1 metodi sperimentali della fisica moderna luca gavioli dipartimento di matematica e fisica...
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1
Metodi sperimentali della fisica moderna
Luca GavioliDipartimento di Matematica e FisicaUniversità Cattolica del Sacro CuoreVia dei Musei 41, I-25121 Brescia, Italy
luca.gavioli@.unicatt.ithttp://www.dmf.unicatt.it/~gavioli/corsi/MSFM/www.dmf.unicatt.it/nano
2
• Introduction
• Basic concepts of vacuum
• Vacuum Hardware (pumps, gauges)
• Mass Spectrometry
OUTLINE
•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
3
GETTERS
Getters are stripes of material adsorbing the gas
NEED OF VACUUM
TV TUBESLCD BACKLIGHTGAS LIGHTS (NEON, HIGH POWER LAMPS)DEWAR (FOR DRINKS)
Active material: alkali (Cs, Rb), rare earths (Yb, Lu), HgSupport: Al2O3, Zr
Interaction of gas (CO2, O) with getter surface (passivation or oxidation)Role of the surface morphology: surface area/bulk
Research applications: impact on everyday life
4
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
5
Ultra High Vacuum Apparatus
6
Ultra High Vacuum Apparatus
7
Tk
mv
B
B
x
eTk
m
V
Nvf 2
x
2
2
Maxwell-Boltzmann distribution 1D
kB = Boltzmann constant
n = Molecular density
N=nNA = total number of molecules
N = Total number of molecules
Gas kinetics
Tk
mv
B
BevTk
m
V
NvvfvF 22
3
2
2
2
124
Maxwell-Boltzmann distribution 3D
Mean number of particles perunit volume between v and v+dv
Tk
mv
B
BeTk
m
V
Nvf 2
3 2
2
2
In polar coordinates
8
Gas kinetics
m
Tk
dv
vdFv B2~
Tk
mv
B
BevTk
m
V
NvF 22
3 2
2
12
Maxwell-Boltzmann distribution 3D
m
Tkv B
8
Average
m
Tkvv B
rms
32
T (°C)
Molecular speed
Quadratic mean
Most likely
Neon @ 300 K
mNe = 20 • 1.67 x 10-27 kg
smvrms / 6101067.12
3001038.1326
23
9
Arrival rate R:number of particles landing at a surface per unit area,unit time
TNKpVmTk
v
B
B
8
Tmk
pvTkpv
VN
RBB 244
cos vdVvFdR
mTk
VN
R B
8
4
dSvdt cos
dVvF
Gas kinetics
dVvvFdRR cos
Tmk
pR
B2
Tkmv
B
BevTkm
VN
vf 22
3 2
212
volume
Mol. per unit volume
0
32
0
22/
0
2
0
2
2
2
2 )( cos sin dvve
Tk
m
V
NdvvvFddR Tk
mv
B
B
dvvdddV 2 sin
10
p = Pressure (torr)
T = Temperature (K)
m = Molecular mass (g)
21-22 s 105.3 cmmoleculemT
pR
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
kB = Boltzmann’s constant (erg/K)
Arrival rate R of atoms at a surface per unit area
Tmk
pR
B2
Gas kinetics
11
Gas kinetics: why the UHV
1 Monolayer ~ 1014 – 1015 atoms/cm2
Residual Gas H2O
CO2
CO
CH4
O2
N2Solid Surface
Bulk SolidAdsorbed Atoms & Molecules
12
Mean free path
Gas kinetics
2r
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
mTk
VN
rvVN
rf B
844 22
13
Mean free path
Gas kineticsFor different molecules A and B
Tk
VN
rf BAB
84 2
rBrA
2BA
AB
rrr
BA
BA
mmmm
fv
224 rN
Vfv
pr
TkB 1
2 2
is so large that the collisions with walls aredominant with respect to molecular collisions
14
15
Sticking probability = 11 monolayer of atoms or molecules fromthe residual gas is adsorbed at the surface in:
1 sec @ p = 1 x 10-6 torr10 sec @ p = 1 x 10-7 torr100 sec @ p = 1 x 10-8 torr1,000 sec @ p = 1 x 10-9 torr10,000 sec @ p = 1 x 10-10 torr100,000 sec @ p = 1 x 10-11 torr
