Spinel-Joined SAW Substrate for Stable Frequency Temperature

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Solution Overview

Problem

Existing surface acoustic wave (SAW) filters using piezoelectric substrates face challenges in maintaining stable frequency temperature characteristics due to thermal expansion, and previous solutions like silicon and sapphire substrates are costly and difficult to shape for downsizing.

Innovation Solution

A joined body comprising a piezoelectric substrate and a polycrystalline spinel substrate, where the ratio of the average thickness of the piezoelectric substrate to the polycrystalline spinel substrate is 0.1 or less, and the polycrystalline spinel substrate has a total thickness variation (TTV) of 1.5 μm or less, is used to create a surface acoustic wave device with improved frequency temperature characteristics.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stability of the object's composition

If silicon substrate is used as support substrate, then thermal expansion suppression is improved, but substrate strength deteriorates (silicon may be broken)

Engineering Contradiction:

Improvethermal expansion suppressionVSAvoidsubstrate strength

Core Design Contradiction:

Stability of the object's compositionVSStrength

Solution Approach 1:

The invention uses a composite structure combining piezoelectric substrate and spinel substrate. The spinel substrate serves as the support substrate with intermediate thermal expansion coefficient between piezoelectric substrate and silicon, providing both sufficient strength and thermal expansion suppression. This composite approach resolves the contradiction by selecting a material that balances both strength and thermal stability requirements.

Inventive Principle:

Principle #40Composite materials

2Stability of the object's composition

If sapphire substrate is used as support substrate, then thermal expansion suppression is improved, but ease of manufacture deteriorates (difficult to shape and downsize)

Engineering Contradiction:

Improvethermal expansion suppressionVSAvoidshaping ease

Core Design Contradiction:

Stability of the object's compositionVSEase of manufacture

Solution Approach 1:

The invention replaces difficult-to-process sapphire with spinel substrate that has comparable thermal expansion properties but superior processability. Spinel can be easily shaped, cut, and downsized while maintaining the thermal expansion suppression function, thus resolving the contradiction between thermal stability and manufacturing ease.

Inventive Principle:

Principle #40Composite materials

3Stability of the object's composition

If single-crystal substrate (silicon or sapphire) is used, then thermal expansion suppression is improved, but cost increases

Engineering Contradiction:

Improvethermal expansion suppressionVSAvoidmanufacturing cost

Core Design Contradiction:

Stability of the object's compositionVSEase of manufacture

Solution Approach 1:

The invention uses spinel substrate which can be produced as polycrystalline material through conventional ceramic processing methods, avoiding the high cost of single-crystal growth. This maintains thermal expansion suppression functionality while significantly reducing substrate cost through easier and cheaper manufacturing processes.

Inventive Principle:

Principle #40Composite materials

4Manufacturing precision

If piezoelectric substrate thickness is increased, then manufacturing precision is improved, but device volume increases

Engineering Contradiction:

Improvefrequency temperature characteristic stabilityVSAvoiddevice volume

Core Design Contradiction:

Manufacturing precisionVSVolume of moving object

Solution Approach 1:

The invention optimizes the thickness ratio between piezoelectric substrate and spinel substrate (T1/T2 ≤ 0.1) to achieve the desired frequency temperature characteristic stability. By carefully controlling this parameter ratio and the spinel substrate thickness uniformity (TTV ≤ 1.5 μm), the device achieves manufacturing precision without excessive volume increase.

Inventive Principle:

Principle #35Parameter changes

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 proposed solution achieves an excellent frequency temperature characteristic with little variation, effectively suppressing thermal expansion and maintaining the stability of the SAW device, while being more cost-effective than traditional high-cost substrates.

Implementation Method 1

a piezoelectric substrate made of a material having a piezoelectric effect is used. Due to an electrical signal inputted to the comb-shaped electrode, the piezoelectric substrate is subjected to stress and deformed, and an acoustic wave according to the pitch is generated.

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

The thermal expansion coefficient of silicon is extremely low, when compared with the thermal expansion coefficient of a material forming the piezoelectric substrate, such as lithium tantalate. Therefore, when the piezoelectric substrate expands due to heat, silicon may be broken.

Methodology Applied

Scientific EffectThermal expansion suppression: Thermal Expansion

Data Source

PatentUS12308817B2Joined body and surface acoustic wave device

Publication Date: 2025.05.20 SUMITOMO ELECTRIC INDUSTRIES LTD

AI summary

Provided is a joined body including a piezoelectric substrate and a polycrystalline spinel substrate provided on one main surface of the piezoelectric substrate, wherein a ratio T1/T2 between T1 and T2 is 0.1 or less, where the T1 represents an average thickness of the piezoelectric substrate and the T2 represents an average thickness of the polycrystalline spinel substrate, and the polycrystalline spinel substrate has a TTV of 1.5 μm or less in a main surface which contacts the piezoelectric substrate.