Electron Induced Ionization Of Molecules With PlasmaTechnological Relevance
Ralf Basner
Institut für Niedertemperatur-Plasmaphysik, F.-L.-Jahn-Strasse 19, 17489 Greifswald, Germany
Abstract. This article describes recent experimental results for the dissociative electron impact ionization of the SiF4
and B2H6 molecules using the time-of-flight mass spectrometric technique. A detailed description of the experimental
method is given. Complete sets of the absolute ionization cross sections for the formation of all ions from SiF4 and from
B2H6 were determined with an overall uncertainty of +15% in the energy range from threshold to 200 eV. Additional
measurements using a sector-field mass spectrometer revealed that fragment ions of SiF4 as well as of B2H6 are formed
with significant excess kinetic energy.
of the ion trajectory. Especially excess kinetic energy
of fragment ions was found to be a cause of ion loss.
Detailed information of the recent progress and the
various experimental techniques is summarized in the
reviews of Märk [1], Matt [2], and Basner [3] and in
the numerous special references quoted therein. The
primary emphasis of this contribution is our
experimental investigation of the formation of positive
ions of molecules for plasma processing. The results
presented here are part of our ongoing study of
ionization cross-section measurements of molecules,
which are important in low-temperature processing
plasmas. A quantitative knowledge of the electronimpact induced direct ionization and dissociative
ionization of these molecules is of crucial importance
in any attempt to investigate, to understand, and to
model the plasma processes.
INTRODUCTION
Low-temperature, non-equilibrium plasmas have
become indispensable in many areas of materials
processing.
They are mainly composed of hot
electrons (average electron energy below 20 eV), cold
ions, and neutral gas molecules, which both have
energies corresponding to temperatures in the range of
300 K to 1000 K. Yet a modest number of the
electrons are responsible for electronic excitation,
dissociation, and ionization of molecules. Electron
impact ionization of the neutral heavy particles in
ground or in excited states is the initial, an important
and in many plasmas the dominant ion formation
process. The dissociative ionization is not only
important for the charge carrier production, but it is
also an essential step in initiating plasma chemical
reactions in the plasma volume and at the boundary
surface. Electron impact ionization has been studied
since the 1930s. The experiments were developed in
two general directions. One is to examine the
ionization process at a very fundamental physical level
and figure out the finer details of the process. The
other one is to determine the probability of ion
formation of the specific ions of a given target as a
function of the electron impact energy, i.e., the partial
ionization cross section on an absolute scale. This
article focuses on the latter scope of activity. A
fundamental new aspect is to avoid or to correct ion
loss at the extraction, transmission, and detection stage
EXPERIMENTAL DETAILS
The experimental determination of partial
ionization cross sections of a molecule by controlled
electron impact under well-defined single-collision
conditions requires a mass selective device. We use
two different experimental devices.
A doublefocusing sector field mass spectrometer, which
provides an accurate way of determining ionization
cross sections as long as the excess kinetic energy of
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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relative partial ionization cross sections were put on an
absolute scale by normalization relative to the total Ar
ionization cross section of 2.77 x 10-16 cm2 at 70 eV
[6]. Typically, the electron gun was operated using
electron pulses of 90 ns width. The amplitude of the
electron beam current is in the range from 1 - 10 µA
with energy spread of about 0.5 eV (FWHM). The
impact energy can be varied from 5 eV to 900 eV.
Extraction fields up to 3 kV/cm with 10 ns rise time
can be applied to the repeller at least 10 ns after the
incident electron pulse passed through the ionization
region.
The output from the MCP-detector is
preamplified and recorded with a 2 GHz multiscaler
using a time resolution of 500 ps. For this set-up the
excess kinetic energy of fragment ions causes the
following effects: (i) The ion source region, which is
normally determined by the spatial dimensions of the
electron beam, is enlarged because of the motion of the
ions during the time interval between their formation
and their extraction. (ii) The divergence of the
extracted “ion beam” is enlarged both spatially as well
as temporally due to the variation in the spatial
positions and energies of the ions when the extraction
pulse is applied. Measurements of the ion extraction
efficiency as a function of the delay time between the
end of the electron pulse and the beginning of the
extraction pulse to the repeller revealed constant ion
currents for all fragment ions as long as the delay
times were below a fixed value. This indicates that all
ions from the extraction region of the ion source are
transported to the detector under these conditions.
