We demonstrate three-dimensional (3-D) nanoimprint lithography using master samples initially structured by two-photon lithography. Complex geometries like micro prisms, micro parabolic concentrators, micro lenses and other micrometer sized objects with nanoscale features are three-dimensionally fabricated using two-photon lithography. Stamps made out of polydimethylsiloxane are then cast using the two-photon lithographically structured samples as master samples. Hereby, expensive serial nano 3-D printing is transformed into scalable parallel 3-D nanoimprint lithography. Furthermore, the transition from two-photon lithography to imprint lithography increases the freedom in substrate and ink choice significantly. We demonstrate printing on textured surfaces as well as residue-free printing with silver ink using capillary action

Atwater, Harry A.

Bukowsky, Colton R.

Saive, Rebecca

arXiv

Caltech Authors - Main

 1 
Three-dimensional nanoimprint lithography using two-photon 
lithography master samples  
Rebecca Saive1*, Colton R. Bukowsky1, and Harry A. Atwater1 
1 Department of Applied Physics and Material Science, California Institute of Technology, Pasadena, CA 91125, USA 
Keywords: 3-D nanoimprint lithography, 3-D printing, two-photon lithography 
  
Abstract 
We demonstrate three-dimensional (3-D) nanoimprint 
lithography using master samples initially structured by 
two-photon lithography. Complex geometries like micro 
prisms, micro parabolic concentrators, micro lenses and 
other micrometer sized objects with nanoscale features 
are three-dimensionally fabricated using two-photon 
lithography. Stamps made out of polydimethylsiloxane are 
then cast using the two-photon lithographically structured 
samples as master samples. Hereby, expensive serial 
nano 3-D printing is transformed into scalable parallel 3-
D nanoimprint lithography. Furthermore, the transition 
from two-photon lithography to imprint lithography 
increases the freedom in substrate and ink choice 
significantly. We demonstrate printing on textured 
surfaces as well as residue-free printing with silver ink 
using capillary action. 
Introduction 
Three-dimensional (3-D) micro- and submicrometer 
structures play an increasingly important role in many 
modern fields of technology such as (opto)electronics[1, 
2] and optics[3, 4]. Conventional structuring methods 
such as deposition and etching are constrained to a 
limited number of geometries. Furthermore, extensive 
process development is needed in order to fabricate 
objects with the desired shape and to modify existing 
geometries. Until recently, there have been few options 
for directly creating 3-D structures, as opposed to 
tweaking process parameters and using multiple 
processing steps to achieve a desired geometry. 
More sophisticated 3-D micro structures can be prepared 
using focused ion beam (FIB) or two-photon lithography 
processing. In two-photon lithography the photoresist is 
polymerized at the focal point of a laser leading to the 
ability to write 3-D structures with feature sizes down to 
150 nm[5]. Therefore, two-photon lithography is also 
referred to as nano 3-D printing. Similar to other 3-D 
printing techniques the desired shape can be drafted with 
a computer assisted design (CAD) program and the 
obtained file can be converted into a print file. Shape 
modifications can be easily performed with few changes 
in the process parameters. Two-photon lithography has 
led to numerous scientific achievements[6] ranging from  
 
