This PDF file contains the front matter associated with SPIE Proceedings Volume 8608, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Laser near-field enhancements underneath transparent microspheres can be used to locally implement new functionalities in materials. Using this technique, we report micro- and nano-structuration on silicon (Si). The Langmuir-Blodgett (LB) technique is primarily used to realize monolayers of C18 functionalized silica (SiO₂) microspheres on a large size area of the substrates. Afterwards, by irradiating the deposited monolayer with single shot UV nanosecond laser pulses in the ablation regime, we demonstrate the formation of dense arrays of craters in silicon substrate. In particular, we describe our works to obtain mono dispersed craters of sub micrometer size using LB technique and taking the fluence and sphere size as variable process parameters. Finite-difference time-domain (FDTD) simulations are presented to estimate the enhancement intensity factor and near-field distribution below the spheres in the experiments.

The pico- and nano-second ablation thresholds and subsequent pulse-energy cut-depth and width relationships of aluminium, polypropylene (PP), polyethylene (PE) and their various thin-film combinations have been determined at 515 nm and 1064 nm. High quality incisions were obtained for all structures within certain parameter ranges. All ablation thresholds were found to be functions of the temporal pulse-width. Numerical simulations revealed the underlying mechanisms as phase explosion and thermal conduction. The presented results provide necessary parameters for the efficient cut and scribe of such materials, allowing the laser to prevail in lieu of more costly and energy intensive methods.

In the laser wafer dicing technique of stealth dicing (SD), a laser beam that is tightly focused inside a silicon wafer is scanned multiple times at different depths. The focused beam creates multilayered cracks that allow dry, debris-free dicing. To reduce the dicing time, it is desirable to produce longer cracks with each scan. However, when the laser beam is focused in a deep region of the wafer, the beam is blurred, and its power density decreases owing to spherical aberration caused by a refractive index mismatch between air and the wafer. Consequently, the generated cracks become shorter. We present an approach to making longer cracks deep within the wafer by correcting the spherical aberration. This correction is made using an SD machine incorporating a phase-only spatial light modulator to apply aberration correction patterns, which are calculated by a method based on inverse ray tracing. Experimental results using 300-µm wafers show that, when the aberration was corrected, the cracks formed during multidepth scans became longer even deep within the wafer and that the dicing speed with correction is more than twice that without correction. This is because each scan produced longer cracks, so fewer scans were necessary. We also demonstrated that the quality of dicing was improved.

Rechargeable lithium-ion batteries have emerged as an attractive power source for a wide variety of applications, in particular for portable electronics. The development and modification of electrode materials is a major issue for the improvement of energy density and power density of lithium-ion batteries. For this purpose, laser-induced self-organizing surface structures were generated using UV-excimer laser radiation with a wavelength of 248 nm and a pulse width of 4-6 ns. The self-organization process was applied for thin films made of lithium cobalt oxide with a thickness of about 3 μm. In order to identify the chemical changes due to laser processing time-of-flight secondary ion mass spectroscopy measurements were performed. It was found that this process can be linked to a decomposition of the electrode material forming a lithium oxide surface layer. Similar self-organized surface structures could also be obtained for thick film electrode materials consisting of LiCoO₂ powder mixed with binder and carbon black which were tape-casted onto aluminium foils. The thickness of these films was in the range of 50 - 100 μm.

The increasing need for long-life lithium-ion batteries requires the further development of electrode materials. Especially on the cathode side new materials or material composites are needed to increase the cycle lifetime. On the one hand, spinel-type lithium manganese oxide is a promising candidate to be used as cathode material due to its non-toxicity, low cost and good thermal stability. On the other hand, the spinel structure suffers from change in the oxidation state of manganese during cycling which is also accompanied by loss of active material into the liquid electrolyte. The general trend is to enhance the active surface area of the cathode in order to increase lithium-ion mobility through the electrode/electrolyte interface, while an enhanced surface area will also promote chemical degradation. In this work, laser microstructuring of lithium manganese oxide thin films was applied in a first step to increase the active surface area. This was done by using 248 nm excimer laser radiation and chromium/quartz mask imaging techniques. In a second step, high power diode laser-annealing operating at a wavelength of 940 nm was used for forming a cubic spinel-like battery phase. This was verified by means of Raman spectroscopy and cyclic voltammetric measurements. In a last step, the laser patterned thin films were coated with indium tin oxide (ITO) layers with a thickness of 10 nm to 50 nm. The influence of the 3D surface topography as well as the ITO thickness on the electrochemical performance was studied by cyclic voltammetry. Post-mortem studies were carried out by using scanning electron microscopy and focused ion beam analysis.

