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A low-power CMOS compatible integrated gas sensor using maskless tin oxide sputtering
Sensors and Actuators B 49 (1998) 81 – 87
A low-power CMOS compatible integrated gas sensor using maskless tin oxide sputtering Lie-yi Sheng a, Zhenan Tang b, Jian Wu a, Philip C.H. Chan a,*, Johnny K.O. Sin a a
Department of Electrical and Electronic Engineering, The Hong Kong Uni6ersity of Science and Technology, Kowloon, Hong Kong b Dalian Uni6ersity of Technology, Dalian, China
Abstract This paper describes a CMOS compatible integrated gas sensor. The device was designed so that the front-end fabrication is fully compatible with the standard CMOS process. The non-CMOS compatible fabrication steps were carried out as post-processing steps. This included the silicon anisotropic etch to create the thermally isolated micro-hotplate (MHP) and the deposition of gas-sensitive thin films using maskless r.f. SnO2 sputtering. The sensors exhibited high sensitivities to gases, such as ethanol and hydrogen. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Integrated gas sensor; Maskless tin oxide sputtering; Lower power
1. Introduction Various integrated gas sensors have been reported [1 – 3]. There are significant advantages for semiconductor gas sensors to be fabricated using the standard CMOS process. It can greatly reduce the manufacturing cost. It also facilitates the integration of signal-conditioning, signal-processing and control circuitry onto the same sensor chip. CMOS compatibility also facilitates the implementation of sensor arrays, which may be used to detect the individual concentrations of a mixture of gases [4]. For our devices, we took care to maintain the CMOS compatible steps as much as possible in the sensor fabrication. Our sensors were designed such that the front-end of the fabrication is fully compatible with the standard CMOS process. The sensorspecific steps were then carried out as post-processing steps, which included an EDP silicon etch and a maskless SnO2 sputtering.
2. Device design Fig. 1 shows the layout and cross-section of our device. The micro-hotplate (MHP) in the center is * Corresponding author. Tel.: + 852 23587041; fax: 23581485; e-mail: [email protected]
+852
0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00092-6
suspended by four micro-bridges. The temperature of the MHP on the active area of the device can be heated to as high as 400°C. Such kind of thermally isolated structure can greatly improve the thermal response and decrease power consumption. The silicon frame surrounding the MHP remains at near ambient temperature, so when the sensor array is used, the thermal cross-talk between the individual sensors can be minimized. This also keeps the temperature of the surrounding regions down so that the sensor peripheral circuits can work reliably under low temperature. It is essential for the effective integration of the sensor support circuits. In our design, the polysilicon ring heater surrounding the MHP is adopted to improve the uniformity of temperature profile on the micro-hotplate [5]. The serpentine polysilicon resistor in the active area is used to monitor the sensor temperature. The sensor electrodes are formed in the center of the MHP. The deep silicon cavity is formed by the EDP (ethylenediamine –pyrocatechol–pyrazine in water solution) silicon anisotropic etching. Compared with the backside micromachined MHP, the fabrication process for the frontside micromachined MHP is more compatible to the standard CMOS process. The common single-side polished silicon wafer is used as the starting substrate. Double-side alignment is not necessary. Finally, the gas-sensitive tin oxide thin film is directly sputtered onto the device surface.
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Fig. 1. Layout and cross-section of the device.
3. Fabrication The cross-sections of the fabrication process are shown in Fig. 2. The fabrication of the device began with a six-mask CMOS compatible process to pattern
all the thin film components and dielectric layers between them. Thermal oxide was grown on 4–7 V cm, n-type, (100)-oriented silicon wafers. Mask c 1 was used to pattern EDP-etch windows in the thermally grown silicon dioxide. Polysilicon was deposited and
Fig. 2. Cross-sections of the fabrication process.
L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
83
Fig. 3. SEM of the device.
doped. Mask c 2 was used to create the two polysilicon resistors, used as heater and temperature sensor, respectively. A LPCVD silicon dioxide layer was deposited. Mask c 3 was used to pattern the contact holes for making the connections between polysilicon resistors and metal leads. An Al/Ti – W multilayer was sputtered. Mask c4 was used to pattern this layer to form the leads between resistors and pads. A second LPCVD silicon dioxide layer was deposited. Mask c 5 was used to pattern the contact holes linking the sensor electrodes on the second LPCVD layer and the metal leads between the two LPCVD dielectric layers. A Ti – W/Au multilayer was sputtered. Mask c6 was used to pattern this layer to form the sensor electrodes. Gold was used to assure good contact to the tin oxide, while Ti – W was used as Au adhesive to SiO2 dielectric layer.