nano and micro mechanics[7-10] to nano and micro 
optics[3, 4, 11, 12].  
However, widespread application will be limited by 
insufficient scalability and constraints in substrate and 
resist choice. Scalability is limited by the writing time 
(serial print method) while the substrate choice is limited 
to smooth, non-reflective surfaces with a refractive index 
different from the photo resist material. Furthermore, the 
development of photoresist is sophisticated and adding 
functionalities such as specific optical or electrical 
properties is non-trivial. A more scalable, flexible and 
robust printing method is represented by nanoimprint 
lithography[13]. In nanoimprint lithography a master 
structure is reproduced through preparation of a 
polydimethylsiloxane (PDMS) stamp and subsequent 
printing using a partially reacted sol or an ink. Different 
inks and sols can provide a wide variety of mechanical, 
thermal, optical and electrical properties[14]. 
Furthermore, the limitations of the substrate are low and 
even printing on textured surfaces can be performed. 
We show a printing process that combines the design 
flexibility of two-photon lithography with the scalability of 
imprint lithography by using two-photon lithographically 
prepared geometries as master structures for stamp 
fabrication[15]. Replicas are a virtually perfect match of 
the originals and the shape quality is solely determined by 
the master structure. 
Furthermore, we demonstrate a high fidelity ink process 
for residue free printing of silver ink lines using capillary 
action[16, 17].  
Methods 
First 3-D structures that later will be used as master 
samples are prepared by two-photon lithography. In two-
photon lithography polymerization of the photoresist 
occurs in the focus point of a laser by a two-photon 
absorption process[18]. The lateral laser position as well 
as the focus height of the laser can be varied such that 3-
D structuring is achieved. We used a Nanoscribe Photonic 
Professional GT[19] and the photoresist IP-Dip (by 
 2 
Nanoscribe). An optional surface functionalization of (3-
(N-ethylamino)isobutyl)trimethoxysilane (4wt% in 
methanol, Gelest SIE4886.0) for 30 minutes was seen to 
increase the reusability of the fabricated polymer master 
stamp by covalently bonding the resist to the silicon 
substrate. The structures were written in Nanoscribe’s 
Galvo mode with a speed between 40000 and 70000 
µm/s. After the writing process unpolymerized resist is 
removed by a 15-20 min bath in propylene glycol methyl 
ether acetate followed by 5 min in isopropanol. In order to 
condition the sample surface for stamp preparation the 
sample was left three hours in a box with a droplet of 
chlorotrimethylsilane. The polydimethylsiloxane (PDMS) 
was prepared by weighing 10 parts of SYLGARD® 184 
silicone elastomer base and one part of the curing agent 
and subsequent mixing for 1 min and defoaming for 5 min. 
The PDMS was then poured on the master sample and 
captured air was removed with a vacuum treatment. 
Sample and PDMS were left in an oven at 80 ˚C for an 
hour in order to polymerize the PDMS. After cooling, the 
PDMS was carefully peeled from the master sample. The 
stamp was then filled with a silica sol-gel and placed on a 
substrate. Alternatively, the silica sol-gel can be drop 
casted or spin casted directly on the substrate and the 
stamp is placed on top afterwards. The stamp is left on 
the sample for at least three hours at room temperature to 
form a rigid silica sol-gel replica and is then peeled off. 
Residual sol-gel is left on the substrate for these 
demonstrations but is known to be on the order of ~15 nm, 
and can be removed via a quick dip in dilute HF with a 
minimal effect on resulting structures[13]. 
Residue-free printing of silver lines was achieved using 
capillary action. Stamps were cut open at their sides and 
put on top of the substrate such that good adhesion was 
achieved. A small amount of ink was brought in contact 
with the openings and pulled-in by capillary action. The 
distance that can be printed depends on the substrate 
surface and its treatment as well as on the stamp 
treatment. We obtain up to 5 mm long structures using 
silicon and indium tin oxide substrates by leaving the 
stamp in isopropanol for at least 4 hours. Furthermore, 
stamps can be filled from both sides in order to achieve 
twice this length. 
Results 
Figure 2 shows scanning electron microscope (SEM) 
images of a) a master structure and b) a replica. The 
master sample was coated with 30 nm aluminum in order 
to facilitate SEM imaging the replica provided sufficient 
conductivity without coating. Six different geometries with 
different scales were explored: from left to right: micro 
lenses, micro volcanos, cones, pyramids, parabolic cross-
section (micro concentrator) lines and triangular cross-
section lines. The white scale bar corresponds to 100 µm.  
Figure 1 Schematic description of the stamp making and 
printing process. a) First, the master sample is prepared 
by two-photon lithograpy, b) then it is developped and 
treated with chlorotrimethylsilane. c) PDMS is poured on 
the master sample and d) carefully pulled off after 
polymerization. e) The PDMS stamp is filled with sol-gel, 
placed on a substrate and f) removed after polymerization 
of the sol-gel. 
 