Utra High Vacuum (UHV): p = 10-10-10-11 torr
Why the UHVO2 at p = 1 x 10-6 torr, 293 KR = 3.61 1014
16
Plots of relevant vacuum features vs. pressure
17
Gas flux through a pipe
dtdV
pQ
d
[Q] = [p][L]3[t]-1
Qdt
dNTK
dtpVd
TKNpVTKNnRT
mNMNKRNN
n
atB
BatBat
AVAVBAV
at
)(
;
; ;
pipe
p = pressure on planedV = volume change across plane
Flux
dtdN
TKQ atB
Volumetric flux: variation of number of molecules through an area
dV/dt= Volumetric flow rate
(Throughput)
18
dtdV
pQ
mat
Bat
AVB
Bat
AV
AVat QQRTM
dtdN
TKRTM
dtdN
NKMK
dtdN
NmN
dtmNd
dtd
)(
QRTM
Qm
dtd
Qm
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=mole mass
M = total mass
AVmNM
Factors affecting the flux
19
Regimes of gas flux through a pipe
For < d viscous
For d intermediate
For > d molecular
dtdV
pQ d
Viscous
laminar
turbulent
pipeFlux
The mol-mol collisions are dominant
dydv
SF xf Friction force = viscosity
S = layer contact areadvx /dy = mol speed gradient
(Throughput)
20
dtdV
pQ Volumetric flux
Laminar: Re<1200
turbulent: Re>2200
mass fluxdtd
Qm
For a pipe with diameter d and section d2/4
Q’ mass flux per unit section 2
4'
dQ
areaQ
Q mm
Reynolds numberd
QRe ' = viscosity
d pipe
Regimes of gas flux through a pipe
21
Laminar: Q < 8 103 (T/M)d [Pa m3/s]
Reynolds numberd
QRe '
QdRT
MddQ
R me
442
eRd
MRT
Q4
Turbulent: Q > 1.4 104 (T/M)d [Pa m3/s]
Regimes of gas flux through a pipe
22
For < d
For d
For > d
dtdV
pQ
d viscous
intermediate
molecular
Knudsen number = d/ Only for intermediate and molecular flux
intermediate
molecular
3 d/ 80
d/ 3
pr
TkB 1
2 2
10-2 p d 0.5
p d 10-2
For air at RT
Regimes of gas flux through a pipe
pr
TkB 1
2 2
23
Pipe conductance:0pp
QC
In parallel
N
iiCC
1
Flux across pipe
Pressures at pipe ends[C] = [L]3[t]-1
Pipe impedance:C
Z1
PCQ
PCQ
22
11
PCCQQQ 2121
21 CCCT
SI: m3s-1
cgs: lt s-1
24
In series
N
i iCC 1
11
222
111
PCQ
PCQ
T
TT C
QCQ
CQ
PPP 2
2
1
121
Q1 = Q2 = QT or gas would accumulate
T
TTTT C
QCQ
CQ
P 21
21
111CCCT
25
Pipe conductance
221
4 ppL
dC
Viscous and intermediate regime
Molecular regime
dL
dC
Ld
C
34
13
3Long cylindrical pipe
Elbow pipe
Laminar Turbulent L
dppC
522
21
The molecules must collide with walls at least once before exiting
Equivalent to a longer piper
For air at 0 C: 11,6 d3/L [lt/s]
26
[S] = [L]3[t]-1
Pumping speed S = Q/p0
Q= flux through aspiration aperturep = Vessel PressureV = Vessel Volume
p0
Relevant physical parameters of a pumping system
SI: m3s-1
cgs: lt s-1In the presence of a pipe
Effective pumping speed
Q at the pump inlet is the same as Q in pipe
SSS
SS
S
pp
Sppp
Qpp
C e
e 1111
1 0
0
00
C
pSSpQ e 0
0pp
SS
e
CSSe
111
S = Volumetric flow rate
27
[S] = [L]3[t]-1
Pumping speed S = Q/p0
Q= flux through aspiration aperturep = Vessel PressureV = Vessel Volume
p0
Relevant physical parameters of a pumping system
if S = C
Effective pumping speed
the Se is halved
C
CSSe
111
CSSC
Se
28
Q= flux through aspiration aperturep = Vessel PressureV = Vessel Volume
p0
Relevant physical parameters of a pumping system
10 QQQ
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
Sources of flux (molecules)
29
