Femtosecond laser micromilling of Si wafers

Femtosecond laser micromilling of silicon is investigated using a regeneratively amplified 775 nm Ti:Sapphire laser with a pulse duration of 150 fs operating at 1 kHz repetition rate. The morphological observation and topological analysis of craters fabricated by single-shot laser irradiation indicated that the material removal is thermal in nature and there are two distinct ablation regimes of low fluence and higher fluence with logarithmical relations between the ablation depth and the laser fluence. Crater patterns were categorized into four characteristic groups and their formation mechanisms were investigated. Femtosecond laser micromilling of pockets in silicon was performed. The effect of process parameters such as pulse energy, translation speed, and the number of passes on the material removal rate and the formation of cone-shaped microstructures were investigated. The results indicate that the microstructuring mechanism has a strong dependence on the polarization, the number of passes and laser fluence. The optimal laser fluence range for Si micromilling was found to be 2-8 J/cm² and the milling efficiency attains its maximum between 10 and 20 J/cm².     

XPS study of silicon surface after ultra-low-energy ion implantation

Ultra-low-energy ion implantation of silicon with a hydrogen-terminated (0 0 1) surface was carried out using a mass-separated ³¹P⁺ ion beam. The ion energy was 30 eV, the displacement energy of silicon, and the ion doses were 6 × 10¹³ ions/cm². Annealing after the implantation was not carried out. The effects of ion implantation on the surface electrical state of silicon were investigated using X-ray photoelectron spectroscopy (XPS). The Si 2p peak position using XPS depends on the doping conditions because the Fermi level of the hydrogen-terminated silicon surface is unpinned. The Si 2p peak position of the specimen after ion implantation at a vacuum pressure of 3 × 10⁻⁷ Pa was shifted to the higher energy region. It suggested the possibility of phosphorus doping in silicon without annealing. In the case of ion implantation at 5 × 10⁻⁵ Pa, the Si 2p peak position was not shifted, and the peak was broadened because of the damage by the fast neutrals. Ultra-low-energy ion doping can be achieved at ultra-high-vacuum conditions.     

First-principles study of indium on silicon (1 0 0)—the structure, defects and interdiffusion

The atomic structures of indium (In) on silicon (Si) (1 0 0)-(2 × 1) surface are investigated by the local density approximation using first-principles pseudopotentials. Total energy optimizations show that the energetically favored structure is the parallel ad-dimer model. The adsorption energy of In on ideal Si(1 0 0)-(1 × 1) surface is significantly higher than that on reconstructed Si(1 0 0)-(2 × 1) surface, suggesting that In adsorption does not break the Si-Si dimer bond of the substrate. When Si surface contains single dimer vacancy defects, In chain will be interrupted, leading to disconnected In nanowires. Displacive adsorption of In on Si(1 0 0) is also considered, and the calculation suggests that interdiffusion of In into Si substrate will not be favorable under equilibrium conditions.     

Investigation on femtosecond laser irradiation energy in inducing hydrophobic polymer surfaces

This study investigates the use of ultrashort femtosecond laser pulses to induce hydrophobic properties on PMMA surfaces. The modification of surface wetting property exhibits a strong dependence on the amount of energy deposited on the PMMA surface. A simple equation has been deduced from the laser parameters to express the energy deposition. It was revealed that water contact angle (WCA) of more than 120°, with a maximum of around 125°, could be achieved when the total energy deposited per unit area on the PMMA surface ranged from 600 J/cm² to 900 J/cm² at an energy deposition rate of around 50 J/cm²/s. Beyond this range, WCA reduced with increasing amount of energy deposition. Furthermore, with higher energy deposition rate or higher laser fluence, total energy required to induce hydrophobic surfaces was reduced. Under different energy deposition, the quantity of polar groups or non-polar groups induced was responsible for the changes in WCA and thus the different surface hydrophobicity.     

