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Submitted on 1 Jan 1979
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ATTACHEMENT AND ION MOLECULE REACTIONS IN SF6 MIXTURES
M. Abdel-Salam, J.L. Moruzzi
To cite this version:
M. Abdel-Salam, J.L. Moruzzi. ATTACHEMENT AND ION MOLECULE REACTIONS IN SF6 MIXTURES. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-53-C7-54.
�10.1051/jphyscol:1979726�. �jpa-00219245�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, J u i l l e t 1979, page C7- 53
.ATTACHEMENT A N 3 ION MOLECULE REACTIONS IN SF6 MD<NRES
M. Abdei-Salam 'and J.L. Moruzzi.
%Department of EZectricaZ Engineer-i.ng, Un2:versity of LiverpooZ, LiverpooZ L69 3BX, U.K.
HochspannuvgsZaboratoriwn TU Miinchen, Arcisstrass 21, 8 kb%chen 2, Vest Gemnany. (present adcZress)
I n t r o d u c t i o n : SF mixtures a r e of p a r t i c u - lar, i n t e r e s t bechuse of t h e i r high d i e l e c -
v - - - .t r i c and t h e r m a l p r o p e r t i e s 11 I .
D i f f e r e n t i n v e s t i g a t o r s 121 have measured t h e r e l a t i v e abundances of t h e n e g a t i v e i o n s p e c i e s i n SF o v e r a wide r a n g e of E/P. A t low E/P ( < ( E / B ) l i m ) , t h e SF- i o n s r e p r e s e n t 95% of t h e t o t a l , w i t h aboue 5% SF- For v a l u e s of E/P > (E/P)lim t h e v a l u e 8; SF-/
SF- b e g i n s t o i n c r e a s e w i t h E/P. The ~ 8 -
anti SF: i o n s a r e produced by t h e Drocess. 6
2
-
SF6 + e SF * / SF;
SF,
2The branching r a t i o was found t o be about 22 :I i n f a v o u r of SF;.
A t E/P v a l u e s g r e a t e r t h a n ( E / ~ ) l i r a , SF- Was assumed t o s t a r t i t s conversion i n t g
SF: by t h e p r o c e s s /2/:
SF; + SF6 -+ SF; + F SF6 ( 2 I n t h i s r e p o r t , t h e n e g a t i v e i o n s p e c i e s and ion-molecule r e a c t i o n s i n v a r i o u s gas m i x t u r e s c o n t a i n i n g SF h a s been s t u d i e d u s i n g a d r i f t t u b e mas8 s p e c t r o m e t e r 1 3 1 . R e s u l t s and D i s c u s s i o n s :
mix u r e s : e main gas flow t o :be%& tub: was th:%troPen t o which Was added a t i t r a n t flow. The t i t r a n t flow was SF / N ~ mixture w i t h 100/760 SF c o n t e n t The p r e s s u r e i n s i d e t h e d r i f e t u b e was maintained c o n s t a n t a t 0.9 t o r r and E/P* Was changed o v e r t h e range 10-45 v o l t / cm . t o m .
Under t h e s e c o n d i t i o n s , o n l y two i o n s were d e t e c t e d , SF; and SF'. By i n c r e a s i n g t h e ambunt of SF added 50 t h e b u l k n i t r o g e n g a s , i t was g o s s i b l e t o observe s a t u r a t i o n i n t h e number of n e g a t i v e i o n s produced, Fig. 1 , i n d i c a t i n g t h a t a l l t h e i n i t i a l e l e c t r o n s has been removed t o form n e g a t i v e i o n s . The r a t e a t which t h e n e g a t i v e i o n s i g n a l i n c r e a s e s w i t h t h e a d d i t i o n of SF6 enabled t h e attachment c o e f f i c i e n t ?I f o r SF6 t o be e s t i m a t e d t o a n accuracy +20%.:
where is f o r SF . Hence, t)/%F can be
c a l c u l a t e d . Over t h e range of 6 E / P ~ 10- 45 v/crn.torr, v/P changes a s shown i n Fig.
2. Here P d e n o t e s t h e SF - p r e s s u r e and P t h e t o t a l p r e s s u r e of t h 6 mixture. The eh- timated v a l u e o f q/I? i s i n agreement w i t h t h e work of Fehsenf e l d 141 b u t v e r y much l a r g e r t h a n o t h e r experiments / 5 , 6 / p e r f or- med on e l e c t r i c a l d i s c h a r g e s . It i s p o s s i - b l e t h a t t h i s d i s c r e p a n c y c a n be e x p l a i n e d by i n v o k i n g a detachment mechanism. Teich /;t/has a l s o o b t a i n e d e x p e r i m e n t a l evidence s u g g e s t i n g t h a t detachment e x i s t s i n SF6.
