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Deposition and characterization of silicon oxynitride for integrated optical applications
Journal of Non-Crystalline Solids 338–340 (2004) 76–80 www.elsevier.com/locate/jnoncrysol
Deposition and characterization of silicon oxynitride for integrated optical applications M.I. Alayo a, D. Criado a
a,*
, L.C.D. Goncßalves b, I. Pereyra
a
LME, EPUSP, University of S~ao Paulo, CEP 5424-970, CP 61548, S~ao Paulo, SP, Brazil b LSI, EPUSP, University of S~ao Paulo, CEP 05508-900, S~ao Paulo, SP, Brazil Available online 19 March 2004
Abstract In this work we present the results of studies on the deposition and characterization of silicon oxynitride films deposited by plasma enhanced chemical vapor deposition technique using N2 , N2 O and SiH4 gaseous mixtures at low temperatures. Rutherford backscattering spectroscopy and refractive index measurements demonstrate that it is possible to transit from silicon dioxide to stoichiometric silicon nitride by varying the N2 /N2 O ratio in the precursor gaseous mixture. Stress measurements and plasma etching experiments show that both the internal stress and the etching rate are very sensitive to the films chemical composition and for specific deposition conditions it is possible to obtain films with very low stress and with high plasma etching rate. These results are very promising for application in low-cost, compact integrated optical devices. Ó 2004 Elsevier B.V. All rights reserved. PACS: 85.40.Sz; 81.15.Gh
1. Introduction The integration of optical and microelectronic devices is receiving nowadays increasing attention mainly due to the advantages that can be attained with this technology, such as: a significant reduction of the devices sensitivity to electromagnetic radiations and increasing packing density in integrated circuits [1–3]. In this way various studies searching for dielectric and semiconductor materials compatible with the optical technology as well as with the microelectronic technology, are being conducted [4,5]. Silicon oxynitride (SiOx Ny ) is considered an excellent candidate for this kind of integration mainly due to its low absorption in the visible range [6,7] and to the possibility of tuning its the refractive index between 1.47 (silicon oxide) and 2.3 (silicon nitride) thus resulting in a
* Corresponding author. Tel.: +55-11 3091 5256; fax: +55-11 3091 5585. E-mail addresses: [email protected] (M.I. Alayo), [email protected] (D. Criado).
0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.025
large degree of freedom for device design in integrated optics [8–11]. Among the variety of techniques available for silicon oxynitride production, plasma enhanced chemical vapor deposition (PECVD) technique is one of the most widely utilized due to the relative high deposition rates and low deposition temperatures [12–15]. On the other hand, for optical applications, besides the tunability of the refractive index of the materials involved, thick films (3–5 lm) with low internal stress are required. However, the literature reports that SiOx Ny films deposited by PECVD tend to scratch for thickness higher than 5 lm [16,17] due to the high residual stress that these materials present. In this work we focus on the correlation among the deposition conditions and the films chemical composition with their internal stress and etching characteristics in order to identify the growth parameters which lead to a material appropriate for integrated optics applications. In this way, a systematic variation of the deposition conditions was done in order to produce series of silicon oxynitride (SiOx Ny ) samples which were characterized by RBS, elipsometry, stress measurements and etching experiments.
