Introduction Basic concepts of vacuum Vacuum Hardware (pumps,
gauges) Mass Spectrometry OUTLINE VACUUM PUMPS AND HARDWARE 1
Slide 2
GETTERS Getters are stripes of material adsorbing the gas NEED
OF VACUUM TV TUBES LCD BACKLIGHT GAS LIGHTS (NEON, HIGH POWER
LAMPS) DEWAR (FOR DRINKS) Active material: alkali (Cs, Rb), rare
earths (Yb, Lu), Hg Support: Al 2 O 3, Zr Interaction of gas (CO 2,
O) with getter surface (passivation or oxidation) Role of the
surface morphology: surface area/bulk Research applications: impact
on everyday life 2
Slide 3
Basic concepts of vacuum UHV Apparatus Gas Kinetics Vacuum
concepts Vacuum Pumps Vacuum Gauges Sample Preparation in UHV
Cleaving Sputtering & Annealing Fracturing Exposure to
gas/vapor Evaporation/Sublimation 3
Slide 4
Ultra High Vacuum Apparatus 4
Slide 5
5 velocity distribution 1D k B = Boltzmann constant Gas
kinetics velocity distribution 3D probability of finding a particle
with speed in the element dv around v probability density of
finding a particle with speed in the element dv around v
Maxwell-Boltzmann distribution
Slide 6
Gas kinetics Mean T (C) Molecular speed Quadratic mean Most
probable Neon @ 300 K m Ne = 20 1.67 x 10 -27 kg f(v) 6
Slide 7
Gas kinetics 7
Slide 8
for ideal gas n = N/V = number density (mol/cm 3 ) N = total
number of molecules 8 Arrival rate R: number of particles landing
at a surface per unit area and unit time how many molecules?
Consider n molecules with speed v moving towards a surface dS on a
surface dS we take the molecules arriving with speed v x in a time
dt volume = vdt cos dS total number of molecules with speed v x
hitting the unit surface in a time dt dS
Slide 9
9 Gas kinetics
Slide 10
10 p = Pressure (torr) T = Temperature (K) m = M amu O 2 at p =
760 torr, 293 KR = 2.75 10 23 molecules s -1 cm 2 O 2 at p = 1 x 10
-6 torr, 293 KR = 3.61 10 14 molecules s -1 cm 2 k B = Boltzmanns
constant (J/K) molecules arrival rate R at a surface (unit area,
time) Gas kinetics M Ox =32 g/mol if M=Molar mass m Ne = 20 1.67 x
10 -27 g
Slide 11
11 Gas kinetics: why the UHV 1 Monolayer ~ 10 14 10 15 atoms/cm
2 Residual Gas H2OH2O CO 2 CO CH 4 O2O2 N2N2 Solid Surface Bulk
Solid Adsorbed Atoms & Molecules
Slide 12
12 Mean free path Gas kinetics 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
Slide 13
13 Mean free path Gas kinetics For different molecules A and B
rBrB rArA is so large that the collisions with walls are dominant
with respect to molecular collisions 2 depends on the fact that we
did not consider the presence of other molecules p in torr
Slide 14
14
Slide 15
15 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 Why the UHV O 2 at p = 1 x 10 -6 torr, 293 K R =
3.61 10 14
Slide 16
16 Plots of relevant vacuum features vs. pressure
Slide 17
17 Gas flux through a pipe d [Q] = [p][L] 3 [t] -1 pipe p =
pressure measured in the plane dV = volume of matter crossing the
plane Volumetric flux: variation of number of molecules through an
area dV/dt= Volumetric flow rate (portata) Throughput
Slide 18
18 Mass flux Variation of mass through an area Volumetric flux
Magnitude of flow rates Pressure drop at the pipe ends Surface and
geometry of pipe Nature of gases Gas flux through a pipe M=molar
mass M = total mass Factors affecting the flux
Slide 19
19 Regimes of gas flux through a pipe For < dviscous For
dintermediate For > dmolecular d Viscous laminar turbulent pipe
The mol-mol collisions are dominant Friction force = viscosity S =
