Spin Transfer MRAM With Separated Write Read Paths
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Solution Overview
Problem
Conventional Spin-RAM designs face challenges in reducing the high write current density required for spin-transfer magnetization switching, which can damage the MTJ tunnel barrier and increase power consumption, especially as memory devices miniaturize, and struggle with differentiating between '0' and '1' states during read operations.
Innovation Solution
A Spin-RAM design featuring two CPP/MTJ sub-cells per bit cell, where the CPP cell and MTJ cell are optimized for separate write and read pathways, with a large anisotropy in the CPP cell to easily switch the MTJ cell's magnetic moment, and a conductive spacer to facilitate spin-transfer switching without high current through the MTJ cell, allowing one MTJ cell to serve as a reference for reading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If spin-transfer magnetization switching is used to reduce write current, then power consumption is reduced, but the write current density becomes too high and damages the MTJ tunnel barrier
Solution Approach 1:
The memory cell is divided into two separate cells: a first MTJ cell optimized for read operations and a second CPP cell optimized for write operations. This segmentation allows the write current to flow through the CPP cell instead of the MTJ cell, protecting the tunnel barrier from high current density damage while maintaining spin-transfer magnetization switching benefits.
Solution Approach 2:
A conductive spacer is introduced as an intermediary component between the first and second cells. This spacer facilitates the flow of write current from the bit line through the CPP cell while providing electrical isolation and structural support, enabling the write path to be separated from the read path.
2Device complexity
If write and read operations use the same cell, then device structure is simplified, but read reliability is reduced due to difficulty in differentiating '0' and '1' states
Solution Approach 1:
The memory cell is divided into two separate cells: a first MTJ cell optimized for read operations and a second CPP cell optimized for write operations. This segmentation allows the read operation to use only the first cell, providing clear resistance states for reliable data reading, while the second cell handles write operations.
Solution Approach 2:
Each cell is optimized for its specific function: the first MTJ cell has properties optimized for read operations (clear resistance differentiation), while the second CPP cell has properties optimized for write operations (efficient spin-transfer switching). This local optimization improves overall device performance.
3Area of stationary object
If address wiring width is reduced for miniaturization, then device density is increased, but sufficient electric current cannot be applied to address wiring for write operations
Solution Approach 1:
The write current path is segmented from the read current path. The CPP cell is specifically designed to handle write operations with optimized current flow through the conductive spacer, allowing sufficient write current to be applied without requiring wider address wiring, thus maintaining device density.
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 design reduces write current density, prevents tunnel barrier breakdown, enhances read reliability and speed by separating write and read paths, and allows for efficient magnetization switching with improved thermal stability and data retention.
Implementation Method 1
The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a CPP configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer.
Implementation Method 2
The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers.
Implementation Method 3
The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons.
Implementation Method 4
The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction.
Data Source
AI summary
A spin-transfer MRAM is described that has two sub-cells each having a conductive spacer between an upper CPP cell and a lower MTJ cell. The two conductive spacers in each bit cell are linked by a transistor which is controlled by a write word line. The two CPP cells in each bit cell have different resistance states and the MTJ cell and CPP cell in each sub-cell have different resistance states. The MTJ free layer rotates in response to switching in the CPP free layer because of a large demagnetization field exerted by the CPP free layer. An improved circuit design is disclosed that enables a faster and more reliable read process since the reference is a second MTJ within the same bit cell. When RMTJ1>RMTJ2, the bit cell has a “0” state, and when RMTJ1<RMTJ2, the bit cell has a “1” state.