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    E L S E V I E R Journal of Non-Crystalline Solids 188 (1995) 46-5 3

    JOURN L OF

    Th in aerog el f i lm s for opt ica l thermal aco ust ic and e lectron icapplications

    L . W . H r u b e s h , J .F . P o c oChemistry and Material Sciences Department Lawrence Livermore National Laboratory Livermore CA 94550 USA

    Received 18 Septem ber 1994; revised 30 Septem ber 1994

    A b s t r a c t

    Aero gels are a special class of continuou sly porous so lid mater ials whic h are character ized by na nom eter s ize part iclesand pores . Typical ly , aerogels are made using so l-gel chemistry to form a so lvent f i l led , h igh porosi ty gel that is dr ied byremo ving the so lvent without co llapsing the tenuous so lid phase. As bulk mater ials, aerogels are kno wn to have ma nyexceptional, a nd even som e uniqu e physica l properties. Ae roge ls prov ide the highest therm al insulation and lowe st dielectricconstant of any o ther material know n. How ever , som e impor tant applicat ions require the aerogels in the form of th in f ilms orsheets. For example, elec tronic applications require micro me ter thin aerogel films bonde d to a substrate, and others requirethicker films, either on a substrate or as free standing sheets. Special methods are required to make aerogel thin films orsheets. In this paper, the special conditions needed to fabricate thin aerogel films are discussed and methods to make filmsand thin sheets are described. A lso, som e specific applications are given for wh ich aerog el film s are being developed.

    1 I n t r o d u c t i o n

    A e r o g e l s a r e u n u s u a l p o r o u s m a t e r i a l s b e c a u s e o ft h e ir u n i q u e m i c r o s t r u c t u r e c o n s i s t i n g o f p o r e s a n dp a r t i c l e s w h i c h a r e i n t h e n a n o m e t e r s i z e r a n g e .A e r o g e l s a r e t y p ic a l l y m a d e u s i n g s o l - g e l c h e m i s t r yt o f o r m s o l v e n t - f i l l e d g e l s w h i c h a r e d r i e d i n a w a yt h a t d o e s n o t s h r i n k o r c o l l a p s e t h e w e a k s o l i ds t r u c t u r e o f t h e s o l i d m a t r i x . T h e d r i e d a e r o g e l i s a na i r -f i ll e d p o r o u s s o l id , u s u a l l y i n m o n o l i t h i c o r g r a n -u l a r f o r m . A s b u l k m a t e r i a l s , a e r o g e l s a r e k n o w n t oh a v e m a n y e x c e p t i o na l a n d e v e n s o m e u n i q u e p h y s i -

    Presented at the 4th International Sym posium on Aerogels,Berkeley, CA, USA , 19-21 Septem ber 1994.* Corresponding author. Tel: + 1-510 423 1 691. Telefax: + 1-510 423 4897 . E-maih [email protected].

    c a l p r o p e r t i e s w h i c h a c c o u n t f o r t h e l a r g e n u m b e ra n d v a r i e t y o f p o t e n t ia l a p p l i c a t i o n s f o r t h e m . T h ee x c e p t i o n a l o p t i c a l , t h e r m a l , a c o u s t i c a l a n d e l e c -t r o n i c p r o p e r t i e s o f a e r o g e l s w h i c h r e s u l t f r o m t h e i ru n i q u e m i c r o s t r u c t u r e h a v e b e e n w e l l e s t a b l i s h e da n d d o c u m e n t e d , p a r t i c u l a r l y i n t h e p r o c e e d i n g s o ft h e t h r e e p r i o r I n t e r n a t i o n a l S y m p o s i a o n A e r o g e l s[ 1 - 3 ] , M o s t o f t h e p r o p e r ti e s w h i c h a r e c h a r a c t e r i s t icf o r b u l k a e r o g e l s a r e a l s o e x h i b it e d i n o t h e r f o r m s o ft h e m a t e r i a l , e . g . , i n t h i n s h e e t s o r f i l m s . A g r o w i n gn u m b e r o f i m p o r t a n t a p p l i c a t io n s r e q u i r e t h e a e r o -g e l s e i t h e r a s f r e e s t a n d i n g s h e e t s o r a s t h i n l a y e r s o no t h e r s u b s t r a t e m a t e r i a l s . F a b r i c a t i n g t h e a e r o g e l s i ns u c h f o r m s p r e s e n t s p r o b l e m s d u e t o th e w e a k , t e n u -o u s n a t u r e o f th e h i g h l y p o r o u s a e r o g e l s a n d b e c a u s es p e c i a l r e q u i re m e n t s a r e n e c e s s a r y t o f o r m a n d d r yt h e m . F o r p u r p o s e s o f d i s c u s s i o n i n t h is p a p e r , w ea r b i t r a r i l y r e f e r t o t h e g e l s a s a e r o g e l s i f t h e i r p o r o s -

    0022-3093/95/$09.50 1995 Elsevier Science B.V. All r ights reservedSSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 0 2 8 - 3

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    L.W. Hrubesh, J.F. Poco Jou rnal of Non-Crystalline Solids 188 1995) 46-5 3 47i t y i s > 75% , xe roge l s i f the i r po ros i t y is < 75%and dense f i lms i f the poros i ty i s < 1%.

