MASKLESS MICRO ETCHING OF GaAs DIRECTLY CONTROLLED BY CALCULATOR

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MASKLESS MICRO ETCHING OF GaAs DIRECTLY CONTROLLED BY CALCULATOR

S. Mottet, L. Henry

To cite this version:

S. Mottet, L. Henry. MASKLESS MICRO ETCHING OF GaAs DIRECTLY CON- TROLLED BY CALCULATOR. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-139-C5-142.

�10.1051/jphyscol:1983523�. �jpa-00223104�

JOURNAL DE PHYSIQUE

Colloque C5, supplkment au nOIO, Tome 44, octobre 1983 page C5-139

MASKLESS MICRO ETCHING OF G a A s DIRECTLY CONTROLLED BY CALCULATOR

S . M o t t e t and L. Henry

C.N.E.Y. Lannion B, Division ICM/TOH, BP 40, 22301 Lannion Cedex, France

R6sum6 - La g r a v u r e c o n t r 6 l Q e des composQs 1 1 1 - V e s t une Qtape i m p o r t a n t e pour l a f a b r i c a t i o n de composants e t de c i r c u i t s i n t Q g r 6 s . Pour c e l a nous avons dSvelopp6 un systsme q u i permet d ' o b t e n i r un spot lumineux de 1 micron de d i a m e t r e . Le deplacement du s p o t sur 1 ' 6 c h a n t F l l o n e t la v i t e s s e s o n t p i l o t & p a r un c a l m l a t a r pour + o u t e f i g u r e 3 r 6 a l i s e r . Le spot lumineux e s t u t i l i s 6 pour g r a v e r p a r v o i e photochimique du GaAs immerge clans des s o l u t i o n s aqueuses.

A b s t r a c t - C o n t r o l l e d e t c h i n g of 1 1 1 - V compound semiconductors i s a n impor- t a n t s t e p f o r t h e f a b r i c a t i o n of d i s c r e t e and i n t e g r a t e d components. For t h i s we developped a s y s t e n which p e r m i t s t o o b t a i n a 1 micron d i a m e t e r l i g h t s p o t . Displacement of t h e s p o t on t h e sample and speed a r e c o n t r o l l e d by a computer f o r any d e s i r e d f i g u r e . The l i g h t s p o t i s used t o e t c h photochemi- c a l l y GaAs immersed i n aqueous s o l u t i o n s .

C o n t r o l l e d e t c h i n g of 1 1 1 - V compounds is a n i m p o r t a n t s t e p f o r t h e f a b r i c a t i o n of i n t e g r a t e d components. S e v e r a l papers d e s c r i b e t h e u s e of a laser t o e t c h photochani- c a l l y GaAs i n aqueous s o l u t i o n s . I n t h e p r e s e n t p a p e r we p r e s e n t t h e r e s u l t s R o b t a i n e d w i t h a n a p p a r a t u s we s p e c i a l l y developped f o r t h e s e t e c h n i c s . W e t r i e d t o g e t t h e b e s t p r e c i s i o n and r e p r o d u c t i b i l i t y of t h e e t c h e d f i g u r e s .

1 -LIGHT BEAM SCANNING APPARATUS

The l i g h t i s i s s u e d from a 632 nm He Ne l a s e r . F i r s t t h e l a s e r bean i s focused i n o r d e r t o i n j e c t t h e l i g h t i n a monanode o p t i c a l f i b e r . Thus a spot of g a u s s i a n d i s t r i b u t i o n of about 2 0 p m d i a m e t e r i s obtained a t t h e o t h e r end of t h e f i b e r . Microscop o p t i c s t h e n reduce t h e beam t o a dimension of about 1.3 p m d i a m e t e r . The o p t i c a l p a t h of a commercial microscop i s used , a l l o w i n g t o i l l u m i n a t e t h e sample w i t h a n e x t e r n a l w h i t e l i g h t s o a s t o observe t h e sample and t h e s p o t , a t t h e same time, through t h e b i n o c u l a r . The l a s e r beam i n t e n s i t y can be g r a d u a l l y d e c r e a s e d , u s i n g a set of n e u t r a l o p t i c a l d e n s i t i e s , over 4 o r d e r of magnitude. The i n c i d e n t power sampled from a f r a c t i o n of t h e beam i s c o n t i m o u s l y measured w i t h a photodiode.

The sample can be moved i n two p e r p e n d i c u l a r d i r e c t i o n s by two s t e p by s t e p motors.

The minimum s t e p d i s p l a c e m e n t These d i s p l a c e m e n t s a r e c o n t r o l l e d by a computer which i s of 0 . 1 P m and t h e displacement p r e c i s i o n of 0 .l p e r m i t s t o performe any p.

d e s i r e d f i g u r e a t any speed a l l m e d by t h e s t e p by s t e p motors.

