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1
“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