Fundamentals and Applications of Chemical Sensors - ACS Publications

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 15, 2014 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch001

1 Recent Advances in Chemically Sensitive Electronic Devices Jay N. Zemel, Jan Van der Spiegel, Thomas Fare, and Jein C. Young Center for Chemical Electronics, Department of Electrical Engineering, University of Pennsylvania, Philadelphia, PA 19104

Significant advances have occurred in microfabricated ion sensitive and Pd gated field effect devices and fiber optic, chemically sensitive elements. These elements are beginning to find their way into commercial development. Recent advances in these devices are discussed and compared. Pyroelectric sensor devices developed here are reviewed. A discussion of the utility of these devices is presented.

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An i m p o r t a n t g o a l o f the r e s e a r c h p r e s e n t e d i n t h i s volume i s t h e development o f i n s t a n t a n e o u s o r near i n s t a n t a n e o u s d a t a a c q u i s i t i o n e l e m e n t s which, w i t h t h e a s s i s t a n c e o f s u i t a b l e c o n t r o l a l g o r i t h m s , c o u l d d e f i n e a system's s t a t e i n r e a l t i m e . The need f o r p r o c e s s c o n t r o l i n f o r m a t i o n h a s become even more u r g e n t a s h i g h speed, low c o s t , s i n g l e c h i p m i c r o c o m p u t e r s become g e n e r a l l y a v a i l a b l e . The development o f new r e a l time s e n s o r s based on contemp o r a r y m a t e r i a l s and phenomena knowledge r e q u i r e s a more s y s t e m a t i c a p p r o a c h t h a n h a s been t h e c a s e t o d a t e . W h i l e t h e need f o r new measurement t o o l s t o advance s c i e n t i f i c and t e c h n o l o g i c a l i n q u i r y i s c e r t a i n l y w e l l a p p r e c i a t e d , s e n s o r and sensor r e l a t e d r e s e a r c h i s f r e q u e n t l y v i e w e d a s " w i d g e t m a k i n g " by t h e s c i e n t i f i c a n d t e c h n o l o g i c a l c o m m u n i t i e s . By and l a r g e , t h e development o f new s e n s o r s has been a byproduct o f t h e normal course o f m a t e r i a l s o r phenomena r e s e a r c h . I t i s o n l y i n t h e l a s t few years t h a t t h e s c i e n t i f i c and t e c h n o l o g i c a l community has taken n o t e o f t h e f a c t t h a t s e n s o r r e s e a r c h i s becoming a d i s t i n c t and important t o p i c .

