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Contribution of plasma generated nanocrystals to the growth of microcrystalline silicon thin films
Journal of Non-Crystalline Solids 338–340 (2004) 86–90 www.elsevier.com/locate/jnoncrysol
Contribution of plasma generated nanocrystals to the growth of microcrystalline silicon thin films S. Kasouit a
a,b
, J. Damon-Lacoste a, R. Vanderhaghen a, P. Roca i Cabarrocas
a,*
Laboratoire de Physique des Interfaces et des Couches Minces (UMR 7647 CNRS), Ecole Polytechnique, 91128 Palaiseau cedex, France b Unaxis France SA, Displays Division, 5 Rue Leon Blum, 91120 Palaiseau, France Available online 19 March 2004
Abstract We report on a particular growth mechanism for microcrystalline silicon thin films deposited using SiF4 /H2 /Ar mixtures. The structure of the films is studied by in situ ellipsometry and atomic force microscopy. The measurements show that growth starts directly from crystallites of various sizes without any detectable amorphous tissue. With increasing deposition time, we observe the formation of columns which grow laterally until their coalescence and the obtaining of a continuous film. Based on plasma impedance measurements, combined with the trapping of particles on a cold substrate, we attribute the early presence of crystallites to the sticking of plasma produced nanocrystals on the substrate, and the columnar growth to the preferential sticking on the top and beside the nanocrystals. Accordingly, modifying the surface of silicon nitride substrates by various plasma treatments allows one to change the density of sticking centers. It therefore gives us a way to control the size of the columns, which grow laterally from the initial crystallites. This is a new way to optimize devices where carriers flow parallel to the substrate, such as thin-film transistors (TFTs). 2004 Elsevier B.V. All rights reserved. PACS: 52.70.Nc; 52.75.Rx; 68.55.-a
1. Introduction Microcrystalline silicon (lc-Si:H) is a widely studied material because of its potential for devices such as thin film transistors [1] and solar cells [2]. The success of such devices is due to the transport properties of the material which are closely related to its structure. Microcrystalline silicon films are known to result from complex growth mechanisms leading to layers with a wide range of crystalline and void fractions [3], grain and column sizes [4], and overall to an inhomogeneous film composition along the growth direction [5]. All these parameters are strongly affected by the growth kinetics of the material, for which surface and bulk processes have been considered. While most studies have focused *
Corresponding author. Tel.: +33-1 69 33 32 07; fax: +33-1 69 33 30
06. E-mail address: [email protected] (P. Roca i Cabarrocas). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.027
on the obtaining of films from silane–hydrogen mixtures, less is known about the growth from SiF4 , even though it has been shown that high quality films can be obtained [6]. We have recently reported a particular growth process using SiF4 /Ar/H2 mixtures in which the films are completely crystallized from the initial stages of deposition [7], in contrast with the kinetics generally observed from silane–hydrogen mixtures [8]. Such specific growth process was attributed to the efficient etching of amorphous phase by fluorine [9]. In this paper, we study the growth and the morphology of lc-Si:H films deposited from mixtures of SiF4 , H2 and Ar. The structural properties are correlated with plasma measurements and thermophoresis experiments which suggest a substantial contribution of plasma generated nanocrystals to the growth. In the case of deposition on silicon nitride (a-SiN:H), changing the stoichiometry of a-SiN:H surface by plasma treatments has a strong effect on the growth kinetics of the material and provides a way to control the lateral size of
S. Kasouit et al. / Journal of Non-Crystalline Solids 338–340 (2004) 86–90
columns. This opens a new way for the optimization of the material for device applications.
