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Rapid thermal chemical vapour deposition of SiOxNy films
Applied Surface Science 54 (1992) 125-129 North-Holland
applied surface science
Rapid thermal chemical vapour deposition of SiOxNy films F. Lebland, C. Licoppe, Y. Gao, Y.I. N i s s i m Laboratoire de Bagneux, Centre National d'Etude des Télécornmunications, France Téléeom, 196 Avenue Henri Ravera, 92220 Bagneux, France
and
S. Rigo Groupe de Physique des Solides de l'Université Paris 7, Tour 23, 2 Place Jussieu, 75251 Paris Cedex 05, France Received 28 May 1991; accepted for publication 31 May 1991
Rapid thermal CVD was utifized to deposit SiOxNy layers on I I I - V materials. The deposition rate and the stoichiometry of the films are controlled by the N 2 0 partial pressure and the temperature. Deposition rates as high as 100 Ä / s can be obtained at 750°C for which the InP substrate is not degraded. The use of N 2 0 as an adjustable reactant gas allows us to control the stoichiometry of the film by varying the O concentration while the Si and N concentration remain unchanged. This effect m a y be explained by the dissociation of nitrogen protoxide into oxygen which is directly incorporated in the films and nitrogen in excess in the reaction chamber which increase the dilution. The use of nitrogen protoxide has allowed us to obtain continuously all compositions between SiO 2 and Si3N 4 with indexes of refraction varying between 1.45 and 2.2. Applications to guiding devices are discussed.
1. Introduetion The development of micro-optoelectronics in I I I - V compounds requires the use of high-grade thin film insulators. Among the I I I - V materials telecommunication systems utilize InP since InPbased alloys have band-gap energies near the minimum loss wavelength in optical fibers (1.3-1.6 /~m). Progress made on optical as weil as electronical devices has allowed recently the optoelectronic integration of devices on InP. Dielectric layers have a prime role in this perspective as a passivation layer or as a guiding film. When CVD is utilized to deposit the dielectric, the quality of the films is improved at high temperature [1] since the chemical decomposition of the reactants is complete. It is observed that at high temperature the stoichiometry and the density of the films are upgraded. Nevertheless, processes of deposition on I I I - V substrates should solve the paradox involved in high deposition temperature and sub-
limation of the Group V element. In this work it is demonstrated that rapid thermal CVD (RTCVD) provides a satisfactory solution to this problem. SiOxNy films are usually obtained in a two-step process: silicon dioxide deposition and nitridation in a N H 3 flow at high temperature. In this work the SiO«Ny films obtained were deposited directly on an InP substrate. The advantages of this process are the suppression of the annealing procedure and the avoidance of inhomogeneous interdiffusion which is especially the case with nitrogen which usually remains in excess at the surface. The study of SiOxNy films has been motivated by the possibility these alloys of fer to modulate dielectric film properties such as density, stoichiometry and index of refraction. Control of the film composition allowed us to obtain indexes of refraction varying continuously between 1.5 and 2.2. Multidielectric layers can now be envisioned on InP-based materials with applications such as guiding, coating, passivation ....
0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
126
F. Lebland et al. / Rapid thermal CVD of S i Q Ny
2. Experimental procedure
velocity of at least 60 Ä / s was necessary to avoid degradation problems.
