Efficient high-resolution surface patterning for 2D and 3D parts

It has been shown in the past that surfaces with controlled topographic characteristics can provide enhanced properties (e.g., low friction and wear, antibacterial behavior, or high absorption of light) compared with surfaces that have ‘random’ roughness.¹ Several examples of surfaces with this kind of ordered topography can be found in nature. For instance, the surfaces of various plants and animals have evolved over thousands of years to meet survival challenges. It is therefore desirable to use this inspiration from nature to design manufacturing methods, with which controlled topography surfaces can be achieved.

Laser interference lithography (LIL) can be used to produce periodic surface structures.² During LIL, the standing wave pattern that exists at the intersection of two or more laser beams is used to expose a photosensitive layer (e.g., a resist). In the case of a negative resist, the positions that correspond to the interference maxima are photopolymerized. After subsequent resist development, a periodic variation in the surface topography can be obtained. The multistep character of LIL, however, gives rise to slow fabrication speeds and only permits the processing of planar surfaces.

We have previously developed an innovative solution for high-speed surface patterning of any periodic structure in one processing step. This technique—direct laser interference patterning (DLIP)—enables the formation of periodic patterns with different features and defined long-range order.³ In DLIP, we make use of the interference between two or more laser beams. This is similar to LIL, but in the DLIP case no development of the irradiated sample is required.^{4, 5} Depending on the number of laser beams that are used, and their geometrical arrangement, we can produce different surface geometries (see Figure 1). For example, two-beam interference produces a 1D line-like geometry, see Figure 1(a). Whereas the use of three—Figure 1(b)—or more—Figure 1(c)—laser beams results in different 2D arrays that depend on the magnitude of the electric field of each beam and their geometric configuration.

Figure 1. Calculated intensity distribution for (a) two-beam, (b) three-beam, and (c) four-beam interference patterning. The geometrical configurations of the beams necessary to achieve the displayed geometries are also shown.

The most important requirement for the fabrication of periodic structures with our DLIP method is that the material being processed must be able to absorb the energy of the laser at the selected wavelength. Furthermore, the laser must provide sufficient pulse energy to ablate or modify the material directly. When we use high-power laser systems, we can achieve fabrication speeds of up to about 1m²/min.⁶ Our structuring process can be based on photothermal, photophysical, or photochemical mechanisms, depending on the material type (see Figure 2). In general, we treat polymers and ceramics with UV laser radiation, whereas we use green or IR lasers to treat metals and coatings. As illustrated in Figure 2, we can obtain a wide range of topographies, with feature sizes down to the nanometer scale (e.g., 80nm for diamond-like coatings).

Figure 2. Examples of periodic surface structures on a transparent conducting oxide (TCO), photoresist (SU8 epoxy), tetrahedral amorphous carbon (Ta-C), i.e., a diamond-like coating, metal (stainless steel), polymer (polystyrene), and hydroxyapatite ceramic (HAP).

In addition, if surfaces are locally treated, we are able to fabricate periodic structures with different spatial periods at different positions. Decorative elements, such as that shown in Figure 3, can also be produced. We obtained this image (of the Church of our Lady in Dresden) by using three different spatial periods.

Figure 3. Optical photograph of a decorative element (of the Church of our Lady in Dresden) fabricated on stainless steel.

DLIP can be used for the treatment of 3D elements, as well as for flat substrates. Indeed, we have recently developed DLIP optical heads for 3D processing at the Fraunhofer IWS (see Figure 4).⁷ These optical heads can be combined with either linear or rotational translation stages—see Figure 4(a)—or they can be mounted on a robotic arm, as shown in Figure 4(b). With the first of these options, the part or the DLIP processing head can be translated vertically, and the relative z-position of the two components can thus be adjusted. This strategy is appropriate for parts with curvature angles that are less than 20° (which corresponds to deviations in the structure period of up to 6.4%).⁸ If it is necessary to treat 3D parts with larger angles of curvature, an industrial robot can provide the required flexibility. The main advantages of this system are the large working space (i.e., 2033 × 2230 × 2429mm) and a maximum linear speed of 2000mm/s. An example of a treated polyethylene terephthalate (PET) bottle is shown in Figure 4(c) and (d).

Figure 4. Direct laser interference patterning (DLIP) optical heads developed at the Fraunhofer IWS and Technische Universität Dresden. The optical heads are integrated (a) into the DLIP μFAB system and (b) with a robotic arm. Both these systems allow the treatment of 3D parts, such as the polyethylene terephthalate (PET) bottle shown in (c) and (d).

For the processing of larger parts, we can construct special DLIP systems. For instance, the system shown in Figure 5(a)—developed at the Technische Universität Dresden—allows the treatment of cylindrical parts that are up to 600mm in length and 300mm in diameter. This DLIP system is also equipped with both nano- and picosecond pulsed lasers that operate at different wavelengths. As with our DLIP μFAB system—see Figure 4(a)—we can operate this large-area DLIP system with different optical heads. Preliminary results from a nickel (Ni) sleeve treated using this system are shown in Figure 5(b). We observe four different treated regions that correspond to the patterns produced through the use of different laser parameters. The possibility of producing dot-like geometries on the Ni sleeve with the use of three laser beams is demonstrated in Figure 5(c). The spatial period in this case is about 5μm, with a structure depth of 1.2μm. We subsequently used this structured sleeve to print a PET polymer foil in a roll-to-roll hot embossing system (with a fixed temperature of 85°C). The structure that we obtained on the PET foil—see Figure 5(d)—shows the negative shape of the initial pattern and that the structure height was 0.6μm.

Figure 5. (a) A large-area DLIP system that can be used for the processing of 2D and 3D parts. (b) A nickel (Ni) sleeve (300mm diameter) that was treated with a nanoscale DLIP processing head. (c) Dot-like geometry on the Ni sleeve that was obtained using three-beam interference. (d) The imprint of the Ni sleeve on a PET foil that was produced using a roll-to-roll hot embossing system. ΔZ: Structure depth.

The results that we have reported here demonstrate the capabilities of our DLIP method for the production of periodic surface patterns on 2D and 3D parts. We can achieve resolutions down to the nanometer range. In addition, our results indicate the high maturity level of our technology and highlight its potential for integration into industrial applications. In our future work, we will focus on improving the homogeneity of our produced nanostructured surfaces and the surface fabrication speed.