Vacuum 55 (1999) 27}37
Combination of a getter pump with turbomolecular pumps
in UHV applications
Roberto Giannantonio*, Magda Bovisio, Andrea Conte
SAES Getters S.p.A., Viale Italia, 77-20020-Lainate (MI), Italy
Received for publication 13 February 1999
Abstract
This work reports on RGA studies relative to pump-down and ultimate pressure experiments made on a 40 l UHV chamber
with a specially designed non-evaporable getter (NEG) pump operated in series with the main pumping group, comprising a
600 l/s hybrid turbo pump backed by a 12 m3/h rotary pump. Experiments were also made with a 50 l/s turbomolecular drag
pump inserted between the exhaust of the hybrid turbo pump and the intake pipe of the rotary pump. The NEG pump, located
on top of the main turbo pump, was realized using "ve NEG elements arranged around the walls of a cylindrical cartridge
inserted inside a stainless-steel nipple having an inner diameter of 150 mm. A single NEG element, consisting in a E 5 mm,
720 mm long rod, shaped as a sinusoid, was prepared by sintering ca. 20 g of pure titanium on a E 0.5 mm nichrome wire. The NEG
pump allowed for a tenfold reduction of H O partial pressure and a reduction of the ultimate pressure from 1]10~9 to
2
1]10~10 mbar. In particular, because of the sorption characteristics for hydrogen of NEG pumps, the H partial pressure could be
2
signi"cantly reduced by e!ectively dealing with the hydrogen backstreaming, typical of turbo pumps. ( 1999 Elsevier Science Ltd. All
rights reserved.
Keywords: Non-evaporable getter pump; Turbomolecular drag pump; Rotary pump; UHV chamber
1. Introduction
Turbomolecular pumps (TMPs) are increasingly being
used as ultrahigh vacuum (UHV) pumping systems mainly because of their high reliability, low system and operating cost, ease of operation, almost oil-free operating
conditions, constant pumping speed, etc [1]. One of the
major limitations of TMPs is due to their rather low
compression ratio for light gases, in particular for H
2
[2}5]. Hydrogen being the main constituent of the residual atmosphere in stainless-steel vacuum chambers operating under UHV conditions, the lowest ultimate
pressure here attainable is mainly determined by the
hydrogen partial pressure. Generally speaking, the partial pressures of the residual gases are determined not
only by surface outgassing from the walls of the vacuum
chamber but also by the backstreaming from the pump
itself (and, for trapped systems, also from the trap) [6].
Back#ow of H from turbo pumps is due both to a rela2
*Corresponding author.
E-mail address: roberto}giannantonio@saes-group.com (R. Giannantonio)
tively high exhaust-to-inlet transmission probability [2]
and to surface desorption from the inner surfaces of the
pump, the latter being di$cult to reduce as the baking
temperature of a TMP never exceeds ca. 1003C [7]. The
reduction of the ultimate partial pressure of H can be
2
achieved by means of: (i) improvement of the compression ratio of the TMP for H [5}7], (ii) reduction of the
2
partial pressure of H in the fore vacuum line (e.g. using
2
baking pumps with higher pumping speed [8}9],
through ballasting [10] or using forepump oils having
low H solubility [5]) or (iii) combination of TMPs with
2
other UHV pumping systems (namely, non-evaporable
getter (NEG) pumps or titanium sublimation pumps
[10]).
Aiming at evaluating the combination of a TMP with
a NEG pump both for industrial and research applications, an in-line NEG pump was specially designed to
operate in series with the turbomolecular pump. In fact,
assembling the NEG pump between the TMP and the
vacuum chamber, i.e. as a trap, allows to increase both
the net pumping speed in the chamber and to cope with
hydrogen backstreaming from the TMP, as suggested by
a previous work [11].
