II. Electron microscopy: TEM and SEM (PDF) - NCLT

What can electrons do? 

Jian-Guo Zheng 

Materials Science & Engineering 

EPIC/NUANCE Center 

Northwestern University 

2220 Campus Drive, 1156 Cook Hall 

Evanston, IL 60208-3108, USA 

Phone: (847) 491-7807, Fax: (847) 491-7820 

E-mail: j-zheng3@northwestern.edu 

NCLT: June-Aug. 2006

What can electrons do? 

outline 

I. Introduction 

II. Electron microscopy: TEM and SEM 

III. Seeing with electrons: imaging 

IV. More than images 

V. Challenges and opportunities 


II Electron microscopy 

II.1 What are electron microscopes? 

II.2 Scanning electron microscope 

II.3 Transmission electron microscope 


II.1 What are electron 

microscopes? 

• Definition 

• First (transmission) electron microscope 

• The brief history of electron microscopy 

• Modern TEMs and SEMs 

• Major components of electron 

microscopes 

• Electron gun 

• Electron lens 


II.1 What are electron 

microscopes? 

Electron 

Microscopes are 

scientific 

instruments that 

use a beam of 

highly energetic 

electrons to 

examine objects 

on a very fine 

scale. 

Electron microscopes: 

scanning electron microscope (SEM) and 

transmission electron microscope (TEM) 

e-beam 

Signals to SEM 

Top 

Bottom 

specimen 

Signals to TEM 


1 st (Transmission) Electron Microscope 

1931 Max Knoll and Ernst Ruska built the first TEM 

1986 Nobel prize for E.Ruska (together with G. Binning and 

H. Rohrer, who developed the Scanning Tunneling Microscope) 


Some Milestones in 

the History of Electron Microscopy 

1897 Discovery of the electron by J.J. Thompson 

1924 P. De Broglie: particle/wave dualism, magnetic 

coil/electron beam-glass/light 

1927 Hans Busch: Electron beams can be focused in an 

inhomogeneous magnetic field. Electron lens relation/ glass lens 

1931 Max Knoll and Ernst Ruska built the first TEM 

1938 Scanning transmission electron microscope (M. von 

Ardenne) 

1939 First commercial TEM by Siemens (Ruska, von Borries) 

1943 Electron energy-loss spectroscopy EELS (J. Hillier) 

~1940 Basic theoretical work on electron optics and electron 

lenses (W. Glaser, O. Scherzer), Cs/Cc 

1951 X-ray spectroscopy 

NCLT: 

(R. Castaing) 

June-Aug. 2006

Some Milestones in 

the History of Electron Microscopy 

1956 First lattice image (J. Menter) 

1957 Multi-slice method (J. Cowley, A. Moodie) 

1964 First commercial SEM by Cambridge Instruments 

(Charles Oatley) 

~1970 HRTEM microscopes with a resolution better than 4 Å 

1986 Nobel prize for E.Ruska (together with G. Binning and H. 

Rohrer, who developed the Scanning Tunneling Microscope) 

1990: Aberration corrector design 

1998: first Cs-corrected transmission electron microscope 

2004: Cc/Cs corrected commercial electron microscope (JEOL, 

FEI, LEO…) 


Modern TEMs 

High voltage TEM 

JEOL 4000EX 

VG HB603 

JEM-2200FS 

NCLT: June-Aug. 2006 

Tecnai G2 F20 ST-twin

Modern SEMs 

Hitachi 4800 

LEO 1550 

FEI Quanta 600F 

JEOL 7700F 

1 st commercial 

AC-SEM 


Strata 400 

FEI dual 

Beam 

(SEM/FIB)

Major components 

of electron microscopes 

• Electron source 

Light Source 


source 

• Illumination system 

• Specimen stage 

Condenser 

lens 

specimen 

Objective 

lens 

specimen 

lens 

Beam deflector 

• Imaging systems 

Projector 

lens 

Image 

on screen 

detector 

• Attachments 

Light 

Microscope 

TEM 

SEM 

EDS, EELS… 

Diagram from: http://www.mih.unibas.ch/Booklet/Lecture/Chapter1/Fig.1-6.gif 


Electron Gun 

Electrons can be emitted 

from a filament (emitter or 

cathode) by gaining 

additional energy from heat 

or electric field. 

Commonly used emitters: 

Tungsten wire 

LaB6 filament 

Field emitter. 

Thermionic emission 

Field emission 

The common properties of 

the emitters are low work 

function, high melting point, 

and high mechanical 

strength 

W wire 


LaB6 

Field emitter

A comparison of electron sources 


Electromagnetic Lens 

An electromagnetic lens can manipulate electron trajectory to form either a small 

electron probe (condenser) or an enlarged image of a specimen. 