Furthermore, extensive studies varying the voltages on
the Einzel lens and on the horizontal and vertical
deflection plates ensured that the diameter of the “ion
beam” at the end of the flight tube is smaller than the
diameter of the MCP for every fragment ion. We
conclude that the experimental conditions necessary
for 100% ion transmission of the ions from the ion
source to the detector were established with the
exception of ion loss at the grids G1 and G2. Since our
techniques relies on measurements of ratios of ions,
the detection efficiency of the MCP for the reference
ion and the various product ions of the gas under study
must be the same. A series of experiments was
performed to measure the ion count rate of a constant
incident ion flux as a function of the ion impact energy
for given operating voltages of the MCP and threshold
levels of the multiscaler. Increasing the ion energy and
the operating voltage of the MCP while decreasing the
threshold level of the multiscaler revealed a saturation
value of the recorded ion count rate. A minimum
threshold level of the multiscaler well above the noise
level was selected in such a way that the ion count rate
was saturated for all singly charged ions and for all
doubly charged ions with twice the ion impact
energies. We assume that the ion impact energy is
fragment ions is close to zero, is employed for high
mass resolution measurements of higher molecular
molecules such as tetramethylsilane, tetraethoxysilane,
and hexamethyldisiloxane [3] to separate the
respective ion signals. All other ionization crosssection measurements are carried out using a time-offlight mass spectrometer (TOF-MS) [4,5] with high
detection efficiency for fragment ions with significant
excess kinetic energy (see the following). A schematic
diagram of the TOF-MS is shown in Fig. 1.
Deflector Lens
Detector II
Reflector Detector I
y
z
x
G3
110
338
182
10 3 10
G4 G5
120
12
Ion Source
Filament
S1-4
Electron Beam
Repeller
G1
G2
Collector
10
Ion Beam
11,5
Deflector
FIGURE 1. Schematic diagram of the time-of-flight mass
spectrometer (TOF-MS) and an expanded view of the
electron impact ion source used in the present study (all
dimensions are in mm): electron beam (tungsten filament, 0 (-900) V); apertures: S1 (0.5 x 4 mm2, 22.4 V preacceleration), S2 /S3 (0.5 x 4 mm2, 22.4 V or 70 V below the
potential of the filament; pulsed), S4 (0.4 x 0.4 mm2,
grounded); repeller (0 - +3 kV); collision chamber exit
aperture (6.5 x 6.5 mm2, molybdenum grids (G1,G2):
transmission 90 %; grounded); flight tube entrance electrode
(diameter 10 mm, 0 - +3 kV); deflector (0 - +500 V); Einzel
lens (0 - +13 kV); reflector (copper grids (G3,G4,G5):
transmission 94%); detector I and II ( Galileo, 40 mm
diameter MCP, active area 12.5 cm2).
It can be operated either in a linear mode using
detector I or in a reflection mode using the reflector
(grids: G3, G4, G5) and detector II. All measurements
described here were performed with the TOF-MS
operated in the linear mode. The ion efficiency curves
were measured simultaneously for Ar and the
molecule under study in a well-defined gas mixture in
an effort to ensure identical operating conditions for
the detection of the ions of each gas. The measured
74
high enough to guarantee a 100% counting efficiency
for each ion hitting the front channel plate.
singly charged fragment ion cross sections are about
one order of magnitude smaller at this impact energy.
See Fig. 2. The value for SiF3+ corresponds to about
72% of the total SiF4 ionization cross section. The
dominance of the SiF3+ partial ionization cross section
becomes even more dominant in the low energy
region, which is of special interest for low-temperature
plasma technology. At 40 eV, SiF3+ ions represent
90% of all ions produced by electron impact on SiF4.
