This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

1. Sample Preparation for Resist Coating
Note: In this work, patterns with single-digit nanometer resolution are defined in PMMA (positive- and negative-tone) and HSQ resists, which are spin-cast onto commercially available TEM windows (approximately 50 &#181;m x 50 &#181;m) with SiNx or SiO2 membranes with thicknesses ranging from 5 nm to 50 nm. One or more TEM windows are fabricated in a 3-mm diameter silicon handling frame (100 &#181;m thick). Throughout this manuscript, we refer to the whole unit as the TEM chip and to the electron-beam transparent membrane as the TEM window.
<ol>
	<li>Remove any organic residue from the TEM chip by performing O2 plasma cleaning for 30 s at 100 W (chamber pressure of 230 mT at approximately 5 sccm O2 flow).</li>
	<li>Cleave a piece of silicon wafer, approximately 2 cm x 2 cm in size, to use as a holder for the TEM chip during resist spinning.</li>
	<li>Place two stripes of double-sided carbon tape approximately equidistant from the center of the silicon holder and separated slightly less than the diameter of the TEM chip (see Figure 1). Rinse the stripes with isopropyl alcohol (IPA) to reduce their adhesive strength. This is necessary to avoid breaking the delicate TEM chip during removal from the Si holder.</li>
	<li>Mount the TEM chip on the silicon holder making sure that it is attached to the carbon tape stripes only at two opposite edges as shown in Figure 1.</li>
</ol>
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58272/58272fig1.jpg" /> 
Figure 1: TEM chip holder for resist spinning. Notice that the TEM chip is attached to the silicon holder only at two edges to diminish the surface area contact, and hence, the adhesion force. <a href="https://www.jove.com/files/ftp_upload/58272/58272fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
2. Spin Coat Parameters for PMMA (Positive and Negative Tone) and HSQ Resists
Note: Resist thickness is not measured directly on the TEM chip, since it is small and usually the resist is cast on other thin layers (e.g., Si film on SiO2 membrane), which complicates the measurement. Instead, resist thickness is determined by the spin speed calibrated using reflectometry measurements from films cast on a bulk Si sample. Reflectometry results were corroborated, usually with a precision better than 20%, by STEM top-down images of collapsed structures.
<ol>
	<li>Mount the silicon holder on the spinner chuck and align the center of the TEM window approximately with the center of the spinner rotor.</li>
	<li>Using a pipette, cover the entire TEM window with one drop (approximately 0.05 mL) of PMMA (A2 950K PMMA diluted in anisole to 0.5-1.0%) or HSQ (1% solids XR-1541).</li>
	<li>Depending on the resist used, follow the spin coating and baking parameters shown in Table 1.</li>
	<li>Carefully remove the TEM chip from the silicon holder. Inspect the resist uniformity over the TEM window using an optical microscope. If the film is homogenous across the central region of the membrane, proceed to the next step; otherwise, repeat the resist coating process on a fresh TEM window.</li>
</ol>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Resist</td>
			<td>Spin speed 
			(x g)</td>
			<td>Film 
			Thickness 
			(nm)</td>
			<td>Baking temperature 
			(&#176;C)</td>
			<td>Baking time 
			(min)</td>
		</tr>
		<tr>
			<td>Positive-tone PMMA</td>
			<td>60</td>
			<td>30</td>
			<td>200a</td>
			<td>2a</td>
		</tr>
		<tr>
			<td>Negative-tone PMMA</td>
			<td>60</td>
			<td>15</td>
			<td>200a</td>
			<td>2a</td>
		</tr>
		<tr>
			<td>HSQ</td>
			<td>107</td>
			<td>10</td>
			<td>Not neededb</td>
			<td>Not neededb</td>
		</tr>
		<tr>
			<td colspan="5">asee Ref.12; bsee Ref. 13</td>
		</tr>
	</tbody>
</table>
Table 1: Resist spin coating and baking parameters. Spin speed units in x g consider a 3-mm diameter TEM chip. Baking is performed on a hot plate for PMMA. No baking is needed for HSQ13.&#160;HSQ resist is stored refrigerated, so it needs to warm up to room temperature before spinning.
3. Load Sample in STEM, Map Window Coordinates, and Perform High-Resolution Focusing
<ol>
	<li>Mount the resist-coated TEM chip on the STEM sample holder, making sure that the resist-vacuum interface faces the incoming beam, since the beam is optimally focused at the top of the sample. Also, make sure that the sides of the TEM window are aligned approximately with the x- and y-axis of the STEM stage. This will facilitate navigating to the TEM window.
	<ol>
		<li>Load the TEM chip into the microscope, and pump overnight to reduce contaminants in the sample chamber.</li>
	</ol>
	</li>
	<li>Move the stage (x, y) coordinates such that the beam is more than 100 &#181;m away from the center of the TEM window (to avoid accidental exposure). Set the STEM probe beam current and energy to 34 pA and 200 keV, respectively.</li>
	<li>In diffraction mode imaging (stationary beam, z-contrast mode and mid-angle annular dark-field detector), set magnification to 30 kX with the beam out of focus, which makes it easier to find an edge of the TEM window. 
	NOTE: The TEM window edges can also be found in imaging mode. We use diffraction mode because it is faster, since the beam does not need to be scanned to form an image.</li>
	<li>Navigate towards the TEM window until an edge of the window is observed on the diffraction image. Navigate along the window edges and record the (x, y) coordinates of the four corners of the TEM window.</li>
	<li>At the last window corner, increase magnification to 50 kX and perform rough focusing on the window membrane by moving the stage z-coordinate (z-height adjustment) until the crossover of the diffraction pattern orientation is observed. Subsequently, perform fine focusing by adjusting the objective lens current.</li>
	<li>Increase magnification to 180 kX. Adjust focus, stigmation and aberration correction settings in order to obtain an aberration-corrected diffraction image of the window membrane as shown in Figure 2B. This focusing method is known as the Ronchigram method14.</li>
</ol>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58272/58272fig2.jpg" /> 
Figure 2: Diffraction image of TEM window membrane. (A) Focused but stigmatic image. The aberration-correction settings for this image are not optimum as evidenced by the closely-spaced diffraction fringes. (B) Exposure-ready non-stigmated image showing a smooth plateau diffraction pattern. <a href="https://www.jove.com/files/ftp_upload/58272/58272fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
