nt-Cu Films Research Articles

Effect of heterogeneous microstructures on mechanical properties of thin gradient nanotwinned copper foils

Strengthening of metals generally weakens ductility, but gradient strengthening is an exception. Heterogeneous microstructures, characterized by variations in grain size and twin spacing (structural gradients), were incorporated into 20-μm thin nanotwinned copper (NT-Cu) foils to enhance their mechanical properties. Four distinct heterogeneous NT-Cu samples, including two-layer (2L) structures and three-layer (3L) structures, were produced through rotary electroplating. The mechanical properties, work hardening rates, and microstructural features of these samples were systematically analyzed and compared. The results showed that an increase of 9.5 %, 6.4 %, and 20.1 % in ultimate tensile strength, yield strength, and work hardening rate, respectively, could be achieved in 3L structures, and the elongation was maintained at 5.5 %. The significant structural gradients and two interfaces of 3L structures were confirmed to stimulate the production of geometrically necessary dislocations (GNDs), which increased strength and work hardening rate. Two interfaces of 3L structures also contributed to the efficient moderation of plastic strains and thus improved the ductility. This study significantly enhances the strength and ductility of thin electroplated NT-Cu foils by introducing heterogeneous microstructures, promoting the application of thin NT-Cu films in electric vehicles, batteries, and the semiconductor industry.

Effects of residual stress and microstructure on extremely anisotropic grain growth in nanotwinned Cu films

In this study, the thermal stability and microstructural evolutions of nanotwinned copper (NT-Cu) films under thermal cycling are investigated. The microstructures and residual stresses were tuned by the electroplating current density. It was found that the thermal stability of the NT-Cu films decreased with an increase in current density. The texture transition from (111) to (100) suggests that the extremely anisotropic grain growth (EAGG) was driven by the minimization of strain energy. Bending beam technic was employed to measure the residual stress in the NT-Cu films. Large residual stress in the NT-Cu films is also considered a main driving force for the transition to the (100)-oriented texture. Additionally, strain energy, surface/interface energy, grain boundary energy, and twin boundary energy were calculated, and the results supported the experimental results.

The mechanical property and microstructural thermal stability of gradient-microstructured nanotwinned copper films electrodeposited on the highly (111)-orientated substrates

Three different substrates with varying (111) textures were used to study the epitaxial growth behavior of nanotwinned copper (nt-Cu) films during direct current electroplating. The (111) proportions for substrates are 78.5%, 97.5%, and 100%, while for the electrodeposits they are 69.3%, 92.7%, and 98.3%, showing a positive correlation between the (111) proportions of the substrates and the electrodeposits. Secondary ion microscopy (SIM) reveals a transition layer consisting of fine grains forms between the columnar nanotwinned grains region and the substrate, exhibiting a gradient microstructure within the nt-Cu films. The thickness of the transition layer shows a reducing tendency as the (111) proportion of substrate increases. Furthermore, to investigate the mechanical properties of the transition layer, a nano-indentation test was performed on the cross-section of gradient-microstructured films. The average hardness of the transition layer near the substrate is 3–4 GPa, which is approximately two times higher than that of the columnar nanotwinned region. The excellent hardness is probably induced by the decrease in grain size and the presence of high-density twin boundaries within the grains. An in-situ annealing test at 300 °C was performed on these three electrodeposits to assess their microstructural thermal stability. It is noticed that the thickness of transition layer would affect the competition of grain growth between substrate grains and columnar nanotwinned grains during aging. Higher microstructural thermal stability could be achieved with fewer grains in the transition layer. Overall, this study reveals the high mechanical properties and microstructural thermal stability of the transition layer, which should be helpful in understanding and promoting the application of nt-Cu thin films.

