Dr. G. Mirjalili, Physics Dept. Yazd University
Vacuum Techniques
Basic vacuum concepts
Dr. G. Mirjalili, Physics Dept. Yazd University
Vapor Pressure
• Terminology:– Evaporation: when a liquid becomes a gas
– Sublimation: when a solid becomes a gas
– Vapor: the gas produced when a liquid or solid is evaporated
– Condensation: when the vapor becomes a liquid or solid again (condensed phases)
– Equilibrium: the state of any system in which opposing forces balance each other
– Volatile: liquids that are easily vaporized - have high vapor pressure
Dr. G. Mirjalili, Physics Dept. Yazd University
Vapor Pressure
• Evaporation occurs when:– the temperature of the material is increased, OR
– the pressure at the surface of the material is decreased
• Vapor Pressure: the pressure at which a liquid or solid becomes a vapor at a given temperature– what happens to the vapor pressure as the temperature is
decreased?
• Outgassing: when a material in its condensed phase becomes a vapor in a vacuum system at low pressure.– Extremely small quantities of water, solvents, or fingerprints left in
a chamber can outgas and increase the time it takes to pump a system down.
Dr. G. Mirjalili, Physics Dept. Yazd University
Vapor Pressure
• Liquids in a closed container will evaporate until:– partial pressure in the air above
the liquid = vapor pressure of the liquid
Dr. G. Mirjalili, Physics Dept. Yazd University
Vapor Pressure
Dr. G. Mirjalili, Physics Dept. Yazd University
Vapor pressure of water at various temperatures
T (O C)
100
25
0
-40
-78.5
-196
P (mbar)
1013
32
6.4
0.13
6.6 x 10 -4
10 -24
(BOILING)
(FREEZING)
(DRY ICE)
(LIQUID NITROGEN)
Dr. G. Mirjalili, Physics Dept. Yazd University
Vapor Pressure• Phase Diagrams:
– determine what state a substance will exist in at a given temperature and pressure
– example: water
Dr. G. Mirjalili, Physics Dept. Yazd University
Vapor Pressure• Phase Diagrams:
– water metals
http://www.lsbu.ac.uk/water/phase.html
Dr. G. Mirjalili, Physics Dept. Yazd University
Vapor Pressure
• Examples:– At what temperature does water boil when the pressure is
reduced to 1 torr?– Are the condensed phases to the left or right of the lines?– If a CVD system runs at 500C and 1 mT, which metals
would be a poor choice to build the chamber out of?• The Moral of the Story:
– Vapor pressure must be considered when selecting EVERYTHING that goes into a vacuum system.
– This includes seals, oil, chamber materials, valves, the wafer, etc.
Dr. G. Mirjalili, Physics Dept. Yazd University
Gas Density (n)Ideal Gas Law
PV = NkT
Gas density at 1 Pascal at room temp.
N/V = n = P/kT = (1 N/m2)/(1.3807x10-23J/K)(300 K)= [1 (kg-m/s2)/m2]/[4.1x10-21 kg-m2/s2]= 2.4x1020 atoms per m3
= 2.4x1014 cm-3 …at 1 Pa
Rule of Thumb
n(T) = 3.2x1013 cm-3 x (300/T) …at a pressure of 1 mTorr
Dr. G. Mirjalili, Physics Dept. Yazd University
Mean free path(1) average distance between molecular collisions in the gas
cm)Torr(p
10×5=
σp2
kT=
nσ2
1=λ
3