The i n c i d e n t beam maximum power on t h e sample i s 0.2 mW. Thus no thermal e f f e c t take p l a c e i n t h e e x p e r i m e n t s we p r e s e n t ( A T L 3 O c ) .

I1 - PREPARATION OF THE SAMPLES

We u s e aqueous s o l u t i o n s a s a n e t c h i n g medium. The sample i s covered by a s m a l l quan- t i t y of aqueous s o l u t i o n s p r e a d i n a t h i n f i l m by c a p i l a r i t y under a cover g l a s s . I l l u m i n a t i o n i s performed t h r o u g h t h e c o v e r g l a s s and t h e taquecus s o l u t i o n f i l m .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983523

JOURNAL DE PHYSIQUE

We f i r s t s t a r t e d our e x p e r i m e n t s w i t h aqueous s o l u t i o n s a l r e a d y mentionned i n t h e l i t t e r a t u r e t o by a c t i v e i n s i m i l a r e x p e r i m e n t s (Ref. 1 - ^3). They a r e :

KOH + ^H20 w i t h d i l u t i o n s of 1 % and 5 % H C l + H 2 0 a t 1 %

H 2 S 0 4 + H 2 0 2 + H 2 0 r e s p e c t i v e l y l O c c , 1 3 c c , 250cc.

111 - EXPERIMENTAL RESULTS

The e t c h i n g c a p a b i l i t i e s of t h e s o l u t i o n s and t h e a p p a r a t u s have been s t u d i e d u s i n g a d i s p l a c e m e n t t e s t drawing canposed of l i n e s of 100,um l e n g h t s e p a r a t e d by s t e p s of 4 m, and t h i s f o r a s e t of displacement speeds ( p i c t u r e 1 ) . The displacement speeds a & i n t h e range of 1 p / s t o 9 p n / s . The e t c h i n g speed i s p r o p o r t i o n n a l t o t h e l i g h t i n t e n s i t y . We o b t a i n e d very c l o s e d r e s u l t s f o r t h e d i f f e r e n t s o l u t i o n s t r i e d . On t h e o t h e r band t h e e t c h i n g speed i s h i g h l y c o r r e l a t e d t o t h e m a t e r i a l type of t h e sample We c a n t y p i c a l l y summarize our r e s u l t s by saying we u s e a bean i n t e n s i t y of 0.5pW

f o r n type GaAs and 0.2 mW f o r s e m i - i n s u l a t i n g m a t e r i a l i n o r d e r t o have t h e same e t c h i n g speed. Concerning p t y p e GaAs we did not succeed t o e t c h i t by t h i s method.

The d i f f e r e n c e s we o b t a i n f o r t h e t h r e e s o l u t i o n s we u s e a r e due t o t h e i r a c t i v i t y on i l l u m i n a t e d GaAs. The s o l u t i o n s of H C 1 and H2S04 + H202 a r e known t o e t c h unillumi- n a t e d m a t e r i a l a t roan t e m p e r a t u r e even f o r t h e high d i l u t i o n s w use t o minimize t h i s e f f e c t . The KOH s o l u t i o n s do n o t a c t on GaAs w i t h o u t i l l u m i n a t i o n which i s f a r more easy t o put t o work. Moreover KOH do not damage AuGe ohmic c o n t a c t which is not

t h e c a s e f o r a c i d s o l u t i o n s . The b e s t s u r f a c e a s p e c t of t h e g r o w e s a r e o b t a i n e d f o r KOH s o l u t i o n s . The a c i d s o l u t i o n s of t e n produce a g r a m l a r a s p e c t of t h e whole s u r - f a c e of t h e sample. An o t h e r major p o i n t is t h e p r e p a r a t i o n delay of t h e s o l u t i o n which may l e a d t o s o l u t i o n q u a l i t y d e g r a d a t i o n . We observed c r i s t a l l i n d e p o s i t s on samples when u s i n g aged s o l u t i o n s .

- 3

P i c t u r e 1 shows t h e grooves o b t a i n e d w i t h KOH on a 10'' m n type GaAs sample f o r t h r e e scanning speeds. The p i c t u r e 2 p r e s e n t s a d e t a i l of t h e grooves shape and p e r m i t t o s e e t h e q u a l i t y of t h e s u r f a c e a f t e r e t c h i n g w i t h KOH.

P i c t u r e 3 i s a view of t h e e t c h i r g o b t a i n e d by a combination of s u c c e s s i v e drawings on a 10'' n t y p e sample w i t h KOH s o l u t i o n . F i r s t was performed a l a r g e groove of 1 0 p m w i d t h and 5 p m depth. Then t h e sample was scanned i n t o a p e r p e n d i c u l a r d i r e c - t i o n i n o r d e r t o make t h i n g r o w e s of about 1 p m w i d t h f o r d i f f e r e n t s c a n n i r g speeds.

