Influence of Diborane Flow Rate on the Structure and Stability of CVD Boron Nitride Films

Article abstract of DOI:10.1021/jp951200d

CVD boron nitride films have been deposited at 800 deg C from diborane, ammonia, and hydrogen gas mixtures, using different B2H6 flow rates.The effect of the / ratio in the gas mixture on the structure, composition, and the stability of the lay

Full text of DOI:10.1021/jp951200d

2
148
J. Phys. Chem. 1996, 100, 2148-2153
Influence of Diborane Flow Rate on the Structure and Stability of CVD Boron Nitride
Films
,
†
‡
†
§
†
C. G o´ mez-Aleixandre,* A. Essafti, M. Fern a´ ndez, J. L. G. Fierro, and J. M. Albella
Instituto Cierncia de Materiales, CSIC, UniVersidad Aut o´ noma C-12, Cantoblanco, 28049 Madrid, Spain, and
Instituto de Cat a´ lisis y Petroleoqu ´ı mica, CSIC, Cantoblanco, 28049 Madrid, Spain
X
ReceiVed: April 28, 1995; In Final Form: October 24, 1995
CVD boron nitride films have been deposited at 800 °C from diborane, ammonia, and hydrogen gas mixtures,
using different B
2
H
6
flow rates. The effect of the [B
2
H
6
]/[NH
3
] ratio in the gas mixture on the structure,
3
]/[NH ]
composition, and the stability of the layers in humid atmospheres has been studied. For low [B
H
2 6
ratios (r e 0.25), the deposition rate is low and some crystalline ordering in the deposit was detected. However,
when ratios >0.25 are used, stable amorphous boron nitride films are deposited at deposition rates four times
-
1
higher (160 nm min ). The partially turbostratic boron nitride films, deposited at r e 0.25, are unstable in
humid atmospheres (80% moisture). The evolution of the unstable films was followed by infrared spectroscopy.
It is observed that the turbostratic component in the film is rapidly attacked by the water molecules present
in the atmosphere, giving rise finally to boron enrichment in the films. After this stage, the attack rate becomes
slower, due to the higher stability of the amorphous component in the films.
1
. Introduction
Boron nitride films have attracted a growing interest for a
few reports have been published about this problem. Economy
et al. reported that the stability of the films depends on the
crystalline structure of the samples and that the instability of
7
variety of technological applications due to their excellent
characteristics, namely hardness, chemical inertness, dielectric
behavior, etc. Because of good step coverage, chemical vapor
deposition (CVD) techniques are widely used for the synthesis
of boron nitride films. The technique also allows precise control
of the deposit characteristics even on substrates with complex
shapes. Diborane (B2H6) and ammonia (NH3) gas mixtures are
turbostratic boron nitride (t-BN) is related to high values of c0/
9
2
. Adams attributed the changes in the film composition after
the exposure to moisture to the hydrolysis of B-N bonds, which
8
give rise to ammonium borate hydrates as final products.
However, at present no systematic studies have been made on
the influence of the chemical composition and structure of the
films on their stability characteristics. In this work, we extend
our previous study on the effect of the reaction temperature,
showing that the relative composition of the gas mixture is a
critical factor in determining the reaction route and, therefore,
the final characteristics of the BN films. For this purpose we
have worked at a fixed temperature of 800 °C in order to obtain
BN films, at a relatively high deposition rate, with a low
hydrogen content. Also in the present study, the stability of
the films is shown to depend strongly on their crystalline
structure.
1-3
commonly used as precursor gases for boron nitride deposition.
The films, when deposited at temperatures T < 1000 °C, are
usually amorphous, although in activated processes (plasma
assisted, high temperature, etc.), crystallographic forms, such
as turbostratic, hexagonal, and even cubic, may also be present.
The reaction between diborane and ammonia is complex,
4
since there are many factors that can alter the overall process.
