Multilayer structure for an elastic wave device

Abstract

The invention relates to a multilayer structure configured for an elastic wave device 11, in particular guided or quasi-guided with losses less than 5mdB / λ, the elastic wave device comprising at least one transducer 19 and operating with an elastic wave having a shear wavelength λ and preferably with a frequency less than 1 GHz, in particular with a frequency between 500 MHz and 1 GHz, the multilayer structure comprising: a piezoelectric layer 3, a dielectric layer 5, and a base substrate 7, the piezoelectric layer 3 being made of lithium tantalate LiTaO3 (LTO), the dielectric layer 5 being made of silicon dioxide (SiO2), and the base substrate 7 being a silicon substrate, in particular silicon Si(100) or Si(111), characterized in that the piezoelectric layer 3 has an orientation (YXl) / θ with θ between 30° and 50°, in particular θ between 42° and 50°, even more preferably θ = 30°, θ = 36°,of θ = 42° or θ = 50° according to the IEEE 1949 std-176 standard definition, and the product f×hLTO of the frequency f and the thickness hLTO of the dielectric layer 3 is preferably between 0.7 GHz.µm and 1.5 GHz.µm, preferably between 1 GHz.µm and 1.3 GHz.µm, and hSiO2 is less than 250 nm, preferably less than 150 nm, hSiO2 being the thickness of the dielectric layer. Fig. 1,

Description

Title of the invention: Multilayer structure for an elastic wave device

[0001] The invention relates to multilayer structures for an elastic wave device and methods for manufacturing such structures.

[0002] Devices using elastic waves have seen their uses increase significantly for various applications such as filtering, signal manipulation and processing, or even surveying.

[0003] In this context, multilayer structures of the piezoelectric-on-insulator or POI type (from the Anglo-Saxon term "piezoelectric-on-insulator") on which interdigitated electrode combs are then arranged have been developed in order to improve the performance of these devices.

[0004] According to the object of the present invention, the aim is to reduce the sensitivity of the devices to temperature variations.

[0005] The object of the invention is realized by a multilayer structure configured for an elastic wave device, in particular guided or quasi-guided with losses less than 5mdB / X, the elastic wave device comprising at least one transducer and operating with an elastic wave having a shear wavelength X and preferably with a frequency less than 1 GHz, in particular with a frequency between 500 MHz and 1 GHz, more particularly between 600 MHz and 900 MHz, the multilayer structure comprising: a piezoelectric layer, a dielectric layer, and a base substrate, the piezoelectric layer being lithium tantalate LiTaO3 (LTO), the dielectric layer being silicon dioxide (SiO2) and the base substrate being a silicon substrate, in particular silicon Si(100) or Si(111), characterized in that, 1 the piezoelectric layer (3) has an orientation (YXZ) / 0 with 0 between 30° and 50°, in particular 0 between 42° and 50°,even more preferably of 0 = 30°, 0 = 36°, 0 = 42° or 0 = 50° according to the standard definition IEEE 1949 std-176, and the product fxhLT0 of the frequency f and the thickness hLT0 of the dielectric layer is preferably between 0.7 GHz.pm and 1.5 GHz.pm, preferably between 1 GHz.pm and 1.3 GHz.pm, and hSiO2 is less than 250 nm, preferably less than 150 nm, hSiO2 being the thickness of the dielectric layer.

[0006] Within this parameter range, it becomes possible to achieve coupling exceeding 10% and reduced sensitivity to temperature variations. Furthermore, for fxhLTO between 1 GHz·pm and 1.3 GHz·pm, the coupling remains high, the sensitivity to temperature variations is reduced, and the device frequency changes only slightly with variations in the thickness of the piezoelectric layer.

[0007] According to one embodiment, the thickness of the piezoelectric layer is determined such that the CTF1 coefficient becomes zero for a given value fxhLTO. Thus, the temperature sensitivity is further reduced.

