WO1996031773A1 - High-temperature chemical sensor - Google Patents

Abstract

A chemical sensor comprising a combination of a sensitive layer (C) and an acoustic wave transducer that is stable at very high temperatures. The acoustic wave transducer includes a piezoelectric GaPO4 crystal, and the sensitive layer (C) may be a semiconductor oxide layer such as an SnO2, TiO2 or ZrO2 layer sensitive to gas molecules such as CO and NOX. Said sensor is useful in motor vehicles for sensing exhaust gases.

Description

 HIGH TEMPERATURE CHEMICAL SENSOR

The field of the invention is that of chemical sensors resistant to high temperatures (of the order of 500 to 800 ° C. approximately). Such sensors are of growing interest in several fields of application and in particular in the automotive sector in which it is sought to control directly at the engine level the emissions of gases such as CO, NO _x and various hydrocarbons to limit the rate of pollution. . Currently, the main gas sensor, marketed, for adjusting the carburetion is based on the determination of the air / fuel ratio. It operates at 800 ° C from a solid electrolyte of the type (Zrθ2, Y2O3), the determination of the current measured during electrochemical reactions makes it possible to follow the evolution of the air / fuel ratio. In addition to this type of oxygen sensor, there are studies on the detection of other gases such as NO _x or CO. These are often resistive sensors based on semiconductor oxide of Sneurs2 type, capable of working around 400 ° C. Some sensors using Tiθ2 or Snθ2 doped with indium are able to operate at temperatures close to 500 ° C. However, the major persistent problem of such sensors is their drift over time accompanied by a difficulty in reproducibility.

In this context, the invention proposes a new type of sensor which is particularly well suited to the detection of gases at high temperature. It is a sensor equipped with a sensitive layer and an acoustic transducer comprising a piezoelectric material resistant to high temperature.

More specifically, the subject of the invention is a chemical sensor comprising a sensitive layer and an acoustic wave transducer characterized in that the transducer comprises a piezoelectric crystal of GaPO

The association of a layer sensitive to the gas which it is sought to detect and of an acoustic wave transducer makes it possible to develop a particularly efficient sensor insofar as a variation of the mechanical (mass), elastic or electrical properties ( conductivity), even low, necessarily induces a variation in the propagation of acoustic waves and therefore information related to the presence of specific molecules in the environment. The choice of GaPθ4 piezoelectric material makes it possible to develop chemical sensors resistant to very high temperatures. Indeed, current sensors comprising an acoustic wave transducer are based on the use of crystals such as quartz which has a phase transition towards 560 ° C, thereby prohibiting any application as piezoelectric at higher temperatures. While the gallium phosphate crystal GaPÛ while having qualities comparable to those of quartz, does not undergo phase transition at high temperature.

Preferably, the chemical sensor according to the invention comprises a sensitive layer resistant to temperatures above about 500 ° C. and adapted to the detection of molecules of the NO _x or CO type.

This sensitive layer can advantageously be produced from a simple semiconductor oxide such as Snθ2, Tiθ2, Zrθ2, Ceθ2, lr> 2θ3, Ga2θ3, Sb2Û5 or even from a ternary semiconductive oxide of the titanate molybdate oxide type.

The transducer used in the chemical sensor according to the invention can be a volume wave transducer. In this case, it may include a GaPθ element, two opposite faces of which are covered at least in part with electrodes, the sensitive layer being deposited on at least one of the electrodes.

The transducer used in the chemical sensor according to the invention can also be a surface wave transducer. In this case, it may comprise a GaPO 4 element on which two series of interdigitated electrodes are deposited, separated by a surface (S), the sensitive layer being deposited at the level of the surface (S) to produce the targeted chemical sensor.

The invention will be better understood and other advantages will appear on reading the description which follows and the appended figures among which:

- Figure 1 illustrates an example of a sensor according to the invention, using a volume wave transducer;

- Figure 2 illustrates an example of a sensor according to the invention, using a surface wave transducer, Rayleigh wave type; - Figure 3 illustrates an example of a sensor according to the invention, using a surface wave transducer, type Lamb waves;

- Figure 4 illustrates an example of a sensor according to the invention, using a surface wave transducer, Love wave type; - Figure 5 illustrates an example of a sensor according to the invention associating an assembly (acoustic transducer, sensitive layer) with an assembly (acoustic transducer, ambient environment).

In the chemical sensor according to the invention, the GaPθ4 crystal is covered with a sensitive layer (C) suitable for the detection of targeted gases.

In general, the sensitive layer (C) used in the chemical sensor according to the invention is chosen for its great interaction with the target gas and for its temperature resistance.

Typically, a layer of Snθ2 type which can be doped with indium or platinum is particularly well suited to the detection of different types of gas, while a layer of Tiθ2 is more specifically adapted to the detection of NO _x .

This sensitive layer (C) can be deposited on the GaPθ4 crystal according to various methods and in particular by conventional sol-gel type method or even by sputtering.

Whatever the type of semiconductor oxide used, analogous adsorption processes take place during the capture of gas molecules, it follows a variation in mechanical (mass), elastic or electrical properties (conductivity) which influences the propagation acoustic waves generated via electrodes, at the level of the GaP04 crystal.

