xiaogan liang, keith j. morton, robert h. austin, and stephen y. chou

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“Single Sub-20 nm Wide, Centimeter-Long Nanofluidic Channel

Fabricated by Novel Nanoimprint Mold Fabrication and Direct Imprinting”

Xiaogan Liang, Keith J. Morton, Robert H. Austin, and Stephen Y. Chou

Nano Lett., 2007, 7 (12), 3774-3780

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What we need…

From microfluidics to nanofluidics…

From random nanopores to nanochannels…

“Single Sub-20 nm Wide, Centimeter-Long Nanofluidic Channel…”

Single channelSub-20 nm widthCentimeter length

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Limitations of writing tools

Electron/ion beam lithography or scanning probeWriting field restricted to ~100 umStitching multiple fields too inaccurate for sub-

20 nm structuresFixed-beam/-probe tools with a moving stage

cannot maintain sub-20 nm over centimeter distances.

Writing tool noise/Line edge roughness (LER)Average size of 5-50 nmClogs channel before width is reduced to 20

nm

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Fabrication – Mold FabricationSiO2 mask layer on SOI

waferPatterned by

photolithographyPreferentially etch

<111> directionRemove mask layerConformal LPCVD of

uniform SiNEtch SiN, selective Si

etchPattern additional

device

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Fabrication – Direct ImprintingRelease agent

treatment

Imprint channel in functional material

Optionally use RIE to transfer channel to substrate

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Key advantages

Atomic smoothness of sidewall over several centimeters Overcomes LER from photolithography

Channel width tightly controlled by LPCVD thickness Limited by thin film deposition not lithography

resolutionChannel uniformity and continuity ensured by

conformal deposition Roughness doesn’t clog channel

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Results

SiO2 LER (3σ): 100’s nm

In contrast, anisotropically etched Si nearly atomically smooth and vertical.

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Results

SiO2 LER (3σ): 100’s nm

In contrast, anisotropically etched Si nearly atomically smooth and vertical.

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Results

Kink shift induced by misalignment with {111} crystallographic axis.

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Results

Mold LER (3σ): 1.6 nm

Imprint LER (3σ): 3 nm

RIE etched SiO2 LER (3σ): 6 nm

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References

Xiaogan Liang, Keith J. Morton, Robert H. Austin, and Stephen Y. Chou, Nano Lett., 2007, 7 (12), 3774-3780

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“Improved nanofabrication through guided transient liquefaction”1

and “Sub-10-nm Wide Trench, Line, and Hole Fabrication Using Pressed Self-Perfection”2

Jong-Sun Yi

1Stephen Y. Chou & Qiangfei Xia, Nature Nanotechnology, 3, 295 - 300 (2008) 2Ying Wang, Xiaogan Liang, Yixing Liang and Stephen Y. Chou, Nano Lett., 2008, 8 (7), pp 1986–1990

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Improving Fabrication

Overcome limitations, including defects, line-edge roughness, and minimum size for feature linewidth.Extrinsic defects (e.g., deviations from intended

design)Intrinsic limitations: caused by the fundamentally

statistical nature of a fabrication method (e.g., noise in photon, electron, or ion generation,

scattering, variations in chemical reaction)

Demonstrate a new method to remove defects, improve and even reshape nanostructures after fabrication: self-perfection by liquefaction (SPEL)

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Not completely new…Lasers have been previously used for similar

applications.e.g., surface planarization, edge roughness

smoothing of optical disks (below), etc.

Nature 421, 925-928 (27 February 2003)

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SPELThree forms demonstrated: open-SPEL, capped-SPEL, guided-

SPELSelective melting of nanostructures for short periods under

different boundary conditions

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ImprovementsLine-edge roughness (LER)

Figures of merit: standard deviation (σ) and correlation length (ξ)

Smoothing to below the red-zone limit (3 nm)Reshaping of structure

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Results – open-SPELSubstantial reduction

of LERDrawback: Grating

lines suffer from rounded sidewalls and top-surface

Near-perfect circular dots

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Results – capped-SPELSimilar improvement

of LERProduces flat top-

surface and vertical sidewalls

May be possible to keep corners sharp

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Results – guided-SPELMolten structures rise

against surface tension until they reach the plate.

Higher aspect ratios due to conservation of material volume

Not clearly understood, as the high surface tension of Si and Cr should require strong pulling forces.

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Limitations and Future work

Cannot be applied when defect dimensions are comparable with dimensions of the structure.

Cannot fix defects where the total materials are insufficient.

Ends of lines become rounded

Effect on complex structures?

Multiple laser pulses to further improve LERExploiting different surface propertiesApplicable to metals, semiconductors, and polymersScale to large-area wafers

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Sub-10 nm trench, line, and holea. nanoimprinted 200

nm period polymer grating

b. after P-SPELc. cross-section shows

possible partial-joining at base of adjacent lines

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Sub-10 nm trench, line, and hole

After removing residual polymer between lines (O2 RIE) with Cr maska) CF4/H2-RIE to

transfer pattern into Si

orb) Cr deposition to

create lines

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References

“Improved nanofabrication through guided transient liquefaction”, Stephen Y. Chou & Qiangfei Xia, Nature Nanotechnology 3, 295 - 300 (2008)

“Sub-10-nm Wide Trench, Line, and Hole Fabrication Using Pressed Self-Perfection”, Ying Wang, Xiaogan Liang, Yixing Liang and Stephen Y. Chou, Nano Lett., 2008, 8 (7), pp 1986–1990