0097-6156/86/0309-O002$10.25/0 © 1986 American Chemical Society

In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


1. ZEMELETAL.

3

Chemically Sensitive -Electronic Devices

I n the past decade, m i c r o f a b r i c a t i o n m e t h o d s d e v e l o p e d i n t h e m i c r o e l e c t r o n i c i n d u s t r y h a v e l e d t o new o p p o r t u n i t i e s f o r d e v i c e r e s e a r c h and development i n v o l v i n g c h e m i c a l l y s e n s i t i v e e l e c t r o n i c s t r u c t u r e s . I n 1980, t h i s s u b j e c t was r e v i e w e d i n depth a t a NATO Advanced Study I n s t i t u t e ( 1 ) . Over t h e l a s t f i v e y e a r s , t h e r e h a v e been t h r e e i n t e r n a t i o n a l conferences ( 2 - 4 ) , devoted t o s e n s o r s w i t h a s t r o n g emphasis on c h e m i c a l s e n s o r s as w e l l as a number o f n a t i o n a l and s p e c i a l i z e d m e e t i n g s on t h e s u b j e c t ( 5 , 6 ) . I n t h i s paper, some r e c e n t d e v e l o p m e n t s t h a t w i l l have l o n g t e r m c o n s e q u e n c e s on t h e s t u d y o f c h e m i c a l l y s e n s i t i v e e l e c t r o n i c d e v i c e s w i l l be reviewed. To s i m p l i f y the d i s c u s s i o n , the t o p i c s are d i v i d e d i n t o the f o l l o w i n g categories: a. Pd based m e t a l - o x i d e - s e m i c o n d u c t o r hydrogen s e n s o r s . b. I n t e g r a t e d e l e c t r o c h e m i c a l s e n s o r s . c. F i b e r o p t i c c h e m i c a l s e n s o r s (FOCS) u s i n g f l u o r e s c e n c e and absorptive behavior. d. T h e r m a l l y based c h e m i c a l s e n s o r s e m p l o y i n g p y r o e l e c t r i c elements. Connecting t h e s e d i s p a r a t e e l e m e n t s i s t h e i r mode o f m a n u f a c t u r e . A l l employ t h e same b a s i c procedures developed by the microelectronic industry to manufacture i t s elements, i . e . planar p h o t o l i t h o g r a p h y . The one c l a s s t h a t has not yet been m i c r o f a b r i c a t e d w i t h p l a n a r p h o t o l i t h o g r a p h y i s the FOCS. I n g e n e r a l , t h e c h o i c e o f phenomena and m a t e r i a l s f o r a p a r t i c u l a r type of measurement r e q u i r e m e n t i s d e t e r m i n e d by t h e i r c o m p a t i b i l i t y w i t h planar p h o t o l i t h o g r a p h y . I n o t h e r words, i f the m a t e r i a l i s not s u i t a b l e f o r p l a n a r p r o c e s s i n g , i t w i l l not be used. W i t h t h e e x c e p t i o n o f FOCS, i f t h e phenomena on w h i c h t h e d e v i c e o p e r a t i o n depends r e q u i r e s a s t r u c t u r e t h a t cannot be m i c r o f a b r i c a t e d w i t h p l a n a r t e c h n o l o g y , i t t o o w i l l not be c o n s i d e r e d . As i t t u r n s o u t , these r e s t r i c t i o n s are not too severe and the f l e x i b i l i t y o f p l a n a r p r o c e s s i n g , c o m b i n e d w i t h i t s p o t e n t i a l f o r low c o s t b a t c h p r o c e s s i n g , makes i t a very a t t r a c t i v e f a b r i c a t i o n methodology. Pd MOS STRUCTURES: The Pd MOS d e v i c e ( c a p a c i t o r and f i e l d e f f e c t t r a n s i s t o r ) has been e x t e n s i v e l y s t u d i e d as a model c h e m i c a l sensor s y s t e m and as a p r a c t i c a l e l e m e n t f o r t h e d e t e c t i o n o f h y d r o g e n m o l e c u l e s i n a gas. There have been two o u t s t a n d i n g r e v i e w s o f t h e s t a t u s of the Pd MOS sensor w i t h primary emphasis on the r e a c t i o n s at the s u r f a c e ( 7 , 8 ) . I n t h i s s e c t i o n , the use o f the d e v i c e as a model c h e m i c a l sensor w i l l be emphasized. As w i l l be seen, the r e s u l t s are a p p l i c a b l e not o n l y t o the Pd based d e v i c e s , they a l s o shed l i g h t on t h e o p e r a t i o n o f c h e m f e t t y p e s y s t e m s as w e l l . B e c a u s e o f i t s s i m p l i c i t y and the c o n t r o l t h a t can be e x e r c i s e d i n i t s f a b r i c a t i o n , t h e d i s c u s s i o n w i l l f o c u s on t h e s t u d y o f the Pd-MOSCAP s t r u c t u r e e x c l u s i v e l y . The i n s i g h t s gained from these s t u d i e s a r e immediately a p p l i c a b l e t o the more u s e f u l Pd-MOSFET. I n a t y p i c a l e x p e r i m e n t a l s t u d y , a Pd-MOSCAP i s kept a t a f i x e d t e m p e r a t u r e i n a c o n t r o l l e d ambient atmosphere of oxygen or an i n e r t gas l i k e pure n i t r o g e n . The gas i s exchanged w i t h a d i l u t e m i x t u r e o f h y d r o g e n i n n i t r o g e n , t y p i c a l l y 10-1,000 ppm of H i n N . A r e a c t i o n o c c u r s at the Pd-gas i n t e r f a c e between the r e s i d u a l a d s o r b e d o x y g e n and t h e h y d r o g e n ( 9 ) . When enough oxygen i s removed as water vapor 2

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FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS

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from t h e Pd s u r f a c e , t h e g a s e o u s h y d r o g e n a d s o r b s and d i s s o c i a t e s i n t o a t o m i c hydrogen. T h i s atomic hydrogen then d i f f u s e s i n t o t h e Pd ( H ) , e v e n t u a l l y r e a c h i n g t h e P d - S i 0 i n t e r f a c e . The d i f f u s i o n t i m e t h r o u g h a t y p i c a l 100 nm t h i c k Pd f i l m i s o f t h e o r d e r o f a m i l l i s e c o n d ( 7 ) . The Hp g e n e r a t e s a d i p o l e l a y e r , i l l u s t r a t e d i n F i g u r e 1 a ) , a t t h i s i n t e r f a c e t h a t l o w e r s t h e e l e c t r o n i c b a r r i e r h e i g h t . The r e s u l t i n g displacement c u r r e n t changes t h e s p a c e c h a r g e d e n s i t y i n t h e s i l i c o n p a r t o f t h e Pd-MOS s t r u c t u r e . The m a g n i t u d e o f t h i s change can be measured by o b s e r v i n g t h e d i s p l a c e m e n t o f t h e a d m i t tance v e r s u s v o l t a g e curve o f Pd-MOSCAP as i n d i c a t e d i n F i g u r e 1 b ) . Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 15, 2014 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch001

p d

2

One o f t h e s t a n d i n g problems i n t h i s P d - S i 0 - S i s y s t e m i s t h e u n c e r t a i n t y a b o u t t h e boundary c o n d i t i o n s r e l a t i v e t o t h e motion o f H i n t o t h e S i 0 and from t h e r e , on t o t h e S i 0 - S i i n t e r f a c e o r t h e S i i t s e l f . P r o b l e m s w i t h l o n g t e r m d r i f t o f t h e steady s t a t e f l a t band v o l t a g e have r a i s e d t h e p o s s i b i l i t y o f H diffusing further i n t o t h e o x i d e . E a r l y work by H o f s t e i n had r a i s e d t h e p o s s i b i l i t y t h a t protons c o u l d be i n j e c t e d i n t o t h e o x i d e ( 1 0 ) . However, a n e x t e n s i v e | e t o f s t u d i e s e v e n t u a l l y d e m o n s t r a t e d t h a t i t was most l i k e l y Na r a t h e r than H t h a t was r e s p o n s i b l e f o r H o f s t e i n ' s o b s e r v a t i o n s ( 1 1 , 1 2 ) . One o f t h e more s i g n i f i c a n t o b s e r v a t i o n s was t h a t Na a p p a r e n t l y enhances t h e d i f f u s i o n o f H i n t h e S i 0 ( 9 ) . These r e s u l t s i m p l y t ^ a t hydrogen does not go i n t o b u l k S i 0 as a charged s p e c i e s , but c o u l d be i n j e c t e d as H a t t h e Pd-oxide o r e l e c t r o l y t e oxide i n t e r f a c e . 2

p d

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p d

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Lundstrom has s u g g e s t e d t h a t most o f t h e H n e a r t h e P d - S i 0 i n t e r f a c e may be n e u t r a l i z e d p r i o r t o d i f f u s i n g i n t o t h e bulk o x i d e , but t h a t enough remain n e a r t h e i n t e r f a c e t o a c c o u n t f o r t h e l o n g t e r m i n s t a b i l i t i e s ( 1 3 ) . The n e u t r a l s p e c i e s (H) i s not c o n s t r a i n e d by t h e image charge a t t h e Pd g a t e and i s a b l e t o move t o a n o t h e r h y d r o g e n a c t i v e s i t e . I n t h e i r i n v e s t i g a t i o n of t h e hydrogen induced d r i f t (HID), N y l a n d e r e t a l . f o u n d t h a t Na c o n t a m i n a t e d s a m p l e s d e m o n s t r a t e d an e n h a n c e d HID ( 1 4 , 1 5 ) . T h i s i s c o n s i s t e n t w i t h t h e o b s e r v a t i o n s o f Holmberg e t a l . ( 1 2 ) . 2

