Surface acoustic wave device comprising thin layer of metallic material

Abstract

A surface wave device (200), the surface wave device comprising a substrate (210); a piezoelectric layer (240) over the top surface of the substrate; a pair of electrodes (250A, 250B) in contact with the piezoelectric layer, the two electrodes including fingers (252A, 252B) extending in the same direction so as to form a periodic structure in which the fingers of the two electrodes alternate with each other, and having an interdigital distance (2p) separating centers of two adjacent fingers of the same electrode; a metal layer (220) interposed between the substrate and the piezoelectric layer; and a dielectric layer (230) interposed between the metal layer and the piezoelectric layer, wherein the metal layer has a thickness of 5 nm to 100 nm and the dielectric layer has a thickness of 25 nm to 600 nm.

Description

Technical Field

[0001] The field of the invention is that of surface acoustic wave (SAW) devices having a composite structure comprising a thin layer of piezoelectric material resting on a semiconductor substrate. Background Art

[0002] Surface acoustic wave devices or SAW devices are used in a wide range of applications, particularly in electronics where they form core elements of filters, oscillators, delay lines and transformers.

[0003] Piezoelectric materials generate voltage when deformed by mechanical stress, and deform conversely when voltage is applied.

[0004] As a result, when an alternating electrical signal is applied to one or more electrodes in contact with the piezoelectric material, a corresponding mechanical signal (ie, oscillation or vibration) is generated at the piezoelectric material: the electrical signal is converted into a mechanical signal.

[0005] The mechanical signal converted into the piezoelectric material exhibits a frequency dependence on the alternating electrical signal that is based on the characteristics of the electrodes, the properties of the piezoelectric material, and other factors such as the shape of the acoustic wave device and other structures that make up the device.

[0006] Surface wave devices exploit this frequency dependence to provide one or more functions by means of surface acoustic wave (SAW) resonators or SAW transducers, which are increasingly used to form so-called "SAW filters" implemented, for example, in the transmission and reception of RF signals in telecommunication applications.

[0007] A SAW filter comprises at least one SAW transducer, possibly connected to other transducers, to perform a filtering function between an input port and an output port.

[0008] The SAW filter generally includes an input SAW transducer formed on the same piezoelectric element, the input SAW transducer generating a surface acoustic wave from an incoming electrical signal and an output SAW transducer receiving the surface acoustic wave and converting it into an outgoing electrical signal.

[0009] The geometry and size of the transducer and the type and shape of the material used determine the SAW filter characteristics such as coupling and reflection factors, Q quality factor of resonance or antiresonance, bandwidth, parasitic responses, suppression of higher order resonances and temperature dependence.

[0010] Patent US10,938,367B2 discloses an interdigital SAW transducer (IDT) 100, such as Figure 1 As shown, A) is a top view, and B) is a cross-sectional view along the XX' plane.

[0011] The interdigital SAW transducer 100 includes a piezoelectric layer 140 resting on a substrate 110 , a pair of electrodes 150A and 150B in contact with a surface of the piezoelectric layer 140 , a metal layer 120 interposed between the substrate 110 and the piezoelectric layer 140 , and a dielectric layer 130 interposed between the metal layer 120 and the piezoelectric layer 140 .

[0012] The electrode 150A and the electrode 150B include fingers 152A and 152B, respectively, extending in the same direction D so as to form a periodic structure with a period of 2p in a direction perpendicular to the direction D, wherein the fingers of the two electrodes are alternately placed in a conventional manner.

[0013] The dielectric and metallic layers inserted between the piezoelectric layer and its substrate improve the behaviour of the transducer and more particularly limit the appearance of parasitic responses, inductive losses related to the properties of the substrate and interface effects within the stack.

[0014] However, the information disclosed by US Pat. No. 10,938,367 B2 is still insufficient for practical applications requiring special features and / or high performance levels of SAW transducers. Summary of the invention

[0015] It is an object of the present invention to characterize surface wave devices in such a way that, in addition to the simple operating principles of the prior art, they are provided with sufficient operating parameters to implement them in practical applications.

[0016] To this end, the present invention relates to a surface acoustic wave device, which includes a substrate; a piezoelectric layer above the top surface of the substrate; a pair of electrodes in contact with the piezoelectric layer, the two electrodes including fingers extending in the same direction to form a periodic structure in which the fingers of the two electrodes alternate with each other and have an interdigital distance separating the centers of two adjacent fingers of the same electrode; a metal layer inserted between the substrate and the piezoelectric layer; and at least one dielectric layer inserted between the metal layer and the piezoelectric layer, wherein the metal layer has a thickness between 5nm and 100nm and the dielectric layer has a thickness between 25nm and 600nm.

[0017] This device represents the result of a compromise solution, suitable for implementation as an acoustic wave device that benefits from the positive electromagnetic shielding effect of the metal layer inserted between the piezoelectric layer and its substrate, while maintaining excellent phase velocity, reflection coefficient and electromechanical coupling coefficient k in the device structure. S 2 aspects of performance.

