- Email: [email protected]
Properties of nitrogen doped silicon films deposited by low pressure chemical vapour deposition from disilane and ammonia
Thin Solid Films 414 (2002) 13–17
Properties of nitrogen doped silicon films deposited by low pressure chemical vapour deposition from disilane and ammonia P. Temple-Boyer*, L. Jalabert, E. Couderc, E. Scheid, P. Fadel, B. Rousset LAAS-CNRS, 7 avenue du colonel Roche, 31077 Toulouse Cedex 4, France Received 20 July 2001; received in revised form 2 February 2002; accepted 15 May 2002
Abstract Nitrogen doped silicon films have been deposited by low pressure chemical vapour deposition from disilane Si2 H6 and ammonia NH3. Deposition kinetics is investigated, pointing out the influences of the deposition temperature, the total pressure and the gas flow rates. According to the Bruggeman theory, variations of the NH3 ySi2 H6 gaseous ratio allow for a wide range of the SiNx stoichiometry as well as a good control of the film nitrogen doping. The different behaviours of the nitrogen atom in silicon films are discussed and an overview of the nitrogen doped silicon physical properties (optical, mechanical and electrical) is proposed for the development of boron-doped polysilicon gates. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapour deposition; Disilane; Nitrogen doped silicon; Physical properties
1. Introduction For the past few decades, miniaturization allows for the improvement of the performances of submicronic MOSFET transistors. As the threshold voltage is linked to the technological process, the control of this nonscalable parameter is of paramount importance in the CMOS structure. The introduction of nitrogen atoms during the oxidation or post-oxidation anneal using ammonia NH3, nitrous oxide N2O or nitric oxide NO gaseous sources improves the behaviour of the silicon oxide as a barrier against boron penetration from the polysilicon gate w1–5x. However, the presence of nitrogen at the SiySiO2 interface reduces the boron concentration in the substrate, but not in the gate oxide. In order to prevent boron penetration into the oxide, one solution consists in the deposition of a thin nitrogen doped silicon (NIDOS) layer at the gateyoxide interface. The development of such layers, either implanted w6x or in-situ doped w7,8x, has led to the study of metaly NIDOSyoxideysilicon or polysiliconyNIDOSyoxidey silicon capacitive structures. However, if NIDOS films *Corresponding author. Tel.: q33-5-61-33-69; fax: q33-5-61-3362-08. E-mail address: [email protected] (P. Temple-Boyer).
are to be successfully used in advanced polysilicon gates, the nitrogen atom behaviour in silicon films must be understood and the varied NIDOS film properties must be studied and optimised accordingly. The aim of this work is to study the deposition kinetics of nitrogen doped silicon films deposited by low pressure chemical vapour deposition (LPCVD) from disilane Si2H6 and ammonia NH3, and to characterise the various physical properties of NIDOS films (optical, electrical and mechanical) for their future use in advanced polycrystalline gate structures. 2. Experiments Growth experiments were carried out in a conventional hot-wall, horizontal, low pressure chemical vapour deposition (LPCVD) furnace. Nitrogen doped silicon films were deposited from disilane Si2H6 and ammonia NH3 on 10 cm, (111), oxidised (approx. 120 nm thermal oxide) silicon wafers. The deposition temperature T and the NH3 ySi2H6 gaseous ratio R ranged from 450 to 480 8C and from 0 to 6, respectively, while the total pressure P was set constant at approximately 200 mtorr. A full load of 12 wafers (set at 20 mm from each other) was included for each deposition experiment. Test wafers
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 4 3 4 - 0
14
P. Temple-Boyer et al. / Thin Solid Films 414 (2002) 13–17
rate Vd of NIDOS films deposited from disilane Si2H6 and ammonia NH3 will be easily obtained from the deposition rate Vd0 of silicon films deposited from pure disilane w10–13x: Vds
Fig. 1. Deposition kinetics of nitrogen doped silicon films for different NH3ySi2H6 gaseous ratios R.
were situated at the positions 4 to 9. Some of the NIDOS samples were boron-doped by implantation (energy: 15 keV-dose: 1015 at.ycm2). Finally, 600–1150 8C thermal anneals were performed in a conventional furnace under nitrogen N2. Deposition thickness and refractive index were measured by ellipsometry at 830 nm wavelength and checked by profilometry. Wafer curvature measurements were performed by profilometry before and after removal of the back side deposition by chemical etching in the solution: HNO3 (50 ml)–CH3COOH (20 ml)–HF (2 ml). Film residual stress was calculated by the formula of Stoney w9x. However, since this stress measurement technique requires high film thickness (100 nm and more) in order to be accurate, the study of the thermomechanical stress has only been performed on NIDOS films deposited at 480 8C (thickness range: 180–220 nm). Finally, the electrical properties of boron-doped NIDOS films were studied and their sheet resistance was measured by 4-point probe experiments.
