Vacuum/volume 44/numbers 5-7/pages 701 to 703/1993 0042-207X/9356 00+ 00 printed m Great Bntam © 1993 Pergamon Press Ltd

Efficient anti-corrosive protection for turbomolecular pumps, the inverted dynamic seal eliminates the purge gas line connection Ik M a t h i e u and J M G r u f f a t , ALCATEL CIT, Industrms Division, Vacuum Technology Dtviston, ~8 avenue de Brogny-BP 69-74009 Annecy, France

The pumpmg of chlorinated and fluorme gases exposes the magnettc bearmgs, the ball bearings and the motor of 1urbomolecular pumps to corrosion The current protectton systems use a neutral gas flow to exhaust the corrostve ~ases from reachmg the bearings The dynamtc seal we tested operates upon the Holweck molecular pump basts A 150 Pa pressure drop m produced across the bearmg chamber The gas parttal pressure m consequently reduced 'Do 10 -9 Pa Its efhcmncy is comparable to a N2 purge of 6x 10 -2 Pa m 3 s -1 The inverted dynamtc seal ts a stmple land rehable built-in anti-corrostve system

1. Introduction

8race the introduction of dry etching (RIE), turbomolecular .pumps have become widely used in the field of semiconductor )manufactunng In this type of process, the use of ehlonne and i§uonne products mvolves many combinations of corroswe gases, for example

]For alumlmum etchmg ~ 2

- - f rom CC14 ~ CCI4, CI2, AICI3, C~C16

For silicon oxide etching

- - f rom CHF3+O2 -* F, F2, HF, COF,_, S1F4 - - f rom C2F 6 --~ CO2, COF2, F, F2, S1F4 - - f rom C3F 8 --~ CO, COF/, F, F 2, SIF4

For polycrystalhne slhcon etching

- - f rom C F 4 + O 2 ~ F2, CO2, COF2, S1F4, F, O - - f rom SF 6 --~ F, F2, SF4, S, S1F4

The mare problem encountered with turbomolecular pumps ~s the resistance of conventional ball beanngs or even magneUc =uspens~on bearings to corrosion In order to prevent the turbo- molecular pumps from being damaged within a few hours, it was necessary to develop special pumps known as CP type {Corrosion Proof) In these pumps, a neutral gas flow is rejected into the chamber containing bearings and estabhshes a barrier against the backflow of reactwe substances produced dunng the process This reduces corrosion significantly 3'4

The neutral gas flow purge system has proved to be effectwe but has a number of &sadvantages

~ t h e ultimate pressure of the pump is reduced by approximately 2 decades, - - i t is necessary to provide a source of dry mtrogen, - - I t ~s necessary to connect a neutral gas hne w~th pressure regulation and controlled valves, - - a n extra gas is introduced into the process which may cause pollution or change the initial mixture,

- - i t is necessary to have a pnmary pump with a higher flow rate in order to pump the purge gas flow

The aim of this study is to compare the use of the purge with a built-in protection system the Inverted Dynamic Seal which eliminates these constraints

2. Experiments

2.1. Summary of the theory of the dynamic seal. The concept of the dynamic seal as applied to vacuum technology was presented by Maunce m Kyoto in 19745 in accordance with the pnnciple of the molecular pump discovered by Gaede m 19126.7 and indus- trialized by Holweck in 19238,9

It should be noted that the dynamic seal (Figure 1) is essentially a cyhndncal multi-threaded screw (A) which turns inside a smooth cyllndncal wall (B) The rotating movement and the small clearance between walls creates the pressure dtfferentml which can be used either to create a dynamic seal or to pump a volume

2.2. Description of the dynamic seal designed for turbomolecular pumps. Because of the operational clearances reqmred" for the industrial manufacture of turbomolecular pumps with ball bear- Ings, we have calculated that the gas was in the wscous state in the dynamic seal The dynamic seal was therefore designed with constant, shallow threads m order to be able to adapt to a laminar gas flow The seal is included in a turbomolecular pump (Figure 1) and is used to pump the volume containing the beanngs

