Integrated Circuits - American Chemical Society

10

Plasma-Assisted Processing The Etching o f Polysilicon i n a D i a t o m i c Chlorine Discharge Herbert H. Sawin, Albert D. Richards, and Brian E. Thompson

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: October 2, 1985 | doi: 10.1021/bk-1985-0290.ch010

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

The fundamental k i n e t i c s and transport properties of plasma processes are reviewed and applied to polys i l i c o n etching in Cl discharges. The r e l a t i v e neutral flux, ion flux, and ion energy i s critical i n c o n t r o l ling the d i r e c t i o n a l i t y of the etching process. The electron density and energy have been estimated from e l e c t r i c a l impedance measurements of Cl discharges. With increasing frequency or pressure; the electron density of the Cl discharge increases while the average electron energy decreases. As the electron energy declines, the f r a c t i o n of energy dissipated in ionizing c o l l i s i o n events decreases while the lower energy d i s sociative processes producing Cl and Cl increase. The larger production of Cl, which chemically etches the doped p o l y s i l i c o n , leads to isotropic etching. The ion flux to the sample does not vary d i r e c t l y with the electron density and i s believed to be strongly affected by the presence of negative ions. 3

2

2

-

I n t h i s paper the k i n e t i c and t r a n s p o r t p r o p e r t i e s o f glow d i s c h a r g e s are r e v i e w e d and used t o d e s c r i b e c u r r e n t work on a p a r t i c u l a r p r o c e s s , the e t c h i n g o f p o l y s i l i c o n w i t h C l . I n plasma p r o c e s s e s f o r the f a b r i c a t i o n o f m i c r o e l e c t r o n i c s , r a d i o f r e q u e n c y glow d i s c h a r g e s are used t o e t c h , d e p o s i t , s p u t t e r , o r o t h e r w i s e a l t e r the wafer s u r f a c e s . Radio f r e q u e n c y ( r f ) d i s c h a r g e s produce h i g h l y r e a c t i v e neut r a l s and i o n s a t low t e m p e r t u r e s b y the i n t r o d u c t i o n o f e n e r g y i n t o the plasma t h r o u g h i t s f r e e e l e c t r o n s . A plasma can be d e f i n e d as a p a r t i a l l y i o n i z e d gas i n w h i c h the charged s p e c i e s have a s u f f i c i e n t c o n c e n t r a t i o n t h a t t h e y i n t e r a c t s i g n i f i c a n t l y t h r o u g h coulombic f o r c e s . The charged s p e c i e s w i t h i n the plasma respond t o oppose any a p p l i e d f i e l d and m a i n t a i n an o v e r a l l n e u t r a l i t y o f the plasma. A glow d i s c h a r g e i s a n o n - e q u i l i b r i u m plasma i n which the e l e c t r o n s have a g r e a t e r average energy t h a n the i o n s and n e u t r a l s . A plasma i s s u s t a i n e d by the i n t r o d u c t i o n o f energy from an e l e c t r i c o r a

0097-6156/85/0290-0164$06.00/0 © 1985 A m e r i c a n C h e m i c a l Society

Stroeve; Integrated Circuits: Chemical and Physical Processing ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


10.

