encapsulation of an integrated thin-film multi-electrode sensor using

8/4/2019 Encapsulation of an Integrated Thin-film Multi-electrode Sensor Using

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E L S E V I E R Sen s o r s and Actuators B 30 (1996) 151-157 B

En c ap su l a t i on o f an i n te gr a ted th in - f il m m u l t i- e le c t r od e se n sor u s i n gp h otoc u r ab l e p o l y ( s tyr e n e )

G w y n W i l li a m s a, C l a u d i u s D ' S i l v a b"Schlumberger Electricity, Langer Road, F elixstowe, Suffolk, IP11 8ER , UKb Department of C hemistry, Manchester Metropolitan University, Faculty of Science and Engineering, John Dalton Building, Chester Street, Manchester,

M1 5GD, UKReceived 15 July 1994; in revised form 19 July 1995; accepted 25 July 1995

AbstractThe direct photolithographic structuring of poly(styrene) by bi-functional azides has been elaborated and applied to the encapsulation ofa disposable thin-film three-electrode device. Effective dielectric encapsulation has b een show n w ith a polystyrene layer only 10/~m thickwhen chem ically bonded to the sensor surface. This process may be generally amenable to the encapsulation of a wide variety of other typesof sensors and electronic devices.

Keywords: Encapsulation; Sensors; Crosslinking; P hotoresist; Integrated thin-film multi-electrode arrays; Pol y(st yre ne) ; Azide; Photo-adhesives

1 . I n t r o d u c t i o nB o t h t h i c k - a n d t h i n - f i l m e l e c t ro d e t e c h n o l o g y h a v eadvan ced as poss ib le rou tes fo r p roduc ing d i sposab le e lec t ro -c h e m i c a l d e v i c e s . T h i n - f i l m d e v i c e s p r e p a r e d u s i n g s t a n d a rd

s e m i c o n d u c t o r p ro c e s s e s o f f e r a d v a n t a g e s o f h i g h s t ru c t u ra ld e f i n it i o n a n d p e r fo rm a n c e c h a r a c t e r i s ti c s m a t c h i n g t h o s e o fs tandard e lec t rodes . These e lec t rodes can a l so be p repared ina s u f f i c i e n t ly r o b u s t f o rm t o a l l o w c o n n e c t i o n s t o b e m a d ed i rec t ly to the e lec t rode su r face u s ing s tandard so lder ing t ech-n iques .

O u r r e s e a r c h i s c o n c e rn e d w i t h t h e d e v e l o p m e n t o f d is -posab le m u l t i -e lec t rode senso rs , in s t rumen ta t ion [ 1 ,2] andb i o s e n s o r s [ 3 ] . T o t h i s o b j e c t i v e w e h a v e e x t e n s i v e l yexp lo red severa l th ree-e lec t rode mu l t i s enso r con f igu ra t ionsfo r e l e c t ro c h e m i c a l a n d e n z y m e a p p l i c a ti o n s , i n c o rp o ra t in ga g o l d c o u n t e r e l e c t ro d e, s e v e r a l w o rk i n g e l e c t ro d es a n d a nA g / A g C 1 r e f e r e n c e e l e c t ro d e . H o w e v e r , o u r w o rk a n d t h a to f o t h e r s i n v o l v e d i n t h e d e v e l o p m e n t o f d i s p o s a b le e l e c t ro -c h e m i c a l s e n s o r s h a s b e e n h i n d e r e d b y t h e l a c k o f a v a i l a b il i tyo f s u i t a b l e e n c a p s u l a n t s [ 4 ] . E n c a p s u l a t i o n o f e l e c t ro c he m -ica l dev ice s i s needed in o rde r to in su la te non -ac t ive e lec t rodea r e a s f r o m t h e a n a l y t e s o l u t i o n t o p r e v e n t d e v i c e f a i l u r e ,co r ros ion and in add i t ion to c rea te a chemica l ly sens i t ivew i n d o w w h e re a n a l y s i s c a n b e u n d e r t a k e n . E n c a p s u l a t i o nm e t h o d s a v a i l a b l e i n c l u d e g l a s s b o n d i n g [ 5 ] , s e m i - a u t o -m a t e d m e t h o d s [ 6 ] , m i c ro m a c h i n i n g [ 7 ] , e l e c tr o c h e m i c a lox ida t ion [ 8 ] , p ho to l i thog raphy [ 4 ,9 ] and m anua l methods .0925-4 005/96 /$15.00 1996 Elsev ier Science S.A. All r ights reservedS S D 1 0 9 2 5 - 4 0 0 5 ( 9 5 ) 0 1 7 9 1 - 5

O f t h e s e , p h o t o li t h o g ra p h y is t h e m o s t a m e n a b l e i n o u r c a s efo r encapsu la t ion . Al though commerc ia l wet - f i lm res i s t s ,p h o t o s il o x a n e a n d p h o t o p o l y b u t y l m e t h a c ry l a t ep o l y m e r s a r eava i lab le fo r encapsu la t ion , they do no t a f fo rd sa t i s fac to ryin su la t ion due to the poo r chemica l s ea l fo rmed wi th thesenso r subs t ra te , thus a l lowing e lec t ro ly te c reep be tw een thee n c a p s u l a n t a n d s u b s t ra t e [ 9 ] , o r h a v e p o o r e l e c t ro c h e m i c a lcharac te r i s t i cs due to the poo r d ie lec t r i c p roper t i es o f thep o l y m e r s u s e d . T h e p h o t o - e p o x y p o l y m e r r e p o r te d b y D u m -scha t e t a l . [ 9 ] appeared em inen t ly su i t ab le fo r th i s t ask basedo n o u r e x p e r i e n c e o f t h e u s e o f h i g h -g r a d e e p o x y r e s i n s i nman ua l encapsu la t ion . However , the a va i l ab i l i ty and s tab i l i tyo f r e a g e n t s u s e d i n t h e p r e p a r a t io n o f t h i s p h o t o p o l y m e r p re -c luded i t s u se .