4. Expose Patterns Using an Aberration-Corrected STEM Equipped with a Pattern Generator System.
Note: The aberration-corrected STEM used in this work is equipped with a pattern generator system (PGS), which controls the electron-beam position to expose patterns defined using computer aided design (CAD) software. Dose is controlled by defining the spacing between exposure points (step size) and the exposure time per point. Table 2 summarizes the exposure parameters used in this protocol. Patterns are exposed at the center of the TEM window in &#34;continuous mode,&#34; since the STEM used in this work does not include a beam blanker. Before and after the exposure, PGS positions the beam at any user-defined point in the field of view (FOV), preferably away from the pattern area. We use in this protocol the top right and bottom right corners of the FOV as the initial and final beam positions, respectively.
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td rowspan="2">Resist</td>
			<td>Dot exposure</td>
			<td colspan="2">Line exposure</td>
			<td colspan="2">Area exposure</td>
		</tr>
		<tr>
			<td>Dose 
			(fC/dot)</td>
			<td>Step size 
			(nm)</td>
			<td>Dose 
			(nC/cm)</td>
			<td>Step size 
			(nm)</td>
			<td>Dose 
			(&#181;C/cm2)</td>
		</tr>
		<tr>
			<td>Positive tone PMMA</td>
			<td>10-100</td>
			<td>0.5</td>
			<td>2&#8211;8</td>
			<td>0.5</td>
			<td>2,000</td>
		</tr>
		<tr>
			<td>Negative tone PMMA</td>
			<td>50-500</td>
			<td>0.5</td>
			<td>20&#8211;40</td>
			<td>0.5</td>
			<td>50,000&#8211;80,000</td>
		</tr>
		<tr>
			<td>HSQ</td>
			<td>10-100</td>
			<td>0.5</td>
			<td>10&#8211;20</td>
			<td>0.5</td>
			<td>20,000&#8211;30,000</td>
		</tr>
	</tbody>
</table>
Table 2: Exposure parameters for PMMA (positive and negative tone) and HSQ resists. The values shown are generic, since optimal dose values depend on the specific pattern design and targeted feature dimensions.
<ol>
	<li>Close the beam gate valve to avoid any accidental exposure of the resist when moving the stage. Verify that the beam current is 34 pA and magnification is 180 kX.</li>
	<li>Use the pre-recorded window corner coordinates to move the stage, so that the FOV center is 5 &#181;m away from the center of the window. Open the beam gate valve and focus at this point using the Ronchigram method described in Step 3.6.</li>
	<li>Close the beam gate valve. Move the stage to place the FOV at the center of the TEM window. Change magnification to 18 kX (corresponding to a 5 &#181;m x 5 &#181;m patterning FOV). Transfer the beam control to PGS and position the beam anywhere away from the pattern area (we use the top right corner in this protocol).</li>
	<li>Perform the following actions in quick succession to avoid overexposing the resist at the initial and final beam positions.
	<ol>
		<li>Open the gate valve and verify, by observing the beam diffraction pattern image, whether the beam is in focus at the initial beam position (as in Figure 2B). Expose the pattern.</li>
		<li>When the exposure is complete, check if the diffraction pattern image remains in focus at the final beam position. Finally, close the gate valve.</li>
	</ol>
	</li>
	<li>Remove the TEM chip from the STEM.</li>
</ol>
5. Resist Development and Critical Point Drying
Note: The development process depends on the resist used. Steps 5.1, 5.2, and 5.3 describe the developing process for positive-tone PMMA, negative-tone PMMA, and HSQ, respectively. However, all resists share the same final critical point drying process, which is necessary to avoid pattern collapse due to the high-aspect ratio of the patterns fabricated with this protocol. Critical point drying (CPD) uses liquid CO2 as working fluid, which is not miscible with water. Consequently, sample dehydration (steps 5.4-5.7) require the use of ACS reagent grade isopropyl alcohol (IPA).
<ol>
	<li>Developing of positive-tone PMMA15: Prepare a 100-mL beaker with 3:1 solution of IPA:methyl isobutyl-ketone (MIBK). Place the beaker in a bath circulator at 0 &#176;C (an ice bath at 0 &#176;C is a lower cost alternative) and wait until the temperature is equalized. Grab the TEM chip with a pair of tweezers and gently stir it in the cold solution for 30 s. Proceed with Step 5.4.</li>
	<li>Developing of negative-tone PMMA16: Gently stir the TEM chip in MIBK at room temperature (24 &#176;C) for 2 min. Transfer the sample to an acetone solution and stir for 3 min. Proceed with step 5.4.</li>
	<li>Developing of HSQ13: Stir the TEM chip in a &#34;salty&#34; deionized water solution, containing 1 wt% NaOH and 4 wt% NaCl, for 4 min at 24 &#176;C. Stir the chip in pure deionized water for 2 min (to rinse off the salty developer). Proceed with Step 5.4.</li>
	<li>Dip the TEM chip in ACS reagent grade IPA and gently stir it for 30 s.</li>
	<li>Quickly place the TEM chip on the special 2&#34; Si wafer shown in Figure 3A. Make sure that the TEM chip is always wet with IPA during the transfer. After approximately 2-3 min, close the CPD wafer holder assembly as depicted in Figure 3B. Leave the whole unit soaking in ACS reagent grade IPA for additional 15 min totally immersed in IPA.</li>
	<li>Quickly transfer the complete CPD wafer holder assembly to a second container with fresh ACS reagent grade IPA and leave it for 15 min totally immersed in IPA.</li>
	<li>Transfer the CPD wafer holder assembly to the CPD instrument process chamber (at all times the TEM chip should be totally immersed in IPA). Run the CPD process following the instrument&#39;s operating instructions.</li>
</ol>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/58272/58272fig3.jpg" /> 
Figure 3: In-house solution for the dehydration of TEM chips in a CPD standard 2&#34; wafer holder. (A) Schematic side view of the TEM chip on a special 2&#34; Si wafer with a small hole drilled in the center (approximately 500 &#956;m in diameter) to allow liquid flow. The wafer fits in a CPD standard 2&#34; wafer holder supplied by the CPD system manufacturer. (B) A second special Si wafer encloses the TEM chip, thus reducing turbulent flow during the CPD process. In A and B, the CPD wafer holder is totally immersed in ACS reagent grade IPA. <a href="https://www.jove.com/files/ftp_upload/58272/58272fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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The authors have nothing to disclose.

