AUTOMATED MEASUREMENTS OF INTEGRATED CIRCUIT LINEWIDTHS BY ELECTRON MICROSCOPY

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AUTOMATED MEASUREMENTS OF INTEGRATED CIRCUIT LINEWIDTHS BY ELECTRON

MICROSCOPY

G. Fritz, J. Mccarthy, J. Benson

To cite this version:

G. Fritz, J. Mccarthy, J. Benson. AUTOMATED MEASUREMENTS OF INTEGRATED CIRCUIT LINEWIDTHS BY ELECTRON MICROSCOPY. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-895-C2-898. �10.1051/jphyscol:19842205�. �jpa-00223882�

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JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment au n02, Tome 45, f6vrier 1984 page C2-895

AUTOMATED MEASUREMENTS OF INTEGRATED C I R C U I T LINEWIDTHS BY ELECTRON MICROSCOPY

G . S . F r i t z , J . J . McCarthy and J . Benson

Tracor Northern Inc., 2551 West B e l t l i n e Highway, Middleton, WI 53562, U. S.A.

Resume - Ce p a p i e r d e c r i t un systPme d ' a u t o m a t i s a t i o n p o u r l a mesure des l a r g e u r s des rubans des c i r c u i t s i n t c g r g s a l ' a i d e d ' u n microscope G l e c t r o - nique a balayage.

A b s t r a c t

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T h i s paper d e s c r i b e s a system which automates i n t e g r a t e d c i r c u i t l i n e w i d t h s measurements i n an e l e c t r o n microscope.

Because o f advances i n i n t e g r a t e d - c i r c u i t ( I C ) technology, c h i p s have become more densely packed and I C l i n e s have become smaller. A t present, most 1 C l i n e w i d t h s a r e measured by o p t i c a l systems, t h e b e s t o t which can o n l y measure l i n e w i d t h s t o 0.5 urn under i d e a l c o n d i t i o n s . As IC l i n e s approach submicron sizes, u s i n g t e c h - niques such as electron-beam l i t h o g r a p h y , o p t i c a l systems w i l l no l o n g e r be a b l e t o measure t h e l i n e s a c c u r a t e l y and r e p r o d u c i b l y . F o r t h e s e geometries, t h e s h o r t e r wavelengths o t t h e scanning e l e c t r o n microscope (SEM) p r o v i d e b e t t e r p r e - c i s i o n t h a n o p t i c a l systems.

I n d e s i g n i n g a l i n e w i d t h measurement system t o r t h e semiconductor i n d u s t r y , t h e major needs a r e based on p r e c i s i o n o r r e p r o d u c i b i l i t y , accuracy, and h i g h throughput. The r e l a t i v e importance o t t h e s e needs depends on t h e a p p l i c a t i o n : t h e needs o t researchers engaged i n p r o d u c t development a r e q u i t e d i t t e r e n t trom t h o s e o f m a n u t a c t u r i n g f a c i l i t i e s . I n manutacturing, r e p r o d u c i b i l i t y has h i g h p r i o r i t y , t o i n s u r e t h a t a process remains t h e same t r o m one p r o d u c t i o n l i n e t o another.

High throughput i s a l s o o t prime concern t o manutacturing, so p r o v i s i o n s h o u l d be made t o r t h e SEN s t a g e t o be a u t o m a t i c a l l y p o s i t i o n e d t o p r e d e t i n e d areas o f i n t e r e s t . I n t h e research l a b o r a t o r i e s , accuracy i s t h e most i m p o r t a n t c o n s i d e r - a t i o n , s i n c e a b s o l u t e dimensions a r e i m p o r t a n t i n o b t a i n i n g a t a r g e t e d d e v i c e p e r - tormance. The problem o t a c c u r a t e measurements has n o t been t o t a l l y solved, s i n c e a u n i v e r s a l l y accepted d e t i n i t i o n o t t h e edge o t a l i n e has n o t been e s t a b l i s h e d . A measurement system s h o u l d t h e r e t o r e have some method o t m i n i m i z i n g t h i s e t t e c t .

