Abstract
A SAW resonator with reduced spurious modes is provided. The resonator comprises a (111) silicon carrier substrate (CS), an electrode structure (ES) and a piezoelectric layer (PIL). The carrier substrate has a crystal orientation with the Euler angles (−45°±10°; −54°±10°; 60°±30°) and the piezoelectric layer comprises LiTaO.sub.3 and has a crystal orientation with the Euler angles (0°; 56°±8°; 0°). There may be intermediate layers (IL1, IL2) of SiO.sub.2 and amorphous or polycrystalline materials. In addition a silicon nitride layer is provided as passivation (PAL). Electrodes are made of aluminum. Thicknesses of all layers are selected in particular ranges to optimize SAW behaviour.
Claims
1. A SAW resonator with reduced spurious modes, comprising a carrier substrate, an electrode structure above the carrier substrate, a piezoelectric layer between the carrier substrate and the electrode structure, wherein the carrier substrate has a crystal orientation with the Euler angles (−45°±1020; −54°±10°; 60°±30°), the piezoelectric layer comprises LiTaO.sub.3 and has a crystal orientation with the Euler angles (0°; 56°±8°; 0°).
2. The SAW resonator of claim 1, wherein the carrier substrate has a crystal orientation with the Euler angles (−45°±5°; −54°±5°; 60°±10°) or (−45°±2°; −54°±5°; 60°±5°) or (45°; −54°; 60°) and/or the piezoelectric layer has a crystal orientation with the Euler angles (0°; 56°±4°; 0°), (0°; 56°±2°; 0°) or (0°; 56°; 0°).
3. The SAW resonator of claim 1, comprising a first intermediate layer arranged between the carrier substrate and the piezoelectric layer, having a thickness t.sub.IL1 with 0.05λ≤T.sub.IL1≤λ where λ, is the wavelength of the acoustic main mode.
4. The SAW resonator claim 3, wherein the first intermediate layer comprises a material selected from polycrystalline Si, an amorphous material, a dielectric material.
5. The SAW resonator of claim 1, comprising a second intermediate layer arranged between the carrier substrate and the piezoelectric layer, having a thickness t.sub.IL2 with 0.05λ≤T.sub.IL2≤0.25λ where λ is the wavelength of the acoustic main mode.
6. The SAW resonator of claim 5, wherein the second intermediate layer comprises a material selected from a silicon oxide and SiO.sub.2.
7. The SAW resonator of claim 1, wherein the piezoelectric layer has a thickness T.sub.PIL with 0.1λ≤t.sub.PIL≤0.3λ where λ is the wavelength of the acoustic main mode.
8. The SAW resonator of claim 1, wherein the electrode structure comprises Al, has a thickness t.sub.EL with 0.05λ≤t.sub.EL≤0.2λ where λ is the wavelength of the acoustic main mode.
9. The SAW resonator of claim 1, further comprising a passivation layer arranged on or above the electrode structure and comprising Si.sub.3N.sub.4 and having a thickness t.sub.PAL with 0.0025λ≤t.sub.PAL≤0.05λ where λ is the wavelength of the acoustic main mode.
10. The SAW resonator of claim 1, wherein the SAW resonator is part of an ectroacoustic filter.
11. The SAW resonator of claim 10, wherein the electroacoustic filter is part of a multiplexer for one or more CA application.
Description
IN THE FIGURES
[0049] FIG. 1 shows a cross-section through a possible layer construction;
[0050] FIG. 2 shows a layer construction comprising a first separation layer;
[0051] FIG. 3 shows a layer construction with a first and a second separation layer;
[0052] FIG. 4 shows—in a top view—a basic layout of the electrode structure;
[0053] FIG. 5 illustrates possible circuit topologies of a duplexer;
[0054] FIG. 6 indicates the definition of the Euler angles;
[0055] FIG. 7 shows the real paths of admittance curves for different Euler angles of the carrier substrate (frequency-dependent);
[0056] FIG. 8 shows the corresponding magnitude values;
[0057] FIG. 9 shows the Euler angle μ dependence of the electroacoustic coupling coefficient κ.sup.2 for LiTaO.sub.3;
[0058] FIG. 10 shows the Euler angle μ dependence of the temperature induced frequency drift ΔTCF for LiTaO.sub.3;
[0059] FIG. 11 shows the real parts of the frequency dependent admittances of a conventional resonator and of a resonator as described above;
[0060] FIG. 12 shows magnitudes of the frequency dependent admittances of the conventional resonator and of the resonator as described.
[0061] FIG. 1 illustrates a cross-section view through the x″′-z′-plane of a possible layer construction of the SAW resonator SAWR. The layer construction comprises a carrier substrate CS on which further layer elements are arranged. Especially the electrode structure ES is arranged above the carrier substrate CS. Between the carrier substrate CS and the electrode structure ES the piezoelectric layer PIL comprising or consisting of a piezoelectric material is arranged. The specifically chosen Euler angles in combination with the two materials establish an interface that can work as a wave mode separator. The layer construction has its layers arranged on one another in the vertical direction (z′). The electrode structure ES has electrode fingers of which the cross-section is shown in FIG. 1. The extension of the electrode fingers is orthogonal to the x′″-z′-plane defining the cross-section shown in FIG. 1 and along the transversal direction y″′.
