PMUT Array With Tunable Embedded Cavity for Resonance Control
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Solution Overview
Problem
Current piezoelectric micromachined ultrasound transducers (PMUTs) face challenges in achieving uniform frequency emission and wide frequency bandwidth, leading to poor axial resolution and insufficient damping of membrane ringing, which are crucial for accurate eye movement tracking in virtual and augmented reality devices.
Innovation Solution
An array of PMUTs is designed with selectively deformable cavities and piezoelectric stacks, where the substrate's thickness between cavities and piezoelectric stacks forms a membrane, allowing for controlled vibration and deformation of cavities to adjust resonant frequencies, thereby enhancing frequency bandwidth and damping ringing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a high Q-factor membrane is used in PMUTs, then the membrane produces strong ultrasound waves, but the membrane ringing is insufficiently damped leading to poor axial resolution and narrow frequency bandwidth
Solution Approach 1:
The patent applies dynamics by making the cavity volume adjustable rather than fixed. The cavity volume is dynamically changed during operation to modify the resonant frequency of the membrane. This allows the system to adapt its characteristics in real-time, enabling both strong ultrasound generation and adequate damping by adjusting the cavity volume to different states.
Solution Approach 2:
The patent changes physical parameters of the system, specifically the cavity volume, to control the resonant frequency and damping characteristics of the membrane. By varying the cavity volume parameter, the system can optimize both the power output and the axial resolution, resolving the contradiction between these two performance metrics.
2Ease of manufacture
If the resonant frequency of PMUT elements is not uniformly tuned, then manufacturing is simplified, but frequency uniformity across the array deteriorates leading to reduced tracking accuracy
Solution Approach 1:
The patent uses dynamics by implementing adjustable cavity volumes for each PMUT element. This allows post-manufacturing tuning of resonant frequencies without requiring precise manufacturing control. Each element's cavity volume can be independently adjusted to achieve uniform resonant frequency across the array, maintaining manufacturing simplicity while ensuring frequency uniformity.
Solution Approach 2:
The patent changes the cavity volume parameter to tune the resonant frequency of each PMUT element. This parameter adjustment enables precise control over the frequency characteristics of individual elements, allowing the array to achieve high frequency uniformity for accurate eye movement tracking while keeping the manufacturing process relatively simple.
3Measurement precision
If wide frequency bandwidth ultrasound waves are desired for high axial resolution, then the frequency bandwidth increases, but the membrane ringing persists longer due to high Q-factor
Solution Approach 1:
The patent applies dynamics by making the cavity volume adjustable during operation. This allows the system to dynamically control the damping characteristics of the membrane. By adjusting the cavity volume, the system can achieve both wide frequency bandwidth for high axial resolution and sufficient damping to limit membrane ringing duration, resolving the contradiction between these two temporal characteristics.
Solution Approach 2:
The patent changes the cavity volume parameter to control both the frequency bandwidth and the damping of the membrane. By optimizing this parameter, the system achieves wide frequency bandwidth for high axial resolution while simultaneously controlling the membrane ringing duration, eliminating the trade-off between these two parameters.
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 solution enables PMUTs to produce ultrasound waves with improved frequency uniformity and bandwidth, increasing axial resolution and reducing ringing, thus enhancing the accuracy of eye movement tracking.
Implementation Method 1
A PMUT element is formed by a membrane suspended over a cavity carrying a layer of piezoelectric material sandwiched between thin electrode layers. In operation, a voltage is applied across the electrodes, resulting in a lateral strain being induced in the membrane via the piezoelectric effect causing movement of the piezoelectric layer.
Implementation Method 2
By applying a suitable AC voltage between the electrodes oscillation of the membrane is induced and an ultrasound wave is generated.
Implementation Method 3
ultrasonic waves are emitted toward the eye and reflections of the ultrasonic waves off the eye are detected. By measuring the time of flight of those ultrasound waves, distance to parts of the eye can be determined.
Implementation Method 4
Challenges in creating such devices arise in that in a high uniformity in the frequency of the ultrasound waves emitted by different elements of the PMUT array is desired for high accuracy, and wide frequency bandwidth ultrasound waves are desired for high axial resolution. Current devices do not address these challenges sufficiently for certain applications—for example, certain designs insufficiently damp ringing of the membrane caused by the high Q-factor of the membrane
Data Source
AI summary
An array of piezoelectric micromachined ultrasound transducers (PMUTs) includes a substrate having first and second cavities buried therein. A first piezoelectric stack is carried by the substrate and at least partially overlays the first cavity. A second piezoelectric stack is carried by the substrate and at least partially overlays the second cavity. A thickness of the substrate between the second cavity and the second piezoelectric stack forms a membrane. Circuitry operates the second piezoelectric stack so as to vibrate the membrane to generate a pulse of ultrasound and to immediately subsequently operate the first piezoelectric stack to cause deformation of the second cavity which results in an increase in a resonant frequency of the membrane.