Implant Angle Control on Optima MD
R. D. Rathmell, B. Vanderberg, A. M. Ray, D. E. Kamenitsa, M. Harris, and K.
Wu
Axcelis Technologies Inc, 108 Cherry Hill Drive, Beverly, MA 01915
Abstract. Implant angle control is increasingly important with each new device node. Some devices have demonstrated
a sensitivity of threshold voltage of about 100 mV/deg for implant angle and require implant angles to be held within +/0.2 for process control. There are many sources of angle variation in single wafer implanters. Mechanical orientation
can usually be controlled to high precision, but an accurate control of the implant angle requires knowledge of the actual
beam angle relative to the surface or crystal planes of the wafer. In-situ methods to measure beam angles in both the
horizontal and vertical planes are required and it is necessary that these methods be calibrated to the surface or crystal
planes of the wafer to achieve the required angle control. Optima MD has incorporated methods to automatically
measure beam angles prior to implant in both planes, and correct for any deviation from the desired implant angle. The
symmetric parallelizing lens that corrects angles without bending the beam enables a method of calibrating the
horizontal angle mask to crystal planes with one or two wafers. This paper discusses the methods of measurement,
calibration, and accuracy of the Optima MD angle control system.
Keywords: ion implant, angle control
PACS: 61.72.Tt, 85.40.Ry, 06.30.Bp
INTRODUCTION
Each succeeding device node has placed more
stringent requirements on angle control, and implant
angle control has become a necessary component of
implanter process control. Early medium current
implanters used raster scanned beams to span wafers
up to 150 mm diameter with scan angles of up to +/2 across the wafer. Channeling effects on profile
depth were tolerated or managed by using nonchanneling crystal orientations.
While this was
adequate angle control for devices at that time and
these tools are still in use today, it became clear that
200 mm wafers would need better angle control.
Parallel beam implanters were introduced to avoid the
scan angle variation [1-3]. These offered implant
angle control of <1, which was satisfactory for
devices until recently. Device structures, which can
lead to asymmetry in source/drain extensions of
transistors due to shadowing of gates or narrowly
spaced photoresist patterns are placing even tighter
limits on angle control. Devices in which Vt varies by
up to 100 mV/degree have been reported requiring
angle control of 0.2 [4]. Such tight control requires
in-situ measurement of the implant angle in horizontal
and vertical planes and is smaller than the present
tolerance on crystal cut error. However, it motivates
the implanter designers to minimize their contribution
to the total variation.
SOURCES OF VARIABILITY
All ribbon beam or ribbon-like beam implanters
today use a magnetic or electrostatic lens to image ions
to infinity after they diverge in one plane from a waist
near the focal point of the lens [1-3,5]. The focal
lengths of these lenses are in the range of 600 to 1000
mm. Magnetic lenses achieve parallelism by bending
the beam in a wedge shaped dipole magnet through an
angle ranging from 40-70 for the central ray.
Electrostatic lenses allow the central ray to continue in
a straight line, while the scanned rays have the scan
angle cancelled by the vector sum of velocities in the
acceleration or deceleration gap between the shaped
electrodes which are symmetric about the central ray.
For some simple effects, the focal properties of
these lenses can be approximated by the thin lens
equation. If the object point of the lens or position of
the scan vertex moves to one side of the focal point of
the lens, the image is still focused to infinity (parallel
rays), but at an angle Ө, where Δx is the lateral
displacement and F is the focal length,
Ө = arctan(Δx/F),
(1)
Also if the implanter focusing elements make the
apparent vertex move backward or forward along the
beam by Δz, the beam will be converging or diverging
at the edge of a 150 mm radius wafer by
φ= arctan(150*Δz/(F2+F*Δz)).
(2)
For small errors of 5 mm in position and F = 800 mm,
these amount to 0.36 and 0.07 angles, respectively.
Electrostatic lenses are designed for a fixed ratio of
lens voltage to extraction voltage and for the Optima
MD lens, a 1% error in the ratio results in a 0.05 error
at the edge of the 300 mm wafer. A magnetic lens is
designed for a fixed bend radius or ratio of
(ME/Q2)½/B. For a 50 bend angle, a 1% error in the
field B due to hysteresis, for example, can result in an
error of just under 0.5 in the average angle. The field
must be set to achieve parallelism across the wafer,
however, and any residual error in the average angle
must be corrected by tilting the wafer about a vertical
axis [6]. Charge exchange of ions that have partially
completed the bend on the corrector magnet may strike
the wafer at the wrong angle, which can also result in
dose non-uniformity [7].
