Influence of Elemental Compositions in Laser Cleaning for Automotive Coating Systems

Laser coating removal is an approach that has the potential to replace conventional chemical-based stripping methods in industry. In addition to the laser parameters, the efficiency of this cleaning technique depends also on the coating system itself. For this reason, we study the influence of the elemental compositions of two Malaysian automotive coated substrates, referred to here as A and B, on laser coating removal mechanisms using the Cynosure Cynergy Nd:YAG laser. The optimum laser coating removal efficiencies for both samples A and B are determined based on the depths of 360 craters obtained using a given formula. Selected crater depths on both of the stipulated samples are then subjected to energy dispersive X-ray analysis to determine their elemental compositions. The results indicate that sample A, which has higher aluminum content, shows greater efficiency in coating system removal, with balanced carbon and oxygen compositions aiding coating reduction during the cleaning process.     

Ultraviolet laser removal of small metallic particles from silicon wafers

Laser removal of small 1 μm sized copper, gold and tungsten particles from silicon wafer surfaces was carried out using ultraviolet radiation at 266 nm generated by Nd:YAG harmonic generation. Successful removal of both copper and gold particles from the surface was achieved whereas tungsten particles proved to be difficult to remove. The cleaning efficiency was increased with an increase of laser fluence. The optimum processing window for safe cleaning of the surface without any substrate damage was determined by measuring the damage threshold laser fluence on the silicon substrate and the required fluence for complete removal of the particles. The different cleaning efficiencies with particle type are discussed by considering the adhesion force of the particle on the surface and the laser-induced cleaning force for the three different particles.     

Time-of-flight measurements of ejected particles during dry laser cleaning

We propose an experimental approach which allows the characterization of the dynamics of the ejected particles in dry laser cleaning. Submicron silica particles on silicon substrates were illuminated by single nanosecond laser pulses at fluences which lead to particle removal. Time- and space-resolved scattered signal detection was demonstrated as a suitable technique to perform time-of-flight analyses of the ejected particles. The determination of the resulting detachment velocity at the particle removal threshold fluence contributes to a better understanding of mechanisms involved in dry laser cleaning. In particular, the present study evidences that the removal efficiency of the laser process is not based on the thermal expansion of materials. PACS 42.62.b; 42.15.Eq     

Matrix laser cleaning: a new technique for the removal of nanometer sized particles from semiconductors

In this paper a new laser-based technique for the removal of nanoparticles from silicon wafers, called matrix laser cleaning, is introduced. In contrast to the already existing technique dry laser cleaning damages of the substrate can be avoided. Furthermore no liquids are used, avoiding problems that occur, e.g. in steam laser cleaning and other wet cleaning techniques. We show that damage free particle removal of polystyrene particles with diameters of at least down to 50 nm is possible with a cleaning efficiency very close to 100% within a single shot experiment. Furthermore the cleaning threshold is independent of the particle size. PACS 64.70.Hz; 68.43.Vx; 81.65.Cf     

Angular laser cleaning for effective removal of particles. from a solid surface

Laser removal of small particles from a metal surface is carried out by changing the incident angle of the laser beam. It has been found that a dramatic improvement of cleaning efficiency in terms of area and energy is observed when using the laser at glancing angle of incidence as compared to perpendicular. Furthermore substrate damage is greatly reduced and probably eliminated at glancing angles. The process mechanism is discussed by considering the adhesion and the laser-induced cleaning forces for different incident angles. It is shown that there are different laser–matter interactions operating. Received: 25 April 2000 / Accepted: 9 May 2000 / Published online: 5 October 2000     

Optical diagnostics for particle-cleaning process utilizing laser-induced shockwave

