BAW Filter Frequency Shifting With Mass-Loaded Lithium Niobate Resonators
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Solution Overview
Problem
Current techniques for manufacturing radio frequency filters using lithium niobate piezoelectric materials face challenges in achieving precise frequency shifting and maintaining electromechanical coupling coefficients due to difficulties in depositing and etching lithium niobate, leading to degradation of piezoelectric properties and unsuitable bandwidth for 5G mobile telephony applications.
Innovation Solution
A method involving a piezoelectric substrate on insulator with a mass overload pattern formed by lift-off of a sacrificial layer, allowing for precise adjustment of electrode thicknesses close to the piezoelectric layer without damaging the material, enabling the use of monocrystalline materials like lithium niobate for broadband filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional deposition and etching techniques are used on lithium niobate, then electrode structures can be formed, but the piezoelectric properties are degraded
Solution Approach 1:
A sacrificial layer is introduced as an intermediary element between the electrode and the lithium niobate piezoelectric layer. This sacrificial layer enables the formation of electrode structures through conventional deposition and etching techniques while protecting the piezoelectric properties of lithium niobate. After the electrode is formed, the sacrificial layer is removed, leaving the electrode structure in place without having directly contacted or degraded the piezoelectric material.
2Ease of manufacture
If aluminum nitride or aluminum scandium nitride is used as piezoelectric material, then manufacturing is easier, but the electromechanical coupling coefficient is too low for broadband filters
Solution Approach 1:
The invention changes the piezoelectric material parameter from conventional aluminum nitride or aluminum scandium nitride to lithium ni obate, which has a significantly higher electromechanical coupling coefficient (greater than six times that of aluminum nitride). This parameter change enables broadband filter performance with coupling coefficients exceeding 45%, while the associated deposition and etching challenges are resolved through the sacrificial layer technique.
3Measurement precision
If the resonance frequency of the parallel resonator is shifted by varying piezoelectric layer thickness, then frequency tuning is achieved, but the process is complex and imprecise
Solution Approach 1:
The invention extracts the frequency shifting function from the piezoelectric layer thickness variation and relocates it to a mass overload layer applied to the electrode. This separation allows the piezoelectric layer to maintain uniform thickness for optimal electromechanical coupling, while the mass overload layer provides precise frequency tuning through its mass effect on the resonator's mechanical properties.
4Ease of manufacture
If aluminum electrode layer is deposited and then etched on lithium niobate, then electrode patterns are formed, but inter-diffusion occurs degrading piezoelectric properties
Solution Approach 1:
The sacrificial layer serves as a protective intermediary between the aluminum electrode material and the lithium ni obate piezoelectric layer during the deposition and etching processes. This intermediary prevents direct contact and inter-diffusion between the aluminum and piezoelectric material, thereby preserving the piezoelectric properties. The sacrificial layer is subsequently removed, leaving clean interfaces without inter-diffusion damage.
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 method preserves the electrodes and piezoelectric properties, achieving the required bandwidth and frequency shifting with improved electromechanical coupling, suitable for 5G filter specifications.
Implementation Method 1
BAW resonators exploit the propagation of acoustic waves in piezoelectric layers
Implementation Method 2
the total thickness of the mass overload pattern(s) being chosen to shift the resonance frequency of the second resonator with the predetermined shift
Data Source
Figure 1~2
Figure 3A~3D
Figure 3E~3F
AI summary
One aspect of the invention relates to a method for making a bandpass filter comprising a first body-wave acoustic resonator (1) and a second body-wave acoustic resonator (2), the resonance frequency of the second resonator being shifted from the resonance frequency of the first resonator by a predetermined shift, the method comprising the steps of: - supplying a piezoelectric substrate (10) on an insulator, - forming a lower electrode (31) of the first resonator (1) and a lower electrode (32) of the second resonator (2), - gluing the donor substrate (10) onto a receiving substrate (50), - removing the donor substrate (10) with a stop on the piezoelectric layer (13), - forming an upper electrode (33) of the first resonator (1) and an upper electrode (34), the formation of the lower electrodes (31,32) being preceded by a step of forming a mass overload pattern (20) at the level of the second zone (134), and/or the formation of the upper electrodes (33,34) being preceded by a step of forming a mass overload pattern (21) at the level of the second zone (134), the total thickness of the mass overload pattern(s) (20,21) being chosen to shift the resonance frequency of the second resonator with the predetermined shift.