Surface acoustic wave device on composite substrate

Abstract

A surface acoustic wave device using a longitudinally polarized guided wave comprises a composite substrate comprising a piezoelectric layer formed over a base substrate, wherein the crystalline orientation of the piezoelectric layer with respect to the base substrate is such that, the phase velocity of the longitudinally polarized wave is below the critical phase velocity of the base substrate at which wave guiding within the piezoelectric layer vanishes. A method of fabrication of such surface acoustic wave device is also disclosed.

Claims

1. A surface acoustic wave device using a longitudinally polarized guided wave, comprising: a composite substrate comprising a piezoelectric layer formed over a base substrate, wherein a crystalline orientation of the piezoelectric layer with respect to the base substrate is such that a phase velocity of the longitudinally polarized guided wave is below a critical phase velocity of the base substrate at which wave guiding within the piezoelectric layer vanishes.

2. The surface acoustic wave device of claim 1, wherein the piezoelectric layer is a single crystal.

3. The surface acoustic wave device of claim 1, wherein a thickness of the piezoelectric layer is on the order of a sub-wavelength range or smaller.

4. The surface acoustic wave device of claim 1, wherein the base substrate has a shear wave velocity of more than 5600 m/s.

5. The surface acoustic wave device of claim 1, further comprising a dielectric layer sandwiched between the base substrate and the piezoelectric layer.

6. The surface acoustic wave device of claim 5, wherein the dielectric layer has a thickness of less than 800 nm.

7. The surface acoustic wave device of claim 1, wherein the piezoelectric layer comprises lithium tantalate (LiTaO.sub.3) or lithium niobate (LiNbO.sub.3).

8. The surface acoustic wave device of claim 7, wherein the piezoelectric layer comprises lithium tantalate (LiTaO.sub.3) with a crystallographic orientation defined as (YX)/θ according to the standard IEEE 1949 Std-176, with θ, an angle of the crystallographic orientation being between 40° and 65°.

9. The surface acoustic wave device of claim 8, wherein a propagation direction of the longitudinally polarized guided wave is at 90° of a crystallographic X-axis.

10. The surface acoustic wave device of claim 7, wherein the piezoelectric layer comprises lithium tantalate (LiTaO.sub.3) and the base substrate comprises silicon.

11. The surface acoustic wave device of claim 1, wherein a relative thickness h/λ and/or a metallization ratio w/p of electrode fingers of a transducer structure on the piezoelectric layer, h and w being the relative thickness and width of the electrode fingers, respectively, and p and λ being an electrode pitch and wavelength of the transducer structure, respectively, are such that electromechanical coupling k.sub.s.sup.2 of the longitudinally polarized guided wave in the piezoelectric layer is between 0.5% and 2.5%.

12. The surface acoustic wave device of claim 7, wherein the piezoelectric layer comprises lithium niobate (LiNbO.sub.3) with a crystallographic orientation (YX/t)/θ/90° according to the standard IEEE 1949 Std-176, with θ, an angle of the crystallographic orientation and a propagation direction of the longitudinally polarized guided wave is at 90° of a crystallographic X-axis, so that θ has a value between 35° and 40° or between 100° and 150°.

13. The surface acoustic wave device of claim 7, wherein the piezoelectric layer comprises lithium niobate (LiNbO.sub.3) and the base substrate comprises diamond.

14. The surface acoustic wave device of claim 7, wherein the piezoelectric layer comprises lithium niobate (LiNbO.sub.3) with a crystallographic orientation (YX/t)/θ/Ψ according to the standard IEEE 1949 Std-176, with θ, an angle of the crystallographic orientation being between 40° and 60″, and a propagation direction of the longitudinally polarized guided wave Ψ is between 80° and 100°.

15. The surface acoustic wave device of claim 7, wherein the piezoelectric layer comprises lithium niobate (LiNbO.sub.3) and the base substrate comprises sapphire.

16. A surface acoustic wave filter device, comprising at least one surface acoustic wave device (SAW) according to claim 1.

17. A surface acoustic wave resonator device comprising at least one surface acoustic wave device (SAW) according to claim 1.

18. A method of fabrication of a surface acoustic wave device as described in claim 1, comprising the steps of: a) providing a piezoelectric layer over a base substrate; and b) providing a transducer structure on the piezoelectric layer; wherein, in step a), the piezoelectric layer is provided such that the piezoelectric layer has a crystalline orientation with respect to the base substrate such that a phase velocity of the longitudinally polarized guided wave is below a critical phase velocity of the base substrate at which wave guiding within the piezoelectric layer vanishes.