[0018] Other non-limiting features according to the invention (alone or in any technically feasible combination):

[0019] - the metal layer may have a thickness between 0.25% and 5% of the interdigital distance;

[0020] - the dielectric layer may have a thickness between 250 nm and 400 nm;

[0021] - the dielectric layer may have a thickness greater than five times the thickness of the metal layer;

[0022] - The dielectric layer can be less than 200nm thick;

[0023] - an optional dielectric layer may be inserted between the substrate and the metal layer;

[0024] - the dielectric layer and the optional dielectric layer may each be less than 300 nm thick;

[0025] - the sum of the dielectric layer thickness and the optional dielectric layer thickness is less than 200 nm;

[0026] - the ratio of the optional dielectric layer thickness to the sum of the dielectric layer thickness and the optional dielectric layer thickness may be between 15% and 30%;

[0027] - the piezoelectric layer may comprise a juxtaposition of a lithium tantalate LiTaO3 layer and a lithium niobate LiNbO3 layer; and

[0028] The metal layer comprises metallized surfaces separated by a distance less than or equal to the interdigital distance.

[0029] The present invention extends to filter devices including surface acoustic wave devices.

[0030] The invention also relates to a method for manufacturing the device, which method comprises a direct bonding step. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other features and advantages of the present invention will become apparent from the following detailed description of the invention with reference to the accompanying drawings, in which:

[0032] [ Figure 1 ] Figure 1 A known interdigital SAW transducer is schematically shown in plan view and in cross section;

[0033] [ Figure 2 ] Figure 2 An interdigital SAW transducer according to the present invention is schematically shown in plan view and in cross section;

[0034] [ Figure 3 ] Figure 3 Includes two graphs showing the effect of metal layers in an interdigital SAW transducer;

[0035] [ Figure 4 ] Figure 4Including showing Figure 2 Four graphs of characteristic quantities of the transducer in as a function of the thickness of the metal layer 220;

[0036] [ Figure 5 ] Figure 5 Includes four graphs, each showing Figure 2 Similar conductance and admittance of structures as a function of facing metal surfaces;

[0037] [ Figure 6 ] Figure 6 Included is shown Figure 2 Six graphs of the harmonic admittance of the transducer for different dielectric layer thicknesses;

[0038] [ Figure 7 ] Figure 7 Including showing Figure 2 Four graphs of characteristic quantities of the transducer in as a function of the thickness of the dielectric layer 230;

[0039] [ Figure 8 ] Figure 8 Shows Figure 2 An alternative transducer structure having an optional dielectric layer 330;

[0040] [ Fig. 9 ] Fig. 9 Shown includes corresponding to Figure 2 or Figure 8 An acoustic wave device with two transducers;

[0041] [ Fig.10 ] Fig.10 Included is shown Figure 8 six graphs of harmonic admittance of the transducer for different thickness combinations of dielectric layers 230 and 330; and

[0042] [ Fig.11 ] Fig.11 Shows Figure 2 and Figure 8 A method for manufacturing a device.

[0043] [ Fig.12 ] Fig.12 A discontinuous metal layer is shown. DETAILED DESCRIPTION

[0044] The inventors of the present invention started with a general interdigital SAW transducer structure and performed extensive numerical modeling to determine certain parameters that critically affect the performance of an interdigital SAW transducer and to define design rules for such a transducer.

[0045] Structure 1 - Single Dielectric Layer

[0046] Figure 2 Shown are A) a plan view of a SAW interdigital transducer 200 according to the present invention, and B) a cross section of the transducer in a plane passing through line segment YY' and perpendicular to the plan view.

[0047] The interdigital SAW transducer 200 includes a piezoelectric layer 240 resting on a substrate 210; a pair of electrodes, the pair of electrodes including a first electrode 250A and a second electrode 250B, each of which is in contact with a surface of the piezoelectric layer 240 and is located on top of the piezoelectric layer, so that the piezoelectric layer 240 is inserted between the substrate 210 and the pair of electrodes; a metal layer 220, which is inserted between the substrate 210 and the piezoelectric layer 240; and a dielectric layer 230, which is inserted between the metal layer 220 and the piezoelectric layer 240.

[0048] In this example, the piezoelectric layer 240 is in direct contact with the dielectric layer 230 , the dielectric layer 230 is in direct contact with the metal layer 220 , and the metal layer 220 is in direct contact with the substrate 210 .

[0049] Electrodes 250A and 250B respectively include fingers 252A and 252B extending in the same direction D so as to form a periodic structure with a period p in a direction perpendicular to direction D, wherein the fingers of the two electrodes are placed alternately, as particularly seen in B), so as to form a regular interdigitated structure.

[0050] The wavelength λ of the acoustic wave of the operating mode is equal to the period 2p and, without limitation, the transducer operates under Bragg conditions for this specific wavelength (the electrode period p of the transducer is half the wavelength λ).

