Dose Measurements of Ultra-Shallow Implanted As and B in
Si by RBS and ERD
P. Pelicon, a,d G.V. Ravi Prasad, a,e M. El Bouanani, b
B. N. Guo, c D. Birt, a J. L. Duggan, a and F. D. McDaniel, a
a
Ion Beam Modification and Analysis Laboratory, Dept. of Physics, University of North Texas,
Denton, TX 76203-1427, USA
b
Laboratory for Electronic Materials and Devices, Dept. of Materials Science, University of North Texas, Denton,
TX 76203-5310, USA
c
Varian Semiconductor Equipment Associates, Inc., 35 Dory Road, Gloucester, MA 01923, USA
d
Institute “Jožef Stefan”, P.O.B. 3000, SI-1001 Ljubljana, Slovenia
e
Institute of Physics, Bhubaneswar-751005, Orissa, India
Abstract. Continuous miniaturization of integrated circuits requires narrower dopant profile depth in the Si channel and
consequently the use of ultra-shallow implants in the manufacturing process. Secondary Ion Mass Spectroscopy (SIMS) is
routinely used to measure the boron depth concentration profiles. However, due to the altered nature of the near-surface
sputtering process inherent to SIMS, it underestimates the B implanted doses for implantation energies below 2 keV. Alternate
ion beam methods for absolute dose measurements of ultra-shallow implanted As and B in Si are presented in this study. The
dopant implant energies ranged from 250 eV, to 5 keV for boron and from 500 eV to 5 keV for arsenic. Implanted doses for both
B and As varied from 2 x 1013 to 1 x 1015 atoms/cm2. The arsenic implants were studied with Rutherford Backscattering
Spectrometry (RBS) using 2 MeV carbon ions. The absolute arsenic implanted doses were measured to an accuracy of better
than 5%. The 1 keV arsenic implants were extensively studied for radiation damage with a 12C beam. No appreciable arsenic
dose loss was observed during C irradiation for an integrated charge of ≤ 80 µC, which was the maximum used for these studies.
For the B implants, Elastic Recoil Detection (ERD) was used with 14 MeV F4+ ions. A 9.4 µm Mylar foil was found to
adequately stop the scattered 19F ions and give good energy separation for the 11B recoiled ions. The absolute dose measurements
are ∼ 5% for the 5 keV 11B implants. Significant radiation damage was observed for the ultra shallow implants and the measured
B dose has been obtained by extrapolation to the zero integrated charge of the beam. The absolute boron dose measurements of
the ultra shallow (250 eV) implants were determined with an accuracy better than 10%.
Spectroscopy (XPS), Auger Electron Spectroscopy
(AES), Glow Discharge Optical Spectroscopy
(GDOS), and the electron microprobe. In some cases,
it is possible to measure concentrations down to 1 x
1015 atoms/cm2. For arsenic implanted into silicon,
RBS with heavier ions is superior to standard 4He
scattering since the Rutherford scattering crosssection increases quadratically with incident ion
atomic number. Doyle et al. demonstrated that
sensitivities better than 1011 atoms/cm2 may be
obtained for heavy trace element contamination in Si
using a 400 keV carbon beam for standard RBS and a
100 keV carbon beam for ToF-RBS 2, 3, 4.
From an analytical point of view, the dose
determination of shallow-implanted boron is much
more challenging.5 The majority of the absolute dose
measurements for 11B have been determined from the
11
B (p, α) 8Be nuclear reaction. Semiconductor
INTRODUCTION
Ion implantation is the preferred method for the
formation of shallow junctions since it can be easily
integrated into the production process. In order to
produce narrow junctions, it is often necessary to use
implantation energies below 1 keV. At these low
implant energies, specific processes start to influence
the implanted dose. Some of these are: selfsputtering of the dopant atoms, dopant loss into the
surface oxide and dopant self-diffusion during the
annealing process.1
For arsenic, several analytical techniques have been
used to study shallow implants. These include
Rutherford Backscattering Spectrometry (RBS),
Particle Induced X-Ray Emission (PIXE), X-Ray
Fluorescence (XRF), Secondary Ion Mass
Spectrometry
(SIMS),
X-Ray
Photoelectron
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
482
manufacturers usually rely on SIMS characterization
for their 11B-implanted wafers.
There are
reproducibility problems with SIMS 6,7, particularly
for ultra-shallow implants. In 1994, Simons wrote a
rather extensive article,8 which outlines the short-and
long-term reproducibility of SIMS measurements.
