Excimer Laser Cleaning Research Articles

Excimer laser cleaning of black sulphur encrustation from silver surface

The process of localized cleaning of silver sulphide from the silver surface has been investigated in this work. An artificial black encrustation was generated on the silver surface and its laser cleaning was performed using an excimer laser (KrF based) working at a wavelength of 248 nm and pulse width 25 ns. Laser process parameters i.e. laser fluence and number of pulses were used to perform the experiment and after different optical, microstructural, elemental, microhardness and surface topography analysis of the laser ablated zone was performed. The parametric window selected for the excimer laser cleaning were laser fluence (200–400 mJ/cm2), repetition rate 4 Hz and number of pulses (60–300). It was observed that excimer laser is very effective in the cleaning of black encrustation having ∼6 wt% of sulphur from the surface without any significant removal of the silver substrate. The fluence value of 260.48 mJ/cm2 was observed as above threshold value for removal of sulphide from the encrusted surface while a fluence of 384.85 mJ/cm2 and 220 pulses showed the maximum removal of sulphur encrustation (∼0.38 wt% of sulphur remaining). Formation of the peripheral rim was observed to occur above 280.17 mJ/cm2 of fluence value. The topography of the ablated surface showed a removal of 5–10 µm of the sulphide layer from the surface whereas the thickness of the sulphide coating was around 8–10 µm. Microhardness value showed a change of microhardness of the silver surface from 75.12 HV0.5 (uncoated parent silver) to 96.07 HV0.5 (centre of the ablated zone). Finally, it was concluded that excimer laser cleaning is a novel method for localized removal of silver sulphide encrustation from silver surface.

Ultraviolet Laser Cleaning of Carbon Fiber Composites

Carbon fiber reinforced plastics (CFRPs) are composite materials that offer a highly desirable combination of physical strength and light weight. Originally developed primarily for aerospace applications, they can now be found in products ranging from automobiles, sailboats, and racing bicycles to golf clubs. Adhesives are often used to join individual CFRP pieces in an assembly because bonding offers several advantages over mechanical fastening methods. However, achieving a high strength adhesive bond can be frustrated by the presence of surface contaminants. A variety of techniques have been employed to clean CFRPs prior to bonding, but each of these methods has limitations in terms of either speed, complexity or the need for subsequent cleaning. Excimer laserbased surface cleaning and ablation now offers a practical alternative which yields a pristine surface with the requisite characteristics for adhesive bonding. This article reviews how excimer laser cleaning is implemented and discusses the results of bond strength testing performed using this technology.

In situ excimer laser irradiation as cleaning tool for solid phase epitaxy of laser crystallized polycrystalline silicon thin films

This study demonstrates an innovative approach to clean a polycrystalline seed layer surface for solid phase epitaxy of amorphous silicon (a-Si). The excimer laser cleaning (ELC) makes use of in situ excimer laser irradiation during the first stages of a-Si deposition leading to melting of the as deposited a-Si and parts of the seed layer. The increased diffusion in liquid silicon allows for “smearing out” of surface contamination species. After liquid phase epitaxy is finished, further a-Si is deposited and crystallized by solid phase epitaxy. Thanks to the “smearing out,” the interface between the crystalline and amorphous silicon exhibits less contamination locally. SIMS measurements demonstrate a reduced carbon concentration at the seed layer surface after ELC. Numerical simulations, taking into account heat transfer and temperature dependent diffusion, support the reduction of carbon concentration. The simulations agree very well with experimental results. To get optimal solid phase growth conditions, the a-Si deposition temperature has to be below 200 °C. Above 200 °C deposition temperature, defective growth occurs resulting in poor crystallinity or even in nonepitaxial growth.