In a previous work we have demonstrated that, for a given
diborane/ammonia flow ratio, the mechanism of the B2H6 +
NH3 reaction is mainly controlled by the deposition temperature
(600-850 °C), which affects the ratio between the concentration
2
. Experimental Section
of active species coming from the decomposition of diborane
and ammonia. Depending on the reaction temperature, different
compounds (i.e. aminodiborane and borazine) can be formed
in the intermediate steps, which in turn determines the final
deposition rate of the film. At temperatures below 700 °C and
low [B2H6]/[NH3] flow ratios (0.055), boron-rich BN films were
obtained through the formation of aminodiborane. On the other
hand, at higher temperatures (g775 °C) the formation of
borazine, as an intermediate compound, led to stoichiometric
partially ordered BN films at lower deposition rates.5 Further-
more, the relative spatial distribution and the bond angle of the
boron and nitrogen atoms in the intermediate compound play
an important role in determining the final structure of the films.
Once deposited, the stability of the films exposed to humid
atmospheres varies with the deposition conditions,6 although
Boron nitride films have been deposited by chemical vapor
deposition (CVD) from diborane and ammonia gas mixtures in
a tubular hot wall reactor, at 800 °C and 267 Pa. The source
gases, high-purity ammonia and diborane diluted in hydrogen
(5% vol), were introduced separately into the hot zone of the
reactor chamber in order to avoid their reaction at room
temperature. The diborane flow rate was varied from 5 to 50
sccm, keeping the ammonia flow rate constant at 100 sccm,
with a total flow rate of 1200 sccm by the addition of hydrogen.
The films were deposited on both sides of polished (100) silicon
substrates.
The composition and stability of the films were evaluated
using transmission infrared (IR) spectroscopy. The IR spectra
were recorded at room temperature using a Hitachi 270-50
-9
-1
infrared spectrophotometer in the 4000-400 cm range with
†
-1
Instituto Ciencia de Materiales.
Permanent address: D e´ partement de Physique, Facult e´ des Sciences-
a resolution of 2 cm . As we shall see later, this technique
‡
has also been used to differentiate between different structures
of the films, since the position of the B-N-B bending peak is
very sensitive to the degree of ordering of the BN deposits.
Semlalia-, BP.S15; Universit e´ Cadi Ayyad, Marrakech, Morocco.
§
Instituto de Cat a´ lisis y Petroleoqu ´ı mica.
Abstract published in AdVance ACS Abstracts, January 1, 1996.
X
0022-3654/96/20100-2148$12.00/0 © 1996 American Chemical Society

Structure and Stability of CVD Boron Nitride Films
J. Phys. Chem., Vol. 100, No. 6, 1996 2149
Figure 1. IR spectra of boron nitride thin films deposited at different
[B H ]/[NH ] ratios.
2 6 3
Figure 2. Deposition rate of CVD boron nitride films deposited at
8
00 °C and 267 Pa Versus the [B
2 6
H
]/[NH
3
] ratio.
TABLE 1: Variation of the 1380 cm- Band Width and the
B-N-B Peak Position in the IR Spectra with the Diborane
Flow Ratio
1
Attempts to use X-ray diffraction to determine the microstruc-
ture of the films were unsuccessful due to the rapid changes, in
both the structure and composition, observed in the partially
ordered materials, during the degradation process (see below
in sections 3.2 and 4). Auger electron (AES) and X-ray
photoelectron spectroscopies (XPS) were also used to determine
the chemical state and atomic composition of the boron nitride
1380 cm-
1
B-N-B
[B
2
H
6
] (sccm)
band width (cm-1)
peak position (cm-1)
5
1
20
2
4
5
110
140
155
140
240
230
80
793
792
788
790
778
778
810
0
-8
films. The Auger spectra were taken in a 1.3 × 10 Pa residual
atmosphere using an electron beam of 3 kV and 1 µA. Argon
ions accelerated at 3 keV were used in the Auger depth profile
5
0
0
-
3
analysis (with an argon pressure of 1.3 × 10 Pa in the
spectrometer chamber). The XPS spectra were obtained by
means of a Fisons Escalab MK II instrument using unmono-
chromated Mg KR radiation (hν ) 1253.6 eV) as X-ray exciting
commercial h-BN
ratios (r > 0.25), the deposition rate abruptly increases (≈160
-1
nm min ). The former films are unstable in a moist atmo-
sphere.
