Multiple layer system, manufacturing method and SAW device formed on multiple layer system

Abstract

A layer system is proposed, in particular for forming a SAW device thereon, the layer system comprising a single-crystal sapphire substrate having a first surface and a crystalline piezoelectric layer comprising AlN deposited onto the first surface and having a second surface. The crystallographic R plane of sapphire is used as the first surface, so that the c-axis of the piezoelectric layer can be oriented parallel to the first surface and the second surface.

Description

Technical Field

[0001] The invention relates to a layer system comprising a thin piezoelectric film, a production method and a SAW device formed on a layer system having a piezoelectric layer. Background Art

[0002] In recent years, standard SAW technology based on lithium tantalate LT wafers for realizing RF filters in the range of 500MHz to 3GHz has been increasingly replaced by advanced microacoustic technologies such as BAW or temperature compensated SAW in order to meet the increasing performance requirements for mobile phone systems.

[0003] Mobile phones based on WCDMA and LTE require the lowest loss RF filters, duplexers and multiplexers to support advanced RF concepts such as carrier aggregation, diversity antennas and MIMO concepts or new modulation schemes. With the new 5G standard, the requirements on microacoustic devices will be further increased with respect to several key characteristics such as lowest losses, reduced temperature drift with temperature, higher linearity and power endurance, new frequency bands between 3 and 6 GHz and larger filter bandwidths. In addition, there is a continuous demand to reduce the cost, size and height of microacoustic devices.

[0004] Today, two microacoustic technologies (SAW and BAW) are essentially used to realize high-performance resonators, filters, duplexers and multiplexers for mobile phone applications. SAW technology mainly uses a single-crystal material (such as lithium tantalate LT or lithium niobate LN) as a piezoelectric substrate, on which electrode structures based on suitable metals (such as interdigital transducers) are realized to excite surface acoustic waves. Additional functional layers (such as passivation layers composed of, for example, silicon nitride, temperature compensation layers composed of, for example, amorphous silicon oxide, or thick metal interconnects) are used to further improve the device performance.

[0005] More advanced SAW devices use a thin piezoelectric single crystal layer bonded to a carrier wafer. In these devices, energy confinement in the piezoelectric layer can be achieved through the waveguide effect, thereby further reducing losses overall. By appropriately selecting the layer system, additional characteristics of the microacoustic device can be enhanced, for example, by the following operations: by introducing additional layers as mentioned above or by using high-resistivity silicon wafers with good thermal conductivity to improve heat dissipation and power endurance. Thin piezoelectric single crystal layers are typically bonded to a carrier substrate by a well-known wafer bonding method, and then the piezoelectric wafer is thinned to the desired layer thickness by a wafer grinding and polishing method. The desired layer thickness is typically in the range of half to one wavelength of a quarter wavelength of the microacoustic wave.

[0006] Achieving highly uniform piezoelectric layers with respect to layer thickness requires advanced thinning methods. Typically, the orientation (crystallographic orientation) of the piezoelectric layer is carefully selected based on advanced simulation and modeling methods such as finite element simulation (FEM) to achieve optimal device performance. Since the piezoelectric wafer material is typically grown from the melt by a single crystal growth method, a wide range of possible crystal orientations is available. The main disadvantage is that large diameter wafer solutions, such as 200 mm and 300 mm wafers, are not available, since, for example, lithium tantalate wafers with such large diameters are currently not available in mass production.

