introduction to ultra high vacuum - unige.itrocca/didattica/laboratorio/s in english... ·...
TRANSCRIPT
Introduction to Ultra High Vacuum
The fundamental parameter is the density of the gas:
2x1019 mol/cm3 at atmospheric pressure
109 mol/cm3 at the orbital height of a satellite .
The characteristics of the gas (viscosity, mean free path, etc…) are
different for such different environments.
Several investigations of condensed matter
samples or of surfaces can take place only
under vacuum conditions.
What is vacuum?
Classification of vacuum
• Low Vacuum (LV) 25 to 760 Torr
• Medium Vacuum (MV) 10-3 to 25 Torr
• High Vacuum (HV) 10-6 to 10-3 Torr
• Very High Vacuum (VHV) 10-9 to 10-6 Torr
• Ultrahigh Vacuum (UHV) 10-12 to 10-9 Torr
• Extreme Ultrahigh Vacuum (XHV) <10-12 Torr
AVS (American Vacuum Society) Reference
Units of pressure: Pascal [Pa], mbar, Torr, atm, …
Conversion factors: 1 Pa = 1 N/m2 10-2 mbar
1 mbar = 0.75006 Torr
What is vacuum used for?
To reduce the concentration of one or more gases below a critical level (e.g. O2
in the electric light bulbs).
To impede chimico-physical processes caused by the action of atmospheric
gases (melting of reactive gases as Mo, W, Ta, maintainment controlled conditions
in gas surface interaction experiments).
Thermal insulation (thermos, dewars for cryogenic liquids, heat capacity
measurements).
Elimination of contaminants in a liquid (degasing of oils and liofilization).
Increase the mean free path of probe particles (molecules, electrons, ions) up to
macroscopic distances (cathodic ray tubes, thermoionic tubes, electron based
spectroscopies, particle accelerators).
Main applications:
(from science to everydays life)
Properties of the gases
Diluited gases can be approximated by
the perfect gas:
P= pressure
m = mass of the particle
T = Temperature [K]
k = Boltzmann constant [1.38 1023 J/K]
d = molecular diameter
n = gas density (molecules/cm3)
Mean velocity (m/s)
Mean free path
m
kTvav
8
PV=nkT equation of state
For air at room Temperature
l~(5x10-3)/P (in cm), l~50 m for P=10-6 torr
nd 22
1
l
4 x 1014
Designing a Vacuum Apparatus: Gas Flux
Volumic Flux:
S=vA or S=V/t
Mass flux:
Q=rS=rvA=rV/t
Assuming thermal equilibrium
Q may be expressed as:
Q=pS (in torr L/s)
Notes:
torr L /s= (N m-2)m3/s = N m/s = J/s =W
1 W = 7.5 torr L/s
Mass flux is associated to energy.
Pumps limited in powerare also limited in flux or pumping speed.
Flux
turbolent flux:
Dominated by the viscosity of the fluid
and by velocity gradients. The
molecules have no homogeneous
direction and velocity.
laminar flux:
One may neglect the friction between
the fluid particles and the pipeline walls.
All molecules have the same velocity.
molecular flux:
Diluited gases, the probability of collision
between gas molecules << collision
probability with the walls of the pipeline.
This is the usual condition for vacuum.
Q~P12-P2
2
Q~(P12-P2
2)0.5
Q~P1-P2
Conductance
p
pvA
PP
QC
21
LF
vAC
3
4 2
L
vRC
3
2 3
in L/s;
or better in torr L/s per torr of pressure gradient
The conductance determines the pressure
drop in a pipeline. For the molecular flux, C is
independent of pressure and is determined
only on the geometry of the pipeline.
Fig. 3.11 pag 53
For a pipeline of uniform geometry:
A area of the section of the pipeline
v mean velocity of the molecules
L length of the pipeline
F perimeter of the pipeline section
which becomes for a cylindrical geometry
Conductances in parallel:
Conductances in series:
nCCCC ...21
nCCCC
1...
111
21
UHV chamber
UHV chamber are made of materials with low
degassing (stainless steel, aluminium). Criticalities
are:
- possibility to outgas the walls (Bake Out)
- appropriate pumping systems and pressure
gauges;
- install flanges with:
Electrical and mechanical feed-throughs;
Instrumentation and facilities to prepare
and analyse the sample.
