Kinetics of Damage Generation on Single-Crystal Silicon Surfaces

Chapter 14

Kinetics of Damage Generation on Single-Crystal Silicon Surfaces The Influence of Lubricant Adsorption 1

2

2

Yuheng Li1, Steven Danyluk , Selda Gunsel , and Frances Lockwood

Downloaded by COLUMBIA UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: March 23, 1992 | doi: 10.1021/bk-1992-0485.ch014

1

Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago, Chicago, IL 60680 Pennzoil Products Company, The Woodlands, TX 77387 2

Single crystal s i l i c o n surfaces ((111) n-type with resistivity of approximately 30-Ωcm) were scratched at room temperature by a dead-loaded spherical diamond indenter (0.49-1.96 N) that was translated in the [110] direction at a speed of 5cm/sec. The scratches were placed between four e l e c t r i c a l pads through which the r e s i s t i v i t y of the damage region can be recorded. The surface of the s i l i c o n was covered with a commercial lubricant additive and the concentration of the additive and the load on the diamond were systematically varied. The change in voltage as a function of time was dependent on the concentration of the lubricant additive. This suggested that the adsorption and e l e c t r o l y t i c properties of the lubricant additive are related to the extent of the subsurface damage produced by the diamond.

The c h e m i s o r p t i o n o f l u b r i c a n t s on s i l i c o n s u r f a c e s h a s a good d e a l o f i n f l u e n c e on t h e m e c h a n i c a l p r o c e s s i n g o f s i l i c o n i n t o complex s h a p e s . F o r example, m e c h a n i c a l damage can occur a result of abrasion during semiconductor p r o c e s s i n g , such as sawing, l a p p i n g , d i c i n g and p o l i s h i n g . T h i s m e c h a n i c a l p r o c e s s i n g i s u s u a l l y done i n t h e p r e s e n c e of a l u b r i c a n t t h a t i s supposed t o reduce b l a d e v i b r a t i o n and a i d i n removing the f r i c t i o n a l heat. L u b r i c a n t s can however r e a c t c h e m i c a l l y w i t h s i l i c o n s u r f a c e s a n d i t h a s been f o u n d t h a t t h e c h e m i c a l r e a c t i o n s a t s i l i c o n s u r f a c e s i n f l u e n c e t h e m e c h a n i c a l damage g e n e r a t i o n a n d p r o p a g a t i o n [l]-[3]. T h e r e have been a number o f e x p e r i m e n t a l methods, s u c h a s e t c h i n g , x - r a y t o p o g r a p h y and e l e c t r o n m i c r o s c o p y t o

0097-6156/92/0485-0231$06.00/0 © 1992 American Chemical Society In Surface Science Investigations in Tribology; Chung, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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SURFACE SCIENCE INVESTIGATIONS IN TRIBOLOGY

