70
W. C. Law and S. De W. Wong
for downstream processes (e.g. chemical mechanical polishing (CMP), etch). CMP
can also be performed prior to seed layer deposition to reduce the initial surface
roughness to help minimize defects and reduce Neel coupling [119, 131, 132].
5.2 Hard Layer
Although rare earth magnets such as NdFeB and TbCoFe are one of the strongest
ferromagnetic materials, it has seen limited success in the earlier designs of MTJs
due to its low Curie temperature and high corrosion factor [133, 134]. The typical
minimum thickness required for ferrimagnetic rare earth-3d transition metal alloy
(RE-TM) alloys to exhibit PMA is >10 nm, increasing the aspect ratio of the patterned
MTJ devices [135].
Other candidates such as chemically ordered L1 0 CoPt and FePt alloys require
at least 500 °C of substrate temperature, falling outside of the temperature window
for typical CMOS BEOL processes [136, 137]. Meta-stable L1 1 phase CoPt with
substrate temperatures between 250 to 400 °C have also been reported [138, 139,
140, 141]. However, this may limit throughput as it takes time for the sample to be
heated up and cooled down within a vacuum chamber.
Other alternatives to the hard layer design include the inner SAF, which had a
reduced HL2 thickness to allow smoother interface, as well as, enhanced control
over the offset field induced to the free layer [142].
5.3 Coupling Layers
To reduce the stray field generated from hard layer 1, an additional thin layer of Ru
(0.4–0.9 nm) is inserted between hard layer 1 and hard layer 2 to create a SAF section
(see Sect. 3.3.3). To prevent accidental switching of the reference layer section, it is
desirable for the induced exchange coupling field H ex to be higher than the coercivity
of the free layer. This can mean that the first peak of IEC with strong J ex has to be
deployed, thus increasing the process challenge.
The IEC effect is also used in subsequent sections within the pMTJ stack to
couple magnetic layers together in a ferromagnetic manner. Examples include
Co/Pt/X/CoFeB in the reference layer section and CoFeB/X/CoFeB free layer
section, where X is the coupling layer. In these cases, the thicknesses of these coupling
layers were tuned to enable ferromagnetic coupling instead. The main aim of such
coupling layers would be to break the crystalline texture so as to minimize lattice
strain that may affect the growth of bcc (001) CoFeB/MgO. Notable candidates
include Ta, W and Mo [124, 125, 126, 143].
W. C. Law and S. De W. Wong
for downstream processes (e.g. chemical mechanical polishing (CMP), etch). CMP
can also be performed prior to seed layer deposition to reduce the initial surface
roughness to help minimize defects and reduce Neel coupling [119, 131, 132].
5.2 Hard Layer
Although rare earth magnets such as NdFeB and TbCoFe are one of the strongest
ferromagnetic materials, it has seen limited success in the earlier designs of MTJs
due to its low Curie temperature and high corrosion factor [133, 134]. The typical
minimum thickness required for ferrimagnetic rare earth-3d transition metal alloy
(RE-TM) alloys to exhibit PMA is >10 nm, increasing the aspect ratio of the patterned
MTJ devices [135].
Other candidates such as chemically ordered L1 0 CoPt and FePt alloys require
at least 500 °C of substrate temperature, falling outside of the temperature window
for typical CMOS BEOL processes [136, 137]. Meta-stable L1 1 phase CoPt with
substrate temperatures between 250 to 400 °C have also been reported [138, 139,
140, 141]. However, this may limit throughput as it takes time for the sample to be
heated up and cooled down within a vacuum chamber.
Other alternatives to the hard layer design include the inner SAF, which had a
reduced HL2 thickness to allow smoother interface, as well as, enhanced control
over the offset field induced to the free layer [142].
5.3 Coupling Layers
To reduce the stray field generated from hard layer 1, an additional thin layer of Ru
(0.4–0.9 nm) is inserted between hard layer 1 and hard layer 2 to create a SAF section
(see Sect. 3.3.3). To prevent accidental switching of the reference layer section, it is
desirable for the induced exchange coupling field H ex to be higher than the coercivity
of the free layer. This can mean that the first peak of IEC with strong J ex has to be
deployed, thus increasing the process challenge.
The IEC effect is also used in subsequent sections within the pMTJ stack to
couple magnetic layers together in a ferromagnetic manner. Examples include
Co/Pt/X/CoFeB in the reference layer section and CoFeB/X/CoFeB free layer
section, where X is the coupling layer. In these cases, the thicknesses of these coupling
layers were tuned to enable ferromagnetic coupling instead. The main aim of such
coupling layers would be to break the crystalline texture so as to minimize lattice
strain that may affect the growth of bcc (001) CoFeB/MgO. Notable candidates
include Ta, W and Mo [124, 125, 126, 143].