19. The method of claim 18, wherein at least one of the steps a) orb) comprises a layer transfer process.

20. The method of claim 18, wherein step b) comprises an I-line lithography patterning step to obtain a transducer structure in the surface acoustic wave device allowing a frequency limit of more than 2 GHz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present disclosure may be understood by reference to the following description taken in conjunction with the accompanying figures, in which reference numerals identify features of embodiments of the present disclosure.

(2) FIG. 1a illustrates the cutting angles defined according to the IEEE Std-176 standard.

(3) FIG. 1b shows a surface acoustic wave device according to the present disclosure and illustrating an elastic wave propagation direction.

(4) FIGS. 2a and 2b show a graph of the effective permittivities calculated for two different propagation directions of the SAW device as illustrated in FIG. 1b.

(5) FIGS. 3a-3d illustrate the characteristics of a surface acoustic wave device using a composite substrate according to the present disclosure with a SiO.sub.2 layer of 200 nm.

(6) FIGS. 4a-4d illustrate the characteristics of a surface acoustic wave device using a composite substrate according to the present disclosure with a SiO.sub.2 layer of 500 nm.

(7) FIG. 5 illustrates the variation of the characteristics of the surface acoustic wave device as a function of the thickness of the dielectric SiO.sub.2 layer.

(8) FIGS. 6a-6d illustrate the characteristics of a surface acoustic wave device using a composite substrate according to the present disclosure with a SiO.sub.2 layer of 200 nm, as a function of the relative thickness and of the metallization ratio of the electrodes of the transducer structure.

(9) FIG. 7 illustrates the plot of the phase velocity (m/s) of the guided wave on the left Y axis and the electromechanical coupling coefficient k.sub.s.sup.2 (%) on the right Y axis, as a function of the rotation angle θ of the cut, on the X axis, for both the longitudinal modes and the Rayleigh modes when LiNbO.sub.3 is used as the piezoelectric material and diamond is used as the base substrate of the composite substrate.

(10) FIG. 8 shows a schematic of the steps of the method for manufacturing a surface acoustic wave device according to the present disclosure.

DETAILED DESCRIPTION

(11) FIG. 1a illustrates the cutting angles defined according to the standard IEEE 1949 Std-176 and FIG. 1b illustrates an elastic wave propagation direction in a surface acoustic wave (SAW) device 5 according to the present disclosure.

(12) A crystal cut is defined by two angles of rotation. A cut is defined in standard IEEE 1949 Std-176 by the axes defined by the thickness t and length l of the blank 1 and by angles of rotation about the axes defined by the width w, the length l and the thickness t of the blank 1. The Z-axis is defined by the width w of the blank 1, the X-axis by the length l and the Y-axis by the thickness t. These axes being understood in principle to be those after rotation, i.e., for a cut involving three rotations the rotation ϕ about the Z-axis (w) is first carried out, leading to the X-axis X′(l) and Y-axis Y′(t). The rotation θ about X′(l) is then carried out leading to the rotated Z-axis Z″(w) and the rotated Y-axis Y″(t). The crystal cut would then be noted as (YXwl)/ϕ/θ.

(13) Furthermore, any cut such that ϕ=0 and θ≠0 is called a single rotation cut, noted then (YX/)/θ, and any cut such that ϕ≠0 and θ≠0 is called a double rotation cut.

(14) As illustrated in FIG. 1b, for the propagation of surface acoustic waves 3, a third angle, denoted Ψ, defines the propagation direction of the surface wave 3. The angle is defined by a rotation about the axis Y″. In this case, the crystal cut would be noted as (YX/t)/θ/Ψ.

(15) FIG. 1b illustrates schematically a SAW device 5 comprising a composite substrate 7 according to the present disclosure. The composite substrate 7 comprises a base substrate 9 and a piezoelectric layer 11.

(16) The base substrate 9 is a silicon base substrate with a surface orientation (100). The silicon base substrate 9 has an acoustic SSBW velocity limit on the order of 5650 m/s. Instead of a silicon substrate, sapphire, or, in general, any substrate material showing a shear wave velocity greater than 5600 m/s would be suitable.

(17) For a silicon substrate, a (100) orientation is used but (100) or (010) could also be used. Indeed, when using propagation directions along the X (±10°) or the Z (±10°) crystallographic axis, the SSBW phase velocity criterion are met. Computations based on Green's functions show that this SSBW velocity can reach a value in excess of 5750 m/s for silicon cut (YX/t)/45°/0° or also noted (YX/)/45°. Also, crystal cut defined as (YX/t)/45/90° is found to maximize the SSBW velocity at 5770 m/s.