d e t e r m i n e t h e amount o f t h e damage g e n e r a t e d due to the m e c h a n i c a l p r o c e s s i n g of s i l i c o n [ 4 ] - [ 6 ] . These s t u d i e s have shown that the damage consists of dislocations and microcracks, and t h e d e n s i t y o f t h e s e d e f e c t s d e p e n d s on doping level, crystallographic orientation and the e n v i r o n m e n t . T h e r e i s not yet a q u a n t i t a t i v e theory to explain or predict the influence of the semiconductor p r o p e r t i e s and e n v i r o n m e n t s on t h e t y p e o r e x t e n t o f damage. D a n y l u k e t . a l . [ 7 ] d e v e l o p e d an e l e c t r i c a l resistivity measurement t e c h n i q u e t o q u a n t i f y the m e c h a n i c a l damage produced in linear scratches and dicing grooves of i n s t r u m e n t e d s p h e r i c a l and V i c k e r s diamonds and diamondi m p r e g n a t e d w h e e l s . The t e c h n i q u e i n v o l v e s fabricating a s p e c i a l l y d e s i g n e d p r i n t e d c i r c u i t on a s i l i c o n w a f e r . The c i r c u i t s i m u l a t e s a f o u r p o i n t p r o b e measurement t h a t i s used in the semiconductor industry to measure sheet r e s i s t a n c e o f w a f e r s . The c i r c u i t can be u s e d t o m e a s u r e changes i n r e s i s t i v i t y o f the s i l i c o n as a d e a d - l o a d e d diamond i n d e n t e r s c r a t c h e s the s i l i c o n s u r f a c e between t h e probes. The p r e s e n t p a p e r d e s c r i b e s a r e a l - t i m e measurement o f t h e change i n r e s i s t i v i t y w h i c h i s r e l a t e d t o t h e k i n e t i c s o f t h e damage g e n e r a t i o n . L u b r i c a n t a d d i t i v e s on t h e s u r f a c e o f t h e s i l i c o n m o d i f y the k i n e t i c b e h a v i o r . We r e p o r t the results of a dynamic measurement of the change in r e s i s t i v i t y d u r i n g the s c r a t c h i n g o f s i n g l e c r y s t a l s i l i c o n as a f u n c t i o n o f l u b r i c a t i n g e n v i r o n m e n t a l c o n d i t i o n s and dead l o a d s . An a d s o r p t i o n model o f d e f o r m a t i o n i s p r o p o s e d . EXPERIMENTAL PROCEDURE Single crystal s i l i c o n w a f e r s (50 mm d i a m e t e r , 0.33 mm t h i c k , (111) n - t y p e ) , s u p p l i e d by t h e Monsanto E l e c t r o n i c M a t e r i a l s Company, were u s e d f o r t h i s s t u d y . The wafer s u r f a c e s a r e p o l i s h e d on one s i d e and l a p p e d on t h e o t h e r s i d e . The r e s i s t i v i t y was a p p r o x i m a t e l y 30 fi-cm. The w a f e r s were p r o c e s s e d as d e s c r i b e d p r e v i o u s l y [8] i n order to produce a s i m u l a t i o n of a four p o i n t probe measurement. F i g u r e 1 shows a s c h e m a t i c d i a g r a m o f the circuit. A constant DC current i s s u p p l i e d at the two o u t s i d e pads and t h e change o f the v o l t a g e was m e a s u r e d a t t h e i n s i d e pads as a f u n c t i o n o f t i m e f o r v a r i o u s s c r a t c h i n g conditions and environmental v a r i a b l e s . The experiment i n v o l v e s d e a d - l o a d i n g a s p h e r i c a l diamond ( n o m i n a l r a d i u s o f 0.33 mm) and s e t t i n g t h e diamond i n m o t i o n a t a s p e e d o f 50 mm/sec i n t h e [110] d i r e c t i o n . The diamond p r o d u c e s a s i n g l e s c r a t c h t h a t i s p o s i t i o n e d midway between t h e two inner m e t a l l i z a t i o n p a d s . The e x p e r i m e n t s a r e c a r r i e d o u t i n a clean room e n v i r o n m e n t where the r e l a t i v e humidity and t e m p e r a t u r e a r e 20% and 78° F r e s p e c t i v e l y . D e - i o n i z e d water and a c o m m e r c i a l l u b r i c a n t were p l a c e d on t h e s i l i c o n s u r f a c e p r i o r t o t h e s c r a t c h f o r m a t i o n , and t h e s e r e s u l t s were compared t o the c a s e w i t h t h e surface

In Surface Science Investigations in Tribology; Chung, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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Damage Generation on Single-Crystal Silicon Surfaces 233

