FBAR Recessed Frame Structure for Spurious Signal Suppression
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Solution Overview
Problem
Acoustic wave devices, particularly bulk acoustic wave (BAW) resonators, face issues with spurious signals due to transverse acoustic waves generated by compression and relaxation of the piezoelectric material, which degrade the frequency response and are difficult to control during manufacturing.
Innovation Solution
A film bulk acoustic wave resonator (FBAR) design with recessed and raised frame regions having different concentrations of defects and electrode thicknesses, creating acoustic velocity discontinuities to prevent transverse waves from entering the central active domain, and a method of forming FBARs involving seed layers and over-etching to control defect distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If transverse acoustic waves are allowed to propagate through the piezoelectric film, then the device structure remains simple, but spurious signals are generated that degrade frequency response
Solution Approach 1:
The piezoelectric film is segmented into distinct regions: a central active region with high-quality piezoelectric material and peripheral frame regions with engineered defects. This segmentation allows transverse waves to be confined and dissipated in the frame regions while maintaining clean acoustic wave propagation in the central region, thereby suppressing spurious signals without requiring complete structural redesign
Solution Approach 2:
Different regions of the piezoelectric film are given different local qualities: the central region maintains high crystalline quality for efficient acoustic wave generation and detection, while the frame regions are intentionally introduced with defects and lower acoustic velocity to act as wave barriers. This local quality differentiation enables selective control of acoustic wave propagation paths
2Reliability
If uniform piezoelectric film quality is maintained across the entire device, then manufacturing is simpler, but transverse waves can enter the central region and create spurious signals
Solution Approach 1:
The invention implements local quality variation by creating distinct defect concentration zones: the frame regions are engineered with higher defect concentrations to reduce acoustic velocity and create wave barriers, while the central active region maintains low defect concentrations for optimal acoustic performance. This localized quality control enables spurious signal suppression while keeping manufacturing processes achievable through targeted deposition and etching techniques
Solution Approach 2:
The acoustic velocity parameter is deliberately changed across different regions of the piezoelectric film. Frame regions are engineered with lower acoustic velocity (through defect introduction) compared to the central high-velocity region. This parameter gradient creates acoustic impedance mismatches that reflect and dissipate transverse waves before they can reach the central active region, effectively suppressing spurious signals
3Reliability
If electrode thickness is uniform across the entire piezoelectric film, then manufacturing is easier, but acoustic velocity discontinuities cannot be created to block transverse waves
Solution Approach 1:
The electrode structure is designed with local quality variation: the top and bottom electrodes have different thicknesses in the frame regions compared to the central active region. This variation creates acoustic velocity discontinuities at the electrode-piezoelectric film interfaces in the frame regions, forming barriers that reflect and contain transverse acoustic waves within the frame regions and prevent their propagation into the central active region
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The FBAR design effectively suppresses spurious signals by refracting transverse waves, reducing manufacturing variability and enhancing the frequency response stability.
Implementation Method 1
A film bulk acoustic wave resonator (FBAR) includes a piezoelectric film disposed in a central region... in which a main acoustic wave is generated during operation
Implementation Method 2
An acoustic velocity in the piezoelectric film in the recessed frame regions may differ from an acoustic velocity in the piezoelectric film in the raised frame regions. The difference in acoustic velocity in the recessed frame regions and raised frame regions may create an acoustic velocity discontinuity sufficient to prevent transverse acoustic waves travelling through the piezoelectric film outside of the central region from entering the central region
Data Source
AI summary
A method of forming a film bulk acoustic wave resonator comprises depositing a bottom electrode on an upper surface of a layer of dielectric material disposed over a cavity defined between the layer of dielectric material and a substrate, depositing a seed layer of piezoelectric material on an upper surface of the bottom electrode, etching one or more openings through the seed layer of piezoelectric material, etching of the one or more openings including over-etching of the seed layer in an amount sufficient to damage portions of the upper surface of the bottom electrode exposed by etching of the one or more openings, and depositing a bulk film of the piezoelectric material on an upper surface of the seed layer, on a portion of the upper surface of bottom electrode including the damaged portions, and on a portion of the upper surface of the dielectric layer.