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SnO2 thin films for gas sensor prepared by r.f. reactive sputtering
Sensors and Actuators B 24-25 (1995) 465-468
Sn02 thin films for gas sensor prepared by r.f. reactive sputtering M. Di Giulio a, G. Micocci a, A. Serra a, A. Tepore a, R. Rella b, P. Sicilian0 b ‘Dipartimento di Sciema dei Material& Zhivetsita de&i Sh4 via Amesano, 73100 Lecce, Italy b lstituto pm lo Sh4o di Nuovi Materiali per I’Elettmrdca {IME-CNR), vi0 Anmano, 73100 Lecce, Italy
Abstract SnOl thin films have been grown by means of the r.f. reactive sputtering method in order to be used as gas sensors. The films are deposited onto heated alumina substrates in an Ar-0, atmosphere starting from an SnO, target. We have optimized the growth parameters in order to achieve the best thin-film properties. The surface structure and the composition of the prepared films are investigated by X-ray diffraction and X-ray photoelectron spectroscopy. The effect on the gas-sensing characteristics of dispersing platinum onto the film surface by sputtering from a Pt target fallowed by a suitable thermal annealing, has also been studied. In particular, Pt-added SnO, thin films show a high sensitivity to carbon monoxide gas at temperatures of about 170 “C. This temperature is lower than the optimum operating temperature (about 350 “C) of SnOz samples without platinum. Kqwords: Gas sensors; Thin films; Tin
oxide
1. Introduction Tin oxide is an n-type semiconductor with attractive characteristics for gas sensors, since it can detect various gases by using the conductivity changes of its surface due to adsorption and desorption processes. In particular, in the last few years SnO, thin films have drawn much interest because of their potential application in microsensor devices [l]. SnO, thin films can be obtained by spray pyrolysis [2,3], chemical-vapour deposition [4], sputtering [S-7] and electron-beam evaporation [8]. The results reported in the literature clearly demonstrate that the physical properties of the fihns depend on the method and the conditions of the film production. In this paper we present the results of the physical properties of SnO, thin films grown in our laboratory by the reactive r.f. sputtering technique by optimizing the deposition parameters in order to achieve the best CO-sensing characteristics, The effect on the electrical properties of Pt-particle dispersion onto the surface of the SnO, films is also reported. 2. Experimental Samples were deposited in an MRC model 86205 sputtering system, equipped with a Cl?I Cl7 CryoTorr 0925~4C05/95/$C?X50 6 1995Elsevier Science SA. Al1 rights reserved SSDI 0925-4005(94)01397-Z
cryopump and a mechanical rotary pump, with a base pressure of 4~ lo-’ mbar. The SnO, target (MRC, purity 99.95%, diameter 5 in, thickness 6 mm) was mounted face down in the top plate of the chamber, while the substrates (alumina plates 1.5 mm thick, ultrasonically cleaned and rinsed with dry nitrogen) were placed on a copper pallet lying on the rotary J arm of the system, 8 cm away from the target. The target was water cooled during the sputtering process. Ultra-high-purity oxygen and argon were introduced into the sputtering chamber through a Matheson mixer-flow-metre; the fluxes were adjusted to obtain the desired percentage composition of the mixture. After admitting the gases at a total sputtering pressure of 5 mtorr, a presputtering process, with the substrates covered by a shutter, was performed in order to clean the target surface of every possible contamination arising from the previous deposition. The plasma discharge was maintained by r.f. power delivered to the target from a MRC 1.5 kW rf. generator via an impedancematching network. The thickness of the films was further measured by using an Alpha-Step 200 stylus profilometer. Various thin-film samples were prepared by changing the r.f. power, the oxygen concentration in the sputtering gas and the substrate temperature. Films with the
466
M. Di Giulio et al. i Semom and Actuators B 24-25 (1995) 465-W
maximum sensitivity to CO were obtained as follows: oxygen content in the mixture 30%, r.f. power about 800 W and substrate temperature between 600 and 650 K. Therefore, only films obtained under these conditions were chosen for our studies. The chemical composition was analysed by X-ray photoelectron spectroscopy (XPS). The morphology was observed by transmission electron microscopy (TEM), while the crystal structure was evaluated by X-ray diffraction. The gas-sensing properties were measured in a dynamic flow system implemented in our laboratory, where dry air at ambient pressure was used as the carrier and the reference gas.
