Cynthia A. Volkert

Size effects in the deformation of sub-micron Au columns

Uniaxial compression tests have been performed on single crystal Au columns ranging in diameter from 180 nm to 8 µm. The columns were machined into the surface of a large-grained Au sheet using a focused ion beam microscope and then mechanically tested using a nanoindenter outfitted with a flat diamond punch. Images of the compressed columns show that deformation occurs by localized shear on the slip systems with the largest resolved shear stresses. After an elastic loading regime, the columns exhibit yielding in discrete strain bursts. The compressive yield stress scales roughly as the inverse square root of the column diameter. The apparent strain hardening rate also increases strongly with decreasing column diameter and stresses as large as 1 GPa are reached. Both of these size effects are attributed to dislocation source-limited behaviour in small volumes.

Effect of sample size on deformation in amorphous metals

Uniaxial compression tests were performed on micron-sized columns of amorphous PdSi to investigate the effect of sample size on deformation behavior. Cylindrical columns with diameters between 8μm and 140nm were fabricated from sputtered amorphous Pd77Si23 films on Si substrates by focused ion beam machining and compression tests were performed with a nanoindenter outfitted with a flat diamond punch. The columns exhibited elastic behavior until they yielded by either shear band formation on a plane at 50° to the loading axis or by homogenous deformation. Shear band formation occurred only in columns with diameters larger than 400nm. The change in deformation mechanism from shear band formation to homogeneous deformation with decreasing column size is attributed to a required critical strained volume for shear band formation.

Stress and plastic flow in silicon during amorphization by ion bombardment

In situ wafer curvature measurements were performed during amorphization of silicon by MeV ion implantation. These measurements provide information about density changes and plastic phenomena in the implanted region. Experiments were performed for a variety of ions, a range of fluxes, and for temperatures between −175 and 200 °C. In all cases, the implanted region expanded due to the creation of damaged crystal, creating compressive stress in the implanted region on the order of 108 N/m2. Once heavily damaged or amorphous regions were formed, radiation‐enhanced plastic flow of material out of the plane of the wafer occurred in order to relieve the stress created by the expansion. The value of the shear viscosity responsible for this phenomena could be measured by comparing samples with the same history but different stresses. For 2‐MeV Xe implantation at room temperature and 1011 ions/cm2 s, the radiation‐enhanced shear viscosity is ∼ 1013 Ns/m2, which is at least four orders of magnitude smaller than the thermally activated shear viscosity. Possible contributions to flow from a homogeneous distribution of broken bonds and from fluid‐like collision cascade regions are discussed.

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.

Approaching the theoretical strength in nanoporous Au

The mechanical properties of nanoporous Au have been investigated by uniaxial compression. Micron-sized columns were machined in the surface of nanoporous Au using a focused Ga+ beam and compressed with a flat punch in a nanoindenter. Using scaling laws for foams, the yield strength of the 15nm diameter ligaments is estimated to be 1.5GPa, close to the theoretical strength of Au. This value agrees well with extrapolations of the yield strength of submicron, fully dense gold columns and shows that in addition to foam density and structure, the absolute size of ligaments and cell walls can be used to tailor foam properties.The mechanical properties of nanoporous Au have been investigated by uniaxial compression. Micron-sized columns were machined in the surface of nanoporous Au using a focused Ga+ beam and compressed with a flat punch in a nanoindenter. Using scaling laws for foams, the yield strength of the 15nm diameter ligaments is estimated to be 1.5GPa, close to the theoretical strength of Au. This value agrees well with extrapolations of the yield strength of submicron, fully dense gold columns and shows that in addition to foam density and structure, the absolute size of ligaments and cell walls can be used to tailor foam properties.

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.

Defect structure in micropillars using X-ray microdiffraction

White beam x-ray microdiffraction is used to investigate the microstructure of micron-sized Si, Au, and Al pillars fabricated by focused ion beam (FIB) machining. Comparison with a Laue pattern obtained from a Si pillar made by reactive ion etching reveals that the FIB damages the Si structure. The Laue reflections obtained from the metallic pillars fabricated by FIB show continuous and discontinuous streakings, demonstrating the presence of strain gradients.

Densification, anisotropic deformation, and plastic flow of SiO2 during MeV heavy ion irradiation

The response of SiO2 thin films and implantation masks to 4.0 MeV Xe irradiation is studied. Trenches in silica deform dramatically after irradiation with 3×1015 ions/cm2. In situ wafer curvature measurements show that thin planar silica films first densify by 3.6% during irradiation. The resulting stress then relaxes viscously by radiation‐enhanced Newtonian flow. At a flux of 3×1010 Xe ions/cm2s the measured shear viscosity was 6×1013Pa s. We find evidence that an irradiation induced anisotropic deformation mechanism is present in the silica films. In equilibrium, this deformation leads to an average compressive saturation stress as large as 4.5×107 Pa.

Damage behavior of 200-nm thin copper films under cyclic loading

Fatigue damage in 200-nm-thick Cu films was investigated and compared with the damage in thicker Cu films. The fatigued 200-nm-thick Cu films exhibited only a few, small extrusions and extensive cracking along twin and grain boundaries, whereas the thicker films showed many extrusions/intrusions and cracks lying along the extrusions rather than along the boundaries. This change in fatigue damage behavior with film thickness is attributed to the inhibition of dislocation mobility and the limited availability and activation of dislocation sources on the small length scale. It is argued that the decrease in film thickness and grain size inhibits the localized accumulation of plastic strain within grains, such as at extrusions/intrusions and in extended dislocation structures, and promotes the formation of damage such as cracks at twin and grain boundaries during fatigue. This effect is suggested as the likely cause for the increase in fatigue life with decreasing specimen dimensions.

Thermal fatigue testing of thin metal films

An experimental method is described for performing thermal fatigue testing of thin films and lines on substrates. The method uses Joule heating from alternating currents to generate temperature, strain, and stress cycles in the metal structures. The apparatus has been installed in a scanning electron microscope and allows in situ observations of the fatigue damage evolution. First observations on Cu films reveal that fatigue damage forms in submicrometer thick films and is strongly affected by the film thickness and grain size. In addition, results from a special test structure confirm that the damage is caused by fatigue and not by electromigration.