Abstract
For a multilayer SAW device arranged on a carrier substrate it is proposed to use a specific material for the carrier substrate. If a silicon material having a selected range of Euler angles is used as a material for the carrier substrat improved suppression of disturbing signals is achieved.
Claims
1. A surface acoustic wave (SAW) device, comprising: a carrier substrate comprising a crystalline silicon with a crystal cut according to Euler angles of (45°±10°, 54°±10°, 0°±10°); a piezoelectric layer arranged on top of the carrier substrate; and an interdigital transducer (IDT) electrode arranged on top of the piezoelectric layer.
2. The SAW device of claim 1, wherein the crystal cut according to Euler angles of (45°±10°, 54°±10°, 0°±10°) is such that wave energy of a spurious mode is radiated predominantly within the carrier substrate while wave energy of a main mode is localized near a top surface of the piezoelectric layer.
3. The SAW device of claim 1, wherein a high acoustic velocity layer is arranged between the carrier substrate and the piezoelectric layer, wherein the acoustic velocity in the high acoustic velocity layer is higher than in the carrier substrate.
4. The SAW device of claim 3, wherein the high acoustic velocity layer has a thickness of x2 according to 0.05λ<x2<λ wherein λ is the wavelength of a main mode of the surface acoustic wave device.
5. The SAW device of claim 3, wherein the high acoustic velocity layer comprises polycrystalline silicon.
6. The SAW device of claim 3, further comprising a temperature coefficient of frequency (TCF) compensating layer having a positive TCF and being arranged between the high acoustic velocity layer and the piezoelectric layer.
7. The SAW device of claim 6, wherein the TCF compensating layer comprises SiO.sub.2, Fluorine doped SiO.sub.2 or other doped SiO.sub.2 based materials.
8. The SAW device of claim 6, wherein the TCF compensating layer has a thickness of x3 according to 0.05λ<x3<λ.
9. The SAW device of claim 1, further comprising a temperature coefficient of frequency (TCF) compensating layer having a positive TCF and being arranged above the IDT electrode.
10. The SAW device of claim 9, wherein the TCF compensating layer comprises SiO.sub.2, Fluorine doped SiO.sub.2 or other doped SiO.sub.2 based materials.
11. The SAW device of claim 9, wherein the TCF compensating layer has a thickness of x3 according to 0.05λ<x3<λ.
12. The SAW device of claim 1, wherein the piezoelectric layer has a thickness of x4 according to 0.1λ<x4<λ.
13. The SAW device of claim 1, wherein the IDT electrode comprises a multilayer structure comprising aluminium.
14. The SAW device of claim 1, wherein the IDT electrode has a thickness of x5 according to 0.05λ<x5<0.2λ.
Description
(1) In the following the invention is explained in more detail with regard to specific embodiments and the accompanying figures.
(2) FIG. 1 shows different examples of SAW devices according to the invention in a schematic cross-section;
(3) FIG. 2 shows the real part of an admittance curve of a SAW device according to the invention;
(4) FIG. 3 shows a respective absolute part in dB of the admittance of the same SAW device;
(5) FIG. 4 shows the wave energy in dependence on the depth under the top surface of the SAW device for the frequency of the main mode;
(6) FIG. 5 shows the amplitude of the spurious mode in dependence on the depth under the top surface of the SAW device;
(7) FIG. 6 shows an admittance curve of a further embodiment according to the invention;
(8) FIG. 7 shows the absolute part in dB of the same admittance curve; and
(9) FIG. 8 shows the amplitude of a bulk mode in dependence on the depth under the top surface of the SAW device.
