Metal Oxide Cross Contamination in Ion Implanters
David C. Sing
Motorola Semiconductor Products Sector Technology and Manufacturing
3501 Ed Bluestein Blvd, MD K-10
Austin, TX 78721
Abstract. Tunneling current and boron penetration increase dramatically as conventional silicon dioxide gate dielectric
films are scaled below 1.5 nm in thickness. Because of these effects, the International Technology Roadmap for
Semiconductors predicts the introduction of new gate dielectric materials within the next few years. High dielectric
constant materials based on metallic oxides of Zirconium, Hafnium, and other transition metals have been proposed as
candidate gate dielectric materials. The introduction of metallic oxides leads to concerns about metallic contamination in
the processing tools. Ion implant tools are of particular concern since the nature of the implantation process is likely to
sputter the exposed metallic oxides and can cause cross contamination to other wafers. Experiments have been
performed to quantify the potential for metallic contamination from metallic oxides.
concerns, since the high-K films are deposited in the
front-end of the MOS production flow. Hence, a new
source of metallic contamination has been introduced
which cannot be controlled by traditional front-end
back-end segregation.
This paper will discuss
experiments performed to evaluate the contamination
potential associated with these high-K materials. Note
that in this paper the terms ‘high-K’ and ‘metal oxide’
are used interchangeably.
INTRODUCTION
In semiconductor manufacturing contamination by
metallic impurities is of great concern. Traditionally,
the contamination source of greatest concern is the
metallic contamination introduced in the ‘back end of
line’ when the metal interconnects are fabricated using
materials such as aluminum, tungsten, copper,
titanium, cobalt, and nickel. Particular concern is taken
to prevent ‘back-end’ wafers from contaminating
processing tools and wafers in the ‘front end of line’
where the MOS and bipolar transistors are being
fabricated. Careful segregation of ‘back-end’ and
‘front-end’ wafers, wafer holders, and processing tools
is used to prevent metallic contamination of the
sensitive front end processes.
ORIGIN OF HIGH-K
CONTAMINATION
Figure 1 shows a simplified process flow which
illustrates the steps involved in high-K gate dielectric
deposition and a common failure mode which can
cause high-K contamination. In Fig. 1 (a) the well
structure of the MOS transistor has been formed and
the structure is ready for gate dielectric deposition. A
high-K film consisting of a metal oxide 30 to 80 A
thick is deposited over the entire wafer in Fig. 2 (b),
followed by a blanket deposition of polysilicon 1000
to 1500 A thick as shown in Fig. 1 (c). In Fig. 1 (d) the
polysilicon has been etched to form the gate dielectric,
the etch has stopped on the high-K film. In Fig. 1 (e)
exposed high-K film is etched away. The film can be
difficult to etch, and a common failure mode is an
The International Technology Roadmap for
Semiconductors predicts the need for new materials
which have a higher dielectric constant (K) than SiO2
to fabricate gate dielectric films for future MOS
transistors [1]. The use of high-K dielectric films
which are physically thicker than electrically
equivalent SiO2 films prevents electron tunneling and
Boron penetration. These films are made of oxides of
metals such as Hafnium, Zirconium, and other
transition metals. However, the introduction of these
metallic oxides as future high-K dielectrics introduces
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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inert beam implant such as 40 keV Argon at a dose of
1015 atoms/cm2 and then analyzing the surface for
contamination using VPD and TXRF. Of the two
methods, VPD is much more sensitive with detection
limits for metals used in high-K films of ~3x108
atoms/cm2, while TXRF detection limits are typically
about ~2x1010 atoms./cm2. Typical levels measured
during routine monitoring range from below the VPD
detection limit to ~1010 atoms/cm2.
incomplete etch which leaves patches of high-K film
exposed on the surface of the wafer, as shown in Fig. 1
(f).
In most process flows the gate formation is
followed by a source/drain extension ion implantation
step. An ion implant using an n-type dopant (such as P
or As for NMOS transistors) or p-type dopant (such as
B or BF2 for PMOS transistors) is used to dope the Si
up to the gate edge, as shown in Fig. 2(a). If an
incomplete metal oxide etch has left exposed high-K
film, as illustrated in Fig. 2(b), then the ion implant
will sputter the exposed metal oxide which will coat
the interior of the ion implanter processing chamber.
