Capillary Forces Studied with Atomic Force Microscopy
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1025-B12-07
Capillary Forces Studied with Atomic Force Microscopy Sahar Maghsoudy-Louyeh, and Bernhard R. Tittmann Department of Engineering Science and Mechanics, The Pennsylvania State University, 212 Earth & Engineering Sciences Building, University Park, PA, 16802 ABSTRACT The deposition of films and coatings is sometimes influenced by the presence of small amounts of moisture, which can affect the nucleation and growth processes. It is important to understand the behavior of coating materials--especially in semiconductors--in terms of hydrophilicity/hydrophobicity along with adhesion forces. Our technical approach centers on the use of the atomic force microscope (AFM) which was found to be a reliable tool for studying the surface characteristics of materials. In addition to obtaining topographic information, the AFM can also probe attractive or repulsive forces between the tip and the sample surfaces. In this research, a systematic study of the influence of humidity on the adhesion forces between different AFM tips (silicon and silicon nitride) and both hydrophilic and hydrophobic materials (quartz, calcite, mica, graphite) has been conducted using atomic force microscopy. Several force-distance curves measured by the M5 AFM have been gathered at a series of different humidity levels and different locations on the samples. In this paper, measurements of the adhesion force for hydrophobic and hydrophilic materials versus humidity are presented. The results show that the adhesion force on graphite which has hydrophobic character is independent of humidity variation. Results also show that the adhesion force for fused quartz, mica, and calcite which are hydrophilic materials, change dramatically with increasing humidity due to capillary forces. This is in good agreement with theoretical calculations. TECHNICAL APPROACH Experiments Force vs. distance curves typically show the deflection of the free end of the cantilever as the fixed end of the cantilever is brought vertically towards and then away from the sample surface. This technique can be used to analyze surface contamination, viscosity, lubrication thickness, local variation of the mechanic properties of the surface such as adhesion and elasticity. Generally speaking, the force-distance curve is a plot of the deflection of the cantilever versus the extension of the piezoelectric scanner, measured with a position sensitive detector [1]. At the left side of the curve, the scanner is fully retracted and the cantilever is undeflected since the tip is not in contact with the sample. As the scanner extends, the cantilever remains undeflected until it comes close to the sample surface and the tip experiences the attractive van der Waals force. The tip snaps into the surface (point d) and the cantilever suddenly bends slightly to the surface. As the scanner continues to extend, the cantilever deflects away from the surface (region b). After full extension, at the top right of the plot, the scanner begins to retract. The cantilever deflection retraces the same curve
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