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High-strength sputter-deposited Cu foils with preferred orientation of nanoscale growth twins
Abstract and Figures
Bulk Cu foils have been synthesized via magnetron sputtering with an average twin spacing of 5 nm. Twin interfaces are of {111} type and normal to the growth direction. Growth twins with such high twin density and preferred orientation have never been observed in elemental metals. These Cu foils exhibited tensile strengths of 1.2 GPa, a factor of 3 higher than that reported earlier for nanocrystalline Cu, average uniform elongation of 1%–2%, and ductile dimple fracture surfaces. This work provides a route for the synthesis of ultrahigh-strength, ductile pure metals via control of twin spacing and twin orientation in vapor-deposited materials.
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... It contains numerous twin boundaries with an average spacing typically about tens of nanometers. Magnetron sputtering [12,13] and electrodeposition [14,15] are the two primary fabrication techniques of nt-Cu, in which direct-current (DC) electrodeposition is favored by the IC industry for its higher efficiency and better compatibility with the present manufacturing techniques of Cu interconnects. ...
... From Eq. (12) to (16), we can obtain ...
... From Fig. 5(b), we can see that plenty of twins with spacing of a few monolayers are present, coinciding with the results in Table. 1, suggesting that twins can form by random stacking. There are already plenty of reports about deposited nt-Cu with twin spacing of about several nanometers [5,12,16,42,43], which can be considered consistent with our simulation result. The aforementioned atom migration-induced dissipation of nucleus was observed and displayed in Fig. 5(c1-c3), where a deposited atom moves along the surface and gets absorbed at the edge of the nucleus below. ...
Direct-current (DC) Electrodeposited nanotwinned copper (nt-Cu) is a promising candidate for interconnection materials in integrated circuit for its excellent mechanical, electrical and thermal properties. However, the formation mechanism of nanotwins during DC electrodeposition of nt-Cu remains unclear. In this study, a two-dimensional nucleation and growth model of nt-Cu is established. The critical nucleation size and average twin spacing were evaluated theoretically. The results demonstrate a relatively high probability of twin nucleation through random stacking with the average twin spacing derived to be only ∼1 nm. The gap between theoretical and observed twinning probability can be ascribed to the surface migration-induced dissipation of nuclei. Molecular dynamics simulation further confirms the formation and subsequent dissipation of nuclei during deposition of Cu. By providing a clear illustration of nanotwin formation process, this study contributes to the understanding of nt-Cu electrodeposition.
... Twin generation is favoured from the low stacking fault energy of brass [27]. When present at the nanoscale, they dramatically increase strength and hardness [28,26]. The higher mechanical strength is linked to the interactions of the dislocations with coherent twin boundaries; the nano-scale distance between the twin boundaries limits slip distance within an individual lamellae. ...
The mechanisms that govern a previously unexplained hardening effect of a single phase Cu-30wt%Zn {\alpha}-brass after heating have been investigated. After cold-work, the alloy possesses an increased yield strength and hardening rate only when heat treated to temperatures close to 220{^\circ}C, and is otherwise softer. Crystallographic texture and microstructure, explored using electron backscatter diffraction (EBSD), describe the deformation heterogeneity including twin development, as a function of heat treatment. When heated, an increased area fraction of deformation twins is observed, with dimensions reaching a critical size that maximises the resistance to dislocation slip in the parent grains. The effect is shown to dominate over other alloy characteristics including short range order, giving serrated yielding during tensile testing which is mostly eliminated after heating. In-situ X-ray diffraction during tensile testing corroborates these findings; dislocation-related line broadening and lattice strain development between as worked and heated {\alpha}-brass is directly related to the interaction between the dislocations and the population of deformation twins. The experiments unambiguously disprove that other thermally-induced microstructure features contribute to thermal hardening. Specifically, the presence of recrystallised grains or second phases do not play a role. As these heat treatments match annealing conditions subjected to {\alpha}-brass during deformation-related manufacturing processes, the results here are considered critical to understand, predict and exploit, where appropriate, any beneficial process-induced structural behaviour.
... The twin structures are bounded by a special type of high-angle grain boundary, the coherent twin boundary (CTB) that stores a minimal boundary energy and renders nanotwinned (NT) metals superior physical and mechanical properties [9,10]. Compared with nanograined and coarse-grained counterparts, the NT metals exhibit combinations of high electrical conductivity [11,12], good thermal stability [13][14][15], outstanding radiation tolerance [16][17][18][19], as well as high strength and ductility [20,21]. Recently, it has been ...
