STT-MRAM Sensing Method Using Selector Element Voltage Modulation
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Solution Overview
Problem
STT-MRAM devices have a relatively small sensing margin due to their low resistance ratio compared to other types of resistance-based memory devices, necessitating an improvement in resistance ratio to enhance sensing capabilities while being inexpensively manufactured.
Innovation Solution
A method for sensing the resistance state of a magnetic memory cell using a magnetic tunnel junction (MTJ) memory element coupled to a two-terminal selector element in series, where the selector element is turned on by raising the cell voltage above a threshold, and the resistance state is determined by measuring currents at different sensing voltages and comparing them to reference values or slopes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If a magnetic tunnel junction (MTJ) memory element is used in STT-MRAM, then the device has unlimited read/write endurance, but the sensing margin is relatively small due to low resistance ratio
Solution Approach 1:
A two-terminal selector element is introduced as an intermediary component coupled in series with the MTJ memory element. This selector element provides voltage-controlled resistance modulation that amplifies the overall resistance ratio of the memory cell, thereby improving the sensing margin while preserving the unlimited read/write endurance of the MTJ device.
Solution Approach 2:
The memory cell is constructed as a composite structure combining the MTJ memory element with the selector element. This composite configuration leverages the complementary characteristics of both components: the MTJ provides non-volatile memory storage with high endurance, while the selector element contributes voltage-dependent resistance modulation to enhance the sensing margin.
2Measurement precision
If the resistance ratio of the memory element is increased to improve sensing margin, then the sensing accuracy improves, but the manufacturing complexity and cost increase
Solution Approach 1:
The resistance modulation function is segmented between two distinct components: the MTJ memory element maintains its simple magnetic tunnel junction structure for reliable manufacturing, while the selector element separately provides the resistance amplification function. This segmentation allows each component to be optimized independently, maintaining manufacturing simplicity while achieving high sensing accuracy.
Solution Approach 2:
The selector element enables dynamic parameter changes in the overall memory cell resistance based on the applied voltage. By controlling the voltage across the selector element, the overall resistance ratio can be modulated to enhance sensing accuracy without requiring complex manufacturing processes, as the parameter change is achieved through electrical control rather than structural complexity.
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 approach significantly improves the sensing margin by providing a five-orders of magnitude increase in the resistance ratio, enabling more accurate resistance state determination with minimal read disturbance and power consumption.
Implementation Method 1
Upon the application of an appropriate current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction of the magnetic reference layer. When the magnetization directions of the magnetic free and reference layers are substantially parallel or oriented in a same direction, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ. Conversely, the electrical resistance of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel or oriented in opposite directions.
Implementation Method 2
When the magnetization directions of the magnetic free and reference layers are substantially parallel or oriented in a same direction, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ.
Implementation Method 3
turning on the selector element by raising a cell voltage across the magnetic memory cell above a threshold voltage for the selector element to become conductive
Data Source
AI summary
The present invention is directed to a method for sensing the resistance state of a memory cell that includes an MTJ memory element coupled to a two-terminal selector element in series. The method includes the steps of raising a cell voltage across the memory cell above a threshold voltage for the selector element to become conductive; decreasing the cell voltage to a first sensing voltage and measuring a first sensing current passing through the memory cell, the selector element being nominally conductive irrespective of the resistance state of the MTJ memory element at the first sensing voltage; and further decreasing the cell voltage to a second sensing voltage and measuring a second sensing current, the selector element being nominally conductive if the MTJ memory element is in the low resistance state or nominally insulative if the MTJ memory element is in the high resistance state at the second sensing voltage.