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- Front Matter: Volume 7202
- Optical Components and Devices
- Direct-write Processing and Surface Modification
- Modeling and Characterization
- Processes for MEMS
- Micro- and Nanomachining
- Joint Session with Conference 7201: Photovoltaics
Front Matter: Volume 7202
Front Matter: Volume 7202
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This PDF file contains the front matter associated with SPIE
Proceedings Volume 7202, including the Title Page, Copyright
information, Table of Contents, Introduction (if any), and the
Conference Committee listing
Optical Components and Devices
Integration of micro-optics and microfluidics in a glass chip by fs-laser for optofluidic applications
Show abstract
A lab-on-a-chip (LOC) is a device that incorporates in a single substrate the functionalities of a biological laboratory, i.e.
a network of fluidic channels, reservoirs, valves, pumps and sensors, all with micrometer dimensions. Its main
advantages are the possibility of working with small samples quantities (from nano- to picoliters), high sensitivity, speed
of analysis and the possibility of measurement automation and standardization. They are becoming the most powerful
tools of analytical chemistry with a broad application in life sciences, biotechnology and drug development. The next
technological challenge of LOCs is direct on-chip integration of photonic functionalities for sensing of biomolecules
flowing in the microchannels. Ultrafast laser processing of the bulk of a dielectric material is a very flexible and simple
method to produce photonic devices inside microfluidic chips for capillary electrophoresis (CE) or chemical
microreactors. By taking advantage of the unique three-dimensional capabilities of this fabrication technique, more
complex functionalities, such as splitters or Mach-Zehnder interferometers, can be implemented. In this work we report
on the use of femtosecond laser pulses to fabricate photonic devices (as waveguides, splitters and interferometers) inside
commercial CE chips, without affecting the manufacturing procedure of the microfluidic part of the device. The
fabrication of single waveguides intersecting the channels allows one to perform absorption or Laser Induced
Fluorescence (LIF) sensing of the molecules separated inside the microchannels. Waveguide splitters are used for
multipoint excitation of the microfluidic channel for parallel or higher sensitivity measurements. Finally, Mach-Zehnder
interferometers are used for label-free sensing of the samples flowing in the microfluidic channels by means of refractive
index changes detection.
Parametric investigation of solder bumping for assembly of optical components
Show abstract
Laser based solder bumping is a highly flexible and fast approach for flux-free soldering of micro-optical components in
complex 3D geometries with localized and time restricted energy input. Solder joints provide superior mechanical
strength, higher radiation stability, humidity resistance and a good thermal and electrical conductivity compared to
adhesive bonding. Due to the good long term stability solder joints are feasible for the integration of optical, mechanical,
electronic, and MEMS/MOEMS devices in multi functional hybrid optical assemblies. Comparative studies of solder
bumping of optical components with sputtered thin film metallization on platforms made of Alumina (Al2O3) and Low
Temperature Cofired Ceramics (LTCC) with both Au and AgPd thick film metallization were carried out using design of
experiment methods (DoE). The influence of the system parameters, laser pulse energy and duration, distance, incidence
angle and nitrogen pressure on targeting accuracy and bond strength were evaluated. The jetting of liquid solder spheres
within a localized nitrogen atmosphere improves wetting on the respective wetting surfaces and simplifies the joining
process due to integration of solder alloy preform handling and reflowing, thus showing great potential for a high degree
of automation.
New packaging concepts for highly stable laser diode modules
Show abstract
For mounting FAC lenses to diode lasers a new technology is introduced. Solder jet ball bumping is demonstrated to
have the potential to replace conventional mounting technologies like adhesive bonding. The advantage of this method is
a thermally and mechanically stable connection of micro optics and laser without drawbacks of outgasing and sensitivity
to UV.
The reached accuracy is within the range of one micrometer.