(10) FIG. 1A shows a SAW device according to the simplest embodiment of the invention in a schematic cross-section. The SAW device comprises a carrier substrate SU, a piezoelectric layer PL arranged on top of the carrier substrate SU and an interdigital transducer electrode IT arranged on top of the piezoelectric layer PL. The material of the carrier substrate SU is chosen from silicon having a specific range of cut angles with Euler angles of (0°±10°, 0°±10°, 45°±10°). A similar working embodiment comprises a substrate material of silicon having Euler angles of (45°±10°, 54°±10°, 0°±10°). The thickness of the carrier substrate is set to provide sufficient mechanical strength to carry the layers and structures disposed on top of the carrier substrate.
(11) The piezoelectric layer PL may comprise any piezoelectric material that is useful for a SAW device. Exemplary embodiments comprise lithium tantalate or lithium niobate. Other piezoelectric materials like AlN and ZnO are possible too.
(12) The interdigital transducer electrode IT is depicted only schematically with a few electrode fingers and may comprise more electrode fingers and one or more interdigital transducer, reflector or any other electrode structure that is commonly used on top of SAW devices. The thickness of the piezoelectric layer PL is chosen depending on resonance frequency and resonator specifications, preferably of about 2 μm or less.
(13) A SAW device according to FIG. 1A is improved in view of guiding of the acoustic main mode and any disturbing bulk mode or another spurious mode. The main mode that yields the desired signal of the SAW device is concentrated and confined near the surface of the piezoelectric layer PL, while most of the spurious or bulk mode leaks into the substrate SU and thus does not contribute to the transfer curve of the SAW device anymore and yields no disturbing signal.
(14) FIG. 1B shows an improved embodiment where a TCF compensating layer TC is disposed between the piezoelectric layer PL and the carrier substrate SU as shown in FIG. 1A. This TCF compensating layer may be chosen from any material having a positive TCF or a TCF that is at least higher than the TCF of the piezoelectric layer PL. A layer that can reduce the temperature coefficient TCF preferably comprises silicon oxide. A preferred thickness X3 of this layer is within the range from 0.1λ and 1.0λ.
(15) In a further improvement the invention is shown in FIG. 1C. A further functional layer HV is introduced into the multilayer structure of the inventive SAW device. A high acoustic velocity layer is arranged between the carrier substrate SU and a TCF compensating layer TC.
(16) A preferred thickness X2 of this layer is within the range from 0.05λ and 1.0λ.
(17) FIGS. 2 and 3 show the admittance curves for a SAW single one port resonator according to FIG. 1B. FIG. 2 shows the real part of the admittance where, as a reference, curve 1 complies to a SAW device as shown in FIG. 1A made on a silicon material with Euler angles of (0°, 0°, 0°). Curve 2 refers to an inventive SAW device with a silicon material having Euler angles of (0°, 0°, 45°). As can best be seen in FIG. 2, a spurious mode signal at about 2300 MHz is substantially reduced at a SAW device according to the invention. The main mode at about 1900 MHz remains unchanged and delivers a sharp signal. FIG. 3 shows the absolute part in dB of the admittance of the same device where curve 1 is the admittance of the reference example while curve 2 complies to the inventive device. It can be seen that disturbing peaks at about 2600 to 2700 MHz are also substantially reduced in amplitude.
(18) The positive effect of the inventive SAW device described above can best be explained with regard to FIGS. 4 and 5. FIG. 4 shows the distribution of the wave energy of the acoustic main mode in dependence on the depth that is the distance to the top surface of the piezoelectric PL (Please provide new figures showing only the curve that supports the invention best. The original figures of your invention report comprise three curves ux, uy and uz. We assume that the curves depict amplitudes along respective coordinate axes. It seems that the greatest impact is along uy). In the left part of FIG. 4 a SAW device with a commonly used silicon substrate is shown while in the right part of FIG. 4 the main mode in an inventive SAW device with a silicon material of a specific Euler angle is depicted. The main mode has a frequency around 1900 MHz.
(19) Comparing the left part according to the art with the right part according to the invention shows that the energy of the main mode is concentrated near the surface of the piezoelectric layer and thus does not intrude into the carrier substrate. Concentrating the energy of the main mode near the top surface guarantees low losses as a high amount of the main mode is recovered as an electrical signal at the interdigital electrode.
