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Organometallic Resists for Improved Pattern Transfer by Nanoimprint Lithography

July 02, 2009
Acikgoz, et. al., reported on their investigation of organometallic polymers as a means to enhance the etch resistivity of NIL resists for pattern transfer. A high molar mass PFMPS resist exhibited high fidelity pattern transfer over 2 cm x 2 cm areas.


Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network

Acikgoz Figure 5
(a) SEM image of sidewalls obtained after etching into the substrate for 1 m wide lines. (b) Large-area SEM image of a sample.
Nanoimprint lithography (NIL) is a demonstrated nanomanufacturing process for a range of applications which can create patterns with feature sizes as small as 10 nm using hard NIL molds. Subsequent process steps enable the transfer of the patterns into the substrate by reactive ion etching (RIE) or the formation of structures on the substrate by electrodeposition, for example.  A hard mold stamp can be used many times, providing a versatile, low cost, high-throughput approach for patterning repeatable features between 10 nm and 1µm.

Of the various imprint lithography techniques, thermal NIL offers the benefits of repeatability, pattern fidelity, and overall versatility. Thermal NIL essentially presses a hard stamp or mold into a thin layer of polymer resist, and heats the polymer to a temperature approximately 80°C above the glass transition temperature (Tg). The mold and resist are then cooled below the Tg, and followed by demolding of the stamp, thereby leaving the pattern in the resist layer. Pattern transfer to the substrate below is accomplished by RIE using appropriate process gases and conditions.

Typical NIL resists suitable for many processes include polystyrene (PS) or poly(methylmethacrylate) (PMMA). For applications requiring pattern transfer into the substrate by RIE, PMMA and PS breakdown readily when exposed to reactive species, and therefore limit the aspect ratio and fidelity of patterns transferred using these resists as an etch mask. To circumvent this problem, alternative approaches which use a hard mask material, such as a metal film, to enhance the etch selectivity of the mask materials have been demonstrated. However, this approach requires extra process steps and still has issues with pattern fidelity and reproducibility.

In a recent issue of Nanotechnology, Acikgoz, et. al., reported on their investigation of organometallic polymers as a means to enhance the etch resistivity of NIL resists for pattern transfer. In this work, the authors synthesized and studied the properties of poly(ferrocenylmethylphenylsilane) (PFMPS) as a NIL resist. PFMPS is a class of organometallic polymers that exhibits diverse and interesting properties. With both silicon and iron atoms in the main chain, PFMPS has low Tg, readily forms homogeneous films, and has high etch resistivity to plasma processes. The authors synthesized both low molar mass and high molar mass resists employing anionic polymerization and transition-metal-catalyzed polymerization methods respectively.

The low molar mass resist showed limitations during NIL patterning due to incomplete filling of the cavity and dewetting during the imprint step. As such, the patterns lift during the demolding step, resulting in incomplete feature definition. In contrast, the high molar mass resist exhibited no dewetting, and high fidelity pattern transfer was achieved of 2 cm x 2 cm areas.

The next issue in achieving reproducible pattern transfer is the removal of the residual layer remaining in the patterns after demolding. The authors showed that the process conditions during stamping can be optimized to minimize the thickness of the remaining layer. Furthermore, the authors demonstrated that physical plasma etching using Argon ion bombardment effectively removes the residual layer without significantly degrading the masking pattern. This is in contrast to traditional RIE processes that react with the polymer surface making it more difficult to uniformly remove the residual layer. The authors then converted to RIE reactants suitable for silicon etching and demonstrated pattern transfer having both anisotropic and high fidelity features.

While additional refinements of the process are necessary to further optimize the performance of organometallic resists for specific applications employing NIL, the development of etch resistant polymers addresses a critical need for enhancing the manufacturing readiness of these techniques by offering a more versatile and diverse materials platform.

Image reproduced with permission from Acikgoz C, Hempenius MA, Vansco GJ, and Huskens J. 2009. Direct surfact structuring of organometallic resists using nanoimprint lithography. Nanotechnology 20(13):135304. DOI:10.1088/0957-4484/20/13/135304. Copyright 2009 Institute of Physics.

Last updated: August 10, 2011

DOI: 10.4053/er222-090702

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Tags: Nanopatterning/Lithogrpahy, Reactive ion etching

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