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165

magnetic f i e l d . The e l e c t r o n s are a c c e l e r a t e d by the f i e l d , t h u s g a i n i n g energy w h i c h i s passed t o the n e u t r a l s t h r o u g h c o l l i s i o n s . The low r a t e o f energy exchange i n e l a s t i c c o l l i s i o n s between the much h e a v i e r n e u t r a l s and the e l e c t r o n s r e s u l t s i n the s i g n i f i c a n t l y h i g h e r average energy o f the e l e c t r o n s . The e l e c t r o n s , t h e r e f o r e , can have s u f f i c i e n t energy t o produce (through i n e l a s t i c c o l l i s i o n s ) s i g n i f i c a n t amounts o f i o n s , f r e e r a d i c a l s , and o t h e r e x c i t e d s p e c i e s w i t h o u t a p p r e c i a b l y h e a t i n g the gas. I n t h i s manner, c o n c e n t r a t i o n s o f f r e e r a d i c a l s and i o n s w h i c h would n o r m a l l y be c r e a t e d a t o n l y a t flame t e m p e r a t u r e s , can be m a i n t a i n e d a t room t e m p e r a t u r e . Since the s u r f a c e and gas-phase r e a c t i o n s o f f r e e r a d i c a l s and e x c i t e d s p e c i e s have lower a c t i v a t i o n e n e r g i e s than s i m i l a r thermally induced r e a c t i o n s , the k i n e t i c s can be g r e a t l y enhanced. Plasmas are used i n t h r e e major m i c r o e l e c t r o n i c s processes; s p u t t e r i n g , plasma enhanced c h e m i c a l vapor d e p o s i t i o n (PECVD), and plasma e t c h i n g . I n each, the plasma i s used as a source o f ions and/or r e a c t i v e n e u t r a l s and i s s u s t a i n e d i n a r e a c t o r so as t o c o n t r o l the f l u x of n e u t r a l s and i o n s t o a s u r f a c e . The typical ranges o f p r o p e r t i e s f o r a glow d i s c h a r g e used i n m i c r o e l e c t r o n i c f a b r i c a t i o n are as shown i n T a b l e I . I n s p u t t e r i n g , i o n s are e x t r a c t e d from a plasma, a c c e l e r a t e d by an e l e c t r i c f i e l d , and bombarded upon a t a r g e t e l e c t r o d e composed of the m a t e r i a l t o be d e p o s i t e d ( 1 ) . The bombarding i o n s d i s s i p a t e t h e i r energy by s p u t t e r i n g p r o c e s s e s i n w h i c h the s u r f a c e atoms are e j e c t e d p r i m a r i l y by momentum t r a n s f e r i n c o l l i s i o n c a s c a d e s . The e j e c t e d atoms are d e p o s i t e d upon w a f e r s w h i c h are p l a c e d w i t h i n 1 i n e - o f - s i g h t o f the t a r g e t e l e c t r o d e , thus i n d u c i n g the vapor t r a n s p o r t of m a t e r i a l w i t h o u t a p p r e c i a b l y h e a t i n g e i t h e r the t a r g e t e l e c t r o d e o r the w a f e r s on w h i c h the f i l m i s d e p o s i t e d . Plasma enhanced c h e m i c a l vapor d e p o s i t i o n uses a d i s c h a r g e t o reduce the temperature a t w h i c h f i l m s can be d e p o s i t e d from gaseous r e a c t a n t s by c r e a t i n g f r e e r a d i c a l s and o t h e r e x c i t e d s p e c i e s w h i c h r e a c t w i t h i n the gas-phase and on the s u r f a c e ( 1 ) . D u r i n g p r o c e s s i n g , the s u r f a c e i s t y p i c a l l y h e a t e d t o a c q u i r e a b e t t e r q u a l i t y o f depos i t i o n f i l m . A l s o , the i o n f l u x from the plasma can be used t o c l e a n the s u r f a c e b e f o r e the d e p o s i t i o n b e g i n s t o improve the f i l m . The i o n f l u x i s a l s o thought t o a l t e r the f i l m d u r i n g d e p o s i t i o n . I n plasma e t c h i n g , the plasma produces b o t h h i g h l y r e a c t i v e neut r a l s (e.g. atomic f l u o r i n e ) and i o n s w h i c h bombard the s u r f a c e b e i n g etched(2). The n e u t r a l s r e a c t w i t h the s u r f a c e t o produce v o l a t i l e s p e c i e s w h i c h desorb and are pumped away. Ion bombardment o f t e n i n c r e a s e s the e t c h i n g r a t e by removing s u r f a c e contaminants w h i c h b l o c k the e t c h i n g o r by d i r e c t l y enhancing the k i n e t i c s o f the e t c h i n g . V e r y l a r g e s c a l e i n t e g r a t i o n (VLSI) r e q u i r e s the e t c h i n g of f i l m s w h i c h have t h i c k n e s s e s t h a t are comparable t o the lateral f e a t u r e d i m e n s i o n s . Plasma e t c h i n g p r o c e s s e s are r e q u i r e d t o p a t t e r n such f e a t u r e s s i n c e t h e y are capable o f n e c e s s a r y a n i s o t r o p y w h i l e wet e t c h i n g p r o c e s s e s (which use aqueous a c i d s or b a s e s ) are t y p i c a l l y i s o t r o p i c , p r o d u c i n g u n d e r c u t t i n g o f the p a t t e r n a t l e a s t equal t o the f i l m t h i c k n e s s . F i g u r e 1 shows the e t c h i n g p r o f i l e s produced by i s o t r o p i c and a n i s o t r o p i c e t c h i n g p r o c e s s e s . An e l e c t r i c f i e l d i s formed by the c o n t a c t o f a plasma w i t h a s u r f a c e c r e a t i n g a plasma sheath i n w h i c h the i o n s are a c c e l e r a t e d along the m a c r o s c o p i c s u r f a c e n o r m a l . Due t o t h i s d i r e c t i o n a l i t y , a l a r g e r f l u x o f ions


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Table I .

T y p i c a l P r o p e r t y Ranges of Glow D i s c h a r g e s Used i n Microelectronic Fabrication Range


Property Pressure Electron density Average e l e c t r o n energy Average n e u t r a l and i o n energy I o n i z e d f r a c t i o n o f gas Neutral d i f f u s i v i t y Free r a d i c a l density Power d i s s i p a t i o n

0.01 t o 1 t o r r _ 10 to 1 0 cm" 1 t o 10 eV 0.025 t o 0_.035 eV 10 t o 10 100 t o 10,000 cm /s l e s s than 30% 0.1 t o 1 W/cm 8

1 1

5

8

7

a

a

Mask

\\\\\\\\

Film

Isotropic Figure

1.

Profiles

Anisotropic produced

by i s o t r o p i c

and a n i s o t r o p i c


etching.

10.

Plasma-Assisted

SAWIN ET AL.

