Multilayer Seed Stack for Smooth MTJ Films at 400°C
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Solution Overview
Problem
Current MTJ structures fail to maintain perpendicular magnetic anisotropy (PMA) and thermal stability of the free and reference layers during high temperature semiconductor processing up to 400°C, which is essential for advanced embedded MRAM devices.
Innovation Solution
A multilayer seed layer stack is used, comprising a high resputtering rate layer followed by a low resputtering rate amorphous layer, to create a smooth top surface that promotes PMA in the overlying magnetic layers, which are then annealed at up to 400°C to enhance thermal stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional single-layer seed layer is used, then the device structure is simple, but the perpendicular magnetic anisotropy (PMA) and thermal stability are lost during high temperature processing up to 400°C
Solution Approach 1:
The seed layer is divided into multiple sub-layers with different materials and functions. The bottom seed layer (e.g., Ta, Ru, Rh) provides adhesion and initial template, while the top seed layer (e.g., Ni, Pd, Pt) provides the final smooth surface and promotes PMA. This segmentation allows each layer to be optimized for its specific function, enabling thermal stability during 400°C processing while maintaining structural manageability
Solution Approach 2:
The patent employs composite seed layer structures combining different materials with complementary properties. For example, combining Ta (high adhesion) with NiCr (smooth surface, PMA promotion) creates a composite system where the synergistic effects of both materials provide both thermal stability and magnetic anisotropy that neither material could achieve alone during high temperature processing
2Manufacturing precision
If the seed layer top surface is rough, then the deposition process is simpler, but the PMA in overlying magnetic layers is reduced and thermal stability is compromised
Solution Approach 1:
The seed layer structure is designed to pre-establish a smooth top surface before magnetic layer deposition. The top seed layer is specifically selected and configured to provide atomic-level smoothness and crystalline orientation that promotes PMA in the overlying magnetic layers, eliminating the need for additional surface preparation steps and ensuring thermal stability
3Stability of the object's composition
If the device is annealed at high temperature (400°C) to enhance thermal stability, then the memory retention is improved, but the PMA in magnetic layers degrades
Solution Approach 1:
The seed layer structure is pre-configured to counteract the harmful effects of high temperature annealing. The specific material composition and interface structure of the seed layer create a protective effect that prevents PMA degradation during 400°C processing, effectively anticipating and neutralizing the thermal damage before it occurs to the magnetic layers
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 multilayer seed layer stack maintains PMA and enhances thermal stability of the magnetic layers, ensuring reliable performance of MTJs in high temperature processing environments.
Implementation Method 1
comprising a high resputtering rate layer followed by a low resputtering rate amorphous layer, to create a smooth top surface that promotes PMA in the overlying magnetic layers
Implementation Method 2
which are then annealed at up to 400°C to enhance thermal stability
Data Source
AI summary
A seed layer stack with a uniform top surface having a peak to peak roughness of 0.5 nm is formed by sputter depositing an amorphous layer on a smoothing layer such as Mg where the latter has a resputtering rate 2 to 30× that of the amorphous layer. The uppermost seed layer is NiW, NiMo, or one or more of NiCr, NiFeCr, and Hf while the bottommost seed layer is one or more of Ta, TaN, Zr, ZrN, Nb, NbN, Mo, MON, TIN, W, WN, and Ru. Accordingly, perpendicular magnetic anisotropy in an overlying magnetic layer is substantially maintained during high temperature processing up to 400° C. and is advantageous for magnetic tunnel junctions in embedded MRAMs, spintronic devices, or in read head sensors. The amorphous seed layer is SiN, TaN, or CoFeM where M is B or another element with a content that makes CoFeM amorphous as deposited.