Nano Pantography

7/26/2019 Nano Pantography

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Overview: Patterning at the sub-10 nm scale is essential for the fabrication of future

integrated circuits and other nanodevices. Although techniques have been demonstrated withsub-10 nm resolution, their integration into manufacturing is hampered by serious technical

and/or economic issues. Several years ago, we demonstrated a nanopantography method

for massively parallel formation of nanosized patterns over large areas.A broad,

monoenergetic ion beam was directed at an array of electrostatic microlenses fabricated on aSi wafer by standard methods. "Beamlets" entering the lenses focused to a spot 100X smaller

than the lens diameter.By tilting the wafer, each beamlet wrote a desired pattern on the wafer

surface. Recently, we improved the throughput and resolution of this nanopatterning methodwith a two-step process, in which nanopatterns are first formed in a very thin masking layer

by nanopantography, and then transferred to the underlying material by highly selective

plasma etching. Using this technique, we were able to write arrays of nano-holes in Si withdiameter down to 3 nm. The proposed work will demonstrate a massively parallel method to

repeatedly write nanopatterns in 2-D materials (graphene and WS2) on a substrate with a

better than the state-of-the-art resolution of 3 nm, using a reusable stencil mask lens array.

Intellectual Merit: While nanopantography can form complex patterns with very highresolution, the process relies on the fabrication of a lens array on each substrate, adding to the

complexity of use in manufacturing. To address this, we propose separating the lens arrayfrom the wafer, so that it can be reused for patterning of subsequent wafers. The lens array

will be fabricated as a stencil mask, to be clamped to the substrate that will be coated with a

2D layer of graphene or WS2. A precise gap of typically 1 m between the lens array and thesubstrate will be set by posts fabricated on the stencil mask. Positive voltage (e.g. 100 V) will

be applied to the lens array, causing ions entering each lens to focus on the groundedsubstrate. Application of this voltage will serendipitously, electrostatically clamp the mask to

the substrate. Feasibility calculations show that mask distortion due to the electrostatic force

will be tolerable, and a beam spot size of 3 nm can be obtained. After processing, voltage

will be turned off and the stencil mask will be released, then placed on sequential substratesto demonstrate a print-and-repeat process. Nano holes, dots, and ribbons formed in the 2Dlayers will be characterized for plasmonic and other optoelectronic properties. Ion trajectory

simulations will provide optimum voltages for best focus, as a function of gap between the

stencil mask and the substrate, and plasma beam conditions. Molecular Dynamicssimulations will be used to study nanofeature etching in graphene on SiO2by O

+ and O2

+,

with an emphasis on the effect of ion energy and mass on feature size. Quality control

measures will be demonstrated for potential pilot line manufacturing.

Broader Impact: The proposed work will provide challenging projects for four Ph.D.

students and several undergraduates, with rich scientific and educational payoffs, as well as

technological advances. Basic knowledge will emerge on nanofabrication, advanced plasmasources, and ion-surface interactions. The project will have broad impact on diverse areas of

nanotechnology, including, but certainly not limited to microelectronic devices.

Nanopatterning of 2-D materials will be incorporated into an NSF-seeded, multidisciplinaryNano-Engineering Minor Option (NEMO), a subset of which is now an integral part of the

undergraduate curricula of our home departments. Also, we will continue to recruit

undergraduate students and local high school teachers with the help of the REU and Research

Experience for Teachers (RET) programs of our Engineering College.


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1.0. INTRODUCTION

Lithography at the sub-10 nm scale is essential for the fabrication of future integrated circuits, aswell as many other nanodevices. A major contender for next generation lithography, extremeultraviolet (EUV) lithography, has been delayed due to many issues, including low throughput

and high cost[1, 2]. Thus, the search for alternatives to photolithographic techniques continues.Block copolymer directed self-assembly (DSA) can produce a limited variety of patterns withdimensions < 10 nm.[3-8] Nanoimprint lithography shows promise for low-cost, high-throughputfabrication, with ultimate resolution in the sub-10nm regime, but because it is a contact process,defect reduction has slowed its introduction to manufacturing. [9-11] Ion or electron beamproximity or projection lithographies can define features finer than 20 nm, but proximity printingrequires stencil masks with equally fine features,[12] and projection printing needs large fieldoptics and is subject to Coulomb interaction limitations[13]. Both proximity and projectionprinting also require extreme measures to be taken to minimize vibrations and misalignment dueto thermal expansion. Multiple e-beam lithography integrates tens of thousands of 5 keV-electron beams for parallel writing, but it has only demonstrated a resolution of 32 nm at highthroughput.[14, 15] Scanning probe lithography can achieve sub-10 nm resolution, but thismethod is too slow for large area fabrication.[16-18]