(18) For sapphire, identical computations have been made, showing that a SSBW velocity of 5750 m/s is the lowest achievable phase velocity. However, the SSBW velocity overcomes 6700 m/s for crystal cut (YX/t)/θ/Ψ with 40°<θ<60° and 80°<Ψ<100°. With such a substrate, it is possible to guide the longitudinal wave excited in a LiNbO.sub.3 layer using adapted layer thicknesses. For instance, considering a longitudinal wave excited on a (YX/t)/41°/90° LiNbO.sub.3 layer of 700 nm onto a SiO.sub.2 layer of 800 nm, both layers being deposited on a (YX/t)/50°/90° sapphire substrate, allows for bulk radiation suppression and the guiding of a longitudinal wave mode with an electromechanical coupling in excess of 6% and potentially up to 10%, considering an operation frequency of 2 GHz.

(19) The piezoelectric layer 11 in this embodiment is a single crystal lithium tantalate (LiTaO.sub.3) with a crystallographic orientation defined as (YX/)/θ according to the standard IEEE 1949 Std-176, with θ, an angle of the crystallographic orientation being between 40° and 65°, more in particular for θ close to 42° or close to 62°. According to a variant of the disclosure, the piezoelectric layer 11 can also be a single crystal lithium niobate (LiNbO.sub.3) layer. The thickness of the piezoelectric layer 11 is on the order of the wavelength λ of the waves 3 of the SAW device 5 or below. Typically it has a thickness in a range between 300 nm and 700 nm, in particular 500 nm, in particular, suitable for an operation in the 2 to 4 GHz frequency range.

(20) According to a variant of the present disclosure, the composite substrate 7 may further comprise a dielectric layer, in particular, a SiO.sub.2 layer (not shown). The dielectric layer may be sandwiched between the base substrate 9 and the piezoelectric layer 11. The presence of the dielectric layer can modify the electrochemical coupling coefficient k.sub.s.sup.2 as well as the temperature stability of the composite substrate 7. This influence depends on the thickness of the layer, which, therefore, represents a parameter to optimize the properties of the composite substrate 7 to obtain the required electrochemical coupling coefficient k.sub.s.sup.2 and the temperature stability of the composite substrate for the desired applications of the SAW device 5. Typically, the thickness of a SiO.sub.2 layer as dielectric layer is less than 800 nm, in particular in a range between 100 nm and 650 nm, more in particular in a range between 600 nm and 650 nm, in particular suitable for an operation in the 2 to 4 GHz frequency range.

(21) The surface acoustic wave device 5 further comprises two transducer structures 21 and 23 on the piezoelectric layer 11. The transducer structures 21 and 23 in this embodiment are the same, but according to a variant, the transducer structures 21 and 23 can be different or only one transducer structure might be present. The surface acoustic wave device 5 can further comprise one or more resonator structures.

(22) The transducer structures 21 and 23 each comprises opposing inter-digitated comb electrodes 25 and 27, each of which has a plurality of electrode fingers. The inter-digitated comb electrodes 25 and 27 and the electrode fingers are formed of any suitable conductive metal, for example, aluminum or aluminum alloy.

(23) An electrical load and/or source potential 29, 31 can respectively be coupled across the inter-digitated comb electrodes 25, 27 of the transducer structures 21, 23, depending upon whether the transducer structures 21, 23 are utilized to excite surface acoustic waves 3 in the composite substrate 7 or to convert received surface acoustic waves to electrical signals, or both. The inter-digitated electrode fingers are then connected to alternating potentials.

(24) The transducer structure 21 and/or 23 excite the surfaces acoustic waves in the electrical field direction, meaning perpendicularly to the extension direction of the electrode fingers of the inter-digitated comb electrodes 25, 27 as shown by the arrow E in the FIG. 1b, thus along the propagation direction defined by Ψ.

(25) The frequency of utilization f.sub.r of the SAW device 5 is defined by f.sub.r=ν/2ρ, ν being the velocity of the acoustic wave and ρ, the electrode pitch of the transducer structures 21, 23, as shown in FIG. 1b. The electrode pitch ρ of the transducer structure is also chosen to be λ/2, λ being the operating wavelength of the acoustic wave. Thus, the electrode pitch ρ defines the frequency of utilization of the transducer structure, and corresponds to the edge-to-edge electrode finger distance between two neighboring electrode fingers connected to different comb electrodes, as illustrated in FIG. 1b. Furthermore, the transducer structure operates at the Bragg condition, defined by ρ=λ/2.

