HIGH Q BAW RESONATOR WITH SPURIOUS MODE SUPPRESSION

Abstract

A BAW resonator is provided wherein the top electrode (TE) has an outer flap (OF). The flap extends away from the active resonator region (AR) and has a projecting section that runs at a level above the piezoelectric layer (PL) that is higher than the level of the top electrode at any of the inwardly located areas enclosed by the outer flap. The higher level is formed by an intermediate step-forming material (SM) arranged between piezoelectric layer and top electrode in the outer flap. The step forming material comprises a structured layer of an acoustic impedance that is low w.r.t. the impedance of the top electrode and the piezoelectric layer.

Claims

1. A BAW resonator, comprising: in a layer sequence from bottom to top of: substrate; a bottom electrode; a piezoelectric layer; and a top electrode; wherein: an active resonator region is formed where all the three layers bottom electrode, piezoelectric layer and top electrode layer overlap each other; the top electrode forms an outer flap extending away from the active resonator region and having a projecting section that runs at a level above the piezoelectric layer that is higher than the level of the top electrode at any of the inwardly located areas enclosed by the outer flap, wherein the higher level is formed by an intermediate step-forming material arranged between piezoelectric layer and top electrode in the outer flap; and the step forming material comprises a structured layer of an acoustic impedance that is low w.r.t. the impedance of the top electrode and the piezoelectric layer.

2. The BAW resonator of claim 1, wherein the margin of the active resonator region comprises an overlap region outwardly adjacent to the underlap in which the thickness of the top electrode is higher than in the the active resonator region wherein the enhanced thickness is achieved by interposing additional material selected from a heavy metal like molybdenium and tungsten between top electrode and piezoelectric layer.

3. The BAW resonator of claim 1, wherein the outer flap extends along the whole perimeter of the active resonator region thereby surrounding the active region.

4. The BAW resonator of claim 1, wherein a dielectric layer comprising at least a SiN layer covers the top electrode.

5. The BAW resonator of claim 1, wherein at a margin of the active resonator region an underlap is formed that extends along the whole perimeter between the active resonator region and the overlap region wherein in the underlap a total thickness of top electrode and dielectric layer is smaller than a respective total thickness of the layer sequence in the center of the active resonator region.

6. The BAW resonator of claim 1, wherein in the underlap the smaller total thickness is achieved by a reduced thickness of the dielectric layer.

7. The BAW resonator of claim 1, wherein in the underlap the smaller total thickness is achieved by enhancing the height in the active region by an additional layer covering the top electrode at the active resonator region exclusively in the area enclosed by the underlap.

8. The BAW resonator of claim 1, wherein in the underlap the smaller total thickness is achieved by a reduced thickness of the top electrode.

9. The BAW resonator of claim 1, wherein the top electrode comprises a bottom layer comprising at least one of W, Pt, Mo, and Ta and a top layer comprising AlCu, wherein at the outer flap the step forming material is arranged between bottom layer and top layer of the top electrode.

10. The BAW resonator of claim 1, wherein the top electrode comprises a bottom layer comprising at least one of W, Pt, Mo, and Ta and a top layer comprising AlCu, wherein the step forming material is arranged under the outer flap between the piezoelectric layer and the bottom layer of the top electrode.

11. The BAW resonator of claim 1, wherein in the overlap region the thickness of the bottom layer of the top electrode is higher than in the underlap and in the active resonator region.

12. The BAW resonator of claim 1, wherein at the overlap and at the outer flap the thickness of the top electrode layer and its sub-layer is the same.

13. The BAW resonator of claim 1, wherein the piezoelectric layer comprises AIN doped with Sc.

14. The BAW resonator of claim 1, wherein the substrate comprises a recess below the active resonator region forming an air-filled cavity.

15. The BAW resonator of claim 1, wherein the substrate comprises a Bragg mirror arranged below the active resonator region.

