Alkyl monolayers on Si(111) as ultrathin electron-beam patterningmedia

Taro Yamada a,*, Nao Takano a, Keiko Yamada b, Shuhei Yoshitomi b,Tomoyuki Inoue b, Tetsuya Osaka a,b

a Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Tokyo 169-0051, Japanb Department of Applied Chemistry, School of Science and Engineering, Waseda University, Tokyo 169-8555, Japan

Received 26 November 2001; received in revised form 4 April 2002; accepted 10 April 2002

Abstract

A process of electron-beam patterning of the surface of a Si(111) wafer was developed by utilizing alkyl monolayers as ultrathin

patterning media. We performed chemical benchmark tests of the electron-beam patterning of alkyl monolayers on Si(111) in

ambient oxygen, followed by the deposition of a metal on bombarded areas by immersion into an aqueous solution containing metal

ions of the metal to be deposited. We investigated practically important issues related to this process, such as the robustness of

organic monolayers against oxidation in aqueous media, the contrast enhancement of the bombarded areas by metal deposition, and

the detectability of electron-bombarded areas of the monolayers by scanning tunneling microscopy (STM). The alkyl-covered

Si(111) surface was significantly resistant to the oxidation by dissolved O2 in pure water, compared to hydrogen-terminated Si(111).

By immersion into a solution containing CuSO4�/HF�/NH4F, electron-bombarded areas were visualized by the presence of the

deposit of Cu. Electron-bombarded areas were also distinguishable from intact areas in terms of height contrast or roughness

measured by STM. These results indicate the usefulness of alkyl monolayers for nano-scale patterning on silicon wafers. Published

by Elsevier Science B.V.

Keywords: Si(111) wafer; Organic monolayer; Oxidation; Electron-beam patterning; Metal deposition; Nanometer-scale fabrication

1. Introduction

The technology of fabricating nanometer-scale struc-

tures is one of the most promising fields of science and

engineering today. Among various materials and meth-

ods being investigated for nano-fabrication, the nano-

scale patterning of silicon wafer surfaces with a resolu-

tion of 10 nm is presently a typical target of develop-

ment [1�/5], which is closely related to the future

microelectronic device industry.

Our original aim was to search for a suitable system of

materials and processes for realizing ultrahigh-density

dot-array planer data-storage media to be used in

vacuum with a facility for writing and reading with a

scannable electron beam. Today, a focused electron

beam with a full-width-at-half-maximum diameter of

about 10 nm is available by field-emission of electrons

and magnetic-field focusing [6], and it can be scanned

rapidly over a large area. The patterning medium to be

prepared on Si wafer plays the most important role in

this process, as it should be sensitive enough for rapid

electron-beam patterning and should not deteriorate on

the scale of nanometers during the course of processing.We recently proposed an entire process of nanometer-

scale electron-beam patterning using monolayers of

alkyl (CnH2n�1�/) groups directly bonded to Si(111)

wafer surfaces as the patterning media [7,8]. As the

thickness of the patterning medium is limited to a

monolayer, there is virtually no depth profile, and hence

the problems of focal depth of the electron beam and

diffusion within the patterning medium should be absent

in delineating ultimately small patterns [5�/8]. Other

important issues are the mobility of adspecies on the

surface, the inertness of the monolayer-covered surfaces

and the chemical reactivity of the monolayer. The

* Corresponding author. Present address: Surface Chemistry

Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198,

Japan. Tel.: �/81-48-467-4578; fax: �/81-48-462-4663.

E-mail address: tayamada@postman.riken.go.jp (T. Yamada).

Journal of Electroanalytical Chemistry 532 (2002) 247�/254

www.elsevier.com/locate/jelechem

0022-0728/02/$ - see front matter. Published by Elsevier Science B.V.

PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 0 8 6 5 - 3

microscopic stability of structures on Si surfaces on the

scale of nanometers is poor at elevated temperatures [9],

and hence a strong linkage of atoms by covalent bonds

is preferred. The monolayers should be stable and

robust in the environment of the patterning processes

in aqueous media, air, and vacuum.

