- Email: [email protected]
Growth and characterisation of boron nitride thin films
Surface
and Coatings
Technology
84 (1996)
383-387
Growth and characterisation of boron nitride thin films M.F. Plass *, W. Fukarek, A. Kolitsch, U. KreiBig Forschungszentrum
Rossendorf e.V., Znstitutfiir
Zonenstrahlphysik
und MateriaJjorschung,
Postfach 51 01 19, 01314 Dresden, Germany
Abstract Boron nitride layers were deposited using ion beam assisted deposition with different ion energies (0.5 or 1.5 keV) and normal angle of incidence on silicon wafers heated to 400 “C. By mounting the electron beam evaporator at an angle of 56” to the normal of the substrate, it was possible to investigate the influence of the continuously changing ion-to-atom ratio on the grown modification and, subsequently, to determine the properties of the BN films. The coatings were characterised using polarised IR reflection spectroscopy, heavy ion elastic recoil detection and microhardness measurement (Vickers indenter). The dynamic hardness of the Si substrate could be almost doubled with an approximately 1000-A thick c-BN coating. The transition determined by IR spectroscopy from non-cubic to cubic growth could be clearly resolved across the 3-inch silicon substrate. As soon as the spectra recorded with p-polarised light exhibited the reflection band of the cubic phase at 1100 cm-‘, an additional feature appeared in the in-plane stretching h-BN mode near 1580 cm-‘. The two features of the h-BN LO band might be due to the layered growth of the BN system corresponding to two different dielectric constants of the h-BN layer near the interface and at the sp’-bonded grain boundaries of the c-BN-containing overlayer. Furthermore, coatings with a high cubic fraction exhibited a stoichiometric boron-to-nitrogen ratio with small oxygen and hydrogen contaminations. Keywords: Boron nitride; Thin films; Polarised
IR reflection
1. Introduction Cubic boron nitride (c-BN) is the second hardest material known, after diamond, and thus has a huge potential for practical applications. The special advantages of c-BN [1,2] with respect to diamond are its inertness to iron, even at high temperatures, and the possibility of both p- and n-type doping. However, successfulsynthesis of well-crystallised c-BN films is still more difficult than for diamond. To gain a better understanding of the deposition process and ultimately to produce high-quality c-BN films, we investigated the influence of deposition parameters on the film properties. In the following, the results of polarised IR spectroscopy (PIRR) and dynamic hardness measurements on BN films deposited with ion beam assisted deposition (IBAD) are presented. The geometry of the deposition set-up was adjusted to study the effect of varying the ion-to-boron ratio on a single silicon wafer. The elemental depth distribution of a c-BN-containing film was determined with heavy ion elastic recoil detection (HIERD). To simplify matters, no distinction is made * Corresponding 591 2703; e-mail:
author. Tel.: +49 [email protected].
0257-8972/96/$15.00 0 1996 Elsevier SSDZ 0257-8972(95)02781-5
(351)
591 3422; fax:
f49
(351)
Science S.A. All rights reserved
between the various non-cubic BN phasesformed during thin film deposition, such as hexagonal, turbostratic or, as recently discovered, rhombohedral [ 31 boron nitride. The term h-BN is applied to the sp2-bonded, layered, and near-stoichiometric BN modifications.
2. Experimental set-up Boron nitride layers were deposited using IBAD with different ion energies &,, (0.5 or 1.5 keV) at normal angle of incidence on 3-inch Si (n-type and (lOO)oriented) wafers heated to 400 “C. A schematic representation of the deposition system with the three main parts (ion source, substrate holder and electron beam evaporator) is shown in Fig. 1. Boron was deposited with spatially varying rates by mounting the Si substrate at a distance of 40 cm and an angle of 56” to the normal of the evaporator. The boron deposition rate R, without ion bombardment (substrate temperature Ts = 400 “C), asmeasuredwith a profilometer on thick films, decreased nearly linearly from 0.055 to 0.035 nm s-l across the sample with increasing distance from the evaporator. This corresponds to an increasing ion-to-atom ratio, as also illustrated in Fig. 1. Using this, the influence of a
384
M. F. Plass et al. JSurface
and Coatings
Technology
84 (1996)
Table 1 Deposition conditions are given in the text IncreasIng to
atom
Ion
quartz
oscillator
?an--,I-rz7zz
10” source
n
illustration
BN samples.
