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Metodi sperimentali della fisica moderna Luca Gavioli Dipartimento di Matematica e Fisica Università Cattolica del Sacro Cuore Via dei Musei 41, I-25121 Brescia, Italy luca.gavioli@.unicatt.it http://www.dmf.unicatt.it/~gavioli/corsi/MSFM/ www.dmf.unicatt.it/nano. OUTLINE. Introduction

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slide1

Metodi sperimentali della fisica moderna

Luca Gavioli

Dipartimento di Matematica e Fisica

Università Cattolica del Sacro Cuore

Via dei Musei 41, I-25121 Brescia, Italy

luca.gavioli@.unicatt.it

http://www.dmf.unicatt.it/~gavioli/corsi/MSFM/

www.dmf.unicatt.it/nano

slide2

OUTLINE

  • Introduction
  • Basic concepts of vacuum
  • Vacuum Hardware (pumps, gauges)
  • Mass Spectrometry
  • References
  • Ferrario: Introduzione alla tecnologia del vuoto: Cap 1-4, 8-11
  • Woodruff – Delchar: Modern techniques of surface science
  • (Cambridge University Press) Chap 2,3
  • Chambers, Modern Vacuum Physics (Chapman & All)
  • Published papers
slide3

Research applications: impact on everyday life

GETTERS

NEED OF VACUUM

TV TUBES

LCD BACKLIGHT

GAS LIGHTS (NEON, HIGH POWER LAMPS)

DEWAR (FOR DRINKS)

Getters are stripes of material adsorbing the gas

Active material: alkali (Cs, Rb), rare earths (Yb, Lu), Hg

Support: Al2O3, Zr

Interaction of gas (CO2, O) with getter surface (passivation or oxidation)

Role of the surface morphology: surface area/bulk

slide4

Basic concepts of vacuum

  • UHV Apparatus
  • Gas Kinetics
  • Vacuum concepts
  • Vacuum Pumps
  • Vacuum Gauges
  • Sample Preparation in UHV
    • Cleaving
    • Sputtering&Annealing
    • Fracturing
    • Scraping
    • Exposure to gas/vapor
    • Evaporation/Sublimation
slide7

Gas kinetics

Maxwell-Boltzmann distribution 1D

N = Total number of molecules

kB = Boltzmann constant

N=nNA = total number of molecules

n = Molecular density

Maxwell-Boltzmann distribution 3D

In polar coordinates

Mean number of particles per

unit volume between v and v+dv

slide8

Gas kinetics

T (°C)

Maxwell-Boltzmann distribution 3D

Molecular speed

Most likely

Average

Neon @ 300 K

mNe = 20 • 1.67 x 10-27 kg

Quadratic mean

slide9

Gas kinetics

Arrival rate R:

number of particles landing at a surface per unit area,unit time

volume

Mol. per unit volume

slide10

Gas kinetics

Arrival rate R of atoms at a surface per unit area

kB = Boltzmann’s constant (erg/K)

T = Temperature (K)

p = Pressure (torr)

m = Molecular mass (g)

O2 at p = 760 torr, 293 K R = 2.75 1023 molecules s-1cm2

O2 at p = 1 x 10-6 torr, 293 K R = 3.61 1014 molecules s-1cm2

slide11

Gas kinetics: why the UHV

H2O

Residual Gas

CO

O2

CO2

CH4

N2

Solid Surface

1 Monolayer ~

1014 – 1015 atoms/cm2

Bulk Solid

Adsorbed Atoms & Molecules

slide12

2r

Gas kinetics

Mean free path

2r

The sphere with 2r is the hard volume

The surface of the sphere is the effective section or cross section for impact

The number of impacts per unit time is

slide13

rA

rB

Gas kinetics

For different molecules A and B

Mean free path

  • is so large that the collisions with walls are

dominant with respect to molecular collisions

slide15

Why the UHV

O2 at p = 1 x 10-6 torr, 293 K

R = 3.61 1014

Sticking probability = 1

1 monolayer of atoms or molecules from

the residual gas is adsorbed at the surface in:

1 sec @ p = 1 x 10-6 torr

10 sec @ p = 1 x 10-7 torr

100 sec @ p = 1 x 10-8 torr

1,000 sec @ p = 1 x 10-9 torr

10,000 sec @ p = 1 x 10-10 torr

100,000 sec @ p = 1 x 10-11 torr

Utra High Vacuum (UHV): p = 10-10-10-11 torr

slide17

d

Gas flux through a pipe

pipe

p = pressure on plane

dV = volume change across plane

dV/dt= Volumetric flow rate

[Q] = [p][L]3[t]-1

Flux

Volumetric flux: variation of number of molecules through an area

(Throughput)

slide18

Gas flux through a pipe

M = total mass

Mass flux

Volumetric flux

Variation of mass through an area

M=mole mass

Factors affecting the flux

  • Magnitude of flow rates
  • Pressure drop at the pipe ends
  • Surface and geometry of pipe
  • Nature of gases
slide19

d

Regimes of gas flux through a pipe

Flux

pipe

(Throughput)

For  < d viscous

For   d intermediate

For  > d molecular

Viscous

S = layer contact area

dvx /dy = mol speed gradient

The mol-mol collisions are dominant

Friction force

 = viscosity

laminar

turbulent

slide20

d

Regimes of gas flux through a pipe

pipe

Volumetric flux

mass flux

For a pipe with diameter d and section d2/4

Q’ mass flux per unit section

 = viscosity

Reynolds number

Laminar: Re<1200

turbulent: Re>2200

slide21

Regimes of gas flux through a pipe

Reynolds number

Laminar: Q < 8 103 (T/M)d [Pa m3/s]

Turbulent: Q > 1.4 104 (T/M)d [Pa m3/s]

slide22

d

Regimes of gas flux through a pipe

viscous

For  < d

For   d

For  > d

intermediate

molecular

Only for intermediate and molecular flux

Knudsen number = d/

intermediate

3  d/  80

d/  3

molecular

10-2 p d  0.5

p d  10-2

For air at RT

slide23

Flux across pipe

Pipe conductance:

[C] = [L]3[t]-1

Pressures at pipe ends

Pipe impedance:

SI: m3s-1

cgs: lt s-1

In parallel

slide24

In series

Q1 = Q2 = QT or gas would accumulate

slide25

Pipe conductance

Viscous and intermediate regime

Laminar

Turbulent

Molecular regime

Long cylindrical pipe

For air at 0 C:

11,6 d3/L [lt/s]

Elbow pipe

The molecules must collide with walls at least once before exiting

Equivalent to a longer piper

slide26

p0

Relevant physical parameters of a pumping system

Q= flux through aspiration aperture

p = Vessel Pressure

V = Vessel Volume

S = Volumetric flow rate

C

Pumping speed S = Q/p0

[S] = [L]3[t]-1

SI: m3s-1

cgs: lt s-1

In the presence of a pipe

Q at the pump inlet is the same as Q in pipe

Effective pumping speed

slide27

p0

Relevant physical parameters of a pumping system

Q= flux through aspiration aperture

p = Vessel Pressure

V = Vessel Volume

C

Pumping speed S = Q/p0

Effective pumping speed

[S] = [L]3[t]-1

if S = C

the Se is halved

slide28

p0

Relevant physical parameters of a pumping system

Q= flux through aspiration aperture

p = Vessel Pressure

V = Vessel Volume

Sources of flux (molecules)

Q1 = True leak rate

(leaks from air,

wall permeability)

Q2 = Virtual leak rate

(outgas from materials,

walls)

Outgas rate for stainless steel after 2 hours pumping: 10-8 mbar Ls-1 cm-2

slide29

Pump-down equation for a constant volume system

Q = Q0 +Q1

S = Pumping speed

p = Vessel Pressure

V = Vessel Volume

Long time limit

Short time limit

True leak rate

Only the gas

initially present

contributes

Virtual leak rate

Other outgassing

sources contribute

slide30

Pump-down equation for a constant volume system

Q = Q0 +Q1

S = Pumping speed

p = Vessel Pressure

V = Vessel Volume

Short time limit

True leak rate

Constant S

Q = 0

Time needed to reduce p by 50 %

Vol of 1 m3 = 103 L to be pumped down from 1000 mbar to 10 mbar in 10 min = 600 s