= molecular cross section ~ projected area of the molecule (the last form of the equation is for air at 20oC)
p (Torr) (molec./cm2-s) 760 2.9x1023 67 nm
1 3.8x1020 51 m
1x10-3 3.8x1017 51 mm
1x10-6 3.8x1014 51 m
1x10-9 3.8x1011 51 km
When > the smallest dimension of the flow path, the flow is free molecule,If < apparatus dimension, the flow is viscous
Or, =.0066/P (P mbar)
Dr. G. Mirjalili, Physics Dept. Yazd University
Mean free path(2) • Mean Free Path:
• MFP increases as the pressure decreases: (cm) ~= .005 / P (torr)
– at 1 atm, MFP = 0.02 microns
– at 1mT, MFP = 5.08 cm– at 10-9 torr (UHV), MFP = 50 km
Mean Free Path vs. Pressure
1.00E-061.00E-041.00E-02
1.00E+001.00E+021.00E+041.00E+061.00E+08
1.00E-10
1.00E-08
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
1.00E+04
Pressure (Torr)
MF
P (
cm
)
MFP
Dr. G. Mirjalili, Physics Dept. Yazd University
Mean free path(3)
Molecular density and mean free path
1013 mbar (atm) 1 x 10-3 mbar 1 x 10-9 mbar
#mol/cm3
MFP
3 x 10 19
(30 million trillion)4 x 10 13
(40 trillion)4 x 10 7
(40 million)
2.5 x 10-6 in6.4 x 10-5 mm
2 inches5.1 cm
31 miles50 km
Dr. G. Mirjalili, Physics Dept. Yazd University
Collisions and Mean Free Path
Gas Densityn = P/ kT
n
Cross-section~ d2
d
Rigorous Hard Sphere Collisions: = kT / 2 d2P
Arcm8 / P (mTorr)15 22.6 10 cm Ar
Dr. G. Mirjalili, Physics Dept. Yazd University
Atom & molecule dimensions
• Atom dimension=3 A0
• Average molecule separation in at atm perrsure :
In 1 cm3 nitrogen gas is 2.5 X1019 molecules so:
1/2.5X10 19= 4X10 -20 cm=34 A0
Dr. G. Mirjalili, Physics Dept. Yazd University
Average molecular separation
• At atmospheric Pressure:
• 2.5X10 19 molecules occupy 1 cm3;
• the volume of one molecules =1/2.5x10 19 =4x10 -20 cm3
• the length of each molecules =(4x10 -20 )1/3=3.4x10 -7 cm=34 A0
3 A0
34 A0
Dr. G. Mirjalili, Physics Dept. Yazd University
Impingement rate/flux, J per unit area, surface incidence rate
21
21
)2(
)2(
14588
4
MRT
PNJ
RkN
mNM
mKT
PJ
M
T
M
RT
m
kTV
VnJ
av
av
av
1cm2
Dr. G. Mirjalili, Physics Dept. Yazd University
Maxwellian Distribution
kT
mv
evkT
mNvN 222
3 2
)2
(4)(
kT
mdvev
kT
m
N
N
N
vdvevkT
mN
N
vdvvN
v v
kT
mv
2)
2(
4)
2(4)(
2
2
0
32
30
222
3
0
20
3
2
12
dvev v
m
kT
m
kT
kT
mv
8
)2
(2
1)
2(4 22
3
The speed of molecules whose speed is between v, v+dv
Dr. G. Mirjalili, Physics Dept. Yazd University
Maxwellian Distributionvrms
0
42
3
222
3
0
2
2
2)
2(
4)
2(4)(
2
2
kT
mdvev
kT
m
N
N
N
dvvevkT
mN
N
dvvvN
v v
kT
mv
m
kTvv
kT
mv
rms
3
8
3)
2(4
2
52
32
Dr. G. Mirjalili, Physics Dept. Yazd University
Maxwellian Distribution
m
kTv
vkT
mvve
kT
mvvve
kT
mN
evdv
d
kT
mN
dv
vdN
dv
vdN
p
kT
mv
kT
mv
kT
mv
2
0)2(0)2
22()
2(4
0)()2
(4)(
0)(
22
222
3
222
3
22
2
Dr. G. Mirjalili, Physics Dept. Yazd University
Surface incidence rate
kT
mv
kT
m
v
vPvf
2exp
24
)()(
22/3
2
0
2_ 8
4)(m
kTdvvvfvcv
vndvvfvnvnZv
zzz 4
1)( 3
0
Maxwellian Distribution
Average speed of an atom:
Flux of atoms to the x-y plane surface:
(Campbell)
Very important!