[0008] According to one embodiment, the multilayer structure may comprise two interdigitated electrode combs with aluminum comb fingers having electrode comb finger thickness hAi, electrode comb finger width a and a pitch p between two directly adjacent fingers belonging to different interdigitated electrode combs, in which hAi / X is fixed between 0.03 and 0.09, in particular between 0.04 and 0.08 and, a / p is fixed between 0.35 and 0.5, in particular a / p is between 0.375 and 0.475.

[0009] Thus it becomes possible to improve the value of the coupling.

[0010] According to one embodiment, hLT0 may be equal to or greater than 600 nm, in particular equal to or greater than 750 nm or equal to or greater than 1 pm and / or in which hSiO2 is equal to or greater than 600 nm, in particular equal to or greater than 1 pm. Thus it becomes possible to work in the frequency range from 600 to 900 MHz.

[0011] According to one embodiment of the invention, the multilayer structure may include a trapping layer, in particular a polycrystalline silicon layer, intercalated between the dielectric layer and the base substrate. This trap-rich layer improves the insulating properties of the multilayer structure and, consequently, the performance of the device.

[0012] According to one embodiment, the silicon substrate has a resistivity greater than 2 kΩ.cm and advantageously greater than 4 kΩ.cm. Thus, leakage currents in the silicon can be reduced so as to reduce mode losses.

[0013] According to a variant of the invention, the thickness of the trapping layer can be between 500 nm and 1 pm. In this thickness range, the insulation effect is satisfactory for most applications, particularly if the silicon substrate has a resistivity greater than 4 kΩ.cm.

[0014] The objects of the invention are also achieved by the method of manufacturing a substrate as described above, comprising the steps of: implanting ions into a donor substrate comprising a piezoelectric layer of lithium tantalate to create a weakened zone within the lithium tantalate layer; assembling the donor substrate with the main face of the lithium tantalate layer with a base substrate via a dielectric layer; and detaching the piezoelectric layer at the weakened zone from the rest of the donor substrate. This method makes it possible to produce the multilayer structures according to the invention with a high yield.

[0015] According to one embodiment of the invention, the thickness of the transferred layer can be greater than or equal to 600 nm, preferably greater than or equal to 750 nm. Using rather thick LTO layers makes it possible to exploit the low-frequency region, in particular between 500 MHz and 1 GHz.

[0016] According to a variant of the invention, the thickness of the transferred layer can be less than or equal to 1 pm, preferably less than or equal to 500 nm. Layer transfer by ion implantation makes it possible to access this range of thicknesses with a thickness homogeneity on the order of ±35 nm, or even ±20 nm or better.

[0017] According to one embodiment of the invention, the detachment step can be followed by epitaxial, in particular homoepitaxial, growth of a piezoelectric layer on the transferred layer. Thus, piezoelectric layers with a desired thickness can be obtained even if the thickness exceeds that achievable by a layer transfer process.

[0018] Preferably the base substrate has a resistivity greater than 2 kQ.cm and advantageously greater than 4 kQ.cm.

[0019] With this process it becomes possible to manufacture multilayer structures according to the invention with less sensitivity to variations in thickness of the piezoelectric layer and / or the dielectric layer.

[0020] According to one embodiment, the determination of the homogeneity of the piezoelectric layer and / or the dielectric layer can be carried out by ellipsometry.

[0021] The objects of the invention are also achieved with a surface and / or bulk elastic wave device, in particular a filter or resonator, comprising a multilayer structure as described above. The elastic wave device is thus characterized in that the coupling of the shear mode is improved compared to its use with known substrates while reducing the impact of the Rayleigh mode. The elastic wave device is therefore also characterized by reduced sensitivity to variations in the thickness of the piezoelectric layer, the dielectric layer, or the electrode layer.

[0022] The objects of the invention are also achieved with an acoustic device comprising at least two elastic wave devices as described above, operating at different wavelengths, in particular elastic shear waves in a wavelength range from 4 pm to 8 pm, and whose transducers of the at least two elastic wave devices are made on the same multilayer structure. By using a multilayer structure according to the invention, it becomes possible to use a single type of substrate for different wavelength ranges while working with high coupling for the shear mode and less than 1% coupling for the Rayleigh mode.