Nanopantography is a relatively new patterning method for massively parallel writing ofnanofeatures over large areas. The method is schematically depicted in Fig. 1. Billions ofelectrostatic lenses are first fabricated on top of a wafer using conventional semiconductormanufacturing processes. A broad area, collimated, monoenergetic ion beam is then directedtowards the wafer surface. By applying an appropriate DC voltage to the lens array with respectto the wafer, the ion beamlet entering each lens converges to a fine spot focused on the wafersurface that can be 100 times smaller than the diameter of the lens.[19, 20] By controlling the tiltof the substrate with respect to the ion beam, the focused ion beamlets can write a desired

pattern in a massively parallel fashion in selected areas of the substrate. Nanopantography hasbeen employed to fabricate


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Graphene and other 2-D materials have recently been synthesized in large sheets, offering anopportunity for fabrication of next generation electronic and photonic devices. This leads to ademand for large-area patterning of devices with sub-10 nm features, a particular challenge formore conventional methods, where rapid prototyping and manufacturing-worthiness is hamperedby their high cost, complexity and/or low throughput.

Broad area collimated ion beamVb

conducting substrate

metal dielectric

+

-

Whatever is written once on the

imaginary plane here

is reproduced at the bottoms of all

the lenses here.

10nm

AFM and SEM images of Ni nano-dots

SEM of 3

nm hole

written in Si

Fig. 1.Top: Schematic depiction of nanopantography, writing massively parallel nanopatterns. Bottom:AFM and SEM images of Ni nanodots deposited by nanopantography and SEMe of one of billions of 6.9nm-wide 41 nm-long trenches, and 3 nm dia. holes etched at the center of each lens by nanopantography.

While nanopantography can achieve sub-10 nm resolution, along with the ability to formintricate patterns, the process relies on the existence of a lens array on each substrate, which addsto the complexity of using the system in manufacturing. This is because (1) the lens array mustbe removed after the patterns are formed, (2) fabrication and removal of the lens arrays adds tothe cost of the system, (3) the fraction of the wafer surface that can be patterned is limited by thegeometry of the lenses, and (4) the lenses are difficult to fabricate on top of fragile materials,

such as graphene sheets. To extend nanopantography to a more general manufacturingmethod, we propose to separate the lens array from the substrate so that it can be reusedfor the patterning of subsequent wafers, much like a mask in optical lithographic printing.

We will use this approach to demonstrate print and repeat fabrication of nanostructures in

graphene and WS22-D films on Si.

2.0. Results from Prior NSF Support

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Award: CMMI 1030620, PI: Vincent M. Donnelly, co-PI: Demetre J. Economou, Amount:$450,000, Period of support: 08/2010 - 07/2014, Title: Large Area, Rapid Manufacturing ofVirtually Any Nanopattern Using Nanopantography.

Intellectual Merit: A two-step

process to improve thethroughput and resolution ofnanopantography was developed(Fig. 2). In the first step, thedesired pattern was defined on aSi wafer using nanopantographywith a short exposure time. Thepurpose was not to etch deepinto silicon, but to break throughthe native oxide on top ofsilicon, creating a shallow

pattern (only 10s of deep) onthe wafer surface. In the secondstep, the patterned Si wafer wasetched in a Cl2plasma, using thepatterned native oxide as a mask.This advance was aided by thediscovery that p-type Si could beetched in chlorine-containingplasmas with ion energies belowthe ion-assisted etchingthreshold (5:1) features to be fabricated (Fig. 3).

Under the conditions of etching stimulated by VUV photons (at extremely low ion bombardmentenergies), a surprising new effect was discovered, in which plasma etching was faster fortrenches with widths smaller than the wavelength of light. In particular, light at wavelength ~100 nm, much greater than the feature size of ~10 nm, penetrated deep (~100 nm) into thetrench. This may be explained by light-guiding effects due to the negative refractive index ofsilicon and/or surface plasmon resonances. The highly selective chlorine plasma etching processused to amplify nanopantography-defined shallow patterns, improved both writing speed andfeature resolution. Instead of etching a 100 nm-deep feature in Si, only a 1-2 nm of SiO2neededto be removed and this reduced the nanopantography process time from 30 minutes to 50seconds. In addition, the resolution was improved because a much thinner layer of material hadto be removed by nanopantography. The depth of focus (DOF) of the ion beam was shallow,only ~20 nm. Etching beyond the range of the DOF would create features larger than the focalspot size, degrading the inherent resolution of the technique. By reducing the thickness to a fewnm, the etched layer fell completely into the DOF of the ion beamlets, resulting in improved

Fig. 2(a) Electrostatic lens structure. (b) A DC potential isapplied between the metal and the silicon wafer to focus the ionbeamlet. A shallow pattern is formed by nanopantographicetching of the native oxide of silicon using focused Ar+ionbeamlets and Cl2gas. (c) Pattern formed in (b) is transferreddeep into silicon by highly selective chlorine plasma etchingusing native oxide as mask.

