-
Extending Microcontact Printing as aMicrol ithograph ic
Technique
Younan Xia and George M. WhitesidesDepartment of Chemistry and
Chemical Biology, Harvard University,
Cambridge, Massach usetts 021 38
The ACS Journal of Surfaces and Colloids
Reprinted fromVolume 13, Numb er 7 , Pages 2059-2067
-
Introduction
Microcontact printing (lCP)l is a very convenient,
non-photolithographic technique that can generate patternedfeatures
of self-assembled monolayers (SAMs)2 on bothplanar and nonplanar
surfaces.s The concept of pCP isquite straightforward. An
elastomeric stamp (usuallymade from poly(dimethylsiloxane), PDMS)
is fabricatedby casting a prepolymer ofPDMS against a master
whosesurface has been patterned with a complementary
reliefstructure using photolithograph/ or micromachinings orfrom a
commercially available relief structure such as adiffraction
grating.o When carrying out prCP on Au, aPDMS stamp is wetted with
an *ink" (typically, arrt -2mM solution ofhexadecanethiol in
ethanol) and is broughtinto contact with the surface of gold for
b-10 s. Thehexadecanethiol (HDT, CH3(CH2)IsSH) transfers from
thestamp to the gold, forms a hexadecanethiolate (CHs-(CH2)1ES-),
and generates patterns of SAMs on the surfaceof gold.
One of the important applications of pCP has been informing
patterned SAMs to be used as ultrathin resists.TSAMs are remarkably
effective as primary resists incontrolling the etching of the
underlying substrates.Patterning of gold with a hydrophobic,
long-chain SAM(typically, derived from HDT), followed by
selectivedis solution of the underiva tized,gold in chemical
etchants(usually an aqueous solution containing
IGSzOy'IeFe-(CN)6/IqFe(CN)o or KCN/O2),8 produced patterned
fea-tures of Au on the surface; these gold features could be
617-495-9857. E-mail:
Lithography: Prirrciples and
2059
subsequently used as secondarymasks to define and directthe
etching ofthe underlying substrates of SiO2 and Si.e,10More
recently, microcontact printing has been extendedto form patterned
SAMs of alkanethiolates on silverlo andcopper, I 1, 12 SAMs of
alkyl sil oxane s on hydroxyl-terminatedsurfaces,ls-16 and Pd
colloids on SilSiOr.t1
Features of patterned SAMs with dimensions largerthan I pm can
be routinely produced using pCP. It ismore difficult to generate
features with sizes less than 1prm, primarily because fabrication
of the conespondingmasters in this range critically depends on the
availabilityof advanced microlithographic techniques (for
example,e-beam writing, deep IfV, and X-ray photolithography)that
are still in development.ls-2O Below 100 nm,
materialproperties-espeeially deformation of the
elastomericstamp-may limit feature size (or at least require
devel-opment of new materials optimized for this regime).
We are developing procedures that can extend thecapabilities
ofprCP into the submicrometer range withoutrequiring masters having
submicrometer-sized featuresthat have to be generated by less
readily availableadvanced lithographic equipment. Basically, our
strateryis to start with an elastomeric stamp havingfeature sizesof
2-4 pm (that is, easily made using routine photo-lithography) and
to frnd non-photolithographic methodsto produce patterned SAMs with
feature sizes srnaller
L220.Chem.
G.; Xia, Y.; Mrksich, M.; Whitesides, G.
Craighead, H. G. Appl. Phys. Lett. 1996, 68,
Langmuir 1997, 13, 2059-2067
Extending Microcontact Printing as a
MicrolithographicTechnique
Younan Xia and George M. Whitesides*
Department of Chemistry and Chem.ical Biology, Haruard
Uniuersity,Carnbridge, Massachusetts 02 1 3 8
Receiued September 27, 19968
^tbit papeT defcribes a number of approaches that have been
employed to reduce the size of
featuresofself-assembledmonolayers(SAMs)generatedusingmicrocontactpriniing(pCP).
In4CP,anelastomericstamp.is used to Plnt p^atteryed SAMs of
alkanethiolates on the surfaces of coinage metals and SAMs
ofalkylsiloxanes on SilSiOz. It is a convenient technique for
generating patterned'microstructures withfeature sizes > 500 nm.
