- Email: [email protected]
Porous silicon layer coupled with thermoelectric cooler: a humidity sensor
Sensors and Actuators 79 Ž2000. 189–193 www.elsevier.nlrlocatersna
Porous silicon layer coupled with thermoelectric cooler: a humidity sensor A. Foucaran a
a,)
, B. Sorli a , M. Garcia b, F. Pascal-Delannoy a , A. Giani a , A. Boyer
a
Centre d’Electronique et de Micro-optoelectronique de Montpellier, UMR 5507 CNRS, UniÕersite´ de Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France b ´ Groupe d’Etude des Semiconducteurs, UniÕersite´ de Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France Received 16 March 1999; accepted 19 August 1999
Abstract In this work, an original humidity sensor is described. It is based on the study of the capacitance variation of a porous silicon layer ŽPSL. during water condensation induced by a commercial small-size thermoelectric cooler ŽTEC.. The measurement principle is to detect the weak increase of capacitance created when water condensation occurs in a PSL stuck on a TEC. This important variation of capacitance is related to the high difference between the dielectric constant of PS Ž ´ r - 12. and water Ž ´ r ( 80.. The dielectric constant of PS ranges from these of silicon oxide Ž ´ r s 3.9. to these of silicon Ž ´r s 12. wH. Mathieu, Physique des semiconducteurs et des composants electroniques, Masson, 1987, p. 36x. Experimental measurements are performed in a climatic chamber for several values of ´ relative humidity from 10% to 95% and for a TEC current equal to 0.43 A for the cooling part of the process. The analysis of the PS capacitance leads to information over the condensation formation during the TEC cooling. A quick increase of the capacitance appears after a delay time, t , of 0.5–2 s from the start of the TEC cooling. The higher the humidity level, the faster the capacitance increase. It is possible to draw the capacitance reached after 1 s, from the start of the TEC cooling as a function of the relative humidity level. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon; Sensor; Capacitance variations; Peltier
1. Introduction Porous silicon ŽPS. is electrochemically formed by anodic dissolution of silicon in a hydrofluoric acid ŽHF. solution. It has been extensively studied since it was discovered by Turner w1x in 1958 and, particularly, since the observations of strong visible luminescence from PS at room temperature Žsuggesting promising applications in silicon-based optoelectronic devices w2x.. Silicon micromachining w3x and microsensors w4x are also applications for PS. As a matter of fact, depending on the electrolysis conditions, this material commonly presents a porous texture with pore diameters varying from 2 to 15 nm, a porosity varying from 20% to 80%, and a very high specific surface w5x in the order of 600 m2 cmy3. The high sensitivity of PS vs. the ambient atmosphere is based on its large internal surface area, which implies possible ab) Corresponding author. Tel.: q33-4-67-14-37-84; fax: q33-4-67-5471-34; e-mail: [email protected]
sorbate effects. This makes the PS layer ŽPSL. a very interesting material for gas sensor applications. Electric characterization of PSL shows that the conductivity is governed by the great concentration of surface states w4,6–10x. Moreover, Anderson et al. w11x and Motohashi et al. w12x described the electrical behavior of a metal PS junction in the presence of different gases. In this way, Stievenard and Deresmes w13x proposed a simple model based on dangling bonds to explain quantitatively the observed phenomenon. Nevertheless, only one author, Anderson w11x, has investigated capacitance variations, rather than conductivity, of PSL under different gas species. The reported capacitance variations were within one order of magnitude only and the temporal responses were in the 5–10 min range. In each case of application, the thickness of the active layer of PSL is higher than a few micrometers because of the poor insulating properties of PS. This induces high response times, important hysteresis phenomenon w10x and desorption times much longer than absorption times. A new concept has been developed at our laboratory. We have suggested a different chemical
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 2 8 5 - X
A. Foucaran et al.r Sensors and Actuators 79 (2000) 189–193
190
approach, based on the realization of an oxidized PSL ŽOPS.. In this way we have simultaneously achieved the conservation of the PS structure and electrical insulation with a total thickness of the active layer less than 1 mm. The aim of this work is to decrease the response time of our sensor Žbased on the active OPS layer measurement capacitance. by sticking it on top of the thermoelectric cooler ŽTEC..
