Formation of Silicon Ultra Shallow Junction by
nonmelt excimer laser treatment
A. Florakis, A. Papadimitriou, N. Chatzipanagiotis,
and D. Tsoukalas
Department of Applied Physics, School of Applied Science
National Technical University of Athens
Athens, Greece
Mail to: anflorak@central.ntua.gr
Abstract- Implementation of Plasma Doping and nanosecond
laser annealing in the non-melt regime has shown to hold great
promise for the realization of Ultra Shallow Junctions, designed
for the sub 45nm node. This work includes extensive simulation
of these two emerging techniques using the Synopsys Sentaurus
Process software tool which are compared with experimental
data after each process step. The results reveal consistency
between simulation and experiment. It is thus concluded that
existing simulation approach based mostly on Kinetic MonteCarlo method allows for sufficient physical understanding of the
underlying mechanisms for these advanced process steps.
I.
INTRODUCTION
The continuous size decrease of Complementary metaloxide-semiconductor (CMOS) devices puts severe limits to
junction formation processes, such as implantation and
thermal activation. The requirements for the upcoming
generation of sub 45nm devices, regarding the critical
parameters of junction depth, sheet resistance and abruptness,
necessitate careful Source/Drain formation engineering.
Several studies [1,2] have shown the merits of combining ultra
low energy dopant implantation alongside to nanosecond laser
annealing techniques able to deliver very limited thermal
budget in Silicon bulk. The minimization of the induced
energy leads to high level of electrical activation, while
retaining the shape of dopant concentration profile.
BF3 PLAsma Doping (PLAD), stands as a promising
candidate for the replacement of the conventional ultra low
energy ion implanters, due it's capability to deliver ions at
energies less than O.2keV and thus creating ultra shallow as
implanted concentration profiles (xj<10nm). Moreover, coimplantation of Fluorine improves junction's morphological
and electrical characteristics both by enhancing the electrical
activation and limiting Boron diffusion [3]. However, there
are some issues that should be dealt, before the introduction of
plasma implantation in full production scale. Primarily, actual
implanted dose is significantly lower than the nominal one. In
addition, induced dopants present a wide range in terms of
energy and kind of species due to the very nature of the
method. Therefore the simulation of the process is vital in
order to achieve the desired as implanted characteristics.
On the other hand, Excimer Laser Annealing (ELA) is
ideal for the formation of shallow junctions, as the delivered
light energy is transformed into heat within the first layers of
978-1-4244-4353-6/09/$25.00 ©2009 IEEE
N. Misra and C. Grigoropoulos
Department of Mechanical Engineering
University of California, Berkeley
Berkeley, CA United States
the lattice due to high absorption coefficient value of Silicon
at this wavelength (248nm). In addition, ultra fast temperature
ramp up and ramp down rates lead to annealing times
significantly smaller than the characteristic times of
phenomena that are associated with diffusion, such as
extended defects dissolution.
This work presents simulation results compared with
experimental data of the three process steps involved, namely
plasma doping, laser induced heating of silicon and
diffusion/activation of dopants. This comparison reveals really
good agreement between the two approaches, supporting a
sufficient level of understanding of physical mechanisms
involved.
II.
EXPERIMENTS AND SIMULATIONS
BF3 plasma was used for implanting boron ion in n-type
silicon wafers. Nominal values of implantation energy and
dose are O.4keV and 3E15cm-2 respectively. During
implantation procedure, a significant amount of damage is
accumulated in the first silicon layers, while the majority of
Boron atoms, are not in substitutional sites, and therefore do
not contribute to conductivity. In order to recrystallize silicon
and activate the dopants, a KrF Excimer laser irradiation
(A=248nm, FWHM=20ns) has been performed. A variety of
annealing conditions, regarding the energy fluency and the
number of pulses has been implemented so as to investigate
the effect of these two parameters in the activation and the
kinetics of boron dopants. Irradiation has been carried out at
room temperature. The use of complete homogenization array
resulted in a top-hut spatial distribution of the energy over a
Smmx Smm area.
In order to investigate the effect of the laser annealing in
dopant concentration profiles, a series of SIMS measurements
were performed.
A possible melting of silicon leads unavoidably to boron
diffusion, as Boron diffusivity in Silicon is several orders
higher in liquid phase than in solid. SIMS analysis was
conducted using an IMS CAMECA instrument with an O2
primary beam at 1.lkeV. The irradiation in every combination
of energy fluency and number of pulses, have led to profile
movement not more than 2nm. As the junction depth Xj of the
as implanted sample is 13nm, irradiations resulted in the
formation of highly abrupt (2.4 nm/decade) and ultra shallow
(xj=15nm) junctions. The morphological characterization of
the samples included Transient Electron Microscopy (TEM)
and Atomic Force Microscopy (AFM).
