Thin-Film BAW Filter Structure for Wider Bandwidth and Steeper Edges
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Solution Overview
Problem
Existing bulk acoustic wave (BAW) filters with lateral acoustic coupling (LBAW) have narrow bandwidth and gently sloping passband edges, limiting their application and competitiveness.
Innovation Solution
A wide-band acoustically coupled BAW filter design utilizing a thin-film structure with interdigital electrodes and a Type 1 dispersion profile, incorporating series and parallel resonators to enhance bandwidth and steepen passband edges, allowing for operation in the GHz frequency range with improved stop-band attenuation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If lateral acoustic coupling (LBAW) is used in BAW filters, then the filter structure is simplified and manufacturing is easier, but the bandwidth becomes narrow and passband edges become gently sloping
Solution Approach 1:
The patent changes the dispersion type parameter from Type 2 (conventional) to Type 1 by adjusting the acoustic mirror design and piezoelectric layer configuration. This parameter change enables wide bandwidth operation while maintaining the simplified LBAW structure, directly resolving the contradiction between manufacturing simplicity and performance precision.
Solution Approach 2:
The patent employs a composite structure combining piezoelectric thin film, electrode layers, and acoustic mirror layers with specific acoustic impedance ratios. This composite design achieves Type 1 dispersion characteristics that enable both wide bandwidth and steep passband edges while maintaining structural simplicity for easy manufacturing.
2Reliability
If Type 2 dispersion is used in conventional BAW filters, then the filter provides basic filtering function, but the bandwidth is limited and passband edges are gentle
Solution Approach 1:
The patent fundamentally changes the dispersion type parameter from Type 2 to Type 1 by modifying the acoustic mirror design and layer configuration. This parameter transformation maintains reliable filtering function while achieving wide bandwidth and steep passband edges, resolving the contradiction between reliability and precision.
3Manufacturing precision
If the number of sections in ladder filter is increased to widen passband, then the bandwidth increases, but the device complexity and fabrication difficulty increase
Solution Approach 1:
The patent changes the dispersion type parameter to Type 1, which fundamentally alters the bandwidth characteristics. This single parameter change achieves wide bandwidth without increasing the number of filter sections, directly resolving the contradiction between passband width and device complexity.
Solution Approach 2:
The patent transitions from electrical coupling in the lateral direction to acoustic coupling through the thickness direction with Type 1 dispersion. This dimensional change in the coupling mechanism enables bandwidth expansion without adding more filter sections, reducing device complexity.
4Ease of manufacture
If conventional LBAW filter design is used, then fabrication process is simple, but out-of-band signal suppression is insufficient
Solution Approach 1:
The patent employs a composite structure with specifically designed acoustic mirror layers and piezoelectric thin film configuration that achieves Type 1 dispersion. This composite design maintains fabrication simplicity while providing enhanced out-of-band signal suppression through improved acoustic isolation and resonance characteristics.
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 design achieves a wide frequency bandwidth of up to 5% relative to the center frequency, with steepened passband edges and enhanced stop-band attenuation, reducing component size and fabrication complexity compared to traditional solutions.
Implementation Method 1
The IDTs are used to transform the electric input signal Vin into an acoustic wave via the piezoelectric effect, as well as to pick up the acoustic signal at the output port and transform it back to an electrical form
Implementation Method 2
an acoustic Bragg mirror composed of alternating high and low acoustic impedance (Z) material layers serves to isolate the vibration in the piezoelectric thin film from the substrate and to prevent acoustic leakage
Implementation Method 3
In BAW devices, the propagation direction of the bulk wave is typically along the thickness axis (z axis). Particle displacement is either perpendicular to the propagation direction (shear wave) or parallel to the propagation direction (longitudinal wave)
Implementation Method 4
The first-order longitudinal (thickness extensional, TE1) vibration mode, in which the thickness of the piezoelectric layer contains approximately half a wavelength of the bulk vibration
Data Source
Figure 1a~1b
Figure 2a~2b
Figure 3(a)~3(c)
AI summary
The invention relates to an acoustically coupled thin-film BAW filter, comprising a piezoelectric layer (1), an input-port (2) on the piezoelectric layer(1) changing electrical signal into an acoustic wave (SAW, BAW), and an output-port (3) on the piezoelectric layer(1) changing acoustic signal into electrical signal. In accordance with the invention the ports (2, 3) include electrodes (5, 6) positioned close to each other, and the filter is designed to operate in first order thickness-extensional TE1 mode.