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

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 NanoLett., 2007, 7 (12), 3774-3780

2. What we need… • From microfluidics to nanofluidics… • From random nanopores to nanochannels… • “Single Sub-20 nm Wide, Centimeter-Long Nanofluidic Channel…” • Single channel • Sub-20 nm width • Centimeter length

3. Limitations of writing tools • Electron/ion beam lithography or scanning probe • Writing field restricted to ~100 um • Stitching multiple fields too inaccurate for sub-20 nm structures • Fixed-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 nm • Clogs channel before width is reduced to 20 nm

4. Fabrication – Mold Fabrication • SiO2 mask layer on SOI wafer • Patterned by photolithography • Preferentially etch <111> direction • Remove mask layer • Conformal LPCVD of uniform SiN • Etch SiN, selective Si etch • Pattern additional device

5. Fabrication – Direct Imprinting • Release agent treatment • Imprint channel in functional material • Optionally use RIE to transfer channel to substrate

6. 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 resolution • Channel uniformity and continuity ensured by conformal deposition • Roughness doesn’t clog channel

7. Results • SiO2 LER (3σ): 100’s nm • In contrast, anisotropically etched Si nearly atomically smooth and vertical.

8. Results • SiO2 LER (3σ): 100’s nm • In contrast, anisotropically etched Si nearly atomically smooth and vertical.

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

10. Results • Mold LER (3σ): 1.6 nm • Imprint LER (3σ): 3 nm • RIE etched SiO2 LER (3σ): 6 nm

11. References • Xiaogan Liang, Keith J. Morton, Robert H. Austin, and Stephen Y. Chou, NanoLett., 2007, 7 (12), 3774-3780

12. “Improved nanofabrication through guided transient liquefaction”1and “Sub-10-nm Wide Trench, Line, and Hole Fabrication Using Pressed Self-Perfection”2 Jong-Sun Yi 1Stephen Y. Chou & QiangfeiXia, Nature Nanotechnology, 3, 295 - 300 (2008) 2Ying Wang, Xiaogan Liang, Yixing Liang and Stephen Y. Chou, NanoLett., 2008, 8 (7), pp 1986–1990

13. 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)

14. 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)

15. SPEL • Three forms demonstrated: open-SPEL, capped-SPEL, guided-SPEL • Selective melting of nanostructures for short periods under different boundary conditions

16. Improvements • Line-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

17. Results – open-SPEL • Substantial reduction of LER • Drawback: Grating lines suffer from rounded sidewalls and top-surface • Near-perfect circular dots

18. Results – capped-SPEL • Similar improvement of LER • Produces flat top-surface and vertical sidewalls • May be possible to keep corners sharp

19. Results – guided-SPEL • Molten 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.

20. 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 LER • Exploiting different surface properties • Applicable to metals, semiconductors, and polymers • Scale to large-area wafers

21. Sub-10 nm trench, line, and hole • nanoimprinted 200 nm period polymer grating • after P-SPEL • cross-section shows possible partial-joining at base of adjacent lines

22. Sub-10 nm trench, line, and hole • After removing residual polymer between lines (O2 RIE) with Cr mask • CF4/H2-RIE to transfer pattern into Si or • Cr deposition to create lines

23. References • “Improved nanofabrication through guided transient liquefaction”, Stephen Y. Chou & QiangfeiXia, 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, NanoLett., 2008, 8 (7), pp 1986–1990