Patent Publication Number: US-2022224306-A1

Title: Strain compensated rare earth group iii-nitride heterostructures

Description:
BACKGROUND 
     The present disclosure is directed to strain compensated heterostructures and acoustic wave resonators incorporating same. 
     Surface acoustic wave resonators made of piezoelectric materials are widely used in communication and timing applications. Aluminum nitride (AlN) as the device material has been used due to its potential of high frequency applications due to its high surface phase velocity compared to materials such as lithium niobate (LiNbO 3 ). Scandium alloyed aluminum nitride (ScAlN) can exhibit high piezoelectric coefficients and is considered suitable for high electromechanical coupling surface acoustic wave and bulk acoustic wave (BAW) resonators. 
     However, development of epitaxial ScAlN BAW resonators has been impeded by difficulties with membrane cracking. The cracking is theorized to occur due to a mix of stress/strain sources, such as Coefficient of Thermal Expansion (CTE) differences; lattice parameter mismatch (Aa); and partial substrate removal. 
     Referring to  FIG. 1 , a schematic of a resonator stack and  FIG. 2  a schematic of related crystal lattice structures are shown. The silicon carbide (SiC) substrate layer A′ is coated with the aluminum nitride (AlN) layer B′ which is coated by a scandium aluminum nitride (ScAlN) layer C′. The schematic illustrates the presence of additional surface tensile forces FT that are apparent from the difference in crystal lattice parameter in the layers A′, B′, C′ respective to the partial removal of substrate A′ where a SiC &lt;a AlN &lt;a ScAlN . As seen in  FIG. 2 , when grown in a single crystal epitaxial structure, the differences in crystal lattice parameters results in the additional compressive forces F C,1  and F C,2  on the upper thin film layers B′ and C′. The substrate A has a smaller lattice constant. As the upper thin film layers B′ and C′ are formed on the substrate A′, additional compressive forces F C,1  and F C,2  act on the upper thin film layers B′, C′ as they are confined by the substrate A′ below. The upper layers B′, C′ compress during formation due to epitaxial arrangement from growth. When the substrate A′ material is removed during processing, the confining forces F C,1  and F C,2  are partially or fully released. The upper thin film layers B′, C′ expand, as they are no longer confined by the substrate confining forces F C,1  and F C,2 . The expansion leads to deformation and/or cracking of the layers B′, C′ of the resonator membrane. When the membrane of the layers B′ and C′ are bowed, the upper layer C′ surface becomes strained in tension and the lower layer B′ surface strained in compression. If the strain gradient between the two layers B′, C′ are sufficient, the membrane cracks. 
     What is needed is a process for eliminating the problems created by the strain/stress between the membrane layers. 
     SUMMARY 
     In accordance with the present disclosure, there is provided a strain compensated heterostructure comprising a substrate comprising silicon carbide material; a first epitaxial layer comprising single-crystal aluminum nitride material formed on a top surface of the substrate; a second epitaxial layer formed on the first epitaxial layer opposite said top surface of the substrate, the second epitaxial layer comprising single-crystal scandium aluminum nitride material; and a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third layer comprising single-crystal aluminum nitride material. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the strain compensated heterostructure further comprises the strain compensated heterostructure being free standing responsive to release from the substrate. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the strain compensated heterostructure further comprises the strain compensated heterostructure being free standing responsive to the substrate being etched from the first layer. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include 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. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the first interfacial strain is responsive to a difference in lattice parameter values and/or coefficient of thermal expansion values between the first epitaxial layer and the second epitaxial layer. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the second interfacial strain is responsive to a difference in lattice parameter values and/or coefficient of thermal expansion values between the third epitaxial layer and the second epitaxial layer. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include 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 neutrally stressed membrane responsive to release from the substrate. 
     In accordance with the present disclosure, there is provided a multilayer membrane structure for a bulk acoustic wave resonator comprising a releasable substrate having a top surface; a first epitaxial layer comprising 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 single-crystal scandium aluminum nitride material; and a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third layer comprising single-crystal aluminum nitride material. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the membrane structure further comprising the multilayer membrane structure being free standing responsive to the releasable substrate being etched from the first epitaxial layer. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the multilayer membrane structure comprise a neutrally stressed membrane responsive to release of the substrate. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the first epitaxial layer and the second epitaxial layer comprise a first interfacial strain; and the third epitaxial layer and the second epitaxial layer comprise a second interfacial strain opposite the first interfacial strain with respect to said second epitaxial layer. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the first interfacial strain equals the second interfacial strain on opposite sides of the second epitaxial layer. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the multilayer membrane structure comprise a suspended membrane having an equal but opposite strain gradient responsive to release of the substrate. 
