WO2022201093A1

WO2022201093A1 - Acoustic resonator

Info

Publication number: WO2022201093A1
Application number: PCT/IB2022/052713
Authority: WO
Inventors: Gianluca Piazza; Gabriel VIDAL ÁLVAREZ
Original assignee: Spectron Microsystems S.R.L.
Priority date: 2021-03-25
Filing date: 2022-03-24
Publication date: 2022-09-29
Also published as: IT202100007364A1; CN117280606A

Abstract

An acoustic resonator, comprising: a layer of piezoelectric material (2) defining one pair of end faces (3, 4) and suited to selectively generate an acoustic wave that propagates from the end faces (3, 4); one pair of electrodes (5) arranged on the respective end faces (3, 4) of the layer of piezoelectric material (2); at least one first acoustic reflector (11) facing towards one face (3) of the piezoelectric material (2) and suited to at least partially reflect the acoustic wave that propagates from said face (3); at least one second acoustic reflector (12) facing towards the other face (4) of the piezoelectric material (2) and suited to at least partially reflect the acoustic wave that propagates from said face (4); the at least one first acoustic reflector (11) has a first predetermined thickness (S₁) and the at least one second acoustic reflector (12) has a second predetermined thickness (S₂). The first thickness (S₁) and the second thickness (S₂) are different from each other.

Description

ACOUSTIC RESONATOR

DESCRIPTION

Field of application of the invention [001] The present invention concerns the technical field of electronic devices for the generation and processing of electrical signals, and the subject of the invention is an acoustic resonator.

State of the art

[002] As is known, in the field of miniaturized electronics there is the need to provide devices suited to allow the filtering of electric signals with especially reduced form factors.

[003] A typical solution for this type of needs is represented by the construction of acoustic resonators based on thin films constituted by layers of piezoelectric material. [004] These acoustic resonators are considerably smaller in size than the circuits based on the electromagnetic counterparts.

[005] The operation of acoustic resonators is based on the generation and processing of acoustic waves, and part of these circuits is used to synthesize radio frequency filters (RF filters).

[006] More specifically, there are two main classes of acoustic resonators currently used in miniaturized electronic circuits, for example used in mobile radio technology (4G and 5G).

[007] A first class of resonators is constituted by resonant acoustic devices with surface acoustic waves (SAW), which are mainly produced by means of simpler structures that can be purchased at a relatively low cost.

[008] SAW resonators are used in electronic circuits operating at frequencies below 2 GHz.

[009] The second class of resonators is constituted by resonant acoustic devices with bulk acoustic waves (BAW). These devices are more complex and expensive than SAW resonators and are mainly used for applications at frequencies above 2 GHz.

[0010] Some BAW devices are used also for frequencies below 2 GHz when the design data require high performance in terms of efficiency and stability with respect to thermal drift.

[0011] 5G standards require acoustic resonators capable of carrying out filtering functions above 3 GHz.

[0012] The new Wi-Fi standards, in fact, also require that filtering systems operating between 5 GHz and 7 GHz are used.

[0013] The thickness selected to define the piezoelectric film layer and the other components of the resonator determines the resonance frequency of the BAW devices. [0014] The new 5G standards create several problems to BAW technology, since very thin piezoelectric films need to be used to obtain filtering devices capable of operating at frequencies above 3 GHz.

[0015] Furthermore, said devices must be able to cover a particularly wide bandwidth, as required by the emerging 5G and Wi-Fi standards.

[0016] In practice, the combination of these requirements results in higher production costs to be borne to control the conformity of the thickness of the films of various materials and the suitable doping phases to which the film of piezoelectric material must be subjected.

[0017] A further negative effect caused by the reduction in thickness of the films of various materials lies in the higher sensitivity of the same to changing environmental conditions.

[0018] In particular, these films are particularly sensitive to temperature and humidity variations and for this reason it is necessary to provide tight packaging inside which the humidity level is controlled so as to maintain the performance of the resonator used as a radiofrequency filter substantially stable over time.

Presentation of the invention

[0019] The present invention intends to overcome the technical drawbacks mentioned above by providing an acoustic resonator that provides high efficiency even when operating within a range of particularly high frequencies.

[0020] More specifically, the main object of the present invention is to provide an acoustic resonator whose production is relatively simple, so as to be easily produced on an industrial scale.

[0021] A further object of the present invention is to provide an acoustic resonator having particularly low production costs.

[0022] Another object of the present invention is to provide an acoustic resonator which can be easily adapted to the new standards emerging in the telecommunications sector, for example 5G, WiFi6, WiFi6E or similar standards. [0023] A further object of the present invention is to provide an acoustic resonator whose electrical characteristics are constant overtime, meaning that it is only minimally affected by the variation of the environmental parameters, for example temperature and humidity. [0024] Another object of the present invention is to provide an acoustic resonator whose structure allows to obtain high attenuation on the spurious frequencies outside the operating band centered on the fundamental frequency of oscillation of the resonator itself.