0042-207X/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 2 - 2 0 7 X ( 9 9 ) 0 0 1 2 0 - 7
28
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
2. Experimental setup and methods
To evaluate the performances of the TMP/NEG
pumping system, the test apparatus was assembled schematically as shown in Fig. 1. It consisted of a stainlesssteel cylindrical chamber having a volume of ca. 40 l and
an inner surface area of ca. 6000 cm2. A blank #ange was
connected at one side of the cylinder, where a ionization
gauge (Leybold IE-414), an extractor gauge (Leybold
IE-514) and a quadrupole mass spectrometer equipped
with 903 SEM (Balzers QMA-125/QMG-421C) were
mounted. The other side of the chamber was connected
through an 8-in Con#at #ange to the NEG pump, isolated from the main TMP (TMP M) by means of the gate
valve 9. The main turbomolecular pump was a leybold
turbovac 600 C with grease-lubricated ceramic ball bearings. Nominal pumping speed for N and H were 620
2
2
and 590 l/s, respectively. Compression ratios for N and
2
H were about 109 and 1.1]103, respectively. The ex2
haust of the main pump could be connected either to
a 12 m3/h rotary vane pump (Edwards RV12) or to
a 50 l/s Pfei!er TMU-064 turbomolecular drag pump
(TMP B), featuring pumping speeds for N and H of 53
2
2
and 31 l/s, respectively and compression ratios for the
same gases of about 109 and 4]103, respectively.
Back-di!usion of oils from the rotary pump was prevented using the catalytic trap 14 (Pfei!er URB-040). All
of the cited instrumentation was interfaced to a computer
to achieve complete automation (i.e. good repeatability)
of measurement cycles.
RGA, pump-down times and ultimate pressure meassurements were performed with: (i) TMP M directly
baked by the rotary pump, (ii) TMP M baked by TMP B,
(iii) TMP M in combination with the NEG pump, and
(iv) TMP M baked by TMP B in combination with the
NEG pump.
The following experimental procedure was used: (i)
venting of the vacuum chamber in ambient air for 15 min,
(ii) pumpdown of the system for 15 min (iii) bake-out at
2003C (mass spectrometer at 1203C, gate valve at 1003C)
for 20 h; if present, the getter pump was activated (heating
from room temperature to 5503C in 1 h and heating at
5503C for 1 h) after 18 h of baking, (iv) cooling of the
system to 503C (the chamber was cooled to 503C instead
of room temperature to enhance the hydrogen desorption rate thus highlighting the role of the NEG pump), (v)
pumpdown of the system for 20 h and measurement of
the pumpdown time to base pressure, (vi) ultimate pressure and RGA measurements, and (vii) isolation of TMP
M from the system (valve 9 was closed) and rate-of-rise
test.
All of the experiments were executed twice to check for
repeatability.
3. In-line NEG pump
Fig. 1. Layout of the experimental apparatus. 1"Bayard}Alpert
gauge; 2"extractor gauge; 3"quadrupole mass spectrometer;
4"vacuum chamber; 5"In-line NEG pump; 6, 10}12, 15, 17}19, 21,
22"valves; 7"micrometric leak valve; 8, 20"Pirani gauges;
9"gate valve; 13"TMP M; 14"catalytic trap; 16"TMP B;
23"rotary vane pump.
A new NEG pump was suitably developed to work in
combination with a turbomolecular pump. The NEG
pump structure here (and elsewhere1) described was optimized in order to operate the pump both as a stand-alone
pump, exploiting its pumping action on the vacuum
chamber, and as a trap for the H back#owing from
2
turbomolecular pumps. In particular, being the Ho coef"cient [12] of a TMP strongly dependent on the pump
inlet conductance, the conductance drop due to the NEG
pump, located on top of the TMP inlet #ange, was
properly minimized.
An assembly of the pump is shown in Fig. 2. A single
NEG element was prepared by sintering ca. 20 g of pure
titanium on a E 0.5 mm nichrome wire. Each NEG
element consists in a E 5 mm, 720 mm long rod, shaped
as a sinusoid. A total of "ve NEG elements, connected in
series, were arranged around the walls of a cylindrical
cartridge, having an outer diameter of 143 mm. The cartridge can easily be inserted inside an AISI 304L stainless
steel nipple having an inner diameter of 150 mm and
1Filed Italian patent application d MI97A 001420.
