Pseudo-Hexagonal Composite Twins in Pulse Electroplated Copper Ya-Wen Lin and Jui-Chao Kuo * ,z Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan The microstructure and crystallographic texture of copper electrodeposited onto a graphite substrate were examined by scanning electron microscopy, and electron backscatter diffraction. A high twin fraction of 80% was achieved using a pulsed electrodeposi- tion technique under the current density of 0.5 A/cm 2 . A new twinning cluster, termed in this paper as “pseudohexagonal compos- ite twins,” was observed in pseudohexagonal grains. The pseudohexagonal composite twins possess the crystallographic feature of a rotation angle of 60  around an axis of h111i. V C 2011 The Electrochemical Society. [DOI: 10.1149/1.3549171] All rights reserved. Manuscript submitted August 19, 2010; revised manuscript received January 5, 2011. Published February 25, 2011. Copper (Cu) has been widely used as an interconnect material in advanced microelectronic technology due to its lower resistivity, higher thermal conductivity, and better electromigration capability. One solution for low-k materials in interconnects lies in improving the air-gap design, in which Cu interconnects are required to support the structure. The mechanical and electrical properties of Cu are, therefore, crucial in the application of interconnect materials. The twin boundary, a special type of grain boundary, is of utmost importance due to its mechanical properties. 1–3 With the develop- ment of nanotwinned copper, the importance of twin boundaries on material deformation has recently increased. A newly discovered nanostructure has been obtained from nanograined copper with ultra- high-density twins. 4 Nanotwinned copper possesses ultrahigh yield strength, ductility, and electrical conductivity. The nanotwinned cop- per structure can be fabricated by magnetron sputtering deposition aside from the pulsed electrodeposition technique. 4–10 The design for both high yield strength and low electrical resistance uses growth twins with nanoscale lamellar spacings and coherent twin bounda- ries. Such a microstructure leads to a combination of high tensile strength and ductility, because it blocks the motion of dislocations effectively, and also retains the strain hardening capability. 5 Com- pared with the conventional coarse-grained Cu, an increased electri- cal conductivity in the nanotwinned Cu is due to the replacement of high-angle grain boundaries by coherent twin boundaries. 4 The preferred orientation can be controlled by current density, chemical potential, film thickness, and electrolyte composition. 11–14 The transition from a dominant {111} to a {100} texture was observed with increasing film thickness. 14 With increasing negative electrochemical potential in the electrochemical deposition (ECD) process, the preferred orientation was changed from {111} texture to {110} and {100} textures. 15 The preferred growth of (100) grains was also observed by various authors in Cu thin films. 16–18 Electrochemical methods have been widely applied because they are less expensive, highly productive, and readily adoptable. Thus, in this study, a pulsed electrodeposition technique was used to syn- thesize nanotwinned Cu. Electron backscatter diffraction (EBSD) was utilized to characterize the crystallographic microstructure of as-deposited Cu (i.e., preferred orientation, grain size, and twin frac- tion) as a function of the current density from 0.25 to 1.5 A/cm 2 . Copper of 99.99% purity grade was prepared as an anode mate- rial with dimensions of 30 mm  30 mm  3 mm. Graphite was pre- pared as a cathode disk material with diameter of 11.3 mm and thickness of 2 mm. Both anode and cathode were polished with 1500, 2000, and 4000 grit SiC abrasive paper, and then with 0.03 mm SiO 2 . These were cleaned ultrasonically for 10 min in acetone, after which they were cleaned again in ethanol. Using the electrode- position technique, Cu was directly deposited onto a graphite sub- strate from a 0.5 M CuSO 4 electrolyte with a pH value of 1.0. The pulsed electrodeposition system consisted of an electroplating cell and an external pulse power supply (Keithley SourceMeter model 2611). During processing, the peak pulse current density (PCD) was kept at 0.5 A/cm 2 with on-time of 0.27 ms and off-time of 2.70 ms. The electrolyte temperature was kept at 20  C by water cooling, and the electrolyte was mechanically stirred at a speed of 200 rpm. The total deposition time was 8 h, and the thickness of the deposited film was about 60 mm. For EBSD sample preparation, the surfaces of the materials were mechanically polished to a final level of 0.03 mm using a standard metallographic procedure; this was followed by electropolishing for 60 s at 1.5 V in a phosphoric acid electrolyte so- lution consisting of 825 ml phosphoric acid and 175 ml deionized water. The EBSD measurement on the as-deposited Cu was per- formed on a field emission scanning electron microscope (JEOL 7001F) with an EDAX/TSL Technology EBSD system operated at 20 kV. The measured area was chosen as 20 mm  20 mm, with a step size of 50 nm. The orientation image maps (OIMs) of the deposits are shown in Fig. 1a. According to the inverse pole figure color coding (see Fig. 1a), the out-of-plane direction (the ND direction) of the Cu deposits is close to the {100}, {110}, and {111} directions. The most grains observed in the Cu deposit with on-time of 0.27 ms and off-time of 2.7 ms, and with current densities of 0.5 A/cm 2 exhibit the out-of- plane direction parallel to the {100} direction. However, the {110} out-of-plane texture has also been observed in the as-deposited Cu films produced through the pulsed electrodeposition technique. 4,5,19 An open question arises as to whether the preferred orientation obtained from the EBSD measurement of 20 mm  20 mm is able to represent the general deposition microstructure. Thus, the (111), (200), and (220) pole figures were measured using XRD to generate the preferred orientation of the general microstructure. The pole fig- ures indicate that the (111) and (220) peaks exhibit a stronger pre- ferred orientation than does the (200) peak (Fig. 2). Therefore, the {111}, {200}, and {220} preferred orientations obtained from XRD are in agreement with those obtained from EBSD. On the basis of these results, concluding that the EBSD results can represent the general deposition microstructure is a reasonable deduction. Figure 1b shows an interesting feature, in which numerous hex- agonal arrangements consisting of six large grains are observed and copper clusters are formed in the center of each hexagonal arrange- ment. During the deposition of copper, many new copper nuclei are initially formed, which then grow to large stable clusters of copper nuclei of a few mm. Subsequently, these large clusters continue to grow and result in coalescence grains that are in hexagonal form. Figure 1b shows the EBSD map of grain boundaries consisting of twin boundaries and high-angle grain boundaries that are considered to comprise the misorientation angle of >15  . Twins observed in the Cu deposits are represented as 60  h111i and are characterized with 60  rotation around an h111i axis with {111} twinning plane. In order to characterize grain boundaries quantitatively from Fig. 1b, * Electrochemical Society Active Member. z E-mail: jckuo@mail.ncku.edu.tw Electrochemical and Solid-State Letters, 14 (5) E18-E20 (2011) 1099-0062/2011/14(5)/E18/3/$28.00 V C The Electrochemical Society E18 Downloaded 25 Feb 2011 to 140.116.30.64. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp