Boundary Acoustic Wave Stack for Energy Trapping and Mode Suppression
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Solution Overview
Problem
Boundary acoustic wave devices face challenges in efficiently trapping vibrational energy due to the use of Al electrodes, leading to higher acoustic velocities and lower trapping efficiency, resulting in larger device sizes and unsatisfactory resonance characteristics, with materials like SiO2 having positive temperature coefficients of acoustic velocity complicating frequency stability.
Innovation Solution
A boundary acoustic wave device with a multilayer structure featuring a piezoelectric first medium, a non-electroconductive second medium, and a third medium with slow transverse waves, using IDTs made of metals with higher densities than Al, such as Pt, Au, Cu, Ag, or their alloys, to reduce propagation loss and enhance electromechanical coupling, while optimizing the thickness of the third medium to minimize higher-order spurious responses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If Al electrodes are used in boundary acoustic wave devices, then the device structure is simple and manufacturing is easy, but the acoustic velocity of transverse waves becomes higher and the trapping efficiency of vibrational energy becomes lower
Solution Approach 1:
The patent changes the material parameter (density) of the electrode from Al to high-density metals (Au, Ag, Cu, Pt) to fundamentally alter the acoustic velocity and impedance characteristics, thereby improving vibrational energy trapping efficiency while maintaining manufacturing feasibility through standard thin-film deposition techniques
Solution Approach 2:
The patent employs composite electrode structures combining high-density metals with specific thickness ratios (electrode thickness to wavelength between 0.006-0.2) to optimize both the trapping efficiency and electromechanical coupling, creating a composite system that balances multiple performance requirements
2Device complexity
If Al electrodes are used in boundary acoustic wave devices, then the device structure remains simple, but the device size becomes larger due to insufficient trapping efficiency
Solution Approach 1:
By changing the electrode material to high-density metals and optimizing the thickness parameter (0.006λ≦H1≦0.2λ), the patent achieves superior trapping efficiency in a compact configuration, reducing the required device volume while maintaining structural simplicity
3Speed
If SiO2 is used as the third medium with positive TCV, then the material provides good acoustic velocity characteristics, but the frequency temperature coefficient TCF becomes difficult to control
Solution Approach 1:
The patent creates a composite multilayer structure (first medium/SiO2 third medium/second medium) where the positive TCV of SiO2 is balanced by the negative TCV of the piezoelectric first medium, achieving near-zero overall TCF while maintaining the beneficial acoustic velocity characteristics of SiO2
Solution Approach 2:
The patent exploits the differential thermal expansion coefficients (TCV) of different materials in the multilayer structure to compensate for frequency drift, using the positive TCV of SiO2 combined with negative TCV materials to achieve temperature-stable frequency operation
4Reliability
If the thickness of the third medium is increased to improve vibrational energy trapping, then the trapping efficiency improves, but higher-order modes are strongly excited producing spurious responses
Solution Approach 1:
The patent optimizes the thickness parameter of the third medium (H2≦0.7λ) to a specific range that simultaneously achieves sufficient vibrational energy trapping while suppressing the excitation of higher-order modes, thereby eliminating spurious responses
Solution Approach 2:
The patent applies the principle of partial action by using a third medium thickness that is sufficient for trapping efficiency (H2≦0.7λ) but deliberately limited to avoid the threshold for higher-order mode excitation, achieving the minimum necessary thickness for effective trapping without excessive thickness that would cause spurious modes
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 solution achieves low-loss boundary acoustic wave propagation with improved resonance characteristics and filter properties, allowing for a reduced device size and enhanced temperature stability by effectively trapping vibrational energy and reducing the excitation of higher-order modes.
Implementation Method 1
a first medium (11) having piezoelectric characteristics
Implementation Method 2
The vibrational energy of boundary acoustic waves is trapped in the third medium made of ZnO in which acoustic velocity becomes low, and thus boundary acoustic waves are propagated
Data Source
AI summary
A boundary acoustic wave device efficiently traps the vibrational energy of boundary acoustic waves and exhibits a high electromechanical coupling coefficient, and is consequently not affected by higher-order modes. The boundary acoustic wave device includes a first medium having piezoelectric characteristics, a non-electroconductive second medium, and a third medium through which slow transverse waves propagate at a lower acoustic velocity than slow transverse waves propagating through the first and second media. The first medium, the third medium, and the second medium are stacked in that order. An IDT is disposed between the first medium and the third medium. The IDT includes a metal layer made of a metal having a density ρ in the range of about 3000 kg/m3 to about 21500 kg/m3. The IDT has electrode fingers arranged at a pitch of λ and has a thickness H1 satisfying the relationship 0.006λ≦H1≦0.2λ, and the third medium has a thickness H2 satisfying the relationship H1<H2≦0.7λ.