Laser Cleaning

The following sections are included:

• List of Contributors

• Contents

• Preface

Laser cleaning was studied in a sequence of experiments that spanned a time frame of 15 years after it had originally been identified as a means to remove sub-micrometer particles from sensitive silicon membrane stencil masks. This paper describes how the subsequent investigations added to the understanding of the basic mechanisms of both, dry laser cleaning and steam laser cleaning, respectively.

A model for nanosecond dry laser cleaning that treats the substrate and particle expansion on a unified basis is proposed. Formulas for the time-dependent thermal expansion of the substrate, valid for temperature-dependent parameters, are derived. Van der Waals adhesion, substrate and particle elasticity, and particle inertia are taken into account for an arbitrary temporal profile of the laser pulse. The characteristic time for the particle on the surface system is deduced. This time is related to the size of the particles as well as the adhesion and elastic constants. Cleaning proceeds in different regimes if die duration of the laser pulse is much shorter or longer than this time. Expressions for cleaning thresholds are provided and compared with experiments on the 248 nm KrF excimer-laser cleaning of Si surfaces from spherical SiO₂ particles with radii between 235 and 2585 nm in vacuum. Discrepancies between the experimental data and theoretical results seem to indicate that nanosecond dry laser cleaning cannot be explained purely on the basis of one-dimensional tiiermal expansion mechanism.

Optical problems, related to the particle on the surface, i.e. optical resonance and near-field effects in laser cleaning are discussed. It is shown that the small transparent particle with size by the order of the wavelength may work as a lens in the near-field region. This permits to focus laser radiation into the area with the sizes, smaller than the radiation wavelength. It leads to 3D effects in surface heating and thermal deformation, which influences the mechanisms of the particle removal.

The main focus of laser cleaning research at Macquarie University, Sydney, Australia, has been developing techniques for cleaning applications in the optics, optoelectronics and photonics industries. Dry/raised humidity cleaning techniques are favoured for small scale optical, integrated-optic and photonic devices. All of wet, damp and dry laser-cleaning methods are applicable to larger scale optics. All of our studies to date have measured the laser cleaning result effected by a single laser pulse. Excellent removal efficiencies of alumina particles (0.3 µm −tens of microns) from glass surfaces have been obtained. Also we have used “dry” particles rather than particles in suspension. Use of dry particles leads to significant agglomeration, thus, our studies evaluate the impact of particle agglomeration on laser cleaning results. In the sections that follow we firstly present some tables summarizing the results from experimental laser cleaning studies, from the research literature published at the time of writing. Then, secondly, we review our own laser cleaning research from the perspectives of development and critical evaluation of the experimental and analysis methods, contrasting laser cleaning results achieved with different UV pulsed laser systems, and accurately measuring laser cleaning threshold fluence.

Nanosecond pulsed laser irradiation is shown to be effective for cleaning contaminant particles as small as 0.3 µm in diameter and a variety of organic/inorganic contaminants from metallic substrates and optical components. In the steam-cleaning approach a micron-thick liquid film is deposited on the surface just before the laser irradiation. Experiments are also done to examine the cleaning of surfaces immersed in liquids. The cleaning threshold and efficiency are investigated for the fundamental, frequency-doubled and frequency-tripled Nd: YAG laser harmonics. The particle removal, rapid phase-change and thin liquid film ablation processes are examined via in-situ non-contact diagnostics.

The removal of dust particles from semiconductor surfaces requires new cleaning strategies such as Steam Laser Cleaning (SLC). It is based on laser-induced explosive evaporation of a liquid layer applied on the surface. We have investigated the laser-induced nucleation and growth of gas bubbles at silicon/water, silicon/isopropanol and silver-film/water - interfaces by light scattering and surface plasmon spectroscopy. The achieved superheating of the liquid before bubble nucleation sets in strongly depends on the substrate roughness. On rough metal films it is only about 30 K in water, compared to about 150 K on smooth silicon surfaces. Isopropanol (IPA) on smooth silicon surfaces could be heated to 116°C, corresponding to a superheating of 36 K. In combination with numerical calculations it was possible to determine the heat transfer coefficients silicon-water (ξ = 3 ·10⁷ W/m² K) and silicon – IPA (ξ = 1 ·10⁷ W/m² K). Using optical techniques we have measured the pressure wave created by the growing bubbles and the bubble growth velocities. For a quantitative study of the efficiency of SLC we deposited spherical colloidal particles on industrial silicon wafers. We observed a sharp threshold for particle removal at 110 mJ/cm² (laser λ = 532 nm, FWHM = 8 ns) which is independent of the size (diameter 800 nm down to 60 nm) and material of the particles and efficiencies above 90% for particle removal. On the basis of our results we discuss the validity of the existing SLC models and the perspective of the application of SLC as an industrial cleaning tool.

Physical mechanisms of laser cleaning of solid surface from soiling particles and films are discussed. Dry laser cleaning of surface from particles is considered to be a result of inertial force, appearing due to thermal expansion of absorbing particles and/or substrate (shaking-off mechanism). Steam laser cleaning is considered for the cases of absorbing particles and absorbing substrate. In the latter case, peculiarities of bubble formation in the liquid layer under the conditions of ns laser action are analyzed. Laser cleaning of surfaces from films by shaking-off and buckling mechanisms (due to thermal tension) is considered. Conditions of the action of each of them are defined. Other physical mechanisms, blasting and evaporation, are briefly discussed.

Laser cleaning of artworks and antiquities is reaching a mature enough stage both in terms of the procedures employed and the understanding of the interaction processes involved. To this effect, a series of studies have been pursued aiming to process optimization. Appropriate model systems have been studied for elucidating the interaction mechanisms both in polymerized phases as well as inorganic encrustation. An overview of these issues presented and their implications for future directions outlined.

The physical mechanism and the theoretical model for pigment discoloration at laser ablation are discussed. The model includes two subsequent steps: 1) examination of ablation in subthreshold (Arrhenius tail) region and 2) examination of the nucleation process and structural modification within the volume of material close to ablation front. The results of calculations show that in the wide range of parameters exists the strong sharpening effect, when the thickness of modified layer (a few nanometers) is significantly smaller than the heat penetration depth or depth of optical absorption. These results are in good agreement with experimental data on discoloration effect.

Laser cleaning of FEA (field emitter array) surfaces with four different wavelengths has been performed to improve the emission characteristics of FEAs. Wavelength dependent (i.e., energy dependent) cleaning effect was found to take place for Si FEAs, when the FEAs were irradiated by a Nd: YLF laser.

Laser cleaning of organic contamination from microelectronic device surfaces has been studied. It was applied to remove plasma debris accumulated on flexible circuit for inkjet printer cartridge and mold flash on IC packages. Compared to conventional cleaning techniques, pulsed laser cleaning has advantages of being fast speed, flexible, easy control, dry and non-contact processing. An audible acoustic wave, plasma-induced electric field and plasma optical signals were used for real-time monitoring of the cleaning process. This permits to perform complete removal of the organic contamination of the device substrates without any damage.

The following sections are included:

• Subject Index