Vertical Beam Angle Control
– An advancement/requirement in modern Ion Implant
Manufacturing
Christian Krueger1, Robert Rathmell2, Dennis Kamenitsa2, and
Bernhard Krimbacher3
1) Fab36, AMD, Dresden, Germany,
2) Axcelis Technologies Ltd., Beverly, Massachusetts,
3) Axcelis Technologies Ltd., Dresden Germany
Abstract. As the industry moves to the new technology nodes of 45 nm and 32 nm devices, implant angle control becomes
even more crucial for consistent device performance. Commercial single wafer ion implanters are able to measure and correct
the horizontal incident angle of the ion beam. But the vertical beam angle (VBA) control has become a very important parameter
as well. In this work the authors demonstrate the impact of a tilt variation for a 65 nm and a 45 nm MOS transistor generated by
different beam setups on one machine. Comparisons are made for each technology with a controlled angle variation of ±4°. The
Vt-distribution should be reduced with better incident angle control allowing faster development of a new transistor node base
line using the new VBA control technique from Axcelis.
Keywords: Angle control, process control, threshold voltage
PACS: 85.40.Ry, 81.05.Ea, 41.85.Qg
INTRODUCTION AND THEORY OF
OPERATION
Manufacturing integrated devices at 45 nm and 32
nm technologies and beyond becomes more
complicated due to the tighter restrictions on transistor
parameter variation. To fabricate a symmetric or an
asymmetric transistor requires exacting process
conditions and results for every manufactured wafer.
Tight control of threshold voltage (Vt) or MillerCapacitance (CM) is needed to improve the
performance of certain devices and achieve the
transistor node requirements with stable results. The
new medium current implanter
Optima MDTM
®
manufactured by Axcelis is using a new technique
for the vertical beam angle (VBA) measurement in
order to compensate for a vertical beam angle
variation from the expected or desired beam angle.
The actual vertical tilt angle between the plane of the
scanned beam and the plane of the wafer is measured
via a profile mask attached to the linear portion of the
mechanical scan arm supporting the wafer. Any offset
in the vertical beam angle is checked against the recipe
parameters and will be corrected automatically.
VBA HARDWARE
The vertical beam angle monitor, as shown in
Figure 1, is rigidly mounted on the arm that supports
the electrostatic chuck (1) such that there is a fixed
offset between the angle of the surface of the VBA
monitor and the surface of the wafer of about 30. The
VBA monitor is a faraday (2) with a thick slotted mask
that limits the current transmitted to the collector as
the slots in the mask are tilted relative to the beam
angle [1]. This will produce a current as a function of
the tilt angle of the VBA monitor that peaks when the
slots are aligned with the beam and that allows
accurate determination of the “center of mass” or flux
weighted vertical angle of the beam. The fixed offset
between the VBA monitor and the wafer surface is
calibrated with a sensitive channeling implant and
allows one to set the chuck tilt relative to the actual
beam angle to the value specified in the recipe.
The vertical beam angle is measured at the start of a
processing job by tilting the chuck so that the VBA
monitor is in line with the ion beam, as shown in
Figure 1a. Data is collected at +/-5 about this point as
the chuck moves down and up through this range of
angles in about 4 seconds. This provides two sets of
data that can be checked for consistency and beam
dropouts. At these tilt angles, the wafer is safely
above the ion beam. After the VBA measurement, the
chuck moves to the implant position, as shown in
Figure 1b, and passes through the scanned beam at the
desired tilt angle relative to the measured beam angle,
while the VBA monitor remains fixed to the tilt
mechanism and is safely out of the beam. The chuck
and wafer are tilted about an axis parallel to the plane
of the scanned beam in order to achieve the desired
tilt.
VBA variation per recipe
1
1
2
2
3
4
5
6
7
8
9
10
11
12
Recipe
FIGURE 2. VBA variation on one implanter for different
recipes over a period of one month (Graph by AMD)
reduce the transistor parameter variation for a stable
transistor node development.
The horizontal beam angles are independently
measured and automatically corrected at seven points
across the wafer [1] to a recipe limit, which in this
study was set to <0.5° (0.3° was typical).
Figure 2 shows a full range-median-average plot of
VBA measurements for 12 different recipes on one of
the Optima MDTM implant tools when the VBA was
measured, but not corrected. The actual VBA varies
somewhat from setup to setup and the amount of
variation depends on the recipe conditions. Naturally,
low energy recipes trend to show higher variations
compared to recipes with higher energies due to a
somewhat larger angle spread and magnification of
small angle errors in deceleration and focusing
elements.
