Journal of The Electrochemical Society, 147 (11) 4329-4332 (2000)
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Ionization and Mass Spectrometry of Decaborane for Shallow
Implantation of Boron into Silicon
M. Sosnowski,a,*,z M. A. Albano,a V. Babaram,a R. Gurudath,a J. M. Poate,a and Dale Jacobsonb
aDepartment of Electrical
bBell laboratories, Lucent
and Computer Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, USA
Technologies, Murray Hill, New Jersey 07974, USA
Future generations of Si electronic devices will need very shallow p-n junctions, in the tens of nanometer range. Implantation of B
to form p-type junctions of such low depth requires very low energies, below 1 keV, where the ion beam formation and transport
are hindered by space-charge effects. Shallow implantation also can be achieved using higher energy beams of ionized large molecules, such as decaborane (B10H14), since the atoms are implanted with only a fraction of the beam energy. Measurements of electron impact ionization and breakup of decaborane in the electron energy range, 25-260 eV, and temperatures up to 3508C are reported here. Ions containing 10 B atoms were found to be the dominant component in all measured mass spectra. In another set of
experiments, the beams of the B10H1
x cluster ions were generated in an electron impact ionization source, mass analyzed, transported through a 2.5 m long ion beam line, and implanted into Si. No significant breakup of the ions and no neutral beam component were found. Beams of ions with ten B atoms were formed more easily and are more robust than initially thought. The results
confirm the potential of decaborane cluster ions for low energy implantation of boron.
© 2000 The Electrochemical Society. S0013-4651(00)03-071-8. All rights reserved.
Manuscript submitted March 15, 2000; revised manuscript received July 31, 2000.
Implantation of energetic ions into solids has played a critical
role in the development of semiconductor technology over the last
25 years. It has made possible precise doping of the semiconductor
layers in various parts of electronic devices, as well as fine adjustment of the threshold voltage of metal-oxide-semiconductor (MOS)
transistors. Fabrication of today’s multilayer silicon integrated circuits may utilize 20 implantation steps with ions of different species,
ranging in energy from 5 keV to a few megaelectronvolts. As the
quest for higher circuit density and smaller device dimensions continues, not only lateral but also vertical dimensions of the implanted
layers become critical. In particular, a further decrease in the size of
MOS transistors requires the creation of very shallow junctions in
the source, drain and the source and drain extension regions. The
International Technology Roadmap for Semiconductors projects the
need for 25 to 43 nm deep junctions in the 0.13 mm semiconductor
devices, expected by the year 2002 and for 20 to 33 nm junctions in
the 0.1 mm devices projected for the year 2005.1 The difficulty of
forming such junctions emerges as a major roadblock in further
progress of silicon MOS technology.
Extension of the standard ion implantation technology to very
shallow depths encounters fundamental problems. The very small
projected ion range requires low ion energy, particularly for light
ions. The energy of B1 ions required for p-type ultrashallow junctions in silicon is of the order of 100 eV. Transport of ion beams of
such low energy is hindered by the coulombic forces (beam spacecharge), to the extent that standard ion implanters cannot deliver sufficient ion currents for commercial semiconductor implantation.
New designs of implanters based, for example, on the deceleration
of energetic beams in front of the target, have been introduced and
are being evaluated by the industry.2 Another solution to the problem
of low energy implantation may be plasma immersion.3 This technique, however, does not discriminate among various ion species,
and its potential for precise dose control remains uncertain. An alternative approach, discussed in this paper, is based on energetic beams
of cluster ions, which produce implantation effects equivalent to
those of monomer ions at lower energy.
In a cluster of n identical atoms impacting on a surface with
kinetic energy E, each of the constituent atoms carries the energy
E/n, which defines its range in the solid. Also, the ion beam of n
atom cluster transports n times the mass of a monomer beam at the
same charge. Thus space-charge problems in the beam transport as
* Electrochemical Society Active Member.
z E-mail: [email protected]
well as problems of target charging are minimized. At the ion source,
the limit imposed by the Child-Langmuir law on the extracted beam
current density is proportional to the extraction voltage to the 3/2
power but inversely proportional to the square root of ion mass.4,5
Since the extraction voltage for a given energy per atom is proportional to n, the extracted fluence increases as n 2. These effects make
cluster ion beams an attractive tool for shallow implantation.
