Reinhold H. Dauskardt

Adhesion and debonding of multi-layer thin film structures

Abstract A fracture mechanics technique to quantitatively measure the adhesion or interfacial fracture resistance of interfaces in thin film structures is described. Adhesion values obtained for the technologically important SiO2/TiN interface in microelectronic interconnect structures are related to a range of material, mechanical and design parameters which include interface morphology and adjacent ductile layer thickness. In addition, the interface was shown to be susceptible to environmentally-assisted subcritical debonding similar to stress corrosion cracking of SiO2 glass in moist air environments. Subcritical debonding behavior was sensitive to a range of material and design parameters, and is expected to have important implications for long term device reliability.

Mean stress effects on flow localization and failure in a bulk metallic glass

Abstract The effect of stress state on strain localization and subsequent failure of a bulk metallic glass alloy is examined. It is shown that failure is associated with a critical tensile mean stress of 0.95 GPa. This is in contrast with previous work utilizing superimposed compressive mean stresses, which found that failure resulted at a critical effective stress. Interestingly, the critical tensile mean stress measured in this study causes the same dilatation as a 274 K temperature increase, nearly to the glass transition temperature. The effect of mean stress on elastic variation of the average free volume is added to a strain localization model. This model describes the compressive mean stress behavior very well, and predicts a strong sensitivity to tensile mean stresses.

Plasticity contributions to interface adhesion in thin-film interconnect structures

The effects of plasticity in thin copper layers on the interface fracture resistance in thin-film interconnect structures were explored using experiments and multiscale simulations. Particular attention was given to the relationship between the intrinsic work of adhesion, G o , and the measured macroscopic fracture energy, G c . Specifically, the TaN/SiO 2 interface fracture energy was measured in thin-film Cu/TaN/SiO 2 structures in which the Cu layer was varied over a wide range of thickness. A continuum/FEM model with cohesive surface elements was employed to calculate the macroscopic fracture energy of the layered structure. Published yield properties together with a plastic flow model for the metal layers were used to predict the plasticity contribution to interface fracture resistance where the film thickness (0.25–2.5 μm) dominated deformation behavior. For thicker metal layers, a transition region was identified in which the plastic deformation and associated plastic energy contributions to G c were no longer dominated by the film thickness. The effects of other salient interface parameters including peak cohesive stress and G o are explored.

Local heating associated with crack tip plasticity in Zr–Ti–Ni–Cu–Be bulk amorphous metals

Deformation in metallic glasses is generally considered to arise from flow in localized shear bands, where adiabatic heating is thought to reduce glass viscosity. Evidence has been inferred from the veined fracture surfaces and molten droplets reported for metallic glasses. In this work, the detailed spatially resolved surface temperature increase and subsequent dissipation associated with crack tip plasticity in a Zr-Ti-Ni-Cu-Be bulk metallic glass is characterized for the first time. Maximum temperatures of up to 54.2 K were estimated from a heat conduction model and shown to be in excellent agreement with a non-hardening plasticity model for the heat generated by a propagating crack. Local cooling was also observed and shown to be consistent with thermoelastic effects.

Adhesion and reliability of copper interconnects with Ta and TaN barrier layers

With the advent of copper metallization in interconnect structures, new barrier layers are required to prevent copper diffusion into adjacent dielectrics and the underlying silicon. The barrier must also provide adequate adhesion to both the dielectric and copper. While Ta and TaN barrier layers have been incorporated for these purposes in copper metallization schemes, little quantitative data exist on their adhesive properties. In this study, the critical interface fracture energy and the subcritical debonding behavior of ion-metal-plasma sputtered Ta and TaN barrier layers in Cu interconnect structures were investigated. Specifically, the effects of interfacial chemistry, Cu layer thickness, and oxide type were examined. Behavior is rationalized in terms of relevant reactions at the barrier/dielectric interface and plasticity in adjacent metal layers. (c) 2000 Materials Research Society.