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made using the standard methodology. Walther et al. [5] have
proposed an indirect method for plasma implantation
simulation, in case that the actually implanted dose is known
beforehand. By means of mass spectroscopy, they have
created a distribution of Boron dose into discrete implantation
energy channels for a given nominal implantation energy
(0.5keV). In their approach, implantation process is divided to
series of sequential Boron implantation steps according to the
above mentioned dose/energy channels. By using proper
normalization to our implantation data, we have calculated the
corresponding concentration profiles for the implantation
conditions presented in Table I (actual dose and Xj for each
sample have been determined using SIMS).
Plasma Implantation Specs
Nominal Implantation
Nominal Dose
Actual Dose
(cm'1)
(em")
Energv (eV)
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Fig.1 Simulated as implan ted boron concentration profiles for the
OAKeV/3El5cm,2(a) and 0.6KeV/3E15 cm,2(b) sampl es. Both Boron and BF2
implantation approaches are presented, alon g with data obtained from SIMS
TEM imaging revealed the presence of an amorphous zone
in the as implanted samples, which have been recrystallized
after thermal treatment. AFM measurements provided
additional evidence about the absence of melting of silicon, as
rms surface roughness values were substantially lower
comparing to that of samples sustained annealing in the
melting regime.
For the determination of the activation level, a series of
sheet resistance measurements were conducted using Van der
Pauw technique. Laser annealing resulted to significant
conductivity enhancement from an initial value of 15k.QJ0, to
6800/0 for the 625mJ/cm 2 -50 pulses condition. This value of
resistivity is not significantly higher from the theoretically
estimated [4] minimum sheet resistance value which is equal
to 550 Q/o.
Both KMC and analytical approaches have been
implemented for comparison reasons. As for the analytical
implantation a multi-parameter analysis was carried out. Mesh
dependency, use of advanced meshing and data optimization
strategies, along with different implantation tables, are some
of the key factors included in our investigation. However, the
calculated concentration profile tail was not in agreement with
SIMS measurements, as it was decaying rapidly after the first
8-9nm, for each implantation condition.
On the other hand, KMC implantation was proved quite
efficient, and the predicted concentration profiles were almost
superimposed to that obtained by SIMS, as it can clearly be
seen in Fig. 1a. and b. Again, several strategies and parameters
were investigated to achieve an optimum level of agreement
between experiment and simulation. However, implantation of
Boron atoms did not lead to the formation of an amorphous
layer as expected according to TEM imaging. Therefore, the
next step was BF2 implantation instead of Boron. Obtained
implantation profiles were in excellent agreement with SIMS
measurements (Fig. 1a. and b). Additionally, the introduction
ofBF2 creates an amorphous layer
The simulation flow process consisted of three different
steps; plasma implantation, laser interaction with matter and
Boron diffusion and activation. Results obtained during the
first two parts, are used as an input to the third. Whenever it
was possible we have followed both analytical and full Kinetic
Monte Carlo (KMC) approaches and compared it with our
experimental data. For our calculations we have used the
Sentaurus Process simulation tool, from Synopsys.
III.
RESULTS AND DISCUSSION
A. Plasma Doping Implantation
Contrary to conventional monoenergetic ion implantation,
plasma doping introduces a variety of species into silicon bulk,
at a wide implantation energy spectrum. As a result, a
prediction of the implanted concentration profile cannot be
Fig. 2 Amorph ized zone th ickness d iagrams obtained by simulat ion along with
correspond ing TEM images for a) OAKeV/3El5cm,2 and b) 0.6KeV /3E15 cm'2
as implanted samples.
(amorphization threshold is considered equal to the one fourth
of Silicon atom density, thus I.15E22cm-2) . The thickness of
the latter is in close proximity to the thickness revealed by
TEM for each condition (Fig. 2a. and b). Thickness and
junction depth values obtained by simulation and experiments
can be found in Table II.
Junction depth and oxidc thicknesses
Sample
lD
Junction Depth (nm)
SIMS
simulated
Oxide thickness (nm)
TEM
simulated
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13
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1.9
2.5
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18
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60
80
100
120
140
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Fig. 3 Simulated surface temperature evolut ion for several energy fluencies.
The peak temperature is reached after 34ns of the beginning of the irradiation,
in every condition.
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As already mentioned, irradiations performed using a KrF
laser able to deliver pulses (FWHM=20ns) with a top-hut
energy fluency spatial distribution over a 5x5mm 2 area. The
energy fluency used was in the 375 to 625mJ/cm 2 range. We
have then created a 2-dimensional mesh of variable density in
depth, in every node of which the heat equation was solved
numerically. An extensive analysis regarding time step and
mesh dependency has been performed. In Fig. 3 and 4a. we
present respectively surface temperature evolution with time
along with the corresponding heating and cooling rates for
each energy fluency condition. Since no direct temperature
monitoring was possible during the experiments only indirect
observations from SIMS and AFM were performed.