Numerical Simulation of Excimer Laser Cleaning of Film and Particle Contaminants

Laser cleaning is a promising surface preparation technique for applications in high value manufacturing industries. However, understanding the effects of laser processing parameters on various types of contaminants and substrates, is vital to achieve the required cleaning efficacy and quality. In this paper, a two-dimensional transient numerical simulation was carried out to study the material ablation characteristics and substrate thermal effects in laser cleaning of aerospace alloys. Element birth and death method was employed to track the contaminant removal on the surface of the material. The result shows that contaminant ablation increases with laser power and number of pulses. The finite element method (FEM) model is capable enough to predict the optimum number of pulses and laser power required to remove various contaminants. Based on the simulation results, the mechanism of the excimer laser cleaning is proposed. Thus, the use of numerical simulation can be faster and cheaper method of establishing the optimum laser cleaning window and reducing the number of experimental tests.

Characterization and removal of co-deposition on the first mirror of HL-2A by excimer laser cleaning

In this study, an excimer laser ablation has was proposed for the first time for removing the co-deposition layer of the first mirrors of HL-2A tokamak. It was found that this approach to be much more effective and homogeneous in the removal of the impurity layer than using Nd:YAG lasers at 532nm and 355nm wavelengths. Time-of-flight mass spectrometry has been used for characterization of the released impurities during laser cleaning. X-ray photoelectron spectroscopy and secondary ion mass-spectrometry was used to evaluate the depth profile and chemical state of the compositions in the deposited layer. The results indicated that the ablation process can be characterized by two well-defined threshold fluences: (a) ablation threshold ≈0.01J/cm2 and (b) a damage threshold ≈0.55J/cm2. SEM has been used to characterize the ablation spot after cleaning at the different laser fluences. The results showed that the deposited layer was almost completely removed after multiple pulses and the repeated spatial scans at 0.3–0.55J/cm2. After the cleaning, the reflectivity of the first mirror was restored to 80% of full capacity in the wavelength range of 600–1100nm.

Excimer laser removal of beeswax from galician granite monuments

Abstract The paper presents the use of laser radiation for cleaning of beeswax treated granite stone. In the early sixties, prestigious restorers decided to apply molten beeswax on the granite surface of valuable monuments with the aim of preventing the erosive action of atmospheric agents. An interesting example of this treatment is the Renaissance Frieze in the Cloister of the Cathedral of Santiago de Compostela in Galicia (Northwest Spain). With time, salt accumulation beneath the beeswax layer has caused an intense surface disintegration of granite. Conventional cleaning methods can destroy sculptured details of these emblematic monuments. For this reason, excimer laser cleaning has been chosen as a promising non contact, selective and environmentally friendly cleaning technique to be studied. The cleaning tests have been carried out using an excimer laser (ArF, 193 nm), with fluences between 0.5 and 2 J cm −2 pulse −1 and a spot area of 0.025 cm 2 . Samples representing beeswax films over Roan granite have been irradiated and their degree of cleaning has been studied as a function of the number of pulses and the laser fluence. Cleaning efficiency has been evaluated by FT-Raman Spectroscopy, allowing to establish the beeswax ablation threshold and the ablation rate. Excimer laser cleaning allows a progressive and controlled removal of a few tenths of micrometers of beeswax per pulse without damaging the underlying granite stone.

Dry excimer laser cleaning applied to nuclear decontamination

Excimer laser ablation is a very powerful tool of dry cleaning. This technique allows the removal or oxide or painting deposited on a material without any modifications of the chemical and physical properties of its surface. This method has been effectively used in many areas. In nuclear industry, there is a great interest to develop in developing an efficient dry decontamination process. A review of the main laser decontamination experiments performed in the world is presented. Our laser cleaning prototype based on excimer laser ablation process is described. This prototype has been tested in nuclear facilities. It is mainly composed of a XeCl laser, a bundle of fibers for beam transmission, optical systems, collection cell with filter for ablated particle recovery, computer control of cleaning efficiency and beam displacement. Different kinds of materials, which are representative of contamination usually found in nuclear field, have been irradiated. Decontamination factors (initial activity/residual activity) higher than 15 for fixed contamination and up to 100 for unfixed contamination have been obtained. These performances demonstrate that the laser-based technique is the most efficient one for dry and fast decontamination.

