Laser surface cleaning of organic contaminants

Laser surface cleaning process has been a useful and efficient technique for various industrial applications. The removal of photoresist contaminants on silicon wafers was investigated with a krypton fluoride (KrF) excimer laser, and the irradiated area was characterized using a profilometer, a scanning electronic microscopy (SEM), an Auger electron spectroscopy (AES) and a Fourier transition infrared spectroscopy (FT-IR). It was found that there exist an optimal number of pulses to remove the contaminant from the substrate surface without any laser-induced damage, depending on the laser density on the surface. A model to predict the optimal number of pulses, which agrees well with Beer–Lambert's law, is proposed and proved to be operable.     

Laser surface cleaning, de-rusting, de-painting and de-oxidizing

Many materials have been tested as substrates and surface products. Typically ferrous (Carbon Steels and Stainless Steels) and non-ferrous (Al and Cu metals and its alloys) materials have been employed. Some epoxy, polyurethane, polyester and acrylic paints with different thickness and colour have been tested. Many types of surface rust and oxide on different bulk material have been undertaken to test. Similarly, some different types of oils and greases commonly used in industry to prevent oxidation, have been studied. Different types of laser sources have been employed: an axial fast flow, 1.5 kW CO₂ c.w. and pulsed laser source emitting a 10.6 7m beam; and a portable Nd:YAG laser, Q-switchedand normal-mode source: 1st harmonic 1.064 7m (6 ns pulse duration), 2nd harmonic 532 nm (120 7s duration pulse, 1 J max per pulse) wavelengths, multi-articulated seven mirror beam guiding device, 20 Hz repetition rate. This provides shots with 600 mJ maximum energy per pulse and 100 MW peak power per pulse with very low beam divergence: 0.5 mrad at full angle.     

Laser cleaning on Roman coins

Ancient metal objects react with moisture and environmental chemicals to form various corrosion products. Because of the unique character and high value of such objects, any cleaning procedure should guarantee minimum destructiveness. The most common treatment used is mechanical stripping, in which it is difficult to avoid surface damage when employed. Lasers are currently being tested for a wide range of conservation applications. Since they are highly controllable and can be selectively applied, lasers can be used to achieve more effective and safer cleaning of archaeological artifacts and protect their surface details. The basic criterion that motivated us to use lasers to clean Roman coins was the requirement of pulsed emission, in order to minimize heat-induced damages. In fact, the laser interaction with the coins has to be short enough, to produce a fast removal of the encrustation, avoiding heat conduction into the substrate. The cleaning effects of three lasers operating at different wavelengths, namely a TEA CO₂ laser emitting at 10.6 m, an Er:YAG laser at 2.94 m, and a 2-Nd:YAG laser at 532 nm have been compared on corroded Romans coins and various atomic and nuclear techniques have also been applied to evaluate the efficiency of the applied procedure. PACS 42.55.-f; 42.55.Ah; 81.05.Bx.     

Laser cleaning of glass

The pulse lasers, YAG-, CO₂-, and N₂-lasers, are examined for use in the cleaning of glass. Cleaning is found to be due to the evaporation and sputtering of stains on the glass by the heat energy of the laser light. Only the N₂ laser can be used for the cleaning of the exit surface of the glass (the opposite side to the laser). A laser with a high peak power of about 10⁶J/s and short-pulse duration below 100 ns is found to be necessary in practice.     

光学元件激光预处理技术

国外学者用1064 nm激光对pickoff镜进行预处理,发现损伤阈值平均提高38.8%。国内研究者用CO2激光中度抛光后,熔石英基片的损伤阈值提到了30%左右,激光波长、扫描方式等对处理效果影响也比较明显。介绍了三种公认的预处理机制,将离线与在线处理方式做了简单地比较,并对国内外激光预处理技术的发展和前景进行了展望。     

Plasma cleaning of multilayer mirrors in EUV lithography from amorphous carbon contaminations

The feasibility of plasma cleaning of the multilayer mirrors used in 13.5-nm EUV lithography from carbon contaminations is studied. Experiments conducted in electrodeless plasma of the surface-wave low-pressure discharge in helium and hydrogen demonstrated the high rate, efficiency, and selectivity of this cleaning without any damage of the mirror’s upper protection layer, even at the atomic level. The optimal working parameters of plasma cleaning are determined and its possible mechanism is discussed.     