Using an in-house developed micro scanner three-dimensional micro components and micro fluidic devices in fused silica are realized using the ISLE process (in-volume selective laser-induced etching). With the micro scanner system the potential of high average power femtosecond lasers (P > 100 W) is exploited by the fabrication of components with micrometer precision at scan speeds of several meters per second. A commercially available galvanometer scanner is combined with an acousto-optical and/or electro-optical beam deflector and translation stages. For focusing laser radiation high numerical aperture microscope objectives (NA > 0.3) are used generating a focal volume of a few cubic micrometers. After laser exposure the materials are chemically wet etched in aqueous solution. The laser-exposed material is etched whereas the unexposed material remains nearly unchanged. Using the described technique called ISLE the fabrication of three-dimensional micro components, micro holes, cuts and channels is possible with high average power femtosecond lasers resulting in a reduced processing time for exposure. By developing the high speed micro scanner up-scaling of the ISLE process is demonstrated. The fabricated components made out of glass can be applied in various markets like biological and medical diagnostics as well as in micro mechanics.

“Stealth Dicing” laser processing is a dry and debris-free semiconductor wafer dicing method achieved by generating thermal micro-cracks inside a wafer with a tightly focused laser beam. This method has two practical issues: (1) the dicing speed is limited by the repetition rate of the pulsed laser, and (2) integrated circuits on the opposite side of the wafer from the laser light are potentially damaged by excessive laser intensity required to compensate for insufficient beam convergence. The insufficient beam convergence is a result of spherical aberration due to a refractive index mismatch between air and the wafer. These problems can be resolved by incorporating a phase-only spatial light modulator (SLM) into the laser dicing system. The SLM produces two types of wavefront configurations simultaneously for two different functions. One is for multi-beam generation with a phase grating pattern. This improves the dicing speed by a factor equal to the number of diffracted beams. The other is for aberration correction of the multiple beams using a pre-distorted wavefront pattern. By correcting aberrations, the focused multiple beams inside the wafer will become sufficiently convergent to avoid undesirable laser damage. We demonstrated these improvements by dicing sapphire wafers with a pulsed laser and a high-numerical-aperture objective lens.

Increasing demand for creating fine features with high accuracy in manufacturing of electronic mobile devices has fueled growth for lasers in manufacturing. High power, high repetition rate ultraviolet (UV) lasers provide an opportunity to implement a cost effective high quality, high throughput micromachining process in a 24/7 manufacturing environment. The energy available per pulse and the pulse repetition frequency (PRF) of diode pumped solid state (DPSS) nanosecond UV lasers have increased steadily over the years. Efficient use of the available energy from a laser is important to generate accurate fine features at a high speed with high quality. To achieve maximum material removal and minimal thermal damage for any laser micromachining application, use of the optimal process parameters including energy density or fluence (J/cm²), pulse width, and repetition rate is important. In this study we present a new high power, high PRF QuasarR 355-40 laser from Spectra-Physics with TimeShift^TM technology for unique software adjustable pulse width, pulse splitting, and pulse shaping capabilities. The benefits of these features for micromachining include improved throughput and quality. Specific example and results of silicon scribing are described to demonstrate the processing benefits of the Quasar’s available power, PRF, and TimeShift technology.

We demonstrated the use of ultraviolet (UV) laser lithography in the production of subwavelength structures. A laser writing system with a 413-nm Kr laser was used to write patterns on a resist-coated fused silica substrate mounted on a rotating table with a linear slider. One- and two-dimensional patterns were written on the resist at a fixed sampling frequency, and then, the substrate was dry etched and coated with Au to obtain metallized gratings. Surface plasmon resonance dips, which appeared in the reflectance spectra of the gratings, shifted for different orientations of the incident linear polarization. However, this dip shift can be considered tolerable for practical purposes, provided that the gratings that couple light with surface plasmons are used as a near-field enhancer. Hence, we concluded that UV laser writing based on polar coordinates is a candidate method for submicron-scale structuring.

