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 low- nap cloths Eliminate visible scratches and pits if possible High polish: 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: 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 Dielectric breakdown at high potential Electron beam will “pile-up” electrons at surface of insulator, building potential Conduction band Empty Valence band Full EgEg Wide bandgap

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

η+δη+δ 1 E1E1 E2E2 E 0 incident charging For insulators: E1 – E2 ~.1 to 5 keV C coat 10  m C coat 5  m Au coat 1  m C coat 1  m Absorbed current (nA) Time (sec) Carbon coat thickness = 300 Å Gold coat thickness = 80 Å Coating and beam diameter

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 torr) Atoms arrives on substrate Migrate, Re- evaporate, collide Form islands Islands grow and coalesce

Choice of evaporant Emissivity vs. Z Z δ Most SEM work: Want coat as thin as possible – small emission range and faithful reproduction of surface features (5-10nm) AuAu-PdPt Pt-C “Wetting”Pre-coat can help nucleation density 60:40 Au-Pd = less granularity Pt-CGood wetting but not great conductivity Finest granularity typically = high T melt metals CBest for X-ray analysis (5-50nm) low absorption does not emit X-rays in energy range of general interest 2.5KeV 25KeV

Important Properties of Selected Coating Elements ElementSymbolThermalResistivity 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, torr) AluminumAl CarbonC ChromiumCr CopperCu GermaniumGe  GoldAu MolybdenumMo NickelNi PalladiumPd PlatinumPt TitaniumTi TungstenW ZirconiumZr Readily oxidizes

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 sample Target atom Gas atom

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: PtAu-PtAu-PdNiCrCu 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 WTaC-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)