3. Results and discussion 3.1. Chemical composition The chemical composition of the films was analysed by XPS using unmonochromatic Al KU radiation as the excitation source (E = 1486.6 eV). Ail XPS spectra showed, within the limits of sensitivity of the experimental apparatus, the presence of oxygen, tin and carbon
only. Corrections of the energy shift due to the steadystate charging effect of the samples were carried out by referencing to the C 1s line (I&,= 284.6 eV) probably resulting from the residual pump-line oil contamination. Fig. 1 shows a typical XPS spectrum of Sn 3d and 0 1s levels obtained from an SnO, thin film. The binding energy of 0 1s is 530.74 eV, while the energy of Sn 3dsn is 486.93 eV with Eb5/2-E,,3n=8.41. These values are in agreement with the observed values in stoichiometric SnO, [9]. 3.2. Crystalstructure The crystal structure was analysed by X-ray dieaction spectroscopy using a Cu Ka source. The results obtained from a sample are reported in Fig. 2. With regard to SnO,, the pattern shows two relatively strong diffraction peaks from SnO, (110) and SnO, (200) and a small peak from SnO, (lOl), which indicate that the SnO, film is polycrystalline. Moreover, the peak from SnO, (200) is very sharp, indicating a larger crystalline size in that particular direction.
4. CO-sensing’ characteristics
500
492 496 BINDING ENERGVW)
480
Fig. 1. XPS spectrum of an ,310~ thin film.
Dynamic changes in the electrical conductance G resulting from CO adsorption were measured on samples with and without a platinum catalyst. The Pt particles were dispersed onto the surface of SnO, films by sputtering from a Pt target immediately after the film deposition. Fig. 3 shows the sensitivities to 180 ppm CO gas of SnO, and Pt-SnO, samples as a function of temperature, where the sensitivity S is defined as (G,,, - G,i,)/G,i,. Two marked differences are evident. The temperature of maximum sensitivity to CO gas of the SnO, sample is at about 300 “C, while the Pt-SnO, sample presents a higher sensitivity at a lower temperature of about 170 “C.
140Pt-snq, z rml = > z z *
60
/
20 .’
0 :I!rY 0
%GLE
(28)
sno,
100
200 Temperature
300
1
I
(T)
40
Fig. 2. XRD pattern of an SnOz thin film grown on alumina substrate.
Fig. 3. Gas sensitivity to 180 ppm of CO as a function of the temperature for SnO, and Pt-Sn02 thin films.
M. Di Giulin et al. / Sensors and Actuotom B 24-25 (1995) 465-468
Table 1 Summary of the average values obtained from electrical measurements Sample
Slope
carried out on various SnOz and Pt-SnOz thin films
G,.l’%, (at 500 ppm)
EO (ev)
461
Sensitivity threshold
Response time (s)
Recovery time (s)
100 180
200 120
(ppm a) SnOz Pt-SIlO~
0.20 0.50
0.489 0.153
2.20 4.82
A typical response curve of a Pt-SnOz thin film to increasing CO concentrations at its optimum operating temperature is reported in Fig. 4. It is clearly seen that the injection of CO leads to a drastic drop in the resistivity. The resistivity reaches its initial value after CO gas is shut off, and this proves that the adsorption process is reversible. Fig. 5 shows the ratio G,,/Gair versus CO concentration for SnO, and Pt-SnO, films obtained at 300 and 170 “C, respectively. As one can see, in both cases the electrical response of the films in a log-log plot
19 20
is linear in a wide range of CO concentration. Such dependence can be explained according to the model proposed by Clifford [lo], from which the following equation for the conductance isotherm could be derived: G,, e G,i~(Kazp,,‘)~
(1)
where PC0 is the partial pressure of the CO present in the mixture, K is the equilibrium constant of the reaction 2CO(ads) + O,(ads) -
2CO,(g)
(2)
o is the Henry law constant, p=kT/E, and E. is a characteristic surface energy parameter. Table 1 reports a summary of the average best-fit values obtained on various samples of the conductance isotherm slope, which is a measure of the film sensitivity, the surface energy parameter E,, the ratio GJGei, measured at 500 ppm of CO in air, the values of the sensitivity threshold and the response and recovery times.
60 1
5. Conclusions 100
ah
IIIII
400
300
200 TIMElm
Fig. 4. Electrical resistance. changes at 170 ‘C due to CO adsorption for a Pt-SnO, sample. The arrows indicate gas injection.
R.f. reactive sputtering of tin dioxide targets gives suitable thin films for CO gas-sensing devices. A comparison between the properties of two types of sensors reported in Table 1 shows that the use of activators such as Pt is necessary in order to reach high sensitivity at relatively low temperature. In fact, the pure SnO, films can detect CO gas at an operating temperature of about 300 “C with a maximum sensitivity of 120% in 500 ppm CO, while the Pt-SnO, samples are capable of detecting CO at a temperature of 170 “C with a maximum sensitivity of about 400% at the same concentration.
