- Email: [email protected]
Detection of gases with arrays of micromachined tin oxide gas sensors
Sensors and Actuators B 65 Ž2000. 244–246 www.elsevier.nlrlocatersensorb
Detection of gases with arrays of micromachined tin oxide gas sensors C. Cane´
a,)
, I. Gracia ` a, A. Gotz ¨ a, L. Fonseca a, E. Lora-Tamayo a, M.C. Horrillo b, b I. Sayago b, J.I. Robla b, J. Rodrigo b, J. Gutierrez ´ a
(IMB-CSIC), Campus UAB, 08193 Bellaterra, Spain Centro Nacional de Microelectronica ´ b Laboratorio de Sensores, IFA-CETEF-CSIC, Serrano 144, 28006 Madrid, Spain Accepted 7 July 1999
Abstract A good detection of NO 2 , CO and toluene at low concentrations has been carried out by using a micromachined gas sensor array composed of three devices working at different temperatures. The structure is fabricated using standard microelectronic technologies and tin oxide layers as sensitive material. The total power consumption of the array is in the range of 150 mW and a good uniformity of temperature is achieved, thanks to a silicon plug placed under the active area of each sensor. With this device type, it is possible to discriminate gases in a mixture when each array microsensor is heated at a proper temperature. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Detection of gases; Micromachined tin oxide; Gas sensors
1. Introduction Metal oxide gas sensors are well-established devices for measuring gas emissions in continuous and in situ modes. Already two decades ago, thick film sensors, based on the sensitivity of a tin oxide layer to gases, were presented w1,2x, mainly based on alumina substrates. However, the requirement of high working temperatures of such devices, has directed the development of gas sensors to the use of new silicon micromachined structures w3x. In this case, the sensitive material is deposited on top of a dielectric membrane that isolates the high temperature area from the bulk silicon. Thus, the power consumption is much lower, as only a small area of the chip is heated. In order to enhance the performance of the sensitive material, a good temperature homogeneity is required and can be achieved by careful and arduous design of the heater or by incorporating some extra structures into the device acting as isothermal plates w4–6x.
) Corresponding author. Tel.: q34-93-580-2625; fax: q34-93-5801496; e-mail: [email protected]
An alternative consisting of a low power thermally isolated structure with a dielectric membrane combined with a silicon plug kept under the sensitive area has been presented elsewhere w7x, and is shown in Fig. 1. In this paper, we present the application of such structure for the implementation of a simple array of sensors that can work at different temperatures, allowing the detection of different gases with high sensitivity and improving the selectivity. In particular, the detection of CO, NO 2 and toluene is presented, despite in can be used for other applications and with other sensitive materials.
2. Experimental A sensor chip with nine devices has been designed and fabricated. The layout includes an array of three different sensors on the same rectangular dielectric membrane of 900 = 3500 mm2 as shown in Fig. 2. The active area of each sensor is of 500 = 500 mm2 , and the heaters are designed to heat each device at a proper working temperature for each gas under test Ž1508C, 3008C and 3508C. while biasing all devices at 5 V. Power consumption is 60
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 1 4 - 7
C. Cane´ et al.r Sensors and Actuators B 65 (2000) 244–246
245
Fig. 1. Cross-sectional view of a micromachined gas sensor with silicon plug.
mW for the device working at 3508C. Devices were fabricated on double side polished p-type ²100: Si substrates, 300 mm thick. A layer of 300 nm of LPCVD Si 3 N4 has been used as a dielectric membrane. The resistor heater is of POCl 3 doped polysilicon and a layer of deposited SiO 2 acts as electrical isolator between the heater and the sensitive layer. The tin oxide was deposited by RF sputtering with thicknesses in the range of 300–600 nm. An annealing at 5008C during 4 h was performed in air ambient to control the material morphology. CrrPt Ž350 nm. electrodes patterned with lift-off have been designed with standard and interdigitated layouts. Prior to the membrane deposition a masked high dose boron diffusion was carried out in order to obtain the desired silicon plug below the active area. Silicon bulk micromachining from the back side was carried out with KOH. An automatic computer-driven set-up composed of mass-flow controllers and DMMs has been used for measuring resistance of the devices and controlling concentrations and mixing processes of gases. The concentrations used for each gas tested were 1–3 ppm for NO 2 , 50–300 ppm for CO and 100–300 ppm for toluene, all in air. The exposure time to each gas was 20 min. The resistance measurements of each microsensor were performed with a constant flow rate of 200 mlrmin. The operating temperatures varied between 1508C and 3508C.