2. Experimental details of the sensors design 2.1. ActiÕe layer fabrication Fig. 2. Sensor-2 design: SENSOR-1 stuck on TEC MARLOW.
The active layer of our sensor is OPS on which two contacts are formed. The first is on the front side of the PSL and the second on the backside of the silicon wafer supporting the OPS. The capacitance of the OPS varies with the species trapped in the OPS. In the following lines, we explain the experimental process of OPS formation and the method used to realize the front side contact. The OPS is formed on the front side of an N q type ²100: silicon wafer with a resistivity of 6 = 10y3 V cm of 300-mm thickness with a gold layer of 200-nm coated on its backside. This gold layer forms an ohmic contact with the silicon wafer. This wafer is then cut into several chips with 15 = 15 mm dimensions. Anodization of samples is performed in a single-tank Teflon circular cell, with a diameter of 0.9 mm, at room temperature. The anodic current density was 30 mArcm2 . The electrolyte used for the PS formation is a mixture of one volume of ethanol Ž98%. and six volumes of HF Ž40 wt.%.. These fabrication conditions led to the formation of a 54% porosity PSL. Desired PSL thickness are obtained by adjusting the anodization time Žthickness varies linearly with time.. Electrochemical oxidation of PSL is performed with the same experimental setup, but with a different electrolyte solution: de-ionized water. As explained by Bsiesy et al. w14x, to prevent the formation of a native oxide which occurs when samples are exposed on air, de-ionized water
covers the sample between PS formation and anodic oxidation. The anodic oxidation current is the same used for PS formation, the oxidation is stopped when the anodization potential reaches 120 V. To make the front side contact to the humidity sensor, gold dots of 30-nm thickness Žto respect the porosity of the OPS. and 0.9-mm diameter are deposited by cathodic pulverization. On this first dot, second dots of 30-nm diameter and 200-nm thickness are also deposited by the same technique to assume the bond wire contact. Then, the sample is diced in chips of 1 mm = 1 mm dimensions around the gold dots. Fig. 1 shows the humidity sensor design ŽSENSOR-1., realized by OPS and based on its capacitance variation vs. humidity level. Fig. 2 shows the second device sensor design ŽSENSOR-2., realized by sticking SENSOR-1 on a TEC device ŽMARLOW S8 1507-TEC — 11.3 = 11.3 = 2.4 mm.. To assume a good electrical and thermal contact between the backside of SENSOR-1 and the top of the TEC, we used silver glue. The sensors testing system is shown Fig. 3. Many devices ensure the creation and regulation control of relative humidity in the SECASI climatic chamber. Other devices ensure the calibration and capacitance measurement of the sensors. The range of RH values tested varies from 10% to 90%. The capacitance of the sensor is measured using a HP-4275A at a frequency of 100 KHz.
3. Experimental results
Fig. 1. OPS Žactive layer. and SENSOR-1 design.
In Fig. 4, a typical graph of capacitance vs. RH is shown for SENSOR-1 type. Anderson et al. w11x obtained important capacitance variations in the RH values ranging from 0% to 100%. Other authors w15,16x made similar measurements but interpolated them with a polynomial function. In this paper, the experimental results show a very high sensitivity of the sensor with varying humidity.
A. Foucaran et al.r Sensors and Actuators 79 (2000) 189–193
191
Fig. 3. The sensor testing system.
The capacitance variation with increasing RH can be related to the modification of the average dielectric constant of the OPS layer in the presence of water. The dielectric constant of water being 80. Hysteresis phenomenon are not observed for these sensors. The typical response time of the sensor is about 3 s. We also observe that with high humidity values, the capacitance increases more rapidly. So to improve the response time of the SENSOR-1 type and entrance reset, we stuck SENSOR-1 to a TEC. Due to its dual function, the TEC allows heating of the OPS to increase water desorption Žto allow reproducibility of measurement.; or to cool the OPS to locally increase the humidity level and then to decrease the signal response time of SENSOR-1. For SENSOR-2 characterization, the typical curves obtained for capacitance variation vs. time Žat a constant humidity rate. for a current pulse applied to the TEC are shown in Fig. 5.
