Overmoded Bulk Acoustic Resonators and Method of Fabricating

Abstract

Disclosed herein is an Overmoded Bulk Acoustic Resonator (OBAR) and a solidly-mounted OBAR (SBAR), which operate in a partially transduced 2nd overtone split between piezoelectric and electrode layers using dual all metal Bragg mirrors. The devices may be deployed in a series configuration. The devices have arbitrarily thick electrodes to minimize ohmic loss and bandwidths high enough to meet filtering requirements of 5G networks. The devices provide sharp filtering which can be performed directly at each antenna element in a form factor much smaller than the half-wavelength separation between adjacent antenna elements required when using electromagnetic resonators.

Claims

1. A process for fabricating an overmoded acoustic resonator comprising: depositing a layer of a piezoelectric material on a first substrate; depositing a first layer of a low acoustic impedance material on a first surface of the piezoelectric layer; forming a first, all-metal Bragg mirror on the first layer of low acoustic impedance material; depositing a first routing layer on the first Bragg mirror; bonding the first routing layer to second substrate using a dielectric material removing the first substrate; depositing a second layer of a low acoustic impedance material on a second surface of the piezoelectric material opposite the first surface; forming a second, all-metal Bragg mirror on the second layer of low acoustic impedance material; and depositing a second routing layer on the second Bragg mirror.

2. The process of claim 1 further comprising: depositing a stiffening layer on the first routing layer; wherein the stiffening layer is bonded to the second substrate instead of the first routing layer.

3. The process of claim 1 wherein the first and second Bragg mirrors are formed with alternating layers of a high acoustic impedance metal and a low acoustic impedance metal.

4. The process of claim 3 wherein the high acoustic impedance metal is Tungsten and the low acoustic impedance metal is Aluminum.

5. The process of claim 1 wherein the first and second layers of a low acoustic impedance material are composed of a material selected from a group consisting of Titanium, Aluminum, Chromium and Indium Tin Oxide.

6. The process of claim 1 wherein the dielectric material is BCB.

7. The process of claim 2 further comprising: milling the second routing layer, the second Bragg mirror and the second layer of a low acoustic impedance material to a level extending into the layer of piezoelectric material to define a top electrode; and milling the first routing layer, the first Bragg mirror and the first layer of a low acoustic impedance material to a level of the stiffening layer to define a bottom electrode.

8. The process of claim 7 further comprising: depositing and patterning a layer of a dielectric material to form a planarization and via layer; depositing and patterning an interconnect layer filling via holes patterned in the dielectric layer.

9. The process of claim 2 wherein the second substrate and the stiffening layer are composed of SiO.sub.2.

10. The process of claim 8 wherein the interconnect layer connects multiple resonators together in series.

11. The process of claim 1 wherein the acoustic resonator is tuned to a specific wavelength, wherein each layer of the first and second Bragg mirrors is approximately ? wavelength in thickness and further wherein the piezoelectric layer is approximately ? wavelength in thickness.

12. A device comprising: first and second structures, each comprising: an active layer of metal; an all-metal Bragg mirror disposed on the active layer of metal; and a routing layer; disposed on the Bragg mirror opposite the active layer; wherein the first and second structures are disposed in an opposing configuration having a layer of a piezoelectric material separating the respective active layers of metal.

13. The device of claim 12 further comprising: a dielectric layer disposed on the routing layer of one of the first or second structures; and a substrate disposed on the dielectric layer opposite the routing layer.

14. The device of claim 12 further comprising: a stiffening layer disposed on the routing layer of one of the first or second structures; a dielectric layer disposed on the stiffening layer; and a substrate disposed on the dielectric layer opposite the stiffening layer.

15. The device of claim 14 wherein other of the first or second structures is milled to define a top electrode and further wherein the one of the first or second structures is milled to define a bottom electrode.

16. The device of claim 15 further comprising: an interconnect layer disposed on the top electrode.

17. The device of claim 16 wherein the interconnect layer connects multiple devices together in series.

18. The device of claim 12 wherein the first and second Bragg mirrors are formed with alternating layers of a high acoustic impedance metal and a low acoustic impedance metal.

19. The device of claim 18 wherein the high acoustic impedance metal is Tungsten and the low acoustic impedance metal is Aluminum.

