FR2827720A1 - Surface acoustic wave device with low surface loss made up of a piezoelectric substrate, an assembly of electrodes and coated with a thin layer of dielectric material - Google Patents

Abstract

The invention relates to a low-loss surface acoustic wave device (or SAW device) and more particularly applies to SAW devices operating in frequency bands above gigahertz. The device according to the invention comprises a piezoelectric substrate (2 ), a set of conductive electrodes (3), and a thin layer (4) of a dielectric material of hardness and dielectric permittivity greater than that of the substrate, deposited substantially uniformly on the surface of said substrate, the electrodes being arranged on said layer. Advantageously, the dielectric material (4) consists of titanium dioxide TiO2 and the substrate (2) is made of Lithium Niobate (LiNbO3) or Lithium Tantalate (LiTaO3).

Description

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LOW-LOSS SURFACE ACOUSTIC WAVE DEVICE WITH HIGH PERMITTIVITY DIELECTRIC LAYER
The invention relates to a surface acoustic wave device (or SAW device according to the abbreviation of the term Surface Acoustic Wave) and more particularly to SAW devices operating in the frequency bands above the Gigahertz.

 During the last twenty years, the growing demand of the mobile telecommunications industry has led to the introduction of a new type of SAW filter, whose characteristics are mainly low losses, a very small size, and which can operate at frequencies up to 2 or 3 GHz, or even beyond.

 SAW devices are generally formed on a mono or polycrystalline piezoelectric substrate. Among the most used, there may be mentioned monocrystalline substrates of LiNbOg (Lithium Niobate) and LiTaO3 (Lithium Tantalate). The most used cuts for producing SAW filters with low propagation losses are usually designated respectively by 64 YX LiNbOg and 36 YX LiTaOg ", where X, Y, Z are the crystallographic axes of the crystal, in effect monocrystalline substrates whose cutting plane is rotated about X respectively by 640 for LiNbOg and 360 for LiTa03 (relative to an orientation of the normal Y-section cutting plane) and in which the direction of propagation of the surface acoustic wave is along the X axis. The type of SAW waves that can propagate in these sections is known as leaky SAW and is characterized by a very strong piezoelectric coupling, a very high wave velocity , which is positive, but also by a weak radiation of the waves of volume, leading to losses.

The 36 Y-X LiTaO3 and 64 Y-X LiNbOg sections were chosen in the prior art embodiments to minimize propagation losses.

However, these cutting angles give an optimal result (in terms of propagation losses) for waves propagating on a homogeneous substrate and neglecting the mechanical effect of a metallization. In the case of modern filters, with minimal losses, the waves no longer propagate on a homogeneous substrate since a metal network is deposited on the surface. The thickness of the aluminum layer forming the electrodes can

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 be 8% to 10% of the wavelength of the acoustic wave. On the other hand, to reduce insertion loss; In general, the reflection of surface waves by metal electrodes is used. The presence of the electrodes on the substrate changes the wave propagation and the optimal cuts are no longer.

 Moreover, in the case of LiNb03, unlike LiTa03, the angle of cut that minimizes the propagation losses is not the same in the case of a free surface and in that of a fully surface. metallized with thin metal layer. In practice, because of the presence of electrodes separated by free spaces, there is therefore no optimal cut for lithium niobate.

 It is known that the increase of the thickness of the electrodes with respect to the wavelength SAW in the sections referred to above is at the origin in particular of the emission of volume waves by the electrodes and diffraction of a portion of the surface waves in volume waves, resulting in increased propagation losses of surface waves. LSAW is the surface wave that is accompanied by a wave radiation of the volume (abbreviation of the Anglesaxon Leaky Surface Acoustic Wave))). As regards the theory of LSAW propagation losses in a SAW filter using a thick electrode on a Y-X LiNbO 3 or Y-X LiTaO 3 type substrate, reference may be made to Plessky et al. (S. S. Plessky and C.S. Hartmann, Proc.V 1993 IEEE Ultrasonics Symp., Pp. 1239-1242) and Edmonson et al.

(Edmonson J. J. and Campbell C. Campbell, Proc., 1994 IEEE Ultrasonics Symp., Pp. 75-79). Thus, for practical applications, the emission of the waves of the volume must be minimized in order to reduce the propagation losses.

