Automated Implanter Endstation for Combinatorial
Materials Science with Ion Beams
I. GroDhans, H. Karl, B. Stritzker
Institut f i r Physik, Universitat Augsburg, 0-86135 Augsburg, Germany
Abstract. The discovery, understanding and optimization of new complex functional materials requires combinatorial
synthesis techniques and fast screening instrumentation for the measurement of the samples. In this contribution the
synthesis of buried 11-V1 compound semiconductor nanocrystals by ion-implantation in SiOz on silicon will be
presented. For that we constructed a computer controlled implanter target end station, in which a 4-inch wafer can be
implanted with a lateral pattern of distinct dose, composition or energy combinations. The chemical reaction of the
constituents is initiated either during the implantation process or ex-situ by a rapid thermal process, where a reactive
atmosphere can be applied. The resulting optical photoluminescence properties of the individual fields of the pattern can
then be screened in rapid succession in an optical ciyostat into which the whole wafer is mounted and cooled down. In
this way, complex interdependences of the physical parameters can be studied on a single wafer and the technically
relevant properties optimized.
depend heavily on the interactions between their
constituents.
INTRODUCTION
To accclcratc tcchnological advanccs in thc ficld of
materials synthesis and modification by ion
implantation a combinatorial method is applied. T h s
kind of approach will promote understanding,
optimization and acceleration of discovery of new
complex functional materials. 'lhe combinatorial
melhod was lirsl applied in biochemislry and drug
research [l-51, but there is also demand and already
progress achieved in the field of solid state physics and
chemistry [6].
A special field of application of ion implantation is
the synthesis of buried semiconductor nanocrystals and
a large number of compound semiconductors have
been already synthesized by ion implantation into
different host materials [7-lo]. The gaussian shaped
concentration depth profiles of the implanted elements
provoke a complex microstructure and non
homogeneous particle size distributions after thermal
treatment. But apart the morphological microstructure
of the resulting nano-composite material formed, their
physical properties like photoluminescence, nonlinear
optical behavior and band structures are altered
drastically and numerous potential applications are
motivating research in t h s field [ 10-141.
The type of implanter target endstation presented in
l h s paper is Lo OUT knowledge h e lirsl Loo1 which
addresses the application of the concept of
combinatorial method in the area of ion implantation
techques. Implantation is a very versatile techque,
since it allows to implant almost every element in h g h
doses and thus allowing to produce a supersaturated
solid state solution of the implanted elements in the
host material. Moreover the implantation of multiple
elements for the modification of materials or synthesis
of buried layers opens a very promising field of
research, but the complexity of the physical and
chemical parameters involved are making a precise
forecast of the resulting properties difficult, since they
In particular, we exemplify
an automated
computer controlled implanter target endstation on the
generation of a Cd,Se, materials library in a layer of
thermally grown SiOz on a (001)-oriented silicon
wafer with a diameter of 4-inch. Furthermore an
outlook on a fast screening instrumentation for the
measurement of the photoluminescence properties of
the materials library specimen at different
tcmpcraturcs is givcn.
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
690
were chosen, since the electrical current induced by
ions impinging directly onto the wafer can not be
measured accurately enough, because efficient and
homogeneous suppression of the emission of
secondary electrons is hard to achieve and electrical
isolation problems at h g h target temperatures arise.
SETUP OF THE TARGET END
STATION
A cross-section of the setup of the target end
station is shown in Fig. 1. T h s system is mounted into
the vacuum chamber at the end of the beam line of a
commercial-type medium current ion implantation
systcm (hcrc thc modcl Eaton NV3206) with an
acceleration voltage of 20 to 200 kV and a beam
current of up to 500pA. The process pressure of the
system is in the range of 10-6mbar.
The wafer is mounted either onto a heatable (up to
9OOOC) or coolable (liquid nitrogen) sample holder
(see Section I11 of Fig. l), which are 7 degrees tilted
against the beam direction in order to avoid channeling
in single crystalline silicon samples during the
implantation process.
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Between the sample holder and the defining
aperture (Section 11) two computer controlled
moveable shutters are installed. A front view of
Section I1 is shown in Fig. 2. The two shutters are
mounted on separate parallel linear drives. Their edges
are cut clockwise respectively counterclockwise 45
degrees towards the horizontal. The integrated
implanted dose is read from the implanter data port by
a computer program whch moves the shutters
corresponding to the desired doselcomposition pattern.
T h s tooling allows the implantation of a checkerboard
pattern of different composition combination into the
silicon wafer.
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FIGURE 1.