Single- and multi-pulse formation of surface structures under static femtosecond irradiation

Femtosecond surface structure modifications are investigated under irradiation with laser pulses of 150 fs at 800 nm, on copper and silicon. We report sub-wavelength periodic structures formation (ripples) with a periodicity of 500 nm for both materials. These ripples are perpendicular to the laser polarization and can be obtained with only one pulse. The formation of these ripples corresponds to a fluence threshold of 1 J/cm² for copper and 0.15 J/cm² for silicon. We find several morphologies when more pulses are applied: larger ripples parallel to the polarization are formed with a periodicity of 1 μm and degenerate into a worm-like morphology with a higher number of pulses. In addition, walls of deep holes also show sub-wavelength and large ripples.     

Polymer self-assembled nano-structures and surface relief gratings induced with laser at 157 nm

Surface relief gratings (SRG) and self-organized nano-structures induced by laser light at 157 nm on the fluoropolymer poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA), films were obtained under well-controlled light exposure conditions. Regular and semi-regular spaced self-organized grating-like structures were created on polymeric films for ∼7.5-100 mJ/cm² laser energy fluence. For lower laser fluence, the surface morphology of light exposed/non-exposed areas exhibited irregular-like structure morphologies, while polymer surface irradiation with energy fluence higher than 150 mJ/cm² causes progressively fading out of the regular patterns. Under the specific experimental conditions, the SRG and self-organization patterning have their origin in the development of a surface thermal instability (Rayleigh's instability), which is resolved itself into regular patterns on the surface of the fluoropolymer film. The thermal instability is due to the explosive polymer surface photo-dissociation at 157 nm and the build up of longitudinal and periodic surface stress, which eventually create the SRG and the self-assembled structures on the polymer.     

Gold coatings on polyethyleneterephthalate nano-patterned by F2 laser irradiation

In this work we present periodic surface structures generated by linearly polarized F₂ laser light (157 nm) on polyethyleneterephthalate (PET). Atomic force microscopy was used to study the topological changes induced by the laser irradiation. The laser irradiation induces the formation of periodic ripple structures with a width of ca 130 nm and a height of about 15 nm in the fluence range 3.80-4.70 mJ/cm² and the roughness of the polymer surface increases due to the presence of these periodic structures. Subsequently, the laser modified PET foils were coated with a 50 nm thick gold layer by sputtering. After Au deposition on the PET foils with ripple structure, the roughness of surface decreases in comparison to PET with ripples without Au coating. For 50 nm thick Au layers, the ripple structure is not directly transferred to the gold coating, but it has an obvious effect on the grain size of the coating. With considerably thinner Au layers, the ripple structures are smoothened but preserved.     

High-power laser interference lithography process on photoresist: Effect of laser fluence and polarisation

High throughput and low cost fabrication techniques in the sub-micrometer scale are attractive for the industry. Laser interference lithography (LIL) is a promising technique that can produce one, two and three-dimensional periodical patterns over large areas. In this work, two- and four-beam laser interference lithography systems are implemented to produce respectively one- and two-dimensional periodical patterns. A high-power single pulse of ∼8 ns is used as exposure process. The optimum exposure dose for a good feature patterning in a 600 nm layer of AZ-1505 photoresist deposited on silicon wafers is studied. The best aspect ratio is found for a laser fluence of 20 mJ/cm². A method to control the width of the sub-micrometer structures based on controlling the resist thickness and the laser fluence is proposed.     

Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses

Interaction of a nanosecond transversely excited atmospheric (TEA) CO₂ laser, operating at 10.6 μm, with tungsten-titanium thin film (190 nm) deposited on silicon of n-type (1 0 0) orientation, was studied. Multi-pulse irradiation was performed in air atmosphere with laser energy densities in the range 24-49 J/cm². The energy absorbed from the laser beam was mainly converted to thermal energy, which generated a series of effects. The following morphological changes were observed: (i) partial ablation/exfoliation of the WTi thin film, (ii) partial modification of the silicon substrate with formation of polygonal grains, (iii) appearance of hydrodynamic features including nano-globules. Torch-like plumes started appearing in front of the target after several laser pulses.     