    S o l -ge l p roc e s se s ha ve be e n w e l l de ve l ope d , e s -pe c i a l l y du r i ng t he pa s t de c a de o r so , t o p roduc eadvanced glasses , g lass-ce ramics and ce ramics , inbo t h bu l k a nd f i l m fo rms . Dur i ng t h i s pe r i od , t he reha ve be e n s e ve ra l symp os i a on t he sub j e c t and m a nyjourna l a r t i c le s ha ve be e n p ub l i she d . The re a re goodbooks and a r t ic les which desc r ibe the advantages ,de t a i l s a nd a pp l i c a t i ons o f t he so l -ge l p roc e s se s[4-6] and the re a re severa l review a r t ic les spec i f ic toso l -ge l f i l ms [7 -9 ] . A n e xc e l l e n t sou rc e o f i n fo rma -t i on fo r a l l a spe c t s o f so l -ge l p roc e s s i ng o f f i l ms ,i nc l ud i ng po rous f i l ms , i s found i n Br i nke r a ndSchere r ' s book, Sol -Gel Sc ience , ch . 13 and 14 [6] .Mos t o f t he l i t e ra t u re on so l -ge l f i l ms de a l s w i t happl ica t ions for fu l l dens i ty coa t ings or pa r t ia l lyde ns i f i e d f il ms , suc h a s A R c oa t i ngs on op t ic s . V e ryl i t t l e w ork ha s be e n re por t e d fo r p roc e s s i ng po rousso l -ge l f i l ms w he re t he f i l ms a re t o r e ma i n po rousfor the i r appl ica t ion. There a re some a r t ic les whichd i sc us s so l -ge l f i l ms up t o a bou t 50% poros i t y[10 -12 ] a nd one a r t ic l e d i s c us se s the p re pa ra t ion o f at rue ae roge l f i lm bu t the poro s i ty i s not reported [13].In th i s paper , we discuss the spec ia l condi t ions andre qu i re me n t s w h i c h a re ne e de d t o ma ke h i gh l y po rousaeroge l f i lms , i .e . , poros i ty > 75%.M os t o f t he me t hod s a l r e ady de ve l ope d fo r a pp l y -i ng so l -ge l t h i n f i l ms a nd c oa t i ngs ge ne ra l l y a pp l y ,w i t h mod i f i c a t i ons , t o ma k i ng t rue a e roge l f i l ms a sl ong a s spe c i a l c ons i de ra t i on i s g i ve n t o p re ve n tra p i d d ry i ng by e va pora t i on du r i ng de pos i t i on . Inth i s paper , we desc r ibe severa l methods tha t havebe e n suc c e s s fu l l y u se d t o fo rm t h i n a e roge l she e tsa nd f i l ms . Me a su re me n t s o f p rope r t i e s w h i c h a reimportant for va r ious appl ica t ions and the appl ica -t ions themse lves a re a l so d i scussed.

    2 Experimental proceduresThe me t hods use d t o ma ke t h i n a e roge l f i l ms

    de pe nd on t he t h i c kne s s de s i re d a nd w he t he r o r no tt he f i l m i s t o be bonde d t o a su r fa c e . H ow e ve r , a l l o ft he se me t hods ha ve t he c om mo n re qu i re me n t tha t t hege l mus t be fo rme d unde r c ond i t i ons i n w h i c h t hera t e o f e va pora t i on i s l i m i t e d , bo t h du r i ng a nd a f t e rt he ge l fo rma t i on . To fa c i l i t a t e t h i s , w e pe r fo rm t he

    c oa t i ng p roc e s se s w i t h i n a n e nc l osu re t ha t i s ma i n -t a i ne d s a t u ra t e d w i t h t he va por o f t he w ork i ng so l -ve n t . The e nc l osu re a t m osphe re a l so c on t a i ns a pa r -t ia l p re s su re o f a m mo ni um h ydrox i de w h i c h he l ps t oc a t a l yz e t he ge l a ti on o f t he f i lms . The mos t c om mo naeroge l f i lms a re s i l i ca , but we have a l so demon-s t ra t e d ou r me t ho d w i t h o t he r me t a l ox i de s fo r w h i c hh y d r o l y s i s / c o n d e n s a t i o n o f t h e m e t a l a l k o x id e i s t h ep re domi na n t c he mi s t ry ( e .g . , z i r c on i a , a l umi na a ndtanta la ) . Genera l ly , depending on the des i red poros-i t y o f t he f i na l a e roge l , w e use d t w o me t hods t oprepare the precursor solut ion for the process . Forge l poros i t ie s > 95%, we used the two-s tep , pa r tia lhyd ro l y s i s / c on de n sa t i on c he m i s t ry r e por t ed p re v i-ous l y fo r s i li c a a e roge l s [14 ] . F o r ge l s w i t h o t he rporos i t i e s , we prepared a s ingle -s tep base -ca ta lyzedhydro l ys i s so l u t i on a c c o rd i ng t o t he t yp i c a l me t hodfo r s i l i c a a s fo l l ow s : mi x t e t r a me t hoxys i l a ne(TMOS ) , w a t e r , me t hy l a l c oho l a nd a mmoni um hy-droxide , in a mola r ra t io of 1 :2 :4:0.01. An addi t iona lamount of a lcohol i s added to th i s mixture to es tab-l i sh the u l t imate poros i ty of the ge l . We discuss thefo l l ow i ng fou r p roc e sse s w h i c h w e ha ve u se d t oma ke h i gh po ros i t y f i l ms : d i p c oa t i ng ; sp i nn i ng ;sp ra y c oa t i ng ; a nd c a p i l la ry f i l m fo rma t i on .2 .1 . S u r fa c e p re p a ra t io n