Description

[0027] In the following the invention will be explained in more detail with reference to specific embodiments and the accompanying figures. The figures are schematically only and are not drawn to scale. For better understanding some detail may be depicted in enlarged form.

[0028] FIG. 1 shows a cross-sectional view through an SMR type BAW resonator with outer flaps and two enlarged sections,

[0029] FIG. 2 shows a cross-sectional view through another SMR type BAW resonator having a modified acoustic mirror,

[0030] FIG. 3 shows a more detailed cross-sectional view through the margin region of the BAW resonator,

[0031] FIG. 4 shows a more detailed cross-sectional view through a modified margin region of the BAW resonator,

[0032] FIG. 5 shows a cross-sectional view through a BAW resonator of the FBAR type,

[0033] FIG. 6 shows a cross-sectional view through a modified margin region of the BAW resonator,

[0034] FIG. 7 shows a cross-sectional view through another modified margin region of the BAW resonator,

[0035] FIG. 8 shows a cross-sectional view through still another modified margin region of the BAW resonator,

[0036] FIG. 9 shows a simplified block diagram of a ladder type filter circuit that may use the BAW resonator,

[0037] FIG. 10 shows a simplified block diagram of a hybrid ladder type filter circuit that may use the BAW resonator as well as resonators made of passive element,

[0038] FIG. 11 shows a simplified block diagram of a lattice filter circuit that may use the BAW resonator.

[0039] FIG. 1 shows a schematic cross-section through the BAW resonator. A stack is formed by layer sequence on a substrate SU of e.g. Si. Any other suitable rigid material can be used too. On top of the Si body a layer of SiO.sub.2 may be formed for isolation purpose.

[0040] Next, an acoustic Bragg mirror is formed and structured on the substrate SU comprising two mirrors M1, M2 that is from two pairs of mirror layers. In the Bragg mirror, high impedance layer HI and low impedance layers LI are alternating. The mirror layers may slightly vary in thickness to set a desired reflection band. High impedance layer HI may comprise W and low impedance layers LI comprise SiO.sub.2. Additional thin adhesion or orientation-promoting layers may be deposited below the mirror pair, e.g. Ti or AIN.

[0041] Optionally CMP planarization of the topmost mirror layer of SiO.sub.2 can be employed for the mirror layers and or the later bottom electrode layers.

[0042] Next the bottom electrode BE is formed using a highly conductive AlCu layer and a high impedance W layer. Again a thin adhesion or orientation-promoting layer may be employed between the bottom electrode and uppermost mirror, e.g. Ti or AlN. Also a capping and/or etch-stop layer such as TiN may be applied to the top of the AlCu layer to allow patterning of additional resonator detuning material located between the Tungsten and AlCu layer of the bottom electrode.

[0043] Atop the bottom electrode W a piezoelectric layer PL of e.g. AlN or AlScN is formed. The thickness thereof is set to lower than half the wavelength of the desired resonance frequency due the additional mass loading effect from being attached to the top/bottom electrodes and mirror.

[0044] All the above layers in the stack are continuous layers extending at least over the later active resonator area.

[0045] On top of the piezoelectric layer PL a step forming material structure SM of e.g. SiO.sub.2 or of SiO.sub.2 and W is arranged that surrounds the active resonator region AR of the resonator. This step forming material SM may be applied just between the tungsten layer of the top electrode TE and the piezoelectric layer PL. A position of the step forming material SM between any other two layers or above the top layer is possible too.

[0046] On top of the already described arrangement and above the step forming material structure SM a stack of layers form the top electrode TE and the top passivating dielectric layer DL, e.g. made of SiN. Starting on the surface of the piezoelectric layer PL a thin adhesive Ti layer, a tungsten layer, an AlCu layer, a thin TiN layer and a dielectric layer of e.g. SiN are deposited. The SiN layer provides device passivation and serves as frequency fine-tuning trimming layer.