Fig. 3. Responses to toluene in air of the three microsensors with 400 nm of SnO 2 .
values. The sensitivity for 1 ppm of NO 2 at 1508C and 50 ppm of CO at 3008C was approximately 100% and for 100 ppm of toluene it was approximately 200% at 3508C and short response times mainly to NO 2 . The design of the polysilicon resistor had some influence on the sensitivity of sensors with simple dielectric membrane. This can be observed in Fig. 3, for toluene detection in air at the temperature of 3508C. However, this effect was avoided when implementing the silicon plug below the active area. Furthermore, in Fig. 4, the CO response in air at 3008C, the temperature of maximum sensitivity to CO, can be seen. At 1508C, good sensitivity is obtained to NO 2 but CO and toluene are not detected at this temperature. In Fig. 5, this can be observed, as CO is not detected in a mixture of NO 2 q CO in air. In this way, it is possible to easily discriminate gases in mixtures. Each array microsensor is tuned to a specific gas of the mixture by heating it to the temperature of maximum sensitivity for this gas. In the case of CO and toluene, the discrimination is not straightforward since at 3008C and 3508C, both CO and toluene
3. Results and discussion Low concentrations of all gases tested were detected for each microsensor of the array obtaining good sensitivity
Fig. 2. Photograph showing three sensor ŽS1, S2, S3. elements on one single membrane.
Fig. 4. Response to CO of S1 heated at 3008C.
246
C. Cane´ et al.r Sensors and Actuators B 65 (2000) 244–246
Fig. 5. Response to NO 2 in air and to NO 2 q CO in air of sensor S1.
can be detected but with different sensitivities and pattern recognition must be used.
4. Conclusions The fabrication of an array of three sensors has been presented. The array is placed on a single dielectric membrane and with three different heater designs that allow to work at different temperatures with the same 5 V voltage bias. Good sensitivities and response times to the tested gases are obtained from these devices. The discrimination of gases in a mixture is improved since it is possible to heat each microsensor to a different operating temperature on the same chip. Thanks to the micromachined structure with low thermal inertia, both continuous and transient measurements can be performed with low power consumption. The structure can be used not only for thin film tin oxide sensors but also for other materials.
Acknowledgements This research has been funded by the Spanish Commission of Science and Technology ŽCICYT. under the programs TIC95-0981 and TIC97-0944.
References w1x P. Tischer, H. Pink, L. Treitinger, J. Appl. Phys. 19 Ž1980. 513–517. w2x P. Van Geloven, J. Moons, M. Honore, J. Roggen, Sens. Actuators 17 Ž1989. 361–368. w3x L. Chambon, C. Maleyson, A. Pauly, J.P. Germain, V. Demarne, A. Grisel, Sens. Actuators B 45 Ž1997. 107–114. w4x S. Majoo, J.L. Gland, K.D. Wise, J.W. Schwank, Sens. Actuators B 35–36 Ž1996. 312–319. w5x J.S. Suehle, R.E. Cavicchi, M. Gaitan, S. Semancik, IEEE EDL-14 Ž1993. 118–120. w6x S. Moller, J. Lin, E. Obermeier, Sens. Actuators B 24–25 Ž1995. ¨ 343–346. w7x A. Gotz, C. Cane, ¨ I. Gracia, ` ´ E. Lora-Tamayo, C. Horillo, J. Getino, C. Garcıa, Sens. Actuators B 44 Ž1997. 483– ´ J. Gutierrez-Monreal, ´ 487.
