This PDF file contains the front matter associated with SPIE Proceedings Volume 8967, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

We propose a control technique for laser induced sub- micron ripples on titanium and silicon using femtosecond laser pulse shaping. This is based on a real-time observation method of nano ripples by diffraction of UV laser beam and programmable pulse temporal design. The feedback diffraction signal provided information of ripples’ period, area, direction, and arrangement uniformity. By using a genetic algorithm optimization, ripples formation was optimized for their period tuning ability and their uniformity. The diffraction signals were validated with scanning electron microscope (SEM) images. At the generation wavelength of 800 nm and depending on the pulse form, ripples on titanium show periods from 610 nm to 680 nm, and ripples on silicon has periods from 710 – 770 nm. Laser pulse energy affects optimization due to transient energy deposit on material with pulse form effects in the threshold fluence and ripple areas.

One common feature of many functional surfaces found in nature is their modular composition often exhibiting several length scales. Prominent natural examples for extreme behaviors can be named in various plant leaf (rose, peanut, lotus) or animal toe surfaces (Gecko, tree frog). Influence factors of interest are the surface’s chemical composition, its microstructure, its organized or random roughness and hence the resulting surface wetting and adhesion character. Femtosecond (fs) laser micromachining offers a possibility to render all these factors in one single processing step on metallic and polymeric surfaces. Exemplarily, studies on Titanium and PTFE are shown, where the dependence of the resulting feature sizes on lasing intensity is investigated. While Ti surfaces show rigid surface patterns of micrometer scaled features with superimposed nanostructures, PTFE exhibits elastic hairy structures of nanometric diameter, which upon a certain threshold tend to bundle to larger features. Both surface patterns can be adjusted to mimic specific wetting and flow behaviour as seen on natural examples. Therefore, fs-laser micromachining is suggested as an interesting industrially scalable technique to pattern and fine-tune the surface wettability of a surface to the desired extends in one process step. Possible applications can be seen with surfaces, which require specific wetting, fouling, icing, friction or cell adhesion behaviour.

This work studied the optothermal response of plasmonic nanofocusing structures under picosecond pulsed laser irradiation. The surface plasmon polariton is simulated to calculate the optical energy dissipation as the Joule heating source and the thermal transport process is studied using a two temperature model (TTM). At the picosecond time scale that we are interested in, the Fourier heat equation is used to study the electron thermal transport and the hyperbolic heat equation is used to study the lattice thermal transport. For comparison, the single temperature model (STM) is also studied. The difference between TTM and STM indicates that TTM provides more accurate estimates in the picosecond time scale and the STM results are only reliable when the local electron and lattice temperature difference is negligible.

The creation of complex three-dimensional (3D) fluidic systems composed of hollow micro- and nanostructures embedded in transparent substrates has attracted significant attention from both scientific and applied research communities. However, it is by now still a formidable challenge to build 3D micro- and nanofluidic structures with arbitrary configurations using conventional planar lithographic fabrication methods. As a direct and maskless fabrication technique, femtosecond laser micromachining provides a straightforward approach for high-precision spatial-selective modification inside transparent materials through nonlinear optical absorption. Here, we demonstrate rapid fabrication of high-aspect-ratio micro- and/or nanofluidic structures with various 3D configurations in glass substrates by femtosecond laser direct writing. Based on this approach, we demonstrate several functional micro- and nanofluidic devices including a 3D passive microfluidic mixer, a capillary electrophoresis (CE) analysis chip, and an integrated micro-nanofluidic system for single DNA analysis. This technology offers new opportunities to develop novel 3D micro-nanofluidic systems for a variety of lab-on-a-chip applications.

A simple and flexible technique for integrating metal micropatterns into glass microfluidic structures based on threedimensional femtosecond laser microfabrication is presented. Femtosecond laser direct writing followed by thermal treatment and successive chemical etching allows us to fabricate three-dimensional microfluidic structures such as microchannels and microreservoirs inside photosensitive glass. Then, the femtosecond laser direct-write ablation followed by electroless metal plating enables space-selective deposition of patterned metal films on desired locations of internal walls of the fabricated microfluidic structures. The developed technique is applied to integrate a metal microheater into a glass microchannel to control the temperature of liquid samples in the channel, which can be used as a microreactor for enhancement of chemical reactions.

