Electrodeposition of Highly-Textured Nanotwinned Copper
Authors: Hasegawa Madoka , Mieszala Maxime , Zhang Yucheng , Michler Johann , Philippe Laetitia 
Published: February 2020

Nanotwinned Cu (nt-Cu) exhibits remarkable strength and high ductility[1, 2], which are very important for wide range of applications. Besides the preferable mechanical properties, there are some noticeable advantages of nt-Cu over nanocrystalline Cu which are also known as a strong material; Firstly the twin boundary is more thermally stable than the grain boundary because of its excess energy being one order of magnitude lower than that of the grain boundary. Moreover, nt-Cu exhibits an electrical resistivity as low as that of bulk pure copper because the electron scattering of twin boundary is much less significant than that of grain boundary. Recently, the mechanical behaviors of highly (111)-oriented nt-Cu films have been intensively investigated in order to understand the deformation mechanism of nanotwinned metals[1-3]. These studies have shown that dense nanotwins typically with the spacing of 100 nm or less have been shown to improve strength and ductility.
In this study, we report on the synthesis of nt-Cu films by pulse plating. By employing appropriate pulse parameters, a highly (111)-textured Cu thin film with well-ordered nanotwins were prepared in the conventional acid Cu sulfate bath at a deposition potential of -0.2V vs SCE. The (111)-oriented nt-Cu film consists of coarse columnar grains (ø1-2 µm) in which twins are uniformly formed along the growing surface (Fig.1a). Furthermore, we observed that an increase in the off-time period results in formation of more uniform and denser nanotwins. The effects of the off-time on formation of well-aligned twins can be explained based on the stress-strain mechanism[4] proposed by Xu et al. for randomly oriented nanotwins in Cu electrodeposits. Twin boundary forms during relaxation of the internal stress because the stress-relaxed nt-Cu deposit is energetically favored than the stressed one. Our experimental observations show the evidence that nanotwins are formed during the off-time, where the adatoms diffuse on the surface to rearrange in order to minimize the stress/surface energy, as assumed in the stress-strain mechanism. In terms of texture, (111) plane possesses the lowest surface energy among other main planes such as (100) and (110). We therefore believe that formation of the well-oriented nt-Cu is brought about by the pulse plating conditions which induce extremely high (111)-texture along with twin formation. The microcompression tests performed on (111)-textured nt-Cu samples showed the yield strength of 600 MPa, which is comparable to nt-Cu films reported previously[1]. More interestingly, we also found that the direction of nanotwin orientation changes with the deposition potential. When Cu deposition is performed at -0.6 V and more negative potentials, Cu electrodeposits consist of coarse grain with dense twin boundaries aligned in the perpendicular direction to the surface (Fig.1b). TEM observation confirmed that the growth direction is (112) and twin boundaries are formed on (111) plane with the nanometer-scale spacing. Mechanical properties of nt-Cu films with different nanotwin orientations are of great interest from the scientific and practical stand points because of the anisotropy of deformation at twin boundary[5]. In this presentation, we will discuss the effects of the orientation of nt-Cu deposits on their deformation mechanism based on the most recent results of our micro mechanical tests of the (111) and (112)-textured nt-Cu deposits.
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