Biogenic surface layers on historical window glass and the effect of excimer laser cleaning

The ablation of biofilms by excimer laser radiation was systematically examined in a series of model studies during which the biofilms originating from different historical panels were simulated on model glasses. The composition of these model glasses was modelled on that of the original historical glasses. Our studies have shown that glass composition, climatic conditions and biofilm formation are factors which interact synergetically. It could be observed that various biofilms grow differently on the same type of model glass and that the same type of biofilm shows a different development on various model glasses. The decisive factors for the effectiveness of biofilm ablation by laser irradiation is the formation of the biofilm on the one hand and its corrosive potential on account of its ability to accumulate moisture and to produce glass-damaging metabolites on the other. Glasses of low chemical stability promote the growth of dense biofilms and can be cleaned only with a high energy density, whereas glasses of high chemical stability merely allow for a slow growth of a biofilm spreading two-dimensionally on the glass surface which can be gently removed using low energy density.

The modeling of excimer laser particle removal from hydrophilic silicon surfaces

We summarize experimental results on the successful removal of submicron-sized polystyrene latex, carboxylate-modified latex, SiO2, and Al2O3 particles from hydrophilic silicon surfaces by excimer laser, using both dry and steam cleaning; the cleaning and damage thresholds have also been determined for these particles. Adhesion and removal models for an ideal sphere particle, that include van der Waals forces, hydrogen bonding, and thermoelastic effects, theoretically explain the laser cleaning results. Two models of the removal force due to the explosive evaporation of liquid film have been calculated and compared. The effects of both asperities on the particle surface and particle aggregation have also been considered. The results of the calculations show that even those surface asperities which are small compared with the particle dimension can cause a large reduction in both adhesion and thermoelastic removal forces. The theoretical predictions are consistent with the experimental observations.

Cleaning of submicrometer structures on Si-masters with pulsed excimer laser and reactive ion etching

The replication of submicrometer-sized structures is accomplished utilizing replica molding of a UV-cured acrylate blend. Atomic force microscopy (AFM), optical, and scanning electron microscope images reveal acrylate residues on the Si-master after replication. The differences between excimer laser and reactive ion cleaning of the Si-masters in terms of their effectiveness and reliability is discussed. Forty to sixty KrF laser pulses at 400 mJ/cm2 are suitable for removing acrylate residues from the Si-masters, while the oxygen plasma etching removes acrylate residues only to a limited degree because of a silicon diacrylate constituent in the acrylate blend.

Excimer laser cleaning of encrustation on Pentelic marble: procedure and evaluation of the effects

This work focuses on the use and control of excimer lasers (KrF, λ=248 nm and XeCl, λ=308 nm) for the removal of encrustation (black crusts, soil-dust and biological deposits) from Pentelic marble. A number of surface analytical techniques, such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) interfaced with microprobe analysis (energy dispersive X-ray analysis: EDX), X-ray diffraction (XRD), laser-induced breakdown spectroscopy (LIBS) and optical microscopy (OM) were used to detect chemical composition and crust morphology, as well as to monitor the effects induced by the laser treatment. SEM and OM providing structural information about the layers of encrustation, LIBS and SEM–EDX providing information on the elemental composition of the ablated material, XRD and FTIR detecting the changes of minerals appear to be particularly effective in assessing the quality of cleaning process. FTIR and XRD showed removal of pollutants from encrustation and partial transformation of calcium sulfate dihydrate (gypsum) to hemihydrate and anhydrite, which have lower specific surface than gypsum. Irradiation conditions creating minimal damaging effects were defined before treatment through surface analysis, ablation rate studies and optimization of laser parameters. On the basis of structural and analytical examinations, both lasers are shown to be appropriate for achieving sufficient removal of unwanted selected layers without modifying the surface morphology and surpassing by far the effectiveness afforded by traditional cleaning techniques.

Excimer laser cleaning of silicon wafer backside metallic particles

An excimer laser cleaning system operating at 248 nm was developed to remove micrometer-sized metallic particles from the backside of silicon wafers. Deliberate iron contamination has been performed using iron-oxide particles having a diameter of 0.5–2 μm. The surface photovoltage (SPV) method was used to characterize the cleaning efficiency through the change in diffusion length and iron concentration in the silicon bulk. Following a rapid thermal annealing at 1050 °C for 4 min, the SPV measures diffusion lengths down to 40 μm for the iron-contaminated wafer, corresponding to an iron concentration up to 1.55×1013 cm−3. The minimum diffusion length increases to 130 μm after two steam laser cleanings done at a laser fluence below the silicon threshold damage of 200 mJ/cm2. The iron concentration measured in the bulk of the iron-contaminated wafers is reduced by more than 91%, to a concentration of 1.4×1012 cm−3.