(20) In FIG. 5 the two devices are regarded at a frequency of about 2600 MHz according to a disturbing mode that is present in the admittance curve of the SAW device according to the art. In this case the known SAW device concentrates energy of the disturbing mode in another area that is near the top of the silicon carrier substrate. When regarding the right part of FIG. 5 according to an inventive SAW device it can be seen that the energy of the disturbing mode can penetrate deeper into the SAW device and the carrier substrate SU and is no longer concentrated near the top surface thereof only. When comparing the curves from FIG. 4 and FIG. 5 it becomes clear that the main mode remains nearly unchanged when implementing the invention. However, the disturbing mode is drawn deeper into the SAW substrate (carrier substrate) than before and is hence prevented from reaching the IDT electrodes and hence prevented from producing a disturbing signal.
(21) In a further improvement according to the structure of FIG. 1C a further functional layer is introduced into the multilayer structure of the inventive SAW device. A high acoustic velocity layer is arranged between the carrier substrate SU and the TCF compensating layer TC. This high acoustic velocity layer provides a high velocity of the acoustic signal that is higher than the acoustic velocity of the same wave in the carrier substrate as well as in the piezoelectric layer. A preferred material for this layer comprises polycrystalline silicon. It is preferred to make this layer as thin as one wavelength or less. A preferred thickness X2 of this layer is within the range from 0.05λ and 1.0λ. It has been found that this embodiment provides a further improvement of mode suppression in a high frequency region above the frequency of the main mode that is above the pass band of the SAW device.
(22) FIG. 6 shows the real part of the admittance curve of this embodiment as curve 2. As a reference, curve 1 shows the admittance of a SAW device according to the art. It can clearly be seen that at a frequency of about 2300 MHz a substantial reduction of a disturbing signal is achieved. Signals according to higher modes of the SAW device at about 2600 MHz and higher remain but are shifted in frequency.
(23) FIG. 7 shows the absolute part in dB of the admittance of the same SAW device. The most advantageous effect of this embodiment occurs at a frequency of 2300 MHz where in the graph (curve 2) a disturbing mode that is visible at curve 1 is totally suppressed. The main resonance of the SAW device according to its pass band frequency remains unchanged. The vanishing of the disturbing mode is a consequence of the changing velocity difference and therefore different boundary conditions due to introduction of an additional fast velocity layer.
(24) FIG. 8 depicts the depth profile of the disturbing mode in an inventive SAW device with an acoustic velocity layer HV compared to a SAW device without this layer. The curve according to the ladder one is depicted in the left part of FIG. 8 while the curve according to the SAW device with functional layer HV is depicted in the right part of FIG. 8. It can be seen that the disturbing mode is drawn deeper into the carrier substrate as the maximum of the curve is deeper within the carrier substrate of the SAW device. This disturbing mode complies to the bulk wave and its deleterious effect is eliminated as the excitement of the bulk mode is minimized with this embodiment.
(25) The invention has been explained with reference to a small number of embodiments only but it is not restricted to these embodiments. Especially, single layers of the multilayer structure of the SAW device can be embodied with other materials and other thicknesses too. It is clear that each new multilayer structure can potentially promote additional specific disturbing modes. As resonance affects at the interface between two adjacent layers can occur, the resonance frequency thereof depends on the the material and specific thickness of these layers. However, the invention allows to reduce any disturbing mode without reducing the amplitude of the main signal. Modes that cannot be sufficiently suppressed can be shifted in their frequency at a location where no other band is present that would be affected by the disturbing signal.
LIST OF REFERENCE SYMBOLS
(26) SU carrier substrate IT IDT electrode PL piezoelectric layer TC temperature compensation layer HV high acoustic velocity layer 1 admittance of reference sample 2 admittance of resonator according to the invention
