Strain-compensated rare-earth III-nitride heterostructures.

Abstract

The strain compensated heterostructure includes a substrate comprising a silicon carbide material, a first epitaxial layer comprising a single crystal aluminum nitride material formed on a top surface of the substrate, a second epitaxial layer formed on the first epitaxial layer opposite the top surface of the substrate, the second epitaxial layer comprising a single crystal aluminum scandium nitride material, and a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third epitaxial layer comprising a single crystal aluminum nitride material.

Description

[Technical field]

[0001] The present disclosure relates to strain-compensated heterostructures and acoustic wave resonators incorporating same. [Background technology]

[0002] Surface acoustic wave resonators made of piezoelectric materials are widely used in communication and timing applications. Aluminum nitride (AlN) as a device material is used due to its potential for high frequency applications due to its high surface phase velocity compared to materials such as lithium niobate (LiNbO3). Scandium alloy aluminum nitride (ScAlN) can exhibit a high piezoelectric coefficient and is considered suitable for surface acoustic wave and bulk acoustic wave (BAW) resonators with high electromechanical coupling.

[0003] However, the development of epitaxial ScAlN BAW resonators has been hampered by issues with film cracking, which is theorized to arise from a combination of stress / strain sources such as differences in coefficient of thermal expansion (CTE), lattice constant mismatch (Δa), and partial substrate removal.

[0004] With reference to FIG. 1 a schematic diagram of a resonator stack is shown, and in FIG. 2 a schematic diagram of the associated crystal lattice structure is shown. A silicon carbide (SiC) substrate layer A' is coated with an aluminum nitride (AlN) layer B', which in turn is coated with a scandium aluminum nitride (ScAlN) layer C'. The schematic shows the additional surface tension F, which is evident from the difference in the crystal lattice constants of layers A', B', C', corresponding to the partial removal of the substrate A'. T This illustrates the existence of SiC AlN ScAlN As can be seen in Figure 2, when grown in a single crystal epitaxial structure, the difference in the crystal lattice constants creates an additional compressive force F C,1 and F C,2 ​​occurs on the upper thin film layers B' and C'. The lattice constant of the substrate A is smaller. When the upper thin film layers B' and C' are formed on the substrate A', they are constrained by the substrate A' below, so an additional compressive force F C,1 and F C,2 acts on the top thin film layers B', C'. The top layers B', C' are compressed during formation due to their epitaxial configuration from growth. If the substrate A' material is removed during processing, the restraining force F C,1 and F C,2 is partially or completely released. The upper thin film layers B' and C' are no longer subjected to the substrate binding force F C,1 and F C,2 Since it is not constrained by the force of the strain, it spreads out. This spread leads to deformation and / or cracking of layers B', C' of the resonator membrane. When the membrane of layers B' and C' is bent, the surface of the upper layer C' is strained in tension and the surface of the lower layer B' is strained in compression. If the strain gradient between the two layers B', C' is sufficient, the membrane will crack.

[0005] What is needed is a process to eliminate the problems caused by strain / stress between film layers. Summary of the Invention

[0006] In accordance with the present disclosure, there is provided a strain-compensated heterostructure comprising: a substrate comprising a silicon carbide material; a first epitaxial layer comprising a single crystal aluminum nitride material formed on a top surface of the substrate; a second epitaxial layer formed on the first epitaxial layer opposite the top surface of the substrate, the second epitaxial layer comprising a single crystal aluminum scandium nitride material; and a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third epitaxial layer comprising a single crystal aluminum nitride material.

[0007] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include wherein the strain-compensated heterostructure further comprises that the strain-compensated heterostructure becomes self-supporting in response to release from the substrate.

[0008] Further embodiments of any of the aforementioned embodiments may additionally and / or alternatively include that the strain-compensated heterostructure further includes that the strain-compensated heterostructure is freestanding in response to the substrate being etched from the first layer.

[0009] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include the first epitaxial layer and the second epitaxial layer including a first interfacial strain, and the third epitaxial layer and the second epitaxial layer including a second interfacial strain equal to the first interfacial strain.

[0010] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include the first interfacial strain being responsive to a difference in lattice constant and / or thermal expansion coefficient values ​​between the first epitaxial layer and the second epitaxial layer.