(26) In this embodiment, the inter-digitated electrode fingers typically all have essentially the same length d, width w and thickness h (not shown). According to variants of the present disclosure, they could also be different.

(27) The propagation characteristics of the surface acoustic waves 3 produced by the SAW device 5 include among others propagation phase velocity, electromechanical coupling coefficient k.sub.s.sup.2 and the temperature coefficient of frequency TCF. The propagation phase velocity affects the relationship between the electrode pitch ρ of the transducer structure and the desired frequency of the device.

(28) Furthermore, in case of a use of the SAW device 5 in a filter device application, the bandwidth of the filter Δf is proportional to the electromechanical coupling coefficient k.sub.s.sup.2 considering the empirical relation Δf/f˜(⅔) k.sub.s.sup.2, Δf corresponding to the bandpass width in frequency of the filter, f.sub.r being the resonant frequency of the transducer structure. The TCF is associated with the influence of a temperature to the frequency changes in filters.

(29) The thermal sensitivity is characterized by the TCF.sub.1 and TCF.sub.2 around the ambient temperature T.sub.0=25° C. This definition was proposed first in J. J. Campbell and W. R. Jones, IEEE Trans. Sonics Ultrasonics 15, 209 (1968), for SAW devices and is widely used and known by the skilled person in the art. Its expression reads as follows:
f=f.sub.0×(1+TCF.sub.1(T−T.sub.0)+TCF.sub.2(T−T.sub.0).sup.2)

(30) This expression corresponds to a polynomial development of the temperature-frequency dependence limited to the second degree as generally for SAW on standard devices. TCF.sub.1 and TCF.sub.2 can be accurately obtained by using best fit procedures considering experimental frequency-temperature measurements for a given magnitude/phase point of the transfer function or the reflection coefficient or self-admittance, transadmittance or self-impedance or transimpedance of the filter.

(31) FIGS. 2a and 2b illustrate the effective permittivities calculated for two different orientations of the composite substrate 7, with the piezoelectric layer 11 being a layer of LiTaO.sub.3 with a thickness of 500 nm on a base substrate 9 of silicon (100).

(32) FIG. 2a shows the permittivity for a 42° Y-cut, X-propagation, thus a (YX/)/42° cut, and FIG. 2b the permittivity in case of a rotation of 90° around Y′, thus a (YX/t)/42/90° cut, according to the standard IEEE 1949 Std-176. FIGS. 2a and 2b plot the imaginary part of the effective/relative permittivity on the right Y axis and the real part of the effective/relative permittivity on the left Y axis against the phase velocity in m/s on the X axis.

(33) The relative permittivity of a material is its absolute permittivity expressed as a ratio relative to the permittivity of vacuum. Permittivity is a material property that affects the Coulomb force between two point charges in the material. Relative permittivity is the factor by which the electric field between the charges, located here under the inter-digitated electrode fingers, is decreased relative to vacuum. Likewise, relative permittivity is the ratio of the capacitance of a capacitor using that material as a dielectric, compared with a similar capacitor that has vacuum as its dielectric. Relative permittivity is also commonly known as dielectric constant.

(34) In FIG. 2a, the graph is shown for LiTaO.sub.3 piezoelectric layer 11 with the (YX/)/42° cut. As can be seen, three acoustic modes can propagate in the piezoelectric layer of lithium tantalate (LiTaO.sub.3) layer. The acoustic modes are present at 2800 m/s, 4300 m/s and 5400 m/s. These three modes correspond to the three types of acoustic waves present in the piezoelectric layer: the Rayleigh acoustic wave, the shear wave and the longitudinally polarized mode, respectively.

(35) In the composite substrate 7, the maximum propagation velocity of the mode corresponding to the longitudinally polarized guided wave in the lithium tantalate piezoelectric layer 11 is actually bounded by the shear acoustic wave velocity of the base substrate. In the present case, the later velocity is the one of silicon (100), thus of a value of about 5650 m/s. Above that threshold, the wave is radiated in the base substrate and consequently the corresponding losses dramatically increase, in proportion with the amount of energy radiated from the surface of the lithium tantalate piezoelectric layer 11 to the base substrate.