T h e r e f o r e , i f t h i s i s t h e c a s e , t h e pub- l i s h e d v a l u e s / 5 , 6 / a r e n o t r e a l a t t a c h - ment c o e f f i c i e n t v a l u e s but a r e n e t v a l u e s
18 1 of ?I/P*
Fig.1. T o t a l i o n s i n t h e mixture a t d i f f e r e n t v a l u e s of E / P ~ w i t h s m a l l a d d i t i o n s of SF6.
The SF- and SF; i o n s a t low E/P ( < ( E / P ~ ) l i m ) iki SF /m m i x t u r e s a r e proiuced by t h e procesg ,(?) in agreement w i t h p r e v i o u s measurements 121 , w i t h a branching r a t i o of about 1 1 :I i n f a v o r of SF;.
A t h i g h e r E/P v a l u e s , i t i s i n t e r e s t i n g t o observe that I n t h e p r e s e n t me'asurements t h e r a t i o SFF/SF- i s independent o f . th e SF a d d i t i o n t o $he mixture,' which. c o n t r a - d i g t s t h e c o n v e r s i o n p r o c e s s g i v e n by e q u a t i o n ( 2 ) .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979726
F i g . 2 . q/P f o r SF i n i t s mixture w i t h n i t r o g e n .
b ) SF N 0 m i x t u r e s : The main g a s f l o w t o the*&$ t u b e was t h e n i t r o u s o x i d e t o which was added a t i t r a n t f l o w . The t i t r a n t f l o w was SF,/N,O m i x t u r e w i t h 77/760 SF,
c o n t e n t . "
VUnder t h e s e c o n d i t i o n s , s e v e n i o n s were d e t e c t e d , N 0- 0- N 0 NO- and NO- from N 0 a g a i n s t ~ ~ 2 J a f ; b bb-2fF;lm SF . ~y i n c r e a s i H g t h e amount of SF zdded t o t h e b u l k N20 g a s t h e i o n c u r r e n t from SF6 ( z SF'+ SF5 )
i n c r e a s e s on the'sF; e r p e n c e of thaf! TNz0- (Z N 0-+O-+N 0-+NO;+NO-) , F i g . 9 .
I n a 2 Hnv$stHg2ted scheme, t h e r e a c t i o n s l e a d i n g t o t h e f o r m a t i o n of f h e n e g a t i v e i o n c u r r e n t s ISF-, I N 2 ~ - a r e g i v e n a s f o l l c w s : 6.
e + N20 N20# a t t a c h m e n t
B20-+SF6 SF;+ N20 c h a r g e - t r a n s f e r e + SF^ % SF; a t t a c h m e n t
By s o l v i n g - t h e e q u a t i o n s which r e l a t e t h e d e n s i t y of t h e i o n s [N20m] and [SF-Q] , one
o b t a i n s t h e c u r r e n t s I and I -. It was
p o s s i b l e t o o b t a i n r e a g a % i b l e f i a P 6 t o b o t h t h e c u r r e n t s I - and I PZ, F i g . 3 o n c h o o s i n g t h e c g g P f i c i e n t 8 dl =O.1, 0$=500.0 and d = 1 5 . 0
The h i z h v a l u e of d i n comparison w i t h d,
a s w e l l a s t h e v e r y h i g h v a l u e of
g i v e a n e v i d e n c e which i n d i c a t e t h a t d2 t h e l a r g e a t t a c h m e n t c o e f f i c i e n t s observed i n SF /N 0 m i x t u r e s i s n o t due complex i o n - mofecgle r e a c t i o n s 191, b u t c a n be accoun- t e d f o r e n t i r e l y by d i r e c t e l e c t r o n a t t a a h - b e n t t o t h e SF6 component of t h e m i x t u r e . C o n c l u s i o n s : W i t h i n t h e r a n g e o f t h e p r e - s e n t measurements, q / ~ f o r SF assumes h i g h v a l u e s t h a n t h o s e p u b l i s h e d ahd do n o t i n - voke a detachment mechanism i n t o a c c o u n t .
-2
