Showing papers by "Zhigang Suo published in 2006"

Mechanisms of reversible stretchability of thin metal films on elastomeric substrates

Abstract: Gold films on an elastomeric substrate can be stretched and relaxed reversibly by tens of percent. The films initially form in two different structures, one continuous and the other containing tribranched microcracks. We have identified the mechanism of elastic stretchability in the films with microcracks. The metal, which is much stiffer than the elastomer, forms a percolating network. To accommodate the large elongation of the elastomeric substrate, the metal network twists and deflects out of the plane but remains bonded to the soft substrate. Consequently, the metal film experiences only small strains and deforms elastically without suffering fatigue.

Stiff subcircuit islands of diamondlike carbon for stretchable electronics

Abstract: Stretchable electronics on elastomeric substrates requires fragile and brittle device materials to be placed on stiff, mechanical distinct subcircuit islands. We deposited a diamondlike carbon (DLC) film at room temperature on a silicone substrate by pulsed laser ablation, and patterned the film into an array of 200×200μm2 islands. When the substrate was uniaxially stretched to a strain of 25%, the islands remained adherent to the substrate and only deformed by ∼5%, while the exposed substrate stretched by more than 30%. A row of 11 DLC islands interconnected with gold stretchable metallization maintained end-to-end electrical conduction during a mechanical cycle to 20% tensile strain. This demonstration of electrically interconnected stiff islands on silicone illustrates two important steps toward fully integrated, elastically stretchable electronics.

Kink formation and motion in carbon nanotubes at high temperatures.

Abstract: We report that kink motion is a universal plastic deformation mode in all carbon nanotubes when being tensile loaded at high temperatures. The kink motion, observed inside a high-resolution transmission electron microscope, is reminiscent of dislocation motion in crystalline materials: namely, it dissociates and multiplies. The kinks are nucleated from vacancy creation and aggregation, and propagate in either a longitudinal or a spiral path along the nanotube walls. The kink motion is related to dislocation glide and climb influenced by external stress and high temperatures in carbon nanotubes.

Thermomechanical criteria for overlay alignment in flexible thin-film electronic circuits

Abstract: A simple mechanical model for a deposited film∕substrate couple is presented to describe how film deposition at an elevated temperature induces change in the substrate’s in-plane dimensions at room temperature. The model provides a quantitative guideline for reducing, or completely eliminating, this elongation, by tailoring the tensile built-in stress in the deposited film. The dimensional stability so achieved is necessary for accurate overlay alignment of photomasks during the fabrication of thin-film electronic circuits.

Method to analyze dislocation injection from sharp features in strained silicon structures

Abstract: A microelectronic device usually contains sharp features (e.g., edges and corners) that may intensify stresses, inject dislocations into silicon, and fail the device. The authors describe a method to analyze dislocation injection on the basis of singular stress fields near the sharp features, and apply the method to interpret available experiments of nitride pads on silicon substrates.

Methodology for avoidance of ratcheting-induced stable cracking (RISC) in microelectronic devices

Abstract: A typical microelectronic device contains a silicon die in a package encapsulated with an epoxy. This polymer and various inorganic materials on the die, such as metal interconnects and ceramic passivation films, have dissimilar coefficients of thermal expansion (CTEs). When such a device is subject to a change in temperature, the mismatch in the CTEs deforms the materials. In particular, as temperature cycles, the plastic deformation in a metal interconnect may accumulate incrementally, a phenomenon known as ratcheting plastic deformation. Ratcheting in the metal film may induce large stresses in an overlaying ceramic film, causing cracks to initiate and grow stably cycle by cycle. In this paper, such ratcheting-induced stable cracking (RISC) is studied using a simplified three-layer model. We describe conditions under which ratcheting will occur in the metal layer, predict the number of cycles for the crack to initiate in the ceramic film, and discuss strategies to avoid RISC in design.

