WO2010122024A1

WO2010122024A1 - Bottom electrode for bulk acoustic wave (baw) resonator

Info

Publication number: WO2010122024A1
Application number: PCT/EP2010/055199
Authority: WO
Inventors: Stephan Marksteiner
Original assignee: Epcos Ag
Priority date: 2009-04-24
Filing date: 2010-04-20
Publication date: 2010-10-28
Also published as: DE102009018879B4; DE102009018879A1

Abstract

For a BAW resonator, it is proposed that a metallic layer (HS1, HS2) of the Bragg reflector (DS, HS1, NS, HS2) used as an acoustic mirror be used to improve the effective conductivity of the bottom electrode. For this purpose, an electrical contact between the bottom electrode (BE) and the upper high-impedance layer (HS1) of the acoustic mirror is established in a connection area (VB), and the connection resistance of the bottom electrode is reduced in the process.

Description

Bottom electrode for BuIk Acoustic Wave (BAW) resonator

BAW (Buick Acoustic Wave) resonator-based filters have secured a significant market share in the field of high-end duplexer applications for mobile applications in recent years. This applies in particular to the US PCS band (or UMTS band 2), which, with its bandwidth of 60 MHz each and its TX-RX band gap of only 20 MHz, poses very high challenges to the effective (piezoelectric) coupling coefficients and grades the one

Filter underlying acoustic resonators. This is where the strength of BAW technology comes into play, whose functional principle allows the realization of resonators with grades> 1500.

A technology variant working with acoustic mirrors (Bragg reflectors) are the so-called solidly mounted resonators or SMRs for short. However, these have a complex structure with a layer comprising a plurality of layers stack. One problem with their design is to achieve sufficient effective coupling coefficients while maintaining high quality and low electrical film resistances.

It is known that hard electrode materials, more specifically materials with high acoustic impedance, e.g. Tungsten, molybdenum, platinum, ruthenium, etc. compress the acoustic wave in the piezoelectric material and thus can produce higher effective couplings.

Furthermore, it is known that there is an optimum thickness for each material for which the effective coupling of the resonators is maximal. The harder the material, the better the maximum achievable effective coupling is greater, but the thinner the optimum layer thickness. However, the conductivity of hard materials is relatively poor, so that for sufficient ohmic conductivity, the electrodes are to be made correspondingly thicker and, for example, at least to be doubled or even tripled. However, the coupling deteriorates by about 4% or 10%. In return, in addition to the constant position of the resonance frequency, the piezo layer would have to be made thinner, whereby the coupling sinks even further.

In practice, there are two different BAW technologies. In addition to the previously mentioned Bragg Spiegeln SMR technology, these are membrane-based BAW resonators (FBAR, originally the name for FiIm-Bulk-Acoustic-Resonator, recently redefined as Free-standing-Bulk-Acoustic-Resonator). These have a higher effective coupling per se due to the lack of the acoustic mirror. One can therefore go beyond the optimal coupling point and make the electrode thickness sufficiently large.

In the case of Bragg reflector-based resonators, this is not so easy. Here one works with optimal layer thicknesses, but supports the conductivity of the electrode by a sandwich construction, which creates a well-conductive soft electrode layer below the hard layer (typically aluminum). The disadvantage of this method, however, is that the thickness of the uppermost silicon oxide layer in the mirror is reduced by the thickness of the soft and conductive electrode layer (eg of aluminum) and therefore the desirable reduction of the temperature response of the resonant frequency by the negative TCF (= Temperature coefficient of frequency) of the oxide not so strong fails. AlN membrane resonators without silicon dioxide typically have a TCF of -28ppm / K. Resonators with Al / W sandwich electrodes typically reach -18 to -22ppm / K. However, resonators with pure W electrodes can be compensated up to -15ppm / K and better.

The object of the invention is therefore to provide a BAW resonator with SMR design, which has at the same time good electrical properties in addition to a high coupling coefficient.

This object is achieved by a BAW resonator with the features of claim 1. Advantageous embodiments of the invention will become apparent from further claims.

The specified BAW resonator is realized in the form of a layer stack on a substrate. In this case, the layer stack comprises, above the substrate, at least one pair of layers comprising a high-impedance layer having a relatively high acoustic impedance and a low-impedance layer having a relatively low acoustic impedance, which together form two partial layers of an acoustic mirror. The acoustic mirror may also comprise one or more sub-layers or one or more layer pairs of high-impedance and low-impedance layers. In each case, a high-impedance layer and a low-impedance layer are always arranged alternately.