1 Vd ŽT,P,«. 1qR 0
(1)
Therefore, for low NH3 ySi2H6 gaseous ratios (R-6), ammonia NH3 can be considered as a neutral gas from the kinetics point of view (even if it is responsible for the nitrogen doping—see hereafter). In a previous work, the Bruggeman theory has been successfully applied to the SiNx material by considering the heterogeneous medium formed by the mix between amorphous silicon a-Si and silicon nitride Si3N4. Thus, a parabolic relation has been exhibited between the SiNx film refractive index n (assessed at 830 nm wavelength) and the NySi ratio x with error lower than 5% w14x: ns4y2.19xq0.51x2
(2)
In the following, and thanks to this relation, the Ny Si ratio, i.e. the SiNx stoichiometry of each deposited NIDOS film, has been calculated from its refractive index measured by ellipsometry at a 830 nm wavelength. In case of the SiH4 yNH3 gaseous source, the formation of nitrogen doped silicon has been previously explained by introducing the influences of radical species like silylene SiH2, monoaminosilane SiH3NH2 and silylamine SiH2NH w15x. This assumption has been kept in the case of the Si2H6 yNH3 gaseous source. Therefore, considering that the influence of silane is negligible at such low temperature (480 8C and less), the following chemical system can be proposed for the deposition of nitrogen doped silicon films: Si2H6™SiH4qSiH2
(3)
SiH2qNH3™SiH3NH2
(4)
3. Results and discussion 3.1. Deposition properties of NIDOS films The deposition kinetic of NIDOS film has been represented on Fig. 1. For NH3 ySi2H6 gaseous ratios R ranging from 0 to 1, the activation energy Ea can be considered as constant, roughly equal to 2.25 eV. In fact, the ammonia flow increase is only responsible for the decrease of the deposition rate for a given temperature. In order to characterise this influence, deposition rates Vd have been represented as a function of the ratio between the ammonia flow and the total gas flow, i.e. as a function of 1y(1qR) (Fig. 2). Since linear curves are evidenced, the influence of the ammonia flow on the deposition rate is only related to the decrease of the partial pressure of disilane Si2H6. Thus, the deposition
Fig. 2. Deposition rate as a function of the NH3ySi2H6 gaseous ratio R for different deposition temperatures T.
P. Temple-Boyer et al. / Thin Solid Films 414 (2002) 13–17
Fig. 3. NySi ratio as a function of the NH3ySi2H6 gaseous ratio for different deposition temperatures T.
15
Fig. 4. NySi ratio as a function of the ratio between the NH3ySi2H6 gaseous ratio R and the deposition rate from pure disilane Vd0 for different deposition temperatures T.
SiH3NH2™SiH2NHqH2
(5)
SiH2™SiqH2
(6)
deposition rate Vd, the SiNx stoichiometry and refractive index n as a function of all the deposition parameters.
SiH2NH™SiqNq3y2H2
(7)
3.2. Thermo-mechanical stress of NIDOS films
Fig. 3 represents the variations of the NySi ratio as a function of the gaseous ratio R for different temperatures. Since ammonia NH3 is responsible for the deposition of nitrogen atoms thanks to the dissociative absorption of SiH2NH wEq. (7)x, the NySi ratio of the deposited NIDOS films increases with the NH3 ySi2H6 gaseous ratio. Furthermore, since silicon atoms adsorption phenomena are related to the very reactive silylene molecule SiH2 wEq. (6)x, the temperature increase leads to the decrease of the NySi ratio for a given gaseous ratio R. All in all, low temperatures and high NH3 y Si2H6 gaseous ratios are necessary for the obtaining of high nitrogen doping (NySi)0.1). However, since reasonable deposition rates, i.e. higher than 1 nmymin, are also highly recommended for the fabrication of polycrystalline gate structure, high temperature and high gaseous ratio should be finally chosen for depositing heavily doped NIDOS films. In order to explain the deposition of nitrogen doped silicon, the NySi ratio has been represented as a function of the ratio between the gaseous ratio R and the deposition rate from pure disilane Vd0 (Fig. 4). Thus, a linear relation can be empirically written: xs
N R f0.32 Si Vd0ŽT,P,«.
