Spin Transfer Torque Magnetoresistive Random Access Memory
71
5.4 Polarizing Layer
In order to induce magnetization reversal within the free layer through electric
current, spin polarization must first be enabled by the reference layer. The spin polarization is defined in Eq. (7), which can be quantified by using the superconductivity
measurement technique described by Tedrow and Meservey [144, 145].
In addition, spin scattering effect must also be considered if there is nonferromagnetic metal present as in the case of Co/X multilayers, where X = Pt or
Pd are common materials as they are able to induce strong MCA due to their high
spin–orbit coupling effect. While Co/Pt multilayer section is an excellent candidate
for hard layers, it has a major drawback of having poor spin polarization efficiency
(~0.46 due to large spin scattering effect at the non-magnetic layers, in contrast to ~
90% for Co/Ni or 65% for amorphous Co 40 Fe 40 B 20 . In addition, Co/Pt multilayers
may translate its fcc crystal structure to the MgO tunnel barrier, which must be in
the bcc (100) crystal structure in order to allow for enhanced coherent tunneling in
the majority 1 state. CoFeB is typically used to solve the above problems simultaneously as it achieves its PMA from the hybridization of iron 3d and oxygen 2p
orbitals, amorphousness when deposited which, upon annealing, crystallizes into bcc
(100) with small lattice mismatch MgO, as well as high spin polarization efficiency.
The CoFeB can be coupled with the Hard Layer 2 through an ultra-thin Ta insertion
layer, which is subsequently referred to as the reference layer (see Fig. 10).
5.5 Tunnel Barrier Layer
The tunnel barrier is arguably one of the most important layers alongside the free
layer, and MgO is typically chosen for high TMR ratio. The stress voltage tolerable
for MgO is typically 1.2 V, which is higher than AlOx barriers posing as an attractive
advantage. To keep RA low, the thickness of MgO is kept to ~1 nm. This may result in
pinhole formation, which can lead to current shunting or reduction of the dielectric
endurance during device operation. As such, deposition techniques such as radio
frequency sputtering of MgO dielectric or Mg deposition followed by controlled
oxidation are often carefully optimized [98, 146].
Another key concern is the Néel coupling effect, which arises due to magnetostatic interactions between the hard and soft layer and may drastically reduce the
TMR. Therefore, the underlayers leading up to the tunnel barrier should ideally
have minimal surface roughness. To improve on the overall device performance, an
optional in-situ annealing during deposition can help to obtain bcc-MgO(100) [21,
98]. This is then followed by an ex-situ annealing with magnetic field applied to
enhance crystallization of both MgO and CoFeB [97, 147, 148].
In addition, having a second MgO over the CoFeB-based free layer can lead to
an improvement in thermal stability as well as symmetrical current switching. This
is due to the additional interfacial anisotropy induced on the other side of CoFeB,
71
5.4 Polarizing Layer
In order to induce magnetization reversal within the free layer through electric
current, spin polarization must first be enabled by the reference layer. The spin polarization is defined in Eq. (7), which can be quantified by using the superconductivity
measurement technique described by Tedrow and Meservey [144, 145].
In addition, spin scattering effect must also be considered if there is nonferromagnetic metal present as in the case of Co/X multilayers, where X = Pt or
Pd are common materials as they are able to induce strong MCA due to their high
spin–orbit coupling effect. While Co/Pt multilayer section is an excellent candidate
for hard layers, it has a major drawback of having poor spin polarization efficiency
(~0.46 due to large spin scattering effect at the non-magnetic layers, in contrast to ~
90% for Co/Ni or 65% for amorphous Co 40 Fe 40 B 20 . In addition, Co/Pt multilayers
may translate its fcc crystal structure to the MgO tunnel barrier, which must be in
the bcc (100) crystal structure in order to allow for enhanced coherent tunneling in
the majority 1 state. CoFeB is typically used to solve the above problems simultaneously as it achieves its PMA from the hybridization of iron 3d and oxygen 2p
orbitals, amorphousness when deposited which, upon annealing, crystallizes into bcc
(100) with small lattice mismatch MgO, as well as high spin polarization efficiency.
The CoFeB can be coupled with the Hard Layer 2 through an ultra-thin Ta insertion
layer, which is subsequently referred to as the reference layer (see Fig. 10).
5.5 Tunnel Barrier Layer
The tunnel barrier is arguably one of the most important layers alongside the free
layer, and MgO is typically chosen for high TMR ratio. The stress voltage tolerable
for MgO is typically 1.2 V, which is higher than AlOx barriers posing as an attractive
advantage. To keep RA low, the thickness of MgO is kept to ~1 nm. This may result in
pinhole formation, which can lead to current shunting or reduction of the dielectric
endurance during device operation. As such, deposition techniques such as radio
frequency sputtering of MgO dielectric or Mg deposition followed by controlled
oxidation are often carefully optimized [98, 146].
Another key concern is the Néel coupling effect, which arises due to magnetostatic interactions between the hard and soft layer and may drastically reduce the
TMR. Therefore, the underlayers leading up to the tunnel barrier should ideally
have minimal surface roughness. To improve on the overall device performance, an
optional in-situ annealing during deposition can help to obtain bcc-MgO(100) [21,
98]. This is then followed by an ex-situ annealing with magnetic field applied to
enhance crystallization of both MgO and CoFeB [97, 147, 148].
In addition, having a second MgO over the CoFeB-based free layer can lead to
an improvement in thermal stability as well as symmetrical current switching. This
is due to the additional interfacial anisotropy induced on the other side of CoFeB,