Multi-layer structure and filter comprising the same

Abstract

There is provided multi-layer structure for a bulk acoustic wave resonator, comprising: a piezoelectric layer (702); a first top electrode layer (704) above the piezoelectric layer; a first bottom electrode layer (714) below the piezoelectric layer (702); a second top electrode layer (706) above the first top electrode layer (704); a second bottom electrode layer (716) below the first bottom electrode layer (714); a third top electrode layer (708) above the second top electrode layer (706); and a third bottom electrode layer (718) below the second bottom electrode layer (716) The first top electrode layer (704) and first the bottom electrode layer (714) are configured to form acoustic confinement layers, and the second top electrode layer (706) and the third top electrode layer (708) are configured to form a first acoustically free surface towards the first top electrode layer (704), and the second bottom electrode layer (716) and the third bottom electrode layer (718) are configured to form a second acoustically free surface towards the first bottom electrode layer (714).

Description

MULTI-LAYER STRUCTURE AND FILTER COMPRISING THE SAMEFIELD

[0001] The invention relates to a structure for a bulk acoustic wave resonator and more particularly to an electrode structure for a bulk acoustic wave resonator that can be scaled to construct filters over a wide frequency range.BACKGROUND

[0002] A mobile communication radio needs to be able to transmit and receive strictly only in the frequency bands allocated to it and be able to reject transmissions and noise at other frequencies. This can be achieved by using bandpass, or passband, filters which in the compact mobile devices are based on the microacoustic technology, i.e. they use the piezoelectric effect to convert the radio signal to a surface acoustic wave (SAW) or bulk acoustic wave (BAW) in acoustically resonating structures. BAW can be a film bulk acoustic wave resonator (FBAR) that comprises a thin-film piezoelectric, which is sandwiched between the top and bottom electrodes, or a solidly mounted resonator (SMR). The FBAR may be based on a suspended membrane or a cavity structure, whereas an SMR is fabricated directly on a substrate. Thin film bulk acoustic wave resonators (FBARs) are used to construct bandpass filters for mobile communication systems operating in dedicated frequency bands within in a wide range of spectrum from below 1 GHz to 6 GHz and extending to even higher frequencies. BAW based technology is better suited for frequencies above 2 GHz than SAW-based.

[0003] 4G mobile communication bands have been mostly allocated below 2.5 GHz but 5G has introduced bands between 3-6 GHz and near 30 GHz. More bands are emerging for frequencies above 6 GHz. This brings great challenges for filters as it is demanding to scale the acoustic filters to over 5 GHz and at present it seems impossible to scale them up to 30 GHz with low losses and high bandwidth. At the 30 GHz range there are no miniature, high performance, and mass producible filters available at present.

[0004] Fig. 1 illustrates an example of a bulk acoustic wave resonator. The BAW resonator 100 can be a FBAR comprising a stack of layers, or a layer stack. The layer stack comprises a thin piezoelectric film, or piezo layer, 102 sandwichedbetween metal electrodes 104. RF signal applied to the electrodes is converted to an acoustic wave with the resonator volume defined by the thickness t of the piezoelectric layer and the electrodes. Dashed line illustrated stress distribution of a fundamental half-wavelength thickness resonance. The fundamental acoustic resonance will occur when the acoustic wavelength at the RF signal frequency effectively corresponds to a half of an acoustic wavelength within the thickness of piezoelectric layer and the electrodes.

[0005] In a simplified example of the BAW resonator 100, the electrodes 104 have the same acoustic velocity as the piezo layer 102, whereby the resonance occurs at frequency f =c<1> where v is the acoustic velocity and t is the total thickness of the layer stack of electrodes and the piezo layer. The resonance forms a frequency-selective function which allows building filters, or BAW filters. The BAW filters are passband filters that are built by connecting BAW resonators of at least two different frequencies f into a network often called the ladder configuration.

[0006] It should be noted that electrically, the BAW / FBAR resonator behaves like a capacitance Cohaving in addition a local low impedance point, corresponding to the local resonance, and a local high impedance point, corresponding to a local antiresonance. The behaviour may be described by a Butterworth van Dyke equivalent circuit.

[0007] Fig. 2 illustrates an example of electrical impedance of a BAW resonator. The magnitude of electrical impedance 202 is illustrated in Ohms over a frequency range from slightly above 0 GHz to 3 GHz. The BAW resonator has a resonance frequency, fr, 204 at 2GHz and an antiresonance frequency, fa, 206 at slightly above 2 GHz. The resonance frequency, fr, is at the lowest point of the electrical impedance over the frequency range and the antiresonance frequency fais at the highest point of the electrical impedance over the frequency range. A frequency difference of the resonance, fr, and antiresonance frequencies, fa, defines an electromechanical coupling coefficient K2of the BAW resonator and how wide bandwidth filters can be constructed from the BAW resonator.

[0008] Definition of K2according to IEEE standard on piezoelectricity iswhere fsis a resonance frequency, fr, and fpis an antiresonance frequency, fa.

[0009] However, a more straightforward indicator for electromechanical coupling strength that is directly related to bandwidth is Af =, expressed in %. As a fr simple rule of thumb, a ladder filter with bandwidth of 1.2Af can be constructed without additional external components.

[0010] Fig. 3 illustrates an example of a Butterworth van Dyke equivalent circuit of a bulk acoustic wave resonator. The Butterworth van Dyke equivalent describes the electrical behaviour of the BAW resonator. Rsis the series resistance, Co-Ro path formed by a capacitor Co connected to a resistor Ro represents the capacitance of the BAW resonator and Lm-Cm-Rm path formed by an inductance Lmconnected to a capacitor Cmand a resistor Rmrepresents the piezoelectric resonating part of the BAW resonator. Rsis the series resistance of the electrodes of the BAW resonator, and Lsis a possible series inductance of the electrodes.

[0011] Fig. 4 illustrates examples of topologies for ladder filters. Each of the topologies comprises BAW resonators of at least two different frequencies f connected into a network. Accordingly, the topologies define BAW filters. The topologies show BAW resonators labelled HI and LO connected into networks. The BAW resonators labelled HI are series resonators having a higher frequency and the BAW resonators labelled LO are parallel resonators having a lower frequency. Topology a) shows a single ladder stage that may be used as a basic building block for ladder filters. Topology b) shows a 3-stage ladder filter comprising 3 basic building blocks according to the topology a) connected in series. Topology c) shows a symmetric 3 14 stage ladder filter comprising 3 basic building blocks according to the topology a) connected in series with a BAW resonator HI.

[0012] It should be noted that properties of an individual resonator dictate the achievable performance of the filter which is constructed from them. Typically, a filter is desired to have at least some of the following properties:- low signal attenuation (insertion loss) in the passband- high signal attenuation outside the passband (stopband)- steep transition from passband to stop-band- desired width of the passband (bandwidth), matching the RF spectrum allocation and radio architecture requirements for the communication band in question.