M.I. Alayo et al. / Journal of Non-Crystalline Solids 338–340 (2004) 76–80
The SiOx Ny films studied in this work were deposited in a standard 13.56 MHz RF PECVD capacitively coupled system described elsewhere [18], from appropriate gaseous mixtures of electronic grade (99.999%) silane (SiH4 ), nitrous oxide (N2 O) and nitrogen (N2 ). The films were obtained utilizing different nitrogen and nitrous oxide gaseous flows but maintaining the total N2 O + N2 flow equal to 75 sccm and with nitrogen gaseous flow varying in 0, 15, 30, 45, 60 and 75 sccm. Also, all samples were deposited with 100 nm thick for RBS and ellipsometry characterization, 800 nm and 2.5 lm thick for stress measurements and 3 lm thick for etching rate measurements. All the samples studied were deposited at 320 °C, since our previous works [19] have shown that this is the optimum temperature to prevent undesirable Si–OH bonds. The RF power density was also kept at 500 mW cm2 in order to obtain high deposition rates. The deposition pressure was kept as low as possible, in order to increase the mean free path of reactant molecules, and consequently, diminishing the variety of heterogeneous and gas phase reactions. The SiH4 flow was fixed at 15 sccm, value high enough to lead to appropriate deposition rates for thick films production [20] but sufficiently low as to prevent undesirable gas phase reactions. For these conditions, the deposition rate was between 15 and 25 nm/min. All the studied films were deposited onto p type, (1 0 0), single crystalline silicon substrates in the 1–10 Xcm resistivity range, for stress, ellipsometry and etching rate measurements. On the other hand, for Rutherford backscattering spectroscopy (RBS) measurements the films were deposited onto ultra dense amorphous carbon. The ellipsometry characterization to obtain the refractive index and the thickness of the deposited films was performed in a Rudolph research auto E1 instrument, having a He–Ne laser (632.8 nm) as light source. The measurements were carried out at different points of the films in order to determine the standard deviation of the results along the sample. The thickness of the samples was also determined with a Tencor 500 profilometry. The amount of Si, N and O per unit area (atoms cm2 ) was obtained by RBS experiments at LAMFI/USP, S~ ao Paulo, using a Heþ beam with energy of 1.7 MeV, charge of 30 lm, current of 30 nA and detection angle of 170°. The stress measurements were made in a Tencor stress measurement (model FLX2410). This system permits one to calculate the residual stress by measuring the radius of curvature of the wafer, before and after the films deposition, using the laser beam reflection method. Finally, the samples were dry etched in a reactive ion etching equipment (model
Table 1 Conditions utilized for the plasma etching studies. In all these experiments the O2 gaseous flow was fixed in 40 sccm Power (W)
CHF3 flow (sccm)
Pressure (mTorr)
Variable parameter
350
40
100
Pressure
150 200 350
20
150
CHF3 flow
150
Power
40 50 250 350
40
Plasmalab 80) using CHF3 and O2 gaseous mixtures and varying the process pressure, the CHF3 gaseous flow, the RF power and processing time. The process parameters for the dry-etching studies are shown in Table 1.
3. Results and discussion In Fig. 1 the nitrogen, oxygen and silicon concentration as a function of the N2 and N2 O gaseous flow are shown. Similar results were reported for films obtained by the remote plasma enhanced chemical vapor deposition technique utilizing SiH4 , N2 , N2 O and He gaseous mixtures [21]. It is interest to point out, however that for the same nitrogen incorporation in the solid phase we utilized a much lower nitrogen concentration in the gaseous phase. This result may be related with the
70
Atomic Concentration (%)
2. Experimental details
77
Si O N
60 50 40 30 20 10 0 -10 -10
0
10
20
30
40
50
60
70
80
N2 Gaseous Flow (sccm)
(
N2O flow)
Fig. 1. Atomic concentration of oxygen ( ), nitrogen (N) and silicon (j) in the films deposited with different N2 and N2 O gaseous flows. The SiH4 gaseous flow was maintained constant in 15 sccm.
