Laser cleaning technology is a successful application of laser technology in the engineering field. Its basic principle is to use the high energy density of the laser to interact with contaminants adhering to the workpiece substrate, causing them to separate from the substrate through thermal expansion, melting, gasification, and other forms. Laser cleaning technology is characterized by high efficiency, environmental protection, and energy conservation, and has been successfully applied in fields such as tire mold cleaning, aircraft paint removal, and cultural relic restoration.
The emergence of laser cleaning technology is a revolution in cleaning technology. Laser cleaning technology takes advantage of the high energy density, high precision, and efficient transmission of lasers. Compared with traditional cleaning technology, it has significant advantages in cleaning efficiency, cleaning accuracy, and cleaning position. It can effectively avoid environmental pollution caused by chemical corrosion cleaning technology and does not damage the substrate.
Principle of laser cleaning
So what is laser cleaning? Laser cleaning is the process of removing material from a solid (or sometimes liquid) surface by irradiating it with a laser beam. At low laser flux, the absorbed laser energy heats the material and causes it to evaporate or sublime. At high laser flux, the material typically converts into plasma. Laser cleaning usually refers to using pulsed lasers to remove material, but if the laser intensity is high enough, continuous wave laser beams can be used to ablate materials. Deep ultraviolet excimer lasers are mainly used for photoablation. The laser wavelength used for photoablation is about 200 nm. The depth of laser energy absorption and the amount of material removed by a single laser pulse depend on the optical properties of the material as well as the laser wavelength and pulse length. The total mass of material ablated from the target by each laser pulse is usually called the ablation rate. Laser radiation characteristics such as scanning speed and scanning line coverage significantly affect the ablation process.
Application of Laser Cleaning Technology
1. Semiconductor field
Semiconductor wafers and optical substrates have similar processes in which raw materials are processed into the required shape by cutting, grinding, and other forms. During this process, particulate contaminants are introduced, which are difficult to remove and can cause repeated contamination problems. Contaminants on the surface of semiconductor wafers can affect the quality of circuit board printing and shorten the service life of semiconductor chips. Contaminants on the surface of optical substrates can affect the quality of optical devices and coatings, which may cause uneven energy distribution and shorten their service life.
Due to the risk of surface damage to the substrate, dry laser cleaning is less commonly used in the cleaning of semiconductor wafers and optical substrates. Wet laser cleaning and laser plasma shock wave cleaning have been successfully applied in this field. Xu Chuanyi et al. studied the deposition of micron-sized special magnetic paint as a medium film on the surface of ultra-smooth optical substrates, followed by cleaning with a pulsed laser. The cleaning effect was good, and although the area of impurity particles per unit area increased, the size and coverage area of impurity particles both decreased significantly. This method can effectively clean micron-sized contaminant particles on the surface of ultra-smooth optical substrates.
Zhang Ping studied the effect of working distance and laser energy on the cleaning efficiency of different-sized contaminant particles in laser plasma cleaning technology. The experimental results showed that for polystyrene particles on conductive glass substrates, the optimal working distance for a laser energy of 240 mJ was 1.90 mm. The cleaning efficiency increased significantly with the increase of laser energy, and larger contaminant particles were easier to clean.
2. Metal Materials Field
Compared to cleaning semiconductor wafers and optical substrates, the cleaning of metal material surfaces involves macroscopic pollutants. The pollutants on metal material surfaces mainly include oxide layers (rust layers), paint layers, coatings, and other attachments. According to the types of pollutants, they can be divided into organic pollutants (such as paint layers and coatings) and inorganic pollutants (such as rust layers). The cleaning of metal material surface pollutants is mainly to meet the requirements of subsequent processing or use. For example, before welding titanium alloy parts, the oxide layer on the material surface of about 10μm thick needs to be removed. During aircraft overhaul, the original paint coating on the skin surface needs to be removed for re-spraying. Rubber tire molds need to be regularly cleaned of attached rubber particles to ensure surface cleanliness and ensure tire quality and mold life.