The most critical step in the protocol is focusing the electron beam before exposure. This is necessary to achieve highest-resolution patterning. When performing multiple exposures (e.g., when a TEM chip has multiple windows and each is being patterned), it is important to refocus the beam before each exposure at a distance of at-most 5 &#956;m from the exposure area. The protocol also includes steps to check the beam focus before and after exposure at two extreme positions of the patterning area (top and bottom corners), which allows a determination of whether some defocusing occurred during patterning, for example due to a membrane being locally tilted in the patterning region.
Another important step in this protocol is using critical point drying (CPD) to dry samples after developing the exposed resist patterns. Without this step, patterns will frequently collapse due to the high aspect ratio of the patterned structures (i.e., patterned resist lateral dimensions smaller than the thickness). Most CPD systems supply a standard 2&#34; wafer holder. However, since TEM chips are very small and the patterned structures are quite delicate, they might be damaged during the CPD process when placed in holders designed for larger samples. Figure 3 shows an in-house solution for CPD of TEM chips using a standard wafer holder. The two wafers, with a flow-enabling hole at the center, enclose the TEM chip and protect it from turbulent flow during the CPD process.
The determination of the optimal resist film thickness tries to balance competing requirements. On the one hand, it should be as thin as possible to achieve the highest resolution and to avoid pattern collapse, but on the other hand, it should be thick enough for pattern transfer applications such as lift-off and etching. This protocol uses 1% HSQ, which is the lowest dilution commercially available and whose further dilution in the lab is not recommended (our experience shows that diluted HSQ often leads to partial crosslinking). However, since diluted PMMA does give reproducible results, this protocol uses 1% for positive-tone PMMA (30 nm thickness) and 0.5% and 1% for negative tone (15 and 30 nm thickness, respectively). We have found that positive-tone PMMA resist does not suffer from pattern collapse as negative-tone PMMA does, thus the use of thinner thickness for negative tone as shown in Table 1. In addition, negative-tone PMMA has ~50% thickness loss after e-beam exposure (and before development), so the final thickness for negative-tone PMMA is ~7 to 15 nm. (The 1.7 and 1.8 nm features from Figure 4 have about 7 nm resist thickness, which is at the limit of pattern collapse.) The PMMA patterns shown in Figure 4 did not use a CPD step; however, if available, this protocol recommends the use of CPD after development of PMMA patterns. In contrast, we found CPD to be critical for HSQ processing due to the fact that it cannot be further diluted (to achieve thinner thickness) and because thicker HSQ patterns are needed to use as an etching mask (e.g., to etch silicon as shown in Figure 5).
The positive-tone PMMA patterns in Figure 4 were coated with a thin metallic film to increase contrast during imaging. The Supporting Information in the work of Manfrinato et al.1 shows that the effect of this metallic coating on the metrology of the patterns is negligible. Similarly, we consider that the results shown in Figure 5 for HSQ resist do not depend drastically on the particular choice of TEM window structure based on the ultra-thin thickness of the underlying Si layer.
To the best of our knowledge, all the measurements described in the Representative Results Section for positive- and negative-tone PMMA1 (Figure 4) are the smallest features reported in the literature to date1,7,12,16,17. Manfrinato et al.1 also demonstrated sub-5 nm pattern transfer, from the resist to a target material, using conventional metal lift-off (for positive-tone PMMA) and sequential infiltration synthesis18 of ZnO (for negative-tone PMMA). The results shown in Figure 5 for HSQ are not the smallest reported features7. However, this protocol is useful for obtaining reproducible features in HSQ at resolutions better than 10 nm, and demonstrates single-digit patterning of silicon structures.
The protocol presented here describes a process for patterning arbitrary structures with single digit nanometer resolution using the conventional electron-beam resists PMMA and HSQ. Additionally, the results shown here and in Ref. 1 demonstrate that such patterns can be transferred with high fidelity to a target material of choice.

