MICROEIECTRONIC ENGINEERING - Harvard …gmwgroup.harvard.edu/pubs/pdf/498.pdfReprinted from MICROEIECTRONIC ENGINEERING An Intemcrtionql Jou-rnal ol Semiconductor Mcu'ruloctu-ring

Reprinted from

MICROEIECTRONICENGINEERINGAn Intemcrtionql Jou-rnal ol Semiconductor Mcu'ruloctu-ring Technologry

Pattern

Micrcrelectronic Engineering 32 (1996) 755-268

transfer: Self-assembled monolayers as ultrathin resists

Younan Xia, Xiao-Mei Zhao, George M. Whitesides*

Department of Chemistr-y', Hurturd Univ'ersin', Cambridge, MA 02138. USA

ET^SEVIER

ELSEVIER

MICROEIESTn$frcENGIN@ITG

Microelectronic Engineering 32 (1996) 2-55-268

Pattern transfer: Self-assembled monolayers as ultrathin resists

Younan Xia, Xiao-Mer Zhao, George M. Whitesides*

Depurtntent o.l' Chemisnt. Hun'urd Uniyersin'. Cambritlge. MA 02l3ll, USA

Abstract

This review includes three sections: ( i) preparation. structure. and propert ies of self ' -assembled monolayers (SAMs); ( i i )techniques fbrpatterning SAMs, including microcontact print ing (pCP). UV-photol i thography, and c-beam writ ing: and ( i i i )use of patterned SAMs as ultrathin resists (2-3 nm thick) in processes fbr pattern transf 'er based on selective chemicaletching and selective deposit ion. Microcontact print ing is a non-l i thographic technique tbr torming patterned f 'eatures withdimensions >100nm; the init ial product of patterning is organized monolayers of alkanethiolates on Au, Ag, Cu and GaAs,and of alkylsi loxanes on Si/SiO. and glass. In this technique, an elastomeric stamp having a surface patterned with a rel iefstructure is used to generate patterned SAMs on the surfhces of solid materials. Thesc pattcrned SAMs are resists that protectthe underlying substrates from dissolut ion in selective etchants (for exarnple, fbr evaporated thin f i lms of Au and Ag,aqueous solut ions of K,S,O., K,Fe(CN),- and K.,Fe(CN),,). Patterned structures of gold or si lver that are produced by thecombination of pCP and selective etching can be used as the secondary masks fbr subscqucnt proccsses such as isotropicetching of SiO,, isotropic or anisotropic etching of Si, anisotropic etching of GaAs. and reactive ion etching (RIE) of Si.Patterned SAMs can also be used as templates for select ive deposit ion o1' nretals by chenrical vapor deposit ion (CVD),electroplat ing, or electroless deposit ion.

Kevworcls: Self ' -assembled monolayers (SAMs); Pattern transf 'er: Ultrathin resists

1. Introduction

Lithography at nanometer scales (<l00nm) requires resists thin enough that they give f-eatureswith reasonable aspect ratios between the horizontal and vertical dimensions. Traditional photoresistshave been used as very thin films []. More recently, self'-assembled monolayers (SAMs) 12-41havebegun to be explored for applications in micro/nanofabrication [5,6].

Self'-assembled monolayers are highly ordered molecular assemblies that form spontaneously bychemisorption of functionalized long-chain alkanes on the surf.aces of appropriate solid materials (Fig.1(a)), their structures are effectively 2-D organic crystals or quasicrystals l2l. Self-assembledmonolayers have, in the past, been extensively studied as model systems for wetting, adhesion,lubrication, corrosion, protein adsorption, and cell attachment 12-11. Well-established systems of

+ Corresponding author. Email : gwhitesides @ gmwgroup.harvard.edu.

0167-9317 1961.$15.00 Copyr ight O 1996 Elsev ier Sc ience BV. A l l r ights reservedS . S D 1 ; 0 t 6 7 - 9 3 1 7 ( 9 5 ) 0 0 l 7 4 - 3

2-56 Y. Xiu et u l . I Mir ' rute lectroni t En,qi r ret , r in,q . j l t 1996) ) -5-5_ )6g

a)

b)

?N

I

head group, X

alkyl chain, (CH2)n

t'-

\--{ , \-l---'1

-*l F0.288 nm $(\

9$s

l l* l -10nm l*

F i - e ' l . ( a ) R c p r e s e n t a t i o t l 0 1 ' a i r i g h l - v - o r c l e r e t l r n o n o l a y c r o 1 ' a l k a n c t h i 0 l a t e . ( X ( C H . ) , , S ) . o n t h c s u r l a c c o l ' A u ( l l l ) . ' t ' h c

head g roL rp . X . a l l ou ' s the su r t ' acc p roper t i cs o l ' t hc n rono laye r to bc con tn r l l e t l : i t s th i ckncss can be changec l b r chang ing thet lun lbe r ' r t , o l ' n l c thv lenc g roups in the po l l n re th - r l ene ch l i n . Thc a l kv l cha ins a rc . on thc a rc ragc . t i l t cc l appru r i rna te l y 30 , -f rom the no rn la l t o the su r f i t ce o f Au . (b ) Represen ta t ion o l ' a c lense lv -packec i SA\ { o1 'a l kane th ie l l t c on thc su r l ' ce .1 ,aco l l o ida l pa r t i c le o f go ld w , i t h a d iamete r o f - l 0 nn t .

