Fluence dependence of femtosecond-laser-induced nanostructure formed on TiN and CrN

The fluence dependence of the nanostructure formation, which has been observed in our recent experiments on femtosecond (fs)-laser ablation, was examined in detail for hard thin films of TiN and CrN. The size D of the periodic fine structure formed with fs-laser pulses can be divided into two regions that depend on the laser fluence F, i.e. the region I where D increases rapidly with increasing F near the ablation threshold, and the region II where D increases slowly with an increase in F and almost saturates. The nanostructure has been observed only in the region I with a narrow width of F. The region II produces a periodic ripple structure whose size is 1/2–4/5 of the wavelength used. The effects of the thermal process and material composition on the nanostructure formation are discussed. PACS 61.80.Ba; 79.20.Ds; 42.62.Cf     

Formation of periodic structures by surface treatments of polyamide fiber: Part I. UV excimer laser irradiation

The processes of UV excimer laser irradiation (both high- and low-fluence) of polyamide fiber were systemically studied, including the surface temperature of the material during the treatment and possible mechanisms for the structure formation. The fluence applied in the high-fluence laser irradiation was above the ablation threshold of the material. The ablation of polymer can be described on the basis of photo-thermal bond breaking within the bulk material. The fluence applied in the low-fluence laser irradiation was far below the ablation threshold of the material. The development of low-fluence laser-induced structures is closely related to the absorption coefficient of the material, the laser fluence used, the polarization of the laser beam, the angle of incidence, and the number of laser pulses applied.     

Non-Thermal Laser Ablation Model for Micro-Surgical Applications

We have developed a non-thermal laser ablation model which may reduce thermal damage to neighboring structures. Based on this model, the three critical parameters for a well controlled non-thermal microsurgery are (1) the laser wavelength with its photon energy matching closely the bond dissociation energy, (2) the energy fluence must be above threshold to avoid thermal process due to non-radiative relaxation from the excited electronic states to vibrational, (3) ultra short laser pulses (few fs) to completely eliminate thermal and direct biomolecular reactions. In this model the UV laser photon dissociates the molecular bonds which leads to the splitting of longer polymer chains into small fragments. The excess energy if any may appear as kinetic energy in the polymer-fragments. The extreme rapidity of the bond breaking process reduces heat conduction. The model establishes a relationship between ablation depth per pulse, the absorption coefficient, the incident laser energy fluence, and the threshold energy fluence. The ablation depths per pulse were calculated for the polymers Polymethyl methacrylate (PMMA) and polyimide for various commercially available UV lasers. It has been found that the minimum ablations depth occurs at 193 nm for both PMMA and polyimide. This assures a well defined incision with minimal thermal damage to the surrounding structures at this wavelength. There exists a definite threshold energy fluence for non-thermal ablation for any given biomolecule and below the threshold the non-radiative relaxation process may cause thermal ablation. New ultra fast lasers (few femtoseconds) (fs) will completely eliminate thermal diffusion as well as direct biomolecular reactions.     

Characterisation of combined positive-negative photoresists by excimer laser ablation

Novel photopolymers containing side groups based on o-methoxycinnamylidenemalonic acid, which undergo selective photo-crosslinking without destruction of the polymer backbone upon irradiation at 5>395 nm, have been developed for potential applications as combined positive-negative resists and multilayer resists. An XeCl excimer laser (5=308 nm, F=20 ns) was used as the irradiation source to study the ablation and microstructuring characteristics of the polymers. The materials were structured before and after crosslinking. The ablation rate was analysed by varying the fluence (0.01-10 J/cm²) and the number of pulses for a given irradiation area. Etch rates of about 2 7m per pulse at a fluence of 9 J/cm² could be achieved for all polymers. The polymer with triazene groups reveals a higher etch rate at low fluences (less than 300 mJ/cm²) than the polymer without a triazene group. The experimentally observed threshold fluence for the triazene-containing polymer is about 30 mJ/cm². Using a Schwarzschild-type reflection objective (152), microstructures with a resolution in the micron range were produced on both polymer films. The quality of the structures was evaluated by scanning electron microscopy. The results indicate that the new polymers could be used as resists for excimer laser ablation lithography.     