In th i s paper w e descr ibe a new techn ique fo r pho toencap -su la t ion tha t u ses mo re read i ly ava i l ab le reagen t s and w hichwe ha ve app l ied to an in tegra ted th in - f i lm mul t i -e lec t rodea r r a y ( I M A ) , u s i n g p h o t o c u ra b l e p o ly ( s t y r e n e ) . T h e m e t h o dcons i s t s o f a fou r -s tep p rocess (F ig . 1 ) : (a ) su r face p re trea t -m e n t ; ( b ) c o a t i n g w i t h a p h o to s e n s i ti v e a d h e s i v e / e n c a p s u -lan t mix tu re ; (c ) exposu re th rough a mas k o f se lec t ive a reas ;( d ) r e m o v a l o f p o l y m e r t h a t i s n o t p h o t o c u re d f r o m t h ee l e c t ro c h e m i c a ll y a c t iv e a r e a s o f t h e I M A d e v i c e .2 . P r i n c i p l e o f p h o t o c u r a b l e p o l y ( s t y r e n e ) e n c a p s u l a n t

E l e c t ro ch e m i c a l d e v i ce s a r e p r e d o m i n a n t l y u s e d i n a q u e -ous med ia , there fo re an e f fec t ive encap su lan t has to have


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152 G. W illiams, C D'Silva / S ensors and Actuators B 3 0 (1996 ) 151-157

U n t r e a t e dsurface

{ i) HzO2/NH3( i l l H202/HC[t OHHOHOHOHOH

Treatedsur face

(OCH3)3SiPhN3/N]-R-N3[ -CH2EHP h-] n

(a) (b )

( c } (d )

Fig. 1. Stepsused n the IMA deviceencapsulation echnology: (a) surfacepretreatment; (b) coatingwith photoeneapsulantmixture; (e) illuminationthrough a m ask; (d) development.goo d dielectric , chem ical and structural stabili ty. On eapproach to the deve lopment of photoencapsulants i s toexplo it the high reactivity of aryl nitrenes generated by pho -todecom posi t ion of a ryl az ides in order to e ross l ink polymers[ 10] . Th is approach w ould resul t in a nega t ive-work ing pho-toresist , which is desirable due to the potential damage ofsens i t ive a reas o f Sem iconduc tor devices by ul t raviole t radi -a t ion . A var ie ty of readi ly prepared b i fune t iona l a ryl az idessuch as 4 ,4 ' -e thy lene- l , l ' -d iph enyla z ide , 1 ,4-diaz idoben-zene and 1,3-diazidobenzene react with poly me rs such aspo ly ( s ty r e n e ) a nd po ly ( su lph one ) [ 11 ] to p r oduc e thinf ilms insolub le in man y organic solvents . P oly(s tyren e) w aschosen as the po lym er for use in this s tudy du e to i t s h ighmo lecular weight, good dielectr ic properties, high reactivityto n i trenes , low cost , go od chem ica l s tab i l ity and process ingproperties. Ho we ver, such f ilms do no t afford effective di-electr ic insulation, unless a chemical seal is achieved withthe sen sor substrate . To achiev e a seal i t is necessa ry to attachthe encapsulant layer chemically to the device surface, af terchemica l pre t rea tment had genera ted the des i red funct iona lg roups (Fig . 1) . 4-Azidophen yl t r imethoxysi lane fu l f i lled therole of a photoad hesive in be ing ab le to reac t chemica l ly bothwith the pretreated surfa ce via the silane functionality andwith the polym er ic encapsulant layer on photoi rradia tion ofthe az ide g roup. Th is compound was a lso expec ted not toalter signif icantly the chem ical and electr ical properties ofthe encapsu lant layer , which w as desirable.

s ilane f rom AB CR , 4 ,4 '-e thylene-1 , l ' -d iphenyla z ide and 4-azidophenyltrimethoxy-silane we re synthe sized usinggenera l procedures [ 12] . U V spec tra and measurements o f e(mo lar decadic extinction coeff icie nt) were undertaken on aHitach i U-2000 UV /Vi s spect rophotometer .3 . 1 . 1 . 4 , 4 ' - E t h y l e n e - l , l ' - d i p h e n y l a z i d ePrepared f rom 4,4 ' -e thylenediani l ine and ob ta ined as apa le ye l low sol id in 82.8% yie ld (m .p . 77 C) ; (Found: C ,63.10, H, 4.53 , N, 31.9. CIaHI2N6 requires C, 63.11, H, 4.58,N , 31 . 8 0 ) ; U V , Am ~ ( E ) 254 nm ( 28 8 5) ; V n~ ( N u j o l ) /c m - 1 2106 ( N 3) .3 . 1 . 2 . 4 - A z i d o p h e n y l t r i m e t h o x y s i l a n ePrepared f rom 4-aminophenyl t r imethoxysi lane andobta ined as a dark coloured oi l in 92.5% yie ld . UV, Am~ (~)254 ( 1003) ; V n~ ( N u j o l ) / c m - 1 2112 ( N 3) .3 . 2. I M A f a b r i c a t i o n a n d s t r u c tu r e

IMA e lec t rodes were fabr ica ted us ing ad vanced th in- fi lmtechnology in a c lass 10 000 c lean room on gold s l ides pre -pared as previously descr ibed [ 13] . Evapo ra ted gold (200r im) on chromium co a ted s l ides were spin coa ted wi th A Z1505 resis t and pa t te rned us ing a contac t photomask . Af te rdeve lopment , non-masked gold a reas were removed wi thaqua- reg ia , the mask rem oved to ex pose the f inger e lec t rodes tructure shown in Fig . 2 and the under ly ing chrom ium key -layer dissolved with alkaline potas sium ferr icyanide. TheAg /Ag CI re fe rence e lec t rode was form ed a f te r encapsula t ionby plating the go ld electrod e with silver at constant current,10 min a t 840 pA , fo l lowed by ox ida t ion a t + 1 .5 V in

3 . M a t e r ia l s a n d m e t h o d s

3 . 1 . M a t e r i a l sAll chemica ls were of reagent g rade and used wi thoutfur ther pur if ica tion. Po ly(s ty rene ) , mol . wt . 280 000, 4 ,4 ' -

e thylenediani line and l i th ium perch lora te w ere ob ta ined f romthe Aldr ich Chemica l Co (UK) . Andnophenyl t r imethoxy- Fig. 2. Thin-film multi-electrode dev ice encapsulated wi th transparentpolystyrene ilm.


