SAW resonator, RF filter, multiplexer and method of

manufacturing a SAW resonator

The present invention refers to SAW resonators, e.g. SAW resonators with a reduced pole-zero distance, to an RF filter comprising a SAW resonator, to a multiplexer and to a method of manufacturing a SAW resonator.

In mobile communication devices RF filters are needed to separate wanted RF signals from unwanted RF signals.

Generally, RF filters should provide a low insertion loss in a passband and a high rejection level outside a passband. Resonators of RF filters should have a high quality factor. Characteristic frequencies should be temperature-independent. Further, specifications with respect to frequency bands should be complied with. Further, corresponding filter components should have small spatial dimensions, a high power durability and be producible with low production costs.

Conventional RF filters may employ surface acoustic waves. Such SAW filters have an electrode structure on a

piezoelectric material. However, the obtainable bandwidth of RF filters based on SAW resonators is limited.

What is wanted is an RF filter that provides an improved flexibility with respect to bandwidth. In particular, what is wanted is an RF filter that allows passbands with a reduced bandwidth .

To that end, a SAW resonator that allows a reduced bandwidth, a corresponding RF filter, a multiplexer and a method of manufacturing a SAW resonator according to the independent claims are provided. Dependent claims provide preferred embodiments .

The SAW resonator (SAW = surface acoustic wave) comprises a piezoelectric material and an electrode structure arranged above the piezoelectric material. Further, the SAW resonator comprises a dielectric adjustment layer arranged between the piezoelectric material and the electrode structure.

The dielectric adjustment layer can be used to adjust the pole-zero-distance of the resonator. The pole-zero-distance of a resonator mainly determines the bandwidth of a

corresponding RF filter. SAW resonators can be electrically connected in a ladder-type like topology to establish an RF filter. Series resonators can be electrically connected in series in a signal path between an input port and an output port. One or more parallel path can electrically connect the signal path to ground. Each parallel path can comprise one or more parallel resonators. A bandpass filter is obtained by providing series resonators that have a resonance frequency that mainly equals the anti-resonance frequency of the parallel resonators.

Similarly, a band rejection filter can be obtained if the anti-resonance frequency of the series resonators mainly equals the resonance frequency of the parallel resonators.

The frequency difference, thus, between the resonance

frequency and the anti-resonance frequency is a measure for the bandwidth of a corresponding bandpass filter or a band rejection filter. Thus, by adjusting the pole-zero-distance of a resonator, in particular by reducing the pole-zero- distance of a resonator, a corresponding width of a passband or of a rejection band can be reduced.

The thickness of the dielectric adjustment layer can be provided such that a desired electroacoustic coupling

coefficient K ² is obtained that provides the desired pole- zero-distance .

Such SAW resonators allow the creation of RF filters that comply with specifications, e.g. of next generation mobile communication systems, with respect to performance,

temperature behaviour and similar parameters.

Conventional SAW RF filters need external circuit elements such as external capacitive elements or inductive elements to reduce the bandwidth. The external circuit elements increase production costs and the geometrical size of the

corresponding filter components.

Further, external circuit elements reduce the effective quality factor of the resonance circuits since their inherent quality factor is typically much smaller than that of the acoustic resonators.

Thus, by employing the dielectric adjustment layer, external circuit elements may become unnecessary and corresponding filter components can be manufactured with reduced costs, with reduced spatial dimensions and with increased quality factors. In such a SAW resonator the electrode structure arranged above the piezoelectric material can be arranged directly on the dielectric adjustment layer. The electrode structure can comprise interdigitated structures with comb like electrode fingers electrically connected to busbars and reflection structures at the distal ends of an acoustic track resulting in an acoustic resonator.

Conventional SAW electrode structure designs, e.g. means for transversal mode suppression like apodizing, slanting or piston mode, can be further used because no adjustments, or only minor adjustments, with respect to parameters such as metallization ratio h, pitch, electrode material and the like are necessary.

It is correspondingly possible that the adjustment layer is provided to adjust the electroacoustic coupling coefficient K ² of the resonator to a predetermined value.

It is possible that the adjustment layer comprises or

consists of a dielectric material selected from aluminium nitride (AIN), a silicon oxide, e.g. silicon dioxide (SiCy) , a doped silicon oxide, e.g. by fluorine, phosphor or boron, an aluminium oxide, e.g. AI ₂ O ₃ , a silicon nitride, e.g. Si _{3 4} , a hafnium oxide, e.g. HfCy, or a metal oxide.