Deceleration/acceleration of a beam with an angle
error magnifies/demagnifies the angle by a factor M,
M = (Ein/Eout) ½,
(3)
where Ein is the energy before decel/accel and Eout is
the energy after. Other effects such as beam blowup
and portions of divergent beams interaction with
apertures also affect the final effective angle at the
wafer.
Vertical angle errors may occur due to
source/extraction alignment or by vertical deflectors or
focusing aberrations [8].
adjusted independently of the beam energy, it is just a
lens. Average angle errors are corrected by steering
the pencil beam before the scanner and P-lens
horizontally by two electric elements of the middle
quadrupole. For a given angle error, the required
change in the P-lens voltage and steerer is computed in
a model based closed-loop control algorithm including
the effects of acceleration or deceleration, and angles
are usually corrected with one iteration.
Angle measurement devices can be calibrated to the
crystal planes of the wafer to provide a proper
reference for channeled implants and to confirm that a
zero degree implant angle is actually normal to the
surface of the wafer.
The axially symmetric
parallelizing P-lens of the Optima MD enables a
unique method for horizontal angle calibration.
During normal operation the P-lens is set to a voltage
that cancels the scan angle to produce parallel rays.
The horizontal beam angles of the scanned beam are
measured using a moving profiler behind a mask with
vertical slots at seven points across the wafer [10]. If
the P-lens voltage is turned off, and with zero voltage
on the accel column, the scanned beam travels at the
extraction energy of 45 keV in straight lines from the
scanner to the wafer, as shown in Figure 1. In that
case, the implant angle ranges linearly approximately
+/- 4 across the wafer, and the beam angle is easily
predicted from the geometry. When a wafer is
implanted in this mode, at some point on the wafer the
ions will have the angle that is parallel to the crystal
planes and a measurement of Therma Wave or sheet
resistance will reach a minimum.
As shown in Figure 2 for a portion of the wafer,
beam angles measured across the wafer form a straight
line plot of angle vs. position while the TW values
form an approximately parabolic shape with a
minimum at the position of maximum channeling.
HORIZONTAL ANGLE
MEASUREMENT AND CALIBRATION
This paper will review the methods of angle
measurement and calibration of those methods for the
Optima MD. The beamline layout can be seen in a
separate paper presenting an overview of the Optima
MD [9].
If a recipe has a limit on maximum implant angle,
Axcelis beam tuning software, Autotune™,
automatically measures and corrects angles to satisfy
the recipe limit. If angles are diverging or converging
as one moves from the center toward the left/right
edges of the wafer, this is corrected by adjusting the
voltage of the Parallelizng lens, P-lens, to make the
lens stronger or weaker. The P-lens voltage can be
FIGURE 1. Illustration of ion rays from the scanner to the
wafer with the P-lens on or off.
2
488
1
482
Mask angle at
-0.8 mm = 0.05
0.5
0
476
-0.5
-1
470
Minimum TW value
at -0.8 mm
-1.5
-2
Therma Wave value
Mask angle, degrees
1.5
464
-60
-40
-20
0
20
40
60
Figure 4. An illustration of the vertical beam angle faraday
Position on wafer (mm)
Figure 2. A horizontal diameter scan of Therma Wave
values compared to horizontal mask angles for an implant of
45 keV, 100 uA, B+ at a dose of 1E12 and tilt/twist of
0/0.with the P-lens off.
The corresponding position on the angle plot is +0.05
for this wafer. If the wafer had no crystal-cut error,
one would calibrate the mask to read 0 at that
position. However, to compensate for crystal-cut error
one typically uses two wafers implanted with 180
difference in twist.
An example of a horizontal angle measurement is
shown in Figure 3.
beam and that allows accurate determination the
“center of mass vertical angle” of the beam.