Laser splicing of end caps: process requirements in high power laser applications
Show abstract
High power fiber laser assemblies require monolithic joining technologies for low loss, mechanically stable and reliable
spliced component interconnection. In contrast to conventional heat sources for splicing a carbon dioxide laser heats
optical fibers and end caps only by radiation. The advantages of laser heating, e.g. precisely defined areas of laser impact
and high process purity, meet the goals for high power applications. Requirements and challenges like tensile strength,
centricity and reproducibility while using the splicing technology for a production line will be shown on behalf of a
special developed CO2 laser splicing device, splicing experiments and respective results.
Direct-write Processing and Surface Modification
Submicron writing by laser irradiation on metal nano-particle dispersed films toward flexible electronics
Show abstract
The requirement for microwiring technology by a wet process has significantly increased recently toward the
achievement of printable and flexible electronics. We have developed the metal microwiring with a resolution higher
than 1 μm by the laser direct writing technique using Ag and Cu nano-particle-dispersed films as precursors. The
technique was applied to the microwiring on a flexible and transparent polymer film. The metallization is caused in a
micro-region by focused laser beam, which reduces the thermal damage of the flexible polymer substrate during the metallization process. The laser direct writing technique is based on the efficient and fast conversion of photon energy to thermal energy by direct excitation of the plasmon absorption of a metal nano-particle, which provides Cu microwiring with a low resistivity owing to the inhibition of the surface oxidation of the Cu nano-particle.
Laser-assisted structuring and modification of LiCoO2 thin films
R. Kohler,
J. Proell,
S. Ulrich,
et al.
Show abstract
The material development of improved lithium ion batteries will play an important role in future mobile applications and
energy storage systems. Electrode materials made of nano-composited materials are expected to improve battery lifetime
and will lead to an enhancement of lithium diffusion and thus improve battery capacity and cyclability. In this study,
research was conducted to further improve the electrochemical properties of thin film cathodes by increasing the surface
to volume ratio and thereby the lithium intercalation rate. Cathode materials were synthesised by r.f. magnetron
sputtering of LiCoO2 targets in a pure argon plasma. LiCoO2 films 3 μm thick and with a grain size of 10 to 500 nm were
deposited on silicon and stainless steel substrates. The deposition parameters (argon pressure, substrate bias) were varied
to create stoichiometric films with controlled nano-crystalline texture and morphology. During laser-assisted surface
treatment, cone-shaped periodic surface structures were produced. For this purpose high repetition excimer laser
radiation at wavelengths of 193 nm and 248 nm and with short laser pulse widths (4-6 ns) were used. Structure sizes
varied with laser and processing parameters, e.g. laser fluences, pulse number, wavelength and processing gas. Laser
annealing in air or furnace annealing in a controlled argon/oxygen environment were then used to create the high
temperature phase of LiCoO2 (HT-LiCoO2). The sputtered films were studied with Raman spectroscopy, x-ray
photoelectron spectroscopy and x-ray diffraction to determine their stoichiometry and crystallinity before and after laser
treatment. The development of HT-LiCoO2 and also the formation of a Co3O4 phase were discussed. By means of
electrochemical cycling, the performance of the manufactured films was investigated.
Controlled-growth of single-walled carbon nanotubes using optical near-field effects
Show abstract
Controlled growth of self-aligned single-walled carbon nanotubes (SWNTs) was realized using optical near-field effects
in a laser-assisted chemical vapor deposition (LCVD) process. Electronic devices containing ultrashort suspended
SWNT channels were successfully fabricated at relatively low substrate temperatures. According to the numerical
simulations using High Frequency Structure Simulator (HFSS), significant local-heating enhancement occurred at
electrode tip apexes under laser irradiation, which was about ten times higher than the rest part of the electrodes.
Experimental results revealed that the localized heating enhancement at the electrode tip apexes significantly stimulates
the growth of SWNTs at a significantly reduced substrate temperature compared with the conventional LCVD process. The near-field enhancement dependence on metallic film thickness and laser polarization was investigated through numerical simulation using HFSS, which provided a guideline for further optimization of maximum near-field enhancement. This technique suggests a viable laser-based strategy for fabricating SWNT-based devices at relatively low substrate temperatures in a precisely controlled manner using the nanoscale optical near-field effects, which paves the way for the mass production of SWNT-based devices using expanded laser beams.