QpSdtdp
V
Pump-down equation for a constant volume system
01 QQQ
True leak rateOnly the gas initially presentcontributes
Virtual leak rateOther outgassing sources contribute
Short time limit Long time limit
Q = Q0 +Q1
S = Pumping speedp = Vessel PressureV = Vessel Volume
30
QpSdtdp
V
Pump-down equation for a constant volume system
True leak rate
Short time limit
tVS
epp
0
Q = Q0 +Q1
S = Pumping speedp = Vessel PressureV = Vessel Volume
Constant S
Q = 0pS
dtdp
V
dtVS
pdp
pp
SV
t 0ln
Time needed to reduce p by 50 %
SV
69,0
V= 1000 LP0 = 133 PaS= 20 L/s
t = 331,6 s 7.5 L/s = 27 m3/h
Vol of 1 m3 = 103 L to be pumped down from 1000 mbar to 10 mbar in 10 min =
600 s sLpp
tV
S /5.710ln6001000
ln 20
31
QpSdtdp
V
Pump-down equation for a constant volume system
Q = Q0 +Q1
S = Pumping speedp = Vessel PressureV = Vessel Volume
SQpu
Ultimate pressure
QpS 0
dp/dt = 0
Virtual leak rateOther outgassing sources contribute
Long time limit
32
Example
2xp
33
Differential pumpingoperate 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 P1 and P2, P1 >> P2
A: gas in with flux QL
gas to B with flux qQ1 = flux pumpedS1 = Q1/p1 QL/p1
B: gas in with flux qTo keep pressure p2
S2 = q/p2
q = C(p1 − p2) C p1
S2 = Cp1/p2
34
ExampleCVD coatings on panels
Antireflective coatings, p-n junction growth for solar panels
P0P1 P2 P1 P0
S1S2 S3
S1 = Cp0/p1
C CC
S2 = Cp1/p2 S3 = Cp2/p1
35
Gas-solid interaction
H2O
CO2
CO
CH4
O2
N2
HeH2
elastic inelastic trapped
physical adsorption (shortened to Physisorption): bonding with structure of the molecule unchanged
Chemisorption:bonding involves electron transferor sharing between the molecule
and atoms of the surfaceCan be thought of as a chemical reaction
36
Gas-solid interaction
H2O
CO2
CO
CH4
O2N2
HeH2
Origin:Van der Waals forces
The well depth is the energy of adsorption
Typical q:6 - 40 kJ/mol = 0,062 - 0,52 eV /molecule
Physisorption
612 rc
rb
zU
37
Gas-solid interaction
H2O
CO2
CO
CH4
O2N2
HeH2
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
612 rc
rb
zU
Chemisorption
38
Gas-solid interactionHow does this affect vacuum?
probability per second that a molecule will desorb
O2
Molecule trapped in the adsorbed state at temp. Tpotential well of depth qDilute layer (no interactions with other mol.)
How long does it stays?
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.
Boltzmann factor
TKq
Beprobability that fluctuations in the energy
sharing will result in an energy q
TKq
Be
39
Gas-solid interaction
probability per second that a molecule will desorb
O2
TKq
Be
p(t) = probability that it is still adsorbed after elapsed t
p(t+dt) = p(t) x (1-dt)
probability of not beingdesorbed after dt
dp = p(t+dt) - p(t) = - dt p(t)
pdtdp
tetp
average time of stayTKq
aBe
11
40
Gas-solid interaction
O2
average time of stay TKq
aBe
11
At RT 1013 s−1
TKq
aBe1310
97 kJ / mol = 1 eV / molecule
Temperature dependance
Molecule dependance
Note: Simple modelNeglects all other interactions, surface diffusion, adsorption sites so a can change
41
DesorptionP = 1000 mbar P = 10-7 mbar
Equilibrium
pumping
Far from equilibrium till….
