Pad Scratching in Chemical-Mechanical Polishing: The Effects of Mechanical and Tribological Properties

In chemical-mechanical polishing (CMP), even the soft pad asperities may, under certain conditions, generate scratches on the relatively hard surfaces being polished. In the present study, contact mechanics models of pad-induced scratching are formulated, and the effects of the hardness of the surface layers and of pad asperities as well as the interfacial friction are elucidated. Additionally, scratch-regime maps are proposed to provide criteria for scratching hard surface layers by the softer pad asperities. Furthermore, scratching indexes are introduced to predict the proportion of asperities in contact that are likely to scratch. The contact mechanics models of scratching have been validated by sliding experiments with two commercial CMP pads (Pad A and IC1000) and various thin-films (Al, Cu, SiO[subscript 2], Si[subscript 3]N[subscript 4], TiN and three low-k dielectrics) using deionized water as a “lubricant.” Both the theoretical models and the experimental results show that the number of scratches increases as the scratching index exceeds 0.33. Al and Cu layers are found to be more susceptible to pad scratching due to their low hardness and high interfacial friction. The scratch-regime maps provide practical guidelines for mitigating pad scratching in CMP.Samsung (Firm

The Effect of Pad-asperity Curvature on Material Removal Rate in Chemical-mechanical Polishing

In chemical-mechanical polishing (CMP), surface asperities of the polishing pad play a key role, for they transmit normal force and impart tangential motion to the hard, nano-scale abrasive particles in the slurry. It has been shown recently, however, that the soft pad asperities themselves often generate micro-scale scratches on the surfaces being polished. To mitigate scratching by pad asperities, therefore, topography control by flattening pad asperities has been proposed and experimentally validated. In this study, the effects of asperity-flattening on pad topography and the material removal rate are investigated. It is found both theoretically and experimentally that even at a relatively high pressures only the tallest of the asperities are flattened and the ratio of asperity radius-to-standard deviation of heights is increased, but the average roughness itself is little affected. Specifically, surface profiles of new and asperity-flattened pads indeed show that the average roughness of about 5 μm is changed less than ten percent. Concurrently, the material removal rate is increased by about 30 percent due in part to the increased real area of contact––the result of increased asperity radius of curvature and decreased standard deviation of asperity heights.Samsung (Firm

Explaining Evaporation-Triggered Wetting Transition Using Local Force Balance Model and Contact Line-Fraction

Understanding wettability and mechanisms of wetting transition are important for design and engineering of superhydrophobic surfaces. There have been numerous studies on the design and fabrication of superhydrophobic and omniphobic surfaces and on the wetting transition mechanisms triggered by liquid evaporation. However, there is a lack of a universal method to examine wetting transition on rough surfaces. Here, we introduce force zones across the droplet base and use a local force balance model to explain wetting transition on engineered nanoporous microstructures, utilizing a critical force per unit length (FPL) value. For the first time, we provide a universal scale using the concept of the critical FPL value which enables comparison of various superhydrophobic surfaces in terms of preventing wetting transition during liquid evaporation. In addition, we establish the concept of contact line-fraction theoretically and experimentally by relating it to area-fraction, which clarifies various arguments about the validity of the Cassie-Baxter equation. We use the contact line-fraction model to explain the droplet contact angles, liquid evaporation modes, and depinning mechanism during liquid evaporation. Finally, we develop a model relating a droplet curvature to conventional beam deflection, providing a framework for engineering pressure stable superhydrophobic surfaces

