Spin Transfer Torque Magnetoresistive Random Access Memory
89
stack materials, resulting in a reduction in re-deposition and improvement in sidewall
profiles, but magnetic degradation is still evident [209, 210].
Me-OH plasma was first proposed in 2004 by Osada et al., due to its high etch
selectivity to Ta hard masks, attributed to the C–O-based etch chemistries [209,
211]. During etching, the hardening of the hard mask surface layer by nitradation,
carbonization, or oxidation from the C–O-based plasma, known as hardening mask
etching, provides a high anisotropy of ~80° without residues, sidewall re-deposition
or corrosion. However, a major issue with C–O-based etch chemistries is the magnetic
degradation of the MTJ magnetic properties due to oxidation from the pattern edge of
the CoFeB free layer, leading to a significant reduction of TMR [210, 212]. Therefore,
a recovery process using reductive He/H 2 plasma treatment has been proposed [209,
212]. The hydrogen radicals from the He/H 2 plasma will reduce the oxidized part of
the pattern edge etched by the C–O-based etch chemistries. During the etching and
recovery processes, both oxidation and reduction progressed from the sidewalls by
diffusion without ion irradiation. The higher energies of the magnetic metal oxides
leads to preferential reduction, with little impact on other parts of the MTJ, such as
the MgO tunnel barrier.
An oxygen-based plasma is not favored for the MTJ etch process due to the risk of
over-oxidation with reaction with the MgO tunnel barrier. However, the advantages of
O 2 gas are isolation of damaged region from the patterned MTJ and the formation of
a self-aligned passivation layer to enclose and protect the MTJ from magnetic degradation during BEOL integration processes. Therefore, a curing process involving a
non-reactive oxygen treatment, known as oxygen showering post-treatment (OSP),
which uses an ozone diffusion chamber to recover the etch damage by selective
oxidation and also improve the electric and magnetic properties of the MTJs [213].
In addition, the OSP has been demonstrated to effectively recover electric short fails
caused by the IBE process through selective oxidation and isolation of the damaged
region from the MTJ [214] (Fig. 19).
Sidewalls Re-deposition.
The self-aligned integration scheme which encompasses a conducting hard mask
between the MTJ and upper wiring layer offers a simple process with fewer
processing steps. However, the tall conducting hard mask limits the beam angle
and also leads to the shadowing of high density arrays of MTJ structures. A short
conducting hard mask is more beneficial in reduction of sidewall re-depositions, but
requires the inclusion of a metal via to connect the MTJ to the upper wiring layer. This
short hard mask is only compatible for STT-MRAM with larger diameters MTJs due
to the difficulty in via alignment and overlay tolerances. For STT-MRAM targeting
sub-nanometer MTJs, the tall conducting hard mask with a tapered profile is more
suitable and can help reduce sidewall re-deposition and MTJ shunting.
A post-etch treatment using a high-angle, low-energy, low-damage IBE cleanup
step to remove the re-deposition materials and damaged layers from the MTJ has been
proposed [201, 215]. M. Gajek et al. fabricated PMA MTJ with diameters of 20 nm
by a combination of RIE and IBE, as shown in Fig. 20. The IBE was performed at
89
stack materials, resulting in a reduction in re-deposition and improvement in sidewall
profiles, but magnetic degradation is still evident [209, 210].
Me-OH plasma was first proposed in 2004 by Osada et al., due to its high etch
selectivity to Ta hard masks, attributed to the C–O-based etch chemistries [209,
211]. During etching, the hardening of the hard mask surface layer by nitradation,
carbonization, or oxidation from the C–O-based plasma, known as hardening mask
etching, provides a high anisotropy of ~80° without residues, sidewall re-deposition
or corrosion. However, a major issue with C–O-based etch chemistries is the magnetic
degradation of the MTJ magnetic properties due to oxidation from the pattern edge of
the CoFeB free layer, leading to a significant reduction of TMR [210, 212]. Therefore,
a recovery process using reductive He/H 2 plasma treatment has been proposed [209,
212]. The hydrogen radicals from the He/H 2 plasma will reduce the oxidized part of
the pattern edge etched by the C–O-based etch chemistries. During the etching and
recovery processes, both oxidation and reduction progressed from the sidewalls by
diffusion without ion irradiation. The higher energies of the magnetic metal oxides
leads to preferential reduction, with little impact on other parts of the MTJ, such as
the MgO tunnel barrier.
An oxygen-based plasma is not favored for the MTJ etch process due to the risk of
over-oxidation with reaction with the MgO tunnel barrier. However, the advantages of
O 2 gas are isolation of damaged region from the patterned MTJ and the formation of
a self-aligned passivation layer to enclose and protect the MTJ from magnetic degradation during BEOL integration processes. Therefore, a curing process involving a
non-reactive oxygen treatment, known as oxygen showering post-treatment (OSP),
which uses an ozone diffusion chamber to recover the etch damage by selective
oxidation and also improve the electric and magnetic properties of the MTJs [213].
In addition, the OSP has been demonstrated to effectively recover electric short fails
caused by the IBE process through selective oxidation and isolation of the damaged
region from the MTJ [214] (Fig. 19).
Sidewalls Re-deposition.
The self-aligned integration scheme which encompasses a conducting hard mask
between the MTJ and upper wiring layer offers a simple process with fewer
processing steps. However, the tall conducting hard mask limits the beam angle
and also leads to the shadowing of high density arrays of MTJ structures. A short
conducting hard mask is more beneficial in reduction of sidewall re-depositions, but
requires the inclusion of a metal via to connect the MTJ to the upper wiring layer. This
short hard mask is only compatible for STT-MRAM with larger diameters MTJs due
to the difficulty in via alignment and overlay tolerances. For STT-MRAM targeting
sub-nanometer MTJs, the tall conducting hard mask with a tapered profile is more
suitable and can help reduce sidewall re-deposition and MTJ shunting.
A post-etch treatment using a high-angle, low-energy, low-damage IBE cleanup
step to remove the re-deposition materials and damaged layers from the MTJ has been
proposed [201, 215]. M. Gajek et al. fabricated PMA MTJ with diameters of 20 nm
by a combination of RIE and IBE, as shown in Fig. 20. The IBE was performed at