BAW Filter Electrode Stack for Epitaxial LiNbO3 on Silicon
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Solution Overview
Problem
Existing methods for producing lithium niobate-based piezoelectric layers on silicon substrates face challenges such as high production costs, loss of stoichiometry, and structural defects, particularly when using zinc oxide buffer layers, which are not compatible with integration into functional devices and result in textured appearances.
Innovation Solution
The use of a first electrode made from electrically conductive refractory transition metal nitrides, such as TiN, ZrN, TaN, HfN, or NbN, as a nucleation layer for epitaxially growing lithium niobate or lithium tantalate layers on silicon substrates, which blocks oxygen and lithium diffusion, maintains stoichiometry, and allows for high-quality crystalline growth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a transfer process (Smart-Cut) is used to fabricate BAW filter with LiNbO3 piezoelectric layer, then high crystalline quality and controlled stoichiometry are achieved, but numerous technical steps and high cost are required
Solution Approach 1:
The patent extracts the LiNbO3 piezoelectric layer from a donor substrate using a transfer process, separating the layer formation from the final device integration. This allows high-quality single-crystal layers to be grown on suitable donor substrates and then transferred to silicon-based substrates for device fabrication, reducing the complexity of directly growing high-quality layers on silicon.
Solution Approach 2:
The patent introduces a donor substrate as an intermediary medium to facilitate the transfer of LiNbO3 layers. The donor substrate serves as a temporary host for growing high-quality piezoelectric layers, which are then transferred to the final silicon-based substrates, enabling decoupling of layer growth conditions from device integration requirements.
2Ease of manufacture
If LiNbO3-based piezoelectric layer is synthesized on silicon substrate via ZnO buffer layer, then layer formation is achieved, but textured appearance, non-single-crystal structure, and adhesion problems occur
Solution Approach 1:
The patent employs a donor substrate that can be discarded after transferring the LiNbO3 layer to the silicon-based substrate. This disposable donor substrate approach allows the use of simple, cost-effective substrates for layer formation without compromising the quality of the final device structure.
Solution Approach 2:
The patent changes the growth conditions and substrate parameters during the epitaxial growth process to achieve single-crystal LiNbO3 layers. By controlling temperature, pressure, and chemical environment parameters during growth on the donor substrate, high crystalline quality is achieved before transfer to the final device substrate.
3Ease of manufacture
If LiNbO3-based piezoelectric layer is synthesized on silicon substrate via ZnO buffer layer, then layer formation is achieved, but compatibility with etching steps and adhesion are compromised
Solution Approach 1:
The patent extracts the problematic ZnO buffer layer from the final device structure by using a transfer process. The LiNbO3 layer is grown on a donor substrate and then transferred directly to the silicon-based substrate, eliminating the need for ZnO buffer layers and their associated adhesion and etching compatibility issues.
Solution Approach 2:
The patent creates a copy of the LiNbO3 layer structure on the donor substrate with the desired properties, then transfers this copy to the silicon-based substrate. This allows optimization of the layer structure during growth without being constrained by the requirements of direct growth on silicon, achieving better adhesion and etching compatibility.
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 enables the production of high-quality, stoichiometric lithium niobate layers directly on silicon substrates, reducing costs and enabling integration into silicon technology, with improved performance for high-frequency BAW filters.
Implementation Method 1
the first electrode is chosen in an electrically conductive refractory material based on nitride... Having a crystalline structure and/or lattice parameter compatible with a silicon-based substrate... Blocking the diffusion of Li atoms into Si
Implementation Method 2
the formation of the piezoelectric layer is done by epitaxy, on said first electrode, of a material of the type ABO3
Implementation Method 3
The core of BAW resonators is composed of a piezoelectric material that influences the final properties of the filter
Data Source
Figure 1~2
Figure 3A~3B
Figure 4A~4C
AI summary
The invention relates to an electro-acoustic device (1) comprising, stacked along a direction (z), a silicon-based substrate (10), a first electrode (21), a piezoelectric layer (30) based on a perovskite selected from lithium niobate LiNbO3, lithium tantalate LiTaO3, or a Li(Nb,Ta)O3 alloy, on the first electrode (21), and a second electrode (22) disposed on the piezoelectric layer (30). Advantageously, the first electrode (21) is made of an electrically conductive refractory material based on a nitride, such as TiN, VN, or TaN. The said electro-acoustic device is a volume acoustic wave filter guided on a Bragg reflector, comprising, between the substrate (10) and the first electrode (21), a stack (50) of layers (51, 52) exhibiting alternately strong and weak acoustic impedances.The low-impedance layers (52) are based on TiN or VN, and the high-impedance layers (51) are based on HfN or TaN. The invention also relates to a method for making such a device (1).