Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

Sub-picosecond UV-laser ablation of Ni films

Laser ablation of thin Ni films on fused silica by 0.5 ps KrF-excimer-laser pulses at 248 nm is reported. The onset of material removal from different film thicknesses (0.1, 0.3, 0.6 and 1.0 m) was measured in a laser ionization time-of-flight mass spectrometer by the amount of Ni atoms vs laser fluence. Significant amounts of metal atoms are already evaporated at laser fluences around 20 mJ/cm², a threshold up to 100 times smaller compared to the one for 14 ns pulses. In contrast to ns laser pulses, the ablation threshold for 0.5 ps pulses is independent of the film thickness. These results reflect the importance of thermal diffusion in laser ablation of strongly absorbing and thermally good conducting materials and prove that for ablation with short pulses, energy loss to the bulk is minimized.     

A study of electrodeposition of CuInSe2 thin films with triethanolamine as the complexing agent

In this article, the electrochemistry of CuInSe₂ and its compositional ingredients CuCl₂, InCl₃ and SeO₂ in aqueous solution were investigated. Triethanolamine was added in the single-step electrodeposition of CuInSe₂ from aqueous solution as the complexing agent in order to improve the crystallinity and uniformity of the layer. The stoichiometry, crystal structure and grain sizes of CuInSe₂ thin films of various deposition conditions were compared. The deposition parameters such as the concentration of complexing agent, deposition potential, deposition time and annealing temperature are found to be important factors in the processes of electrical deposition of CuInSe₂ thin films.     

Characterization of Cu(In,Ga)Se2 thin films prepared by evaporationfrom ternary compounds

Thin films of Cu(In,Ga)Se₂ were fabricated by evaporation from ternary CuGaSe₂ and CuInSe₂ compounds for photovoltaic device applications and their properties were investigated. From XRF analysis, the Cu:(In+Ga):Se atomic ratio in all thin films was approximately 1:1:2. The Ga/(In+Ga) atomic ratio in the thin films changed linearly from 0 to 1.0 with increasing the [CGS]/([CGS]+[CIS]) mole ratio in the evaporating materials. However, for thin films prepared at the [CGS]/([CGS]+[CIS]) mole ratio above 0.4, the composition by EPMA analysis was not consistent with that by XRF analysis. The result of EPMA analysis showed that the surface of a thin film was Cu-rich. XRD studies demonstrated that the thin films prepared at the [CGS]/([CGS]+[CIS]) mole ratio under 0.2 had a chalcopyrite Cu(In,Ga)Se₂ structure and the preferred orientation to the 112 plane. On the other hand, XRD patterns of the thin films produced at the [CGS]/([CGS]+[CIS]) mole ratio above 0.6 showed the diffraction lines from a chalcopyrite Cu(In,Ga)Se₂ and a foreign phase. The separation of a peak was observed near 2θ=27°, indicative the graded Ga concentration in Cu(In,Ga)Se₂ thin film.     

Structural properties of CuInSe2 films prepared by selenization of metallic precursors on MoNx film substrates

The aim of this work was to study the effect of MoN_x film substrates on the structural properties of CuInSe₂ films prepared by selenization of metallic Cu-In alloy precursors. MoN_x films were prepared by reactive dc-magnetron sputtering. All the CuInSe₂ films exhibit single phase chalcopyrite structure with (1 1 2) preferred orientation, which can be explained by the reduction of lattice mismatch between CuInSe₂ and MoN_x. The bulk composition of selenized CuInSe₂ films are near stoichiometric, but the surface composition analysis suggests Cu deficiency on surface area. Furthermore, ordered defect compound, CuIn₂Se_3.5 is found on the surface of CuInSe₂ films. The results will be helpful for fabricating Cd-free ZnO buffer layer CuInSe₂ and Cu(In_1−xGa_x)Se₂ based thin film solar cells.     

Interaction of ultra-short laser pulses with CIGS and CZTSe thin films

The thin-film solar cell technologies based on complex quaternary chalcopyrite and kesterite materials are becoming more attractive due to their potential for low production costs and optimal spectral performance. As in all thin-film technologies, high efficiency of small cells might be maintained with the transition to larger areas when small segments are interconnected in series to reduce photocurrent and related ohmic losses in thin films. Interconnect formation is based on the three scribing steps, and the use of a laser is here crucial for performance of the device. We present our simulation and experimental results on the ablation process investigations in complex CuIn_1?x Ga _x Se₂ (CIGS) and Cu₂ZnSn(S,Se)₄ (CZTSe) cell’s films using ultra-short pulsed infrared (~1 μm) lasers which can be applied to the damage-free front-side scribing processes. Two types of processes were investigated—direct laser ablation of ZnO:Al/CIGS films with a variable pulse duration of a femtosecond laser and the laser-induced material removal with a picosecond laser in the ZnO:Al/CZTSe structure. It has been found that the pulse energy and the number of laser pulses have a significantly stronger effect on the ablation quality in ZnO:Al/CIGS thin films rather than the laser pulse duration. For the thin-film scribing applications, it is very important to carefully select the processing parameters and use of ultra-short femtosecond pulses does not have a significant advantage compared to picosecond laser pulses. Investigations with the ZnO:Al/CZTSe thin films showed that process of the absorber layer removal was triggered by a micro-explosive effect induced by high pressure of sublimated material due to a rapid temperature increase at the molybdenum-CZTSe interface.     

