Nanometer Patterning with IceNanometer Patterning with Ice
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CitationCitationKing, Gavin M., Gregor Schurmann, Daniel Branton, and Jene A.
Golovchenko. 2005. Nanometer patterning with ice. Nano Letters
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Nanometer Patterning with Ice
Gavin M. King,*,†Gregor Schu 1rmann,‡Daniel Branton,‡and
Jene A. Golovchenko†,§
Department of Physics, Department of Molecular and Cellular Biology,
and DiVision of Engineering & Applied Sciences, HarVard UniVersity,
Cambridge, Massachusetts 02138
Received March 3, 2005; Revised Manuscript Received April 12, 2005
Nanostructures can be patterned with focused electron or ion beams in thin, stable, conformal films of water ice grown on silicon. We use
these patterns to reliably fabricate sub-20 nm wide metal lines and exceptionally well-defined, sub-10 nanometer beam-induced chemical
surface transformations. We argue more generally that solid-phase condensed gases of low sublimation energy are ideal materials for nanoscale
patterning, and water, quite remarkably, may be among the most useful.
Electron beam (e-beam) lithography is a powerful established
method used to pattern nanometer scale structures such as
single electron transistors, single molecule detectors, and
nanoelectromechanical devices.1-4E-beam lithography typi-
cally involves applying, chemically transforming, and chemi-
cally dissolving a polymer resist.5The desire to decrease
device size, to enhance the role of quantum mechanical
device characteristics, and to pattern increasingly complex
substrates requires new lithographic approaches to define
nanometer scale structures. Here we demonstrate a new
approach to nanoscale e-beam patterning based on the
condensation and beam stimulated sublimation of water ice.
Electron and ion beam stimulated sublimation of solid
condensed gases has been studied in the context of cryo-
electron microscopy,6plasma confinement,7and astrophys-
ics,8but not, to our knowledge, for high-resolution patterning.
We have discovered that frozen water, a common substance,
can be used to generate nanoscale patterns of metals and
potentially very useful chemical transformations on a sub-
We chose this approach because it is known that electrons
and very high energy ions create electronic excitations,
molecular dissociations, and ionization in ice, but the
consequences of these effects on nanoscale patterning have
not been explored. In insulating surface layers with low
sublimation energies, “Coulomb explosions” can lead to large
localized agitation and ejection of surface atoms.8In addition,
dissociation products of condensed molecular gases can be
ejected after diffusion to the surface from deeper regions
within the beam exposure volume.6,9These processes, as well
as others, have been discussed,7,10,11but we feel it fair to
conclude that the fundamental physics responsible for these
observations is still obscure.
In our experiments, we deposit very thin, conformal layers
of ice from water vapor onto cryogenically cooled silicon
substrates in the chamber of a combined scanning electron
microscope (SEM) and focused ion beam (FIB) apparatus
(FEI Co., Hillsboro, Oregon). Subsequent exposure of the
ice surface to focused energetic electron or gallium ion beams
stimulates local removal of ice and ultimately exposes the
underlying silicon substrate in whatever patterns the beams
are programmed to produce. Afterward, using lower beam
doses, the exposed regions can be inspected nondestructively
with the SEM or FIB operating in its standard scanning
imaging mode. With the sample still cold, the exposed
patterned regions of the substrate can then be metallized or
modified by other techniques (ion implantation, reactive ion
etching, sputter deposition, etc.) in the same vacuum chamber
or an adjoining one. Finally, the ice can be removed either
by in situ sublimation (eliminating liquid surface tension
effects) or by rinsing.
Typically, we deposit ice at a rate of ∼1 nm/s using a
leak valve controlled water vapor flow that is directed onto
the cooled sample (Figure 1). Uniform ice coverage over
∼1 mm2is achieved with a single needle (inner diameter
∼100 µm) water vapor source held ∼5 mm above the sample
surface. At operating sample temperatures of 128 K the
deposited ice is amorphous and sublimes at a rate of only
∼0.3 monolayers/hour with a sublimation energy of 0.45
eV.12,13In practice, no appreciable ice sublimation is observed
over several hours when working at 128 K, even with ice
films <10 nm thick. The ice is promptly removed by
sublimation when the sample temperature is raised to
∼180 K. We have successfully patterned ice with a focused
* Corresponding author. Telephone: 303-492-7818; Fax: 303-492-5235;
†Department of Physics.
‡Department of Molecular and Cellular Biology.
§Division of Engineering & Applied Sciences.
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10.1021/nl050405n CCC: $30.25
Published on Web 04/29/2005
© 2005 American Chemical Society
ion beam (30 keV Ga+, 10 pA, diameter ∼10 nm) or a
focused electron beam (1-30 keV, 30-150 pA, diameter
∼5 nm). All data presented here were patterned with an
electron beam. After patterning, samples were transferred
while cold to a connected chamber with a chromium sputter
An in situ SEM image of a 75 nm thick layer of ice on a
silicon substrate at 128 K immediately after 5 keV e-beam
patterning is shown (Figure 2a). The e-beam dose used to
create each of the 500 nm square patterns was increased from
left to right. The sharp rise in contrast between the fourth
and fifth squares indicates that a dose above 8.8 × 105µC/
cm2is required to remove the entire ice layer and expose
the underlying silicon surface. An atomic force microscope
(AFM) line scan and an SEM image of the same sample are
shown (Figure 2b) after sputter deposition of 40 nm of Cr,
removal from the vacuum chamber, and a rinse in isopropyl
alcohol14to remove the remaining ice and its overlayer. Only
the Cr that comes in contact with the patterned ice-free
regions of the substrate remains after complete ice removal.
