Top electrode material configuration

Abstract

A resonator (100), comprising at least one resonating element (101) having a multilayer top electrode (L1), wherein the multilayer top electrode (L1) comprises at least one pair of alternating layers of a first material layer (13) and an intermediate material layer (12). Herein is further provided an apparatus, such as a resonator array, comprising at least one resonator (100).

Description

[0001] TOP ELECTRODE MATERIAL CONFIGURATION

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to the field of semiconductors and resonators. The disclosure relates particularly, though not exclusively, to top electrode material configurations of the preceding.

[0004] BACKGROUND

[0005] This section illustrates useful background information without admission of any technique described herein representative of the state of the art.

[0006] A key performance parameter in semiconductor apparatuses, such as MEMS resonators, is the stability of the resonance frequency thereof. The characteristics of the chosen top electrode material, such as certain metallic top electrode materials, may contribute to the frequency instability of the resonator.

[0007] SUMMARY

[0008] The appended claims define the scope of protection. Any examples and technical descriptions of apparatuses, products and / or methods in the description and / or drawings not covered by the claims are presented not as embodiments of the invention but as background art or examples useful for understanding the invention.

[0009] It is an object of certain embodiments of the present disclosure to provide improved frequency stability of resonators or at least to provide an alternative to existing technology. Accordingly, certain disclosed embodiments provide for an ingenious resonator and apparatus solving at least one of the problems related to the prior art. According to a first example aspect of the present disclosure there is provided a resonator, comprising at least one resonating element having a multilayer top electrode, wherein the multilayer top electrode comprises at least one pair of alternating layers of a first material layer and an intermediate material layer.

[0010] In certain embodiments, the first material layer is a semiconducting material layer. In certain embodiments, the first material layer is a low resistivity (conductive) material layer. In certain embodiments, the first material layer is a low resistivity or semiconducting material layer. In certain embodiments, the first material layer is of a material having electrical resistivity (resistivity = 1 / electrical conductivity) of less than 10 mQ cm. In certain embodiments, the first material layer is of metallic material (metal), such as gold, Au. In certain embodiments, the first material layer is of gold, Au. In certain embodiments, the first material layer is of gold alloy. In certain embodiments, the first material layer is of aluminium Al, doped gold alloy.

[0011] In certain embodiments, the first material layer is of semiconducting material. In certain embodiments, the first material layer is of Si, Ge, GaAs orany combination of the preceding. In certain embodiments, the first material layer is (is implemented by a layer) of doped silicon. In certain embodiments, the first material layer is of single-crystal silicon. In certain embodiments, the first material layer is (implemented by a layer) of polycrystalline silicon. In certain embodiments, the first material layer is of doped polycrystalline silicon. In certain embodiments, the first material layer is of metallic material, such as Au, or of semiconducting material, such as Si.

[0012] In certain embodiments, the intermediate material layer is (can be called, can be named) a second material layer. In certain embodiments, the intermediate material layer is of a conductive (low resistivity) material, semiconducting material, or of an insulating material (dielectric material, material that does not conduct electricity). In certain embodiments, the intermediate material layer is (implemented by a layer) of a metallic material (metal), a semiconducting material, or of an insulating material.

[0013] In certain embodiments, the intermediate material layer is of a material having electrical resistivity of less than 1 Q cm. In certain embodiments, the intermediate material layer is of a metallic (non-insulating) material. In certain embodiments, the intermediate material layer is of Pt, Ru, Al, Mo, Ta, Ti, W, Cr, TiW, Ni, Pd, AuSi, Nb, Al, or any combination of the preceding. In certain embodiments, the intermediate layer is of polycrystalline silicon. In certain embodiments, the intermediate material layer is of semiconducting material. In certain embodiments, the intermediate material layer is of Si, Ge, GaAs or any combination of the preceding.

[0014] In certain embodiments, the intermediate material layer is of an insulating material. In certain embodiments, the intermediate material layer is of AI2O3, SiO2, AIN, SiN, TiN, NbN, TiO2, or any combination of the preceding.

[0015] In certain embodiments, the intermediate material layer is of a metallic material, such as platinum, Pt, or of semiconducting material, such as Si, or of an insulating material, such as AI2O3.

[0016] In certain embodiments, the first material layer is semiconducting material, such as silicon, and the intermediate layer is of metallic material. In certain embodiments, the first material layer is semiconducting material, such as silicon, and the intermediate layer is of insulating material. In certain embodiments, the first material layer is semiconducting material, and the intermediate layer is of semiconducting material.

[0017] In certain embodiments, the first material layer is of semiconducting material, such as Si with doping level A, and the intermediate material layer is of semiconducting material, such as Si, with doping level B, wherein the doping level A is different from doping level B. In certain embodiments, the first material layer is of semiconducting material, such as Si with doping type A (such as P-type, or N-type doping), and the intermediate material layer is of semiconducting material, such as Si, with doping type B, wherein the doping type A is different from doping type B.

[0018] In certain embodiments, the first material layer is of silicon having a doping type and doping level A. In certain embodiments, the intermediate material layer is of silicon having a doping type and level B. In certain embodiments, the doping type and / or level A and B are different. In certain embodiments, the doping level A and B are different, or the doping type A and B are different, or both are different.

[0019] In certain embodiments, the first material layer is of polycrystalline silicon having a doping type and doping level A. In certain embodiments, the intermediate material layer is of polycrystalline silicon having a doping type and level B. In certain embodiments, the doping type and / or level A and B are different. In certain embodiments, the doping level A and B are different, or the doping type A and B are different, or both are different. In certain embodiments, the intermediate material layer is in between the first material layer and the layer beneath the at least one pair of alternating layers of the first material layer and the intermediate material layer. In certain embodiments, the intermediate material layer is in between the first material layer and the seed layer.

[0020] In certain embodiments, the multilayer top electrode comprises a third layer beneath (below) the at least one pair of alternating layers of the first material layer and the intermediate material layer. In certain embodiments, the third material layer is (can be called, is equal to) a seed layer. In certain embodiments, the multilayer top electrode comprises a seed layer beneath (below) the at least one pair of alternating layers of the first material layer and the intermediate material layer. In certain embodiments, the multilayer top electrode comprises the at least one pair of alternating layers of the first material layer and the intermediate material layer above (on top of, onto, on) the seed layer. In certain embodiments, the seed layer is of a metallic material, such as tantalum Ta, platinum Pt, titanium Ti, titaniumtungsten TiW, aluminium Al, molybdenum Mo, or any combination of the preceding.

