STT-RAM MTJ Nanocurrent Channel for Low Switching Current
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Solution Overview
Problem
Current Magnetic Tunneling Junction (MTJ) technologies face challenges in achieving low switching current density and high thermal stability necessary for advanced STT-RAM devices, particularly for 64 Mbit applications, with existing solutions failing to meet requirements for high tunneling magnetoresistive ratio, low resistance-area values, and sufficient thermal stability factor and coercivity.
Innovation Solution
The development of a MTJ element with a free layer configuration featuring a nanocurrent channel (NCC) layer between ferromagnetic layers, optimized through specific layer compositions and structures such as FL1/NCC/FL2 or FL1/NCC/FL2/NCC/FL3, using materials like CoFeB and MgO, and employing sputtering and annealing processes to enhance spin polarization and reduce switching current density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional MTJ structures with AlOx tunnel barrier and NiFe free layer are used, then the device can be manufactured with existing processes, but the TMR ratio is insufficient and critical current density is too high for high-density STT-RAM
Solution Approach 1:
The patent employs composite material structures including CoFeB/MgO/CoFeB tunnel junctions with engineered interfaces, and introduces nanocurrent channel (NCC) layers composed of granular metal-oxide composites (e.g., CoFeB-SiO2, CoFe-SiO2) to achieve both high TMR ratio and reduced critical current density through optimized material composition and interface engineering
Solution Approach 2:
The patent applies local quality enhancement by creating specific interface structures between ferromagnetic layers and tunnel barriers, introducing NCC layers with controlled granular structures at critical locations, and using gradient compositions in CoFeB layers to locally optimize spin polarization and reduce damping without compromising overall device performance
2Use of energy by moving object
If the free layer thickness is reduced to decrease switching current, then the switching current density decreases, but the thermal stability factor becomes insufficient
Solution Approach 1:
The patent utilizes parameter changes by precisely controlling free layer thickness (10-20 nm range), adjusting MgO tunnel barrier thickness (5-10 nm), and optimizing NCC layer composition and thickness to achieve the optimal balance between low switching current density and sufficient thermal stability factor (Δ > 60)
Solution Approach 2:
The introduction of NCC layers with granular metal-oxide composite structures provides enhanced interfacial perpendicular magnetic anisotropy (IPMA) that stabilizes the magnetic moment orientation perpendicular to the film plane, thereby maintaining thermal stability even in ultra-thin free layers
3Ease of manufacture
If external magnetic field lines are used to switch magnetization in conventional MRAM, then the write mechanism is simple, but half-select disturb and writing disturbance between adjacent cells occur
Solution Approach 1:
The patent replaces the mechanical/electromagnetic field-based write mechanism with a spin-transfer torque mechanism where spin-polarized current directly interacts with magnetic moments through quantum mechanical spin-dependent scattering, enabling cell-isolated switching without affecting adjacent cells
4Area of stationary object
If the MTJ size is reduced to increase memory density, then the areal density increases, but the switching current density becomes difficult to control and thermal stability decreases
Solution Approach 1:
The patent maintains thermal stability in scaled-down MTJs by optimizing the product of free layer thickness and interfacial perpendicular magnetic anisotropy energy density, using NCC layers to enhance IPMA, and carefully controlling the dimensions of the tunnel junction to achieve sufficient energy barrier despite reduced area
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 achieves a significant reduction in switching current density while maintaining high thermal stability and coercivity, meeting the requirements for advanced STT-RAM devices, including a thermal stability factor greater than 55 and coercivity of at least 160 Oe, and reducing undesirable soft breakdown components.
Implementation Method 1
the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer.
Implementation Method 2
Both field-MRAM and STT-RAM have a MTJ element 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.
Implementation Method 3
employing sputtering and annealing processes to enhance spin polarization and reduce switching current density
Implementation Method 4
employing sputtering and annealing processes to enhance spin polarization and reduce switching current density
Data Source
AI summary
A STT-RAM MTJ is disclosed with a MgO tunnel barrier formed by natural oxidation process. A Co10Fe70B20/NCC/Co10Fe70B20, Co10Fe70B20/NCC/Co10Fe70B20/NCC, or Co10Fe70B20/NCC/Co10Fe70B20/NCC/Co10Fe70B20 free layer configuration where NCC is a nanocurrent channel layer made of Fe(20%)-SiO2 is used to minimize Jc0 while enabling higher thermal stability, write voltage, read voltage, Ho, and Hc values that satisfy 64 Mb design requirements. The NCC layer is about 10 Angstroms thick to match the minimum Fe(Si) grain diameter size. The MTJ is annealed with a temperature of about 330° C. to maintain a high magnetoresistive ratio while maximizing Hk⊥(interfacial) for the free layer thereby reducing Heff and lowering the switching current. The Co10Fe70B20 layers are sputter deposited with a low pressure process with a power of about 15 Watts and an Ar flow rate of 40 standard cubic centimeters per minute to lower Heff for the free layer.