Electrochemical Plating System Development of Nanotwinned Cu for Multiple WLP Features

We develop a series of electrochemical plating (ECP) baths and processes, generating nano-twinned Cu (nt-Cu) in wafer-level packaging (WLP) applications to fulfill the expanding market demand for copper-to-copper direct bonding and hybrid-bonding. This newly developed system shows a high density of nt-Cu (close to 100%), controllable grain size (0.10 to 1.0 micron), vertical columnar structure (through the full height), and minimal transition layer between Cu seed and nt-Cu initial position (dozens of nanometers). These systems have expansive operating windows which produce high-density nt-Cu films: from the inorganic electrolyte concentrations, including copper, acid and chloride, to the ECP current density, ECP current waveform, agitation speed and additive concentrations. The new ECP system consists of one, two or three additives, depending on the applications, and we demonstrated that this sytem could fill multiple WLP features with a high density of nt-Cu and a flat surface. Unlike the features without recess, there exist more challenges for the components with PI-recess. The balance between conformal and bottom-up ECP must be well-controlled to achieve a flat surface without affecting the nt-Cu formation. To do so, we used two or three additives in the system, including the one that promotes bottom-up fill. Although having an additional additive in the electrolyte weakens the formation of nt-Cu to a certain level, we achieved the balance successfully by adjusting electrolyte compositions and ECP parameters for a series of WLP features. The working window of ECP parameters for generating nt-Cu structure is relatively wide. To achieve a uniform and flat surface of a high density of nt-Cu for specific WLP features, the additive concentrations and plating current waveform should be well-controlled. They are more critical than other parameters for controlling the ECP nt-Cu performance. We evaluated numerous waveforms, such as constant current, multistep current, ramping current ECP and other waveforms to generate nt-Cu films. At the current stage, both within die (WID) and within feature (WIF) of ECP nt-Cu for WLP features tested can be as low as 5.00 % with high density, columnar orientation, and less transition layer nt-Cu. Contrary to some published literature, our ECP process can produce nt-Cu films with Cu(111) orientation irrespective of the direction of the base substrate. Our ECP nt-Cu process does not appear to exhibit such substrate-dependent preference. We can generate high-density nt-Cu films using Cu (111) dominating seed layer and non-Cu (111) dominating substrates and even using stainless steel substrates. This paper will attempt to discuss additive functions, the mechanism of the balance between conformal and bottom-up ECP process, agitation speed, plating current waveform, multi-bath plating, electrolyte compositions, and substrate influence.

Stress Relaxation and Grain Growth Behaviors of (111)-Preferred Nanotwinned Copper during Annealing

Highly (111)-oriented nanotwinned Cu (nt-Cu) films were fabricated on silicon wafers for thermal-stress characterization. We tailored the microstructural features (grain scale and orientation) of the films by tuning the electroplating parameters. The films were heat-treated and the relaxation behaviors of thermal stresses in the films were explored using a bending beam system. Focused ion beam (FIB) and electron back-scattered diffraction (EBSD) were then employed to characterize the transformations of the microstructure, grain size, and orientation degree of the films. The results indicated that the degree of (111)-preferred orientation and grain size significantly decrease with increasing the current density. The nt-Cu films with a higher degree of (111)-preferred orientation and larger grains exhibit the slower rates of stress relaxation. The film with larger grains possesses a smaller grain boundary area; thus, the grain boundary diffusion for the thermal-stress release is suppressed. In addition, the induced tensile stress in the films with larger grains is smaller leading to the difference in microstructural changes under annealing.

Mechanical strengthening of nanotwinned Cu films with Ag solid solution

This study employs Ag solid solution to enhance the mechanical strength of nanotwinned copper (nt-Cu) films. We deposited a thin silver layer onto the nt-Cu film and annealed it at low temperatures. Depth profile reveals that Ag atoms diffused 100 nm after annealing. The Ag atoms interdiffused into nt-Cu film and strengthened it. A significant enhancement in hardness (45.9%) was obtained using the Ag layer and heat treatment. This could be attributed to the Ag diffusion, grain boundary segregation, and twin density increase. The study delivers a pioneer approach to strengthen the nt-Cu films.

Interfacial Characterization of Low-Temperature Cu-to-Cu Direct Bonding with Chemical Mechanical Planarized Nanotwinned Cu Films.