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resolution. One of the first examples of the writingcapabilities of nanopantography with pattern transferis shown in Fig. 4, where interlocking UH logoswere written over 225nm!250nm areas. The line nearthe crossing point of the right vertical line of letter U

and the horizontal line of letter H is ~10nm-wide.Publications from this work include refs.[21-27]Broader Impacts: The work was the subject of thePhD dissertation of Siyuan Tian. He will start at LamResearch in March 2015. An undergraduate student,Eduardo Hernandez, Dept. of Chemical Engineering,Polytechnic University of Puerto Rico, was supportedby the NSF REU program in the summer 2013,worked on, and co-authored a publication on photo-assisted plasma etching,[28] a key study enabling theadvancement in nanopantography. Eduardo has now

joined our research group to work towards a PhD inChem. Eng. Two more undergraduates worked on theproject: Sean Hensarling who graduated with a Chem.Eng. degree from UH, and Shoshauna Harisson (blackfemale), currently a Chem. Eng. student at UT-Austin.

Award: CBET-1236606, PI: Dong Liu, co-PI: PaulRuchhoeft, Amount: $299,900, Period of Support:07/01/12--06/30/15,Title: Creating Tunable AdaptiveBoiling Heat Transfer Surfaces with Electrowetting.

Intellectual Merit: For this project, we have

developed active surfaces that allow for high thermaltransfer at a much wider range of powerdensities, and we are studying thefundamental principles of this new heattransfer process to design next-generationheat exchangers. The approach takesadvantage of the complimentary roles ofhydrophobicity and hydrophilicity innucleate boiling heat transfer, and we use anelectrowetting (EW) approach todynamically alter the surface wettability at

different thermal loads. At low-to-moderateheat fluxes, the boiling surface remainshydrophobic so that onset of nucleate boilingcommences spontaneously and excellentboiling heat transfer can be obtained. When

the bubble growth and merger intensify at high fluxes, EW is activated to make the surface morehydrophilic, thereby delaying the onset of the critical heat flux (CHF) that can cause a heatexchanger to fail. We have developed tunable boiling surfaces by using silicon wafers with

Fig. 4 Interlocking UH logo etched in siliconusing the present method of pattern transfer. Only12 of the 7.5 million lenses are shown.

Fig. 3. SEM cross sections of holeetched in silicon using the two-stepmethod of pattern transfer. Top: Aftershallow etching, using nanopantographyto break through the native oxide on Si.Bottom: After highly selective plasma

etching to a depth of 80 nm.

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thermal oxide as an electrode, which are coated with a Teflon layer to adapt the wettability.Broader Impacts: We have studied the effects of EW on the overall boiling heat transfercharacteristics as a function of EW signal frequency and have demonstrated a 2.3 timesimprovement over conventional hydrophobic surfaces. We are now adapting our fabricationapproach to allow formation of patterned surfaces with improved nucleation characteristics. This

work involved 3 PhD students (1 female), and 1 has graduated. One journal article has beenpublished,[29] and 3 are in preparation.

Award: ECCS-1240510, PI: Jiming Bao, Amount: $400,000, Period of support: 6/2012 -5/2017,Title: CAREER: Doped Graphene: a Transformative Paradigm for Plasmonics and Two-Dimensional Nanophotonics.

Intellectual Impact:With this support from NSF, we have established a new graphene synthesissystem, we then studied the growth mechanism of bilayer graphene, especially twisted bilayergraphene. We observed and explained G-line resonance and enhanced 2D Raman band due toquantum interference in twisted bilayer graphene. We also investigated unique optical propertiesof graphene oxide liquid crystals, synthesized MoS2, fabricated and characterized MoS2 fieldeffect transistors. Because surface plasmon resonances of large graphene nanodisks and ribbonshave been reported by other groups, we are using the proposed technique to fabricate holes andribbons with smaller size. We are also investigating the effect of graphene on the surfaceplasmon resonance of Au thin film, we discovered that it is the out-of-plane rather than in-planerefractive index of graphene that determine the shift of surface plasmon resonances. BroaderImpacts: This work has involved two PhD students (including one female), and one hasgraduated. Eight journal articleshave been published, and threemore are in preparation.[30-37]

3.0 PROPOSED PRINT AND

REPEATNANOPANTOGRAPHY

METHOD

Figure 5 shows a schematic of thecurrent experimental set up usedfor nanopantography. For ion-assisted patterning of Si bychlorine, ions (Ar+ in this case)were generated in an inductivelycoupled plasma (ICP) source, and

were extracted through a grid.Ions then passed through a 5 mm-diam. limiting aperture and drifteddownstream, in a differentiallypumped high vacuum chamber(drift tube), before entering theprocessing chamber. The Si substrate was housed in the processing chamber on a computer-controlled stage that could vary the tilt of the substrate with respect to the ion beam, as shown

Fig. 5. Schematic of the nanopantography apparatus.