Local modification of ceramic surfaces by a laser induced cladding process
Show abstract
In order to develop a multifunctional material, a laser induced process was applied to change the properties of a glass-ceramic
by introducing a second phase into the surface. Localized melting of the ceramic and/or a melting of a preplaced
powder layer was achieved by the application of laser energy. After solidification a composite with new properties was
developed. The characteristic feature of the process is the option of a local modification, which is restricted to the
substrate surface and can be controlled by adjustment of the laser parameters. Accordingly modified areas with different
geometries and with a complex multiphase microstructure could be fabricated, while the ceramic bulk remains in its
original state.
Sintered LTCC-substrates (Low Temperature Co-fired Ceramic) were modified with powders metal-oxides (WO3, CuO)
with nanosized particles. Powders of metals (Cu, Ni) were used too. Cladding layers located at the top of the substrate or
layers with a thickness up to several hundred microns, which were embedded into the substrate surface, could be
fabricated. The properties of the laser modified regions differ significantly from that of the LTCC-substrate. The
obtained structures offer modified mechanical, thermophysical and electrical properties. In particular an enhanced
thermal conductivity could be detected. The electrical resistivity of the laser modified tracks widely varied depending on
the process parameters and the powder. Tracks made with CuO- and WO3-powders show a negative temperature
coefficient for electrical resistance, i.e. it decreases with increasing temperature, which is typical for semiconductors.
Modeling and Characterization
Multiscale modeling of phase changes during femtosecond laser metal interaction
Show abstract
A multiscale model is developed to study to the femtosecond laser single pulse and pulse train processing of the metal
films. In our model, molecular dynamics simulation combined with the improved two-temperature model is employed in
the ablation area and the improved two-temperature model is applied in heat-affected zone. This paper extends the
improved two-temperature model to describe higher laser fluences processing by introducing the phase change. The
phase change mechanisms of the non-equilibrium thermal melting and vaporization are both analyzed, which has a
strong impact on the lattice temperature evaluation. The model can simulate phase change process of gold with higher
accuracy. It is demonstrated that the pulse train could improve the fabrication accuracy, repeatability, and controllability.
Comprehensive laser beam characterization for applications in material processing
Show abstract
With ever more stringent requirements regarding the quality of laser-supported production
processes, measuring techniques for comprehensive characterization of laser beams and
beam delivery optics are rapidly gaining importance. Of particular interest is precise
knowledge regarding the beam profile, the beam propagation characteristics, and the
wavefront. The latter describes the local direction of energy flux and carries detailed
information about the beam aberrations, including intrinsic ones as well as those introduced
by optical elements along the beam path.
In this paper we give an overview of the status and current developments in the field of laser
beam characterization. Examples from industrial applications are given, including the
diagnostics of 193 nm excimer lasers. Along with a description of measuring procedures
according to ISO standards, emphasis is placed on diagnostics based on Hartmann-Shack
wavefront sensors. From the wavefront and the simultaneously recorded near-field profile
beam parameters such as diameter, divergence, and M2 can be evaluated in real-time. In
addition, the approach also accomplishes prediction of the propagation behavior of the
radiation field.