[0025] Still, not the least object of the present invention is to provide an acoustic resonator configured to allow the filtering of signals associated with high data capacity. [0026] These and other objects that are better explained below are achieved by an acoustic resonator of the type according to claim 1.

[0027] Other objects that are described in greater detail below are achieved by an acoustic resonator according to the dependent claims.

Brief description of the drawings [0028] The advantages and characteristics of the present invention emerge clearly from the following detailed description of some preferred but non-limiting configurations of an acoustic resonator, with special reference to the following drawings:

- Figure 1 shows a schematic top view of a first configuration of an acoustic resonator;

- Figure 2 shows a schematic top view of a second configuration of an acoustic resonator;

- Figure 3 shows a sectional view of the resonator of Figure 1 on plane A-A;

- Figure 4 shows two diagrams intended to illustrate the trend of the admittance as a function of frequency, said diagrams refer to the acoustic resonator that is the subject of the present invention.

Detailed description of the invention

[0029] The subject of the present invention is an acoustic resonator of the type used in the electronics sector to generate and/or filter electrical signals with frequency within a predetermined interval.

[0030] In particular, the acoustic resonator that is the subject of the present invention is particularly suited to promote the generation/filtering of electrical signals falling within the radio frequency band, typically within the band ranging from 1.5 GHz to 30 GHz.

[0031] The expression “acoustic resonator ” used in this context applies to electronic oscillators devices capable of transducing electrical signals into mechanical waves (called acoustic waves) which are generated inside the device itself due to a dimensional deformation (or mechanical vibration) of the components.

[0032] Acoustic resonators, therefore, work according to the operating principle based on the propagation of mechanical waves (also called acoustic waves) inside them, said waves having predetermined width and trend. Through the propagation of said acoustic waves it is possible to generate electrical signals with predetermined characteristics at the ends of corresponding electrodes associated with the resonator.

[0033] It is therefore possible to define the resonator as a device which is suited to transduce the electrical signal present at the ends of the electrodes into acoustic waves that propagate inside the device (and vice versa).

[0034] This behaviour can be exploited to make electronic oscillators, that is, devices suited to generate periodic electrical signals centered on a predetermined frequency band.

[0035] As an alternative, resonators can also be used to make electrical filtering elements suited to provide, at their outputs, a portion of the frequency spectrum associated with the signal applied at the input (for example, a selective pass-band filter, a high-pass filter or a low-pass filter, a stop filter etc.).

[0036] The resonator that is the subject of the present invention, indicated by the reference number 1 in the attached Figures, has been designed to be mainly used as a filtering element in the context of radiofrequency equipment used in 5G technology and in the new standards connected therewith.

[0037] More specifically, Figures from 1 to 5 show a resonator 1 designed to operate in the 2 GHz - 7 GHz frequency band, with special reference to the sub-band between 3 GHz and 4 GHz.

[0038] It is understood, however, that said configurations of the invention are described only by way of example and the innovative technical characteristics described below can be reproduced also in other types of RF resonators operating in bands different from those described above.

[0039] Furthermore, the acoustic resonator 1 described below can be used also as an oscillator and/or as a filtering element for frequency intervals different from those indicated and not necessarily falling within the radio frequency band.

[0040] The acoustic resonator 1 that is the subject of the invention has a layered structure obtained by superimposing layers of materials of different types.

[0041] A cross-sectional view of said structure is visible in Figure 4 and Figure 5 while the shape of said layers in plan view can be different according to design specifications or to the environment where the resonator will be installed.

[0042] Figures from 1 to 3 show different geometrical shapes of resonators obtained by modifying the shape of the various layers in plan view.

[0043] First of all, the resonator 1 comprises a layer of piezoelectric material 2 extending between one pair of end faces 3, 4.

[0044] The geometrical shape of this layer 2 in plan view may correspond to that of a regular polygon with n sides (for example, a square, a rectangle, a trapezium, or any other polygon.

[0045] Conveniently, the piezoelectric material is of the monocrystalline type and can be selected from among the group comprising one of the following materials: aluminium nitride, lithium niobate, lithium tantalum, quartz, zinc oxide, lead zirconate titanate, and other materials with similar electromechanical characteristics.

[0046] Furthermore, the material used to make the piezoelectric layer 2 can be selected also from among materials based on those specified above but obtained with different doping values.

[0047] Conveniently, the layer of piezoelectric material 2 can have a predetermined thickness s_å included between 100 nm and 5000nm.

[0048] Furthermore, the thickness s_å of the piezoelectric layer 2 can be proportional to the wavelength l of the acoustic signal generated by the same, for example this thickness can be equal to half said wavelength (l/2).

