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Atul Dwivedi03/09/10 203/09/10 VNIT 2010 2
Introduction
Nobel laureate physicist, Werner Heisenbergdeclared it turns out that we can no longertalk of the particle apart from the process of
observation[1].
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Introduction
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Topics of Discussion
Why do we do measurement?
What is the need of microscopes?
History
Nanoscale metrology
Microscopy
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History
JOHN QUINCY ADAMS - report to the congress, 18
Weights and measures may be ranked
among the necessaries of life to every individual
of human society. They enter into the
economical arrangements and daily concerns of
every family. They are necessary to everyoccupation of human industry;
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History
1875:-treaty by 17 countries known as meterconvention
1900:-around 35 countries adopted metricsystem.
1938-SEM
1960:-Extensively revised and SI units arestandardized.after that 3-4 times these standardrevised
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History
1983: STM
1986:AFM
After that many versions of AFM are developedin 2009 NIST started project on 4th generationAFM
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Nanoscale Metrology[2]
2. Scale and Line-width metrology
4. Nano-indentation and characterization
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Microscopy
Optical microscopy
Scanning electron microscopy
Scanning probe microscopy
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Optical Microscopy
Limitations:
Can not be used below 100s of nm.
Observation of some characteristic propertieslike electrical and magnetic properties is notpossible
03/09/10 VNIT 2010 10
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Scanning Electron
Microscopy
The first SEM was constructed in1938 by vonArdenne by rastering The electron beam of atransmission Electron microscope (TEM) to
Essentially form a scanning Transmissionelectron microscope(TEM)
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Scanning Electron
Microscopy
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Scanning Electron
Microscopy
Mostly back scatter detector and secondaryelectron detectors are of type Everhart-Thornley detector or a solid statedetector
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Everhart-Thornley
Detector
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Scanning Probe
Microscopy
3. Scanning tunneling microscopy
5. Atomic force microscopy
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Scanning Tunneling
Microscopy In 1981 at IBM Zurich research Laboratory by
Binnig and Rohrer
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Scanning Tunneling
Microscopy
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Scanning Tunneling
Microscopy
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Scanning Tunneling
Microscopy
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Scanning Tunneling
Microscopy Steps:
A: Gradually increase the tunneling current tomove towards the adatom until
interaction energy=activation energy
B: Pull the adatom to desired location
C: Gradually decrease the tunneling current
to move away the tip from the adatom
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Scanning Tunneling
Microscopy
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Scanning Tunneling
MicroscopyIn the presence of a potential U(z),
assuming 1-dimensional case, the energylevels n(z) of the electrons are given bysolutions to Schrdingers equation
where is the reduced Plancksconstant, z is the position, and m is the mass of
an electron. If an electron of energy E is incidentupon an energy barrier of height U(z),
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Scanning Tunneling
Microscopy the electron wave function is a traveling
wave solution of shrodingers equation
where
if E > U(z), which is true for a wavefunction inside the tip or inside the sample. Inside
a barrier, such as between tip and sample ,E < U(z) so the wave functions which satisfy this aredecaying waves,
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Scanning Tunneling
Microscopy
Where
It quantifies the decay of the wave inside the barrier,with the barrier in the +z direction for .
Let us assume the bias is V and the barrier widthis W. This probability, P, that an electron at z=0(left edge of barrier) can be found at z=W (rightedge of barrier) is proportional to the wavefunction squared,
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Scanning Tunneling
Microscopy,
If the bias is small, we can let U E M in the
expression for , where M, the work function,gives the minimum energy needed to bring anelectron from an occupied level.
. The current due to an applied oltage V (assumetunneling occurs sample to tip) depends on two
factors: 1) the number of electrons between EfandeV in the sample, and 2) the number among themwhich have corresponding free states to tunnel
into on the other side of the barrier at the tip.
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Scanning Tunneling
Microscopy. Mathematically, this tunneling current is given by
One can sum the probability over energy differnce toget the number of states available in this energyrange per unit volume The LDOS near someenergy E in an interval is given by
and the tunnel current at a small bias V isproportional to the LDOS near the Fermi level,which gives important information about thesample
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Scanning Tunneling
MicroscopyThus the tunneling current is given by
where s(0,Ef) is the LDOS near the Fermi level of thesample at the sample surface
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Atomic Force
Microscopy
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Example:measurement of aresistance and carrier profile of a
semiconductor
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Atomic Force
Microscopy
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Atomic Force
Microscopy
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Atomic Force
microscopy
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Carrier Conc. By AFM
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Comparison
Optical SEM TEM AFM
Max
Resolution
100s nm 1s nm atomic atomic
Typical cost
(*$1000)
10-50 200-400 500 or higher
100-200
Imagingenvironment
Air,fluid Vacuum Vacuum Air,fluid,vacuum,
special gasSurfaces
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Surfaces and Corresponding
Microscopes1.Atomically smooth surfaces
Natural surfaces- mineral surfaces
Epitaxial growth on a semiconductor Optical surfaces
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Surfaces and Corresponding
Microscopes For atomically smooth surfaces
both SEM and AFM can be usedbut better is AFM because it ishaving vertical resolution of
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Surfaces and Corresponding
Microscopes2.Thin films
Example of rugged polysiliconFilms which are used as capacitors inMemory devices. By making
these films Rough, the surfacearea is increased
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Surfaces and Corresponding
Microscopes3.Rough surfaces
One of the key advantages ofthe SEM with respect to other typesof Microscopy is its large depth offield.This ability makes it possible toimage Very rough surfaces withmillimeters of vertical information
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Surfaces and Corresponding
Microscopes3.Rough surfaces
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Questions? Comments
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[1] Published by: The MIT Press on
behalf ofAmerican Academy of Arts & ScienceStable URL:http://www.jstor.org/stable/20026454
[2] National institute of standardsand technology:http://www.mel.nist.gov/programs/pbo
References
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References
[3] An introduction to STMhttp://www.columbia.edu/~jcc2161/d