The computer t o r t h e system i s e i t h e r an,LSI-11/P o r LSI-11/23 (trademarks o f Dig- i t a l Equipment Corp.). The system i n c l u d e s a t u l l stage automation package con- s i s t i n g o t a x i s p o s i t i o n e r s and s t e p p i n g motors. Up t o 100 stage p o s i t i o n s (X,Y and Z ) can be indexed r e l a t i v e t o a r e t e r e n c e p o s i t i o n and saved on disk. Once a speci t i c s e t o f stage p o s i t i o n s has been r e c a l l e d and t h e r e t e r e n c e p o s i t i o n t o r t h e p r e s e n t water has been indexed, t h e a n a l y s i s can be s t a r t e d . The s t a g e i s a u t o m a t i c a l l q moved t o each d e t i n e d p o s i t i o n and s e l e c t e d IC l i n e s a r e measured i n each t i e l d manually o r a u t o m a t i c a l l y . I t a d i t t e r e n t t y p e o t stage movement and a n a l y s i s i s r e q u i r e d , t h e o p e r a t o r can w r i t e and save on d i s k h i s own s i m p l e sequence o t commands t o p e r t o r m t h e d e s i r e d t y p e o t a n a l y s i s . For t h e SEN w i t h a beam b l a n k i n g device, t h e system i s equipped w i t h a l a t c h b i t t h a t can b l a n k t h e beam under computer c o n t r o l when measurements a r e n o t b e i n g made, t o minimize beam damage.

L i n e w i d t h measurements a r e made w i t h a d i g i t a l scan g e n e r a t o r t o c o n t r o l t h e p o s i - t i o n o t t h e e l e c t r o n beam o t t h e SEk and m o n i t o r i t s video s i g n a l . The d i g i t a l scan g e n e r a t o r c o n s i s t s o t a video processor and an X-Y scan c o n t r o l l e r . Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842205

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C2-896 JOURNAL DE PHYSIQUE

The v i d e o p r o c e s s o r i n c l u d e s an 8 - b i t a n a l o g - t o - d i g i t a l c o n v e r t e r (ADC) capable o t d i v i d i n g t h e SEM v i d e o s i g n a l i n t o 256 d i s t i n c t gray l e v e l s . M u l t i p l e samples may b e o b t a i n e d a t a s i n g l e p i x e l a t a r a t e o t 1 us p e r sample and summed i n t o a 2 0 - b i t a c c u m u l a t i v e r e g i s t e r . I n a d d i t i o n , a 1 2 - b i t d i g i t a l - t o - a n a l o g c o n v e r t e r (DAC) a l l o w s t h e computer t o o u t p u t a v i d e o s i g n a l t o t h e SEM CRT d i s p l a y s .

The programmable X-Y scan c o n t r o l l e r c o n s i s t s o t two 1 2 - b i t DACs t o r p o s i t i o n i n g t h e SEM beam and a j o y s t i c k module. The j o y s t i c k module i s used t o r s e l e c t i v e c o n t r o l o f t h e X-Y c u r s o r s superimposed on t h e d i g i t a l r a s t e r . The p o s i t i o n s o t t h e c u r s o r s can be r e a d by t h e computer and used t o i n d e x areas t o be analyzed.

Several u t i l i t y programs a r e a v a i l a b l e t h a t p r o v i d e c o n t r o l o f beam p o s i t i o n i n g , v i d e o s i g n a l a c q u i s i t i o n , and o u t p u t t o t h e SEM CRT t o r g r a p h i c s i n t o r m a t i o n . DISCUSSION

