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www.elsevier.com/locate/apsusc
Applied Surface Science 247 (2005) 537–544
Laser doping for microelectronics and microtechnology
Thierry Sarnet a,*, Gurwan Kerrien a, Nourdin Yaakoubi a,Alain Bosseboeuf a, Elisabeth Dufour-Gergam a, Dominique Debarre a,
Jacques Boulmer a, Kuniyuki Kakushima b, Cyrille Laviron c,Miguel Hernandez d, Julien Venturini d, Tarik Bourouina e
a IEF Institut d’Electronique Fondamentale Universite Paris Sud Bat. 220, 91405 Orsay, Franceb LIMMS, University of Tokyo, Tokyo 153-8505, Japan
c CEA/DRT – LETI/DTS, 17 avenue des Martyrs, 38054 Grenoble, Franced SOPRA, 26 rue Pierre Joigneaux, 92270 Bois Colombes, France
e ESIEE, Cite Descartes, 2 Bd Blaise Pascal 93162 Noisy-le-Grand Cedex, France
Abstract
The future CMOS generations for microelectronics will require advanced doping techniques capable to realize ultra-shallow,
highly doped junctions with abrupt profiles. Recent experiments have shown the potential capabilities of laser processing of ultra
shallow junctions (USJ). According to the International Technology Roadmap for Semiconductors, two laser processes are able
to reach the ultimate predictions: laser thermal processing or annealing (LTP or LTA) and gas immersion laser doping (GILD).
Both processes are based on the rapid melting/solidification of the substrate. During solidification, the liquid silicon, which
contains the dopants, is formed epitaxially from the underlying crystalline silicon. In the case of laser thermal annealing, dopants
are implanted before laser processing. GILD skips the ion-implantation step: in this case the dopants are chemisorbed on the Si
surface before the laser-shot. The dopants are then incorporated and activated during the laser process. Activation is limited to
the liquid layer and this chemisorption/laser-shot cycle can be repeated until the desired concentration is reached. In this paper,
we investigate the possibilities and limitations of the GILD technique for two different substrates: silicon bulk and SOI. We also
show some laser doping applications for the fabrication of micro and nanoresonators, widely used in the MEMS Industry.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Laser doping; Ultra shallow junctions; Implantation; Microresonator; Nanoresonator; MEMS; LTP; GILD
* Corresponding author. Tel.: +33 1 69 15 40 73;
fax: +33 1 69 15 40 80.
E-mail address: [email protected] (T. Sarnet).
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved
doi:10.1016/j.apsusc.2005.01.172
1. Introduction
The aim of this work is to study a breakthrough
technology based on a laser process. A major
challenge for the future of CMOS technology
concerns the realization of ultra-shallow MOSFET
.
T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544538
source and drain. These junctions need to be ultra
shallow, highly doped with abrupt profiles. The ITRS
(Roadmap for Semiconductors) [1] has predicted that
conventional technologies, based on ion implantation
and rapid thermal annealing (RTP) or spike annealing,
will hardly meet these specifications. Recent experi-
ments have shown the potential capabilities of laser
processing of ultra shallow junctions (USJ) for CMOS
technologies. According to the ITRS, two laser
processes are able to reach the ultimate predictions:
laser thermal processing (LTP) and gas laser immersion
doping (GILD) [2–10]. LTP and GILD are both based
on rapid melting/solidification of the substrate. During
solidification, the liquid silicon, which contains the
dopants, is formed epitaxially from the underlying
crystalline silicon. The main difference between the
two laser processes concerns the implantation step. For
LTP, the dopants are implanted on the surface before the
laser processing, using conventional techniques. GILD
does not require the ion-implantation step: in this case
the dopants are directly chemisorbed on the Si surface
just before the laser-shot. The dopants are then
incorporated and activated during the melting/solidi-
fication cycle. The activation is limited to the liquid
layer and this chemisorption/laser-shot cycle can be
repeated until the desired concentration is reached.
ig. 1. SIMS depth profiles of B doped layers realized by GILD
ith 50, 100 and 200 laser-shots at 670 mJ/cm2.