For dose determination of ultra-shallow implants (∼
250 eV), SIMS has serious limitations that are
believed to be due to the non-equilibrium and altered
nature of the near-surface sputtering process inherent
to SIMS. SIMS measurements do not allow the
extraction of reliable quantitative depth profiles from
ultra-shallow implants and ultra-thin films due to
uncertainties in sputtering probabilities and chargeexchange processes, radiation-enhanced diffusion,
ion mixing effect near the surface, and preferential
sputtering. Recently, it was pointed out by Suzuki et
al. 9 that for ultra-shallow implants, SIMS analysis is
unsuitable for accurate quantification. Their data
shows that 11B concentrations in Si are only 80%
those found by the 11B (p, α) 8Be reaction, for 200 eV
implants. It is widely accepted, in the Ion Beam
Analysis community that NRA can determine 11B
doses with + 5% relative accuracy. In fact, Liu et
al.11 have recently measured the reaction cross
section at the 660 keV resonance. They claim their
cross section values are accurate to 3.3%.
Another technique, which gives excellent results
for 10B analysis, is Neutron Depth Profiling with
NRA using the reaction 10B(n, α) 7Li. This method
was developed in the early 1970’s by Ziegler et al. 10
The method is non destructive and the depth
distribution can easily be determined by measuring
the resultant recoil energy of the alpha particles from
the reaction. By this technique depth resolutions of
10 nm are possible for 100 keV 10B implants with
concentrations of 2 x 1014 atoms/cm2. However,
Neutron Depth Profiling measurements require a
reactor and rather specialized equipment.
For the study presented in this paper, Elastic
Recoiled Detection (ERD) was used to determine
absolute ultra-shallow 11B implanted dose in silicon.
F4+ ions of 14 MeV energy were used to impinge on
the sample at a glancing angle of 10 degrees. The
recoil boron ions were measured at 25 degrees. It
will be shown that this method can be used to
accurately determine 5 keV boron implant doses.
Good results can also be obtained for the ultrashallow 250 eV implants, using the zero incident
beam integrated charge extrapolation method.
arsenic using a high-current ion implanter. The
implanted doses were in the range between 1x 1015
atoms/cm2 and 2 x 1013 atoms/cm2 with intermediate
values of 3 x 1014 atoms/cm2, 1 x 1014 atoms/cm2, and
5 x 1013 atoms/cm2. The implanted energies of
arsenic were 500 eV, 1000 eV, 2000 eV and 5000
eV.
For the boron-implanted samples, the
implantation energies used were 250 eV, 500 eV,
1000 eV and 5000 eV.
DOSE MEASUREMENT OF
SHALLOW-IMPLANTED ARSENIC IN
SILICON BY RBS
Experimental Arrangement
The Arsenic measurements were made using the
RBS beam line of the 3 MV tandem accelerator in the
Ion Beam Modification and Analysis Laboratory at
the University of North Texas. A 2 MeV 12C+ beam
O
10000
As
1000
Yield
Si
100
10
1
50
100
150
200
250
300
350
400
Channel
FIGURE 1. Measured RBS spectrum for an implanted As
dose of 1 x 1015 atoms/cm2 and an implantation energy of
500 eV.
impinged on the arsenic implanted silicon samples,
which were tilted at 10 degrees to avoid channeling.
The backscattered 12C ions were detected at 130
degrees with a silicon surface barrier detector, which
had a solid angle of 3.5 msr.
To normalize the measurements, the number of
impact particles times the solid angle of the detector
was determined with an accuracy better than 3% by
means of a mesh charge collector 12 and RBS
reference standards. Our RBS reference standards
were checked against a thin Rhodium film on
vitreous carbon (thickness accuracy 2 %) supplied by
a European Union precision standards laboratory
(IRMM) in GEEL, Belgium.13
SAMPLE PREPARATION
Silicon wafers with the crystal orientation <100> and
a diameter of 8” were implanted with boron and
483
Results for the Arsenic Measurement
even at arsenic concentrations of 2 x 1013 atoms/cm2.
For concentrations below 2 x 1013 atoms/cm2, the
background uncertainty starts to contribute
significantly to the overall uncertainty.
A typical measured spectrum for 1x 1015 arsenic at
an implant energy of 500 eV is shown in Figure 1. In
order to check for 12C beam damage, consecutive
measurements were made at the same sample
position for different time-integrated current values.