_The capability of this technique cor-rld be extJnded to produce
features smallerthan 500 nm using the following approaches: (L) pCp
with mechanical deformation of the elastomericstamp-tha_t is, with
lateral c^oqapression or uniaxial stretching in the plane of the
stamp and with pressureperpeldicular to the plane-of the stamgi Q)
pCP with physical alternation of the elastomeric stamp-thatis, with
a stamp that has been swelled with a solvenl or a stamp whose
dimensions have been r^educedby extraction of an inert fi.ller;(3)
pCP with reduction in the size of?eatures resultingfrom processes
takingpla99 on the surface-that is, lateral reactive spreading of
hexadecanethiol on fold;
"na (+) pCP witfr
m u ltiple impressions on the same surface. The advantages and
disadvantages ofeach"approach are evaluatedand compared in this
paper.
r996,
Mater.
50743-7463(96)00936-5 CCC: $14.00 @ t99z American Chemical
Societv
P. J. AppL Phys. I*tt.
-
2060 Langmuir, VoL 73, No. 7, 1997
than those ofthe original stamps (preferably, in the range
-
Microcontact Printing
A) 0% and 0%
ffi..,.
B) 0cr6 and 17ara
C) 17a/o and 17a/o
D| 234/a and 23Yo
lpmFigure 2. SEM images of Ag patterns (50 nm thick) that
werefabricated with a PDMS stamp of parallel lines that had
beencompressed in the direction perpendicular to the lines.
Eachpattern was formed by printing twice with the direction of
linesrotated by 90' before making the second printing. The
firstnumber indicates the compressive strain in the
verticaldi_rection. The bright regions are Ag; the dark regions are
Siwhere the unprotected Ag has dissolved in an aqueous
ferri-cyanide solution.
Right now, the smallest features that we have generatedusing
this procedure are -200 nm in dimension; theaccuracy of this
procedure is limited by the mechanics ofthe system used to compress
the stamp laterally.
Since PDMS deforms isotropically under mechanical
+ffi,$s
.&wffi
ffi
ffi
Langmuir, Vol. 73, No. 7, 1997 206L
A) No Compreesion E
Figure 3. SEM images of Ag patterns (50 nm thick) that
werefabricated using microcontact printing with a PDMS stampthat
was under one-dimensional (A-C) and two-dimensional(D) compression.
The one-dimensional compressionwas in thevertical direction. The
bright regions are Ag; the dark regionsare Si where the unprotected
Ag has dissolved in an aqueousfericyanide solution.
compression, both the shapes and sizes of the featurespresent on
the surface of the PDMS stamp change in acontrollable manner. This
procedure can be easily ex-tended to more complex patterns. Figure
3 shows SEMimages of silver patterns that were fabricated using
thisprocedure with a moderately complex pattern that hasfeatures
with acute angles and nonuniform sizes. Thestamp was under one- and
two-dimensional compressionduring pCP, respectively. It is obvious
that the size ofcertain features of a test pattern with such a
complexitycan also be reduced using this procedure, while
retainingthe regularity in the original pattern.
This procedure for generating submicrometer-sizedfeatures is
general. It indicates a new type of microli-thography, in which
dimensions of selected regions ofthepattern can be made smaller or
larger by deforming thestamp mechanically. It also suggests a
potential route tothe fabrication of "smart stamps", that is,
stamps whosesurfaces can be deformed locally with certain
spatialresolution.
Replica Molding against a PDMS Mold underMechanical Compression.
The reconfigurated reliefstructures on the surface of a compressed
PDMS stampcould also be replicated into a second polymeric
materialand then used to cast new PDMS stamps.2a Figure 4outlines
the procedure schematically. The size offeaturesis reduced in the
lateral compression of this stamp. Thecompressed features are
replicated by molding an ultra-violet-eurable, liquid prepolymeric
polyurethane (PU)against the compressed stamp. This procedure is
itera-tive: a new PDMS stamp can be prepared, by castingagainst the
relief structures on the surface of the PUreplica, and then
compressed and replicated. Each time
ffiffi#
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-
2062 Langmuir, Vol. 13, No. 7, 1997
1 . 6 g m 2 [ m
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Microcontact Printing
1.4pm
lpmFigure 6. SEM images ofAu patterns (20 nm thick) that
werefabricated using microcontact printing of HDT with PDMSstamps
that were stretched alongthe direction of the lines (thestrain was
IVo, L00Vo, and 300Vo, respectively), foliowed byselective chemical
etching.
