# Patent

## Title
Bulk acoustic wave resonator and filter

## Abstract

A bulk acoustic wave resonator includes an acoustically active area where an acoustic wave is excitable, and a transition area adjacent to an outside edge of the acoustically active area. A critical frequency of the acoustic wave in the transition area differs from a critical frequency of the acoustic wave in the active area. The transition area includes an additional layer. The bulk acoustic wave resonator includes electrodes for electrically connecting to electrical supply lines. The additional layer is irregular in areas adjacent to junction areas between the electrical supply lines and the resonator.

## Background
CLAIM TO PRIORITY
This patent application claims priority to German Patent Application No. 102006035874.0, which was filed on Aug. 1, 2006. The contents of German Patent Application No. 102006035874.0 are hereby incorporated by reference into this patent application as if set forth herein in full.
BACKGROUND
WO 01/06647 describes a bulk acoustic wave resonator.
SUMMARY
The quality of a bulk acoustic wave resonator can be increased by improving boundary conditions for acoustic waves excited in the resonator such that energy loss resulting from waves escaping from the resonator is reduced. Boundary conditions may be altered in areas that border supply lines to the resonator.
A bulk acoustic wave resonator is described that includes an acoustically active area in which an acoustic wave can be excited, and a transition area that is adjacent to the acoustically active area, in which a critical frequency of the acoustic wave differs from that in the active area. Electrical supply lines are connected to the resonator and/or its electrode layers. The transition area comprises an additional layer, which has irregularities in areas near junctions between the electrical supply lines and the resonator.
The additional layer may have a width or thickness in areas adjacent to the junction areas that is different than the width or thickness of the additional layer in other areas. In this example, the width is measured in the longitudinal direction of the supply line. The width of an irregularity that is measured perpendicular to the supply line may have the same width as the supply line.
The width and/or thickness of the additional layer in areas adjacent to the junction areas may be less than the thickness of the additional layer in its other areas. The additional layer can also be broader and/or thicker in areas adjoining the junction areas of the supply lines than in the other areas.
The width difference of the additional layer in the junction areas and outside of these areas in a single resonator can be, for instance, at least 0.3 μm or least 0.5 μm.
The thickness difference of the additional layer in the junction areas and outside of these areas in a single resonator can be, for instance, at least 5 nm or at least 10 nm.
A critical frequency of an acoustic wave is the frequency at which the acoustic fundamental mode is excited. The wavelength of the acoustic wave excited at the critical frequency is essentially twice the acoustic thickness of the resonator.
The transition area serves as a potential well for adjusting a rectangular wave profile in the active area. With the transition area, it is possible to create favorable boundary conditions for the wave mode to be excited and for attenuating undesired lateral acoustic modes. Thus resonator quality thus can be increased.
In junction areas of supply lines to the resonator, different boundary conditions exist for the acoustic wave than in other areas. Energy losses can result from the escape of lateral modes in the direction of the supply lines. By modifying the transition area . . . in particular the thickness or the width of the additional layer in the junction areas . . . it is possible to create enhanced boundary conditions for the acoustic wave along the entire periphery of the resonator.
The additional layer may be constructed as a frame. The frame can be arranged on an upper electrode of the resonator. The frame can also be arranged underneath the upper electrode of the resonator.
The additional layer may comprise at least one electrically insulating layer made, for instance, of silicon dioxide. In addition, the additional layer can comprise at least one conductive layer.
The thickness of the additional layer can be between 20 nm and 80 nm, for example. This applies, for example, to a design in which only one electrode of the resonator comprises, in addition to an aluminum layer, a heavy layer, having a specific density that exceeds the specific density of Al by at least 50%. This can be, for example, a layer of W, Mo, Ru, Pt, etc. If both electrodes of a resonator comprise a heavy layer in addition to an aluminum layer, the thickness of the additional layer may be between 20 nm and 140 nm.
The resonator described herein may be used in an HF-filter or a duplexer.
Also described herein is a filter which comprises series resonators and parallel bulk acoustic wave resonators. The series resonators are arranged in series arms and the parallel resonators are arranged in transverse branches of a ladder-type arrangement connected to reference potential. The parallel resonators can also be arranged in transverse branches that interconnect two signal paths.
Each resonator has an acoustically active area and a transition area adjacent thereto. The transition area has an additional layer. Supply lines are connected to the resonators. Outside of the junction areas of supply lines to corresponding resonators, at least one parameter, such as the thickness or the width of the additional layer, has a different value for the series resonators than the corresponding parameter for the parallel resonators.
The thickness and/or width of the additional layer may be smaller in the series resonators than in the parallel resonators. The width of the additional layer can be smaller for the series resonators by, for example, at least 0.3 μm than for the parallel resonators. The width difference can also be more than 0.5 μm. The width of the additional layer for the series resonators can be, for example, between 2.0 and 3.5 μm. For the parallel resonators, the width of the additional layer can be between 2.5 and 4.5 μm.
The thickness difference of the additional layer for the series and parallel resonators can be, for example, at least 5 nm or at least 10 nm.
For at least one of the series resonators and the parallel resonators, the additional layer may have a different width and/or thickness in areas adjoining the junction areas of the supply lines than in its other areas.
Example of resonators are described below with respect to the figures.



DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a cross-section of a bulk acoustic wave resonator.
FIG. 2A shows a plan view of a bulk acoustic wave resonator with an additional layer having a reduced width in junction areas to electrical supply lines.
FIG. 2B shows the bulk acoustic wave resonator of FIG. 2A in cross section.
FIG. 3A shows a plan view of a bulk acoustic wave resonator with an additional layer having a reduced thickness injunction areas to electrical supply lines.
FIG. 3B shows the bulk acoustic wave resonator of FIG. 3A in cross section.
FIGS. 4A and 4B show filter arrangements that include hulk acoustic wave resonators.



DETAILED DESCRIPTION
FIG. 1 shows a resonator which comprises a resonator area 1 and an acoustic mirror 2. The resonator is located on a carrier substrate 3. Acoustic mirror 2 is arranged between resonator area 1 and carrier substrate 3.
Resonator area 1 comprises electrodes 11, 12 and a piezoelectric layer 10 arranged between them. The resonator area is divided into an acoustically active area 100, in which a bulk acoustic wave is excited, and a transition area 101 adjacent to the active area. Active area 100 may occupy only a (centrally arranged) region of the surface area of the resonator. The boundary region of this surface area comprises transition area 101.
Transition area 101 differs from active area 100 with respect to structure in that transition area 101 comprises an additional layer 4. Layer 4 is located in the boundary region of resonator area 1, e.g., on upper electrode 12. Additional layer 4 may be limited to transition area 101. For example, active area 100 may be free of the additional layer.
Additional layer 4 may be constructed in the form of a frame. A layer sequence with the additional layer is distinguished from a layer sequence without the additional layer by a lower critical frequency.
At least one supply line 5, 6 is connected to each electrode 11, 12 of the resonator, as shown in FIG. 2A.
FIGS. 2A and 2B show different views of parts of a resonator with an additional layer 4. The width of additional layer 4 is reduced in areas 41 adjoining junction areas of supply lines 5, 6. In FIG. 2B, a cross section along line AA (of FIG. 2A) is shown.
FIGS. 3A and 3B show different views of a resonator with an additional layer 4. The thickness of additional layer 4 is reduced in areas 42 adjoining junction areas of supply lines 5, 6. FIG. 3B shows a cross-section along line AA of the resonator shown in FIG. 3A.
Different thickness ranges of additional layer 4 can be obtained by applying a second sublayer to the first sublayer outside of areas 42. Alternatively, it is possible to etch additional layer 4 in areas 42 to a predetermined depth.
In area 42, the width of the additional layer 4 can be also changed along with the thickness.
There is the possibility of increasing the thickness and/or the width of additional layer 4 in areas 41, 42.
FIG. 4A shows a filter having a ladder-type arrangement. The filter comprises a series arm connecting an input IN and an output Out. At least one series resonator RS is arranged in the series arm.
A parallel resonator RP is arranged in a branch transverse to the series arm, which connects the series arm to a reference potential, which is ground GND in this example. The filter may comprise additional series resonators arranged in the series arm and additional transverse branches, which is/are indicated by the broken line.
FIG. 4B shows another ladder-type arrangement which is useful for symmetric signals and balanced operation. There are two series arms connecting symmetric inputs IN1sym, IN2sym to symmetric outputs Out1sym, OUT2sym. A transverse branch interconnects the two series arms. There may be additional series resonators in the series arms and additional transversal branches interconnecting the series arms.
Components of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims.

