Name: use of sacrificial nanoparticles to remove the effects of shot-noise in contact holes fabricated by e-beam lithography
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Description: Watch this Scientific Journal Video about Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography at JoVE.com

Intel Corporation funded this work through grant number 414305, and the Oregon Nanotechnology and Microtechnology Initiative (ONAMI) provided matching funds. We gratefully acknowledge the support and advice of Dr. James Blackwell in all phases of this work. Special thanks go to Drew Beasau and Chelsea Benedict for analyzing particle positioning statistics. We thank Professor Hall for a careful reading of the manuscript and Dr. Kurt Langworthy, at the University of Oregon, Eugene, OR, for his help with E-beam lithography.

1. Derivatize and Characterize the Surface of the Silicon Wafers

<ol>
	<li>Clean the surface of wafers using Radio Corporation of America (RCA) cleaning solutions SC1 and SC2.</li>
	<li>Prepare SC1 and SC2 by volumetrically mixing the following chemicals: 
	SC1: H2O2:NH4OH:H2O = 1:1:5 v/v and SC2: H2O2:HCl:H2O = 1:1:5 v/v.
	<ol>
		<li>Immerse the wafer in SC1 for 10 min at 70 &#176;C, and then p...

<ol>
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Shot-noise (SN) in lithography is a simple consequence of statistical fluctuations in the number of photons or particles (N) arriving in a given nano-region; it is inversely proportional to the square root of a number of photons/particles:
<img alt="Equation 3" src="/files/ftp_upload/54551/eq3.jpg" />
where A and r are the area and the size of the exposed region, respectively. For example, when using an ArF 193-nm (6.4-eV) excimer laser to pattern ...

The exponential growth in computational power, as quantified by Moore&#39;s law1,2 (1), is a result of progressive advances in optical lithography. In this top-down patterning technique, the achievable resolution, R, is given by the well-known Raleigh theorem3:
<img alt="Equation 1" src="/files/ftp_upload/54551/eq1.jpg" />
Here, &#955; and NA are the light wavelength and numerical aperture, respectively. Note that NA = &#951;&#183;sin&#952;, where &#951; is the refractive index of the medium between the lens and the wafer; &#952; = tan-1(d/2l) for the diameter, d, of the lens, and the distance, l, between the center of the lens and the wafer. Over the last fifty years, the lithographic resolution has improved through the use of (a) light sources, including excimer lasers, with progressively smaller UV wavelengths; (b) clever optical designs employing phase-shift masks4; and (c) higher NA. For exposure in air (&#951; = 1), NA is always less than unity, but by introducing a liquid with &#951; &#62;1, such as water5, between the lens and the wafer, NA can be elevated above 1, thereby improving the resolution of immersion lithography. Currently viable paths to a 20-nm node and beyond include extreme UV sources (&#955; = 13 nm) or patterning techniques using complex double and quadruple processing of a multilayered photoresist6,7.
At nanometer-length scales, statistical fluctuations, caused by shot-noise (SN), in the number of photons arriving within a nano-region cause variation in the dimensions of lithographic patterns. These effects are more pronounced with exposure to high-energy EUV light and E-beams, systems that need orders of magnitude fewer photons/particles compared to normal optical lithography8. Supersensitive chemically amplified (with a quantum efficiency &#62;1) photoresists also introduce a chemical SN caused by a variation in the number of photoreactive molecules in exposed nanoregions9,10. Lower sensitivity photoresists that need longer exposures suppress these effects, but they also reduce throughput.
On the molecular scale, the contribution to line-edge roughness from the molecular size distribution inherent to the photoresist polymers may be reduced by using molecular resists11. An approach that is complementary to this top-down processing of nano-patterning is the use of bottom-up methods12,13 that rely specifically on the directed self-assembly (DSA) of diblock polymers14. The ability of these processes to direct nucleation and to create non-uniform spacing between desired patterns, such as holes or lines, remains challenging. The size distribution of molecular components15,16 also limits the scale and yield of fabrication17,18. Similar problems limit microcontact printing of nanoparticles in soft lithography19.
This paper presents studies of a new hybrid approach (Figure 1) that combines the classic top-down projection lithography with electrostatically directed self-assembly to reduce the effect of SN/line-edge roughness(LER)20. Positively charged amine groups on self-assembled monolayers (SAMs) of N-(2-aminoethyl)-11-amino-undecyl-methoxy-silane (AATMS) underlying the PMMA film are exposed after development. The negatively charged photoresist film of PMMA electrostatically funnels negatively charged gold nanoparticles (GNPs), capped with citrate,21-24 into SN-affected holes25. Re-flow of the PMMA photoresist engulfs predeposited nanoparticles in the film.
<img alt="Figure 1" src="/files/ftp_upload/54551/54551fig1.jpg" /> 
Figure 1: Schematic representation of the strategy to remove the effects of shot-noise and line-edge roughness for the patterning of contact holes using NPs of precise size. Here, the critical dimension (CD) is the desired diameter of the holes. The approach (step 1) begins with depositing a self-assembled monolayer (SAM) of silane molecule bearing positively charged amine groups on the oxide surface of a silicon wafer. Next, E-beam lithography is used to pattern the holes (steps 2 and 3) in PMMA photoresist film, the blue layer, which generates shot-noise, as illustrated in the inset SEM image. Lithography exposes amine groups at the bottom of the holes. Step 4 entails the aqueous phase deposition of controlled-size, citrate-capped (negatively charged) gold nanoparticles (GNPs) in lithographically patterned holes using electrostatic funneling (EF). In step 5, heating the wafer to 100 &#176;C, below the glass transition temperature of the PMMA, 110 &#176;C, causes the reflow of the photoresist around pre-deposited nanoparticles. Etching overlaid PMMA with oxygen plasma (step 6) exposes the GNPs, and subsequent wet-etching (iodine) of the exposed particles (step 7) creates holes corresponding to the size of the GNPs. When coupled with reactive-ion/wet-etching, it is possible to transfer the hole pattern in the photoresist to SiO2 (step 8)31. Reprinted with permission from reference20. <a href="http://cloudflare.jove.com/files/ftp_upload/54551/54551fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The electrostatic interaction between the oppositely charged GNPs and amine groups on the substrate prevents the displacement of the GNPs from the binding site. The reflow step maintains the relative location of the GNPs but erases the holes and the effects of SN/LER. Plasma/wet etching steps regenerate holes that have the size of the GNP. Reactive-ion etching transfers their pattern to SiO2 hard-mask layers. The method relies on using more uniformly sized nanoparticles than a patterned nanohole (NH), expressed as the standard deviation, &#963;, such that &#963;GNP &#60;&#963;NH. This report focuses on steps (4 and 5 described in Figure 1) involving the deposition of nanoparticles from dispersion and the reflow of the photoresist around them to assess the advantages and limitations of the method. Both steps are, in principle, scalable to larger substrates, requiring no extensive modification of the current flow of producing modern integrated circuits on chips.