3.1. Etching amorphous silicon in a SiF4 plasma As indicated above, in an earlier study [7] we attributed the high crystalline fraction and the absence of an amorphous phase in the initial stages of deposition of our films to an efficient etching of the amorphous phase by F atoms. In order to confirm this hypothesis we have studied the etching rate of a-Si:H films at various temperatures by a SiF4 plasma under a total pressure of 0.3 Torr and an RF power of 13 W. Fig. 1 shows the etching rate as a function of the substrate temperature. Surprisingly, the etching rate is quite low, in particular at 473 K which is our standard deposition temperature. As a matter of fact, at this temperature the etching by SiF4 is smaller than that measured for hydrogen plasma under similar conditions [14]. This result, along with the fact that the amorphous fraction was observed to develop with increasing film thickness [7], suggests that etching alone cannot be responsible for the high crystalline fraction in our films.
0.12 0.10 0.08 0.06 300
350
400 450 500 Temperature (K)
550
Fig. 1. Effect of the substrate temperature on the etching rate of a-Si:H films in a SiF4 plasma under an RF power of 13 W and at total pressure of 0.3 Torr.
3.2. Growth kinetics of microcrystalline films Fig. 2 shows the deposition rate of lc-Si:H films obtained from the dissociation of 10 sccm of hydrogen added to the SiF4 /Ar mixture as function of RF power, together with the product of the concentration of hydrogen and SiF2 deduced from OES. While the OES data linearly increase with RF power and so should do the deposition rate [10], the deposition rate is zero for RF powers below 10 W but sharply increases to 0.6 A/s above this power. To get more insight on our plasma, we measured the evolution of the second harmonic of the RF current (J2 ), which provides information on the dynamics of nanoparticle and powder formation [11]. Fig. 3 shows the evolution of J2 for various values of the RF power. While the value of J2 remains constant for an RF power of 5 W, indicating a pristine plasma, the increase of the RF power to 7 W results in a strong decrease of J2 after 20 s of plasma ignition. This evolution has been related to the accumulation of nanocrystals and clusters in the plasma, followed by their agglomeration which strongly changes the impedance of the discharge and results in
0.8 200 0.6
150
0.4
100
0.2 0.0
50
0
5
10
15
20
25
[SiF2]*[H] (a.u)
3. Results
Etching rate (Å/s)
Microcrystalline silicon films were deposited in a parallel plate capacitively coupled PECVD system at a RF frequency of 13.56 MHz. The distance between electrodes was fixed at 4.5 cm, the pressure at 1 Torr and SiF4 and Ar flow rates at 1 and 25 sccm respectively. The hydrogen flow rate was varied between 1 and 15 sccm and the total RF power between 5 and 30 W. The films were either deposited on Corning glass or on crystalline silicon coated with a 300 nm thick silicon nitride layer. A UVISEL, phase-modulated ellipsometer (by JYHoriba group) is integrated into the reactor allowing in situ study of the growth. Optical emission spectroscopy (OES) measurements were performed for various deposition conditions and special attention was paid to Ar, SiF2 and H emission lines, since it has been reported that the product of the intensity of [SiF2 ] · [H] emission intensity is proportional to the deposition rate [10]. The presence of nanoparticles, crystallites and powders in the plasma was studied through the evolution of the second harmonic of the RF current (J2 ) [11,12]. Moreover the reactor was fitted with a liquid nitrogen cooled substrate (cold trap) located on the side-wall between the two electrodes. This cold substrate was used to trap the nanoparticles present in the plasma via the thermophoretic force [13].
0.14
Deposition rate (Å/s)
2. Experiments
87
0 30
RF Power (W) Fig. 2. Deposition rate and [SiF2 ] · [H] product deduced from optical emission spectroscopy measurements as functions of the RF power for a substrate temperature of 200 C, a total pressure of 1 Torr and a hydrogen flow rate of 10 sccm.
