Operation of cryogenic vacuum pumps in severe environmentsP. A. Lessard Citation: Journal of Vacuum Science & Technology A 8, 2874 (1990); doi: 10.1116/1.576641 View online: http://dx.doi.org/10.1116/1.576641 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/8/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Controlled formation of condensed frost layers in cryogenic high vacuum pumps J. Vac. Sci. Technol. A 28, 925 (2010); 10.1116/1.3275748 Cryogenic Subsystem to Provide for Nominal Operation and Fast Regeneration of the ITER Primary Cryosorption Vacuum Pumps AIP Conf. Proc. 710, 184 (2004); 10.1063/1.1774681 Thermal anchors for electrical leads to cryogenic experiments in an ultrahighvacuum environment Rev. Sci. Instrum. 55, 1714 (1984); 10.1063/1.1137608 Operation and maintenance of a diffusionpumped vacuum system J. Vac. Sci. Technol. 16, 71 (1979); 10.1116/1.569870 The Use of Diffusion Pumps for Obtaining Ultraclean Vacuum Environments J. Vac. Sci. Technol. 8, 299 (1971); 10.1116/1.1316316

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Operation of cryogenic vacuum pumps in severe environments P. A. Lessard CTI-CR YOGENICS. Waltham. Massachusetts 02254

(Received 18 August 1989; accepted 4 September 1989)

Because of their high speed, clean vacuum environment, and life cycle cost advantages, cryogenic vacuum pumps have become the pump of choice in many vacuum processes. Since they are concentrating agents rather than throughput pumps, they can pose a potential hazard in some environments. The potentially dangerous environments fall into three categories: first, where severely reactive agents are used as process gases and are cryopumped in significant quantities, second, where hydrogen is used and pumped in large quantities; and third, where hazardous and unsuspected agents are produced by the process itself. Each class of severe environment is discussed and evaluated ii'om a system safety viewpoint.

I. INTRODUCTION

Cryogenic vacuum pumps (Fig. 1) have become the high vacuum pump of choice for many processes because of their minimal life cycle cost, clean vacuum environment, and ease of use. Since the general operating principle is the freezing of gas species in a closed system, they capture and concentrate any process or residual gas. When reactive or dangerous spe­cies are used as process gases or generated by the process, this concentration may present safety problems. Since the cryogenically cooled surfaces of a cryopump are very cold, between 10-100 K, there is little danger during operation. However, periodically the pump must be warmed and flushed with a neutral species. During this regeneration pro­cess, the concentrated species evolve in a relatively short time and must be handled properly.

There are three classes of severe en vironments or problem areas differing principally in the degree of user awareness, The first kind of severe environment is the use of reactive or dangerous species as process gas, for example in reactive etching. User awareness is high, Le" reactive/explosive/ toxic dangers are known. The second general problem area is hydrogen. All high vacuum systems have water which will dissociate into hydrogen and oxygen. Some users use hydro­gen as a process gas; ion implanters in particular generate large amounts of hydrogen in the process. User awareness is moderate. The third, and most pernicious, problem area in­volves the generation of ozone and its subsequent detonation from the liquid state during regeneration. Users are usually unaware of ozone's presence until after a problem.

II. DISCUSSION

A.Class I

As shown in Table I and Ref. 1, there are a large number of toxic hazardous materials used in vacuum systems; they fall into two broad categories: heavy metals and organics. The heavy metals, e.g., antimony, arsenic, selenium, lead, are pri­marily used in optical coatings and ion implanters. Organics such as chlorinated hydrocarbons and halogen compounds are used extensively in plasma etching, reactive ion etching, and chemical vapor deposition (CVD).

The danger associated with the heavy metals is that they are generally toxic and/or carcinogenic. Although cryo­pumps have been used in these environments, users must

recognize that the pump itself represents a hazard after use. On the other hand, eryopumps are generally not suited to pump the extremely reactive and hazardous gases used in etching or CVD processes, especially where high flow rates and high pressures are necessary. The concentration and eventual release of these species pose a potentially severe problem.

B. Class"

Cryopump manufacturers have focused historically on hydrogen as a safety concern for several reasons.