Micro-scale scratching by soft pad asperities in chemical-mechanical polishing

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references.In the manufacture of integrated circuits (IC) and micro-electromechanical systems (MEMS), chemical-mechanical polishing (CMP) is widely used for providing local and global planarization. In the CMP process, polishing pads, typically made of polyurethanes, play a key role. Due to the random, rough surface of the pad, only the tall asperities contact the wafer and transmit the necessary down force and motion to the abrasive particles for material removal. As the applied pressure is concentrated under few asperities, however, the asperities themselves, even though softer, may generate unintended micro-scratches on relatively hard surfaces under certain conditions. This thesis investigates the effects of topographical, mechanical, and tribological properties of the pad and of the wafer surfaces on pad scratching in CMP. The generation and probability of scratching by soft pad asperities on hard monolithic layers are modeled. At single-asperity sliding contact, the asperity contact pressure along with the interfacial friction that can induce surface layer yielding are first derived, for different asperity deformation modes: elastic, elastic but at the onset of yielding, elastic-plastic, and fully-plastic. Under multi-asperity sliding contact, the probability of scratching asperities is determined taking into account the asperity height variation of the rough pad surface. The models are further advanced for scratching of patterned Cu/dielectric layers. As a result, the conditions for and probability of scratching are presented in terms of the asperity-to-layer hardness ratio, friction coefficient, asperity modulus-hardness ratio and ratio of asperity radius to standard deviation of asperity heights. The scratching models are validated by performing sliding experiments using solid polymer pins and CMP pads. For scratch mitigation, especially, a novel, cost-effective asperity-flattening method is introduced to control the pad topography, i.e., to increase the ratio of asperity radius to standard deviation of asperity heights. Finally, the role of asperities in material removal is studied based on contact mechanics and abrasive wear models. A new material removal rate model is developed in terms of pad surface properties, and polishing experiments are conducted on Cu to validate the theoretical prediction that the asperity-flattened pads not only reduce the pad scratching but also improve the material removal rate.by Sanha Kim.Ph. D

Korean Long-Tailed Gorals (Naemorhedus caudatus) are Rare but Successfully Reproducing in Remote Mountains

The long tailed goral (Naemorhedus caudatus) is an elusive species with a low and fragmented total population, mostly occurring in remote and elevated areas. In this study we used infrared automated trap cameras within the Seorak Mountain National Park, Republic of Korea. We documented the occurrence of long tailed gorals at the location of each trap camera, and also the presence of two young gorals within the National Park during the summer 2015. This is an important finding for the conservation of the species, emphasizing the success and benefits of large scale, non-fragmented, national parks for conservation

Ultrathin high-resolution flexographic printing using nanoporous stamps

Since its invention in ancient times, relief printing, commonly called flexography, has been used to mass-produce artifacts ranging from decorative graphics to printed media. Now, higher-resolution flexography is essential to manufacturing low-cost, large-area printed electronics. However, because of contact-mediated liquid instabilities and spreading, the resolution of flexographic printing using elastomeric stamps is limited to tens of micrometers. We introduce engineered nanoporous microstructures, comprising polymer-coated aligned carbon nanotubes (CNTs), as a next-generation stamp material. We design and engineer the highly porous microstructures to be wetted by colloidal inks and to transfer a thin layer to a target substrate upon brief contact. We demonstrate printing of diverse micrometer-scale patterns of a variety of functional nanoparticle inks, including Ag, ZnO, WO[subscript 3], and CdSe/ZnS, onto both rigid and compliant substrates. The printed patterns have highly uniform nanoscale thickness (5 to 50 nm) and match the stamp features with high fidelity (edge roughness, ~0.2 μm). We derive conditions for uniform printing based on nanoscale contact mechanics, characterize printed Ag lines and transparent conductors, and achieve continuous printing at a speed of 0.2 m/s. The latter represents a combination of resolution and throughput that far surpasses industrial printing technologies.Massachusetts Institute of Technology. Department of Mechanical EngineeringNational Science Foundation (U.S.) (Grant CMMI-1463181)United States. Air Force Office of Scientific Research. Young Investigator Program (Grant FA9550-11-1-0089)National Institutes of Health (U.S.) (Grant 1R21HL114011-01A1

Enantioselective Synthesis of Homoisoflavanones by Asymmetric Transfer Hydrogenation and Their Biological Evaluation for Antiangiogenic Activity

