Electrochimica Acta 47 (2001) 891– 897
www.elsevier.com/locate/electacta
Electron-beam induced carbon deposition used as a mask for
cadmium sulfide deposition on Si(100)
T. Djenizian, B. Petite 1, L. Santinacci 1, P. Schmuki *
Department of Material Science, LKO, Uni6ersity of Erlangen – Nuremberg, Martensstraße 7, D-91058 Erlangen, Germany
Received 3 May 2001; received in revised form 30 July 2001
Abstract
The present work investigates the use of carbon masks, deposited on n-type Si(100) surfaces in a scanning electron microscope
(SEM), for electrochemical nanopatterning. Carbon contamination lines were written at different electron doses on the n-type
Si(100) surfaces and characterized by AFM. Subsequently, deposition of CdS was carried out by electrodeposition of Cd from a
1 mM CdF2 +0.05 M NaF solution followed by a chemical treatment in 1 M Na2S. CdS deposits were prepared under various
electrochemical conditions and were characterized by SEM, scanning Auger electron spectroscopy and optical techniques
including fluorescence and photoluminescence measurements. It is demonstrated that under optimized electrochemical conditions
carbon deposits of less than 1 nm thickness and in a width of 100 nm range can act as a negative resist, i.e. can block the
deposition of CdS completely and selectively and thus can be used for nanopatterning. © 2001 Elsevier Science Ltd. All rights
reserved.
Keywords: Electron-beam induced deposition; Negative resist; Electrodeposition; Patterning; Photoluminescence
1. Introduction
Micro- and nanometer scale pattern generation on
semiconductors plays a crucial role in the field of
semiconductor technology, particularly, in view of the
permanent shrinkage of dimensions in integrated circuits [1,2]. To achieve sub-micrometer resolution, a
wide range of processes have been studied, including
lithographic techniques essentially based on the exposure of photoresists to photons, X-rays [3], focused ion
beams (FIB) [3–6] and electron beams (E-beam). In
technological applications, E-beam lithography is
mainly used to fabricate photolithographic masks [3].
In research, however, exploratory direct processes also
are investigated, e.g. to generate ultra-small linewidths
on both Si and SiO2 [7 – 11]. E-beam induced deposition
reactions have also been studied to create 3D nanostructures [12 –14] in the 1 – 100 nm range. In this approach, the E-beam activates gaseous precursors
species, introduced into the vacuum chamber of the
* Corresponding author. Fax: + 49-9131-852-7582.
E-mail address: [email protected] (P. Schmuki).
1
On leave from Department of Material Science, EPF-Lausanne,
LTP, Switzerland.
E-beam instrument, leading to solid deposits on the
irradiated surface. Typically, the precursor vapor species that are used are metallorganic compounds or,
more simply, the residual hydrocarbons from the pump
oil. In the latter case (the so-called contamination writing), the hydrocarbon molecules adsorbed on the surface are decomposed by the E-beam to form an
amorphous structure of carbon, more specifically a
layer of diamond-like carbon (DLC). The predominant
amount of C sp3 in DLC confers to the carbonaceous
deposit electrical and mechanical properties comparable
to diamond [15,16].
Such contamination writing has been applied as a
mask using ion milling [17] or, recently, to achieve high
definition patterning of semiconductor surfaces for subsequent metal electrodeposition [18]. In this previous
work we showed how to use C-masks produced by the
contamination writing in a scanning electron microscope (SEM) to suppress completely and selectively
metal electrodeposition at treated surface locations. The
principle is that, in comparison with the resistivity of
the Si, the DLC layer behaves as an insulator and thus
can block the electrodeposition of the metal.
It was demonstrated that carbon deposits of the
order of only 1 nm thickness can be sufficient to
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T. Djenizian et al. / Electrochimica Acta 47 (2001) 891–897
achieve a negative resist effect, i.e. can block the electrodeposition of gold on a Si(100) surface completely
and selectively [19].
Within the present work, patterned electrodeposition
of cadmium sulfide (CdS) onto Si(100) is explored. Due
to their direct bandgap, CdX type of materials (with
X= S, Se [20], Te [21]) can play an important role for
opto-electronic devices and processes such as solid state
lasers and detectors [22], optical memories [23] and
solar cells [23– 26]. CdS, in particular, with a bandgap
of 2.4 eV has been the subject of a large number of
investigations. Different techniques have been investigated to deposit the II– VI semiconductor material
including chemical vapor deposition (CVD) [27], precipitation [23], plasma sputtering [28] and electrochemical deposition [29–32].