Subsequent implant process steps will re-sputter the
high-K material, which can contaminate other wafers
and transfer the contamination to other tools in the fab.
Experiments were made to quantify the amount of
contamination expected from exposed high-K
materials. Blanket films of Hafnium oxide and
Hafnium oxide aluminate were deposited on bare 200
mm diameter silicon wafers. These wafers were then
implanted with an inert beam to deliberately
contaminate the process chamber. Monitor wafers
were included with some batch implants to measure
the amount of contamination during implant. Other
monitor wafers were implanted afterwards to
determine the amount of residual high-K
contamination.
CONTAMINATION EXPERIMENTS
The processing of wafers containing high-K
materials is restricted to a subset of implant tools
which are routinely monitored for contamination using
Vapor Phase Decomposition (VPD) and Total
Reflectance X-Ray Florescence (TXRF). Monitoring
is performed by implanting a bare Si wafer with an
Batch Implant Tool Experiments
Two types of implant tools are used for implants in
the source/drain extension module: batch tools and
serial tools. The batch tools process 13 wafers at one
time, with the wafers mounted on an aluminum
process disk which is rotated at high speed and
scanned into and out of the ion beam. The serial tools
process one wafer at a time, with the beam being
scanned in the x-axis by an electrostatic scanner and
the wafer is then scanned up and down through the ion
beam plane. Batch implant contamination experiments
are described in this section.
Gate Dielectric
Well
(a)
Polysilicon
(c)
(b)
Polysilicon Gate
Etch
(d)
(a)
Dielectric
Etch
Incomplete
Dielectric
Etch
High-K
Cont.
(e)
(f)
High-K
Cont.
(b)
FIGURE 1. (a) Well formation. (b) Gate dielectric
deposition (c) Polysilicon gate deposition (d) Polysilicon
gate etch (e) Gate dielectric etch. (f) Incomplete dielectric
etch leaves exposed high-K material.
(f)
FIGURE 2. (a) Extension implant into normal wafer.
(b) Contamination sputter of exposed high-K dielectric.
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A series of experiments were performed using
various numbers of wafers coated with high-K films.
In the first experiment, a single high-K wafer was
loaded to the process disc along with an adjacent Si
wafer for TXRF/VPD measurements and 11 dummy
wafers, as shown in Fig. 3. The wafers were implanted
with Ar+ beam at 40 keV and a dose of 1015
atoms/cm2. Immediately after this implant a second set
of wafers was implanted consisting of a bare Si for
TXRF/VPD and 12 dummy wafers. The TXRF/VPD
wafer implanted alongside the high-K wafer received a
sizeable level of Hafnium (Hf) contamination,
1.7x1011 atoms/cm2, while the TXRF/VPD wafer
implanted immediately afterwards did not show a Hf
level above the detection limit of 3x108 atoms/cm2.
disc is insufficient to equalize the contamination
distribution around the disk. The level of residual
contamination for the second multiple wafer
experiment with a full disk of high-K wafer was
8x1010 atoms/cm2, which increased roughly in
proportion to the number of high-K wafers loaded to
the process disk.
A final experiment was performed to determine the
spatial distribution of the Hf contamination from a
single high-K wafer around the process disk.
Comparison of the results from the single wafer vs
multiple wafer experiments indicated that some degree
of localization of the Hf contamination occurs, as
high-K wafers further from a monitor wafer contribute
less Hf contamination. However, it was not known if
the process disk rotation direction had an effect on the
distribution of Hf contamination across the process
disk. A single high-K wafer was loaded to the process
disk with four TXRF/VPD wafers; two wafers were on
either side of the high-K wafer, and two were directly
across from the high-K wafer. Figure 5 shows the
configuration and result of the experiment. The wafers
closest to the high-K wafer received a dose of
approximately 4x1011 atoms/cm2 of Hf, while the
The number of high-K wafers loaded to the
process disk was increased during the next experiment
to determine the level required to get residual
contamination. Figure 4 illustrates the load patterns
and the results observed. A TXRF/VPD wafer loaded
with ten high-K wafers received a 6x1011 atoms/cm2
dose of Hf. A significant level of residual Hf was
measured on the next implant; 5.9x1010 atoms/cm2.
