For cleaning technology, our first thought is to use a variety of cleaning agents and cleaning tools in our lives. However, the traditional cleaning technology without exception will produce different degrees of wear and damage to the cleaning object. With the advancement of science and technology and the ultimate pursuit of precision, the concept of cleaning is no longer limited to simple cleaning such as "washing dishes". People continue to expand the scope of cleaning objects, and the standards for cleaning requirements are also constantly improving. Fragile cultural relics can not withstand polishing, smooth metal surfaces need extreme maintenance, tiny devices need perfect cleaning methods, laser cleaning technology came into being. As early as 1965, Nobel Prize winner Xiao Luo used a pulsed laser to irradiate a piece of paper printed with ink, the ink font on the paper quickly vaporized, and the paper itself was not damaged, and the ink on the paper was successfully "erased". This opens the door to pulsed laser cleaning technology. In 1973, the Asms team first reported the use of lasers to clean artifacts; In 1974, Fox used Q-switched neodymium glass lasers to effectively remove paint layers on plexiglass and metal substrates; In 1982, Zapka and others at IBM's German manufacturing Technology Center irradiated the mask with a focused laser and successfully washed away the particulate contaminants attached to the mask. After more than 40 years of development, laser cleaning technology has made great progress and development.

Principle and mechanism of laser cleaning

Laser cleaning is an advanced cleaning technology that uses a high energy laser beam to irradiate the surface of an object and rapidly evaporate or peel off impurities, pollutants or coatings through optical and thermal effects.

The core component of laser cleaning technology is a pulsed laser with large pulse energy, high average power and peak power. As we all know, laser is a light source with high brightness, high consistency and high orientation. Pulsed laser is a high energy laser beam released in a very short time, with a high peak power and instantaneous power density. Compared with continuous laser, high-power pulsed laser can generate high temperature in an instant, but due to the very short time, the heat is too late to be transmitted to the surrounding material, which greatly reduces the thermal impact of the laser on the substrate material. The high power pulsed laser can also realize the precise control of the laser cleaning process by adjusting the pulse energy and frequency. This adjustability can be tailored to different cleaning needs, ensuring adaptation to different materials and application scenarios. When the laser beam shines on the surface being cleaned, the laser energy is absorbed and has a strong thermal effect on the pollutant in a very short period of time. This thermal effect causes the surface temperature of the contaminant or coating to rise, causing it to evaporate, decompose, or peel off. At the same time, the high energy density of the pulsed laser allows it to directly penetrate certain materials without damaging the substrate surface, making the cleaning process more efficient.

Due to the complex composition and structure of the cleaning materials, the mechanisms of laser action on them are numerous. Therefore, laser cleaning is not just a simple high-energy ablation, but also involves decomposition, ionization, degradation, melting, burning, vaporization, vibration, splashing, expansion, contraction, explosion, peeling, and detachment, etc. Physical and chemical changes. Therefore, the process of pulsed laser cleaning is a complex physical and chemical process involving optics, thermodynamics, and mechanics. As a non-mechanical surface pretreatment method, the laser beam can act on the sample surface according to the set scanning method, allowing the laser to interact fully with the surface dirt, rust layer, or coating. After the surface material absorbs the laser energy, the laser energy is converted into the thermal energy, chemical energy, and mechanical energy needed for cleaning. Currently, the main theoretical explanations for pulsed laser cleaning are the laser ablation mechanism theory and the thermal elastic expansion peeling mechanism theory.

(1) Laser ablation mechanism

The thermal ablation mechanism of pulsed laser cleaning is closely related to the laser power density. In the ablative mechanism, high energy density laser beams are produced due to the high power pulsed laser can release a large amount of energy in a very short period of time. This allows the laser beam to be focused on a small area for a short period of time, enabling rapid heating and evaporation of contaminants or coatings on the target surface. When the energy of the laser is enough to destroy the chemical bonds of the surface substances, the chemical bonds vibrate, bend, and even break, making the molecules decompose, and the surface pollutants are photodecomposed. When the power density of laser cleaning is greater than 10^8 W/cm^2, the contaminated layer on the surface of the material may undergo plastic deformation and produce explosive rebound stress after absorbing the energy of the laser. When the power density of laser cleaning is greater than 10^9 W/cm^2, the contaminated layer on the surface of the material absorbs the high-energy laser and produces gasification or the plasma generated by the optical breakdown forms a plasma explosion shock wave, and these explosive effects will accelerate the separation of pollutants from the substrate surface.