Our process is illustrated schematically in Fig. 1. The

starting substrate is hydrogen-terminated Si(111)(1�/1),

and the process consists of three steps: (1) deposition of

a monolayer of organic molecules covalently bonded to

surface Si atoms; (2) delineating a pattern by chemically

altering the monolayer by a fine electron beam; and (3)

selective deposition of metal atoms onto the bombarded

areas. Step (2) corresponds to the ‘writing’ procedure

when the monolayer is used as a medium for recording

by the electron beam. Step (3) is required to deposit

heavy metal atoms, even in an amount of one monolayer

or less, which will differentiate between the area with

and without the deposit based on the absorbance and

reflectivity of electrons. This will facilitate the ‘reading’

process.

As a benchmark test, all of these processes were

successfully carried out by using an electron beam with a

diameter of 1 mm. The previously reported preparation

method of alkyl monolayers (CH3�/ to C18H37�/) [10]

was verified by Auger electron spectroscopy (AES),

infrared internal-reflection absorption spectroscopy and

scanning tunneling microscopy (STM). As for electron-

beam patterning, the introduction of low-pressure oxy-

gen gas during bombardment facilitated the identifica-tion of bombarded areas because of the formation of a

SiO2 adlayer, which was utilized in subsequent pro-

cesses. Upon brief immersion into an aqueous solution

of NiSO4�/(NH4)2SO4 at room temperature, Ni was

impregnated only over the bombarded areas, as detected

by AES. This is due to a difference in reactivity towards

Ni2� ions between the SiO2 and the alkyl adlayers.

This report describes some critical issues related to thealkyl monolayer used as the patterning medium on

silicon wafer surfaces. One is the stability of alkyl

monolayers in aqueous solutions. The monolayers may

be altered by chemical reaction with solutes, and the

silicon substrate also usually reacts with solutes. The

stability of the monolayer is therefore a measure of the

ability of passivation, which in turn depends on what

species are dissolved in the solutions. The simplestmeasure of the stability is the extent of oxidation of

the surface in pure water, which usually contains

dissolved species, which originated from O2 in the

atmosphere. The robustness of the alkyl monolayers

has been compared with that of hydrogen-terminated

Si(111).

The stability of alkyl monolayers is more critical in

the process of metal impregnation and deposition inaqueous solutions. As will be described later, the

impregnating solution can be basic or acidic and may

contain reagents that can dissolve Si substrate, such as

HF and NH4F. The organic monolayers must be stable

in the presence of those reagents at least until a desired

amount of metal is deposited. This balance was favor-

ably achieved in the Ni impregnation process in the

NiSO4�/(NH4)2SO4 solution. Furthermore, we tried todeposit a much larger amount of metal on the electron-

bombarded areas to enhance the stability of the alkyl

monolayers. A thick layer of metal is not desired in the

fabrication of nanometer-scale structures. A suitable

thickness of deposits on patterns on the scale of 10 nm is

1 nm or less, in view of the sizes of components and the

existence of defects on the substrate. Nevertheless, this

examination is interesting to us, as it should allow us toestimate the maximum contrast between metal deposit

and bare parts of the alkyl monolayer. A single

monolayer of adsorbates with a thickness of several

Angstroms can block the deposition of bulky metal films

with a thickness of more than hundreds of nanometers.

Another important issue is how to observe the minute

patterns formed by electron bombardment. It is difficult

to observe structures with a thickness below 1 nm and asize of tens of nanometers along the surface by conven-

tional scanning electron microscopy (SEM). STM is

sensitive not only to topological height differences below

0.1 nm but also chemical species covering the surface. It

Fig. 1. Schematic illustration of nanometer-scale patterning by

electron beam bombardment using alkyl monolayers on Si(111) wafer

surfaces.

