Presentation Transcript

1. 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
2. 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 low- nap 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
3. 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
4. Grain mounts Micro-drill, press fit, and Ni-epoxy Potting - Casting ceramics
5. 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
6. Most geologic materials are insulators: 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 Conduction band Empty Eg Wide bandgap Valence band Full Electron beam will “pile-up” electrons at surface of insulator, building potential Dielectric breakdown at high potential
7. Charging: Deflects electron beam Can lead to extreme emission of secondary electrons and “bursts” of electrons Ti banding in Si-gel
8. Charging: Lower current density
9. charging 1 η+δ For insulators: E1 – E2 ~ .1 to 5 keV E1 E2 E0 incident Coating and beam diameter C coat 10 mm 150 C coat 5 mm 140 Au coat 1 mm Absorbed current (nA) 130 C coat 1 mm Carbon coat thickness = 300 Å 120 Gold coat thickness = 80 Å 300 600 900 1200 1500 Time (sec)
10. 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
11. 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
12. High vacuum evaporation (10-3 to 10-7 torr) Atoms arrives on substrate Migrate, Re-evaporate, collide Form islands Islands grow and coalesce
13. Choice of evaporant Emissivity vs. Z 2.5KeV 1.0 δ Most SEM work: Want coat as thin as possible – small emission range and faithful reproduction of surface features (5-10nm) Au Au-Pd Pt Pt-C 0.5 25KeV 10 Z 50 “Wetting” Pre-coat can help nucleation density 60:40 Au-Pd = less granularity Pt-C 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
14. Important Properties of Selected Coating Elements Element Symbol Thermal Resistivity Melting Boiling Vaporization cond. at 300 K point point temperature at 300 K ( cm) (K) (K) at 1.3 Pa (W cm–1 K–1) (10-5 atm, 10-2 torr) Aluminum Al 2.37 2.83 932 2330 1273 Carbon C 1.29 3500 4073 4473 2954 Chromium Cr 0.937 13.0 2173 2753 1478 Copper Cu 4.01 1.67 1356 2609 1393 Germanium Ge 0.599 89  103 1232 3123 1524 Gold Au 3.17 2.40 1336 2873 1738 Molybdenum Mo 1.38 5.70 2893 3973 2806 Nickel Ni 0.907 6.10 1725 3173 1783 Palladium Pd 0.718 11.0 1823 3833 1839 Platinum Pt 0.716 10.0 2028 4573 2363 Titanium Ti 0.219 42.0 2000 3273 1819 Tungsten W 1.74 5.50 3669 6173 3582 Zirconium Zr 0.21 40.0 2125 4650 2284 Readily oxidizes
15. Sputter Coating (plasma sputtering) Ion or neutral atom strikes target – imparts momentum to target atoms Some atoms dislodged and carried away Free target atoms deposited on sample target Target atom Gas atom sample
16. Sputtering Methods: Ion beam sputtering Ar gas ionized in cold cathode discharge Ions accelerated 1-30kV Ion beam strikes target and dislodges target atoms Target atoms coat sample
17. Sputtering Methods: Diode (DC) sputtering E field near cathode produces +ions and electrons Ions drawn toward cathode and target Target atoms dislodged Atoms from target coat sample Heating from electrons produced during gas ionization – can use “cool diode sputtering”
18. Sputtering Methods: Plasma magnetron sputtering Chamber evacuated and filled with inert gas (Xe) Apply 1-2kV DC voltage to ionize gas atoms (forming plasma) Permanent magnet behind target focuses plasma onto target (also deflects electrons from the sample) Target atoms dislodged – coat sample Very fine particle size Used in high-resolution applications. Targets = Pt, Cr, W, Ta
19. Sputtering targets: Pt Au-Pt Au-Pd Ni Cr Cu Advantages to sputter coating: Continuous layer even on parts not in “line-of-sight” Short mean free path. Do not need to rotate and tilt the specimen Simple, reproducible protocol Large, reusable target Good for thin metal coatings, not usable for carbon
20. 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
21. 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
22. Measuring thickness During coating: Mass sensing device to determine weight of deposit (change in oscillating frequency of quartz crystal – actively cooled) Measure light absorption Transmittance Reflectance Color change on polished brass Measure resistance across glass slide After coating: Optical techniques Gravimetric measurements X-ray absorption and emission Multiple beam interferometry (very precise)