Fig. 4. Capacitance of SENSOR-1 vs. humidity.
To have a good reproducibility of the results and to avoid water condensation, the TEC is first heated, and then cooled. The different curves obtained for several humidity
Fig. 5. Typical curves of SENSOR-2. Capacitance variations vs. time for pulse current TEC.
192
A. Foucaran et al.r Sensors and Actuators 79 (2000) 189–193
Fig. 6. Capacitance responses vs. time for several humidity.
levels and for the same process TEC current are shown in Fig. 6. We observe three different behaviors. Ž1. For the low humidity value, the capacitance cannot reach the step corresponding to the thermodynamic equilibrium of the system Žsystem constituted by TEC, SENSOR-2 and thin water layer in equilibrium with the atmosphere. because droplets of water cannot supply a continuous thin film layer over the OPS. Ž2. For the high humidity value, the capacitance increases very fast to a peak and then decreases to reach the thermodynamic equilibrium system step. This phenomenon can be explained by the liquid phase change of the water thin layer into ice, inducing a decrease of the capacitance. This is due to the relative electrical permittivity low value of the ice, about 5, against 80 for the liquid water. Ž3. For humidity rate ranging from 30% to 50%, the capacitance increase shows that the equilibrium step is reached more quickly than when the humidity level is high. Fig. 7 shows the maximum capacitance reached for each RH value. For the humidity ranging from 30% to 90%, the variation is linear, on the other hand, the re-
Fig. 8. DCrD t Žfor D t s1 s after the beginning of the cooling. vs. humidity.
sponse time of the sensor is high, more than 8 s to reach the maximum capacitance for humidity levels less than 40%. Finally in Fig. 8, the capacitance variations Ž DC . observed after 1 s Ž D t . cooling for the results presented in Fig. 6 are shown. In this case, the response time of the sensor is about 1 s and it is limited by the TEC rise time for the low humidity ranges.
4. Conclusion In this work, an original humidity sensor is described. It is based on the study of the capacitance variation of a PSL during water condensation induced by a small-size commercial TEC. The measurement principle is to detect the weak increase of capacitance created when water condensation occurs in the PSL stuck on top of the TEC. This important variation of capacitance is related to the high difference existing between the dielectric permittivity constant of PS and water. A humidity sensor with a response time of about 1 s is realized.
Acknowledgements This work is supported by the Direction Generale des ´ ´ Armees. ´
References
Fig. 7. Maximal capacitance reached vs. relative humidity.
w1x D.R. Turner, Electropolishing silicon in hydrofluoric acid solution, J. Electrochem. Soc. 105 Ž1958. 402. w2x L.T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers, Appl. Phys. Lett. 57 Ž1990. 1046. w3x P. Steiner, W. Lang, Micromachining applications of porous silicon, Thin Solid Films 255 Ž1995. 52. w4x R.C. Anderson, R.S. Muller, C.W. Tobias, Investigation of the
A. Foucaran et al.r Sensors and Actuators 79 (2000) 189–193
w5x
w6x
w7x w8x
w9x w10x
w11x w12x
w13x
w14x
w15x
w16x
electrical properties of porous silicon, J. Electrochem. Soc. 138 Ž1991. 3406. R. Herino, G. Bomchil, K. Barla, C. Bertrand, J.L. Ginoux, Porosity and pore size distributions of porous silicon layers, J. Electrochem. Soc. 134 Ž1987. 1994. D. Deresmes, V. Marissael, D. Stievenard, C. Ortega, Electrical behaviour of aluminium porous silicon junctions, Thin Solid Films 255 Ž1995. 258. J.J. Mares, J. Kristofik, J. Pangrac, A. Hospodkova, On the transport mechanism in porous silicon, Appl. Phys. Lett. 63 Ž1993. 180. C. Cadet, D. Deresmes, D. Vuillaume, D. Stievenard, Influence of surface defects on the electrical behavior of aluminium porous silicon junctions, Appl. Phys. Lett. 64 Ž1994. 