TF-SAW resonator with improved quality factor, RF filter and method of manufacturing a TF-SAW resonator

Abstract

A TF-SAW resonator with improved quality factor is provided. The resonator has its piezoelectric material in the form of a thin film and an electrode structure arranged on the piezoelectric layer. Pitch (P) and metallization ratio (n) are chosen to maximize the quality factor (Q).

Claims

1. A thin-film surface acoustic wave (TF-SAW) resonator, comprising: a carrier substrate; a piezoelectric layer on or above the carrier substrate, the piezoelectric layer having a thickness T; a charge reduction layer arranged between the carrier substrate and the piezoelectric layer; and an electrode structure comprising an interdigital transducer (IDT) structure on the piezoelectric layer, the IDT structure having a pitch P and a metallization ratio η, wherein: the piezoelectric layer is a thin film comprising a piezoelectric material, and the pitch P and the metallization ratio η maximize a quality factor Q of the TF-SAW resonator.

2. The TF-SAW resonator of claim 1, wherein P and η depend on the thickness T of the piezoelectric layer.

3. The TF-SAW resonator of claim 1, wherein the piezoelectric material comprises LiNbO.sub.3 or LiTaO.sub.3.

4. The TF-SAW resonator of claim 1, further comprising an intermediate layer between the carrier substrate and the piezoelectric layer, wherein an acoustic velocity in the intermediate layer is smaller than in the piezoelectric layer.

5. The TF-SAW resonator of claim 1, further comprising a temperature compensation layer between the carrier substrate and the piezoelectric layer.

6. An RF filter comprising two or more TF-SAW resonators of claim 1, wherein P and η are chosen for each resonator individually.

7. The TF-SAW resonator of claim 1, wherein P and η depend on the thickness T of the piezoelectric layer, but are independent from an external electric environment of the resonator.

8. A method of manufacturing a thin-film surface acoustic wave (TF-SAW) resonator, comprising: depositing a piezoelectric layer comprising a piezoelectric material on or above a carrier substrate utilizing wafer bonding with thin film processing or a thin film layer deposition technique; arranging a charge reduction layer between the carrier substrate and the piezoelectric layer; and structuring an electrode structure comprising an interdigital transducer (IDT) structure on the piezoelectric layer with a pitch P and a metallization ratio η chosen to maximize a quality factor Q of the TF-SAW resonator.

9. The TF-SAW resonator of claim 8, wherein the charge reduction layer comprises polycrystalline silicon.

10. The method of claim 8, wherein P and η are chosen considering a thickness T of the piezoelectric layer but are independent from an external electric environment of the resonator.

11. The method of claim 8, further comprising locally trimming a thickness T of the piezoelectric layer.

12. The TF-SAW resonator of claim 8, wherein P and η depend on a thickness T of the piezoelectric layer.

13. The TF-SAW resonator of claim 8, wherein the piezoelectric material comprises LiNbO.sub.3 or LiTaO.sub.3.

14. A radio frequency (RF) filter comprising two or more thin-film surface acoustic wave (TF-SAW) resonators, the TF-SAW resonators comprising: a carrier substrate; a piezoelectric layer on or above the carrier substrate, the piezoelectric layer having a thickness T; and an electrode structure comprising an interdigital transducer (IDT) structure on the piezoelectric layer, the IDT structure having a pitch P and a metallization ratio η, wherein: P and η are chosen for each TF-SAW resonator individually, the piezoelectric layer is a thin film comprising a piezoelectric material, and the pitch P and the metallization ratio η maximize a quality factor Q of the TF-SAW resonator.

15. The RF filter of claim 14, wherein P and η depend on the thickness T of the piezoelectric layer, but are independent from an external electric environment of the resonators.

16. The RF filter of claim 14, wherein P and η depend on the thickness T of the piezoelectric layer.

17. The RF filter of claim 14, wherein the piezoelectric material comprises LiNbO.sub.3 or LiTaO.sub.3.

18. The RF filter of claim 14, wherein the TF-SAW resonators further comprise an intermediate layer between the carrier substrate and the piezoelectric layer, wherein an acoustic velocity in the intermediate layer is smaller than in the piezoelectric layer.

19. The RF filter of claim 14, wherein the TF-SAW resonators further comprise a temperature compensation layer between the carrier substrate and the piezoelectric layer.

Description

(1) In the figures:

(2) FIG. 1 shows a perspective view of a TF-SAW resonator;

(3) FIG. 2 illustrates the definitions of the pitch P and the metallization ratio η;

(4) FIG. 3 illustrates a possible layer construction in a cross-section;

(5) FIG. 4 illustrates a possible layer construction with a smaller thickness of the piezoelectric layer;

(6) FIG. 5 shows a layer construction having an intermediate layer;

(7) FIG. 6 illustrates a layer construction having an intermediate layer and a smaller thickness of the piezoelectric layer;

(8) FIG. 7 illustrates quality factors for varying pitches determined with and without de-embedding for a thick piezoelectric layer;

(9) FIG. 8 illustrates quality factors for varying pitches determined with and without de-embedding for a thin piezoelectric layer;

(10) FIG. 9 illustrates an overview over obtained maximum quality factors for both piezoelectric layer thicknesses;

(11) FIG. 10 shows quality factors for varying metallization ratios determined with and without de-embedding for a thick piezoelectric layer; and

(12) FIG. 11 shows quality factors for varying metallization ratios determined with and without de-embedding for a thin piezoelectric layer.

