, he graduated from the Department of Semiconductors, Jilin university and since then has been a teacher in Jilin university. He is a professor now. His research interests cover two main areas—thin film preparation and characterisation and it’s relative device fabrication. His current research work includes production of metal oxide thin films and polymer thin films by means of chemical vapour deposition (PECVD, LPCVD, Laser-CVD and photo-CVD) and fabrication of the film gas sensors and micro gas sensor array and pattern recognition analysis. He is the author of over 100 papers published in scientific magazines and proceedings and has published two teaching manuals, one book and two patents applied in China.
A study of the sensing properties of thin film sensor to trimethylamine Guorui Dai * Department of Electronic Engineering, Jilin Uni6ersity, 79 Jiefang Road, Changchun, 130023, People’s Republic of China Received 12 September 1997; received in revised form 25 May 1998; accepted 2 June 1998
Abstract TiO2 –Fe2O3 sensing thin films have been deposited onto a ceramic substrates using TiCl4 and Fe(CO)5 as the precursors by means of plasma enhanced chemical vapour deposition (PECVD) technique. This type of thin film is used to fabricate the sensors. The gas sensing characteristics of the thin film sensors were measured for trimethylamine (TMA). In order to further improve the sensing properties, a small amounts of In is coated over the surface of TiO2 –Fe2O3 thin films. A high sensitivity and excellent selectivity of the thin film sensors to TMA was found. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Thin film sensor; Trimethylamine; Sensing characteristic
1. Introduction It is well-known that detection of fish freshness is the most important problem in the fish-processing industry. Some gaseous species such as trimethylamine (TMA), dimethylamine and ammonia are given off during the deterioration of fish after death [1]. One of the most widely used methods of testing freshness is chemically measuring the breakdown products of adenosine triphosphate-related compounds in the fish’s tissue [2], which requires a great deal of effort and much time. These facts suggest that the fish freshness can be monitored by a rapid, continuous and non-destructive way using a special gas sensor capable of detecting TMA [3]. Previous studies were directed towards investigating the effects of metal additives on the TMA-sensing properties of TiO2 sensors [4,5]. It was revealed that the sensitivity could be markedly improved by the addition of a small amounts of Ru or In to TiO2 specimen. At present, an interest of research has been shifted to thin-film sensors [6]. This approach has obvious advantages over conventionally fabricated gas sensors on ceramic substrates, e.g. small size, low costs due to automatic and batch production, low energy consumption and the possibility of integration with electronics. * Tel.: +86 431 8923189; fax: + 86 431 8923907; e-mail: [email protected]
Truly thin film gas sensors, deposited by conventional methods, have generally shown poor gas selectivity and poor stability [7]. The author thinks that the sensing property can be detected by a change in the electric conductance of the thin films and which is composed of the bulk conductance and surface conductance, then
&
d
G=
0
s(x)
W W(D− d) dx+ s0 L L
where L, W and D are the length, width and thickness of the sample, respectively. d is the thickness of the surface layer. s0 and s(x) are the bulk and surface conductivity, respectively. Therefore, the surface conductance depends on the concentration of the chemisorbed gas on the semiconductor and the bulk conductance depends on the structure of the double-layer thin films. In the present paper, the TiO2 –Fe2O3 thin films have been deposited by using PECVD technique and the sensing characteristics of the sensors were measured for TMA gas.
2. Experimental
2.1. Thin film preparation and structure analysis Thin film sensing materials, such as titanium dioxide (TiO2) and alpha hematite (a-Fe2O3), have been fabri-
0925-4005/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00286-X
G. Dai / Sensors and Actuators B 53 (1998) 8–12
9
Fig. 1. Experimental set-up for PECVD.