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schematically in Fig. 1. Cl2flooded the Si substrate and etching occurred at the focal points ofthe ion beamlets on the Si surface. Different plasma feed gases will be used for patterning ofother materials such as graphene and WS2 (see below). The proposed printing process withstencil masks would have the potential for processing very large substrates in a single step, butwould be demonstrated on small substrates (10 cm dia.). Also, the step-and-repeat method that

has been developed to extreme precision in photolithography would be applicable, but will notbe implemented in the proposed research because it is mature technology. Instead, the proposedwork will demonstrate amassively parallel method torepeatedly write nanopatterns ona substrate with a better thanstate-of-the-art resolution of 3nm, using the same, removablelens array. This process isshown schematically in Fig. 6.The stencil mask, described in

more detail below, consists ofan electrostatic lens array with aconducting layer encapsulatingan insulting layer of typically500-1000 nm thickness. Themask will be placed in closeproximity (~ 1m) with thesubstrate, and loaded into theprocessing chamber through aloadlock. A positive voltage oftypically 100 V will be applied

to the conducting layer, causingions entering the holes on thelens array to focus at the bottoms of the holes. This also has the serendipitous effect ofelectrostatically holding the stencil mask to the substrate, much as an electrostatic chuck inplasma processing tools, as expanded upon below. After patterning, the voltage on the metallayer will be returned to zero, extinguishing the clamping pressure. The stencil mask will then beremoved and placed on a new substrate, and the print process will be repeated. The details ofhow we propose to do this and target applications are discussed below.

3.1. Reusable Stencil Mask Lens Array3.1.1. Electrode Fabrication Procedure

Membrane Fabrication:Fig. 7 shows a schematic of the membrane fabrication sequence, where(a) a 100 mm diameter, double-side polished silicon wafer, coated with 500 nm of silicon-richsilicon nitride with a tensile stress of 50-100 MPa, will be purchased from Silicon Quest (SanJose, CA) or a similar vendor. (b) Membranes will be formed by lithographically defining a setof windows in a 1 m-thick layer of photoresist (AZ1512, Clariant, Singapore) by contactlithography. We expect to define rectangular regions that are 5!30 mm2in area, covering about50% of the sample surface area; this geometry will be refined as needed. The resist pattern willbe transferred through the silicon nitride layer using CF4+ O2reactive ion etching (RIE). (c) The

Fig. 6Schematic of the print and repeat process.

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Si wafer will then be subjected to crystallographic etching using a solution of 30% (by weight)KOH in water, leaving behind silicon nitride membranes supported by a silicon frame. Tensilestress on the nitride film ensures that the membranes remain taught. As the project progresses,the KOH etch may be replaced by RIE to improve the packing density of the membranes and therigidity of the support frame using a deep RIE tool at the Nanofabrication Center at UH.

Lens Array Fabrication: Fig. 8 shows aschematic of the lens array fabrication: (a) after the membranes are formed, the planar side of the

wafer will be coated with a 20 nm thick layer of copper and a 200 nm thick layer of poly(methylmethacrylate) (PMMA). Next, (b) the lens array consisting of 300 nm-diam. circles on a 600 nmpitch, will be patterned by electron beam lithography. (c) After development, the PMMAfeatures will be transferred through the Cu layer by Ar+milling; the Cu subsequently acts as ahard mask for etching the silicon nitride using SF6+ O2RIE. (d) A Cr electrode will be depositeduniformly over the structure completing fabrication of the electrode array. This approach is verysimilar to the membrane fabrication sequence employed in our lab.[38-41] Also, we can coat themembrane with a thicker metal layer to further reduce the lens size without the need to patternsmaller openings[42].

Spacer Fabrication:To form the support posts that ensure a uniform gap between the lens array

and the wafer, and minimize the contact area (Fig. 9), (a) the planar side of the wafer is (b)coated with a layer of photoresist, whose thickness determines the gap, and (c) a pattern of postsis defined using e-beam lithography. There is much flexibility in this step, as we can explore theuse of various resists with a variety of dielectric constants, breakdown strengths, etc. If thethroughput for spacer fabrication is too low, we have an ion beam proximity lithography tool andreactive ion etch processes similar to those in refs.[43, 44], which would give us the addedadvantage of exploring the use of polymers that are not sensitive to electrons.

Fig. 7.Diagram of the membrane fabrication

sequence.

Fig. 8.Diagram of the lens fabrication sequence.

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Stencil masks very similar to those needed in the proposed work have already been fabricated inour facilities. Fig. 10 shows electron microscope images of 100 nm diameter openings with a 200nm pitch etched through the thickness of a 300 nm-thick silicon nitride membrane, imaged fromthe (a) front side and (b) backside. The size of the features remains the same on both sidesimplying that etching is anisotropic. The size of the openings could be reduced further by coating

the membrane with a 100 nm-thick layer of gold, as shown in (c). Our lens array will typicallyconsist of 300 nm-diam. openings etched through 500-1000 nm thick membranes. The lens arrayfabrication requirements are comfortably within the patterning capabilities of our labs.

