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The Influence of Humidity on the Mechanisms Involved in Laser-Induced Particle Ejection Experiments

We report on experimental investigations on the role of humidity in the laser cleaning technique which simply consists of direct irradiation of the contaminated surfaces. Submicrometer particles are ejected from silicon substrates by single nanosecond laser pulses. The measurements of particle removal efficiency (PRE), laser energy removal threshold, and ejection velocities as a function of the amount of residual moisture trapped on the surface were performed. For the laser parameters we used, we demonstrate that the particle removal results from mechanisms based on the momentum transfer from laser ablated species to the particles. For high laser fluences, a self-limited local ablation of the surface provides the cleaning force. For lower laser fluences, a competition between two humidity-based forces is evidenced: the capillary adhesion force and the explosive evaporation (of water traces) cleaning force. This demonstrates that the humidity plays a major role in the so-called dry laser cleaning technique which aims at meeting the future surface preparation requirements of the emerging nanotechnologies.