Enhanced Toughness Due to Stable Crack Tip Damage Zones in Bulk Metallic Glass

Bulk metallic glass alloys exhibit an impressive range of mechanical properties including large elastic strains to failure ({epsilon}{sub el} {approximately} 2{degree}) and high tensile strengths ({approximately}2 GPa). This paper describes the fracture behavior of a Zr{sub 41.25}Ti{sub 13.75}Ni{sub 10}Cu{sub 12.5}Be{sub 22.5} bulk metallic glass observed using a single edge notched fracture sample loaded in tension (SEN(T)). Fracture toughness values in excess of 130 MPa{radical}m were found. These high values are associated with significant crack tip plastic deformation and blunting. Localized shear bands and branched cracks form an energy dissipating damage zone at the crack tip. Surprisingly, the extent of the damage zone is less than that expected for a polycrystalline metallic material with similar yield strength and toughness.

Characterization of Free Volume in a Bulk Metallic Glass Using Positron Annihilation Spectroscopy

The free volume of metallic glasses has a significant effect on atomic relaxation processes, although a detailed understanding of the nature and distribution of free volume sites is currently lacking. Positron annihilation spectroscopy was employed to study free volume in a Zr-Ti-Ni-Cu-Be bulk metallic glass following plastic straining and cathodic charging with atomic hydrogen. Multiple techniques were used to show that strained samples had more open volume, while moderate hydrogen charging resulted in a free volume decrease. It was also shown that the free volume is associated with zirconium and titanium at the expense of nickel, copper, and beryllium. Plastic straining led to a slight chemical reordering.

On the interpretation of the fractal character of fracture surfaces

Abstract To examine the usefulness of the fractal concept in quantitative fractography, a series of classical fracture surfaces, namely transgranular cleavage, intergranular fracture, microvoid coalescence, quasicleavage and intergranular microvoid coalescence, are analyzed in terms of fractal geometry. Specifically, the five brittle and ductile fracture modes are studied, from three well characterized steels (a mild steel, a low-alloy steel and a 32 wt% Mn-steel) where the salient microstructural dimensions contributing to the final fracture morphology have been measured. Resulting plots of the mean angular deviation, and Richardson (fractal) plots of the lineal roughness, as a function of the measuring step size, are interpreted with the aid of computer-simulated fracture-surface profiles with known characteristics. It is found that the ranges of resolution, over which the fractal dimension is constant, correspond to the pertinent metallurgical dimensions on the fracture surface, and thus can be related to microstructural size-scales.

Improving cutaneous scar formation by controlling the mechanical environment: large animal and phase I studies.

Objective: To test the hypothesis that the mechanical environment of cutaneous wounds can control scar formation. Background: Mechanical forces have been recognized to modulate myriad biologic processes, but the role of physical force in scar formation remains unclear. Furthermore, the therapeutic benefits of offloading cutaneous wounds with a device have not been rigorously tested. Methods: A mechanomodulating polymer device was utilized to manipulate the mechanical environment of closed cutaneous wounds in red Duroc swine. After 8 weeks, wounds subjected to different mechanical stress states underwent immunohistochemical analysis for fibrotic markers. In a phase I clinical study, 9 human patients undergoing elective abdominal surgery were treated postoperatively with a stress-shielding polymer on one side whereas the other side was treated as standard of care. Professional photographs were taken between 8 and 12 months postsurgery and evaluated using a visual analog scale by lay and professional panels. This study is registered with ClinicalTrials.gov, number NCT00766727. Results: Stress shielding of swine incisions reduced histologic scar area by 6- and 9-fold compared to control and elevated stress states, respectively (P < 0.01 for both) and dramatically decreased the histologic expression of profibrotic markers. Closure of high-tension wounds induced human-like scar formation in the red Duroc, a phenotype effectively mitigated with stress shielding of wounds. In the study on humans, stress shielding of abdominal incisions significantly improved scar appearance (P = 0.004) compared with within-patient controls. Conclusions: These results indicate that mechanical manipulation of the wound environment with a dynamic stress-shielding polymer device can significantly reduce scar formation.

An Artificial Solid Electrolyte Interphase with High Li-Ion Conductivity, Mechanical Strength, and Flexibility for Stable Lithium Metal Anodes

An artificial solid electrolyte interphase (SEI) is demonstrated for the efficient and safe operation of a lithium metal anode. Composed of lithium-ion-conducting inorganic nanoparticles within a flexible polymer binder matrix, the rationally designed artificial SEI not only mechanically suppresses lithium dendrite formation but also promotes homogeneous lithium-ion flux, significantly enhancing the efficiency and cycle life of the lithium metal anode.