Theoretical Modeling for Laser Cleaning of Micro-Particles from Solid Surface
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Mat. Res. Soc. Symp. Proc. Vol. 501 ©1998 Materials Research Society
filtering of the liquid at the submicron level. Secondly, the bulk usage of hazardous chemicals and solvents such as trichloroethylene and CFC (carbon fluorochloride) becomes undesirable for environmental and industrial reasons. Other problems associated with the wet techniques are rinsing/drying difficulties and incompatibility with other processes. Hence, dry cleaning techniques such as laser cleaning have emerged in order to overcome these drawbacks.
Table 1.1 Cleaning methods for removal of particles Cleaning methods
Lower limit of diameter of particles removed (pm)
Wiping
5
Brush scrubbing
0.5
Ultrasonic cleaning Etching High-pressure jet spraying (4000 psi) High-pressure jet spraying (10000 psi) High-pressure jet spraying (15000 psi) Megasonic cleaning Laser cleaning
0.5 0.5 0.5 0.3 0.2 0.2 0.1
LASER CLEANING Recently, two types of laser cleaning technique have been reported in the literature,[6-13] relying on pulsed laser heating of the surface without or with the presence of a thin liquid coating. They are referred as "dry laser cleaning" and "steam laser cleaning," respectively. In dry laser cleaning, particles can be ejected from the contaminated surfaces by short-pulse laser irradiation. The proposed mechanism of the ejection is fast thermal expansion of the particle and/or surface. Depending on the wavelength, laser energy is strongly absorbed by the substrate only, or by the particulate only. Substrate Absorption and Particulate Absorption For a transparent particle to the laser irradiation, the most of the laser energy is being absorbed by the substrate. The ejection phenomenon is keyed at the sudden expansion of the substrate surface as a result of the absorption of the laser energy. Tam et. al. found that particle acceleration ( - 1010 cm/s 2 ) caused by laser irradiation is very much larger than the gravitational acceleration. [6] This huge substrate acceleration results in the particle removal from solid surface. Their study qualitatively explained that higher fluence, shorter wavelength and smaller pulsed duration can all result in higher cleaning efficiency. However, further study is required to quantitatively model the laser cleaning process such as determining the cleaning threshold fluence.
400
For a relatively transparent substrate, Kelly et al. came out the average acceleration of particles based on the particulate absorption.[13] Similarly to the case of substrate absorption, rapid thermal expansion of the particle causes the particle to eject from the relatively transparent substrate. The acceleration depends on several parameters such as the peak intensity, absorption coefficient, thermal expansion coefficient and specific heat of the particle. This paper presents a theoretical model developed in our study, which not only can explain the effects of fluence, wavelength, pulsed duration, incident direction and angle, repetition rate, particle properties and sizes, and substrate properties in dry laser cleani
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