Considerable interest has been aroused recently by the reports of
ultrashallow junctions formed by implantation of clusters formed by
ionization of decaborane (B10H14). Experimental MOS devices with
such junctions were made by Fujitsu in collaboration with Kyoto
University.6,7 There was no information, however, about the implanted species, as the ion beam had not been mass analyzed. The ionization and breakup properties of a decaborane molecule subjected to
energetic electron bombardment in an ion source have not been well
known beyond the fact that attempts to use the compound in conventional implanter ion sources have not produced cluster ions. In a
beam of decaborane (B10H14), each boron atom carries approximately one-eleventh of the beam energy, and the boron dose per unit
charge is ten times larger than in the case of a monomer B ion beam.
Thus implantation of decaborane may be an attractive alternative to
ultralow energy atomic B implantation.
These kinetic energy and implantation depth considerations do
not imply that the effects of boron and decaborane ions are the same.
An impact of a cluster of atoms on a surface delivers simultaneously a number of atoms into a limited volume of a solid. Dynamics of
collisions, involving collective motion of the cluster atoms is expected to be quite different from the collision cascades following an impact of a single atom. This may lead to differences in crystal damage, backscattering coefficients, and sputtering yields. Differences in
damage may affect amorphization dose and possibly diffusion in
postimplantation annealing. Backscattering and sputtering may determine the maximum concentration of the implanted species.
Decaborane ions will implant not only boron but also hydrogen.
Since hydrogen is substitutionally insoluble in silicon, it is not expected to contribute to excess self-interstitials and thus to transientenhanced diffusion. If there were any interstitials remaining after
incomplete recombination of Frenkel pairs generated by hydrogen
impacts, they would very likely diffuse to the surface. A hydrogen
atom carries only a small fraction of the decaborane beam energy,
about 40 eV for 5 keV implantation, and will be implanted very
close to the surface. It has been shown that H diffuses out of Si from
even larger depths at temperatures around 7508C.8 Annealing after
implantation should expel the shallow hydrogen implanted by de-
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Journal of The Electrochemical Society, 147 (11) 4329-4332 (2000)
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caborane ions. There is also considerable experience with boron
doping of silicon using BF1
2 ions, which are used in the current shallow junction technology. Despite the presence of the energetic and
chemically aggressive fluorine atoms, excellent device characteristics are obtained. It may be expected that much less energetic and
more diffusive hydrogen atoms implanted close to the surface will
not adversely affect the junction.
These problems need to be addressed by experiments with ion
beams of well-characterized composition, mass, and energy. The primary task, however, was to determine if useful decaborane ion beams
can be effectively generated and implanted. To assess this prospect we
have studied ionization and breakup of the decaborane molecule under energetic electron bombardment at elevated temperature. The
measurements of ion spectra with a quadrupole mass spectrometer
were followed by generation of ion beams in an experimental ion
source and by studies of their properties in a full-scale research ion
implanter. It was found that cluster ions containing 10 B atoms
(B10H1
x ) are the predominant component in the ion mass spectra, even
at elevated temperature, and beams of those ions can be effectively
transported through an ion implanter. No significant neutral beam or
ion breakup components were found in the implanted Si samples. The
results confirm that implantation of B into Si using decaborane ions
has a high potential for successful commercialization.
Ionization of Decaborane
Decaborane at room temperature is a solid white powder with a
melting point of 99.58C, which sublimes easily with the vapor pressure of 0.15 Torr at 208C and 19 Torr at 1008C.9 Thus decaborane
can be introduced into an ion source in the form of a vapor. Ionization of molecules in ion sources occurs mainly through the impact of
electrons, which may also lead to breakup of large molecules into
smaller fragments. A successful decaborane ion source must therefore maximize the extracted current of ions containing ten B atoms.
Dissociation of hydrogen is of secondary concern, but it shifts and
broadens the B10H x mass spectrum.
To understand the effects of electron impacts on the decaborane
molecules, we have performed a series of measurements of mass
spectra of ions generated by the impact of electrons in the energy
range from 25 to 250 eV and source temperatures up to 3508C.10 A
quadrupole mass spectrometer with an electron multiplier detector
was used in the measurements.
Figure 1 shows the mass spectra at three electron impact energies, Ee, of 25, 70, and 255 eV, obtained at a temperature of 2508C.