[0013] It should be noted that filter performance requirements can be propagated to requirements for the constituting resonators in terms of frequency, Q-values, A or K2, and series resistance Rsof the electrodes of the BAW resonator.

[0014] The Q-value of a resonator indicates how low losses it has and how frequency selective it is. The higher the Q-value, the lower the losses in a resonator. Q value together with the series resistance Rsand coupling coefficient K2or f define the minimum insertion loss that can be achieved. The Q-value also defines the steepness of the transition from passband to stop-band. Effects contributing to Q-value (lowering it) include series resistance Rsin the electrodes, acoustic leakage from the resonator to the surroundings, and viscous losses of acoustic waves.

[0015] Q-values at resonance and antiresonance are indicated by the sharpness of the resonance and anti-resonance peaks illustrated in Fig. 2. Series resistance in the electrodes Rsaffects especially the Q-value at resonance. This is clear from expression of the Q-values derived for the equivalent circuit described with Fig. 3: Q-value at resonance, Qr, can be expressed asand Q-value at antiresonance, Qa, aswhere o)r= 2 nfris angular frequency at resonance frequency fr, o)a= 2 nfais angular frequency at antiresonance frequency fa, and Ro, Lm, Rsare as described with Fig. 3. Rois typically very low.

[0016] Scaling a resonator to higher frequencies just by thinning it down, all layers by the same factor, will keep the resonator A and K2the same, assuming materialquality stays the same for the thinner layers. Generally, the Q-value will drop, which means increased losses, when increasing frequency as loss mechanisms typically strengthen with increasing frequency. Most importantly, thinning down the electrodes will increase the series resistance Rsof the electrodes. Increased series resistance makes the resonator and filter composed of resonators to have poor frequency selectivity and high attenuation of the signal. It is desired to have the series resistance of a resonator in the 1 Ohm range or even better below 0.5 Ohms. We can consider sheet resistance or resistance of a single square of the electrode material as the figure of merit for electrode resistance:where peZis the resistivity of the electrode material and te\ it’s thickness. A single resonator typically contributes in the order of ~1 square worth of resistance.There is a maximum amount of electrode resistance per resonator 7?sthat the filter design can allow in order to meet insertion loss and passband edge steepness requirements.

[0017] The Table 1 below shows an example of how the sheet resistance Rsh in an FBAR behaves when an initial design of the FBAR for 5 GHz with RSh of 0.75 Q / sq is scaled up to 10, 15 and 30 GHz by straightforward thinning of the layers. The values are calculated based on typical resistivities of thin film materials, not bulk materials.Table 1: Direct scaling of a Mo-ScAIN-Mo FBAR to higher frequencies, maintaining constant coupling coefficient K2and achievable bandwidth Af.

[0018] The resistance RSh increases as inverse (1 / fe / ) of the electrode thickness (or faster in practice for very thin films) and grows fast to unreasonably high values for higher frequencies, illustrating the bottleneck formed by the electrode resistance.

[0019] Fig. 5 and Fig. 6 illustrate examples of FBAR performance by acoustic field distributions. In Fig. 5, the FBAR performance is illustrated by displacement (u), stress (T) and strain (S) for the FBAR of Table 1 at 5 GHz. The displacement (u), stress (T) and strain (S) are illustrated on top of the FBAR stack that is formed by a top electrode of molybdenum (Mo TOPE), a piezo layer of ScAIN and a bottom electrode of molybdenum (Mo BOTE) that form a resonating dimension of the FBAR. Scaling the structure uniformly keeps the vibration shape unchanged. Top of the device is on the left and bottom on the right.

[0020] If one wishes to keep the resistance of the FBAR equal to the value obtained at 5 GHz, when the FBAR is scaled up to 10, 15 and 30 GHz, one needsto keep the thickness of the electrodes constant and increase the frequency by thinning down the piezoelectric layer only. The Table 2 below shows an example of such approach. The coupling K2drops drastically as at higher frequency a larger fraction of the resonator consists of metal not contributing to the signal conversion from electrical to acoustic and back. There is also an obvious hard limit for upper frequency in this approach: when the piezoelectric layer goes to zero thickness, there is still a finite acoustic length of the two electrodes corresponding to a finite frequency and K2naturally goes to zero, yielding the resonator useless. By this method one is able to design a resonator to 10 GHz but trading off more than half of the achievable bandwidth.Table 2: Scaling the Mo-ScAIN-Mo FBAR to higher frequencies maintaining constant electrode thickness and sheet resistance.

[0021] Fig. 6 illustrates displacement (u), stress (T) and strain (S) for the FBAR of Table 2 at 10 GHz. The displacement (u), stress (T) and strain (S) are illustrated on top of the FBAR stack that is formed by a top electrode of molybdenum (MoTOPE), a piezo layer of ScAIN and a bottom electrode of molybdenum (Mo BOTE) that form a resonating dimension of the FBAR. Scaling the structure keeping the electrode thicknesses constant reduces the piezo thickness and thus capability of electromechanical conversion by the resonator and thus K2. Top of the device is on the left and bottom on the right.

[0022] Frequency of a passband of a filter constructed from BAW resonators, e.g. as described in Fig. 4, is determined by resonance frequencies of the BAW resonators. On the other hand, the resonance frequencies of the BAW resonators are determined by thicknesses, t, of layer stacks of the BAW resonators. Making the layers thicker reduces the frequency and making them thinner increases the frequency, allowing designing filters for different frequencies. Bandwidth of the filter is defined mainly by the coupling coefficients K2of the resonators, which can be affected by the choice of the piezomaterial and thickness ratio of piezoelectric material vs. electrodes and other non-piezoelectric materials in the stack.

[0023] The diminishing electrode thickness and thus too large resistance is a major bottleneck in scaling the BAW / FBAR resonance frequency up to 10 GHz and beyond while maintaining low losses.

[0024] Thus, the problem is that thick electrodes providing a low series resistance reduce the coupling coefficient and thus bandwidth and also set a hard upper limit for highest frequency.

[0025] Passivation is critical for BAW resonators for providing protection from environmental and mechanical degradation, ensuring electrical isolation, and enhancing the overall reliability and performance of the resonator. By providing a stable and protected environment for the sensitive components of the BAW resonator, passivation layers help maintain consistent operation over time, which is essential for their use in demanding applications like RF communication systems. The passivation layer provides tuning of resonator properties: by trimming down the passivation layer, we can affect the frequency response of the resonator, e.g. change the resonance frequency. However, if the passivation layer is on top of a thick electrode stack making the frequency response insensitive to changes at the surface, the tunability of the resonator by modifying the passivation thickness is poor. Too thick of a passivation on the other hand can result in spurious resonancesor mode splitting, and it is not possible to shift the resonant frequency in the order of 1 pole-zero distance (PZD), or Af, without significant drop in coupling.SUMMARY OF THE INVENTION

[0026] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0027] An object of the invention is to alleviate at least part of the above indicated disadvantages.