[0051] This period 2p is understood to be the distance separating the central extension axes of two adjacent fingers of the same electrode (ie the axes each forming an axis of symmetry of the corresponding finger), this axis being parallel to the direction D of extension of the fingers.

[0052] The substrate 210 is preferably made of silicon, and even more preferably of high acoustic quality silicon, but may also be made of glass or ceramic or another semiconductor material, for example.

[0053] Preferably, the metal layer 220 and the electrodes 250A and 250B are independently made of a light metal such as aluminum or an aluminum alloy such as Al-Cu, Al-Si or Al-Ti that is considered to be a good electrical conductor in order to limit the influence of mass loading effects and resistive losses on the frequency response of the transducer.

[0054] However, due to its relatively low melting point, aluminum may cause complications during transducer fabrication, particularly when heat treatments are applied at temperatures above the melting point, for example to allow a LiTaO 3 piezoelectric layer to recover its piezoelectric properties.

[0055] In this case, metals heavier than aluminum can be used, although they have a negative impact on the loading effect: molybdenum, tungsten, platinum or titanium, but also chromium, copper and nickel; alternatively, scandium and vanadium or even conductive carbon can be used because of their favorable density.

[0056] It should be understood that metal layer 220 can be made of a different material than the material that makes up electrodes 250A and 250B. In addition, the metal layer is optionally contacted to, for example, a fixed potential (such as ground). Such contact requires additional manufacturing steps, but makes the layer less sensitive to ambient RF signals.

[0057] The dielectric layer 230 is composed of a dielectric material such as silica, preferably silicon dioxide SiO 2 .

[0058] Other materials are also contemplated, such as ZrO2, Ta2O5, Si3N4, and combinations of these materials.

[0059] Progressive combinations of SiO2 and Si3N4 are also possible in the form of SiON (silicon oxynitride).

[0060] These dielectric materials can be in any crystalline form, for example, polycrystalline or amorphous, as obtained using standard microelectronics deposition methods.

[0061] The piezoelectric layer 240 is preferably composed of lithium tantalate LiTaO3, lithium niobate LiNbO3, or a juxtaposition of a lithium tantalate LiTaO3 layer and a lithium niobate LiNbO3 layer; potassium niobate, gallium nitride, aluminum nitride, zinc oxide or quartz or any other piezoelectric material can also be used to form the piezoelectric layer 240.

[0062] The distance between the first finger 252A and two corresponding portions of the second finger 252B adjacent to the first finger defines an electrode period p of the interdigital SAW transducer 200 .

[0063] The ratio between the width a of fingers 252A and 252B and the electrode period p defines the metallization ratio M of transducer 200 .

[0064] The electrode period p and the metallization ratio M together characterize the interdigital SAW transducer 200 and can determine the operating parameters of the SAW resonator together with other factors (such as the properties of the piezoelectric layer 240, the dielectric layer 230, the metal layer 220 or the substrate 210, the thickness h of the electrode (or its relative form h / 2p)).

[0065] During operation of the SAW resonator, an alternating electrical input signal supplied to the first electrode 250A is converted into a mechanical signal in the piezoelectric layer 240, thereby generating one or more acoustic waves therein.

[0066] The resulting acoustic waves converted from the electrical input signal are primarily surface acoustic waves, which is what the SAW transducer seeks when operating.

[0067] The amplitude and phase of the acoustic wave thus generated in the piezoelectric layer depend on the frequency of the AC input signal, the electrode period p, the relative metal thickness h / 2p and the metallization ratio M, and on the operating parameters of the SAW resonator 200 .

[0068] The frequency dependence is generally described in terms of the variation of the harmonic admittance (ie, harmonic conductance and harmonic susceptance) between the first electrode 252A and the second electrode 252B as the frequency of the AC electrical input signal varies.

[0069] The acoustic waves converted from the input AC electrical signal travel through the piezoelectric layer 240 and eventually reach the second electrode 252B where they are converted into an output AC electrical signal.

[0070] The acoustic waves can also be kept confined underneath the interdigital transducers when the interdigital transducers are surrounded by a reflector consisting of a network of electrodes similar to those of the transducer (preferably but not necessarily limited to electrodes connected to electrical ground), which has a mechanical period close to or the same as that of the transducer and reflects the incident wave in phase towards the interdigital transducer, thereby creating a surface wave resonator. This resonator can form the basic element of a so-called "impedance element" filter, combining poles and zeros to form the desired transfer function.

[0071] Based on the general structure disclosed above, the inventors have performed a number of numerical models to determine the parameters to be set so as to be able to obtain an interdigital SAW transducer meeting precise specifications (particularly the thickness of the metal layer 220 and the dielectric layer 230).

[0072] Other parameters such as phase velocity, reflection coefficient, dielectric constant or electromechanical coupling follow empirical laws based on the thickness of the dielectric layer between the piezoelectric layer and the metal layer, according to the following equation 1:

[0073]

[0074] The coefficients a0 to a3 and b0 to b2 depend on the operating frequency, the chosen materials and the thickness of the stack apart from the thickness of the dielectric layer present between the piezoelectric layer and the metal layer.