Further
details
Ion current Density j,,, (PA cm-*)
N2: Ar gas Nitrogen-to-boron flow ratio arrival ratio range
Ion-to-boron arrival ratio range
A B C D
120 90 90 70
1:2 1:l 1:2 1:l
2.0 1.6-2.1 1.4-1.8 1.3-1.6
1.0 1.1-1.4 0.7-0.9 0.9Sl.O
m shutter
electron evaporator
Fig. 1. Schematic
of the analysed
Sample
ratio
\fllament
383-387
of the deposition
beam
set-up.
continuously changing boron deposition rate RB and the consequently changing ion-to-atom ratio on the BN film properties could be determined. Doing so, no experimental modifications obstructed the attempt to investigate the parameter range for c-BN deposition. The temporal regularity of the boron evaporation was controlled by a quartz oscillator. The base and the working pressure were 5 x 1O-6 and <2 x lop4 mbar, respectively. The large distance of 70 cm between the 5-cm Kaufmann ion source and the substrate resulted in a homogeneous argon and nitrogen ion flux. The ion current density jlon was measured with a Faraday cup biased to 30 V to prevent the emission of secondary electrons. The N,:Ar gas flow ratio was varied between 1:l and 1:2. The subsequently resulting N2+:Ar+ ion ratio was assumed to be the same as the gas flow ratio, as the ionisation energies are similar. Without additional heating the substrate temperature T, increased up to nearly 300 “C for unheated substrates and saturated during the first minute of ion bombardment (E1,,= 1.5 keV, jIon = 100 PA cm-‘). The samples were mounted in the deposition chamber immediately after a HF dip. The BN deposition time was 1 h, except for sample D where it was 2 h. Only sample A was deposited with a lower energy EIon = 0.5 keV. The various deposition conditions and the ion-to-atom ratios of the different samples are listed in Table 1. IR spectroscopy is the established standard analysis technique for BN films, as the resulting peaks are related to either the cubic or the non cubic phase. The BN coatings were investigated with IR spectroscopy with sand p-polarised light in the range between 800 and 7000 cm-‘. With a CaF, wire grid polariser, the out-ofplane bending mode of h-BN could not be investigated. The angle of incidence was 60”. The spectra were recorded relative to the reflection of silicon and corrected afterwards with a calculated Si spectrum. A detailed description of the experimental set-up of
the elastic recoil detection and the theoretical analysis of the measured data to gain information about the elemental depth distributions of the light atoms or the film density is found in Ref. [4]. Dynamic microhardness measurements were carried out with a Vickers indenter with a loading speed of 29 pN s- 1 and a load of 1 or 2 mN. The resulting penetration depth was in the range between 40 and 80 nm, which is up to 80% of the BN layer thickness. Therefore, the absolute values of the measured hardness reflect a strong influence of the underlying Si substrate. Thus, all hardness data for the BN/Si layer system should be only compared with those for non-coated silicon. The hardness of a clean Si wafer was determined to be 1415 f 142 or 1292 f 50 kp mmU2 for loads of 1 or 2 mN, respectively.
3. Results and discussion By comparing the atomic area1 densities measured with HIERD and the layer thickness, it is possible to evaluate the absolute atomic concentrations of the coating. Doing so, the density and the contamination of a boron coating deposited without ion bombardment (T, = 400 “C) were determined. The boron concentration is 1.0 x 1O23cmm3, corresponding to a density of 1.8 g cme3, and the oxygen, carbon and nitrogen contaminations of 3.7%, 2.1% and 0.5%, respectively, are constant throughout the layer. The measured boron density is comparable to the result of Tanabe et al. [S], albeit 25% less than the density of crystalline bulk material. This lower density of boron deposited without ion bombardment has to be considered when the boron arrival rate is estimated from measured film thickness and deposition time. The depth distributions of light elements in sample A was also investigated with HIERD (see Fig. 2). The nitrogen-to-boron ratio of the film is close to one. Nitrogen enrichment can be observed near the surface. No carbon contamination is detectable. The oxygen and hydrogen contaminations are approximately 1%. Fig. 3 shows the corresponding PIRR spectra of sample A. The s-polarised reflection spectrum is very
M. F. Plass et al./Surface
and Coatings
Technology
84 (1996)
383-387
385
boron deposition
rate
I6 60
depth
(rel.
