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Cavity spanning bottom electrode of a substrate-mounted bulk wave acoustic resonator
| Details |
Inventors: Ruby, Richard C.;
Assignee: Agilent Technologies, Inc. (Palo Alto, CA)
Primary Examiner: Pascal; Robert
Assistant Examiner: Takaoka; Dean
Attorney, Agent or Firm:
A filter formed of acoustic resonators, where each resonator has its own cavity and a bottom electrode that spans the entirety of the cavity, so that the bottom electrode has an unsupported interior region surrounded by supported peripheral regions. In the preferred embodiment, the cavity is formed by etching a depression into the substrate, filling the depression with a sacrificial material, depositing the piezoelectric and electrode layers that define an FBAR or SBAR, and then removing the sacrificial material from the depression. Also in the preferred embodiment, the sacrificial material is removed via release holes that are limited to the periphery of the depression. Preferably, the bottom electrode is the only electrode that spans the cavity, thereby limiting the formation of parasitic FBARs or SBARs. In one embodiment, the bottom electrode includes a serpentine edge that leaves a portion of one side of the cavity free of overlap by the bottom electrode, so that a top electrode may overlap this portion. Thus, the top and bottom electrodes can overlap the same side without sandwiching the piezoelectric layer outside of the unsupported interior region. |
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DETAILED DESCRIPTION A filter is formed using robust and high Q acoustic resonators, where each resonator has its own cavity and includes a bottom electrode which extends across substantially the entirety of an open area that is formed within a supporting substrate.
The bottom electrode contacts all sides of the open area, so that it includes an unsupported interior region within supported peripheral regions.
The bottom electrode is one layer of a layer stack that includes at least one piezoelectric layer and at least one other electrode layer.
This results in a significant improvement relative to acoustic resonators in which the bottom electrode has at least one edge within the unsupported region of the layer stack.
In one embodiment of the invention, the open area is formed within the substrate by etching a cavity from the surface on which the acoustic resonator layer stack is to be deposited.
The cavity extends only partially through the substrate, which may be a silicon wafer.
The cavity is then filled with a sacrificial material which is subsequently removed.
The bottom electrode is preferably formed directly atop the sacrificial material, but an intermediate layer may be included.
The bottom electrode extends beyond the sacrificial material at all sides of the cavity.
If the cavity is formed by an anisotropic etching step, the cavity will likely have at least three sides.
Thus, the bottom electrode will have a supported interior region and three or more substrate-supported peripheral regions.
The piezoelectric layer is formed atop the bottom electrode.
Preferably, the piezoelectric layer will extend beyond all edges of the bottom electrode.
The piezoelectric material (e.
g.
, AlN) is a columnar material that readily steps down at the edges of the bottom electrode.
This characteristic of the piezoelectric material causes a reduction in performance when an edge of the bottom electrode terminates within the cavity region, as in the conventional approach.
Using this conventional approach of having an unsupported step down, the distorted and poorly formed columnar piezoelectric membrane at the unsupported step edge will adversely affect electrical performance
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Active Solid-state 