[0049] The resonator 1 also comprises one pair of electrodes 5 arranged on the respective end faces 3, 4 of the piezoelectric material 2.

[0050] These electrodes 5 are placed directly on the faces 3, 4 of the piezoelectric material 2 and are made of an electrically conductive material (typically a metallic material).

[0051] For example, the electrodes 5 can be made of one of the following materials or alloys: aluminium, aluminium-copper, ruthenium, molybdenum, tungsten, titanium, tantalum, platinum, and gold.

[0052] As better illustrated in Figures from 1 to 3, the electrodes 5 can be provided with conveniently shaped lateral extensions 6 suited to be connected to other circuits that are electrically connected to the resonator.

[0053] More specifically, said extensions can be substantially bidimensional and expand along the development thereof between one end 7 with reduced dimensions, suited to be directly superimposed to the layer of piezoelectric material 2, and the opposite end 8 with maximum dimensions, suited to be connected to other electric or electronic circuits. [0054] A first embodiment of the resonator 1 provides, on the electrodes 5, the deposition of layers of a different material suited to form the reflector (or screen) for the acoustic wave generated by the piezoelectric material 2.

[0055] However, in the embodiment illustrated in the Figures, two layers 3, 4 of the same material selected from among the materials with low impedance with respect to the propagation of the acoustic wave (or, shortly, with low acoustic impedance) are deposited on the electrodes 5.

[0056] For example, said layers 9 with low acoustic impedance can be obtained from a thin film of one of the following materials: silicon dioxide, silicon oxide, tellurium oxide, spin-on glass and other materials based thereon but with the addition of doping substances or impurities.

[0057] Conveniently, the thickness S₉ of said layers 9 with low acoustic impedance can be between 50nm and 10pm.

[0058] In particular, the thickness S₉ of the layers 9 with low acoustic impedance is variable according to the thickness s_å of the layer of piezoelectric material 2.

[0059] For example, if the layer of piezoelectric material 2 has a thickness s_å equal to approximately half the length of the acoustic wave (l/2), the layers 9 of low acoustic impedance materials can have a thickness s_å substantially equal to a fraction and to multiples of said wavelength (one quarter of the wavelength l/4, three quarters of the wavelength 3l/4, etc.).

[0060] The function of these layers 9 with low acoustic impedance is first of all to compensate for the thermal drift to which the layer of piezoelectric material 2 is subjected.

[0061] As is known, the oscillation frequency (and/or band) associated with the piezoelectric material 2 varies according to the temperature in the environment in which said layer is located.

[0062] In the technical field of resonators, this condition is defined by the expression “temperature coefficient of frequency”, which indicates the frequency variation as a function of temperature (generally frequency varies by a few tens of millionths of the normalized frequency per each unit of degree centigrade). The temperature coefficient of frequency is, therefore, a parameter that expresses variation in millionths (that is, on a scale equal to 10⁶ of the value of the operating frequency in Hz).

[0063] The frequency at a temperature coefficient is negative when, as the temperature increases, the oscillation frequency decreases.

[0064] On the contrary, the frequency at a temperature coefficient is positive when, as the temperature increases, the oscillation frequency increases.

[0065] In general, piezoelectric materials 2 have a negative frequency at a temperature coefficient within the entire operating range of the material itself (that is, the oscillation frequency can only decrease as the temperature increases).

[0066] For this reason, the pair of layers 9 of low acoustic impedance material has a positive frequency at a temperature coefficient, that is, their internal structure is such as to promote an increase in the operating frequency as the temperature increases.

[0067] In this way, the assembly constituted by the layer of piezoelectric material 2 and the low impedance layers 9 will be such as to have reduced thermal drift (referred to the operating frequency).

[0068] In particular, the variation of the oscillation frequency associated with the piezoelectric layer 2 and caused by the temperature variation are substantially eliminated (or considerably reduced) as a result of a variation of the frequency with opposite sign associated with the behaviour of the layers 9 of low impedance material.

[0069] Furthermore, to carry out this compensation it is necessary to insert two distinct layers of low impedance material 9, each one of which is directed towards the respective face 3, 4 of the layer of piezoelectric material 2.

[0070] The layer of piezoelectric material 2, in fact, has substantially isotropic physical characteristics with respect to the two end faces 3, 4.

[0071] Accordingly, the excitation of the piezoelectric material 2 (for example, through the application of an electrical signal to the pair of electrodes) promotes the generation of two substantially equal mechanical waves that propagate along a direction Z that is perpendicular to the faces 3, 4 but has opposite directions of propagation.

[0072] The acoustic waves generated by the vibration induced on the piezoelectric material 2 tend to be "retained" within the resonator 1 by means of a plurality of reflectors arranged opposite the faces of the piezoelectric material. [0073] The retention of the wave within the layers of the resonator 1 allows the triggering of a resonant condition that promotes the generation of a standing acoustic wave.