[0047] The two enlarged cross sections at the bottom of the figure show a more detailed structure of the step forming material.

[0048] The section shown of the left bottom side of FIG. 1 shows an embodiment wherein the step-forming material SM of low acoustic impedance is arranged between the bottom layer of the top electrode made of W/AlCu and the piezoelectric layer PL. Above the step formed by the step-forming material SM the top electrode or its top sub-layer forms the outer flap OF.

[0049] The section shown of the right bottom side of FIG. 1 shows an embodiment wherein the step-forming material SM of low acoustic impedance is arranged between the bottom layer of the top electrode made of W and the AlCu top layer of the top electrode TE.

[0050] FIG. 2 shows a cross-section through a BAW resonator comprising an acoustic mirror of a high quality low impedance layer HQLI and optionally a high impedance layer HQHI. These two layers form a third mirror M3 deposited above the second mirror M2 of FIG. 1 and just below the bottom electrode. Thickness of the layers of the third mirror M3 are set as usual in view of a desired reflection band. However, deposition process is controlled to allow growth of polycrystalline mirror layers having improved acoustic impedance and quality.

[0051] For the deposition, a CVD, a PECVD or a sputter method may be used. Preferably the condition are set and controlled to achieve a slow and homogeneous crystal growing. Other process parameters too like temperature, gas flow, pressure or BIAS voltage are carefully controlled to support a regular orientation and the formation of large grains within the polycrystalline layers.

[0052] FIG. 3 shows a cross section through a margin region of the BAW resonator with further optional design features. The outer flap is embodied as shown in the right enlarged section of FIG. 1. When starting at the active resonator region AR and going towards the margin an underlap region is formed adjacent to the active resonator region AR. Here, the thickness of top electrode is reduced w.r.t. to the active region. In the overlap region OL adjacent to the underlap region UL a second step forming material of e.g. W is arranged under the top electrode TE above the piezoelectric layer PL. In the overlap region OL the total thickness of the layers above the piezoelectric layer is higher than in regions located more inwardly.

[0053] Adjacent to the overlap region OL the first step-forming material SM is arranged to provide the highest height level for the top electrode that forms the outer flap OF above the frame-shaped structure of the first step-forming material SM.

[0054] FIG. 4 shows cross section through a margin region of another BAW resonator showing the same further optional design features like the embodiment of FIG. 3 but having an inverted order of first step-forming material SM relative to the sub-layers of the top electrode TE. The arrangement accords in this detail with the bottom left cross-section of FIG. 1. This arrangement is believed to be the better choice in terms of performance of the BAW resonator.

[0055] FIG. 5 shows cross section through a BAW resonator of the FBAR type. Here, the layer sequence starting from bottom electrode BE is arranged above a recess RC in the substrate SU. At least below the active resonator region an air-filled cavity provides acoustic reflection due to the high impedance step at the interface to the bottom electrode. All other features and optional features may be embodied as described together with the FIGS. 1 to 4.

[0056] FIG. 6 shows details of a margin at the outer edge of the outer flap OF. The dielectric layer DL of SiN covers the edge and the adjacent surface of the piezoelectric layer PL. Here, the underlap is shown to be patterned in SiN rather than the top electrode layer as shown in FIGS. 3 and 4.

[0057] FIG. 7 shows the dame detail at the outer edge of the BAW resonator. W.r.t. FIG. 6 the sequence of step-forming material SM and bottom sub-layer of top electrode is interchanged. In both FIGS. 6 and 7 the TiN layer below the SiN layer covers only the top surface of the top electrode TE but not the edge.

[0058] FIG. 8 shows a cross-section through a BAW resonator that has the same layer sequence like the resonator of FIG. 6. However, at the outer edge a trench is structured through the dielectric layer DL to expose a stripe of the top surface of the piezoelectric layer as well as the edge of the top electrode. When the trench is properly dimensioned w.r.t. the top-electrode termination/edge a portion of any leaking lateral energy is reflected. The reflected wave improves the acoustic properties and avoids losses if the reflected energy is in phase but is destructive if it is out of phase. The phase thereof can be controlled by positioning suitable offset of trench w.r.t. the edge.

