The Second U.S.-Korea NanoForum
The Development of MEMS-Based
Time-Of-Flight SFM
Dong-Weon Lee
Chonnam National University, Korea
Collaboration with IBM Zurich Research Lab.
Outline
1. Introduction.
2. Concept of MEMS-based TOF-SFM.
3. Experimental results.
Previous systems
MEMS-based switching device
TOF-SFM concept
Design concept
SEM images
Conventional methods for chemical analysis
Atom probe: Erwin Müller in 1968 (FIM+MS)
Basic configuration of the AP with TOF-MS
Probe hole
sample
ions
ion detector
time of flight (L)
voltage pulse (VT)
pulser
timer
Mass analysis (m)
m = 2neVT t2/L2
Where t is the flight time,
e is electric charge of electron,
L is the flight length,
VT is applied voltage pulse,
n # of electrons
50 MΩ 1000 pF
HV
computer
Min. field for ion evaporation: 108 to 109 V/m
Drawback: Sample needs to be a very sharp tip
Recent approaches
Scanning atom probe (SAP): O. Nishikawa in 1994
Scanning tunneling atom probe (STAP): J. Spence in 1995
Previous systems
MEMS-based switching device
TOF-SFM concept
Design concept
SEM images
TOF-SFM based on MEMS technology
Scanning force microscopy (SFM)
SFM is an important tool to make a 3D surface image
of solid surfaces at atomic scale resolution.
Time of flight (TOF) mass spectrometer
Time-of-flight mass spectrometer is a powerful tool to
identify the chemical property of solid surfaces.
Combination of advantages in both techniques allows
chemical and topographical analyses on a nm scale.
Previous systems
MEMS-based switching device
TOF-SFM concept
Design concept
SEM images
Basic of the TOF-SFM concept
TOF-MS
UHV chamber
Signal
TOF
mode
SFM-MS
mode
LEGO-type microstage
EE
SC
100 µm
PZT actuator
m2
m1 m3
t1 t2 t3 time
atom
Image size:100nmx100nm
[NaCl on Al(111)]
Advantages of this approach for TOF-SFM
1. There is no sample limitation.
2. A short tip-electrode distance to minimize the ion extraction voltage.
3. Fast switching between the SFM and TOF mode (msec‘s)
Previous systems
MEMS-based switching device
TOF-SFM concept
Design concept
SEM images
SEM of fully assembled cantilever device
on line
m
m
o
c
To
estance
r
o
z
e
i
p
To
r
To heate
Switchable cantilever
Extraction electrode
In-plane tip
LEGO-type stage
500 nm
A tip-extraction electrode distance of 10 µm
achieved by LEGO microstage
10 µm
Displacement vs. power
Displacement vs. driving Fr.
AFM imaging
Field emission
TOF analysis
Deflection vs. frequency characteristics
CCD
image at 100 Hz
Microscope
Monitor
Lever
Function generator
Stage
Displacement (µm)
Maximum switching speed : ~ 10 msec
60
at 25 mW
50
40
30
20
10
1
10
100
Frequency (Hz)
TOF-SFM measurements can be done orders of magnitude
faster than with currently available systems
1000
Displacement vs. power
Displacement vs. driving Fr.
AFM imaging
Field emission
TOF analysis
Topographic image at dynamic AFM mode
• Regulated on constant amplitude with integrated
piezoresistive detection.
• pressure p < 10−9 mbar.
• preamplifier in vacuum (G = 100UHV x 100air)
2 µm
2; z ∈[0,250nm]
2000nm
topo_2000_41.jpg
0.5 µm
2; z ∈[0,180nm]
1000nm
topo_1000_45.jpg
2; z ∈[0,113nm]
500nm
topo_500_47.jpg
Displacement vs. driving Fr.
AFM imaging
100
Pt-coated tip
Platinum coated TOF Cantilever
Pt-coated tip
I in nA
60
Si
40
0
1000
ln(I/U^2), I [nA]; U [V]
EE
8
pure silicon tip
cantilever
6
4
2
20
-4.0
TOF analysis
10
80
Pt
Field emission
Field emission behaviors
Emission current (nA)
Displacement vs. power
Si TOF Cantilever
0
0
1100
20
40
1300
1500
Voltage
at1400
U (V)
1200
60
1600
U in volts
-4.5
-5.0
-5.5
-6.0
-6.5
-7.0
-7.5
-8.0
Pt-coated tip
Pure silicon tip
0.02 0.03 0.04 0.05 0.06 0.07 0.08
1/U in V^(-1)
Turn-on field emission for the
Pt-coated tip has demonstrated
with an extraction voltage as
low as 15 V
Fowler Nordheim plot
Displacement vs. power
Displacement vs. driving Fr.
AFM imaging
Field emission
TOF analysis
TOF analysis using the SC with the Pt tip
Tip
UDC
UP
30 pulses
at UDC=800 V
and UP=-240 V
Counts (a.u.)
Setup
EE
+
Si
6
5
4
Pt
Ga
C
3
2+
+
+
2
1
0
0
20
40
60
80
Mass in amu
100
120
Si+ and Pt2+ peaks of Pt-coated tip at mass to charge ratio of 28 and 97.5