XBAR Ladder Filter Using LiNbO3 and Rotated LiTaO3 for Low TCF
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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 proposed for future wireless networks, due to limitations in performance parameters like insertion loss, rejection, and temperature coefficient of frequency (TCF).
Innovation Solution
The use of transversely-excited film bulk acoustic resonators (XBARs) made from lithium niobate and lithium tantalate piezoelectric materials, which offer high electromechanical coupling and low TCF, allowing for the design of filters with wide bandwidth and improved temperature stability by combining the features of both materials in a single filter structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional acoustic wave resonators are used, then the filter structure is simple and easy to manufacture, but the filter performance is insufficient for higher frequency bands with limited bandwidth and poor temperature stability
Solution Approach 1:
The patent employs composite piezoelectric material structures, specifically combining lithium niobate and lithium tantalate layers to create XBAR resonators. This composite approach leverages the high electromechanical coupling of lithium niobate with the low TCF of lithium tantalate, achieving superior temperature stability and bandwidth performance that neither material could provide alone in higher frequency bands.
Solution Approach 2:
The patent utilizes controlled rotation of the piezoelectric crystal cut angles (e.g., YX-cut orientations) to optimize the temperature coefficient of frequency and electromechanical coupling characteristics. By adjusting these crystalline orientation parameters, the filter achieves improved temperature stability and bandwidth while maintaining compatibility with standard manufacturing processes.
2Speed
If higher frequency bands are targeted, then wider communication bandwidth is achieved, but existing resonator technologies suffer from increased insertion loss and reduced Q-factor
Solution Approach 1:
The composite lithium niobate-lithium tantalate structure provides high electromechanical coupling coefficients that compensate for increased energy loss at higher frequencies. The lithium niobate layer contributes strong piezoelectric coupling while the lithium tantalate layer reduces temperature-dependent losses, together enabling low insertion loss operation in millimeter-wave bands.
Solution Approach 2:
The XBAR resonator design utilizes bulk acoustic wave modes with specific vibration patterns confined within the piezoelectric film thickness. This mechanical resonance approach, combined with the high coupling of the composite materials, maintains high Q-factors and low insertion loss even at elevated operating frequencies where conventional SAW and BAW resonators degrade.
3Adaptability or versatility
If wider bandwidth filters are designed, then more frequency channels are supported, but temperature coefficient of frequency increases leading to poor frequency stability
Solution Approach 1:
The patent combines lithium niobate with its high electromechanical coupling (enabling wide bandwidth) with lithium tantalate's low temperature coefficient of frequency (providing frequency stability). This composite structure allows the filter to maintain stable center frequency across temperature variations while supporting wide operational bandwidth for multiple frequency channels.
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 the creation of high-performance RF filters capable of operating in higher frequency bands with reduced frequency shifts due to temperature changes, enhancing the performance of wireless communication systems by providing wider bandwidth and improved reliability.
Implementation Method 1
transversely-excited film bulk acoustic resonators (XBARs) made from lithium niobate and lithium tantalate piezoelectric materials
Implementation Method 2
acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR)
Data Source
AI summary
Acoustic filters are disclosed. A bandpass filter has a passband between a lower band edge and an upper band edge. The bandpass filter includes a plurality of transversely-excited film bulk acoustic resonators (XBARs) connected in a ladder filter circuit. The plurality of XBARs includes at least one lithium tantalate (LT) XBAR and at least one lithium niobate XBAR. Each of the at least one LT XBAR includes an LT piezoelectric plate with Euler angles (0°, β, 0°), where β is greater than zero and less than or equal to 40 degrees.