In the following, the stress has been considered as an algebraic value: a compressive stress will be negative, a tensile stress positive. In previous works, the residual stress of silicon films deposited from disilane Si2H6 has been related to their ordering u, their hydrogen content wHx and their crystallisation level x w16x. For the study of nitrogen doped silicon films deposited from disilane Si2H6 and ammonia NH3, this phenomenological study has been kept, the ordering u being replaced by the NH3 ySi2H6 gaseous ratio R. Thus, the residual stress s of NIDOS films is given by: sssRqswHxqsx
Fig. 5 represents the variations of NIDOS film residual stress as a function of the NySi ratio in the following
(8)
Therefore, if ammonia (NH3) can be considered as a neutral gas from the kinetics point of view, this result is no longer true if the SiNx stoichiometry is considered. The NySi ratio is directly related to the NH3 ySi2H6 gaseous ratio R even if it is balanced by the deposition rate from pure disilane Vd0. All in all, Eqs. (1), (3) and (8) allow determination and control of the NIDOS film
(9)
Fig. 5. Residual stress as a function of the NySi ratio.
P. Temple-Boyer et al. / Thin Solid Films 414 (2002) 13–17
16
Fig. 6. Stresses sR, swHx and sx as a function of the NySi ratio.
cases: as-deposited, annealed 15 min at 600 8C and annealed 60 min at 850 8C. However, in order to analyse such curves, it is better to use the sR, swHx, and sx parameters. Since the deposition temperature is lower than 480 8C, no crystallisation phenomenon occurs during the film deposition (sxs0). Thus, it can be written: (10)
sas-depositedssRqswHx
Furthermore, as it has been demonstrated that the 600 8C anneal was responsible for the dehydrogenation of films deposited from disilane Si2H6 w16x, it can be written: s600
(11)
ssR
8Cy15 min
Finally, the 850 8C anneal allows the analysis of the crystallisation phenomena for the different NIDOS films: s850
(12)
ssRqsx
8Cy60 min
From Eqs. (10)–(12), the different components of the residual stress s can be easily calculated: sRss600
3.3. Electrical properties of NIDOS films The sheet resistance of boron-doped NIDOS films has been studied for annealing temperature ranging from 700 to 1150 8C (Fig. 7). For high nitrogen doping (Ny Si)0.05), no conductive properties has been found in spite of the high boron concentration (wBxf5=1019 at.y cm3). This result should be related to the insulative Si–N sp2-like geometry obtained for high NH3 ySi2H6 gaseous ratio (R)1, see below). For low nitrogen doping (NySi-0.05), the conductive Si:N sp3-like geometry is favoured and the sheet resistance is found to increase with the nitrogen doping (from 102 to 105 V and more«). Since the studied NIDOS films are boron-doped, this result demonstrates that, as a group-V atom, nitrogen acts as a n-type, either substitutional, either intersticial, impurity in silicon film deposited in such conditions (480 8C, R-1). However, considering the boron and nitrogen concentrations
(13)
8Cy15 min
swHxssas-depositedys600 sxssas-depositedys850
tion of SiH2NH wEq. (7)x takes over the adsorption of SiH2 wEq. (6)x. Thus, the rearrangement of Si–N–H and Si–H bonds to form Si–N–Si bonds favours the Si–N sp2-like geometry and the NIDOS film hydrogenation. Since these phenomena are shown to be, respectively, responsible for tensile and compressive stress w16,18x, the stress sR increases while the stress swHx decreases as shown on Fig. 6. Finally, the 850 8C–60 min anneal is shown to be responsible for tensile stress for NySi-0.1 and compressive stress for NySi)0.15 (Fig. 6). The tensile behaviour is known to be related to the NIDOS film crystallisation w16x and the stress sx is found to decrease with the NySi ratio. The compressive behaviour demonstrates that the chosen anneal is not enough to crystallise the heavily doped (NySi)0.15) NIDOS and therefore that the nitrogen doping inhibits crystallisation phenomena.
8Cy15 min
8Cy60 min
(14) (15)
Thus, Fig. 6 represents the variations of the stresses sR, swHx, and sx as a function of the NySi ratio. In order to explain such results, the dual behaviour of the nitrogen impurity in silicon should be taken into account w17x. For low NH3 ySi2H6 gaseous ratio (R-1), the adsorption of SiH2NH wEq. (7)x is low compared to the adsorption of SiH2 wEq. (6)x. Therefore, the Si:N sp3like geometry is favoured for low nitrogen doping (Ny Si-0.05). In such conditions, Fig. 6 demonstrates that the stress swHx increases, i.e. the NIDOS film hydrogenation decreases, while the stress sR decreases. For high NH3 ySi2H6 gaseous ratio (R)1), the adsorp-
Fig. 7. Boron-doped NIDOS sheet resistance as a function of the annealing temperature for different NySi ratios.