units)
I
IO00
Fig. 2. Elemental
depth
distribution
of sample
A (R, = 0.049 nm s-i).
,
I
I
I200
I
I400
wsavenumber
I
I600
IO00
2000
3000
wavenumber
(cm-‘)
Fig. 4. PIRR spectra monitoring growth of sample B. The upper ion-(nitrogen and argon)-to-boron approximately 1.8.
(cm-‘)
the resputter threshold for cubic spectra are shifted for clarity. The atoms resputter threshold is
16 65
sample
t.1.
1000 Fig. 3. PIIR
,,,I v
I500
*I.,
wavenumber
(cm-‘)
reflection
spectra
A
--__--_ I$ 2000
of sample
A.
similar to transmission spectra, whereas the p-polarised spectrum proves to be more dynamic and exhibits new features. The transverse optical (TO) bands of c-BN and h-BN can be seen at approximately 1100 and 1400 cm-‘, respectively. The double-peak structure of the h-BN TO mode is due to the anisotropy of the hexagonal crystal. In the spectrum recorded with p-polarised light, the longitudinal optical (LO) modes can be distinguished at 1300 cm-’ and 1600 cm-‘. In the following, the advantage of BN layer deposition with a spatially changing boron deposition rate R,, as described in the second section, is used to demonstrate the influence of the varying ion-to-atom ratio on the grown phase. The change of the reflection spectra at the resputter threshold for high ion-to-atom ratios with changing boron deposition rate is shown in Fig. 4 (sample B). As soon as the c-BN peak at 1100 cm-’ disappears, the film thickness decreases dramatically with increasing ion-to-atom ratio. This lowered BN growth rate is caused by the resputtering of the deposited atoms and can not be explained by decreasing R,. However, there is still growth of a sp2-bonded phase above the resputter threshold for c-BN. No change in the IR spectra between h-BN films and BN films depos-
ited above the c-BN sputter threshold could be detected, in contrast to findings reported recently for x-BN [6]. In Fig. 5 the IR reflection spectra of sample C deposited with various boron rates R, are presented. Only the spectral shape of h-BN-containing films can be seen for R, > 0.047nm s-I. At greater ion-to-atom ratios the c-BN phonon mode appears at 1100 cm-’ and the intensity I1 100 of the c-BN peak increases with decreasing R,. Interestingly, it was not possible to measure any drastic decrease in the layer thickness, as would be expected from the transition of hexagonal to cubic growth with two strongly differing densities for the pure phases. The ratios 11100/(11100 + 11400) of the intensities of
I200
wavenumber
Fig. 5. Evolution decreasing boron 4% for clarity.
I400
1600
(cm-‘)
of the s-polarised PIRR spectra of sample C with deposition rate RB. The upper spectra are shifted
386
M.F.
20
I
. fl..
‘g y” “0 5 i 5
I8
: .,‘\
-
.,;‘;
y.;: ;’
16
-
o.....
.o’ uncoated silicon
0 042
I s’ample c --X-
and Coatings
., ‘ic:., i . k, ‘x : ., ’ :: , : ., kX, ., :. I :
60
- IR peak ratio
0.044 boron
0 046 deposition
0. ‘0.
84 (1996)
eG
40 f 8
2 mN
C 20
5 A-
1 .: E 0 Q fi
0 048 rate R,
0.050
0 052
0
0 1000
(r&s)
1200 wavenumber
Fig. 6. Comparison of the IR intensity of sample C as a function of R,.