[0021] In certain embodiments, the one pair of alternating layers contains a (one) first material layer and a (one) intermediate layer. In certain embodiments, the pair(s) of alternating layers provide the top electrode of the resonator. As used herein, the “multilayer” top electrode refers to the top electrode comprising multiple material layers that together function as a top electrode of the resonator.

[0022] In certain embodiments, the multilayer top electrode comprises N pairs of alternating layers of the first material layer and the intermediate material layer, wherein N is 2, 3 or 4. In certain embodiments, the multilayer top electrode comprises N pairs of alternating layers of the first material layer and the intermediate material layer, wherein N is 2 or more. In certain embodiments, the multilayer top electrode comprises N pairs of alternating layers of the first material layer and the intermediate material layer, wherein N is 3 or more. In certain embodiments, the multilayer top electrode comprises N pairs of alternating layers of the first material layer and the intermediate material layer, wherein N is 4 or more.

[0023] In certain embodiments, the resonating element (the resonator) comprises a bottom electrode. In certain embodiments, the resonating element (the resonator) has a bottom electrode. In certain embodiments, the resonating element (the resonator) comprises a bottom electrode layer. In certain embodiments, the resonating element (the resonator) has a bottom electrode layer. In certain embodiments, the bottom electrode comprises silicon. In certain embodiments, the bottom electrode is implemented by a layer of silicon. In certain embodiments, the resonator comprises a material stack, the material stack comprising the silicon layer (the bottom electrode), the piezoelectric layer on top of the silicon layer, and a multilayer top electrode on top of the piezoelectric layer. In certain embodiments, the resonator is a piezoelectric resonator. In certain embodiments, the piezoelectric layer comprises (is of) aluminum nitride, AIN.

[0024] In certain embodiments, the resonating element comprises a piezoelectric layer, wherein the multilayer top electrode is on the piezoelectric layer, and a bottom electrode on the opposite side of the piezoelectric layer than the multilayer top electrode.

[0025] In certain embodiments, the resonator comprises a silicon layer. In certain embodiments, the resonator comprises the silicon layer below (beneath) a piezoelectric layer. In certain embodiments, the resonator comprises the piezoelectric layer on top of (above, onto, on) the silicon layer. In certain embodiments, the silicon layer is configured to serve (serves, functions) as a bottom electrode of the resonator. In certain embodiments, the resonator comprises a silicon layer serving as a bottom electrode of the resonator.

[0026] In certain embodiments, the bottom electrode comprises silicon. In certain embodiments, the bottom electrode comprises silicon, preferably doped silicon, such as ultra-heavily doped, UHD, silicon. In certain preferred embodiments, the silicon is of single-crystal silicon. In certain embodiments, the bottom electrode (layer) is implemented by a silicon layer. In certain embodiments, the bottom electrode (layer) is implemented by a doped silicon layer. In certain embodiments, the bottom electrode (layer) is implemented by an UHD silicon layer.

[0027] In certain embodiments, the doping level of the silicon is above 1019cm-3. In certain embodiments, the doping level of the silicon is above 102° cm"3. In certain embodiments, the doped silicon is of N-type or P-type doping.

[0028] In certain embodiments, the resonator comprises at least one resonating element. In certain embodiments, the resonating element comprises a plurality of resonating beam elements. In certain embodiments, each beam element is a sub-element of the resonator.

[0029] In certain embodiments, the resonator comprises a plurality of beam elements having a length and a width. In certain embodiments, the plurality of beam elements are positioned adjacent to each other. In certain embodiments, adjacent beam elements are mechanically connected to each other by connection elements. In certain embodiments, the resonator is a stacked beam resonator (a ladder-like configuration). In certain embodiments, the stacked beam resonator comprises a plurality of beam elements positioned side-by-side in a plane. In certain embodiments, the plurality of beam elements are positions adjacent to each other in a width direction thereof. In certain embodiments, the plurality of beam elements are positioned adjacent to each other in a width direction of the resonator. In certain embodiments, the beam elements are separated by trenches. In certain embodiments, the beam elements are connected to each other by connection elements.

[0030] In certain embodiments, the resonator comprises a plurality of beam elements, such as seven, nine, or eleven beam elements. In certain embodiments, said adjacent beam elements are mechanically connected to each other by connection elements. In certain embodiments, the beam elements of the resonator are arranged in a rectangular array configuration.

[0031] In certain embodiments, the resonator is in a shape of a rectangle. In certain embodiments, the resonator is in a shape of an elongated rectangle (beam-shaped). In certain embodiments, the resonator has an aspect ratio (ratio of length to width, when observed from above) different from 1 .

[0032] In certain embodiments, the resonator has a length-to-width aspect ratio of less than 1. In certain embodiments, the resonator is attached (supported, anchored, suspended) to a support structure. In certain embodiments, the resonator is attached to a support structure from the outermost beam elements of the resonator. In certain embodiments, the resonator comprises at least one anchor configured to connect the resonator to, and suspend the resonator from surrounding layers. In certain embodiments the at least one anchor comprises portions of the piezoelectric layer, the top electrode and the bottom electrode.

[0033] In certain embodiments, each beam element is in a shape of a (rectangular) beam. In certain embodiments, each beam element has an aspect ratio (ratio of length to width, when observed from above) different from 1. In certain embodiments, each beam element has a length-to-width aspect ratio of more than 1 .

[0034] In certain embodiments, the resonator is fabricated on a substrate. In certain embodiments, the substrate is a wafer. In certain embodiments, the substrate is a silicon-on-insulator, SOI, wafer. In certain embodiments, the substrate comprises a silicon layer. In certain embodiments, the bottom electrode (layer) is referred to as a substrate or as a silicon layer. In certain embodiments, the resonating element comprises a plurality of resonating beam elements. In certain embodiments, the resonating beam element(s) are longitudinally aligned within 25 degrees of a <100> crystal direction of silicon (of the bottom electrode). Within at least some embodiments the resonating beam element(s) are longitudinally aligned with a <100> crystal direction of the silicon (of the bottom electrode) such that a longitudinal axis of each resonating beam is within 25 degrees of the <100 crystal direction of the silicon (of the bottom electrode).

[0035] In certain embodiments, the resonating beam element(s) are longitudinally aligned within 45 degrees, or 50 degrees of a <100> crystal direction of silicon (of the bottom electrode). Within at least some embodiments the resonating beam element(s) are longitudinally aligned with a <100> crystal direction of the silicon (of the bottom electrode) such that a longitudinal axis of each resonating beam is within 45 degrees, or 50 degrees of the <100> crystal direction of the silicon (of the bottom electrode).