Dr. G. Mirjalili, Physics Dept. Yazd University
Surface incidence rate rate at which molecules of gas strike a unit area of surface (also exit a vessel with a small orifice into a vacuum)
mkT
p
m
kT
kT
pvnJ
2
841
41
scm
surfacestrikingmolecules
MT
)Torr(p105.32
22
• n = molecular density in gas, molecules/cm3
• = mean velocity of Maxwell Boltzmann distribution• k = Boltzmann’s constant• m = mass of molecule• T = absolute temperature in Kelvins (K)
The numerical form is obtained by multiplying numerator and denominator by Avogadro’s number, NAv, and noting that NAvk = R, the gas constant and NAvm = M, the molecular weight of the species in the gas
v
Dr. G. Mirjalili, Physics Dept. Yazd University
Example
A vacuum chamber has a base pressure of 10-6 Torr. Assuming that this is dominated by water vapor, what is the flux of H2O to a substrate placed in this chamber?
n = 3.2x1013 cm-3/mTorr * 10-3 mTorr = 3.2x1010 cm-3
<v> = (8kT/M)1/2 = 59200 cm/s
z = (¼)n<v> = 4.74x1014 molecules per cm2 per sec!
This is approximately one monolayer of H2O every secondat 10-6 Torr base pressure.
Dr. G. Mirjalili, Physics Dept. Yazd University
Equilibrium between a liquid and it vapor
Ec JmkT
PJ
210
)2(
P0= saturation vapor pressure
Dr. G. Mirjalili, Physics Dept. Yazd University
Collision frequency
• V=xt• The average distance traveled in one second :
V=x(1)• Collision frequency=V/l l= mean free path
• For N2 (at room temperature and atm pressure
• C.F=470/6.6x10 -6 =7.1x10 9 per second
• at low pressure C.F increase or decrease?• At which pressure the C.F is 1 second?
Dr. G. Mirjalili, Physics Dept. Yazd University
n, J, l at various P for N2 at 295K• P(mbar) n(m-3) I(M.F.P) J(cm-2 m-1)
103 -1 atm 2.5X10 25 6.6X10-6 2.9X10 23
1 2.5X10 22 6.6X10-3 2.9X10 20
10-3 2.5X10 19 6.6 cm 2.9X10 17
10-6 2.5X10 16 6.6 m 2.9X10 14
10-10 2.5X10 12 660 km 2.9X10 10
Dr. G. Mirjalili, Physics Dept. Yazd University
Outgassing
• The release of gas from internal surface into the vacuum, occurs by desorption of molecules from bound states on the solid surface.
vacuum
Outgassing
Chamber wall molecules
Absorbed molecules
Dr. G. Mirjalili, Physics Dept. Yazd University
adsorption & absorpbtion
Adsorption Absorption
Adsorption Absorption
Desorbtion = removing the molecules
q
q adsorption=15-32 kcal/mol q absorption =2-10 kcal/mol
Dr. G. Mirjalili, Physics Dept. Yazd University
Meam adsorption stay time
• Adsorb molecules have limited lifetimes on the surface
• kT at room temperature is 1/40 eV
• Meam adsorption stay time is the avarege lifetimes of an adsorbed molecules ()
• =10 -13 exp(q/kT)=10 -13 exp(40q)
• “De Boer equation”
Dr. G. Mirjalili, Physics Dept. Yazd University
Meam adsorption stay time
• Values of for various q at 295 K
q(eV) 0.2 3x10 -10
0.4 1s
0.6 20 ms
0.9 400 s
1.1 1.2x 10 6 s (= 2 weeks)
Dr. G. Mirjalili, Physics Dept. Yazd University
