Materials Sample Preparation for TEM

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Materials Sample Preparation for TEM

• Electron Transparency (thickness lt 100 nm)
• Initial form of the material particulate or bulk
  material

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Particulate materials (powders, nanoparticles,
nanowires)

• Desirable particle size is about 500 nm or less.
  Powders that are more coarse than this should be
  ground with a mortar and pestle. The specimen
  preparation consists in transferring a suspension
  of the particles in a solvent such as isopropanol
  to a carbon coated grid and letting the solvent
  evaporate

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Bulk material

• The preparation process can sometimes be
  time-consuming and often requires careful
  grinding and polishing techniques, skills for
  which may take time to develop
• All TEM holders accommodate 3-mm disc. Thus, the
  TEM sample will be in the form of such a disc
• Simple metals or single-phase alloys can often be
  electro polished with an appropriate electrolytic
  solution. Even multiple phase alloys can
  sometimes be prepared in this fashion.
• More commonly, samples from bulk material are
  thinned with an ion beam. Before the final
  ion-beam thinning, however, the sample should be
  first mechanically thinned by lapping and
  polishing so that the final thickness in the
  center of the sample is about 30 microns

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Cross section of thin films

• Cross-section samples can usually be made using
  the same ion-thinning process as for bulk
  samples. However, one must first make a stack by
  gluing together two or more substrate fragments
  film-to-film

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Particulate material If particle size is
small enough to be electron transparent on its
own (lt100nm) Add small amount of powder in
solvent Ultrasonicate to disperse particles
well Place a small drop of suspension on
carbon-coated grids Different types of carbon
amorphous, holey, lacey (increasing pores) If
particle size is larger Grind the particle
with mortar and pestle Disperse particles in
resin Microtome using glass or diamond knives
(depends on hardness of particles)
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Sample Preparation Bulk Material

• Procedures
• Initial thinning to make a slice of material
  between 100-200 mm thick
• Cut the 3-mm disk from the bulk
• Prethin the central region from one or both faces
  of the disk to a few micrometers
• Final thinning of the disk to electron
  transparency
• Note the method to be used will depend on the
  information one desired and the physical
  characteristics of the material (e.g., soft or
  hard, ductile or brittle)

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Initial thinning

• Ductile materials such as metals
• Example
• Chemical wire/string saw
• A wafering saw (non-diamond)
• Spark erosion (electro-discharging machining)
• (is a non-traditional method of removing
  material by a series of rapidly recurring
  electric arcing discharges between an electrode
  (the cutting tool) and the workpiece)
• Roll the material to thin sheet

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Initial thinning

• Brittle materials such as ceramics
• Examples
• Si, GaAs, NaCl, MgO can be cleaved with a razor
  blade
• Ultramicrotomy, allows for immediate
  examinantion (diamond wafering saw)

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Disk Cutting Starting materials is ground or
sliced (cleaved) to slabs about 200 um in
thickness Then the disc can be cut using
ultrasonic disc cutter or the disc punch
Ultrasonic Disc Cutter The Ultrasonic Disc Cutter
is ideal for cutting TEM disks from brittle
materials such as ceramics and semiconductors.
Use an abrasive slurry of either boron nitride
or silicon carbide
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Prethinning the Disk
Mechanical Pre-thinning Disc Grinder produces
high quality parallel-sided thin samples while
reducing the chance of sample damage
Grind with SiC sandpaper (60 - 100 - 240 - 400
- 600 grit sizes) Polish with Al2O3 or diamond
suspensions (30 - 15 - 5 - 1 - 0.1µm)
Rule of thumb abrasive produces damage 3x their
grit size
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Mechanical prethinning dimpling

• The Dimpler provides with the easiest and most
  reliable means to produce many different types of
  samples for TEM and can be used on ceramics, many
  semiconductors, carbons, carbon composites,
  oxides, borides, silicides, glasses and many
  others.
• When prethinning, the Dimpler produces an
  ultra-high area for successful, artifact-free ion
  thinning, while maintaining a greater edge
  thickness to help prevent breakage while
  handling.
• The thickness achieved will depend on the
  material being thinned however, hard specimens
  typically can be dimpled to less than 5 mm with a
  100 mm thick periphery for specimen support.