+

The mechanism f o r t h e H - S i 0 and H - S i 0 - N a i n t e r a c t i o n s was t r e a t e d by Doremus i n a model f o r g e l f o r m a t i o n a t t h e s u r f a c e o f g l a s s e l e c t r o d e s ( 1 7 ) . The i n c o r p o r a t i o n o f water i n S i 0 has been reviewed by N i c o l l i a n and Brews ( 1 6 ) . The water m o l e c u l e may d i f f u s e i n t o t h e o x i d e w i t h o u t i n t e r a c t i n g w i t h t h e bulk o r i t may take p a r t i n an exchange r e a c t i o n w i t h SiOH a l r e a d y p r e s e n t . T h i s r e q u i r e s t h a t H 0 t r a n s f o r m s an e x i s t i n g S i - O - S i bond t o a p a i r o f s i l i n o l groups i n a manner a n a l a g o u s t o t h e Doremus model f o r l o w t e m p e r a t u r e h y d r o g e n t r a n s p o r t (See F i g u r e 2 a ) . I n o t h e r words, hydrogen i s i n t e r s t i t i a l l y t r a n s p o r t e d by w a t e r o r by a c o m p l e x S i - O H - H O - S i e x c h a n g e w i t h w a t e r . N e u t r a l w a t e r i s t h e d i f f u s a n t here and S i H s p e c i e s do n o t t a k e p a r t i n t h e e x c h a n g e r e a c t i o n . The g e n e r a l m e c h a n i s m i s shown i n F i g u r e 2 where t h e m o t i o n o f 0 i s i l l u s t r a t e d . The same p r i n c i p l e s would a p p l y f o r t h e m o t i o n o f H a t t a c h e d t o t h e w a t e r m o l e c u l e . When sodium i s added t o t h e o x i d e , i t s e r v e s as a s i t e f o r Na-OH d i s s o c i a t i o n . T h i s model was e x p a n d e d t o i n c l u d e S i O , SiOH and S i O H s i t e s i n o r d e r t o e x p l a i n s u r f a c e ads o r p t i o n at the e l e c t r o l y t e - o x i d e i n t e r f a c e (18,19). The i n c o r p o r a t i o n o f H i n S i 0 i s almost i m p o s s i b l e t o a v o i d under normal c o n d i t i o n s . The d e n s i t y and n a t u r e o f t h e d e f e c t s a s s o c i a t e d w i t h t h e r m a l l y grown o x i d e s n o t o n l y v a r i e s from sample t o sample, but 2

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ZEMELETAL.

Chemically Sensitive Electronic Devices

ELECTRON PROTON


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APPLIED VOLTAGE F i g u r e 1. a. I l l u s t r a t i o n o f the d i p o l e l a y e r formed a t the p a l l a d i u m - s i l i c o n d i o x i d e i n t e r f a c e as a r e s u l t o f H i n t e r a c t i o n s ; b. Schematic c a p a c i t a n c e v e r s u s a p p l i e d v o l t age, ( O V ) , and p a r a l l e l conductance at f r e q u e n c y 6 v e r s u s a p p l i e d v o l t a g e , (G-V), f o r an n-type s i l i c o n based Pd MOSCAP i n the presence o f oxygen ( s o l i d l i n e ) and hydrogen (dashed l i n e ) .


5



6

F i g u r e 2.

Mechanism f o r the d i f f u s i o n o f hydrogen

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a. Hydrogen atom approaches two n e i g h b o r i n g Si-OH bonds; b. F o r m a t i o n o f water m o l e c u l e from t h e Si-OH s t r u c t u r e and t h e f o r m a t i o n o f a h y d r o x y l b r i d g e between the two S i atoms. Approach o f another hydrogen atom w h i l e the water m o l e c u l e d i f f u s e s away; c. Combination o f another f r e e H w i t h the bound H t o form m o l e c u l a r hydrogen. Both t h e and t h e H 0 2

then d i f f u s e u n t i l they f i n d a s i t e where they can r e a c t w i t h a Si0 m o l e c u l e and r e v e r s e the p r o c e s s . o


1.

ZEMELETAL.


