BAW resonator, RF filter, multiplexer and method of manufacturing a BAW resonator

Abstract

A BAW resonator with an improved lateral energy confinement is provided. The resonator has a bottom electrode in a bottom electrode layer, a top electrode in a top electrode layer and a piezoelectric layer between the bottom electrode layer and the top electrode layer. The piezoelectric layer comprises piezoelectric materials of different piezoelectric polarities.

Claims

1. A bulk acoustic wave (BAW) resonator comprising a bottom electrode layer with a bottom electrode; a top electrode layer with a top electrode above the bottom electrode layer; and a piezoelectric layer with a first piezoelectric material and a second piezoelectric material, wherein: the first piezoelectric material and the second piezoelectric material have different piezoelectric polarities, a first segment of the first piezoelectric material is arranged between a first segment of the second piezoelectric material and a second segment of the second piezoelectric material, the first segment of the second piezoelectric material is arranged between the first segment of the first piezoelectric material and a second segment of the first piezoelectric material or a third segment of the second piezoelectric material, the second segment of the second piezoelectric material is arranged between the first segment of the first piezoelectric material and a third segment of the first piezoelectric material or a fourth segment of the second piezoelectric material, and a top portion of the second segment of the first piezoelectric material or of the third segment of the second piezoelectric material and a top portion of the fourth segment of the second piezoelectric material are below a top portion of the first segment of the first piezoelectric material.

2. The BAW resonator of claim 1, further comprising a growth layer between the bottom electrode layer and the piezoelectric layer.

3. The BAW resonator of claim 2, where an interface between the growth layer and the piezoelectric layer has a first section below the first piezoelectric material and a second section below the second piezoelectric material.

4. The BAW resonator of claim 3, wherein the growth layer in the first section comprises material selected from an Oxide, a Nitride, Ru, RuO.sub.x, MOPVE-AlN, and SiO.sub.2.

5. The BAW resonator of claim 3, wherein the growth layer in the second section comprises material selected from an Oxide, a Nitride, Ru, RuO.sub.x, MOPVE-AlN, and SiO.sub.2.

6. A radio frequency (RF) filter comprising the BAW resonator of claim 1.

7. A multiplexer comprising the RF filter of claim 6.

8. The BAW resonator of claim 1, wherein the second piezoelectric material is a lateral energy barrier.

9. The BAW resonator of claim 1, wherein the second piezoelectric material is provided to generate an interference signal.

10. The BAW resonator of claim 1, wherein the first piezoelectric material and the second piezoelectric material have opposite polarities.

11. The BAW resonator of claim 1, wherein the second piezoelectric material terminates an active region of the resonator.

12. The BAW resonator of claim 1, wherein the first piezoelectric material comprises segments of a single frame or segments of two or more frames of which one is nested in another.

13. The BAW resonator of claim 1, wherein at least one of the first piezoelectric material or the second piezoelectric material comprises AlN.

14. The BAW resonator of claim 1, wherein at least one of the first piezoelectric material or the second piezoelectric material comprises a material selected from Sc-doped AlN and Al.sub.1-xSc.sub.xN with 0≤x≤0.3.

15. The BAW resonator of claim 1, further comprising a cut out or a trench in the piezoelectric layer.

16. The BAW resonator of claim 1, further comprising a trench in the piezoelectric layer enclosing an active region of the resonator.

17. The BAW resonator of claim 1, wherein at least one of the second segment of the first piezoelectric material or the third segment of the first piezoelectric material is disposed entirely outside of an active region of the resonator.

18. The BAW resonator of claim 1, wherein at least a portion of the first segment of the first piezoelectric material is laterally adjacent to at least a portion of the second segment of the first piezoelectric material, to at least a portion of the third segment of the second piezoelectric material, and to at least a portion of the fourth segment of the second piezoelectric material.

19. The BAW resonator of claim 1, wherein the first segment of the first piezoelectric material is isolated from at least one of the second segment of the first piezoelectric material or the third segment of the first piezoelectric material.

