Generation of vacuum (pumps) and measurements...

Generation of vacuum (pumps) and measurementsGeneration of vacuum (pumps) and measurementsG f p pG f p p

Generation of vacuum (pumps):ᴥ pumping systemsp p g y

general considerations on their use and “match” with physical quantities introduced for the dimensioning of vacuum systems

Vacuum (pressure) measurements:(p )ᴥ vacuum gauges

general considerations and use:general considerations and use:– Total pressure gauges,

– Partial pressure measurements.p

AA 2011/2012 Vacuum technology

Classification of pump systems: pressure rangesClassification of pump systems: pressure ranges


Classification of pumps: processesClassification of pumps: processes


Main characteristics of pumpsMain characteristics of pumpsMain characteristics of pumpsMain characteristics of pumpsWorking pressure range

S iStarting pressure

Inlet pressure (max)

Outlet pressure (max)

Ultimate pressure (check the conditions)Ultimate pressure (check the conditions).

Pumping Speed S=S(p, M, or chemical)

Th h Q Q( M )Throughput Q = Q(pin, pout, M, gas).

Compression factor (K=Pout/Pin) K=K(Q,pvv) on datasheet you’ll find K0 for Q = 0 (ideal), direct use for Pu, but for Q ǂ 0 check provided curves


asK(pvv) or K(Q) if available.

Rough or back pumping SystemsRough or back pumping SystemsRough or back pumping SystemsRough or back pumping Systems


Rotary mechanical pumpsRotary mechanical pumps

Hi h i f (1 105 1 2 107) b


High compression factor (1 stage 105 1, 2 stage 107), but …care on water problem and back-streaming

Oil lubricated system:Oil lubricated system:Oil provides also good sealing

and works as cooling liquid, but Oil is in the volume where gas is compressed.

316 mbar316 mbar@ 70 °C

Solution Ballast Solution Ballast


Solution: Ballast, Solution: Ballast, in order to ingiect air at the exaust in order to ingiect air at the exaust

Oil pumps: the backOil pumps: the back--streaming problemstreaming problemStart HV pump, before,pump which compresses

the oil in between HV pump

And rotary pump

Warning: when turbo has to be stopped, air has to be fed


g pp(automatic vent), during the deceleration of the turbo

to avoid back streaming

Characteristics Curves: one stage systemCharacteristics Curves: one stage system

Pumping speedPumping speed Immediate use

S (S back pump)Spb (S back pump)

Time of evacuationTime of evacuation, Immediate use directly connected:

P=Poe -(Sp/V)·t

with bellow or connection:parametrized (D4/L) plot Sp vs t/V


Characteristics Curves: Characteristics Curves: Two or more stage systemTwo or more stage system


Dry Dry SytemSytemDry Dry SytemSytem

OUTLET

• Gas displacement with roto-traslation movement of a

moveing spiral on a fixed spiral.INLET

OUTLET

GAS

g p p

• Pu Pirani measurement :

0,01 mbar

12 3

FIXED SCROLL

• Application: Oil free.

• Limited capacity of pumping vapors and chemical gases,

4

56

7vapors and chemical gases,

liquids are dangerous for the pump.

U l f i d t i lORBITING SCROLL

• Useless for industrial application, escludind load –

lock systems.

Movement of gas in a scroll mechanism

M i f 8 000 10 000 h


Maintenance after 8.000-10.000 hours

Dry pump scroll characteristic curvesDry pump scroll characteristic curvesDry pump scroll: characteristic curvesDry pump scroll: characteristic curves


Dry piston pumpsDry piston pumps


Maintenance after 20.000 h

Membrane dry pumps:Membrane dry pumps:Non return

• Transfer gas pump which make

check-valve

use of the oscillation of a membranes or diaphragms.

• P in system with at least twoPu in system with at least two stage: 1 mbar

• Oil free pumps.

• Available model with teflon protection of volume and parts

exposed to the vacuum for paggressive or corrosive

chemical processes.