Schematic cross section of the target end
station. I. Defining section: (1) defining aperture, (2) corner
cups. 11. Shutter section: (3) moveable aperture, (4) spindle,
(5) stepper motors. 111. Sample section: (6) cooling shield,
(7) wafer, (8) sample holder. (9) Vacuum chamber, (10)
mounting plate & chamber cover.
FIGURE 2.
Schematic front view of the apertures. (1)
Defining aperture, (2) y-aperture, (3) x-aperture, (4) corner
cup. a) Two diagonally cut apertures, b) y-aperture
diagonally cut combined with a diagonal x-slit aperture c)
two slit apertures.
The ion beam is scanned over the defining aperture
with a diameter of 100 mm (see section I of Fig. l), so
that a homogeneous dose distribution across the
opening is achieved. It contains four faraday cups for
the ion beam current measurement and beam
adjustment. They are located in the corners of a square
around the defining aperture (Section I). Corner cups
LIBRARY GENERATION
The generation of a materials libraiy with discrete
implanted sample areas will be exemplified in the
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conventional way, the overall dose D over do needed is
given by
following. The design of the doselcomposition pattern
is illustrated in Fig. 3. For the example, depicted
schematically in Fig. 3, the increase in implanted dose
between adjacent stripes was chosen so, that the
implanted dose is 10 YOhigher than the previous one
(i.e. the dose follows the progression given by di+l =
1.1 di). For the generation of t h s pattern the aperture
geometry and setup depicted in Fig. 2 a) is used.
Calculation for the two sequential implantations of
element A and B gives 3 1.85, which is 6.7 times larger
than that required for the whole wafer by usage of the
hcrcin dcscribc implantcr targct cndstation.
The implantation begins by opening both shutters
completely and then closing shutter x step by step in
order to increase the implanted dose of element A
corresponding to the dose variation chosen. In the
same way element B is implanted by appropriate
movement of shutter y. Here 1 cm wide strips are
implanted, resulting in a checkerboard pattern of 1 x 1
cmz homogeneously implanted samples (see Fig. 3 and
4).
In Fig. 4 a scattered daylight image of a 4-inch
Silicon wafer after rapid thermal processing is shown.
The sample series of the 1:l dose ratio of element A
and B is indicated by the solid line. On the left- and
right-handed side of that line the off-stoichometric
samples are located. The wafer contains 68 fiill size
2
samples (1 x 1 cm ) and some additional bud smaller
ones at the edge.
--
T h s t e c h q u e decreases sample generation time
drastically. The overall implanted dose D over do for
one element is given by
l:istoichiometry
n-1
where do is the dose chosen for the first stripe, a is
the scaling factor (in the example presented here a =
1.1) and n is thc numbcr of stripcs with diffcrcnt doscs
(here is n = 10).
it
FIGURE 4.
Daylight scattered image of a 4-inch
Silicon wafer implanted with Cd and Se with the herein
discussed parameters and dose do of 2.85 x 10l6ern-'.
MATERIALS LIBRARY EVALUATION
v
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For the characterization of the individual samples
of the materials library a fast screening method is
needed. Here the photoluminescence (PL) properties
are of interest. An optical cryostat is used in whch the
whole 4-inch wafer can be mounted for the
measuremen1 or lhe PL speclrum, exciled by an Ar-ion
laser, and measured with a grating spectrometer . The
cryostat is mounted on a xy-stage moveable parallel to
the wafer, so that each field of the library can be
FIGURE 3. Dose profile generated by using the
aperture setup of Fig. 2 a) and the parameters a = 1.1 and n =
10.
Calculation of D over do for these numerical values
gives 2.36 for a single element and 4.72 for two
implanted elements A and B. If one would carry out
the implantation of only the 1: 1 series (i.e. samples of
the diagonal of the materials library) in the
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addressed. The spectrometer is equipped with a cooled
CCD-line camera whch allows fast data acquisition.
ACKNOWLEDGMENTS
The authors are grateful to the DFG for supporting
t h s work.
Grating Spectrometer
CCDCamera
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FIGURE
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4.
Schematic diagram of the
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An implanter target endstation for combinatorial
ion beam synthesis has been presented. The
introduction of a pair of computer controlled moveable
shutters into the ion beam allows a fast and time
saving gcncration of matcrials librarics with cithcr
discrete or continuously varied implantation
parameters like dose, composition or implantation
energy. Subsequent processing of the whole wafer
permits very reproducible studies of its materials
properties.
13 Delerue, C., Allan, G., Lannoo, M., Phys. Rev. B, 48,
11024(1993)
14 Alivastos, A. P., Science, 933 (1996)
Together with a fast screening method the
implanter endstation makes it possible to study a wide
parameter field, whch is not accessible in adequate
time by the conventional sample at a time method.
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