Selective area growth of metal nanostructures

Article abstract of DOI:10.1063/1.115861

Nanometer-scale metal lines are fabricated onto Si(100) substrates by scanning tunneling microscope (STM) based lithography and subsequent chemical vapor deposition. An STM tip is first used to define areas for metal layer growth by electron stimulated desorption of adsorbed hydrogen. Exposure to Fe(CO)5 at 275°C results in preferential deposition of Fe onto Si dangling bond sites (i.e., depassivated areas defined by the STM tip), while the monohydride resist remains intact in surrounding areas. Fe metal lines with widths ～10 nm are constructed using this selective-area, autocatalytic growth technique.

Full text of DOI:10.1063/1.115861

Selective area growth of metal nanostructures
D. P. Adams, T. M. Mayer, and B. S. Swartzentruber
Citation: Applied Physics Letters 68, 2210 (1996); doi: 10.1063/1.115861
View online: http://dx.doi.org/10.1063/1.115861
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Selective area growth of metal nanostructures
D. P. Adams, T. M. Mayer, and B. S. Swartzentruber
Sandia National Laboratories, P. O. Box 5800, Albuquerque, New Mexico 87185
͑Received 20 November 1995; accepted for publication 7 February 1996͒
Nanometer-scale metal lines are fabricated onto Si͑100͒ substrates by scanning tunneling
microscope ͑STM͒ based lithography and subsequent chemical vapor deposition. An STM tip is first
used to define areas for metal layer growth by electron stimulated desorption of adsorbed hydrogen.
Exposure to Fe͑CO͒ at 275 °C results in preferential deposition of Fe onto Si dangling bond sites
5
͑i.e., depassivated areas defined by the STM tip͒, while the monohydride resist remains intact in
surrounding areas. Fe metal lines with widths ϳ10 nm are constructed using this selective-area,
autocatalytic growth technique. ͓S0003-6951͑96͒00816-8͔
The scanning tunneling microscope ͑STM͒ has become
useful not only for the study of surface structure and thin-
film growth but as a lithographic tool for building small de-
vices. In the past, several scanning probe based techniques
have shown potential in patterning sub-0.1 ␮m feature size
components. This includes processes that involve field
evaporation,1 electric-field manipulation of atoms,2 local
chemical reaction under a scanning probe tip,3–5 and expo-
sure of conventional polymeric6,7 and self-assembled mono-
layer resists.8,9 In general, these scanning probe based tech-
niques provide improved resolution compared with
conventional lithography, because tip-sample working dis-
tances are small for operation in both field-emission and tun-
neling modes. Small spacings provide a large current density
(ϳMA/cm2) and a high field strength at low voltages. It is
also apparent from previous work that these techniques are
capable of patterning what may accurately be referred to as
nanostructures. Recently, scanning tunneling microscopes
have been used to build structures of single atom width10 as
well as nanometer-scale test structures, e.g., magnetic arrays
of 10 nm sized Fe particles.11
Despite the progress made using these techniques, prac-
tical fabrication of sub-10 nm sized devices requires signifi-
cant improvements in dimensional control, reproducibility,
and the ability to transfer patterns from a resist to a func-
tional material. In many cases, lack of control and reproduc-
ibility are due to changes in tip geometry that occur during
lithography. Tips are susceptible to growth or deformation
when local chemical reaction4 or scratching of the surface is
involved. Pattern transfer requires robust resists that can
withstand subsequent processing.
In this letter, we discuss a technique for producing
nanometer-scale metal structures that provides better control
over feature size without drastically altering tip geometry.
This technique combines STM-based lithography of atomic-
scale resists12,13 with thermal chemical vapor deposition
͑CVD͒.14,15 Atomic-scale resists are useful for defining small
features because the monolayer thickness prevents signifi-
cant beam scattering. We choose to use hydrogen as a resist
because the conditions ͑i.e., current density and voltage͒ re-
quired to remove H from a Si surface have already been
determined. Becker et al.16 and Boland17 showed that hydro-
gen can be desorbed from a Si surface using very low energy
electron beams ͑ϳ10 eV͒. In more recent work, Shen et al.18
measured the cross section for H removal for various inci-
dent electron energies. Above ϳ6 eV, hydrogen removal oc-
curs by electron stimulated desorption, whereas at lower en-
ergies, hydrogen is desorbed by a multiple-vibrational
excitation process.19 Also, we have characterized the litho-
graphic performance of monohydride resists by determining
the electron doses required for 5–50 nm sized feature
definition.20
The goal of the present study is to test the usefulness of
hydrogen as a resist for pattern transfer selective area CVD
by growth. Selective chemical vapor deposition of metals has
been used in the past, primarily for fabrication of large-scale
structures.21 In this work, we take advantage of our ability to
pattern nanometer-sized features in H-terminated Si͑100͒
surfaces by local desorption using the STM. Metal CVD in-
volves pyrolysis ͑thermal decomposition͒ of Fe͑CO͒5 precur-
sor molecules with the tip withdrawn from the surface. We
choose pyrolysis of Fe͑CO͒5, because previous studies indi-
cate that the nucleation kinetics are dominated by site-
specific reactivity of the precursor with the substrate.22 In
addition, Fe layer growth on Si͑100͒ via pyrolysis occurs at
temperatures below the monohydride desorption temperature
͑520 °C͒ with little C or O contamination.23
All experiments are conducted within an ion-pumped,
ultrahigh vacuum system ͑base pressuresϭ6ϫ10Ϫ11 Torr͒.
Si͑100͒ samples ͓Virginia Semiconductor, n-type ͑P͔͒ are
first heated to 1250 °C in vacuo to remove impurities and
reveal a clean, well-ordered starting surface. Si͑100͒:H 2ϫ1
surfaces are prepared by exposure to an atomic H beam with
the samples maintained at ϳ350 °C. The H doser consists of
a hot ͑1600 °C͒ W filament fixed 1 in. from the sample and a
UHV leak valve attached to a glass bottle of high purity
͑99.9995%͒ H2. Formation of a hydrogenated surface is ac-
complished using exposures of 1200 L.
After hydrogen dosing, the sample is placed into the Ta
clip holder of a variable-temperature STM and equilibrated
at 275 °C for 6 h. In this holder, samples are scanned to
ensure that higher-order hydride species did not form during
dosing. Selected areas of the monohydride terminated sur-
face are then stripped of H using sample-tip biases greater
than 6 V. For CVD, the tip is withdrawn from the surface
and the sample is dosed with 60 L of freshly distilled
Fe͑CO͒5. Fe͑CO͒5 is contained in a covered Pyrex bottle that
is kept cold ͑77 K͒ when not in use to avoid decomposition
to Fe2͑CO͒9.
Scanning tunneling microscopy is used to monitor the
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2210 Appl. Phys. Lett. 68 (16), 15 April 1996 0003-6951/96/68(16)/2210/3/$10.00
140.254.87.149 On: Fri, 19 Dec 2014 23:34:56