78
M.I. Alayo et al. / Journal of Non-Crystalline Solids 338–340 (2004) 76–80
higher SiH4 to N2 O flow ratio, the higher deposition pressures and the Remote Plasma technique utilized by these authors. It is observed that the highest oxygen incorporation in the solid phase occurs, as expected, for the highest N2 O gaseous flow utilized. The material obtained under these deposition conditions presents a chemical composition similar to stoichiometric silicon dioxide (67 at.% for oxygen and 33 at.% for silicon). Furthermore, increasing the nitrogen gaseous flow (and diminishing therefore the nitrous oxide one) increases the nitrogen concentration and diminishes the oxygen concentration monotonically until a composition similar to stoichiometric silicon nitride is attained. Also, we can observe in Fig. 1 that the nitrogen concentration in the material does not present a linear increase with the nitrogen gaseous flow, being more effective when higher nitrogen gaseous flow are utilized. This can be attributed to the different reactivity between nitrogen, oxygen and silane radicals in the plasma. It is known that silicon radicals react preferentially with oxygen radicals to form Si–O bonds [19,22], and only when all oxygen radicals have been consumed that Si–N, Si–H and N–H bonds are formed. In this way, when high nitrous oxide gaseous flows are utilized, many oxygen radicals are present in the plasma, and so, almost all the silane radicals will react with oxygen radicals, resulting in few Si–N bonds in the solid phase. Only when low nitrous oxide gaseous flows (and therefore, low quantity of oxygen radicals), Si–N bonds will be incorporated in the material. All these results are totally compatible with the FTIR characterizations presented in previous works [23]. The refractive index as a function on the nitrogen atomic concentration in the films is shown in Fig. 2 where a linear variation is observed from 1.46 (silicon dioxide) to 2.0 (stoichiometric silicon nitride) depending on the nitrogen atomic concentration. This result demonstrates the viability of using these materials in optical
applications where an accurate control of this parameter is indispensable [8–10]. The results for the stress measurements are shown in Fig. 3. These measurements were performed in films with 800 nm and 2.5 lm thick but non appreciable changes were observed. Other authors [24] have observed that there is a variation of the stress in silicon oxynitride films for different thickness, being tensive for thin films and compressive for thick films. Other authors [22,24,25] considering a small thermal stress in the films concluded that this variation can be related with changes in the intrinsic stress of the films due to the deposition method utilized. In our case, we do not observed changes in the stress of films with different thickness, however, measurements of the thermal stress in the films must be done in order to confirm this result. Even more, a dependence of the stress value with the nitrogen concentration in the material can be appreciated in Fig. 3. It is seen that the residual stress varies from compressive to tensile as the chemical composition of the material varies from silicon dioxide to silicon nitride respectively. Besides it, the stress values are small for films with high concentrations of nitrogen. Other authors have simulated the stress values as a function of the chemical composition of the films [22] using XPS measurements and the behavior obtained by them was similar to our results (compressive for SiO2 and tensile for Si3 N4 ). However, the theoretical stress values obtained by these authors was higher than the values obtained in our films, being this difference more evident for silicon nitride film (a difference of about 750 MPa). This can be due to that the literature uses a random mixture model (RMM) to calculate the stress values. In our case, X-ray absorption near edge structure (XANES) characterization, reported in previous works [26], demonstrated that our material presents a behavior compatible with a random bonding model (RBM) rather than with RMM and so expected that our materials present a different behavior. Among the different parameters that can be varied in the plasma etching technique, the literature commonly
100
1.9
0
Stress (MPa)
refractive index (n)
2.0
1.8 1.7 1.6 1.5
-100 -200 -300 -400
0
10
20
30
40
50
60
N atomic concentration (%)
-500
0
10
20
30
40
50
60
N atomic concentration (%) Fig. 2. Refractive index as a function of the nitrogen atomic concentration.
Fig. 3. Stress values as a function of the nitrogen atomic concentration.
M.I. Alayo et al. / Journal of Non-Crystalline Solids 338–340 (2004) 76–80
etching rate (nm/min)
700 600
55 at.%N 31 at.%N 6 at.%N 0 at.%N
800
(a)
500 400 300 200 100 0
55 at.%N 31 at.%N 6 at.%N 0 at.%N
700 etching rate (nm/min)
800
600
400 300 200 100
100
150
0
200
800
20 30 40 50 CHF3 gaseous flow (sccm)
500 400 300 200 100 0
85
(c)
55 at.%N 31 at.%N 6 at.%N 0 at.%N
etching rate (nm/min)
etching rate (nm/min)
600
(b)
500
Pressure (mTorr)
700
79
(d)
80
75
70 250
300
350
Power (W)
4
6
8 10 12 14 16 18 20 22 Etching Time (min)
Fig. 4. Etching rate as a function of the (a) etching pressure; (b) CHF3 gaseous flow; (c) etching power and (d) etching time.
