Device technologies developed to address specific needs for neuromorphic systems include non-volatile memory (NVM), which can serve as microelectronic "synapses" for low-power mobile computing applications. Desirable memory stack characteristics include, among others, scalability, fast programming, low voltage and power operations, high retention and endurance, multi-state operation and 3D integration capability. Metal-oxide RRAM meets most of these requirements [1]; however, the critical capabilities such as symmetric and gradual conductance update, and potentiation and depression in spike-timing-dependent-plasticity scheme have not been demonstrated. In this study, we show that hafnia-based filamental RRAM devices are capable of gradual multi-level switching at ultra-fast (ps) time scale at low operation voltages (< 1V). This enables high frequency/bandwidth operations with extremely low energy switching.
RRAM devices are formed by crossbar 50x50 nm MIM capacitors fabricated with an ALD polycrystalline 5nm HfO2 film and TiN electrodes. An ultra-fast pulse setup (Fig. 1) provides precise control over the power delivered to the cell and removes the need for a current compliance control (1R configuration). Resistance is shown to stepwise increase or decrease depending on voltage value and polarity (Fig. 2).
To evaluate how synaptic weight can be modulated by the time between the incoming signals, we investigated dependency of the RRAM resistance on time between the consequent pulses (Fig. 3). Reducing the time interval between pulses below 2 ns results in significant amplification of their effect on resistance. It’s consistent with theoretical expectations that, by briefly increasing local temperature [2], the first pulse magnifies the structural changes inducing by the subsequent pulse.
To emulate the effect of time window between the pulses from pre- and post- synaptic neurons, a pair of pulses of positive (pre-synaptic) and negative (post-synaptic) polarity was applied to the same electrode (Fig. 4). In bi-polar switching, a negative polarity pulse induces resistance change in the same direction as a positive pulse applied to the opposite electrode. Potentiation or depression were observed depending on a positive pulse preceding or following the negative one, consistent with the reported brain-inspired operations. A very short time interval between incoming signals enabling potentiation/depression processes implies extremely fast neuromorphic system operations capabilities.
- G. Bersuker et al., J. Comput. Electron. 16, 1085, 2017
- D. M. Nminibapiel et al., EDL 38, 326, 2017
