APPLIED PHYSICS LETTERS 86, 051909 共2005兲
Depth distribution of B implanted in Si after excimer laser irradiation
Giovanni Mannino,a兲 Vittorio Privitera, Antonino La Magna, and Emanuele Rimini
CNR-IMM Sezione Catania, Stradale Primosole 50, 95121 Catania, Italy
Enrico Napolitani
INFM-MATIS and Dipartimento di Fisica, Università di Padova, Via Marzolo 8, 35131 Padova, Italy
Guglielmo Fortunato and Luigi Mariucci
CNR-IFN, Via Cineto Romano 42, 00156 Roma, Italy
共Received 30 August 2004; accepted 30 November 2004; published online 28 January 2005兲
Liquid phase epitaxial regrowth following laser melting significantly modifies the concentration of
point defects in Si, such that peculiar depth distribution of subsequently implanted B arises. At room
temperature, a large fraction of B atoms, ⬃15%, implanted in laser preirradiated Si, migrate up to
the original melt depth. During high temperature annealing, the nonequilibrium diffusion of B is
reduced to ⬃25% of that measured in unirradiated Si. Both these phenomena are conclusively
attributed to an excess of vacancies, induced in the lattice during solidification and to their
interaction with impurities and dopant. © 2005 American Institute of Physics.
关DOI: 10.1063/1.1856696兴
Pulsed laser irradiation of implanted silicon has been
recently revisited as a tool for postimplantation dopant activation, which ensures very shallow and abrupt profiles.1–7
Excimer laser annealing, very likely modifies the density of
point defects,8 which might impact on the diffusion of subsequently implanted dopants. In particular, an excess of vacancies with respect to the equilibrium value has been demonstrated by x-ray diffraction measurements.9
In this letter, we present an experimental investigation,
based on measurements of B implant profiles, to detect the
atomic-scale modification of Si induced by laser beams prior
to implantation. Our data demonstrate that laser irradiated Si
is intrinsically different from nonirradiated material in terms
of its response to an ion implantation process. Excess vacancies, generated by laser irradiations, cause the formation of
peculiar profiles following ion implantation and modify,
moreover, the atomic transport at high temperature.
Irradiations were performed using a XeCl excimer laser
共␭ = 308 nm, 28 ns pulse duration兲. The spot of the laser
beam is rectangular 共3 ⫻ 8 mm兲 with a Gaussian-type intrinsic energy density distribution along the shorter side. Selected areas on Czochralski p-type silicon wafers with a resistivity of 4 ⍀ cm were irradiated by maintaining laser
energy density and beam position over the wafer fixed. Using
this method, we obtained a matrix of separated irradiated
areas, each of them mirroring the whole energy density spectrum of the laser beam. The maximum energy density measured at the center of each spot area was 1.25± 0.06 J / cm2,
where the error refers to energy density peak variation over
the whole number of irradiated areas on the wafer. Within
each spot the silicon melt depth was determined by the
Gaussian shape of the beam energy density profile. The Si
melt depths as a function of energy density and substrate
temperature have been calculated by means of a model based
on heat flow equations as described in Ref. 7. According to
the simulation results, the laser energy density used in this
experiment produces a maximum melt depth of
a兲
Electronic mail: [email protected]
⬃145± 20 nm for crystalline Si irradiated at room temperature. In order to improve the crystalline quality of the regrown layer after irradiation, the irradiation was performed
in a multishot 共ten pulses with frequency of 1 Hz兲 regime.
For comparison, some areas on the wafer were irradiated
with a single pulse. Most of the laser treatments were done in
vacuum at room temperature 共RT兲; just a few were performed on a sample at 450 ° C. At this high temperature irradiation the beam peak energy density was lowered to
0.87 J / cm2 in order achieve the same melt depth.
All the irradiated wafers were subsequently implanted
with B ions at energies in the range 3 – 20 keV, 1
⫻ 1013 / cm2, respectively; some samples were annealed at
800– 1100 ° C for 30 s. The choice of B to probe the presence of defects has been dictated by previous studies, where
we demonstrated that B is very sensitive to monitor small
variations of point-defect concentration10 being the interstitial fraction for B diffusion 艌0.98.11
The chemical profiles measured by secondary ion mass
spectroscopy 共SIMS兲 of as-implanted 5 keV B corresponding to areas irradiated with varying energy densities are
shown in Fig. 1. As a comparison, the B profile measured
outside the laser spot is also shown. A noteworthy feature of
FIG. 1. Chemical profiles of 5 keV B 1 ⫻ 1013 / cm2 implanted into Si preirradiated by laser for ten pulses with different energies.