[0011] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include the second interfacial strain being responsive to a difference in lattice constant and / or thermal expansion coefficient values ​​between the third epitaxial layer and the second epitaxial layer.

[0012] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include that the first epitaxial layer, the second epitaxial layer formed on the first epitaxial layer, and the third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer comprise films that are neutrally stressed in response to release from the substrate.

[0013] In accordance with the present disclosure, there is provided a multilayer structure for a bulk acoustic wave resonator including: a releasable substrate having a top surface; a first epitaxial layer including a single crystal aluminum nitride material formed on the top surface of the substrate; a second epitaxial layer formed on the first epitaxial layer opposite the top surface of the substrate, the second epitaxial layer including a single crystal aluminum scandium nitride material; and a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third epitaxial layer including a single crystal aluminum nitride material.

[0014] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include that the multilayer membrane structure further includes that the membrane structure is freestanding in response to the releasable substrate being etched from the first epitaxial layer.

[0015] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include that the multi-layer film structure includes a film that is neutrally stressed in response to release of the substrate.

[0016] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include the first epitaxial layer and the second epitaxial layer including a first interfacial strain, and the third epitaxial layer and the second epitaxial layer including a second interfacial strain with respect to the second epitaxial layer that is opposite to the first interfacial strain.

[0017] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include the first interfacial strain being equal to the second interfacial strain on either side of the second epitaxial layer.

[0018] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include that the multilayer membrane structure includes a suspended membrane having an equal but opposite strain gradient in response to release of the substrate.

[0019] In accordance with the present disclosure, there is provided a process for fabricating a neutrally stressed released multi-layer structure for a bulk acoustic wave resonator, the process including providing a releasable substrate having a top surface; forming a first epitaxial layer on the top surface of the substrate comprising a single crystal aluminum nitride material; forming a second epitaxial layer on the first epitaxial layer opposite the top surface of the substrate, the second epitaxial layer comprising a single crystal aluminum scandium nitride material; forming a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third epitaxial layer comprising a single crystal aluminum nitride material; and removing the substrate from the first epitaxial layer.

[0020] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include that the substrate comprises a silicon carbide material.

[0021] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include that the process further includes balancing the first interfacial strain with a second interfacial strain.

[0022] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include a first interfacial strain formed between the first epitaxial layer and the second epitaxial layer, and a second interfacial strain formed between the third epitaxial layer and the second epitaxial layer.

[0023] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include where removing the substrate from the first epitaxial layer includes etching the substrate.

[0024] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include where the process further includes mirroring an interface between the first epitaxial layer and the second epitaxial layer and an interface between the third epitaxial layer and the second epitaxial layer.

[0025] Further embodiments of any of the foregoing embodiments may additionally and / or alternatively include a crystal lattice constant of the substrate smaller than a crystal lattice constant of the first epitaxial layer, a crystal lattice constant of the first epitaxial layer smaller than a crystal lattice constant of the second epitaxial layer, and a crystal lattice constant of the third epitaxial layer equal to a crystal lattice constant of the first epitaxial layer.

[0026] Further details of the strain compensated heterostructure and process are set forth in the following detailed description and accompanying drawings, in which like reference numerals refer to like elements. [Brief description of the drawings]