Cost-efficient machining of dielectric surfaces with high-precision and low-roughness for industrial applications is still challenging if using laser-patterning processes. Laser induced back side wet etching (LIBWE) using UV laser pulses with liquid heavy metals or aromatic hydrocarbons as absorber allows the fabrication of well-defined, nm precise, free-form surfaces with low surface roughness, e.g., needed for optical applications. The copper-sulphatebased absorber CuSO₄/K-Na-Tartrate/NaOH/formaldehyde in water is used for laser-induced deposition of copper. If this absorber can also be used as precursor for laser-induced ablation, promising industrial applications combining surface structuring and deposition within the same setup could be possible. The etching results applying a KrF excimer (248 nm, 25 ns) and a Nd:YAG (1064 nm, 20 ns) laser are compared. The topography of the etched surfaces were analyzed by scanning electron microscopy (SEM), white light interferometry (WLI) as well as laser scanning microscopy (LSM). The chemical composition of the irradiated surface was studied by energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FT-IR). For the discussion of the etching mechanism the laser-induced heating was simulated with finite element method (FEM). The results indicate that the UV and IR radiation allows micro structuring of fused silica with the copper-based absorber where the etching process can be explained by the laser-induced formation of a copper-based absorber layer.

Bulk modification and micro-structuring of diamonds using ultra-short laser pulses is of great interest due to its potential in photonic applications, radiation detectors and diamond gem marking. We report on bulk micro-structuring and stimulated Raman scattering (SRS) in type IIa single crystal diamond with multi pulse irradiation by picosecond-laser pulses at the wavelength 532nm (10ps & 44ps). The experiment was expanded by additional setups for on-line video imaging and spectroscopic measurements during laser irradiation and structure growth in the bulk diamond from the backside of the crystal. We discuss the influence of the crystal orientation ({100} and {110}) relative to the laser beam onto (i) the optical breakdown threshold, (ii) the character of the structural modifications and (iii) generation of SRS during irradiation. We show that the formation of bulk microstructures dramatically influences the behavior of the SRS and that the structure growth and the laser-induced breakdown in the bulk are governed by the dielectric breakdown mechanism. We will further present the conditions for efficient SRS lasing depending on the different pulse durations. Based on the Stokes-to-anti-Stokes intensity ratio in the recorded SRS spectra we will finally propose a method of local temperature measurements in the bulk of diamond to determine the “pre-breakdown” temperature.

When a femtosecond laser pulse is focused tightly inside a LiF single crystal, cracks are generated in the <100< directions from the photoexcited region. Previously, we found that cracks of different lengths are formed by simultaneous fs laser irradiation at multiple spots. To elucidate the mechanism of modulation in crack lengths, we observed the transient stress distributions after simultaneous fs laser irradiation at multiple spots inside a LiF single crystal. First, we found that stress amplitude can be doubled by the interference of fs laser induced stress waves. Next, we observed the dynamics of crack formation as well as the transient birefringence distribution. In the case in which one crack became shorter than other cracks, the observation of the crack dynamics showed that the compressive stress by a constrictive interference of stress waves at a crack tip prevented the crack from propagating further. However, In the case in which the elongation of one crack occurred, we could not find any relationship between the elongation of a crack and the interference of stress waves by the observed stress distribution. Based on the time-resolved observation, we discussed the mechanism of the modulation of the laser induced cracks by interference of stress waves.

The use of the femtosecond laser enables generation of small spot sizes and ablation features. Ablation of the small features usually requires only a small amount of laser power to be delivered to the ablation spot. When using only a one beam for the ablation of the small features this process is bound to be time consuming. The spatial light modulator (SLM) together with the computer generated holograms (CGH) can be used for manipulating and shaping of the laser beam in various applications. In laser micromachining, when using laser with relatively high power, the original beam can be divided up to hundreds beams and still have the energy of the individual beam above the ablation threshold of the material. This parallel laser processing enables more efficient use of the laser power regardless of the machining task.

A focus shifting unit integrated with a tunable lens allows for rapid response times, high accuracy, small footprint and simple controllability without the need for any translational mechanics. The focus shifting unit is designed for laser material microprocessing applications where tolerances of a few micrometers are required. However, the optical fluid inside the tunable lens can be severely altered by long term thermal influences from the environment and high powered laser beams. Utilizing the working principles of a cylinder lens, a discrete proportional-integral-derivative controller with an anti-reset windup is simulated and designed for offline regulation of the focal length of the tunable lens. This allows for integration into a three-dimensional scanhead system to reliably deflect the focused laser spot at the workpiece level over long periods of time, i.e. > 8 hours. Deviation of the focal length of the tunable lens is identified by the cylinder lens through ellipticity of a probe laser beam. The focal length is subsequently corrected by altering the input current into the tunable lens by means of the control system which is based on numerical methods. The thermal behavior of the tunable lens, system identification and synthesis of the controller, design of the focus shifting optical system and validation of the controller are studied.