The wafer was diced and packaged in TO-5 metal cans and then subjected to the EDP silicon etching step to create the thermally isolated structure. Only die-attached samples are subjected to the etching step in order to avoid the harsh environment during the dicing process. Finally, the SnO2 thin films were sputtered onto the device. Since no mask is used, we called this maskless sputtering. We sputtered the sensing film at the end of the fabrication process to avoid any chemical or physical damages to the gas-sensitive thin film. The maskless sputtering method avoids defects in the gassensitive film caused by the lithographic and etching steps. The SEM photo of the fabricated device is shown in Fig. 3. Every component on the MHP can be clearly seen. The deep silicon cavity was well defined.
Fig. 4. Power consumption of the device.
Fig. 5. Thermal response of the device.
L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
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nearly symmetrical. The fast thermal response is due to the small thermal mass of the isolated MHP structure. The structure is ideal for temperature-programmed measurements of gas sensitivity [6]. The TCRs (temperature characteristics of resistance) of both polysilicon resistors are shown in Fig. 6. Both curves were nearly linear and the temperature coefficient of the resistance of the polysilicon temperature sensor was about 0.06% per degree.
4.2. Temperature distribution
Fig. 6. TCR of the ring heater and temperature sensor.
4. Thermal characterization
4.1. Power consumption and thermal response The fabricated devices showed good thermal characteristics. Fig. 4 shows the power consumption versus temperature for the device. To obtain a temperature as high as 300°C, only 12 mW was needed. Fig. 5 shows the temperature response, that is, temperature versus time. When a square-wave heating voltage was applied to the MHP. The steady-state temperature was reached within 3 ms (the rise time was measured from 10 to 90%). The heating and cooling responses were fast and
The temperature distribution on the MHP and the surrounding regions is important for integrated gas sensor applications. We used ANSYS Version 5.2 to model the temperature distribution. ANSYS is a powerful finite element analysis program that can be used in many types of engineering analysis, including thermal, structural, mechanical and electrical analyses. For our simulation, the input power to the MHP is 23.4 mW and the resulting temperature distribution is shown in Fig. 7. The temperature non-uniformity on the entire MHP is less than 35°C, while maximum temperature is 407°C. The steady-state temperature at the anchor of the bridges is 58°C, which means the peripheral circuitry can be integrated onto the surrounding regions of the MHP without experiencing excessive temperature. We have also performed the simulation on the transient thermal response of the MHP. The details of
Fig. 7. Temperature distribution on the MHP and the surrounding regions.
L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
Fig. 8. TEM (a) and TED (b) photos of the SnO2 thin film sputtered at room temperature.
the thermal modeling will be reported in a separate paper. The simulated result was 3 ms, which was very close to the measured result.
5. Tin oxide thin film preparation The gas sensitive SnO2 thin films were prepared by sputtering from a SnO2 target (99.99% purity) in a multi-target Denton SJ/24LL system. The r.f. power was 100 W. The pressure was 10 mTorr, The O2 –Ar ratio was set at 10:90. The substrate temperature was set between room temperature to 300°C. We used substrate-heated maskless sputtering instead of MHPheated maskless sputtering [2], which greatly simplified the sputtering process. It is also more suitable for mass production. We sputtered the SnO2 thin film at the end of the fabrication process in order to avoid any chemical or physical damages to the gas-sensitive thin film. The maskless sputtering method avoids any defects in the gas-sensitive film caused by the lithographic and corresponding etching steps [7,8]. It also provided us
Fig. 9. Thermal characteristics of the SnO2 thin films.