V= 1000 L

P0 = 133 Pa

S= 20 L/s

t = 331,6 s

7.5 L/s = 27 m3/h

slide31

Pump-down equation for a constant volume system

Q = Q0 +Q1

S = Pumping speed

p = Vessel Pressure

V = Vessel Volume

Long time limit

Virtual leak rate

Other outgassing

sources contribute

dp/dt = 0

Ultimate pressure

slide33

Differential pumping

operate adjacent parts of a vacuum system at distinctly different pressures

A, B to be maintained at pressures P1 and P2, P1 >> P2

A: gas in with flux QL

gas to B with flux q

Q1 = flux pumped

S1 = Q1/p1 QL/p1

B: gas in with flux q

To keep pressure p2

S2 = q/p2

q = C(p1 − p2)  C p1

S2 = Cp1/p2

The size of the aperture depends by its function  conductance C is determined.

slide34

Example

CVD coatings on panels

Antireflective coatings, p-n junction growth for solar panels

P1

P2

P1

P0

P0

C

C

C

S2

S3

S1

S1 = Cp0/p1

S2 = Cp1/p2

S3 = Cp2/p1

slide35

Gas-solid interaction

H2O

CO

inelastic

trapped

elastic

CO2

physical adsorption (shortened to Physisorption):

bonding with structure of the molecule unchanged

Chemisorption:

bonding involves electron transfer

or sharing between the molecule

and atoms of the surface

Can be thought of as a chemical reaction

CH4

N2

O2

He

H2

slide36

Gas-solid interaction

Physisorption

CO

Origin:

Van der Waals forces

H2O

CH4

CO2

O2

N2

Typical q:

6 - 40 kJ/mol =

0,062 - 0,52 eV /molecule

He

H2

The well depth is the energy of adsorption

slide37

Gas-solid interaction

Chemisorption

CO

Origin:

Electron sharing or transfer

between molecules and surface atoms

H2O

CH4

CO2

O2

N2

Typical q:

40 - 1000 kJ/mol =

0,52 - 10 eV /molecule

He

H2

The well depth is the energy of adsorption

slide38

Gas-solid interaction

How does this affect vacuum?

Molecule trapped in the adsorbed state at temp. T

potential well of depth q

Dilute layer (no interactions with other mol.)

How long does it stays?

O2

Surface atoms have Evib = h = KBT

 = KBT/h

At RT  = 0.025/(6.63 × 10−34 ÷ 1.6 × 10−19) = 6 × 1012 s−1 1013 s−1

 = number of attempts per second to overcome the potential barrier and break free of the surface.

probability that fluctuations in the energy sharing will result in an energy q

Boltzmann factor

probability per second that a molecule will desorb

slide39

Gas-solid interaction

probability per second

that a molecule will desorb

p(t) = probability that it is still adsorbed after elapsed t

p(t+dt) = p(t) x (1-dt)

O2

probability of not being

desorbed after dt

dp = p(t+dt) - p(t) = - dt p(t)

average time of stay

slide40

Gas-solid interaction

average time of stay

At RT  1013 s−1

Molecule dependance

O2

97 kJ / mol = 1 eV / molecule

Temperature dependance

Note: Simple model

Neglects all other interactions, surface diffusion, adsorption sites so a can change

slide41

Desorption

P = 1000 mbar

P = 10-7 mbar

pumping

Equilibrium

Far from equilibrium till….

Experimental

relation

Gas flux /area

 = 0.5

 = 1 for metals

 = 1

 = 0.5 for elastomers

q1 5x10−8 mbar L s−1cm-2

1 mbar L

Outgassing rate  1012 molec s−1cm-2

Nat 2.46x1019

slide42

Desorption

H2O

How important is the molecule/surface interaction energy?

Rate of desorption

N2

fall of pressure at RT

Simple model calculation

idealized UHV system

RT, V= 1 L, A = 100 cm2

S = 1 L/s

only gas source: initially complete ML

of specified binding energy adsorbed

at the wall

q

slide43

Outgassing

Gas is continuously released, (at relatively small rates) from walls

Principally water vapor

Limit to attainable vacuum achievable in reasonable times (hours) ∼10−6 mbar

Origin of fluxes:

