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TEM Sample Preparation for Inorganic Materials

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Title: TEM Sample Preparation for Inorganic Materials


1
TEM Sample Preparation for Inorganic
Materials From transmission electron
microscopy, people need thin (lt100nm) samples
How are we going to make TEM samples that
thin from bulk materials like a brick or an
airplane? Several methods - depends on initial
form of materials and what information is needed
2
Sample Preparation Hard Materials Samples
Powders (particles, fibers) Bulk (stones,
crashed planes) Semiconductors (computer
chips, devices) How to prepare each of these?
3
Sample Preparation Powders 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 Disperse particles in
resin Microtome using glass or diamond knives
(depends on hardness of particles)?
4
Sample Preparation Bulks Some specimens can
be prepared by just using mortar and pestle to
crush the specimen into tiny pieces and then
suspend the small particles in a nonaqueous
solvent, and then catch the particles on a carbon
film TEM grid. Some specimens can be prepared
by cutting the sample into thin slices using a
diamond saw, then cutting 3-mm-diameter disks
from the slice, thinning the disk on a grinding
wheel, dimpling the thinned disk, then ion
milling it to electron transparency.
5
Disk Cutting Starting materials is ground or
sliced (cleaved) to slabs about 500 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.
Disc Punch The Disc Punch is ideal for cutting
TEM disks from Metals
6
General Steps in thinning a bulk sample
7
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)
8
TEM specimen preparation using the Tripod
polishes Wedge polishing Mount sample to
device with micrometer legs or variable angle
head Polish sample into a wedge shape -
possible to produce electron transparent samples
(thickness lt0.5microns) start with coarse
grinds, then move to finer grits Typically use
diamond-impregnated plastic films rather than
sand paper and diamond slurry Used frequently
for cross-sectional samples
9
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.
10
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 Process Once the
area of interest is located, the sample is
polished using the tripod to generate an edged
sample, with thickness of electron transparency.
11
  • Dimple Grinder
  • Advantages
  • 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.

12
  • How to use the Dimple Grinder
  • Generally, we use metallic wheel and 3 um diamond
    paste for dimple grinding stage, followed by

    a. coarse polishing using a felt
    wheel and 3 um diamond paste.

    b. fine
    polishing using a felt wheel and 0.05 um
    alumina suspension.
  • We use boron nitride (BN) paste for dimpling
    metals and alloys.
  • Changes in color of transmitted light through a
    semiconductor (sometimes ceramics) can be used as
    a thickness indicator.

13
How to determine the thickness of Si(110) crystal
by transparency colors?
14
Ion Milling Bombard thin sample with energetic
Ar ions, sputter away material until
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
15
Considerations for the Ion Milling Sputtering
rate and artifact formation are two major factors
in the process of ion milling of TEM specimens
What is sputtering sputtering is the removal
of atoms (atoms are knocked off) from the surface
of a target by incident particles. Sputtering
rate depends on 1). Flux and velocity of the
incident particles 2). Angle of incidence 3).
Relative masses of the specimen atoms 4).
Cohesive energy of the specimen atoms 5).
Chemical interactions between incident particles
and the specimen.
16
Available Techniques and Instrumentation There
are three types of instruments available,
including 1). Conventional broad ion beam
system 2). New generation ion beam system 3).
Focused Ion Beam (FIB) system
17
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
Precision Polishing Ion System (PIPS)
18
Specimen Preparation by Focused Ion Beam (FIB)
Milling There are two different approaches to
TEM sample preparation with FIB lift out and
traditional. Preference between the two depends
on specific analytical conditions such as sample
compositions. The life-out technique uses the
milling action of the ion beam to excavate
materials from both sides of the final thin
section, and then cut the section free from the
bulk specimen. An automated process lifting-out
the delicate specimen and transferring it to a
TEM specimen grid, is available in some FIB
system. The traditional approach requires the
use of manual cleaving and polishing techniques
to create a pre-FIB sample that is small enough
to fit into the TEM specimen chamber and thin
enough (less than 50 mm) to minimize the amount
of material that must be removed by the FIB. This
pre-FIB specimen is then glued to TEM specimen
grid and milled in the FIB to its final
electron-transparent thickness.
19
FIB Instrument This is a dual beam system SEM
FIB. Metal ions are accelerated into solid
sample. Optically it is similar to a SEM. The
system consist of 1). Gun 2). Condenser lens
(positive electrostatic lenses, not magnetic
lens as for electron). 3). Beam defining aperture
(change spot size and beam current). 4). Beam
blanker faraday cage which brings beam out of
optic axis and into bulk materials 5). Objective
lens change focal length 6). Scan coils 7).
Stigmators
FEI Nova 200 Nanolab
20
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!
21
Ex-situ lift-out technique
Extraction of the liberated lamella by
electrostatic glass-needle under an optical
microscope Transfer on TEM grid with carbon grid
22
In-situ lift-out technique a 200 nm lamella






Extraction of the 200 nm lamella using a
microgripper inside FIB (Kleindiek
nanotechnik) Then transfer on TEM grid with
carbon grid
23
Artifact Formation in the sample preparation
  1. Surface topography different thinning rates of
    components of the specimen surface finish and
    cleanliness crystallographic orientation and
    milling angle
  2. Structural artifacts include amorphous surface
    layer composition disproportionation
    crystallographic imperfections like dislocation
    loop structural changes due to excessive
    specimen heating.


24
Artifact Formation in the sample preparation
Ion beam damage (see images at right)
dislocations loop, point defects, amorphous
particles deposition, non-uniform thickness,
overheating, et al.
25
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