APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 12
24 MARCH 2003
The importance of bias pulse rise time for determining shallow implanted
dose in plasma immersion ion implantation
D. T. K. Kwok,a) M. M. M. Bilek, and D. R. McKenzie
Applied and Plasma Physics Group, School of Physics A28, University of Sydney, Sydney 2006,
NSW, Australia
P. K. Chu
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue,
Hong Kong SAR, China
共Received 31 October 2002; accepted 27 January 2003兲
The composition of the low-energy ions arising from the rise and fall time periods of the voltage
pulse in plasma immersion ion implantation 共PIII兲 are simulated by particle-in-cell 共PIC兲 modeling.
It is shown that more than 70% of the low-energy ions with an energy corresponding to less than
half of the applied voltage come from the short rise time period. Although the fall time period is
typically 30 times longer than the rise time, less than 25% of the low-energy ions originate from it.
Based on the PIC results, the depth profile of the implanted ions is derived using the Monte-Carlo
code SRIM2000 关J. F. Ziegler, The Stopping and Range of Ions in Solids 共Pergamon, New York,
1985兲兴. The low-energy ions are found to be implanted to a much shallower depth than ions
introduced during the fall time period the concentration profile which decays more sharply into the
bulk. These results indicate that the most effective way to reduce or increase the surface
concentration is by adjusting the rise time of the PIII voltage pulse. This will require a power supply
capable of fast rise times and good matching between power supply and load. © 2003 American
Institute of Physics. 关DOI: 10.1063/1.1563063兴
energy ions can have pronounced effects on the junction
depth in plasma doping.16 A detailed knowledge of the ion
energy distribution is important in understanding the effects
of ion implantation. A large fraction of low-energy ions, for
example, will cause significant sputtering without deep implantation. The stress relief effects are also strongly dependent on the ion energy since the size of the thermal spike is
energy dependent.17
Here, we calculate the ion energy distributions originating from the rise and fall times of the voltage pulse by performing particle-in-cell 共PIC兲 simulations. Our results illustrate that, although the fall time is much longer than the rise
Since the advent of plasma immersion ion implantation
共PIII兲 in the mid-1980s,1,2 many exciting applications have
been investigated. For example, plasma doping by PIII has
been an alternative technique to beamline ion implantation
for the formation of shallow junctions in deep-submicrometer devices in microelectronics. Previous studies have
shown that plasma doping can produce shallow junctions
similar to those produced by conventional low-energy beamline doping, but with better efficiency.3,4 PIII has also been
combined with deposition techniques to increase the wear
resistance and modify the surface properties of
biomaterials,5– 8 protective layers on polymers in space
applications,9 and tool coatings.10,11 It has been shown that
the compressive stress in coatings produced by cathodic arc
deposition can be relieved by the use of PIII.12–15
In conventional PIII, the sample is immersed in a plasma
and negative high-voltage pulses are applied to the sample.
Electrons are repelled from the sample surface, leaving the
heavy ions to form an ion sheath. An electric field subsequently forms inside the ion sheath, accelerating the ions
towards the sample surface. To maintain the ion flux, the ion
sheath will expand outwards uncovering more ions until the
end of the voltage pulse. This sequence is repeated many
times with a chosen pulsing frequency. An intrinsic difference between beamline and PIII is that the energy spread of
the implanted ions is wider due to rise and fall times of the
voltage pulse, as depicted in Fig. 1. The ions implanted during the rise and fall time periods have an energy less than
that corresponding to the applied pulse voltage. These low-
FIG. 1. A typical voltage pulse used in PIII composed of a linear rise time,
a flat plateau time, and an exponential decay fall time. In our study, rise
time⫽1 ␮sec, plateau time⫽10 ␮s, and fall time with a half-time⫽5 ␮s
given a tail of 30 ␮s. The total simulation time⫽41 ␮s.
a兲
Electronic mail: dkwok@physics.usyd.edu.au
0003-6951/2003/82(12)/1827/3/$20.00
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© 2003 American Institute of Physics
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Kwok et al.