free of lubricants. The commercial lubricant was an alkylated surfactant dissolved in de-ionized water to p r o d u c e c o n c e n t r a t i o n s o f 0.015, 0.029, 0.053, 0.076 and 0.100 wt%. The c o n d u c t i v i t i e s o f t h e s e s o l u t i o n s r a n g e d f r o m 9 t o 150 p.mho/cm and was measured by C o l e - P a r m e r M o d e l 148410 c o n d u c t i v i t y meter. F i g u r e 2 shows t h e results of c o n d u c t i v i t y v e r s u s t h e wt% o f t h e l u b r i c a n t a d d i t i v e . The voltage-time measurements are recorded by a computer system at a sampling rate of 100,000 Hz per c h a n n e l . The v o l t a g e r e s o l u t i o n o f t h e A/D c o n v e r t e r i s 0.01 mV. A s c h e m a t i c d i a g r a m o f the s y s t e m i s shown i n F i g u r e 3. F i g u r e 4 shows a s c h e m a t i c d i a g r a m o f t h e b a l l - o n - f l a t g e o m e t r y and t h e a n t i c i p a t e d change i n r e s i s t i v i t y a s a f u n c t i o n o f p o s i t i o n . The c o m p r e s s i v e s t r e s s e s ahead o f t h e diamond and t h e t e n s i l e s t r e s s e s b e h i n d t h e diamond p r o d u c e damage so t h a t t h e r e s i s t i v i t y i n c r e a s e s as a f u n c t i o n o f t i m e as t h e diamond a p p r o c h e s t h e p r o b e p o s i t i o n s , and a s t e a d y s t a t e change i s p r o d u c e d a f t e r the diamond has p a s s e d the probe p o s i t i o n s . RESULTS F i g u r e 5 shows examples o f CRT d i s p l a y s of the voltage v e r s u s time f o r these d i f f e r e n t s u r f a c e environments ( a i r , DI w a t e r and a l u b r i c a n t a t a c o n c e n t r a t i o n o f 0.100 wt%) and a d e a d - l o a d o f 0.98 N (100 g f ) . In e a c h c a s e , t h e r e i s a rise in voltage when the diamond passes by the m e t a l l i z a t i o n p a d s , and t h e i n c r e a s e i n v o l t a g e r e l a t i v e t o when t h e r e i s no damage. The s l o p e o f t h e v o l t a g e v e r s u s t i m e v a r i e s w i t h e n v i r o n m e n t a l c o n d i t i o n s . F o r example, t h e r e l a t i v e c h a n g e s i n v o l t a g e a r e 3.42, 6.17 and 3.75 % and w i t h s l o p e s o f 106 mv/sec, 93 mv/sec and 70 mv/sec f o r t h e u n l u b r i c a t e d s u r f a c e , d e - i o n i z e d and 0.100 wt% lubricant additive respectively. Other lubricant additive c o n c e n t r a t i o n s gave s i m i l a r r e s u l t s . The r e l a t i v e v o l t a g e change (%) v e r s u s t i m e (ms) f o r a l l the l u b r i c a n t a d d i t i v e c o n c e n t r a t i o n s at a dead-load of 0.49 N (50 g f ) i s shown i n F i g u r e 6. The change i n v o l t a g e shows an i n c r e a s e o f 3.27 % i n a i r e n v i r o n m e n t and 6.07 % i n DI water. As the lubricant additive concentration is i n c r e a s e d f r o m 0 t o 0.100 wt%, t h e v o l t a g e v a r i e s f r o m 6.07 % t o 2.45 %. The s l o p e o f t h e v o l t a g e - t i m e p l o t s f o r e a c h o f t h e l u b r i c a n t a d d i t i v e c o n c e n t r a t i o n s v a r i e d and F i g u r e 7 shows t h e s l o p e o f t h e v o l t a g e change as a f u n c t i o n o f l u b r i c a n t a d d i t i v e c o n c e n t r a t i o n . The d a t a show t h a t t h e slope d e c r e a s e s from 97.4 t o 50.8 mv/sec as t h e l u b r i c a n t a d d i t i v e c o n c e n t r a t i o n i n c r e a s e s f r o m 0 t o 0.100 wt%. F i g u r e 8 shows t h e r e l a t i v e change i n v o l t a g e (%) a s a f u n c t i o n o f t h e l u b r i c a n t a d d i t i v e c o n c e n t r a t i o n (wt%) and l o a d ( N ) . The f i q u r e shows t h a t as t h e l o a d i n c r e a s e s , t h e r e l a t i v e change i n v o l t a g e a l s o i n c r e a s e s but t h e e f f e c t o f lubricant i s smaller.