[0028] At least some embodiments provide a multi-layer structure for a bulk acoustic wave resonator which allows design and fabrication of filters for a wide frequency range.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Fig. 1 illustrates an example of a bulk acoustic wave resonator;

[0030] Fig. 2 illustrates an example of electrical impedance of a bulk acoustic wave resonator;

[0031] Fig. 3 illustrates an example of a Butterworth van Dyke equivalent circuit of a bulk acoustic wave resonator;

[0032] Fig. 4 illustrates examples of topologies for ladder filters.

[0033] Fig. 5 illustrates an example of acoustic field distributions in a film bulk acoustic wave resonator;

[0034] Fig. 6 illustrates another example of acoustic field distributions in a film bulk acoustic wave resonator with thick electrodes and thin piezoelectric layer; and

[0035] Fig. 7 illustrates an example of a multi-layer structure for a BAW resonator in accordance with at least some embodiments;

[0036] Fig. 8 illustrates an example of acoustic field distributions in a film bulk acoustic wave resonator in accordance with at least some embodiments;

[0037] Fig. 9 illustrates examples of bulk acoustic wave resonators in accordance with at least some embodiments;

[0038] Fig. 10 illustrates an example of a multi-layer structure for a BAW resonator in accordance with at least some embodiments; and

[0039] Fig. 11 illustrates examples of bulk acoustic wave resonators according at least some embodiments.EMBODIMENTS

[0040] The object of at least of some embodiments of the present invention is to provide a structure for a bulk acoustic wave resonator which allows design and fabrication of filters for a wide frequency range such as from below 1 GHz frequencies to 6 GHz frequencies and higher frequencies for example 10 GHz and higher frequencies.

[0041] The object is provided by a multi-layer for a bulk acoustic wave resonator, comprising: a piezoelectric layer; a first top electrode layer above the piezoelectric layer; a first bottom electrode layer below the piezoelectric layer; a second top electrode layer above the first top electrode layer; a second bottom electrode layer below the first bottom electrode layer; a third top electrode layer above the second top electrode layer; and a third bottom electrode layer below the second bottom electrode layer. The first top electrode layer and first the bottom electrode layer are configured to form acoustic confinement layers, and the second top electrode layer and the third top electrode layer are configured to form a first acoustically free surface towards the first top electrode layer, and the second bottom electrode layer and the third bottom electrode layer are configured to form a second acoustically free surface towards the first bottom electrode layer.

[0042] Identical or corresponding functional and structural elements which appear in the different drawings are assigned the same reference numerals. When the words first and second are used to refer to different elements, it is to be understood that this does not necessarily imply or mean that the first and second elements are somehow structurally substantially different elements or that their dimensions are substantially different unless specifically stated.

[0043] An acoustically free surface refers to a boundary or interface between stacked layers of solid materials which has effectively an impedance minimum, e.g. zero acoustic impedance, or a very low acoustic impedance, looking from the direction of incoming acoustic waves, here from the direction of the piezolayer at least at or near the operation frequency of the resonator. Solid materials aresubstances that possess a definite shape and volume and maintain their structural integrity under normal conditions of temperature and pressure (NTP). The NTP are commonly used reference conditions in science and engineering to standardize measurements and calculations. The impedance minimum may be a very low acoustic impedance that is less than acoustic impedances of typical solid materials which are in the range from 10 x106kg / m2s to 100 x106kg / m2s. Therefore, very low acoustic impedance is less than 10 x106kg / m2s. The acoustically free surface, or interface, forces the stress field to zero or to a very low value at the interface. In another words the interface behaves as if it was effectively or virtually free to move at least at a certain frequency, e.g. at least at or near the operation frequency of the resonator.

[0044] Fig. 7 illustrates an example of a multi-layer structure for a BAW resonator in accordance with at least some embodiments. The multi-layer structure comprises a piezoelectric layer, or a piezo layer 702, that is sandwiched between top electrode layers 704, 706, 708 and bottom electrode layers 714, 716, 718. The top electrode layers and bottom electrode layers are layers of electrically conductive material, e.g. metallic layers. The top electrode layers and the bottom electrode layers comprise arrangements of at least three layers on top of each other, where a layer 704, 714 that is the closest to the piezo layer 702, e.g. on the piezo layer, has a high acoustic impedance, Zac, and a next layer 706, 716 away from the piezo layer has a low acoustic impedance and a next layer away from the piezo layer has a high acoustic impedance. Accordingly, the electrode layers are arranged to alternate based on their acoustic impedances both above and below the piezo layer. The low acoustic impedance and high acoustic impedance layers together on each side of the piezo layer form a half-wave acoustic resonant volume with an effective free surface boundary condition both at the outer edges of the innermost high acoustic impedance layers 704 and 714 and at the outer edges of the outer most high acoustic impedance layers 708 and 718, regardless of whether outside the third layer lies vacuum, air, or a low acoustic impedance solid material, e.g. Al or SiO2. Outer edge means farthest away from the piezo layer, inner means closer to piezo layer.

[0045] It should be noted that the resonating dimension in the multi-layer structure of Fig. 7 is effectively formed by the piezoelectric layer and the innermost electrode layer that serve for acoustic confinement layers. Tunability of the resonance frequency of the multi-layer structure of Fig. 7 can be achieved by scaling down the total thickness of the layers forming the resonating dimension. Since the layers of the multi-layer structure are metal, the series resistance can be very low.

[0046] In an example in accordance with at least some embodiments, the multilayer structure of Fig. 7 comprises a passivation layer on top of the outer most high acoustic impedance layer 718. In this way the multilayer structure may be protected from environmental factors such as moisture, oxygen, or contaminants that could degrade their performance or reliability. The passivation layer may be of AIN, SiN, AI2O3, for example. It should be noted that tuning of the resonance frequency based on adapting the thickness of the passivation layer on top of the outer most high acoustic impedance layer 718 may result in mode splitting or spurious resonances and reduced coupling, thus deteriorating the performance, at least if the thickness would be adapted, e.g. reduced, for achieving a shift of the resonance frequency in the order of 1 pole-zero distance.

[0047] In an example, at least some of the top electrode layers 704, 706, 708 and bottom electrode layers 714, 716, 718 comprise acoustic confinement / compression layers 704, 714 that are the closest to the piezo layer and at least some of the top electrode layers 704, 706, 708 and bottom electrode layers 714, 716, 718 comprise termination layers 706, 708, 716, 718 that are arranged on the acoustic confinement / compression layers. The acoustic confinement / compression layers are configured to confine and / or compress an acoustic wave at a resonant frequency between a space between the acoustic confinement layers / compression layers. The confinement and / or compression reduces leakage of acoustic energy outside the space between the confinement / compression layers, which facilitates maintaining a high coupling coefficient K2and a high quality Q-factor. The termination layers on both sides of the piezo layer are configured to form an acoustically free surface towards the acoustic confinement layers / compression layers. In this way the acoustic wave sees the outer boundary of the acoustic confinement / compression layer and the termination layers as acoustically free surface.