85
Fig. 10. Sensitivity of the device to ethanol at 250°C (SnO2 thin film is sputtered at room temperature).
with more flexibility in preparing the thin films than the photoresist lift-off techniques [9]. For example, using the maskless method, we can use higher substrate temperatures during SnO2 sputtering. The crystal morphology was obtained through the TEM and TED analyses. The results for the sample sputtered at room temperature are shown in Fig. 8. Fig. 8 indicates the prepared thin film is polycrystalline and ˚ in size. Such consists of microcrystals less than 100 A kind of microcrystal structure is preferable for gas detection. The calibrated thermal characteristics of the SnO2 thin films at different sputtering temperatures are given in Fig. 9. They show extremely high resistance near room temperature, which suggested that only the SnO2 thin film located on MHP will contribute to the high temperature gas response. Comparatively, the resistance for both polysilicon resistors is quite low. So, the maskless sputtered SnO2 thin film has no effect on both the heater and temperature sensor.
Fig. 11. Sensitivity of the device to hydrogen at 275°C (SnO2 thin film is sputtered at 300°C.)
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L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
6. Gas response Using various conditions of preparing the gas-sensitive thin films, we obtained the desired responses to ˚ SnO2 thin films were sputgases. A total of 3000 A tered at two temperatures. The gas response measurements were carried out in an automatic test system for gas sensors [10]. Fig. 10 shows the sensitivity of the room temperature sputtered SnO2 thin film to ethanol, which has the similar response as other ethanol gas sensors. The sensor temperature is 250°C. Fig. 11 shows the sensitivity of the device to hydrogen with high temperature sputtered tin-oxide thin film. And the corresponding sensor temperature is 275°C. Here, the sensitivity (S) to gas is defined as S = (DR/Rair)×100%, where DR is the resistance change of SnO2 thin films in the presence of reducing gases, and Rair is the base resistance under the air ambient.
SnO2 thin film was sputtered using a maskless step at the end of the fabrication sequence in order to avoid any chemical or physical damages to gas-sensitive thin film. The fabricated device showed desirable sensitivities to ethanol and hydrogen.
Acknowledgements This research was supported by the Earmarked Hong Kong Research Council Grants 683/95E, 767/ 96E and Chinese National Science Foundation Grant 69682006. We would like to thank the staff of the Microelectronics Fabrication Facility (MFF) and the Materials Characterization and Preparation Facility (MCPF) of the Hong Kong University of Science and Technology for their assistance in device fabrication.
References 7. Future work The CMOS compatibility of the sensor fabrication process will greatly facilitate the integration of support circuitry. In the future, we plan to integrate the sensor support circuits with the MHP-based gas sensor or sensor array onto the same chip. The support circuitry includes the heater power control circuitry and the signal-processing circuitry. In practice, the resistance of the polysilicon heater can vary due to the process variation and the temperature dependence of the resistor itself. This makes the sensor temperature hard to control. We have designed a resistance tolerant constant power heating circuit, which can maintain the heating power to within 3% with 950% variation of the heater resistance [11]. We are also designing a signal-processing circuit to amplify, modulate and demodulate the sensor signals. It can be used to handle multi-channel signals for the gas sensor array applications. Both of the circuits are realized using a standard 2 m n-well CMOS process.
8. Conclusions We have reported an improved silicon integrated gas sensor. Its front-end fabrication steps were designed to be fully CMOS compatible. The sensor-specific steps were carried out as post-processing steps. The thermally isolated micro-hotplate (MHP) was demonstrated to have good thermal characteristics. It consumed only 12 mW to maintain the sensor active area at 300°C. The micro-hotplate can be heated from room temperature to 275°C within 3 ms. The
