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 sample as well as in AFM and SEM during metrology.</li>
		<li>Protect wafer surface by applying standard tack blue dicing tape, tacky side down.</li>
		<li>Dice wafers into chips using a diamond tipped ceramic dicing saw into sizes appropriate for the specific UHV-STM tool being used to perform HDL. Here, the samples were 8.1 mm x 8.1 mm squares.</li>
	</ol>
	</li>
	<li>After dicing, prepare chip for insertion into UHV-STM patterning tool by gently peeling back the dicing tape, taking care not to touch chip with any nickel-containing tools which will induce an unfavorable surface reconstruction after UHV prep in section 2 below.
	<ol>
		<li>Clean chip by rinsing the front face for 10 sec each with streams of acetone, isopropyl alcohol, methanol, and deionized water, respectively, while gripping the sides of the sample with polytetrafluoroethylene (PTFE) tweezers. Finally, dry with ultrapure N2 or Ar, still gripping with Teflon tweezers.</li>
		<li>Mount sample chip in STM sample holder using methods appropriate for the specific UHV-STM tool to be used for HDL.
		<ol>
			<li>If mounting in a nickel-containing sample holder, cut tantalum foil barrier strips (appx. 4 mm by sample height) using titanium scissors. Sonicate foil strips for 5 min each in ultrapure acetone, isopropyl alcohol, methanol, and DI water, respectively. Dry with ultrapure N2 gas by partially covering a beaker with aluminum foil and injecting an N2 nozzle into the opening. Flow gas until all liquid has evaporated. Using nickel free tweezers, form foil around ends of sample then clamp in sample holder to isolate front and back sides of sample from sample holder.</li>
		</ol>
		</li>
		<li>After mounting, plasma clean sample and sample holder in oxygen plasma to remove carbon contamination.28</li>
	</ol>
	</li>
</ol>
2. UHV Sample Preparation
<ol>
	<li>Introduce sample into UHV system via load-lock or another preferred UHV-safe method so that the UHV processing and HDL can typically remain below 1.3 x 10-9 mbar (except for step 2.5.1.1 below).</li>
	<li>Degas O/N at 650 &#176;C, monitoring the temperature with a pyrometer. Ensure that the chamber pressure is within 25% of background. In the case described here, typical background pressures are approximately 4.5 x 10-10 mbar.</li>
	<li>Degas tungsten hydrogen cracking filament at 1,500 &#176;C for 5 min. Activate titanium sublimation pumps if possible.</li>
	<li>Perform sample &#8220;Flash&#8221; cycle.
	<ol>
		<li>Flash sample by heating to 1,250 &#176;C for 20 sec, monitoring T with pyrometer and using a 10 &#176;C/sec heating gradient. Do not exceed a maximum pressure of 7 x 10-9 mbar. Cool by removing heating current in less than 5 sec.</li>
		<li>Rest sample at 350 &#176;C for 5 min. to allow pressure to recover to baseline. Repeat 3x.</li>
	</ol>
	</li>
	<li>Perform sample passivation.
	<ol>
		<li>Set sample temperature to 350 &#176;C using a pyrometer to monitor the temperature. Introduce 1.3 x 10-6 mbar ultra-pure H2 into prep chamber using leak valve.
		<ol>
			<li>Put a cold trap on the H2 line very near a leak valve in the system in order to further purify H2.</li>
			<li>If possible, pump system with high-velocity turbo pump instead of an ion pump. This is so the ion pump does not contaminate the sample from being exposed to the high pressured and ejecting contaminants. The pumps are put back in the normal state after most the hydrogen has been removed and the sample is still warm (350 &#176;C).</li>
		</ol>
		</li>
		<li>Turn on tungsten hydrogen cracking filament to a temperature of 1,400 &#176;C for 12 min, then turn off filament and H2 gas flow. Cool sample to RT.</li>
	</ol>
	</li>
</ol>
3. Scanning Tunneling Microscopy and Lithography
<ol>
	<li>Transfer sample to STM, and bring sample and STM tip into tunneling range. Using a camera with resolving power of better than a 20 &#956;m spot size, take high-resolution optical image of tip-sample junction. Deskew and resize the optical image so that it represents an undistorted reproduction of the fiducial marks.</li>
	<li>Remove any illumination to reduce system thermal instabilities. Determine surface quality.
	<ol>
		<li>Using conventional STM techniques with sample bias of -2.25 V and 200 pA, identify the number of defects on the surface.</li>
		<li>If surface defects are below acceptable levels, move to the next step. Maximum acceptable defect levels are as followed: Dangling bonds of 1%; Si vacancies of 3%; contaminants of 1%.</li>
	</ol>
	</li>
	<li>Design HDL patterns to be produced, including both experimental patterns and large (1 &#956;m x 1 &#956;m) serp identification patterns. The serp patterns should be drawn with a long vector axis perpendicular to expected AFM fast scan axis, using a pitch of 15-20 nm. Fracture overall pattern into fundamental shapes to define the path followed by the tip when applying HDL conditions.24</li>
	<li>Using vector outputs from the previous step, perform HDL using FE mode lithography for large areas with sample bias of 7-9 V, current of 1 nA, and 0.2 mC/cm and AP mode lithography for small areas or those areas requiring atomic precision edges.24
	<ol>
		<li>Determine optimal AP mode lithography conditions by performing HDL with an variety of conditions with sample bias ranging from 3.5 to 4.5 V, current ranging from 2 to 4 nA, and line dosages ranging from 2 to 4 mC/cm. Choose conditions that produce the narrowest completely depassivated line.</li>
	</ol>
	</li>
	<li>Perform STM metrology on desired HDL patterned areas by imaging at -2.25 V sample bias and 0.2 nA tunneling current.
	<ol>
		<li>[optional] Perform error correction. After STM metrology, compare depassivation pattern in the STM images with the desired pattern from step 3.5. If any areas show insufficient dangling bond formation, repeat the lithography cycle in those areas.</li>
	</ol>
	</li>
	<li>After completing STM HDL, disengage tip from sample, and move sample to load lock.</li>