[0075] Since the thicknesses of the metal layer 220 and the dielectric layer 230 are at least approximately determined, a skilled person will know how to select and adjust the thicknesses and materials (including their respective orientations) in order to optimize the operation of the device based on its technical specifications.

[0076] Model 1 - Metal layer, interest

[0077] An initial series of models were used to verify the relevance of the general structure, particularly the advantages and potential disadvantages of metal layer 220 .

[0078] To this end, two structures equipped with interdigitated SAW electrodes were compared via modeling: on the one hand, a conventional structure with a piezoelectric / silicon oxide / silicon stack, and on the other hand, a structure similar to a conventional “metal layer structure”, but in which a metal layer was introduced at the interface between silicon and silicon oxide, resulting in a piezoelectric / silicon oxide / metal / silicon stack.

[0079] The metal layer structure Figure 2 B) shows that the conventional structure corresponds to Figure 2 The structure of the metal layer 220 is omitted.

[0080] More specifically, the conventional structure consists of a 600 nm thick LiTaO3(YX1) / 42° piezoelectric layer conforming to IEEE Std-176 nomenclature attached via a 500 nm thick silicon oxide layer (modeled as fused silica) to a substrate with orientation (100) and residual conductivity of 100S. m-1 The piezoelectric layer, substrate and silicon oxide layer correspond to Figure 2 layer 240, layer 210 and layer 230.

[0081] The electrodes were modeled as consisting of an infinite array of 2% copper-aluminum alloy electrodes with an electrode period p of 1 μm, a metallization ratio M of 50%, and a relative thickness h / λ (thickness normalized to the acoustic wavelength λ) of 5%.

[0082] In addition to the metal layer structure, it also includes Figure 2 The metal layer structure is the same as the conventional structure except for the metal layer 220 shown in FIG. 1 , which is composed of a 50% copper-aluminum alloy with a thickness of 50 nm between the silicon substrate and the silicon oxide layer. The structure corresponds to Figure 2 Cross-sectional view in .

[0083] The harmonic admittance of both structures is calculated using the finite element method and the boundary element method, more specifically FEM-BIM (finite element method-boundary integral method).

[0084] Reference may be made to the calculation method “Numerical Method for SAW propagation characterization” published by P. Ventura, J. M. Dede, M. Solal, J. Desbois, and J. Ribbe (published in Proceedings of the IEEE Ultrasonic Symposium, pp. 175-186, 1998) and the application of multilayer materials by S. Ballandras, A. Reinhardt, V. Laude, A. Soufyane, S. Camou, W. Daniau, T. Pastureaud, W. Steichen, R. Lardat, M. Solal, and P. Ventura (published in Journal of Applied Physics, Vol. 96, No. 12, pp. 7731-7741, 2004).

[0085] Figure 3 The harmonic conductance and susceptance of the conventional structure in A) and the metal layer structure in B) are shown (respectively, in dB / S. m-1 G of the solid line expressed in units harmo And on the vertical axis on the right side of each figure, S. m-1 The dotted line B is expressed in units harmo Susceptance).

[0086] For conventional structures, the conductance G appears to be always greater than zero, and the primary and parasitic resonances have finite G values, in contrast to real (lossless) modes, for which G can be likened to a Dirac function.

[0087] The losses reflected by the finite conductance (in Figure 3 The lows (less than -150 dB in the figure) are entirely due to parasitic conductance effects in the silicon substrate: the electric field associated with the mechanical displacement penetrates the structure to a depth of more than 2λ (typical values ​​for pure shear waves can reach 10λ or more) and thus generates leakage currents that capture the excitation energy supplied by the input signal and convert it into heat.

[0088] In fact, it can be shown that, even with a high resistivity, a silicon substrate has charge carriers with a sufficiently long lifetime to induce the creation of parasitic capacitance at the interface between the silicon substrate 210 and the dielectric layer 230 of silicon oxide.

[0089] Figure 3 In B), it is shown that the introduction of a metal layer 220 between the silicon substrate 210 and the dielectric layer 230 of silicon oxide keeps the harmonic conductance values ​​below the noise level until the frequency reaches the so-called SSBW (Surface Grazing Body Wave) branching point, at which the acoustic wave is no longer confined to the upper layers of the structure, but penetrates the volume of the silicon substrate in the form of a radiating wave (the model assumes an unrestricted silicon volume in terms of depth, see Figure 2 B).

[0090] The reason for the attenuation of harmonic conductance is that the metal layer forms an electromagnetic shield between the piezoelectric layer and the substrate, eliminating any electric field penetration into the silicon substrate volume, thus suppressing the appearance of parasitic conductance.

[0091] This minimizes or even eliminates the mean free path of the charges, thus significantly limiting or even preventing the presence of leakage currents due to coherent charge propagation at the interface.

