MRAM Sense Amplifier with VCMA Precessional Switching
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Solution Overview
Problem
Magnetoresistive random access memory (MRAM) technologies face challenges in achieving high-speed and low-power precessional switching with small sensing margins and high error rates due to the limitations of existing sense amplifier and writing circuits, particularly in magnetic tunnel junction (MTJ) devices.
Innovation Solution
A high-speed and low-power pre-read and write sense amplifier (PWSA) is developed, incorporating a writing circuit that utilizes precessional dynamics in magnetic tunnel junctions, combining read and write functions in a single power-efficient circuit, leveraging the voltage-controlled magnetic anisotropy (VCMA) effect for reduced power consumption and increased sensing margins, and employing a current feedback circuit to enhance sensing accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If precessional switching is used for high-speed MRAM, then switching speed is improved (down to 100 ps), but sensing margin decreases due to low TMR ratio
Solution Approach 1:
The patent applies preliminary action by performing a pre-read operation before the write operation. The sense amplifier reads the initial state of the MTJ, and a comparison circuit determines whether the MTJ state needs to be changed. This preliminary assessment allows the system to avoid unnecessary write operations and optimize the write timing, thereby improving the effective sensing margin while maintaining high-speed precessional switching.
Solution Approach 2:
The patent implements feedback through a sense amplifier that reads the MTJ state and a comparison circuit that compares the read state with the desired state. This feedback mechanism provides information about the actual MTJ state, enabling the system to make informed decisions about write operations and thereby improve sensing accuracy despite the low TMR ratio inherent in precessional switching.
2Device complexity
If conventional sense amplifier and writing circuit are used, then circuit structure is simple, but write power consumption is high and error rates are high
Solution Approach 1:
The patent merges the sense amplifier and writing circuit into a unified PWSA structure. The sense amplifier serves dual purposes: reading the MTJ state and providing feedback for write operation control. The comparison circuit integrates the decision-making logic for write operations. This merging reduces overall circuit complexity while enabling low-power operation through intelligent write control that avoids unnecessary write pulses.
Solution Approach 2:
The patent changes the operational parameters by using precessional switching with specifically timed voltage pulses instead of conventional current pulses. The write circuit generates voltage pulses with optimized amplitude and duration parameters that exploit the precessional dynamics of the MTJ, achieving low-power switching with reduced energy dissipation compared to conventional approaches.
3Quantity of substance
If 1T-1MTJ cell architecture is used for high density, then storage density is improved, but sensing margin decreases due to series bit line resistance
Solution Approach 1:
The patent substitutes the conventional resistance-based sensing approach with a voltage-based precessional switching approach. Instead of relying on large resistance changes from the MTJ (which are degraded by series bit line resistance in 1T-1MTJ cells), the system uses precisely timed voltage pulses to induce precessional switching. The sense amplifier detects the switching event through voltage transitions rather than resistance ratios, effectively bypassing the limitation imposed by series bit line resistance and maintaining high sensing margin in high-density architectures.
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
The PWSA achieves reduced write power consumption by up to 50%, a 10× reduction in write power and transistor size, and a 2× larger sensing margin, enabling fast read and write operations with minimized bit error rates, suitable for GHz applications with 2 ns read time, 1.8 ns write time, and 8 ns total data program operation time.
Implementation Method 1
the VCMA effect originates from the fact that the interface of oxides with metallic ferromagnets (e.g., CoFeB|MgO) shows a large perpendicular magnetic anisotropy (PMA), which is sensitive to voltages applied across the dielectric layer. This effect is caused by the electric field induced modulation of the relative occupancy of d orbitals at the interface. Since the PMA is modulated due to the applied voltage, a torque is exerted on the free layer magnetization, setting it into a precessional motion, thereby causing switching.
Implementation Method 2
In STT devices, precessional switching is achieved by incorporating an orthogonal combination of free and fixed layers into the device, where the large spin torque from the perpendicular fixed layer sets the free layer magnetization into a precessional motion, resulting in resonant switching.
Implementation Method 3
Magnetic tunnel junctions (MTJs) have become basic building blocks of MRAM, where relatively high tunneling magnetoresistance (TMR) ratios achieve two distinguishable resistive states, generally referred to as parallel (P) and anti-parallel (AP) states.
Data Source
AI summary
A fast and low-power sense amplifier and writing circuit for high-speed Magnetic RAM (MRAM) which provides the long retention times and endurance of magnetic tunnel junction (MTJ) cells, while providing faster access speeds, verified writes, and an increased sensing margin. A high-speed and low-power pre-read and write sense amplifier (PWSA) provide VCMA effect precessional switching of MTJ cells which include pre-read and comparison steps which reduce power consumption. An embodiment of the PWSA circuit is described with write and pre-charge circuit, S and D latches, comparison circuit, and a differential amplifier and control circuit.