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e V C H V e r l a g s g e s e l l s c h a f t m b H , W e i n h e i m / B e r g s t r . 1 9 9 4

R e g i s t e r e d n a m e s , t r a d e m a r k s , e t c . u s e d i n t h i s j o u r n a l , e v e nw i t h o u t s o e c i f i c . i n d i c a t i o n s t h e r e o f . a r e n o t t o b e c o n s i d e r e du n p r o t e c t e d b y l a w . P r i n t e d i n t h e F e d e r a l R e p u b l i c o f G e r m a n y

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Research News

Microfabrication by Microcontact Printingof SeH-Assembled Monolavers **

By James L. Wilbur, Amit Kumar, Enoch Kim,George M. Whitesides *

l . Introduction

Microcontact prir-rt i r-rg (pCP) is a ncw tcchniqr-rc for lornt-

ing pat terns wi th prm d imensions. t t : l I t o f f -ers exper imenta l

s i rnp l ic i ty and f lex ib i l i ty in formin_s cer ta in types of pat terns.

I t re l ies on the rcrnarkable ab i l i ty o f se l f -assemblcd mcrnolar -

ers (SAMs) o f long-chain a lkaneth io la tes on go ld and othermetals to act as ni lnometer resists b1'protcct ing the support ingmetal l iom corrosior-r by appropriately formulated etchants.

Pat terns o f SAMs hav in-s d imensions that can be less than

1 pun are formed by us ing the a lkancth io l as an " ink" . and bypr in t in-u them on thc meta l suppor t us ing an e lastomer ic"s tamp". The s tamp is fabr icated by mold ing a s i l i conc c las-

torrer r"rsing a rnuster preparcd by optical or X-ray microl i tho-graphl ' or by' othcr tcchr-r iqucs.

Microcor . r tac t pr in t in-u o f pat terned SAMs br ings to mi -

crofttbrication a nurnbe r of new capabil i t ies. First. ptCP n-rakcs

it possible to f trrrn pattcrns that arc dist inguishcd only b1'thcir

consti tuent l i rnct i t-n.ral eroups: this capabil i ty ' pennits the con-

trol of sr,rrf i rce pfopert ies such as intcrt ircial f l 'ee energies rvith

-ereat precisi cln. t r l scconcl. becu use r.r-r ic roctr r.r t uct p rr n t i n g re-

l ies on molccr t lar se l l ' -usscnrb l r . i t gcncr i , r tcs r l system that is(a t least loca l l l ) c losc to a thernroc lvn; . rnr ic r l in i rnLr rn anc i isint r i nsical l ,v clef 'ect- rej ect i n g a ncl scl f--hcal i n g. Si mple procc-

dures. w i th min imal protect ion against sur t i rce contan l in l . r -

t ion by adsorbec l matcr ia ls or by par t ic les . can leud to sL l r -prisingly low lcvels of clct-ects in the f inal strr"rctures.t l r l Thcprocedurc can be conducted at i l tmospheric pressurc. in an

unprotccted laboratory ' a t rnosphere. Thus. ; rCP is espec ia l l luscful in laboratories that do not have routinc i . lccess to theecluipment normallv usecl in microftrbrication. or for which

thc cap i ta l cost o f equrpnrr -n t is u ser ious concern. Th i rd .

the patterned SAMs can be dcsi-ened to arct ars resists

wi t l - r a number o f wet -chemica l e tchants . t2 l Work in-s wi th

l iquicl ctchants sul-fers from thc disadvantagcs of handling

Research Ncu s

600

l ' } ro l ' . G. \ ' { . Whi tes ides . Dr . . l . L . Wi lbur . Dr . A . Kunrar . [ . K i rnI )epa11nrcn t o1- Chcrn is t r r ' . Ha lnard Un ivers i t rl l Or l i r rd S t lcc t . Cambr idge. NIA 0 l l lE (US ' \ )-I ' l ' r is

rescurch $as sLlpported in part by thc Adrurrcecl Rcsearch Pro.;ectsAgencr . . lL \ \ ' , ! rn r te lu l l " acknou lcdges a pos tdoc toru l f ' e l lowsh ip f l ' on t thcN a t i o n a l I n s t i t u t c s o l ' H e a l t h ( G r a n t N u r n b c r l - t : . l l ( ; M 1 6 5 1 1 - 0 1 ) . \ \ ' ethank AT & T lb r c lonat ing the muster usec l in f abr ica t ing the PDMS s tumpusec l to genera tc thc pa t tc rns shoun in Frgure 2 . \Ve a lso thank HansBiehLr lck lb r he lp l i t l c l i scuss ions and suqses t rons .