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
29
Fig. 2. Assemblies of the in-line NEG pump. A, detail of the NEG pump cartridge; B, detail of the pump housing; C, detail of the electrical connections;
D, assembled pump.
mounting at each open side a 150 mm Con#at #ange.
Two ends of the NEG elements chain are "xed to a
molybdenum plug located on the cartridge base structure. While inserting the cartridge inside the nipple,
the plug "ts inside a socket placed at the bottom of
the nipple, thus establishing electrical contact between the NEG elements and the external power feedthroughs. The activation of the NEG pump was usually
carried out by means of a DC power supply. An activation temperature of ca. 5503C on the NEG rods could
easily be attained by operating the power supply at
40V/11 A.
In Fig. 3, the sorption characteristics of the pump for
H and CO, measured according to the standard conduc2
tance method,2 are shown.
4. Experimental results and discussion
In Fig. 4 typical total pressure pro"les, measured with
the extractor gauge during pumpdown of the system, are
2ASTM procedures F798-82.
30
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
Fig. 3. H and CO gettering rates vs. sorbed quantities at 25 and 2003C of the tested in-line NEG pump. Activation conditions: 5503C/2 h. Sorption
2
pressure: 4.0]10~6 mbar.
reported. The steep rise of pressure after 18 h of bake-out
is due to the gas, which is almost H , released during the
2
NEG pump activation step. The sudden change of slope
of the three pressure vs. time curves corresponds to the
initial cooling of the chamber, i.e. to a reduced outgassing
throughout. Fig. 4, clearly shows that with the NEG
pump operating in combination with TMP M, a lower
ultimate pressure (5.3]10~10 mbar) could be attained
compared with the pressure reached with TMP M
(1.9]10~9 mbar) and TMP M#TMP B (1.5]
10~9 mbar). Pumpdown times to a reference pressure of
1.9]10~9 mbar could also be greatly reduced when using the NEG pump (23 h) with respect to those related to
the TMP M (40 h) and TMP M#TMP B (26 h) tests.
The slight di!erence between the ultimate pressure attained with TMP M and the one obtained by backing
TMP M with TMP B is due to a higher compression
ratio for H of the tandem TMP con"guration.
2
In Fig. 5 the evolution of the mass spectrometer ion
current i` due to water vapour is shown. As expected,
18
the ultimate partial pressure of water in the chamber
pumped by TMP M baked by TMP B is essentially
identical to the pressure reached when the exhaust of
TMP M is directly connected to the rotary vane pump,
due to a rather high maximum compression ratio of
TMP M for H O [8, 9]. The use of the NEG pump in
2
combination with TMP M results in a net reduction of
the partial pressure of water by almost an order of
magnitude. Similar results were observed for N /CO.
2
Ion current i` pro"les, due to hydrogen, are represent2
ed in Fig. 6. As discussed below, the insertion of TMP
B in series with TMP M reduces the #ux of hydrogen
backstreaming from the exhaust of TMP M to the vacuum chamber, thus halving the "nal hydrogen partial
pressure. A further reduction of the hydrogen partial
pressure could be obtained by using the NEG pump.
After 43.5 h from the start of the experiments, valve 9
was suddenly shut allowing for pressure rate-of-rise
measurements. Some of the results drawn from these tests
are shown in Fig. 7, where the variation of the mass
spectrometer ion current i` due to hydrogen is plotted.
2
A "rst observation will drive the following discussion.
Looking at curve C in Fig. 7, one can clearly see that,
after the sudden shutting of valve 9, i` drops from
2
a steady-state value of ca. 5]10~12 A down to ca.
