EP1000465A1 - Unidirectional transducer etched with surface acoustic waves - Google Patents

Abstract

The invention concerns a transducer with surface acoustic waves wherein an etched grid is overlaid on the standard electrode arrays of acoustic transducers. The overlay of said arrays enables to obtain a preferential direction for the propagation of the acoustic waves while maintaining a high quality factor Q comparable to that of standard birectional transducers. The invention is applicable to mobile radio communication.

Description

UNIDIRECTIONAL ACOUSTIC WAVE TRANSDUCER

OF SURFACE

The field of the invention is that of surface acoustic wave transducers and filters comprising such transducers, used in many fields such as mobile radiocommunication systems for example for filtering of intermediate frequency.

Different types of efficient transducers have emerged over the past ten years.

Unidirectional transducers (Single-Phase Unidirectional

Transducers also known as SPUDT) have replaced bidirectional transducers in many applications due to the reduction in losses that they allow to obtain. This type of transducer, described in published patent application 2 702 899, is produced by inserting in a transducer cells called transduction cells and so-called reflection cells, and by positioning the cells together so as to have rephased the waves. emitted with the waves reflected in the useful direction and have phase opposition in the other direction. These are transducers in which electrodes are distributed designed so that there is a transduction function and a reflection function. It has also been shown in published patent application 2 702 899 that it can be advantageous to produce resonant cavities inside the

SPUDT, a resonant cavity being produced by changing the sign of the reflection function.

For usual substrates, the distance between transduction center and reflection center can be of the form (2n + 1) λ 8 with n integer so that the phases are correct.

However, the quality factor Q relating to the ratio of the capacitance to the conductance of the filter and representative of the bandwidth and loss of insertion of the filter is less effective for unidirectional filters with their specific architectures into which asymmetries are introduced than that conventional, symmetrical bidirectional filters.

To increase these performances, that is to say increase the coupling in a unidirectional transducer (without increasing the capacitance), the invention proposes a transducer into which is introduced an etching network superimposed on the conventional electrode networks of the acoustic transducers. More specifically, the subject of the invention is a surface wave transducer comprising a substrate on which are deposited two networks of interdigitated electrodes and connected at different polarities so as to create acoustic transduction cells defined by at least two consecutive electrodes of different polarities, characterized in that it further comprises at least one network of etchings separated by mesa, the superposition of the networks of electrodes and of the etching network making it possible to obtain a preferred direction of propagation of the acoustic waves.

According to a first variant of the invention, the electrode arrays are symmetrical with respect to an axis located at the center of two consecutive electrodes, of different polarities, the etching network being asymmetrical with respect to said axis. In this configuration, the networks of symmetrical electrodes make it possible to keep a powerful coupling factor while the network of asymmetrical etching makes it possible to create the preferred direction of propagation of the acoustic waves.

Advantageously, the surface wave transducer can comprise a succession of at least two asymmetrical etching networks so as to locally reverse the preferred direction of propagation of the surface waves.

According to another variant of the invention, the electrode networks define sets of 3 electrodes of different width by characteristic wavelength corresponding to the central operating frequency of the transducer in which sets, a first and a second electrode are separated by a distance 3X 16, and are connected to different polarities, the second and a third electrode being separated by a distance λ / 8, so as to define a preferred direction of propagation of the acoustic waves, the electrodes being positioned on the mesa from the engraving network.

The advantage of such a configuration lies in the increase in the reflection coefficients of the electrodes. In conventional structures, to increase these coefficients, it is necessary to increase the thickness of the electrodes; using a mesa structure separated from engravings, the electrodes being deposited on the surface of the mesa, it is possible to increase the reflection coefficient of the electrodes while maintaining a low thickness of metallization.

