MULTIPLE LAYER SYSTEM, METHOD OF MANUFACTURE AND SAW DEVICE FORMED ON THE MULTIPLE LAYER SYSTEM

Abstract

A layer system especially for forming SAW devices thereon is proposed comprising a monocrystalline sapphire substrate having a first surface and a crystalline piezoelectric layer comprising MN, deposited onto the first surface, and having a second surface. As a first surface a crystallographic R-plane of sapphire is used enabling an orientation of c-axis of the piezoelectric layer parallel to the first and second surfaces.

Claims

1. A layer system comprising a monocrystalline sapphire substrate having a first surface a crystalline piezoelectric layer comprising A1N, grown epitaxially on the first surface, and having a second surface facing away from the first surface wherein the first surface is a crystallographic R-plane of sapphire wherein the epitaxial relationship between the sapphire substrate and the piezoelectric layer is as follows: the (11-20)-plane of the piezoelectric layer (x-cut) is parallel to the (1-102)-plane of sapphire (R-plane), the in-plane [1-100]-direction of the piezoelectric layer is parallel to the [1-120]-direction of sapphire, and the in-plane [000-1]-direction (crystallographic c-axis) of the piezoelectric layer is parallel to the [1-10-1]-direction of sapphire.

2. The layer system of claim 1 wherein the piezoelectric layer comprises AlN doped with a dopant that improves the piezoelectric coupling.

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

4. The layer system of claim 1, wherein the piezoelectric layer comprises A1N doped with Sc and wherein a seed layer of pure A1N is arranged between the substrate and the piezoelectric AlScN layer.

5. The layer system of claim 1, comprising a layer of SiO.sub.2 deposited onto the second surface.

6. A SAW device comprising the layer system of claim 1 and having an interdigital electrode structure arranged on top of the second surface.

7. The SAW device of claim 1, wherein the interdigital electrode structure is adapted to excite a SAW in the piezoelectric layer having a given wavelength , wherein the thickness d.sub.P of the piezoelectric layer is chosen according to the relation 0.3d.sub.P3.

8. The SAW device of claim 1, wherein the first surface is tilted against the R-plane by an angle of 0.5 to 6.

9. The SAW device of one of the foregoing claims, wherein the interdigital electrode structures have an in-plane orientation with a rotation angle between 0 and 90 around the surface normal.

10. The SAW device of claim 1, wherein the interdigital electrode structure comprises Cu and/or Al.

11. The SAW device of claim 1, comprising further functional layers chosen from the group of passivation layer, trimming layer of SiN and temperature compensation layer.

12. A method of manufacturing a layer system comprising an A1N layer having a c-axis parallel to the layer, comprising the steps A) providing a sapphire substrate having a plane first surface that is a crystallographic R-plane B) depositing a seed layer of pure A1N onto the first surface C) epitaxial growing a piezoelectric layer onto the seed layer comprising AlScN by using a deposition technique 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.

13. The method of claim 12, comprising the step of D) forming an electrode structure comprising an interdigital electrode on top of the piezoelectric layer, the interdigital electrode being adapted for generating a SAW wave having a mid frequency wherein step C) comprises at the epitaxial growing process controlling the thickness d.sub.P of the epitaxial piezoelectric layer to a value of 0.3d.sub.P3 wherein accords to the wavelength of the SAW at the mid frequency.

Description

[0029] In the following the invention will be explained in more detail with reference to specific embodiments and the accompanying figures. The figures are schematically only and are not drawn to scale. For better understanding some details may be depicted in enlarged form.

[0030] FIG. 1 shows schematically the position of the R-plane within a sapphire basic crystal.

[0031] FIG. 2 shows a schematic cross section through a layer system comprising a sapphire R-plane substrate, an AlScN layer arranged thereon and an electrode structure for a SAW device according to a first embodiment

[0032] FIG. 3 shows a similar layer system with an electrode structure according to a second embodiment

[0033] FIG. 4 shows a schematic top view onto a wafer with the layer system and electrode structure according to the first embodiment

[0034] FIG. 5 shows a schematic top view onto a wafer with the layer system and electrode structure according to the second embodiment

[0035] FIGS. 6A and 6B show the admittance of a SAW resonator build on the layer system of the first embodiment with different amounts of Sc in AlScN

[0036] FIGS. 7A and 7B show the admittance of a SAW resonator build on the layer system of the second embodiment with different amounts of Sc in AlScN

[0037] FIG. 1 shows schematically the position of the R-plane within a sapphire crystal.

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

[0039] For epitaxial growing of an AlScN layer onto the seed layer a 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] Moreover, due to the fact that the sound velocity within this material different to the velocity within AlScN improved acoustical properties like e.g. a wave guiding effect of the layer system is achieved. 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 utilizing e.g. Al or Cu based electrodes are realized with a specific orientation with respect to the crystallographic axes of the AlScN.