2827. I. Schechter, M. Ben-Chorin, A. Kux, Gas sensing properties of porous silicon, Anal. Chem. 67 Ž1995. 3727. A. Foucaran, F. Pascal-Delannoy, A. Giani, P. Combette, A. Boyer, Porous silicon layers used for gas sensor application, Thin Solid Films 297 Ž1997. 317–320. R.C Anderson, R.S. Muller, C.W. Tobias, Investigations of porous silicon for vapor sensing, Sens. Actuators, A 21–23 Ž1990. 835. A. Motohashi, M. Kawakami, H. Aoyagi, Gas identification by a single gas sensor using porous silicon as the sensitive material, Jpn. J. Appl. Phys. 34 Ž1995. 5840. D. Stievenard, D. Deresmes, Are electrical properties of an aluminium porous silicon junction governed by dangling bonds?, Appl. Phys. Lett. 67 Ž1995. 1570. A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, Anodic oxidation of porous silicon layers formed on lightly p-doped substrates, J. Electrochem. Soc. 138 Ž1991. 3450–3456. M.M. Dubinin, V.A. Astakhov, Description of absorption equilibria of vapors on zeolites over wide ranges of temperature and pressure, in: R.F Gould ŽEd.., Molecular Sieve Zeolites, Vol. 2, Adv. Chem. Ser., No. 102, American Chem. Soc., New York, 1971. P.J. Schubert, J.H. Nevin, Polyimide-based capacitance humidity sensor, IEEE Trans. Electron Devices ED-32 Ž7. Ž1985. 1220–1223.
Andre´ Boyer was born in Perpignan. He obtained his Doctor ‘‘es Sciences Physiques’’ degree from Montpellier University in 1975. Since then, he works in the Center of Electronic and Micro-optoelectronic of Montpellier, Montpellier University. He is a specialist in thermocouple temperature measurement, preparation and properties of thin solid films and ultrasonic method in solid state physics. He is involved in the fundamental studies of sensor phenomena and thermal transport processes in small structures.
193
Alain Foucaran was born in Nimes, France. He is a Doctor of Electronics from Montpellier University since 1986. He works on the Center of Electronic and Micro-optoelectronic of Montpellier, Montpellier University. He is a specialist in the preparation and properties of porous silicon and its applications in sensors area. He is involved in thermal sensors for humidity measurements and micropeltier applications. Michel Garcia was born in Sete, France. He is Doctor of Electronics from Montpellier University since 1996. Since then, he works in the ´ ‘‘Groupe d’Etude des Semiconducteurs’’ of Montpellier University. He is a post-doctoral research assistant in the Department of Electronics and Electrical Engineering, University of Glasgow. Alain Giani was born in Arles, France. He is a Doctor of Electronics from Montpellier University since 1992. Since then, he works in the Center of Electronic and Micro-Optoelectronic of Montpellier University, where he is a specialist of vacuum deposition Techniques. He is involved in thermal sensors for flow measurements and optoelectronic applications. Frederique Pascal-Delannoy was born in Avignon, France. She is a ´ ´ Doctor of Electronics from Montpellier University since 1988. Since then, she has been with the laboratory of Electronic and Micro-Optoelectronic Center of Montpellier, Montpellier University where she is a specialist of MOCD growth of antimonide-based materials for optoelectronic applications. Till now, she is involved in the preparation and study of low-size sensors based on thermal transport phenomena like humidity sensors using Peltier device. Brice Sorli was born in Montpellier, France. He received the «Doctorat» degree in Electronics from Montpellier University in 1998. Since then, he is preparing a PhD thesis on «Integration de capteurs d’humidite´ a base de micromodule Peltier» in the Center of Electronic and Micro-optoelectronic of Montpellier, Montpellier University, where he works on electronic measurements, instrumentation, simulation systems and porous silicon.
