High Quality Diffractive Optical Elements(DOEs) Using Nanoimprint Lithography
Simon Drieschner, Fabian Pawlitzek, Vijay R. Kolli, Margarete Zoberbier,Eleonora Storace, Marc Hennemeyer
SUSS MicroTec Lithography GmbHSchleißheimer Str. 90, 85748 Garching, Germany
E-mail: simon.drieschner@suss.com

Augmented reality (AR) enhancing the existing natural environment by overlaying a virtualworld is an emerging and growing market and attracts huge commercial interest into opticaldevices which can be implemented into head-mounted AR equipment. Diffractive opticalelements (DOEs) are considered as the most promising candidate to meet the market’srequirements such as compactness, low-cost, and reliability [1] as they replace large display
headsets for virtual reality (VR) by light-weight glasses. Their structures can vary from
simple optical gratings for diffractive wave guides [2] to binary elements or pyramid-like
multilevel structures [3]. There are many techniques to fabricate DOEs based on, e.g.
greyscale lithography, direct writing using laser or electron beams, and direct machining.
However, these methods exhibit great weaknesses such as low process variability and surface
precision, high equipment costs, and low volumes limiting their suitability for mass
production [3]. In contrast, soft lithography replication offers a pathway to the fabrication oflarge area DOEs with high aspect ratios, multilevel features, and critical dimensions below
the diffractive optical limit down to 50 nm. In combination with UV-curable materials, thefabrication time can be drastically reduced in comparison to, e.g. hot embossing replication
methods, making it very appealing to industrial applications [3].
Here, we present a very flexible approach to fabricate high quality DOEs using nanoimprint
lithography based on porous soft stamp materials (see SEM image in Figure 1). Uniformly
spin-coated substrates approach a flexible working stamp in a radial imprint wave from thecenter to the edge of the imprint substrate. As a result, any enclosure of air is avoided and afull-field imprint can be obtained. High DOE efficiency requires low total thickness variation(TTV) of both the structure height and the residual layer thickness (RLT). To this end,
accurate control of the normal force over the complete substrate area on a uniformly spincoated imprint resist is implemented before UV curing (see Figure 2). The in-plane rigidity ofthe stamp carrier and the repeated alignment during the imprint process yield excellent
precision of the DOE structures on the stamp to the imprint substrate. In-machine separationof the substrate from the stamp enables high production rates and promotes the suitability ofthis technique for mass production. Furthermore, as this technique can also be applied forcurved imprint substrates, it can be a boost in the production of high quality DOEs at lowcost.
References:

[1] Donald C. O'Shea; Thomas J. Suleski; Alan D. Kathman; Dennis W. Prather: Diffractive
Optics: Design,Fabrication, andTest, SPIE (2014),
ISBN:
9780819451712[2] Jingjing Guo,
Yan Tu, Lanlan
Yang, Lili Wang,
and Baoping Wang:“Holographicwaveguide display
with a combinedgrating in-coupler”,
Applied Optics(2016), 55 (32),9293-9298[3] M. T. Gale:“Replicationtechniques fordiffractive opticalelements”,
Microelectronic
Engineering(1997), 34 (3-4),
p.321-339

Figure 1. Top view SEM image of a binary diffractive optical element using nanoimprint lithography.

Figure 2: SEM side view image of a diffractive optical element on a silicon substrate.