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1.11 Silicon films growth in vacuum by pyrolysis of silane
1. Vacuum and thin films pumps
coating processes and
1.11 Silicon films growth in vacuum by pyrolysis of silane L N Alexandrov, F L Edelman and V V V o s k o b o i n i k o v , Institute of Semi-conductor Physics of the Academy of Sciences,
Novosibirsk, 630090, USSR.
The characteristics of growth of polysilicon films on silicon oxide or nitride substrates are discussed. Operative mechanisms of growth are studied, and the epitaxial formation of polysilicon film is given for comparison. The comparison of the observed rates of grain growth with the calculated ones and distribution of grains over sizes and on a surface indicate the action of diffusion processes and processes of building-in of atoms into a lattice. It is shown that recrystallisation growth of grains in the film influence considerably their finite sizes.
1. Introduction Polysilicon films are promising material for electrical engineering. These are used as separating insulation in large integrated circuits and as resistive elements. There are attempts to produce Polysil MIS-transistors, solar batteries, piezoelectric transducers. However, now and in the nearest future Polysil will be used on a large scale as a conductive gate electrode of MIStransistor with MNOS-and MOS-structure as well as in devices with charge coupling. There are several preconditions: (a) the possibility of deposition of Polysil in a united technological cycle with silicon surface decontamination and growth of silicon (or without it), and with deposition of a dielectric; (b) possibility to decrease the stray capacitance and to increase the density of elements of the large integrated circuit and the circuit speed; (c) reduction of threshold voltage of transistors. Figure 1 shows the scheme of the effective three-phase charge coupling device (CCD) on silicon using three-layer metallization with Polysil. The use of Polysil for integrated circuits, especially in the case of multilayer metallization, inevitably requires the control and reproducibility of a shallow developed film texture. In this connection it is necessary to observe a gas phase composition and Polysil grain sizes in a wide temperature range and after repeated heating of films. It is clear that deposition of Polysil on dielectric in the pyrolysis of silane has features similar to those of epitaxial growth in the case of homogeneous origin and different in the case of heterogeneous origin, The purposes of this work are as follows: (1) to find out microscopic phenomena at Polysil formation and after film annealing; (2) estimation of such an important parameter as thickness of a continuous Polysil; (3) comparison of origin of Polysil on an amorphous substrate with that of monosilicon epitaxial film. Vacuum/volume 27/number 3.
Figure I. Schematic diagram of the three-phase charge coupling device on silicon with three-layer metallization on Polysil.
2. Method of obtaining of Polysil The present paper offers mainly (below we shall make reservations illustrating exceptions) the results of deposition of Polysil in a low pressure (10-1-10 -2 torr) reactor which is heated from outside. For substrates we used silicon plates of 0.2 mm thickness and 3 5 4 0 mm diameter which were first chemically treated in a standard way and coated with silicon nitride of 0.1/~m thickness, obtained by ammonolysis of silane at 850-900°C (in particular cases with a silicon oxide layer). The schematic diagram of the reactor and some details of the processes were given before. 1-4 The deposition of Polysil on silicon nitride is one of the most
PergamonPress/Printed in Great Britain
145
L N Alexandrov, F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
widely used operations in microelectronics. It is the process that is carried out in obtaining MNOS-transistors with selfadjusting gate. In CCD IC Polysil is more often deposited on thermal silicon dioxide. We have studied this process in detail and later we shall publish the results. Pyrolysis of silane seems to be a simple reaction. Its equation with no intermediate stages is S i l l 4 ~ 2H2 + Si (Polysil)
(1)
However a quantitative description of the results of pyrolysis is difficult due to imperfect knowledge of a number of secondary factors. For example, in many cases an elementary mechanism of decay of silane is unclear (homogeneous or heterogeneous, with formation of intermediate products, such as SizH6, or without it, and so on). On that ground, in some works the activation energy of decay of silane, which determines the temperature dependence of the rate of Polysil thickness increase (Vp), varies from 10 to 37 and even up to 52 kcal tool -1 (0.52.5 eV). It means that the spread in values can reach almost eight orders of magnitude at various, sometimes badly controllable, conditions of silane decomposition. Of course, these are extreme cases. In reports on heterogeneous silane decomposition the activation energy for Vp(t) is often given at about 0.9-1.6 eV, and in some not numerous investigations on homogeneous decomposition 2-2.4 eV. Some new features of silane decomposition reaction and of silicon growth have been noted in one of the last works on pyrolytic deposition of silicon on a silicon substrate, cleaned from oxygen and carbon at the moment of deposition of a layer in high vacuum. Thus, as it turned out V v ,-~ PsiH," It proved that Si atoms impinging on the substrate re-evaporate considerably, and