[0007] Different methods for achieving thin piezoelectric layers are to deposit such layers by well-known thin film deposition methods such as sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD) including metal organic CVD (MOCVD) and plasma enhanced CVD (PECVD), molecular beam epitaxy (MBE), atomic layer deposition (ALD) or sol-gel deposition). Using these techniques, piezoelectric layers (usually aluminum nitride AlN or scandium-doped aluminum nitride AlScN) for BAW devices can be grown on suitable wafer substrates such as high resistivity silicon. Typically, at present, AlN-based piezoelectric films for BAW devices are grown by sputtering methods, where highly oriented but polycrystalline films are achieved by orienting the crystallographic c-axis perpendicular to the substrate surface. These highly oriented polycrystalline piezoelectric layers well support the propagation of longitudinal waves along the crystallographic c-axis of the AlN-based piezoelectric layer. Using this design and structure, low-loss BAW resonators and devices can be achieved. However, since the epitaxial growth of AlN-based piezoelectric layers cannot be achieved using the commonly used low-temperature sputtering method, the in-plane orientation of AlN-based crystallites is less pronounced compared to a true single crystal or epitaxial layer.

[0008] In general, the realization of thin film piezoelectric layers by thin film deposition methods offers significant advantages over bonding and thinning methods using single crystal wafers, such as excellent thickness control, good layer adhesion, low cost process, low material consumption, full integration into wafer production lines, realization of layers on large diameter wafers and easy variation of chemical layer composition.

[0009] Therefore, several approaches have been made to realize SAW devices using such AlN-based thin films, benefiting from the following approaches: thin film approaches compared to single crystal approaches as mentioned above, and the flexibility of SAW design compared to BAW design. The SAW is patterned with excellent uniformity by photolithographic methods, allowing all resonators with different frequencies to be realized in one process step. In BAW technology, the thickness of the piezoelectric layer is the main frequency-limiting feature. Trimming methods allow high-frequency uniformity to be achieved. The realization of resonators with different frequencies on one wafer requires multiple process steps, such as subsequent layer deposition and layer patterning.

[0010] The main limitation for combining SAW design principles and growing AlN-based piezoelectric layers by thin film deposition methods at this time is the orientation of the AlN-based layers. Since the crystallographic c-axis is always oriented more or less perpendicular to the substrate surface when using low-temperature sputtering, the main piezoelectric coupling is also perpendicular to the substrate surface, while laterally propagating surface acoustic waves on the substrate surface require a significant tilt of the c-axis with respect to the surface normal to achieve a sufficiently large filter bandwidth.

[0011] Moreover, although the crystallographic c-axis of the AlN-based layer is perpendicular to the substrate surface, specific electrode configurations can help excite surface acoustic waves or Lamb / plate wave modes with a lateral propagation direction. However, even when using AlN layers with significant levels of, for example, Sc doping, which ultimately limits the combination of SAW structures and AlN-based thin piezoelectric membranes to some applications that only require a small filter bandwidth, it is almost impossible to achieve an effective coupling coefficient higher than 5% for microacoustic resonators using this design.

[0012] It is therefore advantageous to achieve a layer system which exhibits a piezoelectric film based on AlN grown by a thin-film deposition method in which the crystallographic c-axis is significantly tilted relative to the normal to the substrate surface.

[0013] This object and other objects are achieved by the layer system of the embodiments.Other embodiments provide a manufacturing method and a SAW device formed on the layer system. Summary of the invention

[0014] The present invention provides a different approach on how to realize a layer system with an AlN-based piezoelectric layer having a c-axis which is more or less parallel to the substrate surface.

[0015] It is proposed to use a single crystal sapphire substrate having a crystallographic R plane of sapphire (expressed as a (1-102) plane in the form of Bravais Miller indices) as a first surface. A crystalline piezoelectric layer including AlN can be epitaxially grown onto the first surface according to the law of epitaxy, wherein the (11-20) plane of the AlN-based layer is parallel to the (1-102) plane of sapphire, the in-plane [1-100] direction of the AlN-based layer is parallel to the [-1-120] direction of sapphire, and the in-plane [000-1] direction (crystallographic c-axis) of the AlN-based layer is parallel to the [1-10-1] direction of sapphire. The epitaxial growth of the AlN-based layer produces a preferred orientation in which the crystallographic c-axis is more or less parallel to the substrate surface. Therefore, the second surface, which is the surface of the crystalline piezoelectric layer, is the (11-20) plane.