[1] Wan-Young Chung, Tae-Hoon Kim, Young-Ho Hong, DukDong Lee, Characterization of porous tin oxide thin films and their application to microsensor fabrication, Sensors and Actuators B, 24 – 25 (1995) 482 – 485. [2] J.S. Suehle, R.E. Cavicchi, M. Gaitan, S. Semancik, Tin oxide gas sensor fabricated using CMOS micro-hotplates and in-situ processing, IEEE Electron Device Lett. 3 (14) (1993) 118–120. [3] Qinghai Wu, Wen H. Ko, Micro-gas sensor for monitoring anesthetic agents, Sensors and Actuators B, 1 (1990) 183–187. [4] S. Semancik, R.E. Cavicchi, K.G. Kreider, J.S. Suehle, P. Chaparala, Selected-area deposition of multiple active films for conductometric microsensor arrays, Tech. Digest, 8th Int. Conf. Solid-State Sensors and Actuators (Transducers’95), Stockholm, Sweden, June 25 – 29, 1995, pp. 831 – 834. [5] Samuel K.H. Fung, Zhenan Tang, Philip C.H. Chan, Johnny K.O. Sin, Peter W. Cheung, Design and analysis of a micro-hotplate for integrated gas sensor applications, Sensors and Actuators A 54 (1996) 482 – 487. [6] R.E. Cavicchi, J.S. Suehle, K.G. Kreider, M. Gaitan and P. Chaparala, Optimized temperature pulse sequences for the enhancement of chemically-specific response patterns from microhotplate gas sensors, Tech. Digest, 8th Int. Conf. Solid-State Sensors and Actuators (Transducers’95), Stockholm, Sweden, June 25 – 29, 1995, pp. 823 – 826. [7] V.K. Gueorguiev, L.I. Popova, G.D. Beshkov, N.A. Tomajova, Wet etching of thin SnO2 films, Sensors and Actuators A 24 (1990) 61 – 63. [8] E.S. Braga, A.P. Mammana, C.I.Z. Mammana, R.L. Anderson, Plasma etching of SnO2 films on silicon substrates, Thin Solid Films, 73 (1980). [9] V. Demarne, A. Grisel, A new SnO2 low temperature deposition technique for integrated gas sensors, Sensors and Actuators B, 15 – 16 (1993) 63 – 67. [10] Zhenan Tang, Samuel K.H. Fung, Darwin T.W. Wong, Philip C.H. Chan, Johnny K.O. Sin, An integrated gas sensor based on tin oxide thin film and improved micro-hotplate, Sensors and Actuators B (in press). [11] Sunny S.W. Chan, Philip C.H. Chan, A resistance variation tolerant constant power heating circuit for integrated sensor applications, Tech. Digest, 23rd European Solid-State Circuits Conference (ESSCIRC’97), UK, September 1997 (in press).
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Biographies Lie-yi Sheng received his B.E. and M.E. degrees both in Optical Engineering from Zhejiang University, People’s Republic of China in 1986 and 1989, respectively. Before joining the Hong Kong University of Science and Technology (HKUST) as a PhD student, he was a lecturer in the Department of Optoelectronic and Scientific Instruments Engineering at Zhejiang University. He is now engaged in the research work of integrated gas sensors in HKUST. Zhenan Tang received his B.S. in technology physics in Electronic Science and Technology from Xi’an University in 1982 and his M.Eng. in electronic engineering from Dalian University of Technology in 1990. He is an associate professor and the director of Sensor Technology Laboratory in the Electronic Engineering Department, Dalian University of Technology. At the moment he is a visiting scholar in the Hong Kong University of Science and Technology. Jian Wu received his master degree from Zhejiang University in 1994. After this, he worked as a lecturer in the Department of Biomedical Engineering, Zhejiang University. He is now a research student in the Department of HKUST and his main research area is microsensors. Philip C.H. Chan received his B. S. in electrical engineering from the University of California at Davis and his M.S. and Ph.D. in the electrical engineering
.
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from the University of Illinois at Urbana-Champaign in 1975 and 1978, respectively. He joined Intel Corporation, Santa Clara, CA, 1981, where he was a principal engineer and senior project manager. Dr. Chan had corporate responsibility for circuits simulation tools, VLSI device modeling and process characterization. In 1990, Dr. Chan transferred to the Design Technology Department of the Microproducts Group and developed the first functional 486-based multi-chip module at Intel. In 1991 he joined the Department of Electrical and Electronic Engineering, the Hong Kong University of Science and Technology, Hong Kong. Currently, he is Professor and Head of the Department of Electrical and Electronic Engineering. His research interests are electronic design automation, VLSI devices, circuits and systems, electronic packaging and integrated sensors. Johnny K.O. Sin received the B.A.Sc., M.A.Sc., and Ph.D. degrees in electrical engineering, all from the University of Toronto, Canada, in 1981, 1983, and 1988, respectively. He joined the Microelectronic Devices Research Group of Philips Laboratories, Briarcliff Mantor, New York, USA, in 1988, where he was a senior member of the Research Staff. From 1991 to the present, he has been with the Department of Electrical and Electronic Engineering, the Hong Kong University of Science and Technology, Hong Kong. He is currently an associate professor. His current research is related to the areas of power integrated circuits, integrated chemical gas sensors, thin film transistors, and silicon-on-insulator process and technology.
