4/25/2014

1

Transmission Electron Microscopy (TEM)

In a typical TEM a static beam of electrons at 100-200kV accelerating voltage illuminate a region of an electron transparent specimen which is immersed in the objective lens of the microscope.

The transmitted and diffracted electrons are recombined by the objective lens to form a diffraction pattern in the back focal plane of that lens and a magnified image of the sample in its image plane. A number of intermediate lenses are used to project either the image or the diffraction pattern onto a fluorescent screen for observation.

The uniqueness of TEM is the ability to obtain full morphological (grainsize, grain boundary and interface, secondary phase and distribution,defects and their nature, etc.), crystallographic, atomic structural andmicroanalytical information such as chemical composition (at nm scale),bonding (distance and angle), electronic structure, coordination numberdata from the sample.

A simple analog

4/25/2014

2

An alternative comparison

JEOL 2010F

Electron gun

Probe forming lenses - Cond.

Specimen holder

Magnifying lenses - Int. & Proj.

Objective Lens

HAADF Detector(high angle annular dark-field)Viewing Chamber

Camera Chamber

STEM Detector &/or EELS

XEDS Detector

Basic features ofan analytical electron microscope

4/25/2014

3

FEI Titan

VacuumThe electron microscope is essentially a series of connected vessels separated by valves.

The vacuum near the specimen is around 10-7 Torr. The vacuum in the gun depends on the type of gun, either around 10-7 Torr (tungsten or LaB6) or 10-9 Torr (for a Field Emission Gun).

The pressure in the projection chamber was usually the worst. The projection chamber holds the negatives used to record images. These negatives can outgas, limiting the ultimate vacuum. [digital recording eliminates this!]

4/25/2014

4

The Lenses in TEMCondenser lenses(two)-control howstrongly beam is focused (condensed) onto specimen. At low Mag. spreadbeam to illuminate a large area, at highMag. strongly condense beam.

Objective lens-focus image (imageformation) and contribute most to the magnification and resolution of the image.

Four lenses form magnificationsystem-determine the magnificationof the microscope. Whenever themagnification is changed, the currentsthrough these lenses change.

Image Formation in TEM

Ray Diagram for a TEM

Control contrast

Control brightness,convergence

4/25/2014

5

Why Electrons?

Improved Resolution!In the expression for the resolution

(Rayleigh’s Criterion)

r = 0.61/nsin-wavelength,V-accelerating voltage, n-refractive index

-aperture of objective lens, very small in TEM

sin so r=0.61/ ~0.1 radians (5.5o)Green Light 200kV Electrons~400nm ~0.0025nmn~1.7 oil immersion n~1 (vacuum)r~150nm (0.15m) r~0.02nm (0.2Å)

1/10th size of an atom!unrealistic!

Resolution Limited by Lens Aberrations

point is imagedas a disk.

Spherical aberration is caused by thelens field acting inhomogeneously onthe off-axis rays.

point is imaged

Chromatic aberration is caused by thevariation of the electron energy (PSvoltage) so electrons are notmonochromatic.

rmin0.91(Cs3)1/4

Practical resolution of microscope. Cs–coefficient of spherical aberration of lens (~mm)

as a disk.

4/25/2014

6

Specimen Holder

a split polepieceobjective lens

holder

beam

Heating and strainingTwin specimen holder

Double tilt heating

Rotation, tilting, heating, cooling and straining

Specimen Holder with Electrical Feedthroughs

4/25/2014

7

Beam and Specimen Interaction

(EDS)

(EELS)SAED & CBED

diffraction

BFDFHREM

Imaging

Scanning Transmission Electron Microscopy (STEM)

In STEM, the electron beam is rastered (scan coil) across the surface of a sample in a similar manner to SEM, however, the sample is a thin TEM section and the diffraction contrast image is collected on a solid-state detector.

4/25/2014

8

Imaging in the TEM

• Two principal kinds:

– Diffraction contrast imaging.(bright field / dark field)

Use either a non-diffracted or diffracted beam and remove all other beams from the image by the use of an objective aperture.

– Phase contrast or high resolution imaging.(HREM)

Use all of the diffracted and non-diffracted beams (by using a large objective aperture or none at all) and add them back together (phase and intensity) to form a phase contrast image

Silicon <100> zone axispattern.

Selected Area Diffraction

Parallel Electron Beam

Sample

Diffraction Plane

Selected Area DiffractionParallel Illumination.Lens Aberration limitsresolution to ~1 µm.

Objective Lens

4/25/2014

9

BF & DF Imaging – Diffraction Contrast

Objective aperture

C-filmamorphous

crystal

D

T

BF image

C-film

crystal

D

T

C-film

crystal

DF image

Diffraction + mass/thickness= Contrast

Objective aperture

DDF CDF

Beam tilt

T-transmittedD-diffracted

Hole in OA

OA OA

DDF – displacive DF; CDF – centered DF

Bright Field (BF) and Dark Field (DF) Imaging

Incident beam

specimen

transmitted beam

diffracted beam

objective aperture

hole in objectiveaperture(10-100m)

BF imaging-only transmitted beam is allowedto pass objective aperture to form images.