what is more, a greater part of the atoms reevaporate. As with epitaxy, the film growth is carried out in high vacuum with the aid of a known sequence of steps, the temperature dependence of the motion speed is exponential with an activation energy, which is characteristic of surface diffusion of silicon on silicon, Fsd = 1.5 eV. As can be seen from Farrow, 5 according to the data of different authors, Fsa is measured from 0.2 to 2.5 eV in this case. It is the motion of steps but not the silane decomposition, which determines the rate of growth in the case described (activation energy of the silane decomposition is estimated as 0.7 eV). Ref 6 offers the analysis of the pyrolysis of silane in argon during the deposition of Polysil on the oxidized silicon surface. Of course, under these conditions the silane decomposition rate will be greater and, perhaps, the activation energy in the temperature function of the growth rate will differ from the results obtained at pyrolysis of silane in a mixture with hydrogen. According to the data of Seto 6 the stage determining Polysil growth is the decomposition of silane chemisorbed on the substrate. The activation energy of the Polysil deposition rate is 1 eV. In general, according to a great number of publications the values of the maximum growth rate of about 1-10 tzm m i n - 1 are characteristic of both epitaxial silicon growth and Polysil deposition on an insulating substrate. These values are achieved at high silane concentration and in the range of deposition temperatures of 1000-1200°C.
3. Initial stage of film deposition At the deposition of silicon films on silicon in vacuum Joyce et al 7 observed the formation of three-dimensional nuclei at 146
a contamination of the substrate surface by molecules of O or C in the amount of more than 0.01 of a monolayer. The heating temperature of substrates increased up to 1200°C, the nuclei width-to-height ratio grew, and in 10 min the nuclei became two-dimensional. The use of a getter s and the increase of annealing temperature up to 1300°C 9 permitted to clean the silicon surface from carbide particles, to facilitate the motion of steps at the film growth. The system of steps on the substrate surface added Si atoms, moved, reconstructed and straightened them (Figure 2). The step height was 10-30 A,
Figure 2. A non-stationary surface relief of the homoepitaxial silicon film at 800°C; an effective film thickness is 50 A, growth time is 10 s.
the distance between steps was 0.1 t~m. No pits or growth islands were observed on the films with an effective thickness of more than 25 A, thus the film was continuous. The surface film relief was stabilized at a thickness of hundreds of Angstr6ms, but further optimization of conditions of substrate surface by cleaning and of the angle of its deviation from the (111) plane permits, in principle, to provide a nucleus-free layer growth with stable surface relief at all stages of the film growth. In this case of homoepitaxy, the substrate is fully overgrown with layer growth at a thickness of the order of the height of the initial surface steps. On the atomically clean step-by-step surfaces of silicon, the growth of film, even from another substance, e.g. germanium, can occur with no formation of three-dimensional nuclei from dihedral and trihedral angles of steps by formation of substrate-oriented pseudomorphic coherently conjugate regions, j° At large supersaturations, the work of formation of critical nuclei decreases, the role of point defects of the substrate surface as active centres of atom accumulation grows, and
N Alexandrov. F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
orienting action of the substrate becomes weak. The disorientation of islands apparently causes the formation of a polycrystalline continuous film at the coalescence of islands.~ 1 On amorphous and polycrystalline layers coating the silicon surface, such as silicon nitride or oxide, the polycrystal films forms at any supersaturation) -'~ At the transition from a clean silicon surface to that coated with the oxide or nitride film and, all the more, to the silicon nitride or oxide surface, an increase of the surface energy of the silicon nucleussubstrate interface changes the conditions of formation and growth of islands-grains. Therefore of interest are sufficiently reliable estimates of kinetic and thermodynamic parameters of growth which determine the distribution of islands over sizes at the moment of their coalescence into a continuous layer as well as the kinetics of recrystallization of continuous grain growth. For the homoepitaxial film growth in high vacuum, we calculated an activation energy of surface self-diffusion of silicon atoms, lifetime in an adsorbed state, and time of building-in, including diffusion towards the step, at 840°C to be 6 × 10 -4 and 1 × 10 - 6 S, respectively) At the silicon island growth on a non-orienting substrate, the participation of diffusion was shown by the analysis of a correlation function of distribution of islands on the substrate surface and distribution of distances between islands. 4 The distribution of distances differed from the normal one
0(D)=
( o,22,V
al~/2-------~ exp -
(2)
where D~ determines the deviation of distances between grains l from the mean one 1; ot is the dispersion. At small distances (400-600 A,) nuclei were not always observed, the distribution shifted towards large distances. Each nucleus was surrounded by a region whose dimensions corresponded to a diffusion wave.