tq
qG1Experimental
relation Gas flux /area
hGqq11
ht
t1
= 1 for metals = 0.5 for elastomers
= 0.5
= 1
q1 5x10−8 mbar L s−1cm-2
TKNpV Bat1 mbar L Nat 2.46x1019 Outgassing rate 1012 molec s−1cm-2
42
Desorption
How important is the molecule/surface interaction energy? H2O
N2TKq
aBe1310
Rate of desorption
aa
des
a ndtdn
1
TKq
des
a Bedtdn
1310
Simple model calculationidealized UHV system RT, V= 1 L, A = 100 cm2
S = 1 L/sonly gas source: initially complete ML of specified binding energy adsorbed at the wall
fall of pressure at RT
q
43
Outgassing
Origin of fluxes:
Permeation
Adsorption
Solubility
Desorption
Gas is continuously released, (at relatively small rates) from wallsPrincipally water vapor
Limit to attainable vacuum achievable in reasonable times (hours) ∼10−6 mbar
44
Gas-solid permeation
p1 = 1000 mbar
Residual Gas
H2O
CO2
CO
CH4
O2
N2
p2 = 1x10-8 mbar
HeH2
45
Gas-solid permeationp1 = 1000 mbar
Residual Gas
p2 = 1x10-9 mbar
Permeation is acomplex process Adsorption
Dissociation
Solution into the solid
Diffusion
Recombination
Desorption
46
Gas-solid permeationp1 = 1000 mbar p2 = 1x10-9 mbar
Permeation processcan be quantified troughPhenomenologicalquantities
permeability
=Q/(p1-p2)A
Q=flux trough wallA= unit area
[Q] = [p][L]3[t]-1
=[L]3[t]-1[L]-2
m3s-1m-2 ls-1cm-2
Residual Gas
47
Gas-solid permeation
Kp = Permeability coefficient
For a given gasA = wall area d = wall thickness
dA
pfpfKQ pp 21
m3s-1m-1Pa-1
He
cm3s-
1cm
-2 P
a-1
p = 13 mbard = 1 mm
pppf ,
depending on diffusionmechanisms
48
Gas-solid permeation
Metal – gas Kp
Glass Metals Polymers
He, H2, Ne, Ar, O2
No rare gas All gases
p p p
Table of gas permeability
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 Henry’s law
Valid for low concentrations and for glass and plastic materials
No formation of alloys
psc
50
Solubility
For dissociated gas
Sievert’s law
Valid for low concentrations and for metals
Interstitial or substitutional
psc
H2 on metals
Note the high solubility of H2 in Ti,Zr
51
Vacuum Pumps
Capture pumps
• Pistons• Gears• Turbines• Jet stream
• Cold traps• Ionization• Getters
Throughput pumps
Differences: pressure range, speed, gas selectivity
52
Pressure Ranges Spanned by Different Vacuum Pumps
More than one pump to HV and UHV
53
What pump to use?
S = [L]3[t]-1Pumping speed S = Q/p
pSdtdp
Vfrom
p = inlet pressure
dtdp
ppV
Su
For a pressure range where S does not depend on p, i.e. the pumping speed is constant
dppp
VSdt
u )log( uppVSt
10lnV)log( S
tpp u
Compression ratio: inlet
out
pp
CR
This can be used to measure Sor to estimate the time to reach pu
• Depends on the gas type• S varies with p
54
• Ultimate pressure
• Time to reach the u.p.
• Residual gas composition
• Other (absence of magnetic fields)
What pump to use?
