ffi- >33

CHAPTER

SELF-ASSEMBLED MONOLAYERS AND LTTHOGRAPHY*

GEORGE M. WHITESIDES, Andrew J. Black, Paul F. Nealey and James L. wilbur,

Departrner1t of Chemistry, Harvard University, Cambridge, Massachusefts 02138

Introduction

,,Small,, is a word that increasingly defines a broad set of objectives in lnodem science and technology.

Modern information tecl-rnology has been built on small structures, and the demonstration that it is technologically

practical to rnake systems of the feature size and cornplexity of those that constitute lnicroprocessors and memories

has opened the eyes of a broad range of teclurologists to the potential of small structures. Tltere are now a very Iargc

range of recognized opportunities in technology that rnight be satisfied by rnaking new types of small structures' or

by reconstituting existing structures in down-sized versions; developing practical methods of fabricating these

strucrures often remains complicated, expensive or impossible. At the sarne time' it is clear that there are many

interesting and new phenomena that occur in small strucrures. one useful sub-definition of "small"' wheu

categorizing types of structures that exhibit new phenornena is "nteso scale". Meso-scale structures are those that

have critical dirnensions that match the dirnensions of the phenomena being rnvestigated. These critical dimensions

obviously vary with the appiicati on: 2-20 nrn (the distance an electron travels in ballistic fl ight in solids at modest

temperatures, before coll iding with an atom and changing direction) for quautum dots, 200 - 2000 nm for

interactions involvrng visible and infrared light, for optical structures such as in-fiber diffraction gratings; 10 - 50

prn for surface structures used in studying the biochemistry of mamtnalian cells attached to a surface'

Chernistry has historically been concerned with molecules rather l ltan structures, and although sorne

molecules cal be quite large, most fall below the range of sizes of interest to the makers of smali, electronically or

optically functional systerns. In addition, the properties of molecules are typically not those required for systems

based on electropic conductiviry, although they rnay be very useful itt optrcal system. For a comparison of scale,

benzene is less than I nrn in its largest dimension, a typical protein has a diameter of approximately 4 nm' and a

high-molecular weight polyrner (for example, poly(methyl rncthacrylate) of 106 molecular rveight) extended fully,

might have a length of 1000 nrn or I pm. Making well-defined structures that are morc than a few mn across by

co'ventiopal syrrthetic methods that use current chemical techniqucs would be a rnajor undertaking'

Although there has not been a perfect match between molecular chemistry and the requirements of optical

a'd electro'ic device physics, the need for better methods of fabricatiorl at the lower end of the range of sizes of

interest to the devlce communily, the opportunities to build new fypes of mesostructures, the chance sirnply to build

srnall systerns to conserve space and reagents, and the need for chernistry to find new areas for exploration are now

br inging a number of f ie lds of sc ience and technology toget l rer in unfamihar and product ive cotnbinatrons'

Ul t imately, of course, a l l s t ructures arc rnade of atoms, and chemistry br ings to these act iv i t ies a specia l ized

competence in manipulat ing atorns. With in the areas of rn icroelectronic devicc fabrtcat iot l [ l which

rnicrofabricatiol 5as developed rnost actively, however, chemistry has played an important but secondary role' The

growth of sil icol single crystals, photolithography, etching and doping of setniconductor devices, and device

*Bascd on an address presented by George N{

Confercnce on Chcurical Researclt on Nanophase

I 9 9 5 .

whitcs ides beforc "The Robert A. welsh Foundat ior , 39th

Chemistrv." which was held in Houston, Texas, October 23-24'

packaging alI depend on the use of chemicals, but the intellecnral leadership in microelectronics has clearly rested in

electrical engineering and applied physics. As structures become increasingly stnall, however, they become

i'creasingly difficult ald expensive to produce by simple extensions of existing photolithographic techniques. The

opportulity to extend ideas from molecular chemistry into the production of structures larger than molecules but

srnall by comparison with those that can be routinely produced by photolithography--and especially to structures that

are fulctional i1 meso-scale science and technology--is becorning more attractivc as thc size of these structures

decreases, their complexity and variety grows, and the difficulty of fabricating them increases.