Adatoms diffuse from these regions to islands, decreasing supersaturation and eliminating the possibility of formation of new nuclei in a depletion region. At comparable temperatures (800-840°C) the diffusion length of adatoms on the silicon nitride surface was 4-5 times smaller than that on silicon; it corresponds to a relative increase of an adsorption activation energy, as compared with adatom diffusion activation energy, by 0.2-0.4 eV. Figure 3 shows the Polysil microstructure at the initial stage of growth at 850°C on the silicon nitride surface (a) and the histogram (b) of grain sizes after 6 s of film growth. The Polysil structure and the histograms of grain sizes, which form at the same conditions of deposition on the silicon dioxide surface, are given in Figure 4. It is seen that the film texture on silicon dioxide is rougher, the grain height is higher.
4. Formation of continuous Polysil layer At the deposition of silicon on the silicon oxide or nitride surface, the grain-nuclei have a quasispherical form, the rate of their formation (J) varies from 1 0 7 t o 1 0 9 c m - 2 S - 1 . The rate of growth of grains within the deposition temperature interval of 750-1100°C varies from 1 to 4 nm s - 1. The formation of a continuous layer lasts 40-1 s at a thickness of 40--4 nm, respectively. One can use two approaches when estimating the continuous layer thickness. If the rate of growth of island-nuclei is limited by adatom diffusion on the island surface and decreases with an increase of the size according to the parabolic law, then the critical thickness of the continuous film is determined from the dependence (3)
d m = Kd(Ds/J) 1/4
where D, is the surface diffusion coefficient of adatoms, the kinetic coefficient being Kd ~ 1.
50
N
3o'
IO 1 7OO
(a)
2100
35O0
(b)
Figure 3. The Polysil microstructure at the initial stage of growth at 850°C on the surface of silicon nitride (a), and the histogram of grain sizes (b). Time of grain growth is 6 s. 147
L IVAlexandrov, F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
500
/
i
I
(a)
I OI D,
02 /.Lm
(b)
Figure 4. The Polysil microstructure at the initial stage of growth at 850°C on the surface of thermal silicon dioxide (a), and the histogram of grain sizes (b). Time of grain growth is 6 s.
If the rate of growth of islands is determined by the time of building-in of adatoms, it is independent of island sizes and does n o t change in the course of growth. The rate of island growth can be considered, in the first approximation, to be constant at the small interval of its change. In this case, allowing for the difference between tangential lit and normal I/", components of the island growth rate, we have for arm
dm = gk - ~ t
(4)
where Kb ~ 0.8. A good agreement of the values calculated from (4) with the experimental ones is achieved at V,/V, ~ 2-4. The similar connection was observed with epitaxial growth, and this relation increased depending on the surface cleanliness. As for the dependence (3), the estimation of the diffusion coefficient D~ over this range under the assumption of a limiting role of diffusion in the nuclei growth (D~ ~ d,, 4 J Kd-4) yields evidently too low values; with an increase of the deposition temperature the calculated value decreases, which shows that it is impossible to use this scheme of calculation. Thus, one should consider the building-in of silicon atoms into islands to be a limiting stage of growth. If 10 -6 s is the building-in time, and assuming a maximum possible number of active places, corresponding to the product of a number of nuclei N = Jt 10a -- 109 cm -2 by a number of atoms in the island perimeter, the growth rate is determined as D,/No 106 N~D/TrD= 10-100 n m s -1 for islands of 10 nm diameter. The rate realized in the experiments is somewhat less, which is associated 148
with a smaller number of active growth points of islands. This difference increases for large grains, but the conclusion about a determinative action of the atom building-in under the growth conditions studied is confirmed, in essence. The delay in supply of atoms caused by diffusion processes appears to tell something about less perfect parts of the growth surface.
