Combination of a Getter Pump with Turbomolecular Pumps in UHV

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 m/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 HO partial pressure and a reduction of the ultimate pressure from 1 10 to ; \ 1 10 mbar. In particular, because of the sorption characteristics for hydrogen of NEG pumps, the H partial pressure could be 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 tively high exhaust-to-inlet transmission probability [2] and to surface desorption from the inner surfaces of the Turbomolecular pumps (TMPs) are increasingly being pump, the latter being di$cult to reduce as the baking used as ultrahigh vacuum (UHV) pumping systems main- temperature of a TMP never exceeds ca. 1003C [7]. The ly because of their high reliability, low system and oper- reduction of the ultimate partial pressure of H can be ating cost, ease of operation, almost oil-free operating achieved by means of: (i) improvement of the compres- conditions, constant pumping speed, etc [1]. One of the sion ratio of the TMP for H [5}7], (ii) reduction of the major limitations of TMPs is due to their rather low partial pressure of H in the fore vacuum line (e.g. using compression ratio for light gases, in particular for H baking pumps with higher pumping speed [8}9], [2}5]. Hydrogen being the main constituent of the resid- through ballasting [10] or using forepump oils having ual atmosphere in stainless-steel vacuum chambers oper- low H solubility [5]) or (iii) combination of TMPs with ating under UHV conditions, the lowest ultimate other UHV pumping systems (namely, non-evaporable pressure here attainable is mainly determined by the getter (NEG) pumps or titanium sublimation pumps hydrogen partial pressure. Generally speaking, the par- [10]). tial pressures of the residual gases are determined not Aiming at evaluating the combination of a TMP with only by surface outgassing from the walls of the vacuum a NEG pump both for industrial and research applica- chamber but also by the backstreaming from the pump tions, an in-line NEG pump was specially designed to itself (and, for trapped systems, also from the trap) [6]. operate in series with the turbomolecular pump. In fact, Back#ow of H from turbo pumps is due both to a rela- assembling the NEG pump between the TMP and the vacuum chamber, i.e. as a trap, allows to increase both *Corresponding author. the net pumping speed in the chamber and to cope with E-mail address: roberto}[email protected] (R. Gian- hydrogen backstreaming from the TMP, as suggested by nantonio) a previous work [11].

2. Experimental setup and methods Back-di!usion of oils from the rotary pump was pre- vented using the catalytic trap 14 (Pfei!er URB-040). All To evaluate the performances of the TMP/NEG of the cited instrumentation was interfaced to a computer pumping system, the test apparatus was assembled sche- to achieve complete automation (i.e. good repeatability) matically as shown in Fig. 1. It consisted of a stainless- of measurement cycles. steel cylindrical chamber having a volume of ca. 40 l and RGA, pump-down times and ultimate pressure meas- an inner surface area of ca. 6000 cm. A blank #ange was surements were performed with: (i) TMP M directly connected at one side of the cylinder, where a ionization baked by the rotary pump, (ii) TMP M baked by TMP B, gauge (Leybold IE-414), an extractor gauge (Leybold (iii) TMP M in combination with the NEG pump, and IE-514) and a quadrupole mass spectrometer equipped (iv) TMP M baked by TMP B in combination with the with 903 SEM (Balzers QMA-125/QMG-421C) were NEG pump. mounted. The other side of the chamber was connected The following experimental procedure was used: (i) through an 8-in Con#at #ange to the NEG pump, iso- venting of the vacuum chamber in ambient air for 15 min, lated from the main TMP (TMP M) by means of the gate (ii) pumpdown of the system for 15 min (iii) bake-out at valve 9. The main turbomolecular pump was a leybold 2003C (mass spectrometer at 1203C, gate valve at 1003C) turbovac 600 C with grease-lubricated ceramic ball bear- for 20 h; if present, the getter pump was activated (heating ings. Nominal pumping speed for N and H were 620 from room temperature to 5503C in 1 h and heating at and 590 l/s, respectively. Compression ratios for N and 5503C for 1 h) after 18 h of baking, (iv) cooling of the ; H were about 10 and 1.1 10 , respectively. The ex- system to 503C (the chamber was cooled to 503C instead haust of the main pump could be connected either to of room temperature to enhance the hydrogen desorp- a12m/h rotary vane pump (Edwards RV12) or to tion rate thus highlighting the role of the NEG pump), (v) a 50 l/s Pfei!er TMU-064 turbomolecular drag pump pumpdown of the system for 20 h and measurement of (TMP B), featuring pumping speeds for N and H of 53 the pumpdown time to base pressure, (vi) ultimate pres- and 31 l/s, respectively and compression ratios for the sure and RGA measurements, and (vii) isolation of TMP same gases of about 10 and 4;10, respectively. 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

