Laser Cleaning – The black technology of surface purification

With the advancement of science and technology and the pursuit of extreme precision, the concept of cleaning is no longer limited to simple cleaning such as “washing dishes”. People’s scope of cleaning objects is constantly expanding, and the standards for cleaning requirements are constantly improving. As early as 1965, Nobel Prize winner Schawlow used a pulsed laser to irradiate a piece of paper with ink printed on it. The ink font on the paper quickly vaporized, and the paper itself was not damaged. The ink on the paper was successfully “erased”. Since then, the door to pulsed laser cleaning technology has been opened. In 1973, the Asmus team first reported the use of laser to clean cultural relics; in 1974, Fox used Q-switched neodymium glass laser to effectively remove the paint layer on resin glass and metal substrates; in 1982, Zapka and others from IBM’s German Manufacturing Technology Center used focused lasers to irradiate the mask and successfully cleaned the particulate contaminants attached to the mask. After more than 40 years of development, laser cleaning technology has made great progress and progress.

LASER CLEANING MACHINE RL-CPB

Principle and mechanism of laser cleaning

Laser cleaning is an advanced cleaning technology that uses high-energy laser beams to irradiate the surface of an object, and quickly evaporates or peels off impurities, contaminants 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 high peak power. As we all know, laser is a light source with high brightness, high consistency and high directionality. Pulsed lasers release high-energy laser beams in a very short time, with high peak power and instantaneous power density. Compared with continuous lasers, high-power pulsed lasers can generate high temperatures in an instant, but due to the extremely short time, the heat has no time to be transmitted to the surrounding materials, thereby greatly reducing the thermal impact of the laser on the substrate material. High-power pulsed lasers can also achieve precise control of the laser cleaning process by adjusting the pulse energy and frequency. This controllability can be customized according to different cleaning needs to ensure that it is suitable for different materials and application scenarios. When the laser beam is irradiated on the surface to be cleaned, the laser energy is absorbed and produces a strong thermal effect on the contaminant in a very short 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 pulsed laser allows it to directly penetrate certain materials without damaging the substrate surface, making the cleaning process more efficient.
Due to the complex and diverse composition and structure of the cleaning object, the mechanism of laser action on it is diverse. Therefore, laser cleaning is not just a simple high-energy ablation, but also involves decomposition, ionization, degradation, melting, combustion, gasification, vibration, splashing, expansion, contraction, explosion, peeling, and shedding. Therefore, the process of pulsed laser cleaning is a complex optical, thermal, mechanical and other comprehensive physical and chemical change process. Laser cleaning is a non-mechanical contact surface pretreatment method. The laser beam can act on the surface of the sample in a set scanning mode, so that the laser can fully interact with the surface dirt, rust layer or coating. After the surface material absorbs the energy of the laser, the laser energy is converted into the required thermal energy, chemical energy and mechanical energy for cleaning. At present, there are two main theories about the mechanism of pulsed laser cleaning: laser ablation mechanism and thermoelastic expansion peeling mechanism.

LASER CLEANING MACHINE RL-CW

(1) Laser ablation mechanism

The thermal action ablation mechanism in the pulsed laser cleaning process is closely related to the laser power density. In the ablation mechanism, since high-power pulsed lasers can release a large amount of energy in a very short time, a high-energy-density laser beam is generated. This allows the laser beam to be concentrated in a small area in a short time, and can quickly heat and evaporate the contaminants or coatings on the target surface. When the energy of the laser is sufficient to destroy the chemical bonds of the surface material, the chemical bonds vibrate, bend, or even break, causing the molecules to decompose and the surface contaminants to be decomposed by light. When the power density of laser cleaning is greater than 10^8 W/cm^2, the contamination layer on the surface of the material may undergo plastic deformation after absorbing the energy of the laser and produce explosive rebound stress; when the power density of laser cleaning is greater than 10^9 W/cm^2, the contamination layer on the surface of the material absorbs high-energy lasers and produces gasification or generates plasma due to optical breakdown to form plasma explosion shock waves. These explosive effects will accelerate the separation of contaminants from the substrate surface.