Although the opportunity to develop new, non-photolithographic methods for rnaking stnall structures is a

very attractive one in principle, in practice does chemistry have ideas that would enable it to contribute to the

revolutions in science and technology that are accompanying the investigation of meso-scale phenomena and the

construction of small devices? We believe that the answer to this question is clearly "yes", and that chemistry can

contribute to the fabrication of meso-scale structures in a surprisingly broad range of areas. Among the concepts'

strategies and structures from chemistry that seem to offer particular opportunity in Ineso-scale science and

technology are these:

Non-covalent Synthesis. The most important emphasis in organic chemistry in the last 50 years has been

developing a body of tcchniques that could be used to synthesize complex organic synthesis. This area has been

enonnously productive, but is not a key contributor to the generation of stmcrures tl-rat show useful properties as

rnaterials. or electror-ric or opticai functionalify. The new arca of "non-covalcnt syuthesis"--a strategy for building

large orgapic structures tl.rat are hcld together by weak interactior.rs--hydrogen bonds, van der Waals interactions, the

hydrophobic efl-ect, charge-charge interactiousl-- offers opporrunities lor practical routes to l]ew rypes of useful

structures and functions.

Molecttlar Self-Assenfuly and Self-Assembled Su'uctures. An important new concept in synthesis is that of

molecular self-assembly.2,3 This concept has been extracted from the shrdy of biological sy.t.-r.2-4 In cells,

enormous complexity is developed using rnolecules that asscmble themselves into functional shapes or aggregates'

Familiar examples are the formation of correctly folded, functional proteins from polymers of amino acids, the

fonnation of functional t-RNA's from unfolded ribonucleic acids, and the fonnation of the cell membrane from

phospholipids. This strategy for generating complexity through tnolecular self-assembly, if fully understood and

controlled, offers a fundamentally nerv approaclt to the design and fabrication of non-biologrcal ,y,t"t, '2-6

The key idea in sel f -assernbly is that the f inal st ructure and funct ion of the molecules or molecular

aggregates are c lose to or at thermodynamic equi l ibr ium, and that the indiv idual tnolecules wi l l reach these

structures sirnply by equilibrating to their lorvest-e nergy fomrs: that is. cxpenditure of encrgy' or active interventton

by t6e experirne'ter. is not ncccssary once the rnolecular compoucrlts of the systcr.u havc been correctly designed

ar.rd syntSes i t "d.2,3 T5e inforrnat ion that a l lows scl f -assernbl i t rg systerns of rnolecules to reach their f inal '

functional structures is encoded in tl.re molecular structures, and especially in molecular shapes'

Srnall particlcs atttl lvlicro.rtnrcturcd Solids. Chemistry l.ras bcen the source of rnatly methods of fabncating

a range ot' structures with sizes appropriate for use in thc tneso-dimcnsional world: all-lollg these are colloids,T

- i " " t t . r , 8 -10 l i q r i d - . . ys ta i l i ne s t ru . t , - , r cs ,1 l l i poso rnes . l2 fe r ro f l u id r , l 3 so l -g . l s , l 4 ' l 5 zco l i t es r6 '17 , phase-

l8 ar t i f ic ia l pept ide tubules, ig-22 bubble raf ts23 and many others. Most of these strucruresseparated polymers, '

have bcen dcveloped for othcr reasons, but there is now an attractivc opportunity to recorlsider their use ll l lncso-

scale e lectronic, b io logical and opt ical sc icucc.

Large, Elcctt.onicall l:-functiorral Molecules. An important characteristic of organic tnolecules is that they

are typicaly e lectronical ly insulat ing. This c l raractcr is t ic has l imi ted their appl icat ion in rn icroelectronic devices to

applicatio's such as insulators, adhesion prornoters and thirr-fi lm dielectrics. Iu thc last l0 years, howcver, a range

of organic electrical conductors have been developed, and it is increasingly practical to consider organic conductors

as electrically functional components of certain types of systems (especially sensors and bioelectrical systems),

Organic materials have always been attractive for the ease with which their optical properties can be

tailored, and with the recelt explosion of interest in optical systems for their contribution in optical communications,

there is a clear opportunify to buitd new types of optical systems that rely on the unique characteristics of organic

materials.