Gasses and pumping
Drifting molecules
Adsorb molecules
Outgassing
pump
Dr. G. Mirjalili, Physics Dept. Yazd University
Gas flow and throughput
• Gas flow down a pipe
P1 A Flow A` P2
P1>P2
Pressure is constant across any cross section
We define throughput as:
0PVdt
dVPQ mbar l s-1
Q is constant down the pipe
Dr. G. Mirjalili, Physics Dept. Yazd University
Q is constant down the pipe
Usually, throughput is conserved. (Steady state
Q Q Q
Q = P1V`1=P2 V`2=P3V`3
P1P2 P3
V1
V2 V3
Dr. G. Mirjalili, Physics Dept. Yazd University
Q = P1S1 = P2S2
pump 25 ℓ/s
pump 1500 ℓ/s
P1
P2
P2 = 100 P1
Throughput: quantity of gas removed by pump per unit time:
Q = p(dV/dt) = pS (Torr-liter/s)
Throughput (example)
Dr. G. Mirjalili, Physics Dept. Yazd University
Mass flow rate and throughput
kT
Qm
dt
dNm
kT
Q
dt
dNkT
PV
dt
dV
kT
P
kT
PVdtd
dt
dN
NkTPV
0
))(())((
Example: A fan moves atmospheric air through a room at rate of 0.9 m3 per minute. What is the through in (mbar litter s -1)
Answer: 0.9 m3=900 l and V`=900/60=15 l s-1
Q = PV`=1000x15=15000
Mass flow rate
Dr. G. Mirjalili, Physics Dept. Yazd University
Speed
1
0
lsP
QS
PPVdt
dVPQ
dt
dVP
Where gas enters a pipe from a vessel and where it emerges from the pipe to enter a pump . The volumertic flow rates V` is defined as the speed Pump speed: volume of gas taken in by the pump per unit time
Dr. G. Mirjalili, Physics Dept. Yazd University
S and S*
SS*
vessel
pumpQ
S*>SS/S*=1 Perfect system
Dr. G. Mirjalili, Physics Dept. Yazd University
Conductance
Conductance – gives the capacity of a tube to allow a volume of gas pass from one end to another in unit time
P1 P2Q
C=Q/(P1-P2)
P1-P2
P1>P2
Dr. G. Mirjalili, Physics Dept. Yazd University
Speed of pump at the vessel
S, P
C
S*, P*Q
Q=SP
Q=C(P-P*)=SP=S*P*
S=S*[C/(S*+C)]
The relation between S, S*, C
Vesselpump
Dr. G. Mirjalili, Physics Dept. Yazd University
Pumping Speed
• Pumping Speed and Conductance are related as follows:
Seff is the effective pumping speed at the chamber
Sp is the pumping speed (capability) of the pump
Ctotal is the total conductance of the system between the chamber and the pump.
TPEFF CSS
111 Sp
CT SEFF
Dr. G. Mirjalili, Physics Dept. Yazd University
Vacuum system & electrical system
V1 V2P1 P2
V
Battery =Pump
P V
Q I
P=ZQ
V=RI
P=Q/C
C=1/Z
C=Q/(p1-P2)
P
Dr. G. Mirjalili, Physics Dept. Yazd University
Conductance• Conductance: the ease with which a gas is drawn
through a vacuum component (pipe, valve, etc.)– conductance is dependent on the diameter, length, and shape
of a pipe or orifice
• Conductance Units: volume/time
– liters/second
• Conductance can be the LIMITING factor in a pumping system:
– Example: A pump that has a pumping speed of 400 l/sec, that is pumping through a conductance of 100 l/sec, is reduced to an effective pumping speed at the chamber of less than 100 l/sec.
Answer:S*=400 l/sec, C=100 l/sec S=?