2.3. Experimental assembly. The same assembly was used to com- pare the efficiency of a purge of neutral gas and the inverted dynamic seal (Figure 2) In both cases, the internal volume of the pump located at the beanngs and the motor is connected to a measurement dome where the analyser is located The measure- ment dome is itself pumped by another turbomolecular pump so that the gas analyser filament can be activated To avoid any backstreaming, each turbomolecular pump has its own primary

701

L Mathteu andJ M Gruffat Inverted dynamic seal

IST

Figure 1. The reverted dynamic seal is a bmlt-m system easy to integrate

p u m p A sampling valve is posi t ioned between the p u m p being tested and the measurement dome so as no t to affect the pressure at the bearings and the m o t o r The part ial gas pressure values found at the bearings are compared for the same inlet pressure and when pumping the same gas

3. Results and discussions

The par tml pressure values should not be considered as absolute values as they are obta ined in the measur ing dome and not exactly inside the bear ing volume However, as the measurement c o n d m o n s are the same m bo th cases, it is possible to compare the par t ia l pressure values for the same gas in order to determine the relative efficiency of each of the systems purge, dynamic seal or s t andard p u m p wi thout pro tec t ion Three gases He, Ar and SF 6 were tested (Figure 3)

I OOE-O6 ~c

E I OOE-07

8

~ - I OOE-08

(3-

m I OOE-O!

E I OOE-K

_o 4-'

• • • P u m p w i t h i n v e r t e d d y n ( ] m l c s e a l

D ~ o P u m p w i t h N i t r o g e n p u r g e He

B ~ o - - ~ 3 . - . ~ A r

= ~ u ~

He

"~ '~e S F 6

IOOE- I I I I I I OOE-O5 I OOE-O4 I OOE-05 [ OOE-02 I OOE-OI

I n L e t p ressure ( m b e r )

Figure 3. Residual partial pressure measured m the dome for 1tc, Ar SF, wdh the dynamic seal compared to a 6 × 10 ~" Pa m ~ s ' mtrogen purge

For SF6, the Inverted dynamic seal presents the same efficiency as the purge

For argon, the inverted dynamic seal shows performances equally efficient to the purge up to 10 4 mbal inlet pressure Above this value, the purge allows us to reach a lower argon partial pressure For helium, the purge presents a better efficiency whatever the Inlet pressure range

In addmon , it should be noted that the purge efficiency is the same for any type of the three gases independent of the inlet pressure range

On the contrary , it appears tha t with the Inverted dynamic seal, the higher the gas mola r mass is, the lower the residual partial pressure is This effect was already stated in the past t"

For fur ther experiments, we are going to evaluate the effect o[ the dynamic seal dimensions and peripheral speed The pumping efficiency is described by the formula

(or[ E f f i c l e n c y = K , (1) ( -

with K = cons tan t ~'J = ro ta t ion speed in radlan s ', J = ladJus of the thread, / = length of the thread,

= clearance between ro tor and s ta tor

~ G A S

EXHAUST

t PURGE

Figure 2. Experimental assembly for the measurement of the residual partial pressure

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4. Applications

The advan tage of a protect ion system such as the inverted dynamic seal is tha t it protects the tu rbomolecu la r pump from corrosive gases and commonly encountered reaction products We have demons t ra ted tha t the inverted dynamic seal is very efficient for gases with a molar mass greater than or equal to that of a rgon Mos t corrosive chlor inated gases (C12, BCI~, CC14, etc ) or fluorine gases (CF4, C2[~6, CHFs, SIF4, CHF~, etc ) have a higher mola r mass t han argon The use of the reverted dynamic seal for the dry etching of a lumlnlum, for example, is suitable m this case It should be noted tha t the Inverted dynamic seal is effective only while the tu rbomolecu la r pump is rotat ing, which is the case for most product ion systems These users keep the pumps runn ing 24 h a day, ei ther in process phase or m produc t ion s tandby phase and only stop them for main tenance operat ions