Laser-induced breakdown spectroscopy with high detection sensitivity
Show abstract
Laser-induced breakdown spectroscopy (LIBS) with spatial confinement and LIBS combined with laser-induced fluorescence (LIF) have been investigated to improve the detection sensitivity and selectivity of LIBS. An obvious enhancement in the emission intensity of Al atomic lines was observed when a cylindrical wall was placed to spatially confine the plasma plumes. The maximum enhancement factor for the emission intensity of Al atomic lines was measured to be around 10. Assuming local thermodynamic equilibrium conditions, the plasma temperatures are estimated to be in the range from 4000 to 5800 K. It shows that the plasma temperature increased by around 1000 K when the cylindrical confinement was applied. Fast imaging of the laser-induced Al plasmas shows that the plasmas were compressed into a smaller volume with a pipe presented. LIBS-LIF has been investigated to overcome the matrix effects in LIBS for the detection of trace uranium in solids. A wavelength-tunable laser with an optical parametric oscillator was used to resonantly excite the uranium atoms and ions within the plasma plumes generated by a Q-switched Nd:YAG laser. Both atomic and ionic lines can be selected to detect their fluorescence lines. A uranium concentration of 462 ppm in a glass sample can be detected using this technique at an excitation wavelength of 385.96 nm for resonant excitation of U II and a fluorescence line wavelength of 409.01 nm from U II. The mechanism of spatial confinement effects and the influence of relevant operational parameters of LIBS-LIF are discussed.
Mapping of individual single-walled carbon nanotubes using nano-Raman spectroscopy
Show abstract
Scanning Tunneling Microscope (STM) based Tip-enhanced Raman Spectroscopy (TERS) was used to map Single-walled
Carbon Nanotubes (SWCNTs) dispersed on silicon surfaces. A software program developed with Labview
platform was used to perform the mapping. STM tips made of gold (Au) were fabricated by electrochemical etching and
employed in our TERS system to realize nanoscale spatial resolutions and obtain enhanced signals. Mapping of the
SWCNTs was also performed using a micro-Raman system. It was found that the SWCNTs could be well resolved by
the TERS system but could not be resolved by the micro-Raman system. Further analysis shows that, the ultimate
resolution of the TERS system can reach around 30 nm, while the micro-Raman system shows a resolution around 5 μm.
Processes for MEMS
Low-stress dicing assisted by pulsed laser for multilayer MEMS
Masayuki Fujita,
Yusaku Izawa,
Yosuke Tsurumi,
et al.
Show abstract
We have developed a novel debris-free in-air laser dicing technology, which is expected to give less failure of MEMS
devices and hence improves yields. Our technology combines two processes: a dicing guide fabrication and a wafer
separation process. The first process is internal transformation using a nanosecond Nd:YVO4 laser with high repetition
rate and/or a pulsed fiber laser with 200ns pulsewidth. The laser pulses are focused inside the MEMS wafer without surface ablation. In order to make cross-sectional internal transformation, the laser beam is scanned several times with defocusing. The laser scanning speed per each scanning is 100-700 mm/sec depending on the layer material, the machining time is much faster than the conventional blade dicing. The second process is non-contact separation by thermally-induced crack propagation using a CO2 laser or mechanical separation by bending stress. In the each separation process, the internal transformation fabricated in the first process worked well as the guide of separation, and the processed wafer was diced with low stress. This dicing technology was applied for 4-inch MEMS wafers, e.g. pressure sensors, etc., and the sensor chips were separated without mechanical damages.
High quality laser cleaving process for mono- and polycrystalline silicon
Show abstract
The cleaving process has the potential to replace the dicing of thin wafers. Its inherent advantages are no mechanical
forces to the substrate, no material losses, and high edge quality. In order to determine the fundamental mechanisms
leading to a reliable cleaving process the complex interaction of wavelength and temperature dependent absorption, heat
transfer, material elongation and finally crack formation is theoretically described and experimentally verified. A
successful process observed if sufficient thermal stress can be generated to induce a crack and if no surface deformation
occurs due to overheating. Most relevant parameters determining the process window are irradiated power, cutting speed,
and focus spot size. The results of these parameter variations are presented. Accuracy and reproducibility is
demonstrated by cleaving stripes of different widths fulfilling the requirements of the electronic packaging industry. In
the third section the influence of the crystalline orientation is investigated. As a result mono-crystalline silicon exhibits
an anisotropic behaviour when changing the cutting direction whereas for polycrystalline substrates a permanent change
of the crystal structure is found at the grain boundary. Finally, the obtainable edge quality is presented briefly, which
leads to higher sample strengths compared to conventional laser and mechanical processes.