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G. Williams, C. D'Si lva/Sensors and Actuators B 30 (1996) 151-157 15 3

0 .1 M KC I so lu t ion fo r 2 min . The in tegr i ty o f the encapsu lan tw a s q u a l i t a t i v e ly a s s e s s e d b y t h e f o rm a t i o n o f a w e l l - d e f in e db o u n d a ry b e t w e e n s i l v e r c h l o r i d e a n d e n c a p s u l a n t . T h e s t r u c-t u r e o f t h e I M A d e v i c e i s s h o w n i n F i g . 2 a n d c o n s i s ts o f ar e f er e n c e e l e ct r od e ( f a r l e f t) 2 0 0 0 / z m 1 0 0 0 / z m i n s iz e ,a d j a c e n t t o s e v e n w o rk i n g e l ec t ro d e s , 5 0 0 / zm 5 0 0 / zm i ns ize , su r round ed by a coun te r e lec t rode 500 p ,m wide .3.3. Evaluation o f photoencapsulant

I n t h e o p t i m i ze d p ro c e d u re I M A d e v i c e s w e re r e n d e re dh y d ro p h i l i c p r i o r t o e n c a p s u l a t i o n b y t h e m e t h o d o f M a n d e n -iu s e t a l . [ 14 ] adap ted fo r these s tud ies . Th is p rocedu reinvo lved f i r s t t rea t ing the su r fa ces o f s l ides w i th a so lu t ionc o n t ai n in g 3 m l o f a m m o n i a ( 2 5 % ) a n d 3 m l o f H 2 0 2 ( 3 0 % )i n 1 5 m l o f H 2 0 , a t 6 0 C fo r 2 min . Then the s l ides werer e n d e re d h y d ro p h i l i c b y a f u r t h e r t re a t m e n t w i t h a s o l u ti o nc o n t a i n i n g 3 m l o f H C l ( 3 6 % ) a n d 3 m l o f H 2 0 2 (3 0 % ) i n18 ml o f H20 , a t 60 C fo r 2 min . O therwise s l ides werec l e a n e d b y w a s h i n g i n d e t e rg e n t , w a t er , m e t h a n o l a n d s o x h l e ti n i s o p ro p y l a l c oh o l . T h e p h o t o e n c a p s u l a n t w a s a m i x t u r e o fp o l y ( s t y r e n e ) , 4 , 4 ' - e t h y l e n e - 1,1 - d i p h e n y l a z i d e p h o t o c ro s -s l in k i n g a g e n t a n d 4 - a z i d o p h e n y l t r i m e t h o x y s i l a n e s a p h o t o -a d h e s i o n p ro m o t e r . I n t h e o p t i m i ze d p ro c e d u re t h ee n c a p s u l a n t m i x t u r e c o n t a i n e d p o l y ( s t y r e n e ) ( 2 1 0 -3 M ) ,4 , 4 ' - e t h y l e n e - l , l '- d i p h e n y l a z i d e ( 2 1 0 - 4 M ) a n d 4-a z i d o p h e n y l t r i m e t h o x y s i l a n e (0 .5 X 1 0 -4 M ) i n T H F .

T h e p h o t o e n c a p s u l a n t w a s s p i n - c o a t e d o n t o p r e t r e a t e dI M A d e v i c e s f r o m t e t r a h y d ro fu r a n , m a s k e d , e x p o s e d t o U Vl igh t fo r 10 min a t a d i s t ance o f 36 cm us ing a DEK 107o v e rh e a d e x p o s u re c a b i n e t a n d i n t h e o p t i m i ze d p roc e d u red e v e l o p e d i n a c e t o n e t o r e m o v e u n w a n t e d p o l y m e r f r o m t h ea c t iv e e l e c t ro d e a r e a . C o n n e c t i o n s t o t h e e l e c t ro d e w e re m a d eby so lder ing con nec to rs d i rec t ly to the go ld pads . The e lec -t r o d e s t r u c tu r e a f t e r e n c a p s u l a t i o n a n d fo rm a t i o n o f t h e r e f -e r e n c e e l e c tro d e (d e s c r i b e d a b o v e ) i s s h o w n i n F i g . 2 . T h ee l e c t ro c h e m i c a l l y a c t i v e w i n d o w fo rm e d b y t h e e n c a p s u la n twas 12 .5 mm 3 mm in s ize . Eva lua t ion s tud ies on the encap -s u l a n t to c h e c k fo r u n d e rc u t t i n g a n d q u a l i t y o f t h e s u r f a c ew e re u n d e r t a k e n u s i n g a H i t a c h i $ 5 2 0 s c a n n i n g e l e c t ro nm i c ro s c o p e ( SE M ) . T h e f i l m t h ic k n e s s , o b t a i n e d a t v a ri o u ss p i n s p e e d s, w a s e s t i m a t e d b y S E M fo r a n a v e r a g e o f a t l ea s tth ree samples , u s ing p rev ious ly descr ibed metho ds [ 15 ] . Them e t h o d i n v o l v e d o r ie n t i n g t h e I M A d e v i c e w i t h i n t h e SE Ms o t h a t th e e t c h e d e d g e w a s a n g l e d t o t h e e l e c tro n b e a m a n dm e a s u r i n g t h e a p p a re n t d e p t h f r o m t h e g l a s s b a s e t o t h eenca psu lan t su r face . T r igonom et r ic ca lcu la t ion l ed to a va lueo f t h e f i l m t h i c k n e s s . T h i s p ro c e d u re w a s r e p e a t e d s e v e ra lt i m e s a t d i f f e r e n t lo c a t i o n s a n d a t d i f f e r e n t a n g l e s t o e n s u rerep roduc ib i l i ty .3.4. Electrochemical characterization