20. The device of claim 12 wherein the active layers in the first and second structures are composed of a material selected from a group consisting of Titanium, Aluminum, Chromium and Indium Tin Oxide.

21. The device of claim 13 wherein the dielectric material is BCB.

22. The device of claim 12 wherein the device is tuned to a specific wavelength, wherein each layer of the first and second Bragg mirrors is approximately ? wavelength in thickness and further wherein the piezoelectric layer is approximately ? wavelength in thickness.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic representation of the acoustic cavity for a 1D thickness extensional mode showing stress for fundamental, 1st overtone, and 2nd overtone.

[0013] FIG. 2 shows 1D thickness extensional mode showing stress and piezoelectric charge generation for fundamental, 1st overtone, and 2nd overtone modes.

[0014] FIG. 3 shows 1D thickness extensional mode showing stress and piezoelectric charge generation for fundamental, fully transduced 2nd overtone, and partially transduced 2nd overtone modes.

[0015] FIG. 4 shows fundamental and partially transduced 2nd overtone mode for an Aluminum Nitride (AlN) uniform acoustic property stack consisting of an electrically piezoelectric and conductive region, showing stress distribution and electrode to piezoelectric layer ratio (re-?).

[0016] FIG. 5 shows the results of FEA study sweeping electrode to piezoelectric ratio to determine k.sub.t.sup.2. The stack is composed a piezoelectric layer with AlN properties and a conductor layer with the stiffness and density of AlN but instead of piezoelectric properties is an ideal conductor. Note that the optimal ratio of electrode to piezoelectric material allows enhanced k.sub.t.sup.2.

[0017] FIG. 6 shows the results of an FEA study scaling either acoustic impedance (Z) or phase velocity (v.sub.phase) while fixing the other. This is implemented relative with scaling relative to AlN by scaling material density and stiffness. Graph shows optimal electrode to piezoelectric layer ratio (re-?) for maximum k.sub.t.sup.2 vs. relative scaling of Z or v.sub.phase.

[0018] FIG. 7 shows the partially transduced 2nd overtone mode for a stack with AlN piezoelectric layer, inner low acoustic impedance (Z) metal, and outer high Z metal, showing stress distribution and electrode to piezoelectric layer ratio (re-?). Note the low Z Al, layered with high Z W, flattens the modeshape relative to the uniform.

[0019] FIG. 8 is a schematic showing single vs quad series array with 500 characteristic impedance implemented in Lithium Niobate, providing increased device area for quad series array.

[0020] FIG. 9 is a schematic showing acoustic cavity definition for released and solidly mounted resonators.

[0021] FIG. 10 is a schematic showing Bragg mirror model with incident wave and absorbing layer, overlayed with stress distribution.

[0022] FIG. 11 shows steps of the OBAR fabrication process.

[0023] FIG. 12 is an isometric 3D rendering of an OBAR with cutaway cross section views colored by 1 material, 2 local strain, 3 polarization.

[0024] FIG. 13 shows steps of the SBAR fabrication process.

[0025] FIG. 14 is a schematic of an SBAR showing isometric view of an 8X series arrayed device, dual Bragg mirrors encapsulated by thick metal interconnects confining stress, and the SiO.sub.2/BCB/SiO.sub.2 low loss substrate used for layer transfer process.

[0026] FIG. 15 is a flowchart showing the SBAR Fabrication process.

DETAILED DESCRIPTION

[0027] In an acoustic resonator, resonance occurs when a mechanical stimulus is applied at a specific frequency so that it adds constructively to build up a standing wave. The fundamental frequency for an acoustic resonance is determined by the wave's phase velocity (v.sub.phase) and the round trip path length which, in the case of a thickness mode, is 2 times the thickness, as shown in FIG. 1. If the stimulus is applied at higher multiples of the fundamental frequency, overtones are created as shown below. Using Eq. (1), the fundamental mode is referred to as n=0 and the 2nd overtone as n=2. It can be helpful to compare the frequency ratio between the fundamental and target overtone to ensure understanding of the mode.

