In Situ Charging Potential Monitoring for a High Current Ribbon Beam
Svetlana Radovanov, Reuel Liebert, Phil Corey, James Cummings, Gordon Angel and James Buff
Varian Semiconductor Equipment
35 Dory Road
Gloucester, MA 01930-2297 USA
Abstract - We discuss measurements and modeling associated
with the high current ion beam space charge neutralization. We
show the importance of the electron temperature in maintaining
low beam potential and induced voltage on implanted wafers.
configurations (Configuration A and Configuration B) to
optimize the electron energy and net electron emission
current. A twelve channel multi-pixel scanning Faraday
system was used to measure the width (X), height (Y), and
current density (Z) of the beam. This system incorporates a
high speed X-scan actuator for the multi-pixel Faraday that
traverses the ion beam at constant velocity. In a production
machine the collected data are processed by the control
system for automated ion beam profile optimization. For this
paper, the scanning Faraday system was used to obtain both
ion beam and plasma flood gun net flux distributions.
For measuring the plasma flood gun ion energy
distribution (IED) an electrostatic retarding field energy
analyzer (REA) was mounted in the process chamber along
the diffusion axis of the PFG plasma emerging from an
aperture. The analyzer consisted of three parallel grids and a
collector. The first grid facing the plasma was grounded, the
second was used to repel electrons and the third was swept
from zero or some slightly negative value to 20 V. The
collector was maintained at constant potential of –30 V. The
ions were decelerated by the positive potential; the collector
current was measured as a function of the retarding voltage.
I. INTRODUCTION
Neutralization of the ion beam space charge in a high
current ion implanter is necessary both to optimize beam
transport and to control the positive charge build up on the
surface of the implanted wafer. Electrons formed by ionizing
collisions between the beam and the background gas are
insufficient to achieve this. In order to fully compensate,
further electrons must be introduced into the beam. This is
most efficiently accomplished by a plasma flood gun in
which low energy electrons produced in a controlled
discharge are transported to the positively charged ion beam
via a plasma plume bridge [1-4]. The low energy electrons
formed in the discharge are captured by the potential well of
the beam and their accumulation reduces its magnitude until a
stationary degree of space charge compensation is reached.
The net flux and energy of electrons emerging from the
plasma flood gun aperture, and the ion beam potential and
charge distribution are of fundamental importance in
determining the efficiency of the charge neutralization.
This paper presents the results of measurements of ion beam
parameters, surface charging potentials, net beam flux
distributions, electron temperatures, and electron and ion
energy distributions, introduced into the beam from a plasma
flood gun (PFG) developed here at Varian. Particle–in-cell
simulations with the addition of Monte Carlo collisions (2D
XOOPIC-MCC) were used to model charging effects during
ion implantation [5,6]. A 2D model of plasma interaction
with a high current ion beam was developed. The simulated
results include beam potential distributions, accumulated
charge densities and induced electric fields at the wafer plane.
Beam potential distributions were compared with
experimental surface charging potential measurements.
II. EXPERIMENTAL
The design and configuration of the VIISta 80 Ion
Implanter has been described extensively in previous
publications [7,8]. The physical arrangement of the flood
gun system is shown schematically in Fig.1. A plasma flood
gun uses a magnetic field to confine the discharge and in
these experiments we investigated the efficiency of two field
Fig. 1 PFG Physical arrangement on VIISta 80
1
An axially moveable cylindrical Langmuir probe was
used to estimate the temperature and energy distribution
function (EEDF) of the plasma flood gun electrons The
electron number density could then be estimated using the
thick sheath approximation. The second derivative of the I-V
characteristic was related to the EEDF as defined by the
Druyvesteyn distribution [9].
All measurements reported here used a 60 keV, 10 mA,
As+ beam and a suppression voltage of -1 kV to keep
R from escaping the wafer region.
electrons
Lang
II. MODELING
Fig. 2b. Phase space diagram for electrons as obtained by the
2D particle in cell code XOOPIC
The two-dimensional particle-in-cell (PIC) code
XOOPIC-MCC was used to solve for the ions and electrons
and fields in a self-consistent manner. A Monte Carlo
approach was used to model electron-neutral collisions. A 60
keV, 10 mA arsenic beam was assumed to be directed to a
insulating wafer with floating potential.
The process
chamber wall and the plasma flood gun were grounded. The
suppression electrode was set to –1 kV. Plasma densities and
temperatures were initialized in accordance with
experimental data. An ambient gas of 10-5 Torr was
assumed. Electrons and Xenon ions were injected at the
PFG location.
