Micromachined metal oxide gas sensors: opportunities to improve sensor performance

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Abstract

This review deals with gas sensors combining a metal oxide based sensing layer and a substrate realized by using micromachining. It starts by giving an overview of the design principles and technology involved in the fabrication of micromachined substrates examining thermal and mechanical aspects. Both kinds of micromachined substrates, closed-membrane-type and the suspended-membrane-type, are discussed. The deposition of the sensing layer is complicated by the mechanical fragility of the micromachined substrates. Different approaches used for the formation of the sensing layer such as thin film and thick film deposition techniques are reviewed. Finally, the gas sensing function of the sensitive layer is analyzed and various ways for extracting the information are presented with respect to the improvement of sensor performance brought by this new approach.

Introduction

Gases are linked to life and their odors tremendously influence the ‘image’ of our environment. The human nose serves as a highly advanced sensing instrument which is able to differentiate between hundreds of smells but fails if absolute gas concentrations or odorless gases need to be detected. The demand for gas sensing devices which support the human nose is accordingly large. Support is desired in safety applications where combustible or toxic gases are present and in comfort applications, such as climate controls of buildings and vehicles where good air quality is required. Additionally, gas monitoring is needed in process control and laboratory analytics.

Only for process control and laboratory analytics one can afford high performance, large and expensive gas analyzers. For all other purposes one misses either the money or the place for gas analyzers. Therefore one needs cheap, small and user-friendly gas sensing devices. Accordingly, a lot of research and development is done to design small and cheap gas sensors which possess high sensitivity, selectivity and stability with respect to a given application. This search comes along with a large variety of sensors based on different sensing principles, e.g. semiconductor gas sensors, optical sensors, thermal conductivity sensors, mass sensitive devices like quartz microbalance sensors, catalytic sensors, dielectric sensors, electrochemical sensors and electrolyte sensors.

The focus of this paper is on semiconductor gas sensors based on metal oxides. Their advantages are good sensitivity to some relevant gases like CO, H2, NOx and hydrocarbons, simple signal processing, low production cost and small size. Metal oxide gas sensors are frequently used in gas leakage detection (propane, butane) and ambient air quality monitoring in traffic (CO, NOx). The latter application utilizes a metal oxide sensor to close the air intake in the presence of high concentrations of noxious gases. New application fields are toxic gas detection, like CO, and smoke gas monitoring in houses and buildings.

Well known materials used in metal oxide gas sensors are SnO2, ZnO, TiO2 and WO3, but SnO2 is by far the most popular. All those materials are n-type semiconductors which have, under normal atmospheric conditions and typical working temperatures in 200–400°C range, an electron depleted surface. Electron depletion at the surface is due to adsorption of atmospheric oxygen as O2 or O species which tie up electronic carriers. The electron depleted surface is highly gas sensitive: reducing gases like CO or H2 react with the surface removing the chemisorbed oxygen such that the depletion region decreases, oxidizing gases like NO2 cause an increase of the depletion region. The microscopic changes in the depletion region strongly influence the electrical properties. Measurements of changes in the electric conductivity therefore often serve as sensor signal.

The idea of using semiconductors as gas sensitive devices leads back to 1952 when Brattain and Bardeen first reported gas sensitive effects on Germanium [1]. Later, Seiyama found gas sensing effect on metal oxides [2]. Taguchi finally brought semiconductor sensors based on metal oxides to an industrial product [3], [4], [5]. Taguchi-type sensors are still on the market, a typical example is given in Fig. 1, but most of the commercially available sensors are nowadays manufactured in screen printing technique on small and thin ceramic substrates [6], [7], [8], an example is given in Fig. 2. Screen-printing technique has the advantage that thick-films of metal oxide semiconductor sensors are deposited in batch processing thus leading to a small sensor to sensor distribution within production lots. This technology is nowadays well-established and high performance of screen-printed ceramic sensors is achieved in various field applications.

However, screen-printed ceramic gas sensors are, with respect to power consumption, mounting technology and selectivity still in need of improvement. The power consumption of screen-printed devices is typically in the range of 200 mW to about 1 W [6]. That is too much for applications which allow just the use of battery-driven elements. The mounting of the overall hot ceramic element is difficult. One has to find such designs like the one shown in Fig. 2 which ensure good thermal isolation between sensor element and housing as well as high mechanical stability. Good thermal isolation is thereby not only needed to minimize the overall power consumption but also to enable the integration of signal processing electronics in the same housing. Sufficient selectivity of metal oxide sensors can up to now only be achieved if the sensor is used in an application where the number of gases is limited such that cross-sensitivities can be neglected or if several sensors are put together to an array. In the latter case a lack of selectivity and therefore overlapping sensitivities of different sensors can be turned into an advantage [9]. Even though the use of arrays is very promising with respect to sensor selectivity one has to have in mind that the use of sensor arrays leads at the same time either to an increased size of the sensor element or to the use of several separate sensor elements and thus to an increased power consumption.