Mechanics of a process to assemble microspheres on a patterned electrode

Abstract: A process has been demonstrated recently to assemble microspheres on a patterned electrode under the influence of an applied voltage. Here we examine the mechanics of this process, and describe both the conditions under which excess microspheres jump off the electrode when the voltage is applied, and the forces that attract the remaining microspheres to the desired positions. A quantitative mechanistic understanding of this process rationalizes experimental observations, provides scaling relations, and suggests modifications of the process.

Method of Fabricating Multiple-Frequency Film Bulk Acoustic Resonators in a Single Chip

Abstract: We report experimental results of a novel approach to integrate multiple-frequency film bulk acoustic resonators (FBAR) in a single chip. An additional tuning layer was deposited and patterned on a conventional Metal/AlN/Metal FBAR film stack. By controlling in-plane dimensions of the periodic tuning patterns, resonance frequencies are modulated according to the corresponding loading percentages. To obtain a desirable frequency response of the modulated resonance peaks (a pure frequency shift), the pitch of the tuning patterns needs to be smaller than the membrane thickness. Three thicknesses of the tuning layers are fabricated to demonstrate different tuning ranges and sensitivities. This approach provides a potential solution to integrate multiple-frequency FBAR filters of adjacent bands by only one additional lithographic patterning step.

Interplay between elastic interactions and kinetic processes in stepped Si (001) homoepitaxy

Abstract: A vicinal Si (001) surface may form stripes of terraces, separated by monatomic-layer-high steps of two kinds, ${S}_{\mathrm{A}}$ and ${S}_{\mathrm{B}}$. As adatoms diffuse on the terraces and attach to or detach from the steps, the steps move. In equilibrium, the steps are equally spaced due to elastic interaction. During deposition, however, ${S}_{\mathrm{A}}$ is less mobile than ${S}_{\mathrm{B}}$. We model the interplay between the elastic and kinetic effects that drives step motion, and show that during homoepitaxy all the steps may move in a steady state, such that alternating terraces have time-independent, but unequal, widths. The ratio between the widths of neighboring terraces is tunable by the deposition flux and substrate temperature. We study the stability of the steady-state mode of growth using both linear perturbation analysis and numerical simulations. We elucidate the delicate roles played by the standard Ehrlich-Schwoebel (ES) barriers and inverse ES barriers in influencing growth stability in the complex system containing $({S}_{\mathrm{A}}+{S}_{\mathrm{B}})$ step pairs.

Reliability Study of Interconnect Structures in IC Packages

Abstract: This paper will focus on reliability study of interconnect structure for two areas. The first area is reliability of interconnect structure in thermal cycling test. Major work includes the fundamental study of ratcheting for passivation cracking. By combination with the upper limit of stress intensity factor for a finite crack in the passivation and the ratcheting failure mechanism, the life of passivation in thermal cycling can be obtained in terms of the lower bound of critical number of cycles of the crack initiation. The passivation crack criterion is established to identify the failure modes: no cracking and delayed cracking. Another area is the reliability of bond paid over active (BPOA) with copper bond pads. Major work includes stress analysis of the dielectric layer under probing with different parameters such as the thickness of the copper bond pad, dielectric, metallization and the passivation under different probing loads. The elastic plastic model in copper bond pad and metal lines are introduced. Finally comparison of the results between copper bond pad and aluminum bond pad will be presented

Electric Field-Directed Patterning of Molecules on a Solid Surface

Abstract: Adsorbed on a solid surface, a molecule can migrate and carry an electric dipole moment. A nonuniform electric field can direct the motion of the molecule. A collection of the same molecules may aggregate into a monolayer island on the solid surface. Place such molecules on a dielectric substrate surface, beneath which an array of electrodes is buried. By varying the voltages of the electrodes individually, it is possible to program molecular patterning, direct an island to move in a desired trajectory, or merge several islands into a larger one. This article develops a phase field model to simulate the molecular motion and patterning under the combined actions of dipole moments, intermolecular forces, entropy, and electrodes.