167

Processing


impinges on those s u r f a c e f e a t u r e s w h i c h are p a r a l l e l w i t h the macroscopic s u r f a c e . Plasma e t c h i n g p r o c e s s e s are t y p i c a l l y b r o k e n i n t o two c a t e g o r i e s , "plasma e t c h i n g " and " r e a c t i v e i o n e t c h i n g " ( R I E ) . "Plasma e t c h i n g " p r o c e s s e s are t y p i c a l l y d i s t i n g u i s h e d as having h i g h e r p r e s sures (0.1 t o 1 t o r r ) and e i t h e r h a v i n g symmetric ( e q u a l area) e l e c t r o d e s o r the plasma i s o l a t e d from the w a f e r s . RIE p r o c e s s i n g i s performed a t lower p r e s s u r e s (0.01 t o 0.1 t o r r ) and i n an asymmetric reactor. I n RIE, the w a f e r s are p l a c e d on the s m a l l e r e l e c t r o d e w h i c h i n c r e a s e s the energy o f the i o n s s t r i k i n g the s u r f a c e . Since the power can be i n c r e a s e d t o produce a n i s o t r o p i c e t c h i n g i n a symmetric r e a c t o r , a b e t t e r d i s t i n c t i o n would be a c c o r d i n g t o the dominant e t c h i n g p r o c e s s e s ; c h e m i c a l e t c h i n g by n e u t r a l s w h i c h e t c h i s o t r o p i c a l l y ("plasma e t c h i n g " ) o r i o n - i n d u c e d e t c h i n g w h i c h y i e l d s a n i s o t r o p i c p r o f i l e s (RIE). Plasma K i n e t i c s The k i n e t i c s w h i c h are c h a r a c t e r i s t i c o f glow d i s c h a r g e s are a r e s u l t o f c o l l i s i o n s between e n e r g e t i c e l e c t r o n s and n e u t r a l g a s e s . To c a l c u l a t e the r a t e o f p r o d u c t i o n by e l e c t r o n c o l l i s i o n s , the e l e c t r o n energy d i s t r i b u t i o n f ( e ) must be known. I t can be c a l c u l a t e d u s i n g the Boltzmann t r a n s p o r t equation, a differential concentration b a l a n c e f o r e l e c t r o n s w h i c h c o n s i d e r s the movement of the e l e c t r o n s and the energy d i s t r i b u t i o n caused by t h e i r c o u p l i n g w i t h f i e l d s and t h e i r c o l l i s i o n s w i t h o t h e r p a r t i c l e s ( 3 ) . The energy d i s t r i b u t i o n i s a f u n c t i o n o f b o t h the a p p l i e d f i e l d and the c o m p o s i t i o n of the gas w i t h w h i c h the e l e c t r o n s c o l l i d e . Assuming t h a t 1) the c r o s s - s e c t i o n f o r e l a s t i c c o l l i s i o n s i s i n v e r s e l y p r o p o r t i o n a l t o the e l e c t r o n v e l o c i t y , 2) e l a s t i c c o l l i s i o n s are the dominant mode of energy l o s s , 3) the e l e c t r o n energy r e l a x a t i o n time i s much g r e a t e r t h a n the f i e l d frequency, and 4) the c o l l i s i o n f r e q u e n c y i s much g r e a t e r t h a n the f i e l d f r e q u e n c y ; the time-averaged p o r t i o n o f the d i s t r i b u t i o n f u n c t i o n i s M a x w e l l i a n , where the e l e c t r o n temperature T i s as below. Q

f(e)

o

-— (k»T ) Be r

\ where 2

expT-^^ I B e J L

k

T

2

e E M k T R

=

—

6m

e

y

a L P

Here, e i s the e l e c t r o n energy, kg i s the Boltzmann c o n s t a n t , E is the e f f e c t i v e e l e c t r i c f i e l d , y i s the e l a s t i c c o l l i s i o n f r e q u e n c y , M i s the mass of the gas p a r t i c l e , m i s the e l e c t r o n mass, and p i s the p r e s s u r e . A l t h o u g h these assumptions are n o t e n t i r e l y c o r r e c t , a M a x w e l l i a n d i s t r i b u t i o n f u n c t i o n i s o f t e n used f o r the c a l c u l a t i o n of plasma k i n e t i c s due t o i t s s i m p l i c i t y . The e l e c t r o n temperature i s p r o p o r t i o n a l t o the square o f E /p, which can be p h y s i c a l l y i n t e r p r e t e d as b e i n g p r o p o r t i o n a l t o the average amount of energy a e l e c t r o n a c q u i r e s between c o l l i s i o n s . I n t y p i c a l p r o c e s s e s , however, i n e l a s t i c c o l l i s i o n s are the dominant mode o f energy l o s s w h i c h r e s u l t s i n e x p e r i m e n t a l l y observed average e l e c t r o n e n e r g i e s w h i c h Q

e

0


168

C H E M I C A L A N D P H Y S I C A L PROCESSING O F INTEGRATED CIRCUITS

s c a l e r o u g h l y as t h e square r o o t o f age e l e c t r o n energy i s m o n o t o n i c a l l y The r a t e o f p r o d u c t i o n o f t h e e l e c t r o n impact w i t h i t s p e r c u r s o r r a t e c o e f f i c i e n t k^; r

i "

k

n

E / p ( 3 ) . I n a l l cases t h e a v e r related to E /p. i s p e c i e s w i t h i n t h e plasma by n ^ c a n be c a l c u l a t e d u s i n g t h e 0

