Spectroscopic studies of BN films deposited by dynamic ion mixing

Thin Solid Films, 227 (1993) 44-53

44

Spectroscopic studies of BN films deposited by dynamic ion mixing J. P. Rivi~re, Y. P a c a u d

and M. Cahoreau

Laboratoire de M&allurgie Physique, URA 131 CNRS, 40 avenue du Recteur Pineau, 86022 Poitiers (France)

(Received October 14, 1992; accepted December 15, 1992)

Abstract Boron nitride films were deposited by the dynamic ion mixing (DIM) technique at temperatures 300 K and 820 K on silicon and NaC1 single crystalline substrates. The vapour deposition was obtained by ion sputtering using a Kauffman-type ion source. BN films of nearly equiatomic composition were synthesized when a gas mixture ½Ar + gN 2 2 (volume fraction) was injected in the ion source to make the ion beam which sputtered the BN target. The growing films were simultaneously bombarded with 160 keV Xe + ions and their microstructural state was determined by transmission electron microscopy (TEM) and infrared spectroscopy (IR). Composition depth profiles analysis was carried out by secondary ion mass spectrometry (SIMS) and the chemical binding states of B and N in the films were investigated by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (XAES). The results indicate that the films produced at 300 K without mixing are amorphous, whereas those produced by DIM exhibit the beginning of crystallization and the turbostratic structure (t-BN) was identified. When the substrate temperature was increased to 820 K, a more crystallized state was obtained, consisting of a mixture of turbostratic and hexagonal BN (h-BN) phases. The observed XPS spectra agree with the binding energy between B and N in BN, but the presence of oxygen was also detected. SIMS and XPS spectra reveal that the films where the turbostratic structure is dominant are unstable and easily decompose during air exposure whereas those of hexagonal structure are very stable. The results are discussed and compared with other studies of BN films prepared by different deposition methods.

1. Introduction Boron nitride is a material with many favourable physical and chemical properties for a wide range of applications. For instance, it is transparent to X-rays and the visible spectral region, it is highly resistant to corrosion and exhibits a high hardness and a low coefficient of friction. In addition to direct electronic applications for high-temperature active devices, BN films are potential candidate to realize X-ray lithography masks in order to produce submicron structures for very large scale integrated circuits [1]. There are also promising applications of BN coatings for use as hightemperature, wear-resistant and hard lubricating films [2, 3]. A wide variety of preparation methods have been used, but BN films can be grown in many different phases. The crystalline structures of BN are very similar to those of carbon, since boron nitride exists in the hexagonal (h-BN) and cubic (c-BN) phases, but it must be emphasized that only c-BN presents the most interesting properties for potential applications, because it is a superhard phase analogous to diamond. However, it is difficult to synthesize c-BN and it is only very recently that Yoshida et al. [4, 5] have prepared c-BN films by r.f. sputtering. Iwaki et al. [6] have also real-

0040-6090/93/$6.00

ized c-BN films by ion beam enhanced deposition using the ion sputtering of a boron target. The most commonly obtained phases are the hexagonal or amorphous ones; however, other structures have been also observed, such as the rhombohedric (r-BN) or turbostratic (t-BN), but they are not generally stable [7-9]. Chemical vapour deposition techniques (CVD) usually require high temperatures ( T > 1000 °C) which is a limitation for many applications, particularly when the substrate is a heat treated high strength steel [10, 11]. Physical vapour deposition (PVD) methods [12-14] and ion-assisted techniques have been also applied for BN film synthesis [15, 16]. Another promising preparation method for ceramic coatings at low temperature is the new dynamic ion mixing technique (DIM) [17]. The particularity of the DIM technique is to use a high-energy (100-400 keV) heavy ion (Ar +, Xe +) beam in order to produce atomic mixing, via localized energy deposition, of target atoms. One of the most important features of this surface alloying process is to produce an important mixing of the film/substrate interface in the first steps of the coating deposition, resulting in an intermixed layer of graded composition which improves markedly the adhesion performance of the film [18, 19]. Moreover, the high atomic displacement rate achieved during heavy ion bombardment

