When we think of cleaning technology, the first things that come to mind are various cleaning agents and tools used in daily life. However, traditional cleaning methods invariably cause varying degrees of wear and damage to the objects being cleaned. With the advancement of technology and the pursuit of precision, the concept of cleaning has long gone beyond simple tasks like “washing dishes.” The scope of cleaning objects has expanded, and the standards for cleaning requirements have been continually raised. Fragile artifacts cannot withstand polishing or grinding, smooth metal surfaces need to be meticulously maintained, and small components require a perfect cleaning method. Laser cleaning technology has emerged as a solution. As early as 1965, Nobel laureate Schawlow used pulsed laser irradiation on paper printed with ink to quickly vaporize the ink characters, while the paper itself remained undamaged. This success in removing ink from the paper opened the door to pulsed laser cleaning technology. In 1973, the Asmus team was the first to report using laser cleaning on artifacts. In 1974, Fox successfully removed paint layers from resin glass and metal substrates using Q-switched neodymium-glass lasers. In 1982, Zapka and his colleagues at IBM’s German Manufacturing Technology Center used focused lasers to irradiate mask plates and successfully cleaned the particle contamination adhered to the surface. Since then, laser cleaning technology has developed significantly over the past 40 years.

Principle and Mechanism of Laser Cleaning

Laser cleaning is an advanced cleaning technique that uses high-energy laser beams to irradiate the surface of an object, causing impurities, contaminants, or coatings to rapidly vaporize or be stripped away through optical and thermal effects.

The core component of laser cleaning technology is the pulsed laser with high energy, high average power, and high peak power. As we know, lasers are light sources with high brightness, consistency, and directionality. Pulsed lasers release high-energy laser beams in a very short period of time, with high peak power and instantaneous power density. Compared to continuous lasers, high-power pulsed lasers can generate high temperatures in an instant, but because the duration is extremely short, the heat cannot be transferred to the surrounding material, significantly reducing the thermal impact on the substrate material. High-power pulsed lasers can also be precisely controlled by adjusting pulse energy and frequency, allowing for customized cleaning processes based on different materials and application scenarios. When the laser beam strikes the surface to be cleaned, the laser energy is absorbed, generating intense thermal effects that cause the contaminants or coatings to heat up rapidly, leading to vaporization, decomposition, or stripping. Meanwhile, the high energy density of the pulsed laser enables it to directly penetrate some materials without damaging the substrate surface, making the cleaning process more efficient.

Because the composition and structure of the cleaning material can vary greatly, the mechanisms of laser interaction are also diverse. Therefore, laser cleaning is not just about high-energy ablation; it involves a range of physical and chemical processes such as decomposition, ionization, degradation, melting, combustion, vaporization, vibration, spattering, expansion, contraction, explosion, peeling, and detachment. As a non-mechanical contact surface treatment method, laser cleaning allows the laser beam to scan the object’s surface in a predetermined pattern, ensuring sufficient interaction between the laser and the contaminants, rust layers, or coatings. After the surface material absorbs the laser energy, it transforms into the thermal, chemical, and mechanical energy required for cleaning. Currently, the main theories explaining the pulsed laser cleaning mechanism include the laser ablation mechanism and the thermoelastic expansion stripping mechanism.

(1) Laser Ablation Mechanism

The thermal ablation mechanism during the pulsed laser cleaning process is closely related to the laser power density. In the ablation mechanism, the high-power pulsed laser can release a large amount of energy in an extremely short time, causing the laser beam to concentrate in a small area, which heats up and vaporizes the contaminants or coatings on the target surface. When the laser’s energy is sufficient to break the chemical bonds of the surface material, the chemical bonds vibrate, bend, or even break, leading to molecular decomposition and photodecomposition of the contaminants. When the laser cleaning power density exceeds 10^8 W/cm^2, the contaminated layer on the material surface may undergo plastic deformation, generating explosive rebound stress. When the laser cleaning power density exceeds 10^9 W/cm^2, the contaminants on the surface may vaporize or generate a plasma due to optical breakdown, forming a plasma explosion shockwave. These explosive effects accelerate the removal of contaminants from the substrate surface.