	<li>Vent and remove sample. During vent with ultrapure N2, protect sample by contacting it with inert, flat substrate such as clean sapphire.29 After surface protection, close valves to any pumps then introduce vent gas as quickly as possible. Remove sample from system.</li>
</ol>
4. Sample Transport
<ol>
	<li>Remove sample from load lock. Release sample from sample holder, including removing the foil barrier pieces if possible. Using PTFE (or Ti) tweezers, move sample to transporter, keeping front side of sample protected, attempting to keep atmosphere exposure to less than 10 min.</li>
	<li>Install cover over sample and loosely assemble a pressurized sample transporter. Flush with ultrapure Ar for 1 min. Do not evacuate system at any point, or surface damage may occur.29 Finally, seal sample transporter with a small positive pressure (~50-100 mbar) of Ar so that the sample remains stable for up to a month.</li>
</ol>
5. Atomic Layer Deposition
<ol>
	<li>Ensure the ALD is at the proper deposition temperature (100 &#176;C). Slowly increase the argon pressure in the ALD chamber until atmospheric pressure has been achieved.</li>
	<li>Open ALD chamber.</li>
	<li>Open sample transporter and quickly transfer to ALD chamber using a pair of PTFE tweezers grasping the sample on edge, then close the ALD chamber and purge using a flow of argon at a pressure of &#60; 2 &#215; 10-1 mbar for 1 hr to degas the sample. Set a process to perform 80 repeated cycles of ALD to grow 2.8 nm of amorphous titania.
	<ol>
		<li>Using an unmodified commercial ALD system at a sample temperature of 100-150 &#176;C, with titanium tetrachloride (TiCl4) and water (H2O) as precursors perform deposition with reactants at pressures of 0.3 mbar and 0.8 mbar, with pulse times of 0.1 sec and 0.05 sec, respectively.27</li>
		<li>Following each gas pulse, purge the reactor with Ar for 60 sec at 0.2 mbar to ensure minimal background deposition due to physisorbed reactants. For the masks in this work, 80 ALD cycles are used to grow approximately 2.8 nm of amorphous titania at 100 &#176;C.</li>
	</ol>
	</li>
	<li>Slowly vent ALD chamber with Ar gas and open. Repeat steps 4.1 and 4.2 for sample transport.</li>
</ol>
6. Atomic Force Microscopy (AFM)
<ol>
	<li>Ensure proper calibration of the AFM according to manufacturer&#8217;s protocol. Open sample transporter and gently remove sample using tweezers.</li>
	<li>After removing from transporter, securely install the sample into the AFM using a mechanical mounting method such as a clamping system if possible. Focus the AFM camera onto the sample, and locate the fiducial markings on the sample surface to align the AFM tip to the pattern based on the optical metrology in step 3.2.</li>
	<li>Ensure that the AFM settings will show both the height and phase information and set the scan size to be between 20 and 40 &#956;m wide. Engage the AFM tip onto the sample.</li>
	<li>Using the height and phase information at highest resolution, scan until the locator patterns region are identified. Zoom into the patterned region and locate the region in which an image is desired.</li>
	<li>Take an image of the desired regions using the appropriate image quality and resolution (typically the highest possible). Once all desired regions have been scanned, disengage the tip from the sample. Unload the sample. Repeat steps 4.1 and 4.2 for sample transport.</li>
</ol>
7. Reactive Ion Etching
<ol>
	<li>Chill capacitively coupled RIE reactor to -110 &#176;C, then load the sample into its intro chamber and pump down to 7.5 x 10-6 mbar.
	<ol>
		<li>Stabilize the temperature for 3 min. Then turn the gas on with flow rates of O2 at 8 sccm (standard cubic centimeter per minute), Ar at 40 sccm, and SF6 at 20 sccm.</li>
		<li>Strike plasma using a 150 W RF discharge, then modify the gas flow to etch for 1 min using SF6 at 52 sccm and O2 at 8 sccm. Interaction of these gases with Si will etch at a rate of approximately 20 nm/min.30</li>
		<li>Pump vacuum to 7.5 x 10-6 mbar. Vent RIE system. Repeat steps 4.1 and 4.2 for sample transport.</li>
	</ol>
	</li>
</ol>
8. Scanning Electron Microscopy (SEM)
<ol>
	<li>SEM Sample Mounting and introduction of sample to system.
	<ol>
		<li>Place non-adhesive sample mount into a convenient location to facilitate sample mounting.</li>
		<li>Open sample transporter and gently remove sample using tweezers to grab the sample by the edges and securely install into SEM mount, then introduce sample assembly into SEM.</li>
		<li>Vent and open SEM chamber. Securely install sample assembly to the SEM sample stage. Pump down SEM chamber.</li>
	</ol>
	</li>
	<li>Locate the fiducials.
	<ol>
		<li>Bring the magnification to lowest possible value. Select the accelerating voltage and beam current settings that would give good resolution. Start with lowest acceptable settings. Go up as needed.</li>
		<li>Turn on the electron beam. Bring the general region of interest to recommended working distance and eucentric height.</li>
		<li>Locate and focus on fiducials described in section 1.1. Adjust working distance as necessary. Optimize the focus, brightness and contrast.</li>
	</ol>
	</li>
	<li>Locate and image the patterns.
	<ol>
		<li>Relative to the fiducials, locate the patterns based on the optical metrology of section 3.2 and AFM metrology of section 6. To minimize carbon deposition on patterns, optimize focus using nearby non-essential features. Once optimized, move to the patterns and acquire plan view images and measurements.</li>
		<li>If needed, tilt the sample for 3D images and pattern height measurements. For other patterns, repeat from 8.3 as needed.</li>
	</ol>
	</li>
	<li>Perform SEM system closing routine and dismount sample/s, as prescribed by the SEM manufacturer. Secure the sample back into the transporter.</li>
	<li>Repeat steps 4.3 and 4.4 for sample transport and storage. At this point, the samples are robust and can be stored for an indefinite period of time. Perform post-imaging analyses if necessary.</li>
</ol>