Resul ts

Self-Assenfiled Monolayers. Our work has focused on self-assembled monolayers (SAMs) of

alkanethiolates on gold apd silver.24-26 Tll"r. structures are the best developed examples of non-biological, self-

assembling systerns that are currently available. SAMs are structures in which the constituent molecules associates

with a surface and fonn a monomolecular layer (Figure I ). The lateral interactions between the molecules leads to

ordering, and ttre final structures are essentially trvo-dimensional quasi-crystals supported by the ,urfu"..27 SAMs

exhibit many of the features that are most attractive about self-asscmbled systems: small numbers of defects

(relative to rvhat could be achieved by covalent synthesis of a structure of corresponding complexiry and size); the

ability to serve as the basis for the fabricatron of structures with dimensions in the nm to pm range; ease of

synthesis and practicality in techrlological applicatrons; abil ity to control meso and macroscopic materials properties

of the system.

FIGURE, I

Schematic i l lustration of the structure at the molecular-level of a self-assembled monolayer of n-alkanethiolates on

gold. The head group X (: CF3, CH3, OH, or, COOH, forexample) can bc modifiedto control surface properttes,

and the length of the alkyt chain can be varied to change the thickness of the rnonolayer.

This systern is both the most highly developed of the model systems for self-assembled strucfures, and a

broadly useful system in a range of applications: it thus combines intellectual content and practical application'

SAMs of alkanethiolates (and to a lesser extenr of alkylsiloxanes on glass o, sil i.u,28'29 and of alkyl phosphonates

on z i rconium dioxide30.3 I ; prouide a hig l r degree of contro l over the propert ies of a surface in the di rect ion

perpendicular to the surface. Trvo extensions of the concepts underlying SAMs providc structure in the second and

thi rd d i rncnsrons.

Microcotttact pritrtirrg (ttCP) Many applications of surface chetnistry (and most microfabricated

strucfures) require surfaces or thin fi lms tlrat are patterne<i in the plane of thc surface. Microcontact pnnting32-34 s

a versatile method for accomplishing patteming on tlre scale fi 'ottr sevcral pm and larger to less than 200 nm, without

using photolithography (Figure 2). The basis of the method is the use of an elastomeric stamp having surface relief

features of the appropriatc pattent, to transfer the alkanethiol to the surface of the substrate by contact Most of our

work has bccn based on stamps fabricated from polydirnethylsiloxane (PDMS), although other elastomers such as

polyurethales have been used as well. The starnp is usually prepared by casting the PDMS over a master prepared

by photolithography, although other sorts of masters can also be used. The stamp is "inked" with a solution of the

alkanethiol, and the inked stamp is then brought into contact with the surface of the gold fihn to be pattemed, either

by hand or r-rsing sitnple equiprnent for alignment.

ffi<-Photoresist

I nnntol i thngophy is used

I to create a master

V<- Photoresist pattern

I Si

I ( l -2 pm thickness)

I oonts is poured over

I master and cured

fiDMS I

ffi

<- Photoresist pattern

I nonnt is peeled arvay

I from the master

I PDI\IS is cxposed to aI solut ion containing

I t ts{cH, ), . ctI .

I PDNIS Is*!

- i*"r s.*.r {- alkanethiol

I Stamping onto goldI substrate transfers

t rhiot to form r)!_

,onr,

FIGURE 2

Schernatic of t5e procedure fbr rnicrocontact printing. An elastomeric staurp with three dirnensional relief is formed

by cast ing polydirncthyl s i loxanc (PDMS, Dow-Corning Corporat ion, Sylgard 184) on a tnaster . Masters are

typically prepared by photolithography. The printing procedurc involves several steps: i) the stamp is inked by

applying a dilure solution of alkanethiol to the surface of thc stamp with a cotton swab, i i) the stamp is blown dry in

a strealr of dry .rtrogen; i i i) the stamp is placed on a gold coated substratc (sometimes light pressure is applied to

ersure conformal coutacr befq,een the starnp and the gold); iu) SAN4s are fonned in regions where the stamp

colttacts the surface, v) the starnp is gently peeled frorn the surface.

