Nanoscale Microscopy vnit by atul nano lab vlsi vnit

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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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Atul Dwivedi03/09/10 10

Optical Microscopy

Limitations:

Can not be used below 100s of nm.

Observation of some characteristic propertieslike electrical and magnetic properties is notpossible

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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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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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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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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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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

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Nanoscale Microscopy vnit by atul nano lab vlsi vnit