Patent Application
20210203303
Wide-band filter applications require resonators with a high pole-zero distance (PZD), i.e. frequency spacing between main or series resonance and parallel or antiresonance frequencies. The pole-zero distance PZD is directly related to the effective piezoelectric coupling and hence to intrinsic material properties and the structure of the layer stack the resonator consists of. Especially 5G applications (5th generation wireless systems) require bandwidths far exceeding those bandwidths that are achievable with state of the art micro-acoustic resonators used in a typical ladder type filter design. Hence, non-standard topologies are needed which in many cases require many inductors, often in series or parallel to a micro-acoustic resonator.
To widen the pole-zero distance (PZD) of BAW resonators, inductors can be added in series. Thereby the series resonance can be shifted to a lower frequency position. Alternatively by using a parallel inductor the parallel- or antiresonance can be shifted to a higher frequency position. Usually, these inductors are realized as external elements (e.g. SMDs, POGs) that can be arranged on-chip next to a BAW resonator. Hence, these external elements require additional space. Alternatively the coils can be integrated within a laminate or package the BAW resonator is mounted to or packaged in.
It is an object of the present application to realize the combination of lumped elements like inductors and a micro-acoustic resonator in a compact way and with minimal interconnection lengths.
This and other objects are met by the BAW resonator of claim 1. Advantageous features and embodiments of such a BAW resonator are given by dependent claims.
A BAW resonator of the SMR type (solidly mounted resonator) comprises a substrate, a Bragg mirror, a bottom electrode, a piezoelectric layer and a top electrode. The Bragg mirror serves to keep the acoustic energy inside the resonator and comprises alternating mirror layers of high acoustic impedance and low acoustic impedance. A fundamental reflecting effect is achieved with one pair of mirror layers. Advantageously two pairs of mirror layers or an uneven number of mirror layers is used to completely reflect the wave back into the resonator.
It is proposed to realize an inductor as a planar coil below the active resonator region in a high impedance mirror layers or an additional metal layer arranged between substrate and Bragg mirror. To achieve sufficient reflection at least two high impedance layers are present.
The planar coil is electrically connected to the resonator that is to at least one of the resonator's electrodes.
Such a solution has only minimal space consumption as the integration of a planar coil into an already existing stack of similar layers is easy and synergy effects can be used. Structuring of high impedance mirror layers is necessary too and hence, the structuring of the planar coil can be done the same way.
The BAW resonator comprises at least two high impedance mirror layers. If the coil is structured from one of these layers the reflecting effect of the so-produced coil of high acoustic impedance material can be used advantageously.
However it is preferred to use at least one pair or two pairs of complete mirror layers without coils and to arrange or structure the coil in an additional metal layer. This additional metal layer may be a high impedance layer and may comprise the same material like the high impedance mirror layers. Then, the manufacturing process becomes simpler.
However any other electrically conductive metal of any acoustic impedance can be used for the additional metal layer if the reflecting effect of the complete mirror layers of the Bragg mirror above is sufficiently high. High impedance mirror layers as well as the metal layer with the coil structured therefrom are embedded in a low impedance dielectric material. Then, the planar coil has no detrimental effect onto the acoustic of the resonator and hence on the Q factor thereof.
According to an embodiment the BAW resonator comprises two additional metal layers with a respective first and second planar coil formed therein. First and second planar coil are circuited in series with each other. This can be done by connecting a respective first end of each of the two windings that form the coils by a vertical through contact e.g. a via. The respective other second ends are used to connect the coils in series or parallel to the resonator via at least one of the resonator's electrodes. These connections too can be realized by a respective via. The vias are guided through the mirror layers. Preferably the vias are formed at a position that is outside the active resonator area. An active resonator region is defined to be a region where bottom electrode, piezoelectric layer and top electrode overlap each other. An active resonator area is defined to be the area of the active resonator region when projected normal to the top surface of the substrate. If the vias are arranged outside the active resonator area no acoustic interaction with the resonator and hence, no detrimental effect occurs.
The planar coil is a planar winding that has a first end in the middle of the winding and a second end. The first end is connected by a first via to a first electrode of the resonator and the second end of the planar coil is connected by a second via to the second electrode of the resonator. First and second electrode are selected from bottom electrode and top electrode.
If the coil comprises two planar windings it is preferred to arrange the windings directly one above the other with an intermediate dielectric. The two windings are then coupled and circuited in series by connecting their first ends with a via. The advantage is that the second ends at the respective periphery of the windings can easily be coupled to a first and a second electrode selected from bottom electrode and top electrode directly by a first and a second via or by interposing an outwardly guided conductor line. Then the via is located outside the active resonator area.