Undoped n-type (100) InP substrates polished on one side were used. Prior to the introduction in the reaction chamber the samples were polished in a light Br-methanol solution to limit contamination at the surface and remove the native oxide. After the introduction of the reactive gases in the reaction chamber radiant heat is produced with tungsten halogen lamps to bring the substrate to a proper deposition temperature. With this cold-wall process, temperature acts as a switch to start the reaction [2]. Thermal exposure is minimized by the use of rapid thermal kinetics. In a very short time (3 s) the substrate temperature is increased to 750°C. The first monolayers of SiO«Ny encapsulated the InP substrate, and prevent thermal degradation of its surface. The competition between thermal degradation and encapsulation (deposition rate) implies the obtainment of fast deposition kinetics to protect the substrate. A previous study [2] proved through electrical measurements of the dielectric/semiconductor interface that a
i
i
3. Choice of CVD parameters SiH 4, N H 3 and an oxidant were used as reactant gases. The gases are all diluted in N 2 and the total pressure is kept at 50 Torr. A choice needs to be made between O 2 and nitrogen protoxide as the oxidant precursor. In both cases IR spectroscopy with a 740 SX Nicolet scanning between wavenumbers 4000 and 400 cm-~ was performed to investigate the optical properties of the deposited films obtained. Fig. 1 illustrates the competition between N H 3 and 02 when 02 is chosen. With two different relative flows of oxygen (curve a: 12.7% and b: 1.2%) no peak attributed to silicon nitride or oxynitride could be observed. The absorption peaks around 1055 and 814 cm -1 are both assigned to the S i - O vibration modes. The contribution of N H 3 to the reaction is clearly negligible, except for a residual incorporation of
~
i
Si-O STRETCHING
i
850°C
10
«
o 1350
si-o BENDINü 1266
11'82
10'98
IdIL~ 930 , 8¼6 WAVENUNBER (cm-)
762
678
594
Fig. I. [nfrared absorption spectrum of ]ayers deposited by RTCVD at 750°C uslng 02 as the oxidant mo]ecu]e (dashed lines) and N20 (full line); SiOxNy is only obtained when using N20 as the oxidant gaseous precursor. Several deposition temperatures have
been used.
F. Lebland et aL / Rapid therrnal CVD of SiOxNy
T (°C) 800
900
105
700
l
"~10~"
"~Ea=O.67eV
P:SOT
N2=1OOcm3/min min ~, 103 NH3=2OOcm3/ SiH~=2Ocm°/rnin NzO=2OOcm3/min D
10208
019
IO00/T (K-I}
"---..
-~.
~
"~
11
Fig. 2. Oxynitride Arrhenius plot of the flow rate.
nitrogen in the layers. It is interesting to note the deviation of S i - O stretching bond position towards the lower wavenumbers (in SiO 2 the S i - O streching mode vibrates at 1070 c m - l ) . N - H bonds (3350 cm - a ) could not be detected in the infrared spectrum. On the contrary when N20 is used instead of 02, SiOxNy films are successfully obtained. As can be seen in the I R spectrum of fig. l c very large peaks located at 835 and 1000 cm -1 indicate clearly simultaneous nitrogen and oxygen incorporation in the film. In the light of these results N 2 0 was selected as the oxidant precursor. The second key parameter is the deposition temperature. As can be seen in fig. 2 the deposition rate varies over one order of magnitude when the temperature changes from 650 to 900°C. This indicates a high activation energy E a = 0.67 eV. The positions of the SiOxNymain absorption band for films deposited on InP at temperatures varying between 650 and 850°C are shown in fig. 1. Higher deposition temperatures result in an increase of the growth rate (increasing amplitude of the I R absorption), a change in the compactness and of the stoichiometry of the film as indicated by a displacement of the peak centred at 840 cm-1 (at 650°C) to a higher wavenumber of 873 c m - 1 (at 850°C), and a larger incorporation of oxygen as
127
