PMUT Membrane Stiffness Control via Buried Cavities
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Solution Overview
Problem
Existing piezoelectric micro-machined ultrasonic transducers (PMUTs) face challenges in manufacturing, particularly in achieving optimal membrane stiffness distribution, as techniques like forming holes or adding central masses can affect vibration frequency and acoustic impedance, and are complex to produce.
Innovation Solution
The PMUT design features a buried cavity in the semiconductor body with a membrane having higher stiffness at the center and lower stiffness at the periphery, achieved through epitaxial growth and annealing, allowing for a piston-like movement without through holes, simplifying manufacturing and reducing resonance frequency variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If through holes are formed in edge regions and plugged with polymeric material to reduce stiffness, then piston-like movement is obtained, but the polymeric material affects transducer performance and modifies vibration frequency
Solution Approach 1:
The invention removes the harmful polymeric filling material from the through holes and replaces it with air or vacuum, thereby eliminating the negative impact on transducer performance while maintaining the reduced stiffness effect in edge regions through the through hole structure alone
Solution Approach 2:
The invention changes the filling parameter of the through holes from polymeric material to air or vacuum, which modifies the stiffness characteristics of edge regions while avoiding the performance degradation caused by polymeric materials
2Shape
If a central mass is applied to increase membrane thickness selectively, then piston-like movement is obtained, but resonance frequency varies undesirably and acoustic impedance increases
Solution Approach 1:
The invention removes the central mass application step from the manufacturing process and instead achieves thickness variation through selective etching that creates through holes only in edge regions, allowing the central region to maintain its original thickness and resonance characteristics
Solution Approach 2:
Instead of adding material (central mass) to achieve thickness variation, the invention uses subtractive manufacturing (selective etching to create through holes) to reduce stiffness in edge regions, thereby achieving piston-like movement without adding central mass
3Ease of manufacture
If holes are formed and plugged to reduce edge stiffness, then manufacturing is simplified compared to back etching, but the process becomes more complex than necessary
Solution Approach 1:
The invention removes the hole plugging step from the manufacturing process and replaces it with air or vacuum filling, thereby simplifying the process while maintaining the stiffness reduction effect in edge regions
Solution Approach 2:
The invention applies through holes selectively only in edge regions rather than uniformly across the membrane, achieving local stiffness reduction to enable piston-like movement while maintaining structural integrity in the central region
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 design enhances the transducer's performance by enabling efficient ultrasound generation and reception with improved directivity, simplifying the manufacturing process, and minimizing resonance frequency shifts, while maintaining high production yield and cost-effectiveness.
Implementation Method 1
a piezoelectric actuator, extending over a central region of the membrane
Implementation Method 2
achieved through epitaxial growth and annealing
Implementation Method 3
achieved through epitaxial growth and annealing
Data Source
AI summary
A piezoelectric micro-machined ultrasonic transducer (PMUT) comprising: a semiconductor body having a first cavity and a membrane, which is suspended over the first cavity and faces a front side of the semiconductor body; and a piezoelectric transducer assembly extending at least in part on the membrane, which may be actuated for generating a deflection of the membrane. A second cavity extends buried in a peripheral region of the membrane and delimits a central region of the membrane. Moreover, the peripheral portion has a stiffness lower than the stiffness of the central portion.