XBAR Frequency Trimming Before Cavity Etch Using Electrical Measurements
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Solution Overview
Problem
Current RF filters using acoustic wave resonators are not well-suited for higher frequency communications bands above 3 GHz, such as those proposed for future wireless communication systems, due to limitations in design and performance parameters like insertion loss, rejection, and power handling.
Innovation Solution
The development of transversely-excited film bulk acoustic resonators (XBARs) with a thin film conductor pattern on a piezoelectric plate, utilizing a shear acoustic mode to achieve high piezoelectric coupling and suitable for frequencies above 3 GHz, which includes the use of dielectric layers to set frequency separation between shunt and series resonators and varying the thickness of piezoelectric diaphragms for frequency tuning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional acoustic wave resonators are used for RF filters, then the filters can operate at lower frequencies, but they cannot achieve suitable performance for higher frequency communications bands above 3 GHz
Solution Approach 1:
The patent changes the fundamental operating parameters of the resonator by transitioning from longitudinal acoustic modes to transverse shear acoustic modes. This parameter change enables the resonator to operate at higher frequencies (above 3 GHz) while maintaining suitable performance characteristics including insertion loss, rejection, and power handling capabilities
Solution Approach 2:
The invention replaces the conventional longitudinal acoustic wave mechanism with a transverse shear acoustic wave mechanism. This substitution fundamentally changes how acoustic energy is generated and propagated, enabling high-frequency operation by utilizing shear modes that are better suited for millimeter-wave communications
2Manufacturing precision
If dielectric layers are added to set frequency separation between shunt and series resonators, then frequency control is improved, but device complexity increases
Solution Approach 1:
The patent applies local quality by adding dielectric layers specifically to certain resonators (either series or shunt resonators) to create frequency separation. Instead of modifying all resonators uniformly, the dielectric layers are selectively placed on specific resonator types, achieving the desired frequency control while minimizing overall device complexity
Solution Approach 2:
The dielectric layers change the effective electrical parameters (capacitance, impedance) of the resonators they are applied to. This parameter change creates the necessary frequency separation between series and shunt resonators, enabling precise frequency control in the filter design
3Adaptability or versatility
If piezoelectric plate thickness is varied for frequency tuning, then frequency adaptability is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent utilizes parameter changes by varying the piezoelectric plate thickness across different regions or resonators. This thickness variation directly tunes the resonant frequencies, providing frequency adaptability. The method accepts increased manufacturing precision requirements as a trade-off for achieving the desired frequency tuning range
Solution Approach 2:
Instead of tuning frequency through lateral dimensional changes, the patent exploits the thickness dimension (z-direction) of the piezoelectric plate. By controlling thickness variations, frequency tuning is achieved through a different dimensional parameter, expanding the available tuning range
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
XBARs provide improved performance for high-frequency RF filters, enabling broader bandwidth and better power handling, making them suitable for millimeter-wave communications by minimizing viscous losses and achieving high piezoelectric coupling, thus enhancing the design and implementation of microwave and millimeter-wave filters.
Implementation Method 1
a transducer assembly including interdigital transducers (IDTs) for each of the four series resonators 510A, 510B, 510C, and 510D and three shunt resonators 520A, 520B, and 520C
Implementation Method 2
utilizing a shear acoustic mode to achieve high piezoelectric coupling
Data Source
AI summary
Methods of fabricating acoustic resonators are disclosed. A back surface of a single-crystal piezoelectric plate is bonded to a surface of a substrate. A conductor pattern is formed on the front surface of the piezoelectric plate, the conductor pattern including a plurality of interdigital transducers (IDTs) of a plurality of resonators. A dielectric passivation/tuning layer is formed over the conductor pattern and the front surface of the piezoelectric plate. Electrical measurements are made on at least some of the plurality of resonators. Material is selectively removed from the dielectric passivation/tuning layer in accordance with the electrical measurements. After removing material from the dielectric passivation/tuning layer, cavities are formed in the substrate such that interleaved fingers of each IDT are disposed on a respective diaphragm spanning a respective cavity