FBAR Lower Electrode Grain Size Adjustment
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Solution Overview
Problem
Current film bulk acoustic resonators (FBARs) face challenges in adjusting the effective piezoelectric coupling coefficient (Kt2_eff) to meet the roll-off characteristics of filters, particularly in mobile communication systems, where temperature coefficients and thickness uniformity affect the resonance frequency, limiting their filtering performance.
Innovation Solution
The FBAR design incorporates a lower electrode formed of two layers with different grain sizes and thickness ratios, allowing for adjustable piezoelectric coupling coefficients without additional processing, enabling improved filter characteristics and design flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a conventional single-layer lower electrode is used in FBAR, then the structure is simple and fabrication is easy, but the effective piezoelectric coupling coefficient cannot be adjusted to satisfy filter roll-off characteristics
Solution Approach 1:
The lower electrode is divided into multiple layers (first lower electrode layer and second lower electrode layer) with different grain sizes. This segmentation allows independent control of each layer's properties, enabling adjustment of the effective piezoelectric coupling coefficient while maintaining a relatively simple overall structure that can be fabricated using standard processes.
Solution Approach 2:
Different layers of the lower electrode are given different local qualities, specifically different grain sizes. The first lower electrode layer has a first grain size while the second lower electrode layer has a second grain size different from the first. This local differentiation enables precise control of the piezoelectric coupling coefficient without requiring complex overall structural changes.
2Reliability
If temperature coefficient effects and thickness uniformity variations are considered, then the resonance frequency stability improves, but the filter roll-off characteristic deteriorates due to narrow effective frequency band
Solution Approach 1:
The invention changes the physical parameters of the lower electrode by controlling the grain sizes of different layers. By adjusting the grain size ratio between the first and second lower electrode layers, the effective piezoelectric coupling coefficient can be optimized to achieve both stable resonance frequency (satisfying temperature coefficient requirements) and adequate filter roll-off characteristics with sufficient frequency band width.
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
This approach allows for precise adjustment of the effective piezoelectric coupling coefficient over a wide range, enhancing the filtering performance and roll-off characteristics of FBARs in electronic circuits, such as duplexers and band-pass filters, by modifying the crystallographic and geometrical conditions of the lower electrode.
Implementation Method 1
a resonating unit in which a lower electrode, a piezoelectric film, and an upper electrode are layered in turn on a substrate
Implementation Method 2
a resonator, which filters an oscillation or wave of a predetermined frequency using a resonance phenomenon
Data Source
AI summary
A resonator includes a substrate, and a resonating unit formed on the substrate, and having a lower electrode, a piezoelectric film, and an upper electrode. The lower electrode is formed of at least two layers to adjust a piezoelectric coupling coefficient. The lower electrode includes a first electrode layer and a second electrode layer, which have crystallographic characteristics, particularly, grain sizes or surface roughnesses Ra, different from each other, respectively. The first and the second electrode layers are formed to have a predetermined thickness ratio to each other.