cated onto the silicon wafer or ceramic tube substrates by using PECVD technology. The schematic diagram of the PECVD reaction system is shown in Fig. 1. Titanium tetrachloride (TiCl4) and pentacarbonyl iron [Fe(CO)5] reactants are used as the precursors. The chamber was pumped down to approximately 0.26 Pa and then the desired amounts of oxygen and precursor vapours were introduced into the chamber. The substrate temperature was maintained at 200°C during the thin film deposition. The radio frequency is 13.56 MHz with 300 W power. According to S.P. Mukherjee’s report [8] oxygen may undergo the following reactions with electrons by the resonance capture process: O2 +e − O2 O2− O − +O at 4.53 eV net. The TiO2 –Fe2O3 thin films were deposited by the plasma excited decomposition reaction as follows: TiCl4 +2O TiO2 +2Cl2
tal gold electrodes on the outer wall of a ceramic tube. Electrical contacts were made with 0.05 mm gold wires attached to the gold electrodes (the structure of the film is shown in Fig. 2). The thin film sensors were set up in a 0.18 m3 glass test chamber, kept under a continuous flow of fresh air for 10 min before measurement. The operation temperature was maintained at 170°C, when the heating voltage was 4.0 V. During the electrical measurements, approx. 4.0 V was supplied to either of the coils for heating the sensors and the circuit voltage (V= 10 V) was applied across the sensor and load resistance (RL = 2 KV) connected in series. The signal voltage across the load resistance, which changed with sort and concentration of gas was measured. The gas sensitivities to TMA, H2, C2H5OH, gasoline gas, CO and CH4 were measured. A given amount of each gas was injected into the chamber and mixed by a fan for 30 s. The sensitivity to gases, S, is defined as S= Vg/Va, where Va and Vg are the voltage drop across the load resistance in air and testing gases/air mixture, respectively.
4Fe(CO)5 +26O2Fe2O3 +20CO2 In general, the deposition rate of the thin films is between 10 and 15 nm min − 1 under typical deposition conditions. In-depth profiles of the TiO2 – Fe2O3 films were measured by means of Auger electron spectroscopy (AES) combined with sputtering etching by argon ions. AES measurements were carried out with a PHI model 550 electron spectrometer. According to Yuji Takao’s report, surface modification of the thin film sensors has been carried out by impregnating the thin films with an aqueous solution of In(NO3)3, followed by drying and heating at 400°C in air for 12 h.
3. Results and discussion
3.1. AES analysis of TiO2 –Fe2O3 thin films Fig. 3 shows the surface elemental composition of
2.2. Gas-sensiti6e characteristic measurements TiO2 –Fe2O3 films were deposited between interdigi-
Fig. 2. Scheme of film structure.
10
G. Dai / Sensors and Actuators B 53 (1998) 8–12
Fig. 4. Relationship between gas sensitivity and operating temperature at various TMA concentrations.
Fig. 3. AES spectra of TiO2 –Fe2O3 thin films: (a) Auger electron spectrum of the films; and (b) In-depth profiles of elemental composition in the films
the operating temperature for various TMA concentrations. As shown in Fig. 4, for 100, 500 and 1000 ppm of TMA, the optimum of the operating temperature is 230°C. The gas sensitivity of the sensors increases with an increase in the operating temperature. The operating temperature range of 140 230°C of the thin film sensors is much lower than that of the sintered Ru/TiO2 sensor [9]. This property is useful for us to decrease the heating power in practice. Fig. 5 shows the relationship between the gas sensitivity and TMA concentrations. Furthermore, we have found that the gas sensitivity of the sensor increases with increasing TMA concentration. A low detecting
the TiO2 –Fe2O3 thin films, obtained by means of AES analyses, indicating that the double layer films consist of oxygen, titanium and iron in Fig. 3 (a). An in-depth profile of TiO2 – Fe2O3 thin films can be seen as shown in Fig. 3 (b). An abrupt interface appears between the Fe2O3 thin films and the silicon substrate. In the thin films, TiO2 – Fe2O3, interaction with each other to form a large transition range during the film deposition process is observed. The transition range is considered to be an important factor in controlling charge transfer of the thin film sensors.
3.2. Sensing characteristics of the thin film sensors In general, the sensitivity of the sensors is affected by the operating temperature. The higher temperature enhances surface reaction of the thin films and gives higher sensitivity in a temperature range. Fig. 4 shows the relationship between the gas sensitivity and
Fig. 5. Gas sensitivity as a function of TMA concentration at 170°C.
G. Dai / Sensors and Actuators B 53 (1998) 8–12
11
Fig. 8. Response and recovery characteristics of the thin film sensor to 300 ppm TMA.
