Pranav Shrotriya

On the measurement of the plasticity length scale parameter in LIGA nickel foils

This paper presents measurements of the plasticity length scale for polycrystalline LIGA nickel foils produced by electroplating in a sulfamate bath. The micro-bend test method developed by Stolken and Evans [Acta Mater. 46 (1998) 5109] is used to obtain a composite length scale parameter, lc that is primarily associated with rotational gradients. The micro-bend test utilizes the measurement of the curvature change associated with elastic spring-back of bent micro-beams in the extraction of a composite length scale parameter, lc. The length scale is measured to be ∼5.6 μm for foils between 25 and 175 μm thick. This is in the range (3–5 μm) reported from prior micro-bend experiments on nickel foils (loc. cit.). The measured length scale is also similar to values reported previously for polycrystalline copper wires using torsion tests in which rotation gradients dominate.

Surface topography evolution and fatigue fracture in polysilicon MEMS structures

This paper presents the results of an experimental study of the micromechanisms of surface topography evolution and fatigue fracture in polysilicon MEMS structures. The initial stages of fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO/sub 2/ layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are characterized as functions of fatigue cycling before discussing the micromechanisms of fatigue fracture.

Aptamer Functionalized Microcantilever Sensors for Cocaine Detection

A cocaine-specific aptamer was used as a receptor molecule in a microcantilever-based surface stress sensor for detection of cocaine molecules. An interferometric technique that relies on measuring differential displacement between two microcantilevers (a sensing/reference pair) was utilized to measure the cocaine/aptamer binding induced surface stress changes. Sensing experiments were performed for different concentrations of cocaine from 25 to 500 μM in order to determine the sensor response as a function of cocaine concentration. In the lower concentration range from 25 to 100 μM, surface stress values increased proportionally to coverage of aptamer/cocaine complexes from 11 to 26 mN/m. However, as the cocaine concentration was increased beyond 100 μM, the surface stress values demonstrated a weaker dependence on the affinity complex surface coverage. On the basis of a sensitivity of 3 mN/m for the surface stress measurement, the lowest detectable threshold for the cocaine concentration is estimated to be 5 μM. Sensing cantilevers could be regenerated and reused because of reversible thermal denaturation of aptamer.

Nanoindentation study of plasticity length scale effects in LIGA Ni microelectromechanical systems structures

This paper presents the results of a nanoindentation study of the effects of strain gradient plasticity on the elastic-plastic deformation of lithographie, galvanoformung, abformung (LIGA) Ni microelectromechanical systems (MEMS) structures plated from sulfamate baths. Both Berkovich and North Star/cube corner indenter tips were used in the study to investigate possible effects of residual indentation depth on the hardness of LIGA Ni MEMS structures between the micro- and nanoscales. A microstructural length scale parameter, l = 2.2 μm, was determined for LIGA nickel films. This is shown to be consistent with a stretch gradient length-scale parameter, l s , of approximately 0.9 μm.

On the evolution of surface morphology of polysilicon MEMS structures during fatigue

This paper presents the results of a combined experimental and computational study of surface topology evolution preceding fatigue crack nucleation in polysilicon MEMS structures. The evolution in surface topology observed during the crack nucleation stage is related to the underlying notch-tip stress distributions calculated using the finite element method. Measured changes in surface topography due to the stress-assisted dissolution of silica are shown to be predicted by a linear stability analysis. The implications of the results are discussed for modeling of fatigue in polysilicon MEMS structures.

Iterative control approach to high-speed force-distance curve measurement using AFM: time-dependent response of PDMS example.

Force-distance curve measurements using atomic force microscope (AFM) has been widely used in a broad range of areas. However, currently force-curve measurements are hampered the its low speed of AFM. In this article, a novel inversion-based iterative control technique is proposed to dramatically increase the speed of force-curve measurements. Experimental results are presented to show that by using the proposed control technique, the speed of force-curve measurements can be increased by over 80 times--with no loss of spatial resolution--on a commercial AFM platform and with a standard cantilever. High-speed force curve measurements using this control technique are utilized to quantitatively study the time-dependent elastic modulus of poly(dimethylsiloxane) (PDMS). The force-curves employ a broad spectrum of push-in (load) rates, spanning two-order differences. The elastic modulus measured at low-speed compares well with the value obtained from dynamic mechanical analysis (DMA) test, and the value of the elastic modulus increases as the push-in rate increases, signifying that a faster external deformation rate transitions the viscoelastic response of PDMS from that of a rubbery material toward a glassy one.

Broadband measurement of rate-dependent viscoelasticity at nanoscale using scanning probe microscope: Poly(dimethylsiloxane) example

A control approach to achieve nanoscale broadband viscoelastic measurement using scanning probe microscope (SPM) is reported. Current SPM-based force measurement is too slow to measure rate-dependent phenomena, and large (temporal) measurement errors can be generated when the sample itself changes rapidly. The recently developed model-less inversion-based iterative control technique is used to eliminate the dynamics and hysteresis effects of the SPM hardware on the measurements, enabling rapid excitation and measurement of rate-dependent material properties. The approach is illustrated by the mechanical characterization of poly(dimethylsiloxane) over a broad frequency range of three orders of magnitude (∼1 Hz to 4.5 KHz).

Fatigue damage evolution in silicon films for micromechanical applications

In this paper we examine the conditions for surface topography evolution and crack growth/fracture during the cyclic actuation of polysilicon microelectromechanical systems (MEMS) structures. The surface topography evolution that occurs during cyclic fatigue is shown to be stressassisted and may be predicted by linear perturbation analyses. The conditions for crack growth (due to pre-existing or nucleated cracks) are also examined within the framework of linear elastic fracture mechanics. Within this framework, we consider pre-existing cracks in the topical SiO2 layer that forms on the Si substrate in the absence of passivation. The thickening of the SiO2 that is normally observed during cyclic actuation of Si MEMS structures is shown to increase the possibility of stable crack growth by stress corrosion cracking prior to the onset of unstable crack growth in the SiO2 and Si layers. Finally, the implications of the results are discussed for the prediction of fatigue damage in silicon MEMS structures.

Three-dimensional viscoelastic simulation of woven composite substrates for multilayer circuit boards

Viscoelastic properties of woven composite substrates are essential to design dimensionally stable multilayer printed circuit boards. Unlike most existing numerical work which rely on simplified constitutive (elastic) and geometrical models, this study involves a fully three-dimensional viscoelastic model of a plain weave composite with accurate characterization of the woven geometry. Comparisons between numerical predictions and experimental data clearly indicate that the creep compliance of the composite depends not only on the relaxation of the matrix, but also on the time-dependent flexural deformations of the woven fabric bundles. Predictions of the inhomogeneous deformation fields over the repeating cell agree with experimental observations.

Contact damage in model dental multilayers: an investigation of the influence of indenter size.

This paper presents a combined experimental and computational study of the influence of indenter ball size on contact damage in model multilayered structures with equivalent elastic properties to bonded dentin/crown structures. Following a brief description of restored tooth structures, prior work on the development of model dental multilayered structures is reviewed. The effects of indentation ball size are investigated within a combined experimental and computational/analytical framework. The observed cracking patterns at the onset of crack nucleation are shown to be associated with principal stress contours computed using finite element analysis. The implications of the results are discussed for the design of dental multilayers that are more resistant to crack nucleation and propagation.