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Crystalline Fraction (%)
20 W With cold trap
0.05
J2 (A)
0.04 0.03
7W
20 W
0.02 0.01 0.00
5W 0
20
40 60 Time (s)
80
100
Fig. 3. Time evolution of the second harmonic of the RF current for various RF powers and with the presence of a liquid nitrogen cooled finger which traps nanoparticles via thermophoresis.
the decrease of J2 [12]. For an RF power of 20 W the agglomeration is extremely fast and powders are formed almost immediately. However, the evolution of J2 when a cooled substrate is added shows an almost constant value (except for some periodic oscillations due to the instability of our H2 mass flow controller). Thus, the results shown in Fig. 3 indicate that the increase of deposition rate coincides with the apparition of nanoparticles in the plasma and that the growth with a high crystalline fraction from the beginning of deposition is not due to the efficient etching by F but rather due to the sticking onto the substrate of crystallites produced in the plasma. Accordingly, one would expect a large change in the growth kinetics by changing the chemistry of the substrate. 3.3. Effects of the surface chemistry In applications such as bottom-gate TFTs, lc-Si:H films are usually deposited on silicon nitride (a-SiN:H) which acts as a gate dielectric. We have previously found that plasma treatments of the a-SiN:H strongly affect the structure and the growth kinetics of the microcrystalline silicon films. It was shown that argon plasma treatment leads to a fast densification of the film compared with an untreated a-SiN:H substrate, whereas a nitrogen plasma treatment delays the complete crystallization. Moreover, Raman spectroscopy measurements show that the grain size is more important for the film that exhibits slower increase in crystalline fraction [1], indicating a lateral growth, limited by coalescence between columns. Fig. 4 shows the crystalline fraction as a function of the thickness for films deposited on a-SiN:H with 12 sccm of hydrogen added to the SiF4 –Ar mixture and 20 W for 30 min. Note that the amorphous fraction is negligible in all cases. The duration for the Ar plasma treatment was fixed at 7 min and for the N2 plasma 30 s and 2 min. We can observe that the Ar plasma treatment accelerates the increase of the crystalline fraction as a function of the film thickness, whereas N2 plasma delays
no treatment Ar 7' Ar 7' + N2 1T 30'' Ar 7' + N2 1T 2'
80 60 40 20 0 0
500
1000
1500
Thickness (Å) Fig. 4. Thickness dependence of the crystalline fraction for films deposited under identical conditions on silicon nitride substrates exposed to various plasma treatments prior to the deposition of microcrystalline silicon.
it. The effect is apparent even after a 30 s of a N2 plasma. For the argon treated substrate, the film crystallizes uniformly during its growth. For a N2 plasma treatment of 2 min we can clearly observe a three steps process characterized by (i) a fast increase of the crystalline fraction up to 30%, (ii) the increase in thickness at constant crystalline fraction, corresponding to the formation of columns, and (iii) the increase of the crystalline fraction associated to the filling of the voids between columns (coalescence).
4. Discussion The interpretation of the results presented above is not consistent with previous results on the growth mechanism of amorphous and microcrystalline silicon films from SiF4 , based on a competition of deposition from radicals and etching by either H or F [10]. Rather, based on the plasma impedance measurements and the material characteristics, we suggest that the observed growth kinetics and evolution of the crystalline fraction are due to the contribution of nanocrystals produced in the plasma and contributing to deposition, as already suggested [15]. To elucidate the high void fraction and to check the contribution of nanocrystals to deposition, AFM images were taken on samples at the initial stages of deposition. Fig. 5 gives an example for a 20 nm (top) and a 40 nm (bottom) thick films in which one can see cylindrical structures corresponding to the deposition of nanocrystals (top) which grow laterally as the deposition time increases (bottom). Further support to the contribution of silicon nanocrystals to growth is given by the deposition of films under conditions where nanoparticles are trapped on a substrate cooled by liquid N2 . Indeed, the evolution of the second harmonic of the RF current shown in Fig. 3 indicates that the use of a cold trap is
S. Kasouit et al. / Journal of Non-Crystalline Solids 338–340 (2004) 86–90
89
Without cold trap With cold trap
20 16 < Epsilon 2 >
110 nm 12 8 4 58 nm
0
30' deposition
-4 1.5
2.0
2.5 3.0 3.5 4.0 Photon energy (eV)
4.5
5.0
Fig. 6. Imaginary part of the pseudo-dielectric function of two microcrystalline silicon films deposited with or without a cold trap present in the plasma. Note the decrease of the thickness by a factor of two for the same deposition time when the cold finger is present.
