Portable or mobile devices originally introduced as mobile phones or electronic agendas become more and more ubiquitous. As the processing power of their internal processors grows and equally the bandwidth for communication with stationary processors, such portable devices take on more and more the role of multi-purpose tools available to consumers and specialist users alike.
It has been recognized that portable devices can benefit from the presence of sensors capable of providing a chemical analysis of materials brought into contact or the vicinity of the device. Whilst there are many possible applications for such sensors, it suffices to consider for example the analysis of air surrounding the portable device. Such an analysis can be useful for multiple purposes such as testing for hazardous gases, breath analysis for general medical purposes or driving fitness, and the like.
Known sensors for use as chemical sensor are metal oxide type sensors. In a metal oxide or MOX sensor a sensitive layer of a metal oxide is exposed to a fluid including the analyte. As the analyte is absorbed, the resistance across the layer changes. The change in resistance can be measured and converted into a concentration of the analyte in the fluid.
However integrating such a sensor within the narrow confines of a modern day portable device poses a significant technical challenge. Typically for such devices only a very limited volume is acceded to additional sensors outside the core functionality of the device such as wireless voice or data communication, display, speaker, processors and battery. This means that the real overall dimensions of the sensor, its associated circuitry for control and readout have to be within or close to the submillimeter range.
A sensor with these outer dimensions can only be manufactured, if the active structures, i.e. the size of the metal oxide film between electrodes, are reduced in length to below 50 microns or even less. However, in metal oxide sensors of this size the contact resistance caused by interface effects between the (metallic) contact electrodes and the metal oxide film contributes in ever larger proportion to the measurement, thus making it more difficult to measure actual changes in gas concentrations. The contact resistance may also be less stable over the life time of the sensors and, if not compensated for, increase the error in the measurement of the chemical sensor.
The four-terminal (4 T sensing), 4-wire sensing, or 4-point probes method is a known electrical impedance measuring technique that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal (2 T) sensing. 4 T sensing is used in some ohmmeters and impedance analyzers and in precision wiring configurations for strain gauges and resistance thermometers. 4-point probes are also used to measure the sheet resistance of thin films. The four-terminal method is sometimes replaced by a three-terminal method, where one of each pair of electrodes is combined.
In the field of chemical gas sensors the German patent DE 10147107 C1 describes a four electrode structure within a gas sensitive layer. The resistance change in the layer between the outer electrodes is compared to the potential difference between the inner electrodes to detect the presence of a mixture of different gases.
It can be seen as an object of the invention to improve the chemical sensors using metal oxide films contacted through metallic electrodes, particularly for very small devices.