Detection of gases with arrays of micromachined tin oxide gas sensors C. Cane´
a,)
, I. Gracia ` a, A. Gotz ¨ a, L. Fonseca a, E. Lora-Tamayo a, M.C. Horrillo b, b I. Sayago b, J.I. Robla b, J. Rodrigo b, J. Gutierrez ´ a
(IMB-CSIC), Campus UAB, 08193 Bellaterra, Spain Centro Nacional de Microelectronica ´ b Laboratorio de Sensores, IFA-CETEF-CSIC, Serrano 144, 28006 Madrid, Spain Accepted 7 July 1999
Abstract A good detection of NO 2 , CO and toluene at low concentrations has been carried out by using a micromachined gas sensor array composed of three devices working at different temperatures. The structure is fabricated using standard microelectronic technologies and tin oxide layers as sensitive material. The total power consumption of the array is in the range of 150 mW and a good uniformity of temperature is achieved, thanks to a silicon plug placed under the active area of each sensor. With this device type, it is possible to discriminate gases in a mixture when each array microsensor is heated at a proper temperature. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Detection of gases; Micromachined tin oxide; Gas sensors
1. Introduction Metal oxide gas sensors are well-established devices for measuring gas emissions in continuous and in situ modes. Already two decades ago, thick film sensors, based on the sensitivity of a tin oxide layer to gases, were presented w1,2x, mainly based on alumina substrates. However, the requirement of high working temperatures of such devices, has directed the development of gas sensors to the use of new silicon micromachined structures w3x. In this case, the sensitive material is deposited on top of a dielectric membrane that isolates the high temperature area from the bulk silicon. Thus, the power consumption is much lower, as only a small area of the chip is heated. In order to enhance the performance of the sensitive material, a good temperature homogeneity is required and can be achieved by careful and arduous design of the heater or by incorporating some extra structures into the device acting as isothermal plates w4–6x.
) Corresponding author. Tel.: q34-93-580-2625; fax: q34-93-5801496; e-mail: [email protected]
An alternative consisting of a low power thermally isolated structure with a dielectric membrane combined with a silicon plug kept under the sensitive area has been presented elsewhere w7x, and is shown in Fig. 1. In this paper, we present the application of such structure for the implementation of a simple array of sensors that can work at different temperatures, allowing the detection of different gases with high sensitivity and improving the selectivity. In particular, the detection of CO, NO 2 and toluene is presented, despite in can be used for other applications and with other sensitive materials.
2. Experimental A sensor chip with nine devices has been designed and fabricated. The layout includes an array of three different sensors on the same rectangular dielectric membrane of 900 = 3500 mm2 as shown in Fig. 2. The active area of each sensor is of 500 = 500 mm2 , and the heaters are designed to heat each device at a proper working temperature for each gas under test Ž1508C, 3008C and 3508C. while biasing all devices at 5 V. Power consumption is 60
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 1 4 - 7
C. Cane´ et al.r Sensors and Actuators B 65 (2000) 244–246
245
Fig. 1. Cross-sectional view of a micromachined gas sensor with silicon plug.
mW for the device working at 3508C. Devices were fabricated on double side polished p-type ²100: Si substrates, 300 mm thick. A layer of 300 nm of LPCVD Si 3 N4 has been used as a dielectric membrane. The resistor heater is of POCl 3 doped polysilicon and a layer of deposited SiO 2 acts as electrical isolator between the heater and the sensitive layer. The tin oxide was deposited by RF sputtering with thicknesses in the range of 300–600 nm. An annealing at 5008C during 4 h was performed in air ambient to control the material morphology. CrrPt Ž350 nm. electrodes patterned with lift-off have been designed with standard and interdigitated layouts. Prior to the membrane deposition a masked high dose boron diffusion was carried out in order to obtain the desired silicon plug below the active area. Silicon bulk micromachining from the back side was carried out with KOH. An automatic computer-driven set-up composed of mass-flow controllers and DMMs has been used for measuring resistance of the devices and controlling concentrations and mixing processes of gases. The concentrations used for each gas tested were 1–3 ppm for NO 2 , 50–300 ppm for CO and 100–300 ppm for toluene, all in air. The exposure time to each gas was 20 min. The resistance measurements of each microsensor were performed with a constant flow rate of 200 mlrmin. The operating temperatures varied between 1508C and 3508C.