7

a l s o s t r o n g l y depends on t h e method o f p r e p a r a t i o n . Thus t h e p r e p a r a t i o n o f t h e o x i d e s w i l l p l a y a m a j o r r o l e i n t h e degree t o which d e v i c e s w i l l e x h i b i t v a r i o u s types o f hydrogen induced b e h a v i o r . The s t u d y o f b u l k d e f e c t s i n S i 0 h a s r e l i e d h e a v i l y on ESR measurements. J o h n s o n e t a l . e m p l o y e d a c o m b i n a t i o n o f d e u t e r i u m plasma and ESR t o f o l l o w t h e behavior o f both t h e s o - c a l l e d P s i g n a l c o r r e s p o n d i n g t o a paramagnetic d e f e c t and t h e i n t e r f a c e s t a t e s a t E • 0 . 3 eV a n d E - 0.25 eV, c h a r a c t e r i s t i c o f unannealed samples, w h i l e t h e sample i s s u b j e c t e d t o a s t a n d a r d p o s t - o x i d a t i o n t h e r m a l a n n e a l ( 2 0 , 2 1 ) . I n t h e presence o f t h e deuterium (D) plasma, t h e P^ s i g n a l c o r r e l a t e d w i t h t h e decrease i n t h e i n t e r f a c i a l s t a t e d e n s i t y . SIMS m e a s u r e m e n t s c o r r e l a t e d t h e i n c r e a s e o f D a t t h e S i - S i 0 i n t e r f a c e w i t h t h e decrease i n P. and t h e s u r f a c e s t a t e s , i n a g r e e ment w i t h o t h e r types o f s t u d i e s (22-24). I n one s t u d y , i t was shown t h a t i n f r e s h l y o x i d i z e d S i s p e c i m e n s , t h e r e was no s i g n o f H c o n t a m i n a t i o n . However, a f t e r a p e r i o d o f time i n t h e a i r , hydrogen s i g n a l s a r e observed ( 2 5 ) . T h i s c o r r e l a t e s w i t h o b s e r v a t i o n o f Wen and Zemel on e x p o s i n g o x i d e s t o a c i d - b a s e s o l u t i o n s ( 2 6 ) . O t h e r s t u d i e s showed t h a t CI a d d i t i v e s tended t o i n c r e a s e t h e hydrogen conc e n t r a t i o n spontaneously ( 2 7 ) . These s t u d i e s make i t e v i d e n t t h a t hydrogen and hydrogen b e a r i n g compounds i n t e r a c t s t r o n g l y w i t h S i 0 . As a r e s u l t , i t i s d i f f i c u l t t o o b t a i n unambiguous i n f o r m a t i o n on t h e i n t e r a c t i o n o f hydrogen w i t h t h e Pd-MOSCAP s y s t e m b e c a u s e t h e o x i d e i s n o t a p a s s i v e component. 2

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y

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2

A f u r t h e r c o n c l u s i o n t o be drawn from i n v e s t i g a t i o n s o f hydrogen i n t e r a c t i n g w i t h b u l k and t h i n f i l m S i 0 i s t h a t H-Pd-MOS d e v i c e o p e r a t i o n w i l l be i n f l u e n c e d by t h e type o f o x i d e p r e p a r a t i o n u s e d . I n f a c t , t h e f i r s t use o f a Pd-MOSCAP was i n a study o f hydrogen ann e a l i n g o f i n t e r f a c i a l s t a t e s (28). 2

M o s t r e c e n t l y , Fare e t a l . demonstrated t h a t t h e o x i d e p r e p a r a t i o n can s t r o n g l y i n f l u e n c e t h e behavior o f t h e Pd-MOSCAP (29*31). I n p a r t i c u l a r , t h e i r measurements demonstrated t h a t H c o u l d be i s o t h e r mally i n j e c t e d not o n l y t o the S i - o x i d e i n t e r f a c e , but that the h y d r o g e n w i l l a l s o r e v e r s i b l y e n t e r and l e a v e t h e s i l i c o n s u b s t r a t e (28,29). The data showed t h a t t h e i n t r o d u c t i o n o f H by t h e Pd g a t e would cause v a r i a t i o n s i n t h e r e c o m b i n a t i o n c r o s s s e c t i o n and d e n s i t y o f t h e i n t e r f a c i a l s t a t e s ( 2 9 , 3 1 ) . The o x i d e s used i n t h i s study were q u i t e t h i n (11 nm) a n d were n o t s u b j e c t e d t o any i n t e r f a c i a l s t a t e r e d u c t i o n p r o c e s s i n g . These s t u d i e s p r o v i d e d g e n e r a l c o n f i r m a t i o n t h a t a t o m i c hydrogen i s i n j e c t e d from t h e P d - S i 0 i n t e r f a c e , t h r o u g h the o x i d e , and on i n t o t h e S i - S i 0 i n t e r f a c e where they form i n t e r f a c i a l s t a t e s a t E + 0.3 eV and E - 0.25 eV as o r i g i n a l l y proposed by Keramati and Zemel ( 3 2 , 3 3 ) . However, t h e F a r e e t a l . s t u d i e s a l s o p r o v i d e some c l u e s as t o why t h e e v i d e n c e o f d i f f e r e n t i n v e s t i g a t o r s on t h e i n f l u e n c e o f h y d r o g e n i n j e c t e d by a Pd g a t e v a r i e d s o much (34-36). Most o f these s t u d i e s employed wet o x i d e s t h a t had been w e l l a n n e a l e d , g e n e r a l l y w i t h a hydrogen b e a r i n g ambient gas t o reduce t h e i n t e r f a c i a l s t a t e d e n s i t y . I n t h e absence o f t h e s t r a i n e d bonds o r chemical d e f e c t s , the i n t e r a c t i o n of H w i t h the i n t e r f a c e i s l i k e l y t o be d i f f i c u l t t o o b s e r v e . T h i s data a l s o p o i n t s o u t t h a t c h e m i c a l s h i e l d i n g o f a l l c h e m i c a l l y s e n s i t i v e e l e c t r o n i c e l e m e n t s w i l l be a c r i t i c a l step i n device processing. 2

