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R E P R I N T E D F R O M

Molecular ElectronicsA 'Chemistry for the 2lst Century' Monograph

ISBN 0-632-0.128.+-2

E D I T E D B Y

J . JORTNER & M . RATNER

Blackwel lScience

11 Micro- and Nanofabrication TechniquesBased on Self-assembled MonolayersP . F . N E A L E Y . , A . J . B L A C K t , J . L . w t L B U R f a n d G . M . w H r r E S t D E S t'Department of Chemical Engineering, university of Wisconsin-Madison, 14l S Johnson Dr.. Madison, Wt 53706, USA

lDepartment of Chemistry, Harvard lJniversity, Cambridge. MA 02139, USA

Introduction

New labr icat ion techniques are required to cont inue the trend in the microelectronicsindustry to work at smal ler scale. Opt ical l i thography t l l . the most widespreadtechnique lor the fabr icat ion of microelectronic devices. is approaching the lower l imi t( - l00nm) in the s ize o f fea tures tha t i t can produce. Smal le r f -ea tures (50nm) can beproduced with electron beam wri t ing or atom-b1--atom manipulat ton. These processes.however. are current ly l inear and wi l l require s igni f icant development for h igh-volumecommercial appl icat ions. Opportuni t ies exist to develop labncat ion techniques thatare based on di f ferent pr inciples to reach the scale of 50 nm. Desirable at t r ibutes ofany new labr icat ion technique include low cost (capi ta l and operat ional) . lowenvironmental impact. and the abi l i t .v to make complex structures of appropr iate scalereproducibly wi th low levels of defects.

Molecular- or atomic-scale devices based on the electronic propert ies of s inglemolecules represent a plausible lower l imi t in s ize. Ef forts to labr icate these deviceshave def ined the emerging f ie ld of molecular electronics. Even i f useful devices of scaleI -30 nm can be synthesized. however. a host of problems present themselves for theappl icat ion of the molecular-scale devices. Structures between 100 nm and I nm wi l ls t i l l be needed. for example. to make internal and external interconnects. or for use innew archi tectures such as cel lu lar automata to access the molecular-scale devices.

Molecular self-assembly is a potential strategy for labrication of structures withdimensions that range from nanometres to mi l l imetres t2.3] . In molecular sel f -assembly. subuni ts of molecular dimensions, whose structures can be control led wi thatomic resolution, spontaneously form molecularly ordered aggregates [4.5] with cer-ta in dimensions of I - 100 nm. Sel f -assembly leads to equi l ibr ium structures that are at(or c lose to) thermodynamic minimum [6.7] . and resul t f rom mult ip le. weak. reversibleinteractions such as hydrogen bonds, ionic bonds. and van der Waals forces betweensubunits. The information that determines the l inal structure of the aggregate is codedin the structure and properties of the subunits. As a strategy for fabrication, self,-assembly is thus automatically defect rejecting [5], and self-registering on a scale that istoo small for current techniques for microfabrication and too large for conventionalorganic synthesis. The pr inciples of sel f -assembly are observed in nature [4-6,8,9] ;processes such as protein folding and aggregation I0] and formation of DNA doublehel ices i l l ]serve as biological models for the potent ia l of sel f -assembly in microfabr i -cat ion. Table I l . l shows some examples of sel f -assembl ing systems.

Self-assembled monolayers (SAMs) represent the most widely studied and bestdeveloped class of non-biological self-assembling systems. SAMs are highly ordered

343

344 P . F . N E A L E Y I . T . 1 1 , .

Table 11.1 Sel f -assembl ine svstems.

S-vstem Smal les t d imension Ref .s

i l r - l 8 ll l 4 . l 5 ll l e l[20][ ] 1 .221t23 l[ 4 - 6 . 8 - I l ]

Self-assemblcd monolaycrs

Langmuir-Blodgett hl ms

Liquid crystals

H;,drogen-bonded aggrcgates

Tcmplated crystals

Block copolymers

Svstems in nature

1 - 2 n ml - 2 n ml n m1 . 5 n m0 . 1 n ml - 2 n ml n m

molecular assembl ies of long-chain alkanes that chemisorb on the surfaces of appropr i -

ate sol id mater ia ls. The structure of SAMs. ef fect ively two-dimensional crystals wi th

control lable chemical funct ional i ty. makes them ideal model s) 'stems for the invest iga-

t ion of wett ing. adhesion. iubr icat ion. corrosion. protein adsorpt ion. and cel l at tach-

m e n t [ 2 - I 8 ] .This chapter descr ibes the pr inciples. achievements. and potent ia l of micro- and

nanofabr icat ion techniques that are based on sel f -assembled monola-v-ers: the systems

that are discussed have at least one dimension of molecular scale. We have demon-

strated appl icat ions for SAMs in microelectronics that rnclude passivat ion of surfaces.

use of SAMs for ul t rathin resists and masks. directed depostt ion of mater ia ls on

sur laces patterned with SAMs. and the use of chemical and biochemical funct ional i ty '

on the surface of SAMs for sensors.

Self -assembled monolayers

2.1 Alkanethiols on gold

Self-assembled monolayers of a lkanethiolates on gold (RSH/Au) are the best studied

system of sel f -assembled monolayersl24,25). The RSH/Au system is at t ract ive because

of: ( i ) ease of fabr icat ion: ( i i ) degree of perfect ion: ( i i i ) chemical stabi l i ty under ambient

laboratory condi t ions: ( iv) avai labi l i ty of mater ia ls: and (v) f lexibi l i ty in chemical -

and thus surlace - functionality. Two important factors may limit the use of current

hydrocarbon-based SAMs of alkanethiolates on gold in microelectronic devices: ( i ) they

are thermal ly unstable above 100'C [26] and ( i i ) the high rate of d i f fusion of gold into

Si: th is property makes gold structures on si l icon largely incompat ib le wi th current

techniques of microlabr icat ion [27]. SAMs of a lkanethiolates on gold. however, can

serve as models or prototypes for new techniques or applications that wil l be generally

applicable to new classes of SAMs designed specifically for microprocessing and

microfabrication.