[0036] In certain embodiments, the resonating beam element(s) are longitudinally aligned within 25 degrees of a

[0100] crystal direction of silicon (of the bottom electrode). Within at least some embodiments the resonating beam element(s) are longitudinally aligned with a

[0100] crystal direction of the silicon (of the bottom electrode) such that a longitudinal axis of each resonating beam is within 25 degrees of the

[0100] crystal direction of the silicon (of the bottom electrode).

[0037] In certain embodiments, the resonating beam element(s) are longitudinally aligned within 25 degrees of a

[0110] crystal direction of silicon (of the bottom electrode). Within at least some embodiments the resonating beam element(s) are longitudinally aligned with a

[0110] crystal direction of the silicon (of the bottom electrode) such that a longitudinal axis of each resonating beam is within 25 degrees of the

[0110] crystal direction of the silicon (of the bottom electrode).

[0038] In certain embodiments, the resonating beam element(s) are longitudinally aligned within 25 degrees of a

[0111] crystal direction of silicon (of the bottom electrode). Within at least some embodiments the resonating beam element(s) are longitudinally aligned with a

[0111] crystal direction of the silicon (of the bottom electrode) such that a longitudinal axis of each resonating beam is within 25 degrees of the

[0111] crystal direction of the silicon (of the bottom electrode).

[0039] In certain embodiments, the resonator comprises (obtains, has) mechanical resonance in one or more resonance modes. In certain embodiments, the resonator (the / each resonating element of the resonator) is configured to resonate (operate, oscillate, vibrate) in an in-plane resonance mode (a lateral resonance mode).

[0040] In certain embodiments, the resonator is configured to resonate in an extensional mode (for example such that each or some parts of the resonating element moving in-plane to and from the anchor or the centre of the resonating element). In certain embodiments, extensional mode resonators comprise plate resonating element(s), ring-shaped resonating element(s), or the combination thereof (or other shape(s) and structure(s) of resonating elements).

[0041] In certain embodiments, the resonator is configured to resonate (resonates) in a lengthextensional, LE, resonance mode. In certain embodiments, the resonator is configured to resonate in an in-plane length-extensional, LE, resonance mode. In certain embodiments, the length extensional resonance mode is configured to resonate parallel to the length direction of the resonator. In certain embodiments, the length extensional resonance mode is configured to resonate perpendicular to the width direction of the resonator.

[0042] In certain embodiments, the resonator is configured to resonate in a width-extensional, WE, resonance mode. In certain embodiments, the resonator is configured to resonate in an inplane width-extensional, WE, resonance mode. In certain embodiments, the width extensional resonance mode is configured to resonate parallel to the width direction of the resonator. In certain embodiments, the width extensional resonance mode is configured to resonate perpendicular to the length direction of the resonator.

[0043] In certain embodiments, the resonating element is configured to resonate in a squareextensional, SE, resonance mode. In certain embodiments, the resonating element is configured to resonate in a breathing resonance mode. In certain embodiments, the resonating element is configured to resonate in a transverse resonance mode. In certain embodiments, the resonating element is configured to resonate in a shear resonance mode. In certain embodiments, the resonating element is configured to resonate in a Lame resonance mode. In certain embodiments, the resonating element is configured to resonate in a wineglass, WG, mode. In certain embodiments, the resonator is configured to resonate in a Lamb mode, such as a symmetric Lamb mode. In certain embodiments, the resonating element is configured to resonate in an in-plane flexural (bending) resonance mode.

[0044] In certain embodiments, the resonator is configured to resonate in an out-of-plane resonance mode. In certain embodiments, the resonating element is configured to resonate in a flexural resonance mode (such as operating like a double-ended tuning fork). In certain embodiments, the resonating element is configured to resonate in an out-of-plane flexural resonance mode. In certain embodiments, the resonating element is configured to resonate in a torsional resonance mode. In certain embodiments, the resonator is configured to resonate in a bending resonance mode. In certain embodiments, the resonator is configured to resonate in a longitudinal resonance mode. In certain embodiments, the resonator is configured to resonate in a Lamb mode, such as an antisymmetric (asymmetric) Lamb mode.

[0045] In certain embodiments, the resonator is configured to resonate in a length-extensional, LE, resonance mode, or flexural resonance mode, or a width-extensional, WE, resonance mode. In certain embodiments, the resonator is configured to resonate in a lengthextensional, LE, resonance mode, or a width-extensional, WE, resonance mode. In certain embodiments, the resonator is configured to resonate in a length-extensional, LE, resonance mode, or a flexural resonance mode.

[0046] In certain embodiments, the resonator is configured to resonate in a collective resonance mode. In certain embodiments, each resonating element of the resonator is configured to resonate in the (same) collective resonance mode. In certain embodiments, the resonator is configured to resonate in a desired (main) resonance mode. In certain embodiments, each beam element of the resonator is configured to resonate in the (same) desired resonance mode.

[0047] In certain embodiments, the resonator is a microelectromechanical systems, MEMS, resonator. In certain embodiments, the resonator is (part of) a semiconductor device. In certain embodiments, the resonator is configured to operate in a kilohertz frequency area. In certain embodiments, the resonator is configured to operate at 32 kHz frequency. In certain embodiments, the resonator is configured to operate in a megahertz frequency area. In certain embodiments, the resonator is configured to operate at 16 MHz, 24 MHz, 26 MHz, 27.12 MHz, 38.4 MHz, 40 MHz, 52 MHz, 64 MHz, 76.8 MHz, 128 MHz, or 304 MHz frequency.

[0048] According to a second example aspect of the present disclosure there is provided an apparatus, such as a resonator array (an apparatus), comprising at least one resonator according to the first aspect or any of its embodiments. In certain embodiments, the apparatus comprises (at least) two (more than one) resonators of the first aspect or any of its embodiments coupled to each other. In certain embodiments, the resonators are coupled to each other by a coupler. In certain embodiments, the resonators are coupled to each other by a coupler comprising discontinuity region(s).

[0049] In certain embodiments, the apparatus comprises extensional-mode resonators. In certain embodiments, the apparatus comprises length extensional, LE, mode resonators. In certain embodiments, the apparatus comprises a flexural mode resonator. In certain embodiments, the apparatus comprises a mechanical connector element which connects the flexural resonator to the extensional-mode resonators.

[0050] In certain embodiments, each resonator comprises a plurality of beam elements having a length and a width, wherein the plurality of beam elements are positioned adjacent to each other and adjacent beam elements are mechanically connected to each other by connection elements (a stacked beam resonator, a ladder-like configuration). In certain embodiments, the plurality of beam elements are separated from each other by trenches.