[0074] The fraction of the oscillating standing wave that is progressively transmitted among the layers (and not reflected) represents the losses of the resonator 1.

[0075] Obviously, the resonator 1 is designed to minimize this kind of losses and, instead, to make it easier to maintain the standing condition of the acoustic wave.

[0076] The pair of low acoustic impedance layers 9 positioned directly on the electrodes 5 allow to increase the size of the resonant cavity 10 associated with the layer of piezoelectric material 2 and to promote a first and partial reflection of the acoustic wave generated by the same.

[0077] In the context of the present invention, the expression "resonant cavity" means a geometric space of predetermined dimensions within which most of the energy associated with the mechanical wave produced during the oscillation of the active material, that is, the piezoelectric material 2, resides.

[0078] In general, the energy associated with the acoustic wave residing within the resonant cavity 10 exceeds 95% of the total energy generated by the piezoelectric material and associated with said wave.

[0079] More specifically, it can be analytically and experimentally demonstrated that the introduction of a pair of low acoustic impedance layers 9 increases the size of the resonant cavity 10.

[0080] Therefore, with the introduction of the low acoustic impedance layers 9, it is possible to obtain a resonant cavity 10 whose dimensions are greater than the thickness of the piezoelectric material layer 2 and smaller than the sum of the thickness of the latter and the thickness of the low impedance layers 9.

[0081] In other words, the resonant cavity 10 undergoes an enlargement that is such as to include even a portion of the thickness of the low impedance layers 9, in any case without ever exceeding the latter in size.

[0082] At the same time, the pair of low impedance layers 9 also acts as an acoustic screen, reflecting a portion of the mechanical wave generated within said cavity 10. [0083] In this way, the presence of said low acoustic impedance layers 9 allows to reduce the overall losses of the resonator by favouring the maintenance of the oscillation promoted by the electrodes 5 on the layer of piezoelectric material 2.

[0084] Conveniently, in the low impedance layers 9 the conditions of i) partial reflection of the acoustic wave and ii) compensation for the thermal drift to which the piezoelectric material 2 is subjected all occur simultaneously during the operation of the resonator 1. [0085] Conveniently, the resonator 1 comprises a first acoustic reflector 11 facing towards a face 3 of the piezoelectric material layer 2 and a second acoustic reflector 12 facing towards the other face 4 of the piezoelectric material layer 2.

[0086] As is known, the first acoustic reflector 11 and the second acoustic reflector 12 have the function of partially reflecting the mechanical wave generated by the layer of piezoelectric material 2 (and coming from the faces 3, 4 thereof) so as to maintain the resonator 1 in the active resonance condition as long as possible.

[0087] It is well known from the state of the art in the field of the resonators 1 that the reflector layers 11, 12 are not able to fully reflect the incident acoustic wave, in practice the reflection losses are generally lower than 1% (that is, the reflector is able to reflect more than 99% of the incident mechanical wave and to transmit less than 1% of said wave to the next layer).

[0088] A solution that is often used to increase the overall reflection of the acoustic wave generated by the layer of piezoelectric material 2 consists in providing two stacks of several reflector elements 11, 12, each of which faces towards the corresponding face 3, 4 of the material itself.

[0089] Therefore, contextualizing these known configurations in the resonator 1 that is the subject of the present invention, it is possible to use a single first reflector 11 and a single second reflector 12 or, alternatively, a plurality of first reflectors 11 and a plurality of second reflectors 12 superimposed on one another.

[0090] In general, it is therefore possible to provide a number N of mutually superimposed first reflectors 11 and a number M of mutually superimposed second reflectors 12.

[0091] Furthermore, depending on the design requirements, it is possible to provide the same total number of first reflectors 11 and second reflectors 12 (N = M) or a different number of said reflectors (M ¹ N).

[0092] The acoustic resonator 1 shown in Figure 4 and Figure 5 comprises a plurality of first reflectors 11 and a plurality of second reflectors 12 in different numbers from each other (M ¹ N).

[0093] Regardless of the total number of first reflectors 11 and second reflectors 12, both are arranged directly on the electrodes 5 (in the case where no pair of layers 9 of low acoustic resistance material is provided) or, alternatively, directly on the low acoustic impedance layers 9 (when provided).

[0094] As better illustrated in the Figures, each first reflector 11 and each second reflector 12 may comprise a plurality of sub-layers 13, 14, respectively made of a low acoustic impedance material and a high acoustic impedance material.

[0095] The low acoustic impedance layers 13 may be made up of films of any of the following materials: silicon dioxide, silicon oxide, tellurium oxide, spin-on glass, and other materials based thereon but with the addition of doping substances or impurities.