[0048] It should be noted that the termination layers 706, 708, 716, 718, both above and below the piezo layer 702, comprise a pair of electrode layers that have different acoustic impedances. The electrode layers of each of the termination layers are ordered such that an electrode layer 706, 716 that has a low acoustic impedance is arranged closest to the piezo layer, whereby an electrode layer 708, 718 that has a high acoustic impedance is arranged on top of the low acoustic impedance layer 706, 716. It should be noted that the low acoustic impedance layer 706, 716 is positioned on top of the piezo layer both below the piezo layer and above the piezo layer, whereby the low acoustic impedance layer 706, 716 is positioned towards the piezo layer from the high acoustic impedance layer 708, 718.

[0049] In an example in accordance with at least some embodiments, a multilayer structure for a bulk acoustic wave resonator, comprises: a piezoelectric layer 702; a first top electrode layer 704 above the piezoelectric layer; a first bottom electrode 714 layer below the piezoelectric layer; a second top electrode layer 706 above the first top electrode layer; a second bottom electrode layer 716 below the first bottom electrode layer; a third top electrode layer 708 above the second top electrode layer; and a third bottom electrode layer 718 below the second bottom electrode layer; wherein the first top electrode layer and first the bottom electrode layer are configured to form acoustic confinement layers, and the second top electrode layer and the third top electrode layer are configured to form a first acoustically free surface towards the first top electrode layer, and the second bottom electrode layer and the third bottom electrode layer are configured to form a second acoustically free surface towards the first bottom electrode layer.

[0050] In an example in accordance with at least some embodiments, the first acoustically free surface is between the first top electrode layer 704 and the second top electrode layer 706 and the second acoustically free surface is between the first bottom electrode layer 714 and the second bottom electrode layer 716, and each of the first acoustically free surface and the second acoustically free surface have effectively an impedance minimum at least at or near the operation frequency of the resonator.

[0051] In an example in accordance with at least some embodiments, each of the first top electrode layer 704 and the third top electrode layer 708 have an acousticimpedance (High Z) that is higher than an acoustic impedance (LowZ) of the second top electrode layer 706. The low acoustic impedance and high acoustic impedance layers together form a half-wave acoustic resonant volume with an effective free surface boundary condition both at outer edges of the innermost high acoustic impedance layers 704 and 714 and at the outer edges of the outer most high acoustic impedance layers 708 and 718.

[0052] In an example in accordance with at least some embodiments, each of the first bottom electrode layer 714 and the third bottom electrode layer 718 have an acoustic impedance (High Z) that is higher than an acoustic impedance (Low Z) of the second bottom electrode layer 716. The low acoustic impedance and high acoustic impedance layers together form a half-wave acoustic resonant volume with an effective free surface boundary condition both at outer edges of the innermost high acoustic impedance layers 704 and 714 and at the outer edges of the outer most high acoustic impedance layers 708 and 718.

[0053] In an example in accordance with at least some embodiments, each of the first top electrode layer 704 and the first bottom electrode layer 714 have an acoustic impedance (High Z) that is higher than an acoustic impedance of the piezo layer 702. In this way confinement of an acoustic wave at a resonant frequency between a space between the acoustic confinement layers / compression layers may be supported.

[0054] In an example in accordance with at least some embodiments, each of the first top electrode layer, the third top electrode layer, the first bottom electrode layer and the third bottom electrode layer each have a longitudinal wave acoustic impedance of 60 x106kg / m2s or more.

[0055] In an example in accordance with at least some embodiments, the first top electrode layer 704 and the first bottom electrode layer 714 each have a thickness lower than a quarter of an acoustic wavelength at the resonance frequency of the resonator.

[0056] In an example in accordance with at least some embodiments, the first top electrode layer 704 and the first bottom electrode 714 layer each have a thickness from 0.07 to 0.13 acoustic wavelengths of the acoustic wavelength at the resonance frequency of the resonator.

[0057] In an example in accordance with at least some embodiments, the second top electrode layer 706, the second bottom electrode layer 716, the third top electrode layer 708 and the third bottom electrode layer 718 each have a thickness close to a quarter of an acoustic wavelength at the resonance frequency of the resonator.

[0058] In an example in accordance with at least some embodiments, the multilayer structure comprises an acoustic Bragg reflector on at least one of the third top electrode layer 708 and the third bottom electrode layer 718. The Bragg reflector supports efficiency of the resonator in terms of at least one of: high reflectivity; or frequency selectivity; or acoustic confinement; or temperature stability. In an example, the Bragg reflector comprises an insulator-metal Bragg reflector, or a full metal Bragg reflector, or a full insulator Bragg reflector. The insulator-metal Bragg reflector comprises alternating layers of an insulator and a metal, for example alternating layers of SiO2 and W. The full metal Brag reflector comprises alternating layers of at last two different metals e.g. alternating layers of aluminium, Al, and tungsten, W. The Bragg reflector can also comprise alternating insulators of low and high acoustic impedance, such as SiO2 (Low Z) and AI2O3, AIN or SisN4 (High Z).

[0059] In an example in accordance with at least some embodiments, wherein at least one of the third top electrode layer 708 and the third bottom electrode layer 718 are of high acoustic impedance material. The high acoustic impedance material of the third top electrode layer 708 may have an acoustic impedance that is higher than an acoustic impedance of the second top electrode layer 706. The high acoustic impedance material of the third bottom electrode layer 718 may have an acoustic impedance that is higher than an acoustic impedance of the second bottom electrode layer 716. The high acoustic impedance material may comprise material that has an acoustic impedance ZL=60 x 106kg / m2s or higher for a longitudinal acoustic wave. Some examples of the high acoustic impedance materials are presented below in Table 3 and include for example tungsten (W) or tungstentitanium (TiW).

[0060] In an example in accordance with at least some embodiments, wherein at least one of the first top electrode layer 704 and the first bottom electrode layer 714 are of high acoustic impedance material. The high acoustic impedance material ofthe first top electrode layer 704 may have an acoustic impedance that is higher than at least of the following: an acoustic impedance of the second top electrode layer 706; or an acoustic impedance of the piezo layer 702. The high acoustic impedance material of the first bottom electrode layer 714 may have an acoustic impedance that is higher than at least one of the following: an acoustic impedance of the second bottom electrode layer 716; or an acoustic impedance of the piezo layer 702. The high acoustic impedance material may comprise material that has an acoustic impedance ZL=60 x 106kg / m2s or higher for a longitudinal acoustic wave. Some examples of the high acoustic impedance materials are presented below in Table 3 and include for example tungsten (W) or tungsten-titanium (TiW).

[0061] In an example in accordance with at least some embodiments, wherein at least one of the second top electrode layer 706 and the second bottom electrode layer 716 are aluminium.

[0062] In an example in accordance with at least some embodiments, a BAW resonator comprises the multi-layer structure 700. In an example, the BAW resonator may be an FBAR or SMR.