indicated by a growing contribution on the high energy side of the peak. These results are confirmed by secondary ion mass spectrometry (SIMS) measurements. This technique gave significant results when charge problems were suppressed in the insulators with a 400 Ä thick gold layer evaporated on the samples. The O / S i ratio increased with N20 concentration in the reactive mixture, while the N / S i ratio was observed to remain constant. This means that temperature favoured only the pyrolytic dissociation of nitrogen protoxide, oxygen being incorporated in the layers. The exact determination of nitrogen and oxygen contents has been made by nuclear reaction analysis (NRA). These results agreed with the results obtained with other techniques. The nitrogen concentration remains almost stable at 2.5 x 1022 a t o m s / c m 3 while oxygen varied drastically from 1.8 x 1020 to 1.3 x 1022 a t o m s / c m 3. The infrared spectrum displayed two other absorption peaks around 2220 and 3370 cm 1. The first one was due to absorption by the S i - H stretching vibration. Since the S i - N amplitude varies linearly with the dielectric thickness, the hydrogen concentration in the layers can be determined approximately by the ratio S i - H / S i - N which decreases with temperature (from 25 x 10 -3 at 7 0 0 ° C to 15 x 10 -3 at 850°C). Further information from the I R spectra is only partly understood at the time. In particular the displacement of the S i - H peak with temperature. The S i - H absorption peak was located at 2180 cm 1 in silicon nitride [3] and the shift observed towards higher energies in SiOxNy can be explained by the increase of the average number of oxygen backbonds with S i - H vibration resonance frequency. A peak at 3372 cm -1, close to the N - H resonance has not be assigned in the literature; this peak intensity varies with the oxygen concentration in the films and may be related to N - H bonds whose resonance vibrations are shifted because of oxygen backbonds. The results described here show how critical the kinetics of the reaction are. At 750°C (and above) nitrogen protoxide begins to dissociate and oxygen is incorporated in the layers. The material properties of the resulting SiOxNy films are reported in the following for this deposition temperature.
128
F Lebland et al. / Rapid thermal CVD of SiOxNy
4. Deposition rates and structural properties of the films The kinetics of silicon nitride deposition under the condition fixed above have been already reported previously [3]. The deposition rate is maxim u m when the N H 3 / S i H 4 ratio is adjusted to 10. Below this ratio the reaction rate decreases in the reaction rate controlled regime and above it decreases due to a retardation controlled regime. Deposition rates of SiO«Ny films were studied on the basis of these results and to obtain good uniformity of the film the total pressure was fixed at 50 Torr. In order to incorporate more and more oxygen in the layers, only the N20 flow rate was changed. Plots of deposition rate versus N 2 0 for several N H 3 / S i H 4 ratios are shown in fig. 3. It shows one overcomes the critical rate of 60 Ä / s by a proper adjustment of the flow rates of the reactive gases silane and ammonia. Mechanisms of decomposition of N20 at 750°C were not directly established. In the same CVD conditions, we noticed similar growth rates for silicon nitride films (where N20 = 0) and silicon oxyninitride obtained with a low N20 concentration is kept low (less than 1%). Increasing further the nitrogen protoxide flow rate led to a decrease in the overall deposition rate. Those facts may be explained assuming that primary or by-products of N20 decomposition, such N - O + N or N 2 + O, were emitted during the pyrolytic dissociation. These by-products increase or decrease the partial pressure of non-reactive gas (nitrogen) and therefore
120 750% , 50T -- 100
II~--..~
NH3/SiH~
8O ~ 6O ~ 4C tJ 20
~b 2'0 3'0 Jo s'o go 7'o 8'o 9b 100 N20 FLOW RATE (oma/min) Fig. 3. G r o w t h rate versus N 2 0 for several N H 3 / S i H 4 ratios; d e p o s i t i o n rate higher t h a n 60 Ä / s can be obtained.
i
10
Si-O in SiOz Tp
~-
]
SiHt, fixed NH31
/
///
,,//-~
III
\\\
/,///--"~ \
/
N2 )
0
Si-N in Si3N«
/
!