[00001]$f_n=\left(n+1\right).Math.\frac{1}{2l}{v}_{\mathrm{phase}}$

[0028] Overtones allow a higher resonant frequency in the same size acoustic cavity, making it an attractive approach for scaling to mmWave acoustic resonators. Using a piezoelectric material allows conversion between electrical and mechanical domains with an electromechanical coupling coefficient (k.sub.t.sup.2) dependent on the piezoelectric properties of the material. Exciting an overtone instead of the fundamental mode reduces k.sub.t.sup.2. In a uniform piezoelectric material, only even overtones can be transduced as the charge generated in odd modes fully cancels. For the even modes, k.sub.t.sup.2 reduces quadratically as shown in Eq. (2):

[00002]$\begin{array}{cc}\frac{k_{t_n}^2}{k_{t_{\mathrm{fundamental}}}^2}=\frac{1}{{\left(n+1\right)}^2}\mathrm{for}\mathrm{even}n& \left(2\right)\end{array}$

where n is the overtone number

[0029] This is caused by both increased acoustic load and cancellation of charge from out of phase nodes. For a second overtone in a uniform piezoelectric material, k.sub.t.sup.2 is 1/9 of the fundamental mode, as shown in FIG. 2. This device k.sub.t.sup.2 is too low for many filter applications.

[0030] The OBAR functions in a 2nd overtone, but instead of confining acoustic energy solely to the piezoelectric layer, the mode is split evenly between the piezoelectric layer and each electrode. The piezoelectric layer and each electrode have thicknesses equal to ? the acoustic wavelength (?), causing only the center node to be in the transduced region (piezoelectric layer), leading to a transduced mode shape similar to a fundamental mode. This prevents the typical problem of overtones in which regions of expansion and contraction produce positive and negative polarizations, which cancel net charge, thereby reducing k.sub.t.sup.2. Assuming uniform acoustic properties and layer thickness, k.sub.t.sup.2 is ? of the fundamental mode as shown in FIG. 3.

[0031] This can be further improved to ? of the fundamental mode k.sub.t.sup.2 by optimizing layer thickness and material selection. To understand this impact, it is helpful to evaluate materials based on acoustic impedance (Z) and acoustic phase velocity (v.sub.phase) as defined in Eq. (3)-(4) based on density (?) and Young's modulus (E):

[00003]$\begin{array}{cc}{v}_{\mathrm{phase}}=\sqrt{\frac{E}{?}}& \left(3\right)\end{array}$$\begin{array}{cc}Z=\sqrt{E.Math.?}& \left(4\right)\end{array}$

[0032] To have a second overtone where one node is present in each layer requires equal effective path length which can be determined by layer thickness and the propagation speed (c) through the layer. Therefore, using slower phase velocity materials results in higher thicknesses. k.sub.t.sup.2 is a function of how much charge is captured from the transduced region. Charge density is not uniform, but rather sinusoidal with a maximum in the center. Therefore, not transducing the outermost edges of the mode enhances k.sub.t.sup.2. The mode can be simulated in an infinitely wide plate using 2D COMSOL Finite Element Analysis (FEA) simulations. In this simulation the center piezoelectric layer is AlN and the outer electrode layer modeled as a perfect conductor. Therefore, only the center piezoelectric layer generates and transduces charge. In this simulation, all layers have the same v.sub.phase and Z. By increasing the ratio of electrode to piezoelectric region (re-?), so that only ?s of the center node is transduced. k.sub.t.sup.2 is improved from 2.3%-2.7%. Compare this to the 7.2% k.sub.t.sup.2 of a fundamental mode pure AlN to give relative k.sub.t.sup.2 of 33% and 38% respectively. FIG. 4 shows the difference in mode shape, and the relation between re-? and k.sub.t.sup.2 for a uniform acoustic property stack.

[0033] k.sub.t.sup.2 can be enhanced much further by using a low Z material to decrease acoustic loading from the electrodes. The stiffer and lighter the better, with an unphysical material with 0 Z adding no load and allowing the full k.sub.t.sup.2 of the fundamental mode. Using the same 2D FEA simulations, the p and E of AlN can be scaled to sweep v.sub.phase or Z, related by Eq. (3) and Eq. (4), while keeping the other constant. For each point in these simulations, a secondary sweep of re-? can be run to generate curves similar to FIG. 5 and to determine optimal re-? and k.sub.t.sup.2. It is interesting to note that v.sub.phase has no impact on maximum k.sub.t.sup.2 but impacts the ratio of layer thicknesses needed to reach it. Instead, maximum k.sub.t.sup.2 is determined solely by Z. FIG. 7 shows the results of the simulations with respect to optimal re-? and mode k.sub.t.sup.2.

