FR2754901A1 - Solid state chemical gas sensor with long term stability - Google Patents

Abstract

Production of a chemical gas sensor, having an electrical output, involves deposition, on a main face of a substrate (10) provided with a heating resistor (14), of measuring electrodes (19), which are insulated from the substrate if it is not insulating, and then, on the electrodes, a 10-1,000 nm thick semiconductive metal oxide thin film by inclined ion bombardment-assisted secondary vacuum deposition. Also claimed is a chemical gas sensor for detecting a gas in an atmosphere in contact with a metal oxide thin film (21) on a substrate bearing a resistance measuring electrode array (19), an insulating layer (18) and a resistor (14), the thin film (21) being a 10-1,000 nm thick layer of usually tin, titanium or tungsten oxide having a uniform chemical composition, a cassiterite-type crystallographic structure and a grain size of 5-80 nm. Further claimed is a method of using the above sensor, in which several gases are detected by carrying out measurements at different temperatures.

Description

CHEMICAL GAS SENSOR AND MANUFACTURING METHOD
OF SUCH A SENSOR
The present invention relates to chemical gas sensors with electrical gas outlet making it possible to detect the presence, in an atmosphere in contact with the sensors, of gases such as S02, CO, H2S, NH3, NOx, CH4 and in particular reducing gases, at concentrations which for certain gases may be as low as a few ppm, alone or as a mixture.

Such sensors are known of the type comprising a substrate on which a heating resistor is deposited and comprising a thin layer based on a semiconductor metal oxide deposited on the substrate, as well as electrodes for measuring the resistivity of the thin layer, also carried by the substrate. The oxide is generally
SnO2 or TiO2. However, WO3, ZnO, Fe203 can be used,
Ga203, ZrO2, etc ... for certain gases, even see mixtures of the oxides or superimposed layers of different oxides.

 The invention also relates to the methods of manufacturing sensors of the above type.

 The operation of such sensors is as follows. On the surface or at the grain boundaries of the metal oxide which constitutes the thin layer (depending on whether this thin layer has a monocrystalline or polycrystalline structure) reactions of chemisorption, oxidation-reduction and / or displacement of electrons.

 In the frequent case of the detection of a reducing gas present in an atmosphere containing oxygen, such as clean air, by SnO2 the oxygen, which traps the electrons by its electronic affinity, is adsorbed on the surface of particles of SnO2 and forms a potential barrier at the grain boundaries. This slows down the flow of electrons (which easily flows through the grain boundaries in the absence of o2), which increases the electrical resistance.

If there are reducing gases, they are adsorbed on the SnO2 surface and then oxidized, which consumes oxygen and lowers the potential barrier, allowing the electrons to circulate more easily and thus reducing the electrical resistivity. This decrease is measured using electrodes deposited on the thin layer.

 A chemisorption cycle therefore takes place which can be summarized as follows: there is extraction of electrons from the conduction band by adsorption of oxygen on an available site; then adsorption of the reducing gas on the site which has adsorbed oxygen with redox if the layer is at a sufficient temperature; and finally re-injection of electrons into the conduction band by desorption of the oxidized reducing gas, which releases an adsorption site.

 Chemical sensors using a thin layer of semiconductor oxide have significant advantages, including with respect to electrical sensors having a thick layer of semiconductor oxide. A thick layer, formed by screen printing using an ink containing an organic phase, has a limited content of active material and is very sensitive to certain contaminants. A thin film sensor has better sensitivity, a shorter response time and lower consumption.

 Unfortunately, a thin film sensor manufactured by one of the current methods at present comprises a layer whose stochiometric, and therefore electrical, characteristics are not perfectly homogeneous. The invention aims in particular to provide a thin-film sensor having improved characteristics and in particular characteristics improving the stability, reproducibility and long-term durability of the sensor.

 To this end, the invention proposes in particular a method of manufacturing a chemical gas sensor with an electrical output, according to which electrodes are deposited on a large surface of a substrate provided with a resistant heating track on one side. measurement (isolated from the substrate if the substrate is not insulating), then, on the electrodes, a thin layer of semiconductor metal oxide 10 nm to 1000 nm thick, by deposition under secondary vacuum assisted by a ion bombardment under oblique incidence. In general, we will not exceed a few hundred nm.

