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Hydrogen detection at high concentrations with stabilised palladium
Sensors and Actuators B 78 (2001) 138±143
Hydrogen detection at high concentrations with stabilised palladium K. Scharnagla,*, M. Erikssonb, A. Karthigeyanc, M. Burgmaira, M. Zimmera, I. Eiselea a
Faculty of Electrical Engineering, Institute of Physics, UniversitaÈt der Bundeswehr MuÈnchen, Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany b Department of Physics and Measurement Technology, S-SENCE and Division of Applied Physics, LinkoÈping University, 58183 LinkoÈping, Sweden c DAAD Fellow, Central Electronics Engineering Research Institute, Pilani 333031, India
Abstract In order to improve the stability to high hydrogen concentrations, of hybrid suspended gate ®eld effect transistors (HSGFETs) with thin palladium ®lms as sensitive layer, Pd±Ni and Pd±Ag alloys have been produced by co-evaporation techniques in UHV. In this paper, the preparation methods as well as hydrogen response measurements are presented. The observed results show that the Pd±Ni alloy is an appropriate material for hydrogen sensing at concentrations up to 2% H2, even at room temperature. The response to 2% H2 is around 500 mV at dry conditions. It is reduced to less than half of this value with moistened carrier gas, but at the same time, the desorption time is lowered. In contrast, the Pd±Ag alloy was not stable. A large drift of the sensor signal was observed and the morphology as well as the composition had changed after the test gas exposures. # 2001 Published by Elsevier Science B.V. Keywords: Palladium-alloy; Nickel; Silver; Hydrogen; Low temperature sensor; HSGFET
1. Introduction Hydrogen is predicted to be the main energy source in the future. With regard to the fact that this gas is not sensed by the human olfactory system and its wide range of ignition in air (4±75%), there is a great demand on gas sensors for hydrogen. One possible method to detect hydrogen is to measure the work function change, DW, due to adsorption of hydrogen on a gas sensitive layer. One big advantage of this method is its sensitivity to physisorption as well as to chemisorption at the surface. Because, the response to bulk effects is almost negligible, low temperatures can be used and measurements at room temperature seem to be feasible in contrast to usual conductance sensors, which are working at temperatures of several hundred degrees centigrade. Therefore, it is possible to establish a new type of gas sensor by using a hybrid suspended gate ®eld effect transistor (HSGFET) [1] whose power demand is minimised (below 10 mW). Hydrogen sensors based on Pd-MOS ®eld effect devices have been used during the past 25 years [2]. One problem, which appears at high hydrogen concentrations, is blistering * Corresponding author. Fax: 49-89-6004-3877. E-mail address: [email protected] (K. Scharnagl).
0925-4005/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 8 0 4 - 8
of Pd ®lms [3]. To solve this, several methods have been suggested. One of them is to stabilise the palladium by producing Pd-alloys [4,5]. We have applied this method to the HSGFET by using co-evaporation techniques in UHV. For the current work, Pd±Ni and Pd±Ag have been prepared. 2. Experimental 2.1. Preparation of the alloy layers The Pd±Ni and Pd±Ag ®lms were grown onto Si wafers in an ultra high vacuum chamber which has a base pressure of 1 10 10 Torr. The Si wafer was ®rst cleaned by a mixture of H2SO4 and H2O2, followed HF dipping before introduction to the vacuum chamber. A 20 nm thick layer of Ti was then grown as an adhesion layer. The Pd±Ni ®lm was grown by simultaneous evaporation from two electron beam evaporation sources. An effusion cell was used to evaporate Ag in the case of Pd±Ag alloy. The evaporation rate of each metal was measured by individual quartz micro balances (QMBs), whose sensitivities had been calibrated by X-ray re¯ectivity measurements. The evaporation rates were 0.29 and 0.039 nm/s for Pd and Ni, respectively. The Pd±Ag alloy was grown with
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
Fig. 1. AES depth profile of the UHV grown Pd±Ag layer.