It is crit icaf t6at t6c starnp be elastorncric. If it rs too rigid, it is not able to confortn to the microroughness

on the surfacc of tle substrate, and pattcrn tro'rrf., is iuconrplctc; if it is too plrable or soft, it is not able to maintain

sufficient structural i ltegrity to print rcliably featurcs rvith dimensions of *l ; 'rn-r' Thc tninimum slze of the features

t6at cal be prepared by pCP st i l l has not bccn cornpietc ly def incd, and rv i l l probably dcpend on the degree of

pcrt-cct ion that is rcquircc l in thc f inal systcrn. l t is now rout inc to pr int f 'caturcs at t l rc 500-ru, ' t r .n1. ,32-34 arrd 200-

nm fcaturcs can also bc prcparcd35-37 1olb" i , wi th grcatcr chf- f icul ty) . l t should bc possib lc to go to structurcs bclow

100 nrn, a l though soruc ta i lor ing of thc propcrt ics of ' thc stanrp rnay bc rcquircd. Thc cdgc dcf in i t ion of thc pat tcrns

pro<Juccd by currcnt pCP is on thc ordcr of 50 nm.38

Onc o f thc i rnpor tan t cha rac tc r i s t i cs o f SAMs i s tha t thcy a rc cxcc l l cn t r cs i s t s fb r cc r ta in t ypcs o f

" t .h.r .39 '40 For thc systcnl of a lkancthio latcs on golc l ancl sr lvcr . thc bcst sc lcct iv i ty bctwccn rcgions of tncta l

covcrcd rv i th a C16,, mct l ry l - tcrrn inatcd SAM and rcgions that arc barc is accompl ishcd rv i th an ctoh of aqucous

fcrr icyarr ic lc (Figurc:) .40 - l t ' , "

dcnsi ty of c lcfccts ( in tcrrns ol 'srnal l holcs in SAM covcrcd rcgions) achicvcd by th is

systcrn rs approxlrnatc ly 100 pcr, r , r . l2.4 l th i , dcnsi ty is too high to bc uscful for h igh-rcsolut ion tn icroelcctronic

clcv icc fabr icat ion, but is acccptablc fbr many ot l rcr arcas ol 'appl icat ion-- lbr cxarnplc. opt ics--wl tcrc thc dcf in i t ion of

thc mcso-scalc of s izcs rcquircs largcr st ructurcs.

1FnFIGURE 3

Scalrr i lg c lcctron micrographs ol ' rnctal s tn lcturcs on s i l icon fzrbr icatcd by using l tat tcrncd SAMs ( forrncd by

r l icrocol tact pr int ing) as ul t rath in rcs ists against sc lcct ivc ctcharr ts. (a) 200 nrn-wic lc l incs of golc l scparatcd by 200

nn-widc spaccs oi Si /SiO2 : t |csc arc t |c sntal lcst st ructurcs fabr icatcd to c latc by standarcl pCP. (b) Pat tcrr t tn

coppcr. (c) 200 nrn- th ick ctchcd stnrcturcs of s i lvcr . Thc cdgc rcsolut ion of th is fabr icat iot t tcchniquc is of ordcr 50

nrn for 50 nrn thick r:old strulcturcs, and of ordcr 30 ntn for 50 nm thick silvcr struoturcs.

l l4ic'r 'orrroldittg itr Colti l lut' ics (MIMICT 42 Mi.ru.ontact printing Iras proviclcd a vcry trsclul tcchniquc lor

ferbr icat ing surfhccs pat tcrnccl rv i t l r SAMs. SAMs arc. typical ly , s tnrcturcs t l rat arc approximatc ly 2 nrn th ick-- and

t l tus quasi- t rvo di rncnsional--arrc l l ravc no structural intcgr i ty i r rc lcpcndcnt o l ' t l rc support on wlr ic l r thcy rcst . MIMIC

is a tcchrr iquc t l tat pcrrn i ts thc f i rbr icat ion of st ructurccl arrd rncchanical ly stablc th in- f l l rn s ln lcturcs rv i th t l r ickncsscs

