Composite SAW Substrate for Guided Waves Beyond 2 GHz

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Solution Overview

Problem

Surface acoustic wave (SAW) devices are limited to frequencies up to 2 GHz due to the constraints of existing lithography technology, which restricts their performance and application in higher frequency ranges.

Innovation Solution

A surface acoustic wave device utilizing a composite substrate with a piezoelectric layer oriented relative to a base substrate such that the phase velocity of the longitudinally polarized wave is below the critical phase velocity of the base substrate, allowing for wave guiding and extending the frequency range without requiring advanced lithography techniques.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If standard I-line lithography is used to create inter-digitated comb electrode structures, then manufacturing is simple and cost-effective, but the wavelength is limited to 1.5 μm which restricts operating frequency to 2 GHz

Engineering Contradiction:

Improvelithography process simplicityVSAvoidoperating frequency

Core Design Contradiction:

Ease of manufactureVSSpeed

Solution Approach 1:

The patent changes the fundamental parameter of wave propagation by transitioning from surface waves to bulk acoustic waves. This parameter change allows the use of standard lithography for electrode fabrication while achieving higher operating frequencies, as bulk waves are not constrained by the same wavelength limitations as surface waves in conventional SAW devices.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent employs a composite substrate structure combining a piezoelectric layer with a high-velocity base substrate. This composite material approach enables the device to utilize bulk acoustic waves that propagate faster than surface waves, thereby achieving frequencies above 2 GHz while maintaining compatibility with standard manufacturing lithography processes.

Inventive Principle:

Principle #40Composite materials

2Device complexity

If piezoelectric substrates are used for SAW devices, then the device structure is simple, but temperature sensibility is high and electromechanical coupling is weak

Engineering Contradiction:

Improvesubstrate structureVSAvoidtemperature stability

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent uses a composite substrate consisting of a piezoelectric layer bonded to a base substrate with suitable acoustic and thermal properties. This composite structure provides both strong electromechanical coupling and improved temperature stability, resolving the contradictions of using simple piezoelectric substrates alone.

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The patent applies different material properties to different layers of the substrate structure. The piezoelectric layer provides the necessary electromechanical coupling, while the base substrate provides thermal stability and mechanical support. This local differentiation of material qualities allows the device to overcome the limitations of uniform piezoelectric substrates.

Inventive Principle:

Principle #3Local quality

3Stability of the object's composition

If single crystal piezoelectric materials are used, then the material properties are well-defined, but elastic wave velocities are limited between 3000 and 4000 m/s

Engineering Contradiction:

Improvematerial property consistencyVSAvoidphase velocity

Core Design Contradiction:

Stability of the object's compositionVSSpeed

Solution Approach 1:

The patent combines a piezoelectric layer with a base substrate that supports faster acoustic wave propagation. This composite approach allows the device to maintain the well-defined material properties of single crystal piezoelectrics while achieving higher phase velocities through the coupled structure, exceeding the 3000-4000 m/s limitation of single crystal materials alone.

Inventive Principle:

Principle #40Composite materials

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 enables SAW devices to operate beyond 2 GHz while maintaining temperature stability and electromechanical coupling, enhancing their performance and application in high-frequency filtering and resonator applications.

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

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

the phase velocity of the longitudinally polarized wave is below the critical phase velocity of the base substrate at which wave guiding within the piezoelectric layer vanishes

Methodology Applied

Scientific EffectAcoustic wave guiding: Waveguide

Implementation Method 3

A Rayleigh surface acoustic wave develops on the substrate by electrically exciting the fingers

Methodology Applied

Scientific EffectRayleigh surface acoustic wave: Surface Acoustic Wave

Data Source

PatentUS11437973B2Surface acoustic wave device on composite substrate

Publication Date: 2022.09.06 SOITEC SA

AI summary

A surface acoustic wave device using a longitudinally polarized guided wave comprises a composite substrate comprising a piezoelectric layer formed over a base substrate, wherein the crystalline orientation of the piezoelectric layer with respect to the base substrate is such that, the phase velocity of the longitudinally polarized wave is below the critical phase velocity of the base substrate at which wave guiding within the piezoelectric layer vanishes. A method of fabrication of such surface acoustic wave device is also disclosed.