Pt capacitive structure

trs nnd Actuators

B, 9 (1992) 191-196

low-temperature oxygen sensor based on the LaF, /Pt capacitive structure R Krause boldt University

and Werner Berlin, Department

Moritz of Chemistry,

Bunsenstrasse

I, O-1080 Berlin (Germany)

olf Grohmann .al Institute for Physical :ived September

Chemistry,

Analytical

Centre, Rudower Chaussee 5, O-1199 Berlin (Germany)

11, 1991; in revised form February

3, 1992; accepted

May 2, 1992)

Abstract A new Si/LaF,/Pt oxygen-sensitive structure working on the basis of the field effect in silicon has been developed. It meets the demands of semiconductor technology and is convenient for detecting oxygen both in gas mixtures and in liquids. A low 90% response time of 90 s and a Nemstian sensitivity of 59 mV/lg po2 are obtained. The sensor is shown to be able to detect oxygen partial pressures within a range of eight orders of magnitude. A comparison between the Si/LaF,/Pt and Si/SiO,/Si,N,/Pt structures has been carried out in order to consider whether LaF, is involved in the oxygen-sensing mechanism. The parameters of the platinum layer are changed in order to improve the sensor function. XPS measurements are used to explain the influence of the treatment of LaF, with solutions varying in pH.

ntroduction

he demand for oxygen sensors is growing, rcially for applications in the control of biotechbgical processes or for the solution of environtal problems. Concentration cells containing l2 as a solid electrolyte are widely used for rmination of the air-fuel ratio at high temperes in connection with a three-way catalyst in production of cars. The Clark cell offers a iibility for determining the oxygen concentraat low temperatures, but the liquid electrolyte mtains restricts its application. owever, solid-state devices convenient for suring an oxygen content at room temperature not available. A solution was suggested by lazoe et al. [ 1,2]. There a potentiometric ar:ement using the contact of both a LaF, single tal and sputtered LaF, with a platinum layer a reversible back-side contact consisting of ;nF, has been described. It can be used to sure the oxygen content in gases and solutions oom temperature. A fast response was ob:d by a treatment of the structure with water )ur. A strong influence of the preparation .meters of LaF, on the mechanism seems to It in different sensitivities, as pointed out by rOO5/92/%5.00

investigations in dry gas mixtures. A sensitivity of 30 mV/lgpo, is correlated with a two-electron reduction of oxygen described by reaction ( 1) [2], while in ref. 3 a sensitivity of 59 mV/lg p,, was found, corresponding to reaction (2) : 02+2eO,+e-

=20-

(l)

=o;

(2)

As the reasons for this have not yet been fully understood, a detailed description of the sensing mechanism is not possible and stable functioning for practical use is not guaranteed. MIS structures using the contact between LaF, and platinum, which have been suggested in refs. 4 and 5, do not show a Nernstian behaviour according to sensitivity and the reversibility of the potential shifts is limited. On the other hand, the stable functioning of the fluoride-sensitive Si/LaF, field-effect structure using the direct contact between the solid electrolyte and silicon was shown previously [6]. On the basis of this sensor element, a new oxygen-sensitive Si/LaF,/Pt capacitive structure was developed, and is presented here. It does not need a reversible back-side contact and can easily be produced by semiconductor technology. @ 1992

-

Elsevier

Sequoia.

All rights

reserved

192

2. Experimental A sensor element with the structure shown in Fig. 1 has been prepared. A (111) silicon wafer having a specific resistance of 10 to 12 n cm was etched by means of a 10% solution of HF in order to remove SiO, from the surface. After this, LaF, was thermally evaporated at a pressure below lop3 Pa at a rate of 0.3 rim/s.. In order to obtain a fixed area for capacitance measurements, a 1 pm thick SiOZ layer was grown on top of the polycrystalline LaF3 film by the CVD technique. This process was followed by etching of part of the oxide layer to the surface of LaF, using a 10% solution of HF. The active surface of LaF, had an area of 1.6 mm*. Platinum films were sputtered at a pressure of 7 Pa in an atmosphere of both argon and air. The platinum layer thickness has been determined by talystep measurements. Another MIS structure investigated consisted of a silicon substrate, a Si02 and a S&N4 layer prepared by CVD and a thin sputtered platinum film. CV measurements were carried out at a frequency of 10 kHz using the lock-in principle. In sensitivity measurements the shift of the CV curve on the voltage axis was determined or a capacity near the flat-band point was fixed by a controller. The latter technique provided the possibility to measure directly changes of the flat-band voltage with time. Humid mixtures of oxygen and nitrogen were used to change the oxygen partial pressure for