SAMs include alkanethiolates on Au. Ag. and Cu [2-4]: ancl alkylsi loxanes on hydroxyl-tcrrninatedsur faces ( fbr example, Si /S iO. , A l /A l ,O, , g lass, mica. and p lasma-t reated polymers) I -1 .71. Less-characterized systems of SAMs include: alkanethiolates on GaAs ancl InP [8,9]. Less-characterizedsystems of SAMs inc lude: a lkaneth io lates on GaAs and InP I8.91: a lkanesul f inates on Au t l0 l ,a lky lphosphines on Au t l l l : a lky l groups d i rect ly bound to Si I l2 l : a lcohols . amines and isoni t r i les onPt [4 ,131; carboxyl ic and hydroxamic ac ids on A-e,O or Al .O. V14l : a lky lphosphatcs on ZiO.I l5 , l6 l ; and a lky lphosphonic ac ids on ITO l l7 l . New systems are s t i l l be ing devel .pecl .

Self ' -assembled monolayers as ultrathin resists provide several potential advantage s. ( i) Self-assembly leads to equi l ibr ium structures that are at. or close to. thermodynamic mininrum. As a result.

Y. Xiu et ul. I Mitroelectrottic Engineerin,q -12 ( 1996) 2.5.5-268 257

self-assembling structures tend to be self ' -heal ing and defect-reject ing. ( i i ; The relat ively lowsolid-vapor intertacial free energies of methyl- and f luoroalkyl-terminated SAMs al low them to behandled outside clean-room faci l i t ies without irreversible contamination. ( i i i ) SAMs can be used asvery thin passivating or insulat ing f i lms for control l ing adsorption of impurit ies on surfaces; they maybe useful in fabricatin-s capacitors and molecular electronic devices I l8-201. A typical SAM ofhexadecaneth io late (CH.(CH.)r .S ) on Au is -2.5 nm th ick. and rhe th ickness of the SAM can becontrol led to within 0.1 nm in most cases by control l ing the number of carbon atoms in the alkyl chain(Fig. l(a)).( iv) SAMs can act as ultrathin resists in l i thographic processes. Because SAMs are so thin,some concerns (for example, depth of tbcus: optical transparency in UV and VUV regions, shadowingand undercutt ing) that currently inf luence the pertbrmance of photoresists in hi-eh-resolut ion imagingprocesses are not important in SAMs. The avai labi l i ty of nanometer thick resists also opens the doorto new l i tho-eraphic techniqr,res. For exanrple, in a process that has been suggested that uses metastableargon as the pattern-generating species. the thickness of the resist must be less than -3 nm, since thedamage in the resist by contact with the metastable atom is l imited to a surt-ace layer of <0.5 nm thickt2l l . (v) Fabrications involving SAMs are relat ively low-cost cornpared with conventional l i thog-raphic methods.

2. Preparation, structure, and stability of SAMs

SAMs are usually prepared by immersion of sol id substrates in solut ir-rns containing species reactivetoward the surface, or by exposure of the solid to vapurs of reactive species 12-41. For example,highly-ordered SAMs of hexadecanethiolate on gold can be prepared by immersing a gold substrate ina -2 mM solut ion of hexadecanethiol in ethanol for several minutes. Ordered SAMs have also beenformed on the sur taces of co l lo ida l par t ic les wi th d iameters o l ' ) l50nm (Fig. l (b) ) 122.231. Theprocess of spontaneous formation of an urdered structure that occurs as adsorption and reaction of athiol on gold is an example of molecular self-assembly 1211.

The structure of SAMs has been characterized by transmission electron dif fract ion [25], polarizedinfrared external ref lectance spectroscopy (PIERS) t3l, low-angle X-ray scattering t261. low-energyhelium dif fract ion 1271. and scanning probe microscopy [28]. I t is general ly accepted that long-chainalkaneth io lates have r V: x V3R30 over layer s t ructure on rhe sur facc of Au( l l l ) (F ig. l (a) ) .

Different types of SAMs havc dif ferent stabi l i t ies towards heating and chemicals. SAMs ofalkanethiolates on Au become disordered and/or decompose at elevated temperatures (-100'C);oxidation of alkanethiolates to alkanesulfonates in the presence of UV l ight and ozone also reducestheir stabi l i t ies 13,29,301. In contrast, some SAMs of alkylsi loxanes on Si/SiO. are stable up to-450"C u 8l.

3. Formation of patterned SAMs

Lateral control of SAMs has been achieved by a variety of patterning techniques. Classicaltechniques such as UV-photol i thography [6,3 ]-33] and e-beam writ ing t3a-361 have been used forproducing patterns in SAMs of alkanethiolates on Au. Ag and GaAs, and of alkylsi loxanes onSi/SiO",; new techniques, such as microcontact print ing (pCP) 137-401. atomlithography [21],

258 Y. Xia et al. I Microeler:tronic En,qineerins 32 ( l996) 2-5-j-268

microwrit ing [41], micromachining (using an STM tipft2l or sharp stylus t43l) ancl micromolding incapillaries (MIMIC) t44l have also been developed for generating patterned SAMs. Patterned SAMscan be imaged and visuahzed using a number of techniques, such as SEM [38], AFM [45], SIMS andcondensation figures (CFs) 146).