    F rom e xpe r i e nc e w e found t ha t mos t o f t he me t a lox i de ge l s a dhe re t o g l a s s o r ox i d i z e d su r fa c e s (p rob -a b l y th r o u g h m e t a l - o x i d e - m e t a l b o n d s ) b u t g el s d i dno t s t i c k w e l l t o unpre pa re d m e t a l su r fa c e s . Bond i ngt o a l l ox i d i z e d su r fac e s w a s e nha nc e d by e t c h i ng t hesurface wi th a mi ld a lka l ine solut ion (e .g . , KOH),t he n r i n s i ng w i t h a l c oho l i mme d i a t e l y p r i o r t o f i l mde pos i ti on . F o r t he oppos i t e c a se o f non-bo nd i ng , w et rea ted the surfaces wi th t r imethylchloros i lane , togive a hydrophobic surface tha t se rves as a re leaseagent .2 . 2 . D ip c o a t in g

    Di p c oa t i ng i s t he s i mp l e s t o f t he c oa t i ng p ro -cesses , but i t i s used only when a l l surfaces of asubs t ra te mate r ia l a re to be coa ted. W i th th i s method,f i l m t h i c kne s se s l e s s t ha n a f e w mi c rome t e r s a reob t a i na b le de p e nd i ng on t he v i s c os i t y o f t he p re c u r -sor and the wi thdrawl ra te . In our work, the prec ise

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    48 L.W. Hrubesh, J.F. Po co Jou rna l o f Non-CrystallineSolids 188 1995) 46-5 3t h i c kne s s w a s no t a n i mpor t a n t pa ra me t e r , so t hed i pp i ng a nd w i t hd ra w i ng p roc e dure w a s pe r fo rme dma nua l l y w i t hou t c onc e rn fo r c on t ro l l i ng t he r a t e s .Typ i c a l subs t ra te s w e re P yre x g l a s s s l ide s o f va r i ouss i z e s . The se w e re s i mp l y d i ppe d i n t o t he p re pa re dpre c u rso r so l u t i on , w i t hd ra w n , t he n p l a c e d e dge w i sea nd ve r t i c a l i n a ho l de r w h i c h i s l oc a t e d w i t h i n t hee nc l osu re . The t i me ne c e s sa ry fo r a ge l f i l m t o fo rmw a s found by t r i a l t o be on l y a f e w mi nu t e s . A f t e rthe f i lm i s ge l led , the ent i re holder conta in ing thes l ides i s immersed in a beaker of solvent and i tr e ma i ns su r rounde d by l i qu i d un t i l r e a dy fo r supe r -c r i t ica l dry ing.2.3. Spin coat ing

    Thi s p roc e s s i s e s se n t i a ll y the s a m e a s t ha t u se d t ospin g lass coa t ing s for e lec t ronic appl ica t ions , excepttha t i t i s pe rformed wi th the spin appara tus ent i re lyw i t h i n a n e nc l osu re w h i c h ha s a so l ve n t s a t u ra t e da t mosphe re . F i l m t h i c kne s se s a re t yp i c a l l y le s s t ha n2 i xm. S pe c i a l mod i f i c a t i ons w e re p e r fo rme d on t heappara tus to prevent the poss ib i l i ty of an explos ivehazard . Typica l subs t ra tes a re Pyrex glass s l ides ands i li c on w a fe r s up t o 3 d i a me t e r . The p roc e dure fo rfo rmi ng f i l ms i s t o me t e r a d rop l e t o f p re c u rso rsolut ion onto the spinning subs t ra te whi le i t s sp inra t e i s i nc re a s i ng up t o a ma x i mum spe e d o f 1850rpm. The sp i nne r i s i mme d i a t e l y t u rne d o f f a nds topped wi th a brake , so tha t the sample i s subjec t tomi n i ma l l o s s o f so l ve n t du r i ng ge l a t i on . Typ i c a l l y ,t he ge l w i l l fo rm w i t h i n a f e w mi nu t e s , a f t e r w h i c ht he subs t ra t e i s ma nua l l y r e move d f rom t he sp i na ppa ra t us a nd i mme rse d i n so l ve n t . The subs t ra t e sw i t h f i l ms a re s t o re d subme rse d i n so l ve n t un t i lready for superc r i t i ca l drying.

    2.4. Spray coat ingThi s p roc e s s ha s be e n use d t o pu t t h i c ke r s i ng l elayer coa t ings on subs t ra tes l ike g lass and s i l i conw a fe r s . F i l ms a s t h i c k a s 80 I~m ha ve be e n a c h i e ve dby t h i s me t hod . A n a sp i ra t o r i s u se d t o sp ra y t he

    p re c u rso r so l u t i on d i re c t l y on t o t he subs t ra t e w h i c his supported in a near ly ve r t ica l pos i t ion wi th in thee nc l osu re . Exc e s s so l u t ion d ra i ns by g ra v i t y , l e av i nga t h i c k f i lm w hi c h ge l s w i t h i n a f e w mi nu t e s . The sef i l ms ha ve a va ry i ng t h i c kne s s due t o t he d ra i n i ng ,

    bu t t he su r fa c e o f t he ge l i s smoo t h a nd c on t i nuous .A f t e r ge l a t i on , the subs t ra t e i s ma n ua l l y i mm e rse d i nsolvent unt i l ready for superc r i t i ca l drying.2.5. Surface tension coat ing

