Composite SAW/BAW Substrate for Thermal Stability and Low Spurious Modes

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Solution Overview

Problem

Surface acoustic wave devices and bulk acoustic wave devices are sensitive to temperature variations due to differences in thermal expansion coefficients between piezoelectric materials and silicon substrates, leading to instability, especially at high temperatures, and existing temperature compensation techniques like wafer-bonding introduce spurious resonances.

Innovation Solution

A substrate design with a semiconductor layer between the piezoelectric layer and a stiffening substrate, such as sapphire or glass, with a thermal expansion coefficient closer to the piezoelectric material, and the inclusion of a dielectric and charge-trapping layer, allows for improved thermal stability up to 300°C and facilitates integration of electronic components.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If a silicon substrate is used for wafer-bonding to the piezoelectric layer, then integration of electronic components is facilitated, but thermal expansion mismatch causes instability at high temperatures

Engineering Contradiction:

Improveintegration of electronic componentsVSAvoidthermal stability

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

A semiconductor layer is introduced as an intermediary between the silicon substrate and the piezoelectric layer. This intermediate layer acts as a buffer that accommodates the thermal expansion mismatch between silicon and the piezoelectric material, preventing stress accumulation and device instability at high temperatures while maintaining the silicon substrate's advantage for electronic component integration

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent employs a composite substrate structure consisting of multiple layers: a silicon substrate, a semiconductor layer, and a piezoelectric layer. This composite construction combines the advantages of silicon (ease of electronic integration) with the thermal stability of materials having lower thermal expansion coefficients, creating a multi-functional substrate that addresses both manufacturing ease and thermal reliability

Inventive Principle:

Principle #40Composite materials

2Reliability

If the piezoelectric layer is made thin for bulk acoustic wave devices, then device performance is improved, but mechanical stress and spurious resonances increase

Engineering Contradiction:

Improvedevice performanceVSAvoidspurious resonances

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The semiconductor layer serves as a mechanical buffer between the thin piezoelectric layer and the stiffening substrate. This intermediary layer reduces the mechanical stress transmitted to the thin piezoelectric layer, preventing the development of spurious resonances while allowing the piezoelectric layer to maintain its thin configuration for optimal bulk acoustic wave device performance

Inventive Principle:

Principle #24Intermediary (Mediator)

3Reliability

If temperature compensation is implemented using existing techniques, then frequency stability is improved, but spurious resonances are introduced

Engineering Contradiction:

Improvefrequency stabilityVSAvoidspurious resonances

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The semiconductor layer provides a new approach to temperature compensation by acting as a thermal buffer. Instead of using traditional methods like overlay techniques or direct wafer-bonding to stiff substrates that introduce spurious resonances, this intermediate layer compensates for thermal expansion effects while mechanically isolating the piezoelectric layer, thereby eliminating spurious resonances

Inventive Principle:

Principle #24Intermediary (Mediator)

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 substrate provides enhanced thermal stability and allows for the integration of electronic components while minimizing mechanical stress and spurious resonances, ensuring reliable operation of surface and bulk acoustic wave devices across a broader temperature range.

Implementation Method 1

a charge-trapping layer at the interface between the dielectric layer and the semiconductor layer and/or the interface between the dielectric layer and the piezoelectric layer

Methodology Applied

Scientific EffectCharge trapping:

Implementation Method 2

An electrical signal, such as an electrical voltage change applied to an electrode, is converted into an elastic wave, which is propagated at the surface of the piezoelectric layer. The wave is converted once more into an electrical signal on reaching the other electrode.

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Data Source

PatentUS11711065B2Substrate for a temperature-compensated surface acoustic wave device or volume acoustic wave device

Publication Date: 2023.07.25 SOITEC SA

AI summary

A substrate for a surface acoustic wave device or bulk acoustic wave device, comprising a support substrate and an piezoelectric layer on the support substrate, wherein the support substrate comprises a semiconductor layer on a stiffening substrate having a coefficient of thermal expansion that is closer to the coefficient of thermal expansion of the material of the piezoelectric layer than that of silicon, the semiconductor layer being arranged between the piezoelectric layer and the stiffening substrate.