Among these techniques for generating patterned SAMs, microcontact printing is the one thatseems to off'er the most interesting combination of convenience and new capability (Fig. 2).Microcontact printing involves direct pattern formation by contact of an elastomeric stamp with asurface: this technique forms SAMs when used with appropriate reactants and surfaces [37-40]. Itprovides superior control over the surface chernistry. It is a parallel process - that is, it fbrms the

+- Photoresist(0 .4 -1 .5 pm)

1) Pour PDMS over master2) Cure at 65 oC for -10 h

1) Peel off stamp2) Apply " ink" solut ion

<+ Alkanethiol

+Th in f i lm o f Au(20-1000 nm)

Microcontact print

l-

M (2-3 nm)

F t " . l . l lDepos i tselectivetV | |

sete'cfivelV

t f

Fig. 2. Schematic procedure f i l r carrying out pr.CP of alkanethiols on Au. An elastonrcric stamp was fabricated by castingpoly(dimethylsi loxane) (PDMS)against certain rel iel 'structure that was macle by photol i thography 6r micrgmachining. Afterapplying hexadecanethiol solut ion in ethanol. the stamp was dried in a stream of N,. and bnrught into contact with thesurface of gold fbr 5-10 s. A patterned SAM of hexadecanethiolate was fbrmed on the parts of the eold surface that were incontact with the stamD.

PDMS

S i

Y. Xiu et al. I Microelec'tronic' En,qineering -12 (1996) 25-r-268 259

pattern over the entire area of the substrate in contact with the stamp at the same time - and thus issuitable for patterning of areas of several cm2 on a single contact. At present, pCP has been used topattern SAMs of alkanethiolates on Au 131-101. Ag l4ll, Cu t48l and GaAs l49l and ofalkylsi loxanes on Si/SiO, and glass 1501. Features with dimensions larger than 0.3 pm can beroutinely produced by prCP; smaller features (-l00nm) have also been fabricated with greaterdit frculty and lower rel iabi l i ty [5l-531. The lower l imit fbr the resolut ion in this technique, the upperlimit of the area that can be patterned on one contact. and the degree to which multiple impressionscan be brought into registration, have yet to be established.

4. Pattern transfer from SAMs to the underlying substrates

Patterns in SAMs can be transfered to the underlying substrates either by selective etching or byselective deposit ion (for example, CVD. clectroplat ing and electroless deposit ion).

4.1. Puttern trunsfer bt' selec'tive etc'hins

Table I summarizes selective etchants that have been studied for use with patterned SAMsgenerated by pCP. The most extensively studied systems are patterned SAMs of hexadecanethiolateon evaporated f i lms of Au l3l-401 and Ag t4l l .Our original work focused on the system ofhexadecanethiolate on Au, but hexadecanethiolate on Ag is also attractive. Etching protocols for silverare more convenient to use than those for gold - silver is chemically more reactive than gold, and thussilver dissolves more rapidly than gold in most etchants; the level of defects in SAMs on Ag seems tobe lower than that on Au; silver is an excellent electrical and thermal conductor with useful propertiest541. Fig. 3(a) shows scanning electron micrographs (SEM) of a test pattern of silver (50 nm thick)that was fabricated by pCP with hexadecanethiol. followed by selective chemical etching [171. Theedge resolut ion of these si lver features is <20 nm (Fig. 3(b)). The nanometer thick SAMs probably

Table ISelectrre etchants (al l in H.O) that have been used with patterned SAMs

Surf'ace SAM Etchant (approximate pH ) Ref.

Au

Ag

CuGaAss i / s i o ,glass

RS

RSRSR S i O , / . "RS iO . , , , "

RS

K . S , O , / K . F e ( C N ) , , / K , F e ( C N ) o ( l 4 )K C N / O . ( r 4 )c s ( N H , ) 2 / H . O . ( l )

Fe (NO. ) . ( 7 )

K . S , O , / K , F e ( C N ) o / K r F e ( C N , l , , ( 7 )NHIOH/K ,Fe (CN) " /K *Fe (CN, ) , , ( l 2 )N H + O H / H . O . ( t 2 )

N H + O H / O . ( I 2 1K C N / O , ( 1 4 )F e C l . / H C l ( 1 )NH.1OHNF/NHrF (par t ia l ly se lect ive)HF/NHrF (par t ia l ly se lect ive)

38. 603l -1160/ 1 1a t

11. 6047A 1

1 1

1 '7+ t

'tu3 1 . 1 934, -5050

"These SAMs are fbrmed by contact o f RSiCl . or RSi (OCH.) . w i th the subst ra tes

260 Y. Xiu et ul. I Micrrrclectronit' Engineering, -12 ( t996) 25.j-26g

protects the underlying substrates from dissolution by blockin-e the diffusional access of etchants. Fig.3(c) is a cross-sectional SEM of silver microstructure (0.2 pm thick) that was generated by thecombination of pCP and selective chemical etching. Large si lver l ines (-50 pm wide. -0.2 pm thick,and separated by -50 pm) on Si/SiO, procluced by this method have the meral l ic conductivi tyexpected for bulk metal (5.56 X l0s S/cm); and paral lel l ines o1-si lver are electr ical ly isolated fromeach other.

