Micro-acoustic component having improved temperature compensation

Abstract

For a component operating with acoustic waves, it is proposed to provide a compensation layer on the component for compensating for a negative temperature coefficient of the frequency, which includes a material based on a chemical compound made up of at least two elements, which has a negative thermal expansion coefficient.

Claims

1. A component operating with acoustic waves, having a layer of a piezoelectric material including at least one pair of electrodes for exciting acoustic waves in the piezoelectric material having a compensation layer arranged in the component in such a way that at least a portion of the energy of the acoustic wave is located in the compensation layer, wherein the compensation layer includes dielectric material based on a rare-earth compound made up of at least two elements, the dielectric material having a negative thermal expansion coefficient.

2. The component as claimed in claim 1, in which the compensation layer is applied directly to the layer of the piezoelectric material, wherein the electrodes are arranged on the piezoelectric layer, on the compensation layer, or between these two layers.

3. The component as claimed in claim 1, in which the rare-earth compound is in the form of scandium trifluoride ScF.sub.3.

4. The component as claimed in claim 3, in which the rare-earth compound is in the form of ScF.sub.3 doped with yttrium, having the formula Sc.sub.(1-X)Y.sub.xF.sub.3, wherein the yttrium component expressed by the coefficient x is defined by the relationship 0<x0.25.

5. The component as claimed in claim 4, in which the rare-earth compound is in the form of ScF.sub.3 doped with yttrium having the formula Sc.sub.(1-x)Y.sub.xF.sub.3, where x=0.2.

6. The component as claimed in claim 3, in which the compensation layer including ScF.sub.3 is a glass.

7. The component as claimed in claim 1, in which the compensation layer includes a network former.

8. The component as claimed in claim 1, in which the compensation layer has a positive temperature coefficient of the thermoelastic properties greater than 700 ppm/K.

9. The component as claimed in claim 1, designed as an SAW component, including at least one interdigital transducer on or above the piezoelectric layer including a compensation layer deposited above the piezoelectric layer and the interdigital transducer, which contains ScF.sub.3 in pure form, doped, in the form of a mixed crystal including other oxides or halides, or embedded in a crystalline matrix or a glass wherein the temperature coefficient of the center frequency is fully compensated for by a layer thickness of 5 to 20% relative to the wavelength at the center frequency of the component.

10. The component as claimed in claim 1, designed as an BAW component, including two electrode layers including a compensation layer deposited between the piezoelectric layer and an electrode layer or onto an electrode layer opposite to the piezoelectric layer, wherein the compensation layer contains ScF.sub.3 in pure form, doped, in the form of a mixed crystal including other oxides or halides, or embedded in a crystalline matrix or a glass wherein the temperature coefficient of the center frequency is fully compensated for by a layer thickness of 5 to 20% relative to the wavelength at the center frequency of the component.

11. The component as claimed in claim 2, in which the rare-earth compound is in the form of scandium trifluoride ScF.sub.3, in which the rare-earth compound is in the form of ScF.sub.3 doped with yttrium, having the formula Sc.sub.(1-X)Y.sub.xF.sub.3, wherein the yttrium component expressed by the coefficient x is defined by the relationship 0<x0.25, or in which the rare-earth compound is in the form of ScF.sub.3 doped with yttrium having the formula Sc.sub.(1-X)Y.sub.xF.sub.3, where x=0.2.

12. The component as claimed in claim 11, in which the compensation layer includes a network former.

13. The component as claimed in claim 2, in which the compensation layer has a positive temperature coefficient of the thermoelastic properties greater than 700 ppm/K.

14. The component as claimed in claim 11, in which the compensation layer has a positive temperature coefficient of the thermoelastic properties greater than 700 ppm/K.

15. The component as claimed in claim 2, designed as an SAW component, including at least one interdigital transducer on or above the piezoelectric layer including a compensation layer deposited above the piezoelectric layer and the interdigital transducer, which contains ScF.sub.3 in pure form, doped, in the form of a mixed crystal including other oxides or halides, or embedded in a crystalline matrix or a glass wherein the temperature coefficient of the center frequency is fully compensated for by a layer thickness of 5 to 20% relative to the wavelength at the center frequency of the component.

16. The component as claimed in claim 11, designed as an SAW component, including at least one interdigital transducer on or above the piezoelectric layer including a compensation layer deposited above the piezoelectric layer and the interdigital transducer, which contains ScF.sub.3 in pure form, doped, in the form of a mixed crystal including other oxides or halides, or embedded in a crystalline matrix or a glass wherein the temperature coefficient of the center frequency is fully compensated for by a layer thickness of 5 to 20% relative to the wavelength at the center frequency of the component.

