PMUT Package Structure with Air Cavity and Conductive Posts
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Solution Overview
Problem
Large die size PMUTs face yield issues due to the limitations of eutectic bonding techniques, particularly when the number of bonding sites increases, affecting the packaging efficiency and sensing resolution in applications like medical ultrasonic devices.
Innovation Solution
A eutectic bonding-free PMUT package structure is introduced, utilizing a bumping structure such as solder bumping and/or copper pillar, and an air cavity is formed to cover multiple PMUT cells, replacing the vacuum cavity typically formed by eutectic bonding, which enhances packaging efficiency and sensing capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If eutectic bonding technique is used to bond large die size PMUTs, then electrical connection is achieved, but the number of bonding sites increases causing yield issues
Solution Approach 1:
The patent merges the electrical connection function and mechanical support function into a single integrated structure. The conductive posts serve both as electrical interconnects and as mechanical support structures for the membrane, eliminating the need for separate bonding sites. This consolidation reduces the total number of bonding operations required while maintaining reliable electrical connections across large die sizes.
Solution Approach 2:
The conductive posts perform multiple functions simultaneously: they provide electrical connection between dies, serve as mechanical support for the membrane, and define the cavity structure. This multi-functionality reduces the overall complexity of the packaging structure and decreases the number of discrete bonding sites needed compared to traditional eutectic bonding approaches.
2Productivity
If eutectic bonding is used to form vacuum cavity, then hermetic sealing is achieved, but packaging efficiency decreases due to bonding limitations
Solution Approach 1:
The patent segments the hermetic sealing function from the electrical bonding function. The sealing structure is formed as a separate component that bonds to the package body, independent of the conductive posts that provide electrical connection. This segmentation allows the sealing operation to be optimized separately from the bonding operation, improving overall packaging efficiency while maintaining hermetic integrity.
Solution Approach 2:
The sealing structure is prepared and positioned in advance before final assembly. The conductive posts are first bonded to the package body, then the sealing structure is attached to complete the hermetic seal. This preliminary preparation of sealing components streamlines the packaging process and improves productivity compared to forming seals during the bonding operation itself.
3Measurement precision
If vacuum cavity is formed by eutectic bonding, then PMUT cells are enclosed, but sensing resolution is limited due to noise reflections
Solution Approach 1:
The patent replaces the vacuum environment with an inert gas-filled cavity. The gas filling (such as sulfur hexafluoride or other acoustic matching gases) provides acoustic impedance matching that reduces noise reflections and improves signal quality. This inert gas environment enhances sensing resolution by minimizing spurious reflections while maintaining the enclosed cavity structure needed for PMUT operation.
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 solution improves the yield and sensing resolution of PMUTs by reducing the stress on bonding sites and allowing for more efficient ultrasound propagation and reduced noise reflections, enhancing the performance of ultrasonic devices.
Implementation Method 1
a second die on the active surface of the first die, electrically connected to the first die through a plurality of conductive terminals
Implementation Method 2
PMUTs are MEMS devices which operate in response to flexural motion of a thin membrane coupled with a thin piezoelectric film
Data Source
AI summary
A semiconductor package structure includes a substrate; a first die on the substrate, wherein an active surface of the first die is facing away from the substrate; a second die on the active surface of the first die, electrically connected to the first die through a plurality of conductive terminals; and a sealing structure on the active surface of the first die, surrounding the plurality of conductive terminals and abutting the second die thereby forming a cavity between the first die and the second die. A method for manufacturing the semiconductor package structure is also provided.