2 . t . 1 P R E P A R A T I O N A N D G E N E R A L S T R L J C ' T T ] R E

Self-assembled monolayers of alkanethiolates on gold form by spontaneous adsorption of

alkanethiols IX(CHr),SH] U2-18,25.28-331 or dialkyldisulf ides IX(CH2),,S-S(CHz),,Y]

[33,34] onto gold, either from solut ion or the vapour phase (Eqn I l . l ) .

M I C R O - A N D N A N O F A B R I C A T I O N T E C H N I Q U E S 345

X-R-SH + Au(0), * X-R-S- Au(I) . Au(g),, + l l2H2

l/2 (X-R-S)z + Au(0),, - X-R-S- Au(I) . Au(0),, .

( r 1 .1 )

Dialkylsulfides [X(CH2),,S(CH2),,Y] also form SAMs [35], but are significantly less reac-tive than alkanethiols, and form SAMs of poorer quality. Assembly of thiolates may alsobe induced by electrochemical oxidation of alkanethiols in solution [36] using gold as anelectrode. Thiols are generally preferred over disulphides and sulphides for the prepara-tion of SAMs because they are more soluble, react with the surface of the gold - 103 faster

[33,37], and yield monolayers of better qual i ty. In addit ion. they are easier to synthesize(many are available commercially).

Typically, thin (20-50 nm) films of gold are used as substrates for SAMs; SAMs alsoform on colloidal gold [38-40]. The gold films are usually deposited by electron beamor thermal evaporation onto a solid, flat support such as a polished silicon wafer (witha native oxide), a sheet of mica, or a glass slide. Because gold does not wet thesesurfaces, an adhesion promoter such as titanium, chromium. or an appropriate organicmaterial [41,42] is used.

2 . 1 . 2 s r R L J c ' T U R E A T T H E . A T o M r ( ' s ( ' A L E

The mechanist ic detai ls [32] of the reaction of thiols or disulphides with the goldsurface are not completely understood. The fate of the hydrogen atom in the case ofalkanethiol adsorption, for example, and the exact structure of the resulting monolayerare not known [43]. On Au(l l l ) the sulphur atoms are thought to form an overlayercommensurate with the Au atoms with structure (n6

" f ln:0". Recent X-ray dif frac-tion studies, however, suggested that the species on the surface is a disulphide [aa](rather than a thiolate [ 5,16]). The alkyl chains extend perpendicularly from the planeof the surface in a nearly all-trans configuration. To maximize the van der Waalsinteractions between adjacent methylene groups (-1.5-2 kcal mol- ' per CHr) [5,16],the chains t i l t at an angle of 30" with respect to the surface normal (Figure l l . l (a)).These van der Waals forces (-20-30kcal mol- ' for Cr6SH) and the strength of thesulphur-gold bond (-44 kcal mol-r; drive formation of the monolayer. The result ingstructure on the atomic scale is ordered: for alkyl chains with n > 6. the monolayer isquasi-crystalline in two dimensions. Table I 1.2 summarizes the characterization ofSAMs of alkanethiols on eold.

2 . 1 . 3 D E F E C T s

Figure I 1. I (b) shows high resolution scanning tunnelling microscopy (STM) images(from Delamarche et a\.163,641) of dodecanethiol adsorbed on Au(l I l). The sample inthe large image was annealed at 100"C for l0h in air. A step in the gold surface runslrom the upper right corner to the centre of the image where it disappears with a screwdislocation. The occassional black lines that parallel it are depressions in the monolayerwhere neighbouring thiols have either tilted over or migrated to cover missing lines ofthiols. Although molecular resolution is apparent, the c(4*2) rectangular superlatticestructure is not clearly discernible because of the large scan size. The inset shows asimilar sample that was not annealed. The gold terrace has five depressions: these are

346 P . F . N E A L E Y I : ' 7 . l L .

(a)

-10 nm

Table 11.2 Proper t ies o f se l f ' -assembled monola-vers o f th io ls on go ld .

F igu re 11 .1 (a ) Schema t i c i l l u s t ra t i on o f

thc molccu lar - leve l s t ructure o f a se l l - -

assernblcd monolal-cr of n-alkanethiolates

on go ld . Modi f icat ion o f the head group.

X. a l lows cont ro l o f the monola l -cr 's

sur l -acc propcr l ics . and thc th ickness of

thc monola lcr dcpends on thc length o f

t hc a l kv l cha in . ( b ) Scann ing t unne l l i ng

micrograph o1' clodccancthiol adsorbed on

nu( l I I ) that shows a go ld s tep. a screw

dis locat ion in thc ccnt re . and dcpressed

l i nes i n t hc n rono la rc r duc t o t hc

accommoda t i on o t ' n r i s s i ng l i nes o f t h i o l s

b1 r ' ic ina l rno lccu lcs Thc inset shows f ive

t lcpress ions (go lc1 p i ts 1 . ,1 .A dccp) in the

ccn t r c t ha t a r c l i nkcd b1 doma in

boundar ics . The monola l 'e r pack ing

corresponds to a phase of the c(4*2)

rectangular super la t t icc . These images

wcrc provided b1' Delamarche at ul. [63].See text tbr rcfcrcnces.

"t3'g

(b)

"6:o

Monolal-er property Technique Rcfi

Structure

Composition

Wettabil ity

Thickness of the adsorbed layer(s)

Degree of perfection andelectrical properties

Surface Raman scattering

Transmission (high energy')

c lcc t ron d i f f rac t ion

Low energy hel ium dif i iact ion

X-ray dif fract ion

Infrared spectroscopy

Scann ing tunnel l ing m ic roscopl

X-ray photoelectron spectroscopy

Contact anglc

Ell ipsometry

Electrochem ical methods

[45][46)

[ 4 ] - 5 t l144.47 .48.s2.s31[2e.s4-5e][4e .60-71 ]

[25 .33 .s s .s6 .721

113 .3 t .33 .13-77)

[2e.32)

[29,43,60.7s.78-84]


pits in the gold. 2.4 A deep. l inked by domain boundar ies (s l ight ly depressed regions,where the packing is distorted). It is important to note. however. that the holes are sti l lcovered with SAMs.