[0034] The results of all the simulations show that when re-? is optimal, low Z metals such as Al and Ti are the best choices. k.sub.t.sup.2 can be enhanced to 3.9% with A/electrodes and an AlN center.

[0035] This can be taken one step further by replacing the single material electrodes with dual layers consisting of an inner low Z metal, and an outer high Z metal. An outer layer of a high Z metal serves to better confine acoustic energy to the AlN layer as the modeshape is flattened out, causing a better stress distribution within the transduced region. Effectively the same enhancement effect causing k.sub.t.sup.2 enhancement in the fundamental mode as shown in FIG. 7.

[0036] Using an optimized Al/W pair with AlN, k.sub.t.sup.2 can be further increased to 4.9%, ?? of the fundamental mode. However, using Ti/W allows 4.0% k.sub.t.sup.2 and results in a better Q. This is much better than the 2.3%(? fundamental mode) k.sub.t.sup.2 achievable from a uniform property and thickness stack.

[0037] To avoid the need for external matching networks, the characteristic impedance of a filter needs to be matched to the proceeding and following stages, typically at 50?. With respect to an individual resonator, this means that to be matched, the C.sub.0 must have a specific value dependent on the filter center frequency. While some bandpass filter implementations such as ladder filters can use a higher than 1:1 capacitive ratio between shunt and series devices to improve roll off, this ratio does not typically exceed 4, meaning the match capacitance will be within 0.5-2 times the value set by Eq. (5):

[00004]$\begin{array}{cc}{C}_0=\frac{1}{2?.Math.f.Math.Z0.Math.\sqrt{r}}& \left(5\right)\end{array}$ [0038] where: [0039] f is frequency; [0040] Z0 is the characteristic impedance of the system (normally 50?); and [0041] r is the capacitive ratio between shunt and series filter elements.

[0042] For OBARs, C.sub.0 is formed by two parallel plates which have a separation distance determined by resonant frequency and a dielectric permittivity dependent on the piezoelectric material choice. For a 50? match, the separation distance is much smaller than the device area, so the parallel plate approximation in Eq. (6) is reasonable.

[00005]$\begin{array}{cc}{C}_0=\frac{{?}_r.Math.{?}_0.Math.A}{d}& \left(6\right)\end{array}$ [0043] where: [0044] ?.sub.r is relative dielectric permittivity; [0045] ?.sub.0 is vacuum dielectric permittivity; [0046] A is device area; and [0047] d is separation distance.

[0048] Lateral dimensions have an impact on device performance that go beyond the value of C.sub.0. The mode shape has been discussed for infinitely wide cavities. In real devices, lateral dimensions impact mode shape, lowering k.sub.t.sup.2 and lowering Q through anchor loss. While techniques such as frames and etch trenches can limit anchor loss, ultimately there is still a proportionality between loss and the vertical to lateral cavity ratio.

[0049] One way around this is to array multiple resonators in series, to allow the use of larger areas while still not exceeding the capacitance requirements. This often presents challenges for device interconnects and large footprints. Using the solidly mounted approach with thick electrodes minimizes ohmic loss tradeoffs, allowing more than 8 series devices with less than 10% ohmic loss for expected upper limits of k.sub.t.sup.2 and Q. Using a high dielectric permittivity material like LN and operating at high frequency (thin separation distance) allows small footprints even for large series arrays. A 50? matched series array comprised of 4 resonators still occupies less than 250 ?m.sup.2 of area. This has further advantages for power handling, as having multiple resonators in series, as shown in FIG. 8, reduces the electric field across each device.

[0050] Films can only be made so thin, and scaling up resonator frequency requires scaling down film thickness. In devices where the metal electrodes are in the path of the acoustic wave, the electrode material and thickness also play a role in setting center frequency. Adjusting the ratio of electrode to piezoelectric thickness (re-?) can improve this limit by increasing one layer thickness at the cost of the other. At any given frequency, the OBAR has a 2-3.5 times thicker piezoelectric layer and 5-10 times thicker metal electrodes than a fundamental mode device, allowing practical fabrication of devices in the 30-60 GHz range. Because electrode resistance is inversely proportional to thickness, electrical loading is also greatly reduced in OBAR structures.

[0051] In other words, the OBAR allows for the central piezoelectric layer to have the thickness it would in an unmetalized fundamental mode, while possessing metal much thicker than the most metalized thin film bulk acoustic wave resonators.