st r l r cnt . l rn t l r l i :1 - r t rs r r tg o t - \ \ l rs tcs . but l t l sc l e l t . j t t r s suhst l tn t i l r l

l r t l r ant r . rgc : : l r h igh t lcgt 'cc t r t ' c t tn t r t r l ( ) \ er con lant tn l l t t r t t t , l

sur l r . tcc : : r 'c t l L tccr l r l l t t l r . tgc to thc s t tbs tn t tc - l l 'on t cncrsct te

i n t c rac t i ons r r i t h a l ( ) l l \ o r i t ) n : : t hc ub i l i t r t o nL r r t i pL r l . r l . '

con rp l c r l u r r l \ cns i t i r c o r s l r n i c l unc t i onu l i t r . Bccuu .c t h :

SAMs u rc ( ) n l \ I . l nn r t h i ck . t hc r c i s l i t t l c l oss i n i . i - r . '

de f l n i t i on c l L r c t o t hc t h i ckncss o f t he r cs i s t : t hc n r . r . j o r t i i l . r -

minants o t ' edge rcso lu t i r ' rn scer-n to be the l ldc l i t r t r1 : r r - '

contact pr i r " r t inu anc l thc l in isot rop l o l 'e tch ing thc t rn t i . ' r . -

in -s mcta l . In the cur rent best cases. f -eatures o t ' : izc ( r l . . : r

can be f l rbr icated: t2 l ed-uc rcso lu t ion in sr s tcnrs . ,h , ru in j . : . '

reso lu t ion in feature s ize is less than , i0 t r tn .

A l thor - rgh manv other r -ncthods l i r r thc p i r t tc In i l r - l

S A M s c r i s t l i r r e x i r m p l c . m i c r o r n t t c l t i r t i r t s . l ' i t ' r r . r r r r r . . .

t ron beam l i thographies. ts l micror i r i t ing. [ " - ]

; . in . i : i l

t o l i t hog raph l ox ida t i ve pa t t e rn in -e u i t h UV l r uh r '

p tCP seen-rs to har c thc greatest oppor tun i t r l i r r i r r . r r i i : . : : . , ,

anc l b rouc i upp l i cu t i on . t r l Th i s a r t i c l e l i r cuses on t h . I . r . . .c lurcs l i r r prCP unt l . in par t icu lar . descr ibcs u1 ' r1 ' r11. . ' ' , , " .

a inrcc l tou l i rc l thc tabr icat ion o f rneta l l ic rn ic rost rue t : r r ' : .

2. Discussion

Scl t -assenrb lcd monolayers (SAMs) o f organic cot .n I r , rLr r r . : -

or . r inorganic or meta l sur faces are becoming incrc l r ' rn , : , ' ,

in - t i - lo r t l t t - t t in manv areas of matcr ia ls sc ience. t l I r r lA l th , ' t t l i :

thcrc urc many d i f terent systems of SAMs basec l on d i l ' l c rcnt

orgunic components and supports. the best developecl s\ 5tcnr\

l t r c t h t l s cc l f a l kane th io l a tes .HS(CH2) , ,R .c . l t t go1d f i l r r l s . l r t

T ip ica l l l ' . a go ld f ihn 5 1000 nm th ick is s l rppor tcd ( )n . i

t i tan i r - r rn-pr imed Si ,S iO, waf -er or g lass sheet . The t i t rLnrLrnr

scrvcs as an adhes ion promoter between go ld and thc .111-r -

por t . Thc a lkaneth io ls chemisorb on the go ld sur lacc l l 'onr r r

so lu t ion in which the go ld f i lm is immcrscd. i tnd f i r rnr l r l -

sorbec l a lkaneth io la tes wi th loss o f hydrogen. Ac lsor l . r l i t rn

can alsc'r occul" from the l 'apor.t1' l SAMs formeci on solr l

t }om lor " rg-chain a lkaneth io la tes o f s t ructure X(C' t I . t , , \ ' -(CHr) , , ,S are h igh ly 'ordered[18 ' 1e] and can be cons ic lc rcc l as

crysta l l ine or quas i -crys ta l l ine molecu lar ar ra) ,s . . , \ u ic le

variet l ' of organrc functional groups (X.Y) can bc incorpc-r-

ra ted in to the sur face or in tenor o f the monolaver ' . SAMs

09-15-9618 91 0707-0600 5 - i .1i l / + . l i /) . !dt ' . . l lutcr. 1991. 6. No. 7 8