<table border="0" cellpadding="0" cellspacing="0" width="700">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">AATMS (95%)</td>
			<td style="width:71px;">Gelest Inc.</td>
			<td style="width:89px;">SIA0595.0</td>
			<td style="width:167px;">N-(2-aminoethyl)-11-aminoundecyltrimethoxysilane&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Gold colloids (Ted Pella Inc.)</td>
			<td style="width:71px;">Ted Pella</td>
			<td style="width:89px;">15705-20</td>
			<td>Gold Naoparticles</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">hydrogen peroxide</td>
			<td style="width:71px;">Fisher Scientific&#160;</td>
			<td style="width:89px;">H325-100</td>
			<td>Analytical grade (Used to clean wafer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">hydrochloric acid</td>
			<td style="width:71px;">Fisher Scientific&#160;</td>
			<td style="width:89px;">S25358</td>
			<td>Analytical grade</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Ammonium hydroxide</td>
			<td style="width:71px;">Fisher Scientific&#160;</td>
			<td style="width:89px;">A669S-500SDS</td>
			<td>Analytical grade (Used to clean wafer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">hydrogen fluoride</td>
			<td style="width:71px;">Fisher Scientific&#160;</td>
			<td style="width:89px;">AC277250250</td>
			<td>Analytical grade(used to etch SiO2)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Toluene (anhydrous, 99.8 %)&#160;</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">244511</td>
			<td>Analytical grade (solvent used in Self Assembly of AATMS</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">&#160;Isopropyl alcohol (IPA)</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">W292907</td>
			<td>Analytical grade (Used to make developer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Methyl butyl ketone (MIBK)</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">29261</td>
			<td>Analytical grade(used to make developer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">1:3 MIBK:IPA developer</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;"></td>
			<td>Analytical grade (Developer)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">950 k poly(methyl methacylate (PMMA, 4 % in Anisole)</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">182265</td>
			<td>Photoresist for E-beam lithography</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">Purified Water : Barnstead Sybron Corporation water purification Unit, resistivity of 19.0 M&#937;cm</td>
			<td style="width:71px;"></td>
			<td style="width:89px;"></td>
			<td>Water for substrate cleaning</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Gaertner ellipsometer&#160;</td>
			<td>Gaertner</td>
			<td></td>
			<td>Resist and SAM thickness measurements</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">XPS, ThermoScientifc ESCALAB 250 instrument</td>
			<td style="width:71px;">Thermo-Scientific</td>
			<td style="width:89px;"></td>
			<td>Surface composition</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:373px;">An FEI Siron XL30</td>
			<td style="width:71px;">Fei Corporation</td>
			<td style="width:89px;"></td>
			<td>Characterize nanopatterns</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;">Zeiss sigma VP FEG SEM</td>
			<td style="width:71px;">Zeiss Corporation</td>
			<td style="width:89px;"></td>
			<td>E-beam exposure and patterning</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">MDS 100&#160; CCD camera</td>
			<td style="width:71px;">Kodak</td>
			<td style="width:89px;"></td>
			<td>Imaging drop shapes for contact angle measurements</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Tegal Plasmod</td>
			<td style="width:71px;">Tegal</td>
			<td style="width:89px;"></td>
			<td>Oxygen plasma to etch photoresist</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">I2</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">451045</td>
			<td>Components for gold etch solution</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:373px;">KI</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:89px;">746428</td>
			<td>Components for gold etch solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ellipsometer ( LSE Stokes model L116A);</td>
			<td style="width:71px;">Gaertner</td>
			<td style="width:89px;">L116A</td>
			<td>AATMS self assembled monolayer film thickness measurements</td>
		</tr>
	</tbody>
</table>