55
Rotary Roughing Pump
Pu: 10-2 mbar
Rotor blade
Eccentric rotor
inlet
Exhaust valve
Spring
Cylindricbody
Oil
S: 2,5 ÷ 102 m3/h 0.7 ÷ 28 l/s
1 m3/h = 0.28 l/s
CR: 105
Starting operating pressure: 103 mbar
56
Dual stage Rotary Roughing Pump
Pu: 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
57
Rotary Roughing Pump: gas ballast
CR=105
Op. tempT 70 °C
Pump water vapor at 70 °Cwhen P reaches 3.3 104 Pa
The vapor liquefies and does not reach P > 1 105 PaSo 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 liquefyinside the rotation chamber
Vapor pressure
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 notliquefy
59
Diaphragm Pump
HousingValvesHead coverDiaph. clamping discDiaphragmDiaphragm supp. discConnecting rodEccentric bushing
Pu: ~ 1 mbar
CR: 102 103
Starting operating pressure: 103 mbar
60
Diaphragm Pump
Advantages
Oil-freeNo saturation
Low cost
Disadvantages
High ult.pressure (4 mbar)Low pump speed
Noisy
61
Root Pump
Advantages
Oil-freeNo saturation
High throughput
Disadvantages
Need prevacuumMedium cost
delicate
Eight-shaped rotor turningin opposite direction
•Clearance between rotors ~ 0.3 mm•No lubricants •CR depends on clearance
62
Root Pump
S and CR of a root pump dependon the preliminary pumpinstalled ahead
The gas flux is the same for both pumps
root palette
prpp
patm
SpSr
pprr PSPS
r
pr P
PCR
rpr CRSS
Palette: 60 m3/h = 16,8 l/s Sr = 16,8 x 40 = 672 l/s
63
Turbomolecular Pump
Pu: 10-10 mbar
S: 50 ÷ 5000 l/s
CR: 105 109
Starting operatingpressure: 10-2 mbar
64
Turbomolecular PumpPrinciple of operation
High pressure side
Low pressure side
The pumping action is provided by the collisionsbetween blades and molecules
Molecular regime
The speed distribution (ellipse) depends on the angle betweenV and blade
65
Turbomolecular Pump
Pumping speed: depends on gas type
Residual gas: H2After bake out
66
Compression Ratio of a Turbomolecular Pump
67
Turbomolecular Pump
Advantages
No saturationClean (magnetic)
UHVAny orientation
Disadvantages
CostDelicate
Quite noisy
70 l/s ~5000 €250 l/s ~10000 €
2000 l/s ~23000 €
Rotor suspension
Ball bearings (lubricant required)
Magnetic (lubricant absent)
68
Molecular drag pump
Turbo disk
Threaded stator
Cylindrical Rotor
Forevacuum flange(outlet)
Threaded stator
Safety ball bearing
Gas entry
Magnetic bearing
Lubricant reservoirElectrical socket
Operating principle:Same as turbo but different geometry
No blades but threads
69
Molecular drag pump
Pu: 10-7 mbar
S: 40 ÷ 100 l/s CR:H2: 102 109
He: 103 104
N2: 107 109
Starting operatingpressure: 1-20 mbar
They are use in combination with turboin a single mounting so
Use a low CR backing pump(i.e. membrane for clean
operation)
Higher backing vacuum pressure
70
baffle
vapor diffusion pump
Fluid is heated and ejected from nozzles at high speeddue to the nozzle shape and pressure difference betweeninside and pump cylinder.Fluid speed up to Mach 3-5The gas molecules are compressed to the pump base through collisions with oil vapor
71
vapor diffusion pump
Advantages
No saturationHeavy duty
Low cost
Disadvantages
gas reactionLiquid vapor tension
ContaminationNeeds water cooling
Pu: 10-9 mbarS: 20 ÷ 600 l/sStarting operatingpressure: 10-2 mbar
The pumping speed and the pressure strongly depends on oil type
72
Getter pumps
The active material is sublimatedby 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
73
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 filmon the pump walls that becomesthe active layer
The molecules are chemisorbed on the film
74
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 keptat operating T 300 °C to increasemolecule diffusion
75
sublimation
S strongly depends on gas
> 103 l/s
Zr-Al
Getter pumpsPum
pin
g s
peed (
l/s)
A’= sublimation, A=non evaporable
Non evaporable
800- 2x103 l/s
S depends on active surface saturation
AmT
S
mass
area
Adsorptionprobability
76
Gas-surface weak interactionPhysisorption anddiffusion into the bulk
