NiFeHf Capping Layer for MTJ dR/R Enhancement
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Solution Overview
Problem
Existing Magnetic Tunneling Junction (MTJ) elements face challenges in achieving high magnetoresistive ratio (dR/R) and low magnetostriction (λS) values due to oxygen contamination and inter-diffusion between the free layer and capping layer, particularly with Ta capping layers which diffuse into NiFe, reducing magnetic moment and increasing magnetostriction.
Innovation Solution
A low magnetization capping layer composed of a NiFeHf inner layer, which serves as an oxygen getter and inter-diffusion barrier, is introduced to minimize oxygen contamination and prevent inter-diffusion, formed using a NiFeHf/Ta/Ru trilayer structure to enhance the MTJ's dR/R ratio and maintain acceptable resistance (RA) values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a Ta capping layer is used, then the MTJ structure is simplified and manufacturing is easier, but Ta diffuses into the NiFe free layer reducing magnetic moment and increasing magnetostriction
Solution Approach 1:
A Ru layer is introduced as an intermediary between the Ta capping layer and the NiFe free layer. This Ru intermediary layer prevents direct contact and diffusion between Ta and NiFe, thereby preserving the magnetic moment of the free layer while still allowing the Ta layer to provide its protective capping function.
Solution Approach 2:
The capping structure is changed from a single Ta layer to a composite Ta/Ru/Ta trilayer structure. This composite structure combines the advantages of Ta (protection, ease of manufacture) with Ru (diffusion barrier, low magnetostriction), creating a multi-functional capping layer that resolves the contradiction between manufacturing ease and magnetic stability.
2Ease of manufacture
If a Ta capping layer is used, then the manufacturing process is simpler, but magnetostriction increases due to Ta diffusion into NiFe
Solution Approach 1:
The Ru layer serves as a diffusion barrier intermediary that stops Ta atoms from migrating into the NiFe free layer during thermal processing. This prevents the formation of Ta-NiFe intermetallic compounds that would increase magnetostriction, while the Ta layers continue to provide structural protection.
Solution Approach 2:
The Ta/Ru/Ta trilayer composite structure is designed where the central Ru layer acts as a diffusion barrier with low magnetostriction, while the outer Ta layers provide protective functions. This composite approach maintains compositional stability and controls magnetostriction while preserving manufacturing simplicity.
3Manufacturing precision
If oxygen contamination is present at the free layer interface, then the dR/R ratio decreases, but adding a thicker capping layer to prevent contamination increases device complexity
Solution Approach 1:
The Ru layer acts as an intermediary that provides excellent oxygen barrier properties at the critical NiFe/Ta interface. This single Ru layer is sufficient to prevent oxygen contamination without requiring multiple thick capping layers, thereby maintaining manufacturing precision while minimizing device complexity.
Solution Approach 2:
The Ta/Ru/Ta trilayer composite provides enhanced oxygen barrier properties compared to a single Ta layer, with the Ru layer specifically targeting oxygen diffusion prevention. This composite structure achieves superior interface protection with minimal added complexity, as the trilayer can be deposited using standard sputtering processes.
4Manufacturing precision
If a NiFeHf inner layer is added to the capping structure, then oxygen gettering improves and dR/R increases, but the capping layer structure becomes more complex
Solution Approach 1:
The NiFeHf layer is strategically placed only at the inner position adjacent to the NiFe free layer, where oxygen contamination is most critical. This localized oxygen gettering provides maximum benefit at the critical interface without requiring NiFeHf throughout the entire capping structure, thereby limiting the increase in device complexity.
Solution Approach 2:
The capping structure evolves to a quadrilayer Ta/Ru/Ta/NiFeHf composite where each layer has a specific function: Ta for protection, Ru for diffusion barrier, and NiFeHf for oxygen gettering. This composite approach concentrates complexity only where needed (the NiFeHf oxygen gettering layer) while maintaining simple Ta/Ru outer layers for protection and fabrication.
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 NiFeHf capping layer effectively increases the dR/R ratio and reduces the 'dead layer' between the free and capping layers, achieving higher conductivity and improved magnetic properties, including a significant reduction in magnetostriction, thereby enabling high-density device performance.
Implementation Method 1
the capping layer is a composite with a NiFeHf inner layer formed on an adjacent free layer. The low moment NiFeHf capping layer serves as an oxygen getter agent
Implementation Method 2
The low moment NiFeHf capping layer serves as an oxygen getter agent and an inter-diffusion barrier between the capping layer and free layer
Implementation Method 3
The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons
Implementation Method 4
The pinned layer has a magnetic moment that is fixed in the 'y' direction, for example, by exchange coupling with the adjacent AFM layer that is also magnetized in the 'y' direction
Implementation Method 5
An MTJ element may be based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer
Data Source
AI summary
An MTJ in an MRAM array or TMR read head is disclosed in which a low magnetization capping layer is a composite having a NiFeHf inner layer formed on a NiFe or CoFeB/NiFe free layer, a Ta middle layer, and a Ru outer layer on the Ta layer. For example, a low magnetization NiFeHf layer is achieved by co-sputtering NiFe and Hf targets with a forward power of 400 W and 200 W, respectively. A higher Hf content increases the oxygen gettering power of the NiFeHf layer and the thickness is modified to change dR/R, RA, and magnetostriction values. A so-called dead layer between the free layer and capping layer is restored by incorporating a NiFeHf layer on the free layer to improve lattice matching. The Fe content in the NiFe target used to make the NiFeHf layer is preferably the same as in the NiFe free layer.