Permeation

Adsorption

Solubility

Desorption

slide44

Gas-solid permeation

p2 = 1x10-8 mbar

p1 = 1000 mbar

H2O

CO

CO2

CH4

N2

O2

He

H2

Residual Gas

slide45

Gas-solid permeation

p1 = 1000 mbar

p2 = 1x10-9 mbar

Permeation is a

complex process

Adsorption

Residual Gas

Dissociation

Solution into the solid

Diffusion

Recombination

Desorption

slide46

Gas-solid permeation

p1 = 1000 mbar

p2 = 1x10-9 mbar

Permeation process

can be quantified trough

Phenomenological

quantities

Residual Gas

permeability

=Q/(p1-p2)A

Q=flux trough wall

A= unit area

[Q] = [p][L]3[t]-1

=[L]3[t]-1[L]-2

ls-1cm-2

m3s-1m-2

slide47

Gas-solid permeation

For a given gas

A = wall area

d = wall thickness

depending on diffusion

mechanisms

He

Kp = Permeability coefficient

p = 13 mbar

d = 1 mm

cm3s-1cm-2 Pa-1

m3s-1m-1Pa-1

slide48

Gas-solid permeation

Metal – gas Kp

Table of gas permeability

slide49

Solubility

Is the quantity of substance A that can be dissolved in B at given T and p

For a gas

Gas quantity dissolved in solid volume unit at standard conditions

For undissociated molecular gas (interstitial)

Henry’s law

c = gas concentration

Valid for low concentrations and for glass and plastic materials

No formation of alloys

slide50

Solubility

H2 on metals

For dissociated gas

Interstitial or substitutional

Sievert’s law

Valid for low concentrations

and for metals

Note the high solubility of H2 in Ti,Zr

slide51

Vacuum Pumps

Capture pumps

Throughput pumps

  • Pistons
  • Gears
  • Turbines
  • Jet stream
  • Cold traps
  • Ionization
  • Getters

Differences: pressure range, speed, gas selectivity

slide53

What pump to use?

Pumping speed S = Q/p

p = inlet pressure

S = [L]3[t]-1

  • Depends on the gas type
  • S varies with p

For a pressure range where S does not depend on p, i.e. the pumping speed is constant

This can be used to measure S

or to estimate the time to reach pu

Compression ratio:

slide54

What pump to use?

  • Ultimate pressure
  • Time to reach the u.p.
  • Residual gas composition
  • Other (absence of magnetic fields)
slide55

Rotary Roughing Pump

inlet

Exhaust valve

Oil

Rotor blade

Eccentric rotor

Spring

Cylindric

body

S: 2,5 ÷ 102 m3/h

0.7 ÷ 28 l/s

CR: 105

Starting operating pressure: 103 mbar

Pu: 10-2 mbar

1 m3/h = 0.28 l/s

slide56

Dual stage Rotary Roughing Pump

inlet

Exhaust valve

Rotor blade

Eccentric rotor

Spring

Pu: 10-3÷ 10-4 mbar

  • Advantages
  • No saturation
  • Heavy duty
  • Low cost (2500 €)
  • Disadvantages
  • Oil backstreaming
  • Need traps for oil vapor
  • Noisy
slide57

Rotary Roughing Pump: gas ballast

CR=105

Op. temp

T 70 °C

The gas can liquefy

inside the rotation chamber

Vapor pressure

Pump water vapor at 70 °C

when P reaches 3.3 104 Pa

The vapor liquefies and does not reach P > 1 105 Pa

So the exhaust valve does not open

The vapor remains inside the pump and is mixed with oil

Decrease pump speed, and can damage the rotor by increasing the friction

slide58

Rotary Roughing Pump: gas ballast

Solution: gas ballast

Gas ballast

NO gas ballast

The valve is set to

decrease the CR to 10

liquid

Ballast valve

The vapor does not

liquefy

slide59

Diaphragm Pump

Housing

Valves

Head cover

Diaph. clamping disc

Diaphragm

Diaphragm supp. disc

Connecting rod

Eccentric bushing

CR: 102  103

Starting operating pressure: 103 mbar

Pu: ~ 1 mbar

slide60

Diaphragm Pump

Advantages

Oil-free

No saturation

Low cost

Disadvantages

High ult.pressure (4 mbar)

Low pump speed

Noisy

slide61

Root Pump

Eight-shaped rotor turning

in opposite direction

  • Clearance between rotors ~ 0.3 mm
  • No lubricants
  • CR depends on clearance