Fiber laser microjoining for novel dissimilar material combinations
Show abstract
Today's complexity in packaging of MEMS and BioMEMS requires advanced joining techniques that take the specific
package integration for each device into account. Current focus on reducing investment and operating costs for device
packaging require a flexible and reliable joining approach for similar and dissimilar materials such as metals, polymers,
glass and silicon to manage increasing system complexity. Depending on the application, packaged devices must fulfill
tough requirements regarding strength, thermal stress, fatigue and hermeticity and long-term stability.
This research is focused on laser microjoining of polyimide and PEEK polymers to metals such as nitinol,
chromium and titanium using fiber laser. Our earlier investigations have demonstrated the potential of this unique
joining technique, which successfully addresses the existing microjoining challenges including high precision, localized
processing capability and biocompatibility. Our current study further defines the key processing parameters for joining
novel dissimilar material combinations based on the characterization of such laser joints by means of mechanical failure
tests and the bond area analysis using optical microscope, scanning electron microscopy (SEM) and X-ray photoelectron
spectroscopy (XPS).
The results compare operating windows for generating quality bonds for different material joining
configurations. They also provide an initial approach to characterize laser-fabricated microjoints that can be potentially
used for the optimization of the design process of devices utilizing these materials. Potential packaging applications
include microsystems used for chemical or biological assays (lab-on-a-chip), implantable devices used for pressure or
temperature sensing, neural stimulation and drug delivery.
Laser-based microbonding using hot melt adhesives
Show abstract
This paper presents an alternative adhesive bonding system which is able to join very small parts as well as relatively large parts with high accuracy requirements. The main advantages are the possibility to apply small volumes, to preapply the adhesive with a temporarily delayed joining procedure and extremely short set cycles. The center of micro joining develops suitable joining techniques on the basis of non-viscous adhesive systems (hot melts). The process
development focuses on the suitability for automation, process times and the applicability of batch processes. The article
discusses certain hot melt application techniques that are suitable for batch production e. g. the laser-sintering of hot melt
powder, presents an adapted assembly system and shows an example of an automated assembly process for hot melt
coated micro components. Therefore, using hot melts can be a technologically and economically interesting alternative
for the assembly and packaging of MEMS.
Laser soldering of enameled wires
Show abstract
In electrical connections with enameled copper wires, isolation material residue can be found in the solder area when the
coating is not stripped. This residue can lead to mechanical and electrical problems. In electronic devices and MEMS,
quality requirements increase with rising thermal requirements for electrical contacts made from enameled copper wire.
Examples for this exist in the area of automotive electronics, consumer electronics and in the field of machine design.
Typical products with electrical connecting which use enameled wires include: micro-phones and speakers (especially
for mobile phones), coil forms, small transformers, relays, clock coils, and so on. Due to increasing thermal and
electrical requirements, the manufacturer of enameled wires continuously develops new isolating materials for the
improvement of isolation classes, thermal resistance, etc. When using current bonding and solder processes, there exist
problems for contacting enameled copper wire with these insulation layers. Therefore the Institute of Joining and
Welding, Department Micro Joining developed a laser based solder process with which enamels copper wires can enable
high quality electrical connections without a preceding stripping process.
Micro- and Nanomachining
Large-area plasmonic structures fabricated by laser nanopatterning and their applications
Show abstract
Laser interference lithography is applied to fabricate large-area plasmonic nanostructures. This approach has the
advantages of being non-contact process in air and able to achieve large-area and maskless nanolithography at a high
speed with low system investment. Single layer Au or Ag noble metallic thin film and Ag/Au, Ag/Ni or Au/Ni bimetallic
layer thin films are patterned into nano-dot, nano-rod and nano-nut arrays by laser interference lithography. Plasmonic
effects of the fabricated metallic nanostructures are studied. Tunable and multi-peak surface plasmon resonances of these
nanostructures can be obtained, which have potential applications in solar cells, bio-sensing and photonic circuits.
