Electrochemical Synthesis of High Strength Nanotwin Copper Films

Copper is the common materials for the semiconductor and battery industry. Especially, electrodeposited copper is the most widely used for its excellent electrical, physical properties and low cost. Recently, high strength copper thin film with also high conductivity is urgently required.  Generally, according to the Hall-Petch relation, the grain size decrease causes tensile strength of copper. However, electrical conductivity was significantly decreased by this strengthening method as electron scattering can be caused by grain boundary.

In this study, electrochemical deposition of copper by pulse and pulse reverse current with extremely high peak current density was investigated. Various experiments were carried out to control the microstructure and achieve the high mechanical and electrical property. Pulse reverse current is applied to anodic current instead of off time in the pulse current. This process accelerates stress relaxation of copper film, we expect that reduce the deposition time compared with pulse current and direct current (dc) deposition.

Besides current adjustment, there are two classification of organic additives, are in suppressor which inhibit the copper reduction reaction, such as Polyethylene glycol (PEG), another is accelerator which oppositely increase copper reduction reaction, such as bis-(3-sulfopropyl)-disulfide (SPS).

Moreover, we investigated organic additive behavior in pulse and pulse reverse current electrodeposition copper films, the relation between organic additives and nano scale twin structure. As a results, tensile strength of copper film by pulse current  is significantly higher than dc deposited copper film (Figure 1) and electrical conductivity is similar to dc deposited copper film (~1.9μΩ·cm). Enhanced tensile strength of copper thin film is related with microstructure. Especially, twin structure in the grain is strongly associated with tensile strength of copper films. As shown in the figure 2, high density of twin boundary structure is observed