new technology of metal oxide thin film preparation for chemical sensor application

New technology of metal oxide thin film preparation for chemical sensor application Viacheslav Khatkoa,∗, Josep Caldererb, Eduard Llobeta, Xavier Correiga aDepartament d’Enginyeria Elect

New technology of metal oxide thin film preparation for chemical sensor application Viacheslav Khatkoa,∗, Josep Caldererb, Eduard Llobeta, Xavier Correiga

aDepartament d’Enginyeria Electronica, Universitat Rovira i Virgili, Campus Sescelades, 43007 Tarragona, Spain

bDepartament d’Enginyeria Electronica, Universitat Politecnica de Catalunya, Campus Nord, 08034 Barcelona, Spain

Available online 3 May 2005

Abstract

The reduction of grain size in metal oxide films is one of the key factors to enhance the gas sensing properties of semiconductor layers The basic idea introduced here is to create thin metal oxide films with small grain size by using a special regime of rf sputtering from either metallic or metal oxide targets The regime includes the deposition of thin films with one or several interruptions of the sputtering process The idea has been checked by preparing WO3thin films using reactive rf sputtering from a pure tungsten target Four types of films were prepared For the first type a non-interrupted sputtering was used In the deposition of films type 2, 3 and 4, the sputtering process was interrupted once, two and three times, respectively It was found that the thickness of the WO3films and the sensing properties of WO3based sensors heavily depend on the number of interruptions during the deposition process

© 2005 Elsevier B.V All rights reserved

Keywords: WO3 thin films; Deposition with interruptions; WO3 -based sensors; Rf sputtering

1 Introduction

According to Morrison[1], there exist four general ways

to increase the selectivity of gas sensors These comprise

using catalysts and promoters[2–6], controlling the operating

temperature of the sensors [7–9], including special surface

additives for specific surface adsorption[10] and applying

differential filters[11–14] Nowadays, all these approaches

are being developed very intensively In our opinion, in the

last few years a new way to prepare gas sensors has appeared

This way is connected with the attempt to find methods that

increase the surface area of active layers for chemical sensing

Since the sensor sensitivity is related to the surface–volume

ratio of its sensing film, research can be carried out in three

directions:

• The first one is related to the investigation of active layers

prepared by using nanopowder materials, where

parti-cle size is reduced to nanometers Nanostructure is

ex-pected to have a dramatic influence on sensor performance

∗Corresponding author Tel.: +34 977558653; fax: +34 977559605.

E-mail address: vkhatko@etse.urv.es (V Khatko).

Many different nano-sized powders and technologies for the preparation of active layers are used to investigate the effect of the nanostructure on their sensing characteristics

to different toxic gases[15–18]

• The second direction consists of using special methods of

preparation for the surface patterning of active layers One

of such methods implies a process of anisotropic etching to provide a sensor substrate with a much higher surface area [19] A second method consists of using a porous structure

on the base of a highly ordered nanoporous alumina layer [20,21]

• The third direction includes the technologies via with thin

films of nanometer grain size can be deposited As a rule, these technologies are used to obtain thin film gas sensors For example, active layers with grain size of 1–2 nm could

be deposited using rf sputtering or dc magnetron methods [22,23]

One of the basic ideas to create metal films with small grain size is to use successive step-by-step deposition of ultra-thin films resulting in an island structure of two different mate-rials In this case, at a particular stage of pure metal cluster

0925-4005/$ – see front matter © 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.snb.2005.03.073

V Khatko et al / Sensors and Actuators B 109 (2005) 128–134 129

development, clusters of another material can be formed to

restrict the coalescence of the metal clusters and the

forma-tion of a continuous layer This technology has been applied

with success in the development of catalytic layers for silicon

micromachined calorimetric gas sensors[22,23]

A second basic idea consists of using a special regime of

thin film deposition by dc magnetron, ion-beam or rf

sputter-ing from metallic or metal oxide targets Two different special

regimes can be used In the first one the formation of “extra”

interfaces in the body of the metal film occurs as a result of

the sputtering power density being changed during film

depo-sition As a rule a low deposition rate is set during the initial

stage of film deposition and a high deposition rate is used

during the final stage of the deposition The second regime

implies growing the film with one or several interruptions of

the deposition process In this case the “extra” interfaces are

introduced into the body of the thin film During the

inter-ruption of the sputtering process or when a sudden change in

the deposition rate occurs, an equilibrium surface is formed

due to the free surface bond saturation by the atoms from

residual atmosphere and/or the structural relaxation of the

interface For the subsequent prolongation of the deposition

process, film growth begins over again on the new “extra”

equilibrium surface and the average grain size of the film at

the surface is smaller than in the original film It has been

shown that this leads to metal films with a decreased average

grain size The first regime has been used for the deposition

of Schottky barriers and MOS transistor gates[24,25] The

use of this regime has allowed obtaining the enhancement of

catalytic properties of Pt-SiO2thin films, as well[26]