 In US Pat. No. 6,037, 847 (Fujitsu Limited), the problem is solved by providing a cutting angle greater than that usually used, namely a cutting angle of between 390 and 460 for LiTaO 3 and a cutting angle of between 66 and 74 for LiNbO3. It appears in this document that with these cutting angles, it is possible to optimize the electrode thickness to minimize LSAW surface wave propagation losses. Moreover, the patent DE 197 58 195 A 1 proposes a SAW device with a LiNbO3 or LiTaO3 type substrate whose pure aluminum or composite electrodes are covered with a layer

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 durometer of an aluminum oxide for optimizing the propagation losses for a given cutting angle of the piezoelectric substrate. However, this solution is not completely satisfactory since it requires depositing a layer on the finalized SAW device, controlling the thickness of the layer on the electrodes and between the electrodes, making the manufacturing process much more complicated. .

 The invention proposes a SAW device that can operate in frequency bands up to 2 or 3 GHz with minimal propagation losses of the surface waves, in which it is possible to use piezoelectric substrates of LiNbO3 or LiTaO3 type with common cutting angles. The invention also proposes a means of adjusting the piezoelectric coupling coefficient obtained to adapt it to the filter band that is to be produced. For this, the invention proposes to deposit on the substrate, before the introduction of the electrodes, a uniform thin layer of a material having a high dielectric constant and high hardness, for example of TiO 2 type (titanium dioxide). The applicant has shown that such a device can be optimized to make the minimum propagation losses with given cutting angles, including conventional cutting angles, and is particularly promising with Lithium Niobate, whose potentialities are seen widened thanks to the device according to the invention.

 More specifically, the invention relates to a low-loss surface acoustic wave device comprising a piezoelectric substrate, a set of conductive electrodes, characterized in that it further comprises a thin layer of a dielectric material of hardness and dielectric permittivity greater than that of the substrate, deposited substantially uniformly on the surface of said substrate, the electrodes being disposed on said layer.

 Other advantages and characteristics of the invention will appear on reading the description which follows, illustrated by the figures which represent: - Figures 1A and 1B, diagrams recalling the definition of the cutting angle of a crystal; FIG. 2 is a diagram of an exemplary embodiment of the device according to the invention; - Figure 3, a partial section of the diagram of Figure 2;

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FIG. 4 is a diagram showing curves representing, as a function of the cutting angle, the optimum thickness of the electrodes as a function of the thickness of the firing layer 2 with a LiTaO 3 substrate;
FIG. 5, a diagram showing curves representing, as a function of the cutting angle, the optimum thickness of the electrodes as a function of the thickness of the firing layer 2 with a LiNbOg substrate.

 Figures 1A and 1B illustrate by diagrams the definition of the cutting angles used in the following. The X, Y, Z axes are the crystallographic axes of the crystal. FIG. 1A shows a cutting plate YX, that is to say that the normal to the plane of section is the Y axis and that the direction of propagation is the X axis. In this figure, the directions w are defined ( following the width of the plate), / (along its length, in the direction of propagation), and t, perpendicular to the plate. A section and an angle of propagation are defined starting from the section Y-X and applying three successive rotations. In the case of interest here, shown in Figure 1B, a single rotation is applied around /, an angle 0. The propagation is in the direction 1 of the rotated plate. The initial mark (XYZ) becomes after rotation (XY'Z '). Y-X is a section of a crystal as defined in FIG. 1B.

 The losses in the LSAW surface acoustic wave structures can be explained in particular by propagation losses due to the excitation and diffusion of slow-wave waves (in English), and to other ohmic loss type effects, ie due to resistance of the electrodes Thus, as explained previously, in the case of LSAW devices with lithium tantalate substrate (LiTa03) for example, the section 36 YX was commonly used because it was deemed to minimize the loss of propagation on free surface, but it appears that this cut is not optimal when the mass of the electrodes is no longer negligible compared to the wavelength of the acoustic wave that occurs. propagation (typically, 8% to 12% wavelength thickness).

 The invention proposes a surface acoustic wave device with low propagation losses, including in gigaherz band filtering applications, an exemplary embodiment of which is illustrated in FIG.

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 Fig. 2. An enlarged partial section of the diagram of FIG. 2 is shown in FIG. 3. In these figures, the SAW device 1 comprises a piezoelectric substrate 2 and a set of conductive electrodes 3.