[0096] The high acoustic impedance layers 14 may be made up of films of any of the following materials: aluminium nitride, tungsten, platinum, molybdenum, ruthenium, and other materials based thereon but with the addition of doping substances or impurities. [0097] Conveniently, each single first reflector 11 and/or each single second reflector 12 may have a number X of layers of high impedance material and a number Y of layers of low impedance material.

[0098] In particular, the total number X of sub-layers 14 of high impedance material and the total number Y of sub-layers 13 of low impedance material included in a single reflector 11, 12 may be the same (X = Y) or different (X ¹ Y).

[0099] For example, said sub-layers 13, 14 may, at a minimum, be present singularly (X = Y = 1) while their maximum number may be variable and determined by implementation and/or design requirements.

[00100] However, when there is a number of said sub-layers greater than one (X > 1 and/or Y > 1), the arrangement of the same is always alternated in such a way that between two layers of the same type there is a layer of the opposite type.

[00101] For example, if X = 2 and Y =1, the single low impedance sub-layer 13 is interposed between the two high impedance sub-layers 14, as in the case where X = Y = 4 the corresponding first reflector 11 and/or second reflector 12 is made up of four equal superimposed elements, each of which is constituted by a high impedance sub layer 14 coupled with a low impedance sub-layer 13.

[00102] Conveniently, one or more of the first reflectors 11 (respectively, one or more of the second reflectors 12) may comprise a number X and Y of sub-layers 13, 14 equal to or different from that characterizing the other first reflectors 11 (respectively, equal to or different from that characterizing the other second reflectors 12).

[00103] Furthermore, the low impedance sub-layers 13 and the high impedance sub- layers 14 may have respective thicknesses S₁₃, Si₄ with a predetermined value, the thickness Si, s_å of each first reflector 11 and/or second reflector 12 is therefore determined by the sum of the thicknesses S₁₃, Si₄ of the low and high impedance sub layers 13, 14 that make it up.

[00104] The thickness Si₄ of the high impedance material sub-layers 14 and the thickness S₁₃ of the low impedance sub-layers 13 are variable as a function of the frequency of the acoustic wave generated by the piezoelectric material layer 2.

[00105] In particular, the thickness S₁₃, Si₄ of each sub-layer 13, 14 of high and/or low acoustic impedance material may be chosen so that it is proportional to a fraction of the period of the acoustic wave l that propagates within the same.

[00106] More specifically, each sub-layer 13, 14 has end surfaces 15, 16 spaced from each other by a distance equal to the thickness S₁₃, Si₄ of the same sub-layer 13, 14. [00107] The acoustic wave propagates in the corresponding sub-layer 13, 14 with a predetermined propagation speed, said speed varying according to the type of material used to make the same sub-layer.

[00108] Therefore, each sub-layer 13, 14 defines its own acoustic wave propagation time. This time can be defined as the time interval it takes each point of the acoustic wave to travel through the thickness separating the end surfaces 15, 16 of the sub-layer

13, 14.

[00109] In other words, the propagation time can be calculated as the ratio between the thickness of the corresponding sub-layer 13, 14 and the propagation speed of the acoustic wave within the same layer.

[00110] In the technical field of resonators, the propagation time is also defined with the expression "phase length". By indicating the phase time with the symbol t_p, the thickness of a given reflector layer and/or of the piezoelectric material with s and the propagation speed of the acoustic wave within the same layer with v_p, it is possible to define the following relation: t_p = s / v_p

[00111] As will be better evidenced further on in this description, the thickness S₁₃, Si₄ associated with each sub-layer 13, 14 made of high and/or low acoustic impedance material is chosen as a function of the size of the resonant cavity 10.

[00112] A correlation between the sizes of the various sub-layers 13, 14 defining the reflectors 11, 12 and the size of the resonant cavity allows to minimize the overlap between the reflector bands.

[00113] The expression "reflector bandwidth" used in the present description is intended as referring to the frequency range within which the reflector 11, 12 promotes the reflection of the acoustic signal generated by the piezoelectric material layer 2.

[00114] In other words, the reflector bandwidth represents the frequency range of the reflector 11, 12, within which the incident acoustic signal (that is, the acoustic signal generated by the piezoelectric material 2 and moving away from it along a predetermined propagation direction) is reflected by the reflector so as to be directed back to the piezoelectric material 2.

[00115] The reflector bandwidth may vary according to the number of reflectors used in the same resonator. In the case illustrated in the Figures, the resonator is provided with a plurality of first and second reflectors: the overall bandwidth associated with the first reflector and the overall bandwidth associated with the second reflector will vary according to the number of mutually superimposed first reflectors 11 and second reflectors 12.

[00116] Preferably, the bandwidth associated with the first reflector 11 may be different from the bandwidth associated with the second reflector 12.