III.
In order to evaluate the PFG net current density
distribution, the net flux of the plasma flood gun with and
without a 60 keV, 10 mA, As+ beam was measured using the
multi-pixel Scanning Faraday, as shown on Fig. 3a and 3b.
As can be seen, much higher current densities were measured
with the beam then with the PFG only. The apparent net flux
non-uniformity exists because there is a gradient of current
density inside the beam. Separate regions of higher current
density are visible on the right hand side of the beam.
Figure 4a shows the spatial variation of the wafer surface
potential measurement taken during beam scanning (PFG
configuration A), using the test monitor wafer [10]. A higher
potential is seen at the left side of the wafer (corresponding to
the right side of the beam profile shown in Fig. 3a) agreeing
well with the net flux distribution (See Fig. 3). As the current
density increases a potential barrier (sheath) forms. This
barrier inhibits the flow of electrons to the wafer, thus
building the wafer surface charge. A higher flux of low
energy electrons would significantly reduce this effect.
Extremely low surface charging potentials were measured
(See Fig. 4b) using the same beam with the PFG in
configuration (B) which produced significantly higher flux
of low energy electrons.
RESULTS AND DISCUSSION
The calculated potential distribution after the code had
run long enough for initial transients to decay is shown in
Fig. 2a. The spatial distribution of electrons is shown in Fig.
2b. There is a region of no electrons near the suppression
electrode. There is some variation about the mean value due
to the statistical nature of the PIC approach.
50
40
30
20
10
0
-10
-20
-30
-40
-50
-200
-150
-100
-50
0
50
100
150
200
Beam Width - mm
a
.
50
40
30
20
10
0
-10
-20
-30
-40
-50
-200
500
-150
450
400
-100
350
300
-50
250
0
200
150
50
100
100
50
-1
150
200
Beam Width - mm
b
.
0.0
-15.0
-30.0
-45.0
-60.0
-75.0
-90.0
Fig. 3. Net flux distribution (µA/cm2) -As+, 60 keV, 10 mA
(a) with the beam (b) PFG Only
Fig. 2a. Calculated beam potential distribution-As+, 60 keV,
10 mA obtained by the 2D particle in cell code XOOPIC
2
0.08
20
4.00E-05
dI/dV (Volts/cm2/Ampers)
0.07
Wafer Floating Potential (V)
15
Left Side of the wafer
10
Right Side of the wafer
5
0
3.50E-05
0.06
0.05
3.00E-05
0.04
2.50E-05
0.03
0.02
2.00E-05
0.01
-5
5.00
0.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50
9.00
9.50
1.50E-05
-10
10.00
-8
-6
-4
Scan Position (cm)
Scan is in the wafer center
15
dI/dV(Volts/cm 2/Ampers)
Wafer Surface Potential (V)
2
4
6
8
10
Fig. 5a. Measured ion energy distribution (IED) for the PFG
configuration A.
20
10
Left Side of the Wafer
0
Ion Energy (eV)
Fig.4a. Wafer surface potential- As+, 60keV, 10 mA single
scan (PFG configuration A).
Right Side of the Wafer
5
-2
0.08
5.00E-05
0.07
4.50E-05
0.06
4.00E-05
0.05
3.50E-05
0.04
3.00E-05
0.03
2.50E-05
0.02
2.00E-05
0.01
1.50E-05
0
0.00
-5
0
0.02
0.04
0.06
0.08
0.1
Time (s)
0.12
0.14
0.16
1.00E-05
-10
0.18
-8
-6
-4
-2
0
2
4
6
8
10
Ion Energy (eV)
Fig.4b. Wafer surface potential- As+, 60keV, 10 mA
stationary scan (PFG configuration B).
Fig 5b. Measured ion energy distribution (IED) for the PFG
configuration B.
Retarding potential measurements were used to
determine the PFG ion energy distribution. Fig. 5 shows that
plasma flood gun IED has a Maxwellian-like distribution and
a very low average energy. Examples of the IEDs for PFG
configurations (A) and (B) are given in Fig.5a and 5b. Both
distributions have a similar shape and confirm the existence
of low energy ions along the diffusion axis of the PFG
plume.
The EEDF of the plasma flood gun electrons, as
measured by the cylindrical Langmuir probe in PFG
configurations (A) and (B), are shown in Fig. 6.
PFG configuration (A) produced an EEDF with an
extended tail and very broad bulk electron distribution. The
energy loss caused by wall recombination is very low in
configuration (A). In this configuration, it is possible for
primary electrons from the plasma flood to reach the wafer
surface with the possibility of causing negative charging
damage.