[0063] It should be noted that the resonance frequency of the resonator 700 is defined by the piezo layer 702 and the confinement / compression layers. However, series resistance of the resonator is defined based on electrical conductivity of all the electrode layers of the resonator, thus the confinement / compression layers, the electrically conductive top layers 706, 708 and electrically conductive bottom layers 716, 718.

[0064] Examples of materials for electrode layers and piezoelectric materials of the multi-layer structure are provided in Table 3.Table 3: densities and acoustic impedances Z of various insulators, metals and piezoelectrics. L stands for longitudinal wave and S for shear wave. density ZL(1 x106Zs(1 x106Material Type (kg / m3) kg / m2s) kg / m2s)

[0065] In an example a multi-layer resonator in accordance with at least some embodiments may be defined as follows:- Design the stack of High-Z confinement layers and the piezoelectric layer (piezo and 1stlayers 704, 714) to yield the desired resonance frequency. The thickness ratio of the confinement layer and the piezo-layer can be tuned to yield desired or maximized K2and Af and layer thicknesses thatare manufacturable. The design is done typically by using a 1 D acoustic transmission line or the Mason model.- Calculate quarter-wavelength thicknesses tA / 4at the resonance frequency f for the low-impedance electrode material and high impedance electrode material from tA / 4= where v is the longitudinal acoustic velocity of thematerial in question and f the frequency.- Build the electrode by stacking layers from top to bottom as o high impedance quarter wavelength electrode o low impedance quarter wavelength electrode o thin high impedance compression layer o piezo layer o thin high impedance compression layer o low impedance quarter wavelength electrode o high impedance quarter wavelength electrodeThe sheet resistance Rsflof a three-layer electrode can be calculated fromWhere RsM-3are the sheet resistances of the individual stacked electrode layers

[0066] Example of a multi-layer resonator scaled over a wide frequency bands is given in Table 4.Table 4: Three-layer electrode structure with thin acoustic confinement layer next to the piezo-layer and quarter-wavelength Al and 1 / layers. Layers on one side of the piezo-layer are shown, but the structure is symmetric.

[0067] Fig. 8 illustrates displacement (u), stress (T) and strain (S) of a multi-layer resonator of Table 4 at 30 GHz. The multi-layer resonator comprises a multi-layer structure having a three-layer top electrode and a three-layer bottom electrode and a piezo layer between the bottom and top electrodes in accordance to described with Fig. 7. The frequency is dictated by a resonating dimension defined by the innermost layers: the piezo layer and the confinement / compression layers. Top electrode layers and bottom electrode layers placed on the innermost layers provide a highly reduced resistance and acoustic termination so that the inner part of the resonator is in practice not loaded by them. Each of the confinement / compression layers may be a thin electrically conductive layer of W on which top and bottom electrode layers of Al and W are placed in respective order. Each of the top and bottom electrode layers placed on the confinement / compression layers may have athickness close to a quarter of a wavelength according to the resonance frequency of the resonator. The top and bottom layers provide acoustically free surfaces towards the confinement / compression layers, e.g. at outer surfaces of the confinement / compression layers. The strain and stress are effectively zero at outer edges of the multilayer resonator.

[0068] Fig. 9 illustrates examples of bulk acoustic wave resonators according at least some embodiments. The bulk acoustic wave resonators a), b), c) and d) comprise multi-layer structures described with Fig. 7.

[0069] Referring to the BAW a), the BAW is an FBAR comprising a multi-layer structure suspended over a cavity 903. The FBAR comprises layers from top to bottom: a third top electrode layer 908 of tungsten, a second top electrode layer 906 of aluminium, a first top electrode layer 904, a piezo layer 902, a first bottom electrode layer 914, a second bottom electrode layer 916 of aluminium, a third bottom electrode layer 918 of tungsten, an insulator 905 e.g. of SiO2 and a substrate 907, e.g. of Si.

[0070] Referring to the BAW b), the BAW is an FBAR comprising a multi-layer structure suspended over a cavity 933. The FBAR comprises layers from top to bottom: a third top electrode layer 938 of tungsten, a second top electrode layer 936 of aluminium, a first top electrode layer 934, a piezo layer 932, a first bottom electrode layer 944, a second bottom electrode layer 946 of aluminium, a third bottom electrode layer 948 of tungsten, an insulator 935 e.g. of SiO2 and a substrate 937, e.g. of Si. The bottom electrode layers are under the piezo layer 932 inside the cavity 933. The bottom electrodes may be patterned to a minimum size.

[0071] Referring to the BAW c), the BAW is a solidly mounted resonator, SMR, comprising an acoustic Bragg reflector having an insulator-metal structure. The SMR comprises layers from top to bottom: a third top electrode layer 958 of tungsten, a second top electrode layer 956 of aluminium, a first top electrode layer 954, a piezo layer 952, a first bottom electrode layer 964, a second bottom electrode layer 966 of aluminium, a third bottom electrode layer 968 of tungsten, alternating layers of an low acoustic impedance insulator 955 and high acoustic impedance metal 967 e.g. alternating layers of SiO2 and W, and a substrate 957, e.g. of Si.

[0072] Referring to the BAW d), the BAW is a solidly mounted resonator, SMR, comprising an acoustic Bragg reflector having a full metal structure. The SMR comprises layers from top to bottom: a third top electrode layer 978 of tungsten, a second top electrode layer 976 of aluminium, a first top electrode layer 974, a piezo layer 972, a first bottom electrode layer 984, a second bottom electrode layer 986 of aluminium, a third bottom electrode layer 988 of tungsten, alternating layers of at last two different metals 987, 985 with different acoustic impedances e.g. alternating layers of aluminium and tungsten, and a substrate 997, e.g. of Si. It should be noted that the BAW d) is described above to have a full metal structure, thus the piezo layer is stacked on the substrate between metal layers that have different acoustic impedances. However, it should be noted that insulator layers may be used instead of the metal layers, whereby the insulator layers comprise at least two insulators with different acoustic impedances. The insulator layers are alternated above and below the piezo layer 972. Accordingly, one of the insulator layers may be of a high acoustic impedance material and the other may be of material that has a lower acoustic impedance than the high acoustic impedance material. The high acoustic impedance material may comprise material that has an acoustic impedance ZL=60 x 106kg / m2s or higher for a longitudinal acoustic wave. Some examples of the high acoustic impedance materials are presented below in Table 3 and include for example tungsten (W) or tungsten-titanium (TiW).

