Elastic Wave Filter Structure for Higher SH Wave Coupling
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Solution Overview
Problem
Conventional elastic wave devices have a low electromechanical coupling coefficient for SH waves, making it difficult to achieve good filter characteristics when applied to wide transmission band gaps, such as Band I of UMTS.
Innovation Solution
The elastic wave device features a first dielectric layer with a convex portion over the IDT electrode, where the film thickness of the second dielectric layer is at least 1.6 times the pitch width of the IDT electrode, enhancing the electromechanical coupling coefficient of SH waves.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional elastic wave device structure is used with specified film thickness of first dielectric layer and IDT electrode, then electromechanical coupling coefficient of Stoneley wave is suppressed, but electromechanical coupling coefficient of SH wave is only 0.15 at maximum
Solution Approach 1:
The first dielectric layer is designed with non-uniform thickness, featuring a convex portion (thicker region) positioned over the IDT electrode and thinner regions at the sides. This local variation in thickness creates different acoustic impedance characteristics in different areas, enabling selective enhancement of SH wave coupling while maintaining Stoneley wave suppression through the overall layer structure
Solution Approach 2:
The invention introduces a vertical dimension variation (thickness profile) in the first dielectric layer, transitioning from a two-dimensional uniform layer to a three-dimensional structure with controlled thickness gradients. This dimensional change allows independent optimization of wave coupling characteristics for different wave modes by adjusting the thickness profile parameters
2Adaptability or versatility
If elastic wave device is applied to duplexer with wide transmission band gap such as Band I of UMTS, then wide frequency coverage is achieved, but good filter characteristics cannot be obtained due to low SH wave coupling coefficient
Solution Approach 1:
The convex portion in the first dielectric layer creates localized regions of enhanced acoustic coupling directly over the IDT electrode, where the thicker region increases the electromechanical coupling coefficient for SH waves. This local enhancement enables adequate filter characteristics even when operating in wide band gap applications where uniform coupling would be insufficient
Solution Approach 2:
The invention changes the thickness parameter of the first dielectric layer from a uniform value to a spatially varying profile with a convex portion. By adjusting the height, width, and position of the convex portion, the electromechanical coupling coefficient can be optimized for specific frequency ranges and wave modes, enabling adaptation to wide band gap requirements while maintaining good filter 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
This configuration significantly improves the electromechanical coupling coefficient of SH waves, allowing for effective filter characteristics in wide transmission band gaps, such as Band I of UMTS, by trapping energy within the device.
Implementation Method 1
piezoelectric substrate 2 made of lithium niobate, IDT (Interdigital Transducer) electrode 3 disposed on piezoelectric substrate 2
Implementation Method 2
second dielectric layer 5 made of a medium that propagates transverse waves faster than that on first dielectric layer 4
Data Source
AI summary
An elastic wave device has a convex portion on the top face of a first dielectric layer over an IDT electrode when the elastic wave device has a structure of a boundary wave device in which a film thickness of a second dielectric layer is not less than 1.6 times as much as a pitch width of the IDT electrode. This convex portion increases an electromechanical coupling coefficient of SH wave that is the major wave. Accordingly, good filter characteristics can be easily achieved.