3.2 Optimizing the Gap

Currently, the gap between the electrode of the lens array and the substrate is controlled by auniform silicon dioxide layer that is deposited onto the silicon wafer. This gap was chosen to be


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In nanopantography, an array of lenses simultaneouslyprojects the image of a source (e.g. ion source) onto thesubstrate, as shown schematically in Fig. 11, so that

"

#

$"

%1&

"

%2 (")*

where %1 and %2 are the distances from the lens to theimage and object planes, respectively, and #is the focaldistance. Because the distance to the image is so muchsmaller than the distance to the object, #%2$ +,- and %1+,. /for our previous results. This implies amagnification of about 010 $

ss$ 2 3 "+7. Thus, thediameter of the image 4iof our 5 mm-diameter source(4s) should be about 2.5 nm. The smallest feature wehave been able to form is around 3 nm in diameter.[19,20] The small difference could be attributed to sphericaland chromatic aberrations of the lens itself. Theseparameters can be modeled by their Zernekecoefficients and vary with the numerical aperture (NA)of the lens (56 $ %78 for small angles), where theblur contributed by the spherical aberrations isproportional to 563 and that contributed by chromatic

aberrations is proportional to 56.[45] The overall spot size can be approximated by the squareroot of the sum of the squares of the blur introduced by the source size (i.e., penumbra) and theblur attributed to the spherical and chromatic aberrations

4i$01024s2& 9s22& 9c2

6 (:)*

where 4sis the (ion) source diameter, and 9sand 9care the spherical and chromatic aberrationcoefficients. While the aberrations are quite low for 3nm patterns, this will not be the case forsmaller image (e.g. 1 nm). Increasing the gap between the lens array and the wafer will reducethe numerical aperture and improve the resolution, but then the source diameter has to bereduced (since 010 decreases), which will drop the current, and reduce the throughput of thesystem. However, we not limited to a circular aperture for the source; the use of a beam-shapingsource aperture will allow the lens array to project any desired shape. For example, if we use aplus-shaped source aperture that has 1.7 mm arms and is 10 mm long, we will form a plus-shaped image with a 3 nm wide arm, which is 18 nm long, even when printing with a 1 m gap.Under these conditions, we will print >10X faster than the case where a 3 nm-diam. circularimage is used to write the plus shape. Decreasing the arm width to below 1 mm will result in

sub-2nm features (only possible with the larger gap), with only modest reduction in throughput.Moreover, using an array of plus-shaped openings as a source limiting aperture will allowprinting of multiple images by each lens. We expect that each exposure will take less than 0.5seconds to transfer an image into graphene (estimated below), so that the throughput will be 360100 mm-diameter wafers per hour (ignoring any stage movement overhead) if our lens array is 4cm2in area and we are printing a single image of the source. Even if we print 10 copies of thesource with each lens, in order to increase the pattern density, the throughput remains significantdown to sub-3nm resolution.

Fig. 11.Each lens projects the image

of the source into the wafer surface.

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We must also consider the force that is exerted onto the membrane during printing. The clampingpressure (Pa) between the metal layer of the stencil mask and the substrate is given by

2

0

2)(

gd

VPaP

(3)

where the permittivity of free space, 0 = 8.85 x 10-12 F/m, is the dielectric constant of the

insulating layer, V = the voltage on the metal layer of the lens array, d = dielectric layerthickness, and g = gap size between the top of the substrate and the adjacent surface of thedielectric layer. Since the gap-defining posts occupy a very small fraction of the wafer area, Eq.(3) reduces to

2

0

2)(

g

VPaP

(4)

For V = 100 V,g = 1 x 10-6 m, the pressure exerted on the free-standing membrane is 44 kPa or0.45 atm. The maximum membrane deflection, ;, at the center of a square, free-standingmembrane supported by a frame around its perimeter, was previously estimated to be[46]

; $ +,-2 (2)*where is the pressure across the membrane, is the spacing between the frame supports, isthe modulus of the membrane (290 GPa for silicon-rich silicon nitride), and is the membranethickness. While the geometry for this analysis is not identical to the one proposed here, theanalysis does allow us to estimate how much electrostatic membrane deflection we can expect. A1-m spacing will exert about 44 kPa of pressure on the membrane, causing it to deflect about100 nm if the posts are 16 m apart. We can tolerate this 10% variation in the membrane-wafergap because the focal point remains on the wafer surface, even with small changes in thegap.[47] Using 1 m-diam. posts that are 1 m-tall will allow us to join the patterns formed byeach lens (i.e., form continuous lines) by tilting the substrate by only


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LabVIEW program. This tilting allows the focus spots to be moved on the substrate below, asdepicted in Fig. 12. It should be noted that because the space between the top electrode surfaceand the substrate is free space, the focus of one lens can enter the space below an adjacent lens,so that all areas (except those under the posts, as discussed above) are accessible.