\',
J
\\
,I . ./ y/
° ~ k 1350
126~-11'82-- ~ÓgB 10'14 9}0 8/.6 WAVENUHBER (cm-1)
76}
678
Fig. 4. I R s p e c t r u m for several N 2 0 flows, a p p e a r a n c e of both
contributions.
increase the dilution of the reactive species in the mixture which then decreases the deposition rate. The I R spectra for several values of the N20 flow are reported in fig. 4 showing the effects associated to increased oxidation on the structural properties of these layers. Between the absorption band location in extreme materials, that is the S i - N bond in silicon nitride (835 cm -1 at 750°C) and the S i - O bond in silicon dioxide (1070 cm i at 700°C), a large mixture of both features is observed in oxynitride films. In the alloy, the large peak in the region 1350-600 cm -1 represents the sum of two contributions, one with a S i - O character, and one with a S i - N character, which are more or less important according to the oxygen concentration in the reaction chamber. In fact, with about 1% of N 2 0 gas flow the S i - N peak shifts towards the S i - O peak by only a few wavenumbers. When the N 2 0 flow rate is increased this peak splits in two with the Si-O-like band becoming more and more intense. At high N 2 0 flow rates thin films are mainly composed of oxygen, and nitrogen in the I R spectrum acts as a perturbation of a predominantly Si-O-like band. SIMS depth profiles confirm this observation. As a matter of fact, the N / S i ratio in fig. 5 (dashed lines) did not vary significantly while the O / S i ratio increased when the N20 concentration varied
F. Lebland et al. / Rapid therrnal CVD of SiO~ N,.
regime where the refractive index drops sharply, followed by a slow decrease towards its value in SiO 2. A refractive index varying slowly with oxygen concentration can be used in applications that require a high degree of control of the index of refraction (guiding devices). In the composition regime where the index of refraction varies sharply with oxygen concentration, devices that need a large change in index of refraction (multidielectric layers for mirrors) can be designed. A waveguide on InP using a sandwich of SiOxN », in between two layers of SiO 2 is now being realized. The size of the guiding layer can be chosen at will since the index of the siO~N ~,can be varied continuously.
-- --N/Si --0/Si
BI ~
N20in% [01xlOz2'l[N]xlO22 (at/cm3)! (at/cm3)
lll\,,Y~
/
~
\
~
/ ~
I
I
I
I
i
~~
1.88
o.2~
9
123
2ss
6
0198
2.86
3
0.84
2.98
TINE (s)
i
Fig. 5. SIMS depth profile of SiOxN». films; oxygen concentration increases with increasing N20 flow in the gas stream.
from 3% to 25%. This is the reason why it is assumed that nitrogen coming from N20 is not included in the layers but increases the dilution and thus slows down the reaction. At the same time free oxygen produced by N20 decomposition is incorporated in the films. Nuclear reaction analysis measurements gave a quantitative confirmation of there results (see fig. 5). The index of refraction is directly related to the stoichiometry of the film. This is shown in fig. 6. This curve features two different slopes, with a
2.1
i
I
'
129
i
5. Conclusion
The R T C V D process is unique in its capacity to deposit high-grade dielectrics on I I I - V materials at high temperature. Dielectrics deposited by this technique have a low density of defects and residual impurity, especially hydrogen (less than 1%). Pyrolytic SiOxN », layers using N20 as an oxidant gas have properties that fill the gap between silicon dioxide and silicon nitride. In particular their index of refraction can be varied continuously between 1.45 and 2.2 with the oxygen concentration in the layer. This property allows the use of such materials for optical guiding to be envisioned. Waveguides on InP substrate are now being fabricated on this basis and results will be reported in a forthcoming paper.
mu_l.9 et: i
~z x1.7
- - 1.5
References 5
10
15
20
25
30
35
N20 FLOW RATE (in %) Fig. 6. Index of refraction versus N20 flow rate; n varies continuously between silicon dioxide and silicon index of refraction.