P. Temple-Boyer et al. / Thin Solid Films 414 (2002) 13–17
(wBxf5=1019 at.ycm3 and wNx)5=1020 at.ycm3 ), the obtaining of such high sheet resistances demonstrates that the nitrogen doping efficiency is very low, i.e. that the nitrogen atom prefers intersticial to substitutional sites. For silicon films (NySis0), the influence of the annealing temperature is easily understood: as temperature increases, boron atoms diffuse out of the deposited film and the sheet resistance increases (Fig. 7). For NIDOS films, this phenomenon is still exhibited. However, for a given annealing temperature, the sheet resistance decreases suddenly. This variation should be related to the crystallisation of the boron-doped NIDOS film. As the NySi ratio increases, crystallisation phenomena are inhibited and higher annealing temperatures are required to evidence the sheet resistance decrease. Finally, it is very interesting to show that, for a 1150 8C anneal, the boron-doped NIDOS film sheet resistance is roughly constant (Rf400 V) whatever the nitrogen doping lower than 0.02. 4. Conclusion A complete study of the LPCVD deposition of nitrogen doped silicon (NIDOS) from disilane Si2H6 and ammonia NH3 has been done. Since ammonia has been shown to act as a neutral gas, the influences of the different deposition parameters (temperature, total pressure,«) have been clarified, pointing out the major influence of the NH3 ySi2H6 gaseous ratio. Thus, a wide range and a good control of the film nitrogen doping (according to Bruggeman theory, of the film refractive index) have been obtained and related to the influences of very reactive chemical species like silylene SiH2, monoaminosilane SiH3NH2 and silylamine SiH2NH in the gaseous phase. Mechanical properties of NIDOS films have also been studied, characterising the influences of the film hydrogenation and nitridation and demonstrating that nitrogen inhibits the silicon crystallisation. Finally, the electrical properties of boron-doped NIDOS films have been investigated. This study has
17
pointed out the ambivalent behaviour of the nitrogen atom in silicon, giving preference either to the insulative Si–N sp2-like geometry, either to the conductive Si:N sp3-like one, according to the NH3 ySi2H6 gaseous ratio. Conductive properties have been obtained for low nitrogen doping ratio (NySi-0.05) but the obtaining of high sheet resistances have also demonstrated the low doping efficiency of the nitrogen atom. However, despite their high electrical resistivity (up to 10 V cm for NySi0.05), ‘lightly doped’ NIDOS thin films will be still useable for the realisation of poly-Si:ByNIDOS:By SiO2 ySi capacitive structures. References w1x Y.H. Lin, C.S. Lai, C.L. Lee, T.F. Lei, T.S. Chao, IEEE Trans. Electron Devices 43 (1996) 1161. w2x T. Aoyama, K. Suzuki, H. Tashiro, Y. Tada, K. Horiuchi, J. Electrochem. Soc. 145 (1998) 689. w3x D.A. Buchanan, S.H. Lo, Microelectron. Eng. 36 (1997) 13. w4x M. Navi, S.T. Dunham, J. Electrochem. Soc. 145 (1998) 2545. w5x D. Wristers, L.K. Han, T. Chen, H.H. Wang, D.L. Kwong, M. Allen, J. Fulford, Appl. Phys. Lett. 68 (1996) 2094. w6x A. Yasuoka, T. Kuroi, S. Shimizu, M. Shirahata, Y. Okumura, Y. Inoue, M. Inuishi, T. Nishimura, H. Miyushi, Jpn. J. Appl. Phys. 36 (1997) 617. w7x S. Nakayama, T. Sakai, J. Electrochem. Soc. 144 (1997) 4326. w8x P. Temple-Boyer, F. Olivie, ´ E. Scheid, G. Sarrabayrouse, J.L. Alay, J.R. Morante, Microelectron. Reliabil. 39 (1999) 187. w9x G.G. Stoney, Proc. R. Soc. Lond. 9 (1909) 172. w10x E. Scheid, B. de Mauduit, P. Taurines, D. Bielle-Daspet, Jpn. J. Appl. Phys. 27 (1990) 2105. w11x N. Nakazawa, J. Appl. Phys. 69 (1991) 1703. w12x A.T. Voutsas, M.K. Hatalis, J. Electrochem. Soc. 140 (1993) 871. w13x E. Scheid, J.J. Pedroviejo, P. Duverneuil, M. Gueye, J. Samitier, A. El Hassani, D. Bielle-Daspet, Mater. Sci. Eng. B 17 (1993) 72. w14x E. Dehan, P. Temple-Boyer, R. Henda, J.J. Pedroviejo, E. Scheid, Thin Solid Films 266 (1995) 14. w15x P. Temple-Boyer, L. Jalabert, L. Masarotto, J.L. Alay, J.R. Morante, J. Vacuum Sci. Technol. A 18 (2000) 2389. w16x P. Temple-Boyer, E. Scheid, G. Faugere, ` B. Rousset, Thin Solid Films 310 (1997) 234. w17x M. Saito, M. Miyamoto, Phys. Rev. B 56 (1997) 9193. w18x A.G. Noskov, E.B. Gorokhov, G.A. Solokova, E.M. Trukhanov, S.I. Stenin, Thin Solid Films 162 (1988) 129.
