MILLING AND DRILLING USING CO2-CLUSTERS

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MILLING AND DRILLING USING CO2-CLUSTERS

P. Wolfgang Henkes, R. Klingelhöfer

To cite this version:

P. Wolfgang Henkes, R. Klingelhöfer. MILLING AND DRILLING USING CO2-CLUSTERS. Journal

de Physique Colloques, 1989, 50 (C2), pp.C2-159-C2-161. �10.1051/jphyscol:1989227�. �jpa-00229425�

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JOURNAL DE PHYSIQUE

Colloque C2, supplement au n 0 2 , Tome 50, fgvrier 1989

MILLING AND DRILLING USING C0,-CLUSTERS

P.R. WOLFGANG HENKES and R. KLINGELHOFER

Kernforschungszentrum Karlsruhe GmbH, Institut fiir

Kernverfahrenstechnik, Postfach 36 40, 0-7500 Karlsruhe 1, F.R.G.

R6sum'e

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Un sp6cimen en c u i v r e e s t bomard6 de C02-clusters i o n i s 6 s e t acc616r6s h T 5 5 - k V . Apr6s une i r r a d i a t i o n d'une charge t o t a l e de 7,5 mAs 8 peu pr'es 46 um de l a surface 'etaient hcart'es au dedans d'un c e r c l e de 3 mm diamgtre. Une 50 pm f e u i l l e de c u i v r e ' e t a i t perfor'ee apres 6 minutes d ' i r r a d i a t i o n avec 35 nA. Le diametre du percage ' e t a i t 0,13 mm.

A b s t r a c t

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A copper t a r g e t i s bombarded by i o n i z e d LO2-clusters, which were a c c e l e r a t e d by 155 kV. A f t e r i r r a d i a t i o n w i t h a t o t a l charge o f 7.5 mAs about 46 pm o f t h e i r r a d i a t e d s u r f a c e were removed w i t h i n a f o c a l s p o t o f 3 mm i n d i a - meter. By s t r o n g e r focussing o f t h e c l u s t e r i o n beam a h o l e o f 0.13 mm i n d i a - meter was d r i l l e d i n t o a copper f o i l o f 50 pm thickness a f t e r s i x minutes i r r a d i a - t i o n w i t h 35 nA a t 155 kV.

1

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INTRODUCTION

S p u t t e r i n g by c l u s t e r s was s t u d i e d by Friedman and

coworker^^-^,

who observed s p u t t e r i n g c o e f f i c i e n t s as l a r g e as 105 t a r g e t atoms p e r c l u s t e r i o n . They suggested t o use c l u s t e r i o n s f o r a l t e r i n g s u r f a c e s t r u c t u r e s .

Rescent i n t e r e s t i n microfocused i o n beams f o r m i l l i n g and r e p a i r o f masks f o r l i t h o g r a p h y caused us t o r e v i v e o l d apparatus f o r a p r e l i m i n a r y study o f t h e use o f c l u s t e r beams i n t h i s area o f a p p l i c a t i o n . T h i s n o t e r e p o r t s f i r s t r e s u l t s obtained u s i n g C02-clusters.

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EXPERIMENTAL PROCEDURE AND RESULTS

The p r o d u c t i o n o f a c c e l e r a t e d c l u s t e r i o n s has been described elsewhere 1

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I n c o n t r a s t t o t h e procedure used by Friedman, c l u s t e r s were i o n i z e d by e l e c t r o n impact a f t e r t h e i r forma- t i o n . We used C02 s i n c e i t i s e a s i l y cryopumped u s i n g l i q u i d n i t r o g e n . The apparatus has been used, however, t o produce beams o f hydrogen c l u s t e r s u s i n g l i q u i d He f o r pumping, Thus, any o t h e r gas except helium may be used. The experimental c o n d i t i o n s were chosen t o produce c l u s t e r i o n s o f a mean s i z e o f about 1000 C02-molecules p e r u n i t charge. The c l u s t e r i o n beam, whose diameter a t t h e i o n i z e r was 8 mn, was focused by a system o f electrodes w i t h c i r c u l a r apertures and accelerated by 155 kV on a copper t a r g e t . The e x t r a c t i o n v o l t a g e was adjusted t o produce a f o c a l spot o f 3

nm

i n diameter. Two crossed tantalum w i r e s o f 0.1 nun i n diameter, placed i n f r o n t o f t h e t a r g e t , served as a mask. F i g . l a shows an overview and F i g . l b a d e t a i l o f t h e s t r u c t u r e produced on t h e t a r g e t a f t e r i r r a d i a t i o n w i t h a t o t a l beam charge o f 7.4 MAS. Except f o r t h e areas masked by t h e wires, approximately 46 um of copper have been removed. While t h e t o p o f t h e " w a l l s " shows t h e o r i g i n a l s u r f a c e roughness b e f o r e i r r a d i a t i o n , t h e bottom looks completely smooth and polished. A t t h e upper edges o f t h e w a l l s some deposits can be seen t h a t o b v i o u s l y o r g i n i n a t e from t h e tantalum wires t h a t a r e a l s o sputtered. F i g . 2 shows t h e l o w e r one o f t h e two w i r e s a t t h e c r o s s i n g p o i n t . Again one observes t h e h i g h p o l i s h o f t h e s p u t t e r e d areas i n contrast. t o t h e o r i g i n a l l y rough surface d i s p l a y e d i n t h e area where t h i s w i r e was s h i e l d e d by t h e upper one.

