Micromachined gas sensors for environmental pollutants - Springer Link

Microsystem Technologies 6 (1999) 54±59 Ó Springer-Verlag 1999

Micromachined gas sensors for environmental pollutants G. Faglia, E. Comini, M. Pardo, A. Taroni, G. Cardinali, S. Nicoletti, G. Sberveglieri

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G. Faglia, E. Comini, M. Pardo, A. Taroni, G. Sberveglieri INFM-Gas Sensor Lab, Department of Chemistry and Physics for Engineering and for Materials, University of Brescia, Via Valotti 9, I-25133 Brescia, Italy

possibility of on-line operation. Furthermore the introduction of silicon based micromachined substrates proved the IC-compatibility of these devices [1, 2] and reduced the power consumption of integrated heaters from about 1 W to 100 mW, due to the decrease of size and thermal mass. Reduction of power consumption is a key aspect in order to develop portable equipment and sensor networks inside buildings. As a matter of fact in these networks the supply from a smart unit has to be brought around to very large distances inside the rooms where, among others, sensor for ®re, polluting gases and air conditioning control are placed. The technique presented in this work goes in the direction of developing very low power consumption devices, that could even be battery operated. In fact the sensor heater is periodically supplied for very short terms, hundred of milliseconds, and kept off for long ones, seconds or more. This technique can be applied only to micromachined substrates. Indeed, due to the very low thermal mass of the membrane, the term required to reach steady state conditions when switched from off to operation is very short (about 40 ms). On the contrary, for conventional substrates based on ceramic materials as alumina, it takes about 30 s. The authors have already reported preliminary results [3] that demonstrated that power saving compared with isothermal characterization can be as high as ®ve hundred times. Besides a strong reduction of power consumption, in some con®guration an increase of sensitivity to carbon monoxide is observed, that is related to the surface reactions rate between this gas species and ionosorbed oxygen. The observed drawback is a deterioration of the sensor dynamic rate that is re¯ected into an increase of response and recovery time. Different shapes of the heating wave have been examined. We started from a single square wave and turned to multiple waves with different amplitudes. In the conclusions results are summarized and compared.

G. Cardinali, S. Nicoletti CNR-LAMEL Institute, Via P. Gobetti 101, I-40129 Bologna, Italy

2 Experimental

Abstract An Au doped tin oxide thin ®lm was deposited as base material for carbon monoxide detection over a micromachined substrate. The performances of a recent technique to heat the device, named fast pulsed temperature supply, are presented. This technique exploits the property that, due to the very low thermal mass of the membrane, the term required to reach steady state conditions is very short (about 40 ms). The sensor heater is periodically supplied for very short terms, hundred of milliseconds, and kept off for long ones, seconds or more. Besides a strong reduction of power consumption compared with isothermal characterization, an increase of sensitivity is observed. Different shapes of the heating wave were examined and results are summarized and compared.

1 Introduction Semiconductor gas sensors are devices based on metal oxides which transport properties are modi®ed when they are placed in toxic and dangerous environments. The sensing properties are related to surface reactions between the species to be detected and oxygen or water previously ionosorbed. For these reactions to take place the devices are kept at very high temperatures ranging from 500 K to 800 K, as a rule through an integrated heater based on Pt or poly-Si. The physical quantity normally measured is the resistance, that depends on the adsorbed species through the height of the energy barriers between grains. The application ®eld of semiconductor gas sensors ranges from environmental monitoring and automotive application to domestic and industrial applications. The bene®ts offered by semiconductor gas sensors are related mainly to the low dimension and cost and the Received: 1 June 1999 / Accepted: 2 June 1999

Correspondence to: G. Faglia This work was partially supported by the European Union in the frame of the Esprit Project `Smart air pollution MonitOrinG networks' (SMOG-No21428).

