WO2018111532A1

WO2018111532A1 - Baw resonator device

Info

Publication number: WO2018111532A1
Application number: PCT/US2017/063441
Authority: WO
Inventors: Dr. Thomas POLLARD; Dr. Brian FISHER; Dr. Janardan NATH; Dr. Oscar MENENDEZ NADAL
Original assignee: Snaptrack, Inc.
Priority date: 2016-12-13
Filing date: 2017-11-28
Publication date: 2018-06-21
Also published as: DE102016124236A1; DE102016124236B4; TWI753055B; TW201830695A

Abstract

A BAW resonator device is proposed comprising fully conductive acoustic mirrors. Each of the BAW resonators is electrically contacted by a respective stack (ST1). Two or more BAW resonators may be electrically connected by connection lines (CL) arranged in a plane distant from the electrodes. The connection lines (CL) connect the stacks (ST1) 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

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to German Patent Application No. 102016124236.5, filed December 13, 2016, which is expressly incorporated herein by reference in its entirety.

Description

The invention concerns a BAW device with improved resonator interconnect .

Traditional BAW devices like SMR based resonator filters (SMR = solidly mounted resonator) use thin film electrodes as top and bottom electrodes. Due to an integrated manufacture of the SMR devices the interconnection of different SMR devices for forming the filter are made from the same film like the top or bottom electrodes. As a result, the current in the filter must flow over long distanes through an electrode film section having a small cross sectional area only. This causes significant electrical losses in the filter.

Attempts have been made to reinforce the interconnects by applying an electrode material onto the interconnects thereby improving the electrical conductivity in the interconnects . But it became impossible to directly connect to resonator terminals without severely degrading the performance of the resonators. Further, this degrading is uncontrollable that a compensation thereof is impossible.

Hence, the current runs from the high acoustic energy resonator regions to the low acoustic energy interconnection regions thereby passing the transition regions located between high and low acoustic energy regions. Both, transition regions and interconnection regions are still made of portions of the thin film electrode material. Use of a thin film transition region can have negative impact on resonator performance - showing increased lateral mode leakage (Q reduction) and increased parasitic capacitance (reduced coupling) . Ideally this region would be eliminated, but it has not yet been accomplished to date, and is instead designed with minimum performance impact in mind (e.g. Avago/Broadcom "Air Bridge" Concept).

It is an object of the invention to improve the electrical performance of the resonator electrodes and interconnects and to avoid the above mentioned disadvantages .

This and other objects are met by a device according to claim

1. Further advantageous embodiments are given by the dependent sub-claims .

A device with a microacoustic BAW resonator is disclosed that has a vibrating resonator structure as commonly used. The resonator structure is a layer sequence comprising from top to bottom a top electrode, a piezoelectric layer and a bottom electrode .

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

For improving the electrical conductivity of a terminal of the resonator a conductive structure is arranged between substrate and bottom electrode guiding current away from the bottom electrode towards the substrate. Thus, the above- mentioned problem at the transistion region between the interconnection regions and the high acoustic energy resonator regions is solved by forming the transition region in a completely low acoustic energy region. The lateral area of the conductive structure may comply with that of the resonating resonator portion or may slightly extend the resonator area.

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

According to an embodiment, the conductive structure comprises a first stack of alternating first and second electrically conducting layers. The first layers have a relative low acoustic impedance. The second layers have a relative high acoustic impedance. Therefore, it is possible to construct the first stack to work as an acoustic mirror that can reflect back acoustic waves into the high-energy acoustic region of the resonator. In other words, the acoustic mirror that is necessary for SMR type resonators can be modified by using electrically conducting layers only such that the mirror can be used as a resonator terminal.

If the device comprises more than one BAW resonator that are produced on a common substrate, an electrical short by a common electrical conductive acoustic mirror has to be avoided. According to an embodiment, each first stack under a resonator is embedded in a dielectric. Two first stacks of two neighbored resonators are thus at least partly electrically isolated against each other. Embedding means that each first stack is laterally surrounded be the dielectric. If the substrate is electrically isolating, no additional insulating layer between the stack and the substrate is necessary.