234


Silicon Figure

Wafer

Ohmic

Contact

1. S c h e m a t i c Diagram o f the R e s i s t i v i t y Measurement.

Sample

for

Dynamic

160.0

120.0

80.0

40.0

-

0.0 0.000

0.020

Alkylated Figure

0.040

0.060

Surfactant

0.080

0.100

Concentration

0.120

(wt %)

2. The C o n d u c t i v i t y o f t h e S o l u t i o n a s a F u n c t i o n o f Additive Concentration.


14. LI ET AL.

Damage Generation on Single-Crystal Silicon Surfaces 235 DATA ACQUISITION SYSTEM DAS-20 BOARD CLOCK

_ |

_ J

MOTOR CONTROL


COMPUTER SYSTEM

SOFTWARE PACKAGE

CRT dynamic resistivity profile

Figure

3 . S c h e m a t i c Diagram o f t h e S c r a t c h i n g A p p a r a t u s D a t a A c q u i s i t i o n System.

and

I Force . 5cm/s

damage zone

/

undamage zone

AP «, retistirity of damaged surface

~Z—

/o

Po

resistivity of undamaged surface

Position

Figure

4.

S c h e m a t i c D i a g r a m o f t h e B a l l - o n - F l a t G e o m e t r y and E x p e c t e d Change i n R e s i s t i v i t y a s a F u n c t i o n o f Time.




236

Figure 5. CRT Displays of Surface Environments Effect on Dynamic Voltage. Continued on next page.


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I0.100 wt%

LubricantLoad=100

gm|

a v = 1 . 4 mv At=2 0 ms

slope=70 mv/secI

Figure 5. Continued.

8.0 DI Water

~

6.0

cr>

5

4.0

> 0)

O

2.0

0.053

0.100

0.0

c

-2.0 -40.0

-20.0

0.0 Time

20.0

40.0

60.0

(ms)

Figure 6. The Relative Change in Voltage versus Time.


238


100.0


80.0

60.0

40.0 -0.2 Alkylated

0.02

0.06

0.10

Surfactant Concentration

F i g u r e 7. The S l o p e o f t h e V o l t a g e Change Surfactant Concentration.

(wt%)