Among metal oxide semiconductors, tungsten oxide is

a promising material for gas sensing Several studies have

shown that it can be used for the detection of nitrogen

ox-ide (NO and NO2), carbon monoxide, ammonia vapours, and

hydrocarbons Tungsten oxide films can be deposited by

re-active rf sputtering, thermal evaporation and other methods

The results obtained indicate that the characteristics of the

sensors heavily depend on the conditions and methods used

in their preparation[27–31] Since grain size is one of the key

factors to enhance the gas sensing properties of metal oxide

sensors, the aim of this paper is to study the influence of

in-terrupting the deposition process on the sensing properties of

WO3thin films prepared by rf sputtering

2 Experimental

The tungsten oxide films were deposited on top of silicon

wafers by reactive rf magnetron sputtering using a ESM100

Edwards sputtering system A metal target of 99.95% purity

with a diameter of 100 mm and thickness of 3.175 mm was

used The target to substrate distance was set at 70 mm The

silicon wafers, oxidised in dry oxygen, were held in thermal

contact with a holder during the deposition process The

sub-strate temperature was kept constant during film deposition

at room temperature The sputtering atmosphere consisted of

Ar–O2mixed gas and its flow rate was controlled by separate gas flowmeters to provide an Ar:O2 flow ratio of 1:1 The pressure in the deposition chamber during sputtering was

5× 10−3mbar The rf sputtering power was 200 W These

conditions of deposition gave an average deposition rate up

to 2.12 nm per min

Four types of tungsten oxide films were prepared For the first type a non-interrupted sputtering was used In the de-position of films type 2, 3 and 4, the sputtering process was interrupted once, two and three times, respectively A shutter was used to interrupt the deposition process The total de-position time was 40, 40.5, 41 and 41.5 min, for films type

1, 2, 3 and 4, respectively The interruption time was set to

30 s Film thickness was controlled by ellipsometry (PLAS-MOS 2000) and stylus profilomentry (DEKTAK 3030) and was calculated from AES profiles as well

The samples used to investigate the gas sensing properties

of the films as a function of the number of interruptions during the deposition process were based on silicon substrates The top contacts to the sensing layers were formed using air dry silver paste (Heraeus AD1688-06) Using this paste the test samples were fixed on the ceramic heater prepared according

to the method reported in[32]

The response of the different films to nitrogen dioxide, car-bon monoxide, ethanol and ammonia was investigated The sensors were kept in a temperature and moisture controlled test chamber (27◦C,±1◦C and 41–43% RH) The sensors

were operated at the temperature range from 150 to 300◦C to

analyse the effect of working temperature on their response The resistance of the sensing layers in the presence of either

pure air (Rair) or the different pollutants (Rgas) at the different concentrations was monitored and stored in a PC

The morphology of the sensing layers was determined by AFM The sensing layer surface and the chemical element distributions in the samples were examined with a PHI-660 Auger spectrometer operating at 3 kV and using a probe di-ameter up to 1␮m Auger electron collection depth was up

to 2.0 nm

3 Results and discussion

3.1 Measurement of film thickness

Table 1shows the measured thickness of the WO3 thin films Two basic tendencies in the thickness as a function of

Table 1 Thickness (nm) of the WO3 thin film as a function of the number of interrup-tions of the deposition process as estimated by profilometry and ellipsometry Number of interruptions Measurement method

Profilometry Ellipsometry

Fig 1 AES profiles of rf sputtered WO3 thin films without interruption (a), one (b) and two (c) interruptions of the deposition process.