Defect densi t ies are a pr imary determinant of the sui tabi l i ty of SAMs for use in

micro- and nanofabrication. Estimates for the minimum number of defects for SAMson Au over relatively large areas (several cmr) range from two to several thousandpinholes per cm2; the lat ter value is more real ist ic [16.85: X.M. Zhao, J.L. wi lbur andG.M. Whitesides, unpubl ished resul ts] . A str ingent test using a wet-chemical etchant toamplify the defects in SAMs gives 90 defects mm-2 as a n-rinimum value for the defectdensi ty for a SAM of hexadecanethiolate (HDT) on 20 nm thick gold.

With chemical control over the length of the methylene chains. the height of themonolayer perpendicular to the plane of the surface can be controlled with 0. I nm scaleprecision. Organic and inorganic synthesis also lacil i tates the incorporation of differentfunctional groups into. and at the termini of. the alkyl chains: hydrocarbons. f luorocar-bons. acids. esters. amides. alcohols. n i t r i les. and ethers are a few of the many possible

funct ional groups. Mixed monolayers - e i ther wi th regard to alkyl chain length orchemical functionalit- '" - afford further control over surf-ace modification and areprepared by s imultaneous coadsorpt ion of two di f i 'erent th io ls.

Figure I 1.2 shows how interfacial propert ies such as wett ing and adhesion arecontrol led by SAMs of varying funct ional i ty ' . Water prefere nt ia l l -u- condenses on regionsof a surface patterned with SAMs terminated by hydroxl ' (OH; groups (patterning

techniques are discussed below): the water drops do not form (pr ior to coalescence) onthe regions of the sur lace covered with SAMs terminated by methyl (CH:) groups.

SAMs are capable of passivating certains surfaces. and of protecting these surlacesfrom chemical at tack. SAMs of a lkanethiols on Cu [86] and GaAs [87.88] have beenshown to inhibi t oxidat ion by retarding ox.n-gen transport to the surface. Fe and Ni have

Figure 11.2 Water droplets formed by condensation on regions of a surface (dark) that arecovered with a self-assembled monolayer (SAM) that has a hydrophil ic head group (e.g. -OH).

Water does not condense on other regions of the surface ( l ight) that are covered with an SAMthat has a hydrophobic head group (e.g. -CH.).

348 P.F . NEALEY ET A I , .

Table 11.3 Substrates and l igands that form self-assembled monolayers.

Surface Ligand Refs

SiO,, glassS iS iAgGaAsCuInPAuAuPtPtMetal oxidesMetal oxideszio2In ,O. , /SnO, ( lTO)

RSicl j , RSiORs

lRCOOIz (neat)RCH: CHr , [RCOO] ,RSHRSHRSHRSHR.,PRSO2HRNCRSH. ArSHRCOOHRCONHOHRPO4H.'RPO-.,H,

Ies- 100]l l 0 r ll r 02 l[41 .48.54,103- 105][87 . 106 , l 07 l[ 5 5 , 8 6 . 1 0 8 . 1 0 e ]l l l 0 ll l l l ll l l 2 ll l l 3 l[ 4 s . r l 3 - r r 6 ][52 .7 3 . t t7 - t21 )

l l 2 8 ll l 2 e . r 3 0 ll l 3 l l

also been protected from corrosion by other types of thin films [89]. SAMs are alsoexcel lent insulators both in, and perpendicular to. the plane of the monolayer: awell-formed C r8 SAM reduces the current passing across a gold electrode by roughly I 0o(relative to an underivatized surface), with the residual current attributed to defects.SAMs have also been designed for studies of electrochemistry: ferrocene-terminatedmonolayers have been used in fundamental studies of the dependence of the rate ofelectron transport through SAMs on the thickness of the SAM [90]. Many other thiolligands have been made in order to enhance the electrical properties of SAMs. Theseinclude thiol-terminated polythiophene and polyphenylene chains [91,92) and SAMs ofpolypyrrole-terminated alkanethiols 193,941.

2.2 Other sysfems

Table ll.3 summarizes the wide variety of ligands and substrates that may be used in

the formation of self-assembled monolavers.

3 Patterns in two dimensions: microcontact printing (pCP)

pCP of alkanethiolates on gold

Some applications of SAMs in micro- and nanofabrication require that they can bepatterned in the plane of the substrate. Techniques to pattern SAMs include microcon-tact print ing (pCP) [85,1 32,133), micromachining [37,134,135], photol i thography/liftoff [136], photochemical patterning ll37 ,1381, photo-oxidation [ 39- 142], focusedion beam writ ing [143], electron-beam writ ing [87,144-154], STM writ ing [62,145,| 49 ,15 5, I 5 6], and microwrit ing with a pen [ | 57 , l5 8]. Of these methods, microcontactprinting (pCP) is the most versatile,, and will be discussed in detail for SAMs ofalkanethiolates on gold.

Figure 11.3 shows a schematic of the pCP process. An elastomeric stamp with

3 .1


Si+- Photoresist

Photolithography is usedto create a master

+- Photoresist pattern(1-2 pm thickness)

PDMS is poured overmaster and cured

PDMS

Si+- Photoresist pattern

PDMS is peeled awayfrom the master

PDMS

PDMS is exposed to asolution containingHS(CH2)rsCHs

PDMS

Stamping onto goldsubstrate transfersthiol to form SAM

-.2 SAMs (l - 2 nm)_ r

<- Au (5 - 2000 nm)- \\..- 1; ( 5- l0 nm)

Figure 11.3 Schematic of procedure for microcontact printing.

three-dimensional relief is formed by casting polydimethyl siloxane elastomer (PDMS,Dow-Corning Corporation, Sylgard 184) on a master. Masters are typically prepared byphotolithography. Other techniques for the fabrication of masters include electronbeam lithography and micromachining; commercial objects with existing relief struc-ture (diffraction gratings, for example) can also be used. Microcontact printing involvesseveral steps.1 The stamp is inked by applying a dilute (- I mM) ethanolic solution of alkanethiol tothe surface of the stamp with a cotton swab.2 The stamp is blown dry for l0-20 s with a stream of dry nitrogen.3 The stamp is placed on a gold-coated substrate (light pressure is sometimes applied toassure conformal contact between the stamp and the gold).4 SAMs are formed in the regions of contact between the stamp and the surface.5 The stamp is peeled from the surface.