They are, for example, electrodes made of pure aluminum (AI), aluminum / copper alloy (AI / Cu), pure copper (Cu) or gold (Au).

 According to the invention, a thin dielectric layer 4, of high hardness, is deposited substantially uniformly on the surface 21 of the substrate, the electrodes being disposed on the surface 41 of said dielectric layer 4.

The applicant has shown that it is possible with such a device to minimize the propagation losses, as will be explained later, by choosing for the layer 4 a hard material of high dielectric permittivity, that is to say whose hardness and dielectric permittivity are at least greater than that of the substrate. The hardness is defined here with respect to the propagation velocity of the acoustic waves in the material. By hard material, it is meant here that the velocity of the acoustic waves propagating in the material forming the layer 4 is greater than the speed of the acoustic waves propagating in the substrate.

pour le dispositif selon l'invention est l'oxyde de Titane (TiO,) qui réunit à la fois les conditions requises de dureté et de permittivité diélectrique (la permittivité diélectrique de Ti02 est sensiblement supérieure à celle du Tantalate de Lithium par exemple). D'autres matériaux sont envisageables, comme par exemple l'oxyde de Niobum (Nb2Os)'Ces deux matériaux sont par ailleurs utilisés dans les technologies électroniques, par exemple pour la production de condensateurs, et leur technologie est donc bien maîtrisée. The applicant has shown that a particularly interesting material

for the device according to the invention is titanium oxide (TiO 2) which combines both the required hardness and dielectric permittivity conditions (the dielectric permittivity of TiO 2 is substantially greater than that of lithium tantalate, for example). Other materials are conceivable, such as for example Niobium oxide (Nb 2 O 5). Both materials are also used in electronic technologies, for example for the production of capacitors, and their technology is therefore well controlled.

 Examples of simulation results obtained with a SAW device with a substrate of lithium tantalate (LiTaO 3) and with a lithium niobate substrate (Li NB0 3) on which a thin layer of titanium oxide is deposited, the electrodes arranged on the titanium oxide layer being aluminum. The structure chosen here as an example for carrying out the simulations is a synchronous resonator with a long transducer and relatively short reflectors placed on each side. The calculation method used (FEM / BEM method) is described in particular in Advances in Surface Acoustic Wave Technology, Systems and Applications (Vol.2), Eitors C. C. Ruppel W. & T. A. Fjedly, World

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 Scientific (2001), pages 1-82. The parameters used are the cutting angle θ, the thickness h of the electrodes, the thickness # TiO 2 of the titanium oxide layer and the metalization ratio a / p, where a is the width of a metal finger. the electrodes and p is the period, as illustrated in FIG. 3. In the example described below, the ratio a / p is set to 0.5.

qui présente un intérêt direct dans la définition des filtres à éléments d'impédance (IEFs, ou filtres SAW en échelle appelés ladder filters selon l'expression anglo-saxonne). The calculations made it possible to determine, according to the parameters mentioned, the resonator resonator frequencies f and antiresonance fa, as well as their respective quality factors Qr and Qar. Other characteristics have been determined, such as the propagation velocity of the acoustic wave v, the reflectivity by electrode K, the normalized transductance an (characteristic of electromechanical coupling), the normalized capacitance Cn and the resonance-antiresonance distance RaR defined by the following relation:

which has a direct interest in the definition of impedance element filters (IEFs, or SAW filters in ladder called ladder filters according to the English expression).

 At first, we are interested in simulations carried out with a substrate of Lithium Tantalate (LiTaO3).

fréquence d'anti-resonance). Quatre courbes sont représentées, notées 41 à 44, correspondant respectivement à une épaisseur de la couche de Ti02 de hTo2=0 (pas de couche), hTi02=O, OO5Ào' hTi02=O, 01Bo et h=0, 015Â.. FIG. 4 shows, as a function of the cutting angle # of the crystal of LiTa03, the optimum value h 0 of the thickness of the electrodes normalized with respect to the wavelength # 0 of a surface acoustic wave excited on the surface of the piezoelectric substrate, for different values of the thickness hr, of the titanium oxide layer, also given as a function of the wavelength 0 0. The optimum value shown here is the value that makes the minimum propagation losses at the resonance frequency f, (and at the

anti-resonance frequency). Four curves are represented, denoted 41 to 44, respectively corresponding to a thickness of the TiO2 layer of hTo2 = 0 (no layer), hTiO2 = 0.05o0 hTi02 = 0.01Bo and h = 0.015 ..