Fig. 3. Responses to toluene in air of the three microsensors with 400 nm of SnO 2 .
values. The sensitivity for 1 ppm of NO 2 at 1508C and 50 ppm of CO at 3008C was approximately 100% and for 100 ppm of toluene it was approximately 200% at 3508C and short response times mainly to NO 2 . The design of the polysilicon resistor had some influence on the sensitivity of sensors with simple dielectric membrane. This can be observed in Fig. 3, for toluene detection in air at the temperature of 3508C. However, this effect was avoided when implementing the silicon plug below the active area. Furthermore, in Fig. 4, the CO response in air at 3008C, the temperature of maximum sensitivity to CO, can be seen. At 1508C, good sensitivity is obtained to NO 2 but CO and toluene are not detected at this temperature. In Fig. 5, this can be observed, as CO is not detected in a mixture of NO 2 q CO in air. In this way, it is possible to easily discriminate gases in mixtures. Each array microsensor is tuned to a specific gas of the mixture by heating it to the temperature of maximum sensitivity for this gas. In the case of CO and toluene, the discrimination is not straightforward since at 3008C and 3508C, both CO and toluene
3. Results and discussion Low concentrations of all gases tested were detected for each microsensor of the array obtaining good sensitivity
Fig. 2. Photograph showing three sensor ŽS1, S2, S3. elements on one single membrane.
Fig. 4. Response to CO of S1 heated at 3008C.
246
C. Cane´ et al.r Sensors and Actuators B 65 (2000) 244–246
Fig. 5. Response to NO 2 in air and to NO 2 q CO in air of sensor S1.
can be detected but with different sensitivities and pattern recognition must be used.
4. Conclusions The fabrication of an array of three sensors has been presented. The array is placed on a single dielectric membrane and with three different heater designs that allow to work at different temperatures with the same 5 V voltage bias. Good sensitivities and response times to the tested gases are obtained from these devices. The discrimination of gases in a mixture is improved since it is possible to heat each microsensor to a different operating temperature on the same chip. Thanks to the micromachined structure with low thermal inertia, both continuous and transient measurements can be performed with low power consumption. The structure can be used not only for thin film tin oxide sensors but also for other materials.
Acknowledgements This research has been funded by the Spanish Commission of Science and Technology ŽCICYT. under the programs TIC95-0981 and TIC97-0944.
References w1x P. Tischer, H. Pink, L. Treitinger, J. Appl. Phys. 19 Ž1980. 513–517. w2x P. Van Geloven, J. Moons, M. Honore, J. Roggen, Sens. Actuators 17 Ž1989. 361–368. w3x L. Chambon, C. Maleyson, A. Pauly, J.P. Germain, V. Demarne, A. Grisel, Sens. Actuators B 45 Ž1997. 107–114. w4x S. Majoo, J.L. Gland, K.D. Wise, J.W. Schwank, Sens. Actuators B 35–36 Ž1996. 312–319. w5x J.S. Suehle, R.E. Cavicchi, M. Gaitan, S. Semancik, IEEE EDL-14 Ž1993. 118–120. w6x S. Moller, J. Lin, E. Obermeier, Sens. Actuators B 24–25 Ž1995. ¨ 343–346. w7x A. Gotz, C. Cane, ¨ I. Gracia, ` ´ E. Lora-Tamayo, C. Horillo, J. Getino, C. Garcıa, Sens. Actuators B 44 Ž1997. 483– ´ J. Gutierrez-Monreal, ´ 487.