[0073] Examples of typical thicknesses of layers and materials according to some embodiments:- Piezoelectric layer, or piezo layer: ca. 0.15 - 0.22 wavelength thick aluminium nitride (AIN) or scandium aluminium nitride (ScAIN)- High acoustic impedance (High-Z) confinement / com pression layer: ca. 0.07 - 0.13 wavelength thick tungsten, W, with Zacof 101 x106kg / m2s- Low acoustic impedance (Low-Z) electrode layer: ca. quarter wavelength thickness of Al with Zacof 17 x106kg / m2s.- High-Z electrode termination layer: ca. quarter wavelength thickness of e.g. tungsten, W, with Zacof 101 x106kg / m2s

[0074] Fig. 10 illustrates an example of a multi-layer structure for a BAW resonator in accordance with at least some embodiments. The multilayer structurehas a layer structure in accordance to described with the multi-layer structure described in Fig. 7 with the difference that the outer most high acoustic impedance layer is replaced with a passivation layer 1018. The passivation layer can be trimmed to tune the bulk acoustic wave resonator according to the designed resonator properties. At the same time the remaining top electrode layers provide low resistivity of the structure, for example by making the second bottom electrode layer and the second top electrode layer thick. Examples of passivation layers comprise thin films of material that are applied to the surface of semiconductor devices to protect them from environmental factors, such as moisture, oxygen, or contaminants, that could degrade their performance or reliability. Materials can be AIN, SiN, AI203 and others.

[0075] Accordingly, the multi-layer structure in Fig. 10 may comprise a piezoelectric layer, or a piezo layer 1002, that is sandwiched between top electrode layers 1004, 1006 and bottom electrode layers 1014, 1016, 1018. The top electrode layers and bottom electrode layers are layers of electrically conductive material, e.g. metallic layers. The top electrode layers and the bottom electrode layers comprise arrangements of layers on top of each other, where a layer 1004, 1014 that is the closest to the piezo layer 1002, e.g. on the piezo layer, has a high acoustic impedance, Zac, and a next layer 1006, 1016 away from the piezo layer has a low acoustic impedance. A next layer 1018 below the low acoustic impedance layer 1016 has a high acoustic impedance. A next layer above the high acoustic impedance layer 1006 is a passivation layer 1008. Accordingly, the electrode layers are arranged to alternate based on their acoustic impedances both above and below the piezo layer. The low acoustic impedance and high acoustic impedance layers together on each side of the piezo layer form a half-wave acoustic resonant volume with an effective free surface boundary condition both at the outer edges of the innermost high acoustic impedance layers 1004 and 1014 and at the outer edges of the outer most layers 1008 and 1018, regardless of whether outside the third layer lies vacuum, air, or a low acoustic impedance solid material, e.g. Al or SiO2. Outer edge means farthest away from the piezo layer, inner means closer to piezo layer.

[0076] In an example in accordance with at least some embodiments, the passivation layer 1008 has a thickness for shifting a resonant frequency of the BAWresonator in the order of 1 PZD, when the passivation layer is trimmed. Since the multilayer structure the second top electrode layer 1006, the second bottom electrode layer 1016 and the third bottom electrode layer 1018 that are metallic, low resistivity of the structure can be supported and the shift in the order of 1 PZD can be obtained without significant drop in coupling. The resonant frequency is the frequency at which the BAW resonator naturally oscillates. PZD refers to the frequency difference between a pole (a frequency at which the response is maximized) and a zero (a frequency at which the response is minimized). Shifting the resonant frequency in the order of 1 PZD means altering the resonator’s natural frequency by this specific interval. The thickness of the passivation layer for a shift in the order of 1 PZD shift can be calculated or simulated based on the pole-zero distance of an untrimmed passivation layer in terms of frequency and the specific acoustic and mechanical properties of the resonator and the passivation material.

[0077] In an example in accordance with at least some embodiments, the passivation layer 1008 has a thickness close to a quarter of a wavelength according to the resonance frequency of the resonator. In an example, a thickness of the passivation layer which is a quarter of a wavelength maximizes the PZD. Initially the thickness of the passivation layer may be larger than the quarter of a wavelength at which an ideal coupling is achieved. The passivation layer may be trimmed such that the designed resonance frequency of the resonator is achieved. For example, a low frequency resonator may have a thicker passivation layer, e.g. thicker than the the quarter of a wavelength, and a high frequency resonator may have a thinner passivation layer, e.g. thinner than the the quarter of a wavelength. In this way the two resonators may be manufactured by trimming the passivation layer such that each of the resonators have similar levels of coupling. In an example, the passivation layer may have a thickness from 0.1 up to 0.5 of a wavelength. It should be noted that the trimming may be achieved by removing material, e.g. by etching the passivation layer, from the passivation layer or adding material, e.g. depositing material, to the passivation layer. Table 5 below gives examples of trimmed passivation layers.Table 5 Passivation layer thicknesses and corresponding PZDs0078] In an example in accordance with at least some embodiments, the passivation layer 1008 is designed based on the following requirements:- Fast acoustic velocity -> thick layer (good at high frequencies). - High impedance -> works as acoustic termination layer.- Stable, does not oxidize -> can be the top layer & passivation and is trimmable (smooth dependence of resonator properties with respect to trim depth).- Can be processed on top of the resonators.

[0079] In an example in accordance with at least some embodiments, the passivation layer 1008 comprises: AIN, SiN, AI203, diamond or Ta2Os.

[0080] In an example in accordance with at least some embodiments, there is provided a method for manufacturing a multi-layer structure for a bulk acoustic wave resonator. The multi-layer structure may be in accordance with the multi-layer structure described with Fig. 7 or the multi-layer structure described with Fig. 10.The multi-layer structure may be manufactured to achieve or to perform at least one of the following: resonance frequency of the bulk acoustic wave resonator; fine tune the resonance frequency of the bulk acoustic wave resonator; or provide a shift froma series resonator to a parallel resonator; or provide both a series resonator and a parallel resonator. The method comprises:- adding the first top electrode layer 704 on one side, e.g. above, the single piezoelectric layer 702 and first bottom electrode layer 714 on another side, e.g. below, the single piezoelectric layer;- adding the second top electrode layer 706 on, e.g. above, the first top electrode layer and adding the second bottom electrode layer 716 on, e.g. below, the first bottom electrode layer;- forming a bottom layer of the multilayer structure by: a. adding the third bottom electrode layer 718 on, e.g. below, the second bottom electrode layer;- forming a top layer of the multilayer structure by: a. adding the third top electrode layer 708 on, e.g. above, the second top electrode layer, or b. adding the passivation layer 1008 on, e.g. above, the second top electrode layer.

[0081] It should be noted that in the method the method the sides of the layers, e.g. above and below may be reversed depending on manufacturing method used. Therefore, adding one layer on another layer in the method may refer to adding the one layer above or below the another layer, depending on the manufacturing method for the multi-layer structure, when the layers are stacked upon one another.

[0082] In an example in accordance with at least some embodiments, the method comprises adding a reflector on, e.g. below, the third bottom electrode layer 708. In an example, the reflector is an acoustic Bragg reflector.

[0083] In an example in accordance with at least some embodiments, the method comprises trimming the passivation layer 1008 to a thickness close to a quarter of an acoustic wavelength at the resonance frequency of the resonator.

[0084] In an example in accordance with at least some embodiments, the method comprises trimming the passivation layer 1008 to a thickness from 0.1 up to 0.5 of the wavelength.

[0085] In an example in accordance with at least some embodiments, the method comprises trimming the passivation layer 1008 to a thickness from 0.1 up to 0.4 of the wavelength.

