Piezoelectric Substrate Bonding for Thin Lithium Niobate Wafers
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Solution Overview
Problem
The performance of piezoelectric devices deteriorates when lithium niobate or lithium tantalate substrates are thinned due to the exertion of shearing stress during polishing, leading to separation from the silicon substrate and reduced electromechanical coupling factor k2 in acoustic wave devices.
Innovation Solution
A bonded body is created by irradiating oxygen plasma onto the bonding surfaces of the piezoelectric material substrate and the silicon oxide layer at a temperature of 150° C. or lower to achieve a surface resistivity of 1.7×10^15 Ω/□ or higher, thereby enhancing the bonding strength and preventing performance deterioration during thinning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the piezoelectric material substrate is thinned by polishing, then the processing denatured layer can be removed and device performance is not deteriorated, but a large shearing stress is exerted onto the substrate causing separation from the silicon substrate
Solution Approach 1:
The piezoelectric material substrate is bonded to the silicon substrate before thinning processing. This preliminary bonding action prevents substrate separation during subsequent polishing by providing mechanical support and distributing shear stress, while still allowing the polishing process to effectively remove the processing denatured layer and maintain device performance.
Solution Approach 2:
The silicon substrate acts as an intermediary supporting structure that reinforces the piezoelectric material substrate during thinning. This intermediary provides the necessary mechanical strength to withstand polishing shear stress without compromising the piezoelectric properties or preventing effective removal of the denatured layer.
2Manufacturing precision
If the piezoelectric material substrate is made thinner by polishing, then the processing denatured layer is removed, but the performance of the bonded body deteriorates particularly the electromechanical coupling factor k2
Solution Approach 1:
Bonding the piezoelectric substrate to the silicon substrate before thinning prevents performance deterioration during processing. The supporting substrate provides mechanical stability that preserves the electromechanical coupling factor while still allowing effective removal of the processing denatured layer through polishing.
Solution Approach 2:
The invention changes the processing parameters by performing thinning before complete substrate separation, maintaining the substrate in a supported state. This parameter change in processing sequence allows achieving both thin substrate formation with denatured layer removal and preservation of electromechanical coupling characteristics.
3Length of moving object
If ion injection is used to thin the lithium niobate or lithium tantalate substrate, then the substrate can be thinned, but the crystallinity is deteriorated due to damage during ion injection
Solution Approach 1:
The invention replaces ion injection (a mechanical/physical bombardment process that damages crystallinity) with polishing as the thinning method. By substituting the thinning mechanism, the substrate can be thinned while preserving crystallinity, as polishing mechanically removes material without the damaging effects of ion bombardment.
Solution Approach 2:
The substrate is bonded to the silicon substrate before thinning processing. This preliminary bonding enables the use of polishing instead of ion injection, as the supported substrate can withstand polishing forces while maintaining crystallinity, unlike free-standing substrates that would require damaging ion injection to achieve similar thinning.
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 method effectively suppresses the deterioration of the piezoelectric device performance, particularly the electromechanical coupling factor k2, while maintaining the bonding strength, allowing for the successful fabrication of thinner piezoelectric substrates without separation from the silicon substrate.
Implementation Method 1
Plasma activation is used for realizing the SOI substrate. This is because the bonding can be realized at a relatively low temperature (400° C.).
Implementation Method 2
When Si and SiO2/Si are bonded with each other by plasma activation, a sufficiently high bonding strength is obtained by the formation of Si—O—Si bond at the bonding interface. Further, at the same time, Si is oxidized to Sio2 so that the flatness is improved and the bonding as described above is facilitated at the uppermost surface
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. The surface resistivity of the piezoelectric material substrate on the side of the silicon oxide layer is 1.7×1015Ω/□ or higher.