Appl. Phys. Lett., Vol. 82, No. 12, 24 March 2003
TABLE I. Accumulated doses and percentage contributions of different time
periods to low-energy 共below 12 keV兲 ion dose.
Percentage
Accumulated dose contribution to total
Time period Accumulated dose
below 12 keV
low-energy 共below
共␮s兲
(106 cm⫺2 )
(106 cm⫺2 )
12 keV兲 ion dose
Rise time:
0–1
Plateau time:
1–11
Fall time:
11– 41
Total time:
0– 41
FIG. 2. Ion dose versus impact energy spectra from a single pulse, and
contributions from the rise, plateau, and fall times are plotted separately.
Ions from the rise period have impact energies lower than 12 keV. During
the plateau period, ions acquire higher energy above 12 keV from the ion
sheath. In the fall period, the ion energies are distributed over the whole
energy range from 0 to 20 keV.
time, the majority of the low-energy ions come from the rise
time. During the fall time, the ion density in the ion sheath is
very low and significantly fewer ions are implanted during
this period.
One-dimensional PIC18,19 simulation was carried out to
obtain the ion doses versus impact energy during a single
pulse.16,20 To analyze the energy compositions at different
times, the ion doses are recorded in three distinct temporal
regions: the rise time, the plateau, and the fall time, as shown
in Fig. 1. We simulated a pulse with rise time of 1 ␮s, a
plateau of 10 ␮s, and a fall time consisting of an exponential
decay, with a half-width of 5 ␮s, lasting for a total of 30 ␮s
共30 times the extent of the rise time兲. These conditions are
typical of the pulses used in plasma doping. To simplify the
simulation and to illustrate the low-energy effects of the risetime and fall-time regions, we consider a plasma consisting
only of room-temperature N⫹ ions and electrons with a Boltzmann distribution corresponding to a temperature of 2.5 eV.
The plasma density is 1⫻108 cm⫺3 and the applied voltage
is ⫺20 kV. Four dose versus energy curves, corresponding to
the total dose, the dose implanted during the rise time, the
plateau, and the fall time periods, are shown in Fig. 2. The
total dose curve corresponds to the ion energy spectrum usually observed in PIII simulations.16 The ions implanted
within the rise-time period have an impact energy of less
than 12 keV. The majority of the ions implanted with high
impact energy of more than 12 keV occur in the plateau
period. The impact energies of the ions implanted in the fall
time period range from 0 to 20 keV. A summary of the dose
distribution of the low energy ions 共below 12 keV兲 is given
in Table I. It is found that each pulse contributes a total
implanted dose of 47.5350⫻106 cm⫺2 . About 27% of this
dose consists of ions with an impact energy below 12 keV,
and 72% of these low energy ions came from the rise time
period of 1 ␮s. Only 24% of the low-energy ions come from
the fall time period, even though it is 30 times longer than
the rise-time period. The remaining 4% of the low-energy
ions originate in the plateau period.
The depth profiles of the nitrogen atoms implanted into
9.2583
9.2583
72
31.9417
0.4433
4
6.3350
3.1133
24
47.5350
12.8149
¯
the silicon layer by the simulated pulse have been calculated
using SRIM2000.21 Four hundred thousand ion trajectories
were calculated in SRIM2000, with a dose versus impact energy distribution shown in Fig. 2. Here, we use 77 908 ions
for the rise time, 268 785 ions for the plateau, and 53 308
ions for the fall-time periods. The ions in each of these periods were assigned to energy subranges of 0.8 keV increments to approximate the energy profile in Fig. 2. For example, 7209 ions are assigned to the energy range between 0
to 0.8 keV in the rise time period, and 46 071 ions to the
energy range between 17.6 to 18.4 keV in the plateau period,
etc. The ion trajectories were calculated in the following order: rise time—low to high energies, plateau—low to high
energies, and fall time—high to low energies. The reason we
chose this ordering is that we assume during the fall time that
the high-energy ions will hit the surface before the lowenergy ions, while the reverse is true for the other two periods. The depth profiles of nitrogen atoms implanted during
these phases of the single pulse are depicted in Fig. 3, together with the total depth profile. The depth profile from our
pulse does not resemble a Gaussian distribution due to the
presence of low-energy ions from the rise and fall time periods. The low-energy ions implanted during the rise time
form a shallow distribution, whereas ions implanted during
the fall time extend deeper into the substrate falling off more
FIG. 3. Simulated depth profiles of the implanted nitrogen atoms. Individual
components from the rise, plateau, and fall time periods are shown. The
surface concentration is mainly contributed by the rise time. The depth
profile of the plateau period has a Gaussian shape and the depth profile of
the fall period forms a skewed background towards the surface.
