RRAM-Based Neuromorphic Computing Systems
389
formation. Several approaches have been implemented to enable gradual weight
update in both directions under identical pulse condition, i.e., insertion of an oxide
layer with less defect’s mobility and thermal enhancement layer (TEL) [25, 28].
The first approach interfaced HfO 2 layer with AlO x at the inert electrode side of
the structure [25]. AlO x layer has higher oxygen vacancy diffusion barrier compared
to HfO 2 layer, which induced filament constriction at the AlO x /HfO 2 interface. This
promoted lateral filament modulation during potentiation and depression process.
The devices are able to achieve gradual conductance change in both directions and
further improved their linearity. In comparison with HfO 2 /Ti structure, the improvement on the linearity of the potentiation in AlO x /HfO 2 /Ti system, i.e., from α value
of 16.53 to −0.01, carried a noticeable trade-off in dynamic range of the device
conductance (reduced from 10 to 3). The AlO x /HfO 2 /Ti synaptic device properties
were input into simulated NN implementing multilayer perceptron algorithm. It was
evaluated under the Mixed National Institute of Standards and Technology (MNIST)
dataset to test the NN accuracy in performing pattern recognition. It was shown that
the improvement in linearity of the conductance change of the synapse was translated
into significant increase in pattern recognition accuracy, i.e., from ~10% for HfO 2 /Ti
to ~90% for AlO x /HfO 2 /Ti structure.
The insertion of TEL in HfO x -based device was designed based on gradual SET
process observed during high temperature programming of the device [28]. HfO x /Ti
anion device was observed to exhibit abrupt SET under room temperature programming condition and gradual SET during cell programming at 150 °C. In order to obtain
gradual SET process at room temperature operation, oxygen deficient TaO x layer
was introduced as TEL and oxygen reservoir in the structure replacing Ti electrode.
This layer has significantly lower thermal conductivity compared to Ti electrode,
which induced localized Joule heating effect across the active filament region during
the switching process. This shifted the device switching property from predominantly electric field to thermally induced switching. This resulted in the formation of
multiple weak filaments instead of single filament switching, converting the abrupt
into analog SET process, while maintaining dynamic ratio of 10. While it provides a
promising solution to mitigate abrupt potentiation issue, the multiple weak filaments
system has a trade-off in read disturb and retention of the conductance state. This
will have negative impact on the amount of inferences the NN can perform while
maintaining the weight values within acceptable deviation. This synaptic device has
been experimentally demonstrated on a 1 k-bit 1T1R array to carry out human face
classification.
Tantalum Oxide (TaO x )-based Devices
Another oxide system that has been widely investigated for synaptic device applications is TaO x -based devices. One of the first reports on TaO x -based devices was
Ta 2 O 5−x /TaO 2−x system that demonstrated an excellent digital memory endurance
capability of 10
12 cycles under 10 ns operating speed [11]. Ta 2 O 5−x was implemented
as the oxide switching layer with an oxygen deficient TaO 2−x acted as the oxygen
reservoir in the structure. Multilevel cell capability of this structure was demonstrated
with an improved ON/OFF ratio of ~1000 with well separated 4 conductance levels (2
389
formation. Several approaches have been implemented to enable gradual weight
update in both directions under identical pulse condition, i.e., insertion of an oxide
layer with less defect’s mobility and thermal enhancement layer (TEL) [25, 28].
The first approach interfaced HfO 2 layer with AlO x at the inert electrode side of
the structure [25]. AlO x layer has higher oxygen vacancy diffusion barrier compared
to HfO 2 layer, which induced filament constriction at the AlO x /HfO 2 interface. This
promoted lateral filament modulation during potentiation and depression process.
The devices are able to achieve gradual conductance change in both directions and
further improved their linearity. In comparison with HfO 2 /Ti structure, the improvement on the linearity of the potentiation in AlO x /HfO 2 /Ti system, i.e., from α value
of 16.53 to −0.01, carried a noticeable trade-off in dynamic range of the device
conductance (reduced from 10 to 3). The AlO x /HfO 2 /Ti synaptic device properties
were input into simulated NN implementing multilayer perceptron algorithm. It was
evaluated under the Mixed National Institute of Standards and Technology (MNIST)
dataset to test the NN accuracy in performing pattern recognition. It was shown that
the improvement in linearity of the conductance change of the synapse was translated
into significant increase in pattern recognition accuracy, i.e., from ~10% for HfO 2 /Ti
to ~90% for AlO x /HfO 2 /Ti structure.
The insertion of TEL in HfO x -based device was designed based on gradual SET
process observed during high temperature programming of the device [28]. HfO x /Ti
anion device was observed to exhibit abrupt SET under room temperature programming condition and gradual SET during cell programming at 150 °C. In order to obtain
gradual SET process at room temperature operation, oxygen deficient TaO x layer
was introduced as TEL and oxygen reservoir in the structure replacing Ti electrode.
This layer has significantly lower thermal conductivity compared to Ti electrode,
which induced localized Joule heating effect across the active filament region during
the switching process. This shifted the device switching property from predominantly electric field to thermally induced switching. This resulted in the formation of
multiple weak filaments instead of single filament switching, converting the abrupt
into analog SET process, while maintaining dynamic ratio of 10. While it provides a
promising solution to mitigate abrupt potentiation issue, the multiple weak filaments
system has a trade-off in read disturb and retention of the conductance state. This
will have negative impact on the amount of inferences the NN can perform while
maintaining the weight values within acceptable deviation. This synaptic device has
been experimentally demonstrated on a 1 k-bit 1T1R array to carry out human face
classification.
Tantalum Oxide (TaO x )-based Devices
Another oxide system that has been widely investigated for synaptic device applications is TaO x -based devices. One of the first reports on TaO x -based devices was
Ta 2 O 5−x /TaO 2−x system that demonstrated an excellent digital memory endurance
capability of 10
12 cycles under 10 ns operating speed [11]. Ta 2 O 5−x was implemented
as the oxide switching layer with an oxygen deficient TaO 2−x acted as the oxygen
reservoir in the structure. Multilevel cell capability of this structure was demonstrated
with an improved ON/OFF ratio of ~1000 with well separated 4 conductance levels (2