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Page 1: Features of gold having micrometer to centimeter ... · Features of gold having micrometer to centimeter dimensions can be formed through a combination of stamping with an elastomeric

Features of gold having micrometer to centimeter dimensionscan be formed through a combination of stamping with an elastomericstamp and an alkanethiol " ink" fol lowed by chemical etching

Amit Kumar and George M. Whitesidesa)Haruord Uniuersity, Department of Chemistry, Cambridge, Massachusett.t 02138

(Received 16 February 1993; accepted for publication 22 July 1993)

This letter describes a technique that can be used to produce well-defined features of gold' The

technique involves patterning of a self-assembled monolayer (SAM) on a gold substrate using

an elastomer stamp (fabricated either from a phenol-formaldehyde polymer or

polydimethylsiloxane), followed by selective etching in an aqueous, basic solution of cyanide ion

and dissolved dioxygen (lM KOH, 0.1 M KCN). Electrically conductive structures of gold

with dimensions as small as 1 pm have been produced using this procedure. Once a rubber

stamp is fabricated, patterning and etching of gold substrates is straightforward. This method is

convenient, does not require routine access to clean rooms and photolithographic equipment,

and can be used to produce multiple copies of a pattern.

This report describes a simple method for the fabrica-

tion of patterns of gold, supported on sil icon substrates. A

rubber stamp is used to pattern a self-assembled monolayer(SAM) on a supported gold fi lm. The patterned substrate

is etched in an aqueous, basic solution of cyanide ion and

dissolved dioxygen.lr Features having dimensions as small

as I pm and as large as several square cm have been pro-

duced through this technique. This process has the advan-

tage of simplicity and convenience: the desired stamps can

be fabricated either l i thographically or nonlithographi-

cally, and once the stamp is available, multiple copies of

the patterns can be formed using extremely straightfor-

ward experimental techniques.The rubber stamps were made by two methods. Large

features were made using standard l ithographic fabricationprocedures available as commercial services (Logan StampWorks, Boston, MA). The desired pattern was photo-

graphed, and a positive slide was used as a mask. The

output of a medium pressure mercur!' arc lamp was pro-

lected through the sl ide onto a Novolac resin (a phenol-

tormaldehyde polymer. Hercules. Wi lmington, DE). Re-gions that were exposed to the ultraviolet radiation

dissolved in a mi ld soap solut ion. The remaining, unex-posed polymer was mounted on a base for stamping. Fea-

tures as small as 200 pm were produced using this rela-

tively imprecise photolithographic process. Currently, the

upper l imi t in s ize is dictated by the s ize of the stamp that

can be produced (30 cm x 40 cm ) by the commerciallyavai lable Merr igraph l2 l6 Photopolymer Exposure Uni t(Hercules. Wi lmington, DE).

Ink pads for stamping were prepared hv moistening apiece of l int-free paper with a 0.1-l mM solution of hexa-

decanethiol in either ethanol or diethyl ether. It was inr-portant to keep the ink pad as free of l int as possible.

Alternatively, the hexadecanethiol solution was poured di-rectly onto the stamp. Ink formulations using other sol-vents such as toluene or iso-octane produced features hav-ing poorer resolution. Afier inking and stamping, the

substrate was immediately immersed in the etch solution

described previously'."Figure I depicts representative patterns formed using

stamps made of the Novolac resin polymer. The versati l i ty

of this procedure-stamping followed by etching-is evi-

dent from the range in size and detail of the features that

can be produced. Any feature having dimensions greater

than 200 pm can be photographed, prepared as a rubber

stamp using routinely available commercial services, and

reproduced in gold supported on sil icon.

Stamps for small (1-100 prm) features were fabricated

from polydimethylsiloxane (PDMS); Fig. 2 describes the

process. The masters were made lithographically or were

" ' .A .u thor to whom cor resDondence shou ld be addressed

FIG. i . Phot<tgraphs oi large Au features produced using the Novolac

s tamps . (a ) - (d ) Comp lex and s imp le fea tu res can be eas i l v p roduced .

These representat i re leatures shorv the degree of imperfect ion that resul ts

from the use of commercialh rrrepared stamps.

2002 Appl . Phys. Let t . 63 (14) ,4 October .1993 0003-6951/93/63(1 4) /2002/3/56.00 c 1993 Amer ican Inst i tu te o f Phys ics 2002

Page 2: Features of gold having micrometer to centimeter ... · Features of gold having micrometer to centimeter dimensions can be formed through a combination of stamping with an elastomeric

a) 1 '2pm + $hotcresistpattern

ccmmercrallv available templates, such as transmrssronelectron microscopy grids. The template was placed in aplast ic petr i d ish. A l0:1 (v:v) mixture of PDMS-SylgardSil icone Elastomer 184 and Sylgard Curing Agent I 84(Dow Corning Corp. Midland, MI) was poured into thepetri dish. It was not necessary to put the mixture ofPDMS-elastomer and curing agent under vacuum to re-move dissolved dioxygen. The PDMS cured at room tem-perature in the laboratory ambient for 30-60 min. Thiscure was followed by additional curing at 65 'C for approx-imately I h or until the polymer was rigid. After cooling toroom temperature, the PDMS was carefully peeled fromthe master.

