+;bv pev. -pola t"

Demonstration of a Nanolithographic Technique

using a Self-Assembled Monolayer Resist

for Neutral Atomic Cesium

Ii. K. Berggren, R. Younkin, E. Cheutrg, M. Prentiss.

A. J. Black, G. N' I . Whitesides.

D. C. Ralph, C. T. Black, l l l . Tinkham*

[. ] Prof. M. Prent iss, I ( . I i . Berggren, R. \ounkin, E. Cheung

Prof. M. Tinkham, D. C. Ralph, C. T. Black

Department of Physics, Harvard University

17 Oxford Street, Cambridge, MA 02138 (USA)

Prof . G. M. Whi tes ides, A. J . B lack

Department of Chemistry, Harvard Universi t r .

l2 Oxford St reet , Cambr idge. NIA 02138 (USA )

[**] This rvork rvas supported in part by NSF grant PHY 93L2572 and made use of the

Harvard MRSEC shared facil i t ies. The authors rvould l ike to acknowledge Hans Robinson,

Dr. Hans Biebuyck,, Dr. James L. Wilbur, and Dr. Jabez N{cClelland. for assistance rvith

the init ial stages of this rvork. The authors rvould also l ike to thank Dr. Yuanchang Lu for

assistance in generating SEN'I images, Joseph Thyrvissen for a careful reading of the

manuscr ipt , and I ient Johnson, Dr. Andreas Bard, and Dr. John Gi l laspy for many helpful

discussions.

Abstract

This experiment demonstrates the formation of nanometer-scale features in a gold

substrate using a self-assembled monolayer (SAI{) of nonanethiolate as a resist for a

patterned beam of neutral cesium atoms. The mask used to pattern the atomic beam rvas

a si l icon nitr ide membrane perforated with nm- and plm-scale holes. A dose of ^,3

monolayers of cesium damaged the SAI,I suff iciently to al lorv penetrat ion of the damaged

regions by an aqueous etching solut ion. Etching transferred the pattern of damage in the

SAN,I laver in to the under ly ing gold substrate. Features of -70-nm s ize rvere etched in to

the gold substrate. Invest igat ions of the ref lect iv i ty of samples exposed to the atomic beam

rvi thout a mask and subsequent l r , 'e tched revealed that the res is t -etch svstem exhib i ts a

minimum threshold dose of cesium for damage; at doses lower than ^,3 monola-vers, the

damage was insuf f ic ient to a l lorv penetrat ion of the SAI I { by the etch ing solut ion.

Conventional l i thographic processes incorporate three basic elements: 1) u damaging

agent ( r .S.photons, ions, or e lec t rons) ,2) u pat tern ing mechanism ( r .g .a mask or focus ing

opt ics), and 3) u resist that is sensi t ive to the damaging agent ( typical ly a -1-pm-thick

organic layer). The relatively recent development of optics for neutral atomic beamsl has

yielded a patterning mechanism for l i thographic techniques that use neutral atoms.2-4

There is, therefore, interest in nelv resist systems that are sensit ive to exposure to neutral

atoms.s-8 The experiment described in this paper demonstrates a new method of making

nanostructures that uses a patterned beam of neutral cesium atoms to damage a

^, 1.2-nm-thick sel f -assembled monolayer (SA\1) resist of alkanethiolates on golds'r0 (see

Figure 1). The atomic beam rvas not patterned opt icai ly, but rv i th a contact mask

consist ing of a

;-rm-scale holes.

40 pm x 40pm si l icon ni t r ide (Si3i{4) membrane perforated with nm- and

Exposure to the cesium altered the SAN,{ layer sufficiently to allow

penetrat ion of a rvet -chemical e tch. resul t ing in ^ ,70-nm- lv ide features etched in to a

^,20-nm-thick gold la.ver on a si l icon substrate. With an appropr iate mask and a broad

atomic beam. patterning over larger areas (^,cm2) should be possible rv i thout increasing

exposure t imes or feature size. The pr imarv appl icat ion for th is technique is the fabr icat ion

of nm-scale structures, however, this method might also be used for high-resolution

detection of atomic density.

