Laser ablation of multilayers of ink from a paper substrate for tactile printing

This paper reports the effects of laser ablation upon multiple-layered coloured inks which have been printed on an ordinary white paper. The aim of this work is to examine the feasibility of generating a fully tactile three-coloured image by selectively removing ink layers to reveal underlying layers of a different colour. In this paper laser ablation has been carried out upon four layered ink samples consisting of white/cyan/white/black layers. Ablation was carried out using a Q-switched Nd : YAG laser. The results show that it is possible to selectively remove the inks to expose both the top white and the cyan layers, although charring occurs with deeper ablation. An evaporation/decomposition mechanism is proposed to describe process of ink ablation.     

Frequency-tripled Nd:YAG laser ablation in laser structuring process

With the increasing demand for finer lines/spaces on PCB boards, a new technology—laser structuring—has emerged in recent years. In this research, the frequency-tripled Nd:YAG laser is selected as the laser source in laser structuring; this laser is often used in miniaturization machining. This paper describes in detail the processing parameters’ influences, such as laser power, numbers of repetition, repetition rate and bite size, on laser structuring results. From the research results, it can be concluded that the line width and depth are increased with increases in the laser power and numbers of repetition. Repetition rate, bite size and velocity are related to one another. When the bite size is fixed, the velocity increases with the repetition rate and the depth of the line is decreased at the same time. When the repetition rate is fixed, velocity increases with the bite size.     

Characterization of laser ablation plasmas by laser beam deflection

Laser probe beam and multiple-pass deflection techniques were used for real time and in situ monitoring of laser ablation plasma plumes in the mTorr pressure regime. Intensity and transit time of shock wave fronts were studied as functions of focal lens position, laser energy and pressure. The velocity of the shock wave was determined to be up to 30 km s⁻¹ for a pressure of 40 mTorr and to drop below 4 km s⁻¹ at 1 Torr. For transparent targets rear-side shock wave velocity was on to be slower than the corresponding front one. This method promises a reliable diagnostic tool for pulsed laser deposition processing allowing an increase in the quality of coating technologies.     

A simple model for high fluence ultra-short pulsed laser metal ablation

The ultra-short laser metal ablation is a very complex process, the complete simulation of which requires applications of complicated hydrodynamics or molecular dynamics models, which, however, are often time-consuming and difficult to apply. For many practical applications, where the laser ablation depth is the main concern, a simplified model that is easy to apply but at the same time can also provide reasonably accurate predictions of ablation depth is very desirable. Such a model has been developed and presented in this paper, which has been found to be applicable for laser pulse duration up to 10 ps based on comparisons of model predictions with experimental measurements.     

A simplified cavity analysis for estimating energy coupling during laser ablation and drilling of solids — experiment

The theory for a general departure function, f, for laser-irradiated cavities was previously developed to estimate laser energy coupling to an opaque solid target as a function of heat transfer and the cavity shape and size. In this article, a specific form of f is calculated for ultraviolet (UV) laser ablation of copper (Cu) and aluminum (Al) targets. Methods are also given for calculating the geometric factor, a, and experimentally determining the heat transfer parameter, ν, which is shown for this form of f to be the intensity-dependent effective reflectivity of the material. Experimental results for different gauges of laser energy coupling with a solid target are given and compared to calculations of net absorbed energy based on f and the incident laser energy. Using the simplified cavity analysis, the results demonstrate that the experimental values for f fall within the limits predicted by the theory, and that energy coupling can be predicted to within a mean of 2% of experimental gauges. Neglecting the factors in f from calculations of energy coupling can lead to large errors for laser-irradiated cavities, establishing that both cavity shape and heat transfer should be simultaneously considered. In addition, a first-order sensitivity analysis based on f shows that the initial rate of change in material removal strongly increases with reflectivity, which can lead to runaway cavity formation for highly reflective materials.     

Formation of periodic structures by laser ablation of metals in liquids

Experimental results are presented on ablation of metals (W, Cu, brass and bronze) in a liquid environment (e.g., ethanol or water) by irradiation with either a pulsed copper vapor laser (0.51 μm) or a pulsed Nd:YAG laser (1.06 μm). The target material is ejected into surrounding liquid in the form of nanoparticles. In a certain range of laser parameters (fluence and number of laser shots) the surface of the solid target is composed of micro-cones having a regular structure. The distance between neighboring micro-cones in the structure depends on the laser spot size. The structures allow the observation of up-conversion of the laser frequency due to generation of the second harmonics in the eye retina.     

Two color laser ablation: Enhanced yield, improved machining

Laser ablation of semiconductors with nano- and picosecond lasers can be significantly improved, both in terms of yield and surface quality, by simultaneous irradiation of the sample with the fundamental beam (λ = 1064 nm) and a small amount of its second harmonic (SH) produced in a thin nonlinear crystal. While the total energy fluence is conserved, the small fraction of the second harmonic serves to excite electrons into the conduction band to get the ablation process started. For femtosecond laser pulses, the effect becomes insignificant, since sufficient conduction band population is provided by multiphoton absorption.     