Figure 3 shows some representative scale featuresformed using stamps made of PDMS. We have fabricatedseveral types of features having dimensions ranging from Ipm to several hundred pm. Figure 3(a) shows a fractureprofi le of 2 pm wide l ines of gold on sil icon. Figure 3(b)shows a microelectrode array. The gold microelectrodesare2.5 pm wide. Figure 3(c) shows a regular array of goldsquares on silicon. The squares are I 5 pm on a side. Sucharrays with squares and rectangles having dimensions froml0 to 80 pm have been fabricated. The regularity of thearray was maintained over several centimeters. The micro-

a)

I er .n ins inI c N - o .I

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FIG. -1. SE micrographs of variclus t,\ 'pes i)f- teatures produced using

rubber stamping and chemical etching. (a) Fracture prof i le r t f a ser ies ofgold l ines supported on a s i l icon wafer. Thc *rdth of the gold l ines is 2pm, and the th ickness of the gold is 2000 A. (b) e lectrodes (2.5 pm in

width, separated by 1pm spaces) in a microelectrode array ' . The contactpads are not vts ib le in th is f rame at th is magni f icat ion, (c) A regular

array of squares of gold supported on sil icon. The gold squares are 15 pm

on a side. T'hese arrays exhibited such regularitl ' txer several squarecent imeters.

graphs in Fig. 3 illustrate that many types of features can

be produced using this technique.Figure 4 illustrates one method for increasing the ver-

satility of the stamping method. One stamp consisting ofparallel lines was used to stamp the same pattern twice: forthe second pattern, the stamp was rotated through someangle relative to the first. By multiple impressions, onestamp can be used to produce several types of features.

This procedure is analogous to a procedure used in photo-lithography, where a photoresist is exposed multiple timesthrough the same mask in different orientations.5

Rubber stamping followed by chemical etching, at itscurrent state, is a method for rapid prototyping of /rm-cmscale features. Lithography using rubber stamps has sev-eral advantages over conventional l i thographic techniques

I t l- ArkanerhiolCold (50-200 nm) isupPorted on lO nrn 1 Stamprng cnlo

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2003 Appl. Phys. Lett. , Vol. 63, No. 14, 4 Octobell993 A. Kumar and G. M. Whitesides 2003

Page 3: Features of gold having micrometer to centimeter ... · Features of gold having micrometer to centimeter dimensions can be formed through a combination of stamping with an elastomeric

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FIG. 4. SE micrographs of features produced through multiple stamping.(a) These features were produced using a stamp consisting of parallell ines of 2 prm width. To obtain the features in (b) and (c) one stamp wasused to stamp the same pattern twice. To prepare the patterns in (b) and(c) the stamp in (a) was used twice, wi th the second impression at anangle of45' (b) and 90' (c) re lat ive to the f i rs t .

that require extensivc rnstrLtmentat ion and/or c lean roonls.The primary advantage is the ease with which features canbe produced. Once a substrate is arai lable, a stamp can heprepared for use, and multiple impresslons can be madewithout requinng addi t ional photol i thographl ' .

One of thc: problems u' i th th is method is the relat ivelvinef f ic ient method of del ivery ' of the th io l to the surtace ol 'the substrate. Since the del iverr- of th io l to the surfacerequires physical contact between the stamp and the sur-face, some regions become blurred due to ihe unevennessof the surface topology and due to mechanical movementof the stamp during the stamping procedure. Some regionsdo not becomc derivatized if contact is not made betweenstamp and surface. Subsequent etching produces blurredfeatures or etch pits.

' Ihe number density of these defects.

however, is small enough that l ines extending several cmare conductive and parallel unconnected l ines are electri-cally isolated.

We have demonstrated a l ithographic method lbr rapidprototyping of simple structures on a scale comparable toUV photolithography'. The method incorporates the use ofa select ive chemrcul etch rn combinat ion wi th a techniqueto pattern surt i rces n ' i th rr SAM on a micromcter scale. J 'hepr imar r ld ran tasc o l ' th rs method is the ease o f p repara-t ion of the stanrp and the desired features, and i ts capabi l -i ty for producing mult ip le coples of the desired patternwith only one. in i t ia l l i thographic step.

This research was supported in part by the Office ofNaval Research and the Defense Advanced ResearchProjects Agency.

'A . M . B r i t t an . Anr Sc i . 62 . 102 (1914) .rR . J . I ' u t l c i cphur t . I ' l r t 'Chenrs t r v oJ G t t l d (F . l scv ie r . Amste rdam. 1978) .I r .A, . Ctr t lon rr r r t i ( i \ \ ' i lkrnror- t . . l r l runct 'd Inorqunic Chemi.sty.4th ed.( W i l e l . \ e n Y o r k . 1 ' ) l i O )

tA Kunra r . l { l l r ch r r r ck . and \ . L . Abbo t t . J . Am. Chem. Soc . 114 ,9 1 8 8 ( t q q 2 )

'\.\'. M Mcrreau, St'ntit'onductor Lithography: Principles. Practices and

tVoter ia ls (Plenum. New York. 1988).

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2004 Appl. Phys. Lett. , Vol. 63, No. 14, 4 October .1993 A. Kumar and G. M. Whitesides 2004