V Khatko et al / Sensors and Actuators B 109 (2005) 128–134 131

the number of interruptions during the deposition process can

be derived fromTable 1:

• Differences arise in the thickness measured by

profilomen-try and ellipsomeprofilomen-try for those tungsten oxide thin films

de-posited with interruptions during the deposition process

The formation into the body of the thin film of several

“extra” interfaces made difficult the measurement of film

thickness by ellipsometry Particularly, the method could

not be applied to measure films deposited with three

inter-ruptions

• The total thickness of WO3thin films decreases when the

number of interruptions during the deposition process

in-creases.Fig 1shows AUG depth profiles of chemical

el-ements into the WO3thin films It can be seen decreasing

the film total thickness that is proportionate to total sputter

time

3.2 Gas sensitivity studies

The responses of the different films to nitrogen dioxide,

carbon monoxide, ammonia and ethanol were analysed at

op-erating temperatures 150, 200, 250 and 300◦C.Fig 2shows

Fig 2 Sensor response of rf sputtered WO3 sensing layers to NO2 at 150 ◦C

(a) and 200 ◦C (b) WO3 (0), WO3 (1) and WO3 (3) are sensing layers

prepared without, with one and three interruptions of the deposition process,

respectively.

Fig 3 Sensor response of rf sputtered WO3 sensing layers to ammonia at

250 ◦C WO3(0), WO3(1) and WO3(3) are sensing layers prepared without, with one and three interruptions of the deposition process, respectively.

the response of the different tungsten oxide thin films to NO2

at 150◦C (Fig 2a) and 200◦C (Fig 2b) No matter the

op-erating temperature set, sensors based on tungsten trioxide films prepared with the maximum number of interruptions (i.e three) during their growth show the highest sensitivity to

NO2 For example, at 1 ppm of NO2, the sensitivity for these

sensors calculated using the relation S = (Rgas− Rair)/Rairis 2.156, 0.468, 0.255 and 0.167 at 150, 200, 250 and 300◦C,

respectively None of the WO3sensing layers responded to

CO in the temperature range investigated

The sensors responded to ammonia when operated be-tween 200 and 300◦C.Fig 3shows the response of the

dif-ferent sensing layers to 10 ppm of ammonia at the working temperature of 250◦C Once again, the sensors prepared with

three interruptions during the deposition process show the highest sensitivity.Table 2summarises the sensitivity results

to ammonia for the different films studied It can be seen that 250◦C is the optimal temperature for ammonia sensing

(sensitivity is higher at this operating temperature)

Fig 4 shows the response of the different WO3 sens-ing layers to ethanol at the worksens-ing temperatures of 250◦C

(Fig 4a) and 300◦C (Fig 4b) The sensors responded to

ethanol when operated between 200 and 300◦C At 200 and

250◦C they prepared without interruptions during their

depo-sition process show the highest sensitivities However, when operated at 300◦C, sensors prepared with three interruptions

during their deposition process show the highest sensitivity

Table 2 Sensor sensitivity (S) in the presence of 10 ppm of ammonia as a function

of the working temperature and number of interruptions of the deposition process

Number of interruptions Working temperature ( ◦C)

Fig 4 Sensor response of rf sputtered WO3 sensing layers to ethanol at

250 ◦C (a) and 300◦C (b) WO3(0), WO3 (1) and WO3 (3) are sensing

layers prepared without, with one and three interruptions of the deposition

process, respectively.

to ethanol.Table 3summarises the sensitivity to ethanol for

the different films and operating temperatures studied

3.3 Morphology of the WO 3 films

The surface of the tungsten trioxide films was investigated

by AFM (Nanoscope III).Fig 5clearly points out the

in-fluence of the number of interruptions during the sputtering

process on the morphology of deposited metal oxide films It

can be seen that increasing the number of interruptions leads

to a decrease in the roughness of the film surface

Table 3

Sensor sensitivity (S) in the presence of 10 ppm of ethanol as a function

of the working temperature and number of interruptions of the deposition

process

Number of interruptions Working temperature ( ◦C)

Fig 5 AFM surface morphology of WO3 thin films without interruption (a), one (b) and three (c) interruptions of the deposition process.

3.4 Discussion

As indicated above, the changes in the thickness of the dif-ferent WO3films allow drawing the conclusion that “extra” interfaces could be formed during the process of interruption The prolongation of film growth on the “extra” interface in-volves a new nucleation of the metal oxide film, the formation and growth of a film island structure and, finally, the forma-tion of a continuous layer All the steps in this process of

