Spin Transfer Torque Magnetoresistive Random Access Memory
87
etched by-products are non-volatile, thus decreasing the etching rate and increases
its difficulty of removal from the etching chamber [194, 195, 196]. Early RIE process
chemistry involved Cl 2 /Ar and BCl 3 /Ar plasma, which showed re-deposition of
residues such as Cl-based by-products, which significantly deteriorated the magnetic
properties of the MTJ stack [196, 197, 198]. Figure 19 shows the SEM images
obtained from pre-, post-Cl 2 , post-Cl 2 /H 2 , and post-Ar plasma assisted etching on
patterned TiN/CoFe/Ti on Si wafers [199]. The Ar-only plasma resulted in sidewall
redeposition and/or TiN hard mask corrosion and causes a rough surface due to defect
sites induced by the ion bombardment [200, 201], as shown in Fig. 18c, d. While, the
Cl 2 plasma etched both CoFe and the TiN hard mask, with substantial re-deposition
of non-volatile metal chlorides layers, as shown in Fig. 18e, f. The subsequent exposure to H 2 plasma visibly removed the re-deposited metal chlorides on the sidewalls,
as shown in Fig. 18g, h. Any residual Cl 2 gas may form corrosive HCl on reaction
with moisture/water vapor that will degrade the MTJ magnetic properties. Hence,
a post-etch treatment with O 2 or H 2 O plasma passivation step is mandatory before
exposure to atmospheric conditions [202, 203].
This RIE process has been recently incorporated to etch CoFeB/MgO MTJ stack
[204]. Although the removal of metal chlorides and restoration of magnetic properties
of CoFeB was demonstrated by the H 2 plasma treatment, the risk of corrosion can
be well avoided by using organic chemistries as an alternative, which has shown to
synthesize volatile metallic organic precursors for many known magnetic materials.
87
etched by-products are non-volatile, thus decreasing the etching rate and increases
its difficulty of removal from the etching chamber [194, 195, 196]. Early RIE process
chemistry involved Cl 2 /Ar and BCl 3 /Ar plasma, which showed re-deposition of
residues such as Cl-based by-products, which significantly deteriorated the magnetic
properties of the MTJ stack [196, 197, 198]. Figure 19 shows the SEM images
obtained from pre-, post-Cl 2 , post-Cl 2 /H 2 , and post-Ar plasma assisted etching on
patterned TiN/CoFe/Ti on Si wafers [199]. The Ar-only plasma resulted in sidewall
redeposition and/or TiN hard mask corrosion and causes a rough surface due to defect
sites induced by the ion bombardment [200, 201], as shown in Fig. 18c, d. While, the
Cl 2 plasma etched both CoFe and the TiN hard mask, with substantial re-deposition
of non-volatile metal chlorides layers, as shown in Fig. 18e, f. The subsequent exposure to H 2 plasma visibly removed the re-deposited metal chlorides on the sidewalls,
as shown in Fig. 18g, h. Any residual Cl 2 gas may form corrosive HCl on reaction
with moisture/water vapor that will degrade the MTJ magnetic properties. Hence,
a post-etch treatment with O 2 or H 2 O plasma passivation step is mandatory before
exposure to atmospheric conditions [202, 203].
This RIE process has been recently incorporated to etch CoFeB/MgO MTJ stack
[204]. Although the removal of metal chlorides and restoration of magnetic properties
of CoFeB was demonstrated by the H 2 plasma treatment, the risk of corrosion can
be well avoided by using organic chemistries as an alternative, which has shown to
synthesize volatile metallic organic precursors for many known magnetic materials.