Carrier Illumination as a tool to probe implant dose and
electrical activation.
W.Vandervorsta,1, T.Claryssea, B.Brijsa, R.Looa, Y.Peytiera, B.J. Pawlakb,
E.Budiartoc and P.Bordenc
a
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
also : KULeuen, INSYS, Kasteelpark Arenberg, Leuven, Belgium
b
Philips Research Leuven, Kapeldreef 75,B- 3001 Leuven, Belgium
c
Boxer Cross Inc., Menlo Park, California
1
Abstract. The Carrier Illumination™ (CI) method is an optical technique for non-destructive in-line monitoring of postanneal junction depth, pre-anneal pre-amorphisation implant (PAI) depth, and dose. This work describes the sensitivity of the
CI-signal to the as-implant dose and demonstrates that a universal response function can be derived for doses below the
amorphisation limit. For the implants where the elements/doses cause amorphisation, the CI-signal reflects directly the
thickness of the amorphous depth. In the case of annealed structures, it is shown that CI provides important information on
the electrical activation of the dopant. This is illustrated by the analysis of CVD-layers subsequently annealed and of junction
profiles produced by laser annealing. In both cases nearly identical dopant profiles are observed with secondary ion mass
spectrometry while the electrical activation as derived from sheet resistance measurements is very different. This very
different activation level is clearly reflected in the CI-signal. This indicates that the CI-signal is not solely related to the
junction depth and the profile abruptness but also to the electrical activation of the dopants.
INTRODUCTION
EXPERIMENTAL
The Carrier Illumination™ (CI) method is an
optical technique for non-destructive in-line
monitoring of post-anneal junction depth, pre-anneal
pre-amorphisation implant (PAI) depth, and dose [1].
Originally proposed as an evaluation method for
junction depths of annealed profiles, it has already
been shown that the technique is also sensitive to the
active dopant concentration and the profile
abruptness [2]. Moreover recently it was shown that
the technique can also measure the thickness of the
amorphous depth after a pre-amorphizing implant
[3]. This work further extends these studies and
shows that the CI-method is also very sensitive for
monitoring as-implanted low dose implants. A
universal response function (independent of implant
species/dose/energy) can be found relating the CIsignal to the implant dose. For high doses / heavy
elements, this function is no longer valid and the CIsignal reflects the thickness of the amorphous depth.
With respect to the sensitivity for annealed profiles,
we have studied CVD and laser annealed profiles. It
is shown that although no differences in the dopant
distribution can be observed, different results for the
CI-signal are obtained reflecting differences in
electrical activation.
Wafers were implanted with B, BF2, As and P at
Imec and Sematech. Low energy Sb-implants were
made at Applied Materials. These implants used 8
inch n-type Si (100) wafers at an angle of 7° tilt and
27° twist. All implants were done on the Applied
Materials XR80 LEAP. CI-measurements were made
using a Boxer Cross BX-10 located at Sematech on
wafers implanted at Sematech and at Imec on the
wafers implanted at Imec and AMAT. All signals
reported relate to actual measurement values and no
scaling or calibration between the different systems
was performed. The amorphous depth of the Imec
samples was calculated from the number of displaced
Si-atoms as extracted from a 2MeV channeling RBSmeasurement. CVD-layers were grown at 700°C and
subsequently annealed at 700 (5 to 10 min) and 800
°C (1 and 2 min).
RESULTS
CI-signal response for low dose implants
CI-measurements were done on wafers with
unannealed B, As and P-implants with a wide range
of energies and low to high doses. The results are
CP683, Characterization and Metrology for ULSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
© 2003 American Institute of Physics 0-7354-0152-7/03/$20.00
758
shown in Fig.1 for Boron (top), Phosporus (middle)
and arsenic/antimony (bottom).
3.E+04
B, 0.5 keV
B, 1 keV
B, 2 keV
B 3 keV
B, 5 keV
B
2.E+04
CI signal [uV]
1.E+04
0.E+00
very smooth and follows a well-behaved, predictable
pattern, suggesting a common underlying
mechanism. Therefore, it seems likely that there
should be a single response curve that contains all
the physics.
As the CI-signal is governed by the shape of the
excess carrier profile induced by the pump laser,
recombination of these excess carriers on the defects
produced by the implant seems to be a likely
mechanism to explain the signal dependence on
species, energy, and dose. In fact one could speculate
on a proportionality between the CI-signal and the
total numbers of defects produced during the
implants. Within this frame a proper scaling factor
correlating different species and energies, would then
be the number of defects produced per incident ion
by the implantation process.
Disregarding any in-situ annealing or recombination,
the number of vacancies produced during the implant
can be calculated using TRIM (Transport of Ions in
Matter) [4]. Fig. 2 shows the predicted number of
vacancies per incident ion as a function of implant
energy for the species Boron, Arsenic, Antimony,
Phosphorus and BF2. For the latter case we
calculated the damage as the sum of the vacancies
produced by one B and two F atoms with an energy
scaled to their mass relative to the total mass.