Especially the use of silicon oxide has the advantage that silicon oxide such as silicon dioxide is a material well- known to designers of SAW resonators because silicon dioxide can be used as a material of a TCF compensation layer (TCF = temperature coefficient of frequency) as well. A TCF layer counteracts a temperature dependence of acoustic parameters such as a material's stiffness or wave velocity. When the temperature of a corresponding filter component changes, then due to thermal expansion and due to changing stiffness and wave velocity parameters the characteristic frequencies of the filter, e.g. the position of a passband flank, changes.

By providing a TCF layer that compensates for temperature- induced frequency changes, temperature-induced frequency drifts can be reduced or eliminated.

Thus, by using a TCF material as the material for the

dielectric adjustment layer a reduction in bandwidth and reduced or eliminated temperature-induced frequency drifts can be obtained. The use of aluminium nitride or aluminium oxide has the advantage that additionally an improved power durability can be obtained. Aluminium nitride and aluminium oxide are dielectric. Thus, a short circuit of electrode structures of opposite polarity is prevented. However, the good thermal conductivity of aluminium nitride and aluminium oxide contributes to dissipate RF energy distributed over a large area. Thus, local temperature increases that lead to the local destruction of the material system can be reduced.

It is possible that the piezoelectric material comprises or consists of a material selected from lithium tantalate

(LiTaCy) , lithium niobate (LiNbCy) , quartz and aluminum nitride (AIN) .

Thus, conventional piezoelectric materials, the physical parameters of which are well-known, can be used to establish the corresponding SAW resonator and corresponding RF filters.

It is possible that the piezoelectric material is provided as a thin layer or as a bulk material.

When provided as a bulk material the piezoelectric material can be provided as a monocrystalline material that has a preferred crystal cut. Further, the piezoelectric material can be provided as a thin layer, i.e. by wafer bonding with thin film processing, e.g. mechanical grinding or smart cut, or employing thin-film layer deposition techniques such as sputtering, physical vapour deposition, chemical vapour deposition, molecular beam epitaxy and the like.

It is possible that the SAW resonator further comprises a carrier substrate. The piezoelectric material can be arranged on or above the carrier substrate.

The carrier substrate can comprise or consist of a material selected from silicon, aluminum oxide, sapphire, crystalline carbon (diamond) , silicon carbide SiC, quartz and similar materials including doping of the mentioned carrier substrate materials. In particular, carrier substrates having a

material with a good thermal conductance are preferred.

It is possible that the SAW resonator comprises at least one intermediate layer providing a sagittal acoustic wave guide. The piezoelectric material can be arranged on or above this wave guide.

The wave guide can consist of a single layer. However, it is possible that the wave guide comprises two or more layers. It is preferred that the wave guide has a layer comprising a material that has an acoustic impedance different from the acoustic impedance of a layer above or below the wave guide's layer. Correspondingly, it is possible that the wave guide has two or more layers of different acoustic impedances. An interface between two materials of different acoustic

impedance reflects acoustic waves. Thus, acoustic waves from the surface of the resonator are reflected and acoustic energy is prevented from dissipating in the layer system below. Thus, the wave guide helps to confine the acoustic energy to the surface of the resonator, which improves the quality factor. A layer of high acoustic impedance of the wave guide can comprise aluminium nitride, silicon carbide, crystalline carbon (diamond) or polycrystalline silicon.

A layer of the wave guide having a low acoustic impedance can comprise silicon dioxide, a doped silicon dioxide or

germanium dioxide. Silicon dioxide can be doped by fluorine or phosphorous or boron.

If the SAW resonator has a wave guide and a carrier substrate then it is preferred that the wave guide is arranged between the carrier substrate and the piezoelectric material.

It is possible that the resonator further comprises an intermediate layer providing a temperature compensation layer for TCF reduction. Then, the piezoelectric material is arranged on or above the temperature compensation layer.

The temperature compensation layer may comprise a silicon oxide, e.g. SiCy. In particular, the material of the TCF compensation layer can be selected from the group consisting of SiCy, doped SiCy, GeCy, ScYF, ZrW ₂ Cy, ZrMo ₂ Cy, HfMo ₂ Cy,

SCW ₃ O ₁₂ , AIW ₃ O ₁₂ , Zr(W04) ( PO4) ₂ , Zeolithe and B ₂ O ₃ . These materials all show a positive TCF that can counteract the negative TCF of the used piezoelectric material. Moreover they mostly show a negative coefficient of thermal expansion which is especially true for materials selected from

transition metal compounds and compounds of rare earth metals. Surprisingly such materials show a high positive temperature coefficient of their E-modulus that is an

enhanced stiffness at higher temperatures.

