Copper pillar and memory characteristics using Al2O3 switching material for 3D architecture
© Maikap et al.; licensee Springer. 2014
Received: 19 April 2014
Accepted: 11 July 2014
Published: 26 July 2014
A novel idea by using copper (Cu) pillar is proposed in this study, which can replace the through-silicon-vias (TSV) technique in future three-dimensional (3D) architecture. The Cu pillar formation under external bias in an Al/Cu/Al2O3/TiN structure is simple and low cost. The Cu pillar is formed in the Al2O3 film under a small operation voltage of <5 V and a high-current-carrying conductor of >70 mA is obtained. More than 100 devices have shown tight distribution of the Cu pillars in Al2O3 film for high current compliance (CC) of 70 mA. Robust read pulse endurances of >106 cycles are observed with read voltages of −1, 1, and 4 V. However, read endurance is failed with read voltages of −1.5, −2, and −4 V. By decreasing negative read voltage, the read endurance is getting worst, which is owing to ruptured Cu pillar. Surface roughness and TiO x N y on TiN bottom electrode are observed by atomic force microscope and transmission electron microscope, respectively. The Al/Cu/Al2O3/TiN memory device shows good bipolar resistive switching behavior at a CC of 500 μA under small operating voltage of ±1 V and good data retention characteristics of >103 s with acceptable resistance ratio of >10 is also obtained. This suggests that high-current operation will help to form Cu pillar and lower-current operation will have bipolar resistive switching memory. Therefore, this new Cu/Al2O3/TiN structure will be benefited for 3D architecture in the future.
To succeed the 3D memory architecture with Cu pillar in the future, the via-hole with a size of 4 × 4 μm2 was fabricated in an Al/Cu/Al2O3/TiN M-I-M structure in this study. Tight distribution of the Cu pillars for 100 devices is observed with a low formation voltage of <5 V and high current compliance (CC) of 70 mA. The formation of strong metallic path in Al2O3 layer suggests that Cu pillar could be formed. The Cu pillars have long read pulse endurance of >106 cycles under positive read voltage; however, it has short read endurance under negative read voltages of less than −1.5 V, owing to random read stress-dependent ruptured Cu pillar. On the other hand, bipolar resistive switching memory characteristics are observed by reducing the CC of 500 μA under a small operating voltage of ±1 V. The resistive switching mechanism is formation/dissolution of Cu filament in the Al2O3 film under external bias. The memory device has good data retention of >103 s with acceptable resistance ratio of >10.
Results and discussion
The Cu pillars are formed in Al/Cu/Al2O3/TiN structure under a small voltage of <5 V and a high current of 70 mA. Tight distribution of robust Cu pillars for 100 randomly measured devices with an average current of approximately 50 mA at a Vread of 1 V is observed. The Cu pillars have long read pulse endurance of >106 cycles under positive read voltage. Although, the read pulse endurance under negative read voltage is worst due to Cu dissolution partially. On the other hand, our Al/Cu/Al2O3/TiN memory device shows good bipolar resistive switching behavior at a CC of 500 μA. Good data retention characteristics of >103 s with acceptable resistance ratio of >10 is observed. It is expected that this novel idea to achieve high-density memory through 3D interconnect will have a good alternative of traditional TSV technique owing to a low cost and simple way.
This work was supported by National Science Council (NSC), Taiwan, under contract no. NSC-102-2221-E-182-057-MY2. The authors are grateful to Electronics and Optoelectronics Research Laboratories (EOL)/Industrial Technology Research Institute (ITRI), Hsinchu, for their support.
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