Acknowledgements
1:5
2:0
215
$0
315
~ogWU(w~) Fig. 5. J_&og plots of Gs,,/G.c vs. CO concentration at the optimum operating temperature.
We wish to thank Dr Ziad Ali-Adib from Manchester University for the X-ray diffraction measurements, Dr L. Mirenghi from CNRSM-Brindisi for XPS measurements and Mrs A.R. De Bartolomeo for technical assistance.
468
M. Di GiuIio et al. / Sensors and Actuators B 24-25 (199s) 465-468
@I E. Leja, J. Korecki, K. Krop and K. Toll, Phase composition
[ll G. Advani and A. Jordan, Thin films of SnO, as solid-state gas sensor, I. Electron. Mater., 9 (1990) 29-49.
PI A.F. Carroll and L.H. Slack, Effect of additions to SnOz thin films, I. Electmchem. SIX., 123 (1976) 1889-1893. 131 WM. Sears and MA. Gee, Mechanica of film formation during the spray pyrolysis of tin oxide, Thin Solid Films, 165 (1988) 265-277. 141 J.C. Lou, MS. Lin, J.I. Chyi and J.H. Shieh, Process study of chemically vapor-deposited SnO,(r> 2), Thin Solid F&m, 106 (1983) 163-173. 151 V. Demarne and A. Grisel, An integrated low-power thin film CO gas Sensor on silicon, Sensor and Actuators, 13 (1988) 301313.
of SnO, thin films obtained by reactive d.c. sputtering, 77&t Mid Films, 59 (1979) 147-155. I’1 H. Windischmann and P. Mark, A model for the operation of a thin films SnO, conductance-modulation carbon monoxide sensor, Z. Electmchem. Sot., 126 (1979) 627-633. PI D. Das and R. Banerjee, Properties of electron-beam evaporated tin oxide films, Thin Solid Fibm, 147 (1987) 321-331. PI C.D. Wagner, W.M. Riggs and L.E. Davis, Handbook @X-ray Photoelectron Spectrosco~, Perkin-Elmer Corporation, Physical Electronic Division, USA, 1979. PI P.K. Clifford, Homogeneous semiconducting gas sensors: a comprehensive model, in T. Seiyama, K. Fueki and S. Suzuki (eds.), Rot. Int. Meet. Chem. Sensors, 1983 p. 135.
Sn02 thin films for gas sensor prepared by r.f. reactive sputtering M. Di Giulio a, G. Micocci a, A. Serra a, A. Tepore a, R. Rella b, P. Sicilian0 b ‘Dipartimento di Sciema dei Material& Zhivetsita de&i Sh4 via Amesano, 73100 Lecce, Italy b lstituto pm lo Sh4o di Nuovi Materiali per I’Elettmrdca {IME-CNR), vi0 Anmano, 73100 Lecce, Italy
Abstract SnOl thin films have been grown by means of the r.f. reactive sputtering method in order to be used as gas sensors. The films are deposited onto heated alumina substrates in an Ar-0, atmosphere starting from an SnO, target. We have optimized the growth parameters in order to achieve the best thin-film properties. The surface structure and the composition of the prepared films are investigated by X-ray diffraction and X-ray photoelectron spectroscopy. The effect on the gas-sensing characteristics of dispersing platinum onto the film surface by sputtering from a Pt target fallowed by a suitable thermal annealing, has also been studied. In particular, Pt-added SnO, thin films show a high sensitivity to carbon monoxide gas at temperatures of about 170 “C. This temperature is lower than the optimum operating temperature (about 350 “C) of SnOz samples without platinum. Kqwords: Gas sensors; Thin films; Tin
oxide
1. Introduction Tin oxide is an n-type semiconductor with attractive characteristics for gas sensors, since it can detect various gases by using the conductivity changes of its surface due to adsorption and desorption processes. In particular, in the last few years SnO, thin films have drawn much interest because of their potential application in microsensor devices [l]. SnO, thin films can be obtained by spray pyrolysis [2,3], chemical-vapour deposition [4], sputtering [S-7] and electron-beam evaporation [8]. The results reported in the literature clearly demonstrate that the physical properties of the fihns depend on the method and the conditions of the film production. In this paper we present the results of the physical properties of SnO, thin films grown in our laboratory by the reactive r.f. sputtering technique by optimizing the deposition parameters in order to achieve the best CO-sensing characteristics, The effect on the electrical properties of Pt-particle dispersion onto the surface of the SnO, films is also reported. 2. Experimental Samples were deposited in an MRC model 86205 sputtering system, equipped with a Cl?I Cl7 CryoTorr 0925~4C05/95/$C?X50 6 1995Elsevier Science SA. Al1 rights reserved SSDI 0925-4005(94)01397-Z
