The fabrication of multicomponent devices presents a challenge when facing the densification process. In the case of a microcapacitor, it involves a matrix of a dielectric componenet, such a Barium Titanate (BTO), sandwiched between electrodes for which a metallic material is commonly used. The material phases involved possesses different thermal properties which make impossible to densify at the same temperature and time. The combination of different laser technologies such as Laser-Induced Forward Transfer (LIFT) and Selective Laser Sintering (SLS) could be the key to fabricate these multicomponent devices by using digitazable technologies. In previous works the LIFT deposition and the Selective Laser Sintering of BTO has been prospected. Only superficial densification has been obtained on the dielectric component by SLS. In this work, LIFT and conventional sintering of BTO combined with the LIFT deposition and SLS of silver pastes are proposed for the fabrication of a first prototype.
Laser scribing is a promising technology for thin-film ablation in photovoltaic device manufacturing, particularly with non-conventional materials. This study explores copper oxides as alternative light absorbers due to their abundance and safe sourcing. Transition metal oxide (TMO) layers, like molybdenum oxide (MoO3), vanadium oxide (V2O5), and tungsten oxide (WO3), are investigated as selective contacts for advanced semiconductor devices. The research employs a high-powered fs laser (EKSPLA FemtoLux30, 30W, 1030 nm) with tunable pulse lengths (350 fs~1ps) and various wavelengths (1064, 532, and 355 nm) to determine the threshold ablation fluence and achieve optimal thin-film removal without substrate alteration. Diode isolation and electrical characteristics demonstrate the process's high quality.
We simulate laser ablation process of different metals in both ns and fs regime. Finite element method was implemented to numerically solve the thermal equations. Ablation has been modelled as a normal downwards mesh velocity. Ablation curves have been obtained for metals such as Ag, Cu and Al and oxides such as CuO; in ns regime we have also simulated multipulse operation. For ultrashort fs pulses, the Two-Temperature Model (TTM) needed to be applied. Improvements were made in an important thermal parameter of the TTM, the electron heat capacity, by solving the exact equations which give this parameter within the Free Electron Gas (FEG) model framework.
Transition metal oxide (TMOs) layers have interesting properties as selective contacts for novel semiconductor devices. Especially, oxides of molybdenum (MoO3), vanadium (V2O5), and tungsten (WO3) show good behaviour acting as front hole-selective contacts for n-type crystalline silicon heterojunction solar cells. Laser scribing has been widely used for thin-film ablation and seems the appropriate technology for device manufacturing with such non-conventional materials. In this work, we study the laser scribing of non-stoichiometric evaporated WOx, VOx, and MoOx films with three different wavelengths (1064, 532, and 355 nm) with pulse duration in the ns and ps regimes. The selection of the proper laser source allows a wide parametric window, with complete removal of the TMO films and no alteration of the silicon substrate. The results on the isolation of diodes and their electrical characteristics show the quality of the laser scribing processes.
A study of the of laser peen forming of thin stainless steel metal foils (50 μm thick) using a solid-state ps-pulsed laser, emitting at a wavelength of 1064 nm was conducted. The dependence of the bending angle and the radius of curvature on the energy per pulse, the treated area, the distance between lines, and the repetition rate of the treatment is presented. The study also shows that the bending effect is local, and it cannot be scaled by increasing the repetition rate, because the increase in temperature relaxes the superficial stresses previously induced.
Laser-Induced Forward Transfer (LIFT) is a versatile technique, allowing the transfer of a wide range of materials, with no contact, and high accuracy. Here we show a complete study on the deposition by LIFT, focusing on the deposition of a high viscosity silver paste, from the LIFT process parametrization to the metallization and characterization of heterojunction silicon solar cells.