High vacuum pumping systemsHigh vacuum pumping systems


Root BlowersRoot Blowers

There isThere is no oil in the

systems exposedsystems exposedto the vacuum


Roots pump CharacteristicsRoots pump CharacteristicsHi h i d L i f t b tHigh pumping speed

Compared with rough pumps Low compression factor but:

If operated properly, the roots compress the oil in the outlet,

then allowing oil free (?) vessel.


g ( )Compared with a rough pump,

high speed system and 10 time lower pressure

CryoCryo-- pumpingpumpingyy p p gp p gBased on the principles of

cryocondensation,C tiCryo-sorption,

andCryotrapping:

decreasing the surface temperature of systems exposed to the vacuum

th f dthe processes are favored,depending on the physical and chemical properties of

the gases present in thethe gases present in the vacuum.

At T < 20 K for most

Pgases Pv not higher than 10-11 Torr

At T = 4.2 K H2 gives


At T 4.2 K H2 gives

Pv 10-7 Torr

Terms for Terms for cryopumpingcryopumpingCryo-condensation (interaction between molecules): for gases increasing the coverage of a surface a g g gsaturation equilibrium is reached between adsorption and desorption.p

ᴥ Corresponding gas pressure in vacuum: vapor pressure curve p = Q/S + p tpressure curve. p Q/S + psat

Cryo-sorption (interaction of molecules with f ) b l f isurfaces): submonolayer surface coverage experience

attractive van der Waals forces exerted by cold surfaces:

ᴥ as consequence H2 can be cryosorbed at 20 K, and all gases may be cryosorbed at their own boiling temperature (1 bar).


atio

nco

nd

ensa

id ( j ) ti ( d ti )kTE

tt

Cry

o-c

residence (sojourn) time (see desorption).kTr ett 0=

rpti

onry

-oso

r

Bi di i ti i 2 3 ti bi f

Cr


Binding energy in cryosorption is 2 or 3 times bigger for heavy gases, 10 for H2 and 30 for He.

Conclusions on Conclusions on cryosoprtioncryosoprtiony py p

It may be concluded that at the boiling temperature of N2

(77 K) all gases except He, H2 , D2 and Ne may be(77 K) all gases except He, H2 , D2 and Ne may be cryosorbed.


CryotrappingCryotrappingy pp gy pp gCryotrapping is sorption process by which non-condensables

gases are trapped (buried) in the growing solid-liquid gases are trapped (buried) in the growing solid liquid condensation layer of a condensable gas microcrystallites, while

others are incorporated within the crystallites. This method traps non condensable moleculesThis method traps non-condensable molecules.

CryotrappingH H iH2 or He in

N2, or Ar & CO2


Back pumprequired for

t tistarting pressure (< 10-3 ) mbar and

regeneration.

Careful use of oil pumps?Poisoning of surfaces byPoisoning of surfaces by

Oils.

High pumping speed for H2O and H2


UHV pumpsUHV pumpsp pp p


TurboTurbo--molecularmolecular pumpspumps


Turbo molecular pumping SpeedTurbo molecular pumping Speed

For pressure lower than --- we can assume in our calculation a t t i d d b f f t bl


constant pumping speed, and we can be safe for stable working conditions.

Compression factor (KCompression factor (K00)) MK ∝0

Turbo pumps compress better heavy

molecules molecules(oil ~ 70-75 amu)

For light molecules we have more

backstreaming.

Camparison between turbo pumps and available informationsCamparison between turbo pumps and available informationsTurboVac T1600 TW1600 MagW1500S

SN2 1550 1420 1220Ar 1410 1200 1180He 1300 --- 1150H2 720 --- 920

K0N 2 5· 105 1· 107 1.5 ·108

Ar 1· 106 3· 108 ---He 1·104 --- ---H2 2· 102 --- ---

Pu < 3 ·10-10 mbar < 3 ·10-10 mbar 1 ·10-10 mbar


Pout (N2)< 0.5 mbar < 8 mbar <0.2 mbar (air)

<2.0 mbar (water)

Getter pumps: Sorption pumps Getter pumps: Sorption pumps --TiTi--sublimation pumpssublimation pumps

Ti, sublimated in vacuum, reacts with gases and solidifyg y

On colder surfaces trapping the gases.Cooling the surface at lower temperature,

pumping speed increases 2 or 3 order


pumping speed increases 2 or 3 order.

