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 ( 273.15 K and 101.325 kPa) k B the Boltzmann constant, T the.

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?

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 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

Material evaporation/ sublimation

Do not play games with mercury (Hg) even at room temperature!

Metal atom evaporation sources work at ~ 10-3 – 10-2 Torr vapor pressure

Material depositionmdc-vacuum.com

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