[0051] In accordance with certain embodiments, embodiments of the second aspect are provided, the embodiments comprising subject matter of any single embodiment presented in connection with the first aspect, or the embodiments comprising subject matter of any of the embodiments presented in connection with the first aspect combined with subject matter presented in any other embodiment or embodiments.

[0052] Different non-binding example aspects and embodiments have been illustrated in the foregoing. The embodiments in the foregoing are used merely to explain selected aspects or steps that may be utilized in different implementations. Some embodiments may be presented only with reference to certain example aspects. It should be appreciated that corresponding embodiments may apply to other example aspects as well. In particular, the embodiments described in the context of the first aspect are applicable to each further aspect. Any appropriate combinations of the embodiments may be formed.

[0053] BRIEF DESCRIPTION OF THE FIGURES

[0054] Some example embodiments will be described with reference to the accompanying figures, in which:

[0055] Fig. 1a schematically shows a multilayer top electrode stack according to an example embodiment;

[0056] Fig. 1b schematically shows a multilayer top electrode stack according to another example embodiment; Fig. 1c schematically shows a multilayer top electrode stack according to yet another example embodiment;

[0057] Fig. 1d schematically shows a multilayer top electrode stack according to yet further another example embodiment;

[0058] Fig. 1e schematically shows a multilayer bottom electrode stack according to an example embodiment;

[0059] Fig. 2 schematically shows a material stack of a resonator according to an example embodiment;

[0060] Fig. 3 schematically shows a top view of an example resonator according to an example embodiment;

[0061] Fig. 4 schematically shows a top view of an example of a resonator array according to an example embodiment;

[0062] Fig. 5 shows a graph presenting change in frequency during ageing and reflow steps for a multilayer top electrode resonator and a single top electrode layer resonator;

[0063] Fig. 6a schematically shows a perforated multilayer top electrode stack according to an example embodiment; and

[0064] Fig. 6b schematically shows a perforated multilayer electrode stack in side view according to example embodiments.

[0065] DETAILED DESCRIPTION

[0066] In the following description, like reference signs denote like elements or steps.

[0067] Figs. 1a, 1b, 1c and 1d schematically show a multilayer top electrode stack according to example embodiments. In certain embodiments, the resonator comprises a material stack, the material stack comprising at least the silicon layer L4, the piezoelectric layer L2 on top of the silicon layer, and a multilayer top electrode L1 on top of the piezoelectric layer L2. In certain embodiments, the multilayer top electrode L1 is (is provided, resides, is fabricated) the topmost of the material stack. In certain embodiments, the material layers are deposited by sputtering or by atomic layer deposition, ALD.

[0068] In certain example embodiments, the thickness of the silicon layer L4 is in a range of 5 pm to 15 pm, such as 10 pm. In certain example embodiments, the thickness of the piezoelectric layer L2 is in a range of 0.3 pm to 2.5 pm, in particular 0.5 pm to 1.5 pm, such as 1 m. In certain example embodiments, the total thickness of the multilayer top electrode stack L1 is in a range of 0.2 pm to 0.4 pm, such as 0.3 pm. It should be noted that the thicknesses of the material layers shown in Figs. 1a, 1b, 1c and 1d are not in scale.

[0069] As shown in Fig. 1a, in certain embodiments, the multilayer top electrode (stack) L1 comprises at least one pair of alternating layers of a first material layer 13 and an intermediate material layer 12. As used herein, the terminology “one pair of alternating layers of first material layer 13 and an intermediate material layer 12” should be understood as a pair of material layers: one layer of low resistivity (conductive) material 13 and one layer of insulating material 12. Together the low resistivity material and the intermediate material layers constitute as the pair of alternating layers. One pair contains two layers of material (one layer 12 + one layer 13 = one pair). Analogously, as used herein, the terminology “two / three / four / N pairs of alternating layers of first material layer 13 and an intermediate material layer 12” should be understood as containing 2 / 3 / 4 / N pairs of material layers, each pair containing one layer of low resistivity material 13 and one layer of insulating material 12. By way of an example, as used herein, the terminology “three pairs of alternating layers of first material layer 13 and an intermediate material layer 12” should be understood as comprising three layers of low resistivity material 13, and three layers of intermediate material 12, in an alternating order. Thus, as used herein, “three pairs of alternating layers of first material layer 13 and an intermediate material layer 12” comprises six layers of material in total.

[0070] In certain embodiments, the first material layer 13 is provided (is, resides, is fabricated) onto the intermediate material layer 12. In certain embodiments, the intermediate material layer 12 is provided beneath the first material layer 13 (and vice versa, the first material layer 13 is provided on top of the intermediate material layer 12). In certain embodiments, the multilayer top electrode (stack) L1 comprises at least one pair of alternating layers of a first material layer 13 and an intermediate material layer 12, wherein the intermediate material layer 12 is beneath the first material layer 13. In certain embodiments, the pair of alternating layers comprises the intermediate material layer 12 beneath the first material layer 13. In certain embodiments, the pair of alternating layers comprises the first material layer 13 above (on top of, onto, on) the intermediate material layer 12.

[0071] In certain embodiments, the first material layer 13 is of metallic material, such as gold, Au. In certain alternative embodiments, the first material layer 13 is of polycrystalline silicon. In certain embodiments, the intermediate material layer 12 is implemented by a layer of metallic material, or alternatively by a layer of semiconducting material or a layer of insulating material. In certain embodiments, the intermediate material layer is of (comprises) platinum, Pt. In certain embodiments, the intermediate layer is of polycrystalline silicon. In certain alternative embodiments, the intermediate material layer is of (comprises) AI2O3, SiC>2, AIN, SisN4, TiN, NbN, TiC>2, or any combination of the previously mentioned. It should be noted that the intermediate material layer is not limited to those mentioned in the preceding, and said layer may be comprise other materials as well.

[0072] In certain example embodiment, the first material layer 13 is of polycrystalline silicon (or silicon) having a doping type and doping level A. In certain embodiments, the intermediate material layer 12 is of polycrystalline silicon having a doping type and level B. In certain embodiments, the doping type and level A and B are different.

[0073] In certain embodiments, the multilayer top electrode L1 comprises a seed layer 11 beneath the at least one pair of alternating layers of the first material layer 13 and the intermediate material layer 12. In certain embodiments, the seed layer 11 is provided (is, resides, is fabricated) onto (on top of, on, above) the piezoelectric layer L2. In certain embodiments, the seed layer 11 is provided beneath the intermediate material layer 12. In certain embodiments, the seed layer 11 is of a metallic material, such as tantalum, Ta. In certain embodiments, the seed layer 11 enables improving adhesion and preventing potential interdiffusion between layers.

