SAW RESONATOR WITH IMPROVED POWER DURABILITY AND HEAT RESISTANCE AND RF FILTER COMPRISING AN SAW RESONATOR

Abstract

An improved SAW (SAWR) resonator having an improved power durability and heat resistance and a protection to prevent device failure is provided. The SAW resonator has a carrier substrate (S) and an electrode structure (ES, EF) on a piezoelectric material (PM, PL). Further, the resonator has a shunt path (PCPP) parallel to the electrode structure and provided to enable an RF signal to bypass the electrode structure. The shunt path has a temperature dependent conductance with negative temperature coefficient of resistance.

Claims

1. A SAW resonator, comprising a carrier substrate, an electrode structure and a piezoelectric material arranged between the carrier substrate and the electrode structure, a shunt path parallel to the electrode structure and provided to enable an RF signal to bypass the electrode structure, wherein the shunt path has a temperature dependent conductance.

2. The SAW resonator of claim 1, further comprising reflection structures between which the electrode structures are arranged.

3. The SAW resonator of any one of claims 1-2, wherein the piezoelectric material is contained in a piezoelectric layer and the SAW resonator is a TF-SAW resonator.

4. The SAW resonator of any one of claims 1-3, comprising a protection element, wherein the protection element has a temperature dependent conductance and establishes an element of the shunt path.

5. The SAW resonator of any one of claims 1-4, wherein the carrier substrate comprising silicon.

6. The SAW resonator of any one of claims 1-5, further comprising a shunt layer between the carrier substrate and the piezoelectric material.

7. The SAW resonator of claim 6, wherein the shunt layer comprises polycrystalline silicon.

8. The SAW resonator of claim 7, wherein the shunt layer has a conductivity below 10.sup.3 1/ cm at temperatures below 100 C. and a conductivity above 10.sup.3 1/ cm at temperatures above 200 C.

9. The SAW resonator of any one of claims 1-8, wherein the temperature dependence of the shunt path's conductance is obtained by doping.

10. The SAW resonator of any one of claims 1-9, further comprising a compensation layer between the carrier substrate and the piezoelectric material.

11. The SAW resonator of any one of claims 1-10, wherein the compensation layer comprises a material selected from a silicon oxide, a doped silicon oxide, silicon dioxide, doped silicon dioxide, fluorine doped silicon dioxide.

12. The SAW resonator of any one of claims 1-11, comprising a shunt layer between the carrier substrate and the piezoelectric material, the piezoelectric material in a piezoelectric layer, a compensation layer between the shunt layer and the piezoelectric layer, wherein the carrier substrate comprises silicon, the shunt layer comprises polycrystalline silicon and has a thickness between 0.01, and 1.0, the compensation layer comprises a silicon oxide and has a thickness between 0.01, and 1.0, the piezoelectric layer comprises lithium tantalate and has a thickness between 0.01, and 1.0, the electrode structure comprises Al as its main constituent and has a thickness between 0.02, and 0.2, , is the acoustic wavelength of the resonator's main mode.

13. The SAW resonator of claim 12, wherein the carrier substrate is oriented such that Euler angles are (010, 010, 4510) or (4510, 5410, 010) with respect to the acoustic propagation direction of the resonator's main mode.

14. An RF filter comprising an SAW resonator of any one of claims 1-13.

Description

[0052] In the figures:

[0053] FIG. 1 shows the basic principle of the shunt path;

[0054] FIG. 2 illustrates an implementation in an RF filter;

[0055] FIG. 3 illustrates the possibility of a dedicated protection element;

[0056] FIG. 4 shows possible and preferred paths for the shunt path;

[0057] FIG. 5 illustrates an equivalent circuit diagram of a basic element of a ladder-type like configuration.

[0058] FIGS. 6 to 8 illustrate electric properties of the basic structure shown in FIG. 5 for different temperature and thus conductivity configurations;

[0059] FIGS. 9 to 11 illustrate the reversible effects of a temperature increase.

[0060] FIG. 1 shows a basic possibility of utilizing a tailored conductivity of material below the electrode structures to protect the electrode structures from permanent damage. The SAW resonator SAWR comprises electrode structures ES such as electrode fingers EF shown in sagittal plane of the corresponding component. The electrode structures ES are arranged on a piezoelectric material PM contained in a piezoelectric layer PL. The piezoelectric material PM is arranged on a carrier substrate S. The shunt path follows a parallel conduction protection path PCPP from an electrode element before the electrode structures ES to a location after the electrode structures ES. Thus, an excess of RF power can be shunted to protect the sensitive structures of the resonator.

[0061] FIG. 2 illustrates in a top view perspective a ladder-type like configuration of an RF filter. Between an input port IN and an output port OUT three series resonators SR are electrically connected in series. Further, three parallel paths, each comprising a parallel resonator PR, electrically connect the signal path to ground. The parallel conductance protection path PCPP establishes the possibility to conduct an excess of RF power at higher temperatures from the input port IN directly to the ground potential such that no permanent damage is done to the resonators.

