Basic knowledge of laser cleaning technology

Laser cleaning is to use the characteristics of laser beams such as large energy density, controllable direction and strong convergence ability, so that the bonding force between the pollutants and the substrate is destroyed or the pollutants are directly vaporized to decontaminate and reduce pollutants. The bonding strength with the substrate, and then achieve the effect of cleaning the surface of the workpiece.

There are generally four methods of laser cleaning:

(1) Laser dry cleaning method, which uses pulsed laser direct radiation to decontaminate;

(2) The method of laser + liquid film, that is, first deposit a layer of liquid film on the surface of the substrate, and then use laser to decontaminate the liquid film by explosion;

(3) The method of laser + inert gas, that is, while the laser is irradiating, the inert gas is blown to the surface of the workpiece. When the dirt is peeled off from the surface, it is blown away from the surface by the gas to avoid re-contamination and oxidation of the clean surface;

(4) Use laser to loosen the dirt, and then use non-corrosive chemical methods to decontaminate.

At present, the first three cleaning methods are mainly used in industrial production, among which the laser dry cleaning method and the laser + liquid film cleaning method are widely used; the fourth method is only found in the cleaning and protection of artworks.

(1) It is a “dry” cleaning that does not require cleaning fluid or other chemical solutions, and its cleanliness is much higher than that of chemical cleaning processes;

(2) The range of dirt removal and the range of applicable substrates are very wide;

(3) By adjusting the laser process parameters, the pollutants can be effectively removed without damaging the surface of the substrate, so that the surface can be restored as new;

(4) Laser cleaning can be easily automated;

(5) Laser decontamination equipment can be used for a long time, and the operating cost is low;

(6) Laser cleaning technology is a “green” cleaning process. The waste eliminated is in the form of solid powder, small in size, easy to store, and basically does not pollute the environment.

When using laser for cleaning, the choice of laser is related to the subsequent cleaning work, so the choice of laser is extremely important. At present, there are many types of lasers, such as CO2 lasers, Nd:YAG lasers and excimer lasers, etc. These types of lasers can realize laser cleaning, but there are still rules to follow in the selection of lasers. Excimer lasers are the more appropriate tool when volume clearance or cost are more than required; in other cases CO2 and solid-state lasers are preferred. For the microelectronics industry, excimer lasers are generally used, such as KrF excimer; for mold cleaning, it is better to use CO2 or YAG lasers; for the removal of laser coatings, the effect of CO2 lasers is good, especially TEA CO2 Lasers are very promising in this field; since CO2 lasers cannot be transmitted by optical fibers, remote cleaning applications are greatly limited, which provides a broad space for the application of YAG lasers. In addition, if the composition of surface pollutants is known in advance, the type of laser can be selected according to the best absorption wavelength of the pollutants.

The mechanism of laser cleaning is mainly based on the fact that after the pollutants on the surface of the object absorb the laser energy, they either vaporize and volatilize, or expand instantaneously to overcome the adsorption force of the surface to the particles, so that they are separated from the surface of the object, and then the purpose of cleaning is achieved. At present, there are some differences in the mechanism of laser cleaning, but most of the mechanisms can reasonably explain some phenomena in laser cleaning experiments, which generally include laser vaporization decomposition, laser stripping, thermal expansion of dirt particles, substrate surface vibration and Particles are in four aspects of vibration; and laser cleaning is often the result of multiple mechanisms acting at the same time.

Based on the analysis of a large number of experimental results, it is believed that the laser cleaning mechanism is different according to the difference between the surface attachment and the thermal physical parameters of the substrate. When the thermophysical parameters of the surface attachment and the base material are quite different, the laser cleaning mechanism includes: ablation vaporization, thermal vibration and thermal shock mechanism, and sonic vibration mechanism, such as laser cleaning paint and rubber layers. When the thermophysical parameters of the surface attachment and the base material are not much different, the main mechanism of ablation vaporization is at work, such as laser rust removal.