Abstract
An air-gap structure or through silicon via (TSV) employed in interconnect technology of integrated-circuits requires interconnecting materials of high mechanical strength and low electrical resistivity. Recently, Cu with nano-scaled twins has been intensively researched due to its high yield strength, good ductility and reasonably low electrical resistivity. In addition, electromigration-induced atomic diffusion would be slowed down at the twin-modified grain boundary in Cu line, which may improve electromigration resistance of Cu interconnects. In this study, dense nanoscale twins were introduced in copper films and nanowires through pulse electrodeposition and bombardment of high-energy Ar+ ions at low temperatures. In the electrodeposited nanotwinned Cu films, the nanoindentation hardness increases inversely with the square root of twin lamella width, and follows the Hall-Petch like relationship. In addition, crystallographic texture of nanotwinned Cu films was achieved through adjustment of chloride concentration in copper sulfate electrolyte using direct-current and pulsed-current deposition methods. With increasing chloride concentration in the electrolyte the DC-deposited Cu film showed a monotonically strengthening {110} crystallographic texture, while the PC-deposited one revealed a {111} to {110} transition at the chloride concentration of 10−4 – 10−5 M. We found that change of Cu film texture with varying chloride concentration is attributed to the distinct exchange current density of different Cu crystallographic planes and duty cycle of pulse current. On the other hand, bamboo-like nanotwinned Cu nanowires with 55 nm in diameter were fabricated by pulse electrodeposition at low temperature with anodic aluminum oxide as template. At pulse current density of 0.4 A/cm2, the mean value of twin-lamella width is only 14.6 nm. The formation of high density of twin boundaries (TBs) is attributed to relaxation of coalescence induced stress and twin fault stacking when Cu NWs grow under two-dimensional kinetics. The endurance of electrical current density before breakdown of nanotwinned Cu NWs reaches 2.4 × 108 A/cm2, which is comparable with carbon nanotubes or graphene nanoribbons. The suppression of electromigration induced void growth at triple junction where twin boundaries meet surface or grain boundaries is responsible for the raise of failure current density. Besides, both TB-dislocation and dislocation-dislocation interactions contribute to the strengthening of ion-irradiated Cu films. The strengthened region can be extended to several hundreds of nanometers below the irradiated surface of embedded nanowires and patterned lines. A mechanism based on thermal-spike induced stress is proposed to explain the influences of ion energy and bombardment temperature on the formation of nanotwins. With the advantages of high mechanical strength and good electric endurance, nanotwinned Cu becomes a good candidate of interconnect material for advanced micro- and nano-electronic devices.