cryopump and a mechanical rotary pump, with a base pressure of 4~ lo-’ mbar. The SnO, target (MRC, purity 99.95%, diameter 5 in, thickness 6 mm) was mounted face down in the top plate of the chamber, while the substrates (alumina plates 1.5 mm thick, ultrasonically cleaned and rinsed with dry nitrogen) were placed on a copper pallet lying on the rotary J arm of the system, 8 cm away from the target. The target was water cooled during the sputtering process. Ultra-high-purity oxygen and argon were introduced into the sputtering chamber through a Matheson mixer-flow-metre; the fluxes were adjusted to obtain the desired percentage composition of the mixture. After admitting the gases at a total sputtering pressure of 5 mtorr, a presputtering process, with the substrates covered by a shutter, was performed in order to clean the target surface of every possible contamination arising from the previous deposition. The plasma discharge was maintained by r.f. power delivered to the target from a MRC 1.5 kW rf. generator via an impedancematching network. The thickness of the films was further measured by using an Alpha-Step 200 stylus profilometer. Various thin-film samples were prepared by changing the r.f. power, the oxygen concentration in the sputtering gas and the substrate temperature. Films with the
466
M. Di Giulio et al. i Semom and Actuators B 24-25 (1995) 465-W
maximum sensitivity to CO were obtained as follows: oxygen content in the mixture 30%, r.f. power about 800 W and substrate temperature between 600 and 650 K. Therefore, only films obtained under these conditions were chosen for our studies. The chemical composition was analysed by X-ray photoelectron spectroscopy (XPS). The morphology was observed by transmission electron microscopy (TEM), while the crystal structure was evaluated by X-ray diffraction. The gas-sensing properties were measured in a dynamic flow system implemented in our laboratory, where dry air at ambient pressure was used as the carrier and the reference gas.
3. Results and discussion 3.1. Chemical composition The chemical composition of the films was analysed by XPS using unmonochromatic Al KU radiation as the excitation source (E = 1486.6 eV). Ail XPS spectra showed, within the limits of sensitivity of the experimental apparatus, the presence of oxygen, tin and carbon
only. Corrections of the energy shift due to the steadystate charging effect of the samples were carried out by referencing to the C 1s line (I&,= 284.6 eV) probably resulting from the residual pump-line oil contamination. Fig. 1 shows a typical XPS spectrum of Sn 3d and 0 1s levels obtained from an SnO, thin film. The binding energy of 0 1s is 530.74 eV, while the energy of Sn 3dsn is 486.93 eV with Eb5/2-E,,3n=8.41. These values are in agreement with the observed values in stoichiometric SnO, [9]. 3.2. Crystalstructure The crystal structure was analysed by X-ray dieaction spectroscopy using a Cu Ka source. The results obtained from a sample are reported in Fig. 2. With regard to SnO,, the pattern shows two relatively strong diffraction peaks from SnO, (110) and SnO, (200) and a small peak from SnO, (lOl), which indicate that the SnO, film is polycrystalline. Moreover, the peak from SnO, (200) is very sharp, indicating a larger crystalline size in that particular direction.
4. CO-sensing’ characteristics
500
492 496 BINDING ENERGVW)
480
Fig. 1. XPS spectrum of an ,310~ thin film.
Dynamic changes in the electrical conductance G resulting from CO adsorption were measured on samples with and without a platinum catalyst. The Pt particles were dispersed onto the surface of SnO, films by sputtering from a Pt target immediately after the film deposition. Fig. 3 shows the sensitivities to 180 ppm CO gas of SnO, and Pt-SnO, samples as a function of temperature, where the sensitivity S is defined as (G,,, - G,i,)/G,i,. Two marked differences are evident. The temperature of maximum sensitivity to CO gas of the SnO, sample is at about 300 “C, while the Pt-SnO, sample presents a higher sensitivity at a lower temperature of about 170 “C.
140Pt-snq, z rml = > z z *
60
/
20 .’
0 :I!rY 0
%GLE
(28)
sno,
100
200 Temperature
300
1
I
(T)
40
Fig. 2. XRD pattern of an SnOz thin film grown on alumina substrate.
Fig. 3. Gas sensitivity to 180 ppm of CO as a function of the temperature for SnO, and Pt-Sn02 thin films.
