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Computer simulation of ion beam enhanced deposition of silicon nitride films
182
Nuclear
Instruments
COMPUTER SIMULATION OF ION BEAM ENHANCED OF SILICON NITRIDE FILMS
and Methods
in Physics
Research B39 (1989) 182-184 North-Holland, Amsterdam
DEPOSITION
ZHOU Jiankun, CHEN Youshan, LIU Xianghuai and ZOU Shichang Ion BeamLaboratory, Shanghai Instiiute of ~etufi~r~,
Academia
Sinica, Shanghai ZOOOSU,China
A Monte Carlo computer simulation code SIBL has been developed to describe the growth of silicon nitride films by ion beam enhanced deposition (IBED). A successive and alternate process of deposition of silicon atoms and implantation of nitrogen ions is applied to replace the actual continuous and synchronous process of IBED. The change of the composition and density profiles during film growth is taken into account. The obtained composition profile and the relationship between the film composition and the atomic arrival ratio N/Si are in agreement with the experimental measurements.
Ion beam enhanced deposition (IBED) is a new technique for producing adherent surface coatings with flexibility of composition and property control. A large number of investigations on the physics and applications of IBED have been conducted [l-4]. In recent year, Miiller [5,6] has calculated the density of ZrO, and CeO, films formed by O+-assisted deposition in relation with the atomic arrival ratio (AAR) of implanted ions to deposited atoms and with the ion energy by means of computer simulation. Based on the law of mass conservation and taking account of sputtering yield in the stationary target, Donovan et al. [7] have evaluated the composition of IBED silicon nitride films. In this work a Monte Carlo computer simulation code is developed to describe the growth of IBED film as a concurrent process of implantation and deposition. This code has been applied to calculate the formation of silicon nitride films and the results are compared with the experiments.
2. Model of calculation When an ion strikes the surface of a thin film, it interacts with atoms in its path, suffering energy loss, inducing knock-on atoms which in turn induce additional knock-on atoms, and eventually is trapped or rejected. All these are accompanied by the condensing and stacking of evaporated atoms on surface of a target during IBED. But for modelhng and calculation, a successive and alternate process of deposition and implantation is used instead of the actual concurrent one. The surface region of the substrate to be implanted is preliminarily subdivided into slabs each of 20 A width. The substrate and the deposited layers are regarded as 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
random medium composed of pseudop~ticles, each of which is equivalent to 4 x lOi atoms, and the injecting beam is also composed of pseudoprojectiles of the same size. After deposition of the first layer, the calculation of the cascades induced by projectiles works until a dose of implantation is reached in accordance with the preset atomic arrival ratio of implantated ions to deposited atoms. Then another layer is deposited and calculation continues. At the end of implantation with a certain number of ions the composition and the thickness of the slabs and the deposited layers are adjusted to accommodate the vacancies and interstitials produced in collision cascades, holding the density in a reasonable limit. The primary density of deposited layers is assumed to be 80% of that of the bulk of deposited material in account of porous structure [5]. The thickness of deposited layers is set to a value to make the mimic process nearly the same as the actual one. Atomic N+ ions with half energy are used to replace the molecular N$ ions used in the experiments. The simulation code SIBL, developed from TRIMSP [S], is based on the assumption of a random target, fixed free path of moving particles and binary collisions. The injected ions as well as all knock-on atoms are followed until their energy falls below a preset value. The universal potential and electronic stopping by ZBL (1985) [9] are used in the calculation. The probability for various component to act as a collision partner with the moving particle is assumed to be proportional to their transient and local fractional density. The displacement energy E, for knock-on atoms is assumed to be 25 eV. The bulk binding energy E, = 2 eV is adopted. The cutoff energy of ions is set to 7 eV, whereas that of knock-on atoms is made equal to E, or surface binding energy according to their distance from the surface. The surface binding energy Es is an important parameter because of its direct and appreciable effect
Zhou Jiankun
et al. / Computer simulation
on the sputtering yield which in turn will determine the composition of the film. By reason of the existence of a layer enriched with deposited element at the top of the IBED films, which was indicated by the computer simulation and experimental measurments, the Es for deposited element during IBED process is assumed to be equal to that for stationary target of the same element. As in the case of silicon nitride formation, we determine the E, for silicon by fitting our simulation code to the experimental sputtering yields of silicon reported by Laegroid et al. [lo] and Rosenberg et al. [ll]. The resultant Es for silicon equals 2.3 eV, which plus E, is an approximation to the sublimation energy of silicon. Because of the same reason mentioned above, the sputtering yield of nitrogen is small compared with that of silicon, as an approximation, the Es for nitrogen is made equal to that for silicon.