Lithography using neutral atoms has several desirable features: the theoreticaliy

achievable image resolution of patterns formed with neutral atoms is l imited by the size of

the atom (^,0.3 nm),11 and because neutral atoms are not subject to Coulombic forces, th is

technique avoids some of the instabi l i t ies inherent in ion- and electron-beam l i thography.

Cesium has several attractive features for atomic l i thographv: 1) Cesium has cycling

atomic resonances with large electronic susceptibil i t ies in regions of the spectrum (852 and

895 nm) that are easiiy accessible by relatively high powered diode (-100 m!V) and

solid-state (^,1 W) lasers; it can. therefore, be easily and inexpensively manipulated using

laser l ight . 2) A rvide variety of atomic optical elements have already been developed for

use in patterning atomic cesium, and in preparing high-quality beams.t 31 Thermal effusive

sources of cesium are in widespread use; thel/ are relatively inexpensive and simple to

fabricate.

Current techniques for neutra l a tom l i thography fa l l in to t rvo c lasses: i ) an opt ica l ly

pa t te rned a tomic beam ( r .g .sod ium.2 ch romium,3 and a luminum{ ) i s depos i ted d i rec t l - v

onto a substrate; or 2) the in ternal energ-v s tored in exc i ted metastable s tates of neutra l

atoms ( r .S.argonS and hel iumt ' t ) is used to damage a SAIU. l l le tastable argon can a lso be

used to create a res is t on the substrate us ing contaminant vapor in the vacuum system.6 In

pat tern ing a gold substrate by image t ransfer f rom a res is t o f SAIvI exposed to neutra l

ces ium atorns, lve demonstrate a r le \v technique for neutra i a tom l i thography that uses a

fundamenta l ly ' d i f ferent phy 's ica i mechanism to pat tern a res is t . L in l ike exper iments us ing

the in ternal energy of metastable atoms ( -10 eV) to damage the res is t , the damage

process for cesium is l ikely to be chemical because the energy stored in the thermally

occupied states in the ground state hyperf ine manifold 1^,4 peY ) and the kinetic energy

(-50 meV) together should be insuff icient to effect col l is ionai damage to the chemical

bonds in the res is t ( ^ ,2 eV) .12

This resist-based technique potentiaily incorporates several advantages of resist

systems for appl icat ion to neutra l -atom l i thographl , and h igh-resolut ion atomic detect ion:

1) Unlike direct deposit ion schemes that bui ld features up one atom at a t ime, the etching

process used to transfer the pattern from cesium into gold can ampli fy the depth of the

image, so a few monolayers of material can be used to make a tal ler structure (20 nm) in

Iess t ime. 2) Conventional resists are thick (^,1 pm), and therefore would not be

chemically sensitive to monolayer-level doses of neutral atoms. Because typical beams of

neutral cesium are relat ively low in f lux (^,3 min monolayer t ime), the thinness of SAMs

(-1.2 nm), and therefore thei r sensi t iv i ty to low doses of ces ium, rvas cr i t ica l to

accomplishing exposures in real ist ic t ime scales. 3) I f the dose profi le of a feature inciudes

some undesi red background, res is ts that exhib i t a min imum dose threshold for damage can

be used to narrorv a feature and irnprove i ts contrast after image transfer. We observed

evidence of such a threshold at a dose of ^,3 monolavers for mm-scale features formed in

samples exposed to a broad cesium beam rvithout a mask. 4) I f the amount of gold

remaining after etching depends l inearl l ' on dose,, and i f this l inearity is observabie at the

nm scale, carefu l exposure of a SAl l l res is t in th is dosing regime could detect atomic

densi ty rv i th nm-scale resolut ion: the re lat ive l ,v iner t pat terns created in the res is t or

substrate could be imaged outs ide of vacuum using convent ional techniques of

high-resolut ion microscopy. For mm-scale features,, there was a region, betrveen ̂ "3 and ^,7

monolayers of cesium, where such a l inear response lvas observed.

