Laser cleaning technology is a successful application of laser technology in the engineering field. Its basic principle leverages the high energy density of lasers to enable interaction between laser beams and contaminants adhering to workpiece substrates. Contaminants are separated from substrates via instantaneous thermal expansion, melting, gas volatilization and other mechanisms. Boasting high efficiency, environmental friendliness and energy conservation, laser cleaning technology has been successfully applied in tire mold cleaning, aircraft body paint removal, cultural relic restoration and other fields.
Traditional cleaning technologies include mechanical friction cleaning (sandblasting, high-pressure water jet cleaning, etc.), chemical corrosion cleaning, ultrasonic cleaning, dry ice cleaning and more. These technologies are widely used across industries. For example, sandblasting can remove metal rust spots, surface burrs and conformal coatings on circuit boards by selecting abrasives of varying hardness. Chemical corrosion cleaning is extensively adopted for equipment surface oil scale removal, boiler scale cleaning and oil pipeline unclogging. While mature, traditional methods have notable drawbacks: sandblasting easily damages treated surfaces, and chemical corrosion cleaning causes environmental pollution and may corrode substrates if improperly operated. The emergence of laser cleaning marks a revolution in cleaning technology. Utilizing lasers’ high energy density, precision and efficient transmission, laser cleaning outperforms traditional methods in cleaning efficiency, precision and positioning. It eliminates environmental pollution from chemical cleaning and causes no damage to substrates.
Principles of Laser Cleaning
What exactly is laser cleaning? It refers to the process of removing materials from solid (or occasionally liquid) surfaces via laser beam irradiation. At low laser fluence, absorbed laser energy heats materials, causing evaporation or sublimation. At high laser fluence, materials typically convert into plasma. Laser cleaning usually employs pulsed lasers for material removal, though continuous-wave laser beams can ablate materials at sufficient intensity. Deep ultraviolet excimer lasers, with wavelengths around 200 nm, are primarily used for photoablation.
The depth of laser energy absorption and the amount of material removed per pulse depend on the optical properties of the material, as well as laser wavelength and pulse duration. The total mass ablated from a target per pulse is defined as the ablation rate. Laser radiation characteristics such as scanning speed and line coverage significantly influence the ablation process.
Types of Laser Cleaning Technology
1) Laser Dry Cleaning
Laser dry cleaning involves direct pulsed laser irradiation of workpieces. Contaminants or substrates absorb laser energy, raising their temperature and inducing thermal expansion or substrate thermal vibration, which separates contaminants from substrates. It occurs in two scenarios: either surface contaminants absorb laser energy and expand, or substrates absorb energy and vibrate thermally.
In 1969, S.M. Bedair et al. found that conventional surface treatments (heat treatment, chemical corrosion, sandblasting) all had limitations. They observed that the high energy density of focused lasers could vaporize surface materials without damaging substrates. Experiments confirmed that a Q-switched ruby laser with a power density of 30 MW/cm² could clean contaminants from silicon surfaces without substrate damage, marking the first implementation of laser dry cleaning.
The overall cleaning rate can be expressed via the detachment rate of film debris, as shown below:
(Formula: ε—laser pulse energy index; h—contaminant film thickness index; E—film elastic modulus index)
2) Laser Wet Cleaning
Before pulsed laser irradiation, a liquid film is pre-coated on the workpiece surface. Laser energy rapidly heats and vaporizes the film, generating an instantaneous shockwave that detaches contaminant particles from the substrate. This method requires no chemical reaction between the substrate and liquid film, limiting its applicable materials.
In 1991, K. Imen et al. addressed residual submicron contaminants on semiconductor wafers and metals after conventional cleaning. They coated substrates with a laser-absorbent film and irradiated it with a CO₂ laser. The film absorbed energy, heated rapidly, boiled and underwent explosive vaporization, removing surface contaminants—this defines laser wet cleaning.
3) Laser Plasma Shockwave Cleaning
Laser plasma shockwaves form when lasers ionize air into spherical plasma shockwaves during irradiation. These shockwaves strike substrates, releasing energy to remove contaminants without damaging the substrate (lasers do not directly interact with substrates). This technology cleans particles as small as tens of nanometers and imposes no restrictions on laser wavelength.
The physical principles of plasma cleaning are summarized as follows:
a) Laser beams are absorbed by the contaminant layer on the target surface.
b) High energy absorption forms rapidly expanding plasma (highly ionized unstable gas), generating shockwaves.
c) Shockwaves fragment and remove contaminants.
d) Laser pulses must be short enough to avoid heat accumulation that damages the substrate.
e) Experiments show plasma forms on metal surfaces when oxides are present.
Plasma generation occurs only above an energy density threshold, which depends on the contaminant or oxide layer to be removed. A second higher threshold exists, beyond which the substrate is damaged. To ensure effective cleaning without substrate harm, laser parameters must be adjusted to keep pulse energy density between the two thresholds.