    Thi s p roc e s s i s u se d t o fo rm un i fo rm, r e l a t i ve l ythick planar f i lms on glass and s i l i con subs t ra tes .A e roge l f i l ms w i t h t h i c kne s se s f rom 2 t o 50 Ixmha ve b e e n a c h i e ve d . Eva pora t i on i s e s se n t i a l ly e l i mi -na t e d by us i ng t h i s me t hod a nd t he re fo re it doe s no trequi re the spec ia l enc losure , This method i s s imi la rto d ip coa t ing but u t i l i zes surface tens ion to draw thel iquid onto the sol id surfaces . This i s bes t accom-p l i she d by us i ng a f i x t u re ma de by fo rmi ng a spa c ebe t w e e n t he subs t ra t e t o be c oa t e d a nd a no t he r f l a tsurface . The surface of the f la t i s t rea ted wi th are lease agent to prevent bonding to the ge l so tha t i ti s r e mova b l e a f t e r the d ry i ng p roc es s . S pa c e r s ( sh i msor mic rospheres) a re used to separa te the subs t ra tesby t he de s i re d f il m t h i c kne s s , t he n t he y a re c l a mp e dt oge t her . The spa c e be t w e e n t he subs t ra te s fo rm s a neffec t ive capi l la ry . Liquid precursor f i l l s the ava i l -a b l e vo l um e by c a p i l la r i ty w he n t he f i x t u re i s d i ppe dinto the solut ion. The f i l l ed f ix ture i s e i the r l e f ti mm e rse d w i t h i n t he p re c u rso r so l u t ion t o be e nc a p-sula ted by the ge l , or i t i s removed pr ior to ge la t ionand placed in a solvent sa tura ted a tmosphere .2.6. Supercri t ical dry ing

    A l l o f t he p re pa re d ge l s w e re c onve r t e d t o a e ro -ge l s u s i ng supe rc r i t i c a l d ry i ng me t hods i n o rde r t oprevent dens i f ica t ion of the f i lms . Ei the r d i rec t su-perc r i t i ca l ext rac t ion (SCE) of the solvent was ca r-r ied out a t h igh tempera ture or an a l te rna te lowt e mpe ra t u re e x t ra c t i on o f c a rbon d i ox i de w a s pe r -fo rme d a f t e r e xc ha nge o f t he o r i g i na l so l ve n t . Theme t hod o f d ry i ng c hose n de pe nde d on t he t e mpe ra -ture s tabi l i ty of the subs t ra te mate r ia l . Ge ls on glasssubs t ra t e s w e re ge n e ra l ly c om pa t i b le w i t h h i gh t e m-pe ra t u re S CE, w he re a s s i l i c on w a fe r s r e qu i re d aprotec t ive coa t ing of s i l i con dioxide to survive theh i gh - t e mpe ra t u re S CE o f so l ve n t s. The p roc e dure fo rdi rec t SCE i s to p lace the g lass conta iner holding thesubme rse d (o r e nc a psu l a te d ) s a mpl e s d i re c t ly i n t o a nautoc lave . The autoc lave i s f i l l ed wi th addi t iona lsolvent (usua l ly , a lcohol ) and sea led . The tempera -

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    L.W. Hrubesh, J.F. Poco /Jour nal of Non-Crystalline Solids 188 1995) 46- 53 49tu r e o f t he a u toc l a ve i s i nc r e a se d a t a r a t e o f0 .3 C / m i n , w h i l e t h e p r e s s u re i n c r e a s e s t o a b o u t120 ba r a nd e xc e s s p r e s su r e a bove tha t i s r e l e a se d .Af t e r t he t e m pe r a tu r e r e a c h e s a bo u t 280 C , t he p r e s -su r e i s r e le a se d f r om the ve s se l a t a c on t r o l l e d r a te o f0 . 3 ba r /m in un t i l a p r e s su r e o f a bou t 1 . 5 ba r i sr e a c he d . Th e a u toc l a ve i s t he n pu r ge d w i th a i r as t heve sse l c oo l s . Th i s c y c l e t yp i c a l ly t a ke s 24 h . A l t e r na -t i ve ly , a s im i l a r c yc l e i s pe r f o r m e d a t t e m pe r a tu r e s< 45~ C a f t e r t he so lve n t i n t he ge l f i lm i s f i r ste x c h a n g e d w i t h l i q u i d c a r b o n d i o x i d e . T h e e x c h a n g et im e to r e m ove so lve n t f r om the t h in f i lm s l e s s t ha n50 p , m th i c k wa s t yp i c a l ly 2 h .2 . Z M e t a l l i z a t i o n

    M e ta l l i z a t i on o f t he a e r oge l wa s c a r r i e d ou t byva por o r spu t t e r de pos i t i on d i r e c t ly on to t he a s - p r e -pa r e d a e r oge l f i lm su r f a c e s . F o r som e c a se s , l owr e so lu t ion pa t t e r n ing wa s a c h ie ve d by de pos i t i ng them e ta l t h r ough a n a pp r op r i a t e m a sk . P ho to r e s i s t pa t -t e r n ing t e c hn ique s we r e a pp l i e d on ly a f t e r t he a e r o -g e l s u r f a c e w a s c o a t e d w i t h a p p r o x i m a t e l y 1 0 0 0 . ~o f pa r y l e ne , t o s e a l t he po r e s o f t he a e r oge l .2 . 8 . C h a r a c t e r i z a t i o n

    T h e m o r p h o l o g y o f t h e f i l m s a n d t h e f i l m - s u b -s t r a t e i n t e r f a c e r e ~ ions we r e e xa m ine d us ing a h ighr e so lu t ion ( - - - 20 A) , f i e ld e m is s ion , s c a nn ing e l e c -t r o n m i c r o s c o p e ( H i t a c h i $ 8 0 0 ) .

    T h e f i l m o r s h e e t t h i c k n e s s e s w e r e m e a s u r e d u s -i n g a S l o a n D e c T a c I I - A P r o f i l o m e t e r . T h e D e c T a cI I - A in s t r um e n t m e a su r e s t h i c kne sse s i n t he r a nge0 . 02 - 65 p , m , w i th a ve r t i c a l r e so lu t ion o f 0 . 5 nm ,f o r a s c a n l e n g t h u p t o 3 m m . T h e a c c u r a c y f o rm ic r om e te r t h i c kne ss m e a su r e m e n t s w i th t h i s i n s t r u -m e n t i s be t t e r t ha n 2% .