[0023] The invention will be better understood and other advantages will become apparent upon reading the following description, given by way of non-limiting example, and with reference to the accompanying figures, among which:

[0024] Figure 1 is a diagram illustrating a multilayer structure configured for a elastic wave device according to a first embodiment of the invention.

[0025] Figure 2 illustrates a method for transferring a piezoelectric layer according to the invention.

[0026] Figure 3 illustrates CTFi, the temperature coefficient of the first-order frequency. ppm.K 1 on the right ordinate axis and the shear wave velocity v in m / s on the left ordinate axis in an elastic wave device as a function of the product fxhLT0 of the shear wave frequency f and the thickness hLTO in GHzxpm for different thicknesses hSiO2 from 25nm to 225 nm.

[0027] Figure 4 illustrates the k2 coupling in % on the right-hand ordinate axis and the shear wave velocity v in m / s on the left ordinate axis in an elastic wave device as a function of the product fxhLTO of the shear wave frequency f and the thickness hLT0 in GHzxpm for different thicknesses hS;o2 from 25 nm to 225 nm.

[0028] Figure 5 illustrates an embodiment with four elastic wave devices 11_1 to 11_4 with four different operating wavelengths on the same multilayer structure.

[0029] Fig. 1 illustrates a multilayer structure 1 for an elastic wave device 11, such as a filter or resonator.

[0030] The multilayer structure 1 comprises a piezoelectric layer 3, a dielectric layer 5 and a base substrate 7.

[0031] The piezoelectric layer 3 is made of lithium tantalate LaTiO3 (LTO) with an orientation (YXZ) / 0 according to standard IEEE 1949 std-176. The piezoelectric layer 3 has a thickness hLT0. Preferably 0 is between 30° and 50°, in particular between 42° and 50°, even more preferably between 30°, 36°, 42° or 50° according to standard definition IEEE 1949 std-176.

[0032] The dielectric layer 5 is made of silicon dioxide (SiO2). The dielectric layer 5 has a thickness hSiO2.

[0033] The basic substrate 7 is a silicon wafer, in particular single-crystal silicon with a surface orientation along the (111) direction. According to one embodiment, the substrate may have a surface orientation along the (100) direction. Preferably, the resistivity of the basic substrate is greater than 2 kΩ·cm and advantageously greater than 4 kΩ·cm.

[0034] According to an alternative, a trapping layer can be arranged between the dielectric layer 5 and the base substrate 7. This layer can be a polycrystalline silicon layer. This layer can have a thickness ranging from 500 nm to 1 pm.

[0035] Two electrode combs 13 and 15 are arranged on the surface 17 of the piezoelectric layer. The comb fingers 13a, 13b, 13c of the electrode comb 13 are interdigitated with the comb fingers 15a, 15b, 15c of the electrode comb 15 to form a transducer 19.

[0036] The electrode combs 13 and 15 are made of aluminum or an aluminum-based alloy, such as AlCu (0.5 to 4% Cu), AlTi, or AlSi. The fingers 13a-13c and 15a-15c have a width a and a height hAi. The fingers 13a-13c of the first comb 13 are spaced from the directly adjacent fingers 15a-15c of the second comb 15 by a pitch p.

[0037] The elastic wave device 11 operates preferentially in this way at a wavelength X = 2p, according to the Bragg condition.

[0038] The multilayer structure 1 can be manufactured by a layer transfer process from a donor substrate to a support substrate, here the base substrate 7 with the dielectric layer 5.

