MILLING AND DRILLING USING CO2-CLUSTERS
P. Wolfgang Henkes, R. Klingelhöfer
To cite this version:
P. Wolfgang Henkes, R. Klingelhöfer.
MILLING AND DRILLING USING CO2CLUSTERS. Journal de Physique Colloques, 1989, 50 (C2), pp.C2-159-C2-161.
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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 - 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.
-
Abstract
A copper t a r g e t i s bombarded by i o n i z e d LO2-clusters, which were
accelerated 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 surface were removed w i t h i n a f o c a l spot o f 3 mm i n d i a meter. By stronger focussing o f t h e c l u s t e r i o n beam a hole 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 - INTRODUCTION
Sputtering by c l u s t e r s was studied 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 per c l u s t e r ion. They suggested t o use c l u s t e r
ions f o r a l t e r i n g surface 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 note r e p o r t s f i r s t r e s u l t s obtained using C02-clusters.
2
-
EXPERIMENTAL PROCEDURE AND RESULTS
.
1
The production o f accelerated c l u s t e r ions has been described elsewhere
I n contrast 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 format i o n . We used C02 s i n c e i t i s e a s i l y cryopumped using l i q u i d nitrogen. The apparatus has
been used, however, t o produce beams o f hydrogen c l u s t e r s using 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 ions 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 the 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 voltage was
adjusted t o produce a f o c a l spot o f 3 nm i n diameter. Two crossed tantalum wires 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. Fig. l a shows an overview
and Fig. 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 the areas masked by t h e wires, approximately 46 um of
copper have been removed. While t h e top o f t h e "walls" shows t h e o r i g i n a l surface roughness
before 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 obviously 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 lower one o f t h e two wires a t t h e crossing p o i n t .
Again one observes t h e high p o l i s h o f t h e sputtered areas i n contrast. t o t h e o r i g i n a l l y
rough surface displayed i n t h e area where t h i s w i r e was shielded by t h e upper one.
I n a second experiment the 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 the f o i l and a f t e r another s i x minutes a hole 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, the focal spot of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989227
JOURNAL DE PHYSIQUE
C2-160
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 order o f magnitude. Using s i m i l a r
beam data a 30 pm tungsten f o i l was pierced 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, the 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.
3
-
CONCLUSIONS
From the 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 greater
than 600 copper atoms per i o n . Since t h e i o n beam may contain an unknown f r a c t i o n o f molec u l a r ions 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 ions 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 the 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 - whereas t h e mean energy per
molecule
155 eV - and, consequently, the penetration depth i s low. Thus t h e energy i s
deposited i n a t h i n surface 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
ions 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 the 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 exploratory 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 the t a r g e t
i s smaller by an order o f magnitude compared t o s p u t t e r i n g w i t h atomic ions. 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.
4
-
ACKNOWLEDGEMENTS
We are 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 he1p i n conducting the experiments.
-
Fig. 1 S t r u c t u r e produced on the 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 ions. The
dimensions i n urn are given i n the margins.
a.
Overview
Fig. 1
-
b.
Detail
F i g . 2 - 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 Gasdynamics, 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).