Properties of nitrogen doped silicon films deposited by low pressure chemical vapour deposition from disilane and ammonia P. Temple-Boyer*, L. Jalabert, E. Couderc, E. Scheid, P. Fadel, B. Rousset LAAS-CNRS, 7 avenue du colonel Roche, 31077 Toulouse Cedex 4, France Received 20 July 2001; received in revised form 2 February 2002; accepted 15 May 2002
Abstract Nitrogen doped silicon films have been deposited by low pressure chemical vapour deposition from disilane Si2 H6 and ammonia NH3. Deposition kinetics is investigated, pointing out the influences of the deposition temperature, the total pressure and the gas flow rates. According to the Bruggeman theory, variations of the NH3 ySi2 H6 gaseous ratio allow for a wide range of the SiNx stoichiometry as well as a good control of the film nitrogen doping. The different behaviours of the nitrogen atom in silicon films are discussed and an overview of the nitrogen doped silicon physical properties (optical, mechanical and electrical) is proposed for the development of boron-doped polysilicon gates. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapour deposition; Disilane; Nitrogen doped silicon; Physical properties
1. Introduction For the past few decades, miniaturization allows for the improvement of the performances of submicronic MOSFET transistors. As the threshold voltage is linked to the technological process, the control of this nonscalable parameter is of paramount importance in the CMOS structure. The introduction of nitrogen atoms during the oxidation or post-oxidation anneal using ammonia NH3, nitrous oxide N2O or nitric oxide NO gaseous sources improves the behaviour of the silicon oxide as a barrier against boron penetration from the polysilicon gate w1–5x. However, the presence of nitrogen at the SiySiO2 interface reduces the boron concentration in the substrate, but not in the gate oxide. In order to prevent boron penetration into the oxide, one solution consists in the deposition of a thin nitrogen doped silicon (NIDOS) layer at the gateyoxide interface. The development of such layers, either implanted w6x or in-situ doped w7,8x, has led to the study of metaly NIDOSyoxideysilicon or polysiliconyNIDOSyoxidey silicon capacitive structures. However, if NIDOS films *Corresponding author. Tel.: q33-5-61-33-69; fax: q33-5-61-3362-08. E-mail address: [email protected] (P. Temple-Boyer).
are to be successfully used in advanced polysilicon gates, the nitrogen atom behaviour in silicon films must be understood and the varied NIDOS film properties must be studied and optimised accordingly. The aim of this work is to study the deposition kinetics of nitrogen doped silicon films deposited by low pressure chemical vapour deposition (LPCVD) from disilane Si2H6 and ammonia NH3, and to characterise the various physical properties of NIDOS films (optical, electrical and mechanical) for their future use in advanced polycrystalline gate structures. 2. Experiments Growth experiments were carried out in a conventional hot-wall, horizontal, low pressure chemical vapour deposition (LPCVD) furnace. Nitrogen doped silicon films were deposited from disilane Si2H6 and ammonia NH3 on 10 cm, (111), oxidised (approx. 120 nm thermal oxide) silicon wafers. The deposition temperature T and the NH3 ySi2H6 gaseous ratio R ranged from 450 to 480 8C and from 0 to 6, respectively, while the total pressure P was set constant at approximately 200 mtorr. A full load of 12 wafers (set at 20 mm from each other) was included for each deposition experiment. Test wafers
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 4 3 4 - 0
14
P. Temple-Boyer et al. / Thin Solid Films 414 (2002) 13–17
rate Vd of NIDOS films deposited from disilane Si2H6 and ammonia NH3 will be easily obtained from the deposition rate Vd0 of silicon films deposited from pure disilane w10–13x: Vds
Fig. 1. Deposition kinetics of nitrogen doped silicon films for different NH3ySi2H6 gaseous ratios R.
were situated at the positions 4 to 9. Some of the NIDOS samples were boron-doped by implantation (energy: 15 keV-dose: 1015 at.ycm2). Finally, 600–1150 8C thermal anneals were performed in a conventional furnace under nitrogen N2. Deposition thickness and refractive index were measured by ellipsometry at 830 nm wavelength and checked by profilometry. Wafer curvature measurements were performed by profilometry before and after removal of the back side deposition by chemical etching in the solution: HNO3 (50 ml)–CH3COOH (20 ml)–HF (2 ml). Film residual stress was calculated by the formula of Stoney w9x. However, since this stress measurement technique requires high film thickness (100 nm and more) in order to be accurate, the study of the thermomechanical stress has only been performed on NIDOS films deposited at 480 8C (thickness range: 180–220 nm). Finally, the electrical properties of boron-doped NIDOS films were studied and their sheet resistance was measured by 4-point probe experiments.
