STT-MRAM Flip-Chip Magnetic Shielding via 3D Encasement
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Solution Overview
Problem
Current shielding approaches for spin-transfer torque (STT)-magnetic random access memory (MRAM) structures, particularly perpendicular STT-MRAM, are inadequate as they do not provide complete magnetic shielding from all sides, leading to interference from externally applied electromagnetic fields and inferior magnetic immunity due to open structures and wide openings in shielding layers.
Innovation Solution
A method involving the formation of a three-dimensional magnetic shielding layer with μ-bump openings over a perpendicular STT-MRAM structure within a flip-chip package, including steps such as forming a passivation stack, polymer layer, under bump metallization, T-shaped copper pillars, and magnetic shielding layers to encase the structure from all six sides, connected to a package substrate with ball grid array balls.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If wire-bond packaging with magnetic epoxy layers and protective shield layers is used, then the MRAM structure is protected from damage during processing, but the shielding structure has wide openings that expose the MRAM die to electromagnetic field interference
Solution Approach 1:
The patent transitions from planar shielding layers with openings to a three-dimensional cup-shaped shielding structure that extends vertically along the sidewalls of the MRAM die. This dimensional change allows the shielding to enclose the die more completely, reducing exposed areas to electromagnetic fields while maintaining structural integrity and enabling better magnetic shielding performance
Solution Approach 2:
The cup-shaped shielding structure is formed by depositing magnetic shielding material conformally over the sidewalls of the MRAM die, creating a nested configuration where the shielding layer conforms to and surrounds the die structure. This nesting approach allows the shielding to follow the contours of the die, providing comprehensive protection while minimizing openings
2Ease of manufacture
If the shielding structure is formed with wide openings to accommodate wire-bond access, then manufacturing is simplified, but magnetic immunity deteriorates due to inferior threshold for magnetic shielding
Solution Approach 1:
The patent applies magnetic shielding material selectively to different regions: the cup-shaped structure provides shielding along the sidewalls where it is most needed for protecting the MRAM die, while the bottom surface may have different characteristics. This local application of shielding quality ensures adequate protection without requiring complete enclosure, balancing manufacturing ease with magnetic immunity
Solution Approach 2:
The shielding structure combines magnetic shielding material with the underlying MRAM die structure and packaging materials to create a composite system. The cup-shaped configuration integrates the shielding function with the structural requirements, achieving both ease of manufacture and improved magnetic immunity through the composite nature of the assembly
3Object-affected harmful factors
If complete shielding from all six sides is implemented, then magnetic immunity is improved, but the package cannot support high density input/output required in system on chip applications
Solution Approach 1:
The patent segments the shielding approach by implementing a cup-shaped structure that provides shielding on the sidewalls and top, while leaving the bottom surface with different characteristics. This segmentation allows the shielding to be applied where most needed for magnetic protection while maintaining openness at the bottom for high-density interconnect access, thus balancing magnetic immunity with I/O density requirements
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 enhances magnetic shielding efficiency, reduces exposed areas, and improves magnetic immunity while allowing for high-density input/output requirements, making it suitable for system-on-chip applications.
Implementation Method 1
magnetic shielding layer over the epoxy layer and along sidewalls of each die, the epoxy layer, the passivation stack and the polymer layer
Data Source
AI summary
Methods of magnetically shielding a perpendicular STT-MRAM structure on all six sides within a flip-chip package and the resulting devices are provided. Embodiments include forming a passivation stack over an upper surface of a wafer and outer portions of an Al pad; forming a polymer layer over the passivation stack; forming a UBM layer over the Al pad, portions of the polymer layer and along sidewalls of the polymer layer; forming a T-shaped Cu pillar over the UBM layer; forming a μ-bump over the T-shaped Cu pillar; dicing the wafer into a plurality of dies; forming an epoxy layer over a bottom surface of each die; forming a magnetic shielding layer over the epoxy layer and along sidewalls of each die, the epoxy layer, the passivation stack and the polymer layer; and connecting the μ-bump to a package substrate with a BGA balls.