Sub-wavelength pattern generation by laser direct writing
Sub-micron size periodic structures on surfaces of technologically important materials are known to give rise to new functionalities including field amplification, self-cleaning, holographic appearance, etc., thus opening up a row of novel applications. Special design of the topology can enhance a specific functionality and even multifunctional behavior can be realized. There is a general trend to reduce the size and increase the density of sub-micron features, demanding for the development of new fabrication technologies. Direct laser writing provides a very cheap, fast and flexible means for surface modification. In previous studies we could demonstrate the fabrication of high definition periodic structures by direct laser ablation, using a combination of multiple beam interference and mask projection. However, in such a scheme the achievable pattern density is limited by the numerical aperture of the applied optics and by diffraction, so that the minimum achievable period so far was limited to ~ 1.5 times the applied wavelength. Recently, we proposed a new way to overcome this limitation. The basic idea is to reduce the feature size of the periodic pattern by fluence control and increase the pattern density via repeated irradiation, as explained below.
In every strictly periodic surface structure a ‘unit cell’ can be identified, whose topography is repeated along a one or two dimensional lattice. In the simplest case the unit cell (having a diagonal length of D) contains only one single feature. Our novel technique incorporates three steps: 1) apply laser patterning with an interference scheme, 2) shift the periodic pattern by D/2 (or in general an amount which is less than the size of the unit cell), and 3) repeat the laser patterning. As a result, we obtain a pattern with an increased density. The demagnification, which is inherent to our particular scheme, carries the potential of performing tiny lateral shifts of the projected pattern with extremely high accuracy. Thus, instead of translating the work piece between the subsequent irradiation cycles, the diffractive beam splitter is shifted laterally. This introduces a lateral displacement of the resulting diffraction pattern which experiences a strong demagnification in the plane of the work piece. Consequently, translation of a component in the micrometer range, which is easy to control, converts to a displacement of the illumination pattern on the work piece of the order of 100 nm.
For a feasibility test of the presented technique polyethylene sulfone (PES) samples were irradiated by 500 fs long laser pulses at a wavelength of 248 nm. By reducing the peak fluence from 500 mJ/cm2 down to 50 mJ/cm2 the individual feature sizes could be significantly reduced. After translating the diffraction pattern, the irradiation was repeated for a second time resulting in a topology with a doubled density.
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Laser-Laboratorium Göttingen e.V. (LLG)
Head of the Department
Dr. Peter Simon
"Short Pulses / Nanostructures"
Contact person for
Nano Structure Technology:
Dr. Jürgen Ihlemann