[0092] The suppression of leakage by the metal layer is effective for any type of metal and any thickness of the metal layer, but the thickness is preferably limited in order to avoid the appearance of new modes that degrade the spectral purity of the structure confining the surface acoustic waves.

[0093] Additionally, the use of dense metals and increasing the thickness of the metal layer results in a mass loading effect of the layer, thereby reducing the acoustic wave propagation velocity and modifying the frequency response of the transducer.

[0094] Therefore, in order to limit the disadvantages mentioned in the previous two paragraphs, it is recommended that the metal layer use (i) a light metal, in particular aluminum or an alloy with more than 90% aluminum, such as Al-Cu, Al-Si or Al-Ti, and (ii) a metal with a thickness between 5nm and 100nm, or even between 5nm and 10nm.

[0095] It should be noted that other modeling studies have confirmed that the presence of the metal layer does not significantly change the speed of the mode and its ability to be reflected by the structure, and its electromechanical coupling coefficient k s 2 The electromechanical coupling coefficient in the case of a metal layer is only slightly lower than in the case of a structure without a metal layer.

[0096] It should also be noted that the metal layer 220 is not limited to a continuous layer, but can also consist of a pattern of separate metallized surfaces that perform the same shielding role as a continuous layer, but are able to limit mass loading effects.

[0097] In this case, the distance d separating two consecutive metallized surfaces should ideally be less than λ, preferably less than λ / 2, more preferably less than λ / 4, such as Fig.12As shown, the metal layer 220 consists of a two-dimensional matrix of metallized surfaces 222 separated from each other by a distance d. This condition is by no means limiting, but it does help to reduce the mean free path of the charges induced at the oxide / silicon interface (and more generally at the dielectric / semiconductor interface).

[0098] Without limitation, any type of geometric shape and distribution (square, triangle, hexagonal, other) can be used for the elements of the matrix.

[0099] Model 2 - Metallic Layers and Surface Waves

[0100] The second series of models are used to evaluate the effect of the thickness of the metal layer 220 on the dielectric constant and the characteristic quantities of the surface wave.

[0101] To this end, we consider something like Figure 2 The structure shown corresponds to the metal layer structure of the first series of models, except that this time the metal layer consists of a titanium layer with a thickness varying between 1 nm and 50 nm.

[0102] Figure 4 Four graphs are shown which show the evolution of the characteristic surface wave quantity and the dielectric constant in such a structure as a function of the thickness of the metal layer indicated on the abscissa, varying from 1 nm to 50 nm, wherein in A) the phase velocity is expressed in ms -1 In B), the reflection coefficient is expressed in percentage and varies between 8.86 and 9.06. In C), the electromechanical coupling coefficient k 2 s is expressed in percent and varies between 9.94 and 10.08, and in D), the dimensionless relative permittivity varies between 48.499 and 48.5006.

[0103] These graphs show that increasing the thickness of the metal layer causes only a very slight degradation of the amplitude characteristics of the surface waves (phase velocity, reflection coefficient, electromechanical coupling coefficient).

[0104] The relative permeability is hardly affected by this increase, with a variation of less than 1%, and the variations of the reflection coefficient and the electromechanical coupling coefficient remain at about 1%, so all these variations can be considered negligible.

[0105] For the phase velocity, the mass loading effect of the metal layer is more significant, with a change of about -2 m.s -1 .nm -1 or -50ppm.nm -1 .

[0106] From these findings it can be concluded that the thickness of the metal layer has an influence and that it is advantageous to limit this influence by keeping the thickness of the metal layer below 100 nm, or even below 5%, more preferably 2.5%, of the acoustic wavelength λ.

[0107] However, the thickness of the metal layer is preferably kept greater than 0.25% of the wavelength λ of the acoustic waves in order to maintain the electromagnetic shielding effect provided by the metal layer.

[0108] Model 3 - Metal Layers and Parasitic Capacitance

[0109] In the third model series, using a model based on Green’s functions (described in detail in A. Reinhardt, “Simulation, conception et réalisation de filtres à ondes de volume dans descouches minces piézoélectriques” [Simulation, design and implementation of bulk wave filters in piezoelectric thin films], PhD thesis in Engineering Sciences at the University of Himal, France, 2005), we calculated the admittance, i.e. the conductance and susceptance, of a material stack to attenuate radiated waves from the surface to the core of a stack consisting, from top to substrate, of 100nm aluminum, 600nm LiTaO3, 500nm SiO2 (modeled as fused silica), 50nm molybdenum, 650μm silicon and a semi-infinite polyimide support (kapton (polyimide)).

[0110] For this calculation, the quality factor Q of the metal layer was set to 100, the quality factor Q of SiO2 was set to 1000, and the quality factor Q of LiTO3 and silicon was set to 10000. The quality factor is used to convert the acoustic constant C ij The imaginary part of (the non-conservative part of the problem) is calculated as imaginary part (Cij) = real part (Cij) / Q.