It is possible that the SAW resonator has a passivation and/or temperature compensation layer arranged on or above the electrode structure.

The passivation layer can comprise an oxide, e.g. a metal oxide or a silicon oxide. The metal oxide can be an oxide of the metal of the electrode structure.

Further, the passivation layer can consist of or comprise silicon nitride.

The electrode structure can comprise electrodes based on copper or aluminium or based on a copper-based alloy or on an aluminium-based alloy. The electrodes can comprise a layered structure comprising an adhesion layer and an additional mass loading layer or layered system for transversal mode

suppression. In general, conventional electrode structures can be used on the adjustment layer including appropriate means for transversal mode suppression like apodizing, slanting or piston mode design may be applied.

It is possible that an RF filter comprises one or more such SAW resonators. The SAW resonators can be electrically connected in a ladder-type like configuration and establish a bandpass filter or a band rejection filter.

It is possible that a multiplexer comprises a corresponding filter, e.g. as a transmission filter or as a reception filter. The multiplexer can be a duplexer or a multiplexer of a higher degree such as a triplexer, quadplexer, etc. A method of manufacturing a SAW resonator comprises the steps :

- providing a piezoelectric material,

- arranging a dielectric adjustment layer on the

piezoelectric material,

- structuring an electrode structure on the dielectric material ,

- adjusting the electroacoustic coupling coefficient K ² by adjusting the thickness of the adjustment layer.

The thickness of the adjustment layer can be in a range from 0.5 nm to 50 nm. With respect to the acoustic wavelength l it is possible that the thickness of the adjustment layer is 5% of l or less.

It is also possible to create a TFSAW (Thin film SAW) resonator. To that end, possible steps can comprise the steps :

- providing a carrier substrate,

- arranging a fast velocity layer on top of the carrier substrate

- arranging a slow velocity layer on top of the fast velocity layer

- providing a piezoelectric material on top of the slow velocity layer

- arranging a dielectric adjustment layer on the

piezoelectric material ,

- structuring an electrode structure on the dielectric material

- adjusting the electroacoustic coupling coefficient k ² by adjusting the thickness of the adjustment layer. The slow velocity layer and the fast velocity layer can establish a sagittal acoustic wave guide.

The piezoelectric material can be provided as a piezoelectric thin film created utilizing thin film layer deposition techniques or a monocrystalline material provided with the correct thickness, e.g. provided utilizing a "smart cut" method .

The above-described SAW resonator can have reduced

temperature coefficients of frequency, e.g. by a TCF

compensation layer or a layer that simultaneously reduces the coupling coefficient and thermal dependencies. However, due to a reduced bandwidth a difference in temperature-induced frequency shifts is also reduced. Thus, a difference of thermally induced frequency shifts of left and right passband flanks is reduced.

The reduced electroacoustic coupling coefficient K ² also has the effect that a distribution of acoustic energy towards higher frequencies is reduced.

Central aspects of the SAW resonator and details of preferred embodiments are shown in the accompanying schematic figures.

In the figures:

Fig. 1 shows a basic construction of a SAW resonator;

Fig. 2 shows the use of a passivation or temperature

compensating layer on or above the electrode structure;

Fig. 3 shows the use of a carrier substrate; Fig. 4 shows the use of a sagittal acoustic wave guide;

Fig. 5 shows a SAW resonator comprising a plurality of stacked layers;

Fig. 6 shows the influence of a thickness variation of the dielectric adjustment layer on the electroacoustic coupling coefficient ;

Fig. 7 shows a comparison of admittances of resonators with and without an adjustment layer;

Fig. 8 shows the dependence of the difference in TCF of a resonance and an anti-resonance frequency on the thickness of the adjustment layer;

Fig. 9 shows a possible use of a SAW resonator in a filter and in a duplexer.

Figure 1 illustrates a possible basic construction of a SAW resonator SAWR. Electrode structures ES, e.g. comprising electrode fingers EF and reflection elements are arranged above a piezoelectric material PM. Between the electrode structure ES and the piezoelectric material the dielectric material of the dielectric adjustment layer is arranged.