[0059] In the trench the capping dielectric DL of SiN is removed from the side-wall of the structure allowing the outer flap to be ‘more free’ as the SiN is relatively stiff. This applies less force to where the SiN comes into contact with the piezoelectric and reduces lateral energy leakage/excitation. All these materials can be etched together at once in one step (e.g. the layers of SiN, TiN, AlCu, W, SiO2 and Ti).

[0060] FIGS. 9 to 11 show schematic block diagrams of filter circuits comprising resonators that are circuited to form RF filters. BAW resonators as described above may advantageously be used in these filter circuits.

[0061] FIG. 9 shows a ladder-type arrangement comprising series BAW resonators SR.sub.S and parallel BAW resonators BR.sub.P that may be formed according to the invention with high quality mirror layers. In this embodiment a respective series BAW resonator BR.sub.S and an according parallel BAW resonator BR.sub.P form a basic section BS.sub.LT of the ladder-type arrangement. A ladder-type arrangement comprises a number of basic sections BS.sub.LT that can be circuited in series to achieve a desired filter function.

[0062] FIG. 10 shows a block diagram of a hybrid filter that is depicted with a minimum number of elements. A real circuit may comprise a higher number of such structures. In FIG. 5 a first partial circuit PC1 of the hybrid filter comprises a series impedance element IE.sub.S and a parallel impedance element IE.sub.P. The series impedance element IE.sub.S can be embodied as a capacitor and the parallel impedance elements IE.sub.P can be embodied as a coil. A second partial circuit PC2 comprises at least one series BAW resonator BR.sub.S and at least one parallel BAW resonator BR.sub.P. Within the combined filter circuit first and second partial circuits PC1, PC2, as shown in FIGS. 4 and 6, can alternate or be arranged in an arbitrary sequence.

[0063] The exact design of such a hybrid filter can be optimized according to the requirements of the desired hybrid filter. Such an optimization can easily be done by a skilled worker by means of an optimizing computer program.

[0064] FIG. 11 shows a lattice-type arrangement of BAW resonators comprising series and parallel BAW resonators. In contrast to the ladder-type arrangement, the parallel BAW resonators BR.sub.P are arranged in parallel branches that interconnect two series signal lines with series BAW resonators BR.sub.S. The parallel branches are circuited in a crossover arrangement such that the basic section of the lattice-type arrangement BS.sub.LC comprises a first and a second series BAW resonator SR.sub.S arranged in two different signal lines and two parallel branches circuited to mutually cross over with a respective parallel BAW resonator SR.sub.P arranged therein. A lattice-type filter may comprise a higher number of basic sections according to the filter requirements.

[0065] Two or more of the filter circuits as shown in FIGS. 9 to 11 may form combined filters like duplexers or multiplexers. The filters may be used in RF circuits as band pass, notch or edge filters. The filter circuits may be combined with other circuit elements not shown or mentioned but generally known from the art.

[0066] The invention has been explained by a limited number of examples only and is thus not restricted to these examples. The invention is defined by the scope of the claims and may deviate from the provided embodiments.

LIST OF REFERENCE SYMBOLS AND USED TERMS

[0067]

TABLE-US-00001 BE bottom electrode PL piezoelectric layer TE top electrode AR active resonator region OL overlap region UL underlap OF outer flap DL dielectric layer, comprising a SiN SiN layer BE bottom layer of top electrode comprising tungsten AlCu top layer of top electrode comprising AlCu SM intermediate step forming material RC recess in substrate below active resonator region M1, M2 acoustic mirrors SU substrate HI high impedance layer of M LI low impedance layer of M HQLI high quality low impedance layer HQHI high quality high impedance layer