MgO Clusters in Free Layer for STT-MRAM Thermal Stability
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Solution Overview
Problem
Current spin-transfer torque magnetization switching in MRAMs faces challenges with high critical current density, which can lead to electrical breakdown of thin tunnel barriers and is not compatible with CMOS transistor requirements for high-density devices, necessitating improved spin torque efficiency and thermal stability while maintaining perpendicular magnetic anisotropy and resistance-area values.
Innovation Solution
Incorporating metal oxide clusters or non-stoichiometric metal oxide layers within the free layer of magnetic tunnel junctions to enhance perpendicular magnetic anisotropy, reduce switching current, and maintain acceptable resistance-area values, with interfaces at both surfaces of the free layer to promote thermal stability and preserve the magnetoresistive ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional AlOx tunnel barrier layer is used in STT-MRAM, then the device structure is simple, but the critical current density is too high (several 10^7 A/cm2) causing electrical breakdown and incompatibility with CMOS transistors
Solution Approach 1:
The patent changes the material composition parameters of the tunnel barrier layer by incorporating MgO clusters or non-stoichiometric MgO phases into the AlOx matrix. This compositional modification alters the electrical and magnetic properties, reducing the critical current density from several 10^7 A/cm2 to below 10^6 A/cm2 while maintaining thermal stability through controlled oxidation states and interface engineering with the free layer
Solution Approach 2:
The patent creates a composite tunnel barrier structure combining AlOx and MgO phases, where MgO clusters or non-stoichiometric MgO regions are embedded within the AlOx tunnel barrier layer. This composite structure leverages the beneficial properties of both materials: AlOx provides the base tunnel barrier functionality while MgO enhances perpendicular magnetic anisotropy and reduces switching current through improved spin torque efficiency
2Object-affected harmful factors
If the free layer is made thinner to reduce switching current, then the switching current decreases, but the thermal stability deteriorates
Solution Approach 1:
The patent modifies the interface composition and oxidation state parameters between the free layer and tunnel barrier by incorporating MgO clusters. This changes the perpendicular magnetic anisotropy energy density at the interface, allowing thinner free layers to maintain sufficient thermal stability through enhanced interfacial anisotropy rather than relying solely on volume effects
Solution Approach 2:
The patent creates localized regions of non-stoichiometric MgO or MgO clusters at specific interfaces within the tunnel barrier layer. These localized compositional variations generate enhanced perpendicular magnetic anisotropy at the free layer interface, providing strong thermal stability anchoring that compensates for the reduced thermal mass of thinner free layers
3Reliability
If MgO clusters or non-stoichiometric MgO layers are added to enhance perpendicular magnetic anisotropy, then thermal stability and spin torque efficiency improve, but the resistance-area product increases
Solution Approach 1:
The patent introduces MgO clusters or non-stoichiometric MgO regions as localized compositional modifications within the tunnel barrier layer rather than as a uniform continuous layer. This localized approach provides the necessary perpendicular magnetic anisotropy enhancement at the free layer interface while minimizing the overall resistance contribution, as the MgO clusters occupy only a fraction of the tunnel barrier volume and maintain adequate tunneling pathways
Solution Approach 2:
The patent controls the oxidation state parameter of MgO to be non-stoichiometric (oxygen-deficient) in specific regions, which modifies the local electrical resistance and magnetic properties. This parameter control allows optimization of the trade-off between perpendicular magnetic anisotropy enhancement and resistance-area product, as non-stoichiometric MgO provides stronger spin torque efficiency with lower resistance compared to fully stoichiometric MgO
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 enhances interfacial perpendicular anisotropy and thermal stability, reduces switching current, and maintains the magnetoresistive ratio, making it compatible with CMOS fabrication and suitable for high-density STT-MRAM devices.
Implementation Method 1
enhance perpendicular magnetic anisotropy, reduce switching current, and maintain acceptable resistance-area values
Implementation Method 2
significantly enhances interfacial perpendicular anisotropy and thermal stability
Implementation Method 3
spin-transfer torque (STT) magnetization switching described by C. Slonczewski in 'Current driven excitation of magnetic multilayers'
Implementation Method 4
The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers
Implementation Method 5
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 layer and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic free layer
Implementation Method 6
Both field-MRAM and STT-MRAM have a MTJ element based on a tunneling magneto-resistance (TMR) effect
Data Source
AI summary
A magnetic tunnel junction (MTJ) is disclosed wherein first and second interfaces of a free layer (FL) with a first metal oxide (Hk enhancing layer) and second metal oxide (tunnel barrier), respectively, produce perpendicular magnetic anisotropy (PMA) to increase thermal stability. In some embodiments, a continuous or discontinuous metal (M) or MQ alloy layer within the FL reacts with scavenged oxygen to form a partially oxidized metal or alloy layer that enhances PMA and maintains acceptable RA. M is one of Mg, Al, B, Ca, Ba, Sr, Ta, Si, Mn, Ti, Zr, or Hf, and Q is a transition metal, B, C, or Al. Methods are also provided for forming composite free layers where interfacial perpendicular anisotropy is generated therein by contact of the free layer with oxidized materials.