Sputter Sputter –– getter ion pumps: getter ion pumps: combine penning process and Ti trapping combine penning process and Ti trapping p g p pp gp g p pp g

Diode

Triode


Ion sputter pumpsIon sputter pumpsIon sputter pumpsIon sputter pumps

Diode configuration is better for H2


NEG pump (non evaporable getter)NEG pump (non evaporable getter)Chemical reaction. Reactive alloys of zirconium or titanium, configured in cartridge.y g g

Active gases (O2, N2, H2O, CO, CO2) impinging on the cartridge surface are dissociated and permanently trapped, in the form of stable chemical compounds. Also hydrogen is very effectively pumped. Hydrogen (and its isotopes) atoms diffuse y g y y p p y g ( p )inside the getter bulk and dissolve as a solid solution. Noble gases (and CH4 at room temperature) are not pumped.


Vacuum monitoring and control Vacuum monitoring and control


Vacuum GaugesVacuum Gauges

Direct measurements

Indirect measurements


Direct measurementsDirect measurementsCapacitance Manometer

Baratron

Piezo

Membranovac

Diaphragm:

– High precision commercialHigh precision commercial gauges 0.15.%

» 1100 ‐10‐1 mbar

» 110 10‐2» 110 – 10‐2

» 11 ‐ 10‐3

» 1.1 ‐ 10‐4

» 0.11 ‐ 10‐5

It measures the capacitance variaton due do the deformation of a wall (diaphragm).f th l t di l t f 10 9 th l t bilit i d


for the lowest pressure range displacement of 10-9 cm: thermal stability required.

Indirect measurements (thermal conductivitivty Gauge)Indirect measurements (thermal conductivitivty Gauge)

Thermal conductivity gauge Heat transfer depends on P, linear dependence for 0.01< Kn <10. )(2

2 pd

kT

πλ =

High PressureLow pressure)( pπ

leaks thermal

)( 41

421

+−= TTAH σε

H t t f d d P if T d d ti t t t


Heat transfer depends on P if T and accomodation parameters are constants.

PiraniPirani Gauge Gauge Thermal conductivity gauge, with a Withstone bridge for a higher sensitivity.

Let’s start at a given P with the Vacuum l

gbalanced bridge, if P increases, T of the filament R2 decreases (due to a higher heat exchange), the

vessel

to a higher heat exchange), the bridge is unbalanced.

We can feed more current to keep T =cost (method)

Compensating tube is used f ifor zeroing

at a P < 10-4 mbar.

Thermal conductivity dependes on the gas,


y p g ,

the read-out is calibrated for N2.

If R1 R4 = R2R3, then IM = 0

T constant method: R ↓ if P ↑ therefore V (or V=I ) ↑ then → T ↑ )R2 ↓ if P ↑ , therefore Vdc (or V=Idc) ↑ then → T ↑ )

Accuracy 10%, in the range of ccu acy 0%, t e a ge omore sensitivity, more or less for pressure in the range:

10-2 ÷102 mbar10 2 ÷102 mbar


Thermocouple gaugesThermocouple gauges

Thermocouple gauges also rely on the dependence of P on the

heat transfer.

Constant Current on a filament on which center f

a thermocouple si soldered.

Therefore from the measurementTherefore from the measurement of the T is possible to provide a

measurement of P.

Accuracy 30%.


UHV gauges: UHV gauges: ioniz tion u esioniz tion u es

In HV & UHV the particle density is so low , that it is non

ionization gaugesionization gauges

possible to detect the force exerted on a surface from the molecule impinging on it or the heat transfer.

Hot Cathode gauge

It esploit the ionization of gas by electron bombardment and collection of the positive ion produced in the vacuum vesselp p

The positive ion current collected is proportional to the density of particles in the vacuum vessel, to the electron current and to the ionization cross-section.


Hot cathode ionization gaugesHot cathode ionization gauges

Bombarding e- : ie


Positive ion current i+=ip collected:

PiSi ep ⋅⋅=ie electron bombardment current, P pressure, S sensitivity (depends on

P(x) to be measured ( ) ( )( ) ( )2

2 NPxS

NSxP =

ionization cross-section), Fixing S for nitrogen to 1.