Porous silicon layer coupled with thermoelectric cooler: a humidity sensor A. Foucaran a
a,)
, B. Sorli a , M. Garcia b, F. Pascal-Delannoy a , A. Giani a , A. Boyer
a
Centre d’Electronique et de Micro-optoelectronique de Montpellier, UMR 5507 CNRS, UniÕersite´ de Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France b ´ Groupe d’Etude des Semiconducteurs, UniÕersite´ de Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France Received 16 March 1999; accepted 19 August 1999
Abstract In this work, an original humidity sensor is described. It is based on the study of the capacitance variation of a porous silicon layer ŽPSL. during water condensation induced by a commercial small-size thermoelectric cooler ŽTEC.. The measurement principle is to detect the weak increase of capacitance created when water condensation occurs in a PSL stuck on a TEC. This important variation of capacitance is related to the high difference between the dielectric constant of PS Ž ´ r - 12. and water Ž ´ r ( 80.. The dielectric constant of PS ranges from these of silicon oxide Ž ´ r s 3.9. to these of silicon Ž ´r s 12. wH. Mathieu, Physique des semiconducteurs et des composants electroniques, Masson, 1987, p. 36x. Experimental measurements are performed in a climatic chamber for several values of ´ relative humidity from 10% to 95% and for a TEC current equal to 0.43 A for the cooling part of the process. The analysis of the PS capacitance leads to information over the condensation formation during the TEC cooling. A quick increase of the capacitance appears after a delay time, t , of 0.5–2 s from the start of the TEC cooling. The higher the humidity level, the faster the capacitance increase. It is possible to draw the capacitance reached after 1 s, from the start of the TEC cooling as a function of the relative humidity level. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon; Sensor; Capacitance variations; Peltier
1. Introduction Porous silicon ŽPS. is electrochemically formed by anodic dissolution of silicon in a hydrofluoric acid ŽHF. solution. It has been extensively studied since it was discovered by Turner w1x in 1958 and, particularly, since the observations of strong visible luminescence from PS at room temperature Žsuggesting promising applications in silicon-based optoelectronic devices w2x.. Silicon micromachining w3x and microsensors w4x are also applications for PS. As a matter of fact, depending on the electrolysis conditions, this material commonly presents a porous texture with pore diameters varying from 2 to 15 nm, a porosity varying from 20% to 80%, and a very high specific surface w5x in the order of 600 m2 cmy3. The high sensitivity of PS vs. the ambient atmosphere is based on its large internal surface area, which implies possible ab) Corresponding author. Tel.: q33-4-67-14-37-84; fax: q33-4-67-5471-34; e-mail: [email protected]
sorbate effects. This makes the PS layer ŽPSL. a very interesting material for gas sensor applications. Electric characterization of PSL shows that the conductivity is governed by the great concentration of surface states w4,6–10x. Moreover, Anderson et al. w11x and Motohashi et al. w12x described the electrical behavior of a metal PS junction in the presence of different gases. In this way, Stievenard and Deresmes w13x proposed a simple model based on dangling bonds to explain quantitatively the observed phenomenon. Nevertheless, only one author, Anderson w11x, has investigated capacitance variations, rather than conductivity, of PSL under different gas species. The reported capacitance variations were within one order of magnitude only and the temporal responses were in the 5–10 min range. In each case of application, the thickness of the active layer of PSL is higher than a few micrometers because of the poor insulating properties of PS. This induces high response times, important hysteresis phenomenon w10x and desorption times much longer than absorption times. A new concept has been developed at our laboratory. We have suggested a different chemical
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 2 8 5 - X
A. Foucaran et al.r Sensors and Actuators 79 (2000) 189–193
190
approach, based on the realization of an oxidized PSL ŽOPS.. In this way we have simultaneously achieved the conservation of the PS structure and electrical insulation with a total thickness of the active layer less than 1 mm. The aim of this work is to decrease the response time of our sensor Žbased on the active OPS layer measurement capacitance. by sticking it on top of the thermoelectric cooler ŽTEC..
