It is known that in piezo-electric material an elastic surface wave (a so-called Rayleigh-wave) can be pro¬ duced having a penetration depth of about 2..3 μm. Since the propagation velocity is about 10 m/s, and wave-lengths of the order of magnitude of 1 μm are possible in such a material, vibrations in the order of magnitude of 1 GHz from an electric circuit can be transformed into corresponding mechanical vibrations in this manner. The low propagation velocity allows, then, to construct retardation lines with small dimensions, and also electromechanical filters can be obtained in this manner.

For exciting such waves and recovering again electri¬ cal vibrations, use can be made of so-called interdigital transducers. The latter comprise two comb-shaped electrodes made of metal foil on the surface of a piezo-electric wafer, the teeth of said electrodes being intercaleted said teeth having a width which is equal to 1/4 of the pitch of the teeth. An elastic wave is produced then which is directed perpendicularly to the teeth, the wave-length thereof being equal to said pitch. The amplitude of said vibration and the sharpness of resonance increase with the number of teeth.

For a more detailed description thereof reference can be made to a publication of A. Venema in Tijdschrift NERG 4JJ, 5 (1975) 135. In US 4312228 an apparatus is described comprising a piezo-electric- wafer with two of such composite electrodes, the part between both electrode assemblies being covered with a surface layer adapted to adsorb a gas or a liquid. This layer influences the vibration behaviour of the piezo-electric material, and thus also the propagation velocity of the elastic surface wave. If the characteristics of this layer are changed, e.g. by adsorption of an other substance, this will have an influence on the transit time of a wave between both electrode assemblies. The latter can, for instance, be included in an oscillator circuit which,because of a change

- 2 - in the condition of the layer, and, hus, of the transit time of the elastic wave, will be more or less detuned. Information in respect _, αf the character of the adsorbed substance can be derived therefrom. Such an apparatus can also be used for other changes in condition of the surface layer, or for mutual¬ ly comparing different kinds of surface layers.

This known apparatus has the objection that very small changes in the condition of the surface layer cannot be detected, since small transit time variations of the elastic wave will not lead to a substantial detuning of the oscillator. Although it would be possible as such to measure the transit time of the wave, this requires complicated and expensive measuring apparatuses which, in many cases, will render the apparatus unsuitable for practical purposes. It is an object of the invention to provide such an apparatus having a substantially higher sensitivity than the known apparatus.

The apparatus according to the invention for de¬ termining the condition of a material, and in particular the adsorption of a gas or liquid on this material, comprises a substrate with a plane surface possessing, at least near said surface, piezo-electric characteristics, an electrode assembly provided on said surface and consisting of thin metallic layers in the form of two combs with mutually intercalated teeth, said assembly being adapted for applying thereto an electrical alternating voltage with an adapted frequency, in order to produce an elastic wave in the piezo-electric material directed transversely to said teeth, and at least one surface layer of the material of which the condition is to be determined, which is arranged laterally of said assembly on said surface, said apparatus being characterised in that, at both sides of the comb-shaped electrode assembly. a mirror for the elastic waves produced by said assembly is arranged, said mirrors being, formed by substantially rectilinear surface variations extending parallel to the teeth of the electrodes, and being situated at such a distance from said electrode assembly that the waves reflected between said mirrors are in phase at said electrode assembly if the surface layer is in a given reference condition, said layer being provided at least between said

- 3 - electrode assembly and one of said mirrors.

This structure is comparable to an interferometer known from optrics, and is extremely sensitive for small transit time differences between the electrode assembly and the mirrors. At a variation of the transit time, so that the different waves are no longer in phase, the energy storage in this system will change, which is sensed as an impedance variation by the electrical circuit causing the elastic vibration, and such variation can be easily detected. It is also possible to provide a second and similar electrode assembly at such a distance from the first one that, in the reference condition, the different waves will be in phase also there, which second assembly is connected with an electric measuring circuit. When, in this respect, reference is made to distances and points in which vibrations are in phase, this holds, of course, for points of the electrode assemblies which can be considered as a centre of vibration.

Said mirrors consist, in particular, of a plurality of parallel lines having a mutual distance of a whole number of quarter wave-lengths of the elastic wave in the reference condition of the surface layer, said number being chosen so that a substantially complete reflection is obtained.

Said mirror lines can consist of metallic layers provided on the piezo-electric surface, or of shallow grooves formed in said piezo-electric surface.

In particular the piezo-electric surface can be in the form of a cover layer on a semiconductor substrate in which the associated circuits are integrated.

The invention will be elucidated below in more detail by reference to a drawing, showing in:

Fig. 1 a diagrammatic view of the basic shape of the apparatus of the invention;

Fig. 2 a view at a larger scale of a portion of Fig. 1 ; and Fig. 3 a corresponding representation of a modified embodiment .

