Spontaneous Crystal Transformation and Surface Roughness Regulation in DC Electrodeposited Copper Films

Nanotwined copper (NT-Cu) has gained significant attention due to its unique properties. The high-density coherent twin boundaries within the grains of NT-Cu contribute to its exceptional mechanical properties and electrical conductivity. Direct current (DC) electroplating has emerged as a prominent method for producing copper films with these high-density nanotwins. Compared to alternative methods such as pulse current electroplating and magnetron sputter deposition, DC electroplating offers a more efficient and controllable process. It is capable of continuous operation, which is important for fabricating bulk NT-Cu samples with considerable thickness. Usually the properties of NT-Cu films produced via DC electroplating are linked to their crystal structural characteristics, such as twin thickness and crystal orientation. The mechanical properties of these films exhibit a strong correlation with the angle between twin boundaries and the direction of deformation. The presence of suitable additives in the electrolyte, including various organic additives and complex agents, is essential for achieving the desired crystal orientation during the electroplating process. The most extensively studied crystal structure is the nanotwined columnar grains with a (111) orientation, known for their superior fatigue resistance and the ability to balance strength and toughness effectively. This structure is typically a result of the additive gelatin in the electrolyte. However, the aging of chemical solutions during electrolyte reuse cannot be overlooked. The gradual degradation of organic additives under the influence of strong acids, oxidation, and electric fields leads to changes in the electrolyte’s properties, subsequently affecting the microstructure of the deposited copper. This continuous change in additive content allows for the observation of the transformation rules of the microstructure in the deposited copper during the ongoing electroplating process.

To this end, new study published in the Journal Applied Surface Science and conducted by Dr. Peng Zhang, Professor Lin Zhang, and Professor Xuanhui Qu from the Institute for Advanced Materials and Technology at the University of Science and Technology Beijing, the researchers investigated how the crystal structure and surface roughness of copper films can be regulated during direct current electrodeposition, especially under the influence of electrolyte aging and additive interactions.  The ultimate goal was to enhance the quality and performance of electrodeposited copper films, which will broaden their applications in industries such as electrical interconnects and battery technology.

The team used copper sulfate with a small quantity of additives, including gelatin, sodium polydisulfide dipropane sulfonate, and chloride ions as a fresh electrolyte. To facilitate the study, copper films were fabricated through long-term DC electroplating in the same electrolyte, with the electrolyte aged by continuous use. The copper film was stripped every eight hours, and a new titanium cathode plate was used for the subsequent DC electroplating. During the intervals of replacing the cathode plate, rapid filter paper was used to filter out impurities without renewing the electrolyte. The electroplating parameters remained consistent throughout the process, with a constant current density of 30 mA/cm² and a maintained temperature of 15 ± 0.2 °C, ensured by a magnetic rotor with a rotating speed of 300 r/min. The authors used several advanced characterization techniques including a confocal laser scanning microscope to assess the 3D morphology and roughness of the as-deposited copper films, the scanning electron microscopy (SEM) with backscattering electron (BSE) mode and transmission electron microscopy (TEM) were used to characterize the microstructure and the out-of-plane texture of the copper films was determined by X-ray diffraction (XRD). Electron backscatter diffraction (EBSD) analysis was also conducted to observe the crystal orientation and interface type.

The authors’ findings showed that the repeated use of the electrolyte led to continuous changes in the surface morphology of the electrodeposited copper films. Initially, the copper film obtained from the fresh electrolyte presented a glossy surface with significant metallic luster. However, as the electrolyte aged, matted regions began to appear on the surface, with their coverage area gradually increasing over time. The 3D topography of the surface indicated alternating valleys and peaks in the matted regions, resulting in increased surface roughness. SEM images showed that the matted surface was characterized by a large number of conical bulges, with the density of these bulges decreasing from the matted to the glossy regions. They showed the XRD patterns obtained from different regions of a copper film with a rounded matted region in the middle indicated that the matted regions exhibited a strong (220) texture. In contrast, the glossy regions showed both (111) and (220) textures, with a reduced intensity of the (220) texture compared to the matted regions. Cross-sectional BSE images revealed that the equiaxed grains were predominant in some regions, while lathy structures with high aspect ratios and flat interfaces extended from equiaxed grains to the growth surface in others. Further analysis using EBSD maps confirmed that the lathy structures had a random orientation in the basal plane of the film but exhibited a high (101) orientation along the thickness direction. This high (220) texture detected by XRD was induced by these lathy structures, which were aggregates of highly oriented twins, as verified by TEM images and selected electron diffraction patterns. The spontaneous transformation of crystal structure from equiaxed grains to vertical twins with high (220) texture during continuous electrolyte aging in DC electroplating raised two key questions: why the crystal structure transforms automatically and why it follows the observed transformation sequence. According to the Winand diagram, the transformation sequence of crystal structures during electrodeposition of copper is influenced by two key parameters: inhibition intensity and mass transfer. In this study, the influence of inhibition intensity was primarily considered, given that the DC electroplating was carried out under galvanostatic mode. The decrease in inhibition intensity, reflected by the overpotential, was a key factor driving the spontaneous transformation of the crystal structure.

According to the authors, the transformation sequence observed in their study differed somewhat from that depicted in the Winand diagram. Typically, the crystal structures transform from unoriented dispersion type (UD-type) to field-oriented texture type (FT-type), twinning intermediate type (Z-type), and finally basis-oriented reproduction type (BR-type) as the inhibition intensity decreases. In the study, the initial high inhibition intensity resulted in the formation of UD-type near the substrate. However, with decreasing inhibition intensity, the crystal structure spontaneously changed from UD-type to vertical twins with high (220) texture, bypassing the FT-type structure. Additionally, the researchers observed the vertical twins with high (220) texture which was unique and differed from the commonly studied NT-Cu with (111) textured columnar grains (FT-type). The vertical twins had a very high aspect ratio and exhibited strong twin characteristics. They formed between the UD-type and FT-type structures under specific overpotential conditions, indicating a new transformation sequence. Morevoer, the appearance of (220) textured vertical twins was driven by energy minimization. The generation of (111) texture is typically attributed to the lowest surface energy of the (111) crystal plane. However, the preferential formation of (220) textured vertical twins suggested that minimizing surface energy was not the dominant factor in this case. Instead, the large strain energy in the deposited copper film, which could be coordinated more effectively by the (220) plane, played a significant role.

In conclusion, the new study by Dr. Peng Zhang, Professor Lin Zhang, and Professor Xuanhui Qu provided important data on the regulation of crystal structure and orientation during DC electrodeposition of copper films.  The control of surface roughness through crystal transformation provided potential applications in industries such as lithium battery manufacturing. These findings contribute to the broader understanding of electrodeposited NT-Cu and its applications in advanced material science.