DE102017117870B3 - BAW resonator with reduced spurious modes and increased quality factor - Google Patents

Abstract

An electrically and acoustically improved resonator is provided. The resonator has a center region and a termination region surrounding the center region. A trench in a top electrode, a frame on the top electrode, a wing structure connected to the frame and a notch in a piezoelectric layer reduce spurious modes and increase the quality factor of the resonator.

Description

The present invention relates to BAW resonators, for example for RF filters, with improved acoustic and electrical properties.

Bulk Acoustic Wave (BAW) resonators may be used to construct bandpass or band-stop filters for RF applications, such as wireless communication devices. Such devices may use filters comprising such resonators. Typically, wireless communication devices have a power source with limited energy resources and power saving circuits are preferred.

BAW resonators have a sandwich construction with a piezoelectric material between a bottom electrode and a top electrode. Due to the piezoelectric effect, such resonators convert between RF signals and acoustic waves if an RF signal is applied to the electrodes of the resonators.

An ideal BAW resonator has a certain extent along the thickness direction, characterized by the wave factor of the desired main acoustic mode. However, the ideal resonator is not limited in lateral directions. Thus, vibrations in one dimension are obtained.

However, real resonators have limited lateral dimensions and the resonator is mechanically connected to its environment. Due to the finite lateral dimensions and necessary means for mechanical and electrical connections undesirable spurious modes can occur and the quality factor is reduced. In particular, acoustic energy can be dissipated in the vicinity of the resonator.

BAW resonators are off US 9,571,063 B2 and from US 9,219,464 B2 known.

What is desired is a BAW resonator with reduced spurious modes and an increased quality factor. A BAW resonator according to independent claim 1 is provided. The dependent claims provide preferred embodiments.

The BAW resonator with reduced spurious modes and an increased quality factor comprises a bottom electrode, a top electrode and a piezoelectric material. The bottom electrode is disposed in a bottom electrode layer, the top electrode is disposed in a top electrode layer, and the piezoelectric material is disposed in a piezoelectric layer. The piezoelectric layer is disposed between the bottom electrode layer and the top electrode layer. The BAW resonator further has a center region provided for converting between RF signals and acoustic waves, and an outer region surrounding the center region. Furthermore, the BAW resonator has a termination area between the center area and the outside area. The terminating area is intended for the acoustic decoupling of the outer area from the central area. Furthermore, the BAW resonator has a trench in the top electrode layer. The ditch surrounds the central area. In addition, the resonator has a frame on the top electrode. The frame surrounds the ditch. Furthermore, the BAW resonator has a vane structure that is mechanically connected to the frame and is provided for compensating for vibrations of the central region. In addition, the BAW resonator has a notch in the piezoelectric material. The notch surrounds the center area.

Thus, a BAW resonator is provided where a termination region acting as a termination of the center region acoustically separates the acoustically active center region from the resonator environment to reduce the energy dissipation in the resonator environment. The trench in the top electrode, the frame on the top electrode, the wing structure and the notch are acoustically effective structures of the termination area, which have an essential effect on the acoustic decoupling.

One result of acoustic decoupling is that acoustic energy can not reach the resonator environment. The structures mentioned above help to reflect acoustic energy back into the active center region. Thus, the derivative of acoustic energy is reduced and the quality factor is increased.

Furthermore, the trench, the frame, the wing structure and the notch cooperate to form the vibration mode to avoid spurious modes.

As a result, a BAW resonator with improved electrical and acoustic properties is obtained.

Two or more such BAW resonators may be electrically connected to construct a bandpass filter or a band-stop filter, for example in a ladder-type configuration.

It is possible that the BAW resonator further comprises a signal conductor in the top electrode layer between the center region and the outside region.

The signal conductor electrically connects the top electrode to an external circuit environment, for example to other resonators or to an input port or to an output port of an RF filter.

Similarly, the bottom electrode may be electrically connected to other resonators, to an input port, to an output port, or to the ground terminal of an RF filter.

Generating a BAW resonator involves several fabrication steps to create the layer stack. Means for restricting the acoustic energy to the central region or for avoiding or suppressing spurious modes are preferably arranged on the top side of the resonator, since the resonator itself may be arranged on or above a carrier substrate and producing acoustically active structures under the resonator stack would make the manufacturing steps more difficult ,

However, unavoidable electrical connections, for example the top electrode, can create paths through which acoustic energy can be dissipated. Thus, preferably, the signal conductor electrically connects the cover electrode only in a limited area.

