SAW Transducer Electrode Layout for Parasitic Mode Suppression

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Solution Overview

Problem

Surface acoustic wave (SAW) filter devices on composite substrates suffer from significant degradation in filter passband performance, including higher insertion loss and narrowing of the passband, making them unsuitable for mobile applications, and exhibit parasitic modes due to reflected acoustic waves at the interface between the piezoelectric layer and the substrate.

Innovation Solution

A transducer structure is designed with modified regions where neighbouring electrode means are connected to the same potential, reducing electro-acoustic source density and electromechanical coupling coefficient, thereby controlling the excitation and detection efficiency of acoustic waves and suppressing parasitic modes without altering the characteristic features of the transducer.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Temperature

If a composite substrate with a piezoelectric layer is used to improve temperature stability, then temperature characteristics are improved, but parasitic modes are generated due to reflections at the piezoelectric layer/substrate interface

Engineering Contradiction:

Improvetemperature stabilityVSAvoidparasitic modes

Core Design Contradiction:

TemperatureVSObject-generated harmful factors

Solution Approach 1:

An acoustic matching layer is introduced between the piezoelectric layer and the substrate to act as an intermediary that reduces acoustic impedance mismatch. This matching layer suppresses parasitic mode reflections while preserving the temperature stability benefits of the composite substrate structure

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The acoustic impedance parameters of the interface region are modified by introducing the matching layer with specific acoustic properties. This changes the reflection characteristics at the piezoelectric layer/substrate interface, reducing parasitic modes while maintaining the temperature compensation function

Inventive Principle:

Principle #35Parameter changes

2Power

If the piezoelectric layer thickness is increased to improve coupling, then electromechanical coupling is enhanced, but parasitic modes are excited due to reflections at the substrate interface

Engineering Contradiction:

Improveelectromechanical couplingVSAvoidparasitic modes

Core Design Contradiction:

PowerVSObject-generated harmful factors

Solution Approach 1:

The acoustic matching layer serves as a mediator that allows thicker piezoelectric layers to be used for enhanced coupling without suffering from parasitic mode excitation. The matching layer absorbs and redirects reflected acoustic energy, preventing the formation of parasitic modes

Inventive Principle:

Principle #24Intermediary (Mediator)

3Device complexity

If standard inter-digitated transducer structure is used to achieve simple design, then device complexity is reduced, but significant degradation in filter passband performance occurs with higher insertion loss and passband narrowing

Engineering Contradiction:

Improvetransducer structure simplicityVSAvoidfilter passband performance

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The transducer structure is modified locally by introducing electrode finger pairs with specific geometric parameters (width, spacing, length) that differ from the standard design. These localized modifications create acoustic cancellation effects that reduce parasitic modes and improve passband characteristics while maintaining overall structural simplicity

Inventive Principle:

Principle #3Local quality

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 enhances the performance of SAW filter devices by reducing parasitic modes and improving temperature stability, allowing for more stable filter characteristics and out-of-band rejection, while maintaining the surface wave propagation characteristics.

Implementation Method 1

one or more inter-digitated transducers (IDTs) are used to convert acoustic waves to electrical signals and vice versa by exploiting the piezoelectric effect of certain materials

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

A surface acoustic wave can be established on the substrate by electrically exciting the fingers. Conversely, an electrical signal can be induced across the fingers by a surface acoustic wave propagating in the piezoelectric substrate material beneath the transducer

Methodology Applied

Scientific EffectSurface acoustic wave propagation: Surface Acoustic Wave

Data Source

PatentEP3766176B1Transducer structure for source suppression in saw filter devices

Publication Date: 2022.12.21 SOITEC SA

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AI summary

A transducer structure for a surface acoustic wave device, comprising a pair of inter-digitated comb electrodes (302, 304) wherein said pair of inter-digitated comb electrodes (302, 304) comprises neighbouring electrode means (306, 308) belonging to different comb electrodes (302, 304) and having a pitch p being defined as the edge-to-edge electrode means distance between two neighbouring electrode means (306, 308), the pitch p satisfying the Bragg condition; characterised in that said pair of inter-digitated comb electrodes (302, 304) comprises at least one region (316) in which two or more neighbouring electrode means (306, 308) belong to the same comb electrode (302, 304) while having an edge-to-edge distance to each other corresponding to the pitch p. The invention relates also to a surface acoustic wave filter device.