PIEZOELECTRIC MATERIALS, DEVICES AND METHODS OF FABRICATING SAID DEVICES

Abstract

Disclosed herein are aluminum nitride (AlN) piezoelectric materials, piezoelectric devices and related methods of fabricating said devices. The piezoelectric materials comprise a doping element that enhances the piezoelectric properties of the material and a stiffening element, which enhances the mechanical properties of the piezoelectric material. The incorporation of an enhancing and stiffening element to binary alloys of AlN, results in a quaternary AlN alloy, which reduces current trade-offs between the piezoelectric tensor component (e.sub.33), and stiffness of the material (C.sub.33).

Claims

1. A piezoelectric device, comprising: a substrate material; a piezoelectric layer comprising Al.sub.1-x-yT.sub.xM.sub.yN, wherein 0<x+y<0.5, wherein T is selected from Sc, Cr, Y, and Yb and M is selected from B, In, and Ga; a first electrode layer; and a second electrode layer.

2. The piezoelectric device of claim 1, wherein the piezoelectric layer comprises a wurtzite crystal structure, wherein x<0.5 and y<0.5.

3. The piezoelectric device of claim 1, wherein an atomic content of T ranges from 10-50% and an atomic content of M ranges from 1-25%, and a maximum atomic content of T and M combined is 50% or less.

4. The piezoelectric device of claim 1, wherein x and y are equal.

5. The piezoelectric device of claim 1, wherein the piezoelectric layer has an electromechanical coupling constant, k.sub.t.sup.2, wherein k.sub.t.sup.2 is about 0.1-0.8.

6. The piezoelectric device of claim 1, wherein the piezoelectric layer has a $\sqrt{\frac{C_{33}}{}}$ value of about 7,000-10,000 m/s.

7. The piezoelectric device of claim 1, wherein the piezoelectric layer comprises a stiffness coefficient, C.sub.33, of about 100-400 GPa.

8. The piezoelectric device of claim 1, wherein the piezoelectric layer comprises Al.sub.1-x-yY.sub.xB.sub.yN and/or Al.sub.1-x-yCr.sub.xB.sub.yN and/or Al.sub.1-x-ySc.sub.xB.sub.yN.

9. The piezoelectric device of claim 1, wherein the piezoelectric device is a film bulk acoustic resonator (FBAR) device.

10. The piezoelectric device of claim 1, wherein the piezoelectric layer has a thickness of about 50-2000 nm.

11. The piezoelectric device of claim 1, further comprising a Bragg reflector structure.

12. The piezoelectric device of claim 1, wherein the device is a membrane FBAR, an air gap FBAR, or a solidly mounted resonator (SMR).

13. A method of fabricating a doped piezoelectric device comprising: providing a substrate; and depositing a doped piezoelectric material; wherein the doped piezoelectric material comprises Al.sub.1-x-yT.sub.xM.sub.yN, wherein 0<x+y<0.5, T is selected from Sc, Cr, Y, and Yb and M is selected from B, In, and Ga.

14. The method of claim 13, wherein the doped piezoelectric material is deposited by sputter deposition process, molecular beam epitaxy (MBE), chemical vapor deposition (CVD) or metal-organic chemical vapor deposition (MOCVD).

15. The method of claim 14, wherein the sputter deposition process comprises single target sputtering or multi-target sputtering.

16. The method of claim 13, wherein deposition of the piezoelectric layer results in a piezoelectric layer having an atomic content of T from 10-50% and an atomic content of M of 1-25%, and a maximum atomic content of T and M combined of 50% or less.

17. The method of claim 13, wherein the piezoelectric layer is deposited at a thickness of 50-2000 nm.

18. The method of claim 13, wherein the piezoelectric layer comprises Al.sub.1-x-yY.sub.xB.sub.yN and/or Al.sub.1-x-yCr.sub.xB.sub.yN and/or Al.sub.1-x-ySc.sub.xB.sub.yN.

19. The method of claim 13, wherein the piezoelectric device is a film bulk acoustic resonator (FBAR) device.

20. An electrical filter comprising: a film bulk acoustic resonator (FBAR) device, which comprises: a piezoelectric layer comprising Al.sub.1-x-yT.sub.xM.sub.yN, wherein 0<x+y<0.5, wherein T is selected from Sc, Cr, Y, and Yb and M is selected from B, In, and Ga.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 depicts a cross-sectional view of an embodiment of a piezoelectric device having a piezoelectric layer deposited thereon, in accordance with embodiments disclosed herein.

