SOT MRAM Cell Layout Using a Selector Pillar to Shrink Bit Cells
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Solution Overview
Problem
Prior art SOT MRAM cells require two transistors for control, leading to larger bit-cell footprint and reduced device density, complicating fabrication and increasing write error rates due to the need for large overdrive currents.
Innovation Solution
Incorporating a combination of a transistor and a selector element, or two selector elements, instead of two transistors for each SOT MRAM cell, to reduce the number of transistors and increase device density, with a two-dimensional array configuration that includes a magnetic tunnel junction and a nonmagnetic metallic assist plate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If two transistors are used for control in prior art SOT MRAM cells, then the device can be operated, but the bit-cell footprint increases and device density decreases
Solution Approach 1:
The patent extracts one transistor from the control circuitry and replaces it with a selector element (such as a diode or threshold switch). This reduction in transistor count directly decreases the bit-cell footprint while maintaining operational control through the combined transistor-selector element circuit configuration.
Solution Approach 2:
The selector element (diode or threshold switch) is introduced to perform control functions that were previously exclusively handled by transistors. This multi-functional approach allows the circuit to maintain full operational control with fewer active components, reducing overall cell area.
2Reliability
If two transistors are used for control in prior art SOT MRAM cells, then the device can be operated, but fabrication becomes more complex
Solution Approach 1:
By removing one transistor from the control circuit and replacing it with a simpler selector element, the fabrication process is simplified. Selector elements like diodes or threshold switches have fewer fabrication steps and lower process complexity compared to transistors, making the overall device easier to manufacture.
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 increases device density, reduces write error rates, and simplifies fabrication by eliminating the need for large overdrive currents, while maintaining high operational speed and endurance.
Implementation Method 1
use switching of magnetization direction of a free magnetic layer by injection of an in-plane current in an adjacent conductive layer, which is referred to as a spin-orbit-torque (SOT) layer
Implementation Method 2
The read operation of a SOT memory cell is performed by passing electrical current through the magnetic tunnel junction of the SOT memory cell
Data Source
AI summary
A spin-orbit-torque (SOT) magnetoresistive random access memory (MRAM) device includes a SOT MRAM cell containing a first two terminal selector element, a nonmagnetic metallic assist plate, and a magnetic tunnel junction located between the first two terminal selector element and the nonmagnetic metallic assist plate, and a circuit selection element selected from a transistor or a second two terminal selector element electrically connected to the nonmagnetic metallic assist plate of the SOT MRAM cell.