383-387
7s 0
hardness: load I mN load
Technology
I
o:,
‘h’
-
I
: :
,:
‘..., ‘\
I
0
P/ass et al. JSurface
ratio
and the dynamic
1400 (cm-‘)
I600
l4OO
1500
wavenumber
1600 (cm”)
hardness
the c-BN and h-BN peak at 1100 cm-’ and 1400 cm-r are shown in Fig. 6. The mechanical properties of sample C were investigated, too. The results of the hardness measurements for different rates R, are also presented for comparison in Fig. 6. The dynamic hardness of the h-BN coated silicon (R B 2 0.045 nm s-i) was even smaller than for the uncoated substrate. Hardness values between 2000 and 5000 kp mmM2, as already reported [ 7-91 for approximately l-pm thick c-BN coatings, were not attained by our thin coatings. However, the dynamic hardness of the coated substrate almost doubled for I1 100/(11100 + 11400) > 40%. These estimated dynamic hardness values are comparable to published results [ 10,111 for approximately lOO-nm thick c-BN coatings. The determination of the hardness of a thin coating with a microindenter and the resulting problems, like substrate effects or pop-in [12], might be the reasons for the poor correlation of the hardness and the IR measurements. However, the mechanical properties depend on the deposited phase, in contrast to the deposition rate. The hexagonal part of the PIRR spectra of sample D recorded with p-polarised light exhibits an attractive feature (Fig. 7). At the onset of c-BN growth a clearly discernible shoulder appears at the low energy edge of the in-plane stretching h-BN TO mode. Two-phonon contributions [13,14] or rearrangement of the h-BN crystal planes [ 141 owing to the c-BN growth could be possible origins, but these effects seem to be too small to explain the observed feature. Next, c-BN could institute changes of the stress, the oscillator strength, the damping, or the eigenfrequency. However, according to Drude model, the TO mode should be most affected. Thus, this effect can be excluded, as the TO band remains almost constant. Finally, the layered growth of c-BN must be mentioned. As h-BN appears near the substrate interface [ 23 and at the c-BN grain boundaries [ 151, one can, in principle, distinguish both domains and assign them a specific dielectric constant according to the effective medium theory. The energetic position
Fig. 7. Evolution of the p-polarised PIRR (sample D). The upper spectra are shifted for clarity. The righthand section is enlarged to show the appearance of an additional shoulder of the h-BN LO mode that occurs as soon as c-BN growth begins,
of the LO band is affected by the dielectric constant and consequently by the electronic bulk density, as described by the LST relation. Thus, the h-BN LO mode separates in two bands, as soon as the c-BN growth occurs. A detailed discussion of this matter will be presented elsewhere. It is planned in the future to gain information about the layered growth of c-BN containing films from the p-polarised reflection spectra.
Acknowledgements
The authors acknowledge S. Mandl for discussions. This work is supported by the SMWK (No. 4-7514.83-FZR/405).
References [l] [Z] [3] [4] [S] [6] [7] [S] [9]
G.L. Doll, J.A. Sell, CA. Taylor II and R. Clarke, Phys. Reo. B, 43 (1991) 6816. D.J. Kester, KS. Ailey, D.J. Lichtenwalner and R.F. Davis, J. Vat. Sci. Technol. A, I2 (1994) 3074. D.L. Medlin, T.A. Friedmann, P.B. Mirkarimi, M.J. Mills and K.F. McCarty, Phys. Rev. B, 50 (1994) 7884. U. Kreissig, R. Grotzschel and B. Behrisch, Nucl. Instrum. Meth. B, 85 (1994) 71. N. Tanabe, T. Hayashi and M. Iwaki, Diamond Relat. Mater., 1 (1992) 883. M. Kuhr, S. Reinke and W. Kulisch, Diamond Relat. Mater., 4 (1995) 375. K. Inagawa, K. Watanabe, K. Saitoh, Y. Yuchi and A. Itoh, Surf Coat. Technol., 39/40 (1989) 253. Y. Andoh, K. Ogata, Y. Suzuki, E. Kamijo, M. Satou and F. Fujimoto, Nucl. Instrum. Meth. B, 19120 (1987) 787. S. Mineta, M. Kohata, N. Yasunaga and Y. Kikuta, Thin Solid Films, 189 (1990) 125.