To enable the direct-spinning process of super-micro fibres (< 0.5 dtex) suitable for novel medical, hygienical and technical products microhole arrays with diameters down to 25 μm in very high quality are required. Using ultrashort pulses together with a helical drilling optics microholes with high accuracy were manufactured in metals of a thickness in the range of 0.3 mm. However, the required process time for a single microhole ranges up to several ten seconds. Simple energy balance considerations show that higher averaged powers - either achieved with larger pulse energies or an increased repetition rate - considerable reduce the process time. In this case plasma formation and heat accumulation show an increased formation of melt and recast. Thus, the objective is to increase the productivity while maintaining consistent quality of the microholes. With this aim, the influence of pulse energy and repetition rate on the borehole geometry, processing quality and process efficiency was investigated for helical drilling. In the present research work a TruMicro 5250 laser source (t_p = 8 ps, λ=515 nm, f_R=800 kHz) was used. To determine the process time of the microhole the transmitted laser radiation was recorded. A systematic evaluation of the process quality and process time dependent on pulse energy and repetition rate will be presented in this contribution. First laser manufactured spinning nozzles with microhole diameters down to 25 μm processed in 0.24 mm thick AuPt alloy were used to fabricate unique super-micro fibres with yarn counts down to 0.2 dtex.

We report on the laser cutting of carbon fiber reinforced thermo-plastics (CFRTP) with a cw IR fiber laser (single-mode fiber laser, average power: 350 W). CFRTP is a high strength composite material with a lightweight, and is increasingly being used various applications. A well-defined cutting of CFRTP which were free of debris and thermal-damages around the grooves, were performed by the laser irradiation with a fast beam galvanometer scanning on a multiple-scanpass method.

The dual beam of cw-350 W single-mode near-IR fiber laser and ns-pulsed-35 W UV laser were used in the experiments for cutting. The laser beam on the sample surface was scanned with a galvanometer scanner and focused with the f-theta lens of 400 mm focal length for IR and UV laser irradiations. A prototype remote scanner head for the multiple laser irradiations has been developed for a high-quality and high-speed laser processing of carbon fiber reinforced plastics (CFRP). In this paper, we report on the laser trepanning of circular patterns on CFRP.

Carbon fibre reinforced plastics (CFRP) have a large potential in the automotive lightweight construction due to their low density and high mechanical stability. Compared with today’s laser processing methods of metals the main issues in laser processing of CFRP are the very differing thermal, optical and mechanical properties of the components. To understand the process in detail, the ablation process of CFRP with ultrashort laser pulses was investigated. The shock wave and the vapor resulting from processing with single laser pulses were recorded. Shadow photography and luminescence photography with an ultra-high-speed camera was used to show the ablation process with a temporary resolution of up to 3 ns. The field of view was 250 μm × 250 μm. An ultrashort laser pulse with pulse duration of 4 ps and a wavelength of 800 nm was focused onto the workpiece. The energy content of the shock wave was calculated from the resulting images. The energy content of the shock wave was about 20 % of the incident energy and the speed of propagation of the shock wave was more than 2000 m/s. The high intensities in the range of 10¹³ W/cm² lead to formation of a plasma plume which was clearly seen in the shadow photography images.

A carbon fiber reinforced plastic [CFRP], which has high strength, light weight and weather resistance, is attractive material applied for automobile, aircraft and so on. The laser processing of CFRP is one of suitable way to machining tool. However, thermal affected zone was formed at the exposure part, since the heat conduction property of the matrix is different from that of carbon fiber. In this paper, we demonstrated that the CFRP plates were cut with UV nanosecond laser to reduce the heat affected zone. The ablation plume and ablation mass were investigated by laser microscope and ultra-high speed camera. Furthermore, the ablation model was constructed by energy balance, and it was confirmed that the ablation rate was 0.028 μg/ pulse in good agreement with the calculation value of 0.03 μg/ pulse.