[0052] Beyond manufacturability, thinner metal layers present a series challenge for ohmic loss. Ohmic loss refers to loss caused by the finite conductivity of the metal electrodes. In brief, ohmic loss (R.sub.S) is the electrical resistance due to the finite conductivity of metal electrodes, and is proportional to the device geometry as expressed by Eq. (7):

[00006]$\begin{array}{cc}{R}_s?t\frac{l}{w}& \left(7\right)\end{array}$ [0053] where: [0054] t is thickness; [0055] l is length; and [0056] w is width.

[0057] The electrical loading (impact on Qs) caused by this resistance is dependent on its ratio to resonator impedance at series resonance. For a resonator matched to a specific Z.sub.0 such as 50?, meeting a target k.sub.t.sup.2 and Q, this series resonance impedance is frequency independent. Because R.sub.m is in this case frequency independent, but R.sub.s is inversely proportional to frequency, ohmic loss scales as 1/f.

[0058] For a resonator to function, an acoustic cavity must be formed with boundaries to reflect elastic waves. To do this, an acoustic impedance mismatch is needed at the cavity edge to reflect energy. The high acoustic impedance mismatch between the solid materials composing a resonator and air is ideal for this and often used to define a resonator's cavity and prevent energy from propagating beyond its boundaries. Energy can instead be confined without an air interface by using the acoustic impedance mismatch between different solid materials. However, this acoustic impedance mismatch is much lower than air, resulting in poor confinement. To improve reflection, Bragg mirrors, also called Bragg reflectors, can be used. Bragg mirrors function using a series of interfaces with mismatched acoustic impedance precisely spaced so that the reflections constructively interfere. A common approach is to alternate ? ? thick layers of high and low Z materials, as seen in the FIG. 9. The better the Z mismatch between the materials, the more reflective the Bragg mirror. The reflectivity of the Bragg Mirror can be increased further by adding more layers.

[0059] Not releasing the devices has benefits. First, fabrication and packaging can often be simplified if the final structure is not a suspended thin film. Additionally, the robustness of resonators with respect to shock is better when they are not composed of a suspended thin film. Finally, power handling in resonators is often limited by the build-up of heat. Suspended thin films devices must sink heat laterally through the thin film, which has much higher thermal resistance than unreleased devices that sink heat straight down. These stability improvements can be taken further by additionally encapsulating the top electrode.

[0060] Frequency scaling of thickness mode devices results in extremely thin metal electrodes, which have high ohmic loss. Most Bragg mirror implementations are not conductive. However, some devices have been made using high and low Z metals to form the Bragg mirror. The disclosed device uses top and bottom all metal Bragg mirrors to allow arbitrarily thick metal electrodes, resulting in minimal ohmic loss. Because the separation distance between interfaces is dependent on metal thickness, and to reflect needs to be ? ?, reflection is not broadband but rather at specific frequencies. For the SBAR, this is matched to the 2nd overtone within the active region. At the fundamental mode frequency (i.e., ?? the 2nd overtone frequency), Bragg mirror spacing is too far from ? ? to be reflective, suppressing this mode. In other words, the fundamental mode at ? device target frequency is now suppressed by being leaked into the substrate, as shown in FIG. 10. This is advantageous for filter implementations as it results in a resonator with a clean broadband response and no unwanted resonant modes.

[0061] The fabrication techniques will now be disclosed in the context of specific, exemplary devices. As would be realized, the invention is not meant to be limited by the specific exemplary embodiments, which are offered only to illustrate the fabrication process. Two embodiments are described, a released OBAR embodiment and a SBAR embodiment.

[0062] The fabrication of the OBAR embodiment will now be disclosed. The exemplary device is a released OBAR functioning at 33 GHz. The device exhibits ?50? matched PtAlNAl OBAR operating at 33 GHz with 1.7% k.sub.t.sup.2 and series resonance Q>100.

[0063] For the thickness dimension, the stack consists of a 70 nm Pt bottom electrode with 10 nm Cr adhesion layer, 140 nm AlN piezoelectric layer, and 90 nm Al top electrode. While a Pt bottom electrode is suboptimal for k.sub.t.sup.2, the process for depositing very thin AlN on Pt layers can be tuned.