B 10
B 31
B 50
B 200
B 500
-1.E+04
-2.E+04
-3.E+04
-4.E+04
-5.E+04
-6.E+04
1.E+11
1.E+12
1.E+13
1.E+14
1.E+15
1.E+16
B dose (cm-2)
Phosphorus
0
CI signal (uV)
-20000
-40000
Energy
(keV)
10
50
200
500
1000
2000
-60000
-80000
-100000
-120000
1.E+11
1.E+12
1.E+13
1.E+14
-2
10000
Dose (cm )
Vacancies per ion
As/Sb
0.0E+00
-2.0E+04
CI- signal (µ V)
-4.0E+04
-6.0E+04
-8.0E+04
-1.0E+05
-1.2E+05
-1.4E+05
-1.6E+05
-1.8E+05
-2.0E+05
1E+11
As 5 KeV
As 10 KeV
As 10 KeV, BX
As 30 KeV
As 50 KeV,BX
1000
100
Boron
Arsenic
Phosphor
BF2
10
As 200 KeV,BX
Sb
Sb 2 KeV
Sb 5 KeV
1E+12
1E+13
1E+14
1E+15
1E+16
1
1E+17
0.1
Dose (cm -2)
1
10
100
1000
10000
Energy (keV)
FIGURE 2. The number of vacancies produced per
FIGURE 1. CI-signal vs. implant dose and energy for as-
implanted ion in silicon as a function of implant energy.
Data are generated by SRIM-2000 [4].
implanted Boron, Arsenic, Sb and Phosphorus-doped
samples. The As-measurements done at Sematech are
labeled as BX.
Not too surprisingly, similar energies lead to the
same number of vacancies regardless of the implant
species, as most of the energy is dissipated in nuclear
collisions absorbing the same amount of energy per
displaced atom. Only at high energies and for light
ions, electronic energy loss becomes important as
well and the curves start to depend on the implant
species.
Although the response is non-linear, the data in Fig.1
show a very high sensitivity. Even in the region with
the weakest response (dose 1012 – 1013 cm-2), the
sensitivity is as high as 500-1000 µV/dec. Relative to
the noise in the present systems this suggests a
resolution in the 1010 ions/cm2 range. For all dopant
species the signal response to dose and energy is
759
becomes amorphous. When the continuous
amorphous film has been formed, a thermal
mechanism determines the CI-signal instead of the
electronic interactions with the defects [5]. Under
these conditions a direct correlation with the
thickness of the amorphous layer can be found [3].
The data in Fig.5 combine CI-results on Si
amorphized by As, BF2 and Ge-implants [3] with
different doses and energies. A sensitivity of
~2.5x103 µV/nm can be seen, implying an ultimate
resolution better than 0.02 nm! Note that in [3] the
CI-signals are plotted without sign and shown as
positive whereas they are negative. For the
comparison, in Fig.5 we have inverted them and
multiplied by 3. The latter factor probably reflects
differences in the two measurement set ups and, in
particular, the pump laser intensity. It is clear that
there is no relation to the element species and the
signal solely depends on the thickness of the
amorphous layer. Intrinsically there is of course a
dependence on the implant dose/energy/species as
for each condition the efficiency of the
amorphization process and the amorphous film
growth are different.
When the CI-signal is plotted as a function of the
vacancy “dose”, as given by the product of implant
dose and the number of vacancies per ion, the result
is a single universal response curve for the preannealed sample, as shown in Fig. 3. The
convergence is most evident in the low and medium
vacancy dose range.
Energy (keV)
20000
B 3 keV
B 10
B 50
B 200
B 500
As 10
As 50
As 200
P 10
P 50
P 200
P 1000
10000
CI signal (uV)
0
-10000
-20000
-30000
-40000
-50000
-60000
-70000
-80000
1.E+13 1.E+14 1.E+15 1.E+16 1.E+17 1.E+18 1.E+19
Dose x Vacancies per ion (cm-2)
FIGURE 3. Universal response curve of the CI-signal as
a function of the product of dose and number of vacancies
per ion, demonstrating a common underlying mechanism
for the as-implanted samples, regardless of implant
species, energy. The curve is valid for low doses that do
not cause any overlap between atomic displacements and
amorphisation..