A mass spectrum of a B10 cluster with the natural isotope ratio (20%
10B, 80% 11B), calculated from the binomial distribution, is shown
for reference. Breakup of the large molecules is clearly seen. The
mass spectrum at Ee 5 70 eV, for example, shows the formation of
BnHx clusters with n ranging from 1 to 10. Hydrogen dissociation is
also clearly visible even at the lowest electron energy at which
almost all boron atoms are in the B10 cluster. Loss of different numbers of hydrogen atoms (14 2 x) is responsible for broadening of the
peaks beyond the width expected from the presence of the two B isotopes. Loss of hydrogen also shifts the positions of different Bn
peaks toward lower masses. For example, the position of the B10
peak corresponds to the loss of six hydrogen atoms (6 AMU) from
the original 14 in the molecule. There is more dissociation of both B
and H at Ee 5 70 eV than at the higher and the lower energies. A
shoulder on the left side of the B10Hx peak is seen only at Ee 5
70 eV. It can be interpreted as B10Hx (x < ,4) or possibly B9Hx (x >
,8). The latter is unlikely, in the light of considerable hydrogen dissociation seen in all other peaks.
Enhanced breakup at the intermediate energies, near 70 eV, is
demonstrated in Fig. 2, where the relative abundance of B10Hx, with
respect to the total integrated mass spectrum, is plotted as a function
of the electron impact energy. The intensity of the B10Hx peak as a
function of electron energy, shown in Fig. 3, resembles the characteristic electron impact ionization cross section of many gasses. The
data show that while the B10Hx intensity increases initially with the
electron impact energy to the peak at about 70 eV, and then decreas-
Figure 1. Ion mass spectra at 2508C and at the electron energies of (a) 25, (b)
70, and (c) 255 eV. Mass spectrum of a B10 cluster calculated from the natural isotope abundance ratio (20% 10B, 80% 11B) is shown in the inset of (a).
es sharply above 100 eV, the dissociation of B follows a similar but
less steep energy dependence curve. The highest intensity of the B10
cluster is near 70 eV, even though the dissociation of B is higher at
this energy than at 25 or 250 eV. At any energy used in the meas-
Figure 2. Fraction of B10H1
x ions in mass spectra from decaborane as a function of electron energy.
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Journal of The Electrochemical Society, 147 (11) 4329-4332 (2000)
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S0013-4651(00)03-071-8 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 3. Normalized intensity of the B10Hx peak as a function of electron
energy.
urements, most ions (70-95%) contain ten B atoms. It is interesting
that this was also found when the source temperature was increased
from 250 to 3508C (Fig. 4). The data indicate that the B10Hx cluster
is more stable than may have been expected, which is promising for
the prospect of using decaborane in ion sources for shallow boron
implantation.
Properties of B10H1
x Ion Beams
To asses the feasibility of using B10H1
x clusters for shallow implantation of B into Si, we built an experimental ion source and a
beam line with an analyzing magnet. The source, based on electron
impact ionization, resembles the design of a Bayard-Alpert ionization gauge.11 Electrons emitted by a hot filament are accelerated
toward an anode, made in the form of an open spiral, and make several passes through the ionization volume inside the spiral before
being collected. An advantage of this design is that the electron energy is well defined by the potential difference between the anode and
the cathode and thus can be tuned for maximizing the generated
B10H1
x ion current. The source is equipped with an asymmetric
Einzel lens, which focuses the ion beam at the entrance to a 708 mass
analyzing magnet. The source and the lens structure are electrically
isolated from the magnet chamber and the rest of the apparatus, and
are biased by an adjustable high voltage power supply (0-20 kV),
which provides the ion acceleration voltage. The value of the magnetic field defines the mass energy product of ions passing through
an aperture at the exit of the magnet while the width of the aperture
defines the mass range. Ion mass spectra were obtained by measuring the ion current, passing through the resolving aperture, as a function of the magnet current. The relation between the magnet current
and ion mass was established by a calibration procedure in which the
Figure 4. Temperature dependence of the fraction of B10H1
x ions in mass
spectra from decaborane at different electron energies.
magnet currents corresponding to argon ions Ar1 and Ar21 were
measured.
The ion beam emerging from the aperture passes between electrostatic deflection plates and enters a sample chamber. The plates
were used for the ion deflection experiments, described below, and
for scanning the beam over Si samples during implantation. Another aperture, in the sample chamber, defines the implanted area. A
plate electrode with an opening larger than the last aperture is placed
in front of the sample and is maintained at 2200 V with respect to
ground. The role of this electrode is to prevent electrons emitted at
the edges of the apertures from impinging on a sample and also to
suppress the emission of secondary electrons from the sample itself.