A l l c y c l i c v o l t a m m e t ry e x p e r im e n t s w e re p e r fo rm e d u s i n ga n E C P 1 5 0 b ip o t e n t i o s ta t i n te r f a c e d to a V i g l e n -A T m i c ro -com pu ter [ 15 ] . T he s tan dard e lec t rode con f igu ra t ion u sed

wi th so l id e lec trodes cons i s ted o f a go ld w ork ing e lec t rode ,p la t inum coun te r e lec t rode and sa tu ra ted ca lomel re fe rencee l e c tro d e (S C E ) [ 1 5 ,1 6] . A l l c y c l i c v o l ta m m e t ry e x p e ri -men ts were per fo r med in aqueous 0 .1 M l i th ium perch lo ra tein a t empera tu re -con t ro l l ed room a t 22 + 1 C . IM A dev iceswere eva lua ted in the th ree-e lec t rode con f igu ra t ion u s ing oneo f t h e s e v e r a l g o ld w o rk i n g e l e c t ro d e s, t h e c o u n t e r a n d A g /AgC1 re fe rence e lec t rodes . The res i s t iv i ty o f the f i lms wase v a l u a te d b y t h e m e t h o d o f P o t j e -K a m l o t h e t a l . [ 8 ] f r o m t h es l op e o f t h e c u r r e n t -v o l t a g e r e l a t io n s h i p m e a s u re d a t z e rocu rren t c ro ss ing u s ing a t r i angu la r sweep o f 150 mV s - 1.

4 . R e s u l t s a n d d i s c u s s i o n

4.1. Photo-properties o f azidesT h e p h o t o l y ti c p ro p e rt ie s o f 4 , 4 ' - e t h y l e n e - I , l ' - d i p h e n y l -

a z i d e ( 4 .3 1 0 -5 M ) a n d 4 - a z i d o p h e n y l t r i m e t h o x y s i l a n e(3 .0 10 -5 M ) in ch lo ro fo rm were inves t iga ted a f te r per i -od ic i r rad ia t ion a t a d i s t ance o f 36 cm us ing a DEK 107o v e rh e a d e x p o s u re c a b in e t as s h o w n i n F i g . 3 ( a ) a n d (b ) ,r e s p e c ti v e ly . 4 , 4 ' -E t h y l e n e - l , l ' - d i p h e n y l a z i d e o b e y s f ir st -o rder k ine t ics ( see in se t , F ig . 3 ( a ) ) w i th an apparen t ra tecons tan t o f 2 .1 10 -4 s - l ( t l / 2 = 5 5 m i n ) w i t h a r e a s o n a b l yt igh t i so sbes t i c po in t cen t red a t 273 nm. Pho to lyses o f 4 -a z i d o p h e n y l t ri m e t h o x y s i la n e (3 .0 X l O -5 M ) u n d e r t h esame c ond i t ions showed tw o re la t ive ly t igh t i so sbes t i c po in t s

1 . 0 0

(a )IoglAt - Ac~]

0 1 0000 2 0 40 60 804O

~ r n e ( ~ n )- - 6 0

3 0 0 3 5 0L / n m

1 . 0 0 - I o g I A ~ - A , ~ ]

( b ) 1 o ~

,o

0 i i i i v2 4 0 2 8 0 3 2 0 3 6 0

~ . / n mFig. 3. Sp ectra] changes of (a ) 4,4'-ethylene-l,l'-diphenylazide (4.3 X1 0 - 5 M ) a n d ( b ) 4 - a z i d o p h e n y l t r im e t h o x y s i l a n e ( 3 . 0 x 1 0 - 5 M ) . T h ei n s e t s s h o w t h e f i rs t - or d e r p l o t s o f t h e c h a n g e a b s o r b a n c e a t 2 5 4 n m v s .t i m e fo r ( a ) 4 , 4 ' - e t h y l e n e - l , l ' - d i p h e n y l a z i d e a n d ( b ) 4 - a z i d o p h e n y l -t r i m e t h o x y s i l a n e .


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15 4 G. Wil l iams, C. D'Si lva / Sensors and Actuators B 30 (1996) 15 1-15 7

T a b l e 1E f f e c t o f s p i n - c o a t i n g s p e e d o n e n c a p s u l a n t ( p o l y ( s t y r e n e ) ( 2 X 1 0 - 3 M ) ,4 , 4 ' - e t h y l e n e - l , l ' - d i p h e n y l a z i d e ( 2 I 0 - 4 M ) a n d 4 - a z i d o p h e n y l t r im e -t h o x y s i l a n e ( 0 .5 1 0 - 4 M ) i n T H F )S pe ed ( r p m ) T h i c k n e s s ( / a n ) M i n i m u minewidth (/~m)1000 10 4001500 10 4002500 10 1003000 10 100Table 2Effectof varying he mass ratios of polystyrene:diazideRatio Properties20:1 film did not withstand stirring10:1 etching 400 /zm10:2 etching 400/~mT a b l e 3E f f e c t o f p o l y ( s t y r e n e ) c o n c e n t r a t i o n o n f i l m t h i c k n e s s a n d m i n i m u m e t c h -i n g l in e w i d t h . A c o n s t a n t m a s s r at i o o f p o l y ( s t y r e n e ) : 4 , 4 ' - e t h y l e n e - l , l ' -d i p h e n y l a z i d e = 1 0: 1 w a s m a i n t a i n e d t h r o u g h o u tC o n c e n t r a t i o n ( M ) T h i c k n e s s ( p . m ) M i n i m u m l i n e w i dt h ( / ~ m )1 .20 x 10 -4 3 502 . 3 0 x 1 0 - 4 9 1 0 02 . 9 0 x 1 0 - 4 1 1 . 5 4 0 03 . 5 0 x 1 0 - 4 1 3 4 0 0

w a s t e s t e d a n d p r i m a r i l y i d e n t if i e d to a f f e c t t h e t h i c k n e s s a n dr e s o l u ti o n o f e t c h i n g ( s e e T a b l e 3 ) . T h e f i l m th i c k n e s si n c r e a s e d w i t h c o n c e n t r a t i o n , a s d i d t h e m i n i m u m e t c h i n gl in e w i d t h . F i g . 4 s h o w s t h e r e s o l u t io n o f a n e t c h e d a r e a 5 0/~ m i n w i d t h a n d e n c a p s u l a n t t h i c k n e s s o f 1 0 ~ m .