layer contact area dv x /dy = mol speed gradient Throughput
Slide 20
20 Volumetric flux Laminar: R e 2200 mass flux For a pipe with
diameter d and section d 2 /4 Q mass flux per unit section Reynolds
number = viscosity d pipe Regimes of gas flux through a pipe
Slide 21
21 Laminar: Q < 8 10 3 ( T/M)d [Pa m 3 /s] Reynolds number
Turbulent: Q > 1.4 10 4 ( T/M)d [Pa m 3 /s] Regimes of gas flux
through a pipe
Slide 22
22 For < d For d For > d d viscous intermediate molecular
Knudsen number = d/ Only for intermediate and molecular flux
intermediate molecular 3 d/ 80 d/ 3 10 -2 p d 0.5 p d 10 -2 For air
at RT Regimes of gas flux through a pipe
Slide 23
23 Pipe conductance: In parallel Flux across pipe Pressures at
pipe ends [C] = [L] 3 [t] -1 Pipe impedance: SI: m 3 s -1 cgs: lt s
-1
Slide 24
24 In series Q 1 = Q 2 = Q T or gas would accumulate
Slide 25
25 Pipe conductance Viscous and intermediate regime (Poiseuille
law) Molecular regime Long cylindrical pipe Elbow pipe Laminar
Turbulent The molecules must collide with walls at least once
before exiting Equivalent to a longer pipe For air at 0 C: 11,6 d 3
/L [lt/s]
Slide 26
26 [S] = [L] 3 [t] -1 Pumping speed S = Q/p 0 Q= flux through
aspiration aperture p = Vessel Pressure V = Vessel Volume p0p0
Relevant physical parameters of a pumping system SI: m 3 s -1 cgs:
lt s -1 In the presence of a pipe Effective pumping speed in the
vessel Q at the pump inlet is the same as Q in pipe C p 0 =
pressure at pump inlet Volumetric flow rate
Slide 27
27 [S] = [L] 3 [t] -1 Pumping speed S = Q/p 0 Q= flux through
aspiration aperture p = Vessel Pressure V = Vessel Volume p0p0
Relevant physical parameters of a pumping system if C = S Effective
pumping speed the S e is halved C
Slide 28
28 Q= flux through aspiration aperture p = Vessel Pressure V =
Vessel Volume p0p0 Relevant physical parameters of a pumping system
Q 1 = True leak rate (leaks from air, wall permeability) Q 2 =
Virtual leak rate (outgas from materials, walls) Outgas rate for
stainless steel after 2 hours pumping: 10 -8 mbar Ls -1 cm -2
Sources of flux (molecules)
Slide 29
29 Pump-down equation for a constant volume system True leak
rate Only the gas initially present contributes Virtual leak rate
Other outgassing sources contribute Short time limit Long time
limit Q = Q 0 +Q 1 S = Pumping speed p = Vessel Pressure V = Vessel
Volume
Slide 30
30 Pump-down equation for a constant volume system True leak
rate Short time limit Q = Q 0 +Q 1 S = Pumping speed p = Vessel
Pressure V = Vessel Volume Suppose: Constant S Q = 0 Time needed to
reduce p by 50 % V= 1000 L P 0 = 133 Pa S= 20 L/s t = 331,6 s 7.5
L/s = 27 m 3 /h Vol of 1 m 3 = 10 3 L to be pumped down from 1000
mbar to 10 mbar in 10 min = 600 s
Slide 31
31 Pump-down equation for a constant volume system Q = Q 0 +Q 1
S = Pumping speed p = Vessel Pressure V = Vessel Volume Ultimate
pressure dp/dt = 0 Virtual leak rate Other outgassing sources
contribute Long time limit
Slide 32
32 Pressure versus distance
Slide 33
33 Differential pumping operate adjacent parts of a vacuum
system at distinctly different pressures The size of the aperture
depends by its function conductance C is determined. A, B to be
maintained at pressures P 1 and P 2, P 1 >> P 2 A: gas in
with flux Q L gas to B with flux q Q 1 = flux pumped S1 = Q 1 /p 1
Q L /p 1 B: gas in with flux q To keep pressure p 2 S 2 = q/p 2 q =
C(p 1 p 2 ) C p 1 S 2 = Cp 1 /p 2 Modern Vacuum Physics, Ch.
