[Show abstract][Hide abstract] ABSTRACT: This article presents an experimental procedure to perform highly localized compression tests on nanoscale structures/features, such as nanospheres and nanopillars, via standard nanoindentation equipment. Current manufacturing capabilities, such as focused ion beam (FIB), lend themselves well to the creation of micron-spaced nanostructures, but it is fundamental to target an individual instance with little or no damage to the surrounding ones. The procedure successfully addresses the problem of locating and testing purposely designed nanostructures of order of 50 nm or less. The technique is illustrated for the case of closely spaced arrays of nanopillars, which were successfully manufactured, characterized, and tested through several pieces of equipment. For the purposes of compression, along with a traditional Berkovich tip, a new multifunctional (MF) tip was devised. This last tip is endowed with a complex contact geometry enabling both atomic force microscope (AFM) scanning and flat punch compression of the nanostructure. The levels of accuracy in tip positioning as well as robustness to alignment errors are unprecedented in comparison with previous in situ compression tests. As a consequence, the MF tip becomes a versatile tool that can be used beyond uniform compression. As an example, ancillary shear tests in controlled conditions are reported. Such results may lay the bases for metal-forming processes at the nanoscale, such as nanoforging or cutting operations, which are relevant to MEMS design and manufacturing.
Journal of Materials Research. 02/2009; 24(03):768 - 775.
[Show abstract][Hide abstract] ABSTRACT: The compressive plastic strength of nanometer-scale single-crystal metallic pillars is larger than that found in conventionally sized samples. This behavior is generally associated with a change in the length scale that determines plastic behavior and the consequent inability of nanoscale samples to store dislocations. Here, we show in the case of nanocrystalline nickel pillars, for which there is a fixed microstructural length scale set by the grain size, that smaller is still stronger and find that this behavior derives from statistical expec-tations that have long been used to understand the size-dependent strength of brittle solids such as glass.
[Show abstract][Hide abstract] ABSTRACT: Thin film stress evolution during physical vapor deposition is highly sensitive to the exact nature of the growth environment. It has been previously observed that during the growth of Cu films, stress evolution is acutely affected by oxygen partial pressure. However, changes in partial pressure imply a number of changes to the growth environment and to the condition of the growth surface. To specifically examine the role of adsorption on growth stresses, in this work we have grown Cu Volmer-Weber films, adsorbed a known quantity of oxygen, and continued growth once ultrahigh vacuum conditions were again achieved. This enabled the study of stress evolution as a function of adsorbate coverage, independent of background pressure. We found that even at low coverages, adsorbed oxygen has a profound impact on stress evolution. Additionally, we found that the adsorbed oxygen is consumed by the growing film over a significant thickness of growth and we have extracted the rate of oxygen consumption. We also observed an epitaxial stress associated with the continued growth of Cu on the underlying strained O-dosed Cu film, and show that the reversible stress is apparently unaffected by the oxygen adsorption step.
Journal of Applied Physics 03/2007; 101(5):054904-054904-4. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Significant effects of sample dimension on the yield strength of metallic crystals have been known for more than 50 years when researchers identified this phenomenon in metallic whiskers. These sample-size effects are once again attracting great interest with the discovery of the indentation size effect and the enhanced yield strength found for sub-micrometer diameter focused ion beam (FIB)-machined metallic pillars. Here, we discuss these issues and suggest mechanisms that may be responsible for the observed behaviors. In the case of FIB-machined pillars we draw an analogy between the yield strength of these structures and the fracture strength of glass rods and suggest that the experimentally observed yield behavior in these pillars is consistent with that expected from extreme value statistics. Additionally, we revisit the topic of surface effects in crystal plasticity and suggest a new mechanism via which a free surface could act as a measurable source of hardening for a crystal that has a bulk interior free of defects such as dislocations or grain boundaries. Finally we suggest experimental approaches that can be used to test the ideas discussed herein.
[Show abstract][Hide abstract] ABSTRACT: A Comment on the Letter by R. Koch, Dongzhi Hu, and A. K. Das, Phys. Rev. Lett. 94, 146101 (2005)PRLTAO0031-900710.1103/PhysRevLett.94.146101. The authors of the Letter offer a Reply.