(2) thermoelastic expansion stripping mechanism

It includes thermoelastic vibration, steam pressure, photoinduced pressure, phase explosion, shock wave and so on. When the laser is irradiated on the surface of the material, the base material and the material to be cleaned first undergo thermal expansion. The release stress generated by this thermoelastic expansion will first remove part of the surface material, which is the thermal vibration mechanism. In the vibration mechanism, the thermal effect of the laser will also increase the temperature of the pollutant and the substrate, but because the laser energy used is much lower than the laser energy in the ablative mechanism, the pollutant will not be directly ablated, but there will be mechanical fracture, vibration breakage and other phenomena. Contaminants are removed by jet or stripped off the substrate surface. The pulsed laser can also ionize the air around the particles on the surface of the contaminant or substrate material, forming a plasma shock wave to remove the surface contaminant. In wet laser cleaning, a liquid film (water, ethanol or other liquid) is pre-covered on the surface of the cleaning object and then irradiated with a laser. The liquid film absorbs the laser energy, resulting in a strong explosion of the liquid medium. The explosive boiling liquid moves at high speed, transferring the energy to the surface to be cleaned, and removing the surface dirt with the help of high transient explosive force to achieve the purpose of cleaning.

Typical application of laser cleaning

For more than 40 years, laser cleaning, as a new and efficient environmental cleaning technology, has been rapidly developed, and has been widely used in the fields of electronic component cleaning and paint and rust removal.

(1) Laser cleaning electronic components

In the development of the semiconductor industry, the cleaning of the pollution particles on the surface of the silicon wafer mask has always been a big problem. Traditional chemical cleaning will cause great pollution, and mechanical cleaning and ultrasonic cleaning methods can not achieve the required cleaning effect. With the development of science and technology, semiconductor and microelectronics equipment is getting smaller and smaller, the size of the particles that need to be cleaned is getting smaller and smaller, and the cleaning difficulty is getting greater and greater, and the emergence of laser cleaning technology provides a new solution to this problem, and related research and applications have been rapidly developed.

Due to the fragile surface of the electronic components and the frequent coating of the device surface, traditional laser ablation cleaning has the risk of damaging the device. To solve this problem, the scientists used a new type of highly efficient cleaning technology. This technology uses a high-intensity laser, focusing through a convergent lens, to induce the breakdown of air into a high-temperature and high-density laser plasma. As the resulting plasma expands rapidly, it compresses the surrounding air, creating a powerful plasma shock wave. In this process, the mechanical effect of the high-intensity shock wave enables the nanoparticles to overcome the adhesion with the substrate surface, so that the particles are quickly "flushed" away, achieving an efficient cleaning of the surface particles. Different from the traditional method, the laser plasma shock wave produces spherical plasma shock wave through the air medium in the process of laser irradiation, which only acts on the surface of the matrix to be washed without affecting the matrix itself, thus avoiding the damage to the device. It is encouraging that the entire cleaning process does not need to introduce the assistance of chemical reagents, effectively avoiding negative harm to the natural environment. This cleaning technology performs well for nanoparticle contamination, a common problem in microelectronic substrates, and provides a feasible, efficient and environmentally friendly way to solve this problem.

(2) Laser rust removal

Laser rust removal is an important application of laser cleaning technology, which uses pulsed laser with peak power to irradiate the rust layer. In this process, the laser energy is absorbed, causing the temperature of the rust layer to rise sharply, triggering changes such as expansion, thermal shock and phase transition, and finally effectively removing the rust layer. Compared with the traditional rust removal process, laser rust removal has a series of significant advantages. First of all, laser rust removal is a non-mechanical contact process that will not cause mechanical damage to the surface of the workpiece, thus protecting the integrity of the workpiece. Its equipment has a high degree of integration, flexible operation, easy to achieve automatic control, improve production efficiency and ease of operation. The good directivity of laser technology enables the rust removal process to achieve accurate positioning, which is suitable for dealing with complex surfaces and improves the accuracy of cleaning. In addition, the laser rust removal process produces lower noise and no dust pollution, helping to create a cleaner working environment. In general, laser rust removal technology shows many advantages in the process of rust removal, such as high efficiency, precision and environmental protection, providing advanced solutions for the field of industrial cleaning. This innovative technology not only improves traditional cleaning methods, but also provides a more sustainable and environmentally friendly option for industrial production.

One of the main mechanisms of laser rust removal is to achieve the removal of rust layer by vaporizing the material heated by the laser beam. However, for the rust layer generated by the oxidation of the iron substrate, due to its loose porous surface, the thickness of tens to hundreds of microns, the gasification depth of the pulsed laser is relatively limited. Therefore, the removal mechanism of laser rust removal is not a single gasification ablation, but also involves other cleaning mechanisms, such as plasma shock wave and phase explosion. This means that in addition to removing the rust layer through gasification, the laser will also produce a strong plasma shock wave, as well as phase explosion and other effects, which further synergies on the rust layer to ensure a more comprehensive and thorough cleaning effect.

With the continuous development of laser cleaning technology, I believe it can bring more innovation and convenience to the cleaning industry. In the future, we are expected to witness laser cleaning technology in various fields to bring greater benefits to the production process, while making a more positive contribution to environmental protection. Laser cleaning has become the bright choice of cleaning technology, leading us into a new era in the field of cleaning.