3. Results and discussion The SIBL code was used to simulate the growth of silicon nitride films with the assistance of 1 keV nitrogen ions (N+ and N:) bombardment at incident angle of 20 ‘. The component ratio N/Si as a function of AAR obtained by computer simulation with SIBL code was compared with the experimental data and theoretical calculation given by Donovan et al. [7]. From fig. 1 it can be seen that the results of simulation fit experiment better than Donovan’s calculation because the sputtering yield during IBED is different from that for a stationary target. It seems that the method of mass conservation in combination with the sputtering yield for stationary target of deposited element is not quite suitable for evaluating the relationship between film
ofSi nitride films
of growth
183
0.8.
,
car10
-
Monte
. .
Experimental
Cal.
,’
*’ 0
I’ 0
0.4
0.2
Atomic
0.6 arrival
I."
0.8 rat10
(NISI)
Fig. 2. Composition ratio N/(N + Si) in film as a function arrival ratio N/Si at ion energy of 40 keV.
of
composition and AAR in the case of IBED with low energy ion bombardment, especially for high AAR values. Owing to the short projected range of obliquely injected nitrogen ions of low energy, most nitrogen ions are remained in the near-surface region during IBED processing. With the increase of AAR(N/Si), the sputtering yield of nitrogen is raised while that of silicon is lowered. Then, the SIBL code has been used to establish the relationship between the arrival ratio N/Si and the nitrogen incorporation fraction in silicon nitride films formed by IBED with 40 keV nitrogen ion bombardment at incident angle of 45” [12]. The theoretical relationship between film composition and AAR (N/Si) is in agreement with experiment, which is shown in fig. 2. It is noted that the sputtering yield of silicon almost does not change with AAR in this case and is nearly the same as in the stationary silicon target. The sputtering yield of nitrogen is always near zero during IBED
1.0 Etching
3
0
0.8 t
time
(mln)
40
60 I
P
5 ;
2” Si
0.b.
(Monte-Carlo)
7
I
Substrate
0
0.4 Atomic
0.8 arrival
1.2 rat10
1.6
(N/.51)
Fig 1. The incorporated N fraction N/(N + Si) versus atomic arrival ratio N/Si at an ion energy of 1 keV. 0 Experimental (Donovan et al.); - - - calculated by Donovan et al.; calculated by E.P. Donovan et al., supposing 20% additional Monte Carlo calculation; -. -. Monte neutral beam; Carlo calculation, considering 20% additional neutral beam.
.
Fig. 3. Depth profiles of N and Si measured by AES and calculated by Monte Carlo simulation. II. ION BEAM MIXING
Zhou Jiankun et al. / Computer simulation
184
of growh of Si nitride firms
4. Conclusions IBED
layer
,
Substrate
J1iJ”=o.7
(1) The developed simulation code SIBL is available to calculate the fundamental parameters of IBED to obtain films with desirable composition ratio and make the IBED process predictable. The calculated results are in good agreement with experiment in the case of synthesis of silicon nitride films. (2) The silicon nitride films formed by IBED consisted of at least four layers: the surface layer with enrichment of silicon, the layer with constant composition, the intermixed layer and the implanted layer.
---Energy deposition -Deposited Si Substrate Si
Depth
(61,
Fig. 4. Depth distribution of energy deposition (nuclear) and intermixing of deposited Si and substrate Si atoms at interface region.
processing
because
there
are
surface.
