Ultrashort pulsed lasers are increasingly used in micromachining applications. Their short pulse lengths lead to well
defined thresholds for the onset of material ablation and to the formation of only very small heat affected zones, which
can be practically neglected in the majority of cases. Structure sizes down to the sub-micron range are possible in almost
all materials - including heat sensitive materials. Ultrashort pulse laser ablation - even though called "cold ablation" - in
fact is a heat driven process. Ablation takes place after a strong and fast temperature increase carrying away most of the
heat with the ablated particles. This type of heat convection is not possible when reducing the laser fluence slightly
below the ablation threshold. In this case temperature decreases slower giving rise to heat-induced material deformations
and melt dynamics. After cooling down protruding structures can remain - ablation-free laser surface structuring is
possible. Structure formation is boosted on thin metal films and offers best reproducibility and broadest processing
windows for metals with high ductility and weak electron phonon coupling strength. All approaches to understand the
process formation are currently based only on images of the final structures. The pump-probe imaging investigations
presented here lead to a better process understanding.
A parallel processing of two-photon polymerization structuring is demonstrated with spatial light modulator. Spatial light
modulator generates multi-focus spots on the sample surface via phase modulation technique controlled by computer
generated hologram pattern. Each focus spot can be individually controlled in position and laser intensity with computer
generated hologram pattern displayed on spatial light modulator. The multi-focus spots two-photon polymerization
achieves the fabrication of asymmetric structure. Moreover, smooth sine curved polymerized line with amplitude of 5
μm and a period of 200 μm was obtained by fast switching of CGH pattern.
In this contribution, we demonstrate multi-photon femtosecond laser lithography for the fabrication and rapid prototyping of plasmonic components. Using this technology different dielectric and metallic SPP-structures can be fabricated in a low-cost and time-efficient way. Resolution limits of this technology will be discussed. Investigations of the optical properties of the fabricated SPP-structures by far-field leakage radiation microscopy will be reported.
Conventional lithography is a leading high-throughput patterning method for mass production. But the dramatically increasing cost of lithographic equipment and mask sets, which is a consequence of pushing optical lithography to its limits, makes alternative, maskless lithographic techniques attractive. Femtosecond lasers have been found suitable for processing of a wide range of materials with sub-micrometer resolution. The limit of achievable structure sizes is predicted to be below 100 nm. Therefore, it is attractive to use this technique for maskless lithography. In this paper, first results on super-resolution femtosecond laser lithography showing great potential for future applications are presented.
Rapid progress in ultrafast laser systems opened many exciting possibilities for high-resolution material processing. These laser systems allow to control and deliver optical energy and laser pulses in time and space with unprecedented precision. It is not surprising that these high-quality optical pulses have revolutionized microfabrication technologies. Femtosecond lasers enabled processing of a wide range of materials (including heat sensitive and thermo reactive) with a sub-micrometer resolution. At present, nearly arbitrary shaped 2D and 3D structures can be produced by direct write photofabrication techniques using femtosecond laser pulses. In this paper we present a brief review of our recent progress in femtosecond (maskless, direct-write, nonlinear) laser lithography and 3D photofabrication technique.
Direct-write micro- and nanostructuring laser technologies are very important for the fabrication of new materials and multifunctional devices. Using tightly focused femtosecond laser pulses one can produce submicrometer holes and periodic structures in metals, semiconductors, and dielectrics on arbitrarily shaped surfaces. The achievable structure size is not restricted by the diffraction limit. It is determined by material properties and the laser pulse stability. We report investigations of possibilities to use femtosecond laser pulses for nanostructuring of different materials.
Micro- and nanostructuring are very important for the fabrication of new materials and multifunctional devices. Existing photo-lithographic technologies can only be applied to a limited number of materials and used on plane surfaces. Whereas, microstructuring with femtosecond laser pulses has established itself as an excellent and universal tool for micro-processing, it is still unclear what are the limits of this technology. It is of great interest to use this technique also for nanostructuring. With tightly focused femtosecond laser pulses one can produce sub-micrometer holes and structures whose quality depends on the material. We present new results on nanostructuring of different materials with femtosecond laser pulses in an attempt to make this an universal technology, and discuss its reproducibility, and further prospects for quality control.