The high density inductively coupled plasma (ICP)source allows us to achieve a higher beam currentthan in earlier studies.[19, 20] The source hasmagnetic field confinement and an electronemitting-filament assembly below the extractiongrid to neutralize the positive ion space charge.Electron density ne, electron temperature Te andplasma potential Vp at the plasma center and nearthe edge were measured in pulsed plasma mode. Apower modulation frequency of 10 kHz with 20%duty cycle produced a time-average electron density

of 1.4!

10

11

cm

-3

on axis 0.3 cm above the beamextraction aperture during the afterglow. During the initial stage of the afterglow, high energyelectrons in the tail of electron energy distribution function were quickly lost by diffusion to thewalls, causing the Teto drop rapidly to ~0.2 eV at center and edge of the plasma.

The angular distribution of ions entering the lenses is a major factor limiting resolution. Toreduce the ion angular spread, the diameter of the beam-defining aperture, downstream of theextraction grid, was reduced from 11.4 mm to 5.0 mm, and the extraction grid-sample distancewas increased from 40 cm to 60 cm. Thus, the ion acceptance angle was reduced from 1.64 to0.47. In the case of a 650 nm-diam. lens (NA=0.28), this resulted in improvement of thetheoretical resolution from ~16 nm to ~5 nm. In fact, the modified system produced features with3 nm diameter (Fig. 1).

Though the focus here is in patterning 2-D materials, we will also pattern Si because of itsobvious importance and because we wish to compare the results with the removable mask tothose obtained previously with the integral mask. For Si patterning, Ar is fed to the plasma beamsource and Ar+ in the presence of Cl2 is focused in the lenslets. For graphene patterning withnano holes and ribbons O2will be fed to the source, perhaps with added Ar, and a mix of O

+andO2

+will be extracted. It should be noted that the focusing of ions is independent of mass, so thatboth ions will come to a focus. For WS2 nano-dots, either the WS2 around each dot will beetched away with rastered beamlets from a fluorine-containing plasma source, or metal (e.g. Cuor Ni see Fig. 1 showing Ni nanodots) will be deposited to form a nano-dot mask and theunmasked WS2will be removed with a short fluorine-containing plasma etch. The metal maskwill then be dissolved away (e.g. by FeCl3solution in the case of Cu).

The etching rate for a 0.2 nm-thick single graphene layer, can be estimated based on the presentsystem parameters and the time it took (~50 s) to sputter away a 2 nm native oxide on Si withAr+at 100 eV. The native oxide sputtering rate was 0.04 nm/s. This is twice the reported SiO2sputtering rate of ~0.02 nm/s for Ar+ at 100 eV,[48] and therefore seems reasonable. If theetching rate of graphene with an O+/O2

+ beam is greater than ~2 nm/s, typical for such a

Fig. 12.By tilting the wafer, the focal

spot moves to write any desired pattern.

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chemically-assisted etching process, then it would take 50 x (0.2/2) x (0.02/2) = 0.05 s to removethe layer. Consequently, even with a 10-fold reduction in beam current, it should take about 0.5s. Thus, even with an order of magnitude margin for error, the processing time will be very short.We expect similar etching times for a single layer of WS2, using a fluorine-containing plasma.

4.0 SIMULATIONS

4.1 Ion Trajectory Simulations

The degree of collimation and the spread of the energy distribution of ions focused by the lensarray on the wafer, in close proximity to the array, affects the size of the smallest focal spot thatcan be formed. To identify conditions for minimum possible spot size, ion trajectory simulationswill be performed at two different length scales. First, in a macroscalesimulation, ions will befollowed in their flight (Fig. 5) from the extraction grid (at the bottom of the plasma source),through the drift tube, to the entrance of the lens array. This simulation will provide the angulardistribution of ions approaching the lenses. Second, in the microscale simulation, ions will befollowed as they are focused by the lenses to form a spot on the wafer surface. The angular

distribution of ions from the first simulation, and the measured ion energy distribution (IED),will be used as input to the second simulation. In a further cascade down the length scales,molecular dynamics simulations will be performed to understand modification of the wafersurface at the atomic scale.

The goal of the macroscale simulation is to predict the angular spread of ions entering the lenses.The ion angular distribution can be affected by several factors, including: (a)divergence of ionsas they traverse the holes of the extraction grid, (b)the penumbra effect originating from the factthat the ion source aperture has a finite size, (c) possible divergence of ions due to the spacecharge of the ion beam, (d)possible divergence of ions due to stray electric fields.