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J., Pok, W., 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=Realization+of+Atomically+Controlled+Dopant+Devices+in+Silicon.">Realization of Atomically Controlled Dopant Devices in Silicon.</a> Small. 3 (4), 563-567 (2007).</li><li>Li, K., Namboodiri, P., 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=Controlled+formation+of+atomic+step+morphology+on+micropatterned+Si+(100).">Controlled formation of atomic step morphology on micropatterned Si (100).</a> J. Vac. Sci. Tech. B. 29 (4), 041806 (2011).</li><li>Ballard, J. B., Owen, J. H. G., 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=Pattern+transfer+of+hydrogen+depassivation+lithography+patterns+into+silicon+with+atomically+traceable+placement+and+size+control.">Pattern transfer of hydrogen depassivation lithography patterns into silicon with atomically traceable placement and size control.</a> Journal of Vacuum Science and Technology B. 32 (4), 041804 (2014).</li><li>Gusev, E. P., Cabral, C., Copel, M., D&#8217;Emic, C., Gribelyuk, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=U+ltrathin+HfO+2+films+grown+on+silicon+by+atomic+layer+deposition+for+advanced+gate+dielectrics+applications.">U ltrathin HfO 2 films grown on silicon by atomic layer deposition for advanced gate dielectrics applications.</a> Microelectronic Engineering. 69, 145-151 (2003).</li></ol>