0003-6951/2005/86共5兲/051909/3/$22.50
86, 051909-1
© 2005 American Institute of Physics
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051909-2
Mannino et al.
the profiles in the preirradiated samples is the concentration
plateau at ⬃2 ⫻ 1017 / cm3, independent of the irradiation energy density that extends up to a depth coincident with that
calculated by the melting model.7 For the energies reported
in Fig. 1 the calculated melt depths are ⬃125, ⬃100, and
⬃90 nm. At room temperature, a large amount of atoms, as
high as ⬃15% of the nominal implanted dose, migrate up to
the melt depth; this fraction decreases with the irradiation
energy density, being the profile confined in a progressively
shallower layer. The changes in the shape of the B asimplanted profiles cannot be attributed to uncontrolled variations 共e.g., ion channeling兲 during the implant. All the SIMS
measurements reported in Fig. 1 are taken in a small 共3
⫻ 1 mm兲 region within a single irradiated area of the same
wafer. Furthermore, we verified that the shape and energy
density dependence of all the profiles is well reproducible,
regardless of the spot in which they have been measured.
Chemical profiles identical to those reported in Fig. 1, in
terms of shape and depth reached, were also found for a
3 keV B implant performed in irradiated Si. In the case of a
20 keV implant, the profile is hardly distinguishable from
that of the same implant in nonirradiated Si. The peculiar
shape of the implant profile hence originates when the implant is confined within the originally melted layer. Finally,
after irradiation performed in single shot regime, the B profile measured in these irradiated regions coincides with that
measured outside the laser spot. This result implies that the
atomic-scale modification, which causes the B profile plateau, arises from the superimposition of a number of
“events” occurring after each laser pulse. The number of ten
pulses represents a threshold above which the modification
becomes experimentally measurable. These experimental
facts indicate that the Si lattice keeps the memory of the laser
irradiation. Moreover, the modification under investigation is
certainly of point-like nature, being any irradiated samples
identical to the nonirradiated Si in terms of observation by
high-resolution transmission electron microscopy.
Assuming an alteration of the crystal point-defect concentration, we investigated the high temperature diffusion of
B, 共retarded diffusion兲. Figure 2 shows the B profiles implanted in Si, previously laser-irradiated at room temperature
关Fig. 2共b兲兴 or at 450 ° C 关Fig. 2共c兲兴, before and after annealing at 900 ° C 30 s; for comparison the reference profile in
nonirradiated Si is also reported 关Fig. 2共a兲兴. The experimental profiles are compared with simulations, using as input the
as-implanted profile and assuming a fixed fraction of interstitials surviving to Frenkel pair recombination. These simulations rely on the pairing effect of doping diffusing atoms
and defects.12 The simulation shown in Fig. 2共a兲 well reproduces the enhanced diffusion of B in nonirradiated Si by
assuming 1 interstitial/implanted-ion not recombined.13 The
same procedure, applied to the diffused profiles in Figs. 2共b兲
and 2共c兲, significantly overestimates the B diffusion both in
the peak and in the tail of the profile, suggesting that the net
amount of interstitial available to the diffusion process has to
be reduced in irradiated Si. The easiest way to reduce the
number of interstitials in the simulation consists in the assumption that vacancies in excess are present in the sample
after regrowth. This hypothesis has been implemented into
the model including a Gaussian-type vacancy profile, having
a dose of ⬃8 ⫻ 1012 / cm2 for both samples 共irradiated at RT
and 450 ° C兲, this value being the only fit parameter. Based
on this hypothesis, the simulation of B diffusion in irradiated
Appl. Phys. Lett. 86, 051909 共2005兲
FIG. 2. Chemical profiles of 5 keV B, 1 ⫻ 1013 / cm2 implanted and diffused
at 900 ° C 30 s, measured in 共a兲 nonirradiated Si, 共b兲 in preirradiated Si for
ten pulses at room temperature, and 共c兲 in preirradiated Si for ten pulses at
450 ° C.