The protocol presented in this manuscript provides guidance for defining patterns with single-digit nanometer resolution in poly (methyl methacrylate) (PMMA) and hydrogen silsesquioxane (HSQ), which are two common electron-beam resists used in high-resolution patterning by electron-beam lithography. We achieve these results using an aberration-corrected scanning transmission electron microscope (STEM) as the exposure tool, outfitted with a pattern generator for controlling the electron beam. After resist exposure, the nanoscale patterns can be transferred to a variety of target materials1, thus enabling fabrication of novel devices at single-digit nanometer resolution.
Previous studies have shown that electron-beam lithography (EBL) is capable of defining patterns in resist materials with dimensions in the sub-10 nm scale2,3,4,5,6. However, for dimensions around 4 nm, these demonstrations have required non-standard procedures such as use of assist structures7 or long-exposure times for self-developing resists8. Other nanopatterning techniques, such as electron-beam induced deposition9 or scanning probe lithography10,11, have proven capable of achieving sub-4 nm resolution, although these require significantly longer exposure times compared to EBL.
Modern dedicated EBL systems produce electron beams with spot sizes in the few nanometer length scale (2-10 nm), which makes defining patterns with sub-10 nm resolution very difficult. In contrast, our protocol implements EBL using an aberration-corrected STEM, which is a highly optimized instrument for material characterization at angstrom length scales. This difference allows routine patterning of record-breaking lithographic features with single nanometer resolution1. While state-of-the-art, commercial aberration-corrected STEM systems cost in the range of millions of dollars, they are available for use in several national user facilities, and some are accessible without cost.

<table><tbody><tr><td>Plasma asher</td><td>Plasma Etch</td><td>PE-75</td><td>Located in class 100 cleanroom</td></tr><tr><td>Silicon Nitride 5 nm thick TEM Windows (9 SMALL Windows)&amp;nbsp;</td><td>TEM windows.com</td><td>SN100-A05Q33A</td><td></td></tr><tr><td>TEM chip holder for resist coating</td><td></td><td></td><td>Home made</td></tr><tr><td>27 nm thick c-Si TEM Windows</td><td>TEMwindows.com</td><td></td><td>Custom order</td></tr><tr><td>A2 950K PMMA diluted in anisole to 0.5-1.0% by weight</td><td>MicroChem</td><td>M230002</td><td></td></tr><tr><td>HSQ (1% solids XR-1541) e-beam resist in MIBK</td><td>Dow Corning</td><td>XR-1541-001</td><td></td></tr><tr><td>Spinner</td><td>Reynolds Tech</td><td>ReynoldsTech Flo-Spin system</td><td>Located in class 100 cleanroom</td></tr><tr><td>Hot plate</td><td>Brewer Science</td><td>CEE 1300X</td><td>Located in class 100 cleanroom</td></tr><tr><td>Spectral reflectometer</td><td>Filmetrics</td><td>F20</td><td>Located in class 1000 cleanroom</td></tr><tr><td>Bath circulator</td><td>Thermo Scientific</td><td>Neslab RTE 740</td><td>Located in class 100 cleanroom</td></tr><tr><td>Optical microscope</td><td>Nikon</td><td>Eclipse L200N</td><td>Located in class 1000 cleanroom</td></tr><tr><td>MIBK/IPA 1:3 developer</td><td>MicroChem</td><td>M089025&amp;nbsp;</td><td></td></tr><tr><td>Sodium hydroxide</td><td>Sigma-Aldrich</td><td>221465</td><td></td></tr><tr><td>Sodium chloride</td><td>Sigma-Aldrich</td><td>31434</td><td></td></tr><tr><td>Isopropyl Alcohol, ACS Reagent Grade</td><td>Fisher Scientific</td><td>MK303202</td><td></td></tr><tr><td>TEM chip holder for critical point drying</td><td></td><td></td><td>Home made</td></tr><tr><td>Critical point drying system</td><td>Tousimis</td><td>Autosamdri-815B, Series C</td><td>Located in class 100 cleanroom</td></tr><tr><td>Aberration-corrected STEM</td><td>Hitachi</td><td>HD 2700C</td><td></td></tr><tr><td>Pattern generation system</td><td>JC Nabity Lithography Systems</td><td>NPGS v9&amp;nbsp;</td><td></td></tr><tr><td>Scanning Electron Microscope (SEM)</td><td>Hitachi</td><td>S-4800</td><td></td></tr><tr><td>Reactive ion etcher</td><td>Oxford Instruments</td><td>Plasmalab 100&amp;nbsp;</td><td>Located in class 1000 cleanroom</td></tr></tbody></table>