((~ 1 9 9 3 - Elsevier Sequoia. All rights reserved

d. P. Rioidre et al. / Spectroscopic studies of BN fihns

can be also considered as a modification process itself to control the microstructural state of the film. For instance, the crystallization of a TiB: coating can be obtained at temperatures much lower than those required in a conventional thermal treatment [20]. The identification of the parameters that determine the growth process and the resulting structure of the films are important for potential applications. On the other hand, the stability of the films after exposure to air is also an important parameter. Effectively, it has been observed during previous experimental studies that a considerable amount of oxygen and carbon is often present in BN films whatever the preparation method: ion beam deposition [21], r.f. sputtering [10] or the pulse plasma method [22]. In addition, it has been shown that some BN films could react with a moist atmosphere, particularly those of turbostratic structure

[13]. The objective of the present work is to investigate the structure and stability of BN films grown by DIM with respect to their preparation conditions: deposition temperature and ion mixing during film growth. Infrared spectroscopy, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) studies have been carried out, and further characterization of the films has been performed by transmission electron microscopy (TEM).

2. Experimental details 2.1. Film preparation The BN films were deposited by a sputtering method using a broad beam ion source of Kauffman type. The experimental apparatus has been described previously [17, 18] and only a schematic diagram is presented in Fig. 1. The base pressure in the sputtering chamber is better than 5 x 10-5 Pa before deposition and increases to 10-2-10 -3 Pa during deposition. A 99.9% pure hot pressed BN target of 10 cm in diameter is used in these experiments. The energy of sputtering ions is 1.2 keV and the beam intensity is 75 mA. We used a gas mixture ½Ar + ~N2 (volume fraction) in order to produce BN films of nearly equiatomic composition as previously verified by electron energy loss spectrometry (EELS) [23]. The deposition rate monitored via calibrated quartz crystal oscillator is very low (about 2 x 10 -2 nm s-1). In addition, the film thickness and its density are measured after deposition by a grazing X-ray reflection technique consisting of measuring the critical angle or total reflection, which are directly related to the film density [24]. The films are deposited on different substrates: either silicon or freshly cleaved NaCI single crystals. The Si substrates are etched in diluted H F and cleaned by ultrasonic agitation in ethyl alcohol followed by rinsing in de-ionized water and drying under a dry

THICKNESS

MONITOR

Xe +

~j

t60 K e V

memm~lilh b'~ , ~ L 1

ROTATING or HEATING SUBSTRATE HOLDER

I 1 | ! ~

FKAUFHAN SOURCE GAS MIXTURE

BN TARGET

AP+N 2

VACUUM I SYSTEM "

Fig. 1. Schematic representation of the DIM apparatus.

45

J. P. Rivid,reet al. / Spectroscopic studies of BN films

46

TABLE 1. Deposition conditions and corresponding structure (mixing ions: Xe ~ 160 keV, ion per atom arrival rate ratio Q _~ 3.5 × 10 3) Series

Deposition conditions

Structural state

Thickness e (rim)

At

Without D I M 300 K

Amorphous

149

A~

DIM 300 K

Turbostratic t-BN

134

A3

Without DIM 820 K

h-BN + t-BN (dominant)

122

A4

D I M 820 K (Xe ÷ 160 keV)

t-BN + h-BN (dominant)

134

nitrogen flow. The ion mixing was performed using 160 keV Xe + ions accelerated by a 200 kV ion implanter operating in line with the deposition chamber. Very low ion currents, in the range of 20-50 nA cm -2, were used in order to prevent beam heating of the growing film. The characteristic parameters of ion implantation were calculated by Monte Carlo simulations using the T R I M computer code [25]. We have also calculated the damage profile in the films as a function of depth, and it is found that a uniform damage level of about 7 dpa (displacements per target atom) is obtained throughout half the film thickness but decreases toward the outer surface. The preparation conditions of the different series of BN films are summarized in Table 1. 2.2. Methods of characterization The microstructural state of the films was first analysed by T E M with a JEOL 200 CX electron microscope using the specimens deposited on NaC1. Additional information was also obtained by transmission IR spectroscopy with a Perkin-Elmer 125 spectroscope. The chemical composition of the films was determined from specific techniques for light element analysis, such as EELS [26]. Nearly stoichiometric films were produced with B/N - 0.98, but the films contain in general significant amounts of oxygen ( ~ I0 at.%) and carbon ( 3 - 4 at.%) [20]. SIMS and XPS experiments were carried out in an ISA RIBER instrument equipped to allow alternative SIMS and XPS analyses in the same chamber (vacuum ~ 2 × 10-8 Pa) by a simple 90 ° sample rotation. The SIMS system was a microprobe MIQ 156 with a quadrupole mass filter. The XPS system was equipped with a MAC2 analyzer. Depth profile analysis was undertaken using an Xe + primary ion beam with an energy of 5 keV and a current density of 8.5 × 10 . 3 mA cm -2 at an incident angle of 45 °. The sputtered area was ~- 340 x 480 gm ~, and positive secondary ions were detected. A 1253.6 eV Mg Kc~ radiation source was used in XPS studies to obtain the binding energies of Cjs, O~s,