[0074] As shown in Fig. 1a, in certain embodiments, the multilayer top electrode L1 comprises a top electrode stack, the top electrode stack comprising the seed layer 11 , the intermediate material layer 12 on top of the seed layer 11 , and a first material layer 13 on top of the intermediate material layer 12. In certain example embodiments, the thickness of the seed layer is in a range of 0.002 pm to 0.15 pm, in particular in the range of 0.02 pm to 0.07 pm, such as 0.05 pm. In certain example embodiments, the thickness of the intermediate material layer is in a range of 0.002 pm to 0.15 pm, in particular in the range of 0.02 pm to 0.07 pm, such as 0.05 pm. In certain example embodiments, the thickness of the first material layer is in a range of 0.002 pm to 0.3 pm, in particular in the range of 0.1 pm to 0.3 pm, such as 0.2 pm. It should be noted that the thicknesses of the top electrode material layers shown in Figs. 1a, 1 b, 1c and 1d are not drawn in scale.

[0075] Accordingly, herein is provided a resonator, comprising at least one resonating element having a multilayer top electrode L1 , wherein the multilayer top electrode L1 comprises at least one pair of alternating layers of a first material layer 13 and an intermediate material layer 12.

[0076] As shown in Fig. 1 b, in certain embodiments, the multilayer top electrode L1 comprises at least two pairs of alternating layers of a first material layer 13 and an intermediate material layer 12 (a double pair top electrode stack). Accordingly, the multilayer top electrode L1 comprises (at least) two first material layers 13 and two intermediate material layers 12, in an alternating order (sequence). As described in detail in the context of Fig. 1a, the multilayer top electrode L1 further comprises a seed layer 11 in between the pair(s) of alternating layers and the piezoelectric layer L2 in accordance with certain embodiments.

[0077] As shown in Fig. 1c, in certain embodiments, the multilayer top electrode L1 comprises at least three pairs of alternating layers of a first material layer 13 and an intermediate material layer 12 (a triple pair top electrode stack). Accordingly, the multilayer top electrode L1 comprises (at least) three first material layers 13 and three intermediate material layers 12, in an alternating sandwich configuration.

[0078] As shown in Fig. 1 d, in certain embodiments, the multilayer top electrode L1 comprises at least four pairs of alternating layers of a first material layer 13 and an intermediate material layer 12 (a quadruple pair top electrode stack). Accordingly, the multilayer top electrode L1 comprises (at least) four first material layers 13 and four intermediate material layers 12, in an alternating sandwich sequence / configuration.

[0079] In certain embodiments, the multilayer top electrode L1 comprises N pairs of alternating layers of the first material layer 13 and the intermediate material layer 12, wherein N is 2, 3, 4 or even more. In certain embodiments, the number of first material layers 13 within the multilayer top electrode L1 is equal to the number of intermediate material layers 12 within the multilayer top electrode L1 .

[0080] In certain embodiments, the multilayer top electrode L1 comprises at least the layers of: the seed layer 11 , the intermediate material layer 12, and the first material layer 13 (in this order / sequence). In certain embodiments, the multilayer top electrode L1 comprises at least the seed layer 11 , the intermediate material layer 12, and the first material layer 13, and optionally N times further the intermediate material layer 12, and the first material layer 13 (in this order / sequence). The preceding order(s) of layers applies when observing from bottom towards the top, starting from the piezoelectric layer towards the top of the resonator. In certain embodiments, the N-pair multilayer top electrode L1 comprises the layer sequence of: seed layer / N x [intermediate material layer / first material layer]. Therein N x [intermediate material layer I first material layer] stands for intermediate material layer and first material layer pairs being repeated N times. In certain embodiments, N is at least 1 , preferably 2, more preferably 3 or more. The preceding sequence applies when observing from bottom towards the top, starting from the piezoelectric layer towards the top of the resonator.

[0081] By way of an example, the above sequence may be formulated to the following according to certain preferred embodiments: Ta / N x [Pt / Au], wherein N x [Pt / Au] stands for Pt I Au pairs being repeated N times. In certain embodiments, N is at least 1 , preferably 2, more preferably 3 or more.

[0082] According to certain embodiment, the sum of the thicknesses of the first material layer(s), the intermediate material layer(s), and the seed layer of the multilayer top electrode (is designed to, is configured to) match the thickness that a (conventional) single-layer top electrode would have. In accordance with certain embodiments, the multilayer top electrode L1 enables an improvement in the frequency stability of the resonator, even 2-3 times improvement in comparison to a single-layer top electrode.

[0083] In certain embodiments, the multilayer top electrode L1 is a sandwich (sandwiched) top electrode (wherein the material layers are in an alternating order / sequence). In certain embodiments, the multilayer top electrode L1 is a sandwich of alternating material layers. In certain embodiments, the sandwich of alternating layers (the multilayer top electrode) comprises (very, extremely) narrow layers of materials. In certain embodiments, the sandwich of alternating layers comprises narrow layers of materials, at least in comparison to the thickness of the other material layers of the resonator. In certain embodiments, the seed layer, the intermediate material layer and / or the first material layer have thicknesses in a micrometer range. In certain embodiments, the seed layer, the intermediate material layer and / or the first material layer have thicknesses in a nanometer range. In certain embodiments, even very narrow material layers enable providing the targeted increase in frequency stability.

[0084] Fig. 1e schematically shows a multilayer bottom electrode stack according to an example embodiment. In certain embodiments, the material stack comprises, in addition to the multilayer top electrode L1 , a multilayer bottom electrode L7. Accordingly, in certain embodiments, the material stack of the resonator 100 comprises the multilayer bottom electrode L7, the piezoelectric layer L2 on top of the multilayer bottom electrode L7, and the multilayer top electrode L1 on top of the piezoelectric layer L2. What is disclosed in context of multilayer top electrode L1 , is equally applicable to the multilayer bottom electrode L7 as well.

[0085] In certain embodiments, the multilayer top electrode L1 and the multilayer bottom electrode L7 are different from each other (they comprise different types of material layers, and / or materials within said layers). In certain embodiments, the multilayer top electrode L1 and the multilayer bottom electrode L7 are identical with each other.

[0086] Fig. 2 schematically shows a material stack of a resonator 100 according to an example embodiment. Fig. 2 schematically shows an example cross section (sectional view, side view) of the resonator 100 residing on a substrate.

