Chad S. Korach

Nanoscale Defect Generation in CMP of Low-k/Copper Interconnect Patterns

Chemical mechanical polishing CMP can lead to nanoscale damage in films and surface features due to normal and lateral deformations of the surface. Defects arise due to the synergistic effects of chemical and mechanical mechanisms. This occurrence increases with severity as feature sizes are on the same order as abrasive polishing particles and materials decrease in stiffness, as with advanced low-k dielectrics. Here, relationships describing the response of normal and lateral surface deformations have been developed experimentally and from contact modeling. This is achieved by atomic force microscopy nanoscratching with diamond tips in a KOH environment to simulate CMP processing conditions. The deformations are related to the applied normal load, friction coefficient, film properties, and line densities. The deformations are observed to have critical loads of 1 and 5 N associated with normal and lateral deformations, respectively, which are on the same order as actual CMP particle pressure estimates.

Effective property of tooth enamel: Monoclinic behavior

Human tooth enamel possesses a unique morphology characterized by a repeated cell arrangement, which is composed of varying orientations of hydroxyapatite crystals. In the past, various investigators have reported diverse mechanical properties based on isotropic or orthotropic mechanical models in their experimental and numerical studies. However, these models are insufficient to capture the accurate microstructural effects on the enamel mechanical response. In this paper, a monoclinic anisotropic model, which offers correct descriptions of enamel deformation behaviors, is introduced. The model takes into account the 3D orientation changes of the hydroxyapatite crystals and their spatial elastic property variations. The proposed approach is based on a unit-cell and periodic boundary conditions, and it utilizes the collective deformation characteristics of many rods to determine 13 independent material constants required for the monoclinic model. These constants are necessary to utilize the effective property model to study various mechanical conditions such as abrasion, erosion, wear and fracture of whole tooth enamel.

Mechanical Properties of Tooth Enamel: Microstructural Modeling and Characterization

Human tooth enamel possesses unique morphology characterized by repeated cell arrangement. Due to its complex structure, various investigators have reported diverse mechanical models and properties in their experimental and numerical studies. In this paper, the proper behavior described by the monoclinic model is reported and the effects of hydroxyapatite fibers and prism rods on the effective properties of tooth enamel are presented. The results are obtained from 3D finite element analysis with a novel procedure to construct periodic cell models and impose boundary conditions. This specialized approach allows determinations of 13 independent material constants needed for the monoclinic model. These constants may be used to construct homogenized models to study the mechanical behavior of entire tooth under abrasion, erosion, wear and fracture. In addition, a large scale 3D analysis was also performed to simulate instrumented micro-indentations of tooth enamel. The computed results are compared with experimentally obtained load-displacement measurements to verify the proposed model for the tooth enamel.

Experimental Study and Modeling of Lapping Using Abrasive Grits with Mixed Sizes

In this paper, the lapping process of wafer surfaces is studied with experiments and contact modeling of surface roughness. In order to improve the performance of the lapping processes, effects of mixed abrasive grits in the slurry of the free abrasive machining (FAM) process are studied using a single-sided wafer-lapping machine. Under the same slurry density, a parametric experimental study employing different mixing ratios of large and small abrasive grits and various normal loadings on the wafer surface applied through a jig is conducted. Observations and measurements of the total amount of material removed, material removal rate, surface roughness, and relative angular velocity are presented as a function of various mixing ratios and loadings and discussed in the paper. The experiments show that the 1:1 mixing ratio of abrasives removes more material than other mixing ratios under the same conditions, with a slightly higher surface roughness. Modeling of the mixed abrasive particle distributions correspondingly indicates that the roughness trend is due to the abrasive size distribution and the particle contact mechanics. The results of this study can provide a good reference to the FAM processes that practitioners use today by exploiting different abrasive mixing ratios in slurry and normal loadings in the manufacturing processes. [DOI: 10.1115/1.4004137]

Modeling the Tribochemical Aspects of Friction and Gradual Wear of DLC Films

Problems in nanomechanics often need to combine mechanical approaches together with methods of physics and chemistry that are outside of the traditional mechanics scope. Recent experimental studies of dry sliding between two hydrogenated DLC (diamond-like carbon) coated counterparts in low oxygen environment showed that adsorbates have considerable influence on friction and the friction coefficient increases with the increasing of the time interval between contacts. The observed friction phenomena are assumed caused by a reaction between the adsorbate and carbon atoms of the coatings, and when the slider passes a point on the track, it removes mechanically some adsorbate from the surface. The mechanical action leads to re-exposure of the surface to gases in the environment. This paper focuses on physical and tribochemical processes that occur in sliding contact between the DLC coated slider and the counterpart. We develop further our recently presented model of the process and assume that there is a transient short-life high temperature field at the vicinities of contacting protuberances that may cause various transformations of the surface. The model helps to explain how microscopic processes, such as the breaking and forming of interatomic bonds, may affect macroscopic phenomena, such as friction and wear.