Subwavelength photoresist patterning using liquid-immersion interference exposure with a deep-UV hologram mask
Show abstract
Interference exposure using a deep-UV laser in combination with dry etching is instrumental in manufacturing subwavelength patterns used at visible wavelengths. For well resolved patterns, interference fringes must be held still during exposure to achieve a high fringe contrast. Two-beam interference exposure requires a lot of space and equipment to build stable optics and produce patterns on an industrial scale. On the other hand, hologram mask exposure is
theoretically far more robust in unfavorable surrounding conditions since a resist layer is placed directly beneath the
mask. To produce good-quality resist patterns by using hologram masks, two issues need to be addressed. First, light reflections occurring at interfaces between the mask, the air gap and the resist need to be reduced to secure a high uniformity of exposure intensity. Second, only two diffraction beams should be generated to make an interference field with a high fringe visibility. What mask configurations should be chosen depends on what patterns are to be made. The best answer to produce sub-100-nm patterns is using a hologram mask in Bragg geometry and filling the air gap with a
high-index liquid.
Micro-scale large-area UV laser processing
Show abstract
UV lasers are well-established sources for a wide variety of micro-machining applications. The small wavelength makes
them ideal for processing of small features or to modify thin surfaces. Especially short pulse UV lasers are ideal for
ablation of various materials, e. g., polyimide, parylene, PMMA, copper, gold and diamond. Furthermore these lasers are
used for silicon annealing and patterning of fine circuitries to various substrates. The demand for smaller feature sizes of
micro-mechanical and micro-electronic devices set new requirements in regard to resolution, throughput and overall cost
efficiency of the process.
In this paper, high-power excimer laser micro-machining and annealing relevant applications will be presented and
discussed.
Micro ID marking for semiconductor chips: recent progress and future prospects
Show abstract
When the frequency, pulse width, the beam profile, and energy density of the laser were controlled, then irradiated
onto the silicon wafer with a beam of 15μm diameter or less, we observed that convex dot with a height of 100-300
nano- meters was formed. [1] The laser energy density through which the convex dot was formed was below
3.8J/cm2. In the semiconductor excitation laser, the pulse width was 40nsec-150nsec; the wavelength was 532 nm.
[2]We developed equipment by using convex dots that was able to form 2D minute code of 16x16 dots in 100μm
x100μm area in each IC chip on the silicon wafer without particle generation .
Micromachining with picosecond double pulses on silicon and aluminium
Show abstract
Laser double pulses offer interesting opportunities to increase the ablation performance of ultra short laser pulses. In
recent published and performed experiments we have presented an optical setup that covers delay times from some
picosecond up to 20 ns as well as first experimental results of ablating aluminium and silicon. In this paper we present
further results of especially interesting time domains for both materials. The ablation efficiency on silicon with inter
pulse delays from 6.3 ns to 15 ns was investigated. In this range the double pulse effect was mainly depending on the
fluency. The double pulse efficiency increase is connected with a higher thermal impact on the work piece. The change
of delay and repetition rate has no influence on the ablation efficiency for both single and double pulses. The
experiments on aluminium concentrated on the pulse delays of 50 ps to 400 ps. The ablation depth per pulse is lower
than for single pulse ablation in this range. Double pulse efficiency decreases up to a pulse delay of 150 ps.