evaporation rates of 0.16 and 0.065 nm/s for Pd and Ag, respectively. The total thickness of both alloy layers was 50 nm and the pressure during evaporation was 5 10 9 Torr. By conversion to atomic concentrations, through the lattice constants of the metals, the alloy compositions are Pd85Ni15 and Pd74Ag26. The quality of the alloy layers was checked by depth pro®ling with auger electron spectroscopy (AES) in a VG Scienti®c Microlab 310-F Instrument with 10 keV primary energy and a constant retard ratio of 4. Fig. 1 shows the depth pro®le of the Pd±Ag layer before the gas exposures. As can be seen, the composition is constant throughout the alloy layer (except for some unavoidable carbon on the surface) and the same is valid for the Pd±Ni layer. The Ni and Ag contents, respectively, are lower, according to AES, than expected from the QMB readings, which might be due to the sensitivity factors used (0.612 for Pd MN1, 0.641 for Ni LM2 and 1.0 for Ag MN2). 2.2. Measurement setup The work function measurements have been carried out by means of a HSGFET sensor setup. The sensor is based on an ISFET together with a gate chip lying on top of it. It is a piece (about 1:5 mm 2:5 mm) of the above mentioned wafer whose gas sensitive layer is facing the channel. Between ISFET and gate is an air gap of 2±3 mm which provides free access for the ambient air to the Pd±Ni and Pd±Ag layer, respectively. The sensor is mounted on a TO8 header. A work function change due to hydrogen adsorption on the sensitive-®lm is recognised by the sensor which is electrically operated in the so-called feedback mode by means of a matching circuit. This means that the source drain current is kept constant during measurement by regulating the gate potential. The resulting signal is directly proportional to the adsorption induced work function change [6]. The adjustment and monitoring of the total gas ¯ow as well as the concentration of hydrogen in the carrier gas was
139
done by a computer. Synthetic air (20% oxygen, 80% nitrogen, hydrocarbon free) was used as carrier gas. The temperature of the sensor and sensitive layer were kept constant by means of a Pt-heater which is located between ISFET and header. The measurements have been carried out at around 25 and 1008C, under dry and humid conditions. After exposing to synthetic air for several hours Ð in order to reach equilibrium at the surface Ð the samples were exposed to several concentrations of hydrogen followed by afresh ¯ushing with synthetic air. In order to test the reliability and reproducibility, this cycle was repeated several times. Step like gas pro®les with concentrations up to 2% H2 were done, too. In order to investigate the cross sensitivity of our alloys, additional work function measurements have been done with a commercial Kelvin probe [7]. With this computer controlled setup, it is possible to expose the sample to several cross gases, which are commonly occurring during environment applications. The used gases were 100 ppm NO2, 1000 ppm CO, 70 ppm NH3, 100 ppm SO2 and 1 ppm Cl2. These measurements have been performed at 1308C under dry and humid (30% RH) conditions. 3. Results and discussion Fig. 2 shows the response of the Pd±Ni alloy to 0.1% hydrogen. The signal is fast and it follows the transient characteristic of the MFC (needle shaped peaks in Fig. 2). In the case of 0.1% H2, 80% of the signal height was reached within 10 s. Saturation as well as recovery take place during 40 min gas cycles. The measured responses are 315, 290, 292 mV (®rst, second, third pulse). At concentrations exceeding 0.1% H2, full recovery takes more than 40 min. Additional measurements with moistened carrier gas proved that the desorption rate is higher for humid conditions. Fig. 3 presents a step pro®le at humid conditions and concentrations of 0.5, 1 and 2% H2. The duration of one step is 10 min. As can be seen, the response is strongly reduced (more than twice-shared) during humid conditions. One can see that the step pro®le is well mapped by the sensor response. The baseline, however, drifts in this case. The responses for dry and humid conditions, acquired with the step pro®le, are listed in Table 1. Sensor measurements which were performed at room temperature show results as good as at elevated temperatures. The sensor reaction of Pd±Ni to 1% H2 is depicted in Fig. 4. This measurement has been carried out at 258C and under dry conditions. At gas exposure, a quick sensor reaction takes place, which reaches 50% of the maximum signal within 20 s. During the following duration of 20 min gas exposure, a stable and almost drift free equilibrium is reached. After termination of the gas exposure, the sensor recovers completely and the baseline is reached within 1 h. Additional step shaped gas pro®les express the concentration dependence
140
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
Fig. 2. Time response of Pd±Ni to 0.1% H2 at 0% RH and approximately 1008C.