versus A l k y l a t e d


14. LI ET AL.



DISCUSSION I t i s w e l l known t h a t t h e s c r a t c h i n g damage i n s i l i c o n i s composed of microcracks and dislocations [9]-[10]. D i s l o c a t i o n s a r e formed a s a r e s u l t o f c o m p r e s s i v e s t r e s s e s and m i c r o c r a c k s a r e formed a s a r e s u l t o f t e n s i l e s t r e s s e s [ 1 1 ] . D i s l o c a t i o n s and m i c r o c r a c k s s c a t t e r e l e c t r o n s so t h a t r e s i s t i v i t y i n c r e a s e s a s t h e number o f m i c r o c r a c k s a n d d i s l o c a t i o n s i n c r e a s e s [ 1 2 ] - [ 1 4 ] . T h i s means t h a t e l e c t r i c a l r e s i s t i v i t y i s r e l a t e d t o t h e amount o f m e c h a n i c a l damage. Environmental effects are also known t o have an i n f l u e n c e on t h e t r i b o l o g i c a l b e h a v i o r o f s e m i c o n d u c t i n g m a t e r i a l s and a number o f r e s e a r c h e r s have i n v e s t i g a t e d t h e a d s o r p t i o n e f f e c t s on s u r f a c e m e c h a n i c a l p r o p e r t i e s [ 1 5 ] [ 1 6 ] . I t h a s been p r o p o s e d t h a t a d s o r p t i o n o f g a s e s a n d f l u i d s e i t h e r p i n d i s l o c a t i o n cores to the surface or modify the s u r f a c e energy so t h a t crack p r o p a g a t i o n i s f a v o r a b l e . T h e r e i s a l s o some e v i d e n c e t h a t s p a c e c h a r g e e f f e c t s c a n i n f l u e n c e t h e s u r f a c e d e f o r m a t i o n i f t h e e x t e r n a l l y imposed l o a d s g e n e r a t e s t r e s s e s t h a t have a s p a t i a l e x t e n t o f t h e same s i z e a s t h e s p a c e c h a r g e r e g i o n [ 1 7 ] . S u r f a c e c h a r g e s affect t h e d i s l o c a t i o n m o t i o n and damage f o r m a t i o n and g e n e r a t i o n [ 1 8 ] - [ 1 9 ] b e c a u s e t h e s i z e and m a g n i t u d e o f t h e s u r f a c e c h a r g e d e t e r m i n e s t h e s p a c e c h a r g e f i e l d s . The m a g n i t u d e and e x t e n t o f t h e s p a c e c h a r g e d e p e n d s on t h e s u r f a c e c h a r g e , b u l k d o p i n g and t e m p e r a t u r e , and f o r s i l i c o n a t room t e m p e r a t u r e i t c a n range f r o m 0.5 t o 10 um b e l o w t h e surface [ 2 0 ] . These depths of the space charge a r e comparable to that of the y i e l d stress surfaces f o r spherical indenters contacting silicon. Lubricating e n v i r o n m e n t s c a n m o d i f y t h e s u r f a c e c h a r g e and c o n s e q u e n t l y t h e d e f o r m a t i o n mode. The e x p e r i m e n t a l r e s u l t s p r e s e n t e d i n t h i s p a p e r show t h a t t h e v o l t a g e v e r s u s t i m e i s c o r r e l a t e d w i t h t h e movement o f t h e diamond and e x h i b i t s a t i m e d e p e n d e n t and s t e a d y s t a t e v a l u e . The f o r m e r depends on t h e a d s o r p t i o n k i n e t i c s and damage p r o p a g a t i o n a s t h e s t r e s s e s v a r y , a n d t h e l a t t e r i s r e l a t e d t o t h e t o t a l damage. The model f o r t h e s t e a d y state change i n voltage has been previously e x a m i n e d and f o u n d t o be r e l a t t h e damage s i z e a n d t h e c o n d u c t i v i t y o f t h e damage zone f o r a l i n e a r s c r a t c h made by a diamond i n d e n t e r [ 7 ] :

(1)

where V and V a r e t h e measured v o l t a g e w i t h and w i t h o u t damage, b i s t h e s u b s u r f a c e damage c r o s s s e c t i o n a l r a d i u s , d i s t h e i n n e r p r o b e s p a c i n g , and a and «^ a r e t h e c o n d u c t i v i t i e s o f t h e s u b s u r f a c e damage zone and t n e s i l i c o n s u b s t r a t e , r e s p e c t i v e l y . E q u a t i o n (1) e x p r e s s e s t h e v o l t a g e Q

2


240


i n t e r m s o f measured q u a n t i t i e s and i t i s e x p e c t e d t h a t v o l t a g e w i l l change a s t h e damage s i z e i n c r e a s e s o r t h e c o n d u c t i v i t y decreases. The e f f e c t i v e s i z e o f t h e damage z o n e , b, c a n be m o d i f i e d by a d s o r p t i o n i f t h e y i e l d s u r f a c e i s w i t h i n t h e space charge fields. The space charge fields in semiconductors i n contact with electrolytic fluids i s d e s c r i b e d by a Debye l e n g t h , w h i c h h a s been f o u n d t o v a r y a s [21]:

e.kT


2e

a n (

e

a

r

e

t

h

1

V

/

o

x

e

where ^ * ? d i e l e c t r i c constants of semiconductor and e l e c t r o l y t e , q and a r e t h e e l e c t r o n c h a r g e and d o p i n g density, V , V and U are the surface p o t e n t i a l o f the semiconductor, the a p p l i e d p o t e n t i a l across the d i f f u s i o n l a y e r o f e l e c t r o l y t e and s p a c e c h a r g e p o t e n t i a l a t t h e interface and k and T a r e B o l t z m a n n ' s constant and temperature r e s p e c t i v e l y . The above e q u a t i o n s u g g e s t s t h a t t h e t h i c k n e s s o f t h e space charge region v a r i e s with ^ d i e l e c t r i c constant o f t h e e l e c t r o l y t e , t e m p e r a t u r e and d o p i n g l e v e l o f t h e s e m i c o n d u c t o r . As a r e s u l t , m e c h a n i c a l damage w i l l depend on the l u b r i c a n t a d d i t i v e c o n c e n t r a t i o n because the d i e l e c t r i c constant w i l l vary with c o n c e n t r a t i o n . A d s o r p t i o n w i l l i n f l u e n c e damage i f b^x. I n t h i s c a s e : s