    F i l m a d h e r e n c e o n g l a s s a n d s i l i c o n w a f e r s w a squa l i t a t i ve ly de t e r m ine d us ing a t a pe s t i c k t e s t w i thS c o tc h* t a pe . The t a pe wa s l i gh t ly p r e s se d on to asu r f a c e o f a e r oge l c ov e r ing a t l e a s t 1 c m 2 a r e a , t he npe e l e d o f f . The t a pe i s e xa m ine d f o r r e s idue tode t e r m in e i f t he a e r oge l ha s be e n l i f t e d o f f o r i f t hea d h e s i v e h a s b e e n r e m o v e d f r o m t h e t a p e.

    R e f r a c t i v e i n d e x e s o f f i l m s w e r e m e a s u r e d u s i n ga n Au toEL- I I Au tom a t i c E l l i p som e te r . The th i c k -n e s s m e a s u r e d b y t h e D e k T a c p r o f i l o m e t e r i s e n t e re d

    a s i npu t t o t he e l l i p som e te r to c a l c u l a t e t he r e f r a c t iveinde x o f a f i lm . G e n e r a l ly , i t is d i f f i c u l t to a pp ly t h i st e c hn ique be c a use t he su r f a c e r e f l e c t i v i t y o f a e r oge l si s t yp i c a l ly < 1% a t a wa v e le ng th o f 632 . 8 nm .Ho we ve r , su f f i c i e n t r e f le c t i v i t y i s u sua l ly ob t a ine d a tsha l low a n g le s o f i nc ide nc e ( i . e ., < 45 ) . Oth erphys i c a l p r ope r t i e s o f t he a e r oge l f i lm s we r e de t e r -m i n e d f r o m t h is m e a s u r e m e n t a s f o l lo w s .

    T h e f i lm d e n s i ti e s w e r e d e t e r m i n e d f r o m t h e m e a -su r e m e n t o f t he op t i c a l r e f r a c t ive i nde x , n , u s ing ther el a ti o n, p = ( n - 1 ) / 0 . 2 0 9 , r e p o rt e d b y H e n n i n ga nd S ve nsson f o r s i l i c a a e r oge l s [ 15 ] . The d i e l e c t r i cc o n s t a n t a n d t h e p o r o s i t y o f t h e f d m s w e r e d e t e r -m i n e d i n d i r e ct l y fr o m t h e d e n s i t y , a f t e r m e a s u r e m e n to f t he op t i c a l r e f r a c t ive i nde x . The po r os i ty i s de t e r -m i n e d f r o m th e re l a t io n / 7 = 1 - P / P s , w h e r e P s i sthe de ns i ty o f t he so l id . S o f o r s i l i c a a e r oge l , w i thP s = 2 . 1 9 g / c m 3 , / 7 = [ (1 . 45 8 - n ) / 0 . 4 5 8 ] i s t h epe r c e n t po r os i ty . S im i l a r ly , t he d i e l e c t r i c c ons t a n t sf o r t he s i li c a a e r oge l f i lm s o r she e t s we r e d e t e r m ine da f t e r m e a su r e m e n t o f t he op t i c a l r e f r a c t ive i nde xus ing K '= 1 + 1 . 6p [ 16 ], so K ' = 1 + 7 . 7 ( n - 1 ).

    3 Results

    U s i n g t h e m e t h o d s d e s c r i b e d , w e h a v e s u c c e s s -f u l ly f a b r i c a t e d th in , f l a t, un i f o r m s i l i c a a e r oge l f i lm s ,ha v ing va r ious po r os i t i e s , w i th m e a su r e d th i c kne sse sf r om < 1 t o 20 p , m on g l a s s a nd s i l i c on wa f e rsubs t r a t e s . The a dhe r e nc e o f t he f i lm s to t he sub -s t r a t e s ha s be e n qua l i t a t i ve ly de t e r m ine d by a t a pes t i c k t e s t . G e ne r a l ly , good f i lm a dhe r e nc e wa s ob -t a ine d f o r a l l a e r oge l s w i th a p o r os i ty l e s s t ha n a bo u t8 6 % a n d m i x e d r e s u lt s w e r e o b s e r v e d f o r p o r o s i ti e sb e t w e e n 8 6 a n d 9 5 % . F o r p o r o s i t ie s > 9 5 % , t h er e su l t s we r e i nva l id be c a use t he a e r oge l i s t oo we a kto su r v ive t he a pp l i c a t i on o f t he t a pe . W e ha ve a l sof a b r i c a t e d una t t a c he d a e r oge l she e t s w i th m e a su r e dth i c kne sse s be twe e n 30 a nd 100 pxm us ing the c a p i l -l a r y f i l l m e thod . The se f i lm s a nd she e t s ha ve be e ne x a m i n e d u s i n g s c a n n i n g e l e c t r o n m i c r o s c o p y ( S E M )to ve r i f y t he i r a e r oge l - l i ke s t r uc tu r e . As a n e xa m pleo f a t yp i c a l fi lm , F ig . 1 shows a s e r i e s o f S EM s o f a44 txm th i c k a e r oge l f i lm de pos i t e d on a 7 . 6 c mdia m e te r s i l i c on wa f e r . The m e a su r e d r e f r a c t ive i n -de x o f t h i s f i lm i s 1 . 049 , so i t s c a l c u l a t e d de ns i ty i sa b o u t 0 . 2 3 5 g / c m 3 , i ts p o r o s i t y is a b o u t 8 9 % a n d i t s

