Transcript Electrochromic Nanocrystal Quantum Dots

Laser-Assisted Direct Imprint
(LADI) Technology
S. Y. Chou, C. Keimel, and
J. Gu, Ultrafast and direct
imprint of nanostructures in
silicon, Nature, 417 (2002)
835-837.
Yingqi Jiang
Outline
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Fabrication process
Experimental results
Technology features
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Quartz mould
Laser beam fluence
Surface monitoring
Applications
Summary
Fabrication process
a)
Quartz mould is brought into
contact with the silicon substrate
with external force.
b)
XeCl (308 nm wavelength)
excimer laser pulse (20 ns pulse
width) melts a thin surface layer of
Si.
c)
Molten silicon is embossed in the
liquid phase.
d)
Silicon rapidly solidifies.
e)
The mould and silicon substrate
are separated, leaving a negative
profile of the mould.
Experimental result (I)——Prototype
Quartz Mould
Imprinted silicon substrate
(a)
(b)
500nm
2μm
Scanning electron microscope (SEM) images. a, The mould after the two LADI
processes showing no visible damage. b, A uniform 300 nm period silicon grating
patterned by LADI. The grating has 140 nm linewidth and is 110 nm deep.
Experimental results (II)——10nm resolution
SEM image of the cross-section of samples patterned using LADI. a, A quartz mould. b, Imprinted
patterns in silicon. The imprinted silicon grating is 140 nm wide, 110 nm deep and has a 300 nm
period, an inverse of the mould. We note that the 10 nm wide and 15 nm tall silicon lines at each top
corner of the silicon grating are the inverted replicas of the small notches on the mould
(the notches were caused by the reactive ion etching trenching during
mould fabrication). This indicates the sub-10-nm resolution of the LADI process.
Technology features
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Direct imprint
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Rapid process
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Only one step! No etching to generate the final structures
The embossing time is less than 250 ns.
High resolution
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molten silicon has a viscosity of 0.003 cm2 s-1, which is onethird that of water (0.01 cm2 s-1). This low viscosity
enables the molten silicon to flow rapidly into all crevasses,
filling them completely and conforming to the mould.
A variety of structures with resolution better than 10 nm
have been imprinted
Features (cont.)——Quartz Mould
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Feasibility
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Quartz has a melting temperature over 200 oC higher than Si
higher than
Si (used
Si-melting
furnaces)
(quartz
is used
as theincrucible
material
in Si-melting
Quartz
1650 (±75) °C
Silicon
1414 °C
——Data and icture from wikipedia
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Quartz does not absorb the laser energy because it has a
bandgap larger than the photon energy (93% measured
transmittance)
Quartz conducts heat much less well—two orders of
magnitude more poorly—than Si.
Pre-fabrication method
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Fused quartz, 1.5mm×1.5mm, and 1mm thick
Conventional nanoimprinting + RIE
Features (cont.)——Laser beam fluence
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Low fluence
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High fluence
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a fluence lower than 0.8 J cm-2 does
not melt the silicon surface
a fluence higher than 2 J cm-2, laser
ablation of silicon will occur
Final choice:
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a laser pulse of 20 ns duration and
1.6 J cm-2 fluence melts a silicon
surface sufficiently without ablation.
http://www.earthsci.unimelb.edu.au/isotope/res
earch/index.html
Features (cont.)——Monitoring of the
melting of silicon surface
solidifying
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Method
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melting
Principle
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Measuring the time-resolved
reflectivity of a HeNe laser
beam (wavelength λ= 633 nm)
from the silicon surface.
saturating
The silicon theoretically starts to melt immediately in less than
picoseconds after the laser hit the surface.
When silicon melts, it changes from a semiconductor to a metal,
hence its surface reflectivity to visible light increases by about a
factor of two.
Melting depth
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On the basis of theoretical calculations and the melting depths
experiments, the melting depth was estimated to be about 280 nm.
Applications
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LADI can be extended to large areas, other materials,
and other processes.
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Pattern areas as large as a whole wafer (4” or 8”), or a 1”square die (that die can be used to cover an entire wafer by
step and repeat),
Used for other materials beyond crystalline silicon and
polysilicon (gates of MOSFETs).
Help to crystallize polysilicon further.
Well suited for three-dimensional patterning
Unique method to fill tiny holes in a dielectric (for example,
SiO2) with silicon and a unique means of flattening the
surface of a semiconductor deposited on a dielectric.(Both are
difficult issues in integrated circuit fabrication.)
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Summary
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Demonstrate a rapid technique ——Laser-Assisted
Direct Imprint (LADI) technology——for patterning
nanostructures in silicon that does not require
etching.
Experimentally show the intriguing characteristics of
LADI such as sub-10-nm resolution, sub-250-ns
processing time, and excellent imprint of large
isolated patterns.
Analyze three features of LADI technology including
quartz mould, laser influence, and surface
monitoring.
Thank you for your attention!