Sample Preparation

Report
Sample Preparation
Electron Microprobe samples must be:
1) Solid
2) Flat
3) Well polished (1 micron polish or better)
4) Low vapor pressure
5) Conductive
SEM samples - preferably:
1) Solid
2) Low vapor pressure
3) Conductive
Electron Microprobe Samples:
Petrographic thin sections or polished sections
Use mounting epoxy with low vapor pressure
Buehler Epoxide, Epo-thin
Petropoxy 154
Struers EpoFix
Important to polish surface flat (minimum relief)
Flatness generally achieved with diamond polishing on lownap cloths
Eliminate visible scratches and pits if possible
High polish: 0.3-0.05 μm
Generally finish with alumina – low nap
Can use colloidal silica polishing (chemical-mechanical)
- Essential for EBSD
Electron Microprobe Samples:
Thick specimens
Generally encapsulated in low vapor pressure, hard-curing epoxy
Buehler Epo-Thin
Struers EpoFix, SpeciFix
(can use conductive fillers)
cut, and polished as above
Porous materials can be vacuum-impregnated with low-viscosity epoxy
Grain mounts
Potting - Casting ceramics
Micro-drill, press
fit, and Ni-epoxy
Cleaning:
All samples should be as clean and dry as possible
1) 2-stage ultrasonic cleaning in clean water followed
by isopropyl alcohol preferable
2) Quick acetone rinse
3) Final rinse in methanol, be sure there is no residue
(use lint-free cloth)
4) Dry in oven, on hot plate, or in vacuum
Most geologic materials are insulators:
Conduction band
Empty
Valence band full or nearly full
Wide band gap with empty conduction
band
Essentially no available energy states to
which electron energies can be increased
Eg
Wide bandgap
Valence band
Full
Electron beam will “pile-up”
electrons at surface of insulator,
building potential
Dielectric breakdown at high potential
Charging:
Deflects electron beam
Can lead to extreme emission of secondary electrons and “bursts”
of electrons
Ti banding in Si-gel
Charging:
Lower current
density
charging
η+δ
1
For insulators:
E1
E1 – E2 ~ .1 to 5 keV
E2
E0 incident
Coating and beam diameter
Absorbed current (nA)
C coat 10 mm
150
C coat 5 mm
140
Au coat 1 mm
130
C coat 1 mm
Carbon coat thickness = 300 Å
120
Gold coat thickness = 80 Å
300
600
900
Time (sec)
1200
1500
Goals:
Improve conductivity and emissivity (for SEM)
Conductors:
Conduction bands and valence bands overlap
Easy to energize electrons to the continuum = secondary electrons
For biological specimens, can load metals into surface
For most samples - Coating required
Coating techniques:
Thermal evaporation
Many metals and some inorganic insulators
evaporate to mono-atomic state when heated in a
vacuum
How to heat:
Resistive heating - current used to heat support or
unsupported C rods
Electric arc method - Arc between two conductors
Conductor surface evaporates
Electron beam evaporation - Evaporant is anode
target - Heated by 2-3 keV cathode
High vacuum evaporation (10-3 to 10-7 torr)
Atoms arrives
on substrate
Migrate, Reevaporate,
collide
Form islands
Islands grow
and coalesce
Choice of evaporant
2.5KeV
Emissivity vs. Z
1.0
δ
Most SEM work:
0.5
Want coat as thin as possible – small
emission range and faithful reproduction
of surface features (5-10nm)
Au
Au-Pd
“Wetting”
Pt-C
25KeV
10
Z
50
Pt Pt-C
Pre-coat can help nucleation density
60:40 Au-Pd = less granularity
Good wetting but not great conductivity
Finest granularity typically = high Tmelt metals
C
Best for X-ray analysis (5-50nm)
low absorption
does not emit X-rays in energy range of general interest
Important Properties of Selected Coating Elements
Element
Symbol
Resistivity Melting
at 300 K
point
(m cm)
(K)
Boiling
point
(K)
Aluminum
Carbon
Chromium
Copper
Germanium
Gold
Molybdenum
Nickel
Palladium
Platinum
Titanium
Tungsten
Zirconium
Al
C
Cr
Cu
Ge
Au
Mo
Ni
Pd
Pt
Ti
W
Zr
2.83
3500
13.0
1.67
89  103
2.40
5.70
6.10
11.0
10.0
42.0
5.50
40.0
2330
4473
2753
2609
3123
2873
3973
3173
3833
4573
3273
6173
4650
Readily oxidizes
Thermal
cond.
at 300 K
(W cm–1 K–1)
2.37
1.29
0.937
4.01
0.599
3.17
1.38
0.907
0.718
0.716
0.219
1.74
0.21
932
4073
2173
1356
1232
1336
2893
1725
1823
2028
2000
3669
2125
Vaporization
temperature
at 1.3 Pa
(10-5 atm, 10-2 torr)
1273
2954
1478
1393
1524
1738
2806
1783
1839
2363
1819
3582
2284
Sputter Coating (plasma sputtering)
1) Ion or neutral atom strikes target –
imparts momentum to target atoms
2) Some atoms dislodged and carried
away
3) Free target atoms deposited on sample
target
Target atom
Gas atom
sample
Sputtering Methods:
Ion beam sputtering
1) Ar gas ionized in cold cathode
discharge
2) Ions accelerated 1-30kV
3) Ion beam strikes target and dislodges
target atoms
4) Target atoms coat sample
Sputtering Methods:
Diode (DC) sputtering
1) E field near cathode produces +ions and
electrons
2) Ions drawn toward cathode and target
3) Target atoms dislodged
4) Atoms from target coat sample
Heating from electrons produced during gas
ionization – can use “cool diode
sputtering”
Sputtering Methods:
Plasma magnetron sputtering
1) Chamber evacuated and filled with
inert gas (Xe)
2) Apply 1-2kV DC voltage to ionize
gas atoms (forming plasma)
3) Permanent magnet behind target
focuses plasma onto target (also
deflects electrons from the sample)
4) Target atoms dislodged – coat
sample
Very fine particle size
Used in high-resolution applications.
Targets = Pt, Cr, W, Ta
Sputtering targets:
Pt
Au-Pt
Au-Pd
Ni
Cr
Cu
Advantages to sputter coating:
1) Continuous layer even on parts not in “line-of-sight”
Short mean free path.
2) Do not need to rotate and tilt the specimen
3) Simple, reproducible protocol
4) Large, reusable target
5) Good for thin metal coatings, not usable for carbon
High resolution coating
Braten (1978)
Thermally evaporated Au-Pd or C+Au-Pd
Echlin et al. (1980)
Electron-beam evaporation of refractory metal
W
Ta
C-Pt
2-3 nm resolution
Good mid resolution coating (5-8nm resolution)
Sputter Pt or Au-Pd
cooled specimen
slow sputter rate
Coating thickness
Too thin = charging
Too thick = obscure details and absorb X-rays
Flat surface:
can get continuous layer 0.5nm thick
Irregular surface: requires at least 5nm thickness for continuity
Use the thickness that gives you the best, most informative
image
Measuring thickness
During coating:
1) Mass sensing device to determine weight of
deposit (change in oscillating frequency of
quartz crystal – actively cooled)
2) Measure light absorption
Transmittance
Reflectance
Color change on polished brass
3) Measure resistance across glass slide
After coating:
1) Optical techniques
2) Gravimetric measurements
3) X-ray absorption and emission
4) Multiple beam interferometry (very precise)

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