Fig. 3. Time response of Pd±Ni to 0.5, 1 and 2% H2 with moistened carrier gas at 1008C.
of the palladium-alloy at low temperatures. The time response of the sensor is shown in Fig. 5 and the work function changes are listed in Table 2. The observed responses at 1008C with Pd±Ag as sensitive layer are not as large as with Pd±Ni. Both layers give a similar fast response to hydrogen. The most obvious Table 1 Sensor response under dry and humid conditions in mV H2 concentration (%) Dry Humid
0.5 343 116
1
2 420 152
1 483 191
0.5 437 158
384 126
differences between Pd±Ni and Pd±Ag are the signal shape and the baseline drift. As depicted in Fig. 6, no stable saturation level could be achieved during 40 min gas exposure. In contrast to Pd±Ni (see Fig. 2), the sensor signal shows a positive slope during hydrogen exposure: each gas exposure results in a baseline shift which depends neither on temperature changes nor other external in¯uences. Only if hydrogen is completely desorbed, as can be seen out of the third (longer) desorption phase in Fig. 6, the positive drift stops and it seems that the sensor will ®nd a new baseline. Analyses by scanning electron microscopy (SEM) and AES, revealed that the structure of Pd±Ag has changed drastically. In Fig. 7, the SEM picture of the Pd±Ag surface
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
141
Fig. 4. Sensor signal of Pd±Ni at 258C and 0% RH.
Fig. 5. Time response of Pd±Ni to 0.2, 0.5, 1 and 2% H2 at 258C and dry conditions.
after gas treatment is shown. The surface has restructured and is decorated with ``particles'' which have diameters between 50 and 100 nm. Some of them are of regular shape. They are located on a nearly unstructured surface. The AES measurements have shown that a rearrangement Table 2 Sensor response at room temperature under dry conditions in mV H2 concentration (%) Dry
0.2 327
0.5 406
1
2 480
512
of the Pd±Ag alloy took place. From Fig. 8, one can see in comparison to Fig. 1 that Ag has segregated towards the surface of the Pd±Ag layer (see arrow). The composition changed from Pd rich in the bulk to almost equal concentrations at the surface. This Ag segregation is accompanied with a rather high Cl concentration, which is probably due to the exposure to other gases during cross sensitivity measurements. Segregation of Ag towards the surface of Pd±Ag ®lms has been observed previously and has been shown to be thermally induced for temperatures above 508C [8]. The studies were made in ultrahigh vacuum. One interpretation of the hydrogen induced drift is that
142
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
Fig. 6. Time response of Pd±Ag to 0.1% H2 at 0% RH and approximately 1008C.
adsorbed oxygen inhibits the segregation, whereas during hydrogen exposure, the segregation can proceed at 1008C. Neither segregation nor surface clusters could be found with an unexposed reference sample. The same analysis has also been done with the Pd±Ni alloy layer. Gas exposure did not change the alloy composition and morphology as compared to the unexposed sample: the composition is almost constant at Pd90Ni10 throughout the whole layer and the surface looks still smooth and unstructured.
In order to determine the cross sensitivity of our alloys, additional Kelvin probe measurements have been carried out. Each material was exposed to the test gases at 1308C for 40 min followed by synthetic air for 40 min. This cycle was repeated three times. The average signal height of the second and third reaction is noti®ed in Table 3. The cross sensitivities are small with respect to the elevated temperature and the high gas concentrations, which are several times higher than the relevant German MAK value (maximum working place concentration) [9].
Fig. 7. SEM image of the Pd±Ag layer after exposure to hydrogen.
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
143
References
Fig. 8. AES depth profile image of the hydrogen treated Pd±Ag alloy. Note the vertical arrow that indicates the surface region where segregation has occurred.