(36) In FIG. 2b, the graph shown corresponds to a crystalline orientation of the lithium tantalate (LiTaO.sub.3) being a 42° Y-cut, X-propagation with a 90° rotation around Y′, thus a (YX/t)/42/90° cut. As can be seen, only two acoustic modes are now present, the ones at 2800 m/s and at 5400 m/s. The acoustic mode at 4300 m/s, corresponding to the shear waves, has disappeared.

(37) Basically, by choosing the propagation direction at 90° with respect to the crystallographic X axis, the shear wave mode is no longer excited, and only the Rayleigh wave mode and the longitudinally polarized volume mode are propagating inside the piezoelectric layer 11.

(38) Therefore, in the composite substrate 7 according to the present disclosure, with a silicon base substrate (100) 9 and a (YX/)/42° or a (YX/t)/42°/90° cut piezoelectric LiTaO.sub.3 layer 11 of 500 nm, it is possible to guide modes that have a propagation velocity on the order of the base substrate, here namely 5640 m/s inside the piezoelectric layer 11. Due to the special crystalline orientation of the piezoelectric layer 11, the phase velocity is such that it remains below the critical phase velocity at which the longitudinally polarized volume wave would not be confined inside the piezoelectric layer 11 but would vanish into the base substrate 9.

(39) FIGS. 3a to 3d illustrate properties of a composite substrate with a silicon (100) base substrate, a 200 nm thick SiO.sub.2 dielectric layer and a 500 nm thick LiTaO.sub.3 piezoelectric layer.

(40) As already illustrated in FIGS. 2a and 2b, in a composite substrate according to the present disclosure, it is possible to guide modes that have a propagation velocity close but smaller than the SSBW velocity limit of the substrate, here namely 5400 m/s inside the piezoelectric layer. These modes will be referenced to as “high velocity” modes.

(41) Indeed, since the temperature expansion coefficient of silicon is in the vicinity of 2.6 ppm/° C., while, for example, that of (YX/)/42° LiTaO.sub.3 is approximately 16 ppm/° C., the combined temperature expansion coefficient of the SAW device will be generally within the range of 2.6 to 16 ppm/° C., depending upon the thickness of the piezoelectric layer and the stress level at the bonding interface. The effective lowering of the temperature expansion coefficient of the composite substrate results in a reduced temperature coefficient of frequency (TCF) of the transducer structure deposited on top of the composite substrate.

(42) Furthermore, the propagation velocity of this “high velocity” mode is particularly attractive for direct band synthesis, eliminating the frequency multiplication stages necessary to obtain the target value starting from a low (<30 MHz) or intermediaries (between 300 and 600 MHz, for example) frequency source multiplied to reach Ultra High Frequency (UHF) or S or C bands. The maximum frequency achievable with such a mode by using an I-line type of lithography (UV 365 nm) is around 4 GHz; i.e., considering a velocity of 5500 m/s and a wavelength of 1.4 μm, this gives an electrode width of about 350 nm. Therefore, it is possible to directly design the resonator at the final operating frequency using standard SAW-industry technology.

(43) FIGS. 3a to 3c respectively illustrate the phase velocity of the longitudinally polarized guided wave, the electromechanical coupling and the TCF value as a function of the rotation angle θ of the cut of the piezoelectric layer and the propagation direction angle Ψ.

(44) FIG. 3d shows the variations of the propagation velocity (m/s) on the left Y axis and of the TCF (ppm/K) on the right Y axis, for varying angles of crystal orientation θ plotted on the X axis for Ψ=90°. The propagation velocities for both the metallized surface, thus with the transducer structure, and the free surface, without metallic surface, are shown.

(45) FIGS. 3a to 3c illustrate that the three desired parameters, high velocity, TCF close to zero and high electromechanical coupling cannot be optimized at the same time. The frequency is set to 2 GHz. Nevertheless the propagation direction Ψ=900 is favorable to minimize the thermal effects around 0 values in the range of 40° to 65°. In this angular range, electromechanical coupling k.sub.s.sup.2 values of over 1% can be observed.

(46) As can be further seen in FIG. 3d, the variation of the propagation velocity (m/s) and of the TCF (ppm/K) are actually almost in opposition of phase, so that when the propagation velocity (m/s) is at a high value, the TCF is at a high negative value (ppm/K). However, it is possible to select the cut of the crystal orientation in order to find a trade-off between the TCF and the propagation velocity value desired. For example, at around θ=42°, a TCF of about −20 ppm/K and a velocity of over 5600 m/s can be observed or at around θ=62°, a TCF of about −5 ppm/K and a velocity of about 5350 m/s can be observed. Thus, both (YX/t)/42/90° and (YX/t)/62/90° are advantageous crystallographic orientations. With such values, improved SAW devices as frequency sources, for example, filter or resonators, can be obtained for which a stable temperature and sufficiently high electrochemical coupling are necessary. In this particular example, SAW filters with bandpass of the order of 0.1% to 1% can been achieved with improved temperature stability.