(13) FIG. 1 illustrates a possible construction of a thin film-SAW resonator TFSAWR in a perspective view. The elements of the resonator are arranged on a carrier substrate CS. In particular, a piezoelectric layer PL comprising a piezoelectric material or consisting of a piezoelectric material is arranged and deposited on the carrier substrate CS. On the piezoelectric layer PL an interdigital structure IDT is arranged and structured. The interdigital structure comprises electrode fingers EF that are electrically connected to one of two busbars BB. Thus, the interdigital transducer has its electrode fingers arranged in a comb-like pattern to convert between RF signals and acoustic waves via the electroacoustic effect.

(14) In the longitudinal direction the interdigital transducer IDT is flanked by reflectors RF comprising reflection fingers for confining acoustic energy longitudinally to the acoustic track.

(15) FIG. 2 illustrates a possible IDT geometry. The pitch P is defined as the distance between two edges of adjacent electrode fingers that point in the same direction. Thus, the pitch P is defined as the sum of the width W of an electrode finger and the distance between the electrode finger and the adjacent electrode finger. The metallization ratio η is defined as W/P.

(16) FIG. 3 illustrates a cross-section in the sagittal plane through the layer construction. The piezoelectric layer PL with its piezoelectric material is arranged on the carrier substrate CS. On the top side of the piezoelectric layer PL the electrode fingers EF are arranged. T denotes the thickness of the piezoelectric layer in a vertical direction.

(17) In contrast to the layer construction of FIG. 3, FIG. 4 illustrates a layer construction where the thickness T of the piezoelectric layer PL is smaller.

(18) FIGS. 5 and 6 show corresponding layer constructions for a thicker piezoelectric layer PL (FIG. 5) and a thinner piezoelectric layer PL (FIG. 6), each having an intermediate layer IL between the piezoelectric layer PL and the carrier substrate.

(19) The intermediate layer can comprise or consist of a material having a smaller acoustic velocity compared to the piezoelectric layer. Thus, a waveguide confining acoustic energy to the piezoelectric layer is obtained.

(20) Further, it is possible that the intermediate layer IL or an additional layer comprises material of a TCF layer for reducing or eliminating frequency drifts of characteristic frequencies as a result of temperature changes.

(21) FIG. 7 illustrates a plurality of measured quality factors for a layer construction having a specific thickness of the piezoelectric layer. The curves corresponding to the higher quality factors are measured utilizing a de-embedding method for neglecting measuring artefacts caused by the resonator's electric environment outside the acoustic track. The quality factors with the lower Q value, however, are obtained utilizing conventional means for determining the quality factor without de-embedding of the electric environment.

(22) It can be clearly seen that the real quality factors are different from the quality factors obtained by conventional measuring means. Further, it can be seen that the frequency range of the optimal quality factors Q.sub.opt for the real values is shifted compared to the maximum quality factor that would be obtained by conventional measuring means.

(23) The plurality of quality factors correspond to different pitches, thus, illustrating the effect of pitch variation on maximum quality factors.

(24) It can be clearly seen that conventional measuring means would suggest a pitch that has its highest quality factor at around 2000 MHz or slightly below 2000 MHz while the real optimum quality factor is obtained at around 2200 MHz for a different pitch.

(25) Thus, FIG. 7 clearly shows that the provided methods for establishing resonators provide resonators with improved quality factors.

(26) The same arguments hold true for a layer construction with a thinner piezoelectric layer as shown in FIG. 8. Similarly to FIG. 7, FIG. 8 would suggest an optimum Q factor when the pitch is varied for around 2000 MHz while the actual optimum Q value is obtained at frequencies above 2500 MHz.

(27) The results of the above considerations are shown in FIG. 9. Two frequency-dependent quality factors for the optimum quality factor are shown. For a layer construction having a thicker piezoelectric layer the optimum quality factor is obtained at a lower frequency. For the layer construction based on a thinner piezoelectric layer the frequency range is shifted to higher frequencies. However, if only conventional means for determining the quality factor would be applied, then the frequency range for the optimum quality factor would be nearly independent of the piezoelectric layer thickness since electromagnetic artefacts from the environment outside of the acoustic track dominate the quality factor and the real quality factor of the acoustic track itself is clouded. In particular for the layer construction having the smaller thickness of the piezoelectric layer, the frequency for the best quality factor would be outside the interval and would not be considered.

(28) FIG. 10 shows a plurality of measured quality factors for a thick piezoelectric layer. The curves with the higher quality factors correspond to results with de-embedding while the curves with the lower quality factors correspond to results without de-embedding. The plurality of quality factor measurements correspond to different metallization ratios η.

(29) Similarly, FIG. 11 shows the measured quality factors corresponding to FIG. 10 while the results shown in FIG. 11 are based on a layer construction with a thinner piezoelectric layer.

(30) In both cases it can be seen that the quality factor can be maximized by optimizing the metallization ratio.

(31) Thus, FIGS. 7 to 9 show that the real quality factor has a strong pitch dependence. FIGS. 10 and 11 show that the quality factor has a η dependence. FIGS. 7 to 11 show that conventional methods for determining the quality factor do not provide the actual quality factor and only when the preferred measures for determining the quality factor by means of de-embedding are performed, then the real quality factors can be determined and correspondingly improved resonators and RF filters can be obtained.

(32) The resonator, the filter and the method are not limited to the technical details shown and explained above. The resonator can comprise further structures. Further means, e.g. apodization, slanting or the structuring of further means for establishing a transversal acoustic waveguide, e.g. FINEA (FINger-Enden-Aufdickung) piston mode, are also possible.

LIST OF REFERENCE SIGNS

(33) BB: busbar

(34) CS: carrier substrate

(35) EF: electrode finger

(36) IDT: interdigital transducer structure

(37) IL: intermediate layer

(38) Q.sub.opt: optimal quality factor

(39) PL: piezoelectric layer

(40) P: pitch

(41) REF: reflector

(42) T: thickness of the piezoelectric layer

(43) TFSAWR: thin film-SAW resonator

(44) W: width of electrode finger