7A). The raised features on the PDMS stamp deformedin this
process of compression: the vertical dimensions ofthe raised
features decreased while the lateral dimensionsincreased. As a
result, the lateral dimensions of therecessed regions decreased
(Figure 78 and C).
This approach does not reduce the period ofthe pattern;it can
only be used to reduce the lateral dimensions of thebare regions of
gold (that is, regions not derivatized withSAMs). Much effort is
required to measure and controlthe vertical pressure; therefore,
this procedure cannotgenerate features \Mith the required size
reproducibly,unless we invest in a certain amount of
mechanicalapparatus.
Microcontact Printing with a Starnp That HasBeen Swelled in a
Solvent. Cross-linked PDMS can beswelled in a number of "good"
solvents, such as toluene
Langmuir, Vol. 73, No. 7, 1997 2063
gla$$
Dfus
0.5pm
1pmFigure 7. (A) Illustration of size reduction by
mechanicallypressing a PDMS stamp against the gold substrate
whileconducting microcontact printing. The dashed lines
representthe original profrles of the microstructures on the stamp.
(Band C) SEM images of Au patterns (20 nm thick) that
werefabricated using microcontact printing with PDMS stampswithout
and with vertical pressure, followed by selective etchingin an
aqueous cyanide solution.
and hexane.26-28 A swelled PDMS stamp still has goodmechanical
strength to be used inpCP. For normal PDMSstamps, the dimensions
ofboth recessed and raised regionsincrease after swelling in the
solvent. We only observedsize reduction for certain features for
those thin stampsthat are grafted to a rigid support (Figure 8A).
Thesespecial stamps were fabricated using a similar procedureas
described for the fabrication of stamps used for pCPunder vertical
pressure. Figure 88 and C shows SEMimages ofAupatterns that were
fabricatedusinga PDMSstamp without and with swelling in toluene for
-30 min.
(26) Bueche, A. M. J. Polym. Sci. 1955, )fi/,97.(27)Hene, L. E.
ST.;Dewhurst, H. A.; Bueche, A. M. J. Polyrn. Sci.
1959. -ICCrW. 105.(28) DeBoIt , L. C.; Mark, J . E.
Macromolecules lg87 , 20, 2369 .
rAu
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I
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tI 0
300"/o
Force
-
2064 Langrnuir, VoL 73, No. 7, 1997 Xia and Whitesides
Filler
PDM$
AA glassPDMS
II swutec in toluene* I
I
$Extract the Filler
0.7 pn
lpmFigure 8. (A) Illustration of size reduction by swelling
thePDMS stamp in toluene for -30 min before conductingmicrocontact
printing. (B and C) SEM images of Au patterns( 20 nm thick) that
were fabricated using a PDMS stanr p withoutand with swelling in
toluene, followed by selective chemicaletching in a cyanide
solution.
After being swelled in toluene, the raised features on thestamp
increased in size, and this increase caused areduction in the
distance between the raised features.
The swelling process was reversible: the PDMS blockreturned to
its original shape after the solvent hadevaporated. The shape of a
swelled PDMS stamp couldbe locked into place by using a solvent
that can be graftedto the PDMS network or that can be cross-linked
into asolid material via thermal or ultraviolet treatment.
Ingeneral, this approach to reducing sizes is not as conve-nient as
others.
Microcontact Printing with a Stamp Whose Di-mensions Have Been
Reduced by Extraction of anInert Filler. In making a PDMS stamp, we
added aninert filler into the prepolymer mixture before curing
thesystem. After cross-linking (at 65 "C for -10 h) the
PDMSprepolymer into a solid material, the inert filler was
t*?d
1pmFigure 9. (A) Illustrations of size reduction by extraction
ofthe inert filler that was added into the PDMS prepolymer
beforecuring. (B and C) SEM images of Au patterns (20 nm thick)that
were fabricated using a PDMS stamp before (B) and after(C) removing
the filler. T,he bright regions are Au; the darkregions are Si
where the unprotected Au has dissolved in anaqueous cyanide
solution. In this example, -54Vo (dw) of PS04 Iwas added into the
Sylgard 184 mixture (A:B : 1:10).
