Characterization of Missing-poly Defects in Ion Implantation in
ULSI Manufacturing
Brian Dunham, Rick Anundson, Z. Y. Zhao
Fab25, AMD, Mail Stop 608, 5204 E. Ben White Blvd., Austin, TX 78741
Ion implantation is one of the most critical processes in the front-end-of-the-line for ULSI technology. In the device defining
process, a polysilicon gate is formed between the source and the drain of the transistors. After the poly gate formation, the
silicon wafers are exposed to high dose implants such as source/drain and extension implants. These are usually done by batch
implanters which host the wafers on a large wheel that spins at high RPM in vacuum during processing. Although some are
worse than others, such implanters are known to generate particles leading to the damage of the poly gate. For instance, the highspeed spin of the wafer wheel (1200 RPM) may provide strong enough force so that the impact of a small particle can be
detrimental to a poly line structure. This destruction of the gate is classified as a missing-poly defect. This work shows that
implanter defectivity increases with the increase of wheel spin speed. Several other factors may also contribute to the missing
poly issue, which includes wafer handling, damage from the plasma flood neutralization device and arcing from ion beam
focusing elements near the wafer plane.
This work presents the results of a systematic approach to characterize the defects in subsequent high dose implants. The defects
are inspected on a KLA 2138 patterned wafer inspection tool, and the results are stored online for future references. The wafers
are then placed on a JEOL 9855S SEM system so that the compositions of killer defects are examined by moving the probe to the
exact location of defects. The combination of metrology tools enables us to determine how much defectivity is added to the
product wafer and composition of the defects. From this we are able to categorize the defects and trace them to the originating
sources based on the defect location and the composition. The data shows that the high-speed spin of the wafer disc substantially
increases the destruction of poly due to particles. Other effects of secondary importance, and the implanter effects on particle
generation are also included in the paper. Possible preventative measures are discussed to eliminate or reduce this detrimental
defect.
source/drain and extension implants.
These are
usually done by batch implanters which host wafers on
a large wheel which spins at high RPM in vacuum
during processing. Although some are worse than
others, such implanters are known to have defects
leading to the damage of the poly gate. The ion optics
elements in the beamline and process chamber may
add defects to the device wafers. Ion focusing lenses
may arc, causing a spray of particles. The plasma
flood gun (PFG) is located in a small chamber that is
connected and open to the process chamber.
Secondary electrons are released from the PFG and
move to the wafers on the disc to prevent wafer
charging. The PFG may work adversely and generate
particles that could damage poly lines. In addition, the
wheel is spun with 1200 RPM that may provide strong
enough force so that the impact of a small particle can
be detrimental to a poly line structure. It is the interest
of this work to investigate some of the major possible
causes on ion implanters.
INTRODUCTION
In semiconductor device manufacturing, ion
implanters are used in the front end of line to define
the transistors and other electrical components in the
integrated circuit. In the device defining process, a
polysilicon gate is formed between the source and the
drain of the transistors. The ULSI circuits scaling is
putting hundreds of millions of transistors on a single
die. This requires the polysilicon gate line to become
thinner and thinner. The manufacturing process faces
a very challenging situation to deal with the small
polysilicon lines. All the processes after polysilicon
deposition may cause the polysilicon line to break in
the front-end-of-line processes. This may include the
plasma strip to remove the photoresist, the chemical
cleaning process and the ion implantation. In this
work, we will only concentrate on the influence of ion
implantation on the polysilicon line breakage.
Undoubtedly the combination of other processes can
make the missing-poly case more tangled and difficult
to interpret.
