Spin Transfer Torque Magnetoresistive Random Access Memory
81
sheet resistance, the voltage measured can be extracted by integrating E T over the
probe distances.
In order to ensure that the current flows through both the top and bottom electrodes
for TMR to be measured, a few considerations have to be made when using the CIPT
tool. Firstly, the mean probe spacing distance x has to be chosen to ensure results
follow a good fit to the model. A general rule of thumb described by CAPRES states
that in order for the current flowing through the outer probes to be distributed amongst
the top and bottom electrodes equally, the characteristic length scale λ =
R A
R T +R B
should be between λ ≤ x ≤ 5λ. Thus, ultra-small probe spacings are required for
MTJ thin films which typically have very small RA values.
Secondly, the top and bottom electrodes have to be designed carefully as the
R T /R B ratio is important. As current prefers to flow in the path of least resistance,
the current effectively shunts through the top electrode if R B is much higher than R T ,
resulting in minimal tunneling through the tunnel barrier. This can be interpreted as
a suppression of the measured TMR. In most electrode designs, the R T /R B ratio is
set to be ~18.
Lastly, at least 7 nm of Ru is required as a capping layer in order for the MTJ stack
to remain conductive after exposure to native oxidation. Otherwise, the software is
programmed to repeatedly drive in the probes, resulting in probe breakage.
6.3 Sputtering
Magnetron sputtering deposition is a physical vapor deposition process used extensively in semiconductor industry for thin film growth, due to its ultra-smooth deposition, good film adhesion and high uniformity at an acceptable throughput. As shown
in Fig. 14, a high voltage is applied to generate plasma by ionizing inert gas (usually
Ar, Kr or Xe) introduced into the chamber, which is maintained at 20 mTorr. The Ar
+
ions created will accelerate towards the cathode, bombarding atoms from the high
purity target (typically >99.9%) with high energy. The ejected atoms from the target
will then impinge onto the surface of the substrate, provided that the mean free path of
the material is longer than the target-to-substrate distance. At the same time, released
electrons during ionization may also collide with additional Ar atoms, creating more
secondary Ar
+ ions and free electrons to create a self-sustaining process. The chamber
pressure is lowered to 2 mTorr before opening the shutter gate for the actual deposition. To avoid charge build-up on dielectric materials such as MgO, RF sputtering is
performed at 13.56 MHz frequency and 100 W power.
The popularity of magnetron sputtering arises from its flexibility in tuning several
parameters in order to optimize and ensure smooth and uniform coating of the target
material. A low base chamber pressure is often required to minimize interfacial
oxidation and H 2 O contamination that may be detrimental to hydrophilic materials,
such as MgO. Since the generation of plasma is usually inefficient (<0.1% ionization
rate), permanent magnets can help to confine the secondary electrons into a helical
81
sheet resistance, the voltage measured can be extracted by integrating E T over the
probe distances.
In order to ensure that the current flows through both the top and bottom electrodes
for TMR to be measured, a few considerations have to be made when using the CIPT
tool. Firstly, the mean probe spacing distance x has to be chosen to ensure results
follow a good fit to the model. A general rule of thumb described by CAPRES states
that in order for the current flowing through the outer probes to be distributed amongst
the top and bottom electrodes equally, the characteristic length scale λ =
R A
R T +R B
should be between λ ≤ x ≤ 5λ. Thus, ultra-small probe spacings are required for
MTJ thin films which typically have very small RA values.
Secondly, the top and bottom electrodes have to be designed carefully as the
R T /R B ratio is important. As current prefers to flow in the path of least resistance,
the current effectively shunts through the top electrode if R B is much higher than R T ,
resulting in minimal tunneling through the tunnel barrier. This can be interpreted as
a suppression of the measured TMR. In most electrode designs, the R T /R B ratio is
set to be ~18.
Lastly, at least 7 nm of Ru is required as a capping layer in order for the MTJ stack
to remain conductive after exposure to native oxidation. Otherwise, the software is
programmed to repeatedly drive in the probes, resulting in probe breakage.
6.3 Sputtering
Magnetron sputtering deposition is a physical vapor deposition process used extensively in semiconductor industry for thin film growth, due to its ultra-smooth deposition, good film adhesion and high uniformity at an acceptable throughput. As shown
in Fig. 14, a high voltage is applied to generate plasma by ionizing inert gas (usually
Ar, Kr or Xe) introduced into the chamber, which is maintained at 20 mTorr. The Ar
+
ions created will accelerate towards the cathode, bombarding atoms from the high
purity target (typically >99.9%) with high energy. The ejected atoms from the target
will then impinge onto the surface of the substrate, provided that the mean free path of
the material is longer than the target-to-substrate distance. At the same time, released
electrons during ionization may also collide with additional Ar atoms, creating more
secondary Ar
+ ions and free electrons to create a self-sustaining process. The chamber
pressure is lowered to 2 mTorr before opening the shutter gate for the actual deposition. To avoid charge build-up on dielectric materials such as MgO, RF sputtering is
performed at 13.56 MHz frequency and 100 W power.
The popularity of magnetron sputtering arises from its flexibility in tuning several
parameters in order to optimize and ensure smooth and uniform coating of the target
material. A low base chamber pressure is often required to minimize interfacial
oxidation and H 2 O contamination that may be detrimental to hydrophilic materials,
such as MgO. Since the generation of plasma is usually inefficient (<0.1% ionization
rate), permanent magnets can help to confine the secondary electrons into a helical