Research Ner,r's .{DVANCED

S.A\ ls cun thcrc t i r rc bc ta i lo red to prov ide a widc var ie ty o fI t t r t tc l ' i l t l 1 ' r1 '1r1 ' r r '1 ' l lg . ' : g e t t lh i l i ty and protcc t ion uguinst cor -ros ion hr chcnr ica l e tchants are espec ia l ly re levant to pCP.

Ficurc I out l incs the procedure used for prCP. An e las-tonrcric st i .rnrp is r-rsed to transfer alkanethiol " ink" to a gold

sLrr tLrcc br contact ; i f the s tamp is pat ternec l . a pat terned

S.' \ \ l lbnns. The stamp is labricated by casting polydimcthyl-

s i lo runc (PDMS) on a master hav ing the des i red pat tern.

\ lusters are prepared using standard photol i thographic tcch-n iques. or const ructed f rom ex is t ing mater ia ls hav ing mi-croscalc surfbcc feartures.

r7r7>r7r?2r71zl <- P h o t o re s i s t

l s i lI ef,otolitt ography is used

I to create a master

Y<-- Photoresisl partern

I Si I t l -2 sm th icknesst

II PDI\IS is poured overI master and cured

VI PDMS I

<- Photores is t pat tern

l s i III PDNIS is peeled awav

I from the mastcr

I PDMS I-r-t-t-j-t-tr

I envrs is exposed to aI solution containingV HS(CH2) rsCHr

-

I nDMS I_1--5LFL-F <- alkanethiol

_- Au (5 - 2000 nm)

-... Ti ( 5- l0 nm)

Metal not protected by SAM isremoved by exposure toselective chemical etchant

F ig . L Schc 'n ra l rc o1- the procedure tb r n t i c r t t con tuc t p r in t ing ( f rcP) o l - p l t -t c rned se l l ' -asscn- rb lec l n rono lave ls (SAMs) o f a lkaneth io la tes on go ld sur l 'aces .Photo l i thogruphr i s one o f ' severa l tec l rn iques tha t can be usec l to labr ica te umaster . , r ' i th thc c les i red pa t te rn . The s tamp is f abr ica tcc i b r cas t ing po l l -d i rne thy ' l s i lo ranc tP I ) \4S) on the master . A f ie r cur ing . the t ) l )MS is pcc ledl iom the mastc r anc l " inked" i v i th a lkanc th io l The s tanrp is app l i cd to thesurface o1'a golcl l i lnt {sLtl .rportecl on a si l icon wal-er that is prrrned n,i th anludhes ion pronro tc r . l i r l e ra rnp lc . t i tan iun t ) . anc l a lkaneth i t t l t rans le rs andl irrms a pattcrned SA\l . I . iquicl etchants rerlove ntctals not protectecl bv thel l kaneth io la te la1 ,er ' .

In a typical experimental procedure. a photol i thographical-

ly ' produced master is placed in a glass or plast ic Petr i dish.and a 10 : 1 rzr . t io (w: w or v : v ) mix ture o f SYLGARD s i l iconeelastomer 184 and SYLGARD s i l icone e lastomer 184 cur ins

