Laser ablation and micropatterning of thin TiN coatings

Laser ablation of thin TiN films deposited on steel substrates has been studied under wide-range variation of irradiation conditions (pulsewidth, wavelength, energy density and spot size). It has been demonstrated that both picosecond (150–300 ps) and nanosecond (5–9 ns) laser pulses were suitable for controllable ablation and microstructuring of a 1-μm-thick TiN film unlike longer 150-ns pulses. The ablation rate was found to be practically independent of the wavelength (270–1078 nm) and pulsewidth (150 ps–9 ns), but it increased substantially when the size of a laser spot was reduced from 15–60 μm to 3 μm. The laser ablation technique was applied to produce microstructures in the thin TiN films consisting of microcraters with a typical size of 3–5 μm in diameter and depth less than 1 μm. Tests of lubricated sliding of the laser-structured TiN films against a steel ball showed that the durability of lubricated sliding increased by 25% as compared to that of the original TiN film. Received: 28 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

Self-rolled polymer microtubes with engineered hidden walls

Micro and nanotubes are manufactured by self-rolling of thin bilayer polymer films (polystyrene/poly(4-vinyl pyridine, PS/P4VP) gradually released from a solid substrate. Swelling of the P4VP layer in acidic water opposed by a stiff PS layer leads to rolling of the film into tubings of micrometer diameters. The position and the arrangement of the tubes on the substrate are defined by UV photolithography structuring of the polymer film. A few nanometer thick metallic patterns, such as arrays of conductive stripes and simple electrical circuits, are formed on the top of the polymer film by magnetron sputtering and transferred to the tube interior by rolling. Intra-tubular rolled-up microsolenoids are applied to generation of localized magnetic fields inside the tubes and in their vicinity.     

The Self-organization of Metal Loaded Micelles - An Approach to Prepare Ordered Arrays of Metallic Nanoislands

The preparation and characterization of hexagonally ordered pure Au nanoparticles is described. Self-assembly of diblock copolymers in solution is the driving force which leads to micellar structures. Starting, for example, with Au-salt loaded inverse micelles, monomicellar arrays exhibiting a significant hexagonal order can be prepared with taylored intermicellar distances and structure heights on top of various substrates. In order to remove the polymer matrix and to finally obtain arrays of pure Au nanoparticles, the micelles are first exposed to an oxygen plasma followed by an annealing process. Special attention is given to the chemical state of the nanoparticles applying photoelectron spectroscopy to control all preparational steps.     

Single-pulse drilling study on Au,Al and Ti alloy by using a picosecond laser

Picosecond laser single pulse ablation of Au, Al and Ti alloy (Ti6Al4V) was experimentally investigated with a laser pulse width of 10 ps at a wavelength of 1064 nm for potential industrial micromachining applications. The diameters, depths and morphologies of the drilled craters were studied. Two novel phenomena were found: as hole diameters decreased with fluence, a change of slope of the trend line indicated a change in ablation mechanism for Al and Ti alloy, metallic materials with short electron-phonon coupling times (<10 ps), while Au showed no such transition: an isolated island structure was also observed on Au due to significant melt expulsion. A one-dimensional two-temperature model has been used to discriminate different ablation phenomena. It is shown that metallic materials with different electron–phonon coupling constant have different ablation characteristics in the ps regime. This study could be very helpful for metallic material micromachining with high repetition rate ps lasers pulses which indicates that high throughput may be achieved as well as good machining quality.     

Formation of the nanodots and change of the characteristics of thin magnetic films under laser irradiation

Results of experimental investigations of laser radiation interaction with the Ti, Co, NiFe, TbFe and LaSrMnO₃ films are presented. It is shown, that it is possible to improve magnetic characteristics and to obtain magnetic films with regular distribution of nanodots by the laser radiation. It is observed an increasing of magnetic permeability and the reduction of the coercive force after irradiation of the NiFe films by the nanosecond laser pulses that are induced by the substantial growth of the size of the nanodots in the process of recrystallization. Magnetic nanodots of 100–200 nm size are produced by the method of the laser cutting of the continuous magnetic films, or are formed in a nonmagnetic matrix by the diffusion in the multilayered films and oxidization of TbFe and LaSrMnO_3−x films at the irradiation of the nanosecond laser pulses.     