[0028] FIG. 2 depicts a cross-sectional view of an embodiment of a piezoelectric device having a piezoelectric layer deposited thereon, in accordance with embodiments disclosed herein.

[0029] FIG. 3 depicts a cross-sectional view of a piezoelectric device having a piezoelectric layer deposited thereon, in accordance with embodiments disclosed herein.

[0030] FIG. 4 shows graphical results of relation between dopant addition x, y, to piezoelectric material AlN and the resulting k.sub.t.sup.2 and v.sub.s values.

DETAILED DESCRIPTION

[0031] Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative bases for teaching one skilled in the art to variously employ the embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical application. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.

[0032] Disclosed herein are aluminum nitride (AlN) piezoelectric materials, piezoelectric devices and related methods of fabricating said devices. The piezoelectric materials comprise a doping element that enhances the piezoelectric properties of the material and a stiffening element, which enhances the mechanical properties of the piezoelectric material. The incorporation of an enhancing and stiffening element to binary alloys of AlN, results in a quaternary AlN alloy, which reduces current trade-offs between the piezoelectric tensor component (e.sub.33), and stiffness of the material (C.sub.33).

[0033] In one embodiment, a piezoelectric device is disclosed. The piezoelectric device can be a bulk acoustic wave (BAW) resonator. The BAW resonator can be a film bulk acoustic resonator (FBAR). Shown in FIGS. 1-3 are various embodiments of an FBAR piezoelectric device 10, comprising a substrate 100, a piezoelectric layer 300, a first electrode layer 200A and a second electrode layer 200B.

[0034] The piezoelectric layer 300 comprises a quaternary alloy, Al.sub.1-x-yT.sub.xM.sub.yN, wherein 0<x+y<0.5. The element T can be selected from at least one of Sc, Cr, Y, and Yb and the element M can be selected from at least one of B, In, and Ga. In this piezoelectric layer 300, T is incorporated as the piezoelectric enhancing dopant (or alloy), and M is incorporated for purposes of increasing the stiffness and mechanical properties of the alloyed piezoelectric material.

[0035] In one embodiment, the piezoelectric material has a wurtzite crystal structure, wherein x<0.5 and y<0.5. In other embodiments, the combination of x and y totals 0.5 or less. 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or less. In other words, the combination of the enhancing dopant and the stiffening dopant (M and T combined) in the AlN alloy of layer 300, has an atomic percent of 50% or less, 40% or less, 20% or less, or 10% or less. In one embodiment, the atomic content of the enhancing dopant, M, can be 1-50%, or 1-45%, or 1-35%, or 1-30%, or 1-25%, or 1-20%. In further embodiments, the enhancing dopant M is incorporated in an atomic content of 10-50%, or 20-50%, or 30-50%, or 40-50%. The atomic content of the stiffening dopant, T, is 1-40%, or 1-35%, or 1-30%, or 1-25%.

[0036] In one embodiment, the piezoelectric layer material incorporates Al.sub.1-x-yT.sub.xM.sub.yN, where x and y are equal, resulting in Al.sub.1-2xT.sub.xM.sub.xN.

[0037] In certain embodiments, the piezoelectric layer comprises Al.sub.1-x-yY.sub.xB.sub.yN and/or Al.sub.1-x-yCr.sub.xB.sub.yN and/or Al.sub.1-x-ySc.sub.xB.sub.yN.

[0038] In one embodiment, the piezoelectric material of layer 300 has an electromechanical coupling constant, k.sub.t.sup.2, where k.sub.t.sup.2 is about 0.1-0.8., and a

[00004]$\sqrt{\frac{C_{33}}{}}$

value of about 7,000-10,000 m/s, and a stiffness coefficient, C.sub.33, of about 100-400 GPa. In further embodiments, the k.sub.t.sup.2 value is about 0.2-0.7, or 0.3-0.6, or 0.4-0.5, or 0.2-0.8, or 0.3-0.8, or 0.4-0.8, or 0.5-0.8, or 0.6-0.8, or 0.7-0.8, or any value or range therebetween. In other embodiments, the

[00005]$\sqrt{\frac{C_{33}}{}}$

value is about 7,500-10,000, or 8,000-10,000, or 8,500-10,000, or 9,000-10,000, or 7, 000-9,500, or 7,500-9,000, or 8,000-8,500, or any value or range therebetween. In further embodiments, the stiffness coefficient, C.sub.33, has a value of about 150-350 GPa, or 200-300 GPa, or any value or range therebetween.