In a typical ULSI device manufacturing process, the
deeper and low dose implants are done before the
poly-gate deposition. After the polysilicon deposition
step, the ion implants are all high dose implants, i.e.,
EXPERIMENTAL METHOD
Silicon wafers with polysilicon line patterns were
used during the experiment. A sample of three wafers
CP683, Characterization and Metrology for ULSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
© 2003 American Institute of Physics 0-7354-0152-7/03/$20.00
278
TABLE 1. Tool X Experimental Trials
Run Spin Speed (RPM) PFG Settings
1
Standard
1250
2
Standard
800
3
Standard
400
4
Off
1250
5
High Settings
1250
6
Standard
1250
was inspected prior to the wafers processing in the
implanter in order to obtain an average baseline defect
level. The wafers were scanned using a KLA 2138
and then characterized (missing polysilicon, surface
particle, pattern defect, etc.) using a JEOL 9855S. The
KLA 2138 directs a laser onto the wafer, which
reflects light that is collected and given an intensity
value between 1 and 256. This value is then compared
to adjacent die in the same location. If the difference
in intensities is outside a threshold, then the tool labels
that area as having a defect. The JEOL 9855S is a
scanning electron microscope (SEM) that collects the
secondary electrons from the top of the wafer and uses
them to generate an image. The defect wafer map
created by the KLA 2138 can be used to drive
automatically to the defects. The JEOL 9855S also
has the capability for Electron Dispersive
Spectroscopy (EDX). An electron beam generated by
a Tungsten filament is used to excite the electrons in
the atoms on the wafer surface. When the electrons go
through de-excitation, they give off X-rays that are
element specific. The emitted X-rays are collected by
a germanium crystal detector. The X-rays act as a
fingerprint for the molecules in the defect, allowing us
to determine the atomic species present. After the
trials were run, all of the experimental wafers were
scanned on the KLA and reviewed on the SEM. A
sample of the surface particles reviewed on the SEM
also received EDX to determine their chemical makeup.
Attempts were made to investigate the importance of
different aspects relevant to missing poly. The first
was to use silicon wafers with different polysilicon
line width. All polysilicon lines were approximately
0.16 micron in height. The average widths of the lines
were 60 nanometer (nm), 90 nm, and 130 nm. The
purpose for generating three groups of line widths was
an attempt to determine if thinner line widths were
more prone to damage due to the ion implant process.
The second attempt was to study the effect with
different implanters. Two different types of ion
implanters were used during the experiment. Ion
implanter X and Y are manufactured by two different
vendors. Under normal conditions, implanter X has
higher defectivity. Ion implanter Y uses a highvoltage (up to 20 kilovolt) ion focusing lens near the
wafer surface which may generate particles when
improperly maintained.
For purposes of this
experiment, the lens was placed in a condition where
arcing was likely. This was an attempt to generate
enough particles to reduce the statistical uncertainty
with significant numbers of missing polysilicon lines.
Several ion implant condition factors were included in
the testing of Tool X. These factors included
polysilicon line width, spin speed of the wheel holding
TABLE 2. Tool Y Experimental Trials
Run Spin Speed (RPM) PFG Settings
1
Standard
1250
2
Standard
1250
4
Standard
800
5
Standard
900
6
Standard
1000
7
Standard
1100
8
Standard
1300
Tilt
0°
0°
0°
0°
0°
10°
Lens
Off
On
On
On
On
On
On
tilt angle of the spinning wheel. The settings used for
the trials appear in Table 1. Run 1 through 5 had
wafers of 60 nm, 90 nm, and 130 nm polysilicon line
width. All other trials on Tool X and all trials on Tool
Y used the 90 nm polysilicon line width.
2,5
i
mm
1250 RPM (Standard)
4QO:RRty1
800 RPM
'
o.Q5
FIGURE 1. Tool X spin speed impact on surface particles
and missing poly.
279
TABLE 3. Tool X PFG setting results
Missing Poly
Surface Particle
PFG Settings
Counts
Counts
Standard
3.3
11
High Settings
1
5.5
20QQ
15001
1
<f 1QPQ-
Off
2.5
6
•Q-xx : x
OJ : x • •'•.
.
^ :500H
A second set of trials was run on Tool Y using
different test conditions, as illustrated in Table 2.
Based on the results of Tool X testing, the effect of the
spin speed was tested again, this time using the high
voltage lens in an exaggerated particle producing
condition in an attempt to produce in large quantity the
missing poly defect.
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FIGURE 2. Voltage lens impact on missing poly counts
3 shows that a slight correlation may exist between
spin speed and missing poly if the first four trials with
worse arcing conditions are removed from the data set.
The average lens current is an indication of frequency
and magnitude of arcs occurring during the ion
implant process. Figure 4 depicts the correlation
between average integrated lens current and implanter
defectivity.
Under normal operating conditions, tool X generates
on average more missing poly defects than tool Y,
which could be attributed to the fact that Tool X is an
older generation tool.
Figure 4 illustrates the
differences in missing poly between the tools
The physical and chemical characteristics of the
surface particles varied across both tools and trials.
The particle sizes ranged from less than a micron to
greater than ten microns. They also varied in shape
and roughness. EDX analysis revealed that several of
the defects contained carbon, oxygen, and fluorine.