Comparison of Sea Water Exposure Environments on the Properties of Carbon Fiber Vinylester Composites

Composites used in infrastructure and structural applications can be exposed to environmental conditions initiating degradation in the composite due to stress, UV radiation, moisture and chemical effects. Combined exposure of UV radiation and sea water creates synergistic degradation, and is generated from cyclic exposure to the individual conditions. Here, three separate exposure systems are used to age carbon fiber-reinforced vinylester composites: UV radiation, salt spray, and humidity environmental chambers; full sample immersion in salt and sea water conditions; and outdoor exposure in a tidal pond. Characterization of the time-dependent changes in the mechanical strength and modulus of the coupons is performed for each environment and IR spectroscopy is used to assess chemical changes in the vinylester matrix. Comparison between the conditions will be discussed in the context of long-term outdoor exposure with accelerated laboratory conditions.

Measurement of Structural Variations in Enamel Nanomechanical Properties using Quantitative Atomic Force Acoustic Microscopy

Quantitative Atomic force acoustic microscopy (AFAM) was used to measure the nanomechanical properties in human dental enamel associated with microstructural locations. AFAM measurements were made on samples from human molars at locations near the occlusal surface and the dentine enamel junction (DEJ). Within these locations, single point AFAM testing was performed on individual prism and sheath regions within the enamel microsctructure. Orientation changes in the enamel properties were observed by measurements in the perpendicular and parallel directions to enamel prisms. From quantitative AFAM, elastic modulus of the sample surfaces is calculated based on the measured cantilever frequency and probe tip geometry. Mean elastic modulus of the prismatic enamel was 109 ± 1 GPa and the enamel sheath was 96 ± 2 GPa, measured at the occlusal surface. Elastic modulus was found to decrease by 49% when measured in the direction normal to the prism parallel axis, and decrease by ~6.5% between the locations at the occlusal surface and those near the DEJ. The property variation of the prism and sheath is associated to the differences in the mineral to organic content, with the orientation differences due to the apatite crystal directions within the enamel microstructure.

Effects of Processing Conditions on Chitosan-Hydroxyapatite Biocomposite Mechanical Properties

Hydroxyapatite (HAp) based composites are used in numerous biomedical applications from bone pastes and cements to tissue scaffolds. Chitosan (Ch) is a ubiquitous biocompatible polymer derived from chitin which is soluble in acidic solutions and can be found in natural organisms such as shellfish. Past work has demonstrated that composites of HAP-Ch have high mechanical strength when processing conditions are optimized. Here, we present the effects of changes in the HAP/Ch mass fractions, the use of malic and citric acids on chitosan processing, and of nanoscale HAp particles on the composite strength and modulus. SEM is used to analyze the fracture surfaces and identify matrix-particle bonding morphology. Chemical variations are measured within the material structure by the use of FTIR and EXAFS and related to the composite processing changes.

Stresses between 3D fractures in infinite and layered elastic solids

Periodically distributed opening mode fractures are often found in layered sedimentary rocks. The stress analysis related to opening mode fractures in layered solids is solved by a new numerical approach combining (3D) fast Fourier transform with the theory of periodic eigenstrain and the conjugate gradient method. Results show that the fracture spacing to layer thickness ratio for embedded opening mode fractures, using a three-dimensional (3D) model, is in good comparison with that of the plane strain case (two-dimensional model). The critical value of the fracture spacing to layer thickness ratio increases for a stiff layer case and decreases for a stiff surrounding solid case. Out-of-plane fracture length is also studied as a parameter in the 3D modeling. Opening mode surface fractures in a layered half-space were also studied. The results show that a critical fracture saturation ratio exists for this case and occurs when the normal surface stress transitions from tensile to compressive. This stress state is shown to be caused by a bending effect in the layer.

Measurement of Epoxy Stiffness by Atomic Force Acoustic Microscopy

The mechanical characteristics of the epoxy matrix found in filler reinforced polymer composites is important for determining strength and performance. Locally, property variations in regions surrounding fillers can influence the overall macroscopic composite response to loading. We investigate local nanomechanical stiffness of reinforced epoxy composites by using atomic force acoustic microscopy. The effects of tip shape on the contact mechanics at the epoxy interface are found to influence the reported results significantly and will be discussed in context of different tip models. The results have direct correlation to the effect of near-filler interphase regions and the long-term influence of environmental conditions on the polymer composites.© 2009 ASME