Figure 1 shorvs the processing steps used in this experiment. 20-nm-thick gold f i lms

were deposited by electron-beam evaporation onto a Si (100) substrate rvith a ^,2-nm-thick

native oxide la-ver using 1.5 nm of Ti as an adhesion promoter. The SAlvls were formed on

a solut ion of ̂ ,1 mN'I nonanethiol (CHr(CHr)rSH) inthe gold bv overnight immersion in

absolute ethanol ( t r toH ). After the

several t imes rvith EIOH, and rvere

samples \\ 'ere remove(l from solut ion, they \\ 'ere r insed

subsequently dried using nitrogen gas rvith part iculates

T2

Figure 1 : Schematic diagram of the experimental procedure (not to scale). A) A

Au/Ti/SiO2/Si substrate rvas first coated with a SAIvI of nonanethiolate. B) The substrate

was then exposed to a beam of neutral cesium atoms through a mask that lvas in close

contact (^,g pm) with the surface. C) In a f inal etching step, the pattern of damage

caused by the cesium rvas transferred into the gold layer.

/SAM(-1 nm)

+ Au(20 nm)/Ti(1 .5 nm)!-SiO2(-2 nm)/Si(500 pm)

I expose the SAM to a beam of neutralCs atoms through a physical mask

oooo

. t o t l t ooI O

. . . t o V: :- a o o o o

o '. . -+- physical mask

ooo

t j ./ affected region

etching removes gold in regionswhere the SAM was damaged

B. ^I or ' . t f

ot - t o

o

- o ta oo

: oo o o

-

o :.+'."IO _i , U :Yo :oo ooo

o o t a.:fljr

oa O

oO O

oo

o

o = Cs atoms

c

larger than 0.2 pm removed by f i l t rat ion. A patterned 50-nm-thick Sigl{.r membranel3 was

placed in contact (^,5 pm spacing) rvi th the substrate. The mask and substrate lvere held

in place with a steel cl ip. The samples were then put into the vacuum svstem and exposed

to the beam of cesium for t imes varying betrveen 5 min and 2 hours. Immediateiy upon

removal from the vacuum system (( 5 min delay) the exposed samples were etched for 7

min in a wet-chemical etch.14 When outside of vacuum, the samples tvere exposed to

ambient laboratory condit ions. The samples rvere anaVzed using either scanning electron

microscop-v (SEM) or optical ref lectometrt ' .

F igure 2 shorvs a schemat ic d iagram of the atomic beam apparatus used to expose

the sampies. The atomic beam \vas created b1' heating cesium in an oven to ^'290oC and

extracting atoms through a 1.6-mm-diameter circular aperture in the oven lvall. The beam

rvas col l imated using a 1.S-mm-diameter c ircularaperture placed 27.0 cm away from the

along the path of the atomic beam.oven, and the sample rvas p laced 17.0 cm fur ther

Sample entry was accompl ished through a load- lock chamber pumped rv i th a LN2-cooled

sorpt ion pump. Pressures in the main chamber dur ing exposure \ \ 'ere t1 'p ica l ly

5 x 10-8 torr . The f lux in the atomic beam n 'as determined b"v measur ing the opt ica l

th ickness of the beam for c i rcu lar ly polar ized laser l ight tuned to the 6St / , F-4 to 6P312

F-5 transit ion in atomic Cs. The typical f lux was 20 monolavers per hour, rvhere we define

a monolaver to have the surface ciensity of the SANI molecuies ( '1.6 x 10ra atoms/cm2).

Figure 3 shorvs a SENI of ^,70-nm- and ^,500-nm-wide features formed in a gold

surface by cesium exposure, as rvel l as the masks used to form these features. Typical

exposure t imes \ \ ,ere -15 minutes. corresponding to a dose of ^ ,5 monolavers. The

roughness of the edge of the damaged r lgions was -20 nm, rvhich rvas comparable to the

13

Figure 2 : Schematic diagram of the apparatus used for the exposure of samples of SAMs

on a gold surface to a measured dose of cesium from a neutral atomic beam. The sample

holder could be removed to al lorv for the determination of cesium f lux b-v the measurement

of absorption of resonant laser l ight by the atomic beam.

oo9.cI

3oo-tJ

E Oo=3='= 0 ,o =;.

