Two-Substrate PMUT Bonding for Acoustic Output
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Piezoelectric micromachined ultrasonic transducers (PMUTs) face challenges in packaging due to the need for the front side of the membrane to be free from contact, control of sound reflections, and increased sensitivity to packaging stress, which complicates fabrication and reduces acoustic output.
Innovation Solution
A PMUT design and fabrication method involving two substrates bonded together using a conductive metallic bond or solder balls, eliminating the need for through-silicon vias and allowing for improved mechanical rigidity and acoustic performance by designing the tube structure and gap between substrates for enhanced sound pressure and reduced mechanical cross-talk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the wafer is thinned before etching the acoustic tube, then the etching process can be performed, but the final die becomes thin and perforated, increasing sensitivity to packaging stress and cross-talk between neighboring PMUTs
Solution Approach 1:
The device is divided into two separate substrates: a first substrate containing the PMUT membrane and acoustic tube, and a second substrate providing mechanical support. This segmentation allows the PMUT to be etched and released while the second substrate maintains structural integrity, reducing sensitivity to packaging stress.
Solution Approach 2:
A bonding interface between the first substrate (PMUT) and second substrate (support structure) acts as an intermediary that transfers and distributes mechanical stresses, preventing stress concentration on the thin PMUT membrane and reducing cross-talk between neighboring devices.
2Ease of manufacture
If the front side of the membrane is contacted or coated by another material during packaging, then packaging can be performed, but the membrane vibration is restricted, reducing acoustic output
Solution Approach 1:
Instead of contacting the front side of the membrane (which would restrict vibration), the packaging structure contacts the back side of the membrane through the bonding interface. This inversion allows the front side to remain free for optimal vibration while still providing mechanical support and enabling packaging.
3Power
If sound emitted from the front side of the membrane reflects off surfaces, then acoustic energy is generated, but the reflections reduce the acoustic output from the tube on the back side
Solution Approach 1:
The acoustic tube structure extracts and directs the acoustic energy generated by membrane vibration preferentially toward the back side output, separating the front-side acoustic energy generation from the back-side acoustic output to minimize the negative effects of reflections.
4Reliability
If complicated fabrication steps are used to realize through-wafer vias, then electrical connections can be established, but the fabrication process becomes complex and costly
Solution Approach 1:
Electrical connection structures are formed on the first substrate before bonding to the second substrate. This preliminary action eliminates the need for complex through-wafer via fabrication, as connections are established in advance on the surface where they are needed.
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 simplifies the fabrication process, reduces mechanical cross-talk between PMUTs, and enhances acoustic performance by allowing the PMUTs to vibrate freely without significant squeeze-film damping, thereby increasing acoustic output pressure.
Implementation Method 1
A typical PMUT is a multilayer membrane structure that is excited into flexural vibration using piezoelectric actuation
Implementation Method 2
The substrates are bonded such that the metallized top surface of the PMUT array faces the second substrate
Implementation Method 3
two substrates that are bonded together using either a conductive metallic bond or solder balls
Implementation Method 4
The membrane structure is often formed by etching through a silicon wafer to remove the material beneath the membrane, thereby allowing it to vibrate. This etch forms a hollow tube beneath the back-side of the membrane. Sound is emitted from the tube when the membrane vibrates and the tube may be designed as an acoustic resonator to improve acoustic performance of the PMUT
Data Source
AI summary
A piezoelectric micromachined ultrasound transducer (PMUT) is disclosed. The PMUT consists of a flexural membrane that is piezoelectrically actuated. These membranes are formed on a first substrate that is bonded to a second substrate. The two substrates are separated by an air gap to allow the PMUT to vibrate. Several methods for joining the two substrates are described.