single-digit nanometer electron-beam lithography with an aberration-corrected scanning transmission electron microscope

We demonstrate extension of electron-beam lithography using conventional resists and pattern transfer processes to single-digit nanometer dimensions by employing an aberration-corrected scanning transmission electron microscope as the exposure tool. Here, we present results of single-digit nanometer patterning of two widely used electron-beam resists: poly (methyl methacrylate) and hydrogen silsesquioxane. The method achieves sub-5 nanometer features in poly (methyl methacrylate) and sub-10 nanometer resolution in hydrogen silsesquioxane. High-fidelity transfer of these patterns into target materials of choice can be performed using metal lift-off, plasma etch, and resist infiltration with organometallics.

Figure 4&#160;shows lithographic patterns on positive-tone PMMA (resist removed from exposed regions after development) and negative-tone PMMA (resist removed from unexposed areas). TEM windows consisted of approximately 30 nm thick PMMA resist for positive-tone PMMA (15 nm thick for negative-tone PMMA) spin cast on a 5 nm thick SiNx membrane. A thin metallic film (10 nm AuPd over 5 nm Ti) was deposited after development of positive-tone PMMA to enhance contrast during STEM imaging. For positive-tone PMMA, the average smallest isolated feature is 2.5 &#177; 0.7 nm (Figure&#160;4C, 4D), while the smallest pitch pattern is 17.5 nm (Figure 4F). For negative-tone PMMA, the average smallest isolated feature is 1.7 &#177; 0.5 nm (Figure 4G), while the smallest pitch pattern is 10.7 nm (Figure 4J).&#160;
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/58272/58272fig4.jpg" /> 
Figure 4: Aberration-corrected electron beam lithography of positive- and negative-tone PMMA. (A thin film of 10 nm AuPd over 5 nm Ti was deposited on all positive-tone PMMA patterns shown in this figure.) (A) SEM image of arbitrary patterns in positive-tone PMMA. (B) TEM image of arbitrary patterns in negative-tone PMMA. (C,D) SEM images of the smallest patterned holes in positive-tone PMMA, with average hole diameter of 2.5 &#177; 0.7 nm. (E,F) SEM images of hole arrays defined on positive-tone PMMA with a pitch of 21.5 nm (E) and 17.5 nm (F). (G,H) TEM images of pillar arrays in negative-tone PMMA with 20 nm pitch and with average pillar diameter of 1.7 &#177; 0.5 nm (G) and 1.8 &#177; 0.5 nm (H). (I,J) TEM images of negative-tone PMMA pillar arrays of 15.2 and 10.7 nm pitch, respectively. All scale bars are 40 nm. This figure has been reproduced from Manfrinato, V.R., Stein, A., Zhang, L., Nam, C.-Y., Yager, K.G., Stach, E.A, and Black, C.T. Aberration-Corrected Electron Beam Lithography at the One Nanometer Length Scale. Nano Lett. 17 (8), 4562-4567 (2017). <a href="https://www.jove.com/files/ftp_upload/58272/58272fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Figure 5 displays patterns defined on HSQ resist.&#160;The TEM window used for HSQ lithography consisted of approximately 10 nm thick HSQ resist spin cast on a 27 nm thick Si membrane. After exposure and resist development, 3-4 nm of the ultra-thin Si layer in the HSQ-free regions (unexposed areas) of the window were removed by inductive coupled plasma (ICP) etching using a mixture of 50 sccm HBr and 20 sccm Cl2 gases at a chamber pressure of 10 mT (bias and ICP power of 60 W and 250 W, respectively). Figure 5A consists of four rows of short vertical lines. The upper two rows were exposed with a line dose stepped exponentially from 2 to 120 nC/cm (0 nm designed width for these lines). The lower two rows were exposed with an area dose stepped exponentially from 3,000 to 60,000 &#956;C/cm2 (5 nm wide and 200 nm long designed rectangles). Figure 5B is a zoomed image of the center region of the bottom row in Figure 5A. The two leftmost, the four center, and the four rightmost lines were exposed with an area dose of 23,300, 27,300, and 32,000 &#956;C/cm2, respectively. The four center lines have an average measured width of 7 nm.
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/58272/58272fig5.jpg" /> 
Figure 5: Aberration-corrected electron beam lithography of HSQ resist. (The TEM window used in this figure was made of 27 nm-thick Si. After HSQ development, inductive coupled plasma etching was used to remove 3-4 nm of Si from the areas not covered by HSQ.) (A) TEM image of four rows of vertical lines exposed with doses varying exponentially from 2 to 120 nC/cm (top two rows) and 3,000 to 60,000 &#956;C/cm2 (bottom two rows). The beam step size was 0.5 nm for all lines. (B) High magnification TEM image of the central area of the bottom row in (A). The group of 4 lines in the center have an average measured width of 7 nm and were exposed with an area dose of 27,300 &#956;C/cm2.&#160;<a href="https://www.jove.com/files/ftp_upload/58272/58272fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope at JoVE.com

Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope

We use an aberration-corrected scanning transmission electron microscope to define single-digit nanometer patterns in two widely-used electron-beam resists: poly (methyl methacrylate) and hydrogen silsesquioxane. Resist patterns can be replicated in target materials of choice with single-digit nanometer fidelity using liftoff, plasma etching, and resist infiltration by organometallics.

We demonstrate extension of electron-beam lithography using conventional resists and pattern transfer processes to single-digit nanometer dimensions by ...

single-digit-nanometer-electron-beam-lithography-with-an-aberration

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Engineering

10.1K Views.  Brookhaven National Laboratory. Dit protocol biedt richtlijnen voor het definiëren van patronen met single-digit nanometer resolutie in twee gemeenschappelijke elektronenbundel weerstaan met behulp van een scanning transmissie elektronenmicroscoop of STEM als de blootstelling instrument. Het gebruik van een aberratie-gecorrigeerde STEM in dit protocol maakt routinematige patronen van lithografische functies met enkele nanometerresolutie mogelijk. Hoewel zeer gespecialiseerde en dure gereedschappen, deze instrumenten zijn s...

Video: Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope

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.

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 ...

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications of semiconductor materials. However, it is still under debate how the defect structure determines the band structure, and therefore, the recombination behavior of electron-hole pairs responsible for the optical and electrical properties of the extended defects. The present paper is a survey of procedures for the spatially resolved investigation of structural and of physical properties of extended defects in semiconductor materials with a scanning electron microscope (SEM). Representative examples are given for crystalline silicon. The luminescence behavior of extended defects can be investigated by cathodoluminescence (CL) measurements. They are particularly valuable because spectrally and spatially resolved information can be obtained simultaneously. For silicon, with an indirect electronic band structure, CL measurements should be carried out at low temperatures down to 5 K due to the low fraction of radiative recombination processes in comparison to non-radiative transitions at room temperature. For the study of the electrical properties of extended defects, the electron beam induced current (EBIC) technique can be applied. The EBIC image reflects the local distribution of defects due to the increased charge-carrier recombination in their vicinity. The procedure for EBIC investigations is described for measurements at room temperature and at low temperatures. Internal strain fields arising from extended defects can be determined quantitatively by cross-correlation electron backscatter diffraction (ccEBSD). This method is challenging because of the necessary preparation of the sample surface and because of the quality of the diffraction patterns which are recorded during the mapping of the sample. The spatial resolution of the three experimental techniques is compared.

The optical, electrical, and structural properties of dislocations and of grain boundaries in semiconductor materials can be determined by experiments performed in a scanning electron microscope. Electron microscopy has been used to investigate cathodoluminescence, electron beam induced current, and diffraction of backscattered electrons.

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications ...

Studying Dynamic Processes of Nano-sized Objects in Liquid using Scanning Transmission Electron Microscopy

Samples fully embedded in liquid can be studied at a nanoscale spatial resolution with Scanning Transmission Electron Microscopy (STEM) using a microfluidic chamber assembled in the specimen holder for Transmission Electron Microscopy (TEM) and STEM. The microfluidic system consists of two silicon microchips supporting thin Silicon Nitride (SiN) membrane windows. This article describes the basic steps of sample loading and data acquisition. Most important of all is to ensure that the liquid compartment is correctly assembled, thus providing a thin liquid layer and a vacuum seal. This protocol also includes a number of tests necessary to perform during sample loading in order to ensure correct assembly. Once the sample is loaded in the electron microscope, the liquid thickness needs to be measured. Incorrect assembly may result in a too-thick liquid, while a too-thin liquid may indicate the absence of liquid, such as when a bubble is formed. Finally, the protocol explains how images are taken and how dynamic processes can be studied. A sample containing AuNPs is imaged both in pure water and in saline.

This protocol describes the operation of a liquid flow specimen holder for scanning transmission electron microscopy of AuNPs in water, as used for the observation of nanoscale dynamic processes.