The invention will be better understood and other advantages will appear on reading the description which follows given without limitation and thanks to the appended figures among which:

- Figure 1a illustrates a first example of a transducer according to the invention with 4 electrodes per λ, having a preferred direction of propagation of surface waves; - Figure 1b illustrates a second example of a transducer according to the invention with 4 electrodes per λ, having a preferred direction of propagation of surface waves opposite to that of the first example of transducer;

- Figure 2 illustrates a third example of a transducer according to the invention with 4 electrodes per λ locally comprising a first directivity of the surface waves in one direction and locally a second directivity of the surface waves opposite to the first;

- Figure 3 illustrates a fourth example of a transducer according to the invention with 4 electrodes per λ in which the phase shift between the transduction center and the reflection center is equal to λ / 16;

- Figure 4 illustrates a fifth example of a transducer according to the invention with 4 electrodes per λ in which the mesa and the engravings do not have the same width; - Figure 5a illustrates a transducer with two electrodes by λ according to the prior art;

- Figure 5b illustrates a first example of a transducer according to the invention with 2 electrodes per λ, in which the electrodes are positioned astride on etching flanks, of an etching network of period λ2;

- Figures 6a and 6b illustrate a second example of a transducer according to the invention with 2 electrodes per λ, in which the electrodes are positioned on mesa or engravings of an etching network of period λ; - Figure 7a illustrates an example of a conventional SPUDT type transducer with 3 electrodes per λ;

FIG. 7b illustrates an example of a SPUDT type transducer with 3 electrodes per λ, in which the network of electrodes is superimposed on the etching network;

- Figure 7c illustrates a variant of the example of an undirectional transducer illustrated in Figure 7b.

In general, the surface wave transducer according to the invention comprises the superposition of networks of electrodes and at least one etching network. Conventionally, the substrates used can in particular be quartz, the electrodes can be obtained by metallization, for example of aluminum. The substrates used can also advantageously be of the LiNbθ3, LiTaOs or even Li2B4Û7 type. Furthermore, the etching techniques are well mastered on such quartz type substrates and in particular the technique known as ICP (Inductive Coupled Plasma) using a high energy plasma and allowing mass production at low cost of etched devices. It should be noted that the width of the etchings can be different from the width of the mesa and in particular smaller, according to certain variants, the etching width being equal to the electrode width.

According to a first variant of the invention, to obtain a high coupling coefficient conferred by networks of symmetrical electrodes, a preferred direction of propagation of surface waves is created by virtue of the presence of the etching network. We will describe several examples of possible implementation to implement this type of configuration.

Examples of transducers with 4 electrodes per wavelength λ

In this type of transducer, the electrodes are distributed symmetrically on the substrate with a period λ / 4. Such a structure has a good coupling coefficient but remains bidirectional. In fact, the reflections created by the electrodes cancel each other out and no preferred direction of propagation of the acoustic waves is generated with such a configuration. To disrupt this bidirectionality, the invention proposes to superimpose an etching network to reveal additional reflections and this asymmetrically with respect to a central axis C defined between two electrodes connected to different polarities and symbolized in the diagram by a + sign and a - sign , in Figure 1a.

By making the centers of the electrodes coincide with the centers of the mesa or engravings, we obtain if the distance between mesa and consecutive engraving is λ / 4, reflections of the engraving center positioned in λ / 8 or 3λ / 8, relative in the center of an electrode. These etching flanks generate the reflections necessary to obtain a preferred direction of propagation of the surface acoustic waves. FIG. 1 b illustrates a configuration in which the preferred direction of propagation of the acoustic waves is opposite to that of FIG. 1a. In this first variant, the distance between consecutive electrodes can advantageously be equal to the distance between a mesa and a consecutive etching.

As mentioned above, it can be interesting to create resonant cavities. To achieve this type of configuration, in which the preferred direction of propagation of the acoustic waves is locally reversed, the transducer according to the invention can comprise a succession of implantation according to FIG. 1a and of implantation according to FIG. 1b, as illustrated in FIG. 2. In this type of installation at the level of the axis AA ′, there is a break in the periodicity of the etching network so as to pass continuously from an etching network of first type Rj as shown in Figure 1a, to a second type of etching network Rj + i, as shown in Figure 1b.

Such a configuration has the advantage of a much easier technology than that conventionally used in this type of transducer for which it is difficult to locally move the position between a transduction center and a reflection center in order to obtain the directional reversal. wish.

In the previous described configurations of transducer with 4 electrodes per wavelength λ, a transduction center located at the center of an electrode (for example referenced +) is separated from an etching flank corresponding to a reflection center of a distance λ / 8, corresponding to the ideal case. In certain applications and taking into account the substrates employees, it can be interesting to create a phase shift different from 45 ° (corresponding to λ / 8). For this and according to the invention, it is advantageously possible to offset the etching network with respect to the electrode network. FIG. 3 illustrates a configuration in which a transduction center is distant from an engraving flank by a distance of λ 16, ie a phase shift of 22.5 °. In this configuration, the electrodes are aligned with the flanks of etchings which can represent technological ease.

