Solar Absorber by Nanoimprint Lithography
and Sputter Deposition
Tina Mitteramskoglera, Michael J. Haslingera, Ambiörn Wennbergb, Iván
Fernandes-Martínezb, Michael Muehlbergera, Matthias Krausec and Elena GuillénaaPROFACTOR GmbH, Steyr-Gleink, 4407, Austria
bNano4Energy SL, Madrid, 28006, SpaincHelmholtz-Zentrum Dresden-Rossendorf e.V., Dresden, 01328, Germany
E-mail: tina.mitteramskogler@profactor.at

For harvesting solar power efficiently, an ideal absorber is characterized by a high absorption
in order to capture the incoming energy while suppressing thermal re-radiation. Selective solar
absorbers comprised of plasmonic structures offer a great flexibility in design along withunmatched optical performance [1]. However, the nanopattern generation, typically done withelectron beam writing, is a very time-intensive step. In this work, we present a fast, scalableand flexible method for the fabrication of plasmonic absorbers by the combination of adeposition mask prepared by nanoimprint lithography and subsequent thin film deposition bymagnetron sputtering.
The deposition mask is a two-layer system. The underlying lift-off layer aids the subsequentmask lift-off, whereas the NIL fabricated top layer is defining the location and shape where thenanodisks are created during metal deposition. The chosen layout, shown in Figure 1 is not
optimized for a solar absorber application, but shows feature sizes similar to what is reported
in literature [2–5]. Due to the versatility of this process, the absorber features could be tailoredby choosing an optimized nanopattern for the nanoimprint step. After calibrating the deposition
time and the maximal deposition height, the process was transferred onto polished Inconel
NiCr-alloy plates used in high temperature solar absorbers. Two different sputtering methods
are compared: direct current sputtering and high-power pulsed magnetron sputtering (HiPIMS).
A schematic drawing of the fabrication procedure is shown in Figure 2. First, a substrate having
an insulating layer was spin-coated with a lift-off layer (LOR 1A, MicroChem) followed by an
UV-curable resist (mr-NIL_212_FC XP, micro resist technology), which is imprinted with ahybrid h-PDMS-PDMS stamp containing the ellipsoidal pattern. After plasma etching andchemical etching to create undercut features in the mask, the material is deposited via
magnetron sputtering. The subsequent wet chemical mask lift-off step, shown in Figure 3,results in the desired nanoparticle array on the surface. Figure 4 shows a SEM image of thefinished structure.
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[2] Bermel, P.; Lee, J.; Joannopoulos, J.D.; Celanovic, I.; Soljacie, M.; "SELECTIVE
SOLAR ABSORBERS" Annual Review of Heat Transfer 2012 15,(15) 231–254.
[3] Wang, H.; Prasad Sivan, V.; Mitchell, A.; Rosengarten, G.; Phelan, P.; Wang, L.; "Highly
efficient selective metamaterial absorber for high-temperature solar thermal energy
harvesting" Solar Energy Materials and Solar Cells 2015 137,235–242.
[4] Han, X.; He, K.; He, Z.; Zhang, Z.; "Tungsten-based highly selective solar absorber usingsimple nanodisk array" Optics Express 2017 25,(24) A1072.
[5] Cheng, C.-W.; Abbas, M.N.; Chiu, C.-W.; Lai, K.-T.; Shih, M.-H.; Chang, Y.-C.; "Wideangle polarization independent infrared broadband absorbers based on metallic multi-sized
disk arrays" Optics express 2012 20,(9) 10376–10381.

Funding from the multiLINK project (funded by the Austrian ministry for transport, innovationand technology, bmvit) and the LINK project is gratefully acknowledged. A part of thisresearch is carried out within the ”FTI-Project DigiManu – Labor für die Digitalisierung derProduktion” project funded by the State of Upper Austria through the Strategic Economic andResearch Program ”Innovatives OÖ 2020”.

Figure 1. Plasmonic absorber design used for our experiments. An array of ellipses with axes a=100 nm,b=200 nm, t=45 nm up to 100 nm and unit cell of 400x600 nm² sits on top of an insulating layer withheight h=100 nm.

Figure 2. Fabrication of metallic nanoparticles based on UV-NIL. 1) spin coating the lift-off and imprint
layer 2) UV-NIL to copy the nanopattern from a stamp to the substrate 3) reactive ion etching forcontacting the lift-off layer 4) chemical development of the lift-off layer to create undercut features 5)
sputtering of the desired material 6) mask lift-off.

Figure 3. Left: Mask lifting-off and curlingup after immersion in etchant solution. Right:
Finished absorber structure with layout asshown in Figure 1. The nanostructured area is
1x1cm2.

Figure 4. SEM image of a sample fabricated on aSi-wafer after partial mask lift-off and complete
removal of the lift-off layer (step 5-6 of Figure 2).