M.F.
[lo]
Plass et al. JSurface
and Coatings
R.J. Bricault, P. Sioshansi and S.N. Bunker, Nucl. Instrum. Meth. B, 21 (1987) 586. [11] D.C. Cameron, M.Z. Karim and M.S.J. Hashmi, Thin Solid Films, 236 (1993) 96. [12] N.G. Chechenin, J. Bottiger and J.P. Krog, Thin Solid Films, 261 (1995) 228.
Technology
84 (1996)
383-387
387
[ 131 P.J. Gielisse, S.S. Mitra, J.N. Plendl, R.D. Griffis, L.C. Mansur, R. Marshall and E.A. Pascoe, Phys. Rev., 155 (1967) 1039. [14] R. Geick, C.H. Perry and G. Rupprecht, Phys. Reo., 146 (1966) 543. [15] W.-L. Zhou, Y. Ikuhara, M. Murakawa, S. Watanabe and T. Suzuki, Appl. Phys. Lett., 66 (1995) 2490.
and Coatings
Technology
84 (1996)
383-387
Growth and characterisation of boron nitride thin films M.F. Plass *, W. Fukarek, A. Kolitsch, U. KreiBig Forschungszentrum
Rossendorf e.V., Znstitutfiir
Zonenstrahlphysik
und MateriaJjorschung,
Postfach 51 01 19, 01314 Dresden, Germany
Abstract Boron nitride layers were deposited using ion beam assisted deposition with different ion energies (0.5 or 1.5 keV) and normal angle of incidence on silicon wafers heated to 400 “C. By mounting the electron beam evaporator at an angle of 56” to the normal of the substrate, it was possible to investigate the influence of the continuously changing ion-to-atom ratio on the grown modification and, subsequently, to determine the properties of the BN films. The coatings were characterised using polarised IR reflection spectroscopy, heavy ion elastic recoil detection and microhardness measurement (Vickers indenter). The dynamic hardness of the Si substrate could be almost doubled with an approximately 1000-A thick c-BN coating. The transition determined by IR spectroscopy from non-cubic to cubic growth could be clearly resolved across the 3-inch silicon substrate. As soon as the spectra recorded with p-polarised light exhibited the reflection band of the cubic phase at 1100 cm-‘, an additional feature appeared in the in-plane stretching h-BN mode near 1580 cm-‘. The two features of the h-BN LO band might be due to the layered growth of the BN system corresponding to two different dielectric constants of the h-BN layer near the interface and at the sp’-bonded grain boundaries of the c-BN-containing overlayer. Furthermore, coatings with a high cubic fraction exhibited a stoichiometric boron-to-nitrogen ratio with small oxygen and hydrogen contaminations. Keywords: Boron nitride; Thin films; Polarised
IR reflection
1. Introduction Cubic boron nitride (c-BN) is the second hardest material known, after diamond, and thus has a huge potential for practical applications. The special advantages of c-BN [1,2] with respect to diamond are its inertness to iron, even at high temperatures, and the possibility of both p- and n-type doping. However, successfulsynthesis of well-crystallised c-BN films is still more difficult than for diamond. To gain a better understanding of the deposition process and ultimately to produce high-quality c-BN films, we investigated the influence of deposition parameters on the film properties. In the following, the results of polarised IR spectroscopy (PIRR) and dynamic hardness measurements on BN films deposited with ion beam assisted deposition (IBAD) are presented. The geometry of the deposition set-up was adjusted to study the effect of varying the ion-to-boron ratio on a single silicon wafer. The elemental depth distribution of a c-BN-containing film was determined with heavy ion elastic recoil detection (HIERD). To simplify matters, no distinction is made * Corresponding 591 2703; e-mail:
author. Tel.: +49 [email protected].