Laser cleaning of polymer surfaces

We have investigated the removal of small spherical particles from polymer surfaces by means of 193-nm ArF and 248-nm KrF laser light. Polystyrene (PS) particles with diameters in the range of 110 nm to 1700 nm and silica particles (SiO₂) with sizes of 400 nm and 800 nm are successfully removed from two different substrates, polyimide (PI) and polymethylmethacrylate (PMMA). Experiments were performed in air (23 °C, relative humidity 24–28%) and in an environment with a relative humidity (RH) of about 90%. Received: 13 July 2000 / Accepted: 14 July 2000 / Published online: 9 November 2000     

Laser cleaning of artificially aged textiles

In order to analyse ancient textiles to identify the fibre, a proper cleaning of the sample is necessary. The anaerobic environment (i.e., the peatery), aiding the preservation of handmade textiles, often impregnates the textiles in such a way that the usual chemical procedures for cleaning are not sufficient while making the sample brittle. The use of laser pulses may offer an alternative method of cleaning. Four different textiles (wool, silk, flax, and cotton) have been investigated. As a first step we used laser irradiation on untreated samples to investigate the laser induced effects on textile materials. As a second step, consolidated samples, samples artificially aged (i.e., samples exposed to a lacustrine environment), and samples both artificially aged and consolidated were irradiated. Several combinations of the main laser parameters (energy density and number of pulses) were tested in order to find the best irradiation conditions. Scanning electron microscopy (SEM) was used to analyze the induced morphological changes. The obtained results are well reproducible while proving the very high efficiency in material removal of the laser pulses: in the four investigated textiles, the laser cleaning procedure permitted the identification of the fibres. PACS 79.20.Ds; 61.80.Bd     

The physical optics integral on the scatterer's unlit surface

The scattering integrals of the modified theory of physical optics are redefined according to the illuminated and unlit surfaces of the scattering object. With this aim the canonical problem of wedge diffraction is taken into account. It is shown that the new scattering integral contain two geometrical optics and diffracted fields. One of the geometrical optics waves is the reflected field component that propagates in the real space. The other one transmits to an imaginary space through the scattering surface and does not have any influence in the real space. The diffracted waves exist in the real space and satisfy the related boundary condition on the scattering surfaces. The resultant field expressions are compared with the exact series solution of the problem numerically.     

Spatial resolution of imaging contaminations on the GaAs surface by scanning tunneling microscope-cathodoluminescence spectroscopy

We obtained the luminescence image of the GaAs (1 1 0) surface by scanning tunneling microscope-cathodoluminescence (STM-CL) spectroscopy, where low-energy (∼100 eV) electrons field emitted from the STM tip were used as a bright excitation source. The STM-CL image with high photon signal (1.25 × 10⁴ cps) showed the dark image corresponding to the surface contamination in the STM image working as the nonradiative recombination centers of carriers. This dark image demonstrated the spatial resolution of about 100 nm in STM-CL spectroscopy of the GaAs (1 1 0) surface, which was determined by the field-emitted electron beam diameter.     

Simulation study and experiment on laser-ablation surface cleaning

A study of the ablation effects of an ultra-short pulsed Nd : YAG laser was carried out. Using a 21 μm Zn-coated carbon steel plate as the target, the relationship between ablation rate and laser fluence was investigated through computer simulation and experiment, and the optimal processing conditions were determined. The tendency of the scanning operation curve was confirmed and the data obtained were taken as a guide for the practical utilization of this technique. Finally, a decontamination factor was introduced and satisfactory cleaning was obtained.     

Effect of silicon surface cleaning procedures on oxidation kinetics and surface chemistry

The effect of surface cleaning procedures on the kinetics of thermal oxide growth on silicon is presented. The goal is to relate the properties of the cleaned surface (composition, chemistry, impurity content) to the changes in oxide growth mechanisms. Experimentally, silicon (100) wafers were given different variations of an RCA clean, and then oxidized in dry O₂ at 900°C producing oxides with thicknesses between 170 and 3900 Å. The results, in general agreement with earlier studies, show that the percentage difference in thickness is strongly dependent on oxide thickness. The data, which are explained in terms of the predictions of a linear-parabolic and a parallel oxidation model, suggest that the surface cleans do not alter the diffusion of molecular oxygen in the oxide. Auger analysis of the surfaces shows that there is a substantial carbon contamination on the HF stripped wafer which is considerably reduced after a 5 min N₂ anneal.     

