Laser Cleaning

Understanding the Principles and Applications of Laser Cleaning Technology

Understanding The Principles And Applications Of Laser Cleaning Technology | Laserchina

Discover the revolutionary world of laser cleaning technology with this comprehensive guide. Learn about the principles, types, and diverse applications of this high-precision cleaning method that’s changing the industry.

Introduction to Laser Cleaning Technology

The advent of laser cleaning technology represents a revolutionary leap in the field of cleaning methods. This innovative technique leverages the high energy density, precision, and efficient conductivity of lasers, offering clear advantages over traditional cleaning methods in terms of efficiency, precision, and the ability to clean specific locations. One of its most significant benefits is the avoidance of environmental pollution typically associated with chemical cleaning methods, all while causing no damage to the substrate.

The Principle of Laser Cleaning

Understanding The Principles And Applications Of Laser Cleaning Technology | Laserchina

Laser cleaning involves the removal of materials from solid (or sometimes liquid) surfaces by exposing them to a laser beam. At low laser fluences, the absorbed laser energy heats and evaporates or sublimates the material. At high fluences, the material often turns into plasma. Typically, laser cleaning refers to pulsed laser applications for material removal, but with sufficient intensity, continuous-wave laser beams can also ablate material. Excimer lasers operating in the deep ultraviolet are primarily used for photoablation, with wavelengths around 200nm. The depth of laser energy absorption and the amount of material removed by a single laser pulse depend on the material’s optical properties, as well as the laser’s wavelength and pulse duration. The total mass ablated by each pulse, commonly referred to as the ablation rate, is significantly influenced by laser characteristics like the beam scanning speed and scan line overlap.

Types of Laser Cleaning Technology

  • Dry Laser Cleaning: This method involves direct pulsed laser irradiation on the workpiece, causing the substrate or surface contaminants to absorb energy and increase in temperature, resulting in thermal expansion or substrate vibration, leading to their separation. This can occur in two ways: either the surface contaminants expand upon laser absorption or the substrate vibrates due to laser-induced heat.
  • Wet Laser Cleaning: Prior to pulsed laser irradiation, a liquid film is applied to the surface of the workpiece. The rapid temperature increase of the liquid film under the laser’s influence causes it to vaporize, creating a shockwave that impacts the contaminant particles and dislodges them from the substrate. This method requires that the substrate and liquid film do not react, thus limiting the range of applicable materials.
  • Laser-Induced Plasma Shockwave Cleaning: A spherical plasma shockwave is generated when the laser beam ionizes the air during irradiation. The shockwave impacts the surface of the workpiece to be cleaned, releasing energy that removes contaminants without affecting the substrate. This technique can clean particulate contaminants down to tens of nanometers in diameter and is not limited by laser wavelength.

The physical principles of plasma cleaning can be summarized as follows:

  1.  The laser beam emitted by the laser is absorbed by the contaminant layer on the surface to be treated.
  2.  The high energy absorption creates rapidly expanding plasma (a highly ionized, unstable gas), generating a shockwave.
  3.  The shockwave fragments the contaminants, which are then ejected.
  4.  The pulse width of the light must be short enough to avoid thermal accumulation that could damage the treated surface.
    e) Experiments show that when there are oxides on a metal surface, the plasma forms at the metal interface.

Plasma is only generated when the energy density exceeds a threshold, which depends on the contaminant or oxide layer being removed. This threshold effect is essential for effective cleaning while ensuring the safety of the substrate material. A second threshold exists for plasma formation; exceeding it could damage the substrate. To ensure effective cleaning without harming the substrate, laser parameters must be adjusted so that the pulse energy density is strictly between the two thresholds.

Initially, these three types of laser cleaning technologies were developed to clean microscopic particles from semiconductor wafers, emerging alongside the advancement of semiconductor technology. However, laser cleaning has since been applied in other fields, such as cleaning tire molds, removing paint from aircraft skins, and restoring surfaces of cultural relics.

Applications of Laser Cleaning Technology

Understanding The Principles And Applications Of Laser Cleaning Technology | Laserchina

Semiconductor Field

Cleaning semiconductor wafers and optical substrates involves similar processes: shaping raw materials through cutting, grinding, and other methods. During these processes, particulate contaminants are introduced, which are difficult to remove and pose a serious risk of recontamination. Contaminants on the surface of semiconductor wafers can affect the quality of circuit board printing and shorten the lifespan of semiconductor chips. Contaminants on optical substrates can impact the quality of optical devices and coatings, potentially leading to uneven energy distribution and reduced lifespan.

Because dry laser cleaning can easily cause damage to the substrate, its use in cleaning semiconductor wafers and optical substrates is limited. Wet laser cleaning and laser-induced plasma shockwave cleaning have been more successfully applied in this field.

Metal Material Field

Cleaning metal material surfaces involves contaminants in the macroscopic range, in contrast to the microscopic ones found on semiconductor wafers and optical substrates. Contaminants on metal surfaces typically include oxide layers (rust), paint layers, coatings, and other adhesions, which can be organic (paint, coatings) or inorganic (rust).

Cleaning metal surface contaminants mainly serves to prepare for subsequent processing or use. For example, before welding titanium alloy parts, it’s necessary to remove an oxide layer about 10μm thick, or when overhauling an aircraft, removing the original paint layer from the skin for repainting. Regular cleaning of rubber tire molds to remove attached rubber particles is also essential to maintain surface cleanliness, which ensures the quality of the produced tires and the lifespan of the molds. Since the damage threshold of metal materials is higher than the laser cleaning threshold for their surface contaminants, selecting a laser cleaning machine with appropriate power can achieve good results, and this has been successfully applied in various fields.

Laser cleaning technology is an advanced technique with broad research and application prospects in high-end fields such as aerospace, military equipment, and electronics. Its applications are expanding thanks to its efficiency, environmental friendliness, and effective cleaning results. The technology has not only been well-established for paint removal and rust removal but has also been reported in recent years for cleaning the oxide layers on metal wires. The expansion of current applications and exploration of new fields lay the foundation for the development of laser cleaning technology. The development and diversification of new laser cleaning equipment, including machines that cover multiple applications and those designed for specific purposes, are ongoing. Future integration with industrial robots to achieve fully automated laser cleaning is also a promising direction.

Trends in the Development of Laser Cleaning Technology

  • Strengthening theoretical research in laser cleaning to guide its application. A review of extensive literature reveals that there is no mature theoretical system supporting laser cleaning technology, with most research being experimental. Establishing a theoretical system is fundamental for the further maturation of laser cleaning technology.
  • Expanding applications in existing fields and exploring new ones. Laser cleaning technology has matured in applications such as paint removal and rust removal, and recent reports have highlighted its use in cleaning the oxide layers on metal wires. The growth of its applications in both existing areas and new fields is fertile ground for the technology’s development.
  • The development of new laser cleaning machines. Future equipment will likely diversify, with some machines covering several applications, such as a single machine capable of both paint removal and rust removal, and others designed for specific tasks, perhaps requiring custom jigs or fiber optics for cleaning contaminants in small spaces. Collaborating with industrial robots to achieve fully automated laser cleaning is another hot application direction.

Conclusion

Laser cleaning technology, represented by the laser cleaning machine, is a shining example of modern innovation, offering a range of benefits that traditional cleaning methods cannot match. With the ongoing development of new laser cleaning equipment and the expansion of its applications into new sectors, the future of cleaning processes looks bright. As LASERCHINA engineers continue to pioneer this field, we can expect to see laser cleaning become a staple in high-precision industries, setting a new standard for cleanliness, efficiency, and environmental responsibility.

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