The thickness of the dielectric adjustment layer (mainly equal to the distance between the electrode structure ES and the piezoelectric material PM) adjusts the electroacoustic coupling coefficient k ² . Thus, by having the possibility of adjusting the electroacoustic coupling coefficient a measure to adjust the bandwidth of a passband or of a rejection band is obtained. Figure 2 illustrates the possibility of providing a

passivation layer PL above the material of the electrode structure. This layer may also act as a temperature

compensation layer for TCF reduction. Alternatively, an additional TCF layer may be applied resulting in a

combination of TCF and passivation layer. Thus, top surfaces and side surfaces of the electrode structure are covered to protect the electrode structure against external

environmental influences.

Figure 3 shows the possibility of using a carrier substrate CS for piezoelectric thin films to provide carrier

functionality, i.e. to support an easy integration of the corresponding SAW resonator in a filter module or in a duplexer module. To that end, the carrier substrate CS provides the necessary mechanical stability. Further, the carrier substrate CS can be used as a base for arranged signal lines that should electrically connect the electrode structure to an external circuit environment.

Figure 4 illustrates a possible sagittal acoustic wave guide WG. The wave guide WG comprises a first layer LI and a second layer L2. The first layer LI can have a low acoustic

impedance. The second layer L2 can have a high acoustic impedance .

The acoustic impedance depends on the wave velocity and on the stiffness parameters. A higher wave velocity leads to a higher impedance. Higher stiffness values lead to higher frequencies. Further, a lower specific density leads to a lower acoustic impedance. Figure 5 illustrates a possible configuration of a SAW resonator comprising a passivation and/or temperature

compensation layer on the electrode structures, a wave guide comprising two layers below the piezoelectric material PM and a carrier substrate CS below the lower layer of the wave guide WG.

Figure 6 illustratess the dependence of the coupling

coefficient K ² on the thickness of the adjustment layer in a SAW resonator having the layer configuration as shown in Figure 5.

Typically, this arrangement leads to very large pole-zero distance, which is unfavourable for many applications. By proper choice of thickness and material of DM, the pole-zero distance can be adjusted appropriately.

An increase in layer thickness causes a decrease of the coupling coefficient. The layer configuration of the SAW resonator provides a near linear dependence. Thus, Figure 6 provides the preferred thickness for a desired coupling coefficient needed for a desired bandwidth.

Figure 7 illustrates the effect of the adjustment layer on the pole-zero distance given by the frequency difference between the resonance frequency and the anti-resonance frequency: curve (1) corresponds to a conventional SAW resonator. Curve (2) corresponds to the admittance of a SAW resonator comprising the adjustment layer between the

piezoelectric material and the electrode structure. The presence of the dielectric adjustment layer has a small impact on resonance frequency which can be readjusted by pitch modification. However, the anti-resonance frequency exhibits a larger shift to a lower frequency resulting in a pole-zero distance reduction. The frequency difference mainly determines the bandwidth of the corresponding bandpass filter or band rejection filter. Thus, the provision of the

adjustment layer having a defined layer thickness allows to adjust the bandwidth of the corresponding band rejection filter or bandpass filter.

Figure 8 shows that an increasing thickness of the dielectric adjustment layer leads to a reduced difference in TCF of resonance and anti-resonance frequencies.

Figure 9 shows a possible topology of a duplexer. The

duplexer comprises a transmission filter TXF and a reception filter RXF. Between the transmission filter TXF and the reception filter RXF an antenna connection AN can be

provided. Between the antenna connection AN and the reception filter RXF an impedance matching circuit can be provided. The transmission filter and the reception filter can have a ladder-type like topology with series resonators SR and parallel resonators PR. Series resonators SR are electrically connected in series in the signal path between an input port and an output port. Parallel resonators PR in corresponding parallel paths electrically connect the signal path to ground .

One or more resonators of a transmission filter and/or one or more resonators of a reception filter can be realized as discussed above.

The SAW resonator is not limited to the embodiments or technical features shown in figures and explained above. The resonator can comprise further circuit elements or further layers. RF filters can comprise further resonators and circuit elements. Also, the multiplexer can comprise further components, e.g. for protecting sensitive MEMS structures from unwanted external effects. The method of manufacturing the SAW resonator can comprise further steps, in particular steps and measures for obtaining a homogenous thickness of the adjustment layer and a high crystalline quality of the adjustment layer.

List of Reference Signs

AN: antenna connection

CS : carrier substrate

DAL: dielectric adjustment layer

DM: dielectric material

DU: duplexer

EF: electrode finger

ES : electrode structure

LI, L2 : layers of a wave guide MUL: multiplexer

PR: parallel resonator

PL: passivation layer

PM: piezoelectric material RXF : reception filter

SAWR : SAW resonator

SR: series resonator

TXF : transmission filter

WG: wave guide