In the present work, we investigate the negative mask
effect of E-beam deposited carbonaceous structures created on Si(100) on the electrochemical deposition of
CdS.
2. Experimental
Experiments were carried out on n-type silicon
wafers (100) with a resistivity of 1– 10 Vcm. Prior to the
experiments, a layer of 1 mm thermal SiO2 was grown
on the wafers. Into this SiO2 layer, arrays of square
openings of 400× 400 mm were etched by classical
photolithography. (This pre-patterning was carried out
to facilitate locating the nanostructures that were subsequently deposited into the openings.) The patterned
wafers were cleaved to samples of 2×2 cm, each one
carrying five square openings. The samples were degreased by sonicating in acetone, isopropanol and
methanol, rinsed with deionized water and dried in an
argon stream. Subsequently, the samples were then
immersed into 1% HF for 1 min. (This treatment is
used to hydrogen passivate the surface within the
square openings while still leaving a sufficiently thick
thermal oxide layer on the rest of the surface.) To
achieve E-beam induced deposition of C-line patterns,
the samples were treated within the open squares with
the ‘horizontal’ single line mode in a Philips XL-30
FEG SEM. During the deposition the pressure of the
chamber was 3×10 − 6 mbar, the voltage was set to 30
kV and the E-beam current was 35 pA. Different
electron dose exposures were achieved by varying the
exposure time under a constant electron current.
Rectangle-shaped carbon patterns were produced using
the full frame mode of the SEM. In this case, the
surface was irradiated for 30 min within a frame of 35
mm ×25 mm.
After the E-beam treatment, contact to the Si samples was established by smearing an InGa eutectic on
the backside of the samples. The samples were pressed
against an O-ring of an electrochemical cell leaving a
surface containing five 400× 400 mm openings exposed
to the electrolyte. The electrochemical set-up consisted
of a conventional three-electrode configuration with a
platinum gauze as a counter electrode and a Haber–
Luggin capillary with Ag/AgCl electrode as a reference
electrode. Polarization curves (with a rate of 2 mV/5 s)
and potential step experiments were carried out using a
Jaissle potentiostat–galvanostat (IMP 88PC-100V). For
electrochemical CdS-deposition various approaches
were explored in the preliminary experiments. Direct
deposition of CdS according to the procedures given in
Refs. [31,32] did not lead to satisfactory results. Therefore, we followed an approach given in Ref. [33]. In the
first step, electrochemical deposition of Cd was carried
out from the electrolyte consisting of 1 mM CdF2 +
0.05 M NaF. In the second step, the metallic Cd was
converted to Cd(OH)2 by maintaining the sample at
open circuit conditions for 3 min in the above electrolyte. To convert the Cd(OH)2 into CdS, the sample
was then immersed for 90 s in a 1 M Na2S solution. All
solutions were prepared from reagent grade chemicals
and deionized water.
Chemical characterization of the deposits was carried
out by Auger electron spectroscopy (AES) using the C
peak at 270 eV, the Cd peak at 372 eV and the S peak
at 148 eV. AES spectra were acquired in a Perkin–
Elmer 660 AES spectrometer. Optical characterization
of the deposits was carried out by measuring the photoluminescence (PL) spectra at room temperature. The
PL spectrum was excited using 100 mW of the 400 nm
line of a krypton laser and the spectra were acquired by
two SPEX 1250 M monochromators in combination
with a Hamamatsu photomultiplier.