[1] G.E. Morosanu, Thin Solid Films 65 (1980) A1-208. [2] Y.I. Nissim, C. Licoppe, J.M. Moison, J.L. Regolini, D. Bensahel and G. Auvert, Proc. SPIE 1033 (1988) 273. [3] Y.I. Nissim, J.M. Moison, F. Houzay, F. Lebland, C. Licoppe and M. Bensoussan, Appl. Surf. Sci. 46 (1990) 175.
applied surface science
Rapid thermal chemical vapour deposition of SiOxNy films F. Lebland, C. Licoppe, Y. Gao, Y.I. N i s s i m Laboratoire de Bagneux, Centre National d'Etude des Télécornmunications, France Téléeom, 196 Avenue Henri Ravera, 92220 Bagneux, France
and
S. Rigo Groupe de Physique des Solides de l'Université Paris 7, Tour 23, 2 Place Jussieu, 75251 Paris Cedex 05, France Received 28 May 1991; accepted for publication 31 May 1991
Rapid thermal CVD was utifized to deposit SiOxNy layers on I I I - V materials. The deposition rate and the stoichiometry of the films are controlled by the N 2 0 partial pressure and the temperature. Deposition rates as high as 100 Ä / s can be obtained at 750°C for which the InP substrate is not degraded. The use of N 2 0 as an adjustable reactant gas allows us to control the stoichiometry of the film by varying the O concentration while the Si and N concentration remain unchanged. This effect m a y be explained by the dissociation of nitrogen protoxide into oxygen which is directly incorporated in the films and nitrogen in excess in the reaction chamber which increase the dilution. The use of nitrogen protoxide has allowed us to obtain continuously all compositions between SiO 2 and Si3N 4 with indexes of refraction varying between 1.45 and 2.2. Applications to guiding devices are discussed.
1. Introduetion The development of micro-optoelectronics in I I I - V compounds requires the use of high-grade thin film insulators. Among the I I I - V materials telecommunication systems utilize InP since InPbased alloys have band-gap energies near the minimum loss wavelength in optical fibers (1.3-1.6 /~m). Progress made on optical as weil as electronical devices has allowed recently the optoelectronic integration of devices on InP. Dielectric layers have a prime role in this perspective as a passivation layer or as a guiding film. When CVD is utilized to deposit the dielectric, the quality of the films is improved at high temperature [1] since the chemical decomposition of the reactants is complete. It is observed that at high temperature the stoichiometry and the density of the films are upgraded. Nevertheless, processes of deposition on I I I - V substrates should solve the paradox involved in high deposition temperature and sub-
limation of the Group V element. In this work it is demonstrated that rapid thermal CVD (RTCVD) provides a satisfactory solution to this problem. SiOxNy films are usually obtained in a two-step process: silicon dioxide deposition and nitridation in a N H 3 flow at high temperature. In this work the SiO«Ny films obtained were deposited directly on an InP substrate. The advantages of this process are the suppression of the annealing procedure and the avoidance of inhomogeneous interdiffusion which is especially the case with nitrogen which usually remains in excess at the surface. The study of SiOxNy films has been motivated by the possibility these alloys of fer to modulate dielectric film properties such as density, stoichiometry and index of refraction. Control of the film composition allowed us to obtain indexes of refraction varying continuously between 1.5 and 2.2. Multidielectric layers can now be envisioned on InP-based materials with applications such as guiding, coating, passivation ....
0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
126
F. Lebland et al. / Rapid thermal CVD of S i Q Ny
2. Experimental procedure
velocity of at least 60 Ä / s was necessary to avoid degradation problems.
