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T h e Synergism o f C o m p l e m e n t a r y Techniques J. N. Ramsey IBM Corporation, Hopewell Junction, NY 12533 It has been almost 25 years since Prof, David Wittry presented the first application of the electron probe microanalyzer to the solution of semiconductor/microelectronics problems(1). The need for small area chemical analysis has been strong in this field and is growing because of continuing miniaturization(2,3). The electron probe microanalyzer (and later the scanning electron microscope with energy dispersive analysis capability) has developed markedly over these 25 years. This small area analysis, almost exclusively elemental, has been essential for process development and problem solving. Elemental information, however, is often not sufficient. For example, finding hydrogen, carbon, oxygen and nitrogen in a particle does not add measurably to the identification of a material and its elimination from a process. Thus, the search has been for small area molecular analysis techniques to complement the elemental analysis. X-ray or electron diffraction allows identification of crystalline species by the long-used Hannawalt-Dow-ASTM-JCPDS system(4). Small particles can be removed for analysis in a small rotating specimen X-ray powder camera, or by extraction replication and selected area diffraction in a Transmission Electron Microscope (5). For those specimens where a residue of reactant or corrosion product is too adherent, the material may be removed for analysis by micro-bulldozing (with a microhardness indentor), micro-jack hammering (with a needle attached to a small piezoelectric crystal on a pencil-like rod), and micro-boring (with a precision controlled dental drill)(5). While these techniques are very useful, they fail with amorphous materials, organics and polymers. Polarized light microscopy has long been used in chemical and mineralogical studies, and was raised to a high level of applicability by Walter McCrone and associates, culminating in the extensive Particle Atlas(6), which included innumerable glasses, organics and polymers. Small area molecular spectroscopies are obviously desirable, but there have been problems of coupling a microscope and a spectrometer and retaining high efficiency energy transfer. This was solved for Raman spectroscopy by Dalhaye and Dhamelincourt(7), which resulted in 1 micron lateral-dimension 0097-6156/86/0295-0398$07.75/0 © 1986 American Chemical Society In Microelectronics Processing: Inorganic Materials Characterization; Casper, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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Raman i m a g i n g c a p a b i l i t y i n a n i n s t r u m e n t ( t h e MOLE) p r o d u c e d b y Jobin-Yvon(8). T h i s t e c h n i q u e has been w i d e l y a p p l i e d ( s e v e r a l symposia have been h e l d ) ( 9 , 1 0 ) i n c l u d i n g the f i e l d o f microelectronics(11,12). S m a l l a r e a i n f r a r e d a n a l y s i s has been made p o s s i b l e w i t h t h e i n t r o d u c t i o n o f a l l - r e f l e c t i n g i n f r a r e d microscope o p t i c s f o r use w i t h e i t h e r d i s p e r s i v e o r FT i n f r a r e d instruments(13). We h a v e a p p l i e d t h i s t e c h n i q u e e x t e n s i v e l y t o process c o n t r o l problems i n m i c r o e l e c t r o n i c s manufacturing(14-16). The v i b r a t i o n a l t r a n s i t i o n e n e r g i e s o f t h e m o l e c u l e s , w h i l e i n the i n f r a r e d p o r t i o n o f the spectrum, are "seen" a s the d i f f e r e n c e ( D e l t a ) between the impinging v i s i b l e l a s e r l i g h t and t h e s c a t t e r e d Raman l i g h t , a l s o i n t h e v i s i b l e . Both i n f r a r e d a n d Raman g i v e n o t o n l y a f i n g e r p r i n t o f e a c h m o l e c u l e , b u t a l s o information on i t s f u n c t i o n a l u n i t s ands t r u c t u r e . However, b e c a u s e t h e two m e t h o d s r e s p o n d t o d i f f e r e n t q u a n t u m m e c h a n i c a l s e l e c t i o n r u l e s , the i n t e n s i t i e s o