A new NEG pump was suitably developed to work in combination with a turbomolecular pump. The NEG pump structure here (and elsewhere) 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 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 " Fig. 1. Layout of the experimental apparatus. 1 Bayard}Alpert steel nipple having an inner diameter of 150 mm and 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. Filed Italian patent application C 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. In Fig. 3, the sorption characteristics of the pump for Two ends of the NEG elements chain are "xed to a H and CO, measured according to the standard conduc- molybdenum plug located on the cartridge base struc- tance method, are shown. ture. While inserting the cartridge inside the nipple, the plug "ts inside a socket placed at the bottom of the nipple, thus establishing electrical contact be- 4. Experimental results and discussion tween the NEG elements and the external power feed- throughs. The activation of the NEG pump was usually In Fig. 4 typical total pressure pro"les, measured with carried out by means of a DC power supply. An activa- the extractor gauge during pumpdown of the system, are tion temperature of ca. 5503C on the NEG rods could easily be attained by operating the power supply at 40V/11 A. ASTM 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 pressure: 4.0;10\ mbar.

> reported. The steep rise of pressure after 18 h of bake-out Ion current i pro"les, due to hydrogen, are represent- is due to the gas, which is almost H, released during the ed in Fig. 6. As discussed below, the insertion of TMP NEG pump activation step. The sudden change of slope B in series with TMP M reduces the #ux of hydrogen of the three pressure vs. time curves corresponds to the backstreaming from the exhaust of TMP M to the vac- initial cooling of the chamber, i.e. to a reduced outgassing uum chamber, thus halving the "nal hydrogen partial throughout. Fig. 4, clearly shows that with the NEG pressure. A further reduction of the hydrogen partial pump operating in combination with TMP M, a lower pressure could be obtained by using the NEG pump. ultimate pressure (5.3;10\ mbar) could be attained After 43.5 h from the start of the experiments, valve 9 compared with the pressure reached with TMP M was suddenly shut allowing for pressure rate-of-rise (1.9;10\ mbar) and TMP M#TMP B (1.5; measurements. Some of the results drawn from these tests 10\ mbar). Pumpdown times to a reference pressure of are shown in Fig. 7, where the variation of the mass ; \ > 1.9 10 mbar could also be greatly reduced when us- spectrometer ion current i due to hydrogen is plotted. ing the NEG pump (23 h) with respect to those related to A "rst observation will drive the following discussion. the TMP M (40 h) and TMP M#TMP B (26 h) tests. Looking at curve C in Fig. 7, one can clearly see that, > The slight di!erence between the ultimate pressure at- after the sudden shutting of valve 9, i drops from tained with TMP M and the one obtained by backing a steady-state value of ca. 5;10\ A down to ca. TMP M with TMP B is due to a higher compression 2;10\ A. In steady-state conditions, where the usual " ratio for H of the tandem TMP con"guration. relation P & F&/S2 & holds (F& is the net #ow of In Fig. 5 the evolution of the mass spectrometer ion hydrogen into the chamber and S2 & is the total pumping > current i due to water vapour is shown. As expected, speed in the chamber), this abrupt variation of the hydro- the ultimate partial pressure of water in the chamber gen partial pressure can only be due to a sudden reduc- pumped by TMP M baked by TMP B is essentially tion of the hydrogen #ow. As valve 9 separates the line identical to the pressure reached when the exhaust of comprising the chamber and the NEG pump from the TMP M is directly connected to the rotary vane pump, turbomolecular pump(s), this extra #ow of hydrogen can due to a rather high maximum compression ratio of only come from the turbomolecular pump(s) itself. A part TMP M for HO [8, 9]. The use of the NEG pump in of the total amount of hydrogen in the chamber is combination with TMP M results in a net reduction of therefore due to the backstreaming of H from the turbo- the partial pressure of water by almost an order of molecular pump(s). In these conditions, the turbomolecu- magnitude. Similar results were observed for N/CO. lar 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 hydrogen. All of the pumping speeds S appearing in the H ultimate partial pressure attainable in the vacuum Table 1 are e+ective pumping speeds in the vacuum chamber, material balances for hydrogen can be made chamber, a!ected by the "nite conductance of the line around di!erent regions of the experimental apparatus, separating TMP M from the vacuum chamber itself, this indicated by dashed closed lines as R1}R4 in Fig. 8 (sys- line comprising also the NEG pump. Hydrogen partial " > tem with the NEG pump fully passivated, thus represent- pressures are expressed as p& ki , where k is a constant ed without the NEG pump) and Fig. 9 (system with the containing both the calibration factor for the extractor NEG pump fully activated, thus represented with the gauge and the calibration factor for the mass spectrom- NEG pump). The equations originating from these mass eter. Aiming at determining only relative estimates for balances are collected in Table 1. Pressures, #uxes and pumping speeds and #uxes, the explicit value for k will pumping speeds appearing in Table 1 are referred to 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 (HO) vs. time pro"les for di!erent system con"gurations.