Table 3 Results in mV of the cross sensitivity measurements with Pd±Ni and Pd± Ag at 1308C and 0% RH Gas (ppm)
100 NO2
Pd±Ni Pd±Ag
81 71
1000 CO 53 6
70 NH3 54 26
100 SO2 24 11
1 Cl2 16 22
4. Conclusion Gas measurements have shown that it is possible to detect hydrogen concentrations up to 2% without any blister formation, by means of work function measurements in combination with palladium alloys as gas sensitive material. However, the Pd±Ag alloy could not withstand the gas treatment at 1008C. Changes of the morphology as well as rearrangement of the layer composition took place as has been veri®ed via SEM and AES analysis. The baseline drift throughout the gas measurements indicates that this layer decomposition is a hydrogen induced and gradual process. According to the observed result, Pd74Ag26 is no appropriate sensor material at elevated temperatures. On the other hand, it seems possible to overcome the problem of blistering with the Pd±Ni alloy. Despite the gas exposure to high hydrogen concentrations, the layer remained as it was prepared: no change in morphology and no segregation could be found. Additional measurements which were performed at temperatures of 258C proved that this material is well suited even at this low temperature as well as at elevated temperatures of 1008C. Considering in addition the low cross sensitivity, Pd±Ni seems to be a very promising material for room temperature hydrogen monitoring.
[1] B. Flietner, T. Doll, J. Lechner, M. Leu, I. Eisele, Reliable hybrid GasFETs for workfunction measurements with arbitrary materials, Sens. Actuators B 22 (1994) 109. [2] I. LundstroÈm, M.S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogen-sensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55. [3] M. Armgarth, C. Nylander, Blister formation in Pd gate MIS hydrogen sensors, IEEE Electron Device Lett. 3 (1982) 384±386. [4] P. Janega, L. Young, Palladium±silver alloy gates in metal-oxidesemiconductor structures for use as hydrogen sensors, J. Electrochem. Soc. 34 (1) (1986) 252±253. [5] R.C. Hughes, W.K. Schubert, Thin films of Pd±Ni alloys for detection of high hydrogen concentrations, J. Appl. Phys. 71 (1992) 542±544. [6] M. BoÈgner, A. Fuchs, K. Scharnagl, R. Winter, T. Doll, I. Eisele, Electrical field impact on the gas adsorptivity of thin metal oxide films, Appl. Phys. Lett. 73 (17) (1998) 2524±2526. [7] K. Besocke, S. Berger, Piecoelectric driven Kevin probe for contact potential difference studies, Rev. Sci. Instrum. 47 (7) (1976). [8] R. Anton, H. Eggers, J. Veletas, Auger electron spectroscopy investigations of segregation in Au±Pd and Ag±Pd alloy thin films, Thin Solid Films 226 (1993) 39±47. [9] Sorbe, Sicherheitstechnische Kenndaten chemischer Stoffe, Ecomed, 60 ErgaÈnzungslieferung, Vol. 2, 1997.
Biographies K. Scharnagl obtained his Diploma in Physics from the Technische UniversitaÈt MuÈnchen in 1998. He is currently working towards his PhD thesis at the UniversitaÈt der Bundeswehr MuÈnchen. His subjects include microsystem technology, chemical sensors and their applications. M. Eriksson received his PhD in 1997 from LinkoÈping University. He has studied fundamentals of hydrogen sensing with Pd-MOS structures in ultrahigh vacuum. He has also studied adsorption processes and catalytic reactions on catalysts with different degrees of dispersion. He is now a project leader at S-SENCE, a centre of excellence at LinkoÈping University, working with improving the properties of field-effect gas sensors. A. Karthigeyan obtained his MSc degree in Physics from Madras University in 1987. In 1990, he joined in Central Electronics Engineering Research Institute, Pilani, India. Since then he has been engaged in the research and development of CMOS process and micro strip detectors. He is currently working towards his PhD degree on preparation and characterisation of gas sensing materials. M. Burgmair received his Diploma in Physics at the University ErlangenNuÈrnberg in 1994 in the field of optical spectroscopy of rotational tunneling phenomena. After 3 years practical experience in the semiconductor industry, he is now working towards his PhD thesis at the UniversitaÈt der Bundeswehr MuÈnchen. His subject include microsystem technology and chemical sensors. M. Zimmer received his Diploma in Physics from the Technische UniversitaÈt MuÈnchen in 1998. He is currently working towards his PhD thesis at the UniversitaÈt der Bundeswehr MuÈnchen in the field of microsystem technology and chemical sensors. I. Eisele obtained his PhD from the Technische UniversitaÈt MuÈnchen. From 1968 until 1971, he was working as a research associate at Wayne State University in Detroit. From 1972 until 1980, he joined the Siemens Research Laboratories in Munich. In 1980, he became a regular Professor of Material Science and in 1994 of Microsystem Technology at the UniversitaÈt der Bundeswehr MuÈnchen.