Plus: Wall cooling
Pressure limit:10-10 ÷ 10-12 mbar
Advantages
Pump H2
Heavy dutyLow cost
No contamination
Disadvantages
SaturationMetal vapours
No rare gas pumping
Stripes of active material
Getter pumps
With a number of panelsone can obtain S > 1x104 l/s
But if warmed it releases the gas
77
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
78
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 intocathode material
Produce cathode vaporsDepositing also on anodesto work as getters
H2: accumulates into the cathodes Need regeneration by annealing
79
Ion-getter pump
Advantages
Heavy dutyNo traps
No contaminationAny mounting position
Silent
Disadvantages
High magnetic fieldsLow pump S for H2
Medium - high cost
Pressure limit:10-11 ÷ 10-12 mbar
S: 4 ÷ 1000 l/s Starting operatingpressure: 10-3 10-4 mbar
80
Adsorbing pumps
Liquid N2 cooledAdsorbing material
- Gas – cold surface interaction- Physisorption
Pumping mechanism
Adsorbing porous materialHigh surface/volume ratio
ZeolitesAl2O3, SiO2
H2O and N2 pumping
Liquid He cooledCold walls- Gas – cold surface interaction
- Physisorption, condensationPumping mechanism
Cryogenic pumps
81
Cryopump
- Gas – cold surface interaction- Physisorption and condensation
Pumping mechanism
Metal wall
82
Cryopump
Pressure limit:10-10 ÷ 10-11 mbar
Advantages
Heavy dutyNo contamination
Low cost
Disadvantages
SaturationNoisy
Needs other UHV pumps
The gas condensationif gas pressure > vapor pressureat wall T
S: 4 ÷ 100 l/s
Starting operatingpressure: 10-9 mbar
vapor pressure
83
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 +
--
84
Ionization in gases Ionization energy
eV
Ion +Electron affinityIon -
-
+
-
-
-
Less probableMore probable
85
Collision type:- elastic- atom excitation- molecule dissociation- Ionising ( e)
Relative energy loss
Atom or neutral molecule – electron collision
collision after 2
collision before 2
22
21
2
1
vmE
vmE
ek
ek
ek
kk KE
EE
1
21
2
2vm
Egask
21
1mmmm
E
EK
e
e
k
k
egas
mm
K ee
e
kk
mm
EEgas
1
64 1010 mme
Very small energy losses
Elastic collision
for gas molecules
86
Total energy lossik
L
ieEK
/
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
eetL
number of collisions tL
ee
Elastic collision
L
87
Apply external electric field E
e
ek K
eEE
2
prTkB
e
12
Maximum kinetic energy of an e-
moving in a gas
pE
rTk
K
eE B
ek 22 Depends on electric field
and pressure
Elastic collision
eemeE
v
maxIf e- has vin~ 0
88
Ionization
Ionization energyie
eB
ek W
K
eEpE
rTk
K
eE
22 2
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
-
-
+
89
Ionization
Long path to produce more ions
But it can also- e- trapped inside atomwith formation of negative ions -
pe
ii
-
e- with Ek --- constant pressure --- unit lenght
electron incidention) (e of number -
i
Specific ionization coefficient
collisions number tl
ee
Due to practical measurements
e- can ionize an atom
90
Vacuum measurement
Different types of vacuometers depending on pressure range
Mechanical, thermal, ionization
91
Vacuum measurement
MechanicalBourdon
To vacuum
Membrane
Pin wheel
tube
index
105 102 Pa(103 1 mbar)
The tube curvature changeswith pressure
Needs calibrationPrecision: 1-2% fsr
105 102 Pa(103 1 mbar)
The membrane or bellow bendswith pressure
Needs calibrationPrecision: 1-2% fsr
pR
x T4
2
0
92
Thermal conductivity vacuometers
Pirani
heated filament
The filament temperature, and hence the resistancedepends 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
TKTTKpRR
RRV
W fgasf
4
2
32
32
93
Thermal conductivity vacuometers
unbalanced
Hence
Thermaldissipation
radiativedissipation
contactdissipation
For small p, the reference bridge is
TKTRR
RRV
W ff
4
2
32
32
00
TKpRR
RR
VVWW gas
f
2
32
32
02
0
The pressure is obtained by measuring the Wheatstone voltage
)(cos 20
2 VVtp
In general it dependson the gas type
= cost Stephan-Boltzmann=wire emissivity
Kgas= gas thermal conductivityKf= wire thermal conductivity=coefficient
TKTTKpRR
RRV
W fgasf
4
2
32
32
94