Samples fully embedded in liquid can be studied at a nanoscale spatial resolution with Scanning Transmission Electron Microscopy (STEM) using a ...

Correlative Super-resolution and Electron Microscopy to Resolve Protein Localization in Zebrafish Retina

We present a method to investigate the subcellular protein localization in the larval zebrafish retina by combining super-resolution light microscopy and scanning electron microscopy. The sub-diffraction limit resolution capabilities of super-resolution light microscopes allow improving the accuracy of the correlated data. Briefly, 110 nanometer thick cryo-sections are transferred to a silicon wafer and, after immunofluorescence staining, are imaged by super-resolution light microscopy. Subsequently, the sections are preserved in methylcellulose and platinum shadowed prior to imaging in a scanning electron microscope (SEM). The images from these two microscopy modalities are easily merged using tissue landmarks with open source software. Here we describe the adapted method for the larval zebrafish retina. However, this method is also applicable to other types of tissues and organisms. We demonstrate that the complementary information obtained by this correlation is able to resolve the expression of mitochondrial proteins in relation with the membranes and cristae of mitochondria as well as to other compartments of the cell.

This protocol describes the necessary steps to obtain subcellular protein localization results on zebrafish retina by correlating super-resolution light microscopy and scanning electron microscopy images.

We present a method to investigate the subcellular protein localization in the larval zebrafish retina by combining super-resolution light microscopy and ...

Developmental Biology

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape method or individual WS2 nanotubes by dispersing the suspension of WS2 nanotubes. The selected samples have been fabricated into devices by the use of the electron beam lithography and electrolyte is put on the devices. We have characterized the electronic properties of the devices under applying the gate voltage. In the small gate voltage region, ions in the electrolyte are accumulated on the surface of the samples which leads to the large electric potential drop and resultant electrostatic carrier doping at the interface. Ambipolar transfer curve has been observed in this electrostatic doping region. When the gate voltage is further increased, we met another drastic increase of source-drain current which implies that ions are intercalated into layers of WS2 and electrochemical carrier doping is realized. In such electrochemical doping region, superconductivity has been observed. The focused technique provides a powerful strategy for achieving the electric-filed-induced quantum phase transition.

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

Here, we present a protocol to control the carrier number in solids by using the electrolyte.

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape ...

Magnetic Field Mapping using Off-Axis Electron Holography in the Transmission Electron Microscope

Off-axis electron holography is a powerful technique that involves the formation of an interference pattern in a transmission electron microscope (TEM) by overlapping two parts of an electron wave, one of which has passed through a region of interest on a specimen and the other is a reference wave. The resulting off-axis electron hologram can be analyzed digitally to recover the phase difference between the two parts of the electron wave, which can then be interpreted to provide quantitative information about local variations in electrostatic potential and magnetic induction within and around the specimen. Off-axis electron holograms can be recorded while a specimen is subjected to external stimuli such as elevated or reduced temperature, voltage, or light. The protocol that is presented here describes the practical steps that are required to record, analyze, and interpret off-axis electron holograms, with a primary focus on the measurement of magnetic fields within and around nanoscale materials and devices. Presented here are the steps involved in the recording, analysis, and processing of off-axis electron holograms, as well as the reconstruction and interpretation of phase images and visualization of the results. Also discussed are the need for optimization of the specimen geometry, the electron optical configuration of the microscope, and the electron hologram acquisition parameters, as well as the need for the use of information from multiple holograms to extract the desired magnetic contributions from the recorded signal. The steps are illustrated through a study of specimens of B20-type FeGe, which contain magnetic skyrmions and were prepared with focused ion beams (FIBs). Prospects for the future development of the technique are discussed.

Guidelines are presented for recording and interpreting off-axis electron holograms to provide quantitative images of magnetic fields in nanoscale materials and devices in the transmission electron microscope.

Off-axis electron holography is a powerful technique that involves the formation of an interference pattern in a transmission electron microscope (TEM) by ...

Quantitative Atomic-Site Analysis of Functional Dopants/Point Defects in Crystalline Materials by Electron-Channeling-Enhanced Microanalysis

A novel elemental and chemical analysis scheme based on electron-channeling phenomena in crystalline materials is introduced, where the incident high-energy electron beam is rocked with the submicrometric pivot point fixed on a specimen. This method enables us to quantitatively derive the site occupancies and site-dependent chemical information of impurities or intentionally doped functional elements in a specimen, using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy attached to a scanning transmission electron microscope, which is of significant interest to current materials science, particularly related to nanotechnologies. This scheme is applicable to any combination of elements even when the conventional Rietveld analysis by X-ray or neutron diffraction occasionally fails to provide the desired results because of limited sample sizes and close scattering factors of neighboring elements in the periodic table. In this methodological article, we demonstrate the basic experimental procedure and analysis method of the present beam-rocking microanalysis.

We provide a general outline of quantitative microanalysis methods for estimating the site occupancies of impurities and their chemical states by taking advantage of electron-channeling phenomena under incident electron beam-rocking conditions, which reliably extract information from minority species, light elements, oxygen vacancies, and other point/line/planar defects.