Fig. 5. AFM images of microcrystalline silicon films deposited on a thermal oxide substrate. At the initial stage of deposition (top) the film is not continuous and results from the direct deposition of plasma generated nanocrystals. As deposition proceeds (bottom), nanocrystals deposit on top and beside initial ones leading to a lateral growth and the formation of large columns.
efficient to suppress powder formation by trapping the powder precursors. As a consequence, the concentration of nanoparticles cannot reach the critical concentration for agglomeration to occur and only nanoparticles are present in the discharge. Further evidence for the strong influence of nanocrystals on the growth process is given in Fig. 6, where we plot the imaginary part of the pseudo-dielectric function of two films deposited under the same plasma conditions but with or without trapping of the clusters and nanocrystals by the cooled substrate. As shown in Fig. 6, the deposition rate decreases by more than 50% in the case of trapping of nanoparticles on the cold trap, which is consistent with the absence of powders in the plasma and thus a lower power coupling to the plasma. Moreover the analysis of the ellipsometric spectra indicates that the roughness
and void fraction are higher in the case of deposition with the cold trap. Finally, the effect of the plasma treatment on the aSiN:H can be related to surface chemistry. Ar plasma produces silicon dangling bonds which enhance the density of nucleation sites and thus leads to a fast coalescence of the nucleation centers leading to a continuous film. On the contrary, N2 plasma passivates the surface of a-SiN:H leading to a small density of nucleation centers and the formation of columns due to the preferential sticking of nanocrystals on top of each other. This leads to a higher thickness compared with the case of the deposition on Ar treated substrates. The columns grow laterally by the deposition of other particles, or from the contribution of radicals. The size of grains for Ar treated substrates suggests that the dimensions of clusters contributing to the growth is less than 4 nm, which fits well with the particles being mainly neutral at such size [16]. The reduction of the density of sticking sites is an efficient way to increase the lateral size of the columns and overall to improved thin-film transistors (TFTs) characteristics [1]. 5. Conclusion We have studied the growth of microcrystalline silicon films from SiF4 , H2 and Ar mixtures. In situ growth studies by spectroscopic ellipsometry were performed and the results have been correlated to the presence of nanoparticles in the plasma and their evolution as a function of different plasma parameters, monitored through the measurement of the second harmonic of the RF current. These results, combined with AFM images of the films in the initial stages of deposition, along with the effects of thermophoresis on the dynamics of powder formation as well as on the properties of the deposited films, lead us to the conclusion that plasma generated
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silicon nanocrystals are responsible for the growth of microcrystalline films free from any amorphous phase.
References [1] S. Kasouit, P. Roca i Cabarrocas, R. Vanderhaghen, Y. Bonassieux, M. Elyaakoubi, I. French, J. Rocha, B. Vitoux, Thin Solid Films 427 (2003) 67. [2] A. Shah, J. Meier, E. Vallat-Sauvain, C. Droz, U. Kroll, N. Wyrsch, J. Guillet, U. Graf, Thin Solid Films 403&404 (2002) 179. [3] R. Brenot, R. Vanderhaghen, B. Drevillon, P. Roca i Cabarrocas, Appl. Phys. Lett. 74 (1999) 58. [4] E. Vallat-Sauvain, U. Kroll, J. Meier, A. Shah, J. Pohl, J. Appl. Phys. 87 (2000) 3137. [5] B. Kalache, A.I. Kosarev, R. Vanderhaghen, P. Roca i Cabarrocas, J. Appl. Phys. 93 (2003) 1262.
