Y. W. Zheng

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

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 ...

Dry laser cleaning of particles from solid substrates: Experiments and theory

The experimental analysis of dry laser cleaning efficiency is done for certified spherical particle (SiO2, 5.0, 2.5, 1.0, and 0.5 μm) from different substrates (Si, Ge, and NiP). The influence of different options (laser wavelength, incident angle, substrate properties, i.e., type of material, surface roughness, etc.) on the cleaning efficiency is presented in addition to commonly analyzed options (cleaning efficiency versus laser fluence and particle size). Found laser cleaning efficiency demonstrates a great sensitivity to some of these options (e.g., laser wavelength, angle of incidence, etc.). Partially these effects can be explained within the frame of the microelectronics engineering (MIE) theory of scattering. Other effects (e.g., influence of roughness) can be explained along the more complex line, related to examination of the problem “particle on the surface” beyond the MIE theory. The theory of dry laser cleaning, based on one-dimensional thermal expansion of the substrate, demonstrates a great...

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

Spherical 0.5-μm silica particles were placed on a silicon (100) substrate. After laser illumination with a 248-nm KrF excimer laser, hillocks with size of about 100 nm were obtained at the original position of the particles. The mechanism of the formation of the subwavelength structure pattern was investigated and found to be the near-field optical resonance effect induced by particles on the surface. Theoretically calculated near-field light intensity distribution was presented, which was in agreement with the experimental result. The method of particle-enhanced laser irradiation has potential applications in nanolithography.

Laser induced removal of spherical particles from silicon wafers

Laser-induced removal of spherical silica particles from silicon wafers was investigated. The cleaning efficiency and laser cleaning thresholds for particles with diameters of 0.5, 1.0, 2.5, and 5.0 μm were carefully measured. It is found that the cleaning efficiency is more sensitive to laser fluence than laser pulse number and repetition frequency. The particle ejecting energies were found to increase with laser fluence. The threshold laser fluences for removing particles with sizes of 0.5 and 1.0 μm are 225 and 100 mJ/cm2, respectively, when KrF excimer laser is used. The threshold laser fluences are only a value below 5.0 mJ/cm2 for particles with a size of 2.5 and 5 μm. A model including both linear expansion and elastic deformation model was proposed to explain the experimental results. With this model, the particle movement and deformation in laser cleaning process were calculated. The expressions for threshold laser fluences were derived. The theoretical predictions are found to be greater than the experimental results. The difference can be explained by the enhancement of light intensity near the contacting area, due to the focusing and scattering by spherical particles. This model is useful to the study of laser cleaning as well as particle adhesion and deformation on solid surfaces.

Laser surface cleaning and potential applications in disk drive industry

Laser cleaning has emerged as an effective cleaning technique for removing contaminants from solid surfaces. Dry laser cleaning and steam laser cleaning have been developed recently, relying on pulsed laser heating of the surface without and with the presence of a thin liquid coating. A cleaning model was established for removal of particles from solid surfaces by taking Van der Waals force, capillary force and cleaning force into account. The model can not only explain the influence of laser fluence on cleaning efficiency, but also predict the cleaning thresholds. Laser-induced removal of film-type contaminants from solid surfaces has been studied. Laser cleaning mechanisms and its applications in disk drive industry will be discussed in this article.

Nanostructure fabrication using pulsed lasers in combination with a scanning tunneling microscope: Mechanism investigation

Nanostructure fabrication using lasers in combination with a scanning tunneling microscope has been reported in the past several years. Different mechanisms have been discussed for the formation of these nanostructures. However, they are controversial. In this study, we investigated the mechanism of nanostructure fabrication on both gold films and hydrogen-passivated Ge surfaces. Current-distance curves for a gold film and for an H-passivated Ge surface under an electrochemically etched tungsten tip were measured to determine the tip-sample distance. An analytical model was proposed to explain different mechanisms for nanostructure fabrication on gold films and on H-passivated Ge surfaces. Thermal expansion of the tip under laser irradiation was calculated. With comparison between the tip-sample distance and the thermal expansion of the tip, we can determine whether the mechanism is based on optical enhancement or on thermal mechanical indentation.

Laser-induced removal of plate-like particles from solid surfaces

Laser-induced removal of plate-like particles from solid surfaces has been studied both experimentally and theoretically. A theoretical model has been established by taking into account adhesion force, cleaning force and heat equation. The cleaning condition and threshold can be obtained by comparing adhesion force and cleaning force. The theoretical analysis shows that the cleaning force increases with increasing laser fluence, which leads to high cleaning efficiency. Laser cleaning from the reverse-side is more effective in removing plate-like particles than from the front-side. Theoretical predictions have been verified by the experimental results for removing plate-like particles from quartz substrates. A comparison of cleaning two types of particles is addressed.

Characterization of ejected particles during laser cleaning

Laser cleaning is a prospective cleaning method that can be widely used in microelectronics fabrication, archive restoration, and optical apparatus cleaning. Removal of particles from a solid substrate is an important aspect of laser cleaning. Although many studies have been carried out on this subject, few of them are objected to the characterization of the ejected particles in laser cleaning. In this article, a method was developed to “capture” the particles ejected from the substrate after laser irradiation. Detection of both angular distribution and ejection energies was achieved with this method. It was found that the angular distribution of the ejected particles fitted to a Gaussian curve when the laser irradiated normally to the substrate. The distribution curve for the particles ejected from a rough surface has a wider full width at half maximum than that from a smooth substrate. It was also found that the particle ejection energy increased obviously with laser fluence, therefore the laser cleanin...

Laser surface cleaning and real-time monitoring

Laser cleaning has emerged as an effective cleaning technique for removing contaminants from solid surfaces. Dry laser cleaning and steam laser cleaning have been developed recently, relying on pulsed laser heating of the heating of the surface without and with the presence of a thin liquid coating. Two cleaning models for the viewpoint of force and energy for dry laser cleaning and a cleaning model for steam laser cleaning were established for removal of particles from solid surfaces by taking Van der Waals force, capillary force and cleaning force into account. The models not only explain the influence of laser fluence on cleaning efficiency, but also predict the cleaning thresholds. Laser- induced removal of film-type contaminants has been studied and the surface cleanliness has been studied and the surface cleanliness can be monitored in real time by acoustic and electric means.

Angular effect in laser removal of spherical silica particles from silicon wafers

In this study, laser cleaning efficiencies to remove 2.5 μm particles have been investigated with different incident angles ranging from 0° to 60°. It is found that when the laser light irradiated normally to the substrate surface, the particle could be removed most efficiently. In this direction, the cleaning efficiency was also most sensitive to the light intensity. A sharp drop of cleaning efficiency occurred with a small change of the incident angle. Theoretical calculations based on the Lorentz–Mie theory and an accurate solution of the boundary problem, indicate that the light intensity near the contacting point is sensitive to the incident angle even though the incident light is uniform.