o f ' a f ' c r v p t rn . I n MIMIC (F igu rc 4 ) . t l r c sa rnc PDMS s ta rnps uscd in p tCP a rc b rough t i r r t o con tac t r v i t h a r r

appropr iatc sur lhcc (rnctal . ccranr ic. or polynrcr) Wlrcn in corr tact rv i t l r a support . t l rcy forrn a bccl o l 'srnal l -

d inrcrrs ional capi l lar ics. l l 'a l iqurd prcpolyrncr is brouglr t in contact rv i t l r arr cntrar)cc to th is nct ' ,vork. capi l lary

l i r rccs pul l thc prcpolymcr into t l rc bccl ; {br sui tablc d imcnsiorrs. t l rc capr l lary nct ' "vork can I i l l cunrplctc ly in pcr iods

ol-a t t r inutcs to hours. Pcl lynrcr izat iorr o l ' t l rc prcpolyrncr in s i tu to a solrd, fb l lorvccl by rcrnoval of thc PDMS starnp,

rcsul ts in gcncrat ion ol 'polyrncr ic rn icrostructurcs that carr havc substant ia l rncclrarr ical s tabi l i ty .

PDMS s tamp is p lacedon a substrate

Place a drop of prepolymera t one end

Channe ls i i l l bycapil lary action

I cure; Remove PDMS

Polymer

I r l ( l [ . ]R t i 4

S c l r c n t a t i c o f t l r c c a p i l l a r y n r i c r o n r o l c l i n q p r o c c s s ( l v l I N , l t C ' ) A r r c l a s t o r n c r i c s t a r t r p ( s i n r i l a r t o t l t o s c t t s c d i t t

rn ic rocontac t p r in t ing) i s p laccc l in cor r f i r l rna l cor r tac t * , i t l r a subs t ra tc to f i r r tn a r t r< l l t l tha t c t lns is ts o f a I t c t rvork o { '

channc ls . A lo rv -v iscos i ty po ly rncr -p rccursor i s p laccc l in cor r tac t rv i th < l l l cn t t tgs to thc nc t rvo t ' k . a r tc l thc c l ranne ls { l l l

by cap i l la ry ac t ion . A l i c r cur i r rg thc po lynrc r p rccursor . t l r c c las to r r rc r rc s tu t t rp i s tc t t tovcc l a t td ca t r bc t rscc l aga i t t .

- i l r c i rnagcs arc scann ing c lcc t ron rn ic rographs o l 'po ly rncr ic s tn lc tu rcs fnbr ica tcc l by MIMIC.

t lJ4t l ic 'al iotr.s. SAMs. both tn largc-arca. lrornogcncous fbnn antl as pattcr l ts, l tavc tro' ,v bcclt uscd in a rattgc

c l f - app l i ca t ions . An cxanrp lc o l 'a usc lb r a homogcncor . rs SAM is as a s t t r {hcc l t rcsc t r t ing l igar tc ls t l ta t sc lcc t i vc ly

in tc rac t rv i t l r anc l ac lsorb p .o t " i , , s .4J T l rcsc typcs o l ' sur laccs arc tc la t i vc ly cas i l y p rcparcd by syn t l rcs is , cvc t l when

thc l iganc l rcqu i rcc l i s s t ruc tu ra l l y co tnp lcx , bccausc thc t i r l t powcr o l 'o rgan ic sy r t thcs is ca t t l r c b ro t rg l t t to bcar o r l thc

l t rcpara t io r r o f thc inc l i v idua l rno lccu lcs tha t a rc thc p rccursors o f ' thc SAM. Invcs t iga t io r ts o l ' thc p ropcr t i cs o f thcsc

b io log ica l l y func t iona l SAMs havc gcucra tcd a nurnbcr o l ' t ypcs o f sur fhccs tha t in tc rac t rv i th p ro tc ins ; pcrhaps as

iu rpor tan t ly , thcy havc a lso dcmonst ra tcd sur fhccs tha t c lo r ro l adsorh pro tc ins and arc rcac l i l y p rcparcd by

appropr ia tc rnan i l tu la t ion c l f t l re f i rnc t io r ra l g roups prcscnt a t thc tc r t t r i t t i o f thc a lkaneth io la tcs : thcsc non-adsorb i r rg

sur fhccs arc i rnpor tan t as cont ro l r .44 A, , ana ly t i ca l t cc l tn iquc tha t i s cspcc ia l l y t rsc l i r l i r t con j t r r tc t io l l l v i th SAMs

that are active in biological recognition is surface plasmon resonance (SPR) spectroscopy.43 In this technique, a

beam of l ight is reflected from a thin, semitransparent gold fi lm. The angle of minimum intensity in reflection is

related to the index of refraction of the interface, and thus to the quantify of protein adsorbed at that interface

(Figure 5) .