Periodic patterning of silicon by direct nanosecond laser interference ablation

The production of periodic structures in silicon wafers by four-beam is presented. Because laser interference ablation is a single-step and cost-effective process, there is a great technological interest in the fabrication of these structures for their use as antireflection surfaces. Three different laser fluences are used to modify the silicon surface (0.8 J cm⁻², 1.3 J cm⁻², 2.0 J cm⁻²) creating bumps in the rim of the irradiated area. Laser induced periodic surface structures (LIPSS), in particular micro and nano-ripples, are also observed. Measurements of the reflectivity show a decrease in the reflectance for the samples processed with a laser fluence of 2.0 J cm⁻², probably caused by the appearance of the nano-ripples in the structured area, while bumps start to deteriorate.     

Interaction of divalent copper with two diaminealkyl hexagonal mesoporous silicas evaluated by adsorption and thermochemical data

Hexagonal mesoporous silicas chemically modified with ethylenediamine moieties were synthesized through the co-condensation of tetraethylorthosilicate (TEOS) with two different silylating agents: (i) N-[3-(trimethoxysilyl)propyl]-ethylenediamine and (ii) the new agent prepared from the incorporation of the ethylenediamine molecule into the epoxide group of the precursor 3-glycidoxypropyltrimethoxysilane. From these silylating agents under neutral n-octylamine template methodology, the respective MNN and MGNN inorganic-organic hybrids were synthesized. Elemental analysis showed that the number of pendant groups in these hybrids were 1.69 and 1.62 mmol g⁻¹, with pore diameters and surface areas of 1.81 and 1.53 nm and 663 ± 14 and 614 ± 16 m² g⁻¹. Infrared spectroscopy, nuclear magnetic resonance for ¹³C and ²⁹Si nuclei and X-ray diffraction patterns are in agreement with the success of the proposed synthetic methods, as confirmed for the formation of the mesoporous hybrids. Both mesoporous materials have been used for divalent copper adsorption from aqueous solution at 298 ± 1 K. The series of adsorption isotherms were adjusted to a modified Langmuir equation. The maximum number of moles adsorbed gave 1.4 ± 0.1 and 1.4 ± 0.2 mmol g⁻¹ for MNN and MGNN, respectively. The same interactions were calorimetrically followed and gave exothermic enthalpy, negative Gibbs free energy and positive entropy values. These favorable thermodynamic data indicate cation/nitrogen basic center interactions on the new mesoporous materials at the liquid/solid interface for both systems.     

Femtosecond photoemission microscopy

We developed a novel experiment for time-resolved photoemission microscopy by combining a commercial photoemission electron microscope (PEEM) with a pulsed Ti:sapphire laser oscillator. The laser system, the setup of the delay stage for pump-probe experiments, and the interface between the PEEM and the laser system are discussed. We use self-organization of Ag islands and nanowires on Si(1 1 1) and 4° vicinal Si(0 0 1) to generate structures with a plasmon resonance that matches the photon energy of our laser (?ω = 3.1 eV after frequency doubling). In two-photon photoemission (2PPE) the photoemission yield then directly visualizes the plasmons in the nanostructures. Accordingly, the photoemission yield depends on the size and shape of the nanostructures, and on the polarization of the laser pulses as well. In Ag nanowires, we observe surface plasmon polariton (SPP) waves by a beating that is formed by interference of the SPP wave and the incident laser light. In a pump-probe experiment, we can directly visualize the propagation of the SPP on a femtosecond time scale.     