0

n

i e i

where k^ i s c a l c u l a t e d by t h e i n t e g r a t i o n o v e r a l l e l e c t r o n e n e r g i e s of t h e r e a c t i o n p r o b a b i l i t y f o r c o l l i s i o n s between e l e c t r o n s and n.;


k

i

-

J. J i f

where t h e f i r s t term i n t h e i n t e g r a l i s t h e e l e c t r o n v e l o c i t y and cr^Ce) i s t h e c o l l i s i o n c r o s s - s e c t i o n as a f u n c t i o n o f e l e c t r o n energy. T h e r e f o r e , i f t h e e l e c t r o n energy d i s t r i b u t i o n f u n c t i o n and the c o l l i s i o n c r o s s - s e c t i o n a r e known, t h e r a t e f o r t h e g e n e r a t i o n o f s p e c i e s w i t h i n a plasma c a n be c a l c u l a t e d as second-order gas-phase reactions. However, t h e c r o s s - s e c t i o n s a r e t y p i c a l l y n o t a v a i l a b l e and the e l e c t r o n d i s t r i b u t i o n f u n c t i o n i s n o t known, making the above a n a l y s i s u s e f u l i n u n d e r s t a n d i n g t h e plasma p r o c e s s e s , b u t n o t y e t used i n t h e d e s i g n o f plasma r e a c t o r s . T r a n s p o r t o f Charged S p e c i e s N e u t r a l s p e c i e s w i t h i n a plasma undergo d i f f u s i v e and c o n v e c t i v e t r a n s p o r t i d e n t i c a l t o t h a t i n a gas, however, charged s p e c i e s have an a d d i t i o n a l r e s t r i c t i o n as a r e s u l t o f t h e c o n s e r v a t i o n o f charge n e u t r a l i t y o v e r d i s t a n c e s w h i c h a r e g r e a t e r than t h e Debye l e n g t h o f the plasma ( o f t h e o r d e r o f 100 urn). T h e r e f o r e , i n a plasma w h i c h c o n t a i n s an e q u a l number o f p o s i t i v e ions and e l e c t r o n s , t h e i r t r a n s p o r t i s c o u p l e d t h r o u g h t h e coulombic i n t e r a c t i o n s f o r c i n g them t o have an e q u a l d i f f u s i v i t y c a l l e d the a m b i p o l a r d i f f u s i v i t y , D ( 3 ) . The f l u x e s T^'s w i t h i n a plasma o f charged s p e c i e s i n terms o f t h e d e n s i t i e s n^'s, d i f f u s i t i v i e s D^'s, and m o b i l i t i e s u^'s a r e r

D

e

T

- - e +

V n

n

E

e " e"e sc

= -D Vn +

+

+ n

+ | l +

E

s c

where E i s t h e space charge f i e l d induced b y t h e charge s e p a r a t i o n . Assuming n = n , t h e above e x p r e s s i o n s c a n be s o l v e d y i e l d i n g s c

+

e

D u +

e

+ D u e

+

Vn H

= - D Vn

* »»+

a

Us i n g the f a c t t h a t u >> u and t h e E i n s t e i n r e l a t i o n between d i f f u s i v i t y and m o b i l i t y (D/u = k T / e ) , t h e a m b i p o l a r d i f f u s i v i t y can be s i m p l i f i e d t o +

fi


10.

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169

Processing

T h e r e f o r e , t h e d i f f u s i v i t y o f i o n s i s enhanced b y t h e f a c t o r T / T (2100) w h i l e t h a t o f e l e c t r o n s i s r e d u c e d . A s u r f a c e w h i c h i s i n c o n t a c t w i t h a plasma, b u t w h i c h i s e l e c t r i c a l l y i s o l a t e d , must r e a c h a s t e a d y - s t a t e p o t e n t i a l w i t h time such t h a t t h e f l u x e s o f p o s i t i v e and n e g a t i v e charge t o t h e s u r f a c e a r e e q u a l . Under such c o n d i t i o n s , t h e s u r f a c e p o t e n t i a l i s s a i d t o be a t the f l o a t i n g p o t e n t i a l and i s n e g a t i v e w i t h r e s p e c t t o t h e plasma w h i c h reduces t h e f l u x o f e l e c t r o n s b u t enhances t h e f l u x o f p o s i t i v e ions p r o v i d i n g a net n e u t r a l f l u x . This d i f f e r e n c e i s manifested i n an e l e c t r i c f i e l d w h i c h i s analogous t o t h e space charge f i e l d w i t h i n the b u l k o f t h e plasma w h i c h e n f o r c e s t h e e q u a l i t y o f d i f f u s i v e fluxes. S i n c e t h e e l e c t r i c f i e l d p o i n t s i n t o t h e s u r f a c e , a sheath w h i c h i s d e p l e t e d i n e l e c t r o n s i s formed a t t h e s u r f a c e . Simplistic a l l y , t h e i o n s d i f f u s e i n t o t h e f i e l d formed a t t h e s u r f a c e and a r e captured. Therefore, the f l u x o f p o s i t i v e ions i s equal t o that o f the e l e c t r o n s t o a f l o a t i n g s u r f a c e and i s determined b y the r a t e o f ion d i f f u s i o n t o the sheath(4)• Due t o t h e c o u p l i n g o f t h e p o s i t i v e i o n s w i t h t h e e l e c t r o n s , t h e r a t e o f d i f f u s i o n i s enhanced b y t h e h i g h e r energy o f t h e e l e c t r o n s and t h e f l u x t o t h e sheath i s g i v e n by