A CW laser heterodyne spectrometer has been assembled with time resolved data acquisition for probing surface acoustic waves (SAWs) generated by the interaction of a pulsed laser and surface. Literature suggests that SAWs can enhance chemical catalysis, nucleation and surface chemical mobility. Pulsed lasers are known to induce SAWs with bandwidth that is inversely proportional to the pulse width. The goal of this experiment is to apply laser heterodyne spectroscopy to understand the photophysical interactions that promote the formation of laser induced SAWs. The experiment explores the effects produced by a 100 Hz repetition rate UV (355nm) laser with a 6 ns pulse width. Silicon (111) is used as the substrate material, making it is less likely for propagating non linear waves to experience anisotropy. The development of the time-resolved heterodyne spectrometer includes the development of specific data acquisition and software analysis tools to monitor sub-nanometer surface displacements. In addition, to insure that the pulsed laser irradiated material remains within the thermoelastic regime as opposed to ablation, a 2D laser thermal heating model is used to define the duty cycle of the repetitive pulsed laser. Results show that it is possible to measure and analyze laser induced SAWs many centimeters away from the source and substrate dispersion affects the spectral properties of the propagating SAWs. Under controlled conditions, we have measured surface vertical displacements approaching 0.1nm.

The commercial availability of fiber lasers based on MOPA architectures with arbitrary temporal pulse shaping capabilities offers completely new possibilities for laser material processing. In this study, based on numerical modeling results in the nanosecond regime for the case of silicon at 1064 nm wavelength, we show that not only the single pulse laser ablation efficiency depends on the temporal pulse shape but, we also demonstrate how a stochastic approach can be applied in order to reach an optimized pulse shape maximizing the material vaporization rate for given laser pulse energy and duration. Experimental results are compared to the numerical modeling results, and the discrepancies are discussed in terms of the role played by plasma shielding effects and melt ejection at high intensity.

For thin film ablation one can often not take full advantage of the relatively high output power and high pulse energy of lasers. A solution might be the use of parallel processing technique with multiple beams which help to increase process speed and to save process costs. Within this contribution we demonstrate thin film scribing of GZO and ITO which shows the potential of parallel processing combined with Top-hat beam shaping. The beam shaping optic provides process optimized beam profiles leading to a more efficient process and an improved ablation quality.

To be competitive in laser micro machining, high throughput is an important aspect. One possibility to increase productivity is scaling up the ablation process i.e. linearly increasing the laser repetition rate together with the average power and the scan speed. In the MHz-regime high scan speeds are required which cannot be provided by commercially available galvo scanners. In this work we will report on the results by using a polygon line scanner having a maximum scan speed of 100 m/s and a 50 W ps-laser system, synchronized via the SuperSync™ technology. We will show the results concerning the removal rate and the surface quality for working at the optimum point i.e. most efficient point at repetition rates up to 8.2 MHz.

We present results from laser annealing experiments in Si using a passively Q-switched Nd:YAG microlaser. Exposure with laser at fluence values above the damage threshold of commercially available photodiodes results in electrical damage (as measured by an increase in photodiode dark current). We show that increasing the laser fluence to values in excess of the damage threshold can result in annealing of a damage site and a reduction in detector dark current by as much as 100x in some cases. A still further increase in fluence results in irreparable damage. Thus we demonstrate the presence of a laser annealing window over which performance of damaged detectors can be at least partially reconstituted. Moreover dark current reduction is observed over the entire operating range of the diode indicating that device performance has been improved for all values of reverse bias voltage. Additionally, we will present results of laser annealing in Si waveguides. By exposing a small (<10 um) length of a Si waveguide to an annealing laser pulse, the longitudinal phase of light acquired in propagating through the waveguide can be modified with high precision, <15 milliradian per laser pulse. Phase tuning by 180 degrees is exhibited with multiple exposures to one arm of a Mach-Zehnder interferometer at fluence values below the morphological damage threshold of an etched Si waveguide. No reduction in optical transmission at 1550 nm was found after 220 annealing laser shots.

In this work, a picosecond DPSS and a nanosecond Nd:YAG laser have been used for the annealing and the partial nanocrystallization of an amorphous silicon layer. These experiments were conducted in order to improve the characteristics of a micromorph tandem solar cell. The laser annealing was attempted at 1064nm in order to obtain the desired crystallization’s depth and ratios. Preliminary annealing-processes, with different annealing parameters, have been tested, such as fluence, repetition rate and number of pulses. Irradiations were applied in the sub-melt regime, in order to prevent significant diffusion of p- and n-dopants to take place within the structure. The laser experimental work was combined with simulations of the laser annealing process, in terms of temperature distribution evolution, using the Synopsys Sentaurus Process TCAD software. The optimum annealing conditions for the two different pulse durations were determined. Experimentally determined optical properties of our samples, such as the absorption coefficient and reflectivity, were used for a more realistic simulation. From the simulations results, a temperature profile, appropriate to yield the desired recrystallization was obtained for the case of ps pulses, which was verified from the experimental results described below. The annealed material was studied, as far as it concerns its structural properties, by XRD, SEM and micro-Raman techniques, providing consistent information on the characteristics of the nanocrystalline material produced by the laser annealing experiments. It was found that, with the use of ps pulses, the resultant polycrystalline region shows crystallization’s ratios similar to a PECVD developed poly-Silicon layer, with slightly larger nanocrystallite’s size.