0257-8972/96/$15.00 0 1996 Elsevier SSDZ 0257-8972(95)02781-5
(351)
591 3422; fax:
f49
(351)
Science S.A. All rights reserved
between the various non-cubic BN phasesformed during thin film deposition, such as hexagonal, turbostratic or, as recently discovered, rhombohedral [ 31 boron nitride. The term h-BN is applied to the sp2-bonded, layered, and near-stoichiometric BN modifications.
2. Experimental set-up Boron nitride layers were deposited using IBAD with different ion energies &,, (0.5 or 1.5 keV) at normal angle of incidence on 3-inch Si (n-type and (lOO)oriented) wafers heated to 400 “C. A schematic representation of the deposition system with the three main parts (ion source, substrate holder and electron beam evaporator) is shown in Fig. 1. Boron was deposited with spatially varying rates by mounting the Si substrate at a distance of 40 cm and an angle of 56” to the normal of the evaporator. The boron deposition rate R, without ion bombardment (substrate temperature Ts = 400 “C), asmeasuredwith a profilometer on thick films, decreased nearly linearly from 0.055 to 0.035 nm s-l across the sample with increasing distance from the evaporator. This corresponds to an increasing ion-to-atom ratio, as also illustrated in Fig. 1. Using this, the influence of a
384
M. F. Plass et al. JSurface
and Coatings
Technology
84 (1996)
Table 1 Deposition conditions are given in the text IncreasIng to
atom
Ion
quartz
oscillator
?an--,I-rz7zz
10” source
n
illustration
BN samples.
Further
details
Ion current Density j,,, (PA cm-*)
N2: Ar gas Nitrogen-to-boron flow ratio arrival ratio range
Ion-to-boron arrival ratio range
A B C D
120 90 90 70
1:2 1:l 1:2 1:l
2.0 1.6-2.1 1.4-1.8 1.3-1.6
1.0 1.1-1.4 0.7-0.9 0.9Sl.O
m shutter
electron evaporator
Fig. 1. Schematic
of the analysed
Sample
ratio
\fllament
383-387
of the deposition
beam
set-up.
continuously changing boron deposition rate RB and the consequently changing ion-to-atom ratio on the BN film properties could be determined. Doing so, no experimental modifications obstructed the attempt to investigate the parameter range for c-BN deposition. The temporal regularity of the boron evaporation was controlled by a quartz oscillator. The base and the working pressure were 5 x 1O-6 and <2 x lop4 mbar, respectively. The large distance of 70 cm between the 5-cm Kaufmann ion source and the substrate resulted in a homogeneous argon and nitrogen ion flux. The ion current density jlon was measured with a Faraday cup biased to 30 V to prevent the emission of secondary electrons. The N,:Ar gas flow ratio was varied between 1:l and 1:2. The subsequently resulting N2+:Ar+ ion ratio was assumed to be the same as the gas flow ratio, as the ionisation energies are similar. Without additional heating the substrate temperature T, increased up to nearly 300 “C for unheated substrates and saturated during the first minute of ion bombardment (E1,,= 1.5 keV, jIon = 100 PA cm-‘). The samples were mounted in the deposition chamber immediately after a HF dip. The BN deposition time was 1 h, except for sample D where it was 2 h. Only sample A was deposited with a lower energy EIon = 0.5 keV. The various deposition conditions and the ion-to-atom ratios of the different samples are listed in Table 1. IR spectroscopy is the established standard analysis technique for BN films, as the resulting peaks are related to either the cubic or the non cubic phase. The BN coatings were investigated with IR spectroscopy with sand p-polarised light in the range between 800 and 7000 cm-‘. With a CaF, wire grid polariser, the out-ofplane bending mode of h-BN could not be investigated. The angle of incidence was 60”. The spectra were recorded relative to the reflection of silicon and corrected afterwards with a calculated Si spectrum. A detailed description of the experimental set-up of
the elastic recoil detection and the theoretical analysis of the measured data to gain information about the elemental depth distributions of the light atoms or the film density is found in Ref. [4]. Dynamic microhardness measurements were carried out with a Vickers indenter with a loading speed of 29 pN s- 1 and a load of 1 or 2 mN. The resulting penetration depth was in the range between 40 and 80 nm, which is up to 80% of the BN layer thickness. Therefore, the absolute values of the measured hardness reflect a strong influence of the underlying Si substrate. Thus, all hardness data for the BN/Si layer system should be only compared with those for non-coated silicon. The hardness of a clean Si wafer was determined to be 1415 f 142 or 1292 f 50 kp mmU2 for loads of 1 or 2 mN, respectively.