2

V



8

The c o n c l u s i o n s t o be drawn from these s t u d i e s a r e t h a t t h e comp o s i t i o n and past h i s t o r y o f both t h e P d - S i 0 and S i - S i 0 i n t e r f a c e s , the method o f p r o d u c i n g t h e o x i d e and t h e post-growth a n n e a l i n g s t e p s p l a y k e y r o l e s i n d e t e r m i n i n g t h e n a t u r e o f t h e Pd-MOS response t o hydrogen. As a model system f o r a c h e m i c a l s e n s o r , i t p o i n t s o u t t h e i m p o r t a n t r o l e t h a t c a t a l y t i c m a t e r i a l s l i k e Pd can p l a y i n f u t u r e sensor d e s i g n s .


2

2

INTEGRATED SILICON-BASED ELECTROCHEMICAL SENSORS: The i d e a o f m a r r y i n g i n t e g r a t e d c i r c u i t f a b r i c a t i o n t e c h n o l o g y w i t h membrane s c i e n c e t o g e n e r a t e new c l a s s e s o f c h e m i c a l l y s e n s i t i v e e l e c t r o n i c s d e v i c e s began i n t h e e a r l y 70*s (37~39). The r a t i o n a l e f o r t h i s e f f o r t was t h e e x p e c t a t i o n t h a t p l a n a r m i c r o f a b r i c a t i o n t e c h n o l o g y would produce t h e same d r a m a t i c impact on i n f o r m a t i o n s e n s i n g systems as i t h a d on i n f o r m a t i o n p r o c e s s i n g systems. I n p a r t i c u l a r , i t was hoped t h a t t h e s e new s e n s o r s y s t e m s w o u l d embody t h e f o l l o w i n g advantages: 1. 2. 3. 4. 5.

s m a l l , rugged s o l i d - s t a t e s t r u c t u r e s low impedance o u t p u t s high dimensional p r e c i s i o n low c o s t batch f a b r i c a t i o n p r o s p e c t o f m u l t i - s p e c i e s sensors w i t h o n - c h i p e l e c t r o n i c s