The underivatized regions of gold can be selectively etched, after pCP, to producestructures of gold. These gold structures can be used subsequently as resists for etchingthe underlying substrates. Alternatively, SAMs of the same or a different alkanethiolate

<- alkanethiol

Si

Si

3s0 P . F . N E A L E Y L T , 1 1 , .

can be formed on the underivatized regions by a second pCP step, or by washing the

surface with a di lute solut ion of an alkanethiol .

Several properties of the elastomeric PDMS stamp are crit ical for the high resolution

and the degree of perfection of the patterned SAMs formed b-v- pCP [2. ] 59].

1 In most cases. SAMs are only formed on the surlace where there is conformal contact

of the stamp with the gold ( for appropr iate alkanethiols and contact t imes. see below).

PDMS is deformable enough such that contact is achieved even on surfaces with

significant roughness I I 60].

2 The elastomer is suf f ic ient l -v r ig id that the rel ief structure of the stamp retains i ts

shape.

3 PDMS has a low in te r lac ia l f ree energy (22 .1 dynes cm-r ) [61 ]so i t does no t adhere

irreversibly to the sur lace of the gold.

4 PDMS swel ls in ethanol ; th is character ist ic al lows the stamp to absorb the alkane-

th io l ink .

The detai ls of the process by which alkanethiol t ransfers f rom the stamp to the

substrate are not completel-v- understood. Because the stamps are elastomeric in nature.

pCP offers some immediate advantages over t radi t ional pat terning techniques such as

photol i thography: patterns can be introduced on curved surf-aces ! 62] .

A number of a lkanethiols can be used for prCP on gold surfaces. Hexadecanethiol

(HDT) i s par t i cu la r lv we l l sur ted fo r p rCP because i t i s au tophob ic and has lou ' r 'o la t i l i t y .

Non-autophob ic a lkaneth io ls I I 63- I 66 ] such as HS(CH:) r :COOH can spread reac t ive ly

af ter appl icat ion to the gold in air and do not y ' ie ld high resolut ion patterns smal ler than

l-2 pm. To reduce react ive spreading. non-autophobic alkanethiols have been stamped

under water [67] . Alkanethiols shorter than HDT can be too volat i le for use in pCP:

SAMs can form from alkanethiol vapour in regions not in contact wi th the stamp.

Figure I 1.4(a) shows a scanning electron microscopl ' (SEM) image of a master that

was prepared by photol i thography. Figure I 1.4(b) shows an SEM image of patterned

SAMs formed with prCP using an elastomeric stamp that was cast f rom the master

shown in Fig. I L4(a). These images represent the level of complexi t l ' . perfect ion. and

scale of features that can be produced rout inely bv pCP. Figure I I .4(c) shows a lateral

force micrograph [ 60] of a test pat tern of SAMs terminated by two di f ferent alkane-

thiols. SAMs terminated by CHr (dark stars in the image) were formed b1' pCP with

HDT: SAMs terminated by COOH were deposi ted ( in regions of the gold not der iva-

t ized by pCP) by washing the substrate wi th a di lute. ethanol ic solut ion of

HS(CHr)r 5COOH. The edge resolut ion between regions terminated by CH j and COOH

was 50 nm [ 60] . The smal lest pat tern generated to date wi th pCP is 200 nm l ines

separated by 200 nm spaces I I 68] .

Microcontact pr int ing is an at t ract ive technique because i t is s imple. inexpensive.

and f lexible. The cost of PDMS elastomer is negl ig ib le. many stamps can be cast f rom

one master. and each stamp can be used hundreds of t imes. The process is inherent ly

parallel in that large areas (many cmr) can be patterned at once. Routine access to a

clean room is not required. although occasional use of a clean room is convenient to

make the photolithographic masters. Areas of future research in the development of

prCP include the generat ion of smal ler scale patterns (< l00nm). minimizat ion of the

number of defects in the patterns. and the registration and alignment of patterns over

large areas or in multiple printing steps.

M I C R O - A N D N A N O F A B R I C A T I O N T E C H N I Q L ] E S 351

Master

(a)

COOHi

CHsf

ffi(c)

5| tm

(b) n

100 pm

Figure 11.4 (a) Scanning electron micrograph (SEM) of a master with three-dimensional rel iefthat was used to cast stamps for microcontact print ing (pCP). The contrast in the image resultsfrom height dif ferences between regions. and also from dif ferent materials on the surface in therel ief structure. (b) SEM image of a patterned self-assembled monolayer (SAM) formed bymicrocontact print ing. The dark regions of the surface are covered with SAMs. and the l ightregions are underivatized gold. The scale bars in the inserts correspond to l0 pm. (c) Lateralforce micrograph of a patterned SAM. Relat ively high fr ict ional forces between the probe t ipand the surface are detected in regions ( l ight) covered with a carboxy-terminated SAM. andrelat ively low fr ict ional forces are detected over regions (dark) covered with a methyl-terminated SAM.

352 P . F . N E A L E Y E T A L .

Feature sizes of approximately 100 nm in patterned SAMs have been fabricated by

adapting the pCP process. In one procedure, mechanical compression of the stampproduced features ca five times smaller than those originally cast from the master

[ 69]. In another process, stamps with small feature sizes were cast lrom mastersprepared by anisotropic etching of si l icon [168]. Alternatively, control led reactive

spreading of 4 non-autophobic alkanethiols also produced features of dimensionl00 nm ! 701.

3.2 pCP of other materials

Microcontact printing (pCP) has been used in systems other than alkanethiols on gold.