This chemical signature is seen in photoresist. This
signature is common due to photoresist being present
on wafers during normal production. Figure 5 depicts
one of the suspected resist particles.
RESULTS
The experimental results from testing on Tool X
indicate missing poly has no correlation to tilt, poly
line width, or PFG settings. A correlation was seen
between lower than standard spin speed and reduced
missing poly. Standard spin speed on Tool X is 1250
RPM.
Figure 1 demonstrates that the 400 and 800
RPM spin speeds are significantly lower for missing
poly and surface particle counts.
Due to the closeness of the PFG to the wafer surface,
it isforeseeable that the PFG operation conditions may
have an impact on the defectivity density on the
wafers. As can be seen in Table 3, variation of PFG
conditions did not affect the missing poly count much.
This leads us to believe when the PFG is maintained
well, it does not have much impact on the total
defectivity. Another possible cause of missing poly
may come from the different tilt angle of wafers in
implantation since the wafer wheel position change
may change the particles' striking path. However, our
test showed the effect from this is minimal. This is
also partly due to the limitation of high current
implanters' capability of tilt. They usually can only
tilt +/- 10°.
90.00% j----------
Results from Tool Y testing indicate a strong
correlation to the high voltage lens, as shown in Figure
2. This would be expected as the lens was placed in
an arc producing state in an attempt to generate
particles. As a result, the correlation between the
wheel's spin speed and the missing poly was
overwritten because of the strong correlation of lens
current and the number of surface defects. The first
four trials of the test had higher missing poly and
surface defect counts due to higher arc rate of the ion
focusing lens. It would have been good if the lens
arcing could be controlled. However, this was not
possible due to the accidental nature of particle
generation by arcing effect of lens electrodes. Figure
Spin Speed (RPM)
FIGURE 3. Tool Y run order and spin speed missing poly
as a percentage of total surface defects.
280
CONCLUSIONS
From these studies, several factors were discovered
to have an impact on the quantities of surface particles
and missing poly. The results from Tool X showed a
substantial decrease in defects when running a lower
spin speed. By lowering the disc spin speed, the poly
lines on the wafer would be traveling at a lower
velocity in the spinning disc reference system. When
a particle then collided with a poly line, less shear
pressure was placed on the line.
No strong
correlations were seen between the thickness of the
poly line and the quantity of missing poly.
As the poly width decreases, the stress that it can take
should also decrease. As seen in the Tool X spin
speed experiments, a low thickness wafer was the only
wafer run
at 800 RPM that had missing poly. This observation
was not seen at the other two spin speeds, which raises
question to the impact of thickness on missing poly.
The data suggests that spin speed acts as a larger
driving force in generating missing poly.
The tilt on Tool X showed no impact on missing poly
or surface particle creation. It was postulated that
changing the tilt of the disc would also change the
poly line, a force is applied from the wafer to rapidly
decelerate the particle. If the path of the colliding
particle is perpendicular to the surface, then the full
normal force of the wafer is used to help decelerate.
As the path of the particle becomes parallel to the
surface, the force the wafer applies to decelerate is
lowered. This means that the force that the poly line
has to counteract is increased, which will also increase
the pressure placed on the poly. Since no impact on
missing poly was seen when changing the tilt, it is
possible that the pressure placed on a polysilicon line
during impact was great enough to break it, regardless
of the incident angle, incident angle of a colliding
surface particle relative to the wafer surface. When a
particle collides with a
The impact of the PFG on missing poly surface
0.09
0.1
0.11
0.12
0.13
TdolY
TqblX.