Long-term variation in nuclide activities under laser ablation of metallic targets in aqueous solutions of uranium salt

The long-term variation in the activities of nuclides belonging to the uranium series after laser irradiation of metallic targets (Au, Be, Al, Ti) in aqueous solutions of UO₂Cl₂ has been experimentally investigated. It is found that the activity of ²³⁴Th, in contrast to unirradiated solution, continues to increase for 50 to 60 days after laser irradiation and, having reached a stationary level, remains at it for a long time (about 1 yr). The activity dynamics is modeled theoretically on the assumption that a fraction of ²³⁸U in the irradiated solution is under peculiar conditions, so that its half-life ismuch shorter than that of spontaneous α decay of this nuclide under normal conditions. Possible mechanisms of variation in the shape of the potential barrier for α-particles in the presence of nanoparticles formed by laser ablation of targets are discussed.     

Self-influence of a femtosecond laser beam upon ablation of Ag in liquids

Formation of nanoparticles of either Au or Ag is reported under ablation of metallic targets exposed to radiation of a femtosecond Ti:sapphire laser (wavelength of 810 nm, pulse width of 120 fs) in either water or ethanol. Nanoparticles are characterized by UV-visible spectroscopy and transmission electron microscopy. Nanoparticles of Ag are several times smaller than those of Au at otherwise equal conditions. The effect is attributed to the self-influence of a laser beam via generation of the second harmonics of the laser radiation on Ag clusters and its good matching to the plasmon resonance of Ag. PACS 42.62.-b; 61.46.+w; 78.66.-w     

Enhancement of pulsed laser removal of metal oxides by electrochemical control

Pulsed laser irradiation of oxidized metallic surfaces in an electrolytic cell under proper voltage conditions is demonstrated to be a promising new approach for effective removal of oxide films. Systematic measurements on simulated corrosion-product films by optical reflectance profile and energy dispersive X-ray spectroscopy are used to study the physical mechanisms of this novel phenomenon, the physical conditions for its observation and its possible generality. It was observed that the utilization of a basic electrolyte solution and the imposition of a certain cathodic potential prior to laser irradiation is an essential requirement for a high removal efficiency. This new technique has potential applications in metallurgy, semiconductor fabrication technology and decontamination of nuclear power plans and is suitable for maskless patterning of oxidized surfaces.     

Luminescence properties of femtosecond-laser-activated silver oxide nanoparticles embedded in a biopolymer matrix

Strong visible luminescence is observed from silver clusters generated by femtosecond-laser-induced reduction of silver oxide nanoparticles embedded in a polymeric gelatin matrix. Light emission from the femtosecond-laser-activated matrix areas considerably exceeds the luminescence intensity of similarly activated bare silver oxide nanoparticle films. Optical spectroscopy of the activated polymer films supports the assignment of the emissive properties to the formation of small silver clusters under focused femtosecond-laser irradiation. The size of the photogenerated clusters is found to sensitively depend on the laser exposure time, eventually leading to the formation of areas of metallic silver in the biopolymer matrix. In this case, luminescence can still be observed in the periphery of the metallic silver structures, emphasizing the importance of the organic matrix for the stabilization of the luminescent nanocluster structures at the metal–matrix interface. PACS 78.66.Qn; 78.20.-e; 78.40.-q; 78.67.Bf; 78.68.+m     

Properties of conductive polymer films deposited by infrared laser ablation

Thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) were deposited by resonant infrared laser vapor deposition (RIR-LVD). The PEDOT:PSS was frozen in various matrix solutions and deposited using a tunable, mid-infrared free-electron laser (FEL). The films so produced exhibited morphologies and conductivities that were highly dependent on the solvent matrix and laser irradiation wavelength used. When deposited from a native solution (1.3% by weight in water), as in matrix-assisted pulsed laser evaporation (MAPLE), films were rough and electrically insulating. When the matrix included other organic “co-matrices” that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good electrical conductivity (0.2 S/cm), but only when irradiated at certain wavelengths. These results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process.     