S=S*[C/(S*+C)]
S=400[100/(400+100)]= 80 l/sec
Dr. G. Mirjalili, Physics Dept. Yazd University
Viscous and Molecular Flow
Viscous Flow(momentum transferbetween molecules)
Molecular Flow(molecules moveindependently)
Dr. G. Mirjalili, Physics Dept. Yazd University
Flow regims
• Viscous Flow: “mob mentality”– the type of gas flow that occurs when
gas molecules are so close together that there are constant collisions
– mean free path is relatively short– gas flows like a liquid from high
pressure to lower pressure– predominantly in the rough vacuum
regime
Dr. G. Mirjalili, Physics Dept. Yazd University
Flow regims • Molecular Flow: “loner mentality”
– the type of gas flow that occurs when gas molecules’ direction of movement is completely random (not necessarily towards lower pressure)
– there are few collisions between molecules in the chamber
– mean free path is long
– predominantly in the high and ultra-high vacuum regimes
• Knudsen Flow (or Transition Range)– transition region between viscous and molecular flows
(some behaviors from both)– medium vacuum regime
Dr. G. Mirjalili, Physics Dept. Yazd University
Flow Regimes
Mean Free PathCharacteristic Dimension
Viscous Flow: is less than 0.01
Mean Free PathCharacteristic Dimension
Molecular Flow: is greater than 1
Mean Free PathCharacteristic Dimension
Transition Flow: is between 0.01 and 1
()
(d)
VT
M
P
C
Dr. G. Mirjalili, Physics Dept. Yazd University
Knuden`s Number
/d=10.01Viscous flow Molecular flow
T.F
Dr. G. Mirjalili, Physics Dept. Yazd University
Flow Regims • Which flow regime you are in determines the type of vacuum
hardware you will be using:• Viscous Flow:
– displacement pumps: rotary vane, roots, diaphragm
– direct pressure gauges: manometers, bourdon tubes
– elastomer seals
– less limited by the conductance of the system
• Molecular Flow:– momentum transfer or entrapment pumps: turbo, ion, cryo
– indirect pressure gauges: ion gauges
– metal seals
– VERY limited by the conductance of the system
• More on these later ...
Dr. G. Mirjalili, Physics Dept. Yazd University
Conductance in Viscous Flow
Under viscous flow conditions doubling thepipe diameter increases the conductance Sixteen(16) times.The conductance is INVERSELY related to the pipe length
Dr. G. Mirjalili, Physics Dept. Yazd University
Conductance in Molecular Flow (round long tubes)
Under molecular flow conditions doublingthe pipe diameter increases the conductanceeight times(8).The conductance is INVERSELY related tothe pipe length.
Dr. G. Mirjalili, Physics Dept. Yazd University
Calculation the usal pipes conductance
Conductance in usual pipes
(molecular flow)
CL
C0
L
Lpipe CC
CCC
0
0
Dr. G. Mirjalili, Physics Dept. Yazd University
Molecular flow conductance of an aperture(1)
P1, J1, n1
P2, J2,n2
J1 J2
)(2
)(2
)2(
)(
)(
2121
210
21
PPAm
RTPPA
m
kTQ
mkT
PJ
dt
dNkTQ
AJJdt
dN
Dr. G. Mirjalili, Physics Dept. Yazd University
Molecular flow conductance of an aperture(2)
Am
RTC
PPAm
RTQ
PPCQ
2
)(2
)(
0
21
21
Conductance of an aperture
C0=9.3 D2 l/s -1 for a circular aperture
Maximums speed of a pump:
S*=C inlet=C0= 9.3D2
Dr. G. Mirjalili, Physics Dept. Yazd University
Transmission probability
WJ1 A
J1
WJ2 A
J2
0210
0
2121
21
21
)(2
)(2
)(2
)(
)(
WCCPPWCQ
AM
RTC
PPAWM
RTPPAW
m
kTQ
AWJJkTQ
kT
AJJW
pipe
Dr. G. Mirjalili, Physics Dept. Yazd University
Calculation of the C for Long and usual pipes
sec/81.33
litM
T
l
dCL
sec/2
1038.1 214
2 litpp
l
dCL
Conductance in viscous flow (long pipes)
Conductance in molecular flow (long pipes)
Conductance in usual pipes (molecular flow)
L
Lpipe CC
CCC
0
0
Dr. G. Mirjalili, Physics Dept. Yazd University
Calculation the transmission probability
M
RT
L
dCL
26
3
AM
RTC
20
L
Lpipe CC
CCC
0
0
Ld
C
dL
C
CC
CC L
L
Lpipe 3414311
0
0
0WCC pipe dL
W431
1
Dr. G. Mirjalili, Physics Dept. Yazd University
Conductance
Dr. G. Mirjalili, Physics Dept. Yazd University
SYSTEM
PUMP
C1
C2
Series Conductance
RT = R1 + R2
1 = 1 + 1C1 C2CT
1 = C1 + C2
C1 x C2CT
CT = C1 x C2
C1 + C2
Dr. G. Mirjalili, Physics Dept. Yazd University
Pumping Speed
• Pumping Speed and Conductance are related as follows:
Seff is the effective pumping speed at the chamber
Sp is the pumping speed (capability) of the pump
Ctotal is the total conductance of the system between the chamber and the pump.