f f u n c t i o n a l groups and s t r u c t u r e s can v a r y , g i v i n g sometimes markedly d i f f e r e n t f i n g e r p r i n t s o f t h e same m a t e r i a l . T h e u s e o f v i s i b l e l i g h t i n Raman t o s t i m u l a t e i n f r a r e d t r a n s i t i o n s o f f e r s s e v e r a l a d v a n t a g e s (11,17): S m a l l e r a n a l y t i c a l area (because the d i f f r a c t i o n l i m i t ( f o c u s s i n g ) o f v i s i b l e l i g h t l e s s t h a n lum v s . a b o u t 15-20ym f o r i n f r a r e d ) Heavy m e t a l compounds w i t h v e r y l o w v i b r a t i o n a l f r e q u e n c i e s c a n b e s e e n ( b e c a u s e w i t h Raman, t h e r a n g e e x t e n d s d o w n w a r d t o b e l o w 100cm , whereas i n f r a r e d stops a t about 500-600cm ) The s p e c t r u m c a n u s u a l l y b e e x a m i n e d i n - s i t u ( b e c a u s e t h e Raman c a n b e i n t h e r e f l e c t i v e mode, w h e r e a s w i t h i n f r a r e d , i n t h e t r a n s m i s s i o n mode , t h e s p e c i m e n m u s t b e t r a n s f e r r e d to an i n f r a r e d transparent substrate - f o r t u n a t e l y f o r s i l i c o n d e v i c e w o r k , s i l i c o n i s t r a n s p a r e n t and u s u a l l y p a r t i c l e s can be analyzed i n s i t u ) . [*N0TE: ADDED I N PROOF: S i n c e t h i s c o n f e r e n c e , c o n s i d e r a b l e p r o g r e s s h a s b e e n made i n r e f l e c t i o n microscope attachments on FTIR instruments(30).] Raman, h o w e v e r , a l s o h a s s e v e r a l d i s a d v a n t a g e s t h e a t n e e d t o be w o r k e d a r o u n d . Raman s c a t t e r i n g i s w e a k , r e q u i r i n g h i g h l a s e r o p t i c a l p o w e r d e n s i t i e s t h a t may " b u r n " o r v o l a t i l i z e t h e s a m p l e ( a l t h o u g h most specimens can be immersed i n a drop o f w a t e r , u n d e r a c o v e r g l a s s ( 1 7 ) ) . I n a d d i t i o n , t h e s p e c i m e n may f l u o r o s e c e , w h i c h w i l l o v e r w h e l m t h e Raman s i g n a l , a n d a l l m a t e r i a l s do not have Raman-active v i b r a t i o n a l bands. Thus, Raman a n d i n f r a r e d a r e v e r y c o m p l e m e n t a r y t e c h n i q u e s . L a s e r d e s o r p t i o n / i o n i z a t i o n and mass a n a l y s i s has b e e n a v a i l a b l e i n t h e t r a n s m i s s i o n mode f o r s e v e r a l y e a r s (18,19). T h i s t e c h n i q u e h a s r e c e n t l y b e e n e x t e n d e d t o t h e much m o r e u s e f u l r e f l e c t i o n mode a s t h e LAMMA 1000 b y L e y b o l d H e r a e u s ( 1 8 , 2 0 ) a n d t h e LIMA b y Cambridge Mass S p e c t r o s c o p y Co.(21,22) I n these instruments, the r e g i o n o f i n t e r e s t i s l o c a t e d under a microscope. T h e l a s e r beam i s f o c u s s e d a n d p u l s e d t h r o u g h a microscope, producing a plasma that fragments and i o n i z e s the m o l e c u l e s , w h i c h a r e t h e n a n a l y z e d i n a t i m e o f f l i g h t mass spectrometer. One o f t h e m a j o r p a r a m e t e r s t o b e c o n t r o l l e d a n d measured i s the l a s e r power, t o get r e p r o d u c i b l e f r a g m e n t a t i o n :
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t h e f r a g m e n t p a t t e r n s h o u l d be r e c o g n i z a b l e t o a n o r g a n i c m a s s s p e c t r o s c o p i s t and c l o s e t o t h o s e f r o m e l e c t r o n i m p a c t so t h a t t h e e x t e n s i v e l i t e r a t u r e and s p e c t r a l i b r a r i e s c a n be u t i l i z e d . I f t h e l a s e r p o w e r i s t o o h i g h a n d f r a g m e n t a t i o n e x t e n d s t o H, C, 0 a n d N, t h e n , o f c o u r s e , l i t t l e m o l e c u l a r i n f o r m a t i o n i s obtained. The m o s t p r o d u c t i v e i n t e r a c t i o n o f a n a n a l y t i c a l / c h a r a c t e r i z a t i o n g r o u p and t h e p r o c e s s d e v e l o p m e n t and m a n u f a c t u r i n g p e o p l e who h a v e t h e p r o b l e m , i s t o o p e r a t e i n a p r o b l e m - s o l v i n g mode. When y i e l d s , r e l i a b i l i t y , i n - p r o c e s s i n g t e s t i n g , o r some o t h e r c r i t i c a l p r o d u c t p a r a m e t e r s t a r t t o d r i f t , whether i t i s i n development or manufacturing, the analyst should n o t a c c e p t a l a r g e number o f s a m p l e s f o r e x h a u s t i v e elemental analysis. I t s h o u l d be d e t e r m i n e d what t h e n a t u r e o f t h e p r o b l e m i s , o r what a t t r i b u t e s of the p r o c e s s have s h i f t e d , o r what c h a n g e s w e r e made r e c e n t l y . (Every s o l u t i o n to a problem c a r r i e s w i t h i n i t the seeds of the next problem.)