Furthermore, in the aforementioned examples, the mesa and the engravings have the same widths, however the latter may also advantageously be of different width.

Typically, the engravings can have a width equal to λ / 8 while the mesa have a width equal to 3λ / 8. This allows the metallization etchings to be completely filled as illustrated in FIG. 4. The sensitivity in production can thus be increased. In addition, with this lattitude, it becomes very simple to change the width or the position of the etchings locally in the transducer, so as to modify the phase and the amplitude of the reflection coefficient.

Examples of transducers with 2 electrodes per wavelength λ

The examples described above all relate to transducers of 4 electrodes per λ, in which the preferred directivity of the surface waves is easily obtained. According to the prior art, the transducers with two electrodes by λ, illustrated in FIG. 5a, have very high and stronger coupling coefficients than in the transducers with 4 electrodes by λ but are nevertheless bidirectional, the reflections between electrodes being in phase and this symmetrically. The superposition of an etching network in this type of transducer advantageously overcomes this drawback. In addition, this type of transducer has a technological advantage since it makes it possible to fabricate electrode networks with a pitch twice as large as the pitch required in transducers with 4 electrodes per λ.

Conventionally, transducers with 2 electrodes per λ, include electrodes of width λ / 4 separated by a pitch λ / 2, as illustrated in Figure 5a. The waves emitted at an electrode Ej are in phase with the waves reflected by the consecutive electrode Ej + 1 and vice versa for the waves emitted at Ej + 1 and those reflected by the electrode Ej. To disturb this symmetry and the phasing of reflections at the electrodes, the invention proposes to position the electrodes on the etching flanks as illustrated in FIG. 5b.

To understand the functioning of such a structure and how it is possible to optimize this type of layout, we will consider the reflection coefficients respectively relating to an etching flank, to an electrode and located at the center of the electrode.

In the upward engraving -> mesa direction, the reflection coefficient is referenced rg.

In the descending direction mesa -> engraving, the reflection coefficient is referenced -rg ^* . In the upward direction, substrate - electrode, the reflection coefficient is referenced -re *.

In the downward direction, electrode -> substrate, the reflection coefficient is referenced re.

If the center of the electrode is moved a distance d from the engraving side, the reflection coefficient r located at the center of the electrode is given by the following formula

= -2. Re (r _g le ^A + j. 2 Re (r _e )

To obtain a unidirectional transduction, we want the reflection coefficient r to be a pure real, which implies:

-2.Refr _g i ^λ + j. Re (r _β ) = 0 where Re real part

r = -2.Re/r _g Ycosl 4.π.—

Therefore

Equations (1) and (2) impose the condition | Re (re) | <| Re (rg) | , which can always be obtained for a judiciously chosen value of the distance d.

The first example of a transducer with 2 electrodes per λ, which has just been described still requires a technology in which the electrodes must be deposited at the intersection of mesa and of etching, which is not very easy.

This is why, the invention also proposes another configuration of transducer with 2 electrodes per λ, but of more direct technology.

It is a unidirectional transducer in which the network of 2 electrodes per λ is superimposed on a network of etchings of pitch λ, as illustrated in FIG. 6a or 6b.

This type of transducer works at the second harmonic. It has the advantage of offering a wider geometry than the geometries described above and is particularly advantageous for applications at very high frequencies.

By considering the same parameters rg, re and r, we obtain for the central reflection coefficient r: -2.jk

-jAπ.

= 2lm (4r „.e λ j..2Re (r _e )

As before, to obtain a preferred direction of propagation, we seek to obtain a pure real reflection coefficient, that is:

with Im: imaginary part

As in the previous example, it is possible to determine a value d, such that it allows | Re (re) | <| lm (rg) | . And as for transducers with 4 electrodes per λ, it can be very advantageous to produce transducers in which the mesa and the etchings do not have the same width.

Examples of transducer with 3 electrodes per wavelength λ

According to another variant of the invention, it is possible to use a conventional surface wave transducer, of the SPUDT type, using an asymmetrical array of electrodes and in which the coupling performance is increased due to the improvement of the reflection coefficients of the electrodes used for a transducer wavelength. FIG. 7a illustrates a transducer with 3 electrodes per λ, two electrodes are spaced by λ / 4, to cancel the sharp reflections of said electrodes; indeed, a wave emitted by an electrode is in phase opposition with respect to the wave reflected by the consecutive electrode separated from the distance λ / 4. The third electrode separated by a distance of 3λ / 8 from the consecutive electrode, acts as a reflector.