the conversion factor k should be considered as a mean streaming #uxes F 1 and F *1) are represented by Eqs. (10) value, averaged over the whole set of experiments, its and (11). Eqs. (10) and (11) are related to the hydrogen $ " # dispersion being 10%. F) F" F* is the (steady- mass balance in region R4, when TMP M is directly state) throughput due to outgassing (F") and to a leak baked by the rotary pump and when TMP M is baked by found to be present in the vacuum line (F*). S+ and S+* are TMP B, respectively (these equations apply also when the apparent pumping speeds of TMP M, when the pump the NEG pump is under operation i.e. also for system is baked by the rotary pump and by TMP B (with or con"gurations C and D). Eqs. (10) and (11) express the 0 without the NEG pump), respectively. S+ is the real well-known relation used by Kruger and Shapiro to pumping speed of TMP M. The relationships de"ning model their experimental single-rotor turbomachine [2]. 0 S+, S+ and S+* (together with the corresponding back- 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.

(herein after called KS equation) reads, using the same conductance), the KS equation can be rewritten as " " & ! & symbols adopted by the two researchers, as: =p Sp S! p S! p. Comparing this equation & !& 0& & & p p, where p is the pressure at the high- with Eq. (10), we can see that S+ S! , S+ S and & & & & & vacuum side of the compressor, p is the pressure at the F 1 S! p (i.e. S+* S and F *1 S! p, if Eq. (11) & & exhaust, and are transmission probabilities and is considered), thus highlighting the dependence of " = is the Ho coe$cient of the pump. As = S/S!, where F 1 (and F *1)onp. Expressions similar to the KS equa- S is the apparent pumping speed of the pump and S! is tion and to Eqs. (10, 11) can also be found elsewhere in the pumping speed of an aperture having the same size of the literature [13]. the free entrance of the pump (i.e. S! is the maximum Under system con"guration C (and D), the NEG theoretical pumping speed of the pump, acting as a pure pump reduces the hydrogen partial pressure p in the 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.