     In accordance with the present disclosure, there is provided a process for making a neutrally stressed released multilayer membrane structure for a bulk acoustic wave resonator comprising providing a releasable substrate having a top surface; forming a first epitaxial layer comprising 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 single-crystal scandium aluminum nitride material; forming a third epitaxial layer formed on the second epitaxial layer opposite the first epitaxial layer, the third layer comprising single-crystal aluminum nitride material; and removing the substrate from the first epitaxial layer. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the substrate comprises silicon carbide material. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the process further comprising balancing a first interfacial strain with a second interfacial strain. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the first interfacial strain is formed between the first epitaxial layer and the second epitaxial layer; and the second interfacial strain is formed between the third epitaxial layer and the second epitaxial layer. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include removing the substrate from the first epitaxial layer comprises etching the substrate. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the process further comprising 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. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include a crystal lattice parameter of the substrate is less than a crystal lattice parameter of the first epitaxial layer and the crystal lattice parameter of the first epitaxial layer is less than a crystal lattice parameter of the second epitaxial layer; and a crystal lattice parameter of the third epitaxial layer is equal to the crystal lattice parameter of the first epitaxial layer. 
     Other details of the strain compensated heterostructures and process are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a prior art resonator stack. 
         FIG. 2  is a schematic of related crystal lattice structures. 
         FIG. 3  is a schematic of an exemplary crystal lattice structure. 
         FIG. 4  is a process diagram for the exemplary process. 
         FIG. 5  is a schematic diagram illustrating the exemplary net stress state. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 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  can possess material characteristics, such as a particular lattice constant a SiC . The substrate  12  can include a silicon carbide (SiC) material, silicon, sapphire, GaN, or other suitable epitaxial template substrate upon which the first layer  14  can be nucleated. The substrate  12  can be single crystal material. The first layer  14  can possess material characteristics different from the substrate  12 , such as a lattice constant a AlN , or a coefficient of thermal expansion CTE α AlN . The first layer  14  can be an aluminum nitride (AlN) material. The first layer  14  can be an epitaxial single-crystal film. The first layer  14  can be a nucleation layer. A second layer  18  can be formed on the first layer  14  opposite the substrate  12 . The second layer  18  can possess material characteristics that are different from the substrate  12  and the first layer  14 , such as a lattice constant of a ScAlN  and/or CTE α ScAlN . The second layer  18  can be an epitaxial single-crystal film. The second layer  18  can be a scandium aluminum nitride (ScAlN) material. A third layer  20  can be formed on the second layer  18  opposite the first layer  14 . The third layer  20  can possess the same material characteristics of the first layer  14 , such as a lattice constant a AlN  and/or CTE α AlN . The third layer  20  can be an epitaxial single-crystal film. The third layer  20  can be an aluminum nitride (AlN) material. In an exemplary embodiment, the third layer  20  can be a scandium aluminum nitride (ScAlN) composition that includes a composition with a lower scandium concentration than the second layer. For example, the second layer  18  can include a composition of Sc 0.30 Al 0.70 N and the third layer  20  can be a composition of Sc 0.05 Al 0.95 N. The third layer  20  can include a layer that is a thicker Sc 0.05 A 10.95 N layer than if AlN was used as it has a closer lattice match to Sc 0.30 Al 0.70 N than AlN. The goal is to balance the stress on either side of the second layer  18  that comprises Sc 0.30 Al 0.7 0N. The compositions and thicknesses in the layers  14 ,  18  and  20  can be configured to also correct for the CTE differences based on thermal growth from process temperatures. 
     After the first layer  14  and the second layer  18  are formed on the substrate  12 , a resultant compressive force will act on the stack  10 . As explained above, the mismatch of the material characteristics of the substrate  12 , the first layer  14  and the second layer  18  will impart compressive forces on the stack  10 . Particularly, when growing epitaxial single-crystal films, large interfacial strains can occur due to the difference in lattice parameter and coefficient of thermal expansion between the heteroepitaxial materials. If the difference is small enough and if the films are 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 first layer  14  comprising AlN has a smaller lattice parameter than the second layer  18  comprising ScAlN. When the substrate  12  is released from the first layer  14 , the AlN/ScAlN bilayer  24  bows or distorts in an attempt to relieve the interfacial strain  25 . When the bilayer  24  is bowed, the second layer  18  upper surface  26  becomes strained in tension and the lower surface  28  of the first layer  14  AlN becomes strained in compression. 