[0027] [Figure 1] FIG. 1 is a schematic diagram of a prior art resonator stack. [Diagram 2] FIG. 1 is a schematic diagram of a related crystal lattice structure. [Diagram 3] FIG. 1 is a schematic diagram of a typical crystal lattice structure. [Figure 4] FIG. 1 is a process diagram for an exemplary process. [Diagram 5] FIG. 1 is a schematic diagram illustrating a typical net stress state. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] 3, there is illustrated a stack 10 including a substrate 12 having a first layer 14 formed on a substrate surface 16. The substrate 12 has a particular lattice constant a SiCThe substrate 12 may have material properties such as: lattice constant a 1 , lattice constant b 2 , lattice constant c 3 , lattice constant d 2 , lattice constant d 3 , lattice constant s 1 , lattice constant s 2 ... AlN or coefficient of thermal expansion CTEα AlN The first layer 14 may have different material properties than the substrate 12, such as a lattice constant a. The first layer 14 may be an aluminum nitride (AlN) material. The first layer 14 may be an epitaxial single crystal film. The first layer 14 may be a nucleation layer. A second layer 18 may be formed on the first layer 14 opposite the substrate 12. The second layer 18 may have a lattice constant a. ScAlN and / or CTEα ScAlN The second layer 18 may have different material properties than the substrate 12 and the first layer 14, such as a lattice constant a . The second layer 18 may be an epitaxial single crystal film. The second layer 18 may be a scandium aluminum nitride (ScAlN) material. A third layer 20 may be formed on the second layer 18 opposite the first layer 14. The third layer 20 may have a lattice constant a AlN and / or CTEα AlN The third layer 20 may have the same material properties as the first layer 14, such as SiO 2 , SiO 2 , SiO 3 , SiO 4 , SiO 5 , SiO 6 , SiO 7 , SiO 8 , SiO 9 , SiO 10 , SiO 11 , SiO 12 , SiO 13 , SiO 14 , SiO 15 , SiO 16 , SiO 17 , SiO 18 , SiO 2 , SiO 2 , SiO 3 , SiO 4 , SiO 2 , SiO 3 , SiO 4 , SiO 3 ... 0.30 Al 0.70 N, and the third layer 20 may include Sc 0.05 Al 0.95 The third layer 20 can contain a thicker ScN composition than when AlN is used. 0.05 Al 0.95 It can contain a layer that is a ScN layer rather than an AlN layer. 0.30 Al 0.70 The goal is to develop Sc 0.30 Al0.70 The goal is to balance the stress on both sides of the second layer 18, which contains N. The compositions and thicknesses in layers 14, 18, and 20 can be configured to compensate for CTE differences based on thermal growth from process temperatures.

[0029] After the first layer 14 and the second layer 18 are formed on the substrate 12, a resultant compressive force acts on the stack 10. As previously mentioned, the mismatch in material properties of the substrate 12, the first layer 14, and the second layer 18 impose a compressive force on the stack 10. In particular, when growing epitaxial single crystal films, the difference in lattice constants and thermal expansion coefficients between the heteroepitaxial materials can result in large interfacial strains. If the difference is small enough and the film is below the relaxation critical thickness, a coherent (or semi-coherent) interface 22 can form, where the crystal structure is distorted at the interface 22 to allow for bond matching. The lattice constant of the first layer 14, which comprises AlN, is smaller than that of the second layer 18, which comprises ScAlN. When the substrate 12 is released from the first layer 14, the AlN / ScAlN bilayer 24 bends or distorts in an attempt to relieve the interfacial strain 25. As the bilayer 24 bends, the upper surface 26 of the second layer 18 is strained under tension, and the lower surface 28 of the AlN of the first layer 14 is strained under compression.

[0030] The third layer 20, after being formed on the second layer 18, has the same material properties as the first layer 14 and exerts an opposing bending force on the second layer 18 opposite the force that the first layer 14 exerts on the second layer 18. If the third layer 20 comprises similar materials but is of a different thickness or has a different concentration of materials in the composition, the third layer 20 may exert an opposing force on the second layer 18 opposite the force that the first layer 14 exerts on the second layer 18. When the substrate 12 is released from the film 30, the bending force exerted by the first layer 12 will be equal to the bending force exerted by the third layer 20, resulting in an equilibrium of strain / stress on the film 30. The unique layering disclosed takes advantage of the epitaxial nature of the growth of the materials in the layers 14, 18, 20 and mirrors the strain on both sides of the second layer 18 of ScAlN, thereby reducing the lattice constant (a AlN and a ScAlN ) and CTE(α AlN and αScAlN ) can be eliminated. An equal but opposite strain gradient on the surface 32 is provided by the third layer 20 to match the ScAlN / AlN interface 22 underneath. Thus, when the substrate 12 is removed, the suspended membrane 30 remains neutrally stressed and does not crack, allowing for further processing and device fabrication. With the improvement of providing an additional third layer 20 on the stack 10 to counterbalance the first layer 14, upon release the resonator membrane 30 does not crack or deform. It is believed that a range of net stress values ​​can be obtained where the residual stress does not cause sufficient deformation to crack the membrane 30.