[0016] With the novel layer system, a high coupling parallel to the c-axis and therefore parallel to the layer plane can be generated. Therefore, the piezoelectric layer is suitable for forming SAW devices thereon that achieve a high coupling coefficient. The layer can be formed by the commonly used layer growth techniques, which can be controlled to achieve epitaxial growth.

[0017] Sapphire, with its high thermal conductivity, low electrical conductivity and low RF losses as well as high acoustic velocity, is an ideal substrate material for realizing microacoustic RF devices and is available in wafer form with the R-plane as the first surface. In addition, such sapphire wafers are available with larger wafer diameters than single crystal piezoelectric wafers in LT or LN cut from melt-drawn ingots. However, despite the availability of R-plane sapphire and its previous uses, such as as a substrate for growth layers for optoelectronic applications, R-plane sapphire has hardly been used as a substrate for microacoustic devices.

[0018] A layer system with additionally improved high voltage electrical coupling parallel to the substrate surface comprises: an AlN-based piezoelectric layer doped with a suitable dopant such as Sc. The dopant such as Sc can be used to improve the piezoelectric coupling of the AlN and increase the piezoelectric response of the piezoelectric layer.

[0019] AlScN can also be epitaxially grown on the newly proposed R-plane sapphire by means of suitable deposition methods such as high-temperature sputtering, PLD, MOCVD, ALD or MBE.

[0020] Depending on the use of the layer system, different amounts of doping may be used. Thus, according to an embodiment, the piezoelectric layer comprises AlScN, wherein the amount of Sc contained in the piezoelectric layer is between 5 at % and 45 at %. However, any dopant that improves the piezoelectric coupling may also be advantageous.

[0021] In a preferred embodiment, the layer system includes a crystallographic orientation of the piezoelectric AlN layer, wherein the (11-20) plane of the AlN layer is parallel to the (1-102) plane of sapphire, the in-plane [1-100] direction of the AlN layer is parallel to the [-1-120] direction of sapphire, and the in-plane [000-1] direction (crystallographic c-axis) of the AlN layer is parallel to the [1-10-1] direction of sapphire.

[0022] In a preferred embodiment, the piezoelectric layer comprises AlN doped with Sc and is arranged on an epitaxial seed layer of pure undoped AlN. Thus, the seed layer is arranged between the substrate and the piezoelectric AlScN layer. The AlN seed layer also contributes to the waveguide effect, since the speed of sound in AlN is greater than that in, for example, Sc-doped AlN. The waveguide piezoelectric layer allows the SAW to propagate with lower losses and therefore with higher efficiency. The appearance of parasitic modes is also suppressed in the waveguide layer system.

[0023] Furthermore, in order to clearly define the growth direction of the polar c-axis of the piezoelectric layer, a slight tilt in the range of 0.5 to 6 degrees from the R-plane sapphire surface may be necessary.

[0024] The layer system can further comprise a passivation layer of SiO 2 deposited onto the second surface and / or a temperature compensation layer.

[0025] Using this wafer with an AlN-based layer system epitaxially grown on top, electrode structures for exciting acoustic waves (such as interdigital transducers) can be realized on the surface of the AlN-based layer system, wherein the orientation of these structures relative to the crystal orientation can be selected in a way to achieve the best performance with respect to the following parameters: available wave modes, piezoelectric coupling, absence of parasitic modes, frequency temperature coefficient, loss mechanisms and other critical parameters. By rotating the IDT orientation around the surface normal of the layer system, this can be changed for a given layer system. The interdigital transducer IDT can have any in-plane orientation, wherein the in-plane orientation has a rotation angle between 0 and 90° around the surface normal.

[0026] The acoustic velocity along the propagation direction and spacing of the interdigitated electrode structure defines the frequency and wavelength of the SAW so excitable.The preferred thickness of the doped AlN layer can then generally be set within the range of 0.3 to 3.0 times the wavelength λ of the surface acoustic wave SAW.