It is found on the curve 41 of FIG. 4 that, in the absence of the TriO 2 layer, no thickness value of the electrodes makes it possible to make the propagation losses around 6 = 36 minimal, while the choice of very thin electrodes corresponds to an optimum for a cut of 36, 7 approximately. On the other hand, the calculations made by the applicant show that with the insertion of a layer of Tri02, the optimum value of the thickness

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 electrodes is finished, even for angles around 36. Moreover, the greater the thickness of the TiO 2 layer, the better the thickness of the electrodes. It thus appears from the results shown in FIG. 4 that for a usual cutting angle θ around 36, and a thickness of the TiO 2 layer approximately between 0.1% and 2% of the wavelength of a surface acoustic wave excited on the surface of the piezoelectric substrate, it is possible to find the optimum electrode thickness h (between 3% and 10% of the wavelength λ) so as to obtain a device whose propagation losses are minimal.

 The applicant thus shows that, thanks to the device according to the invention, it is possible to find the combination which minimizes the losses by the choice of 3 parameters (cutting angle 6, thickness of the electrodes ho, and thickness of the TiO 2 hT layer, ). Two of these parameters can be chosen arbitrarily (within the limits indicated), the third is then determined by the curves of Figure 4. Figure 4 thus shows a possible range of cutting angles from 34 to 380, to illustrate that the cut said standard from 36 to 370 can be optimized. However, it is understood that the range of angles making it possible to limit the propagation losses, thanks to the device according to the invention, can be wider, typically between 300 and 450. Calculations show that at the antiresonance frequency, the behavior is similar.

 Table 1 presented in the appendix gives for a substrate of lithium tantalate, the values of certain parameters of the filter according to the invention (defined above), calculated under the same conditions as above (using the FEM / BEM model) with a dielectric layer of TiO2, aluminum electrodes, and a ratio a / p = 0.5. This table shows the precise numerical results obtained for some particular configurations, denoted A to F, each corresponding to a cutting angle value and an electrode thickness value, for different values of the thickness of the layer of Tri02. .

 Table 1 provides in particular the same conclusions as those drawn from FIG. 4, namely that the choice of the three cutting angle 0 parameters, the thickness of the hop electrodes and the thickness of the TiO 2 h-layer makes it possible, thanks to the device according to FIG. invention, to find the optimal combination to limit the propagation losses. Thus, the applicant

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 showed that there is not a single optimum angle but that for each angle (between 340 and 380, for example, as shown in Figure 4) and for a reasonable thickness of the electrodes (about 8% of the length of acoustic wave), we can find the thickness of the layer Tif ,, which minimizes propagation losses. Furthermore, Table 1 above also shows that the difference between the resonant and antiresonance frequency decreases when the thickness of Shot 2 is increased. This can be used when trying to reduce the band. relative pass-through of the filter.

 We now consider the simulations performed with a lithium Niobate substrate (LiNbO3). As before, the dielectric layer is formed of titanium oxide and the electrodes are aluminum.

résonance f,. Cinq courbes sont représentées, notées 51 à 55, correspondant respectivement à une épaisseur de la couche de Ti02 de h-, =0, 005, hTlo2=0, 010Ào' hW2=0, 015Ào et hW2=0, 020Ào. et hTlo2=0, 025Ào'
On vérifie que sans couche de Ti02 la coupe de 41 est optimale pour une épaisseur d'Aluminium des électrodes très faible. Il apparaît sur ces courbes que grâce au dispositif selon l'invention, il est possible même en utilisant des coupes usuelles du Niobate de Lithium (8 = 410 et 8 = 64 ) de trouver des valeurs d'épaisseur d'électrodes qui minimisent les pertes de propagation. FIG. 5 shows, as a function of the cutting angle θ of the LiNbOg crystal, the optimum value of the electrode thickness normalized with respect to the wavelength λ o of a surface acoustic wave excited on the surface of the electrode. piezoelectric substrate, for different values of the thickness hT102 of the titanium oxide layer (data also as a function of the wavelength S0). As in Figure 4, the optimum value shown here is the value that makes the minimum propagation losses at the frequency of

resonance f ,. Five curves are shown, 51 to 55, respectively corresponding to a thickness of the TiO 2 layer of h -1 = 0.005, hTlo 2 = 0.010 → hW 2 = 0.015 Å and hW 2 = 0.020 Å. and hTlo2 = 0, 025Ao '
It is verified that without a TiO 2 layer, the cut of 41 is optimal for a very low aluminum electrode thickness. It appears on these curves that thanks to the device according to the invention, it is possible even by using conventional cuts of lithium niobate (8 = 410 and 8 = 64) to find electrode thickness values which minimize losses. of propagation.