[0064] A 3-mask process is used for fabrication as shown in steps (1)-(5) of FIG. 11. In step (1) a 70 nm Pt layer with a 10 nm Cr adhesion layer is blanket DC-sputtered onto a high-resistivity Si wafer to form the bottom electrode. Pattering is then done by ion milling of the Pt and Cr layers with a dielectric mask (e.g., photoresist). This results in a more continuous piezoelectric film, compared to liftoff. In step (2) the piezoelectric material, (e.g., AlN, ScAlN, Ln and many others) is deposited at low power (3 KW) using an AMS sputtering tool. In step (3) the piezoelectric material is wet etched in heated CD26 (TMAH). In step (4) 90 nm of Al is DC sputtered and patterned by liftoff. Finally in step (5) the Si is undercut to release the active region using XeF.sub.2. Note that, because the top and bottom electrodes of this device are routed at 90? instead of 180?, changing aspect ratio increases resistance of one electrode and decreases resistance of the other, making ohmic loss mainly independent of the aspect ratio. For these OBARs this loss is calculated to be <10% of the total loss for the overtone mode. An isometric 3D rendering of the device is shown in FIG. 12.

[0065] The fabrication of the SBAR embodiment will now be disclosed. SBAR uses all of the features previously described to implement an implementable mmWave acoustic resonator. First, the active region of the device functions in a 2nd overtone thickness mode split between the piezoelectric and active metal layers. This approach allows a piezoelectric layer thickness almost the same as an unmetallized fundamental mode. Second, the use of optimized active metal layers consisting of a low acoustic impedance inner layer and higher acoustic impedance outer layer allows a k.sub.t.sup.2 of almost ? the fundamental mode (?4.7% for pure AlN, 15.6% for Y-36? LN). Third, the use of all metal Bragg mirrors to define the primary acoustic cavity allows the electrodes to be arbitrarily thick making ohmic loss minimal even for 50? matched high k.sub.t.sup.2*Q devices, as well as providing full encapsulation. Fourth and finally, using a layer transfer based fabrication process enables design freedom for choice of materials.

[0066] For sputtered films such as AlN or ScAlN this allows optimal selection of seed layer for growth, or in the case of single crystal materials such as Lithium Niobate this allows fabrication of devices starting from commercially available thin films on silicon.

[0067] Fabrication plays a critical role for these devices. For optimal k.sub.t.sup.2 the metal layer contacting the piezoelectric must be low Z. Based on modeling of the quality factor, Ti appears to be the best choice for this layer, although other metals may be used.

[0068] Additionally, roughness of the layers closest to the center piezoelectric has the largest impact on device performance. Instead of attempting to grow high quality AlN on top of a thick metal stack capped by Ti, a layer transfer process is used. This has the added benefit of allowing compatibility with single crystal piezoelectric materials such as Lithium Niobate or Lithium Tantalate which would allow for higher k.sub.t.sup.2.

[0069] Devices are fabricated using a 9-step, 3-mask process shown schematically in FIG. 13. And in flowchart form in FIG. 15. In step 1502 (1), a layer of a piezoelectric material (e.g., a 110 nm layer of AlN, ScAlN, LN, and many others), is deposited on a substrate (e.g., a Si handler die) in one embodiment using a reactive co-sputter tool with a low power recipe optimized for very thin films. Preferable, yet piezoelectric layer will be ?? wavelength in thickness.

[0070] In step 1504 (2), the bottom structure is deposited, comprising an active metal layer, an all-metal Bragg mirror and thick routing layer (preferably 400 nm thick Al), which is sputter deposited on the piezoelectric layer, optionally followed by a SiO.sub.2 (1.6 um) stiffening layer at Step 1506. The stiffening layer is extremely useful for preventing the spread of cracks from any bubbles or other defects in the bonding process. The active metal layer is also a low acoustic impedance metal, preferably Ti, which has lower mechanical loss, but higher electrical resistance than Al. Because more mechanical energy is present at the inner layer, using Ti instead of Al will reduce loss in the device (i.e., will improve Q). In other embodiments, other low acoustic impedance materials (e.g., Al) may be used as the active metal layer.