use of sacrificial nanoparticles to remove the effects of shot-noise in contact holes fabricated by e-beam lithography

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has been attributed to statistical fluctuations in the number of photons/electrons arriving at a given nano-region arising from shot-noise (SN). The SN varies inversely to the square root of a number of photons/electrons. For a fixed dosage, the SN is larger in EUV and E-beam lithographies than for traditional (193 nm) optical lithography. Bottom-up and top-down patterning approaches are combined to minimize the effects of shot noise in nano-hole patterning. Specifically, an amino-silane surfactant self-assembles on a silicon wafer that is subsequently spin-coated with a 100 nm film of a PMMA-based E-beam photoresist. Exposure to the E-beam and the subsequent development uncover the underlying surfactant film at the bottoms of the holes. Dipping the wafer in a suspension of negatively charged, citrate-capped, 20 nm gold nanoparticles (GNP) deposits one particle per hole. The exposed positively charged surfactant film in the hole electrostatically funnels the negatively charged nanoparticle to the center of an exposed hole, which permanently fixes the positional registry. Next, by heating near the glass transition temperature of the photoresist polymer, the photoresist film reflows and engulfs the nanoparticles. This process erases the holes affected by SN but leaves the deposited GNPs locked in place by strong electrostatic binding. Treatment with oxygen plasma exposes the GNPs by etching a thin layer of the photoresist. Wet-etching the exposed GNPs with a solution of I2/KI yields uniform holes located at the center of indentations patterned by E-beam lithography. The experiments presented show that the approach reduces the variation in the size of the holes caused by SN from 35% to below 10%. The method extends the patterning limits of transistor contact holes to below 20 nm.

Figure 2 shows an SEM image of 20-nm GNPs deposited in 80-nm diameter holes patterned in a 60-100 nm-thick PMMA film driven by electrostatic funneling. As observed by others22, the process resulted in about one particle per hole. The distribution of particles around the center of the holes was Gaussian (top right inset). Most holes (93%) contained one GNP, and 95% of these particles occurred within 20 nm of the center. Further optimization, discuss...

Watch this Scientific Journal Video about Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography at JoVE.com

Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

Uniformly sized nanoparticles can remove fluctuations in contact hole dimensions patterned in poly(methyl methacrylate) (PMMA) photoresist films by electron beam (E-beam) lithography. The process involves electrostatic funneling to center and deposit nanoparticles in contact holes, followed by photoresist reflow and plasma- and wet-etching steps.

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has ...