Hydrogen detection at high concentrations with stabilised palladium K. Scharnagla,*, M. Erikssonb, A. Karthigeyanc, M. Burgmaira, M. Zimmera, I. Eiselea a
Faculty of Electrical Engineering, Institute of Physics, UniversitaÈt der Bundeswehr MuÈnchen, Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany b Department of Physics and Measurement Technology, S-SENCE and Division of Applied Physics, LinkoÈping University, 58183 LinkoÈping, Sweden c DAAD Fellow, Central Electronics Engineering Research Institute, Pilani 333031, India
Abstract In order to improve the stability to high hydrogen concentrations, of hybrid suspended gate ®eld effect transistors (HSGFETs) with thin palladium ®lms as sensitive layer, Pd±Ni and Pd±Ag alloys have been produced by co-evaporation techniques in UHV. In this paper, the preparation methods as well as hydrogen response measurements are presented. The observed results show that the Pd±Ni alloy is an appropriate material for hydrogen sensing at concentrations up to 2% H2, even at room temperature. The response to 2% H2 is around 500 mV at dry conditions. It is reduced to less than half of this value with moistened carrier gas, but at the same time, the desorption time is lowered. In contrast, the Pd±Ag alloy was not stable. A large drift of the sensor signal was observed and the morphology as well as the composition had changed after the test gas exposures. # 2001 Published by Elsevier Science B.V. Keywords: Palladium-alloy; Nickel; Silver; Hydrogen; Low temperature sensor; HSGFET
1. Introduction Hydrogen is predicted to be the main energy source in the future. With regard to the fact that this gas is not sensed by the human olfactory system and its wide range of ignition in air (4±75%), there is a great demand on gas sensors for hydrogen. One possible method to detect hydrogen is to measure the work function change, DW, due to adsorption of hydrogen on a gas sensitive layer. One big advantage of this method is its sensitivity to physisorption as well as to chemisorption at the surface. Because, the response to bulk effects is almost negligible, low temperatures can be used and measurements at room temperature seem to be feasible in contrast to usual conductance sensors, which are working at temperatures of several hundred degrees centigrade. Therefore, it is possible to establish a new type of gas sensor by using a hybrid suspended gate ®eld effect transistor (HSGFET) [1] whose power demand is minimised (below 10 mW). Hydrogen sensors based on Pd-MOS ®eld effect devices have been used during the past 25 years [2]. One problem, which appears at high hydrogen concentrations, is blistering * Corresponding author. Fax: 49-89-6004-3877. E-mail address: [email protected] (K. Scharnagl).
0925-4005/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 8 0 4 - 8
of Pd ®lms [3]. To solve this, several methods have been suggested. One of them is to stabilise the palladium by producing Pd-alloys [4,5]. We have applied this method to the HSGFET by using co-evaporation techniques in UHV. For the current work, Pd±Ni and Pd±Ag have been prepared. 2. Experimental 2.1. Preparation of the alloy layers The Pd±Ni and Pd±Ag ®lms were grown onto Si wafers in an ultra high vacuum chamber which has a base pressure of 1 10 10 Torr. The Si wafer was ®rst cleaned by a mixture of H2SO4 and H2O2, followed HF dipping before introduction to the vacuum chamber. A 20 nm thick layer of Ti was then grown as an adhesion layer. The Pd±Ni ®lm was grown by simultaneous evaporation from two electron beam evaporation sources. An effusion cell was used to evaporate Ag in the case of Pd±Ag alloy. The evaporation rate of each metal was measured by individual quartz micro balances (QMBs), whose sensitivities had been calibrated by X-ray re¯ectivity measurements. The evaporation rates were 0.29 and 0.039 nm/s for Pd and Ni, respectively. The Pd±Ag alloy was grown with
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
Fig. 1. AES depth profile of the UHV grown Pd±Ag layer.
