TWI753055B - Baw resonator device - Google Patents

Abstract

A BAW resonator device is proposed comprising fully conductive acoustic mirrors . Each of the BAW resonators is electrically contacted by a respective stack. Two or more BAW resonators may be electrically connected by connection lines arranged in a plane distant from the electrodes. The connection lines connect the stacks of the respective resonators. The fully conductive acoustic mirrors may be arranged on top of the resonators as well such that the BAW device has a structure symmetric with respect to horizontal mirror plane.

Description

BAW resonator device

This patent application claims priority from German Patent Application No. 102016124236.5, filed on December 13, 2016, the entire content of which is expressly incorporated herein by reference.

The present invention relates to BAW devices with improved resonator interconnections.

Conventional BAW devices such as SMR based resonator filters (SMR=fixed mounted resonator) use thin film electrodes as top and bottom electrodes. Due to the integrated fabrication of SMR devices, the interconnections of the different SMR devices used to form the filter are made of the same film as the top electrode or the bottom electrode. As a result, the current in the filter has to flow over long distances via electrode film portions having only a small cross-sectional area. This causes significant electrical losses in the filter.

Attempts have been made to strengthen the interconnect by applying electrode materials to the interconnect, thereby improving conductivity in the interconnect. However, it becomes impossible to connect directly to the resonator terminals without severely degrading the performance of the resonator. Furthermore, this degradation is uncontrollable, so it is impossible to compensate for it.

Thus, current flows from the high acoustic energy resonator region to the low acoustic energy interconnection region, via the transition region located between the high acoustic energy region and the low acoustic energy region. Both the transition region and the interconnect region are still made of parts of the thin film electrode material. The use of thin-film transition regions may have a negative impact on resonator efficacy - exhibiting increased lateral mode leakage (reduced Q) and increased parasitic capacitance (reduced coupling). This area should ideally be eliminated, but this has not been done so far, and it is designed with minimal performance impact in mind (eg, Avago/Broadcom "air bridge" concept).

The aim of the present invention is to improve the electrical performance of the resonator electrodes and interconnections and to avoid the above-mentioned drawbacks.

This and other objects are met by a device according to claim 1 . A further advantageous embodiment is provided by the dependent claims.

A device with a micro-acoustic BAW resonator is disclosed with a general vibrating resonator structure. The resonator structure is a layer sequence including a top electrode, a piezoelectric layer and a bottom electrode from top to bottom.

Below the bottom electrode between the resonator structure and the substrate carrying the resonator structure, a Bragg mirror is arranged for maintaining the acoustic energy within the resonator.

To improve the conductivity of the resonator terminals, a conductive structure is arranged between the substrate and the bottom electrode to direct current away from the bottom electrode to the substrate. Thus, the above-mentioned problem at the transition region between the interconnect region and the high acoustic energy resonator region is solved by forming the transition region in the region of completely low acoustic energy. The lateral area of the conductive structure may coincide with the lateral area of the resonating resonator portion, or may extend slightly beyond the resonator area.

The height of the conductive structure exceeds the height of the bottom electrode. Thus, the series resistance of the bottom resonator terminals is significantly reduced when using materials with a conductivity comparable to or exceeding that of the bottom electrode. This is because current flows in a top-to-bottom direction from a region of high acoustic energy to a region of low acoustic energy with a significantly enlarged conductor cross-sectional area and over a smaller distance when compared to conventional BAW resonator devices.

According to one embodiment, the conductive structure includes a first stack of alternating first and second conductive layers. The first layer has relatively low acoustic impedance. The second layer has relatively high acoustic impedance. Thus, the first stack can be configured to operate as an acoustic mirror capable of reflecting sound waves back into the high energy acoustic region of the resonator. In other words, the acoustic mirrors necessary for SMR-type resonators can be modified via the use of only conductive layers, so that the mirrors can be used as resonator terminals.

If the device includes more than one BAW resonator fabricated on a common substrate, electrical shorting by a common conductive acoustic mirror must be avoided. According to one embodiment, each of the first stacks below the resonator is embedded in an electrical medium. The two first stacks of two adjacent resonators are thus at least partially electrically isolated from each other. Embedded means that each first stack is laterally surrounded by an electrical medium. Additional insulating layers between the stack and the substrate are not necessary if the substrates are electrically isolated.