1 Vd ŽT,P,«. 1qR 0
(1)
Therefore, for low NH3 ySi2H6 gaseous ratios (R-6), ammonia NH3 can be considered as a neutral gas from the kinetics point of view (even if it is responsible for the nitrogen doping—see hereafter). In a previous work, the Bruggeman theory has been successfully applied to the SiNx material by considering the heterogeneous medium formed by the mix between amorphous silicon a-Si and silicon nitride Si3N4. Thus, a parabolic relation has been exhibited between the SiNx film refractive index n (assessed at 830 nm wavelength) and the NySi ratio x with error lower than 5% w14x: ns4y2.19xq0.51x2
(2)
In the following, and thanks to this relation, the Ny Si ratio, i.e. the SiNx stoichiometry of each deposited NIDOS film, has been calculated from its refractive index measured by ellipsometry at a 830 nm wavelength. In case of the SiH4 yNH3 gaseous source, the formation of nitrogen doped silicon has been previously explained by introducing the influences of radical species like silylene SiH2, monoaminosilane SiH3NH2 and silylamine SiH2NH w15x. This assumption has been kept in the case of the Si2H6 yNH3 gaseous source. Therefore, considering that the influence of silane is negligible at such low temperature (480 8C and less), the following chemical system can be proposed for the deposition of nitrogen doped silicon films: Si2H6™SiH4qSiH2
(3)
SiH2qNH3™SiH3NH2
(4)
3. Results and discussion 3.1. Deposition properties of NIDOS films The deposition kinetic of NIDOS film has been represented on Fig. 1. For NH3 ySi2H6 gaseous ratios R ranging from 0 to 1, the activation energy Ea can be considered as constant, roughly equal to 2.25 eV. In fact, the ammonia flow increase is only responsible for the decrease of the deposition rate for a given temperature. In order to characterise this influence, deposition rates Vd have been represented as a function of the ratio between the ammonia flow and the total gas flow, i.e. as a function of 1y(1qR) (Fig. 2). Since linear curves are evidenced, the influence of the ammonia flow on the deposition rate is only related to the decrease of the partial pressure of disilane Si2H6. Thus, the deposition
Fig. 2. Deposition rate as a function of the NH3ySi2H6 gaseous ratio R for different deposition temperatures T.
P. Temple-Boyer et al. / Thin Solid Films 414 (2002) 13–17
Fig. 3. NySi ratio as a function of the NH3ySi2H6 gaseous ratio for different deposition temperatures T.
15
Fig. 4. NySi ratio as a function of the ratio between the NH3ySi2H6 gaseous ratio R and the deposition rate from pure disilane Vd0 for different deposition temperatures T.
SiH3NH2™SiH2NHqH2
(5)
SiH2™SiqH2
(6)
deposition rate Vd, the SiNx stoichiometry and refractive index n as a function of all the deposition parameters.
SiH2NH™SiqNq3y2H2
(7)
3.2. Thermo-mechanical stress of NIDOS films
Fig. 3 represents the variations of the NySi ratio as a function of the gaseous ratio R for different temperatures. Since ammonia NH3 is responsible for the deposition of nitrogen atoms thanks to the dissociative absorption of SiH2NH wEq. (7)x, the NySi ratio of the deposited NIDOS films increases with the NH3 ySi2H6 gaseous ratio. Furthermore, since silicon atoms adsorption phenomena are related to the very reactive silylene molecule SiH2 wEq. (6)x, the temperature increase leads to the decrease of the NySi ratio for a given gaseous ratio R. All in all, low temperatures and high NH3 y Si2H6 gaseous ratios are necessary for the obtaining of high nitrogen doping (NySi)0.1). However, since reasonable deposition rates, i.e. higher than 1 nmymin, are also highly recommended for the fabrication of polycrystalline gate structure, high temperature and high gaseous ratio should be finally chosen for depositing heavily doped NIDOS films. In order to explain the deposition of nitrogen doped silicon, the NySi ratio has been represented as a function of the ratio between the gaseous ratio R and the deposition rate from pure disilane Vd0 (Fig. 4). Thus, a linear relation can be empirically written: xs
N R f0.32 Si Vd0ŽT,P,«.
In the following, the stress has been considered as an algebraic value: a compressive stress will be negative, a tensile stress positive. In previous works, the residual stress of silicon films deposited from disilane Si2H6 has been related to their ordering u, their hydrogen content wHx and their crystallisation level x w16x. For the study of nitrogen doped silicon films deposited from disilane Si2H6 and ammonia NH3, this phenomenological study has been kept, the ordering u being replaced by the NH3 ySi2H6 gaseous ratio R. Thus, the residual stress s of NIDOS films is given by: sssRqswHxqsx
Fig. 5 represents the variations of NIDOS film residual stress as a function of the NySi ratio in the following
(8)
Therefore, if ammonia (NH3) can be considered as a neutral gas from the kinetics point of view, this result is no longer true if the SiNx stoichiometry is considered. The NySi ratio is directly related to the NH3 ySi2H6 gaseous ratio R even if it is balanced by the deposition rate from pure disilane Vd0. All in all, Eqs. (1), (3) and (8) allow determination and control of the NIDOS film
(9)
Fig. 5. Residual stress as a function of the NySi ratio.