/UATERIATS

a-qent (Dor i ( 'o rn ing C 'orporat ion) is poured over i t . The

elastomcr is a l loucc l to s i t fbr approx imate ly 30 minutcs a t

room tempcnr turc anc l prcssure to de-uas. then cured f or I 2

hours a t 60 C ' . anc l gcnt l \ pcc lec l f rom thc master . " lnk ing"

of the c lastonrer ic s t l rnrp is accompl ished by expos ins thc

s tamp to a 0 .1 t o 1 .0n rN I so lu t i on o f ' a l kane th io l i n i , l nh ) ' -

drous et l iano l . e i thcr b1 pour ing the so lu t i r - rn over the sur -

face of the s tamp. or b) nrbb ing the s tamp gent ly wi th a

Q-t ip that has been satunr tec l u i th the ink ing so lLr t ion. The

stamp is al lo'nl 'ecl to clry' unti l no l icluicl is visible by eye on thc

sur face of thc s tamp ( t1- 'p ica l ly about 60 seconds) . e i ther

L lnder ar rb icnt condi t ions. or bv c \pr )s l l rc to a s t ream of

n i t rogcn gas. Fo l lowin-s ink in-q. 1hc s tamp is lpp l icd ( typ ica l -

ly 'by 'hanc i ) to a go ld s l l r f t rce. Vcry l ight hunc l prcss l l rc is Lrsed

to aid in completc contact betu,een the stan-rp ancl the sLlr-

face. The stanrp is then gcntly pceled l l 'onr the surluce. Fol-

lowin-s removal cl l- thc stamp. thc pattcrnecl golcl sLlr l i lce cirn

be subjectcc l to chcmica l c tchants (sec be lou ' ) that se lec t i rc l r , '

remove unclcrivatizcd arcas ol- the gold sr-rrface. and i1-clc-

s i red. the r - rnc lcr ly ing suppor t (s) . A l tc rnat ivc ly . fur thcr c lc r i r -

i t i za t ion o f unstumped areas can bc accompl ishec ' l . e i thcr br

us ing a scct - rnc l s tamp. or b1 'washin-s the cnt i rc sur lacc u i th

a d i f f e ren t a l kane th io l .

The e lastomer ic churacter o f the s tamp is csscnt ia l to thc

success of the process. Polyd i rnethy ls i loxane (PDMS). u 'hcn

cured. is sufT ic ient ly e lastomer ic to a l low good cont t rnra l

contact of. the stamp ancl the surf ace . even fbr surfaccs u' i th

si-unif icant rel ief. this contlrct is essential fbr ef f icicnt c()nt i lct

t ranst -er o f t l ie a lkancth io l " ink" to the go ld f i l rn . Thc c las-

tomer ic proper t ies o1 ' PDMS are a lso impor tant rvhcn t l rc

s tamp is remove c l f rom the master : i f the s tamp wcrc r ig ic l ( ls

is the master ) i t w 'ou ld be d i f f lcu l t to separate the s tamp anc l

master a f ic r cur ing wi thout darnaging one of the tu 'o sub-

s t ra tes. PDMS is a lso suf f ic ient ly r i -e id to re ta in i ts sha; rc .

even for tcaturcs with sub-micron dirnensions: we havc sr.rc-

cessful ly gcncratccl patterns with l incs as small as 100 nnr inwidth . t r l The sur tace of PDMS has a low in ter l lc ia l f l 'ee

ene rgy t20 l ( " ' : 22 .1 dvnescm r l and t he s ta rnp c l oes no t ac l -here to thc go ld f i lm. t ' lThe s tamp is durab le : we havc uscc l

the same stamp Llp to 100 t imes over a period of sevcnrl

months wi thout s ien i t lcant degradat ic ' rn in per fbrmancc. Thcpolymer ic n l r ture o f PDMS a lso p lays a cr i t ica l ro le in theink ing proccc lurc . by enabl ing the s tamp to absorb the a lka-neth io l ink br su 'e l l ing.

Microcontact pr in t ing on -eo ld sur laces cun be conductedwi th a var ie t r o f a lkaneth io l " inks" . A lkaneth io ls that dcrnot undergo rcact ive spreadin-ut2r r4 l (a f ier app l icat ion to

the golcl l l lnr) are required for fbrmation of small l -eatures

wi th h igh rcso lu t ion. For s tampins in ar i r . we use autophobic

a lkancth ic- r ls such as heradecancth io l . Microcontact pr in t ing

of o thcr non-aL l tophobic a lkaneth io ls . for cxample.HS(CH2)r .COOH. can bc conc ' luc ted by s tamping unc ler al iqu id such as water . t25 l

Pat terned SAMs of a lkaneth io ls on go lc l prov idc cxce l lentresist character wrth a number of wet-chcmical etchanlq.Ir ' 2' r ' ]

