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