e

+

I f t h e p o t e n t i a l o f a s u r f a c e which i s i n c o n t a c t w i t h a plasma i s p o s i t i v e w i t h respect t o the f l o a t i n g p o t e n t i a l , a large increase i n the e l e c t r o n f l u x o c c u r s w h i c h opposes t h e change i n p o t e n t i a l . S i m i l a r l y , a surface w i t h a p o t e n t i a l negative w i t h respect t o the f l o a t i n g p o t e n t i a l r e c e i v e s a g r e a t l y reduced f l u x o f e l e c t r o n s . The f l u x o f ions i s l i m i t e d b y t h e d i f f u s i o n o f i o n s t o t h e sheath region, but i s not influenced g r e a t l y by v a r i a t i o n s o f electrode p o t e n t i a l from t h a t o f t h e f l o a t i n g p o t e n t i a l . The p r i m a r y response i s t o enhance o r reduce t h e e l e c t r o n f l u x t o t h e s u r f a c e due t o t h e h i g h e r m o b i l i t y o f the e l e c t r o n s . Because o f the l a r g e e l e c t r o n f l u x induced by p o s i t i v e b i a s i n g o f t h e s u r f a c e b u t r e l a t i v e l y s m a l l v a r i a t i o n s i n i o n f l u x e s w i t h n e g a t i v e b i a s e s , t h e plasma sheath a c t s e l e c t r i c a l l y as a l e a k y d i o d e . I n r f d i s c h a r g e s , t h e diode n a t u r e o f t h e s h e a t h c a n induce r e c t i f i c a t i o n o f the r f s i g n a l c a u s i n g t h e average sheath p o t e n t i a l to approach one q u a r t e r o f t h e peak-to-peak v o l t a g e V i n a symmet r i c e l e c t r o d e r e a c t o r , w h i l e v o l t a g e s i n an a s s y m e t r i c r e a c t o r can approach V 1 2 a t t h e s m a l l e r e l e c t r o d e ( 4 ) . I o n bombardment e n e r g i e s can, t h e r e f o r e , v a r y from a few eV t o o v e r 1000 eV. However, i o n n e u t r a l c o l l i s i o n s w i t h i n t h e sheath c a n cause s i g n i f i c a n t r e d u c t i o n s i n t h e energy o f most i o n s s t r i k i n g t h e s u r f a c e from e n e r g i e s c a l c u l a t e d u s i n g t h e sheath p o t e n t i a l . A l s o , t h e h i g h e r sheath p o t e n t i a l s induce a g r e a t e r charge s e p a r a t i o n and sheath t h i c k n e s s . Surface K i n e t i c s The p r o d u c t i o n o f f r e e r a d i c a l s and o t h e r e x c i t e d n e u t r a l s p e c i e s w i t h i n the plasma c a n enhance t h e r a t e o f r e a c t i o n i n t h e gas and w i t h t h e s u r f a c e b y r e d u c i n g t h e a c t i v a t i o n energy f o r r e a c t i o n and/or d i s s o c i a t i v e a d s o r p t i o n on the s u r f a c e . F o r example, C F does not adsorb on s i l i c o n , however, b o t h o f t h e major plasma p r o d u c t s 4