The authors have nothing to disclose.

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 ...

atomically-traceable-nanostructure-fabrication

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Engineering

8.6K 조회수.  Zyvex Labs. 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. 

비디오: Atomically Traceable Nanostructure Fabrication

Fabrication of Silica Ultra High Quality Factor Microresonators

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the quality factor (Q) of the cavity, can be in excess of 500ns for cavities with Q factors above 100 million. As a result of their low material losses, silica microcavities have demonstrated some of the longest photon lifetimes to date1-2. Since a portion of the circulating light extends outside the resonator, these devices can also be used to probe the surroundings. This interaction has enabled numerous experiments in biology, such as single molecule biodetection and antibody-antigen kinetics, as well as discoveries in other fields, such as development of ultra-low-threshold microlasers, characterization of thin films, and cavity quantum electrodynamics studies.3-7
The two primary silica resonant cavity geometries are the microsphere and the microtoroid. Both devices rely on a carbon dioxide laser reflow step to achieve their ultra-high-Q factors (Q&gt;100 million).1-2,8-9 However, there are several notable differences between the two structures. Silica microspheres are free-standing, supported by a single optical fiber, whereas silica microtoroids can be fabricated on a silicon wafer in large arrays using a combination of lithography and etching steps. These differences influence which device is optimal for a given experiment.
Here, we present detailed fabrication protocols for both types of resonant cavities. While the fabrication of microsphere resonant cavities is fairly straightforward, the fabrication of microtoroid resonant cavities requires additional specialized equipment and facilities (cleanroom). Therefore, this additional requirement may also influence which device is selected for a given experiment.
Introduction
An optical resonator efficiently confines light at specific wavelengths, known as the resonant wavelengths of the device. 1-2 The common figure of merit for these optical resonators is the quality factor or Q. This term describes the photon lifetime (&tau;o) within the resonator, which is directly related to the resonator's optical losses. Therefore, an optical resonator with a high Q factor has low optical losses, long photon lifetimes, and very low photon decay rates (1/&tau;o). As a result of the long photon lifetimes, it is possible to build-up extremely large circulating optical field intensities in these devices. This very unique property has allowed these devices to be used as laser sources and integrated biosensors.10
A unique sub-class of resonators is the whispering gallery mode optical microcavity. In these devices, the light is confined in circular orbits at the periphery. Therefore, the field is not completely confined within the device, but evanesces into the environment. Whispering gallery mode optical cavities have demonstrated some of the highest quality factors of any optical resonant cavity to date.9,11 Therefore, these devices are used throughout science and engineering, including in fundamental physics studies and in telecommunications as well as in biodetection experiments. 3-7,12
Optical microcavities can be fabricated from a wide range of materials and in a wide variety of geometries. A few examples include silica and silicon microtoroids, silicon, silicon nitride, and silica microdisks, micropillars, and silica and polymer microrings.13-17 The range in quality factor (Q) varies as dramatically as the geometry. Although both geometry and high Q are important considerations in any field, in many applications, there is far greater leverage in boosting device performance through Q enhancement. Among the numerous options detailed previously, the silica microsphere and the silica microtoroid resonator have achieved some of the highest Q factors to date.1,9 Additionally, as a result of the extremely low optical loss of silica from the visible through the near-IR, both microspheres and microtoroids are able to maintain their Q factors over a wide range of testing wavelengths.18 Finally, because silica is inherently biocompatible, it is routinely used in biodetection experiments.
In addition to high material absorption, there are several other potential loss mechanisms, including surface roughness, radiation loss, and contamination loss.2 Through an optimization of the device size, it is possible to eliminate radiation losses, which arise from poor optical field confinement within the device. Similarly, by storing a device in an appropriately clean environment, contamination of the surface can be minimized. Therefore, in addition to material loss, surface scattering is the primary loss mechanism of concern.2,8
In silica devices, surface scattering is minimized by using a laser reflow technique, which melts the silica through surface tension induced reflow. While spherical optical resonators have been studied for many years, it is only with recent advances in fabrication technologies that researchers been able to fabricate high quality silica optical toroidal microresonators (Q&gt;100 million) on a silicon substrate, thus paving the way for integration with microfluidics.1
The present series of protocols details how to fabricate both silica microsphere and microtoroid resonant cavities. While silica microsphere resonant cavities are well-established, microtoroid resonant cavities were only recently invented.1 As many of the fundamental methods used to fabricate the microsphere are also used in the more complex microtoroid fabrication procedure, by including both in a single protocol it will enable researchers to more easily trouble-shoot their experiments.

