p=n 0 k B T or n 0 =p/k B T ideal gas atom density 2.69 × m −3 at normal conditions ( K and kPa) k B the Boltzmann constant, T the temperature and p the pressure. Loschmidt constant 2.69 × cm −3 Atom flux atoms /sec on cm 2 at Torr Atom density of the solid state surface :: atoms/cm 2 One monolayer of residual gas may be adsorbed each second at Torr To perform 1hour experiment under clean conditions - achieve Torr vacuum How and where to get it?? How to measure?? What is UHV?

Vacuum in real time

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 conditions How good is UHV ? - count single atoms adsorbed on a surface Scanning (electron) tunneling microscope – single atom count-integral method  I tun ~ exp(-k  z)  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 atoms Adsorption 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 250 o C-450 o C (instruments inside permitting)  =n/n max -surface coverage Adsorption isotherms of CO on Pd(111) N – atom flux depend on gas preasure

Heat of adsorption Carbon mono-oxide and oxygen on 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 ability to permanently remove gas from the chamber. measured in units of volume per unit time (mech pumps) Lifetime

SAES getters 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, CO 2, N 2, O 2, H 2 O, H 2 Outgasing occurs from the tube components during shelf life and operation Exothermic reaction BaAl 4 + 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 chamber N=n s 4  r 2 - total number of atoms adsorbed on the surface, n s - surface atom density V=4/3  r 3 - chamber volume Atom density in the chamber, when everything is desorbed : n=N/V = 3n s /R Average n s for almost all solid and liquid surfaces ~ atoms/cm 2 Even in the small volume with R~10cm  n~ atoms/cm 3 – causes increase of a pressure by Torr 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 appropriate materials for UHV Oxide Ceramics – aluminum oxide, zirconium oxide … insulators

Material evaporation/ sublimation Do not play games with mercury (Hg) even at room temperature!

Metal atom evaporation sources work at ~ – Torr vapor pressure

Material deposition mdc-vacuum.com

Fortunately, materials for UHV application have been thoroughly studied in last 60years