170

C H E M I C A L A N D P H Y S I C A L PROCESSING OF INTEGRATED CIRCUITS

(CF and F) have h i g h s t i c k i n g p r o b a b i l i t i e s and chemisorb w i t h low activation energies(5). I n a d d i t i o n , when the s u r f a c e r e a c t i o n i s r a t e l i m i t i n g , a s e n s i t i v i t y t o w a f e r temperature i s o f t e n o b s e r v e d . The c h e m i c a l n a t u r e o f f r e e r a d i c a l r e a c t i o n s p e r m i t s the c h o i c e o f gases w h i c h s e l e c t i v e l y e t c h the d e s i r e d f i l m s , but not o t h e r f i l m s . I o n bombardment can b o t h induce s p u t t e r i n g and enhance c h e m i c a l processes. Ions bombarding the s u r f a c e w i t h an e n e r g i e s g r e a t e r t h a n a p p r o x i m a t e l y 20 eV can s p u t t e r m a t e r i a l from the s u r f a c e as d e s c r i b ed p r e v i o u s l y f o r the s p u t t e r i n g p r o c e s s . This etching process i s p h y s i c a l i n n a t u r e and i s not v e r y s e l e c t i v e t o the c o m p o s i t i o n o f the s u r f a c e . I n plasma e t c h i n g p r o c e s s e s , the s i m u l t a n e o u s f l u x of n e u t r a l s and i o n s can have a s t r o n g s y n e r g i s t i c e f f e c t i n w h i c h the r a t e o f e t c h i n g by the c o m b i n a t i o n i s s i g n i f i c a n t l y h i g h e r t h a n the sum o f i n d i v i d u a l p r o c e s s e s . Enhancements o f g r e a t e r than 25 have been found i n the f l u o r i n e - s i l i c o n s y s t e m ( 6 ) . The ion-enhancement mechanism i s not y e t c l e a r l y u n d e r s t o o d , but i t i s b e l i e v e d t o be a r e s u l t o f i n c r e a s e d s p u t t e r i n g y i e l d s w i t h the weakening of s u r f a c e bonds caused by the h a l o g e n a t i o n o f the s u r f a c e . There i s evidence t h a t h a l o g e n a t i o n of the s u r f a c e extends a number o f atomic l a y e r s i n t o the s u r f a c e ( 7 ) . Ion bombardment can a l s o induce d i r e c t i o n a l i t y by removing s p e c i e s from the s u r f a c e w h i c h e i t h e r b l o c k the n e u t r a l s p e c i e s from r e a c t i n g w i t h the s u r f a c e o r p r e f e r e n t i a l l y r e a c t w i t h the n e u t r a l e t c h i n g s p e c i e s r e d u c i n g the n e u t r a l e t c h i n g . I f the ion-enhanced e t c h i n g p r o c e s s e s are r a p i d w i t h r e s p e c t t o the n e u t r a l e t c h i n g p r o c e s s e s , the e t c h i n g p r o c e s s i s a n i s o t r o p i c .


3

Reactor Design Plasma r e a c t o r s are d e s i g n e d t o c o n t r o l b o t h n e u t r a l and i o n f l u x t o the w a f e r s b e i n g p r o c e s s e d , and t h u s , the p r o c e s s k i n e t i c s . Many d i f f e r e n t d e s i g n s have been d e v e l o p e d , but they can be c a t e g o r i z e d i n t o two major t y p e s : volume l o a d i n g and s u r f a c e l o a d i n g ( 7 ) . In a volume l o a d i n g r e a c t o r , the w a f e r s are s t a c k e d w i t h a gas space between w a f e r s . The term volume l o a d i n g r e f e r s t o the number o f w a f e r s b e i n g p r o c e s s e d as p r o p o r t i o n a l t o the volume o f the r e a c t o r . The plasma i s c r e a t e d n e a r the w a f e r s and the n e u t r a l r e a c t a n t s d i f f u s e from the d i s c h a r g e , between the w a f e r s , and t o the w a f e r surfaces. The f l u x o f i o n s t o the wafer s u r f a c e s i s m i n i m i z e d by t h i s d e s i g n , t h u s , the p r o c e s s i s c h e m i c a l and i s o t r o p i c . I n the s u r f a c e l o a d i n g r e a c t o r s , the w a f e r s are p l a c e d on e l e c t r o d e s u r f a c e s , t h e r e f o r e , the number o f wafers i s l i m i t e d by the a v a i l a b l e electrode area. The d i s c h a r g e i s c r e a t e d between the e l e c t r o d e s and c o n t a c t s the w a f e r s . The w a f e r s r e c e i v e b o t h a p p r e c i able i o n and n e u t r a l f l u x e s from the plasma. I f the a r e a o f one e l e c t r o d e i s s m a l l e r than t h a t o f the opposing e l e c t r o d e , the r e c t i f y i n g b e h a v i o r of the sheath formed a t the s u r f a c e causes the s m a l l e r e l e c t r o d e t o be bombarded by h i g h e r energy i o n s . By i n s e r t i n g a d d i t i o n a l e l e c t r o d e s and/or a l t e r i n g the geometry, the i o n f l u x t o the w a f e r can be c o n t r o l l e d somewhat i n d e p e n d e n t l y of the n e u t r a l f l u x . Plasma Impedance A n a l y s i s To a n a l y z e the p r o c e s s e s w i t h i n a d i s c h a r g e , knowledge of the e l e c t r o n d e n s i t y and energy d i s t r i b u t i o n i s needed. Langmuir probes have



10.

SAWIN ET AL.