-
7
source at a pressure ≈1.3 × 10 Pa in the analyzer. The O
1
s, C 1s, B 1s, and N 1s regions were studied in an energy
Table 1 shows the band width of the B-N stretching band
at 1380 cm- for the boron nitride films deposited at different
diborane concentrations. The band becomes narrower when the
diborane flow rate decreases, that is, at low deposition rates
(see also Figure 1). The narrowing of the band can be caused
by a higher ordering of the atoms as well as by a lower impurity
content in the boron nitride network when the deposition process
is slower. Also in the table, the position of the peak at ≈780
1
interval of 10 eV. The core level binding energies were
calibrated to the C 1s line at 285 eV due to residual hydrocar-
bon.1 The thickness and the refractive index of the deposited
films were measured using a Gaertner ellipsometer at a
wavelength of 632.8 nm with a He-Ne laser beam at 70°
incidence angle.
0
-1
cm corresponding to the B-N-B bending vibration is shown.
3
. Results
.1. Influence of Diborane Flow Rate on the Composition
-1
Note that the broader the 1380 cm band width, the lower the
wavenumber corresponding to the B-N-B vibration peak. It
is important that according to Rozenberg et al. the shift of the
780 cm- peak toward higher wavenumbers can be explained
by some crystallization in the boron nitride films deposited with
3
of the Films. Figure 1 shows the IR spectra of the boron nitride
films deposited at 800 °C and 267 Pa for different diborane
concentrations ([B2H6] ) 10-50 sccm). For comparison, the
spectrum of commercial Merck hexagonal boron nitride (h-BN)
is also included in the same figure. The most characteristic
features of the boron nitride spectrum is the strong asymmetric
1
1
2
low diborane flow rates.
In order to analyze the IR spectra, the band at 1380 cm-
was deconvoluted into Gaussian curves centered at the positions
of the maxima in the negative second derivative of the
1
-
1
band centered at ≈1380 cm , which corresponds to the B-N
-1
bond stretching vibration, along with the sharper band at around
experimental curve. Besides the B-N peak at 1372 cm ,
-
1
11
-1
7
80 cm attributed to the B-N-B bending vibration. In
another peak in the 1400-1500 cm range is included in the
-
1
addition, an absorption peak at 2500 cm associated with the
B-H bond stretching is present in the spectra of the films
deposited with high diborane flow rates (g40 sccm). As can
be seen in the figure, the intensity of the B-N band increases
with the diborane flow rate, thus indicating an increase in the
broad band. The new peak has been traditionally associated
1
3
with the C-C vibration by Weber et al. and with the B-O
stretching vibration by Adams. The former assignment may
be disregarded, since, as we will see below, no carbon atoms
have been detected by AES in the films. Finally, the intensity
9
2
-1
deposition rate. The variation of the deposition rate with the
of this peak at 1465 cm (I ), normalized to the B-N peak
am
at 1372 cm- (IBN), has also been related to the structure of the
1
[B2H6]/[NH3] flow ratio is given in Figure 2. It can be observed
that in the region of lower [B2H6]/[NH3] ratios (r e 0.25) the
deposition rate is low (≈40 nm min ). However, for higher
BN film: the highest value of the relative intensity peak at 1465
cm (Iam/IBN) corresponds to the amorphous structure whereas
-
1
-1

2
150 J. Phys. Chem., Vol. 100, No. 6, 1996
G o´ mez-Aleixandre et al.
-1
Figure 3. Intensity of the band at ≈1465 cm normalized to the band
-
1
2 6 3
at 1372 cm (Iam/IBN) Versus the [B H ]/[NH ] ratio.