 It thus appears that for a usual cutting angle 8, around 410 and a thickness of the Tir2 layer substantially between 0.1% and 3% of the wavelength of a surface acoustic wave excited on the surface of the piezoelectric substrate, it is possible to set the electrode thickness h between 2% and 10% of the wavelength λ so as to obtain a device whose propagation losses are minimal.

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 Moreover, for a cutting angle around 640 and a thickness of the TiO 2 layer approximately between 0.1% and 3% of the wavelength at, it is also possible to set the thickness of the electrode. between 2% and 10% of the wavelength λ 0 so as to obtain a device whose propagation losses are minimal. Of course, as before, the same optimization can be done in a much broader range of cutting angles (typically between 350 and 650).

0 = 410 et pour un angle de coupe 0 = 64 , traditionnellement les plus utilisés en pratique. Les différentes configurations correspondant à des couples de valeurs épaisseur de TiO/épaisseur d'électrode sont notées de A'à F'. Toutes les combinaisons des paramètres choisis (A'à F') donnent d'excellents résultats pour les facteurs de qualité Q, à la fréquence de résonance et à la fréquence d'anti-résonance. Ce qui est remarquable notamment, c'est que le coupe 41 Y-XliNbO3 permet des pertes de propagation minimales avec une très forte constante de couplage piézoélectrique. Ainsi, dans l'art antérieur, cet angle de coupe était peu utilisé car était réputé présenter des pertes de propagation élevées. Grâce au dispositif selon l'invention, en choisissant des angles de coupe inférieurs à 410 on peut obtenir un couplage encore plus fort, sans pour autant subir des pertes de propagation fortes, en choisissant correctement l'épaisseur de la couche de Tri02, grâce par exemple aux résultats présentés sur la figure 5 ou dans les tableaux 2 et 3. Tables 2 and 3 presented in the appendix give the values of the parameters obtained by simulation, respectively for a cutting angle.

0 = 410 and for a cutting angle 0 = 64, traditionally the most used in practice. The different configurations corresponding to pairs of TiO thickness / electrode thickness values are denoted from A'to F '. All combinations of the chosen parameters (A'-F ') give excellent results for the quality factors Q, the resonant frequency and the anti-resonance frequency. What is remarkable in particular is that the 41 Y-XliNbO3 cut allows minimal propagation losses with a very strong piezoelectric coupling constant. Thus, in the prior art, this angle of cut was little used because was deemed to have high propagation losses. Thanks to the device according to the invention, by choosing cutting angles lower than 410, an even stronger coupling can be obtained, without however suffering strong propagation losses, by correctly choosing the thickness of the layer of TriO 2, thanks to example to the results presented in Figure 5 or in Tables 2 and 3.

 The applicant has thus shown that in the case of lithium tantalate (LiTaO 3), as in the case of lithium niobate (LiNbO 3), it is possible thanks to the SAW device according to the invention to minimize the propagation losses, even in the case of using conventional cutting angles, and with sufficient electrode thicknesses (beyond 5% of the propagation wavelength of the acoustic wave) to reduce ohmic resistance including in high frequencies, order of the gigaherz and beyond.