20. A method of manufacturing a bulk acoustic wave (BAW) resonator, comprising: providing a bottom electrode layer; structuring a bottom electrode in the bottom electrode layer; depositing a first piezoelectric material and a second piezoelectric material in a piezoelectric layer on or above the bottom electrode layer, wherein: the first piezoelectric material and the second piezoelectric material have different piezoelectric polarities, a first segment of the first piezoelectric material is arranged between a first segment of the second piezoelectric material and a second segment of the second piezoelectric material, the first segment of the second piezoelectric material is arranged between the first segment of the first piezoelectric material and a second segment of the first piezoelectric material or a third segment of the second piezoelectric material, the second segment of the second piezoelectric material is arranged between the first segment of the first piezoelectric material and a third segment of the first piezoelectric material or a fourth segment of the second piezoelectric material, and a top portion of the second segment of the first piezoelectric material or of the third segment of the second piezoelectric material and a top portion of the fourth segment of the second piezoelectric material are below a top portion of the first segment of the first piezoelectric material; and depositing a top electrode layer on or above the piezoelectric layer.

21. The method of claim 20, further comprising selectively removing material selected from the first piezoelectric material and the second piezoelectric material.

22. The method of claim 21, wherein the selective removal comprises fully removing a selected segment of the first piezoelectric material or a selected segment of the second piezoelectric material.

23. The method of claim 20, further comprising depositing a growth layer with a first section and a second section.

24. The method of claim 20, further comprising selectively removing material selected from the second piezoelectric material.

25. A bulk acoustic wave (BAW) resonator, comprising: a bottom electrode layer with a bottom electrode; a top electrode layer with a top electrode above the bottom electrode layer; and a piezoelectric layer with a first piezoelectric material and a second piezoelectric material, wherein: the first piezoelectric material and the second piezoelectric material have different piezoelectric polarities, a first segment of the first piezoelectric material is arranged between a first segment of the second piezoelectric material and a second segment of the second piezoelectric material, the first segment of the second piezoelectric material is arranged between the first segment of the first piezoelectric material and a second segment of the first piezoelectric material or a third segment of the second piezoelectric material, the second segment of the second piezoelectric material is arranged between the first segment of the first piezoelectric material and a third segment of the first piezoelectric material or a fourth segment of the second piezoelectric material, and a top portion of the first segment of the second piezoelectric material and a top portion of the second segment of the second piezoelectric material are below a top portion of the first segment of the first piezoelectric material.

26. The BAW resonator of claim 25, wherein at least one of the first segment of the second piezoelectric material or the second segment of the second piezoelectric material is disposed entirely outside of an active region of the resonator.

Description

(1) Basic concepts and details of preferred embodiments are shown in the schematic accompanying figures.

(2) In the figures:

(3) FIG. 1 shows a BAW resonator having a basic construction;

(4) FIG. 2 shows the use of a growth layer;

(5) FIG. 3 shows the use of an acoustic mirror;

(6) FIG. 4 shows the use of a trench in the piezoelectric layer;

(7) FIG. 5 and correspondingly FIG. 6 show frame-like structures comprising the second piezoelectric material;

(8) FIG. 7 shows the use of a Bragg-mirror like frame structure surrounding the active resonator area;

(9) FIG. 8 shows a simulated displacement of a correspondingly improved resonator; and

(10) FIG. 9 shows the equivalent circuit diagram of a basic duplexer circuit;

(11) FIG. 10 shows the use of a trench in the piezoelectric layer.