[6] S. Ishihara, D. He, M. Tanaka, I. Shimizu, Jpn. J. Appl. Phys. 32 (1993) 1539. [7] S. Kasouit, S. Kumar, R. Vanderhaghen, P. Roca i Cabarrocas, I.D. French, J. Non-Cryst. Solids 299–302 (2002) 113. [8] S. Hamma, P. Roca i Cabarrocas, J. Appl. Phys. 81 (1997) 7282. [9] Y. Okada, J. Chen, I.H. Campbell, P.M. Fauchet, S. Wagner, J. Appl. Phys. 67 (1990) 1757. [10] G. Bruno, P. Capezzuto, G. Cicala, J. Appl. Phys. 69 (1991) 7256. [11] A.V. Kharchenko, V. Suendo, P. Roca i Cabarrocas, Thin Solid Films 427 (2003) 236. [12] L. Boufendi, J. Gaudin, S. Huet, G. Viera, M. Dudemaine, Appl. Phys. Lett. 79 (2001) 4301. [13] N. Cha^abane, A.V. Kharchenko, H. Vach, P. Roca i Cabarrocas, New J. Phys. 5 (2003) 37.1. [14] A. Fontcuberta i Morral, P. Roca i Cabarrocas, J. Non-Cryst. Solids 299–302 (2002) 196. [15] P. Roca i Cabarrocas, J. Non-Cryst. Solids 266–269 (2000) 31. [16] A. Gallagher, Phys. Rev. E 62 (2000) 2690.
Contribution of plasma generated nanocrystals to the growth of microcrystalline silicon thin films S. Kasouit a
a,b
, J. Damon-Lacoste a, R. Vanderhaghen a, P. Roca i Cabarrocas
a,*
Laboratoire de Physique des Interfaces et des Couches Minces (UMR 7647 CNRS), Ecole Polytechnique, 91128 Palaiseau cedex, France b Unaxis France SA, Displays Division, 5 Rue Leon Blum, 91120 Palaiseau, France Available online 19 March 2004
Abstract We report on a particular growth mechanism for microcrystalline silicon thin films deposited using SiF4 /H2 /Ar mixtures. The structure of the films is studied by in situ ellipsometry and atomic force microscopy. The measurements show that growth starts directly from crystallites of various sizes without any detectable amorphous tissue. With increasing deposition time, we observe the formation of columns which grow laterally until their coalescence and the obtaining of a continuous film. Based on plasma impedance measurements, combined with the trapping of particles on a cold substrate, we attribute the early presence of crystallites to the sticking of plasma produced nanocrystals on the substrate, and the columnar growth to the preferential sticking on the top and beside the nanocrystals. Accordingly, modifying the surface of silicon nitride substrates by various plasma treatments allows one to change the density of sticking centers. It therefore gives us a way to control the size of the columns, which grow laterally from the initial crystallites. This is a new way to optimize devices where carriers flow parallel to the substrate, such as thin-film transistors (TFTs). 2004 Elsevier B.V. All rights reserved. PACS: 52.70.Nc; 52.75.Rx; 68.55.-a
1. Introduction Microcrystalline silicon (lc-Si:H) is a widely studied material because of its potential for devices such as thin film transistors [1] and solar cells [2]. The success of such devices is due to the transport properties of the material which are closely related to its structure. Microcrystalline silicon films are known to result from complex growth mechanisms leading to layers with a wide range of crystalline and void fractions [3], grain and column sizes [4], and overall to an inhomogeneous film composition along the growth direction [5]. All these parameters are strongly affected by the growth kinetics of the material, for which surface and bulk processes have been considered. While most studies have focused *
Corresponding author. Tel.: +33-1 69 33 32 07; fax: +33-1 69 33 30
06. E-mail address: [email protected] (P. Roca i Cabarrocas). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.027
on the obtaining of films from silane–hydrogen mixtures, less is known about the growth from SiF4 , even though it has been shown that high quality films can be obtained [6]. We have recently reported a particular growth process using SiF4 /Ar/H2 mixtures in which the films are completely crystallized from the initial stages of deposition [7], in contrast with the kinetics generally observed from silane–hydrogen mixtures [8]. Such specific growth process was attributed to the efficient etching of amorphous phase by fluorine [9]. In this paper, we study the growth and the morphology of lc-Si:H films deposited from mixtures of SiF4 , H2 and Ar. The structural properties are correlated with plasma measurements and thermophoresis experiments which suggest a substantial contribution of plasma generated nanocrystals to the growth. In the case of deposition on silicon nitride (a-SiN:H), changing the stoichiometry of a-SiN:H surface by plasma treatments has a strong effect on the growth kinetics of the material and provides a way to control the lateral size of
S. Kasouit et al. / Journal of Non-Crystalline Solids 338–340 (2004) 86–90
columns. This opens a new way for the optimization of the material for device applications.