The ability to generate arrays of microstructures of coinage metals with controlled shapes anddimensions is directly usef ul in fabricating arrays of microelectrocles f or sensors ancl otherelectrochemical devices. These patterned f l lms of Au and Ag can also be used as the secondary masksfor the etching of underlying layers of SiO. and Si t47.551 (Fig. 3(d)). The t 'eatures shgwn in Fig. 3were generated in the open laboratory, without access to clean room facilities. Bccause proccsses forformation of SAMs rely on molecular self-assembly, they resist clef-ects ancl contamination.Microcontact print ing can, in principle. be used for many micro/nanofabricatisn tasks in low-costprocesses. The quality of the patterns produced are not yet compatible u,ith that required formicrofabrication of complex electronic devices, but the technology is str l l in an early stage of

cw

1F;

Wt : I ,

ffid ffi

:

W

ffiffi

ffi5Fm

Fig. 3 . (a , b) SEMs of a tes t pat tern o f Ag ( -50nnr th ick) that was rec luced by se lect i rc e tch in-u l i r r - l -5 s in an aqucousso lu t i on con ra in i ng K .S .O , (0 .1M) . K .Fe (CN) , . ( 0 .01 M) and K*Fe (CN, l . , t 0 .001 M l . us ing a pa r r c rned SAM o fhcxadecanthiolate as the resist. The bright regions are si lver. the dark regrons are Si/SiO- where thc unclerivatized si lver hasbeen removed bi ' etching. (c) A cross-sectional SEM 0f a test pattcrn that was f 'abricatecl in Ag f i lms (0.2 prn thick) using thecombination of pCP and selective wet etching in the above etchant for - '15 s. (d) SEM of a si l iggn pattern that was f irrmeclby an isot rop ic e tch ing of s i l i con in KOH/ i -propanol a t 65 'C f i r r - l0min us ing a pa lerned s i lver l i lm as n task. Thc s i lverrnask that was fbrmed by pCP and selective etching remainecl on the surtace ol ' the Si substrate.

-

100 nm

Y. Xitt et ul. I Microelectrrnit' Etrgineering _32 1tt)r)61 2-i-5-268

development. I t is direct ly appl icable to many problems in fabricating sensors and oprical devices,where the requirements fbr continuity, isolation, and unitormity in the final patterns are less serious.

A potential advantage of pCP over photolithography is that pCP can generate features with a rangeof ditf-erent sizes using a single chrome-mask. PDMS is an elastomer. and conforms to the surf'ace ofthe substrate during pCP. This characteristic of PDMS provides an opportunity to generate f'eatureswith reduced sizes by physical ly manipulat in-r the stamps. Fig.4(b) shows an example in which areduction of features from -2 p^ to -0.2 pm was achieved simply by compressing the PDMS stampwhi le conduct ing pCP t5 l l . In a second example (F ig.4(c)) manipulat ion of the chemisrry offormation of the SAM provides another strategy lbr reduction in feature size: here a reduction from-2 pm to -0. I pm was accomplished by using control led reactivc spre ading of the hexadecanethiolon gold 1521.

Photolithography cannot be easily applied to curved surfaces fbr a number ef reasons. Because pCp

Ar/SAMttr

I

2 6 1

Compressivemicroprinting

AFM

10pm

Reactivespreading

30 nm0

30 nm0

Fig' -1. (a) SEM of a test pattern of golcl that was produccd by thc standard procedurc ol ' p( iP ancl selcct ive etching i1oxygen-saturated cyanide solut i t lns. (b) SEM of a test pattern ol 'golcl that was proth-rcecl with thc sanre PDMS stamp as in(a), but undcr mechanical conlpression. Size reduction fronr -2 p-nr t() -0.2 pnr uas achievecl in t lre one-cl irnensrgnalcompression. Two-dimensional compressic)l t wits also possible. (c) SEM o1'a test pattern o1'golcl that was procluced with thesame PDMS stamp as in (a). but print ing was carr ied under water and the starnp was al lowed to renrain in contact with thegold surface fbr -5 min. The reactive spreacl ing of ' hexadecanthiols l ionr the eclges of the sri l l r . lp caused a reduction indimension fbr the bare regions f l 'ont -2 pnt to -0. I prn.

2 p m

262 Y. Xiu et ul. I Microelectronic' Engineerinp -72 ( 1996) 2-t5-268

involves conformal contact using an elastomer stamp, it can be usecl to print patterns on even sharplycurved surfaces t56l.Fig. 5 shows SEM image of gold microstructures (Fig. 5(a)) and patternedSAMs (Fig. 5(c)) on curved surfaces. Microcontact printing generated patterns on planar and curvedsubstrates with similar resolution (Figs. 5(b) and (c)).

4.2. Pattern transfer by selective deposition

In addit ion to selective etching, select ive deposit ions of metals using CVD [57-591, electroplat ing[36], and electroless deposit ion [6,38] have also been accomplished with patterned SAMs astemplates. Fig. 6(a) shows an SEM image of Cu lines that had been deposited on a Si/SiO, surface byCVD using (hexafluoroacetylacetonato)copper(I)(vinyltrimethylsilane) as the source gas; the patternon the surface was defined and directed by u patterned SAM of octadecylsiloxane generated by pCPt581. Figs. 6(b) and (c) show SEM micrographs of microstructures of copper that were produced byselective CVD; the top surfaces of the substrates had been derivatized by SAMs of octaclecylsiloxaneusing contact print ing. Copper deposited only in the recessed regions unclerivatized by SAMs.Procedures based on surf.ace-selective CVD may be useful in microfabrication. For example. the

a) b)

CH cooH

50 pm rf,IrFig. 5. (a) SEM image of a gold pattern on a curved surface. The pattern was formed on a gold-cgated glass fiber by pCPwith hexadecanethiol and selective etching of gold. (b. c) SEM images of patterned SAMs f irrmecl by pCp on a planar and acurved surf 'aces. Light regions are covered with CH.-terminated thiolate (CH,(CH.),.S ); dark reglons are derivatized withCOOH-terminated th io la te (HOOC(CH,) , .S ) .

c)

planar

curved

Y. Xia et ul. I Microelectronic Encineerine -12 (1996) 2-5-5-268 263

CuI

SAM+

Fig.6. (a) SEM image of copper l ines produced by selective CVD on Si/SiO. surface that had been patterned with SAMs ofoctadecylsi loxane using pCP. (b, c) SEM images of copper microstructures that were fabricated on textured Si/SiO.surfaces; the top surface of the substrates were covered by SAMs of octadecysi loxanc dclrvered by a f lat PDMS stamp. Theside walls of the r idges in (b) and the 0.7 pm dri l led holes in (c) were f iee f iom the SAMs. Copper nucleated and depositedonly on those regions that were not derivatized by the SAM.

