A Moire ´ method for high accuracy alignment in nanoimprint lithography M. Mu ¨ hlberger a, * , I. Bergmair a,c , W. Schwinger a , M. Gmainer a , R. Scho ¨ ftner a , T. Glinsner b , Ch. Hasenfuß c , K. Hingerl c , M. Vogler d , H. Schmidt e , E.B. Kley e a Profactor GmbH, Im Stadtgut A2, 4407 Steyr, Austria b EV Group, DI E. Thallner Str. 1, 4782 St. Florian/Inn, Austria c CD Laboratory of Surface Optics, Altenbergerstr. 69, 4040 Linz, Austria d Micro Resist Technology GmbH, Ko ¨ penicker Str. 325, 12555 Berlin, Germany e Friedrich-Schiller-Universita ¨ t, Institute of Applied Physics, Max-Wien Platz 1, 07743 Jena, Germany Available online 1 February 2007 Abstract Nanoimprint lithography (NIL) is a cost efficient technique for the mass production of nanostructures. We demonstrate alignment accuracies in the range of 100 nm and below in UV-based nanoimprint lithography (UV-NIL) using a simple optical technique. The advantages of this technique are the relative simplicity of the marker-design and the whole setup combined with the possibility of an upgrade of existing equipment and still ultra-high precision alignment capabilities. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Nanoimprint lithography; Alignment; Moire ´ 1. Introduction Our goal is to use nanoimprint lithography to fabricate several layers of a photonic crystal in woodpile structure on top of each other [1,2]. Fig. 1 shows a schematic sketch of such a structure. The period d of the structure designed for a wavelength of 1.5 lm is around 600 nm. In order to achieve a photonic bandgap the 3rd layer has to be aligned to the 1st with a precision of better than d/4 i.e. 150 nm. In a first step it is therefore essential to achieve precise align- ment of the subsequent layers. This should be done by a simple method which can be implemented in commercially available UV-NIL equipment. 2. UV-NIL setup In the UV-NIL process a nanostructured stamp is brought into contact with a UV-curable polymer under well controlled conditions. UV hardening of the polymer and removal of the stamp transfer the pattern into the polymer for possible further processing. The stamp is usu- ally structured by e-beam lithography and reactive ion etching and coated with an anti-adhesive layer. UV-NIL therefore allows the rapid production of nanoscale struc- tures in a fast and parallel process. We work with an EVG Ò 620 nanoimprinter using a 10· flat objective video optics setup and a motorized alignment stage with a minimal step width of 100 nm. The experi- ments have been performed using SiO 2 stamps and glass substrates. The stamps with a size of 25 · 25 mm 2 typically contain structures with 400 nm or 200 nm depth and 200– 600 nm width. The stamp is 2400 lm thick. A scanning electron micrograph of an imprint can be seen in Fig. 2. For the woodpile photonic structure we need line and space patterns as shown. This imprint was made on a spin coated 100 mm Si wafer using UVCur06xp from micro resist technology GmbH, the residual layer thickness h r is smaller than 10 nm, which is a prerequisite for subsequent pattern transfer into the substrate by reactive ion etching. 0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.081 * Corresponding author. E-mail address: michael.muehlberger@profactor.at (M. Mu ¨ hlberger). www.elsevier.com/locate/mee Microelectronic Engineering 84 (2007) 925–927