An increase of industrial needs for micro-ablation and surface structuration using sub-picosecond laser working at high repetition rate is required. In this context, new industrial lasers were recently commercialized for such a type of purpose. The potential of a new industrial femtosecond laser source (Tangerine model from Amplitude Système) is investigated in this work for different etching purposes. Our experimental results will be also compared to those obtained when using Ti:Sa laser source, with the help of numerical simulations.

A technique for structuring the surface of a bulk metallic glass (BMG) via scanning with a beam of laser pulses in the pico- and femtosecond time regime is presented. Specimens were characterized by various techniques to analyze the effects of ultrashort laser pulses on the amorphous matrix. Broadly varying surface structures, with roughness parameters in the range of R_a = 0:066 to 0:329 μm, measured using white light interferometry (WIM) and optical 3D microscopy, were produced. These techniques could be useful for fabricating biomedical implants from BMGs. As proof of principle, a patterned grid, designed for evaluating bone cell response to different surface structures, are produced.

Laser-induced Forward Transfer (LIFT) is a 3D direct-write method suitable for precision printing of various materials. As the ejection mechanism of picosecond LIFT has not been visualized in detail, the governing physics are not fully understood yet. Therefore, this article presents an experimental imaging study on the ejection process of gold-based LIFT. The LIFT experiments were performed using a 6.7 picosecond Yb:YAG laser source equipped with a SHG. The beam was focused onto a 200 nm thick gold donor layer. The high magnification images were obtained using bright field illumination by a 6 ns pulsed Nd:YAG laser source and a 50× long-distance microscope objective that was combined with a 200 mm tube lens. For laser fluence levels up to two times the donor-transfer-threshold, the ejection of a single droplet was observed. The typical droplet radius was estimated to be less than 3 μm. A transition of ejection features towards higher fluence, indicates a second fluence-regime in the ejection process. For higher laser fluence, the formation of an elongated gold jet was observed. This jet fragments into multiple relatively small droplets, resulting in a spray of particles on the receiving substrate.

Femtosecond lasers are gaining industrial interest for surface patterning and structuring because of the reduced heat effects to the surrounding material, resulting in a good quality product with a high aspect ratio. Analysis of the plasma plume generated during ablation can provide useful information about the laser-material interactions and thereby the quality of the resulting surface patterns. As a low-cost alternative to rather complicated ICCD camera setups, presented here is an approach based on filming the laser machining process with a high speed camera and tuning the frame rate of the camera to slightly lower than the laser pulse frequency. The delay in frequency between the laser and camera results in frames taken from sequential pulses. Each frame represents a later phase of plume expansion although taken from different pulses. Assuming equal plume evolution processes from pulse to pulse, the sequence of images obtained completes a plume expansion video. To test the assumption of homogeneity between sequential plumes, the camera can be tuned to the frequency of the laser, as to capture consecutive plumes at the same phase in their evolution. This approach enables a relatively low-cost, high resolution visualization of plasma plume evolution suitable for industrial micromachining applications with femtosecond lasers. Using this approach we illustrate differences in plume expansion at the example of machining homogeneous surface patterns in different liquid and gaseous processing environments.

Nonlinear propagation of intense ultrafast laser pulses inside transparent materials has a strong influence on the fabrication quality and accuracy for 3D laser-material processing. Due to their ability to maintain near-constant fluence profiles over an appreciable distance along the propagation direction in linear and nonlinear media, ultrafast Bessel beams are ideal sources for high aspect ratio sub-micron structuring applications. We report here on the interaction of transparent materials, especially fused silica, with ultrafast non-diffractive beams of moderate cone angle at various laser energies and pulse durations and define their impact on photoinscription regimes, i.e. formation of isotropic and non-isotropic (positive and negative) refractive index structures. The laser pulse duration was observed to be key in deciding the type of the structures via excitation efficiency. To understand the significant mechanisms for forming these different structures, the free carrier behavior as a function of laser pulse duration and energy was studied by capturing instantaneous excitation profiles using time-resolved microscopy. Time-resolved imaging and simulation studies reveal that low carrier densities are generated for ultrashort pulses leading to soft positive index alterations via presumably non-thermally induced structural transitions via defects. On the other hand, the high free carrier density generation in the case of longer pulse durations leads to a hydrodynamic expansion resulting in high aspect ratio sub-micron size wide voids. Delayed ionization, carrier defocusing and lower nonlinear effects are responsible for the confinement of energy, resulting in efficient energy deposition on-axis.