No aggressive methods such as ultrasonication or mechanical
scrubbing were needed to assist the liftoff. Figure 2b confirms
that 5 keV e-beam doses greater than 8.8 × 105µC/cm2are
required to remove the 75 nm ice layer thickness and ensure
subsequent pattern transfer to the deposited Cr. This critical
dose for water ice resist is roughly 3 orders of magnitude
larger than that required for a typical exposure of the polymer
resist polymethyl methacrylate (PMMA). Assuming the
amorphous ice layer has a density130.91 gm/cm3, we
calculate the sputter yield (i.e., H2O molecules ejected per
incident electron) S ) 0.03 for 5 keV electrons. S decreases
by over an order of magnitude as the beam energy increases
from 1 to 30 keV, but it does not vary significantly with
temperature between 128 and 158 K. In contrast to the
relatively low electron sputter yield, we found S ≈ 12 for
30 keV gallium ions in an FIB!15
Figures 2c and 2d present results of varying the line dose
under the same conditions as previously described for the
area dose study, except with a thinner (20 nm) ice layer.
Figure 2c is an in situ cryogenic SEM image of micron-
long, single pixel lines of varying doses. Here the line dose
is increased from 1.1 µC/cm (left) to 5.6 µC/cm (right). After
6 nm Cr deposition, an SEM image and an AFM line scan
(Figure 2d) yield the critical line dose under these conditions
of ∼2.2 µC/cm. We note the patterned metal dot at the
bottom end of each line allows for easy identification of
The AFM scans in Figures 2b and 2d also reveal the
growth of a layer of material ∼ 1 nm thick in the region
where the beam hit the ice even with doses too low to eject
all of the ice down to the underlying substrate. We attribute
this thin layer to a beam-induced surface chemical transfor-
mation that involves both the ice layer and the underlying
silicon substrate because (a) the effect is observed with no
metallization step; (b) the effect is specific to silicon, i.e.,
we see no similar growth of material on SiO2 or Si3N4
Figure 1. Schematic of the apparatus. Within the main vacuum
chamber at <1 × 10-6Torr, water from the vapor phase is directly
condensed onto cryogenically cooled substrates, typically at 128
K. The water vapor pressure is controlled by leak valves (not shown)
connected to a MgSO4‚7H2O water vapor source. The coldfinger
(located ∼5 mm above the sample surface) is held at 80 K, ensuring
a sufficient thermal gradient to keep unwanted species from
condensing on the substrate. Ice deposition rates of ∼1 nm/s are
easily achieved. Substrate temperatures can be controlled to (1 K
and raised to sublime off unwanted ice. An Ar sputter gun allows
for metallization in the loadlock chamber.
Figure 2. Patterning a silicon surface. (a) In situ cryogenic (sample
T ) 128 K) SEM image of 500 nm × 500 nm squares formed in
a 75 nm thick ice layer by a 5 keV, 112 pA electron beam. The
electron beam patterning dose was increased from 2.2 (left side,
square barely visible) to 35.2 × 105µC/cm2(right). (b) Room-
temperature SEM image of the same sample after metallization (40
nm Cr). A topographic AFM line scan thru a region of the surface
(dashed line) is shown above the SEM image. (c) In situ cryogenic
SEM image of single pixel lines defined in a 20 nm thick ice layer
on silicon (e-beam doses as stated). (d) Post metallization SEM
and AFM line scan of the same sample.
Nano Lett., Vol. 5, No. 6, 2005
substrates; (c) the material grows only to a self-terminating
thickness above the silicon (111) surface; and (d) the final
(self-terminated) thickness of the material scales with the
amount of ice initially deposited on the silicon surface. When
5 nm of ice was deposited, subsequent e-beam exposure
produced a ∼0.5 nm high chemical transformation; a 75 nm
ice thickness produced a 3.0 nm high structure. Further
e-beam exposure did not increase the material’s height. In
contrast to contamination lithography,16observations (b) and
(c) imply that we can rule out deposition of unspecified (e.g.,
hydrocarbon) vacuum contaminants in the chamber. Since
the electron range in our experiments is larger than the ice
layer thickness, observation (d) suggests that the stimulated
surface chemistry required for the material growth persists
only as long as the ice layer that is being removed still exists.