The embedded dielectric may be applied after forming and constructing the first stack. Alternatively, the deposition of the dielectric layer can be done in local layers. Such partial layers embedded in the dielectric may be applied and structured after the deposition and structuring of the respective partial layers of the stack, ie after the deposition and structuring of the respective one of the first or second conductive layers.

BAW devices such as filters include multiple BAW resonators fabricated on the same substrate. According to another embodiment, each BAW resonator of such a plurality of BAW resonators is arranged over a separate first stack. Connection lines electrically connect two or more of the first stacks in a plane remote from the bottom electrode. This results in a series or parallel connection of the two first stacks, and thus a series or parallel connection of the two resonators which are electrically connected to and arranged above the two first stacks.

The connection lines may be constructed in at least one of the lowermost first conductive layer and the second conductive layer. Alternatively, it may be advantageous to construct the connection lines from a separate conductive layer arranged below the lowest layer of the first stack. Materials that provide sufficient electrical conductivity can be selected for this layer without having to use materials with relatively high or low acoustic impedance. Furthermore, this single conductive layer may have a greater thickness than the lowest first or second layer, as this layer need not act as a mirror layer, and thus this layer may be arbitrarily thick as it is at very low acoustic levels in the energy area.

Since the connecting wires contact the respective first stacks in the low acoustic energy region, no adverse acoustic changes in the resonator performance are caused. Better conductivity connecting lines and their placement in low acoustic energy regions lead to overall better filter performance through reductions in electrical losses. This technique can also lead to an improved Q-factor of the resonator, as more uniform excitation of the resonator is possible due to the fact that the first stack applies a more uniform voltage across the lateral area of the bottom electrode of the resonator.

The first conductive layer preferably includes one of polysilicon, graphite, aluminum, conductive oxide, and doped semiconductor. Further preferred conductive materials may be selected from Alcoa based alloys (eg, AZ91, AE42 and AS41), SC, La, Y, Yb, Be, LaN, Ga, magnesium based alloys, tin based alloys, bismuth based alloys, Mg2C3 and Mg3N2. These materials have relatively low acoustic impedance and can be used in acoustic mirrors that operate according to the Bragg principle.

The second conductive layer may include one of W, WC, WN, SiC, Mo, Mo2N, Ir, Au, Pt, Rh, Re, Ru, Ta, HfN, and a copper-based alloy. Some of these materials have been used as high impedance layers in common SMR resonators.

In another embodiment, the device has a second acoustic mirror on top of each resonator. The mirror includes a second stack of conductive first and second layers constructed as the first stack. Thus, the second stack may have the same structure as the first stack, but arranged over the top electrode of each resonator. Structures that are symmetrical with respect to the horizontal mirror layer via the resonator are preferred.

Also over the second stack, second connection lines are constructed from the topmost first and second layers of the second stack to electrically connect two or more BAW resonators in series or in parallel.

Alternatively, it may also be advantageous to construct the second connection line from a separate conductive layer arranged above the topmost layer of the second stack.

Of course, each second stack is advantageously embedded in another layer of electrical media. This ensures mutual electrical isolation between the two resonators not connected via the second connecting line.

With such a second stack, the electrical terminals of the respective top electrodes can be improved in terms of their electrical performance. Thus, similar improvements can be achieved as already mentioned by the first stack.

With the proposed BAW device, a filter can be constructed. For this purpose, several resonators are circuit-connected in a ladder or lattice arrangement. A first number of BAW resonators of the device are electrically connected in series with the signal line. A second number of BAW resonators of the device are circuit-connected in shunt lines that are circuit-connected in parallel with the signal line and are connected to different nodes in the signal line. The filter circuit itself conforms to commonly used filter circuits and therefore does not require a more detailed explanation.

The filter thus constructed can be used to form one filter of the duplexer. The second filter can also have the same structure of the present invention. Nevertheless, it is possible to use a SAW filter as the second filter. The first filter and the second filter of the duplexer are selected from Rx filters and Tx filters. Preferably, the Tx filter of the duplexer is constructed according to the new principles as an inventive filter with improved power handling capability, because for higher signal levels or power amplitudes of the Tx signal applied to the Tx filter This is required.