The doubly charged fragment ions appear above an
impact energy of 40 eV. The cross section values of
the doubly charged ions with the exception of SiF2++
are about two orders of magnitude smaller. We found
no evidence for the thermal decomposition of SiF4 at
the hot surface of the filament used in the electron gun.
The mass spectrum at 70 eV is in good agreement with
known mass spectral cracking patters of SiF4 for the
higher ion masses. Differences with previously
published data at lower ion masses, in particular for
SiFx+ (x = 0 - 2) and F+ can be explained in terms of
the excess kinetic energy of these fragment ions,
which may have affected the earlier measurements.
RESULTS AND DISCUSSION
In the following paragraphs we will discuss our
results of recent experimental studies relating to the
ionization of tetrafluorosilane (SiF4) and of diborane
(B2H6).
Electron Impact Ionization of the SiF4
Molecule
SiF4 is a volatile reaction product of fluorine-based
plasmas for silicon etching and is used also as a
precursor in the plasma-enhanced deposition of thin
silicon layers. The absolute partial cross sections for
the formation of various singly charged (SiFx+ (x = 1 –
4), Si+, F+) and doubly charged (SiFx++ (x = 1 – 3),
Si++) positive ions produced by electron impact on SiF4
were measured from threshold to 900 eV [5].
1.0
1
Normalized ion current
-16
2
Ionization cross section [ 10 cm ]
10
0
10
-1
10
0.8
0.6
0.4
0.2
-2
10
0.0
-500
-3
-400
-300
-200
-100
0
100
200
300
400
Horizontal deflection voltage [ V ]
10
0
20
40
60
80
100
120
140
160
180
200
Electron energy [ eV ]
FIGURE 3. Normalized ion beam currents of the ions SiF4+
(filled circles), SiF3+ (open diamonds), SiF+ (filled triangles),
Si+ (stars), and Ar+ (open circles) as a function of the
horizontal deflection voltage at 70 eV impact energy. These
data were obtained with a double-focusing sector-field mass
spectrometer using a modified ion extraction stage [3].
FIGURE 2. Absolute partial (counting) SiF4 ionization
cross sections for the ions SiF4+ (filled squares), SiF3+ (open
circles), SiF2+ (open diamonds), SiF+ (filled inverted
triangles), Si+ (open squares), F+ (filled triangles), SiF3++
(open triangles), SiF2++ (open inverted triangles), SiF++
(filled circles), Si++ (filled diamonds), F++/Si+++ (crosses),
and the total (charge-weighted) ionization cross sections
(stars).
We carried out qualitative determinations of the
excess kinetic energy for all fragment ions by
performing a full horizontal sweep of the extracted ion
beam using a double-focusing mass spectrometer [3].
The results of these measurements are presented in
Fig. 3. The shape of the SiF4+ parent ion signal is
representative of an ion beam with no excess kinetic
energy. The shape of the ion signal for the SiF3+
fragment ion shows an ion distribution indicative of a
small amount of excess kinetic energy. All other ions
The corresponding cross section curves (partial
counting) and the curve of the total (charge weighted
sum of all singly and doubly charged ions) ionization
cross section for impact energies up to 200 eV are
shown in Fig. 2. The SiF3+ fragment ion has the
largest partial ionization cross section with a
maximum value of 4.3 x 10-16 cm2 at 90 eV. All other
75
value of 10.74 x 10-16 cm2. It is obvious from the cross
section curves shown in Fig. 4 that dissociative
ionization is the dominant process for impact energies
above 40 eV. Only the four fragment ions H2+, H3+,
BH3+, and B2+ have maximum cross section values of
less than 0.1 x 10-16 cm² which is comparable to the
maximum value of the B2H6+ parent ionization cross
section. The largest maximum cross section values
were obtained for the B2H5+ fragment ion (2.87 x 10-16
cm² at 70 eV) followed by B2H2+ (2.18 x 10-16 cm² at
70 eV), B2H4+ (1.59 x 10-16 cm² at 70 eV), and BH2+
(1.07 x 10-16 cm² at 70 eV). These ions account for
about 72% of the total ionization cross section of B2H6
at 70 eV. The ion spectrum in the lower energy region
changes with increasing impact energy due to different
appearance potentials for the various ions, but is also
dominated by B2H5+, B2H4+, and B2H2+. For example,
their contribution amounts to 88% of the total
ionization cross-section value at 20 eV. No ion signals
were found that can be attributed to the formation of
doubly charged ions. Our qualitative determinations
of the excess kinetic energy (see above) for the
fragment ions revealed that all fragment ions
containing one boron atom (B+, BHy+, y=1-3) and H+
are formed with significant excess kinetic energy.