TPEFF CSS
111 Sp
CT SEFF
Dr. G. Mirjalili, Physics Dept. Yazd University
pump500 ℓ/s
P1,
P2
connecting tube, conductance
S1
S2
21 S
1
C
1
S
1
L
D12C
3
D = diameter, in cmL = length, in cmC = conductance, in ℓ/s
example 1D = 15 cmL = 20 cmC = 2025 ℓ/sS1= 401 ℓ/s
example 2D = 10 cmL = 20 cmC = 600 ℓ/sS1= 273 ℓ/s
Pump is expensive. Tube is cheap.
@ molecular flowC
Dr. G. Mirjalili, Physics Dept. Yazd University
Sample vacuum situations and calculations
500 l/sec pump + “infinite” conductance
EPS = 500 l/sec
500 l/sec pump + 500 l/sec conductance
1/EPS = 1/500 + 1/500 = 2/500 l/secor EPS = 250 l/sec
two 500 l/sec pumps connected in parallel
EPS = 500 + 500= 1000 l/sec
S1
S2 C
1/S1=1/C+1/S2
Dr. G. Mirjalili, Physics Dept. Yazd University
Sample vacuum situations and calculations
EPS = 100 l/sec
maximum pressure0.03 torr
throughput pressure pumping speed
Problem:If the effective pumping speed from a chamber is 100 l/sec and the chamber pressure must not exceed 0.03 torr, what must the gas flow into (or the throughput out of) the chamber be ?
Solution: maximum throughput = (100 l/sec)(0.03 torr),or 3 torr-liter/second
throughput 3 torr-liter/second
gas flow3 torr-liter/sec
Q=PS
Dr. G. Mirjalili, Physics Dept. Yazd University
Sample vacuum situations and calculations
EPS = 250 l/sec
steady-state pressure410-4 torr
Problem:Suppose the effective pumping speed from a chamber is 250 l/sec and we wish to inject a gas flow of 0.1 torr-liter/second flow of gas into the chamber. What will the steady-state pressure be?
Solution: 0.1 torr-liter/second 250 liter/second
throughput 0.1 torr-liter/second
gas flow0.1 torr-liter/sec
= 410-4 torr
P=Q/S
Dr. G. Mirjalili, Physics Dept. Yazd University
Sample vacuum situations and calculations
EPS = 67 l/sec
chamber pressure1.510-6 torr
Problem:A calibrated N2 leak of 810-3 sccm is attached to a chamber and the measured pressure is 1.5 10-6 torr. What is the effective pumping speed of the chamber in liters/sec?
Solution:810-3 sccm = (8/60)10-3 standard cc/sec
= (8/60)10-6 standard liter/sec = 760(8/60)10-6 torr-liter/sec = 1.01 10-4 torr-liter/sec
We divide by the indicated pressure to get:1.01 10-4 torr-liter/sec 1.5 10-6 torr
throughput 810-3 sccm
N2 flow810-3 sccm
= 67 liters/sec
standard cm3 per minute
“standard” = “atmospheric pressure”
S=Q/p
1.01x10-4/1.5x10-6
Dr. G. Mirjalili, Physics Dept. Yazd University
pump 2100 ℓ/s
pump 1500 ℓ/s
Chamber 1
gas inlet, N2
1x10-3 torr ℓ/s
connecting tube1 cm inner diameter10 cm length
gas inlet, O2
1x10-4 torr ℓ/s
Chamber 2
Estimate:
P(N2) in chamber 1
P(N2) in chamber 2
P(O2) in chamber 1
exercise
Dr. G. Mirjalili, Physics Dept. Yazd University
Conductance Maximum or Minimum?
• Obviously, in most cases, we want to maximize conductance.