(2) Thermoelastic Expansion Stripping Mechanism

This mechanism includes thermal elastic vibration, vapor pressure, photomechanical pressure, phase explosion, shock waves, and more. When the laser irradiates the material surface, both the substrate and the contaminants undergo thermal expansion. The stripping stress generated by this thermal elastic expansion first removes some surface material, which is the thermal vibration mechanism. In this vibration mechanism, the thermal effect of the laser will cause both the contaminants and the substrate to heat up. However, since the laser energy used is far lower than that in the ablation mechanism, the contaminants are not directly ablated but instead experience mechanical fracture, vibration, and fragmentation. The contaminants are ejected or stripped from the substrate surface. Pulsed lasers can also ionize the air surrounding the contaminants or substrate surface particles, creating a plasma shockwave that helps remove the contaminants. In wet laser cleaning, a liquid film (water, ethanol, or other liquids) is pre-applied to the surface of the cleaning object, and then the laser is irradiated. The liquid film absorbs the laser energy, causing the liquid medium to undergo explosive boiling. The rapidly moving boiling liquid transfers energy to the contaminants on the surface, using the high transient explosive force to remove them.

Typical Applications of Laser Cleaning

Over the past 40 years, laser cleaning has developed rapidly as a new, efficient, and environmentally friendly cleaning technology, with widespread applications in electronic component cleaning, paint stripping, rust removal, and more.

(1) Laser Cleaning of Electronic Components

During the development of the semiconductor industry, cleaning contaminants from the surface of silicon wafer masks has always been a significant challenge. Traditional chemical cleaning methods cause considerable contamination, while mechanical and ultrasonic cleaning methods fail to achieve the desired cleaning effects. As technology advances, electronic devices are becoming smaller, and the size of the particles that need to be cleaned is decreasing, making cleaning more difficult. Laser cleaning technology has emerged as a new solution, and research and application in this field have rapidly developed.

Due to the fragility of electronic component surfaces and the common presence of coatings, traditional laser ablation cleaning methods carry the risk of damaging the components. To address this issue, scientists have adopted a new, efficient cleaning technique that utilizes high-intensity lasers focused through lenses to induce air breakdown and form high-temperature, high-density laser plasma. As the plasma rapidly expands, it compresses the surrounding air, creating a strong plasma shockwave. The mechanical effect of this shockwave allows nanoparticles to overcome their adhesion to the substrate surface, thus “blasting” them away, efficiently cleaning the surface particles. Unlike traditional methods, the laser plasma shockwave is generated during the laser irradiation by ionizing the air medium and only affects the substrate surface without harming the substrate itself. Encouragingly, the entire cleaning process does not require any chemical agents, effectively avoiding negative environmental impacts. This cleaning technology has shown great potential in solving the issue of nanoparticle contamination on microelectronic substrates, offering an efficient and environmentally friendly solution.

(2) Laser Rust Removal

Laser rust removal is an important application of laser cleaning technology. It uses high peak power pulsed lasers to irradiate rust layers. During this process, the laser energy is absorbed, causing the temperature of the rust layer to rise rapidly, resulting in expansion, thermal shock, and phase changes that ultimately remove the rust layer. Compared to traditional rust removal methods, laser rust removal offers several significant advantages. First, it is a non-contact process, so it does not cause mechanical damage to the surface of the workpiece, thereby preserving its integrity. The equipment is highly integrated, flexible to operate, and easily automated, improving both production efficiency and operational convenience. The precise directionality of the laser allows for accurate positioning during rust removal, making it suitable for handling complex surfaces and improving cleaning precision. Additionally, the laser rust removal process produces less noise and generates no dust, creating a cleaner working environment. Overall, laser rust removal demonstrates high efficiency, precision, and environmental friendliness, providing an advanced solution for industrial cleaning. This innovative technology not only improves traditional cleaning methods but also offers a more sustainable and environmentally friendly option for industrial production.

One of the main mechanisms for laser rust removal is through laser beam-induced vaporization of the material to remove the rust layer. However, for iron-based substrates with rust layers, the surface is often loose and porous, and the thickness ranges from tens to hundreds of micrometers. As a result, the vaporization depth of the pulsed laser is relatively limited. Therefore, laser rust removal involves more than just vaporization and ablation; other cleaning mechanisms, such as plasma shockwaves and phase explosions, also come into play. This means that in addition to vaporizing the rust layer, the laser also generates strong plasma shockwaves and phase explosion effects, working together to ensure a more thorough and complete cleaning.

With the continued development of laser cleaning technology, it is expected to bring more innovation and convenience to the cleaning industry. In the future, we are likely to witness laser cleaning technology creating greater benefits across various fields while making a more positive contribution to environmental protection. Laser cleaning is rapidly becoming a shining star in cleaning technology, leading us into a new era of cleaning innovation.

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