[0039] Such a method for transferring the piezoelectric layer 3 is illustrated in [Fig. 2]. In step A, it involves providing a donor substrate 21 composed of an assembly of a thick layer 23, typically with a thickness between 5 and 400 µm, formed of the piezoelectric material, here lithium tantalate, and a manipulator substrate 25. The coefficient of thermal expansion of the manipulator substrate 25 is similar to that of the base substrate 7 with the dielectric layer 5, or at least closer to that of the base substrate 7 than the coefficient of thermal expansion of the thick layer 23 with respect to the base substrate 7. Here, silicon is preferably used for the manipulator substrate 25, as with the base substrate 7. The preparation of such a donor substrate 21 is known from WO2019002080A1 or WO2019186032A1. To carry out the invention, the correct orientation 0 is first chosen for the thick layer 25.

[0040] Next, in step B, light species 27, in particular hydrogen ions H+ or helium ions He2+, are introduced into the thick layer 23 to generate a weakening plane 29 and define the piezoelectric layer 3 between the weakening plane 29 and the main free face 31 of the thick layer 23. This step is known as ion implantation.

[0041] Then, during step C, the main free face 31 of the thick layer 23 is assembled with the base substrate 7 via the dielectric layer 5 present on a main face 33 of the base substrate 7.

[0042] Finally, during step D, the detachment of the piezoelectric layer 3 at the level of the embrittlement plane 29 is obtained by an input of mechanical energy and / or thermal to obtain the multilayer structure 1. The thickness hLT0 of the transferred piezoelectric layer 3 is preferably greater than or equal to 600 pm, in particular 750 nm and preferably less than or equal to 1 pm.

[0043] The process described above can be repeated with the manipulator substrate 25 and the remainder of the thick layer 23' still present on the manipulator substrate 25.

[0044] According to an alternative, the manufacturing process may include an epitaxial growth step, preferably homoepitaxial, to increase the thickness hLT0 of the transferred piezoelectric layer 3. Thus, the desired thickness can be obtained even if the upper limits of the process for the thickness of the transferred layer are exceeded.

[0045] According to another alternative, a layer-free process using a embrittling layer can be used. In this alternative, a lithium tantalate piezoelectric substrate is bonded to a base substrate 7 via a dielectric layer 5. Then, one or more thinning steps are carried out, for example by using one or more of the following methods: grinding, chemical-mechanical polishing (CMP), local thickness adjustment, and potentially reactive and plasma-assisted dry etching, in order to obtain the desired thickness.

[0046] Figure 3 illustrates CTFi, the temperature coefficient of the first-order frequency. ppm.K 1 on the right ordinate axis and the shear wave velocity v in m / s on the left ordinate axis in an elastic wave device 1 as a function of the product fxhLT0 of the shear wave frequency f and the thickness hLTO in GHzxpm for different thicknesses hSiO2 from 25nm to 225 nm.

[0047] Figure 4 illustrates the coupling k2 in % on the right-hand ordinate axis and the velocity of the shear wave v in m / s on the left ordinate axis in an elastic wave device 1 as a function of the product fxhLTO of the frequency f of the shear wave and the thickness hLT0 in GHzxpm for different thicknesses hS;o2 from 25 nm to 225 nm.

[0048] A multilayer structure 1 with a Si (111) base substrate, defined as (XZwlt) / -45° / -54.7° / 45°, with the flat (fiat in Anglo-Saxon terminology) of the substrate at 45° to that of the LTO layer, and a 1 pm polycrystalline silicon trapping layer was used for the simulation. The orientation of the LTO layer was 0 = 42°. The numerical simulation was performed using the method described in S. Ballandras et al., “Finite element analysis of periodic piezoelectric transducers”, Journal of Applied Physics 93, 702 (2003).

[0049] For the calculations the thickness of the piezoelectric layer 3 was 1 pm, the frequency varied between 0.3 GHz and 2.5 GHz.

[0050] The simulations were carried out taking into account the effective permittivity of the surface derived from the Green's function of the surface. In other words, only the shear basis guided by the stacking of materials in the multilayer structure 1 was considered.

[0051] For SiO2, the modified material constants TC11 = 4.78*10⁴ K⁻¹, TC12 = 1.168*10³ K⁻¹ and TC44 = 3.02*10⁴ K⁻¹ were used. For Si, the modified material constants TC11 = -5.40*10⁵ K⁻¹, TC12 = -7.33*10⁵ K⁻¹ and TC44 = -4.20*10⁵ K⁻¹ were used.

