DESCRIPTION

Field of application of the invention

[001] The present invention is related to the technical field of electronic devices for the generation and processing of electrical signals and the subject of the invention is a resonator device.

State of the art

[002] As is known, in the field of miniaturized electronics it is necessary to construct devices suited to enable the filtering of electrical signals having particularly reduced form factors.

[003] A typical solution to this kind of requirement consists in the construction of acoustic resonators based on thin films of piezoelectric material arranged in layers.

[004] These acoustic resonators have a considerably reduced size compared to circuits based on the electromagnetic counterparts.

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

[006] In particular, 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 represented by surface acoustic wave (SAW) resonant acoustic devices, which are predominantly made with simpler structures that can be obtained at a relatively low cost.

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

[009] The second class of resonators consists of bulk acoustic wave (BAW) resonant acoustic devices. These devices are more complex and expensive than SAW resonators and are mainly used at frequencies higher than 2 GHz.

[0010] Some BAW devices are used also for frequencies lower than 2 GHz in the case where the design data require high performance in terms of efficiency and stability with respect to thermal drifts.

[0011] 5G standards require acoustic resonators that can perform filtering functions above 3 GHz. [0012] The new Wi-Fi standards, in fact, also require the use of filtering systems operating between 5 GHz and 7 GHz.

[0013] The thickness selected to define the layer of piezoelectric film and the other components of the resonator determines the resonance frequency of BAW devices. [0014] The new 5G standards create several problems for BAW technology as extremely thin piezoelectric films must be used to make filtering devices able to operate at frequencies higher than 3 GHz.

[0015] Furthermore, in current electronic devices radio frequency filtering must be performed with very small form factors.

[0016] For this reason, acoustic resonators based on thin films of piezoelectric material arranged in layers are used, which have the advantage of being smaller than electromagnetic devices themselves. There is a large class of acoustic resonators that have been marketed to synthesize RF filters.

[0017] The new communication standards therefore require the construction of BAW devices based on very thin films suited to allow them to be used at higher frequencies than those presently used.

[0018] For example, resonator devices working at operating frequencies higher than 5 GHz require the use of piezoelectric films with a thickness of around 300 nm.

[0019] In addition, the scaling of electrodes requires the use of metals with thicknesses of less than 100 nm.

[0020] This extreme reduction in the thicknesses of the layers that make up the piezoelectric device is associated with an increase in electrical losses and a significant reduction of the power of the signal with which the device can operate.

[0021] Further size scaling for operation above 10 GHz would be prohibitive, in fact with such extremely reduced thicknesses it is difficult to set the operating frequency of the device accurately.

[0022] Consequently, electrical losses would increase significantly to the detriment of the overall efficiency of the resonator device.

Presentation of the invention

[0023] The present invention intends to overcome the aforementioned technical drawbacks by providing a resonator device that offers high efficiency even when operating within a range of particularly high frequencies.

[0024] In particular, the main object of the present invention is to provide a resonator device made up of layers whose thickness can be easily obtained with the current production technologies.

[0025] A further object of the present invention is to provide a resonator device with particularly low manufacturing costs.

[0026] Another object of the present invention is to provide a resonator device that is capable of operating in a particularly precise manner also at very high frequencies, even higher than 5 GHz or 10 GHz.

[0027] A further object of the present invention is to provide a resonator device whose electrical characteristics are constant over time, that is, a device which is minimally affected by changes in environmental parameters such as temperature and humidity. [0028] Still, another and yet not the least object of the present invention is to provide a resonator device configured to allow the filtering of signals associated with high data capacity.

[0029] These objects, together with others that are better illustrated below, are achieved by a resonator device of the type according to claim 1.

[0030] Other objects that are described in greater detail below are achieved by a resonator device according to the dependent claims.

Brief description of the drawings

[0031] The advantages and the characteristics of the present invention will become clear from the following detailed description of some preferred but not limiting embodiments of an acoustic device, which refers in particular to the following drawings, wherein:

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

- Figure 2 shows a sectional view of a device according to the invention in a first embodiment;

- Figure 3 shows a sectional view of a device according to the invention in a second embodiment.

Detailed description of the invention

[0032] The present invention concerns an acoustic resonator device of the type used in the sector of electronics to generate and/or filter electrical signals whose frequency falls within a predetermined range.

[0033] More specifically, the acoustic resonator device that is the subject of the present invention is particularly suited to promote the generation/filtering of electrical signals that fall within the radio frequency band, typically within the band between 1.5 GHz and 30 GHz.