J

(cI-o-FJ

ot-tsoCL6'CLoa

f taoq ,33rn ?t

o l 'o. -

go.o-

f

- a

r{ r{rJ rr+r

E O= i l or?r J= . +

g

rzo r l -A =1 a = .- =qJ rcroq)-l

J

rq gEgqil

'rI ==' q):: (oo.3

o!3.o

T4

Figure 3 : A) SEM image of 500-nm- and 5O-nm-scale holes in a Si3N4 mask that rvas

used to pattern the atomic beam. The fabrication of the mask is described elservhere.r3 B)

Image of features formed in Au after exposure to a dose of ^-,5 monolayers of monatomic

neutral cesium through the mask shown in A and subsequent etching. C) Cross-section of

SEM images shown in B. The secondary electron intensity across a typical feature is shorvn

in arbitrary units, averaged over I nm in the vertical axis for the 50O-nm-wide feature, and

11 nm in the vertical axis for the 7O-nm-rvide feature.

sis N4-l pmHoles

.t-,(-

a

Oq)

tU\-Cg

1OcoC)c)a

500 nm+

500 1 000 1500Position (nm)

N4 Holes

siq/si'..

70 nm{--*

100 200

frr. -'1.00

Position (nm)

granularity of the gold surface.

In order to investigate the damage to the surface as a function of dose, samples were

also exposed to the atomic beam without using a mask. Using the known flux profi le of the

atomic beam and the measured reflectivity profi le of damaged spots to normally incident

l ight at 632 nm, the damage to the surface was determined as a function of the dose of

cesium atoms. The reflectivity of the gold has previously been correlated, using atomic

force microscopy, to the amount of gold remaining on the surface.T Figure 4 shorvs the

transfer function from cesium dose into the reflectivity of the etched gold layer. Below ̂ ,3

monoiayers, no visible damage rvas observable. After ̂ ,3 monola,vers, increased dose

resul ted in further etching. Above -10 monolayers, , increased dose induced protect ion

against etching.

Control experiments demonstrated that damage lvas caused by atomic cesium and

not by some other agent , e .g .molecu lar ces ium, o ther chemica l contaminants , or thermal

radiation from the oven. Radiation pressure from laser l ight resonant oniy rvith atomic

cesium rvas used to distort the circular prof i le of the atomic beam.l None of the other

possible beam constituents rvould be deflected by the laser l ight. Figure 5 shorvs the

distorted damaged spot on an etched substrate. Damage observed outside of the or iginal

beam spot - where cesium had been pushed by the i ight - established that the cesium

atoms did damage. The lack of damage in regions that originally rvere in the beam

- rvhere cesium had been removed - indicated that cesium atoms rvere predominantly

responsible for the damage seen in these experiments. When longer exposures were used,

radiat ion pressure establ ished that the protect ion ef fect that rvas observed at h igh doses

(> 10 monolayers) was also due to atomic cesium.

l c

Figure 4 : Reflect ivi ty of an etched gold surface rvhose protective SAlvl resist had been

damaged by a measured dose of cesium atoms from an atomic beam rvithout a mask' The

reflect ivi ty rvas measured rvith incident laser i ight at 632 nm. Different symbois refer to

data obtained for dif ferent exposure t imes on the same sample. Reflect ivi t ies for a range of

doses were extracted for each exposure t ime bv examining the ref lect ivi ty along the

gradient of the dose profi le. The inset graph shorvs that for doses above -10 monolavers' a

slow increase in ref lect ivi ty rvas observed rvith increasing dose. \Ve define a monolaver to

have the su r face dens i t y o f the SA\ l ( ' 1 .6 x i0 ra a toms/cm?) '

A 20 min . 30 min o 40 min

80f ^r?