evaporation rates of 0.16 and 0.065 nm/s for Pd and Ag, respectively. The total thickness of both alloy layers was 50 nm and the pressure during evaporation was 5 10 9 Torr. By conversion to atomic concentrations, through the lattice constants of the metals, the alloy compositions are Pd85Ni15 and Pd74Ag26. The quality of the alloy layers was checked by depth pro®ling with auger electron spectroscopy (AES) in a VG Scienti®c Microlab 310-F Instrument with 10 keV primary energy and a constant retard ratio of 4. Fig. 1 shows the depth pro®le of the Pd±Ag layer before the gas exposures. As can be seen, the composition is constant throughout the alloy layer (except for some unavoidable carbon on the surface) and the same is valid for the Pd±Ni layer. The Ni and Ag contents, respectively, are lower, according to AES, than expected from the QMB readings, which might be due to the sensitivity factors used (0.612 for Pd MN1, 0.641 for Ni LM2 and 1.0 for Ag MN2). 2.2. Measurement setup The work function measurements have been carried out by means of a HSGFET sensor setup. The sensor is based on an ISFET together with a gate chip lying on top of it. It is a piece (about 1:5 mm 2:5 mm) of the above mentioned wafer whose gas sensitive layer is facing the channel. Between ISFET and gate is an air gap of 2±3 mm which provides free access for the ambient air to the Pd±Ni and Pd±Ag layer, respectively. The sensor is mounted on a TO8 header. A work function change due to hydrogen adsorption on the sensitive-®lm is recognised by the sensor which is electrically operated in the so-called feedback mode by means of a matching circuit. This means that the source drain current is kept constant during measurement by regulating the gate potential. The resulting signal is directly proportional to the adsorption induced work function change [6]. The adjustment and monitoring of the total gas ¯ow as well as the concentration of hydrogen in the carrier gas was
139
done by a computer. Synthetic air (20% oxygen, 80% nitrogen, hydrocarbon free) was used as carrier gas. The temperature of the sensor and sensitive layer were kept constant by means of a Pt-heater which is located between ISFET and header. The measurements have been carried out at around 25 and 1008C, under dry and humid conditions. After exposing to synthetic air for several hours Ð in order to reach equilibrium at the surface Ð the samples were exposed to several concentrations of hydrogen followed by afresh ¯ushing with synthetic air. In order to test the reliability and reproducibility, this cycle was repeated several times. Step like gas pro®les with concentrations up to 2% H2 were done, too. In order to investigate the cross sensitivity of our alloys, additional work function measurements have been done with a commercial Kelvin probe [7]. With this computer controlled setup, it is possible to expose the sample to several cross gases, which are commonly occurring during environment applications. The used gases were 100 ppm NO2, 1000 ppm CO, 70 ppm NH3, 100 ppm SO2 and 1 ppm Cl2. These measurements have been performed at 1308C under dry and humid (30% RH) conditions. 3. Results and discussion Fig. 2 shows the response of the Pd±Ni alloy to 0.1% hydrogen. The signal is fast and it follows the transient characteristic of the MFC (needle shaped peaks in Fig. 2). In the case of 0.1% H2, 80% of the signal height was reached within 10 s. Saturation as well as recovery take place during 40 min gas cycles. The measured responses are 315, 290, 292 mV (®rst, second, third pulse). At concentrations exceeding 0.1% H2, full recovery takes more than 40 min. Additional measurements with moistened carrier gas proved that the desorption rate is higher for humid conditions. Fig. 3 presents a step pro®le at humid conditions and concentrations of 0.5, 1 and 2% H2. The duration of one step is 10 min. As can be seen, the response is strongly reduced (more than twice-shared) during humid conditions. One can see that the step pro®le is well mapped by the sensor response. The baseline, however, drifts in this case. The responses for dry and humid conditions, acquired with the step pro®le, are listed in Table 1. Sensor measurements which were performed at room temperature show results as good as at elevated temperatures. The sensor reaction of Pd±Ni to 1% H2 is depicted in Fig. 4. This measurement has been carried out at 258C and under dry conditions. At gas exposure, a quick sensor reaction takes place, which reaches 50% of the maximum signal within 20 s. During the following duration of 20 min gas exposure, a stable and almost drift free equilibrium is reached. After termination of the gas exposure, the sensor recovers completely and the baseline is reached within 1 h. Additional step shaped gas pro®les express the concentration dependence
140
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
Fig. 2. Time response of Pd±Ni to 0.1% H2 at 0% RH and approximately 1008C.