The characteristics of laser-induced breakdown spectroscopy (LIBS) such as short measurement time and no sample preparation provide clear advantages over other analytical techniques for rapid elemental analysis at manufacturing sites where the composition of products need to be determined in real-time for process monitoring or quality control. Thin film solar cells based on CuIn_1-xGa_xSe₂ (CIGS), polycrystalline compound semiconductor material, have unique advantages of high efficiency (>20%), long-term stability, and low manufacturing cost over other types of solar cell. The electrical and optical properties of the thin CIGS films are closely related to the concentration ratios among its major constituent elements Cu, In, Ga and Se such as Ga/(Ga + In) and Cu/(Ga + In), and thus an accurate measurement of the composition of CIGS thin films has been an issue among CIGS solar cell researchers, requiring a fast and reliable technique for composition analysis. This paper presents the results of nanosecond (ns) and femtosecond (fs) laser based LIBS analysis of thin CIGS films. The critical issues for LIBS analysis of CIGS thin films such are discussed in comparison with ns- and fs-LIBS measurement results. The calibration of LIBS signal intensity ratios with respect to reference concentration data is carried out and the results of optimal line selection for LIBS analysis, depth profiling capability, and reproducibility are discussed.

We report on theoretical models of the interaction of ultra-short laser pulses with multilayer structures used in thin-film solar cells. A finite-difference based optical model of light propagation within the thin-film system is used to determine the 3D-distribution of absorbed laser power in the structure. The model includes the evolution of the density of charge carriers which may be driven either by direct absorption of the laser radiation or multi-photon absorption and impact ionization of highly excited carriers. Depending on the excitation wavelength and pulse energy absorption occurs in different depths of the structure which has a large effect on the efficiency of the laser ablation process.

In emitter wrap through (EWT) solar cells, laser drilling is used to increase the light sensitive area by removing emitter contacts from the front side of the cell. For a cell area of 156 x 156 mm², about 24000 via-holes with a diameter of 60 μm have to be drilled into silicon wafers with a thickness of 200 μm. The processing time of 10 to 20 s is determined by the number of laser pulses required for safely opening every hole on the bottom side. Therefore, the largest wafer thickness occurring in a production line defines the processing time. However, wafer thickness varies by roughly ±20 %. To reduce the processing time, a coaxial camera control system was integrated into the laser scanner. It observes the bottom breakthrough from the front side of the wafer by measuring the process emissions of every single laser pulse. To achieve the frame rates and latency times required by the repetition rate of the laser (10 kHz), a camera based on cellular neural networks (CNN) was used where the images are processed directly on the camera chip by 176 x 144 sensor–processor–elements. One image per laser pulse is processed within 36 μs corresponding to a maximum pulse rate of 25 kHz. The laser is stopped when all of the holes are open on the bottom side. The result is a quality control system in which the processing time of a production line is defined by average instead of maximum wafer thickness.

We investigate a laser welding process for contacting aluminum metallized crystalline silicon solar cells to a 10-μm-thick aluminum layers on a glass substrate. The reduction of the solar cell metallization thickness is analyzed with respect to laser induced damage using SiN_x passivated silicon wafers. Additionally, we measure the mechanical stress of the laser welds by perpendicular tear-off as well as the electrical contact resistance. We apply two types of laser processes; one uses one to eight 20-ns-laser pulses at 355 nm with fluences between 12 and 40 J/cm² and the other single 1.2-μs-laser pulses at 1064 nm with 33 to 73 J/cm². Ns laser pulses can contact down to 1-μm-thick aluminum layers on silicon without inducing laser damage to the silicon and lead to sufficient strong mechanical contact. In case of μs laser pulses the limiting thickness is 2 μm.