3. Results and discussion
Fig. 1a shows an AFM top view of a silicon surface
that was E-beam line exposed at different equidistantly
spaced locations for 300, 180, 120, 60 and 30 s, respectively. The C distribution across the surface was verified
by measuring the Auger C peak intensity perpendicular
to the deposit lines. The results clearly confirmed a
strongly increased presence of C at the E-beam treated
locations, with an increasing C intensity the longer the
E-beam exposure time. The topography of the carbon
lines was obtained from an AFM cross-section shown
in Fig. 1b, taken through the image of Fig. 1a. From
Fig. 1b it is apparent that independent of the exposure
time, the lines are distinctly separated. The height and
width of the C-lines as a function of the exposure time
are shown in Fig. 2. An increase in the exposure time
results primarily in an increased width of the carbon
lines. Height values were taken at the maximum and
linewidths were taken as FWHM of the nearly Gaus-
T. Djenizian et al. / Electrochimica Acta 47 (2001) 891–897
893
sian cross-section. The linewidth increases continuously
with the exposure time whereas the height of the de-
Fig. 3. Potentiodynamic polarization curves in the cathodic direction
from − 0.1 to −2 V (Ag/AgCl) in 1 mM CdF2 +0.05 M NaF for an
untreated Si sample and a sample carrying the C-line pattern as in
Fig. 1a.
Fig. 1. (a) AFM top view of an array of five contamination lines
E-beam deposited on silicon with 300, 180, 120, 60 and 30 s exposure
time (the exposure time increases from the right to the left). (b) AFM
cross-section. The increase of width of the lines corresponds to 300,
180, 120, 60 and 30 s E-beam exposure.
Fig. 2. Dependence of the carbon line height and width on the
E-beam exposure time.
posit tends to reach a limit value of around 3 nm at 120
s E-beam irradiation time. The fact that the height of
the carbon deposit is limited may be ascribed to the
local heating and the evaporation of C due to high
energy E-beam irradiation that becomes more significant the longer the exposure time [14].
Fig. 3 shows cathodic potentiodynamic polarization
curves from −0.1 to −2 V (Ag/AgCl) in 1 mM
CdF2 + 0.05 M NaF for a n-type silicon sample carrying the same carbon line patterns as in Fig. 1a. A
cathodic polarization curve of an untreated sample is
included in the figure as a reference. For both the
samples, the current increases steeply at approximately
−0.8 to −0.9 V (Ag/AgCl) corresponding to nucleation and growth of cadmium on silicon. The second
current increase occurs at approximately − 1.5 V (Ag/
AgCl) and can be attributed to H2 evolution. It should
be noted that in order to achieve metal electrodeposition on the semiconductor surface under cathodic polarization, n-type Si was used. (For p-type material a
blocking Schottky junction would be established at the
semiconductor/electrolyte interface, which would complicate the electrochemical processes [34].) In order to
perform a chemical analysis of the deposits and particularly to properly assess the selectivity of the process,
larger C-patterns (rectangles) had to be created to
match the lateral resolution capabilities of the AES
system.
Fig. 4 shows a SEM image of such a C-patterned
silicon sample after the polarization in 1 mM CdF2 +
0.05 M NaF from − 0.1 to −1.5 V (Ag/AgCl) followed by the hydration of Cd and the chemical
treatment in the Na2S solution. The rectangle-shaped
carbon pattern produced using the full frame mode of
the SEM has a nominal width of 25 mm and a length of
35 mm. From Fig. 4 it is apparent that the surface is
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T. Djenizian et al. / Electrochimica Acta 47 (2001) 891–897
Fig. 4. SEM image of a sample with a rectangle-shaped carbon film
after the cathodic polarization from − 1 to − 1.5 V (Ag/AgCl) in 1
mM CdF2 +0.05 M NaF followed by the hydration of Cd and
chemical treatment in a 1 M Na2S solution. The scan rate was 2 mV/5
s. The C-masked area acts as negative resist.
covered by a deposit except for the rectangle in the
center corresponding to the location of the carbon
layer. Within the center area, absolutely no electrochemically deposited features can be observed.
To confirm the high degree of selectivity that was
achieved with this masking effect, an AES line scan was
acquired through the rectangle as a function of distance
(Fig. 5). The cadmium and sulfur intensities show the
same behavior and reach zero within the C-masked
area. The width of the cadmium and sulfur free area is
approximately 30 mm, i.e. corresponds almost to the
nominal width of the initial C-rectangle. Outside the
masked area the ratio of Cd/S is close to 1:1 (Cd: 45%,
S: 55%) as expected for a stoichiometric deposition.