V Khatko et al / Sensors and Actuators B 109 (2005) 128–134 133

film growth take a specific time to complete Therefore, the

total film thickness decreases by increasing the number of

interruptions during the deposition process The analysis of

AUG depth profiles shows that the formation of “extra”

in-terfaces is not related to the free surface bond saturation by

the atoms from residual atmosphere Obviously, the “extra”

interfaces are formed due to the surface relaxation during

the interruption time This differs from the results obtained

for metal films deposited in an atmosphere containing

oxy-gen[33], where the sputtered films adsorb oxygen from the

residual atmosphere

The enhancement of the sensing properties observed for

WO3films deposited with three interruptions is due to the

de-crease in grain size into the metal oxide films This conclusion

is confirmed by AFM data Certainly, further confirmation by

alternative methods would be necessary Work is in progress

to perform more analyses, in particular transmission electron

microscopy, on the samples

4 Conclusions

WO3 thin films were deposited by reactive rf

sputter-ing from a pure tungsten target The deposition process

was conducted without interruption and with one, two and

three interruptions On the base of these films, sensing

lay-ers were prepared and their response to NO2, CO, ammonia

and ethanol was investigated It was shown that the sensing

layers prepared with the maximum (i.e three) number of

in-terruptions show the best sensing properties The increase

in sensitivity is related to the decrease of grain size in the

WO3thin films observed as the number of interruptions was

increased This conclusion is based on AFM data

References

[1] S.R Morrison, Selectivity in semiconductor gas sensors, Sens

Ac-tuators 12 (1987) 425–440.

[2] G Sberveglieri, S Groppelli, P Nell, V Lantto, H Torvela, P

Romp-painen, S Lepp¨avuori, Response to nitric oxide of thin and thick

SnO2 films containing trivalent additives, Sens Actuators, B Chem.

1 (1990) 79–82.

[3] J Mizsei, V Lantto, Simultaneous response of work function and

resistivity of some SnO2-based samples to H2 and H2 S, Sens

Ac-tuators, B Chem 4 (1991) 163–168.

[4] K.D Shierbaum, U Weimar, W Gopel, Comparison of ceramic,

thick-film and thin-film chemical sensors based upon SnO2 , Sens.

Actuators, B Chem 7 (1992) 709–716.

[5] E Llobet, G Molas, P Molinas, J Calderer, X Vilanova, J Brezmes,

J.E Suegras, X Correig, Fabrication of highly selective tungsten

oxide ammonia sensors, J Electrochem Soc 147 (2000) 776–779.

[6] C Bittencourt, R Landers, E Llobet, G Molas, X Correig, M.A.P.

Silvaa, J.E Sueiras, J Calderer, Effect of oxygen partial pressure

and annealing temperature on the formation of sputtered tungsten

oxide films, J Electrochem Soc 149 (2002) H81–H86.

[7] R Ionescu, E Llobet, Wavelet transform-base fast feature

extrac-tion from temperature modulated semiconductor gas sensors, Sens.

Actuators, B Chem 81 (2002) 289–295.

[8] R Ionescu, E Llobet, S Al Khalifa, J.W Gardner, J Brezmes, X.

Vilanova, X Correig, Response model for thermally-modulated tin

oxide based microhotplate gas sensors, in: Proceedings of the 16th European Conference on Solid State Transducers EUROSENSORS XVI, Prague, 2002, pp 247–248.

[9] J Puigcorbe, A Vila, J.R Morante, Thermal fatigue modelling in mi-cromachined gas sensors, in: Proceedings of the 16th European Con-ference on Solid State Transducers EUROSENSORS XVI, Prague,

2002, pp 257–258.

[10] S.R Morrison, Semiconductor gas sensors, Sens Actuators 2 (1982) 329–343.

[11] G.K Flingelli, M.M Fleischer, H Meixner, Selective detection of methane in domestic environments using a catalyst sensor system based on Ga2O3, Sens Actuators, B Chem 48 (1998) 258–262 [12] M Frietsch, F Zudock, J Goschnick, M Bruns, CuO catalytic mem-brane as selectivity trimmer for metal oxide gas sensors, Sens Ac-tuators, B Chem 65 (2000) 258–262.

[13] M Fleischer, S Kornely, T Weh, H Meixner, Selective gas detection with high-temperature operated metal oxides using catalytic filters, Sens Actuators, B Chem 69 (2000) 205–210.

[14] M Frietsch, V Trouillet, I Kiselev, J Goschnick, The influence of different gradient coating on the detecting properties of a metal oxide gas sensor microarray, in: Proceedings of the 16th European Con-ference on Solid State Transducers EUROSENSORS XVI, Prague,

2002, pp 595–596.

[15] G.-J Li, X.-H Zhang, S Kawi, Relationships between sensitivity, catalytic activity, and surface areas of SnO2 gas sensors, Sens Ac-tuators B Chem 60 (1999) 64–70.

[16] G Zhang, M Liu, Effect of particle size and dopant on properties

of SnO2-based gas sensors, Sens Actuators B Chem 69 (2000) 144–152.

