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A simple technique is presented allowing the fabrication of high density periodic patterns via direct laser ablation. Applying fluence control for reducing the ablated feature sizes combined with lateral translation of an interference pattern between two (or more) irradiation cycles, sub-wavelength period patterns (< 200 nm) are created. Variation of the amount and direction of translation and the applied intensities during subsequent irradiation steps leads to variable pattern design as demonstrated for polymeric and silicon samples.
©2013 Optical Society of America
1. Introduction
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 [1]. 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. Lithographic techniques are known to provide an extremely high precision of better than 50 nm and are therefore well suited for the fabrication of high resolution textures. However, these methods require very expensive equipment and long processing time. Moreover they carry some restrictions in terms of material choice and surface geometry. Alternatively, direct laser writing provides a very cheap, fast and flexible means for surface modification. However, the achievable pattern density is limited by the numerical aperture of the applied optics and by diffraction, so that sub-wavelength feature sizes are hard to realize. Even with short wavelength (ultraviolet) laser sources, the resulting texture density cannot compete with that obtained by lithography. Self organization effects, e.g. the so called ‘ripples’ (also known as LIPSS, Laser Induced Periodic Surface Structures) appearing under certain irradiation conditions, may be used to generate sub-wavelength periods [2–5]. However, the resulting topology achieved by LIPSS is rather irregular and not fully deterministic, as it is when interference techniques are applied.
In previous studies, applying UV ultrashort pulses, we could demonstrate the fabrication of high definition periodic structures by direct laser ablation, using a combination of multiple beam interference and mask projection [6, 7]. However, the minimum achievable period was limited by the available optics to ~1.5 times the applied wavelength (typically 248 nm). Also with other far field techniques applying laser direct writing over large sample areas no sub-wavelength periods (with strictly repeatable patterns) have been reported in the literature.
The present publication proposes a new way to overcome the above mentioned limitations and demonstrates the feasibility of fabricating deterministic, high definition sub-wavelength structures using a laser direct writing technique. The basic idea is to reduce the feature size of the periodic pattern by fluence control and increase the pattern density via repeated irradiation.
2. The irradiation technique
In previous work we used a combination of multiple beam interference and mask projection to fabricate various textures with sub-micron precision [6]. In this case a diffractive beam splitter, placed in the object plane of an imaging objective, is used to create a number of partial beams (at least two) illuminating a diaphragm, which is placed directly behind the beam splitter (Fig. 1 ). This diaphragm is then imaged onto the surface of the work piece with a demagnification ranging typically between 10x to 50x. The imaging optics is designed to transmit most of the diffracted partial beams which can be individually blocked or unblocked to increase the variability of the resulting interference pattern. The surface of the work piece is placed in the image plane where all unblocked partial beams overlap and create an interference pattern depending on their intersection angle, relative intensity, and phase relationship. These parameters can easily be varied by appropriate adjustment of the optical setup, thus resulting in a vast variety of different textures [7].
The demagnification, inherent to this particular scheme, carries the potential of performing tiny lateral shifts of the projected pattern with extremely high accuracy. This recognition forms the basis of the novel approach, as explained below.
In every strictly periodic surface structure a ‘unit cell’ can be identified, whose features are typically repeated along a two dimensional lattice. In the simplest case, which will be discussed as an example, 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 key steps of the method are illustrated in Fig. 2 .
For an easy applicability of this technique, it is a fundamental question how shifting of the pattern is accomplished. Obviously, the work piece could be translated between the successive irradiation steps. However, if sub-wavelength periods shall be achieved, a translation of ~100-200 nm with a precision of 10-20 nm would be necessary. Evidently, these strict requirements to the applied translation stages result in difficult handling and high costs. The main idea to overcome this problem is to make use of the demagnification of the optical setup. In case of a demagnification of 10x – 50x a lateral displacement of the diffractive beam splitter in the object plane corresponds to a 10 – 50 times reduced translation of the diffraction pattern on the surface of the work piece. Consequently, a 100 nm displacement of the resulting pattern on the work piece can be obtained by a translation of the beam splitter by a few microns with a necessary precision of only several 100 nanometers. Evidently, standard components can be used for this adjustment, thus turning this scheme to an enabling technology for cheap and easy fabrication of high density sub-micron structures
3. Results and discussion
To validate the idea, corresponding experiments were performed. In our experiments ultraviolet laser pulses of subpicosecond duration (0.5 ps) at a wavelength of 248 nm were applied, since such pulses have been demonstrated to provide superior ablation results in terms of resolution and pattern definition [6, 7]. As a diffractive beam splitter a crossed phase grating made of fused silica was used, generating eight first order beams. Four of these beams were transmitted through the imaging optics, creating an interference pattern that resembled a two dimensional array of sine2 peaks. Performing laser ablation with a beam having such an intensity distribution, the resulting ablation pattern is a two dimensional array of holes. Considering the threshold-like nature of the ablation process, only those parts of the intensity profile will actively contribute to material removal which are above the ablation threshold, as already discussed for single Gaussian beams in the literature [8]. As a consequence, variation of the intensity results in a variation of the active beam profile and thus in a variation of the diameter and shape of the ablated holes. This makes it possible to change the effective spot size within the unit cell. Figure 3 shows the theoretical predictions of the ablation results. The regarded irradiation pattern is the intensity of the interference pattern of the four overlapping beams. For a typical ablation behavior, the ablation depth is obtained by taking the logarithm of the spatially varying normalized intensity folded with a step function representing the ablation threshold. In the simulations the ablation threshold was set to 25 mJ/cm2, which is a typical value for many polymers under the regarded irradiation conditions (wavelength and pulse duration). The result of the calculations is displayed in form of contour plots. Figure 3(a) shows the calculated ablation pattern obtained at a peak fluence of 500 mJ/cm2, resulting in an array of large holes with nearly rectangular boundaries. The diameter of the holes can strongly be reduced by reducing the peak fluence down to 50 mJ/cm2 as shown in Fig. 3(b). Figure 3(c) shows the simulated pattern after a shift of the pattern by half the diagonal length of the unit cell (D/2) followed by successive irradiation.
