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Research Papers

Effect of Silanization Film Thickness in Soft Lithography of Nanoscale Features

[+] Author and Article Information
Lucas H. Ting, Shirin Feghhi, Sangyoon J. Han, Marita L. Rodriguez, Nathan J. Sniadecki

Department of Mechanical Engineering,  University of Washington, Seattle, WA 98195

J. Nanotechnol. Eng. Med 2(4), 041006 (Apr 04, 2012) (5 pages) doi:10.1115/1.4005665 History: Received June 04, 2011; Revised June 19, 2011; Published March 30, 2012; Online April 04, 2012

Soft lithography was used to replicate nanoscale features made using electron beam lithography on a polymethylmethacrylate (PMMA) master. The PMMA masters were exposed to fluorinated silane vapors to passivate its surfaces so that polydimethylsiloxane (PDMS) did not permanently bond to the master. From scanning electron microscopy, the silanization process was found to deposit a coating on the master that was a few hundreds of nanometers thick. These silane films partially concealed the nanoscale holes on the PMMA master, causing the soft lithography process to produce PDMS features with dimensions that were significantly reduced. The thickness of the silane films was directly measured on silicon or PMMA masters and was found to increase with exposure time to silane vapors. These findings indicate that the thickness of the silane coatings is a critical parameter when using soft lithography to replicate nanoscale features, and caution should be taken on how long a master is exposed to silane vapors.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

SEM micrograph of a PMMA master. Holes were created in a 1.9 μm thick PMMA photoresist on a silicon wafer using e-beam lithography. The holes are arranged hexagonally, spaced 3 μm apart, and had an average diameter of 290 nm.

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Figure 2

SEM micrograph of PDMS nanoposts. PDMS was cast from the PMMA master shown in Fig. 1 after silanizing it for 1 h. The replica nanoposts had an average height of 465 nm and diameter of 227 nm, which are smaller than the dimensions of the master. View is at 45 deg off-vertical.

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Figure 3

SEM micrographs of PMMA masters after silanization process. Starting with (a) 200 nm deep PMMA resist, e-beam lithography was used to create 58 nm diameter holes. (b) The masters were then exposed to plasma for 90 s, which widened the holes to 177 nm. (c) They were then silanized for 1 h, which backfilled the holes and narrowed their diameters to 119 nm. The cracks observed on the surface are due to the sputtered metal film, which was needed for SEM imaging.

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Figure 4

Silanization time reduced nanopost diameter. A set of masters were created using e-beam lithography on 1.9 μm thick PMMA photoresist. The masters were plasma treated for 90 s and then silanized for 30 s, 1 h, or 18 h. The diameters of the nanoposts were measured from the SEM micrographs.

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Figure 5

Silanization time reduced nanopost height. The same set of PMMA masters as described in Fig. 4 were treated with plasma for 90 s and then silanized for 30 s, 1 h, or 18 h. The heights of the nanoposts were measured from the SEM micrographs.

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Figure 6

SEM micrograph of PDMS nanoposts cast from a PMMA master silanized for 18 h. Due to silanization, nanoposts were small bumps, had reduced diameters, and almost negligible height. View is at 45 deg off-vertical.

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Figure 7

Measurement of silane film thickness. Flat samples of silicon and PMMA were half-covered with a glass coverslip and silanized for 30 s, 1 h, or 18 h. The height of the deposited silane film was measured using a profilometer.

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