5. Recrystallization changes in grain sizes and finite Polysil structure Enlargement of grains in the polysilicon films occurs due to accumulative recrystallization in the grown continuous film during the time of further growth. Investigations of the temperature dependence of the rate of recrystallization growth of grains4 yielded the values for an activation energy of 1.2-1.7 eV, which corresponds to both the activation energy of the surface (interface) self-diffusion in silicon and the bond energy per atom. The growth of individual grains at the expense of neighbouring ones occurs via the transition of atoms from one grain to another, overcoming the interface potential barrier, however the supply of silicon atoms is facilitated by their diffusion on the grain interfaces. The limiting grain sizes corresponded to the film thickness.ll Special prolonged annealings (30 min and 2 h at 1350°C) have shown that further growth of individual grains proved to be possible due to film decomposition. Grains reach 8-15 tzm in length and film free regions appear on the substrate surface. The change of free energy of grain interfaces is a moving force of the reerystallization growth of grains. This change is far less than that of the free energy during deposition,4
L NAlexandrov, F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
Figure 5. The Polysil microstructure on silicon nitride used as a MIS-transistor gate.
however the rates of grain growth at recrystallization exceed these during deposition. At 1000°C the grain size for the deposition time varied from 0.2 to 1.6 ~m; it corresponded to the growth rate of 0.5/~m rain- 1, but at deposition this growth was only 0.16 ~m rain = 1. Here a considerably high density of an atomic flow at recrystallization, as compared with a flow of adsorbed atoms, takes place on the grain surfaces. The characteristic microstructure of Polysil on silicon nitride used as a MIS-transistor gate is shown in Figure 5. The results of electron-diffraction examinations show that polysilicon films are textured as a rule, The texture axis is 110. However the causes of texturing are unclear, and the formation of texture is not associated with recrystallization, since often textures arose at the early stage of film growth before coalescence of individual grains into a continuous layer. At the deposition of films in a outside reactor heated from outside, textures appear more often than in the case of induction heating of the substrate on a graphite pedestal. The most important factor, which complicates the reproducibility of obtaining and the successful use of Polysil in MIS gates and metallization, is imperfect knowledge of microscopic processes following film doping. The grain interfaces gives a large contribution to a specific resistance, reaching 10 6 to 108 ohm cm (at Polysil thickness of 1 ~m it corresponds to 10T . 1 0 4 = 1011 ohm/D). In order to lower the resistance down to a value acceptable for metallization of 102 ohm/[] at deposition the Polysil is doped from the gas phase (or other ways) by up to a concentration of 1019-1021 atoms cm -3, i.e. at the limit of solubility. In this connection, there is a danger of precipitation of phase-compounds with silicon in the doped Polysil. The most undesirable are boron-silicon phases insoluble in usual etches which remain on the plate surface after photolithography on Polysil and are detrimental to further treatment (diffusion, metallization by aluminium and so on). The investigation of the phase composition of these insoluble deposits 2 has showed the presence of SiB4, SiB6 and elementary boron (Figure 6).
Figure 6. The deposit microstructure after etching of Polysil.
Conclusion The investigations of growth and recrystallization of silicon films on surfaces of silicon nitride, silicon dioxide and silicon have shown that the rate of film growth is sensitive to an initial state of the substrate surface and growth conditions in vacuum. In particular, we have found some peculiarities of surface diffusion and of building-in of adatoms during the growth of grain-nuclei. Phenomena of accumulative recrystallization are essential at the initial stage of growth and after formation of a continuous layer. Of special interest are investigations of peculiarities of Polysil origin and mechanisms of growth on the surface of silicon dioxide (pyrolytic and thermal), mainly in the connection with problems of multilevel metallization and of CCD-structures.
References i F L Edelman, V V Voskoboinikovand V E Latuta, Mikroelektronika 3, 1974, 418. 2 F L Edelman, Elektron tekhnika Ser Materlaly, 8, 1974, 102. 149
L NAlexandrov, F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
a F L Edelman, V V Voskoboinikov, V V Smirnov and V V Pavlov,
Mikroelektronika, 3, 1974, 554. 4 L N A1exandrov, F L Edelman and V V Voskoboinikov, Thin Solid Films, 32, 1975, 241. 5 R F C Farrow, JElectrochem Soc, 121, 1974, 899. 6 j Seto, J Electrochem Soc, 122, 1975, 701. B A Joyce, J N Neave and B E Watts, SurfSci, 15, 1969, 1.
150
s L N Alexandrov and R N Lovyagin, Thin Solid Films, 20, 1974, 1. 9 L N Alexandrov and R N Lovyagin, Japan J Appl Phys, Suppl 2, 1974, 609. 1o L N Alexandrov, R N Lovyagin, O P Pchelaykov and S I Stenin, J Cryst Growth, 24/25, 1974, 298. 11 L N Alexandrov, Kinetika Obrazovaniya i Struktura Tvoyrdykh Sloyov, Nauka, Novosibirsk (1972).
coating processes and
1.11 Silicon films growth in vacuum by pyrolysis of silane L N Alexandrov, F L Edelman and V V V o s k o b o i n i k o v , Institute of Semi-conductor Physics of the Academy of Sciences,
Novosibirsk, 630090, USSR.