## Claims

1. A bulk acoustic wave resonator, comprising:
an acoustically active area where an acoustic wave is excitable;
a transition area adjacent to an outside edge of the acoustically active area, the transition area comprising an additional layer; and
electrodes to electrically connect to electrical supply lines;
wherein the additional layer is irregular in areas adjacent to junction areas between the electrical supply lines and the bulk acoustic wave resonator; and
wherein a critical frequency of the acoustic wave in the transition area differs from a critical frequency of the acoustic wave in the active area.

2. The bulk acoustic wave resonator of claim 1, wherein, in the areas adjacent to the junction areas, the additional layer has a width that is different from a width of the additional layer in areas that are not adjacent to the junction areas; and
wherein the width in the areas adjacent to junction areas is measured in a longitudinal direction of the supply lines.

3. The bulk acoustic wave resonator of claim 1, wherein, in the areas adjacent to the junction areas, the additional layer has a thickness that is different from a thickness of the additional layer in areas that are not adjacent to the junction areas.

4. The bulk acoustic wave resonator of claim 3, wherein the thickness of the additional layer is between 20 nm and 80 nm.

5. The bulk acoustic wave resonator of claim 1, wherein the electrodes comprise two electrodes, each of the two electrodes comprising a conductive layer having a specific density that exceeds a specific density of Al by at least 50%; and
wherein a thickness of the additional layer is between 20 nm and 140 nm.

6. The bulk acoustic wave resonator of claim 1, wherein the additional layer comprises silicon dioxide.

7. The bulk acoustic wave resonator of claim 1, wherein an irregularity of the additional layer in one of the areas adjacent to one of the junction areas has a width that corresponds to a width of one of the electrical supply lines.

8. The bulk acoustic wave resonator of claim 1, wherein the additional layer is frame-shaped.

9. The bulk acoustic wave resonator of claim 2, wherein, in the areas adjacent to the junction areas, the additional layer has a thickness that is different from a thickness of the additional layer in areas that are not adjacent to the junction areas.

10. The hulk acoustic wave resonator of claim 9, wherein the additional layer comprises silicon dioxide.

11. The bulk acoustic wave resonator of claim 3, wherein, in the areas adjacent to the junction areas, the additional layer has a width that is different from a width of the additional layer in areas that are not adjacent to the junction areas.

12. The bulk acoustic wave resonator of claim 11, wherein the additional layer comprises silicon dioxide.

13. The bulk acoustic wave resonator of claim 1, further comprising:
a piezoelectric layer between the electrodes; and
an acoustic mirror below the electrodes and the piezoelectric layer.

14. A filter comprising:
series bulk acoustic wave resonators; and
parallel bulk acoustic wave resonators;
wherein each of the series bulk acoustic wave resonators and each of the parallel bulk acoustic wave resonators comprises:
an acoustically active area where an acoustic wave is excitable;
a transition area adjacent to an outside edge of the acoustically active area, the transition area comprising an additional layer; and
electrodes to electrically connect to electrical supply lines;
wherein the additional layer is irregular in areas adjacent to junction areas between the electrical supply lines and the resonator; and
wherein a critical frequency of the acoustic wave in the transition area differs from a critical frequency of the acoustic wave in the active area; and

wherein a size of a first irregularity in an additional layer in a series bulk acoustic wave resonator is different than a size of a second irregularity in an additional layer in a corresponding parallel bulk acoustic wave resonator.

15. The filter of claim 14, wherein the first and second irregularities comprise a same type of irregularity.

16. The filter of claim 14, wherein the first and second irregularities comprise thicknesses of the additional layers in the series bulk acoustic wave resonator and in the parallel bulk acoustic wave resonator.

17. The filter of claim 14, wherein the first and second irregularities comprise widths of additional layers in the series bulk acoustic wave resonator and in the parallel bulk acoustic wave resonator.

18. A bulk acoustic wave resonator, comprising:
an acoustically active area where an acoustic-wave is excitable;
a transition area adjacent to an outside edge of the acoustically active area, the transition area comprising an additional layer and having a different acoustic property than the acoustically active area; and
electrodes to electrically connect to electrical supply lines;
wherein the additional layer is irregular in an area adjacent to a junction area between an electrical supply lines and the bulk acoustic wave resonator.

19. The bulk acoustic wave resonator of claim 18, wherein, in the area adjacent to the junction area, the additional layer has a width that is different from a width of the additional layer in areas that are not adjacent to the junction area.

20. The bulk acoustic wave resonator of claim 18, wherein, in the area adjacent to the junction area, the additional layer has a thickness that is different from a thickness of the additional layer in areas that are not adjacent to the junction area.