Saw device with composite substrate for ultra high frequencies

Abstract

A SAW device having a stacked design of functional layers is proposed that is build up on a carrier substrate (SUB) that is chosen to provide a high acoustic velocity. The stack further comprises a thin TCF compensation layer (TCL), a thin film piezoelectric layer (PEL) and a set of interdigital electrodes (IDE) on top of the piezoelectric layer. Energy of the desired mode mainly in the high acoustic velocity material. Despite the high possible operating frequencies the SAW device can reliably be manufactured with present lithographic techniques.

Claims

1. A surface acoustic wave (SAW) device, comprising a carrier substrate; a dielectric temperature coefficient of frequency (TCF) compensating layer having a positive temperature coefficient of frequency arranged on the carrier substrate; a thin film piezoelectric layer arranged on the dielectric TCF compensating layer; and an interdigital transducer (IDT) electrode structure arranged on the thin film piezoelectric layer, wherein: the carrier substrate comprises a thick layer or a bulk material, and the IDT electrode has an operating frequency band above 2.5 GHz.

2. The SAW device of the claim 1, wherein a wave velocity of the carrier substrate is higher than that of a standard silicon (Si) wafer.

3. The SAW device of claim 1, wherein the TCF compensation layer comprises one or more of: silicon dioxide (SiO.sub.2), doped SiO.sub.2, germanium dioxide (GeO.sub.2), scandium fluoride (ScF.sub.3), yttrium fluoride (YF.sub.3), zirconium tungstate (ZrW.sub.2O.sub.8), zirconium molybdate (ZrMo.sub.2O.sub.8), hafnium molybdate (HfMo.sub.2O.sub.8), ScW.sub.3O.sub.12, AlW.sub.3O.sub.12, zirconium tungsten phosphate (Zr(WO.sub.4)(PO.sub.4).sub.2), Zeolithe or boric oxide (B.sub.2O.sub.3).

4. The SAW device of claim 1, wherein the carrier substrate is selected from a material having a wave velocity of an acoustic wave greater than that of a standard silicon (Si) wafer, wherein the material comprises at least one of sapphire, graphene, diamond, silicon carbide (SiC), polycrystalline silicon, diamond like carbon, or aluminum nitride (AlN).

5. The SAW device of claim 1, wherein the thin film piezoelectric layer comprises at least one of lithium tantalate (LT) or lithium niobate (LN).

6. The SAW device of claim 1, wherein the thin film piezoelectric layer has a thickness between about 5 nanometers (nm) to 300 nm.

7. The SAW device of claim 1, wherein the TCF compensating layer has a thickness between about 5 nanometers (nm) to 200 nm.

8. The SAW device of claim 1, wherein: the carrier substrate comprises the bulk material including at least one of sapphire, graphene, diamond, SiC, polycrystalline silicon, diamond like carbon, or AIN; the dielectric TCF compensating layer has the positive temperature coefficient of frequency arranged on the carrier substrate and comprises at least one of silicon dioxide (SiO.sub.2), doped SiO.sub.2, germanium dioxide (GeO.sub.2), scandium fluoride (ScF.sub.3), yttrium fluoride (YF.sub.3) zirconium tungstate (ZrW.sub.2O.sub.8), zirconium molybdate (ZrMo.sub.2O.sub.8), hafnium molybdate (HfMo.sub.2O.sub.8), ScW.sub.3O.sub.12, AlW.sub.3O.sub.12, zirconium tungsten phosphate (Zr(WO.sub.4)(PO.sub.4).sub.2, Zeolithe or boric oxide (B.sub.2O.sub.3), the TCF compensating layer having a thickness of 5 nanometers (nm) to 200 nm; the thin film piezoelectric layer arranged on the TCF compensating layer comprises at least one of lithium tantalate or lithium niobate, the thin film piezoelectric layer having a thickness of 5 nm to 300 nm; and the IDT electrode structure arranged on the piezoelectric layer comprises a resonator operating at an operating frequency between 3 GHz and 8 GHz.