The layer stack further comprises in the order given a bottom electrode, a piezoelectric layer and a top electrode. In order to improve the connection resistance of the bottom electrode, it is now proposed to make a partial layer of the acoustic mirror electrically conductive and to connect it electrically to the bottom electrode, in particular outside the acoustically active region. In this way it is possible to improve the electrical conductivity of the bottom electrode without additional layer in the stack of layers.

One advantage of the proposed BAW resonator is that all layers of the layer stack can be optimized with regard to material selection and layer thickness to their actual function and, overall, to optimum acoustic properties. Thus, in order to improve the conductivity, it is not necessary to deviate from such an optimized layer stack of a BAW resonator known per se and thereby to change the material and / or layer thicknesses of individual or multiple layers to increase the conductivity, while still significantly improving the electrical properties of the BAW resonator to achieve.

In particular, the bottom electrode can thus be optimized with regard to its acoustic properties with regard to the selection of the material and in particular the selection of a particularly hard electrode material and the layer thickness. The connection of the bottom electrode to the electrically conductive sub-layer of the acoustic mirror reduces the sheet resistance of the bottom electrode by a factor of 2 to 4, even if a relatively poorly conductive material (eg tungsten) is used for the sub-layer. Due to the relatively high layer thickness of the sub-layer, which is required for optimal functioning of the acoustic mirror in the form of a Bragg reflector (about one quarter of the wavelength of the longitudinal acoustic wave), has This sub-layer has an electrical sheet resistance which is substantially lower than that of a set to an optimum layer thickness and high hardness bottom electrode.

For the bottom electrode, a hard electrode material is preferred. This may be selected from tungsten, titanium, titanium / tungsten, tantalum, molybdenum, platinum, iridium and ruthenium. The low specific conductivity of these electrode materials is compensated for particularly advantageously by the invention and thus improves the overall conductivity of the bottom electrode.

It is advantageous if the sub-layer of the acoustic mirror following the bottom electrode is a low-impedance layer which has a high impedance discontinuity with respect to the bottom electrode. The sub-layer of the acoustic mirror following the bottom electrode is therefore, in particular, a dielectric sub-layer with low acoustic impedance, such as, for example, silicon dioxide.

A further advantage is achieved if the dielectric sub-layer directly below the bottom electrode comprises a material which has a lower or even opposite temperature coefficient of the frequency than the piezoelectric layer. Since the acoustic wave in the BAW resonator at least partially penetrates into the acoustic mirror, the acoustic and other parameters of the partial layers also determine the behavior of the BAW resonator. By means of a sub-layer having a low or opposite temperature coefficient as set out, it is possible to reduce the temperature coefficient of the frequency for the entire BAW resonator. This leads to the BAW resonator has a lower drift of the resonance frequency with temperature fluctuations and is therefore particularly well suited for frequency-accurate applications or for applications with a long operating temperature interval or in an environment with strongly fluctuating temperatures.

Such a dielectric layer may also be applied alternatively or additionally above the top electrode. Since in this case the two temperature coefficient reducing dielectric layers co-operate, it may be advantageous to increase the total layer thickness of the first dielectric sub-layer of the acoustic mirror and the dielectric layer deposited on the top electrode with respect to the desired effect on the temperature coefficient optimize.

A preferred material with which the temperature coefficient can be effectively reduced is SiO 2, which is accordingly preferred as the material for the dielectric sub-layer of the acoustic mirror.

In a further embodiment of the invention, the acoustic mirror has at least one further electrically conductive partial layer. Since the electrical conductivity usually only with layers of relatively high acoustic

Impedance is still arranged between the first and second electrically conductive sub-layer of the acoustic mirror, a low-impedance layer, usually a further dielectric layer. Advantageously, both electrically conductive partial layers can now be electrically conductively connected to the bottom electrode and thus further reduce the surface resistance of the bottom electrode or further improve its electrical conductivity. In this way It is also possible to use materials for these high-impedance layers which have relatively poor specific conductivity, since this effect is compensated for by the inventively increased total layer thickness of the electrically conductive layers.

Theoretically, further pairs of layers of low-impedance layer and electrically-conductive high-impedance layer can be used in an analogous manner for the construction of the mirror. However, since the production costs increase with each additional partial layer, the acoustic mirror is optimized to a minimum number of partial layers, so that usually only one or at most two pairs of layers are used in the acoustic mirror. This has the further advantage that with fewer partial layers, a higher bandwidth of the mirror is achieved.