(47) FIGS. 4a to 4d illustrate the same parameters as in FIGS. 3a to 3d, but for the case of a SiO.sub.2 layer thickness of 500 nm, also for the frequency of 2 GHz. It is important to notice that the effect of the thickness of the SiO.sub.2 layer is particularly visible in the TCF (FIG. 4c). By thickening the dielectric layer, it becomes thus possible to improve the temperature stability without influencing the electromechanical coupling and the propagation velocity in the same amount.

(48) FIG. 4d, in particular, illustrates that it is even possible to cancel the thermal coefficient of frequency 1 (TCF.sub.1) for several configurations. Indeed, for a crystallographic orientation with θ values close to 50° and close to 70°, the TCF.sub.1 is cancelled while the propagation velocity is around 5400 m/s and 5150 m/s, respectively. However, the minimization of the effects of temperature is obtained for crystalline orientations that do not have the maximum of coupling.

(49) However, the electromechanical coupling is substantially sensitive to a SiO.sub.2 passivation layer deposited on the excitation surface and for SAW devices with a composite substrate, it is possible to reach values close to 8% with phase velocities remaining greater than 5000 m/s.

(50) As can be seen from FIGS. 3a through 4d, a guided wave with a propagation velocity on the order of 5500 m/s, a TCF between −20 and 0 ppm/K with an electrochemical coupling coefficient varying between 0.5 and 2% can be achieved.

(51) FIG. 5 illustrates the variations of the electromechanical coupling coefficient k.sub.s.sup.2 (%) on the left Y axis and of the TCF (ppm/K) on the right Y axis, as a function of the thickness of the passivation or dielectric layer on the X axis, for a surface acoustic wave (SAW) device as described above, i.e., for the case of a LiTaO.sub.3 layer with a (YX/)/42°/90° cut and for the frequency of 2 GHz.

(52) In FIG. 5, one can see that for a thickness of about 630 nm or below, the TCF is cancelled. At this value of 630 nm, the electromechanical coupling coefficient k.sub.s.sup.2 is on the order of 2.1%. Thus, the presence of the dielectric layer maintains the temperature stability while improving the electromechanical coupling.

(53) FIGS. 6a to 6d illustrate the characteristics of a surface acoustic wave device using a composite substrate according to the present disclosure with a SiO.sub.2 layer of 200 nm, as a function of the relative thickness and of the metallization ratio of the electrodes of the transducer structure, at the Bragg condition. The composite substrate here is a 500 nm thick LiTaO.sub.3 layer with a (YX/t)/42°/90° crystal cut, on 200 nm of SiO.sub.2 on silicon (100) base substrate.

(54) In particular, FIG. 6a shows the variation of the phase velocity (m/s), FIG. 6b shows the variation of the electromechanical coupling coefficient k.sub.s.sup.2 (%), FIG. 6c shows the variation of the reflection coefficient (%) (bandwidth of the stop band) and, finally, FIG. 6d shows the variation of the directivity for the high speed mode in such composite substrate. The directivity is defined as the ratio between the amplitude of the resonances at the input and the output of the stop band.

(55) The calculation of the mode characteristics is essential for any SAW component design operation using this “high velocity” mode. In particular, it is important to know the evolution of the propagation velocity, the electrochemical coupling coefficient k.sub.s.sup.2 and the diffraction effects as a function of the shape of the electrodes, namely the relative thickness h/λ and the metallization ratio w/ρ with ρ and λ being the electrode pitch and the wavelength of the electrode array, respectively, and h and w being the thickness and the width of the electrode fingers, respectively. The transducer structure operates at the Bragg condition, so that ρ=λ/2.

(56) The reflection coefficient values achieved, with values up to 1.5, as can be seen in FIG. 6c, are appropriate for the use of the SAW device as resonator.

(57) FIG. 6a shows that phase velocities of the order of 5500 m/s can be reached, and that the phase velocity varies between 5500 m/s and 5400 m/s with increasing relative thickness h/λ of the electrode fingers of the transducer structure.