L i n e w i d t h measurements c o n s i s t o f a c q u i r i n g v i d e o p r o f i l e s across t h e I C l i n e o t i n t e r e s t e i t h e r h o r i z o n t a l l y o r v e r t i c a l l y . Once a c q u i r e d , t h e l i n e w i d t h measurements a r e made manually by t h e a d j u s t m e n t o f c u r s o r s on t h e v i d e o p r o t i l e o r by one o t s e v e r a l a u t o m a t i c methods. The a u t o m a t i c methods a r e peak-to-peak, m i n i - mum-to-minimum, maximum slopes, peak-percentage, and v i d e o - t h r e s h o l d i n g . D i t - f e r e n t methods a r e employed because one can o b t a i n s e v e r a l t y p e s o t p r o f i l e s when examining p h o t o r e s i s t s , a l u m i n i z e d l i n e s , m e t a l l i c l i n e s , and t h e same l i n e s a t d i f f e r e n t stages o f manufacturing. Two o t t h e a u t o m a t i c t e c h n i q u e s a r e i l l u s - t r a t e d i n F i g . 1. Peak-to-peak measurement i s shown i n F i g . l ( a ) . I n t h e peak percentage t e c h n i q u e t h e program l o c a t e s t h e peaks and t h e n determines t h e p o i n t a t which t h e d e s i g n a t e d peak i n t e n s i t y percentage i s found. I n F i g . l ( b ) a peak .percentage o t 50% was.used t o determined t h e edges o t t h e l i n e .

9

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d t h . r e f

.(a) Example o f peak-to-peak 1 measurement d i s p l a y e d on SEM 15: v i d e o p r o t i l e i s shown a1

i ne- CRT onq

Fig. l ( b ) peak percentage l i n e w i d t h measurement: p o i n t a t which i n t e n - s i t y a t 50% o t peak i s d e f i n e d as w i t h two v e r t i c a i l i n e s d e p i c t i n g t h e - edge o t l i n e . ^'

peaks as determined by t h e software.

Several advantages a r e r e a l i z e d i n t h i s method o t a n a l y s i s . F i r s t , by u s i n g an automated process t o determine t h e l i n e w i d t h s , t h e o p e r a t o r i s removed f r o m t h e d e c i s i o n making process, so t h a t o p e r a t o r b i a s i s removed and r e p r o d u c i b i l i t y i s enhanced. Second, v i d e o p r o f i l e s can be s i g n a l averaged a t each p o i n t t o r any s p e c i f i e d p e r i o d o t time. l h i s procedure improves t h a t s i g n a l - t o - n o i s e r a t i o and t h e measurement s t a t i s t i c s . F i g u r e 2 i l l u s t r a t e s t h e e t t e c t o f s i g n a l averaging.

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I n t h e p r o t i l e o t Fig. 2(a), 5 v i d e o samples were used a t each p o i n t ; i n F i g . 2(b), 1000 v i d e o samples. One c o u l d a l s o t a k e m u l t i p l e scans across t h e r e g i o n o f i n t e r e s t and average t h e scans t o g e t h e r ; however, i t i s d i f f i c u l t t o assure t h a t t h e e l e c t r o n beam remains a t t h e same D o s i t i o n d u r i n q m u l t i p l e scans because o f s amp

F i g . 2 ( a ) Video p r o f i l e across I C l i n e F i g . 2 ( b ) p r o f i l e a c r o s s same l i n e i n which 5 v i d e o samples a t each p o i n t w i t h 1000 v i d e o samples

i n p r o f i l e were used t o determine average v i d e o s i g n a l : l i n e measured w i t h a peak percentage a n a l y s i s s e t as 50% o t peak

The t h i r d advantage o t t h i s system i s t h e c o m p u t e r - c o n t r o l l e d a c q u i s i t i o n o f t h e v i d e o p r o f i l e s . The beam i s d i g i t a l l y p o s i t i o n e d t o each p o i n t i n t h e scan and a l l o w e d t o s e t t l e b e t o r e t h e a c t u a l v i d e o s i g n a l i s acquired. Most SEM scan gen- e r a t o r s use analog e l e c t r o n i c s , which may produce a n o n l i n e a r image a t v i d e o r a t e s . The n o n l i n e a r i t y o f t h e image a t v i d e o scan r a t e s i s due t o t h e response t i m e o f t h e scan c o i l s o t t h e SEM. Some SEMs may p e r t o r m b e t t e r t h a n o t h e r s i n t h i s regard. however, i t i s l i k e l y t h a t any SEM w i l l have some n o n l i n e a r i t y a t v i d e o r a t e s .