The thin-film solar cell market has seen a period of consolidation during the last years and many involved companies were forced to stop production due to increasing price pressure from competing cell technologies. Today, thin-film solar industry is gaining momentum again. Especially Cu(In,Ga)Se2 technology evolves at high pace fired by recently achieved record efficiencies of 20.4 percent on flexible polyimide substrate [1] and 20.8 percent on glass substrate [2]. Fresh companies are preparing market entry with matured products and manufacturing technology suitable for high-volume and high-throughput production. Among these key-enabling technologies is laser patterning for cell-to-cell interconnects. Several research groups worked on efficient and reliable laser processes that are now ready for the industrial assessment. Here we present a set of work-horse processes for P1, P2 and P3 scribing of CIGS cells on glass substrate. Optimized parameters are presented for 532 nm and 1064 nm using 50 ps pulses from an all-in-fiber laser system. We further demonstrate the successful realization of functional 8-cell modules with a reduced “dead-zone” width of 70±5 μm and high efficiencies. The certified efficiency of 16.6 percent for our low-dead-zone champion module confirms the observation that shrinking of interconnects has no adverse effects on their electrical quality.

This work examines an attenuator of laser radiation based on four Dove prisms, which is much more efficient in comparison with two cascades of a di-prismatic attenuator. In this case, not only is its alignment and assembly significantly simplified, but also the consistency of the construction becomes evident. It turns out that the most optimal configuration of a four-prismatic attenuator is a pair-parallel one. This configuration, compared with a pair-perpendicular one, provides exchangeability of the di- and the four-prismatic model of the attenuator, maintains the direction of propagation of the radiation, and has a reference beam to control an attenuation coefficient in the operation process of the attenuator.

For assembly of micro-devices, such as photonic devices, the precision alignment of components is often critical for their performance. Laser forming, also known as laser-adjusting, can be used to create an integrated microactuator to align the components with sub-micron precision after bonding. In this paper a so-called three-bridge planar manipulator was used to study the laser-material interaction and thermal and mechanical behavior of the laser forming mechanism. A 3-D Finite Element Method (FEM) model and experiments are used to identify the optimal parameter settings for a high precision actuator. The goal in this paper is to investigate how precise the maximum occurring temperature and the resulting displacement are predicted by a 3-D FEM model by comparing with experimental results. A secondary goal is to investigate the resolution of the mechanism and the range of motion. With the experimental setup we measure the displacement and surface temperature in real-time. The time-dependent heat transfer FEM models match closely with experimental results, however the structural model can deviate more than 100% in absolute displacement. Experimentally, a positioning resolution of 0.1μm was achieved, with a total stroke exceeding 20μm. A spread of 10% in the temperature cycles between several experiments was found, which was attributed to a spread in the surface absorptivity. Combined with geometric tolerances, the spread in displacement can be as large as 20%. This implies that feedback control of the laser power, in combination with iterative learning during positioning, is required for high precision alignment. Even though the FEM models deviate substantially from the experiments, the 3-D FEM model predicts the trend in deformation sufficiently accurate to use it for design optimization of high precision 3-D actuators using laser adjusting.

In this work we investigate the use of a picosecond (ps) laser used for monolithic connection to pattern glass substrates to achieve light in-coupling in silicon thin film solar cells. We present our results on the patterning of three commercially available and frequently used multi-component glasses Corning EAGLE XG®, Schott BOROFLOAT® 33 and Saint-Gobain SGG DIAMANT®. We find that the different glass structural components influence the degree of texturing obtained. This can be attributed to the different laser induced electron collision times and recombination rates, and thus the critical electron density evolution leading to ablation. Thus the ablated crater profile is glass composition dependent. The surface texture is altered from periodic to random with decreasing scribing speed. The transmission of the textured substrates gradually decreases while the reflection increases as a consequence of the topological and morphological changes. The angular resolved measurements illustrate that highly textured substrates scatter the light towards greater angles. This demonstrates potential for the application in substrate configuration (nip) thin film solar cells, as the scattering can increase the optical path, and hence the absorption in the absorber layer. Simulations of periodically textured glass substrates demonstrate a focused optical generation rate near the front contact and absorber layer interface. The influence of the modified refractive index region on the optical generation rate and reflection depends on the crater profile. The reflection is generally reduced when a periodic texture in the micrometre range is implemented.

Proposed is a smart optical writing head design suitable for high precision industrial laser based machining and manufacturing applications. The design uses an Electronically Controlled Variable Focus Lens (ECVFL) which enables the highest achievable spatial resolution of writing head spot sizes for axial target distances reaching 8 meters. A proof-of-concept experiment is conducted using a visible wavelength laser with a collimated beam that is coupled to beam conditioning optics which includes an electromagnetically actuated deformable membrane liquid ECVFL cascaded with a bias convex lens of fixed focal length. Electronic tuning and control of the ECVFL keeps the laser writing head far-field spot beam radii under 1 mm that is demonstrated over a target range of 20 cm to 800 cm. Applications for the proposed writing head design, which can accommodate both continuous wave and pulsed wave sources, include laser machining, high precision industrial molding of components, as well as materials processing requiring material sensitive optical power density control.