 The ionic assistance makes it possible to fix the stochiometric composition of the deposit and therefore its electrical characteristics and to give the layer a stable structure almost identical to that of solid oxide; thanks to the cleaning of the substrates by ion bombardment, the substrate is pickled, which makes it possible to obtain a high purity of the deposit. In addition, the deposit obtained is very dense because the ion bombardment causes a rearrangement of the oxide grains. The grain size can be adjusted by adjusting the deposition conditions and prior heat treatment.

 By controlling the pressure and the composition of the atmosphere of the enclosure in which the physical deposition is carried out by evaporation under vacuum, known as PVD, the ionic flow can be adjusted in order to adjust the stochiometric composition of the thin layer and its structure. Before starting the vacuum deposition operation, it is possible to carry out an ion bombardment of pickling of the substrate.

 The subject of the invention is also a sensor of a kind capable of being obtained by the process defined above, comprising a flat substrate, one large face of which successively carries a resistant heating track terminated by connection terminals, an insulating layer (generally made of SiO2 or Si3N4), metallic measuring electrodes deposited and finally a thin layer, 10 to 1000 nm thick, based on metal oxide (tin, titanium or tungsten), of regular chemical composition, having a crystallographic structure of the pure cassiterite type, with grains of size between 5 and 80 nm.

 Such a sensor has great stability over time and high reproducibility. As the thin layer has a low thermal inertia, the sensor lends itself well to the detection of several gases by carrying out several successive measurements at different temperatures. It is indeed possible to quickly switch the sensor from one temperature to another and to take advantage of the fact that the oxide of the thin layer generally exhibits a maximum marked sensitivity to the presence of the gases to be detected which corresponds to a different temperature for different gases.

 The sensor can include low-grade additives (up to 10%), which increase stability or sensitivity and selectivity for particular gases.

Among the possible additives, mention may especially be made of platinum, palladium, aluminum, silver. These additives can be in a surface layer or dispersed in the sensitive layer.

 The substrate can have various natures. I1 can be made of an insulating material such as silica or alumina. It can also be constituted by an intrinsic semiconductor, such as silicon, the heating resistance of which is then separated by a surface layer of silicon oxide.

 It is advantageous, in order to carry out batch manufacturing, to use silicon substrates, for which there are photogravure and deposition methods making it possible to produce several tens of sensors simultaneously on the same wafer.

The above characteristics as well as others will appear better on reading the following description of a particular embodiment, given by way of nonlimiting example. The description refers to the accompanying drawings, in which
- Figure 1 is a block diagram of a sensor, shown in section, and a block diagram of electronics that can be associated with the sensor
- Figures 2A to 2E show the successive stages of a possible method of manufacturing a sensor
- Figure 3 is a block diagram showing a possible constitution of layer deposition apparatus for simultaneously making several sensors.

 The sensor, the principle of which is shown in FIG. 1, comprises a silicon substrate 10, one face of which is oxidized to form an insulating film 12. This film carries a heating resistor 14 terminated by supply pads 16 of which only one is visible in FIG. 1. The resistor 14, generally consisting of a thin track folded back on itself, which can in particular be made of titanium or tungsten, is covered with an insulating layer of silica 18, which can be produced by a conventional technique of deposit.

 The silica insulating layer carries the electrodes 19 for measuring resistivity, which can be constituted by two sets of interdigitated electrodes. Each set is connected to an electrical output contact 20, only one of which is shown in FIG. 1. Finally, the sensitive layer 21 is deposited on the electrodes.

When using the sensor, the pads 16 are connected to a heating source 22 and the electrodes are connected to a bridge for measuring resistance or leakage currents. A calculating member 26 may be provided to provide an indication of the presence of gas to be detected on an outlet
S. This calculation unit can also control the heating means 22 so as to produce temperature steps corresponding to a sensitivity to different gases.

 The sensor can be provided with a cover 28 made of porous material, intended to protect the sensitive layer 21 against external aggressions and to stop undesirable products such as hydrocarbons.

 In alternative embodiments, the electrodes on the sensitive layer, and / or the heating element is on the back of the substrate.

 The manufacturing of the sensor can be carried out by the sequence of operations shown in FIG. 2A to 2E.