2.1 Sensing layer The sensing layer is a tin oxide thin ®lm deposited by DC sputtering through the Rheotaxial Growth and Thermal

Oxidation (RGTO) technique [4]. The lateral dimensions and the nominal ®lm thickness are 0.7 mm by side and 300 nm, respectively. On top of the SnO2 sensing layer, a very thin ®lm (4 nm) of gold was deposited to catalyze CO detection. The desired geometry was achieved by shadow masking.

2.2 Substrate The substrate was a low thermal mass micromachined Si one reported in Fig. 1. The key elements of the device structure are represented by: ± 200 nm thin stoichiometric Si3 N4 membrane on Si substrate; ± 25 nm TiN adhesion layer ± double spiral heater resistor and temperature sensor made of 300 nm Pt; ± passivation layer realized combining a spin-on-glass and LTO layers; In the micromachined Si substrate of Fig. 1, the Si3 N4 membrane dimension is 1.5 mm by side while the device size is 4:5  4:5 mm2 . To reduce the thermal power consumption towards the Si substrate, the ratio between the membrane size and the active area of the device (e.g. the heated area) has been kept equal to 2. To achieve a better temperature uniformity all over the device active area, the heater geometry has been improved using the SOLIDIS ± FEM simulation program developed at the ETH-Zurich (CH). Rather than a meander shaped heater geometry, characterized by a very high radial temperature gradient, a double spiral heater con®guration has been used. Thermographic analysis performed at the premises of the DBDASA Munich (D) have con®rmed the highest temperature uniformity achievable with the new heater design [2]. The temperature of the active area is obtained by

DT ˆ

RT ÿ RTamb a‰RTamb ÿ R1 …1 ÿ k†Š

where: DT is the difference between the heater and the room temperatures, RT is the Pt resistance at the tem-

Fig. 1. Exploded view of the LAMEL micromachined Si substrate device for gas sensor application

perature T, Tamb the Pt resistance at room temperature, a the positive temperature coef®cient of Pt at room temperature, R1 is the contribution to the overall heater resistance given by the Pt interconnection pad-heater, k is the coef®cient that relates the temperature of the interconnecting part (T1 ) to DT through the relation T1 ˆ kDT. As it has been evidenced by the thermographic analysis, the interconnecting part is at an average temperature much lower than that of the heater. This means that the k value is reasonably small. The temperature values obtained from the above relation have been plotted in ®gure 2 as a function of the applied power, using k ˆ 0:2 and k ˆ 0:4. As we can see, a small dependence of power consumption from the k value is noted. Moreover, looking at Fig. 2, we can point out that a gas sensor working temperature of 673 K is maintained applying 0.1 mW rather than 0.4±0.8 W of most commercial devices.

2.3 Set-up for characterization Sensor performances have been evaluated exposing the devices to traces of CO and of other environmental pollutants in synthetic air, at different R.H. values in normal conditions (T = 293 K and atmospheric pressure), by placing them in a test gas facility described elsewhere [5]. The set-up used for the measurement of the sensor response is reported in Fig. 3. The temperature waveform has been generated by a D/A converter (Keithley PC card DAS1800AO with 100 KHz bandwidth) coupled with a unity gain current ampli®er to drive the substrate heater. The sensing layer response has been evaluated measuring the voltage drop on a resistance in series using an A/D converter (Keithley PC card DAS1800AO with 100 KHz bandwidth). An example of the temperature waveform is reported in Fig. 4 for a period lasting 1 s. The temperature as measured by the integrated temperature sensor is reported as well. As we can see, a time as short as 40 ms is suf®cient to rise the temperature of the device active area from room temperature up to 720 K. Moreover thermal stability and sturdiness was determined by cycling every second the substrates temperature

Fig. 2. Active area temperature as a function of the applied power for 2 different k values

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Fig. 3. Electrical experimental set-up for sensor characterization

from RT to 700 K for a month without provoking any failure or leaking.