If the image rotation is ignored, the behavior of the electromagnetic lens can be 

described by the formula used for optical lens: 1/f=1/p+1/q 

The focal length of the lens (f) can be changed by the electric current supplied to 

the lens (f α 1/(NI) 2 ) 

Object 

Lens 

Image 


Picture from: http://www.matter.org.uk/tem/lenses/thin_lens_optics.htm

II.2 Scanning electron microscope 

• Signals and functions of a normal SEM 

• How does SEM work? 

• Optimizing SEM Image 

• Interaction volume and resolution 

• Stigmatism and Resolution 

• Depth of Field 

• Secondary electron image- Topography 

• Backscattered electron image: Z-contrast 

• X-ray Energy Dispersive Spectroscopy (EDS) in 


• What else can we do in the SEM? 


Signals and functions of a 

normal SEM 

Secondary electrons 

Backscattered Electrons 

X-rays 


How does SEM work? 

A SEM image is built up point by point, which is similar to TV, and the 

intensity of each pixel is proportional to the signal intensity. 

Electron gun generates an electron 

beam; (Condenser and objective) 

lens focuses the beam onto a 

sample; The beam deflector 

controls the beam scan onto the 

sample; At each point where the 

beam hits the sample, 

backscattered and secondary 

electrons are ejected from the 

specimen; Detectors collect these 

electrons, and convert them into a 

signal that is sent to a video screen. 

SEM Diagram 


gun 

Condenser 

lens 

Bean 

deflector 

Objective 

lens 

Specimen 

stage 

Detector 


Scanning electron microscope 

Forming an 

electron probe 

source 

size do 

CL 

Beam scanning, beam/specimen 

Interaction and signal detection 

Beam scan 

Detector 

OL 

S 

specimen 

Signals 

Secondary electrons 

Backscattered electrons 

X-ray 

CL 

NCLT: Modified June-Aug. from: H:\2006research\nclt\SEM\SEM01.htm

Optimizing SEM Image 

•High resolution (ability to differentiate between small features on 

the surface: small probe diameter 

•Large depth of field (Ability for an image to be in focus over 

large changes in surface topography): a low convergence angle 

•High quality (ability to see details without interference from noise 

signal): high beam current 

An optimized SEM image is obtained through a control of the electron 

beam, that is, probe diameter, probe current and convergence angle, 

involving electron gun, condenser lens, objective aperture, and 

working distance. A high resolution high quality image with a large 

depth of field is made by a beam with a high current and a low probe 

diameter and a low convergence angle. As the probe current is 

increased, both probe diameter and convergence angle increase. This 

is not desirable for imaging, so the parameters must be controlled in a 

way to produce the best mix of image quality, resolution, and depth of 

field depending on the features of interest on the sample itself. 


Interaction volume and resolution 

The Scanning Electron Microscope (SEM) was developed mainly because of 

the limitations of optical microscopy (low resolution and poor depth of field). 

SEM resolution is 

dependent on the 

interaction volume resulted 

from the interaction 

between incident electrons 

and specimen. The 

interaction volume is 

associated with the beam 

size/shape, electron 

energy and specimen 

material. 

Please note: Small beam 

size may improve the 

resolution, but decrease 

the signal to noise ratio, 

that is, image quality or 

sensitivity. 

Resolution: Size/Shape of Interaction Volume 

Sensitivity: Signal/Noise Ratio 

It is necessary to make a balance between the 

resolution and sensitivity 


Interaction 

volume 

Diagram from: H:\2006research\nclt\SEM\SEM01.htm

Stigmatism and Resolution 

The shape of electron beam 

affects SEM image 

resolution: when the beam is 

round, or without stigmatism, 

the image shows small 

features (high resolution) as 

seen in Fig. a; when the 

beam is not round, or with 

stigmatism, the image 

details become vague (lower 

resolution) as seen in Fig. b. 

without stigmatism 

a 

1 µm 

c 

with stigmatism 

b 

1 µm 

d 

When the image has 

stigmatism, changing beam 

focus may result in elongated 

feature: Figs. c and d were 

recorded when the beam 

were under and over focus, 

respectively. 

1 µm 

1 µm 

Under focus 

Over focus 


Picture from: H:\2006research\nclt\SEM\SEM01.htm

Depth of Field 

The Scanning Electron Microscope (SEM) was developed mainly because of 

the limitations of optical microscopy (low resolution and poor depth of field). 

SEM image taken with secondary electrons (left) shows topographical features 

clearly. As compared with corresponding optical micrograph (right), the SEM 

image shows three-dimensional appearance of the specimen image which is a 

direct result of the large depth of field (DOF) of the SEM, the biggest 

advantage. 


