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Sub-5-nm-spatial resolution in scanning spreading resistance microscopy
using full-diamond tips
D. Álvarez, J. Hartwich, M. Fouchier, P. Eyben, and W. Vandervorst
Citation: Appl. Phys. Lett. 82, 1724 (2003); doi: 10.1063/1.1559931
View online: http://dx.doi.org/10.1063/1.1559931
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Sub-5-nm-spatial resolution in scanning spreading resistance microscopy
using full-diamond tips
D. A
´lvareza)
Corporate Research Nanodevices, Infineon Technologies AG, D-81730 Munich, Germany, and Materials
and Components Analysis, IMEC Kapeldreef 75, B-3001 Leuven, Belgium, and KU Leuven, INSYS,
Kard Mercierlaan 92, B-3001Leuven, Belgium
J. Hartwich
Corporate Research Nanodevices, Infineon Technologies AG, D-81730 Munich, Germany
M. Fouchier
Materials and Components Analysis, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
P. Eyben and W. Vandervorst
Materials and Components Analysis, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium,
and KU Leuven, INSYS, Kard. Mercierlaan 92, B-3001Leuven, Belgium
共Received 13 August 2002; accepted 19 January 2003兲
Scanning spreading resistance microscopy is a two-dimensional carrier profiling technique now
widely used for the characterization of silicon 共Si兲devices as well as other semiconductor materials.
Whereas the state-of-the-art spatial resolution for this technique using commercial-diamond-coated
silicon probes is limited to 10–20 nm, enhanced resolution is demonstrated through the use of
full-diamond tips integrated in Si cantilevers. Sub-5-nm-spatial resolution is obtained on fully
depleted silicon on isolator devices, putting the technique closer to the characterization requirements
of the forthcoming semiconductor dimensions. Resistance and scanning electron microscope
measurements clearly show that this enhanced resolution results from a smaller effective radius for
full diamond tips as compared to the diamond-coated Si probes. © 2003 American Institute of
Physics. 关DOI: 10.1063/1.1559931兴
The International Technology Roadmap for Semiconduc-
tors 共ITRS兲emphasizes the need for sub-10-nm resolution
for the characterization of carrier concentration profiles with
good quantification accuracy and sufficient sensitivity.1
Scanning spreading resistance microscopy 共SSRM兲is a two-
dimensional 共2D兲carrier profiling technique based on the
atomic force microscope 共AFM兲. In this technique, a conduc-
tive probe in contact mode is biased relative to a semicon-
ductor sample.2The resulting current 共measured with a loga-
rithmic current amplifier兲is then a measure for the local
spreading resistance. The latter can be correlated to the local
carrier concentration through the formula:
R⫽
/4a,共1兲
where Ris the measured resistance (V/I),
is the local
resistivity of the sample, and ais the contact radius which
also determines the spatial resolution. SSRM has a good spa-
tial resolution 共10–20 nm兲,2,3 a wide dynamic range
(1015–1020 cm⫺3)4and excellent concentration sensitivity.
Routines for fast quantitative interpretation of SSRM images
require minimal calculation and are now readily available.5–7
Silicon-on-insulator 共SOI兲devices offer numerous ad-
vantages with respect to bulk semiconductor devices8,9 and
represent one of the most promising upcoming
technologies.10 In this study, fully depleted pMOS SOI 共FD-
SOI兲test samples were measured using SSRM 共see Fig. 1兲.
The measurements were performed on a Digital Instruments
Nanoscope D-3000 AFM equipped with an SSRM module.
Two types of conducting probes were used: diamond-doped
coated Si tips11 with a spring constant that ranged from 20 to
40 N/m, and pyramidal, full-diamond tips at the end of Si
cantilevers with a spring constant of 60 N/m 共fabrication
process previously reported兲.12 The tips were inspected with
a LEO1560 thermal field emission scanning electron micro-
scope 共SEM兲with Gemini column at 5 keV prior to the mea-
surement and those with the smallest radius were selected.