5.8
Slide 34
34 Example CVD coatings on panels Antireflective coatings, p-n
junction growth for solar panels P0P0 P1P1 P2P2 P1P1 P0P0 S1S1 S2S2
S3S3 S 1 = Cp 0 /p 1 CCC S 2 = Cp 1 /p 2 S 3 = Cp 2 /p 1
Slide 35
35 Gas-solid interaction H2OH2O CO 2 CO CH 4 O2O2 N2N2 He H2H2
elastic inelastictrapped 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
Slide 36
36 Gas-solid interaction H2OH2O CO 2 CO CH 4 O2O2 N2N2 He H2H2
Origin: Van der Waals forces The well depth is the energy of
adsorption E to be supplied to desorb the molecule Typical q: 6 -
40 kJ/mol = 0,062 - 0,52 eV /molecule Physisorption
Slide 37
37 Gas-solid interaction H2OH2O CO 2 CO CH 4 O2O2 N2N2 He H2H2
Origin: Electron sharing or transfer between molecules and surface
atoms The well depth is the energy of adsorption Typical q: 40 -
1000 kJ/mol = 0,52 - 10 eV /molecule Chemisorption P is a precursor
state the molecules have to overcome
Slide 38
38 Gas-solid interaction How does this affect vacuum?
probability per second that a molecule will desorb O2O2 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? Surface atoms have E vib = h = K B T = K B T/h At RT =
0.025/(6.63 10 34 1.6 10 19 ) = 6 10 12 s 1 10 13 s 1 = number of
attempts per second to overcome the potential barrier and break
free of the surface. Boltzmann factor probability that fluctuations
in the energy will result in an energy q
Slide 39
39 Gas-solid interaction probability per second that a molecule
will desorb O2O2 p(t) = probability that it is still adsorbed after
elapsed t p(t+dt) = p(t) x (1- dt) probability of not being
desorbed after dt dp = p(t+dt) - p(t) = - dt p(t) average time of
stay
Slide 40
40 Gas-solid interaction O2O2 average time of stay At RT 10 13
s1 97 kJ / mol = 1 eV / molecule Temperature dependance Molecular
dependance Note: Simple model Neglects all other interactions,
surface diffusion, adsorption sites so a can change
Slide 41
41 Desorption P = 1000 mbarP = 10 -7 mbar Equilibrium pumping
Far from equilibrium till. Experimental relation Gas flux /area = 1
for metals = 0.5 for elastomers = 0.5 = 1 q 1 5x10 8 mbar L s 1 cm
-2 1 mbar L N at 2.6x10 19 Outgassing rate 10 12 molec s 1 cm
-2
Slide 42
42 Desorption How important is the molecule/surface interaction
energy? H2OH2O N2N2 Rate of desorption Simple model calculation
idealized UHV system RT, V= 1 L, A = 100 cm 2 S = 1 L/s only gas
source: initially complete ML of specified binding energy adsorbed
at the wall fall of pressure at RT q integrate
Slide 43
43 Outgassing Origin of fluxes: Permeation Adsorption
Solubility Desorption 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
Slide 44
44 Gas-solid permeation p 1 = 1000 mbar Residual Gas H2OH2O CO
2 CO CH 4 O2O2 N2N2 p 2 = 1x10 -8 mbar He H2H2
Slide 45
45 Gas-solid permeation p 1 = 1000 mbar Residual Gas p 2 = 1x10
-9 mbar Permeation is a complex process Adsorption Dissociation
Solution into the solid Diffusion Recombination Desorption
Slide 46
46 Gas-solid permeation p 1 = 1000 mbarp 2 = 1x10 -9 mbar
Permeation process can be quantified trough phenomenological
quantities permeability =Q/(p 1 -p 2 )A Q=flux trough wall A= unit
area [Q] = [p][L] 3 [t] -1 = [L] 3 [t] -1 [L] -2 m 3 s -1 m -2 ls
-1 cm -2 Residual Gas
Slide 47
47 Gas-solid permeation K p = Permeability coefficient For a
given gas A = wall area d = wall thickness m 3 s -1 m -1 Pa -1 He
cm 3 s -1 cm -2 Pa -1 p = 13 mbar d = 1 mm depending on diffusion
mechanisms
Slide 48
48 Gas-solid permeation Metal gas K p GlassMetalsPolymers He, H
2, Ne, Ar, O 2 No rare gasAll gases p p p p p p Table of gas
permeability
Slide 49
49 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) c = gas concentration Henrys law Valid
for low concentrations and for glass and plastic materials No
formation of alloys
Slide 50
50 Solubility For dissociated gas Sieverts law Valid for low
concentrations and for metals Interstitial or substitutional H 2 on
metals Note the high solubility of H 2 in Ti,Zr
52 Pressure Ranges Spanned by Different Vacuum Pumps More than
one pump to HV and UHV
Slide 53
53 What pump to use? S = [L] 3 [t] -1 Pumping speed S = Q/p p =
inlet pressure For a pressure range where S does not depend on p,
i.e. the pumping speed is constant Compression ratio: This can be
used to measure S or to estimate the time to reach p u Depends on
the gas type S varies with p Q=Q 0 cost p u S = Q 0
Slide 54
54 Ultimate pressure Time to reach the u.p. Residual gas
composition Other (absence of magnetic fields) What pump to
use?
