Applied Surface Science 127–129 Ž1998. 304–308
Mass spectroscopic studies on the laser ablation of boron nitride
Heebyung Chae, Seung Min Park
)
Department of Chemistry, Kyunghee UniÕersity, Seoul 130-701, South Korea
Abstract
The laser ablation of pyrolytic boron nitride ŽpBN. target was investigated by quadrupole mass spectroscopy. The
q
laser-correlated ion mass spectra and time-of-flight ŽTOF. spectra were obtained. Bq and Bq
2 ions were detected but N ,
q
q
q
N2 , or BN ions were not observable. Instead, neutral N2 molecules were detected. The velocity of B ions and
temperature of the plume were derived by shifted Maxwell–Boltzmann fitting of the TOF spectra of Bq ions. At the laser
fluence of 0.7 Jrcm2 , the velocity of Bq ions was 21.7 kmrs and the temperature was 15 765 K. Also studied were the
effects of the retarding field on the energetics of Bq ions. The space-charge effect was found to influence the dynamics of
plume propagation significantly. The reactive laser ablation under N2 atmosphere has been attempted by using a pulsed
valve synchronized with the laser pulse. q 1998 Elsevier Science B.V.
PACS: 52.20.y j; 35.20.Wg; 42.62.Hk; 78.90.q t
Keywords: Laser ablation; Boron nitride; Time-of-flight
1. Introduction
The interaction of laser light with solid targets has
been widely studied since the invention of lasers in
the early 1960s. Recently, there has been a growing
interest in the laser ablation of solid targets by high
power pulsed lasers aiming at the deposition of high
quality thin films including superconductors, semiconductors, ferroelectrics and dielectrics w1x. In particular, laser ablation is now considered to be one of
the most useful techniques in the stoichiometric deposition of multi-element materials, as in YBCO
superconductors, due to its congruent melting characteristics. More recently, pulsed laser ablation has
also proved to be a powerful tool in the deposition of
refractory materials like boron nitride w2x, silicon
)
Corresponding author. Fax: q82-2-966-3701; e-mail:
[email protected].
nitride w3x, carbon nitride w4x, and titanium nitride w5x
since high temperatures well above melting point for
each of these, can easily be obtained. Among these
nitride thin films, cubic boron nitride ŽcBN. is one of
the most promising materials with diverse applications in electronics, optics, and hard coatings w6x.
In this work, we have used quadrupole mass
spectroscopy and investigated the laser ablation
mechanism of pyrolytic boron nitride ŽpBN.. Laser
ablation technique has been attempted by many research groups for the deposition of cBN thin films
w7–9x. However, there has been no successful report
regarding deposition of cBN films except a few
cases which employed ion beam w10x or RF plasma
technique w11x together with laser ablation. For cBN,
the nitrogen deficiency is one of the major problems
in the deposition of high quality thin films w11x. The
characteristics of the laser-generated plasma plume
have a crucial effect on the quality and properties of
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 6 4 7 - 8
H. Chae, S.M. Park r Applied Surface Science 127–129 (1998) 304–308
the films deposited by pulsed laser ablation. In this
respect, information on the dynamics of laser-induced plume and the transport of the ablated species
to the substrate will be of critical importance to
understanding the mechanisms of pulsed laser depositions of thin films. Therefore, more studies have to
be performed to reveal the material ejection process
in the laser ablation of boron nitride.
Also, gas phase condensation which forms clusters and associative chemical reactions that give
molecular oxides or nitrides in the reactive laser
ablation, take place in the real deposition condition.
In spite of the extensive experimental work carried
out so far, many phenomena produced by the gas
dynamics of the plume, as well as the laser interaction with boron nitride surfaces, are far from having
satisfactory explanations and still need intensive investigations w12x. Laser ablation of boron nitride is a
very complicated process and there is no reliable
model to describe the optical and thermal attributes
of the laser ablation as yet.
305
averaged into a single TOF spectrum. A boxcar
averager ŽSR250. was used to get laser-correlated
mass spectra.
To investigate the influence of retarding field on
the energetics of Bq ions, two parallel grids ŽR.M.
Jordan, 95% transmission. were installed. A ground
grid and a voltage-controlled retardation grid were
placed 3 and 5.5 cm away from the target, respectively. The region between the target and the entrance hole to the quadrupole was field free when the
retardation grid was removed and the small radial
oscillation field in the quadrupole had negligible
effect on the measured TOF spectra for the ion
energies observed here. The distance from the target
to the detector was 30 cm. To study the effect of
pressure on the formation of boron dimers or boron
nitride molecules, a pulsed valve ŽGeneral Valve
Series 9. was mounted inside the chamber toward
the target. Nitrogen gas was blown to the target by
the pulsed valve synchronized with the laser pulse.