^' The apparatus of the invention comprises a wafer 1 of a piezo-electric material, or at least a piezo-electric covering layer on a suitable substrate, said covering layer 1

- 4 - having a sufficient thickness for being adapted to form there¬ in so-called Rayleigh-waves with a penetration depth of several μm.

On this wafer 1 a so-called interdigital electrode assembly 2 is provided. A portion of such an assembly is shown in Fig. 2 at a larger scale. Beyond the portion of the wafer 1 below said assembly, said wafer 1 does not need to be piezo- electrical, so that, if the wafer 1 is not piezo-electrical itself, only at said assembly 2 a piezo-electric covering layer is to be provided.

As shown in Fig. 2 the assembly 2 consists of two electrodes 3 and 4 each comprising a number of teeth 5. The width of said teeth 5 is equal to the width of the interspaces between two teeth 5 of different electrodes 3 and 4. If a suitable alternating voltage is applied to said electrodes, in the surface portion of the wafer 1 an elastic wave is pro¬ duced having a propagation direction which is directed per¬ pendicularly to the direction of the teeth 5. The excitation frequency should be such that the wave-length, in view of the propagation velocity in the piezo-electric material, will be equal to four times the tooth width or the tooth pitch, and, in this respect, reference can be made to the literature, e.g. the publication of A. Venema in Tijdschrift NERG 40_, 5 (1975) 135. The wave-lengths obtainable with such an assembly will, therefore, depend on the precision of manufacturing such an assembly. The etching techniques developed for integrated semiconductor circuits allow the production of very fine electrode assemblies, so that wave-lengths in the order of magnitude of 1 μm, i.e. frequencies of several GHz (at a pro¬ pagation velocity of 3.10 m/s in the piezo-electric material) are possible. This means that a relatively very small path length on the wafer 1 already comprises a very large number of wave-lengths of the elastic wave. At both sides of the assembly 2 a plurality of lines

7 is provided on the surface of the wafer 1, which consist either of vapour-deposited metal layers or of fine grooves formed into the surface. The distance between the lines nearest to the assembly 2 and the centre point of this assembly 2

- 5 - equals a whole number αf half wave-lengths, and the distance between the parallel lines of each set equals a whole number of quarter wave-lengths. Said lines operate as a mirror for the elastic wave. A single line 7 will not produce a complete reflection, but a number of parallel lines at a distance of a whole number of quarter wave-lengths will provide an accor¬ dingly better reflection. In practice some tens of lines can provide a substantially complete reflection. The elastic waves emitted by the electrode assembly 2 return, after reflection against the mirror lines 7, in phase with the emitted waves in the centre of the assembly 2. This will, then, operate as an interferometer, in which, in the tuned condition, a maximum energy storage occurs. However, as soon as the reflected waves do not arrive in phase in the centre point of the assembly 2, this energy is sharply reduced, which, in the electric circuit supplying the electrode assembly 2, will be sensed as a cor¬ responding impedance change, which can be detected in a simple manner. The assembly 2 is shown in Fig. 2 half-way between the line assemblies 7; this is, of course, not necessary, as long as the distances in respect both line assemblies 7 are a whole multiple of the half wave-length.

On the surface of the wafer 1, at least between the lines 7 and the electrode assembly 2, a surface layer 8 is provided which is adapted to influence the elastic behaviour of the piezo-electric material. The said interferometer tuning is, of course, adapted to the wave-length which occurs in the piezo-electric material provided with this layer.

When the condition of the layer 8 changes, also the vibration behaviour of the piezo-electric material is changed, leading to a disturbance of the said vibration condition. Such a change will, for instance, occur if the layer can adsorb an other substance so that its mass will increase. Use can be made thereof for detecting gases or liquids which can be adsorbed on the layer 8, and already very small quantities thereof will be sufficient for producing a substantial dis¬ turbance of the vibration equilibrium.

Although it is possible to provide only one surface layer 8 at one side of the electrode assembly 2, the symmetric¬ al embodiment of Fig. 1 is preferred since, then, the sensi-

- 6 - tivity will be higher.

Fig. 3 shows still an other embodiment in which a second electrode assembly 2' is provided satisfying the same phase requirements as the first assembly 2. The assembly 2' is connected with an electric measuring circuit, and the assembly 2 only serves for excitation of the elastic vibration.

In this manner it becomes possible to obtain an extremely sensitive gas and/or liquid detector by means of which very small concentrations can be detected. Of course also other phenomena can be detected which influence the behaviour αf the surface layer 8.

Sometimes it will be advisable to use a piezo- electric layer 1 provided on a semiconductor substrate, which semiconductor substrate can be used, then, for forming therein integrated circuits which can be used for generating the electric vibrations and for measuring the occurring deviations.