Patterned SAMs have been formed with alkanethiols on copperl lT l ] , alkanethiols on

si lver U721, and with alkyltr ichlorosi lanes on Si/SiO, and glass [ 73]. The latter system

is particularly useful because: (i) coinage metals (which are often incompatible materi-

als for silicon microelectronic devices) are not necessary; (ii) SAMs formed from

alkyltr ichlorosi lanes have higher thermal stabi l i ty than SAMs of alkanethiolates ongold; (ii i) patterned SAMs on glass are desirable for optical applications. SAMs formed

lrom alkyltrichlorosilanes take longer to form and are not as highly ordered as SAMs of

alkanethiolates on gold; they are also not as effective as chemical resists ! 73]. The edge

resolut ion in pCP of alkyltr ichlorosi lanes on Si/SiO2 or glass is currently -200 nm

ll l4l compared with the edge resolut ion of 20-50 nm [ 75] observed in thealkanethiolateigold system.

Microcontact print ing of l iquids containing suspended pal ladium col loids [ 75] has

also been demonstrated. Features with dimensions on the micron- and submicron-scalewere prepared with edge resolution of approximately 100 nm. Colloids can be depos-ited on a variety of substrates including glass, Si/SiO2, and polymers. Following

deposition. the palladium can act as a catalyst for electroless deposition of metals.

These experiments indicate that pCP may be a general technique that can be used in

systems other than SAMs.

4 Fabricationof three-dimensional structures

4.1 SAMs as ultrathin resists

Problems in optical lithography concerning depth of focus, optical transparency,shadowing and undercutting for the fabrication of features of sub-50-nm dimensionsresult from the inability to produce sufficiently thin films of photoresist (<20 nm). The

use of SAMs as resists potentially resolves these problems because the thickness of themonolayer is -1-2 nm. SAMs are capable of protecting the underlying substrate from

corrosion by wet-chemical etchants U7 61. Relative capabilities are determined by thelength of the alkyl chain and the nature of the terminal functional group [134].

Figure I 1.5 shows a series of metal structures that were fabricated using patterned

SAMs of HDT formed by pCP as resists. The underivatized regions were etched by a

solut ion of ferr icyanide, or with a basic, oxygenated cyanide etch [3,172,176]. The

derivatized sections were barely affected by the etch in the time required to remove theunderivatized gold (Figs I l .5a-c) or si lver (Fig. I l .5d). Si lver etches more quickly and


1CI0 pnr

Figure 11.5 Meta l s t ructures [ (a . b . c ) go ld . (d) s i lvcr ] labr icated us ing pat tcrned sc l f - -assembledmonolayers ISAMs: lbrmed by ' microcontact pr in t ing lprCP)J as rcs is ts . The under ivat ized meta lis e tched wi th a so lu t ion o f f -er r ic c-van idc (a . d) or w i th a bas ic ox\genated cy 'an ide so lu t ion(b. c ) . The 200 nm l ines o f go ld separatcd b1 '200 nm wides spaccs of S i /S iO. arc the smal les ts t ructures tabr icatcd to datc b l uCP

with better resolut ion than gold [ ]21. This technique is capable of producing 200 nm

features (F ig . I l .5c ) [ 68 ] w i th edge reso lu t ion o f -20 nm.

Microcontact pr int ing of pat terned SAMs and select ive etching was used to labr icate

gold features on non-planar sur laces ! 62] ; the capabi l i t r , ' to work on non-planar

substrates is beyond the capabi l i t ies of rout ine photol i thographl ' . Figure I 1.6 shows

clear ly resolved features of gold wi th dimensions of a few microns on curved sur laceswi th d iameter 500 pm (F ig . I l .6a-b) and d iameter 50 prm (F ig . I l .6c ) .

The gold (or silver) features can act as secondary resists for further substrate etching

[85 .168.1 ]4 . l7 l . l l 8 l . Complex s i l i con s t ruc tu res were fabr ica ted w i th fea tures as

smal l as 100 nm. A typical fabr icat ion process involves the fol lowing steps: ( i ) t i tanium( 1 . 5 - 5 n m ) a n d t h e n g o l d ( 1 5 - 1 0 0 n m ) a r e e v a p o r a t e d o n t o a S i [ 9 9 ] w a f e r : [ i i ] apatterned SAM is formed by pCP: ( i i i ) under ivat ized regions of gold are etched with a

basic ferr icyanide solut ion to create a gold structure based on the pattern of the SAM:

toi

ffi,:$,t*$:r*$

#{d--lr,1

:$

i :i-qe

NiWi;Fi4:,:.

€a+,::s-::;,il':.s

"$i

f=

;;i

6."i;'R.s$

ffiw

ffi

354

Lrt::r:q\wB

Figure 11.6 (a. b) Scanning electron micrograph (SEM) image of gold microstructures lbrmedb-v microcontact printing (pCP) with HDT on a gold-coatcd capil lary (r = ,500 mm). followed b1'etching. A number of defects are apparent in (b) (whi tc arrow): these defects or ig inated in themaster used to cast the stamp. (c) SEM image of patterned gold structures lormed on a 50 mmdiameter gold-coated glass fibre. There was a stripe (white arrow) where the capil lar! ' waspr inted twice when i t ro l led more than 360' . L ight rcgions in these images correspond to goldldark regions are Ti lTiO, lg lass where etching removed the gold. IRepr intcd wi th permissionfrom Jackman RJ. Wi lbur JL. Whitesides GM. St ' i t ' r t t ' t , 1995: 269: 661. Copl ' r ight ( 1995)American Associat ion for the Advancement of Sciencel .

( i v ) the t i tan ium layerand na t ive S iO. , layerare removed by d isso lu t ion in 106 HF: (v )

the s i l icon wafer is etched anisotropical ly ( I 50/o solut ion by volume of 4 M KOH in

l-PrOH, at 60'C for t ime per iods proport ional to the desired etch depth) using the gold

pattern as a resist ; and (v i ) the remaining gold and t i tanium are removed by exposure to

aqua reg ia ( l : I HNO3:HCl ) . F igure I1 .7 (a) shows a go ld g r id fabr ica ted by pCP and

subsequent etching of regions not covered with SAMs. Figure I 1.7(b) shows the

resul t ing pyramid-shaped 'p i ts ' in the substrate af ter anisotropic etching of s i l icon.