2]10~12 A. In steady-state conditions, where the usual
relation P
"F /S
holds (F is the net #ow of
6-5,H
H T,H
H
hydrogen into the chamber and S is the total pumping
T,H
speed in the chamber), this abrupt variation of the hydrogen partial pressure can only be due to a sudden reduction of the hydrogen #ow. As valve 9 separates the line
comprising the chamber and the NEG pump from the
turbomolecular pump(s), this extra #ow of hydrogen can
only come from the turbomolecular pump(s) itself. A part
of the total amount of hydrogen in the chamber is
therefore due to the backstreaming of H from the turbo2
molecular pump(s). In these conditions, the turbomolecular pump acts as a source of hydrogen instead of acting as
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
31
Fig. 4. Total pressure vs. time pro"les for different system con"gurations.
a sink. To quantify the in#uence of H backstreaming on
2
the H ultimate partial pressure attainable in the vacuum
2
chamber, material balances for hydrogen can be made
around di!erent regions of the experimental apparatus,
indicated by dashed closed lines as R1}R4 in Fig. 8 (system with the NEG pump fully passivated, thus represented without the NEG pump) and Fig. 9 (system with the
NEG pump fully activated, thus represented with the
NEG pump). The equations originating from these mass
balances are collected in Table 1. Pressures, #uxes and
pumping speeds appearing in Table 1 are referred to
hydrogen. All of the pumping speeds S/ appearing in
.
Table 1 are e+ective pumping speeds in the vacuum
chamber, a!ected by the "nite conductance of the line
separating TMP M from the vacuum chamber itself, this
line comprising also the NEG pump. Hydrogen partial
pressures are expressed as p "ki`, where k is a constant
2
H
containing both the calibration factor for the extractor
gauge and the calibration factor for the mass spectrometer. Aiming at determining only relative estimates for
pumping speeds and #uxes, the explicit value for k will
not be reported here. It is however worth noticing that
32
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
Fig. 5. Mass spectrometer ion current i` (H O) vs. time pro"les for di!erent system con"gurations.
18 2
the conversion factor k should be considered as a mean
value, averaged over the whole set of experiments, its
dispersion being $10%. F "F #F is the (steadyK
D
L
state) throughput due to outgassing (F ) and to a leak
D
found to be present in the vacuum line (F ). S and S* are
M
L M
the apparent pumping speeds of TMP M, when the pump
is baked by the rotary pump and by TMP B (with or
without the NEG pump), respectively. SR is the real
M
pumping speed of TMP M. The relationships de"ning
SR , S and S* (together with the corresponding backM
M M
streaming #uxes F and F* ) are represented by Eqs. (10)
BS
BS
and (11). Eqs. (10) and (11) are related to the hydrogen
mass balance in region R4, when TMP M is directly
baked by the rotary pump and when TMP M is baked by
TMP B, respectively (these equations apply also when
the NEG pump is under operation i.e. also for system
con"gurations C and D). Eqs. (10) and (11) express the
well-known relation used by Kruger and Shapiro to
model their experimental single-rotor turbomachine [2].
The mass-balance equation used by Kruger and Shapiro
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
33
Fig. 6. Mass spectrometer ion current i` (H ) vs. time pro"les for di!erent system con"gurations.
2
2
(herein after called KS equation) reads, using the same
symbols adopted by the two researchers, as: =p "
1
& p !& p , where p is the pressure at the high12 1
21 2
1
vacuum side of the compressor, p is the pressure at the
2
exhaust, & and & are transmission probabilities and
12
21
= is the Ho coe$cient of the pump. As ="S/S , where
C
S is the apparent pumping speed of the pump and S is
C
the pumping speed of an aperture having the same size of
the free entrance of the pump (i.e. S is the maximum
C
theoretical pumping speed of the pump, acting as a pure
conductance), the KS equation can be rewritten as
Sp "S & p !S & p . Comparing this equation
1
C 12 1
C 21 2
with Eq. (10), we can see that SR &S & , S &S and
M
C 12 M
F &S & p (i.e. S* &S and F* &S & p , if Eq. (11)
BS
C 21 2
M
BS
C 21 2
is considered), thus highlighting the dependence of
F (and F* ) on p . Expressions similar to the KS equaBS
2
BS
tion and to Eqs. (10, 11) can also be found elsewhere in
the literature [13].
Under system con"guration C (and D), the NEG
pump reduces the hydrogen partial pressure p in the
1
34
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
Fig. 7. Mass spectrometer ion current i` (H ) vs. time pro"les during rate-of-rise tests for di!erent system con"gurations.