Nanotwinned metals have been intensely investigated due to their unique microstructures and superior properties. This work aims to investigate the nanovoid formation mechanism in sputter-deposited nanotwinned Cu. Three different types of epitaxial or polycrystalline Cu films are fabricated by magnetron sputtering deposition technique. In the epitaxial Cu (111) films deposited on Si (110) substrates, high fractions of nanovoids and nanotwins are formed. The void size and density can be tailored by varying deposition parameters, including argon pressure, deposition rate, and film thickness. Interestingly, nanovoids become absent in the polycrystalline Cu film deposited on Si (111) substrate, but they can be regained in the epitaxial nanotwinned Cu (111) when deposited on Si (111) substrate with an Ag seed layer. The nanovoid formation seems to be closely associated with twin nucleation and film texture. Based on the comparative studies between void-free polycrystalline Cu films and epitaxial nanotwinned Cu films with nanovoids, the underlying mechanisms for the formation of nanovoids are discussed within the framework of island coalescence model.
Graphical abstract
... To further enhance the capabilities of this approach, researchers have explored the deposition of high-density (111)-textured Cu nanotwins onto SiC wafers. Zhang et al. conducted a series of studies involving the deposition of nanotwinned 330 stainless steels and copper (Cu) using magnetron sputtering [14][15][16][17]. They exercised precise control over ...
This study focuses on the analyses of nano-twinned copper (Cu) films deposited through magnetron sputtering on silicon carbide (SiC) chips. The investigation encompasses the utilization of a chromium (Cr) adhesive layer coupled with varying voltage bias conditions. The goal is to comprehensively examine the influence of the adhesive layer and negative bias voltages, contributing to an enhanced understanding of materials engineering and bonding technologies for advanced applications. The formation of a nano-twinned structure and (111) surface orientation can be properly controlled by applied substrate bias. High-density nanotwinned structures were introduced into Cu films sputtered on SiC substrates with 82.3% of (111) orientation proportion at −150 V, much higher than the Cu film sputtered with another substrate bias. It is concluded that the sputtered Cu nanotwinned film formed with −150 V bias voltage has the potential to be employed as the interlayer for low-temperature direct bonding.
Ultrafine-grained and heterostructured materials are currently of high interest due to their superior mechanical and functional properties. Severe plastic deformation (SPD) is one of the most effective methods to produce such materials with unique microstructure-property relationships. In this review paper, after summarizing the recent progress in developing various SPD methods for processing bulk, surface and powder of materials, the main structural and microstructural features of SPD-processed materials are explained including lattice defects, grain boundaries and phase transformations. The properties and potential applications of SPD-processed materials are then reviewed in detail including tensile properties, creep, superplasticity, hydrogen embrittlement resistance, electrical conductivity, magnetic properties, optical properties, solar energy harvesting, photocatalysis, electrocatalysis, hydrolysis, hydrogen storage, hydrogen production, CO2 conversion, corrosion resistance and biocompatibility. It is shown that achieving such properties is not limited to pure metals and conventional metallic alloys, and a wide range of materials are currently processed by SPD, including high-entropy alloys, glasses, semiconductors, ceramics and polymers. It is particularly emphasized that SPD has moved from a simple metal processing tool to a powerful means for the discovery and synthesis of new superfunctional metallic and nonmetallic materials. The article ends by declaring that the borders of SPD have been extended from materials science and it has become an interdisciplinary tool to address scientific questions such as the mechanisms of geological and astronomical phenomena and the origin of life.
In this study, heterostructured copper films containing different proportions of highly (220)-oriented vertical nanotwins were prepared by direct current (DC) electroplating. Uniaxial tensile tests showed that the mechanical properties improved notably when the volume ratio of the vertical nanotwins increased—the film with the 88% vertical twins exhibited the ultimate tensile strength of 455 MPa, approximately 83% higher than those of the equiaxed Cu films. Postmortem electron microscopy characterizations, assisted by molecular dynamics simulations, revealed that these special heterostructures of the equiaxed grains and vertical nanotwins facilitated the activation of multi-mode slip systems. The activation of multi-mode slip systems leads to higher dislocation density, thus endowing the material with enhanced mechanical strength and work hardening rate without significant loss of ductility.
We have explored the influence of sputtering parameters on the structural, mechanical, and electrical properties of nanoscale twinned 330 stainless steel thin films. As the residual stress in the film is changed from tensile to compressive by varying the growth conditions, the nanoscale twinned structure, the average columnar grain size and texture of the film show little or no change. Hardness of the film in compression reaches 7 GPa, compared to about 5.5 GPa in films with high residual tension, and an order of magnitude higher than that of bulk 330 stainless steel. Molecular dynamics simulations indicate that twin boundaries pose a strong barrier to glide dislocation transmission under applied in-plane biaxial loading, consistent with the GPa level strengths measured in these films. The increase in the room temperature electrical resistivity of these films, compared to bulk 330 stainless steel, is found to be small, indicating that nanoscale twinned structures may provide the best combination of high mechanical strengths and high electrical conductivity.