A low-power CMOS compatible integrated gas sensor using maskless tin oxide sputtering Lie-yi Sheng a, Zhenan Tang b, Jian Wu a, Philip C.H. Chan a,*, Johnny K.O. Sin a a
Department of Electrical and Electronic Engineering, The Hong Kong Uni6ersity of Science and Technology, Kowloon, Hong Kong b Dalian Uni6ersity of Technology, Dalian, China
Abstract This paper describes a CMOS compatible integrated gas sensor. The device was designed so that the front-end fabrication is fully compatible with the standard CMOS process. The non-CMOS compatible fabrication steps were carried out as post-processing steps. This included the silicon anisotropic etch to create the thermally isolated micro-hotplate (MHP) and the deposition of gas-sensitive thin films using maskless r.f. SnO2 sputtering. The sensors exhibited high sensitivities to gases, such as ethanol and hydrogen. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Integrated gas sensor; Maskless tin oxide sputtering; Lower power
1. Introduction Various integrated gas sensors have been reported [1 – 3]. There are significant advantages for semiconductor gas sensors to be fabricated using the standard CMOS process. It can greatly reduce the manufacturing cost. It also facilitates the integration of signal-conditioning, signal-processing and control circuitry onto the same sensor chip. CMOS compatibility also facilitates the implementation of sensor arrays, which may be used to detect the individual concentrations of a mixture of gases [4]. For our devices, we took care to maintain the CMOS compatible steps as much as possible in the sensor fabrication. Our sensors were designed such that the front-end of the fabrication is fully compatible with the standard CMOS process. The sensorspecific steps were then carried out as post-processing steps, which included an EDP silicon etch and a maskless SnO2 sputtering.
2. Device design Fig. 1 shows the layout and cross-section of our device. The micro-hotplate (MHP) in the center is * Corresponding author. Tel.: + 852 23587041; fax: 23581485; e-mail: [email protected]
+852
0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00092-6
suspended by four micro-bridges. The temperature of the MHP on the active area of the device can be heated to as high as 400°C. Such kind of thermally isolated structure can greatly improve the thermal response and decrease power consumption. The silicon frame surrounding the MHP remains at near ambient temperature, so when the sensor array is used, the thermal cross-talk between the individual sensors can be minimized. This also keeps the temperature of the surrounding regions down so that the sensor peripheral circuits can work reliably under low temperature. It is essential for the effective integration of the sensor support circuits. In our design, the polysilicon ring heater surrounding the MHP is adopted to improve the uniformity of temperature profile on the micro-hotplate [5]. The serpentine polysilicon resistor in the active area is used to monitor the sensor temperature. The sensor electrodes are formed in the center of the MHP. The deep silicon cavity is formed by the EDP (ethylenediamine –pyrocatechol–pyrazine in water solution) silicon anisotropic etching. Compared with the backside micromachined MHP, the fabrication process for the frontside micromachined MHP is more compatible to the standard CMOS process. The common single-side polished silicon wafer is used as the starting substrate. Double-side alignment is not necessary. Finally, the gas-sensitive tin oxide thin film is directly sputtered onto the device surface.
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L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
Fig. 1. Layout and cross-section of the device.
3. Fabrication The cross-sections of the fabrication process are shown in Fig. 2. The fabrication of the device began with a six-mask CMOS compatible process to pattern
all the thin film components and dielectric layers between them. Thermal oxide was grown on 4–7 V cm, n-type, (100)-oriented silicon wafers. Mask c 1 was used to pattern EDP-etch windows in the thermally grown silicon dioxide. Polysilicon was deposited and
Fig. 2. Cross-sections of the fabrication process.
L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
83
Fig. 3. SEM of the device.
doped. Mask c 2 was used to create the two polysilicon resistors, used as heater and temperature sensor, respectively. A LPCVD silicon dioxide layer was deposited. Mask c 3 was used to pattern the contact holes for making the connections between polysilicon resistors and metal leads. An Al/Ti – W multilayer was sputtered. Mask c4 was used to pattern this layer to form the leads between resistors and pads. A second LPCVD silicon dioxide layer was deposited. Mask c 5 was used to pattern the contact holes linking the sensor electrodes on the second LPCVD layer and the metal leads between the two LPCVD dielectric layers. A Ti – W/Au multilayer was sputtered. Mask c6 was used to pattern this layer to form the sensor electrodes. Gold was used to assure good contact to the tin oxide, while Ti – W was used as Au adhesive to SiO2 dielectric layer.