sensitivity measurements at room temperature. Different humidities were produced using various saturated solutions of inorganic salts. To obtain the dependence of the flat-band voltage on pressure, a high-vacuum system was used for successive evacuation. XPS was carried out with an ESCALAB 200 X at CRR = 40 using Al KCYradiation. The charging correction is related to EB(C 1s) = 285 eV.

3. Results and discussion Previously published results have shown that with a fluoride-sensitive capacitive Si/SiO,/Si,N,/ LaF,/electrolyte structure sensing characteristics like a Nernstian sensitivity, a good lower detection limit and good stability are obtained [7]. The electrical properties could be improved by direct evaporation of LaF3 on silicon [6], e.g., a steepness of the high-frequency CV curves that is ten times higher than that of a structure with an insulator involved could be determined. The Si/LaF, contact was used in this paper. The Si/LaF, interface can be prepared in a very reproducible way. Even for varying conditions of preparation, the flat-band voltage was shown to be within a range of f 100 mV. This was surprising, because for the well-known Si/Si02 system the flat-band potential shifts by several volts as a function of preparation conditions. The ion conductivity of LaF, makes sure that there is no potential drop within the bulk. There is no electron conductivity in the LaF, and hence no charge transfer at the Si/LaF, interface. Therefore the contact is not reversible but capacitive. Furthermore, the Si/LaF, interface is not a source of sensor drift, as demonstrated by a value of 0.07 mV/h for the fluoride ion sensor [6], which shows the stability of the back-side contact of LaF, .

ohmic contact Fig. 1. Schematic presentation oxygen-sensitive structure.

of top view and cross section

of the

Thus sputtering a platinum layer on top of the LaF, film resulted in a sensor without any need for a reference system on the back-side as for the sensor proposed by Yamazoe et al. [2]. Both d.c. and a.c. voltages are applied between platinum and the ohmic back-side contact on the silicon wafer. Hence a reference system on the sensing interface of LaF, is also not necessary. Our investigations proved the Si/LaF,/Pt structure to have a similar electrical behaviour to the

193

-5.6

VW1

Fig. 2. CV curves of the Si/LaF,/Pt Si/SiOJSi,N,/Pt (b) structures.

(b)

(a) (voltage

-3

axis on top) and

fluoride-sensitive structure, as the comparison between CVcurves of Si/LaF,/Pt and Si/Si02/Si3Nq/ Pt in Fig. 2 shows clearly. For the Si/LaF,/Pt structure the flat-band voltage was shown to be 1090 f 100 mV. The extraordinarily high steepness of the high-frequency CF/ characteristics of 300 to 500 nF/cm2 V offers the possibility to determine capacitance changes at a constant d.c. voltage with a high resolution instead of potential shifts, which are usually determined as a measure for changes in concentration. While a change in oxygen content from 10 to 100% causes a shift of flat-band potential of 59 mV, corresponding to 5% of the absolute value of the voltage, the same change in oxygen partial pressure will result in a shift of 47% in capacity, i.e., 24 nF/cm2. In our investigations we did not utilize this advantage of the sensor structure, i.e., we determined the potential continuously by fixing the capacity at a value near the flat-band point by means of a controller, because potential shifts can be correlated more easily to theoretical values of sensitivity resulting from the Nernst equation. The platinum layer on top of LaF, resulted in completely new functions concerning sensitivity compared with the fluoride sensor, as will be discussed in the following. The strong dependence of the sensor properties on the preparation parameters of the LaF, and platinum layers that has been mentioned above was confirmed by our investigations. In the course of an optimization process, the state of LaF3 in particular was influenced by means of varying the preparation parameters. Investigations by other authors showed that a treatment of the potentiometric Sn/SnF2/LaF,/Pt structure with water vapour improves the response time [2] and changes the sensitivity [3]. However,