2. Experimental procedure
GILD is performed in a high vacuum chamber
(10�7 mbar) on Si and SOI wafers, using a homo-
genized XeCl excimer laser (308 nm, 30 ns, 200 mJ per
pulse, 1 to 25 Hz). After cleaning and removing the
native oxide the substrate is introduced in the chamber.
The dopant precursor gas (BCl3) is injected and
chemisorbed on the substrate before each laser pulse.
LTP process is performed using a VEL15 SOPRA laser
(308 nm, 200 ns, 0.6 Hz). The samples have been
characterized using four-point probes, SIMS, IR
spectrometry and in situ optical characterization at
675 nm [9].
These laser processes are also carried out through an
oxide mask in order to selectively treat a particular zone
on the wafer. The masks are obtained after a series of
steps involving photolithography (for features with
lateral size>2–3 mm) or electron beam lithography (for
sub-micron features).
3. Experimental results
3.1. Results on Si bulk
The dopant profiles have been measured after
GILD laser implantation by SIMS [11]. Fig. 1 presents
boron concentration profiles obtained with 50, 100 and
200 laser-shots at 670 mJ/cm2. Box-like and abrupt
profiles have been obtained, with high dopant
concentration and very low sheet resistance. For the
same laser energy density, the junction thickness
increases with the number of shots due to the boron
incorporation, which changes the thermodynamic
parameters and the optical absorption at 308 nm.
Thanks to the optical characterization (in situ
reflectivity at 675 nm), we have been able to follow the
junction evolution during the GILD process. Fig. 2a
presents the linear evolution of the junction thickness
as a function of the number of laser-shots for three
different laser energy densities. SIMS data are also
plotted on the same graph and show a good agreement
with the optical diagnostic.
The boron incorporation remains constant during
the process, for the same laser energy density. The
linear evolution of the boron concentration (at/cm2)
versus the number of laser-shots at different energy
densities is shown in Fig. 2b. We have measured with
an IR spectrometer the transmission coefficient of
doped layer. By using the Drude theory for doped layer
index measurement we have been able to find the ratio
of the activated dose over the effective mass for each
sample. We have chosen an effective mass of 0.37 to
find the best fit between the SIMS and FT-IR
F
w
T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544 539
Fig. 2. Evolution during GILD process of the junction depth (a) and
the boron concentration (number of B atoms/cm2) (b), as a function
of the number of laser-shots for different laser energy densities.
Fig. 4. Evolution of sheet resistance vs. laser energy density for
different laser-shots on SOI 200/1000 nm and Si substrates.
measurements. The evolution of boron concentration
incorporated by shots on silicon substrate versus the
laser energy density is shown in Fig. 3.
3.2. Results on SOI
Additional GILD experiments have been carried
out on SOI (200/1000) substrate. Due to the 1000 nm
Fig. 3. Boron dose incorporation (number of B atoms/shots/cm2),
comparison on silicon bulk and SOI 200/1000 substrates.
of silicon oxide the heat is confined to the 200 nm of
the c-Si layer, decreasing the melting threshold to
260 mJ/cm2. An amorphization threshold also
appears above 610 mJ/cm2 because the liquid phase
reaches the oxide layer. Sheet resistances evaluated
from FT-IR (effective mass of 0.37) are in good
agreement with those measured by the four-point
probe. We have observed that the junction depth
increases with the number of laser-shots, like in the
GILD experiment on c-Si. In Fig. 3, we compare the
evolution of the boron concentration incorporated by
shots as measured by FT-IR versus the laser energy
density on SOI and c-Si substrates. For SOI, the
boron dose is much higher than the one on silicon
bulk and the boron incorporation can reach up to 30%
of the total chemisorbed sites on the silicon surface.
We explain this fact by the important difference
during the heating of the substrate before melting.