(12) FIG. 1 shows a cross-section through a basic layer construction establishing a BAW resonator BAWR. The resonator has a bottom electrode layer BEL, a top electrode layer TEL arranged above the bottom electrode layer BEL. Further, the resonator has a piezoelectric layer PEL arranged between the bottom electrode layer and the top electrode layer. The bottom electrode layer exceeds the overlap area of the bottom electrode BE in the bottom electrode layer and the top electrode TE in the top electrode layer TEL. The top electrode TE mainly defines the resonator's active area. The piezoelectric layer PEL comprises a first segment S1 of the first piezoelectric material PM1. Further, the piezoelectric layer PEL comprises a first segment S1 and a second segment S2 of the second piezoelectric material PM2 in the piezoelectric layer PEL. The first section S1 of the first piezoelectric material PM1 is arranged between the first section S1 and the second section S2 of the second piezoelectric material PM2. The first piezoelectric material PM1 and the second piezoelectric material PM2 have different polarities. Thus, when an RF signal is applied to the bottom electrode BE and to the top electrode TE, then the first piezoelectric material PM1 and the second piezoelectric material PM2 have a phase shift in their oscillation. The phase shift causes a negative interference such that acoustic waves are prevented from leaving the active area of the resonator. Consequently, energy losses are reduced and the quality factor of the resonator BAWR is increased.

(13) The distance between the bottom electrode BE in the bottom electrode layer BEL and the top electrode TE in the top electrode layer TEL mainly determines the resonance frequency and the thickness of the piezoelectric layer.

(14) It is preferred that the second piezoelectric material PM2 is arranged in the vicinity of the rim region of the active area where the energy barrier has the largest impact on confining energy.

(15) FIG. 2 illustrates the use of a growth layer GL arranged between the bottom electrode layer BEL and the piezoelectric layer PEL. The growth layer can comprise different materials. In particular below the first piezoelectric material PM1 a first growth material GM1 can be arranged and below the second piezoelectric material PM2 a second growth material GM2 can be arranged. Thus, regarding the corresponding footprints, the footprint of the first section of the growth layer corresponds to the footprint of the first segment and of the first piezoelectric material and the footprint of the second section of the growth layer corresponds to the footprint of the second segment and of the second piezoelectric material.

(16) The first material of the growth layer and the second material of the growth layer can be chosen such that the corresponding polarities of the first piezoelectric material PM1 and of the second piezoelectric material PM2 are obtained.

(17) The thickness of the growth layer can be between 1 nm and 50 nm, e.g. 5 nm.

(18) The material of the growth layer can be deposited after the material of the bottom electrode layer has been deposited and before the material of the piezoelectric layer is deposited.

(19) FIG. 3 shows the possibility of providing an acoustic mirror AM below the bottom electrode BE. The acoustic mirror can have one or several first layers L1 and one or several second layers L2. The first layers L1 and the second layers L2 have different acoustic impedances such that a Bragg-mirror structure for acoustic waves having a vertical wave vector is obtained.

(20) FIG. 4 shows the possibility of providing a trench TR in the piezoelectric layer surrounding the active area of the resonator BAWR. In the trench a remaining part of the first piezoelectric material or of the second piezoelectric material can be arranged. The trench can be characterized in that it is arranged between piezoelectric materials of the same piezoelectric material on both sides. In particular, it is possible that a remaining part of a piezoelectric material differs from the piezoelectric material at the flanks of the trench. Such trenches can easily be manufactured by utilizing a different etching rate for the two different piezoelectric materials. A trench provides a large difference in the acoustic impedance and thus provides a good lateral energy barrier for lateral wave modes.

(21) FIG. 5 shows a cross-section through a BAW resonator BAWR and FIG. 6 shows a horizontal cut through the piezoelectric layer. FIGS. 5 and 6 show corresponding relationships between segments and frame structures and the corresponding energy barrier functionality of the different piezoelectric materials.

(22) In the piezoelectric layer two frames consisting of the second piezoelectric material PM2 are embedded in the first piezoelectric material PM1. Thus, one frame consisting of the first piezoelectric material is arranged between the two frames consisting of the second piezoelectric material. A one-dimensional Bragg-mirror structure like an energy barrier is obtained because at least in the vicinity of the rim region of the top electrode TE the first piezoelectric material and the second piezoelectric material have a phase shift in their oscillation. By arranging the first segment S1 of the first piezoelectric material between the first section S1 and the second section S2 of the second piezoelectric material an energy barrier for lateral wave modes LM is obtained and lateral energy leaking is reduced or prevented.

