Implant Angle Repeatability on Optima MD
Robert Rathmell, Jonathan David, and Mark Harris
Axcelis Technologies, Inc., 108 Cherry Hill Dr, Beverly, MA 01915 USA
Abstract. A very sensitive channeling implant has been used to quantify the repeatability of implant angles for the Optima MD
over a period exceeding one month. Sensitivity of sheet resistance to variation in vertical and horizontal beam angles was
measured for 500 keV P++ at a dose of 5E13 /cm2 near orientations tilt/twist of 0/0 and 35.26/0 for (100) wafers. Sensitivity
to tilt in the vertical plane at both conditions was at least a 10% change in Rs for a 0.5 change in tilt at a tilt of 0.75 away from
the channel (i.e. at 0.75/0 and 34.5/0), where the variation in Rs was linear with tilt. Horizontal angle sensitivity resulted in
a 7.5% change in Rs for a 0.5 change in angle, but was less sensitive to smaller angle variation, since Rs varied roughly as a
second order function near the channel. Sensitivity to tilt is reduced by about a factor of 3 for implants at 0/0 compared to
0.75/0 with a similar response at 35.26/0 compared to 34.5/0. Implants were carried out using wafers from a single boule
over a period exceeding one month at the orientation of 0.75/0. Angle control was enabled by in-situ measurement and
correction of the beam angles in both the vertical and horizontal planes prior to implant. The repeatability of sheet resistance
was used to infer the variation in angle. Attributing all Rs variation to be the result of beam angle variation showed that the
standard deviation of beam angles for over 30 wafers is 0.03.
Keywords: Implant angle control
PACS: 85.40.Ry, 81.05.Ea, 41.85.Qg
HARDWARE DESCRIPTION
Optima MD angle control hardware was described
previously [1 - 3]. A brief summary of the techniques
will be given here. The Optima MD is a medium dose,
serial implant tool that uses a hybrid scanning system.
The beam is scanned electrostatically in the horizontal
plane and angles are “corrected” by a curved
acceleration gap to form rays that are parallel to the
central, unscanned ions.
The wafer is scanned
mechanically “vertically” in the tilted plane of the
implant to pass through the ribbon-like beam to
uniformly dose the entire wafer. Beam angles are
measured in the horizontal and vertical planes prior to
implant and automatically corrected to ensure the
implant is performed at the angles specified in the
recipe.
Horizontal Angle Measurement
Horizontal beam angles are measured by a moving
profiler behind a mask with 7 slots. The angle
corrections ensure that the 7 beamlets spanning the
width of the wafer are parallel to each other and that
the largest angle satisfies the recipe limit of Maximum
Horizontal Beam Angle.
Corrections are
automatically made by steering the beam prior to the
scan plates to correct the average angle or adjusting
the parallelizing lens voltage, if necessary, to correct
parallelism.
.
Vertical Angle Measurement
The beam is deflected vertically by nominally 15
by an electrostatic energy filter just before entering the
end station. Measurement in the vertical plane of the
angle of the scanned beam verifies that the actual
Vertical Beam Angle, VBA, is known prior to implant.
The measurement is made by the VBA monitor, which
is a faraday with a thick mask with narrow slots that
admits more current when it is aligned with the
direction of the ions. The VBA monitor is mounted on
the scan arm of the wafer so that it has a fixed offset
from the wafer surface as shown in Figure 1. At the
start of a process job, the scan arm tilts so that the
VBA monitor collects current as it rotates from 5
above to 5 below the nominal angle and then again on
the way back to generate two plots of current vs. angle.
The two plots are compared to detect noise or beam
drop-outs. The flux weighted average angle of the two
data sets is the Vertical Beam Angle. The wafer angle
is adjusted about an axis parallel to the scanned beam,
if necessary.
Both measurement schemes are calibrated to
crystal planes during machine setup to ensure that a 0
implant is normal to the surface of the wafer.
+5
320
-5
Rs, ohms/sq
Wafer
300
290
280
30
Tilt Angle, deg
FIGURE 2. Average sheet resistance for a series of wafers
implanted with 500 keV P++ at a dose of 5E13/cm2 near the
0/0, <100> axis at a range of vertical tilt angles.