Kwok et al.
Appl. Phys. Lett., Vol. 82, No. 12, 24 March 2003
gradually with depth. The depth profile created by the ions
from the pulse plateau has the Gaussian shape expected for a
more narrow energy distribution because it is produced
mainly by ions with energies in the vicinity of 20 keV.
From a postural point of view, the most important result
of our calculation is the identification of the extreme dominate of the short rising edge of the pulse 共temporal extent
only 1 ␮s兲 in accounting for the bulk of the low-energy ions
implanted into the substrate 共75%兲. The relatively long fall
time 共⬃30 ms兲 is responsible for only 25% of low-energy
implanted ions. This can be understood by the fact that the
density of ions at the edge of the expand sheath present while
the pulse is rising is high 共comparable to the bulk plasma
density兲, while the density of ions at the edge of the contracting sheath present during the fall time is very much lower.
The rate of ion implantation would be expected to be roughly
proportional to the density of ions at the sheath edge. Although intuitively sensible, this effect is not easily measured
experimentally. Techniques that probe depth distributions of
implanted dose cannot distinguish between low-energy ions
arriving during the pulse rise time and those arriving during
the fall time. We have calculated the concentration profiles of
nitrogen atoms implanted into the silicon. However, plasma
treatment consists of several reactions processed simultaneously. The surface of the silicon will absorb a certain
amount of nitrogen molecules forming silicon nitride. The
implanted nitrogen atoms will diffuse and drive into a deeper
region below the surface. If reactive gas is used, the plasma
will etch away the surface layer in a constant rate. An analysis including all the reactions will completely describe the
plasma treatment. In general, in the PIII process, the sample
temperature is controlled by a cooling sample stage and the
diffusion process is minimized. The formation rate of silicon
nitride is low under a low working pressure and low temperature, and nitrogen gas is essentially nonreactive. Thus,
we expect that the calculated implanted depth profiles will
predict reasonably the distribution of nitrogen atoms.
In this work, we investigated the low-energy ions from
the rise and fall time periods of a high-voltage pulse in
plasma immersion ion implantation by one-dimensional
particle-in-cell simulation. Although the fall time is 30 times
longer than the rise time, 72% of the low-energy ions are
implanted during the rise time and only 25% come from the
fall time. During the rise time, the ion sheath is full of positive ions and the sheath expands into the bulk plasma. On the
other hand, during the fall time, the ion sheath contracts into
a more or less empty region of ions. Therefore, it is reasonable that the ions flux is much higher in the rise time than in
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the fall time and a larger part of low-energy ions come from
the rise time. Monte-Carlo SRIM2000 was employed to calculate the depth profile resulting from a single pulse. By separating the contributions from the three regions of the pulse
共rise, plateau, and fall periods兲, the rise time period is found
to contribute most heavily to the shallow part of the profile.
Implanted ions from the plateau period exhibit the Gaussian
shape expected for monoenergetic implantation. The fall period manifests as a more gradual trajectory of implanted ion
concentration from the surface into the bulk. Hence, our results indicate that the proportion of shallow implanted dose
can be most effectively controlled by adjusting the relative
temporal extent of the rise time.
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