A novel SnO2-based gas sensor

A novel SnO2-based gas sensor

Pergamon Talannra,Vol 41 No IO. pp 1735-1740 1994 CopyrIght Cc 1994 Elsewcr Scrcncc Lrd Pnnted tn Great Br~utn All nghts rexrvcd 0039.9140194 57 00 +...

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Pergamon

Talannra,Vol 41 No IO. pp 1735-1740 1994 CopyrIght Cc 1994 Elsewcr Scrcncc Lrd Pnnted tn Great Br~utn All nghts rexrvcd 0039.9140194 57 00 + 0 00

0039-9140(94)E011&B

A NOVEL SnO,-BASED KAREL

SIROK?

and JANA

GAS SENSOR JIRESOVA

Instttute of Chemical Technology. Prague 166 28, Czech Repubhc (Recetred 9 September

1993

Reused

10

November 1993

Accepted 4 March

1994)

novel ‘two-termmal’ semiconductor gas sensor was developed based on a heavdy Sb-doped SnO: film prepared by cathodic sputtermg The sensor 1s heated at Its operattonal temperature by the gas sensltlve film itself A device for detectmg the leakage of flammable gases. some noxious or hazardous gases can be made m this way Summary-A

INTRODUCTiON

In recent years gas sensors using oxide semiconductors have been subjected to extensive research and development The attraction of semiconductmg gas sensors comes from their high sensmvuy and theu low fabrication cost. In principle such sensor usually consists of a film (or of a smtered body) of gas sensitive metal oxide and a heater. If the oxide which is heated to a high temperature by the heater comes into contact wuh combusuble gas, its electric resistance changes’” Little attention has been pald up to now to the posslbllity of heatmg the sensor by the gas sensitive oxide itself A much simpler construcuon of the sensor could be achieved in this way (Fig. 1) To use the oxide itself as a heating element some Important conditions must be fulfilled. The resistance of the oxide must be rather low so that the applied voltage need not be very high, and the temperature coefficient (TC) of the oxide resistance should be positive. It 1s well known that tm oxide can be prepared with low resistlvity which could be controlled by doping, and on the other hand its electric resistance IS greatly affected by the presence of combustible gas in the ambient atmosphere’-6. Tin oxide has n-type conductivity. When exposed to a combustible gas its conductivity increases. If the SnO, is used simultaneously as the heating element then the current flowmg through the heater increases, consequently the temperature of the oxide increases (the applied voltage is kept constant), the conductivity further increases (because pure SnO, has a negative TC of renstivity), etc. Thus, SnOz behaves as a

system with posttive feedback and such a sensor would be unstable. A negative feedback is desired if the sensor is to be stable after expositton of a combustible gas. Therefore, an oxide with the positive TC of resistance must be used which can be obtained when SnO, IS heavily doped with Sb [7] The purpose of this paper 1s to describe a novel SnO,-based sensor wuh positive TC of resistance prepared by cathodic sputtering.

EXPERIMENTAL

Sensor structure and fabrication process

The source material was high purity SnO, powder which was thoroughly mixed with Sb,OJ powder in weight ratio 4.3: 1 and pressed into a 5 cm diameter by 0.4 cm disc. This disc (target) was smtered at 1250°C for 2 hr An alumina plate 5 x 5 x 0.6 mm was used as a substrate. A pair of comb-like Pt electrodes was sputtered in advance on the substrate. The SnO,(Sb) film was deposited over the electrodes by rf sputtering (27 MHz) in a two electrode system. A mixture of oxygen and argon was used as a sputtering gas (I 1 vol.% 0 in Ar) at a pressure of 1.5 Pa. At I hr deposition time the film thickness was about 1 pm A shadow mask was used to control the region of deposition (Fig. 2a). As-sputtered SnOz(Sb) film was treated at 500°C for 4 hr (sensor Sl). Alternatively, a sensor wuh a Pt heater was prepared. A Pt film and Pt electrodes were sputtered in one step on an alumina substrate 10 x 10 x 0.6 mm (Fig. 2b). The Pt film at the edge of the substrate could be used as a heating element or as a Pt resistance thermometer,

1735

1736

KAREL SIROKYand JA\A JIRESOVA 1

POWER

oxide

film

Fig I ‘Two-termmal’ semlconductor gas sensor The sensor contams only a pair of electrodes both for heatmg and gas concentration measurement (from the change of current)

respectively The SnO,(Sb) thin film was deposited over the comb-like electrodes as before (sensor S2)