The dynamics of laser-induced plasma/shockwave and the interaction with a surface in the laser shock cleaning process are analyzed by optical diagnostics. Shockwaves are generated by a Q-switched Nd:YAG laser in air or with N₂, Ar, and He injection into the focal spot. The shock velocity is measured by monitoring the photoacoustic probe–beam deflection signal under different conditions. In addition, nanosecond time-resolved images of shockwave propagation and interaction with the substrate are obtained by the laser-flash shadowgraphy. The results reveal the effect of various operation parameters of the laser shock cleaning process on shockwave intensity, energy-conversion efficiency, and flow characteristics. Discussions are made on the cleaning mechanisms based on the experimental observations. PACS 81.65.Cf; 42.62.-b; 47.40.Nm     

In situ monitoring of laser cleaning by coupling a pulsed laser beam with a scanning electron microscope

Lasers have proved to be effective tools for material processing at the micron and nanometer scales. In particular, laser interaction with nanostructures offers the unique advantage of highly localized excitation and heating. In this study, a short-pulsed laser beam is coupled to a scanning electron microscope, without disturbing the microscopy function, in order to study in situ laser cleaning of individual submicron particles from a silicon substrate. The substrate conditions before and after particle removal were inspected by electron microscopy. The mechanisms of particle removal and the underlying dynamic coupling of the laser radiation associated with particle cleaning are investigated. PACS 42.62.Cf; 42.82.Gw; 81.65.Cf; 07.78.+s     

Removal mechanisms of micro-scale particles by surface wave in laser cleaning

This paper is to investigate the mechanisms of micro-scale particle removal by surface wave, which was induced by a short pulse laser in a cleaning process. The authors analyzed the adhesive forces of particles on substrate surface and the clearance force produced by surface wave in laser cleaning. The physical model of particle removal by laser-induced surface wave was established to predict the removal area and the processing conditions of laser cleaning. In this research, a KrF excimer laser was applied to irradiate 304 stainless steel specimen distributed with copper particles to generate surface wave for copper particle removal. Considering that a time-varying and uniformly distributed heat source irradiates on material surface with thermao-elastic behavior, the displacement and acceleration of substrate induced by a pulsed laser were solved by an uncoupled thermal–mechanical analysis based on the finite element method. The processing parameters such as laser energy, laser spot size are discussed, respectively. A series of laser cleaning experiments were designed to compare with computation results. The results show that the removal area by surface wave beyond the laser spot increases with the laser energy and that, the surface acceleration decreases with the increase of the laser spot size.     

Universal threshold for the steam laser cleaning of submicron spherical particles from silicon

The efficiency of the "steam laser cleaning" process is examined. For the investigation of the physics of particle removal from the particularly interesting surface of silicon we have deposited well-characterized spherical polymer and silica particles of different diameters ranging from several tens to hundreds of nanometers on commercial wafers. As a result of our systematic study we observe a sharp threshold of the steam cleaning process at 110 mJ/cm² (5=532 nm, FWHM=7 ns) which is independent of the size (for particles with diameters as small as 60 nm) and material of the particles. An efficiency above 90% after 20 cleaning steps is reached at a laser fluence of 170 mJ/cm². Experiments with irregularly shaped alumina particles exhibit the same threshold as for spherical particles.     

Laser removal of copper particles from silicon wafers using UV, visible and IR radiation

Laser removal of small copper particles from silicon wafer surfaces was carried out using Nd:YAG laser radiation from near-infrared (1064 nm) through visible (532 nm) to ultraviolet (266 nm). It has been found that both 266 nm and 532 nm are successful in removing the particles from the surface whereas 1064 nm was shown to be ineffective in the removal of particles. The damage-threshold laser fluence at 266 nm was much higher than other wavelengths which provides a much wider regime for safe cleaning of the surface without causing any substrate damage. The cleaning efficiency was increased with a shorter wavelength. The effect of laser wavelength in the removal process is discussed by considering the adhesion force of the particle on the surface and the laser-induced cleaning forces for the three wavelengths. Received: 31 May 2000 / Accepted: 14 July 2000 / Published online: 20 June 2001     

Removal of doped poly(methylmetacrylate) from tungsten and titanium substrates by femto- and nanosecond laser cleaning