The damage threshold of metal materials is higher than the laser cleaning threshold of surface pollutants. By selecting an appropriate power laser, good cleaning effects can be achieved, and it has been widely applied in some fields. Wang Lihua et al. studied the application of laser cleaning technology in the treatment of oxide skin on aluminum alloy and titanium alloy surfaces. The research results showed that a laser with an energy density of 5.1 J/cm2 can clean the oxide layer on the surface of A5083-111H aluminum alloy while maintaining the good quality of the substrate. A pulsed laser with an average power of 100 W can effectively clean the oxide layer on the surface of titanium alloy and improve the surface hardness of the material by scanning.
3. Cultural Relics Field
Due to their long history, metal cultural relics and stone cultural relics will have pollutants such as dust and ink stains on their surfaces, which need to be cleaned to restore them. Paper-based cultural relics such as calligraphy and painting will grow mold and form spots on their surfaces when stored improperly, which seriously affects their original appearance, especially for paper-based cultural relics with high cultural or historical value, which will affect their appreciation and protection.
Zhao Ying et al. studied the feasibility of using UV laser cleaning to remove mold spots on Xuan paper. The experimental results showed that a laser with an energy density of 3.2 J/mm2 can remove thin spots after scanning once, and scanning twice can completely remove the spots. However, if the laser energy used is too high, it will damage the Xuan paper while removing the spots. Zhang Xiaotong et al. successfully repaired a gilded bronze cultural relic using laser vertical irradiation liquid film method. Zhang Licheng et al. used laser cleaning technology in the restoration of a painted Han Dynasty female pottery figurine. Yuan Xiaodong et al. studied the effect of laser cleaning technology in cleaning stone cultural relics, comparing the damage to sandstone before and after cleaning, as well as the effects of ink stain cleaning, smoke pollution cleaning, and paint pollution cleaning.
Conclusion
Laser cleaning technology is an advanced technology with broad research and application prospects in high-precision fields such as aerospace, military equipment, electronics, and electrical engineering. Currently, laser cleaning technology has been maturely applied in some fields, and its application areas are gradually expanding thanks to its high efficiency, environmental friendliness, and good cleaning effects.
The development of laser cleaning technology has not only been maturely applied in fields such as paint removal and rust removal but also has seen reports in recent years of using laser cleaning to remove oxide layers on metal wire surfaces. The expansion of existing application areas and the application of new fields are the basis for the development of laser cleaning technology. The development of new laser cleaning equipment will also lead to differentiation and the development of various functions. In the future, full-automatic laser cleaning can be achieved by cooperating with industrial robots.
The development trends of laser cleaning technology are as follows:
- Advancing Theoretical Research: Strengthening the theoretical research of laser cleaning is essential to guide its practical application. Currently, most research in this field is based on empirical studies, and there is a lack of a comprehensive theoretical framework. Establishing a robust theoretical system for laser cleaning will provide a solid foundation for its further development and maturation.
- Expanding Application Areas: Laser cleaning technology has already found success in applications such as paint and rust removal. However, there is significant potential for expanding its use in both existing and new fields. For example, recent reports have showcased the use of laser cleaning to remove oxide layers from metal wire surfaces. By exploring and diversifying the application areas, laser cleaning technology can continue to grow and find new avenues for implementation.
- Developing Specialized Equipment: The development of new laser cleaning equipment will contribute to the advancement of this technology. There are two main directions for equipment development. The first is creating versatile devices that can cater to multiple application areas, such as equipment capable of simultaneously performing paint removal and rust removal functions. The second direction involves specialized equipment designed for specific needs, such as fixtures or fibers that can effectively clean pollutants in small and confined spaces. Additionally, the pursuit of fully automated laser cleaning systems through collaboration with industrial robots is another exciting direction for equipment development.