Bls and Nls core photoelectrons and Auger lines in the series KVV (B and N). The instrument was used at high resolution and the peak position was determined with an uncertainty of 0.2 eV. The characteristic XPS spectra of the chemical elements of the films were obtained after sputtering the thin oxidized surface layer ( < 5 nm) and successive erosions also allowed in-depth XPS analysis to be performed.

3. Results

3.1. Microstructural study by TEM and IR spectroscopy When deposited at 300 K without ion mixing (sample A0, the BN films exhibit an amorphous structure, since only two diffuse rings are visible in the diffraction pattern of Fig. 2. The morphology of the as-deposited film is also presented in the bright field micrograph of Fig. 2 and we can see a highly uniform contrast, characteristic of an amorphous structure (a-BN). Figure 3 illustrates the structure of a film (A2) produced by DIM at 300 K using 160 keV Xe + ions, and one can see from the diffraction pattern that the rings are less diffuse, suggesting the beginning of crystallization. However, the contrast in the bright field picture is still highly uniform. The interplanar distances deduced from the measurements of the successive ring diameters correspond reasonably well with those of the turbostratic phase (t-BN) determined by Dugne et al. [9]. The deposition temperature has been increased to 820 K in series A 3 and A 4. The microstructural state of BN films deposited without mixing at 820 K (A3) is presented in Fig. 4. The contrast is not uniform, and small disoriented crystallized grains are observed; the diffraction

Fig. 2. Bright field TEM image and diffraction pattern of a BN film deposited without DIM at 300 K (sample A 0.

47

J. P. Rivibre et al. / Spectroscopic studies o f B N fihns

Fig. 3. Bright field TEM image and diffraction pattern of a BN film deposited with DIM (Xe ÷ 160 keV) at 300 K (sample A2).

U Fig. 4. Microstructural aspect of a BN film deposited at 820 K without DIM (sample A3). pattern exhibits very sharp concentric rings, mostly corresponding to a crystallized state of hexagonal structure. However, additional rings are also detected, some corresponding to the turbostratic phase (t-BN), but others being very probably due to carbides (B4C) or oxides (B1302). These impurities are probably due, for a great part, to adsorption of oxygen or fixation of carbon compounds after deposition. Film A4 was produced by D I M at 820 K and Fig. 5 illustrates the structure of this film. The presence of crystallized grains a few nanometres in diameter is observed in the bright field image. The diffraction pattern indicates that the dominant phase is the turbostratic one, but some rings can be attributed at the hexagonal structure, and the presence of carbides and oxides is also detected. H o w ever, the lattice spacings of the different phase~ h-BN, t-BN and also c-BN are very close and the measurement accuracy is not sufficient to attribute unambiguously some rings to one phase rather than to another

Fig. 5. Microstructural aspect of a BN film deposited at 820 K with DIM (Xe + 160 keV) (sample A4). one; in addition, the presence of carbides and oxides complicates the determination. Complementary information on the film structure can be deduced from IR transmission spectra. Experiments have been performed with films deposited on both Si and NaC1 substrates; however, owing to the presence of a n additional S i - O peak at 1000-1100 cm -~ with Si substrates we will present only the I R spectra obtained on NaC1 substrates. Effectively, h-BN spectra exhibit two peaks, one in the range 1370-1400 cm -~ attributed to B~-N bond stretching (in-plane) and the other in the range 750-780 c m - ~ associated with B - N - B bending (out of plane) [27]. In the case of t-BN or a-BN, there is generally a broadening and shifting o f these two peaks [23]. In contrast, c-BN exhibits only one peak in the range 1050-1100 c m - t [28]. The I R spectra recorded for the different films are presented in Fig. 6. We can

I

I

I

~

I

I

I

I

A1 A2 A

u

3

~

< 1370 I

1600

II

780 I

1400

I

1200

J

I

1000

I

J[

800

I

600

WAVENUMBER (cm -1 ) Fig. 6. IR spectra of the different BN films deposited on NaCI substrates.