The embedding dielectric can be applied after forming and structuring the first stack. Alternatively, deposition of the dielectric layer can be done in partial layers. Such a partial layer of the embedding dielectric can be applied and structured after deposition and structuring a respective partial layer of the stack, that is after deposition and structuring of a respective one of the first or the second conductive layer.

A BAW device such as a filter comprises a number of BAW resonators manufactured on the same substrate. According to a further embodiment, each of such a number of BAW resonators is arranged over a separate first stack. A connection line electrically connects two or more of the first stacks in a plane distant from the bottom electrode. Thereby, a series or parallel connection of the two first stacks and thus, a series or parallel connection of the two resonators electrically connected thereto and arranged above the first stacks results.

The connection line may be structured out of at least one of the lowest first and second conducting layers. Alternatively, it may be advantageous to construct the connection line from a separate conducting layer that is arranged under the lowest layer of the first stacks. A material may be chosen for this layer that provides sufficient conductivity without being obligated to use a material of relative high or low acoustic impedance. Further, the separate conducting layer can have a greater thickness than the lowest first or second layer as this layer need not function as a mirror layer and thus can be arbitrarily thick because it is in a region of very low acoustic energy.

As the connection line contacts the respective first stacks in a low acoustic energy region it does not cause detrimental acoustic changes in the resonator performance. Better conductivity connection line and location thereof at a low acoustic energy region results in an overall better filter performance via reduction of electrical losses. This technique may also result in an improved Q factor of the resonators because more uniform excitation of the resonator is possible due to a more uniform voltage applied by the first stacks across the lateral area of the bottom electrode of the resonators.

The first electrically conducting layers preferably comprise one of poly-Si, graphite, aluminum, a conducting oxide and a doped semiconductor. Further preferred electrically conducting materials may be chosen from Mg-Al-based alloys (e.g. AZ91, AE42, and AS41) , SC, La, Y, Yb, Be, LaN, Ga, Mg- based alloys, Sn-based alloys, Bi-based alloys, Mg₂C₃, and Mg₃N₂. These materials have relatively low acoustic impedance and may be used in an acoustic mirror working according to the Bragg principle .

The second electrically conducting layers may comprise one of W, WC, WN, SiC, Mo, Mo₂N, Ir, Au, Pt, Rh, Re, Ru, Ta, HfN,

and Cu-based alloys . Some of these materials are already used in common SMR resonators as high impedance layers.

In a further embodiment, the device has a second acoustic mirror on top of each resonator. This mirror comprises second stacks of electrically conductive first and second layers construed like the first stacks. Hence, the second stacks may have the same structure like the first stacks but are arranged above the top electrode of each resonator. A structure that is symmetrical relative to a horizontal mirror layer through the resonator is preferred.

Above the second stacks too, second connection lines are structured from the topmost first and second layers of the second stacks to electrically connect two or more BAW resonators in series or in parallel.

Alternatively, it may be advantageous to construct the second connection line too from a separate conducting layer that is arranged above the topmost layer of the second stack.

Of course, each second stack is advantageously embedded in a further layer of dielectric. This way mutual electrical isolation is guaranteed between two resonators that are not connected by a second connection line.

With such second stacks, the electrical terminals of the respective top electrode can be improved with respect to their electrical performance. Hence, similar improvements like the already mentioned ones that are resulting from the first stacks can be achieved.

With the proposed BAW device, a filter can be constructed. For doing this, several resonators are circuited in a ladder type or a lattice type arrangement. A first number of the device' BAW resonators are circuited in series in a signal line. A second number of the device' BAW resonators are circuited in shunt lines that are circuited in parallel to the signal line and are connected to different nodes in the signal line. The filter circuit per se complies with commonly used filter circuits and thus, needs not be explained in more detail.