Figures I1.7(c)-(d) are SEM images of more complex s i l icon structures that were

formed with this same process.

Microlithography using neutral metastable argon atoms U79) has been investigated

as a technique for patterning and labricating surlace features with high resolution(< 100 nm); beams of the neutral atoms have de Brogl ie wavelengths of order 0.01 nm,

and can in pr inciple be focused to a spot that is l imi ted by the s ize of the atom.

Conventional resists cannot be patterned with these beams because damage from the

metastable argon atoms is restricted to a surface layer less than 0.5 nm thick. This

depth, however, is sufficient to damage SAMs of HDT on gold such that an aqueous


-

.----

---

IITrII

ttil

il;

t 2 F m

Figure 11.7 (a) Scanning e lect ron micrograph (SEM) of a gr id o f go ld labr icated b-vmicrocontact pr in t ing (pCP) and se lect ivc c tch ing of go ld . Thc dark rcg ions arc thc under ly ings i l i con . ( b ) Se lec t i ve an i so t rop i c c t ch ing o f t hc s i l i con i n (a ) r esu l t s i n ' p i t s ' t ha t a r c i nvc r t edpyramids. (c -d) SEMs of e tched s i l icon s t ructures us ing go ld pat terns ( lormed by pC-P andetching) as resists.

ferr icyanide solut ion wi l l etch the metal under the exposed regions. Figure I 1.8 shows a

schematic of the process. an SEM image of a copper TEM gr id used as a mask ( in

contact wi th the sur lace of the substrate). and an SEM image of the gold structure thatwas fabr icated using the TEM gr id as a mask.

4.2 Templatedadsorption

SAMs are essential ly two-dimensional structures. The techniques for labrication de-scribed above focused on the use of SAMs as resists: the pattern in the SAMs wastransf,erred to the underlying substrate b.v selective etching to produce a structure withthree-dimensional features. Materials can also be assembled on the surlace of patternedSAMs to bui ld three-dimensional structures.

Patterned SAMs that consist of regions of bare gold and regions covered by SAMhave been used as templates for assembling polymeric structures [ 80] and metalstructures [ 50.151] with electrochemistry. and metal structures with chemical vapour

{b}

ia)

(d)(c)

356 P . F . N E A L E Y E ' T A I , .

mask

) snrr,r\ go td

exposure tometastable Ar*damagesSAM

Figure 11.8 The scheme represents exposure of self-assembled monolayers of dodecanethiol

( - 1 .5 nm th ick) on go ld (20 nm wi th 1 .5 nm of t i tan ium) to a bcam of metastab lc Ar . fo l lowed

by wet-chemica l e tch ing. Thc SEM p ic tures are o f the copper TEM gr id ( -10 nm th ick) used as

a mask ( in contact with the surlace of thc substrate). and the result ing fabricated gold structure.

deposi t ion [56.181-183] and e lect ro less deposi t ion [85] . The insulat ing proper t ies of

SAMs inhibit deposit ion in these processes in selected regions. Figure I L9(a) shows a

schematic of a procedure to pattern a si l icon structure with SAMs of CHr(CH:)r7SiCl3

by pCP with a flat stamp on a surlace with relief. Copper is deposited on regions not

covered with SAMs by chemical vapour deposit ion (Fig. l l .9b) | 831.

Other assembly processes take advantage of, our abilit-v-- to pattern SAMs with

different organic functional groups. Different regions of the substrate can be tailored to

have different surface free energies and different wettabilities. For example. a surfacepatterned with SAMs terminated by a methyl group and SAMs terminated b-v a carboxygroup has regions that are extremely hydrophobic (CH.) and hydrophil ic (COOH).

respectively. When such a surlace is exposed to water vapour at low temperatures or at

high humidity, water droplets preferential ly condense on the hydrophil ic regions (see

Fig. I 1.2). These droplets or 'condensation l igures' have been used to image patterned

SAMs [ 58], as sensors for monitoring humidity and temperature [85]. and as optical

diffraction gratings I I 8a].Figure 11.10 shows a schematic of a process to fabricate sol id three-dimensional

structures based on the different wetting properties of patterned SAMs. Hydrophilic

regions are formed on a gold surface by microcontact printing with HS(CH.)r 5COOH.All other regions of the gold surface are made hydrophobic by immersing the sample

into a di lute solut ion of HDT in ethanol. The sample is then slowly pul led from aprepolymer liquid (for example. UV-curable polyurethanes. Norland Optical Adhesives

60, 6l or 8l ) through an interface into air. The prepolymer l iquid preferential ly wets

I

i

,S i

I etching removesI gold in regionsI where SAM wasv daraged by Ar*

M I C R O . A N D N A N O F A B R I C A T I O N T E C H N I Q U E S 357

'rtrrttnttttttttnrtrrmrzzttff(il3",iifffri1r,",.

in hexane

sio2(> 1000 nm)

ffiffiffi

.,@

I microcontact printing

<- SAM (2-3nm)

Figure 11.9 (a) Schemat ic out l ine o f thc procedure used fbr pat tern ing a lky ls i loxancs on a non-p lanar subst ra te . Nuc leat ion and depos i t ion o f coppcr on l ) ' occurs on those reg ionsunder ivat ized b-v thc se l f -asscmbled monolaver in the fo l lowing process of se lect ive CVD. (b)

Scanning clectron micrograph of a microstructure of copper fabricatcd bv CVD on a substratewhose surlacc was derivatized by a monolayer of octadecyltr ichlorosi lane. The specimen hasbeen fractured in cross-section to reveal the copper morphology and coveragc - here -200 nm.Copper deposits only in recessed regions not covered by alkylsi loxane (the tops of the r idges).

the hydrophi l ic reg ions of the sur face. assumes a shape to min imize i ts f ree energy. and

does not adhere to the hydrophobic reg ions. The s t ructure is so l id ihed by cur ing the

polymer in place by exposure to UV l ight . The smal lest structures we have fabr icated in

this way have lateral d imensions of -3 pm, and r ise above the plane of the sample by afew microns. Optical waveguides with lengths as long as 2-3 cm have been fabricated

us ing a s imi la r p rocess [185] . F igure l l . l l (a ) and (b ) shows these wavegu ides incross-sect ion, and Fig. I l . l l (c) shows the mult imode output f rom a waveguide.