Simulation reveals that even for the highest fluency
(625mJ/cm 2) there is no melting occurrence (Tsimclting=1683K), however we are close to melting. Both SIMS
and AFM measurements reveal no melting, and sheet
resistance analysis showed that at this fluency, high dopant
activation can be achieved. As an additional checkpoint we
have used our simulation to compare our results with
literature data [6,7] and a good agreement was observed.
From fig. 3 it is clear that the peak temperature for each
temperature is reached at about 34ns from the beginning of
the irradiation. Finally, in fig. 4b. we are presenting snapshots
of the temperature distribution inside the Silicon bulk, for
four different times.
Depth (m ICro n s)
Fig. 4 a) Surface temperature ramp up and down rates. b) Temperature
distribution within Silicon bulk, in four different time snapshots . The t=34ns
frame corresponds to the maximum surface tempera ture obtained .
B. Laser Annealing
One of the key features ofKrF ELA is the high absorption
coefficient of Silicon at this wavelength (1.6E6cm-').
Combined to the ability of this kind of laser to deliver large
number of nanosecond pulses, ultra high ramp up and down
temperature gradients can be achieved. As annealing times
are significantly lower than the characteristic constants for
diffusion, by keeping silicon in the solid phase, Boron
diffusion can be retarded or even eliminated. Irradiations at
several different energy fluency/number of pulse
combinations are necessary for the achievement of optimum
tradeoff between sheet resistance and diffusion. Therefore, it
is important to be able to predict the influence of each
condition to the evolution of the temperature distribution
along the surface and the volume of the material. Moreover,
the knowledge of the thermal behavior for a given condition
is necessary for calculating the effect of laser annealing to
boron kinetics and activation.
C. Boron diffusion and activation
Our analysis concluded with the modeling of the boron
diffusion and activation. For that we have used as an input the
temperature evolution distribution obtained by the previous
analytical calculations as well as the KMC simulated
implanted profile. For space economy we present here the
diffusion and activation simulation results following a KMC
approach which show more consistent results with
experiments than analytical simulations.
For the purpose of non-lattice KMC analysis, all the
species involved into the diffusion and activation mechanisms,
such as dopants, point defects, impurities (alone or combined),
are considered as particles. In accordance to the assigned
values for the occurrence of each event (prefactors and
migration or binding energies), these particles can perform
jumps in order to migrate, agglomerate into clusters and then
remit again. For our calculation several models where
involved following the computational approach of MartinBragado et al. [8]. Among them are, diffusion (through kickout mechanism [9]), activation/deactivation (either by
clustering with Interstitials and Vacancies, or dopant
precipitation) and clustering. In addition, formation and
dissolution of extended defects, such as {3 II} defects or
even Dislocation Loops and finally amorphization and
recrystallization are also taken into consideration. The
implementation of KMC for the simulation of multipulse
annealing effect led to limited dopant profile movement (up to
2nm), for every energy fluency/number of pulses condition, in
total agreement with the experimental data. Fig. 7 presents an
example of the diffusion behavior for the case of 10 pulses
irradiation at 625mJ/cm 2 •
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Authors would like to thank Dr. A. Halimaoui from LETI for
providing the implanted wafers.
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This work was funded by the European Union
framework of the 1ST project 026828 PULLNANO.
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AKNOWLEGDEMENTS
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REFERENCES
Fig. 6 Boron concentration profiles obtained by SIMS and simulation for the
625mJ/cm 2-I Opulses cond ition.
Our analysis revealed significant enhancement of the
electrical activation (about 1 order of magnitude difference
from the lowest to the highest fluency condition). Generally,
sheet resistance is reduced by increasing primarily the energy
fluency and secondly the number of pulses, as it can clearly be
seen in Fig. 7. Simulation results are in excellent agreement
with the measured values through the Van der Pauw technique
for the 50 pulses condition, however our calculations for the
10 pulses, are not in the same agreement.
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450
500
550
Eng. B, Volumes 154-155, 5 December 2008 , Pages 39-42 .
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2
SOOmJ/cm 10p
2
SOOmJ/cm SOp
2
562mJ/cm 10p
2
562mJ /cm SOp
2
625mJ/cm 10p
625mJ/cm2 SOp
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E
· S
(/)
[I]
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. S
E·
S
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recrystallization of the amorphous zone has been achieved,
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50 pulses) in accordance with images obtained by TEM.
IV. CONCLUSIONS
In this work we investigate the effect of KrF laser
annealing to the formation of ultra shallow junction. Both
experimental and simulation data show significant
enhancement of the electrical and morphological
characteristics, along with very limited dopant concentration
profile movement. Combination of analytical and Kinetic
Monte Carlo simulation techniques, allowed a consistent
modeling description of every step in the process flow, from
plasma implantation to dopant diffusion and activation.
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Energy Fluency (mJ/cm')
Fig.7 Calculated (S) and experimental (E) sheet resistance values for
different energy fluency/number combinations. Minimum sheet resistance
value is obtained at the 625mJ /cm 2 - 50 pulses condition
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