The so-construed filter may be used to form one filter of a duplexer. The second filter too may be of the same inventive structure. Nevertheless, it possible too to use a SAW filter as a second filter. First and second filter of the duplexer are chosen from an Rx filter and a Tx filter. It is preferred to construct the Tx filter of a duplexer according to the new principle as an inventive filter has an improved power Handling capability as it is necessary for higher signal level or power amplitude of the Tx signals applied to the Tx filter.

The invention will be explained in more detail with respect to some embodiments and the accompanying figures . The figures are schematically only and not drawn to scale. Hence, neither relative nor absolute dimensions can be taken the figures. Short description of the figures

Figure 1 shows a conventional BAW device known from the art.

Figure 2 shows a BAW device according to a first embodiment of the invention.

Figure 3 shows a BAW device according to a first embodiment of the invention.

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

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

Embodiments

FIG 1 shows a conventional device comprising three BAW resonators of the SMR type. An SMR resonator comprises a top electrode TE, a piezoelectric layer PS and a bottom electrode BE. The active resonator region is the volume where all three layers TE, PS and BE are overlapping each other. By structuring the electrode layers, three single SMR resonators are achieved. These three resonators are electrically connected in series between a terminal Tl and a third terminal T3 by interconnections IC that are formed of sections of bottom electrode BE or top electrode TE. The interconnects IC are overlapping two or more stacks each. Then, each interconnect IC is electrically connecting two adjacent resonators.

Below each resonator, a stack of alternating layers of low and high acoustic impedance is arranged. Each stack thus forms an acoustic mirror for reflecting back acoustic waves to the active region to keep the acoustic energy within the active resonator region. The low impedance layers LI are formed from a dielectric like S1O2 for example. High impedance layers are formed from a heavy stiff metal like tungsten W for example. The bottom electrode may be part of the acoustic mirror and is thus formed of a heavy metal too .

According to an embodiment, a hybrid bottom electrode may be used. "Hybrid" electrode refers to the electrode's impedance behavior that is between low and high. Such a bottom electrode may comprise several layers to yield an optimized trade-off between electrical conductivity and (high) acoustic impedance. As a working example a multilayer structure of Ti, AlCu, and W can be used. A thin layer of about lOnm Ti is for adhesion purpose. A high conductivity but only medium acoustic impedance layer of about 150nm AlCu and a high impedance layer of about 80nm W are completing the electrode structure. The AlCu adds more conductivity without reducing too much the coupling, e.g. if the W layer thickness would be reduced acoustic performance would increase but electrical conductivity would drop.

With the exception of the interconnection, the resonators are electrically isolated by an embedding dielectric. Moreover, the electrically conductive high impedance layers are structured and restricted to the lateral area of the respective active resonator region. Such a conventional BAW device has the mentioned disadvantages as the conductivity of the interconnects IS is restricted to the thin film electrode layers of bottom and top electrode.

FIG. 2 shows a first embodiment of the invention. Shown is a single resonator RES formed by a layer sequence comprising a top electrode TE, a piezoelectric layer PS and a bottom electrode BE. So far, the embodiment complies with a conventional resonator. But contrary to the lateral interconnect IC of the conventional resonator formed in the plane of the bottom electrode there is an electrically conductive first stack ST1 allowing to form an electrical terminal that guides the current from the bottom of the resonator towards a substrate that may be arranged somewhere below the stack and carries the whole device. Hence, the effective cross-section of the electrical terminal complies with the lateral resonator area that is equal to the lateral cross- sectional area of the first stack. The first stack may comprise alternating layers of low and high acoustic impedance HI, LI that are both formed of electrical conductive material. The stack hence functions as an acoustic mirror AS.

The resonator can thus be contacted by a first terminal Tl at the top electrode TE and a second terminal T2 at the bottom of the first stack ST1.

High impedance layers HI may be formed of W. Low impedance layers HI may be formed of Al .