In the last years the above-mentioned difficulties lead to new developments in substrate technology and strong research in preparation of sensing materials and signal evaluation. The integration of gas sensitive metal oxide layers in standard microelectronic processing was achieved and lead together with the use of micromachining steps to micromachined metal oxide gas sensors like the ones shown in Fig. 3, Fig. 4.1 This technology is very promising to overcome the difficulties of screen-printed ceramic sensors due to the following facts. The sensitive layer of micromachined metal oxide gas sensors is deposited onto a thin dielectric membrane of low thermal conductivity which provides good thermal isolation between substrate and the gas-sensitive heated area on the membrane. In this way the power consumption can be kept very low (typical values obtained lie in the range between 30 and 150 mW [10], [11], [12]) and the substrate itself stays nearly at ambient temperature. The mounting of the sensor element becomes therefore much easier than for an overall hot ceramic sensor element, and control and signal-processing electronics can be integrated on the same substrate if desired. Using standard microelectronic steps to pattern electrode structures results in a further advantage. The minimal structure sizes get much smaller, a minimal width between electrodes lying in the μm range can be achieved [13]. The gas sensitive area can in this way be tremendously reduced and the use of interdigitated electrodes with a high length-to-width ration allows even the evaluation of sensing films with very high sheet resistivity. Sensor arrays which are often needed to overcome the bad selectivity of single sensor elements can be easily implemented in this technology. Beyond that, the small thermal mass of each micromachined element allows rapid thermal programming which can be used to study the kinetics of surface processes and to achieve kinetically controlled selectivity [14].

This paper reviews the research and developments established so far in the field of micromachined metal oxide gas sensors. This includes a description of frequently used technologies, a discussion of design parameters which influence the thermal and mechanical properties of such devices and finally a description of the actual gas sensing principles. All these aspects are interconnected. Fig. 5 visualizes the various interactions. The properties of the sensor element such as mechanical stability, power consumption and thermal response are mainly determined by the different parts of the sensor element. Details of these interactions are given in 2 Micromachining substrates for gas sensors, 3 Sensitive layer. Additionally the sensor parts have a strong influence on the actual gas sensing. The choice of sensing film is responsible for the receptor function of the sensor, i.e. the specific reaction between gas species and the film, and determines together with other sensor parts how such gas reactions are transduced into a measurable sensor signal. The gas sensing by itself is very complex. By combination of various transducer and operation modes one can create a large variety of sensor signals. Some possibilities and their effect on the properties of the sensor element are described in Section 4.

Before going into details it should be emphasized that not only the proper design of the micromachined gas sensor element but also the proper choice of packaging solutions and mounting technologies is absolutely necessary to obtain finally a gas sensor systems which can withstand the harsh environment of practical applications, e.g. shock, vibration, ambient temperature changes and so on. Even though this paper deals only with the micromachined gas sensor chip one should have in mind that this chip has to be designed such that it can be mounted on a substrate, electrically connected and packaged.

Section snippets

Micromachining substrates for gas sensors

To fabricate micromachined gas sensors, sensor substrate materials have to be chosen and functional elements have to be designed. This is complicated by the lack of reliable material constants for thin films. This section presents typical designs found in literature and describes some criteria for the design of substrates with low power consumption, well-controlled temperature distribution across the sensing layer, and high mechanical strength. Well-controlled temperature distributions over the

Formation of tin oxide layer

The deposition of the sensing layer is the most crucial part in the preparation of gas sensors. Normally the deposition is carried out as the last process step in the fabrication of a micromachined gas sensor. This way poisoning of standard equipment with tin oxide can be avoided and the gas sensing film can be protected from uncontrollable modifications during later process steps. Some possible deposition techniques are listed in Table 5. Whereas the various chemical and physical vapor

Gas sensing: receptor, transducer and operation modes

The actual gas sensing consists of three different parts: receptor, transducer and operation mode. These parts are presented in Fig. 19. For details one might refer to [45], [46], [47], [48], [49], [50], [51], [52]. In this context only a short description of microstructures is given to demonstrate the different transducer function of thin film versus thick film sensing layers. The former are frequently used for micromachined gas sensors whereas the later are applied to ceramic gas sensors and

Conclusions

A variety of possibilities to fabricate micromachined substrates compatible with thin as well thick film metal oxide layers were presented. The advantages compared with the well-established ceramic sensors are: reduction of power consumption, faster thermal time constants which allow faster temperature modulations, the possibility to integrate the sensitive layer with control and signal evaluation electronic on one chip and easy integration of sensor arrays.

Thin and thick film sensing layers

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