 We start with a silicon substrate 10 which may belong to a wafer in the case of batch production. The silicon is oxidized on the surface to form the oxide film 12, intended to electrically isolate the resistance of the substrate and also to create a thermal insulation reducing the thermal inertia of the sensor. The heating resistor 14 is then formed by depositing a thin layer and photoengraving. In practice, it is often necessary to bring this resistance to a temperature exceeding 3500C and, for this reason, a TiW / Pt resistance will be used. The layer of electrical insulator 18 is then formed, by a conventional technique (FIG. 2B).

 Holes are opened in the insulator to the right of the ends of the resistor 14 (Figure 2C). The areas 16 of connection with the power source 22 are then formed, for example in the form of a gold deposit (FIG. 2B).

Then the two interdigitated sets of electrodes are formed, by deposition and photoengraving, for example in TiW with platinum coating (FIG. 2B). Finally, the sensitive layer 21 of tin oxide is deposited, leaving the output contacts of the electrodes uncovered. Detection is then carried out by measuring the inter-electrode resistance, for example by indirect measurement of the leakage current between the fingers of the two sets.

 The sensitive layer 21 can be deposited in an apparatus of the type shown in FIG. 3. It comprises an enclosure 30 maintained under secondary vacuum and containing a tin oxide vaporization capsule 32. The capsule comprises an electron gun 34 and means for creating an electro-magnetic or electric field bringing the electrons to the tin oxide. The blanks of the kind shown in FIGS. 2E are distributed at the upper part of the enclosure, on a support 36 and under heating resistors 38. The support 36 can be rotated to ensure the uniformity of the deposit.

The pressure in the chamber is kept at around 10-9
Pa, by means not shown, in spite of the ion supply which is produced by an ion gun 40. Conventionally, the ion gun can comprise a housing closed by a screen grid 42 placed behind a grid d acceleration 44 brought to a high negative potential. A photo-emissive cathode 46 is placed inside a tubular anode 48.

 The ion gun is oriented so that the substrates receive, at least during part of the deposition operation, the ions at an oblique angle, which provides better deposition characteristics.

Claims (9)

 1. A method of manufacturing a chemical gas sensor with an electrical outlet according to which measuring electrodes (19 are deposited on a large surface of a substrate (10) provided with a resistant heating track (14) on one side ), isolated from the substrate if the substrate is not insulating, then, on the electrodes, a thin layer of semiconductor metal oxide 10 to 1000 nm thick, by deposition under secondary vacuum assisted by ion bombardment under incidence oblique.  2. Method according to claim 1, characterized in that precedes the deposition of a step of etching the substrate by ion bombardment.  3. Method according to claim 1 or 3, characterized in that the substrate is rotated during the deposition of metal oxide.  4. Method according to any one of claims 1 to 3, characterized in that, the substrate being made of silicon, a batch of sensors is simultaneously produced on the same wafer by surface oxidation, production of a heating resistance by deposition and photoengraving , deposition of silicon oxide, formation of feedthroughs, manufacture of interdigitated electrodes (19) by deposition and photoengraving, and deposition of the thin layer (21).  5. Chemical sensor of gas present in an atmosphere surrounding the sensor, with electrical output, comprising a thin layer of metal oxide (21), one side of which is intended to be in contact with the atmosphere and the other side of which is carried by a large face of a substrate via an array of resistivity measuring electrodes (19), an insulating layer (18) and an electrical resistance (14), said thin layer having 10 to 1000 nm thick and consisting of metal oxide, generally tin, titanium or tungsten, of regular chemical composition, having a crystallographic structure of the cassiterite type, with grains of size between 5 and 80 nm .  6. Sensor according to claim 5, characterized in that the tin, titanium or tungsten oxide contains an additive for increasing selectivity.  7. Sensor according to claim 5 or 6, characterized in that the substrate is made of silica, alumina or an intrinsic semiconductor, the heating resistance of which is then separated by a surface layer of silicon oxide.  8. Sensor according to claim 5, 6 or 7, characterized in that it comprises a housing (28) forming a filter for retaining impurities, protecting the thin layer.  9. Method of using a sensor according to any one of claims 5 to 8, characterized in that several gases are detected by carrying out several successive measurements at different temperatures.