Metallurgical study of flip-chip bonding
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the d u r a t i o n of the v i b r a t i o n was within 0.5 sec. The e l e c t r o n m i c r o f r a c t o g r a p h y and o r d i n a r y r e p l i c a o b s e r v a t i o n were c a r r i e d out on a Hitachi HU-11 m i c r o s c o p e . The a r g o n - i o n - b o m b a r d m e n t etching was given at an a c c e l e r a t i n g voltage of 5 kv f o r the d u r a tion of 2.5 hr. F o r the t e n s i l e t e s t , the chips were pulled off f r o m the s u b s t r a t e s on an I n s t r o n - t y p e t e s t ing machine. The method of the t e s t i n g is shown in Fig. 1. The p l a s t i c d e f o r m a t i o n of p e d e s t a l s was obs e r v e d by using an i n t e r f e r o m e t r i c m i c r o s c o p e . The contact e l e c t r i c a l r e s i s t a n c e was m e a s u r e d on s p e c i ally made chips and s u b s t r a t e s by u s i n g a c a l i b r a t e d circuit. 2) E X P E R I M E N T A L RESULTS 2.1) M i c r o s c o p i c O b s e r v a t i o n . Upon e x a m i n i n g the chips pulled off f r o m the s u b s t r a t e s , we s o m e t i m e s found that the p e d e s t a l s c a m e off sticking to the pads and vice v e r s a , but in many c a s e s , f r a c t u r e o c c u r r e d at the i n t e r f a c e s b e t w e e n pads and p e d e s t a l s . The f o r m e r case i m p l i e s that the bonding b e t w e e n pads and p e d e s t a l s is s t r o n g e r than that between p e d e s t a l s and s p u t t e r e d g l a s s (or that between pads and s i l i c a on the chips). Fig. 2 shows an example of the f o r m e r case.
I Spring for Buffer
Dia CopperWire Adhesive Chip Substrate ,CopperBlock lmm
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(> Fig. 1--Method for pulling off chips from a substrate by a tensile testing machine. VOLUME 1,MARCH 1970 679
(a)
(b)
(c)
Fig. 2--The aluminum pillar. Magnification 345 times. (a) Focused at the bottom (surface of the pad); (b) focused at the point 9 p higher from the bottom; (c) focused at the point 19/~ higher from the bottom (equal to the piller height).
Fig. 3--A pulled-off pad and its pedestal. Magnification 410 times. (a) Pedestal; (b) pad.
(b)
(a) shows the pad s u r f a c e , (b) shows the side s u r f a c e of the elongated p e d e s t a l , and (c) shows the i n t e r f a c e b e t w e e n the p e d e s t a l and the s p u t t e r e d g l a s s . In the l a t t e r c a s e , we can see a white p a r t and a d a r k , dirty p a r t a s shown in Fig. 3. We o b s e r v e d these white and d a r k a r e a s u s i n g an e l e c t r o n m i c r o f r a c t o g r a p h i c technique. The r e s u l t s a r e shown in F i g s . 4 and 5. Fig. 4(a) shows the whole view of a pulled-off pad as seen by an optical m i c r o s c o p e . (b) shows the m a j o r a r e a of the pad on a m i c r o f r a c t o g r a p h . A r e a A is the o r i g inal s u r f a c e of the pad which r e m a i n s untouched by the pedestal. B is an a r e a swept by the d e f o r m i n g f r o n t of the pedestal. It c o n t a i n s m a n y b l a c k spots, which a r e identified as a l u m i n a f o r m e r
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