70

60

Q. aL

III

AIIID

=

()ooE

o()a)o_

70

65

60

55

a

a Oa

aa

a a o

A n

; €

t0

A 1 I

a . A o :

"r^h{:tII

..Er"r.l 3 Zc!'l

50

nodamageobserved

l inearregime

t 4 l

5

Dose (Monolayers

saturated damage reglme

10

of Atomic Cesium)

16

Figure b: Photograph of an etched sample made in a control experiment where the SAIvI

resist lvas exposed to an unmasked cesium beam that had been distorted by radiation

pressure. For exposures (not shorvn) without deflecting laser l ight., the resulting etched

spot rvas circular, as indicated by the dashed l ines in the figure. The observed distortion of

the damaged region for the sampie shorvn here proved that atomic cesium was responsible

for the damage effect seen in this paper.

Def lectingLight

Cesium isdeflected intothis region

Border of cesium exposurewithout deflecting light

The central result of this paper is that nm-scale patterns can be fabricated in a gold

surface by exposing a nonanethiolate SAM resist to a patterned beam of neutral cesium

atoms. Relatively low doses (-3 monolayers) of atomic cesium were sufficient to sensit ize

the SAM surface to penetration by u subsequent wet-chemical etch. When studied using

the reflectivity of the gold substrate as an assay, this technique showed a crit ical

thresholding effect. This effect might be used to improve the contrast or narrorv features

rvi th broad backgrounds.

The damage mechanism of the SANI rvas not explored in this paper; i t is l ikely that

the damage is caused by o chemicai in teract ion of the cesium rv i th some component of the

SAIVI surface or the SAIVI-Au interface.l5 Because SANIs incorporating a variety of surface

chemistr ies can be formed, this chemical damage mechanism suggests that the technique

time, or increasedescr ibed in this paper couid be exploi ted ei ther to opt imize the exposure

the range and ut i l i ty of possible structures created rv i th neutral atoms.

In control experiments that establ ished that only atomic cesium effected the surface

da,mage, the substrate was pat terned on the macroscopic length-scale by opt ica l def lect ion

of the cesium beam using radiat ion pressure forces. In past experiments, l ight forces from

an optical standing-rvave have been used to pattern similar materials on the nm

length-scale.2-4 The technique presented here could be used to transfer nm-scale features

formed by optical standing-wave focusing into gold features.

layer can then be

in th i s paper i s a

used as a mask to etch in to the under ly ing

f irst step torvard fabricating nanostructures

Because the patterned gold

si l icon.5 the resul t presented

in si l icon using opt ical ly

pat terned neutra l a torn ic beams.

Erperimental

The optical thickness of the cesium beam was determined b.v measuring absorption of

852-nm laser l ight16 on resonance. The incident laser was shaped by expanding the laser

beam to a ^,l-cm-diameter spot, and then aperturing to a diameter of 1.8 mm and l0pW

of power. Upon entering the a+-polarized laser beam, the atoms in the F-4 ground state

were quickly optically pumped into the mp - 4 Zeeman sublevel. A -5 Gauss magnetic

field lvas used to establish the quantization axis for the atoms. ltfeasurements of the

absorption were made before and after each exposure and averaged (dri f ts were tSrpical ly

less than I0% over I hour) . Anal -vs is of absorpt ion data a l iowed calcu lat ion of f lux to a

combined systematic and stat ist icai error " f i i ln. An oven temperature of ^,290"C

(measured using thermocoupies outside the oven) was used to calculate the longi tudinal

velocity distr ibution, rvhich lvas assumed to be thermal. The geometrical factor from the

non-uniform radial density prof i le of the atomic beam rvas included in the analysis. This

profi le lvas veri f ied experimental l-v by probing the atomic beam rvith resonant iaser l ight.

and re-imaging the f luorescence onto a CCD-arra.v.

The ref lect ivi ty of mm-scale regions of damage in the gold surface from the unmasked

atomic beam was measured using l ight from a hel ium-neon laser at 632 nm. The laser

beam was spatial ly f i l tered by focusing through a 100- pm pin-hole, and then focused using

a 5-cm lens to a I f e2 d iameter on the substrate of 20 pm. A por t ion of the specular ly

ref lected l ight rvas directed into a photodetector (assumed to be l inear over the porver

ranges used - 0.1 - 4 pIV) and compared to the incident laser polver. The 100%

reflect ir . ' i ty levei rvas cal ibrated using a high-quaii t .v mirror.

r0

References

[1] For a review of atomic optics and optical forces, see C. S. Adams, M. Sigel, J. I l ' l lynek,

Physics Reports 1994, 240,, 143.

