Featuresof gold havingmicrometerto centimeterdimensions
can be formedthrougha combinationof stampingwith an elastomeric
stampand an alkanethiol"ink" followedby chemicaletching
Amit Kumarand GeorgeM. Whitesidesa)
02138
Haruord Uniuersity,Departmentof Chemistry,Cambridge,Massachusett.t
(Received16 February 1993;acceptedfor publication22 July 1993)
This letter describesa techniquethat can be used to produce well-definedfeaturesof gold' The
monolayer (SAM) on a gold substrateusing
technique involves patterning of a self-assembled
an elastomer stamp (fabricated either from a phenol-formaldehyde polymer or
followedby selectiveetchingin an aqueous,basicsolutionof cyanideion
polydimethylsiloxane),
(lM KOH, 0.1 M KCN). Electrically conductivestructuresof gold
dioxygen
and dissolved
small
as 1 pm have been produced using this procedure. Once a rubber
as
with dimensions
patterning
and etching of gold substratesis straightforward. This method is
stamp is fabricated,
convenient,does not require routine accessto clean rooms and photolithographic equipment,
and can be used to produce multiple copiesof a pattern.
This report describesa simple method for the fabrication of patternsof gold, supportedon silicon substrates.A
monolayer
rubber stamp is usedto patterna self-assembled
(SAM) on a supportedgold film. The patternedsubstrate
is etchedin an aqueous,basic solution of cyanideion and
dissolveddioxygen.lr Featureshaving dimensionsas small
as I pm and as large as severalsquare cm have been produced through this technique.This processhas the advanthe desiredstampscan
tage of simplicity and convenience:
be fabricated either lithographically or nonlithographically, and once the stamp is available,multiple copiesof
the patterns can be formed using extremely straightforward experimentaltechniques.
The rubber stamps were made by two methods. Large
featureswere made using standardlithographicfabrication
proceduresavailableas commercialservices(Logan Stamp
Works, Boston, MA). The desired pattern was photographed, and a positive slide was used as a mask. The
output of a medium pressuremercur!' arc lamp was prol e cte d t hr ough t he s l i d e o n to a N o v o l a c re s i n (a phenol to rm aldehy depoly m e r. H e rc u l e s .Wi l mi n g to n , D E ). R egions that were exposed to the ultraviolet radiation
d i ssolv edin a m ild s o a p s o l u ti o n .T h e re ma i n i ng,unexposedpolymer was mounted on a basefor stamping.Features as small as 200 pm were produced using this relatively imprecisephotolithographicprocess.Currently, the
u p p e rlim it in s iz eis d i c ta te db y th e s i z eo f th e s ta mp that
can be produced (30 cm x 40 cm ) by the commercially
a va i lableM er r igr ap h l 2 l 6 P h o to p o l y m e rEx p o s ureU ni t
(He rc ules .W ilm ing to n , D E ).
I nk padsf or s t a m p i n gw e re p re p a re dh v m o i steni nga
pieceof lint-freepaper with a 0.1-l mM solution of hexadecanethiolin either ethanol or diethyl ether. It was inrportant to keep the ink pad as free of lint as possible.
solution was poured diAlternatively,the hexadecanethiol
rectly onto the stamp. Ink formulations using other solvents such as tolueneor iso-octaneproducedfeatureshaving poorer resolution. Afier inking and stamping, the
"'.A.uthor to whom corresDondence should be addressed
2002
substratewas immediatelyimmersedin the etch solution
describedpreviously'."
patternsformed using
Figure I depicts representative
The versatility
polymer.
resin
Novolac
stampsmade of the
evietching-is
by
of this procedure-stamping followed
features
that
dent from the range in size and detail of the
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
reproducedin gold supportedon silicon.
Stampsfor small (1-100 prm) featureswere fabricated
from polydimethylsiloxane(PDMS); Fig. 2 describesthe
process.The masters were made lithographically or were
F I G . i . P h o t < t g r a p hos i l a r g e A u f e a t u r e sp r o d u c e d u s i n g t h e N o v o l a c
s t a m p s .( a ) - ( d ) C o m p l e x a n d s i m p l e f e a t u r e sc a n b e e a s i l vp r o d u c e d .
T h e s er e p r e s e n t a t i r lee a t u r e ss h o r vt h e d e g r e eo f i m p e r f e c t i o nt h a t r e s u l t s
from the use of commercialh rrreparedstamps.
. 1 9 9 3 0 0 0 3 - 6 9 5 1 / 9 3 / 6 34()1/ 2 0 0 2 / 3 / 5 6 . 0 0 c 1 9 9 3A m e r i c a nI n s t i t u t o
ef Physics
A p p l .P h y s .L e t t .6 3 ( 1 4 ) , 4O c t o b e r
2002
a)
a) 1' 2pm
+ $ h o tc re s i s tp a tte rn
tl-
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Cold (50-200 nm)
supPorted on lO nrn
Ti adhesionliyer.
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ccmmercrallv available templates,such as transmrssron
electron microscopy grids. The template was placed in a
p l as t icpet r i dis h. A l 0 :1 (v :v ) mi x tu re o f PD M S-S yl gard
Silicone Elastomer 184 and Sylgard Curing Agent I 84
(Dow Corning Corp. Midland, MI) was poured into the
petri dish. It was not necessaryto put the mixture of
PDMS-elastomer and curing agent under vacuum to remove dissolveddioxygen. The PDMS cured at room temperature in the laboratory ambient for 30-60 min. This
'C
for approxcure was followed by additional curing at 65
imately I h or until the polymer was rigid. After cooling to
room temperature, the PDMS was carefully peeled from
the master.