Silicon nanoparticles suspended in deionized water were obtained by the laser ablation of solids in liquids technique. A silicon wafer target was ablated with a Nd:YVO4 pico-second laser emitting at 1064 nm with 10 ps pulse width at a repetition rate of 402 kHz with an energy per pulse of 106 µJ. The effect of fluence changes in nanoparticle size and optical properties was studied. The fluence was varied from 2 to 6 J/cm2 by attenuating the beam by means of optical attenuators. Results show a strong relation between nanoparticles size and fluence values. Optical characterization gives bad gap values higher than that of bulk Si, indicating the existence of quantum confinement effects produced by size reduction of Si nanocrystals.
LIPSS as well as hierarchical structures were generated by applying picosecond laser irradiation on a stainless steel-304 probe in a confinement liquid medium. Periodicity modification from high to low spatial surface frequency LIPSS was observed by modifying the pulse repetition rate from 1.3 to 402 kHz at a constant fluence. One of the outcomes reported in this study was the wettability analysis of the processed area which yielded significative changes on the contact angle between a water drop and the treated surface showing a wetting transition from hydrophilicity to hydrophobicity as an effect of the multiple impact pulses.
Laser-Induced Forward Transfer (LIFT) is a direct-write laser technique for the transference of material in an enormous range of viscosities and rheological behaviors, from solid-state to low-density inks. Furthermore, LIFT enables the transference of small volumes of material (as low as picoliters) with a high lateral spatial resolution (down to a few micrometers) to produce printed patterns with great flexibility.
In this work, simulations using a finite-element model involving Phase Field tracking method are presented and compared with experimental results.
Specifically, two LIFT processes are studied: a modified model is used to reproduce the secondary effects (such as bulgy shapes and secondary jets) observed after several ms in Blister-Actuated LIFT (BA-LIFT) of glycerol/water mixtures, and a model for LIFT transference of high-viscosity metallic pastes employed to study the different regimes observed in experiments (non-transference, explosive, cluster, dot, and bridge transfer)
This work presents results on laser texturing of aluminum-doped zinc oxide (ZnO:Al) films, a transparent conductive oxide (TCO), for improving light management in thin-film solar cells. Surface texturing is a fundamental step in solar cell manufacturing, especially in thin-film technologies, in order to decrease surface reflectance and enhance light scattering to improve light absorption in the PV absorbent. The ZnO:Al films used in this work were deposited by RF magnetron sputtering on glass substrates, and we used a Diode Pumped Solid State (DPSS) laser source emitting at 355 nm for texturing. All textures were obtained using a direct scribing technique and two different geometrical approaches for patterning were tested: The first consists in a simple linear pattern of equally-spaced parallel grooves while the second approach defines a crisscross pattern obtained by performing a second array of laser scribes orthogonal to the one defined in the first approach. We discuss the results attending at the morphological, optical and electrical characteristics of the samples, measuring the haze behavior and discussing the contribution of two possible scattering sources: The welldefined geometrical pattern formed by the grooves, acting as a diffraction grating, and a random roughness of low amplitude created during the laser process. Finally we deposited amorphous silicon solar cells onto the textured ZnO:Al films and studied the effect in the spectral response and short-circuit current (Jsc). We found, with the appropriate process parameters, an increment of 15% in Jsc compared to non-textured solar cells.
Laser-induced forward transfer (LIFT) is a fast, clean, non-contact metallization technique that allows the deposition of small volumes (down to picoliters) of a wide range of materials in a very precise and controlled way. In this work, we show a complete description of the metallization by LIFT using pulsed laser sources and a commercial silver-based paste. We include a description of the transference process and discuss the influence of the paste viscosity and the acceptor substrate roughness. To avoid the use of a standard paste-curing process in a furnace, a second laser process is used to cure the silver paste in a selective way, preventing any thermal damage in the materials below the paste or in other parts of the device. In summary, we show that the use of a LIFT technique allows the metallization of wide areas with high aspect ratio lines, that can be cured selectively, and present very good mechanical properties, being suitable for its use in flexible electronics applications.
Laser Direct Write techniques for printing applications of living material is currently a hot topic in different biomedical and engineering fields like tissue engineering, drug delivery, biosensing, etc. specialized groups in the field have been done to find and control the ideal conditions of printability, in order to maintaining unchanged the properties of the biological transferred material.