[2] G. Timp, R. E. Behringer, D. M. Tennant, J. E. Cunningham, NL Prentiss, I(. K.

Berggren, Phys. Reu. Lett. 1992,69, 1636.

[3] J. J. NlcClelland, R. E. Scholten, E. C. Palm, R. J. Celotta. Science 1993, 262,887.

[4] R. W. N{cGowan, D. M. Giltner, and S. A. Lee, Opt. Lett. 1995, 20,2535.

[5] K. I(. Berggren. A. Bard, J. L. Wilbur, J. D. Gillaspy, A. G. Helg, J. J. IllcClelland, S. L.

Rolston. W. D. Phi l l ips, M. Prentiss, G. NI. Whitesides, Science 1995,269, 1255.

[6j K. S. Johnson, I(. K. Berggren, A. J. Black, C. T. Black, A. P. Chu, N. H. Dekker, D. C.

Ralph, J. H. Thywissen, R. Younkin, M. Prentiss, M. Tinkham, G. M. Whitesides, in press,

Appl. Phys. Lett.

[7] A. Bard, I(. K. Berggren. J. L. lVilbur, J. D. Gillaspy, S. L. Rolston, J. J. NfcClelland. !V.

D. Phillips, M. Prentiss, G. M. Whitesides, submitted to J. Vac. Sci. Tech. B,

[8] S. Norvak, T. Pfau, J. Mlynek. in press, J. Appl. Phys. B.

[9] R. C. Nuzzo, F. A. Fusco, D. L. Allara, J. Am. Chem. Soc. 1987, 114, 1990.

[10] C. D. Bain, J. Eval l , G. N{. Whitesides, ibid.,7155.

[11] Diffraction does not play a role in neutral atom lithograph-'- because the deBroglie

wavelength is - .01 nm. Electron and ion beam lithography exploit the same advantage to

make features as small as 5 nm.

f12] Sodium atoms have also been used to pattern a SAM resist of hexadecanethiolate on a gold

substrate. The pattern rvas subsequently transferred into the gold using a rvct-chemical

etch. H. Robinson, I(. I(. Berggren, H.'Biebuyck, M. Prentiss, G. M. Whitesides,

1 l

unpublished results.

[13] I(. S. Ralls, R. A. Buhrman, R' C. Tiberio. Appl. Phvs' Lett' 1989,55,2459'

[14] 0.001 M KaFe(CN)6, 0.01 M K3Fe(CN)6, 0.1 M I(zSzO:' and I i r l KOH; see Y. Xia, X.-NI.

zhao, E. Kim, G. M. lVhitesides, chem. IlIater. Lgg', 12,2332. For discussion of the

quality of the selectivity of sAMs against this etch, see also Y. Xia, X. -M. Zhao, and G.

M. lVhitesides, in press, Microelectronic Engineering'

[15] Prior to etching, exposed samples rvith mm-scale patterns of damage were analyzed using a

SSX-100 (Surface Science) X-ray photoelectron spectrometer rvith an Al Ka X-ray source,

and an analyzer pass-energy of 150 eV. Both cesium and oxygen rvere observed in the

exposed regions but not on the surrounding surface. Subsequent rvashing of the samples

using water and EtoH significantly reduced both the cs (3d5) and o (1s) photoelectron

peak intensit ies. Differences in the C (1s), S (2s.2p), and Au (4f) peak intensit ies between

damaged and undamaged rvashed regions rvere not observed. Ii. I(. Berggren, R. Younkin,

E. Cheung, M. Prentiss. A. J. Black. G. I I . Whitesides, unpublished results'

[16] For a description of the design of the diode lasers used in these experiments see C.

Wieman, L . Ho lberg, Reu. Sc i . Ins t rum. 1991, 62, I .