[0086] In an example in accordance with at least some embodiments, the method comprises:- Adding an acoustic compression layer, i.e. a top compression layer, on a top surface of a piezoelectric layer and adding an acoustic compression layer, i.e. a bottom compression layer, on a bottom surface of the piezoelectric layer. The acoustic compression layers may be of high-Z material, for example tungsten.- Adding a layer of low acoustic impedance material, i.e. top low acoustic impedance layer, on a surface of the top compression layer and adding a layer of low acoustic impedance material, i.e. bottom low acoustic impedance layer, on a surface of the bottom compression layer. In this way the acoustic compression layers are added symmetrically around the piezoelectric layer. The layers of low acoustic impedance material may be of Low-Z material, for example Al.- Bottom layer of the multilayer structure may be formed by: adding a layer of high acoustic impedance material, i.e. bottom high acoustic impedance layer, on a surface of the bottom low acoustic impedance layer; or adding a reflector on a surface of the bottom low acoustic impedance layer. In an example the reflector may comprise alternating layers of Low-Z material and High-Z material. For example, the reflector may comprise a first layer on top of Low-Z material on the surface of the bottom low acoustic impedance layer and a second layer of High-Z material on the surface of the first layer of the reflector. Further layers may be added to the reflector such that the High-Z and Low-Z layers are alternated. In an example the bulk acoustic wave resonator is an SMR and the Low-Z material may be SiO2 and the High-Z material may be tungsten.- Top layer of the multilayer structure may be formed by: adding a passivation layer on top of the top low acoustic impedance layer; or adding a layer of high acoustic impedance material, i.e. top high acousticimpedance layer, on a surface of the top low acoustic impedance layer. The passivation layer provides that the bulk acoustic wave resonator can be tuned according to the designed resonance frequency by trimming the passivation layer. In an example the bulk acoustic wave resonator is used for one or more resonators of an RF filter, for example a ladder filter. The RF resonator has designed properties, such as resonance frequency, coupling and frequency response. Then, each of the one or more resonators have the passivation layer on top, which may be tuned to achieve the designed properties for the RF filter.

[0087] In an example in accordance with at least some embodiments, the method for manufacturing a multilayer structure for a bulk acoustic wave resonator comprises adding alternating layers of Low-Z material and High-Z material on a surface of the bottom layer. The Low-Z material may be an insulator and the High- Z material may be a metal. In an example, the Low-Z material may be SiO2 and the High-Z material may be tungsten.

[0088] In an example in accordance with at least some embodiments, the method comprises trimming the passivation layer 1008 on top of the top low acoustic impedance layer 1006 for satisfying the designed resonator properties for the multilayer structure. In an example, the passivation layer may be trimmed, e.g. by etching, for tuning the multi-layer structure and satisfying the designed resonator properties. Examples of the designed resonator properties comprise required frequency shift between resonators for a needed bandwidth e.g. in a ladder filter RF design.

[0089] In an example in accordance with at least some embodiments the method comprises adding a reflector, e.g. an acoustic Bragg reflector on the third bottom electrode layer.

[0090] In an example in accordance with at least some embodiments, the method may comprise trimming and / or depositing material to one or more layers of the multilayer structure, whereby the designed resonance frequency of the multilayer structure may be achieved.

[0091] Fig. 11 illustrates examples of bulk acoustic wave resonators according at least some embodiments. The bulk acoustic wave resonators e) and f) comprise multi-layer structures in accordance to described with Fig. 10.

[0092] Referring to the BAW e), the BAW is an FBAR comprising a multi-layer structure suspended over a cavity 1103. The FBAR comprises layers from top to bottom: a passivation layer 1 108, a second top electrode layer 1106 of aluminium, a first top electrode layer 1104, a piezo layer 1102, a first bottom electrode layer 1114, a second bottom electrode layer 1116 of aluminium, a third bottom electrode layer 1118 of tungsten, an insulator 1105 e.g. of SiO2 and a substrate 1107, e.g. of Si or Ti.

[0093] Referring to the BAW f), the BAW is a solidly mounted resonator, SMR, comprising an acoustic Bragg reflector having an insulator-metal structure. The SMR comprises layers from top to bottom: a passivation layer 1158 of tungsten, a second top electrode layer 1156 of aluminium, a first top electrode layer 1154, a piezo layer 1152, a first bottom electrode layer 1164, a second bottom electrode layer 1166 of aluminium, a third bottom electrode layer 1168 of tungsten, alternating layers of an low acoustic impedance insulator 1155 and high acoustic impedance metal 1167 e.g. alternating layers of SiO2 and W, and a substrate 1157, e.g. of Si. It should be noted that an SMR having a full metal structure is also viable. The discussion of high acoustic impedance materials, examples of high acoustic impedance materials and thicknesses of high impedance materials presented with the discussion of Fig. 9 and Tabel 9 apply also to the structures of Fig. 11.DEFINITIONS

[0094] Let’s define the layer thicknesses in dimensionless units of acoustic wavelength at a given frequency (the resonance frequency fQof the resonator (let’s use the resonance frequency f0= frhere, but antiresonance faor a value between frand fa could also be chosen based on optimization target)) as follows:where t is the absolute thickness, Ao= — is the acoustic wavelength of the layer fo material in question at frequency f0, v is the acoustic wave velocity of the layermaterial and t' is dimensionless thickness. For longitudinal wave resonators the longitudinal bulk wave velocity vLis used but the same would apply to shear wave resonators and in that case the shear wave velocity vswould be used.

[0095] Converting from the dimensionless thickness t’ for a resonator designed for0to absolute thickness t in meters is done asUsing the dimensionless thicknesses for all layers makes the description of the structures independent of frequency.

[0096] Acoustic impedance is denoted with Zacand can be calculated from the density and acoustic wave velocity as Zac= pv, where p is the density of the material and v is the longitudinal bulk acoustic velocity of the material. (For shear wave devices the shear wave velocity Is used to calculate the shear wave impedance).

[0097] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0098] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0099] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any othermember of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and examples of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0100] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0101] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0102] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.REFERENCE SIGNS LIST100 BAW resonator in Fig. 1102 piezo layer in Fig. 1104 electrodes in Fig. 1202 electrical impedance in Fig. 2204 resonance frequency in Fig. 2206 antiresonance frequency in Fig. 2Co, Cm capacitor in Fig. 3Lm, Lsinductance in Fig. 3Ro, Rs, Rm resistor, in Fig. 3HI high frequency resonator in Fig. 4LO low frequency resonator in Fig. 4 u displacement in Figs. 5, 6 and 8T stress in Figs. 5, 6 and 8S strain in Figs. 5, 6 and 8702 piezo layer in Fig. 7704, 706, 708 top electrode layers in Fig. 7714, 716, 718 bottom electrode layers in Fig. 7High Z high acoustic impedanceLow Z low acoustic impedance a), b), c), d) bulk acoustic wave resonators in Fig. 9902, 932, 952, 972 piezo layer in Fig. 9904, 906, 908, 934, 936, 938, 954, top electrode layers in Fig. 9956, 958, 974, 976, 978914, 916, 918, 944, 946, 948, 964, bottom electrode layers in Fig. 9966, 968, 984, 986, 988903, 933 cavity in Fig. 9905, 935, 955 insulator in Fig. 9967, 985, 987 metal in Fig. 9907, 937, 957, 997 substrate in Fig. 91002 piezo layer1014, 1016, 1018 bottom electrode layers1004, 1006 top electrode layers1102, 1152 piezo layer in Fig. 111104, 1106, 1154, 1156, 1158, 1174, top electrode layers in Fig. 111114, 1116, 1118, 1164, 1166, 1168, bottom electrode layers in Fig. 111108 passivation layer in Fig. 111103 cavity in Fig. 111105, 1155 insulator in Fig. 111167 metal in Fig. 111107, 1157 substrate in Fig. 11