[Show abstract][Hide abstract] ABSTRACT: Experimental results are presented for stress evolution, in vacuum and electrolyte, for the first monolayer of Cu on Au(111). In electrolyte the monolayer is pseudomorphic and the stress-thickness change is -0.60 N/m, while conventional epitaxy theory predicts a value of +7.76 N/m. In vacuum, the monolayer is incoherent with the underlying gold. Using a combination of first-principles based calculations and molecular dynamic simulations we analyzed these results and demonstrate that in electrolyte, overlayer coherency is maintained owing to anion adsorption.
[Show abstract][Hide abstract] ABSTRACT: Stress evolution during intermittent homoepitaxial growth of (111)-oriented Cu and Ag thin films has been studied. A tensile stress change is observed when growth is stopped, but the change is reversed when growth is resumed. Reflection high energy electron diffraction analysis of the atomic scale surface roughness during intermittent growth demonstrates a strong correlation between the surface structure and reversible stress evolution. The results are discussed in terms of an evolving surface defect population.
[Show abstract][Hide abstract] ABSTRACT: Stress evolution during the intermittent growth of Volmer-Weber Cu thin
films and homoepitaxial growth of (111)-oriented Cu and Ag thin films
has been studied. In all systems a tensile stress change is observed
when growth is stopped, but the change is reversed when growth is
resumed. In addition to direct experimental evidence, thermodynamic and
kinetic arguments are employed to show that the reversible stress
phenomenology is due to the entire ensemble of surface defects evolving
during the growth process. In the earliest stages of growth a direct
correlation is made between the stress evolution and an increase in the
adatom population. At longer timescales, when a more complex set of
defects are expected, reflection high energy electron diffraction
analysis of the atomic scale surface roughness is used to explain the
stress evolution phenomenology. These observations are correlated to
stress evolution in nanometer scale thin film islands to demonstrate the
importance of surface defect densities on stress in nanostructures.
[Show abstract][Hide abstract] ABSTRACT: From in situ stress measurements, we have observed that a large component of the precoalescence compressive stress that develops during Volmer-Weber growth of polycrystalline Cu films relaxes reversibly. This phenomenon is similar to the reversible stress relaxation previously observed in the postcoalescence regime. We have also observed that less than a tenth of a monolayer of deposition leads to an instantaneous stress of order 1 GPa. The stress changes in both the precoalescence and postcoalescence regimes of growth are explained by changes in the adatom population during and after deposition.
[Show abstract][Hide abstract] ABSTRACT: There are two fundamental excess thermodynamic parameters that characterize a surface, the surface free energy and the surface stress. The surface free energy is the reversible work per unit area to form new surface while maintaining a constant equilibrium density of surface atoms. The surface stress is the reversible work per unit area required to form new surface by elastic deformation of a preexisting surface, and thus the atom density is altered. For a fluid surface the surface free energy is equal to the surface stress, but for a solid this is in general not true. We develop thermodynamic arguments that describe proper interpretations of wafer curvature experiments that are typically used in electrocapillarity experiments of solid electrodes. Additionally, we consider stress evolution during underpotential deposition. The sources of stress relate to electrocapillarity differences between overlayer and substrate, interface stress, and coherency stress. Experimental results are presented for the systems Pb2+/Au(111), Pb2+/ Ag(111), and Ag+/Au(111). We show how it is possible to use the experimental data to extract results for the interface stresses in each of these systems. The following values of interface stress were determined: for the incommensurate Pb/Au(111) interface, 1.76 ± 0.04 N/m; for the incommensurate Pb/Ag(111) interface, 0.9 ± 0.04 N/m; and for the coherent Ag/Au(111) interface, −0.08 ± 0.04 N/m. Finally, we employ the thermodynamic arguments developed to consider two important problems in the electrocapillarity of solids. The first is a comparison of the magnitude of the change in surface free energy and surface stress that result from pure double − layer effects. The second is the potential-induced 23 × √3 (111) reconstruction that occurs on Au surfaces. Here, we calculate the difference in surface stress between the reconstructed and unreconstructed surface, obtaining −0.43 N/m, which compares favorably with recently published experimental results.