Nanoimprint lithography: An old story in modern times? A review

Schift, Helmut

doi:doi:10.1116/1.2890972

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Journal of Vacuum Science & Technology B / Volume 26 / Issue 2 / Review Article

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J. Vac. Sci. Technol. B 26, 458 (2008); http://dx.doi.org/10.1116/1.2890972 (23 pages)

Nanoimprint lithography: An old story in modern times? A review

Helmut Schift

Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

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(Published online 27 March 2008)

Abstract

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Nanoimprint lithography (NIL) is a high throughput, high-resolution parallel patterning method in which a surface pattern of a stamp is replicated into a material by mechanical contact and three dimensional material displacement. This can be done by shaping a liquid followed by a curing process for hardening, by variation of the thermomechanical properties of a film by heating and cooling, or by any other kind of shaping process using the difference in hardness of a mold and a moldable material. The local thickness contrast of the resulting thin molded film can be used as a means to pattern an underlying substrate on wafer level by standard pattern transfer methods, but also directly in applications where a bulk modified functional layer is needed. Therefore it is mainly aimed toward fields in which electron beam and high-end photolithography are costly and do not provide sufficient resolution at reasonable throughput. The aim of this review is to play between two poles: the need to establish standard processes and tools for research and industry, and the issues that make NIL a scientific endeavor. It is not the author’s intention to duplicate the content of the reviews already published, but to look on the NIL process as a whole. The author will also address some issues, which are not covered by the other reviews, e.g., the origin of NIL and the misconceptions, which sometimes dominate the debate about problems of NIL, and guide the reader to issues, which are often forgotten or overlooked.

ACKNOWLEDGMENTS

The basis for writing this review was the development of thermal NIL over the past ten years in Paul Scherrer Institut (PSI). Already before, L. Baraldi developed a hot microembossing process for integrated optics.^{11 , 49} This had already many of the “ingredients” of NIL, including low residual layer printing under vacuum and the buildup of a setup with real-time observation of polymer flow. Many thanks are due to all those researchers, engineers, technicians, and students, who contributed to the continuous development of the NIL technology in the Laboratory for Micro- and Nanotechnology (LMN) at PSI, particularly to K. Vogelsang, L. Heyderman, R. Jaszewski, and S. Park. Within NaPa,²⁴² special thanks go to the NaPaNIL committee consisting of C. Sotomayor-Torres, A. Kristensen, and M. Tormen; furthermore to J. Ahopelto, D. Mendels, S. Zaitsev, F. Reuther, I. Garcia Romero, and K. Seunarine, and to all contributors to the NaPa Library of Processes.²⁴⁷ The author also profited much from exchanges with H.-C. Scheer, Y. Hirai, and F. Houle, and particularly indebted to S. Y. Chou who opened this exciting field and always pushed him toward new scientific challenges. This review was done within the framework of the INKA-Institut (Institute of Nanotechnological Applications in Polymers), co-founded by the University of Applied Sciences Nordwestschweiz and the PSI. The partial funding of the Swiss Federal Office for Science and Education in the framework of the EC-funded project NaPa (Contract No. NMP4-CT 2003-500120 and OFES No. 03.0424-4) is gratefully acknowledged.

Article Outline

INTRODUCTION
VARIANTS OF NANOIMPRINT LITHOGRAPHY
1. Nanoimprint lithography: A definition
2. Characteristics of mechanical deformation
3. Dimensional issues
SPECIFIC QUESTIONS
1. Thermal NIL processes
  1. Thermoplastic resist materials
  2. Implications of molecular weight and viscosity
2. UV-NIL processes
  1. Spin coating and multilayer films
  2. Coating by droplet dispensing
  3. UV-NIL materials
  4. Reverse tone NIL
  5. Thermoplastic UV-curable materials
3. Hard and soft tool concepts
  1. High-pressure tools in thermal NIL
  2. Low-pressure tools in UV-NIL
  3. Stamp copies and contamination control
  4. Intermediate stamps and combined thermal and UV-NIL
4. Surfaces and interfaces
  1. Antiadhesive coatings based on fluorinated silane chemistry
  2. Template contamination in UV-NIL
  3. Assessment of the release-layer quality
5. Demolding
  1. Parallel and peel demolding
  2. Adhesion, friction, and local geometry effects
  3. Lateral shrinkage and global geometry effects
6. Unwanted effects and unsolved questions
  1. Zero residual layer imprint
  2. Room temperature imprint
CONCLUSION
1. Relevance
2. Improving the NIL process

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KEYWORDS and PACS

Keywords

cooling, curing, electron beam lithography, hardness, mechanical contact, nanolithography, nanopatterning, photolithography, surface hardening, thermomechanical treatment, thin films, transfer moulding, silicon, polymer, fused silica, silane

PACS

81.16.Nd

Micro- and nanolithography
81.16.Rf

Micro- and nanoscale pattern formation
85.40.Hp

Lithography, masks and pattern transfer

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History

Received 30 July 2007
Accepted 13 February 2008
Published online 27 March 2008

Digital Object Identifier

http://dx.doi.org/10.1116/1.2890972

PUBLICATION DATA

ISSN

1071-1023 (print)

1520-8567 (online)

Publisher

American Vacuum Society

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