Presented by Darsen Lu (3/19/2007) Fabrication of high-aspect-ratio silicon nanostructures using near-field scanning optical lithography and silicon anisotropic wet-etching process by S. J. Kwon, Y. M. Jeong, and S. H. Jeong Gwangju Institute of Science and Technology (Korea) Applied Phys. A – Material Science and Processing, vol. 86, pp. 11-18, 2007) Presented by Darsen Lu (3/19/2007) EE235 Class Presentation on Lithography (Spring 2007)

EE235 Class Presentation on Lithography (Spring 2007) Outline Introduction Description of the tools and experimental setup Fabrication Results Conclusion EE235 Class Presentation on Lithography (Spring 2007)

EE235 Class Presentation on Lithography (Spring 2007) Introduction Scanning probe lithography (SPL) SPL is a nanopatterning technique based on scanning probe microscope technologies Various forms of SPL techniques include Atomic Force Microscope (AFM) Lithography Dip-pen lithography Near Field Scanning Optical lithography (NSOL) As opposed to the electron beam (e-beam) lithography, SPLs can be exercised under ambient conditions This Paper The NSOL is used to pattern nanochannels with high aspect ratio EE235 Class Presentation on Lithography (Spring 2007)

EE235 Class Presentation on Lithography (Spring 2007) Introduction Dip Pen Nanolithography Atomic Force Microscope http://www.nanoink.net/WhatisDPN.htm http://stm2.nrl.navy.mil/how-afm/how-afm.html Near Field Scanning Optical Microscopy In near field scanning optical microscopy (NSOM) a very small light source very close to the sample is scanned. Light passes through a sub-wavelength diameter aperture and illuminates a sample that is placed within its near field, at a distance much less than the wavelength of the light. The resolution achieved is far better than that which is attainable in conventional optical microscopes. http://en.wikipedia.org/wiki/Near-field_scanning_optical_microscope EE235 Class Presentation on Lithography (Spring 2007)

The Near-field scanning optical lithography (NSOL) system 442nm, 130mW, He-Cd Laser Source ND Filter Shutter Fiber Position Feedback System Light goes through the apertured cantilever nanoprobe and expose the photoresist (PR). The nanoprobe moves along a trajectory to pattern the PR Power Meter Motorized Stage Apertured Cantilever Nanoprobe Wafer + PR EE235 Class Presentation on Lithography (Spring 2007)

EE235 Class Presentation on Lithography (Spring 2007) Fabrication process Goal: fabricate high-aspect-ratio (HAR) nanochannels Procedure: Start with a 110 wafer Deposit 10nm nitride using LPCVD A commercial photoresist is deposited on the wafer The 111 direction of the wafer is aligned precisely to the y-axis of the microscope The NSOL scans through the programmed trajectory PR Development RIE etch of PR and nitride in CF4/O2 Plasma KOH wet etch EE235 Class Presentation on Lithography (Spring 2007)

Results: After PR Development Good linewidth uniformity is achieved (Fig. 1, 2) Reproducibility over the same sample has been proved (Fig. 3) Reproducibility between different samples requires further study (Fig. 4) Solid lines: first sample Dotted lines: second sample 13% Fig. 3 7.7% Fig. 1 Fig. 2 Fig. 4 EE235 Class Presentation on Lithography (Spring 2007)

Result: After KOH wet etching Wet Etching Result 1. Width 200nm / Depth 800nm 2. Width 230nm / Depth 1560nm Compare with theoretical KOH selectivity Reason for width increase: Nonzero etching in the {111} plane Thinning of Si3N4 at the edge of the channel during the dry-etching The. {111} : {110} = 1 : 600 Exp. {111} : {110} = 1 : 26 NSOL effect was proven by closing the shutter during the scanning process (see right figure) EE235 Class Presentation on Lithography (Spring 2007)

EE235 Class Presentation on Lithography (Spring 2007) Summary An NSOL system has been set up for nanofabrication Lines have been patterned and good linewidth uniformity has been demonstrated. However, further study is needed to improve the reproducibility over different samples. Nanochannel has been produced after wet etching. A linewidth of ~200nm and an aspect ratio of 1:6.8 has been achieved. This nanofabrication technology could be potentially applicable to the fabrication of micro/nanofluidic channels, substrates for biomolecular analysis, etc. EE235 Class Presentation on Lithography (Spring 2007)