[0039] Piezoelectric layer 300 has a thickness of about 50-2000 nm, or 100-2000 nm, or 200-2000 nm, or 300-2000 nm, or 400-2000 nm, or 500-2000 nm, or 600-2000 nm, or 700-2000 nm, or 800-2000 nm, or 900-2000 nm, or 50-1000 nm, or 100-1000 nm, or 200-1000 nm, or 300-1000 nm, or 400-1000 nm, or 500-1000 nm, or 600-1000 nm, or 700-1000 nm, or 800-1000 nm, or 900-1000 nm. In certain embodiments, the piezoelectric layer 300 has a thickness of about 50-500 nm, or 100-400 nm, or 200-300 nm, or any value or range therebetween.

[0040] The substrate layer 100 can be a ceramic material such as alumina, sapphire, glass, single-crystalline or polycrystalline aluminum nitride, gallium nitride, silicon carbide or a silicon, Si (100) or Si (111) substrate. Silicon wafers are the most common substrate for BAW devices due to their scalability towards mass manufacturing and compatibility with various manufacturing process steps.

[0041] In certain embodiments, portions of the substrate layer 100 can be removed. such as the configuration shown in FIG. 1. This removal step and geometry will depend on the type of piezoelectric device being fabricated. The depicted embodiment of FIG. 1 is referred to as a membrane FBAR resonator. In other embodiments, the device is an air gap FBAR, such as the device depicted in FIG. 3, which incorporates an air gap 150 between the substrate and the optional passivation layer 400 or the electrode layer 200A. In one embodiment, the piezoelectric device is a solid mounted resonator (SMR), having an acoustic mirror (Bragg reflector) 500, as shown in FIG. 2.

[0042] As can be seen in FIGS. 1 and 3, in certain embodiments a passivation layer 400 is deposited on a top surface of the substrate material 100. Additionally, a seed layer (not shown) is deposited onto electrode 200A, also sometimes referred to as a nucleation layer or a buffer layer. In one embodiment, the deposition of seed layer is an optional step. The purpose of this layer is to provide improved crystal growth and enhance crystal orientation for deposition of the functional piezoelectric layer. It is to be understood that the seed layer deposition, while referenced as a single step for purposes of simplicity and brevity, can include multiple seed layers deposited in succession. In one embodiment, the seed layer comprises AlN and is deposited at a film thickness of about 10-50 nm. The seed layer is deposited on the substrate via molecular beam epitaxy, chemical vapor deposition (CVD), pulsed laser deposition, reactive sputtering, or other appropriate methods that are known to those skilled in the art. In one example, an AlN seed layer can be deposited on a Si substrate, using an Al target in a sputtering chamber, at a temperature of 350 C. Base pressure of the sputtering chamber during deposition can be about 210.sup.3 mbar. Gas flows introduced during sputtering can be for example, 10 sccm of Ar and 40 sccm of N.sub.2.

[0043] As shown in FIG. 4, k.sub.t.sup.2 coefficient values and

[00006]$\sqrt{\frac{C_{33}}{}}$

(km/s) values are graphed for different doping level percent variations of the proposed quaternary AlN alloys. The piezoelectric tensor and electromechanical coupling constant are computed with first principles calculations. The structural parameters are computed by density functional theory (DFT) as implemented in the Vienna ab-initio Simulation Package (VASP), using the standard set of Perdew-Burke-Ernzerhof (PBE) functional and plane augmented wave (PAW) pseudopotentials. In a case of multiple possible structures for a given composition, low-energy structures are selected with the assistance of an alloy cluster expansion. In the event multiple structures have similar energies of formation at a given composition, they are each plotted on the graph. The piezoelectric properties are computed with the density functional perturbation theory (DFPT) natively implemented in VASP. Since Cr-doped AlN is magnetic, to correctly reproduce the magnetic state, the piezoelectric tensor is calculated with the HSE hybrid functional.

[0044] The values depicted in FIG. 4 show a randomized pattern, with no clear observable trend lines. This suggest that the choice of dopant and doping level compositions are not predictable or obvious. This is partially due to the key values of k.sub.t.sup.2 and v.sub.s being nonlinearly dependent on the underlying physical stress, dielectric, and piezoelectric tensors. Therefore, it may be expected that even if the underlying physics were to change linearly with doping, the KPI of interest, to wit k.sub.t.sup.2 and v.sub.s, are much more difficult to predict.