Material properties and layer thicknesses of the layer stack of the BAW resonator are very well controlled for optimal acoustic behavior which is more demanding than the electromagnetic properties. Inductors (i.e. the planar coils) are shaped using the same photolithographic steps that are anyway needed to pattern the high-impedance mirror layers. High impedance mirror layers may be restricted in area to the active resonator area such that mirror layers of neighbored resonators are electrically isolated against each other to avoid EM crosstalk between these resonators that would otherwise ultimately reduce the filter selectivity.
The manufacture of the proposed BAW resonator requires only low process variation compared to other solutions and processes where external lumped elements need to be realized and coupled e.g. integrated into laminates, or embodied as PoG (passives on glass).
Bragg mirror as well as electrode, piezoelectric layer and package if required can be embodied according to the art as these components do not interact with the proposed planar coil. The material of the high impedance layers can comprise a high impedance metal chosen from W and Mo. As a material of the low impedance and dielectric layers silicon dioxide is a preferred choice due to its proved properties and easy handling.
Independent therefrom the materials of the electrodes of the resonator can be chosen from a group comprising W and Mo. Manufacture of the complete layer stack may be simplified if the same metal is used for mirror layer and electrodes. However, better electrical conductivity of molybdenum Mo or Al may make Mo or Al a preferred choice for the electrodes. If a high impedance mirror layer is targeted W may be preferred in view of the higher_impedance of tungsten W.
The piezoelectric layer may consist of AlN. However, ZnO and AlN doped with Sc may be used too.
On top of the top electrode a passivation layer of SiN may be deposited. If necessary a mechanically stable capping may complete the BAW resonator. Such a capping may comprise a capping layer integrally formed on the top surface thereby keeping an air-filled cavity above the active resonator region. The cavity may be pre-formed as a sacrificial layer that is structured that sacrificial material remains only on those surface areas that need to be protected in a cavity under a capping layer. The cavity can be released after depositing the capping layer and removing the structured material of the sacrificial layer through release holes made in the capping layer.
A BAW resonator is mainly used for creating RF filters by circuiting such resonators in a ladder type arrangement according to the art. The resonators of such an arrangement are circuited in series and parallel by top electrode connection and/or bottom electrode connection. According to the specifications the filter must attend to, the bandwidth of the resonators need to be adapted by coupling inductors to the resonators as proposed. In a filter circuit, at least as many inductors as BAW resonators can be realized within one filter die i.e. on a single substrate chip.
Measures can be taken to avoid crosstalk between different resonators on the same chip. For doing so metal layers can be grounded to shield the coil in a vertical direction. A kind of fence of long vias arranged at the perimeter of the active resonator area may shield the coil in a horizontal direction.
In the following the invention will be explained in more detail with reference to preferred embodiments and the accompanied figures. The figures are schematic only and are not drawn to scale. Hence, neither relative nor absolute geometry parameters can be taken from the figures.
At least one of the high impedance layers HI comprises a planar coil that is structured as a winding WG in the high impedance layer HI.
An active resonator region AR is the region where all three layers of the sandwich overlap each other. Only in the active resonator region AR acoustic waves can be excited and propagate.
The windings are arranged under the active resonator region AR. Depending on the required inductance of the planar coil the area the windings WG occupy may be smaller than the active resonator region AR, equal or, in an extreme case, may extend over the active resonator region AR. In all cases the high impedance layer HI the windings are formed to function as a mirror layer and have a respective thickness of about a quarter wavelength of the acoustic wave.
When interconnecting both windings of
Depending on the circuiting with the acoustic resonator (series, parallel) and the needed value of the inductor, it may be decided whether to use “aiding” or “opposing” inductors. Furthermore, the inductor design may depend on size constraints and optimal integration with acoustics.
The two windings of the two additional metal layers are circuited in series similar as those shown in
As the desired widening of the pole zero distance is higher with a parallel inductance having a smaller value only one winding may be sufficient to achieve the desired area that complies with a respective inductance value.
The diagram of
The invention has been shown with reference to selected embodiments only but is not restricted to these embodiments. Materials of the layers, thickness, area and size of the windings may deviate from the depicted or described embodiments. The Bragg mirror may be formed by a deviating number of mirror layers using other high or low impedance materials. The at least one planar coil can be embodied in a high impedance mirror layer or in an additional metal layer below the Bragg mirror. Other substrate materials than silicon may be used too. Besides the shown layers the BAW resonator may comprise further functional layers like thin adhesion supporting layers at the interfaces between two adjacent layers. Depositing at least a passivation layer of e.g. SiN on top of the top electrode according to the art is also self-evident. Further, the BAW resonator may be used in a circuit of several BAW resonators that form a filter circuit in a ladder type arrangement for example. These circuits may be formed by integrally interconnecting neighbored BAW resonators via top electrode or bottom electrode connection which can be done by respective structuring of the electrode layer after deposition.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102018121689.0 | Sep 2018 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/071571 | 8/12/2019 | WO | 00 |