TABLE 2: Atomic Composition, As Determined from AES
and XPS, of the As-Deposited Boron Nitride Films Obtained
at Different [B
2
H
6
]/[NH
3
] Ratios
Figure 4. N 1s and B 1s core level spectra of the BN films ([B
0 sccm).
2 6
H ] )
5
r
[O]
5.5
[N]
[B]
B/N
0
0
0
.1
.4
.5
43
43
37.5
51.5
57
62.5
1.197
1.325
1.670
the reordering of the material leads to a lowering of the
14
normalized intensity. The variation of the normalized intensity
with the gas composition is displayed in Figure 3. For [B2H6]/
[
NH3] e 0.25, the Iam/IBN values remain practically constant in
the 0.34-0.41 range, while in the films deposited with ratios
0.25 the relative intensity increases to higher values (≈0.8).
Therefore, the change in the normalized intensity of the 1465
>
-1
cm band, when the [B2H6]/[NH3] increases above 0.25, can
be attributed to the different structure in the deposited film. This
result is in agreement with the observed shift of the B-N-B
peak position.
Additional analysis of the films was conducted using AES
and XPS spectroscopic techniques, which provide information
about the surface composition and depth profile of the layers.
In Table 2 the atomic bulk composition (O, B, N) obtained by
AES of the as-deposited samples for different [B2H6]/[NH3]
ratios is given. The B/N ratio increases when the [B2H6]/[NH3]
ratio gets higher; therefore, a boron enrichment in the films is
produced when B2H6 flow rate increases. For low ratios (Table
Figure 5. Evolution of the CVD boron nitride deposited at r ) 0.1 as
a function of the exposure time in a moist atmosphere (80%).
398.0 eV attributed to the B-N bond.15 These results indicate
that the presence of both boron and nitrogen atoms in the
material is in an atomic ratio B/N ) 1.67. Under these
conditions (r ) 0.5), the boron enrichment of the films seems
to be due to the low relative concentration of active nitrogen
ions or radicals in the gas phase.
2
, r ) 0.1), the Auger depth profile indicates a homogeneous
film composition with a small oxygen content (≈5.5%) detected
across the entire film thickness. However, the film obtained at
high values (r ) 0.4 and 0.5) contains oxygen contamination
and carbon atoms in the outer layer with only nitrogen and boron
atoms being detected throughout the bulk.
3.2. Stability of CVD Boron Nitride Films. As mentioned
above, boron nitride films deposited with higher diborane flow
rates are stable in the ambient atmosphere. However, the
composition of the films obtained using low diborane flow rates
([B2H6] e 25 sccm) continuously changes with the time of
exposure in moist atmospheres (80% moisture). The evolution
of unstable boron nitride films ([B2H6] ) 10 sccm) has been
followed using IR spectroscopy (Figure 5). As can be seen in
Using XPS spectroscopy, both the elemental composition and
atomic chemical state of the films were studied. Some carbon
and oxygen contamination was detected on the surface. Figure
4
shows the binding energy of B 1s and N 1s core levels at the
surface of the boron nitride films deposited with a high gas
ratio (r ) 0.5). The deconvolution of the peak allows us to
distinguish between two forms of boron, the B 1s signal at 188.0
eV corresponding to the B-B bond15 and the other component
at 190.6 eV being characteristic of boron nitride.15 No peak
-1
Figure 5, apart from the BN peak, centered at 1380 cm , some
-1
other features in the 1100-900 cm range in the IR spectrum
16
was observed at 193.2 eV assigned to boron oxide, B2O3. The
N 1s signal shows a shoulder at high energy (399.4 eV) due to
contamination by NO groups in the surface16 and the peak at
are observed, although no systematic variation of the features
8
has been found. These new peaks, already detected by Matsuda
17
and Cholet, correspond to ammonium borate hydrates, such

Structure and Stability of CVD Boron Nitride Films
J. Phys. Chem., Vol. 100, No. 6, 1996 2151
Figure 6. Variation of the total IR stretching band area (≈1380 cm-1)
of boron nitride deposited at r ) 0.1 Versus the exposure time in the
moist atmosphere.