The influence of different laser pulse lengths on the removal of a polymer layer from metal substrates was investigated. As model systems, doped poly(methylmetacrylate) (PMMA) on titanium and tungsten substrates were selected.The ablation threshold and irradiation spot morphology of titanium and tungsten were compared for femtosecond (fs) and nanosecond (ns) laser irradiation and different pulse numbers. Nanosecond laser treatment resulted in a non-homogeneous surface morphology for both titanium and tungsten substrates. Femtosecond irradiation of tungsten revealed a homogeneous ablation spot with little changes in the surface morphology. For titanium, the formation of columnar structures within the irradiation spot was observed.Two different dopant concentrations were used for PMMA to achieve an equal linear absorption coefficient for the femto- and nanosecond laser wavelengths of 790 and 1064 nm. The best results were achieved for the removal of doped PMMA by femtosecond laser irradiation, where only a minimal modification of the metal surface was detected. In the case of nanosecond laser exposure, a pronounced change of the structure was observed, suggesting that damage-free cleaning of the selected metal may only be possible using femtosecond laser pulses. Different experimental parameters, such as laser fluence, pulse repetition rate and sample speed were also investigated to optimize the cleaning quality of doped PMMA from tungsten substrates with femtosecond laser pulses.     

Laser plasma acceleration of quasi-monoenergetic beams of light multicharged ions

Experimental results on the spectra of residual impurity ions from a plasma created on the surface of a tungsten target that is irradiated by a femtosecond laser pulse and is subjected to preliminary pulsed laser cleaning are reported. It is shown that deep modulation corresponding to the formation of a quasi-monoenergetic ion beam appears in the energy spectra of protons and light multicharged carbon ions in this case. The position of a “dip” at the scale of velocities closely correlates with the position of the front of heavier ions.     

Removal of graffiti from the mortar by using Q-switched Nd:YAG laser

This paper presents part of the larger study on microstructural features of mortars and it's effects on laser cleaning process. It focuses on the influence of surface roughness, porosity and moisture content of mortars on the removal of graffiti by Nd:YAG laser. The properties of this laser are as follows: wavelength (λ) 1.06 μm, energy: 500 mJ per pulse, pulse duration: 10 ns. The investigation shows that the variation of laser fluence with the number of pulses required for the laser cleaning can be divided into two zones, namely effective zone and ineffective zone. There is a linear relationship observed between number of pulses required for laser cleaning and the laser fluence in the effective zone, while the number of pulses required for the laser cleaning is almost constant even though the laser fluence increases in the ineffective zone. Moreover, surface roughness, porosity and moisture content of mortar samples have influence on the laser cleaning process. The effect of these parameters become however negligible at the high level of laser fluence. The number of pulses required for the laser cleaning is low for smooth surface or less porous mortar. Furthermore, the wetness of the samples facilitates the cleaning process.     

Comparison of mechanisms and effects of Nd:YAG and CO2 laser cleaning of titanium alloys

The paper compares laser cleaning trials performed using a Q-switched Nd:YAG laser, λ = 1.064 μm and a continuous wave (CW) CO₂ laser, λ = 10.6 μm, applied to aerospace-grade, contaminated titanium alloys. The mechanisms for cleaning using each laser system are modelled to determine the mode and extent of contaminant removal. The model results are then compared with the surface chemistry and micro-structural results from the cleaning trials performed. The results show the dominant cleaning process for Nd:YAG cleaning to be by evaporation of the contaminant via conduction through surface heating, while for CO₂ laser cleaning the small fraction of the beam coupling directly with the contaminant is sufficient for direct heating and selective evaporation. The results for experimental cleaning, electron beam (EB) welding and diffusion bonding align well with the model, particularly when secondary reactions are taken into account.     