OM 


Depth of field 

Depth of field (D) varies with the 

objective aperture size, working 

distance and magnification, and is 

given by the following expression: 

Depth of field (D) 

1 

D ∝ αM 

Where M is the magnification and α 

is the divergence angle which can 

be calculated from the aperture 

radius (RAp) and working distance 

(DW): 

α = R Ap 

D W 

SEM: small α 

Optical microscope: large α 

Depth of field (D) is the height over 

which the sample can be clearly 

focused. 


Secondary electron image- 

Topography 

SE contrast mechanism: Edge effect 

SE image of Al2O3/Ni 

Secondary electrons escape from sample areas close to the surface. The edge effect takes 

place when many electrons escape from areas with sharp edges or peaks to be deflected by 

the Faraday Cage (B). Some electrons escape from areas without sharp edges (A and C). Few 

secondary electrons are detected when generated secondary electrons are only deflected 

partially by the Faraday Cage due to the location on the sample they are generated (A). 


Picture: Courtesy of Prof. T. Sekino, ISIR, Osaka Univ. 

Diagram from: H:\2006research\nclt\SEM\SEM01.htm

Backscattered electron images: 

Z-contrast 

In BSE image, the signal is dependent on atomic number 

In BSE image, Ni precipitates is much brighter than Al2O3; as 

comparison, In SE image, Ni and Al2O3 have similar contrast 

Backscattered electron (BSE) image Secondary electron (SE) image 

BSE 

SE 

Al2O3/Ni composite 

Courtesy of Prof. T. Sekino, ISIR, Osaka Univ. 


X-ray Energy Dispersive 

Spectroscopy (EDS) in SEM 

RGB 

EDS spectrum: Characteristic X-ray peaks on continuum bk 

http://www.geosci.ipfw.edu/cgi-bin/sem/techinfo.cgi?choice=xmap 


EDS elemental maps

What else can we do in the SEM? 

•High Resolution SEM 

•Low kV imaging 

•STEM 

•Stereomicroscopy 

•Voltage contrast 

•Magnetic field contrast 

•Crystallographic contrast 

•Electron channeling diffraction 

•Electron backscattered diffraction (EBSD) 

•Cathodluminiscence (CL) 

•E-beam lithography 


II.3 Transmission Electron 

Microscopy (TEM) 

• Signals and Functions of Modern TEMs 

• An example: ASTAR TEM @ EPIC 

• Basic TEM diagram 

• Illumination system 

• Imaging system 

• Basic TEM: Image and Diffraction 

• TEM Imaging: mass-thickness contrast, diffraction contrast, phase contrast 

• TEM diffraction: SAED & CBED 

• X-ray Energy Dispersive Spectroscopy (EDS) 

• Electron Energy Loss Spectroscopy (EELS) 

• Energy-filtered TEM 

• EFTEM spectrum imaging 

• Scanning Transmission Electron Microscopy (STEM): BF/ADF 

• STEM: HAADF Z-contrast 

• STEM: EDS and EELS 

• Other TEM techniques 


Signals and Functions of 

Modern TEMs 

Incident electrons 

X-ray 

specimen 

Imaging 

Diffraction 

Inelastically 

scattered 

electrons 

Transmitted 

electrons 

Elastically 

scattered 

electrons 


Spectroscopy 

Spectrum imaging

For example: ASTAR TEM @ EPIC Northwestern University: 

Analytical Scanning Transmission Atomic Resolution 

JEOL 2100F (S)TEM 

High Voltage: 200kV, 

Gun: Schottky FEG emitter 

Pole piece: Ultra high resolution 

Stages: Double tilt, Single tilt 

Camera: Film/TV rate CCD/Slow 

scan CCD 

Attachments: Oxford EDS, 

GIFTridiem, BF/ADF/HAADF 

STEM 

Functions: BF/DF, HRTEM, 

STEM, CBED, Nanodiffraction, 

PEELS, EDS, EFTEM, Z-Contrast 

Imaging 

Lattice resolution: 0.1 nm 

Point resolution: 0.19 nm 

STEM spot: 0.136 nm 

Sub-nano-analysis: 0.2 nm-0.5 nm 

Cs=0.5 mm 

Cc=1.1mm 

Sample tilt: 25° 

Environment is important to achieve high 

performance ! 