Advantages

Oil-free

No saturation

High throughput

Disadvantages

Need prevacuum

Medium cost

delicate

slide62

Root Pump

patm

pp

pr

S and CR of a root pump depend

on the preliminary pump

installed ahead

root

palette

Sr

Sp

The gas flux is the same for both pumps

Palette: 60 m3/h = 16,8 l/s

Sr = 16,8 x 40 = 672 l/s

slide63

Turbomolecular Pump

S: 50 ÷ 5000 l/s

CR: 105 109

Starting operating

pressure: 10-2 mbar

Pu: 10-10 mbar

slide64

Turbomolecular Pump

Principle of operation

Molecular regime

Low pressure side

High pressure side

The speed distribution (ellipse) depends on the angle between

V and blade

The pumping action is provided by the collisions

between blades and molecules

slide65

Turbomolecular Pump

Pumping speed: depends on gas type

After bake out

Residual gas: H2

slide67

Turbomolecular Pump

Rotor suspension

Ball bearings (lubricant required)

Magnetic (lubricant absent)

Advantages

No saturation

Clean (magnetic)

UHV

Any orientation

Disadvantages

Cost

Delicate

Quite noisy

70 l/s ~5000 €

250 l/s ~10000 €

2000 l/s ~23000 €

slide68

Molecular drag pump

Safety ball bearing

Turbo disk

Magnetic bearing

Threaded stator

Cylindrical Rotor

Operating principle:

Same as turbo but

different geometry

Threaded stator

No blades but threads

Forevacuum flange

(outlet)

Gas entry

Lubricant reservoir

Electrical socket

slide69

Molecular drag pump

CR:

H2: 102 109

He: 103 104

N2: 107 109

S: 40 ÷ 100 l/s

Starting operating

pressure: 1-20 mbar

Pu: 10-7 mbar

They are use in combination with turbo

in a single mounting so

Higher backing vacuum pressure

Use a low CR backing pump

(i.e. membrane for clean

operation)

slide70

vapor diffusion pump

Fluid is heated and ejected from nozzles at high speed

due to the nozzle shape and pressure difference between

inside and pump cylinder.

Fluid speed up to Mach 3-5

The gas molecules are compressed to the pump base through collisions with oil vapor

baffle

slide71

vapor diffusion pump

The pumping speed and the pressure strongly depends on oil type

Starting operating

pressure: 10-2 mbar

S: 20 ÷ 600 l/s

Pu: 10-9 mbar

Disadvantages

gas reaction

Liquid vapor tension

Contamination

Needs water cooling

Advantages

No saturation

Heavy duty

Low cost

slide72

Getter pumps

Pumping mechanism

- Gas-surface chemical interaction

- Chemisorption

- Solution of gas inside material

Sublimation getters

Non evaporable getters

The active material is sublimated

by thermal heating

The active material is

constituted by porous medium

slide73

Sublimation getter pumps

Pumping mechanism

- Gas-surface chemical interaction

- Chemisorption

- Solution of gas inside material

Sublimation getters

Ti or Ti – Mo filaments

The material form a thin film

on the pump walls that becomes

the active layer

The molecules are chemisorbed on the film

slide74

Non evaporable getter pumps

Pumping operation

Cartridge of porous material (Zr-16%Al)

Activated by heating (750 °C) and kept

at operating T 300 °C to increase

molecule diffusion

Problem: saturation of getter material requires cartridge change

slide75

Getter pumps

area

Adsorption

probability

mass

Pumping speed (l/s)

S strongly depends on gas

sublimation

Non evaporable

800- 2x103 l/s

> 103 l/s

A’= sublimation, A=non evaporable

Zr-Al

S depends on active surface saturation

slide76

Getter pumps

With a number of panels

one can obtain S > 1x104 l/s

Stripes of active material

Plus: Wall cooling

Gas-surface weak interaction

Physisorption and

diffusion into the bulk

But if warmed it releases the gas

Advantages

Pump H2

Heavy duty

Low cost

No contamination

Pressure limit:

10-10÷ 10-12 mbar

Disadvantages

Saturation

Metal vapours

No rare gas pumping

slide77

Ion-getter pump

Pumping mechanism

- Gas-surface chemical interaction

- Chemisorption

- Solution of gas inside material

- Ionization of gas molecules

- Burying inside the active material

Ion-getter with cathodic grinding

Ti

~1 Tesla

7 KV

slide78

Basic processes occurring within a single cell

  • e- ionize molecules
  • Secondary e- ionize molecules

Ions are accelerated to cathodes

  • produce secondary e-
  • grind up cathode material
  • make craters

Ions buried into

cathode material

Produce cathode vapors

Depositing also on anodes

to work as getters

Need regeneration by annealing

H2: accumulates into the cathodes

slide79

Ion-getter pump

Starting operating

pressure: 10-3 10-4 mbar

S: 4 ÷ 1000 l/s

Pressure limit:

10-11÷ 10-12 mbar

Advantages

Heavy duty

No traps

No contamination

Any mounting position

Silent

Disadvantages

High magnetic fields

Low pump S for H2

Medium - high cost

slide80

Cryogenic pumps

Liquid He cooled

Cold walls

Pumping mechanism

- Gas – cold surface interaction

- Physisorption, condensation

Liquid N2 cooled

Adsorbing material

Adsorbing pumps

Pumping mechanism

- Gas – cold surface interaction

- Physisorption

Adsorbing porous material

High surface/volume ratio

Zeolites

Al2O3, SiO2

H2O and N2 pumping

slide81

Cryopump

Pumping mechanism

- Gas – cold surface interaction

- Physisorption and condensation

Metal wall

slide82

Cryopump

vapor pressure

The gas condensation

if gas pressure > vapor pressure

at wall T

S: 4 ÷ 100 l/s

Starting operating

pressure: 10-9 mbar

Pressure limit:

10-10÷ 10-11 mbar

Advantages

Heavy duty

No contamination

Low cost

Disadvantages

Saturation

Noisy

Needs other UHV pumps

slide83

Ionization in gases

  • Type of collisions:
  • neutral Molecule – electron
  • neutral Molecule – ions
  • neutral molecule – neutral molecule (Penning)
  • radiation absorption
  • neutral Molecule – hot metal surface

Ionization of a molecule (atom) from collisions with e-

-

-

+

-

-

Ion -

Ion +

slide84

Ionization in gases

Ionization energy

Electron affinity

Ion +

Ion -

-

-

-

+

Less probable

-

More probable

eV

slide85

Atom or neutral molecule – electron collision

  • Collision type:
  • elastic
  • atom excitation
  • molecule dissociation
  • - Ionising ( e)

Elastic collision

Relative energy loss

for gas molecules

Very small energy losses

slide86

Elastic collision

e- suffers very small energy loss for each elastic collision

e- mean free path e = average space between two elastic collisions

e- collision rate e = collisions number per unit time

number of collisions

Total energy loss

slide87

Elastic collision

Apply external electric field E

If e- has vin~ 0

Maximum kinetic energy of an e-

moving in a gas

Depends on electric field

and pressure

slide88

Ionization

if

Ionization energy

e- can ionize an atom

-

But it can also

- Increase the atom kinetic energy

- Excite an e- to unoccupied bound states

+

Ionization probability i = ionizing collisions/total collisions

-

slide89

Ionization

e- can ionize an atom

  • e- trapped inside atom
  • with formation of negative ions

-

But it can also

Due to practical measurements

collisions number

e- with Ek --- constant pressure --- unit lenght

Specific ionization coefficient

-

Long path to produce more ions

slide90

Vacuum measurement

Different types of vacuometers depending on pressure range

Mechanical, thermal, ionization

slide91

Vacuum measurement

Mechanical

Bourdon

Membrane

tube

Pin wheel

index

To vacuum

105 102 Pa

105 102 Pa

(103 1 mbar)

(103 1 mbar)

The tube curvature changes

with pressure

Needs calibration

Precision: 1-2% fsr

The membrane or bellow bends

with pressure

Needs calibration

Precision: 1-2% fsr

slide92

Thermal conductivity vacuometers

Pirani

heated filament

The filament temperature, and hence the resistance

depends on heat dissipation in the gas,

i.e. on the gas pressure

Pressure variation means T variation i.e.

resistance variation.

