C. Imawan, H. Steffes, F. Solzbacher, E. Obermeier

Technical University of Berlin, Microsensor & Actuator Technology (MAT),
Sekr. TIB 3.1, Gustav-Meyer-Allee 25, D – 13355 Berlin, Germany
 
 
 
 

Keywords: Sputtering; MoO₃ multilayer; H₂ gas sensors

1. Introduction
Sputtering deposition is finding ever increasing applications in high technology solutions for preparing thin films [1,2]. It is generally accepted that the films prepared by the sputtering method show a wide range of microstructures and properties which are highly dependent on the deposition parameters. Establishing optimum parameters or a special technique for thin film preparations may permit films to be prepared with specific microstructures that are relevant to a particular application [3,4]. In the present paper we propose a new method for preparing sputtered MoO₃ multilayer thin films for the application in gas sensors. The conventionally sputtered MoO₃ thin films have a whisker-like structure with a large grain size [5,6]. Even though the structure exposes usually high sensitivity because of the high surface-to-volume ratio, it can also cause a bad long-time stability of the sensor.

For the preparation of the MoO₃ multilayer successive processes of sputtering and back-sputtering are developed. A major advantage of this technique is that a back-sputtering process at each interface during the multilayer deposition leads to interfacial diffusion of atoms but not leading to any substantial increase in crystal structure development. Therefore, the films show a dense structure and a smooth surface with reduced crystallite sizes. It is evident that the structure can improve the long-term stability of the sensors without reducing their gas sensitivity. 

The microstructure of the films was characterized using XRD and SEM. Gas sensing properties were measured using various test-gases. Effects of the deposition method on the correlated microstructures and sensing properties of the films will be discussed. 

 
Table 1. Summarized sputtering parameter of MoO₃ thin films.

Sample	Layer	Sputtering				Back-sputtering			Total thickness
		Power (W)	Thickness of each layer (nm)			Power (W)	Duration (min)		(nm)
A	Single-layer	350	-			-	-		450
B	Multilayer	350	60			250	3.5		450
C	Multilayer	350	80			750	3.5		450

In order to compare structural and gas-sensing properties between conventionally sputtered MoO₃ single-layer thin films and new MoO₃ multilayer thin films, both of the films were prepared and investigated. The deposition conditions of the preparation of the films are summarized in Table 1. Successive processes of sputtering and back-sputtering were used to prepare the MoO₃ multilayer films. A schematic structure of the multilayer thin films which were back-sputtered at each of their interfaces and an illustration of the behavior of the atoms during the deposition are shown in Fig. 1. Each thickness of the Mo-O layers was about 60 nm (sample B) and 80 nm (sample C), respectively. After the deposition of one layer of Mo-O was completed, Ar ions bombarded the surface of the layer during back-sputtering process. After a definite time (3.5 min.) the back-sputtering was stopped and than the deposition for further layers started. The resulting films were built up by ten layers. The back-sputtering power was 250 W or 750 W, respectively. The two powers (low and high) were chosen in order to study the effect of the back-sputtering power on the properties of the films.

The thickness of the deposited single-layer and multilayer films was about 450 nm as measured by a profilometer. For stabilizing the sensing properties, the films were annealed at 500°C in synthetic air flow 4 l/min. for 5 h.

Gas-sensing properties were investigated by measuring the DC electrical resistance of the films. A sensor chip made from SiO₂/Si wafer with Pt-IDC structures is used as a substrate for the films. The gas-sensing characterization was carried out in a computer-controlled gas measurement system as described previously [7]. To investigate the gas-response and cross-sensitivity of the sensors, they were tested under exposure of various gases such as NO₂, NH₃, H₂, CO, CH₄ and SO₂. The sensor response is specified as (R_o - R_g)/ R_g for reducing gases and (R_g – R_o)/ R_o for oxidizing gases, where R_o and R_g indicate the sensor resistances in dry synthetic air and in the presence of testing gases, respectively.

Microstructures of the films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). An X-ray diffractometer using a monochromatized Co-Ka (l = 1.7889 Å) radiation source, was employed to obtain diffraction patterns of the films. The surface morphology was investigated by using a SEM (Hitachi, S-2300) operating at 25 kV.

After the single-layer film had been annealed in synthetic air, the phase transformations from the b -MoO₃ and Mo₉O₂₆ to orthorhombic MoO₃ occurred, as seen in Fig. 2. top. It is clear when comparing the XRD pattern of the as-deposited film and the annealed film that during the annealing the intensity of the (110) preferred orientation was decreased. The crystallites undergo a reorientation, with increasing the intensity of the (010) peak and then this orientation is the preferred one.

The multilayer films (sample B and C) show a similar behavior. The XRD pattern indicate that the as-deposited multilayer film has a major phase of MoO₃ and also indicates the formation of b -MoO₃ (Fig. 3. bottom). The intensity of the peaks is very low and they show a strong broadening effect. This suggests that the film consists of very small crystallites and the particles are spreading to form a little amorphous contribution to the film on the substrate. The annealing process caused the growth of the crystal, as shown by the increasing intensity of the reflections (Fig. 3. top). The b -MoO₃ transformed to orthorhombic MoO₃ and the (010) preferential orientation is formed.