The individual layers of the layer structure of the BAW resonator are usually structured. At least in the laterally defined area, which corresponds to the base area of the acoustically active area, each of the individual layers of the layer stack is unstructured. Outside this area, layer areas may be removed. The active area is defined by the common overlap area of top electrode, piezoelectric

Layer and bottom electrode. The two electrodes are therefore structured so that the active area is optimally dimensioned. The optimum base area of the active area is determined by the desired capacitance or electrical impedance of the BAW resonator. Also, for a desired higher power stability of the resonator, the area of the active area may be increased. It is advantageous if the electrical connection between the electrically conductive partial layer and the bottom electrode takes place outside the laterally defined region, which is defined by a vertical projection of the active region onto the substrate. Alternatively, it is of course also possible to provide an electrical connection in the form of plated-through holes within the active region or its projection.

Advantageously, the electrically conductive connection between the conductive partial layer and the bottom electrode is made via a connection region whose area is optimized for a low line resistance. The connection region is arranged close to the active region and encloses it from as many as possible, in particular from at least three sides.

The electrically conductive sub-layer may comprise a high-impedance material selected from tungsten, titanium, titanium / tungsten, tantalum, molybdenum, platinum, iridium and

Ruthenium, therefore, therefore, from the same material selection, which is advantageous for the preparation of the bottom electrode.

At least one of the dielectric sublayers of the acoustic mirror may comprise silicon oxide. However, it is also possible to use other low-impedance material, for example low-k dielectrics, which at the same time have a particularly low acoustic impedance in addition to the low dielectric constant. It is also possible to carry out the first dielectric sub-layer, which is directly adjacent to the bottom electrode, because of the achievable temperature compensation of silicon oxide, and in the case of another dielectric sub-layer of these To perform a material with even lower acoustic impedance.

The dielectric sublayers or low impedance layers, which are arranged between the electrically connected layers such as bottom electrode and electrically conductive sublayer, are therefore preferably structured so that the material of these dielectric sublayers is outside the active region or outside the projection of the active region, but at least in the connection area.

A preferred material for the piezoelectric layer is aluminum nitride. This grows with good layer properties and especially high quality on high-impedance electrode materials, as they are preferred for optimum acoustic properties.

The top electrode may comprise a material having good electrical conductivity. Well suited are standard electrode materials such as aluminum, copper or gold. However, it is also possible for the top electrode to manufacture these from a high-impedance material or at least to select the partial layer of the top electrode made of such a material which is in direct contact with the piezoelectric layer. In this case, further partial layers can be applied, for which electrically better conductive materials can be used.

In a preferred use, the indicated BAW resonator is used to construct an RF bandpass filter. Such filters are made by electrical interconnection of individual BAW resonators in a ladder-like structure get, called laddertype or latticetype structures.

In the following the invention will be explained in more detail by means of exemplary embodiments and the associated three figures. The figures are only illustrative of the invention and are therefore designed only schematically and not to scale. In addition, individual parts may be displayed distorted to scale, so that the figures are also no relative dimensions can be found.

FIG. 1 shows a first BAW resonator in schematic cross section,

FIG. 2 shows a second BAW resonator in schematic cross section,

FIGS. 3A to 3D show various process stages in the production of a BAW resonator in a schematic plan view.

FIG. 1 shows a first exemplary embodiment of a BAW resonator according to the invention. The BAW resonator has a layer stack ST, which is applied to a substrate S. It is possible, each of the individual layers of the

Layer stack to apply over the entire surface and then to structure. This is advantageous, in particular in the case of electrically conductive layers, in order to minimize unwanted capacitive couplings to adjacent components and in particular to adjacent further BAW resonators. However, the layer stack ST can also comprise unstructured layers which extend over the entire surface of the BAW resonator or over the entire base area of the layer stack. Dielek- As a rule, trical layers of the lower pairs of mirrors can remain unstructured.