17. The component as claimed in claim 2, designed as an BAW component, including two electrode layers including a compensation layer deposited between the piezoelectric layer and an electrode layer or onto an electrode layer opposite to the piezoelectric layer, wherein the compensation layer contains ScF.sub.3 in pure form, doped, in the form of a mixed crystal including other oxides or halides, or embedded in a crystalline matrix or a glass wherein the temperature coefficient of the center frequency is fully compensated for by a layer thickness of 5 to 20% relative to the wavelength at the center frequency of the component.

18. The component as claimed in claim 11, designed as an BAW component, including two electrode layers including a compensation layer deposited between the piezoelectric layer and an electrode layer or onto an electrode layer opposite to the piezoelectric layer, wherein the compensation layer contains ScF.sub.3 in pure form, doped, in the form of a mixed crystal including other oxides or halides, or embedded in a crystalline matrix or a glass wherein the temperature coefficient of the center frequency is fully compensated for by a layer thickness of 5 to 20% relative to the wavelength at the center frequency of the component.

19. A component operating with acoustic waves, having a layer of a piezoelectric material including at least one pair of electrodes for exciting acoustic waves in the piezoelectric material having a compensation layer which is arranged in the component in such a way that at least a portion of the energy of the acoustic wave is located in the compensation layer, in which the compensation layer includes a dielectric material having a negative thermal expansion coefficient the material being chosen from one of the following compounds: ZrW.sub.2O.sub.8, ZrMo.sub.2O.sub.8, HfW.sub.2O.sub.8, HfMo.sub.2O.sub.8, ScW.sub.3O.sub.12, AlW.sub.3O.sub.12, Zr(WO.sub.4)(PO.sub.4).sub.2, ScF.sub.3BaF.sub.2YF.sub.3, ScF.sub.3BaF.sub.2ZnF.sub.2, ScF.sub.3BaF.sub.2InF.sub.3, ScF.sub.3MgF.sub.2, YbF.sub.3ScF.sub.3, LuF.sub.3ScF.sub.3, Zn(CN).sub.2, BeF.sub.2, B.sub.2O.sub.3, and zeolite.

Description

(1) The present invention will be described in greater detail below based on exemplary embodiments and the associated figures. The figures have merely been drawn schematically, and serve only for better understanding of the present invention. The figures are therefore in particular not true to scale, since individual portions may be depicted enlarged or reduced. Accordingly, neither relative nor absolute dimensions are to be derived from the figures.

(2) FIGS. 1 and 2 each show a schematic cross section of an SAW component including a compensation layer in a different arrangement in each case;

(3) FIG. 3 shows a BAW component having a compensation layer;

(4) FIG. 4 shows a GBAW component having a compensation layer;

(5) FIG. 5 shows an additional BAW component,

(6) FIG. 6 shows an SAW component;

(7) FIGS. 7a and 7b each show an SAW component having a structured compensation layer;

(8) FIGS. 8a to 8c show SAW or GBAW components which have one or multiple additional dielectric layers DS;

(9) FIG. 9 shows the profile of the modulus of elasticity as a function of the temperature in the system Sc.sub.(1-X)Y.sub.xF.sub.3 having different yttrium content levels x.

(10) FIG. 1 shows the simplest embodiment of an SAW component provided with a compensation layer KS. A first electrode layer EL1, which is designed in the form of comb electrodes which are intermeshed in a comb-like manner, is arranged on a substrate which includes at least one thin piezoelectric layer. The substrate SU is made up in particular of lithium tantalate having a cut which is suitable for the SAW generation and propagation. For example, LT42 has a temperature coefficient of the elastic properties in the x-direction of approximately 40 ppm. To compensate for this, the compensation layer KS is arranged above the electrode layer EL1 in a suitable thickness which is measured in thickness corresponding to the desired degree of compensation.

(11) FIG. 2 shows a similar component in which, however, the compensation layer KS is applied to a surface of the substrate which is opposite the surface provided with the electrode layer. If the thickness of the piezoelectric layer is chosen to be suitably thin, good compensation for the temperature coefficient of the frequency may also be achieved via this arrangement.

(12) FIG. 3 shows a component operating with bulk acoustic waves (BAW component), in which a compensation layer KS is applied directly to a piezoelectric substrate SU. A first electrode layer EL1 is arranged on the exposed surface of the substrate SU, and a second electrode layer EL2 is arranged on the exposed surface of the compensation layer KS. The thickness of the compensation layer KS and the substrate SU together determine the wavelength of the BAW component, so that a thicker compensation layer KS results in a thinner substrate at a given wavelength, in order to set the same resonance frequency in the BAW component.

