MTJ Free Layer Drain Coupling for MRAM Write Current Alignment

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Solution Overview

Problem

The inherent write current characteristics of magnetic tunnel junctions (MTJs) used in magnetic random access memory (MRAM) bitcells are not aligned with the write supply current capability, requiring more current to switch from a parallel to anti-parallel state than from an anti-parallel to parallel state, leading to a design conflict.

Innovation Solution

The MTJ design is modified by coupling the drain of the access transistor with the free layer, allowing more write current to be supplied to switch from a parallel to an anti-parallel state, aligning the inherent write current characteristics with the write supply current capability, and using an anti-ferromagnetic layer to pin the magnetization of the pinned layer, ensuring it retains its orientation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If the drain of the access transistor is coupled to the pinned layer in a conventional MTJ design, then the write current characteristics are simplified, but the write supply current capability is insufficient to switch from parallel to anti-parallel state

Engineering Contradiction:

ImproveMTJ structure configurationVSAvoidwrite supply current capability

Core Design Contradiction:

Device complexityVSPower

Solution Approach 1:

The patent inverts the conventional MTJ configuration by coupling the drain of the access transistor to the free layer instead of the pinned layer. This inversion changes the current flow path and spin transfer torque direction, enabling the write circuitry to supply sufficient current for switching from parallel to anti-parallel state while maintaining proper magnetization control

Inventive Principle:

Principle #13The other way round (Inversion)

2Productivity

If more write current is supplied to switch from parallel to anti-parallel state, then the switching capability is improved, but the magnetization of the pinned layer may be disturbed

Engineering Contradiction:

Improveswitching capabilityVSAvoidmagnetization stability

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent applies local quality by providing different magnetic properties to different layers: the pinned layer has high magnetic anisotropy and is pinned by an anti-ferromagnetic layer to maintain stable magnetization, while the free layer has lower anisotropy allowing it to be switched by spin transfer torque. This local differentiation enables high switching capability without compromising magnetization stability

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The anti-ferromagnetic layer is deposited beforehand to pin the pinned layer's magnetization, creating a protective cushion that prevents the pinned layer from being disturbed by the write current. This prior cushioning ensures that high write currents can be applied for switching without risking magnetization stability

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

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 enables more efficient switching of memory states in MRAM bitcells by providing sufficient write current to switch from a parallel to an anti-parallel state while maintaining the magnetization of the pinned layer, addressing the design conflict and ensuring data retention.

Implementation Method 1

Electrons can tunnel through the tunnel barrier 16 if a bias voltage is applied between two electrodes 20, 22 coupled on ends of the MTJ 10. The tunneling current depends on the relative orientation of the free and pinned layers 12, 14.

Methodology Applied

Scientific EffectTunneling:

Implementation Method 2

using an anti-ferromagnetic layer to pin the magnetization of the pinned layer, ensuring it retains its orientation

Methodology Applied

Scientific EffectExchange coupling:

Implementation Method 3

When using a spin-torque-transfer (STT) MTJ, the difference in the tunneling current as the spin alignment of the free and pinned layers is switched between P and AP

Methodology Applied

Scientific EffectSpin transfer torque:

Data Source

PatentUS8889431B2Magnetic tunnel junction (MTJ) and methods, and magnetic random access memory (MRAM) employing same

Publication Date: 2014.11.18 QUALCOMM INC

AI summary

Magnetic tunnel junctions (MTJs) and methods of forming same are disclosed. A pinned layer is disposed in the MTJ such that a free layer of the MTJ can couple to a drain of an access transistor when provided in a magnetic random access memory (MRAM) bitcell. This structure alters the write current flow direction to align the write current characteristics of the MTJ with write current supply capability of an MRAM bitcell employing the MTJ. As a result, more write current can be provided to switch the MTJ from a parallel (P) to anti-parallel (AP) state. An anti-ferromagnetic material (AFM) layer is provided on the pinned layer to fix pinned layer magnetization. To provide enough area for depositing the AFM layer to secure pinned layer magnetization, a pinned layer having a pinned layer surface area greater than a free layer surface area of the free layer is provided.