TABLE 3: Atomic Composition of the Degraded Boron
Nitride Films
Figure 7. N 1s and B 1s core level spectra of unstable boron nitride
[B
2 6 3
H ]/[NH ]
[O]
[C]
[N]
[B]
B/N
film after exposure to the humid atmosphere.
0
0
0
0
.05
.1
.2
5.3
5.6
7.3
5.6
27.7
25.2
21.8
23.5
26.3
26.3
26.4
28
40.7
42.9
44.5
42.9
1.55
1.63
1.68
1.53
carbon atoms incorporated across the entire thickness. The
increase in the B/N ratio (measured by AES and XPS) and the
decrease in the amount of B-N bondings when the degradation
proceeds (as detected by IR) suggest that, during the exposure
of the films to moisture, the BN structure is attacked with loss
.25
as (NH4)2O‚5B2O3‚8H2O. Matsuda reported that this compound
is formed by hydrolysis of BN samples and later is thermally
decomposed to boron oxide, water, and ammonia molecules.
Figure 5 also indicates that, during the degradation process
of the BN films, the peak position of the broad IR band centered
1
8
of nitrogen atoms, probably by volatilization as ammonia.
In Figure 7, the X-ray photoelectron spectra on the surface
of an unstable boron nitride film with a ratio r ) 0.1 after
exposure to atmosphere are given. The B 1s and N 1s core
level spectra exhibit a shoulder at a high binding energy at 191.6
and 399.6 eV assigned to B-O and N-O, respectively. No
contribution at 193.2 eV corresponding to boron oxide was
detected. In addition, the position of the peaks for B 1s (190.6
eV) and N 1s (398.0 eV) agrees with those given in the literature
for boron nitride. The XPS analysis of the C 1s signal shows
two components, the intense one with a binding energy of
around 285.0 eV characteristic of carbon as the neutral species
(i.e., graphite) and the other signal at a binding energy of 287.0
eV assigned to carbon bonded to oxygen. The absence of the
other component in the B 1s peak at 186.7 eV, which would
at 1380 cm-1 does not change even after long exposure periods,
but its intensity strongly decreases with the exposure time. This
fact can be better appreciated in Figure 6, where the total IR
band area at 1380 cm Versus the exposure time is shown. In
this figure, a sharp decrease in the band area during the first 15
days after the deposition, followed by a slower decay, is
observed. This behavior is similar to that reported by Adams
in the films formed by pyrolysis of borazine at 300-450 °C.
By deconvolution of the broad band centered at 1380 cm ,
the changes in the film composition during the exposure to
humid atmospheres were followed. As mentioned in section
.1, the BN stretching band has been resolved by fitting the
experimental spectra to a sum of Gaussian curves corresponding
to the B-N vibration at 1372 cm and the peak at 1465 cm ,
attributed either to the B-O vibration or to the amorphous
structure of the film (see above). The variation of the Iam/IBN
-
1
9
-
1
1
9
correspond to the B-C bond, indicates that no carbon is
3
bonded to boron in the degraded films.
-
1
-1
4
. Discussion
In Figure 2, a pronounced influence of the [B H ]/[NH ] ratio
2
6
3
-
1
values, that is, the intensity of the band centered at 1465 cm
on the boron nitride deposition rate is observed. Two regions,
with low (r e 0.25) and high deposition rates (r > 0.25), can
be distinguished in the curve, and these can be associated with
two different growth mechanisms for the boron nitride films.
In a previous work concerning the temperature effect on the
boron nitride deposition (600-850 °C range) we considered
borazine and aminodiborane as the two most probable inter-
-1
normalized to the B-N band at 1372 cm , Versus the exposure
time is displayed in the inset of Figure 6. As can be seen, this
ratio increases during the first two weeks followed by its
stabilization after long exposures.