[0034] The expression “acoustic resonator device” used in this context applies to electronic oscillator devices capable of transducing electrical signals into mechanical waves (called acoustic waves) that are generated within the device itself due to dimensional deformation (or mechanical vibration) of the components.

[0035] Acoustic resonator devices, therefore, work according to the operating principle based on the propagation of mechanical waves (also called acoustic waves) within them, said mechanical waves having a predetermined amplitude and pattern trend. Through the propagation of these acoustic waves, electrical signals with predetermined characteristics can be generated at the ends of corresponding electrodes associated with the resonator.

[0036] It is therefore possible to define the resonator device as a piece of apparatus suited to transduce the electrical signal present at the ends of the electrodes into acoustic waves that propagate inside the device (and vice versa).

[0037] Such behaviour can be exploited to make electronic oscillators, that is, devices suited to generate periodic electrical signals centered on a predetermined frequency band.

[0038] Alternatively, resonator devices 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 band-pass filter, a high-pass or low-pass filter, a band-stop filter, etc.).

[0039] The resonator device that is the subject of the present invention, indicated in the attached figures with the reference number 1, is designed to be mainly employed as a filtering element in radio frequency equipment used in 5G technology and in the related new standards.

[0040] In particular, Figures from 1 to 3 show a resonator 1 designed to operate in the frequency band above 5 GHz and in particular above 10 GHz.

[0041] However, it is understood that these embodiments of the invention are provided only by way of example and the innovative technical characteristics described below may also be reproduced in other types of RF resonators operating in bands that are different from those mentioned above.

[0042] In addition, the acoustic resonator 1 described below can also be used as an oscillator and/or filtering element for frequency intervals different from those specified and not necessarily falling within the radio frequency band.

[0043] The acoustic resonator 1 that is the subject of the invention has a layered structure obtained by superimposing layers of materials of various types.

[0044] A sectional view of said structure can be seen in Figure 2 and Figure 3, while the shape of these layers in plan view, visible in Figure 1, may be different, depending on the design specifications or the installation environment of the device 1.

[0045] Figures 2 and 3 show different geometric shapes of resonator devices obtained by modifying the plan shape of the various layers.

[0046] The device 1 first of all comprises a layer of piezoelectric material 2 that extends between one pair of end faces 3, 4.

[0047] This layer 2 may have a geometric shape in plan view corresponding to a regular polygon with n sides (for example, a square, a rectangle, a trapezoid or any other polygon).

[0048] Conveniently, the piezoelectric material 2 is of the monocrystalline or polycrystalline type and can be selected from 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 electro-mechanical properties.

[0049] In addition, the material used to make the piezoelectric layer 2 can also be selected from among materials based on those specified above but obtained with different doping values.

[0050] The resonator device 1 also comprises one or more additional layers placed directly in contact with a corresponding face 3, 4 (or with the corresponding faces 3, 4) of the layer of piezoelectric material 2.

[0051] The main characteristic of this additional layer, indicated in the drawings by the reference number 5, lies in that it is made of a metallic material or an alloy of metallic materials.

[0052] This characteristic, combined with other properties mentioned below, allows this layer of metallic material 5 to serve two functions: firstly, said layer 5 constitutes an electrode for the resonator device, capable of exciting the layer of piezoelectric material 2 in such a way as to promote the generation of an acoustic wave; secondly, said layer 5 also constitutes a reflector capable of (at least partially) reflecting the acoustic wave generated by the layer of piezoelectric material 2. [0053] Advantageously, the two functions specified above (electrode and reflector) are performed simultaneously by the layer of metallic material 5 placed directly on one (or both) of the faces 3, 4 of the layer of piezoelectric material 2.

[0054] The presence of a layer of metallic material 5 conveniently designed to be placed directly in contact with a face 3, 4 of the material forming the piezoelectric layer 2 provides some important advantages.

[0055] In general, in the resonators known in the art, the electrode has no reflective properties and therefore this element is incorporated in the resonant cavity of the device. [0056] In the context of the present invention, the expression "resonant cavity" means a geometric space with predetermined dimensions, inside which there is most of the energy associated with the mechanical wave produced during the oscillation of the active material, that is, the piezoelectric material 2.

[0057] In general, the energy associated with the acoustic wave residing within the resonant cavity exceeds 95% of the total energy generated by the piezoelectric material and associated with said wave.