2. Experimental details of the sensors design 2.1. ActiÕe layer fabrication Fig. 2. Sensor-2 design: SENSOR-1 stuck on TEC MARLOW.
The active layer of our sensor is OPS on which two contacts are formed. The first is on the front side of the PSL and the second on the backside of the silicon wafer supporting the OPS. The capacitance of the OPS varies with the species trapped in the OPS. In the following lines, we explain the experimental process of OPS formation and the method used to realize the front side contact. The OPS is formed on the front side of an N q type ²100: silicon wafer with a resistivity of 6 = 10y3 V cm of 300-mm thickness with a gold layer of 200-nm coated on its backside. This gold layer forms an ohmic contact with the silicon wafer. This wafer is then cut into several chips with 15 = 15 mm dimensions. Anodization of samples is performed in a single-tank Teflon circular cell, with a diameter of 0.9 mm, at room temperature. The anodic current density was 30 mArcm2 . The electrolyte used for the PS formation is a mixture of one volume of ethanol Ž98%. and six volumes of HF Ž40 wt.%.. These fabrication conditions led to the formation of a 54% porosity PSL. Desired PSL thickness are obtained by adjusting the anodization time Žthickness varies linearly with time.. Electrochemical oxidation of PSL is performed with the same experimental setup, but with a different electrolyte solution: de-ionized water. As explained by Bsiesy et al. w14x, to prevent the formation of a native oxide which occurs when samples are exposed on air, de-ionized water
covers the sample between PS formation and anodic oxidation. The anodic oxidation current is the same used for PS formation, the oxidation is stopped when the anodization potential reaches 120 V. To make the front side contact to the humidity sensor, gold dots of 30-nm thickness Žto respect the porosity of the OPS. and 0.9-mm diameter are deposited by cathodic pulverization. On this first dot, second dots of 30-nm diameter and 200-nm thickness are also deposited by the same technique to assume the bond wire contact. Then, the sample is diced in chips of 1 mm = 1 mm dimensions around the gold dots. Fig. 1 shows the humidity sensor design ŽSENSOR-1., realized by OPS and based on its capacitance variation vs. humidity level. Fig. 2 shows the second device sensor design ŽSENSOR-2., realized by sticking SENSOR-1 on a TEC device ŽMARLOW S8 1507-TEC — 11.3 = 11.3 = 2.4 mm.. To assume a good electrical and thermal contact between the backside of SENSOR-1 and the top of the TEC, we used silver glue. The sensors testing system is shown Fig. 3. Many devices ensure the creation and regulation control of relative humidity in the SECASI climatic chamber. Other devices ensure the calibration and capacitance measurement of the sensors. The range of RH values tested varies from 10% to 90%. The capacitance of the sensor is measured using a HP-4275A at a frequency of 100 KHz.
3. Experimental results
Fig. 1. OPS Žactive layer. and SENSOR-1 design.
In Fig. 4, a typical graph of capacitance vs. RH is shown for SENSOR-1 type. Anderson et al. w11x obtained important capacitance variations in the RH values ranging from 0% to 100%. Other authors w15,16x made similar measurements but interpolated them with a polynomial function. In this paper, the experimental results show a very high sensitivity of the sensor with varying humidity.
A. Foucaran et al.r Sensors and Actuators 79 (2000) 189–193
191
Fig. 3. The sensor testing system.
The capacitance variation with increasing RH can be related to the modification of the average dielectric constant of the OPS layer in the presence of water. The dielectric constant of water being 80. Hysteresis phenomenon are not observed for these sensors. The typical response time of the sensor is about 3 s. We also observe that with high humidity values, the capacitance increases more rapidly. So to improve the response time of the SENSOR-1 type and entrance reset, we stuck SENSOR-1 to a TEC. Due to its dual function, the TEC allows heating of the OPS to increase water desorption Žto allow reproducibility of measurement.; or to cool the OPS to locally increase the humidity level and then to decrease the signal response time of SENSOR-1. For SENSOR-2 characterization, the typical curves obtained for capacitance variation vs. time Žat a constant humidity rate. for a current pulse applied to the TEC are shown in Fig. 5.