Table 3 gives the composition, as determined by AES and
XPS, of different samples deposited at low [B2H6]/[NH3] ratios,
after exposure to the ambient atmosphere for two months. One
can observe that all the unstable films (r ) 0.05-0.25)
decompose to films with a similar composition, due to a unique
degradation mechanism for all the samples. The composition
of the films after atmosphere exposure, given in Table 3,
presents the following characteristics: (a) high B/N ratio (1.53-
5
mediates for the BN synthesis. In the temperature range of
the study and for a [B2H6]/[NH3] ratio of 0.056, we concluded
that an increase in the temperature produces a change from
aminodiborane to borazine as an intermediate compound.
Moreover, the activation energy determined for the formation
of boron nitride from borazine as the intermediate compound
-1
1
.68), (b) oxygen content of ≈5.6% with no variation in relation
was 35 kcal mol , which corresponds to a mechanism
controlled by a surface reaction. Likewise, when aminodiborane
to the “as-deposited” sample (see also Table 2, for r ) 0.1), (c)

2
152 J. Phys. Chem., Vol. 100, No. 6, 1996
G o´ mez-Aleixandre et al.
is considered as the precursor in boron nitride deposition (T e
B H
+
NH3
f
B H NH
+ H2
(1)
2
6
2
5
2
-1
7
00 °C), 5 kcal mol was obtained for the apparent activation
diborane ammonia aminodiborane
5
energy. This low activation energy could be related to a
diffusion process, which would be the limiting step in the total
reaction. Changes in the step controlling process of the boron
nitride deposition reaction, when the boron source gas flow rate
However, in a medium with an excess of ammonia (or,
equivalently, a low concentration of diborane), the aminodi-
borane may react with other ammonia molecules, giving
different adduct compounds and aminoborane, which further
rearranges to yield finally borazine, according to the following
varies, also have been pointed out by Prouhet et al.20 These
authors, using boron trifluoride and ammonia gas mixtures,
showed that for low BF3 flow rates the total deposition rate is
determined by chemical reaction kinetics, whereas mass transfer
becomes the limiting phenomenon of the deposition process for
high BF3 flow rates. In the present study, the two zones
observed in the deposition rate (Figure 2) can be explained by
two different reaction mechanisms when the [B2H6]/[NH3] flow
ratio changes. For low r ratios, with an excess of ammonia
molecules, borazine molecules are likely formed as an inter-
mediate step, whereas for high ratios, the formation of amino-
diborane becomes preferential.
2
2
scheme:
B H NH
+
NH3 f B H NH ‚NH
3
(2)
(3)
2
5
2
2
5
2
aminodiborane ammonia
ammonia adduct
of aminodiborane
heat
B H NH ‚NH
8
H BNH3
+
H BNH2
2
2
5
2
3
3
ammonia adduct
ammoniaborane aminoborane
(adduct)
of aminodiborane
In association with the changes in the reaction mechanisms,
different phases of boron nitride can also be distinguished from
heat
3
H BNH3
8
2
H BNH2
+ H2
(4)
(5)
-
1
ammoniaborane
aminoborane
the IR spectra. The peak at 1080 cm characteristic of the
cubic phase has not been detected, which means that this phase
is not present in the films.21 As stated above the intense BN
ring condensation
1
H BNH2
aminoborane
8 / (HBNH) + H
2
2
3
3
-
1
stretching peak at 1372 cm
indicates the formation of
borazine
amorphous or hexagonal phases, with a band-width reduction
when the diborane flow rate decreases (Table 1), indicating an
increase in the degree of ordering of the film structure. It is
also observed in Table 1 that the band corresponding to the
Therefore, the formation of borazine molecules is then favored
at low [B2H6]/[NH3] ratios, whereas for higher ratios all the
ammonia molecules are practically consumed in the formation
of aminodiborane, impeding the reaction toward the borazine.