Magnetron-sputter-deposited austenitic 330 stainless steel (330 SS) films, several microns thick, were found to have a hardness ∼6.5 GPa, about an order of magnitude higher than bulk 330 SS. High-resolution transmission electron microscopy revealed that sputtered 330 SS coatings are heavily twinned on {111} with nanometer scale twin spacing. Molecular dynamics simulations show that, in the nanometer regime where plasticity is controlled by the motion of single rather than pile-ups of dislocations, twin boundaries are very strong obstacles to slip. These observations provide a new perspective to producing ultrahigh strength monolithic metals by utilizing growth twins with nanometer-scale spacing. © 2004 American Institute of Physics.
It is well known that plastic deformation induced by conventional forming methods such as rolling, drawing or extrusion can significantly increase the strength of metals. However, this increase is usually accompanied by a loss of ductility. For example, with increasing plastic deformation, the yield strength of Cu and Al monotonically increases while their elongation to failure (ductility) decreases. The same trend is also true for other metals and alloys. We report an extraordinary combination of high strength and high ductility produced in metals subject to severe plastic deformation (SPD). We believe that this unusual mechanical behavior is caused by the unique nanostructures generated by SPD processing. The combination of ultrafine grain size and high-density dislocations appears to enable deformation by new mechanisms. This work demonstrates the possibility of tailoring the microstructures of metals and alloys by SPD to obtain both high strength and high ductility. Materials with such desirable mechanical properties are very attractive for advanced structural applications. (17 refs.)
Optical characterization is used to study spontaneously formed polytypic
quantum well structures in lightly doped epilayers on heavily doped ([N]
> 3×1019 cm-3) 4H-SiC substrates. Low
temperature (1.8 K) photoluminescence (PL) shows emission from the wells
in both epilayer and substrate, the latter occurring at higher energy. A
self-consistent model of the quantum wells including polarization charge
is used to explain the results. Raman scattering data provide direct
evidence of depletion of the 4H barriers in the substrate due to
modulation doping into these wells.
An electro-deposited Cu sample with a high density of nano-scale growth twins shows an ultrahigh tensile strength (∼1GPa) with a considerable plastic strain (>13%). Both the strength and the ductility increase with a decreasing twin lamellae thickness. The yield strength values follow the empirical Hall–Petch relationship for conventional polycrystalline Cu.
Nanocrystalline solids, in which the grain size is in the nanometre range, often have technologically interesting properties such as increased hardness and ductility. Nanocrystalline metals can be produced in several ways, among the most common of which are high-pressure compaction of nanometre-sized clusters and high-energy ball-milling. The result is a polycrystalline metal with the grains randomly orientated. The hardness and yield stress ofthe material typically increase with decreasing grain size, a phenomenon known as the Hall-Petch effect,. Here we present computer simulations of the deformation of nanocrystalline copper, which show a softening with grain size (a reverse Hall-Petch effect,) for the smallest sizes. Most of the plastic deformation is due to a large number of small `sliding' events of atomic planes at the grain boundaries, with only a minor part being caused by dislocation activity in the grains; the softening that we see at small grain sizes is therefore due to the larger fraction of atoms at grain boundaries. This softening will ultimately impose a limit on how strong nanocrystalline metals may become.
We present an experimental approach to systematically produce nanostructures with various grain sizes and twin densities in
the Ni-Co binary system. Using electrodeposition with various applied current densities and organic additive contents in the
deposition bath, we synthesize nanostructured fcc and hcp solid solutions with a range of compositions. Due to the low stacking
fault energy (SFE) of these alloys, growth twins are readily formed during deposition, and by adjusting the deposition conditions,
a range of twin boundary densities is possible. The resulting nanostructured alloys cannot be described by a single characteristic
length scale, but instead must be characterized in terms of (1) a true grain size pertaining to general high-angle grain boundaries
and (2) an effective grain size that incorporates twin boundaries. Analysis of Hall-Petch strength scaling for these materials
is complicated by their dual length scales, but the hardness trends found in Ni-80Co are found to be roughly in line with
those seen in pure nanocrystalline nickel.
The elastic and tensile behavior of high-density, high-purity nanocrystalline Cu and Pd was determined. Samples with grain sizes of 10–110 nm and densities of greater than 98% of theoretical were produced by inert-gas condensation and warm compaction. Small decrements from coarse-grained values observed in the Young's modulus are caused primarily by the slight amount of porosity in the samples. The yield strength of nanocrystalline Cu and Pd was 10–15 times that of the annealed, coarse-grained metal. Total elongations of 1–4% were observed in samples with grain sizes less than 50 nm, while a sample with a grain size of 110 nm exhibited > 8% elongation, perhaps signifying a change in deformation mechanism with grain size. Hardness measurements followed the predictions of the Hall-Petch relationship for the coarse-grained copper down to ≈ 15 nm, and then plateaued. Hardness values (divided by 3) were 2–3 times greater than the tensile yield strengths. Processing flaws may cause premature tensile failure and lower yield strengths. The size and distribution of processing flaws was determined by small-angle neutron scattering. Tensile strength increased with decreasing porosity, and may be significantly affected by a few large processing flaws.