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Overview of Scanning Electron Microscope,

Transmission Electron Microscope,

Scanning Transmission Electron Microscope,

Low Energy Electron Microscope

P.E. Batson

with help from K.A. Mkhoyan, U. Minnesota• Why electrons rather than light? • What physical processes do we use for obtaining image contrast? • How are the instruments above related? • What are the results like?

KA Mkhoyan

rest mass kinetic energy

Total Energy

Wavelength of light -> 3800 -7500 Å -> not very interesting for materials characterization where regions of materials are less than 10 nanometer (100 Å) or smaller, and atomic distances, where much new physics occurs, are smaller than 1 nanometer.

KA Mkhoyan

1

0.1V -> 38 Å

1.0V -> 12 Å

100V -> 1.2 Å

1kV -> 0.38 Å

10kV -> 0.12 Å

Now, what happens when electrons

interact with the specimen and how do we collect them?

Large angle,

relatively chaotic processes

Small angle,

relatively well behaved processes

Image plane, or detector

Source

Optics

Apertures optimized for signal

http://www.purdue.edu/rem/rs/sem.htm#2

SEM System is

largely a probe forming optical system that images by scanning and collecting serial, time varying signals that are fed to a

TV-like display

http://www.tasc.infm.it/research/tem/images/semimage.jpgElectron Column Gun

Optics

Specimen

PumpsKnobs and displays

Detectors

Computers!

The ORION™ Helium ion Microscope from Carl Zeiss SMT Currently one of these is being installed at Rutgers:

Torgny Gustaffson - Len Feldman

50ȝm

Anthophyllite asbestos

Size of fibers - about 50ȝm

Alveoli Size 140 ȝm

Cell Size 10-100 ȝm

Result: Cancer

http://usgsprobe.cr.usgs.gov/picts2.htmlTopographical Contrast Probe

Forming

The first 50 Years of Electorn Microscopy

The first Next 50 Years of Electorn Microscopy

7'6"Yao

Ming

Many signals, one microscope: Nion UltraSTEM

Aberration

corrector 1

Aberration

corrector 2 Described in: Krivanek et al. Ultramicroscopy 108(2008) 179-195 and Dellby et al. EPJAP 2011. More info at www.nion.com.

UltraSTEM20

0* UltraSTEM20

0* *instrument shown:

CNRS Orsay, France*instrument shown:

CNRS Orsay, France

UltraSTEM20

0* *instrument shown:

CNRS Orsay, France

Fully modular (all

lenses, the corrector, etc., are independent modules, with identical mechanical interfaces) and thus very flexible.

Ultra-stable,

friction-free sample stage.Operating voltage range 20-200 kV.

Efficiently

coupled EELS.UHV at the sample (<10 -9 torr; <10 -7 Pa). http://www.specs.de/cms/upload/PDFs/SPECS_Prospekte/LEEM.pdfand IBM Corp (Ruud Tromp)

Probe forming

Image forming

Low Energy Electron Microscopy

http://www.research.ibm.com/leem/

2nm resolution today

Deceleration

http://www.specs.de/cms/front_content.php?idcat=209Electron Column Gun

Optics

Specimen

PumpsKnobs and displays

Detectors

Computers!

SpecimenSource

SEM TEM/STEM LEEM Energy 2-10 KeV 50-200 keV 0-10 eV Probe Size 1-10 nm 0.1-1 nm 2-5 nm

Probe Forming

Optics

Imaging Optics

Variations on a Theme!

Energy width -> 0.8 - 3 eV 0.25-0.40 eV++

Tungsten thermionic total current-> 0.1mA/sr

Field Emission total current-> 1.0 ȝA/sr

so where small probe is not needed, tungsten wins.

Cylindrical Electron Lens

Lorentz Force

F =(-e/c) vx B

so F = (-e/c) v o B r producing a spiral:v finally F r = (-e/c)v B z so that electron beam is deflected towards axis Br BzV

Cylindrical Lens

Focusing

SpecimenSource

SEM TEM/STEM LEEM Energy 2-10 KeV 50-200 keV 0-10 eV Probe Size 1-10 nm 0.1-1 nm 2-5 nm

Probe Forming

Optics

Imaging Optics

Reiterating variations on a theme.

Now, what happens when electrons

interact with the specimen and how do we collect them?

First: THICK Specimens

Large angle,

relatively chaotic processes

Small angle,

relatively well behaved processes

Image plane

Source

Optics

Apertures optimized for signal

With thick specimens, most of the signal comes out the front side.

0 Eo - Incident beam Energy

Primary Imaging Mechanism for the SEM and HIM: Secondary and Backscattered Electrons

Fred Cosandey

Why does Secondary Electron Imaging

look like such a "natural" rendering of the specimen?

Secondary electron image of

red blood cells shows surface topography in a very easily understood image. Unlit areas in the common situation on the right are dark in the scanning electron case. The image looks "natural" because it matches a situation we encounter nearly every moment of the day. Important result: If we reverse the ray paths, and exchange the source and detectors we will get the same contrast. eye

Surface

Surface

Light

SourceElectronSource

detector

Shadow

Shadow

Illustrates a general principle: Reciprocity.

If we interchange the source and detector

and reverse the particle paths, we get identical results. Secondaries BackscatteredBackscattered electrons are higher energies: penetrate deeper, see less surface contrast

Fred Cosandey

Suppose your specimen thickness is much less than a few times Ȝ? Then the large plume of scattered electrons below the surface does not exist !! The secondary electron resolution can then be as good as the probe size.

Zhu, Y., Inada, H., Nakamura, K. & Wall, J. Nature Mater. 8, 808-812 (2009).Secondary Electron Image using Hitachi STEM at Brookhaven

Annular Dark

Field

Backside

particleBackside particle Zhu, Y., Inada, H., Nakamura, K. & Wall, J. Nature Mater. 8, 808-812 (2009). Secondary Electron Image using Hitachi STEM at Brookhaven Atomic resolution with secondary (and backscattering) electrons

Backside atom

visible in Annular

Dark Field

Backside atom

missing in secondaries

Now, what happens when electrons

produce xrays?

Large angle,

relatively chaotic processes

Small angle,

relatively well behaved processesquotesdbs_dbs6.pdfusesText_12

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