Determination of the Subpicosecond Laser Pulse Chirp in the Middle IR Range Based on the Fourth Harmonic Noncollinear Generation

A new method is proposed to determine the subpicosecond laser pulse chirp in the middle IR range at the central wavelength of 10 μm, based on the generation of the second-harmonic pulses both by the bandwidth-limited and frequency-modulated subpicosecond pulses and the subsequent noncollinear generation of the fourth-harmonic radiation by the corresponding second-harmonic pulses. The time dependences are given of the instantaneous frequency of the frequency-modulated second-harmonic pulse at the central wavelength of 5 μm, generated in the field of the frequency-modulated subpicosecond IR pulse, propagating in the negative uniaxial AgGaS₂ crystal along the direction of 61°36′ relative to the optical axis. These results can be used in designing a nonlinear optical phase correlator to determine the phase and time profile of the subpicosecond laser pulse in the middle IR range.     

Preparation of CuInSe2/CuGaSe2 two layers absorber film by metal–organic chemical vapor deposition

Deposition of CuInSe₂ thin film on CuGaSe₂ thin film and vice versa has been studied by a low pressure metal–organic chemical vapor deposition technique with three precursors without additional Se. The properties of the resultant films have been examined by scanning electron microscopy, X-ray diffraction, and micro-Raman scattering. Good quality and well demarcated films are obtained only in the case of CuInSe₂ grown on CuGaSe₂. When CuGaSe₂ was grown on top of CuInSe₂ diffusion of Ga into CuInSe₂ was found to produce an alloy film instead.     

Nanosecond and femtosecond UV laser ablation of polymers: Influence of molecular weight

We examine the nanosecond and femtosecond UV laser ablation of poly(methyl methacrylate) (PMMA) as a function of molecular weight (M_w). For laser ablation with nanosecond laser pulses, at the excimer wavelengths 248 nm and 193 nm, we show that high temperatures develop; yet the dynamics of material ejection differs depending on polymer M_w. The results on the nanosecond ablation of polymers are accounted within the framework of bulk photothermal model and the results of molecular dynamics simulations. Turning next to the 248 nm ablation with 500 fs laser pulses, the ablation threshold and etching rates are also found to be dependent on polymer M_w. In addition, ablation results in morphological changes of the remaining substrate. Plausible mechanisms are advanced.     

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.     

Influence of heat treatment on the electrophysical properties of thin films of copper-indium diselenide

An investigation of the structural, electrical, optical, and thermophysical properties is carried out on thin polycrystalline films of the ternary semiconductor CuInSe₂, which is potentially useful for fabricating solar cells. The thin films were obtained by thermal evaporation of CuInSe₂ and Se powder from two independent sources and by high-vacuum deposition in a closed cell (quasiequilibrium deposition). The influence of annealing in air on the parameters of the thin films is analyzed, and the dynamics of variation in the properties of the films are investigated as a function of the annealing time. Temperature dependences of the electrical conductivity, mobility, and thermal conductivity of CuInSe₂ thin films are given together with the spectral dependence of the short-circuit photocurrent of a photosensitive Au-CuInSe₂-Au structure. Zh. Tekh. Fiz. 67, 34–38 (March 1997)     

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.     

Ultrashort-pulse laser ablation of indium phosphide in air

Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be φ_th(1)=0.16 J/cm². The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 23 August 2000     

Simulation of KrF laser ablation of Al2O3-TiC

Previous work by the authors on micromachining of Al₂O₃-TiC ceramics using excimer laser radiation revealed that a columnar surface topography forms under certain experimental conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations show that the columns develop from small globules of TiC, which appear at the surface of the material during the first laser pulses. To understand the mechanism of formation of these globules, a 2D finite element ablation model was developed and used to simulate the time evolution of the temperature field and of the surface topography when a sample of Al₂O₃-TiC composite is treated with KrF laser radiation. Application of the model showed that the surface temperature of TiC rises much faster than that of Al₂O₃, but since TiC has a very high boiling temperature, its vaporization is significant only for a short time. By contrast, the surface temperature of Al₂O₃ rises above its boiling temperature for a much longer period, leading to a greater ablation depth than TiC. As a result, a small TiC globule stands above the Al₂O₃ surface. The results of the model are compared with experimental measurements performed by AFM. After three pulses, the height of the globules predicted by the model is about 340 nm, in good agreement with the height measured experimentally, about 400 nm.     

