ULTRA-HIGH FREQUENCY MICRO-ACOUSTIC DEVICE

Abstract

A micro-acoustic wave device is proposed for application in ultrahigh frequency range. The device uses a thin film piezoelectric material stacked on a carrier substrate. Additionally, a material is embedded between carrier substrate and piezoelectric thin film that decouples the acoustic of these layers. With this approach it is possible to achieve very high Q factor even for longitudinal waves, which are required for high frequency applications.

Claims

1. A device operating with acoustic waves realized in a layer stack, comprising: a carrier (SU); a decoupling layer (DCL); a piezoelectric layer (PL); and an electrode structure (ES) for exciting acoustic longitudinal waves, wherein the decoupling layer acoustically decouples the layers above from the layers below the decoupling layer that no bulk wave can acoustically couple to the carrier.

2. The device of claim 1, wherein the decoupling layer has a Young's modulus of less than 1 GPa and a density of less than 500 kg/mJ.

3. The device of claim 1, wherein the decoupling layer comprises aerogel.

4. The device of claim 1, wherein the decoupling layer comprises silica aerogel.

5. The device of one of claim 1, wherein the device is realized as a SAW device comprising an interdigital transducer (IDT) as an electrode structure.

6. The device of claim 1, comprising a TCF compensating layer (TCL) arranged between decoupling layer and piezoelectric layer.

7. The device of claim 6, wherein the TCF compensating layer comprises one of SiO.sub.2, doped SiO.sub.2 and GeO.sub.2.

8. The device of claim 1, comprising a shielding layer (SHL) arranged between carrier and decoupling layer.

9. The device of claim 8, wherein the shielding layer comprises one of poly-silicon, Si.sub.3N.sub.4, AlN, Al.sub.2O.sub.3, SiC, diamond like carbon and diamond.

10. The device of claim 1, wherein the device is realized as a SAW resonator (R); wherein the material of the piezoelectric film has a cut-angle selected to support excitement and propagation of longitudinal waves; wherein the SAW resonator comprises an interdigital transducer as an electrode structures; and wherein the pitch of interdigital transducer is set to correspond to a longitudinal wave and a resonance frequency between 2.5 GHz and 6 GHz.

Description

[0020] In the following the invention will be explained in more detail with respect to embodiment and the accompanied figures. The figures are schematic only and not drawn to scale. The same elements or elements having the same or comparable function are referenced by the same reference symbols.

[0021] FIG. 1 shows a schematic cross section through a stacked device according to a first embodiment

[0022] FIG. 2 shows a schematic cross section through a stacked device according to a second embodiment

[0023] FIG. 3 shows schematic electrode structures of a SAW resonator

[0024] FIG. 4 shows a schematic block diagram of a filter built out of resonators as shown in FIG. 3

[0025] FIG. 5 shows the admittance of a resonator according to an embodiment and of a reference example

[0026] FIG. 6 shows the absolute value of the admittance of a resonator around the resonance frequency according to an embodiment and of a reference example

[0027] FIG. 7 shows the absolute value of the impedance of a resonator around the resonance frequency according to an embodiment and of a reference example.

[0028] FIG. 1 shows a schematic cross section through a layer stack according to a SAW device of a first embodiment of the invention. The stack is based on a carrier SU. A decoupling layer DCL is arranged above the carrier SU. Thereon a piezoelectric thin film PL is arranged. On top of the stack ab electrode structure realizing e.g. a SAW filter comprising resonators is applied.

[0029] FIG. 2 shows a schematic cross section through a layer stack according to a SAW device of a second embodiment of the invention comprising further optional layers. Compared to the stack of FIG. 1 two additional layers are introduced. A shielding layer of e.g. poly silicon is arranged between carrier SU and decoupling layer DCL. The decoupling layer DCL may comprise an aerogel. Shielding layer may have a thickness of 0.2 μm to 2.5 μm.

[0030] Further, a TCF compensating layer TCL of about ioonm to 800 nm SiO.sub.2 is arranged between the decoupling layer DCL and the piezoelectric thin film PL. Thin film PL consists of LN for example that is applied with a cut angle that supports longitudinal wave excitation and propagation.

[0031] The electrode structures ES consist of an Al based metallization and comprise interdigital transducers. The pitch of the interdigital transducers is set to a value according the desired wavelength and amounts to a half wavelength of the longitudinal wave propagating in the piezoelectric thin film PL. In a later process step of device manufacture a passivation layer of a dielectric or a resist may be applied (not shown in the figure).

[0032] A concrete embodiment comprises from bottom to top the following layers:

[0033] a Si carrier SU,

[0034] a shielding layer of .sub.5oonm of poly-Si

[0035] a decoupling layer DCL of about 25 nm to 75 nm silica aerogel

[0036] a piezoelectric thin film PL of LN.sub.170Yrot.sub.90X or LN20 having a thickness of 100 nm to 500 nm

[0037] an Al based electrode structure ES that may comprise Cu with a height of about 70 nm to 150 nm.

[0038] A schematic electrode structure ES of a SAW resonator R is shown in FIG. 3. The resonator R has a commonly known metallization structure consisting of an interdigital transducer IDT arranged in an acoustic track between two reflectors REF.

[0039] FIG. 4 shows a schematic block diagram of a ladder type filter that may be built from the new resonators. The filter comprises at least series resonator RS in a series signal line and a parallel resonator RP arranged in a shunt line extending from the series signal line to ground. A pair of these two resonators forms a basic section BS that already has a filter function. Real ladder type filters comprises n such basic sections BS wherein the number n of which is dependent on the desired degree of achievable filter selectivity. In the figure three basic section and a further series resonator RS are shown exemplarily.

[0040] FIG. 5 shows the absolute value of the admittance of a resonator as shown in FIG. 3 and based on a layer stack according to FIG. 3 and designated as curve 1. For reference, a curve 2 shows the admittance of a comparable stack that is missing the decoupling layer DCL. In the embodiment the resonators resonates at about 5GHz. It lo has a sufficiently high k2 for high frequency applications but is slightly reduced compared to curve 2. The real part shows the outstanding improvement in loss situation that is achieved by adding a decoupling layer DCL which decouples the acoustic in the piezoelectric thin film PL from the layer stack below. Additional volume waves, e.g. at 5 and 5.8 GHz are visible, which can be suppress by further optimizing the layer stack. Coupling factor k2 can be further optimized by thickness and cut angle optimization.

[0041] Besides the improved resonance peaks of curve 2 a spurious mode that appears at about 5200 MHz in curve 2 has nearly disappeared and is shifted to a lower frequency of about 5000 MHz as can be seen from curve 1.

[0042] FIG. 6 shows the improvement in loss level at resonance and FIG. 7 at anti-resonance frequency of a one-port resonator based on a layer stack according to FIG. 2. Similar like in FIG. 5 curve 1 accords to the invention while curve 2 accords to a respective reference example. The resonance frequency was plotted as absolute value of admittance |Y| and the anti-resonance peak as absolute value of impedance |Z|. In both figures the curves are scaled to the same frequencies for better comparison. It can be seen that the quality factor Q is clearly improved due to sharper and higher peaks

LIST OF USED TERMS AND REFERENCE SYMBOLS

[0043] 1,2 Curves assigned to new resonator and reference

[0044] BS Basic section of a ladder type filter

[0045] DCL Decoupling layer

[0046] ES Electrode structure

[0047] IDT Interdigital transducer

[0048] PL Piezoelectric thin film

[0049] REF Reflector

[0050] RP,RS,R Resonator

[0051] SHL Dielectric layer/shielding layer

[0052] SU Carrier

[0053] TCL TCF compensating layer