The wafer was diced and packaged in TO-5 metal cans and then subjected to the EDP silicon etching step to create the thermally isolated structure. Only die-attached samples are subjected to the etching step in order to avoid the harsh environment during the dicing process. Finally, the SnO2 thin films were sputtered onto the device. Since no mask is used, we called this maskless sputtering. We sputtered the sensing film at the end of the fabrication process to avoid any chemical or physical damages to the gas-sensitive thin film. The maskless sputtering method avoids defects in the gassensitive film caused by the lithographic and etching steps. The SEM photo of the fabricated device is shown in Fig. 3. Every component on the MHP can be clearly seen. The deep silicon cavity was well defined.
Fig. 4. Power consumption of the device.
Fig. 5. Thermal response of the device.
L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
84
nearly symmetrical. The fast thermal response is due to the small thermal mass of the isolated MHP structure. The structure is ideal for temperature-programmed measurements of gas sensitivity [6]. The TCRs (temperature characteristics of resistance) of both polysilicon resistors are shown in Fig. 6. Both curves were nearly linear and the temperature coefficient of the resistance of the polysilicon temperature sensor was about 0.06% per degree.
4.2. Temperature distribution
Fig. 6. TCR of the ring heater and temperature sensor.
4. Thermal characterization
4.1. Power consumption and thermal response The fabricated devices showed good thermal characteristics. Fig. 4 shows the power consumption versus temperature for the device. To obtain a temperature as high as 300°C, only 12 mW was needed. Fig. 5 shows the temperature response, that is, temperature versus time. When a square-wave heating voltage was applied to the MHP. The steady-state temperature was reached within 3 ms (the rise time was measured from 10 to 90%). The heating and cooling responses were fast and
The temperature distribution on the MHP and the surrounding regions is important for integrated gas sensor applications. We used ANSYS Version 5.2 to model the temperature distribution. ANSYS is a powerful finite element analysis program that can be used in many types of engineering analysis, including thermal, structural, mechanical and electrical analyses. For our simulation, the input power to the MHP is 23.4 mW and the resulting temperature distribution is shown in Fig. 7. The temperature non-uniformity on the entire MHP is less than 35°C, while maximum temperature is 407°C. The steady-state temperature at the anchor of the bridges is 58°C, which means the peripheral circuitry can be integrated onto the surrounding regions of the MHP without experiencing excessive temperature. We have also performed the simulation on the transient thermal response of the MHP. The details of
Fig. 7. Temperature distribution on the MHP and the surrounding regions.
L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
Fig. 8. TEM (a) and TED (b) photos of the SnO2 thin film sputtered at room temperature.
the thermal modeling will be reported in a separate paper. The simulated result was 3 ms, which was very close to the measured result.
5. Tin oxide thin film preparation The gas sensitive SnO2 thin films were prepared by sputtering from a SnO2 target (99.99% purity) in a multi-target Denton SJ/24LL system. The r.f. power was 100 W. The pressure was 10 mTorr, The O2 –Ar ratio was set at 10:90. The substrate temperature was set between room temperature to 300°C. We used substrate-heated maskless sputtering instead of MHPheated maskless sputtering [2], which greatly simplified the sputtering process. It is also more suitable for mass production. We sputtered the SnO2 thin film at the end of the fabrication process in order to avoid any chemical or physical damages to the gas-sensitive thin film. The maskless sputtering method avoids any defects in the gas-sensitive film caused by the lithographic and corresponding etching steps [7,8]. It also provided us
Fig. 9. Thermal characteristics of the SnO2 thin films.
85
Fig. 10. Sensitivity of the device to ethanol at 250°C (SnO2 thin film is sputtered at room temperature).
with more flexibility in preparing the thin films than the photoresist lift-off techniques [9]. For example, using the maskless method, we can use higher substrate temperatures during SnO2 sputtering. The crystal morphology was obtained through the TEM and TED analyses. The results for the sample sputtered at room temperature are shown in Fig. 8. Fig. 8 indicates the prepared thin film is polycrystalline and ˚ in size. Such consists of microcrystals less than 100 A kind of microcrystal structure is preferable for gas detection. The calibrated thermal characteristics of the SnO2 thin films at different sputtering temperatures are given in Fig. 9. They show extremely high resistance near room temperature, which suggested that only the SnO2 thin film located on MHP will contribute to the high temperature gas response. Comparatively, the resistance for both polysilicon resistors is quite low. So, the maskless sputtered SnO2 thin film has no effect on both the heater and temperature sensor.