[0052] As a first approximation, the change in frequency of an elastic wave device 11 as illustrated in [Fig. 1] as a function of temperature is Af / f (T) = CTFi(T-To), with CTFi (TCF in Anglo-Saxon terminology) the temperature coefficient of the first order frequency and To the reference temperature set at 25°C corresponding to so-called ambient conditions.

[0053] In [Fig. 3], curves 40 to 44 illustrate the shear wave velocity v as a function of f*hLTO for hSiO2 thicknesses of 225 nm, 175 nm, 125 nm, 75 nm, and 25 nm, respectively. Curves 45 to 49 illustrate CTFi, the temperature coefficient of the first-order frequency, in ppm.K, on ​​the right-hand y-axis as a function of f*hLTO for hSiO2 thicknesses of 225 nm, 175 nm, 125 nm, 75 nm, and 25 nm, respectively.

[0054] In [Fig. 4], curves 40 to 44 illustrate the velocity v as a function of f*hLT0 for hSiO2 thicknesses of 225 nm, 175 nm, 125 nm, 75 nm and 25 nm respectively, as in [Fig. 3]. Curves 55 to 59 illustrate the coupling k2 as a function of f*hLTO for hSiO2 thicknesses of 225 nm, 175 nm, 125 nm, 75 nm and 25 nm respectively.

[0055] Figure 4 illustrates that the k2 coupling exceeds 10% for values ​​of fxhLT0 rather below 1.5 GHz.pm for the different thicknesses of the dielectric layer and typically reaches its maximum between 0.7 and 1 GHz.pm, while the velocity stability, i.e., the region with the derivative dv / d(fxhLT0) is low or even zero, is towards values ​​of fxhLT0 rather than above 1 GHz.pm. Thus, couplings greater than 10% with velocities that change only slightly as a function of fxhLT0 are achievable for values ​​of fxhLT0 above 1 GHz.pm, particularly for values ​​between 1 and 1.3 GHz.pm. This reduces the sensitivity to variations in the thickness of the piezoelectric layer and lessens the constraints on the homogeneity of the thickness during manufacturing.

[0056] Figure 3 illustrates that the CTFireste remains below 10 ppm.K1 for fxhLT0 above 1 GHz.pm, particularly for values ​​between 1 and 1.3 GHz.pm, and remains even less than 10 ppm.K 1 for fxhLTO between 0.7 and 1 GHz.pm for dielectric layer thicknesses 5 less than 150 nm, especially 125 nm, see curves 47, 48 and 49.

[0057] It even becomes possible to choose the thickness of the dielectric layer 5 such that CTFi is zero for a given value fxhLT0.

[0058] This makes it advantageous to be able to reduce the undesirable effects of thermal variations when using the device in an industrial context while maintaining a high k2 coupling and speed stability with respect to variations in the thickness of the piezoelectric layer 3.

[0059] Similar values ​​are obtained when the piezoelectric layer 3 has an orientation (YXZ) / 0 with 0 between 30° and 50°, in particular 0 between 42° and 50°, even more preferably 0 = 30°, 0 = 36°, or 0 = 50° according to the standard definition IEEE 1949 std-176. The contribution of the Rayleigh mode or the mode due to the presence of the trapping layer remains small for these angles 0.

[0060] Figure 5 illustrates an embodiment with four elastic wave devices. 11_1 to 11_4 with four different operating wavelengths on the same multilayer support 1. Thus, the multilayer support 1 and the parameters a and p of the transducer 19 are chosen such that elastic wave devices 11_1 to 11_4 work with wavelengths between 4 pm and 8 pm allow operation between 500 MHz and 1 GHz.

[0061] Thus, for different spacing p between adjacent electrodes of interdigitated combs, and therefore for different shear wavelengths X, the same multilayer structure can be used even if it is used for a shear wavelength X different from that for which it was initially designed. The only drawback to using a multilayer structure at a frequency f different from that for which it was designed is a relative decrease in the coupling coefficient.