T. Yamada et al. / Journal of Electroanalytical Chemistry 532 (2002) 247�/254248

would be suitable for monitoring each stage of proces-

sing of nanometer-scale patterns. In the present study

we used a metal grid to print a lattice pattern on alkyl-

covered Si(111) by electron bombardment with ambientO2 to observe the image contrast by STM. The pattern

of dots (25 mm2) generated by this grid is macroscopic

compared to our final goal of nano-dot arrays; however,

this experiment was useful as it revealed how electron-

and non-bombarded areas can be distinguished from

each other. Also, it was easy to find the position of

fabricated features as the square patterns are arrayed

within a circle with a diameter greater than 1 mm. Thisis one crucial step in achieving our goal of realizing and

observing nanometer-scale structures fabricated over

organic monolayers.

2. Experimental

The preparation of solutions and the rinse of speci-

mens were all done with ‘DI’ water purified by a Milli-Qapparatus. The substrate silicon wafers used were n-type

Si(111) (single-side mirror-finished, oriented within 9/

0.58 from the (111) plane, thickness 0.35�/0.60 mm, and

resistivity 3�/8 V cm). Prior to all experiments, Si(111)

pieces were subjected to the hydrogen-termination

procedure [11]. First, they were sonicated in trichlor-

oethylene, C3H6O and MeOH. Then they were treated

in ‘SPM’ solution (four parts of concd. H2SO4�/onepart of 30% H2O2, heated at 120 8C) for 10 min and

stored in DI water. Immediately before use, they were

immersed in a 40% NH4F solution for 10 min.

Preparation of alkyl layers by Grignard reagents was

performed by the method reported by Boukherroub et

al. [10]. We used a commercial tetrahydrofuran (THF)

solution of CH3MgBr or a Et2O solution of n -

C10H21MgBr. A specimen of hydrogen-terminatedH:Si(111) was placed in a Schlenk tube with a water-

cooled condenser purged with ultrapure Ar, and one of

the Grignard reagents was added. The temperature was

then raised and kept at about 65 8C for the THF

solution and at 35 8C for the Et2O solution for 12�/18 h

by using an oil bath. To stop the reaction, the specimen

was removed into the air and briefly rinsed successively

in THF containing 3% CF3COOH, ultrapure water and1,1,2-trichloroethane.

To deposit Cu on the electron-bombarded spots, aq.

‘BHF’ solutions composed of 0.1 M CuSO4�/1 M HF�/

1 M NH4F or 0.01 M CuSO4�/0.11 M HF�/0.11 M

NH4F were used.

Surface elementary analysis by AES and electron-

bombardment patterning were done in an ultrahigh

vacuum (UHV) chamber equipped with an electron gunfor electron bombardment and a two-stage load-lock

sample introductory system. The electron gun integrated

in the cylindrical mirror analyzer for AES had a full-

width-at-half-maximum spot diameter of about 1 mm

on the specimen surface, which corresponded to the

detection area of AES. The electron gun for bombard-

ment was of an electrostatic-type with a beam diameterof 1 mm, and it was used to generate a spot area

sufficiently large for the surface analysis thereafter. The

maximum acceleration energy was 3 keV, and the

maximum beam current 100 mA.

SEM was performed to observe the deposit of metals

using a Hitachi S4500S field-emitter-type microscope.

The STM images were recorded by a Digital Instrument

‘Nanoscope E’ setup. Sharpened PtIr alloy tips (‘Nano-tip’) were used without any special pretreatment. The

scanner base/sample assembly was housed in a Plexi-

glass purge box (volume ca. 6 l) purged with ultrapure

Ar.

3. Results and discussion

3.1. Oxidation of H:Si(111), CH3:Si(111) and

C10H21:Si(111) in DI water

The robustness of the CH3:Si(111) and C10H21:Si(111)

surfaces was evaluated in comparison with the H:Si(111)

surface by tracing adsorbed O2 in DI water. Specimens

of H:Si(111), CH3:Si(111) and C10H21:Si(111) prepared

by the Grignard reaction were simply immersed in DI

water stored in a Teflon container with the lid off for 1day or more. The dissolution of O2 is believed to be

equilibrated with the ambient atmosphere at a concen-

tration of about 0.2 mM at room temperature. The

samples were immersed in the DI water for specific

periods; then were subjected to AES analysis.