0.E+00
CI-signal (µ V)
As, 5e15 cm-2
CI response for amorphizing implants
The situation becomes different when heavy
elements and/or high doses are used. As shown in
Fig.4, the response curves for As, Sb and BF2 deviate
significantly from this universal curve for higher
doses (cfr the top axis for the 5 keV-As-implant). In
this case the simple calculation of the number of
vacancies produced, based on the results in Fig.2, no
longer holds as they start to overlap and the Si
CI signal (uV)
1.E+13
1.E+14
10000
-10000
-30000
-50000
-70000
-90000
Non overlapping
-110000
vacancies
-130000
-150000
1.E+14 1.E+15 1.E+16
1.E+15
1.E+16
-1.E+05
BF2, 1e15 cm-2
BF2 5e15 cm-2
IIT-ref.3
-2.E+05
-3.E+05
-4.E+05
0
50
100
Amorphous depth (nm)
150
FIGURE
5. Correlation between CI-signal and
amorphous depth for As (5, 10, 30 keV), BF2 (2,5,10 keV)
and Ge-implants (ref.). The absolute CI-signals from ref
[3] have been changed of sign and multiplied by 3.
5keV
As-dose (5 kev)(at/cm2)
1.E+12
As, 1e15 cm-2
10 keV
1.E+17
10
30 keV
CI as a probe for electrical activation
50
200
Originally conceived as a method to probe junction
depths of annealed samples [1], considerable effort
has been devoted to the correlation between SIMS)junction depths and the CI-signal [6]. For an ideal
box-like profile, it can theoretically be shown that
the signal response (CI versus junction depth)
follows a cosine behavior. Of course a deviation for
very thin profiles is observed (Fig.6), as all the
curves need to converge to the same bulk value for
zero thickness. For more gradual profiles it has been
shown that the technique is not measuring the actual
junction depth (i.e. the depth at which a p/n or
Sb 5 keV
Sb 2 KeV
amorphization
1.E+17
1.E+18
Vacancy Dose (cm -2 )
BF2, 2 keV
BF2 5 keV
1.E+19
BF2 10 keV
FIGURE 4. CI-response plotted as a function of the
number of vacancies produced during the implant for
different doses and energies. All As-implants except
otherwise indicated. The corresponding dose for 5 keV As
is indicated at the top. The solid dashed line refers to the
“universal” response curve as can be deduced from in
fig.3.
760
p+/p: 1.00E+19
p+/p: 1.00E+20 (4e19 actual)
p+/p: 5.00E+19
p+.p: CVD1:/9e19
p+/p: CVD1 1e20
p+.p: CVD1:/1e20
p+.p: CVD1-2/1.2e20
p+.p: CVD1/1.5e20
p+/p: 3.00E+20
p+/n/p: CVD1 1e20 (refer)
i/p+/n: 1e19
p+/n: 5e19
i/p+/n: 5e19
4.0E+04
3.0E+04
CI- signal uV
no anneal
CVD
2.0E+04
anneal
1.0E+04
LTA
1&2
0.0E+00
0
-1.0E+04
200
400
600
CVD
LTA
800
1000
CVD2: level 5e19
-2.0E+04
CVD1: nom. 1e20
4e20 (?)
-3.0E+04
nom 3e20
-4.0E+04
SIMS@1e19 Depth (A)
FIGURE 6 : CI-response as a function of junction depth (SIMS) for box like profiles grown with different concentrations
levels. Grown by low temperature CVD, the indicated concentration levels may not be entirely active (cfr Fig 8-9). Arrows
indicate the measurement values for the samples discussed in Fig.8-10 together with their corresponding junction depths (dashed
lines.
n/p junction occurs) but rather the depth of a
particular concentration level [2]. This level is
determined by the amount of excess carrier
generation induced by the generation laser. Probing
the sample with different excitations levels thus
provides a way to determine profile abruptness [2].
As pointed out in Fig.6 the CI-response depends not
only on the junction depth (here taken as the 1e19
at/cm3 in the SIMS profiles) but also very strongly
on the concentration. It should be noted that the
calibration curves in Fig.6 are based on the SIMSconcentration levels which may not be entirely (7080%?) active as the CVD-layers were grown at low
temperature (700°C). Hence some uncertainty in the
correlation to dopant concentration may exist (see
also the following section on annealed CVD-layers).
However the data sufficiently support the
observation (Fig.7) that for a fixed junction depth the
concentration sensitivity is as high as 2.5 104 µV/dec
for concentrations in excess of 1 1019 at/cm3 or a
sensitivity in the order of 0.1 % is present.
CI-signal (µ V)
4.E+04
3.E+04
2.E+04
1.E+04
0.E+00
1.E+18
1.E+19
1.E+20
1.E+21
3
Concentration (at/cm )
FIGURE 7. Concentration dependence of CI-signal for a
box like profile (junction 12 nm).
This makes CI an extremely sensitive tool to probe
dopant activation. The sensitivity to electrical
activation is exemplified in a study on a series of
box-profiles (grown by CVD at 700°C) which after
growth, were subjected to a further thermal treatment
i.e. an anneal at 700°C (5-10 min) and 800°C (1-2
761
layers. However again the observed strong decrease
in signal illustrates the excellent sensitivity of the CItechnique to the electrical properties of the samples
(change in CI-signal by 50% for a 20% change in
concentration).