FIG. 1 illustrates a conventional device including three SMR-type BAW resonators. The SMR resonator includes a top electrode TE, a piezoelectric layer PS, and a bottom electrode BE. The active resonator region is the volume where all three layers of TE, PS and BE overlap each other. By structuring the electrode layers, three individual SMR resonators are realized. The three resonators are electrically connected in series between the terminal T1 and the third terminal T3 via an interconnection IC formed by portions of the bottom electrode BE or the top electrode TE. The interconnect ICs each cover two or more stacks. Subsequently, each interconnect IC electrically connects two adjacent resonators.

Below each resonator, a stack of alternating layers of low and high acoustic impedance is arranged. Thus, each stack forms an acoustic mirror for reflecting acoustic waves back to the active area to keep the acoustic energy within the active resonator area. The low-resistance layer LI is formed of, for example, a dielectric such as SiO 2 . The high resistance layer is formed of a heavy hard metal such as tungsten W. The bottom electrode may be part of the acoustic mirror and thus also be formed of heavy metals.

According to embodiments, mixed bottom electrodes may be used. A "hybrid" electrode refers to the impedance behavior of the electrode between low and high. Such bottom electrodes may comprise several layers to create an optimal compromise between electrical conductivity and (high) acoustic impedance. As an example, a multilayer structure of Ti, AlCu, and W may be used. A thin layer of Ti of about 10 nm was used for adhesion purposes. A high conductivity, but only about 150 nm AlCu acoustic impedance layer and about 80 nm W high impedance layer complete the electrode structure. AlCu adds more conductivity without reducing coupling much, eg if the W layer thickness is reduced, the acoustic performance increases but the conductivity decreases.

In addition to interconnection, the resonators are electrically isolated via the embedded electrical medium. Furthermore, conductive high impedance layers are constructed and confined to the lateral area of the corresponding active resonator region. Such conventional BAW devices suffer from the above-mentioned disadvantages because the conductivity of the interconnect IS is limited to the thin film electrode layers of the bottom and top electrodes.

Figure 2 illustrates a first embodiment of the present invention. A single resonator RES formed from a layer sequence comprising a top electrode TE, a piezoelectric layer PS and a bottom electrode BE is shown. So far, this embodiment conforms to conventional resonators. But in contrast to the lateral interconnects IC of conventional resonators formed in the plane of the bottom electrode, there is a conductive first stack ST1 that allows the formation of electrical circuits that direct the current from the bottom of the resonator to the substrate Terminals, the substrate can be placed somewhere under the stack and carry the entire device. Thus, the effective cross-section of the electrical terminal conforms to a lateral resonator area equal to the lateral cross-sectional area of the first stack. The first stack may comprise alternating layers of low acoustic impedance LI and high acoustic impedance HI, each formed of a conductive material. The stack thus acts as an acoustic mirror AS.

Therefore, the resonator can be contacted via the first terminal T1 at the top electrode TE and the second terminal T2 at the bottom of the first stack ST1.

The high resistance layer HI may be formed of W. The low resistance layer L1 may be formed of Al.

The bottom electrode BE may be part of the acoustic mirror AS.

Figure 3 illustrates a second embodiment of the present invention. It comprises three resonators, each of which is arranged above a first stack ST1 implemented as an acoustic mirror AS. Between the first resonator shown on the left-hand side of FIG. 1 and the laterally adjacent second resonator, an electrical connection is formed via a connecting line CL. The connection lines CL connect the two first stacks ST1 at their respective bottom sides, which are at the lowermost layers of the respective first stacks ST1. The connection line CL is formed of a conductive metal such as Al, and may have a thickness of, for example, 1 μm to 2 μm. Alternatively, the connection line CL may be formed of a conductive layer extending from the bottommost portion of the first stack ST1.

Another electrical interconnect touches the bottom of the stack at the third resonator (ie the outermost resonator at the right hand side of Figure 3). It is formed as another connection line CL' connecting the first stack on the right-hand side with the terminal T3.

On top of this arrangement, the middle and right-hand resonators are interconnected via an enlarged top electrode TE common to both resonators. The first terminal T1 contacts the top electrode of the left-hand resonator. As a result, the three resonators are circuit-connected in series between the first terminal T1 and the third terminal T3. The second terminal T2 may contact a common part of the top electrodes TE of the second (=middle) and third (=right-handed) resonators to allow different ways of circuit connection. Between the terminals T1, T2 and the top electrode TE, thick metal pads are applied around the terminal area. Preferably, the pad is arranged in an area of low acoustic energy, ie between two adjacent resonator areas as shown for terminal T2 in FIG. 3 or beside the resonator area as shown for terminal T1 .