T h e s e p r o s p e c t s have s t i m u l a t e d a s i g n i f i c a n t amount o f r e s e a r c h . D e s p i t e these e f f o r t s , p r o g r e s s h a s been s l o w t o d a t e . I t i s o n l y r e c e n t l y t h a t a commercial s i n g l e s p e c i e s c h e m i c a l l y s e n s i t i v e f i e l d e f f e c t t r a n s i s t o r ( c h e m f e t ) h a s become a v a i l a b l e ( 4 0 ) . Among t h e r e a s o n s f o r t h e slow development a r e : t h e l a c k o f p l a n a r t e c h n o l o g i e s f o r m i c r o m a c h i n i n g t h e c h e m i c a l l y s e n s i t i v e membrane t h a t a r e s t i l l c o m p a t i b l e w i t h s t a n d a r d m i c r o e l e c t r o n i c t e c h n o l o g y and use o f uns h i e l d e d f i e l d e f f e c t based d e v i c e s where i o n i c motion c r e a t e s almost i n s u r m o u n t a b l e p a c k a g i n g problems. I n the chemfet, there i s a membrane i n t h e gate r e g i o n which i s i n d i r e c t c o n t a c t w i t h t h e f l u i d t o be a n a l y z e d . As a r e s u l t , t h e e l e c t r o n i c a l l y a c t i v e r e g i o n o f t h e chemfet i s d i r e c t l y exposed t o t h e e n v i r o n m e n t . Any i o n i c l e a k a g e paths from t h e s o l u t i o n , around t h e c h e m i c a l l y s e n s i t i v e membrane and along the surface o f the g a t e - d i e l e c t r i c w i l l discharge the p o t e n t i a l g e n e r a t e d i n t h e c h e m i c a l l y s e n s i t i v e membrane by t h e i o n s e n t e r i n g from t h e s o l u t i o n . T h i s i s i l l u s t r a t e d i n F i g u r e 3. A n o t h e r m a j o r p r o b l e m produce J by inadequate p a c k a g i n g o f t h e c h e m i c a l l y s e n s i t i v e i o n i c d e v i c e i s i o n i c d r i f t s i m i l a r t o t h a t shown i n F i g u r e 3 w h i c h p r o d u c e s an u n a c c e p t a b l e c r o s s t a l k between n e i g h b o u r i n g s e n s o r s . A l s o , because t h e chemfet i s a c a p a c i t i v e s t r u c t u r e i t i s u n f o r g i v i n g to even very s m a l l leakage c u r r e n t s i n t h e d i e l e c t r i c medium c o n s i s t ing o f t h e gate i n s u l a t o r and t h e c h e m i c a l l y s e n s i t i v e membrane. The i o n c o n t r o l l e d d i o d e was an i n i t i a l attempt t o i s o l a t e t h e a c t i v e e l e c t r o n i c s from t h e c h e m i c a l s o l u t i o n by p r o d u c i n g a meta l l i c - l i k e v i a that allows the i s o l a t i o n of the chemically s e n s i t i v e r e g i o n from an a r e a where e l e c t r o n i c c o m p o n e n t s c o u l d be d e p o s i t e d (41,42). However, t h e l i m i t e d p r e c i s i o n o f the non-standard m i c r o f a b r i c a t i o n t e c h n i q u e s made t h i s process d i f f i c u l t and c o s t l y . S i n c e t h i s d e v i c e i s s t i l l e s s e n t i a l l y a c a p a c i t i v e membranei n s u l a t o r - s e m i c o n d u c t o r s t r u c t u r e l i k e t h e chemfet, t h e same problems of h e r m e t i c i s o l a t i o n o f t h e gate remain.



ZEMEL ET AL.


F i g u r e 3 . I o n i c l e a k a g e paths i n chemfet s t r u c t u r e s : a.Schematic i l l u s t r a t i o n o f i o n i c leakage paths around t h e c h e m i c a l l y s e n s i t i v e membrane. Leakage through t h e membrane a l s o o c c u r s but i s not i l l u s t r a t e d ; b. Schematic i l l u s t r a t i o n o f l e a k a g e a t the s u r f a c e o f a s t a n d a r d i o n s e n s i t i v e f i e l d effect transistor.