F i gu res I and J shon ' r ep resen t l r t i ve ex lmp l cs o f l ne tu l m i -

r L'C'II Varluss.qe.sL'll.rthu/t rnhH. D-69469 lft'inltt'int, 1991 ()9-15-9618 91 07()7-060 1S -t.//i1l+ -r-t 1/ 6 0 1

.4DVANCED Research Ncws

,YIATERIALS

+

2.5 pmH

I : - r I I i e . t r ' r ' r r n r i e l o - u n t p l . t : o f c o r - n p l e x p a t -

l ! r 1 r . r n r r r l r l n r e k e l \ u r l ' i r c c s I i r r n r e d b l a c o m b r -

n . L i r r , n r r l n r e f r ) e r ) n t | r c t p r i n t i n u ( f r C P ) a n c l r i c t -

. h . ' r ' r r . . L l l t u l t i n g . r l ) . . \ t c s t p a t t e r n o 1 - t h c t r p cu .J ( i n r l r i c r ( )c lcc t r t rn ics app l i ca t rons : b l ' ) Sub-|c -L l ( rn \ ( ) l { l ) u t h igher n ragn i l i c r t t ions . I ) r r Ik u r ' -d i r \ r rn rc \cn t rcg ions w 'here unc lc r i r l r t i zc t l go lc l

{ r rn t l unc lc r ly ing n icke l ) was renr ( ) \ c ( l by thechern ic l r l e tc l r . ( 'on t ras t be t rveen nr ic ron-sca leI t i l tu rcs i s c lear l l e r , i c ' len t . Sec tc r t l i r r e rper i -

n rcn tu l de ta i l s .

crostnlcturcs that can bc fabricated usins uCP fol lor,r, 'ed bvchen"rical ctching.

F igure 2 shows an etched test pat tern. The PDMS stamp

Llscd to create the pattern was cast fiom a commercial master.

Hexadecaneth io l was pr in ted on a go ld sur face ( - 1000 At .

which was supported on a si l icon wat'er with nickel 1- 250 A;as an adhesion promoter. The substrate was submcrgcdin a soh - r t i on o f KCN (0 .1 rnM to 1 .0 M)and KOH (1 M1 . t : r ' lThe so lu t ion was s t i r red. and oxy-qen wrrs cont inut - rus lybubbled thror-rgh the solut ion during etchin-e. Undcri 'n 'at izedareas of the gold surtace were removed in approximately15 minutes. expos ing the under ly ing n icke l . N icke l wasremoved by submerg ing the subst ra te in a mix ture o f HrSO*,

3 0 ' % H , O . . t t . , l ' O * . a n d 3 0 % N i S O I ( 5 : 5 : l : - l r c s p e c t r v e -

ly ' ) . tz - I

F igLrrc i s i rows e lect ron micrographqt :s1 o l ' pat terned

SAMs l irrnrcd by prCP (a c), and electron nricrographs of

the corresponding si l icon microstructures labricutcd b1' chem-

ica l c tch ing of the go ld and s i l icon layers (d l ' ) . In F igure 3a.

thc grid pattern was produced using a PDMS stunrp consist ing

of para l lc l l ines. The s tamp was appl ied tu ' icc . u i th the sec-

onci appl icat ion oriented at 90 relat ir , 'e to thc t irst. Fig-

ure 3b shows a f iac ture prof i le (a t h i -uhcr mugni l icu t ion) o f

a samplc made with the same stamp and procedllre as in

Fi-sure 3a. In Fieure 3c. thc pattern was crci. l tcd b1' appl ica-

t ion o f a s ing lc PDMS stamp. In a l l cases. hcrac lccancth io l

602 t ' I ' ( ' l l I i ' r lug.sge.sal l . t thult nrl tH, D-69469 Ll ' t i t r l tL' inr. 1 994