Plasma-Assisted

171

Processing

been s u c c e s s f u l l y used i n DC d i s c h a r g e s but are not e a s i l y a p p l i e d t o the r f d i s c h a r g e s because o f the r a p i d p o t e n t i a l o s c i l l a t i o n s ( 9 ) . An e s t i m a t e o f the e l e c t r o n d e n s i t y and the average e l e c t r i c field w i t h i n a d i s c h a r g e can made from the measurement o f the e l e c t r i c a l impedance o f the plasma. R e s e a r c h e r s have measured the impedance by a number of t e c h n i q u e s , i n c l u d i n g the a n a l y s i s o f the impedance matching network used i n c o u p l i n g the power t o the d i s c h a r g e ( 1 0 ) . I n t h i s s t u d y , the impedance o f the d i s c h a r g e was measured by a c q u i r i n g the v o l t a g e and c u r r e n t waveforms t o the powered e l e c t r o d e . C o n s i d e r a p a r a l l e l p l a t e r e a c t o r i n w h i c h the plasma i s c o n f i n e d between the e l e c t r o d e s as shown i n F i g u r e 2. A t f r e q u e n c i e s g r e a t e r t h a n 5 MHz, the e l e c t r i c a l response i s p r i m a r i l y determined by the e l e c t r o n s s i n c e the l a r g e r mass o f the i o n s l i m i t t h e i r r e s p o n s e . The plasma sheaths are modeled as c a p a c i t o r s s i n c e they have a low c o n - c e n t r a t i o n o f e l e c t r o n s due t o the f o r m a t i o n o f the sheath e l e c t r i c f i e l d w h i c h r e p e l s the e l e c t r o n s . The b u l k of the plasma a c t s as a r e s i s t o r where the r e s i s t a n c e i s determined by the c o n c e n t r a t i o n and m o b i l i t y of the e l e c t r o n s . U s i n g the a p p r o x i m a t i o n t h a t the e l e c t r o n d e n s i t y i s c o n s t a n t throughout, the plasma impedance i s modeled as a r e s i s t a n c e R and the plasma sheath as a capacitance C . Therefore, $

|v|

|v|

2

R =

cos 0

and

—77-

=

sin 6

|I|

|!|

where 0 i s the phase l a g , |v| i s the r f v o l t a g e a m p l i t u d e , and | l | i s the r f c u r r e n t a m p l i t u d e . I n a d d i t i o n , the r a t i o o f the average e f f e c t i v e e l e c t r i c f i e l d t o p r e s s u r e E / p , the sheath v o l t a g e V , and the e l e c t r o n c o n c e n t r a t i o n n can be c a l c u l a t e d as e

g

e

E

e

M

R

ll J

d

where d i s the e l e c t r o d e s p a c i n g , A i s the e l e c t r o d e a r e a , and e i s the charge of an e l e c t r o n . The above impedance model makes a number o f s i m p l i f y i n g a p p r o x i mations which are n o t p h y s i c a l l y c o r r e c t . F i r s t , the e l e c t r o n conc e n t r a t i o n i s not u n i f o r m but v a r i e s a p p r o x i m a t e l y as shown i n F i g u r e 3. The e l e c t r i c f i e l d a t the i n t e r f a c e between the plasma and the sheath i n c r e a s e s c a u s i n g a l o c a l l y h i g h e r E /p t h a n the E /p w h i c h the model c a l c u l a t e s w h i c h r e p r e s e n t s o n l y an average v a l u e , however, i t s h o u l d be q u a l i t a t i v e l y c o r r e c t . Due t o the h i g h e r E /p i n the sheath, the h i g h e r energy p r o c e s s e s are f a v o r e d , e.g. ionization. T y p i c a l l y , the e l e c t r o n c o n c e n t r a t i o n i s g r e a t e s t a t the c e n t e r of the d i s c h a r g e s i n c e e l e c t r o n s are produced throughout the dsicharge and gas-phase e l e c t r o n l o s s p r o c e s s e s are o f t e n much l e s s t h a n e l e c t r o n l o s s by d i f f u s i o n t o the e l e c t r o d e s u r f a c e s . The power d i s s i p a t e d by i o n bombardment of the s u r f a c e s a t these f r e q u e n c i e s (4 to 12 MHz) i s r e l a t i v e l y minor ( l e s s t h a n 3 % ) , as can be shown by the product o f the i o n f l u x and sheath v o l t a g e (which were experiment a l l y measured and are d i s c u s s e d b e l o w ) . However, the model assumes a c o n s t a n t sheath c a p a c i t a n c e and does n o t i n c l u d e the motion o f the i o n s and e l e c t r o n s w i t h i n the plasma s h e a t h w h i c h l e a d s t o n o n l i n e 0

e

Q


172

C H E M I C A L A N D P H Y S I C A L PROCESSING OF INTEGRATED CIRCUITS

a r i t i e s t h a t can be r e a d i l y observed a t the lower f r e q u e n c i e s . The model does a reasonable j o b o f i d e n t i f y i n g the t r e n d s i n e l e c t r o n d e n s i t y , e l e c t r o n energy, s h e a t h v o l t a g e , and sheath capacitance w h i c h are n e c e s s a r y t o the u n d e r - s t a n d i n g o f the plasma k i n e t i c s . P o l y s i l c o n Etching i n CI,

