Transcript Low Pressure -- Vacuum generation, gauging and use.

What is UHV?
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??
Vacuum in real time
How clean is a surface ? - single atom count
Field emission
microscope
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
Atomically clean
tungsten tip
0.01Ml of adsorbed gas
Resolution limited by tangential velocity component of an emitted electron
Atoms Imaged with ions:
Field ion microscope
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
dItun~ exp(-kdz) dI~ 10 orders of magnitude/nm
Surface reactions : competition between molecular and atomic
chemosorption
• Dissociative chemisorptioncrossing below zero energy :
spontaneous molecule dissociation
More time
for desorption
• 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
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
N – atom flux depend on gas preasure
Q=n/nmax -surface coverage
Adsorption isotherms of CO on Pd(111)
To desorp it/them – break a bond –
Anneal adsorption system
UHV chambers : bake them to 250oC-450oC
(instruments inside permitting)
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
Lifetime
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)
Simple principle and construction-just mount
and combine with another ion pump
SAES getters
http://www.saesgetters.com
Exothermic Barium Ring Getters for CRT
(Cathode ray tube)
• 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
SAES Flat panel display technologies: Alloy of Ti, Zr, V and Fe).
high-porosity – large adsorption-active surface area:
non-dispensable getter
screen printing and sintering of the getter material onto the substrate,
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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
Spherical UHV chamber
N=ns4Pr2 - total number of atoms adsorbed on the surface, ns- surface atom density
V=4/3 Pr3 - chamber volume
Atom 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 Dn~ 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
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Degreasing
Etching
Sand or grit-blasting
Electroplating (gold is perfect but expensive)
Polishing – better electro-polishingone 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 ~ 10-3 – 10-2 Torr vapor pressure
Material deposition
mdc-vacuum.com
Fortunately, materials for UHV application have been thoroughly
studied in last 60years