These findings clearly indicate that the contamination
area (surrounded by the film) blocks the electrochemical reactions and constitutes a negative resist for the
deposition of CdS. The slight overgrowth of CdS at
the edges shows that electrochemical parameters (de-
Fig. 6. SEM image of CdS deposits on Si surfaces carrying the C-lines
pattern with the same dose sequence as in Fig. 1a. CdS deposition
was carried out by: (a) a potential sweep (2 mV/5s) from − 0.1 to
−1.5 V (Ag/AgCl) and (b) by a potential step to − 1.45 V for 7 min
in 1 mM CdF2 +0.05 M NaF followed by a chemical treatment in a
1 M Na2S solution.
Fig. 5. AES line scan for Cd and S across the rectangle of Fig. 4.
position stop) were not entirely optimal in this experiment.
In order to optimize the electrochemical parameters
and to study the lateral resolution of the masking effect
of such carbon for the deposition of Cd a set of n-Si
samples with arrays of contamination lines were prepared and electrochemically treated using different potential step and sweep parameters. Typical results of
such experiments are shown in Fig. 6.
A first set of experiments was carried out by polarizing samples in the potential sweep experiments from
− 0.1 to − 1.5 V at different scan rates. Fig. 6a shows
the SEM image of a sample polarized with a scan rate
of 2 mV/5 s after the chemical conversion of Cd to
CdS. It can be pointed out that in this case C-lines are
around 5 mm spaced (this space is larger than in Fig. 1).
Clearly, deposition has taken place on the sample ex-
T. Djenizian et al. / Electrochimica Acta 47 (2001) 891–897
cept for the location of the four C-lines. The surface
is covered with a homogeneous deposit. The typical
CdS crystallite size is in the range of 75 nm. The
lateral resolution corresponds to the nominal Clinewidth, however, the lowest E-beam dose location
has disappeared (not shown in Fig. 6a) suggesting
that in this case overgrowth of the C-line has occurred. For a faster sweep rate of 5 mV/s a significantly lower surface density of crystallites on the
non-irradiated surface was obtained, i.e. Cd is not
entirely covering the silicon surface. Lower values of
sweep rates (e.g. 1 mV/5 s) resulted in an overgrowth
of the C-lines and a rough deposit with a typical
feature size of clusters in the 100 nm range. As these
potential scanning results were not absolutely satisfactory (comparably large size crystallites, overgrowth of
the smallest C-lines). Potential step experiments were
also performed. Fig. 6b shows the result of a potential step experiment where Cd deposition onto the
C-line patterned substrate was carried out by applying a potential step of −1.45 V for 7 min, and after
the conversion procedure to CdS was performed.
It is clear that for these conditions no cluster is
visible within the treated surface locations and the
C-line deposits block the electrodeposition of Cd
completely. More remarkably, the result shows that
the electrodeposition of Cd is blocked even for the
shortest E-beam exposure time, i.e. the smallest C-line
with a thickness of less than 1 nm is able to block
the electrochemical reaction completely and selectively. Furthermore, it is apparent that the film is
smooth and homogeneous, the size of CdS grains is
in the range of 35 nm and thus also the sharpness of
the C-edgelines is excellent. Deposition of Cd at less
negative potentials (−0.9 V (Ag/AgCl) for 7 min) led
to only a few clusters in the 100 nm size. Using a
higher cathodic potential of deposition (− 1.6 V (Ag/
AgCl) for 7 min) results in a high density of small
crystallites but overgrowth of the irradiated silicon
surface is observed. As a result, even though the
smallest contamination line blocks the direct electrodeposition of Cd, a poor lateral resolution is
achieved due to this overgrowth of the C-lines. Additionally, at these higher cathodic potentials a very
inhomogeneous deposit can form due to hydrogen
evolution and the formation of H2 bubbles that stick
to the surface and have a masking effect for electrodeposition. Experiments performed with shorter deposition times than 7 min at −1.45 V showed no
coalescence of nuclei but the presence of single isolated clusters. For significantly longer deposition
times than 7 min at − 1.45 V (Ag/AgCl) again overgrowth of the C-lines occurs and the thicker Cd deposit layers show some degree of disbonding. To
reveal the effect of the conversion treatment on the
morphology of the deposit, SEM images were also
895
taken of samples removed from the electrolyte directly after the Cd deposition. The results of these
investigations showed that the morphology, i.e. the
average cluster size, is not affected by the conversion
to CdS. Thus, as expected, the electrochemical
parameters used for the Cd-metal deposition determine the final coverage and grain size of the CdS
deposit. In accordance with the literature, electrodeposition of metals on semiconductor surfaces mostly
follows a 3D-island growth mechanism [35] implying
growth kinetics of the Volmer–Weber type [36]. This
suggests that directly applying a high cathodic potential creates a high density of nuclei, which leads to an
early coalescence of islands and thus a desired fine
grain structured film would be obtained at higher cathodic applied potentials. However, the upper potential limit is given by the above-mentioned onset of
hydrogen evolution that causes an inhomogeneous deposit due to H2 bubbling.