J. P. Rivi#re et al./ Spectroscopic studies of BN films

48

see that all of them exhibit two peaks, but that it is only in the case of film A 3 that the absorption peaks are sharp and correspond well with the theoretical values of the hexagonal phase. The peaks of spectra A1, A2 and A 4 are very broad and slightly shifted towards lower wavenumbers, suggesting that the amorphous or turbostratic phases are the dominant phases, which is in good agreement with the preceding TEM observations [23]. The principal results of the microstructural study are summarized in Table 1. 3.2. SIMS and XPS results of as.deposited films For this study, the samples were transferred immediately after preparation to the analysis chamber to prevent the formation of surface oxides. Representative depth profiles recorded for the different BN films deposited on Si substrates are shown in Figs 7 and 8 for samples Az and A4 prepared by DIM either at 300 K or at 820 K. The SIMS profiles corresponding to samples A~ and A3 deposited without mixing are very similar to the previous ones and are

.

7~

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6~

~

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25

Z9 0

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.

.

.

1000

2000 Time (s)

Fig, 7. SIMS depth profiles of different elements in a BN film deposited with DIM at 300 K (sample A2).

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29

not shown here. Thus the intensity profiles of "B ~- and ~4N* secondary ions are very uniform down to the substrate for samples A3 and A4 deposited at 820 K (Fig. 8). The only difference we have noticed between A3 and A4 concerns the '60+ profile which exhibits an important increase in sample A3 at the film/substrate interface. In the films prepared by DIM, this oxygen peak at the Si interface is smaller (Figs. 7 and 8) because of the interface mixing effect, which produces a graded interface. The signals of all ions under study appear very low (Fig. 7) at the beginning of the sputtering and increase after an erosion time of t = 350 s in the case of sample A2 for '1B + and '2C+. This result can be correlated with measurements of the sample current performed on the four samples, indicating that A~ and A2 samples appear to be gobd insulators, particularly near the surface where the conductivity is low. The sample current then increases with the erosion time in the same way as the signal of the profiled ions in Fig. 7. Such behaviour could be explained by the difference in the structural states of the different films (Table 1), but is more likely to be due to the presence of important amounts of oxides and carbides in the surface layer of samples Az and Az. In all the samples, we observed the enhancement of the 29Si+ signal by the presence of native oxygen on the Si wafer, which was then modified by the mixing conditions so that the nature of the Si-O bonding at the interface may be different from one to another sample. In order to characterize qualitatively the Si-O bonding, we have calculated the ratio r of the enhanced 295i+ signal at the interface over the unenhanced one in the Si substrate, where there is no oxygen. The results are given in Table 2, together with the erosion times ts necessary to reach the Si substrate. The values of r are not the same for the different samples, suggesting that the Si-O chemical bonding is not identical in all the samples. Conversely, very comparable values of ts are found, since the different films have nearly the same thickness (see Table 1). One can conclude that the average rate of erosion is the same for all the films, whether they are insulating or not. In addition, we have made several profiles for each sample at different places on the film surfaces and verified each time the reproducibility of the analysis; thus the films are homogeneous in the lateral dimensions.

0

o

11

~~0

~2

I000

2000 Time

(s

)

Fig. 8. SIMS depth profiles o f different elements in a BN film deposited with DIM at 820 K (sample A4).

TABLE 2. Interface analysis: r is the ratio of29Si + at the interface and in the "wafer" Series

Erosion time to reach Si wafer ts (s)

r

AI A2 A3 A4

1406 1373 1426 1413

45.5 45 11.63 3.56

J. P. Rioigre et al. / Spectroscopic studies of BN films

positions were identified by comparison with the energy values reported in the Handbook of X-ray photoelectron spectroscopy [29]. These values are given in Table 3. Figure 9 shows the plasmon loss structure associated with the N,, and B,, photoelectron lines in samples A,, A2, A3 and A4. The peak positions, their full width at half maximum (FWHM) and their identification are summarized in Table 4. If we consider first the binding energy of N in BN, deduced from the Nts spectra, there is excellent agreement with the expected value corresponding to N in h-BN for sample A3, whereas for all the other samples (At, A2 and A4) this peak is shifted toward higher values, which are closer to the binding energy of N in t-BN. This peak shift in samples A1, A2 and A4 could also be attributed to the presence of N - O and N - O H bonds, since the FWHM are also larger than expected. In all the N,, spectra, the bulk plasmon peak (BP) is present; on the other hand the Nis n plasmon peak (nP) is very weak in sample A2 (mainly of t-BN structure) and is not detected at all in sample AI (amorphous). The energy losses of BP and nP in samples A 3 and A4 are in good agreement with the values of 25 eV and 9 eV found previously by Trehan et al. [30]. We have noticed in all the spectra a small hump between the BP and nP which also exists in the spectra