The characteristics of growth of polysilicon films on silicon oxide or nitride substrates are discussed. Operative mechanisms of growth are studied, and the epitaxial formation of polysilicon film is given for comparison. The comparison of the observed rates of grain growth with the calculated ones and distribution of grains over sizes and on a surface indicate the action of diffusion processes and processes of building-in of atoms into a lattice. It is shown that recrystallisation growth of grains in the film influence considerably their finite sizes.
1. Introduction Polysilicon films are promising material for electrical engineering. These are used as separating insulation in large integrated circuits and as resistive elements. There are attempts to produce Polysil MIS-transistors, solar batteries, piezoelectric transducers. However, now and in the nearest future Polysil will be used on a large scale as a conductive gate electrode of MIStransistor with MNOS-and MOS-structure as well as in devices with charge coupling. There are several preconditions: (a) the possibility of deposition of Polysil in a united technological cycle with silicon surface decontamination and growth of silicon (or without it), and with deposition of a dielectric; (b) possibility to decrease the stray capacitance and to increase the density of elements of the large integrated circuit and the circuit speed; (c) reduction of threshold voltage of transistors. Figure 1 shows the scheme of the effective three-phase charge coupling device (CCD) on silicon using three-layer metallization with Polysil. The use of Polysil for integrated circuits, especially in the case of multilayer metallization, inevitably requires the control and reproducibility of a shallow developed film texture. In this connection it is necessary to observe a gas phase composition and Polysil grain sizes in a wide temperature range and after repeated heating of films. It is clear that deposition of Polysil on dielectric in the pyrolysis of silane has features similar to those of epitaxial growth in the case of homogeneous origin and different in the case of heterogeneous origin, The purposes of this work are as follows: (1) to find out microscopic phenomena at Polysil formation and after film annealing; (2) estimation of such an important parameter as thickness of a continuous Polysil; (3) comparison of origin of Polysil on an amorphous substrate with that of monosilicon epitaxial film. Vacuum/volume 27/number 3.
Figure I. Schematic diagram of the three-phase charge coupling device on silicon with three-layer metallization on Polysil.
2. Method of obtaining of Polysil The present paper offers mainly (below we shall make reservations illustrating exceptions) the results of deposition of Polysil in a low pressure (10-1-10 -2 torr) reactor which is heated from outside. For substrates we used silicon plates of 0.2 mm thickness and 3 5 4 0 mm diameter which were first chemically treated in a standard way and coated with silicon nitride of 0.1/~m thickness, obtained by ammonolysis of silane at 850-900°C (in particular cases with a silicon oxide layer). The schematic diagram of the reactor and some details of the processes were given before. 1-4 The deposition of Polysil on silicon nitride is one of the most
PergamonPress/Printed in Great Britain
145
L N Alexandrov, F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
widely used operations in microelectronics. It is the process that is carried out in obtaining MNOS-transistors with selfadjusting gate. In CCD IC Polysil is more often deposited on thermal silicon dioxide. We have studied this process in detail and later we shall publish the results. Pyrolysis of silane seems to be a simple reaction. Its equation with no intermediate stages is S i l l 4 ~ 2H2 + Si (Polysil)
(1)
However a quantitative description of the results of pyrolysis is difficult due to imperfect knowledge of a number of secondary factors. For example, in many cases an elementary mechanism of decay of silane is unclear (homogeneous or heterogeneous, with formation of intermediate products, such as SizH6, or without it, and so on). On that ground, in some works the activation energy of decay of silane, which determines the temperature dependence of the rate of Polysil thickness increase (Vp), varies from 10 to 37 and even up to 52 kcal tool -1 (0.52.5 eV). It means that the spread in values can reach almost eight orders of magnitude at various, sometimes badly controllable, conditions of silane decomposition. Of course, these are extreme cases. In reports on heterogeneous silane decomposition the activation energy for Vp(t) is often given at about 0.9-1.6 eV, and in some not numerous investigations on homogeneous decomposition 2-2.4 eV. Some new features of silane decomposition reaction and of silicon growth have been noted in one of the last works on pyrolytic deposition of silicon on a silicon substrate, cleaned from oxygen and carbon at the moment of deposition of a layer in high vacuum. Thus, as it turned out V v ,-~ PsiH," It proved that Si atoms impinging on the substrate re-evaporate considerably, and what is more, a greater part of the atoms reevaporate. As with epitaxy, the film growth is carried out in high vacuum with the aid of a known sequence of steps, the temperature dependence of