The layer stack ST has, above the substrate, at least one pair of layers each consisting of a low-impedance layer NS and a high-impedance layer HS. In the illustrated case, the acoustic mirror in the layer stack comprises two such pairs of layers. The low-impedance layer NS is formed of SiO 2 in all cases and remains unstructured. The high-impedance layers are made of a hard metal such as tungsten, titanium,

Titanium / tungsten, tantalum, platinum, molybdenum or rubidium formed. Preferably, however, tungsten is used. The high-impedance layers HS1, HS2 are applied in a structured manner and extend at least over the active region AB. The thickness of the low-impedance layers between the substrate and the second high-impedance layer, between the second high-impedance layer and the first high-impedance layer and between the first high-impedance layer and the bottom electrode BE is in each case approximately ¹ ° of the wavelength of the longitudinal acoustic wave at the resonant frequency of the BAW resonator. The high-impedance layers HS1, HS2 also have a corresponding thickness. For a resonance frequency of the BAW resonator of, for example, 1900 MHz, an optimum layer thickness of a tungsten W-existing high-impedance layer is approximately 700 nm. To optimize the acoustic behavior of the mirror, however, these layer thicknesses can also be chosen differently.

Above the uppermost low-impedance layer, a bottom electrode BE is applied, preferably again made of a material which is as hard as possible. The same preferred choice of material applies to the bottom electrode BE as for the high-impedance layer HS. The thickness for the bottom electrode BE is chosen so that optimum acoustic properties of the resonator arise (eg a maximum effective coupling or minimum TCF). Such an optimum thickness is for example at 150-250 nm.

Above the bottom electrode BE, the piezoelectric layer PS is applied, for example a piezoelectric material with high coupling, in particular aluminum nitride, which can be readily deposited by means of thin-film methods. An exemplary layer thickness for the piezoelectric layer is approximately one quarter of a wavelength, which corresponds to approximately 1400 nm for aluminum nitride. The uppermost layer is a top electrode TE is applied, wherein an electrically good conductive material of any, but good electrical sufficient layer thickness is applied. When

Standard material can be used aluminum in sufficient thickness.

The bottom electrode comprises in terms of area at least the active region AB and a corresponding supply line, which leads to an electrical connection pad or to another BAW resonator, which is electrically connected via the bottom electrode BE to the illustrated BAW resonator. The top electrode is also structured in such a way that it covers at least the active region AB and, moreover, like the

Bottom electrode BE terminates in an electrical supply line, which also leads to a connection pad or to an electrode of an adjacent BAW resonator, which is electrically connected to the illustrated. The piezoelectric layer PS may also be limited to the active region. Advantageously, however, it is applied over a large area and serves as a common layer for several BAW resonators. The active area is the common overlapping area of top electrode TE, piezoelectric layer PS and bottom electrode BE. In FIG. 1, the active region AB is shown by its dashed lines over its lateral boundaries.

Not shown are further layers which may be applied over the top electrode or generally as the uppermost layer above the layer stack ST. Such layers may include passivation layers, trimming layers, and frequency detuning layers, particularly for differentially adjusting parallel and serial resonators in ladder type or lattice type reactance filters constructed of BAW resonators.

While the high-impedance layers HS are usually structured and limited to the active region, the BAW resonator according to the invention has a lateral extension (in the figure on the left-hand side) in at least one high-impedance layer HS1, in which it forms a connection region VB in which the bottom electrode is electrically connected to the first high-impedance layer HS1. In the connection region, the low-impedance layer NS or the partial layer, which is applied over the first high-impedance layer HS1, form a recess. However, it is also possible to structure the topmost low-impedance layer NS in such a way that it extends substantially over the active region AB and is limited to this, or exceeds it with a small tolerance.

FIG. 2 likewise shows, in schematic cross section, a BAW resonator modified in contrast thereto. In contrast to 1, the connecting region VB between the bottom electrode BE and the upper high-impedance layer HS1 is widened and extends in the figure on both sides of the active region AB. This is possible if the base area of the uppermost high-impedance layer HS1 is chosen to be correspondingly large and the uppermost low-impedance layer, which here is a dielectric layer DS, is structured in such a way that it is limited to the active region AB. In this way, it is possible for the connection region VB to surround the active region AB in an annular manner. Optionally, however, the connection region saves a small section in which the lead for the top electrode is led out of the layer stack or the active region AB of the layer stack, so that the top electrode outside the active region does not overlap with the bottom electrode BE. In this area, the high-impedance layer HS1 can be structured accordingly, so that it does not extend or only insignificantly beyond the limits of the active region AB.

Exemplary lateral dimensions for the individual layers to be structured of the BAW resonator are shown in FIGS. 3A to 3C. The illustrated layer structure begins with the uppermost high-impedance layer HS1, which extends in FIG. 3A over the active region AB plus the connection region VB. In addition, a dielectric layer DS is applied as the uppermost low-impedance layer of the acoustic mirror, at least in the active region.