The signal conductor may comprise material of the top electrode layer. In particular, the signal conductor may consist of a material of the cover electrode layer remaining after a patterning process in which material of the cover electrode layer in the surroundings of the termination region is removed.

It is possible that the BAW resonator comprises an acoustic mirror arranged in the termination area. The acoustic mirror is provided for suppressing a lateral leak.

The relationship between the different layers is such that the different layers are in one direction z stacked parallel to the wave vector of the main acoustic mode. In contrast, the terms central region, outer region, and termination region define lateral regions viewed in a plan view when the viewing direction is parallel to the wave vector.

Thus, the termination region surrounds the center region in a lateral direction, and the outer region surrounds the termination region and the center region in a lateral direction.

It is possible that the acoustic mirror comprises the notch and a Bragg structure on the signal conductor.

The notch is an effective means for suppressing leakage of acoustic energy in a vicinity of the top surface of the piezoelectric layer. For electrically connecting the top electrode, the signal conductor bridges the notch and generally provides a path for acoustic energy to the resonator environment. To prevent the dissipation of acoustic energy in the resonator environment, the Bragg structure may include forming a pattern of stripes that act as a Bragg acoustic wave mirror. The Bragg structure has a periodic structure along the lateral direction x in which sections with high acoustic impedance and low acoustic impedance are iteratively arranged. For this purpose, strips of a material, for example the material of the cover electrode, are arranged on the signal conductor.

It is possible that material of the signal conductor is thinner at an acoustic node to reduce the bending mode excitation. An acoustic node is a point at which acoustic vibrations have a smallest amplitude, that is, a node. By thinning the signal conductor at this position compared to the environment of the acoustic vibration node, the bending mode excitation is reduced and a lateral leak is further suppressed.

It is possible that the piezoelectric material comprises AlN (aluminum nitride) or AlScN (aluminum scandium nitride) or scandium-doped aluminum nitride. The use of ZnO (zinc oxide) and PZT (lead zirconate titanate) is also possible.

The material of the bottom electrode or the top electrode may include materials selected from tungsten, an aluminum-copper alloy, titanium, titanium titanium, molybdenum, gold, silver, copper, an alloy comprising silver and copper, iridium, ruthenium, and platinum.

It is possible that sidewalls of the notch are an interface between areas of differing acoustic impedance.

The acoustic impedance is a characteristic of a material and depends on the speed of sound and the density of the material. The acoustic impedance Z increases with the density ρ and with the speed of sound v. The speed of sound v depends on parameters such as stiffness parameters c of the materials and the density p: e.g. v = square root (c / p) and Z = square root (c * ρ)).

An interface between regions of differing acoustic impedance at least partially reflects acoustic waves.

It is possible that the notch is empty, filled with a gas or filled with a dielectric material.

It is possible that the piezoelectric layer includes an etching stopper layer with respect to the piezoelectric material.

Thus, the etch stop layer includes a material that differs in etching properties from the piezoelectric material otherwise contained in the piezoelectric layer.

The etch stop layer facilitates generating the notch. In this case, the depth of the notch is equal to the thickness of the piezoelectric material in the piezoelectric layer over the etching stopper layer.

It is possible that the etch stop layer comprises wurtzite ((Zn, Fe) S) or a material having the structure of wurtzite or a similar structure. The structure of the etch stop layer may have a hexagonal lattice symmetry such as 4H-SiC (silicon carbide). Crystal growth of the piezoelectric over it is promoted while achieving good etch selectivity.

By providing such an etching stopper layer, the depth of the notch can be adjusted with high precision.

It is possible that the piezoelectric layer comprises a first piezoelectric material on the bottom electrode and a second, different piezoelectric material on the first piezoelectric material.

In particular, it is preferable that the first piezoelectric material and the second piezoelectric material may be selectively etched with each other. Then, the surface of the first piezoelectric material may act as an etch stop surface to produce the notch disposed in the second piezoelectric layer and made by locally completely removing material of the second piezoelectric material.

It is accordingly possible that the depth of the notch is equal to the thickness of the second piezoelectric material.

