Pulsed laser cleaning relies on the characteristics of the light pulses generated by the laser, based on a photophysical reaction caused by the interaction between a high-intensity beam, short-pulse laser, and the contaminant layer. Its physical principle can be summarized as follows:
(a) The laser beam emitted by the laser is absorbed by the contaminant layer on the surface to be treated.
(b) The absorption of high energy forms a rapidly expanding plasma (a highly ionized, unstable gas), generating a shock wave.
(c) The shock wave fragments and removes the contaminants.
(d) The pulse width must be short enough to avoid heat buildup that could damage the treated surface.
(e) Experiments show that plasma is generated on metal surfaces when oxides are present.
Plasma is generated only when the energy density is above a threshold, which depends on the level of contaminant or oxide being removed. This threshold effect is crucial for effective cleaning while ensuring the safety of the substrate material. There is also a second threshold for plasma generation. If the energy density exceeds this threshold, the substrate material will be damaged. To ensure effective cleaning while maintaining the safety of the substrate material, the laser parameters must be adjusted to keep the energy density of the light pulse strictly between these two thresholds. Each laser pulse removes a contamination layer of a certain thickness. If the contamination layer is thick, multiple pulses are required for cleaning. The number of pulses needed to clean the surface depends on the degree of surface contamination. An important consequence of the two thresholds is the self-control of the cleaning process. Light pulses with energy densities higher than the first threshold will continue to remove contaminants until they reach the substrate material. However, because their energy densities are below the substrate material's damage threshold, the substrate will not be damaged.