Figure 2 shows that for the 1 keV arsenic implants,
there is no measurable difference in the integrated
charge.
Sample AF03852: 1e15 As at/cm
Experimental Arrangement
2
after 17
34
51
68
85
400
Counts
300
200
As peak
area does
not change
with accumulated
beam charge
100
0
280
300
DOSE MEASUREMENT OF
SHALLOW BORON IMPLANTS BY
HEAVY ION ERD
µC
µC
µC
µC
µC
The ERD method was first proposed in 1976 by
L’Ecuyer et al. and reported in the Handbook of
Modern Ion Beam Materials Analysis. 14 The first
practical experiments were done in 1979 by Doyle, et
al. 15 The experimental arrangement used in this study
is similar to that outlined in the Handbook. 14 The
ERD detector was positioned at 25 degrees relative to
the beam direction. The sample was tilted 10 degrees
with respect to the beam. A 9.4 µm Mylar foil was
placed in front of the ERD detector together with a
rectangular aperture 3 mm wide and 10 mm high.
The thickness of the stopping foil and the ERD recoil
angle are optimized to stop the scattered 14 MeV F4+
incident ions and yield an isolated boron peak in the
high-energy part of the detected spectra. Figure 3
shows a typical measured spectrum with the 11B peak
clearly resolved from H, O, and C. An RBS detector
at 150 degrees was used to calibrate the incident ion
integrated charge during the measurements. The
solid angles of the RBS and the ERD detectors were
measured by a 2% NIST 241Am standard. The major
problem with all heavy ion experiments is
determining the number of incident ions. This was
accomplished by using an “80 % transmission
tungsten Mesh-beam integration monitor” in the
beam line prior to the scattering chamber.12 This
mesh is insulated from its surroundings (floating
electrically) and enclosed in a cylindrical tube biased
at -300 volts, so that secondary electrons do not
escape the mesh. This device is used for all of our
high-precision RBS measurements. In order to
normalize the 14 MeV F4+ ions, RBS measurements
were made on a 2% RBS reference standard. In this
particular case, the 14 MeV F4+ ions were
backscattered from a Au standard. Since the RBS
cross section, solid angle of the detector, and
thickness of the Au target are all known, the
integrated charge from the mesh can be accurately
scaled to the true current on the target. With this
technique, the true charge which impinges on the
target for the ERD and RBS measurements could be
determined to better than 3%.
Shift due to
C deposition
associated with
the vacuum
conditions
320
Channel No.
FIGURE 2. Arsenic peak in the RBS spectrum measured
in the intervals of accumulated charge of 17 µC for a 2
MeV 12C1+ beam at the same irradiated sample spot. The
sample was implanted to 1 x 1015 atoms/cm2 at an energy
of 1 keV. The observed energy shift of the peak is a result
of beam-associated contamination build-up on the sample
surface during the measurements.
The area of the
irradiation spot is ~ 1 mm2. The vacuum in the sample
chamber during the measurements was better than 5 x 10-7
mbar.
For the lowest arsenic implant dose (2 x 1013
atoms/cm2) this method is still applicable. It was
found that for 70 µC, which was the integrated value
similar to Figure 2, the arsenic peak showed 500
counts. At values below 2 x 1013 atoms/cm2, the
solid angle of the detector would have to be increased
and longer run times would be required to minimize
the statistical error contribution and obtain reasonable
accuracy.
The relative accuracy of the As measurement
described here is determined by the sum of the
normalization uncertainty and the statistical
uncertainty in the number of accumulated As counts.
The normalization error is ~ 3% and consists of ~ 1%
instability of the charge integration and ~ 2% in the
absolute error in the thickness of the RBS reference
sample. Since there is no appreciable radiation
damage, the statistical uncertainty could be kept well
below 2% by extending the data collection times
484
Results of the B Implantation
Measurements
peaks, which come from contaminants on the surface.
This good separation was seen for all of the 11B
implanted samples with implant energies up to 5 keV.
For higher implant energies, the B peak would move
to the lower channels and overlap with the surface
hydrogen and carbon peaks.
A major concern, for all ERD measurements with
heavy ions, is the induced damage of the sample. In
order to study this effect, a series of measurements
were made on the same target spot. Figure 4 shows a
plot of the boron fluence as a function of
accumulated charge for different implant energies.
