Name: precision milling of carbon nanotube forests using low pressure scanning electron microscopy
Uploaded: 2017-02-05T15:00:00
Description: Watch this Scientific Journal Video about Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy at JoVE.com

This work was supported by the Air Force Office of Scientific Research grant FA9550-16-1-0011 and University of Missouri startup funds. The authors would like to thank the University of Missouri Electron Microscopy Core facility for assistance with SEM imaging and use of patterning equipment and software.

1. Preparation of CNT Forest Sample for Milling
<ol>
	<li>CNT Synthesis
	<ol>
		<li>Deposit 10 nm of aluminum oxide (alumina) on a thermally oxidized silicon wafer using atomic layer deposition13 or other physical vapor deposition methods.</li>
		<li>Deposit 1 nm of iron on the alumina support layer by sputtering14 or other physical vapor deposition method.</li>
		<li>Synthesize CNTs using an established process, such as thermal chemical vapor deposition<...

<ol>
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G., Rack, P. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoscale+electron+beam+induced+etching:+a+continuum+model+that+correlates+the+etch+profile+to+the+experimental+parameters.">Nanoscale electron beam induced etching: a continuum model that correlates the etch profile to the experimental parameters.</a> Nanotechnology. 19 (45), 455306 (2008).</li><li>Amama, P. B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influence+of+Alumina+Type+on+the+Evolution+and+Activity+of+Alumina-Supported+Fe+Catalysts+in+Single-Walled+Carbon+Nanotube+Carpet+Growth.">Influence of Alumina Type on the Evolution and Activity of Alumina-Supported Fe Catalysts in Single-Walled Carbon Nanotube Carpet Growth.</a> ACS Nano. 4 (2), 895-904 (2010).</li><li>Almkhelfe, H., Carpena-Nunez, J., Back, T. C., Amama, P. 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W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Force+sensitive+carbon+nanotube+arrays+for+biologically+inspired+airflow+sensing.">Force sensitive carbon nanotube arrays for biologically inspired airflow sensing.</a> Smart Mater Struct. 21 (9), 094024 (2012).</li><li>Maschmann, M. R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+situ+SEM+Observation+of+Column-like+and+Foam-like+CNT+Array+Nanoindentation.">In situ SEM Observation of Column-like and Foam-like CNT Array Nanoindentation.</a> ACS Appl Mater Inter. 3 (3), 648-653 (2011).</li><li>Pathak, S., Raney, J. 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The protocol details best practices for milling relatively large (micron-scale) features in CNT forests. In general, the material removal rate may be reduced by reducing the acceleration voltage, spot size, and aperture diameter. To trim a specific CNT within a forest, recommended conditions include 5 kV, a spot size of 3, and an aperture that is 50 &#956;m or less in diameter. Note that the milling technique using reduced area rectangles is detailed such that the electron beam rasters the enclosed region only one time. ...