Figure 2 SEM images taken under a 45˚ angle of a) the 
master structures that were prepared by two-photon 
lithography and b) printed replicas. The white scale bar 
corresponds to 100 µm.  
 
 3 
 
Figure 3 SEM images taken under a 45˚ angle of a), b) 
and c) master structures and b), d) and f) replicas. The 
white scale bar corresponds to 10 µm. k) and l) show SEM 
images of triangular cross-section lines printed on 
structured silicon. Scale bar: k) 20 µm, l) 3 µm. 
Figure 3 shows higher resolution images of the structures. 
a), c), e), g) and i) are master structures while b), d), f), h) 
and j) show the replicas. The white scale bars correspond 
to 10 µm. All shapes are reproduced with high fidelity. 
Moreover, even the sidewall textures are replicated 
showing that the quality of the printed shape is limited by 
the quality of the master structure. 
Although the shapes chosen here all have a constant or 
monotonically increasing profile approaching the 
substrate it has been shown that structures with 
overhanging parts can be fabricated in this fashion as 
well[15]. 
Figure 3 k) and l) show SEM images of triangular cross-
section lines printed on a textured silicon substrate. The 
printed structure experienced only small disturbance 
although this presents an extreme case with texture 
roughness being at similar dimensions as the printed 
features. 
A known limitation of imprint lithography is the occurrence 
of residues in between printed features. While sol-gel 
residues are ~15 nm, and can be removed via a quick dip 
in dilute HF with a minimal effect on resulting 
structures[13], silver ink residues can lead to significant 
disturbance of the desired structure as shown in Figure 
4c. 
 
Figure 4 Triangular cross-section lines a) master 
structure, b) printed with sol-gel and c) printed with silver 
ink.  
As described in the Methods section it is possible to 
overcome this issue by first placing the stamp on the 
substrate and then infilling the lines by capillary action. 
SEM images of such silver lines are shown in Figure 5. 
 
Figure 5 Triangular cross-section lines printed with silver 
ink. Residue-free structures were achieved by infilling the 
PDMS stamp using capillary action. 
Conclusion 
We have shown that 3-D nanostructures fabricated by 
two-photon lithography can be replicated by nanoimprint 
lithography. We have also demonstrated that 3-D 
structures can be transferred to non-standard substrates. 
Sub micrometer features are reproduced with high fidelity 
and the quality of the replicated structure exclusively 
depends on the master structure.  
 