[0058] It is known that the resonator device functions properly only in the case where the resonant cavity has predetermined dimensions selected according to the wavelength (usually multiples of half a wavelength).

[0059] Therefore, the thicknesses of the electrodes are reduced as much as possible in order to minimize their contribution within the resonant cavity; in this way the thickness of said resonant cavity is mainly obtained through the contribution of the layer of piezoelectric material.

[0060] In the resonator device that is the subject of the present invention, the electrode has been incorporated in a metallic layer 5 which serves the additional function of reflecting the acoustic wave generated by the layer of piezoelectric material 2.

[0061] In this configuration, the electrode is no longer part of the resonant cavity (which is entirely defined by the thickness of the piezoelectric material), rather this component is “buried” within the layer 5, which also serves the function of an acoustic reflector. [0062] The advantages of this solution are many: firstly, with respect to the known state of the art, the thickness s _å of the piezoelectric material can be greater due to the fact that the entire resonant cavity is constituted and defined only by the latter (and no longer by the coupling of the piezoelectric material with the electrode). [0063] In addition to the above, the electrode is significantly thicker than those present in the resonators known up to now, since the dimensions of these components must now conform to those of the reflectors which, as is known, have thicknesses equal to a quarter wavelength or multiples thereof.

[0064] In a first configuration of the resonator device 1 , the layer of metallic material 5 suited to constitute both the electrode and the acoustic reflector can be a single layer and can therefore be in contact with a face 3, 4 of the layer of piezoelectric material 2. [0065] This example is illustrated in the device shown in Figure 2, where a single metallic layer 5, positioned on the lower face 3 of the piezoelectric material 2, is used. [0066] For the device to work properly, the layer of piezoelectric material 2 must always be interposed between a pair of electrodes.

[0067] In this case, the resonator 1 has an additional electrode, indicated by the reference number 6 and consisting of a layer of conductive material.

[0068] This configuration is clearly illustrated in Figure 2 and Figure 3.

[0069] Said electrode 6, however, is not suited to define an acoustic reflector and its function is therefore completely similar to that of the electrodes already known in the state of the art.

[0070] The electrode 6 is placed directly in contact with the face 4 of the layer of piezoelectric material 2 opposite the face on which the metallic layer 5 is placed.

[0071] In the case illustrated in the Figures, the electrode 6 is placed directly in contact with the upper face 4 of the layer of piezoelectric material 2.

[0072] Conveniently, the electrode 6 can have a predetermined thickness S ₆ .

[0073] In a different embodiment of the resonator device 1 a plurality of layers of metallic material 5 may be used.

[0074] For example, there may be two layers of metallic material 5, each placed opposite the layer of piezoelectric material 2.

[0075] The layer of piezoelectric material 2 is then sandwiched between two layers of metallic material 5 placed directly in contact with the respective faces 3, 4 of the same. [0076] This configuration is not illustrated in the figures.

[0077] Alternatively, the resonator device 1 may comprise a stack of layers of metallic materials 5, that is, a plurality of mutually superimposed metallic layers 5.

[0078] The first layer 5 of the stack is thus placed directly in contact with one face 3 of the layer of piezoelectric material 2. [0079] This configuration is illustrated in Figure 3.

[0080] In this case, the electrode is substantially distributed throughout the thickness of the stack of layers of metallic material 5, as each of them has conductive properties. [0081] Furthermore, each metallic layer 5 of the stack also serves the function of a reflector, and therefore by superimposing the layers it is possible to obtain a cascade of acoustic mirrors, each suited to reflect a portion of the incident acoustic wave that propagates from the piezoelectric material 2.

[0082] As is better illustrated in Figure 1, the layer of conductive metallic material 5 serving as an electrode may have such dimensions as to completely or partially cover the end face 3, 4 of the piezoelectric material 2 on which it is placed in contact.

[0083] Conveniently, as better illustrated in Figure 1 , the layer of metallic material 5 may have such dimensions as to cover only part of the end face 3, 4 of the piezoelectric material 2.

[0084] If an electrical signal is applied to the layer of metallic material 5 serving as an electrode, said signal is suited to promote the excitation only of the area of the piezoelectric material 2 which is subtended by the layer of metallic material 5, that is, the area of the piezoelectric material 2 which lies under the metallic layer 5.