264 Y. Xiu er ul. I Mit'roeleL'trrnit En,qineering -12 (199(t) J-j5-l(tS

topology of the substrates (especially that in (c)) is directly related ro f'eed-throushs and relatedstructures in many rnicroelectronic devices.

5. Conclusions and future work

Self-assembled monolayers provide an effective ancl low-cost strategy firr pattern transf'er in micro-and nanofabrication. A variety of techniques have been developed fbr thc fbrmation of patternedSAMs. These patterned SAMs serve direct ly as nanometer-thick resists in protcct ing the underlyingsubstrates from dissolut ion in wet etchant [37 --40,4],601. Several issues rerlain to be solved. however.before these patterning techniques based on SAMs f ind real appl icat iclns in rnicrselectlrnics. First, thesmallest feature that can be produced by these patterning techniques has yet te be cstabl ished.Currently, using SAMs as resists, the srnal lest f-eature that has been generi i tcd by UV-photol i thggraphyis -0.4 pLm [6] . -25 nm for e-beam wr i t ing [34] , and -100 nm for microconract pr in t ine I52l Secgncl .the formation and distr ibution of def-ects in SAMs. especial ly under thc concl i t ions of chemicaletchin-e. must be undcrstood. Using thc combination ot ' pCP with hexadecancthiol and sclect ive wctetching, we have bcen able tt t generate pattterned, def-ect-free, si lver structu;es (50nm thick) with anarea of -0.4 mm' t4l .6l l . Third. the compatibi l i ty of thcse parrerning rechniques with the preduqienof microelectronic circuitry must be explored. The coinage metals (Au. Ag and Cu) thar are widclyused as substrates for thc fbrmation of SAMs of alkanethiolates cannot bc usecl in the production ofmicroelectronic devices based on si l icon. because these atoms dif f lse into the si l icon ancl act as traps162l .Systems that form SAMs di rect ly on semiconduuurs s t i l l mLrsr bc dcrc loped [121.

ln more general terms, however. SAMs ancl patterning by pCP i l lustrurtc a new appreach tgmicrofabrication. SAMs are representative self-assembling systents. ancl dcrnonstrate the attract ive-ness of self-assembly as a strategy f irr torming small . hi-uh-quali ty struct l l rcs with remarkably, l i t t le ofthe investment required by the more farni l iar clcan-roolrr technolosies r.rscd in micrclf l lbr icat ion. Inpart icular, the farct that self-assembling systems are therntodynantic nt inirngrn structu;es. ancl therefbretend to reject def.ects. Ir leans that problems such as part iculate cctntaminatign (with is Ll constantproblem in photol i tho-eraphy and other l ine-of '-sight techniques) wil l be lcss seriels preblems withSAMs (in which the SAM-forming component can spread spontaneously uncler rhe part icle) than inphotol i thography (where part icles cost shaclows).

Microcontact print ing i l lustrates the largely unexpkrrecl potential of non-1'rhotol i thr)eraphic pattern-ing techniques. This technique is capable, in a research sett in-t. of -eenerating l(X)- l(X) nm features: i tsl imitat ions after seri t lus development rernain to be clef ined. The elastonreric character ef the masterprovides both problems and opportunit ies in registrat ion. The capabil i ty f i rr large-area patterning bypCP is substantial, but st i l l remains to be developed.

Acknowledgments

This work was supported in part by ONR ancl ARPA. This work made use of MRSEC SharedFaci l i t ies supported by the National Science Founclat ion (DMR-9400396). We woulcl l ike to thankEnoch Kim. Andrew Black and Drs Paul Nealey, Jirn Wilbur. Andrew Zhuk for cgmmenrins on thispaper.

Y. Xiu er ul. I Microelectronic Engineerins, 32 ( 1996) 2-5.5-268 265

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(t992) 137-463.

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f6l J.M. Calvert, Lithographical ly patterned self ' -assembled f i lnrs. in: A. Ulman. ed.. Thin Fi lm.s.Vol. 20, Acadentic Press.Boston, 1995, pp. 109- l ,1 l .

[7f For recent studies. see A.N. Parikh. D.L. Al lara. I .B. Azctuz. and F. Rondelez. An intr insic relat ionship bctwccnmolecular structure in self ' -assembled n-alkylsi loxane monolayers and deposrt ion temperature. . / . Pftr '^s. Chen.98(1994 ) 1577 -7590 .

IU] C.W. Sheen. J.-X. Shi, J. Martensson, A.N. Parikh and D.L. Al lara. A new class of organrzecl self-assembledmonolayers : a lkane th io ls on GaAs (100) . .1 . Amer . Chem. Sot ' . l14 (1992) l5 l -1- l -51-5.

t9l Y. Gu, Z. Lin, R.A. Butera. V.S. Smentkowski and D.H. Waldeck, Prepart ion ol ' sclf-assenrbled r lonolavers on InP.Lansmu i r l l ( 199 -5 ) 18 .19 -18 -51 .

I l0 l J .E. Chadwick. D.C. Myles and R.L. Garre l l . Se l f -assembl l 'o f su l f inate monolavcrs on -uo ld : Nerv membrane minter ics .J. Amer. Chent. 5or' . 115 ( 1993) 1036-+- 1036-s.