The maximum variation for a 10 keV Boron (11)
recipe is ~ 0.5° compared to a very low variation of
less than 0.1 from setup to setup for Arsenic (2 & 3)
or Phosphorus (1 & 8) recipes with energies between
50 keV and 250 keV. However, the data also show
that there can be over 1 variation from one recipe to
another if no correction for VBA is used. In addition,
the setup for a given recipe may be different on every
tool as well. Therefore, such factors could lead to
significant angle variation and make it very difficult to
Active correction of the vertical tilt of the wafer by
adjusting the wafer orientation about an axis parallel
the scanned beam plane in order to compensate for the
measured VBA offset could be one of the key factors
to get the transistor parameter distribution better under
control. The new capabilities with the Optima MDTM
are able to do this very well. Figure 3 shows a graph
for a number of different beam setups. The data show
the measured Vertical Beam Angle and the
corresponding mechanical wafer tilt. The sum of these
is the actual angle between the beam and the surface of
VBA active correction for P+ at 30 keV
7.5
0
7.4
-0.1
7.3
-0.2
7.2
-0.3
7.1
-0.4
7
6.9
-0.5
1
4
7 10 13 16 19 22 25 28 31 34 37 40 43
Batch
Wafer Tilt
Implant tilt
VBA
FIGURE 3: VBA active correction for a 30 keV Phosphorus
recipe with 7° Tilt requirement (Graph by Axcelis)
VBA, deg
STARTING CONDITIONS
ACTIVE INCIDENT ANGLE
CORRECTION
Tilt, deg
FIGURE 1.
a) Scan arm at tilt angle to measure
vertical beam angle of the accelerated ion beam. (1)
electrostatic chuck with wafer (2) VBA-Faraday-Cup
b) Scan arm at an implant position with VBA out of the
beam (Figure by Axcelis)
the wafer and is shown to be very constant for all
processing jobs.
A separate paper at this conference uses a sensitive
channeling condition to show that the angle
measurement of the Optima MDTM is repeatable
relative to the planes of the crystal from setup to setup.
[2].
It was expected that the angle compensation would
reduce the transistor parameter distribution for a
certain device in a long term trend.
65nm
45nm
FIGURE 4. Vt-sensitivity for a NMOS transistor at different
technology levels (Graph by AMD)
DEVICE DATA
In order to determine the sensitivity of device
parameters, Vt and CM, to wafer tilt, a wafer split
experiment with a relatively high tilt angle variation
was carried out using a Halo-and a Source Drain
Extension, SDE-implant. The tilt of the wafer (the
orientation of the wafer about an axis parallel to the
plane of the scanned beam) was purposely varied ± 4
for the Halo implant, compared to the Process of
Record, POR. A separate implant using a +5 offset in
the SDE tilt was done with the Halo implant at the
normal tilt angle. The resultant device data show a
clear Vt-dependency on the incidence angle for the
Halo implants. Surprisingly, variations of the tilt angle
of the Halo implant influence the results much more
than the Extension implant tilt variation. Figure 4
shows the influence the tilt angle changes for 65 nm
and 45 nm technologies. The Vt-sensitivity of that
particular NMOS transistor is different for each
technology level, yielding 14 mV/deg and 21 mV/deg,
respectively, with the higher sensitivity for the smaller
node, as expected. Theoretically this effect would
influence the transistor node development significantly
if there are variations larger than ~0.5 at the 45 nm
node and would make a clear interpretation of
“controlled” experiments very difficult.
In reality there is a relatively small reduction in
the Vt parameter distribution on long term trends
comparing data before active correction of VBA to
those after active correction, as shown in Figure 5 for
4 months of data. The red bar shows when VBA autocorrection was turned on. That means that this
parameter distribution is more influenced by other
parameters like perhaps a variation in gate length, CM,
or some other implant condition than by the implant
incident angle itself. It is also possible that the implant
angle was not ranging very far even before turning on
active correction as was seen for some recipes in
Figure 2.
FIGURE 5: Long term Vt behavior prior to and after the
use of the VBA unit to correct angles on all Optima MD
(Graph by AMD)
While the effect on the long term distribution is
relatively small, Figure 4 shows that there is a high
sensitivity to beam angle for halo implants, and it is
more and more important to minimize all relevant
sources of variation for an effective device
development.
observed variation in device electrical parameters of
real product so much suggests that either there was not
much angle variation in the critical implants prior to
implementing active control or parameters other than
beam implant angle are causing lot-to-lot variations
that may be seen.
CONCLUSION
REFERENCES
The Optima MD Vertical Beam Angle measurement
allows precise control of the orientation of the wafer
about an axis parallel to the plane of the scanned beam
during implant. Some devices studied show a high
sensitivity such that a variation of ~0.5 could result in
significant changes in Vt. As expected, the sensitivity
is increasing as the node size decreases. The fact that
controlling the VBA very tightly did not change the
1.
R.D. Rathmell, et al, “Implant Angle Control on Optima
MD”, AIP Proc. of 16th Intl. Conf. on Ion Implantation
Tech., Marseille, France 349-352 (2006)
2.
R. D. Rathmell and Jonathan David, “Implant
Angle Repeatability on Optima MD”, these
proceedings