[0027] According to an embodiment, the interdigitated electrode structure comprises Cu and / or Al.

[0028] Further functional layers selected from the group of a passivation layer, a trimming layer of SiN and a temperature compensation layer can be incorporated into the layer sequence of the layer system, preferably above the piezoelectric layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In the following, the present invention will be described in more detail with reference to specific embodiments and the accompanying drawings. The drawings are schematic only and are not drawn to scale. For better understanding, some details may be depicted in an enlarged form.

[0030] Figure 1 Schematic illustration of the location of the R-plane within the basic crystal structure of sapphire.

[0031] Figure 2 A schematic cross section through a layer system according to a first exemplary embodiment is shown, the layer system comprising a sapphire R-plane substrate, an AlScN layer arranged on the sapphire R-plane substrate, and an electrode structure for a SAW device.

[0032] Figure 3 A similar layer system with an electrode structure according to a second exemplary embodiment is shown.

[0033] Figure 4 A schematic plan view is shown on a wafer having a layer system and an electrode structure according to a first exemplary embodiment.

[0034] Figure 5 A schematic plan view is shown on a wafer having a layer system and an electrode structure according to a second exemplary embodiment.

[0035] Fig. 6A and 6B The admittance of a SAW resonator built on the layer system of the first exemplary embodiment is shown, with different amounts of Sc in the AlScN.

[0036] Fig. 7A and 7B The admittance of a SAW resonator built on the layer system of the second exemplary embodiment is shown, with different amounts of Sc in the AlScN. DETAILED DESCRIPTION

[0037] Figure 1 The position of the R-plane within the sapphire crystal is schematically shown.

[0038] An AlScN layer with a Sc content of 40 mol% can be epitaxially grown directly onto this R-plane sapphire wafer. In this case, the [11-20] direction of the AlScN layer is perpendicular to the substrate surface (x-cut AlScN). According to an advantageous embodiment, a seed layer system made of, for example, pure and undoped AlN can be grown as a bottom layer onto the sapphire substrate. Such an AlN layer can support epitaxial growth. The thickness of the seed layer can be as thin as 30 nm, but can be adjusted as required.

[0039] For epitaxially growing the AlScN layer onto the seed layer, the deposition technique is selected from metal organic CVD (MOCVD), plasma enhanced CVD (PECVD), molecular beam epitaxy (MBE), atomic layer deposition (ALD), sol-gel deposition, high temperature sputtering and pulsed laser deposition PLD.

[0040] Furthermore, due to the fact that the speed of sound within this material is different from that within AlScN, improved acoustic properties are achieved, like for example a waveguiding effect of the layer system.The c-axis of the grown AlScN layer is oriented parallel to the first surface of the sapphire substrate.

[0041] On top of the AlScN layer, interdigital transducers are realized using, for example, Al or Cu electrodes based with a specific orientation relative to the crystallographic axes of the AlScN.

[0042] Figure 2A schematic layer stack with a thin AlN seed layer, a thin AlScN layer, and an electrode structure IDT according to a first embodiment is shown, which has a first possible orientation with respect to the crystallographic axes of both Al2O3 and AlScN. Figure 2 The electrode structure in the IDT realizes a SAW device that excites a primary acoustic wave with shear properties. The propagation direction is the crystallographic [1-100] direction of the AlScN. Depending on the intermediate frequency set by the spacing of the electrode structure, the thickness of the piezoelectric AlScN layer is selected to be in the range of 0.5 to 1.5 times the wavelength λ. Higher thicknesses are possible but not required. The thickness ratio of the different layers can be modified in such a way that a maximum waveguide effect can be achieved.

[0043] Figure 3 The same layer stack is shown, but with an electrode structure IDT according to a second embodiment having a second possible orientation with respect to the crystallographic axes of both Al2O3 and AlScN. Figure 2 The IDT orientation in is rotated 90° around the surface normal. Figure 3 In this embodiment, the SAW device excites a primary acoustic wave with Rayleigh characteristics. The propagation direction is the crystallographic

[0001] direction of AlScN.

