SOT-MRAM Cell With Integrated Diode for Single Transistor Control
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Solution Overview
Problem
Spin-transfer torque MRAM devices face longevity issues due to high write energies passing through the magnetic tunnel junction, leading to material degradation and increased writing errors, while existing spin-orbit torque MRAM cells require additional die area for two transistors to control read and write operations.
Innovation Solution
Incorporating a cylindrical diode structure around the spin-orbit-torque MRAM cell stack, allowing control of both read and write operations through a single transistor, with the diode's voltage drop and current tuned by adjusting its positioning and materials to optimize device performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If spin-transfer torque MRAM devices use high write energies to switch magnetization state, then writing speed is improved, but material degradation occurs and device longevity decreases
Solution Approach 1:
The patent introduces a spin-orbit torque layer (SOTL) as an intermediary component between the current source and the magnetic tunnel junction. Write current flows through the SOTL which generates spin-orbit torque to switch the magnetization state of the free layer, rather than passing current directly through the MTJ. This mediator approach enables fast writing while preventing material degradation in the tunnel junction, thus extending device longevity.
2Reliability
If spin-orbit torque MRAM cells use a spin-orbit torque layer to reduce write energy through the MTJ, then device longevity is improved, but additional die area is required for control transistors
Solution Approach 1:
The patent merges the read control and write control functions into a single transistor. The transistor gate controls both the read current path through the MTJ and the write current path through the spin-orbit torque layer. This consolidation eliminates the need for separate read and write transistors, reducing die area while maintaining the longevity benefits of SOT-MRAM.
Solution Approach 2:
The single transistor is designed with multi-functionality to perform both read and write operations. By controlling the voltage applied to the transistor gate, the same device can route current through different paths: through the MTJ for read operations and through the SOTL for write operations. This universal control mechanism reduces the number of components needed while maintaining full functionality.
3Reliability
If write current is passed through the spin-orbit torque layer instead of the MTJ, then material degradation is reduced, but device complexity increases
Solution Approach 1:
The patent transitions from a planar two-terminal MTJ structure to a three-dimensional stack including the SOTL, MTJ, and integrated diode. This vertical integration allows the write current to flow through the SOTL layer while the read current flows through the MTJ, enabling material durability improvements without significantly increasing lateral footprint. The cylindrical diode structure further optimizes space utilization in the vertical dimension.
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 cell density, reduces energy consumption, and extends device longevity by minimizing write energy through the MRAM cell stack, while maintaining reliable read and write operations with improved integration and reduced error rates.
Implementation Method 1
Spin-orbit-torque (SOT) MRAM cells include a spin-orbit torque, or spin-Hall-effect (SHE), layer in contact with the magnetic tunnel junction (MTJ) structure of the MRAM. Current is passed through the SHE layer, but not through the MTJ structure, to write to the cell
Implementation Method 2
MRAM is a type of solid state, non-volatile memory that uses tunneling magnetoresistance (TMR) to store information. Each MTJ includes a free layer and fixed layer that each include a layer of a magnetic material, and that are separated by a non-magnetic insulating tunnel barrier.
Data Source
AI summary
A spin-orbit torque magnetoresistive random-access memory device formed by fabricating a spin-Hall-effect (SHE) layer above and in electrical contact with a transistor, forming a spin-orbit-torque (SOT) magnetoresistive random access memory (MRAM) cell stack disposed above and in electrical contact with the SHE rail, wherein the SOT-MRAM cell stack comprises a free layer, a tunnel junction layer, and a reference layer, forming a cylindrical diode structure above and in electrical contact with the SOT-MRAM cell stack, forming a write line disposed in electrical contact with the SHE rail, and forming a read line disposed above and adjacent to an outer cylindrical electrode of the diode structure.