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

FIG. 3 shows a second embodiment of the invention. It comprises three resonators each arranged above a first stack ST1 embodied as an acoustic mirror AS. Between a first resonator shown at the left hand side of FIG 1 and a lateral adjacent second resonator an electrical interconnection is formed by a connection line CL. The connection line CL connects both first stacks ST1 at their respective bottom sides that is at the lowest layer of the respective first stacks ST1. The connection line CL is formed from an electrically conducting metal like Al and may have a thickness of 1 to 2 urn for example. Alternatively, the connection line CL may be formed from an extended bottom most conducting layer of the first stacks ST1.

Another electrical interconnection contacts the bottom of the stack at the third resonator (that is the outermost resonator at the right-hand side of FIG. 3) . It is formed as another connection line CL' that connects the right-hand first stack with a terminal T3.

On the top of the arrangement, the middle resonator and the right-hand resonator are interconnected by enlarged top electrode TE that is common to both resonators. A first terminal Tl contacts the top electrode of the left-hand resonator. As a result, the three resonators are electrically circuited in series between the first terminal Tl and the third terminal T3. A second terminal T2 may contact the common section of the top electrode TE of the second (= middle) and third (= right-hand) resonator to allow a different way of circuiting. Between terminals Tl, T2 and the top electrode TE a thick metal pad is applied aound the terminal area. This pad is preferably arranged in a low acoustic energy region, that is between two adjacent resonator regions as shown for terminal T2 in figure 3 or beside a resonator region as shown for terminal Tl.

The structure shown may be continued by interconnecting more resonators in series or parallel thereto. Instead of terminal T3 connection line CL' may connect a neighbored stack of another resonator .

It is preferred that a layer of dielectric is arranged between the connection lines CL, CL' and the top surface of the substrate SU for isolation purposes. This dielectric may be the same as the dielectric DE the first stacks are embedded in. The dielectric DE may comprise SiC-2.

Figure 4 shows a BAW device with a circuit of several BAW resonators according to a second embodiment of the invention. In this embodiment, on top of the top electrodes TE of the BAW resonators second stacks ST2 are arranged. The second stacks are of the same structure as the first stacks ST1 and comprise alternating first and second conducting layers of low and high impedance. As a result, the second stack ST2 also forms an acoustic mirror AS that reflects acoustic waves back into the resonators RES. Further, as the stacks comprise only electrically conducting layers the current is guided away from the top electrode in a vertical direction by the second stacks ST2. The second stacks ST2 are embedded too within a dielectric DE like the first stacks.

On top of the dielectric DE further connection lines are arranged to connect the top sides of two neighbored second stacks ST2 in a plane above and distant from the piezo- electric layer. The connection lines CL have a higher thickness and a lower resistance compared to the top electrodes and the lateral connections known from the art. Hence, this embodiment further lowers the electric resistance of the interconnections on the top side of the BAW arrangement. Hence, the loss of the BAW arrangement is further decreased. Additional profit is yielded by the symmetry of the structure with respect to a horizontal symmetry plane in the middle of the piezoelectric layer PS. The excitation within the resonators is more uniform in the symmetric arrangement and results in a higher Q factor.

Moreover, lateral losses can be reduced to devices having a normal transistion region.

A BAW device according to or similar to the embodiment shown in Figure 4 provides an improved hermeticity at the high acoustic energy regions especially at the piezoelectric layer.

Another advantage is that this invention can enable higher performance for filters operating at a higher frequency (>3GHz) . To keep the impedance of a higher resonance frequency filter at the same level as at a lower frequency filter, the geometric parameters of the acoustically active layers in the resonator stack must be reduced by the factor the frequency increases. While the impedance of the filter resonators are designed to be the same as a resonators operating at lower frequency (e.g. < 3GHz) , the resonators are just shifted up in frequency .

To illustrate how this impacts filter performance, the following example is given to show the difference between filters operating at lower frequency fl and higher frequency f2. The higher operating frequency resonator is scaled down in size relative to the lower frequency resonator by the same factor f2/fl as the frequency increases. Thereby, all linear dimensions of the resonator are scaled.