Investigations of possibilities for nanostructuring with femtosecond laser pulses of different materials are reported. The aim is to develop a simple laser-based technology for the fabrication of two- and three-dimensional nanostructures with structure sizes on the order of several hundred nanometers. This is required for many applications in photonics, for the fabrication of photonic crystals and microoptical devices, for data storage, displays, etc. Sub-wavelength structuring of metals by direct femtosecond laser ablation is performed. The band gap dependence of the minimum structure size for transparent materials is identified.
The development of a simple laser-based technology for the fabrication of two-dimensional nanostructures with a structure size down to one hundred nanometers is reported. The ability to micro- and nano-structure is very important for the fabrication of new materials and multifunctional microdevices. Photolithographic technologies can be applied only for plane surfaces. Using femtosecond laser pulses one can fabricate 100 nm structures on arbitrary 3D-surfaces of metals and dielectrics. In principle, the minimum achievable structure size is determined by the diffraction limit of the optical system and is of the order of the radiation wavelength. However, this is different for material processing with ultrashort laser pulses. Due to a well-defined threshold character of material processing with femtosecond lasers one can beat the diffraction limit by using tightly focused femtosecond laser pulses and by adjusting laser parameters slightly above the processing threshold. In this case only the central part of the beam can modify the material and it becomes possible to produce sub-wavelength structures. In this presentation, sub-wavelength microstructuring of metals and fabrication of periodic nanostructures in transparent materials are demonstrated as promising femtosecond laser-based nanofabrication technologies.
The applications of conventional infrared lasers running cw or quasi-sw for drilling, cutting and shaping are limited in the precision achievable due to the long interaction time which leads to heat affected zones. The necessity to use a gas jet to blow the molten material out of the cut kerf will damage fragile workpieces like thin foils. Short laser pulses of sufficient intensity remove the material directly by evaporation and minimize the amount of heat transferred into the solid. Classical infrared laser sources generate a shielding air plasma within some ns at power densities above some 10<SUP>7</SUP>W/cm<SUP>2</SUP>. The optical breakdown threshold value in air can be shifted to higher intensities by using visible light as well as reducing the focal diameter. An alternative way is to shorten the pulse duration to less than 10 ps that a plasma is generated only after the pulse. Thus, the material removal process begins after the deposition of the pulse energy into the material. But such short pulses will generate a pressure wave due to the sudden thermal expansion and can damage or destroy microscopic components. For industrial production the productivity is a further aspect. Hence, a certain mean power is required in order to obtain the desired production rate. Considering the above aspects, copper vapor lasers (CVLs) with ns pulse duration are well suited for precision machining of metals and ceramics. Processing with CVLs is an advantage in that its wavelength is highly absorbed by metallic targets and the probability for the optical breakdown in air is low. CVLs in an oscillator-amplifier-setup incorporate diffraction limited beam quality and high average power. The present paper outlines the potential of the CVL for the industrial use regarding high processing speed and precision. Under these aspects the limiting mechanisms on the material removal process and the necessary processing strategies for scaling up the productivity are shown. The relevant laser parameters for increasing the working speed and the relationship to the achievable precision are given. The design aspects of a copper vapor laser system with high mean output power and repetition rate are outlined. To conclude, several typical machining tasks, e.g. cutting of green foils, drilling of scimmer holes for thermal analysis are presented.
Copper vapor lasers in a MOPA-chain (MOPA, master-oscillator- power-amplifier) configuration with low divergence can be used for the high precision machining of metals and ceramics. The fundamental interaction phenomena, ablation process and possible industrial applications are presented. The following paper relates the results and experiences in the operation of a copper vapor laser MOPA chain, consisting of an oscillator and up to three amplifiers, with the triggering points for these lasers exactly variable through a master-timing-system. In principle, a low-divergent laser beam is generated (511 and 578 nm wavelengths) via an off-axis unstable resonator scheme, with precise synchronization of the amplifiers producing average powers of over 140 W. Due to the excellent beam focusability, peak power densities of some 10<SUP>10</SUP>W/cm<SUP>2</SUP> are achievable in a 50 ns pulse duration, which provides almost material-independent precision machining at high velocities. Beginning from the principles of beam-target reciprocation, the removing and cutting of metallic as well as non-metallic materials with copper vapor lasers is described. Additionally, the potential of copper vapor lasers for industrial applications is illustrated through precision machining examples.