( )xS

( )

If normalized to Nitrogen S(N2) = 1.00

Relative Sensitivity respect to N

( )( )xgasofsensititylative

readingmeterPxP

Re)( =

H2 0.42 - 0.45

H 0 18

Relative Sensitivity respect to N2

He 0.18

H2O 0.9

N2 1 00


N2 1.00

Acetone 5

Penning GaugeCold Cathode ionizzation gaugesCold Cathode ionizzation gauges

Penning Gauge

High voltage (1 kV) discharge induced bycosmic rayscosmic rays.

Thanks to the magnetic field the effective path length of the bombarding e- is larger.

Disadvantages: higher self-pumping due to sputtering 0.1 ~ 0.5 l/sec.Advantages: pressure range that connect hot cathode and TC.


Vacuum Gauges calibrationVacuum Gauges calibrationHV gauges are companies calibrated at the order 10 %,

and there is also sensitivity problem on gas type.

UHV gauges are calibrated in the order of 10-20%.

Calibration procedure will be followed in the lab.

12 5 3 from12.5.3 fromM.H. Hablanian

High-Vacuum Technology a practical Guide


High Vacuum Technology a practical Guide(Marcel Dekker Inc., New York 1997)

Back to the practice in labBack to the practice in lab

Purpose is to measure the conductance of

pp

pthe gaseous polarized Hydrogen, and check wich gauge to use for its stablecheck wich gauge to use for its stable working conditions.

We have to work with Hydrogen then we have to to design the system for H2g y 2

Estimated through-put for target thickness measurements: H in the range of 2 10-3measurements: H in the range of 2 10 3

mbar l/s of H it meand1 10-3 mbar l/s of recombined H2.


Ma

Partial pressure measurementsPartial pressure measurementsa ss s Sppec t rometr


ry

Ion sourceIon sourceAn electron beam is required in order to ionized the gas in the vacuum systemionized the gas in the vacuum system.

Ion optic pieces in order to extract the ions from the ionizing volume and focus it in the filtering system.filtering system.

AA 2011/2012 Vacuum technologyAA 2011/2012 Vacuum technology

Ion sourceIon source


At 70 eV max σιonization

Ion Ion seperationseperation ((fileringfilering m (m (amuamu)/q(e)))/q(e))

Dynamic or static :

pp f gf g qq

Dynamic or static :

),(),(),(2

2

tttdm

rBrvrEr

∧+=

the second member of the ion motion equation is function of time or not.

),(),(),(2

tttdte

v

q

Dynamic E(r,t) quadrupole Static B( r ) sector


Most frequently used in vacuum, compact and easyMost frequently used in vacuum, compact and easy


By By sostitutionsostitution: Mathieu: Mathieu--type equationtype equation

where+ for x

fwhere

- for y


In the stability vertex: q = 0.706 we have

)/(10813 20

26 rfVm ⋅=

m in amu, ro in m, V in V and f = ω/2π Hz.

but

)/(108.13 0rfVm

but

q

a

V

U

2

1=

then only a given ratio m/e is stable (stay on the axis of the quadrupole spectrometer.quadrupole spectrometer.


Phenomenological description (fundamentals Leybold) Phenomenological description (fundamentals Leybold)

U t tU constant

Adding Vcos( ωt)Adding Vcos( ωt)

Gi MGiven M, ions on the axis f(V)

Given U/V, i+ f(M)


FragmentationFragmentation--crackingcracking

By the electron excitation of molecule we have also fragmantation effects:

F i ki f H O


Fragmantation-cracking pattern for H2O

Cracking pattern and isotopesCracking pattern and isotopesxxx/yyy:

xxx M(amu),yyy: relative abundance.

Max is 100

Isotopic abundance


Mass Spectra parent and sonsMass Spectra parent and sons


Air mass spectrumAir mass spectrum


Ion Detection Ion Detection a) Faraday Cup

and and b) Secondary Electron Multiplier



ion


Or in other words sensitivity of detection Or in other words sensitivity of detection Or in other words, sensitivity of detection Or in other words, sensitivity of detection



Leak detectionLeak detection


Leak detectionLeak detection(vacuum method)(vacuum method)

He

Mass spectrometer usually


Mass spectrometer usuallyB static tuned to He+

Leak check without Leak check without the leak detectorthe leak detector


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Generation of vacuum (pumps) and measurements...

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