In a first variant of the invention, the sensor comprises a volume wave transducer consisting of a GaPO4 element, provided with two electrodes as illustrated in FIG. 1. The sensitive layer (C) is placed on one electrodes, or even both. The device thus developed constitutes a resonator whose frequency can be measured. The adsorption of the gas molecules that one seeks to detect, results in an increase in mass Δm and leads to a variation in resonant frequency of the resonator thus formed. This frequency variation is given as a first approximation by the following Sauerbrey equation:

Δf = -2F ² Δm / pq v _q A with F frequency of the resonator (Hz) pq density of the piezoelectric material

Vq propagation speed of acoustic waves (m / s)

Area of sensitive surface (m ² )

In a second variant of the invention, the chemical sensor comprises a surface wave transducer consisting of a GaP04 element on which are deposited two series of interdigitated electrodes SE-j and SE2 between which the sensitive layer (C) is deposited .

The whole constitutes a delay line, the acoustic waves emitted by the first series of electrodes SE- | propagate to the second set of electrodes SE2 with a certain delay, resulting in a phase variation. By recombining the output signal with the input signal, the output signal being amplified and put in phase with the input signal, an oscillating circuit is produced having a characteristic resonant frequency f-j. When the conditions of propagation of the surface acoustic waves are modified between the two series of electrodes, the induced phase variation is no longer the same and the oscillating circuit no longer resonates at the frequency f | but resonates at the frequency f-j. Such a Rayleigh wave type surface wave device propagating at the sensitive layer / GaP04 interface in a plane perpendicular to the plane of the layers is illustrated in FIG. 2. When the GaPU4 material has a small thickness, it is possible to produce a transducer with surface waves such as

Lamb. The sensitive layer (C) can be deposited on the surface of the thin film of GaPθ4 on the electrode series side or on the opposite side to the electrode series. This last variant has the advantage of making it possible to encapsulate the entire device so that only the layer (C) is in contact with the medium to be analyzed. Thus the metal electrodes, the GaPθ4 material and possibly the associated electronics are protected.

FIG. 3 illustrates an example of a device obtained by anisotropic etching on the rear face of a silicon substrate on which a nitride etching stop layer and a GaPU4 layer have previously been deposited. The thickness of the membrane thus produced can be of the order of a few microns.

In a fourth variant of the invention, the chemical sensor comprises a Love wave type surface acoustic wave transducer, propagating within an intermediate material (layer C- |) located between the piezoelectric material GaPÛ4 and the sensitive layer (C), intermediate material in which the propagation speed of the acoustic waves is lower than that of the acoustic waves in the piezoelectric material. This intermediate material can typically be silica, gold or even aluminum. This type of sensor is illustrated in Figure 4.

In order to avoid parasitic effects, linked for example to temperature fluctuations, or to aging problems, it is possible to integrate an internal reference at the level of the sensor according to the invention. As illustrated in FIG. 5, two sets of electrodes (SE-j) and (SE2) are deposited on the crystal of GaPO4, in parallel with which two other sets of electrodes (SE'-j) and ( SE'2), the surface (S) separating (SE- |) from (SE2) being covered with a sensitive layer (C), the surface (S ') located between (SE'-]) and (SE'2 ) remaining in contact with the surrounding environment.

Claims

1. Chemical sensor comprising a sensitive layer (C) and an acoustic wave transducer, characterized in that: - the transducer comprises a piezoelectric crystal of GaPθ4; - the sensitive source (C) withstands temperatures above about 500 ° C and is suitable for the detection of gas molecules of the CO, NO _{x type} . 2. Chemical sensor according to claim 1, characterized in that the sensitive layer (C) is of the simple semiconductor oxide type such that Snθ2, Tiθ2, ZrC> 2, CeC> 2, ln2θ3, Ga2θ3, SD2O5 or ternary semiconductor oxide of the mixed oxide type titaπate, molybdate. 3. Chemical sensor according to one of claims 1 or 2, characterized in that the acoustic wave transducer comprises a GaPθ4 element of which two opposite faces are covered at least in part with electrodes, the sensitive layer being deposited on at least one of the electrodes. 4. Chemical sensor according to one of claims 1 or 2, characterized in that the transducer is a surface wave transducer, comprising an element of GaPθ4 on which are deposited two series of interdigitated electrodes, separated by a surface (S ) on which the sensitive layer is deposited. 5. Chemical sensor according to claim 4, characterized in that it comprises a thin film of GaPθ4, the sensitive layer being deposited on the surface of the film, so as to generate acoustic waves like Lamb waves. 6. Chemical sensor according to claim 4, characterized in that the transducer comprises an intermediate layer (C- |) between the sensitive layer (C) and the GaPθ4 crystal, layer (C- |) in which the acoustic waves are confined . 7. Chemical sensor according to one of claims 1 or 2, characterized in that it comprises, in parallel, on the GaPθ crystal: - two sets of electrodes (SE-i) and (SE2) separated by a surface (S) on which the sensitive layer (C) is deposited; - two sets of electrodes (SE'- |) and (SE'2) separated by a surface (S ') identical to (S) in contact with the ambient medium.