PropertIes of SnOz (Sb) thm jilm sensor The resistance of the sensor at the temperature of 25OC was as low as 44 ohms and increased with mcreasmg temperature wtth an average temperature coeffictent of reststance (TCR) 0.07 ohm/C. The gas-sensmg charactertsttc of the sensors were measured m a glass chamber. A gas or vapor was Injected mto the chamber through a septum usmg varrous srzed synnges. Ftgure 3 shows the dependence of sensor reststance on concentration of some combusttble gases. The sensor wtth the heater (sensor S2) was used for these measurements and the sensor temperature was adjusted by varymg the current through the heater and measured by using the heater resistance as a platmum resistance thermometer. The gas sensitive properties of the SnO,(Sb) films are charactertzed by a decrease m electrtcal resistance due to the contact with a reducing gas. These features are attrtbuted to the n-type semtconducttvtty of tin oxide. Figure 4 shows the dependence of electrical current flowmg through the sensor Sl on the concentratton at a constant apphed voltage. The current can be used tmmedtately as a measure of the gas concentratron The mnral temperature was estimated m this case according to known melting points of salts as descrrbed elsewhere 8 To estimate the response ttme the sensor was placed m a through-flow cell whrch was contmuously supplied wtth au. The gas flow was controlled using a rotary valve whtch enabled gas concentratton to be changed raptdly. Figure 5 shows the typtcal response and restoring charactensttcs. When the sensor was exposed to the au contaming 300 ppm of ethylalcohol the sensor

current Increased rapidly and then reached the constant value. Then. the sensor current recovered by removmg the reducmg gas and finally reached the unttal value. Alternattvely the sensor was used m a brtdge accordmg to Fig 6. The reststors R,, R2 and R, were chosen so that they were approxtmately equal m value to the reststance of the sensor at uuttal operating temperature. The bridge output stgnal was proporttonal to the change m sensor reastance. Ftgure 7 shows the dependence of output voltage on the gas concentratton. MODEL

At low gas-concentratton levels and at a specttied temperature, an approxtmatton for the reststtvtty of an oxtde sensor IS usually expressed by the followmg formula9 R,=R,(l

+aC)“,

(1)

where Q and n are constants. C IS the gas concentratton m vol % or ppm R, and R, are electrtcal reststance m clean au and m au- contauung gas, respecttvely For the sake of stmphctty, m our model the relattonshrp between the sensor reststance and gas concentratton rangmg from 0 to 0.5.vol % (5000 ppm) we have approxtmated by R,=R,-C/(a-t+C). Au

(2)

wtre

/

Sn02

(Sbl

thin

film

‘substrate -Pt

(b)

\

electro

,Au

des

wire

Pt

heating

SnO2 (Sb)

film thin

film

‘substrate ‘Pt

electrodes

Au wire Fig 2 Schematic diagram of sensor structure (a) WIthout a heater (sensor Sl). (b) with a Pt-thm film heater (sensor S2)

1737

SnO,-based gas sensor

E

s

L

‘\\;---

f-

lOi

I

I

105

loL

10’

lo*

Concentration,

ppm

Fig 3 Dependence of sensor resistance on gas concentration

Initial

oporatlng

(sensor S2)

LOOOc

temperature

/ 180

/ -/ q E

o Ethanol 0 Heptane .

/

Hydrogen

@ Methane

/ I/

1

__ __ __--I..-. Concentration,

C2H50H 300 ppm in air

--

1 lo5

ppm

Fig 4 Dependence of sensor current on gas concentration

Clean air _ , _

_-_--

(sensor SI)

Clean air

Time (min) Time (s) Rg 5. Reponse and restonng characteristics (sensor SI)

KAREL SIROK+and JANAJIRESOVA

1738

I

0

the sensor temperature increases and consequently R, Increases because of positive TCR of the film

UOPP

Rl

R, = r. + rl P,

R2

I

Fig 6 Bridge connexton of the sensor

where a and /? are constants if the operating temperature is kept constant. If the heating of the sensor is carrted out through the gas sensitive oxide film, R,, tl and p are no longer constant but depend on temperature, i.e on the current flowing through the film. At a constant applied voltage U,,, the current Z flowing through the film IS Z = U,,/R,

(3)

and the heating power P IS P = u,z.