2
2
chamber, with respect to the value reached under system
con"guration A (and B), due to the added pumping speed
S . As the compression ratio K (or K* )"p /p and
M
2 1
NEG
M
the pumping speed S (or S* ) of TMP M are strictly
M
M
correlated, the H partial pressure p "K p at the
2
2
M 1
exhaust of TMP M for system con"guration C (and D)
should be lower than the pressure found for system
con"guration A (and B). Nevertheless, the partial pressure of hydrogen in equilibrium with hydrogen dissolved
inside the rotary pump oil is almost constant during the
whole duration of the experiments, the oil being saturated
with hydrogen (the rotary pump oil can thus be regarded
as a reservoir of hydrogen). This means that the partial
pressure of hydrogen at the exhaust of TMP M (and at
the exhaust of TMP B) is almost constant throughout the
experiments. Therefore, a #ux F (and a #ux F*) , due to
R
R
outgassing of the rotary pump oil, logically represented as a #ux entering into the system, is introduced
in the equations so that the hydrogen partial pressure at
the exhaust of TMP M is the same both in system
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
35
Fig. 8. Layout of the system con"gurations without the NEG pump (NEG pump fully passivated): TMP M (A) and TMP M#TMP B (B). Mass
balances around system regions R1, R2 and R4, surrounded by dashed closed lines, give rise to the equations in Table 1.
Fig. 9. Layout of the system con"gurations with the NEG pump (NEG pump fully activated): TMP M#NEG (A) and TMP M#TMP B
#NEG (B). Mass balances around system regions R1}R4, surrounded by dashed closed lines, give rise to the equations in Table 1.
36
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
Table 1
Equations derived from hydrogen mass-balances around regions R1}R4 de"ned in Figs. 8 and 9. Eqs. (12)}(19) are the solution of the set of Eqs. (1)}(9).
F "F #F (F "H #ow due to outgassing of vacuum chamber, manifolds, etc.; F "H #ow due to leaks); F , F* "backstreaming H #ows,
K
D
L D
2
L
2
BS BS
2
recycled by TMP M; F , F*"backstreaming H #ows, due to outgassing of the rotary pump oil (non recycled by TMP M); S , S* "apparent,
R R
2
M M
e!ective pumping speeds of TMP M; SR "real, e!ective pumping speed of TMP M; S "e!ective pumping speed of the NEG pump. Symbols
M
NEG
containing the asterisks are related to the series combination TMP M#TMP B. Symbols without the asterisks are related to TMP M directly baked
by the rotary pump
System con"guration
Mass-balance
region
Mass-balance equations
Main results
Valve 9 open
(A) TMP M
(B) TMP M#TMP B
(C) TMP M#NEG
(D) TMP M#TMP B#NEG
(R2)
(R1)
(R4)
(R2)
(R1)
(R4)
(R2)
(R1)
(R2)
(R1)
P "(F #F )/SR +k2]10~11
6-5,H
K
BS M
P "F /S +k2]10~11
6-5,H
K M
P SR "P S #F
6-5,H M
6-5,H M
BS
P "(F #F* )/SR +k9]10~12
6-5,H
K
BS M
P "F /S* +k9]10~12
6-5,H
K M
P SR "P S* #F*
6-5,H M
6-5,H M
BS
P "(F #F #F )/(SR #S )+k5]10~12
6-5,H
K
BS
R M
NEG
P "(F #F )/(S #S )+k5]10~12
6-5,H
K
R M
NEG
P "(F #F* #F*)/(SR #S )+k2]10~12
6-5,H
K
BS
R M
NEG
P "(F #F*)/(S* #S )+k2]10~12
6-5,H
K
R M
NEG
(1)
(2)
(10)
(3)
(4)
(11)
(5)
(6)
(7)
(8)
S /S +10
NEG M
S /S* +4.5
NEG M
S /SR +4.5
NEG M
S* /S +2.2
M M
F /F +0.7
R K
F*/F +0.2
R K
F /F +1.2
BS K
F* /F +0
BS K
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
Valve 9 closed
(C) TMP M#NEG and
(D) TMP M#TMP B#NEG
(R3)
P "F /S +k2]10~12
6-5,H
K NEG
con"gurations A and C (and the same both in system
con"gurations B and D). Of course, both hydrogen
streams F (or F* ) and F (or F*) pass through the
R
BS
R
BS
exhaust of TMP M towards the vacuum chamber but, in
the present model, the former should be considered as
being e!ectively recycled by the turbopump while the
latter should be regarded as just crossing TMP M to be
trapped by the NEG pump.