the motion speed is exponential with an activation energy, which is characteristic of surface diffusion of silicon on silicon, Fsd = 1.5 eV. As can be seen from Farrow, 5 according to the data of different authors, Fsa is measured from 0.2 to 2.5 eV in this case. It is the motion of steps but not the silane decomposition, which determines the rate of growth in the case described (activation energy of the silane decomposition is estimated as 0.7 eV). Ref 6 offers the analysis of the pyrolysis of silane in argon during the deposition of Polysil on the oxidized silicon surface. Of course, under these conditions the silane decomposition rate will be greater and, perhaps, the activation energy in the temperature function of the growth rate will differ from the results obtained at pyrolysis of silane in a mixture with hydrogen. According to the data of Seto 6 the stage determining Polysil growth is the decomposition of silane chemisorbed on the substrate. The activation energy of the Polysil deposition rate is 1 eV. In general, according to a great number of publications the values of the maximum growth rate of about 1-10 tzm m i n - 1 are characteristic of both epitaxial silicon growth and Polysil deposition on an insulating substrate. These values are achieved at high silane concentration and in the range of deposition temperatures of 1000-1200°C.
3. Initial stage of film deposition At the deposition of silicon films on silicon in vacuum Joyce et al 7 observed the formation of three-dimensional nuclei at 146
a contamination of the substrate surface by molecules of O or C in the amount of more than 0.01 of a monolayer. The heating temperature of substrates increased up to 1200°C, the nuclei width-to-height ratio grew, and in 10 min the nuclei became two-dimensional. The use of a getter s and the increase of annealing temperature up to 1300°C 9 permitted to clean the silicon surface from carbide particles, to facilitate the motion of steps at the film growth. The system of steps on the substrate surface added Si atoms, moved, reconstructed and straightened them (Figure 2). The step height was 10-30 A,
Figure 2. A non-stationary surface relief of the homoepitaxial silicon film at 800°C; an effective film thickness is 50 A, growth time is 10 s.
the distance between steps was 0.1 t~m. No pits or growth islands were observed on the films with an effective thickness of more than 25 A, thus the film was continuous. The surface film relief was stabilized at a thickness of hundreds of Angstr6ms, but further optimization of conditions of substrate surface by cleaning and of the angle of its deviation from the (111) plane permits, in principle, to provide a nucleus-free layer growth with stable surface relief at all stages of the film growth. In this case of homoepitaxy, the substrate is fully overgrown with layer growth at a thickness of the order of the height of the initial surface steps. On the atomically clean step-by-step surfaces of silicon, the growth of film, even from another substance, e.g. germanium, can occur with no formation of three-dimensional nuclei from dihedral and trihedral angles of steps by formation of substrate-oriented pseudomorphic coherently conjugate regions, j° At large supersaturations, the work of formation of critical nuclei decreases, the role of point defects of the substrate surface as active centres of atom accumulation grows, and
N Alexandrov. F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
orienting action of the substrate becomes weak. The disorientation of islands apparently causes the formation of a polycrystalline continuous film at the coalescence of islands.~ 1 On amorphous and polycrystalline layers coating the silicon surface, such as silicon nitride or oxide, the polycrystal films forms at any supersaturation) -'~ At the transition from a clean silicon surface to that coated with the oxide or nitride film and, all the more, to the silicon nitride or oxide surface, an increase of the surface energy of the silicon nucleussubstrate interface changes the conditions of formation and growth of islands-grains. Therefore of interest are sufficiently reliable estimates of kinetic and thermodynamic parameters of growth which determine the distribution of islands over sizes at the moment of their coalescence into a continuous layer as well as the kinetics of recrystallization of continuous grain growth. For the homoepitaxial film growth in high vacuum, we calculated an activation energy of surface self-diffusion of silicon atoms, lifetime in an adsorbed state, and time of building-in, including diffusion towards the step, at 840°C to be 6 × 10 -4 and 1 × 10 - 6 S, respectively) At the silicon island growth on a non-orienting substrate, the participation of diffusion was shown by the analysis of a correlation function of distribution of islands on the substrate surface and distribution of distances between islands. 4 The distribution of distances differed from the normal one
0(D)=
( o,22,V
al~/2-------~ exp -
(2)
where D~ determines the deviation of distances between grains l from the mean one 1; ot is the dispersion. At small distances (400-600 A,) nuclei were not always observed, the distribution shifted towards large distances. Each nucleus was surrounded by a region whose dimensions corresponded to a diffusion wave.
