WO2006064029A1

WO2006064029A1 - Thin film semiconductor gas sensor with improved selectivity

Info

Publication number: WO2006064029A1
Application number: PCT/EP2005/056798
Authority: WO
Inventors: Elisabetta Comini; Nicola Poli; Giorgio Sberveglieri
Original assignee: Consiglio Nazionale delle Richerche CNR; Istituto Nazionale per la Fisica della Materia INFM CNR
Current assignee: Consiglio Nazionale delle Richerche CNR; Istituto Nazionale per la Fisica della Materia INFM CNR
Priority date: 2004-12-17
Filing date: 2005-12-14
Publication date: 2006-06-22
Anticipated expiration: 2007-06-17
Also published as: ITTO20040883A1

Abstract

A thin film semiconductor gas sensor comprising an insulating substrate (2) provided with a thin film of semiconductor material (6) and with an associated heating element (4), capable of heating the semiconductor film to an operating temperature of the sensor, said sensor comprising a second insulating substrate (12) equipped with a respective heating element (14) and with a catalyst layer (10), arranged at a distance from the semiconductor film (6) facing the latter, in such a manner as to define between the catalyst layer (10) and the semiconductor film (6) a gap for the gas subjected to measurement.

Description

Thin film semiconductor gas sensor with improved selectivity

The present invention relates to a thin film semiconductor gas sensor device of the type comprising an insulating substrate on which is deposited a film of gas-sensitive semiconductor material, and with which is associated a heating element suitable for heating the sensitive film to the operating temperature thereof.

This type of sensor measures the variation in the resistance of the sensitive film brought about by the chemical and/or physical absorption of the gas to be measured which is present in the ambient atmosphere; in order to detect the gases present in the environment, such sensors must generally be maintained at temperatures of above 200°C.

Recently, sensors have been developed which are based on measuring the variation in the work function of the sensitive film as a function of the concentration of the gas present in the atmosphere. Such sensors comprise a sensitive film, the work function of which may be modified following adsorption of molecules of the gas to be detected onto the sensitive film. The interest in this type of sensor primarily arises from the fact that they are capable of detecting both chemically adsorbed species and physically adsorbed species, which are weakly bound, in an operating range extending from ambient temperature to slightly higher temperatures.

Although thin film semiconductor sensors which measure the variation in the resistance of the sensitive film are widely used as gas sensors, in the light of the low production cost thereof, they generally suffer from the problem of poor selectivity for the gas to be measured, which compromises the use thereof as a single device. One way of overcoming their poor selectivity is to use more than one sensor simultaneously and to post-process the acquired data by various methods.

Highly selective gas sensors which may be mentioned include "wet" electrolyte electrochemical cells, which are characterised by low energy consumption, since they operate at ambient temperature, and by high selectivity. However, these sensors are costly due to the sensitive element and they have a limited service life of around one year due to the presence of the liquid electrolyte.

They have a restricted market in practical applications which include industrial gas detection and professional detectors.

EP-A-O 853 762 and EP-A-O 857 966 describe gas detectors having a porous membrane in the sensor structure which is capable of filtering the gaseous species to be monitored from other species present in the atmosphere.

Catalytic gas detectors, as described for example in US 5 902 556, are also known. These detectors are used in continuous monitoring of atmospheres for detecting the presence of low molecular weight inflammable gases, such as for example methane, and operate by a principle which differs completely from that of the thin film sensors provided by the present invention.

Their principle of operation is based on the fact that low concentrations of hydrocarbon gases are oxidised in air at elevated temperature over an appropriate catalyst such as palladium, rhodium, platinum or iridium. The resultant exothermic reaction produces an increase in temperature which is detected by the sensor. There is accordingly a direct correlation between the level of the increase in temperature and the concentration of the combustible gas in the atmosphere subjected to measurement.

The primary object of the present invention is to overcome the poor selectivity of thin film semiconductor gas sensors by providing a novel type of sensor which is economic to produce and particularly convenient.

Said object is achieved thanks to a thin film sensor device having the features claimed in the following claims.

The features and advantages of the device according to the invention will emerge clearly from the following detailed description, which is provided with reference to the attached drawings, in which:

Fig. 1 is a schematic sectional diagram of the device;

Fig. 2 is a schematic diagram of an alternative embodiment of the sensor device according to the invention; and

Figs. 3 and 4 are graphs showing the response of a sensor device according to the invention, described in the following exemplary embodiment, under increasing concentrations of NO₂ and, respectively, CO with the catalyst activated and not activated.

With reference to the schematic diagram of Fig. 1, the sensor device comprises a thin film sensor element, of a type known per se, comprising an insulating substrate 2, a sensitive semiconductor layer 6, deposited on one face of the substrate, in electrical contact with interdigitated contacts 8 and a resistive heating element 4 applied onto the other face of the substrate 2. It is understood that the illustrated configuration is purely exemplary and that other arrangements should not be excluded, for example of the heating element, which could be provided on the same face of the substrate bearing the sensitive film.

In a position facing the sensitive film and at a distance therefrom is provided a thin layer of catalyst 10, deposited on an insulating substrate 12 which bears, on the opposite face thereof relative to the catalyst, a resistive heating element 14.

The device according to the invention is suitable for miniaturisation to form a complete sensor system and catalyst in a single "case", in such a manner as to have a catalyst directly facing the sensitive material, which catalyst may be activated and convert a specific gaseous species, or may not be activated and so have no influence on measurement.

