1. 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 , 2007)
Presented by Darsen Lu (3/19/2007)
EE235 Class Presentation on Lithography (Spring 2007)

2. 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)

3. 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)

4. EE235 Class Presentation on Lithography (Spring 2007)
Introduction
Dip Pen Nanolithography
Atomic Force Microscope
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.
EE235 Class Presentation on Lithography (Spring 2007)

5. 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)

6. 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)

7. 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)

8. 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)

9. 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)