Organic semiconductors (OSC) are solution processable synthetic materials with high carrier mobility that promise to revolutionise flexible electronics manufacturing due to their low cost, lightweight and high volume low temperature printing in reel-to-reel (R2R) [1] for applications such as flexible display backplanes (Fig.1), RFID tags, and logic/memory devices. Despite several recent technological advances, organic thin film transistor (OTFT) printing is still not production-ready due to limitations mainly with printing resolution on dimensionally unstable substrates and device leakage that reduces dramatically electrical performance. OTFTs have the source-drain in ohmic contact with the OSC material to lower contact resistance. If they are unpatterned, a leakage pathway from source to drain develops which results in non-optimum on/off currents and not controllable device uniformity (Fig.2). DPSS lasers offer several key advantages for OTFT patterning including maskless, non-contact, dry patterning, scalable large area operation with precision registration, well-suited to R2R manufacturing at overall μm size resolutions. But the thermal management of laser processing is very important as the devices are very sensitive to heat and thermomechanical damage [2]. This paper discusses 343nm picosecond laser ablation trimming of 50nm thick PTAA, TIPS pentacene and other semiconductor compounds on thin 50nm thick metal gold electrodes in a top gate configuration. It is shown that with careful optimisation, a suitable process window exists resulting in clean laser structuring without damage to the underlying layers while also containing laser debris. Several order of magnitude improvements were recorded in on/off currents up to 10⁶ with OSC mobilities of 1 cm²/Vsec, albeit at slightly higher than optimum threshold voltages which support demanding flexible display backplane applications.

Processing of thin and ultra-thin glass displays is becoming more important in the fast increasing market of display manufacturing. As conventional technologies such as mechanical scribing followed by manual breaking mostly lead to bad edge quality and thus to a huge amount of reject, other processes like ablation processes [1] with picosecond lasers are getting more and more interesting. However processing with ultrashort pulsed lasers partially leads to unwanted effects which should be understood in a better way by means of intensive basic research. Therefore the ablation mechanism of ultrashort pulses on transparent materials was investigated in this research project. On the one hand the ablation mechanism was analyzed in a simulative way by means of rate equations on the other hand by laboratory experiments.

Trepanning heads are well known to be efficient in high aspect drilling and to provide a precise control of the hole geometry. Secondly, femtosecond lasers enable to minimize the heat effects and the recast layer on sidewalls but are typically used on thin sheet. The combination of both present a high potential for industrial applications such as injector or cooling holes where the bore sidewall topology has a major influence on the dynamics of the gas flow. In this paper we present results using this combination. The effect of pulse energy, repetition rate and revolution speed of the head on both geometry and roughness are discussed. The quality of the sidewall is checked by roughness measurement and by metallographic analysis (SEM; chemical etching, micro hardness).

Direct fabrication of graphene patterns on SiO₂/Si substrates was demonstrated using a single-step laser-induced chemical vapor deposition (LCVD) process. A laser beam was used to irradiate a nickel-coated SiO₂/Si substrate in a methane-hydrogen environment to induce a local temperature rise on the laser focused area. Followed by a rapid cooling process by moving the laser beam, graphene patterns were formed on the laser scanning pathway. Nickel (Ni) layer under graphene patterns was removed by the Ni etchant diffused into the area under the graphene. Laser direct writing graphene patterns on SiO₂/Si substrates was achieved. Energy dispersive X-ray diffraction spectroscopy was used to confirm the removal of Ni layers. The discovery and development of the LCVD growth process provide a route for the rapid fabrication of graphene-based electronic devices.

Laser forward transfer of arbitrary and complex configurable structures has recently been demonstrated using a spatial light modulator (SLM). The SLM allows the spatial distribution of the laser pulse, required by the laser transfer process, to be modified for each pulse. The programmable image on the SLM spatially modulates the intensity profile of the laser beam, which is then used to transfer a thin layer of material reproducing the same spatial pattern onto a substrate. The combination of laser direct write (LDW) with a SLM is unique since it enables LDW to operate not only in serial fashion like other direct write techniques but instead reach a level in parallel processing not possible with traditional digital fabrication methods. This paper describes the use of Digital Micromirror Devices or DMDs as SLMs in combination with visible (λ = 532 nm) nanosecond lasers. The parallel laser printing of arrayed structures with a single laser shot is demonstrated together with the full capabilities of SLMs for laser printing reconfigurable patterns of silver nano-inks Finally, an overview of the unique advantages and capabilities of laser forward transfer with SLMs is presented.

Ultrathin flip-chip semiconductor die packaging on paper substrates is an enabling technology for a variety of extremely low-cost electronic devices with huge market potential such as RFID smart forms, smart labels, smart tickets, banknotes, security documents, etc. Highly flexible and imperceptible dice are possible only at a thickness of less than 50 μm, preferably down to 10-20 μm or less. Several cents per die cost is achievable only if the die size is ≤ 500 μm/side. Such ultrathin, ultra-small dice provide the flexibility and low cost required, but no conventional technology today can package such die onto a flexible substrate at low cost and high rate. The laser-enabled advanced packaging (LEAP) technology has been developed at the Center for Nanoscale Science and Engineering, North Dakota State University in Fargo, North Dakota, to accomplish this objective. Presented are results using LEAP to assemble dice with various thicknesses, including 350 μm/side dice as thin as 20 μm and less. To the best of our knowledge, this is the first report of using a laser to package conventional silicon dice with such small size and thickness. LEAP-packaged RFID-enabled paper for financial and security applications is also demonstrated. The cost of packaging using LEAP is lower compared to the conventional pick-and-place methods while the rate of packaging is much higher and independent of the die size.