[0085] In other words, the portion of the piezoelectric material 2 covered by the layer of metallic material 5 serving as an electrode represents the only part of the material itself suited to generate the acoustic wave. This acoustic wave, therefore, will propagate outwards starting from the only portion of the face of the piezoelectric material 3, 4 which is in the excited state (and which corresponds to the area of the face 3, 4 covered with the layer of metallic material 5).

[0086] In general, the layer of metallic material 5 serving as an electrode has lateral extensions greater than those of the end face 3, 4 of the piezoelectric material 2 in such a way as to define appropriate projections 7 suited to be connected to other circuits electrically connected to the resonator.

[0087] In particular, the projections may be substantially two-dimensional and extent along their development between a small end 8 (suited to be superimposed on face 3, 4 of the layer of piezoelectric material 2) and an opposite end 9, protruding from the layer of piezoelectric material 2 and having maximum dimensions, intended to be connected to other electrical or electronic circuits.

[0088] The shape of the layer of metallic material 5 in plan view may be substantially polygonal (for example, a regular polygon or an irregular polygon) or it may include at least one circular or semicircular portion, as better visible in Figure 1.

[0089] Conveniently, the layer of piezoelectric material 2, in fact, has substantially isotropic physical properties with respect to the two end faces 3, 4.

[0090] Consequently, exciting the piezoelectric material 2 (for example, by applying an electrical signal to the pair of electrodes) means promoting the generation of two substantially equal mechanical waves that propagate along a longitudinal direction Z (which extends perpendicularly to faces 3, 4) but with opposite directions of propagation. [0091] Furthermore, the propagation direction Z passes through all the layers of other material that make up the resonator 1 and that are facing or superimposed on the two faces 3, 4 of the layer of piezoelectric material 2.

[0092] As already mentioned above, the acoustic waves generated by the vibration applied to the piezoelectric material 2 tend to be “retained” within the device 1 by means of a plurality of reflectors arranged on the opposite side with respect to the faces of the piezoelectric material.

[0093] Retaining the wave within the layers of the device 1 makes it possible to trigger a resonance condition that is such as to promote the generation of a stationary acoustic wave.

[0094] The fraction of the oscillating stationary wave that is progressively transmitted between the various layers (and is not reflected) represents the losses of the resonator device 1.

[0095] Obviously, the device 1 will be designed in such a way as to minimize this kind of losses and, instead, make it easier to maintain the stationary condition of the acoustic wave.

[0096] As is known, acoustic reflectors serve 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 device 1 in the active resonance condition as long as possible.

[0097] In this description, in Figures 1 - 3, the reference number 5 indicates both the metallic layer and the corresponding electrode and acoustic reflector defined by the same.

[0098] Additional reflectors constituted by additional layers of materials other than metal are indicated by the reference number 10 and are better described below in the present description.

[0099] It is well known in the state of the art of resonators 1 that the reflective layers 5, 10 are not able to fully reflect the incident acoustic wave, in practice reflection losses are generally less 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 that wave to the next layer). [00100] The layers made of metallic material 5 also serve as reflectors and for this reason they can consist of a plurality of sub-layers 11, 12 respectively constituted by a low acoustic impedance metallic material and a high acoustic impedance metallic material.

[00101] The metallic layers 11 with low acoustic impedance can be made from films or alloys of one or more of the following materials: copper and aluminium.

[00102] The layers 12 with high acoustic impedance can be made from films or alloys of one or more of the following materials: tungsten, molybdenum, tantalum, tantalum nitride, gold, platinum, ruthenium, iridium and alloys of these materials.

[00103] Conveniently, each layer of metallic material suited to define a reflector can have a number X of layers of high impedance material 12 and a number Y of layers of low impedance material 11.

[00104] This embodiment can be seen in the device illustrated in Figure 3.

[00105] In particular, the total number X of sub-layers 12 of high impedance material and the total number Y of sub-layers 11 of low impedance material included in one single metallic reflector 5 may be the same (X = Y) or different (X ¹ Y).

[00106] For example, there may be just one single sub-layer 11, 12 (X = Y = 1) while the maximum number of said sub-layers 11, 12 may be variable and determined by construction and/or design requirements.

[00107] However, when one of said sub-layers 11 , 12 is present in a number higher than one (X > 1 and/or Y > 1), the arrangement thereof is always alternated in such a way that between two layers of the same type there is a layer of the opposite type.