reports that only some of them have significant effect in the etching rate values [27,28], so in this work we study the dependence of the etching rate as a function of these significant parameter such as: (a) pressure, (b) CHF3 gaseous flow, (c) RF power and, (d) etching time. The results are despite in Fig. 4 where it can be appreciated that the etching rate is almost constant for all the films with nitrogen atomic concentrations lower than 18% (films deposited with nitrogen gaseous flows lower than 60 sccm). For higher nitrogen concentrations, the etching rate increases attaining the maximun value for when silicon nitride like films, deposited with a nitrogen gaseous flow of 75 sccm. This result was expected in some way since other authors report higher etching rates for silicon nitride than for silicon dioxide [29]. The variation of the pressure (Fig. 4(a)) shows that for the intermediate pressure value studied (150 mTorr) the larger etching rate is obtained. Also, when a CHF3 gaseous flow of 40 sccm is utilized (Fig. 4(b)) larger etching rate values are obtained. Furthermore, Fig. 4(c) shows that for a RF power of 350 W an etching rate practically twice that obtained for 250 W is obtained for all samples. This result is compatible with the literature,
where larger etching rates for higher powers are reported [27]. With these results it was possible to determine the optimal conditions for etching silicon oxide, silicon oxynitride and silicon nitride materials. Even more, the results show that it is possible to obtain a selectivity ratio between Si3 N4 and SiO2 materials of up to 4, also compatible with the literature [27]. Finally, in Fig. 4(d) is shown the etching rate as a function of the time, it is seen that the etching rate decreases with time, result which can be related with the formation of polymers during the etching process which may originate from involatile etching products or from film-forming precursors that were adsorbed during the etching process [28].
4. Conclusions Silicon oxynitride films with optical and physical characteristics appropriated for optical applications were deposited by the PECVD technique. The results show the possibility of tuning the refractive index between 1.46 and 2.0 resulting in a large degree of freedom
80
M.I. Alayo et al. / Journal of Non-Crystalline Solids 338–340 (2004) 76–80
for device design in integrated optics. Besides it, the stress results demonstrated that the material varies from compressive to tensile, depending of the chemical atomic concentration. Finally, the plasma etching studies allow to observe the dependence of the etching rate as a function of different etching parameters, and a selectivity of up to 4 between the Si3 N4 and the SiO2 material.
Acknowledgements The authors acknowledge to Barbara D. Silva by the help in the etching process and characterization and Manfredo H. Tabacniks for the RBS measurements. The authors are grateful also to Brazilian agencies FAPESP (Process numbers: 00/10027-3, 03/04523-6 and 01/ 06516-1) and CNPq for financial support.
References [1] M.P.Y. Desmulliez, Mater. Sci. Eng. B-Solid 74 (2000) 269. [2] A.V. Krishnamoorthy, IEEE J. Sel. Top. Quantum Electron. 4 (1998) 899. [3] Y.T. Kim, S.M. Cho, Y.G. Seo, H.D. Yoon, Y.M. Im, S.J. Suh, D.H. Yoon, Surf. Coat. Technol. 171 (2003) 34. [4] M. Modreanu, M. Gartner, N. Tomozeiu, J. Seekamp, P. Cosmin, Opt. Mater. 17 (2001) 145. [5] S.R. Walker, J.A. Davies, P. Mascher, S.G. Wallace, W.N. Lennard, G.R. Massoumi, R.G. Elliman, T.R. Ophel, H. Timmers, Nucl. Instrum. Meth. Phys. Res. B 170 (2000) 461. [6] K.C. Mohite, Y.B. Khollam, A.B. Mandale, K.R. Patil, M.G. Takwale, Mater. Lett. 4494 (2003) 1. [7] R. Machorro, E.C. Samano, G. Soto, F. Villa, L. Cota-Araiza, Mater. Lett. 45 (2000) 47.
