Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. One type of MRAM cell is a spin torque transfer MRAM (STT-MRAM) cell, which includes a magnetic cell core supported by a substrate. The magnetic cell core includes at least two magnetic regions, for example, a “fixed region” and a “free region,” with a non-magnetic region between. The free region and the fixed region may exhibit magnetic orientations that are either horizontally oriented (“in-plane”) or perpendicularly oriented (“out-of-plane”) with the width of the regions. The fixed region includes a magnetic material that has a substantially fixed (e.g., a non-switchable) magnetic orientation. The free region, on the other hand, includes a magnetic material that has a magnetic orientation that may be switched, during operation of the cell, between a “parallel” configuration and an “anti-parallel” configuration. In the parallel configuration, the magnetic orientations of the fixed region and the free region are directed in the same direction (e.g., north and north, east and east, south and south, or west and west, respectively). In the “anti-parallel” configuration the magnetic orientations of the fixed region and the free region are directed in opposite directions (e.g., north and south, east and west, south and north, or west and east, respectively). In the parallel configuration, the STT-MRAM cell exhibits a lower electrical resistance across the magnetoresistive elements (e.g., the fixed region and free region). This state of low electrical resistance may be defined as a “0” logic state of the MRAM cell. In the anti-parallel configuration, the STT-MRAM cell exhibits a higher electrical resistance across the magnetoresistive elements. This state of high electrical resistance may be defined as a “1” logic state of the STT-MRAM cell.
Switching of the magnetic orientation of the free region may be accomplished by passing a programming current through the magnetic cell core and the fixed and free regions therein. The fixed region polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the core. The spin-polarized electron current exerts the torque on the free region. When the torque of the spin-polarized electron current passing through the core is greater than a critical switching current density (Jc) of the free region, the direction of the magnetic orientation of the free region is switched. Thus, the programming current can be used to alter the electrical resistance across the magnetic regions. The resulting high or low electrical resistance states across the magnetoresistive elements enable the write and read operations of the MRAM cell. After switching the magnetic orientation of the free region to achieve the one of the parallel configuration and the anti-parallel configuration associated with a desired logic state, the magnetic orientation of the free region is usually desired to be maintained, during a “storage” stage, until the MRAM cell is to be rewritten to a different configuration (i.e., to a different logic state).
A magnetic region's magnetic anisotropy (“MA”) is an indication of the directional dependence of the material's magnetic properties. Therefore, the MA is also an indication of the strength of the material's magnetic orientation and of its resistance to alteration of the magnetic orientation. A magnetic material exhibiting a magnetic orientation with a high MA strength may be less prone to alteration of its magnetic orientation than a magnetic material exhibiting a magnetic orientation with a lower MA strength. In case of free regions with comparable total magnetic moments, the amount of programming current required to switch the free region from the parallel configuration to the anti-parallel configuration is affected by MA strength in that a free region with a stronger (i.e., a higher) MA strength may require a greater amount of programming current to switch the magnetic orientation thereof than a free region with a weaker (i.e., a lower) MA strength. However, a free region with a weak MA strength is also often less stable during storage such that it may be prone to premature alteration out of its programmed configuration (i.e., the programmed parallel or anti-parallel configuration).
A magnetic material's MA strength may be impacted by interaction (e.g., contact) between the magnetic material and a neighboring nonmagnetic material (e.g., an oxide material). Contact may induce MA (e.g., increase MA strength) along the interface between the magnetic material and the nonmagnetic material, adding to the overall MA strength of the magnetic material and the MRAM cell. Generally, the greater the ratio of the magnetic material in contact with the surface/interface MA-inducing material to the non-contacted portion of the magnetic material, the higher the MA strength of the magnetic region.
Often, design and fabrication of MRAM cells involves a tradeoff between achieving high MA strength in the free region and other often-desirable characteristics of the cell. For example, a thin (i.e., short height) free region, adjacent to an MA-inducing material, may have a high ratio of contact-to-non-contacted portions and, therefore, high MA strength. However, a thin free region may have a low “energy barrier ratio” (Eb/kT, wherein, Eb represents the cell's energy barrier, k is the Boltzmann constant, and T is the temperature of the cell), compared to a thick free region. The Eb and energy barrier ratio are indications of the cell's thermal stability and, therefore, its data retention. The lower the Eb and the lower the energy barrier ratio, the more prone the cell may be to premature switching. A thin free region may also have low tunnel magnetoresistance (“TMR”). TMR is a ratio of the difference between the cell's electrical resistance in the anti-parallel stage (Rap) and its resistance in the parallel stage (Rp) to Rp (i.e., TMR=(Rap−Rp)/Rp). Low TMR may lower a cell's read-out signal and may slow the reading of the MRAM cell during operation. Low TMR may also necessitate use of high programming current. Thus, there is often a tradeoff between, on the one hand, forming a free region to be thin so as to achieve a high MA strength and, on the other hand, forming the free region to be thick so as to achieve a high Eb, high energy barrier ratio, high thermal stability, high data retention, and use of low programming current.
Efforts have been made to form thick free regions that have high MA strength by positioning the free region between two MA-inducing materials, which doubles the surface area of the magnetic material exposed to the MA-inducing material. However, MA-inducing material may be prone to structural defects in its microstructure when formed over conventional base materials of MRAM cell structures (e.g., base materials such as tantalum (Ta) or ruthenium (Ru)). The structural defects in the MA-inducing material may lead to the overlying magnetic material of the free region being formed with structural defects or to the structural defects propagating from the MA-inducing material to the magnetic material after the magnetic material is formed. Moreover, where the free region is thick, the structural defects may be more pronounced, having more volume in which to form and propagate. The defects in the free region may degrade the magnetic properties of the region and, also, the MRAM cell as a whole. Therefore, fabricating MRAM cells with dual surface/interface MA-inducing regions to achieve high MA strength without degrading other properties often presents challenges.
In addition, attempts to increase MA strength, decrease programming current, increase TMR, increase thermal stability, or increase the energy barrier ratio Eb/kT are also often met with the challenges of maintaining consistency from cell to cell in an array of MRAM cells and of selecting materials that are not prone to degradation during the fabrication processes. For example, some materials that may be conducive to forming magnetic regions with high MA strength may have a low thermal tolerance or may have a tendency to be formed with structural defects, leading to inconsistent characteristics or degradation of characteristics within a memory array. Variations in physical, chemical, or other characteristics of the MRAM cells, may lead to increased electrical resistance variation between cells and variations in other magnetic properties, which then lead to less reliable operation and functioning of the array, overall. Thus, fabricating arrays of MRAM cells with precision and consistency from cell to cell and without sacrificing performance has often presented challenges.
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