Undoped n-type (100) InP substrates polished on one side were used. Prior to the introduction in the reaction chamber the samples were polished in a light Br-methanol solution to limit contamination at the surface and remove the native oxide. After the introduction of the reactive gases in the reaction chamber radiant heat is produced with tungsten halogen lamps to bring the substrate to a proper deposition temperature. With this cold-wall process, temperature acts as a switch to start the reaction [2]. Thermal exposure is minimized by the use of rapid thermal kinetics. In a very short time (3 s) the substrate temperature is increased to 750°C. The first monolayers of SiO«Ny encapsulated the InP substrate, and prevent thermal degradation of its surface. The competition between thermal degradation and encapsulation (deposition rate) implies the obtainment of fast deposition kinetics to protect the substrate. A previous study [2] proved through electrical measurements of the dielectric/semiconductor interface that a
i
i
3. Choice of CVD parameters SiH 4, N H 3 and an oxidant were used as reactant gases. The gases are all diluted in N 2 and the total pressure is kept at 50 Torr. A choice needs to be made between O 2 and nitrogen protoxide as the oxidant precursor. In both cases IR spectroscopy with a 740 SX Nicolet scanning between wavenumbers 4000 and 400 cm-~ was performed to investigate the optical properties of the deposited films obtained. Fig. 1 illustrates the competition between N H 3 and 02 when 02 is chosen. With two different relative flows of oxygen (curve a: 12.7% and b: 1.2%) no peak attributed to silicon nitride or oxynitride could be observed. The absorption peaks around 1055 and 814 cm -1 are both assigned to the S i - O vibration modes. The contribution of N H 3 to the reaction is clearly negligible, except for a residual incorporation of
~
i
Si-O STRETCHING
i
850°C
10
«
o 1350
si-o BENDINü 1266
11'82
10'98
IdIL~ 930 , 8¼6 WAVENUNBER (cm-)
762
678
594
Fig. I. [nfrared absorption spectrum of ]ayers deposited by RTCVD at 750°C uslng 02 as the oxidant mo]ecu]e (dashed lines) and N20 (full line); SiOxNy is only obtained when using N20 as the oxidant gaseous precursor. Several deposition temperatures have
been used.
F. Lebland et aL / Rapid therrnal CVD of SiOxNy
T (°C) 800
900
105
700
l
"~10~"
"~Ea=O.67eV
P:SOT
N2=1OOcm3/min min ~, 103 NH3=2OOcm3/ SiH~=2Ocm°/rnin NzO=2OOcm3/min D
10208
019
IO00/T (K-I}
"---..
-~.
~
"~
11
Fig. 2. Oxynitride Arrhenius plot of the flow rate.
nitrogen in the layers. It is interesting to note the deviation of S i - O stretching bond position towards the lower wavenumbers (in SiO 2 the S i - O streching mode vibrates at 1070 c m - l ) . N - H bonds (3350 cm - a ) could not be detected in the infrared spectrum. On the contrary when N20 is used instead of 02, SiOxNy films are successfully obtained. As can be seen in the I R spectrum of fig. l c very large peaks located at 835 and 1000 cm -1 indicate clearly simultaneous nitrogen and oxygen incorporation in the film. In the light of these results N 2 0 was selected as the oxidant precursor. The second key parameter is the deposition temperature. As can be seen in fig. 2 the deposition rate varies over one order of magnitude when the temperature changes from 650 to 900°C. This indicates a high activation energy E a = 0.67 eV. The positions of the SiOxNymain absorption band for films deposited on InP at temperatures varying between 650 and 850°C are shown in fig. 1. Higher deposition temperatures result in an increase of the growth rate (increasing amplitude of the I R absorption), a change in the compactness and of the stoichiometry of the film as indicated by a displacement of the peak centred at 840 cm-1 (at 650°C) to a higher wavenumber of 873 c m - 1 (at 850°C), and a larger incorporation of oxygen as
127