Carbon nanotubes (CNTs) and graphene are carbon-based nanomaterials that have attracted significant attention because of their superior strength, durability, thermal, and electrical properties. Precision machining of carbon nanomaterials has become an emerging topic of research and offers the potential to engineer and manipulate these materials towards a variety of engineering applications. Machining CNTs and graphene requires nanoscale spatial precision to first locate a nanoscale area of interest and then to selectively remove only the material within the area of interest. As an example, consider the machining of vertically oriented CNT forests (also known as CNT arrays). The cross section of CNT forests may be precisely defined by lithographic patterning of catalyst films. The top surface of the vertically oriented forests, however, are frequently poorly ordered with non-uniform height. For surface-sensitive applications such as thermal interface materials, the irregular surface may impede optimal surface contact and reduce device performance. Precision trimming of the irregular surface to create a uniform flat surface could potentially offer better, more repeatable performance by maximizing the available contact area.
Precision machining techniques for nanomaterials frequently do not resemble conventional macroscale mechanical machining technologies such as drilling, milling, and polishing by means of hardened tooling. To date, techniques using energetic beams have been most successful at site-selective milling of carbon nanomaterials. These techniques include laser, electron beam, and focused ion beam (FIB) irradiation. Of these, laser machining techniques provide the most rapid material removal rate1,2; however, the spot size of laser systems is on the order of many microns and is too large to isolate nanometer-scale entities such as a single carbon nanotube segment within a densely populated forest. By contrast, electron and ion beam systems produce a beam that may be focused to a spot that is several nanometers or less in diameter.
FIB systems are specifically designed for nanoscale milling and deposition of materials. These systems utilize an energetic beam of gaseous metal ions (typically gallium) to sputter material from a selected area. FIB milling of CNTs is achievable, but often with unintended byproducts including gallium and carbon redeposition in surrounding regions of the forest3,4. When the technique is used for CNT forests, the redeposited material masks and/or alters the morphology of selected milling region, altering the native appearance and behavior of the CNT forest. The gallium may also implant within the CNT, providing electronic doping. Such consequences often make FIB-based milling prohibitive for CNT forests.
Transmission electron microscopes (TEMs) utilize a finely focused beam of electrons to probe the internal structure of materials. Acceleration voltages for TEM operation typically range from 80-300 kV. Because the knock-on energy of CNTs is 86.4 keV5, the electron energy produced by TEM is sufficient to directly remove atoms from the CNT lattice and induce highly localized milling. The technique mills CNTs with potentially sub-nanometer precision5,6,7; however, the process is very slow &#8211; often requiring minutes to mill a single CNT. Importantly, TEM-based milling approaches require CNTs to first be removed from a growth substrate and dispersed onto a TEM grid for processing. As a result, TEM-based methods are generally not compatible with CNT forest milling in which the CNTs must remain on a rigid substrate.
Milling of CNT forests by scanning electron microscopes (SEMs) has also received attention. In contrast to TEM-based techniques, SEM instruments are typically unable to accelerate electrons with sufficient energy to impart the knock-on energy required to directly remove carbon atoms. Rather, SEM-based techniques utilize an electron beam in the presence of a low-pressure gaseous oxidizer. The electron beam selectively damages the CNT lattice and may dissociate the gaseous ambient into more reactive species such as H2O2 and the hydroxyl radical. Water vapor and oxygen are the most commonly reported gases to achieve selective area etching. Because the SEM-based techniques rely on a multiple-step chemical process, numerous processing variables may influence the milling rate and precision of the process. It has been previously observed that increasing acceleration voltage and beam current directly increase the milling rate because of an increased energy flux, as expected11. The effect of chamber pressure is less obvious. A pressure that is too low suffers from a deficiency of the oxidizing agent, decreasing the milling rate. Further, an over-abundance of gaseous species scatters the electron beam and decreases the electron flux in the milling region, also decreasing the material removal rate.
To estimate the carbon removal rate, an approach similar to that used by Lassiter and Rack12 was employed, whereby electrons interact with precursor molecules near the surface to generate reactive species that etch the substrate surface. From this model, the etch rate is estimated as
<img alt="Equation" src="/files/ftp_upload/55149/55149eq1.jpg" />
where NA is the surface concentration of the etchant species, Z is the surface concentration of available reaction sites, x is a stoichiometry factor relating the volatile etching products generated relative to the reactants, A&#963; represents the probability of generating the desired etching species from an electron-water vapor collision, and &#915;e is the electron flux at the surface. The factors of x and A&#963; are assumed to be unity, while Z is assumed to be nearly constant and significantly larger than NA. Further details may be found in our previous work.11
In this article, a procedure is explored that uses low-pressure water vapor within a SEM to mill regions ranging from individual CNTs to large volume (tens of cubic micrometers) material removal. Here we demonstrate the technique used to mill CNT forests using an ESEM by the use of reduced area rectangles, horizontal line scans, and software-controlled rastering of the electron beam. Additional software and hardware are required for pattern generation, as outlined in the Materials List. Emphasis is placed on removing relatively large (100&#39;s of cubic microns) material volume from a CNT forest, so the following processing conditions are relatively aggressive.
When handling the sample and the sample stub, it is important to wear disposable nitrile gloves. This will prevent oils from being transferred to the stub or sample and consequently deteriorating the effectiveness of the pumps.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>100 mm diameter silicon wafer with 1 micron thermal oxide</td>
			<td>University Wafer</td>
			<td></td>
			<td>Beginning substrate</td>
		</tr>
		<tr>
			<td>Iron sputter target</td>
			<td>Kurt J. Lesker</td>
			<td>EJTFEXX351A2</td>
			<td>Sputter target&#160;</td>
		</tr>
		<tr>
			<td>Savannah 200</td>
			<td>Cambridge</td>
			<td></td>
			<td>For atomic layer deposition of alumina</td>
		</tr>
		<tr>
			<td>Quanta 600F Environmental SEM</td>
			<td>FEI</td>
			<td></td>
			<td>Environmental scanning electron microscope used to support a low-pressure water vapor ambient environment for CNT forest milling</td>
		</tr>
		<tr>
			<td>xT Microscope Control software</td>
			<td>FEI</td>
			<td>4.1.7</td>
			<td>Control software used on Quanta 600F ESEM</td>
		</tr>
		<tr>
			<td>Nanometer Pattern Generation System - Software</td>
			<td>JC Nabity Lithography Systems</td>
			<td>Version 9</td>
			<td>Software used for electron-beam lithography</td>
		</tr>
		<tr>
			<td>Dedicated computer with PCI516 Lithography board</td>
			<td></td>
			<td></td>
			<td>Equipment used for electron-beam lithography</td>
		</tr>
		<tr>
			<td>DesignCAD software</td>
			<td></td>
			<td>V 21.2</td>
			<td>Optional equipment used to generate patterns for electron-beam lithography</td>
		</tr>
		<tr>
			<td>E-beam lithography mount</td>
			<td>Ted Pella</td>
			<td>16405</td>
			<td>Electron beam lithography mount with a Faraday cup and gold nanoparticles on carbon tape</td>
		</tr>
		<tr>
			<td>Picoammeter</td>
			<td>Keithley</td>
			<td>6485</td>
			<td>Used with the Faraday cup to quantify beam current</td>
		</tr>
		<tr>
			<td>12.7 mm diameter SEM stub</td>
			<td>Ted Pella</td>
			<td>16111</td>
			<td>SEM stub</td>
		</tr>
		<tr>
			<td>45 degree pin stub holder</td>
			<td>Ted Pella</td>
			<td>15329</td>
			<td>Optional equipment used to mill the cross section of a CNT forest</td>
		</tr>
	</tbody>
</table>