[8] K. Worhoff, A. Driessen, P.V. Lambeck, L.T.H. Hilderink, P.W.C. Linders, Th.J.A. Popma, Sens. Actuators 74 (1999) 9. [9] C. Dominguez, J.A. Rodriguez, F.J. Munoz, N. Zine, Vacuum 52 (1999) 395. [10] M. Modreanu, M. Gartner, J. Mol. Struct. 565&566 (2001) 519. [11] J. Viard, R. Berjoan, J. Durand, J. Eur. Ceram. Soc. 17 (1997) 2001. [12] C. Gorecki, Opt. Lasers Eng. 33 (2000) 15. [13] Y. Ma, G. Lucovsky, J. Vac. Sci. Technol. B 12 (4) (1994) 2504. [14] H. Niimi, K. Koh, G. Luckosky, Nucl. Instrum. Meth. Phys. Res. B 127&128 (1997) 364. [15] D.L. Smith, J. Vac. Sci. Technol. A 11 (4) (1993) 1843. [16] M.N.P. Carre~ no, M.I. Alayo, I. Pereyra, A.T. Lopes, Sens. Actuators, A 100 (2002) 295. [17] D.A.P. Bulla, N.I. Morimoto, Thin Solid Films 334 (1998) 60. [18] M.N.P. Carre~ no, J.P. Bottechia, I. Pereyra, Thin Solid Films 308 (1997) 219. [19] I. Pereyra, M.I. Alayo, J. Non-Cryst. Solids 212 (1997) 225. [20] M.I. Alayo, I. Pereyra, M.N.P. Carre~ no, Thin Solid Films 332 (1998) 40. [21] S.V. Hattangady, H. Niimi, G. Lucovsky, J. Vac. Sci. Technol. A 14 (6) (1996) 3017. [22] P. Temple-Boyer, B. Hajji, J.L. Alay, J.R. Morante, A. Martinez, Sens. Actuators 74 (1999) 52. [23] D. Criado, I. Pereyra, M.I. Alayo, Mater. Charact. 50 (2003) 167. [24] Y. Shima, H. Hasuyama, T. Kondoh, Y. Imaoka, T. Watari, K. Baba, R. Hatada, Nucl. Instrum. Meth. Phys. Res. B 148 (1999) 599. [25] K. Danaie, A. Bosseboeuf, C. Clerc, C. Gousset, G. Julie, Sens. Actuators, A 99 (2002) 78. [26] W.L. Scopel, M.C.A. Fantini, M.I. Alayo, I. Pereyra, Thin Solid Films 413 (2002) 59. [27] N.G. Einspruch, D.M. Brow, VLSI Electronics – Microstructure Science, in: Plasma Processing for VLSI, vol. 8, Academic Press, 1984. [28] M. Madou, Fundamentals of Microfabrication – The Science of Miniaturization, 2nd Ed., CRC, 2002. [29] T. Sugano, Applications of Plasma Processes to VLSI Technology, Wiley Interscience, 1985.
Deposition and characterization of silicon oxynitride for integrated optical applications M.I. Alayo a, D. Criado a
a,*
, L.C.D. Goncßalves b, I. Pereyra
a
LME, EPUSP, University of S~ao Paulo, CEP 5424-970, CP 61548, S~ao Paulo, SP, Brazil b LSI, EPUSP, University of S~ao Paulo, CEP 05508-900, S~ao Paulo, SP, Brazil Available online 19 March 2004
Abstract In this work we present the results of studies on the deposition and characterization of silicon oxynitride films deposited by plasma enhanced chemical vapor deposition technique using N2 , N2 O and SiH4 gaseous mixtures at low temperatures. Rutherford backscattering spectroscopy and refractive index measurements demonstrate that it is possible to transit from silicon dioxide to stoichiometric silicon nitride by varying the N2 /N2 O ratio in the precursor gaseous mixture. Stress measurements and plasma etching experiments show that both the internal stress and the etching rate are very sensitive to the films chemical composition and for specific deposition conditions it is possible to obtain films with very low stress and with high plasma etching rate. These results are very promising for application in low-cost, compact integrated optical devices. Ó 2004 Elsevier B.V. All rights reserved. PACS: 85.40.Sz; 81.15.Gh
1. Introduction The integration of optical and microelectronic devices is receiving nowadays increasing attention mainly due to the advantages that can be attained with this technology, such as: a significant reduction of the devices sensitivity to electromagnetic radiations and increasing packing density in integrated circuits [1–3]. In this way various studies searching for dielectric and semiconductor materials compatible with the optical technology as well as with the microelectronic technology, are being conducted [4,5]. Silicon oxynitride (SiOx Ny ) is considered an excellent candidate for this kind of integration mainly due to its low absorption in the visible range [6,7] and to the possibility of tuning its the refractive index between 1.47 (silicon oxide) and 2.3 (silicon nitride) thus resulting in a
* Corresponding author. Tel.: +55-11 3091 5256; fax: +55-11 3091 5585. E-mail addresses: [email protected] (M.I. Alayo), [email protected] (D. Criado).
