Oliver Kraft

Ultrahigh Strength Single Crystalline Nanowhiskers Grown by Physical Vapor Deposition

The strength of metal crystals is reduced below the theoretical value by the presence of dislocations or by flaws that allow easy nucleation of dislocations. A straightforward method to minimize the number of defects and flaws and to presumably increase its strength is to increase the crystal quality or to reduce the crystal size. Here, we describe the successful fabrication of high aspect ratio nanowhiskers from a variety of face-centered cubic metals using a high temperature molecular beam epitaxy method. The presence of atomically smooth, faceted surfaces and absence of dislocations is confirmed using transmission electron microscopy investigations. Tensile tests performed in situ in a focused-ion beam scanning electron microscope on Cu nanowhiskers reveal strengths close to the theoretical upper limit and confirm that the properties of nanomaterials can be engineered by controlling defect and flaw densities.

Deformation behavior of thin copper films on deformable substrates

Abstract The role of strain hardening for the deformation of thin Cu films was investigated quantitatively by conducting specialized tensile testing allowing the simultaneous characterization of the film stress and the dislocation density as a function of plastic strain. The stress–strain behavior was studied as a function of microstructural parameters of the films, such as film thickness (0.4–3.2 μm), grain size and texture. It was found that the stress–strain behavior can be divided into three regimes, i.e. elastic, plastic with strong strain hardening and plastic with weak hardening. The flow stresses and the hardening rate increase with decreasing film thickness and/or grain size, and are about two times higher in (111)-grains compared to the (100)-grains. These effects will be discussed in the light of existing models for plastic deformation of thin films or fine grained metals.

High-strength cellular ceramic composites with 3D microarchitecture.

Significance It has been a long-standing effort to create materials with low density but high strength. Technical foams are very light, but compared with bulk materials, their strength is quite low because of their random structure. Natural lightweight materials, such as bone, are cellular solids with optimized architecture. They are structured hierarchically and actually consist of nanometer-size building blocks, providing a benefit from mechanical size effects. In this paper, we demonstrate that materials with a designed microarchitecture, which provides both structural advantages and size-dependent strengthening effects, may be fabricated. Using 3D laser lithography, we produced micro-truss and -shell structures from ceramic–polymer composites that exceed the strength-to-weight ratio of all engineering materials, with a density below 1,000 kg/m3. To enhance the strength-to-weight ratio of a material, one may try to either improve the strength or lower the density, or both. The lightest solid materials have a density in the range of 1,000 kg/m3; only cellular materials, such as technical foams, can reach considerably lower values. However, compared with corresponding bulk materials, their specific strength generally is significantly lower. Cellular topologies may be divided into bending- and stretching-dominated ones. Technical foams are structured randomly and behave in a bending-dominated way, which is less weight efficient, with respect to strength, than stretching-dominated behavior, such as in regular braced frameworks. Cancellous bone and other natural cellular solids have an optimized architecture. Their basic material is structured hierarchically and consists of nanometer-size elements, providing a benefit from size effects in the material strength. Designing cellular materials with a specific microarchitecture would allow one to exploit the structural advantages of stretching-dominated constructions as well as size-dependent strengthening effects. In this paper, we demonstrate that such materials may be fabricated. Applying 3D laser lithography, we produced and characterized micro-truss and -shell structures made from alumina–polymer composite. Size-dependent strengthening of alumina shells has been observed, particularly when applied with a characteristic thickness below 100 nm. The presented artificial cellular materials reach compressive strengths up to 280 MPa with densities well below 1,000 kg/m3.

Electromigration failure by shape change of voids in bamboo lines

The behavior of electromigration‐induced voids in narrow, unpassivated aluminum interconnects is examined, using scanning electron microscopy. Some electromigration tests were interrupted several times in order to observe void nucleation, void growth, and finally the failure of the conductor line. It is found that voids which opened the line have a specific asymmetric shape with respect to the electron flow direction. Besides void nucleation and void growth, void shape changes can consume a major part of the lifetime of the conductor line. A first attempt to model these processes on the basis of diffusion along the void surface shows that voids with a noncircular initial shape tend to produce the fatal asymmetry due to electron wind effects, with the anisotropy of surface energy possibly playing only a minor role.

Fatigue in thin films: lifetime and damage formation

Two new techniques, developed for studying the fatigue behavior of thin metal films on substrates, are presented: The first technique involves deposition of Cu films onto elastic polyimide substrates. During cyclic tensile testing of the film–substrate composite, the film is subjected to tension–compression cycles, since it is plastically deformed, while the substrate undergoes only elastic deformation. Using this technique, it was found that, for 3 μm thick Cu films, the number of cycles to failure follows the phenomenological Coffin–Manson relationship. For the other technique, thin films, here Ag films with thicknesses ranging from 0.2 to 1.5 μm, are deposited onto micromachined SiO2 cantilever beams. The beams are then deflected with a frequency of 45 Hz using a nanoindentation system. A detailed investigation of the damage formation in both fatigued Cu and Ag films revealed surface roughening prior to failure. Extrusions and cracks are formed inside large grains and between small grains, respectively.