P. Temple-Boyer et al. / Thin Solid Films 414 (2002) 13–17
16
Fig. 6. Stresses sR, swHx and sx as a function of the NySi ratio.
cases: as-deposited, annealed 15 min at 600 8C and annealed 60 min at 850 8C. However, in order to analyse such curves, it is better to use the sR, swHx, and sx parameters. Since the deposition temperature is lower than 480 8C, no crystallisation phenomenon occurs during the film deposition (sxs0). Thus, it can be written: (10)
sas-depositedssRqswHx
Furthermore, as it has been demonstrated that the 600 8C anneal was responsible for the dehydrogenation of films deposited from disilane Si2H6 w16x, it can be written: s600
(11)
ssR
8Cy15 min
Finally, the 850 8C anneal allows the analysis of the crystallisation phenomena for the different NIDOS films: s850
(12)
ssRqsx
8Cy60 min
From Eqs. (10)–(12), the different components of the residual stress s can be easily calculated: sRss600
3.3. Electrical properties of NIDOS films The sheet resistance of boron-doped NIDOS films has been studied for annealing temperature ranging from 700 to 1150 8C (Fig. 7). For high nitrogen doping (Ny Si)0.05), no conductive properties has been found in spite of the high boron concentration (wBxf5=1019 at.y cm3). This result should be related to the insulative Si–N sp2-like geometry obtained for high NH3 ySi2H6 gaseous ratio (R)1, see below). For low nitrogen doping (NySi-0.05), the conductive Si:N sp3-like geometry is favoured and the sheet resistance is found to increase with the nitrogen doping (from 102 to 105 V and more«). Since the studied NIDOS films are boron-doped, this result demonstrates that, as a group-V atom, nitrogen acts as a n-type, either substitutional, either intersticial, impurity in silicon film deposited in such conditions (480 8C, R-1). However, considering the boron and nitrogen concentrations
(13)
8Cy15 min
swHxssas-depositedys600 sxssas-depositedys850
tion of SiH2NH wEq. (7)x takes over the adsorption of SiH2 wEq. (6)x. Thus, the rearrangement of Si–N–H and Si–H bonds to form Si–N–Si bonds favours the Si–N sp2-like geometry and the NIDOS film hydrogenation. Since these phenomena are shown to be, respectively, responsible for tensile and compressive stress w16,18x, the stress sR increases while the stress swHx decreases as shown on Fig. 6. Finally, the 850 8C–60 min anneal is shown to be responsible for tensile stress for NySi-0.1 and compressive stress for NySi)0.15 (Fig. 6). The tensile behaviour is known to be related to the NIDOS film crystallisation w16x and the stress sx is found to decrease with the NySi ratio. The compressive behaviour demonstrates that the chosen anneal is not enough to crystallise the heavily doped (NySi)0.15) NIDOS and therefore that the nitrogen doping inhibits crystallisation phenomena.
8Cy15 min
8Cy60 min
(14) (15)
Thus, Fig. 6 represents the variations of the stresses sR, swHx, and sx as a function of the NySi ratio. In order to explain such results, the dual behaviour of the nitrogen impurity in silicon should be taken into account w17x. For low NH3 ySi2H6 gaseous ratio (R-1), the adsorption of SiH2NH wEq. (7)x is low compared to the adsorption of SiH2 wEq. (6)x. Therefore, the Si:N sp3like geometry is favoured for low nitrogen doping (Ny Si-0.05). In such conditions, Fig. 6 demonstrates that the stress swHx increases, i.e. the NIDOS film hydrogenation decreases, while the stress sR decreases. For high NH3 ySi2H6 gaseous ratio (R)1), the adsorp-
Fig. 7. Boron-doped NIDOS sheet resistance as a function of the annealing temperature for different NySi ratios.