in pretreating the whole structure it is impossible to decide whether changes in the platinum layer or the LaF, film occur. Therefore a treatment was carried out both before and after sputtering the platinum. Furthermore, another pretreatment was chosen, i.e., the layers were exposed to solutions with varying pH. Previously it was shown that a reversible exchange between fluoride ions in the LaF, layer and hydroxide ions in the solution occurs [8]. Therefore it could be expected that a treatment of LaF, with solutions having different pH values results in a change of concentration of oxygen species in the LaF, layer. Treatment with acid solutions both before and after Pt sputtering gave rise to an increase in response rate. Thus it can be pointed out that mainly the LaF, surface is modified by a pretreatment. Furthermore, a permeability of the platinum layer to electrolyte has to be provided in order to explain the effect at the sensor structure with sputtered platinum. In Fig. 3 the potential shifts obtained within 15 min after a change in oxygen partial pressure as a measure for response rate are presented as a function of temperature for a 1 M hydrochloric acid pretreatment of the LaF3 before platinum sputtering. The higher the temperature of pretreatment, the faster is the response function of the sensor. Another method for optimization of the LaF, surface was the modification of the evaporation parameters of LaF,. It could be proved that there is a strong influence of the substrate temperature during vacuum vapour deposition on response time. The temperature of the silicon substrate was varied between 300 and 600 K. While only a slow sensor response was found after preparation at

20

WCI

80

Fig. 3. Presentation of potential shifts within 15 min after a change in oxygen concentration from 10 to 100% as a function of the tempera_ . _ ture of the hydrochloric acid used for pretreatment of LaF,.

194 1200

1100

7 9 if] >

\

-3770

E i? > z

E iz > 3 59 mvng poz

950 0.5

b Po2@total

-2.5

Fig. 4. Oxygen sensitivity in humid gas mixtures (T = 298 K). 900

-4070 -2

400 to 600 K, a deposition at room temperature resulted in a fast response. The choice of convenient conditions of thermal vapour deposition gives rise to sensors with higher response rates than could be achieved by means of a treatment. A pretreatment of such LaF, layers did not cause a further improvement. The best sensor elements, which can be prepared with reproducible sensing properties, contain a 200 nm thick LaF, film thermally evaporated at a silicon substrate temperature of 300 K and a 40 nm thick platinum layer sputtered at a rate of 100 nm/min in an argon plasma. The voltage varies logarithmically with oxygen partial pressure. In humid gas mixtures reversible changes of flat-band potential corresponding to a Nernstian sensitivity of 59 mV/lg ps were observed (Fig. 4). A low 90% response time of about 90 s was obtained. Further improvement can be expected by optimization of the evaporation parameters of LaF,, like rate and pressure. Thermal vapour deposition seems to be advantageous compared to the sputtering technique used by Yamazoe et al. [2] because a further treatment of the LaF, surface is not necessary to achieve a fast response. As a look at the different sensitivities obtained with the proposed capacitive structure and by other authors shows, not only the quality but also the mechanism depend on the preparation. In order to test the efficiency of the sensor element, potential shifts corresponding to changes in pressure within a range of eight orders of magnitude were determined. The sensor can in principle be used to measure oxygen partial pressures from 10F3 to 10’ Pa. A detection limit has not

lg P

5.5

Fig. 5. Influence of changes in pressure on flat-band poteniial of Si/LaF,/F’t (a) and Si/SiO,/Si,N_,/Pt (b) for pressures ranging from IO-* to lOsPa.

been reached. The results are presented for the range lo-’ to 10’ Pa in Fig. 5 (curve a). Because of the high response time in vacuum, equilibrium values of the potential were not obtained within the time the sensors were exposed to a certain pressure (30 min). Therefore potential shifts could not be related with a Nernstian sensitivity and a hysteresis in the sensor response was observed when the pressure was increased after evacuation. It was shown that the oxygen-sensitive structure works in gas mixtures as well as in solutions. In water a sensitivity corresponding to a two-electron process was found. Influence of platinum layer characteristics on sensor function

