Millimeter-Wave Acoustic Resonators With Suspended Piezoelectric Films
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Solution Overview
Problem
Implementing high figure of merit (FoM) acoustic resonators at millimeter-wave frequencies is challenging due to short wavelength, leading to reduced quality factor (Q) and electromechanical coupling (k2), and alternative methods like higher-order Lamb modes result in minimal k2 and Q.
Innovation Solution
The use of intermediate layers in the fabrication of piezoelectric devices based on thin-film lithium niobate/lithium tantalate or aluminum nitride/scandium aluminum nitride, employing higher order first and third antisymmetric bulk acoustic tones, allows for high k2, Q, and FoM at millimeter-wave frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If acoustic resonators are frequency scaled into millimeter-wave to achieve smaller size, then device compactness is improved, but quality factor and electromechanical coupling deteriorate due to short wavelength leading to sub-50 nm lateral feature size
Solution Approach 1:
The patent changes the operational mode from fundamental acoustic modes to higher-order Lamb modes (specifically third-order antisymmetric A3 mode), which allows the resonator to achieve millimeter-wave frequencies (57 GHz) without requiring sub-50 nm lateral feature sizes. This parameter change in the acoustic mode order enables maintaining larger feature sizes while operating at higher frequencies, thus preserving quality factor.
Solution Approach 2:
The patent employs a composite piezoelectric film structure consisting of multiple layers (110 nm 128° Y-cut lithium niobate with aluminum nitride/scandium aluminum nitride) on a substrate. This composite material approach enhances electromechanical coupling (k2 = 7.3%) and quality factor (Q = 56) at millimeter-wave frequencies, achieving a figure of merit of 4.1 that is an order of magnitude higher than state-of-the-art acoustic resonators.
2Device complexity
If higher-order Lamb modes are used for frequency scaling without thin films, then device structure is simplified, but electromechanical coupling decays dramatically due to internal charge cancellation
Solution Approach 1:
The patent applies local quality by using periodically poled piezoelectric film (P3F) lithium niobate with specific crystallographic orientation (128° Y-cut) and thickness (110 nm). This localized structural optimization at the film level enables the third-order antisymmetric Lamb mode to achieve high electromechanical coupling (k2 = 7.3%) at 57 GHz, counteracting the typical decay associated with higher-order modes.
Solution Approach 2:
The patent changes the piezoelectric film parameters including material composition (lithium niobate with aluminum nitride/scandium aluminum nitride), thickness (110 nm), and crystal orientation (128° Y-cut) to optimize performance. These parameter changes enable the resonator to achieve high figure of merit (4.1) at millimeter-wave frequencies while using higher-order Lamb modes, resolving the trade-off between mode order and coupling efficiency.
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 record-breaking performance at millimeter-wave frequencies with high FoM, compact size, and low losses, facilitating more compact 5G/6G signal processing components.
Implementation Method 1
acoustic resonators, where electromagnetic (EM) energy is converted into and stored as mechanical vibration
Implementation Method 2
employing higher order first and third antisymmetric bulk acoustic tones
Data Source
AI summary
Exemplary multimeter-wave acoustic resonators and methods employing intermediate layers in the fabricating of piezoelectric devices based on thin-film lithium niobate/lithium tantalate or aluminum nitride/scandium aluminum nitride on a substrate in a film stack that can use higher order first and third antisymmetric bulk acoustic tones. In some embodiments, the acoustic resonator comprises a base substrate of a material with low electromagnetic loss, a piezoelectric film layer that is suspended over the base substrate, and an interdigitated electrode (IDE) array formed by a pair of independently addressable microelectrode arrays disposed on a top surface of the piezoelectric film layer to resonantly vibrate at a range of frequencies greater than 6 GHz. The piezoelectric film layer was formed over an intermediate or sacrificial layer positioned between the base substrate and the piezoelectric film layer and then etched to provide the piezoelectric film layer suspended over the base substrate.