3. Results and discussion By comparing the atomic area1 densities measured with HIERD and the layer thickness, it is possible to evaluate the absolute atomic concentrations of the coating. Doing so, the density and the contamination of a boron coating deposited without ion bombardment (T, = 400 “C) were determined. The boron concentration is 1.0 x 1O23cmm3, corresponding to a density of 1.8 g cme3, and the oxygen, carbon and nitrogen contaminations of 3.7%, 2.1% and 0.5%, respectively, are constant throughout the layer. The measured boron density is comparable to the result of Tanabe et al. [S], albeit 25% less than the density of crystalline bulk material. This lower density of boron deposited without ion bombardment has to be considered when the boron arrival rate is estimated from measured film thickness and deposition time. The depth distributions of light elements in sample A was also investigated with HIERD (see Fig. 2). The nitrogen-to-boron ratio of the film is close to one. Nitrogen enrichment can be observed near the surface. No carbon contamination is detectable. The oxygen and hydrogen contaminations are approximately 1%. Fig. 3 shows the corresponding PIRR spectra of sample A. The s-polarised reflection spectrum is very
M. F. Plass et al./Surface
and Coatings
Technology
84 (1996)
383-387
385
boron deposition
rate
I6 60
depth
(rel.
units)
I
IO00
Fig. 2. Elemental
depth
distribution
of sample
A (R, = 0.049 nm s-i).
,
I
I
I200
I
I400
wsavenumber
I
I600
IO00
2000
3000
wavenumber
(cm-‘)
Fig. 4. PIRR spectra monitoring growth of sample B. The upper ion-(nitrogen and argon)-to-boron approximately 1.8.
(cm-‘)
the resputter threshold for cubic spectra are shifted for clarity. The atoms resputter threshold is
16 65
sample
t.1.
1000 Fig. 3. PIIR
,,,I v
I500
*I.,
wavenumber
(cm-‘)
reflection
spectra
A
--__--_ I$ 2000
of sample
A.
similar to transmission spectra, whereas the p-polarised spectrum proves to be more dynamic and exhibits new features. The transverse optical (TO) bands of c-BN and h-BN can be seen at approximately 1100 and 1400 cm-‘, respectively. The double-peak structure of the h-BN TO mode is due to the anisotropy of the hexagonal crystal. In the spectrum recorded with p-polarised light, the longitudinal optical (LO) modes can be distinguished at 1300 cm-’ and 1600 cm-‘. In the following, the advantage of BN layer deposition with a spatially changing boron deposition rate R,, as described in the second section, is used to demonstrate the influence of the varying ion-to-atom ratio on the grown phase. The change of the reflection spectra at the resputter threshold for high ion-to-atom ratios with changing boron deposition rate is shown in Fig. 4 (sample B). As soon as the c-BN peak at 1100 cm-’ disappears, the film thickness decreases dramatically with increasing ion-to-atom ratio. This lowered BN growth rate is caused by the resputtering of the deposited atoms and can not be explained by decreasing R,. However, there is still growth of a sp2-bonded phase above the resputter threshold for c-BN. No change in the IR spectra between h-BN films and BN films depos-
ited above the c-BN sputter threshold could be detected, in contrast to findings reported recently for x-BN [6]. In Fig. 5 the IR reflection spectra of sample C deposited with various boron rates R, are presented. Only the spectral shape of h-BN-containing films can be seen for R, > 0.047nm s-I. At greater ion-to-atom ratios the c-BN phonon mode appears at 1100 cm-’ and the intensity I1 100 of the c-BN peak increases with decreasing R,. Interestingly, it was not possible to measure any drastic decrease in the layer thickness, as would be expected from the transition of hexagonal to cubic growth with two strongly differing densities for the pure phases. The ratios 11100/(11100 + 11400) of the intensities of
I200
wavenumber
Fig. 5. Evolution decreasing boron 4% for clarity.