The mechanism described through reactions 1-5 also explains
our previous observations on the effect of temperature on the
overall reaction, since the increase in the temperature is supposed
to activate reaction 2, leading to a depletion of the aminodi-
borane molecules and a parallel increase of borazine. In any
case, the formation of borazine is a complex process involving
several steps, which explains the low reaction rate observed in
this route. On the other hand, the borazine molecule, with a
B/N atomic ratio equal to unity, is a planar hexagonal ring with
alternating B and N atoms and equal bond distances (1.44 Å).
This atomic distribution and the distance between the atoms
agrees with what is found in the turbostratic structure of the
boron nitride films. From these comments we derive the
following picture: at low [B2H6]/[NH3] ratios (r e 0.25) borazine
is preferentially formed in the intermediate stages, giving a film
with a nearly stoichiometric composition and turbostratic
structure. In contrast, at high ratios (r > 0.25) all the reactants
are consumed in the formation of aminodiborane as intermediate
species, ending up in amorphous boron-rich films.
Related to the stability of boron nitride films, as mentioned
in section 3.2, amorphous boron nitride films deposited at high
rates are stable after exposure to humid atmospheres for at least
six months. However, unstable BN films are obtained at low
[B2H6]/[NH3] ratios (r e 0.25). In the as-deposited unstable
films, the oxygen concentration, as measured by AES, is ≈5.5%
and remains constant during the degradation of the films. The
presence of oxygen in the films is not understood, since no
oxygen was introduced into the reactor during the deposition
process. We assume that the oxygen atoms incorporated in the
films come from other contamination sources after deposition.
Following the evolution of the film deposited with [B2H6]/
-
1
B-N-B bending vibration appears at 778 cm at high flow
-
1
rates with a shift to higher frequencies (793 cm ) when the
diborane flow rate decreases. Rozenberg et al.12 found a strong
correlation between the position of this peak and the structure
of the film. They showed that the B-N-B peak, which appears
in the 760-790 cm- range for amorphous boron nitride, shifts
toward higher wavenumbers when the BN network is becoming
more ordered in a turbostratic structure. Therefore, from the
shift observed in the B-N-B peak along with the abrupt change
1
-1
in the relative intensity of the 1465 cm band (shown in Figure
), it can be concluded that a change in the structure of the
3
films, from turbostratic (t-BN) to amorphous, is taking place
when the diborane flow rate increases in the range studied.
In addition to the above results, an increase in the refractive
index (n ) 1.83 for r ) 0.1) with increasing diborane flow rate
(n ) 2.12 for r ) 0.5) is also observed. The increase in the
refractive index can be explained by the boron enrichment of
the films, as has been detected by AES (Table 2). In fact, the
B/N ratio of the films increases from 1.19 to 1.67 when the
[
B2H6] varies in the 10-50 sccm range. These results agree
4
with those reported by Murarka et al. Obviously, changes in
the refractive index can also be induced by modification of the
film structure.
As mentioned above, for a given ammonia concentration in
the gas mixture, the diborane flow rate determines the inter-
mediate compound and thus the reaction mechanism for boron
nitride deposition. Both intermediate compounds aminodiborane
(B2H5NH2) and borazine (B3N3H6) show significant differences
in their chemical structure as well as in their B/N ratio. The
aminodiborane results from the formation of a donor-acceptor
bond between the ammonia molecule (weak donor) and the
electron-poor B-Hb-B double bridge of the diborane molecule:
[NH3] ) 0.1 (see Figure 6), the sharp decrease in the IR
Ht
Hb
Ht
stretching band during the first days in the humid atmosphere
can be explained by the rapid attack of the preferentially formed
Ht = terminal hydrogen
Hb = bridge hydrogen
B
B
Ht
Hb
Ht
7
turbostratic boron nitride structure. The slower decay detected
The subsequent loss of one hydrogen molecule22 thus results
in a diborane-like structure with a B/N atomic ratio equal to 2:
for high exposure periods (>15 days) has been associated with
the higher stability of the remaining amorphous component of

Structure and Stability of CVD Boron Nitride Films
J. Phys. Chem., Vol. 100, No. 6, 1996 2153
the films. According to this interpretation, the increase in the
Iam/IBN ratio with the exposure time in the first region (see inset
in Figure 6) would correspond to the continuous attack of the
BN turbostratic structure and the consequent increase in the
amorphous component of the film. For long exposure periods,
the stabilization of the Iam/IBN ratio at a high value (Iam/IBN ≈
and AES techniques. Nearly stoichiometric films result from
low diborane gas mixtures ([B2H6] e 25 sccm), whereas boron-
rich nitride films are deposited when high [B2H6]/[NH3] flow
ratios are used. The former present ≈5.5% of oxygen atoms
homogeneously incorporated in their network. The presence
of oxygen atoms in these films has been attributed to the
instability in moist atmospheres of the turbostratic structure
preferentially formed under these experimental conditions.