indicated by a growing contribution on the high energy side of the peak. These results are confirmed by secondary ion mass spectrometry (SIMS) measurements. This technique gave significant results when charge problems were suppressed in the insulators with a 400 Ä thick gold layer evaporated on the samples. The O / S i ratio increased with N20 concentration in the reactive mixture, while the N / S i ratio was observed to remain constant. This means that temperature favoured only the pyrolytic dissociation of nitrogen protoxide, oxygen being incorporated in the layers. The exact determination of nitrogen and oxygen contents has been made by nuclear reaction analysis (NRA). These results agreed with the results obtained with other techniques. The nitrogen concentration remains almost stable at 2.5 x 1022 a t o m s / c m 3 while oxygen varied drastically from 1.8 x 1020 to 1.3 x 1022 a t o m s / c m 3. The infrared spectrum displayed two other absorption peaks around 2220 and 3370 cm 1. The first one was due to absorption by the S i - H stretching vibration. Since the S i - N amplitude varies linearly with the dielectric thickness, the hydrogen concentration in the layers can be determined approximately by the ratio S i - H / S i - N which decreases with temperature (from 25 x 10 -3 at 7 0 0 ° C to 15 x 10 -3 at 850°C). Further information from the I R spectra is only partly understood at the time. In particular the displacement of the S i - H peak with temperature. The S i - H absorption peak was located at 2180 cm 1 in silicon nitride [3] and the shift observed towards higher energies in SiOxNy can be explained by the increase of the average number of oxygen backbonds with S i - H vibration resonance frequency. A peak at 3372 cm -1, close to the N - H resonance has not be assigned in the literature; this peak intensity varies with the oxygen concentration in the films and may be related to N - H bonds whose resonance vibrations are shifted because of oxygen backbonds. The results described here show how critical the kinetics of the reaction are. At 750°C (and above) nitrogen protoxide begins to dissociate and oxygen is incorporated in the layers. The material properties of the resulting SiOxNy films are reported in the following for this deposition temperature.
128
F Lebland et al. / Rapid thermal CVD of SiOxNy
4. Deposition rates and structural properties of the films The kinetics of silicon nitride deposition under the condition fixed above have been already reported previously [3]. The deposition rate is maxim u m when the N H 3 / S i H 4 ratio is adjusted to 10. Below this ratio the reaction rate decreases in the reaction rate controlled regime and above it decreases due to a retardation controlled regime. Deposition rates of SiO«Ny films were studied on the basis of these results and to obtain good uniformity of the film the total pressure was fixed at 50 Torr. In order to incorporate more and more oxygen in the layers, only the N20 flow rate was changed. Plots of deposition rate versus N 2 0 for several N H 3 / S i H 4 ratios are shown in fig. 3. It shows one overcomes the critical rate of 60 Ä / s by a proper adjustment of the flow rates of the reactive gases silane and ammonia. Mechanisms of decomposition of N20 at 750°C were not directly established. In the same CVD conditions, we noticed similar growth rates for silicon nitride films (where N20 = 0) and silicon oxyninitride obtained with a low N20 concentration is kept low (less than 1%). Increasing further the nitrogen protoxide flow rate led to a decrease in the overall deposition rate. Those facts may be explained assuming that primary or by-products of N20 decomposition, such N - O + N or N 2 + O, were emitted during the pyrolytic dissociation. These by-products increase or decrease the partial pressure of non-reactive gas (nitrogen) and therefore
120 750% , 50T -- 100
II~--..~
NH3/SiH~
8O ~ 6O ~ 4C tJ 20
~b 2'0 3'0 Jo s'o go 7'o 8'o 9b 100 N20 FLOW RATE (oma/min) Fig. 3. G r o w t h rate versus N 2 0 for several N H 3 / S i H 4 ratios; d e p o s i t i o n rate higher t h a n 60 Ä / s can be obtained.
i
10
Si-O in SiOz Tp
~-
]
SiHt, fixed NH31
/
///
,,//-~
III
\\\
/,///--"~ \
/
N2 )
0
Si-N in Si3N«
/
!
