Upload
others
View
13
Download
1
Embed Size (px)
Citation preview
Vacuum Science and TechnologyVacuum Science and Technology
! Most CVD , Epitaxy and Plasma processes use vacuum
! Vacuum : < 1 atm = 760 Torr
! ~ 0.1-760 Torr : Rough Vacuum
! ~ 10-4-0.1 Torr : Medium Vacuum
! ~ 10-8- 10-4 Torr : High Vacuum
! < 10-8 Torr : Ultrahigh Vacuum (UHV)
! Base pressure: the lowest pressure the chamber can be pumped down to without any gases flowing.
! Processing pressure (with gases flowing) maybe higher than the “base pressure”
Why doesn’t pressure go to 0?Why doesn’t pressure go to 0?
pump
chamber
virtual leaks
leaks
! Even if gas flow is 0, there are always leaks. They may be very small but they are always there.
! Virtual leaks are leaks that are not associated with transport across the chamber wall but act as a gas source (e.g. , desorption from gas walls, slow leak of gas trapped inside crevices and cracks.)
! Pumps are characterized by pumping speed (liters/second, m3/h, etc.)
! They are called vacuum pumps but they are really compressors.
! Think of pumping speed as volume captured per unit time and displaced out of the chamber. Amount of gas depends on how much gas is inside that volume (Pressure).
Gas flow in
Pumping Speed, Throughput and Flow ratePumping Speed, Throughput and Flow rate
PSQ =Throughput (torr liters/sec)
Pressure (torr) Pumping speed (liters/s)
qRTPV
RTPn
RTPVn ==⇒= &&
ss
s VRTPn && =
Specifying flow rate in std cm3/min (sccm) is equivalent to specifying molecular (or molar) flow rate
0 oC
760 Torr
sccm.s/littorrutemin/molecules.sccm
05791106921 19
=×=
RTnPSQ &==
Residence time and chamber pressureResidence time and chamber pressure
SPQdt
)PV(d R −=−= τ/to
R
ePPdtVS
PdP −=⇒−=
SVR=τResidence time
SPQQQQdt
)PV(dleakleakgas
R −=−+=
Steady state no gas flow SQP leak
min =
Steady state with gas flowS
QQP leakgas +
=
Typical pump down curveTypical pump down curve
“Mechanical” Roughing Pumps“Mechanical” Roughing Pumps
Pumping speedPumping speed
QS
P
Turbomolecular Turbomolecular pump (pump (turbopumpturbopump))
Turbomolecular Turbomolecular pump speed curvespump speed curves
Typical Vacuum ApparatusTypical Vacuum Apparatus
Vacuum SealsVacuum Seals
PumpsPumps
PumpsPumps
ConductanceConductance!A connecting passage such as a tube,
elbow, constriction or expansion impedes the gas flow (induces ∆P) and said to have resistance to flow.
!Analogous to Ohm’s law in electronics: ∆V=IR
!Frequently conductance is used to characterize the ∆P where C=1/Z
!Has same units of pumping speed but it is not the same thing
ZQPP =− 21
21 PPQC−
=
321
32141
343
232
121
1111
111
CCCC
CQQ
CCCPP
CQPP
CQPP
CQPP
t
t
++=
=
++=−
=−=−=−
components connected in series
ConductanceConductance
)inches(L)inches(D)torr(P)s/(C
viscoustube
4
3000=l)inches(L)inches(D)s/(C
moleculartube
3
80=l
Pressure GaugesPressure Gauges
Pressure GaugesPressure Gauges
Thermocouple gauge capacitance manometer Ion Gauge
Flow Control DevicesFlow Control Devices! Mass flow controllers – senses
mass flow via exchanging small amounts of heat with the gas stream.
! Active feedback control. Automatic.
TCmQ p∆&& =
! Needle (or metering) valve –slow turning valve adjusts flow by changing the conductance over many turns. No feedback control. All manual