Dependence of ablation threshold and LIPSS formation on copper thin films by accumulative UV picosecond laser shots

The ablation threshold and Laser-induced periodic surface structure (LIPSS) formation on copper thin film were investigated using a picosecond laser (Nd:YAG laser: 266 nm, 42 ps, 10 Hz). We show that the ablation threshold varies with respect to the number of laser shots (N) on two different substrates. The single-shot ablation threshold was estimated to be close to 170 ± 20 mJ/cm². The incubation coefficient was estimated to be 0.68 ± 0.03 for copper thin films on silicon and glass substrates. In addition, morphology changes of the ablated regions, in the same spot area, were studied as a function of fluence and number of laser shots. An intermediate structure occurred with a mix of low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL) and regular spikes at a fluence F < 250 mJ/cm² and 1,000 < N ≤ 10.000 shots. LSFL was observed with a spatial period close to the irradiation wavelength and an orientation perpendicular to the laser polarization, and HSFL with a spatial period of ～120 nm and a parallel orientation. Lastly, the global relationship between the laser parameters (i.e. fluence and number of shots) and LIPSS formation was established in the form of a 2D map.     

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.     

Effect of fluence and ambient environment on the surface and structural modification of femtosecond laser irradiated Ti

Under certain conditions, ultrafast pulsed laser interaction with matter leads to the formation of self-organized conical as well as periodic surface structures(commonly reffered to as, laser induced periodic surface structures, LIPSS). The purpose of the present investigations is to explore the effect of fsec laser fluence and ambient environments(Vacuum O_2) on the formation of LIPSS and conical structures on the Ti surface. The surface morphology was investigated by scanning electron microscope(SEM). The ablation threshold with single and multiple(N = 100) shots and the existence of an incubation effect was demonstrated by SEM investigations for both the vacuum and the O_2 environment. The phase analysis and chemical composition of the exposed targets were performed by x-ray diffraction(XRD) and energy dispersive x-ray spectroscopy(EDS), respectively. SEM investigations reveal the formation of LIPSS(nano micro). FFT d-spacing calculations illustrate the dependence of periodicity on the fluence and ambient environment. The periodicity of nano-scale LIPSS is higher in the case of irradiation under vacuum conditions as compared to O_2. Furthermore, the O2 environment reduces the ablation threshold. XRD data reveal that for the O_2 environment, new phases(oxides of Ti) are formed. EDS analysis exhibits that after irradiation under vacuum conditions, the percentage of impurity element(Al) is reduced. The irradiation in the O_2 environment results in 15% atomic diffusion of oxygen.     

Mechanism and control of periodic surface nanostructure formation with femtosecond laser pulses

Fundamental mechanism of femtosecond-laser-induced periodic surface nanostructure formation has been investigated under the condition using superimposed multiple shots at lower fluence than the single-pulse ablation threshold. With increasing the shot number of low-fluence fs-laser pulses, the periodic nanostructure develops through the bonding structure change of target material, the nanoscale ablation with optical near-fields induced around the high curvatures, and the excitation of surface plasmon polaritons (SPPs) to create the nano-periodicity in the surface structure. It is confirmed that non-thermal interaction at the surface plays the crucial role in the nanostructure formation. Based on the mechanism, we have demonstrated that the periodic nanostructure formation process can be controlled to fabricate a homogeneous nanograting on the target surface, using a two-step ablation process in air. The experimental results obtained represent exactly the nature of a single spatial standing SPP wave mode that generates periodically enhanced near-fields for the nanograting formation. The calculated results for a model target reproduce well the nanograting period and explain the characteristic properties observed in the experiment.     

Excimer-laser-induced permanent electrical conductivity in solid C60 films

After being irradiated in air by a XeCI (308 nm) excimer laser, the electrical conductivity of solid thin-film C₆₀ has been improved by more than six orders of magnitudes. The products resulting from laser irradiation of C₆₀ films have been investigated by Raman scattering and the onset of conductivity can be attributed to laser-induced oxygenation and disintegration of the fullerene. Irradiated by 40 ns laser pulses with different fluence, products with different microstructure were observed. At lower fluence, the Raman features of microcrystalline graphite and fullerene polymer were observed. At a fluence just below the ablation threshold (36 mJ/cm²), the fullerene molecules in the film were disintegrated completely and transformed to amorphous graphite.     