(2) Thermoelastic expansion peeling mechanism
It includes thermoelastic vibration, vapor pressure, photoinduced pressure, phase explosion, shock waves, etc. When the laser is irradiated on the surface of the material, both the base material and the object to be cleaned undergo thermal expansion first. The detachment stress generated by this thermoelastic expansion will first remove some 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 contaminant and the substrate, but because the laser energy used is much lower than the laser energy in the ablation mechanism, the contaminant will not be directly ablated, but will be mechanically fractured, vibrated and broken. The contaminant is removed or peeled off the substrate surface in a jetting manner. The pulsed laser can also ionize the air around the contaminant or the surface particles of the substrate material to form a plasma shock wave to remove the surface contaminants. In wet laser cleaning, a liquid film (water, ethanol or other liquid) is pre-covered on the surface of the object to be cleaned, and then irradiated with a laser. The liquid film absorbs the laser energy, causing the liquid medium to explode strongly. The boiling liquid of the explosion moves at high speed, transferring energy to the surface of the object to be cleaned, and using the high transient explosive force to remove surface dirt to achieve the purpose of cleaning.

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

(1) Laser cleaning of electronic components
During the development of the semiconductor industry, cleaning of contaminated particles on the surface of silicon wafer masks has always been a major problem. Traditional chemical cleaning causes great pollution, while mechanical cleaning and ultrasonic cleaning methods cannot achieve the desired cleaning effect. With the development of science and technology, semiconductors and microelectronic devices are getting smaller and smaller, and the size of particles that need to be cleaned is also getting smaller and smaller, making cleaning more and more difficult. The emergence of laser cleaning technology has provided a new solution to this problem, and related research and applications have developed rapidly.
Due to the fragility of the surface of electronic components and the frequent coating on the surface of devices, traditional laser ablation cleaning has the risk of damaging the device. To solve this problem, scientists have adopted a new type of efficient cleaning technology. This technology uses high-intensity lasers, which are focused by converging lenses to induce air breakdown to form high-temperature and high-density laser plasma. As the generated plasma rapidly expands to the surroundings, it compresses the surrounding air and forms a strong plasma shock wave. In this process, the mechanical effect of the high-intensity shock wave enables the nanoparticles to overcome the adhesion to the substrate surface, thereby quickly “washing” the particles away, achieving efficient cleaning of surface particles. Unlike traditional methods, laser plasma shock waves generate spherical plasma shock waves by breaking through the air medium during laser irradiation. They only act on the surface of the substrate to be cleaned without affecting the substrate itself, thus avoiding damage to the device. Encouragingly, no chemical reagents are required to assist in the entire cleaning process, effectively avoiding negative damage to the natural environment. This cleaning technology has performed well in solving the common nanoparticle contamination problem of microelectronic substrates, providing a feasible, efficient and environmentally friendly method to solve this problem.

(2) Laser rust removal
Laser rust removal is an important application of laser cleaning technology. It uses high peak power pulsed laser 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 change, and finally effectively removing the rust layer. Compared with traditional rust removal processes, laser rust removal has a series of significant advantages. First, laser rust removal is a non-mechanical contact process and will not cause mechanical damage to the surface of the workpiece, thereby protecting the integrity of the workpiece. Its equipment is highly integrated, flexible in operation, and easy to realize automatic control, which improves production efficiency and convenience of operation. The good directionality of laser technology enables precise positioning of the rust removal process, adapts to the processing of complex curved surfaces, and improves the accuracy of cleaning. In addition, the laser rust removal process generates low noise and no dust pollution, which helps to create a cleaner working environment. Overall, laser rust removal technology shows many advantages in the rust removal process, such as high efficiency, precision and environmental protection, and provides 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 remove the rust layer by vaporizing the material heated by the laser beam. However, for the rust layer generated by oxidation of the iron substrate, due to its loose and porous surface and thickness ranging from tens of microns to hundreds of microns, the vaporization depth of the pulsed laser is relatively limited. Therefore, the removal mechanism of laser rust removal is not a single vaporization ablation, but also involves other cleaning mechanisms, such as plasma shock waves and phase explosions. This means that in addition to removing the rust layer by vaporization, the laser will also produce strong plasma shock waves, as well as phase explosions and other effects, which further synergistically act on the rust layer, ensuring a more comprehensive and thorough cleaning effect.

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

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