(3) The a t t i t u d e o f t h e a n a l y s t s h o u l d b e "We a r e h e r e t o h e l p y o u b y g e t t i n g t o k n o w y o u r p r o c e s s , and i t s i n t e r a c t i o n s w i t h p r e v i o u s and s u b s e q u e n t process steps . The a n a l y s t d e c i d e s t h e s p e c i m e n s m o s t l i k e l y t o show d i f f e r e n c e s and t h e p r o p e r f o r m f o r t h e t e c h n i q u e s t o be e m p l o y e d ( f o r e x a m p l e , A u g e r o r ESCA s h o u l d n o t b e p e r f o r m e d o n a s a m p l e t h a t has been c a r r i e d a r o u n d i n a someone's h a n d s ) . C o n s t a n t and i t e r a t i v e i n t e r a c t i o n i s g e n e r a l l y r e q u i r e d , l e a d i n g t o d e s i g n o f e x p e r i m e n t s , c l a r i f i c a t i o n s , more (and d i f f e r e n t ) a n a l y s e s , u n t i l t h e p r o c e s s i s u n d e r s t o o d and t h e p r o b l e m i s solved. Sometimes t h e p r o c e d u r e i s s h o r t ; more o f t e n , w i t h t h e c o m p l e x p r o c e s s i n t e r a c t i o n s s o common i n t h e m i c r o e l e c t r o n i c s f i e l d , i t i s not. T h i s problem s o l v i n g approach i s always productive because e v e r y o n e " w i n s " : t h e p r o c e s s e n g i n e e r has more u n d e r s t a n d i n g f o r b e t t e r p r o c e s s c o n t r o l , and t h e a n a l y s t i s n o t j u s t an a n a l y s t b u t , by u s i n g b r o a d s c i e n c e and e n g i n e e r i n g c o n c e p t s , t h e a n a l y s t i s an i n t e g r a l p a r t o f t h e p r o c e s s i n g team. 1 1
Examples of P r o b l e m S o l v i n g by M o l e c u l a r
Analysis
A n a l y s i s o f I n t e r m e t a l l i c s - Need f o r M o l e c u l a r A n a l y s i s . The IBM c h i p j o i n i n g s y s t e m u s e s s o l d e r b o n d i n g i n a new l a r g e m o d u l e , c a l l e d T h e r m a l C o o l e d M o d u l e , w h i c h r e q u i r e s an a b i l i t y t o remove and r e p l a c e c h i p s f o r e n g i n e e r i n g c h a n g e s . Stress t e s t s made d u r i n g t h e d e v e l o p m e n t p e r i o d o f t h e p r o g r a m r e q u i r e d i n - s i t u a n a l y s i s and c h a r a c t e r i z a t i o n o f t h e p h a s e s f o r m e d i n t h e r o u g h l y 50 m i c r o n d i a m e t e r v i a s o n t h e c h i p s . F i g u r e 1 shows e l e c t r o n microprobe video scans of the i n t e r m e t a l l i c phases r e m a i n i n g on t h e Cr/Cu/Au bond pad v i a m e t a l l u r g y a f t e r s e v e r a l r e f l o w s a n d s u b s e q u e n t e t c h i n g away o f t h e s o l d e r . T h e r e a p p e a r t o b e two t y p e s o f i n t e r m e t a l l i c s , w i t h d i f f e r e n t m o r p h o l o g i e s a n d d i f f e r e n t l e v e l s o f Cu a n d S n ( i t i s n o t p o s s i b l e t o do a q u a n t i t a t i v e a n a l y s i s on r e g i o n s as s m a l l and i r r e g u l a r as t h e s e p a r t i c l e s s i n c e t h e e l e c t r o n beam c a n n o t b e c o n t a i n e d i n a p a r t i c l e to match c o n d i t i o n s w i t h elemental standards). M i n i a t u r i z a t i o n i s progressing w i t h X-ray d i f f r a c t i o n : a l o c a t a b l e 1 0 - m i c r o n d i a m e t e r a r e a c a n now b e a n a l y z e d w i t h a microdiffractometer(23,24). F i g u r e 2 shows t h a t b o t h Cu~Sn and