SEM micrographs of the annealed single layer and multilayer films are shown in Fig. 4. It is obvious that the deposition methods have a strong influence on the surface morphology of the MoO₃ films. The structure of the single-layer film can be characterized as whisker-like (Fig. 4.a). It forms grains which are arranged in almost independently clusters. The grain length is about the same for all grains, but the grain thickness is varied and widely distributed. The structure exposes a very high surface-to-volume ratio. 
 

The multilayer films show a smooth surface and dense morphology (Fig. 4.b and c). An interpenetrating microstructure was likely developed, as seen on the film B (deposited using a back - sputtering power of 250 W). For the film C (deposited using a back - sputtering power of 750 W), the microstructure consists of fine and several elongated grains with straight grain boundaries. It also shows a smoothening effect on the surface by the increased rf-power of the back-sputtering.
 
 

The dense structure and smoothening effect on the surface of the multilayer films is as a result of a back-sputtering process at each interface during the multilayer deposition. The effect is much more closely associated with the energy of the Ar-ions bombarding the surface of the layers. As seen from XRD pattern, the as-deposited multilayer films have very low peaks intensity and are likely to have amorphous-like structure. However, the bombardment with Ar ions at relatively higher energy during back-sputtering seems to promote interfacial diffusion of atoms. Therefore, the adatoms tend to form a more closely-packed construction through random-walk processes and finally, the periodic structures of the atoms are destroyed. The effect of ion bombardment on short-range disorder has been demonstrated previously [9].

3.2. Gas-sensing properties
Gas-sensing properties of the MoO₃ films were characterized at the temperature range of 200-400°C in dry synthetic air. The measurement results indicate that the sputtered MoO₃ thin films can detect well H₂ at an operating temperature of 300°C. The response towards various test-gases at this temperature from single and multilayer films is shown in Fig. 5. The general behavior of the sensors is that they have very low responses towards CO, CH₄, and SO₂. The MoO₃ single-layer exhibits a high response to H₂ and also a considerable response to NH₃ and NO₂. The MoO₃ multilayer film deposited using a back-sputtering power of 250 W (sample B) shows a decreased response towards H₂, NH₃, and NO₂. For the MoO₃ multilayer sputtered using a back-sputtering power of 750 W, the response to H₂ is still high, though the response to NO₂ is strongly reduced. From this results, it can be concluded that the best selectivity for H₂ detection is exhibited by the MoO₃ multilayer film sample C.
 
 

A comparison of the dynamic response to 1000 ppm H₂ at 300°C for the three different sensors is shown in Fig. 6. The corresponding data have been used for evaluating the response time of the sensors (see Fig. 7). The response time is defined as the time required for the electrical resistance of the sensors to reach 50% of the equilibrium value following a step increase in H₂ gas concentration. As seen in Fig. 6, the single-layer film (sample A) has the best signal shape during exposure to H₂. When looking at the result for the response time at Fig. 7, the film shows a slower response time than the multilayer films which have a very fast response time. This very fast response causes an abrupt decrease in resistance, followed by a gradual increase in resistance to reach the equilibrium value. The fastest response time is found for the multilayer film sample C.

The H₂ response of the sensors at 300°C is plotted as a function of concentration, in Fig. 8. It is obvious that the H₂ response is influenced by the microstructure of the films. The MoO₃ single-layer shows a saturated response to H₂ in a concentration of about 2000 ppm. A good linearity of the response is exposed by the MoO₃ multilayer sample B, but the H₂ response is low. The MoO₃ multilayer sample C shows a very high response to H₂ for concentrations ranging from 100 to 9000 ppm. The linearity of the curve is very good in the range of high concentrations between 2000 and 9000 ppm H₂. These results prove that the new deposition method for the preparation of MoO₃ multilayers can obviously improve the H₂ sensing properties compared to conventional MoO₃ single-layer thin films. 

A long-term unenergized storage of a sensor after the first characterization can influence its stability. Figure 9 shows the effect of the unenergized storage period of 95 days on the stability of the H₂ response. The single-layer with a very porous structure shows bad stability. After 95 days storage period from its first characterization, the H₂ response is reduced from 17 to 10.3. An enhanced stability is found for the multilayer films. A small increase of the H₂ response is shown after the storage period. The improvement of the stability may be caused by the dense structure of the multilayer film.

Studies of the reduction mechanism for MoO₃ and the surface chemistry of the material during pretreatment in hydrogen can explain why the MoO₃ has a good sensitivity to H₂. Choi et al. [10] have investigated the electronic structure of MoO₃ using XPS before and after hydrogen reduction. After the reduction by H₂ at 400°C, the surface concentration of M⁺⁶ decreased drastically. While 3 h in hydrogen treatment the Mo⁺⁶ concentration was reduced from 100% to 24%. The reduction of MoO₃ involves the reduction from Mo⁺⁶ to lower oxidation states Mo⁺⁵ and Mo⁺⁴. This result indicates that the surface of MoO₃ is very sensitive to the reduction by hydrogen.
 