T h e i n c o r p o r a t i o n o f t h e p h o t o - a d h e s i v e i n t o t h e e n c a p -s u l a ti o n m i x t u r e h a d v e r y l i t tl e e f f e c t o n s u r f a c e a d h e s i o n o ff il m s w i t h o u t p r i o r s u r f a c e p r e t r e a t m e n t t o i n t r o d u c e s i l a n o lo r h y d r o x y g r o u p s f o r s u r fa c e a t t a c h m e n t [ 1 4 , 1 7 ]. T h e i n t r o -d u c t i o n o f s u r f a c e s il a n o l g ro u p s e n h a n c e d a d h e s i o n o f t h ep h o t o - e n c a p s u l a n t e v e n i n t h e a b s e n c e o f p h o t o - a d h e s i v e , a sv i s u a l ly a s s e s s e d b y t h e q u a l i t y o f s p i n - c o a t e d f i lm s w h e nc o m p a r e d t o u n t r e a te d g l a s s s li d e s c l e a n e d b y w a s h i n g i nd e t e r g e n t , w a te r , m e t h a n o l a n d s o x h l e t i n i s o p r o p y l a l c o h o l .In h i nds i gh t t h i s observa t i on i s no t su rp r i s i ng , as hyd rogenb o n d i n g b e t w e e n s u r f a c e s il a n o l g ro u p s a n d t h e p h e n y l s u b -s t it u e n ts o f t h e p o l y s t y r e n e b a c k b o n e w a s n o w p o s s i b l e a n dt h e s t re n g t h o f t h i s i n te r a c t i o n w o u l d b e e x p e c t e d t o b e g r e a t e ri n med i a o f l ow d i e l ec t r i c permi t t i v i t y , as i n t h i s case . S t ud i eso n f i lm s p r e p a r e d o n p r e t r e a t e d g l as s s u r f a c e s w i t h a n d w i th -o u t 4 - a z i d o p h e n y l t r im e t h o x y s i l a n e c l e a r l y s h o w t h a t f i lm sl a c k i n g t h i s c o m p o u n d w e r e p r o n e t o e d g e p e e l i n g , w h i l ef il m s i n c o r p o r a t i n g it r e m a i n e d t i g h tl y b o u n d t o t h e s u r f a c e ,a s e x p e c t e d b a s e d o n i ts r e a c t i o n w i t h s u r f a c e s i la n o l g r o u p s( s e e F i g . 1 ) . T h e c o n c e n t r a t i o n o f th e p h o t o - a d h e s i v e u s e di n s t u d i e s w a s m a i n t a i n e d e q u a l t o t h a t o f d i a z i d e i n t h em i x t u r e a s a t t h is c o n c e n t r a t i o n v e r y l it tl e e f f e c t w a s o b s e r v e don t he p roper t i es o f t he fi l m. F i g . 2 show s a t h i n - f i lm e l ec t ro de

a t a b o u t 2 7 0 a n d 2 3 0 n m w i t h a n a p p a r e n t f ir s t -o r d e r r a t ec o n s t a n t o f 6 . 6 1 0 - 4 s - ~ ( t~ /2 = 1 7 .6 m i n ) i n p r o p a n - 2 - o l( s e e F i g . 3 ( b ) ) .4.2. Encapsulation o f the IMA devices

I n i ti a l ly a n u m b e r o f s t u d i e s w e r e p e r f o r m e d i n th e a b s e n c eo f p h o t o - a d h e s i v e t o i d e n t i f y t h e e f f e c t o f s p i n s p e e d , m i x t u r ec o m p o s i t i o n a n d c o n c e n t r a t i o n o n t h i c k n e s s a n d e t c h i n g o ft h e e n c a p s u l a n t u s i n g a f i x e d v o l u m e o f t h e p h o t o - e n c a p s u -l a n t m i x t u r e s p u n f r o m t e t r a h y d r o f u r a n . S p i n c o a t i n g w a su n d e r t a k e n i n a l a m i n a r f l o w h o o d t o p r e v e n t e x c e s s i v e e x p o -s u r e t o s o l v e n t v a p o u r s a n d t h e t h ic k n e s s , m e a s u r e d a t v a r i o u ss p i n s p e e d s u s i n g a t e s t p a t t e r n m a s k w i t h l i n e w i d th s o f 5 0 ,1 00 , 4 0 0 a n d 1 0 0 0 / x m , f r o m e t c h e d l a y er s b y S E M . F o r af i x e d c o m p o s i t i o n o f e n c a p s u l a n t m i x t u r e , s p i n s p e e d h a dv e r y l i tt le e f f e c t o n t h e t h ic k n e s s , a s s u m m a r i z e d i n T a b l e 1 .T h e a v e r a g e t h i c k n e s s o f l a y e r s a t d i f f e r e n t s p e e d s w a s 1 0/ zm . H o w e v e r , a n e f f e c t w a s o b s e r v e d i n e tc h i n g r e s o l u t io n ,w h i c h w a s g r e a t e r a t h i g h e r s p e e d s : 1 0 0 / x m a t < 2 5 0 0 r p mc o m p a r e d to 4 0 0 / ~ m a t > 2 5 0 0 r p m . T h e e f f e c t o f e n c ap s u -l a t io n m i x t u r e c o m p o s i t i o n o n f i l m p r o p e r t ie s i s s u m m a r i z e di n T a b l e 2 . S t a b l e f il m s w e r e f o r m e d a f t e r i rr a d i a ti o n a n de t c h i n g o n l y f o r p o l y s t y r e n e : d i a z i d e c o m p o s i t i o n s > 1 0 :1 .B a s e d o n t h e s e r e s u l ts f u r t h e r w o r k w a s u n d e r t a k e n u s i n g ap o l y s t y r e n e : d i a z i d e r a t i o o f 1 0 :1 a n d a s p i n s p e e d o f 3 0 0 0r p m . T h e e f f e c t o f m i x t u r e c o n c e n t r a t i o n o n f i lm f o r m a t i o n F ig . 4 . S EM p ic tu r e s o f a r e so lu t i o n p a t t e r n f r o m a 1 0 /~ m th i c k l a y e r( l i n e w i d t h 5 0 p . m ) .