The level of Hf contamination during implant
increased by a factor of 3.5 while the number of highK wafers loaded was increased by a factor of 10,
which indicates the high speed rotation of the process
High-K
Wafer
(a)
VPD
Monitor
Ar+ 40 keV 1E15
6E11 Hf
Ar+ 40 keV 1E15
5.9E10 Hf
(b)
40 keV Ar+ 1E15
1.7E11 Hf
Ar+ 40 keV 1E15
FIGURE 3. Batch Implant tool Hf contamination
from one high-K blanket wafer. No residual hf
was detected during the next implant.
Ar+ 40 keV 1E15
8E10 Hf
FIGURE 4. Multiple wafer high-K contamination
tests. (a) 10 high-K wafers. (b) 13 high-K wafers.
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3.7E11Hf
4.0E11Hf
9.7E10 Hf
80 keV Ar+
1E15
5.9E11 Hf
1.0E11Hf
Ar+ 40 keV 5E15 Implant
FIGURE 5. Spatial distribution of Hf contamination on
a batch implant disk.
FIGURE 6. Serial tool contamination experiment.
wafers on the opposite side of the disk received a
factor of four less contamination. The contamination
levels were approximately the same with respect to the
process disc rotation direction. This result is consistent
with a contamination transport mechanism which is
fast relative to the characteristic mechanical velocities.
A sputtered Hf ion with 1 eV of energy has a velocity
of approximately 1x103 m/sec, which is approximately
a factor of 20 faster than the velocity of the wafers on
the implant disk rotating at 1200 RPM. Therefore it is
not surprising that the implant disk rotation has only a
minor effect on the distribution of sputtered Hf
contaminant.
CONCLUSIONS
High-k contamination experiments were performed
on batch and serial implant tools. Experiments showed
that multiple (~10) wafers coated with a blanket film
of high-K material were required to contaminate a
batch implant tool with Hf levels detectable on
subsequent implants. The Hf contamination from a
single high-K wafer is not evenly distributed around
the process disk. The Hf levels are approximately a
factor of four higher at locations adjacent the high-K
wafer compared to locations opposite from the high-K
wafer. There is no strong dependence of the
contamination distribution with respect to the process
disk rotation direction.
Serial Implant Tool Experiments
The serial implant tool processes one wafer at a
time. A contamination test was performed in which
five high-K wafers were each implanted with an 80
keV Argon beam at a dose of 1015 atoms/cm2, as
shown in Fig. 6. A 30 degree tilt quad recipe was used
to simulate the implant conditions used on product.
Experiments with a serial implant tool showed that
higher levels of contamination occur when blanket
high-K wafers are implanted. Initial contamination
levels were about a decade higher following
implantation of only five blanket coated high-K wafers
compared to a full load of 13 wafers on a batch tool.
Following the high-K wafer implants a TXRF/VPD
implant was performed. The expectation was that the
serial implant tool would exhibit less high-K
contamination, since there are fewer surfaces which
can collect Hf contamination and are struck by the ion
beam. However, the level of residual contamination
following the high-K implants was 6x1011 Hf
atoms/cm2, an order of magnitude larger than the
highest levels of contamination measured in the batch
tools. The high contamination levels suggests that Hf
accumulated with each of the five successive implants,
with a significant fraction of the Hf contaminant that
was deposited from the first high-K wafer remaining
after the fifth high-K wafer was implanted.
ACKNOWLEDGMENTS
The author wishes to thank the Dan Noble Center
Physical Analysis Lab for VPD and TXRF
measurements.
REFERENCES
1. Semiconductor Industry Association, International
Technology Roadmap for Semiconductors: 2001 Edition,
Austin, Texas, USA, International SEMATECH, 2001.
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