Adatoms diffuse from these regions to islands, decreasing supersaturation and eliminating the possibility of formation of new nuclei in a depletion region. At comparable temperatures (800-840°C) the diffusion length of adatoms on the silicon nitride surface was 4-5 times smaller than that on silicon; it corresponds to a relative increase of an adsorption activation energy, as compared with adatom diffusion activation energy, by 0.2-0.4 eV. Figure 3 shows the Polysil microstructure at the initial stage of growth at 850°C on the silicon nitride surface (a) and the histogram (b) of grain sizes after 6 s of film growth. The Polysil structure and the histograms of grain sizes, which form at the same conditions of deposition on the silicon dioxide surface, are given in Figure 4. It is seen that the film texture on silicon dioxide is rougher, the grain height is higher.
4. Formation of continuous Polysil layer At the deposition of silicon on the silicon oxide or nitride surface, the grain-nuclei have a quasispherical form, the rate of their formation (J) varies from 1 0 7 t o 1 0 9 c m - 2 S - 1 . The rate of growth of grains within the deposition temperature interval of 750-1100°C varies from 1 to 4 nm s - 1. The formation of a continuous layer lasts 40-1 s at a thickness of 40--4 nm, respectively. One can use two approaches when estimating the continuous layer thickness. If the rate of growth of island-nuclei is limited by adatom diffusion on the island surface and decreases with an increase of the size according to the parabolic law, then the critical thickness of the continuous film is determined from the dependence (3)
d m = Kd(Ds/J) 1/4
where D, is the surface diffusion coefficient of adatoms, the kinetic coefficient being Kd ~ 1.
50
N
3o'
IO 1 7OO
(a)
2100
35O0
(b)
Figure 3. The Polysil microstructure at the initial stage of growth at 850°C on the surface of silicon nitride (a), and the histogram of grain sizes (b). Time of grain growth is 6 s. 147
L IVAlexandrov, F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
500
/
i
I
(a)
I OI D,
02 /.Lm
(b)
Figure 4. The Polysil microstructure at the initial stage of growth at 850°C on the surface of thermal silicon dioxide (a), and the histogram of grain sizes (b). Time of grain growth is 6 s.
If the rate of growth of islands is determined by the time of building-in of adatoms, it is independent of island sizes and does n o t change in the course of growth. The rate of island growth can be considered, in the first approximation, to be constant at the small interval of its change. In this case, allowing for the difference between tangential lit and normal I/", components of the island growth rate, we have for arm
dm = gk - ~ t
(4)
where Kb ~ 0.8. A good agreement of the values calculated from (4) with the experimental ones is achieved at V,/V, ~ 2-4. The similar connection was observed with epitaxial growth, and this relation increased depending on the surface cleanliness. As for the dependence (3), the estimation of the diffusion coefficient D~ over this range under the assumption of a limiting role of diffusion in the nuclei growth (D~ ~ d,, 4 J Kd-4) yields evidently too low values; with an increase of the deposition temperature the calculated value decreases, which shows that it is impossible to use this scheme of calculation. Thus, one should consider the building-in of silicon atoms into islands to be a limiting stage of growth. If 10 -6 s is the building-in time, and assuming a maximum possible number of active places, corresponding to the product of a number of nuclei N = Jt 10a -- 109 cm -2 by a number of atoms in the island perimeter, the growth rate is determined as D,/No 106 N~D/TrD= 10-100 n m s -1 for islands of 10 nm diameter. The rate realized in the experiments is somewhat less, which is associated 148
with a smaller number of active growth points of islands. This difference increases for large grains, but the conclusion about a determinative action of the atom building-in under the growth conditions studied is confirmed, in essence. The delay in supply of atoms caused by diffusion processes appears to tell something about less perfect parts of the growth surface.