3 Results and discussion When the heater is periodically supplied as reported in Fig. 4, the sensor conductance is also periodic as shown in Fig. 5. According to the thermionic theory [6, 7] the conductivity of semiconductor devices is related to the capability of conduction band electrons to exceed double surface Schottky barriers that form at the interface between grains. The conductivity is thermally activated by the mean voltage barrier value

more electrons are able to exceed the barrier and the conductivity increases almost immediately. The height of the energy barrier in 1 is

where G0 is the bulk conductivity and qVS the energy barrier. Therefore when temperature increases, much

q2 Nt2 …2† 2s Nd where Nt is surface density of ionosorbed oxygen, s the tin oxide dielectric constant and Nd the volume dopant density due to oxygen lattice vacancies [8]. The introduction of carbon monoxide consumes ionosorbed oxygen reducing their steady state density Nt at equilibrium and therefore the height of the energy barrier. According to Eq. (1) the conductance increases reaching the equilibrium value showed in Fig. 5. Anyway surface reactions take place with a very slow rate compared to an heating period. From the instant carbon monoxide is introduced, it takes about 30 minutes for conductivity response in a period to evolve from the

Fig. 4. Shape of the square voltage wave applied to the device heater and corresponding layer temperature detected by the integrated temperature sensor

Fig. 5. Steady state tin oxide layer conductance produced by periodic heating as in Fig. 4. Value in ambient humid air (R.H. = 40% at T ˆ 293 K) and 30 minutes after the introduction of 250 ppm of carbon monoxide are reported

qVS

G ˆ G0 eÿ kT

…1†

qVS ˆ

shape it has in air to the new equilibrium one. Therefore to represent the evolution of the device response it is worth identifying a signi®cant instant for each temperature cycle. One chooses the instant when the device has reached the steady state temperature and the conductance a plateau (see Fig. 5), that is 90 ms after the rising wave front. By reporting the signi®cant points during the transient as a function of time, device conductance evolves towards new equilibrium with a kinetic very similar to the one observed when isothermal characterization is performed. Three shapes of fast pulsed supplying have been examined as summed up in Fig. 6. For each cycle the signi®cant instants are pointed out. First approach was performed through type A heating wave: the device is kept at 4 V (about 720 K) for tH ˆ 100 ms each period P ranging from 0.5 s to 180 s (not represented in the ®gure). The relation between power consumption WP and period is

tH WDC …3† P where WDC is the power consumption for isothermal characterization at the same voltage. Therefore there is a strong reduction of power consumption when period is increased. For example when P ˆ 20 s power consumption is two hundred times lower and equal to 500 lW. Furthermore the authors have observed an increase of sensor response (relative change of conductance) compared to isothermal characterization for periods shorter WP ˆ

than 20 s [3]. Interference by humidity and some environmental polluting gases as nitrogen dioxide and benzene does not increase in comparison with isothermal characterization and the device response is stable during one week measurements [3]. The main drawback appears to be the increase in response and recovery time due to a worsening of sensor dynamic response. As a matter of fact, when period augments at constant length of the heating term, on the whole is reduced the term during which the sensor is heated and carbon monoxide oxidation and oxygen absorption reactions take place [8]. Type B shape was applied to investigate the possibility to enhance carbon monoxide detection. The sensor heater is supplied at 4 V (720 K), 2 V (500 K) and 1.5 V (450 K) respectively for 100 ms every 400 ms. The cycle duration is equal to 1.5 s. This particular cycle was chosen to increase the reaction rates for carbon monoxide reactions, which are promoted at lower temperatures compared to the ones required for oxygen absorption. Figure 7 reports the kinetic response to 50 ppm of carbon monoxide depending on the pulses (4 V, 2 V and 1.5 V) at which signi®cant points are taken. For comparison the isothermal response at 4 V (DC) is reported too. The main features extracted from Fig. 7 are reported in Table 1. Compared to isothermal response, fast pulsed temperature heating strongly improves the sensor response of the device from about one at DC to 17.2 at the 1.5 V pulse. Moreover power consumption is reduced tenfold with

Fig. 6. Shapes of fast pulsed supplying that have been examined. For each cycle the signi®cant instants are pointed out ( )