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    50 L. W. Hrubesh, J .F . Poco / Journ al o f Non-C rys ta l li ne Sol ids 188 1995) 46 -53

    F ig . 1 . SEMs o f a typ ica l s i l i ca ae roge l f i lm on a s i l i con subs t ra te , p repa red by the cap i l l a ry f i l l me thod . The f la t and un i fo rmly th ickae roge l f i lm i s shown bonded to a th in (= 1 .5 p ,m) laye r o f s i l i con d iox ide wh ich was vapo r depos i ted on the s i l i con wafe r . The g ranu la rappea rance o f the ae roge l i s an a r t i f ac t due to cha rg ing by the beam and i t s su r face i s ac tua l ly much s moo the r than i t appea rs .

    die lec t r ic cons tant i s 1 .38. The f i lm th ickness uni -fo rmi t y ove r t he fu l l 7 .6 c m d i a me t e r , me a su re d byt he De kTa c , w a s w i t h i n t he a c c u ra c y o f t he i n s t ru -ment ( i .e . , 2%).

    W e ha v e m e a su re d t he r e f ra c ti ve inde xe s o f t he sef i l ms a nd she e t s a nd w e ha ve c a l c u l a t e d t he i r bu l kde ns i t i e s t o c onf i rm t ha t t he y a re a e roge l s ha v i ngde ns i ti e s in t he r a nge f rom a b ou t 0 .04 t o 0 .46 g / c m 3 ,a nd po ros i ti e s in t he r a nge f rom 77 t o 98% . W e ha vea lso ca lcula ted the d ie lec t r ic cons tants of these f i lmsba se d on t he me a su re d re f ra c t i ve i nde xe s . The d i -e lec t r ic cons tants span the range from about 1 .06 to1.7 . Table 1 shows charac te r i s t ic da ta for somerepresenta t ive th in ae roge l f i lms .W e de ve l ope d p roc e s se s to s e a l , pa tt e rn a nd m e t -a l l i z e t h i n a e roge l f i l ms . A n e xa mpl e o f a h i gh

    re so l u ti on pa t te rn o f go l d m e t a l on a n a e roge l f i l m i ssho wn in F ig . 2 . Th is pa t te rn of 4 p~m wid e s t r ipswas achieved wi th the fo l lowing s teps : (1) fabr ica tea 10 Ixm thick s i l i ca ae roge l f i lm on a 7 .6 cmdi a me t e r s i l i c on w a fe r u s i ng c a p i l l a ry f i l l i ng a ndsupe rc r it i ca l d ry i ng ; (2 ) va por de pos i t - -1 00 nmpa ry l e ne on t o a e roge l f i l m ; (3 ) va por de pos i t 40 nmt i t a n i um me t a l t he n 500 nm go l d ; (4 ) sp i n on 1500nm pho t o re s i s t a nd d ry a t 90C ; (5 ) ma sk a nde xpose t o u l t r a v i o l e t ; (6 ) r e move ma sk , w a sh w i t hKOH ; (7 ) e t c h a w a y go l d ; (8 ) p l a sma e t c h t i t a n i umdow n t o pa ry l e ne ; (9 ) a sh a w a y t he re ma i n i ng re s i st ,l e a v i ng a me t a l l i z e d pa t t e rn on t he pa ry l e ne c oa t e daeroge l .W e ha ve a l so spu t te re d t h in (< 0 .5 ~ m ) m e t a llayers d i rec t ly onto the ae roge l surfaces and e lec t ro-

    Tab le 1Charac te r i s t i c da ta fo r some rep resen ta t ive th in ae roge l f i lms on g la ssF i lm type Meth od Ave rage th ickness (p ,m) Ref rac t ive index Po ros i ty (%) D ie lec t r ic cons tan t

    (measu red ) (measu red ) (ca lcu la ted ) (ca lcu la ted )Sil ica capil lary 18.1 1 .062 86 1 .48Sil ica spin 3 .4 1 .103 78 1 .79Sil ica spray 44.1 1 .049 89 1 .38Zirco nia capil lary 24.7 1 .059 95 -Sil ica capil lary 113.5 1 .009 98 1 .07

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    L.W. Hrubesh, J.F. Poco Journal of Non-Crystalline Solids 188 1995) 46-53 51

    4 xm

    F ig . 2 . Pho tog raph o f 4 p ,m w ide go ld m e ta l s t r ip s pa t te rned on a th in s i l i ca ae roge l f i lm ( = 1 .3 ~ m ) w h ich i s on a s i l i con wafe r . Th ispa t te rn was p roduced by pho to res i s t p rocess ing a f te r the ae roge l was f i r s t coa ted wi th abou t 100 nm th ick pa ry lene po lyme r wh ich rema insa f te r the p rocess .

    pla ted th icker (> 1 .0 ixm) layers on the sput te redlayers .