It is possible that the wing structure is oriented at an angle with respect to the wave vector of a main mode of the resonator. The angle can be chosen below 0 °, 45 °, 90 °, 135 °. In this case, the wing structure faces the top when the angle is 45 °. If the angle is equal to 135 °, the wing structure points to the bottom side. However, other angles are possible. For example, the angle may be between 0 ° and 45 ° or between 45 ° and 90 ° or between 90 ° and 135 ° or between 135 ° and 180 °.

It is possible that the wing structure has a side wall. The sidewall may be disposed directly over a sidewall of the notch or within a region surrounding the notch.

Thus, it is possible that the wing structure and the notch are aligned. However, it is possible that the wings are arranged in a region surrounded by the notch.

It is possible that the BAW resonator further comprises a passivation layer. The passivation layer may cover segments of the top of the top electrode, the bottom of the top electrode, the surface of the frame, the surface of the vane structure, the surface of the trench, and / or the surface of the signal conductor.

The passivation layer may comprise or consist of aluminum nitride or silicon nitride. Such a layer may also act as a trim layer.

It is possible that the BAW resonator is an FBAR-type resonator or an SMR-type resonator.

In a FBAR (FBAR = Film Bulk Acoustic Resonator) type resonator, a cavity below the bottom electrode is designed to acoustically decouple the resonator in a vertical direction.

In an SMR-type resonator, an acoustic mirror is disposed below the bottom electrode. The acoustic mirror includes layers of high and low acoustic impedance that function as a Bragg reflector to confine acoustic energy to the active region of the resonator and to suppress a vertical leak of acoustic energy.

It should be noted that the trench in the cover electrode is characterized by a recess in the cover electrode. This means that the trench is located at a region where the thickness of the cover electrode is smaller than the thickness of the cover electrode in the center region.

In contrast, the frame is characterized by additional material. The thickness of the material over the piezoelectric layer is greater than the thickness of the top electrode in the central region.

Accordingly, the notch is characterized as a recess in the piezoelectric layer.

Note that the notch may be empty or may include a dielectric material such that the parasitic capacitance of the resonator, that is, the static capacitance of the resonator outside of the center region (e.g., in the top electrode interconnect), is reduced.

Bandpass filters made with at least one such resonator may have reduced waviness in the passband. The quality factor is increased and thus the energy dissipation from a battery, for example a mobile communication device, is reduced. The insertion loss of the corresponding RF filter is reduced. Central operating principles and details of preferred embodiments are shown in the attached schematic figures.

• 1 und 3 Querschnitte durch einen Resonator,
• 2 eine Draufsicht auf einen Resonatorstapel,
• 4 einen Querschnitt durch einen Resonatorstapel mit einer Bragg-Struktur in der Deckelektrodenverbindung,
• 5 die Möglichkeit der Verwendung eines anderen piezoelektrischen Materials,
• 6 die Verwendung einer Ätzstoppschicht,
• 7 die Verwendung unterschiedlicher piezoelektrischer Materialien,
• 8 bis 11 verschiedene mögliche Formen der Flügelstruktur,
• 12 eine mit einem dielektrischen Material gefüllte Kerbe,
• 13 die auf die Seitenwand der Kerbe ausgerichtete Flügelstruktur,
• 14 die Anwendung einer Passivierungsschicht,
• 15 eine vertikal ausgerichtete Flügelstruktur,
• 16 einen Resonator vom SMR-Typ und
• 17 die Effizienz der Bragg-Strukturen auf dem Signalleiter.

In the figures show:

• 1 and 3 Cross sections through a resonator,
• 2 a top view of a resonator stack,
• 4 a cross section through a resonator stack with a Bragg structure in the cover electrode connection,
• 5 the possibility of using another piezoelectric material,
• 6 the use of an etch stop layer,
• 7 the use of different piezoelectric materials,
• 8th to 11 various possible shapes of the wing structure,
• 12 a notch filled with a dielectric material,
• 13 the wing structure aligned with the side wall of the notch,
• 14 the application of a passivation layer,
• 15 a vertically oriented wing structure,
• 16 an SMR type resonator and
• 17 the efficiency of the Bragg structures on the signal conductor.