I l l l K . Uvdal . I . Person and B. L iedberg. Tnc l 'c lohcxy lphosphine absorbed on go l r l . I t tngmtr i r l l (1995) l2-52-12,56.[12] M.R. L in fbrd. P. Fenter . P.M. E isenberger and C.E.D. Chidscy ' . A lky l monoluvcrs on s i l icon prepared t iom l -a lkanes

and hydrogen-rerminared s i l icon, J . Amer . Chem. Sor , . l l7 (199-5) 3145-315.5.I l3 l J .J . H ickman. P.E. La ib in is . D. I . Auerbach. C. Zou. T.J . C]ardner . G.M. Whi tcs ides and M.S. Wr i -qhton. Tou 'ard

orthogonal sclf-assembly of redox active molecules on Pt and Au: Selective rcaction ol ' disulf ide with Au andisocyanide wi th Pt . Lungmuir 8 (1992) 3-57-3-59.

I l4 l J .P. Fo lkers . C.B. Gorman. P.E. La ib in is , S. Buchholz . G.M.Whi tes ides and R.G. Nuzzo. Sel f -assenrb led rnonolayers o1 'long-chain hydrorarn ic ac ids on the nat ivc ox ides c l f meta ls . L tmgntu i r l l (1995) 813-81-1.

t l5 l G. Cao, H. -G. Hong and T.E. Mal louk. Layered meta l phosphates and phosphonates: FRonr crvs ta ls to monolayers ,At 'c. Chem. Rr,.r. 25 (1992) 420-421.

[16] H.E. Katz , Mul t r layer depos i t ion o l 'nove l organophosphonates 'nr i th z i rcon iurn( lV ) . C l tent . Muter .6 (1994) 2227-2232.[17] T.J. Gardner. C.D. Frisbie and M.S. Wri irhton, Systems t irr ortho-sonal self-assenrbl i 'of electroacrive monolayers on Au

and ITO: an approach to molecular elccrronics. . / . Amer. Chem. Srrr. . l17 (199-5 ) 6921-6933.[18] P. Fontaine. D. Gogeuenheim. D. Dcresncs. D. Vai l laume, M. Garet ancl F. Rondelez. Octaclecyltr ichlorosi lane

monolayers as u l t ra th in gate insu la t ing f i lms in meta l - insu la tor -semiconductor dev ices. Appt . Pht , .s . Let t .62 (1993)2256-2258.

[19] A.S. Mar t in and J .R. Sambles. Molecu lar rect i f ie r . Phr ' .s . Rer ' . Ler r .70 (1993\ 218-221.[20i D.R. Jung and A.W Czanderna. Chernical and physical intcractions at metal/sel l-assenrblecl organic nronolayer

interf'aces. Critit'ul Reurar'.s in Solid Stute untl Muteriul.s St'ience.s l9 ( 199-+) l--5.1.[ 2 l l K . K . B e r g g r e n . A . B a r d . J . L . W i l b u r . J . D . G i l l a s p y . A . G . H e l g . J . J . M c C l c l l a n d . S . L . R o l s t o n . W . D . P h i l l i p s , M . P r e n t i s s

and G.M. Whi tes ides. Micro l i thography us ing neut ra l metastab le a toms and sc l l -assernb led monolavers . St ' ient 'e ,269( 1995) t255-1257.

l22l C. Weisbecker and G.M. Whitesides, Langmuir. to appear.l23l M. Ueda, K. Kudo and K. Ichrmura. Photochromic behavior of a spirobenzopvran chemisorbed un a col loiclal si l ica

su r face . J . Mu te r . Chem.5 (7 ) ( 199 -5 ) 1007 - l 0 l l .[24] G.M. Whitesides, J.P. Mathias and C.T. Seto, Molecular self ' -assembly and nanochemistry: A chemical strategy lbr the

syn thes i s o f nanos t ruc tu res , Sc ien t ' e 254 (1991 ) l 3 l 2 -1319 .t25l L. Strong and G.M. Whitesides, Structures of self-assembled monolayer f i lms ol 'organosulfur compounds aclsorbed on

gold single crystals: electron di l ' l iact ion studies, Lurtgntuir 4 ( 1989) 5'+6--5-5t3.

266 Y. Xia et ul. I Mic'roelec'tronic' En,qineering j2 ( 1996) 2-5-5-2(t8

t26l N. Camil lone II I . C.E.D. Chidsey, G.-Y. Liu and G. Scoles. Substrate depenclence ot ' the surl i ice srrucrure ancl chainpack ing of docosy l mercaptan se l f -assembled on the ( l l l ) , ( l l0 ) , anct (100) f i rces o1 ' s ing le crysra l so ld . . / . Chent .Pftr ' .r . 98 (1993) 1234-4245.

t27l N. Carni l lone II I . C.E.D. Chiclsey, G.-Y. Liu, T.M. Putvinski ancl G. Scoles. Surtace structure and thermal motion of 'r r -a lkanc th io ls sc l f ' -assembled on Au( I I l ) s tud ied by low energy he l iurn c l i f f rac t ion. .1 . Chem. P l t ts . 94 ( lgg l )11,193-8502.

[28] C.A. A lves, E.L. Smi th and M.D. Por ter . Atomic sca le imaging of a lkaneth io la tc monolavers a t go ld sur face wi thatcrmic fbrce microscopy. J. Anrcr. Chem. sor' . l14 (1992) 1222_122]' .

l29l A. Ulman, Thermal stabit i ty of Langntuir-Blodgett and sel l-assentbled f l lnts: A possible scenarig f irr grder-cl isordert rans i t ions. Adv. Murer . 3 (1991) 29t t -303.