Fig. 4. Capacitance of SENSOR-1 vs. humidity.
To have a good reproducibility of the results and to avoid water condensation, the TEC is first heated, and then cooled. The different curves obtained for several humidity
Fig. 5. Typical curves of SENSOR-2. Capacitance variations vs. time for pulse current TEC.
192
A. Foucaran et al.r Sensors and Actuators 79 (2000) 189–193
Fig. 6. Capacitance responses vs. time for several humidity.
levels and for the same process TEC current are shown in Fig. 6. We observe three different behaviors. Ž1. For the low humidity value, the capacitance cannot reach the step corresponding to the thermodynamic equilibrium of the system Žsystem constituted by TEC, SENSOR-2 and thin water layer in equilibrium with the atmosphere. because droplets of water cannot supply a continuous thin film layer over the OPS. Ž2. For the high humidity value, the capacitance increases very fast to a peak and then decreases to reach the thermodynamic equilibrium system step. This phenomenon can be explained by the liquid phase change of the water thin layer into ice, inducing a decrease of the capacitance. This is due to the relative electrical permittivity low value of the ice, about 5, against 80 for the liquid water. Ž3. For humidity rate ranging from 30% to 50%, the capacitance increase shows that the equilibrium step is reached more quickly than when the humidity level is high. Fig. 7 shows the maximum capacitance reached for each RH value. For the humidity ranging from 30% to 90%, the variation is linear, on the other hand, the re-
Fig. 8. DCrD t Žfor D t s1 s after the beginning of the cooling. vs. humidity.
sponse time of the sensor is high, more than 8 s to reach the maximum capacitance for humidity levels less than 40%. Finally in Fig. 8, the capacitance variations Ž DC . observed after 1 s Ž D t . cooling for the results presented in Fig. 6 are shown. In this case, the response time of the sensor is about 1 s and it is limited by the TEC rise time for the low humidity ranges.
4. Conclusion In this work, an original humidity sensor is described. It is based on the study of the capacitance variation of a PSL during water condensation induced by a small-size commercial TEC. The measurement principle is to detect the weak increase of capacitance created when water condensation occurs in the PSL stuck on top of the TEC. This important variation of capacitance is related to the high difference existing between the dielectric permittivity constant of PS and water. A humidity sensor with a response time of about 1 s is realized.
Acknowledgements This work is supported by the Direction Generale des ´ ´ Armees. ´
References
Fig. 7. Maximal capacitance reached vs. relative humidity.
w1x D.R. Turner, Electropolishing silicon in hydrofluoric acid solution, J. Electrochem. Soc. 105 Ž1958. 402. w2x L.T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers, Appl. Phys. Lett. 57 Ž1990. 1046. w3x P. Steiner, W. Lang, Micromachining applications of porous silicon, Thin Solid Films 255 Ž1995. 52. w4x R.C. Anderson, R.S. Muller, C.W. Tobias, Investigation of the
A. Foucaran et al.r Sensors and Actuators 79 (2000) 189–193
w5x
w6x
w7x w8x
w9x w10x
w11x w12x
w13x
w14x
w15x
w16x
electrical properties of porous silicon, J. Electrochem. Soc. 138 Ž1991. 3406. R. Herino, G. Bomchil, K. Barla, C. Bertrand, J.L. Ginoux, Porosity and pore size distributions of porous silicon layers, J. Electrochem. Soc. 134 Ž1987. 1994. D. Deresmes, V. Marissael, D. Stievenard, C. Ortega, Electrical behaviour of aluminium porous silicon junctions, Thin Solid Films 255 Ž1995. 258. J.J. Mares, J. Kristofik, J. Pangrac, A. Hospodkova, On the transport mechanism in porous silicon, Appl. Phys. Lett. 63 Ž1993. 180. C. Cadet, D. Deresmes, D. Vuillaume, D. Stievenard, Influence of surface defects on the electrical behavior of aluminium porous silicon junctions, Appl. Phys. Lett. 64 Ž1994. 