[0044] Additional functional layers, such as passivation layers, temperature compensation layers or frequency tailoring layers, may be applied on top of the SAW electrode structure.

[0045] The benefit of such a microacoustic device with the proposed layer system is that the advantages related to the design flexibility of SAW devices are combined with the advantages of the easy production of BAW devices. In SAW devices, the planar structure defined by the main frequency is patterned with excellent uniformity by photolithographic methods, allowing all resonators with different frequencies to be realized in one process step. The benefits provided by BAW technology are due to the possible thin film processing. These benefits are, for example, excellent thickness control, good layer adhesion, low cost processing, low material consumption, full integration into the wafer production line, realization of layers on large diameter wafers, and easy variation of chemical layer composition. Compared with the previous process for manufacturing thin film SAW devices by bonding and thinning single crystal piezoelectric wafers, the benefits of thin film technology make the new layer system and the SAW devices produced thereon superior to the "old" technology.

[0046] An additional benefit associated with the use of sapphire wafers is the reduction of RF losses, without the need for complex trap-rich layer technology that is usually necessary when using high-resistivity Si wafers. Furthermore, the excellent thermal conductivity that improves the power endurance of the microacoustic device must be emphasized. Furthermore, the high sound velocity in the layer system supports the waveguiding of the microacoustic layer. With AlN-based material systems, relatively high sound velocities can be obtained, also enabling the realization of high-frequency surface acoustic wave devices with relaxed requirements regarding the photolithographic technology used.

[0047] Figure 4 and 5 Two exemplary orientations of the SAW electrode structure IDT relative to the crystallographic axes of the AlScN layer are shown in top view.

[0048] exist Figure 4 In the example, the c-axis [000-1] of the AlScN layer is tilted 90° relative to the surface normal, and the electrode structure IDT is oriented so that the main SAW propagation is along the [1-100] direction.

[0049] exist Figure 5 In the embodiment, the c-axis [000-1] of the AlScN layer is tilted 90° relative to the surface normal, and the electrode structure IDT direction is oriented so that the main SAW propagation direction can be along the crystallographic c-axis ([000-1] direction). Figure 4 The electrode structure shown in FIG. 1 is rotated 90° compared to the IDT.

[0050] Fig. 6A , 6B Shown according to Figure 4 Admittance curves of SAW resonators of given configuration in . For the simulations, the SAW resonators with 7% Sc content ( Fig. 6A ) and 37.5% Sc content ( Figure 6B ). The layer thickness of the AlScN is about 1200nm / 3700nm (the first value is for low Sc content, the second value is for high Sc content). The electrode structure IDT is embodied by Cu electrodes with a height of about 100nm. In both cases, the corresponding spacing of the transducers of the electrode structures is set to 0.8μm. The metallization ratio a / p is set to about 0.45, where a is the finger width and p is the distance between the centers of adjacent electrode fingers.

[0051] The propagation direction of the SAW is parallel to the [-1100] direction of the AlScN. With this configuration, the shear horizontal SAW mode can be excited.

[0052] Fig. 7A and 7B Shown according to Figure 5Admittance curves of SAW resonators of given configuration in . Again, for the simulations, the SAW resonators with 7% Sc content ( Fig. 7A ) and 37.5% Sc content ( Figure 7B ) with the same disclosed material properties of the AlScN piezoelectric layer. The layer thickness of AlScN is about 1000nm / 800nm ​​(the first value is for low Sc content, the second value is for high Sc content). The electrode structure IDT is embodied by a Cu electrode with a height of about 150nm. In both cases, the corresponding spacing of the transducers of the electrode structure is set to 0.8μm. The metallization ratio a / p is set to about 0.5 / 0.4 (the first value is for low Sc content, the second value is for high Sc content), where a is the finger width and p is the distance between the centers of adjacent electrode fingers. The propagation direction of the SAW is parallel to the crystallographic c-axis ([000-1] direction). With this configuration, pure Rayleigh mode SAW can be excited. Smaller piezoelectric coupling can be achieved by reducing the Sc content of the AlScN layer (as in Figure 7B Compared with the higher Sc content of 37.5%, Fig. 7A The embodiment is set to have 7% Sc).