TABLE 3. XPS binding energies of N and B in BN form [29] XPS peak

Compound

Binding energy (eV)

FWI-IM (eV)

h-BN t-BN

397.6 398.5

1.6

BI,

h-BN B B203 B4C B(OH)3

190.3 187.0 193.1 186.3 192.8

1.45

Si2p

Si SiO2

99.15 103.4

1.4 2.20

Na,

The kinetic energies of the C~,, O,~, N1, and Sizp photoelectron lines were recorded for all the films either in the as-deposited state or after removing the contaminated surface layer. The analysis was performed in a relatively large energy range in order to obtain both the plasmon n peaks (bulk: BP; r~ plasmons: riP) and the N(KVV) and B(KVV) Auger transitions. The binding energies are determined by reference to the C,, binding energy due to adventitious carbon that is always in the film, which is known to occur at 284.6 eV. In the absence of a high-purity BN standard sample, the peak

Nls

Bls

Nls

B1s

Iol

~'l BP

49

(c) BP

BP o

BP

S. i

kO0 BindingenergyleVI

260

~oo

Binding energy(eV)

Nls

Nls

Bls

(d)

(b} BP

BP

o i.d

Bls

BP

BP

i

.oo

BindingenergyleVI

Binding energy leVI

Fig. 9. XPS spectra of NI, Bls and related plasmon structures of the different BN films: (a) sample AI, (b) sample A2, (c) sample A3, (d) sample A4.

J. P. Rivibre et al. / Spectroscopic studies o f B N films

50

TABLE 4. XPS data of the BN films with identification of the different peaks XPS data

A 1 binding energy (eV)

A 2 binding energy (eV)

A 3 binding energy (eV)

A 4 binding energy (eV)

Identification

N I~

420.17 413.69 400.45 398.33 ( F W H M = 1.85)

421.02 413.38 409.54 3 9 8 . 3 1 ( F W H M 1.85)

419.60 412.16 405.10 397.70 ( F W H M 1.85)

420.60 412.31 407.02 398.09 ( F W H M 1.85)

BP SP nP N in BN

Bis

212.78 -203.15 192.06 ( F W H M = 2.65)

214.22 -203.06 192.50 ( F W H M = 2.9)

214.88 206.75 199.18 190.33 ( F W H M = 2.5)

214.44 207.81 199.27 191.03 ( F W H M = 3)

BP SP nP B in BN

of Trehan et al. [30]; it had not been identified before, but it could correspond to surface plasmons (SP). Concerning the Bls peak, the value of the binding energy is still in excellent agreement with the theoretical one of B in h-BN for sample A 3, but the F W H M of the peak is larger. In all the other samples the binding energy of the Bls line is shifted towards higher values, as has also been observed for the Nts peak. In addition, the F W H M values are larger than expected in all the samples. This behaviour is very likely to be due to the presence of both the turbostratic phase in significant amounts and B - O bonds, as previously suggested by Dugne et al. [9], Aleskin et al. [31] and Lacrambe [32]. The BP and the nP are detected in all spectra even if the nP peak is weaker for samples A t and A 2. Since this later corresponds to sp 2 hybridization (graphite-like behaviour), the observation of well-defined n P peaks in s a m p l e s A 3 and A 4 confirms the presence of h-BN in these films. 3.3. Film stability after e x p o s u r e to air The three films A2, A3 and A 4 w e r e analysed after exposure to air for times varying between 30 h and 240 h. However, film At, which is of amorphous structure and which contains a significant amount of oxygen, as indicated by the above XPS spectra, exhibits a similar behaviour to A2 (t-BN), as indicated by a Rutherford back-scattering study [33], and its behaviour during exposure to air is analogous. We have performed the SIMS analysis after exposure to air with the same experimental conditions as on the as-deposited films. Very different results were observed, as one can see in Fig. 10. First, when examining the depth profiles of sample A3, we cannot detect any modification even after two air exposures of 30 and 72 h successively; the SIMS profiles are strictly identical to those presented in Fig. 8 obtained just after deposition. We can conclude that this film of hexagonal structure i s v e r y stable during exposure to air in ambient atmosphere. On the contrary, the SIMS profiles of films A2 and A 4 a r e completely modified (Fig. 10(b,c)) in comparison with those of Fig. 7 before exposure to air. We see that