removed by extraction with a solvent to reduce the volume(and,
therefore, the dimension in each direction) of thePDMS block.26-28
We have used a number of linear, lowmolecularweight oligomers
ofPDMS such as silicone fluids(PS039, PS040, PS041, Hiils) as the
fillers. These fluidsmix well with the Sylgard silicone prepolymer
and can beextracted easily with toluene. After extraction ofthe
flller,the volume of the PDMS block as well as the dimensionsofthe
microfeatures on its surface decreased isotropicallyin all three
directions (Figure 9A). We have been able toload as much as -687o
(dw) of PS041 to the mixture ofSylgard prepolymer. The calculated
reduction in the sizeof features was inconsistent with that
measured using alight diffraction method. Figure 9B shows lines of
goldthat were fabricated using microcontact printing with aPDMS
stamp that has -54Vo (w/w) PS041 in it. Figure9C shows gold lines
that were fabricated using this samestamp after the PS041 has been
extracted by immersingin toluene for -30 min. The period of the
pattern and thedimensions of both lines and the spacings between
lineswere reduced in this process. However, afber the extrac-
-
Microcontact Printing
tion of the frller, the stamp became very soft. When webrought
the stamp into contact with the gold surface forprinting, the
raised areas of the stamp deformed againand resulted in the
characteristics observed in the Snntphotographs shown here, that
is, less reduction in thedimensions of the lines than in the
dimensions of thespacings between lines.
This approach can only be used to achieve size reductionby a
factor of -1.4. It was the only procedure that allowedus to reduce
the size of the microfeatures in all threedirection. we can design
an iterative procedure (such asthat shown in Figure 4) that will
allow us to achieve moresignificant reduction (by a factor of
>b) in the size of thefeatures.
Microcontact Printing Using Controlled ReactiveSpreading. This
process is shown in Figrrre 1 schemati-cally. The lateral spreading
ofHDT liquid (predominatelyfrom the edge of the patterned SAM
beyond the area ofthe stamp in contact with the substrate) on the
surfaceof gold reduces the dimensions ofthe bare regions (Figure10A
and B). The alkanethiol used in this p"ocess mustbe insoluble in
water;thus, the water acts ai a barrier tothe diffirsion ofHDT from
the recessed regions ofthe stampto the surface of gold. The
distance d (pm) over which thledge of the SAM advances through
reactive spreading isrelated to the concentration of HDT tCl (mM)
and theprinting time t (min) by the empirical equation d.2
=(0.L6lcluz)t. The dependence of d on r is consistent witha model
proposed for the spreading of a liquid on a solidsur{'ace.29,30
Conducting pCP under water is critical for the successof this
procedure. If printing was carried out in air, HDTliquid did not
spread on gold: it formed an autophobicsystem-a liquid in contact
with a surface modifies thechemistry of the surface and lowers its
solid-vapor andsolid-liquid surface tension, and the liquid
ietractsspontaneously.sl In this case, the stamp cannot be keptin
contact with the gold surface for intervals longer than30 s;
otherwise, disordered SAMs also formed on thoseregions not in
contact with the stamp by the diffusion ofHDT from the recessed
regions of the stamp to the goldsurface through the vapor
phase.
This method only reduces the dimensions of certainfeatures-bare
regions of gold underivatizedwith SAMs.The dimensions of the whole
patterned area cannot besh_runk by using this technique alone. In
combining withselective etching of gold, this method can be used
tofabricate arrays of nanometer-sized trenches or grids ingold that
are separated by several micromete., (Figur"10C and D).
Microcontact Printing with Multiple Impressionson the Same
Surface. Muttiple printings ol SAMs onthe same surface were also
possible: we could use thisapproach to reduce the size of certain
features of SAMsorto generate more complex patterns from a single
stamponly having simple patterns on its surface. Figure 11Aand B
illustrates the use of this procedure for sizereduction. The
dimensions ofthe printed regions (that is,regions derivatized with
sAMs ) increased aft er the secondprinting while the dimensions of
the bare regions de-creased. Figure llC shows the application of
multipleprintings in formingnew patterns. The gold patterns (20nm
thick) were generated by a double-printing proce-dure: a PDMS stamp
having parallel lines on its surfacewas first contacted with the Au
surface for -10 s: it wasthen removed, cleaned with ethanol,
rewetted with HDT
, Lz-g) Lapez, J.; Miller, C. A.; Ruckenstein, E. J.
