Nanoimprint Lithography for Next-Generation Carbon Nanotube-Based Devices

Abstract

This research reports the development of 3D carbon nanostructures that can provide unique capabilities for manufacturing carbon nanotube (CNT) electronic components, electrochemical probes, biosensors, and tissue scaffolds. The shaped CNT arrays were grown on patterned catalytic substrate by chemical vapor deposition (CVD) method. The new fabrication process for catalyst patterning based on combination of nanoimprint lithography (NIL), magnetron sputtering, and reactive etching techniques was studied. The optimal process parameters for each technique were evaluated. The catalyst was made by deposition of Fe and Co nanoparticles over an alumina support layer on a Si/SiO₂ substrate. The metal particles were deposited using direct current (DC) magnetron sputtering technique, with a particle ranging from 6 nm to 12 nm and density from 70 to 1000 particles/micron. The Alumina layer was deposited by radio frequency (RF) and reactive pulsed DC sputtering, and the effect of sputtering parameters on surface roughness was studied. The pattern was developed by thermal NIL using Si master-molds with PMMA and NRX1025 polymers as thermal resists. Catalyst patterns of lines, dots, and holes ranging from 70 nm to 500 nm were produced and characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Vertically aligned CNTs were successfully grown on patterned catalyst and their quality was evaluated by SEM and micro-Raman. The results confirm that the new fabrication process has the ability to control the size and shape of CNT arrays with superior quality.

Keywords: carbon nanotubes; chemical vapor deposition; magnetron sputtering; nanoimprint lithography; reactive ion etching.

Figure 1

Catalyst patterning processes: (A) “negative” and (B) “positive” pattern replica/transfer.

Figure 2

SEM images of imprinted substrate (a) and (b) catalyst pattern (Process A).

Figure 3

Imprinted substrate (a) and (b) catalyst pattern (Process B).

Figure 4

Spin-curve for NRX1025 (a) 2.5% and (b) 7% solution.

Figure 5

Spin-curve for PMMA mr-I 35k thermal resist.

Figure 6

AFM study of catalyst deposition. DC sputtering at working pressures: (a) 1 mTorr, (b) 2 mTorr, (c) 4 mTorr, and (d) 6 mTorr, respectively.

Figure 7

Plasma etching rates for NRX1025: (a) effect of plasma power, (b) effect of oxygen content.

Figure 8

Plasma etching rates for PMMA.

Figure 9

AFM study of plasma etching effect on thermal resist.

Figure 10

SEM study of plasma etching effect: (a) imprinted; (b) etched for 5 min; (c) etched for 8 min.

Figure 11

SEM images of short CNTs arrays fabricated by first approach (Process A) with the catalyst patterned by stamp with holes of 290 nm diameter.

Figure 12

SEM images of short CNTs arrays fabricated by first approach (Process A) with the catalyst patterned by stamp with lines of 500 nm width.

Figure 13

SEM images of short CNTs arrays fabricated by a second approach (Process B) with the catalyst patterned by stamp with dots of 160 nm diameter.

Figure 14

SEM images of long CNTs arrays fabricated by the first approach (Process A) with the catalyst patterned by stamp with lines of 70 nm width, 140 nm pattern period. Top view of a CNT arrays at high (a) and low (b) magnifications. Catalyst patterns under CNT arrays (c) SEM image and (d) AFM topography map.

Figure 15

SEM images of long CNTs arrays fabricated by the second approach (Process B) with the catalyst patterned by stamp with lines of 455 nm width, 843 nm pattern period. Top view of a CNT arrays at (a) low and (b) high magnifications.

Figure 16

HR-SEM images of individual CNTs in array at (a) 200k and (b) 500k magnifications.

Figure 17

Raman spectra of reference and patterned CNT arrays.