[0031] Also referring to FIG. 4, a process map is shown. Process 100 generally describes steps used to prevent the resonator membrane 30 from deforming during fabrication. Process 100 includes step 110 of providing a substrate. A next step 112 includes forming a first layer on the substrate. The first layer may be epitaxially grown. In an exemplary embodiment, the first layer may be an AlN material. A next step 114 includes forming a second layer on the first layer. The second layer may be epitaxial. The second layer may be an ScAlN material. A next step 116 includes forming a third layer on the second layer. The third layer may be epitaxial. The third layer may be the same material as the first layer. The third layer may be AlN. The resulting stack results in a first layer and a third layer with an equal but opposite strain gradient relative to the second layer in the stack. A next step includes removing the substrate from the first layer. The substrate can be etched to allow electrical contact with the electrodes. When the substrate is removed, the membranes in the stack are suspended, but the stack does not deform due to the strain gradients accumulated from the epitaxial growth of the first and second layers in the substrate. The third layer provides a balancing force that matches the forces of the first and second layers formed during layer formation.

[0032] Referring also to FIG. 5, a schematic diagram illustrates the relationship of the net stress state between layers 14, 18, 20. The net stress state can be understood as a series of force equilibria between layers 14, 18, 20. The forces on layers 14, 18, 20 can be balanced such that cracks do not occur in membrane 30. The diagram in FIG. 5 illustrates that there are regions where the forces are balanced on either side of a zero value. The diagram shows a region near zero that is less than the critical strain level for cracking (in either the compressive or tensile direction). A membrane with force equilibria in this region near zero is said to be neutrally stressed. The closer the forces can be balanced to zero, the better the processing tolerances and durability of the resonator. The forces do not have to be balanced at exactly zero net force, instead there are several forces that can exist on either side of the zero equilibrium value. There are values ​​beyond equilibrium that will cause cracking. The absolute numbers at equilibrium depend on the combination of material composition and thickness in layers 14, 18, 20. The disclosed process includes the idea of ​​manipulating the blemishes / stresses formed during fabrication to avoid deformation of the film after the substrate is removed. It is contemplated that various materials can be used as the substrate. However, using different substrate materials may change the initial stress state between layers 14, 16, 18 and substrate 12. For example, in the case of a SiC substrate, the epilayers are initially compressed (since SiC has a smaller lattice constant). The opposite occurs in the case of silicon (since silicon has a larger lattice constant). In either case, however, the problem and solution of film stress balance described herein remains the same. The film stack can be used with resonators such as bulk acoustic wave (BAW) resonators.

[0033] Technical advantages of the exemplary disclosure include a neutrally stressed and released membrane for use as a resonator.

[0034] Another technical advantage of the exemplary epitaxial single crystal AlN / ScAlN / AlN films includes the suppression of bending of the released films and the subsequent prevention of cracking.

[0035] Another technical advantage of typical epitaxial single crystal AlN / ScAlN / AlN films includes the introduction of a strain-matching surface layer (third layer) to compensate for the strain present at the interface between the nucleation layer (first layer) and the second layer.

[0036] Another technical advantage of the exemplary processes includes the application of the disclosed processes to other combinations of materials for rare earth III-nitrides (IIIA, IIIB, lanthanides, e.g., AlGaN, InGaN, ScAlN, YAlN, etc.) multilayer structures.

[0037] Another technical advantage of the exemplary process includes the deformation-free suspended membrane that allows for a free-standing device.

[0038] Another technical advantage of the exemplary process includes mirroring the interface between the first epitaxial layer and the second epitaxial layer and the interface between the third epitaxial layer and the second epitaxial layer.

[0039] Another technical advantage of the exemplary process is that it provides that the first interfacial strain is equal in magnitude to the second interfacial strain on either side of the second epitaxial layer.

[0040] Another technical advantage of the exemplary process is that it results in a multi-layer film structure that includes a film that is neutrally stressed in response to the release of the substrate.

[0041] Strain-compensated heterostructures and processes are provided. Although the strain-compensated heterostructures and processes have been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art upon reading the foregoing description. It is therefore intended to cover such alternatives, modifications, and variations that fall within the broad scope of the appended claims.

Claims

1. A strain-compensated heterostructure, comprising: a substrate comprising a silicon carbide material; a first epitaxial layer comprising a single crystal aluminum nitride material formed on an upper surface of the substrate; a second epitaxial layer formed on the first epitaxial layer opposite the top surface of the substrate, the second epitaxial layer comprising a single crystal aluminum scandium nitride material; and a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third epitaxial layer comprising a single crystal aluminum nitride material; and Including, the strain-compensated heterostructure does not deform and crack in response to the substrate being removed; Strain-compensated heterostructures.