The illustrated structure may continue by interconnecting more resonators in series or parallel with it. Instead of terminal T3, a connecting line CL' can connect an adjacent stack of another resonator.

For isolation purposes, a dielectric layer is preferably arranged between the connection lines CL, CL' and the top surface of the substrate SU. The electrical medium may be the same electrical medium DE in which the first stack is embedded. The electrical medium DE may contain SiO2.

Figure 4 illustrates a BAW device with a circuit of several BAW resonators according to a second embodiment of the present invention. In this embodiment, the second stack ST2 is arranged on top of the top electrode TE of the BAW resonator. The second stack has the same structure as the first stack ST1 and includes alternating first and second conductive layers with low and high impedance. As a result, the second stack ST2 also forms an acoustic mirror AS that reflects the acoustic waves back to the resonator RES. Furthermore, since the stack comprises only conductive layers, the current is directed away from the top electrode by the second stack ST2 in a vertical direction. The second stack ST2 is also embedded in the electrical medium DE like the first stack.

A further connection line is arranged on top of the dielectric DE to connect the top sides of two adjacent second stacks ST2 in a plane above and away from the piezoelectric layer. The connection line CL has a higher thickness and lower resistance than the top electrodes and lateral connections known from the art to which the present invention pertains. Therefore, the present embodiment further reduces the resistance of the interconnects on the top side of the BAW arrangement. Therefore, the losses of the BAW arrangement are further reduced. Additional benefits arise via the symmetry of the structure with respect to the horizontal plane of symmetry in the middle of the piezoelectric layer PS. The excitation within the resonator is more uniform in a symmetrical arrangement and results in a higher Q factor.

Furthermore, lateral losses can be reduced for devices with normal transition regions.

A BAW device according to or similar to the embodiment shown in Figure 4 provides improved sealing at regions of high acoustic energy, especially at piezoelectric layers.

Another advantage is that the present invention enables higher performance for filters operating at higher frequencies (>3GHz). In order to keep the impedance of the higher resonant frequency filter at the same level as the lower frequency filter, the geometry of the acoustically active layer in the resonator stack must be reduced by a factor that increases in frequency. Although the impedance of the filter resonator is designed to be the same as the resonator operating at lower frequencies (eg, < 3GHz), the resonator only changes upward in frequency.

To illustrate how this affects filter performance, the following example is provided to illustrate the difference between filters operating at lower frequency f1 and higher frequency f2. Relative to the lower frequency resonators, the higher operating frequency resonators scale down in size by the same factor f2/f1 as the frequency increases. Thus, all linear dimensions of the resonator are scaled.

This resonator will then have an effective capacitive Q-factor Q2 (the ratio of the size of the capacitor's impedance to the equivalent series resistance), which is reduced by the same factor f2/f1. The present invention can alleviate the challenge of getting into high frequencies because thick interconnects can be used without compromising acoustic performance, and because the direction of current flow in the resonator region will be in a top-to-bottom direction rather than a left-to-right direction. fact. Furthermore, the cross-sectional area of the conductive structure (ie, the stack) is now larger and the path length is shorter compared to prior art resonators.

A further advantage arises via routing the top connection line CL. Routing can be done unconstrained from most geometric constraints. Furthermore, the material used for the top connection wire can be freely selected as long as the conductivity is good.

Figure 5 illustrates a block diagram of a BAW device with a circuit of several BAW resonators. The block diagram itself is known in the art to which the present invention pertains, and may also be used in a BAW device according to the present invention.

The first number of BAW resonators SR are circuit-connected in series in the signal line connecting the input SE to the output SA. A series connection can be done between terminals T1 and T2 like the three resonators illustrated in FIG. 4 .

A second number of BAW resonators PR are circuit-connected in shunt lines that are circuit-connected in parallel with the signal lines and connected to nodes in the signal lines. The encircling dashed line indicates that the resonators of this arrangement are realized on the common wafer CH.

The scope of the invention is defined by the claims and may not be limited to the embodiments and drawings, which illustrate only a single implementation, many other embodiments and modifications are possible and fall within the scope of the invention.