3.1. Etching amorphous silicon in a SiF4 plasma As indicated above, in an earlier study [7] we attributed the high crystalline fraction and the absence of an amorphous phase in the initial stages of deposition of our films to an efficient etching of the amorphous phase by F atoms. In order to confirm this hypothesis we have studied the etching rate of a-Si:H films at various temperatures by a SiF4 plasma under a total pressure of 0.3 Torr and an RF power of 13 W. Fig. 1 shows the etching rate as a function of the substrate temperature. Surprisingly, the etching rate is quite low, in particular at 473 K which is our standard deposition temperature. As a matter of fact, at this temperature the etching by SiF4 is smaller than that measured for hydrogen plasma under similar conditions [14]. This result, along with the fact that the amorphous fraction was observed to develop with increasing film thickness [7], suggests that etching alone cannot be responsible for the high crystalline fraction in our films.
0.12 0.10 0.08 0.06 300
350
400 450 500 Temperature (K)
550
Fig. 1. Effect of the substrate temperature on the etching rate of a-Si:H films in a SiF4 plasma under an RF power of 13 W and at total pressure of 0.3 Torr.
3.2. Growth kinetics of microcrystalline films Fig. 2 shows the deposition rate of lc-Si:H films obtained from the dissociation of 10 sccm of hydrogen added to the SiF4 /Ar mixture as function of RF power, together with the product of the concentration of hydrogen and SiF2 deduced from OES. While the OES data linearly increase with RF power and so should do the deposition rate [10], the deposition rate is zero for RF powers below 10 W but sharply increases to 0.6 A/s above this power. To get more insight on our plasma, we measured the evolution of the second harmonic of the RF current (J2 ), which provides information on the dynamics of nanoparticle and powder formation [11]. Fig. 3 shows the evolution of J2 for various values of the RF power. While the value of J2 remains constant for an RF power of 5 W, indicating a pristine plasma, the increase of the RF power to 7 W results in a strong decrease of J2 after 20 s of plasma ignition. This evolution has been related to the accumulation of nanocrystals and clusters in the plasma, followed by their agglomeration which strongly changes the impedance of the discharge and results in
0.8 200 0.6
150
0.4
100
0.2 0.0
50
0
5
10
15
20
25
[SiF2]*[H] (a.u)
3. Results
Etching rate (Å/s)
Microcrystalline silicon films were deposited in a parallel plate capacitively coupled PECVD system at a RF frequency of 13.56 MHz. The distance between electrodes was fixed at 4.5 cm, the pressure at 1 Torr and SiF4 and Ar flow rates at 1 and 25 sccm respectively. The hydrogen flow rate was varied between 1 and 15 sccm and the total RF power between 5 and 30 W. The films were either deposited on Corning glass or on crystalline silicon coated with a 300 nm thick silicon nitride layer. A UVISEL, phase-modulated ellipsometer (by JYHoriba group) is integrated into the reactor allowing in situ study of the growth. Optical emission spectroscopy (OES) measurements were performed for various deposition conditions and special attention was paid to Ar, SiF2 and H emission lines, since it has been reported that the product of the intensity of [SiF2 ] · [H] emission intensity is proportional to the deposition rate [10]. The presence of nanoparticles, crystallites and powders in the plasma was studied through the evolution of the second harmonic of the RF current (J2 ) [11,12]. Moreover the reactor was fitted with a liquid nitrogen cooled substrate (cold trap) located on the side-wall between the two electrodes. This cold substrate was used to trap the nanoparticles present in the plasma via the thermophoretic force [13].