P. Temple-Boyer et al. / Thin Solid Films 414 (2002) 13–17
(wBxf5=1019 at.ycm3 and wNx)5=1020 at.ycm3 ), the obtaining of such high sheet resistances demonstrates that the nitrogen doping efficiency is very low, i.e. that the nitrogen atom prefers intersticial to substitutional sites. For silicon films (NySis0), the influence of the annealing temperature is easily understood: as temperature increases, boron atoms diffuse out of the deposited film and the sheet resistance increases (Fig. 7). For NIDOS films, this phenomenon is still exhibited. However, for a given annealing temperature, the sheet resistance decreases suddenly. This variation should be related to the crystallisation of the boron-doped NIDOS film. As the NySi ratio increases, crystallisation phenomena are inhibited and higher annealing temperatures are required to evidence the sheet resistance decrease. Finally, it is very interesting to show that, for a 1150 8C anneal, the boron-doped NIDOS film sheet resistance is roughly constant (Rf400 V) whatever the nitrogen doping lower than 0.02. 4. Conclusion A complete study of the LPCVD deposition of nitrogen doped silicon (NIDOS) from disilane Si2H6 and ammonia NH3 has been done. Since ammonia has been shown to act as a neutral gas, the influences of the different deposition parameters (temperature, total pressure,«) have been clarified, pointing out the major influence of the NH3 ySi2H6 gaseous ratio. Thus, a wide range and a good control of the film nitrogen doping (according to Bruggeman theory, of the film refractive index) have been obtained and related to the influences of very reactive chemical species like silylene SiH2, monoaminosilane SiH3NH2 and silylamine SiH2NH in the gaseous phase. Mechanical properties of NIDOS films have also been studied, characterising the influences of the film hydrogenation and nitridation and demonstrating that nitrogen inhibits the silicon crystallisation. Finally, the electrical properties of boron-doped NIDOS films have been investigated. This study has
17
pointed out the ambivalent behaviour of the nitrogen atom in silicon, giving preference either to the insulative Si–N sp2-like geometry, either to the conductive Si:N sp3-like one, according to the NH3 ySi2H6 gaseous ratio. Conductive properties have been obtained for low nitrogen doping ratio (NySi-0.05) but the obtaining of high sheet resistances have also demonstrated the low doping efficiency of the nitrogen atom. However, despite their high electrical resistivity (up to 10 V cm for NySi0.05), ‘lightly doped’ NIDOS thin films will be still useable for the realisation of poly-Si:ByNIDOS:By SiO2 ySi capacitive structures. References w1x Y.H. Lin, C.S. Lai, C.L. Lee, T.F. Lei, T.S. Chao, IEEE Trans. Electron Devices 43 (1996) 1161. w2x T. Aoyama, K. Suzuki, H. Tashiro, Y. Tada, K. Horiuchi, J. Electrochem. Soc. 145 (1998) 689. w3x D.A. Buchanan, S.H. Lo, Microelectron. Eng. 36 (1997) 13. w4x M. Navi, S.T. Dunham, J. Electrochem. Soc. 145 (1998) 2545. w5x D. Wristers, L.K. Han, T. Chen, H.H. Wang, D.L. Kwong, M. Allen, J. Fulford, Appl. Phys. Lett. 68 (1996) 2094. w6x A. Yasuoka, T. Kuroi, S. Shimizu, M. Shirahata, Y. Okumura, Y. Inoue, M. Inuishi, T. Nishimura, H. Miyushi, Jpn. J. Appl. Phys. 36 (1997) 617. w7x S. Nakayama, T. Sakai, J. Electrochem. Soc. 144 (1997) 4326. w8x P. Temple-Boyer, F. Olivie, ´ E. Scheid, G. Sarrabayrouse, J.L. Alay, J.R. Morante, Microelectron. Reliabil. 39 (1999) 187. w9x G.G. Stoney, Proc. R. Soc. Lond. 9 (1909) 172. w10x E. Scheid, B. de Mauduit, P. Taurines, D. Bielle-Daspet, Jpn. J. Appl. Phys. 27 (1990) 2105. w11x N. Nakazawa, J. Appl. Phys. 69 (1991) 1703. w12x A.T. Voutsas, M.K. Hatalis, J. Electrochem. Soc. 140 (1993) 871. w13x E. Scheid, J.J. Pedroviejo, P. Duverneuil, M. Gueye, J. Samitier, A. El Hassani, D. Bielle-Daspet, Mater. Sci. Eng. B 17 (1993) 72. w14x E. Dehan, P. Temple-Boyer, R. Henda, J.J. Pedroviejo, E. Scheid, Thin Solid Films 266 (1995) 14. w15x P. Temple-Boyer, L. Jalabert, L. Masarotto, J.L. Alay, J.R. Morante, J. Vacuum Sci. Technol. A 18 (2000) 2389. w16x P. Temple-Boyer, E. Scheid, G. Faugere, ` B. Rousset, Thin Solid Films 310 (1997) 234. w17x M. Saito, M. Miyamoto, Phys. Rev. B 56 (1997) 9193. w18x A.G. Noskov, E.B. Gorokhov, G.A. Solokova, E.M. Trukhanov, S.I. Stenin, Thin Solid Films 162 (1988) 129.