A f o u r t h advantage i s t h e a b i l i t y t o mimifiize t h e e t t e c t o t d e t i n i n g t h e edge o f a l i n e . The system a l l o w s t h e o p e r a t o r t o c a l i b r a t e t h e scan i n b o t h t h e X and Y d i r e c t i o n s i n d e p e n d e n t l y t o a known "standard". The o p e r a t o r s i m p l y measures t h e

" s t a n d a r d " l i n e u s i n g t h e t y p e o t a u t o m a t i c measurement t h a t w i l l b e used on t h e unknowns and t h e n s u p p l i e s t h e computer w i t h t h e " t r u e " value. Using t h i s t e c h - nique, one can m i n i m i z e t h e problem o f d e t i n i n g t h e edge o t t h e l i n e .

What p r e c i s i o n and accuracy can be expected f r o m a l i n e w i d t h measurement system?

T h i s i s n o t a t r i y i a l q u e s t i o n , s i n c e many f a c t o r s a f f e c t p r e c i s i o n and accuracy.

One i s t h e l e v e l o f n o i s e on t h e v i d e o p r o t i l e . A study was pertormed i n w h i c h an 8 urn IC l i n e was measured a t a m a g n i t i c a t i o n o t 5400x i n t h e peak-to-peak mode.

The v i d e o p r o f i l e s f o r t h i s s t u d y were a c q u i r e d a t d i t t e r e n t l e v e l s o f s i g n a l averaging. As expected, t h e u n c e r t a i n t y i n t h e video s i g n a l d e c l i n e s as 1 n, where n i s t h e number o t samples averaged i n t h e measurement. The p r e c i s i o n o t t h e l i n e w i d t h measurements versus t h e amount o t s i g n a l a v e r a g i n g and t h u s t h e l e v e l o f n o i s e on t h e s i g n a l i s shown i n T a b l e 1. The u n c e r t a i n t y i n t h e v i d e o l e v e l e v i d e n t l y a f f e c t s t h e u n c e r t a i n t y o f t h e measurement. k i t h a r e l a t i v e v i d e o u n c e r t a i n t y o f 4% we were a b l e t o o b t a i n l i n e w i d t h measurements w i t h an uncer- t a i n t y o f 0.008 um.

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TABLE 1 . - - C o r r e l a t i o n o f v i d e o u n c e r t a i n t y i n l i n e w i d t h measurement o t 8 um l i n e

bo. o f R e l a t i v e U n c e r t a i n t y Video U n c e r t a i n t y i n L i n e w i d t h

Samples o f Video measurement

I n o r d e r t o o b t a i n i n f o r m a t i o n on t h e p r e c i s i o n expected as a f u n c t i o n o f t h e s i g - n a l t o n o i s e l e v e l , a study was performed i n w h i c h d i f f e r e n t l e v e l s o f n o i s e were added t o a s i m u l a t e d waveform. The w i d t h s o t t h e s i m u l a t e d p r o f i l e s were measured u s i n g b o t h t h e peak-to-peak and peak percentage modes t o r a n a l y s i s . The r e s u l t s o t 50 measurements a t each n o i s e l e v e l a r e shown i n F i g u r e 4. As can be seen t r o m t h e data, p r e c i s i o n o f l e s s t h a n 1 p e r c e n t i s o b t a i n e d even when s i g n a l t o n o i s e r a t i o s a t t h e peak a r e as l o w as 10/1.

Among o t h e r t a c t o r s t h a t a t t e c t t h e p r e c i s i o n and accuracy o t t h e measurements, t h e e f f e c t o f t h e dynamic range o f t h e v i d e o p r o t i l e and t h e r a t e o t change i n t h e v i d e o s i g n a l a t t h e edge o t t h e l i n e must be examined i t one i s t o o b t a i n a b e t t e r u n d e r s t a n d i n g o t what p r e c i s i o n and accuracy can be expected under d i f f e r i n g con- d i t i o n s .

F i g . 4 S i m u l a t e d v i d e o p r o f i l e s a t d i t t e r e n t F i g . 5 R e l a t i o n s h i p o f u n c e r t a i n t y l e v e l s o t noise: a ) n o noise; b) 5% n o i s e i n t h e l i n e w i d t h measurment versus t h e s i g n a l t o n o i s e l e v e l o f t h e v i d e o p r o f i 1 e.