[00117] The first reflector 11 and the second reflector 12 may be selected so that they have a respective bandwidth B suited to allow the reflection of the main mode generated by the piezoelectric material layer 2.

[00118] However, as is known in resonator theory, the piezoelectric material layer 2 can generate unwanted acoustic waves, that is, acoustic waves associated with spurious or secondary modes.

[00119] The use of different bandwidths B respectively associated with the first reflector 11 and the second reflector 12 allows to increase the attenuation and "damping" effect of spurious or secondary modes, that is, modes other than the main mode.

[00120] To achieve this effect, it is possible to design first reflectors 11 and second reflectors 12 whose bandwidths B cannot be superimposed as said bandwidths B are delimited by different frequency intervals. Said bandwidths B, however, have a common zone centered on the frequencies of the main mode of the acoustic wave generated by the piezoelectric material 2.

[00121] In this way, it is possible to combine the joint action of said bandwidths B to promote the attenuation of a wide range of acoustic waves whose frequency falls outside the overlapping bandwidth portion (suited to promote the reflection of the main mode) and associated, respectively, with the first reflector 11 and the second reflector 12. [00122] Such configuration allows to eliminate from the beginning the generation of spurious modes which do not coincide with the main mode produced by the layer of piezoelectric material 2; the formation of these undesired modes can be due to various reasons well known in the field of resonators (for example, the presence of impurities inside the piezoelectric material 2 and/or the sub-layers forming the reflectors 11, 12, the geometrical shape of the resonator and/or of the electrodes, etc.).

[00123] The formation of reflectors 11, 12 operating at partially different bandwidths allows to "cut" most of the spurious modes generated by the piezoelectric material layer 2 as they promote the absorption of these unwanted modes while allowing the reflection of the main mode only (so as to trigger the oscillation of this specific mode).

[00124] Conveniently, the thickness S₁₃, Si₄ of the sub-layer 13, 14 may then be selected in such a way that, during the propagation time calculated as described above, a predefined portion of the period of the acoustic wave propagates through the end surfaces 15, 16 of the material.

[00125] For example, the thickness Si₄ of the low acoustic impedance sub-layer 13 and the thickness Si₄ of the high acoustic impedance sub-layer 14 can be selected in such a way as to allow the propagation of one quarter of the period (TT/2) or three quarters of the period (3/2TT).

[00126] The thicknesses S₁₃, Si₄ of the sub-layers 13, 14 forming the same reflector 11, 12 can be the same or different from each other, just as the sub-layers 13, 14 forming different reflectors 11, 12 can have the same thickness S₁₃, Si₄ or different thickness. [00127] According to a specific aspect of the invention, the first thickness Si associated with the first reflector 11 is different from the second thickness s_å associated with the second reflector 12.

[00128] Thanks to this configuration, it is possible to obtain an acoustic resonator capable of considerably attenuating the spurious components associated with frequencies other than those falling within the oscillation bandwidth.

[00129] In fact, it has been analytically found during the design stage as well as experimentally that a different value of the first and second thicknesses si, s_å allows to reduce the spurious frequency components by a value generally between 20 dB and 60 dB. [00130] For example, in the configuration of the invention illustrated in Figures 4 and 5, the value of the first thickness Si (associated with the first reflectors 11) is greater than the value of the second thickness s_å (associated with the second reflectors 12).

[00131] Furthermore, the increased thickness Si associated with a corresponding type of reflectors (in the case illustrated in the Figures, of the first reflector 11) can be obtained by using sub-layers 13, 14 having a unit thickness S₁₃, Si₄ greater than that of the sub layers 13, 14 used to make the reflector 12 having a lower thickness.

[00132] Alternatively, an increased thickness Si can be obtained by superimposing a total number of sub-layers 13, 14 greater than the total number of sub-layers 13, 14 used to make the reflector having a lower thickness S₂.

[00133] In the version of the resonator illustrated in the Figures, the thickness Si of each single first reflector 11 (arranged on the piezoelectric material layer 2) is greater than the thickness s_å of each second reflector 12 (arranged under the piezoelectric material layer 2).

[00134] As described above, the thickness si; s_å of each reflector 11, 12 is obtained from the sum of the thicknesses S₁₃, Si₄ of the high impedance and low impedance sub layers 13, 14 composing it.

[00135] More specifically, the thickness S₁₃, Si₄ of the sub-layers 13; 14 comprising each first reflector 11 and/or second reflector 12 is variable so as to satisfy a dimensional relationship that varies according to the phase length t_p associated with the excited mode crossing the same layer.

[00136] The expression "mode of vibration", also used earlier in this description, is intended as referring to the characteristic vibration mode related to a system or structure that defines a resonator 1 and has several points with different vibration amplitudes. [00137] A mode of vibration comprises i) a temporal variation of the vibration and ii) a spatial variation of the amplitude of movement through the structure.