2827. I. Schechter, M. Ben-Chorin, A. Kux, Gas sensing properties of porous silicon, Anal. Chem. 67 Ž1995. 3727. A. Foucaran, F. Pascal-Delannoy, A. Giani, P. Combette, A. Boyer, Porous silicon layers used for gas sensor application, Thin Solid Films 297 Ž1997. 317–320. R.C Anderson, R.S. Muller, C.W. Tobias, Investigations of porous silicon for vapor sensing, Sens. Actuators, A 21–23 Ž1990. 835. A. Motohashi, M. Kawakami, H. Aoyagi, Gas identification by a single gas sensor using porous silicon as the sensitive material, Jpn. J. Appl. Phys. 34 Ž1995. 5840. D. Stievenard, D. Deresmes, Are electrical properties of an aluminium porous silicon junction governed by dangling bonds?, Appl. Phys. Lett. 67 Ž1995. 1570. A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, Anodic oxidation of porous silicon layers formed on lightly p-doped substrates, J. Electrochem. Soc. 138 Ž1991. 3450–3456. M.M. Dubinin, V.A. Astakhov, Description of absorption equilibria of vapors on zeolites over wide ranges of temperature and pressure, in: R.F Gould ŽEd.., Molecular Sieve Zeolites, Vol. 2, Adv. Chem. Ser., No. 102, American Chem. Soc., New York, 1971. P.J. Schubert, J.H. Nevin, Polyimide-based capacitance humidity sensor, IEEE Trans. Electron Devices ED-32 Ž7. Ž1985. 1220–1223.
Andre´ Boyer was born in Perpignan. He obtained his Doctor ‘‘es Sciences Physiques’’ degree from Montpellier University in 1975. Since then, he works in the Center of Electronic and Micro-optoelectronic of Montpellier, Montpellier University. He is a specialist in thermocouple temperature measurement, preparation and properties of thin solid films and ultrasonic method in solid state physics. He is involved in the fundamental studies of sensor phenomena and thermal transport processes in small structures.
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Alain Foucaran was born in Nimes, France. He is a Doctor of Electronics from Montpellier University since 1986. He works on the Center of Electronic and Micro-optoelectronic of Montpellier, Montpellier University. He is a specialist in the preparation and properties of porous silicon and its applications in sensors area. He is involved in thermal sensors for humidity measurements and micropeltier applications. Michel Garcia was born in Sete, France. He is Doctor of Electronics from Montpellier University since 1996. Since then, he works in the ´ ‘‘Groupe d’Etude des Semiconducteurs’’ of Montpellier University. He is a post-doctoral research assistant in the Department of Electronics and Electrical Engineering, University of Glasgow. Alain Giani was born in Arles, France. He is a Doctor of Electronics from Montpellier University since 1992. Since then, he works in the Center of Electronic and Micro-Optoelectronic of Montpellier University, where he is a specialist of vacuum deposition Techniques. He is involved in thermal sensors for flow measurements and optoelectronic applications. Frederique Pascal-Delannoy was born in Avignon, France. She is a ´ ´ Doctor of Electronics from Montpellier University since 1988. Since then, she has been with the laboratory of Electronic and Micro-Optoelectronic Center of Montpellier, Montpellier University where she is a specialist of MOCD growth of antimonide-based materials for optoelectronic applications. Till now, she is involved in the preparation and study of low-size sensors based on thermal transport phenomena like humidity sensors using Peltier device. Brice Sorli was born in Montpellier, France. He received the «Doctorat» degree in Electronics from Montpellier University in 1998. Since then, he is preparing a PhD thesis on «Integration de capteurs d’humidite´ a base de micromodule Peltier» in the Center of Electronic and Micro-optoelectronic of Montpellier, Montpellier University, where he works on electronic measurements, instrumentation, simulation systems and porous silicon.