[0045] Also disclosed are methods for fabricating a doped piezoelectric device, such as those shown in the embodiments of FIGS. 1-4. The method comprises the steps of providing a substrate and depositing a doped piezoelectric material on the substrate. The doped piezoelectric material comprises Al.sub.1-x-yT.sub.xM.sub.yN, wherein 0<x+y<0.5, T is selected from Sc, Cr, Y, and Yb and M is selected from B, In, and Ga.

[0046] The doped piezoelectric material disclosed above can be deposited by sputter deposition, molecular beam epitaxy (MBE), or metalorganic chemical vapor deposition (MOCVD). In one embodiment, the sputter deposition process comprises single target sputtering or multi-target sputtering. In embodiments, the deposition of the piezoelectric layer results in a piezoelectric layer having an atomic content of T from 20-50% and an atomic content of M of 1-25%, and a maximum atomic content of T and M combined of 50% or less.

[0047] The piezoelectric layer can be deposited at a variety of thickness, including the in the range of about 50-2000 nm, or any range or value therebetween.

[0048] The substrate layer can comprise a material such Si (100) with a thickness of about 200 m to 1.5 mm, or 380 m to 725 m, or 675 m or 725 m and a diameter in the area of 100 mm-300 mm, or 150 mm-200 mm, or 300 mm, or 100 mm. Optionally, an electrically insulating passivating layer can be deposited onto substrate 100. This layer can be comprised of, e.g., SiO.sub.2 or SiN. An electrode layer 200A is then deposited. The electrode layer comprises, e.g., Mo, W, Al, or doped Si, and is deposited at thickness ranging from 0.05 to 0.5 um.

[0049] An optional seed layer, or buffer layer can be deposited on or below electrode 200A, prior to the deposition of the piezoelectric layer 300. The seed layer comprises AlN or doped AlN with graded composition layers, at total thickness of about 10-50 nm. Graded AlN seed, buffer or nucleation layers are known to those skilled in the art and will not be discussed in further details here. Their purpose is generally to relax strain or lattice mismatch and enhance crystal orientation for the main functional piezoelectric layer 300, deposited thereafter.

[0050] The deposition step of the piezoelectric layer 300, at a thickness of 50-2000 nm, can be deposited through deposition techniques including chemical vapor deposition MOCVD, molecular beam epitaxy (MBE), or sputter deposition.

[0051] In one embodiment, the deposition method is sputtering, including multi-target or single-target sputtering. In one embodiment, where multi-target sputter deposition is utilized, multiple separate targets are used, such as AlSc and GaN targets, in the sputtering system. The frequencies and pattern of the pulses can be used to achieve the desired crystal quality and composition (e.g., by using a higher sputtering power on one target as compared to the other). In another embodiment, all materials are combined into a single target, such as melting AlScGa or AlScGaN.sub.1-y into a single alloy. The target's composition may not be identical to the final composition, due to the different mobility and sputtering yields of each element within the sputtering chamber, which yield loss or accumulation of atoms in the thin films as compared to the target. These effects can be adjusted for by increasing or lowering the corresponding atoms in the target. The piezoelectric film growth is monitored for a specific desired composition, using in-situ techniques or post-deposition analysis to ensure the desired target composition and properties. Similarly, deposition parameters (power, pressure, N2 or Ar partial pressures, wafer temperature, DC-bias, distance target to wafer, etc.) are adjusted as needed to achieve the desired film characteristics.

[0052] Once the piezoelectric layer 300 is deposited, the second electrode layer 200B can then be deposited thereon, also by sputtering or other deposition techniques well known in the art. An optional annealing step can further be conducted to improve crystal quality, optionally with an applied AC or DC electric field.

[0053] Also disclosed is an electrical filter comprising the film bulk acoustic resonator (FBAR) device described in the foregoing embodiments. In one embodiment, the electrical filter comprises a piezoelectric layer of Al.sub.1-x-yT.sub.xM.sub.yN, wherein 0<x+y<0.5, wherein T is selected from Sc, Cr, Y, and Yb and M is selected from B, In, and Ga. It is to be understood that the piezoelectric devices disclosed in all prior embodiments can be incorporated in various embodiments related to an electrical filter, which comprises said devices. The previously described attributes of the piezoelectric devices disclosed are incorporated herein and not reiterated for purposes of brevity.

[0054] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, to the extent any embodiments are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics, these embodiments are not outside the scope of the disclosure and can be desirable for particular applications.