Wafer scale patterning by soft UV-Nanoimprint Lithography

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Abstract

We present first results on wafer scale patterning within one imprint step only, using Soft UV-Nanoimprint Lithography (UV-NIL). In this process, flexible transparent stamps, fabricated by cast moulding ensure a conformal contact, whereas the usage of low viscosity UV-curable resists allows three-dimensional pattern generation at low pressure (<1 bar) and room temperature. Technical aspects that will be covered in this paper include the master and stamp fabrication and a new imprint tool, developed by EV Group, Austria. To demonstrate the applicability of this technique, imprints on 100 mm silicon wafers have been analysed with special emphasis to the residual resist thickness hr. In addition, data on the dimensional stability of the imprint process are presented.

Introduction

The reliable and cost effective lithographic patterning of nanostructures is not only essential for integrated nano-circuits, it also paves the way for the development of new nano-scale devices which will impact wide areas of science and technology. Especially, imprint lithography has the potential to build the bridge between the two contrary requirements of low cost fabrication and high resolution. Although there is a wide variation of imprint techniques, a low pressure approach at room temperature to avoid heating cycles seems favourable to realize short imprint times.

In this field, two main techniques are outstanding in research and development: Soft Lithography [1], [2] and UV-Nanoimprint Lithography (UV-NIL) [3], [4]. Due to the use of flexible stamps, Soft Lithography has demonstrated the potential for full scale wafer patterning in a single step. Soft Lithography modifies only the surface of a monolayer with limitations in fabricating high quality three-dimensional features. On the other hand, UV-NIL utilizes rigid stamps and low viscosity UV-curable resists, thereby enabling high contrast three-dimensional patterning. However, UV-NIL is limited by practicable reasons in imprint area due to the waviness of the substrates.

Soft UV-NIL combines the advantages of these two techniques in order to achieve a uniform three-dimensional pattern relief on wafer scale, allowing short process times and accurate structure transfer. Flexible stamps, made of polycarbonate in combination with UV-curable resist systems, were presented in the Step and Flash technique [5]. Soft UV-NIL, on the other hand, utilizes Polydimethylsiloxane (PDMS) as stamp material due to its attractive properties like low interfacial free energy and high optical transparency [1]. The stamps were fabricated by cast moulding of a rigid master that can be used many times for the fabrication of soft stamps. The usage of low-viscosity UV-curable resists allows three-dimensional patterning at low pressure without any heating cycles [3], [4], [6], [7]. To minimize the risk of deformation of the soft stamps, a low pressure is essential, too. In this paper, first results of this technique will be presented. This includes the demonstration of the process flow up to a wafer size of 100 mm imprinted in a single step as well as evaluations on the residual resist thicknesses of different imprinted samples. Furthermore, the dimensional stability of the stamp and imprinted features is demonstrated by SPM analysis.

Section snippets

Experimental

The developed process flow, shown in Fig. 1, can be divided into the steps outlined below. First, a thin film of photopolymerizable material is deposited via spin-coating on a silicon substrate (Fig. 1(a)). This material is then patterned by imprinting at low pressure using a structured flexible stamp (Fig. 1(b)). Due to its elastomeric character, the stamp adapts its shape to the waviness of the substrate and thereby contacts the whole spin-coated area. In the subsequent process step, the

Results

For all investigations PDMS stamps with a diameter of 100 mm, containing multiple test structures with lateral feature sizes varying between 280 nm up to 20 μm and a depth of 127 nm, have been used. The structures were evenly spread over the stamp surface with a ratio of elevated to recessed pattern of 2:1. A pressure of 500 mbar was chosen for imprinting all 100 mm diameter samples.

A first quantitative analysis of the Soft-UV-NIL process has been carried out on the basis of SPM-scans in

Conclusion

We have developed and qualified an innovative imprint technique for patterning nanostructures on wafer scale with one single imprint. The Soft UV-NIL is based on the usage of transparent PDMS stamps and low viscosity photopolymerizable resist systems. Using a new imprint tool, developed by EV Group, Austria, imprints on 100 mm silicon wafers were performed with a high degree of the residual resist thickness uniformity, essential for accurate structure transfer. The, up to now, demonstrated

Acknowledgments

This work is supported by the European Community under the “Information Society Technologies” Programme IST – 2001-37472. The contribution from Prof. Y. Chen and Dr. E. Roy (LPN-CNRS) is highly acknowledged for their support in the field of the PDMS stamp fabrication. We also thank Dr. W. Henschel (AMO GmbH) for carrying out the EBL process.

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