0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.025
large degree of freedom for device design in integrated optics [8–11]. Among the variety of techniques available for silicon oxynitride production, plasma enhanced chemical vapor deposition (PECVD) technique is one of the most widely utilized due to the relative high deposition rates and low deposition temperatures [12–15]. On the other hand, for optical applications, besides the tunability of the refractive index of the materials involved, thick films (3–5 lm) with low internal stress are required. However, the literature reports that SiOx Ny films deposited by PECVD tend to scratch for thickness higher than 5 lm [16,17] due to the high residual stress that these materials present. In this work we focus on the correlation among the deposition conditions and the films chemical composition with their internal stress and etching characteristics in order to identify the growth parameters which lead to a material appropriate for integrated optics applications. In this way, a systematic variation of the deposition conditions was done in order to produce series of silicon oxynitride (SiOx Ny ) samples which were characterized by RBS, elipsometry, stress measurements and etching experiments.
M.I. Alayo et al. / Journal of Non-Crystalline Solids 338–340 (2004) 76–80
The SiOx Ny films studied in this work were deposited in a standard 13.56 MHz RF PECVD capacitively coupled system described elsewhere [18], from appropriate gaseous mixtures of electronic grade (99.999%) silane (SiH4 ), nitrous oxide (N2 O) and nitrogen (N2 ). The films were obtained utilizing different nitrogen and nitrous oxide gaseous flows but maintaining the total N2 O + N2 flow equal to 75 sccm and with nitrogen gaseous flow varying in 0, 15, 30, 45, 60 and 75 sccm. Also, all samples were deposited with 100 nm thick for RBS and ellipsometry characterization, 800 nm and 2.5 lm thick for stress measurements and 3 lm thick for etching rate measurements. All the samples studied were deposited at 320 °C, since our previous works [19] have shown that this is the optimum temperature to prevent undesirable Si–OH bonds. The RF power density was also kept at 500 mW cm2 in order to obtain high deposition rates. The deposition pressure was kept as low as possible, in order to increase the mean free path of reactant molecules, and consequently, diminishing the variety of heterogeneous and gas phase reactions. The SiH4 flow was fixed at 15 sccm, value high enough to lead to appropriate deposition rates for thick films production [20] but sufficiently low as to prevent undesirable gas phase reactions. For these conditions, the deposition rate was between 15 and 25 nm/min. All the studied films were deposited onto p type, (1 0 0), single crystalline silicon substrates in the 1–10 Xcm resistivity range, for stress, ellipsometry and etching rate measurements. On the other hand, for Rutherford backscattering spectroscopy (RBS) measurements the films were deposited onto ultra dense amorphous carbon. The ellipsometry characterization to obtain the refractive index and the thickness of the deposited films was performed in a Rudolph research auto E1 instrument, having a He–Ne laser (632.8 nm) as light source. The measurements were carried out at different points of the films in order to determine the standard deviation of the results along the sample. The thickness of the samples was also determined with a Tencor 500 profilometry. The amount of Si, N and O per unit area (atoms cm2 ) was obtained by RBS experiments at LAMFI/USP, S~ ao Paulo, using a Heþ beam with energy of 1.7 MeV, charge of 30 lm, current of 30 nA and detection angle of 170°. The stress measurements were made in a Tencor stress measurement (model FLX2410). This system permits one to calculate the residual stress by measuring the radius of curvature of the wafer, before and after the films deposition, using the laser beam reflection method. Finally, the samples were dry etched in a reactive ion etching equipment (model
Table 1 Conditions utilized for the plasma etching studies. In all these experiments the O2 gaseous flow was fixed in 40 sccm Power (W)
CHF3 flow (sccm)
Pressure (mTorr)
Variable parameter
350
40
100
Pressure
150 200 350
20
150
CHF3 flow
150
Power
40 50 250 350
40
Plasmalab 80) using CHF3 and O2 gaseous mixtures and varying the process pressure, the CHF3 gaseous flow, the RF power and processing time. The process parameters for the dry-etching studies are shown in Table 1.