After the immersion, Auger spectra of the surfaces

consisted of signals for Si, C and O. Fig. 2 shows the

Auger peak height ratios of O(KLL, 520 eV)/Si(LMM,100 eV) for H:Si(111), CH3:Si(111) and C10H21:Si(111)

as a function of immersion time in pure water. It is clear

that H:Si(111) was rapidly and continuously oxidized,

whereas CH3:Si(111) and C10H21:Si(111) retained smal-

ler amounts of O. C10H21:Si(111) gained a small amount

of O in the initial stage, and no increase of O was seen

for an extended period of time. On H:Si(111), the peak

position and the fine structure of the Si(LMM) transi-tion were gradually altered to those of silicon oxide. The

substrate of H:Si(111) was oxidized by dissolved O2

present in equilibrium with the atmosphere to form

SiO2. The fine structure of the AES Si(LMM) signal for

CH3:Si(111) indicated that the surface consisted of a

mixture of Si0 and SiO2. On the other hand, the

Si(LMM) peak remained unchanged on the alkyl-

covered Si(111) during the entire immersion time. TheO signal seen in the spectra of alkyl-covered Si(111) is

probably due to water trapped in the brush-like adlayer

composed of linear C10H21�/ chains, and/or due to

T. Yamada et al. / Journal of Electroanalytical Chemistry 532 (2002) 247�/254 249

partial oxidation of alkyl moieties. The oxidation of

substrate Si is attributed to the effect of dissolved O�2 in

water [12]. The CH3 species also significantly delays

oxidation of the substrate Si. This result shows unequi-

vocally that alkyl-covered Si(111) has a robustness

distinguished from H:Si(111).

3.2. Enhancing the contrast between metal-deposited and

non-deposited areas on Si(111) using Cu BHF solutions

The present process of patterning involves electron-

beam-assisted oxidation of alkyl-covered Si(111) and

retention of metal species after immersion into aqueous

solutions containing metal ions in the oxidized area

[7,8]. The oxidized area is composed of SiO2 in an

amount close to that corresponding to a monolayer witha significant amount of included C, which originated

from the alkyl moieties. Within aqueous media, the SiO2

surface can be modified with OH groups and becomes

interactive with metal cations, whereas the intact alkyl

monolayer is indifferent to cations. This is the mechan-

ism of position-selective impregnation of metal.

We found that Ni atoms were selectively deposited in

the electron-bombarded SiO2 areas in a NiSO4�/

(NH4)2SO4 solution at room temperature [7,8]. The

result was the impregnation of Ni only over the oxidized

spot, with an uptake of Ni less than a monolayer, as

detected by AES. The deposition spot of Ni was

indistinguishable by visual observation. The mechanism

of deposition seems to be different from the reduction of

Ni2� to bulk Ni metal coupled with the oxidation ofsubstrate Si, which is known to proceed at elevated

temperatures around 80 8C, in the same solution [13�/

15]. The Ni species is considered to be impregnated as

Ni2� coordinated to the Si�/O or Si�/OH species in the

bombarded areas. A small amount of Ni is preferable

for nanometer-scale patterning, as the thickness of the

deposit cannot be large compared to the dimensions of

the patterns along the surface. The position selectivity ofNi deposition originates from the chemistry of the Si�/O

deposit, and therefore we anticipate a sharp contrast

with minimum extraneous deposition in non-bom-

barded areas.