- SAM (-2 nm)

a)

-Au (40 nm)

fr i f t nml' 'Glass (0.2 mm)

---------+ ---------e. \r So lu t i on o l

\

Coupl ingPrism

Flow Cell

Diode AnayDetector

Polarized Light

AdsorbedProtein

F ICURE 5

a) Schernat ic of the exper imental set-up for using surface plasmon resonance spectroscopy (SPR) to moni tor

interactions between proteins and the surface of a self-assernbled monolayer (SAMs). The adsorption of proteins to

SAMs is measured in situ and in real time. p-polarized light is incident on the subs[rate and the angle at which the

reflected light shows a minimum in intensify is recorded. This angle (Qm) is characteristic of the opticalproperties

of the interface and depends on the average thickness of a protein layer. b) Typical data recording Qp versus time

(solid curve). The change in the index of refraction of the protein-containing solution also causes a changein Qrn;

this contribution to the measured signal is indicated by the dashed curve. The difference between the two curves

measures the amount of protein adsorbed to the SAM. The extent of protein adsorbtion can be controiled (0Vo-100V"

protein adsorption) by choosing an appropriately functionalized SAM; selective adsorption of specific proteins by

using a SAM functionalized with a biospecific l igand is also possible. Reprinted rvith permission llom reference 43.

Copyright 1995 American Chemical Sociery.

The use of SAMs that are specifically functionalized to interact with proteins is valuable in exarnining the

inf luence of cel l morphology on cel l metabol ism.45-47 In one appl icat ion of pCP, a surface is part i t ioned into

regions of SAM that are approximately l0-30 pm in s ize and that interact wi th proteins in the rnedium, arrd

'Kb)

su l ' foun( l i l tg req io l ts l l ra t do no t adsorb pro te i r rs . - fhc

pa t te rns o f 'ac lsorbec l p ro te i r rs t l ta t resu l t u , l te t t t l tese pa{ tc t ' r tec l

s r r r lhccs arc cxposec l to t l re rned iu rn used lb l t i ssue cu l tu rc thcn c le te r rn ine t l re shapes o l 'n tan t tna l ian cc l l s tha t a t tac l t

t o l l r c s r r r l i r c c ( l ; i e u r e 6 ) . U s i n g s i r n p l e p a t t e r n i n g . i t i s t h u s p o s s i b l e t o d i c t a t e t h c s h a p e a s s t t t t r e c l b y a c e l l t l r i r t

a t t a c h e s t o a s u r l h c c . a n c l t o e x a r n i n e l l r e i r r l l u e r r c e o l ' t h a t s h a p c u n t h e r n c t a t r o l i s r t t o f ' t l t e c c l l . l ' h c r e s t t l t s o l ' t h c s e

ly , l tes o l ' s tuc l ies a re i rn l to r lan t i r r suggest i r rg connect i r . r r rs l rc tu ,ecr r ce l l rnorp l to logy a t rc l ce l l t t te tabo l i s t t t , a t tc l t t ta t '

c v c n t u a l l v s h e c l l i g l r t o n c o n r p l e x p l r e n o r n e n a s u c h a s c o n l a c t i n l r i b i t i o n o f ' c e l l p r o l i f ' c r a t i o n .