• But sometimes, we DO want to limit conductance:– Slow pumping to minimize pressure “shock” to the system.– Throttling to maintain desired pressure in system.
Dr. G. Mirjalili, Physics Dept. Yazd University
Pumping Speed• Pump speed: volume of gas taken in by the pump per unit time • at the pressure of the pump inlet:
• S = dV/dt (in lit/sec), S f(p)
• Pumping Speed: the rate at which a vacuum pump removes gasses from a system.– Also known as volumetric flow rate
• Pumping Speed Units: volume/time– liters/second or ft3/minute (CFM)
• Pumping Speed and Conductance are NOT synonymous– Conductance is a property of
a component in a vacuum system.
– Pumping Speed refers to the flow of gas across a plane in a system.
Dr. G. Mirjalili, Physics Dept. Yazd University
Basic equation of flow and pump speed
T
T
QSPdt
dPV
dtQSPdtVdP
)(S
QP Tu
)}(exp{
)(
0 SVtPP
dtV
S
p
dP
Steady state
If QT= 0
)ln()( 0 PPSVt Re-expressed
The time necessary for the pressure to fall from P0 to P
Dr. G. Mirjalili, Physics Dept. Yazd University
Pump down time
SPt
PV
d
d
S
Vτ
e PP
PV
S
t
P
t/τ-0
d
d
equation for the throughput
exampleV = 1000 ℓS = 500 ℓ /s = 2 severy 2.3, 10 x pressure drop
Why in the real world, it takes much longer from 10-6 torr to 10-7 torr?
Surface outgas
P
t
Dr. G. Mirjalili, Physics Dept. Yazd University
Examples
1:
In a vacuum chamber, V=40 lit, and S=0.5 lit/s. what is the time taken for the pressure to fall from P0=1000 mbar to 1 mbar?.
t= (40/0.5)ln 10 3=552 s= 9 min
2:
If a volume of 1m3 has to be pumped down from 1000 mbar to 10 mbar in 5 min what is the pump speed?
S=(V/t)ln(P0/P)
S=(1000/300)ln(10 2)=900 lit min -1 =5.4 m3 h-1
Dr. G. Mirjalili, Physics Dept. Yazd University
Gas Sources• Outgassing: the natural evolution of
species inside the chamber, at low pressure, contributing to the gas load– Sublimation of solid chamber surfaces– Desorption from the walls of physically
adsorbed molecules– Out-Diffusion of gas that has been
absorbed into the grain boundaries of the metal
– Vaporization of liquids or solids in the chamber with relatively high vapor pressure
Dr. G. Mirjalili, Physics Dept. Yazd University
Backstreaming(1)
•Movement of gases (including pump oil vapor) from pumps into the vacuum chamber. It can be an important issue with diffusion pumps.
•Design of diffusion pumps can make some difference. Placement of a continuous operation cold plate over the diffusion would be the best solution, but it is rarely included in microprobe design.
•Oil diffusion pumps have a long history and are considered by many to be less costly and easier to use in a multiple user facility.
Dr. G. Mirjalili, Physics Dept. Yazd University
Oil back streaming
2
PRESSURE LEVELS: LESS THAN 0.2 mbar
Dr. G. Mirjalili, Physics Dept. Yazd University
Backstreaming(2)
– an ideal pump only removes molecules and does not give any back
– real pumps “regurgitate” some gasses back into the system
• oil from diffusion pumps and rotary vane pumps (draw)
– oil-based pumping systems are designed with ballast, anti-suckback valves, and cold traps to minimize backstreaming
– this is the primary reason for the gradual replacement of oil-based pumps with “dry” pumps
Dr. G. Mirjalili, Physics Dept. Yazd University
Dr. G. Mirjalili, Physics Dept. Yazd University
Pumpdown CurveP
ress
ure
(m
bar
)
Time (sec)
10-11
10 1 10 3 10 5 10 7 10 9 10 11 10 13 10 15 10 17
10+1
10-1
10-3
10-5
10-7
10-9
Volume
Surface Desorption
Diffusion
Permeation
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