Fig. 3 Theoretical prediction of ablation patterns at a fluence of 500 mJ/cm2 (a), 50 mJ/cm2 (b) and after shifting the pattern by D/2 followed by successive irradiation (c).
For a demonstration of the feasibility of the presented technique polymer and silicon samples were irradiated by 500 fs long laser pulses at a wavelength of 248 nm. A description of the laser system delivering these pulses is given in [9].
Figure 4 shows an example on polyethylene sulfone (PES). Using the configuration described above and irradiating the sample with 5 pulses having a peak fluence of 500 mJ/cm2, the surface structure displayed in Fig. 4(a) was obtained. It consists of an array of holes with an average diameter of 280 nm. In order to increase the structure density, the size of the individual holes has to be reduced. As discussed above, this can be accomplished by reducing the peak fluence down to 50 mJ/cm2. Using this approach, the diameter of the holes was reduced to 140 nm, as shown in Fig. 4(b). In our experiment we applied a demagnification of 36x. Given a period of 500 nm as seen in Fig. 4(a) and 4(b), a shift of the pattern by half the diagonal of the unit cell corresponds to a translation of the diffractive beam splitter by 9 µm. After translating the pattern, the irradiation was repeated for a second time resulting in a topology of doubled density shown in Fig. 4(c). For all ablated topographies there is excellent agreement with the theoretical predictions validating the feasibility of the proposed technique.
Fig. 4 Hole patterns generated in PES at a peak fluence of 500 mJ/cm2 (a) and 50 mJ/cm2 (b). In c) the resulting pattern is shown after shifting the diffraction pattern and repeating the irradiation with 50 mJ/cm2.
Besides doubling the pattern density the technique also allows the formation of a variety of new patterns. By applying various lateral shifts, all being smaller than the size of the unit cell, new unit cells can be designed. Figure 5 displays further examples.
As a further extension of the possibilities, different fluences or pulse numbers can be applied during the two irradiation cycles (before and after shifting the pattern), for which an example in Silicon is shown in Fig. 6 .
4. Conclusion
In conclusion, we demonstrated the ability of creating sub-wavelength patterns with strict periodicity via direct laser ablation. The presented method utilizes fluence control for reducing the feature sizes combined with lateral displacement of an interference pattern used for irradiation between successive irradiation steps. In this way periodic patterns with reproducible structural details with dimensions well below 200 nm can be created as was demonstrated for PES and silicon samples. Variation of the irradiation parameters results in a great variability of the resulting patterns.
Acknowledgments
The authors are indebted to Jürgen Ihlemann for stimulating discussions.
References and links
1. K.-C. Park, H. J. Choi, C. H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity,” ACS Nano 6(5), 3789–3799 (2012). [CrossRef] [PubMed]
2. M. Huang, F. Zhao, Y. Cheng, N. Xu, and Z. Xu, “Mechanisms of ultrafast laser-induced deep-subwavelength gratings on graphite and diamond,” Phys. Rev. B 79(12), 125436 (2009). [CrossRef]
3. F. Liang, R. Vallée, and S. L. Chin, “Physical evolution of nanograting inscription on the surface of fused silica,” Opt. Mater. Express 2(7), 900–906 (2012). [CrossRef]
4. A. K. Sharma, J. Smedley, T. Tsang, and T. Rao, “Formation of subwavelength grating on molybdenum mirrors using a femtosecond Ti:sapphire laser system operating at 10 Hz,” Rev. Sci. Instrum. 82(3), 033113 (2011). [CrossRef] [PubMed]
5. A. Borowiec and H. K. Haugen, “Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses,” Appl. Phys. Lett. 82(25), 4462–4464 (2003). [CrossRef]
6. J. Bekesi, J.-H. Klein-Wiele, and P. Simon, “Efficient submicron processing of metals with femtosecond UV pulses,” Appl. Phys., A Mater. Sci. Process. 76(3), 355–357 (2003). [CrossRef]
7. J.-H. Klein-Wiele and P. Simon, “Fabrication of periodic nanostructures by phase-controlled multiple-beam interference,” Appl. Phys. Lett. 83(23), 4707–4709 (2003). [CrossRef]
8. F. Korte, J. Serbin, J. Koch, A. Egbert, C. Fallnich, A. Ostendorf, and B. N. Chichkov, “Towards nanostructuring with femtosecond laser pulses,” Appl. Phys., A Mater. Sci. Process. 77, 229–235 (2003).
9. G. Marowsky, P. Simon, K. Mann, and C. K. Rhodes, Femtosecond Excimer Laser Pulses, Träger Ed., (Springer-Verlag Berlin Heidelberg 2012) p. 842.