10

A new approach t o semiconductor based s e n s o r s , d e v e l o p e d i n o u r l a b o r a t o r i e s , i s t h e i n t e g r a t e d s h i e l d , e l e c t r o c h e m i c a l s e n s o r . The g o a l o f t h i s r e s e a r c h was t o develop m u l t i s p e c i e s s e n s o r s which c o u l d be c o m p l e t e l y f a b r i c a t e d by wafer l e v e l p r o c e s s i n g , w h i l e a t t h e same t i m e c o m p l e t e l y i s o l a t i n g t h e e l e c t r o n i c a l l y a c t i v e zone f r o m t h e c h e m i c a l l y s e n s i t i v e l a y e r s which a r e i n c o n t a c t w i t h t h e f l u i d s . The s t r u c t u r e i s s c h e m a t i c a l l y shown i n F i g u r e 4 ( 4 3 ) . The d e v i c e c o n s i s t s o f two s e c t i o n s . The f i r s t p a r t i s the c h e m i c a l l y s e n s i t i v e layer i n d i r e c t contact t o t h e s h i e l d e d s i g n a l l i n e which forms a complete e l e c t r o c h e m i c a l e l e c t r o d e t o t h e second p a r t , t h e i n p u t o f a h i g h impedance b u f f e r . C e n t r a l t o t h i s d e s i g n i s t h e i n s u l a t e d s h i e l d around t h e t h e l i n e and s e n s o r s . T h i s p r o v i d e s t h e n e c e s s a r y e l e c t r i c a l i s o l a t i o n and c h e m i c a l p a s s i v a t i o n t o r e d u c e t h e c r o s s - t a l k between d i f f e r e n t s e n s o r s . I n a d d i t i o n , t h e s h i e l d i s b o o t s t r a p p e d i n o r d e r t o m i n i m i z e t h e c o u p l i n g between t h e s h i e l d and l i n e . I n f a c t , t h i s s t r u c t u r e i s an i n t e g r a t e d m o n o l i t h i c v e r s i o n o f the c l a s s i c a l i o n s e l e c t i v e membrane e l e c t r o d e c o n t a c t e d v i a a w i r e t o an e l e c t r o m e t e r . The s e n s o r s a r e d e s i g n e d as f a r a d a i c elements. One o f t h e most important a s p e c t s o f t h e i n t e g r a t e d , s h i e l d e d e l e c t r o d e s t r u c t u r e i s that i t lends i t s e l f t o l a r g e s c a l e chemical s e n s o r i n t e g r a t i o n , i . e . f o u r o r more chemical s e n s o r s on a c h i p . A w a f e r i s p r o c e s s e d t o produce a l a r g e number o f uncoated s e n s o r s by the same s t a n d a r d m i c r o f a b r i c a t i o n t e c h n o l o g y u s e d t o m a n u f a c t u r e i n t e g r a t e d c i r c u i t s . The c h e m i c a l l y s e n s i t i v e membranes a r e then added f o l l o w i n g t h e t e s t i n g o f the e l e c t r o n i c c i r c u i t r y . T h i s p r o c e d u r e makes i t q u i t e f e a s i b l e t o p r o d u c e an a r r a y o f s e n s o r s , each r e s p o n d i n g t o a d i f f e r e n t c h e m i c a l q u a n t i t y . By s e p a r a t i n g t h e p r e p a r a t i o n o f t h e s e n s o r m a t e r i a l s and s i l i c o n d e v i c e s t r u c t u r e s , the sensor s c i e n t i s t can o b t a i n wafers c o n t a i n i n g h i s d e s i g n o f t h e s i l i c o n s t r u c t u r e a t t h e wafer l e v e l through a s i l i c o n f o u n d r y . Use o f s i l i c o n f o u n d r i e s has become s t a n d a r d i n t h e custom IC d e s i g n i n d u s t r y b e c a u s e i t h a s been amply d e m o n s t r a t e d t h a t q u i t e complex s i l i c o n i n t e g r a t e d c i r c u i t s can be f a b r i c a t e d t h e r e a t r e l a t i v e l y low c o s t and w i t h a h i g h degree o f r e l i a b i l i t y . Not o n l y can t h e sensor c i r c u i t r y be p r e p a r e d t h i s way, i t i s a l s o p o s s i b l e t o i n t e g r a t e o n c h i p s i g n a l p r o c e s s i n g c i r c u i t s w h i c h i m p r o v e and c o n d i t i o n t h e s e n s o r s i g n a l . T h i s i s p a r t i c u l a r l y important because the s e n s o r s i g n a l s a r e o f t e n weak and s u s c e p t i b l e t o n o i s e d u r i n g t r a n s m i s s i o n . The d e p o s i t i o n o f t h e c h e m i c a l l y s e n s i t i v e l a y e r s by a p p r o p r i a t e p l a n a r p r o c e s s i n g s t e p s i s the l a s t phase p r i o r t o p a c k a g i n g (43,44). An important development i n chemfet r e s e a r c h was t h e d i s c o v e r y t h a t t h e p r o t o n i c conductor I r O , p r e p a r e d by DC r e a c t i v e s p u t t e r i n g , i s an e x c e l l e n t f a r a d a i c pH s e n s i t i v e m a t e r i a l (45-47). When p r o p e r l y p r e p a r e d , t h e s e m a t e r i a l s show no s i g n i f i c a n t redox i n t e r f e r e n c e s ( 4 7 ) . Because t h e m a t e r i a l s a r e f a r a d a i c and h i g h l y c o n d u c t i n g , t h e o u t e r p o t e n t i a l o f t h e I r O obey the N e r n s t r e l a t i o n over t h e range ~0

Fundamentals and Applications of Chemical Sensors - ACS Publications

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