CoIIoi.d,Interface Sci.1979,56, 460.
(30) Greenspan, H. P. J. Fluid Mech. lg79, 84,IZS.(31) Biebuyck,
H. A.; Whitesides, G. M. Langrnuir lgg4, 10,45g1.
Langmuir, Vol. 13, No. 7, 1997 20GE
Al 5 s8c
B)
-if,Ljitrl';alj!;:
F,{,rfi
C) 5X5 sec
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$ffi$ffiFigure 10. SEM images of Au patterns (20 nm thick)
thatwere fabricated using microcontact printing of HDT underwater,
followed by selective chemical etching in an aqueouscyanide
solution. By carrying out microcontacl printing underwater for
several minutes, it was possible to acfiieve asubstantial reduction
in dimension for the bare Au (that is, notcovered by the SAMs) from
-2.5 to -0.1 pm (A and B). Bycro ss - stamping with this proce
dure, it was p ossible to fabricategn tlray ofAu gnds with
submicrometer-sized features (C andD). The bright_regions are Au
covered by sAMs; the dark regionsare si where the underivatized Au
has been removed by etJhingin CN-/O2.
ink, and recontacted with the Au surface for -10 s, withthe
orientation of the lines rotated by -2. The printedsamples were
then etched in an aqueous cyanide solution
-
2066 Langrnuir, VoI. 73, No. 7, 1997
10 prn 1 pmFigure 11. (R and B) Illustration of size reduction
by double-printing. The gold patterns (20 nm thick) were fabricated
by(A) stamp once and (B) stamp twice (after the first printing,
thestamp was removed from the Au surface, it was translated inthe
direction perpendicular to the lines for a certain distance,and the
second printing was made). (C) Illustration of makingnew patterns
using double-printing. The lines were rotated bya small angle (-2")
before making the second printing.
to remove the unprotected gold. Note that the dimensionsof the
features etched in the gold film are only -200 nmat the tip.
By rotating for different angles between these twoprintings, we
could produce patterns with a variety of
Xia and Whitesid.es
.F
1 prmFigure 12. TWo more examples to illustrate the
applicationof double-printing in making new patterns. (A) SEM image
ofa test pattern ofAu (20 nm thick) generated by
double-printing,followed by seiective wet etching in an aqueous
ferricyanidesolution. (B) SEM image of another test pattern ofAu
that wasfabricated using a modified procedure for double-printing
(seethe text for details). The bright regions are Au covered by
SAMs;the dark regions are Si where the underivatized Au
hasdissolved. The stamp used here only had parallel lines on
itssurface.
shapes and sizes (for example, Figure 1^2A, rotated by-45).
Moreover, we could remove hexadecanethiolateSAMs on Au by immersing
the patterned sample inpiranha solution (a mixture ofHzSO+ (987o)
and HzOz (30qo)at the ratio of 7:3). This property opens the door
togenerate new forms of patterns that cannot be producedsimply by
double-printing. Figure 128 show the SEMimage of a gold pattern
that was fabricated using a frve-stage procedure: (1) lines of
hexadecanethiolate SAMswere printed on a Au surface; (2) this
sample was etchedin an aqueous ferricyanide etchant for -7 min; (3)
thehexadecanethiolate SAMs on the remaining Au wereremoved by
immersing the sample in piranha solution for-5 s; (4) lines of
hexadecanethiolate SAIVIs were printedon this Au surface, with the
orientation ofthe lines rotatedby -45"; (5) this sample was etched
in the fericyanideetchant for another -7 min. The Au pattern
generatedin this way was complementary to that fabricated usingthe
normal procedure of double-printing (Figure l2A).
At present we conduct microcontact printing by hand,and it is
difficult to achieve accurate alignment between
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Microcontact Printing
the stamp and the substrate. Once a good procedure
forregistration is available, we believe that this approachcan be
used to generate features with sizes of