Investigation of Surface Acoustic Wave Propagation Characteristics in New Multilayer Structure - SiO2/IDT/LiNbO3/Diamond/Si

At a Glance

Metadata	Details
Publication Date	2021-10-21
Journal	Micromachines
Authors	Hanqiang Zhang, Hongliang Wang
Institutions	North University of China
Citations	28
Analysis	Full AI Review Included

Executive Summary

This research presents a theoretical investigation using Finite Element Method (FEM) simulation for a novel multilayer Surface Acoustic Wave (SAW) structure designed for high-frequency, wideband, and temperature-stable applications.

Novel Structure: The device utilizes a SiO₂/IDT/128°YX-LiNbO₃/diamond/silicon layered stack, leveraging the complementary advantages of high-velocity diamond and high-piezoelectricity LiNbO₃, capped by temperature-compensating SiO₂.
High Velocity: The structure achieved a high phase velocity (Vp) of up to 10 km/s, significantly increasing the potential operating frequency compared to traditional LiNbO₃ single crystals (144% increase).
High Coupling: A maximum electromechanical coupling coefficient (k²) of 6.47% was obtained in the Sezawa mode, enabling wide bandwidth filters.
Temperature Stability: By optimizing the thickness ratio of the SiO₂ cover layer, a zero-Temperature Coefficient of Frequency (TCF) (0 ppm/°C) was achieved, crucial for stable operation.
Frequency Range: The design supports operation above the GHz range (f₀ ≥ 1 GHz), making it suitable for 5G and next-generation communication systems.
Methodology: The performance parameters (Vp, k², TCF) were accurately calculated by sweeping the normalized film thicknesses (h/a) of the LiNbO₃, Al electrode, and SiO₂ layers using 2D FEM simulation (COMSOL).

Technical Specifications

Parameter	Value	Unit	Context
Maximum Phase Velocity (Vp)	10	km/s	Sezawa mode (h₁/a = 0.1, SiO₂-free structure)
Maximum Coupling Coefficient (k²)	6.47	%	Sezawa mode (Optimized h₁/a = 0.4 and h₅/a = 0.2)
Temperature Coefficient of Frequency (TCF)	0	ppm/°C	Achieved through SiO₂ compensation layer
Operating Frequency (f₀)	≥1	GHz	Calculated for λ = 6 µm
Simulated Resonant Frequency (f₀)	1023	MHz	Specific LCRF design (h₁/a=0.7, h₄/a=0.1, h₅/a=0.2)
Rayleigh Wave Velocity (Simulated)	6138	m/s	At f₀ = 1023 MHz
Wavelength (λ)	6	µm	Standard design parameter
Metallization Rate (MR)	0.5	Ratio	Electrode width (a) / Pitch (p)
LiNbO₃ Density (ρ)	4700	Kg/m³	Material constant
Diamond Longitudinal Sound Velocity	18	km/s	Material property
LiNbO₃ TCF (Single Crystal)	-75	ppm/°C	Reference value
Minimum Insertion Loss (S₂₁)	less than 10	dB	Simulated LCRF frequency response

Key Methodologies

The propagation characteristics and filter performance were analyzed using 2D Finite Element Method (FEM) simulations via COMSOL Multiphysics 5.6.

Model Construction: A 2D unit model of the SiO₂/IDT/LiNbO₃/diamond/Si layered structure was established. The substrate thickness was set to greater than 1.5λ to ensure accurate confinement of surface wave energy.
Boundary Conditions:
- A Perfectly Matched Layer (PML) of 1λ thickness was applied to the bottom boundary to absorb propagating waves and prevent spurious reflections.
- Periodic boundary conditions were applied to the IDT structure to reduce computational time.
- The top surface (SiO₂) was set as a free boundary.
Geometric Parameter Optimization: The simulation involved sweeping the normalized thickness ratios (h/a, where ‘a’ is electrode width) to determine optimal performance:
- LiNbO₃ thickness ratio (h₁/a) varied from 0.1 to 1.0.
- Al electrode thickness ratio (h₄/a) varied from 0.1 to 0.5.
- SiO₂ cover layer thickness ratio (h₅/a) varied from 0.1 to 1.0.
Analysis Types:
- Modal Analysis: Used to calculate the phase velocity (Vp) and electromechanical coupling coefficient (k²) for both Rayleigh and Sezawa modes.
- Transient Analysis: Conducted over 50 ns (timestep 0.1 ns) to observe the time-domain propagation of acoustic signals and particle displacement.
- S-Parameter Analysis: Used to determine the frequency response (S₂₁) and insertion loss of the designed Longitudinal Coupled Resonator Filter (LCRF).
Meshing: The model was finely meshed using quadrilateral elements, with the minimum unit size set to 1.26 nm to maintain calculation accuracy.

Commercial Applications

The high-performance characteristics (high Vp, high k², zero TCF) of the SiO₂/IDT/LiNbO₃/Diamond/Si structure make it ideal for demanding RF and sensor applications.

5G and 6G Mobile Communication: Enables the design and manufacture of high-frequency (GHz range) and wideband SAW filters and duplexers required for modern mobile networks.
High-Power RF Electronics: The diamond substrate provides exceptional thermal conductivity, allowing the devices to operate reliably under high-power conditions, crucial for base stations and radar systems.
Phased Array Radar Systems: Requires components with high frequency stability and low insertion loss, directly addressed by the zero-TCF and high Q-factor potential of this structure.
Industrial Control and Environmental Sensors: The small size, low cost, and high temperature stability of SAW devices make them suitable for robust sensing applications.
High-Performance Resonators: The high k² value allows for the creation of wideband LCRFs with low insertion loss, improving signal integrity in complex RF front-ends.

View Original Abstract

Surface acoustic wave (SAW) devices are widely used in many fields such as mobile communication, phased array radar, and wireless passive sensor systems. With the upgrade of mobile networks, the requirements for the performance of SAW devices have also increased, and high-frequency wideband SAW devices have become an important research topic in communication systems and other application fields. In this paper, a theoretical study for the realization of a layered SAW filter based on a new SiO2/IDT/128°YX-LiNbO3/diamond/silicon layered structure using the modeling software COMSOL Multiphysics is presented. The effects of lithium niobate (LiNbO3), an interdigital transducer (IDT), and SiO2 thin films on the evolution of the phase velocity, electromechanical coupling coefficient (k2), and temperature coefficient of frequency were studied by employing a finite element method simulation. Furthermore, a longitudinal coupling resonator filter was designed. To investigate the SAW characteristics of the filter, a transient analysis was conducted to calculate the electrical potential and particle displacement under the resonance condition and to analyze the frequency response. The study concluded that this new multilayer structure can be applied to design and manufacture a variety of high-frequency and wideband SAW filters with a temperature compensation function, for operation above the GHz range.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi12111286

References

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