the erosion time required to reach the Si substrate in sample A 2 is very short: t s ~ 2 2 0 s instead of 1373 s before e~posure (Table 2). In addition, we can see that the maximum signal levels of tzc+ and 160+ ions are markedly increased. These results indicate that film A 2 is unstable and progressively decomposes in air, its thickness decreasing until after 30 h of exposure it is only a few nanometres thick. An XPS analysis was also carried out with this film (A2) and the results are summarized in Table 5. The first important point to notice is the presence of a n Sizp peak arising from both SiO2 and Si; thus we can estimate the remaining thickness of the film to be about 3 nm, since the depth resolution of XPS is about 3 nm. The N~s peak is shifted toward a binding energy higher than the value of N in h-BN; however, it could correspond to N in t-BN, given by Rother et al. [34]. The large F W H M indicates also the presence of N - O or N - O H bonding. The Bts peak was decomposed into three gaussian peaks at, respectively, 186.06 eV, 192.02 eV and 187.32 eV. The second one is attributed to B203 or B(OH)3, and is the most important, suggesting that the majority of B atoms are bonded with O and that there are likely very few B atoms bonded with N. The B(KVV) Auger spectrum of film A2 after 30 hours of air exposure is presented in the derivative mode in Fig. 11. In comparison with the results of Hanke et al. [35] we can identify unambiguously the presence of B203 and also of some BN. For film A4, the SIMS profiles are also strongly modified after 240 h of air exposure (Fig. 10(c)) since they are very similar to those of film A 2 (Fig. 10(a)) and the erosion time to reach the Si substrate is also very short (t s ,~ 180 s); thus the BN film has practically disappeared in the same way as for film A2.

4. Discussion

The different experimental results presented in this study demonstrate the influence of both temperature deposition and dynamic ion mixing on the structure of BN films, since they are in a structureless amorphous

J. P. Rivi~re et al. / Spectroscopic studies o f BN films 7

I

7 6

,

*>I

6,

51

% o 7,16~.

*

°ll

i#1

~S

% o

Lt'

+ %

u

o~3. %

._J

2'

2' I-

..in.

bl

(al -

,

-

.

.

1000 Time i s )

.

Z000

.

.

.

2000

1000 Time (sI

•o

--5. u~

:-..

%4

g

+

C+

iZ

0

a

0+

16

0

x

N+

It,

0

o

B-

11

-it0

-,,-

Si ~

29

0

~3, .ll. ...l

2'

(c} 0

:000

1000 Time (s)

Fig. 10. SIMS depth profiles for films A2, A3 and A4 after air exposure. (a) film A2 after 30 h, (b) film A 3 after 102 h, (c) film A4 after 10 days.

TABLE 5. XPS data of film A2 after air exposure (30 hr) XPS peak

Binding energy (eV)

FWHM

.o

(eV) N],

Bt~

422.04 413.79

8203

Identification

398.52 399.75 400.33 404.84 (w) a

2.29 2.49 2.98

BP SP N in t-BN NH4NO3 (possible) 9.

189.06 (w) 192.06 192.02 187.32 (w)

2.54 2.34 1.34

B in t-BN B(OH)3 or B:O 3 B

1S8) 3Z /'~

flz03

i

I

lz,8.67

I

-SO.-'/

b.uger KVY

/ \ B,0,

s

/

,, ,v

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i~ /

a(w) = weak intensity. -lO0

state (a-BN) after deposition without mixing at 300 K, whereas the crystallization is induced either by increasing the substrate temperature or by bombarding the growing film. With the experimental conditions used, the BN films consist generally of a mixture of two phases h-BN and t-BN with additional contaminants, such as oxides or carbides, due probably to the absorption of oxygen and fixation of carbon compounds after deposition. The possible influence of the substrate on the microstructure of the film cannot be ruled out.

175-5~, i

130

,

0

,

J

,

J

.