e

Q

t

n

e

so t h a t t h e r e s i s t i v i t y o f t h e damage r e g i o n i s r e l a t e d t o t h e t e r m s i n t h e c u r l y b r a c k e t s . S i n c e t h e Debye l e n g t h i s proportional to the resistivity of the electrolytic s o l u t i o n , t h e r e l a t i v e change i n v o l t a g e w i l l d e c r e a s e a s the concentration of the l u b r i c a n t a d d i t i v e increases. F i g u r e 2 shows t h a t t h i s c o r r e l a t i o n h o l d s t r u e w i t h i n t h e concentration ranges used f o r the experiments i f the temperature and d o p i n g level are held constant. The c o r r e l a t i o n between t h e s e v a r i a b l e s and t h e s i z e o f t h e damage zone h a s r e c e n t l y been r e p o r t e d [ 1 7 ] . The kinetics of the voltage change i s r e l a t e d t o a d s o r p t i o n and t h e v a r i a b i l i t y w i t h t i m e o f t h e c o n d u c t i v i t y o f t h e damage r e g i o n . The v o l t a g e v a r i e s w i t h t i m e a s c r a c k s p r o p a g a t e and e x p o s e new s u r f a c e t o t h e e l e c t r o l y t e . The mechanisms f o r t h e k i n e t i c b e h a v i o r a r e t h e r e f o r e t i e d t o t h e i n c r e a s e o f b and a w i t h t i m e . T h e s e mechanisms w i l l be t h e s u b j e c t o f upcoming p u b l i c a t i o n s . 2


14. LI ET AL.



CONCLUSIONS 1.

The i n - s i t u e l e c t r i c a l measurement t e c h n i q u e p r o v i d e s i n f o r m a t i o n on t h e time d e p e n d e n t a n d s t e a d y state generation o f mechanical damage i n s i l i c o n during s c r a t c h i n g by a d e a d - l o a d e d s p h e r i c a l diamond. The r e l a t i v e changes i n v o l t a g e (0.98 N) a r e 3.42, 6.17, and 3.75 % and t h e s l o p e s a r e 106, 93 a n d 70 f o r t h e u n l u b r i c a t e d s u r f a c e , d e - i o n i z e d water a n d 0.100 wt% lubricant additive respectively.

2.

The change o f v o l t a g e w i t h t i m e a l s o depends on t h e dead-load on t h e diamond. The s t e a d y s t a t e v o l t a g e change increases as the l o a d increases but the i n f l u e n c e of l u b r i c a n t i s decreased.

3.

The s l o p e o f t h e v o l t a g e v e r s u s t i m e c u r v e versus alkylated surfactant concentration i s negative. This d a t a i s c o n s i s t e n t w i t h t h e model t h a t t h e d i e l e c t r i c c o n s t a n t o f t h e l u b r i c a n t d o m i n a t e s t h e Debye l e n g t h i n the s i l i c o n .

4.

The s t e a d y s t a t e v a r i a t i o n i n t h e v o l t a g e - t i m e p l o t h a s been modeled a s a damage region whose s i z e and c o n d u c t i v i t y determines the v o l t a g e .

ACKNOWLEDGEMENTS T h i s work was s u p p o r t e d p a r t i a l l y by t h e N a t i o n a l S c i e n c e F o u n d a t i o n - T r i b o l o g y Program under g r a n t No. MSM-8714491. We thank Dr. Larsen-Basse f o r h i s s u p p o r t . Thanks a r e a l s o e x t e n d e d t o Dr. Jang Y u l Park o f Argonne N a t i o n a l L a b o r a t o r y for the conductivity measurements of the alkylated surfactants. REFERENCES

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RECEIVED November 27, 1991


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