Optima HD Imax: Molecular Implant
D.R. Tieger, P.R. Splinter, T.J. Hsieh, W.P. Reynolds
Axcelis Technologies Inc., 108 Cherry Hill Drive, Beverly, Ma. 01915, USA
Abstract.
Molecular implantation offers semiconductor device manufacturers multiple advantages over traditional high
current ion implanters. The dose multiplication due to implanting more than one atom per molecule and the transport of beams at
higher energies relative to the effective particle energies result in significant throughput enhancements without risk of energy
contamination. The Optima HD Imax is introduced with molecular implant capability and the ability to reach up to 4.2 keV
effective 11B from octadecaborane (B18H22). The ion source and beamline are optimized for molecular species ionization and
transport. The beamline is coupled to the Optima HD mechanically scanned endstation. The use of spot beam technology with
ionized molecules maximizes the throughput potential and produces uniform implants with fast setup time and with superior
angle control. The implanter architecture is designed to run multiple molecular species; for example, in addition to B 18H22 the
system is capable of implanting carbon molecules for strain engineering and shallow junction engineering. Source lifetime data
and typical operating conditions are described both for high dose, memory applications such as dual poly gate as well as lower
energy implants for source drain extension and contact implants. Throughputs have been achieved in excess of 50 wafers per
hour at doses up to 1x1016 ions/cm2 and for energies as low as 1keV.
Keywords: Ion implantation, high current implanter, wafer scan
PACS: 85.40.Ry
INTRODUCTION
Molecular Implantation Overview
After more than a decade of research and
development [1,2], ion implantation with molecular
ions is becoming accepted as a solution to
semiconductor
manufacturing
productivity
bottlenecks in low energy implantation and for
enhanced device performance through material
modifications. To this end, production equipment
is being designed to optimize performance for
molecular ionization and transport [3,4]. The Optima
HD Imax is a production ion implanter which is
designed to maximize the productivity offered by
molecular implant and subsequently minimize the
Cost per Wafer manufacturing of the end user. The
implanter is a mass analyzed beamline with the exact
mechanics of a traditional high current machine.
These mechanics are ionization, beam extraction,
beam analysis and beam transport.
While all the
components are familiar to the experienced user of
ion implant equipment, the incorporation of
molecular species allows the tool to be productive at
both high doses (>1x1016 ions/cm2) and at the lowest
energies of interest (<500eV).
Molecular implant for p-type dopants is being
realized with the use of octadecaborane (B18H22).
The implant of 18 atoms per ion charge enables the
high dose capability with demonstrated performance
on the Optima HD Imax of greater than 3mA
electrical current or 54mA of effective 11B current
(see equation 1).
I11B = 18.0 x IB18
(1)
As depicted in Figure 1, the ionization process of
the B18H22 molecule results in a left skew spectrum
spread of molecules starting with B18Hx and
following with decreasing boron atoms (i.e. B17Hx,
B16Hx, etc.). Depending upon the mass resolution,
this results in a molecular weight of approximately
210.
When compared to elemental Boron, this
average molecular weight translates to an energy
ratio of 19.1:1 (see equation 2).
E11B = 0.0524 x EB18
(2)
As can be concluded, the heavy molecular ion
limits the practical maximum energy range of the
machine by requiring a very high magnet field in the
mass analysis electromagnet. The Imax has been
designed to allow for a maximum 11B effective of
4.2keV at 80 keV extraction.
FIGURE 1. B18H22 Beam Spectrum
OPTIMA HD IMAX ARCHITECTURE
The Optima HD Imax shares a design architecture
with the Optima HD which is considered a traditional
high current ion implanter [5]. Wherever possible,
design choices were made in such a way as to
maximize commonality with the standard HD but not
if such choices would compromise the performance
of the molecular beams that the Imax delivers. The
ion source, extraction system, aspects of the
analyzing magnet and resolving slit have all been
modified for the molecular implant version but the
process chamber and two dimensional mechanical
scan end-station are the same between the two
Optima HD models. Shown in Figure 2 is the Optima
HD Imax beamline.
into the ionization chamber creating a stream of
electrons parallel to the chamber extraction slit. The
source optics are designed with an extended
extraction slot to increase extraction area relative to
standard high current tools. The molecular species
used on the Imax are distributed in solid form,
therefore a vaporizer system has been developed by
SemEquip and ATMI [7]. Due to the vapor pressure
of B18H22, deposits readily form in the ionization
chamber, electrode and surrounding vacuum
housings.
To prevent degradation of source
performance and source lifetime, these deposits are
automatically cleaned at regular intervals.
To
enable the cleaning, the source system encompasses
the hardware needed to perform in-situ cleaning by
reaction with atomic fluorine. The gas cabinet has
the capability to house both Argon (for both process
and warm-up of the source) and NF3 cleaning gases.
To prevent condensation and clogging, the delivery
path of the vaporized B18H22 to the ionization
chamber is heated and kept in the temperature range
of 90 -130 °C.
Extraction Region
The extraction electrode consists of a typical
ground and suppression aperture pair. Unique to
this design, the plates of the electrodes sandwich a
heating coil which is elevated to approximately 150
°C to eliminate condensation of the vaporized
B18H22.
The ground and suppression apertures feature an
extended extraction area geometry that is matched to
the source ionization chamber slit. Due to the wide
electrical operating range of 2kV to 80kV, care has
been taken to design aperture geometries that are
effective at all energies. In addition to the aperture
optimization of size and curvature, the electrode
manipulator must allow for a sufficient gap axis
travel to match the operating range [8].
Analyzer Magnet Design
FIGURE 2. Optima HD Imax Beamline
Ion Source System
The ion source of the Optima HD Imax is the
ClusterIon® Model 350 from SemEquip, Inc. [6]. The
ClusterIon® source is an electron impact source
which consists of an emitter and a 90 degree bend of
the resultant electron beam which is then directed
The analyzer electromagnet shares many features
with that of the Optima HD, for example, the magnet
yoke and coils are identical. However, to allow for
the large magnetic rigidity that is required to bend
mass 220 at 80 keV (17,600 AMU-keV mass x
energy product) with a field of ~ 1 Tesla, the bend
angle has been decreased to 70 degrees and the
radius has been enlarged. In addition, a pole index
has been applied and the gap modified but the
resultant ion beam trajectory is very similar for the
two machines. Finally, it should be noted that the
optical path length from ion source to wafer of the
HD Imax, 1.7 m, is approximately the same as the
standard HD.
Resolving Aperture
To maximize beam current for applications where
throughput is extremely important, such as in the
dual poly gate (DPG) process, a relatively large
resolving aperture is employed. However, for
applications that require more stringent mass
resolution to limit the contribution from the B 17Hx, a
smaller resolving aperture is desirable [9]. In fact, the
Imax system has been designed to allow for multiple
resolving apertures such that there are three, linearly
actuated apertures: a large and small process aperture
and a very small (3 mm) aperture for optical setup
and AMU calibration.
Scanning System
The implanter beamline generates and transports
a fixed spot beam which is coupled to a two
dimensionally scanned mechanical endstation. This
two dimensional scanning results in uniform
exposure of the fixed ion beam over the wafer as
every point on the wafer is translated through every
point of the beam over multiple scans. During
processing, the wafer is translated through the ion
beam with a pendulum.
The pendulum is capable
of effective linear speeds of up to 3m/s while a
second stage perpendicular to the pendulum moves
the entire assembly through a slow scan of
approximately 0.1m/s. With proper choice of scan
parameters (number of slow scans and fast scan
speed) the typical uniformities are < 1% (sigma). In
addition the use of beam profiling has demonstrated
that the horizontal angle can be known to better than
0.5 degrees [5].
MULTIPLE SPECIES
The ability to run multiple species on an ion
implanter is key to its versatility. In addition to
supporting octadecaborane, the Imax machine is
capable of running multiple forms of carbon
molecules. Two process applications have emerged
for carbon, the first being the traditional use of
carbon for controlling boron diffusion and the second
is to induce strain in the n-type channel. Research
and development has been conducted using two
molecules, ionized C7H7 as resolved from C14H14 and
ionized C16H10. While potentially both molecules
can be used for boron diffusion retardation and to
produce tensile strain, one may be selected over the
other due to the required energy, dose or resultant
damage. As stated, the starting material for the C7
implants is C14H14. A typical spectrum of carbon
ionization for C14H14 on the Imax is shown in Figure
3. The maximum effective carbon energy for C16
(mass 202) is ~4keV while the top energy for C7
(mass 91) is ~10keV.
FIGURE 3. C14H14 Beam Spectrum
SYSTEM PERFORMANCE
The beam current multiplication of Equation 1
means, of course, that the electrical doses for an HD
Imax are much lower than a standard high current
tool: even ultra high dose, 2x1016 atom/cm2 effective
are only ~1x1015 atom/cm2 electrical and the majority
of implant applications are in the 1013-1014 range.
This leads to high throughputs relative to standard
high current machines. The overall productivity gain
of the Imax incurs a reduction of 12% due to the
requirement for periodic in-situ cleaning, however,
the 3x to 4x beam current premium over traditional
High Current machines more than makes up for the
lost availability due to the cleaning step.
For
example, as can be seen in Figure 4, effective beam
currents in excess of 45ma can be sustained for
effective 11B energies of 4keV.
In comparison,
traditional high current ion implanters provide beam
currents in the 10mA range for the analogous 11B
monomer implant.
Source Lifetime
The HD Imax has been operating at customer
sites for almost two years. In that time, multiple
source life marathons have proven that the
technology is robust during manufacturing operating
conditions. Shown in Figure 4 is such a source life
demonstration of 200 hours of continuous operation.
During this marathon the average time between
cleans was 7:10 hours and the average time of clean
(beam-off to beam-on) was 59 minutes. Finally, the
average beam current for this marathon was 45.7 mA
effective Boron current.
advantage translates into lower Cost per Wafer [8]
and demonstrates that the HD Imax is a superior
technological advance for high dose implantation.
FIGURE 4. Beam current during a 200 hour marathon at a
customer site demonstrating effective average 11B current
45.7ma at 4keV.
Operating Conditions
The time duration between cleans is a direct
contributor to the effective duty factor of the Imax
tool. Additionally, the beam current stability will
also contribute directly to the productivity equation.
Shown in Figure 5 is one such period of 8 hours from
late in the same customer source life marathon and
where the beam current remained constant to 94% of
the starting value.
TABLE 1. Throughput comparisons between STD high
current tool and Imax for three different processes.
SUMMARY
The Optima HD Imax is a two dimensional
mechanical scan machine with molecular implant
capability. The dose multiplication of the borohydride molecules allows for superior beam current
and throughput compared to traditional high current
implanters. Field operating results have shown that
molecular implantation is a viable and productive
solution for device manufacturing.
REFERENCES
D. Takeuchi, N. Toyoda, et al., “Shallow Junction
Formation by Polyatomic Cluster Ion Implanation”,
Technical Report of IEIC, SDX 1995, pp 83-89.
2 A. Perel, et al., “Decaborane Ion Implanation”,
proceedings, IIT 2000, pp 304 – 307.
3 H. F. Glavish, T.N. Horsky, et al., ”A Beam Line
System for a Commercial Borohydride Ion Implanter”,
Proceedings, IIT 2006, pp 167 – 170.
4 A. Renau, “A Better Approach to Molecular Ion
Implantation”, 7th International Workshop on Junction
Technology, IEEE 2007. pp 107 – 112.
5 P. Splinter, M. Graf, et al., “Optima HD: Single Wafer
Mechanical Scan Ion Implanter”, Proceedings, IIT
2006, pp 601-604.
6 T. Horsky et al., “Boron Beam Performance and in-situ
Cleaning of the ClusterIon® Source”, Proceedings, IIT
2006, pp 198 – 201.
7. D. Adams et al., “A Vaporizer for Decaborane and
Octadecaborane”, Proceedings, IIT 2006, pp 178 – 181.
8. D.R. Tieger et al., “Beam Current Improvements on the
Axcelis Optima HD Imax Implanter”, this Proceedings.
9. D.R. Tieger et al., “ClusterBoronTM Implants on a High
Current Implanter”, Proceedings, IIT 2006, pp 206 –
209.
10. M.A. Harris et al., “Dose Retention Effects in Atomic
Boron ClusterBoronTM (B18H22) Implant Processes”,
Proceedings, IIT 2006, pp 155 – 158.
1
FIGURE 5. Beam current through an 8 hour cycle
between cleans during a 200 hour marathon at a customer
site.
Throughput Results
The HD Imax can be compared to standard high
current tools’ throughput over a variety of
applications. Shown in Table 1 are the following: a
comparison for 4.2 keV 11B monomer equivalent
DPG running at one customer site, a comparison of 2
keV 11B monomer equivalent S/D and a comparison
for 10 keV BF2 replacement contact implant being
run at another customer site. In all cases the
throughput from Imax, even with the downtime for
cleaning and a trim correction for dose retention
[10], is far superior to the throughput from
conventional high current tools. This throughput