\',
J
\\
,I . ./ y/
° ~ k 1350
126~-11'82-- ~ÓgB 10'14 9}0 8/.6 WAVENUHBER (cm-1)
76}
678
Fig. 4. I R s p e c t r u m for several N 2 0 flows, a p p e a r a n c e of both
contributions.
increase the dilution of the reactive species in the mixture which then decreases the deposition rate. The I R spectra for several values of the N20 flow are reported in fig. 4 showing the effects associated to increased oxidation on the structural properties of these layers. Between the absorption band location in extreme materials, that is the S i - N bond in silicon nitride (835 cm -1 at 750°C) and the S i - O bond in silicon dioxide (1070 cm i at 700°C), a large mixture of both features is observed in oxynitride films. In the alloy, the large peak in the region 1350-600 cm -1 represents the sum of two contributions, one with a S i - O character, and one with a S i - N character, which are more or less important according to the oxygen concentration in the reaction chamber. In fact, with about 1% of N 2 0 gas flow the S i - N peak shifts towards the S i - O peak by only a few wavenumbers. When the N 2 0 flow rate is increased this peak splits in two with the Si-O-like band becoming more and more intense. At high N 2 0 flow rates thin films are mainly composed of oxygen, and nitrogen in the I R spectrum acts as a perturbation of a predominantly Si-O-like band. SIMS depth profiles confirm this observation. As a matter of fact, the N / S i ratio in fig. 5 (dashed lines) did not vary significantly while the O / S i ratio increased when the N20 concentration varied
F. Lebland et al. / Rapid therrnal CVD of SiO~ N,.
regime where the refractive index drops sharply, followed by a slow decrease towards its value in SiO 2. A refractive index varying slowly with oxygen concentration can be used in applications that require a high degree of control of the index of refraction (guiding devices). In the composition regime where the index of refraction varies sharply with oxygen concentration, devices that need a large change in index of refraction (multidielectric layers for mirrors) can be designed. A waveguide on InP using a sandwich of SiOxN », in between two layers of SiO 2 is now being realized. The size of the guiding layer can be chosen at will since the index of the siO~N ~,can be varied continuously.
-- --N/Si --0/Si
BI ~
N20in% [01xlOz2'l[N]xlO22 (at/cm3)! (at/cm3)
lll\,,Y~
/
~
\
~
/ ~
I
I
I
I
i
~~
1.88
o.2~
9
123
2ss
6
0198
2.86
3
0.84
2.98
TINE (s)
i
Fig. 5. SIMS depth profile of SiOxN». films; oxygen concentration increases with increasing N20 flow in the gas stream.
from 3% to 25%. This is the reason why it is assumed that nitrogen coming from N20 is not included in the layers but increases the dilution and thus slows down the reaction. At the same time free oxygen produced by N20 decomposition is incorporated in the films. Nuclear reaction analysis measurements gave a quantitative confirmation of there results (see fig. 5). The index of refraction is directly related to the stoichiometry of the film. This is shown in fig. 6. This curve features two different slopes, with a
2.1
i
I
'
129
i
5. Conclusion
The R T C V D process is unique in its capacity to deposit high-grade dielectrics on I I I - V materials at high temperature. Dielectrics deposited by this technique have a low density of defects and residual impurity, especially hydrogen (less than 1%). Pyrolytic SiOxN », layers using N20 as an oxidant gas have properties that fill the gap between silicon dioxide and silicon nitride. In particular their index of refraction can be varied continuously between 1.45 and 2.2 with the oxygen concentration in the layer. This property allows the use of such materials for optical guiding to be envisioned. Waveguides on InP substrate are now being fabricated on this basis and results will be reported in a forthcoming paper.
mu_l.9 et: i
~z x1.7
- - 1.5
References 5
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35
N20 FLOW RATE (in %) Fig. 6. Index of refraction versus N20 flow rate; n varies continuously between silicon dioxide and silicon index of refraction.
[1] G.E. Morosanu, Thin Solid Films 65 (1980) A1-208. [2] Y.I. Nissim, C. Licoppe, J.M. Moison, J.L. Regolini, D. Bensahel and G. Auvert, Proc. SPIE 1033 (1988) 273. [3] Y.I. Nissim, J.M. Moison, F. Houzay, F. Lebland, C. Licoppe and M. Bensoussan, Appl. Surf. Sci. 46 (1990) 175.