(4)

In clean air C = 0 and R, = R, (initial state). When the sensor is exposed to air containmg gas, its electrical resistance decreases (the second term in equation 2), the sensor current Increases,

Inltlol

operottng

(5)

where r, and r, are constants if the range of heating power IS not too large Together with the negative feedback caused by the positive TCR a positive feedback occurs as z and ~3 decrease with temperature Increase (the gas sensitivity of the film increases with temperature). This simple model was applied on the system SnO,(Sb) film (sensor SZ)-ethanol m the concentration range O-200 ppm. Figure 8 shows the dependence of sensor resistance on ethanol concentration at different heater power Dependence of a and B on the heatmg power was determined from these measurements as a =k,,-k,P

(6)

and p = K. - K, P,

(7)

where k,, k,, K, and K, are approximately constant. Makmg use of equations (2)-(7) one can calculate the change of sensor resistance caused by different concentratron of ethanol m air In the ‘two-terminal’ regime. The solution starts by putting the mitral power which corresponds to the chosen operating temperature m equations (j)-(7) and solving equation (2) for R, The second iteration starts by using the value of R, m equation (3) and successrve solvmg equations

temperoture

0.6

300 ‘C

o

Ethanol

l

Heptone

e

Hydrogen

o

Methone

10‘

10’ Concentrotbon,

ppm

Rg 7 Dependence of bndge output voltage on gas concentratton. Sensor resistance m clean air. R,= 47 3 R, v,,, = 14 V (sensor SI)

SnO,-based gas sensor

LOI

0

I 50 Ethanol

I739

I

r

8

100

150

200

concentration,

ppm

Fig 8 Dependence of sensor resistance on ethanol concentration at dlfferent heatmg power (sensor S2)

o -

Ethanol Fig

CALCULATED

concentrotlon,

POINTS DEPENDENCE

ppm

9 Dependence of sensor reststance (sensor S2) when heated by SnO,(Sb) film on ethanol concentratton Companson of the calculated dependence with the expenmental data

for R,,a and /3, and equation (2) for R, The solution usually converges after four iterations. Ftgure 9 shows the calculated dependence of the sensor resistance on the ethanol concentration when the sensor (S2) 1s heated by the SnO,(Sb) film (full hne) together with several experimental points. Initial conditions were: P = 3.0 W, t z 35O”C, R,= R,= 61 Q. In au containing 200 ppm of ethylalcohol they were: P = 4.0 W, t 2 410°C R,= 46 R. The difference between the expenmental and the calculated values rarely exceeded expenmental errors. Nevertheless, the described model is too stmplified to be used in larger temperature and/or concentratton range but explains the basic mechanism of the ‘two-terminal’ sensor. (4)-(7)

EXPERIMENTAL

CONCLUSIONS

Heavily Sb-doped SnO, thin film can be used not only for sensing of low concentration of combusttble gases but can serve at the same time for heating the sensor at its operating temperature The film has good short-term stability In the first week after preparation some agmg of the sensor was observed followed by a nearly constant current as a function of time. When the sensor was energized contmuously the zero-point stabihty was +2% of steady state current m clean au which was obviously caused by changes of ambient temperature and humidity. The long-term stability 1s now under investigation. These sensors enable fabrtcatton, e.g. of simple alarm devices for hazardous gas levels with high enough

KAREL

1740

SIROK+

sensltwity The sensors are nonselective other oxide-based semlconductor sensors.

like

REFERENCES

and JAYAJIR&OVA

4 J F McAleer, P T Moseley, J 0 W Norris and D E Wllhams, J Chem Sot. Faradar Trans I, 1987. 83, 1323 5 K Nomura. Y Ujlhlra. S S Sharma. A Fueda and T Murakaml, J Mafer SCI . 1989. 24, 937

6 N Yamazoe. Y, Kurokawa and T Selyama, Sensors I P T Moseley and B C Tofield (eds), Sohd Stare Gas Sensors Adam Hdger, Bnstol, 1987 2 T Selyama (ed ), Chemtcal Sensor Technology, Vol I Kodansha, Tokyo, 1988 3 W Gbpel, J Hesse and J N Zemel (eds), SENSORS, A Comprehenswe Sumey, Vol 2, Part 1, Chemwal and Btochemwa! Sensors VCH, Wemhelm. 1991

Acruators. 1983, 4, 283 7 W R Smclalr. F G Peters, D S E Koonce, J Electrochem

W Stdhnger and Sot, 1965. 112.

1069 8 K Slroki, Sensors .4cruarors E, 1993. 17, I3 9 P K Chfford and D T Tuma, Sensors Acruarors, 1982/83, 3, 233