Indeed, the gas adsoption is principally influenced by the microstructure of the film surface. The full width at half maximum (FWHM) of a peak of the XRD-reflection gives an indication of the mean crystallite size. The FWHM of the (040) reflection for the sample A, B, and C after annealing is 0.345, 0.404, and 0.433 deg. The correlated crystallite sizes of the films are 77, 54, and 47 nm, respectively. These results indicate that the single-layer film (sample A) has the largest crystallite size and the multilayer film deposited using a back-sputtering power of 750 W (sample C) has the smallest crystallites. A remarkable observation is that the crystallite size is much smaller than the principal physical structure size as seen on the SEM micrographs (Fig. 4). This indicates that each of the grains (column tops) seen by SEM may consist of subgrains [11].
 

Considering the mechanism of conduction by a porous body like a thin film needs to take account of the structural inhomogeneity, which implies a lower resistance path through the bulk of crystallites alternating with higher resistance constrictions at the point contacts. That means, the grain boundary contacts are most resistive for conducting electrons so that they govern electric resistance of the films as well as the gas sensitivity [12]. The effects of crystallite size on the sensitivity were also shown by Suzuki et al. [13]. Ogawa et al. [14] have proposed that the mobility modulation effect is strongly related to the crystallite size of the thin films and has an effect on the sensitivity of the film towards reduced gas. 

Based on the microstructure of the MoO₃ thin films which each of their grains consist of subgrains, we propose two models for conductance limited by intergrain constrictions as shown in Fig.10. In each of both the depletion layer represents a substantial fraction of the material comprising the intergrain contact. For the single-layer film (sample A), the depletion zones from the two surfaces are quasi-overlap, leaving a high resistance ohmic path across the grain boundary. Within the two subgrains the depletion zone shows a wide channel across the boundary and causes a large conductivity. So, the electric resistance of the single-layer film is dominated by the intergrain contact. On the other hand, for the multilayer film the subgrain boundary contacts are very resistant to electron conduction through the chain, so that they govern the electric resistance as well as the gas sensitivity of the film.
 

4. Conclusions
A new method for the preparation of sputtered MoO₃ multilayer thin films has been developed. The films show an orthorhombic structure of MoO₃ with a preferential (010) orientation after annealing. The single-layer film has a whisker-like structure. On the other hand, the multilayer films have a dense and smooth structure. This structure is as a result of a back-sputtering process at each interface during the multilayer deposition. 

Gas-sensing characteristics of the films prove that this new preparation method can improve the H₂ sensing properties such as sensitivity, selectivity, response time and long-term stability of the conventionally sputtered MoO₃ single-layer film. The cross-sensitivity towards NO₂, NH₃, CO, CH₄, and SO₂ is low. The response time (t₅₀) to H₂ can be drastically reduced from 40 s for the single-layer film to 10 s for the multilayer films. The sensor exposes a very high response to H₂ with a good signal linearity in the range of high concentrations between 2000 and 9000 ppm.
 

Acknowledgements
The authors wish to thank Dr. D. Mutschall for many useful ideas and Dr. P. Fricke at the Institute Fresenius, Dresden for the XRD measurements. Financial support through the German Ministry of Education, Research and Technology BMBF is gratefully acknowledged.
 

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Biographies
Cuk Imawan received his B.Sc. degree in physics from Gadjah Mada University in 1991 and his M.Sc. degree in Physics in 1995 from University of Indonesia. In 1991 he joined the department of physics of the University of Indonesia. He is now studying towards his Ph.D. in Electrical Engineering on a DAAD-scholarship at the Microsensor and Actuator Technology Center of the Technical University Berlin.

Harald Steffes received his Dipl.-Phys. degree (M.Sc. in physics) from the University of Kaiserslautern in1997. Since May of 1998 he has been a member of the Microsensor & Actuator Technology Center of the Technical University of Berlin. He is currently working towards his Ph.D. in the field of solid state gas sensors.

Florian Solzbacher studied in U.K., Germany and Utah, USA. He received his M.Sc. degree in Electrical Engineering from the Technical University Berlin in 1997. Since August 1997 he has been studying as a Ph.D. student at the Microsensor and Actuator Technology Center at the Technical University Berlin. His field of interest covers chemical sensors, biomedical engineering and III-V semiconductors.

Ernst Obermeier received his M.Sc. and Ph.D. degrees in electrical engineering from the Technical University of Munich in 1977 and 1983, respectively. He worked as a research scientist at the Fraunhofer Institute in Munich from 1977 to 1988. Since 1988 Professor Obermeier has been the head of the Microsensor and Actuator Technology Center at the Technical University Berlin. His research is focused on silicon technology, simulation and modeling of micromachined sensors and the development of chemical and high-temperature sensors.
 

This paper is accepted for publishing in Journal of Sensors and Actuators, January 2001

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