Strained Piezoelectric Resonators Using Stress-Layer Strain Transfer
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Solution Overview
Problem
The challenge in using scandium-doped aluminum nitride (Al1-xScxN) for piezoelectric resonators is the difficulty in achieving a uniform distribution of scandium, leading to preferential phase separation and rocksalt formation, which affects the electromechanical coupling coefficient kt2, and there is a need for maintaining tensile biaxial stress after deposition.
Innovation Solution
A method involving the formation of a stress layer on the substrate before depositing the piezoelectric layer, followed by its removal to induce compressive strain along the z-axis, enhancing the electromechanical coupling coefficient kt2.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If scandium content in AlN is increased to improve electromechanical coupling coefficient, then kt2 value increases, but rocksalt phase formation increases and uniform distribution becomes difficult to achieve
Solution Approach 1:
The patent applies tensile biaxial strain as a parameter change to stabilize the wurtzite phase and prevent rocksalt formation in high-scandium-content AlScN layers. This strain modification allows achieving high kt2 values while maintaining phase purity and uniform scandium distribution
Solution Approach 2:
The patent creates a composite structure by combining AlScN piezoelectric layer with AlN buffer layers and DBR (distributed Bragg reflector) layers. This composite approach enables control over phase formation and improves uniformity while maintaining high electromechanical coupling
2Manufacturing precision
If strict process control is applied to prevent rocksalt phase formation, then phase purity improves, but fabrication complexity and difficulty increase
Solution Approach 1:
The patent introduces AlN buffer layers and stress control layers during the deposition process to pre-establish conditions that stabilize the wurtzite phase. This preliminary action prevents rocksalt formation before it occurs, simplifying subsequent process control
Solution Approach 2:
The patent modifies deposition parameters including applying tensile biaxial strain during growth and controlling substrate temperature to create thermodynamic conditions that favor wurtzite phase formation, thereby reducing the need for strict post-deposition control
3Stability of the object's composition
If AlScN layer is allowed to form equilibrium lattice constant during deposition and annealing, then material stability improves, but tensile biaxial stress is lost which is needed for wurtzite stabilization
Solution Approach 1:
The patent introduces DBR layers and stress control layers that compensate for lattice relaxation during deposition and annealing. These layers cushion against the loss of tensile biaxial stress, maintaining wurtzite phase stability even when the AlScN layer attempts to form its equilibrium lattice constant
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 induced compressive strain along the z-axis increases the electromechanical coupling coefficient kt2, improving the piezoelectric performance of the resonators.
Implementation Method 1
A piezoelectric material is a material that couples mechanical strain and electric fields. Due to a polarizable non-symmetric atomic structure, an applied electric field will induce a mechanical strain, and conversely a mechanical strain will induce an electric field in the material.
Implementation Method 2
The stress layer induces a z-axis compressive strain in the piezoelectric layer, which remains even after removal of the stress layer.
Data Source
AI summary
Disclosed are methods of fabrication and related piezoelectric devices comprising a piezoelectric layer with an induced compressive strain along a z-axis. The methods include formation of a stress layer on a bottom side of a wafer substrate, after deposition of the piezoelectric layer. Stress layer removal results in an induced compressive strain along the z-axis which increases the electromechanical coupling coefficient, and thereby piezoelectric performance. Related devices are also disclosed, including bulk acoustic wave (BAW) thin film resonators (FBAR), which are fabricated in accordance with the disclosed methods.