Embedded Interdigitated Transducer Structure for SAW Above 3 GHz
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Solution Overview
Problem
Surface acoustic wave (SAW) devices are limited to operating frequencies below 3 GHz due to stability issues with comb electrode miniaturization and electric losses, making it difficult to further miniaturize transducers using standard I-line lithography.
Innovation Solution
An inter-digitated transducer structure with embedded piezoelectric electrodes, where the electrode pitch satisfies the Bragg condition, allowing for higher phase velocities and reduced electro-mechanical coupling, enabling operation above 3 GHz without requiring advanced lithography tools. The electrodes have a smaller acoustic impedance than the piezoelectric layer, confining energy within the electrodes and allowing for a shear mode excitation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If comb electrode dimensions are miniaturized to achieve operation above 3 GHz, then operating frequency is improved, but structure stability deteriorates
Solution Approach 1:
The comb electrodes are embedded within grooves formed in the piezoelectric substrate, creating a nested structure where the electrodes are housed inside the substrate material. This embedding provides mechanical support and stability to the miniaturized electrodes, allowing them to maintain structural integrity at frequencies above 3 GHz while being confined within the substrate's groove structures.
Solution Approach 2:
The invention uses a composite structure combining the piezoelectric substrate material with the embedded comb electrodes. The substrate provides mechanical strength and stability, while the embedded electrodes maintain their electrical functionality. This composite approach allows the miniaturized electrodes to operate stably at high frequencies by leveraging the mechanical properties of the surrounding piezoelectric material.
2Speed
If comb electrode pitch is reduced to achieve higher frequencies, then operating frequency is improved, but electric losses increase
Solution Approach 1:
By nesting the comb electrodes within grooves in the piezoelectric substrate, the structure reduces parasitic capacitance and electromagnetic interference that cause electric losses. The embedding isolates the closely-spaced electrodes from each other and from the substrate surface, allowing smaller pitch dimensions to be used for higher frequency operation while minimizing energy loss through reduced coupling and radiation effects.
3Ease of manufacture
If standard I-line lithography is used for electrode fabrication, then manufacturing complexity is reduced, but minimum feature size increases
Solution Approach 1:
The groove embedding structure provides physical confinement and definition for the comb electrodes, allowing the lithography process to focus on patterning the electrodes within the pre-formed grooves rather than defining absolute positions from scratch. This nested approach relaxes the minimum feature size requirements by using the groove structure as a mechanical template, enabling standard I-line lithography to achieve the necessary precision for high-frequency 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
This configuration enables SAW devices to operate at frequencies above 3 GHz with improved stability and reduced electric losses, achieving higher equivalent phase velocities and allowing for the formation of acoustic wave devices with enhanced spectral purity and bandwidth.
Implementation Method 1
one or more inter-digitated transducers (IDTs) are formed over a surface propagating substrate and are used to convert acoustic waves to electrical signals and vice versa by exploiting the piezoelectric effect of the substrate
Implementation Method 2
A Rayleigh surface acoustic wave develops on the substrate by electrically exciting the fingers
Data Source
Figure 1
Figure 2
Figure 3a~3b
AI summary
A transducer structure (200) for an acoustic device comprising a piezoelectric layer (212), a pair of inter-digitated comb electrodes (206, 208), comprising a plurality of electrode means (206-1, 206-2, ..., 208-1, 208-2, ...) with a pitch p satisfying the Bragg condition given by p= lambda/2, lambda being the operating acoustic wavelength of said transducer, characterized in that the inter-digitated comb electrodes are embedded in the piezoelectric layer (212) said pair of inter-digitated comb electrodes comprises neighbouring electrode means belonging to different comb electrodes wherein the electrode means all have the same geometry; and wherein said pair of inter-digitated comb electrodes comprises one region (218) or more regions (218) in which two or more neighbouring electrode means (208-4, 208-5) belong to the same comb electrode while having an edge-to-edge distance to each other corresponding to the pitch p and having the same geometry such that, in use, the excitation of a wave propagating mode in the volume of the electrode means is taking place and is the predominant propagating mode of the structure. The invention relates also to a acoustic wave device comprising at least one transducer structure as described above and to a method for fabricating said transducer structure. The invention relates also to the use of the frequency of the bulk wave propagating in the electrode means of said transducer structure in an acoustic wave device to generate contribution at high frequency, in particular above 3GHz.