This is measured through the W. bridge V variation

slide93

Thermal conductivity vacuometers

Thermal

dissipation

unbalanced

contact

dissipation

radiative

dissipation

  • = cost Stephan-Boltzmann

=wire emissivity

Kgas= gas thermal conductivity

Kf= wire thermal conductivity

=coefficient

For small p, the reference bridge is

Hence

The pressure is obtained by measuring

the Wheatstone voltage

In general it depends

on the gas type

slide94

Ionization vacuum gauges

Hot cathode

Cold cathode

Based on gas ionization and current measurements

slide95

Ionization vacuum gauge

I+ = ion current

i = specific ionization coefficient

I- = electron current

from filament

Sensitivity K = σi · λe

e = electron mean free path

I+ = I-iep

Directly proportional to pressure

Sensitivity K =ie

The gauge measure the total pressure

K depends on gas, gauge geometry,

gauge potential

Range: 10-4 – 10-12 mbar

Usually one increases  by designing the gometry

slide96

Ionization vacuum gauge

Cold cathode

1 tesla

electrons from gas or field emission

similar to the behavior inside the

ion getter pumps

Less precise due to problem of

discharge current at low pressure

No filament so less subject to

Filament faults

Note: discharge starts only by mag field

to avoid high E field - induced currents

Range: 10-4 – 5 x10-10 mbar

slide97

Mass Spectrometry

Need to distinguish the intensity of specific gas molecules

Collect molecules

Molecule ionization

Separation of different molecules

Current measurement

Specific mass = ion mass (a.u.)/ion charge =

n = ion ionization multiplicity

For a single molecule there are

many peaks, depending on n

Specific mass of Ar+ = 40

Specific mass of Ar++ = 20

slide98

Mass Spectrometry

Specific mass table

slide99

Mass Spectrometry

detector

To remove

secondary electrons

Faraday cup

All ions measured

No filaments

Low sensitivity

sturdy

Channeltron - electron multiplyer

High sensitivity

Delicate

Fast response

Amplifier time constant large

slide100

Quadrupole Mass Spectrometry (QMS)

Storing

system

Detector

(Channeltron)

Analyser

(Quadrupole field)

Ion source

(filament)

Vacuum

Chamber

Quadrupole field between the rods

Ions of varying mass are shot axially into the rod

The applied quadrupole field deflects the ions in the X and Y directions, causing them to describe helical trajectories through the mass filter.

slide101

Quadrupole Mass Spectrometry (QMS)

The forces are uncoupled along x,y,z axis

-U-Vcos(t)

Quadrupole potential

U+Vcos(t)

r0 = rod separation

Superimpose an oscillating field Vcos(t)

slide102

Quadrupole Mass Spectrometry (QMS)

ion equation of motion

Constant speed along z

Stability parameters

slide103

Quadrupole Mass Spectrometry (QMS)

Solved numerically for different a and q

Ions oscillate in the xy plane

Only some e/m values reach detector

Solutions inside are real (stable trajectory)

All solutions outside are imaginary

and give increasing oscillation amplitudes

Neutralization of the ions on the rods

slide104

Quadrupole Mass Spectrometry (QMS)

Zoom to region I

fixed U, V and  the overall ion motion

can (depending on the values of a and q)

result in a stable trajectory

causing ions of a certain m/z value

to pass the quadrupole

Stable solutions

V=V0cos(t)

for

The line shrink to one point

Only one ion with m/e ratio can reach detector

slide105

Quadrupole Mass Spectrometry (QMS)

Reducing U relative toV, an increasingly wider m/z

range can be transmitted simultaneously.

Zoom to region I

Work line

q

for

The line enter the stable solutions region

V=V0cos(t)

All the ions with a/q on the line will reach detector

the width q of the stable region determines the resolution.

By varying the magnitude of U and V at constant U/V ratio

an U/V = constant scan is obtained

ions of increasingly higher m/e values to travel through the quadrupole

slide106

Quadrupole Mass Spectroscopy (QMS) profiles of the residual gas

H2O

p ≈ 3x10-7 mbar

Before bake-out

H2

H2O

CO+N2

p ≈ 5x10-11 mbar

After bake-out

CO2

slide107

VACUUM SEALING

Low Vacuum

Clamps

Viton rings

No bake at high temperatures

Reusable

slide108

VACUUM SEALING

UHV

HV

Plastic deformation

and shear

Bake at high temperatures

Reusable (maybe once)

slide109

VALVES

Diaphragm

Butterfly

slide110

VALVES

Stem

All metal

Dynamometric sealing

slide111

VALVES

Leak

Gate

High conductance

UHV to air compatible

Large clearance for instruments

Bakeable

slide112

FEEDTHROUGH

Multi-pin for high currents

Multi-pin for signal or

Low currents

slide113

MANIPULATION

Rotation