In this work we present a comprehensive study of the printability map of two of the most used biocompatible hydrogels, Sodium Alginate and Methylcellulose. We discuss the effect of hydrogel density, laser parameters influence, etc. using a blister assisted laser writing technique. In our approach we use a thick polyamide layer for blister generation, this presents huge advantages to limit the direct laser irradiation of the living material to be transferred.
In addition the physics of blister dynamics and droplet-jet formation is discussed by means of a combined study using numerical modeling of the process fluid dynamics and high speed imaging of the transfer.
Finally a particular example of advantages of the approach from the biological point of view is discussed presenting a cell viability study of Jurkat cell printing in the conditions discussed in the study
Laser induced forward transfer (LIFT) technique has been used for printing of various materials ranging from flexible metallic contacts to conductive silver lines. In this study, we are focusing on the printing of an industrial-grade silver paste formulated for the metalization of the front side of solar cells.
Printing of industrial silver pastes using the LIFT technique is challenging because the high viscosity of the silver paste allows only a small window of process parameters for reproducible and well-defined material transfer. In this work, we are examining the multiple-pulse effects during the printing of silver paste. Time-resolved imaging and characterization of the ejected silver paste voxels are performed to examine the influence of process parameters on the morphology of transferred paste dots and lines.
We have observed that by firing repeating laser pulses below the transfer energy threshold it is possible to print smaller volumes of paste, which yields an opportunity to print lines with higher resolution.
We also show that it is possible to print well-defined dots (voxels) of the paste using pulse energies near transfer threshold values. However, regarding the printing of lines, there is a strong interaction effect between adjacent voxels. This influence is so important that a distance between adjacent laser pulses threshold has been evaluated to print lines. The printing of single voxels has been achieved above the evaluated threshold value, while no printing could be achieved below the threshold. This distance threshold represents a limitation to the LIFT process of high viscosity pastes, which indicates that a compromise must be done between voxel size and laser frequency.
An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. In the present work, thin-film amorphous-silicon samples were irradiated with a continuous-wave green laser source. Laser irradiated spots were produced by using different laser powers and irradiation times. Micro-Raman spectroscopy was used to study the crystallization induced on the irradiated surface. Both laser peak power density and irradiation time are identified as key variables in the crystallization process, but within the parametric window considered, the enhancement of the crystalline factor, is more sensitive to the power density than to the irradiation time. The optimum parameters are then used for crystallizing a large sample area by means of overlapped laser scanned lines. Ellipsometric data experimentally show that the whole volume of a micron-thick sample is crystallized.
The main objective of this work is to adapt Laser Induced Forward Transfer (LIFT), a well-known laser direct writing technique for material transfer, to define metallic contacts (fingers and busbars) onto c-Si cells. A layer of a commercial silver paste (viscosity around 30-50 kcPs), with thickness in the order of tens of microns, is applied over a glass substrate using a coater.. The glass with the silver paste is set at a controlled gap over the c-Si cell. A solid state pulsed laser (532 nm) is focused on the glass/silver interface producing a droplet of silver that it is transferred to the acceptor substrate. The process parameters (silver paste thickness, gap and laser parameters -spot size, pulse energy and overlapping of pulses) are modified and the morphology of the voxels is studied using confocal microscopy. Long lines are printed with a scanner and their uniformity, width, and height are studied. Examples of metallization of large areas (up to 10 cm x 10 cm) over c-Si cells are presented.
