CoFeB MTJ Nanopillar for Low Switching Current STT-RAM
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Solution Overview
Problem
Current Magnetic Tunneling Junction (MTJ) devices for Spin-RAM face challenges in achieving high dR/R ratios and low critical current densities, which are essential for high-density memory applications, as they often result in electrical breakdown and resistance variations that affect read operations and thermal stability.
Innovation Solution
A MTJ nanopillar structure is designed with a CoFe fixed layer having large shape anisotropy, a free layer with a nanocurrent channel FeSiO layer, and a thin Ru capping layer, along with a crystalline MgO tunnel barrier, to enhance dR/R ratios and reduce critical current densities, facilitating magnetization switching without damaging the tunnel barrier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional MTJ structures are used to achieve high dR/R ratios, then read operation performance improves, but critical current density increases causing electrical breakdown
Solution Approach 1:
The patent changes material parameters by replacing the free layer material from NiFe to CoFeB, which has different magnetic properties including lower damping and higher spin polarization. This material substitution enables achieving high dR/R ratio with lower critical current density, resolving the contradiction between read performance and electrical reliability
Solution Approach 2:
The patent employs composite material structures including CoFeB free layer combined with MgO tunnel barrier and Ru capping layer. This composite approach optimizes both the tunneling magnetoresistance effect for high dR/R and the overall device stability for reduced electrical breakdown, simultaneously addressing both contradictory requirements
2Speed
If high critical current density is applied for magnetization switching, then switching speed improves, but tunnel barrier integrity deteriorates
Solution Approach 1:
The patent optimizes the thickness parameter of the MgO tunnel barrier to a specific range (2-4 nm) and adjusts the CoFeB free layer thickness to control the switching current density. These parameter optimizations enable achieving fast magnetization switching while maintaining tunnel barrier integrity by preventing electrical breakdown at the optimized current density levels
Solution Approach 2:
The Ru capping layer serves as an intermediary between the CoFeB free layer and the MgO tunnel barrier, improving the overall device performance and protecting the tunnel barrier from degradation during high current density switching operations
3Reliability
If low critical current density is achieved, then electrical reliability improves, but thermal stability deteriorates
Solution Approach 1:
The patent carefully balances the thickness parameters of the CoFeB free layer and MgO tunnel barrier to achieve an optimal operating point where the critical current density is low enough for electrical reliability but the magnetic anisotropy energy remains high enough for thermal stability. This parameter optimization resolves the contradiction between electrical and thermal performance
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 proposed structure achieves a high dR/R ratio of about 100% and a critical current density of less than 2×10^6 A/cm², improving magnetization switching and thermal stability while maintaining the integrity of the tunnel barrier, thus enhancing the performance of Spin-RAM devices.
Implementation Method 1
a CoFe fixed layer having large shape anisotropy
Implementation Method 2
spin transfer (Spin-RAM) device that achieves low switching current
Implementation Method 3
a thin Ru capping layer
Implementation Method 4
The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons
Implementation Method 5
The AFM layer holds the magnetic moment of the pinned layer in a fixed direction
Implementation Method 6
Both MRAM and STT-RAM have a MTJ element based on a tunneling magneto-resistance (TMR) effect
Data Source
AI summary
A STT-RAM MTJ that minimizes spin-transfer magnetization switching current (Jc) while achieving a high dR/R is disclosed. The MTJ has a MgO tunnel barrier formed by natural oxidation to achieve a low RA, and a CoFeB/FeSiO/CoFeB composite free layer with a middle nanocurrent channel layer to minimize Jc0. There is a thin Ru capping layer for a spin scattering effect. The reference layer has a shape anisotropy and Hc substantially greater than that of the free layer to establish a “self-pinned” state. The free layer, capping layer and hard mask are formed in an upper section of a nanopillar that has an area substantially less than a lower pedestal section which includes a bottom electrode, reference layer, seed layer, and tunnel barrier layer. The reference layer is comprised of an enhanced damping constant material that may be an insertion layer, and the free layer has a low damping constant.