Thin Film Deposition Methods for CuInSe 2 Solar Cells

CuInSe₂ and its alloys with Ga and/or S are among the most promising absorber materials for thin film solar cells. CuInSe₂-based solar cells have shown long-term stability and the highest conversion efficiencies of all thin film solar cells, above 19%. Solar cells based on these materials are also very stable, thus allowing long operational lifetimes. The preparation of a thin film solar cell is a multistage process where every step affects the resulting cell performance and the production cost. CuInSe₂ and other Cu chalcopyrites can be prepared by a variety of methods, ranging from physical vapor deposition methods such as evaporation and sputtering to low-temperature liquid phase methods such as electrodeposition. The present review discusses first the concept and operation principle of thin film solar cells, as well as the most important thin film solar cell materials. Next, the properties of CuInSe₂ and related compounds, as well as features of solar cells made thereof are reviewed. The last part of the text deals with deposition methods used for the preparation of CuInSe₂ and Cu(In,Ga)Se₂ thin film absorbers and solar cells. Although the emphasis here is on absorber preparation methods, buffer and conducting oxide preparation are discussed as well.     

Single-shot ablation threshold of chromium using UV femtosecond laser pulses

Single-shot ablation threshold for thin chromium film was studied using 266 nm, femtosecond laser pulses. Chromium is a useful material in the nanotechnology industry and information on ablation threshold using UV femtosecond pulses would help in precise micromachining of the material. The ablation threshold was determined by measuring the ablation crater diameters as a function of incident laser pulse energy. Absorption of 266 nm light on the chromium film was also measured under our experimental conditions, and the absorbed energy single-shot ablation threshold fluence was \(46 \pm 5\)  mJ/cm². The experimental ablation threshold fluence value was compared to time-dependent heat flow calculations based on the two temperature model for ultrafast laser pulses. The model predicts a value of 31.6 mJ/cm² which is qualitatively consistent with the experimentally obtained value, given the simplicity of the model.     

Femtosecond uv excimer laser ablation

Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient _eff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.     

Low-cost 3D nanocomposite solar cells obtained by electrodeposition of CuInSe2

Thin CuInSe₂ films have been prepared by electrodeposition from a single bath aqueous solution on both dense and nanoporous TiO₂. The films are deposited potentiostatically using a N₂-purged electrolyte at different potentials. Various deposition times and solution compositions have been employed. The effect of annealing in air and in argon at different temperatures and times is also investigated. Thin films and nanocomposites of TiO₂ and CuInSe₂ have been studied with electron microscopy, X-ray diffraction, Raman spectroscopy, and optical absorption spectroscopy. After a thermal anneal in argon at 350 °C for 30 min excellent CuInSe₂ is obtained. In particular the nominal crystal structure and the bandgap of 1.0 eV are found. Although pinholes are present occasionally, good samples with diode curves showing a rectification ratio of 24 at ±1 V are obtained. Upon irradiation with simulated solar light of 1000 W m⁻² a clear photoconductivity response is observed. Furthermore, also some photovoltaic energy conversion is found in TiO₂|CuInSe₂ nanocomposites.     

Elimination of cracking during UV laser ablation of SrTiO3 single crystals by employing a femtosecond laser

We have performed a comparative study of UV laser ablation of SrTiO₃ with nanosecond- and sub-picosecond sources, respectively. The experiments were performed with lasers at a wavelength of 248 nm and pulse durations of 34 ns and 500 fs. Femtosecond ablation turns out to be more efficient by one order of magnitude and eliminated the known problem of cracking of SrTiO₃ during laser machining with longer pulses. In addition, the cavities ablated with femtosecond pulses display a smoother surface with no indication of melting and well-defined, sharp edges. These effects can be explained by the reduced thermal shock effect on the material by using ultrashort pulses.     

Processing of W/Si and Si/W bilayers and multilayers with single and multiple excimer-laser pulses

Interdiffusion phenomena, thermal damage and ablation of W/Si and Si/W bilayers and multilayers under XeCl-excimer laser (=308 nm) irradiation at fluences of 0.15, 0.3 and 0.6 J/cm² were studied. Samples were prepared by UHV e-beam evaporation onto oxidized Si. The thickness of W and Si layers and the total thickness of the structures were 1–20 nm and 40–100 nm, respectively. 1 to 300 laser pulses were directed to the same irradiation site. At 0.6 J/cm² the samples were damaged even by a single laser pulse. At 0.3 J/cm² WSi₂ silicide formation, surface roughening and ablation were observed. The threshold for significant changes depends on the number of pulses: it was between 3–10 pulses and 10–30 pulses for bilayers with W and Si surfaces, respectively, and more than 100 pulses for multilayers with the same total thickness of tungsten. At 0.15 J/cm² the periodicity of the multilayers was preserved. Temperature profiles in layered structures were obtained by numerical simulations. The observed differences of the resistance of various bilayers and multilayers against UV irradiation are discussed.