A l l t h e o p e r a t i o n s have s e e n performed i n a same experiment, t h e c o n t r o l of t h e whole o p e r a t i o n beeing achieved by t h e computer. We r a n a r k a h o l e of some microns d e p t h a t t h e top of t h e p i c t u r e which correspond t o t h e i n i t i a l s p o t p o s i t i o n b e f o r e scanning s t a r t e d .

I n o r d e r t o show t h e r e s o l u t i o n c a p a b i l i t i e s of t h e method we performed s e v e r a l o p t i - c a l g r a t i n g s of 1250 grooves p e r m i l l i m e t e r ( s t e p s of 0.8 ~ m ) . We have choosen t o p r e s e n t on p i c t u r e 4 a view of a g r a t i r g performed w i t h H C 1 s o l u t i o n . The purpose was t o o b t a i n r e g u l a r grooves of s m a l l depth. We have a l s o choosen t h i s f i g u r e t o show t h e s u r f a c e g r a m l a r d e f f e c t s observed when u s i n g HC1. G r a t i r g s performed w i t h KOH have a b e t t e r s u r f a c e a s p e c t .

IV - INTERPRETATION OF THE RESULTS

We t r i e d o t h e r e x p e r i m e n t s o n samples on which AuGe ohmic c o n t a c t s had a l r e a d y been e v a p o r a t e d . The KOH s o l u t i o n h a s no e f f e c t on AuGe, s o t h a t only uncovered m a t e r i a l c a n be e t c h e d . Scanning t h e s p o t on AuGe do not produce any damage. Close t o t h e c o n t a c t edge t h e r e i s no e t c h i r g and t h e e t c h i n g rate d e c r e a s e s as t h e d i s t a n c e t o t h e c o n t a c t d e c r e a s e s . T h i s must b e r e l a t e d t o t h e phenanena i n v o l w d i n t h e e t c h i n g p r o c e s s .

The p r o c e s s i s based on a n e l e c t r o c h e m i c a l e f f e c t . F o r t h i s we have t o c o n s i d e r GaAs t o have a n a t u r a l s u r f a c e d e p l e t i o n l a y e r which l e a d t o a s u r f a c e e l e c t r i c f i e l d . T h i s d e p l e t i o n l a y e r i s induced by s u r f a c e c h a r g e s which may be due t o many e x t e r n a l f a c t o r s : oxydes, p o l u t i o n s , e t c . . . As e t c h i n g goes on over microns we have t o admit t h a t a f t e r t h e e t c h i n g h a s s t a r t e d , s u r f a c e s t a t e s due t o e x t e r n a l f a c t o r s have been

removed and a r e no l o n g e r t h e o r i g i n of t h e s u r f a c e e l e c t r i c f i e l d . On t h e o t h e r hand we must n o t i c e t h a t t h e pH of t h e s o l u t i o n has no p r e d h i n a n t e f f e c t . We conclude t h a t t h e s u r f a c e charges a r e d i r e c t l y c o r r e l a t e d t o t h e s u r f a c e def f e c t i t s e l f ( b r o k e n atomic b i n d i n g s ?). Anyway i t i s admitted t h e Fermi l e v e l t o be c l o s e t o t h e midgap a t s u r f a c e of a t y p e GaAs. So t h a t t h e p r o c e s s c a n be d e s c r i b e d a s :

aqueous s o l u t i o n I n t y p e A S C ~

s u r f a c e s t a t e s

i o n exchanges

-

I*

- C r e a t i o n of e l e c t r o n - h o l e p a i r s i n t h e m a t e r i a l by l i g h t a b s o r p t i o n .

- S e p a r a t i o n of t h e e l e c t r o n - h o l e p a i r s by t h e s u r f a c e e l e c t r i c f i e l d .

- Accumulation of h o l e s a t t h e s u r f a c e which p e r m i t s t o a c t i v e t h e i o n exchanges w i t h t h e aqueous s o l u t i o n i n a n e l e c t r o & e m i c a l process.

I n such a p h o t o v o l t a f c e f f e c t we have t o c o n s i d e r t h e d i f f u s i o n of t h e m i n o r i t y f r e e c a r r i e r s i n t h e m a t e r i a l , r e l a t e d t o t h e d i f f u s i o n l e n g t h and t h e h i g h of t h e s u r f a c e b a r r i e r . T h i s e x p l a i n s t h e f a c t t h a t n e a r a n ohmic c o n t a c t , a s t h e high of t h e b a r r i e r d e c r e a s e s , t h e e t c h ing c a p a b i l i t i e s d e c r e a s e s .

