Elastic Wave Stack Structure for Piezoelectric Substrate Warpage
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Solution Overview
Problem
Elastic wave devices face issues with warpage and deterioration of characteristics due to stress generated during the bonding process of multiple films on the piezoelectric substrate, particularly when using methods described in existing technologies like WO 2012/086441 A1 and WO 2012/086639 A1.
Innovation Solution
The elastic wave device incorporates a supporting substrate, an acoustic multilayer film with at least four acoustic impedance layers, and a piezoelectric substrate with a bonding layer positioned between the acoustic impedance layers to reduce stress and prevent warpage, where the bonding layer functions as an insulating layer with a thickness of about 5 nm or less, and the substrates are made of materials like Si and LiNbO3 or LiTaO3 to enhance thermal expansion coefficient matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If multiple films are formed on the piezoelectric film side to bond the supporting substrate, then the bonding strength is improved, but stress is generated in the films causing the piezoelectric film to warp and characteristics to deteriorate
Solution Approach 1:
A bonding layer is introduced as an intermediary component between the piezoelectric substrate and the acoustic impedance layers. This bonding layer serves as a mediator that provides adequate bonding strength while minimizing stress transmission to the piezoelectric film, thereby preventing warpage and characteristic deterioration.
Solution Approach 2:
The thickness of the bonding layer is precisely controlled to be 5 nm or less. By changing the dimensional parameter of the bonding layer to such a thin dimension, sufficient bonding strength is achieved while the stress-induced warpage of the piezoelectric film is effectively suppressed.
2Reliability
If the bonding layer thickness is increased to improve bonding strength, then the bonding reliability is improved, but the stress on the piezoelectric film increases causing warpage
Solution Approach 1:
The thickness of the bonding layer is optimized to be 5 nm or less. This parameter change achieves the optimal balance where the bonding layer is thick enough to provide reliable bonding between layers, yet thin enough to minimize the stress it imposes on the piezoelectric substrate, thereby preventing warpage.
3Reliability
If many acoustic impedance layers are formed to improve wave confinement, then the wave confinement performance is improved, but the manufacturing complexity and stress increase
Solution Approach 1:
The acoustic multilayer film is segmented into distinct acoustic impedance layers with alternating high and low acoustic impedance values. This segmentation creates effective acoustic reflection and confinement without requiring an excessive number of layers, thereby managing manufacturing complexity while achieving wave confinement performance.
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 effectively prevents warpage of the piezoelectric substrate and maintains the characteristics of the elastic wave device, ensuring efficient confinement of plate waves and reducing the likelihood of deterioration.
Implementation Method 1
An absolute value of a thermal expansion coefficient of the piezoelectric substrate is larger than an absolute value of a thermal expansion coefficient of the supporting substrate
Implementation Method 2
The at least four acoustic impedance layers include at least one low acoustic impedance layer and at least one high acoustic impedance layer having an acoustic impedance higher than the low acoustic impedance layer
Data Source
AI summary
An elastic wave device includes a supporting substrate, an acoustic multilayer film on the supporting substrate, a piezoelectric substrate on the acoustic multilayer film, and an IDT electrode on the piezoelectric substrate. An absolute value of a thermal expansion coefficient of the piezoelectric substrate is larger than an absolute value of a thermal expansion coefficient of the supporting substrate. The acoustic multilayer film includes at least four acoustic impedance layers. The elastic wave device further includes a bonding layer provided at any position in a range of from inside the first acoustic impedance layer from the piezoelectric substrate side towards the supporting substrate side, to an interface between the third acoustic impedance layer and the fourth acoustic impedance layer.