[0111] Electrically, the loss angle tg(δ) is set to 10 for SiO2 -2 , for silicon is set to 10 -3 , and for LiTaO3 is set to 10 -4 As previously mentioned, this coefficient makes it possible to calculate the imaginary part of the dielectric constant from the real value of the dielectric constant, according to methods known to those skilled in the art.

[0112] The aluminum layer is configured to represent the metallization of the SAW filter in the frequency range of 1 GHz to 2 GHz, i.e., it is considered to occupy no more than 1 mm 2 surface area.

[0113] Figure 5 Four graphs A) to D) are shown, each showing a graph with respect to Figure 2 The conductance G, admittance B and static capacitance C0 of similar structures correspond to 500 μm 2 , 250μm 2 , 100μm 2 and 50 μm 2 Aluminum surface.

[0114] In addition to demonstrating the HBAR (high-harmonic bulk acoustic resonator) effect above 3 GHz, we also note the linearity of the effect on the conductance of the static capacitance C0 formed by the opposing conductive elements (metal layer 120 and electrodes 150A and 150B) and the dielectric layer 130 separating them.

[0115] Therefore, in order to limit the parasitic capacitance, the metallization of the piezoelectric layer should be limited, for example by limiting the surface area occupied by the layer defining the resonator electrodes and their respective connection pads to less than 10 4 μm 2 The value of .

[0116] Preferably, the cumulative surface area of ​​the individual connection pads should be kept below 10 4 μm 2 , each connection pad is defined as a region of the layer defining the electrode, which forms a rectangle with sufficient surface area, which is designed to enable micro welding by wedge bonding and ball bonding, respectively.

[0117] Dielectric layer

[0118] Initially, it should be noted that the presence of the dielectric layer 230 is necessary in order to reduce the HBAR (High Overtone Bulk Acoustic Resonator) effect due to the presence of the metal layer 220 , and thereby reduce the parasitic responses induced thereby in the interdigital SAW resonator 200 .

[0119] Model 4 - Dielectric Layer and Admittance

[0120] Still in Figure 2 In the illustrated metal layer structure, the fourth model series evaluates the effect of the thickness of the dielectric layer 230 interposed between the piezoelectric layer 240 and the metal layer 220 on the characteristics of the interdigital SAW resonator 200 .

[0121] For this evaluation, we considered Figure 2 A structure similar to the structure shown in , which corresponds to the metal layer structure of the first series of models, except that the metal layer is made of molybdenum and the dielectric layer is made of a silicon oxide SiO2 layer of varying thickness.

[0122] For the calculations, a quality factor Q of 10000 was considered for the LiTaO 3 layer and the silicon substrate; a quality factor Q of 1000 was considered for SiO 2 ; and a quality factor Q of 100 was considered for molybdenum.

[0123] Figure 6 Six graphs are shown which show the evolution of the modulus and susceptance of the harmonic admittance as a function of the frequency indicated on the abscissa (in dB / S.μm on the ordinate on the left-hand side of each graph). -1 The harmonic modulus of the solid line expressed in units |Y| harmo , and on the right-hand side of each graph the vertical axis is Sm -1 The dotted line susceptance B is expressed in units harmo ), wherein the SiO2 thickness on the graphs indicated by A) to F) are 0nm, 20nm, 50nm, 100nm, 200nm and 500nm respectively.

[0124] It can be seen from these graphs that the distance between the resonant frequency (maximum harmonic conductance) and the anti-resonant frequency (minimum harmonic conductance) increases with increasing SiO2 thickness.

[0125] These two frequencies increase as the mass effect decreases, and the coupling between the metal layer and the piezoelectric layer decreases as the SiO2 thickness increases.

[0126] Therefore, it may be advantageous to maximize the thickness of the SiO2 layer.

[0127] It can also be seen that the parasitic response near 2.7 GHz only appears when the SiO2 thickness is greater than 200 nm, as can be seen in graphs E) and F), but not in graphs A) to D).

[0128] Therefore, it is advantageous to use an oxide layer with a thickness less than 200 nm, for example between 50 nm and 200 nm, preferably between 100 nm and 200 nm, to avoid such parasitic responses.

[0129] Model 5 - Dielectric Layer and Surface Waves

[0130] The fifth series of models is used to evaluate the effect of the thickness of the dielectric layer 230 on the dielectric constant and the characteristic quantities of the surface wave.

[0131] For this evaluation, we consider something like Figure 2 The structure shown corresponds to the metal layer structure of the first series of models, except that aluminum alloy Al-Cu with 2% copper, titanium Ti, molybdenum Mo, copper Cu and nickel Ni are used for the metal layers, wherein the thickness of the dielectric layer varies between 0 nm and 400 nm.

[0132] Figure 7 Four graphs are shown which show the evolution of the characteristic surface wave quantity and the dielectric constant in such a structure as a function of the thickness of the dielectric layer indicated on the abscissa, varying from 0 nm to 0.4 μm, wherein in A) the phase velocity is expressed in ms -1 In B), the reflection coefficient is expressed in percentage and varies between 8 and 9.6, and in C), the electromechanical coupling coefficient k 2 s is expressed in percent and varies between 6.5 and 10, and in D) the dimensionless relative permittivity varies between 51 and 48.5.