1.E+21
2.5-700C-5min
2.6-700-6min
2.7-700-7min
2.8-700-9min
2.10-800-1min
2.11-800-2min
1.5- unannealed (p+/p)
sheet res matched
Concentration (at/cm 3)
1.E+20
In a similar study we have analysed an unannealed
CVD-layers versus a box profile produced by laser
thermal annealing. Whereas the SIMS-profiles are
almost identical (Fig.10) the LTA-sample has a sheet
resistance of 178 Ohm/sq and the CVD-layer of only
356 Ohm/sq, corresponding respectively to an active
concentration level of 1 1020 and 4 1019 at/cm3. The
CI-signal changes from –13000 (LTA) to –7100 µV
(CVD). Again such a large difference in CI-signal
for the same junction depth (40 nm) can only be
explained as the result of a very different activation
level (~1 1020 versus 3 1020 at/cm3, Fig.6).
1.E+19
apparent electrical
profile before anneal
(70% activation)
1.E+18
1.E+17
0
5
10
15
20
25
30
Depth (nm)
FIGURE 8 : SIMS profiles of annealed B-doped CVDlayers.
1.E+21
LTA
min). The corresponding SIMS profiles, shown in
Fig.8, indicate the absence of any diffusion upon
those treatments. On the other hand sheet resistance
measurements on these wafers show a clear
reduction from 1800 Ohm/sq to 1400 Ohm/sq
implying an increased activation of the Boron in the
annealed samples from 4x1019 at/cm3 to 5x1019
at/cm3. CI-measurements on these samples show a
similar change (Fig.9). In this case the signal changes
from ~28,000 to 16-17,000. Based on the data in
Fig.6 the active dopant concentration appears to
increase by a factor, which is larger than the
observed decrease in sheet resistance. This
discrepancy indicates that the calibration levels in
Fig.6 are inaccurate (all based on non-annealed
CVD-layers) and need to be remeasured on annealed
CVD-
Concentration (cm-3)
1.E+20
0
CI signal (µ V)
10
20
30
40
50
Depth (nm)
60
70
80
FIGURE 10. SIMS–profiles of unannealed CVD-layer
and laser thermal annealed implant profile.
In a final example we used the CI-technique to study
different laser anneal cycles. Again the resulting
SIMS profiles are very similar with a minor
difference on the junction depth (13 and 15 nm)
(Fig.11). Sheet resistance measurements show values
as low as 187 and 246 Ohm/sq, which would
correspond to an active concentration as high as 4
1020 at/cm3. As this is much above the solid
solubility and at the same time extremely shallow,
concerns on the correctness of the sheet resistance
values (and thus the concentration levels) could be
raised as probe penetration (potentially as high as
100 nm) would lead to a sampling of the substrate
and an equally low sheet resistance value [7]. CImeasurements show a dramatic difference between
both samples 14000 µV for the shallowest 246
Ohm/sq-sample versus -17000 µV for the deeper
one. Again based on Fig.6 one must conclude that
increased
activation
24000
22000
20000
18000
16000
14000
12000
1500
1.E+18
1.E+16
28000
10000
1000
1.E+19
1.E+17
30000
26000
CVD
2000
sheet resistance (ohm/sq)
FIGURE 9. CI-signal versus sheet resistance for
annealed and unannealed CVD-layers. Two different
batches of CVD-layers/anneals lead to the same increase in
activation upon anneal.
762
such a drastic change in CI-signal for a 2 nm
difference in junction depth, can only occur for
extremely high doping levels (~ 4-5 1020 at/cm3 )
confirming the validity of the sheet resistance
measurements.
4.
5.
6.
concentration (at/cm3)
1E+22
1E+21
7.
187 ohm/sq
1E+20
1E+19
246 ohm/sq
1E+18
1E+17
0
10
20
30
40
depth (nm)
FIGURE 11. SIMS profiles of laser annealed implants.
Based on the indicated sheet resistance values active
concentrations would be as high as 4 1020 at/cm3.
CONCLUSIONS
A detailed study on the response function of
Carrier Illumination has been performed as a probe
for unannealed and annealed profiles. For low dose
implants where no amorphization occurs, a universal
response function can be defined relating the CIsignal to the number of vacancies produced during
the implants. In the case of amorphization, a one-to
one relation between the CI-signal and the
amorphous depth can be observed. Finally it is
shown that the CI-technique is extremely sensitive to
the degree of activation and strong signal differences
can be found for samples with nearly identical
dopant profiles.
ACKNOWLEDGMENTS
The authors are grateful to E.Collart (AMAT) for
the Sb-implants.
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M.J.Rendon, MRS Symposium Proceedings Vol 717,
Warrendale, Pennsylvania, 2002, pp.285-290.
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