Samples are mounted on an aluminum block at the center of the sample chamber.
A mass spectrum of 6 keV ions impinging on the sample block is
shown in Fig. 5. The broad peaks correspond to those found in the
quadrupole mass spectrometry experiments (Fig. 1) and are identified as corresponding to cluster ions with different number of B
atoms. Again, the B10H1
x ion group is the most prominent. The integrated current of cluster ions with 10 B atoms is 3.3 mA, larger than
the current of other ions. This signifies that the B10H1
x ions are not
only successfully generated in the ion source but also survive the
2.5 m long trip through the magnet and the beam line in a vacuum of
1026 Torr. Charge exchange and breakup of the large cluster ions
upon collision with residual gas molecules might be expected but
was found not to be significant, as described below.
A beam of B10H1
x selected by an appropriate choice of the magnet current and the aperture width was implanted into a Si sample.
The long rectangular sample extended in the direction perpendicular
to the beam and parallel to the electrostatic field between the deflection plates. In this way, the voltage applied to the plates deflected the
beam from its normal straight trajectory to different points on the
sample. The point where the deflected beam impinged on the sample
could be precisely determined from the ion energy and the deflection
voltage. If there was a neutral beam component, due to charge exchange in collisions with residual gas molecules, then some implanted boron would be found in the sample at the position of an undeflected beam. Breakup of the large ions into smaller fragments would
also result in implantation at points on the sample between the deflected and undeflected beam positions. Implanted boron was detected in the sample by measurements of a particles from the reaction of
the predominant boron isotope, 11B, with 650 keV protons
11
5B
1 p r 84Be 1 a
a particles were detected at a scattering angle of 1708 by a silicon
surface-barrier detector, covered by a 10 mm Mylar foil to block
scattered protons. The yield of a particles for a given proton beam
charge is directly proportional to the number of 11B atoms in the
sample. The proportionality factor was established experimentally
by a measurement on a sample with a known boron concentration.
The sample implanted with B10H1
x ions was analyzed with the proton beam, 1 mm in diam, at 1 mm intervals along the line of deflection. The results are shown in Fig. 6. The vertical axis scale is given
Figure 5. Ion current measured on the sample as a function of ion mass at the
beam energy of 6 keV.
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4332
Journal of The Electrochemical Society, 147 (11) 4329-4332 (2000)
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The results confirm the potential of decaborane cluster ions for
low energy implantation of boron. We are now engaged in studying
the effects of implantation of these ions in Si.12 An understanding of
differences between implantation effects of cluster and monomer
ions is of considerable scientific interest. It is also essential for the
application of cluster ions in shallow junction formation. The success of this method in semiconductor circuit manufacturing will also
depend on the availability of ion sources capable of generating sufficient beam currents of cluster ions. Such sources are currently
being developed.
Figure 6. Boron dose measured in a Si sample implanted with a deflected ion
beam. The arrow shows the position of the undeflected beam where neutralized ions would be implanted.
in the implanted B dose calculated from the a particle yield, which
agrees well with the dose from the integrated cluster ion current. The
peak at 2.25 cm is exactly at the calculated position of the deflected
beam. An arrow indicates the position of the undeflected beam (at
5.3 cm), where neutralized ions would be implanted. The number of
a particles detected there is equal to the background and is below
1% of the peak intensity.
Conclusions
Decaborane molecules are effectively ionized by impact of electrons in the energy range of 20 to over 200 eV, with the maximum
yield near 70 eV. The generated ions contain from one to ten B atoms
with various numbers of H atoms but most of them contain ten boron
atoms, even when the source temperature is increased to 3508C.
Beams of the B10H1
x cluster ions were generated in an electron
impact ionization source, mass analyzed, transported through a 2.5 m
long ion beam line, and implanted into Si. No significant breakup of
the ions and no neutral beam component were found. There was also
no vapor-phase transport of boron between the solid decaborane at
the ion source and the sample.
Acknowledgments
The authors acknowledge the support of the National Science
Foundation (GOALI program) and Sematech. The technical assistance of Anthony Mujsce of Bell Laboratories, Lucent Technologies,
in quadrupole mass spectroscopy measurements is gratefully
acknowledged.
New Jersey Institute of Technology assisted in meeting the publication
costs of this article.
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