XBAR Filter Dielectric Layer Split for Spurious Mode Suppression
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Solution Overview
Problem
Current RF filters using acoustic wave resonators are not well-suited for higher frequency communications bands, such as those defined in the 5G NR standard, particularly for bands n77 and n79, and millimeter wave frequencies, due to limitations in handling transmit power and achieving sufficient frequency separation between shunt and series resonators, leading to inefficient spurious mode excitation.
Innovation Solution
The use of transversely-excited film bulk acoustic resonators (XBARs) with a back-side dielectric frequency-setting layer, which helps in reducing spurious modes by adjusting the thickness of the dielectric layer on the back side, thereby improving the electromechanical coupling and frequency separation between shunt and series resonators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single dielectric layer is used for frequency setting, then the structure is simple, but the frequency separation between shunt and series resonators is insufficient and spurious modes are excited
Solution Approach 1:
The single dielectric layer is divided into two separate dielectric layers: a first dielectric layer on the front side and a second dielectric layer on the back side of the piezoelectric substrate. This segmentation allows independent optimization of each layer's thickness to achieve sufficient frequency separation between shunt and series resonators while preventing spurious mode excitation, thereby resolving the contradiction between structural simplicity and frequency separation performance.
Solution Approach 2:
The frequency setting function is extended from a single-dimensional approach (one dielectric layer) to a two-dimensional approach by adding dielectric layers on both the front and back sides of the piezoelectric substrate. This dimensional expansion provides additional degrees of freedom in frequency control, enabling independent adjustment of resonant frequencies for both shunt and series resonators to achieve the required frequency separation.
2Reliability
If the dielectric layer thickness is increased to improve frequency separation, then frequency separation improves, but spurious modes are excited
Solution Approach 1:
By dividing the total dielectric thickness into two separate layers on opposite sides of the piezoelectric substrate, the frequency setting function is distributed. This allows the first and second dielectric layers to be independently optimized such that their combined effect achieves the required frequency separation while each individual layer maintains a thickness that prevents spurious mode excitation.
Solution Approach 2:
Different dielectric layers are positioned at different locations (front side and back side) with potentially different thicknesses optimized for their local functions. The first dielectric layer on the front side and the second dielectric layer on the back side can have different thickness values tailored to control specific resonant modes, allowing frequency separation improvement without triggering spurious modes.
3Speed
If higher frequency bands are used for 5G communications, then bandwidth increases, but transmit power handling capability decreases
Solution Approach 1:
The dual-sided dielectric layer configuration provides additional control dimensions for resonator design at higher frequencies. By independently adjusting the thickness of the first and second dielectric layers, the resonant frequencies can be precisely controlled to achieve the required frequency separation even at millimeter-wave frequencies, enabling 5G bandwidth requirements to be met while maintaining power handling capability through optimized electromechanical coupling.
Solution Approach 2:
The invention changes the physical parameters of the resonator structure by introducing two dielectric layers with different thicknesses on opposite sides of the piezoelectric substrate. This parameter modification allows precise control of resonant frequencies and electromechanical coupling coefficients, enabling the resonator to operate effectively at higher 5G frequency bands while maintaining adequate transmit power handling capability.
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 implementation of XBARs with a back-side dielectric frequency-setting layer effectively reduces spurious modes and enhances the electromechanical coupling, enabling the design of high-performance RF filters capable of handling higher frequency bands with improved bandwidth and reduced spurious mode excitation.
Implementation Method 1
a plate of piezoelectric material
Implementation Method 2
adjusting the thickness of the dielectric layer on the back side, thereby improving the electromechanical coupling and frequency separation between shunt and series resonators
Data Source
AI summary
Acoustic filters and methods of fabrication are disclosed. A filter device includes a substrate and a single-crystal piezoelectric plate, a back surface of the piezoelectric plate attached to a surface of the substrate. The filter device includes a plurality of acoustic resonators including one or more shunt resonators and one or more series resonators. Each of the plurality of acoustic resonators includes an interdigital transducer (IDT) formed on the front surface of the piezoelectric plate, interleaved fingers of the IDT disposed on a respective diaphragm formed by a respective portion of the piezoelectric plate that spans a respective cavity in the substrate. A divided frequency setting layer is formed on at least some of the one or more shunt resonators but not on the one or more series resonators.