[0071] The all-metal Bragg mirrors are composed of multiple mirror pairs comprising alternating layers of a low acoustic impedance metal and a high acoustic impedance metal. In a preferred embodiment, the low acoustic impedance material is Al and the high acoustic impedance metal is W. Each mirror pair (low impedance metal layer and high impedance metal layer) is ?? wavelength thick, with each layer being ?? wavelength thick. However, there are other mirror thickness schemes to reflect shear modes. The Bragg mirror may have any number of mirror pairs. Additional reflections from each layer provides better confinement of the acoustic energy, but at a higher fabrication cost.

[0072] In step 1508 (3), the initial Si die with deposited layers is flip chip bonded to a new second substrate (glass, silicon, sapphire, silica, etc.) using a dielectric interface layer. Any dielectric material may be used, however, in certain embodiments, divinylsiloxane-bis-benzocyclobutene (BCB) is used for the interface layer. First, both dies are wet cleaned, undergo O.sub.2 plasma barrel ashing, and are then dehydrated in an oven. Then the second substrate is spin coated with 3.8 ?m BCB (e.g., Cyclotene 3022-46) and after a 3 minute, 85? C. post application bake, bonded to the initial Si substrate using ramped temperature and pressure in a flip chip bonding tool. The BCB should not be baked in atmosphere to ensure that an SiO.sub.2 layer does not form at the surface, and using ramped temperature and pressure allows formation of a good bond layer with minimal bubbles or cracks. Also, using substrate (e.g., a fused silica handler die) larger than our initial Si die 2.5?2.5 cm vs 1.1?1.3 cm and only spinning BCB on the substrate prevents failed contact due to edge bead effects. After the final hard bake, the BCB is resistant to >12 hour exposure to wet solvents such as acetone, IPA, and 1165 heated to 80? C.

[0073] In step 1510 (4), the structure may be flipped for ease of fabrication and the substrate is removed. The Si handler is thinned from 500 to 50 ?m using SF.sub.6/O.sub.2 RIE, cleaned of etch residue, and then the final Si is removed using XeF.sub.2 to prevent surface damage to the AlN film. Any remaining F-based residue from the Si can be removed through a wet clean process consisting of immersion in acetone, IPA, then deionized water.

[0074] In step 1512 (5), the top structure is deposited comprising an active metal layer, all metal Bragg mirror and a thick routing layer (preferably 400 nm thick Al) which is sputter deposited. Note that it is not necessary that the Bragg mirrors in the top structure and bottom structure be identical. The Bragg mirrors may be composed of different materials and/or may have different numbers of mirror pairs.

[0075] In step 1514 (6), ion milling is used to define the top electrode. Etching is stopped in the middle of the piezoelectric layer to prevent any shorting between top and bottom electrode due to redeposition. Additionally, instead of using a fixed 22.5? angle, the first third of the total etch time is milled at a shallow 45? angle and the final two thirds of the time at a sharp 5? angle to prevent redeposition of material in the trench between series top electrodes. In step 1516 (7), a second ion milling step with the same methodology is used to define the bottom electrode, stopping on the SiO.sub.2 stiffening layer.

[0076] In step 1518 (8), a dielectric material, for example, Photoresist, is used as a planarization and via layer by spinning, pattering, and then hard baking to fully cross link. Finally, in step 1520 (9), a thick (1.5 ?m) Al interconnect layer is deposited and patterned using liftoff. This A/layer fills the via holes left in the photoresist, connects devices together for series arrays, and is contacted by probes to allow measurements. Multiple SBAR devices are shown connected in series in schematic and isometric views in FIG. 14.

[0077] The disclosed acoustic resonators have a role to play in filtering at mmWave frequencies. While frequency dependence of intrinsic loss mechanisms mean Q of these devices will not equal that of sub-6 GHz resonators, Q requirements are not as stringent for mmWave applications.

[0078] The OBAR uses a set of key features including a 2nd overtone thickness mode split between the piezoelectric and active metal layers, series arrays of devices, and all metal Bragg mirrors if solidly mounted to allow a practical to manufacture device capable of meeting k.sub.t.sup.2 and Q requirements. k.sub.t.sup.2 can be kept up to ?s of a fundamental mode meaning up ?4.7% for pure AlN, and 15.6% for Y-36? LN. The disclosed OBAR device presents a practical approach to acoustic resonator design for mmWave filtering.

[0079] As would be realized by those of skill in the art, the specific examples discussed herein have been provided only as exemplary embodiments and the invention is not meant to be limited thereby. Modifications and variations are intended to be within the scope of the invention, which is given by the following claims: