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| Ion implantation in silicon technology |
| Leonard Rubin and John Poate |
Without implanters, today's integrated
circuits would be impossible
Ion implanters are essential to modern integrated-circuit
(IC) manufacturing. Doping or otherwise modifying
silicon and other semiconductor wafers relies on the
technology, which involves generating an
ion beam and steering it into the substrate
so that the ions come to rest
beneath the surface. Ions may be allowed
to travel through a beam line at the energy
at which they were extracted from a
source material, or they can be accelerated
or decelerated by dc or radio-frequency
(RF) electric fields.
Semiconductor processors today use ion implantation for almost
all doping in silicon ICs. The most commonly implanted species
are arsenic, phosphorus, boron, boron difluoride, indium,
antimony, germanium, silicon, nitrogen, hydrogen, and helium. Implanting
goes back to the 19th century, and has been continually refined
ever since. Physicist Robert Van de Graaff of the Massachusetts
Institute of Technology and Princeton University helped pioneer
accelerator construction, and the high-voltage technology
that
emerged from this effort was instrumental in building High
Voltage Engineering Corp. (HVEC) in the late 1940s and 1950s.
HVEC served as an incubator for the technology essential
to building the first commercial ion implanters and the individuals
who pioneered the field.
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| Figure 1. Some of the
most commonly implanted species highlighted on the periodic
table, along with typical concentration-versus-depth traces
for various implant energies. |
William Shockley first recognized the potential of ion implantation
for doping semiconductor materials, and his 1954 patent application
demonstrates a remarkable understanding of the relevant process
issues long before implantation entered mass production. However,
the patent expired in 1974, just as the commercial ionimplantation
market began taking off. So although Shockley demonstrated visionary
insight, his patent earned few royalties.
Ion-implantation equipment and applications
gradually came together in the 1960s.
Experience gained in building research
accelerators improved hardware reliability
and generated new techniques for purifying
and transporting ion beams. Theorists
refined the hypothesis of ion stopping,
which enabled the precise placement of ions
based on the energy and angle of implantation,
and experimenters determined that
high-temperature postimplant annealing
could repair implantation-induced crystal
damage. Initially, these anneals were done at
a temperature of 500 to 700° C, but after
several years, semiconductor processors
found that the optimum annealing temperature
ranged from 900 to 1,100° C. After the
resolution of process integration issues, ion
implantation rapidly displaced thermal diffusion
of deposited dopants as the dominant
method of semiconductor doping because it
was more precise, reliable, and repeatable.
IC manufacturers, especially IBM and
Western Electric, designed and built many of
the early ion implanters, almost exclusively
for in-house use. But in the early 1970s, the
market for commercial ion implanters began
opening as start-up companies tapped the
technology spun off from HVEC and the
technology developed by IC manufacturers, who
became their customers.
Some memory circuits now sell for less than 20
nanodollars/transistor. Today, implanters and other fabrication
hardware must meet aggressive productivity targets
to achieve this minuscule cost. A large wafer fabricator
may process up to 50,000 wafers/month, with
each wafer requiring 20 to 30 implants. This output
requires the use of about 20 implanters, each with the
capacity to implant more than 200 wafers/h. In practice,
maximum implanter throughput typically ranges from
250 to 270 wafers/h, including placing the wafers into
and removing them from sealed cassettes used by automated
material-handling systems. This throughput is
achieved for wafer sizes of 150, 200, and 300 mm.
Depending on the configuration of the beam line and the
end station (the wafer-processing chamber), an implanter
occupies an area of 16 to 28 m2. Thus, fabrication space
poses almost as significant a barrier as capital cost
against compensating for poor throughput by installing
additional implanters.
Applications
Among semiconductor-processing techniques, ion
implantation is nearly unique in that process parameters,
such as concentration and depth of the desired dopant, are specified
directly in the equipment settings for implant dose and energy,
respectively (Figure 1, above). This differs
from chemical vapor deposition, in which desired
parameters such as film thickness and density
are complex functions
of the tunable-equipment
settings, which
include temperature
and gas-flow
rate. The number
of implants needed
to complete an
IC has increased
as the complexity
of the chips has
grown. Whereas
processing a simple
n-type metal
oxide semiconductor
during the
1970s may have
required 6 to 8 implants, a modern complementarymetal-
oxide-semiconductor (CMOS) IC with embedded
memory may contain up to 35 implants.
The technique’s applications require doses and energies
spanning several orders of magnitude. Most
implants fall within one of the boxes in Figure 2, below. The
boundaries of each box are approximate; individual
processes vary because of differences in design tradeoffs.
Energy requirements for many applications have
fallen with increased device scaling. A shallower dopant
profile helps keep aspect ratios roughly constant as lateral
device dimensions shrink. As energies drop, ion doses
usually, but not always, decline as well. The width of the
statistical distribution of the implanted ions decreases
with energy, and this reduces the dose required to produce
a given peak dopant concentration. The result is the sloping lines
in Figure 2. Implantation is actually extremely inefficient at
modifying material composition.
The highest ion dose implanted with an economical
throughput is about 1016/cm2, yet this corresponds
to but 20 atomic layers. Only the extreme sensitivity of
semiconductor conductivity to dopant concentration
makes ion implantation practical.
 |
| Figure 2. Dose and energy
requirements of major implantation applications (species shown
roughly in order of decreasing usage). |
Ion energy requirements vary from less than 1 keV to
more than 3,000 keV. Accelerating ions to higher energies
requires a longer beam line, yet low-energy beams
are difficult to transport intact over longer distances
because the beam cross section expands to a point where
it can no longer travel down the beam tube. This fundamental
physics makes it nearly impossible to construct a
beam line capable of all required ion energies. Figure 2
indicates that the largest magnitude in required doses
occurs in the middle of the energy range. Because dose is
essentially the beam’s charge multiplied by the implantation
time, available beam currents in the 5- to 200-keV
range must vary by at least 4 orders of magnitude to perform
all required implants efficiently. This level is difficult
to reproduce repeatedly in a single ion-source/beamline
configuration.
Market segments
Consequently, the commercial ion-implanter market
long ago evolved into three segments. The red, black,
and blue regions of Figure 2 indicate high-current, medium-
current, and high-energy applications, respectively.
As the name suggests, high-current implanters produce
the highest beam currents, up to 25 mA (Figure 3, below). For
high-dose applications, the greater the beam current, the
faster the implantation, which means the output of more
wafers per hour. Implanter makers have invested a great
deal of effort in maximizing beam current, especially at the
lowest energies, where Child’s law limits the flux of ions
extractable from a source. Although high-current implanters
can produce beams in the 10-µA range, source instabilities
make these beams unsuitable for low-dose applications.
The short beam line of these implanters allows an energy
range from <1 keV up to 100 to 200 keV.
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| Figure 3. Schematic of
the electron confinement technology necessary to transport
several milliamperes of beam at energies below 10 keV in a
modern high-current beam line. |
Medium-current implanters are designed for maximum
dose uniformity and repeatability.
Their beam currents are in the range of
1 µA to 5 mA, at energies of 5 to ~600 keV.
The wafer-processing end stations can
implant ions at angles up to 60° from the
perpendicular to the wafer surface. This is
essential for certain applications, such as
anti-punchthrough implants, for example,
in which dopants must be implanted partially
underneath a previously formed gate
structure. The lower operational cost of
medium-current implanters when used for
lower-dose applications and their ability to
do high-tilt implants distinguish them from
high-current implanters.
Last, only high-energy implanters can
generate megaelectron volt ion beams.
Commercial high-energy implanters produce
beam currents for singly-charged ions
up to ~1 mA. Energies for multiply-charged
ions can be up to ~4,000 keV, with beam
currents of ~50 µA. High-energy implanters
can produce beams down to 10 keV, making them suitable
for many medium-current applications as well. This
additional functionality justifies the capital cost of these
machines. High-energy implanters using both RF linear
acceleration (Figure 4, below) and dc acceleration are used
widely today in semiconductor manufacturing.
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| Figure 4. Schematic of
a radio-frequency linear accelerator used in a high-energy
ion implanter. |
A modern ion implanter costs about $2–5 million,
depending on the model and the wafer size it processes. Of
the three classes of implanters, the high-current machines
have traditionally been the biggest market in terms of revenue
and unit volume. Revenue is increasing faster than
unit volume because implanters have become more expensive.
However, both revenue and volume are subject to the
severe boom-and-bust cycles that have affected the entire
semiconductor capital-equipment industry in the past
decade, a pattern that will likely continue.
Process requirements
The tiny transistor dimensions that allow the fabrication
of microprocessors with clock speeds exceeding
3 GHz necessitate strict doping accuracy. For the most
sensitive devices, the implanted dose must be as uniform
as possible. Typically, a 3-standard-deviation (s) variation
of 1.5% is the acceptable upper limit. This uniformity
must be consistently achieved across wafers as large
as 300 mm in diameter. Wafer-to-wafer and lot-to-lot
repeatability is equally crucial. The energy of the ion
beam should not exceed a 3s variation of 3.0% across all
wafers. The angle of the ion beam to the wafer must also
be carefully controlled to prevent variations in dopant
position at the edge of device features. Variations in
dopant depth are also a concern. Implant angle control
with a 3s variation of ~1.0° is usually enough to create
reproducible device electrical characteristics.
Designers of modern implanters have largely achieved
the desired dose, energy, and angle accuracy. The greatest
challenge remaining is to increase the productivity of the
beam current at energies below 10 keV. Device scaling has
greatly increased the demand for implants in this energy
range. Designers also need to reduce implanter-induced
contamination as much as possible. Atoms of previously
implanted dopants can be sputtered onto the wafer surface
(cross-contamination), or ions of the right species
but wrong energy or charge state can be implanted (energy
contamination). Particles can be deposited onto the
wafer surface either by ion-beam transport or during
wafer handling. Even particles as small as 120 nm may
cause yield losses. Finally, metallic contaminants can be
deposited onto the wafer surface, usually from sputtering
of beam-line components, or worse, implanted into the
wafer (energetic metallic contamination). Modern devices
are so sensitive to these problems that many customers
demand levels of particle and metallic contamination
below those detected by metrology equipment.
Modern end stations implant either one wafer at a
time or a small batch of wafers—typically 13 or 17—
mounted on a rapidly rotating disk. Only single-wafer
end stations are capable of high-tilt implants because of
the mechanical complications of supplying water and
other necessities to a rotating disk. However, the ability
to implant multiple wafers simultaneously gives batch
end stations a productivity advantage. Consequently,
both end stations have found successful commercial
niches. Nearly all medium-current implanters dope a
single wafer at a time, and most high-current and high-energy
implanters process wafers in batches.
Makers have shipped more than 6,000 ion implanters
worldwide since 1980. Assuming that about 4,000 of
them remain in service, all ions implanted worldwide at
any given time represent a mass transfer of only about
5 mg/s. Two companies located 25 km apart on Boston’s
North Shore, Varian and Axcelis, manufacture roughly
70% of all ion implanters (Table 1, below). This siting is not
entirely coincidental, because many of the same people
founded and/or helped build the two companies, Varian
in 1971 and Axcelis in 1978. It is unlikely that a significant
amount of ion-implanter design will migrate to new
areas because of the machines’ complexity. The expertise
required to design a new beam line is not easily duplicated,
which makes ion implanters the Swiss analog watches
of the semiconductor industry. They are an elegant, complex
product designed and made mostly by a small number
of skilled craftspeople in a small corner of the world.
Further reading
Dearnaley, G.; Freeman, J. H.; Nelson, R. S.; Stephen,
J. Ion Implantation; American Elsevier Publishing Co.:
New York, 1973; 802 pp.
Rimini, E. Ion Implantation: Basics to Device Fabrication;
Kluwer Academic Publishers: Boston, 1995; 393 pp.
Rubin, L.; Morris, W. High-Energy Ion Implanters and
Applications Take Off. Semiconductor International 1997,
20, 77–85. Ryssel, H.; Ruge, I. Ion
Implantation; John Wiley and
Sons: New York, 1986; 350 pp.
Ziegler, J. F., Ed. Ion Implantation: Science and Technology;
Ion Implant Technology Co.: Edgewater, MD, 2000; 687 pp.
Chason, E.; et al. Ion beams in silicon processing and
characterization. J. App. Phys. 1997, 81 (10), 6513–6561.
Current, M. I. Ion implantation for silicon device manufacturing:
A vacuum perspective. J. Vac. Sci. Tech. A:
Vacuum, Surfaces, and Films 1996, 14 (3), 1115–1123.
Biography
Leonard Rubin is
a senior scientist and John
Poate is
vice president and chief technology officer at Axcelis
Technologies in Beverly, Massachusetts.
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