AUTHOR INFORMATION 
Corresponding Author 
*rebsaive@caltech.edu 
 4 
 
ACKNOWLEDGMENTS  
The information, data, or work presented herein was funded in part by the U.S. Department of Energy, Energy Efficiency 
and Renewable Energy Program, under Award Number DE-EE0006335 and by the Bay Area Photovoltaics Consortium 
under Award Number DE-  
EE0004946. We thank Lucas Meza for two-photon lithography advice.
REFERENCES 
1 Schmid, M., and Manley, P.: ‘Nano-and microlenses as 
concepts for enhanced performance of solar cells’, Journal of 
Photonics for Energy, 2015, 5, (1), pp. 057003-057003 
2 Möller, S., and Forrest, S.: ‘Improved light out-coupling in 
organic light emitting diodes employing ordered microlens 
arrays’, Journal of Applied Physics, 2002, 91, (5), pp. 3324-
3327 
3 Ergin, T., Stenger, N., Brenner, P., Pendry, J.B., and 
Wegener, M.: ‘Three-dimensional invisibility cloak at optical 
wavelengths’, Science, 2010, 328, (5976), pp. 337-339 
4 Muller, N., Haberko, J., Marichy, C., and Scheffold, F.: 
‘Silicon hyperuniform disordered photonic materials with a 
pronounced gap in the shortwave infrared’, Advanced Optical 
Materials, 2014, 2, (2), pp. 115-119 
5 http://www.nanoscribe.de/en/products/ip-photoresists/ 
accessed 02/29/2016  
6 Phillips, D., Padgett, M., Hanna, S., Ho, Y., Carberry, D., 
Miles, M., Simpson, S., Mu, J., Li, J., and Li, W.: Shape-
induced force fields in optical trapping’, Nanoscale, 2014, 6, 
pp. 282 
7 Jang, D., Meza, L.R., Greer, F., and Greer, J.R.: 
‘Fabrication and deformation of three-dimensional hollow 
ceramic nanostructures’, Nature materials, 2013, 12, (10), pp. 
893-898 
8 Meza, L.R., Das, S., and Greer, J.R.: ‘Strong, lightweight, 
and recoverable three-dimensional ceramic nanolattices’, 
Science, 2014, 345, (6202), pp. 1322-1326 
9 Bückmann, T., Stenger, N., Kadic, M., Kaschke, J., Frölich, 
A., Kennerknecht, T., Eberl, C., Thiel, M., and Wegener, M.: 
‘Tailored 3D mechanical metamaterials made by dip‐in direct‐
laser‐writing optical lithography’, Advanced Materials, 2012, 
24, (20), pp. 2710-2714 
10 Röhrig, M., Thiel, M., Worgull, M., and Hölscher, H.: ‘3D 
Direct Laser Writing of Nano‐and Microstructured Hierarchical 
Gecko‐Mimicking Surfaces’, Small, 2012, 8, (19), pp. 3009-
3015 
11 Atwater, J., Spinelli, P., Kosten, E., Parsons, J., Van Lare, 
C., Van de Groep, J., De Abajo, J.G., Polman, A., and Atwater, 
H.: ‘Microphotonic parabolic light directors fabricated by two-
photon lithography’, Applied Physics Letters, 2011, 99, (15), 
pp. 151113 
12 Soukoulis, C.M., and Wegener, M.: ‘Past achievements 
and future challenges in the development of three-dimensional 
photonic metamaterials’, Nature Photonics, 2011, 5, (9), pp. 
523-530 
13 Verschuuren, M.A.: ‘Substrate conformal imprint 
lithography for nanophotonics’, 2010 
14 Radha, B., Lim, S.H., Saifullah, M.S., and Kulkarni, G.U.: 
‘Metal hierarchical patterning by direct nanoimprint 
lithography’, Scientific reports, 2013, 3 
15 LaFratta, C.N., Baldacchini, T., Farrer, R.A., Fourkas, J.T., 
Teich, M.C., Saleh, B.E., and Naughton, M.J.: ‘Replication of 
two-photon-polymerized structures with extremely high aspect 
ratios and large overhangs’, The Journal of Physical Chemistry 
B, 2004, 108, (31), pp. 11256-11258 
16 Saive, R., Borsuk, A.M., Emmer, H.S., Bukowsky, C.R., 
Lloyd, J.V., Yalamanchili, S., and Atwater, H.A.: ‘Effectively 
Transparent Front Contacts for Optoelectronic Devices’, 
Advanced Optical Materials, 2016 
17 Saive, R., Bukowsky, C.R., Yalamanchili, S., Boccard, M., 
Saenz, T., Borsuk, A.M., Holman, Z., and Atwater, H.A.: 
‘Effectively transparent contacts (ETCs) for solar cells’, (IEEE, 
2016, edn.), pp. 3612-3615 
18 Wu, E.-S., Strickler, J.H., Harrell, W.R., and Webb, W.W.: 
‘Two-photon lithography for microelectronic application’, 
(International Society for Optics and Photonics, 1992, edn.), 
pp. 776-782 
19 Anscombe, N.: ‘Direct laser writing’, Nature Photonics, 
2010, 4, (1), pp. 22-23 
 


Three-dimensional nanoimprint lithography using two-photon lithography master samples

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main