Piezoelectric Substrate Bonding With Nitrogen-Rich Oxide Interfaces
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Solution Overview
Problem
The characteristics of piezoelectric devices, particularly the electro-mechanical coupling factor k2, are deteriorated when lithium niobate or lithium tantalate substrates are thinned, leading to reduced performance in acoustic wave devices due to crystallinity damage and insufficient bonding strength.
Innovation Solution
A bonded body is created with a silicon oxide layer having a higher nitrogen concentration between the piezoelectric material substrate and the supporting substrate, achieved by providing a first silicon oxide film on the piezoelectric material and a second silicon oxide film on the supporting substrate, then irradiating plasma at a temperature of 150° C or lower for direct bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the piezoelectric material substrate is thinned to improve device performance, then the device characteristics are improved, but the crystallinity is deteriorated and the electro-mechanical coupling factor is reduced
Solution Approach 1:
The patent applies preliminary action by performing plasma activation treatment on the bonding surfaces before bonding. This preliminary treatment creates a nitrogen-enriched layer at the bonding interface, which prevents crystallinity deterioration during subsequent substrate thinning processes. The activated surface forms a protective structure that maintains crystal integrity even when the substrate is thinned to improve device characteristics.
Solution Approach 2:
The patent changes the chemical composition parameters at the bonding interface by introducing nitrogen through plasma activation. This parameter change creates a nitrogen-enriched layer that stabilizes the crystalline structure during thinning. The nitrogen concentration is controlled to be 1×10^19 to 1×10^21 atoms/cm³, which optimizes both the maintenance of crystallinity and the electro-mechanical coupling factor during substrate thinning.
2Length of moving object
If the piezoelectric material substrate is thinned to reduce size, then the device miniaturization is achieved, but the electro-mechanical coupling factor k2 is reduced
Solution Approach 1:
The plasma activation treatment is performed as a preliminary step before substrate thinning. This treatment creates a nitrogen-enriched protective layer that preserves the electro-mechanical coupling factor even when the substrate is thinned. The preliminary activation prevents the degradation that would normally occur during thinning processes, allowing miniaturization without sacrificing performance.
Solution Approach 2:
The nitrogen-enriched layer acts as an intermediary between the piezoelectric substrate and the external environment during thinning. This intermediate layer protects the crystal structure from damage during processing, maintaining the electro-mechanical coupling factor. The nitrogen layer serves as a buffer that prevents direct exposure of the piezoelectric material to damaging conditions during thinning.
3Strength
If plasma activation is applied to bond the substrates, then the bonding strength is improved, but the substrate temperature increases which may damage the piezoelectric material
Solution Approach 1:
The patent changes the plasma parameters by using low-temperature plasma activation at temperatures of 150°C or lower. This parameter change allows sufficient nitrogen incorporation into the bonding interface to achieve strong bonding (bonding strength of 10 MPa or more) without exposing the piezoelectric substrate to temperatures that would cause crystallinity damage or material degradation.
Solution Approach 2:
The plasma activation is performed in a controlled inert or low-reactivity environment that enables nitrogen transfer to the bonding surface without requiring high temperatures. The plasma state provides an inert yet reactive environment that facilitates chemical modification of the surface at low temperatures, achieving strong bonding while protecting the temperature-sensitive piezoelectric material.
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 method suppresses the deterioration of crystallinity and maintains high electro-mechanical coupling factors even at thinner substrate thicknesses, enhancing the performance of acoustic wave devices.
Implementation Method 1
irradiating a plasma onto a bonding face of a first silicon oxide film and a bonding face of a second silicon oxide film at a temperature of 150° C. or lower to activate the bonding faces
Implementation Method 2
an average value of a nitrogen concentration of the silicon oxide layer is higher than a nitrogen concentration at an interface between the silicon oxide layer and the supporting substrate and higher than a nitrogen concentration at an interface between the silicon oxide layer and the piezoelectric material substrate
Data Source
AI summary
A bonded body includes a supporting substrate, a silicon oxide layer provided on the supporting substrate, and a piezoelectric material substrate provided on the silicon oxide layer and composed of a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate. An average value of a nitrogen concentration of the silicon oxide layer is higher than a nitrogen concentration at an interface between the silicon oxide layer and supporting substrate and higher than a nitrogen concentration at an interface between the silicon oxide layer and piezoelectric material substrate.