Piezoelectric Substrate Bonding With Trenches and CTE Buffer Layer

Solution Overview

Problem

The existing methods for manufacturing substrates with piezoelectric materials like lithium tantalate (LiTaO3) for acoustic surface wave filters face issues with thermal expansion differences between LiTaO3 and support substrates like silicon, leading to bonding interface degradation and cracking during heat treatment, which affects the resonance frequency and quality factor of the filters.

Innovation Solution

A method involving a support substrate with parallel trenches on its surface, an intermediate layer with a thermal expansion coefficient between the support and the piezoelectric layer, and a thinning process to form the piezoelectric layer, which reduces the constraint field at the interface and maintains thermal compensation by inserting a glass material or other intermediate layers like TEOS, BPSG, or PSG.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a LiTaO3 layer is transferred onto a silicon substrate, then thermal compensation is achieved due to the low CTE of silicon, but the bonding interface degrades and cracks appear during heat treatment due to CTE mismatch

Engineering Contradiction:

Improvebonding interface integrityVSAvoidheat treatment temperature

Core Design Contradiction:

ReliabilityVSTemperature

Solution Approach 1:

The support substrate is segmented by forming a network of trenches that divide the substrate into multiple regions. This segmentation reduces the continuous constraint field at the bonding interface, allowing the interface to better withstand thermal expansion differences during heat treatment without degradation or cracking.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

An intermediate layer is inserted between the LiTaO3 useful layer and the silicon support substrate. This intermediate layer acts as a buffer that accommodates the thermal expansion coefficient difference between the two materials, reducing mechanical constraints at the bonding interface while maintaining thermal compensation effects.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If trenches are formed on the support substrate to reduce constraints, then bonding interface integrity is improved, but the device complexity increases

Engineering Contradiction:

Improvebonding interface integrityVSAvoidsubstrate structure complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The support substrate is segmented by forming a network of trenches that divide the substrate into multiple regions. This segmentation reduces the continuous constraint field at the bonding interface, allowing the interface to better withstand thermal expansion differences during heat treatment without degradation or cracking.

Inventive Principle:

Principle #1Segmentation

3Reliability

If an intermediate layer is inserted between the support substrate and useful layer, then thermal expansion mismatch is reduced, but the manufacturing process complexity increases

Engineering Contradiction:

Improvebonding interface integrityVSAvoidmanufacturing process simplicity

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

An intermediate layer is inserted between the LiTaO3 useful layer and the silicon support substrate. This intermediate layer acts as a buffer that accommodates the thermal expansion coefficient difference between the two materials, reducing mechanical constraints at the bonding interface while maintaining thermal compensation effects.

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

This approach ensures the integrity of the substrate during heat treatment while maintaining thermal compensation, reducing the impact of thermal expansion differences and preventing cracking, thus stabilizing the resonance frequency and quality factor of the acoustic surface wave filters.

Implementation Method 1

insertion of an intermediate layer between the front face of the support substrate and the useful layer, the intermediate layer having a coefficient of thermal expansion between the first and second coefficients of thermal expansion

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Implementation Method 2

The formation of the trenches makes it possible to limit the constraints at the bonding interface and, thus, ensures the integrity of the useful layer

Methodology Applied

Scientific EffectStress distribution:

Implementation Method 3

The low coefficient of thermal expansion of silicon, the latter being equal to 2.6×10−6/° C., makes it possible to limit the expansion of the LiTaO3 layer and consequently to limit the variations of the resonance frequency and the quality factor of the F-SAW filter. This effect is referred to hereinafter as 'thermal compensation.'

Methodology Applied

Scientific EffectThermal compensation: Thermal Expansion

Data Source

PatentUS11837463B2Method for manufacturing a substrate

Publication Date: 2023.12.05 SOITEC SA

AI summary

A method for manufacturing a substrate includes the following steps: (a) providing a support substrate with a first coefficient of thermal expansion, having on one of its faces a first plurality of trenches parallel to each other in a first direction, and a second plurality of trenches parallel to each other in a second direction; (b) transferring a useful layer from a donor substrate to the support substrate, the useful layer having a second coefficient of thermal expansion; wherein an intermediate layer is inserted between the front face of the support substrate and the useful layer, the intermediate layer having a coefficient of thermal expansion between the first and second coefficients of thermal expansion.