Zero Transistor Bitcell Using Spin Hall Metal for MRAM
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Solution Overview
Problem
Conventional spin torque transfer (STT) magnetic random access memory (MRAM) faces limitations such as slow write speeds, high write currents that lead to oxide breakdown, and limited die density due to the need for access transistors per bitcell, as well as the risk of MRAM cell flipping during read operations.
Innovation Solution
A transverse current bi-directional bitcell architecture utilizing spin orbit torque, which eliminates the need for access transistors and employs a spin hall metal with a large spin hall angle adjacent to the free layer of a magneto tunnel junction (MTJ) stack, allowing for a diffusive spin torque current to write and store charge states without flowing through the MTJ's tunnel barrier, thereby reducing write current and improving endurance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional STT MRAM uses charge current through MTJ to write data, then the magnetic moment of free layer can be switched, but write speed is slow (10's of ns) and write current is large
Solution Approach 1:
The patent introduces a spin hall metal layer as an intermediary between the charge current source and the free layer. The charge current flows through the spin hall metal (not directly through MTJ), generating spin current via the spin hall effect that then acts on the free layer. This intermediary mechanism enables faster switching (100's of ps) with reduced write current compared to direct STT through MTJ.
Solution Approach 2:
The patent replaces the direct spin torque transfer mechanism (where charge current directly exerts torque on free layer through MTJ) with a spin orbit torque mechanism. The charge current generates spin current through the spin hall metal, and this spin current then exerts torque on the free layer. This substitution of the physical mechanism enables faster and more energy-efficient writing.
2Reliability
If large write current is passed through MTJ to switch magnetic moment, then data can be written, but oxide breakdown occurs and endurance is limited
Solution Approach 1:
The spin hall metal layer serves as a mediator that decouples the charge current path from the MTJ tunnel barrier. The charge current flows laterally through the spin hall metal to generate spin current, which then vertically acts on the free layer. This intermediary approach eliminates direct charge current flow through the oxide barrier, preventing breakdown and improving endurance.
Solution Approach 2:
The patent changes the dimension of charge current flow from vertical (through MTJ stack) to lateral (through spin hall metal). By transitioning the current path to a different spatial dimension, the charge current no longer stresses the oxide barrier, thereby improving device reliability and endurance.
3Quantity of substance
If access transistor is included per bitcell to control read/write operations, then selective access is enabled, but die density is limited
Solution Approach 1:
The patent extracts and removes the access transistor from the bitcell structure. Selective access to the MTJ is achieved through diode-based current path control and timing sequences rather than transistor switching. This extraction of the transistor reduces the bitcell area significantly, enabling higher die density.
Solution Approach 2:
The spin hall metal layer performs multiple functions: it serves as the write current path, generates spin current for switching, and acts as part of the read current path. This multi-functionality eliminates the need for separate access transistors to control different current paths, reducing device complexity and increasing density.
4Measurement precision
If read current is sufficiently large to read MTJ state, then read functionality is achieved, but MRAM cell may undesirably flip
Solution Approach 1:
The spin hall metal layer acts as an intermediary that enables read current to flow laterally through the spin hall metal rather than vertically through the MTJ stack. This intermediary path allows sufficient read current to flow for signal detection while preventing direct current stress on the MTJ that could cause unwanted switching.
Solution Approach 2:
The patent changes the read current path from vertical (through MTJ) to lateral (through spin hall metal). By measuring the voltage drop or current characteristics in the lateral path, the read operation achieves sufficient signal detection without exposing the MTJ to high current stress that could cause data flipping.
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 solution enhances write speed, reduces power consumption, and increases memory density by minimizing the number of transistors required per block of cache/memory while maintaining non-volatile storage and read functionality without disturbing the magnetization state.
Implementation Method 1
The MTJ is written by a diffusive spin torque current injected from an interface across which a charge current flows
Implementation Method 2
The charge current affects the magnetic moment of the free layer to either be aligned or anti-aligned with the magnetic moment of the pinned layer
Implementation Method 3
As the alignment of magnetic moments remains unchanged in the absence of the charge current, the MTJ stack behaves as a bi-stable system suitable for memory storage
Implementation Method 4
a pair of write diodes having opposite polarity, the first and second write bitlines being configured to source the charge current through the spin hall metal
Data Source
AI summary
A bitcell for storing a charge state, the bitcell including a spin hall metal for passing through a charge current, a magneto tunnel junction (MTJ) stack for generating and storing a non-volatile spin state corresponding to a binary bit in response to passage of the charge current through the spin hall metal, and for inducing the charge current corresponding to the non-volatile spin state in response to application of a read voltage, first and second write bitlines for sourcing the charge current through the spin hall metal in response to a write voltage being applied to both of the first and second write bitlines, and a read bitline for inducing the charge current through the spin hall metal in response to the read voltage being applied to the read bitline, and a first wordline and a second wordline for permitting a flow of the charge current through spin hall metal.