Slide 55
55 Rotary Roughing Pump P u : 10 -2 mbar Rotor blade Eccentric
rotor inlet Exhaust valve Spring Cylindric body Oil S: 2,5 10 2 m 3
/h 0.7 28 l/s 1 m 3 /h = 0.28 l/s CR: 10 5 Starting operating
pressure: 10 3 mbar
Slide 56
56 Dual stage Rotary Roughing Pump P u : 10 -3 10 -4 mbar
Advantages No saturation Heavy duty Low cost (2500 ) Disadvantages
Oil backstreaming Need traps for oil vapor Noisy Rotor blade
Eccentric rotor inlet Exhaust valve Spring
Slide 57
57 Rotary Roughing Pump: gas ballast CR=10 5 Op. temp T 70 C
Pump water vapor at 70 C when P reaches 3.3 10 4 Pa The vapor
liquefies and does not reach P > 1 10 5 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 The gas can liquefy inside the rotation chamber Vapor
pressure
Slide 58
58 Rotary Roughing Pump: gas ballast Ballast valve Solution:
gas ballast NO gas ballast Gas ballast liquid The valve is set to
decrease the CR to 10 The vapor does not liquefy
Slide 59
59 Diaphragm Pump Housing Valves Head cover Diaph. clamping
disc Diaphragm Diaphragm supp. disc Connecting rod Eccentric
bushing P u : ~ 1 mbar CR: 10 2 10 3 Starting operating pressure:
10 3 mbar
Slide 60
60 Diaphragm Pump Advantages Oil-free No saturation Low cost
Disadvantages High ult.pressure (4 mbar) Low pump speed Noisy
Slide 61
61 Root Pump Advantages Oil-free No saturation High throughput
Disadvantages Need prevacuum Medium cost delicate Eight-shaped
rotor turning in opposite direction Clearance between rotors ~ 0.3
mm No lubricants CR depends on clearance
Slide 62
62 Root Pump S and CR of a root pump depend on the preliminary
pump installed ahead The gas flux is the same for both pumps
rootpalette prpr p p atm SpSp SrSr Palette: 60 m 3 /h = 16,8 l/sS r
= 16,8 x 40 = 672 l/s
64 Turbomolecular Pump Molecular speed distribution without
blades (only v) Molecular speed distribution plus blade speed
Slide 65
65 Turbomolecular Pump Principle of operation High pressure
side Low pressure side The pumping action is provided by the
collisions between blades and molecules Molecular regime The speed
distribution (ellipse) depends on the angle between V and
blade
Slide 66
66 Turbomolecular Pump Pumping speed: depends on gas type
Residual gas: H 2 After bake out
69 Molecular drag pump Turbo disk Threaded stator Cylindrical
Rotor Forevacuum flange (outlet) Threaded stator Safety ball
bearing Gas entry Magnetic bearing Lubricant reservoir Electrical
socket Operating principle: Same as turbo but different geometry No
blades but threads
Slide 70
70 Molecular drag pump P u : 10 -7 mbar S: 40 100 l/s CR: H 2 :
10 2 10 9 He: 10 3 10 4 N 2 : 10 7 10 9 Starting operating
pressure: 1-20 mbar They are use in combination with turbo in a
single mounting so Use a low CR backing pump (i.e. membrane for
clean operation) Higher backing vacuum pressure
Slide 71
71 baffle 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
Slide 72
72 vapor diffusion pump Advantages No saturation Heavy duty Low
cost Disadvantages gas reaction Liquid vapor tension Contamination
Needs water cooling P u : 10 -9 mbarS: 20 600 l/s Starting
operating pressure: 10 -2 mbar The pumping speed and the pressure
strongly depends on oil type
Slide 73
73 Getter pumps The active material is sublimated by thermal
heating Sublimation getters - Gas-surface chemical interaction -
Chemisorption - Solution of gas inside material Pumping mechanism
Non evaporable getters The active material is constituted by porous
medium
Slide 74
74 Sublimation getter pumps Sublimation getters - Gas-surface
chemical interaction - Chemisorption - Solution of gas inside
material Ti or Ti Mo filaments Pumping mechanism The material form
a thin film on the pump walls that becomes the active layer The
molecules are chemisorbed on the film
Slide 75
75 Non evaporable getter pumps Cartridge of porous material
(Zr-16%Al) Pumping operation Problem: saturation of getter material
requires cartridge change Activated by heating (750 C) and kept at
operating T 300 C to increase molecule diffusion
Slide 76
76 sublimation S strongly depends on gas > 10 3 l/s Zr-Al