Neovascular eye diseases are a major cause of blindness. Excessive angiogenesis is a feature of several conditions, including wet age-related macular degeneration, proliferative diabetic retinopathy, and retinopathy of prematurity. Development of novel anti-angiogenic small molecules for the treatment of neovascular eye disease is essential to provide new therapeutic leads for these diseases. We have previously reported the therapeutic potential of anti-angiogenic homoisoflavanone derivatives with efficacy in retinal and choroidal neovascularization models, although these are racemic compounds due to the C3-stereogenic center in the molecules. This work presents asymmetric synthesis and structural determination of anti-angiogenic homoisoflavanones and pharmacological characterization of the stereoisomers. We describe an enantioselective synthesis of homoisoflavanones by virtue of ruthenium-catalyzed asymmetric transfer hydrogenation accompanying dynamic kinetic resolution, providing a basis for the further development of these compounds into novel experimental therapeutics for neovascular eye diseases

Tropical Data: Approach and Methodology as Applied to Trachoma Prevalence Surveys

PURPOSE: Population-based prevalence surveys are essential for decision-making on interventions to achieve trachoma elimination as a public health problem. This paper outlines the methodologies of Tropical Data, which supports work to undertake those surveys. METHODS: Tropical Data is a consortium of partners that supports health ministries worldwide to conduct globally standardised prevalence surveys that conform to World Health Organization recommendations. Founding principles are health ministry ownership, partnership and collaboration, and quality assurance and quality control at every step of the survey process. Support covers survey planning, survey design, training, electronic data collection and fieldwork, and data management, analysis and dissemination. Methods are adapted to meet local context and needs. Customisations, operational research and integration of other diseases into routine trachoma surveys have also been supported. RESULTS: Between 29th February 2016 and 24th April 2023, 3373 trachoma surveys across 50 countries have been supported, resulting in 10,818,502 people being examined for trachoma. CONCLUSION: This health ministry-led, standardised approach, with support from the start to the end of the survey process, has helped all trachoma elimination stakeholders to know where interventions are needed, where interventions can be stopped, and when elimination as a public health problem has been achieved. Flexibility to meet specific country contexts, adaptation to changes in global guidance and adjustments in response to user feedback have facilitated innovation in evidence-based methodologies, and supported health ministries to strive for global disease control targets

Additive manufacturing of flexible 3D surface electrodes for electrostatic adhesion control and smart robotic gripping

Abstract Mechanically flexible surface structures with embedded conductive electrodes are attractive in contact-based devices, such as those used in reversible dry/adhesion and tactile sensing. Geometrical shapes of the surface structures strongly determine the contact behavior and therefore the resulting adhesion and sensing functionalities; however, available features are often restricted by fabrication techniques. Here, we additively manufacture elastomeric structure arrays with diverse angles, shapes, and sizes; this is followed by integration of conductive nanowire electrodes. The fabricated flexible three-dimensional (3D) surface electrodes are mechanically compliant and electrically conductive, providing multifunctional ability to sense touch and to switch adhesion via a combined effect of shear- and electro adhesives. We designed soft, anisotropic flexible structures to mimic the gecko’s reversible adhesion, which is governed by van der Waals forces; we integrated nanowires to further manipulate the localized electric field among the adjacent flexible 3D surface electrodes to provide additional means to digitally tune the electrostatic attraction at the contact interface. In addition, the composite surface can sense the contact force via capacitive sensing. Using our flexible 3D surface electrodes, we demonstrate a complete soft gripper that can grasp diverse convex objects, including metal, ceramic, and plastic products, as well as fresh fruits, and that exhibits 72% greater electroadhesive gripping force when voltage is applied

Functional equations in matrix normed modules

In this paper, we prove the Hyers-Ulam stability of the Cauchy additive functional equation in matrix normed modules over a C*-algebra