I : I ( I T J I { I : 6

Select ivc at tacl r rncnt ancl spreacl ing of ' t rovine capi l lar-v crrc lothel ia l cc l ls ot t i t sur l i tcc coverccl rv i th l tat ternecl SAMs

l i l ln tecl b l , rn ic locontact pr int ing. - l ' l rc

SAMs \vcfc pat tcrnccl bv ptC' [ ) s t rc l r t l tat ccr ta in regions of the str r lhcc \ \ 'c t 'e

t c r t t l i t t a t cc lby r r r c t l r y |q ro t l pS . rvh i l eo t l t e rs r r ,e rc te r ln i t t a t cc l [ r l ' o | i eo (c t l t r , | c t teu I1 ,co l )g r ( ) |pS .

\ \ / a S c X | ) ( ) S c ( l t o a s o | t t t i o t t c t r t t l a i t t i n g i t r t t a l r i x p r . o t c i r r ( l i b r t l n e c t i n ) : ( l t i s 1 l r o t e i t

SAMs ancl c l id r rot aclsorb to o l igo(et l ry lene glycol) - tcrnr inatccl SAMs. I :nc lot l re l ia l cel ls at tacl tecl orr ly to rcgions ol -

the str r l i rcc rv l rcre thc rnatr ix yt rote in u,as adsorbecl : t l re pat tcnl o l 'cel l at tachnrcnt thus lef lectecl t l te geontet t '1 ' ' , ' . ' , t

t l i r r rcrrs iorrs o l ' t l rc p i r t terrrcc l SAM fbrnrecl by ' ; tCP.

I r r non-b iok rg i ca l app l i ca t i ons . p rCP l rave a l so begun to f i n r l usc . One tha t i s cspec ia l l y p ro tn i s i r r t i s

l i l r r r ra t i on o l ' pa t ten ls on non-p lanar su r lhces . Nor r -p lanar l r t l r og raphy i s a ve ry subs tan t ia l cha l l e l rgc l b r

photol i thoeraphr, . arrc l cvcn vcry gcnt lv curvccl sur fhccs ol icn cart r tot bc pal ternecl st rcccssf t t l ly t ry photol i t l toeral thy

bccause o l ' l i r n i t a t i o r r s to c l cp th o l ' l i l cus . M ic roco r r tac t p r i r r t i ng . b ) , con t ras t . r vo rks as s r t too t l t l y r v i t l t t na r t y cu rvcc l

sul laccs as rv i th p lanar surthccs t l ' rgLrrc 7 y.48

100prm

F I G U R E 7

Scar t r r i ng c l cc t ro r r n r i c rou rap l r o l ' uo l r l n r i c ros t ruc tu rcs on a g lass cap i l l a ry ( r - -500rn rn ) . - l - l t c s t ruc tu rcs wcrc

f i rb r i ca tc r l by scvc ra l s t cps : i ) coa t in { .1 , t hc cap i l l a ry r v i t l r go ld . i i ) ro l l rns t l r c {o l c l - coa tcc l cap i l l a ry ac ross i l n

c lastorncr ic stanrp that rvas i r rkcd u, i th a lkancthio l : t l r is stcp lbrrncd a l lat tcnrc( l SAN,I on thc sur lhcc ol ' thc capi l lary,

ancl i r i ) c tc l r ing ol - thc golr l rv i th a lbrr . icyanrclc solut ion i11 rcgions not protcctc( l [ ry t l rc pat tcrnccl SAMs.

Conc lus ions

As l 'abr icat ion ol 'srnal l s t ructurcs grorvs in inrportant in a rv idc rangc ol 'arcas-- l l -or t1 r t . r icroclcctronics.

tho rough op t i cs a r r i l l n l c roc l cc t ro rncchan ica l sys tcn rs (MITMS) to cc l l b io logy - - thc u t i l i t y o f r t c rv tnc thods o l '

carry ing c lut r r r icrc l fhbr icatrun u, i l l i r rcrcasc. Most st ructurcs u, i t l r p l rn anc[ nnr c l i tncnsions arc now rnaclc by

pho to l i t hoe rap l r y . P l ro to l i t hou raphy rv i l l co r r t i nuc as thc unqucs t ioncd t l o t t r i nan t t cc l tnc l l ogy in fab r i ca t i on o l -

rn icroclcctrorr ic r lcv iccs lbr thc l i r rcsccablc l i r turc. Photol i thography l ras. l rorvcvcr. i r t rportant l i rn i tat ions fbr ccr ta i r t

typcs of 'appl icat ions. I t c locs not provi i lc rnuch vcrsat i l i ty in tcrnrs of ' t l rc rnatcr ia ls propcrt ics of- thc polyrr lcrs that

can bc uscd. nor is i t casi ly appl icablc to thc gcncrat ion of sur lhccs that prcscnt coruplcx organic Iunct ional groups

o f ' t hc so r t nccc l cc l i n b ioc l r c rn i s t r y anc l b io logy . I t i s no t , i n gcnc ra l . app l i cab lc to cu rvcd o r i t r t c r i o r su r faccs .