2. A strain-compensated heterostructure, comprising: a substrate comprising a silicon carbide material; a first epitaxial layer comprising a single crystal aluminum nitride material formed on an upper surface of the substrate; a second epitaxial layer formed on the first epitaxial layer opposite the top surface of the substrate, the second epitaxial layer comprising a single crystal aluminum scandium nitride material; and a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third epitaxial layer comprising a single crystal aluminum nitride material; and Including, the strain-compensated heterostructure does not deform and crack in response to the substrate being etched from the first epitaxial layer; Strain-compensated heterostructures.

3. 3. The strain-compensated heterostructure of claim 1, wherein the first epitaxial layer and the second epitaxial layer include a first interfacial strain, and the third epitaxial layer and the second epitaxial layer include a second interfacial strain equal to the first interfacial strain.

4. 4. The strain-compensated heterostructure of claim 3, wherein said first interfacial strain is responsive to differences in lattice constant and thermal expansion coefficient values ​​between said first epitaxial layer and said second epitaxial layer.

5. 4. The strain-compensated heterostructure of claim 3, wherein the second interfacial strain is responsive to differences in lattice constant and thermal expansion coefficient values ​​between the third epitaxial layer and the second epitaxial layer.

6. 3. The strain compensated heterostructure of claim 1, wherein the first epitaxial layer, the second epitaxial layer formed on the first epitaxial layer, and the third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer comprise a stress balanced film in response to removal of the substrate.

7. A multilayer structure for a bulk acoustic wave resonator, comprising: a removable substrate having an upper surface; a first epitaxial layer comprising a single crystal aluminum nitride material formed on the top surface of the substrate; a second epitaxial layer formed on the first epitaxial layer opposite the top surface of the substrate, the second epitaxial layer comprising a single crystal aluminum scandium nitride material; and a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third epitaxial layer comprising a single crystal aluminum nitride material; the multi-layer structure does not deform or crack in response to the removable substrate being etched from the first epitaxial layer; Membrane structure.

8. The membrane structure of claim 7 , wherein the multi-layer membrane structure includes a membrane that is stress balanced in response to removal of the substrate.

9. 8. The membrane structure of claim 7, wherein the first epitaxial layer and the second epitaxial layer include a first interfacial strain, and the third epitaxial layer and the second epitaxial layer include a second interfacial strain with the second epitaxial layer that is equal to the first interfacial strain.

10. The membrane structure of claim 9 , wherein the first interfacial strain is equal to the second interfacial strain on either side of the second epitaxial layer.

11. 8. The membrane structure of claim 7, wherein the multi-layer membrane structure includes a membrane having an equal but opposite strain gradient with respect to the second epitaxial layer in response to removal of the substrate.

12. 1. A process for manufacturing a multilayer structure for a bulk acoustic wave resonator, comprising: Providing a removable substrate having an upper surface; forming a first epitaxial layer comprising a single crystal aluminum nitride material on the top surface of the substrate; forming a second epitaxial layer on the first epitaxial layer opposite the top surface of the substrate, the second epitaxial layer comprising a single crystal aluminum scandium nitride material; forming a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third epitaxial layer comprising one of a single crystal aluminum nitride material or a single crystal aluminum scandium nitride material; removing the substrate from the first epitaxial layer.

13. 13. The process of claim 12, wherein the first epitaxial layer and the second epitaxial layer include a first interfacial strain, and the third epitaxial layer and the second epitaxial layer include a second interfacial strain equal to the first interfacial strain.

14. The process of claim 12 , wherein removing the substrate from the first epitaxial layer comprises etching the substrate.

15. The process of claim 12 , wherein the substrate is selected from the group consisting of silicon carbide, silicon, sapphire, and gallium nitride.

16. 16. The process of claim 15, wherein a crystal lattice constant of the substrate is smaller than a crystal lattice constant of the first epitaxial layer, the crystal lattice constant of the first epitaxial layer is smaller than a crystal lattice constant of the second epitaxial layer, and the crystal lattice constant of the third epitaxial layer is equal to the crystal lattice constant of the first epitaxial layer.

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