This resonator will then have an effective capacitance Q- factor Q2 (ratio of capacitor impedance magnitude to equivalent series resistance) that is reduced by the same factor f2/fl. The invention can alleviate the challenge of going to high frequency as thick interconnects can be used without compromising the acoustic performance and the fact that the current flow direction will be in a top-to-bottom direction rather than a left-to-right direction in resonator region. Moreover, the cross sectional area of the conducting structure (i.e. the stack) is now much larger and path length is much shorter compared to prior art resonators.

A further advantage results from routing the top connection lines CL. The routing can be done free from the most geometrical restrictions. Furhter, the material for the top connection lines can be chosen freely with respect only to good electrical conductivity.

Figure 5 shows a block diagram of a BAW device with a circuit of several BAW resonators. The block diagram per se is known from the art and can be used for BAW device according to the invention too.

A first number of BAW resonators SR are circuited in series in a signal line that connects an input SE with an output SA. The series connection may be done like for the three resonators depicted in FIG. 4 between terminals Tl and T2.

A second number of BAW resonators PR are circuited in shunt lines that are circuited in parallel to the signal line and are connected to nodes in the signal line. The circumferring broken line signals that the resonators of the arrangement are realized on a common chip CH.

The scope of the invention is defined by the claims and may not be limited by the embodiments and the accompanied figures that can only show single accomplishments where numerous other embodiments and modifications are possible and are lying within the scope of the invention.

List reference symbols

RES microacoustic BAW resonator

TE top electrode

BE bottom electrode

PS piezoelectric layer

SU substrate conductive structure

ST1 first stack

HI, LI first and second electrically conducting layers

AS acoustic mirror

DE dielectric

CL connection line

ST2 second stack

Claims

1. Device with a microacoustic BAW resonator, comprising

— a top electrode

— a piezoelectric layer

— a bottom electrode

further comprising

— a substrate below the bottom electrode

— a conductive structure arranged between substrate and bottom electrode guiding current away from the bottom electrode towards the substrate.

2. The device of claim 1, wherein the conductive structure comprises a first stack of alternating first and second electrically conducting layers, the first layers having a relative low acoustic impedance, the second layers having a relative high acoustic impedance.

3. The device of claim 2, wherein the first stack forms an acoustic mirror for reflecting back acoustic waves into the high energy acoustic region of the resonator.

4. The device of one of the foregoing claims,

wherein the first stack is embedded in a layer of a

dielectric that at least the side surfaces of the first stack are laterally surrounded by the dielectric.

5. The device of one of the foregoing claims,

comprising two or more BAW resonators, each BAW resonator arranged over a separate first stack, and a connection line structured out of one or more of the lowest electrically conducting layers of the first stack and electrically

connecting two first stacks in a plane distant from the bottom electrode.

6. The device of one of claims 1 to 4,

wherein a connection line structured out of a separate conducting layer that is arranged directly below the lowest layer of the stack, the connection line being thicker than the lowest first or second conducting layer.

7. The device of one of the foregoing claims,

wherein first electrically conducting layers comprise one of poly-Si, graphite, aluminium, a conducting oxide and a doped semiconductor .

8. The device of one of the foregoing claims,

wherein second electrically conducting layers comprise one of W, WC, WN, SiC, Mo, Mo₂N, Ir, Au, Pt, Rh, Re, Ru, Ta, HfN,and

Cu-based alloys.

9. The device of one of the foregoing claims,

comprising second stacks of electrically conductive first and second layers, arranged above and in direct contact with the top electrode of each resonator and forming a top acoustic mirror.

10. The device of claim 9, wherein

— each second stack is laterally embedded in a further layer of dielectric

— a second connection line structured from a top one of the electrically conducting layers of the second stack for electrically connecting two stacks at a plane above and distant from the top electrode .

11. A filter comprising the device of one of the foregoing claims,

wherein a first number of the device' BAW resonators are

circuited in series in a signal line

wherein a second number of the device' BAW resonators are circuited in shunt lines in parallel to the signal line.

12. A duplexer comprising a first and a second filter, wherein one of filters is a filter of one of the foregoing claims .

13. The duplexer of claim 11 wherein the second filter is a SAW filter, an FBAR filter or Lamb Wave device..