3. Results and discussion
2. Experimental setup
Laser ablation was performed by using the fourth
harmonic of Nd:YAG laser Ž l s 266 nm, SpectraPhysics GCR150-10. with pulse duration of 5 ns.
Pyrolytic boron nitride ŽUnion Carbide. target of size
10 mm = 10 mm was used as purchased, without
further treatment. The laser beam was focused using
a lens onto the target which was mounted on a
standard rotary motion feedthrough. To avoid the
aging effect of the target by repetitive irradiation
from a high powered laser, targets were frequently
replaced to ensure signal reproducibility.
The ablation chamber was pumped by a turbo
pump with base pressure of 2 = 10y8 Torr and
equipped with a quadrupole mass spectrometer ŽVG
SX300.. To detect ions, the ionizer was turned off
and the ion optics in front of the quadrupole rods
were grounded during the experiment. Time-of-flight
ŽTOF. spectra at fixed mass as well as ordinary mass
spectra were obtained. To record TOF spectra, a
digital storage oscilloscope ŽLeCroy 9361, 300 MHz.
was used. A total of 100 laser shots were irradiated
on the target surface under the same condition, each
giving a TOF spectrum. These TOF spectra were
By using quadrupole mass spectrometer and gated
integrator, we have obtained laser-correlated mass
spectra of the species produced by laser ablation of
pBN. With the ionizer off, only ions were detected
and a typical mass spectrum is shown in Fig. 1.
Boron isotopes were detected but Nq ions were not
Fig. 1. A typical laser-correlated ion mass spectrum of the species
produced by the laser ablation of pyrolytic boron nitride target at
the laser fluence of 0.52 Jrcm2 . Only boron isotopes are observed.
306
H. Chae, S.M. Park r Applied Surface Science 127–129 (1998) 304–308
observed at the laser fluence of 0.52 Jrcm2 . At
q
ions
lower fluences, Bq
2 were also observed. N
appeared in the mass spectra at laser fluences above
1 Jrcm2 , but the intensity of Nq ions peak was
much smaller than that of Bq ions. BNq ions were
not detected. According to Angleraud et al. w13x,
emission from N2 was not observed in the plume
generated by laser ablation of BN at fluences above
3 Jrcm2 . They assumed that all N2 and BN molecules
are dissociated in the plasma. However, we have
found that nitrogens evolve as neutral N2 molecules
in the plume at laser fluences at least up to 1.7
Jrcm2 . This can be explained because the dissociation energy of N2 is 9.76 eV while that of BN is just
4.0 eV. Molecular species with small dissociation
energy may not survive the high temperature in the
plume at higher laser fluences. Fig. 2 displays the
correlation of Nq
and Bq ion signals with laser
2
irradiation of BN target. The signal was obtained
with the ionizer on. The current of the filament was
0.5 mA and the electron energy was 70 eV.
According to Hastie et al. w14x, the vaporization
thermochemistry of BN is controlled by the relatively simple reaction:
BN Ž s . s B Ž l . q 0.5N2 Ž g . ,
at temperatures above 2500 K and the amounts of
gaseous B, B 2 , and BN species are much smaller.
Similar results have been obtained by di Palma et al.
w15x. They studied the composition and gas dynamics
of laser ablated AlN plumes. With laser condition of
Fig. 2. The laser-dependence of N2 and B signals as measured by
quadrupole mass spectrometer with ionizer on. The laser fluence
was 0.65 Jrcm2 . With the ionizer off, no N2 signal was observed.
Fig. 3. Integrated N2 signal vs. laser fluence. The nonlinear
dependence of N2 signal magnitude on the laser fluence implies a
multiphoton process for the production of N2 molecules by laser
ablation of pBN.
l s 1064 nm and fluences 20 Jrcm2 , they observed N2 as neutrals and Alq as ions. They could
also find N, AlN, and Al as neutrals but the abundance of each species was negligibly lower than that
of N2 .
The integrated N2 signal intensity which corresponds to the flux of N2 into the ionizer region
increased nonlinearly with laser fluence, i.e., I s
aF n, where I is the integrated N2 signal, a is a
constant, and F is the laser fluence. The log–log
plot is shown in Fig. 3. n s 3.5 was obtained according to the best linear fit. However, it can not be
concluded that N2 molecules are produced by a
direct multi-photon process since they are expected
to be formed by some complicated nonlinear way
including the effects of laser–target, laser–plasma,
and plasma–target couplings w16x. For the integrated
Bq ion signals, n s 2.9 was obtained. Therefore, the
nitrogen deficiency is expected to increase with laser
fluence.