However, in order to define the limit of robustness of

the alkyl monolayers, it is interesting to investigate the

effect of increasing the thickness of the metal deposit by

using powerful reagents under severe conditions such ashigh reaction temperatures, long deposition times and

high reagent concentrations. During the build-up of the

metal deposit in the bombarded areas, the alkyl mono-

layers might deteriorate to some extent, resulting in

extraneous deposition outside the bombarded areas. We

attempted to control this trade-off by adjusting the

concentration and the reaction time to form a thick

layer of metal only in bombarded areas.To avoid experimental difficulties in using heated

Ni2� solutions, we chose a Cu ‘BHF’ solution at room

temperature, which was composed of CuSO4, HF and

NH4F [16,17]. The process of deposition of bulk Cu

metal on a Si surface in the BHF solution is considered

to involve the oxidative dissolution of Si to form SiF2�6

and the coupled reduction of Cu2� into metallic Cu

[16,17]. The dissolution of SiOx in the bombarded areaprecedes the reduction of Cu2�, and the alkyl-covered

areas will be inert as long as the erosion of Si substrate

by the fluoride components is negligible.

A specimen of C10H21-covered Si(111) was subjected

to electron-beam bombardment in ambient O2 (incident

electron energy�/2 keV, beam current�/100 mA, beam

spot diameter�/ca. 1 mm, irradiation time�/300 s, O2

pressure�/4�/10�4 Pa). No pattern was visible aroundthe bombardment spot. A drop of a 0.1 M CuSO4�/1 M

HF�/1 M NH4F solution was placed on the specimen at

room temperature. In 5 s, a dark matt area with a

diameter of about 1 mm was visible to the eye at the

bombardment spot. The drop of solution was then

washed away by DI water, and the deposited pattern

was examined. The spot was dark and clearly distin-

guishable from the surrounding area that was wetted bythe BHF solution and lightly colored with metallic Cu.

In the area away from that wetted by the BHF solution,

the color was that of the original Si(111). The contrast

achieved was clearly visible to the eye. However, the

Fig. 2. Auger peak height ratios O(KLL, 520 eV)/Si(LMM, 100 eV) of

H:Si(111) (filled circles), CH3:Si(111) (filled triangles) and

C10H21:Si(111) (filled squares) as a function of immersion time in pure

water exposed to the atmosphere. H:Si(111) was prepared by immer-

sing pre-oxidized n-Si(111) in a 40% aqueous NH4F solution for 10

min. CH3:Si(111) and C10H21:Si(111) were prepared by the reaction of

H:Si(111) with n -CH3MgBr (1 M in tetrahydrofuran at 65 8C) and n -

C10H21MgBr (1 M in diethylether at 35 8C) for 18 h. Incident electron

energy for AES�/2 kV.

T. Yamada et al. / Journal of Electroanalytical Chemistry 532 (2002) 247�/254250

C10H21-covered area was also covered with a certain

amount of Cu. The F� species within the solution seems

primarily responsible for the oxidation. The deposition

of Cu in the alkyl-covered area is considered to have

been nucleated at the defects that existed in that area,

which might include a small amount of O2 invisible to

AES. It is possible that the nucleation of Cu islands took

place on the defects in the C10H21-monolayer, such as

pinholes in the monolayer itself, as well as the steps and

voids of the substrate.

A dilute BHF solution (0.01 M CuSO4�/0.11 M HF�/

0.11 M NH4F) was employed to reduce the Cu deposit

off the bombardment spot. The same procedure as

described above with 30 s of deposition time made the

bombardment spot colored, whereas the alkyl-covered

part wetted by the solution did not change color. This

specimen was introduced into the UHV and subjected to

AES. Fig. 3 shows the Auger spectrum recorded at the

bombardment spot and that recorded off the spot within

the wetted area. The bombarded area gave a spectrum of

Cu without the Si(LMM) signals. Clearly, the bombard-

ment spot was covered with a thick layer of Cu. The

signals of C and O indicate the presence of surface

impurities overlaid on the Cu deposit. These impuritiesmust naturally originate in the deposition solution and/

or in contact with the atmosphere during the specimen

transfer. The spectrum for the area away from the spot

did not exhibit the signal of Cu, indicating that there

was no homogeneous deposition of Cu. The contrast

enhancement was thus successful as was observed both

visually and spectroscopically.