Joint Session with Conference 7201: Photovoltaics
Novel laser technologies for crystalline silicon solar cell production
Show abstract
Laser processes have penetrated into the crystalline silicon solar cell production market some time ago already, but are
still far from reaching the status they probably will achieve one day. As the largest fraction of state-of-the-art production
lines still produces conventional screen-printed aluminum back surface field (Al-BSF) cells, the applicability of lasers is
currently limited mainly to the process step of laser edge isolation, while only a few other companies use lasers for
groove formation (fabrication of laser grooved buried contact solar cells) or via hole drilling. Due to the
contactless nature as well as the possibility to process a wide variety of materials with fine structures, lasers can be used
for a vast field of production steps like ablating, melting and soldering different materials. Within this paper several
applications of laser processes within the fabrication of various next-generation silicon solar cell structures are presented.
These processes are for example laser via hole drilling, which is inevitable for MWT and EWT (metal and emitter wrap
through) solar cells, LFC (laser-fired contacts) as a fast and easy approach for the production of passivated emitter and
rear solar cells as well as laser ablation of dielectric layers and laser doping which offer the chance for industrial
production of several different high efficiency solar cell structures.
Thin layer ablation with lasers of different beam profiles: energy efficiency and over filling factor
Show abstract
Structuring and patterning of thin layer via selective laser ablation is one of the key technologies in
production of display and photovoltaic. Concurrently, there are two ablation processes used in
production of thin film solar cells: Scribing via selective ablation and edge isolation via deletion.
The common laser beams have circular cross section. Furthermore, the most currently lasers of
high beam quality have Gaussian beam profile. Because of threshold behaviour Gaussian beam
profile is not favourable for ablation process. On the other side there are emerging laser concepts
which deliver rectangular or saqure top-hat beams with high beam quality. In this paper we will
discuss the fundamentals of ablation processes with circular Gaussian beams, one dimensional
top-hat beams and two dimensional square top-hat beams. The major issues will be the energy
efficiency of the process and the area over filling aspect for the different beam profiles. The
corresponding experimental results will be presented.
Optical characterization of the heat-affected zone in laser patterning of thin film a-Si:H
Show abstract
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 processing for high-efficiency silicon solar cells
Show abstract
The main goal in PV research is a significant reduction of Watt-peak costs of PV systems and thus of solar cells.
Innovative cell concepts including robust and reasonable process technologies are necessary to provide highest
efficiencies and low process complexity. Laser technology with its excellent features for material machining offers many
opportunities to make economical manufacturing processes feasible for solar cell production. To benefit from these
advantages of laser technology the knowledge of methods for a relevant process characterization is required. This paper
reviews experimental investigations of laser processes concerning laser related machining of silicon for PV application.
The processes of interest are laser ablation of diffusion barriers and passivating dielectric layers from silicon surfaces to
realize local contact openings. The impact of important laser source parameters, such as pulse energy, pulse duration and
laser wavelength, on a silicon substrate in terms of crystal damage is investigated by means of contactless charge carrier
lifetime measurements. From these measurements important conclusions can be drawn considering the final solar cell
performance. This paper describes the extraction of relevant electrical parameters of laser treated silicon wafers like local
saturation current densities deduced from lifetime measurements. These investigations allow the evaluation of different
laser sources for the high-efficiency approach.
Optimizing laser beam profiles using micro-lens arrays for efficient material processing: applications to solar cells
Show abstract
High power laser sources are used in various production tools for microelectronic products and solar cells, including the
applications annealing, lithography, edge isolation as well as dicing and patterning. Besides the right choice of the laser
source suitable high performance optics for generating the appropriate beam profile and intensity distribution are of high
importance for the right processing speed, quality and yield.
For industrial applications equally important is an adequate understanding of the physics of the light-matter interaction
behind the process. In advance simulations of the tool performance can minimize technical and financial risk as well as
lead times for prototyping and introduction into series production. LIMO has developed its own software founded on the
Maxwell equations taking into account all important physical aspects of the laser based process: the light source, the
beam shaping optical system and the light-matter interaction.
Based on this knowledge together with a unique free-form micro-lens array production technology and patented micro-optics
beam shaping designs a number of novel solar cell production tool sub-systems have been built. The basic
functionalities, design principles and performance results are presented with a special emphasis on resilience, cost
reduction and process reliability.