Claims

CLAIMS1 . A multi-layer structure for a bulk acoustic wave resonator, characterized in that the multi-layer structure comprises:- a single piezoelectric layer (702);- a first top electrode layer (704) above the single piezoelectric layer (702);- a first bottom electrode layer (714) below the single piezoelectric layer;- a second top electrode layer (706) above the first top electrode layer;- a second bottom electrode layer (716) below the first bottom electrode layer;- a third top electrode layer (708) above the second top electrode layer, or a passivation layer above the second top electrode layer (1008); and- a third bottom electrode layer (718) below the second bottom electrode layer; wherein the first top electrode layer (704) and first the bottom electrode layer (714) are configured to form acoustic confinement layers, and the second top electrode layer (706), and the third top electrode layer (708) or the passivation layer are configured to form a first acoustically free surface towards the first top electrode layer (704), and the second bottom electrode layer (716) and the third bottom electrode layer (718) are configured to form a second acoustically free surface towards the first bottom electrode layer (714).

2. The multi-layer structure of claim 1 , wherein the first top electrode layer (704) and the third top electrode layer (708) have an acoustic impedance that is higher than an acoustic impedance of the second top electrode layer (706).

3. The multi-layer structure of any claim of claim 1 or 2, wherein each of the first bottom electrode layer (714) and the third bottom electrode layer (718) have an acoustic impedance that is higher than an acoustic impedance of the second bottom electrode layer (716).

4. The multi-layer structure of any of claims 1 to 3, wherein each of the first top electrode layer (704) and the first bottom electrode layer (714) have anacoustic impedance that is higher than an acoustic impedance of the piezoelectric layer (702).

5. The multi-layer structure of any of claims 1 to 4, wherein each of the first top electrode layer (704), the third top electrode layer (708), the first bottom electrode layer (714) and the third bottom electrode layer (718) each have a longitudinal wave acoustic impedance of 60 x106kg / m2s or more.

6. The multi-layer structure of any of claims 1 to 5, wherein the first top electrode layer (704) and the first bottom electrode layer (714) each have a thickness lower than a quarter of an acoustic wavelength at the resonance frequency of the resonator.

7. The multi-layer structure of claims 1 to 6, wherein the first top electrode layer (704) and the first bottom electrode layer (714) each have a thickness from 0.07 to 0.13 acoustic wavelengths of the acoustic wavelength at the resonance frequency of the resonator.

8. The multi-layer structure of any of claims 1 to 7, wherein the second top electrode layer (706), the second bottom electrode layer (716), the third top electrode layer (708) and the third bottom electrode layer (718) each have a thickness close to a quarter of an acoustic wavelength at the resonance frequency of the resonator.

9. The multi-layer structure of any of claims 1 to 8, comprising an acoustic Bragg reflector on at least one of the third top electrode layer (708) and the third bottom electrode layer (718).

10. The multi-layer structure of claim 9, wherein the acoustic Bragg reflector is an insulator-metal Bragg reflector or a full metal Bragg reflector or a full insulator Bragg reflector.11 . The multi-layer structure of any of claims 1 to 10, wherein at least one of the first top electrode layer (704) and the first bottom electrode layer (714) are ofa high acoustic impedance material for example tungsten (W) or tungstentitanium (TiW).

12. The multi-layer structure of any of claims 1 to 11 , wherein at least one of the second top electrode layer (706) and the second bottom electrode layer (716) are aluminium.

13. The multi-layer structure of any of claims 1 to 12, wherein at least one of the third top electrode layer (708) and the third bottom electrode layer (718) are of a high acoustic impedance material, for example tungsten (W) or tungstentitanium (TiW).

14. The multi-layer structure of any of claims 1 to 13, wherein the first acoustically free surface is between the first top electrode layer (704) and the second top electrode layer (706) and the second acoustically free surface is between the first bottom electrode layer (714) and the second bottom electrode layer (716), and each of the first acoustically free surface and the second acoustically free surface have effectively an impedance minimum at least at or near the operation frequency of the resonator.

15. The multi-layer structure of any of claims 1 to 14, wherein the passivation layer (1008) has a thickness close to a quarter of an acoustic wavelength at the resonance frequency of the resonator.

16. The multi-layer structure of any of claims 1 to 15, wherein the passivation (1008) layer is trimmed to a thickness from 0.1 up to 0.5 of the wavelength.

17. The multi-layer structure of any of claims 1 to 16, wherein the passivation layer (1008) is trimmed to a thickness from 0.1 up to 0.4 of the wavelength.

18. A bulk acoustic resonator comprising the multi-layer structure of any of claims 1 to 17.

19. The bulk acoustic resonator of claim 18 comprising a film bulk acoustic wave resonator or a solidly mounted resonator.20.A filter for radio frequency bands from 6 GHz to 30 GHz, comprising the multilayer structure of any of claims 1 to 17; or the bulk acoustic resonator of claim 18; or the bulk acoustic resonator of claim 19.21 .A method for manufacturing a multilayer structure according to any of claims 1 to 17, comprising:- adding the first top electrode layer (704) on one side of the single piezoelectric layer (702) and first bottom electrode layer (714) on another side of the single piezoelectric layer;- adding the second top electrode layer (706) on the first top electrode layer and adding the second bottom electrode layer (716) on the first bottom electrode layer;- forming a bottom layer of the multilayer structure by: a. adding the third bottom electrode layer (718) on the second bottom electrode layer;- forming a top layer of the multilayer structure by: a. adding the third top electrode layer (708) on the second top electrode layer, or b. adding the passivation layer (1008) on the second top electrode layer.

22. The method according to claim 21 , comprising:- adding a reflector on the third bottom electrode layer (718).

23. The method according to claim 21 , comprising:- trimming the passivation layer (1008) to a thickness close to a quarter of an acoustic wavelength at the resonance frequency of the resonator.

24. The method according to any of claims 21 to 23, comprising:trimming the passivation layer (1008) to a thickness from 0.1 up to 0.5 of the wavelength.

25. The method according to any of claims 21 to 24, comprising: - trimming the passivation layer (1008) to a thickness from 0.1 up to 0.4 of the wavelength.

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