[00138] The temporal variation defines the frequency of the oscillations.

[00139] The spatial variation defines the different vibration amplitudes from one point to another of the structure comprising the resonator 1.

[00140] Specifically, the sub-layers 13, 14 made of high or low impedance material that define the reflector with greater thickness Si (for example, the first reflector 11) have a thickness Si_{3; Si4} varying according to the following relationship: six = (2n + 1)*(TT/2) wherein:

Si_x is the thickness of the corresponding sub-layer 13,14; n is a positive integer;

TT/2 corresponds to one quarter of the period referred to the excited mode (where 2*TT = period of the excited mode).

[00141] According to this relation, therefore, the sub-layers 13,14 making up the largest reflector (in the case of the resonator illustrated in the Figures, this reflector corresponds to the first reflector 11) may present, with respect to the period 2*TT of the mode excited by the layer of piezoelectric material 2, a thickness Si3, Si4 selected according to the following series:

3*p/2; 5* p/2; 7* p/2 ; 9* p/2; 11* p/2; 13* p/2 ...

[00142] Conveniently, the sub-layers 13,14 made of high impedance or low impedance material defining the reflector with lower thickness s_å (for example, the second reflector 12) have a thickness S13; S14 substantially equal to one quarter of the period referred to the excited mode, that is, satisfy the following relation:

Six = TT/2 wherein

Si_x is the thickness of the corresponding sub-layer 13,14;

TT/2 corresponds to one quarter of the period referred to the excited mode (where 2*p = period of the excited mode).

[00143] Basically, therefore, the acoustic resonator 1 that is the subject of the present invention is provided with a first reflector 11 and a second reflector 12 having thicknesses si;S2 different from each other and constituted by alternating sub-layers 13,14 made of high intensity material, in which:

- the sub-layers 12;13 of one between the first reflector 11 and the second reflectorS13, Si4 each have the same thickness substantially equal to six = (2n + 1)*(TT/2), where TT/2 represents the quarter wave of the mode excited by the layer of piezoelectric material 2;

- the sub-layers 12; 13 of the other reflector 12 each have the same thickness substantially equal to six = TT/2, that is, equal to the quarter wave of the mode excited by the layer of piezoelectric material 2.

[00144] In the configuration of the resonator1 illustrated in the Figures, the first reflector 11 comprises sub-layers 13,14 having a higher thickness Si3,Si4 while the second reflector comprises sub-layers having a lower thickness. However, the resonator configuration may differ from this one and feature a first reflector comprising sub-layers having a lower thickness and a second reflector comprising sub-layers having a higher thickness.

[00145] In a particular configuration of the invention illustrated in Figure 5, the thickness Si of the first reflector 11 positioned closest to the layer of piezoelectric material can be greater or smaller than the thickness Si of the remaining first reflectors 11.

[00146] In other words, the first thickness Si associated with the reflector 11 positioned closest to the layer of piezoelectric material 2 can be the absolute maximum (or the absolute minimum) with respect to the first thicknesses Si associated with the remaining reflectors 11 used in the resonator.

[00147] Similarly, the thickness s_å of the second reflector 12 positioned closest to the layer of piezoelectric material 2 can be greater or smaller than the thickness s_å of the remaining second reflectors 12.

[00148] The second thickness s_å associated with the reflector 12 positioned closest to the layer of piezoelectric material 2 can be the absolute maximum (or the absolute minimum) with respect to the second thicknesses s_å associated with the remaining reflectors 12 embedded in the resonator 1.

[00149] Conveniently, the first thicknesses Si and/or the second thicknesses s_å can progressively decrease starting from the respective maximum thickness associated with the reflector 11, 12 positioned closest to the piezoelectric material layer 2.

[00150] The expression " rogressive decrease " of the thicknesses si, s_å refers to that particular configuration in which the thickness si, s_å of a given reflector 11, 12 is always smaller than the thickness si, s_å of the previous reflector (that is, the reflector which, with respect to the direction of propagation of the acoustic wave, is the one that is affected by the wave immediately before the reflector in question).

[00151] Figure 6 shows the frequency response of the admittance of two acoustic resonators, both designed to operate approximately at a frequency of 3.45 GHz.

[00152] The first of these resonators - associated with the upper graph - has a single lower reflector (formed by layers that are substantially one-quarter wavelength thick). [00153] The other resonator - associated with the lower graph - has a single lower reflector (formed by layers that are substantially one-quarter wavelength thick) and a single upper reflector (formed by layers that are substantially three-quarters wavelength thick). [00154] From the comparison shown in the graph in Figure 4, it is clear that the presence of the upper reflector allows for considerable attenuation of the spurious frequency components generated as main mode harmonics.