5/7

G. Williams, C. D'Silva / Sensors and Actuators B 30 (1996) 151-157 155d e v i c e e n c a p s u la t e d w i t h a 1 0 / z m p o l y s t y r e n e la y e r w i th a na c t iv e w i n d o w a r e a o f 1 2 . 5 m m X 3 m m e x p o s e d f o r a n a ly s is .4.3. Electrochemical characterization of lMA devices

A d e m a n d i n g t e s t o f e n c a p s u l a t i o n in t e g r it y is t h e m e a s -u r e m e n t o f t h e l e a k a g e c u r r e n t o f a f u l l y e n c a p s u l a t e d e le c -t r o d e . T h i s w a s c a r d e d o u t b y c o n t i n u o u s i m m e r s i o n o f af u l l y c o a t e d e l e c t r o d e i n 0, 1 M N a C I ( r e s i s t iv i t y > 1 0 0 12c m ) [ 8 ] a n d s c a n n in g th e a p pl ie d v o lt a g e b e t w e e n + 2 a n d- 2 V a t v a r i o u s i n te r v a l s . T h e a v e r a g e r e s i s ti v i ty f o r s e v e n

e l e c t r o d e s d e t e r m i n e d p e r i o d i c a l l y o v e r a 1 4 d a y p e r i o d w a s( 4 . 2 + 2 . 2 ) 1 0 H I I c m , w h i c h d i d n o t c h a n g e s i g n if i c a n tl yo v e r t h e t e s t p e r i o d . T h i s v a l u e w a s c o m p a r a b l e t o t h a to b s e r v e d b y P o t j e - K a m l o t h e t a l . [ 8 ] , w h o r e p o r t e d f o re n c a p s u l a t i o n s t u d i e s w i t h p o l y ( o x y p h e n y l e n e ) f i l m s a n i n i-t ia l v a l u e o f 4 .5 1 01 2 l ~ c m w h i c h d e c a y e d a f t e r t h e fi r st 1 0d a y s o f i m m e r s i o n t o 7 . 3 X 1 01 1 1 2 c m .

T h e t h i n - fi l m e l e c t r o d e e n c a p s u l a t e d a s s h o w n i n F i g. 2w a s t e s t e d a g a i n s t c o n v e n t i o n a l s t a n d a r d g o l d m a c r o - e l e c -t r o d e s ( 0 . 0 2 c m 3 ) b y o b t a i n i n g c y c l i c v o l t a m m o g r a m s f o rb o t h t y p e s i n 0 .1 M s u l p h u r i c a c id . T h e s h a p e s o f th e c y c l i cv o l t a m m o g r a m s f o r b o t h e l e c t r o d e s a r e s im i l a r ( s e e F i g . 5 ) .H o w e v e r , t h e c u r r e n t d e n s i t i e s o b t a i n e d f o r t h e t h i n - fi l m e le c -t r o d e s w e r e a p p r o x i m a t e l y 1 0 t i m e s t h a t s e e n w i t h p o l is h e dm a c r o - e l e c t r o d e s , r e f le c t i n g a d i f f e r e n c e i n s u r f a c e t e x t u r ea n d a r e a . C y c l i c v o l ta m m o g r a m s w e r e o b t a i n e d f o r t h e I M Ad e v i c e i n t h e t h r e e - e l e c t r o d e c o n f i g u r a t i o n w i t h t h e s o d i u ms a lt o f f e r r o c e n e c a r b o x y l i ca c i d ( 1 1 0 - 3 M ) i n 0 .1 M a q u e -o u s l i t h i u m p e r c h l o r a t e . T h e c y c l i c v o l t a m m o g r a m s ( s e e

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F i g . 5 . C y c l i c v o l t a m m o g r a m s m e a s u r e d i n O . l M H 2 S 0 4 , s c a n r a t e100 mV s-I: (a) polishedAu disk electrode; (b) thin-filmAu electrodewith external Pt c o u n t e r e l e c tr o d e ( s o l i d c u r v e ) ; thin-filmAu electrode ndcounterelectrode dashed curve .

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F i g . 6 . C y c l i c v o lt a m m o g r a m s m e a s u r e d w i t h a t h i n - f i lm A u e l e c t r o d e inthe three-electrode onfiguration n an aqueou s sodium salt of ferroce ne-carboxylicacid +0 .1 M LiCIO4. Sca n rate: 1, 25; 2, 50; 3, 100; 4, 2 00; 5,400; 6, 800 (mV s- I) .0 .8

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Fig. 7. Relationshipbetween peak current (ip) and the square root of scanrate for a thin-filmAu electrode n a three-electrode onfiguration.F i g . 6 ) h a v e t h e c h a r a c t e r is t i c s o f a r e v e r s i b l e o n e - e l e c t r o nt r a n s f e r p r o c e s s w i t h t h e p r o d u c t i o n o f s o l u b l e , s t a b le p r o d -u c t s. T h e p e a k s e p a r a t io n A E p (Ep~-Ep) c a l c u l a t e d f r o mF i g . 6 i s 9 2 m V f o r th e t h i n - fi l m e l e c t r o d e c o m p a r e d t o 8 2m V d e t e r m i n e d u s i n g a s t a n d a rd g o l d m a c r o - e l e c t r o d e v e r s u sSCE, i nd i ca t i ng s l i gh t l y s l ugg i sh e l ec t ron t r ans fe r be t weent h i s c o m p o u n d a n d e v a p o r a t e d g o l d e l e c t r o d e s , w h i c h m a yr e p r e s e n t a d i f f e r e n c e in s u r f a c e t e x t u r e .