[0042] FIG. 2 shows the principle layer stack with a thin seed layer of AlN, a thin AlScN layer and an electrode structure IDT having a first possible orientation with respect to the crystallographic axes of both Al.sub.2O.sub.3 and AlScN according to the first embodiment. In this embodiment the SAW device that is achieved by the electrode structure IDT in FIG. 2 excites a main acoustic wave with a shear character. The propagation direction is the crystallographic [1-100] direction of AlScN. The thickness of the piezoelectric AlScN layer is chosen in dependence of the mid frequency set by the pitch of the electrode structure to be within a range of 0.5 to 1.5 times the wavelength X. Higher thicknesses are possible but not required. The thickness ratios of the different layers can be modified in a way that a maximum wave guiding effect can be achieved.

[0043] FIG. 3 shows the same layer stack but provided with an electrode structure IDT having a second possible orientation with respect to the crystallographic axes of both Al.sub.2O.sub.3 and AlScN according to the second embodiment. Actually the IDT is rotated by 90 around the surface normal with respect to the IDT orientation in FIG. 2. In this embodiment of FIG. 3 the SAW device excites a main acoustic wave with a Rayleigh character. The propagation direction is the crystallographic direction of AlScN.

[0044] Additional functional layers like passivation layers, temperature compensation layers or frequency trimming layers can be applied on top of the SAW electrode structures.

[0045] The benefit of such a micro-acoustic device with the proposed layer system lies in the combination of the advantages related to the design flexibility for SAW devices and that of the easy production of BAW devices. In SAW devices the main frequency defining planar structures are patterned by lithography methods with an excellent uniformity allowing the realization of resonators all having different frequencies in one process step. The benefits provided by BAW technology are due to possible thin film processing. These are e.g. a very good thickness control, good layer adhesion, low cost processing, low material consumption, full integration into a wafer line, realization of layers on large diameter wafers and easy variation of the chemical layer composition. Compared to the former process for manufacturing thin film SAW devices by bonding and thinning single crystal piezo wafers the benefits of the thin film technology make the new layer system and the SAW devices produced thereon superior to the old technology.

[0046] Additional benefits related to the use of sapphire wafers are reduced RF losses not requiring a complex trap rich layer technology typically necessary when using high resistivity Si wafers. Further, an excellent thermal conductivity improving the power durability of the micro-acoustic devices has to be stressed. Moreover, the high sound velocity in the layer system supports wave guiding of micro-acoustic layers. The relatively high sound velocity achievable with the AlN based material system also enables the realization of high frequency surface acoustic wave devices with relaxed requirements concerning the photolithography technology used.

[0047] FIGS. 4 and 5 show a top view of two exemplary orientations of the SAW electrode structure IDT with respect to the crystallographic axes of the AlScN layer.

[0048] In FIG. 4 the c-axis [000-1] of the AlScN layer is inclined by 90 with respect to the surface normal and the orientation of the electrode structure IDT enables a main SAW propagation along the [1-100]-direction.

[0049] In FIG. 5 the c-axis [000-1] of the AlScN layer is inclined by 90 with respect to the surface normal and the orientation of the electrode structure IDT direction enables a main SAW propagation direction along the crystallographic c-axis ([000-1]-direction). In this second embodiment the electrode structure IDT is rotated by 90 compared to the electrode structure IDT shown in FIG. 4.

[0050] FIGS. 6A, 6B show admittance curves of a SAW resonator according to the configuration given in FIG. 4. For the simulation, published material properties for AlScN piezoelectric layers with 7% Sc content (FIG. 6A) and 37.5% Sc content respectively (FIG. 6B) have been used. Layer thickness of AlScN is about 1200 nm/3700 nm (first value for low Sc content, second value for high Sc content). The electrode structure IDT is embodied by Cu electrodes with a height of about 100 nm. The respective pitch of a transducer of the electrode structure is in both cases set to 0.8 m. The metallization ratio a/p where a is finger width and p is a distance between centers of neighbored electrode fingers is set to be about 0.45.

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

[0052] FIG. 7A and 7B show admittance curves of SAW resonators according to the configuration given in FIG. 5. Again, for the simulation the same published material properties for AlScN piezoelectric layers with 7% Sc content (FIG. 7A) and 37.5% Sc content respectively (FIG. 7B) have been used. Layer thickness of AlScN is about 1000 nm/800 nm (first value for low Sc content, second value for high Sc content). The electrode structure IDT is embodied by Cu electrodes with a height of about 150 nm. The respective pitch of a transducer of the electrode structure is in both cases set to 0.8 m. The metallization ratio a/p where a is finger width and p is a distance between centers of neighbored electrode fingers is set to be about 0.5/0.4 (first value for low Sc content, second value for high Sc content). The propagation direction of the SAW is parallel to the crystallographic c-axis ([000-1]-direction). With this configuration, a pure Rayleigh mode SAW can be excited. A smaller piezoelectric coupling can be achieved by reducing the Sc content of the AlScN layer(as set for embodiment of FIG. 7A with 7% Sc when compared with the higher Sc content of 37.5% in FIG. 7B)

[0053] Due to the limited number of embodiments the invention shall not be limited to these embodiments. The layer system may be used for realizing other devices with other electrode structures, differing layer thicknesses and combinations with additional layers that may be helpful for special purposes. Realization and effects of such variations are known per se from the art. A full scope of the invention is given by the claims.