The overall goal of this protocol is to remove the effect of shot-noise from lithographic patterns using nanoparticles and resist reflow techniques. This nanopatterning technique allows the removal of Shot-noise from extreme UV and E-Beam lithography's currently planned in fabricating advanced semiconductor devices, such as, micro-processors and memory chips. The main advantage of this technique is that the steps are easily implemented.In current semiconductor processing fabs, without extensive modification of existing duel sets. The implication of this technique extends toward our ability to craft sub-20 nanometer patterns because it will deduce dimensional fluctuation from optical and chemical shot-noise effect. Generally, individuals new to this method will struggle because it requires familiarity with bottom-up and top-down processing methods, such as, sub fact self assembly and projection lithography.Demonstrating the procedure will be Dr.Moshood Morakinyo currently at Intel. And he will be assisted by Srikar Rao, a Graduate student in my lab. To being the procedure, prepare SC-1 and SC-2 solutions for the RCA cleaning method.Immerse the silicon wafer in the SC-1 and SC-2 solutions in turn for 10 minutes each at 70 degrees Celsius. Rinse the wafer with deionized water after each immersion and then dry the wafer in a stream of Nitrogen gas. Next, soak the clean, dry silicon wafer in a 0.05 molar solution of AATMS in dry toluene at 80 degrees Celsius for 20 minutes to derivatize the wafer surface.Sonicate the wafer in toluene at room temperature for five minutes at 100 watts and dry the wafer with Nitrogen gas. Apply a photo-resist film of poly methyl methacrylate to the prepared wafer by spin coating. And then pattern holes in it with electron beam lithography.Prepare a suspension of citrate capped gold nanoparticles of a smaller diameter than the patterned holes. The GNP concentration may range from 5.7 times 10 to the 9th to 10 to the 12th nanoparticles per milliliter depending on the GNP size. To deposit GNPs in the patterned contact holes by immersion, soak the patterned wafer in the suspension for 24 to 48 hours, depending on the GNP size.Alternatively, to deposit GNPs by evaporation, first place the patterned wafer on the flat surface, spread drops of GNP suspension over the entire surface of the wafer. Keep the wafer on a hot plate at 30 to 35 degrees Celsius for 10 minutes to evaporate the solvent. After GNP deposition, ultrasonicate the wafer in a deionized water bath for 50 seconds at 100 watts.Dry the wafer with Nitrogen gas. To perform top-down scanning electron microscopy of the wafers, set the electron beam to the lowest possible acceleration voltage and current to reduce the damage to the photo resist film during imaging. Heat the wafer on a hot-plate at 100 degrees Celsius for three minutes to facilitate reflow of the PMMA photo resist around the deposited GNPs.Dry etch the wafer with Oxygen plasma for about 55 seconds to expose the GNPs in the contact holes without completely removing the PMMA film. Monitor the etching rate throughout the process to ensure that the PMMA film is not completely removed. Then, wet etch the GNPs for 10 minutes with a solutions of 1.0 grams of Iodine crystals and 4.0 grams of Potassium Iodide and 40 milliliters of deionized water to remove the GNPs from the film.Image the wafer with scanning electron microscopy. Calculate the hole centers, GNP displacement, particle count, particle density, and fill fraction from the SCM images before and after reflow and etching. Using this method, 20 nanometer GNPs were deposited in 80 nanometer holes patterned in a PMMA coated silicon wafer.The PMMA photo resist was heated to just below it's glass transition temperature to enable photo resist reflow around the GNPs, erasing the nano-hole patterns, created by electron bean lithography. After re-exposing the GNPs by dry etching, the GNPs were removed by wet etching, leaving 20 nanometer diameter holes in the PMMA film and the same arrangement as the original pattern. When the immersion deposition method was used, 93%of the holes were singularly occupied and 95%of the particles were deposited within 18 nanometers of the hole centers.The evaporation method resulted in many holes being occupied by multiple GNPs, indicating that the process requires further optimization. Extraction of the contribution of photo resist reflow on displacement of the 20 nanometer GNPs, indicated that the reflow process had a negligible effect on GNP positioning. The coefficient of variation of the 20 nanometer holes created by this method was determined to be about 9%which was comparable to the coefficient of GNPS size variation.This was roughly a six fold improvement over the predicted CV of electron beam lithography patterned 20 nanometer holes and about a 60%improvement over the predicted CV of the original 80 nanometer holes. Once mastered, this technique can be done in 24 hours if it is performed properly. While attempting this procedure, remember to use mono-dispersed nanoparticles and mild ultrasonication to remove loosely bound nanoparticles on the resist surface.Though this method has been demonstrated using E-Beam lithography, it can also be applied to other systems based on extreme UV, X-ray, or other high energy exposure systems. I developed this idea for this method during an Intel sponsored project on lithography. Now this technique provides a new approach to remove effects of shot-noise and line edge roughness in lithography.After watching this video, you should have a good understanding of how to deposit nanoparticles, reflow resist and etch nanoparticles to reduce shot-noise in nano fabrication.

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