The solution of the set of Eqs. (1)}(9) gives the results (12)}(19), also summarized in Table 1. Even if
the hydrogen partial pressure is greatly reduced by
chaining TMP M with TMP B, the apparent pumping speed of TMP M being almost equal to its real
pumping speed, a net reduction in hydrogen concentration can be attained only using the NEG pump, exploiting a pumping speed S +4.5SR . However, despite its
M
NEG
impact on the hydrogen ultimate pressure, the NEG
pump does not act as a perfect trap. If so, the ultimate
pressure attained under system con"guration C should
be very close to the one reached under system con"guration D. On the contrary, a rather high fraction of the
total hydrogen back#ow, represented by both ratios
F /F and F /F , pass through the NEG pump, thus
BS K
R K
contributing to the observed value of the hydrogen ultimate pressure. The NEG pump structure described in the
present work was chosen in order to maximize both
the net pumping speed of the NEG pump itself and the
conductance of the line separating TMP M from
the vacuum chamber (i.e. the e!ective pumping speed
(9)
of TMP M). Due to the cylindrical symmetry of the
NEG pump, obtained by arranging the NEG elements
around the inner walls of the pump cartridge, the performances of TMP M were maintained as high as possible
but the trapping e.ciency of the NEG pump could not be
optimized.
It is worth remarking that an independent estimate for
the ratio S* /S can be obtained by means of the KS
M M
equation. In fact, for any given #ux F of hydrogen passing through TMP M and its baking pump, we have, in
steady-state conditions, that F"S P "S P , where
M 1
B 2
S and S are the (e+ective and apparent) pumping speeds
M
B
for H of TMP M and the baking pump, respectively.
2
Therefore, the compression ratio of TMP M, de"ned as
K "p /p can also be written as K "S /S . Generally
M
2 1
M
M B
speaking, S depends on K in accordance with the KS
M
M
equation, written as = /=3 "S /S3 "(K3 !K )/
M
M
M M
M M
(K3 !1)"(K3 !S /S )/(K3 !1) where = , =3,
M
M
M
M
M B
M
S3 and K3 are the Ho coe$cient, the maximum Ho
M
M
coe$cient, the maximum pumping speed and the maximum compression ratio of TMP M, respectively [7]. As
in the present case K3 <1, we have that 1/S +
M
M
1/S3 #1/(K3 S ), so that the ratio S* /S can be calM M
M B
M
culated by simply setting in the above expression the
nominal pumping speeds of the rotary pump (for S ) and
M
of TMP B (for S* ). A ratio S* /S +2.2, in accordance
M M
M
with the value (15), is obtained, provided that a value
S +0.5 l/s is used for the rotary pump. The oil of the
B
rotary pump being saturated with hydrogen, this "gure,
R. Giannantonio et al. / Vacuum 55 (1999) 27}37
compared with a nominal pumping speed of 3.3 l/s, seems
to be reliable.
37
elements arranged horizontally, thus intercepting backstreaming hydrogen, are now running.
5. Conclusions
References
The combination of a specially designed in-line NEG
pump with a TMP was evaluated by means of residual
gas analysis, ultimate pressure and pump-down experiments. The use of the NEG pump allows for a reduction
of both H partial pressure and total ultimate pressure
2
greater than that obtainable by the insertion of a second
turbopump on the exhaust line of the primary turbo
pump. The use of a NEG pump, in combination with
a turbomolecular pump, seems to be a suitable solution
to cope with H back-streaming, typical of all TMP2
based pumping systems (in particular, when TMPs not
equipped with molecular-drag stages are used). When
great amounts of hydrogen are present in the vacuum
chamber, NEG pump structures having higher trapping
e$ciencies than that observed for the pump model discussed in this work, should probably be preferred. Further experimental work on a NEG pump having NEG
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