It can be seen from Figure 4 that there is very little
ion-induced damage for the 5 keV implants for an
integrated beam charge up to 220 µC. For implant
energies below 1 keV, considerable 11B loss was
observed as can be seen in the figure. For heavy ion
bombardment, the surface layers of the target are
usually eroded away by sputtering and ion induced
erosion (desorption). 16 This damage versus
accumulated beam studies have usually been fitted by
one or two exponentially decreasing functions. 17,18
The curves shown in Figure 4 are seen to fit the
experiment quite well. These theoretical curves were
generated using the exponential equations outlined in
reference 16 for ion-induced damage.
In Figure 4, for the case of the 250 eV and 500 eV
implants, the absolute implant dose may be
determined by extrapolating the accumulated beam
charge to zero using the exponential functions
outlined by Behrisch, et al. 16 As mentioned earlier,
the accepted value is in good agreement with the 5
keV implant data. The error with the accepted values
for the 5 keV implant measurements is + 5%. The
extrapolated value for the 500 eV data is within 3%
of the 5 keV implant measurements. The worse case
extrapolation is for the 250 eV data where the
implanted boron is mostly on the surface. However,
even for these very shallow implants, the
extrapolation is within 10% of the accepted value.
Figure 3 shows a typical ERD spectrum for 11B. This
spectrum was made for 1 x 1015 boron atoms/cm2
implanted in Si(100) at an energy of 250 eV. It can
be seen from the spectrum that the B is clearly
separated from the hydrogen, carbon and oxygen
19 4+
Eimpl = 250 eV, ERDA 14.2 MeV F , θ=25 deg, 9.4 µm mylar
O
Beam charge per
measurement 46 µC,
cumulative:
after 46 µC
after 184 µC
H
C
100
Yield
B
10
1
50
100
150
200
250
300
Channel
FIGURE 3. ERD spectra measured for an implanted B
dose of 1 x 1015 atoms/cm2 and implantation energy of 250
eV. The first and fourth consecutive spectra were obtained
at the same irradiation spot, which accumulated 46 µC of
integrated beam charge.
10
2
B fluence [1e14 at/cm ]
9
8
7
6
5
CONCLUSIONS
250 eV
500 eV
5000 eV
4
For Arsenic implanted into silicon, RBS
measurements were made with 2 MeV 12C ions. The
RBS spectra showed a well-separated arsenic peak
with no measurable radiation damage for up to 84 µC
of charge on the same beam spot. The measurements
were made with an accuracy of ± 5% for shallow
implants as low as 2 x 1013 arsenic atoms/cm2. The
data indicate that lower concentrations could be
measured by increasing the solid angle of the RBS
detector.
3
0
50
100
150
200
250
Accumulated beam charge [µC]
FIGURE 4. The resulting boron fluence measured by the
14.2 MeV 19F4+ beam as a function of integrated charge on
the measured spot. Extrapolation, using exponential
functions justified in the work of Behrisch et al., 16 has been
used to evaluate the implanted fluence.
485
15. Doyle, B.L. and Peercy, P.S., Appl. Phys. Lett. 34, 811
(1970).
16. Behrisch, R., vonder Linden, W., Von Toussaint, V.,
and Grambole, D., Nucl. Inst. Meth. B155, 440-446 (1999).
17. Scherzer, B.M.U., Bleuer, R.S., Behrisch, R., Schuls,
R., Roth, J., Borders, J., and Langley, R., J. Nucl. Mater.
85, 102 (1979).
18. Roth, J., Scherzer, B.M.U., Bleuer, R.S., Brice, D.K.,
Picraux, S.T., and Wampler, W.R., J. Nucl. Mater. 93/94,
601(1980).
For boron implanted into silicon, the ERD
measurements were made with 14 MeV 19F4+ ions.
The measured boron absolute dose is in excellent
agreement (+5%) with the nominal value for the 5
keV implant data.
Furthermore, there is no
measurable ion induced damage for the 5 keV data.
For the 250 eV and 500 eV implants, ion beam
damage was observed. It was, as expected, seen to
increase as the accumulated charge was increased on
the same beam spot. Well established zero integrated
charge methods such as those proposed by Behrisch
et al.16 were used to fit experimental dose loss data
versus accumulated incident ion integrated charge.
The calculated boron dose agreed with the nominal
dose to better than 10 %.
ACKNOWLEDGEMENTS
The work at UNT supported in part by the National
Science Foundation, the Office of Naval Research,
the Texas Advanced Technology Program, and the
Robert A. Welch Foundation.
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