     The third layer  20 , after being formed on the second layer  18 , having the same material properties as the first layer  14 , will impart counter bowing forces on the second layer  18  opposite to the forces imparted by the first layer  14  on the second layer  18 . If the third layer  20  includes a similar material but has a different thickness or a different concentration of materials in the composition, the third layer  20  can impart counter forces on the second layer  18  opposite to the forces imparted by the first layer  14  on the second layer  18 . When the substrate  12  is released from the membrane  30 , the bowing forces imparted by the first layer  12  will equal the bowing forces imparted by the third layer  20 , resulting in a strain/stress balance on the membrane  30 . The unique layering disclosed leverages the epitaxial nature of the growth of the materials in the layers  14 ,  18 ,  20 , and by mirroring the strain on either side of the second layer  18  of ScAlN, the impact of the difference between lattice parameter (a AlN  and a ScAlN ) as well as CTE (α AlN  and α ScAlN ) can be eliminated. An equal but opposite strain gradient on the surface  32  is imparted by the third layer  20  in order to match the lower ScAlN/AlN interface  22 . Therefore, when the substrate  12  is removed, the suspended membrane  30  remains neutrally stressed and does not crack, enabling further processing and device fabrication. With the improvement of the additional third layer  20  on the stack  10  to counterbalance the first layer  14 , upon release, the resonator membrane  30  will not crack or deform. It is contemplated that a range of net stress values can be obtained, where the residual stress will not cause sufficient deformation to crack the membrane  30 . 
     Referring also to  FIG. 4  a process map is shown. The process  100  generally describes the steps used to prevent the resonator membrane  30  from deforming during production. The process  100  includes the step  110  of providing a substrate. The next step  112  includes forming a first layer on the substrate. The first layer can be epitaxially grown. In an exemplary embodiment, the first layer can be AlN material. The next step  114  includes forming a second layer on the first layer. The second layer can be epitaxial. The second layer can be a ScAlN material. The next step  116  includes forming a third layer on the second layer. The third layer can be epitaxial. The third layer can be the same material as the first layer. The third layer can be AlN. The resultant stack results in the first layer and the third layer having equal but opposite strain gradient with respect to the second layer in the stack. The next step includes removing the substrate from the first layer. The substrate can be etched to allow for electrical contact with electrodes. With the substrate removed, the membrane in the stack is suspended, however the stack will not deform due to strain gradient built up from the epitaxial growth of the first and second layers in the substrate. The third layer provides a balancing force to match the first and second layer forces formed during layer formation. 
     Referring also to  FIG. 5 , a schematic diagram illustrates the net stress state relationship between the layers  14 ,  18 ,  20 . The net stress state can be understood as a set of force balances between the layers  14 ,  18 ,  20 . The forces on the layers  14 ,  18 ,  20  can be balanced such that there is no cracking in the membrane  30 . The diagram at  FIG. 5  illustrates that there is a region of force balancing on either side of a value of zero. The diagram shows a region near zero which is less than the critical strain level for cracking (in either the compressive or tensile direction). Membranes with force balances within this region near zero are termed neutrally stressed. The closer to zero the forces can be balanced, the more processing tolerance and resonator durability will result. The forces do not have to balance at exactly zero net force, instead there are some forces that can exist on either side of the zero balanced value. There will be a value beyond the balance that results in cracking. The absolute numbers in the balance will depend on the combination of material compositions and thicknesses in the layers  14 ,  18 ,  20 . The disclosed process includes the concept of manipulating the stain/stress formed during production to avoid the deformation of the membrane after removal of the substrate. It is contemplated that a variety of materials can be utilized as substrates. However utilizing different substrate materials, may change the initial stress conditions between the layers  14 ,  16 ,  18  and the substrate  12 . For instance for SiC substrate, there will be an initial compressive force on the epi-layers (as SiC has a smaller lattice parameter). The opposite would happen for Silicon (as it has a larger lattice parameter). However, in either case, the membrane stress balance problem and solution described herein remains unchanged. The membrane stack can be utilized with a resonator, such as a bulk acoustic wave (BAW) resonator. 
     A technical advantage of the exemplary disclosure includes a neutrally stressed released membrane for use as a resonator. 
     Another technical advantage of the exemplary epitaxial single crystal AlN/ScAlN/AlN membrane includes inhibition of the released membrane from bowing and subsequently prevents cracking. 
     Another technical advantage of the exemplary epitaxial single crystal AlN/ScAlN/AlN membrane includes 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. 
     Another technical advantage of the exemplary process includes applying the disclosed process to other combinations of materials with respect to Rare-Earth III-Nitride (IIIA, IIIB, Lanthanides, for example: AlGaN, InGaN, ScAlN, YAlN, and the like) multilayer membrane structures. 
     Another technical advantage of the exemplary process includes a suspended membrane free of deformation allowing a free standing device. 
     Another technical advantage of the exemplary process 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. 
     Another technical advantage of the exemplary process results in the first interfacial strain equaling the second interfacial strain in magnitude on opposite sides of the second epitaxial layer. 
     Another technical advantage of the exemplary process results in the multilayer membrane structure comprising a neutrally stressed membrane responsive to release of the substrate. 
     There has been provided a strain compensated heterostructures and process. While the strain compensated heterostructures and process has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.