(a) Its presence is Ubiquitous. All vacuum systems op­erating above ~ 10 - 1] Torr have water present on the

GAS FLOW FROM USER'S VACUUM SYSTEM INTO INLET or CRYOPU\lP

++HHHf

FIG. L Cryopump cutaway.

MOUNTING FLANG!

eOK CCHOENSING ARRAY

15K ARRAY

- 60 K RADIATION SHIELO

VACUUM VESSEL

COlO-HEAO CYLINDER

r GAS RETURN

/

/ CONNECTION

'l GAS SUPPLY CONNEcnON

DRIVE-UNiT OISPlACER ASSEMBLY

ACCESSORY PORT CONNECTION

2874 J. Vac. Sci. Technol. A 8 (3), May/Jun 1990 0734-2101/90/032874-03$01.00 @ 1990 American Vacuum Society 2874

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2875 P. A. Lessard: Cryogenic vacuum pumps In severe environments 2375

TABLE I. Typical process gases.

Name Formula Process"

Ammonia NH, D

Arsine AsH, I,D

Boron trichloride BCI, I, E

Boron trifluoride BFI, I, E

Carbon/chlorine/fluorine CCl.,CF4 ,CC12 ,F2 E mixes Chlorine el2 E

Diborane B2H2 D

Dichlorosilane H 2 SiCl, D

Hydrogen bromide HBr E Hydrogen chloride HCI E Hydrogen selenide H,Se Phosphine PH, I, D

Silane SiH. D,E Silicon tetrachloride SiCl. D Sulfur hexafluoride SFo E

Trichlorosilane SiHCI, D

aD = deposition; E = etch; and I =. ion implant.

chamber walls which readily dissociates into H; and O2 • This is particularly true jf a source of high energy is present, Le., nearly always. (b) Hydrogen and oxygen require very little energy to ignite.2

(c) Hydrogen and oxygen combine with significant en­ergy release. 2

The presence of hydrogen and oxygen (or any other oxi­dant) is a potential source of explosion during regeneration of a cryopump. In a fortunate turn of events, this potential problem can be addressed by the cryopump manutllcturer ill a straightforward manner.

Hydrogen is one ofthree species (helium and neon being the others) that does not condense appreciably at normal cryopump temperatures. A suitable adsorbent, usually char­coal, must be provided at the coldest temperature achievable by the cryopump in order to cryosorb hydrogen. 3 By limit­ing the amount of charcoal present in the pump, the manu­facturer can limit the hydrogen in the pump and ensure that a predetermined safe pressure limit cannot be exceeded un­der reasonable "worst case" conditions, At CTI-Cryogenics, for example, the hydrogen capacity is limited so that if igni­tion of a room temperature stoichiometric mixture of hydro­gen and oxygen should occur, the resultant overpressure would not cause rupture of the cryopump vessel with an appropriate safety factor.

While limiting the hydrogen pumping is straightfor­ward, determining the safe overpressure limit is not. It is vital to determine conclusively whether detonation will oc­cur if the H 2 /02 is ignited. Compared to defiagration, or burning, in which the ratio of final to initial pressure might be 6-8: 1 or so, a detonation will have a final to initial pres­sure ratio of as much as 20: 1. The transformation from defla­gration to detonation is highly dependent on initial condi­tions and geometry. Another vital factor is refrigerator and

J. Vac. Sci. Techno!, A, Vol. 8, No.3, May/Jun 1990

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balling arrangement. A conventional eryopump and a fiat cryopump have significantly different dynamic loading fae­tors4 hecause a much different amount of internal space is filled with structure.

Therefore, while hydrostatic testing can determine a safe pressure limit, the actual overpressure that could be re­alized in a hydrogen-oxygen burn is a strong function of detonation tendencies of a particular design and the dynamic loading factor of that design. Shown in Table II are the re­sults of extensive explosion testing for two arrangements, straight (CT-8) and flat (CT-8F), of the most popular size of cryopump, 6 in. ANSI flange.

c. Class ill

A very small number of users have experienced what is almost certainly ozone detonations in eryopumps during re­generation. Of the few incidents identified, all have been sputtcrers using high oxygen partial pressures and rfplasma excitation. Several had operated with no problem for months before making some change to the operating parameters or sputter chamber geometry. The sequence of events is com­monly: routine shutdown of the pump for regeneration, with or without purge flow (a neutral gas introduced to the cold pump as warm-up aid and diluent); a waiting period ofsev­era! minutes; a series of loud pops or bangs as the second stage temperature reaches 60-65 K, building in number and intensity, then going away; and the clearly evident smell of ozone exiting the relief valve.