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Dimpling chemically
Viewing mirror
etchant
specimen
Jet orifice
light
Example Dimpling Si using HF HNO3 GaAs
using Br methanol
13
How to determine the thickness of Si(110) crystal
by transparency colors?
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Advantages of the dimpler

• Large thin area with thicker rims
• Easier to handle fragile samples
• shorter preparation times
• easier location of the region of interest to be
  thinned
• large thin area in the center surrounded by thick
  rim eases handling of the thin samples.

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TEM specimen preparation using the Tripod
polishers A Tripod Polisher was designed by
scientists at IBM, which is used to prepare
accurately micro sizes of TEM and SEM samples.
For TEM samples, the Tripod Polisher has been
used successfully to limit ion milling times to
less than 15 minutes, and in some cases, has
eliminated the need for ion milling. It can be
used to prepare both plan-view and cross-sections
from a variety of sample materials, such as
ceramics, composites, metals and, geological
specimens.
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Tripod polishes The point of interest is aligned
with the back feet of the Tripod, to ensure the
point of interest is coplanar with the back
feet. Once the point of the interest has been
reached, the polishing plane should be parallel
with the back feet.
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Tripod polishes General polishing sequence a. 30
um diamond film b. 15 um diamond film c. 3 um
diamond film d. 1 um diamond film e. 0.5 um
diamond film f. 0.05 um silica
Reducing grit size
In wedge polishing mode, could generate edge thin
enough for TEM
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Final thinning
Ion Milling Bombard thin sample with energetic
Ar ions, sputter away material until electron
transparent Ar is introduced into an electric
field, ionized, accelerated at the sample as a
plasma Variables - ion current, angle of
incidence, sample temperature, sample rotation,
High ion current more damage Smaller angle of
incidence - less implantation, less damage, less
preferential sputtering (for composite material
or cross sectional interface specimen comprised
of with radically dissimilar milling rates
For inspection of the specimen
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Low Angle Ion Milling (down to 1
degree) Advantages of low angle milling using
the single post 1). Improved surface finish
(ion polishing) 2). Larger thin areas while
reducing artifacts 3). Less contamination from
specimen surroundings 4). Reduced artifacts
(less amorphous layer, less contamination).
Milling rates in the PIPS Below list some
typical milling rates at 4º obtained in the PIPS
for various materials using one ion gun at 5 keV
and no specimen rotation (um/hour for each
gun) 1) Copper 18 2) Silicon 15 3)
Silicon carbide 8 4) Stainless steel 7
Gatans Precision Polishing Ion System (PIPS)
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• Why rotate the sample?
• Rotate 360o (at a few rpm) to reduce surface
  structure (e.g. grooves) generating

For cross section TEM specimen, limit the
rotation (not 360o) to protect the interface

• Why cool the sample?
• minimizing atom migration in or on the specimen
• limiting phase change at low temperature

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Artifact Formation in ion milling
Many defects found on Argon ion milled CdTe
surface
Only growth defects found in iodine-thinned
specimen
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Final thinning electropolishing

• Electropolishing only be used for electrically
  conductive samples such as metals and alloys

Twin-jet electropolisher
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Focused Ion Beam (FIB) milling
Similar to SEM?
A schematic diagram for the liquid metal ion
source FIB system
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FIB ions source --- Liquid Metal Ion System
(LMIS) 1). heat Ga metal above melting
temperature Ga flows to a W tip with radius 2-5
um Others sources Au, Be, Si, Pd, Ni 2). use
field emission to form 2-5nm Ga tip (Taylor
cone) 3). extract Ga ions and accelerate them
down the column 4). Ga flow continuously
replenishes source. The principal is a strong
electromagnetic field causes the emission of
positively charged ions.
Why do we need ions instead of electrons ions
are bigger than electrons they have high
interaction probability since they have high
mass, they have slow speed but high momentum and
this is good for milling!
25
d
Conventional H-bar technique
Find out more on recent advances in FIB from Li,
J. et al. Materials Characterization, 57, 64
(2006)
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Lift-out technique
No need for prethinning procedures
Risk of losing the sample is also high
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In-situ lift-out technique a 200 nm lamella




Extraction of the 200 nm lamella using a
microgripper inside FIB (Kleindiek
nanotechnik) Then transfer onto TEM grid with
carbon film