Single Asperity Chemical Mechanical Wear Studied by Atomic Force Microscopy
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Single Asperity Chemical Mechanical Wear Studied by Atomic Force Microscopy Forrest Stevens, Steve Langford, and J. Thomas Dickinson Physics Department, Washington State University Pullman, WA 99164-2814 ABSTRACT Nanometer scale wear caused by a single asperity of silicon nitride was examined by measuring the wear caused by atomic force microscope tips translated against sodium trisilicate glass, soda-lime glass, or fused silica in aqueous solutions. As a function of contact force, FN, and scan duration, t, the wear to both tip and substrate scales approximately as (FNt)0.5. Substrate wear was independent of temperature from 5˚C to 60˚C, whereas tip wear showed a temperature dependence on soda-lime glass corresponding to an activation energy of 60 kJ/mol on soda-lime glass. INTRODUCTION Wear of silicates and other hard surfaces is important for polishing optics, planarization of oxide layers on silicon wafers, creating parts of silicon nitride and other ceramics, and for micromachining. Chemical Mechanical Polishing/Planarization (CMP) has traditionally been accomplished by moving the object to be polished across a flat surface or pad in the presence of a polishing material such as cerium oxide, and a polishing fluid, commonly water. The precise nature and detailed mechanisms of the polishing process is a continual area of study. The technique of atomic force microscopy (AFM) uses a sharpened tip to probe a surface with high spatial resolution. This offers the opportunity to study polishing and wear at very small scales in a simplified system involving a single abrasive particle (the AFM tip), which can slide across the surface in the presence of controlled chemical solutions.1,2 Typically, low contact forces are used to minimize damage to the tip and the substrate. However, by operating at higher forces, an AFM can be used to measure the effect of high-force scanning on both the tip and the substrate. In the AFM geometry, the tip serves as an idealized single asperity interacting with a nearly flat substrate with a precisely controlled normal force, sliding speed and direction. With care, the same tip can be used to image the change in the substrate and/or the tip itself can be imaged to examine how it is being modified. EXPERIMENTAL Atomic force microscopy images were acquired with a Molecular Imaging PicoScan AFM with fluid cell, hot stage, and Peltier cold stage. Commercial CVD silicon nitride cantilevers were obtained from Digital Instruments. These cantilevers have a nominal force constant of 0.58 Nm-1. Wear was induced by scanning a small area (~500-3000 nm) at high force, and the effect of high-force scanning could be determined by taking a larger scan at low force. Each wear measurement required a new AFM tip and a previously unscanned portion of the substrate. The tip shape before and after each wear experiment was characterized by scanning a silicon substrate which had been etched to form sharp spikes ~600 nm tall with a tip radius of curvature of < 10 nm (MikroMasch, TGT01). Because the spikes ar
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