[0062] Further, FIG. 2 illustrates a possible layer construction including the second intermediate layer IL.sub.2.
[0063] FIG. 3 indicates the possibility of arranging both the first IL.sub.1 and the second IL.sub.2 intermediate layer. The first intermediate layer IL.sub.1 can be arranged between the second intermediate layer IL.sub.2 and the carrier substrate. However, it is also possible that the order of the first and second intermediate layers is inverted.
[0064] FIG. 4 illustrates a basic configuration of a SAW resonator in a top view. The surface of the SAW resonator is parallel to the x″′-Y″′-plane. The direction of propagation of the acoustic waves is parallel to the longitudinal (x″′) direction. The electrode fingers EFI have their extension along the y′″-direction. The busbars BB have an extension along the longitudinal direction x″′. Electrode fingers EFI are electrically connected to one of two busbars BB and establish interdigitated structures IDS. The interdigitated structures IDS establish the electrode structure ES and are arranged between acoustic reflectors R to confine acoustic energy in the acoustic track. The electrode structure ES together with the reflectors R are arranged on the piezoelectric material PM.
[0065] FIG. 5 illustrates a possible circuit topology of a duplexer as an example of a multiplexer. The duplexer comprises a transmission filter TXF and a reception filter RXF. Each of the two filters has electro acoustic resonators, e.g. SAW resonators. The resonators can be series resonators SR electrically connected in series in a signal path. Parallel resonators PR electrically connect the signal path to ground. A common port CP can be electrically connected to an antenna AN. An impedance matching circuit IMC can be provided between the transmission filter TXF and the reception RXF to match input and output impedances of the filters in accordance with the corresponding frequencies.
[0066] FIG. 6 illustrates the definition of the Euler angles. The resulting axes x″′, y″′ and z″′ correspond to the axes denoted by x, y, z in the figures above.
[0067] FIG. 7 illustrates a comparison of the real paths of admittance curves of two resonators. Curve 1 corresponds to a resonator where the silicon substrate has Euler angles (0°, 0°, 0°). Curve 2 corresponds to a resonator where the carrier substrate has the Euler angles (0°, 0°, 45°). It can clearly be seen that the orientation of the crystallographic axes of the carrier substrate substantially determines the performance of the resonator.
[0068] Correspondingly, FIG. 8 shows the frequency-dependent magnitudes of the admittance curves of the resonators corresponding to FIG. 7.
[0069] FIG. 9 shows the Euler angle μ dependence of the electroacoustic coupling coefficient κ.sup.2. Specifically, the coefficient is increased when the Euler angle μ is decreased. The—when compared to conventional resonators—reduced angle μ would lead to an unwanted high coupling coefficient. The too high coupling coefficient would—in conventional resonators—be compensated by additional circuit components, e.g. for impedance matching. However, in the material and layer composition as described above an intrinsic compensation of the too high coupling coefficient can be obtained and the need for additional circuit components is reduced or even eliminated.
[0070] Similarly, FIG. 10 shows the Euler angle μ dependence of the temperature induced frequency drift ΔTCF. ΔTCF represents the difference in TCF between resonance and antiresonance frequency of a resonator. This corresponds to a difference in TCF of left and right skirt of a filter.
[0071] FIG. 11 shows the real parts of the frequency dependent admittances of a conventional resonator (curve 1) showing a plurality of spikes associated with unwanted modes and of a resonator as described above (curve 1) with a much smoother course due to the reduction of unwanted modes.
[0072] Similarly, FIG. 12 shows the magnitudes of the frequency dependent admittances of the conventional resonator (curve 1) and of the resonator as described above (curve 1) with a much smoother course.
[0073] The resonator is not limited to the details and configurations shown above. Additional elements such as TCF layers, passivation layers, wave guiding elements and similar elements can be present. Despite the possibility of the presence of a plurality of additional layers—that would lead to potential sources of unwanted spurious modes—spurious modes are reduced and the performance is improved.
LIST OF REFERENCE SIGNS
[0074] AN: antenna [0075] BB: busbar [0076] CP: common port [0077] CS: carrier substrate [0078] EFI: electrode finger [0079] ES: electrode structure [0080] IDS: interdigitated structure [0081] IMC: impedance matching circuit [0082] PAL: passivation layer [0083] PIL: piezoelectric layer [0084] PM: piezoelectric material [0085] PR: parallel resonator [0086] R: acoustic reflector [0087] RXF: reception filter [0088] SAWR: SAW resonator [0089] IL.sub.1, IL.sub.2: first, second intermediate layer [0090] SR: series resonator [0091] TXF: transmission filter
