Principles, types and applications of laser cleaning technology

 Laser cleaning technology is a successful application of laser technology in the field of engineering, and its basic principle is to utilize the characteristics of high laser energy density to make the laser interact with the pollutants attached to the workpiece substrate and separate it from the workpiece substrate in the form of instantaneous thermal expansion, melting, and gas volatilization. Laser cleaning technology has high efficiency, environmental protection, energy saving, and other characteristics, and has been successfully applied to tire mold cleaning, aircraft fuselage paint removal, cultural relics restoration, and other fields.

Traditional cleaning technologies include mechanical friction cleaning (sandblasting cleaning, high-pressure water gun cleaning, etc.), chemical corrosion cleaning, ultrasonic cleaning, dry ice cleaning, etc., these cleaning technologies have been widely used in various industries, such as sandblasting cleaning through the selection of different hardness of abrasives can be cleaned metal embroidery spots, metal surface burrs, circuit board surface three-proof paint, etc., chemical corrosion cleaning technology is widely used in equipment surface scale cleaning, Chemical corrosion cleaning technology is widely used in equipment surface oil scale cleaning, boiler scale cleaning, oil pipeline cleaning, and other fields. Although these cleaning technologies have matured, but there are still some problems, such as sandblasting cleaning is easy to deal with the damage caused by the surface, chemical corrosion cleaning will cause environmental pollution, and improper treatment leads to cleaning surface corrosion.

 

The emergence of laser cleaning technology is a revolution in cleaning technology. Laser cleaning technology utilizes the laser's high energy density, high precision, efficient conduction, and other advantages, compared with the traditional cleaning technology in the cleaning efficiency, cleaning precision, clean up the location of the obvious advantages, can effectively avoid chemical corrosion cleaning and other cleaning technology will cause environmental pollution, and will not produce damage to 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 fluences, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser fluences, the material is usually converted to plasma. Typically, laser cleaning refers to the removal of material with a pulsed laser, but if the laser intensity is high enough, the material can be ablated with a continuous wave laser beam. Excimer lasers in the deep ultraviolet are mainly used for photoablation. The wavelength of the laser used for photoablation is about 200 nm. The depth of absorption of the laser energy and the amount of material removed by a single laser pulse depends on the optical properties of the material as well as the wavelength of the laser and the pulse length. The total mass ablated from the target per laser pulse is often referred to as the ablation rate. Laser radiation characteristics such as laser beam scanning speed and scan line coverage significantly affect the ablation process.

Types of laser cleaning technology

(1) Laser dry cleaning: dry laser cleaning that is, pulsed laser direct irradiation cleaning workpiece, so that the substrate or surface contaminants absorb the energy temperature rise, resulting in thermal expansion or substrate thermal vibration, and then separate the two. The method is roughly divided into 2 cases: one is the surface contaminants absorbing laser expansion; the other is the substrate absorbing laser thermal vibration.

In 1969, S.M. Bedair et al. found that including heat treatment, chemical corrosion, sandblasting cleaning and other surface treatment methods have different shortcomings, at the same time, the use of laser focusing after the high energy density can make the material surface evaporation of the phenomenon of the existence of non-destructive cleaning of the surface of the material may be found through experiments, the use of the power density of 30 MW/cm2 ruby tuned Q laser can be realized without damaging the Through experiments, it was found that the use of power density of 30 MW/cm2 ruby Q laser can realize the cleaning of silicon surface pollutants without damaging the substrate, the first time to realize the use of laser cleaning of material surface pollutants, that is, laser dry cleaning.

 

The overall rate can be expressed by the film debris detachment rate as follows:

ε-indicator of laser pulse energy, h-contaminant membrane layer

thickness index, E-elastic modulus index of the film layer.

 

(2) Laser wet cleaning: before the pulse laser irradiation of the workpiece to be washed, the surface is pre-coated with a liquid film, the liquid film under the action of the laser temperature rises rapidly, and gasification, the gasification of the instantaneous generation of shock waves, the role of the pollutant particles, so that it is detached from the substrate. This method requires that the substrate and the liquid film can not react, so it limits the range of application materials.

 

In 1991, K. Imen et al. for the use of traditional cleaning methods for the treatment of semiconductor wafers, metal materials, and other surfaces with submicron particles of pollutants remaining, researched in the material substrate surface coating a highly efficient laser absorption film, followed by the use of CO2 laser irradiation, the film absorbs the laser energy, the temperature rises rapidly and boils, resulting in explosive vaporization of the substrate surface of the pollutants away. This is called laser wet cleaning. This cleaning method is laser wet cleaning.

 

(3) Laser plasma shock wave cleaning: laser plasma shock wave is in the laser irradiation process to break through the air medium and produce spherical plasma shock wave, shock wave action on the surface of the substrate to be washed and release energy to remove the pollutants; the laser does not act on the substrate, so the substrate does not produce damage. Laser plasma shock wave cleaning technology can now clean tens of nanometers of particle-size contaminants, and there is no limit to the laser wavelength.

 

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

a) The beam emitted by the laser is absorbed by the contaminated layer on the surface to be treated.

b) The absorption of large amounts of energy creates a rapidly expanding plasma (highly ionized unstable gas), which generates a shock wave.

c) The shock wave causes the contaminants to fragment and be removed.

d) The width of the light pulse must be short enough to avoid a damaging heat build-up on the treated surface.

e) Experiments have shown that plasma is generated on metal surfaces when oxides are present on the surface.