Direct laser interference patterning of poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS) thin films

We have developed a patterning procedure based on selective ablation using interference patterns with ns-laser pulses to fabricate periodic arrays on large areas of poly(3,4-ethylene dioxythiophene)-poly(4-styrene sulfonic acid) (PEDOT-PSS) thin films over a metallic gold–palladium layer. Single pulse laser-ablation experiments were performed to study the ablation characteristics of the thin films as a function of the film thickness. The ablation threshold fluence of the PEDOT-PSS films was found to be dependent on thickness with values ranging from 43 mJ/cm² to 252 mJ/cm². Additionally, fluences at which the PEDOT-PSS films could be ablated without inducing damage in the underlying metallic films were observed (128 mJ/cm² and 402 mJ/cm² for film thicknesses of 70 nm and 825 nm, respectively). Linear periodic arrays with line spacings of 7.82 μm and 13.50 μm were also fabricated. The surface topography of these arrays was analyzed using scanning electron and atomic force microscopy. For thicker polymeric layers, several peeled sub-layers of the conjugated polymer with average thicknesses of about 165–185 nm were observed in the ablation experiments. The size and scale of structures produced by this technique could be suitable for several biomedical applications and devices in which controlling cell adhesion, promoting cell alignment, or improving biocompatibility are important.     

Femtosecond laser-induced damage morphologies of crystalline silicon by sub-threshold pulses

The effect of sub-threshold pulses of circularly polarized Ti:sapphire femtosecond laser system on crystalline (1 0 0) silicon wafer was investigated. Surface damage morphologies were studied by irradiating the test silicon surface with pulses (peak fluence of 0.25 J/cm²) in succession. These pulses were below the single-pulse surface damage threshold. After the few initial pulses, the observed surface damage morphologies were found to be characterized by a minor phase change region and a major surface damage area at the center, corresponding to the well-known laser-induced periodic surface structure (LIPSS). Further increase in the number of pulses resulted in the formation of new surface morphologies with different features such as ablation, modification, and re-deposited materials. These features were reproducible and more distinguishable at higher number of pulses.     

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.     

Efficient micromachining of poly(vinylidene fluoride) using a laser-plasma EUV source

In this paper an efficient micromachining of poly(vinylidene fluoride) (PVDF) by direct photo-etching with a laser-plasma EUV (extreme ultraviolet) source was demonstrated for the first time. Mass spectroscopy was employed to investigate the ablation products and revealed emission of numerous molecular species of C-containing fragments of the polymer chain. Chemical surface changes after irradiation were investigated using X-ray photoelectron spectroscopy (XPS). The XPS spectra obtained for PVDF samples, irradiated with low and high EUV fluence, indicate significant differences between chemical structures in near-surface layers. It was shown that irradiation with low fluence results in defluorination and thus carbon enrichment of the polymer in near-surface layer. In contrary, irradiation with high fluence leads to intense material ablation and hardly modifies the chemical structure of the remaining material.     

Effect of irradiation history on the preparation of laser induced periodic microstructure on polyimide surface

Although UV laser is proved to be an effective tool to prepare microstructure on polymer surface, laser ablation accompanied by the formation of laser induced periodic surface structure (LIPSS) limits its application in many fields. The purpose of this report is to investigate the effect of pre-irradiation in advance, using a low-fluence laser, on the LIPSS formation. The properties of pre-irradiated PI films were characterized by X-ray photoelectron spectroscopy (XPS), surface tension based on the contact angle measurements and UV-vis spectra. It was found that pre-irradiation at low fluence led to the changes in surface property such as chemical components though no LIPSS was formed. As a result, threshold of LIPSS formation on such pre-irradiated PI film decreased and fine LIPSS with deeper amplitude was obtained.     

Fabrication of high-aspect-ratio microstructures using excimer laser

An excimer laser micromachining system is developed to study the ablation of high-aspect-ratio microstructures. The study examines the ablation efficiency, specifically, the impact of changing major laser operating parameters on the resulting microstructural shapes and morphology. The study focuses on glass, although results on silicon and aluminum are also included for comparison. In ablating grooved structures, the ablation depth has been observed to be linearly proportional to the operating parameters, such as the pulse number and fluence. The results specifically indicate that ablation at low fluence and high repetition rates tends to form a V-shaped cross-section or profile, while a U-shaped profile can be obtained at high fluence and low repetition rate. The ablation rate or ablated volume has then been quantified based on the ablation depth measured and the ablated profile observed. The threshold fluence has also been obtained by extrapolating experimental data of ablation rate. The extrapolation accuracy has been established by the good agreement between the extrapolated value and the one predicted by Beer's law. Moreover, a one-dimensional analytical solution has been adopted to predict the ablated volume so as to compare with the experimental data. The reasonable agreement between the two indicates that a simple analytical solution can be used for guiding or controlling further laser operations in ablating glass structures. Finally, the experimental results have shown that increasing the repetition rate favors the morphology of ablated surfaces, though the effect of repetition rate on ablation depth is insignificant.     