The boron incorporation efficiency increases with the
temperature gradient before melting. As a conse-
quence, the measured sheet resistance on SOI
substrate is lower than the one on Si substrate, but
the evolution of sheet resistance with the laser-shots
is the same. Fig. 4 shows the evolution of the sheet
resistance versus the laser energy density for
different laser-shots on SOI 200/1000 nm and Si
substrate.
4. ITRS comparison
According to the ITRS predictions, no conventional
techniques for USJ formation can meet the CMOS
T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544540
sub-0.1 mm specifications, mainly because of the
limitation due to channel implantation effects and
annealing dopant diffusion.
Laser processing can solve these technological
problems. Thanks to optical characterization of laser
processing, we have been able to determine the
evolution of junction thickness and boron incorpora-
tion as a function of the laser energy density on c-Si
bulk [9]. Therefore, we can extrapolate a theoretical
limit of GILD process for USJ.
In Fig. 5, we present these limitations and
characteristics of junctions made by GILD as
measured by four-point probes and SIMS. We
compare the universal Rs/X j tradeoff of implanted
and thermally annealed boron with recent conven-
tional techniques [2], and with laser thermal proces-
sing (LTP) studies from our previous work and from
Felch et al. [12].
The measurements show that GILD and LTP meet
the ITRS 2002 specifications for the 45 nm technology
node and laser processing is more efficient than any
other usual technique. In the case of LTP, transient
enhanced diffusion (TED) does not occur but the
implanted profile determines the doping profile.
Because GILD skips the implantation step, problems
coming from this implantation step can be avoided.
Junctions can reach depths as thin as 6 nm, and
because the activation rate is very high, the sheet
resistance can decrease down to 400 V/& with only
10 laser-shots and with an energy density just above
the melting threshold. The corresponding abruptness
is then close to 2 nm/decade.
Fig. 5. Sheet resistance vs. junction depth according to the ITRS
2002 specifications. Comparison of GILD results with conventional
techniques and laser annealing.
5. Application of laser doping for MEMS
5.1. Introduction
Recently, there has been a great deal of interest in
micromechanical resonators for a wide range of appli-
cations including wireless communications, thanks to
its superior characteristics, such as high Q-factor,
temperature stability, and aging properties. Moreover,
the integration compatibility with CMOS circuits
enables smaller chip size and low-cost mass production.
However, fabricating small structures of low mass and
high frequency becomes challenging. We have used the
potentiality of gas immersion laser doping (GILD) to
fabricate such micro silicon bridges [13].
Generally, boron-doped silicon has residual tensile
stresses, since the boron atom is smaller than silicon: the
stress largely depends on the concentration of boron.
Huang and Najafi have reported tensile stresses ranging
from 13 to 140 MPa with boron concentration of
9 � 1019 cm�3, depending on subsequent annealing
processes [14]. In our case the dopant concentration is
very high, which means that we have the possibility to
increase the resonant frequency for the same bridge
dimension.
5.2. Fabrication
A single polished (1 0 0) wafer (n-type 50 V cm)
was cleaned by acid chemicals and was thermally
oxidized to form a 100 nm mask layer of SiO2. This
mask thickness was chosen to limit the absorption of
laser light during the subsequent laser irradiation
(Fig. 6).
We used both photolithography and electron beam
lithography to pattern the SiO2 mask in order to open a
bridge shape along the h1 0 0i direction. Boron doping
was done by GILD using a homogenized XeCl
excimer laser (l = 308 nm, 30 ns, 700 mJ/cm2, 2 Hz)
and BCl3 as a precursor gas into a high vacuum
chamber. The thickness of the p-layer is tuned within
the 10–200 nm range with a boron concentration up to
3 � 1021 at/cm3 [3,9], by using different energy
densities and numbers of shots. The wafer was then
annealed at 600 8C for 1 h in order to measure the
residual stresses. The sheet resistance has been
measured using four-point probes, before and after
annealing (Fig. 7).