[00108] For example, if X = 2 and Y =1 , the single low impedance metallic sub-layer 11 is interposed between the two high impedance metallic sub-layers 12, just as in the case where X = Y = 4 the corresponding reflector 5 is constituted by the superimposition of four equal pairs, each consisting of a high impedance metallic sub-layer 12 coupled with a low impedance metallic sub-layer 11.

[00109] In addition to the above, the low impedance sub-layers 11 and the high impedance sub-layers 12 can have respective thicknesses Sn, Sn with a predetermined value; the metallic layer 5 thus has a predetermined thickness Ss obtained from the sum of the thicknesses Sn, Si _å of the low and high impedance metallic sub-layers 11, 12 that make it up (Ss = Sn + S12).

[00110] The thickness S12 of the metallic sub-layers 12 made of high impedance material and the thickness Sn of the metallic sub-layers 11 made of low impedance material vary according to the frequency of the acoustic wave generated by the layer of piezoelectric material 2.

[00111] More specifically, the thickness Sn, S12 of each sub-layer 11, 11 of high and/or low acoustic impedance material can be selected so that it is proportional to a fraction of the period of the acoustic wave l propagating within them.

[00112] The acoustic wave propagates in the corresponding metallic sub-layer 11, 12 with a predetermined propagation speed, said speed varies according to the type of material used to make the sub-layer itself.

[00113] Therefore, each sub-layer 11, 12 defines its own propagation time for the acoustic wave. Said time can be defined as the time interval it takes for each point of the acoustic wave to travel through the thickness separating the end surfaces of the corresponding sub-layer 11, 12.

[00114] In other words, the propagation time can be calculated as the ratio between the thickness Sn, Si2 of the corresponding sub-layer 11, 12 and the propagation speed of the acoustic wave within the same layer.

[00115] Conveniently, the thickness sn, S12 of the metallic sub-layer 11, 12 can thus 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 of the material.

[00116] For example, the thickness Sn of the sub-layer 11 with low acoustic impedance and the thickness S12 of the layer 12 with high acoustic impedance can be selected in such a way as to allow the propagation of a quarter of a period (TT/2) or three quarters of a period (3/2TT).

[00117] Conveniently, in addition to the layer/s of metallic material, the device may also comprise one or more reflective layers 10 made of an insulating material.

[00118] Also in this case, the reflective layers 10 made of an insulating material may comprise a first sub-layer 13 made of low acoustic impedance insulating material and a second sub-layer 14 made of high acoustic impedance insulating material.

[00119] Conveniently, the resonator 1 can be configured to include a plurality of reflectors 10 made of insulating material and mutually superimposed.

[00120] The thickness Sio of each individual reflector 10 made of insulating material can be obtained as the sum of the thicknesses S ₁₃ ; S ₁₄ of the high and low impedance sub layers 13, 14 that make it up.

[00121] More specifically, the thickness S ₁₃ ; S ₁₄ of the sub-layers 13; 14 that make up each reflector 10 can be selected in such a way as to satisfy a variable dimensional relation as a function of the phase length t_p associated with the excited mode passing through the same layer.

[00122] Conveniently, in the technical field of resonators the propagation time is also referred to as “phase length”. By indicating the phase time with the symbol t_p, the thickness of a given reflective layer and/or piezoelectric material with s and the propagation speed of the acoustic wave within the same layer with v_p, the following relationship can be defined: t_p = s / v_p

[00123] Furthermore, the expression “vibration mode” or “excited mode” used in this description is intended to refer to the characteristic mode of vibration related to a system or structure having several points with different vibration amplitudes.

[00124] A vibration mode comprises i) a time variation of the vibration and ii) a space variation of the amplitude of movement through the structure.

[00125] The time variation defines the frequency of the oscillations.

[00126] The space variation defines the different vibration amplitudes from one point of the structure to another.

[00127] The sub-layers 13, 14 made of high impedance or low impedance material which define each individual reflector 10 of the plurality used in a resonator 1 can be selected so that they have a thickness S ₁₃ ; S ₁₄ that varies according to the following relation: six = (2N + 1 )*(TT/2) wherein:

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

N is a positive integer (1, 2, 3, ...);

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

[00128] According to this relation, therefore, the sub-layers 13, 14 constituting one or more reflectors 10 made of insulating material may have, with respect to the period 2* p of the mode excited by the layer of piezoelectric material 2, a thickness S ₁₃ ; S ₁₄ selected according to the following series:

3*77/2 ; 5* TT 2; 7* 77/2; 9* 77/2; 11* p/2; 13* 77/2 ...