[30] J. Huang and J.C. Hentmingcr. Photooxiclat ion of ' thiols in self-assembled nronolavers on golcl. .1. Anter. Chem. Sot ' .115 ( 1993) 3312-33:13.

[3 l l N ' I .J . Tar lov . D.R.F. Burgess Jr and G. Gi l len UVphotopat tern ing of a lkaneth io la te monolayers assembled 11n go ld anc ls i lver , J . Amer . Chem. Soc ' . l l5 (1993) -530-5- -5306.

l32 l J . Huang. D.A. Dahlgren and J .C. Hemminger . Photopat tcrn inc o f sc l f -asscntb led a lkaneth io la tc prgnolayers en gs ld : as imple n lonolayer photoresrs t u t i l i z ing aqueous chcnr is t r l ' . L tn ,qr r t r r i r l0 (199-1) 616 6 lB.

t33 l K.C. Chan, T. K i rn . J .K. Schoer and R.M. Crooks, Polymer ic se l l -assenrb lec l r19n9l1 \crs . -1 . pat tc ' rn t ransf 'er by use ef 'photcrl i thography. electrochemical methods. and an ultrathin. self-assemblecl diacetvlenic rcsists. .1. Arrtt ' r . Cltem. Sor..l l7 ( t99-5 ) 5875--5U76.

l3 ' t l M. Lerce l . R.C. T iber io . P.F. Chapman. H.G. Cra ighead. C.W. Sheen. A.N. Par ikh ant l D.L. A l la l r . Se l t -assemblec lmonolayer electron beant resist on CiaAs and SiO., J. Vrtc. St. i . Technol. B l l ( l9t)3r l tJl3-21t28.

t35 l G. Gi l len ' S. Wight , J . Bennet t and M.J. Tar lov . Pat tern ing of se l f -assemblec l a lkancth io l monola l ,c rs on s i lver hvmicrofbcus ion and electron beam bombardment, Appl. Ptn.t. Lett.65 ( lg9-l) 53-l-.536.

[36] J .A.M. Sondag-Huethors t . H.R.J . van Hel leput te and L.G. Fokk ink. Generat ion u t e lec t rochemica l lv c lcpes i tcc l l re ta lpat terns by means of e lect ron beam (nant ) ) l i thography of se l l -asscnth lec l monolaver res is ts . Appt . Pht . r . L t , t t .64 f I99- l t285-287.

t37l A. Kumar and G.M. Whitesides, Features o1'gold having micronreter to cenrinretcr i l imc-nsit)ns can be f irrr led threugh acombination of stamping with an elastonteric stamp and an alkanetiol " ink" t i r l lowed br chemical etching. Appl. plt .s.Letr. 63 ( 1993) 2002-2001.

t38 l A. Kumar ' H.A. B iebuyck and G.M. Whi tes ic les . Pat tern ing se l l -asscnrb led monolarcrs : Appl icat igns in pater ia lsc i ence , Lungmu i r l 0 ( 1994 ) 1498 - l 5 l l .

[39] J 'L . Wi lbur . A. Kumar. E. K im and G.M. Whi tes ides, Microcontact pr in t ing o1 'se l f -asscmblec i monolayers : A f lex ih lenew techniquc fbr microfabrication, Adv. Muter. 6 0gg4) 600-60-1.

t10l A. Kumar, N.L. Abbott, E. Kim. H.A. Brebuyck and G.M. Whiresides. Patterned sclt-asserlhled monolavers andmeso-scale phenomena. Acr.. Chent. Re.i . 28 ( 199-5 ) 219_226.

[ .11] A. Kumar. H.A. B iebuyck. N.L. Abbot t and G.M. Whi tes idcs. The use of se l l - r isser lb led rnonolayers and a se lect iveetch to generate patterned gold f 'eatures. .1. Amer. Cherrt. Sot.. I 14 (1992) 9188-9199.

[42] C.B. Ross. L . Sun and R.M. Crooks. Scanning probe l i thography. l . Scanning tunnc l ing micrescgpe ipc lucedl i thography o1 'se l f -assentb led n-a lkaneth io l rnonolayer res isrs . L tn ,Enui r 9 (1993) 631- t r36.

l43 l N.L. Abbot t , J .P. Fo lkers and G.M. Whi tes ides, Manipu la t ion u f the wet tab i l i t l ,o l 'sur t 'aces on the 0.1- l - rn ic rontererscale through micromachining and molecular self-assembly, Sc' ience 257 |gg\ 13lt0- 13U2.

t44l E. Kirn. Y. Xia and G.M. Whitesides. Polymeric structures lbrrned by molcl ing in capi l lar ies. Nrrlrrr.e 376 (1995)58 r -58.1.

[4-51 J .L . Wi lbur , H.A ' B iebuyck. J .C. MacDonald and G.M. Whi tes ides. Scanning t i r rce nr ic roscopies can image parrernedself-assembled monolayers, Lungmuir l l ( 199-5 ) 82-5-l l3 I .

[ ' t6l G.P. Lopez, H.A. Biebuyck. C.D. Frisbie and G.M. Whitesicles. Imaging of f 'eatures on surfaces by condensarion f igures,Science 260 (1993) 617-619.

l l l l Y. Xia' E. Kirn and G.M. Whitesides, Microcontact print ing of alkanethiols on si lvcr and i ts apl icat ion inmicrofabrication, .1. Eler:trochem. Sor.. , 143 (1996) 1070_1079.