Size effects in the fatigue behavior of thin Ag films

Abstract Thin film dimensions and microstructure affect the microscopic processes responsible for fatigue. This work focuses on the characterization of such mechanisms and the resulting fatigue behavior. The fatigue behavior of 0.2–1.5 μm thick, Ag films on SiO 2 was investigated. The films were tested using cantilever microbeam deflection with respect to the influence of loading conditions. Extrusions similar to those observed in bulk material were found at the Ag film surfaces after cyclic loading. Voids observed beneath the extrusions, close to the film-substrate interface, contributed significantly to fatigue failure. Fatigue damage was observed to occur predominantly in (100)-oriented grains. Thinner films were more fatigue resistant and contained fewer, smaller extrusions than thicker films.

Mechanical testing of thin films and small structures

Thin films, as well as small structures, are used in many technical applications, such as semiconductor devices, information storage media, microelectromechanical systems (MEMS), and biomedical devices. The knowledge of materials properties is essential for the design and fabrication of these devices, particularly since small structures often have different properties than their bulk counterparts. This article describes the most common experimental techniques used to measure mechanical properties in small dimensions.

Approaching theoretical strength in glassy carbon nanolattices

The strength of lightweight mechanical metamaterials, which aim to exploit material-strengthening size effects by their microscale lattice structure, has been limited by the resolution of three-dimensional lithography technologies and their restriction to mainly polymer resins. Here, we demonstrate that pyrolysis of polymeric microlattices can overcome these limitations and create ultra-strong glassy carbon nanolattices with single struts shorter than 1 μm and diameters as small as 200 nm. They represent the smallest lattice structures yet produced--achieved by an 80% shrinkage of the polymer during pyrolysis--and exhibit material strengths of up to 3 GPa, corresponding approximately to the theoretical strength of glassy carbon. The strength-to-density ratios of the nanolattices are six times higher than those of reported microlattices. With a honeycomb topology, effective strengths of 1.2 GPa at 0.6 g cm(-3) are achieved. Diamond is the only bulk material with a notably higher strength-to-density ratio.

Cyclic deformation of polycrystalline Cu films

Fatigue impairs the reliability of macroscopic metallic components utilized in a variety of technological applications. However, the fatigue behaviour of thin metal films and small-scale components used in microelectronics and mechanical microdevices has yet to be explored in detail. The fatigue behaviour in submicrometre thin films is likely to differ from that in bulk material, since the volume necessary for the formation of dislocation structures typical of cyclic deformation in bulk material is larger than that available in thin films. The thin-film dimensions and microstructure, therefore, affect the microscopic processes responsible for fatigue. The fatigue behaviour of Cu films 0.4, 0.8 and 3.0 µm thick on polyimide substrates was investigated. The specimens were fatigued at a total strain amplitude of 0.5% using an electromechanical tensile-testing machine. This work focuses on the characterization of fatigue mechanisms and the resulting fatigue damage of thin Cu films. Extrusions similar to those observed in bulk material were found at the film surfaces after cyclic loading. Voids observed beneath the extrusions, close to the film-substrate interface, contributed significantly to thin-film failure. Thinner films were more fatigue resistant and contained fewer and smaller extrusions than thicker films did. A small thickness appears to inhibit void nucleation. This observation is explained in terms of vacancy diffusion and annihilation at free surfaces or grain boundaries. Transmission electron microscopy investigations confirmed that no long-range dislocation structures have developed during fatigue loading of the films investigated.

Electromigration mechanisms in conductor lines : Void shape changes and slit-like failure

Abstract A detailed analysis of electromigration damage in unpassivated Al-based conductor lines was conducted. Damage observations revealed that slit-like voids in lines having a bamboo structure develop from equi-axed voids through a shape change driven by the “electron wind”. In order to simulate this behavior a numerical model was developed in which electromigration-driven diffusion on the void surfaces is assumed to act as the primary transport mechanism. This theoretical treatment considers the influence of the current density distribution and the temperature field in the vicinity of the void, while at the same time taking account of the finite line width as well as surface tension effects. In addition, the often observed facetting of voids is discussed on the basis of simulations in which anisotropic surface diffusivity was assumed. It will be shown that void shape changes and slit formation have a major influence on the performance of lines with bamboo structure and that the modeling has the potential of predicting their life times.