[0087] In certain embodiments, the resonator 100 is fabricated on a substrate. In the example embodiment of Fig. 2, a silicon on insulator (SOI) substrate (wafer) 450 is used. The reference numerals 401 and 402 denote bottom electrode and top electrode contacts, respectively.

[0088] In the example embodiment shown in Fig. 2, the multilayer top electrode is implemented in layer L1. In this example embodiment, layer L2 is a piezoelectric layer for piezoelectric actuation of the resonator residing in the area of denoted by 100. In certain embodiments, the piezoelectric layer L2 comprises doping, such as scandium Sc, yttrium Y, lanthanum La, titanium Ti, zirconium Zr, hafnium Hf, erbium Er, dysprosium Dy doping, or any combination of the preceding. An opening in L2 is denoted by 420. In this example embodiment, layer L4 is a silicon layer for the resonator (for example resonating beam elements and their connecting elements according to certain embodiments). In this example embodiments, layer L5 is a buried oxide layer (SiO2) of the SOI wafer, and layer L6 is a silicon handle layer. In certain embodiments, layer L6 comprises a cavity C1. In certain embodiments, the layer L5 follows the cavity C1 shape as shown in Fig. 2.

[0089] In this example embodiment, layer L3 denotes an optional layer for the bottom electrode. In certain embodiments, when a doped silicon layer is used as L4, it is possible to leave out the separate L3 bottom electrode. In such embodiments, the conductive (low resistivity) doped silicon layer L4 acts as the bottom electrode. In certain embodiments, the silicon layer L4 comprises degenerately doped silicon. In certain embodiments, more than 50 % of the silicon layer L4 mass consists of degenerately doped silicon. In certain embodiments, the silicon layer L4 is doped to an average impurity concentration of at least 2*1019cm-3, such as at least 102° cm-3. In certain embodiments, the doped silicon is of N-type or P-type doping. In certain embodiments, the bottom electrode comprises ultra-heavily doped, UHD, silicon. In certain embodiments, the doping comprises scandium Sc, yttrium Y, lanthanum La, titanium Ti, zirconium Zr, hafnium Hf, erbium Er, dysprosium Dy doping, or any combination of the preceding.

[0090] In certain embodiments, the silicon layer L4 comprises single crystalline silicon. In certain embodiments, the silicon layer L4 consists essentially of single crystalline silicon. In certain embodiments, the silicon layer L4 comprises degenerately doped single crystalline silicon.

[0091] In certain preferred embodiments, the resonator 100 comprises a material stack, the material stack comprising the silicon layer L4 (the bottom electrode), the piezoelectric layer L2 on top of the silicon layer L4, and a multilayer top electrode L1 on top of the piezoelectric layer L1. The multilayer top electrode is described in detail in context of Figures 1a-1d. In certain embodiments, the resonator 100 is a piezoelectric resonator. In certain embodiments, the piezoelectric layer L2 comprises aluminum nitride.

[0092] In certain embodiments, the longitudinal axis L of a resonating element (or all resonating elements) 101 is aligned with <100> crystal direction of the silicon (of the bottom electrode), such as aligned with

[0100] crystal direction of silicon (of the bottom electrode), or deviates less than 25 degrees therefrom, or less than 15 degrees therefrom in certain embodiments. In certain preferable embodiments, the longitudinal axis L of the resonating element (or all resonating elements) 101 is aligned with <100> crystal direction of the silicon (of the bottom electrode), such as aligned with

[0100] crystal direction of the silicon (of the bottom electrode), or deviates less than 5 degrees therefrom, or less than 2 degrees therefrom in certain embodiments.

[0093] Fig. 3 schematically shows a top view (from above, from up to down) of an example resonator demonstrating features and dimensions thereof according to an example embodiment. In certain embodiments, the resonator 100 comprises at least one resonating element 101. In certain embodiments, the resonator 100 comprises a plurality of resonating elements 101.

[0094] The resonator 100 according to embodiment shown in Fig. 3 comprises a plurality of resonating elements 101 having a length L and a width W. In certain embodiments, the resonating elements 101 are beam elements (beam-shaped). In the embodiment shown in Fig. 3, the resonator 100 comprises seven resonating elements 101 (the number of elements 101 may vary depending on the embodiment). In certain embodiments, the resonating elements 101 are longer L than they are wide W. In certain embodiments, the coordinate system is selected so that the x-axis resides in the width direction W of the resonating elements 101 and the y-axis in the longitudinal direction L of the resonating elements 101.

[0095] According to the example embodiment shown in Fig. 3, the plurality of resonating elements 101 are positioned adjacent to each other. In certain embodiments, the plurality of resonating beam elements 101 form a ladder-like configuration (stacked beam resonator). In certain embodiments, the plurality of resonating beam elements 101 are positioned adjacent to each other in a width direction thereof. The adjacent resonating beam elements

[0096] 100 are mechanically connected to each other.

[0097] In certain embodiments, the resonator 100 is formed of the plurality of resonating elements

[0098] 101 and a plurality of connection elements 102. In certain embodiments, said adjacent resonating elements 100 are mechanically connected to each other by connection elements 102. In certain embodiments, the adjacent resonating elements 101 are separated by trenches 104. In certain embodiments, the trenches 104 have a length TL (trench length). In certain embodiments, the length L of the beam element 101 comprises at least the length of the trench TL and the length of at least one connection element 102.

[0099] In certain preferred embodiments, the resonator 100 is a stacked beam resonator (a ladderlike configuration) comprising a plurality of resonating beam elements 101 positioned side- by-side in a plane, separated by trenches 104 and connected by connection elements 102. In at least some stacked beam resonators, the resonating beam elements 101 are positioned in the same plane. In certain stacked beam resonators, no two resonating beam elements 101 are positioned atop each other.

[0100] In certain embodiments, the resonating elements 101 of the resonator 100 are arranged in a rectangular array configuration. In certain embodiments, the resonator has a length L (which is equal to the length of the beam element). In certain embodiments, the resonator has a width RW (resonator width).

[0101] In certain embodiments, the resonator 100 is attached to a support structure (not shown in Fig. 3). In certain embodiments, the resonator 100 is attached to the support structure 110 from the outermost resonating elements 101 of the resonator 100 by anchoring point(s) 103. In certain embodiments, the resonator 100 comprises electrical terminal(s) at anchoring points 103. In certain embodiments, the resonating element(s) are separated from the support structure 110 by (an external) trench 104’.

