sem and tem - personal webpages at ntnufolk.ntnu.no/jonathrg/fag/tfe4180/slides/jenkins 7-2 sem and...

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TFE4180 Semiconductor Manufacturing Technology, TEM and SEM

SEM and TEMChapter 7.2 : Semiconductor Science by Tudor E. Jenkins

Saroj Kumar Patra,Department of Electronics and Telecommunication,

Norwegian University of Science and Technology ( NTNU )

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Contents Why use electrons?

• De-Broglie Hypothesis• Electromagnetic lenses and Numerical Aperture

Transmission Electron Microscopy (TEM)• Structure• Use Mode• Practical complications

Scanning Electron Microscopy (SEM)• Structure• Use Mode

Scanning Transmission Electron Microscopy (STEM) Energy Dispersive X-ray Spectroscopy (EDS) Electron Energy Loss Spectroscopy (EELS)


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Why use electrons?


• Resolution of the microscope dependson wavelength .

• Optical microscopy is limited bywavelength to visible light.

• Maximum resolution of opticalmicroscopes ~ 200nm.

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Why use electrons?


• Material-wave : Wave particle duality.

• Short wavelength

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Why use electrons?


For V= 100 keV:

Non-relativistic consideration:

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Electromagnetic Lenses


• Lenses focus poorly even atmoderate angels.

• This causes NAEM ~ 0.01

• This is far worse than opticalmicroscopy where NAOP ~ 1.0

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Transmission Electron Microscopy (TEM)


• Send electrons through a thin (~100 nm) sample.

• Get information about the sample based on whathas happened to the electrons when they come outthe other side of the sample.

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TEM column with components


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TEMSchematicDiagram


Energy-loss spectrometer

Aperture

Sample stage

Detector

CCD video camera

Fluorescent screen

CRT

Condenser lens

Anode

Lenses

Electron gun

X-ray detector

Objective aperture

Displayed sample image

Liquid N2Dewar

Figure 7.36

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Bright Field (BF)


• Separating out the electrons that arescattered. i.e., those who have gonestraight through the sample.

• Dark and Bright fields for areas withhigh and low degree of electronabsorption respectively (Mass-thicknesscontrast).

• Dark and Bright fields for areas withhigh and low degree of electronabsorption respectively (Diffractioncontrast).

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Bright Field (BF)


• Separating out the electrons that arescattered. i.e., those who have gonestraight through the sample.

• Dark and Bright fields for areas withhigh and low degree of electronabsorption respectively (Mass-thicknesscontrast).

• Dark and Bright fields for areas withhigh and low degree of electronabsorption respectively (Diffractioncontrast).

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Dark Field (DF)


• Separating out the electrons that havebeen scattered in a particular direction.

• Light fields for areas in which electronshave been scattered in the directionchosen (Diffraction contrast).

• Greater chance of spreading to areaswith high mass and thickness ( Mass-thickness contrast).

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Dark Field (DF)


• Separating out the electrons that havebeen scattered in a particular direction.

• Light fields for areas in which electronshave been scattered in the directionchosen (Diffraction contrast).

• Greater chance of spreading to areaswith high mass and thickness ( Mass-thickness contrast).

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HRTEM ( Lattice Images )


• Why ? : To take the “image” of the crystal planes in thematerial.

• How ? : Setting out both the non-diffracted beam andone of the diffracted rays with lens aperture and letthem interfere in the image plane.

Lets go back a little…..

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Test

EM Lens

Diffraction planeObjective aperture

Image planeSelected area aperture

More lenses

Screen / Camera

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Test

EM Lens



More lenses

Screen / Camera

Objective aperture ( in Diffraction plane)

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Test

EM Lens



More lenses

Screen / Camera


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Test

EM Lens



More lenses

Screen / Camera


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Test

EM Lens



More lenses

Screen / Camera

Two images of the sample are brought together

Interference

Phase contrast image of the crystal plane

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Two images of the sample are brought together

Interference

Phase contrast image of the crystal plane

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Electron Diffraction


• Why ? : To study the crystal structure of the material.

• How ? : Adjusting last lens to diffraction plane (insteadof image plane) which is focused on the screen /camera

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Test

EM Lens



More lenses

Screen / Camera

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Test

EM Lens



More lenses

Screen / Camera

Selected Area Aperture ( in Image plane)

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Test

EM Lens



More lenses

Screen / Camera

Much information about the material

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TEM Practical Complications


• Must have thin samples (e.g., ~ 100 nm) Difficult

Time consuming

Can change the sample

• Damage to the sample

• Narrow view

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Scanning Electron Microscope (SEM)


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Why use SEM?


• Visual : High Resolution Image of the sample ( but TEM has even better resolution)

• Versatile : Not too picky on the samples. Bad idea with live test

Samples can be inserted very quickly.

With modern sample holders, more than one sample can be

loaded at the same time.

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SEM Structure


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Simple Schematic of SEM


X-scan signal

Specimen

Scanning magnets

Stage

Detector

Y-scan signal

X-scan

Y-scan

Z-axis signal CRT

Condenser lens

Anode

Electron extractor

Filament (electron emitter)

Electron beam

Figure 7.20

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Electron Beam System


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WD:

• Important Parameters : Condenser aperture and working distance (WD)

• What about Resolution and Depth of Field !

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Aperture Size:

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Interaction volume(Typically ~ 100 nm to 5μm

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Secondary Electrons


- Contrast : Topography

- Secondary – not the same electrons

that were injected.

- Coming from the second upper

layer of the interaction volume.

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SEM Image


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Back Scattered Electrons


Contrast : Element

Cause : Coulomb Force ∗∗ ∗

Resolution : Lower than SE

(Due to large interaction volume)

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SEM Image using BSE


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Scanning Transmission Electron Microscopy (STEM)


http://www.ipme.ru/e-journals/RAMS/no_1100/browning/Brow.pdf

• Combining the two techniques

:TEM and SEM.

• Provides opportunity for

element analysis EDS

EELS

• Annular Dark Field Imaging (not in syllabus)

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Energy Dispersive X-ray Spectroscopy (EDS)


• X-rays emitted from atoms

that have been excited by

the electron radiation.

• The wavelengths of X-rays

are characteristic of atomic

number.

• It works best on heavier

elements.

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Energy Dispersive X-ray Spectroscopy (EDS)


• X-rays emitted from atoms

that have been excited by

the electron radiation.

• The wavelengths of X-rays

are characteristic of atomic

number.

• It works best on heavier

elements.

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Electron Energy Loss Spectroscopy(EELS)


Plot: Relative intensity against Energy loss.NB most electrons lose no energy ( elasticcollision)• Some electrons lose energy due to

inelastic shock.

• The shape of the curve to the right

is characteristic for each element.

• It works best for low mass.

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