T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544 541
Fig. 6. Reflection coefficient R of the substrate at l = 308 nm as a
function of the silicon mask thickness.
Fig. 8. Description of the process: (a) (1 0 0) Si substrate cleaning
and native oxide removal, (b) oxide layer formation (100 nm), (c)
oxide patterning (photolithography or e-beam lithography), (d) laser
doping, (e) oxide mask removal by HF and (f) TMAH selective
etching of undoped silicon to release the doped bridge.
After the heat treatment, the junction is still very
electrically activated (R < 100 V/sq.). After removing
the SiO2 mask with HF, an anisotropic etching of the
undoped silicon was performed with tetra methyl
ammonium hydroxide (TMAH) to selectively release
the doped bridges. The GILD process and the
anisotropic etching are summarized in Fig. 8.
5.3. Results and measurements
A typical SEM image of a laser-doped bridge (4 mm
in width and 50 mm in length) is shown in Fig. 9. The
thickness of the bridges were measured on cross-
Fig. 7. Evolution of the square sheet resistance before and after vs.
number of laser-shots.
sectional views using a high resolution FEG-SEM, as
shown in Fig. 10. The resonance was obtained by laser
illumination (l = 780 nm) modulated by network
analyzer. The vibration was measured using a laser
Doppler vibrometer (l = 632.5 nm). All these char-
acterizations were done under vacuum (5 � 10�3 Pa)
[13].
Fig. 11 shows a typical spectrum of a 100 mm
bridge, with a resonance at 2.77 MHz and a Q-factor
of 1835. This frequency is 25 times higher than that of
stress-free single crystalline Si bridge with the same
dimensions. This resonant frequency tends to a
Fig. 9. SEM micrograph of a GILD boron-doped single-crystalline
Si bridge. Dimensions of the bridge (length � width � thickness):
50 mm � 4 mm � 120 nm.
T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544542
Fig. 10. SEM–FEG micrograph of a GILD boron-doped single-
crystalline Si bridge. Thickness of the bridge: 90 nm.ig. 12. Evolution of the resonant frequency as a function of the
umber of laser-shots. The fstressed/fstress-free ratio is shown on the right.
ig. 13. AFM image of a laser treated zone showing a roughness of
.5 nm RMS. Sample: pre-amorphized Si (BF25 keV, 2e 15 at/cm2.
aser irradiation conditions: 2.2 J/cm2, 308 nm, 200 ns, single shot.
maximum for 100 laser-shots and slightly decreases
above this value (Fig. 12). We also compared in
Fig. 12 this frequency to the stress-free bridge, which
has a resonant frequency of 100 kHz (100 mm bridge)
and 400 kHz (50 mm bridge): the laser doping largely
increases the resonant frequency (up to 40 times
higher).
The RMS roughness of the laser treated areas has
been analyzed with an AFM. In most cases the
roughness after the laser treatment is maintained at
very good levels, usually lower than 5 A RMS
(Fig. 13). However we have observed a slight increase
in this roughness when working with pre-amorphized
silicon wafers (Fig. 14). In that particular case we have
Fig. 11. Output spectra of a 100 mm bridge at 1st mode measured in
a vacuum chamber. The resonant frequency of 2.77 MHz is more
than 20 times higher than that of stress-free bridge (100 kHz). The
Q-factor is 1835.
ig. 14. Evolution of the roughness and melted depth as a function
f the laser energy density. Laser: 308 nm, 20 ns. Sample: pre-
morphized Si wafer (Ge+ 10 keV, B+ implanted 0.5 keV) processed
ith LTP. Zone 1: below melting threshold, no change in roughness;
one 2: partial melting of the pre-amorphized layer, increased
oughness, zone 3: total melting of the pre-amorphized layer, low
oughness.
F
n
F
0
L
F
o
a
w
z
r
r
T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544 543
Fig. 16. Measured resonant frequencies and fitting to calculations.