5. Recrystallization changes in grain sizes and finite Polysil structure Enlargement of grains in the polysilicon films occurs due to accumulative recrystallization in the grown continuous film during the time of further growth. Investigations of the temperature dependence of the rate of recrystallization growth of grains4 yielded the values for an activation energy of 1.2-1.7 eV, which corresponds to both the activation energy of the surface (interface) self-diffusion in silicon and the bond energy per atom. The growth of individual grains at the expense of neighbouring ones occurs via the transition of atoms from one grain to another, overcoming the interface potential barrier, however the supply of silicon atoms is facilitated by their diffusion on the grain interfaces. The limiting grain sizes corresponded to the film thickness.ll Special prolonged annealings (30 min and 2 h at 1350°C) have shown that further growth of individual grains proved to be possible due to film decomposition. Grains reach 8-15 tzm in length and film free regions appear on the substrate surface. The change of free energy of grain interfaces is a moving force of the reerystallization growth of grains. This change is far less than that of the free energy during deposition,4
L NAlexandrov, F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
Figure 5. The Polysil microstructure on silicon nitride used as a MIS-transistor gate.
however the rates of grain growth at recrystallization exceed these during deposition. At 1000°C the grain size for the deposition time varied from 0.2 to 1.6 ~m; it corresponded to the growth rate of 0.5/~m rain- 1, but at deposition this growth was only 0.16 ~m rain = 1. Here a considerably high density of an atomic flow at recrystallization, as compared with a flow of adsorbed atoms, takes place on the grain surfaces. The characteristic microstructure of Polysil on silicon nitride used as a MIS-transistor gate is shown in Figure 5. The results of electron-diffraction examinations show that polysilicon films are textured as a rule, The texture axis is 110. However the causes of texturing are unclear, and the formation of texture is not associated with recrystallization, since often textures arose at the early stage of film growth before coalescence of individual grains into a continuous layer. At the deposition of films in a outside reactor heated from outside, textures appear more often than in the case of induction heating of the substrate on a graphite pedestal. The most important factor, which complicates the reproducibility of obtaining and the successful use of Polysil in MIS gates and metallization, is imperfect knowledge of microscopic processes following film doping. The grain interfaces gives a large contribution to a specific resistance, reaching 10 6 to 108 ohm cm (at Polysil thickness of 1 ~m it corresponds to 10T . 1 0 4 = 1011 ohm/D). In order to lower the resistance down to a value acceptable for metallization of 102 ohm/[] at deposition the Polysil is doped from the gas phase (or other ways) by up to a concentration of 1019-1021 atoms cm -3, i.e. at the limit of solubility. In this connection, there is a danger of precipitation of phase-compounds with silicon in the doped Polysil. The most undesirable are boron-silicon phases insoluble in usual etches which remain on the plate surface after photolithography on Polysil and are detrimental to further treatment (diffusion, metallization by aluminium and so on). The investigation of the phase composition of these insoluble deposits 2 has showed the presence of SiB4, SiB6 and elementary boron (Figure 6).
Figure 6. The deposit microstructure after etching of Polysil.
Conclusion The investigations of growth and recrystallization of silicon films on surfaces of silicon nitride, silicon dioxide and silicon have shown that the rate of film growth is sensitive to an initial state of the substrate surface and growth conditions in vacuum. In particular, we have found some peculiarities of surface diffusion and of building-in of adatoms during the growth of grain-nuclei. Phenomena of accumulative recrystallization are essential at the initial stage of growth and after formation of a continuous layer. Of special interest are investigations of peculiarities of Polysil origin and mechanisms of growth on the surface of silicon dioxide (pyrolytic and thermal), mainly in the connection with problems of multilevel metallization and of CCD-structures.
References i F L Edelman, V V Voskoboinikovand V E Latuta, Mikroelektronika 3, 1974, 418. 2 F L Edelman, Elektron tekhnika Ser Materlaly, 8, 1974, 102. 149
L NAlexandrov, F L Edelman and V V Voskoboinikov: Silicon film growth in vacuum by pyrolysis of silane
a F L Edelman, V V Voskoboinikov, V V Smirnov and V V Pavlov,
Mikroelektronika, 3, 1974, 554. 4 L N A1exandrov, F L Edelman and V V Voskoboinikov, Thin Solid Films, 32, 1975, 241. 5 R F C Farrow, JElectrochem Soc, 121, 1974, 899. 6 j Seto, J Electrochem Soc, 122, 1975, 701. B A Joyce, J N Neave and B E Watts, SurfSci, 15, 1969, 1.
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s L N Alexandrov and R N Lovyagin, Thin Solid Films, 20, 1974, 1. 9 L N Alexandrov and R N Lovyagin, Japan J Appl Phys, Suppl 2, 1974, 609. 1o L N Alexandrov, R N Lovyagin, O P Pchelaykov and S I Stenin, J Cryst Growth, 24/25, 1974, 298. 11 L N Alexandrov, Kinetika Obrazovaniya i Struktura Tvoyrdykh Sloyov, Nauka, Novosibirsk (1972).