The sample was prepared by cleavage and subsequent me-
chanical polishing to expose the cross section of the device.
A low surface roughness (rms⬃0.2 nm) was achieved in or-
der to avoid topography induced artefacts in the measure-
ments. To collect the current, all the layers of interest were
contacted by the deposition of Pt using a focused ion beam
共FIB兲.
Figure 2共a兲shows a SSRM image of a pMOS FD-SOI
device 共original scan area 2⫻2
m2) measured with a
a兲Electronic mail: david.alvarez@infineon.com
FIG. 1. Schematic of the cross section of a FD-SOI transistor. Implant doses
were boron 7 keV, 5⫻1015 cm⫺2and phosphorus 12 keV 6.5⫻1012 cm⫺2
for the source/drain and channel, respectively.
APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 11 17 MARCH 2003
17240003-6951/2003/82(11)/1724/3/$20.00 © 2003 American Institute of Physics
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diamond-coated tip. The applied voltage between the tip and
the sample was 0.6 V, the force throughout the measurement
40
N and the scan rate 1 Hz. No significant difference in
the measured resistance was observed for a range of 25–40
N in the applied force and of 0.5–0.6 V in the applied
voltage. It is clear that SSRM can successfully resolve the
different parts of the device. The buried oxide exhibits a
much higher resistance than the source/drain and gate. The
outdiffusion of the source/drain implants into the channel
establishing the effective channel length of the transistor,
could also be resolved.
Figure 2共b兲shows another transistor from the same
sample but measured with a full diamond tip. A scan area of
400⫻400 nm2was selected in this case. The chosen param-
eters were 0.5 V for the applied voltage, 60
N for the force
and 1 Hz for the scan rate. No significant difference for the
measured resistance was observed over a range of 30–60
N
for the force. The outdiffusion from the source into the chan-
nel can be clearly resolved.
Figure 2共c兲shows a comparison of the cross section of
the region where the gate and the source overlap for the two
different measurements. The section taken with a full dia-
mond tip shows a much steeper transition from the conduc-
tive to the isolating region. The variation from 10% to 90%
of the log(R) measured in the source with respect to the
buried oxide takes 10 nm for the full diamond tip, whereas it
requires 51 nm for the coated tip. The result from a full
diamond tip shows a very flat profile within the channel with
only a limited increase when approaching the buried oxide.
This is indicative of the minimal 2D interaction with nearby
layers and supports the calculations by DeWolf et al.4,13 who
have shown that current confinement and the need to intro-
duce 2D-correction factors only arises when approaching an
insulating boundary within two tip radii. In our case, an ef-
fective tip radius of ⬃5 nm can be inferred. Within the same
reasoning an influence extending over 25 nm can be seen for
the diamond coated tips. The use of full diamond tips leads
to a more accurate measurement of the thickness for gate and
channel/source layer, the actual values being in close agree-
ment with the nominal values 共see Fig. 1兲. Additionally, in
the case of full diamond tips, a peak in the measured resis-
tance between gate and source is observed, which is attrib-
uted to the gate oxide 共3nm兲. Finally, one can observe that
the measured resistance for the same layers is in every case
larger for full diamond tips. This suggests, according to Eq.
共1兲, a smaller effective radius for the second probe. To check
the reproducibility, two more pairs of tips were tested for
similar structures from the same sample. Coated tips gave
distances of 78 and 50.7 nm for the same transition from
source to buried oxide, while full diamond tips gave 39 and
19 nm. Full diamond tips always gave a higher resistance as
well. Four images were taken of the same device with each
probe under similar conditions and neither decrease in the
resolution nor change in the measured resistance was ob-
served, indicating a good wear behavior for small scan areas
(1–4
m2).
Figures 3共a兲–3共c兲show three high magnification SEM
images of the apex of three different tips. Tip 3 belongs to
the set of probes selected for the measurement of the FD-SOI
devices shown in Fig. 1. Figure 3共d兲shows the apex of a
coated Si tip. Higher magnification was not possible due to
the rapid deposition of a carbon contamination layer and to
the movement of probe when focusing at the very end of the
tip apex due to the cantilever bending by the electron beam.