Si well reproduces the entire experimental profiles.
The center of the Gaussian vacancy profile has been positioned at the liquid/solid interface but it has to be pointed
out that the position, as well as the shape of the vacancy
profile, does not play a role in the simulation. The B profiles
remain unchanged, and the agreement with the experimental
profile is maintained, regardless of the width and position of
the vacancy Gaussian profile used because of the high diffusivity of vacancies at room temperature.
Taking into account this vacancy excess, the amount of
interstitials interacting with diffusing B in irradiated and subsequently implanted samples was drastically reduced to 0.2
and 0.3 interstitial/implanted-ion for RT and 450 ° C sample
temperature during irradiation, respectively.
The scenario drawn so far can be summarized as follows: after each laser pulse, vacancies are formed under nonequilibrium conditions at the liquid/solid interface 关see the
dotted line in Fig. 3共a兲兴. The vacancy distribution just formed
broadens toward the “cold” Si lattice beyond the interface, as
well as toward the surface in the part of the sample at higher
temperature, close to the molten regrowing layer 关see the
solid line in Fig. 3共a兲兴. The vacancy diffusion, after the
sample solidification, reflects the temperature field and therefore is asymmetric with respect to the solid/liquid interface.
The subsequent laser pulse, occurring with a frequency of
1 Hz melting up to the same depth as the previous pulse,
removes those vacancies that have survived within the layer
solidified. In contrast, those vacancies diffused beyond the
liquid/solid interface are not be annealed anymore and join
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051909-3
Appl. Phys. Lett. 86, 051909 共2005兲
Mannino et al.
effect was not detected, the profile peak lies in the vacancyrich region, We guess that, since more complex defects are
generated in a high concentration region, room temperature
migration near the peak of the profile is inhibited. The portion of the profile, where the observed effect is clearly detectable 共e.g., the low concentration tail兲 for the 3 – 5 keV
implants, is instead well beyond the vacancy rich region.
Moreover, the vacancies stabilized at room temperature
into complexes are promptly released in the very early stages
of the heating process due to their low binding energy.14
Therefore, the vacancies released from complexes partially
reduce the enhanced diffusion of B by annihilating the interstitial 共the “+1” is reduced down to “+0.2– 0.3”兲 defects that
otherwise promote B enhanced diffusion.
The structural modification occurring after laser irradiation is well correlated with a vacancy accumulation after
each laser pulse. The as-implanted profile is significantly different from a conventional implant profile, with a pronounced plateau extending up to the melt depth, due to the
excess of vacancies generated. Moreover, the diffused profile
is shallower from that obtained in conventional diffusion experiment, simply because the vacancies still stored in the
solidified layer reduce the amount of interstitials sustaining
the B nonequilibrium diffusion.
The authors thank S. Whelan for valuable discussions
and R. Storti for technical assistance. This work has been
partially supported by the IST project FLASH 共IST-200138901兲 and FIRB 共RBNE012N3X-002兲.
1
FIG. 3. Schematic view of the excess-V formation and diffusion processes.
the vacancies formed during this subsequent shot. In that
way, after each laser pulse, vacancies are stored in the cold
Si beyond the interface 关see Fig. 3共b兲兴. When the laser is
finally switched off, the solidified layer is filled with an increased concentration of vacancies sufficient to affect B migration, provided the number of laser pulses is high enough
共ten laser pulses are observed to be needed at least兲 as shown
in Fig. 3共c兲. These vacancies are then stored in the regrown
layer, either trapped by the impurities present in the Cz substrate or by themselves, and form a stable distribution uniformly distributed over the entire melted thickness. The vacancy complexes distribution, due to trapping that occurs
while migrating toward the surface, has a peak at the melt
depth where the initial vacancy concentration is the highest.
In close relation with such vacancy complexes distribution,
B migrates and accumulates along the V distribution at room
temperature. The B profile, increasing from the plateau region up to the melt depth, is a unique signature of the vacancy distribution. In the case of 20 keV implant, where the
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