360
The primary goal of this experiment was to
evaluate the ability of the angle control scheme of
Optima MD to accurately and repeatably measure and
correct implant angles from batch to batch over a
significant period of time. Prior experience has shown
that high energy phosphorus implants at a dose of
5E13 atoms/cm2 are very sensitive to channeling
effects [3]. Two different crystal orientations were
considered for the test. For (100) wafers, an implant at
a tilt/twist of 0/0 is aligned with the <100> axis,
while an implant at 35.26/0 is aligned with the
<112> axial channel.
Vertical Angle Sensitivity
Implants were made at a range of tilt angles near
these two axes to determine the sensitivity. The
wafers were measured with a Therma-Wave ThermaProbe and a sheet resistance, Rs, probe. Results for Rs
are shown in Figures 2 and 3. Therma Wave results
(not shown) showed comparable sensitivity.
One can see that the sensitivity to small angle
changes is greater at a tilt of about 0.75 away from
the channeling axis and has a roughly linear slope.
Sensitivity was comparable at both conditions and the
0.75/0 orientation was selected for the repeatability
test.
Rs, ohms/sq
EXPERIMENT
Test Conditions
1.5% per
1/4 deg
270
-0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00
VBA
FIGURE 1. Scan arm is shown in the position to measure
vertical beam angle. The VBA monitor is mounted to the
scan arm with a fixed angle to the wafer.
5% per
1/4 deg
310
340
320
6% per
1/4 deg
2% per
1/4 deg
300
280
260
33.50 34.00 34.50 35.00 35.50 36.00 36.50
Tilt Angle, deg
FIGURE 3. Average sheet resistance for a series of wafers
implanted with 500 keV P++ at a dose of 5E13/cm2 near the
35.26/0, <112> axis at a range of vertical tilt angles.
Horizontal Angle Sensitivity
In the horizontal plane on the Optima MD, one does
not have to implant a series of wafers to produce the
“V-Curve” of Rs vs. angle. One can implant a single
wafer with the Parallelizing Lens off so that the
scanned beam spans the wafer with angles that vary
linearly by +/-1.2 degrees at this energy, as shown in
Figure 4. The sensitivity of sheet resistance to angle is
about the same for the <100> and <112> axes. The Rs
increases about 7% at 0.5 from the channel, which, at
this angle, is about 70% as sensitive as the vertical
axis. Horizontal sensitivity was about the same at a
tilt/twist of 0.75/0 (not shown), because the
channeling feature is largely planar.
Horizontal
sensitivity is less for small angle errors near 0.
1.5
1
+/- 0.5
16
12
0.5
0
8
-0.5
4
0
-150 -100
-50
0
50
100
-1
-1.5
150
Angle Repeatability Implants
Beam Angle, deg.
Change in Rs, %
24
20
Position, mm
Rs 0/0
Rs 35.26/0
Angle
FIGURE 4. Variation of sheet resistance horizontally
across the wafer implanted with 500 keV P+ at a dose of
5E13/cm2 with the P-lens off so that angles vary linearly
from one side to the other.
Test Angle Correction Accuracy
Rs Ohm/Sq
320
+/- 0.55
uncorrected
310
Results of the Angle Repeatability Implants
Sheet resistance repeatability of the implants is
shown in Figure 6. Dashed lines above and below the
data show the equivalent change in Rs corresponding
to +/- 0.1 in angle variations. If one assumes that all
of the variation is due to angle variation, the results
show that the maximum angle variation was less than
+/-0.1. It is possible that some of the variation is
caused by finite dose repeatability and metrology.
310
Rs, ohms/sq
In order to confirm that angle corrections worked,
the beam was tuned to cause a significant angle error,
and wafers were implanted with and without automatic
corrections enabled. Since the beam is deflected in the
vertical plane by nominally 15 at the Angular Energy
Filter of the Optima MD, the AEF can be adjusted to
change the beam angle in the vertical plane. Wafers
were implanted with the AEF adjusted to cause the
beam to enter the end station at angles approximately
+/-0.55 from the nominal 15 bend. In each case, one
wafer was implanted with VBA corrections off, and
one with corrections enabled. Figure 5 shows that the
wafers implanted with corrections enabled fall within
<0.1 to the expected Rs value for 0.75/0 orientation.