Claims

Demands

1. A multilayer structure configured for an elastic wave device (11) in particular guided or quasi-guided with losses less than 5mdB / X, the elastic wave device comprising at least one transducer (19) and operating with an elastic wave with a shear wavelength X and with a frequency less than 1GHz, in particular with a frequency between 500 MHz and 1 GHz, the multilayer structure comprising: a piezoelectric layer (3), a dielectric layer (5), and a base substrate (7), the dielectric layer being interposed between the base substrate and the piezoelectric layer;the piezoelectric layer (3) being made of lithium tantalate LiTaO3 (LTO), the dielectric layer (5) being made of silicon dioxide (SiO2) and the base substrate (7) being a silicon substrate, in particular silicon Si(100) or Si(111), characterized in that the piezoelectric layer (3) has an orientation (YXZ) / 0 with 0 between 30° and 50°, in particular 0 between 42° and 50°, even more preferably 0 = 30°, 0 = 36°, 0 = 42° or 0 = 50° according to the standard definition IEEE 1949 std-176, and the product fxhLT0 of the frequency f and the thickness hLT0 of the piezoelectric layer (3) is between 0.7 GHz.pm and 1.5 GHz.pm, preferably between 1 GHz.pm and 1.3 GHz.pm, and hSiO2 is less than 250 nm, preferably less than 150 nm, hSiO2 being the thickness of the dielectric layer (5).;

2. The multilayer structure according to claim 1 comprising two interdigitated electrode combs (13, 15) with comb fingers (13a-13c, 15a-15c) of Aluminium with a thickness of electrode comb fingers (13a-13c, 15a-15c) hAi, a width of electrode comb fingers (13a-13c, 15a-15c) a and a pitch p between two directly adjacent fingers (13a, 15a) belonging to different interdigitated electrode combs (13, 15), in which hAi / X is between 0.03 and 0.09, in particular between 0.04 and 0.08 and, a / p is between 0.35 and 0.5, in particular a / p is between 0.375 and 0.

475.

3. The multilayer structure according to claim 1 or 2, wherein hLT0 is equal to or greater than 600 nm, in particular equal to or greater than 750 nm or equal to or greater than 1 pm.

4. The multilayer structure according to any one of claims 1 to 3 wherein a trapping layer, in particular a polycrystalline silicon layer, is intercalated between the dielectric layer (5) and the base substrate (7) with in particular a thickness between 500 nm and 1 pm.

5. The multilayer structure according to any one of claims 1 to 4, wherein the resistivity of the base substrate (7) is greater than 2 kQ.cm and advantageously greater than 4 kQ.cm.

6. A method for manufacturing the multilayer structure according to any one of claims 1 to 5 comprising the steps of: - implanting ions into a donor substrate comprising a piezoelectric layer of lithium tantalate to create a weakening zone inside the lithium tantalate layer, - assembling the donor substrate with the main face of the lithium tantalate layer with a base substrate via a dielectric layer, and - detaching the piezoelectric layer at the weakening zone from the rest of the donor substrate.

7. A manufacturing method according to claim 6, wherein the thickness of the transferred layer is greater than or equal to 600 nm, preferably greater than or equal to 750 nm.

8. A manufacturing method according to claim 6 or 7, wherein the thickness of the transferred layer is less than or equal to 1 pm.

9. A manufacturing method according to any one of claims 6 to 8, the detachment step is followed by epitaxial growth of a piezoelectric layer on the transferred layer.

10. Surface and / or volume elastic wave device, in particular a filter or resonator, comprising a multilayer structure according to any one of claims 1 to 5.

11. Acoustic device comprising at least two elastic wave devices according to claim 10 which operate at different wavelengths, in particular elastic shear waves in a wavelength range from 4 pm to 8 pm and whose transducers of at least two elastic wave devices are made on the same multilayer structure.