show beam profiles indicative of a broad distribution
of excess kinetic energies as shown for SiF+ and Si+ in
Fig. 3.
Electron Impact Ionization of the B2H6
Molecule
Diborane, B2H6, is used as a precursor for the
deposition of cubic boron nitride (BN) films of high
hardness and high chemical resistance using B2H6plasmas with admixtures of H2-NH3 and N2-He-Ar.
Furthermore, doping of silicon with boron for
applications in semiconductor devices is successfully
achieved by different plasma-enhanced methods in
diborane containing gas mixtures. We measured the
absolute partial ionization cross sections of all singly
charged ions of B2H6 in the energy range from
threshold to 200 eV for the first time. The results are
depicted in Fig. 4.
-16
2
Ionization cross section [ 10 cm ]
1
10
0
10
REFERENCES
-1
10
1. Märk, T. D., “Partial Ionization Cross Sections,” in
Electron Impact Ionization, edited by T. D. Märk, and G.
H. Dunn, Wien: Springer Verlag, 1985, pp. 137-197.
-2
10
0
20
40
60
80
100
120
140
160
180
2. Matt, S., Fiegele, T., Hanel, G., Muigg, D., Denifl, G.,
Becker, K., Deutsch, H., Echt, O., Mason, N.,
Stamatovic, A., Scheier, P., and Märk T. D., “Kinetics
and Energetics of Electron Impact Ionization of
Molecules: Ionization Cross Section, Appearance
Energies and Kinetic Energy Release” in Atomic and
Molecular Data and Their Applications-2000, edited by
K. A. Berrington et al., AIP Conference Proceedings 543,
New York: American Institute of Physics, 2000, pp. 191208.
200
Electron energy [ eV ]
FIGURE 4. Absolute partial B2H6 ionization cross
sections for B2H6+ (open diamonds), B2H5+ (open squares),
B2H4+ (filled circles), B2H3+ (open triangles), B2H2+
(crosses), B2H+ (half filled diamonds), B2+ (filled stars),
BH3+ (open circles), BH2+ (filled triangles), BH+ (open
inverted triangles), B+ (filled inverted triangles), H3+ (half
filled squares), H2+ (open stars), H+ (filled diamonds), and
absolute total B2H6 ionization cross section (filled squares).
3. Basner, R., Schmidt, M., Becker, K., and Deutsch, H.,
“Electron Impact Ionization of Organic Silicon
Compounds,” in Advances in Atomic, Molecular, and
Optical Physics, Volume 43, edited by M. Inokuti, San
Diego: Academic Press, 2000, pp. 147-185.
The cross-section curves of all ions show a very
similar shape as a function of impact energy. The cross
sections increase rapidly from threshold to a maximum
and then decrease slightly with higher impact energy.
The maximum for the B2-containing ions was found to
be in the range between 40 and 70 eV and at higher
energies around 80 eV for the B-containing ions and
H+. The total ionization cross-section curve of
diborane exhibits a maximum at 70 eV with a peak
4. Basner, R., Schmidt, M., Becker, K., Tarnovsky, V., and
Deutsch, H., Thin Solid Films 374, 291-297 (2000).
5. Basner, R., Schmidt, M., Denisov, E., Becker, K., and
Deutsch, H., J. Chem. Phys. 114, 1170-1177 (2001).
6. Rapp, D., and Englander-Golden, P., J. Chem. Phys. 43,
1464-1479 (1965).
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