In 2001, J.M. Lee et al. leveraged plasma shockwaves from high-power focused lasers. A pulsed laser with an energy density of 2.0 J/cm² (far exceeding silicon’s damage threshold) irradiated silicon wafers parallelly, successfully removing 1 μm tungsten particles. Strictly speaking, laser plasma shockwave cleaning is a subset of dry cleaning.
Initially developed to remove microscopic particles from semiconductor wafers, these three laser cleaning technologies have expanded to tire mold cleaning, aircraft skin paint removal, cultural relic restoration and more. Inert gas can be blown onto substrates during laser irradiation to instantly remove detached contaminants, preventing recontamination and oxidation.
Applications of Laser Cleaning Technology
1) Semiconductor Industry: Cleaning of Semiconductor Wafers and Optical Substrates
Semiconductor wafers and optical substrates undergo identical processing steps (cutting, grinding) to form desired shapes, introducing particulate contaminants that are difficult to remove and prone to recontamination. Contaminants on wafers impair circuit printing quality and shorten chip lifespans. On optical substrates, they degrade optical device and coating performance, causing uneven energy distribution and reduced service life.
Laser dry cleaning is rarely used here due to substrate damage risks, while wet cleaning and plasma shockwave cleaning have numerous successful applications. Xu Chuanyi et al. deposited micron-scale magnetic paint as a dielectric film on ultra-smooth optical substrates, achieving effective pulsed laser cleaning. Although total impurity particles increased, their size and coverage decreased significantly. Zhang Ping studied the effects of working distance and laser energy on cleaning efficiency for particles of varying sizes. Experiments showed a 240 mJ laser achieved optimal cleaning of polystyrene particles on conductive glass at a 1.90 mm working distance. Cleaning efficiency improved with higher laser energy, and larger particles were easier to remove.
2) Metal Industry: Metal Surface Cleaning
Metal surface cleaning targets macroscopic contaminants: oxide/rust layers, paint, coatings and other attachments, categorized as organic (paint, coatings) or inorganic (rust) contaminants. Cleaning meets subsequent processing/usage requirements: e.g., removing 10 μm-thick oxide layers from titanium alloys before welding, stripping paint from aircraft skins for repainting, and cleaning rubber residue from tire molds to ensure product quality and mold lifespan.
Metals have higher damage thresholds than their contaminant cleaning thresholds, enabling effective cleaning with appropriately powered lasers. Mature applications include: Wang Lihua et al. demonstrated that a 5.1 J/cm² laser removed oxide layers from A5083-111H aluminum alloy while preserving substrate quality, and a 100 W pulsed laser effectively cleaned titanium alloy oxide layers and enhanced surface hardness. Domestic manufacturers (Raycus Laser, Han’s Laser, Shenzhen Chuangxin) widely supply laser cleaning equipment for rubber molds, metal rust and part oil removal.
3) Cultural Relics Conservation: Cleaning of Cultural Relics and Paper Artifacts
Metal and stone cultural relics accumulate dirt, ink stains and other contaminants over time, requiring removal to restore original appearances. Paper artifacts (paintings, calligraphy) develop mold and plaques during improper storage, severely impairing their condition and cultural/historical value.
Zhao Ying et al. verified UV laser cleaning of mold plaques on rice paper: a single scan at 3.2 J/mm² removed thin plaques, while two scans achieved complete removal; excessive laser energy damaged the paper. Zhang Xiaotong successfully restored a gilded bronze artifact using the laser wet method. Zhang Licheng applied laser cleaning to a painted female pottery figurine from the Han Dynasty. Yuan Xiaodong et al. evaluated laser cleaning efficacy for stone relics, comparing substrate damage and removal efficiency for ink, smoke and paint stains on sandstone.
Conclusion
Laser cleaning is an advanced technology with broad research and application prospects in aerospace, military equipment, electronics and other high-precision fields. Mature in multiple industries due to its efficiency, environmental friendliness and superior cleaning results, its applications continue to expand. Beyond established paint and rust removal, recent advances include laser cleaning of oxide layers on metal wires. Future development hinges on expanding existing applications, entering new fields and innovating equipment:
- Strengthen theoretical research to guide practical applications. Current research relies heavily on experiments, lacking a mature theoretical framework. Establishing such a framework is critical for technological maturity.
- Expand applications in existing and new fields. Mature in paint/rust removal, emerging uses include metal wire oxide cleaning, providing fertile ground for growth.
- Develop new laser cleaning equipment, diverging into multi-purpose universal devices (e.g., combined paint/rust removal) and specialized tools (e.g., custom fixtures/fibers for confined spaces). Full automation via integration with industrial robots is a promising direction.
Post time: May-14-2026