0.14
Deposition rate (Å/s)
2. Experiments
87
0 30
RF Power (W) Fig. 2. Deposition rate and [SiF2 ] · [H] product deduced from optical emission spectroscopy measurements as functions of the RF power for a substrate temperature of 200 C, a total pressure of 1 Torr and a hydrogen flow rate of 10 sccm.
88
S. Kasouit et al. / Journal of Non-Crystalline Solids 338–340 (2004) 86–90 100
Crystalline Fraction (%)
20 W With cold trap
0.05
J2 (A)
0.04 0.03
7W
20 W
0.02 0.01 0.00
5W 0
20
40 60 Time (s)
80
100
Fig. 3. Time evolution of the second harmonic of the RF current for various RF powers and with the presence of a liquid nitrogen cooled finger which traps nanoparticles via thermophoresis.
the decrease of J2 [12]. For an RF power of 20 W the agglomeration is extremely fast and powders are formed almost immediately. However, the evolution of J2 when a cooled substrate is added shows an almost constant value (except for some periodic oscillations due to the instability of our H2 mass flow controller). Thus, the results shown in Fig. 3 indicate that the increase of deposition rate coincides with the apparition of nanoparticles in the plasma and that the growth with a high crystalline fraction from the beginning of deposition is not due to the efficient etching by F but rather due to the sticking onto the substrate of crystallites produced in the plasma. Accordingly, one would expect a large change in the growth kinetics by changing the chemistry of the substrate. 3.3. Effects of the surface chemistry In applications such as bottom-gate TFTs, lc-Si:H films are usually deposited on silicon nitride (a-SiN:H) which acts as a gate dielectric. We have previously found that plasma treatments of the a-SiN:H strongly affect the structure and the growth kinetics of the microcrystalline silicon films. It was shown that argon plasma treatment leads to a fast densification of the film compared with an untreated a-SiN:H substrate, whereas a nitrogen plasma treatment delays the complete crystallization. Moreover, Raman spectroscopy measurements show that the grain size is more important for the film that exhibits slower increase in crystalline fraction [1], indicating a lateral growth, limited by coalescence between columns. Fig. 4 shows the crystalline fraction as a function of the thickness for films deposited on a-SiN:H with 12 sccm of hydrogen added to the SiF4 –Ar mixture and 20 W for 30 min. Note that the amorphous fraction is negligible in all cases. The duration for the Ar plasma treatment was fixed at 7 min and for the N2 plasma 30 s and 2 min. We can observe that the Ar plasma treatment accelerates the increase of the crystalline fraction as a function of the film thickness, whereas N2 plasma delays
no treatment Ar 7' Ar 7' + N2 1T 30'' Ar 7' + N2 1T 2'
80 60 40 20 0 0
500
1000
1500
Thickness (Å) Fig. 4. Thickness dependence of the crystalline fraction for films deposited under identical conditions on silicon nitride substrates exposed to various plasma treatments prior to the deposition of microcrystalline silicon.
it. The effect is apparent even after a 30 s of a N2 plasma. For the argon treated substrate, the film crystallizes uniformly during its growth. For a N2 plasma treatment of 2 min we can clearly observe a three steps process characterized by (i) a fast increase of the crystalline fraction up to 30%, (ii) the increase in thickness at constant crystalline fraction, corresponding to the formation of columns, and (iii) the increase of the crystalline fraction associated to the filling of the voids between columns (coalescence).