Surface modifications of a titanium implant by a picosecond Nd:YAG laser operating at 1064 and 532 nm

Interaction of an Nd:YAG laser, operating at 1064 or 532 nm wavelength and pulse duration of 40 ps, with titanium implant was studied. Surface damage thresholds were estimated to 0.9 and 0.6 J/cm² at wavelengths 1064 and 532 nm, respectively. The titanium implant surface modification was studied by the laser beam of energy density of 4.0 and 23.8 J/cm² (at 1064 nm) and 13.6 J/cm² (at 532 nm). The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following titanium/implant surface morphological changes were observed: (i) both laser wavelengths cause damage of the titanium in the central zone of the irradiated area, (ii) appearance of a hydrodynamic feature in the form of resolidified droplets of the material in the surrounding outer zone with the 1064 nm laser wavelength and (iii) appearance of wave-like microstructures with the 532 nm wavelength. Generally, both laser wavelengths and the corresponding laser energy densities can efficiently enhance the titanium/implant roughness. This implant roughness is expected to improve its bio-integration. The process of the laser interaction with titanium implant was accompanied by formation of plasma.     

Fabrication and application of highly ordered mesoporous Co3O4, NiO, and their metals

Highly ordered mesoporous Co₃O₄, NiO, and their metals were synthesized by nanocasting method using there corresponding mesoporous SBA-15 silica as a template. The obtained porous metal oxides have high surface areas, large pore volume, and a narrow pore size distribution. The N₂-adsorption data for mesoporous metal oxides have provided the BET area of 257.7 m² g⁻¹ and the total pore volume of 0.46 cm³ g⁻¹. The mesoporous metals were employed as a catalyst in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained on the mesoporous Co metal catalyst.     

Multipulse irradiation of silicon by femtosecond laser pulses: Variation of surface morphology

The multipulse interaction of ultraviolet femtosecond laser pulses with silicon and generation of surface structures in a large area spot (?1 mm²) has been studied. The evolution of multiscale structures at the constant fluence strongly depends on the number of pulses, N. For N < 200, the “carpet-like” pattern of nano-, and micro-spikes is generated by the bubble explosion in a thin surface foam layer. The accumulation of bubbles and their explosion due to repetition of laser pulses cause damped membrane-like oscillations of the silicon surface. For 200 ≤ N, bifurcation of surface morphology takes place: (i) the surface tension waves of the wavelength ∼200 μm appear in the peripheral region of the spot. Generated by the surface thermal gradient in the liquid foam layer, they spread from the hot centerline towards the periphery of the spot. The change of their wavelength with propagation distance indicates onset of the Eckhaus instability caused by the phase modulation in multipulse interaction. (ii) Deep caverns appear in a highly superheated silicon layer in the central region of the spot due to the fast gas-liquid phase separation and the fragmentation process.     

Towards the synthesis of MAX-phase functional coatings by pulsed laser deposition

Pulsed laser deposition with a Nd:YAG laser was used to grow thin films from a pre-synthesized Ti₃SiC₂ MAX-phase formulated ablation target on oxidized Si(1 0 0) and MgO(1 0 0) substrates. The depositions were carried out in a substrate temperature range from 300 to 900 K, and the pressure in the deposition chamber ranged from vacuum (10⁻⁵ Pa) to 0.05 Pa Argon background pressure. The properties of the films have been investigated by Rutherford backscattering spectrometry for film thickness and stoichiometric composition and X-ray diffraction for the crystallinity of the films. The silicon content of the films varied with the energy density of the laser beam. To suppress especially the silicon re-sputtering from the substrate, the energy of the incoming particles must be below a threshold of 20 eV. Therefore, the energy density of the laser beam must not be too high. At constant deposition energy density the film thickness depends strongly on the background pressure. The X-ray diffraction measurements show patterns that are typical of amorphous films, i.e. no Ti₃SiC₂ related reflections were found. Only a very weak TiC(2 0 0) reflection was seen, indicating the presence of a small amount of crystalline TiC.     