precision milling of carbon nanotube forests using low pressure scanning electron microscopy

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning electron microscope (ESEM) operating with a low pressure water vapor ambient. In this technique, a portion of the electron beam interacts with the water vapor in the vicinity of the CNT sample, dissociating the water molecules into hydroxyl radicals and other species by radiolysis. The remainder of the electron beam interacts with the CNT forest sample, making it susceptible to oxidation from the chemical products of radiolysis. This technique may be used to trim a selected region of an individual CNT, or it may be used to remove hundreds of cubic microns of material by adjusting ESEM parameters. The machining resolution is similar to the imaging resolution of the ESEM itself. The technique produces only small quantities of carbon residue along the boundaries of the cutting zone, with minimal effect on the native structural morphology of the CNT forest.

The ESEM technique was used to mill a CNT forest synthesized using thermal CVD15,16. Selected area removal of a few CNTs from within a forest is shown in Figure 211. For this demonstration, parameters include 5 kV, spot size of 3, 11 Pa, 170,000X magnification, 2 ms dwell time, and an aperture of 30 &#181;m.
To demonstrate a large...

Watch this Scientific Journal Video about Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy at JoVE.com

Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy

Low pressure scanning electron microscopy in a water vapor ambient is used to machine nanoscale to microscale features in carbon nanotube forests.

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning ...

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