Fig. 4 shows a digital-camera image of a bombard-ment spot (a), and SEM images recorded at the borders

of a deposited Cu spot and an area away from the

electron-bombarded area (b, c). The diameter of the Cu

deposit was about 1 mm, and the perimeter of the spot

was rather wrinkled. The SEM images show that the

perimeters of Cu islands are apparent with a sharp

cutoff of the distribution of Cu. We could not detect

small Cu islands grown on the intact C10H21-monolayer.The Cu spot is seen to consist of grains with diameters

much larger than 50 nm. The thickness of the Cu deposit

must have been greater than this, as can be inferred from

the AES spectra.

The in-plane distribution of the electronic current

density generated by the electron gun is circular and

follows ca. a Gaussian function of the distance from the

center of the bombardment spot. Proportionally, thedensity of O2 on the surface follows a function similar to

this. This function contains values between the max-

imum and zero within an extensive annular area. We

should therefore consider a certain mechanism of the

sharp stepwise discrimination of the beam current

density manifested by Cu deposition. The process of

deposition depends on the density of silicon oxide

varying as a function of radius, the rate of depositionof Cu, the time of deposition treatment, and the growth

mechanism of the metallic Cu island. The wrinkled

border of the Cu deposit seems to have been formed by a

mechanism of dynamic growth of the deposit along the

surface, which was abruptly terminated when the

deposition solution was washed away.

3.3. Detection of electron-bombarded patterns on alkyl-

covered Si(111) by STM

As we reported previously, the intrinsic corrugation of

CH3-covered Si(111) and that of H:Si(111) are smaller

than 1 nm (peak-to-peak) observed by STM. In the

present study we scanned the electron-bombarded area

of these surfaces in an O2 atmosphere. The oxidized area

exhibited a corrugation between 5 and 10 nm. The

features of the oxidized surfaces are apparently differentfrom those before bombardment in microscopic images

with a frame size of ca. 20 nm2. To estimate the contrast

between the bombarded and non-bombarded areas with

Fig. 3. Auger spectra recorded on and away from the Cu-deposited

from the BHF solution on an electron-bombarded spot on

C10H21:Si(111) with ambient O2. The spectra were recorded at the

ultimate vacuum of the chamber at 2 kV of the incident energy and 1�/

5 mA of the sample current. The electron bombardment condition was:

incident energy�/2 keV, sample current�/300 mA, spot diameter ca. 1

mm, O2 pressure�/4�/10�4 Pa, irradiation time�/300 s. Cu was

deposited by immersion into a 0.01 M CuSO4�/0.11 M HF�/0.11 M

NH4F solution for 30 s. (a) AES recorded ca. 3 mm away from a

bombardment spot; (b) AES exactly on the bombardment spot.

T. Yamada et al. / Journal of Electroanalytical Chemistry 532 (2002) 247�/254 251

a large frame size, electron irradiation was performed

through a mask to project a pattern on the sample

surface.

The mask used was a Cu grid (#800, thickness 0.005

mm) manufactured for sample holding in transmission

electron microscopy. The pattern on this grid was a two-

dimensional array of holes (25 mm2) with a spacing of 10

mm (see Fig. 5). This grid was fixed at an aperture (1 mm

in diameter) made on a stainless-steel plate, and this

plate was placed in front of the Si(111) specimen so as to

separate the grid and the specimen surface by a space of

about 0.1 mm. The electron bombardment in ambient

O2 was performed under the standard conditions for

H:Si(111), CH3:Si(111) and C10H21:Si(111).

