Name: atomically traceable nanostructure fabrication
Uploaded: 2015-07-17T15:00:00
Description: Watch this Scientific Journal Video about Atomically Traceable Nanostructure Fabrication at JoVE.com

This work was supported by a Contract from DARPA (N66001-08-C-2040) and by a grant from the Emerging Technology Fund of the State of Texas. The authors would like to acknowledge Jiyoung Kim, Greg Mordi, Angela Azcatl, and Tom Scharf for their contributions related to selective atomic layer deposition, as well as Wallace Martin and Gordon Pollock for ex-situ sample processing.

1. Ex-Situ Sample Preparation
<ol>
	<li>Prepare chips
	<ol>
		<li>Design appropriate etch mask to put identifying markers in the Si(100) wafer. Using standard optical lithography and RIE, etch a grid of lines as fiducial marks into the wafer from which STM samples will be taken. The lines should be 10 &#956;m wide, 1 &#956;m deep, and at pitch of 500 &#956;m. After etching, strip remaining photoresist from sample. 
		Note: The fiducial marks must be identifiable in-situ for tip location on the sa...

<ol>
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Performing metrology on the nanostructures described above requires the ability to bridge the tip positioning during HDL and pattern location using other tools such as AFM and SEM. In contrast to other well-developed patterning tools with high-resolution position encoding such as e-beam lithography, the HDL performed here was performed with an STM without well controlled coarse positioning, so extra position identification protocols were used, as shown in Figure 3. First, a long-focal-length microscope i...