The images clearly show however that tip 3 exhibits a geo-
metrical radius 共radius of curvature of the tip apex兲much
smaller than that of the coated tip. Full diamond tips are
fabricated in a molding process, in which the diamond is
deposited in inverted pyramids etched onto Si. The diamond
grows according to the shape of the pyramidal molds, which
fully determines the final geometry of the tip. For coated tips
the diamond is deposited on an existing Si tip without any
mold to externally shape the coating, resulting in a rough
surface. It is this roughness which finally provides the effec-
tive radius of these tips. Aspatial resolution of 10–20 nm for
topographic measurement has been reported for these
probes.14 To evaluate the effective radius of the tips 共tip–
sample contact radius兲,I–Vcurves in point contact on bulk
Pt were performed in order to measure the total resistance of
FIG. 2. 共a兲SSRM measurement of a FD-SOI as obtained with a diamond
coated silicon tip; 共b兲Transistor from the same sample measured with a full
diamond tip; 共c兲comparison of two cross sections from both reveals a better
spatial resolution for the full diamond tip.
1725Appl. Phys. Lett., Vol. 82, No. 11, 17 March 2003 A
´lvarez
et al.
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the probes. Figure 3共e兲shows the obtained curves and the
calculated resistance.
The coated tip showed a resistance of 10 k⍀. The resis-
tance of full diamond tips exhibits a large dependence on the
tip radius. For tip 3 a slight deviation from the ohmic behav-
ior is observed. This could be due to the presence of a con-
tamination layer adsorbed on the sample from the SEM15 or
to a lower B concentration in the very first layers of the
deposited diamond due to the pretreatment of the substrate to
enhance the nucleation density.16 Both factors should be
more critical for very sharp tips, where the contact area with
the conductive substrate is ⬃nm2. This would also explain
the more noisy SSRM image obtained with the very sharp
full diamond tip for the SOI device. Taking into account the
pyramidal shape of the tip and a nominal bulk resistivity for
the diamond of 0.01 ⍀cm,8the effective tip radius can be
calculated from the measured resistance using the formula:17
R⫽
/共4rtan
␣
兲,共2兲
where ris the tip radius and
␣
⫽35.3° the apex angle for
these molded probes. This computes to 50, 25, and 3.5 nm
contact radius for tip 1, tip 2, and tip 3, respectively.
The first calibration measurements have shown a good
sensitivity and dynamic range (⬃1015–1020 cm⫺3) for full
diamond tips using epitaxial staircase calibration
structures.12,18 For the sharpest tips obtained, scanning over
the large areas of our calibration structures (⬃40
m
depth兲18 led to tip wear 共change in tip radius兲that did not
make a reliable calibration possible.
In conclusion, full diamond tips can achieve a smaller
effective radius in single contact mode than that of diamond
coated Si tips. In scanning mode, full diamond tips can show
a spatial resolution of less than 5 nm for SSRM measure-
ments as demonstrated by the analysis of FD-SOI devices.
This brings SSRM one step further to meeting the ITRS
requirements for carrier profiling spatial resolution. Opti-
mum quantification requires the availability of thin calibra-
tion structures in order to limit tip wear during the calibra-
tion measurements.
D.A
´. and M.F. gratefully acknowledge financial support
by the EU-Project HERCULAS and P. Eyben by the Institute
for Science and Technology 共IWT兲.
1The International Technology Roadmap for Semiconductors 共ITRS兲, 2001
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FIG. 3. 共a兲,共b兲,共c兲Molded full diamond tips with different radii as a result
of different filling of the Si pits during the diamond deposition; 共d兲diamond
coated Si tip; 共e兲I–Vcurves and measured resistance for the probes.
1726 Appl. Phys. Lett., Vol. 82, No. 11, 17 March 2003 A
´lvarez
et al.
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