chamber, with respect to the value reached under system whole duration of the experiments, the oil being saturated con"guration A (and B), due to the added pumping speed with hydrogen (the rotary pump oil can thus be regarded " S,#%. As the compression ratio K+ (or K+*) p/p and as a reservoir of hydrogen). This means that the partial the pumping speed S+ (or S+*) of TMP M are strictly pressure of hydrogen at the exhaust of TMP M (and at " correlated, the H partial pressure p K+p at the the exhaust of TMP B) is almost constant throughout the exhaust of TMP M for system con"guration C (and D) experiments. Therefore, a #ux F0 (and a #ux F0*) , due to should be lower than the pressure found for system outgassing of the rotary pump oil, logically repre- con"guration A (and B). Nevertheless, the partial pres- sented as a #ux entering into the system, is introduced sure of hydrogen in equilibrium with hydrogen dissolved in the equations so that the hydrogen partial pressure at inside the rotary pump oil is almost constant during the 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 1, F* 1 backstreaming H #ows, " " recycled by TMP M; F0, F*0 backstreaming H #ows, due to outgassing of the rotary pump oil (non recycled by TMP M); S+, S*+ apparent, 0" " e!ective pumping speeds of TMP M; S+ real, e!ective pumping speed of TMP M; S,#% e!ective pumping speed of the NEG pump. Symbols 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 Mass-balance equations Main results region

Valve 9 open " # 0 ; \ (A) TMP M (R2) P & (F) F 1)/S+ k2 10 (1) S,#%/S+ 10 (12) " ; \ (R1) P & F)/S+ k2 10 (2) S,#%/S*+ 4.5 (13) 0" # 0 (R4) P &S+ P &S+ F 1 (10) S,#%/S+ 4.5 (14) # " # 0 ; \ (B) TMP M TMP B (R2) P & (F) F* 1)/S+ k9 10 (3) S*+/S+ 2.2 (15) " ; \ (R1) P & F)/S*+ k9 10 (4) F0/F) 0.7 (16) 0" # (R4) P &S+ P &S*+ F* 1 (11) F*0/F) 0.2 (17) # " # # 0# ; \ (C) TMP M NEG (R2) P & (F) F 1 F0)/(S+ S,#%) k5 10 (5) F 1/F) 1.2 (18) " # # ; \ (R1) P & (F) F0)/(S+ S,#%) k5 10 (6) F* 1/F) 0 (19) # # " # # 0# ; \ (D) TMP M TMP B NEG (R2) P & (F) F* 1 F*0)/(S+ S,#%) k2 10 (7) " # # ; \ (R1) P & (F) F*0)/(S*+ S,#%) k2 10 (8)

Valve 9 closed (C) TMP M#NEG and # # " ; \ (D) TMP M TMP B NEG (R3) P & F)/S,#% k2 10 (9)