BF

DF

DF

DF imagingonly diffractedbeams areallowed to passthe aperture toform images.

Particles in Al-CuAlloy.

4/25/2014

10

Phase Contrast ImagingHigh Resolution Electron Microscopy

(HREM)

Use a large objectiveAperture (get both beams). Phases and intensities of diffracted and transmitted beams are combined to form a phase contrast image.

TD

Si

Objectiveaperture

Electron diffraction pattern recordedFrom both BN film on Si substrate.

BN

Electron Diffraction

Specimen foil

T D

e-

L 2

r

dhkl

[hkl] SAED pattern

L -camera lengthr -distance between T and D spots1/d -reciprocal of interplanar distance(Å-1)SAED –selected area electron diffraction

Geometry fore-diffraction

Bragg’s Law: = 2dsin

=0.037Å (at 100kV)=0.26o if d=4Å

= 2dr/L=sin2as 0r/L = 2

r/L = /d or

r = Lx 1d

hkl

Reciprocal lattice

4/25/2014

11

Why are there so many spots?

Reciprocal lattice

k – wave vectorlkl = 1/ – wavelength of electron

SAED Patterns of Single Crystal, Polycrystalline and Amorphous Samples

a b c

a. Single crystal Fe (BCC) thin film-[001]b. Polycrystalline thin film of Pd2Sic. Amorphous thin film of Pd2Si. The diffuse

halo is indicative of scattering from anamorphous material.

r1 r2200

020

110

4/25/2014

12

Diffraction Spot Intensity

Spot intensity: Ihkl lFhkll2

Fhkl - Structure Factor

Fhkl = fn exp[2i(hu+kv+lw)]N

n=1

fn – atomic scattering factor

fn Z, sin/

h,k,l are Miller indices and u,v,w fractional coordinates

Specimen Preparation-DestructiveDispersing crystals or powders on a carbon film on a grid

3mm

Mechanical Thinning

Grind, Lap

Machine & Slice

4/25/2014

13

Cross-Sections

Substrate

Four pieces of a specimenformed from thin film(s) on a

substrate.

Thin Film

The four pieces areglued together ( face-to-face ,

and face-to-back ) to forma cross section.

Glue

Cross-Sections...

A 2.8mm diameter pieceis drilled from the

cross section.

The 2.8mm diameter rodis placed within a 3mm

external diameter metal tube.

4/25/2014

14

Cross-Sections...

Thin slices are cut from the rod,and mechanically thinned to

~100 m.

~100 m

3.0mm

Mechanical Thinning

Planar Thinning Dimple GrindingHolderHolder

DimpleWheel

HolderBeveled Plate

Specimen

ThinRegion Planar Thinning

DiscSpecimen

V

PumpedElectrolyte Jet

PumpedElectrolyte Jet

Electro-Chemical Thinning

Ion Gun

Ion Gun

Specimen

Ion Milling

4/25/2014

15

Focused Ion Beam (FIB) MillingFIB beam

Sample epoxied to grid

Grid

Sample milled by FIB beam

TEM beam

Focused Ion Beam (FIB) System

• focus a Ga ion beam to a few tens of nanometers to mill the specimen

• interaction of Ga ion beam with the specimen also generatessecondary electrons that can be used for SEM imaging. So wecan observe the area under milling during the milling process.FIB permits selected area milling.

• high specimen milling rates as well as high positional accuracy for milling of the area(s) of interest.

4/25/2014

16

FIB procedure

a. Select an area of interest,

Coat with a layer of metal (Pt)

b. Make “trench” using

high current ion beam

c. Thinning the wall d. Cut the wall and removehttp://www.labcompare.com/623-Videos/139165-AURIGA-Laser-FIB-SEM-Microscope-from-ZEISS/

Applications of TEM

TEM

Conventional TEMMicrostructure, morphology (grain size, orientation), phase distribution and defect analysis (point defects, dislocations and grain boundaries)

In situ TEMIrradiation and deformation experimentsEnvironmental cells (corrosion)Phase transformations(hot- and cold-stage, electric field)

Analytical TEM (Z-contrast imaging)Chemical composition-EDS, EELS, ELNES, EXELFS, Z-contrast imagingCBED-lattice strain, thickness, charge density

HRTEMLattice imaging, structure of complex materialsand atomic structure of defects (interfaces)

4/25/2014

17

Sub-Nanometric EDS Analysis (JEOL-2010F Field-emission TEM)

MBE-grown InGaAsP/InPMulti-quantum well structure

EDS spectra taken with a 5ÅProbe. A.1nm InGaAsP layerB.~3nm away from interfaceWithin InP matrix.

A

B

A

B

InGaAsP

InP

HREM

In-situ nano-indentation

See movie

4/25/2014

18

Limitations of TEM

• Sampling• Interpretation of image

• Beam damage

• Specimen preparation