Cleaning force in laser cleaning of silicon substrates

Abstract

A laser cleaning model was established for removal of non-absorbing particles from an absorbing solid surface by taking adhesion force and cleaning force into account. The cleaning force per unit area due to laser-induced thermal expansion of a substrate surface is γE AT(0, t), where γ, E, and δT(0, t) are the linear thermal expansion coefficient, the elastic modulus and temperature rise at the substrate surface, respectively. The cleaning condition and threshold fluence can be obtained by comparing the cleaning force and the adhesion force. The theoretical analysis shows that cleaning force increases with increasing laser fluence, deducing the pulse duration, or decreasing laser wavelength, which leads to a higher cleaning efficiency at higher laser fluence, smaller pulse duration or shorter laser wavelength. The experimental results show that the cleaning threshold fluence for laser removal of quartz particles from silicon surfaces is about 135 mJ/ cm2, which is in good consistency with the theoretical threshold fluence of 120 mJ/cm2. With increasing laser fluence, the cleaning efficiency increases, which has been predicted by our theoretical analysis.