Evacuation time
)QQ(Q-SPdt
dPV pdl
Gas entering into the pump
S pumping speed
P pressure on the gas inlet
Ql = leaks Qd = degassing of internal walls Qp = backstreaming from the pumps
Se Ql = Qd = Qp = 0, e S=const
Pf= limit pressure reached after a long operation time
UHV chamber
Gas quantity leaving the chamber (minus
sign indicates decrease in pressure)
f
0
P
Pln
S
Vtime) ont(evacuati
UHV compatible materials
Mechanical preperties
Thermodynamic properties
Dissolved Gases
Resistance to high pressure;
Can be sealed with weldings or gaskets.
Low vapour pressure.
Thermal expansion compatible
with the other materials
Non porous;
No cracks;
Low desorption rate.
IN GENERAL
METALS UHV chambers
AUSTENITIC
STAINLESS
STEEL
(AISI 304/316)
Within the chamber
Mo
Ta
W
Cu (OFHC)
...
Sample holder
Dismountable joints
Cu gaskets,
Al ring
Swagelok
(stainless
steel) ...
GLAS CERAMIC
• windows
• PIRANI AND
IONIZATION GAUGES
• CATHODE RAY TUBE
•...
Buna-N, Viton, Kalrez, ...
High permeability and degassing.
POLYMERS
• INSULATION
• UHV
CONNECTORS
•...
• GASKETS
•
•...
How to seal an UHV chamber?
The different parts of the chamber need to be joined together and sealed
DEMOUNTABLE JOINTS
LOW VACUUM:
Flange ISO-KF with O-ring (metallic ring + teflon gasket)
COPPER GASKET
HIGH AND ULTRA HIGH VACUUM:
CONFLAT Flange with copper gasket
PERMANENT JOINTS
-Different welding techniques,
-TIG (arch welding in an inert gas) to avoid oxidation.
- Glas – Metal and Ceramic metal joints
PUMPING SYSTEMS
Chemical pumps
(getters)
LOW VACUUM: ROTATIVE PUMP
Pressure limit:
k
P
S
Q
S
QP die
ult
Qe, Qi gas charge (external, internal).
S pumping speed
Pd pressure at the exhaust port
k compression ratio
Pult is given by the sum of three terms for each gas moiety present in
the gas. Pult is moreover determined by:
- Internal degasing (oil);
- Affidability of the pressure gauge (condensation of the
vapour)
- Efficiency for the different gases (Helium is pumped
worse)
Work principle :
• the gas enters into the inlet;
• the rotor compresses it;
• the gas is expelled through the exhaust
tube.
single /double stage
Typical performances:
• pumping speed: 3-5 l/s
• pressure limit: 10-3 / 10-4 mbar
LOW VACUUM: ROTATIVE PUMP
FUNCTION:
- Seal the space between blade and rotor and stator.
- Lubrificate the pump.
- Dissipate the heat
DISADVANTAGE:
Backflux of oil in the vacuum chamber
Special Oils with
-Low vapour pressure;
-Chemical inertness.
ZEOLITE TRAP Zeolite: mineral with high surface area and pores of molecular dimensions. The trap is placed between Vacuum vessel and rotatory pump to reduce backflux If well designed the pumping speed is reduced by the trap only by 10%
Use of oil free pumps
-Scroll;
- piston
-Etc..
Roots Pump (large gas fluxes)
UHV: turbomolecular pump
Turbine with axial flux maximizes the volumetric efficiency for given
diameter and volume.
Consists of a series of concentric rotors(13) and stators (12).
The external forces act symmetrically on the perimeter the good
balancing allows for high rotaional speeds.
The pumping takes place by transfer of momentum from the spinning
blades to the gas.
There are no surfaces subject alternatively to high and low vacuum
(important to decrease degasing) .
Lubrification with oil or with grease; cooling by water or air flux.
Inclination of the blades:
small inclination maximizes the pumping speed .
High inclination maximizes the compression ratio.
Different inclinations are chosen for the different
rotor - stator pairs
How does it work?
The inclination of the blades maximizes the probability
that the molecules are pushed in the direction of the
outlet port and “backstreaming” is minimized.
UHV: turbomolecular pump
Typical preformances
S = 50 - 15000 L/S
LOW PRESSURE LIMIT
< 2 x 10-10 mbar
S = S(P,GAS)
TURBO STAGES
DRAG STAGES
To increase the exhaust pressure above
20 mbar combined turbomolecular-
molecular pumps have been developed.
The molecular stadium consists of a
rotating cylinder and a wall with
appropriate grooves
10-2 mbar
Ion pumps: Starcell Varian
Titanium sublimation pump
Getter pumps: NEXTorr SAES Getters
Getter pumps are based on the chemical removal of
the residual gas. They work fine to maintain very low
pressures, but saturate rapidly if exposed to external
gas sources.