It is well known that lasers have helped to increase efficiency and to reduce production costs in the photovoltaic (PV) sector in the last two decades, appearing in most cases as the ideal tool to solve some of the critical bottlenecks of production both in thin film (TF) and crystalline silicon (c-Si) technologies. The accumulated experience in these fields has brought as a consequence the possibility of using laser technology to produce new Building Integrated Photovoltaics (BIPV) products with a high degree of customization. However, to produce efficiently these personalized products it is necessary the development of optimized laser processes able to transform standard products in customized items oriented to the BIPV market. In particular, the production of semitransparencies and/or freeform geometries in TF a-Si modules and standard c-Si modules is an application of great interest in this market. In this work we present results of customization of both TF a-Si modules and standard monocrystalline (m-Si) and policrystalline silicon (pc-Si) modules using laser ablation and laser cutting processes. A discussion about the laser processes parameterization to guarantee the functionality of the device is included. Finally some examples of final devices are presented with a full discussion of the process approach used in their fabrication.
An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to
thin surface layers. Laser energy is used to heat the a-Si thin film to change the microstructure to poly-Si.
Thin film samples of a-Si were irradiated with a CW-green laser source. Laser irradiated spots were produced by using
different laser powers and irradiation times.
These parameters are identified as key variables in the crystallization process. The power threshold for crystallization is
reduced as the irradiation time is increased. When this threshold is reached the crystalline fraction increases lineally
with power for each irradiation time.
The experimental results are analysed with the aid of a numerical thermal model and the presence of two crystallization
mechanisms are observed: one due to melting and the other due to solid phase transformation.
Laser processing has been the tool of choice last years to develop improved concepts in contact formation for high efficiency crystalline silicon (c-Si) solar cells. New concepts based on standard laser fired contacts (LFC) or advanced laser doping (LD) techniques are optimal solutions for both the front and back contacts of a number of structures with growing interest in the c-Si PV industry. Nowadays, substantial efforts are underway to optimize these processes in order to be applied industrially in high efficiency concepts. However a critical issue in these devices is that, most of them, demand a very low thermal input during the fabrication sequence and a minimal damage of the structure during the laser irradiation process. Keeping these two objectives in mind, in this work we discuss the possibility of using laser-based processes to contact the rear side of silicon heterojunction (SHJ) solar cells in an approach fully compatible with the low temperature processing associated to these devices. First we discuss the possibility of using standard LFC techniques in the fabrication of SHJ cells on p-type substrates, studying in detail the effect of the laser wavelength on the contact quality. Secondly, we present an alternative strategy bearing in mind that a real challenge in the rear contact formation is to reduce the damage induced by the laser irradiation. This new approach is based on local laser doping techniques previously developed by our groups, to contact the rear side of p-type c-Si solar cells by means of laser processing before rear metallization of dielectric stacks containing Al2O3. In this work we demonstrate the possibility of using this new approach in SHJ cells with a distinct advantage over other standard LFC techniques.
This work investigates the influence of the laser wavelength on laser doping (LD) and laser-fired contact (LFC)
formation in solar cell structures. We compare the results obtained using the three first harmonics (corresponding to
wavelengths of 1064 nm, 532 nm and 355 nm) of fully commercial solid state laser sources with pulse width in the ns
range. The discussion is based on the impact on the morphology and electrical characteristics of test structures.
In the case of LFC the study includes the influence of different passivation layers and the assessment of the process
quality through electrical resistance measurements of an aluminium single LFC point for the different wavelengths.
Values for the normalized LFC resistance far below 1.0 mΩcm2 have been obtained, with better results at shorter
wavelengths.
To assess the influence of the laser wavelength on LD we have created n+ regions into p-type c-Si wafers, using a dry
LD approach to define punctual emitters. J-V characteristics show exponential trends at mid-injection for a broad
parametric window in all wavelengths, with local ideality factors well below 1.5.
In both processes the best results have been obtained using green (532 nm) and, specially, UV (355 nm). This indicates
that to minimize the thermal damage in the material is a clear requisite to obtain the best electrical performance, thus
indicating that UV laser shows better potential to be used in high efficiency solar cells.
Profiting by the increasing availability of laser sources delivering intensities above 109 W/cm2 with pulse energies in the
range of several Joules and pulse widths in the range of nanoseconds, laser shock processing (LSP) is being
consolidating as an effective technology for the improvement of surface mechanical and corrosion resistance properties
of metals and is being developed as a practical process amenable to production engineering.
