Method of fabricating a magnetic tunnel junction (MTJ) device with reduced switching current

#The present disclosure generally relates to spin-transfer-torque magnetic random access memory (STT-MRAM). More specifically, the present disclosure relates to directing internal strain and stress to reduce STT-MRAM switching current.

#Unlike conventional random access memory (RAM) chip technologies, in magnetic RAM (MRAM) data is not stored as electric charge but is instead stored by magnetic polarization of storage elements. The storage elements are formed from two ferromagnetic layers separated by a tunneling barrier layer. One of the two layers, called the fixed layer, has at least one reference magnetic polarization set to a particular polarity. The magnetic polarity of the other magnetic layer, called the free layer, is altered to represent either a “1” (e.g., anti-parallel to the fixed reference layer) or “0” (e.g., parallel to the fixed reference layer). One such device having a fixed layer, a tunneling barrier layer, and a free layer is a magnetic tunnel junction (MTJ). The electrical resistance of an MTJ is dependent on the magnetic polarity of the free layer compared to the magnetic polarity of the fixed layer. A memory device such as MRAM is built from an array of individually addressable MTJs.

#To write data in a conventional MRAM, a write current is applied through an MTJ. The write current has a magnitude exceeding a level called the critical switching current level which is sufficient to change the orientation of spin or magnetization in the free layer. When the write current flows in a first direction, the MTJ can be placed into or remain in a first state, where its magnetizations are in a parallel orientation. When the write current flows in a second direction, opposite to the first direction, the MTJ can be placed into or remain in a second state, where its magnetizations are in an anti-parallel orientation.

#For a more complete understanding of the present disclosure, reference is now made to the following description taken in conjunction with the accompanying drawings.

#FIG. 1 is a top perspective diagram of a magnetic tunnel junction (MTJ) with directed stress/strain according to aspects of the present disclosure.

#FIGS. 2A-2D are cross-sectional diagrams of a magnetic tunnel junction fabricated according to aspects of the present disclosure.

#Aspects of the present disclosure increase perpendicular magnetic anisotropy of layers in an MTJ by applying a mechanical strain/stress to the MTJ. According to aspects of the present disclosure, an MTJ may be fabricated using processes and structural configurations that induce a directed static strain on an MTJ to increase the perpendicular magnetic anisotropy and as a consequence, reduce switching current of the MTJ. The directed static strain/stress on the MTJ may be induced in a controlled direction and/or with a controlled magnitude during fabrication according to aspects of the present disclosure. An MTJ fabricated according to aspects of the present disclosure is permanently subject to a predetermined directed stress and permanently includes the directed static strain/stress that provides reduced switching current.

#Anisotropy may be induced by application of mechanical stress and strain on an MTJ. The magnitude of anisotropy that may be induced by stress and strain also depends on the magnetostriction of the material under stress as described by the equation for magnetoelastic energy:

#E = 3 2 ⁢ λσ ⁢ ⁢ sin 2 ⁢ θ