[0133] Each diagram contains five curves corresponding to the nature of the metal layer considered (Al-Cu, Ti, Mo, Cu, Ni).

[0134] It can be seen from these graphs that for SiO2 thicknesses less than 600 nm, and even less than 400 nm, the phase velocity, reflection coefficient, and electromechanical coupling coefficient k S 2 Each shows a maximum: the phase velocity is around 50nm; the reflection coefficient is between 80nm and 180nm, depending on the nature of the metal layer; the electromechanical coupling coefficient k S 2 Between 250nm and 400nm.

[0135] Therefore, in order to strike a balance between the interest in maximizing the SiO2 thickness and the interest in maximizing one or more of the parameters mentioned in the previous paragraph, it is advantageous to select a SiO2 thickness between 25nm and 2μm, preferably between 50nm and 600nm, more preferably between 50nm and 400nm, and even more preferably between 250nm and 400nm.

[0136] Alternatively, the preferred value of the SiO2 thickness can be expressed relative to the thickness of the metal layer. In this case, the dielectric layer thickness 230 can be advantageously selected to be between 0.5 times and 40 times the thickness of the metal layer 220, preferably between 10 times and 40 times, and even more preferably between 20 times and 40 times.

[0137] We can also seek to exploit the existence of a maximum, for example by choosing a SiO2 thickness between 250nm and 400nm to maximize the electromechanical coupling k S 2 , maximizing the phase velocity by selecting a SiO2 thickness between 25nm and 100nm, or maximizing the reflectance by selecting a SiO2 thickness between 80nm and 180nm.

[0138] Structure 2 - Double dielectric layer

[0139] An alternative to placing the metal layer 220 in direct contact with the substrate 210 is to insert an optional dielectric layer 330 between them in order to obtain Figure 8 The layered structure shown in is referred to as an "optional dielectric layer structure".

[0140] The mass loading effect is related to the increase in mass per unit volume at the location of wave propagation.

[0141] The wave can be considered to be more sensitive to the properties of the medium because this coincides with the location of its maximum energy density.

[0142] Therefore, by comparison (for example, a molybdenum metal layer is 10.22 g.cm -3 Compared to the silica dielectric layer of 2.65g.cm -3 ), the heavier the metal layer and the closer it is to the piezoelectric layer where the energy maximum of the wave is located, the more the phase velocity will be slowed down due to the aforementioned mass loading effect.

[0143] Removing the high density metal layer from the piezoelectric layer by the presence of the lower density dielectric layer actually results in an increase in the phase velocity.

[0144] In the case of the optional dielectric layer, mass loading effects are naturally considered since two dielectric layers (e.g. silica) are always present in the stack with a constant sum of thicknesses from one calculation to the next, only the position of the metal layer changes and therefore its effect on the phase velocity is also the same.

[0145] for Figure 2 and Figure 8 For common components, please refer to the previous description.

[0146] Model 6

[0147] about Figure 8 The optional dielectric layer structure shown in the sixth series of models is used to evaluate the influence of the thickness of the dielectric layer 230 inserted between the piezoelectric layer 240 and the metal layer 220 and the thickness of the optional dielectric layer 330 inserted between the substrate 210 and the metal layer 220 on the characteristics of the interdigital SAW transducer 200.

[0148] For this evaluation, we considered Figure 8 A structure similar to the structure shown in , which corresponds to the metal layer structure of the series 1 model, except that this time the metal layer is made of molybdenum, there is an optional dielectric layer 330, and both dielectric layer 230 and optional dielectric layer 330 are made of silicon oxide SiO2 layers of varying thickness.

[0149] For the calculations, a quality factor Q of 10000 was considered for the LiTaO 3 layer and the silicon substrate; a quality factor Q of 1000 was considered for SiO 2 ; and a quality factor Q of 100 was considered for molybdenum.

[0150] Fig.10 Six graphs are shown which show the evolution of the harmonic admittance (in dB / S.μm on the ordinate on the left-hand side of each graph). -1 G of the solid line expressed in units harmo conductivity, and on the right-hand ordinate of each graph Sm -1 The dotted line B is expressed in units harmo In the graphs indicated in A) to F), the SiO2 thickness of the dielectric layer 230 is 0 nm, 20 nm, 50 nm, 100 nm, 200 nm and 500 nm, respectively, and the SiO2 thickness of the optional dielectric layer 330 is 520 nm, 500 nm, 470 nm, 420 nm, 320 nm and 20 nm, respectively.

[0151] It can be seen that the combined effect of the two oxide layers 230 and 330 gives results similar to a single oxide layer and may encompass the thickness of both layers.

[0152] For example, it may be advantageous to insert dielectric layer 230 and optional dielectric layer 330 each having a thickness of less than 300 nm, preferably less than 100 nm each.