We have proposed the ZnO film annealing method using CO₂ laser. We fabricated the layered structure to enhance the annealing effect. The sample structure was SiO₂/ZnO/Quartz substrate. The ZnO film was deposited on quartz by a pulsed laser deposition and the SiO₂ film was deposited on ZnO film by physical vapor deposition. We used photoluminescence measurement to investigate the optical property of ZnO film. We found that the optical property was improved in two steps. The first step was surface passivation effect of SiO₂ coating and the second one was the annealing effect of CO₂ laser. We analyzed the fabricated film for surface state by XPS and for crystallinity by TEM measurements. The change of ZnO surface state was observed when the SiO₂ film was deposited on the ZnO film. The change of crystallinity of ZnO was observed after CO₂ laser annealing. The crystallinity of ZnO before laser annealing seemed to be polycrystal, while the crystallinity of ZnO after laser annealing seemed to be single crystal.

We report on selective realignment of the magnetization direction of the exchange biased ferromagnetic layer in two different spintronic layer stacks using laser radiation. The exchange bias effect occurs in an antiferromagnetic/ferromagnetic bilayer system when cooled in an external magnetic field below the Néel temperature and results in a shift of the ferromagnetic hysteresis loop with increased coercivity. The effect is utilized to pin the magnetization direction of the reference ferromagnetic layer in spin valve systems. We investigated the realignment of the pinned magnetization direction in a spin valve system with in plane exchange bias and in a Co/Pt multilayer with perpendicular exchange bias. The layer stacks were heated above the Néel temperature in a defined lateral area by using rapidly deflected laser radiation. Two different laser assisted annealing techniques were investigated applying either continuous or pulsed laser radiation. During laser annealing, the sample was subjected to an external magnetic field in order to selectively realign the magnetization direction of the pinned ferromagnetic layer. Magnetic structuring was performed by heating narrow single tracks as well as irradiating single pulses. By using a magneto optical sensor in combination with a polarization microscope, the magnetic structures have been visualized. After laser annealing of larger-scaled areas, the exchange bias field strength and the coercive field strength were analyzed using a magneto optical Kerr effect set up (MOKE). The impact of the processing parameters laser peak intensity, laser pulse duration, scan speed (continuous wave) and magnetic field strength on the resulting reversed exchange bias field was evaluated.

ZnO nanocrystals have received much attention as building blocks for nano/micro-devices due to their unique morphologies, electronic and optoelectronic properties. In order to apply the ZnO nanocrystals to the practical optoelectronic applications, control of the growth density, shape and position is required. We have achieved density-controlled ZnO nanowires and periodic ZnO nanostructures using laser interference patterning. Various shapes of ZnO nanostructure, such as microcylinder and aligned nanowall, were grown using interference patterned film and substrate. In addition, optically pumped ultraviolet lasing from a piece of the ZnO nanocrystal was achieved.

Electronics circuits are susceptible to production defects. Yield improvement can be obtained by in-process inspection and repair of these defects. We describe an automated laser repair system, and discuss how to optimize it to obtain best performance in terms of throughput and quality. The laser repair system can repair excess conductor defects (e.g., shorts) in printed circuit boards by an ablation process. Moreover, it includes a feedback loop by capturing images of the repaired area using a co-aligned imaging system. Here we present the automated laser system design and repair process as well as repair performance optimization.

We report on a fast machining process for cutting silicon wafers using laser radiation without melting or ablating and without additional pretreatment. For the laser induced cutting of silicon materials a defocused Gaussian laser beam has been guided over the wafer surface. In the course of this, the laser radiation caused a thermal induced area of tension without affecting the material in any other way. With the beginning of the tension cracking process in the laser induced area of tension emerged a crack, which could be guided by the laser radiation along any direction over the wafer surface. The achieved cutting speed was greater than 1 m/s. We present results for different material modifications and wafer thicknesses. The qualitative assessment is based on SEM images of the cutting edges. With this method it is possible to cut mono- and polycrystalline silicon wafers in a very fast and clean way, without having any waste products. Because the generated cracking edge is also very planar and has only a small roughness, with laser induced tension cracking high quality processing results are easily accessible.