4. Discussion The interpretation of the results presented above is not consistent with previous results on the growth mechanism of amorphous and microcrystalline silicon films from SiF4 , based on a competition of deposition from radicals and etching by either H or F [10]. Rather, based on the plasma impedance measurements and the material characteristics, we suggest that the observed growth kinetics and evolution of the crystalline fraction are due to the contribution of nanocrystals produced in the plasma and contributing to deposition, as already suggested [15]. To elucidate the high void fraction and to check the contribution of nanocrystals to deposition, AFM images were taken on samples at the initial stages of deposition. Fig. 5 gives an example for a 20 nm (top) and a 40 nm (bottom) thick films in which one can see cylindrical structures corresponding to the deposition of nanocrystals (top) which grow laterally as the deposition time increases (bottom). Further support to the contribution of silicon nanocrystals to growth is given by the deposition of films under conditions where nanoparticles are trapped on a substrate cooled by liquid N2 . Indeed, the evolution of the second harmonic of the RF current shown in Fig. 3 indicates that the use of a cold trap is
S. Kasouit et al. / Journal of Non-Crystalline Solids 338–340 (2004) 86–90
89
Without cold trap With cold trap
20 16 < Epsilon 2 >
110 nm 12 8 4 58 nm
0
30' deposition
-4 1.5
2.0
2.5 3.0 3.5 4.0 Photon energy (eV)
4.5
5.0
Fig. 6. Imaginary part of the pseudo-dielectric function of two microcrystalline silicon films deposited with or without a cold trap present in the plasma. Note the decrease of the thickness by a factor of two for the same deposition time when the cold finger is present.
Fig. 5. AFM images of microcrystalline silicon films deposited on a thermal oxide substrate. At the initial stage of deposition (top) the film is not continuous and results from the direct deposition of plasma generated nanocrystals. As deposition proceeds (bottom), nanocrystals deposit on top and beside initial ones leading to a lateral growth and the formation of large columns.
efficient to suppress powder formation by trapping the powder precursors. As a consequence, the concentration of nanoparticles cannot reach the critical concentration for agglomeration to occur and only nanoparticles are present in the discharge. Further evidence for the strong influence of nanocrystals on the growth process is given in Fig. 6, where we plot the imaginary part of the pseudo-dielectric function of two films deposited under the same plasma conditions but with or without trapping of the clusters and nanocrystals by the cooled substrate. As shown in Fig. 6, the deposition rate decreases by more than 50% in the case of trapping of nanoparticles on the cold trap, which is consistent with the absence of powders in the plasma and thus a lower power coupling to the plasma. Moreover the analysis of the ellipsometric spectra indicates that the roughness
and void fraction are higher in the case of deposition with the cold trap. Finally, the effect of the plasma treatment on the aSiN:H can be related to surface chemistry. Ar plasma produces silicon dangling bonds which enhance the density of nucleation sites and thus leads to a fast coalescence of the nucleation centers leading to a continuous film. On the contrary, N2 plasma passivates the surface of a-SiN:H leading to a small density of nucleation centers and the formation of columns due to the preferential sticking of nanocrystals on top of each other. This leads to a higher thickness compared with the case of the deposition on Ar treated substrates. The columns grow laterally by the deposition of other particles, or from the contribution of radicals. The size of grains for Ar treated substrates suggests that the dimensions of clusters contributing to the growth is less than 4 nm, which fits well with the particles being mainly neutral at such size [16]. The reduction of the density of sticking sites is an efficient way to increase the lateral size of the columns and overall to improved thin-film transistors (TFTs) characteristics [1]. 5. Conclusion We have studied the growth of microcrystalline silicon films from SiF4 , H2 and Ar mixtures. In situ growth studies by spectroscopic ellipsometry were performed and the results have been correlated to the presence of nanoparticles in the plasma and their evolution as a function of different plasma parameters, monitored through the measurement of the second harmonic of the RF current. These results, combined with AFM images of the films in the initial stages of deposition, along with the effects of thermophoresis on the dynamics of powder formation as well as on the properties of the deposited films, lead us to the conclusion that plasma generated
90
S. Kasouit et al. / Journal of Non-Crystalline Solids 338–340 (2004) 86–90
silicon nanocrystals are responsible for the growth of microcrystalline films free from any amorphous phase.
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