Download TEM - SNS College of engineering

OUTLINE
INTRODUCTION TO TEM
COMPONENTS &
WORKING OF TEM
SAMPLE PREPARATION
TYPES OF TEM
INTRODUCTION
ERNST RUSKA
MAX KNOLL
IN 1931,WHILE CONDUCTING RESEARCH FOR
HIS MASTERS AT THE TECHNICAL COLLEGE OF
BERLIN,ERNST RUSKA & MAX KNOLL DESIGNED
THE FIRST TEM
TEM IS MUCH LIKE SLIDE PROJECTOR BUT THE BASIC
DIFFERENCE IS LIGHT MICROSCOPE USES BEAM OF LIGHT
WHEREAS TEM USES BEAM OF ELECTRON
MICROSCOPE
RESOLUTION
MAGNIFICATION
OPTICAL
200 nm
1000x
TEM
0.2 nm
500000x
SPECIMEN MUST BE ULTRATHIN
SIMPLE DIAGRAM TO SHOW THE DIFFERENCE
BETWEEN SLIDE PROJECTOR & TEM
DIAGRAM TO REPRESENT TEM’S WORKING
Virtual Source
First Condenser Lens
Second Condenser Lens
Condenser Aperture
Sample
Objective Aperture
Objective Lens
Selected Area Aperture
First Intermediate Lens
Second Intermediate Lens
Projector Lens
Main Screen (Phosphor)
DIFFERENT COMPONENTS OF TEM
1.
2.
ELECTRON EMITTER
4.
CONDENSER
5.
APERTURE CONTROLS
6.
SPECIMEN HOLDER
7.
OBJECTIVE LENS
8.
PROJECTOR LENS
9.
OPTICAL LENS
10.
FLUORESCENT SCREEN
HIGH TENSION CABLE
3.
STEPPER MOTORS FOR CENTERING
THE ELECTRON BEAM
11.
VACUUM PUMP LEADS
12.
GONIOMETER
13.
VACUUM AND MAGNIFICATION
CONTROL
14.
FOCUSING CONTROL
SOME TYPICAL TEMs
ELECTRON BEAM
SOURCE(ELECTRON GUN)
TUNSTEN(W)
FILAMENT
LANTHANUM
HEXABORIDE
(LaB6 )
ELECTRICAL CONNECTION
OF THE GUN
TYPE OF ELECTRON
EMISSION
W FILAMENT
TC = 2500-3000 K
Øw = 4.5 EV
THERMOIONIC
EMISSION(TE)
IN CASE OF (TE)
ACCORDING TO
RICHARDSON’S LAW
Jc = ATc2 exp(-øw/ kTc)
WHERE,
Jc=CURRENT
DENSITY(Am-2 )
k =1.38 ×10 -23 J K-1
(BOLTZMANN’S
CONSTANT)
TC=CATHODE
TEMPERATURE
A≈ 12×10⁵ AK-2m-2
Øw =WORK FUNCTION
FIELD
EMISSION(FE)
IN CASE OF (FE) ACCORDING TO
FOWLER NORDHEIM FORMULA
Jc = k1 |E|2/øw exp (-k2 øw 3/2/|E|)
WHERE,
E= ELECTRIC FIELD
REST OF THE TERMS BEAR MEANING
AS USUAL.
JC≈(1-3)×10⁴ A/m2
LaB6
TC =1400-2000 K
Øw = 2.7 ev
JC≈(2-5)×10⁵ A/m2
Condenser lenses
IT CONTROLS HOW STRONGLY THE BEAM IS FOCUSED
( CONDENSED) ONTO THE SAMPLE.IT DETERMINES THE
SIZE OF THE SPOT THAT STRIKES THE SAMPLE
ALIGNMENT
CONDENSER LENSES CONTINUED
CHANGING THE STRENGTH OF THE TWO LENSES
WE CAN CHANGE THE POSITION OF THE FOCUS
CONDENSER LENSES CONTINUED
AS WE CHANGE THE EXCITATION OF THE TWO
LENSES,THE MAGNIFICATION OF THE IMAGE
CHANGES.THIS WAY ADJUSTING THE EXCITATION
OF THE TWO LENSES WE CAN VARY THE SPOT
SIZE.
HERE WE SEEM TO HAVE
BROKEN A RULE, IN THIS
DIAGRAM. WE HAVE BENT
THE RAYS IN FREE SPACE AT
THE PLANES, WHERE THEY
REACH FOCUS ACCORDING TO
THE PREVIOUS DIAGRAM.
SURELY BEAMS JUST CAN’T
BEND, WITHOUT HAVING A
LENS OR DEFLECTION COIL.
TRUE. IN FACT, WHAT WE ARE
DOING IS CHANGING OUR
ATTENTION FROM ONE SET
OF BEAMS THAT PASS
THROUGH THE FIRST LENS, TO
A 2ND SET OF BEAMS THAT
PASS THROUGH THE 2ND LENS.