So, the method
few
nitrogen
atoms
near
IBED film composition by means of the sputtering yield of stationary target is applicable in this case. Fig. 3 shows the composite profile of a sample at AAR N/Si = 0.7. The coincidence of the calculated profile with the experimental results of AES measurements is satisfactory. This implies that the redistribution of components after cascade collision is neglegible. It can be seen that at the surface of the sample the concentration of silicon is high and the nitrogen concentration is low. Then the concentration of both components changes in opposite direction and the composition reaches a constant value at a depth of about 800 A. The interface transitional region can be further divided into a intermixed layer and an implanted layer, as shown in fig. 4. The thickness of the implanted layer is dependent on the energy of ions and is approximately equal to the projected range. The intermixed layer determines the adhesion between film and substrate. Fig. 4 also shows the profile of energy deposition due to nuclear stopping. Owing to the physical and chemical effect induced by energy deposition and atomic displacement, there would be more complicated layer structure in the interface region than that indicated by computer simulation. the
for
The authors would like to thank Dr. J.P. Biersack for placing TRIM at the authors’ disposal.
evaluating
References [l] T. Takagi, Thin Solid Films 92 (1982) 1. [2] Ion Bombardment Modifications of Surfaces, eds. 0. Auciello and R. Kelly (Elsevier, Amsterdam, 1984). [3] M.E. Harper, J.J. Cuomo, H.R. Kaufman, J. Vat. Sci. Technol. 21 (1982) 737. [4] 3. Martin, J. Mater. Sci. 21 (1986) 1. [5] K.H. Miiller, Appl. Phys. A40 (1986) 209. 161 K.H. Miiller, J. Appl. Phys. 59 (1986) 2803. [7] E.P. Donovan, D.R. Brighton, G.K. Hubler and D. Van Vechten, Nucl. Instr. and Meth. B19/20 (1987) 983. [8] J.P. Biersack and W. Eckstein, Appl. Phys. A34 (1984) 73. [9] The Stopping and Range of Ion in Solids, eds. J.F. Ziegler, J.P. Biersack and U. Littmark (Pergamon. New York, 1985). [IO] N. Laegreid and G.K. Wehner, J. Appl. Phys. 32 (1961) 365. (111 D. Rosenberg and G.K. Wehner, J. Appl. Phys. 33 (1962) 1842. [12] Liu Xianghuai, Xue Bin, Zheng Zhihong, Zhou Zuyao and Zou Shichang, these Proceedings (IBMM ‘88) Nucl. instr. and Meth. B39 (1989) 185.
Nuclear
Instruments
COMPUTER SIMULATION OF ION BEAM ENHANCED OF SILICON NITRIDE FILMS
and Methods
in Physics
Research B39 (1989) 182-184 North-Holland, Amsterdam
DEPOSITION
ZHOU Jiankun, CHEN Youshan, LIU Xianghuai and ZOU Shichang Ion BeamLaboratory, Shanghai Instiiute of ~etufi~r~,
Academia
Sinica, Shanghai ZOOOSU,China
A Monte Carlo computer simulation code SIBL has been developed to describe the growth of silicon nitride films by ion beam enhanced deposition (IBED). A successive and alternate process of deposition of silicon atoms and implantation of nitrogen ions is applied to replace the actual continuous and synchronous process of IBED. The change of the composition and density profiles during film growth is taken into account. The obtained composition profile and the relationship between the film composition and the atomic arrival ratio N/Si are in agreement with the experimental measurements.
Ion beam enhanced deposition (IBED) is a new technique for producing adherent surface coatings with flexibility of composition and property control. A large number of investigations on the physics and applications of IBED have been conducted [l-4]. In recent year, Miiller [5,6] has calculated the density of ZrO, and CeO, films formed by O+-assisted deposition in relation with the atomic arrival ratio (AAR) of implanted ions to deposited atoms and with the ion energy by means of computer simulation. Based on the law of mass conservation and taking account of sputtering yield in the stationary target, Donovan et al. [7] have evaluated the composition of IBED silicon nitride films. In this work a Monte Carlo computer simulation code is developed to describe the growth of IBED film as a concurrent process of implantation and deposition. This code has been applied to calculate the formation of silicon nitride films and the results are compared with the experiments.
