The performances of conductive-bridging random access memory (CBRAM) have been reviewed for different switching materials such as chalcogenides, oxides, and bilayers in different structures. The structure consists of an inert electrode and one oxidized electrode of copper (Cu) or silver (Ag). The switching mechanism is the formation/dissolution of a metallic filament in the switching materials under external bias. However, the growth dynamics of the metallic filament in different switching materials are still debated. All CBRAM devices are switching under an operation current of 0.1 μA to 1 mA, and an operation voltage of ±2 V is also needed. The device can reach a low current of 5 pA; however, current compliance-dependent reliability is a challenging issue. Although a chalcogenide-based material has opportunity to have better endurance as compared to an oxide-based material, data retention and integration with the complementary metal-oxide-semiconductor (CMOS) process are also issues. Devices with bilayer switching materials show better resistive switching characteristics as compared to those with a single switching layer, especially a program/erase endurance of >10⁵ cycles with a high speed of few nanoseconds. Multi-level cell operation is possible, but the stability of the high resistance state is also an important reliability concern. These devices show a good data retention of >10⁵ s at >85°C. However, more study is needed to achieve a 10-year guarantee of data retention for non-volatile memory application. The crossbar memory is benefited for high density with low power operation. Some CBRAM devices as a chip have been reported for proto-typical production. This review shows that operation current should be optimized for few microamperes with a maintaining speed of few nanoseconds, which will have challenges and also opportunities for three-dimensional (3D) architecture.

Impact of the device size and thickness of Al₂O₃ film on the Cu pillars and resistive switching memory characteristics of the Al/Cu/Al₂O₃/TiN structures have been investigated for the first time. The memory device size and thickness of Al₂O₃ of 18 nm are observed by transmission electron microscope image. The 20-nm-thick Al₂O₃ films have been used for the Cu pillar formation (i.e., stronger Cu filaments) in the Al/Cu/Al₂O₃/TiN structures, which can be used for three-dimensional (3D) cross-point architecture as reported previously Nanoscale Res. Lett.9:366, 2014. Fifty randomly picked devices with sizes ranging from 8 × 8 to 0.4 × 0.4 μm² have been measured. The 8-μm devices show 100% yield of Cu pillars, whereas only 74% successful is observed for the 0.4-μm devices, because smaller size devices have higher Joule heating effect and larger size devices show long read endurance of 10⁵ cycles at a high read voltage of -1.5 V. On the other hand, the resistive switching memory characteristics of the 0.4-μm devices with a 2-nm-thick Al₂O₃ film show superior as compared to those of both the larger device sizes and thicker (10 nm) Al₂O₃ film, owing to higher Cu diffusion rate for the larger size and thicker Al₂O₃ film. In consequence, higher device-to-device uniformity of 88% and lower average RESET current of approximately 328 μA are observed for the 0.4-μm devices with a 2-nm-thick Al₂O₃ film. Data retention capability of our memory device of >48 h makes it a promising one for future nanoscale nonvolatile application. This conductive bridging resistive random access memory (CBRAM) device is forming free at a current compliance (CC) of 30 μA (even at a lowest CC of 0.1 μA) and operation voltage of ±3 V at a high resistance ratio of >10⁴.

Diode-like threshold switching and high on/off ratio characteristics by using an Al/Ag/Al₂O₃/TiN conductive bridge resistive random access memories (CBRAM) have been obtained. The 5 nm-thick Al₂O₃ device shows superior memory parameters such as low forming voltage and higher switching uniformity as compared to the 20 nm-thick switching layer, owing to higher electric field across the material. Capacitance-voltage (CV) characteristics are observed for the Ag/Al₂O₃/TiN devices, suggesting the unipolar/bipolar resistive switching phenomena. Negative capacitance (NC) at low frequency proves inductive behavior of the CBRAM devices due to Ag ion migration into the Al₂O₃ oxide-electrolyte. Thicker Al₂O₃ film shows diode-like threshold switching behavior with long consecutive 10,000 cycles. It has been found that a thinner Al₂O₃ device has a larger on/off ratio of >10⁸ as compared to a thicker one. Program/erase (P/E) cycles, read endurance, and data retention of the thinner Al₂O₃ oxide-electrolyte shows superior phenomena than the thicker electrolyte. The switching mechanism is also explored.