The use of IS type (with Inverted dynamic seal) or CP (corrosion p roof ) type pumps can be classzfied according to

Math/eu and J M Gruffat Inverted dynamic seal

-+-the molar mass of the gas being pumped, ---the flow rate of the corrosive gas, ---the operating conditions of the pump non stop 24 h a day or With halts at weekends or at night

~ . Pumping gases lighter than Ar(40), e.g. HF(20), HCi(36), 3(17). We have seen that the purge is more effective for these

~ases Therefore the use of a CP type pump is generally recom- mended For a few sccm of corrosive gases, the IS type pump

may be used

4.2. Pumping gases heavier than Ar(40), e.g. CIz(70), BCI3(117), ~CI4(170), SiF4(104), CF4(88), C2F6(58), C3Fs(188), CHF3(ll8). l~ the pump continues rotating 24 h a day the inverted dynamic Seal IS the optimum choice IS type pumps are the most advan- tageous in this case, even for large gas flows

If the pump is to be stopped for more than 6 h, at mght or at l i e weekend for instance, with large flows of pumped gas, the inverted dynamic seal does not protect the pump It is then ~aecessary to use a CP IS type (with both purge and inverted ~lynamic seal) pump and to operate the purge when the pump is Stopped For small quantities of gas or for stops of short dburatlon, IS type pumps may be suitable

5. Conclusion

We compared two techniques for protecting turbomolecular pump bearings against corrosive gases

- - a 6 x 10 2 Pa m 3 s- ~ nitrogen purge, - - a built-in Holweck type inverted dynamic seal at 71 m s- i and producing a differential pressure of 150 Pa

We measured the residual partial pressures at the bearings for

lhe three gases He, Ar and SF6 Under our experimental conditions, with a tested turbo-

molecular pump without a protection system, the partial pres- imre of argon is 8 x 10 -4 Pa This is brought down to 2 x 10 -6 IPa with the inverted dynamic seal and to 3 x l0 7 Pa with the ,purge A factor of 100 or 1000 is therefore gained over the

standard pump For gases lighter than Ar(40) such as helium, the partial pres-

sure is 9 x 10-s Pa with the inverted dynamic seal and 8 x 10-8 Pa with the purge, i e a factor of 1000 m favour of the purge

For gases heavier than Ar(40) such as SF6(146), irrespective of the protection system, the partial pressure is very low and less than 1 × 10 -8 Pa

This new protection system, the inverted dynamic seal offers many advantages

--built-in system, thus reducing the s~ze of the lnstallaUon, ----efficient protection of the turbomolecular pump, --improvement of ultimate pressure of two decades, --savings on the nitrogen used for the purge, --savings on the gas hne with piping, control valve, shut-offvalve and control system, --savings on the size of the primary pump as there is no N2 flow to be pumped

According to the size of the gas flows to be pumped and the operating regimes of the pump, and m order to obtain the best service life/cost ratio, there are now several alternatives between the CP, IS or CP IS type pumps

Acknowledgements

The authors would like to thank ALCATEL CIT Vacuum Tech- nology and especially the company management for giving per- mission to publish our results We thank also all the techmoans and engineers involved in this study

References

~J W Coburn, Plasma Etching and Reactive Ion Etchmy American Vacuum Socaety Monograph Series, New York 0982) 2D W Hess, Plasma Chem Plasma Process 0982) 3p Duval, High Vacuum Productzon tn the Mtcroelectromcs Industry Elsevier, Amsterdam (1988) 4Gotz, Hennlng and Augustine, J Vac Sct Technol, A2, 182 (1984) S L Maurlce, Joint d'&anchelt6 dynamlque, 6th International Congress of Vacuum Technology, Kyoto, 25 March (1974) 6W Gaede, Z Phys, 13, 864-870 (1912) 7W Gaede, Annln Phys, 41(2), 337 (1913) 8F Holweck, CR Acad Sct Parts, 117, 43 (1923) 9F Holweck, L'Onde Electrtque, 21(2), 497 503 (1923) ~°T Selsakisho, Molecular Pumps, Patent 59-267264

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