One way of increasing the reflection coefficient of such an implantation consists in increasing the thickness of said electrodes. In general, beyond a certain electrode metallization value, said electrode loses its properties and the technology becomes delicate. This is why, the invention proposes a variant of transducer, in which the reflection coefficients of the electrodes are improved by increasing the thickness of electrodes without increasing the thickness of metallization as illustrated in FIG. 7b. Such technology also makes it possible to use a single mask to produce the etching network and the electrode metallization network.

To perfect such a structure, it is advantageous not to etch the substrate uniformly as illustrated in FIG. 7c. Indeed, by not etching at a transduction center, it is possible to increase the distribution of reflectivity to selectively render certain electrodes, reflection center and thus lead to a decrease in transduction length. In particular, by not etching at a transduction center located between an electrode referenced + and an electrode referenced -, we are able to phase a wave of surface at this transduction center and a wave reflected by the electrode constituting a reflection center placed in 3λ / 8.

Claims

1. Surface wave transducer comprising a substrate on which are deposited two networks of interdigitated electrodes and connected to different polarities so as to create acoustic transduction cells defined by at least two consecutive electrodes of polarities 5 different, characterized in that it further comprises at least one network of etchings separated by mesa, the superposition of the electrode networks and the etching network making it possible to obtain a preferred direction of propagation of the acoustic waves. 2. Surface wave transducer according to claim 1, characterized in that the electrode networks are symmetrical with respect to an axis located at the center of two consecutive electrodes of different polarities, the etching network being asymmetrical with respect to said axis . 3. Surface wave transducer according to claim 2, characterized in that it comprises a succession of at least two etching networks 5, asymmetrical so as to locally reverse the preferred direction of propagation of the surface waves. 4. Surface wave transducer, according to one of claims 2 or 3, characterized in that the etching network comprises an alternation of etchings and mesa, the width of the etchings being 0 different from the width of the mesa. 5. Surface wave transducer according to one of claims 2 to 4, characterized in that the distance between two consecutive electrodes is equal to the distance between a mesa and a consecutive etching. 5 6. Surface wave transducer according to one of claims 2 to 5, characterized in that the distance between consecutive electrodes is equal to a quarter of the characteristic wavelength corresponding to the central operating frequency of the transducer, the networks d 'electrodes comprising pairs of interdigitated electrodes. 0 7. Surface wave transducer according to one of claims 2 to 6, characterized in that the distance between a mesa and a consecutive etching is equal to a quarter of the characteristic wavelength. 8. A surface wave transducer according to claims 5, 6 and 7, characterized in that the width of the electrodes is equal to one eighth of the characteristic wavelength. 9. Surface wave transducer according to one of claims 1 to 8, characterized in that the central axis coincides with an etching flank of the etching network. 10. Surface wave transducer according to one of claims 2 to 8, characterized in that the central axis is located at a non-zero distance from an etching flank of the etching network. 11. Surface wave transducer according to one of claims 2 to 5, characterized in that the distance between two consecutive electrodes is equal to a characteristic half-wavelength. 12. Surface wave transducer according to claim 1 1, characterized in that the width of the electrodes is of the order of a quarter of the characteristic wavelength. 13. Surface wave transducer according to one of claims 1 1 or 12, characterized in that the distance between an etching and a consecutive mesa is equal to a quarter of the characteristic wavelength, the electrodes being positioned astride said mesa and engravings. 14. Surface wave transducer according to one of claims 1 1 or 12, characterized in that the distance between an etching and a consecutive mesa is equal to the characteristic wavelength. 15. A surface wave transducer according to claim 1, characterized in that the electrode networks define sets of 3 electrodes per characteristic wavelength, in which together a first and a second electrode are separated by a distance equal to a quarter of characteristic wavelength, and are connected to different polarities, the second and a third being separated by a distance equal to 3 eighth of a characteristic wavelength so as to define a preferred direction of propagation of the acoustic waves, the electrodes being positioned on the mesa of the etching network. 16. A surface wave transducer according to claim 15, characterized in that the first and second electrodes of a set of 3 electrodes per wavelength are positioned on the same mesa.