Micromachined metal oxide gas sensors: opportunities to improve sensor performance
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
References (96)
- et al.
Multicomponent analysis: an analytical chemistry approach applied to modified SnO2 sensors
Sens. Actuators B
(1990) - et al.
Thermal analysis and design of a micro-hotplate for integrated gas-sensor applications
Sens. Actuators A
(1996) - et al.
A micromachined solid state integrated gas sensor for the detection of aromatic hydrocarbons
Sens. Actuators B
(1997) - et al.
Multi-electrode substrate for selectivity enhancement in air monitoring
Sens. Actuators B
(1997) - et al.
Integrated array sensor for detecting organic solvents
Sens. Actuators B
(1995) - et al.
Low-power micro gas sensor
Sens. Actuators B
(1996) - et al.
A low-power CMOS compatible integrated gas sensor using maskless tin oxide sputtering
Sens. Actuators B
(1998) - et al.
Tin oxide microsensor for LPG monitoring
Sens. Actuators B
(1994) - et al.
An integrated gas sensor on silicon substrate with sensitive SnOx layer
Sens. Actuators B
(1990) - et al.
Photo-assisted silicon micromachining: opportunities for chemical sensing
Sens. Actuators B
(1996)
A heated membrane for a capacitive gas sensor
Sens. Actuators A
Diode-based microfabricated hot-plate sensor
Sens. Actuators A
Material and design considerations for lowpower microheater modules for gas-sensor applications
Sens. Actuators B
Optimization of an integrated Sn02 gas sensor using a FEM simulator
Sens. Actuators A
Si-planar-pellistor: design for temperature modulated operation
Sens. Actuators B
Thin-film gas sensor implemented on a low-power consumption micromachined silicon structure
Sens. Actuators B
Realization and performance of thin SiO2/SiNx membrane for microheater applications
Sens. Actuators A
SnO2 sensors: current status and future prospects
Sens. Actuators B
Semiconducting oxides as gas-sensitive resistors
Sens. Actuators B
Recent developments in semiconducting thin-film gas sensors
Sens. Actuators B
A.c. measurements on tin oxide sensors to improve selectivities and sensitivities
Sens. Actuators B
Effect of electrode material on sensor response
Sens. Actuators B
Diode-like SnO2 gas detection devices
Sens. Actuators B
Highly sensitive NO2 sensor device featuring a JFET-like transducer mechanism
Sens. Actuators B
Thin-film SnO2 sensor arrays controlled by variation of contact potential a suitable tool for chemometric gas mixture analysis in the TLV range
Sens. Actuators B
Contact and sheet resistances of SnO2 thin films from transmission-line-measurements
Sens. Actuators B
Field-effect-induced gas sensitivity changes in metal oxides
Sens. Actuators B
Carbon monoxide gas detector with shortened detecting cycle
Sens. Actuators B
Micromachined thin film SnO2 gas sensors in temperature pulsed operation mode
Sens. Actuators B
Algorithms to improve the selectivity of thermally-cycled tin oxide gas sensors
Sens. Actuators
Temperature modulation in semiconductor gas sensing
Sens. Actuators B
Optimization of temperature programmed sensing for gas identification using micro-hotplate sensors
Sens. Actuators B
Optimized temperature-pulse sequences for the enhancement of chemically specific response patterns from micro-hotplate gas sensors
Sens. Actuators B
Gas identification by modulating temperatures of SnO2-based thick film sensors
Sens. Actuators B
Detection of a sample gas in the presence of an interferant gas based on a nonlinear dynamic response
Sens. Actuators B
Strategies to avoid VOC cross-sensitivity of SnO2-based CO sensors
Sens. Actuators B
Use of the Seebeck effect for sensing flammable gas and vapours
Sens. Actuators B
Combined Seebeck and resistive SnO2 gas sensors, a new selective device
Sens. Actuators B
Tin dioxide gas sensors: use of the Seebeck effect
Sens. Actuators
A technique for supressing ethanol interference employing Seebeck effect devices with carrier concentration modulation
Sens. Actuators B
The semistor: a new concept in selective methane detection
Sens. Actuators B
A low power integrated catalytic gas sensor
Sens. Actuators B
Surface properties of germanium
Bell. Syst. Tech. J.
A new detector for gaseous components using semiconductive thin films
Anal. Chem.
Cited by (617)
Hydrogen sensor with a thick catalyst layer anchored on soda-lime glass
2024, International Journal of Hydrogen EnergyMetal oxide -based electrical/electrochemical sensors for health monitoring systems
2024, TrAC - Trends in Analytical ChemistryStructural and impedance spectroscopy characterization of Stannic oxide electronic system
2023, Materials Today: ProceedingsDecoration of laser-ablated ZnO nanoparticles over sputtered deposited SnO<inf>2</inf> thin film based formaldehyde sensor
2022, Sensors and Actuators B: Chemical