Finally, the evolution of an unstable boron nitride film has
been followed by infrared spectroscopy. The degradation
process of the film can be explained by the attack of the
turbostratic boron nitride phase, with the formation of am-
monium borates hydrates, which further decompose to other
compounds, such as boron oxide, ammonia, and water. Con-
sidering our previous study on the effect of temperature, we
can conclude that higher deposition temperatures require greater
diborane/ammonia flow ratios to improve the stability of the
films.
0
.6) supports the highly amorphous character of the films after
removal of the turbostratic component.
The AES analysis of the boron nitride films after depletion
of the turbostratic structure (see Table 3) indicates that the films
have a high carbon content. The carbon atoms may not be
present as carbonate ions because, in the IR spectral region
around 1400 cm- (stretching vibration mode of carbonate ions),
no increase in the absorption is detected during the degradation
process. Thus, taking into account the detection by XPS of
carbon neutral species in the graphite form, we conclude that
carbon atoms are present in the films as a graphite phase.
The attack of the turbostratic structure is easily understand-
able, since the t-BN phase is a partially ordered structure with
two-dimensional layers of hexagonal rings at regular intervals,
although there is a complete lack of orientation from layer to
layer. The properties of t-BN depend on the size of the
1
Acknowledgment. We are indebted to CYCIT for the
0
support given to this work (project n MAT92-0093).
7
crystallites and the interlayer spacing. A t-BN material with
References and Notes
a high interlayer spacing (c0/2) will present a high porosity,
which favors the penetration of water molecules in the network,
thus producing weakness of the bonds and consequently an
increase in its reactivity.18 The t-BN will be easily hydrolyzed
when water molecules are present, giving as a result ammonium
borate hydrates, such as (NH4)2O‚5B2O3‚8H2O. Our IR results
indicate that ammonium borates are actually produced in the
hydrolysis process. However, this compound can decompose
to boron oxide, water, and ammonia, thus making it impossible
to detect its IR peaks along some of the degradation stages.
The attack of the film continues until the film is mostly
amorphous, with oxygen (5.6%) and carbon atoms (25%)
incorporated in the network.
(
(
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. Conclusions
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We have studied the effect of diborane flow rate on the
composition, structure, and stability of CVD boron nitride films
deposited at 800 °C and 267 Pa from ammonia and diborane
gas mixtures, keeping the ammonia flow rate constant at 100
sccm. This work confirms our previous view that the reaction
mechanisms for the formation of the boron nitride films for a
given deposition temperature are controlled by the relative
composition of the gas mixture.5 For low [B2H6]/[NH3] ratios,
turbostratic boron nitride films were preferentially obtained at
low deposition rates (≈40 nm/min), whereas amorphous films
were formed at high deposition rates (≈160 nm/min) when high
(
(16) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moalder, J. F.;
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[
B2H6]/[NH3] ratios are used. The change in the structure and
1
in the deposition rate has been associated with the formation
of different intermediate species during the reaction, that is,
hexagonal planar ring molecules of borazine or, in the second
case, aminodiborane molecules.
The composition and the atomic chemical state in the
deposited boron nitride films have been investigated using XPS
(
J. Solid State Inorg. Chem. 1993, t.30, 971.
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JP951200D