Channeling Doping Profiles Studies for Small Incident Angle
Implantation into Silicon Wafers
B.N. Guo1, N. Variam1, U. Jeong1, S. Mehta1, M. Posselt2, and A. Lebedev2
1
Varian Semiconductor Equipment Associates, 35 Dory Road Gloucester, MA 01930,USA
2
Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314, Dresden, Germany
Abstract. Traditional de-channeling dopant profiles in the silicon crystal wafers have been achieved by tilting the wafer
away from the incident beam. As feature sizes of device shrink, the advantages for channeled doping profiles for
implants with small or near zero degree incident angles are being recognized. For example, high-energy CMOS well
spacing limitations caused by shadowing and encroachment of the ion beam by photoresist mask can be avoided for near
zero degree incident implants. Accurate models of channeled profiles are essential to predict the device performance.
This paper mainly discusses the damage effect on channeled dopant profiles. Especially, damage effects on channeled
dopant profiles are correlated to ThermaWave (TW) measurements. It is demonstrated that there is a critical dose at
which the damage effects have to be considered for channeled dopant profile evolvements.
implemented in the Crystal-TRIM can be found
elsewhere [5,6].
INTRODUCTION
Traditional de-channeling dopant implants are
achieved by tilting and rotating the wafer normal so
that the incident ions are implanted into relative
random crystal lattice network. As feature sizes of
device shrink, the advantages of implants with small or
near zero degree incident angles are being recognized.
For implants at higher incidence angle, shadowing and
encroachments of the ion beam by the photoresist
mask or the gate stack cause pattern shifts, well
spacing constraints for high-energy well implants, and
loss of lateral abruptness and degraded drive current
performance for the low energy source/drain extension
implants. A precise control of implant incident angle is
thus necessary to produce both uniform electrical
characteristics across the wafer and enhanced
transistor performance [1,2,3].
In an earlier paper, authors discussed briefly the
dependence of channeling in the areas of the
acceptance angle and incident angle, dopant species,
energy, dose and extent of damage induced in the
crystal [7]. It was demonstrated that ThermaWave
(TW) and Secondary Ion Mass Spectrometry (SIMS)
results could be used to align the wafer relative to the
incident beam to achieve perfectly channeled dopant
profiles. Small beam incident deviation from the wafer
normal will lead to observable differences in SIMS
dopant profiles and such differences cannot be fully
compensated with thermal dopant activation process,
typical of advanced CMOS fabrication [7].
To understand the channeled dopant profiles,
damage effects along with dose accumulation should
be fully understood. In the present work, TW and
SIMS data are presented to discuss the evolving
dopant profiles along with accumulated implanted
dose. Simulated dopant profiles based on CrystalTRIM will be provided elsewhere [8].
Near zero incident angle implants result in
channeled dopant profiles rather than traditional dechanneled profiles. From a device engineering
perspective, accurate models of channeled profiles are
becoming more important. Process simulators such as
TSUPREM relied on a comprehensive database to
generate and predict de-channeled dopant profiles [4].
Atomistic computer simulation codes, such as CrystalTRIM, are of great importance to generate information
such as as-implanted range and damage profiles for
near zero channeled implants into crystalline silicon
substrate. Extensive description of the physical models
EXPERIMENTAL DETAILS
All SIMS samples are 200mm wafers with only
native oxide as the top-layer. Wafers are implanted on
VIISta 810 medium-current ion implanter. In the
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
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As observed in Figure 1, the excellent fitting
correlation (R2>0.99) can be obtained for wafers
implanted with B at 70, 270, and 540keV. For As and
P implants, two different set parameters were used to
fit implants with low dose (R2>0.99) and high dose
(R2>0.99). For As implants, it seems that the saturation
of TW reached for implants above 5e13/cm2 implants.
VIISta platform of ion implanters, beam parallelism
and beam incident angles are both measured and
controlled with Faraday systems, magnetic beam angle
corrector system, and software. The wafer position
accuracy is controlled in a precise fashion to ±0.1°.
A series of wafers were implanted at 0° tilt and 0°
rotation relative to <100> wafer normal with B at 70,
270, and 540keV, P at 270 and 540keV, As at 120 and
270keV, with doses range from 5e11 to 1e14 /cm2. All
implants with dose less than 5e13/cm2 were completed
within the same beam setup, and beam currents were
raised for higher dose implants. Careful alignment was
performed before implanting sample wafers. All
wafers were measured with ThermaProbe TP320.
Selected wafers were measured with SIMS for doping
profiles.