The main acknowledged advantage of the laser shock processing technique consists on its capability of inducing a
relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical
behaviour, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion
cracking.
Following a short description of the theoretical/computational and experimental methods developed by the authors for
the predictive assessment and experimental implementation of LSP treatments, experimental results on the residual stress
profiles and associated surface properties modification successfully reached in typical materials (specifically Al and Ti
alloys) under different LSP irradiation conditions are presented. In particular, the analysis of the residual stress profiles
obtained under different irradiation parameters and the evaluation of the corresponding induced surface properties as
roughness and wear resistance are presented.
In this paper we present an original approach to estimate the heat affected zone in laser scribing processes for photovoltaic applications. We used high resolution IR-VIS Fourier transform spectrometry at micro-scale level for measuring the refractive index variations at different distances from the scribed line, and discussing then the results obtained for a-Si:H layers irradiated in different conditions that reproduce standard interconnection parameters. In order to properly assess the induced damage by the laser process, these results are compared with measurements of the crystalline state of the material using micro-Raman techniques. Additionally, the authors give details about how this technique could be used to feedback the laser process parametrization in monolithic interconnection of thin film photovoltaic devices based on a-Si:H.
Laser shock processing (LSP) or laser shock peening has been proposed as a competitive alternative technology to
classical treatments for improving fatigue and wear resistance of metals. This process induces a compressive residual
stress field which increases fatigue crack initiation life and reduce fatigue crack growth rate. We present a configuration
and results in the LSP concept for metal surface treatments in underwater laser irradiation at 1064 nm with and without a
thin surface paint layer. A convergent lens is used to deliver 1, 2 J/cm2 with a 8 ns laser FWHM pulse produced by a 10
Hz, Q-switched Nd:YAG laser with a spot diameter of a 1,5 mm moving forward along the workpiece. A LSP
configuration with experimental results using a pulse density of 5000 pulses/cm2 in 6061-T6 aluminum samples are presented.
Following its first developments in the 1970's, and profiting by the increasing availability of laser sources delivering intensities above 109 W/cm2 with pulse energies in the range of several Joules and pulse widths in the range of nanoseconds, laser shock processing (LSP) is being consolidating as an effective technology for the improvement of surface mechanical and corrosion resistance properties of metals and is being developed as a practical process amenable to production engineering. The main acknowledged advantage of the laser shock processing technique consists on its capability of inducing a relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behaviour, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking.
Although significant work from the experimental side has been contributed by several authors to explore the optimum conditions of application of the treatments and to assess their ultimate capability to provide enhanced mechanical behaviour to work-pieces of typical materials, only limited attempts have been developed, however, in the way of full comprehension and predictive assessment of the characteristic physical processes and material transformations with a specific consideration of real material properties.
In the present paper, a review on the physical issues dominating the development of LSP processes from a high intensity laser-matter interaction point of view is presented along with the theoretical and computational methods developed by the authors for their predictive assessment, and practical results at laboratory scale on the application of the technique to different materials are shown along with corresponding results on the mechanical properties improvement induced by LSP treatments.
Although significant work has been conducted in order to explore the optimum conditions of application of LSP
treatments and to assess their capability to provide enhanced mechanical properties, only limited attempts have been
developed in the way of predictive assessment of the characteristic physical processes and transformations with a specific
consideration of the associated laser interaction dynamics. For this reason, additionally to the authors' efforts in the line
of the numerical predictive assessment of the effects induced by the LSP technique from a predominantly mechanical
point of view, the observation and analysis of the plasma dynamics following laser interaction have been envisaged as a
means for the proper assessment of the shocking process relative to the material target and also as a way of plasma
dynamics control in view of process optimization. In the present paper, the basis for the plasma spectroscopic observation
of LSP experiments in combination to numerical dynamics simulations are proposed as a means for the extraction of
relevant guidelines for process design optimization.