Copper-to-copper (Cu-to-Cu) direct bonding is a promising approach to replace traditional solder joints in three-dimensional integrated circuits (3D ICs) packaging. It has been commonly conducted at a temperature over 300 °C, which is detrimental to integrated electronic devices. In this study, highly (111)-oriented nanotwinned (nt) Cu films were fabricated and polished using chemical mechanical planarization (CMP) and electropolishing. We successfully bonded and remained columnar nt-Cu microstructure at a low temperature of 150 °C thanks to the rapid diffusion of Cu on (111) surface. We employed a new microstructural method to characterize quantitatively the interfacial bonding quality using cross-sectional and plan-view microstructural analyses. We discovered that CMP nt-Cu bonding quality was greater than that of electropolished nt-Cu ones. The CMP nt-Cu films possessed extremely low surface roughness and were virtually free of pre-existing interface voids. Thus, the bonding time of such CMP nt-Cu films could be significantly shortened to 10 min. We expect that these findings may offer a pathway to reduce the thermal budget and manufacturing cost of the current 3D ICs packaging technology.

Kinetic study of grain growth in highly (111)-preferred nanotwinned copper films

We investigated the microstructure changes in abnormal grain growth of nanotwinned Cu (nt-Cu) upon annealing by electron backscattered diffraction (EBSD) and focused ion beam (FIB). The nt-Cu films were produced by electrodeposition on a Si wafer with a barrier/seed layer of TiW/Cu. When a 3 μm thick nt-Cu film was annealed at 350 °C for 45 min, its microstructure was found to have changed from the (111) oriented twins to the (200) oriented grains with an average grain size larger than 100 μm. When the nt-Cu film is thicker than 5 μm, the change can occur at a lower temperature of 300 °C. We observed that there is a critical abnormal grain growth temperature, below which no microstructure change can occur. We propose that it is due to nucleation limitation. After nucleation, the growth is highly anisotropic, where the lateral growth is at the least one order of magnitude faster than the normal growth. We have attempted to use JMAK theory of phase transformation to analyze the abnormal grain growth kinetics, yet we obtained very little success.

Ultra-high annealing twin density in -oriented Cu films

We reported the fabrication of densely twinned Cu films with highly (211) texture developed from the annealing of <111>-oriented nanotwinned copper (nt-Cu) films. During the annealing at 300 °C, abnormal grain growth occurred in the nt-Cu film and resulted in a microstructure dominated by the anomalously large <211>-oriented grains. Moreover, densely-intersected annealing twin bands with <221>, <611>, and <931> orientations are embedded in the large <211> grains. The measured annealing twin density is 0.2 µm−1, which is highest among the reported values in previous literatures with similar grain size. Extensive multiple twinning process may occur during the annealing to form the unique microstructure.

Fabrication and Characterization of ⟨100⟩-Oriented Quasi-single Crystalline Cu Lines

Nanotwinned copper (nt-Cu) films with ⟨111⟩ crystal orientation were electroplated on Si wafers by pulse plating, with original grain size of ∼1.4 μm. By patterning and annealing the nt-Cu film at ...

Electrodeposition of Highly-Textured Nanotwinned Copper

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. [1] M. Dao, L. Lu, Y. F. Shen, S. Suresh, Acta Materialia 2006, 54, 5421.[2] L. Lu, X. Chen, X. Huang, K. Lu, Science 2009, 323, 607.[3] H. Y. Hsiao, C. M. Liu, H. W. Lin, T. C. Liu, C. L. Lu, Y. S. Huang, C. Chen, K. N. Tu, Science 2012, 336, 1007.[4] D. Xu, V. Sriram, V. Ozolins, J.-M. Yang, K. N. Tu, G. R. Stafford, C. Beauchamp, Journal of Applied Physics 2009, 105.[5] J. C. Ye, Y. M. Wang, T. W. Barbee, A. V. Hamza, Applied Physics Letters 2012, 100, 5.