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ANNEX

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Tableau 1 : Simulations avec LiTa03

Table 1: Simulations with LiTa03

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<tb> hTiO2 / # 0 <SEP> h / # 0 <SEP> Qr <SEP> Qar <SEP> v (m / s) <SEP> K <SEP> a <SEP> C
<tb> 0.015 <SEP> 0.06 <SEP> 10000 <SEP> 53000 <SE> 4495 <SEP> 0.183 <SEP> 100. <SEP> 10-5 <SEP> 50.10-5
<tb>E'0.020<SEP> 0.07 <SEP> 4800 <SEP> 90000 <SEP> 4497 <SEP> 0.197 <SEP> 98. <SEP> 10 ''<SEP> 52. <SEP> 10-5
<tb> F '<SEP> 0.025 <SEP> 0.08 <SEP> 3687 <SEP># 105 <SEP> 4497 <SEP> 0, <SEP> 216 <SEP> 95. <SEP>10'<SEP> 54. <SEP> 10 '
<Tb>
Table 3: Simulations with a cut 64 YX LiNbO3

Claims (16)

1-Low-loss surface acoustic wave device (1) comprising a piezoelectric substrate (2), a set of conductive electrodes (3), characterized in that it further comprises a thin layer (4) of a dielectric material of hardness and dielectric permittivity greater than that of the substrate, deposited substantially uniformly on the surface of said substrate, the electrodes being disposed on said layer.  2-Device according to claim 1, characterized in that the dielectric material consists of titanium dioxide Tri02.  3-Device according to claim 1, characterized in that the dielectric material consists of Niobum oxide NbOg.  4-Device according to one of the preceding claims, characterized in that the thickness of the dielectric material layer is substantially between 0.1% and 5% of the wavelength of a surface acoustic wave excited on said surface of said piezoelectric substrate.  5-Device according to one of the preceding claims, characterized in that the electrodes are made of pure aluminum (AI), aluminum alloy / copper (AI / Cu), pure copper (Cu) or gold (Au).  6-Device according to one of the preceding claims, characterized in that the thickness of the electrodes is substantially between 1% and 15% of the wavelength of a surface acoustic wave excited on said surface of said piezoelectric substrate.  7-Device according to one of the preceding claims, characterized in that the piezoelectric substrate is formed from lithium tantalate crystal LiTa03.  8-Device according to claim 7, characterized in that the cutting angle (8) of the LiTaOg crystal is substantially between 34 and 380, in that the thin dielectric layer is formed of titanium dioxide and has a thickness of substantially between 0.1% and 3% of the wavelength of a surface acoustic wave excited on the surface of said substrate and in that the thickness of the electrodes is determined, for a given angle of cut, as a function of the thickness of the TiO 2 layer so as to minimize propagation losses. <Desc / Clms Page number 13>  9-Device according to claim 8, characterized in that the cutting angle (8) of the LiTaO3 crystal is substantially between 360 and 370, in that the thickness of the TiO 2 layer is substantially between 0.1 % and 2% of said wavelength and in that the thickness of the electrodes is substantially between 3% and 10% of said wavelength.  10- Device according to claim 9, characterized in that the thickness of the TiO 2 layer is substantially between 1. 0% and 2% of said wavelength and in that the thickness of the electrodes is substantially between 6% and 10% of said wavelength.  11-Device according to one of claims 1 to 6, characterized in that the piezoelectric substrate is formed from lithium Niobate crystal LiNbO.  12-Device according to claim 11, characterized in that the cutting angle (8) of the LiNbOg crystal is substantially between 350 and 700, in that the thin dielectric layer is formed of titanium dioxide and has a thickness of substantially between 0.1% and 3% of the wavelength of a surface acoustic wave excited on the surface of said substrate and in that the thickness of the electrodes is determined, for a given angle of cut, as a function of the thickness of the TiO 2 layer so as to minimize propagation losses.  13-Device according to claim 12, characterized in that the cutting angle (0) of the crystal of LiNb03 is around 41, in that the thickness of the TiO 2 layer is substantially between 0.1% and 3% of said wavelength and in that the thickness of the electrodes is substantially between 2% and 10% of said wavelength.  14- Device according to claim 13, characterized in that the thickness of the TiO2 layer is substantially between 2.0% and 3% of said wavelength and in that the thickness of the electrodes is substantially between 6% and 10% of said wavelength.  15-Device according to claim 11, characterized in that the cutting angle (8) of the LiNbOg crystal is around 640, in that the thickness of the TiO 2 layer is substantially between 0.1% and 3% of said wavelength and in that the thickness of the electrodes is substantially between 2% and 10% of said wavelength. <Desc / Clms Page number 14>  . 16- Device according to claim 15, characterized in that the thickness of the TiO 2 layer is substantially between 2.5% and 3% of said wavelength and in that the thickness of the electrodes is substantially between 8% and 10% of said wavelength.