FIG. 3B shows, as a further layer applied above, the bottom electrode BE, which extends over the active region AB. In the connection region VB, not only that arranged in FIG. 3A to the left of the dielectric layer Area includes, but also an edge region around the dielectric layer thin film, the bottom electrode BE contacted the exposed in the connection area uppermost high-impedance layer HSL of the acoustic mirror electrically. In the area of the later connection pad AP, a further metallization can already be applied here. In addition, a piezoelectric layer PS is applied, which can be structured at least similar to the dielectric layer. However, the base area of the piezoelectric layer PS can also be chosen substantially larger than that of the dielectric layer DS.

FIG. 3C shows, as the topmost layer, the top electrode TE, which is applied over the piezoelectric layer PS. Their overlap with piezoelectric layer PS (not shown in the figure) and bottom electrode BE forms the active region AB. At one point (here on the side of the active region AB opposite the connection pad), an extension of the top electrode TE is shown, which connects a connection line VL to another

Terminal pad or an adjacent BAW resonator represents. The connecting line is insulated from the bottom electrode BE by the piezoelectric layer PS and from the high-impedance layer HS1 by a correspondingly structured dielectric layer.

The top electrode TE does not extend over the connection region VB. In the connection region VB, a connection pad AP is provided above the bottom electrode, which optionally may comprise an additional and, for example, solderable metallization. However, it is also possible that the bottom electrode BE - similar to the top electrode TE - extends further to the left and there a further electrode, in particular a further bottom electrode for an adjacent BAW resonator is formed.

In FIG. 3C, the connection region VB is shown hatched. As can be seen, the connecting region, that is to say the overlapping and contact region between the bottom electrode and the upper high-impedance layer HS1, extends nearly closed around the active region AB and only saves the region in which the connecting conductor VL runs. In this way, it is possible, the conductivity of the

Base electrode BE to support from multiple sides and thus effectively increase by a factor of 2 to 4. This is achieved, although effectively alone the supply resistance to the bottom electrode BE is reduced. However, the current can flow not only from the left, but from three or more sides on the active bottom electrode.

In FIG. 3D, in addition to the hatching of the connecting region VB, the active region BE is also highlighted by another hatching.

Not shown are further dielectric layers which can be used for temperature compensation, trimming, detuning or passivation of the BAW resonator and can be applied over the entire layer stack ST, for example. Also not shown are embodiments in which in the connection region VB additionally a contact to deeper high impedance layers HS2 is made, so that the electrical connection resistance of the bottom electrode BE is further reduced by the conductivity of this additional high impedance layer. In all cases, the design or the base of the proposed BAW resonator is compared to a known resonator only slightly increased by the base of the connection region VB. For the BAW resonator according to the invention, however, no additional layer is required compared with a known BAW resonator. Nevertheless, all layers can be embodied in an optimal layer thickness and a metal can be used for the bottom electrode BE, which has a relatively poor electrical conductivity per se. By supporting additional conductivity of at least one of the metallic sub-layer of the mirror (high impedance layers) this disadvantage is easily compensated because the high impedance layers in the acoustically optimized mirror always have a sufficiently high layer thickness and thus good conductivity.

In the connection region VB, the uppermost low-impedance layer, that is to say the dielectric layer DS, can be provided by structuring with recesses (via trenches) in which the connection region VB can form.

However, as stated, it is possible to limit the dielectric layer DS exclusively to the active region AB plus a tolerance range surrounding it and to structure it accordingly.

The invention is not limited to the embodiments shown and described in the embodiments and in particular the figures. In particular, the design of a BAW resonator may differ significantly from the embodiments.

It is possible, for example, further pairs of layers of high and low impedance layers in the acoustic mirror and to form these from any high and low impedance materials. The bottom electrode BE may be electrically conductively connected in the connection region to the uppermost high-impedance layer HS2, to the lower high-impedance layer HS1 or to both high-impedance layers.

All materials used in the layer stack ST can be varied, but are also optimized for variation back to an optimum layer thickness with regard to acoustics. Preferably, the layer thickness for bottom electrode, piezoelectric layer and top electrode is selected as a function of an optimal coupling and leads in particular to low layer thicknesses of the bottom electrode. The layer thickness of the piezoelectric layer PS is chosen such that the main mode is formed at the desired resonant frequency of the BAW resonator. Since the structuring required for the BAW resonator according to the invention can take place outside the active region AB, they have no influence whatsoever on the acoustic properties of the BAW resonator in the active region.