I n a second experiment t h e c l u s t e r i o n beam was focused, as w e l l as possible, on a copper f o i l o f 50 pm thickness. Using a beam c u r r e n t o f 35 nA a t 155 kV i t took o n l y s i x minutes t o p i e r c e t h e f o i l and a f t e r another s i x minutes a h o l e was d r i l l e d , whose diameter o f 0.13 mm d i d n o t change s u b s t a n t i a l l y a f t e r prolonged i r r a d i a t i o n . Thus, t h e focal spot of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989227

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C2-160 JOURNAL DE PHYSIQUE

t h e s p u t t e r i n g c l u s t e r i o n beam seems t o have been o f t h i s o r d e r o f magnitude. Using s i m i l a r beam data a 30 pm tungsten f o i l was p i e r c e d w i t h i n 26 minutes. A1203 ceramic has been d r i l l e d 0.2 mm deep w i t h i n s i x hours. However, t h e beam c u r r e n t f o r t h i s case i s n o t w e l l known.

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CONCLUSIONS

From t h e amount o f t a r g e t m a t e r i a l removed t h e s p u t t e r c o e f f i c i e n t i s estimated t o be g r e a t e r than 600 copper atoms p e r i o n . Since t h e i o n beam may c o n t a i n an unknown f r a c t i o n o f mole- c u l a r i o n s t h i s f i g u r e can be regarded as a lower l i m i t f o r c l u s t e r i o n s o f 155 keV. The l a r g e s p u t t e r c o e f f i c i e n t can be explained by t h e f a c t t h a t t h e t o t a l energy t r a n s f e r r e d t o t h e t a r g e t by a c l u s t e r i o n upon impact i s l a r g e

-

155 keV

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whereas t h e mean energy p e r molecule

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^{155 eV}

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and, consequently, t h e p e n e t r a t i o n depth i s low. Thus t h e energy i s deposited i n a t h i n s u r f a c e l a y e r . A t h e o r e t i c a l tr atment o f t h e i n t e r a c t i o n o f c l u s t e r i o n s w i t h surfaces i s given by Friedman and Vineyardg. As observed by Friedman e t a1.3, t h e s p u t t e r c o e f f i c i e n t i s predominantly determined by t h e energy o f c l u s t e r s and l e s s by t h e i r size. From t h e i r data one gathers t h a t t h e s p u t t e r i n g y i e l d grows w i t h a h i g h power o f t h e energy. I n comparison t o machining w i t h microfocused beams o f atomic i o n s t h e t o t a l energy expended t o s p u t t e r one copper atom i s much lower, i n our case l e s s than 260 eV. T h i s i s t o be compared, f o r example, t o 2000 eV f o r 40 keV ~ e + on copper5.

The e x p l o r a t o r y experiments described above show t h e usefulness o f c l u s t e r i o n beams f o r m i l l i n g purposes. Because o f the l a r g e s p u t t e r c o e f f i c i e n t t h e thermal l o a d on t h e t a r g e t

i s s m a l l e r by an order o f magnitude compared t o s p u t t e r i n g w i t h atomic i o n s . The a b i l i t y t o p o l i s h may be i t s e l f have i n t e r e s t i n g a p p l i c a t i o n s i n micromachining.

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ACKNOWLEDGEMENTS

We a r e g r a t e f u l t o Prof. E. W. Becker f o r supporting t h i s work and t o M r . D i e t z e l f o r valuable he1 p i n conducting t h e experiments.

Fig. 1

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S t r u c t u r e produced on t h e copper t a r g e t by i r r a d i a t i o n w i t h Cop c l u s t e r i o n s . The dimensions i n urn a r e given i n t h e margins.

a. Overview

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F i g . 1

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^b. D e t a i l

F i g . 2

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D e t a i l of t h e lower tantalum w i r e t h a t served as a mask a t t h e c r o s s i n g p o i n t w i t h t h e upper one.

REFERENCES

/1/ O.F. Hagena, P.R.W. Henkes, U. P f e i f f e r , Proc. o f t h e

loth

I n t . Symp. on R a r e f i e d Gasdyna- mics, Progress i n A s t r o n a u t i c s and Aeronautics, Vol. 51, p a r t 11, 1123-1135 (1976).

/2/ R. Beuhler, L. Friedman, Chem. Rev. 86, 521-537 (1986).

/3/ M.W. Matthew, R.J. Beuhler, M. Ledbetter, L. Friedman, J.Phys.Chem.@, 3152-3159 (1986).

/4/ Lewis Friedman, George H. Vineyard, Comments a t Mol .Phys.

15,

251-259 (1984).

/5/ M. Kaminsky, Atomic and I o n i c Impact Phenomena on Metal Surfaces, 155-157, Springer(l965).