[48] Y. X ia , E. K im, M. Mrks ich and G.M.Whi tes ides. Chem. Marer . S (1996) 601-603.t19 l E. K im and G.M. Whi tes ides. unpubl ished resu l ts .

Y. Xin et ul. I Microelec'tronic' Engineerins 32 ( 1996) 2-55-268 261

t50l Y. Xia, M. Mrksich, E. Kim and G.M. Whitesides, Microcontact print ing of octadecylsi loxane on the surf 'ace of si l icondioxide, and i ts appl icat ion in microfabrication, J. Amer. Chem. Sor' . , l17 (199-5) 9516-9571.

[51] Y. Xia and G.M. Whitesides, Reduction in the size of t 'eatures of patterned SAM generated by microcontact print ingwith mechanical compression of the stamp. Adv,. Mater. T (1995) 1l l-473.

t52l Y. Xia and G.M. Whitesides, Use of control led reactive spreading of l iquid alkanethiol on the surface of gold to modifythe size of f 'eatures produced by microcontact print ing, J. Arner. Chem. Sr;r: . l17 (1995) 3274-3275.

[53] J.L. Wilbur, E. Kim, Y. Xia and G.M. Whrtesides. Lithographic molding: a convenient route to structures withsubmicron dimensions, Ady. Muter. 7 (1995) 649-652.

l-511 C. Kit tel, Introdut ' t ion to Solid Stute Pht 'sir :s.6th ed., John Wiley & Sons. Inc.. New York. 1986.[-55] E. Kim, A. Kumar and G.M. Whitesides, Combining patterned self-assenrbled ntonolayers of alkanethiolates on gold

with anisotropic etching of si l icon to generate control led surf-ace morphologies. "/ . F, let ' trot 'hem. Sot ' . 142 (1995)628-633.

[56] R.J. Jackman, J.L. Wilbur and G.M. Whitesides. Fabrications of submicron f 'eatures on curved substrates bvmicrocontact printing, Sc:ience 269 (1995\ 661*666.

[57] S.J. Potochnik, P.E. Pchrsson. D.S.Y. Hsu and J.M. Calvert, Selective copper chemical vapor desposit ion usingPd-act ivated organos i lane f i lms, Lungrnu i r 11 ( 1995) 18.11-1845.

[58] N.L. Jeon. R.G. Nuzzo, Y. Xia, M. Mrksich and G.M. Whitesides, Patternecl self-assemblecl monolayers fbrmed bymicrocontact print ing direct select ive metal l izat ion by chemical vapor deposrt ion on planar ancl non-planar substrates.Lctngmuir, to appear.

[59] M.J. Hampden-Smith and T.T. Kodas. Chemrcal vapor deposit ion of nrctals: Part 1. An overview of CVD processes,Chern. Vtp. Depo.sir ion 1 (1995) 8-23.

t60l Y. Xia. X.-M. Zhatt. E. Kim and G.M. Whitesides. An etching solut ion for use with patterned self-assernbledmonolayers of alkanethiolates on Au. Ag. and Cu. Chem. Mtf ier., 12 (1995) 2.332-2337.

l6 l l Y . X ia , X. -M. Zhao and G.M. Whi tes ides. unpubl ished resu l ts .t62l J. Li, T.E. Seidel and J.W. Mayer, Copper-based metal lrzation in ULSI structures. Muter. Re.s. Sor' . Bul letrrr XIX(tt)

0994 ) r5 - t 8 .

Younan Xia was born in Jiangsu. China. in 1965. He rece' ivcd a B.S. in chemical physics fromUnivers i ty o f Sc ience and Technology"of China (USTC) in 1987. anc l then worked at F i j ian Ins t i tu teof Research on the Structure of Mattcr. Academia Sinica. l i rr fbur years. He earned a M.S. inchemist ry f rom Univers i ty o f Pennsv lvan ia (wi th A lan G. N{acDiarmid) in 1993. He is now a Ph.D.student working under Prof 'essur Gcorge M. Whitesides at Harvard University. His rnterests includeself-assembled monolayers, micro/nanofabrication. nanomaterials. conducting polymers. inorganicnonlinear optical materials. and the mechanrsm of conrbustion of sol id propellants.

Xiao-Mei Zhao was born in Bei j ing. China. in 1969. She received A.B. and M.S. degrees inPhysical Organic Chemrstry t iom Unrversity of Rochester in 1991 and 1993. Her research work atUniversity of Rochester fbcused on Langmuir-Blodgett f i lms. She is now working under Prof-essorGeorge M. Whitesides on self--assembled monolayers. optical waveguides. micro- and nano-f abrications.

ffi.,,;fi%i',i-*

268 Y, Xiu et al. I Microeler:tronit. Engineering 32 ( l996) 2-5-5_26g

(ieorge M. Whitesides was born August 3, 1939 in Louisvi l le. KY. He receivecl an A.B. clegreefiorn Harvard University in 1960 and a Ph.D. f iom the Cali fbrnia Inst i ture of Technology (with J.D.Roberts) in 1961. He was a ntember of the faculty of the Massachusetts Inst i tute of Technokrgyfbrm 1963 to l9ft2- He joined the Department of Chemistry of Harvard University in lgg2. ancl wasDepartment Chairman (1986-l9tt9). He is now Mall inckroclt prof-essur of Chemistry at HarvardUniversity. Present research interests inclucie biochemistry. surfacc chemistry. materials science.molecular virology, optics, self-assembly ancr organic synthesis.