In the microscale simulation, ions are deflected by the spatially varying potentials of the lens,and with the proper conditions come to a focus on the substrate surface. A single lens will beconsidered. Ion trajectory simulations will be performed to optimize the focal characteristics ofthe lens. Because of the exceedingly small dimensions (microns), the volumetric charge densityin the region around a lens is negligibly small. Thus, the 2-D Laplace equation will be used todetermine the potential and electric field profiles in the domain. A uniform flux of ions will belaunched at the entrance plane with the angular distribution obtained by the macroscalesimulation, and the measured IED. Using the electric field profile, the 3-D trajectory of ions willbe computed by integrating Newtons equation of motion with a leap-frog method. Integrationwill continue until ions strike the wafer surface. The flux distribution of ions on the surface willthus be obtained. The main control parameters will be the potential applied to the electrode of thelens array, and the gap between the lens array and the wafer. Simulations will be performed forwafers at different angles with respect to the ion beam axis to predict the effect of substratetilting on focal point displacement.

4.2 Molecular Dynamics SimulationsMolecular dynamics (MD) simulations will be employed to study near surface processes at theatomic scale during surface modification by nanopantography, and to guide experimentation. InMD, Newtons equation of motion is integrated for each atom in the system. The force is

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calculated as the gradient of the interatomic potential, usually obtained empirically or incombination with ab-initio calculations. The interatomic potentials for the system of interest(graphene on Si or SiO2) are known[49]. Since the ion bombardment energy is ~100 eV or less, amoderate size computational cell (~ 1000s of atoms) should be adequate. Since we are interestedin features with characteristic size less than 10 nm, the size of the computational cell can actually

be the physical size of the system under consideration. Simulations will be extensions of ourprevious works with MD[50-52]. Graphene cutting and patterning will be studied as a functionof the energy and the angle of impact of the focused ion beamlets.[53]

5.0. APPLICATIONS

There are many applications for high volume nanomanufacturing, including ultrasmall transistorsin integrated circuits, sensors such as those for DNA sequencing, metamaterials that exhibitcontrolled transmission or reflectance of incident electromagnetic waves, and quantum dots thatemit light with tunable colors. Here we will focus on 2D materials such as graphene andatomically thin transition metal dichalcogenides (TMDs) that have attracted much attention in

recent years. One reason is that such 2D structure is compatible with current silicon technology:a large number of devices can be printed on a single 2D material layer using lithography.Miniaturization has been the driving force of silicon technology. It is also important for devicesmade of 2D nanomaterials because the functionality and performance of the device is stronglydependent on size. Nanopantography is a cheap and high throughput technique that enables thefabrication of nanostructures over a large area. To demonstrate the unique capability ofnanopantography, we will fabricate three types of 2D nanostructures: graphene nanohole arrays,graphene nanoribbons, and WS2nanodisks.

Doped graphene is ahigh-mobility freeelectron gas, andgraphene nanostructuresexhibit localized surfaceplasmon resonances asnoble metals. Theresonance frequencystrongly depends onsize[54]. Two methodshave been used tofabricate graphene nano-disks or dots: electron-beam lithography and nanosphere lithography.[55-58]. For both techniques, it is difficult to achieve resolution better than ~20 nm.Furthermore, electron-beam lithography is not suitable for scalable synthesis at a low cost, andnanosphere lithography cannot provide arrays of nanostructures with a long range order. Fig.13(a) shows an array of nanoholes that we propose to make in graphene. The nanohole size willbe systematically varied from ~5 nm to 100 nm. Nanoholes will be created by O+/O2

+ beametching. Nanopantography allows us to rapidly etch such nanoholes without using any resist andsubsequent liftoff. Infrared/UV-visible spectroscopy will be used to characterize the nanoholesurface plasmon resonances. By varying the gate voltage and hole size, surface plasmonresonances can be continuously tuned. Smaller graphene nanoholes are needed to tune the

Fig. 13. Three examples of nanostructures fabricated using direct ionetching of nanopantography. (a) Nanohole array in graphene. (b)Graphene nanoribbons (dark regions). (c) WS2nanodisks (blue dots).

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resonance from infrared to near-infrared and even visible. Such tunability is important to observeunique quantum phenomena in confined 2D system and will find numerous applications such assurface plasmon based sensing, and ultrathin 2D metamaterials.

The high electron mobility of graphene also enables the development of high-speed transistors.

The zero energy bandgap of graphene, however, makes transistors impossible to turn off,resulting in large leakage current and high power consumption. One way to solve this problem isto make graphene nanoribbons, which can have a bandgap due to quantum confinement andsurface edge states.[59] Like graphene nanodisks, graphene nanoribbons can also be used forphotodetectors[60]. A variety of methods have been developed to fabricate graphenenanoribbons. They can be classified as top-down and bottom-up approaches[61]. Examples oftop-down approach include electron-beam lithography[59], nanowire lithography[62], helium ionbeam lithography[63] and copolymer lithography[64]. The bottom-up approach uses chemicalsynthesis from small molecules[65]. Both approaches have limitations[61]. Nanostructures frombottom-up approach are difficult to be assembled in a two-dimensional array. Fig. 13(b) showsone set of nanoribbons; the width of each ribbon is as small as 5 nm. Nanopantography will

allow us to create millions of copies of nanoribbons in parallel. We will first characterize ribbonarrays with optical transmission as done for graphene nanoholes. We will then use a probestation to measure their I-V characteristics.