(58) As can be seen in FIG. 6b, the electromechanical coupling coefficient k.sub.s.sup.2 actually increases with the thickness of the metal. Values of k.sub.s.sup.2 up to 2.5% can be reached. It can be also noted that for a metallization ratio w/ρ above 0.7, in particular, close to 0.8, the high speed mode exhibits an electromechanical coupling close to zero.

(59) According to FIG. 6d, it can be seen that the directivity reaches values up to 6. However, for a metallization ratio w/ρ of the order of 0.5, it can be seen that the directivity is close to zero. This is an important and valuable aspect for the fabrication of devices close to the resolution limit of the technology, for example, when the transducer structure comprises similar electrodes width and similar interelectrode distance.

(60) Although not shown here, the impact of the thickness of the electrode is to lower the TCF when increased.

(61) The influence of the relative thickness h/λ and the metallization ratio w/ρ on the values of the electromechanical coupling coefficient k.sub.s.sup.2, the phase velocity, and the TCF, therefore, represents another parameter to optimize the properties of the composite substrate 7 to obtain the required electrochemical coupling coefficient k.sub.s.sup.2 and the temperature stability of the composite substrate for the desired applications of the SAW device 5.

(62) FIG. 7 illustrates the dispersion curves for the Rayleigh modes and the longitudinal modes of a composite substrate with a diamond base substrate and a 500 nm thick LiNbO.sub.3 piezoelectric layer. The frequency of operation is 2 GHz.

(63) In particular, the phase velocity (m/s) of the guided wave on the left Y axis and the electromechanical coupling coefficient k.sub.s.sup.2 (%) on the right Y axis are plotted as a function of varying angles of crystal orientation θ on the X axis, for a propagation direction Ψ=90° along the crystallographic X-axis, for both the longitudinal modes and the Rayleigh modes, when LiNbO.sub.3 is used as the piezoelectric material and diamond is used as the base substrate of the composite substrate.

(64) A favorable configuration for the use of longitudinal modes with lithium niobate corresponds to cutting angles θ between 100° and 150°, in particular, close to 120 with phase velocity in excess of 7000 m/s together with electromechanical coupling coefficient k.sub.s.sup.2 as high as 20%. For this configuration, the computation of the TCF of this mode with a 4 μm wavelength on a stack composed of (YX/t)/120/90° LiNbO.sub.3 and silicon dioxide SiO.sub.2, 100 nm thick, and semi-infinite C-oriented diamond substrate yields values ranging from −12 to +12 ppm/K for lithium niobate thickness, respectively ranging from 350 to 500 nm.

(65) It can be also noted that for θ between 350 and 40°, in particular, close to the (YX/t)/36/90° cut LiNbO.sub.3, the Rayleigh wave exhibits a minimum coupling, below 0.5% whereas the longitudinal mode reaches a 10% electromechanical coupling coefficient k.sub.s=, making this configuration advantageous for the purpose of spectral purity.

(66) Actually, the operation of the SAW device should be used for angles θ between 35° and 400 or between 100° and 150°, in particular, close to 360 or close to 120°, with a propagation direction at 90° along the crystallographic X-axis, given the fact that for both these configurations, the coupling of the Rayleigh modes is at its minimum, below 0.5% and while an electromechanical coupling coefficient k.sub.s.sup.2 of about 11% is observed for the first configuration, values over 20% can be reached in the second configuration.

(67) FIG. 8 illustrates a schematic of the steps of the method for manufacturing a surface acoustic wave device according to the present disclosure, as illustrated in FIG. 1b.

(68) The method comprises a first part of providing a piezoelectric layer over a base substrate.

(69) To realize this part of the process, a base substrate 510 is provided in step a). The base substrate 510 is a monocrystalline silicon substrate with an orientation (100), or any other substrate material with high acoustic wave propagation velocity, such as diamond, sapphire, silicon carbide or aluminum nitride.

(70) In step a), a handle substrate 530 is also provided that, in this embodiment, is a single crystal piezoelectric substrate. In this embodiment, the handle substrate 530 is LiTaO.sub.3, but lithium niobate (LiNbO.sub.3) could also be used. A predetermined splitting area 550 is provided inside the handle substrate 530 to form a to-be-transferred piezoelectric layer 570 with a thickness t.

(71) Furthermore, in this embodiment, the LiTaO.sub.3 substrate has a (YX)θ cut according to the standard IEEE 1949 Std-176, with θ, an angle of the crystallographic orientation being between 400 and 65°. Here in particular, the LiTaO.sub.3 substrate has a (YX/)/42° orientation. In the variant where LiNbO.sub.3 would be used, the LiNbO.sub.3 layer would then have a (YX/t)/θ/90° orientation, with θ between 350 and 40° or between 1000 and 150°. The predetermined splitting area can be realized by ion implantation as known in the art.