[00129] In addition, further reflectors 10 made of insulating material and used in the same resonator 1 may have sub-layers 13, 14 made of high or low impedance material and having a respective thickness S ₁₃ ; S ₁₄ selected so as to be substantially equal to a quarter of a period referred to the excited mode, that is, suited to satisfy the following relation:

Si _x = tt/2 wherein

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

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

[00130] In essence, therefore, the acoustic resonator 1 that is the subject of the present invention may comprise a stack of reflectors 10 made of metallic material.

[00131] The reflectors of said stack which are arranged in proximity to the layer of piezoelectric material 2 can be constituted by sub-layers 13, 14 whose thickness S ₁₃ ; S ₁₄ is selected so as to satisfy the following series:

3*77/2 ; 5* TT 2; 7* TT/2; 9* p/2; 11* p/2; 13* p/2 ...

[00132] The remaining reflectors 10 made of insulating material which complete the stack and are arranged at a greater distance from the layer of piezoelectric material (compared to the distance of the reflectors 10 described in the previous paragraph) may consist of sub-layers 13, 14 whose thickness S ₁₃ ; S ₁₄ is selected in such a way as to meet the following series:

Six = TT/2 that is, equal to the quarter wave of the mode excited by the layer of piezoelectric material 2.

[00133] In this way, the thickness of the insulating layers 10 made is greater near the layer of piezoelectric material and decreases at a certain distance from thereof.

[00134] In this way, by appropriately adjusting the thicknesses of the sub-layers according to the relations set out above, the reflectors 10 made of insulating material make it possible to effectively reflect, with reduced losses, different types of acoustic waves, namely i) shear stress waves (which also have a propagation component along a direction substantially perpendicular to Z) and ii) longitudinal stress waves (whose propagation is therefore substantially parallel to Z).

[00135] The same considerations made in the previous paragraphs (regarding the embodiment of the metallic reflector 5) with respect to the positioning of the sub-layers 13, 14, the number and thickness of said sub-layers 13, 14 apply also for the reflectors 10 made of insulating material.

[00136] Conveniently, the sub-layer of insulating material 13 with low acoustic impedance can be selected from among the group comprising silicon dioxide, spin-on glass, tellurium oxide, and silicon oxycarbide.

[00137] In addition, the sub-layer of insulating material 14 with high acoustic impedance can be selected from the group comprising aluminium nitride and respective oxides of tungsten, platinum, molybdenum, ruthenium.

[00138] The device may comprise at least one layer 15 of insulating material which, depending on the case, can have limited low acoustic impedance or considerable high acoustic impedance. This layer 15 can have a predetermined thickness Sis.

[00139] The layer 15 does not have reflective properties and, for example, 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 on these but with the addition of dopants or impurities.

[00140] The function of said low or high acoustic impedance layers 15 is primarily to compensate for the thermal drifts to which the layer of piezoelectric material 2 is subjected.

[00141] As is known, the frequency (and/or band) of oscillation associated with the piezoelectric material 2 varies according to the temperature in the environment in which said layer is placed.

[00142] In the technical field of resonators this condition is referred to with the expression “temperature coefficient of frequency”, which indicates the frequency variation as a function of temperature (generally the frequency varies by a few tens of millionths of the normalized frequency per unit of a degree centigrade). The temperature coefficient of frequency, therefore, is a parameter that expresses the variation in millionths (that is, 10 ⁶ of the value of the working frequency in Hz).

[00143] The frequency at a temperature coefficient is negative when, as the temperature increases, the oscillation frequency decreases.

[00144] In contrast, the frequency at a temperature coefficient is positive when, as the temperature increases, the oscillation frequency increases.

[00145] 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 exclusively decreases as the temperature increases).

[00146] For this reason, the layer 15 of material with low or high acoustic impedance has a positive frequency at a temperature coefficient, that is, its internal structure is such that it promotes an increase in operating frequency as the temperature increases. [00147] In this way, the assembly constituted by the layer of piezoelectric material 2 and the low or high impedance layer 15 is such that it has a reduced thermal drift (referred to the operating frequency).

[00148] In particular, the change in the oscillation frequency associated with the piezoelectric layer 2 (caused by the temperature change) is substantially cancelled (or strongly reduced) due to a variation in the frequency with opposite sign associated with the behaviour of the layers 15 made of low or high impedance material.