;;Student'st
Imptantet
:
' '
FIGURE 5. Tool type impact on missing poly counts
particles was also very minimal. Within the PFG,
arcing can occur While the voltage lens may help to
focus the ion beam, the experiments on Tool Y
demonstrate that it also has a significant impact on
surface particles and missing poly when improperly
used or maintained. The initial testing done on the
voltage lens compared only Tool Y at 1200 RPM with
the lens on and off. The standard wafer with the lens
off had minimal missing poly counts, while the wafer
with the lens on had missing poly numbers ranging in
the thousands. During these trials, the current of the
lens was monitored. While the voltage lens was on,
the lens current spiked several times, indicating arcing
between the lens and the chamber wall. These arcs
generated a substantial number of particles that
traveled to the wafer and eventually knocked off poly
lines. When the Tool Y spin speed was changed with
the lens on, no correlation was observed between spin
speed and missing poly. The data shown in Figure 3
indicates variation in lens arcing may have washed out
a spin speed response. Figure 4 demonstrates that the
increasing integrated lens current drove the missing
poly and surface particle quantities. This current is
related to the quantity and intensity of arcs from the
lens, which acted as a particle source. From these
results, the number of missing poly defects appears to
be influenced more by the number of particles in the
chamber than by the disc spin speed. The spin speed,
as shown from the Tool X results, influences the
0.14
Average Lens Current
FIGURE 4. Tool Y lens current impact on missing poly
and surface particles
FIGURE 6. Surface particle seen on wafer from Tool X at
1250 RPM
281
arcing. The type of implanter used should also be
considered, as the inherent defectivity of the tool can
promote missing poly.
DISCUSSION
One would naturally ask how much force would be
needed to break a poly line. And the thinner the poly
line the easier it may be broken. In ref. [1], the
authors presented their results on polysilicon with The
Young's modulus E= 169 +/- 6.15 GPa and the
Poison's ratio v = 0.22 +/- 0.011. The shear modulus
G, by which the polysilicon will break, can be
calculated as follows:
FIGURE 7. Missing poly example from Tool Y at 1100
RPM with voltage lens on
probability that a colliding particle will damage the
polysilicon lines. Figures 7 and 8 show two examples
of missing polysilicon observed during the Tool X and
Tool Y testing.
The type of tool used for wafer implantation can also
dictate the quantity of missing poly. In the experiment
described above, two different batch implanters were
tested with identical conditions. Between the two,
Tool X generated the higher number of missing
polysilicon lines. Tool X is an older generation tool,
which is not as inherently clean as the newer Tool Y.
The types of surface particles seen during testing
ranged from very small to very large. The varying
compositions of the particles made it difficult to trace
their root causes. One of the more common particle
types seen was dark, jagged, and composed of carbon,
oxygen, and phosphorus. The chemical make-up of
the particle suggests photoresist, which is used to
block the implant in areas where implantation is not
desired. The resist could break off during implant and
redeposit on the wafers at a later time.
(1)
From the data above, one can derive that G = 69.3
GPa. The specimens used in that work had polysilicon
deposited on a one-centimeter square single crystal
silicon. The polysilicon lines in this work, however,
range from 0.05 to 0.28 microns in width. Logically,
one would not expect such thin poly lines to stand a
Giga-Pascal stress. As a rough estimate, suppose the
wafer disc is one meter in radius and spun at 1200
round per minute. At the rim of the disc the linear
speed is 120 meters/second. The time for a particle at
rest to go over a 0.2 micron length is 2E-5 second.
Now assume the contamination particle is ten micron
in size and is composed of silicon. We further assume
only one jiim2 contact the poly line on the first contact
when the particle hits the poly line and it takes 2E-5
second for the particle to come to rest. In other words,
all the mechanical energy of this particle exerts on the
polysilicon line using the rotating disc as the reference
system. We can estimate the stress of the particle on
the poly line to be approximately 10 Pascal. We see
this is far less than the aforementioned shear modulus
value cited in the literature. Due to the equipment
difficulty in this work, we cannot demonstrate the
mechanical strength of the thin polysilicon lines. But
it is the authors' intention to bring this topic to
readers' attention.
Several recommendations can be made to lower the
risk of damaging the polysilicon lines during implant.
The spin speed of the disc should be kept at the lowest
level possible that does not hinder the implant. If
dealing with a voltage lens that focuses the beam,
make certain that the lens parameters do not promote
ACKNOWLEDGEMENT
The authors thank James Hussey and Sally Lawrence
for their support in etching the polysilicon lines on the
tested device wafers. We also thank Advanced Micro
Devices (AMD) management for the support of this
project and the Contamination Free Module (CFM)
module in Fab25, AMD for the data collection.
FIGURE 8. Missing poly example from Tool X at 1250
RPM with high polysilicon thickness
282
REFERENCES
W. Sharpe, Jr., B. Yuan, R. Vaidyanathan and R.
Edwards, Measurements of Young's Modulus,
Poission's Ratio, and Tensile Strength of
Polysilicon, Proc. Tenth Intern. Workshop on
Microelectromechanical Systems, p. 424, 1997.
283