    4 DiscussionA e roge l f i l ms c a n s e rve i n ma ny use fu l a pp l i c a -

    t ions . Here we desc r ibe severa l spec i f ic appl ica t ionsfo r w h i c h w e ha ve p roc e s se d t h i n a e roge l f i l ms . Inmo s t c a se s, t he a e roge l f i l ms a re u se d o n p ro t o t ype sw hi c h a re c u r re n t l y be i ng t e s t e d fo r func t i ona l i t y a ndpe r fo rma nc e .4 1 O p t i c a l

    Thi c k a e roge l f i l ms ( - - 0 .2 mm) ha ve be e n fa b r i -ca ted for use as covers l ips on sola r ce l ls . In th i s case ,t he a e roge l f i l ms a re fo rm e d , t he n t r a ns fe r re d t o t heso l a r c e ll su r fa c e a nd bo nde d t o i t , i n s te a d o f be i ngfo rme d d i re c t l y on i t . By re p l a c i ng t he h i gh i nde xg l a s s c ove rs l i p s no rma l l y use d , t he l ow re f ra c t i vei nde x a e roge l c a use s l e s s r e f l e c ti ve F re sne l l o s s fo ri nc i de n t li gh t a t i t s ou t e r su r fa c e , a l low i ng more l i gh tto reach the ac t ive surface , thus inc reas ing the sola rce l l e ff ic iency.

    T h i n a e r o g e l f i lm s ( = 30 ~ m ) h a v e b e e n d e -

    pos i t e d a s a c l a dd i ng on t he ou t s i de o f g l a s s l a se rpump tubes . In th i s appl ica t ion, the ae roge l ac t s as are f ra c t i ve i nde x ma t c h be t w e e n t he pump t ube g l a s sa nd a i r , t o mi n i mi z e re f l e c t i on o f i n t e rna l l y ge ne r -a ted l ight a t the oute r enve lop of the tube , thusi nc re a s ing t he a m oun t o f e xc i t a ti on l i gh t r e a c h i ng t helas ing mediu m. Th e ae roge l is pre fe rred to ant i re f lec -t ion coa t ings in th i s appl ica t ion because i t provides abe t t e r ma t c h fo r a b roa de r ba nd o f l i gh t w a ve s .4 2 Th e r m a l

    Thi n a e roge l f i lms ( - - - 25 Ixm) ha ve b e e n fo rm e don g l a s s subs t ra t e s fo r u se i n ' c oo l ' i n f ra re d ( IR)de tec tors . Here , the ae roge l f i lm se rves as a the rmalbarr ie r to shie ld the IR de tec tor e lements f rom thehea t radia ted by the subs t ra te mate r ia l s .

    M a ny t h i c k she e ts ( ~ 1 mm ) o f a e roge l a re c oa t e dw i t h t h i n l a ye r s o f me t a l , t he n l a m i na t e d t oge the r t oform a super- insula t ing the rmal hea t sh ie ld b lock.4 3 A c o u s t i c

    Thi c k a e roge l f i l ms ( -~ 0 .5 m m) a re fo rme d ont he su r fa c e o f c e ra m i c t r a nsduc e rs t o s e rve a s a c ous -t i c i mpe da nc e ma t c h i ng l a ye r s . S e nd i ng a nd re c e i v -

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    52 L. W. Hrubesh, J.F. Poco /Jo urn al of Non-Crystalline Solids 188 1995) 46-53i n g a i r b o r n e a c o u s t i c w a v e s i s s u b s t a n t i a l l y i n c r e a s e db y u s i n g a e r o g e l s i n t h i s a p p l i c a t i o n .

    4 . 4 . E l e c t r o n i cT h i n a e r o g e l f i l m s ( = 2 pL m ) a r e f o r m e d o n

    s i l i c o n w a f e r s t o p r o v i d e a l o w d i e l e c t r i c c o n s t a n ts u b s t r a t e i n i n t e g r a t e d c i r c u i t s . T h e d i e l e c t r i c c o n -s t a n t f o r a l l a e r o g e l s i s l e s s t h a n 2 . 0 a n d i t s v a l u ed e p e n d s o n t h e p o r o s i t y , a c o n t r o l l a b l e p a r a m e t e r f o ra e r o g e l f i l m s . D i e l e c t r i c c o n s t a n t s o f th e a e r o g e l sw e l l b e l o w 2 . 0 w i l l e n a b l e s i g n i f i c a n t i m p r o v e m e n t si n t h e s p e e d o f i n t e g r a t e d c i r c u i t s .

    S e v e r a l t h i c k o r g a n i c a e r o g e l f i l m s ( - ~ 0 .5 m m )a r e f o r m e d b y c a p i l l a r y f i ll , t h e n p y r o l y z e d t o c a r b o na e r o g e l f i l m s f o r u s e i n a e r o c a p a c i t o r s . T h e s e d e -v i c e s c a n p r o v i d e s p e c i f i c c a p a c i t a n c e s i n e x c e s s o f4 5 F a r a d s p e r g r a m o f m a t e r i a l [ 1 7 ] .

    5 C o n c l u s i o n s

    W e h a v e s h o w n t h a t i t i s p o s s i b l e a n d p r a c t i c a l t of o r m h i g h l y p o r o u s , t r u e a e r o g e l f i l m s o n s u b s t r a t e su s i n g a v a r i e t y o f c o m m o n d e p o s i t i o n m e t h o d s , i fc a r e i s t a k e n t o s l o w e v a p o r a t i o n o f s o l v e n t s d u r i n gg e l a t i o n , a n d i f s u p e r c r i t i c a l d r y i n g i s d o n e t o p r e -s e r v e t h e i r t e n u o u s s t r u c t u r e . T h e a e r o g e l f i l m s a r eg o o d q u a l i t y , b o n d w e l l t o t h e s u b s t r a t e s a n d a r es t r o n g e n o u g h t o s u r v i v e o t h e r p r o c e s s i n g s t e p s u s e dt o p r e p a r e t h e m f o r s p e c i f i c a p p l i c a t i o n s . M o s t i m -p o r t a n tl y , t h e a e r o g e l f i l m s e x h i b i t m a n y o f th ee x c e p t i o n a l p r o p e r t i e s w h i c h a r e c h a r a c te r i s ti c o f t h eb u l k m a t e r i a l a n d t h e r e f o r e t h e y c a n b e c o n s i d e r e df o r u s e i n a v a r i e t y o f o p ti c a l , a c o u s t i c a l , t h e r m a l ,a n d e l e c t r o n i c a p p l i c a t i o n s . W e a n t i c i p a t e t h a t , w i t hf u r t h e r d e v e l o p m e n t , a e r o g e l f i l m s a n d s h e e t s w i l l b eu s e d i n s u c h a p p l i c a t i o n s i n t h e n e a r f u t u r e .

