Transcript Total view of the AFM - Electrical engineering

Microcantilevers III
Cantilever based sensors:
The cantilever based sensors can be classified into three groups
(i) General detection of any short range forces: Scanning probe microscopy
(ii) Detection of mass attachment: based on changes in frequency response
(iii) Detection of molecular adsorption: based on changes in surface stress of a
functionalized cantilever. This can also be detected based on resonance
amplitude change.
(iv) Detection of radiation (IR or nuclear): based on deflection of a bimaterial
cantilever, or from deflection changes
(v) Detection of charges or electric or magnetic fields: Based on resonance
frequency or amplitude changes
Note that although cantilevers are highly sensitive to stress changes, but
resonance frequency based changes are the most sensitive due to quality factor
enhancements.
1
Kelvin probe technique
Kelvin probe technique measures surface charge, surface potential,
and surface work function. The advantages are quantitative nature,
and ease of operation.
Evac
qVcon = qVdc
semi
Si
+ Silicon
p
Eg,Si
Probe Tip
Ec
qs
qVcon
Ev,EF,
Si
G. Koley and M. G. Spencer, J. Appl. Phys. 90, 337 (2001)
=  Si + Eg,Si - (semi +
qs)
EC
EF,se
mi
Semiconductor Sample
Goutam Koley
Mathematical Model for SKPM
Qs Qtip
1 C
2
Vdc  Vac sin t  Vcon  
Ftotal 
2
2 d
40 d
 Ftotal  Fdc  F  F2
 C

F  
(Vdc  Vcon )Vac sin t
 d

G. Koley and M. G. Spencer, J. Appl. Phys. 90, 337 (2001)
Goutam Koley
SKPM Measurement System
AMPLITUDE
DETECTOR
LASER
POSITION
DETECTOR
OSCILLATOR
LOCK-IN
CIRCUITRY
CONTROLLER
Z
CONTROLLER
Vdc
SCANNER
Vacsint
SAMPLE
Vdc
SURFACE POTENTIAL
IMAGE
G. Koley and M. G. Spencer J. Appl. Phys. 90, 337 (2001)
MORPHOLOGY
IMAGE
Goutam Koley
Electronic characterization of dislocations
10 nm
/Div
0.1 V
/Div
Morphology
G. Koley and M. G. Spencer, Appl. Phys. Lett. 78, 2873 (2001)
Potential
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Surface potential patterning using mask
UV light
20 m circle
quartz mask
HFET Sample
(35% Al in barrier,
44 nm AlGaN layer)
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Spatial resolution of charge storage
• UV exposure through
a mask of 1, 2, 5, 10
and 20 m squares
• Spatial resolution on
the order of ~1-2 m
Relative inverse surface barrier (eV)
0.38
0.37
0.36
0.35
0.34
3.5 m
0.33
0.32
G. Koley et al. JAP (2004)
0.31
0.3
60
62
64
66
68
70
72
Distance(m)
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Measurements in GaN based transistors
AFM scanning probe
Biasing Probes
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Surface morphology and potential profiles in
dc biased transistors
Drain
Gate
Morphology
Source
Vd = 2V,
Vg = -1.5
V
Surface Potential
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The Cantilever effect
Vm eas  Vdc  K


 Vsurf  Vsurf R 1  R 

R  Ccanti z  Ctip z

Smaller R results in larger measurement accuracy
G. Koley et al. Appl. Phys. Lett. 79, 545 (2001)
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Measurement of transients
AFM scanning probe
Measurement setup schematic
Probe tip
Gate
Drai
n
Source
Biasing Probes
G. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
Source
20 
resistor
Gate
Drai
n
A
A
+ve dc bias
-ve dc bias
or square pulse
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Drain current variations due to stress
Type I: pulsed stress at Vd = 5V,
Vg = pulsed from 0 to -10 V
Type II: continuous stress at Vd
= 20V, Vg = -12 V for 2 mins
27
11
25
25
24
23
20
10
15
9
10
5
0
22
21
0
0.5
1
1.5
Time (s)
2
2.5
100 m HFET
20
Drain current (mA)
Drain current (mA)
Drain current (mA)
26
8
6
200
400
600
100 m HFET
5
4
0
Vd = 1 V, Vg = 0 V
during measurement.
7
800
1000
Time (s)
G. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
0
100
200
300
400
500
Time (s)
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Correlation between drain current and
surface potential
Drain current
Surface potential
9
-1.5
Relative surface potential (V)
Drain current (mA)
8
7
6
Vd = 1 V, Vg = 0 V
during measurement.
5
4
3
100 m HFET
2
1
0
100
200
300
400
500
Time (s)
-2
-2.5
-3
-3.5
100 m HFET
0
100
200
300
400
500
Time (s)
Device stressed at Vd = 20 V, Vg = -12 V for 2 mins
G. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
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Potential variation with distance and time
Surface Potential (V)
• Stressed at Vg = -12V,
Vd = 20 V for 2 mins
• Maximum variation
observed ~0.3 m
from the gate edge
• Charges take a long
time to reach
equilibrium value
-0.5
150 m HFET
-1
0 min
0.5 min
1 min
1.5 min
2 min
3 min
4 min
5 min
8 min
11 min
18 min
28 min
before stress
-1.5
-2
-2.5
-3
-3.5
0
0.2
0.4
0.6
0.8
1
Distance from gate edge (m)
G. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
Goutam
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Surface conductivity measurements
(a) Morphology, (b)
conductivity, and (c)
overlap of the surface
morphology and
conductivity images
Goutam Koley
Scanned gate microscopy
Scanned gate microscopy is useful to determine the variation of conductivity
along a thin channel, and where direct measurement of conductivity is
difficult
(a) Experimental set up for SGM, (c)
the SGM image of a single-walled CNT
bundle for Vtip = 1 V; Black
corresponds to very high resistance
Refer to handouts for scanning thermal microscopy
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Scanning capacitance microscopy
Scanning capacitance technique actually measures the dC/dV signal which is
inversely proportional to doping. The advantages of this technique include a large
measurement range (1015 – 1020 cm-3), and resolution of <10 nm
N C V 
C3
q 0 AlGaN
dV
dC
zC V 
 0 AlGaN
C
For capacitance measurement a low frequency ac voltage is applied to the sample. The ac voltage
periodically changes the tip-sample capacitance. The sensor produces a high frequency signal to
measure very small capacitance changes.
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Application of capacitance microscopy
Cross-sectional measurement in a
MOSFET under actual operation
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Applications to GaN samples
Morphology image
Capacitance image
C-V curve
• The dC/dV decreases around the dislocations indicating the reduction in
the background carrier concentration
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