The 1 . 2 and 2 show different cross sections ( 1 and 3 ) and plan views ( 2 ) of a BAW resonator. The resonator has a bottom electrode BE and a cover electrode TE , Between the two electrodes is a piezoelectric material PM in the piezoelectric layer PL arranged. The layer stack describes the construction of the resonator in the z-direction, ie parallel to the wave vector of the main acoustic mode. The layers extend along the xy plane orthogonal to the z direction. The middle area CR of the resonator is provided so that it is the active resonator region where the conversion between RF signals and acoustic waves is substantially performed. The middle area CR is from the final area TER Surrounded, the middle area CR acoustically from the outside area OR should decouple. Within the final area are a ditch TR in the cover electrode and a frame structure FR and a wing structure FS provided in the cover electrode. There is also a score NO provided in the piezoelectric material under the cover electrode. The ditch TR , the frame FR , the wing FS and the score NO act as acoustically effective means for suppressing unwanted spurious modes and limiting the acoustic energy within the resonator. To the cover electrode TE electrically connect, although the central area CR from the notch NO surrounded, the signal conductor bridges SC the score. Accordingly shows 3 a cross section through the layer stack at position BB, while 1 shows a cross section at the position AA.

4 shows the possibility of limiting a lateral leak by applying a manufacturing structure in the form of a Bragg structure BS on the signal conductor SC ,

5 shows the possibility of using different piezoelectric materials between the two electrodes.

The dimensions (depth [extension along the direction z ], Width [extension over the direction x or y ]) are adapted to the characteristic acoustic properties of the piezoelectric material.

6 illustrates the use of an etch stop layer ESL inside the piezoelectric layer, so that the notch can have a high-homogenous depth.

7 shows the use of intermediate layers of different piezoelectric materials within the piezoelectric layer. Thus, on the bottom electrode is a piezoelectric sub-layer with the first piezoelectric material PM1 arranged. On this sublayer is a second piezoelectric material PM2 comprehensive second sub-layer arranged. With respect to an etchant, eg, hydrofluoric acid, are the etch rates of the first piezoelectric material PM1 and the second piezoelectric material PM2 different. Thus, the two piezoelectric materials are practically selectively etchable to each other and an etch stop interface IT I is obtained having a homogeneous depth of the notch in the second piezoelectric material PM2 allowed.

The first piezoelectric material PM1 and the second piezoelectric material PM2 can be selected from aluminum nitride and scandium-doped aluminum nitride, which have different etch rates with respect to hydrofluoric acid.

8th shows an embodiment where the wing structure is arranged in a 90 ° orientation with respect to the wave vector of the main acoustic mode.

9 illustrates an embodiment where the angle between the wing structure and the wave vector is 45 °. The wing structure faces the exterior of the resonator.

10 illustrates the possibility of a 45 ° angle between the wave vector and the wing structure. However, the wing structure faces the center region of the resonator.

11 shows the possibility of an angle of 135 °, while the wing structure faces the outer area.

12 illustrates the possibility of filling the notch NO with the dielectric material DM , In this case, the notch NO can be made by applying a recess made with the dielectric material DM is filled before the material of the cover electrode is deposited. Thus, after polishing the top of the piezoelectric material and the dielectric material, the top electrode and the signal conductor can be deposited and formed on a smooth surface and with high quality.

13 shows the possibility of vertical alignment of the wing structure FS , in particular a side wall of the wing structure FS and a side wall SW the score.

14 illustrates the possibility of protecting the resonator by arranging a passivation layer PAS on the accessible surfaces and under the material of the cover electrode.

15 Figure 11 illustrates a preferred embodiment where the wing structure FL is disposed orthogonal to the wave vector and extends over the notch in the piezoelectric material.

While the 1 to 15 show by way of example the resonator structure over the cavity shows 16 the use of an electroacoustic mirror EAM under the bottom electrode.

17 illustrates the efficiency of a Bragg structure BS for acoustic decoupling of the central area CR from the resonator environment. In particular shows 17 a simulated section of the resonator, the vibrations OSC in the middle area CR and in the section of the signal conductor SC show that to the center area CR wise, and over the notch NO , Acoustic waves propagate in the corresponding segment of the signal conductor SC out. The Bragg structures BS however, the acoustic waves efficiently reflect back to the center region CR and there are virtually no vibrations on the signal conductor SC beyond the Bragg structure. Thus, the middle area CR acoustically well decoupled and the signal conductor SC does not contribute to the leak.

The BAW resonator is not limited to the technical details explained above and shown in the figures. BAW resonators comprising further layers, reflective structures, and acoustically effective means for confining the acoustic energy to the central region are also covered.