Rc:c l t r ch \ cu s ADVANCED

f)

vn'as transf 'erred by pCP to a _eold surfacc that was sl lpportc(i( ) n i . t t c \ t gn rdc . / l 00 . s i l i eon w l f ' e r . App ro r i r n l r t e ' l r l 0 A t r l 't i taniunr lvas used rrs i . ln aclhesion promotcr. Figurcs 3cl Ishou s i l icon microst ructures fabr icated by cher- r - r ica l c lch ingof thc surl i iccs not protected by thc patternecl SAMs shou,rrin Figurcs 3u c. Unclcrivatized _eold was removed as c' lc-scribed pre'r ioLrslr. The t i tar"r iunt layer wits removcd by wash-ing wi th d i lLr tc HF. Anisot rop ic c tch ing of the s i l i con wafer .in areas crposecl br thc rcr-t-toval o1'thc -qold and t i taniun-rlayers. was Llccolnpl ishecl br strbrner-ein-e the substrnte in ast i r red so lu t ion o1- KOH ( -+ M ) in isopropunol (1 ,5 , ,1 , by r , 'o l -ume) a t 60 C lor uppror inra tc l r 30 t r - r inutcs . t2" l Af ter re-moval o f s i l i con by ' e tch ing. thc rc t ra in ing go lc l layer . whichserved to protect the unc ier ly in-u s i l i con ( in the pat ternedregior-rs) from thc chemical etch. was dissolved in aqua re-gi l( 1 : 1 HNO, ,HCI ) . Sma l l va r i a t i ons i n e t ch ing t imes r csu l t edin s l i -eht ly d i f ferent featurcs. Each of the images shown inFi-eures 3 is from a dif ferent sample. Al l SEM irnages inFignre 3 were obtained with the sample t i l ted by approxi-

/UATERIALS

l j i g . -3 . E lec t ron nr ic rog laphs o l ' pa t te rnec lS . \ \ {s l i r rn rec l bv r r rc locontuc l p r in t ing ( l c )l rnd . c lcc t ron nr ic lographs o1 's i l rcon n t i c ros l ruc-t u r c s r c s u l t i n u l r o n r c h e n r i c u l c t c h i n g o l ' t h c ' s ep l t te rnc( l SA\ ' l s (d l ' ) . r \n iso t lop ic e tch ing o1 'thc s i l i c r rn ,1 l (X) ) ua f c r u 'us accor .np l i shcd h1subnrcr -g ing the s r . rL rs t la tc in a s t i r red so lu t ion o1 'K O I I ( l 6 N ' { ) i n i s o p r o p a n o l ( 1 5 ' l u b r r o l -r .mc) . u t ( r0 C ' . f i r r uppror in ra te l r ' 30 nr inu tcs .T h e t r t a n i u n r l a v c r u l s r e n r o r e d b y n a s h i n uwi th d i lu te HI - ' . The l inu l shapcs o1 ' the lesu l t ings i l i con nr ic los t rL lc t r . l r cs re l lec t bo th thc o r is in l r l

n i l l l c r l l ( ! c l l t ' t i l t f t l h \ I l l l L l ( r e ( r n t i r L l l l f i n l i n . lanr l thc i ln iso t fop \ o l ' the s i l i con e tch . Snr r l lr u r - iu t ions in e tch ing t in re s rcsu l tec l in s l ig l t t l lr l i l ' f c rcn t l cu tu rcs . I : l r ch o l - the in raqcs shou n inF igure 3 i s l l ' on ' r a d i f l t rcn t sun ip lc . A l l SE\1i n r a g e s i n F i g r : r c . l w e r e o b t l i n e d * i t l i t h esar rp le l i l te r l b \ appror imate l r 70 to in rpror .ernag ing o f - thc sur tucc rc l ie f 'o l ' the sarnp le . Thct i l t o1 ' thc sunrp le c l i s to r ts the in rage: thc rcc t i ln -gu lu r -pu t te r - r ts in F igures - la .b .c . unc i I 'uor , r ld ap-pc i r l l s s r luurcs i1 ' the sarnp le $ 'e le n ( ) t t i l tec l . Sectc r t l i r r l c lc l r t ion l l de ta i l s .

rratel), ' 70 l l 'on-r horizontul to implo'n e

t l ie sL l r lacc rc l ic f o l ' the sample. Thesample c l is tor ts thc in l rgc : thc rectangLl lar

Figr,rres 3a.b.cl. uncl c uoLrlcl i rppear as sqLl i tres

\\ 'crc l tot t i l tecl.

