Hermetic MEMS Packaging via Low-Temperature Intermetallic Bonding
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Solution Overview
Problem
Conventional hermetic packaging of MEMS devices is costly and faces technical challenges such as high temperatures during sealing, which can affect the reliability of silicon integrated circuits and the functioning of MEMS components, and existing passivation and lubrication methods are prone to degradation due to exposed copper layers.
Innovation Solution
A hermetic package is created using a vertical stack of metal layers with gold-indium intermetallic compounds, where the metal layers extend over the sidewalls of seed metal piles to encapsulate copper, and a low-temperature transient liquid phase process is used to form intermetallic compounds that maintain a stable seal at higher temperatures, allowing for lubrication and passivation within the package.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional hermetic packaging is used with high-temperature sealing, then hermetic seal is achieved, but reliability of silicon integrated circuits and MEMS components deteriorates
Solution Approach 1:
The patent changes the temperature parameter of the sealing process by using a low-temperature transient liquid phase bonding process at approximately 150°C instead of conventional high-temperature sealing. This is achieved by forming intermetallic compounds (such as Cu6Sn5, Cu3Sn, Ni3Al, or CoAl) that enable hermetic sealing at low temperatures, thereby protecting temperature-sensitive MEMS components and silicon integrated circuits from thermal damage while maintaining seal reliability
Solution Approach 2:
The patent employs composite material structures consisting of multiple metal layers with different properties. The stack includes a first metal layer (e.g., Cu, Ni, Co) and a second metal layer (e.g., Sn, Al) that form intermetallic compounds during bonding. This composite structure enables low-temperature hermetic sealing by leveraging the specific metallurgical properties of the metal combinations, achieving both hermeticity and thermal compatibility
2Ease of manufacture
If copper layers are exposed in the metal stack, then manufacturing is simplified, but passivation and lubrication methods degrade over time
Solution Approach 1:
The patent applies local quality by selectively exposing copper layers only in specific regions where electrical connection or bonding is required, while encapsulating copper layers in other regions with protective metal layers. The vertical stack structure provides localized protection where needed, maintaining manufacturing simplicity in exposed areas while ensuring passivation stability in encapsulated areas
Solution Approach 2:
The patent introduces intermediate metal layers (such as Ni or Co) that act as mediators between the copper layer and the external environment. These intermediate layers form stable intermetallic compounds that prevent direct exposure of copper to degrading substances, thereby maintaining passivation stability while allowing the copper layer to remain part of the manufacturing structure
3Reliability
If metal layers are extended to encapsulate copper, then passivation is improved, but device complexity increases
Solution Approach 1:
The patent segments the metal stack into distinct functional layers: a first metal layer for electrical connection and bonding, and a second metal layer for encapsulation and passivation. This segmentation allows each layer to perform its specific function efficiently, improving copper encapsulation while keeping the overall structure manageable through clear functional division rather than a monolithic complex structure
4Temperature
If low-temperature transient liquid phase process is used, then component reliability is maintained, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies preliminary action by pre-forming the metal layers with controlled thicknesses and compositions before the bonding process. The metal layers are deposited with specific parameters (thickness, purity, surface preparation) that are optimized to form the desired intermetallic compounds during the low-temperature transient liquid phase bonding. This preliminary preparation reduces the precision requirements during the actual bonding process by ensuring the materials are ready to react as intended
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 results in a cost-effective, fully hermetically sealed MEMS device that maintains a stable seal at temperatures above the assembly temperature, preventing lubricant degradation and ensuring reliable operation of MEMS components.
Implementation Method 1
A hermetic package is created using a vertical stack of metal layers with gold-indium intermetallic compounds
Implementation Method 2
a low-temperature transient liquid phase process is used to form intermetallic compounds that maintain a stable seal at higher temperatures
Implementation Method 3
the metal layers extend over the sidewalls of seed metal piles to encapsulate copper
Implementation Method 4
This approach results in a cost-effective, fully hermetically sealed MEMS device that maintains a stable seal at temperatures above the assembly temperature, preventing lubricant degradation
Data Source
AI summary
In described examples, a hermetic package of a microelectromechanical system (MEMS) structure includes a substrate having a surface with a MEMS structure of a first height. The substrate is hermetically sealed to a cap forming a cavity over the MEMS structure. The cap is attached to the substrate surface by a vertical stack of metal layers adhering to the substrate surface and to the cap. The stack has a continuous outline surrounding the MEMS structure while spaced from the MEMS structure by a distance. The stack has: a first bottom metal seed film adhering to the substrate and a second bottom metal seed film adhering to the first bottom metal seed film; and a first top metal seed film adhering to the cap and a second top metal seed film adhering to the first top metal seed film.