Fig. 3. Time response of Pd±Ni to 0.5, 1 and 2% H2 with moistened carrier gas at 1008C.
of the palladium-alloy at low temperatures. The time response of the sensor is shown in Fig. 5 and the work function changes are listed in Table 2. The observed responses at 1008C with Pd±Ag as sensitive layer are not as large as with Pd±Ni. Both layers give a similar fast response to hydrogen. The most obvious Table 1 Sensor response under dry and humid conditions in mV H2 concentration (%) Dry Humid
0.5 343 116
1
2 420 152
1 483 191
0.5 437 158
384 126
differences between Pd±Ni and Pd±Ag are the signal shape and the baseline drift. As depicted in Fig. 6, no stable saturation level could be achieved during 40 min gas exposure. In contrast to Pd±Ni (see Fig. 2), the sensor signal shows a positive slope during hydrogen exposure: each gas exposure results in a baseline shift which depends neither on temperature changes nor other external in¯uences. Only if hydrogen is completely desorbed, as can be seen out of the third (longer) desorption phase in Fig. 6, the positive drift stops and it seems that the sensor will ®nd a new baseline. Analyses by scanning electron microscopy (SEM) and AES, revealed that the structure of Pd±Ag has changed drastically. In Fig. 7, the SEM picture of the Pd±Ag surface
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
141
Fig. 4. Sensor signal of Pd±Ni at 258C and 0% RH.
Fig. 5. Time response of Pd±Ni to 0.2, 0.5, 1 and 2% H2 at 258C and dry conditions.
after gas treatment is shown. The surface has restructured and is decorated with ``particles'' which have diameters between 50 and 100 nm. Some of them are of regular shape. They are located on a nearly unstructured surface. The AES measurements have shown that a rearrangement Table 2 Sensor response at room temperature under dry conditions in mV H2 concentration (%) Dry
0.2 327
0.5 406
1
2 480
512
of the Pd±Ag alloy took place. From Fig. 8, one can see in comparison to Fig. 1 that Ag has segregated towards the surface of the Pd±Ag layer (see arrow). The composition changed from Pd rich in the bulk to almost equal concentrations at the surface. This Ag segregation is accompanied with a rather high Cl concentration, which is probably due to the exposure to other gases during cross sensitivity measurements. Segregation of Ag towards the surface of Pd±Ag ®lms has been observed previously and has been shown to be thermally induced for temperatures above 508C [8]. The studies were made in ultrahigh vacuum. One interpretation of the hydrogen induced drift is that
142
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
Fig. 6. Time response of Pd±Ag to 0.1% H2 at 0% RH and approximately 1008C.
adsorbed oxygen inhibits the segregation, whereas during hydrogen exposure, the segregation can proceed at 1008C. Neither segregation nor surface clusters could be found with an unexposed reference sample. The same analysis has also been done with the Pd±Ni alloy layer. Gas exposure did not change the alloy composition and morphology as compared to the unexposed sample: the composition is almost constant at Pd90Ni10 throughout the whole layer and the surface looks still smooth and unstructured.
In order to determine the cross sensitivity of our alloys, additional Kelvin probe measurements have been carried out. Each material was exposed to the test gases at 1308C for 40 min followed by synthetic air for 40 min. This cycle was repeated three times. The average signal height of the second and third reaction is noti®ed in Table 3. The cross sensitivities are small with respect to the elevated temperature and the high gas concentrations, which are several times higher than the relevant German MAK value (maximum working place concentration) [9].
Fig. 7. SEM image of the Pd±Ag layer after exposure to hydrogen.
K. Scharnagl et al. / Sensors and Actuators B 78 (2001) 138±143
143
References
Fig. 8. AES depth profile image of the hydrogen treated Pd±Ag alloy. Note the vertical arrow that indicates the surface region where segregation has occurred.