We describe the use of a carbon dioxide laser reflow technique to fabricate silica resonant cavities, including free-standing microspheres and on-chip microtoroids. The reflow method removes surface imperfections, allowing long photon lifetimes within both devices. The resulting devices have ultra high quality factors, enabling applications ranging from telecommunications to biodetection.

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the ...

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Patterning via Optical Saturable Transitions - Fabrication and Characterization

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.4 Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization5,6, beam pen lithography (BPL)7, scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities1-3, opening the door to massive parallelization.

We report that the diffraction limit of conventional optical lithography can be overcome by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.1-3 This paper outlines our fabrication technique and two locking mechanisms, namely: dissolution of one photoisomer and electrochemical oxidation.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing in combination with new nanofabrication techniques. A plasmon is a quantum of charge density oscillation that lends nanoscale metals such as gold and silver unique optical properties. In particular, gold and silver nanoparticles exhibit localized surface plasmon resonances-collective charge density oscillations on the surface of the nanoparticle-in the visible spectrum. Here, we focus on the fabrication of periodic arrays of anisotropic plasmonic nanostructures. These half-shell (or nanocup) structures can exhibit additional unique light-bending and polarization-dependent optical properties that simple isotropic nanostructures cannot. Researchers are interested in the fabrication of periodic arrays of nanocups for a wide variety of applications such as low-cost optical devices, surface-enhanced Raman scattering, and tamper indication. We present a scalable technique based on colloidal lithography in which it is possible to easily fabricate large periodic arrays of nanocups using spin-coating and self-assembled commercially available polymeric nanospheres. Electron microscopy and optical spectroscopy from the visible to near-infrared (near-IR) was performed to confirm successful nanocup fabrication. We conclude with a demonstration of the transfer of nanocups to a flexible, conformal adhesive film.

We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films.

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing ...

Fabrication of Superhydrophobic Metal Surfaces for Anti-Icing Applications

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in industry. The wettability of the produced surface was analyzed by bouncing drop experiments and the topography was analyzed by confocal microscopy. In addition, we show various methodologies to measure its durability and anti-icing properties. Superhydrophobic surfaces hold a special texture that must be preserved to keep their water-repellency. To fabricate durable surfaces, we followed two strategies to incorporate a resistant texture. The first strategy is a direct incorporation of roughness to the metal substrate by acid etching. After this surface texturization, the surface energy was decreased by silanization or fluoropolymer deposition. The second strategy is the growth of a ceria layer (after surface texturization) that should enhance the surface hardness and corrosion resistance. The surface energy was decreased with a stearic acid film.
The durability of the superhydrophobic surfaces was examined by a particle impact test, mechanical wear by lateral abrasion, and UV-ozone resistance. The anti-icing properties were explored by studying the ability to repeal subcooled water, freezing delay, and ice adhesion.