9. A filter circuit comprising the SAW device of claim 1, embodied as SAW filter, a duplexer or a multiplexer.

Description

(1) In the following the invention is explained in more detail with regard to specific embodiments and the accompanying figures. In the figures, some details may be depicted enlarged for better understanding and thus, the figures are not drawn to scale.

(2) FIG. 1 shows a stacked design of a SAW device according to the invention in a schematic cross-section,

(3) FIG. 2a shows a dependency of frequency on piezoelectric thin film thickness using a SAW resonator according to the invention,

(4) FIG. 2b shows a dependency of k2 on piezoelectric thin film thickness using a SAW resonator according to the invention,

(5) FIG. 3 shows the absolute value and real part of the admittance of a SAW resonator according to the invention,

(6) FIG. 4 shows an embodiment similar to that of FIG. 3 but including a TCL layer for different ambient temperatures according to the invention.

(7) FIG. 1 shows a stacked design of a SAW device according to the invention in a schematic cross-section. A carrier substrate SUB comprises a material having a high acoustic velocity. The thickness is sufficient to be mechanically stable for further processing and operation of the SAW device. A thin TCF compensating layer TCL is applied on the substrate and has a positive TCF suitable to compensate for the negative TCF of other materials in the stack.

(8) The next layer is a thin film piezoelectric layer PEL which needs to provide appropriate wave excitation and shows a suitably high electromechanical coupling. On top interdigital electrodes IDE are arranged comprising a metallization that is suitable to provide the SAW device function that is to excite a SAW and to recover an electric signal therefrom. Preferably the interdigital electrodes IDE form a resonator. A multiple of resonators can form a filter of a ladder type or lattice type arrangement. However, the interdigital electrodes IDE may realize another SAW device e.g. a DMS filter, a duplexer or a multiplexer.

(9) According to a more specified embodiment a SAW resonator is formed having a piezoelectric thin film using LiTaO3 or LiNbO.sub.3 on top of a carrier substrate using sapphire. Aluminum based electrodes are used for SAW excitation.

(10) FIGS. 2a and 2b show the dependency of frequency (FIGS. 2a) and k2 (FIG. 2b) on thickness of thin film LiTaO3 and thin film LiNbO3 compared to a resonator having a LiTaO3 bulk material. Usage of piezoelectric film using thin relative thicknesses provides increased velocities compared to LiTaO3 bulk material. This can be combined with an increased k2 by choosing the appropriate material and piezoelectric thin film thickness.

(11) According to a more specified embodiment a SAW resonator is formed having the following features and dimensions: a carrier substrate SUB of sapphire having a specified cut, a thin piezoelectric layer PEL of LiTaO3 having a thickness of about 5 nm-300 nm and a specified cut.

(12) IDT electrode structures IDE are made of Al and/or copper and form a one-port resonator with a resonance frequency of about 5 GHz.

(13) A simulation of the real part and the absolute value of the admittance of this embodiment is shown in FIG. 3. This SAW device shows a high k.sup.2 and has low losses due to the wave guiding of the layer stack suitable for high frequency application as realized with the present embodiment. A Rayleigh type spurious mode is visible above the main resonance at 5 GHz. Further optimization of this raw design will surely suppress this spurious excitation.

(14) Apart from this spurious signal, no additional spurious mode is excited and no further spurious signal is visible in a wide frequency range. This is beneficial for possible sophisticated SAW solutions like carrier aggregation.

(15) The embodiment is further extended by a TCF compensating layer TCL of about 5 nm-200 nm SiO.sub.2. A SAW device with improved temperature stability of operating frequency is achieved. FIG. 4 shows the absolute value of the admittance of this embodiment in a narrow band depiction for two different ambient temperatures.

(16) Despite the restricted number of described embodiments the scope of the invention is not limited to the embodiments of figures. Deviations from the proposed materials and dimensions are conceivable and are lying within the skills of an experienced expert. Possible deviations that are within the scope of the invention are solely defined by the claims where claim 1 provides the broadest scope.