Laser shock processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results in the LSP concept for metal surfaces treatments in underwater laser irradiation at 1064 nm. A convergent lens is used to deliver 2.5 J/cm2 in a 8 ns laser FWHM pulse produced by a Q-switch Nd:YAG Laser. Experimental results using a pulse density of 5 000 pulses/cm2 and spots of 0.8 mm in diameter in 1040 steel samples are presented. Compressive residual stress distribution as a function of depth is assessed by the hole drilling method. High level compressive residual stresses are produced using 1064 nm wavelength. This method can be applied for surface treatment of final metal products.
Laser micromachining techniques are among the most promising fabrication processes in strategic industrial fields. Although lasers systems have been widely applied last twenty years in semiconductor industry for microfabrication process development, the current availability of new excimer and Diode Pumped Solid State Lasers (DPSS) sources are extending the applications fields of laser microprocessing. Nowadays MEMs, fluidic devices, advanced sensors and biomedical devices and instruments are among the more promising developments of this technology. Nevertheless the fast progress of this technology has brought as a consequence the building up of specific laser based machines for each process of interest (most of them until now strictly 2D), and an important gap has been generated, from the fabrication point of view, in fully 3D potential applications. In this work, the conception, design and first results of a fully automatized 3D laser micromachining workstation, based on the main concept of flexibility, is presented. This system integrates two UV laser sources, excimer and DPSS in ns pulse regime, and an advanced positioning system (with six degrees of freedom) for complex parts machining. Several examples of first results obtained with this system, including processing of semiconductors for sensoring and photovoltaic applications, organic materials for biomedical devices and metallic materials for different strategic industrial sectors are presented.
Laser shock processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results in the LSP concept for metal surfaces treatments in underwater laser irradiation at 535 nm. A convergent lens is used to deliver 0.9 J/cm2 in an 8 ns laser FWHM pulse produced by 10 Hz Q-switched Nd:YAG Laser with spots of a 1.5 mm in diameter moving forward along the workpiece. A LSP configuration with experimental results using a pulse density of 2 500 pulses/cm2 and 5 000 pulses/cm2 in 6061-T6 aluminum samples are presented by 532 nm. High level of compressive residual stresses are produced (600 MPa-750 MPa) with a depth of 1.2 mm. It has been shown that surface residual stress level is comparable to that achieved by conventional shot peening, but with greater depths. This method can be applied to surface treatment of final metal products.
Although valuable experimental work has been performed in order to explore the optimum conditions of application of the treatments and to assess their capability to provide enhanced mechanical properties, there is little work done on the theoretical prediction of these optimum parameters. In the present paper, a model is presented to provide an estimation of the residual stresses and surface deformation in order to see the influence of the different parameters in the process. The influence of pulse duration, pulse pressure peak, spot radius, number of shots, overlapped shots and material properties are studied. The great influence of 3D deformation effects in the process is clearly shown as one of the most important limiting factors of the process traditionally neglected in previous literature. Additionally, from the experimental point of view, in the present paper a summary is provided of different results obtained from the most recent LSP experiments carried out by the authors along with some conclusions for the assessment of LSP technology as a profitable method for the extension of fatigue life in critical heavy duty components.
In the present paper, a model is presented aiming to provide a physical basis for the theoretical estimation of both the plasma expansion dynamics following laser irradiation and the shock wave propagation into the treated material with a specific consideration of its constitutional properties (i.e. taking into account its real mechanical and E.O.S. parameters). Although initially limited to a 1D description, the proposed model aims to overcome the difficulties existing for an accurate theoretical estimation of the process due to the treated material behavior not directly amenable to analytical solutions and sometimes modeled through empirical approaches, and to provide a detailed treatment of the plasma behavior (i.e. ionization, breakdown, etc.). Additionally, and as a direct consequence of its analytical-numerical character, the model can provide a fully time dependent representation of the processes, including the laser pulse temporal profile, what is a real advance over previous theoretical studies.
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