p=n 0 k b t or n 0 =p/k b t ideal gas atom density 2.69 × 10 25 m −3 at normal conditions (...
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p=n0kBT or n0=p/kBT ideal gas atom density
2.69 × 1025 m−3 at normal conditions ( 273.15 K and 101.325 kPa) kB the Boltzmann constant, T the temperature and p the pressure.
Loschmidt constant 2.69 × 1019 cm−3
•Atom flux 1015 atoms /sec on cm2 at 10-6 Torr•Atom density of the solid state surface :: 1015 atoms/cm2 •One monolayer of residual gas may be adsorbed each second at 10-6 Torr
To perform 1hour experiment under clean conditions - achieve 10-10 Torr vacuum
How and where to get it??How to measure??
What is UHV?
An image is formed at the detector-phosphorescent screen due to the different current densities, which originates from the work functions on the emitter surface
Field emission microscope
Resolution limited by tangential velocity component of an emitted electron
Atoms Imaged with ions:Field ion microscope
How clean is a surface ? - single atom count
Atomically clean tungsten tip
0.01Ml of adsorbed gas
Real surface: how to get well controlled conditions?Study under ultra high vacuum conditionsHow good is UHV ? - count single atoms adsorbed on a surface
Scanning (electron) tunneling microscope – single atom count-integral method
Itun~ exp(-kz) I~ 10 orders of magnitude/nm
Surface reactions : competition between molecular and atomic chemosorption
• Dissociative chemisorption- crossing below zero energy : spontaneous molecule dissociation
• Molecular physadsorption crossing above zero energy : molecular adsorption at low T, at higher T - thermally activated dissociation
• Molecular chemisorption Adsorbate in the molecular chemosorbed state
Potential energy of the molecule (atom)- substrate complex
More time for desorption
Sticking coefficient - the ratio of the number of adsorbate atoms (or molecules) that do adsorb, or "stick", to a surface to the total number of atoms that impinge upon that surface during the same period of time
Adsorption: chemical reaction with surface atomsAdsorption heat- energy released to form adsorption bond:
Adsorption/Desorption kinetics : flux from/to the surface at temperatureT:
N~ n exp(-q/kT) n -surface atom density, q – adsorption heat
To desorp it/them – break a bond – Anneal adsorption system
UHV chambers : bake them to 250oC-450oC (instruments inside permitting)
=n/nmax -surface coverage
Adsorption isotherms of CO on Pd(111)
N – atom flux depend on gas preasure
Heat of adsorption
Carbon mono-oxide and oxygenon polycrystalline surfaces
Mono-crystalline surfaces
Sc, Y, La V, Hf -to expensive for hetters (sorbents in sublimation pumps)::Cheap Titanium Sublimation Pumps (TSR) are used to achieve UHV
Titanium Sublimation Pumps
Simple principle and construction-just mount and combine with another ion pump
pumping speed is a measure of the pump's abilityto permanently remove gas from the chamber. measured in units of volume per unit time (mech pumps)
Lifetime
SAES getters http://www.saesgetters.com
• barium getter devices produce a film of pure elemental barium deposited on the internal surfaces of evacuated tubes. • The chemical activity of the barium film is extremely high, • permanently absorbing active gas molecules such as CO, CO2, N2, O2, H2O, H2
• Outgasing occurs from the tube components during shelf life and operation• Exothermic reaction BaAl4 + 4Ni --> Ba + 4NiAl temperature 800C rises to 1200C
Exothermic Barium Ring Getters for CRT(Cathode ray tube)
SAES Flat panel display technologies: Alloy of Ti, Zr, V and Fe).
• Plasma HPTF Getter is used in a PDP as an in-situ pump• Plasma displays operate at around half an atmosphere pressure, not at high
vacuum• It dramatically shortens process time and lowers gaseous impurities in the display.• Internal gas atmosphere purification
screen printing and sintering of the getter material onto the substrate,
high-porosity – large adsorption-active surface area:non-dispensable getter
Spherical UHV chamberN=ns4r2 - total number of atoms adsorbed on the surface, ns- surface atom density
V=4/3 r3 - chamber volumeAtom density in the chamber, when everything is desorbed : n=N/V = 3ns/R
• Average ns for almost all solid and liquid surfaces ~ 1015 atoms/cm2
• Even in the small volume with R~10cm n~ 1014 - 1015 atoms/cm3 – causes increase of a pressure by 10-2Torr
• Just seconds to out pump it?• Real surface of an UHV chamber is many orders larger : porosity !!• It takes weeks to desorb residual gases from the several micron sub-surface layers
High porosity – drawback!!Hunt for smooth and solid surface for UHV chamber
Surface treatment of metals for UHV components• Degreasing• Etching• Sand or grit-blasting• Electroplating (gold is perfect but expensive)• Polishing – better electro-polishing- one order better vacuum in
shorter time
• Glass – (silica glass, Pyrex) are perfectly clean appropriatematerials for UHV
• Oxide Ceramics – aluminum oxide, zirconium oxide … insulators