The shifted Maxwell–Boltzmann ŽMB. fitting
with the stream velocity, Õs , and temperature, T, as
two fitting parameters has been attempted for the
TOF mass spectrum of Bq ions. Fig. 4 shows a
typical TOF spectrum and the MB fit. The stream
velocity of Bq ions obtained by MB fit was about
the same as the mean velocity of Bq ions. The TOF
data agree well with the MB fit, implying that the
laser ablation of BN has a thermal nature. But the
H. Chae, S.M. Park r Applied Surface Science 127–129 (1998) 304–308
Fig. 4. The shifted Maxwell–Boltzmann fit of the TOF spectra of
Bq ions. The open circles are experimental data and the solid line
is the best fit. Best fit parameters for the stream velocity, Õs and
temperature, T are Õs s 21.7 kmrs and T s15 765 K.
high temperature of the ejected particles is thought to
originate from the interaction of the plume and the
laser pulse w17x. The plume formed in front of the
target will further absorb the laser energy, forming a
highly excited plasma and the possibility of photochemical process including photodissociation of
molecular species can not be excluded. Also, the
broad distribution of Bq ions, giving a high plume
temperature above 15 000 K at laser fluence of 0.7
Jrcm2 , may stem from the space-charge effect w17x
in the plasma as well as the laser–plasma interaction
mentioned above. The Coulomb interaction of the
ions with electrons that escape at the plume boundary, produces a space-charge field. For the ions, the
boundary effect of laser produced plume may have
significant effect on the energetics.
Fig. 5a shows the effect of the retardation voltage
on the transmission of Bq ions through the grids at
three different laser fluences. The voltage required to
reduce the transmission down to the half maximum
at a given laser fluence was considerably higher than
the mean translational energy measured by the TOF
mass spectrum, which presumably results from the
space-charge effect. Also, the electrons at the plume
boundary interact with the retardation field and make
the ion trajectories directed toward the quadrupole.
This may have caused the enhancement of the signal
at low retardation voltages. The energy spread of the
Bq ions which survived the retarding potential be-
307
came much narrower and the mean translational
energy increased with the retardation voltage as depicted in Fig. 5b, which is expected to be useful to
produce energy-controlled ion beams with narrow
energy distributions from refractory solid targets.
Reactive laser ablation has been attempted in high
vacuum by using a pulsed valve synchronized with
the laser pulse. Nitrogen jet from the pulsed valve
was directed toward the pBN target at 608 to the
surface normal to supply nitrogen molecules at the
ablation zone instead of filling the whole chamber.
Mass spectra were obtained with pulsed valve either
on or off. Even with pulsed valve on, BN molecules
were not detected although formation of CN
molecules had been confirmed in a separate work
w18x, where the laser ablation of graphite had been
Fig. 5. Ža. The effect of the retarding voltage on the Bq signal at
three different laser fluences. The integrated signals were normalized to 1 at the retarding voltage of 0 V. Žb. The mean translational energy of the Bq ions which survived the retarding field
and reached the detector.
308
H. Chae, S.M. Park r Applied Surface Science 127–129 (1998) 304–308
done in a similar condition. This can be explained by
the fact that the dissociation energy of CN is 7.75 eV
while that of BN is just 4.0 eV. Therefore, reactive
ablation in nitrogen atmosphere without assistance of
ion beam or RF plasma does not seem be of great
help in the deposition of high quality cubic boron
nitride thin films.
Also, the increased local pressure at the ablation
zone had no effect on the formation of B 2 dimers. B 2
dimers will certainly be formed more by recombination process at higher pressures but do not seem to
survive the high temperature of the plume because
the dissociation energy of B 2 is as small as 2.9 eV. It
is of note that a great increase of C 2 signal was
observed with increase of local pressure in the laser
ablation of graphite w18x. Due to the large dissociation energy of C 2 , 6.25 eV, C 2 may survive the
harsh environment of the plume.
4. Conclusion
Laser ablation of pBN results in nitrogen atoms
constituting the solid target to evolve as nitrogen
molecules while boron atoms are detected as Bq
ions ionized in the plasma, which clearly shows the
cause of nitrogen deficiency of the BN thin films
deposited by laser ablation technique. Therefore,
supplementary techniques like RF plasma or ion
beam may be required for the efficient activation of
nitrogen atoms.
BN molecules were not detected, but a small
amount of B 2 dimers were found. The formation of
B 2 might be enhanced by recombination as pressure
increases, but the enhancement was not observable
presumably due to the high temperature of the
plasma. The temperature of the plume derived by the
MB fitting of the TOF spectrum of Bq ions was
above 15 000 K, which is attributed to the interaction
of the plasma and laser pulse. We also found that
space-charge influences the energetics of the ions
present in the plume significantly.
Acknowledgements
This work was supported by the Basic Science
Research Institute Program, Ministry of Education,
1996, Project No. BSRI-96-6401 and the Korea Science and Engineering Foundation ŽGrant No. 950501-09..
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