A BAW resonator according to the invention may deviate from the lateral dimensions exemplified in FIG. Thus, in particular, the connection area can be distributed more uniformly around the active area AB. The bottom and top electrodes can have more than one supply line, in particular if they are electrically connected via this supply line to other elements and in particular to other BAW resonators. LIST OF REFERENCE NUMBERS

ST layer stack

S substrate

NS low impedance layer

HS1, HS2 first and second high impedance layer

DS Dielectric layer

BE bottom electrode

PS piezoelectric layer

TE top electrode

VB connection area

AB active area

AP connection pad

VL connection line

Claims

claims

1. BAW resonator, which is applied in the form of a layer stack (ST) on a substrate (SU), comprising

- At least one pair of layers of a high impedance layer (HS) relatively high acoustic impedance and a

Low-impedance (NS) layer having a relatively low acoustic impedance forming two sub-layers of an acoustic mirror, wherein in the acoustic mirror, the high-impedance layer and the low-impedance layer are alternately arranged

a bottom electrode (BE)

a piezoelectric layer (PS) and a top electrode (TE) in which a sub-layer of the acoustic mirror is electrically conductive and electrically connected to the bottom electrode.

2. BAW resonator according to claim 1, wherein the electrically conductive sub-layer has a higher surface conductivity than the bottom electrode (BE).

3. BAW resonator according to claim 1 or 2, wherein the bottom electrode (BE) comprises a material or consists of a material which is selected from tungsten, Ti, titanium / tungsten, tantalum, molybdenum, platinum, iridium and ruthenium.

4. BAW resonator according to one of claims 1-3, wherein between the electrically conductive sublayer and the bottom electrode (BE) at least one dielectric sub-layer of the acoustic mirror is arranged.

5. BAW resonator according to claim 4, wherein the dielectric sublayer comprises a material having a lower or an opposite acting temperature coefficient of frequency as the piezoelectric layer (PS).

6. BAW resonator according to one of claims 1-5, wherein over the top electrode (TE), a dielectric layer (DS) is applied, which comprises a material having a temperature coefficient of frequency, which is lower or counteracted as the temperature coefficient of piezoelectric layer (PS).

7. BAW resonator according to claim 5 or 6, wherein the dielectric sublayer or the applied over the top electrode dielectric layer (DS) comprises silicon oxide.

8. BAW resonator according to one of claims 1-7, wherein the sub-layers of the acoustic mirror each having a thickness in the range of a quarter-wavelength of the longitudinal resonant wave propagatable in the sub-layer at longitudinal acoustic wave.

9. BAW resonator according to one of claims 1-8, wherein a further pair of layers of the acoustic mirror comprises an electrically conductive sub-layer, which is electrically connected to the bottom electrode (BE).

10. BAW resonator according to any one of claims 1-9, wherein the common overlap region of

Bottom electrode (BE), piezoelectric layer (PS) and top electrode (TE) forms the active region (AB), in which the at least one electrically conductive sub-layer of the acoustic mirror is galvanically connected to the bottom electrode (BE) outside the active region (AB) ,

11. BAW resonator according to one of claims 1-10, wherein the at least one electrically conductive sub-layer of the acoustic mirror and the

Bottom electrode (BE) in a connection region (VB) are electrically connected to each other, wherein the connection region encloses the active region (AB) on at least three sides.

The BAW resonator according to any one of claims 1-11, wherein the at least one electrically conductive sub-layer comprises a material or consists of a material selected from tungsten, Ti, titanium / tungsten, tantalum, molybdenum, platinum, iridium and ruthenium ,

The BAW resonator of any of claims 1-12, wherein the at least one dielectric sub-layer of the acoustic mirror comprises silicon oxide.

14. BAW resonator according to one of claims 1-13, wherein the sub-layers of the acoustic mirror extend at least over the entire active region (AB), in which one or more of the dielectric sub-layers, which are arranged between the electrically conductive sub-layer and the bottom electrode (BE), in the connection region (VB) are removed.

15. BAW resonator according to one of claims 1-14, wherein the piezoelectric layer (PS) AlN and the top electrode (TE) comprises Al, Cu or Au.

16. Use of a BAW resonator according to one of claims 1-15 for constructing an HF bandpass filter.