The entire device may be produced by depositing the sensitive material and the catalyst using a conventional sputtering method. It is possible to use two untreated insulating substrates, preferably of aluminium, and the sensitive layer is deposited onto one of the substrates; this substrate is then provided with the resistive heating element 4 and with the interdigitated contacts 8 which enable measurement of the resistance of the sensitive element. The substrate 12 with the catalyst, on the other hand, is provided with the heating element 14 which enables thermal activation of the catalysis process.

Although reference is made in this context to measurement made by means which measure the resistance of the sensitive element, it is understood that the constructive principle of the invention is not limited to this type of measurement and that devices comprising means for measuring the variation in the work function of the sensitive film are also conceivable.

It will furthermore be understood that the constructive principle is not limited to specific choices for the material of the sensitive film and the catalyst. The sensitive film used may comprise any semiconductor material known for this function. The chemical nature of the sensitive film is naturally selected as a function of the gaseous species to be measured. Once the nature of the sensitive film and the gaseous species to be measured have been determined, the catalyst is preferably selected such that it is capable of catalysing the conversion of a gaseous species likely to have an influence on the response of the sensitive film into a gaseous species which does not interfere with the response of the sensitive film to the species to be measured.

The distance between the two substrates, or better between the catalyst layer 10 and the semiconductor film 6 may be variable, i.e. means permitting adjustment of said distance may be provided. Typically, said distance is of the order of some hundreds of microns or less.

Preferably, the dimensions of the substrate 12 on which the catalyst is deposited are greater than those of the substrate 2 on which the semiconductor film is deposited, in such a manner as to ensure that the catalyst is operational in the zone occupied by the sensitive film.

The schematic diagram of Fig. 2, in which elements corresponding those in Fig. 1 are denoted by the same reference numeral, relates to the production of a device according to the invention directly onto micromachined silicon. This solution uses two micromachined substrates, adhesively bonded to one another in such a manner as to maintain a fixed distance between the catalyst and the sensitive film. The bottom substrate is composed of sensitive film, contacts and heating element, while the top substrate is composed of the catalyst layer and the heating element.

In trials carried out in the context of the invention for the purpose of testing the effectiveness of the device, a device was used comprising tungsten oxide as the sensitive film, deposited on an aluminium substrate by means of magnetron sputtering, while molybdenum oxide was deposited as the catalyst material.

As is known, when maintained at an operating temperature of approx. 320°C, molybdenum oxide catalyses the conversion of NO₂ into NO. The device was accordingly tested on nitrogen dioxide with the catalyst both activated and not activated and with a gas such as CO which ought not to be influenced by the presence of the catalyst.

The tungsten oxide proves to be sensitive to NO₂, but insensitive to the gas converted into NO.

Fig. 3 shows the results of some tests carried out with this device, with an NO2 concentration in the ppb range, with the catalyst activated (maintained at 320°C) and not activated (maintained at ambient temperature) . It may be seen how the presence of the catalyst brings about an appreciable reduction in the response to the interfering gas.

Fig. 3, in contrast, shows the response of the tungsten oxide sensor, maintained at a temperature of 300⁰C, towards increasing concentrations of CO (in the ppm range) with the catalyst activated (320°C) or not activated (ambient temperature) .

It will noted how the response is only slightly reduced, but not substantially, by the presence of the thermally activated catalyst.

The device according to the invention thus makes it possible, by activating or deactivating the catalyst layer, to vary the sensor's response to the ambient atmosphere. This makes it possible to eliminate or reduce the influence of the interfering gas on sensor response and furthermore facilitates the identification of the gaseous species present in the atmosphere.

Claims

1. A thin film semiconductor gas sensor comprising an insulating substrate (2) provided with a thin film of semiconductor material (6) and with an associated heating element (4) , capable of heating the semiconductor film to an operating temperature of the sensor, characterised in that said sensor comprises a second insulating substrate (12) equipped with a respective heating element (14) and with a catalyst layer (10), arranged at a distance from the semiconductor film (6) facing the latter, in such a manner as to define between the catalyst layer (10) and the film semiconductor (6) a gap for the gas subjected to measurement.

2. A sensor device according to claim 1, characterised in that said catalyst (10) is capable of catalysing the conversion of a gaseous species likely to have an influence on the response of the semiconductor film (6) to the gas to be measured into a gaseous species which does not interfere with said response.

3. A sensor device according to claim 1 or 2, characterised in that the semiconductor film (6) is deposited on one face of the substrate (2) and the associated resistive element (4) is associated with the other face of the substrate (2) .

4. A sensor device according to any one of claims 1 to 3, characterised in that said catalyst layer (10) is deposited on one face of the respective substrate (12) and the associated resistive element (14) is applied onto the other face of the substrate (12) .

5. A sensor device according to any one of claims 1 to 4, characterised in that the catalyst layer (10) facing the semiconductor film (6) is of a greater area than the area of said semiconductor film (6) .

6. A sensor device according to any one of claims 1 to 5, characterised in that said semiconductor film (6) is an oxide capable of measuring a gas selected from among carbon monoxide, ethanol, methanol, benzene and isoprene.

7. A sensor device according to any one of claims 1 to 6, characterised in that said catalyst layer (10) is a catalyst intended to catalyse the conversion of NO₂ into NO.

8. A sensor device according to claim 7, characterised in that said catalyst is tungsten oxide.

9. A sensor device according to any one of the preceding claims, characterised in that it comprises adjustment means capable of varying the distance between said catalyst layer (10) and the semiconductor film (6) .