[0062] FIG. 3 illustrates the possibility of providing a dedicated protection element PE integrated in the parallel conductance protection path PCPP. The protection element has a specific temperature dependent conductance characteristic. The protection element can be provided as arranged on the piezoelectric material, arranged above the piezoelectric material or embedded below the surface of the piezoelectric material.

[0063] FIG. 4 illustrates a preferred embodiment where a shunt layer SL is provided between the carrier substrate S and the piezoelectric layer PL. The shunt layer has a thickness and a temperature dependent conductivity such that the characteristic desired conductance is obtained.

[0064] Optionally the resonator has a compensation layer CL. The compensation layer can be arranged between the carrier substrate S and the piezoelectric layer PL. In particular, it is possible that the compensation layer CL is arranged between the shunt layer SL and the piezoelectric layer PL. However, it is also possible that the compensation layer CL is arranged between the carrier substrate S and the shunt layer SL.

[0065] Due to finite resistivity of the piezoelectric material, of the material of the compensation layer and the material of the carrier substrate corresponding further parallel partial shunt paths may also exist. However, it is possible that a special dedicated shunt path is provided which carries most of the excess of RF power, e.g. 90% or more of the excess of RF power that should be led around the sensitive structures.

[0066] FIG. 5 illustrates an equivalent circuit diagram of a basic element of a ladder-type like configuration. In the signal path between the input port IN and the output port OUT in the basic element one series resonator SR is connected. Further, one parallel resonator PR is connected in a parallel path electrically connecting the signal path SP to ground. The environment of the corresponding resonator is modelled by a series configuration comprising a capacitive element CE, a resistive element RE and a further capacitive element CE. The series configuration is electrically connected parallel to the series resonator and to the parallel resonator, respectively.

[0067] The equivalent circuit diagram shown in FIG. 5 is the basis for the considerations with respect to FIGS. 6 to 11.

[0068] In FIG. 6 matrix elements S.sub.ij (transmission: S.sub.21 and reflection: S.sub.11) and corresponding values for power, current and voltage are shown. In particular, curve 1 illustrates the insertion loss (S.sub.21). Curve 2 illustrates the reflectivity (S.sub.11) Curves 3 and 4 represent the power losses P where curve 3 represents the power losses in the series resonator and curve 4 represents the power losses in the parallel resonator.

[0069] In the lower part of FIG. 6 corresponding value for current (solid lines) and voltage (dashed lines) are provided. Curves 3 represent the series resonator and curves 4 represent the parallel resonator. The characteristics shown in FIG. 6 represent a configuration of the low conductivity of the shunt path represented by the resistive element and the capacitive elements in FIG. 5. Thus, FIG. 6 illustrates a mode of normal operation and normal operation temperatures.

[0070] In contrast, FIG. 7 illustrates the corresponding curves for an increased temperature resulting in an increased conductivity of the shunt paths around each resonator. It is clearly visible that the dissipated power is significantly reduced.

[0071] Furthermore, FIG. 8 illustrates the characteristics at a high temperature scenario where the conductivity is high. Dissipated power is mainly zero and the reflection coefficient mainly equals 100%.

[0072] FIG. 9 illustrates the insertion loss in an unloaded circuit configuration (S.sub.21) and in a loaded circuit configuration (S.sub.21) for a normal operation temperature.

[0073] In contrast, FIG. 10 illustrates the same situation for an increased temperature due to self-heating of the resonator clearly shown by the substantially increased insertion loss for the loaded circuit configuration S.sub.21.

[0074] Furthermore, FIG. 11 illustrates the insertion losses after the resonators have cooled down. The initial filter characteristics shown in FIG. 9 have been nearly restored. Permanent damage has been prevented.

[0075] The SAW resonator and the RF filter are not limited to the details explained above and shown in the figures. The resonator can comprise further elements. In particular the electrode structure can comprise a multilayer construction that helps shape the main acoustic mode and that helps to suppress unwanted acoustic modes. The RF filter can comprise further circuit elements such as further filter stages and in particular a cascading of resonators is possible to further reduce the stress on the resonators.

LIST OF REFERENCE SIGNS

[0076] CE: capacitive element [0077] CL: (temperature) compensation layer [0078] EF: electrode finger [0079] ES: electrode structure [0080] GND: ground potential [0081] IN: input port [0082] OUT: output port [0083] PCPP: shunt path, parallel conductance protection path [0084] PE: protection element [0085] PL: piezoelectric layer [0086] PM: piezoelectric material [0087] PP: parallel path [0088] PR: parallel resonator [0089] RE: resistive element [0090] S: carrier substrate [0091] SAWR: SAW resonator [0092] SL: shunt layer [0093] SP: signal path [0094] SR: series resonator