On t h e o t h e r hand f o r s e m i - i n s u l a t i n g m a t e r i a l t h e Fermi l e v e l i s a l r e a d y c l o s e t o t h e midgap, so t h a t i f a s u r f a c e s p a c e change r e g i o n e x i s t s i t must be very s m a l l . I n such a c a s e t h e p r o c e s s i s mainly a c t i v a t e d by t h e f r e e c a r r i e r s d i f f u s i o n c u r r e n t r a t h e r t h a n by a s u r f a c e e l e c t r i c f i e l d induced c u r r e n t . T h i s would be c o n s i t e n t w i t h t h e d i f f e r e n c e i n e t c h i n g speed observed f o r s e m i - i n s u l a t i n g m a t e r i a l compared t o n t y p e m a t e r i a l .

F o r p t y p e m a t e r i a l , a s u r f a c e s p a c e charge r e g i o n would l e a d t o a n accxlmulation of e l e c t r o n s a t t h e s u r f a c e . Noone succeded i n e t c h i n g p type m a t e r i a l by t h i s process.

We conclude t h a t t h e a c t i v a t i o n of t h e e l e c t r o c h e n i i c a l need a n accumulation of h o l e s on t h e s u r f a c e of t h e semiconductor.

To summarize : f o r a g i v e n i n c i d e n t l i g h t power t h e e t c h i n g r a t e depends upon t h e c a p a b i l i t y of t h e m a t e r i a l t o s e p a r a t e e l e c t r o n - h o l e p a i r s b e f o r e r e c a n b i n a t i o n and t o d r i v e h a l e s , t o t h e s u r f a c e of t h e c r y s t a l . The p r e c i s i o n of t h e e t c h i n g around t h e l i g h t s p o t depends of t h e d i f f u s i o n l e n g t h of t h e h o l e s .

V - CONCLUSION

We developped a s y s t e n t h i c h p e r m i t s t o e x e c u t e m s t of t h e e t c h i n g s f o r p l a n a r t e c h n o l o g y on GaAs w i t h o u t masks. The scanning speed and t h e l i g h t power can b e c o n t r o l l e d by t h e computer f o r any programmed f i g u r e . I n p a r t i c u l a r , a s e t c h i n g r a t e s r a t i o of n type and s e m i - i n s u l a t i n g is i n t h e o r d e r of 1 0 , t h e r e 'is no d i f f i c u l t y t o s t o p e t c h i r g of a n a c t i v e n l a y e r t o i t s i n t e r f a c e w i t h a s e m i - i n s u l a t i n g s u b s t r a t e ( o r p type u n d e r l a y i n g m a t e r i a l ) . We showed t h a t aqueous s o l u t i o n s can be used which p r e s e n t no a c t i o n on GaAs without i l l u m i n a t i o n and moreover no damage f o r GeAu ohmic c o n t a c t s .

C5-142 JOURNAL DE PHYSIQUE

The method s t u d i e d w i t h GaAs m a t e r i a l should be e x t r a p o l a t e d t o 1 1 1 - V compounds. The b e s t aqueous s o l u t i o n has c e r t a i n l y not y e t been found a s w e have t o i n v e s t i g a t e s o l u t i o n s t h a t a r e normally i n a c t i v e f o r 1 1 1 - V compounds.

Moreover t h e r e i s no l i m i t a t i o n t o aqueous mediums, gas can be used a s w e l l without c o m p l i c a t i o n of t h e method.

ACKNOWLEDGMENTS

The a u t h o r s a r e g r a t e f u l t o C. BOISROBERT, M. PASSARET and P.N. FAVENNEC f o r t h e i r h e l p f u l s u g g e s t i o n s . The a u t h o r s a r e a l s o g r a t e f u l t o D. RIVIERE and J.P. EERCIER f o r t h e q u a l i t y of t h e p i c t u r e s t h e y g o t from the e l e c t r o n i c microscop.

P i c t u r e 1 P i c t u r e 2

P i c t u r e 3 P i c t u r e 4

REFERENCES

1 - R.M. OSGOOD, Jr, A. SANCHEZ-RUBIO, D.J. EHRLICH and V. DANEU 'Appl. Phys. L e t t . 2 (5) (March 1982), 391.

2 - D. J. EHRLICH, R.M. OSGOOD, J r , and T.F. DEUTSCH

IEEE J o u r n a l of Quantum E l e c t r o n i c s , QE-16 n o 11 (November 1980), 1233.

3 - T.J. CHUANG

J . Var. S c i . Technol., 21(3) ( S e p t . / O c t . 1982), 798.