[00155] The resonator can be anchored to the base substrate 17 preferably made of a material suited to have a high acoustic impedance. For example, said base substrate 17 can be selected from among the following materials: silicon, silicon carbide, sapphire, lithium niobate, lithium tantalate, glass, quartz, aluminium nitride, and diamond.

[00156] Conveniently, the teachings described with reference to the present invention can also be applied to resonators designed to operate with harmonics of the fundamental mode (wherein in this case the fundamental mode corresponds to one of the attenuated modes).

[00157] The present invention can be carried out in other variants, all within the scope of the inventive features claimed and described herein; these technical features may be replaced by different technically equivalent elements and the materials used; the shapes and dimensions of the invention may be any, provided that they are compatible with its use.

[00158] The reference numbers and signs included in the claims and in the description have only the purpose of making the text clearer and should not be considered as elements limiting the technical scope of the objects or processes they identify.

Claims

1. An acoustic resonator, comprising:

- a layer of piezoelectric material (2) defining one pair of end faces (3, 4) and suited to selectively generate an acoustic wave that propagates from said end faces (3, 4);

- one pair of electrodes (5) arranged on the respective end faces (3, 4) of said layer of piezoelectric material (2);

- at least one first acoustic reflector (11) oriented towards a face (3) of said piezoelectric material (2) and suited to at least partially reflect the acoustic wave that propagates from said face (3);

- at least one second acoustic reflector (12) oriented towards the other face (4) of said piezoelectric material (2) and suited to at least partially reflect the acoustic wave that propagates from said face (4), wherein said at least one first acoustic reflector (11) has a first predetermined thickness (si); wherein said at least one second acoustic reflector (12) has a second predetermined thickness (S2); characterized in that said first thickness (si) and said second thickness (S₂) are different from each other.

2. Resonator as claimed in claim 1, characterized in that it comprises one pair of layers of low acoustic impedance material (9), each layer of low acoustic impedance material (9) being interposed between said layer of piezoelectric material (2) and a respective first or second reflector (11, 12).

3. Resonator as claimed in claim 2, characterized in that each layer of low acoustic impedance material (9) of said pair is arranged on a respective electrode (5).

4. Resonator as claimed in claim 3, characterized in that said at least one first reflector (11) is directly arranged on a layer of low acoustic impedance material (9) of said pair and said at least one second reflector (12) is directly arranged on the other low acoustic impedance layer (9) of said pair.

5. Resonator as claimed in claim 1 , characterized in that said at least one first reflector (11) and said at least one second reflector (12) are respectively constituted by one or more sub-layers of high acoustic impedance material (14) and by one or more sub-layers of low acoustic impedance material (13), said low acoustic impedance sub layers (13) and said high acoustic impedance sub-layers (14) being arranged in a mutually alternating manner.

6. Resonator as claimed in claim 5, characterized in that each high acoustic impedance sub-layer (14) and each low acoustic impedance sub-layer (13) have respective thicknesses (s^, SM) with a predetermined value, said first thickness (si) and said second thickness (S2) being substantially equal to the sum of the thicknesses of the low acoustic impedance sub-layers (S₁₃) and the high acoustic impedance sub-layers (SM) which make up the respective first (11) or second reflector (12).

7. Resonator as claimed in claim 6, characterized in that the thickness (SM) of each of said high acoustic impedance sub-layers (14) and/or the thickness (sn) of each of said low acoustic impedance sub-layers (13) are selected in such a way as to allow, during the propagation of the acoustic wave generated by said layer of piezoelectric material (2), the crossing of a predetermined fraction of the acoustic wave period.

8. Resonator as claimed in claim 4, characterized in that it comprises a plurality of said first reflectors (11) and/or a plurality of said second reflectors (12), at least one among said high acoustic impedance sub-layers (14) and/or at least one among said low acoustic impedance sub-layers (13) respectively associated with said first reflectors (11) and/or said second reflectors (12) having a thickness value (S13, SM) that progressively decreases as the distance from said layer of piezoelectric material (2) increases.

9. Resonator as claimed in one or more of the preceding claims, characterized in that the thickness (si) of said first reflector (11) and the thickness (S₂) of said second reflector (12) are variable according to the thickness (S₂) of said layer of piezoelectric material (2).

10. Resonator as claimed in one or more of the preceding claims, characterized in that said pair of layers of low acoustic impedance material (9) has a temperature coefficient of frequency that is the opposite of the temperature coefficient of frequency associated with said layer of piezoelectric material (2).

11. Resonator as claimed in one or more of the preceding claims, characterized in that said low acoustic impedance sub-layers (13) and said high acoustic impedance sub-layers (14) making up said at least one first reflector (11) and said at least one second reflector (12) have a temperature coefficient of frequency that is the opposite of the temperature coefficient of frequency associated with said layer of piezoelectric material (2).