As already mentioned, the preparation parameters have a strong influence on the sensor characteristics and sensing mechanism. In order to get information about the latter, a few of these parameter changes will be discussed below. Platinum layers have been sputtered using both an argon plasma and an air plasma. The latter resulted in a rather slow response of the flat-band voltage to changes in oxygen partial pressure. This can be attributed to a modification of the LaF, surface by highly reactive oxygen in the air plasma. So the composition of LaF,, especially as regards the oxygen content of the surface, is probably changed. The platinum layer thickness has also been varied. While the films sputtered in argon showed

195

Comparison between Si/LaF,lPt and Si/SiO,/Si,N,lPt structures There are no experimental results concerning the function of LaF, in the oxygen-sensitive LaF,/Pt structure, i.e., it has not yet been investigated whether LaF, or only the catalytic metal film is involved in the potential-determining process. Platinum and palladium MIS structures also were reported on, mainly regarding their hydrogen sensitivity, which was explained as being caused by a dipole layer at the metal/insulator interface, so that the hydrogen-sensing properties should be independent of the insulator used [lo]. However, only indirect oxygen sensitivities in the presence of hydrogen were determined

15

z 3 L

0 0

1[ml

Fig. 6. Dependence of the square root platinum layer thickness.

800 of 60% response

time on

a systematic dependence of response time on thickness (Fig. 6), as for air-sputtered films no influence of Pt layer thickness was found. Therefore in the case of sputtering in air, the response rate seems to be determined by a process at the LaF,/Pt interface. On the other hand, the response kinetics are obviously determined by a diffusion process of oxygen species through the platinum layer using argon as the medium for sputtering. Crank [9] proposed a model for sorption and desorption by a membrane, which can be simplified to W -=1--$exp (3) [ (%>I K where Mt is the total amount of oxygen that has entered the platinum layer at time t, M, is the corresponding quantity after infinite time, D is the diffusion coefficient, 1 the Pt layer thickness and t the time. At 60% response time M,/M, is constant. According to eqn. (3), Fig. 6 shows that there is a linear correlation between the square root of the 60% response time and the platinum film thickness. Because extrapolation to I = 0 nm results in a value for the response time %O, a description of the response mechanism as a mere diffusion process in the range of thickness investigated is not possible. A complex mechanism caused by the superposition of kinetics of the LaFJPt interface and diffusion kinetics has to, be considered. That is why an exact calculation of the diffusion coefficient is not possible. Its value has been estimated to be about 5 x lo-” crn’/s.

[Ill. Therefore we compared the Si/LaF,/Pt and Sil SiOz/Si3N4/Pt structures. CV measurements carried out on exposing the devices to different N2jH2 mixtures resulted, as expected, in similar potential shifts of 26 to 28 mV/lgp,, for both sensor elements. As regards the oxygen sensitivity, the behaviour of the two structures differed considerably. Si/SiO,/Si,N,/Pt was proved to be insensitive to oxygen, while Si/LaF,/Pt showed the sensing characteristics mentioned above. Thus it can be concluded that not only the catalytic metal film but also the specific properties of LaF, are responsible for the formation of a reversible potential in OJNz mixtures. LaF, cannot easily be replaced by another insulating or ion-conducting material. Si/LaF3/Pt and Si/Si0JSi3N,/Pt were also compared with respect to their pressure sensitivities. Curves (a) and (b) in Fig. 5 show that changes in flat-band potential of both structures have opposite signs. This fact confirms that there are two completely different mechanisms of formation of potential in the absence of hydrogen. Potential shifts with decreasing pressure of the Si,N,-containing structure cannot be interpreted as hydrogen sensitivity, because the concentration of hydrogen to be expected in air is lower than the detection limit pointed out by Lundstrom et al. for platinum MOS structures [lo]. Therefore the response of this structure to changes in humidity in a nitrogen carrier-gas stream was investigated within a range 32 to 100%. Potential shifts of 19.5 mV/lg (relative humidity), which correlate to the changes in flat-band voltage with decreasing pressure, were obtained.

196

_

La3d5

844.6

834.6 Binding Energy / eV

Fig. 7. La 3d,,, XPS spectra of an LaF, single crystal (I) and two evaporated layers of LaF,, one of them not treated (3) and the other pretreated with hydrochloric acid (2).

Influence of oxygen in the LaF3 surface As mentioned above, the treatment of LaF3 with acids has a strong influence on the sensor function. It is expected to change the oxygen content in LaF, .