To confirm the mask effect of the contamination
lines an AES scan line for Cd and S was carried out
perpendicularly to the lines for the samples shown in
Fig. 6. As previously observed for the sample in Fig.
5 the intensities of Cd and S reach zero within the
C-treated locations. This verifies the ‘absolute’ mask
effect of the C-deposits also for the C-line samples.
To assess the thickness and the depth distribution of
the composition, argon etching was carried out in the
Auger spectrometer. The results showed that the deposited film next to the C-lines in the case of Fig. 6a
was approximately 50 nm thick and in the case of
Fig. 6b was approximately 35 nm thick. For both the
samples the Cd/S ratio remains 1:1 over the entire
depth of the film. This result confirms that the conversion of Cd to Cd(OH)2 as well as the subsequent
conversion to CdS penetrate the entire film thickness.
In order to verify the presence of functional (luminescent) CdS, the samples were further characterized by optical techniques. Fig. 7a shows a room
temperature PL spectrum that is typical for all the
investigated samples. A wide peak in the green region
of the spectrum with an onset between 600 and 700
nm and a peak at approximately 490 nm is obtained.
As reported widely in literature this PL response is
typical of CdS [33,37] and corresponds to a direct
band-to-band recombination of the excited charge
carriers. To assess the selectivity of the optical response, an image in fluorescence mode of a laser confocal
microscope
(ZEISS-LSM-310)
using
a
wavelength of 488 nm was taken on the sample of
Fig. 6a. Clearly, four black lines corresponding to the
four C-masks are apparent (Fig. 7b) due to the lack
of fluorescence of the C-deposit lines, the rest of the
surface shows a uniform PL. Therefore, selectivity
also in view of the PL is confirmed.
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T. Djenizian et al. / Electrochimica Acta 47 (2001) 891–897
4. Conclusions
The results clearly show that E-beam induced C-deposition can act as a negative resist for a subsequent
selective deposition of CdS on a silicon surface. C-lines
were written in a SEM chamber and deposited by the
decomposition of residual hydrocarbon molecules issued from the pump oil. In comparison with silicon, the
C-deposit behaves as an insulator. Therefore, electrochemical reactions are completely blocked at E-beam
treated surface locations. It has been demonstrated that
carbon of the order of less than 1 nm thickness can be
sufficient to achieve a mask effect with an excellent
lateral resolution and sharpness at the C-edgelines.
Cd was electrochemically deposited and converted to
CdS by hydration and chemical treatment. It was found
that selectivity of the process depends not only on
C-line deposition parameters but also strongly on the
electrochemical parameters. The typical feature size of
the Cd clusters, coverage and occurrence of undesired
overgrowth depend on the sweep rate, the cathodic
potential applied and the time of deposition.
In accordance with the Volmer– Weber approach, the
higher the cathodic potential step the finer the grain
size leading to an early coalescence of nuclei and subse-
Fig. 7. (a) Photoluminescence spectrum taken on sample of Fig. 6b
measured at room temperature. The wide peak with an onset between
600 and 700 nm and a peak at approximately 490 nm are typical for
CdS. (b) Fluorescence-image of CdS deposit on sample of Fig. 6a
using excitation at 488 nm.
quently an excellent lateral resolution. The CdS deposit
has been characterized by scanning AES. The results
confirm the selectivity of the process and show that the
deposition process leads to a stoichiometric Cd/S ratio.
Optical properties were performed by fluorescence and
PL measurements at room temperature and show the
typical CdS response for the surface not masked with
C-patterns. This negative lithographic process confirms
perspectives for selective electrodeposition of a large
variety of materials in the nanometer range.
Acknowledgements
The authors would like to acknowledge the Swiss
National Science Foundation for financial support of
this work. For help with the experiments we would like
to thank Nicolas Xanthopoulos (EPFL) for the AES
measurements.
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