Basic TEM diagram 

Gun 

Illumination system 

Specimen 

Imaging system 

CL 

CA 

SP 

OL(OA) 

SA 

IL 

PL 

S 


Illumination system 

Gun 

crossover 

Condenser 

Lens 1 

Condenser 

Lens 2 

Upper 

Objective 

Lens 

Specimen 

Parallel beam 


Convergent beam

Imaging system 

Specimen 

Diffraction pattern 

Image 

Intermediate 

Lens 

Projector 

Screen 

Diffraction 


Image

Basic TEM: 

Image and Diffraction 

Thin specimen

TEM Imaging: Mass-thickness 

BF TEM 

image of 

silica: 

Thickness 

contrast 

TEM Contrast 

mechanisms: 

Amorphous material 

SiO2 

•Mass-thickness 

•Diffraction contrast 

•Phase contrast 

Thickness map: EELS 



YBaCuO stacking faults 

TEM Imaging: 

Diffraction Contrast 

Defects 

Phase 

Strain fields 

g 

DF TEM 

TiAl lamellar structure 

BF image 


DF image

High Resolution TEM 

Growth twin in SnO2 

TEM imaging: 

Phase contrast 

Interference pattern of 

diffraction beams 

•Crystal structure 

•Defect 

Journal of Applied Physics, 

79 (10): 7688-7694 May 15 1996 


Diffraction: SAED 

Selected Area Electron Diffraction 

--crystal structure, orientation relationship 

SAED 

SAED pattern: CdO/MgO 


g=200 

Parallel 

electron 

beam 

CdO Film 

Cubic/cubic 

MgO substrate 

__ 

200 nm 

J. Am. Chem. Soc., Vol 126 (27), 8477-8492, 2004 


Diffraction: CBED 

Crystal symmetry 

precipitate 

Point group 

Convergent beam 

sample 

Phil. Mag. A, 80 (2): 493-500 Feb 2000 


Spectroscopy and 

Spectrum Imaging 

•EDS 

• EELS 


X-ray Energy Dispersive 

Spectroscopy (EDS) 

EDS: 

•Composition information 

•Easy to operate the system 

•Convenient to identify 

elements 

•Good for elements with 

higher atomic number 

•Complementary to EELS 

•Low background 

For STEM: 

•High spatial resolution 

•Low signal (counts) 

•FEG gun needed 

Operation modes: 

•Spot 

•Line scan 

•mapping 

Electron probe 

X-ray 

Thin sample 

a 

b 

ADF STEM image 

NCLT: (I nm probe) June-Aug. 2006 

EDS spectra from a) ZnO tetrapod 

and b) CuO particles. Ni signal is 

from Ni supporting grid. 

O 

O 

Zn 

Cu 

Ni 

Ni 

Cu 

Zn 

Zn 

Cu 

a 

b

Electron Energy Loss 

Spectroscopy 

•EELS signals reflect elemental concentration 

•EELS adds another dimension for generating TEM 

image contrast 

•EELS signal can be used to rapidly identify and map 

elements and phases 

•EELS isolates pure elastic signal for quantitative 

imaging and diffraction 

•EELS provides information about electronic 

structure and properties 

What does an EELS spectrum look like? 

Two types of Energy Filter 


Diagram of EELS 

Courtesy:Dr. Mike Kundmann, Gatan

PEELS spectra from ZnO tetrapod and CuO particles 

•TEM 

•STEM 

ZnO 

CuO 

O-K edge 

Zn L23 edge 

1020 eV 

O-K edge 

Cu L23 edge 

931 eV 


Energy-filtered TEM: elemental-specific imaging 

EELS elemental map: three-window method 

Pre-edge 1 Pre-edge 2 Post-edge 

Cu-L23 

Cupper Map 


EFTEM spectrum imaging 

•3D data set is built up image by image 

ZnO-CuO 

Cu-L edge 

Zn-L edge 


Scanning Transmission Electron 

Microscopy (STEM) 

STEM: BF/ADF 

STEM diagram 

Au/Ag nanoparticles 

BF STEM 

ADF STEM 

Annular Dark 

field detector 

Bright field 

detector 

Compare with TEM: 

-BF: Similar 

-DF: More efficient 

Detector ring/ aperture (TEM) 


Advanced Materials, 12 (9): 640-643, May 3 2000

STEM: HAADF Z-contrast 

-Actual atomic position 

-Contrast changing with Z 

-Small probe for analysis 

Ge/Si 

Si Dumb-Bell 

0.136 nm 

STEM of Si [110] 

Advantage: HAADF STEM image vs. HRTEM 


STEM: EDS and EELS 

Spectrum: spot, line, area 

Spectrum imaging 


Other TEM techniques 

• Lorentz microscopy 

• Reflection electron microscopy 

• Electron holography 

•Cryo-TEM 

• In-situ TEM 

• TEM tomography 

•…

II. Electron microscopy: TEM and SEM (PDF) - NCLT

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