We illustrate several methodologies to produce superhydrophobic metal surfaces and to explore their durability and anti-icing properties.

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in ...

Fabrication of Soft Pneumatic Network Actuators with Oblique Chambers

Soft pneumatic network actuators have become one of the most promising actuation devices in soft robotics which benefits from their large bending deformation and low input. However, their monotonous bending motion form in two-dimensional (2-D) space keeps them away from wide applications. This paper presents a detailed fabrication method of soft pneumatic network actuators with oblique chambers, to explore their motions in three-dimensional (3-D) space. The design of oblique chambers enables actuators with tunable coupled bending and twisting capabilities, which gives them the possibility to move dexterously in flexible manipulators, to become biologically inspired robots and medical devices. The fabrication process is based on the molding method, including the silicone elastomer preparation, chamber and base parts fabrication, actuator assembly, tubing connections, checks for leaks, and actuator repair. The fabrication method guarantees the rapid manufacturing of a series of actuators with only a few modifications in the molds. The test results show the high quality of the actuators and their prominent bending and twisting capabilities. Experiments of the gripper demonstrate the advantages of the development in adapting to objects with different diameters and providing sufficient friction.

Here we present a fabrication method of soft pneumatic network actuators with oblique chambers. The actuators are capable of generating coupled bending and twisting motions, which broadens their application in soft robotics.

Soft pneumatic network actuators have become one of the most promising actuation devices in soft robotics which benefits from their large bending ...

Scalable Stamp Printing and Fabrication of Hemiwicking Surfaces

Hemiwicking is a process where a fluid wets a patterned surface beyond its normal wetting length due to a combination of capillary action and imbibition. This wetting phenomenon is important in many technical fields ranging from physiology to aerospace engineering. Currently, several different techniques exist for fabricating hemiwicking structures. These conventional methods, however, are often time consuming and are difficult to scale-up for large areas or are difficult to customize for specific, nonhomogeneous patterning geometries. The presented protocol provides researchers with a simple, scalable, and cost-effective method for fabricating micro-patterned hemiwicking surfaces. The method fabricates wicking structures through the use of stamp printing, polydimethylsiloxane (PDMS) molding, and thin-film surface coatings. The protocol is demonstrated for hemiwicking with ethanol on PDMS micropillar arrays coated with a 70 nm thick aluminum thin-film.

A simple protocol is provided for the fabrication of hemiwicking structures of varying sizes, shapes, and materials. The protocol uses a combination of physical stamping, PDMS molding, and thin-film surface modifications via common materials deposition techniques.

Hemiwicking is a process where a fluid wets a patterned surface beyond its normal wetting length due to a combination of capillary action and imbibition. ...

Fabrication and Design of Wood-Based High-Performance Composites

Delignified densified wood is a new promising and sustainable material that possesses the potential to replace synthetic materials, such as glass fiber reinforced composites, due to its excellent mechanical properties. Delignified wood, however, is rather fragile in a wet state, which makes handling and shaping challenging. Here we present two fabrication processes, closed-mold densification and vacuum densification, to produce high-performance cellulose composites based on delignified wood, including an assessment of their advantages and limitations. Further, we suggest strategies for how the composites can be re-used or decomposed at the end-of-life cycle. Closed-mold densification has the advantage that no elaborate lab equipment is needed. Simple screw clamps or a press can be used for densification. We recommend this method for small parts with simple geometries and large radii of curvature. Vacuum densification in an open-mold process is suitable for larger objects and complex geometries, including small radii of curvature. Compared to the closed-mold process, the open-mold vacuum approach only needs the manufacture of a single mold cavity.

Delignified densified wood represents a new promising lightweight, high-performance and bio-based material with great potential to partially substitute natural fiber reinforced- or glass fiber reinforced composites in the future. We here present two versatile fabrication routes and demonstrate the possibility to create complex composite parts.

Delignified densified wood is a new promising and sustainable material that possesses the potential to replace synthetic materials, such as glass fiber ...