Getter pumps Pumping speed (l/s) A= sublimation, A=non evaporable
Non evaporable 800- 2x10 3 l/s S depends on active surface
saturation Molecular weight (g) area Adsorption probability
Slide 77
77 Gas-surface weak interaction Physisorption and diffusion
into the bulk Plus: Wall cooling Pressure limit: 10 -10 10 -12 mbar
Advantages Pump H 2 Heavy duty Low cost No contamination
Disadvantages Saturation Metal vapours No rare gas pumping Stripes
of active material Getter pumps With a number of panels one can
obtain S > 1x10 4 l/s But if warmed it releases the gas
Slide 78
78 Ion-getter pump 7 KV ~1 Tesla Ti - Gas-surface chemical
interaction - Chemisorption - Solution of gas inside material
Pumping mechanism - Ionization of gas molecules - Burying inside
the active material Ion-getter with cathodic grinding
Slide 79
79 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 H 2 : accumulates into
the cathodes Need regeneration by annealing
Slide 80
80 Ion-getter pump Advantages Heavy duty No traps No
contamination Any mounting position Silent Disadvantages High
magnetic fields Low pump S for H 2 Medium - high cost Pressure
limit: 10 -11 10 -12 mbar S: 4 1000 l/s Starting operating
pressure: 10 -3 10 -4 mbar
Slide 81
81 Adsorbing pumps Liquid N 2 cooled Adsorbing material - Gas
cold surface interaction - Physisorption Pumping mechanism
Adsorbing porous material High surface/volume ratio Zeolites Al 2 O
3, SiO 2 H 2 O and N 2 pumping Liquid He cooled Cold walls - Gas
cold surface interaction - Physisorption, condensation Pumping
mechanism Cryogenic pumps
Slide 82
82 Cryopump - Gas cold surface interaction - Physisorption and
condensation Pumping mechanism Metal wall
Slide 83
83 Cryopump Pressure limit: 10 -10 10 -11 mbar Advantages Heavy
duty No contamination Low cost Disadvantages Saturation Noisy Needs
other UHV pumps The gas condensation if gas pressure > vapor
pressure at wall T S: 4 100 l/s Starting operating pressure: 10 -9
mbar vapor pressure
Slide 84
84 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 + - -
Slide 85
85 Ionization in gases Ionization energy eV Ion +Electron
affinity Ion - - + - - - Less probable More probable
Slide 86
86 Collision type: - elastic - atom excitation - molecule
dissociation - Ionising ( e) Considering the relative speed and
energy conservation Atom or neutral molecule electron collision
Very small Elastic collision In the collision the kinetic energy of
electrons (and of the molecules) remains almost unchanged relative
energy loss for electrons
Slide 87
87 Total energy loss 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 Elastic collision
Slide 88
88 Apply external electric field E Maximum kinetic energy of an
e - moving in a gas Depends on electric field and pressure Elastic
collision If e - has v in ~ 0
Slide 89
89 Ionization Ionization energy e - can ionize an atom if But
it can also - Increase the atom kinetic energy - Excite an e - to
unoccupied bound states Ionization probability i = ionizing
collisions/total collisions - - +
Slide 90
90 Ionization Long path to produce more ions But it can also -
e - trapped inside atom with formation of negative ions - - e -
with E k unit pressure unit lenght Specific ionization coefficient
Due to practical measurements e - can ionize an atom
Slide 91
91 Vacuum measurement Different types of vacuometers depending
on pressure range Mechanical, thermal, ionization
Slide 92
92 Vacuum measurement Mechanical Bourdon To vacuum Membrane Pin
wheel tube index 10 5 10 2 Pa (10 3 1 mbar) The tube curvature
changes with pressure Needs calibration Precision: 1-2% fsr 10 5 10
2 Pa (10 3 1 mbar) The membrane or bellow bends with pressure Needs
calibration Precision: 1-2% fsr
Slide 93
93 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
Slide 94
94 Thermal conductivity vacuometers unbalanced Hence Thermal
dissipation radiative dissipation contact dissipation For small p,
the reference bridge is The pressure is obtained by measuring the
Wheatstone voltage In general it depends on the gas type = cost