Table 3 Results in mV of the cross sensitivity measurements with Pd±Ni and Pd± Ag at 1308C and 0% RH Gas (ppm)
100 NO2
Pd±Ni Pd±Ag
81 71
1000 CO 53 6
70 NH3 54 26
100 SO2 24 11
1 Cl2 16 22
4. Conclusion Gas measurements have shown that it is possible to detect hydrogen concentrations up to 2% without any blister formation, by means of work function measurements in combination with palladium alloys as gas sensitive material. However, the Pd±Ag alloy could not withstand the gas treatment at 1008C. Changes of the morphology as well as rearrangement of the layer composition took place as has been veri®ed via SEM and AES analysis. The baseline drift throughout the gas measurements indicates that this layer decomposition is a hydrogen induced and gradual process. According to the observed result, Pd74Ag26 is no appropriate sensor material at elevated temperatures. On the other hand, it seems possible to overcome the problem of blistering with the Pd±Ni alloy. Despite the gas exposure to high hydrogen concentrations, the layer remained as it was prepared: no change in morphology and no segregation could be found. Additional measurements which were performed at temperatures of 258C proved that this material is well suited even at this low temperature as well as at elevated temperatures of 1008C. Considering in addition the low cross sensitivity, Pd±Ni seems to be a very promising material for room temperature hydrogen monitoring.
[1] B. Flietner, T. Doll, J. Lechner, M. Leu, I. Eisele, Reliable hybrid GasFETs for workfunction measurements with arbitrary materials, Sens. Actuators B 22 (1994) 109. [2] I. LundstroÈm, M.S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogen-sensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55. [3] M. Armgarth, C. Nylander, Blister formation in Pd gate MIS hydrogen sensors, IEEE Electron Device Lett. 3 (1982) 384±386. [4] P. Janega, L. Young, Palladium±silver alloy gates in metal-oxidesemiconductor structures for use as hydrogen sensors, J. Electrochem. Soc. 34 (1) (1986) 252±253. [5] R.C. Hughes, W.K. Schubert, Thin films of Pd±Ni alloys for detection of high hydrogen concentrations, J. Appl. Phys. 71 (1992) 542±544. [6] M. BoÈgner, A. Fuchs, K. Scharnagl, R. Winter, T. Doll, I. Eisele, Electrical field impact on the gas adsorptivity of thin metal oxide films, Appl. Phys. Lett. 73 (17) (1998) 2524±2526. [7] K. Besocke, S. Berger, Piecoelectric driven Kevin probe for contact potential difference studies, Rev. Sci. Instrum. 47 (7) (1976). [8] R. Anton, H. Eggers, J. Veletas, Auger electron spectroscopy investigations of segregation in Au±Pd and Ag±Pd alloy thin films, Thin Solid Films 226 (1993) 39±47. [9] Sorbe, Sicherheitstechnische Kenndaten chemischer Stoffe, Ecomed, 60 ErgaÈnzungslieferung, Vol. 2, 1997.
Biographies K. Scharnagl obtained his Diploma in Physics from the Technische UniversitaÈt MuÈnchen in 1998. He is currently working towards his PhD thesis at the UniversitaÈt der Bundeswehr MuÈnchen. His subjects include microsystem technology, chemical sensors and their applications. M. Eriksson received his PhD in 1997 from LinkoÈping University. He has studied fundamentals of hydrogen sensing with Pd-MOS structures in ultrahigh vacuum. He has also studied adsorption processes and catalytic reactions on catalysts with different degrees of dispersion. He is now a project leader at S-SENCE, a centre of excellence at LinkoÈping University, working with improving the properties of field-effect gas sensors. A. Karthigeyan obtained his MSc degree in Physics from Madras University in 1987. In 1990, he joined in Central Electronics Engineering Research Institute, Pilani, India. Since then he has been engaged in the research and development of CMOS process and micro strip detectors. He is currently working towards his PhD degree on preparation and characterisation of gas sensing materials. M. Burgmair received his Diploma in Physics at the University ErlangenNuÈrnberg in 1994 in the field of optical spectroscopy of rotational tunneling phenomena. After 3 years practical experience in the semiconductor industry, he is now working towards his PhD thesis at the UniversitaÈt der Bundeswehr MuÈnchen. His subject include microsystem technology and chemical sensors. M. Zimmer received his Diploma in Physics from the Technische UniversitaÈt MuÈnchen in 1998. He is currently working towards his PhD thesis at the UniversitaÈt der Bundeswehr MuÈnchen in the field of microsystem technology and chemical sensors. I. Eisele obtained his PhD from the Technische UniversitaÈt MuÈnchen. From 1968 until 1971, he was working as a research associate at Wayne State University in Detroit. From 1972 until 1980, he joined the Siemens Research Laboratories in Munich. In 1980, he became a regular Professor of Material Science and in 1994 of Microsystem Technology at the UniversitaÈt der Bundeswehr MuÈnchen.