[00149] Furthermore, it is possible/necessary to insert two distinct layers of low impedance or high impedance material 15, 15', each facing the respective face 3, 4 of the layer of piezoelectric material 2.

[00150] Conveniently, a layer of low or high impedance insulating material 15 is always superimposed on the electrode 6 which is directly in contact with a face 3, 4 of the layer of piezoelectric material 2.

[00151] The other layer of low or high impedance material 15' (optional), instead, is superimposed on the last reflector 10 made of insulating material.

[00152] Conveniently, any layer of low impedance or high impedance insulating material 15, 15' can be made from the same material or from different materials with similar chemical-physical properties; in this way, said layers 15, 15' have substantially the same dynamic behaviour when the acoustic wave generated by the layer of piezoelectric material 2 passes through them.

[00153] According to a peculiar aspect of the invention, the thicknesses of the material layers making up the resonator 1 that is the subject of the present invention are selected so as to satisfy the present dimensional relation:

- the thickness s _å of the layer of piezoelectric material 2 substantially corresponds to a multiple of half the wavelength of the acoustic wave generated by the same layer following its excitation (s _å = N*l/2);

- the thickness Ss of the metallic reflective layer/electrode 5 placed directly in contact with one face 3, 4 of the piezoelectric material 2 is substantially a multiple of half the wavelength of the acoustic wave generated by the layer of piezoelectric material (Sis = N*l/2);

- it is possible to define with the reference letter (S ₆ _is) the sum of the thickness S ₆ of the electrode 6 (placed on a face 3, 4 of the layer of piezoelectric material 2) and the thickness Sis of the low or high acoustic impedance insulating layer 15 superimposed on said electrode 6; the value of the thickness S ₆ _is substantially corresponds to a multiple of half the wavelength of the acoustic wave generated by the layer of piezoelectric material (S ₆ _is = N ^* l/2).

In the above relations, the letter N represents a natural integer greater than or equal to one (N = 1, 2, 3, ...).

[00154] According to the above, the thicknesses S2, Ss and S ₆ _is can have a value equal to l/2 (minimum value) or a value equal to multiples of l/2 (l, 3l/2, ...).

[00155] For example, the layer of piezoelectric material 2 can have a predetermined thickness s _å included between 100nm and 5000nm, and the thickness Sis of these layers 15 with low or high acoustic impedance can be included between 50nm and 10pm.

[00156] The thickness S ₆ of the electrode 6 and the thickness Sis of the layer 15 of non- reflective insulating material with low acoustic impedance are both less than l/2 (if considered individually).

[00157] However, these thicknesses S ₆ , Sis are selected in such a way as to maintain their sum S ₆ _is constant at a value equal or close to l/2 (or multiples of l/2).

[00158] Furthermore, in the case where a resonator 1 has an additional non-reflective layer 15' (with low acoustic impedance or high acoustic impedance) superimposed on the insulating reflector 10 (or superimposed on the last insulating reflector 10 of the stack), even this additional layer 15' has a corresponding thickness Sis’ equal or close to N*l/2.

[00159] The expressions “high acoustic impedance layer” and “low acoustic impedance layer” used in this description are to be understood in a relative sense. In fact, a layer made of a low impedance material has a first predetermined impedance value (with respect to the passage of the acoustic wave) which is lower than the impedance value of a different layer made of a high acoustic impedance material.

[00160] Conversely, a layer made of a high impedance material has a second predetermined impedance value (with respect to the passage of the acoustic wave) which is higher than the impedance value of a different layer made of a low acoustic impedance material.

[00161] In other words, the first impedance value associated with the layers made of a low acoustic impedance material is always lower than the second impedance value associated with the layers made of a high acoustic impedance material (and vice versa). [00162] Conveniently, the resonator can be anchored to a base substrate 16 preferably made of a high acoustic impedance material. For example, said base substrate 16 can be chosen from among the following materials: silicon, silicon carbide, sapphire, lithium niobate, lithium tantalate, glass, quartz, aluminium nitride and diamond. [00163] Conveniently, the teachings provided in relation 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).

[00164] The present invention can be carried out in other variants, all falling within the scope of the inventive features claimed and described herein; these technical features can be replaced by different technically equivalent elements and materials; the shapes and dimensions of the invention can be any, provided that they are compatible with its use.

[00165] The numbers and reference signs included in the claims and in the description are only intended to increase the clarity of the text and must not be considered as elements limiting the technical interpretation of the objects or processes identified by them.