Multilayer Seed Layer Stack for Thermal Stability in MTJs
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Solution Overview
Problem
Current magnetic tunnel junction (MTJ) structures fail to maintain thermal stability and perpendicular magnetic anisotropy (PMA) at elevated temperatures typical of high-temperature semiconductor processes, such as those up to 400°C, which is crucial for advanced embedded MRAM devices.
Innovation Solution
A multilayer seed layer stack with a high resputtering rate layer and a low resputtering rate amorphous layer is used to create a smooth template layer, enhancing the thermal stability of the MTJ by maintaining PMA in the reference and free layers during high temperature processing. This stack includes a bottommost layer for adhesion, a high resputtering rate layer with a smooth top surface, and an amorphous layer with reduced roughness, followed by a template layer promoting PMA, along with a tunnel barrier and free layer configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional single-layer seed layer is used, then the device structure is simple, but perpendicular magnetic anisotropy (PMA) and thermal stability are lost at high temperatures up to 400°C
Solution Approach 1:
The seed layer is divided into multiple distinct layers including a bottommost layer for adhesion, a high resputtering rate layer, an amorphous layer, and a template layer. Each layer performs a specific function: the bottommost layer ensures substrate adhesion, the high resputtering rate layer provides a smooth top surface, the amorphous layer reduces roughness, and the template layer promotes perpendicular magnetic anisotropy. This segmentation allows the system to maintain PMA and thermal stability at high temperatures while managing the complexity through functional specialization.
Solution Approach 2:
The seed layer structure employs a composite multilayer stack combining materials with different properties: a bottommost layer material for adhesion, a high resputtering rate material, an amorphous material with low resputtering rate, and a template layer material. This composite structure leverages the unique properties of each material layer to collectively achieve thermal stability and PMA maintenance at 400°C, resolving the contradiction between reliability improvement and structural complexity.
2Manufacturing precision
If the seed layer top surface is rough, then deposition is easier, but the template layer cannot effectively promote PMA in the overlying magnetic layer
Solution Approach 1:
The high resputtering rate layer and amorphous layer are deposited beforehand to create a smooth top surface on the template layer. This preliminary smoothing action occurs during the seed layer formation process, preparing an ideal substrate for the subsequent template layer deposition. The smooth surface ensures effective PMA promotion in the overlying magnetic layer while the entire process remains integrated into the deposition sequence, maintaining ease of manufacture.
3Reliability
If the amorphous layer thickness is increased to reduce roughness, then PMA is better maintained, but the resputtering effect during deposition is reduced
Solution Approach 1:
The thickness of the amorphous layer is optimized to a specific range that balances two competing requirements: sufficient thickness to reduce surface roughness and maintain PMA, but not so thick as to excessively reduce the resputtering rate during deposition. By carefully controlling the amorphous layer thickness parameter, the system achieves optimal PMA maintenance while preserving adequate deposition productivity through the resputtering effect.
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 solution effectively maintains or enhances PMA in MTJs after high temperature processing up to 400°C for at least 30 minutes, improving thermal stability and coercivity, thereby addressing the limitations of existing MTJ structures.
Implementation Method 1
a second seed layer contacts a top surface of the bottommost layer and is selected because of a high resputtering rate property
Implementation Method 2
The tunnel barrier is typically about 10 Angstroms thick so that a current through the tunnel barrier can be established by a quantum mechanical tunneling of conduction electrons
Implementation Method 3
Magnetoresistive Random Access Memory (MRAM) has a read function based on a tunneling magnetoresistive (TMR) effect in a MTJ stack
Implementation Method 4
a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry
Data Source
AI summary
A laminated seed layer stack with a smooth top surface having a peak to peak roughness of 0.5 nm is formed by sequentially sputter depositing a first seed layer, a first amorphous layer, a second seed layer, and a second amorphous layer where each seed layer may be Mg and has a resputtering rate 2 to 30X that of the amorphous layers that are TaN, SiN, or a CoFeM alloy. A template layer that is NiCr or NiFeCr is formed on the second amorphous layer. As a result, perpendicular magnetic anisotropy in an overlying magnetic layer that is a reference layer, free layer, or dipole 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 laminated seed layer stack may include a bottommost Ta or TaN buffer layer.