Laser cleaning of particle and grease contaminations on the surface of optics

In the high power laser facility, surface contaminations on the optics will worsen the laser beam quality and damage the optics. Particle and grease contaminations are two of the usual contaminations on the surface of optics. In this work, the 1064-nm laser induced plasma shockwave cleaning is utilized to remove SiO₂ particle contaminations on the K9 glass surface. The results indicate the removal ratio can reach above 95%. The effects of parameters (particle position, laser gap distance and laser energy) on the cleaning efficiency have been studied in the case of single pulse laser cleaning. In addition, CO₂ laser (10.6 μm) is utilized to remove the dimethylsilicone oil contaminations on the gold-coated K9 glass surface. The results show that CO₂ laser can effectively remove the dimethylsilicone oil by properly controlling the laser parameters. The cleaned area increases with the increased laser power or irradiation time when the other parameters are constant.     

Laser plasma shockwave cleaning of SiO2 particles on gold film

A Nd:YAG laser (1064 nm) induces optical breakdown of the airborne above the gold-coated K9 glass surface and the created shockwave removes the SiO₂ particles contaminated on the gold films. The laser cleaning efficiency has been characterized by optical microscopy, dark field imaging, ultraviolet-visible-near infrared spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and the Image-pro software. The relationships between removal ratio and particle position and laser gap distance have been studied in the case of single pulse laser cleaning. The results show that the 1064 nm laser induced plasma shockwave can effectively remove the SiO₂ particles. The removal ratio can reach above 90%. The effects of particle position and laser gap distance on the cleaning efficiency are simulated for the single pulse laser cleaning. The simulated results are consistent with the experimental ones.     

激光等离子体去除微纳颗粒的热力学研究

微杂质污染一直是影响精密器件制造质量和使用寿命的关键因素之一.对于微纳米杂质颗粒用传统的清洗方式(超声清洗等)难以去除,而激光等离子体冲击波具有高压特性,可以实现纳米量级杂质颗粒的去除,具有很大的应用潜力.本文主要研究了激光等离子体去除微纳米颗粒过程中的热力学效应:实验研究了激光等离子体在不同脉冲数下对Si基底上Al颗粒去除后的颗粒形貌变化,发现大颗粒会发生破碎而转变成小颗粒,一些颗粒达到熔点后发生相变形成光滑球体,这源于等离子体的热力学效应共同作用的结果.为了研究微粒物态转化过程,基于冲击波传播理论研究,得到冲击波压强与温度特性的演化规律;同时,利用有限元模拟方式研究激光等离子冲击波压强和温度对微粒作用规律,得到了颗粒内随时间变化的应力分布和温度分布,并在此基础上得到等离子体对颗粒的热力学作用机制.     

热退火、激光束和电子束等作用对纳米硅制备及其局域态发光特性的影响

在纳米晶体硅制备的过程中, 晶化处理是影响和提高纳米硅发光效率的重要制备环节. 热退火、激光退火和电子束辐照是使纳米硅样品晶化的不同方式. 实验表明: 选取适当的晶化方式和参量对制备纳米硅晶体结构至关重要, 特别是在制备硅量子点和量子面的过程中控制好参量, 可以得到较高的发光效率. 有趣的是, 在实验中发现: 当晶化时间较短(如低于20 min)时, 可以获得较好的纳晶硅结构(如量子点结构), 对应于较好的纳晶硅光致发光(PL)和掺杂局域态发光; 当晶化时间较长(如超过30 min)时, 纳米晶体硅结构被破坏, 致使PL谱逐渐减弱与消失. 结合热退火、激光退火和电子束辐照对纳米硅晶化过程, 本文建立起晶化时间对纳米硅局域态发光影响机理的物理模型, 解释了晶化时间对纳米硅局域态发光的影响.     

Complete removal of paint from metal surface by ablation with a TEA CO2 laser

For high-speed metal surface cleaning, we applied TEA CO₂ laser pulses to ablate painted materials on metal surfaces and examined the efficiency of removal under different surface and irradiation conditions. Surfaces treated with a line-focused laser beam were analyzed with an energy dispersive X-ray analyzer and inspected with optical microscopic observation. Although paints were selectively ablated from the metal surface, the cleaning efficiency was found to depend on surface conditions of substrates. An application of a small amount of dimethyl formamide was effective for completely removing of resin without scorching the surface.