Metal and polymer melt jet formation by the high-power laser ablation

The laser-induced metal and polymer melt jets are studied experimentally. Two classes of physical phenomena of interest are: first, the process of explosive phase change of laser induced surface ablation and second, the hydrodynamic jetting of liquid melts ejected from a beamed spot. We focus on the dynamic link between these two distinct physical phenomena in a framework of forming and patterning of metallic and polymer jets using a high-power Nd:YAG laser. The microexplosion of ablative spot on a target first forms a pocket of hot liquid melt and then it is followed by a sudden volume change of gas-liquid mixture leading to a pressure-induced spray jet ejection into surrounding medium.     

Nd:YAG laser ablation characteristics of thin CIGS solar cell films

This work reports that the ablation characteristics of thin CuIn_1?x Ga _x Se₂ (CIGS) solar cell film differ significantly with elemental composition and laser pulse energy. From in situ shadowgraphs measured during Nd:YAG laser (1,064 nm) irradiation of CIGS films and crater morphologies, it was found that strong surface evaporation is dominant for low Ga concentration films of which band gap is well below the photon energy. As the band gap of CIGS film becomes close to or over the laser photon energy due to increased Ga content, surface absorption diminishes and at low laser energy, laser heating of the film plays an important role. It is demonstrated that for the CIGS films with Ga/(Ga + In) ratio being approximately over 0.2, the laser irradiation leads to solid phase removal of the film due to thermomechanical fracture at low laser energy but to ablative evaporation at elevated energy.     

激光等离子体效应对细胞烧蚀特性研究

研究对比了激光直接辐照、聚焦辐照以及激光等离子体辐照三种辐照方式下,洋葱表皮细胞的烧蚀特征,并基于激光辐照的热力学特性对细胞的温升以及相变过程进行分析。观察发现： 直接辐照对细胞的杀伤效果很不明显;聚焦辐照会引起焦点附近细胞的断裂以及脱水;激光等离子体辐照作用下,细胞会呈现大面积的去除,断裂边缘粗糙,且细胞层有叠加现象。理论分析发现,激光等离子体具有热效应、辐射电离及冲击波效应等,会增加激光脉冲能量到细胞的沉积、以及对细胞冲击剥离等,从而会大大增加细胞的杀伤范围和效率,可用于对细胞进行大面积杀伤。     

Laser-assisted modification of polystyrene surfaces for cell culture applications

Laser-assisted patterning and modification of polystyrene (PS) was investigated with respect to applications in micro-fluidics and cell culture. For this purpose the wettability, the adsorption of proteins and the adhesion of animal cells were investigated as function of laser- and processing parameters. The change of surface chemistry was characterized by X-ray photoelectron spectroscopy. The local formation of chemical structures suitable for improved cell adhesion was realized on PS surfaces by UV laser irradiation. Above and below the laser ablation threshold two different mechanisms affecting cell adhesion were detected. In the first case the debris deposited on and along laser irradiated areas was responsible for improved cell adhesion, while in the second case a photolytic activation of the polymer surface including a subsequent oxidization in oxygen or ambient air is leading to a highly localized alteration of protein adsorption from cell culture media and finally to increased cell adhesion. Laser modifications of PS using suitable exposure doses and an appropriate choice of the processing gas (helium or oxygen) enabled a highly localized control of wetting. The dynamic advancing contact angle could be adjusted between 2° and 150°. The hydrophilic and hydrophobic behaviour are caused by chemical and topographical surface changes.     

Enhanced adhesion of metal films on PET after UV-laser treatment

The adhesion-force of thin metal films on PET foils can be significantly improved by UV excimer-laser irradiation of the polymer surface prior to metal deposition. The laser fluences required are well below the ablation threshold.     