Excimer laser restoration of painted artworks: procedures, mechanisms and effects

Excimer laser ablation is demonstrated to afford a novel, highly effective method of restoration of painted artworks. The application relies on the strong UV-absorptivity of these substrates ensuring efficient material removal, with minimal light penetration to the sublayers. On the basis of structural and analytical examinations, 248 nm is shown to be appropriate for achieving sufficient etching rates in combination with good surface morphology. With proper optimization of the irradiation parameters, excimer laser cleaning can be a highly selective process, surpassing by far the degree of selectivity afforded by traditional restoration methods. Furthermore, a number of different laser analytical techniques can be used for on-line monitoring and control, thereby safeguarding against damage. Broadband reflectography, providing structural information about the uncovered layers, and laser-induced breakdown spectroscopy, providing information on the elemental composition of the ablated material, appear to be particularly effective in this respect. Furthermore, the plausible consequences of the laser irradiation on the state of the paintings are investigated in experiments involving model and realistic systems. In particular, the importance of photochemical effects is addressed by chromatographic analysis of irradiated realistic samples for the detection of photoproducts, and by examining the degree of photolysis of photosensitive dopants incorporated in model samples. Potential consequences of laser-induced photomechanical effects are addressed via the use of holographic interferometry. In all, these experiments indicate that optimal fluence ranges can be defined in which damaging effects to the substrate are minimal or insignificant.

Removal of Plasma-Etch-Induced Polymers from Submicron Via Holes by Excimer Laser Ablation

AbstractA relatively new approach in removing the sidewall and bottom polymers resulting from reactive ion etching of via holes, using a non-contact dry excimer laser cleaning technique has been investigated. Pulsed excimer laser ablation at 248 nm has been found to be capable of removing the via-etch-induced polymers at fluences limited by the damage threshold of the underlying Al-Cu metal film with TiN anti-reflective coating of 250-280 mJ cm-2. A fluence window of 150-200 mJ cm-2 for efficient laser cleaning is also determined from the ablation rate data using the relation in the limit of Beer's law absorption. Experimental results have also shown that the ablation rate when irradiating at an angle is not only comparable to that at vertical incidence, but even register higher values for most of the ablation rate data obtained. An optimum incident angle for laser cleaning of 45° can be determined from the results.

Automatic Control and Real-Time Monitoring of Laser Cleaning and Laser Ablation

AbstractClosed-loop control and real-time monitoring of pulsed excimer laser cleaning and ablation have been realised by software and hardware design and development. The audible acoustic wave generated during excimer laser surface cleaning and ablation has been used as the feedback signal for the automatic control system of laser-surface interaction. Man–machine interface for laser operation and acoustic monitoring has been developed with LabVIEW 4.1 under Windows 95 environment. This system has been used to control and monitor laser cleaning and ablation processes in real-time and proven to successfully meet the requirements in our study. By database exchange, this automatic control system can also be applied to control drilling and materials removing.

Excimer laser cleaning of mold-contaminated paper: Sterilization and air quality considerations

The Excimer laser can be used to remove biological debris from underlying substrates. The ability of the krypton fluoride (KrF) Excimer laser to removeAspergillus niger mold, which had been grown on filter paper, was evaluated in the present study. The air in the immediate vicinity of the laser cleaning apparatus was sampled to determine if viable mold fragments were released into the air during treatment. It was found that filter paper could be sterilized with the Excimer laser wavelength with the proper selection of parameters. During this process, viable spores and mold fragments are released into the atmosphere, and pose a relative risk unless proper measures to prevent contamination of the work area are taken.

