MRAM Cell Lifespan Extension via Polarity-Reversed Heating Current
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Solution Overview
Problem
The endurance of MRAM cells is limited due to the degradation of the thin oxide layer in the magnetic tunnel junction caused by repeated heating current cycling during thermally assisted write operations, which restricts the voltage magnitude and affects the lifespan of the device.
Innovation Solution
A method involving a bipolar transistor to control the heating current in the magnetic tunnel junction, reversing the polarity of the heating current between each writing step, and using a sequence of writing steps that heat the junction to a high temperature threshold followed by adjusting the second ferromagnetic layer's magnetization, while cooling to freeze the written state.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If heating current is passed through the magnetic tunnel junction to heat it to high temperature for writing operations, then the write operation can be performed, but the thin oxide layer degrades and the lifespan of the MRAM cell is limited
Solution Approach 1:
A heating element is introduced as an intermediary component between the current line and the magnetic tunnel junction. This heating element serves as a mediator that generates heat when current passes through it, thereby heating the magnetic tunnel junction without requiring high voltage across the vulnerable oxide layer. The heating element acts as a buffer that converts electrical energy to thermal energy externally, protecting the oxide layer from direct high-voltage stress while still enabling the necessary heating for write operations.
Solution Approach 2:
The patent replaces the direct electrical heating method (passing current through the oxide layer) with a thermal field approach using a dedicated heating element. Instead of using the electrical field to directly heat the junction through the oxide, a separate heating element generates thermal field that diffuses to the magnetic tunnel junction, achieving the same heating effect while avoiding oxide layer degradation from direct current stress.
2Temperature
If high voltage is applied across the oxide layer to enable heating current flow, then heating can be achieved, but the oxide layer experiences increased stress and degradation
Solution Approach 1:
The heating element serves as an intermediary that decouples the voltage application from the oxide layer. The heating element is positioned such that it can be heated by current flow without requiring high voltage across the oxide layer. This mediator allows thermal energy to be generated and transferred to the magnetic tunnel junction while the oxide layer is shielded from direct high-voltage exposure, thus achieving temperature increase without proportional increase in oxidative stress.
Solution Approach 2:
The heating element is strategically positioned adjacent to the magnetic tunnel junction but electrically isolated from the oxide layer. This local placement allows the heating element to generate heat that diffuses locally to the junction region, creating a localized thermal field that heats the magnetic tunnel junction without requiring the entire oxide layer to withstand high voltage. The thermal energy is concentrated where needed while the harmful electrical stress is confined to the heating element rather than the oxide layer.
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 increases the lifespan of the MRAM cell by extending the number of writing steps before breakdown, as demonstrated by experimental results showing a two-order magnitude increase in the number of writing steps without tunnel junction failure.
Implementation Method 1
passing the heating current in the magnetic tunnel junction such as to heat the magnetic tunnel junction to a high temperature threshold
Implementation Method 2
the transistor is a bipolar transistor being arranged for controlling the passing of a heating current in the magnetic tunnel junction and changing the heating current polarity
Implementation Method 3
adjusting a second magnetization of the second ferromagnetic layer for writing a write data
Data Source
AI summary
Method for writing to a MRAM cell including a magnetic tunnel junction including a first and second ferromagnetic layer, and a tunnel barrier layer; and a bipolar transistor in electrical connection with one end of the magnetic tunnel junction, the bipolar transistor being arranged for controlling the passing and polarity of a heating current in the magnetic tunnel junction. The method includes a sequence of writing steps, each writing step including passing the heating current in the magnetic tunnel junction such as to heat it to a high temperature threshold; and once the magnetic tunnel junction has reached the high temperature threshold, adjusting a second magnetization of the second ferromagnetic layer for writing a write data; wherein during one of the writing steps, the polarity of the heating current is reversed from one during the subsequent writing step. The method allows for an increased lifespan of the MRAM cell.