2. Model of calculation When an ion strikes the surface of a thin film, it interacts with atoms in its path, suffering energy loss, inducing knock-on atoms which in turn induce additional knock-on atoms, and eventually is trapped or rejected. All these are accompanied by the condensing and stacking of evaporated atoms on surface of a target during IBED. But for modelhng and calculation, a successive and alternate process of deposition and implantation is used instead of the actual concurrent one. The surface region of the substrate to be implanted is preliminarily subdivided into slabs each of 20 A width. The substrate and the deposited layers are regarded as 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
random medium composed of pseudop~ticles, each of which is equivalent to 4 x lOi atoms, and the injecting beam is also composed of pseudoprojectiles of the same size. After deposition of the first layer, the calculation of the cascades induced by projectiles works until a dose of implantation is reached in accordance with the preset atomic arrival ratio of implantated ions to deposited atoms. Then another layer is deposited and calculation continues. At the end of implantation with a certain number of ions the composition and the thickness of the slabs and the deposited layers are adjusted to accommodate the vacancies and interstitials produced in collision cascades, holding the density in a reasonable limit. The primary density of deposited layers is assumed to be 80% of that of the bulk of deposited material in account of porous structure [5]. The thickness of deposited layers is set to a value to make the mimic process nearly the same as the actual one. Atomic N+ ions with half energy are used to replace the molecular N$ ions used in the experiments. The simulation code SIBL, developed from TRIMSP [S], is based on the assumption of a random target, fixed free path of moving particles and binary collisions. The injected ions as well as all knock-on atoms are followed until their energy falls below a preset value. The universal potential and electronic stopping by ZBL (1985) [9] are used in the calculation. The probability for various component to act as a collision partner with the moving particle is assumed to be proportional to their transient and local fractional density. The displacement energy E, for knock-on atoms is assumed to be 25 eV. The bulk binding energy E, = 2 eV is adopted. The cutoff energy of ions is set to 7 eV, whereas that of knock-on atoms is made equal to E, or surface binding energy according to their distance from the surface. The surface binding energy Es is an important parameter because of its direct and appreciable effect
Zhou Jiankun
et al. / Computer simulation
on the sputtering yield which in turn will determine the composition of the film. By reason of the existence of a layer enriched with deposited element at the top of the IBED films, which was indicated by the computer simulation and experimental measurments, the Es for deposited element during IBED process is assumed to be equal to that for stationary target of the same element. As in the case of silicon nitride formation, we determine the E, for silicon by fitting our simulation code to the experimental sputtering yields of silicon reported by Laegroid et al. [lo] and Rosenberg et al. [ll]. The resultant Es for silicon equals 2.3 eV, which plus E, is an approximation to the sublimation energy of silicon. Because of the same reason mentioned above, the sputtering yield of nitrogen is small compared with that of silicon, as an approximation, the Es for nitrogen is made equal to that for silicon.
3. Results and discussion The SIBL code was used to simulate the growth of silicon nitride films with the assistance of 1 keV nitrogen ions (N+ and N:) bombardment at incident angle of 20 ‘. The component ratio N/Si as a function of AAR obtained by computer simulation with SIBL code was compared with the experimental data and theoretical calculation given by Donovan et al. [7]. From fig. 1 it can be seen that the results of simulation fit experiment better than Donovan’s calculation because the sputtering yield during IBED is different from that for a stationary target. It seems that the method of mass conservation in combination with the sputtering yield for stationary target of deposited element is not quite suitable for evaluating the relationship between film
ofSi nitride films
of growth
183
0.8.
,
car10
-
Monte
. .
Experimental
Cal.
,’
*’ 0
I’ 0
0.4
0.2
Atomic
0.6 arrival
I."
0.8 rat10
(NISI)
Fig. 2. Composition ratio N/(N + Si) in film as a function arrival ratio N/Si at ion energy of 40 keV.
of
composition and AAR in the case of IBED with low energy ion bombardment, especially for high AAR values. Owing to the short projected range of obliquely injected nitrogen ions of low energy, most nitrogen ions are remained in the near-surface region during IBED processing. With the increase of AAR(N/Si), the sputtering yield of nitrogen is raised while that of silicon is lowered. Then, the SIBL code has been used to establish the relationship between the arrival ratio N/Si and the nitrogen incorporation fraction in silicon nitride films formed by IBED with 40 keV nitrogen ion bombardment at incident angle of 45” [12]. The theoretical relationship between film composition and AAR (N/Si) is in agreement with experiment, which is shown in fig. 2. It is noted that the sputtering yield of silicon almost does not change with AAR in this case and is nearly the same as in the stationary silicon target. The sputtering yield of nitrogen is always near zero during IBED
1.0 Etching
3
0
0.8 t
time
(mln)
40
60 I
P
5 ;
2” Si
0.b.