In Microelectronics Processing: Inorganic Materials Characterization; Casper, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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Cu,Sn^ a r e p r e s e n t . The Au i n the Cr/Cu/Au f i l m attachment scheme on the S i c h i p a l s o forms Au-Sn i n t e r m e t a l l i c s , which m a i n l y f l o a t i n t o the s o l d e r and have been etched away i n p r e p a r i n g these samples. I n a d d i t i o n t o phase a n a l y s i s , the d e p a r t u r e from e q u i l i b r i u m l a t t i c e p o s i t i o n s can be i n t e r p r e t e d as a measure o f the s t r e s s i n a c r y s t a l l i n e s u b s t a n c e . Figure 3 shows the d e c r e a s e i n s t r e s s i n the Cu^Sn l a y e r as a f u n c t i o n of the number o f r e f l o w s : the d e c r e a s e i n s t r e s s was a t t r i b u t e d t o s t r e s s - r e l i e f by c r a c k i n g and s p a l l i n g i n t o the s o l d e r (which was v e r i f i e d by SEM). E x t r a c t i o n R e p l i c a t i o n ( C r y s t a l s i n F i r e d M o l y ) . The m u l t i l a y e r ceramic package t h a t i s used by IBM has a s i n t e r i n g o p e r a t i o n i n w h i c h the a l u m i n a / g l a s s body undergoes s i n t e r i n g - w i t h - a - l i q u i d phase s i m u l t a n e o u s l y w i t h the f i r i n g o f the molybdenum l a n d s . T h i s i s a complex p r o c e s s , r e q u i r i n g c l o s e c o n t r o l o v e r incoming m a t e r i a l s and p r o c e s s e s ( 2 5 ) . The molybdenum s u r f a c e i s e l e c t r o l e s s n i c k e l p l a t e d and e l e c t r o l e s s " f l a s h " g o l d p l a t e d t o p r o v i d e a s o l d e r a b l e o r b r a z e a b l e s u r f a c e . I n the e a r l y days of the program, t h e r e was an o c c u r r e n c e of b l i s t e r s of the Ni/Au p l a t i n g a f t e r a subsequent t h e r m a l o p e r a t i o n . F i g u r e 4a shows an SEM m i c r o g r a p h o f such a b l i s t e r , which has been opened and l a i d back w i t h a n e e d l e t o expose m a t e r i a l f o r a n a l y s i s . A s m a l l e r b l i s t e r i s seen towards 12 o ' c l o c k from the b i g b l i s t e r . Figure 4b, an enlargement, shows a c a v i t y network under the b l i s t e r and F i g u r e 4c shows t h a t the c a v i t y i s l i n e d w i t h n e e d l e o r rod c r y s t a l l i t e s and some p l a t e l e t s . Energy d i s p e r s i v e X-ray a n a l y s i s showed p r i m a r i l y aluminum, magnesium and s i l i c o n . The c r y s t a l l i t e s were e x t r a c t e d on a r e p l i c a ( 5 ) , and e l e c t r o n d i f f r a c t i o n was done: the n e e d l e s ( r o d s ) a r e m u l l i t e w h i l e the p l a t e s a r e a n o r t h i t e . These unexpected phases were n e c e s s a r y c l u e s t o i n s t i t u t e p r o c e s s development changes and c o n t r o l s t o e l i m i n a t e the v o i d s , w h i c h had trapped p l a t i n g s o l u t i o n . These, i n t u r n , expanded and loosened o r b u r s t the p l a t i n g d u r i n g the n e x t t h e r m a l e x c u r s i o n . Thus, a p p r o p r i a t e s m a l l a r e a a n a l y s i s gave a p r o p e r d i a g n o s i s (what appeared t o be a p l a t i n g problem was r e a l l y a s i n t e r i n g problem) and l e d t o g r e a t e r p r o c e s s u n d e r s t a n d i n g and c o n t r o l s . P a r t of t h i s was a t i g h t e n i n g o f the c o m p o s i t i o n l i m i t s on the g l a s s , w h i c h i s c o n t r o l l e d by a M a t e r i a l s E n g i n e e r i n g S p e c i f i c a t i o n . Such s p e c i f i c a t i o n s are seen t o be " l i v i n g " documents, b e i n g t i g h t e n e d o r loosened as t r a d e - o f f s o c c u r i n y i e l d / c o s t of p r o d u c t / c o s t of m a t e r i a l s / c o s t of c o n t r o l s . Au-Sn B r a z e M a t e r i a l - Why Doesn't I t Work? I n the e a r l y development s t a g e s o f a b r a z i n g o p e r a t i o n , Au-Sn b r a z i n g a l l o y preforms were used t o o b t a i n a h e r m e t i c s e a l . A problem arose when l e a k e r s o c c u r r e d p e r i o d i c a l l y . The p r o c e s s , good and bad product and the m a t e r i a l s ( t h e r e was no