Figure 3 shows some representative scale features
formed using stamps made of PDMS. We have fabricated
severaltypes of features having dimensions ranging from I
pm to severalhundred pm. Figure 3(a) shows a fracture
profile of 2 pm wide lines of gold on silicon. Figure 3(b)
shows a microelectrode array. The gold microelectrodes
are2.5 pm wide. Figure 3(c) showsa regular array of gold
squareson silicon. The squaresare I 5 pm on a side. Such
arrays with squaresand rectangleshaving dimensionsfrom
l0 to 80 pm have been fabricated. The regularity of the
array was maintained over severalcentimeters.The micro2003
Appl.Phys.Lett.,Vol. 63, No. 14, 4 Octobell993
FIG. -1. SE micrographs of variclus t,\'pesi)f- teatures produced using
r u b b e r s t a m p i n ga n d c h e m i c a le t c h i n g .( a ) F r a c t u r e p r o f i l er t f a s e r i e so f
g o l d l i n e s s u p p o r t e do n a s i l i c o n w a f e r . T h c * r d t h o f t h e g o l d l i n e s i s 2
p m , a n d t h e t h i c k n e s so f t h e g o l d i s 2 0 0 0 A . ( b ) e l e c t r o d e s( 2 . 5 p m i n
w i d t h , s e p a r a t e db y 1 p m s p a c e s )i n a m i c r o e l e c t r o d ea r r a y ' .T h e c o n t a c t
p a d s a r e n o t v t s i b l e i n t h i s f r a m e a t t h i s m a g n i f i c a t i o n ,( c ) A r e g u l a r
array of squaresof gold supportedon silicon. The gold squaresare 15 pm
on a side. T'hesearrays exhibited such regularitl' txer several square
centimeters.
graphs in Fig. 3 illustrate that many types of featurescan
be produced using this technique.
Figure 4 illustrates one method for increasingthe versatility of the stamping method. One stamp consisting of
parallel lines was usedto stamp the samepattern twice: for
the second pattern, the stamp was rotated through some
angle relative to the first. By multiple impressions,one
stamp can be used to produce several types of features.
This procedureis analogousto a procedure used in photolithography, where a photoresistis exposedmultiple times
through the same mask in different orientations.5
Rubber stamping followed by chemical etching, at its
current state,is a method for rapid prototyping of /rm-cm
scalefeatures.Lithography using rubber stamps has several advantagesover conventionallithographictechniques
A. Kumarand G. M. Whitesides
2003
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FIG. 4. SE micrographsof featuresproduced through multiple stamping.
(a) These features were produced using a stamp consisting of parallel
l i n e so f 2 p r m w i d t h . T o o b t a i n t h e f e a t u r e si n ( b ) a n d ( c ) o n e s t a m p w a s
used to stamp the samepattern twice. To preparethe patterns in (b) and
( c ) t h e s t a m p i n ( a ) w a s u s e d t w i c e , w i t h t h e s e c o n di m p r e s s i o na t a n
a n g l e o f 4 5 ' ( b ) a n d 9 0 ' ( c ) r e l a t i v et o t h e f i r s t .
2004
and/or cl eanr oonls.
that requi reextensi vcrnstrLtmentati on
The primary advantageis the easewith which featurescan
be produced.Once a substratei s arai l abl e,a stampcan he
preparedfor use, and multiple impresslonscan be made
w i thout requi nngaddi ti onalphotol i thographl ' .
One of thc:probl emsu' i th thi s methodi s the relat ivelv
i neffi ci entmethodof del i very'of the thi ol to the surt aceol'
the substrate.S i nce the del i verr-of thi ol to the sur f ace
requiresphysicalcontact betweenthe stamp and the surface, some regionsbecomeblurred due to ihe unevenness
of the surfacetopology and due to mechanicalmovement
of the stamp during the stampingprocedure.Someregions
do not becomcderivatizedif contact is not made between
stamp and surface.Subsequentetching producesblurred
'Ihe
number densityof thesedefects.
featuresor etch pits.
however,is small enough that lines extendingseveralcm
are conductiveand parallel unconnectedlines are electrically isolated.
We havedemonstrateda lithographicmethodlbr rapid
prototyping of simple structureson a scalecomparableto
The method incorporatesthe useof
UV photolithography'.
a sel ecti vechemrculetch rn combi nati onw i th a technique
n' i th rr S A M on a mi cromcterscale.J'he
to patternsurti rces
p r i m a r r l d r a n t a s c o l ' t h r s m e t h o di s t h e e a s eo f p r e p a r a ti on of the stanrpand the desi redfeatures,and i ts c apabili ty for produci ng mul ti pl e copl esof the desi redpat t er n
w i th onl y one. i ni ti al l i thographi cstep.
This researchwas supportedin part by the Office of
Naval Research and the Defense Advanced Research
ProjectsAgency.
Appl. Phys.Lett.,Vol. 63, No. 14, 4 October.1993
' A . M . B r i t t a n .A n r S c i . 6 2 . 1 0 2 ( 1 9 1 4 ) .
r R . J . I ' u t l c i c p h u.r tI ' l r t ' C h e n r s t r ov J G t t l d( F . l s c v i e A
r . m s t e r d a m .1 9 7 8 ) .
. t r t l o n r r r r t (i i \ \ ' i l k r n r o r - t..l r l r u n c t ' dI n o r q u n i cC h e m i . s t y . 4 t he d .
I r . A ,C
( W i l e l. \ e n Y o r k . 1 ' ) l i O )
tA
Kunrar. l{ llrchrrrcka
. nd \. L. Abbott. J. Am. Chem.Soc.114,
9 1 8 8( t q q 2 )
'\.\'.
M Mcrreau, St'ntit'onductor Lithography: Principles. Practices and
t V o t e r i a l s( P l e n u m . N e w Y o r k . 1 9 8 8 ) .
A. Kumarand G. M. Whitesides
2004