Since the damages are related to the energy loss
through nuclear stopping power, the damage
accumulation for the same energy projectiles is much
more rapid for heavy ions such as As than that for
relative light ions such as P and B. Damage effects on
channeling will enter into play significantly at the
critical dose above which the TW behavior deviates
from that described by Eq. 2. This is further illustrated
in the SIMS results described in Figure 2-Figure 4
(B/P/As SIMS profiles at 270keV for selected doses).
In Figure 2 and Figure 3, all SIMS profiles for B
and P at 270keV are scaled relative to 1e13/cm2. If
there were no effect of damage accumulation, then the
SIMS profiles should have similar shapes after scaling.
For B 270keV at dose below 1e14/cm2, it is clearly
shown that there is no significant profile evolvement
with dose accumulation. Scaled SIMS profile for
1e12/cm2 is different from profiles with higher dose
due to the SIMS detection limits and measurement
statistics.
RESULTS
ThermaWave (TW) measurement is a widely used
metrology to characterize dopant uniformity across
wafers, especially at lower implanted dose for dechanneling implants. In TW measurements, in-depth
defects or implant-induced damages were correlated
with variations of laser reflectance through between
laser generated thermal-wave (photo-induced heat) and
plasma wave (electron-hole pairs) interaction with
defects or damages. TW sensitivity is define as
(TW2 − TW1 )
TW Sensitivity =
( Dose2 − Dose1 )
(TW2 + TW1 )
For P, 270keV, accumulated dose leads to less
channeled profiles and damage-induced effects are
observed for implanted dose above 1e13/cm2. As
previous data shown with implants P 800keV, 0° tilt at
various doses [7], the damage below dose of 1e13/cm2
for P implant between 270 to 800keV is not
significant. Damage effects on SIMS profiles should
be considered for P implants with dose above
1e13/cm2.
(1)
( Dose2 + Dose1 )
Studies on TW sensitivities for no-channeling
implants were reported previous [9]. However, there
are no extensive studies on TW sensitivities at normal
incident channeling implants.
Figure 4 is a plot of SIMS profiles for As 270keV
implants at 0° tilt. By scaling SIMS profile at dose of
1e12/cm2 (negligible damage effects), it is observed
that damage effect on the SIMS profiles should be
considered for As dose as low as 3e12/cm2.
TW results for 0° implants as described above are
plotted against implanted doses in Figure 1. Since
lattice damage accumulates with implanted dose, the
channeled dopant profiles evolve as a function of
implant dose. Assumption had been made that TW
sensitivity should be constant if there were no
significant damage induced effects. The TW response
curve can be fitted with a simple power function,
TW = a * Dose b
From SIMS profiles as shown in Figure 2-Figure 4
and TW response with accumulated dose in Figure 1,
there is a correlation for damage induced dopant
profile evolvements and implant sensitivity for 0°
implants. Similar correlations are also observed in the
other implants shown in Figure 1 and results will be
presented elsewhere [8]. TW sensitivity for 0°
implants is expected to be a constant for low dose
implants. Accumulated dose will lead to evolving
SIMS profiles and changing TW sensitivity
(2)
Mathematically, TW sensitivity is equal to power
index, b, for such power function.
659
established by low dose implants. For high-energy B
implants, it is not necessary to consider the damageinduced effects for implants below 1e14/cm2. Since all
implants above 5e13/cm2 were performed at higher
beam currents (~5 times higher than others), similar
SIMS profiles imply that 0° tilt B implants are not
sensitivity to beam current variations. For P and As
implants, damage induced profile evolvements should
be considered for implant doses above 1e13/cm2 and
3e12/cm2, respectively.
1E+18
<=1e14
Scaled to 1e13
1e12
Concentration (atoms/cc)
1E+17
1.0E+04
1E+16
1E+15
1E+14
1E+13
TWU
0
1.0E+03
As
0.2
0.4
0.6
0.8
1
Depth (um)
1.2
1.4
1.6
Figure 2. Damage effects on evolving SIMS profiles for B,
270keV, 0° tilt with implant doses from 1e12 to 1e14/cm2.
All SIMS profiles are scaled relative to 1e13/cm2 for
comparison.
120keV
270keV
270keV
P 540keV
B
1.0E+02
1.0E+11
1E+18
70keV
270keV
540keV
1e13
1E+17
1.0E+12
1.0E+13
1.0E+14
<5e12
Concentration (atoms/cc)
Dose
Figure 1. TW plot for 0° tilt implants as a function of
accumulated implant doses (from 5e11 to 1e14/cm2). Implant
conditions include B at 70, 270, 540keV, P at 270, 540keV,
and As at 120, 270keV.