Hydrocarbon liquids can also be assembled on hydrophobic regions of patterned

SAMs by pul l ing the substrate f rom the l iquid through an interface into water ! 59,1861( instead of a i r ) . Again, the process of assembly is control led by the minimizat ion of f ree

energies. In the case of assembling the polyurethane prepolymers on patterned SAMs,the final structures rise higher above the surface when labricated on hydrophobicregions in water than when labricated on hydrophil ic regions in air because the surfacetension of water is greater than the surface tension of a i r . Figure I l . I I (d) shows anatomic force micrograph of an array of polymeric lenses [ 59] that were assembled onhydrophobic regions of a patterned SAM in water. The pattern in this case was circles(- l0 prm diameter) of SAMs of HDT in a background of SAMs of HS(CH,), ,OH.

As discussed earlier, palladium colloids can be patterned by pCP on a variety ofsurfaces [75]. Figure l l . l2 shows a schematic of the procedure for patterning thecolloids, and an SEM image depicting the scale and perfection of the copper structures

358 P . F . N E A L E Y E T . 1 1 , .

SAM of-s(cH2)1scH3 Patterned Substrate

SAM of-s(cH/15cooH

1-2 nm SAM40 nm Ag3 n m T i

Immerse in liquidPull from liquid into airat 2 mm/min

Prepolymer liquidonly wets hydrophilicregions

Prepolymer Liquid

Polymer, max. thickness I ;.rm Curewidth 50 um

Cross-Section(not to scale)

1-2 nm SAM40 nm Ag3 n m T i

Substrate

Figure 11.10 Schematic of the process of assembl ing l iquids on patterned sel f -asscmblcdmonola;-ers (SAMs). Thc l iquid prel 'erent ia lh 'wets thc hi 'drophl ' l ic rcgions (carboxy'- tcrminatedSAM)of the surface. and de-wets f iom the hy'drophobic rcgions (nicth) l - terminated SAM) ofthe surt-ace. Liquid prepolymcrs are cured af icr assembl l ' to fabr icatc sol id thrcc-dimcnsionalstructures.

that can be labr icated by using the col lo ids as templates for e lectroless deposi t ion of

copper.

4.3 Micromoulding in capillaries (MlMlC)

Patterning techniques are wel l establ ished for semiconductors and metals. but arerelat ively underdeveloped for organic polymers. wi th the notable except ion of the

special ized polymers used in photol i thography. Methods to pattern polymers based on

the preferent ia l wett ing of pat terned SAMs by l iquid polymer precursors were discussed

above. Micromoulding in capi l lar ies (MIMIC) [ 87] is another technique to patternpolymers that uses many of the mater ia ls descr ibed above. but achieves patterning bycomple te ly d i f fe ren t p r inc ip les . F igure 11 .13 shows a schemat ic o f the MIMIC process .

An elastomeric stamp is placed in conformal contact wi th a substrate to form a mould

that consists of a network of channels. A low viscosi ty polymer precursor is placed in

contact with openings to the network. and the channels fi l l b-v capil lary action. Aftercur ing the polymer precursor. the elastomeric stamp is removed. The patterned

M I C R O - A N D N A N O F A B R I C A T I O N T E C H N I Q L J E S 359

50 pm

o p m(d )

F igu re 11 .11 (a . b ) Scann ing c l cc t ron m ic rog raph (SEM) i n i agcs o l ' po l vmc r i c wavcgu idcs t ha twerc fabr icatcd b1 assembl ing prepol l 'mcr I iqu ids (Nor land Opt ica l .Ac lhes i re 60. po l l ,ure thane)

on pat tcrned se l [asscmbled monolavcrs (SAMs) (as in F ig . l l .9 ) . (c ) Pro. jcc t ion o f thc laseroutput f iom a war, 'eguidc l cm in length. (d) AFM imagc (constant fbrcc nrodc) of an arra;- ofpo l l 'u rc thane lenscs that were asscmbled on a pat tcrned SAM.

polymer does not adhere to the s tamp. and remains on the suppor t . Under cer ta in

condit ions. the patterned polymer was subsequentl ;- detached from the substrate toproduce a free standing l i lm. Figure I l . l4 shows examples of pol l 'meric structures thatwere labricated using MIMIC.

As a method to produce polymeric microstructures. MIMIC ! 87] holds severaladvantages over conventional photol i thography. Photol i thography requires the use ofspecial ized polymers and photosensit izers: MIMIC is appl icable to most polymers withlow viscosity. Photol i thography requires three steps: ( i) spin coating to form a f i lm: ( i i )patterning (by exposure to l ight): and ( i i i ) developing exposed (or unexposed) regions tofabricate microstructures. MIMIC forms and patterns polymeric microstructures in asingle step. In photol i thography. the polymeric microstructure formed in each exposuremust be the same thickness: with MIMIC. patterning structures with mult iple thick-nesses is possible. Like the masks employed in conventional photol i thography. thestamps used as templates for MIMIC can be reused.

4.4 Micromachining of electrodes

Gold electrodes were labr icated using a combinat ion of micromachining and SAMs

[ 34]. In one procedure, SAMs of HDT were formed over the ent i re sur lace of a

360 P . F . N E A L E Y E T . 1 L .

PDMS

PDMS is exposed to a solutioncontaining Pd - Colloid

Selective electroless depositionof copper catalyzed try thedeposited palladium colloids

Pd_coroi,l --m

II Stamping onto the pretreated

I substrate transfers the coltoid

I to form the patternpd-conoid f I

Functionalized \siloxane laver + ?'m",m",wf(adhesion promorer)

| Substrate

I

Deposited Cu \

ru - uuffuru -r , # .A,rE,

Funct ionat izedJl Substrate Istloxane laver

(a)

F igure 11.12 (a) Schemat ic i l lus t ra t ion o f the proccss to pat tern sur faces wi th Pd co l lo ids wi thmicrocontact pr in t ing. The Pd co l lo ids are used to cata l l ' se the c lect ro lcss depos i t ion o f Cu onspec i f ic reg ions of the subst ra te . (b) Scanning e lect ron micrograph (SEM) image of Cu s t ructurcsthat were labricated with the proccss described above.