Removal of doped poly(methylmetacrylate) from tungsten and titanium substrates by femto- and nanosecond laser cleaning

The influence of different laser pulse lengths on the removal of a polymer layer from metal substrates was investigated. As model systems, doped poly(methylmetacrylate) (PMMA) on titanium and tungsten substrates were selected.The ablation threshold and irradiation spot morphology of titanium and tungsten were compared for femtosecond (fs) and nanosecond (ns) laser irradiation and different pulse numbers. Nanosecond laser treatment resulted in a non-homogeneous surface morphology for both titanium and tungsten substrates. Femtosecond irradiation of tungsten revealed a homogeneous ablation spot with little changes in the surface morphology. For titanium, the formation of columnar structures within the irradiation spot was observed.Two different dopant concentrations were used for PMMA to achieve an equal linear absorption coefficient for the femto- and nanosecond laser wavelengths of 790 and 1064 nm. The best results were achieved for the removal of doped PMMA by femtosecond laser irradiation, where only a minimal modification of the metal surface was detected. In the case of nanosecond laser exposure, a pronounced change of the structure was observed, suggesting that damage-free cleaning of the selected metal may only be possible using femtosecond laser pulses. Different experimental parameters, such as laser fluence, pulse repetition rate and sample speed were also investigated to optimize the cleaning quality of doped PMMA from tungsten substrates with femtosecond laser pulses.     

Pulsed laser deposition of polymers doped with fluorescent molecular sensors

Pulsed laser deposition (PLD) has been used to obtain thin films of poly(methyl-methacrylate), PMMA, and polystyrene, PS, doped with fluorescent probes based in the amino aromatic compounds S6, DMA-2,4, Dans and Acrid-acryl. These compounds, both in solution and inserted in polymeric films, show solvatochromic emission band shifts upon changes of pH, polarity and viscosity in their micro-environment and, prepared in thin films, could prove advantageous for use as sensors of the presence of contaminating environmental agents. Deposits, obtained by irradiating targets consisting in films of doped PMMA and PS with a Ti:sapphire laser (800 nm, 120 fs pulse), were analyzed by optical and environmental scanning electron microscopy, fluorescence microscopy, laser-induced fluorescence, micro-Raman spectroscopy and flow injection analysis-mass spectrometry. Results show the effective transfer of the polymer and the probe to the substrate and the high dependence on film structure on the polymer used in the targets. Irradiation with a KrF laser (248 nm, 20 ns) of Dans-doped targets only produced deposits when using PMMA for target preparation. Results are discussed in terms of the participation of multiphoton processes in the fs IR irradiation, the influence of the linear absorption coefficient of the targets and of the different contributions of thermal and chemical effects in the IR femtosecond and UV nanosecond domains. PACS 81.15.Fg; 61.82.Pv     

Change in optical path length of saturable absorbing dye-doped polymer films under laser irradiation

The cause of the change in the optical path lengthΔ(nd) of the saturable absorbing dye-doped polymer films under laser irradiation is investigated using polyvinyl alcohol and gelatin films doped with erythrosin B. The temperature rise of the dye-doped films, the changes in optical path length, film thickness, the refractive index and the optical density of the dye-doped films are measured under laser irradiation. The thermal expansion due to the temperature rise of the dye-doped films caused by the irradiation has greater effect on theΔ(nd) than the fading of dye molecules due to the irradiation of laser beams.     

Porphyrin-sensitized laser swelling and ablation of polymer films

Laser-induced morphological changes of poly(methyl methacrylate), poly(N-vinylcarbazole), and gelatin films doped with porphyrins have been studied by etch depth measurement and scanning electron microscopy. An irreversible swelling of the irradiated surface was observed for all films in the case of low laser fluence. The swelling was replaced by ablation when the fluence was increased. The etch depth depends on the irradiation fluence and the dye concentration in the polymer. The observation of the irradiated surfaces suggests that the thermal effect is predominant both for swelling and ablation. The surface temperature at which swelling or ablation is initiated was estimated, assuming that these morphological changes take place at a certain temperature for any dye concentration in each polymer film.