(13) FIG. 4 shows an additional type of components operating with acoustic waves, i.e., a component operating with guided acoustic waves, a so-called GBAW component. In the case of this component, electrodes are again arranged in an in particular structured first electrode layer EL1 on a piezoelectric substrate SU. Thereover, the compensation layer KS is arranged in a desired layer thickness.

(14) The completion of the component is formed by a cladding layer ML applied above the compensation layer KS, which has a higher velocity v(ML) of the acoustic wave than the compensation layer v(KS):
v(ML)>v(KS).

(15) The velocity in turn may be correspondingly set according to
v=(c/)
via the thickness or the rigidity c of the materials used. It is thus ensured that the guidance of the acoustic wave takes place predominantly within the substrate and the compensation layer. In addition, the thickness of the cladding layer is set high enough that practically no acoustic motion or vibration is able to occur at the surface of the cladding layer pointing away from the piezo layer or pointing away from the compensation layer.

(16) FIG. 5 shows a BAW component having a first electrode layer EL1, a piezoelectric layer SU, and a second electrode layer EL2, in which the compensation layer KS is applied on the outside to one of the two electrode layers EL1, EL2.

(17) Of course, it is possible to arrange the compensation layer KS anywhere between the first electrode layer EL1 and the second electrode layer EL2. As another option, multiple compensation layers KS of different thickness may be used. BAW components having one or multiple such compensation layers may be formed as an SMR (solidly mounted resonator) resting directly on the substrate, or having a membrane design.

(18) FIG. 6 shows an additional GBAW component, in which the compensation layer is arranged between the piezoelectric substrate and the first electrode layer EL1. A cladding layer ML may also be arranged above the electrode layer EL1, as depicted in FIG. 6.

(19) FIGS. 7a and 7b show options of how the acoustic properties of an SAW component provided with a compensation layer KS may be improved further. The reduced reflectivity of the electrodes due to the low acoustic impedance difference between the electrodes and the compensation layer material is restored by means of an additional reflection created via structuring of the compensation layer KS. For this purpose, recesses (FIG. 7a) or bulges (FIG. 7b) are introduced into the surface of the compensation layer KS in parallel with the electrode fingers, which form reflection areas for the acoustic wave and are arranged in the same grid as the electrode fingers, and therefore amplify the reflectivity of the at the electrode fingers. In connection with the present invention, additional dielectric layers DS are also possible between the piezo crystal/piezo layer and the electrodes, or above the compensation layer. FIGS. 8a to 8c show such exemplary embodiments. Thus, in FIG. 8a, a dielectric layer DS is arranged between the first electrode layer EL1 and the compensation layer KS. In FIG. 8b, a dielectric layer DS is arranged above the cladding layer ML. FIG. 8c shows an embodiment which simultaneously has two dielectric layers DS1 and DS2, as are already depicted individually in FIGS. 8a and 8b.

(20) FIG. 9 shows the profile of the modulus of elasticity as a function of the temperature in the system Sc.sub.(1-X)Y.sub.xF.sub.3 for different parameters x corresponding to an yttrium component between 0 and 25%. It will be seen that for an yttrium content of 20% (x=0.2), the greatest increase in the modulus of elasticity results in the temperature range of 300 to 500 K, so that this material has the highest positive temperature coefficient of the modulus of elasticity and is best suited for use in a compensation layer in a component operating with acoustic waves. The pure scandium trifluoride demonstrates a negative thermal expansion coefficient, but only a temperature coefficient of the modulus of elasticity approaching zero.

(21) From the diagram and the underlying experiments, a temperature coefficient of the center frequency of approximately 1500 ppm/K results for the mixed scandium-yttrium trifluoride having an yttrium component between 20 and 25%. On the other hand, fluorine-doped SiO.sub.2 demonstrates a coefficient <700 ppm/K, while undoped SiO.sub.2 demonstrates a temperature coefficient <300 ppm/K. In comparison to compensation layers commonly used today made up of undoped SiO.sub.2, an improvement of the compensation by a factor of 5 thus results.

(22) The material properties, in particular of the mixed scandium-yttrium trifluoride, for example, rigidity, lie within a range comparable to the SiO.sub.2 layers which have been used to date. At a somewhat higher density than SiO.sub.2, it may be expected that the other component properties are also not negatively affected by the new compensation layer. Since only a lower layer thickness of the compensation layer is required due to the improved compensation, in fact, a significant improvement in the acoustic properties may be expected.

(23) The present invention is not limited to the embodiments described in detail in the exemplary embodiments, which only specify exemplary embodiments of components having a compensation layer which function by means of acoustic waves. In principle, components are also conceivable which have more than one compensation layer, or components which have other means for reducing the temperature coefficient of the center frequency, in particular, stress layers.