Wide-range STM images of these bombarded surfaces

are shown in Fig. 5. The grid patterns were clearly

copied on the Si(111) surfaces. In these images, the

electron-bombarded areas were indented. The apparent

average depths of indentation in these images were 30

nm for H:Si(111) (Fig. 5a), 3 nm for CH3:Si(111) (b) and

5 nm for C10H21:Si(111) (c) for an extended bombard-

ment time of 120 min. In general, O2 species on the

surface yield a lower tunneling current in STM imaging

than a non-oxygenated area; hence features in oxyge-

nated areas look darker in the constant-current topolo-

gical mode. It is difficult to tell whether the O2-deposited

areas were projected or receded topographically in the

present case. It should be noted that the difference of

height was estimated to be less than 10 nm for the alkyl-

covered surfaces within a frame of 50 mm2. Nearly all

ultimately small differences of height, which are actually

associated with chemically significant differences, have

been mapped for the processed patterns.It is expected that nanometer-scale patterns prepared

under the same principle as described above can be

imaged by STM. Based on this knowledge, we are

attempting to proceed to the final stage of this develop-

ment, using a nanometer-focused electron gun or

nanometer-pored masks, to demonstrate that the use

of organic monolayers is a viable approach for nano-

scale fabrications.

4. Conclusion

1) Alkyl-covered Si(111) is more resistant to oxidation

than hydrogen-terminated Si(111) in DI water with

O2 dissolved at equilibrium with atmospheric O2.

2) Electron-bombarded areas on alkyl-covered Si(111)

in ambient O2 was visualized by wet deposition ofCu selectively on areas of bombardment in a

CuSO4�/HF�/NH4F solution. The metallic Cu

deposit consisted of grains with diameters larger

Fig. 4. A digital-camera picture of Cu-deposited Si(111) (a) and SEM images of the perimeter of Cu deposit (b, c). The C10H21:Si(111) surface was

subjected to electron bombardment (incident electron energy�/2 keV, sample current�/100 mA, beam spot diameter�/ca. 1 mm, irradiation time�/

300 s) with ambient O2 (4�/10�4 Pa), and immersed into a 0.01 M CuSO4�/0.11 M HF�/0.11 M NH4F solution at room temperature for 30 s and

rinsed in DI water. The spot pattern composed of a metallic Cu deposit was clearly visible, and two areas at the perimeter of the deposit were

magnified by SEM (incident electron energy 25 keV).

T. Yamada et al. / Journal of Electroanalytical Chemistry 532 (2002) 247�/254252

than 50 nm when the maximum contrast was

achieved.

3) The electron-bombarded areas of alkyl-coveredSi(111) in ambient O2 and the remaining intact

areas were differentiated in STM observation of

specimens bombarded through a Cu mask (#800,

array of 25mm2 holes). This result represents a step

towards the realization of 10-nm-scale patterns

using alkyl-covered Si(111).

Acknowledgements

This work was financed by the Research for the

Future Project ‘Wafer-Scale Formation Process of Nano

Dots’, the Japan Society for the Promotion of Science,

Iketani Science and Technology Foundation, the Mur-

ata Science Foundation and the Yazaki Memorial

Foundation for Science and Technology. The authors

are grateful for the generous donation of the UHVapparatus by Matsushita Research Institute, Tokyo,

Inc. Thanks are also due to Kitano Seiki Co., Ltd., who

leased valuable instruments to us. The authors are

grateful to Professor Yutaka Okinaka for his help in

preparing this article.

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Fig. 5. STM images of H:Si(111), CH3:Si(111) and C10H21:Si(111) after electron bombardment with ambient O2 with a grid placed in front of the

surface. The dimensions of the grid were as indicated. The electron bombardment was performed at 2 kV of incident energy, 300 mA of sample

current within a beam diameter of ca. 1 mm. The ambient O2 pressure was 1�/10�6 Torr. STM was performed in a demoisturized purge box

supplied with ultrapure Ar. A PtIr tip was employed. Tunneling gap voltage�/�/1.5 V (tip negative), tunneling current�/1 nA. (a) H:Si(111),

bombarded for 20 min, frame size�/5�/5 mm2, height span�/40 nm (lightest to darkest), (b) CH3:Si(111), bombarded for 20 min., frame size�/5�/5

mm2, height span�/10 nm, (c) C10H21:Si(111), bombarded for 120 min, frame size�/5�/5 mm2, height span�/10 nm.

T. Yamada et al. / Journal of Electroanalytical Chemistry 532 (2002) 247�/254 253

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