5**


PDMSmaster

Cut off ends

I er""" on a substrate

J cure; Remove PDMS

Place a drop of prepolymerat one end

Channels f i l l bycapillary action

Polymer

F igure 11 .13 Schemat ic d iagram o1 'cap i l lan 'mic romould ing o f a pa t tc rn o1 'para l le l .rcctangular channels. I t is not necessar l ' fbr both ends of thc channels to be open: even i l 'oneend is c losed. the channcls l i l l wi th thc prcpolvmcr. The trapped air sccms to cscape bydi f fusing through the pol l -d imeth;- l s i loxanc (PDMS) mastcr.

gold-coated substrate. and bare gold was exposed in patterns of l ines on the substrate b1,

scratching the surf 'ace using the t ip of a surgical scalpel b lade. Large area (>0.1 mmr)

arrays of band type electrodes ( l pm wide) were labr icated in th is wa) ' : the exposed gold

forms the electrochemical ly ' act ive surf-ace of the electrode. In a second procedure.

SAMs of HS(CH.).OH were deposi ted on the gold surface. l ines were micromachined.

and the exposed gold was der ivat ized with HDT. The regions of gold covered with

HS(CHr).OH were subsequent ly etched with an aqueous solut ion of CN- saturatedwith O- ' . and the gold wires that remained were micrometre wide. cent imetre long. and100 nm thick. Paired electrodes with electrochemical l l ' act ive areas as smal l as100 nm x I pm were labr icated from these structures by machining a I pm gap into thesupported gold wire (st i l l covered with an electr ical ly insulat ing SAM of HDT). Theexposed gold on ei ther s ide of the gap is electrochemical l l . act ive.

362 P . F . N E A L E Y E 7 - . 1 L .

a)(,7 tF:fr,d

F.l

iflirlri:ff$Figure 11.14 Pat terns labr icated b l ,micromould ing in cap i l la r ies (MIMIC) . Pat tcrns weresput tered wi th go ld before imaging by 'scanning c lcc t ron microscopv. The po lymer uscd wasphotocured po l i r (meth1 ' lmethacry la te) . (a) An ob l ique imagc of a f rac turcd samplc showingrectangular s labs of po lvmer on a go ld f i lm suppor ted on Si /S iO. . (b) Imagc (captured at 30")o fmore complex pat terns on a s i l i con wafer . (c ) A f iec-s tanding f i lm: thc same h ln t as in (b) wasrc leased f rom the sur lace b-v- d isso lv ing the SiO. la1,er in NHrF/HF. The fo ld ing in th is reg ionof the sample was acc identa l . but i l lus t ra tes the f lex ib i l i t l - o f the f i lm. (d) A f rcc-s tandingpolymeric structure labricated in channels formed b1- conformal contact of two polydimeth;- l

s i loxane (PDMS) masters l ikc those in (a) . us ing an or ienta t ion in which the grooves of thc twomasters were perpendicular. Thc two la-v. 'ers of the channcls lbrmed one interconnectedpolymer ic s t ructure ( inset ) . [Repnnted wi th permiss ion f iom Kim E. X ia Y. Whi tes ides GM.l {a tu r c 1995 : 376 : 581 . copy r i gh r ( 1995 ) Macmi l l an Magaz incs L im i t cd . l

Conclusions and future directions

Molecular self-assembly is a useful strategy for micro- and nanoflabrication. andpotential ly for other appl icat ions in molecular electronics. Equil ibr ium structures areformed that are at. or close to. thermodynamic minimum [6.7]. These structures tend tobe self-heal ing and defect reject ing [5].and the size of the structures can be in the rangeof l- l00nm. a range di l icult to address with current labrication techniques. Inpart icular, systems that by definit ion have at least one dimension of molecularscale - self-assembled monolayers - have demonstrated applications in microelectron-ics and micro- and nanofabrication that include passivation of surflaces. use of SAMsfor ultrathin resists and masks. directed deposition of materials on surlaces patternedwith SAMs. and the use of surlace functionality for sensors.

Several issues remain to be resolved, however. before SAMs find real applications inmicroelectronics. Alkanethiolates on gold are the most developed system of SAMs: thepotential applications and techniques for fabrication that we described are based on


this system. Unfortunately. gold is incompatible with si l icon processingt2Tl. and SAMsof alkanethiolates on gold are not sulficientl5r thermally stable for many applications.New systems of SAMs need to be developed that are more compatible with currentprocesses and materials for the fabrication of microelectronic devices. At present, thedensity of defects in metal structures formed by chemical etching using patterned SAMsas resists is too high for applications in microelectronics. although it is acceptable formany optical systems. The formation and distr ibution of these defects must beunderstood to reduce their numbers. To take ful l advantage of SAMs as ultrathin resistsor to use SAMs to labricate quantum devices ( lateral dimensions less than 50 nm

il881). techniques to pattern SAMs at sub-5O-nm scales ( in the plane of the monolayer)must be developed. SAMs are patterned routinely by microcontact print ing

[85,I 32.176] with features of micron dimensions. and these structures form the basis oflabrication techniques that are immediately suitable for appl icat ions in optics

[ 59.185] and b iotechnology [ 89- l9 l ] . i f not in microelect ronics.

.'lcknov'ledgements We thank Dr Emmanuel Delamarche and Dr Hans Biebuyck forproviding the images in Fig. I l . l . The research was supported in part by the Off ice ofNaval Research. ARPA and NSF (PHY 9312572). This work made use of MRSECShared Faci l i t ies supported by the National Science Foundatron (DMR-9400396).J.L.W. grateful ly acknowledges a postdoctoral fel lowship from the NIH.

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