The fixed offset between the VBA and the wafer
surface 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
batch by tilting the chuck so that the VBA is in line
with the ion beam, as shown in Figure 5a. The chuck
and VBA are tilted +/- 5 about this point to collect the
data. At these tilt angles, the wafer is safely above the
ion beam. Then the chuck moves to the implant
position, as shown in Figure 5b, and scans through the
Angle, deg.
1.0
0.5
0.0
-0.5
-1.0
-150
-100
-50
0
50
Position, mm
100
150
5a
Figure 3. Horizontal angles for 2 keV, 750 uA B+.
VERTICAL ANGLE MEASUREMENT
The vertical beam angle monitor, VBA, is rigidly
mounted on the arm that supports the electrostatic
chuck such that there is a fixed offset between the
angle of the surface of the VBA and the surface of the
wafer of about 30. The VBA is a faraday with a thick
slotted mask as shown in Figure 4 that limits the
current transmitted to the collector as the slots in the
mask are tilted relative to the beam angle. This will
produce a current as a function of the tilt angle of the
VBA that peaks when the slots are aligned with the
5b
Figure 5. a) Scan arm at tilt angle to measure vertical beam
angle. b) Scan arm at an implant position with VBA out of
the beam.
VERTICAL BEAM ANGLE
CALIBRATION
2000
1.2
1900
1
1800
0.8
1700
0.6
1600
0.4
1500
0.2
0.54
1400
-4
-3
-2
-1
0
1
2
Normalized VBA current
Average TW value
As with horizontal angles, it is preferred to calibrate
the vertical beam angle measurement to the crystal
planes of a wafer as the best way to verify that a zero
degree tilt is indeed perpendicular to the wafer. In this
case implants are done at a range of tilt angles with
wafers from a single boule. The VBA is used to
measure the beam angle, then implants are performed
at tilts ranging +/- 1.5 and the average TW data are fit
with a second order trendline to determine the
minimum accurately. The data in Figure 6 show an
offset of 0.54 between the average angle of the
uncalibrated VBA and the crystal planes. Results are
not shown, but these implants are also run with 180
twist to compensate for crystal-cut error. The angle
offset is then zeroed so that subsequent angle
measurements are referenced to the ideal crystal plane
orientation, normal to the surface of the wafer.
Implants are then performed with constant focal length
scanning ensuring the wafer has the same beam focus
and angle content at all positions regardless of tilt.
Using these angle control schemes, an implant of
450 keV P++ was made at a tilt/twist of 0/0 and the
implanted profile was measured on center and at a
radius of 140 mm at four points, 3, 6, 9, and 12
o’clock. As shown in Figure 7, the profiles match well
with each other indicating all points of the wafer were
implanted with ions at the same angle. Profiles from
wafers implanted at tilts of 0.2 and 0.5 show the
sensitivity of this implant to small angle errors.
0
3
4
Tilt angle, degrees
Figure 6. Calibration of VBA (or beam tilt angle)
measurement to crystal planes using 370 keV P++, 5E13 at
near 0/0 orientation.
1.E+18
tilt = 0.2 deg
Conc., at/cm3
beam at the desired tilt angle relative to the measured
beam angle, while the VBA remains fixed to the tilt
mechanism and is safely out of the beam.
1.E+17
Center
140-3
140-6
140-9
140-12
+0.2deg
+0.5deg
1.E+16
1.E+15
tilt = 0.5 deg
1.E+14
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Depth, microns
Figure 7. SIMS profiles of channeled implants of 450 keV
P++, 5E13 at 0/0 in {100} wafers at 5 points compared to
implants at 0.2 and 0.5 tilts.
SUMMARY
Optima MD has complete angle control in horizontal
and vertical planes of the scanned beam.
Measurements are made at the start of a batch and are
automatically corrected in the horizontal plane by
adjusting the voltage of the parallelizing lens or
horizontal steerer and in the vertical plane by tilting
the wafer to the specified recipe tilt as referenced to
the actual vertical beam angle. This method allows
angles to be controlled within 0.2 degrees at most
conditions. The angle measurement devices can be
calibrated to crystal planes to ensure beam orientation
is actually referenced to the wafer. This calibration is
needed only once as long as the wafer chuck or
measurement devices are not moved from the
calibrated positions by a disassembly of components.
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