Fig. 11. Sensitivity of the device to hydrogen at 275°C (SnO2 thin film is sputtered at 300°C.)
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L. Sheng et al. / Sensors and Actuators B 49 (1998) 81–87
6. Gas response Using various conditions of preparing the gas-sensitive thin films, we obtained the desired responses to ˚ SnO2 thin films were sputgases. A total of 3000 A tered at two temperatures. The gas response measurements were carried out in an automatic test system for gas sensors [10]. Fig. 10 shows the sensitivity of the room temperature sputtered SnO2 thin film to ethanol, which has the similar response as other ethanol gas sensors. The sensor temperature is 250°C. Fig. 11 shows the sensitivity of the device to hydrogen with high temperature sputtered tin-oxide thin film. And the corresponding sensor temperature is 275°C. Here, the sensitivity (S) to gas is defined as S = (DR/Rair)×100%, where DR is the resistance change of SnO2 thin films in the presence of reducing gases, and Rair is the base resistance under the air ambient.
SnO2 thin film was sputtered using a maskless step at the end of the fabrication sequence in order to avoid any chemical or physical damages to gas-sensitive thin film. The fabricated device showed desirable sensitivities to ethanol and hydrogen.
Acknowledgements This research was supported by the Earmarked Hong Kong Research Council Grants 683/95E, 767/ 96E and Chinese National Science Foundation Grant 69682006. We would like to thank the staff of the Microelectronics Fabrication Facility (MFF) and the Materials Characterization and Preparation Facility (MCPF) of the Hong Kong University of Science and Technology for their assistance in device fabrication.
References 7. Future work The CMOS compatibility of the sensor fabrication process will greatly facilitate the integration of support circuitry. In the future, we plan to integrate the sensor support circuits with the MHP-based gas sensor or sensor array onto the same chip. The support circuitry includes the heater power control circuitry and the signal-processing circuitry. In practice, the resistance of the polysilicon heater can vary due to the process variation and the temperature dependence of the resistor itself. This makes the sensor temperature hard to control. We have designed a resistance tolerant constant power heating circuit, which can maintain the heating power to within 3% with 950% variation of the heater resistance [11]. We are also designing a signal-processing circuit to amplify, modulate and demodulate the sensor signals. It can be used to handle multi-channel signals for the gas sensor array applications. Both of the circuits are realized using a standard 2 m n-well CMOS process.
8. Conclusions We have reported an improved silicon integrated gas sensor. Its front-end fabrication steps were designed to be fully CMOS compatible. The sensor-specific steps were carried out as post-processing steps. The thermally isolated micro-hotplate (MHP) was demonstrated to have good thermal characteristics. It consumed only 12 mW to maintain the sensor active area at 300°C. The micro-hotplate can be heated from room temperature to 275°C within 3 ms. The
[1] Wan-Young Chung, Tae-Hoon Kim, Young-Ho Hong, DukDong Lee, Characterization of porous tin oxide thin films and their application to microsensor fabrication, Sensors and Actuators B, 24 – 25 (1995) 482 – 485. [2] J.S. Suehle, R.E. Cavicchi, M. Gaitan, S. Semancik, Tin oxide gas sensor fabricated using CMOS micro-hotplates and in-situ processing, IEEE Electron Device Lett. 3 (14) (1993) 118–120. [3] Qinghai Wu, Wen H. Ko, Micro-gas sensor for monitoring anesthetic agents, Sensors and Actuators B, 1 (1990) 183–187. [4] S. Semancik, R.E. Cavicchi, K.G. Kreider, J.S. Suehle, P. Chaparala, Selected-area deposition of multiple active films for conductometric microsensor arrays, Tech. Digest, 8th Int. Conf. Solid-State Sensors and Actuators (Transducers’95), Stockholm, Sweden, June 25 – 29, 1995, pp. 831 – 834. [5] Samuel K.H. Fung, Zhenan Tang, Philip C.H. Chan, Johnny K.O. Sin, Peter W. Cheung, Design and analysis of a micro-hotplate for integrated gas sensor applications, Sensors and Actuators A 54 (1996) 482 – 