We will also fabricate quantum dots of 2D transition metal dichalcogenides (TMDs) which offernew opportunities, not provided by graphene. A monolayer of a TMD is a direct bandgapsemiconductor, with numerous applications in photovoltaics, light-emitting devices, andphotodetectors[66-69]. Using nanodisks of TMDs, we can further tune their bandgap and electro-optic properties, leading to new potential applications. However, only chemical synthesis hasbeen used to fabricate TMD nanostructures up to date [70, 71]. Because of high luminescenceand spin-valley interaction, they will find new applications in biomedical labeling, spintronicsand quantum bits [70, 71] [72]. Fig. 13(c) shows how TMD nanodisks will be carved out of largearea WS2 films. Micro-photoluminescence will be used to characterize their light emittingproperties.

6.0. QUALITY CONTROL

6.1 Integrity of the stencil mask and mask-to-substrate gap.

As one measure of quality control, the average gap, g, between the dielectric layer and thesubstrate will be determined each time a pattern is printed by measuring the capacitance. Thiswill also signal a failure of the stencil mask, a clear concern in such a process.

6.2 Reproducibility in printing over the full area of the stencil mask.

Laser diffractometry will be evaluated for monitoring yields in printing over the full patterndefined by the stencil mask. Feature sizes and separation between features written in each lensare much smaller than UV-visible wavelengths, , of convenient lasers, but the separationbetween lenses is comparable to , thus a diffraction pattern will emerge that is the reciprocalspace image of the lens pattern, provided there is enough optical contrast between etched andunetched regions.

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7.0. BROADER IMPACT OF THE PROPOSED WORK

The proposed work will provide challenging projects for four PhD students and a comparablenumber of undergraduates, with rich scientific and educational payoffs, as well as technologicaladvances. Basic knowledge will emerge in nanostructure fabrication, ion beams, and advancedelectronic and photonic devices. A thorough understanding of these subjects will rely on acombination of experiments and simulations.

The College of Engineering continues to support an NSF-seeded, multidisciplinary Nano-Engineering Minor Option (NEMO), a subset of which is now an integral part of a concentrationarea in the undergraduate curricula of our home departments. The program consists of a three-course sequence, each with laboratory, and an independent, mentored research project. A newundergraduate laboratory space has recently been dedicated to this program by the ECEdepartment to support experiments in lithographic techniques, thin-film deposition, and materialsynthesis. We will integrate our research with the laboratory of each of these courses, and wewill develop lecture and laboratory components, based on the proposed work, to complement andenrich the course.

Further outreach activities to attract students and inform the public of the benefits of plasmascience and engineering are as follows:

More than half of the undergraduate students at the University of Houston are minorities. Infact, in 2012, the U.S. Department of Education granted UH its official Hispanic-ServingInstitutiondesignation. Minority undergraduate students will aggressively be recruited for thisproject. Undergraduate students will also be recruited via. the Research Experience forUndergraduates(REU) program of our Engineering College, as well as through the UH SummerUndergraduate Research Program supported by the UH Provosts office. Our labs have hostedseveral undergraduate students who went on to graduate school. Some of them have alreadyfinished their PhD (one is now a ChE faculty member, the rest work for industry). Two of our

recent REU students joined our department and are now PhD candidates in our research group.One of them (E. Hernandez) was a co-author in a publication. We will continue to recruit high school AP science teachers to work in our labs during the

summer months, funded by the Colleges Research Experience for Teachers (RET) program.The RET program provides a mentored research experience culminating in improved lessonplans for High School teachers to bring back to their classroom. In the summer 2008, werecruited Mr. Jose Arroyo, a Houston Independent School District (HISD) teacher, who isteaching at an inner city school with 90% Hispanic students. In the summer 2009, we recruitedMs. Kellie Simon (African American female) of Goose Creek Consolidated ISD. In the summersof 2010 and 2011 we recruited Ms. Irene Fong, from Lanier Middle School, HISD. Mr. Jarrod G.Collins (African American), Klein Forest High School Chemistry Teacher, worked in our lab in

the Summer 2012. In the summer 2013 we hosted Meghan Keefe, Physics and ChemistryTeacher, Cristo Rey Jesuit College Preparatory School of Houston. While at UH, these teachersdeveloped modules that were taken to their school and presented to their science classes. Aprior RET participant in PRs lab was recognized with the Presidential Award for Excellence inMathematics and Science Teaching (PAEMST), a top award K-12 mathematics or scienceteachers, for lessons plans developed through this program.

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