L

,

~

150 Kin.

.

l

.

i

,

energyleVI

~

170

.

i

•

l

.

l

*

190

Fig. I 1. Typical Auger spectrum of the B(KVV) transition for film A 2 after 30 h of air exposure, indicating the presence of B203.

However, the IR studies have been performed on both Si and NaCI substrates and give identical results; moreover, all the analysis techniques have shown that sample A3 deposited at 820 K without DIM is essentiallyof

52

J. P. Rividre et al. / Spectroscopic studies o f BN filrns

hexagonal structure and both B and N XPS spectra exhibit clearly the rc plasmon peak characteristic of sp 2 hybridization, which is the fingerprint of h-BN. Even if some amount of oxides and carbides is present in film m3, it appeared to be very stable during lengthy air exposure at room temperature. Conversely, all the other films, where the dominant phase is not the hexagonal but the turbostratic or amorphous phase, are not stable in air and are easily decomposed. ~uch behaviour has been observed many times in previous studies with BN films prepared by the ion beam deposition method [21], pulse plasma [22] or r.f. sputtering [13]. The intrinsic cause of the reaction of BN films with a moist atmosphere has been given by Rand et al. [36] and Smolla et al. [37]. They have reported that the presence of oxygen during deposition leads to the formation of boron oxide, which is very sensitive to moisture according to the hydrolysis reaction: B 2 0 3 q- H 2 0

' 2B(OH)3 (volatile)

(1)

Thus it appears that a low oxygen content is important for producing stable BN films. However, this condition is not enough, since Kikkawa et al. [13] have also previously observed that BN films exhibiting the t-BN structure are very unstable compared with h-BN ones. They measured that the amount of t-BN gradually decreases with increasing the air exposure. The present results (SIMS and XPS) about the film stability in air of films A2 and A4, which consist respectively of t-BN and a mixture of t-BN and h-BN, are very similar to those of Kikkawa et al. It can be concluded that the presence of oxygen and the turbostratic phase are the two factors leading to BN film instability in air. However, the exact mechanisms of the t-BN phase instability in air are not well understood. The turbostratic boron nitride is a two-dimensional variant of the h-BN structure [38, 39], i.e. the B3N3 hexagons are disposed in layers parallel to the (0001) crystal axis. The h-BN structure consists of interlocking hexagonal B and N sublattices displaced by c/2 (unit cell a = 2.504 A and c = 6.661 A). The interlayer spacing is equal to 3.331 A in h-BN, whereas in t-BN it is greater by up to 7% than the ideal value of h-BN [40]. It is possible that stacking disorder does not contribute only to expansion of the interlayer spacing, as suggested by Aita [39], but also to an increased retention of oxygen during deposition. There is another, result which could appear contradictory: the fact that film A4 produced by DIM at 820 K is not of completely crystallized h-BN structure, whereas film A3, prepared at the same temperature, but without DIM, is mainly of h-BN crystallized structure. Effectively, it is known that the crystallization of amorphous films under ion irradiation can occur at temperatures lower than those required by a thermal process alone. For instance, it has been recently shown that

crystallization of TiB2 coatings can be induced by DIM at room temperature, while they are amorphous when deposited without DIM [20]. The reduction of the crystallization temperature is considerable in the case of TiB2, since the amorphous states crystallizes by a thermal process only at 1170 K. However, ion irradiation of a growing film produces two opposite effects: (1) a disordering effect due to atomic displacements and replacements in collision cascades, and (2) an ordering effect, which is a consequence of the point defect mobility when the irradiation temperature is higher enough. Thus, there is competition between these two processes: irradiation-induced disordering and thermally activated crystallization. Depending on the irradiation temperature, one process can predominate and there is a critical temperature above which the second process dominates. This temperature is not known for BN, but according to our results, it is certainly higher than 820 K. In order to verify this assumption, further experiments are in progress using a heating substrate holder reaching a temperature higher than 820 K.

5. Conclusion

BN films of nearly equiatomic composition have been synthesized by sputtering a BN target with Ar ÷ and N2+ ions. We have shown that the deposition temperature and the ion mixing effect during film growth have important consequences for the resulting structure. However, the films obtained are either of hexagonal structure or a mixture of h-BN and t-BN, and this last one is not stable in air. It is suggested that to obtain BN films of good quality with the h-BN structure, a temperature higher than 820 K is necessary with our method of preparation.

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