FIG. 1. STM micrograph of a Si͑100͒:H 2ϫ1 surface. The micrograph is
taken using a sample bias of Ϫ2.30 V and a tip-sample current of 0.15 nA.
sample surface after depassivation and selective area growth.
The images presented in this work are acquired using a
sample bias of Ϫ2.30 V and a tunneling current of 0.15 nA.
These particular operating conditions do not result in addi-
tional hydrogen desorption.
Figure 1 shows a typical H-passivated Si͑100͒ surface
used in this study. This surface is monohydride terminated
and contains a small defect density. As demonstrated in pre-
vious work, H-passivated dimers are aligned in rows, with
each dimer having an oblong shape.24 After depassivation,
bright features appear due to the H-free Si dangling bonds.
This is shown in Fig. 2 for a line drawn onto a single terrace
using a sample bias of ϩ8 V and a line dose of 100 ␮C/cm.
For this particular line, the depassivated area is continuous
over a width of ϳ8 nm; a limited amount of electron stimu-
lated desorption occurs outside the central region.
FIG. 3. STM micrographs taken after Fe͑CO͒ dosing of the patterned Si
5
monohydride surface. Tgrowthϭ275 °C. Two separate lines are shown. In
each, the metal has been selectively deposited onto more reactive Si dan-
gling bond sites. In the bottom micrograph, the metal line is drawn ϳ25°
with respect to the dimer row direction. Monohydride-terminated Si dimer
rows are seen in neighboring areas on the surface. The micrograph is taken
using a sample bias of Ϫ2.30 V and a tip-sample current of 0.15 nA. The
contrast is line adjusted.
Metal is selectively deposited onto the patterned areas by
pyrolysis of Fe͑CO͒ at 275 °C, as demonstrated in Fig. 3.
5
Metal lines formed on clean Si have a rough surface mor-
phology and are most likely polycrystalline. STM and heavy
ion backscattering spectrometry ͑HIBS͒ confirm that ϳ2
monolayers of Fe are locally deposited during a 60 L dose.
Figure
4
shows representative HIBS spectra for
Fe͑CO͒5-dosed Si͑100͒ and Si͑100͒:H 2ϫ1 surfaces. In sepa-
rate experiments, x-ray photoelectron spectroscopy indicates
very small C and O impurity levels, characteristic of pyroly-
sis onto Si substrates at elevated temperatures.25,26
These results show that metal nanostructures can be
grown on patterned H-terminated Si substrates. STM con-
firms that the adsorbed hydrogen is still intact as a monohy-
dride in the unexposed areas of the surface and is not dis-
rupted by the Fe͑CO͒5. There are a few bright objects present
on H-terminated areas after Fe͑CO͒ dosing that have an
5
FIG. 2. STM micrograph showing a single line stripped of H in the ͗110͘.
Electron beam stimulated desorption involved a sample bias of ϩ8 V and a
line dose of 100 ␮C/cm. Depassivated Si dimer rows are seen within the
central portion of line. Scattered bright sites in the vicinity of the patterned
line, in part, represent areas ‘‘exposed’’ by the electron beam. The micro-
graph is taken using a sample bias of Ϫ2.30 V and a tip-sample current of
0.15 nA.
areal density approximately equal to the density of defects on
the as-prepared H-terminated surface. These may be metal
clusters that have nucleated at specific defect sites, such as
higher order hydrides or metal impurities, but additional
work is required to confirm this.
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Appl. Phys. Lett., Vol. 68, No. 16, 15 April 1996 Adams, Mayer, and Swartzentruber 2211
140.254.87.149 On: Fri, 19 Dec 2014 23:34:56