Vacuum measurements
• 15 orders of magnitude (10-12 - 103
mbar) No one single gauge can span
such a large range.
• hence at least two gauges are needed
for any vacuum system.
• The smaller the pressure the more
difficult it is to reach accuracy. Often
what matters in UHV is the density of
the gas rather than its pressure. The
latter quantity is then determined from
temperature and guess of the gas
species.
Pressure gauges
Measure the total pressure of the gas.
4 functioning principles:
- Force
- Transferred Momentum
- Dissipated heat
- Ionization
They measure the partial pressure of the
different gases in a given volume.
Mass spectrometers with ionization
source, often Quadrupole Mass
Spectrometers (QMS). Needed to analyze
the vacuum and reactants composition.
Needed to discover leaks
Gas Analyzers
Pressure gauges
The transducer depends on the pressure
range of interest
Direct reading
Measurement of the pressure from the
force exerted by the flux of particles
incident on a surface:
DIAPHRAGM – BOURDON –
CAPACITANCE – SPINNING ROTOR
Pressure measurement through a property of the
gas which depends on its density.
ION GAUGE,TC, PIRANI
Indirect reading
Heat transfer gauges: TC e PIRANI
Both make use of the thermal conductivity of a
gas. One measures the heat transferred by a heat
source to a wall at RT.
Working condition: l ≥ d (distance from the heat source to the
wall at RT)
Low vacuum
PIRANI: (1000-1 or 0.1 mtorr)
The heat loss of a filament is determined by a
Wheatstone bridge used both to heat the
filament and to determine its resistivity
The meter and the compensation element have to be as
similar as possible. The former is mounted in a sealed
chamber with known pressure (P<1 mtorr), the other in a
chamber communicating with the vacuum system. V is
maintained constant. A variation of P in the open shell
causes a variation of its T and an unbalancing of the bridge
Heat transfer gauges: TC e PIRANI
Thermocouple sensor: (5000-1 mtorr)
Similar to Pirani gauge but the heat variation
is evaluated via a thermocouple.
For P<10 mtorr the accuracy is limited by:
- variation of the composition of the gas;
- contaminants;
- variation of the external temperature.
Some models include a thermisor to balance
the external temperature variations
Both make use of the thermal conductivity of a
gas. One measures the heat transferred by a heat
source to a wall at RT.
Working condition: l ≥ d (distance from the heat source to the
wall at RT)
Low vacuum
Heat transfer gauges: TC e PIRANI
l d heat transfer is proportional
to the density of molecules (i.e. to
P)
< d non linearity caused by the
collisions between molecules
Both make use of the thermal conductivity of a
gas. One measures the heat transferred by a heat
source to a wall at RT.
Working condition: l ≥ d (distance from the heat source to the
wall at RT)
Low vacuum
Ionization gauges
To measure high and ultrahigh vacuum.
COLD CATHODE
HOT CATHODE
COLD CATHODE IONIZATION GAUGES (Penning)
10-2 – 10-9 torr
V (typically 4kV) is applied and one measures the total
discharge through the gas. The current is caused by ions
already present in the gas (produced e.g. by cosmic rays).
Advantages:
- No hot filament which may cause degasing of the walls
near to it;
- high sensitivity.
-Disadvantages:
- Discontinuity of the calibration (reduction of the accuracy);
- Delayed ignition at low pressure.
Ionization gauges
HOT CATHODE IONIZATION GAUGE
TRIODE:
- A filament emits electrons. They are accelerated to a grid and
hit against the gas molecules ionizing them. The ions are
collected by a filament. The pressure is evaluated by reading the
ionization current.
- Range: 10-2 – 10-8 torr.
- Sensitivity is limited by the electron which hit the grid since they
generate X rays which hit the collector causing photoemission
Bayard-Alpert gauge:
-Its a modified triode minimising the photoelectron current by
placing the collector within a grid composed of very tiny
filaments.
3 main advantages:
1) The dimension of the collector is reduced to minimize the
collected X rays
2) The potential difference between grid and collector makes so
that the whole volume is efficiently ionized
3) The efficiency of the collector is augmented by its central
position.
Collector
Filament
Grid
B-A gauge with
spiral grid
P≥1 10-9 torr
B-A gauge UHV 24
Closed grid and tiny collector.
S=Icoll/(IelxP)=24 A/(Axtorr)
P≥ 10-11 torr