ReRAM is one of promising nonvolatile memories with excellent properties such as low power, high switching speed, and scalability against downsizing. Recently, ReRAM has been attracted attention as artificial neuron devices such as synapse which changes resistance according to input voltage or current signals.

Recently we observed multiple resistance states in the RESET process of Ti/HfO_x/Pt ReRAM devices. Then we found that the resistance of the reset state changed according to input of voltage pulses. In this study, we focused resistance change response of the SET as well as RESET processes of the Ti/HfO₂/Pt ReRAM device. The size of the cross-point device area was 1×1 μm². The HfO_x layer was deposited by reactive sputtering using Ar and O₂ mixture gases.

Fig.1 shows the typical bipolar resistance switching characteristic. Compliance current was SET to 500 μA. We applied voltage pulse train of constant voltage with 1 μsec pulse width and 2 μsec interval. Fig.2 shows resistance changes of the device during 200 pulses at various voltages. There was an initial abrupt increase of resistance followed by gradual increase. Pulse numbers before initiating abrupt increase of resistance was smaller as the absolute voltage value was larger. Fig.2 shows voltage dependence of the r₅₀₀₀ (resistance after 5000 voltage pulses were applied), and r₅₀₀₀ was continuously changed with applied negative voltage.

On the other hand, resistance change behaviors in the set process is completely different as shown in Fg.4. Resistance was high around 10⁶ W when the applied voltage was lower than 1.1 V, ad it changed to low around 10³ W when the applied voltage was higher than 1.5 V . In the voltage pulse ranges between 1.2 and 1.4 V, the resistance state indicated only two values of high and low. There were binary resistance states existed. From these experimental results, it maybe suggested that voltage pulse response of the RESET process is suitable for application to artificial synapse.