with thermal CVD can be used to deposit metal nanostruc-
tures. Fe is grown selectively onto depassivated regions of
Si͑100͒:H 2ϫ1 surfaces by Fe͑CO͒5 pyrolysis at 275 °C. The
monohydride resist remains intact during gas exposure, with
only limited nucleation occurring on H-terminated Si. The
effect of hydrogen is to increase the barrier to nucleation
compared with growth on clean Si͑100͒. This is demon-
strated with the construction of 10 nm wide lines. In general,
we feel that this technique could be useful for locally depos-
iting other metals via CVD or for fabricating novel quantum-
effect electronic devices. The success of this technique relies
upon choosing a CVD gas system in which nucleation is
dominated by site-specific chemical reactions.
The authors thank J. Banks and J. Knapp of Sandia Na-
tional Laboratories for the HIBS work. This study was sup-
ported by the U.S. Department of Energy under Contract No.
DE-AC04-94AL85000.
FIG. 4. HIBS spectra showing Fe metal content after 60 L Fe͑CO͒ dose at
275 °C for ͑a͒ a clean Si͑100͒ surface and ͑b͒ a Si͑100͒:H 2ϫ1 surface.
5
Nucleation during Fe CVD growth is dominated by site-
specific chemical reactivity of the precursor with the sub-
strate. Previous work has shown that decomposition of the
Fe͑CO͒5 molecule occurs predominantly at Si dangling bond
sites.22 Passivation of the dangling bond sites with hydrogen
removes the active site for pyrolysis and effectively raises
the activation barrier for nucleation. Previous studies have
also shown that Fe CVD is autocatalytic on Si—i.e., the
barrier to dissociation of the precursor on an existing Fe
cluster is smaller than on the clean Si surface ͑about 0.14 eV
on Fe and 0.40 eV on Si͒.22,23 Therefore, once a nucleus is
created, it grows rapidly compared to the formation of addi-
tional clusters. This highly nonlinear growth rate is advanta-
geous in maintaining the area selectivity of metal deposition.
This technique appears to be well-suited for fabrication
of sub-10 nm sized metal structures. Control over feature
size at this level is evident. The two Fe metal lines shown in
Fig. 3 have approximately the same width as the depassi-
vated areas defined by the STM, ϳ10.0 nm. Further reduc-
tion in linewidth may be achieved by defining even smaller
areas during H removal. In this work, we have produced
continuous depassivated lines of Si with a width as small as
2.5 nm. Other studies have demonstrated continuous lines
with widths as small as 1.0 nm, i.e., of single dimer width.19
In general, depassivated Si linewidths can be tailored by
changing line dose, tip voltage, or tip-sample separation.20
Future work will focus on how the width of metal lines is
influenced by continued metal precursor exposure and
growth. It is expected that lateral, i.e., in-plane, growth will
occur as the line is thickened. We note that the lines shown
in Fig. 3 are only partially continuous. Since growth is auto-
catalytic, however, metal deposits may easily be thickened
without nucleating an appreciable number of additional clus-
ters on H-covered areas.
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