3. Results and discussion In Fig. 1 the nitrogen, oxygen and silicon concentration as a function of the N2 and N2 O gaseous flow are shown. Similar results were reported for films obtained by the remote plasma enhanced chemical vapor deposition technique utilizing SiH4 , N2 , N2 O and He gaseous mixtures [21]. It is interest to point out, however that for the same nitrogen incorporation in the solid phase we utilized a much lower nitrogen concentration in the gaseous phase. This result may be related with the
70
Atomic Concentration (%)
2. Experimental details
77
Si O N
60 50 40 30 20 10 0 -10 -10
0
10
20
30
40
50
60
70
80
N2 Gaseous Flow (sccm)
(
N2O flow)
Fig. 1. Atomic concentration of oxygen ( ), nitrogen (N) and silicon (j) in the films deposited with different N2 and N2 O gaseous flows. The SiH4 gaseous flow was maintained constant in 15 sccm.
78
M.I. Alayo et al. / Journal of Non-Crystalline Solids 338–340 (2004) 76–80
higher SiH4 to N2 O flow ratio, the higher deposition pressures and the Remote Plasma technique utilized by these authors. It is observed that the highest oxygen incorporation in the solid phase occurs, as expected, for the highest N2 O gaseous flow utilized. The material obtained under these deposition conditions presents a chemical composition similar to stoichiometric silicon dioxide (67 at.% for oxygen and 33 at.% for silicon). Furthermore, increasing the nitrogen gaseous flow (and diminishing therefore the nitrous oxide one) increases the nitrogen concentration and diminishes the oxygen concentration monotonically until a composition similar to stoichiometric silicon nitride is attained. Also, we can observe in Fig. 1 that the nitrogen concentration in the material does not present a linear increase with the nitrogen gaseous flow, being more effective when higher nitrogen gaseous flow are utilized. This can be attributed to the different reactivity between nitrogen, oxygen and silane radicals in the plasma. It is known that silicon radicals react preferentially with oxygen radicals to form Si–O bonds [19,22], and only when all oxygen radicals have been consumed that Si–N, Si–H and N–H bonds are formed. In this way, when high nitrous oxide gaseous flows are utilized, many oxygen radicals are present in the plasma, and so, almost all the silane radicals will react with oxygen radicals, resulting in few Si–N bonds in the solid phase. Only when low nitrous oxide gaseous flows (and therefore, low quantity of oxygen radicals), Si–N bonds will be incorporated in the material. All these results are totally compatible with the FTIR characterizations presented in previous works [23]. The refractive index as a function on the nitrogen atomic concentration in the films is shown in Fig. 2 where a linear variation is observed from 1.46 (silicon dioxide) to 2.0 (stoichiometric silicon nitride) depending on the nitrogen atomic concentration. This result demonstrates the viability of using these materials in optical
applications where an accurate control of this parameter is indispensable [8–10]. The results for the stress measurements are shown in Fig. 3. These measurements were performed in films with 800 nm and 2.5 lm thick but non appreciable changes were observed. Other authors [24] have observed that there is a variation of the stress in silicon oxynitride films for different thickness, being tensive for thin films and compressive for thick films. Other authors [22,24,25] considering a small thermal stress in the films concluded that this variation can be related with changes in the intrinsic stress of the films due to the deposition method utilized. In our case, we do not observed changes in the stress of films with different thickness, however, measurements of the thermal stress in the films must be done in order to confirm this result. Even more, a dependence of the stress value with the nitrogen concentration in the material can be appreciated in Fig. 3. It is seen that the residual stress varies from compressive to tensile as the chemical composition of the material varies from silicon dioxide to silicon nitride respectively. Besides it, the stress values are small for films with high concentrations of nitrogen. Other authors have simulated the stress values as a function of the chemical composition of the films [22] using XPS measurements and the behavior obtained by them was similar to our results (compressive for SiO2 and tensile for Si3 N4 ). However, the theoretical stress values obtained by these authors was higher than the values obtained in our films, being this difference more evident for silicon nitride film (a difference of about 750 MPa). This can be due to that the literature uses a random mixture model (RMM) to calculate the stress values. In our case, X-ray absorption near edge structure (XANES) characterization, reported in previous works [26], demonstrated that our material presents a behavior compatible with a random bonding model (RBM) rather than with RMM and so expected that our materials present a different behavior. Among the different parameters that can be varied in the plasma etching technique, the literature commonly
100
1.9
0
Stress (MPa)
refractive index (n)
2.0
1.8 1.7 1.6 1.5
-100 -200 -300 -400
0
10
20
30
40
50
60
N atomic concentration (%)
-500
0
10
20
30
40
50
60
N atomic concentration (%) Fig. 2. Refractive index as a function of the nitrogen atomic concentration.