Discharges

The e t c h i n g o f h e a v i l y P-doped p o l y s i l i c o n f i l m s has been s t u d i e d as a f u n c t i o n of f r e q u e n c y i n a s i n g l e - w a f e r , p a r a l l e l p l a t e e t c h e r w i t h anodized aluminum e l e c t r o d e s . The e t c h i n g r a t e was measured as a f u n c t i o n o f f r e q u e n c y , F i g u r e 4, w h i l e m a i n t a i n i n g a c o n s t a n t p r e s sure o f 0.6 t o r r , an e l e c t r o d e s p a c i n g o f 2.5 cm, a C l f l o w r a t e o f 5 seem, and a d i s c h a r g e power o f 0.8 W/cm . S i m i l a r t r e n d s were a l s o observed a t 0.3 t o r r . Below about 5 MHz, the e t c h i n g was a n i s o t r o p i c s u g g e s t i n g t h a t the e t c h i n g p r o c e s s was c o n t r o l l e d by ion-induced processes. Above 5 MHz, the e t c h i n g was i s o t r o p i c i n d i c a t i n g the dominance of c h e m i c a l e t c h i n g , w i t h CI l i k e l y b e i n g the primary e t c h a n t . The i n c r e a s e i n r a t e above 5 MHz suggests t h a t the f r a c t i o n o f power d i s s i p a t e d i n CI p r o d u c t i o n i n c r e a s e s w i t h f r e q u e n c y . The d i a g n o s t i c measurements d e s c r i b e d below were performed i n a s i m i l a r a p p a r a t u s , but w i t h s t a i n l e s s s t e e l e l e c t r o d e s . The e t c h i n g r a t e s i n t h i s system a l s o i n c r e a s e d w i t h f r e q u e n c y above 1 MHz, in c o n s t r a s t t o the d e c r e a s i n g r a t e s observed by B r u c e ( 1 1 ) . Bruce, however, was e t c h i n g undoped, s i n g l e c r y s t a l s i l i c o n w h i l e i n t h i s s t u d y n - t y p e p o l y s i l i c o n was used w h i c h i s expected t o be a t t a c h e d by C I . The amplitude of the r f v o l t a g e a t the powered e l e c t r o d e i s a l s o shown i n F i g u r e 4. U s i n g |v|/2 t o s c a l e the i o n energy, the energy s h o u l d s i g n i f i c a n t l y decrease w i t h i n c r e a s i n g f r e q u e n c y . In a d d i t i o n , at low f r e q u e n c i e s the i o n s are able t o pass t h r o u g h the sheath i n l e s s than one r f c y c l e , t h e r e b y i n c r e a s i n g the maximum i o n enegry. The d e c r e a s i n g i o n energy w i t h frequency should reduce the ion-enhanced etching rate which could partially explain the t r a n s i t i o n from a n i s o t r o p i c t o i s o t r o p i c e t c h i n g . The o p t i c a l e m i s s i o n f o r peaks a s s o c i a t e d w i t h C l (255 nm), Cl (386.3 nm), and CI (837.6 nm) are shown as a f u n c t i o n of f r e q u e n c y i n F i g u r e 5 where the e m i s s i o n has been n o r m a l i z e d f o r a l l the peaks at 100 KHz. The t r e n d s f o l l o w those o f the e t c h i n g r a t e . S i n c e the t o t a l p r e s s u r e i s held constant, the i n c r e a s i n g C l e m i s s i o n s u g g e s t s t h a t the e l e c t r o n c o n c e n t r a t i o n w i t h i n the plasma i n c r e a s e s w i t h frequency o r t h a t the e l e c t r o n energy changes c a u s i n g g r e a t e r e m i s s i o n . The d e c l i n e i n the CI e m i s s i o n r e l a t i v e t o t h a t o f Cl does not n e c e s s a r i l y i n d i c a t e the r e d u c t i o n i n CI c o n c e n t r a t i o n s i n c e the a l t e r a t i o n o f the e l e c t r o n energy d i s t r i b u t i o n and the d i f f e r e n c e i n e x c i t a t i o n t h r e s h o l d s c o u l d dominate. The p o s i t i v e i o n f l u x t o the w a f e r e l e c t r o d e , shown i n F i g u r e 6 as a f u n c t i o n o f f r e q u e n c y , was determined u s i n g an e l e c t r o m e t e r w h i c h measured the i o n c u r r e n t p a s s i n g through a 50 urn o r i f i c e i n the w a f e r e l e c t r o d e . S i n c e the d i a m e t e r o f the o r i f i c e i s l e s s t h a n t h a t o f the mean-free-path o f the p a r t i c l e s w i t h i n the plasma, the f l o w i s c o l 1 i s i o n l e s s and e q u i v a l e n t t o t h a t i m p i n g i n g upon the w a f e r . A t lower f r e q u e n c i e s , the observed f l u x c o r r e l a t e s w e l l w i t h the observed e t c h i n g r a t e s s u g g e s t i n g t h a t the r a t e i s c o n t r o l l e d by i o n bombardment. However, i n the h i g h frequency r e g i o n , the f l u x does not c o r r e l a t e w i t h the e t c h i n g r a t e . The l a r g e v a r i a t i o n of the 2


2

+

2

+

2

2

2


Plasma-Assisted

SAWIN ET AL.

Processing

r f Plasma


F i g u r e 2.

E l e c t r i c a l Analog

E l e c t r i c a l a n a l o g f o r c a p a c i t i v e l y powered

plasma.

Intra-electrode Position F i g u r e 3. Expected n o n - u n i f o r m i t y p o s i t i o n between e l e c t r o d e s .

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w i t h i n a plasma as a f u n c t i o n

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Integrated Circuits - American Chemical Society

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