[0153] It is also possible to insert these layers 230 and 330 in such a way that the sum of their respective thicknesses remains below 600 nm.

[0154] Additionally, one of the parasitic responses has disappeared, corresponding to the second mode at approximately 2.7 GHz in the structure shown in curve D).

[0155] Therefore, in order to eliminate this second mode, it is advantageous to choose the oxide layer thickness such that the ratio of the optional dielectric layer thickness to the sum of the dielectric layer thickness and the optional dielectric layer thickness may be between 15% and 30%.

[0156] Surface Acoustic Wave Devices and Filters

[0157] Applications of the surface wave device according to the present invention include such as Figure 2 A filter device for a pair of transducers shown in FIG.

[0158] Those skilled in the art are able to combine such resonators to obtain a filtering function.

[0159] Fig. 9 is a combination of two SAW transducers 200_1 and 200_2 (such as Figure 2 or Figure 8 Non-limiting depiction of a quadrupole device 900 ) shown in FIG. 9 , which contacts the same piezoelectric layer 940 supported by a common substrate.

[0160] As described above, one of the two SAW transducers is an input SAW transducer, and the other is an output SAW transducer.

[0161] Manufacturing method

[0162] Figure 2 and Figure 8 The devices shown in can be manufactured using methods each comprising one or more direct bonding steps, in particular molecular bonding, between two layers constituting the device structure.

[0163] therefore, Fig.11 The manufacturing method is shown by directly bonding two elements A and B 1150. Figure 2 and Figure 8 Method 1100 for any of the devices shown in , wherein A may be the dielectric layer 230 and B may be the metal layer 220, A may be the piezoelectric layer 240 and B may be the dielectric layer 230, A may be the metal layer 220 and B may be the substrate 210, A may be the metal layer 220 and B may be the optional dielectric layer 330, A may be the optional dielectric layer 330 and B may be the substrate 210.

[0164] Each of element A and element B can be a separate element or have been bonded to other layers, for example, a first component including a piezoelectric layer 240 and a dielectric layer 230 (the latter representing element A) can be bonded to a second component including a substrate 210 and a metal layer 220 (the latter representing element B).

[0165] Of course, the invention is not limited to what is disclosed herein and variants can be added without departing from the scope of the invention as defined by the claims.

Claims

1. A surface acoustic wave device (200), comprising: -Substrate(210); - a piezoelectric layer (240) above the top surface of the substrate (210); - a pair of electrodes (250A, 250B) in contact with the piezoelectric layer, the two electrodes comprising fingers (252A, 252B) extending in the same direction (D) so as to form a periodic structure in which the fingers of the two electrodes alternate with each other and having an interdigital distance (2×p) separating the centers of two adjacent fingers of the same electrode; - a metal layer (220) interposed between the substrate and the piezoelectric layer; as well as - at least one dielectric layer (230) interposed between the metal layer and the piezoelectric layer, in: - the metal layer (220) has a thickness between 5 nm and 100 nm; and - said at least one dielectric layer (230) has a thickness between 25 nm and 600 nm.

2. The surface acoustic wave device according to claim 1, wherein: The metal layer has a thickness between 0.25% and 5% of the interdigital distance.

3. The surface acoustic wave device according to any one of the preceding claims 1 and 2, wherein: The dielectric layer has a thickness between 250 nm and 400 nm.

4. The surface acoustic wave device according to any one of the preceding claims 1 and 2, wherein: The dielectric layer has a thickness greater than five times the thickness of the metal layer.

5. The surface acoustic wave device according to any one of the preceding claims 1 and 2, wherein: The dielectric layer has a thickness less than 200 nm.

6. The surface acoustic wave device according to any one of the preceding claims 1 to 5, further comprising an optional dielectric layer (330) interposed between the substrate (210) and the metal layer (220).

7. The surface acoustic wave device according to claim 6, wherein: The dielectric layer (230) and the optional dielectric layer (330) each have a thickness less than 300 nm.

8. The surface acoustic wave device according to claim 6, wherein: The sum of the thickness of the dielectric layer (230) and the thickness of the optional dielectric layer (330) is less than 200 nm.

9. The surface acoustic wave device according to claim 6, wherein: The ratio of the thickness of the optional dielectric layer (330) to the sum of the thickness of the dielectric layer (230) and the thickness of the optional dielectric layer (330) is between 15% and 30%.

10. The surface acoustic wave device according to any one of the preceding claims 1 to 9, wherein: The piezoelectric layer includes a juxtaposition of a lithium tantalate LiTaO3 layer and a lithium niobate LiNbO3 layer.

11. The surface acoustic wave device according to any one of the preceding claims 1 to 10, wherein: The metal layer (220) includes metallized surfaces separated by a distance less than or equal to the interdigital distance.

12. A surface acoustic wave filter device (900), comprising the surface acoustic wave device according to any one of the preceding claims 1 to 11.

13. A method (1100) of manufacturing a device according to any one of the preceding claims 1 to 12, the method comprising a direct bonding step (1150).

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