Femtosecond pulsed laser ablation of thin gold film

Laser micromachining on 1000 nm-thick gold film using femtosecond laser has been studied. The laser pulses that are used for this study are 400 nm in central wavelength, 150 fs in pulse duration, and the repetition rate is 1 kHz. Plano-concave lens with a focal length of 19 mm focuses the laser beam into a spot of 3 μm (1/e² diameter). The sample was translated at a linear speed of 400 μm/s during machining. Grooves were cut on gold thin film with laser pulses of various energies. The ablation depths were measured and plotted. There are two ablation regimes. In the first regime, the cutting is very shallow and the edges are free of molten material. While in the second regime, molten material appears and the cutting edges are contaminated. The results suggest that clean and precise microstructuring can be achieved with femtosecond pulsed laser by controlling the pulse energy in the first ablation regime.     

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.     

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.     

Surface ablation of lithium tantalate by femtosecond laser

We report measurements of the laser induced breakdown threshold in lithium tantalate with different number of pulses delivered from a chirped pulse amplification Ti: sapphire system. The threshold fluences were determined from the relation between the diameter D² of the ablated area and the laser fluence F₀. The threshold of lithium tantalite under single-shot is found to be 1.84 J/cm², and the avalanche rate was determined to be 1.01 cm²/J by calculation. We found that avalanche dominates the ablation process, while photoionization serves as a free electron provider.     

Experimental study of infrared nanosecond laser ablation of silicon: The multi-pulse enhancement effect

Experimental study has been performed on nanosecond (ns) laser ablation of silicon at 1064 nm, through which a so-called “multi-pulse enhancement effect” has been revealed, which has been rarely reported in literature. The major features of this effect are: (1) for multi-pulse laser ablation at the same spatial location, the ablation efficiency increases as the pulse number increases and the pulse-to-pulse temporal distance decreases; (2) for multi-pulse ablation performed sequentially at a group of locations, the ablation quality and efficiency starting from the second location can be significantly enhanced if the distance between adjacent locations is sufficiently small. Further study is needed to confirm and understand the underlying physical mechanism for the multi-pulse enhancement effect, which can be utilized to significantly improve the quality and efficiency of laser silicon micromachining using the low-cost and low-energy-consumption infrared ns lasers. This may decrease the cost and energy consumption of many relevant areas, such as the solar cell industry.     

Preparation of polyynes by laser ablation of graphite in aqueous media

Polyynes were prepared by liquid-phase laser ablation of a graphite target at 1064 nm and identified by analyzing UV absorption spectra in deionized water and various aqueous solutions. We observed that major UV absorption peaks coincide with the electronic transitions corresponding to linear hydrogen-capped polyynes (C_nH₂: n = 6, 8, 10). The peak intensities increased when polyynes were produced by irradiating the target immersed in acidic media, while those were relatively weak in basic media. This leads us to conclude that OH⁻ or H⁺ ions play a certain role in the formation of polyynes.     

Femtosecond laser ablation of polypropylene for breathable film

A polypropylene (PP) film was ablated using a femtosecond laser with a center wavelength of 785 nm, a pulse width of 184 fs and a repetition rate of 1 kHz. Increments of both the pulse energy and the shot number of pulses lead to co-occurrence of photochemical and thermal effect, demonstrated by the spatial expansion of rim on the surface of PP. The shapes of the laser-ablated PP films were imaged by a scanning electron microscope (SEM) and measured by a 3D optical measurement system (NanoFocus). And, the gas and water vapor transmission rate, mechanical properties of PP film micropatterned by fs laser pulses was characterized. Our results demonstrate that a femtosecond pulsed laser is an efficient tool for breathable packaging films in modifying the flow of air and gas, where the micropatterns are specifically tailored in size, location and number of which is easily controlled by laser processing conditions.     

Femtosecond pulsed laser ablation with spatial filtering

With the rise in demand for miniaturized features with better acute edge acuity and negligible thermal damage zone, one of the key vital areas lies in the refinement of the quality of the laser beam itself. Spatial filter is routinely used in optical micromachining systems to smoothen the Gaussian profile of the machining spot in order to obtain a feature of the desired quality. However, its profile smoothening effect has never been investigated for femtosecond pulsed laser micromachining process since the extremely high peak power of femtosecond pulses will cause damage on the filtering aperture of spatial filter. During the development of an acousto-optical micromachining system using femtosecond pulses, we found that if the damage of the filtering aperture can be circumvented, spatial filter can improve the machining quality of femtosecond pulse ablation, especially when ablation is conducted at low-fluency range (just above the ablation threshold fluency). In this paper, we investigate and demonstrate both the improvement and potential that beam refinement can bring about. In our experiment, a series of test patterns were ablated with a 400 nm second-harmonic Ti:Sapphire femtosecond laser of 150 fs duration at varying pulse energy ranging from 31 to 39 nJ. The specimen used in the experiment is a platinum- (Pt) sputtered coating of 100 nm thickness on a quartz substrate. The results show a significant improvement in the constancy of the shape as well as the size of ablated feature, revealing an improved beam profile and beam energy distribution due to spatial filtering.     