con"gurations A and C (and the same both in system of TMP M). Due to the cylindrical symmetry of the con"gurations B and D). Of course, both hydrogen NEG pump, obtained by arranging the NEG elements streams F 1 (or F *1) and F0 (or F0*) pass through the around the inner walls of the pump cartridge, the perfor- exhaust of TMP M towards the vacuum chamber but, in mances of TMP M were maintained as high as possible the present model, the former should be considered as but the trapping e.ciency of the NEG pump could not be being e!ectively recycled by the turbopump while the optimized. latter should be regarded as just crossing TMP M to be It is worth remarking that an independent estimate for trapped by the NEG pump. the ratio S+* /S+ can be obtained by means of the KS The solution of the set of Eqs. (1)}(9) gives the re- equation. In fact, for any given #ux F of hydrogen pas- sults (12)}(19), also summarized in Table 1. Even if sing through TMP M and its baking pump, we have, in " " the hydrogen partial pressure is greatly reduced by steady-state conditions, that F S+P S P, where chaining TMP M with TMP B, the apparent pump- S+ and S are the (e+ective and apparent) pumping speeds ing speed of TMP M being almost equal to its real for H of TMP M and the baking pump, respectively. pumping speed, a net reduction in hydrogen concentra- Therefore, the compression ratio of TMP M, de"ned as " " tion can be attained only using the NEG pump, exploit- K+ p/p can also be written as K+ S+/S . Generally 0 ing a pumping speed S,#% 4.5S+. However, despite its speaking, S+ depends on K+ in accordance with the KS " " ! impact on the hydrogen ultimate pressure, the NEG equation, written as =+/=+3 S+/S+3 (K+3 K+)/ ! " ! ! pump does not act as a perfect trap. If so, the ultimate (K+3 1) (K+3 S+/S )/(K+3 1) where =+, =+3, pressure attained under system con"guration C should S+3 and K+3 are the Ho coe$cient, the maximum Ho be very close to the one reached under system con"gura- coe$cient, the maximum pumping speed and the max- tion D. On the contrary, a rather high fraction of the imum compression ratio of TMP M, respectively [7]. As < total hydrogen back#ow, represented by both ratios in the present case K+3 1, we have that 1/S+ # F 1/F) and F0/F), pass through the NEG pump, thus 1/S+3 1/(K+3 S ), so that the ratio S+*/S+ can be cal- contributing to the observed value of the hydrogen ulti- culated by simply setting in the above expression the mate pressure. The NEG pump structure described in the nominal pumping speeds of the rotary pump (for S+) and present work was chosen in order to maximize both of TMP B (for S+*). A ratio S+*/S+ 2.2, in accordance the net pumping speed of the NEG pump itself and the with the value (15), is obtained, provided that a value conductance of the line separating TMP M from S 0.5 l/s is used for the rotary pump. The oil of the the vacuum chamber (i.e. the e!ective pumping speed rotary pump being saturated with hydrogen, this "gure, R. Giannantonio et al. / Vacuum 55 (1999) 27}37 37 compared with a nominal pumping speed of 3.3 l/s, seems elements arranged horizontally, thus intercepting back- to be reliable. streaming 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 [1] La!erty JM, editor. Foundations of vacuum science and techno- gas analysis, ultimate pressure and pump-down experi- logy. New York: Wiley, 1998. Chap 9. ments. The use of the NEG pump allows for a reduction [2] Kruger CH, Shapiro AH. Trans Natl Vac Symp 1960;7:6. [3] Bernhardt KH, J Vac Sci Technol A 1983;1:136. of both H partial pressure and total ultimate pressure [4] Becker W. Vacuum 1966; 16:625. greater than that obtainable by the insertion of a second [5] Henning J. Vacuum 1971; 21:523. turbopump on the exhaust line of the primary turbo [6] Santeler DJ. J Vac Sci Technol 1971;8:299. pump. The use of a NEG pump, in combination with [7] O'Hanlon JF. A user's guide to vacuum technology. New York: a turbomolecular pump, seems to be a suitable solution Wiley, 1980. Chap 7 and 11. [8] Ishimaru H. J Vac Sci Technol A 1989;7:2439. to cope with H back-streaming, typical of all TMP- [9] Cho B, Lee S, Chung S. J Vac Sci Technol A 1995;13:2228. based pumping systems (in particular, when TMPs not [10] Bernardini M, Bradaschia C, Pan HB, Pasqualetti A, Torelli G, equipped with molecular-drag stages are used). When Zhang Z. Problem of H back-stream from turbo pumping sys- great amounts of hydrogen are present in the vacuum tems. Paper Presented at the XIV3 National Congress on Vacuum chamber, NEG pump structures having higher trapping Science and Technology, Vicenza (Italy), 5}8 May, 1998. [11] Pozzo A, Bo$to C, Mazza F, Vacuum 1996;47:783. e$ciencies than that observed for the pump model dis- [12] Ho TL. Physics 1932;2:386. cussed in this work, should probably be preferred. Fur- [13] Roth A. Vacuum technology. 2nd ed. Amsterdam: North-Hol- ther experimental work on a NEG pump having NEG land, 1982. Chap 7.