[0102] In certain embodiments, the resonator 100 is of an elongated shape (having the length L smaller than their width RW). In certain embodiments, the resonator 100 is in the shape of a rectangle (the resonator 100 has a shape of a rectangle). In certain embodiments, the resonator 100 has an aspect ratio (ratio of length L to width RW, when observed from above) different from 1. In certain embodiments, the resonator 100 has a length-to-width, L-to-RW, aspect ratio of less than 1 .

[0103] In certain embodiments, the resonating beam elements 101 are of an elongated shape (having their length L larger than their width W). In certain embodiments, each resonating beam element 101 is in the shape of a rectangular beam (beam-shaped). In certain embodiments, each resonating beam element 101 has an aspect ratio (ratio of length L to width W, when observed from above) different from 1. In certain embodiments, each resonating beam element 101 has a length-to-width, L-to-W, aspect ratio of more than 1. In certain example embodiments, each resonating beam element 101 has a length-to-width, L-to-W, aspect ratio of more than 2, such as 5, 8, or 10.

[0104] Fig. 4 schematically shows an example of a resonator array (an apparatus) according to an example embodiment. Herein is provided an apparatus, such as a resonator array, comprising at least one resonator 100 in accordance with certain embodiments. In certain embodiments, as shown in Fig. 4, the apparatus comprises (at least) two resonators 100 (stacked beam resonators). In certain embodiments, the apparatus comprises a plurality of resonators 100 positioned adjacent to each other. In certain embodiments, the apparatus comprises a plurality of resonators 100 coupled to each other by a coupler 310. All embodiments described in context a single resonator 100 apply herein as well for the resonators 100 of the apparatus.

[0105] Fig. 4 schematically shows an apparatus comprising two resonators 100 coupled to each other by a coupler 310 in accordance with certain embodiments. In certain embodiments, the coupler 310 comprises a layer of gold. In certain embodiments, the coupler 310 is a flexural coupler. The apparatus shown in Fig. 4 comprises two (width) extensional-mode resonators 100, and a (one or more) flexural mode resonator coupler 310. Further, the apparatus comprises a (one or more) mechanical connector element 320 which connects the flexural resonator coupler 310 to the extensional mode resonators 100. In certain embodiments, at least one of the extensional-mode resonators 100 of the apparatus comprises a piezoelectric thin-film actuator for exciting the said extensional-mode resonator to a resonance mode and thereby the whole apparatus to a collective resonance due to mechanical coupling of the extensional-mode resonators 100.

[0106] In certain alternative embodiments, the coupler 310 is a length-extensional coupler (not shown). In certain embodiments, the coupler comprises discontinuity regions to render the coupler electrically inert (not shown). In certain embodiments, more than 50% of the mass of the apparatus comprise material portions of single-crystalline silicon. In certain embodiments, the apparatus 100 comprises an electrostatic actuator for exciting at least one of the extensional-mode resonators 100 to a resonance mode and thereby the whole resonator to a collective resonance due to mechanical coupling of the extensional-mode resonators 100.

[0107] In certain embodiments, the resonators 100 are implemented in the form of stacked beam resonators having a plurality of adjacent resonating beam elements 101 connected by connection element(s) 102 and separated by trenches 104 (thus forming a ladder-like structure). In certain embodiments, (all, at least two of) the resonators 100 of an apparatus are identical with one another (each other). In certain alternative embodiments, (all, at least two of) the resonators 100 of an apparatus are different from one another.

[0108] Fig. 5 shows a graph presenting change in frequency, Af (ppm), during ageing and reflow steps 0->3, for a multilayer top electrode resonator A and a single top electrode layer resonator B. As stated, the line A in the graph presents results for a multilayer top electrode solution in accordance with certain embodiments, and the line B presents results for a conventional single top electrode resonator for comparison. The results of the graph show results of packaged resonators.

[0109] The graph shows a starting point 0, wherein the results represent the initial measurement values right after the packaging of the resonator. In the starting point, the multilayer top electrode resonator A has the same initial value (i.e. zero) as the conventional single top electrode layer resonator B. The graph shows three steps 1 , 2 and 3, following the starting point 0. The steps 1 , 2 and 3 proceed chronologically, such that they follow one another as shown in the graph via arrows. Each step 1 , 2 and 3 shows the measurement results of the frequency change for both the multilayer top electrode resonator A and the conventional single top electrode layer resonator B.

[0110] Step 1 denotes the measurement results after 15 hours of ageing at 125 °C. As shown in the graph, the multilayer top electrode resonator A according to certain embodiments shows less change in frequency in comparison to the conventional single top electrode layer resonator B. The multilayer top electrode resonator A shows only a minor change in frequency after the step 1 of ageing, whilst the conventional single top electrode layer resonator B shows a significant change in resonance frequency after the same ageing.

[0111] Step 2 denotes the measurement results after reflowing process that follows the ageing. As shown in the graph of Fig. 5, at this stage both resonators A and B present a decrease in the frequency change.

[0112] Step 3 denotes the measurement results after 120 hours of ageing at 125 °C that follows the aforementioned first ageing (step 1) and the reflowing (step 2). As shown in the graph, the resonator A having the multilayer top electrode according to embodiments of the present solution has significant improvement in comparison to the conventional resonator B. The conventional resonator B’s frequency drifts at the ageing step 3 at values above 10 ppm, whilst the multilayer top electrode resonator A shows only a frequency drift of less than 5 ppm.

[0113] Figs. 6a and 6b schematically show a perforated multilayer top electrode stack in top view and in side view (cross-sectional view), respectively, according to example embodiments. In certain embodiments, the resonating element 101 comprises a perforated top electrode L1. In certain embodiments, the resonating element 101 comprises a perforated top electrode L1 , wherein the perforations reach through one or more material layers of the top electrode L1. Fig. 6a shows a schematic top view of the resonating element 101 comprising perforations 150 within the top electrode L1 thereof.

[0114] In at least some embodiments, the perforations of the perforated resonating element are uniformly spaced, creating a mesh, as shown in Fig. 6a. In certain embodiments, the perforations are comprised in columns along the perforated resonating element and adjacent columns of perforations are off-set from each other. In some embodiments, the perforations are equally spaced throughout the perforated resonating element.

[0115] In certain embodiments, the perforations of the perforated resonating element have the shape of circles, hexagons, hexagons with triangle extensions, crosses, rectangles or ellipses. In certain embodiments, all the perforations of the resonating element 101 are identical. In certain alternative embodiments, the resonating element 101 comprises different kinds of perforations.