In t e r es t i ng l y t he E~/z v a l u e f o r t h i s c o m p o u n d i s 5 7 m Vw i t h t h e t h in - f il m t h r e e - e l e c t r o d e s t r u c tu r e a n d 3 0 9 m V f o rt h e s t a n d a rd g o l d e l e c t r o d e m e a s u r e d v e r s u s S C E . N o t e t h eu s e o f a t h i n - fi lm r e f e r e n c e e le c t r o d e r e s u l ts i n a b o u t a 2 5 2m V n e g a t i v e s h if t i n p e a k p o t e n t ia l . A 2 0 0 m V n e g a t i v e s h i f to f p e a k p o t e n t ia l w a s o b s e r v e d b y W a n g a n d T i a n [ 18 ] u s i n gd i s p o s a b l e s c re e n - p r i n t e d A g / A g C I r e f e r e n c e e l e c t r o d e s v e r -s u s a c o n v e n t i o n a l A g / A g C I r e f e r e n c e e le c t r o d e . T h i s n e g -a t i ve sh i f t i n t he E~/2 v a l u e b r i n g i n g i t c l o s e t o z e r o m a y b ed e s i r a b le , a s i t m i n i m i z e s i n t e r f e r e n c e s f r o m e l e c t r o a c t i v es p e c i e s , e s p e c ia l l y in a p p l i c a t i o n s s u c h a s e n z y m e e l e c t r o d e sw h e r e f e r r o c e n e c a r b o x y l i c a c i d i s c o n v e n t i o n a l l y u s e d a s a ne l e c t r o n - t ra n s f e r m e d i a t o r . A p l o t o f p e a k c u r r e n t ( i p ) v e r s u st h e s q u a re r o o t o f t h e s c a n ra t e ( v t /2 ) ( s e e F i g . 7 ) s h o w e d al i near re l a t i onsh i p , s i mi l a r t o r esu l t s ob t a i ne d wi t h a s t andard


6/7

15 6 G. Williams, C D'Silva / Senso rs and Actuators B 30 (1996) 151-15 7

g o l d e l e c t r o d e a n d c o n s i s t e n t w i t h a d i f f u s io n - l i m i t e d e le c -t r o n - t r a n s f e r p r o c e s s . S i m i l a r i d e a l c h a r a c t e r i s t i c s w e r eo b s e r v e d f o r th e s e I M A d e v i c e s w h e n u s e d u n d e r a c i d ( p HI ) o r b a s i c ( p H 1 1 ) c o n d i t i o n s , a s n o n o t i c e a b l e v a r i a ti o n si n r e s p o n s e w e r e o b s e r v e d . C o r r e s p o n d i n g s t u d i e s o n I M Ad e v i c e s p r e p a r e d w i t h o u t p h o t o - a d h e s i v e a d d e d t o th e e n c a p -s u la n t m i x t u r e s h o w e d t h e m t o h a v e e x t r e m e l y p o o r e l e c t ro -c h e m i c a l c h a r a c t e r is t i c s . T h e l i f e ti m e o f d e v i c e s w a s f o u n dn o t t o b e d e p e n d e n t o n f a i l u r e o f t h e e n c a p s u l a n t b u t o n f a i l u r eo f t h e g o l d e l e c t r o d e s . T h e e l e c t r o d e s d e s c r i b e d h e r e w e r ef o u n d t o f a i l o n r e p e a t e d v o l t a g e c y c l i n g a f t e r le s s t h a n 2 0s c a n s d u e t o s t r i p p i n g o f g o l d f r o m t h e e l e c t r o d e s u r f a c e .E l e c t r o d e s p r e p a r e d w i t h g o l d f i l m s o f t w i c e t h e t h i c k n e s sw e r e f o u n d t o b e h a v e r e p r o d u c ib l y in e x c e s s o f 5 0 o r m o r escans .

5. ConclusionsT o s u m m a r i z e , t he I M A d e v i c e b e h a v e s a s a n i d ea l e l ec -

t r o c h e m i c a l s e n s o r w h e n p a t t e r n e d w i t h p h o t o c u r a b l ep o l y ( s t y r e n e ) . E f f e c t i v e d i e l e c t ri c i n s u la t i o n o f t h e e l e c t r o d es t r u c t u r e h a s b e e n s h o w n p o s s i b l e w i th o n l y a 1 0 / x m t h i c kp o l y m e r l a y e r, w h e n c h e m i c a l l y b o n d e d t o t h e e l e c t ro d e s u r-f a c e . T h e s e m a t e r i a l s h a v e b e e n t e s t e d f o r u s e o n a o n e - s h o td i s p o s a b l e d e v i c e s u c h a s F i g . 2 , d e s i g n e d f o r u s e a s a m u l t i-f u n c t i o n e l e c t r o c h e m i c a l s e n s o r o r b i o s e n s o r . T h e h a r s h c o n -d i t i ons u sed i n r e nde r i ng t he s l i des hyd roph i l i c a t f i rs t s i gh tm a y a p p e a r t o p r e c l u d e t h e u s e o f t h i s t e c h n i q u e f o r o t h e rd e v i c e s . H o w e v e r , t h e s e c o n d i t i o n s a r e n o t d i s s im i l a r t o t h o s eu s e d i n t h e s e m i c o n d u c t o r in d u s t r y f o r t h e c l e a n i n g o f s i li c o nw a f e r s , s o i t is m o s t l i k e l y th a t n o f u r t h e r t re a t m e n t o f t h ed e v i c e w i ll b e r e q u i r e d p r i o r t o e n c a p s u l a t i o n u n d e r c l e a n -r o o m c o n d i t io n s . T h i s t e c h n i q u e o f f e r s a n a l t e rn a t i v e e n c a p -s u l a ti o n p r o c e d u r e t o t h a t r e p o r t e d b y D u m s c h a t e t a l . [ 9 ]b u t u s e s m o r e r e a d i l y a v a i la b l e re a g e n t s . H o w e v e r , w e e x p e c tt h a t a n y l o n g - t e r m u s a g e o f t h e e n c a p s u l a n t w o u l d r e q u i r et h e d e v i c e t o b e p o t t e d t o p r e v e n t f il m d a m a g e b y a b r a s i o n .F u r t h e r w o r k h a s b e e n u n d e r t a k e n t o c h a r a c t e r i z e t h e p o t e n -t ia l o f o t h e r b i - a n d t r i- f u n c t i o n a l a zi d e s a n d p o l y m e r s t o a c ta s e n c a p s u l a n t s a n d t h e u s e o f e n c a p s u l a t e d e l e c t r o d e s i nenv i ro nm en t a l mon i t o r i ng [ 1 ,2 ,11 ] .

