Underwater and water-assisted laser processing: Part 1—general features, steam cleaning and shock processing

When a high-power laser beam is focused into liquid, it results in a shock wave emission and cavitation bubble generation. Upon inserting a rigid substrate into the liquid, the bubbles migrate towards the substrate due to the Bjerknes attractive force. Due to bubble–substrate and/or bubble–free-surface interaction, a high-speed liquid jet is formed during bubble collapse, and a collapse shock wave is generated at the moment of bubble collapse near the substrate. These shock waves and liquid jet induce large forces acting on the substrate to remove particles from it. For a substrate several millimeters away from the laser focus point, the collapse shock wave and liquid jet play key roles in removal of particles. The cleaning efficiency increases with an increase of laser fluence and decreases with an increase of distance between substrate surface and laser beam focus point or depth below liquid surface. In a case of bubbles close to substrate and liquid-surface boundaries, implosion of the bubbles will give rise to shock waves and liquid jets oblique to the substrate surface with the parallel and perpendicular components of the forces onto the particles. These oblique liquid jets and shock waves result in high cleaning efficiency. A liquid, such as alcohol and commercial washing solution, as the surrounding medium, rather than air or vacuum, can reduce adhesion force and enhance cleaning efficiency.

Laser-induced cavitation bubbles for cleaning of solid surfaces

The surface properties of silica and titania are mainly determined by the presence, density, and type of terminal hydroxyl groups (Si−OH “silanol” and Ti−OH “titanol”). Thermal treatment at elevate...

Thermally- and photoinduced changes in the water wettability of low-surface-area silica and titania.

The field of nanoelectronics has evolved into a major area of investigation. Nanolithographic techniques such as atom beams [1, 2], electron beams [3], scanning probe tunneling [4‐6], and scanning nearfield optical lithography [7‐9] are expected to be potential methods in the fabrication of present and future nanodevices. However, due to their incompatibility with the present fabrication processing and their low throughput, the application of these methods is presently confined to the experimental stage. Meanwhile, traditional optical lithography is limited to the diffraction effect and always relates to complex system and high cost. In this letter, we report a novel, low-cost, and simple optical lithography technique using particleenhanced laser irradiation. Standard spherical silica particles (Duke [10]) packaged as low-residue aqueous suspensions were used in our experiment. The diameter of the particles is 0.5 μ m, with a deviation limited in a range of ∠ 5%. Silicon

Laser writing of a subwavelength structure on silicon (100) surfaces with particle-enhanced optical irradiation

The effects of optical resonance and near field in the interaction of transparent particles on a substrate with laser light have been examined experimentally and theoretically. It is found that pits can be created at the contacting point between the particle and the metallic surface by laser irradiation (KrF,λ=248 nm) with a single pulse. The influence of the particle size and the laser fluence on the structuring of the surface has been investigated. The size of the particle ranges from 1.0 μm to 140 nm in diameter. The morphologies of the holes created have been characterized by an atomic force microscope and a scanning electron microscope. For constant laser fluence, the created hole is sensitive to the particle size. For higher-laser fluence, the corresponding hole becomes larger and deeper. With a low fluence of 300 mJ/cm2 and for 140 nm particles, the lateral dimensions of created pits can be down to 30 nm. With a high fluence of 750 mJ/cm2 and 1.0 μm particles, the diameter and the depth of created holes are about 350 and 100 nm, respectively. Theoretical calculations and an accurate solution of a boundary problem indicate that incident light could excite some resonance modes inside the particle and produce enhanced light intensities on the contacting area (substrate surface). The light intensity on the contacting area is nonuniform and sensitive to the particle size parameter. Experimental results are explained and are very consistent with those of theoretical calculations. The experimental results also provide direct evidence of the optical resonance and near-field effects in the interaction of transparent particles on the substrate with laser light.

Pulsed laser-assisted surface structuring with optical near-field enhanced effects

Particles as small as 0.3 μm in diameter have been successfully removed from a glass surface using a single ultraviolet pulse from a frequency doubled copper vapor laser (255.3 nm). Quantitative analysis of the particle density before and after laser irradiation shows that laser cleaning occurs after a fluence threshold is reached. The cleaning efficiency after threshold follows a nonlinear trend with respect to fluence. A model is presented which reveals that the cleaning efficiency is a function of the irradiance distribution of the beam used. Results of modeling thermal expansion of the substrate and particles, and particle adhesion do not confirm a thermal expansion mechanism for laser cleaning in this study, in contrast with other recent reports.

A quantitative analysis of single pulse ultraviolet dry laser cleaning

A recent study of ultraviolet laser cleaning of silica glass surfaces contaminated with medium density alumina particles has shown a systematic type of laser-induced surface damage. This is characterized as a pit which increases in diameter and depth with increasing irradiating fluence. The damage pit occurs at lower fluence levels when smaller contaminant particles are used. The pits are due to laser ablation of the particle coated glass surface. The threshold for this laser ablation is well below that for a sample of the glass in its uncontaminated condition and the efficiency of ablation is estimated as being about 1000 times greater than that for silica. This laser damage is difficult to visualize by standard optical microscopy and could easily go undetected in laser cleaning studies. It may have general implications in the application of the new laser cleaning technologies to a variety of surfaces, including dielectrics and semiconductors, where medium to high densities of contaminant particles are p...

Reduced threshold ultraviolet laser ablation of glass substrates with surface particle coverage: A mechanism for systematic surface laser damage

A frequency-doubled copper vapour laser was used to irradiate the surface of silica with ultraviolet light at fluences of between 0.1 and 0.5 J/cm2 per pulse for several hundred pulses. Analysis of the surface composition using time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) shows that the relative surface-hydroxyl (OH) concentration decreases with increasing laser irradiance. This dehydroxylation results in silica surfaces with final-state hydroxyl concentrations similar to those obtained through the thermal treatment of silica at around 1000 °C. The mechanism for the dehydroxylation reported here, however, is more likely to be photolytic than thermal. Laser dehydroxylation allows for the selective and rapid dehydroxylation of silica offering many practical advantages over thermal dehydroxylation. The modified surface has significantly increased hydrophobicity compared to the untreated silica surface.

Surface modification of silica with ultraviolet laser radiation

The cleaning action of the cleaning arc of two optical-fiber fusion-splicing devices is investigated. Interferometric analysis of the optical-fiber endfaces before and after exposure to the cleaning arcs reveals damage to the endfaces caused by one of the devices. The position of the fiber relative to the cleaning arc is thought to cause this damage.

Electric-arc cleaning of optical-fiber endfaces

We report results that show that particles as small as 0.1 /spl mu/m diameter can be successfully removed from the glass surface. In order to obtain quantitative data from such images, we measured the difference in surface particle density before and after irradiation for a range of fluences and particle sizes. These results reveal that: (1) laser cleaning occurs after a definite threshold is reached (2) after the cleaning threshold is reached, the difference in surface particle density before and after irradiation follows a definite trend with respect to fluence, and (3) the area cleaned depends on the beam area above the threshold fluence for cleaning.

D. R. Halfpenny

Papers

A quantitative analysis of single pulse ultraviolet dry laser cleaning

Reduced threshold ultraviolet laser ablation of glass substrates with surface particle coverage: A mechanism for systematic surface laser damage

Surface modification of silica with ultraviolet laser radiation

Electric-arc cleaning of optical-fiber endfaces

Removing particles from glass with ultraviolet laser cleaning