Fig. 6. Gas sensitivity as a function of operating temperature to different gases at the same concentration of 300 ppm.
concentration of TMA is 30 ppm and the gas sensitivity is approx. 3 at 170°C. This type of thin film sensors was also tested for their sensitivity to other gases. Fig. 6 shows their sensitivity to TMA. NH3, C2H5OH, gasoline gas, CH4, CO and H2 at the same concentration of 300 ppm as a function of operating temperature. These experimental results indicate that the TiO2 – Fe2O3 thin film sensors are very sensitive to TMA at 140 – 230°C, but not sensitive to CH4, CO and H2. The discrimination between TMA gas and NH3, C2H5OH, gasoline gas was expressed by a separation coefficient, C, calculated as follows
C= (VTMA − Va)/(Vx − Va) where VTMA, Va and Vx are voltage drop across the load resistance in TMA, air and test gases. Fig. 7 shows the relationship between the separation coefficient and the operating temperature at 300 ppm of the test gases. The separation coefficient is over 4.0 at 140°C. Therefore, the sensors of TiO2 –Fe2O3 thin films exhibits both high sensitivity and excellent selectivity to TMA gas. Fig. 8 shows the typical gas-response characteristic of the sensors of TiO2 –Fe2O3 thin films. After an introduction of 300 ppm of TMA gas, the response appears immediately. The 90% response time is within 5 s. The 90% recovery time is 20 s.
4. Conclusions Based upon the present analysis, we have shown that the sensor of TiO2 –Fe2O3 thin films prepared by PECVD technique provides the best sensitivity, selectivity and response to TMA gas. Thus it is concluded that the method based on double layer thin films modified by the addition of In may be useful for TMA detection. Studies of the sensing mechanism of the surface modification and TMA gas detection are in progress.
Acknowledgements
Fig. 7. Relationship between separation coefficient of TMA with respect to C2H5OH, NH3, gasoline and operating temperature.
The author wishes to thank Professor Kaiji Zhen for helpful discussion and associate Professor Guan Yuguo, Nan Jin, Han Zheng and Xu Xiulai who took some part in the work.
12
G. Dai / Sensors and Actuators B 53 (1998) 8–12 by a semiconductive Ru/TiO2 sensor, J. Electrochem. Soc. 5 (10) (1988) 2539 – 2540.
References [1] M. Egashira, Y. Shimizu, Y. Takao, Enhancement of trimethylamine sensitivity of semiconductor gas sensors by ruthenium, Chem. Lett. 195 (3) (1988) 389–392. [2] T. Saito, K. Arai, M. Matsuyoshi, A new method for estimating the freshness of fish, Bull. Jpn. Soc. Sci. Fish 24 (1959) 749 – 750. [3] Y. Takao, Y. Iwanaga, Y. Shimizu, M. Egashira, Trimethylamine-sensing mechanism of TiO2-based sensors 1, Sensors and Actuators B 10 (1993) 229–234. [4] Y. Takao, K. Fukuda, Y. Shimizu, M. Egashira, Trimethylaminesensing mechanism of TiO2-based sensors 2, Sensors and Actuators B 10 (1993) 235 –239. [5] M. Egashira, Y. Shimizu, Y. Takao, Trimethylamine sensor based on semiconductive metal oxides for detection of fish freshness, Sensors and Actuators B 1 (1990) 108–112. [6] V. Demarne, A. Grisel, An integrated low-power thin-film CO gas sensor on silicon, Sensors and Actuators 13 (1988) 301–313. [7] J.C. Anderson, Thin film transducers and sensors, J. Vac. Sci. Technol. A 4 (3) (1986) 610–616. [8] S.P. Mukherjee, P.E. Evans, The deposition of thin films by the decomposition of tetra-ethoxy silane in a radio frequency glow discharge, Thin Solid Films 14 (1972) 105–118. [9] Y. Shimizu, Y. Takao, M. Egashira, Detection of freshness of fish
.
Biography Guorui Dai, was born in February 1940 in Liaoning province. In July 1963, he graduated from the Department of Semiconductors, Jilin university and since then has been a teacher in Jilin university. He is a professor now. His research interests cover two main areas—thin film preparation and characterisation and it’s relative device fabrication. His current research work includes production of metal oxide thin films and polymer thin films by means of chemical vapour deposition (PECVD, LPCVD, Laser-CVD and photo-CVD) and fabrication of the film gas sensors and micro gas sensor array and pattern recognition analysis. He is the author of over 100 papers published in scientific magazines and proceedings and has published two teaching manuals, one book and two patents applied in China.