Laser micro/nano patterning of hydrophobic surface by contact particle lens array

Direct laser surface micro/nanopatterning by using Contact Particle Lens Array (CPLA) has been widely utilized. The method involves laser scanning of a monolayer of transparent particles arranged on the substrate to be patterned. Despite the different techniques available for CPLA deposition; the particles monolayer can only be formed on hydrophilic surfaces, which restrict the range of substrates that could be patterned by this method. In this study, a technique for patterning of hydrophobic surfaces by using CPLA has been proposed. In the proposed technique, monolayer of CPLA is formed on a hydrophilic substrate and then transported to a hydrophobic substrate by using a flexible sticky plastic. The transported CPLA is then scanned by a laser for patterning the hydrophobic substrate. The plastic pre-selected for this work was transparent to the laser. Experimental investigations were carried out to generate bumps and bowl shaped patterns using transported particles. Features smaller than the diffraction limit have been generated. The optical near field and associated temperatures around the particles were numerically simulated with a coupled electromagnetic and thermal modelling technique.     

Nonlinear optics of chiral surface systems

Differential SHG signals from chiral surfaces are recorded using right and left circularly polarized incident light. We show that chiral information from the surface is conveyed through this SHG process. This study focuses on the model protein system, cytochrome c, adsorbed on various self-assembled mono- and bi-layers at the solid/liquid interface. SHG-CD signals are shown to be related to the oxidation state of the protein and are utilized to observe changes in this feature of the protein in situ at surfaces of varying chemical properties. Received: 20 September 1998 / Revised version: 9 December 1998     

Near-field optics of a dielectric surface

The problem of classical optics related to the electromagnetic-field distribution near the surface of a dielectric medium with allowance made for its discrete structure is solved. The effect of a probe on the near-field optical observations is taken into account, and the condition is obtained under which this effect can be neglected. It is shown that the near-field observations can be used to get information not only on the surface structure but also on the effective fields in the near-surface region. A method for optically controlling the probe position over the surface is proposed.     

Laser cleaning of steel for paint removal

Paint removal is an important part of steel processing for marine and offshore engineering. For centuries, a blasting techniques have been widely used for this surface preparation purpose. But conventional blasting always has intrinsic problems, such as noise, explosion risk, contaminant particles, vibration, and dust. In addition, processing wastes often cause environmental problems. In recent years, laser cleaning has attracted much research effort for its significant advantages, such as precise treatment, and high selectivity and flexibility in comparison with conventional cleaning techniques. In the present study, we use this environmentally friendly technique to overcome the problems of conventional blasting. Processed samples are examined with optical microscopes and other surface characterization tools. Experimental results show that laser cleaning can be a good alternative candidate to conventional blasting.     

Laser cleaning of contamination on sandstone surfaces in Yungang Grottoes

Laser cleaning is firstly used to remove the contaminations on the sandstone surfaces in Yungang Grottoes. A Q-switched Nd:YAG laser operated at 1064 nm with a pulse width of 10 ns is used to perform the experiments. Before experiments in Yungang Grottoes, the laboratory experiments on sandstone samples obtained from Shanxi province have been conducted. The laser induced damage thresholds of the uncontaminated sandstones and the laser cleaning thresholds of ink contaminated sandstones have been obtained in the laboratory. The difference between the cleaning effects of dry laser cleaning and steam laser cleaning has been compared. On the basis of the laboratory experimental data, the experiments in Yungang Grottoes have been conducted and the results indicate that laser cleaning is safe and effective for removal of graffiti ink and black smudges of smoke.     

Visualization of particle trajectories in the laser shock cleaning process

The laser shock cleaning (LSC) method has recently attracted substantial attention since it can remove micro/nano-scale contaminant particles from a solid surface without direct exposure of the surface to laser irradiation. However, despite the importance of the particle detachment and redeposition mechanisms in the LSC process, the behavior of the particles during the cleaning process has never been analyzed experimentally. In this work, the motion of the micrometer-scale particles detached by a laser-induced plasma/shock wave is visualized by a photoluminescence imaging technique. The technique yields time-resolved particle trajectories under typical conditions of the LSC process, with and without a gas jet blowing. Discussions are made on the behavior of the detached particles and redeposition mechanisms.     

Addition of surfactants in ozonated water cleaning for the suppression of functional group formation and particle adhesion on the SiO2 surface

Various kinds of surfactants were added to a cleaning solution and deionized (DI) water, and their effect on the suppression of organic function group formation and particle adhesion to a SiO₂ surface was analyzed using multi-internal reflection Fourier transform infrared spectroscopy. The results implied that attached organic functional groups are affected by the chemical structure of a surfactant in DI water. Furthermore, the addition of anionic glycolic acid ethoxylate 4-tert-butylphenyl ether (GAE4E) is the most effective in terms of preventing organic group attachment and particle adhesion to the SiO₂ surface, whether it was added to the cleaning solution or post-cleaning rinse water, with or without polystyrene latex particles. Moreover, it was possible to completely prevent particle adhesion to the SiO₂ surface with the proper addition of GAE4E in DIO₃ solution.