(Monte-Carlo)
7
I
Substrate
0
0.4 Atomic
0.8 arrival
1.2 rat10
1.6
(N/.51)
Fig 1. The incorporated N fraction N/(N + Si) versus atomic arrival ratio N/Si at an ion energy of 1 keV. 0 Experimental (Donovan et al.); - - - calculated by Donovan et al.; calculated by E.P. Donovan et al., supposing 20% additional Monte Carlo calculation; -. -. Monte neutral beam; Carlo calculation, considering 20% additional neutral beam.
.
Fig. 3. Depth profiles of N and Si measured by AES and calculated by Monte Carlo simulation. II. ION BEAM MIXING
Zhou Jiankun et al. / Computer simulation
184
of growh of Si nitride firms
4. Conclusions IBED
layer
,
Substrate
J1iJ”=o.7
(1) The developed simulation code SIBL is available to calculate the fundamental parameters of IBED to obtain films with desirable composition ratio and make the IBED process predictable. The calculated results are in good agreement with experiment in the case of synthesis of silicon nitride films. (2) The silicon nitride films formed by IBED consisted of at least four layers: the surface layer with enrichment of silicon, the layer with constant composition, the intermixed layer and the implanted layer.
---Energy deposition -Deposited Si Substrate Si
Depth
(61,
Fig. 4. Depth distribution of energy deposition (nuclear) and intermixing of deposited Si and substrate Si atoms at interface region.
processing
because
there
are
surface.
So, the method
few
nitrogen
atoms
near
IBED film composition by means of the sputtering yield of stationary target is applicable in this case. Fig. 3 shows the composite profile of a sample at AAR N/Si = 0.7. The coincidence of the calculated profile with the experimental results of AES measurements is satisfactory. This implies that the redistribution of components after cascade collision is neglegible. It can be seen that at the surface of the sample the concentration of silicon is high and the nitrogen concentration is low. Then the concentration of both components changes in opposite direction and the composition reaches a constant value at a depth of about 800 A. The interface transitional region can be further divided into a intermixed layer and an implanted layer, as shown in fig. 4. The thickness of the implanted layer is dependent on the energy of ions and is approximately equal to the projected range. The intermixed layer determines the adhesion between film and substrate. Fig. 4 also shows the profile of energy deposition due to nuclear stopping. Owing to the physical and chemical effect induced by energy deposition and atomic displacement, there would be more complicated layer structure in the interface region than that indicated by computer simulation. the
for
The authors would like to thank Dr. J.P. Biersack for placing TRIM at the authors’ disposal.
evaluating
References [l] T. Takagi, Thin Solid Films 92 (1982) 1. [2] Ion Bombardment Modifications of Surfaces, eds. 0. Auciello and R. Kelly (Elsevier, Amsterdam, 1984). [3] M.E. Harper, J.J. Cuomo, H.R. Kaufman, J. Vat. Sci. Technol. 21 (1982) 737. [4] 3. Martin, J. Mater. Sci. 21 (1986) 1. [5] K.H. Miiller, Appl. Phys. A40 (1986) 209. 161 K.H. Miiller, J. Appl. Phys. 59 (1986) 2803. [7] E.P. Donovan, D.R. Brighton, G.K. Hubler and D. Van Vechten, Nucl. Instr. and Meth. B19/20 (1987) 983. [8] J.P. Biersack and W. Eckstein, Appl. Phys. A34 (1984) 73. [9] The Stopping and Range of Ion in Solids, eds. J.F. Ziegler, J.P. Biersack and U. Littmark (Pergamon. New York, 1985). [IO] N. Laegreid and G.K. Wehner, J. Appl. Phys. 32 (1961) 365. (111 D. Rosenberg and G.K. Wehner, J. Appl. Phys. 33 (1962) 1842. [12] Liu Xianghuai, Xue Bin, Zheng Zhihong, Zhou Zuyao and Zou Shichang, these Proceedings (IBMM ‘88) Nucl. instr. and Meth. B39 (1989) 185.