Composite SAW Substrates for Stable Shear-Wave RFID Sensing
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Solution Overview
Problem
Surface acoustic wave (SAW) tag devices using bulk Lithium Niobate have limited electromechanical coupling coefficient and temperature stability, restricting their application to short-range RFID and temperature measurement, and are not suitable for measuring mechanical parameters due to strong temperature drift.
Innovation Solution
A composite substrate with a piezoelectric layer, such as Lithium Niobate or Lithium Tantalate, oriented at specific crystallographic angles on a base substrate like Silicon, promotes shear wave propagation, enhancing electromechanical coupling coefficient and temperature stability, allowing for extended range RFID and measurement of mechanical parameters like force and pressure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If bulk Lithium Niobate is used as substrate, then the device structure is simple, but the electromechanical coupling coefficient is limited to 5-6%
Solution Approach 1:
The patent uses a composite substrate consisting of a piezoelectric layer (Lithium Niobate or Lithium Tantalate) deposited on a base substrate (Silicon, Sapphire, or Quartz). This composite structure enables shear wave propagation with electromechanical coupling coefficients exceeding 20%, compared to 5-6% for bulk Lithium Niobate, while maintaining structural integrity and enabling remote interrogation at extended distances.
2Speed
If bulk Lithium Niobate is used, then the device can operate at 2.45GHz, but the temperature drift is strong at 70ppm/K
Solution Approach 1:
The composite substrate of piezoelectric layer on base substrate enables shear wave propagation with significantly improved temperature stability. The patent reports temperature coefficients of delay (TCD) reduced to below 10ppm/K, compared to 70ppm/K for bulk Lithium Niobate, while maintaining 2.45GHz operation capability.
3Device complexity
If Rayleigh wave mode is used, then the device structure is conventional, but the electromechanical coupling coefficient cannot exceed 6%
Solution Approach 1:
The patent changes the wave propagation mode from Rayleigh wave to shear wave by adjusting the crystallographic orientation of the piezoelectric layer. This parameter change enables electromechanical coupling coefficients above 20%, compared to the 6% limit of Rayleigh wave mode, while maintaining the same device structure configuration.
4Reliability
If the electromechanical coupling coefficient is increased, then the remote interrogation distance is extended, but the temperature stability deteriorates
Solution Approach 1:
The composite substrate structure simultaneously achieves high electromechanical coupling coefficient (above 20%) and improved temperature stability (TCD below 10ppm/K). The combination of piezoelectric layer with specific crystallographic orientation on a stable base substrate enables both high coupling for extended range and low temperature drift for stable operation.
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 solution increases the electromechanical coupling coefficient by a factor of 2 to 5, improves temperature stability, and extends the range of remote interrogation, enabling SAW tag devices to measure mechanical parameters effectively while reducing temperature-related interference.
Implementation Method 1
a piezoelectric layer, such as Lithium Niobate or Lithium Tantalate, oriented at specific crystallographic angles on a base substrate like Silicon, promotes shear wave propagation, enhancing electromechanical coupling coefficient
Implementation Method 2
the shear waves are the predominant waves propagating in the piezoelectric layer and not the Rayleigh waves like in the state of the art
Data Source
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Figure 3a~3b
AI summary
The invention relates to a surface acoustic wave tag device, comprising an acoustic wave propagating substrate (202), at least one transducer structure (204) comprising inter-digitated comb electrodes (216), and at least one reflecting means, the reflecting means comprising at least one reflector (206, 208, 210), characterised in that the acoustic wave propagation substrate (202) is a composite substrate (202) comprising a base substrate (226) and a piezoelectric layer (224), wherein the crystallographic orientation of the piezoelectric layer (224) with respect to the base substrate (226) is such that the propagation of a shear wave inside the piezoelectric layer (224) and in the direction of propagation corresponding to the acoustic wave is enabled. The invention relates also to a physical quantity determining device and to a fabrication method of such surface acoustic wave tag device (200).