    4 . 5. O t h e r a p p l i c a t i o n s Referen cesO t h e r a p p l i c a t i o n s i n t h e m i c r o e l e c t r o n i c s a n d

    e l e c t r o - o p t i c s i n d u s t r i e s i n c l u d e h i g h - s p e e d e l e c -t r o n i c c o n d u c t o r s f o r b o t h u l t r a l a r g e s c a l e i n t e g r a t e dc i r c u i t s a n d f o r i n t e r c o n n e c t i o n s b e t w e e n c o m p u t e rc h i p s .A p p l i c a t i o n s f o r t h e t h i n f i l m d i e l e c t r i c s i n c l u d e :m i c r o w a v e s t r i p l i n e s ; m i c r o w a v e c i r c u i t s s u c h a su s e d i n r a d a r s a n d c o m m u n i c a t i o n s ; l o w c a p a c i t a n c ec h i p c o n n e c t o r s ; e x t r e m e l y l i g h t w e i g h t e l e c t r o n i cp a c k a g e s ; p o w e r t r a n s m i s s i o n h i g h v o l t a g e i n s u l a -t o r s ; s p a c e r s f o r e l e c t r o d e s i n v a c u u m t u b e s .

    S o m e o t h e r a p p l i c a t i o n s o f t h i n a e r o g e l f i l m si n c l u d e : m e m b r a n e s f o r g a s s e p a r a t i o n s ; f l a t p a n e ld i s p l a y s ; w i n d o w g l a z i n g ; e l e c t r i c a l s t a n d o f f s ; l i g h tb u l b s f o r d i f f u s i n g l i g h t ; p a s s i v a t i o n l a y e r f o r o x i d a -t i v e m e t a l s ; i n s u la t i v e l a y e r b e t w e e n s t a c k s o f m e t a l so r p o l y m e r s ; a n t i - r e f l e c t i v e f i l m s ; c l a d d i n g o n o p t i -c a l f i b e r s ; o p t i c a l , c h e m i c a l s e n s o r s .

    T h i s i s o n l y a p a r t i a l l i s t i n g o f p o t e n t i a l a p p l i c a -t i o n s f o r a e r o g e l f i l m s . H i g h p o r o s i t y f i l m s a p p e a r t oh a v e a s m a n y a p p l i c a t i o n s a s t h e b u l k a e r o g e l s d o .H o w e v e r i n b o t h c a s e s , c o n t i n u e d d e v e l o p m e n t i sn e e d e d t o r e f i n e t h e p r o c e s s e s , t o r e d u c e p r o c e s s i n gc o s t s , a n d t o d e m o n s t r a t e t h e p r a c t i c a l u s e s o f t h et e c h n o l o g y . I n t he c a s e o f t h i n a e r o g e l f i l m s , t h ee f f o r t h a s j u s t b e g u n .

    [1] J. Frick e, ed., Aerogels, Sprin ger Proceedings in Physics,Vol. 6 (Springer, Heidelberg, 1986).[2] R. Vacher, J. Phalippou, J. P elous and T. Woignier, ed.,Proc. 2nd Int . Syrup. on Aerogels, Rev . Phys. Appl . 24, C4(1989).[3] J. Fricke, ed., Aerogels 3, Proc. 3rd Int. Sym p. on A erogelsJ. N on-Cryst. Solids 145 (1992).[4] D .R. Ulrich, J. Non-Cryst. Solids 100 (1988) 174.[5] L.C . Klein, ed., Sol -gel Technology for Thin Film s, Fibers,Preforms, Electronics and Specialty Shap es (Noyes, ParkRidge, NJ, 1988).[6] C.J. Brinker and G.W . Scherer, Sol -Ge l Science (AcademicPress, New Y ork, 1990).[7] H. Dislich, in: Sol-gel Technology for Thin Films, Fibers,Preforms, Electronics and Specialty Shapes, ed. L .C. Klein(Noyes, Park Ridge, NJ, 1988) p. 50.[8] R.B . Pettit, C.S. Ashby, S.T. R eed and C.J. Brinker, in:Sol-g el T echnology for Th in Films, Fibers, Preforms, Elec-tronics and Specialty Shapes, ed. L.C. Klein (Noyes, ParkRidge, NJ, 1988) p. 80.[9] C.J. Brinker, A.J. Hurd, P.R. Shrunk, G.C. Frye and C.S.Ashley, J. Non-Cryst. Solids 147 (1992) 424.[10] C.J. Brinker, A.J. Hurd and K.J. Ward, in: UltrastruetureProcessing of Advanced Ceramics, ed. J.D . Mackenzie andD.R. Ulrich (Wiley, New Y ork, 1988) p. 223.[11] H. Schm idt, G. Rinn , R. N as s and D. Spo rn, in: BetterCeramics Through Chem istry III , ed. C.J. Brinker, D.E.Clark and D.R. Ulrich (Materials Research Society, Pitts-burgh, PA, 1988) p. 743.[12] H. Hirashima and K. Sudoh, J. Non-Cryst. Solids 145 (1992)51.

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