Stephan-Boltzmann =wire emissivity K gas = gas thermal conductivity
K f = wire thermal conductivity =coefficient
Slide 95
95 Ionization vacuum gauges Hot cathodeCold cathode Based on
gas ionization and current measurements
Slide 96
96 Ionization vacuum gauge I + = I - i e p Sensitivity K = i e
Directly proportional to pressure Sensitivity K = i e I + = ion
current i = specific ionization coefficient I - = electron current
from filament e = electron mean free path The gauge measure the
total pressure Range: 10 -4 10 -12 mbar K depends on gas, gauge
geometry, gauge potential Usually one increases by designing the
gometry
Slide 97
97 Ionization vacuum gauge 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 1 tesla Range:
10 -4 5 x10 -10 mbar Cold cathode No filament so less subject to
Filament faults Note: discharge starts only by mag field to avoid
high E field - induced currents
Slide 98
98 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
Specific mass of Ar + = 40 Specific mass of Ar ++ = 20 For a single
molecule there are many peaks, depending on n
Slide 99
99 Mass Spectrometry Specific mass table
Slide 100
100 Mass Spectrometry detector Faraday cup All ions measured No
filaments Low sensitivity sturdy Channeltron - electron multiplyer
High sensitivity Delicate Fast response To remove secondary
electrons Amplifier time constant large
Slide 101
101 Quadrupole Mass Spectrometry (QMS) Vacuum Chamber Ion
source (filament) Analyser (Quadrupole field) Detector
(Channeltron) Storing system 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.
Slide 102
102 Quadrupole Mass Spectrometry (QMS) r 0 = rod separation
(~3mm) U+Vcos( t) -(U+Vcos( t)) Superimpose an oscillating field
Vcos( t) The forces are uncoupled along x,y,z axis Quadrupole
potential
Slide 103
103 Quadrupole Mass Spectrometry (QMS) ion equation of motion
Constant speed along z Stability parameters
Slide 104
104 Quadrupole Mass Spectrometry (QMS) Solved numerically for
different a and q All solutions outside are imaginary and give
increasing oscillation amplitudes Neutralization of the ions on the
rods Ions oscillate in the xy plane Only some e/m values reach
detector Solutions inside are real (stable trajectory)
Slide 105
105 Quadrupole Mass Spectrometry (QMS) Zoom to region I The
line shrink to one point Only one ion with m/e ratio can reach
detector Stable solutions 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 for
Slide 106
106 Quadrupole Mass Spectrometry (QMS) Zoom to region I The
line enter the stable solutions region Work line All the ions with
a/q on the line will reach detector V=V 0 cos( t) for Reducing U
relative toV, an increasingly wider m/z range can be transmitted
simultaneously. qq 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
Slide 107
107 Quadrupole Mass Spectrometry (QMS) How to select different
m/e ratios Change U, V keeping the ratio constant This results in
different m/e values allowed to pass the quadrupole In the U,V
space a changing in m/e ratio means moving along the straight
line
Slide 108
108 Quadrupole Mass Spectrometry (QMS) V AC > U DC the two
pairs act as a band pass filter, low and high mass ions outside the
band being rejected, with limits determined by the values of U, V,
and . U+Vcos( t) (x) -(U+Vcos( t)) (y) Ion traveling in the z
direction X Motion will tend to be stable for (t) >0 and
unstable (t)
Slide 109
109 Quadrupole Mass Spectroscopy (QMS) profiles of the residual
gas p 3x10 -7 mbar Before bake-out p 5x10 -11 mbar After bake-out
H2OH2O CO+N 2 CO 2 H2OH2O H2H2
Slide 110
110 VACUUM SEALING Clamps Low Vacuum No bake at high
temperatures Reusable Viton rings
Slide 111
111 UHV VACUUM SEALING HV Bake at high temperatures Reusable
(maybe once) Plastic deformation and shear
Slide 112
112 VALVES Diaphragm Butterfly
Slide 113
113 VALVES Dynamometric sealing Stem All metal
Slide 114
114 VALVES Gate Leak High conductance UHV to air compatible
Large clearance for instruments Bakeable
Slide 115
115 FEEDTHROUGH Multi-pin for signal or Low currents Multi-pin
for high currents