Cassettes of wafers from the same boule used for
the implants in Figures 2 to 5 were selected for a
repeatability test. Using wafers from a single boule
ensured that orientation of the crystal matched the
sensitive region near a tilt of 0.75 shown in Figure 2
and minimized changes in the crystal cut error through
the test. No attempt was made to compensate for
crystal cut error in the selected material, although
wafers with a reported crystal cut error of <0.1 were
chosen, which is consistent with the curve in Figure 2.
Implants were performed daily for over a month
using 500 µA of 500 keV P++, at a dose of 5E13
atoms/cm2 and an orientation of 0.75/0 tilt/twist.
Recipe limits for horizontal beam angles were set to
0.1 to ensure that horizontal steering or P-lens voltage
would be automatically adjusted if any of the seven
measured peaks were >0.1. Vertical beam angle
auto-correction was enabled so that the wafer angle
was adjusted about an axis parallel to the scanned
beam to correct for variation in the VBA.
300
305
300
295
290
285
290
280
+/- 0.55
corrected
280
270
-1.00
-0.50
0.00
0.50
1.00
1.50
1
4
7 10 13 16 19 22 25 28 31 34 37
Wafer
Rs Ave
Average
+0.1 deg
-0.1 deg
Implant angle, deg
FIGURE 5. Wafers implanted at a recipe orientation of
0.75/0 with the beam steered by the AEF to cause a
vertical angle error of +/- 0.55 with automatic angle
corrections on and off are compared with the V-curve.
FIGURE 6. Sheet resistance results of daily implants of
500 keV P++ at 5E13 atoms/cm2 at an orientation of
0.75/0 tilt/twist. Upper and lower lines show the change in
Rs equivalent to +/- 0.1 degree in tilt.
Implant Data Logs for these implants report the
measured Horizontal Beam Angle, HBA, and VBA for
these implants. Table 1 shows the average value and
standard deviation of these angles taken from the
IDL’s for all implants. It also shows the average value
and standard deviation of the sheet resistance. If the
standard deviation of the sheet resistance is all
attributed to angle variations, it would convert to a
standard deviation of 0.032 for all of the implants.
TABLE 1. Data summary for repeatability implants
Angle
Average
Standard
standard
value
deviation
deviation
based on Rs
HBA
0.005°
0.019°
NA
VBA
0.76°
0.023°
NA
Rs
295.3
0.74%
0.032°
SUMMARY
Optima MD has the ability to measure and
automatically correct implant angles in the horizontal
scan plane and the vertical plane the start of a batch of
wafers. A very sensitive channeling implant condition
was selected to test repeatability of the implant angle
using daily implants for over a month. The orientation
of the implant was chosen as 0.75/0 tilt/twist because
of the sensitivity (5% change in Rs for 0.25 change in
angle) and the near linearity of the response. It was
also shown that comparable sensitivity could be
obtained by implanting at 34.5/0. The variation in
Rs was consistent with total angle variation of less
than +/-0.1 with a standard deviation of 0.03 for all
implants.
ACKNOWLEDGMENTS
The authors would like to thank Fernando Silva
and Khen Ung for running the implants.
REFERENCES
1. K. W. Wenzel, A. M. Ray, B. H. Vanderberg, and R. D.
Rathmell, “Optima MD: Mid-Dose, Hybrid-Scan Ion
Implanter”, Proc. of 16th Intl. Conf. on Ion Implantation
Tech., Marseille, France, AIP Conf. Proc. 866, 2006, pp.
637-640.
2. R. D. Rathmell, B. H. Vanderberg, A. M. Ray, D. E.
Kamenitsa, M. Harris, and K. Wu, “Implant Angle
Control on the Optima MD”, Proc. of 16th Intl. Conf. on
Ion Implantation Tech., Marseille, France, AIP Conf.
Proc. 866, 2006, pp. 349-352.
3. R. D. Rathmell, D. E. Kamenitsa, M. L. King, and A. M.
Ray, IEEE Proc. of Intl. Conf. on Ion Implantation Tech.,
Kyoto, Japan, 1998, pp. 392-395