487. [6] R.E. Cavicchi, J.S. Suehle, K.G. Kreider, M. Gaitan and P. Chaparala, Optimized temperature pulse sequences for the enhancement of chemically-specific response patterns from microhotplate gas sensors, Tech. Digest, 8th Int. Conf. Solid-State Sensors and Actuators (Transducers’95), Stockholm, Sweden, June 25 – 29, 1995, pp. 823 – 826. [7] V.K. Gueorguiev, L.I. Popova, G.D. Beshkov, N.A. Tomajova, Wet etching of thin SnO2 films, Sensors and Actuators A 24 (1990) 61 – 63. [8] E.S. Braga, A.P. Mammana, C.I.Z. Mammana, R.L. Anderson, Plasma etching of SnO2 films on silicon substrates, Thin Solid Films, 73 (1980). [9] V. Demarne, A. Grisel, A new SnO2 low temperature deposition technique for integrated gas sensors, Sensors and Actuators B, 15 – 16 (1993) 63 – 67. [10] Zhenan Tang, Samuel K.H. Fung, Darwin T.W. Wong, Philip C.H. Chan, Johnny K.O. Sin, An integrated gas sensor based on tin oxide thin film and improved micro-hotplate, Sensors and Actuators B (in press). [11] Sunny S.W. Chan, Philip C.H. Chan, A resistance variation tolerant constant power heating circuit for integrated sensor applications, Tech. Digest, 23rd European Solid-State Circuits Conference (ESSCIRC’97), UK, September 1997 (in press).
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Biographies Lie-yi Sheng received his B.E. and M.E. degrees both in Optical Engineering from Zhejiang University, People’s Republic of China in 1986 and 1989, respectively. Before joining the Hong Kong University of Science and Technology (HKUST) as a PhD student, he was a lecturer in the Department of Optoelectronic and Scientific Instruments Engineering at Zhejiang University. He is now engaged in the research work of integrated gas sensors in HKUST. Zhenan Tang received his B.S. in technology physics in Electronic Science and Technology from Xi’an University in 1982 and his M.Eng. in electronic engineering from Dalian University of Technology in 1990. He is an associate professor and the director of Sensor Technology Laboratory in the Electronic Engineering Department, Dalian University of Technology. At the moment he is a visiting scholar in the Hong Kong University of Science and Technology. Jian Wu received his master degree from Zhejiang University in 1994. After this, he worked as a lecturer in the Department of Biomedical Engineering, Zhejiang University. He is now a research student in the Department of HKUST and his main research area is microsensors. Philip C.H. Chan received his B. S. in electrical engineering from the University of California at Davis and his M.S. and Ph.D. in the electrical engineering
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from the University of Illinois at Urbana-Champaign in 1975 and 1978, respectively. He joined Intel Corporation, Santa Clara, CA, 1981, where he was a principal engineer and senior project manager. Dr. Chan had corporate responsibility for circuits simulation tools, VLSI device modeling and process characterization. In 1990, Dr. Chan transferred to the Design Technology Department of the Microproducts Group and developed the first functional 486-based multi-chip module at Intel. In 1991 he joined the Department of Electrical and Electronic Engineering, the Hong Kong University of Science and Technology, Hong Kong. Currently, he is Professor and Head of the Department of Electrical and Electronic Engineering. His research interests are electronic design automation, VLSI devices, circuits and systems, electronic packaging and integrated sensors. Johnny K.O. Sin received the B.A.Sc., M.A.Sc., and Ph.D. degrees in electrical engineering, all from the University of Toronto, Canada, in 1981, 1983, and 1988, respectively. He joined the Microelectronic Devices Research Group of Philips Laboratories, Briarcliff Mantor, New York, USA, in 1988, where he was a senior member of the Research Staff. From 1991 to the present, he has been with the Department of Electrical and Electronic Engineering, the Hong Kong University of Science and Technology, Hong Kong. He is currently an associate professor. His current research is related to the areas of power integrated circuits, integrated chemical gas sensors, thin film transistors, and silicon-on-insulator process and technology.