Laser welding by means of multi-kilowatt solid state lasers can be considerably improved if the focused welding spot is embedded in a pre-heating spot generated e.g. by an additional laser. To improve the compactness of the optical system, the same functionality can be effectively achieved by means of diffractive diffusers. Because such a diffractive optical solution may suffer from the presence of speckles, a comprehensive characterization of the laser source is performed. The paper includes the design, the compensation of the difference in the intensity levels, the fabrication and the optical performances of the fabricated DOEs. We furthermore present the functionality of the DOEs in the welding process.
Photomasks contain geometric information that will be transferred to substrates or pre-structured surfaces. Conventional mask aligner lithography in the sense of shadow printing of the photomask suffers from limited achievable resolution. Photomask and substrate are typically separated by an air gap causing diffraction effects and hence affecting the minimum structure size. Even though contact lithography offers a resolution in the wavelengthscale, yield problems and contamination of the photomask are its drawbacks. Using proximity lithography, these problems can be avoided since it profits from a contact-free exposure process. To overcome the resolution limitation of the shadow printing mode more advanced diffraction based photo masks need to be used.
Mask-aligner (MA) lithography is a well-known method for the fabrication of micrometer sized structures on a substrate
with a diameter up to 300 mm. In spite of a theoretical resolution below 200 nm, the minimum printable feature sized
remained above 1μm due to diffraction effects and limit its utilization to advanced packaging, or MEMS fabrication.
Recently, developments in the illumination system and mechanical parts (known as AMALTIH for Advanced MA
LITHography) as well as mask design, have permitted to used diffractive based photo-mask, and then reach the
resolution limit mentioned above. This opens the possibility to fabricate smaller structures, usually accessible only by ebeam
lithography. We propose here to demonstrate a fast and robust fabrication method of large area plasmonic absorber
structures based on 2D sub-micrometric (350 nm period) nano-needles in a transparent polymer on a glass substrate and
coated with a 50 nm thick gold layer. The interaction of the incoming light with metallic structured surface leads to the
small total reflections of the 0th order below 5 %, over a large spectral band (460-660 nm) and a large set of incidence
angles with TE and TM polarizations. Those results demonstrate that our fabrication process is a step toward the
implementation of plasmonic based effect structures for a wide range of application.
Modern optical applications have special demands on the lithographic fabrication technologies. This relates to the lateral shape of the structures as well as to their three dimensional surface profile. On the other hand optical nano-structures are often periodic which allows for the use of dedicated lithographic exposure principles. The paper briefly reviews actual developments in the field of optical nano-structure generation. Special emphasis will be given to two technologies: electron-beam lithography based on a flexible cell-projection method and the actual developments in diffractive mask aligner lithography. Both offer a cost effective fabrication alternative for high resolution structures or three-dimensional optical surface profiles.
Diffractive mask-aligner lithography allows printing sub-micrometer resolution structures by using non-contact mode. For such a purpose, binary diffraction gratings are used as masks and are designed to transmit solely the ±1st diffraction orders. The high resolution interferogram is realized by the overlapping and the interference of the propagating beams. By applying the techniques known as Self-Aligned Double Patterning (SADP), it´s possible to decrease the period of the fabricated grating (350 nm) by a factor of two, and thus reaching the 90nm structure width. As application, metallic gratings have been fabricated operating as wire grid polarizer (WGP).
Diffractive mask-aligner lithography is capable to print structures that have a sub-500-nanometer resolution by using non-contact mode. This requires the use of specially designed phase-masks and dedicated illumination conditions in the Mask-Aligner to obtain the optimal exposure conditions, a spectral filter and a polarizer needs to be placed in the beam path. We introduce here mask designs that includes a polarizer on the top side of a photo-mask and a diffractive element on the bottom one. This enables printing of high resolution structures of arbitrary orientation by using a classical mask-aligner in proximity exposure mode.
A method is demonstrated for writing long grating phase masks, which can be used for patterning large-area (square meter size) submicron-period gratings. The method consist of illuminating a small area transmission grating phase mask by a continous wave transverse-electric-polarized collimated laser beam under the −1st order Littrow mounting to define a high-contrast interferogram composed of fringes. By sliding a long photoresist-coated substrate under this small area phase mask, gratings of arbitrary length may be written, with grating lines oriented in the scan direction. The patterning of uninterrupted gratings with lengths exceeding 300 mm is demonstrated.
A new type of achromatic phase mask is presented which creates an interferogram of single spatial frequency regardless
of the ratio between the interferogram period and the exposure wavelength. The functional demonstration of this
monolithic phase mask was made in the case of a long grating of period as large as 2 μm by mean of an exposure beam
at 442 nm wavelength, i.e., more than four times smaller. The monolithic element performs one first splitting function
exerted by a central transmission grating of period Λ1 which diffracts the incoming beam in two diffracted beams in the
substrate which are then reflected to the backside of the substrate. The element performs a second diffractive function by
means of two identical side-grating of period Λ2 placed at either side of the first grating. This function is the redirection
of the two said beams under the monolith substrate at an angle which creates an interferogram of the desired period.
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