Carbon nanocrystals produced by pulsed laser ablation of carbon

Plasma laser ablation experiments were performed irradiating glassy-carbon targets placed in vacuum through a pulsed Nd:YAG laser operating at the second harmonic (532 nm), 9 ns pulse width and 10⁹ W/cm² density power.

Thin films of ablated carbon were deposited on silicon oxide substrates placed at different distances and angles with respect to the target.

The analysis of the deposited material was carried out by using surface profiler, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and Raman spectroscopy.

Results show the evidence of carbon nanocrystals and nanostructures with dimension of the order of 100 nm deposited on the substrates together with a large amount of amorphous phase. The spectroscopic investigations and the SEM images indicate the formation of nanodiamond seeds as a nucleation process induced on the substrate surface. Nanostructures were investigated as a function of the laser intensity and angle distribution. Experimental results were compared with the literature data coming from nanodiamonds growth with different techniques.

Experiments performed at Instituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (INFN-LNS) of Catania (Italy) and data analysis conducted at Dipartimento di Fisica and DFMTA of the Università of Messina (Italy), CNR-ITIS of Messina and ST-Microelectronics of Catania will be presented and discussed.     

Metal thin film ablation with femtosecond pulsed laser

Micromachining thin metal films coated on glass are widely used to repair semiconductor masks and to fabricate optoelectrical and MEMS devices. The interaction of lasers and materials must be understood in order to achieve efficient micromachining. This work investigates the morphology of thin metal films after machining with femtosecond laser ablation using about 1 μm diameter laser beam. The effect of the film thickness on the results is analyzed by comparing experimental images with data obtained using a two-temperature heat transfer model. The experiment was conducted using a high numerical aperture objective lens and a temporal pulse width of 220 fs on 200- and 500-nm-thick chromium films. The resulting surface morphology after machining was due to the thermal incubation effect, low thermal diffusivity of the glass substrate, and thermodynamic flow of the metal induced by volumetric evaporation. A Fraunhofer diffraction pattern was found in the 500-nm-thick film, and a ripple parallel to the direction of the laser light was observed after a few multiple laser shots. These results are useful for applications requiring micro- or nano-sized machining.     

Picosecond and femtosecond pulsed laser ablation and deposition of quasicrystals

A Nd:glass laser with pulse duration of 250 fs and 1.3 ps has been used to evaporate a Al₆₅Cu₂₃Fe₁₂ quasicrystalline target. The gaseous phase obtained from the ablation process has been characterised by several techniques such as emission spectroscopy, quadrupole mass spectrometry and ICCD imaging, used to study the plume composition, energy and morphology. The results show that the ablation processes in the short-pulse regimes are very different to the nanosecond one. In particular the plume angular distribution shows a characteristic high cosine exponent and the composition is completely stoichiometric and independent from the laser fluence. Furthermore the mass spectra indicate the presence of clusters, both neutral and ionised and the emission from the target suggest a rapid thermalisation leading to the melting of the surface. To clarify the ablation process some films have been deposited, on oriented silicon, at different experimental conditions and analysed by scanning electron microscopy, atomic force microscopy, energy dispersive X-ray analysis and X-ray diffraction. The analyses show the presence of nanostructured films retaining the target stoichiometry but consisting of different crystalline and non crystalline phases. In particular the nanostructure supports the hypothesis of the melting of the target during the ablation and a mechanism of material ejection is proposed for both picosecond and femtosecond regimes.     

Multi-criteria optimization in CO2 laser ablation of multimode polymer waveguides

High interconnection density associated with current electronics products poses certain challenges in designing circuit boards. Methods, including laser-assisted microvia drilling and surface mount technologies for example, are being used to minimize the impacts of the problems. However, the bottleneck is significantly pronounced at bit data rates above 10 Gbit/s where losses, especially those due to crosstalk, become high. One solution is optical interconnections (OI) based on polymer waveguides. Laser ablation of the optical waveguides is viewed as a very compatible technique with ultraviolet laser sources, such as excimer and UV Nd:YAG lasers, being used due to their photochemical nature and minimal thermal effect when they interact with optical materials. In this paper, the authors demonstrate the application of grey relational analysis to determine the optimized processing parameters concerning fabrication of multimode optical polymer waveguides by using infra-red 10.6 µm CO₂ laser micromachining to etch acrylate-based photopolymer (Truemode^™). CO₂ laser micromachining offers a low cost and high speed fabrication route needed for high volume productions as the wavelength of CO₂ lasers can couple well with a variety of polymer substrates. Based on the highest grey relational grade, the optimized processing parameters are determined at laser power of 3 W and scanning speed of 100 mm/s.     

Micromachining of a thin film by laser ablation using femtosecond laser with masks

A gold thin film was machined by laser ablation using a femtosecond laser with mask patterns in the shape of lines and numbers. The patterns were successfully transferred with proper focusing and laser fluence. The optimal femtosecond laser fluence to keep the line width was about 5.2 mJ/cm² on the mask, and 99 mJ/cm² on the film. The processing resolution was 13 μm, and the narrowest line width was about 4 μm.