AS‧‧‧Acoustic mirror BE‧‧‧Bottom electrode CH‧‧‧Common chip CL‧‧‧Connecting line CL'‧‧‧Connecting line DE‧‧‧Electrical medium HI‧‧‧High impedance layer IC‧‧‧Interconnect Connect LI‧‧‧low impedance layer PR1‧‧‧BAW resonator PR3‧‧‧BAW resonator PS‧‧‧piezoelectric layer RES‧‧‧micro-acoustic BAW resonator SA‧‧‧output SE‧‧‧input SR1‧ ‧‧BAW resonator SR2‧‧‧BAW resonator SR3‧‧‧BAW resonator ST1‧‧‧First stack ST2‧‧‧Second stack SU‧‧‧Substrate T1‧‧‧Terminal T2‧‧‧ Terminal T3‧‧‧Terminal TE‧‧‧Top Electrode

The invention will be explained in more detail with respect to some embodiments and figures. The figures are only schematic and not drawn to scale. Therefore, neither relative nor absolute dimensions can be derived from the drawings.

Figure 1 illustrates a conventional BAW device known from the prior art.

FIG. 2 illustrates a BAW device according to a first embodiment of the present invention.

FIG. 3 illustrates a BAW device according to a first embodiment of the present invention.

Figure 4 illustrates a BAW device with a circuit of several BAW resonators according to a second embodiment of the present invention.

Figure 5 illustrates a block diagram of a BAW device with a circuit of several BAW resonators.

Domestic storage information (please note in the order of storage institution, date and number) None

Foreign deposit information (please note in the order of deposit country, institution, date and number) None

T1‧‧‧terminal

TE‧‧‧Top Electrode

RES‧‧‧Microacoustic BAW Resonator

LI‧‧‧Low impedance layer

HI‧‧‧High impedance layer

AS‧‧‧Acoustic Mirror

ST1‧‧‧First stack

BE‧‧‧Bottom Electrode

PS‧‧‧piezoelectric layer

Claims (12)

A device having a micro-acoustic BAW resonator comprising: - a top electrode; - a piezoelectric layer; - a bottom electrode; - a substrate, below the bottom electrode; and - a conductive structure, the conductive structure Disposed between the substrate and the bottom electrode, the conductive structure conducts current away from the bottom electrode to the substrate, wherein the conductive structure includes a first stack of alternating first and second conductive layers layer, and wherein the device further comprises two or more BAW resonators, each BAW resonator being arranged over a separate first stack, and a connecting line constructed on the first stack in one or more of the lowest conductive layers in and electrically connect the two first stacks in a plane remote from the bottom electrode. The device of claim 1, wherein the first conductive layer has a relatively low acoustic impedance, and the second conductive layer has a relatively high acoustic impedance. The apparatus of claim 1, wherein the first stack forms an acoustic mirror for reflecting sound waves back into a high energy acoustic region of the microacoustic BAW resonator. The device of claim 1, wherein the first stack is embedded in a layer of dielectric, at least the side surfaces of the first stack are The dielectric surrounds laterally. The apparatus of claim 1, wherein a second connection line is constructed in a separate conductive layer disposed directly below the lowermost conductive layer of the first stack, the second connection line being longer than the lowermost first conductive layer or the second connection line The conductive layer is thick. The device of claim 1, wherein the first conductive layer comprises one of polysilicon, graphite, aluminum, a conductive oxide, and a doped semiconductor. The device of claim 1, wherein the second conductive layer comprises one of W, WC, WN, SiC, Mo, _Mo2N , Ir, Au, Pt, Rh, Re, Ru, Ta, HfN, and a copper-based alloy. The apparatus of claim 1, further comprising: a second stack having a conductive layer, the second stack being disposed over and in direct contact with the top electrode and forming a top acoustic mirror . Apparatus according to claim 8, wherein - the second stack is laterally embedded in a further dielectric layer, and - a second connection is constructed from a top conductive layer of the conductive layers of the second stack Wires for electrically connecting the two second stacks at a plane above the top electrode and remote from the top electrode. A filter comprising the device according to one of the preceding claims, wherein a first number of BAW resonators of the device are circuit-connected in series in a signal line, and wherein a second number of BAW resonators of the device are circuit-connected in a shunt line in parallel with the signal line. A duplexer comprising a first filter and a second filter, wherein the first filter is the filter according to claim 10 above. The duplexer of claim 11, wherein the second filter is a SAW filter, a FBAR filter or a Lamb wave device.

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