Scanning Force Microscope Studies of Detachment of Nanometer Adhering Particulates
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humidity on crack growth in soda lime glass has been well studied (Ref. 4 and references therein). Stress is applied to the particles with the tip of a scanning force microscope (SFM), which may be compared to a well-characterized asperity in polishing processes. SFM tips can be instrumented to monitor the normal and lateral forces as the tip encounters the particle. Similar microscopies have been previously employed to study particle adhesion 5 7 in a number of model systems, often involving metal particles on semiconductor substrates. In this work, we show that small amounts of water vapor dramatically lower the lateral force required to fracture the salt-glass bond as the SFM tip is drawn across the particle. We model this decrease in terms of the effect of water vapor on the interfacial surface energy. Particle size also affects the interfacial shear strength, presumably due to variations in the size of interfacial flaws relative to the total interface area. EXPERIMENT Submicron-sized NaCI crystals were deposited on soda lime glass substrates by dissolving a few grains (- 1 mm 3) of commercial salt in a drop of de-ionized water on a clean microscope slide. The solution was spread across the slide with a cotton swab and allowed to evaporate to dryness at ambient humidity and temperature. Both evaporation and sample storage were under ambient laboratory atmosphere conditions-typically 20-40% relative humidity (RH). Particle observation and manipulation were performed on the stage of a Digital Instruments (Santa Barbara, CA) Nanoscope III scanning force microscope mounted in a controlled environment chamber. The humidity was adjusted by introducing a controlled mixture of dry and humidified air. The RH in the chamber was continuously monitored with a BioForce 273 Mat. Res. Soc. Symp. Proc. Vol. 566 ©2000 Materials Research Society
Laboratory humidity sensor. Humidity variations in the course of an experiment were controlled to + 1% absolute RH, with an estimated uncertainty of± 2% absolute RH. This work employed triangular, 115-jim long, "wide" Si 3N 4 cantilevers from Digital Instruments of Santa Barbara, CA. Particle detachment experiments were initiated by adjusting the RH in the experimental chamber and scanning the surface in the contact mode to find particles of the appropriate size. High scan rates (typically 42 gim/s) and low applied normal forces (5-20 nN) during this stage of the experiment were employed to minimize the effect of imaging on particle adhesion. When an appropriate particle was found, the tip was positioned for a linear scan that would intersect the NaCl particle at right angles, as close as possible to the center of an edge. At the beginning of the linear scan, the scan speed was reduced to 0.20 jim/s and the normal force increased (up to 320 nN). Particles of an appropriate size for a given humidity would detach from the substrate upon the tip's first encounter with the particle. Subsequent large area, low contact force scans over the region surrounding the initial particle position wer
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