LIST OF REFERENCE NUMBERS

BAWR BAW

resonator

BE

bottom electrode

BEL

Bottom electrode layer

BS

Bragg structure

CR

middle area

DM

dielectric material

IT I

Ätzstoppgrenzfläche

ESL

etch stop layer

FR

frame

FS

wing structure

NO

score

OR

outside area

OSC

vibration

PAS

passivation

PL

piezoelectric layer

PM

piezoelectric material

PM1

first piezoelectric material

PM2

second piezoelectric material

SC

signal conductor

SW

Side wall

TE

cover electrode

TEL

Top electrode layer

TER

termination region

TR

dig

x, y

lateral directions

z

vertical direction

Claims (17)

BAW resonator (BAWR) with reduced spurious modes and increased quality factor, comprising: a bottom electrode (BE) in a bottom electrode layer (BEL), a top electrode (TE) in a top electrode layer (TEL) and a piezoelectric material (PM) in a piezoelectric layer (PL) disposed between the bottom electrode layer (BEL) and the top electrode layer ( TEL) a center area (CR) provided for converting between RF signals and acoustic waves, an outer area (OR) surrounding the center area (CR), and a termination area (TER) between the center area (CR) and the outer area (OR) wherein the termination area (TER) is provided for the acoustic decoupling of the outer area (OR) from the central area (CR), a trench (TR) in the top electrode layer (TEL), the trench (TR) surrounding the center region (CR), a frame (FR) on the top electrode (TE), the frame (FR) surrounding the trench (TR), a wing structure (FS) mechanically connected to the frame (FR) and intended to compensate for vibrations of the central area (CR), a notch (NO) in the piezoelectric material (PM), wherein the notch (NO) surrounds the center region (CR). A BAW resonator according to the preceding claim, further comprising a signal conductor (SC) in the top electrode layer (TEL) between the center region (CR) and the outer region (OR). BAW resonator according to one of the preceding claims, comprising an acoustic mirror arranged in the termination area (TER), wherein the acoustic mirror is provided for suppressing a lateral leak. A BAW resonator according to the preceding claim, wherein the acoustic mirror comprises the notch (NO) and a Bragg structure (BS) on the signal conductor (SC). A BAW resonator according to any one of the three preceding claims, wherein the material of the signal conductor (SC) is thinner at an acoustic node to reduce bending mode excitation. A BAW resonator according to any one of the preceding claims, wherein the piezoelectric material (PM) comprises AlN or AlScN. BAW resonator according to one of the preceding claims, wherein sidewalls of the notch (NO) are an interface between regions of different acoustic impedance Z. BAW resonator according to one of the preceding claims, wherein the notch (NO) is empty, filled with a gas or filled with a dielectric material (DM). A BAW resonator according to any one of the preceding claims, wherein the piezoelectric layer (PL) comprises an etch stop layer (ESL) with respect to the piezoelectric material (PM). A BAW resonator according to the preceding claim, wherein the etch stop layer comprises wurtzite or a material having a hexagonal lattice or a silicon carbide. A BAW resonator according to any one of the preceding claims, wherein the piezoelectric layer (PL) comprises a first piezoelectric material (PM1) on the bottom electrode (BE) and a second, different piezoelectric material (PM2) on the first piezoelectric material (PM1). A BAW resonator according to the preceding claim, wherein the depth of the notch (NO) is equal to the thickness of the second piezoelectric material (PM2). A BAW resonator according to any one of the preceding claims, wherein the wing structure (FS) is oriented at an angle with respect to the wave vector of a main mode of the resonator, the angle being selected from the values 0 °, 45 °, 90 °, 135 °. BAW resonator according to one of the preceding claims, wherein the wing structure (FS) has a side wall which is arranged - directly over a side wall of the notch (NO) or within a region surrounded by the notch (NO). BAW resonator according to one of the preceding claims, further comprising a passivation layer (PAS), the segments covering the - cover side of the cover electrode (TE), - the bottom side of the cover electrode (TE), - the surface of the frame (FR), - the surface of the wing structure (FS), - the surface of the trench (TR) and / or - the surface of the signal conductor (SC). BAW resonator according to one of the preceding claims, wherein the resonator is an FBAR-type resonator or an SMR-type resonator. RF filter comprising one or more BAW resonators according to one of the preceding claims.

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