I400
1600
(cm-‘)
of the s-polarised PIRR spectra of sample C with deposition rate RB. The upper spectra are shifted
386
M.F.
20
I
. fl..
‘g y” “0 5 i 5
I8
: .,‘\
-
.,;‘;
y.;: ;’
16
-
o.....
.o’ uncoated silicon
0 042
I s’ample c --X-
and Coatings
., ‘ic:., i . k, ‘x : ., ’ :: , : ., kX, ., :. I :
60
- IR peak ratio
0.044 boron
0 046 deposition
0. ‘0.
84 (1996)
eG
40 f 8
2 mN
C 20
5 A-
1 .: E 0 Q fi
0 048 rate R,
0.050
0 052
0
0 1000
(r&s)
1200 wavenumber
Fig. 6. Comparison of the IR intensity of sample C as a function of R,.
383-387
7s 0
hardness: load I mN load
Technology
I
o:,
‘h’
-
I
: :
,:
‘..., ‘\
I
0
P/ass et al. JSurface
ratio
and the dynamic
1400 (cm-‘)
I600
l4OO
1500
wavenumber
1600 (cm”)
hardness
the c-BN and h-BN peak at 1100 cm-’ and 1400 cm-r are shown in Fig. 6. The mechanical properties of sample C were investigated, too. The results of the hardness measurements for different rates R, are also presented for comparison in Fig. 6. The dynamic hardness of the h-BN coated silicon (R B 2 0.045 nm s-i) was even smaller than for the uncoated substrate. Hardness values between 2000 and 5000 kp mmM2, as already reported [ 7-91 for approximately l-pm thick c-BN coatings, were not attained by our thin coatings. However, the dynamic hardness of the coated substrate almost doubled for I1 100/(11100 + 11400) > 40%. These estimated dynamic hardness values are comparable to published results [ 10,111 for approximately lOO-nm thick c-BN coatings. The determination of the hardness of a thin coating with a microindenter and the resulting problems, like substrate effects or pop-in [12], might be the reasons for the poor correlation of the hardness and the IR measurements. However, the mechanical properties depend on the deposited phase, in contrast to the deposition rate. The hexagonal part of the PIRR spectra of sample D recorded with p-polarised light exhibits an attractive feature (Fig. 7). At the onset of c-BN growth a clearly discernible shoulder appears at the low energy edge of the in-plane stretching h-BN TO mode. Two-phonon contributions [13,14] or rearrangement of the h-BN crystal planes [ 141 owing to the c-BN growth could be possible origins, but these effects seem to be too small to explain the observed feature. Next, c-BN could institute changes of the stress, the oscillator strength, the damping, or the eigenfrequency. However, according to Drude model, the TO mode should be most affected. Thus, this effect can be excluded, as the TO band remains almost constant. Finally, the layered growth of c-BN must be mentioned. As h-BN appears near the substrate interface [ 23 and at the c-BN grain boundaries [ 151, one can, in principle, distinguish both domains and assign them a specific dielectric constant according to the effective medium theory. The energetic position
Fig. 7. Evolution of the p-polarised PIRR (sample D). The upper spectra are shifted for clarity. The righthand section is enlarged to show the appearance of an additional shoulder of the h-BN LO mode that occurs as soon as c-BN growth begins,
of the LO band is affected by the dielectric constant and consequently by the electronic bulk density, as described by the LST relation. Thus, the h-BN LO mode separates in two bands, as soon as the c-BN growth occurs. A detailed discussion of this matter will be presented elsewhere. It is planned in the future to gain information about the layered growth of c-BN containing films from the p-polarised reflection spectra.
Acknowledgements
The authors acknowledge S. Mandl for discussions. This work is supported by the SMWK (No. 4-7514.83-FZR/405).
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