As nanotechnology becomes more important in a wide variety of arenas, understanding the structures being formed gains importance, especially in fields of lithography and electronics. To emphasize the importance of metrology at the nanoscale, specifically at scales below 10 nm, it should be pointed out that a variation in feature size of only 1 nm indicates a fractional variation at least 10%. This variation can have significant implications for device performance and material character.1,2&#8211;4 Using synthetic methods, very precisely formed individual features such as quantum dots or other complex molecules can be fabricated,2,5,6 but generally lacking the same precision in feature placement and orientation, despite work toward improving size and placement control. This paper demonstrates an approach for fabricating nanostructures with near atomic size precision and atomic precision in feature placement, as well as with atomic metrology in feature placement. Using the atomic precision of Scanning Tunneling Microscope (STM) induced Hydrogen Depassivation Lithography (HDL), atomically precise patterns with chemically sensitive contrast are formed on a surface. Selective Atomic Layer Deposition (ALD) then applies a hard oxide material in the patterned areas, with Reactive Ion Etching (RIE) ultimately transferring the patterns into the bulk material, as shown schematically in Figure 1. Combining the highly precise HDL process with the standard ALD and RIE processes results in a flexible method to produce nanostructures on a surface with arbitrary shape and positioning.
<img alt="Figure 1" src="/files/ftp_upload/52900/52900fig1.jpg" /> 
Figure 1. Primary Nanofabrication Process Steps. As an example, a 200 nm x 200 nm square is shown. Each circled arrow indicates a step of atmospheric exposure and transport between sites. After UHV sample prep, the sample is patterned using UHV HDL followed by STM metrology (top left). ALD is then performed, followed by AFM metrology (right). RIE transfers the patterns into Si(100), followed by SEM metrology (bottom left). <a href="https://www.jove.com/files/ftp_upload/52900/52900fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The most precise lithography to date usually involves scanned probe techniques, specifically STM-based patterning where atomic resolution patterning and functionalization has been demonstrated for many applications.7 Previously, atom manipulation has produced nanostructures with ultimate precision by using individual atoms as building blocks,8,9,10 but the nanostructures required cryogenic conditions and thus lacked long-term robustness. RT atom manipulation by removal of hydrogen atoms from the surface has been shown, specifically HDL.11,12,13 HDL promises to enable new classes of electronic and other devices based on the spatial localization of surface contrast. Using HDL without further processing, various device architectures are possible including dangling bond wires or logic devices.14,15,16 In addition to providing electrical contrast, HDL can introduce chemical contrast on the surface where the passivating H layer has been removed, in effect creating a template for further chemical modification. This chemical modification has been demonstrated on silicon and other surfaces, showing selectivity for deposition of metals,17 insulators,18 and even semiconductors.16,19 Each of these examples produces two dimensional structures, so other processing steps must be used to produce true three dimensional structures with the atomically resolved control promised by HDL. Previously, this has required repeated patterning,19,20,21 annealing,22 or less well resolved processes such as tip-based e-beam induced deposition.23
Similar to e-beam lithography, HDL uses a localized flux of electrons to expose a resist. Several similarities exist such as the capacity to perform multi-mode lithography with variable spot size and patterning efficiency.24 However, the true power of HDL arises from how it differs from e-beam lithography. First, the resist in HDL is a monolayer of atomic hydrogen so that resist exposure becomes a digital process; the resist atom either is or is not present.25 Since the H atom placement corresponds to the underlying Si(100) lattice the HDL process can be an atomically precise process, although it should be noted that in this paper the HDL has nanometer precision as opposed to having atomic perfection and thus is not digital in this case. Since the electron source in HDL is local to the surface, the various modes of STM operation facilitate both throughput optimization as well as error checking. At tip-sample biases below ~4.5 V, lithography may be performed at the single atom level with atomic precision, known as Atomically Precise mode (AP mode). In contrast, at biases above ~7 V, electrons are emitted directly from the tip to the sample with wide linewidths and high depassivation efficiencies, known here as Field Emission mode (FE mode). HDL throughputs can then be optimized by careful combination of these two modes, although the overall throughputs remain small relative to e-beam lithography with patterning up to 1 &#956;m2/minute possible. When the bias is reversed so that the sample is held at ~-2.25 V, electrons tunnel from the sample to the tip with extremely low depassivation efficiency, thus permitting inspection of the atomic structure of the surface both for error correction and for atomic scale metrology.
This nanostructure fabrication process shown in Figure 1 starts with an UHV-HDL step, as described above. Following HDL, the sample is vented to atmosphere, at which time the patterned areas become saturated with water, forming a thin (i.e., ~1 monolayer) SiO2 layer.26 After transport, the sample is inserted into an ALD chamber for deposition of titania (TiO2), with thicknesses around 2-3 nm deposited here, as measured by AFM and XPS.27 Since the titania reaction depends upon a water saturation of the surface, this process is possible despite atmosphere exposure which saturates the surface with water. Next, to transfer the ALD mask pattern into the bulk the sample was etched using RIE so that 20 nm of Si is removed, with the etch depth determined by AFM and SEM. In order to facilitate metrology steps, an Si(100) wafer is patterned with a grid of lines which are designed to be visible after UHV preparation by a long working distance optical microscope, AFM plan-view optical imaging, and low-magnification plan-view SEM imaging. To help identify the nanoscale structures, 1 &#956;m2 serpentine patterns (serps) are patterned onto the samples with the most isolated nanopatterns located at fixed locations relative to the serps.
This combination of HDL, selective ALD, and RIE can be an important process for nanostructure fabrication, and it includes an atomic scale metrology as a natural byproduct of the process. Below, we include a detailed description of the steps involved to fabricate sub-10 nm nanostructures in Si(100) using HDL, selective ALD, and RIE. It is assumed that one is skilled in each of these processes, but information will be included related to how to integrate the various processes. Particular emphasis will be given to those unexpected difficulties experienced by the authors in order to prevent the same difficulties, especially related to transport and metrology.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>Si Wafer</td>
			<td>VA Semiconductor</td>
			<td></td>
			<td>P type (Boron) Si&#60;100&#62; +/- 2 degrees, 280 mm +/- 25 mm thick, 0.01-0.02 ohm-cm</td>
		</tr>
		<tr>
			<td>Ta foil</td>
			<td>Alfa Aesar</td>
			<td>335</td>
			<td>0.025mm (0.001in) thick, 99.997% (metals basis)</td>
		</tr>
		<tr>
			<td>Methanol</td>
			<td>Alfa Aesar</td>
			<td>19393</td>
			<td>Semiconductor Grade, 99.9%</td>
		</tr>
		<tr>
			<td>2-Propanol</td>
			<td>Alfa Aesar</td>
			<td>19397</td>
			<td>Semiconductor Grade, 99.5%</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Alfa Aesar</td>
			<td>19392</td>
			<td>Semiconductor Grade, 99.5%</td>
		</tr>
		<tr>
			<td>Argon</td>
			<td>Praxair</td>
			<td></td>
			<td>Ultra high purity (grade 5.0)</td>
		</tr>
		<tr>
			<td>Deionized water</td>
			<td>Millipore</td>
			<td>Milli-Q Water Purification System</td>
			<td>&#62;18 MW resistance water produced on demand.</td>
		</tr>
		<tr>
			<td>TiCl4</td>
			<td>Sigma Aldrigh</td>
			<td>254312</td>
			<td>&#8805;99.995% trace metals basis</td>
		</tr>
		<tr>
			<td>O2</td>
			<td>Matheson</td>
			<td>G2182101</td>
			<td>Research Grade</td>
		</tr>
		<tr>
			<td>SF6</td>
			<td>Matheson</td>
			<td>G2658922</td>
			<td>Ultra high purity (grade 4.7)</td>
		</tr>
		<tr>
			<td>Blue Medium Tack Roll</td>
			<td>Semiconductor Equipment Corporation</td>
			<td>18074</td>
			<td>Thickness 75 um / .003&#8221;&#160; Length 200 M / 660&#8217;&#160;</td>
		</tr>
	</tbody>
</table>

atomically traceable nanostructure fabrication

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

In the cases described here, HDL is performed using multi-mode lithography.24 In FE mode, performed with 8 V sample bias, 1 nA, and 0.2 mC/cm (equivalent to 50 nm/sec tip speed), the tip moves over the surface either parallel or perpendicular to the Si lattice, producing lines of depassivation. While this lineshape is very tip dependent, in the case here, the completely depassivated portion of the lines was approximately 6 nm wide, with tails of partial depassivation extending another 2 nm on either side of th...

Watch this Scientific Journal Video about Atomically Traceable Nanostructure Fabrication at JoVE.com

Atomically Traceable Nanostructure Fabrication

We report a protocol for combining the atomic metrology of the Scanning Tunneling Microscope for surface patterning with selective Atomic Layer Deposition and Reactive Ion Etching. Using a robust process involving numerous atmospheric exposures and transport, 3D nanostructures with atomic metrology are fabricated.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process ...

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