Nitride-Capped STT-MRAM MTJ for Diffusion and RA Control
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Solution Overview
Problem
In magnetic tunnel junctions (MTJs), particularly in STT-MRAM, there is a challenge in maintaining the integrity of the Hk enhancing layer to prevent metal and nitrogen diffusion to the free layer and oxygen migration out of this layer, which degrades the magnetoresistive ratio (DRR) and increases the resistance-area (RA) product, especially during high-temperature CMOS fabrication processes.
Innovation Solution
A nitride or oxynitride capping layer is introduced between the Hk enhancing layer and the hard mask or seed layer, along with a metallic buffer layer to act as a barrier, minimizing diffusion and oxygen migration, and conductive pathways are formed within the capping layer to maintain the magnetoresistive ratio and reduce the RA product.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional MTJ structure without a nitride capping layer is used, then the device structure is simpler, but metal and nitrogen diffuse to the free layer and oxygen migrates out of the Hk enhancing layer, degrading the magnetoresistive ratio (DRR) and increasing the resistance-area (RA) product
Solution Approach 1:
A nitride or oxynitride capping layer is introduced as an intermediary barrier between the Hk enhancing layer and the hard mask or seed layer. This intermediate layer prevents direct interaction between the Hk enhancing layer and surrounding materials, blocking metal diffusion to the free layer, preventing nitrogen diffusion from the capping layer, and stopping oxygen migration out of the Hk enhancing layer, thereby maintaining the magnetoresistive ratio (DRR) and reducing the resistance-area (RA) product
Solution Approach 2:
The patent employs composite material structures by combining the nitride or oxynitride capping layer with the Hk enhancing layer and free layer. This composite structure leverages the specific properties of each material: the nitride/oxynitride layer provides diffusion barrier properties, while the Hk enhancing layer maintains perpendicular magnetic anisotropy, creating a synergistic effect that simultaneously protects against multiple degradation mechanisms
2Reliability
If the Hk enhancing layer is made thicker to prevent diffusion, then diffusion barriers are improved, but the resistance-area (RA) product increases and thermal stability is compromised
Solution Approach 1:
The diffusion barrier function is segmented into two distinct layers: a thin nitride or oxynitride capping layer that provides the primary diffusion barrier, and the Hk enhancing layer that maintains magnetic properties. This segmentation allows each layer to be optimized for its specific function—the capping layer for diffusion prevention and the Hk enhancing layer for thermal stability—avoiding the need to increase the thickness of the Hk enhancing layer, which would compromise thermal stability while providing adequate diffusion protection
3Reliability
If a conductive capping layer is used to reduce the RA product, then electrical conductivity is improved, but metal diffusion to the free layer is enhanced
Solution Approach 1:
The nitride or oxynitride capping layer exhibits local quality differentiation: it is conductive enough to allow spin-polarized current transport for STT-MRAM operation, yet sufficiently dense and chemically stable to block metal diffusion to the free layer and prevent nitrogen diffusion from the capping layer. This local quality optimization allows the same layer to simultaneously provide electrical conductivity and diffusion barrier functions
4Reliability
If high-temperature annealing is applied to improve magnetic properties, then magnetic performance is enhanced, but oxygen migrates out of the Hk enhancing layer and metal diffusion increases
Solution Approach 1:
The nitride or oxynitride capping layer is deposited on the Hk enhancing layer before high-temperature annealing processes. This preliminary action creates a protective barrier that prevents oxygen migration out of the Hk enhancing layer during subsequent high-temperature annealing, allowing the annealing process to proceed at temperatures sufficient to develop the desired magnetic properties without compromising the oxygen content and composition stability of the Hk enhancing layer
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 configuration enhances the thermal stability and magnetic performance of the free layer, improving the DRR and reducing electrical shorts, while maintaining a low RA product, thus supporting high-performance advanced memory designs.
Implementation Method 1
a nitride or oxynitride barrier layer that serves as a barrier between a Hk enhancing layer and a hard mask or seed layer, and that minimizes diffusion of metals or nitrogen through the Hk enhancing layer to an adjoining free layer
Implementation Method 2
enhances the thermal stability and magnetic performance of the free layer, improving the DRR and reducing electrical shorts
Implementation Method 3
conductive pathways are formed within the capping layer to maintain the magnetoresistive ratio and reduce the RA product
Implementation Method 4
along with a metallic buffer layer to act as a barrier, minimizing diffusion and oxygen migration
Data Source
AI summary
A magnetic tunnel junction (MTJ) is disclosed wherein first and second interfaces of a free layer (FL) with a first metal oxide (Hk enhancing layer) and second metal oxide (tunnel barrier), respectively, produce perpendicular magnetic anisotropy (PMA) to increase thermal stability. In some embodiments, a capping layer that is a conductive metal nitride such as MoN contacts an opposite surface of the Hk enhancing layer with respect to the first interface to reduce interdiffusion of oxygen and nitrogen compared with a TiN capping layer and maintain an acceptable resistance×area (RA) product. In other embodiments, the capping layer may comprise an insulating nitride such as AlN that is alloyed with a conductive metal to minimize RA. Furthermore, a metallic buffer layer may be inserted between the capping layer and Hk enhancing layer. As a result, electrical shorts are reduced and the magnetoresistive ratio is increased.