(72) The thickness t of the piezoelectric layer 570 to be transferred is on the order of the operating wavelength of the final SAW device, in particular, smaller than the operating wavelength of the final SAW device. The thickness t is, in particular, in a range of 300 nm to 700 nm, such as 500 nm.

(73) In step b), the handle substrate 530 and the base substrate 510 are attached to sandwich the to-be-transferred piezoelectric layer 570 between the remainder of the handle substrate 590 and the base substrate 510. Attachment can occur byway of bonding.

(74) As known in the art, the piezoelectric layer 570 can be detached from the remainder of the handle substrate 590 by applying energy, in particular thermal or mechanical energy. Detachment occurs at the predetermined splitting area 550.

(75) In the composite substrate 600, as illustrated at step c), the piezoelectric layer 570 is provided such that the piezoelectric layer 570 has a crystalline orientation with respect to the base substrate 510, such that the phase velocity of a longitudinally polarized wave travelling in the piezoelectric layer 570 is below the critical phase velocity SSBW of the base substrate 510 at which wave guiding within the piezoelectric layer 570 vanishes.

(76) In a variant, a thin SiO.sub.2 layer (not shown) can be provided on top of the base substrate 510 prior to providing the piezoelectric layer 570 to improve the coupling while keeping the temperature stability as explained above. The SiO.sub.2 layer may be naturally present on the Si base substrate 510. As a preferred embodiment, the SiO.sub.2 layer has a thickness less than 800 nm, in particular, in a range between 100 to 650 nm, more in particular, in a range between 600 to 650 nm.

(77) Prior to the attachment, additional processing steps can be added, such as polishing of the side of the piezoelectric layer 570 and/or of the side of the base substrate 510 at which attachment will take place.

(78) According to step d), a transducer structure 610 is formed on the piezoelectric layer 570, using a combination of layer deposition and patterning steps. As can be seen in FIG. 3a, the transducer structure 610 comprises two inter-digitated comb electrodes comprising each a plurality of electrode means 630 and 650, respectively. The inter-digitated comb electrodes and their respective electrode means 630 and 650 are formed of a conductive metal, for example, aluminum, or aluminum alloys or molybdenum or tungsten.

(79) According to a particular advantageous embodiment, the inter-digitated comb electrodes are arranged such that the direction of propagation 670 of the longitudinally polarized guided wave is in the direction Ψ=900 to ensure a high propagation velocity together with a high temperature stability as described above.

(80) According to the present disclosure, the patterning step is realized using an I-line lithography. Using the longitudinally polarized guided wave inside the piezoelectric layer 570 allows realizing SAW devices 690 for frequencies above 2 GHz, in particular, above 3 GHz with electrode dimensions that can be imaged using I-line lithography.

(81) The choice of the layers, their material, their thickness and crystal orientation used for a SAW device 5, 690 is made so as to satisfy a certain number of criteria, namely the electromechanical coupling coefficient k.sub.s.sup.2, the temperature coefficient of frequency (TCF) and the acoustic wave propagation velocity of an acoustic wave travelling in the piezoelectric layer 11, 570.

(82) According to the present disclosure, a composite substrate 7, 600 can be obtained that provides a predetermined level of electromechanical coupling of at least 0.5% of up to over 2%, in particular, in the order of up to 2.5% and a temperature stability of less than |20 ppm/K| can be achieved by combining materials with different temperature coefficient of frequency (TCF) and, if necessary, by adding a dielectric layer like SiO.sub.2. For such substrates, higher acoustic wave propagation velocity of the order of 5500 m/s of a guided mode of the longitudinally polarized guided wave can be observed than for devices using the Rayleigh mode. More in particular, a composite substrate can also achieve electromechanical coupling up to 20%, with acoustic wave propagation velocity of the order of 7000 m/s or more of a guided mode of the longitudinally polarized guided wave can be observed.

(83) Using such a composite substrate, the performance of SAW devices as well as their application ranges can be improved compared to bulk piezoelectric substrates by using I-line lithography or any lithography means allowing for high quality electrode patterning with submicron dimensions.

(84) A number of embodiments of the present disclosure have been described. Nevertheless, it is understood that various modifications and enhancements may be made without departing from the scope of the following claims.