In order to demonstrate the decrease in the amount of oxygen in the LaF, lattice by the interaction with an acid, XPS measurements were carried out. In Fig. 7 La 3d,,, XPS spectra of an LaF, single crystal (curve 1) and of polycrystalline LaF, layers treated with hydrochloric acid (curve 2) and untreated (curve 3) are presented. A comparison between the spectra of the single crystal and the polycrystalline layer, which has not been treated after evaporation, shows that the peak of the latter has a shoulder towards lower binding energies, which is caused by oxygen bound to lanthanum in the lattice of LaF,. On treatment with acid, the amount of lanthanum influenced by oxygen decreased (curve 2). Thus another proof for the influence of oxygen in LaF, on the response time has been obtained. However, an optimum concentration of oxygen in the LaF, lattice has not yet been found. Investigations to find out whether there is an optimum oxygen content in LaF, are in progress. This should be very important for an explanation of the sensor mechanism as well as for further optimization of the sensor response time.

References 1 N. Yamazoe, J. Hisamoto, N. Miura and S. Kuwata, Solid state oxygen sensor operative at room temperature, Proc. 2nd Int. Meet. Chemical Sensors, Bordeaux, France, July 7-10, 1986.

and N. Yamazoe, Solid state oxygen 2 N. Miura, J. Hisamoto sensor using sputtered LaF, film, Sensors and Actuators, 16 ( 1989) 301-310. 3 S. Harke, H.-D. Wiemhofer and W. Giipel, Investigations of electrodes for oxygen sensors based on lanthanum trifluoride as solid electolyte, Sensors and Actuators, BI (1990) 188- 194. 4 T. Katsube, M. Hara, 1. Serizawa, N. Ishibashi, N. Adachi, N. Miura and N. Yamazoe, MOS-type micro-oxygen sensor using LaF, workable at room temperature, Jpn. J. Appl. Phys., 29 (1990) L1392-L1395. 5 T. Ito, H. Inagaki, M. Takeuchi and I. Igarashi, FET type oxygen Sensors operative at low temperatures using LaF,, Proc. 3rd Int. Meet. Chemical Sensors, Cleveland, OH, USA, Sept. 24-26, I9W, p. 47. J. Szeponik and W. Moritz, A new structure for chemical sensor devices, Sensors and Actuators B, 2 (1990) 243-246. W. Moritz, I. Meierhofer and L: Miiller, Fluoride-sensitive membrane for ISFETs, Sensors and Actuators, 15 ( 1988) 21 l-219. W. Moritz and L. Miiller, Mechanistic study of fluoride ion sensors, Aualyst, 116 (1991) 589-593. J. Crank, The Mathematics of DifSusion, Clarendon Press, Oxford, 1956. I. Lundstrom, A. Spetz, F. Winquist, U. Ackelid and H. Sundgren, Catalytic metals and field-effect device-a usefu1 combination, Sensors and Actuators, BI (1990) 15-20. 11 M. Armgarth, D. Siiderberg and I. Lundstrom, Palladium and platinum gate metal-oxide-semiconductor capacitors in hydrogen and oxygen mixtures, Appl. Phys. Left., 41 (1982) 6544655.

Biographies

Stefi Krause received a diploma in chemistry at Humboldt University in 1991. She has been working at the Institute for Physical and Theoretical Chemistry and is currently engaged in the development of electrochemical semiconductor sensors. Werner Moritz received diploma, Dr. rer. nat. and Dr. SC.nat. degrees in chemistry in 1977, 1981 and 1989, respectively, from Humboldt University, Berlin. From 1977 to 1981 he was an assistant in the Physical Chemistry Department, working in the field of electrochemistry and radiochemistry. From 1981 to 1984 he was engaged in chemical sensor research in industry. Since 1985 he has been with the Department of Physical and Theoretical Chemistry of Humboldt University. His research interests include the electrochemistry of chemical sensor devices and the development of new sensor materials. Ingolf Grohmann received a diploma and a Dr. rer. nat. in chemistry at Humboldt University in 1977 and 1982. Since 1982 he has been working in the field of surface analysis of catalysts, zeolites and thin films at the Central Institute of Physical Chemistry in Berlin.