Selective laser melting, as a rapid manufacturing technology, is uniquely poised to enforce a paradigm shift in the manufacturing industry by eliminating the gap between job- and batch-production techniques. Products from this process, however, tend to show an increased amount of defects such as distortions, residual stresses and cracks; primarily attributed to the high temperatures and temperature gradients occurring during the process. A unit cell approach towards the building of a standard sample, based on literature, has been investigated in the present work. A pseudo-analytical model has been developed and validated using thermal distributions obtained using different existing scanning strategies. Several existing standard and non-standard scanning methods have been evaluated and compared using the empirical model as well as a 3D-thermal finite element model. Finally, a new grid-based scan strategy has been developed for processing the standard sample, one unit cell at a time, using genetic algorithms, with an objective of reducing thermal asymmetries.

The laser sintering of gold nanoparticles on a copper substrate was studied to develop an alternative to gold plating. The major problem in the gold coating on a copper substrate using gold nanoparticle ink is that copper atom easily diffuses to the surface through the gold layer and to form an oxide layer during the thermal sintering of gold nanoparticles. The depth profile of elemental composition of laser-sintered gold layer showed that the effective reduction of the diffusion of copper atom through the gold layer is possible because the laser sintering of metal nanoparticles is extremely fast process due to the nano-heater effect caused by the laser excitation of plasmon band.

Molybdenum (Mo) feedthrough pins (with a diameter of smaller than half of a millimeter) are commonly used for rechargeable batteries because of their inert nature and close CTE match with glass. Pure Mo has a very high melting temperature, and is not conducive to soldering. Due to the small geometry, the attachment of electrical conducting wire/ribbon to the pin is very challenging if conventional attachment methods were possible. Solid-state bonding by resistance welding is marginally feasible, but often results in moderate bond strength. In this work, fine spot welding using a pulsed Nd:YAG laser for the attachment of a conductive ribbon to a Mo pin is reported. The effect of the conductive ribbon materials was investigated. The weld condition was studied with the aim of determine the best set of laser processing parameters, including the angle of laser beam incidence, the laser power and the pulse duration. Weld strength testing on ribbon-pin weld structures was conducted. The laser fine spot welding resulted in a three times higher bond strength than resistance welding.

Line-focus diode laser is applied to advance crystalline silicon thin-film solar cell technology. Three new processes have been developed: 1) defect annealing/dopant activation; 2) dopant diffusion; 3) liquid phase crystallisation of thin films. The former two processes are applied to either create a solar cell device from pre-crystallised films or improve its performance while reducing the maximum temperature experienced by substrate. The later process is applied to amorphous silicon films to obtain high crystal and electronic quality material for thin-film solar cells with higher efficiency potential. Defect annealing/dopant activation and dopant diffusion in a few micron thick poly-Si films are achieved by scanning with line-focus 808 nm diode laser beam at 15-24 kW/cm² laser power and 2~6 ms exposure. Temperature profile in the film during the treatment is independent from laser power and exposure but determined by beam shape. Solar cell open-circuit voltages of about 500 mV after such laser treatments is similar or even higher than voltages after standard rapid-thermal treatments while the highest temperature experienced by glass is 300C lower. Amorphous silicon films can be melted and subsequently liquid-phase crystallised by a single scan of line laser beam at about 20 kW/cm2 power and 10-15 ms exposure. Solar cells made of laser-crystallised material achieve 557 mV opencircuit voltage and 8.4% efficiency. Electronic quality of such cells is consistent with efficiencies exceeding 13% and it is currently limited by research-level simplified cell metallisation.

Femtosecond laser texturing of silicon yields micrometer scale surface roughness that reduces reflection and enhances light absorption. In this work, we study the potential of using this technique to improve efficiencies of amorphous silicon-based solar cells by laser texturing thin amorphous silicon films. We use a Ti:Sapphire femtosecond laser system to texture amorphous silicon, and we also study the effect of laser texturing the substrate before depositing amorphous silicon. We report on the material properties including surface morphology, light absorption, crystallinity, as well as solar cell efficiencies before and after laser texturing.

The growing demand for high productivity in the thin-film photovoltaic module industry, together with the request for more and more efficient devices, needs high-performance laser-scribing. The results of scribing tests on CdTe and CIGS solar cells samples are here presented. A comparison between the scribes obtained with ns regime fiber lasers, and a ps regime diode pumped solid state laser will be also reported.