Seed Layer Stack for Magnetic Tunnel Junctions
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Solution Overview
Problem
Current magnetic tunnel junctions (MTJs) face challenges in achieving high thermal stability and perpendicular magnetic anisotropy due to rough seed layers, which degrade magnetic properties and allow Ta diffusion from the bottom electrode, affecting the performance of memory devices like MRAM.
Innovation Solution
A seed layer stack comprising a laminated structure with high resputtering rate elements like Mg, Al, and low resputtering rate NiX layers, topped with NiCr, which reduces surface roughness and enhances diffusion resistance, promoting a face-centered cubic crystal structure and improving thermal stability in the reference layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional seed layer is used, then the crystal growth in the reference layer is promoted, but the surface roughness increases and thermal stability decreases
Solution Approach 1:
The seed layer is divided into multiple sub-layers with different materials and functions: a first seed layer (Ta/Ru laminate) for crystal structure promotion and diffusion barrier, and a second seed layer (NiCr) for surface smoothing. This segmentation allows each layer to optimize its specific function without compromising the other, resolving the contradiction between crystal growth promotion and surface roughness control.
Solution Approach 2:
The invention uses composite material structures: Ta/Ru laminated composite in the first seed layer for enhanced diffusion resistance and crystal promotion, and NiCr composite in the second seed layer for surface smoothing. These composite structures provide properties that single materials cannot achieve alone, simultaneously improving crystal growth and reducing surface roughness.
2Device complexity
If a single-layer seed layer is used, then the structure is simple, but the diffusion resistance and surface uniformity cannot be simultaneously optimized
Solution Approach 1:
The seed layer is segmented into functionally distinct layers: the first seed layer handles diffusion barrier and crystal promotion, while the second seed layer handles surface smoothing. This functional segmentation allows optimization of reliability without excessive complexity, as each layer has a specific role.
Solution Approach 2:
The first seed layer (Ta/Ru laminate) serves multiple functions: it acts as a diffusion barrier against Ta migration, promotes FCC crystal structure formation, and provides a template for subsequent layers. This multi-functionality reduces the need for additional layers, balancing complexity and reliability.
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 seed layer stack enhances thermal stability and perpendicular magnetic anisotropy, leading to improved data retention and reproducible tunnel barrier properties, while preventing Ta migration and maintaining high magnetoresistive ratios.
Implementation Method 1
A seed layer stack and method of forming the seed layer stack are disclosed that has a smooth top surface with a reduced peak to peak roughness
Implementation Method 2
promotes a face centered cubic (FCC) texture in the reference layer
Implementation Method 3
good diffusion resistance to prevent materials such as Ta from the bottom electrode (BE) or an underlayer from migrating to the tunnel barrier
Data Source
AI summary
A seed layer stack with a smooth top surface having a peak to peak roughness of about 0.5 nm over a range of 100 nm is formed by sputter depositing an X layer such as Mo on a Ni layer where the X layer has one or both of a larger bond energy and a greater atomic number than Ni. A (Ni/X)m laminate is formed and then an uppermost NiCr seed layer is deposited to enhance perpendicular magnetic anisotropy (PMA) in an overlying ferromagnetic layer. A <111> NiCr crystal structure promotes <111> texture in the ferromagnetic layer. X layers serve as a diffusion barrier to Ta migration from a bottom electrode and have good lattice matching with the adjoining Ni layer and uppermost NiCr layer. As a result of the smooth seed layer stack in a magnetic tunnel junction (MTJ), MTJ properties are improved and more reproducible.