M. Di Giulin et al. / Sensors and Actuotom B 24-25 (1995) 465-468
Table 1 Summary of the average values obtained from electrical measurements Sample
Slope
carried out on various SnOz and Pt-SnOz thin films
G,.l’%, (at 500 ppm)
EO (ev)
461
Sensitivity threshold
Response time (s)
Recovery time (s)
100 180
200 120
(ppm a) SnOz Pt-SIlO~
0.20 0.50
0.489 0.153
2.20 4.82
A typical response curve of a Pt-SnOz thin film to increasing CO concentrations at its optimum operating temperature is reported in Fig. 4. It is clearly seen that the injection of CO leads to a drastic drop in the resistivity. The resistivity reaches its initial value after CO gas is shut off, and this proves that the adsorption process is reversible. Fig. 5 shows the ratio G,,/Gair versus CO concentration for SnO, and Pt-SnO, films obtained at 300 and 170 “C, respectively. As one can see, in both cases the electrical response of the films in a log-log plot
19 20
is linear in a wide range of CO concentration. Such dependence can be explained according to the model proposed by Clifford [lo], from which the following equation for the conductance isotherm could be derived: G,, e G,i~(Kazp,,‘)~
(1)
where PC0 is the partial pressure of the CO present in the mixture, K is the equilibrium constant of the reaction 2CO(ads) + O,(ads) -
2CO,(g)
(2)
o is the Henry law constant, p=kT/E, and E. is a characteristic surface energy parameter. Table 1 reports a summary of the average best-fit values obtained on various samples of the conductance isotherm slope, which is a measure of the film sensitivity, the surface energy parameter E,, the ratio GJGei, measured at 500 ppm of CO in air, the values of the sensitivity threshold and the response and recovery times.
60 1
5. Conclusions 100
ah
IIIII
400
300
200 TIMElm
Fig. 4. Electrical resistance. changes at 170 ‘C due to CO adsorption for a Pt-SnO, sample. The arrows indicate gas injection.
R.f. reactive sputtering of tin dioxide targets gives suitable thin films for CO gas-sensing devices. A comparison between the properties of two types of sensors reported in Table 1 shows that the use of activators such as Pt is necessary in order to reach high sensitivity at relatively low temperature. In fact, the pure SnO, films can detect CO gas at an operating temperature of about 300 “C with a maximum sensitivity of 120% in 500 ppm CO, while the Pt-SnO, samples are capable of detecting CO at a temperature of 170 “C with a maximum sensitivity of about 400% at the same concentration.
Acknowledgements
1:5
2:0
215
$0
315
~ogWU(w~) Fig. 5. J_&og plots of Gs,,/G.c vs. CO concentration at the optimum operating temperature.
We wish to thank Dr Ziad Ali-Adib from Manchester University for the X-ray diffraction measurements, Dr L. Mirenghi from CNRSM-Brindisi for XPS measurements and Mrs A.R. De Bartolomeo for technical assistance.
468
M. Di GiuIio et al. / Sensors and Actuators B 24-25 (199s) 465-468
@I E. Leja, J. Korecki, K. Krop and K. Toll, Phase composition
[ll G. Advani and A. Jordan, Thin films of SnO, as solid-state gas sensor, I. Electron. Mater., 9 (1990) 29-49.
PI A.F. Carroll and L.H. Slack, Effect of additions to SnOz thin films, I. Electmchem. SIX., 123 (1976) 1889-1893. 131 WM. Sears and MA. Gee, Mechanica of film formation during the spray pyrolysis of tin oxide, Thin Solid Films, 165 (1988) 265-277. 141 J.C. Lou, MS. Lin, J.I. Chyi and J.H. Shieh, Process study of chemically vapor-deposited SnO,(r> 2), Thin Solid F&m, 106 (1983) 163-173. 151 V. Demarne and A. Grisel, An integrated low-power thin film CO gas Sensor on silicon, Sensor and Actuators, 13 (1988) 301313.
of SnO, thin films obtained by reactive d.c. sputtering, 77&t Mid Films, 59 (1979) 147-155. I’1 H. Windischmann and P. Mark, A model for the operation of a thin films SnO, conductance-modulation carbon monoxide sensor, Z. Electmchem. Sot., 126 (1979) 627-633. PI D. Das and R. Banerjee, Properties of electron-beam evaporated tin oxide films, Thin Solid Fibm, 147 (1987) 321-331. PI C.D. Wagner, W.M. Riggs and L.E. Davis, Handbook @X-ray Photoelectron Spectrosco~, Perkin-Elmer Corporation, Physical Electronic Division, USA, 1979. PI P.K. Clifford, Homogeneous semiconducting gas sensors: a comprehensive model, in T. Seiyama, K. Fueki and S. Suzuki (eds.), Rot. Int. Meet. Chem. Sensors, 1983 p. 135.