Ionization vacuum gaugesHot cathode Cold cathode
Based on gas ionization and current measurements
95
Ionization vacuum gauge
I+ = I- i e p
Sensitivity K = σi · λe
Directly proportional to pressure Sensitivity K = i e
I+ = ion current i = specific ionization coefficient
I- = electron currentfrom 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
96
Ionization vacuum gauge
electrons from gas or field emissionsimilar to the behavior inside the ion getter pumps
Less precise due to problem ofdischarge current at low pressure
1 tesla
Range: 10-4 – 5 x10-10 mbar
Cold cathode
No filament so less subject toFilament faults
Note: discharge starts only by mag fieldto avoid high E field - induced currents
97
Mass Spectrometry
Need to distinguish the intensity of specific gas molecules
Collect molecules
Molecule ionizationSeparation of different molecules
Current measurement
Specific mass = ion mass (a.u.)/ion charge =
n = ion ionization multiplicity
nem
Specific mass of Ar+ = 40
Specific mass of Ar++ = 20
For a single molecule there aremany peaks, depending on n
98
Mass Spectrometry
Specific mass table
99
Mass Spectrometry
detector
Faraday cupAll ions measured
No filamentsLow sensitivity
sturdy
Channeltron - electron multiplyerHigh sensitivity
Delicate
Fast response
To remove secondary electrons
Amplifier time constant large
100
Quadrupole Mass Spectrometry (QMS)
VacuumChamber
Ion source(filament)
Analyser(Quadrupole field)
Detector(Channeltron)
Storing system
Quadrupole field between the rodsIons of varying mass are shot axially into the
rodThe applied quadrupole field deflects the ions
in the X and Y directions, causing them to describe helical trajectories through the mass
filter.
101
Quadrupole Mass Spectrometry (QMS)
r0 = rod separation
U+Vcos(t)
-U-Vcos(t)
20
22 )(r
yxU
Superimpose an oscillating field Vcos(t)
The forces are uncoupled along x,y,z axis
Quadrupole potential
102
Quadrupole Mass Spectrometry (QMS)
ion equation of motion2
0
22 ))(cos(r
yxtVU
Constant speed along z0
0)cos(2
0)cos(2
20
20
zm
ry
tVUeym
rx
tVUexm
Stability parameters
20
2
20
2
4
8
rmeV
q
rmeU
a
2t
0
zm
yeym
xexm
103
Quadrupole Mass Spectrometry (QMS)
Solved numerically for different a and q
0)2cos2(
0)2cos2(
2
2
2
2
yqad
yd
xqad
xd
All solutions outside are imaginaryand give increasing oscillation amplitudes
Neutralization of the ions on the rods
Ions oscillate in the xy planeOnly some e/m values reach detector
Solutions inside are real (stable trajectory)
104
Quadrupole Mass Spectrometry (QMS)
Zoom to region I
0)2cos2(
0)2cos2(
2
2
2
2
yqad
yd
xqad
xd
The line shrink to one pointOnly one ion with m/e ratio can reach detector
336,02
qa
VU
Stable solutions
20
2
4rq
Vem
fixed U, V and the overall ion motion can (depending on the values of a and q)
result in a stable trajectorycausing ions of a certain m/z value
to pass the quadrupole
20
220
2
4
8rm
eVq
rmeU
a
V=V0cos(t)for
qa
105
Quadrupole Mass Spectrometry (QMS)
Zoom to region I
The line enter the stable solutions region
336,02
qa
VU
Work line
All the ions with a/q on the line will reach detector
20
220
2
4
8rm
eVq
rmeU
a
V=V0cos(t)
for
Reducing U relative toV, an increasingly wider m/z
range can be transmitted simultaneously.
q
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
106
Quadrupole Mass Spectroscopy (QMS) profiles of the residual gas
p ≈ 3x10-7 mbarBefore bake-out
p ≈ 5x10-11 mbarAfter bake-out
H2O
CO+N2
CO2
H2OH2
107
VACUUM SEALING
Clamps
Low Vacuum
No bake at high temperatures
Reusable
Viton rings
108
UHV
VACUUM SEALING
HV
Bake at high temperatures
Reusable (maybe once)
Plastic deformationand shear
109
VALVES
Diaphragm
Butterfly
110
VALVES
Dynamometric sealing
StemAll metal
111
VALVES
Gate
Leak
High conductanceUHV to air compatible
Large clearance for instrumentsBakeable
112
FEEDTHROUGH
Multi-pin for signal or
Low currents
Multi-pin for high currents
113
MANIPULATION
Rotation
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