 

The plasma is only generated at energy densities above a threshold, which depends on the contamination or oxide layer being removed. This threshold effect is important for effective cleaning while keeping the substrate material safe. There is also a second threshold for the appearance of plasma. If the energy density exceeds this threshold, the substrate material is destroyed. In order to perform effective cleaning with the safety of the substrate material, the laser parameters must be adjusted to the situation so that the energy density of the optical pulse is strictly between the two thresholds.

 

In 2001, J.M. Lee et al. successfully cleaned 1 μm tungsten particles adsorbed on the surface of silicon wafers by irradiating the wafers in parallel with a pulsed laser with an energy density of 2.0 J/cm2, which is much higher than the damage threshold of the wafers, by taking advantage of the fact that a plasma shockwave will be generated by the focusing of a high-power laser. This type of cleaning is known as laser-plasma shock wave cleaning, which is strictly speaking a type of dry laser cleaning.

 

The initial purpose of the above three laser cleaning technologies was to clean the tiny particles on the surface of semiconductor wafers, and it can be said that laser cleaning technology emerged with the development of semiconductor technology. But laser cleaning technology is constantly applied to other fields, such as tire mold cleaning, aircraft skin paint removal, cultural relics surface restoration.

 

In the laser radiation at the same time, available inert gas blowing to the substrate surface, when the dirt from the surface will be stripped immediately after the gas blowing away from the surface, in order to avoid the surface again contamination and oxidation.

Application of laser cleaning technology

1) Semiconductor field

Semiconductor wafers and optical substrate cleaning semiconductor wafers and optical substrates in the processing of the same process, that is the raw material in the form of cutting, grinding, and other forms of processing for the desired shape. This process introduces particulate contaminants, which are difficult to remove and cause serious problems of recurring contamination. Contaminants on the surface of semiconductor wafers affect the quality of printed circuit boards, which in turn shortens the life of semiconductor chips. Contaminants on the surface of optical substrates affect the quality of the optics and coatings and can lead to energy inhomogeneity and shortened service life. Since laser dry cleaning is prone to substrate surface damage, this cleaning method is less used in the cleaning of semiconductor wafers and optical substrates, and laser wet cleaning and laser-plasma shock wave cleaning have more successful applications in this field. Xu Chuanyi et al. studied the deposition of micron-level special magnetic paint as a dielectric film on the surface of an ultra-smooth optical substrate, followed by the use of pulsed laser cleaning, the cleaning effect is better, although its impurity particles per unit area of the particulate matter increased, the size of the impurity particles and the coverage of the area have decreased significantly, the method can be effective in cleaning the micron-level contaminant particles on the surface of the ultra-smooth optical substrate. Ping Zhang studied the laser plasma cleaning technology in the working distance and laser energy on the cleaning effect of pollutant particles of different particle sizes, The test results show that for the conductive glass substrate on the polystyrene particles, the energy of the 240 mJ laser's optimal working distance of 1.90 mm, with the increase in laser energy to clean up the effect of the increase is obvious, and large particles of pollutants are easier to clean up.

 

2）Metal material field

Metal material cleaning metal material surfaces compared to semiconductor wafers and optical substrate cleaning, cleaning pollutants belong to the macro category. Pollutants on the surface of metal materials are mainly oxidized layer (corrosion layer), paint, coatings, other attachments, etc, The type of pollutants can be divided into organic pollutants (such as paint, coatings) and inorganic pollutants (such as corrosion layer). Metal material surface contaminants cleaning mainly to meet the subsequent processing or use requirements, such as titanium alloy parts welding before the need to remove the surface of the material about 10μm thick oxide layer, aircraft overhaul need to remove the original paint coating on the surface of the skin in order to facilitate the re-spraying, rubber tire molds need to be cleaned up regularly to ensure that the surface of the rubber particles to ensure that the surface cleanliness to ensure that the production of the quality of the tires and the life of the molds. The damage threshold of the metal material is higher than the laser cleaning threshold of the surface contaminants, so choosing the appropriate power laser can achieve better cleaning results, in some areas that have mature applications.

Conclusion

Laser cleaning technology is a more advanced technology, in the aerospace, military equipment, electronics, electrical, and other high-precision fields that have broad research and application prospects. At present, laser cleaning technology has been maturely applied in some fields, thanks to its high efficiency, environmental protection, good cleaning effect, and other characteristics, its application fields are also gradually broadened.

 

The development of laser cleaning technology has not only been in the removal of paint, rust, and other areas of mature applications but in recent years and the use of laser cleaning of metal wire surface oxide layer of the report. The expansion of existing areas of application and the application of new areas is the basis for the development of laser cleaning technology. The development of new laser cleaning equipment and the development of new laser cleaning equipment will diverge and develop a variety of functions. In the future, fully automated laser cleaning can also be realized by cooperating with industrial robots.

The development trend of laser cleaning technology is as follows:

(1) strengthen the laser cleaning theory research, and guide the application of laser cleaning technology. Check a large number of literature found that the laser cleaning technology has no mature theoretical system support, most of the research is based on the test. The establishment of a laser cleaning theory system is the basis for the further development of laser cleaning technology.

 

(2) Expanding the application of existing fields and the application of new fields. Laser cleaning technology has been used in paint removal, rust removal, and other areas of maturity, and in recent years there have been laser cleaning of metal wire surface oxide layer reports. The expansion of existing areas of application and new areas of application is fertile ground for the development of laser cleaning technology.

 

(3) the development of new laser cleaning equipment. The development of new laser cleaning equipment will be divided, a class is to cover a number of applications with a certain degree of universality of the equipment, such as a device that can be achieved at the same time in addition to paint and rust removal function, the other is for specific needs of special equipment, such as in order to clean the pollutants inside the small space may require the design of a specific tooling or optical fiber to achieve the function. Fully automated laser cleaning by cooperating with industrial robots is also a popular application direction.