Bulk acoustic wave filter device and method of manufacturing the same
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Solution Overview
Problem
Bulk acoustic wave filter devices suffer from spurious resonances and noise due to lateral wave vibrations, which reduce the quality factor (Q factor) and coupling coefficient, making it challenging to achieve uniform insertion loss characteristics within the pass band.
Innovation Solution
A bulk acoustic wave filter device is designed with a density reduction layer on the upper electrode, formed by selective oxidation, which suppresses lateral wave excitation by reducing the density of specific portions of the electrode, thereby minimizing spurious resonances and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a uniform upper electrode is used, then the manufacturing process is simple, but spurious resonances and noise occur due to lateral wave vibrations
Solution Approach 1:
The upper electrode is designed with non-uniform thickness, where the central portion has a first thickness and the peripheral portion has a second thickness greater than the first. This local variation in electrode structure suppresses lateral wave vibrations at the periphery while maintaining the desired resonance characteristics in the central region, thereby reducing spurious resonances and noise without significantly complicating the manufacturing process.
2Object-affected harmful factors
If the upper electrode density is reduced, then lateral wave excitation is suppressed, but the electrode's electrical conductivity may be affected
Solution Approach 1:
The electrode structure utilizes controlled thickness variation as a parameter change to achieve density reduction in the peripheral portion. By making the peripheral portion thinner, the mass density is reduced which suppresses lateral wave excitation, while the central portion maintains sufficient thickness to ensure adequate electrical conductivity for the piezoelectric actuation.
3Object-affected harmful factors
If the electrode thickness is varied, then lateral wave vibrations are suppressed, but the manufacturing precision requirements increase
Solution Approach 1:
The upper electrode is segmented into two distinct regions: a central portion with a first thickness and a peripheral portion with a second thickness. This segmentation allows each region to be optimized for its specific function while using standard thin-film deposition techniques to achieve the thickness variation, balancing the suppression of lateral wave vibrations with manufacturability.
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 implementation of the density reduction layer effectively reduces spurious resonances and noise, ensuring high Q factor and coupling coefficient, and securing even, low insertion loss characteristics within the pass band of the filter device.
Implementation Method 1
piezoelectric dielectric materials may be deposited on semiconductor substrates, such as silicon wafers. A bulk acoustic wave filter device is demonstrated when such thin film type elements with the BAWR configuration are implemented as a filter.
Implementation Method 2
the density reduction layer may be a result of a select oxidation of the conductor of the upper electrode
Data Source
AI summary
A bulk acoustic wave filter device includes a substrate, a lower electrode on the substrate, a piezoelectric layer covering at least a portion of the lower electrode, and an upper electrode covering at least a portion of the piezoelectric layer. The upper electrode has a density reduction layer disposed on at least a portion thereof, except a central portion of a resonance region of the bulk acoustic wave filter device that deforms and vibrates with the piezoelectric layer during activation of the piezoelectric layer. The density reduction layer has a density lower than a density of other portions of the upper electrode.