Moclcrn pl roto l i thography is a lso cxpcnsivc s incc i t cntai ls s ignf icant proccssing ancl capi ta l costs.

Molccular sc l f ' -asscrnbly ant l sc l f ' -asscrnblccl rnonolaycrs (SAMs). couplct l rv i th tcchniqt tcs for pat tcrni t rg

sucl t as ptCP arrc l MIMIC ol ' f 'cr a l tcw stratcgy l i l r lhbr icat ing rn icro- ancl r tat tostructurcs. - i l tcsc

tcc l t r t iqucs c io not

rcplacc photol i t l rography. but instcacl cornplcrnent i t . T l rcy r / r . r o{ ' lcr a l r igh c lcgrcc ol 'cc lntro l ovcr t l tc dctai lcc l ,

nrolccular- lcvcl s t r t rcturc o l ' thc intcr lhccs. ' l -hc

th ickucss ol ' thc sur lhcc structurcs gcncratccl rv i th SAMs (2 r t rn) is

substant ia l ly t l r inncr t l ran t l rosc that can bc prcparccl by photol i thography. - l ' l t is

thrnr tcss can bs cxplo i tcd fbr t l tc

r l cvc lop rncn t o f -nc rv fb rn rs o l - l i r hog raphy4 t ) . , , t , , ach icvc vc ry s rna l [ (<50 n rn ) sca lc f c ' a t t r t cs 50 -5 ] T l r cy a rc

appl icablc to a broat lcr ra l t tc o l -nratcr ia ls tharr photol r t l rography, ar tc l t t ray bc t rscf i r l in gcr tcrat ing largc-arca

slructurcs. Whcn appl icablc. t l rcy arc probably lcss cxpcnsivc t l tar t photol i thography. SAMs havc t l t rcc i tnportant

l i rn i tat ions at t l r is t i rnc: i ) Thcrc arc only l i rn i tcd c lcrnonstrat ions ol ' rcgist rat ion in Intr l t i lcvcI f i rbr icat ion. i i ) - fhc

ntost l r ighly c lcvclopccl systc lns of 'SAMs arc cornbinat ions sucl t as gol t l or s i lvcr ot t t i tat r i t t t t r -pr i rncd Si /SiO2; the

presencc of the heavy metals may limit their use in microelectronic device fabrication. i i i) The levels of defects rn

etched structures that are too high to allorv the yield required in fabrication of cornplex tnicroelectrontc systems.

Where wi l l SAMs and other sel f -assenbled organic structures ul t imately be used, and how wi l l they

contribute to science and technology? Certain parts of the answer to this question are clear because the applications

have alrcady been dernolstrated; others remain speculative. It now appears that the types of methods of fabricahon

used for SAMs of alkanethiolates on gold are among the best available for preparing surfaces having the complex

functionality required for studying biosurface interacrions. The scale of strucfures prepared by pCP and MIMIC are

also that required for studying and using meso-scale phenomena with l ight (e.g., diffraction).54 SAMs are very well

suited for fundamental studies of the physical-organic chemistry of adhesion und fri.t ion-38'55

More speculative technological uses of SAMs as electrically functional components of very small devices

will require learnilg lrorv to manipulate and use electrical conductivity on a rnolecular scale. The prospects for this

type of appl icat ion are very at t ract ive: in a certa i t i sensc, a wel l -def ined molecular redox center such as

ferrocene/ferroceniurn are a single-electron capacitor. The practical applicatiou of this idea rvil l. however, requlre

thc development of the speculative field of rnolecular electronics, and will constitute a major undertaking.

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