(23) The position of the first and of the first and the second segments of the first and of the second piezoelectric materials are chosen in FIG. 6 to explain the working principle. Of course, other sections of the first and of the second piezoelectric material can work as the first and as the second segments, correspondingly.

(24) The width of the frame and the ratio of the width of adjacent frames determine the effectiveness of the energy barrier with respect to different wave vectors. Thus, with the number of the frame structures, the width of the frame structures and the ratio of width of adjacent frame structures can be chosen according to a desired impact on a certain spectral width.

(25) For example for a resonance frequency of approximately 2.5 GHz a width of a frame structure can be in a range between 1 μm and 10 μm. A preferred width may be 2 μm for a resonance frequency of 2.5 GHz when aluminium nitride is employed as piezoelectric material.

(26) FIG. 7 illustrates a Bragg-like mirror structure surrounding the active area of the resonator. The Bragg-like structure comprises a plurality of four or more frames comprising the second piezoelectric material and trenches between the frames having a depth of approximately 50% of the thickness of the piezoelectric layer.

(27) The remaining 50%, i.e. the material below the trench, is filled with the first piezoelectric material.

(28) FIG. 8 illustrates a simulation of the displacement in the active area of a resonator having the structure shown in FIG. 3. The active area is surrounded by a frame comprising N-polarized aluminium nitride while the active area comprises Al-polarized aluminium nitride. It can be clearly seen that oscillations outside the active area are practically eliminated. Thus, an effective energy confinement is obtained. Parameter x denotes the lateral position.

(29) FIG. 9 illustrates how corresponding BAW resonators can be used in filters and multiplexers. Thus, FIG. 9 illustrates a basic circuit topology of a duplexer DU having a transmission filter DXF and a reception filter RXF. The transmission filter TXF and the reception filter RXF have a ladder-type like circuit topology with series resonators SR electrically connected in series in a signal path and with parallel resonators PR electrically connected in parallel paths electrically connecting the signal path to ground. Between the transmission filter TXF and the reception filter RXF an impedance matching circuit, a phase shifting circuit and/or an antenna connection AN can be provided. The antenna connection AN establishes a common port via which transmission signals can be transmitted and via which reception signals can be received. The impedance matching circuit or phase shifting circuit electrically isolates the transmission filter TXF from the reception filter RXF and provides an open impedance and a short circuit impedance for the corresponding frequency ranges.

(30) FIG. 10 shows the result of processing steps where the selective removal comprises the full removal of the second piezoelectric material in the vicinity of the active region, e.g. to establish a trench at the perimeter of the active region.

(31) The resonator, the filter, the duplexer and the method are not limited to the features described above and shown in the schematic figures. Resonators can comprise further structures such as conventional frame structures on the top electrode and further mirror structures below the bottom electrode. Filter circuits can comprise further resonators and multiplexers can comprise further filters.

(32) Further, manufacturing methods can comprise further manufacturing steps for providing and processing the necessary elements.

LIST OF REFERENCE SIGNS

(33) AM: acoustic mirror AN: antenna connection BAWR: BAW resonator BE: bottom electrode BEL: bottom electrode layer d: displacement DU: duplexer GM1: first growth layer material PM1: first piezoelectric material L1, L2: first, second layer of acoustic mirror S1, S2: first, second segment FR: frame GL: growth layer LM: lateral acoustic mode x: lateral position MUL: multiplexer PR: parallel resonator PEL: piezoelectric layer RXF: reception filter GM2: second growth layer material PM2: second piezoelectric material SR: series resonator TE: top electrode TEL: top electrode layer TXF: transmission filter TR: trench