Disassembly of the pump typically shows bent and man­gled arrays but no evidence of destruction of charcoal or the epoxy which bonds the charcoal to the second stage array. There is some evidence, namely spotting and bend patterns, to indicate the explosions arc initiated between the bottom of the radiation shield and the vacuum vessel. The severity of the incidents ranges from a barely noticeable popping sound to actual physical damage to the vacuum vessel.

Although the extremely limited number of incidents and the lack of instrumentation and data allow no definite scc­nario to be identified, examination of available anecdotal ancl physical evidence yields the following most likely chain of events: the rapidly varying rf excitation mimics a commer­cial ozonator and produces a relatively large fraction of ozone from the oxygen process gas; the ozone/oxygen

TABLE n. Explosion test results, 6 in. ANSI flange cryopump.

Explosion test Hydrogen capacity Hydrogen Peak pressure

Model limita quantity" following ignition

CT-8 12 std. liters 12 std. liters 260 psia 18 std. liters 500psia 24 std. liters 640 psia"

CT-8F 8 std. liters 8 std. liters 260 psia 12 std. liters 430 psia 14 std. liters 595 psia"

a Limited by amount of charcoal on low temperature stage of cryopump. b Stoichiometric mix of hydrogen and oxygen. e Resulted in permanent vessel deformation but not rupture.

• ....•• ? ••.• - •.•.• ".-.-••.• -........... ' •••••••••••••.•• ,:-. •• - •.•.• -.• - •• -.............. ;-•••••. ,.~" •.•.• - ...... -.,' ••••• ' ............................... -••••.•••••.. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.247.166.234 On: Sun, 23 Nov 2014 16:15:05

2876 P. A. Lessard: Cryogenic vacuum pumps in severe environments 2876

T ABLE III. Ozone action items.

Design for 0, duty (roughing pumps, scals, etc.)

Minimize oxygen feed flow Regenerate as often as possible--automate Change rf power Carefully evaluate all chamber geometry changes Change to alternate pump means

(03 /02 ) freezes on the second stage arrays of the cryo­pump; upon regeneration, the 0, /02 ice melts and separates into two layers, one of which is rich in ozone" ; the OJ /02

drips through the radiation shield purge tube penetration (Fig. 1); and the ozone detonates/rom the liquid state, i.e., purge gas as a diluent cannot interfere with process.

Table III lists possible action items. The main point is that the user must minimize O} production because ozone inter­acts negatively with all other pump types, e.g., by attacking the working fluid or lubricant. Note, too, that oxygen alone supports combustion vigorously and appropriate handling tcchniques must be used, in particular allow no hydrocarbon in a system using oxygen.

J. Vac. Sci. Techno!. A, Vol. S, No.3, May/Jun 1990

III. SUMMARY

Although cryogenic vacuum pumps have found wide ac­ceptance in coating and semiconductor manufacturing be­cause of their performance and cost advantages, there are severe environments where their use must be carefully evalu­ated. When used to pump severely reactive gases or heavy metals, the appropriate safety problems must be carefully evaluated by the system designer; there are some processes that should not use cryopumps. When pumping relatively large amounts of hydrogen, appropriate care (e.g., proper purge flow and venting) must be used, but manufacturers minimize the risk by limiting the pump's capacity for hydro­gen. Finally, a small number of reactive sputterers using rf excitation have produced ozone which is particularly dan­gerous if cryopumped and not regenerated frequently.

'1. F. O'Hanlon and D. B. Fraser, J. Vac. Sci. Techno!' A 6,1226 (1988). 2 B. Lewis and G. von Elbe, Combustion, Flames, and Explosions of Gases

(Academic, New York, 1961). J P. Lessard, J. Vac. Sci. Techno!. A 7,2373 (1989). 4 J. Biggs, Introduction to Structure Dynamics (McGraw-HilI, New York,

1964). 5 Kirk-Othmer, Encyclopedia of Chemical Technology (Wiley, New York,

1978), Vol. 14.

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