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Chem. Sensors, Bordeaux, France, 7-10 July, 1986, pp. 403-404.[6] A. Sibbald and J.E.A. Shaw, A flow-through ion-selective field-effecttransistor, Sensors and Actuators, 12 (1987) 297-301.[7] R.L. Smith and S.D. Collins, Micromachined packaging for chemicalmicrosensors, 1EEE Trans. Electron Dev. , ED-35 (1988) 787-792.[8] K. Potje-Kamloth, P. Janata, J. Janata and M. Josowicz, Electro-chemical encapsulation fo r sensors, Sensors and Actuators, 18 (19 88)415-425.[9] C. Dumschat, H. Milller, H. Rantschek, H.-J. Timpe, W. Hoffmann,

M.T. Pham and J. Hiiller, Encapsulation of chemically sensitive field-effect transistors with photocurable epoxy resins, Sensors andActuators B, 2 (19 90) 271-276.

[ 10] M. Steppan, G. Buhr and H. Vollmann, The resis t technique: aehem icalcontribution to electronics, Ang ew. Chemic, 21 (1982) 455-554.[ 11 ] G. W illiams and C. D 'Silv a, Photolithographic encapsulation of anintegrated electrochemical multielectrode de vice using thin films,Fourth Fur. Conf. Organised Thin Films (EC OF 1992), U niversity ofWales, Bangor, 10-12 Sept., 1992, OCA .

[12] C. D'S ilv a, A.P. Seddon and K.T. Douglas, Photoaffinity labels forenzymes capable of binding glutathione or its disulphide, J. Chem.Soc. Perkin Trans. I, (1981) 3029-3033.

[ 13] H. M organ, D.M. Taylor and C. D 'Silva , Surface plasmon resonancestudies of chemisorbed biotin-strepavidin multilayers, Thin SolidFilms, 209 (1992) 122-126.[ 14] C.F. M andenius, S. Welin, B. Da nielsson, I. Lund strt~m and K.Mosbach, The interaction of proteins and cells with affinity ligandscovalently coupled to silicon surfaces and monitored by ellipsometry,

Anal. Biochem., 137 (19 84) 106-114.[15] C. D'Silv a, S, Afeworld, O.L. Pard, P .K. Baker and A.E. Underhill,Electrochemical characterisation of polyvinylferrocene polymers:suhstituent effects on the redox reaction, J. Mater. Chem., 2 (1992)225-230.[ 1 6 ] C . D ' S i l v a , X.-B. Wang and R. Pethig, Physical and Electrochemicalcharacterisation of polyvinylferrocene film s, J. Phys. D, 22 (1989)1591-1597.[17] C. Barnes, C. D'S ilva , J.P. Jones and T.J. Lewis, A Concanavilin, Acoated piezoelectric crystal biosensor, Sensors and Actuators B, 3( 1991 ) 295-304.[18] J . Wang and B. Tian, Screen-printed str ipping voltam me tric/potentiometric electrodes for decentralized testing of trace lead, Anal.

Chem., 64 (1992) 1706-1709.

T h e a u t h o r s t h a n k s E P S R C f o r s u p p o r t u n d e r G r a n tG R / F 1 6 4 2 4 a n d G R / J 2 0 9 37 . G .W . a c k n ow l e d ge s E P S R Cf o r th e a w a r d o f a s t u d e n t sh i p .

References[ 1 G. Williams and C . D'Si lva, in K.T.V. Grattan and A .T. Augousti

(eds . ) , Sensors VI. Technology System s and Applications, Institute ofPhysics, Bristol, UK, 1993, pp. 161-166.

BiographiesGwyn Williams o b t a i n e d a B . E n g . ( H o n s ) a n d P h . D . in

e l e c t r o n i c e n g i n e e r i n g f r o m t h e U n i v e r s i t y o f W a l e s , B a n g o ri n 1991 and 1995 , r espec t i ve l y . Hi s Ph .D. i nv o l ve d t he s t udyo f e l e c t r o c h e m i c a l n s t r u m e n t a ti o n . H e w a s a s c i e n t if i c o f f i c e rw i th t h e C o u n t r y s id e C o u n c i l fo r W a l e s ( 1 9 9 4 - 1 9 9 5 ) ,r e s e a r c h i n g th e e f f e c t s o f l o n g - t e r m c l i m a t e c h a n g e o n v e g -e t a ti o n . H e i s c u r r e n t ly e m p l o y e d a t S c h l u m b e r g e r E l e c t r i c -i ty , d e v e l o p i n g m e t e r i n g i n s t r u m e n t a t i o n .


7/7

G. Wil l iams, C. D'Si lva / Sensors and Actuators B 30 (1996) 151 -15 7 157C l a u d i u s D ' S i l v a obta ined a B .A. (Hons) and Ph .D. inchemis t ry f rom the Un ivers i t ie s of Lancas te r and Birming-

ham, respec t ive ly . He was a research execut ive wi th Proc te ra nd Ga m b le , U K ( 197 9- 19 8 0) , a f e l low in the D e pa rtm e n tof Chemis t ry , Univers i ty of Essex (1980- -1983) , senior fe l -low in the Depar tmen t of Biolog ica l Chemis t ry , Univers i tyof Mich igan, A nn Arbor , US A (19 83- 198 6) , research fe l lowin the Depar tment of Biotechnology and Biophysics , Uni-

vers i ty of Leeds (198 6-19 87) and the Fe l lowsh ip of Eng i-neer ing / GE C senior fe l low in molecula r and b iomolecula re lec t ronics , Univers i ty of Wales , Bangor (1 987 -19 92 ) . HeWas a temp orary sen ior lecturer in chemistry, U niversity ofSunderland (19 92 -19 93 ) and was appointed sen ior lecturerin organic chemistry, Manchester Metropolitan University,in 1 993. He currently heads an interdisciplinary group w ork-ing on mo lecular materials, sensors and d rug design.

encapsulation of an integrated thin-film multi-electrode sensor using

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