Considering the possible species present (H2O, its atomic
or molecular fragments, silicon and hot electrons) on or near
the silicon-ice interface when the beam impacts, we postulate
that the thin layer of material is likely silicon oxide. This is
further supported by the observation that the thin layers of
material can be removed with hydrofluoric acid, leaving
behind a depression in the silicon surface (Figure 4f). The
electronic properties of the thin layer of material are currently
The spatial resolution that we have achieved for ice
patterned Cr lines on silicon using ∼20 nm thick ice layers
is shown in Figure 3 with beam exposures (a) at 30 keV
and (b) at 20 keV. We note that this apparent difference in
line width is potentially related to aperture alignment,
astigmatism adjustment, and focus optimization steps neces-
sary after changing the beam acceleration voltage. The
narrowest dimensions of the material tentatively identified
as silicon oxide (Figure 4) are, remarkably, much smaller
than those obtained for Cr (Figure 3). A study of the lateral
dimensions of these putative oxide structures as a function
of beam energy illustrates the evident trend toward higher
resolution with increasing electron beam energy. At 10 keV
(Figure 4a), the structure’s full width at half-maximum
(fwhm) measures 18 nm. The fwhm is reduced to 14 nm at
25 keV and to 10 nm at 30 keV (Figures 4b and 4c). The
measurements listed in Figure 4 are values obtained without
subtracting the effective diameter of the AFM probe tip.
When we account for the AFM tip effective diameter of ∼5
nm (Figure 4d), the true width of these lines are of order
13, 9, and 5 nm for the 10, 25, and 30 keV exposures,
respectively. Figure 4e shows a 2 µm × 1.5 µm AFM image
of two lines defined by a 30 keV electron beam. Here we
see that the 3.0 µC/cm line measures ∼1.2 nm in height
above the Si, while the 1.5 µC/cm line on the left is only
∼0.8 nm high.
Although the “record” line widths of e-beam exposed
PMMA are in the single digit nanometer range, such results
were achieved with specialized higher energy beams (∼100
keV) and attentive ultrasonication during resist develop-
ment.17-19A more typical minimum line width achieved with
e-beam exposed PMMA on bulk silicon substrates with
commercial e-beam lithography tools is of order 30 nm.20
To reliably achieve thinner line widths, angled depositions
of metals or other materials on exposed resists are common.
Our demonstrated sub-20 nm metal lines are achievable with
an ice resist on bulk silicon with e-beam energies <30 keV
and without directional deposition of metal. We speculate
that proximity effects21,22are minimized with ice resist
because the excitation of ice by backscattered electron
exposure away from the point of beam incidence is not an
additive process. Instead, excited ice that is not ejected from
the surface can relax to its initial unexposed state.
Patterning with ices of any condensed gas is a straight-
forward and practical process. Ice resist does not require
Figure 3. SEM analysis of sub-30 nm wide Cr lines on a bulk
silicon substrate. Both structures were defined on 20 nm ice resist
layers and subsequently metallized with 8 nm Cr. (a) 30 keV line,
4.5 µC/cm dose. (b) 20 keV line, 4.4 µC/cm dose.
Figure 4. AFM analysis of the local electron beam induced
chemical transformation of a silicon substrate. All data except panel
(d) are tapping mode AFM images with associated AFM linescans.
All results in (a-c) and (e) were produced with beam doses too
low to eject all of the ice down to the silicon substrate. (a-c) Line
width as a function of beam energy (energies as stated). (d) TEM
image of the AFM tip taken after acquiring the data in (a-c). (e)
Line height as a function of dose (30 keV beam). Left line, 1.5
µC/cm; right line 3.0 µC/cm. (f) Trenches are formed when raised
lines similar to that shown in (e) (right line) are treated for 15 s
with 48% w/w HF.
Nano Lett., Vol. 5, No. 6, 20051159
spinning or baking. All processing and patterning steps can
occur in a single evacuated chamber and be monitored at
high resolution. The final removal of unexposed resist leaves
minimal residues. Environmentally harmful solvents are not
required and complete dry removal of the ice layer can be
performed by in situ sublimation. Although in our current
apparatus dry resist removal by sublimation after Cr metal-
lization causes large loose Cr flakes to settle on the substrate,
an inverted configuration may avoid this problem. Com-
pletely dry resist processing will be particularly useful for
preparing delicate micro- and nanoelectromechanical devices
on a variety of substrates, especially those exhibiting complex
It will be important to discover the resolution limits and
to minimize the critical dose requirements by testing a wide
range of beam energies, beam diameters, and other condensed
gases. To avoid induced interface chemistry, rare gases are
clearly called for. Alternatively, to yield desired chemical
transformations, other gases can be selected to react with
the substrate and produce precisely defined thickness
tunnel junctions and gate insulators needed for nanoscale
electronic devices. The gate insulator needed for nanoscale
field effect transistors is an important example for which
the material grown with water may well be suited. We
anticipate that the rich and varied chemical, electronic, and
mass transport properties of energetic beam stimulated solid
condensed gases will provide many opportunities for dis-
covery and innovation in connection with nanoscale pat-
Acknowledgment. This work was supported by awards
from the NSF (#DMR-0073590), DOE (#DE-FG02-
01ER45922), NIH (#RO1HG02338), and Agilent Technolo-
gies. We thank the Harvard Center for Imaging and
Mesoscale Structures for facilities support as well as Damon
Farmer and Trygve Ristroph for assistance and discussions.
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