A cone microstructure has been used as a template to generate nanotips and to promote nanoparticle alignment. A quasi-periodic array of nanotips is produced when the laser-induced cone microstructure is subject to chemical etching due to tapering of the cone tips. Nanoparticles can be produced on the surface of a silicon specimen by irradiating it in the presence of an inert gas atmosphere. The backscattered material that is re-deposited on the substrate, upon irradiation at fluences close to the melting threshold, is composed of a thin film intermixed with extremely small nanoparticles. Further irradiation promotes film clustering and nanoparticle formation. In the presence of cones, the nanoparticles become aligned into straight and long (~1 mm) lines whose spacing is close to the laser wavelength. This result suggested an ordering mechanism similar to that occurring for laser-induced periodic surface structures. The relation between nanoparticle line spacing and angle of incidence of the radiation supported this similarity. Nanoparticle ordering also was promoted by laser-enhanced chemical vapor deposition (LCVD) using polarized light, when a laser-induced periodic surface nanostructure was present in the substrate.
Laser-induced surface structuring of silicon was studied using fluences close to the metering threshold and He gas background atmosphere. The effects of an initial surface microstructured region and of light polarization on the evolution of the surface topography were investigated. The microstructured surface topology consisted of an array of microholes surround by microcones of 2-3 micrometers tip-diameter and over 20 micrometers high. Pulsed laser irradiation of laser- microstructured silicon induces the formation of nanostructures. Nanocolumns having a diameter of 100 to 200 nm and reaching a height of up to 3 micrometers upon cumulative laser pulses grow on top of every microcone. The mechanisms of nanocolumn origin and growth are analyzed. Periodic undulations approximately 10 nm-high are formed when flat silicon substrates are irradiated with polarized laser light. These periodic structures have a wavelength that is a function of the light wavelength and the angle of incidence of the laser beam. At a slightly higher laser fluence, approximately 30 nm-diameter nanoparticles form on the surface of laser irradiated flat silicon specimens. Linear arrays of silicon nanoparticles with fairly uniform size that extend up to a millimeter are formed if the irradiation is performed using polarized light or the irradiated area contains a microstructured region. These nanostructures are analyzed within the frame of the theory of laser induced surface periodic structures.
Excimer laser irradiation of insulators produces structural and chemical modifications in the near-surface region of these materials. These changes have lead to the usage of excimer lasers to engineer the surface of insulators for various applications, as illustrated in four examples presented here: (1) Laser-enhanced bonding of deposited metallic films. A very strong bonding between metallic films and Al2O3 can be achieved if the substrates are pulsed-laser treated prior to deposition. AES reveals that strong bonding occurs when an intermediate interfacial compound forms as a metallic film is deposited on a laser-irradiated substrate. (2) Laser encapsulation of metallic particles in silica. Thin films of gold, copper and iron deposited on silica can become encapsulated as small particles upon pulsed laser irradiation XTEM indicates two distinctive stages in the encapsulation process, during one laser pulse. In the first stage, the film melts and clusters into small particles, and in the second stage, the particles are driven into the substrate. (3) Laser- induced surface activation for electroless deposition. In this process, a pattern is imprinted on a substrate by laser irradiating its surface through a mask. Upon immersion of the substrate in an electroless solution, a metallic film is deposited only on the laser-exposed area. Auger emission spectroscopy (AES) and cross sectional transmission electron microscopy (XTEM) indicate that electroless deposition is promoted by the presence of metallic aluminum in AlN and in Al2O3, and of substoichiometric oxide in Al2O3, as well. Other laser irradiation effects that also could induce activation are analyzed. (4) Laser-induced deactivation of a previously activated area.
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Laser Applications in Microelectronic and Optoelectronic Manufacturing IX