5e13
1E+16
1e14
1E+15
DISCUSSIONS
To achieve uniformly channeling dopant profiles
across the wafer, the beam incident angles should be
tightly controlled. Small angle deviation from normal
<100> direction will lead to differential channeled
profiles across the wafer, and such differences cannot
be smoothed out by thermal dopant activation process.
TW measurement can be used as a means to align
beam direction with wafer incident angle [7].
1E+14
0
0.2
0.4
0.6
0.8 1 1.2
Depth (um)
1.4
1.6
1.8
2
Figure 3. Damage effects on evolving SIMS profiles for P,
270keV, 0° tilt with accumulated implant doses from 1e12 to
1e14/cm2. All SIMS profiles are scaled relative to 1e13/cm2
for comparison.
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REFERENCES
1E+19
1e14
1. U. Jeong, J.-Y. Jin, and Sandeep Mehta, “Devices dictate
control of implant-beam incident angle,” Solid State
Technology, Oct. 2001.
5e13
1E+18
Concentration (atoms/cc)
1e13
5e12
1E+17
2. T. Yamashita, M. Kitazawa, Y Kawasaki, H. Takashino,
T. Kuroi, Y. Inoue, and M. Inuishi, “Advanced
retrograde well technology for 90-nm-node embedded
static random access memory using high-energy parallel
beam”, Jpn. J. Appl. Phys., Vol. 41, Part 1, No. 4B,
2399, (2002).
1e12
Scaled to
3e12
1E+16
1e12
3. U. Jeong, S. Mehta, C. Campbell, R. Lindberg, Z. Zhao,
B. Cusson, and J. Buller, “Effects of beam incident
angle control on NMOS source/drain extension
applications,” 14th International Conference on Ion
Implanatation Technology, Tao, New Mexico, USA,
September 22-27, 2002. (to be published)
1E+15
1E+14
1E+13
0
0.5
1
1.5
2
4. TSUPREM Process simulator, Avant! Corporation.
2.5
Depth (um)
5. M. Posselt, B. Schmidt, C.S. Murthy, T. Feudel, K.
Suzuki, “Modeling of damage accumulation during ion
implantation into single-crystalline silicon,” J.
Electrochem. Soc., Vol. 144, No. 4, 1496, (1997).
Figure 4. Damage effects on evolving SIMS profiles for As,
270keV, 0° tilt with accumulated implant doses from 1e12 to
1e14/cm2. SIMS profile with implanted dose of 1e12/cm2
was scaled to 3e12/cm2 to match SIMS channeling tails.
6. M. Posselt, B. Schmidt, T. Feudel , and N. Strecker,
“Atomistic simulation of ion implantation and its
application in Si technology,” Materials Science and
Engineering B71, 128, (2000).
Damage effects on the evolving channeled dopant
profiles with accumulated dose are correlated with TW
results. For B implants at 0° tilt, damage effects on
SIMS profiles can be negligible or SIMS profiles can
be scaled for implants with dose below 1e14/cm2. For
P and As implants at 0° tilt, damage effects on SIMS
profiles have to be considered for implanted dose
above 1e13/cm2 and 3e12/cm2, respectively.
7. B.N. Guo, N. Variam, U. Jeong, S. Mehta, M. Posselt,
and A. Lebedev, “Experimental and simulation studies of
the channeling phenomena for high energy
implantation,” 14th International Conference on Ion
Implanatation Technology, Tao, New Mexico, USA,
September 22-27, 2002. (to be published)
8. B.N. Guo, N. Variam, U. Jeong, S. Mehta, M. Posselt,
and A. Lebedev, “Studies of channeled dopant profile
evolvements with damage accumulation,” to be
submitted to Applied Physics Letters.
Crystal-TRIM simulation results will be reported
with suitable empirical parameters and compared
against SIMS measurements [8]. Such simulations can
be used to generate more accurate look-up database for
channeled dopant and damage profiles to facilitate
process development after calibrated with SIMS
dopant profiles.
9. S. Falk, R. Callahan, P. Lindquist, “Accurate dose
matching measurements between different ion
implanters,” Proceedings of 11th International
Conference on Ion Implanatation Technology, Austin,
Texas, USA, June 16-21, 1996. IEEE Cat. No.
96TH8182, 268, (1996).
ACKNOWLEDGMENTS
Authors would like to thank our demonstration and
strategic applications team and Application Lab of
VSEA for their technical suggestion and supports.
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