The thicknesses of the bridges are depicted. The stress-free silicon
bridge with a thickness of 120 nm is also shown.
seen that it is necessary to melt the entire pre-
amorphized layer in order to obtain a good recon-
struction of the surface, and, consequently, a low
roughness (<5 A RMS).
5.4. Discussion
The design of the resonator is based on a vertical
resonating bridge, where the motion of the bridge is
perpendicular to the substrate (Fig. 15). If we apply a
stress (s) to this structure the resonant frequency tends
to increase. This dependence is generally described by
the following equation:
fRstressed¼ 1:028
t
L2
ffiffiffiffiE
r
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 þ 12L2
ap2Et2s
r
where k and m are the spring constant and its mass, E
the Young’s modulus, r the mass density and s the
applied in-plane tensile stress.
We have estimated the residual stress by fitting the
previous equation to the experimental resonant
frequencies. The highest stress value obtained was
190 MPa at 100 laser-shots; these values are repre-
sented in Fig. 16.
By increasing the number of shots, the resonant
frequency tends to decrease while the thickness still
increases. This can be explained by a decrease in the
crystallinity and mechanical properties of the Si
bridge when using excess laser exposure (over 100
shots). From our previous reports [3,9], the boron
concentration at more than 200 laser-shots exceeds the
solubility limit of boron in silicon at room tempera-
ture: we anticipated that some defects of this super-
Fig. 15. Schematic layout showing the resonance frequency
increase due to in-plane tensile stresses on a laser-doped bridge.
doped silicon or maybe inactive boron incorporation
[15] might have slightly decreased the resonance.
Also, one might suggest that the annealing (1 h at
600 8C) tends to release the stress and decrease the
resonant frequencies. Additional laser-doped bridges
have already been realized, without annealing, with
length between 1 and 500 mm, which corresponds to a
wide resonant frequency range that should cover
numerous MEMS applications (estimated range
1 MHz to 1 GHz). These unique features suggest
several MEMS applications, currently under investi-
gation (e.g. RF filter, mass detection sensors, etc.).
Smaller features have also been realized by
combining electron-beam lithography and GILD. In
that case we have spin-coated a thin PMMA layer on
top of a pre-oxidized Si wafer. The mask was directly
written in the PMMA layer using a high resolution
FEG–SEM and, after development, has been trans-
ferred to the oxide layer. The wafer has been laser-
doped through this mask. This process is still under
development and should eventually allow us to make
very promising nanoresonators and nanostructures.
Fig. 17 shows a SEM photograph of two micro bridges
that were obtained using GILD laser doping through a
mask that was designed by electron beam lithography:
the width of the bridges is 900 and 800 nm, with a
thickness below 100 nm and a typical RMS roughness
below 10 A.
T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544544
Fig. 17. SEM micrograph of two micro bridges obtained using
GILD laser doping through a mask designed by electron beam
lithography. Dimensions of the bridges: 9 mm � 900 nm (up) and
8 mm � 800 nm (down). Thicknesses of the bridges: <100 nm,
typical RMS roughness <10 A.
6. Conclusions
In this work, we present the GILD process which
could be an alternative for implantation and rapid
annealing in the making of ultra shallow junctions. By
studying GILD on Si and SOI 200/1000 nm substrates,
we have seen that the number of boron atoms
incorporated by shots depends on laser energy density
and temperature gradient. Thanks to optical char-
acterizations, we have been able to estimate the
theoretical limit curve for the Rs/X j on Si bulk.
Therefore, we have been able to determine the laser
energy density and the number of laser-shots to make
an ultra thin junction with excellent activation rate,
which can meet the final ITRS specifications. Finally
we have realized micro and nanoresonators for MEMS
applications using both laser doping and photolitho-
graphy or electron beam lithography.
Acknowledgements
This work has been supported by the French
Ministry of Research and Technology in the frame-
work of the RMNT DOLAMI project. The authors
would like to thank all DOLAMI participants for their
contribution to this work.
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