Lamb Wave Device with Rotation Twin Crystal Structure
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Solution Overview
Problem
Conventional Lamb wave devices using piezoelectric thin films suffer from spurious responses and limited bandwidth due to lower electromechanical coupling coefficients, which hinder their performance as resonators and filters, especially in high-frequency regions.
Innovation Solution
A Lamb wave device with a piezoelectric thin film of LiTaO3 or LiNbO3, where the c-axis is aligned with the normal to the film surfaces and features a rotation twin crystal structure, and an IDT electrode made of Al or its alloy, with specific thickness ratios that suppress spurious responses by optimizing the electromechanical coupling coefficient.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Lamb wave device uses a piezoelectric thin film with IDT electrode as disclosed in Non-Patent Document 1, then the electromechanical coupling coefficient is increased and bandwidth is improved, but spurious responses are generated in the pass band or attenuation band
Solution Approach 1:
The patent applies parameter changes by rotating the crystal orientation of the piezoelectric thin film from the conventional 90°-rotated Y-X LiNbO3 to a 45°-rotated Y-X LiNbO3 configuration. This parameter change in crystal orientation fundamentally alters the wave propagation characteristics, enabling the excitation of the A0 mode with different dispersion properties that suppress spurious responses while maintaining high electromechanical coupling coefficient and bandwidth.
Solution Approach 2:
The patent employs local quality by creating a floating portion structure where the piezoelectric thin film is suspended above the base substrate at specific regions. This local structural modification allows the Lamb wave to propagate in a controlled manner with enhanced coupling to the IDT electrode while isolating certain areas from spurious mode generation, thereby improving the signal-to-noise ratio and suppressing unwanted responses.
2Reliability
If the thickness of piezoelectric thin film and IDT electrode is increased to enhance electromechanical coupling, then bandwidth increases, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The patent optimizes the thickness parameters of both the piezoelectric thin film and IDT electrode to achieve maximum electromechanical coupling with minimal complexity. By carefully selecting the thickness ratio between the piezoelectric film and the IDT electrode, the design achieves high coupling efficiency without requiring excessive thickness that would complicate manufacturing. The floating portion height is also optimized to enhance coupling while maintaining 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 device achieves superior frequency properties with reduced spurious responses and increased bandwidth, enabling central frequencies of 2 to 10 GHz and fractional band widths of 1% to 10%, suitable for both filters and resonators, while maintaining environmental resistance.
Implementation Method 1
devices using various waves excited using a piezoelectric effect have been proposed and have also been used
Implementation Method 2
a device using a Lamb wave propagating in an elastic body, which is different from a Rayleigh wave, has been disclosed. The Lamb wave is one type of bulk waves, that is, one type of so-called plate waves, and is an elastic wave which propagates in a piezoelectric body while reflecting on two primary surfaces thereof
Implementation Method 3
an elastic wave which propagates in a piezoelectric body while reflecting on two primary surfaces thereof
Implementation Method 4
the A1 mode, which is an asymmetric mode having a high acoustic velocity and a large electromechanical coupling coefficient, is excited
Data Source
Figure 1~2
Figure 3A~3B
Figure 3C~4
AI summary
A device using a Lamb wave is provided which has the structure in which spurious by undesired modes can be effectively suppressed. A Lamb wave device (1) has a base substrate (2); a piezoelectric thin film (3) which is formed on the base substrate (2) and which has a floating portion floating above the base substrate (2), the floating portion having a first surface (3a) facing the base substrate (2) and a second surface (3b) opposite to the first surface (3a); and an IDT electrode (4) disposed on at least one of the first and the second surfaces of the piezoelectric thin film (3). The piezoelectric thin film (3) is formed of LiTaO3 or LiNbO3, and the c axis of the piezoelectric thin film (3) is set in approximately the same direction as that of the normal line to the first and the second surfaces of the piezoelectric thin film (3), and the crystal structure of the piezoelectric thin film (3) is a rotation twin crystal having the c axis functioning as the rotation axis.