[17] A Chiorino, G Ghotti, F Prinetto, M.C Carotta, C Malagu, G Martinelli, Preparation and characterization of SnO2 and WOX –SnO2 nanosized powders and thick films for gas sensing, Sens Actuators

B Chem 78 (2001) 89–97.

[18] P Ivanov, E Llobet, X Vilanova, J Brezmes, X Correig, J Hubalek,

K Malysz, I Gracia, C Cane, Screen-printed nano-grain tin ox-ide films for micro-hotplate gas sensors, in: Proceedings of the fourth Conferencia de Dispositivos Electronicos, Calella de la Costa, Barcelona, 2003, p 145.

[19] B Lillis, E Hurley, S Galvin, A Mathewson, H Berney, A novel, high surface area, capacitance based silicon sensor for DNA hybridi-sation detectiondkjdot, in: Proceedings of the 16th European Con-ference on Solid State Transducers EUROSENSORS XVI, Prague,

2002, pp 693–694.

[20] O.K Varghese, D Gong, M Paulose, K.G Ong, C.A Grimes, E.C Dickey, Highly ordered nanoporous alumina films: Effect of pore size and uniformity on sensing performance, J Mater Res 17 (2002) 1162–1171.

[21] E.C Dickey, O.K Varghese, K.G Ong, D Gong, M Paulose, C.A Grimes, Room temperature ammonia and humidity sensing using highly ordered nanoporous alumina films, Sensor 2 (2002) 91– 110.

[22] V Khatko, E Logothetis, R Soltis, J Hangas, J McBride, Develop-ment of highly active catalyst for Si-microcalorymetric gas sensor, in: Sven Kruger, Wolfgang Gessner (Eds.), Advanced Microsystems for Automotive Application, Springer, Berlin, 2000, pp 27–37 [23] V Khatko, R Soltis, J McBride, K Nietering, Catalytic properties

of Pd/SiO2 and Pt/SiO2 multilayer stacks, Sens Actuators, B Chem.

77 (2001) 548–554.

[24] N Rumak, V Khatko, S Vasiliev, Formation of metal-semiconductor contacts by ion-beam methods, in: Proceedings of the Belarusian Academy of Sciences, Ser Physical-Technical, 4, 1990, pp 82–84 (in Russian).

[25] S Vasiliev, N Rumak, V Khatko, Formation of MOS-structures with quasilayer metal gate, Electron Eng Ser Microelectron 3 (1990) 10–14, in Russian.

[26] V Khatko, Catalytic property variation of sputtered Pt–SiO2 thin film, in: Proceedings of International Conference “New Technologies

of Multicrystal Module Production”, Minsk-Naroch, Belarus, 2000,

pp 71–74 (in Russian).

[27] G Sberveglieri, L Depero, S Groppelli, P Nelly, WO3 sputtered

thin films for NOx monitoring, Sens Actuators, B Chem 26–27

(1995) 89–92.

[28] M Penza, M.A Tagliente, L Mirenghi, C Gerardo, C Martucci,

G Cassano, Tungsten trioxide (WO3) sputtered thin films for a NOx

gas sensor, Sens Actuators, B Chem 50 (1998) 9–18.

[29] C Lemire, D.B.B Lollman, A Al Mohammad, E Gillet, K Aguir,

Reactive rf magnetron sputtering deposition of WO3 thin films, Sens.

Actuators, B Chem 84 (2002) 43–48.

[30] C Bittencourt, R Landers, E Llobet, X Correig, J Calderer, Role of

oxygen partial pressure and annealing temperature on the formation

of W O bonds in WO3 films, Semicond Sci Technol 17 (2002) 522–525.

[31] R Ionescu, E Llobet, X Vilanova, J Brezmes, J.E Sueiras, J Calderer, X Correig, Quantitative analysis of NO2 in the presence of

CO using a single tungsten oxide semiconductor sensor and dynamic signal processing, Analyst 127 (2002) 1237–1246.

[32] J Hub´alek, K Malysz, J Pr´aˇsek, X Vilanova, P Ivanov, E Llobet,

J Brezmes, X Correig, Z Sv˘er´ak, Pt-loaded Al2O3 catalytic filters for screen-printed WO3 sensors highly selective to benzene, Sens Actuators, B, Chem 101 (2004) 277–283.

[33] T.T Bardin, J.C Pronko, R.C Budhan, J.S Lin, R.F Bunshah, The effect of oxygen concentration in sputter-deposited molybde-num films, Thin solid films 165 (1988) 243–247.