Transcript Total view of the AFM

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.
Goutam Koley
Application of capacitance microscopy
Cross-sectional measurement in a
MOSFET under actual operation
Goutam Koley
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
Goutam Koley
Problems 2
• Calculate the amplitude of the 17 KHz force acting on the cantilever for an
applied ac voltage of 10 V rms (frequency 17 KHz). The cantilever
dimensions are 30 and 100 microns respectivley. It is 2 microns away from
the ground. The work function of the cantilever is 5.65 eV, and that of the
sample is 5.15 eV. Assuming that this force is applied at the edge of the
cantilever, what is the deflection if the spring constant is 0.1 N/m.
 C

F  
(Vdc  Vcon )Vac sin t
 d

SEM Microcharacterization
• SEM characterization modes:
– Microscopy
– Electron Beam Induced Current
– Cathodoluminescence
– Energy dispersive X-Ray Spectrum Analysis
– Electron beam lithography
Fundamental Physics I
Trivia: SEM working principles were outlined in 1942 by Zworykin, but it was not until 10 years later that a working
machine was assembled in Cambridge University.
h
h
h
1.22
e 



(nm)
mv
2mE
2mqV
V
The SEM operates with electrons having energy 20
– 30 keV. For 20 KeV, the De Broglie wavelength
e = 0.0087 nm.
The interaction of the electrons with a given material produces secondary electrons, backscattered electrons,
characteristic and continuum X-Rays, Auger electrons, photons, and electron-hole pairs
Fundamental Physics II and Applications
Re can be found out from the empirical expression
Re 
4.28 106 E1.75

(cm)
Where  is the sample density, and E is the energy in keV
The interaction of the electrons with a given material produces secondary electrons, backscattered electrons,
characteristic and continuum X-Rays, Auger electrons, photons, and electron-hole pairs
SEM imaging parameters
• Magnification M = (length of CRT display) / (length of sample area scanned).
In modern machines magnifications up to 200, 000 can be achieved.
• Resolution as low as 1 nm can be achieved, which is usually limited not by the
wavelength of the electrons but by the diameter of the focused electron beam
and electron scattering in the sample from the valence and the core electrons.
Due to electron scattering the original collimated beam gets broadened.
• Contrast of the SEM depends mostly on the sample topography since most of
the secondary electrons are emitted from the top 10 nm of the sample. The
contrast C depends on angle as:
C = tan d, where  is the angle from normal incidence. At 45° angle, d = 1°
causes change in contrast by 1.75%.
The contrast in backscattered electrons can come from the difference in atomic
number Z.
• The SEM operates in a very different manner from optical microscope, in that
electrons even away from the detector are attracted, amplified, and displayed
on the CRT. Thus the image displayed in the CRT is not a true image of the
sample.
SEM working parts I
Trivia: SEM was discovered in 1942 by V. K. Zworykin, but it was not until 10 years later that a fully
functional microscope was developed by researchers at Cambridge University
• The basic SEM consist of an Electron gun,
and a few focusing lenses, and detector. For
EDS an X-Ray detector is also used
• The pressure inside the chamber is
maintained at ~10-8 Torr vacuum.
• Microscopes are usually operated in the
voltage range of 20 – 30 keV, but for
insulating samples 1 kV or less can be used.
For insulating samples a thin metal coating
can also be used.
• The standard electron detector is an EverhartThornley design that is capable of amplifying
electron currents by almost a million times.
SEM working parts II: Electron sources
• There are mainly three types of
electron sources
– Tungsten hairpin filament: This is simply a tip
that is heated to an extremely high temperature
of ~2500 C to make electrons have high
enough energy to overcome the surface work
function of ~4.5 eV
– To get higher electron current stable materials
with lower work function is preferred. LaB6 as
polycrystalline powder is used to reduce the
work function to about half that of the tungsten
metal and significantly increasing the current
– In field-emission guns, an extremely high
electric field is applied to have the electrons
“tunnel” through the barrier into vacuum.
These could be operated as “cold” or they
could be operated at higher temperature, when
they are called Schottky emitters. The later
ones are easier to clean and maintain