[0116] Fig. 6b shows alternative embodiments of the cross section of the resonating element 101 along the line C-C’ of Fig. 6a. Fig. 6b cross-sections correspond with the cross-sections shown in Fig, 1a. It should be noted that the perforations are not limited to only this crosssection but may be applied to any embodiments of the present solution.

[0117] Perforations according to certain embodiments may be provided in a variety of fashions. For example, certain embodiments employ lithographic patterning. Etching may be employed with at least some embodiments. For example, after lithography, one or more (successive) etching steps are employed to achieve certain embodiments. In certain embodiments, the etching may be wet etching, or dry etching. In certain embodiments, the etching is a deep reactive ion, DRIE, etching.

[0118] As shown in Fig. 6b, the perforations 150 may reach at different depths within the material stack of the resonating element. In certain embodiments, the perforations 150 reach through the first material layer 13 (exposing the intermediate layer 12), as shown in the top left in Fig. 6b. In certain embodiments, the perforations 150 reach through the first material layer 13 and the intermediate material layer 12 (exposing the seed layer 12), as shown in the top right in Fig. 6b. In certain embodiments, the perforations 150 reach through the at least one pair, or the plurality of pairs of alternating layers of a first material layer 13 and an intermediate material layer 12.

[0119] In certain embodiments, the perforations 150 reach through the first material layer 13, the intermediate material layer 12, and the seed layer 11 (exposing the piezoelectric layer L2), as shown in the bottom left in Fig. 6b. In certain embodiments, the perforations 150 reach through the at least one pair, or the plurality of pairs of alternating layers of a first material layer 13 and an intermediate material layer 12, as well as through the seed layer 11. In certain embodiments, the perforations 150 reach through the multilayer top electrode L1. In certain embodiments, the perforations 150 reach through the first material layer 13, the intermediate material layer 12, and the seed layer 12, and the piezoelectric layer L2 (exposing the silicon layer L4), as shown in the bottom right in Fig. 6b. In certain embodiments, the perforations 150 reach through the at least one pair, or the plurality of pairs of alternating layers of a first material layer 13 and an intermediate material layer 12, as well as through the seed layer 11 and the piezoelectric layer L2. In certain embodiments, the perforations 150 reach through the multilayer top electrode L1 and the piezoelectric layer L2.

[0120] Without limiting the scope and the interpretation of the patent claims, certain technical effects of one or more of the example embodiments disclosed herein are listed in the following. A technical effect is enhancing the stability of the frequency and reduced frequency drift of a resonator, such as a MEMS resonator. In accordance with certain embodiments, the multilayer top electrode enables an improvement in the frequency stability of the resonator, even 2-3 times improvement in comparison to a single-layer top electrode.

[0121] More specifically, a technical effect is to enhance the frequency stability and reduced frequency drift with respect to thermal cycling and with respect to ageing, through optimization of the materials used for the resonator. In particular, the optimization of materials concerns the top electrode materials. These effects are demonstrated in the preceding Fig. 5. A technical effect of the instant ‘sandwich’ top electrode layer configuration is enabling “locking” the grain boundary movement within the ‘sandwiched’ layers, thus reducing the structural changes of the top electrode materials that negatively affect the frequency stability of the resonator.

[0122] In the present solution it has been found that the top electrode of the resonator contributes to the frequency instability due to the material characteristics of the top electrode. The conventional manner of using a single continuous top electrode layer, typically of metal, may lead to frequency instability of the resonators, especially in MEMS resonators, which are more sensitive to electrode material characteristics than quartz crystal resonators.

[0123] A further technical effect is enabling using same or similar materials as before (but in the ingenious manner disclosed herein), thus eliminating the need for extensive material change procedures in the production. Yet further technical effect is enabling usage of same or similar methods for trimming the resonator as before, such as frequency trimming via ion-beam trimming, thus eliminating the need for any method changes or introduction of new processes in the production.

[0124] Various embodiments have been presented. It should be appreciated that in this document, words comprise, include, and contain are each used as open-ended expressions with no intended exclusivity.

[0125] The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented in the foregoing, but that it can be implemented in other embodiments using equivalent means or in different combinations of embodiments without deviating from the characteristics of the invention.

[0126] Furthermore, some of the features of the afore-disclosed example embodiments may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.

Claims

CLAIMS1. A resonator (100), comprising at least one resonating element (101) having a multilayer top electrode (L1), wherein the multilayer top electrode (L1) comprises at least one pair of alternating layers of a first material layer (13) and an intermediate material layer (12).

2. The resonator (100) of claim 1, wherein the intermediate material layer (12) is of a metallic material, such as platinum, Pt, or of semiconducting material, such as Si, or of an insulating material, such as AI2O3.

3. The resonator (100) of claim 1 or 2, wherein the first material layer (13) is of metallic material, such as Au, or of semiconducting material, such as Si.

4. The resonator (100) of any preceding claim, wherein the multilayer top electrode (L1) comprises a seed layer (11) beneath the at least one pair of alternating layers of the first material layer (13) and the intermediate material layer (12).

5. The resonator (100) of claim 4, wherein the seed layer (11) is of a metallic material, such as tantalum, Ta.

6. The resonator (100) of any preceding claim, wherein the multilayer top electrode (L1) comprises N pairs of alternating layers of the first material layer (13) and the intermediate material layer (12), wherein N is 2 or more.

7. The resonator (100) of any of the preceding claims, wherein the resonating element (101) comprises a piezoelectric layer (L2), wherein the multilayer top electrode (L1) is on the piezoelectric layer (L2), and a bottom electrode (L4) on the opposite side of the piezoelectric layer (L2) than the multilayer top electrode (L1).

8. The resonator (100) of claim 7, wherein the bottom electrode (L4) comprises silicon, preferably doped silicon, such as ultra-heavily doped, UHD, silicon.

9. The resonator (100) of any preceding claim, wherein the resonating element (101) comprises a plurality of resonating beam elements.

10. The resonator (100) of claim 9, wherein the resonating beam elements are longitudinally aligned within 25 degrees of a <100> crystal direction of silicon.

11. The resonator (100) of any of the preceding claim, wherein the resonating element (101) is configured to resonate in a length-extensional, LE, resonance mode, or flexural resonance mode, or a width-extensional, WE, resonance mode.

12. The resonator (100) of any of the preceding claim, wherein the resonator (100) is a microelectromechanical systems, MEMS, resonator.

13. The resonator (100) of any of the preceding claim, wherein the resonating element (101) comprises a perforated multilayer top electrode (L1), wherein the perforations reach through one or more material layers of the multilayer top electrode (L1).

14. An apparatus, such as a resonator array, comprising at least one resonator (100) according to any of claims 1-13.

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