LiTaO3 Bonded Substrate with Surface Li Diffusion for Stable SAW
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Solution Overview
Problem
Conventional methods for manufacturing lithium tantalate single crystal substrates for surface acoustic wave devices result in high warpage, cracking, and increased costs due to high processing temperatures, and lack of single polarization treatment leads to low Q values and temperature-dependent frequency shifts.
Innovation Solution
A lithium tantalate single crystal substrate with a 36° Y-49° Y cut orientation undergoes vapor phase Li diffusion to create a Li concentration profile with higher concentrations near the surface and lower concentrations towards the core, followed by single polarization treatment, and is bonded to a base substrate with a pseudo-stoichiometric LiTaO3 surface layer removed from the bonding surface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If high temperature vapor phase Li diffusion treatment is applied to create uniform Li concentration throughout the substrate, then frequency stability against temperature change is improved, but warpage and cracking increase significantly
Solution Approach 1:
The patent applies local quality by creating a non-uniform Li concentration distribution where the surface region (0-18μm depth) has higher Li concentration than the bulk substrate. This localized modification at the surface layer improves frequency stability without requiring uniform transformation throughout the entire substrate, thereby avoiding excessive warpage and cracking that would result from high-temperature bulk treatment.
Solution Approach 2:
The patent changes the concentration parameter of Li by controlling the vapor phase diffusion process to achieve a specific concentration gradient. The surface region reaches Li-rich composition (Li:Ta ratio of 50-α:50+α where α=0.5-2.0) while the bulk remains stoichiometric, creating the desired parameter distribution that balances frequency stability with manufacturing quality.
2Stability of the object's composition
If the entire substrate is transformed to Li-rich composition by gas phase equilibrium method, then frequency stability is improved, but productivity decreases due to high manufacturing temperature
Solution Approach 1:
The patent applies local quality by limiting the Li diffusion treatment to only the surface region (0-18μm depth) rather than transforming the entire substrate. This localized approach reduces the manufacturing temperature requirement and processing time compared to bulk transformation, thereby improving productivity while still achieving the necessary frequency stability through surface layer modification.
Solution Approach 2:
The patent applies partial action by performing Li diffusion treatment only on the surface region (0-18μm depth) rather than the entire substrate thickness. This partial transformation achieves the critical frequency stability requirement through surface layer modification without the excessive processing time and high temperatures needed for complete substrate transformation, thus improving productivity.
3Stability of the object's composition
If Li diffusion treatment is applied to create Li-rich surface layer, then frequency stability is improved, but warpage occurs due to high processing temperature
Solution Approach 1:
The patent applies local quality by concentrating the Li diffusion treatment in the surface region (0-18μm depth) rather than treating the entire substrate uniformly. This localized modification creates minimal thermal stress and compositional gradient throughout the bulk material, significantly reducing warpage while still achieving improved frequency stability through the modified surface layer.
4Stability of the object's composition
If vapor phase Li diffusion is used to modify the substrate, then frequency stability is improved, but quality fluctuation occurs due to varying distance from Li source
Solution Approach 1:
The patent applies local quality by defining a specific treatment depth (0-18μm from surface) where Li concentration is controlled to be substantially uniform. This localized surface treatment approach ensures consistent quality across the substrate by limiting the diffusion zone to the surface region, avoiding the quality fluctuation that would occur in bulk treatment where distance from the Li source varies significantly.
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 reduces warpage and cracking, enhances temperature stability, and achieves high electromechanical coupling coefficients and Q values, making the substrate suitable for broadband applications in smartphones with improved productivity and cost-effectiveness.
Implementation Method 1
undergoes vapor phase Li diffusion to create a Li concentration profile with higher concentrations near the surface and lower concentrations towards the core
Implementation Method 2
followed by single polarization treatment
Implementation Method 3
A surface acoustic wave (SAW) device formed with a comb-like electrode (IDT: Interdigital Transducer) for exciting a surface acoustic wave on a piezoelectric substrate
Data Source
AI summary
[Object]It is an object of the present invention to provide a lithium tantalate single crystal substrate which undergoes only small warpage, is free from cracks and scratches, has better temperature non-dependence characteristics and a larger electromechanical coupling coefficient than a conventional Y-cut LiTaO3 substrate.[Means to solve the Problems]The lithium tantalate single crystal substrate of the present invention is a rotated Y-cut LiTaO3 single crystal substrate having a crystal orientation of 36° Y-49° Y cut characterized in that: the substrate is diffused with Li from its surface into its depth such that it has a Li concentration profile showing a difference in the Li concentration between the substrate surface and the depth of the substrate; and the substrate is treated with single polarization treatment so that the Li concentration is substantially uniform from the substrate surface to a depth which is equivalent to 5-15 times the wavelength of either a surface acoustic wave or a leaky surface acoustic wave propagating in the LiTaO3 substrate surface.