Cross-Point MRAM Selector Stack for Low Leakage Switching
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Solution Overview
Problem
Conventional bidirectional selector devices for memory applications suffer from high current leakage, limiting the miniaturization of STT-MRAM memory cells and increasing manufacturing costs.
Innovation Solution
A two-terminal bidirectional selector device is introduced, comprising a magnetic tunnel junction connected in series with a load-resistance layer and volatile switching layers made of tantalum oxide and hafnium oxide, which reduces current leakage by forming a conductive filament upon voltage application, allowing for low resistance states and self-compliance functionality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional bidirectional selector devices are used in STT-MRAM memory cells, then the memory cell structure is simpler, but current leakage increases significantly
Solution Approach 1:
The selector device is divided into multiple functional layers including a first volatile switching layer with metal dopant and second tantalum oxide, and a second volatile switching layer with stoichiometric hafnium oxide. Each layer performs a specific function in controlling current flow and reducing leakage, thereby solving the current leakage problem while maintaining structural organization.
Solution Approach 2:
The patent employs composite material structures combining different oxide materials (tantalum oxide with varying oxygen content, hafnium oxide) and metal dopants. These composite materials create distinct resistance states and enable self-compliance functionality that reduces current leakage while maintaining the benefits of a simplified selector-based memory cell structure.
2Object-generated harmful factors
If access transistors are used in STT-MRAM memory cells, then current leakage is controlled, but the memory cell area increases
Solution Approach 1:
The patent extracts the transistor gate control functionality and replaces it with a two-terminal selector device that achieves similar current control without requiring the additional gate electrode and control circuitry. This extraction of the essential switching function while removing unnecessary components reduces the memory cell area while maintaining current leakage control through the selector's inherent resistance characteristics.
3Object-generated harmful factors
If selector devices with high resistance are used, then current leakage is reduced, but switching efficiency decreases
Solution Approach 1:
The selector device utilizes dynamic resistance switching between high resistance state (HRS) and low resistance state (LRS) through applied voltage. The volatile switching layers can dynamically adjust their resistance based on applied voltage pulses, enabling efficient switching operation. The metal dopant in the first volatile switching layer facilitates controlled filament formation and dissolution, achieving both low leakage in HRS and high switching efficiency when transitioning to LRS.
Solution Approach 2:
The patent changes the oxygen content parameter in tantalum oxide to create different resistance states. The first volatile switching layer contains second tantalum oxide with higher oxygen content than the load-resistance layer, while the second volatile switching layer contains stoichiometric hafnium oxide. These parameter changes enable the device to achieve both low current leakage and efficient switching by controlling the oxidation state and filament formation.
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 proposed selector device achieves low current leakage and efficient switching with a significant reduction in electrical resistance, enabling smaller memory cell sizes and cost-effective manufacturing while maintaining reliable operation.
Implementation Method 1
reduces current leakage by forming a conductive filament upon voltage application, allowing for low resistance states
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
Data Source
AI summary
The present invention is directed to a magnetic memory cell including a magnetic tunnel junction (MTJ) memory element and a two-terminal bidirectional selector coupled in series between two conductive lines. The MTJ memory element includes a magnetic free layer; a magnetic reference layer; and an insulating tunnel junction layer interposed therebetween. The two-terminal bidirectional selector includes a bottom electrode; a top electrode; a load-resistance layer interposed between the bottom and top electrodes and comprising a first tantalum oxide; a first volatile switching layer interposed between the bottom and top electrodes and comprising a metal dopant and a second tantalum oxide that has a higher oxygen content than the first tantalum oxide; and a second volatile switching layer in contact with the first volatile switching layer and comprising a third tantalum oxide that has a higher oxygen content than the first tantalum oxide.