Near Field Scanning Optical Microscopy

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- Near Field Scanning Optical Microscopy -
Electrostatic Force Microscopy - Magnetic Force
Microscopy
Yongho Seo Near-field Photonics Group Leader
Wonho Jhe Director School of Physics and
Center for Near-field Atom-photon
technology, Seoul Nation University in South Korea
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Scanning probe Microscopy
NSOM (near-field scanning optical microscopy) EFM
(electrostatic force microscopy) MFM (magnetic
force microscopy)
Scanning Probe Microscope ? nano-scale resolution
? Slow scanning
Optical Microscope ? low resolution ?
diffraction limit ? real time
- self oscillating - self sensing - chip and
simple design
Quartz Crystal Resonator probe
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High Frequency Dithering Probe for Shear Force
Detection
Bimorph, tuning fork Low dithering frequency 10
100 kHz Slow response time 100 ms
High frequency (rf) dithering ? fast
scanning Small dithering amplitude (ltlt 1 nm) ?
high lateral resolution Large Signal voltage (gt
0.1 V) ? high signal to noise ratio
High Frequency Quartz Crystal Resonator Thickness
Shear mode 2 100 MHz dithering frequency fast
response time 1 ms
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Quartz Crystal Resonators
AT-cut QCR
BT-cut trident QCR
Z-cut Tuning fork
High Frequency (rf) Thickness Shear
or Extensional mode k 105 - 106 N/m
Low Frequency (10 kHz) Flexural Mode k 104 -
105 N/m
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QCR based NSOM
Shear mode NSOM - 2 MHz dithering frequency -
make a hole to insert optical fiber tip - easy to
replace tip - increased the stability - high
Q-value gt 103
Perforated QCR probe
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QCR probe Feedback Scheme
high frequency dithering tip
induced signal
Phase detection
Function Generator
Tube scanner
Simple design Low cost No lock-in amp
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High Frequency Dithering Shear Force Microscopy
Topographic Image of CD surface
Total time 20 s
Large dithering amplitude
White dots dust
Amplitude mode
Phase mode
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Schematics for high speed NSOM
Near field detection scheme
Laser Diode 650 nm PMT Optical Signal
measurement Reflection mode
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Fastest Scanning NSOM Image
Near field Scanning Probe Microscopy Image of
grating surface in reflection mode
Total time 0.5 s 7x7 mm2
Total time 0.5 s 1x1 mm2
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Electrostatic Force Microscopy
Tuning Fork (32.768 KHz)
L 2.2 mm, t 190 mm, w 100 mm k 1300
N/m. The tip is electrically shorted to an
electrode.
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Force Sensitivity of Tuning Fork
Force sensitivity ? (k/Qf)-1/2
Si Cantilever
Quartz Tuning Fork
f 10 - 100 kHz k 1 - 100 N/m Q 102 - 103
10 nm dithering
f 10 - 100 kHz k 104 - 105 N/m Q 103 - 105
smaller than 1 nm dithering
In this experiment, L 2.2 mm, t 190 mm, w
100 mm k 1300 N/m. Q 1800, f 32 kHz
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Electrochemical Etching
Reduce the diameter of Co and Ni wire
H3PO4
Pt
Co, Ni
D 100 mm
10 mm
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Make tip and Attach it to the tuning fork
-Attach the wire to the tuning fork and make a
tip -use home-made micromanipulator
Pt
Silver paint
Co, Ni
H3PO4
Tuning fork
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PZT Thin Film
PZT thin films (Zr/Ti 20/80) by INOSTEK Inc.
and Crystalbank
The property requirements of PZT thin films for
high quality nano storage devices ? smooth
surface roughness ? high piezoelectric properties
even in the case of very thin films ? long term
stability and reliability
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Electrostatic Force Microscopy
Approach Curve with Bias voltage
Tip
PZT
Pt
Bias voltage applied between the tip and Pt
substrate
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Electrostatic Force Microscopy
Minimum Detectable Capacitance due to thermal
noise
? 2 x 10-20 F
Frequency shift due to surface charge
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Electrostatic Force Microscopy
Tuning Fork based EFM - polarization images
After poling of square area
Line drawing
7 x 7 mm2
0.9 x 0.9 mm2
-long time stable
-High resolution (50 nm)
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Electrostatic Force Microscopy
Tuning Fork based EFM - polarization images
4 x 4 mm2
7 x 7 mm2
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Magnetic Force Microscopy
MFM contrast - magnetic force gradient between
tip and sample
Frequency shift
Phase shift
Force gradient
Magnetic force - very weak force (pN)
Lift mode - keep constant gap between tip and
sample (10 nm) - to avoid the
strong short range topographic contrast
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Magnetic Force Microscopy
Approach Curve
Approach Withdraw
Shear force
attractive force
high S/N ratio high frequency Sensitivity lt 3
mHz
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Magnetic Force Microscopy
Tuning Fork Co tip attached
L 2.2 mm, t 190 mm, w 100 mm spring
constant, k 1300 N/m (smallest one
commercially available)
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Magnetic Force Microscopy
Advantage of the shear mode MFM
-Perpendicularly recorded sample and
longitudinally polarized tip
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Magnetic Force Microscopy
(a) shear mode, Co tip, perpendicular (b) shear
mode, Co tip, parallel dithering (c) shear mode,
Ni tip (d) tapping mode
100 Mb hard disk
30 x 30 mm2
30 x 30 mm2
30 x 30 mm2
30 x 30 mm2
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Magnetic Force Microscopy
Tuning Fork based MFM height and amplitude
dependency
3 x 1 mm2
13 x 3 mm2
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Magnetic Force Microscopy
High resolution Tuning Fork based MFM
1 Gbit/inch2 hard disk Dithering Amplitude 20
nm lift height 50 nm Spatial resolution 50
nm 2 x 2 mm2
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Atomic Layer of HOPG with trident QCR (1MHz)
Atomic layer (3Å)
160 x160 nm2
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NSOM, EFM, and MFM using Quartz Crystal Resonator
In summary, Quartz Crystal Resonator based
NSOM, -high resonance frequency, and small
dithering amplitude. -facilitates high-speed
scanning -obtained atomic scale AFM Tuning fork
based EFM and MFM - EFM obtained with high
resolution, for the first time. - MFM shear
mode MFM improved resolution. - Tuning fork
Force sensitive SPM sensor
Published results Y. Seo, J.H. Park, J.B. Moon
and W. Jhe, Appl. Phys. Lett. 77 4274 (2000). Y.
Seo, Wonho Jhe, Rev. Sci. Instrum. 73 (2002).