Fig. 3. Stress values as a function of the nitrogen atomic concentration.
M.I. Alayo et al. / Journal of Non-Crystalline Solids 338–340 (2004) 76–80
etching rate (nm/min)
700 600
55 at.%N 31 at.%N 6 at.%N 0 at.%N
800
(a)
500 400 300 200 100 0
55 at.%N 31 at.%N 6 at.%N 0 at.%N
700 etching rate (nm/min)
800
600
400 300 200 100
100
150
0
200
800
20 30 40 50 CHF3 gaseous flow (sccm)
500 400 300 200 100 0
85
(c)
55 at.%N 31 at.%N 6 at.%N 0 at.%N
etching rate (nm/min)
etching rate (nm/min)
600
(b)
500
Pressure (mTorr)
700
79
(d)
80
75
70 250
300
350
Power (W)
4
6
8 10 12 14 16 18 20 22 Etching Time (min)
Fig. 4. Etching rate as a function of the (a) etching pressure; (b) CHF3 gaseous flow; (c) etching power and (d) etching time.
reports that only some of them have significant effect in the etching rate values [27,28], so in this work we study the dependence of the etching rate as a function of these significant parameter such as: (a) pressure, (b) CHF3 gaseous flow, (c) RF power and, (d) etching time. The results are despite in Fig. 4 where it can be appreciated that the etching rate is almost constant for all the films with nitrogen atomic concentrations lower than 18% (films deposited with nitrogen gaseous flows lower than 60 sccm). For higher nitrogen concentrations, the etching rate increases attaining the maximun value for when silicon nitride like films, deposited with a nitrogen gaseous flow of 75 sccm. This result was expected in some way since other authors report higher etching rates for silicon nitride than for silicon dioxide [29]. The variation of the pressure (Fig. 4(a)) shows that for the intermediate pressure value studied (150 mTorr) the larger etching rate is obtained. Also, when a CHF3 gaseous flow of 40 sccm is utilized (Fig. 4(b)) larger etching rate values are obtained. Furthermore, Fig. 4(c) shows that for a RF power of 350 W an etching rate practically twice that obtained for 250 W is obtained for all samples. This result is compatible with the literature,
where larger etching rates for higher powers are reported [27]. With these results it was possible to determine the optimal conditions for etching silicon oxide, silicon oxynitride and silicon nitride materials. Even more, the results show that it is possible to obtain a selectivity ratio between Si3 N4 and SiO2 materials of up to 4, also compatible with the literature [27]. Finally, in Fig. 4(d) is shown the etching rate as a function of the time, it is seen that the etching rate decreases with time, result which can be related with the formation of polymers during the etching process which may originate from involatile etching products or from film-forming precursors that were adsorbed during the etching process [28].
4. Conclusions Silicon oxynitride films with optical and physical characteristics appropriated for optical applications were deposited by the PECVD technique. The results show the possibility of tuning the refractive index between 1.46 and 2.0 resulting in a large degree of freedom
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for device design in integrated optics. Besides it, the stress results demonstrated that the material varies from compressive to tensile, depending of the chemical atomic concentration. Finally, the plasma etching studies allow to observe the dependence of the etching rate as a function of different etching parameters, and a selectivity of up to 4 between the Si3 N4 and the SiO2 material.
Acknowledgements The authors acknowledge to Barbara D. Silva by the help in the etching process and characterization and Manfredo H. Tabacniks for the RBS measurements. The authors are grateful also to Brazilian agencies FAPESP (Process numbers: 00/10027-3, 03/04523-6 and 01/ 06516-1) and CNPq for financial support.
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