Excimer laser cleaning of Si(100) surfaces at 193 and 248 nm studied by LEED, AES and XPS spectroscopies

AES, XPS and LEED were used to study the role of the laser energy density and total photon dose on the efficiency of KrF ( λ=248 nm) and ArF ( λ=193 nm) excimer laser induced cleaning of Si(100) surfaces. Samples having the native oxide layer were first investigated. It was found that significant oxide removal takes place only at Atomically clean, damage free Si(100) surfaces were obtained irradiating pre-etched samples, which had only a thin SiO x ( x<2) layer and F, C and O containing adsorbed species. At 248 nm, when using 15 pulses, complete contaminant elimination was achieved in regions exposed to 0.8 J/cm 2 without provoking any morphological modification of the surface. At 193 nm clean surfaces were produced using average energy densities as low as 0.2 J/cm 2, if the number of laser shots was properly increased.

Excimer laser cleaning and processing of Si(100) substrates in ultrahigh vacuum and reactive gases

We have examined the effect of KrF excimer laser irradiation (248 nm, 20–25 ns) on Si(100) substrates under ultrahigh vacuum conditions and in controlled partial pressures of reactive gases. Surface compositions were determined before and after irradiation using in situ angle resolved x-ray photoelectron spectroscopy. We varied peak power, pulse energy, and halocarbon (CF4) or carbon monoxide (CO) pressures to investigate their effects on oxygen and carbon removal. Energy densities near 500 mJ/cm2 were sufficient to eliminate the carbon contamination; higher energies, which depended on the silicon preparation conditions, were required for oxygen removal. Samples were irradiated by pulses at 480–740 mJ/cm2 in molecular oxygen (O2) environment after cleaning to determine pulse energy and gas pressure requirements for oxide regrowth.

Mechanisms of excimer laser cleaning of air-exposed Si(100) surfaces studied by Auger electron spectroscopy, electron energy-loss spectroscopy, reflection high-energy electron diffraction, and secondary-ion mass spectrometry

The structure and composition of air-exposed Si(100) surfaces irradiated by a KrF (248 nm, 5.0 eV, 22 ns pulses) excimer laser in ultrahigh vacuum (UHV) have been studied using Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and reflection high-energy electron diffraction (RHEED). The near-surface region was compositionally profiled using secondary-ion mass spectrometry (SIMS). Pulsed laser irradiation at energies below the melting threshold (≊500 mJ cm−2 ) led to significant reductions in surface C and O concentrations but detectable contamination remained even after 200 pulses. However, surfaces exposed to several laser pulses at energies just above the melting threshold were atomically clean (less than 0.01 monolayer of impurities) as judged by AES and EELS and exhibited a 2×1 RHEED pattern. The peak in the EELS spectrum associated with surface dangling bond states was slightly lower in intensity and higher in energy than results obtained from thermally cleaned samples, while other surface states associated with the 2×1 reconstruction and noninteger RHEED streak intensities were also slightly less intense. SIMS profiles of the irradiated wafers revealed C and O impurities distributed throughout the melt volume. Thus, excimer laser irradiation produces clean Si(100) surfaces by a combination of desorption (accounting for approximately 1/3 to 1/2 of the decrease in impurity concentration) and dissolution leaving a disordered (2×1) surface. Annealing irradiated wafers at 600 °C for 1 min in UHV produced sharp 2×1 RHEED patterns and EELS spectra identical to those obtained from thermally cleaned (≥900 °C) surfaces.

Excimer laser cleaning, annealing, and ablation of β–SiC

The effects of ArF excimer laser irradiation on β–SiC in UHV were examined for a variety of laser intensities and pulse densities. The samples were analyzed in situ with Auger and electron loss spectroscopies, and ex situ with x-ray photoelectron spectroscopy. With progressively higher laser intensities, the SiC surface was initially cleaned of carbon and oxygen surface contaminants, and the Si–LVV Auger lineshape changed from the oxide to the carbide. Still higher laser intensities then partially reordered the surface. A carbon surface layer developed, and the C-KLL lineshape transformed from carbidic to graphitic. Finally, the surface segregated into an almost pure Si layer and an underlying carbon-rich layer, followed by ablation and pitting. Bulk heating during laser exposure may enhance reordering of sputtered or implanted material.