A novel elemental and chemical analysis scheme based on electron-channeling phenomena in crystalline materials is introduced, where the incident ...

Chemistry

Picometer-Precision Atomic Position Tracking through Electron Microscopy

The modern aberration-corrected scanning transmission electron microscopes (AC-STEM) have successfully achieved direct visualization of atomic columns with sub-angstrom resolution. With this significant progress, advanced image quantification and analysis are still at the early stages. In this work, we present the complete pathway for the metrology of atomic resolution scanning transmission electron microscopy (STEM) images. This includes (1) tips for acquiring high-quality STEM images; (2) denoising and drift-correction for enhancing measurement accuracy; (3) obtaining initial atomic positions; (4) indexing the atoms based on unit cell vectors; (5) quantifying the atom column positions with either 2D-Gaussian single peak fitting or (6) multi-peak fitting routines for slightly overlapping atomic columns; (7) quantification of lattice distortion/strain within the crystal structures or at the defects/interfaces where the lattice periodicity is disrupted; and (8) some common methods to visualize and present the analysis.
Furthermore, a simple self-developed free MATLAB app (EASY-STEM) with a graphical user interface (GUI) will be presented. The GUI can assist in the analysis of STEM images without the need for writing dedicated analysis code or software. The advanced data analysis methods presented here can be applied for the local quantification of defect relaxations, local structural distortions, local phase transformations, and non-centrosymmetry in a wide range of materials.

This work presents a workflow for atomic position tracking in atomic resolution transmission electron microscopy imaging. This workflow is performed using an open-source Matlab app (EASY-STEM).

The modern aberration-corrected scanning transmission electron microscopes (AC-STEM) have successfully achieved direct visualization of atomic columns ...

﻿Analyzing the Heterogeneous Performance of Single Nanoparticles with Electrochemical Methods

Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This nanoscale heterogeneity remains hidden during the ensemble-averaged characterization of nanoparticles. Electrochemical techniques have been developed to measure currents from single nanoparticles but do not provide information about the structure and identity of the molecules that undergo reactions at the electrode surface. Optical techniques such as surface-enhanced Raman scattering (SERS) microscopy and spectroscopy can detect electrochemical events on individual nanoparticles while simultaneously providing information on the vibrational modes of electrode surface species. In this paper, a protocol to track the electrochemical oxidation-reduction of Nile Blue (NB) on single Ag nanoparticles using SERS microscopy and spectroscopy is demonstrated. First, a detailed protocol for fabricating Ag nanoparticles on a smooth and semi-transparent Ag film is described. A dipolar plasmon mode aligned along the optical axis is formed between a single Ag nanoparticle and Ag film. The SERS emission from NB fixed between the nanoparticle and the film is coupled into the plasmon mode, and the high-angle emission is collected by a microscope objective to form a donut-shaped emission pattern. These donut-shaped SERS emission patterns allow for the unambiguous identification of single nanoparticles on the substrate, from which the SERS spectra can be collected. In this work, a method for employing the SERS substrate as a working electrode in an electrochemical cell compatible with an inverted optical microscope is provided. Finally, tracking the electrochemical oxidation-reduction of NB molecules on an individual Ag nanoparticle is shown. The setup and the protocol described here can be modified to study various electrochemical reactions on individual nanoparticles.

Author Spotlight: Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy  

The protocol describes how to monitor electrochemical events on single nanoparticles using surface-enhanced Raman scattering spectroscopy and imaging.

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This ...

Focused Ion Beam Lithography to Etch Nano-architectures into Microelectrodes

With advances in electronics and fabrication technology, intracortical microelectrodes have undergone substantial improvements enabling the production of sophisticated microelectrodes with greater resolution and expanded capabilities. The progress in fabrication technology has supported the development of biomimetic electrodes, which aim to seamlessly integrate into the brain parenchyma, reduce the neuroinflammatory response observed after electrode insertion and improve the quality and longevity of electrophysiological recordings. Here we describe a protocol to employ a biomimetic approach recently classified as nano-architecture. The use of focused ion beam lithography (FIB) was utilized in this protocol to etch specific nano-architecture features into the surface of non-functional and functional single shank intracortical microelectrodes. Etching nano-architectures into the electrode surface indicated possible improvements of biocompatibility and functionality of the implanted device. One of the benefits of using FIB is the ability to etch on manufactured devices, as opposed to during the fabrication of the device, facilitating boundless possibilities to modify numerous medical devices post-manufacturing. The protocol presented herein can be optimized for various material types, nano-architecture features, and types of devices. Augmenting the surface of implanted medical devices can improve the device performance and integration into the tissue.

We have shown that the etching of nano-architecture into intracortical microelectrode devices may reduce the inflammatory response and has the potential to improve electrophysiological recordings. The methods described herein outline an approach to etch nano-architectures into the surface of non-functional and functional single shank silicon intracortical microelectrodes.

With advances in electronics and fabrication technology, intracortical microelectrodes have undergone substantial improvements enabling the production of ...

Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope

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