Ablation experiments on polyimide with femtosecond laser pulses

Some applications of polymer films require the microstructuring of partly uneven substrates. This cannot be achieved by conventional photolithography, usually performed with ultraviolet short-pulse lasers (excimer, fourth harmonic Nd:YAG). When processing thermally sensitive or undoped polymers with low optical absorption, the use of femtosecond laser pulses can improve the ablation precision, also reducing the heat-affected zone. Therefore, a Ti:sapphire laser system was employed to perform ablation experiments on polyimide (PI). The irradiated areas were evaluated by means of optical and scanning electron microscopy. Highly oriented ripple structures, which are related to the polarization state of the laser pulses, were observed in the cavities. The relationship between the ablation threshold fluence and the number of laser pulses applied to the same spot is described in accordance with an incubation model.     

Polymer jacket stripping of optical fibres by laser irradiation

Laser jacket stripping of the two-layer polymer jacket coating of Corning SMF-28 silica fibres has been studied as an alternative approach to chemical and mechanical techniques. These polymer outer layers, although chemically similar, become discernable through laser ablation and depth per pulse experiments. Etch rate measurements using nanosecond UV excimer laser sources (F₂, ArF, KrF and XeCl lasers) reveal that, as expected, the threshold fluence (energy per unit area) for significant material removal drops as the laser wavelength becomes shorter. For some wavelengths and fluences, spontaneous cone formation has been observed, thus providing additional threshold data through apex angle determination. The possible occurrence of deleterious damage to the silica cladding has been assessed using electron microscopy and optical transmission measurements. A wide fluence range over which damage was not observed characterised all the interactions. Irradiation techniques for producing apertures in the polymer coating or complete jacket removal are demonstrated and discussed. Briefly, polymer jacket stripping using a femtosecond laser source (800 nm) has been demonstrated. PACS 42.62.Cf; 52.38.Mf     

Influence of film thickness on laser ablation of hydrogenated amorphous carbon films

Single-pulse damage thresholds of hydrogenated amorphous carbon (a-C:H) films were measured for 8-ns laser pulses at 532-nm wavelength. Layer thicknesses from below the optical penetration depth to above the thermal diffusion length (60 nm–13 μm) were examined. After correction of the damage-threshold values for the fraction of energy effectively absorbed by the material, the damage threshold was found to increase linearly with the layer thickness, also for film thicknesses below the optical penetration depth of a-C:H. The threshold fluence reached the bulk value for a layer thickness equal to the thermal diffusion length. The thermal diffusion coefficient was obtained from fitting the experimental data. Several phenomena like graphitization, blistering, exfoliation, and ablation were observed for different fluence regimes and film thicknesses. PACS 79.20.Ds; 06.60.Jn; 78.66.Jg     

Morphology and oxidation of Zr-based amorphous alloy ablated by femtosecond laser pulses

Femtosecond laser ablation of an amorphous alloy in air, including single-pulse ablation, multi-pulse drilling and trenching has been investigated. Laser-induced ablation and related effects were examined by means of scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy and electron diffraction. Oxidation was observed in the multi-pulse ablation region surface. With selected parameters, molten trace, spatter and crystallization can be avoided in the vicinity of the machining area. The results show that femtosecond laser ablation with selected parameters is a promising method for micromachining amorphous alloys. PACS 79.20.Ds; 87.80.Mj; 81.05.Bx; 81.65.Mq; 61.82.Bg     

Underwater excimer laser ablation of polymers

In this paper, we study the photoablation kinetic of poly (ethylene terephthalate) (PET), polycarbonate (PC), polyimide (PI) and polystyrene (PS) in both air and water. Compared to the results obtained in air, we highlight the decrease of the ablation threshold (AT) of polyesters in contact with water as a function of polymer chemical structure. In order to check the expected hydrolytic reaction of polyesters near the ablation threshold, the chemical modification of the polymer surfaces, as well the composition of the ablation products, were investigated after irradiation near the fluence of ablation threshold in air (air-F _t ) by X-ray photoelectron spectroscopy (XPS) and confocal Raman microspectroscopy. The morphology of polymers obtained by underwater irradiation and near the air-F _t was also examined using scanning electron microscopy (SEM). To understand the process and its dynamics in contact with water, we consider the model of temperature at the polymer-water interface based on the semi-analytical solution of the transit heat-diffusion equation.