The pat terns unc l micros l ructurcs shown in F i -uurcs 2 and3 arc rcpresentati" 'c cr lnt l-r lcs of the fcatr-rres that can befabr icated us ing pr ( 'P. Complex pat terns and shapes. wi thdimcnsions rangir- lg t l 'on-t < 1 pm to scveri l l hundred pm arewel l rcso lvcd. l -catur res o f th is s ize (and cornp lex i ty ) suggestthat thc prCP tccl inique may'be uscfr-r l for the f irbrication ofmicrostnlcturcs that i i rc normll l11, ' labricatcd usin-s electron-bcam l i thouraphl , ' anc l opt ica l l i thography.

3. Conclusions and Summarv

Microcontact pr in t ing L ls ing an e lastomer ic s tamp is themost versat i lc method ava i lab lc for prepur in*c pat terned

pat tcrns ln

i f the samplc

| l u t t : r . | 9 9 4 . f i . N o ' 7 8 t | ' C ' I I I . ' t ' r l l . q ' s ' g t : ' s c | / ' t c | u t l ' t t t t h I ! ' D . 6 9 1 6 9 603

4 -4 - : t ' ' - I

,4DVANCED Rcscarch \ c r i .

lUATERIALS

SAMs of a lkaneth io la tes on go ld . The capac i ty to for rn pat -

terned SAMs on -so ld . co l rp led wi th thc ab i i i ty o f SAMs tcr

act as 1 2 nm thick rcsists lor chetnical etches. provicles thc

f oundat ion lor a neu ' tcchnique fbr micro lhbr icat ion. Micro-

c ( )n tuc t p r i n t i ng h i . r \ I n i r r r y usc l ' u l ch l r n r c te r i s t i c s : i t i s cxpc r i -

rnenta l ly ' s implc . and can be conductcd in a convent iona l

chcmica l labc l ra torv no t 'out ine acccss to c lcan rooms or

p l - ro to l i thographic equipment is re 'qu i rcd (a l thou-eh a micro-

fabr icat ion tcchnique typ ica l ly photo l i thography is re-

quired to rnake thc master): the technique rel ies on self-asscm-

b11,. and thus tends to minin'r izc c' lcl ' :cts: with care. i t is capable

of producing sub-micron fcaturcs^ and clf routinely fbrm-

ing micron-sca le f eatures wi th h igh f ide l i ty : thc e lastomet ' ic

stamps urc cusi ly prepared und cun be used hundreds of t i rnes(whilc stored under ambient lab condit ions) without de-eracla-

t ior.r i r .r perlormancc. At this early stage of development.

the ch ie f weakness of the rnethod. re la t ivc to o ther ver . " 'h igh-

ly developed microl i thographic methocls such as photol i thog-

raphy. is the relat ivcly' high dcnsity of defects in thc

f inal structure. The de-sree of pcrfcct ion thzit can bc achicvccl

by prCP. al ier careful dcvelopment. remains to be cstab-

l ished.

Thc sirnpl ici ty ancl l ler ibi l i ty of this techniqr,re providcs thc

opportunity fbr r lanl ' other appl icat ions. Microcontact print-

ing has bccn usecl fbr the tabrication of microstructures by

methods othcr than chemica l c tch ing. fbr example. the for -

mati c-r 'r o f ' nr icror.r-sca le di f f iact i on grati ngs by condensation

of watcr on hrc l rophi l ic reg ions. ts l for the format ion of rn i -

croelectrodss.l ' l t t l uncl f or the patterned fbrmation of micro-

crystals ancl rr icrocrl 'stal arrays.t2l Patterned SAMs have

also been used to lbrnr patterns of adsorbed proteins and

t r t tached mamnra l ian ce l ls . t3 l lThc successes of the prCP tech-

n ique in these appl icut ions. and the success of the procedurc

fbr rn ic ro fabr icat ion in s i l i con and other matcr ia ls . ausers

wcl l for fu turc appl icat ions.

f l l A . K L r r r r r r . ( r \ l \ \ l r i t e : r r l e s . . 1 1 t 1 t 1 . p l 1 . v 1 . 1 , , 7 ,p l A . K L r r r r r r . l l \ I : | r . ' h L r r c k . G . M . \ \ ' h i t c s i d c s .

l 5 1 l

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Ncu York 1991.

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[ , ( t t 1 ' ! t 1 ] t t i ' l g 9 l . / / / 1 l 9 E

l l 8 l[ 1 e ][]olf t 1 l| 1 1 1

[]r lt r j l

t? stIr6]P7l

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