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58 Fig. 7. Kinetic response to 50 ppm of CO as a function of the sampling instant at 4 V, 2 V and 1.5 V. Isothermal response at 4 V (DC) is reported too. Instabilities are ascribable to the resolution of the measurement system Table 1. As a function of the instant when conductance is sampled are reported the sensor conductance in air ÿGairÿ and in 50 ppm of CO ÿGgasÿ , the sensor response S and the response tresp and recovery time trec necessary to reach the 90% and the 70% of the step during rising and recovery. The isothermal response at DC is reported for comparison Instant (ms)

Gair (S)

90 (4 V) 590 (2 V) 1090 (1.5 V) DC (4 V)

6.6 2.2 5.1 1.1

10)7 10)8 10)9 10)7

Ggas (S) S = DG/Gair tresp (s) 2.3 3.5 9.2 2.2

10)6 2.5 10)7 14.9 10)5 17.2 10)7 1.0

280 150 250 70

trec (s) 210 120 140 40

respect to isothermal characterization (from 100 mW to 10 mW). The drawback is the reduction in response and recovery rates. A good trade-off seems the 2 V heating wave, for which dynamic worsening is still acceptable and is counterbalanced by a great increase for response. Recent activity has dealt with the study of the in¯uence of the length of the high temperature wave at 4 V, that is responsible for oxygen ionosorption, over the sensor response extracted during the low temperature wave at 2 V, where carbon monoxide reactions are enhanced. The length of the high temperature term is varied in the range 50±300 ms while the length of the low temperature term is kept constant at 100 ms (see type C in Fig. 6). The choice of 2 V as value of the low temperature wave was compulsory as a trade-off between the requirement of a satisfying response and the worsening of dynamic. Figure 8 reports the kinetic response as a function of time due to the introduction of 100 ppm of CO for heating cycles of type C with increasing length of the high temperature wave. Two remarks can be deduced: ± When the length of the high temperature wave increases, the dynamic rate is enhanced, that is response and recovery time are reduced. This effect is easily understandable since a longer length means an overall greater heating of the device. ± The amplitude of the sensor response displays a maximum for cycle length in the range between 100 ms and

Fig. 8. Kinetic response due to the introduction of 100 ppm of CO as a function of the length of the ®rst square wave (see Fig. 6)

200 ms as summarized in Table 2 where is reported the response calculated from Fig. 8 as a function of the length of the high temperature cycle. Therefore results validates the focusing on multiple waves cycles with high temperature terms at T ˆ 720 K for 100 ms. For comparison Fig. 9 shows the kinetic response for a sample supplied with an high temperature wave lasting 100 ms with respect to the same response during an isothermal characterization at V ˆ 4 V and V ˆ 2 V. If the sample is kept isothermally at 2 V, the coverage with oxygen active ions does not take place. On the contrary, when the device is kept at 4 V the reaction between CO and the ionosorbed oxygen molecules is unfavoured. The fast pulsed supplying with a double square wave induces oxygen ionosorption during the term at 4 V and enhances CO detection during the term at 2 V. Table 2. Sensor response to 100 ppm of carbon monoxide calculated with the data extracted from Fig. 8 versus the length of the ®rst square wave TH (ms)

S(DG/G)

50 100 200 300

0.5 1.4 1.5 1.1

Fig. 9. Comparison of the kinetic responses due to the introduction of 100 ppm CO for a device supplied with a double heating wave (tH ˆ 100 ms) and isothermally at 2 V and 4 V

4 Conclusions We have studied a new technique to operate semiconductor gas sensors deposited over micromachined substrates, named fast pulsed temperature supplying. By this technique it is possible to reduce the power consumption to lW level and increase tenfold and more the sensor response to carbon monoxide, as compared to isothermal characterization. The shape of the heating wave has been optimized in order both to reduce power consumption and to increase sensor response. Future work will deal with further optimization of the heating wave, especially to avoid the worsening of the sensor dynamic response. References

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