[0053] Due to the limited number of embodiments, the present invention should not be limited to these embodiments. The layer system can be used to realize other devices with other electrode structures, different layer thicknesses, and combinations with additional layers that may be helpful for specific purposes. The implementation and effects of such variations are known in the art. The full scope of the invention is defined by the claims.

Claims

1. A layer system comprising: A single crystal sapphire substrate having a first surface; a crystalline piezoelectric layer comprising AlN, epitaxially grown on the first surface and having a second surface facing away from the first surface; wherein the first surface is the crystallographic R plane of sapphire; The epitaxial relationship between the sapphire substrate and the piezoelectric layer is as follows: The (11-20) plane of the piezoelectric layer is parallel to the (1-102) plane of sapphire which is the crystallographic R plane. The in-plane [1-100] direction of the piezoelectric layer is parallel to the [-1-120] direction of the sapphire, and The in-plane [000-1] direction of the piezoelectric layer is parallel to the [1-10-1] direction of sapphire.

2. The layer system according to claim 1, The piezoelectric layer includes AlN doped with a dopant that improves piezoelectric coupling.

3. The layer system according to claim 2, The piezoelectric layer comprises AlScN, and the amount of dopant Sc contained in the piezoelectric layer is between 5 at % and 45 at %.

4. The layer system according to claim 2, Wherein the piezoelectric layer comprises AlN doped with Sc, and wherein a seed layer of pure AlN is arranged between the substrate and the piezoelectric AlScN layer.

5. The layer system according to claim 1, Includes a SiO2 layer deposited onto the second surface.

6. A SAW device comprising the layer system of any one of claims 1 to 5 and having an interdigitated electrode structure arranged on top of the second surface.

7. The SAW device according to claim 6, The interdigital electrode structure is suitable for exciting a SAW in the piezoelectric layer with a given wavelength λ, wherein the thickness d of the piezoelectric layer is P According to the relationship 0.3λ≤d P ≤3λ to select.

8. The SAW device according to claim 6, The first surface is inclined at an angle δ of 0.5° to 6° relative to the R plane.

9. The SAW device according to claim 6, Wherein the interdigitated electrode structure has an in-plane orientation with a rotation angle between 0° and 90° about the surface normal.

10. The SAW device according to claim 9, The interdigital electrode structure comprises Cu and / or Al.

11. The SAW device according to claim 6, The invention also includes at least one functional layer selected from the group consisting of a passivation layer, a SiN trimming layer, and a temperature compensation layer.

12. A method for producing a layer system comprising an AlN layer having a c-axis parallel to the layer, the method comprising the following steps: A) providing a sapphire substrate having a planar first surface, wherein the planar first surface is a crystallographic R plane, wherein the crystallographic R plane is a (1-102) plane of the sapphire substrate; B) depositing a seed layer of pure AlN onto the first surface; C) epitaxially growing a piezoelectric layer onto the seed layer comprising AlScN by using a deposition technique selected from one or more of the following: metal organic CVD (MOCVD), plasma enhanced CVD (PECVD), molecular beam epitaxy (MBE), atomic layer deposition (ALD), sol-gel deposition, high temperature sputtering and pulsed laser deposition (PLD).

13. The method according to claim 12, comprising the steps of: D) forming an electrode structure comprising interdigitated electrodes on top of the piezoelectric layer, the interdigitated electrodes being suitable for generating a SAW wave having an intermediate frequency, Step C) comprises: during the epitaxial growth process, the thickness d of the epitaxial piezoelectric layer is increased to P Control to satisfy 0.3λ≤d P ≤3λ, where λ is consistent with the wavelength of the SAW wave at the intermediate frequency.

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