RESOLUTION IS LIMITED BY LENS ABERRATION
Spherical Aberration
ABERRATION
Marginal Focus
Axial Focus
SPHERICAL
CHROMATIC
ABERRATION
ABERRATION
ASTIGMATISM
Optic
Axis
Disc of minimum
confusion
Chromatic Aberration
Focus A
Lens
Focus B
Optic
Axis
Disc of minimum
confusion
Point is imaged as disc
SPHERICAL ABERRATION IS CAUSED BY THE
LENS FIELD ACTING INHOMOGENOUSLY
ON THE OFF AXIS RAYS.
Lens
CHROMATIC ABERRATION IS CAUSED BY THE
VARIATION OF THE ELECTRON ENERGY & THUS
ELECTRON ARE NOT MONOCHROMATIC
ASTIGMATISM
CORRECTION OF ASTIGMATISM
ASTIGMATISM MEANS THAT THE STRENGTH
OF THE LENS IS DIFFERENT IN TWO
DIFFERENT
DIRECTIONS.THAT
MEANS
THERE ARE NOW TWO FOCUS POINTS.
y
ASTIGMATISM CAN BE COMPENSATED
FOR BY PLACING A SIMPLE STIGMATOR
IN THE POLEPIECE BORE OF THE LENS.
STIGMATOR WORK BY ADDING A SMALL
QUADRUPOLE DISTORTION TO THE
LENSES.
x
Line focus in y direction
Line focus in x direction
THE OVAL IS MEANT TO REPRESENT A
PERSPECTIVE VIEW OF THE TOP OF THE LENS.
IN ORDER TO COPE WITH EVERY
POSSIBLE ORIENTATION OF
ASTIGMATISM,WE NEED TWO SETS OF
QUADRUPOLES MOUNTED AT 45⁰
DEGREES TO ONE ANOTHER.
CORRECTION OF SPHERICAL &
CHROMATIC ABERRATION
SPHERICAL ABERRATION CAN BE
COMPENSATED FOR BY A COMBINATION
OF MAGNETIC QUARDRUPOLE &
OCTOPOLE LENSES, WHEREAS A COMBINATION OF ELECTROSTATIC & MAGNETIC
QUADRUPOLES IS NECESSARY FOR THE CHROMATIC ABERRATION.
BRIGHT FIELD
MODE(BF)
IMAGING
DARK FIELD
MODE(DF)
IN BF ONLY THE
TRANSMITTED
PRIMARY BEAM IS
ALLOWED TO PASS
OBJECTIVE
APERTURE TO FORM
IMAGES
IN DF ONLY
DIFFRACTED BEAMS
ARE ALLOWED TO
PASS THE APERTURE
PREPARATION OF SAMPLE
SAMPLE MUST BE THIN ENOUGH,SHOULD BE OF THE ORDER OF 100-200 nm, SO THAT IT CAN
TRANSMIT AN ELECTRON BEAM
THE THINNER THE SAMPLE,LESS IS THE SCATTERING OF THE ELECTRON BEAM AND BETTER THE
IMAGE & ANALYTICAL RESOLUTION.
IN CASE OF MATERIALS OF SMALL DIMENSION LIKE POWDERS OR NANOTUBES,A DILUTE
SAMPLE CONTAINING THE SPECIMEN IS DEPOSITED ONTO SUPPORT GRID OR FILMS.
IN CASE OF METALS & SEMICONDUCTORS DIFFERENT TECHNIQUES LIKE
ELECTROPOLISHING,CHEMICAL ETCHING ARE USED.
IN CASE OF BIOLOGICAL SAMPLES DIAMOND KNIFE OR ULTRAMICROTOME IS USED TO CUT
THIN SECTIONS.
SOMETIME TO INCREASE THE CONTRAST & TO ISOLATE A CERTAIN AREA OF INTEREST STAINING
METHOD IS USED.
MORE RECENTLY FOCUSSED ION BEAM METHOD HAVE BEEN USED TO PREPARE SAMPLES.THIS
TECHNIQUE MAKES IT POSSIBLE TO MILL VERY THIN MEMBRANES FROM A SPECIFIC AREA OF
INTEREST IN A SAMPLE LIKE SEMICONDUCTOR OR METAL
GRID IS A SIEVE WOVEN FROM A THIN METAL WIRE,USUALLY NICKEL OR
COPPER GRIDS OF 3 mm DIAMETER ARE COMMERCIALLY AVAILABLE WITH
DIFFERENT MESH SIZES(GENERALLY OF 100-200 µm SIZE)
TYPES OF TEM
CONVENTIONAL TEM
HIGH RESOLUTION TEM
ANALYTICAL TEM
HIGH VOLTAGE TEM