1
F(e) (a.u.)
10-1
10-2
-3
10
0
5
10
Electron Energy (eV)
15
20
Fig. 6a. Measured electron energy distribution (EEDF) for the
PFG in a configuration A.
3
in VIISta 80. The simulations are in good agreement with
experiment. .
New real time charge monitoring devices are necessary to
optimize the charge control systems for the high current
ribbon beam technology. We have recently developed a test
wafer prepared with 2D array of probes. Each probe can
monitor floating potential, electron energy and current
density associated with spatially large ion beams and
plasmas. This test wafer requires no hard wiring and can be
loaded using the standard wafer handling subsystem of the
VIISta 80. Local charge non-uniformity can be evaluated in
situ using the test wafer, without impacting the throughput of
the single wafer high current implanter.
1
F(e) (a.u.)
10-1
10-2
10-3
0
5
10
Electron Energy (eV)
15
20
ACKNOWLEDGMENT
Fig. 6b. Measured electron energy distribution (EEDF) for
the PFG in a configuration B.
We would like to thank G. Unger for constructing the
equipment and for his help in acquiring the data.
REFERENCES
1.4
Electron Temperature (eV)
1.2
[1] M.F. Mack, M. Pharand, M.S. Ameen, M. Graf, W.
Sawyer, P. Lustiber, D. Fish, B.G. Moser, M. Kabasawa,
K. Okada, H. Kawaguchi, P. Mason, E. Persson and R.
Santiesteban, “Optimized Charge Control for High
Current ion Implantation”, IIT’98, June 1998.
[2] M.I. Current, M.C. Vella and W. Lukaszek, “ Beamplasma Concepts for Wafer Charging Control During Ion
Implantation, IIT’96, June 1996.
[3] H. Ito, H. Asechi, Y. Matsunaga, M. Niwayama, K.
Yoneda, M. Vella, M. Reilly and W. Hacker,” High
Density Plasma Flood System for Wafer Charge
Neutralisation”, IIT’98, June 1998.
[4] I.A. Soloshenko, “ Space Charge Compensation of
Technological Ion Beams”, IEEE Transactions on
Plasma Science, Vol. 27, No.4, pp. 1097 (1999).
[5] J.P.Verboncoeur, A.B. Langdon and N.T. Gladd,
Comp.Phys.Comm 87, May 1995.
[6] C.K. Birdsall, “Particle-in-Cell Charged-Particle
Simulations, Plus Monte Carlo Collisions With Neutral
Atoms, PIC-MCC”, IEEE Transactions on Plasma
Science, Vol. 19, N0.2,, pp. 65 (1991).
[7] G.Angel, E.Bell, D.Brown, J.Buff, J.Cummings,
W.Edwards, C.McKenna, S.Radovanov and N.White, “A
Novel beam Line for Sub-keV Implants with reduced
Energy Contamination”, IIT’98, June 1998.
[8] G.Angel, E.Bell, D.Brown, J.Buff, M. Collins,
J.Cummings, W.Edwards, S.Radovanov and N.White “
Enhanced Low Energy Drift-mode Beam Currents in a
High Current Ion Implanter, IIT’98, June 1998.
[9] F.F. Chen, Plasma diagnostic Techniques (Academic,
New York, 1965, pp105.
[10] S.B. Radovanov, P. Corey, G. Angel and D. Brown,
“Wafer Floating Potential for a High Current serial ion
Implantation System”, IIT’98, June, 1998.
1
U-shaped filament
0.8
Xe @0.6 sccm
0.6
0.4
0.2
Hair-pin filament
0
0
1
2
3
4
5
6
7
Arc Current (A)
Fig 7. Electron temperatures measured using Langmuir Probe
for different arc currents (PFG configuration B)
In configuration B however, the EEDF is
Maxwellian and is peaked below 1eV, as shown in Fig 6b.
This low temperature is maintained for the full range of arc
currents, as shown in Fig 7. Fig. 4b also shows that this field
configuration produces sufficient electron flux to maintain
low surface potentials on implanted wafers. Clearly this is
the configuration that would be chosen to eliminate the
danger of device damage from either positive or negative
wafer charging.
IV. CONCLUDING REMARKS
In critical applications, such as charge control with very
thin gate oxide and inter-poly thicknesses, the plasma flood
gun system must meet new requirements to achieve uniform
charge distribution and avoid local charging at the wafer
surface.
2D XOOPIC-MCC simulation has provided a powerful
tool in the optimization of the plasma flood gun systems used
4