Microstructural modification by laser surface remelting and its effect on the corrosion resistance of an Al-9 wt%Si casting alloy

Aluminum alloys with silicon as a major alloying element constitute a class of materials, which provides the most significant part of all shaped castings manufactured. Such alloys have a wide range of applications in the automotive and aerospace industries. The literature presents contradictory results and no satisfactory explanations concerning to resulting microstructures provided by laser surface remelting (LSR) and its effect on the electrochemical behavior of Al-Si alloys. The aim of this study was to investigate the effect of microstructural refinement by LSR on corrosion resistance of an Al-9 wt%Si casting alloy. As-cast samples were subjected to a continuous 1 kW CO₂ laser. Corrosion resistance has been analyzed by an electrochemical impedance spectroscopy (EIS) technique and polarization curves carried out in both 0.5 M NaCl and 0.5 M H₂SO₄ solutions at 25 °C. An equivalent circuit has also been proposed and impedance parameters were simulated by the ZView^® software. It was found that the structural modification provided by the LSR process induces a decreasing effect on the corrosion resistance when compared to that of the untreated sample.     

Property improvement of pulsed laser deposited boron carbide films by pulse shortening

Growth characteristics and surface morphology of boron carbide films fabricated by ablating a B₄C target in high vacuum with a traditional KrF excimer laser and a high brightness hybrid dye/excimer laser system emitting at the same wavelength while delivering 700 fs pulses are compared. The ultrashort pulse processing is highly effective. Energy densities between 0.25 and 2 J cm⁻² result in apparent growth rates ranging from 0.017 to 0.085 nm/pulse. Ablation with nanosecond pulses of one order of magnitude higher energy densities yields smaller growth rates, the figures increase from 0.002 to 0.016 nm/pulse within the 2-14.3 J cm⁻² fluence window. 2D thickness maps derived from variable angle spectroscopic ellipsometry reveal that, when ablating with sub-ps pulses, the spot size rather than the energy density determines both the deposition rate and the angular distribution of film material. Pulse shortening leads to significant improvement in surface morphology, as well. While droplets with number densities ranging from 1 × 10⁴ to 7 × 10⁴ mm⁻² deteriorate the surface of the films deposited by the KrF excimer laser, sub-ps pulses produce practically droplet-free films. The absence of droplets has also a beneficial effect on the stoichiometry and homogeneity of the films fabricated by ultrashort pulses.     

Ion implantation induced by Cu ablation at high laser fluence

High energy laser plasma-produced Cu ions have been implanted in silicon substrates placed at different distances and angles with respect to the normal to the surface of the ablated target. The implanted samples have been produced using the iodine high power Prague Asterix Laser System (PALS) using 438 nm wavelength irradiating in vacuum a Cu target. The high laser pulse energy (up to 230 J) and the short pulse duration (400 ps) produced a non-equilibrium plasma expanding mainly along the normal to the Cu target surface. Time-of-flight (TOF) technique was employed, through an electrostatic ion energy analyzer (IEA) placed along the target normal, in order to measure the ion energy, the ion charge state, the energy distribution and the charge state distribution. Ions had a Boltzmann energy distributions with an energy increasing with the charge state. At a laser fluence of the order of 6 × 10⁶ J/cm², the maximum ion energy was about 600 keV and the maximum charge state was about 27+.In order to investigate the implantation processes, Cu depth profiles have been performed with Rutherford backscattering spectrometry (RBS) of 1.5 MeV helium ions, Auger electron spectroscopy (AES) with 3 keV electron beam and 1 keV Ar sputtering ions in combination with scanning electron microscopy (SEM). Surface analysis results indicate that Cu ions are implanted within the first surface layers and that the ion penetration ranges are in agreement with the ion energy measured with IEA analysis.     

High power single-shot laser ablation of silicon with nanosecond 355 nm

We report on high intensity single-shot laser ablation of monocrystalline silicon with a nanosecond Nd:YAG at 355 nm. It is shown that for incident laser intensities exceeding ∼11.5 GW/cm² on the silicon surface, unusually high etch depths can be achieved reaching values up to 60 μm. The results support previous observations of dramatic increase in etch rates in single-shot laser ablation at 266 nm. A laser-induced explosive boiling mechanism together with secondary plasma heating is believed to be associated with this effect.